the subretrofacial vasomotor nucleus: anatomical, chemical and pharmacological properties and role...

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Progress in Neurobiology Vol. 42, pp. 197-227, 1994 Copyright © 1994 Elsevier Science Ltd

Printed in Great Britain. All rights re,reed 0301-0082/94/$24.00

THE SUBRETROFACIAL VASOMOTOR NUCLEUS: ANATOMICAL, CHEMICAL AND PHARMACOLOGICAL PROPERTIES AND

ROLE IN CARDIOVASCULAR REGULATION

R. A. L. DAMPNEY Department of Physiology, University of Sydney, NSW 2006, Australia

CONTENTS I. Introduction 2. Definition of subretrofacial (SRF) nucleus

2.1. Cytoarchitecture 2.2. Spinal projections

3. Physiological properties of SRF neurons 3.1. Effects of stimulation

3. I.I. Specificity of action on cardiovascular effectors 3.1.2. Effects on regional vascular beds---viscvrotopic organization

3.2. Effects of inhibition 3.3. Role of SRF nucleus in cardiovascular reflexes 3.4. Respiratory modulation of activity of SRF neurons 3.5. Origin of spontaneous activity of SRF neurons

4. Chemical properties of SRF neurons 4.1. Comparison of catecholamine and non-catecholamine SRF neurons 4.2. Relationship between chemical properties and target vascular bed of SRF neurons 4.3. Identification of neurotransmitter(s)

5. Inputs to SRF neurons 5.1. Nuclei of origin 5.2. Putative neurotransmitters and neuromodulators

5.2.1. Glutamate 5.2.2. GABA 5.2.3. Acetylcholine 5.2.4. Opiates 5.2.5. Scrotonin 5.2.6. Angioteusin 5.2.7. Other peptides

5.3. Actions of antihypertvnsive drugs on SRF neurons 6. Pole of SRF nucleus in regulating cardiovascular changes associated with integrated responses

6.1. Defensive behavior 7. Concluding remarks

Acknowledgements References

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1. INTRODUCTION

Over a century ago, Dittmar demonstrated that bilateral destruction of a restricted portion of the ventral medulla, near the level of the facial nucleus, causes a profound fall in blood pressure (Dittmar, 1873). In the early part of this century, however, more attention was paid to the role of the dorsal medulla in cardiovascular regulation, when Ranson and Billingsley (1916) showed that electrical stimulation of sites on the floor of the floor ventricle could elicit large changes in blood pressure. Subsequently, with the introduction of the stereotaxic method, the entire brainstom was mapped for sites which elicited either increases or decreases in blood pressure. Presser responses could be elicited from sites which occupied a large part of the dorsolateral reticular formation in the rostral medulla and caudal pens, while depressor responses were evoked from sites located more

medially and caudally (Alexander, 1946; Wang and Ranson, 1939).

These classic observations led to the view, which is still put forward in some textbooks, that the central neurons controlling the sympathetic outflow to the heart and blood vessels are distributed diffusely throughout the dorsolateral reticular formation of the medulla and caudal pens. In the last 20 years, however, a large body of information gained from physiological, pharmacological and anatomical observations has led to a completely different view, which is that central cardiovascular neurons are organized into discrete groups. In particular, one of these cell groups, located within the rostrai part of the ventral medulla, is believed to be of crucial importance in the tonic and phasic control of the sympathetic vasomotor outflow.

A number of key observations led to the discovery

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of this cell group. In 1958, Loeschcke and Koepchen reported that injection of procaine into the lateral recesses of the fourth ventricle, such that it could diffuse to the ventral surface of the medulla, led to a profound fall in blood pressure as well as a de- pression of ventilation. No such effects were observed when procaine was applied directly to the dorsal surface. Subsequently, it was demonstrated that these effects on blood pressure and ventilation could be produced by cooling of a circumscribed area on the ventral surface of the medulla located rostromedial to the most rostral rootlet of the hypoglossal nerve (Schlfifke and Loeschcke, 1967).

These observations left unanswered the question as to whether the cardiovascular and respiratory effects of applying procaine or cold block to the ventral surface results from an action on cell bodies or fibers of passage. Studies by Feldberg and Guertzenstein and co-workers, however, established that application of the inhibitory amino acids ~,-aminobutyric acid (GABA) and glycine directly to the area on the ventral surface first described by Schl~ifke and Loeschcke (1967) led to a marked fall in blood pressure (Guertzenstein and Silver, 1974; Feldberg and Guertzenstein, 1976). Since GABA and glycine act on receptors that are confined to the cell bodies and dendritic processes of neurons (Curtis and John- ston, 1974), these observations demonstrated for the first time that the tonic activity of neuronal cell bodies located close to the ventral surface of the rostral medulla is essential for the maintenance of resting blood pressure, at least in the anesthetized cat.

The introduction of the method of retrograde transport of horseradish peroxidase for tracing neuronal connections within the central nervous system led to the next crucial observation concerning the role of the ventral medulla in cardiovascular regulation, when Amendt and co-workers (1978, 1979) identified a group of neurons within this region that project to the intermediolateral cell column in the thoracolumbar spinal cord of the cat. More importantly, most of these neurons were found to be located close to the ventral surface and particularly in an area that appeared to correspond closely to the so-called "glycine-sensitive area" as defined by Feldberg and Guertzenstein. This led Amendt et al. (1979) to propose that the cardiovascular effects evoked by application of drugs to the glycine-sensitive area may be mediated by sympathoexcitatory neurons within the ventral medulla that project directly to sympathetic preganglionic neurons in the spinal cord.

These seminal discoveries stimulated a period of intense research on the role of the ventral medulla in cardiovascular regulation. It has become clear that, at least in the cat and rabbit, the sympathoexcitatory neurons close to the glycine-sensitive area are concentrated in a discrete subregion of the rostral ventrolateral medulla (RVLM) which has been termed the subretrofacial (SRF) nucleus. Moreover, these neurons have highly characteristic functional, anatomical, chemical and receptor-binding properties, which shall be considered in detail in this review. First, I shall define the SRF nucleus according to its morphological and anatomical properties. The effects of stimulation of SRF neurons on cardiovascular and other effectors, and their role in cardiovascular reflexes, will

then be considered. This is followed by a discussion of the chemical properties of SRF cells anti whether they are chemically coded according to ttwir target vascular bed. The origin of afferent inputr; to SRF' neurons, the receptors that they act on and the functional significance of such inputs, is ~hen discussed. Finally, I shall consider the general question as to the role of the SRF nucleus in regulating cardiovascular changes associated with integrated somatomotor and autonomic responses, with particular reference to defensive behavior. Since tt~e SRF nucleus is best defined in the cat, particular attention will be paid to studies carried out in this species. Results obtained in other species, however, will also be included where appropriate.

Apart from the SRF nucleus, there are other groups of neurons in the ventral medulla that can also influence cardiovascular function, such as those located in the caudal ventrolateral medulla (Li and Blessing, 1990), the rostral ventromedial medulla (Cox and Brody, 1991) and the midline raphe (Mor- rison and Gebber, 1982). For a more general discussion of the role of ventral medullary neurons in cardiovascular regulation, the reader is referred to reviews by Ciriello et al. (1986b), Calaresu and Yardley (1988) and Guyenet (1990).

2. DEFINITION OF SUBRETROFACIAL (SRF) NUCLEUS

The method of microinjection of excitatory amino acids such as L-glutamate has been used in the cat, rabbit and rat to map the precise location of the region within the RVLM that contains sympathoexcitatory pressor neurons. These compounds excite cell bodies of neurons but do not affect fibers of passage (Zieglg~insberger and Puil, 1973; Goodchild et al., 1982). In the cat, significant increases in blood pressure are elicited by microinjection of L-glutamate into a narrow region located just ventral to the retrofacial nucleus, extending from the level of the caudal pole of the facial nucleus to the level approximately 1.5 mm more caudal (McAtlen and Dampney, 1989, 1990). In the rabbit, the pressor region in the rostral ventrolateral medulla is also located just ventral to the retrofacial nucleus, although it appears to extend further in the mediolateral direction, as compared to the cat (Dampney et al., 1985). Similarly, in the rat, Ruggiero et al. (1989) reported that the pressor region corresponds to a narrow longitudinal strip just ventral to the rostral part of the nucleus ambiguus (homologous to the retrofaciat nucleus of other species).

The method of glutamate microinjection has proven to be a useful method of defining the region within the rostral ventrolateral medulla which contains a relatively high density of pressor neurons. The main limitation of the method, however, is that any effects observed are likely to be the result of activation of a relatively large number of neurons, which may not all have uniform properties. In addition, it is difficult to determine the precise boundaries of the pressor region using this method, as pressor effects can result from diffusion of glutamate from the injection site to pressor neurons some distance away.

THE SUBRETROFACIAL VASOMOTOR NUCLEUS 199

Nevertheless, as shall be described in more detail below, the pressor region in the RVLM of the cat and rabbit, as mapped by glutamate microinjection, is remarkably consistent with the results of studies that have mapped putative sympathoexcitatory pressor cells in the RVLM according to their electrophysiological, anatomical and receptor-binding properties.

2.1. CYTOARCHITECTURE

In the cat, the pressor region in the RVLM corresponds very closely to a dense longitudinal column of cells, 0.2-0.5 mm wide and approximately 1.5 mm long, located ventral to and separate from the retrofacial nucleus (Poison et al., 1992). It is this column of cells that has been termed the subretrofaciai (SRF) nucleus. The SRF nucleus can be clearly seen in both sagittal (Fig. 1) and coronal (Fig. 2A) Nissl-stained sections through the rostral medulla. The nucleus is bounded ventrally by fiber tracts (Figs 1, 2A) and can be distinguished from the regions lateral and medial to it by the fact that these regions have a cell density between one quarter and one half of that within the column itself (Poison et al., 1992). The rostral boundary of the SRF nucleus has been defined as the level of the caudal pole of the facial nucleus, because rostral to this level the nucleus cannot be clearly separated from the surrounding reticular formation (Poison et al., 1992). Similarly, the caudal boundary of the SRF nucleus has been defined as the point where it can no longer be distinguished from the surrounding reticular formation and is approximately 0.5 mm rostral to the rostral pole of the lateral reticular nucleus (Poison et al., 1992).

The cell column corresponding to the SRF nucleus is not defined in the standard atlases of the cat brain by Taber (1961) and Berman (1968). In the atlas by Taber, it is part of the region defined as the nucleus paragigantocellularis lateralis. This latter nucleus, however, extends further rostrally and medially than the SRF nucleus as defined above (Taber, 1961). Moreover, as shall be discussed in detail below (see Section 3.1.1), the nucleus paragigantocellularis lateralis, unlike the SRF nucleus, also subserves various non-cardiovascular functions. In the atlas by Berman, the region corresponding to the SRF nucleus lies within a very extensive area referred to as the lateral tegmental field.

The cells within the SRF nucleus of the cat show considerable variation in both shape and size, although the majority are medium-sized (Fig. 2C), having an average diameter in the range 15-25 #m (Poison et al., 1992). Furthermore, there are also variations in the morphology of SRF cells according to their rostrocaudal position: those located in the rostral part of the nucleus tend to be larger than those located more caudally (Poison et al., 1992).

In other species, distinct cell groups corresponding to the pressor region in the RVLM cannot be so readily identified on the basis of their cytoarchitecture. In the rabbit, the pressor region lies within the region defined by Meesen and Olszewski (1949) as the nucleus reticularis lateralis, which encompasses an area bounded medially by the rostrai pole of the inferior olive, laterally by the spinal trigeminal nucleus and dorsally by the nucleus ambiguus and

retrofacial nucleus. The rostral and caudal borders of the nucleus reticularis lateralis are not clearly defined, but the entire extent of this nucleus in the rabbit is much greater than that of the pressor area in this species (Dampney et al., 1985). In the rat, the pressor neurons lie within a region termed the nucleus reticularis rostroventrolateralis (Ruggiero et al., 1989), which has a similar extent to the rostral part of the nucleus reticularis lateralis in the rabbit. As pointed out by Ruggiero et aL (1989), the nucleus reticularis rostroventrolateralis in the rat, like the equivalent region in the rabbit, has a relatively indistinct cytoarchitecture. Even so, Newman (1985), in his detailed morphological study of reticulospinal nuclei in the brain stem of the rat, described a "conspicuous cluster of medium neurons" just ventral to the retrofacial nucleus. It is therefore possible that this cell group is homologous to the SRF nucleus in the cat.

2.2. SPINAL PROJECTIONS

In the cat, the precise distribution of cells within the RVLM that project directly to the thoracic or upper lumbar segments of the spinal cord has been mapped, using the method of retrograde transport of horseradish peroxidase (HRP) or wheat germ agglutinin-conjugated HRP (Dampney et al., 1987b; Miura et al., 1983; Poison et al., 1992). As shown in Fig. 2B, a dense cluster of bulbospinal cells is located at all levels of the SRF nucleus, but not in the reticular region surrounding the SRF nucleus. Furthermore, a quantitative study has shown that approximately 50% of SRF cells project to a single segment, L3, of the cord (Poison et al., 1992). Since it is highly probable that many SRF cells project only to thoracic and/or the upper two lumbar segments of the cord, the total proportion of SRF cells that project to one or more segments of the thoracolumbar spinal cord must be considerably higher than 50% and may even be close to 100%.

The sites of termination of axons arising from SRF bulbospinai neurons has been studied using the method of anterograde transport of WGA-HRP. Injections of this tracer into the pressor region in the RVLM of the cat, centered on the SRF nucleus, resulted in labeling in the intermediolateral cell column and, to a lesser extent, the central autonomic area in both the thoracic and lumbar cords (Damp- ney et al., 1987a, Fig. 3). No labeling, however, could be detected in the dorsal or ventral horns. Since the intermediolateral cell column and central autonomic area contain the cell bodies of sympathetic preganglionic neurons (Cabot, 1990; Coote, 1988), these observations indicate that SRF bulbospinal neurons project directly to the origin of the sympathetic outflow to the heart and blood vessels.

In the rabbit, a dense group of neurons projecting to the thoracolumbar spinal cord has also been identified within the RVLM just ventral to the retrofacial nucleus (Dampney et al., 1982, 1985). Although this cell group extends further in the mediolateral direction than SRF bulbospinal neurons in the cat, they have a similar limited distribution in the rostrocaudal direction. Like SRF bulbospinal neurons in the cat, they are a quite separate group from other bulbospinal cells located more medially. Further-

200 R.A.L. DAMPNEY

more, as in the cat, there is a remarkable correlation between the location of this bulbospinal cell group and the pressor region in the RVLM of the rabbit, as mapped by L-glutamate microinjection (Dampney et al., 1982, 1985). It is therefore suggested that this group of cells in the rabbit is homologous to the SRF nucleus in the cat. Further evidence regarding this point is discussed in later Sections.

In the rat, RVLM neurons projecting to the sympathetic preganglionic nuclei do not form a circumscribed compact group as is the case in the cat or rabbit. In this species, neurons in the RVLM that project directly to spinal sympathetic neurons have been identified by the method of transneuronal retrograde transport of viruses, injected into the adrenal gland or sympathetic ganglia (Strack et al., 1989a,b; Wesselingh et al., 1989). Presympathetic neurons labeled in this way have a rather scattered distribution throughout the RVLM, although there is some functional evidence (see Section 3.2 below) that they have a relatively high concentration within a subregion equivalent to the SRF nucleus of the cat or rabbit. The differences between the three species, therefore, appear to be in the degree to which RVLM neurons projecting to sympathetic preganglionic nuclei are concentrated within a specific subregion.

3. PHYSIOLOGICAL PROPERTIES OF SRF NEURONS

3.1. EFFECTS OF STIMULATION

The increase in blood pressure elicited by excitation of SRF cells has been shown to be due to an activation of sympathetic vasoconstrictor nerves innervating blood vessels throughout the body, in skeletal muscle, skin and the renal and mesenteric vascular beds (Dampney et al., 1985; McAllen, 1986c; McAllen and Dampney, 1989). In addition, stimulation of the cells also causes an increase in heart rate and the release of adrenomedullary catecholamines (McAUen and Dampney, 1989).

3.1.1. Specificity o f action on cardiovascular effectors

Despite the widespread actions of SRF neurons on cardiovascular effectors, the same neurons do not appear to influence non-cardiovascular functions. In the cat, McAllen (1986c) made a thorough study of the range of non-cardiovascular sympathetic actions of SRF neurons. Excitation of these cells by highly localized mieroinjeetions of excitatory amino acids had no observable effect on the pupils and nictitating membrane and did not elicit piloerection or sudomo- tor responses. Only small and inconsistent effects on gut motility were observed, suggesting that motility- regulating neurons are located close to but separate from the SRF nucleus.

Similarly, both anatomical and functional studies indicate that respiratory neurons in the RVLM are located close to but separate from the SRF nucleus. Smith et aL (1989) found that respiratory neurons belonging to the B6tzinger complex, which were retrogradely labeled by WGA-HRP injected into the

caudal part of the ventral respiratory group, were located just dorsomedial to the SRF nucleus and in the retrofacial nucleus. Consistent with this, strong respiratory effects are evoked by microinjection of excitatory amino acids into this region but not by focal injections into the SRF nucleus itself (Dampney and McAllen, 1988; McAllen, 1986b). Finally, single unit recording studies have noted that neurons with respiratory rhythmicity are consistently located dorsal to neurons with electrophysiological properties indicative of cardiovascular neurons (Barman and Gebber, 1985; McAllen, 1986a).

A study has also been carried out to determine the relationship between cardiovascular and antinociceptive neurons within the RVLM of the cat. Micro- injection of excitatory amino acids elicited strong antinociceptive responses from sites located medial to the SRF nucleus but not usually from the SRF nucleus itself (Siddall and Dampney, 1989). Finally, a study by Kamiya et al. (1988) described neurons within the RVLM of the cat that receive inputs from medullary auditory and somatosensory nuclei. These neurons form a column, referred to by the authors as the ventrolateral medullary nucleus, which extends from the caudal pole of the facial nucleus to the level of the rostral one-third of the inferior olive. This column therefore has a very similar rostrocaudal extent to the SRF nucleus, but it is located more medially and closer to the ventral surface. Further- more, its pattern of afferent innervation is quite different to that of the SRF nucleus, which receives no input from medullary auditory and somatosensory nuclei (see Section 5.1 below).

Fewer studies have been made in the rabbit of the relationship between the SRF nucleus and neurons subserving non-cardiovascular functions. Neverthe- less, in this species, as in the cat, functional mapping studies have demonstrated that respiratory neurons are located close to but separate from the SRF nucleus (Dampney et al., 1985). In summary, then, the observations to date indicate that the SRF nucleus does not subserve any function apart from the regulation of the sympathetic output to the cardiovascular system. At the same time, they have emphasized the fact that cells surrounding the SRF nucleus subserve a variety of different functions and thus highlight the importance of defining the SRF nucleus as a distinct entity.

3.1.2. Effects on regional vascular beds--viscerotopic organization

There is now very clear evidence that SRF vasomotor cells do not all exert a uniform control over the sympathetic vasomotor outflow. The first indication that this may be the case was when Barman et al. (1984) found that the firing pattern of single neurons in the RVLM of the cat were not all correlated to the same degree with the activity of sympathetic nerves innervating different vascular beds. Some were most highly correlated with one particular output, while others appeared to be more closely linked to another output. Consistent with these observations, Dean and Coote (1986) found that following bilateral application of glycine to the ventral surface adjacent to the SRF nucleus, the time course of inhibition of

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FIG. 1. Parasagittal sections showing the relationship of the subrctrofacial nucleus to surrounding structures in the ventral part of the rostral medulla and caudal pons in the cat. The line drawing in (A) shows the outlines of the nuclear structures in the low-power photomicrograph of the Nissl-stained section shown in (B). The section is 4.0 nun lateral to the midline. (C) is a magnification of the area bounded by the rectangular box in (B), with the boundaries of the SRF nucleus indicated. The scale bar represents 1.0 mm in (A) and (B) and 0.25 turn in (C). Abbreviations: 7, facial nucleus; Amb, nucleus ambiguus; LR, lateral reticular nucleus; RF, rctrofacial nucleus; SOL, lateral nucleus of the superior olive; SOM, medial nucleus of the superior olive; SRF, subretrofacial nucleus. From Polson et al. (1992), with

permission.

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FIG. 2. (A) Photomicrograph of a Nissl-stained coronal section through the rostral ventral medulla showing the location of the SRF nucleus (arrowed) with respect to surrounding structures. The larger nucleus located more dorsally is the retrofacial nucleus. (B) Distribution of cells projecting to the lumbar spinal cord in one experiment, Each dot represents one spinally-projecting cell. The panels represent sections through the rostral ventral medulla at three different rostrocaudal levels. The number in the bottom left corner represents the distance in mm of each section caudal to the caudal pole of the facial nucleus. Photomicrographs at the bottom of the figure shows cells within (C) the SRF nucleus and (D) the reticular formation medial to the SRF nucleus, that were retrogradely labeled with WGA-HRP injected into the lumbar spinal cord. The scale bar represents 0.5 mm in (A) and (B) and 50/~m in (C)

and (D). From Polson et aL (1992), with permission.

202

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FIG. 3. Axonal labeling within the spinal cord at the level of the third thoracic segment, following microinjection of the tracer wheat germ agglutinin-conjugated horseradish peroxidase into the SRF nucleus of the cat. The left side is ipsilateral to the injection site. (A) Each large dot represents a single labeled axon, while the fine dots represent terminal labeling within the intermediolateral cell column and central autonomic area in a single 50 #m thick section. (B) Low-power photomicrograph taken from a section at the same level in another experiment, in which the terminal labeling was particularly dense. (C) and (D) are high-power photomicrographs of the areas bounded by the rectangles on the left and right sides of (B), corresponding to the ipsilateral and contralateral intermediolateral cell columns, respectively.

From Dampney et al. (1987a), with permission.

203

SUBRETROFACIAL VASOMOTOR NUCLEUS 205

sympathetic nerves innervating renal and skeletal muscle vascular beds was different, indicating that the cells controlling these outputs are separate subgroups with different locations in the nucleus.

More detailed information concerning the viscerotopic organization of vasomotor SRF cells has come from studies which have examined the pattern of regional vasomotor effects evoked by microinjections of excitatory amino acids into different sites in and surrounding the SRF nucleus of the cat. In particular, Dampney and McAllen (1988) found that postgangiionic vasoconstrictor nerves innervating blood vessels

in skeletal muscle or skin were preferentially activated from sites located lateral or medial, respectively, to the SRF nucleus (Fig. 4). The largest inc~ases in both skin and muscle vasoconstrictor activity, however, were elicited from sites within the SRF nucleus itself. These results therefore suggest that SRF neurons controlling skeletal muscle and skin vascular beds overlap within the nucleus, although they tend to be located on the lateral and medial sides, respectively. Similarly, SRF neurons located at different rostrocaudal levels also differ with respect to their target vascular beds. In particular, SRF neurons controlling

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Fxo. 4. Above: ventral view of the brain stem showing three glutamate injection sites (labeled A, B and C) in an anesthetized cat, corresponding to the recordings shown below. Below: recordings showing blood pressure (B.P.) in mmHg and the activity (calibrated in spikes/see) of single sympathetic fibers supplying skin and muscle vascular beds. The times and volumes of the injections of the glutamate solution (0.5 M) are indicated below by arrows. Note that large increases in both skin and muscle sympathetic activity were elicited by injection into the middle point, but selective increases in either skin or muscle sympathetic activity were elicited by injections into the medial and lateral points, respectively. From Dampney and

McAllen (1988), with permission. Abbreviations: Pyr, pyramid; Trap, trapezoid body.

206 R.A.I.. DAMPNIEY

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Fro. 5. Examples, from the same experiment, of the cardiovascular effects of microinjection of sodium glutamate (0,2 M solution) into sites within the SRF nucleus of an anesthetized cat. The volume of the injection was 1 nl in (A) and 4 nl in (B). Note the parallelism between the changes in hindlimb and forelimb resistance, but the dissociation between changes in renal sympathetic nerve activity and either forelimb or hindlimb resistance. The arrows indicate the times of the glutamate injections. From McAllen and

Dampney (1990), with permission.

the renal bed are distributed more rostrally than those controlling the hindlimb and mesenteric beds (Lovick, 1987; McAilen and Dampney, 1990: Dean et aL, 1992).

Given that SRF neurons controlling different vascular beds are organized topographically within the nucleus, the question arises as to what is the principle governing this organization. One possibility is that such sub-groups are arranged rostrocaudally, according to the rostrocaudal level of the sympathetic preganglionic neurons that they control. This arrangement would be similar to the general segmental arrangement of sympathetic preganglionic neurons themselves within the spinal cord, which is related to the rostrocaudal location of their target ganglia (Strack et al., 1988)... it l~as been shown, however, that the location of $II.F neuror~ is not related to the segmental level of the the sympathetic preganglionic neurons that they innervate (McAllen and Dampney, 1990). For example, as shown in Fig. 5, L-glutamate microinjections into sites within the SRF nucleus located at different rostrocaudal levels elicited paral- lel effects on the hindlimb and forelimb vascular resistance, although there was little correlation between renal vasomotor responses and either hindlimb or forelimb effects. These observations suggest that SRF neurons controlling the sympathetic outflow to the hindtimb and forelimb vascular beds have a similar distribution within the nucleus, despite the fact that the sympathetic outflows to the hindlimb and forelimb originate from widely-separated seg-

ments in the spinal cord. McAllen and Dampney (1990) therefore proposed an alternative hypothesis, which is that the topographical distribution of SRF neurons within the nucleus is related to the type of their target vascular bed (e.g. skeletal muscle, or kidney). The experimental observations are consistent with this hypothesis, since the hindlimb and forelimb beds are both mainly of the same type (skeletal muscle).

Although there is good evidence that some SRF neurons have specific target vascular beds. it is not clear whether there are also SRF neurons that provide a generalized non-specific excitatory input to sympathetic preganglionic neurons innervating different vascular beds. Barman and Gebber (1985) first suggested the possibility that there are both specific and non-specific neurons, on the basis of their finding that the axons of some SRF neurons innervate the intermediolateral cell column in only a very restricted part of the thoracic cord, while others project to widely separated thoracic segments. As pointed out above, however, even SRF neurons with collateral axons terminating in widely separated segments may still exert a specific control over sympathetic preganglionic neurons controlling vascular beds of a specific type (e.g. skeletal muscle in hindlimb and forelimb). Therefore, whether some SRF neurons are capable of exerting a generalized effect over the entire sympathetic vasomotor outflow remains an unresolved question.

Another important question is whether all sympa-


thoexcitatory vasomotor neurons within the RVLM are confined to the SRF nucleus. In the cat, McAllen and Dampney (1990) found that microinjection of very small volumes (1-4 nl) of 0.2 M L-glutamate solution into histologically identified sites ventral to the facial nucleus, up to 0.9 mm beyond the rostral pole of the SRF nucleus as defined in Section 2.1, could elicit significant increases in renal sympathetic activity. Consistent with this, neurons with projections to the intermediolateral cell column have been identified in this region, although the number of such neurons is much less than in the SRF nucleus itself (Miura et al., 1983). It is therefore very likely that the subfacial region also contains vasomotor neurons, although these are more scattered in their distribution. Similarly, the fact that pressor responses are not elicited by microinjection of excitatory amino acids into sites medial, lateral or caudal to the SRF nucleus does not rule out the possibility that these regions also contain vasomotor neurons; such neurons may exist, but in a low concentration compared with the SRF nucleus itself.

3.2. EFFECTS OF INHIBITION

As mentioned in the Introduction, inhibition or destruction of the region containing the SRF nucleus in the anesthetized cat leads to a profound fall in blood pressure, similar to that observed after transec- tion of the spinal cord (Guertzenstein, 1973; Feldberg and Guertzenstein, 1976; Guertzenstein and Silver, 1974; Dean and Coote, 1986; Stein et al., 1989). Similar effects have also been described in the anesthetized rabbit (Dampney and Moon, 1980; Pilowsky et aL, 1985) or rat (Ross et al., 1984). The fall in blood pressure is accompanied by a decrease in the activity of sympathetic nerves to the kidney, splanchnic bed and skeletal muscle (Dean and Coote, 1986; Pilowsky et al., 1985; Stein et al., 1989), but the magnitude of the decrease varies between the different sympathetic outflows. For example, inhibition of cells in the SRF region causes a much greater decrease in renal than in mesenteric or splenic sympathetic activity (Yardley et al., 1989; Hayes and Weaver, 1990; Beluli and Weaver, 1991).

The fall in blood pressure that occurs immediately following bilateral lesions that include the SRF nucleus is not sustained, at least in rats. Cochrane and Nathan (1989) found that several days following bilateral placement of lesions in rats, the resting blood pressure of the lesioned animals was similar to that in control animals. It can be concluded, therefore, that tonic inputs arising from sources other than the SRF nucleus are capable of maintaining resting blood pressure when SRF cells are disabled. One possible source of such inputs is a group of neurons in the rostral ventromedial medulla, just lateral to the pyramids but medial to the SRF nucleus (Helke et aL, 1989). Cells in the rostral ventromedial medulla, at least in the rat and rabbit, have been shown to project directly to sympathetic preganglionic nuclei in the spinal cord (Helke et al., 1989; Strack et al., 1989a; Li et al., 1992b). Moreover, inactivation of cells in this region results in a fall in blood pressure, indicating that they exert a tonic excitatory effect on the

sympathetic vasomotor outflow (Cox and Brody, 1991).

3.3. ROLE OF SRF NUCLEUS IN CARDIOVASCULAR REFLEXES

Inhibition or destruction of the RVLM region containing the SRF nucleus causes a loss of baroreceptor reflexes (Dampney, 1981; Granata et al., 1983, 1985). However, as pointed out by Kumada and co-workers (1990), these observations are com- plicated by the fact that these lesions also cause a profound fall in blood pressure and a loss of sympathetic tone (see Section 3.2). To circumvent this problem, Granata and co-workers (1985) carried out a series of experiments in which they combined a lesion of the nucleus of the solitary tract (NTS) on one side with a lesion of the RVLM on the opposite side. Since the connections between the NTS and the RVLM in the rat are entirely ipsilateral (Ross et al., 1985), this procedure isolates the RVLM (including the SRF nucleus) from baroreceptor inputs, without abolishing tonic sympathetic vasomotor activity. These lesions did, however, abolish the baroreceptor reflex. Similarly, in the rabbit, Terui and co-workers (1986b) interrupted the connections between baroreceptor inputs and the SRF nucleus on the contralateral side by splitting the medulla in the midsagittal plane. This procedure did not affect the level of tonic sympathetic vasomotor activity. Subsequent destruction or inhibition of the region corresponding to the SRF nucleus abolished the sympathoinhibitory response to stimulation of baroreceptor afferent fibers on the ipsilateral side, but not the contralateral side. It therefore can be concluded that the SRF nucleus plays a critical role in the expression of the baroreceptor reflex.

Inhibition or destruction of the RVLM region containing the SRF nucleus also causes a loss of reflexes arising from many other peripheral receptors, including chemoreceptors, cardiopulmonary receptors and somatic receptors (Dampney, 1981; Granata et al., 1983, 1985; McAllen, 1985; Dean and Coote, 1986; Stornetta et al., 1989). It therefore follows that, at least in anesthetized animals, the SRF nucleus is a critical component of central pathways mediating a wide range of cardiovascular reflexes.

Electrophysiological studies have provided more precise information on the role of the SRF nucleus in cardiovascular reflexes. In particular, there have been several detailed investigations of the response of SRF neurons to inputs from baroreceptors. In the cat, Barman and Gebber (1985) studied the electrophysiological properties of neurons in the RVLM whose activity was related to the sympathetic nerve discharge and which could be antidromically activated from the intermediolateral cell column. These neurons were located in a highly circumscribed region just ventral to the respiratory neurons in the RVLM, indicating that they were SRF bulbospinal neurons (see Section 3.1.1). All neurons tested were powerfully inhibited by baroreceptor inputs, typical of sympathoexcitatory vasomotor neurons. Further, the conduction velocities of their axons had a similar range to that of descending sympathoexcitatory pathways in the spinal cord, as estimated by Coote and

208 R.A.L. DAMPNEY

McLeod (1984). These observations were confirmed by McAllen (1986a), who also mapped the precise location of the bulbospinal barosensitive neurons within the SRF region (Fig. 6). There is a remarkable correspondence between the location of these neurons as identified by electrophysiological criteria and the location of the SRF nucleus as defined according to cytoarchitectural and anatomical criteria.

Neurons within the RVLM but outside the SRF nucleus have quite different electrophysiological properties from those within the nucleus. For example, a recent study in the cat of a sample of 80 neurons, all of which were located in the RVLM but medial or dorsal to the SRF nucleus, found that none of these neurons had electrophysiological properties characteristic of sympathoexcitatory vasomotor neurons (Blair, 1991). This observation therefore supports the view that vasomotor neurons in the RVLM of the cat are concentrated within the SRF nucleus.

Very similar findings have been reported in the

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FIG. 6. (A) Recording of the activity of a single bulbospinal SRF cell in an anesthetized cat. Note that stimulation of the carotid sinus baroreceptors (by increasing the pressure in the isolated carotid sinus) elicits powerful inhibition. (B) The locations of SRF bulbospinal barosensitive neurons in the RVLM of the cat. Note that they are clustered in the restricted region just ventral to the retrofacial nucleus (RF). Other abbreviations: 5ST, spinal trigeminal tract; IO, inferior olive; PYR, pyramidal tract. Adapted ~from McAllen and Dampney (1989), with permission. Scale bar: 1 mrn.

rabbit by Terui e t al. (1986a). Of 161 bulbospinal neurons within the RVLM that responded to stimulation of baroreceptor afferent fibers, all were inhibited, indicating that they are sympathoexcitatory vasomotor neurons. Furthermore, as in the cat, these neurons were found within a circumscribed portion of the RVLM, just ventral to the rostral portion of the retrofacial nucleus, which corresponds to the location of the SRF nucleus. Eighty-six per cent of the SRF bulbospinal neurons had conduction velocities in the range 1.5-10 m/sec, similar to the range reported for the cat (Barman and Gebber, 1985; McAllen, 1986a). This range of conduction velocities corresponds to that for small myelinated fibers. A minority (14%) of bulbospinal SRF neurons in the rabbit, however, had estimated conduction velocities below 1.5 m/sec, which is indicative of unmyelinated fibers.

In the rat, detailed studies of the electrophysiological properties of bulbospinal barosensitive neurons in the RVLM have been carried out by several groups of workers (Brown and Guyenet, 1985; Guyenet and Brown, 1986; Sun and Guyenet, t985, 1987; Morrison et al. , 1988; Chan et al. , 1991a). The studies by Guyenet and co-workers have focussed in a narrow strip within the RVLM, 0.3 mm wide and extending from the caudal pole of the facial nucleus to the level 400/~m more caudal. This region has been called the retrofacial portion of the nucleus paragigantocellularis lateralis (Andrezik et al. , 1981) and appears to correspond very closely to the region in the RVLM which Ruggiero et al. (1989) identified as being a highly sensitive pressor region. The anatomical location of this region, together with its physiological properties, strongly suggests that it is homologous to the SRF nucleus of the cat and rabbit. Neurons within this region in the rat can therefore also be referred to as SRF neurons.

The bulbospinal barosensitive SRF neurons in the rat have electrophysiological properties very similar to those of SRF neurons in the cat and rabbit. Virtually all are spontaneously active, have a pulse- synchronous discharge, are inhibited by stimulation of arterial baroreceptors and project to sympathetic preganglionic nuclei in the spinal cord (Brown and Guyenet, 1985; Sun and Guyenet, 1985, t986, 1987: Morrison et al. , 1988; Chan et al. , 1991a). As in the rabbit, they can be divided into two sub-groups on the basis of the conduction velocity of their descending axons.

Apart from their sensitivity to baroreceptor afferent inputs, SRF neurons also respond to inputs from a variety of other receptors, including chemoreceptors, cardiopulmonary receptors, renal receptors, somatic receptors and vestibular receptors (McAllen, 1985, 1987; Terui e t aL, 1986a; Sun and Guyenet, 1987; Saeki e t al. , 1988; Morrison and Reis, 1989; Yates e t al. , 1991; Sun and Spyer, 1991a,b). In fact, there is a remarkable convergence of different inputs on to individual SRF neurons. For example, Terui e t aL (1986a) found that over 90% of SRF neurons in the rabbit that were inhibited by stimulation of A-fibers in the aortic nerve (which arise from baroreceptors) also responded to stimulation of C-fibers in the aortic nerve, which consist of both baroreceptor and non-baroreceptor afferents. A similar proportion

Tim 8UBRETROFACIAL VASOMOTOR NUCLEUS 209

of these neurons were excited by chemoreceptor stimulation. Furthermore, other studies have shown that SRF barosensory neurons also receive inputs arising from renal and somatic receptors (Terui et al., 1987; Saeki et al., 1988).

Similarly, in the rat, Sun and Guyenet (1987) found that that was a convergence of vagal excitatory, vagal inhibitory and arterial baroreceptor inputs on to bulbospinal SRF neurons. Excitatory inputs to SRF neurons could in all cases be blocked by iontophoretic application of kynurenic acid, a glutamate receptor antagonist, while inhibitory inputs were in all cases blocked by iontophoretic application of bicuculline, a GABA receptor antagonist.

Inputs to the SRF nucleus from peripheral baroreceptors, chemoreceptors and cardiopulmonary receptors are relayed via the NTS. Unlike SRF neurons, however, NTS neurons are relatively specific in their responsiveness to particular inputs. For example, Donoghue et al. (1985) found that the large majority (approximately 85%) of cells in the the NTS that were excited by carotid sinus, aortic or vagal stimulation only responded to one of these inputs. Furthermore, even where convergence of such inputs occurred, there was no evidence that these originated from receptors of different types (e.g. baroreceptors and chemoreceptors), rather than receptors of the same type but with different locations in the cardiovascular system.

In summary, then, signals from different receptor types are transmitted through the NTS largely via separate channels, but then ultimately converge on to individual neurons within the SRF nucleus. In addition, as discussed in more detail below (see Section 5.1), SRF neurons also receive inputs arising from nuclei within the brain itself. At the same time, even though SRF neurons are a site of convergence of inputs arising from many different peripheral and central sources, it should not be concluded that all SRF neurons respond to different inputs in a uniform way. Since there are subgroups of SRF neurons which differ according to the type of vascular bed or beds that they control (see Section 3.1.2 above), one would expect that such subgroups would differ in the degree to which they are influ- enced by particular inputs, as is the case with sympathetic preganglionic neurons controlling different types of vascular beds (Jiinig, 1988). Further studies will be needed to determine whether SRF neurons can be subdivided according to the pattern of their response to inputs from different peripheral receptors.

In the rat, at least some bulbospinal SRF neurons maintain a tonic discharge even when all excitatory inputs are blocked by application of kynurenic acid (Sun et al., 1988a). Furthermore, these neurons display a highly regular pacemaker-like discharge under these conditions. All such "pacemaker" neurons belong to the sub-group with fast axonal conduction velocities, as described above. In contrast, other SRF bulbospinal neurons, which have slower axonal conduction velocities, do not exhibit a pacemaker-like activity in the absence of excitatory inputs (Haselton and Guyenet, 1989b). Further- more, as shall be discussed in more detail below (see

Sections 3.5 and 4.1) the pacemaker and non-pacemaker cells also differ with respect to their pharmacological and chemical properties. Despite these differences both types of cells are inhibited by baroreceptor inputs, indicating that they are sympathoexcitatory (Sun et al., 1988a; Haselton and Guyenet, 1989b).

3.4. RESPIRATORY MODULATION OF ACTIVITY OF SRF NEURONS

The central respiratory rhythm generator, which is responsible for the production of the rhythmic activity of motorneurons innervating respiratory muscles, also influences the firing pattern of sympathetic vasomotor nerves (for review see Richter and Spyer, 1990). This effect of the central respiratory rhythm generator is quite independent of cardiorespiratory reflexes, since it persists after vagotomy and sinoaortic denervation (Bainton et al., 1985; Barman and Gebber, 1976; Haselton and Guyenet, 1989a). Different patterns of respiratory modulation of sympathetic vasomotor activity have been described (Gilbey et al., 1986; Numao et al., 1987), depending on the target vascular bed of the sympathetic outflow (Numao et al., 1987).

The activity of SRF neurons in the cat, rabbit and rat also exhibits a respiratory rhythmicity, which arises from the central respiratory rhythm generator (Terui et al., 1986a; McAllen, 1987; Haselton and Guyenet, 1989a). Moreover, distinctly different patterns of respiratory modulation of SRF neurons have been observed, similar to the different patterns observed in the sympathetic outflow itself (McAllen, 1987; Haselton and Guyenet, 1989a). This has led to the suggestion that the respiratory modulation of sympathetic vasomotor discharge could be generated entirely at the level of the SRF neurons (McAilen, 1987). Consistent with this hypothesis, Guyenet et al. (1990) reported that injections of kynurenic acid into the RVLM at the level of the caudal pole of the facial nucleus virtually abolished the respiratory modulation of sympathetic activity while caus- ing only a slight reduction of phrenic activity. In fact, the authors emphasized that this effect was only observed when the injections were made into this highly restricted region, which, as pointed out above, corresponds to the location of the SRF nucleus. This finding therefore suggests that respiratory modulation of sympathetic activity depends upon afferent inputs acting on glutamate receptors located on or close to SRF neurons.

The fact that SRF neurons exhibit different patterns of respiratory modulation, which appear to correlate with the different patterns observed in regional sympathetic outflows, is interesting in view of the evidence summarised above that there are subgroups of SRF neurons, each of which is specific for a particular vascular bed (Section 3.1.1). As suggested by Haselton and Guyenet (1989a), it is possible that each functionally distinct subgroup of SRF neurons may be characterized by a respiratory pattern that is characteristic for that subgroup. In any case, these observations provide further evidence that SRF neurons are a site of integration of inputs from many different sources.

211) R.A.L. DAMPNEY

3.5. ORIGIN OF SPONTANEOUS ACTIVITY OF SRF NEURONS

As discussed in Section 3.1 above, it is well established that SRF neurons are tonically active and generate a significant component of resting sympathetic vasomotor activity, at least in anesthetized animals. Three different mechanisms have been proposed to explain how this tonic activity is generated. One of the earliest suggestions was that SRF vasomotor cells are chemosensitive and are tonically excited even at normal levels of blood pH, PO2 and pCO2, like chemosensitive cells in the carotid body (Dampney and Moon, 1980). Consistent with this hypothesis, the blood flow and capillary density within the SRF nucleus is significantly greater than in surrounding areas (G6bel et al. , 1990). In addition, ultrastructural studies have shown that the somata and dendrites of at least some neurons in the SRF region are closely apposed to small capillaries (Milner et al. , 1987). Furthermore, there is some functional evidence in support of the chemosensitivity theory. Seller et al. (1990) demonstrated that perfusion of the RVLM region containing the SRF nucleus with a hypercapnic solution elicits a marked and rapid increase in sympathetic vasomotor activity, even after blockade of synaptic inputs to RVLM neurons by local injection of cobalt chloride. At the cellular level, Sun et al. (1992) have shown that cyanide-induced hypoxia rapidly excites SRF bulbospinal neurons, which further supports the view that these neurons function as oxygen sensors.

As pointed out above (see Section 3.3) some bulbospinal SRF neurons exhibit a highly regular pacemaker-like discharge when all excitatory synaptic inputs are blocked (Sun e t al. , 1988a). It has been suggested, therefore, that resting activity in sympathetic vasomotor neurons is due in large part to the intrinsic activity of these pacemaker cells (Sun e t al., 1988a). Even if pacemaker cells are capable of maintaining resting sympathetic activity under conditions when synaptic inputs to the SRF nucleus are blocked, however, this activity is not necessarily the principal source of tonic excitation under normal conditions. Pacemaker cells are insensitive to clonidine (Guyenet et al., 1989) and yet bilateral microinjection of clonidine into the RVLM region including the SRF nucleus produces a marked fall in blood pressure (Bousquet and Schwartz, 1983). Thus, it follows that the hypotensive effect of clonidine is due to its inhibitory action on non-pacemaker sympathoexcitatory cells in the SRF nucleus, indicating that these cells are also tonically active and contribute to the resting sympathetic vasomotor tone.

A third hypothesis explaining the origin of the tonic activity of SRF neurons in the RVLM has been proposed by Barman and Gebber (1989). They have shown in the anesthetized cat that a large component of the resting activity of sympathetic vasomotor nerves is characterized by a 2~i Hz rhythm (Barman and Gebber, 1980; Gebber and Barman, 1980) and that this sympathetic rhythm is generated by supraspinal neural mechanisms (Ardell e t al. , 1982). Although this rhythm is exhibited by SRF sympathoexcitatory cells, it is not generated exclusively by pacemaker cells, since it persists even under con-

ditions when the firing patterns of pacemake! cells a r¢ dissociated from those of sympathetic vasomotor neurons (Barman and Gebber, 1989). Instead, it has been suggested that the 2-6 Hz rhythm o1" SRI- sympathoexcitatory cells is derived from inputs originating from antecedent neurons (Barman and Geb- ber, 1989). Similarly, Morrison et aL ~1988) also concluded, on the basis of the fact that individual SRF cells fire in synchrony, that these cells probably receive a common tonic excitatory input. This input is not essential, however, for maintaining resting sympathetic vasomotor activity, since this is ~lot reduced even after blockade of all synaptic inputs to SRF neurons by microinjection of cobalt ions (Trzebski and Baradziej, 1992)

In summary, there is strong experimental evidence that the resting activity of SRF cells is due, at least in part, to tonic excitatory inputs arising from various sources. The pacemaker activity of some neurons may also contribute to this activity. It also possible that the chemosensitivity of at least some SRF cells is also a factor. The relative importance of these three mechanisms under normal conditions in conscious animals, however, remains to be determined.

4. CHEMICAL PROPERTIES OF SRF NEURONS

There have been many studies, mainly in the rat and cat, of the histochemical properties of cells within the RVLM (for reviews see Ciriello et al., 1986b: Ciriello and Caverson, 1989; Milner et al., 1989b): These studies demonstrate that the RVLM as a whole contains a wide variety of chemically-distinct neuronal types. In particular, there are neurons immunoreactive for phenylethanolamine N-methyltransferase (PNMT, the converting enzyme for adrenaline synthesis), serotonin and many neuropeptides including neuropeptide Y, substance P, somatostatin and enkephalin. In addition, recent studies have demonstrated a high density of neurons immunoreactive for phosphate activated glutaminase, a glutamate-synthesizing enzyme (Minson et al., 1991) as well as for glutamic acid decarboxylase, a GABA-synthesizing enzyme (Jones et al. , 1991) within the RVLM of the rat.

In some cases two or more of the above putative transmitters, or their synthesizing enzymes, have been identified within the same RVLM neurons. In particular, Minson et al. (1991) found that all PNMT and serotonin immunoreactive neurons within the RVLM also contained the synthesizing enzyme tbr glutamate. Similarly, PNMT and neuropeptide Y have been shown to be frequently co-localized in RVLM neurons in the rabbit (Blessing et al., 1986) and rat (Helke e t al., 1989) while PNMT and substance P are occasionally co-localized within RVLM neurons in the rat (Pilowsky e t al., 1986; Milner et aL, 1988; Helke e t al. , 1989).

Since RVLM neurons subserve several different functions, the studies referred to above provide little information about the chemical properties of SRF cardiovascular neurons. A recent study by Poison e t al. (1992), however, has focussed specifically on the immunohistochemical properties of cells in the SRF


nucleus in the cat. This study showed that over half (estimated 57%) of the population of SRF cells are immunoreactive for tyrosine hydroxylase, a marker of catecholamine cells. Moreover, catecholamine cells in the RVLM are largely confined to the SRF nucleus, although they do extend more rostrally and caudally (Fig. 7). The more rostral catecholamine cells are located just ventral to the facial nucleus, in the region which appears to be a rostral extension of the SRF nucleus (see Section 2.1 above). The catecholamine cells located caudal to the SRF nucleus are much more scattered than within the nucleus itself. Thus, the dense longitudinal column of catecholamine cells within the RVLM of the cat corresponds very closely to the SRF nucleus as defined on the basis of cytoarchitecture (Poison et al., 1992).

Apart from catecholamine cells, the SRF nucleus in the cat also contains cells immunoreactive for neuropeptide Y (estimated 11% of the total), enkephalin (16% of total) and serotonin (10% of total), but no cells immunoreactive for substance P, galanin or somatostatin (Poison et al., 1992). It is therefore clear that SRF cells are quite heterogeneous with respect to their chemical properties. Furthermore, Poison et al. (1992) found that different chemically-identified cells within the SRF nucleus are unevenly distributed along the rostrocaudal axis of the nucleus. For example, in the most caudal part of the nucleus, virtually all cells were catecholamine cells and the vast majority (estimated 80%) also were immunoreactive for neuropeptide Y. This suggests that at this level of the nucleus many catecholamine cells also contained neuropeptide Y. In contrast, only approximately 30% of cells in the rostral part of the SRF nucleus synthesize catecholamines and no NPY immunoreactive cells were observed.

The study by Poison et al. (1992) did not demonstrate whether the catecholamine cells in the SRF nucleus of the cat synthesize noradrenaline or adrenaline. Other workers, however, have demonstrated in the cat a dense cluster of PNMT immunoreactive cells in a region which corresponds to the SRF nucleus, as well as in other parts of the RVLM (Ciriello et al., 1986a ; Kitahama et al., 1986; Reiner and Vincent, 1986; Ruggiero et al., 1986). Thus, it may be concluded that at least many of the catechoi- amine cells in the SRF nucleus synthesize adrenaline and are therefore part of the CI group of neurons, as first designated by H6kfelt et al. (1974). It follows that some SRF cells in the cat are part of the C1 group, but the C1 group also includes cells located outside the SRF nucleus.

It should be pointed out, however, that there are species differences in the chemical properties of cells within the SRF nucleus. Although cells containing catecholamine-synthesizing enzymes have been identified in the SRF nucleus or equivalent region of several different species, not all of these necessarily synthesize adrenaline. For example, while neurons immunoreactive for PNMT have been identified in the rat (H6kfelt et al., 1974; Armstrong et aL, 1982), cat (e.g. Kitahama et al., 1986) and dog (Dormer et al., 1986), PNMT immunoreactivity cannot be demonstrated in the rabbit (Blessing et al., 1986) or guinea pig (McLachlan et al., 1989; Cumming et al., 1986). Consistent with this, Fuller and Hemrick-

Luecke (1983) found that the level of adrenaline in both the rabbit and guinea pig brainstem was below detectable levels ( < 1 pmol/g) compared with 37 pmol/g in the rat. It is therefore likely that the catecholamine cells, in the SRF nucleus of these species do not synthesize adrenaline. Similarly, there are also differences between species in the relative numbers and distribution of catecholamine neurons and neurons immunoreactive for neuropeptide Y within the RVLM (Hailiday and McLachlan, 1991).

4.1. COMPARISON OF CATECHOLAMINE AND NON-CATECHOLAMINE SRF NEURONS

As mentioned above, catecholamine cells comprise over half of the cells in the SRF nucleus in the cat. Similarly, in the rat, it has been estimated that over 70% of all bulbospinal cells within the SRF region are catecholamine cells of the Cl group (Ruggiero et al., 1989), while in the rabbit the proportion has been estimated to be 65% (Li et al., 1992b). Further- more, recent studies using the method of transynaptic transport of viruses have confirmed the conclusions of earlier studies (Ross et al., 1981) that a high proportion of SRF neurons projecting directly to sympathetic preganglionic neurons in the spinal cord of the rat are immunoreactive for PNMT (Strack et al., 1989b; Wesselingh et al., 1989). Despite the relative abundance of bulbospinal catecholamine cells in the SRF nucleus, however, the question as to whether these cells play an important role in the vasomotor functions of the nucleus has been debated for some time.

One major reason for this controversy is that several earlier studies showed that sympathetic preganglionic neurons in the spinal cord of the rat or cat are usually inhibited by iontophoretically applied adrenaline (Coote et al., 1981; Guyenet and Cabot, 1981; Guyenet and Stornetta, 1982), which, as mentioned above, is believed to be synthesized by C I neurons within the SRF nucleus of these species. Furthermore, it has also been shown that inhibition of adrenaline synthesis by pre-treatment with a PNMT inhibitor does not affect the pressor response to stimulation of the SRF nucleus (Connor and Drew, 1987; Head and Howe, 1987). These observations therefore strongly indicate that adrenaline is not the neurotransmitter mediating the excitatory actions of SRF neurons on sympathetic preganglionic vasomotor neurons.

On the other hand, as discussed in more detail below (see Section 4.3), recent studies indicate that adrenaline as well as other monoamines have a modulatory action on neurotransmission in the spinal cord. It does not necessarily follow, therefore, that adrenaline-synthesizing neurons in the SRF nucleus do not have a sympathoexcitatory action. In fact, a combined anatomical and electrophysiological study by Haselton and Guyenet (1989b) provided strong evidence that such neurons are sympathoexcitatory. The authors found that nearly all bulbospinal neurons in the RVLM of the rat that provide a collateral innervation of supramedullary regions are immunoreactive for PNMT (i.e. C1 cells) and many are located in the region just caudal to the facial nucleus, which is homologous to the SRF nucleus in the cat

212 R, A. L. DAMPNEY

and rabbit. The authors then used extracellular recording to identify neurons within this region that could be antidromically activated from both the spinal cord and supramedullary structures, and which were thus presumptive C i cells. These neurons were found to be tonically active and strongly inhibited by baroreceptor inputs, indicative of sympathoexcitatory neurons.

Another electrophysiological study in the rat by Morrison e t al. (1988) used a different approach to examine the question as to whether C1 cells are sympathoexcitatory. In their study, neurons in the RVLM that project to the intermediolateral cell column were first identified antidromically. All of these neurons were localized to a small region just ventral to the retrofacial nucleus and therefore can be regarded as SRF neurons. These neurons could be sub-divided into two groups on the basis of the conduction velocity of their axons, but both types had electrophysioiogical characteristics indicative of sympathoexcitatory neurons. In particular, the prob- ability of their discharge was related to the cardiac cycle, they were strongly inhibited by baroreceptor activation and their conduction velocities were in the same range as that of descending sympathoexcitatory axons. Careful mapping of the recording sites demonstrated that all of these lay within 100 #m of cell bodies of C1 neurons, identified by their immunoreactivity to PNMT. Furthermore, Morrison et al. (1988) studied the ultrastructural properties of the lateral funiculus in the spinal cord (which contains descending sympathoexcitatory axons) and found PNMT immunoreactivity in both myelinated and unmyelinated axons. The estimated conduction velocities of these axons were in the same range as that of identified sympathoexcitatory neurons within the SRF nucleus.

In the cat, there is indirect evidence in support of the hypothesis that catecholamine neurons within the SRF nucleus are sympathoexcitatory. As discussed above, McAllen (1986a) recorded the activity of a randomly selected sample of spinally-projecting neurons within the SRF nucleus. Nearly all of these displayed a cardiac rhythmicity and were powerfully inhibited by baroreceptor inputs, indicating that they were sympathoexcitatory neurons. Since over half of the neurons within the SRF nucleus of the cat are catecholamine cells, it seems highly likely that the sample of recorded neurons would have included catecholamine cells. In conclusion, then, the weight of evidence in both the rat and cat strongly supports the view that catecholamine CI neurons within the SRF nucleus are sympathoexcitatory.

There is also evidence indicating that non-catecholamine neurons within the SRF nucleus are sympathoexcitatory. As discussed above (see Section 3.3), SRF neurons in the rat can be sub-divided into two groups, according to whether they demonstrate a pacemaker-like activity or not. The pacemaker cells are not immunoreactive for catecholamine-synthesizing enzymes (Sun et al. , 1988b), but have functional properties characteristic of sympathoexcitatory neurons. That is, they are tonically active, display a cardiac rhythmicity and are inhibited by baroreceptor inputs (Sun et al. , 1988a).

In summary, it may be concluded that both cat-

echolamine and non-catecholamine cells in the SRF nucleus are sympathoexcitatory and participate in the baroreceptor reflex control of sympathetic vasomotor activity. These two groups of cells, however, appear to differ in other respects, at least in the rat. As reviewed by Guyenet (1990), only non-catecholamine cells exhibit a pacemaker-like discharge in the absence of excitatory inputs, while only the catecholamine cells are inhibited by clonidine. Further, SRF cells which provide a collateral innervation of both the spinal cord and supramedullary structures are predominantly of the catecholamine type (Haselton and Guyenet, 1989b, 1990). The functional significance of these differences between the two types of SRF cells with respect to their electrophysiological, pharmacological and anatomical properties remains to be determined.

4.2. RELATIONSHIP BETWEEN CHEMICAL PROPERTIES AND TARGET VASCULAR BED OF SRF NEURONS

As described above (see Section 4.1), there are clear differences in the chemical properties of cells at different levels of the SRF nucleus in the cat. The question then arises as to whether there is any relationship between these properties and the target vascular bed of SRF neurons, since this also varies according to rostrocaudal location (see Section 3.1.2). Poison e t al. (1992) tested this possibility by compar- ing the immunohistochemical properties of SRF cells at different rostrocaudal levels with the regional vasomotor effects of stimulating SRF cells at the corresponding levels. Their data indicated that cells in the most caudal part of the SRF nucleus, the majority of which contain both catecholamines and NPY, regulate the sympathetic output to the hindlimb but not to the renal vascular bed. At the same time, the data also showed that the sympathetic output to the hindlimb was also excited by stimulation of cells in the rostral part of the nucleus, which does not contain catecholamine/NPY cells. Thus, while there may be some correlation between the chemical type and target vascular bed of SRF neurons, there does not appear to be a clear one-to-one correspondence between the two. A definitive determination of this question will require studies in which the target vascular bed of individual SRF neurons can be defined by some electrophysiologicat criteria and then correlated with the chemical properties of the same cells.

4.3. IDENTIFICATION OF NEUROTRANSMITTER(S)

For the reasons discussed above (Section 4.1), it is unlikely that adrenaline is the excitatory neurotransmitter released from the terminals of SRF bulbospinal neurons. Recent evidence, however, indicates that glutamate, or a glutamate-like compound, may be the principal excitatory transmitter. First, application of the glutamate antagonist kynurenate to sympathetic preganglionic neurons projecting to splanchnic ganglia blocks the excitatory response normally elicited by stimulation of the RVLM region containing the SRF nucleus (Morrison et al. , 1989); Secondly, the pressor response elicited by stimulation of this region also causes an immediate increase in the

Rostral (0.25) Caudal (1.25)

FIG. 7. Low-power photomicrographs showing immunoreactivity for tyrosine hydroxylase in the SRF nucleus of the cat in (A) a parasagittal section 4.0 mm lateral to the midline and in coronal sections at a rostral (B) and caudal (C) level of the nucleus, which are 0.25 and 1.25 mm caudal to the caudal pole of the facial nucleus, respectively. The scale bars all represent 0.5 mm. Note that the tyrosine hydroxylase-labeled (presumably catecholamine) cells are highly concentrated within the SRF nucleus,

particularly its caudal part. From Poison et al, (1992), with permission.

213

0

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. ,o i • AMAP > 20 mmHg ~ ~ } • AMAP= 10mmHg ~ ~ i o noresponse ~ . ~

, ",Q.~ FIG. 9. (A) Diagram showing distribution of angiotensin II receptor binding sites (shaded area) in the SRF nucleus. (B) High-power photomicrograph of the autoradiograph corresponding to the rectangle bounded by the dashed lines in (A). (C) Diagram showing the locations of the centers of sites in the RVLM at which angiotensin II was injected in the anesthetized cat. Note that the pressor sites corresponded precisely to the region of the SRF nucleus, which has a particularly high density of angiotensin II receptors.

Adapted from Allen et al. (1988), with permission.

214


release of glutamate and of aspartate, measured using microdialysis (Kapoor et al., 1992). Thirdly, a recent study has shown that a large majority (between 71 and 83%) of bulbospinal neurons within the RVLM of the rat, including the SRF region, is immunoreactive for phosphate activated glutaminase, a glutamate-synthesizing enzyme (Minson et al., 1991). Finally, glutamate-like immunoreactivity has been demonstrated in axon terminals and synapses within the intermediolateral cell column (Morrison et al., 1989; Llewellyn-Smith et al., 1992). In fact, approximately two-thirds of synapses on identified preganglionic neurons in the thoracic spinal cord of the rat are immunoreactive for glutamate (Llewellyn-Smith et al., 1992).

There is conflicting evidence, however, regarding the identity of the particular glutamate receptor sub-type that is activated by SRF bulbospinal neurons. Bazil and Gordon (1991) found that the pressor response to stimulation of the SRF region in the rat was blocked by spinal administration of a specific antagonist of the N-methyl-l)-aspartate (NMDA) receptor sub-type. Similarly, Sundaram and Sapru (1991) reported that the cardiac sympathoexcitatory response was blocked by injection of an NMDA antagonist directly into the intermediolateral cell column. On the other hand, Morrison et al. (1989) found that iontophoretic application of a specific NMDA antagonist to individual sympathetic preganglionic neurons projecting to the splanchnic nerve did not affect their usual excitatory response to SRF stimulation. Further, these authors also found using receptor autoradiography that there is a relatively high density of glutamate receptors of the kainic sub-type within the intermediolateral cell column. In addition, Nishi et al. (1987) recorded from sympathetic preganglionic neurons in tissue slices and found that focal electrical stimulation elicited fast EPSPs that were inhibited by D-glutamylglycine, which is a glutamate antagonist with a preference for receptors of the kainate or quisqualate sub-type. As pointed out by Guyenet (1990), however, there are technical difficulties associated with the in vitro preparation, such as uncertainty as to whether the recordings are made from sympathetic preganglionic vasomotor neurons.

In summary, although the question as to which specific receptor sub-type or sub-types mediating the excitatory response to SRF stimulation remains unresolved, there is nevertheless agreement that glutamate or a glutamate-like compound is likely to be the principal neurotransmitter released by the terminals of SRF neurons. The question remains, however, as to whether other compounds identified within SRF neurons also play a role in neurotransmission. In particular, the role of catecholamines, which as mentioned above are believed to be synthesized by a high proportion of SRF neurons, has been studied in detail. In the case of adrenaline, recent studies in the rat have shown that its iontophoretic application to sympathetic preganglionic neurons can either increase or decrease their activity (Miyazaki et al., 1989; Lewis and Coote, 1990). Of greater significance, however, is the observation from an intracellular recording study that the effects of applied adrenaline, whether it be hyperpolarization or depolarization, are

very slow, of the order of seconds or minutes (Miyazaki et al., 1989). This observation confirms that adrenaline is not a fast-acting neurotransmitter, but could have a modulatory action with a much slower time course. Similarly, other compounds that have been identified within the SRF nucleus, such as neuropeptide Y, may also have a modulatory action. For example, as McCall (1988a) has suggested, these compounds may act to set the level of excitability of sympathetic preganglionic neurons. Clearly, many questions remain with regard to the precise functions of putative neurotransmitters and neuromodulators released by axon terminals arising from SRF neurons.

5. INPUTS TO SRF NEURONS

5.1. NUCLEI OF ORIGIN

Studies in the cat (Lovick, 1985, 1986) and rat (Van Bockstaele et al., 1989) have demonstrated that afferent inputs to the RVLM arise from a wide variety of nuclei, located at all levels of the central nervous system, which have autonomic, respiratory and sensory-related functions. It is likely that many of these inputs synapse with RVLM neurons involved in non-cardiovascular functions, such as antinocicep- tion, respiratory regulation, or arousal and vigilance (Lovick 1985, 1986; Van Bockstaele et al., 1989). A clearer picture of the likely origin of afferent inputs specifically to the SRF nucleus, however, has come from studies in which a retrogradely transported tracer was injected into the site which was first identified physiologically as the SRF pressor region.

These studies have shown that in the cat, rabbit and rat, major projections to the SRF pressor region arise from the NTS, caudal ventrolateral medulla, K611iker-Fuse nucleus in the pons, the periaqueductal grey (PAG) in the midbrain, the paraventricular nucleus and the lateral hypothalamic area (Dampney et al., 1982, 1987a; Ross et al., 1985; Carrive et ai., 1988, 1989) as illustrated schematically in Fig. 8. Smaller projections from the central amygdaloid nucleus (Takayama and Miura, 1991) and medullary raphe nuclei (Dampney et al., 1987a) have also been demonstrated. There is not yet direct evidence that any of these afferent inputs synapse directly with SRF neurons. Nevertheless, studies using the method of anterograde transport of tracers have confirmed that the projections from the NTS, caudal ventrolateral medulla and midbrain PAG terminate within the SRF nucleus (Ross et al., 1985; Dampney et al., 1987a; Carrive et al., 1988; Li et al., 1992b).

Physiological studies have thrown some light on the likely functional significance of some of these pathways. In particular, electrophysiological studies in both the rat (Agarwai et al., 1989) and rabbit (Li et al., 1991) have shown that SRF cardiovascular neurons are inhibited by stimulation of the caudal ventrolateral medulla. This region of the medulla contains a group of depressor syrnpathoinhibitory neurons (Li and Blessing, 1990), many of which can be activated by stimulation of peripheral baroreceptors and which can also be antidromically activated by focal stimulation of the SRF region in the rat

216 R.A. L, DAMPNEY

PVN

'', ,-, ",. " ....... NTS ~ ..... .... .,, ', ..... " \ ~ ' ~!!i~.~:i~

......... ?..~ ..... ".., / / CVLM . . . . . ' " " I N P U T S ................... ~ _ .. " ' ~ ~ ~ _ _ , ~ " "

O U T P U T SRF

FIG. 8. Diagram showing the main connections of the SRF nucleus. Abbreviations: CVLM, caudal ventrolateral medulla; IML, intermediolateral cell column; KF, Kflliker-Fuse nucleus; LHA, lateral hypothalamic area; NTS, nucleus traetus solitarius; PAG, periaqueductal grey; PVN, paraventrieular

nucleus. From Dampney (1990), with permission.

(Agarwal and Calaresu, 1991) and rabbit (Terui et al., 1990). Therefore, taken together, the anatomical and physiological evidence is consistent with the hypothesis that neurons in the caudal ventrolateral medulla transmit inhibitory inputs from peripheral baroreceptors to sympathoexcitatory neurons in the SRF nucleus. At the same time, there is also evidence that inhibitory inputs to SRF neurons arising from some depressor neurons in the caudal ventrolateral medulla are independent of the peripheral baroreceptors (Guyenet et al., 1987; Crave et aL, 1991).

As discussed in Section 3.4, SRF neurons exhibit a respiratory rhythmicity, indicating that they receive inputs from the neurons that comprise the central respiratory rhythm generator (Terui et aL, 1986a; McAllen, 1987; Haselton and Guyenet, 1989a). Con- sistent with this, some of the cells that are retrogradely labeled following injection of tracer into the SRF nucleus are located within regions corresponding to the ventral respiratory group (in and surrounding the nucleus ambiguus and retroambigualis) or dorsal respiratory group (in the ventrolaterat sub- nucleus of the NTS) (Ross et aL, 1985; Dampney et al., 1987a). Similarly, the projection from the Kfltiker-Fuse nucleus in the pens could also confer a respiratory rhythmicity on SRF cells, since cells in this nucleus also are part of the central respiratory network (Smith et al., 1989).

One of the largest projections to the SRF nucleus arises from the medial and commissural subnuclei of the NTS (Ross et al., 1985; Dampney et aL, 1987a). The functional significance of these inputs rerrmins unknown, They are not likely to represent inhibitory inputs arising from peripheral baroreceptors, since these are mediated by a GABAergic synapse on to SRF neurons (see Section 5,2.2 below) and there appear to be no GABAergic projections from the NTS to the RVLM region containing the SRF nucleus (Meeley et aL, 1985). It is possible that these inputs could mediate the reflex excitation of SRF vasomotor neurons arising from stimulation of receptors such as the peripheral chemoreceptors (Sun and

Spyer, 1991a), but there appears to be no direct evidence on this point.

Inputs to the SRF nucleus also originate from several groups of cells in the midbrain PAG (Carrive et aL, 1988, 1989; Carrive and Bandler, 1991), Stimu- lation of these different cell groups elicits different patterns of cardiovascular effects, which are a part of more complex integrated behavioral responses. In particular, the connections between the PAG and the SRF nucleus appear to mediate the cardiovascular components of different types of defensive behavior; these are discussed in more detail in Section & 1.

Much less is known about the functional role of inputs to the SRF nucleus from the paraventricular nucleus, lateral hypothalamic area and the central amygdaloid nucleus. The paraventricular nucleus is one of the major sources of afferent inputs to other autonomic nuclei such as the sympathetic preganglionic nuclei in the spinal cord, midbrain PAG, parabrachial region in the pens and NTS as well as the SRF nucleus (Luiten et al., I985; Dampney et al., 1987a; Strack et aL, 1989a,b). Functional studies of the effects of stimulation of cells in the paraventricular nucleus on cardiovascular function has produced conflicting results (Yamashita et aL, 1987; Katafuchi et al., 1988; Gelsema et aL, 1989). Therefore, the specific functional significance of the paraventricular- SRF pathway is unknown. In the case of the lateral hypothalamic area and the central amygdaloid nucleus, both of these regions have been implicated in the cardiovascular changes associated with particular behaviors (for reviews see Allen and Cechetto, 1992; Kapp et aL, 1982).

It has already been pointed out (see Section 3.3.) that the SRF nucleus is essential for the expression of a wide range of cardiovascular reflexes arising from peripheral receptors. In addition, it has been shown that cardiovascular responses arising from stimulation of a number of supramedullary regions such as the lateral hypothalamus, the fastigial nucleus in the cerebellum, or the posterior cerebellar cortex are dependent upon synaptic inputs to the SRF region


(McAUen, 1985; Dean and Coote, 1986; Silva- Carvalho et al., 1991). The neural connections between each of these regions and the SRF nucleus that subserve these cardiovascular responses may be monosynaptic, but could also include relays in other nuclei such as the midbrain PAG, pontine Krlliker- Fuse nucleus, or the NTS (Paton and Spyer, 1990; Paton et al., 1990; Allen and Cechetto, 1992), all of which project directly to the SRF nucleus (Fig. 8).

5.2. PUTATIVE NEUROTRANSMITTERS AND NEUROMODULATORS

Anatomical and pharmacological studies have indicated that there is a wide range of putative neurotransmitters or neuromodulators that can influence the activity of SRF vasomotor neurons. These studies will be briefly reviewed in this section.

5.2.1. Glutamate

There is good evidence that at least many of the excitatory inputs to SRF vasomotor neurons utilize an excitatory amino acid neurotransmitter. For example, as described in Section 3.3, application of kynurenic acid, a non-specific blocker of glutamate receptors, blocks the excitation of SRF neurons normally elicited by stimulation of unmyelinated vagal afferent fibers (Sun and Guyenet, 1987). Som- ogyi et aL (1989) have provided anatomical evidence that many of the NTS neurons that project directly to the SRF nucleus (Section 5.1) use an excitatory amino acid (such as glutamate) as a neurotransmitter and it has been suggested that such neurons mediate the excitatory actions on SRF neurons that can be elicited by stimulation of some vagal and other afferent fibers (Urbanski and Sapru, 1988). In addition, excitatory inputs to the SRF nucleus arising from stimulation of sympathoexcitatory sites in the hypothalamus also appear to use glutamate as a neurotransmitter (Sun and Guyenet, 1986). Similarly, a double-labeling study has demonstrated the existence of glutamate-immunoreactive neurons within the amygdala that project directly to the SRF nucleus (Takayama and Miura, 1991). It appears that the glutamatergic inputs to SRF neurons are not tonically active, however, since the resting level of blood pressure is unaffected by blockade of glutamate receptors within the SRF region (Urbanski and Sapru, 1988).

It has been shown that stimulation of both NMDA and non-NMDA glutamate receptor subtypes causes excitation of SRF neurons (Miura et al., 1991). Further, the threshold of stimulation of both types was reduced in spontaneously hypertensive rats as compared to normal controls, suggesting that in the former case the hypertension may be partly due to the abnormal properties of glutamate receptor subtypes in the SRF nucleus (Miura et aL, 1991).

5.2.2. G A B A

SRF neurons are tonically inhibited by GABAergic inputs. Blockade of GABA receptors in the immediate vicinity of the neurons, but not surrounding regions, results in an increase in their firing rate as

well as in blood pressure (Willette et al., 1983, 1984a; Sun and Guyenet, 1985; Kubo and Kihara, 1987; Dampney et al., 1988; Smith and Barron, 1990). GABA receptors on the neurons also mediate the inhibition arising from stimulation of arterial baroreceptors or cardiopulmonary receptors CYamada et al., 1984; Sun and Guyenet, 1987; Dampney et aL, 1988). In addition, a substantial part of the tonic GABAergic inhibition of SRF neurons is independent of baroreceptor inputs (Kubo and Kihara, 1987; Dampney et al., 1988; McCall, 1988b).

In normotensive rats and rabbits, the GABAergic inhibition of SRF pressor neurons is to a large extent dependent upon tonic inputs arising from depressor neurons in the caudal ventrolateral medulla (Blessing, 1988; Smith and Baron, 1990). In spontaneously hypertensive rats, however, depressor neurons in the caudal ventrolateral medulla appear to have little or no resting tonic activity, so that the GABAergic inhibition of SRF pressor neurons in these animals is accordingly much less than in normotensive animals (Smith and Baron, 1990). This may therefore be another factor contributing to the higher level of sympathetic activity and blood pressure in these animals.

5.2.3. Acetylcholine

Microinjection into the RVLM of cholinergic agonists, or of agents which release acetylcholine from nerve terminals, increases blood pressure (Willette et al., 1984b; Sundaram and Sapru, 1988; Lee et al., 1991). It is likely that endogenous acetylcholine is tonically released in the RVLM, since blockade of muscarinic receptors or inhibition of acetylcholine synthesis within the RVLM decreases blood pressure (Willette et al., 1984b; Arneric et al., 1990; Lee et al., 1991). Further, a pressor response is evoked by systemic administration of the drug physostigmine (a cholinesterase inhibitor), which is presumably due to the augmented action of acetylcholine released from cholinergic terminals within the RVLM (Giuliano et al., 1989). The specific receptor involved is of the muscarinic M 2 type (Sundaram et al., 1988; Giuliano et al., 1989), which has a high density within the RVLM (Arneric et al., 1990).

It is not clear whether the pressor actions of acetylcholine in the RVLM are due to a direct or indirect effect on SRF neurons. In the rat, the region corresponding to the SRF nucleus contains a high density of nerve terminals immunoreactive for the acetylcholine-synthesizing enzyme choline acetyltransferase (Giuliano et al., 1989), which is consistent with the hypothesis that these terminals directly innervate SRF vasomotor neurons. On the other hand, an ultrastructural study has shown that the majority of synaptic contacts made by cholinergic terminals on RVLM neurons appear to be of the inhibitory type (Milner et al., 1989a). Furthermore, only a small minority (8°/.) of these synapses were with catecholamine neurons, which constitute the majority of SRF neurons (see Section 4.1). To account for these observations, Giuliano et al. (1989) have suggested that cholinergic terminals in the RVLM inhibit local inhibitory (for example, GABA- ergic) neurons which then synapse with SRF neurons.

JPN 42/2---C

218 R.A.L. DAMPNEY

There is, however, no direct evidence for this hypothesis.

In spontaneously hypertensive rats, injection of cholinergic drugs into the RVLM causes a greater presser response than in normotensive rats (Lee et al., 1991). Similarly, the depressor response elicited by cholinergic antagonists in the RVLM was also greater in spontaneously hypertensive rats (Lee et al., 1991). There is, however, no significant difference in either the basal or evoked rate of release of acetylcholine in the RVLM of the two strains (Yamada et al., 1987; Arneric et al., 1990), suggesting that an increased sensitivity of cholinergic receptors on RVLM neurons may account for the greater responsiveness to cholinergic stimuli in the spontaneously hypertensive rat (Lee et al., 1991). In contrast to the genetic model of hypertension, hypertension induced by chronic stress is associated with an increased level of choline acetyltransferase activity in the RVLM, indicative of an increased rate of release of endogenous acetylcholine in this situation (Lin and Li, 1990).

5.2.4. Opiates

Activation of opiate receptors in the RVLM, by microinjection of stable analogs of enkephalin, produces a decrease in blood pressure and heart rate, while naloxone, an opiate antagonist, prevents these effects (Punnen et al., 1984; Sapru et al., 1987). Within the RVLM, there is a particularly high density of enkephalin-immunoreactive terminals in the SRF region (Ruggiero et aL, 1989). An ultrastructural study of transmitter-identified neurons in the RVLM has shown that one of the primary targets of these terminals is CI catecholamine cells in the SRF nucleus (Milner et al., 1989c). Taken together, these observations suggest that enkephalin is released from terminals in the RVLM and directly inhibits SRF vasomotor neurons.

It has been shown that one source of the enkephalin-immunoreactive axon terminals is the NTS, particularly in the medial and commissural nuclei (Morilak et al., 1989). Although the functional significance of this opiate pathway is not known, it has been hypothesized that it has a modulatory effect on baroreceptor or other cardiovascular reflexes (Morilak et al., 1989). In support of this view, it has been shown that the decompensatory phase of severe hemorrhage, whereby the reflex vasoconstriction is reversed to a vasodilatation, is prevented by central administration of an opiate receptor antagonist (Ludbrook and Rutter, 1988). It is therefore possible that the opiate pathway from the NTS to the SRF nucleus subserves the reversal of vasoconstriction under these conditions.

It has been suggested that opiate receptors and adrenoceptors of the ~t 2 subtype in the brain may be linked (Kunos et al., 1987). Consistent with this, putative CI neurons in the SRF nucleus, many of which are innervated by enkephalin-immunoreactive terminals, also contain ~t 2 adrenoceptors (Hasetton and Guyenet, 1989b). Further, microinjections of enkephalin into the RVLM elicits cardiovascular effects very similar to those elicited by adrenergic agonists such a-methyl noradrenaline (Bousquet and Schwartz, 1983; see Section 5.2.7).

5.2.5. Serotonin

The SRF region contains a high density of serotonin-immunoreactive nerve terminals (Ruggiero et al., 1989), some of which synapse directly with CI catecholamine cells (Nicholas and Hancock, 1988). Lovick (1989) showed that microinjection of serotonin into the RVLM resulted in a long-lasting fall in blood pressure, but the decrease was relatively mod- est (mean 16 mmHg) and had a very long onset latency (mean 41 sec). Like serotonin, microinjection of the serotoninjA receptor agonist 8-OHDPAT elicits a fall in blood pressure and sympathetic activity (Dabire et al., 1990; Mandal et al., 1990; Nosjean and Guyenet, 1991). It is possible, however, that at least a portion of the action of this drug is due to activation of ~2 adrenoceptors (Nosjean and Guyenet, 1991). Thus, both the functional significance and pharmacology of the serotonergic innervation of SRF cells remains to be determined.

5.2.6. Angiotensin

Studies in the rabbit and cat, using the method of in vitro autoradiography, have shown that there is a very high density of angiotensin receptors in the SRF nucleus (Allen et aL, 1987, 1988a,b; Mendelsohn et al., 1988). In fact, angiotensin receptors within the RVLM appear to be associated exclusively with the SRF nucleus (Fig. 9). Activation of these receptors by microinjection of angiotensin II into the SRF nucleus, or its application to the nearby ventral surface, leads to a rise in blood pressure and sympathetic vasomotor activity (Allen et al., 1988a; Andreatta et aL, 1988; Sasaki and Dampney, 1990; Li et al., 1992a; Fig. 9). In contrast, microinjection of a specific antagonist to angiotensin receptors has opposite cardiovascular effects (Sasaki and Dampney, 1990): An- giotensin has no cardiovascular effect, however, when injected into sites outside the SRF nucleus (Fig. 9). These observations therefore indicate that endogenous angiotensin, or an angiotensin-like compound, is tonically released from nerve terminals in the SRF nucleus and excites vasomotor neurons. Nerve terminals immunoreactive for angiotensin have been demonstrated in the RVLM of the rat (Lind et al., 1985), although their precise distribution has not been described, nor have their cells of origin been identified.

When compared with the response elicited by glutamate microinjections into the SRF nucleus, the presser response elicited by angiotensin II microinjections is much slower in onset and in latency to peak response (Allen et aL, 1988a; Muratani et al., 1991). Such a slow time course indicates that angiotensin II is not a fast-acting neurotransmitter and therefore suggests that the functional role of angiotensin II is more likely to be a modulatory one.

Although angiotensin II excites SRF vasomotor neurons, it does not have any detectable effect on respiratory neurons within the RVLM, even when injected in a high concentration or volume (Li et al., 1992a). This, together with the receptor-binding studies described above, indicates that angiotensin receptors are associated specifieallywith SRF vasomotor neurons. In fact, it is possible that they may be


associated only with a subgroup of vasomotor neurons, since Chan et al. (1991b) found that iontophoretic application of angiotensin II to identified sympathoexcitatory neurons in the RVLM of the rat excited some but had no effect on others. Further- more, Sun and Guyenet (1989) found that sympathocxcitatory neurons with electrophysiological properties indicative of non-catecholamine neurons were insensitive to angiotensin II. Thus, it has been suggested that angiotensin II may exert its actions specifically on catecholamine neurons within the SRF nucleus (Sun and Guyenet, 1989; Li et aL, 1992a). Further studies will be needed, however, to test this hypothesis.

Angiotensin II excites SRF neurons in spontaneously hypertensive as well as normotensive rats. In the former group of animals, however, the sensitivity and responsiveness of SRF neurons to angiotensin II was relatively greater (Chan et al., 1991b).

5.2.7. Other peptides

A variety of other peptides can alter cardiovascular function via actions on neurons within the RVLM. In particular, vasopressin, substance P and neuropeptide Y all elicit increases in blood pressure when injected into the RVLM, or when applied to the nearby ventral surface (Tseng et al., 1988; Urbanski et al., 1989; Andreatta-Van Luyen et al., 1990). Sun and Guyenet (1989) have shown that all of these compounds directly excite putative sympathoexcitatory SRF neurons in vitro, identified by their intrinsic pacemaker-like discharge. Similarly, they found that thyrotropin and calcitonin gene-related peptide also excite SRF neurons, whereas the opiates enkephalin and morphine had an inhibitory effect, in agreement with other studies on the actions of opiates (see Section 5.2.4.). Immunohistochemical studies have demonstrated fibers and terminals immunoreactive for these peptides in the SRF region in the rat (Nilaver et al., 1980; Hrkfelt et al., 1975; Skofitsch and Jacobowitz, 1985; Yamazoe et al., 1985; Milner et al., 1988).

The time course of action of these peptides on putative SRF sympathoexcitatory neurons is very slow (Sun and Guyenet, 1989). Furthermore, Agar- wal and Calaresu (1992) have shown that, at least in the case of substance P, there is a facilitatory interaction between its effects on neuronal firing rate and that of glutamate. These observations therefore indicate that these peptides probably act as neuromodulators in the SRF nucleus rather than as fast-acting neurotransmitters.

5.3. ACTIONS OF ANTIHYPERTENSlVE DRUGS ON SRF NmmoNs

There is evidence that two classes of antihypertensive drugs (the ~ adrenoceptor antagonists and clondine and its analogs) decrease blood pressure, at least in part, by acting on bulbospinal SRF neurons. The/~ adrenoceptor antagonists such as propranolol are known to inhibit sympathetic activity via a central action (for review see Meier et aL, 1980). Sun and Guyenet (1990) found that putative sympathoexcita-

tory SRF neurons are excited/n vitro by/~ adrenoccp- tor agonists and by local application of tyramine, which stimulates the release of catecholamines from nerve terminals. These effects were blocked by /~ adrenoceptor antagonists. It has therefore been suggested that at least some SRF neurons in vivo are subject to a tonic/J receptor-mediated excitation, an effect which could be inhibited by ~ blockers such as propranoioi which can cross the blood-brain barrier (Sun and Guyenet, 1990).

The drug clonidine and its analogs have also long been known to decrease blood pressure as a conse- quence of a centrally-induced sympathoinhibition (Kobinger, 1978). Microinjections of clonidine or its oxazoline analog rilmenidine into the RVLM of the rat or cat decreases blood pressure (Bousquet et al., 1984; Punnen et al., 1987). Both of these drugs are believed to exert their antihypertensive effects via an action on imidazole receptors in the RVLM rather than ~t 2 adrenoceptors, since their effects are blocked by prior microinjection of idazoxan, a ligand for both imidazole and ~2 adrenoceptors, but not by the drugs yohimbine or SKF-86466 which have a selective affinity for ~2adrenoceptors (Gomez et al., 1991; Tibirica et al., 1991b). This finding may be of clinical importance, since the affinity of clonidine for ~t 2 adrenoceptors in the locus coeruleus and frontal cortex may account for the sedative actions of this drug (Gomez et al., 1991; Tibirica et al., 1991a). In contrast, rilmenidine, which has a much higher selectivity than clonidine for imidazole receptors (Gomez et al., 1991; Tibirica et al., 1991a), produces hypotension without significant sedative effects in animals or humans (Koenig-Berard et al., 1988; Filastre et al., 1988),

There is also good evidence that the RVLM is the critical, and possibly even the only site, at which clonidine and rilmenidine exert their antihypertensive effects. The depressor response to ¢lonidine is not reduced by decerebration, or by destruction or inhibition of neurons in the NTS (Antonaccio and Hal- ley, 1977; Punnen et al., 1987). Furthermore, bilateral microinjections of idazoxan into the RVLM blocks the depressor effects of systemically-administered cionidine (Punnen et al., 1987).

There is indirect evidence that clonidine specifically inhibits SRF catecholamine neurons in the RVLM. First, Haselton and Guyenet (1989b) found that putative C1 neurons with electrophysiological properties indicative of SRF sympathoexcitatory neurons were powerfully and uniformly inhibited by intra- venous administration of clonidine, whereas pacemaker SRF neurons in the rat, which do not appear to be catecholaminergic (see Section 4.1) are insensitive to clonidine (Guyenet et al., 1989). Furthermore, measurements of changes in the metabolic activity of catecholamine neurons, using an in vivo electrochem- ical technique, have indicated that there is a correlation between the hypotensive action of clonidine and the decrease in the overall activity of catecholamine neurons in the RVLM (Tibirica et al., 1989). The same also applies in the case of rilmenidine (Tibirica et al., 1991a), suggesting that this drug also may be specific for SRF catecholamine neurons.

In summary, then, the evidence supports the general conclusion that the SRF vasomotor nucleus is a

220 R.A.L . DAMPNEY

major site of convergence of many pharmacologically distinct synaptic inputs, originating from many different peripheral cardiovascular receptors, as welt as other autonomic nuclei located at all levels of the central nervous system. It is not surprising, then, that the same nucleus appears to be a crucial site of action of some centrally-acting antihypertensive drugs.

6. ROLE OF SRF NUCLEUS IN REGULATING CARDIOVASCULAR CHANGES ASSOCIATED

WITH INTEGRATED RESPONSES

As discussed above, the SRF nucleus receives afferent inputs from the amygdala, the lateral hypothalamic region and the midbrain periaqueductai gray (PAG). All of these regions are thought to be involved in the integration of cardiovascular changes associated with particular behavioral responses (Kapp et al., 1982; Carrive, 1991; Allen and Cechetto, 1992). In general, little is known of the details of the neural substrates for these integrated responses. In the last few years, however, much progress has been made in defining the role of the PAG in such responses. In particular, recent studies have thrown considerable light on the anatomical and functional properties of the connections between the PAG and the SRF nucleus which mediate the cardiovascular changes associated with different types of defensive behavior. These studies are discussed in the following section.

6.1. DEFENSIVE BEHAVIOR

The midbrain PAG plays an important role in defensive behaviors, which are associated with different patterns of cardiovascular changes, according to the type of somatomotor response. For example, as reviewed by Bandler (1988), one pattern of changes in the cat consists of a threat display (e.g. hissing, paw striking) accompanied by a pressor response, while another pattern consists of immobility accompanied by a depressor response. Carrive and co-workers have shown that, in the decerebrate cat, stimulation of cells within the lateral part of the PAG elicits motor changes typical of a threat display, which is accompanied by a rise in blood pressure. In contrast, stimulation of cells within the ventrolateral part of the PAG elicits motor changes typical of immobility, which is accompanied by a fall in Mood pressure (Carrive, 1991). Thus, it has been suggested that neuronal subgroups within different PAG subregions are capable of eliciting different co-ordinated patterns of autonomic and somatomotor responses, each of which is appropriate for a particular behavior (Carrive, 1991).

Anatomical studies using the methods of retrograde and anterograde transport of tracers have shown that PAG projections to the SRF nucleus originate from cells within both the lateral and ventrolateral parts of the PAG, corresponding to the subregions from which pressor and depressor responses can be elicited (Carrive et al., t988, 1989; Carrive and Bundler, 1991). Therefore, it has been proposed that these pathways represent excitatory

and inhibitory inputs, respectively, to SRF neurons (Carrive, 1991).

One very interesting feature of both the pressor and depressor subregions within the PAG is that they are viscerotopically organized. For example, while a pressor response is elicited by stimulation of cells at both rostral and caudal levels of the lateral PAG, the response from the rostral part is accompanied by skeletal muscle vasoconstriction, with little change in renal vascular resistance, while that from the caudal part is accompanied by strong renal vasoconstriction with little neurogenic effect on skeletal muscle resistance (Carrive et al., 1989). Similarly, in the case of the ventrolateral PAG, stimulation of its rostral and caudal parts preferentially affects the vascular resistance of the skeletal muscle and renal vascular beds, respectively (Carrive and Bandler, 1991). Thus, as in the SRF nucleus, there are different groups of neurons in the PAG which have different effects on regional vascular beds.

The anatomical connections between the PAG and SRF nucleus show a similar specificity. As summar- ized in Fig. 10, neurons in the lateral and ventrolateral parts of the rostral PAG, that respectively increase and decrease skeletal muscle vascular resistance, project directly to the caudal part of the SRF nucleus, which preferentially controls the sympathetic vasomotor output to skeletal muscle beds (see Section 3.1.2). The same organizational principle also applies to the connections between renal vasomotor neurons in the PAG and in the SRF nucleus (Fig. 10). In summary, these observations indicate that vasomotor neurons within the PAG are organized viscerotopically according to the vascular bed or beds that they control and make highly specific connections with SRF neurons (Carrive, 1991).

7. CONCLUDING REMARKS

The results of many studies using a variety of experimental approaches have demonstrated that the SRF nucleus has highly distinctive functional, anatomical, chemical and pharmacological properties. Functional studies have shown that the nucleus is a crucial component of the central pathways subserving cardiovascular responses either reflexly elicited by stimulation of peripheral receptors, or originating from autonomic nuclei at higher levels of the brain, at least in anesthetized animals. Further, the fact that the nucleus projects directly to sympathetic preganglionic nuclei in the thoracolumbar spinal cord and in turn receives inputs from other central autonomic nuclei, underlies its pivotal role in cardiovascular regulation.

At the same time, several important questions remain. First, it should be emphasized that nearly all the studies on the functional role of the SRF nucleus have been conducted in anesthetized animals. This therefore raises the question as to the importance o f the SRF nucleus in cardiovascular regulation in conscious animals during normal behaviors. It is possible that future experiments using more novel techniques may provide more information on this point. In particular, the method of labeling active neurons by their expression of the proto-oncogene

T ~ SUBRETROFACIAL VASOMOTOR NUCLEUS 221

c-fos (Hunt et al., 1987) provides a means of identi- fying neurons that are activated by particular stimuli in conscious animals. For example, a recent study has shown that hemorrhage in the conscious cat causes c-fos expression in SRF bulbospinal neurons (McAilen et al., 1992). It is also possible to combine the c.fos labeling method with retrograde labeling (McAllen et al., 1992), or with immunohistochemical

labeling of neurotransmitters or their synthesizing enzymes (Li and Dampney, 1992). Thus, provided such experiments are conducted under appropriately controlled conditions, it should be possible to define the population of neurons within the brain that are activated by particular stimuli that induce cardiovascular responses in the conscious state and at the same time determine the neurotransmitter content and

I ~"~ : project|on mediating I an exci ta tory effect

PAG

I

: project ion m e d i a t i n g [ an Inhibitory effect I

PAG

PAG : Periaqueductal grey - - 1 - ~ N SRF : Subretrofacial nucleus ~ ~ IML : Intermediolateral column

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

\ Y . . . . . . . I ....... ; ~ l l l I F / ............... [ [ OutflOW to:

FIG. 10. Schematic diagram showing proposed descending pathways from the midbrain periaqueductal gray that regulate the sympathetic vasomotor output to the hindlimb and renal vascular beds. Neurons (indicated by open circles) in the rostral and caudal parts of the lateral PAG provide an excitatory input to neurons in the caudal and rostral parts, respectively, of the subretrofacial (SRF) vasomotor nucleus in the rostral ventrolateral medulla. Similarly, neurons in the rostral and caudal parts of the ventrolateral PAG (indicated by filled circles) provide an inhibitory input to neurons in the caudal and rostral parts, respectively, of the subretrofacial (SRF) vasomotor nucleus. Neurons in the caudal and rostral parts of the SRF nucleus project preferentially to the sympathetic vasomotor outflows controlling the hindlimb

and renal vascular beds, respectively. From Carrive and Bandler (1991), with permission.

222 R . A . L . DAMPNEY

connections of such neurons. Such information may provide more insight into the role of the SRF nucleus under these conditions.

As discussed in Section 5.2, recent experiments suggest that an excitatory amino acid such as glutamate is the principal neurotransmitter released from the terminals of SRF bulbospinal neurons. At the same time, it is also clear that most SRF neurons contain one or more of a variety of other compounds (including catecholamines and neuropeptides) that may act as co-transmitters or neuromodulators. At the present time, very little is known about the functions of these compounds within SRF neurons, or indeed any other group of central neurons. It is nevertheless intriguing that the SRF neurons that contain catecholamine synthesizing enzymes differ, in terms of both their electrophysiological, pharmacological and anatomical properties, from SRF cells that lack these enzymes (see Section 4.1). Thus, there appears to be some link between the chemical properties of SRF cells and their particular functions, but the significance of this relationship is unknown.

Recent studies have established that there are also sub-groups of SRF vasomotor neurons that differ according to the vascular bed that they control (see Section 3.1.2). It is not known, however, whether there are also SRF neurons that exert a more global effect on the sympathetic vasomotor outflow and future studies may address this question. In addition, other studies have also shown that SRF neurons are heterogeneous in terms of their pharmacological properties. For example, some cells are inhibited by cionidine, whereas others are not (see Section 4.1). Similarly, there is indirect evidence that angiotensin may excite only a subgroup of SRF vasomotor neurons (see Section 5.2.6). Further information about the pharmacological properties of SRF vasomotor neurons, and specifically whether SRF neurons controlling particular vascular beds may have different receptors, is also a challenge for future research.

A c k n o w l e d g e m e n t s - - T h e work of the au tho r ' s l abo ra to ry is suppor t ed by the Na t iona l Hea l th and Medica l Research Counc i l o f Aus t ra l i a , the N a t i o n a l Hea r t F o u n d a t i o n of Aus t r a l i a and the Clive and Vera Ramac io t t i Founda t ions . I t h a n k D r Rob in McAl len for m a n y discuss ions on the funct ions of the S R F nucleus which have helped me great ly in wr i t ing this manuscr ip t .

REFERENCES

AGARWAL, S. K. and CALARESU, F. R. (1991) Monosynaptic connec- tion from caudal to rostral ventrolateral medulla in the barorecep- tot reflex pathway. Brain Res. 555, 70-74.

AGARWAL~ S. K. and C^L^Ra:SO, F. R. (1992) Interaction of putative neurotransminers in rostral ventrolateral medullary cardiovascular neurons. J. auton. Nerv. Syst. 38, 159-166.

AGARW^L, S. K., GELSEMA, A. J. and CALARESU, F. R. (1989). Neur- ons in rostral VLM are inhibited by chemical stimulation of caudal VLM in rats. Am. J. Physiol. 25"1, R265-R270,

ALEXAI'qVER, R. S. (1946) Tonic and reflex functions of medullary sympathetic cardiovascular centers. J. Neurophysiol. 9, 205-217.

ALLEN, A. M., CHAI, S. Y., SEX'ION, P. M., LEWlS, S. J., VLm~RNE, A. J. M., JARROT'r, B., Lotus, W. J., CLEVERS, J., MCKINLEY, M. J., PAXlNOS, G. and MErOELSOHN, F. A. O. 0987) Angiotensin II receptors and angiotensin converting enzyme in the medulla oblong, am. Hypertension 9, Suppl. IH, III-198--III-205.

ALLEN, A. i . , DAMPNEY, R. A. L. and MENDELSOHN, F. A. O. (19gga)

Angiotensin receptor binding and pressor effects in cat subretrofacial nucleus. Am. J. PhysioL 255, HI01 I-HI017.

ALLEN, A. M., McKINLEY, M. J., OLDFIELD, B. J., DA~PNEY, R. A. L. and MENDELSOHN, F. A. O. (1988b) Angiotensin II receptor binding and the haroreflex pathway. Clin. Exp. Hypertens. A -Theor. 10 (Suppl. l), 63-78.

ALLEN, G. V. and CECrI~TTO, D. F. (1992) Functional and anatomical organization of cardiovascular pressor and depressor sites in the lateral hypothalamic area. 1. Descending projections. J. comp. Neurol. 315, 313-332.

AMENDT, K.~ CZACHURSKI, J., DEMBOWSKY, K. and SELLER, H. (1978) Neurones within the "chemosensitive area" on the ventral surface of the brainstem which project to the intermediolaleral column. Pfliigers Arch. ges. Physiol. 375, 289-292.

AMENDT, K., CZACHURSKI, J., DEMBOWSKY, K. and SELLER, H. (1979) Bulbospinal projections to the intermediolateral cell column; a neuroanatomical study. J. auton. Nerv. Syst. !, 103-d 17.

ANOREATTA, S. H., AVER1LL, D. B., SANTOS, R. A. S, and FERRARIO, C. M. (I 988) The ventrolateral medulla: A new site of action of the renin-angiotensin system. Hypertension 11 (Suppl. I), 1-163-I-166.

ANDREATTA-VAN LEYEN, S., AVERILL, D. B. and FERRARIO, C. M. (1990) Cardiovascular actions of vasopressin at the ventrolateral medulla. Hypertension 15 (Suppl. I), 102-106.

ANDREZIK, J. A., CHAN-PALAY, V. and PALAY, S. L. (1981) The nucleus paragigantocellularis lateralis in the rat. Conformation and cytology. Anat. EmbryoL 161, 355-371.

ANTONACCIO, M. J. and HALLEY, J. (1977) Clonidine hypotension: lack of effect of bilateral lesions of the nucleus solitary tract in anesthetized cats. Neuropharmacology 16, 431-433.

ARDELL, J. L., BARMAN, S. M. and GEBBER, G. L. (1982) Sympathetic nerve discharge in chronic spinal cat. Am. J. Physiol. 243, H463-H470.

ARMSTRONG, D. M., Ross, C. A., PICKEL, V. M., JOH, T. H. and REIS, D. J. (1982) Distribution of dopamine-, noradrenaline- and adrenaline-containing cell bodies in the rat medulla oblongata: Demonstrated by the immunocytochemical localization of catecholamine biosynthetic enzymes. J. comp. Neurol. 212, 173-187.

ARNERIC, S. P., GIULIANO, R., ERNSBERGER, P., UNDERWOOD, M. D. and REIS, D. J. (1990) Synthesis, release and receptor binding of acetylcholine in the CI area of the rostral ventrolateral medulla: contributions in regulating arterial pressure. Brain Rcs. 511, 98-112.

BAINTON, C. R., RICHTER, D. W., SELLER, H., BALLANTYNE, D. and KLEIN, J. P. (1985) Respiratory modulation of sympathetic activity. J. auton. Nerv. Syst. 12, 77-90.

BANDLER, R. (1988) Brain mechanisms of aggression as revealed by electrical and chemical stimulation; suggestion of a central role for the midbrain periaquedoctal gray region. In: Progress in Psychobi- ology and Physiological Psychology, Vol. 13, pp. 67-153. Eds. A. EPSTEIN and A. MORRISON, Academic Press: New York.

BARMAN, S. M. and GEBlaER, G. L. (1976) Basis for synchronization of sympathetic and phrenic nerve discharges. Am. J. Physiol. 231, 1601-1807.

BARMAN, S. M. and GEaaER, G. L. (1980) Sympathetic nerve rhythm of brain stem origin. Am. J. Physiol. 239, R42-R47.

BARMAN. S. M. and GERBER, G. L. (1985) Axonal projection patterns of ventrolateral medallospinal sympathoexcitatory neurons. J. Neurophysiol. 53, 1551-1566.

BARMAN, S. M. and GERBER, G. L. (1989) Basis for the naturally occurring activity of rostral ventrolateral medullary sympathoexcitatory neurons. In: Central Neural Organization of Cardiovascu- lar Control. Progress in Brain Research, Vol. 81, pp. 117-129. Eds. J. CIRIELLO, M. M. CAVERSON and C. POLOSA. Elsevier: Amster- dam.

BARMAN, S. M., GEBnER, G. L. and CAL^R~U, F. R. (1984) Differen- tial control of sympathetic nerve discharge by the brain stem. Am. J. Physiol. 247, R513-R519.

BAZlL, M. K. and GORDON, F. J. (t991) Effect of blockade of spinal NMDA receptors on sympathoexcitation and cardiovascular responses produced by cerebral ischemia. Brain Res. 5 ~ , 149-152.

BELULI, D. J. and WEAVER, L. C. (1991) Areas of rostral medulla providing tonic control of renal and splenic nerves. Am. J. Physiol. 261, HI687-HI692.

BERMAN, A. L. (1968) The Brainstem of the Cat; a Cytoarchitectonic Atlas with Stereotaxic Coordinates. University of Wisconsin Press: Madison, WI.

BLAIR, R. W. (1991) Convergence of sympathetic, vagal and other


sensory inputs onto neurons in feline ventrolateral medulla. Am. J. Physiol. 260, HI918-HI928.

B ~ N o , W. W. (1988) Depressor neurons in rabbit caudal medulla act via GABA receptors in rostral medulla. Am. J. PhysioL 284, H686-H692.

Bt~,~snqo, W. W., How~, P. R. C., JoH, T. H., Ouv~R, J. R. and WILLOUOmtY, J. O. (1986) Distribution of tyrosine hydroxylas¢ and neuropeptide Y-like immunoreactive neurons in rabbit medulla oblongata, with attention to colocalization studies, presumptive adrenaline-synthesizing perikarya and vagal preganglionic cells. J. comp. Ncurol. 248, 285-300.

BOUSQUI~T, P. and SCt~VARTZ, J. (1983) Alpha-adrenergic drugs: pharmacological tools for the study of the central vasomotor control. Biochem. Pharmac. 32, 1459-1465.

BOUSQU~T, P., FELDMAN, J. and SCHWARTZ, J. (1984) Central cardiovascular effects of alpha adrenergic drugs: differences between catecholamines and imidazolines. J. Pharmac. exp. Ther. 230, 232-236.

BROW~, D. L. and GU'~N~T, P. G. (1985) Electrophysiological study of cardiovascular neurons in the rostral ventrolateral medulla in rats. Circ. Res. 56, 359-369.

CAm)T, J. B. (1990) Sympathetic preganglionic neurons: tyro- architecture, ultrastructure and biophysical properties. In: Central Regulation of Autonomic Functions, pp. 43-67. Eds. A. D. Lo~vY and K. M. SPYER. Oxford University Press: New York.

CALAt~SU, F. R. and YARDLEY, C. P. (1988) Medullary basal sympathetic tone. Ann. Rev. Physiol. 50, 511-524.

C^RRIVE, P. (1991) Functional organization of PAG neurons controlling regional vascular beds. In: The Midbrain PAG, pp. 67-100. Eds. A. DePAULtS and R. BANDt~R. Plenum: New York.

CAP, PaVE, P. and B^~,a)t~R, R. (1991) Viscerotopic organization of neurons subserving hypotensive reactions within the midbrain periaqueductal grey: a correlative functional and anatomical study. Brain Res. 541, 206-215.

CARRtVE, P., BANDt~, R. and DAMPI,~'~, R. A. L. (1988) Anatomical evidence that hypertension associated with the defence reaction in the cat is mediated by a direct projection from a restricted portion of the midbrain periaqueductal gray to the subretrofacial nucleus of the medulla. Brain Res. 460, 339-345.

CARRIW, P., BA~I~R, R. and DAMP~EY, R. A. L. (1989) Viscerotopic control of regional vascular beds by discrete groups of neurons within the midbrain periaqueductal gray. Brain Res. 493, 385-390.

CHAN, R. K. W., CleAN, Y. S. and WONG, T. M. (1991a) Electro- physiological properties of neurons in the rostral ventrolateral medulla of normotensive and spontaneously hypertensive rats. Brain Res. ~49, 118-126.

CH^~, R. K. W., CH^N, Y. S. and WONO, T. M. (1991b) Responses of cardiovascular neurons in the rostral ventrolateral medulla of the normotensive Wistar Kyoto and spontaneously hypertensive rats to iontophoretic application of angiotensin II. Brain Res. 556, 145--150.

C-hR~ELLO, J. and CAV~X~SON, M. M. (1989) Relation of enkephalin-like immunoreactive neurons to other neuropeptide and monoamine- containing neurons in the ventrolateral medulla. In: Central Neural Organization of Cardiovascular Control. Progress in Brain Research, Vol. 81, pp. 3-15. Eds. J. C~RI~LLO, M. M. CAVERSON and C. PoLos~. Elsevier: Amsterdam.

Cmn~LLO, J., CAWRSON, M. M. and PARK, D. H. (1986a) Immunohis- tochemical identification of noradrenaline- and adrenaline-synthesizing neurons in the cat ventrolateral medulla. J. comp. Ncurol. 253, 216-230.

CIRn~U~3, J., CAv~ttsoN, M. M. and POLOSA, C. (1986b) Function of the ventrolateral medulla in the control of the circulation. Brain Res. Rev. I1, 359-391.

COCH~E, K. L. and NA~AN, M. A. (1989) Normotension in conscious rats after placement of bilateral electrolytic lesions in the rostral ventrolateral medulla. J. auton. Nerv. Syst. 26, 199-21 I.

Co~,~oR, H. E. and DREW, G. M. (1987) Do adrenaline-containing neurones from the rostral ventrolateral medulla excite pregangiionic sympathetic cell bodies? J. auton. Pharmac. 7, 87-96.

Coo~, J. H. (1988) The organization of cardiovascular neurons in the spinal cord. Rev. Physiol. Biochem. Pharmac. I10, 147-285.

Coo3~, J. H. and MACLEOD, V. H. (1984) Estimation of conduction velocity in bulbospinal excitatory pathways to sympathetic outflows in cat spinal cord. Brain Res. 311, 97-107.

Coo~, J. H., M^¢~3D, V. H., F~TWOOD-WALKER, S. and G~LnEY, M. P. (198 I) The response of individual sympathetic pregangiionic

neurons to microelectrophoretically applied endogenous monoamines. Brain Res. 215, 135--145.

COX, B. F. and BRODY, M. J. (1991) Tonic control of arterial pressure and regional hemodynamics by supra-medullary sites. Clin. exp. Hypertens. AI3, 197-218.

CRAVO, S. L., MORRISON, S. F. and P-~ts, D. J. (1991) Differentiation of two cardiovascular regions within caudal ventrolateral medulla. Am. J. Physiol. 261, R985-R994.

CUMM~O, P., VoN Kt~OSIGK, M., R£U,~R, P. B., McGv.Et, E. G. and Vn~NT, S. R. (1986) Absence of adrenaline neurons in the guinea pig brain--a combined immunohistochemical and high- performance liquid chromatography study. Neurosci. Lett. 63, 125-130.

CtntTIS, D. R. and JOHNSTON, G. A. R. (1974) Amino acid transmitters in the mammalian central nervous system. Ergebn. Physiol. 69, 97-188.

DAnIR~, H., LAUeIE, M. and SCHMtTT, H. (1990) Hypotensive effects of 5-HTIA receptor agonists on the ventrolateral medullary pressor area in dogs. J. cardiovasc. Pharmac. 15 Suppl. 7, $61-$67.

DAMPS, R. A. L. (1981) Brain stem mechanisms in the control of arterial pressure. Clin. exp. Hypertension 3, 379-391.

DAMPN]~', R. A. L. (1990) The subretrofacial nucleus: its pivotal role in cardiovascular regulation. News Physiol. Sci. 5, 63-67.

D^MI,~Y, R. A. L. and MCALtXN, R. M. (1988) Differential control of sympathetic fibres supplying hindlimb skin and muscle by subretrofacial neurones in the cat. J. Physiol., Lond. 395, 41-56.

DAMP~', R. A. L. and MOON, E. A. (1980) Role of ventrolateral medulla in vasomotor response to cerebral ischemia. Am. J. Physiol. 239, H349-H358.

DAMPLY, R. A. L., GOODCHILD, A. K., ROnERTSON, L. G. and MOI~rrGO~RY, W. (1982) Role of ventrolateral medulla in vasomotor regulation: a correlative anatomical and physiological study. Brain Res. 249, 223-235.

DAMPr,~Y, R. A. L., GOODCmLD, A. K. and TAN, E. (1985) Vasopres- sor neurons in the rostral ventrolateral medulla of the rabbit. J. auton. Nerv. Syst. 14, 239-254.

DAMPNEY, R. A. L., CZACHURSKI, J., DEMBOWSKY, K., GOODCHILD, A. K. and SELLER, H. (1987a) Afferent connections and spinal projections of the pressor region in the rostral ventrolateral medulla of the cat. J. auton. Nerv. Syst. 20, 73-86.

DAMPI,~¥, R. A. L., GOODCI4tLD, A. K. and MCALLEN, R. M. (1987b) Vasomotor control by subretrofacial neurones in the rostral ventrolateral medulla. Can. J. Physiol. Pharmac. 65, 1572-1579.

DAMPLY, R. A. L., BLESSING, W. W. and TAN, E. (1988) Origin of tonic GABAcrgic inputs to vasopressor neurons in the subretrofacial nucleus of the rabbit. J. auton. Nerv. Syst. 24, 227-239.

DEAN, C. and COOTE, J. H. (1986) A ventromedullary relay involved in the hypothaiamic and chemoreceptor activation of sympathetic postgangiionic neurons to skeletal muscle, kidney and splanchnic area. Brain Res. 377, 279-285.

DEAN, C., SEAGARD, J. L., HoPe, F. A. and KAMPU,~, J. P. (1992) Differential control of sympathetic activity to kidney and skeletal muscle by ventral medullary neurons. J. auton. Nerv. Syst. 37, 1-10.

DITTMAR, C. (I 873) Ueber die Lage des sogenannten Gefasszentrums in der Medulla oblongata. Bet. Verb. siichs Ges. Wiss. Leipzig, Math. Phys. KI. 25, 449--469.

DONOGHUE, S., FELDER, R. B., GILBEY, M. P., JORDAN, D. and Sp'~'R, K. M. (1985) Post-synaptic activity evoked in the nucleus tractus solitarius by carotid sinus and aortic nerve afferents in the cat. J. Physiol., Lond. 360, 261-273.

DOR~,~R, K. J., RUOO~RO, D. A., ANWAR, M. and ASHt.OfK, S. R. (1986) Functional mapping of the canine rostral ventrolateral medulla (RVL) with L-glutamate and phenylethanolamine N- methyltransferase. Soc. Neurosci. Abstr. 14, 191.

FELDBEP, G, W. and GUERTZENS~IN, P. G. (1976) Vasodepressor effects obtained by drugs acting on the ventral surface of the brain stem. J. Physiol., Lond. 258, 337-355.

FILLASTRE, J. P., LETAC, B., GALIN~, F., L~ BIn^N, G. and SCtlWARTZ, J. (1988) A multicenter double blind study of rilmenidine and clonidine in 333 hypertensive patients. Am. J. Cardiol. 61, 81D-85D

FULLER, R. W. and HEMKICK-LUECKE, S. K. (1983) Species differences in epinephrine concentration and norepinephrine N-methyltransferase activity in hypothalamus and brainstem. Comp. Physiol. Biochem. 74C, 47-49.


GEBBER, G. L. and BARMAN, S. M. (I 980) Basis for 2-6 cycle/s rhythm in sympathetic nerve discharge. Am. J. Physiol. 239, R48-R56.

GELSEMA, A. J., ROE, M. J. and CALARESU, F. R. (1989) Neurally mediated cardiovascular responses to stimulation of cell bodies in the hypothalamus of the rat. Brain Res. 482, 67 -77.

GILBEY, M. P., NUMAO, Y. and SPYER, K. M, (1986) Discharge patterns of cervical sympathetic preganglionic neurones related to central respiratory drive in the rat. J. Physiol., Lond. 378, 253-265.

GIULIANO, R., RUGGIERO, D. A., MORRISON, S.~ ERNSBERGER, P. and REIS, D. J. (1989) Cholinergic regulation of arterial pressure by the C1 area of the rostral ventrolateral medulla. J. Neurosci. 9, 923-942.

GOBEL, U., SCHROCK, H., SELLER, H. and KUSCHINSKY, W. (1990) Glucose utilization, blood flow and capillary density in the ventrolateral medulla of the rat. Pfliigers Arch. ges. Physiol. 416, 477-480.

GOMEZ, R. E., ERNSBERGER, P., FEINLAND, G. and REIS, D. J. (1991) Rilmenidine lowers arterial pressure via imidazole receptors in brainstem CI area. Eur. J, Pharmaeol. 195, 181-191.

GOOOCHILD, A. K., DAMPNEY, R, A. L. and BANDLER, R. (1982) A method for evoking physiological responses by stimulation of cell bodies, but not axons of passage, within localized regions of the central nervous system. J. Neurosei. Meth. 6, 351-363.

GRANATA, A. R., RUC, GmRO, D. A., PARr, D. H., JOH, T. H. and RElS, D. J. (1983) Lesions of epinephrine neurons in the rostral ventrolateral medulla abolish the vasodepressor components of baroreflex and cardiopulmonary reflex. Hypertension 5 (Suppl.), V80-V84.

GRANATA, A. R., RUGGIERO, D. A., PARK, D. H., JOH, T. H. and REIS, D. J. (1985) Brain stem area with C1 epinephrine neurons mediates baroreflex vasodepressor responses. Am. J. Physiol. 248, H547-H567.

GUERTZENSTEIN, P. G. (1973) Blood pressure effects obtained by drugs applied to the ventral surface of the brain stem. J. Physiol., Lond. 229, 395-408.

GUERTZENSTE1N, P. G. and SILVER, A. 0974) Fall in blood pressure produced from discrete regions of the ventral surface of the medulla by glycine and lesions. J. Physiol., Lond. 242, 489--503.

GUYENET, P. G. (1990) Role of the ventral medulla oblongata in blood pressure regulation. In: Central Regulation of Autonomic Functions, pp. 145-167. Eds. A. D. LOEWY and K. M. SPYER. Oxford University Press: New York.

GUYErCET, P. G. and BROWN, D. U (1986) Nucleus paragigantocellularis lateralis and lumbar sympathetic discharge in the rat. Am. J. Physiol. 250, R1081-R1094.

GUYENET, P. G. and CABOT, J. B. (1981) Inhibition of sympathetic preganglionic neuron by catecholamines and clonidine-mediation by an alpha-adrenergic receptor. J. Neurosci. I, 908-917.

GUYENET, P. G. and STORNETTA, R. L. (1982) Inhibition of sympathetic preganglionic discharges by epinephrine and :~-methyl- epinephrine. Brain Res. 235, 271-283.

GUYE~T, P. G., SUN, M.-K. and BROWN, D. L. (1987) Role of GABA and excitatory aminoacids in medullary baroreflex pathways. In: Organization o f the Autonomic Nervous System: Central and Peripheral Mechanisms, pp. 215-225. Eds. J. CIRIELLO, F. R. CALARESU, C. POLOSA and L. P. RENAUD. Alan R. Liss: New York.

GUYENET, P. G., HASEL'FON, J. R. and SLr~, M.-K, (1989) Sympathoex- citatory neurons of the rostroventrolateral medulla and the origin of the sympathetic vasomotor tone. In: Central Neural Organiz- ation of Cardiovascular Control. Progress in Brain Research, Vol. 81, pp. 105-116. Eds. J. CIRIELLO, M. M. CAVERSON and C. POLOSA. Elsevier: Amsterdam.

GUYENET, P. G., DARNALL, R. A. and RILEY, T. A. (1990) Rostral ventrolateral medulla and sympathorespiratory integration in rats. Am. J. Physiol. 259, RI063-R1074.

HALLIDAY, G. M. and MCLACrlLAN, E, M. (1991) A comparative analysis of neurons containing catecholamine-synthesizing enzymes and neuropeptide Y in the ventrolateral medulla of rats, guinea pigs and cats. Neuroscience 43, 531-550.

HASELTON, J. R. and GUYEI~rE'r, P. G. (1989a) Central respiratory modulation of medullary sympathoexcitatory neurons in rat. Am J. Physiol. 256, R739-R750.

HASELTON, J. R. and GUVEh'ET, P. G. (1989b) Electrophysiological characterization of putative CI adrenergic neurons in the rat. Neuroscience 30~ 199-214.

HASEETON, J. R. and GUWNET, P. G. (1990) Ascending collaterals of

medullary barosensitive neurons and CI cells in rats Am. 7 Physiol. 258, RI051-RI063.

HAYES, K. and WEAVER, L. C. (1990) Selective control of sympathetic pathways to the kidney, spleen and intestine by the ventrolateral medulla in rats. J. Physiol., Lond. 428, 371-385.

HEAD, G. A. and HOWE, P. R. C. (1987) Effects of 6-hydroxydopa- mine and the PNMT inhibitor LY134046 on pressor responses to stimulation of the subretrofacial nucleus in anesthetized stroke- prone spontaneously hypertensive rats. J. auton. Nerr. Syst. lg, 213 224.

I-IELKE, C. J., T,OR, K. B, and SASEK, C. A. (1989) Chemical neu- roanatomy of the parapyramidal region of the ventral medulla in the rat. In: The Central Neural Organization o f Cardiovascular Control. Progress in Brain Research, Vol. 81, pp. 17-28. Eds. J. CIRIELLO, M. M, CAVERSON and C. POLOSA. Elsevier: Amsterdam.

HOKFELT, T., FUXE, K., GOLDS'rEIN, M. and JO,ANSSON, O. (1974) Immunohistochemical evidence for the existence of adrenaline neurons in the rat brain. Brain Res. 66, 235-251.

H()KFELT, T., FUXE, K., JOHANSSON, D., JEFFCOATE, S. and WHtr~, N. (1975) Distribution of thyrotropin,reteasing hormone (TRH) in the central nervous system as revealed with immunocytochemistry. Eur. J. Pharmac. 34, 389-392.

HUNT, S. P., PINI, A. and EVAN, G. (1987) Induction of c-fos-like protein in spinal cord neurons following sensory stimulation. Nature (Lond.) 328, 632-634.

J,~NIG, W. (1988) Pre- and postganglionic vasoconstrictor neurons: Differentiation, types and discharge properties. A. Retd. Physiol. 50, 525-540.

JONES, B. E., HOLMES, C. J., RODRIGUEZ-VEIGA, E. and MAINVILLE, L. (1991) GABA-synthesizing neurons in the medulla--their relationship to serotonin-containing and spinally projecting neurons in the rat. J. comp. Neurol. 313, 349-367.

KAMIYA, H., ITOH, K., YASUI, Y., lso, T. and MtzuNo, N. (1988) Somatosensory and auditory relay nucleus in the rostral part of the ventrolateral medulla: a morphological study in the cat. J. camp. Neurol. 273, 421-435.

KA~3OR, V., MtNSON, J. and CHALMERS, J. (1992) Ventral medulla stimulation increases blood pressure and spinal cord amino acid release. Neuroreport 3, 55-58.

KAPP, B. S., GALLAGHER, M., UNDERWOOD, M. D., MCNALL, C. L. and W,II"E.ORN, D. (1982) Cardiovascular responses elicited by electrical stimulation of the amygdala central nucleus in the rabbit. Brain Res. 234, 251-262.

KATAFUCHI, T., OOMURA, Y. and KUROSAWA, M. (1988) Effects of chemical stimulation of paraventricular nucleus on adrenal and renal nerve activity in rats. Neurosci. Lett. 86, 195-200.

KtTAHAMA, K., .DENOROY, L., BEROD, A. and JOUVET, M. (1986) Dis- tribution of PNMT-immunoreactive neurons in the cat medulla oblongata. Brain Res. Bull. 17, 197-208.

KORINGER, W. (1978) Central ~t-adrenergic systems as targets for hypotensive drugs. Rev. PhysioL Biochem. Pharmae. 81, 39-100.

KOENIG-BERARD, E., TIERNEY, C., BEAU, B., DELBARRE, G., LI-K~TE, F. and LAaRtD, C. (1988) Cardiovascular and central nervous system effects of rilmenidine (S 3341) in rats. Am. J. Cardiol. 61, 22D-31D

KuBo, T. and KItfARA, M. (1987) Studies on GABAergic mechanisms responsible for cardiovascular regulation in the rostral ventrolateral medulla of the rat. Arch. int. Pharmacodyn. 285, 277-287.

KUMADA, M., TERUI, N. and KUWAKI, T. (I 990) Arterial baroreceptor reflex--its central and peripheral neural mechanisms. Prog. Neurobiol. 35, 331-361.

KUNOS, G., MOSQUEDA-GARCIA, R. and MASTRIANNI, J. A. (1987) Endorphinergic mechanism in the central cardiovascular and analgesic effects of clonidine. Can. J. Physiol. Pharmac. 65, 1624-1632.

LEE, S. B., K1M, S. Y. and StinG, K. W. (1991) Cardiovascular regulation by cholinergic mechanisms in rostral ventrolaterat medulla of spontaneously hypertensive rats. Eur. J. Pharmaeol. 205, 117- 123.

LEWIS, D. I. and COOrE, J. H. (1990) Excitation and inhibition of rat sympathetic preganglionic neurons by catecholamines. Brain Res. 530, 229-234.

L¿, Y.-W. and BLESSING, W. W. (1990) Localization of vasodepressor neurons in the caudal ventrolateral medulla in the rabbitl Brain Res. 517, 57-63.

LL Y.-W. and DAMPNEY, R. A. L. (1992) Expression ofe-/os protein


in the medulla oblongata of conscious rabbits in response to baroreceptor activation. Neurosci. Lett. 144, 70-74.

LI, Y.-W., G~ROBA, Z. J., MCALLEN, R. M. and BLr:SSlNO, W. W. (1991) Neurons in rabbit caudal ventrolateral medulla inhibit bulbospinal barosensitive neurons in rostral medulla. Am. J. Physiol. 261, R44-R51.

L[, Y.-W., POLSON, J. W. and DAMPt, n~V, R. A. L. (1992a) Angiotensin II excites vasomotor neurons but not respiratory neurons in the rostral and caudal ventrolateral medulla. Brain Res. 577, 161-164.

Lt, Y.-W., WESSELINGH, S. L. and BL~SING, W. W. (1992b) Projec- tions from rabbit caudal medulla to C1 and A5 sympathetic premotor neurons, demonstrated with phascolus leucoagglutinin and herpes simplex virus. J. camp. Neural. 317, 379-395.

LIN, Q. and LI, P. (1990) Rostral medullary eholinergic mechanisms and chronic stress-induced hypertension. J. auton. Nerv. Syst. 31, 211-218.

LIND, R. W., SWANSON, L. W. and GANTLm, D. (1985) Organization of angiotensin immunoreactive cells and fibers in the rat central nervous system. Neuroendocrinology 40, 2-24.

LLEWELLYN-SMITH, I. J., Pa~D, K. D., MINSON, J. B., P1LOWSKY, P. M. and CHALMEI~, J. P. (1992) Glutamate-immunoreaetive synapses on retrogradely-labeled sympathetic preganglionic neurons in rat thoracic spinal cord. Brain Res. 581, 67-80.

LoEsotcr~, H. H. and KOEPC~, H. P. (1958) Beeinflussung van Atmung und vasomotorik durch einbringen van novocain in die Liquorraume. Pfliigers Arch. ges. Physiol. 266, 61 I--627.

LOVlCK, T. A. (1985) Projections from the diencephalon and mesen- cephalon to nucleus paragigantocellularis lateralis in the cat. Neuroscience 14, 853--861.

LOVlCK, T. A. (1986) Projections from brainstem nuclei to the nucleus paragigantocellularis lateralis in the cat. J. auton. Nerv. Syst. 16, l - l l .

LOVlCK, T. A. (1987) Differential control of cardiac and vasomotor activity by neurones in nucleus paragigantocellularis lateralis in the cat. J. Physiol., Land. 389, 23-35.

LOVlCK, T. A. (1989) Cardiovascular responses to 5-HT in the ventrolateral medulla of the rat. J. auton. Nerv. Syst. 28, 35-41.

LUDnROOK, J. and RurrER, P. C. (1988) Effect of naloxone on hemodynamic responses to acute blood loss in unanesthetized rabbits. J. Physiol., Land. 400, 1-14.

LUITEN, P. G. M., HORST, G. J. TrY, KARST, H. and SrwreNS, A. B. (1985) The course of paraventricular hypothalamic efferents to autonomic structures in medulla and spinal cord. Brain Res. 329, 374-378.

MANDAL, A. K., ZHONG, P., KELLAR, K. J. and GILLIS, R. A. (1990) Ventrolateral medulla--an important site of action for the hypotensive effect of drugs that activate serotonin-lA receptors. J. cardiovasc. Pharmac. 15, $49-$60.

MCALLEN, R. M. (1985) Mediation of the fastigial pressor response and a somatosympathetic reflex by ventral medullary neurones in the cat. J. Physiol., Land. 368, 423-433.

MCALLEN, R. M. (1986a) Identification and properties of sub-retrofacial bulbospinal neurones: a descending cardiovascular pathway in the cat. J. auton. Nerv. Syst. 17, 151-164.

MCALLLm, R. M. (1986b) Location of neurones with cardiovascular and respiratory function, at the ventral surface of the cat's medulla. Neuroscience 18, 43-49.

MCALLlrN, R. M. (1986c) Action and specificity of ventral medullary vasopressor neurones in the cat. Neuroscience 18, 51-59.

MCALLEN, R. M. (1987) Central respiratory modulation of subretrofacial bulbospinal neurones in the cat. J. Physiol., Land. 388, 533-545.

MCALLEN, R. M. and DAMPNEY, R. A. L. (1989) The selectivity of descending vasomotor control by subretrofacial neurons. In: Central Neural Organization of Cardiovascular Control. Progress in Brain Research, Vol. 81, pp. 233-242. Eds. J. CmIELLO, M. M. CAV~RSON and C. POLOSA. Elsevier: Amsterdam.

McALLEN, R. M. and DAMI'NEY, R. A. L. (1990) Vasomotor neurons in the rostral ventrolateral medulla are organized topographically with respect to type of vascular bed but not body region. Neurosci. Lett. I10, 91-96.

MCALLEN, R. M., BADOER, E., SHAVTON, A. D., OLDFIELD, B. J. and MCKINLEY, M. J. (1992) Hemorrhage induces c-fos immunoreactivity in spinally projecting neurons of cat subretrofacial nucleus. Brain Res. 575, 329-332.

MCCALL, R. B. (1988a) Effects of putative neurotransmitters on

sympathetic preganglionic neurons. Ann. Rev. Physiol. 50, 553-564.

MCCALL, R. B. (1988b) GABA-mediated inhibition of sympathoeaci- tatory neurons by midline medullary stimulation. Am. J. Physiol. 255, R605-R615.

McLACHLAN, E. M. ANDERSON, C. R. and Sn~CLAIR, A. D. (1989) Are there bulbospinal catecholaminergic neurons in the guinea pig equivalent to the CI cell group in the rat and rabbit? Brain Res. 481, 274-285.

MI~LFt, M. P., RUGG~RO, D. A., ISHtTSUKA, T. and i~Is, D. J. (1985) Intrinsic ~,-aminobutyric acid neurons in the nucleus of the solitary tract and the rostral ventrolateral medulla of the rat: an immunn- cytochemical and biochemical study. Neurosci. Lett. 58, 83-89.

MEESEN, H. and OLSZL~.'SICI, J. (1949) A Cytoarchitectonic Atlas of the Rhombencephalon of the Rabbit. S. Karger: Basel.

MEIER, M., ORWlN, J., ROGG, H. and BltUWN~R, H. (1980) fl-adrenoceptor antagonists in hypertension. In: Pharmacology of Anti- hypertensive Drugs, pp. 179-194. Ed. A. SCRIAnINE. Raven: New York.

MENDEtSOm~, F. A. O., ALLEN, A. M., CLEVERS, J., DENTON, D. A., TARJAN, E. and McKINLEY, M. J. (1988) Localization of angiotensin II receptor binding in rabbit brain by in vitro autoradiography. J. comp. Neurol. 270, 372-384.

MILER, T. A., [hCt~L, V. M., PARK, D. H., JOH, T. H. and l~lS, D. J. (1987) Phenylethanolamine N-methyltransferase-containing neurons in the rostral ventrolateral medulla of the rat. I. Normal ultrastructure. Brain Res. 411, 28-45.

MILER, T. A., lhCKEL, V. M., AB^TI~, C., JOH, T. H. and l~ts, D. J. (1988) Ultrastructural characterization of substance P-like immunoreactive neurons in the rostral ventrolateral medulla in relation to neurons containing catecholamine-synthesizing enzymes. J. camp. Neural. 270, 427-445.

MtL~ER, T. A., PlCg£L, V. M., GtULIANO, R. and RExs, D. J. (1989a) Ultrastructural localization of choline acetyltransferase in the rat rostral ventrolateral medulla: evidence for major synaptic relations with non-catecholaminergic neurons. Brain Res. 500, 67-89.

MILER, T. A., IhCKEL, V. M., MORRISON, S. F. and REIS, D. J. (1989b) Adrenergic neurons in the rostral ventrolateral medulla: ultrastructure and synaptic relations with other transmitter-identified neurons. In: Central Neural Organization of Cardiovascular Con- trol. Progress in Brain Research, Vol. 81, pp. 29-47. Eds. J. CIRIELLO, M. M. CAWRSON and C. POLOSA. Elsevier: Amsterdam.

MIL~,~R, T. A., I~CKEL, V. M. and REtS, D. J. (1989¢) Ultrastructural basis for interactions between central opioids and catecholamines. I. Rostral ventrolateral medulla. J. Neurosci. 9, 2114-2130.

MINSON, J., PILOWSKY, P., LLEWELLYN-SMITH, I., KANEKO, T., KAPOOR, V. and CtlALM~S, J. (1991) Glutamate in spinally projecting neurons of the rostral ventral medulla. Brain Res. 555, 326-331.

MIURA, M., ONAI, T. and TAKAYAMA, K. (1983) Projections of upper structure to the spinal cardioacceleratory center in cats: an HRP study using a new microinjection method. J. auton. Nerv. Syst. 7, 119-139.

MIURA, M., TAKAYAMA, K. and OKADA, J. (1991) Difference in sensitivity of cardiovascular and pulmonary control RVLM neurons to glutamate and agonists of four glutamate receptor subtypes between SHR rats, WKY rats and cats. J. auton. Nerv. Syst. 36, 1-12.

MIYAZAKI, T., COOTE, J. H. and DUN, N. J. (1989) Excitatory and inhibitory effects of epinephrine on neonatal rat sympathetic preganglionic neurons in vitro. Brain Res. 497, 108-116.

MORILAK, D. A., SOMOOYL P., MCILHINNEY, R. A. J. and CHALMERS, J. (1989) An enkephalin-containing pathway from nucleus tractus solitarius to the pressor area of the rostral ventrolateral medulla of the rabbit. Neuroscience 31, 187-194.

MORRISON, S. F. and GEBBER, G. L. (1982) Classification of raphe neurons with cardiac-related activity. Am. J. Physiol. 243, R49-R59.

MOt~tRISON, S. F. and l~ts, D. J. (1989) Reticulospinal vasomotor neurons in the RVL mediate the somatosympathetic reflex. Am. J. Physiol. 256, RI084-RI097.

MORRISON, S. F., MILNER, T. A. and I~ls, D. J. (1988) Reticulospinal vasomotor neurons of the rat rostral ventrolateral medulla: Relationship to sympathetic nerve activity and the CI adrenergic cell group. J. Neurosci. 8, 1286-1301.

MORRISON, S. F., ERNSaERGER, P., MILNER, T. A., CALLAWAY, J., GONG, A. and R~is, D. J. (1989) A glutamate mechanism in the


intermediolateral nucleus mediates sympathoexcitatory responses to stimulation of the rostral ventrolateral medulla. In: Central Neural Organization of Cardiovascular Control. Progress in Brain Research, Vol. 81, pp. 159-169. Eds. J. CIRIELLO, M. M. CAVERSON and C. POLOSA. Elsevier: Amsterdam.

MURATANI, H., AVERILL, D. B. and FERRARIO, C. M. (1991) Effect of angiotensin II in ventrolateral medulla of spontaneously hypertensive rats. Am. J. Physiol. 260, R97%R984.

NEWMAN, D. B. (1985) Distinguishing rat brainstem reticulospinal nuclei by their neuronal morphology. 1. Medullary nuclei. J. Hirnforsch. 26, 187-226.

NICHOLAS, A. P. and HANCOCK, M. B. (1988) Immunocytochemical evidence for substance P and serotonin input to medullary bul- bospina! adrenergic neurons. Neuroscienee 27, 569-576.

NILAVER, G., ZIMMERMAN, E. A., WILKINS, J., MICHAELS, J., HOFFMAN, D. and SILVERMAN, A.-J. (1980) Magnocellutar hypothalamic projections to the lower brain stem and spinal cord of the rat. Neuroendocrinology 30, 150-158.

NISHI, S., YOSHIMURA, M. and POLOSA, C. (1987) Synaptic potentials and putative transmitter actions in sympathetic preganglionic neurons. In: Organization of the Autonomic. Nervous System: Central and Peripheral Mechanisms, pp. 15-26. Eds. J. CIRIELLO. F:. R. CALARESU, C. POLOSA and L. P. RENAUD. Alan R. Liss: New York.

NOSJEAN, A, and GUYENET, P. G. (199 I) Role of ventrolateral medulla in sympatholytic effect of 8-OHDPAT in rats. Am. d. Physiol. 260, R600-R609.

NUMAO, Y., KOSHIYA, N., G1LBEY, M. P. and SPYER, K. M. (1987) Central respiratory drive-related activity in sympathetic nerves of the rat: the regional differences. Neurosci. Left. 26S, 279-284.

PATON. J. F. R. and SPYER, K. M. (I 990) Brain stem regions mediating the cardiovascular responses elicited from the posterior cerebellar cortex in the rabbit. J. Physiol., Lond. 427, 533-552.

PATON, J. F. R., SILVA-CARVALHO, L., GOLDSMITH, G. E. and SPYER, K. M. (1990) Inhibition of barosensitive neurons evoked by Iobule IXb of the posterior cerebellar cortex in the decerebrate rabbit. J. Physiol., Lond. 427, 553-565.

PILOWSKY, P., WEST, M. and CHALMERS, J. P. (1985) Renal sympathetic nerve responses to stimulation, inhibition and destruction of the ventrolateral medulla in the rabbit. Neurosci. Lett. 60, 51-55.

PILOWSKY, P., MINSON, J., HODGSON, A., HOWE, P. and CHALMERS, J. (I 986) Does substance P coexist with adrenaline in neurons of the rostral ventrolateral medulla in the rat? Neurosci. Lett. 71, 293-298.

POLSON, J. W., HALLIDAY, G. M., MCALLEN, R. M., COLEMAN, M. J. and DAMPNEY, R. A. L. (1992) Rostrocaudal differences in morphology and neurotransmitter content of cells in the subretrofacial vasomotor nucleus. J. auton. Nerv. Syst. 38, 117-138.

PUNNEN, S., URBANSK1, R., KRIEGER, A. J. and SAPRU, H. N. (1987) Ventrolateral medullary pressor area: site of hypotensive action of clonidine. Brain Res. 422, 336-346.

PUNNEN, S., WILLETrE, R., KRIEGER, A. J. and SAPRU, H. N (1984) Cardiovascular response to injections of enkepbalin in the pressor area of the ventrolateral medulla. Neuropharmacology 23, 939-946.

RANSON, S. N. and BILLINGSLEY, P. R. (1916) Vasomotor reactions from stimulation of the floor of the fourth ventricle. Am. d. Physiol. 41, 85-90.

REtNER, P. B. and VINCENT, S. R. (1986) The distribution of tyrosine hydroxylase, dopamine-//-hydroxylase and pbenylethanolamine- N-methyltransferase immunoreactive neurons in the feline medulla oblongata, d. comp. Neurol. 248, 518-531.

RICHTER, D. W. and SPYER, K. M. (1990) Cardiorespiratory control. In: Central Regulation of Autonomic Functions, pp. 189-207. Eds. A. n. LOEWY and K. M. SPYER. Oxford University Press: New York.

Ross, C. A., ARMSTRONG, D. M., RuoomgO, D. A., PICKEL, V. M., JOH, T. H. and REIS, D. J. 0981) Adrenaline neurons in the rostral ventrolateral medulla innervate thoracic spinal cord: a combined immunocytochemical and retrograde transport demonstration. Neurosci. Lett. 25, 257-262.

ROSS, C. A., RUGGIERO, D. A., PARK, D. H., JOH, T. H., SVEO, A. F., PARDAL, J. F., SAAYEDRA, J. M. and PEIS, D. J. (1984) Tonic vasomotor control by the rostral ventrolateral medulla: effect of electrical or chemical stimulation of the area containing CI adrenaline neurons on arterial pressure, heart rate and plasma catecholamines and vasopressin. J. Neurosci. 4, 474-494.

Ross, C. A., RUGG1ERO, D. A. and REIs, D. J. (1985) Projections fi'om the nucleus tractus solitarii to the rostral ventrolateral medulla. J. comp. Neurol. 242, 511-534.

RUCrGIERO, D. A., GATTI, P. J., G1LLIS, R. A., NORMAN, W. P., ANWAR, M. and REIS, D. J. (1986) Adrenaline-synthesizing neurons in the medulla of the cat. J. camp. NeuroL 2.$2, 532-542.

RUGGIERO, D. A., CRAVO, S. L., ARANGO, V. and REIS, D. J. (I 989) Central control of the circulation by the rostral ventrolateral reticular nucleus: anatomical substrates. In: Central Neural Organ - ization of Cardiovascular Control Progress in Brain Research, Vol. 81, pp. 49-79. Eds. J. CIRIELLO, M. M. CAVERSON and C. POLOSA. Elsevier: Amsterdam.

SAEK1, Y., TERU1, N. and KUMADA, M. (1988) Participation of ventrolateral medullary neurons in the renal-sympathetic reflex in rabbits. dap. J. PhysioL 38, 267-281.

SAPRU, H. N., PUNNEN, S. and WILLETTE, R. N. (1987) Role of enkephalins in ventrolateral medullary control of blood pressure. In: Brain Peptides and Catecholamines in Cardiovascular Regu- lation, pp. 153-167. Eds. J. P. BUCKLEY and C. M. FERRARIO. Raven Press: New York.

SASAKI, S. and DAMPNEY, R. A. L. (1990) Tonic cardiovascular effects of angiotensin-II in the ventrolateral medulla. Hypertension 15, 274-283.

SCULl.EKE, M. and LOESCHCKE, H. H. (1967) Lokalisation Pines ander Regulation von Atmung und Kreislauf beteiligten Gebietes an der ventralen Oberfl~che der Medulla oblongata durch Kalteblockade. Pfliigers Arch. ges. Physiol. 297, 201-220.

SELLER, H., KONIG, S. and CZACHURSKt, J. (1990) Chemosensitivity of sympathoexcitatory neurones in the rostroventrolateral medulla of the cat. Pfliigers Arch. ges. Physiol. 416, 735-741.

SIDDALL, P. J. and DAMPNEY, R. A. U (1989) Relationship between cardiovascular neurones and descending antincociceptive pathways in the rostral ventrolateral medulla of the cat. Pain 37, 347-355.

SILVA-CARVALHO, L., PATON, J. F. R., GOLDSMITH, G. E. and SPYER, K, M. (1991) The effects of electrical stimulation of Iobule IXb of the posterior cerebellar vermis on neurons within the rostral ventrolateral medulla in the anesthetised cat. J. auton. Nerv. Syst. 36, 97-106.

SKOFITSCH, G. and JAcoaowtvz, D. M. (1985) Calcitonin gene-related peptide: detailed immunohistochemical distribution in the central nervous system. Peptides (Fayetteville) 6, 721-745.

SMITH, J. C., MORRISON, D. E., ELLENaERGER, H. H., OTTO; M. R. and FELDM^N, J. L. (1989) Brainstem projections to the major respiratory neuron populations in the medulla of the cat. J, comp. Neurol. 281, 69-96.

SMITH, J. K. and BARRON, K. W. (1990) GABAergic responses in ventrolateral medulla in spontaneously hypertensive rats. Am: J. Physiol. 258, R450-R456.

SOMOGYI, P., MINSON, J. B., MORILAK, D., LLEWELLYN-SMITH, I., MCILHtNNEY, J. R. A. and CHALMERS, J. (t989) Evidence for an excitatory amino acid pathway in the brainstem and for its involvement in cardiovascular control. Brain Res. 496, 401-407.

STEIN, R. D., WEAVER, L. C. and YARDLEY, C. P. (1989) Ventrolateral medullary neurones: effects on magnitude and rhythm of discharge of mesenteric and renal nerves in cats. d. Physiol., Lond. 408, 571-586.

STORNETTA, R. L., MORRISON, S. F., RUGGIERO, D. A. and PEts, D. J. (1989) Neurons of rostral ventrolateral medulla mediate somatic pressor reflex. Am. ,L PhysioL 2,~, R448--R462.

STRACK, A. M., SAWYER, W. B., MARLImO, L. M. and LOEWY, A. D. (1988) Spinal origin of sympathetic preganglionic neurons in the rat. Brain Res. 455, 187-191.

STRACK, A. M., SAWYER, W. B., HUGHES, J. H., PLATT, K. B. and LOEWY, A. D. (1989a) A general pattern ofCNS innervation of the sympathetic outflow demonstrated by transneuronal pseudorabies viral infections. Brain Res. 491, 156-162.

STRACK, A. M., SAWYER, W. B., PLATT, K. n. and LOEWY, A. D. (1989b) CNS cell groups regulating the sympathetic outflow to adrenal gland as revealed by transneuronal cell body labeling with pseudorabies virus. Brain Res. 491, 274-296.

SUN, M.-K. and GUYENET, P. G. (1985) GABA-mediated barorecep- tot inihibition of reticulospinal neurons. Am. J. Physiol. 24% R672-R680.

SuN, M.-K. and GUYENET, P. G. (1986) Hypothalamic glutamatergic input to medullary sympathoexcitatory neurons in rats. Am. J. Physiol. 251, R798-RHI0.

Tin,: SUBRETROFACIAL VASOMOTOR NUCLEUS 227

SUN, M.-K. and G u ~ , P. G. (1987) Arterial baroreceptor and vagal inputs to sympathocxcitatory neurons in rat medulla. Am. J. Physiol. 252, R699-R709.

SUN, M.-K and G u ~ , P. G. (1989) Effects of vasopressin and other neuropeptides on rostral medullary sympathoexcitatory neurons 'in vitro'. Brain Res. 492, 261-270.

SUN, M.-K. and GUYEqET, P. G. (1990) Excitation of rostral medullary pacemaker neurons with putative sympathocxcitatory function by cyclic AMP and /Ladrenoceptor agonists 'in vitro'. Brain Res. 511, 30--40.

SUN, M.-K. and SPYER, K. M. (1991a) Responses of rostroventrolateral medulla-spinal vasomotor neurons to chemoreceptor stimulation in rats. J. auton. Nerv. Syst. 33, 79-84.

SuN, M.-K. and SPYI~, K. M. (1991b) Nociceptive inputs into rostral ventrolateral medulla-spinal vasomotor neurons in rats. J, Phys- iol., Lend. 436, 685-700.

SUN, M.-K., HACI~TT, J. T. and GUVENET, P. G. (1988a) Sympathoex- citatory neurons of rostral ventrolateral medulla exhibit pacemaker properties in the presence of a glutamate-receptor antagonist. Brain Res. 438, 23-40.

SUN, M.-K, YOUNG, B. S., HACKL"rT, J. T. and GUYENET, P. G. (1988b) Rostral ventrolateral medullary neurons with intrinsic pacemaker properties are not catecholaminergic. Brain Res. 451, 345-349.

SUN, M.-K., JESI~, I. T. and REIs, D. J. (1992) Cyanide excites medullary sympathoexcitatory neurons in rats. Am. J. Physiol. 262, RI82-RI89.

SUNDARAM, K. and SAPRU, H. (1988) Cholinergic nerve terminals in the ventrolateral medullary pressor area: pharmacological evidence. J. auton. Nerv. Syst. 22, 221-228.

SUNDARAM, K. and SAPRU, H. (1991) NMDA receptors in the intermediolateral column of the spinal cord mediate sympathoexcitatory cardiac responses elicited from the ventrolateral medullary pressor area. Brain Res. 544, 33-41.

SUNDARAM, K., KRIEGER, A. J. and SAPRU, H. (1988) M 2 muscarinic receptors mediate presser responses to cholinergic agonists in the ventrolateral medullary presser area. Brain Res. 449, 141-149.

TABI~R, E. (1961) The cytoarchitecture of the brain stem of the cat. I. Brain stem nuclei of the cat. J. comp. Neurol. !16, 27-69.

TAKAYAMA, K. and MIUXA, M. (1991) Glutamate-immunoreactive neurons of the central amygdaloid nucleus projecting to the subretrofacial nucleus of SHR and WKY rats--A doubleqabeling study. Neurosci. Lett. 134, 62~o6.

T~-RU1, N., SA~Kh Y. and KUMADA, M. (1986a) Barosensory neurons in the ventrolateral medulla in rabbits and their responses to various afferent inputs from peripheral and central sources. Jap. J. Physiol. 36, 1141-1164.

Tl~,ta, N., SAEKI, Y., KUWAKI, T. and KUMADA, M. (1986b) Role of neurons of the ventrolateral medulla oblongata in the arterial baroreceptor reflex in rabbits. In: Brain and Blood Pressure Control, pp. 49-54. Ed. K. NAKAMURA. Excerpta Medica: Amster- dam.

TElttn, N., SAEKI, Y. and KUMADA, M. (1987) Confluence of barosensory and nonbarosensory inputs at neurons in the ventrolateral medulla in rabbits. Can. J. Physiol. Pharmac. 65, 1584-1590.

TERtn, N., MASUDA, N., SAEKI, Y. and KUMADA, M. (1990) Activity of barosensitive neurons in the caudal ventrolateral medulla that send axonai projections to the rostral ventrolaterai medulla in rabbits. Neurosci. Lett. 118, 211-214.

TIBIRICA, E., MERMET, C., FELDMAN, J., CJONON, F. and BousQu~T, P. (1989) Correlation between the inhibitory effect on catecholaminergic ventrolateral medullary neurons and the hypotension evoked by clonidine: a voltammetric approach. J. Pharmac. exp. Ther. 250, 642-647.

TIBIRICA, E., FELDMAN, J., MERMET, C., CrONON, F. and BOUSQUET, P. (1991a) An imidazoline-speciflc mechanism for the hypotensive effect of clonidine: a study with yohimbine and idazoxan. J. Pharmac. exp. Ther. 7,$6, 606-613.

TIBIRICA, E., FELDMAN, J., MERMET, C., MONASSlER, L., GONON, F. and BOUSQUET, P. (1991 b) Selectivity of rilmenidine for the nucleus retieularis lateralis, a ventrolaterai medullary structure containing imidazoline-preferring receptors. Fur. J. Pharmacol. 209, 213-221.

TXZEBSKI, A. and BARADZlV.J, S. (1992) Role of the rostral ventrolateral medulla in the generation of synchronized sympathetic rhythmicities in the rat. J. auton. Nerv. Syst. 41, 129-140.

TSENQ, C.-J., MOSQUEDA-GARCIA, R., APPALSA~rV, M. and ROBOT- SON, D. (1988) Cardiovascular effects of neuropeptide Y in rat brainstem nuclei. Circ. Res. 64, 55-61.

UXnANSKI, R. W. and SAPRU, H. N. (1988) Putative neurotransmitters involved in medullary cardiovascular regulation. J. auton. Nerv. Syst. 25, 181-193.

URnANSKI, R. W., MURUGA1AN, J., KRmG~, A. J. and SAPRU, H. N. (I 989) Cardiovascular effects of substance P receptor stimulation in the ventrolateral medullary presser and depressor areas. Brain Res. 491, 383-389.

VAN BOCKSTAELE, E. J., PIERIBONE, V. A. and ASTON-JONE$, G. (1989) Diverse afferents converge on the nucleus paragigantocellularis in the rat ventrolateral medulla: retrograde and anterograde tracing studies. J. comp. Neurol. 290, 561-584.

WANe, S. C. and RANSON, S. W. (1939) Autonomic responses to electrical stimulation of the lower brain stem. J. comp. Neurol. 71, 437-455.

WFA.~LINGH, S. L., LI, Y.-W. and BLV.SSlNO, W. W. (1989) PNMT- containing neurons in the rostral medulla oblongata (CI, C3 groups) are transneuronally labeled after injection of herpes simplex virus type I into the adrenal gland. Neurosci. Left. 106, 99-104.

WILLETrE, R. N., KRmGES., A. J., BARCAS, P. P. and SAPXU, H. N. (1983) Medullary ~/-aminobutyric acid (GABA) receptors and the regulation of blood pressure in the rat. J. Pharmac. exp. Ther. 226, 893-899.

WILLETTE, R. N., BARCAS, P. P., KRIEGER, A. J. and SAPRU, H. N. (1984a) Endogenous GABAergic mechanisms in the medulla and the regulation of blood pressure. J. Pharmac. exp. Ther. 230, 34-39.

WILLETTE, R. N., ~ , S., KR~.GER, A. J. and SAPRU, H. N. (1984b) Cardiovascular control by cholinergic mechanisms in the rostrai ventrolateral medulla. J. Pharmac. exp. Ther. 231,457-463.

YAMADA, K. A., MCALLL~, R. M. and LoEwY, A. D. (1984) GABA antagonists applied to the ventral surface of the medulla oblongata block the baroreceptor reflex. Brain Res. 297, 175-180.

YAMADA, S., KAGAWA, Y., USHIJIMA, H., TAKAYANAGI, N., TOMITA, T. and HAYASHI, E. (1987) Brain nicotinic cholinoceptor binding in spontaneous hypertension. Brain Res. 410, 212-218.

YAMASmTA, H., KAt,~AN, H., KASAh M. and OSAKA, T. (1987) Decrease in blood pressure by stimulation of the rat hypothalamic paraventricular nucleus with L-glutamate or weak current. J. auton. Nerv. Syst. 19, 229-234.

YAMAZO~, M., StnOSAKA, S., EMSON, P. C. and TOHYAMA, M. 0985) Distribution of neuropeptide Y in the lower brainstem: an immunohistocbemical analysis. Brain Res. 335, 109-120.

YARDt~V, C. P., STEIN, R. D. and WEAVER, L. C. (1989) Tonic influences from the rostral medulla affect sympathetic nerves differen- tially. Am. J. Physiol. 256, R323-R331.

YATES, B. J., YAMAGATA, Y. and BOLTON, P. S. (1991) The ventrolateral medulla of the cat mediates vestibulosympathetic reflexes. Brain Res. 552, 265-272.

ZmGLGXNSBI~RGER, W. and PUIL, E. A. (1973) Actions of glutamic acid on spinal neurones. Expl Brain Res. 17, 35-49.

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