internal jugular venous spillover of noradrenaline and metabolites and their association with...

Internal jugular venous spillover of noradrenaline and

metabolites and their association with sympathetic

nervous activity

G . W . L A M B E R T , 1 D . M . K A Y E , 1 J . M . T H O M P S O N , 1 A . G . T U R N E R , 1

H . S . C O X , 1 M . V A Z , 1 G . L . J E N N I N G S , 1 B . G . W A L L I N 2 and M . D . E S L E R 1

1 Human Autonomic Function Laboratory and Alfred and Baker Medical Unit, Baker Medical Research Institute,

Commercial Road, Prahran Victoria, Australia

2 Department of Clinical Neurophysiology, Sahlgrenska Hospital, G �oteborg, Sweden

ABSTRACT

It is recognized that the brain plays a pivotal role in the maintenance of blood pressure and the control

of myocardial function. By combining direct sampling of internal jugular venous blood with a

noradrenaline isotope dilution method, for examining neuronal transmitter release, and microneuro-

graphic nerve recording, we were able to quantify the release of central nervous system

noradrenaline and its metabolites and investigate their association with efferent sympathetic nervous

outflow in healthy subjects and patients with pure autonomic failure. To further investigate the

relationship between brain noradrenaline, sympathetic nervous activity and blood pressure regulation

we examined brain catecholamine turnover, based on the internal jugular venous overflow of

noradrenaline and its principal central nervous system metabolites, in response to a variety of

pharmacological challenges. A substantial increase was seen in brain noradrenaline turnover

following trimethaphan, presumably resulting from a compensatory response in sympathoexcitatory

forebrain noradrenergic neurones in the face of interruption of sympathetic neural traffic and

reduction in arterial blood pressure. In contrast, reduction in central nervous system noradrenaline

turnover accompanied the blood pressure fall produced by intravenous clonidine administration, thus

representing the blood pressure lowering action of the drug. Following vasodilatation elicited by

intravenous adrenaline infusion, brain noradrenaline turnover increased in parallel with elevation in

muscle sympathetic nervous activity. While it is difficult to assess the source of the noradrenaline

and metabolites determined in our studies, available evidence implicates noradrenergic cell groups of

the posterolateral hypothalamus, amygdala, the A5 region and the locus coeruleus as being involved

in the regulation of sympathetic outflow and autonomic cardiovascular control.

Keywords brain, clonidine, ganglion blockade, isotope dilution, microneurography.

Received 17 June 1997, accepted 7 January 1998

Previous reports, conducted in human subjects, have

illustrated some dependence of sympathetic out¯ow on

cerebral noradrenergic activity (Esler et al. 1990, Ferrier

et al. 1992) and have provided evidence of a relation-

ship between subcortical noradrenergic neuronal ac-

tivity and renal (Ferrier et al. 1993) and cardiac

(Lambert et al. 1995a, b) sympathetic activation. These

results are supported by those from experiments con-

ducted in animals, where stimulation of noradrenergic

pressor regions of the hypothalamus and amygdala have

been shown to increase renal nerve ®ring and renal

vascular resistance (Koepke et al. 1987, Huangfu et al.

1991). In the present study we estimated brain nor-

adrenaline turnover by measuring the internal jugular

venous over¯ow of noradrenaline and its major lipo-

philic central nervous system metabolites, and studied

their relation to human sympathetic nervous system

activity.

Measurements were made, ®rst, with a pharma-

cological challenge which entailed reducing mean

blood pressure via a peripherally acting hypotensive

agent, namely the ganglion blocking drug,

Correspondence: Gavin W. Lambert, FaculteÂ de MeÂdecine, Necker-Enfants Malades, Laboratoire de Pharmacologie, 156 rue de Vaugirard, 75015

Paris, France.

Acta Physiol Scand 1998, 163, 155±163

Ó 1998 Scandinavian Physiological Society 155

trimethaphan. Changes in central nervous system

monoamine turnover in this context we believed

would probably be a consequence of a re¯ex stim-

ulation of pressor, sympathoexcitatory brain regions.

In a second experiment, a similar reduction in supine

blood pressure was elicited by the centrally acting

agent, clonidine. Here it was thought that any

changes in central nervous system monoamine turn-

over might be indicative of the central mode of ac-

tion of the drug. The ®nal pharmacological challenge

involved measuring the internal jugular venous

over¯ow of monoamines following an infusion of

adrenaline. While sympathetic activation post-adrena-

line administration has been described and attributed

to a decrease in central venous pressure following

vasodilatation in skeletal muscle beds (Persson et al.

1989), we sought to investigate the central nervous

system response to such a stimuli.

To further examine the dependence of sympathetic

nervous out¯ow on central monoaminergic neuronal

activity we also studied (1) patients with pure auto-

nomic failure, who have sympathetic nerve degenera-

tion (Meredith et al. 1991) and hence no contribution

by cerebrovascular sympathetic nerves to internal jug-

ular venous monoamine over¯ow and (2) the relation

of muscle sympathetic nervous activity, measured by

microneurography, to central nervous system nor-

adrenaline turnover in healthy subjects.

METHODS

Subjects

The participants comprised 33 healthy volunteers

(aged 18±74 years) and seven patients with protracted

histories of symptomatic postural hypotension attrib-

utable to pure autonomic failure (aged 62 � 5 years).

The healthy subjects were recruited by local adver-

tisement from the general community and underwent

a comprehensive clinical and physical examination to

screen for any previously undiagnosed medical con-

ditions prior to their acceptance in any of the exper-

imental protocols. Exclusion criteria for the healthy

subjects included a history of major illness, cardio-

vascular disease, current drug medication and previous

psychiatric therapy. The screening procedure included

a full blood examination including white cell differ-

ential analysis, serum biochemistry and tests for pre-

vious exposure to the hepatitis B and human

immunode®ciency viruses.

Patients with pure autonomic failure, without central

nervous system de®cit, provided a clinical model of

whole body sympathetic denervation and were used to

examine the potential confounding in¯uence of cere-

brovascular sympathetic nerves on internal jugular ve-

nous noradrenaline and noradrenaline metabolite

over¯ow determinations. These patients were referred

to the Alfred Baker Medical Unit after protracted his-

tories of symptomatic postural hypotension. None of

the patients present had diabetes mellitus, amyloidosis,

autoimmune disease, metabolic disorders or carcinoma.

There were no cases of dopamine-b-hydroxylase de®-

ciency and in none of the patients was there evidence of

peripheral neuropathy. The diagnosis of autonomic

failure was made using a series of both invasive and

non-invasive tests of autonomic function (Meredith

et al. 1991).

The studies reported here conformed to the relevant

guidelines of the National Health and Medical Research

Council of Australia and were approved by the Alfred

Hospital Human Research Ethics Committee. All pa-

tients and healthy volunteers gave written informed

consent prior to their participation in the experimental

procedures.

General procedure

All studies were performed with subjects in the su-

pine position. Caffeinated beverages, alcohol and to-

bacco smoking were prohibited for the 12 h

preceding the catheter study. Blood samples were

obtained from central venous and arterial catheters

percutaneously inserted under strict aseptic conditions

in the cardiac catheterization laboratory of the Alfred

and Baker Medical Unit according to previously de-

scribed methods (Hasking et al. 1986, Lambert et al.

1991). Internal jugular vein catheterization was per-

formed under direct ¯uoroscopic vision with the

catheter tip's position, in the internal jugular vein

beyond the mandibular angle, being veri®ed using

radiopaque contrast media. This catheter was used

for internal jugular vein blood sampling and for the

determination of internal jugular vein blood ¯ow by

thermodilution. In the patients with pure autonomic

failure, and their age-matched healthy counterparts,

the catheter was positioned in the internal jugular

vein following the assessment of cardiac noradrena-

line kinetics (Hasking et al. 1986). In some of the

healthy subjects, the internal jugular venous catheter

placement was combined with microneurographic

peroneal nerve recording to enable examination of

the possible relationship between brain noradrenaline

turnover and muscle sympathetic activity. The blood

pressure and heart rate were continuously monitored

during the experimental protocols. Throughout the

course of the catheter studies levo-[7-3H]-noradrena-

line [speci®c activity of 11±25 Ci mmol)1, New En-

gland Nuclear (Boston, MA, USA)] was infused into

the subjects for the assessment of total body and

cerebral noradrenaline spillover rate determinations.

Central control of sympathetic activity � G W Lambert et al. Acta Physiol Scand 1998, 163, 155±163

156 Ó 1998 Scandinavian Physiological Society

An analogous method utilizing an infusion of tritium

labelled adrenaline (levo-[N-methyl-3H]-adrenaline,

speci®c activity 69±78) was performed in conjunction

with the tritiated noradrenaline infusion to measure

rates of adrenaline secretion.

In all studies, 10-mL blood samples for plasma

neurochemical evaluation were obtained simultaneously

from the arterial and venous catheters and immediately

placed in chilled tubes containing an anticoagulant/

antioxidant mixture of ethyleneglycol and reduced

glutathione in 200 lL of water. At the completion of

the catheter study and within 15±75 min of sampling,

the blood samples were centrifuged and the plasma

stored at )80 °C until assayed. Arterial haematocrits

were determined for each subject.

Ganglion blockade with trimethaphan

In a subset of the healthy subjects (n � 4, aged

33 � 8 years), immediately after the resting internal

jugular vein blood samples were obtained, an intra-

venous infusion of the ganglion blocker, trimethap-

han (Arfonad, trimethaphan camsylate, Roche

Products Pty Ltd, NSW, Australia) was commenced.

This drug was given slowly, at a dose suf®cient to

produce a reduction of »10±20 mmHg in supine

systolic blood pressure (0.4±1.2 mg min)1). Blood

sampling for internal jugular venous neurochemical

evaluation was repeated at 30 and 60 min following

initiation of trimethaphan administration. Heart rate

and blood pressure were monitored continuously

throughout trimethaphan administration and the ef-

fect of ganglion blockade on jugular venous mono-

amine over¯ow, and on sympathetic nervous function

was examined.

Central suppression of sympathetic nervous out¯ow with clonidine

In a further subset of the healthy subjects (n � 5, aged

24 � 4 years), following resting internal jugular venous

blood sampling, an intravenous infusion of the centrally

acting sympathoinhibitory agent, clonidine (Catapres,

clonidine hydrochloride, Boehringer Ingelheim, NSW,

Australia), was commenced. This drug was given

slowly, over »15 min, at a dose suf®cient to produce a

reduction of »15±20 mmHg in supine systolic blood

pressure (150±225 lg total dose). Blood sampling for

neurochemical evaluation was repeated at 30 and

60 min following initiation of clonidine administration.

The heart rate and blood pressure were monitored

continuously throughout clonidine treatment and, as

for trimethaphan, the effect of the drug on sympathetic

nervous function was examined by determining the rate

of spillover of noradrenaline into plasma for the body

as a whole.

Sympathetic nervous activity following adrenaline infusion

In another subset of the healthy individuals (n � 7,

aged 21 � 1 years), resting blood samples were ob-

tained from either the right or left internal jugular vein.

Muscle sympathetic nervous activity was concurrently

measured using the microneurographic technique of

Valbo et al. (Valbo et al. 1979, Esler et al. 1991). After

resting blood samples and microneurographic record-

ings were obtained, an intravenous infusion of adren-

aline (2±3 lg min)1 over 30±40 min) was administered.

Muscle sympathetic nervous activity was recorded

throughout the infusion and post-infusion period.

Twenty minutes after the termination of the adrenaline

infusion internal jugular venous blood sampling was

repeated.

Neurochemical assays

Plasma neurochemical concentrations were determined

by high performance liquid chromatography coupled

with electrochemical detection according to previously

published techniques (Medvedev et al. 1990, Lambert

et al. 1994). The interassay coef®cients of variation,

determined from »80 consecutive assay runs, were

�11% for noradrenaline, �8% for dihydroxyphenyl-

glycol (DHPG), �4% for 3-methoxy-4-hydroxyphen-

ylglycol (MHPG) and �3% for adrenaline. The intra-

assay coef®cients of variation, determined between ®ve

and eight repeated measurements of pooled venous

plasma, were �3% for noradrenaline, �2% for DHPG,

�5% for MHPG and �6% for adrenaline. All assays

were linear within the physiological range with a sen-

sitivity (signal-to-noise ratio of 3) of 0.1 pmol for the

catechols and 1.0 pmol for MHPG.

Assessment of central nervous system monoamine turnover

Veno-arterial plasma concentration differences com-

bined with an appropriate internal jugular vein ¯ow

measurement were used, according to the Fick Princi-

ple, to determine metabolite over¯ows from the brain

and were calculated according to the following general

formula:

Overflow � �Venousconc ÿ Arterialconc� � Q

where Venousconc and Arterialconc are the plasma

concentrations of the compound of interest in the ve-

nous ef¯uent and the arterial blood supply, respectively,

and Q refers to the plasma or blood ¯ow. For the

catecholamines, noradrenaline and adrenaline, a further

adjustment was made allowing for the fractional ex-

traction of tritium labelled catecholamine across the

brain during a constant rate infusion of radiolabelled

noradrenaline and adrenaline. As there is no evidence

of extraction of other metabolites across the brain


Acta Physiol Scand 1998, 163, 155±163 G W Lambert et al. � Central control of sympathetic activity

during transcerebral passage (Goldstein et al. 1991), the

net over¯ow was calculated without recourse to isotope

dilution methodology.

Assessment of sympathetic nervous activity

Sympathetic nervous system function was evaluated

with recording of efferent post-ganglionic sympathetic

nerve ®ring rates by microneurography (Valbo et al.

1979), and with measurement of noradrenaline spillover

by isotope dilution (Esler et al. 1979). For noradrenaline

spillover measurements, after steady state arterial plas-

ma tracer concentrations of 3H-noradrenaline had been

reached, the overall release rate into plasma of endog-

enous noradrenaline was determined according to

methods developed in our laboratory (Esler et al. 1979)

and calculated according to the formula:

Total Spillover rate � �3H� Catechol Infusion Rate

Plasma Catechol SpecificRadioactivity

For regional noradrenaline spillover (Esler et al.

1984a,b, Esler et al. 1988) at steady state:

Regional spillover � �NAven ÿNAart�� NAart �NAex�� plasma flow

where NAven and NAart are the venous and arterial

noradrenaline concentrations, respectively, and NAex is

the fractional extraction of tritiated noradrenaline at

steady state in a single passage through the organ in

question. Analogous methods utilizing an infusion of

tritium labelled adrenaline were used to measure whole

body and regional adrenaline spillover rates (Esler et al.

1990).

For microneurographic recording of muscle sym-

pathetic nerve activity a sterile tungsten electrode

with an uninsulated 1-mm diameter tip (Titronics

Medical Instruments, Iowa City, Iowa, USA) was

inserted percutaneously into the peroneal nerve pos-

terior to the ®bular head according to the technique

of Valbo et al. (Valbo et al. 1979). Raw neurograms

were ampli®ed by 50 000±99 000 times, ®ltered (700±

2000 Hz bandwidth) and integrated using the 662C-3

Nerve Traf®c Analysis System (Bioengineering De-

partment, University of Iowa, USA). Pulse synch-

ronicity and low signal-to-noise ratio con®rmed burst

activity as being of muscle sympathetic efferent ori-

gin.

Statistical analysis

All results, unless otherwise speci®ed, are expressed

as means � standard error of the mean (SEM). With

normally distributed data the effects of pharmaco-

logical interventions were evaluated using two-way

analysis of variance. For data showing a non-Gauss-

ian distribution, paired observations were evaluated

with Wilcoxon signed rank test. Relationships be-

tween variables were evaluated by least squares linear

regression analysis. The null hypothesis was rejected

at P < 0.05.

Figure 1 Total body (a) and cardiac (b) noradrenaline (NA) spillover

into plasma and brain noradrenaline turnover (c), as estimated from

the combined internal jugular venous over¯ows of noradrenaline and

its principal lipophilic metabolites, in healthy subjects and in patients

with pure autonomic failure (PAF). ** P < 0.01 signi®cantly lower

than the corresponding value in the healthy subjects.



RESULTS

Central nervous system noradrenaline turnover

in patients with pure autonomic failure

The cerebrovascular circulation is subject to a rich

sympathetic innervation. As such, the actual source of

neurochemicals washing into the cerebral ef¯uent may

be open to some conjecture. To elucidate the origin of

the over¯ow of noradrenaline and its metabolites into

the internal jugular vein we studied patients with pure

autonomic failure. The spillover of noradrenaline into

plasma, for both the body as a whole and the heart was,

as expected, substantially reduced in patients with pure

autonomic failure (Fig. 1) yet the estimated central

nervous system turnover of noradrenaline was no dif-

ferent to that of the healthy, age-matched subjects

(Fig. 1). Unilateral internal jugular vein blood ¯ows

were similar in the two groups studied (445 �

56 mL min)1 in the healthy subjects and 506 �

70 mL min)1 in the patients with pure autonomic

failure).

Relation of muscle sympathetic nerve activity to brain

noradrenaline turnover in healthy subjects

In a subset of the healthy individuals (n � 12) resting

muscle sympathetic nervous activity was recorded in

parallel with estimates of central nervous system nor-

adrenaline turnover. Muscle sympathetic nerve im-

pulses occurred at irregular bursts in synchrony with

the cardiac rhythm. The mean burst frequency was

19 � 3 bursts min)1 but there was substantial vari-

ability between the degree of muscle sympathetic acti-

vation in the subjects examined, with the range being

4±34 bursts min)1. Linear regression analysis of the

cerebral noradrenaline turnover data, irrespective of the

internal jugular vein sampled, revealed a signi®cant

positive relationship between the estimated central

nervous noradrenaline turnover and the level of muscle

sympathetic nervous activity, as assessed from mic-

roneurographic nerve recordings (y � 6.0x + 13.8;

r � 0.64, P � 0.02, Fig. 2).

Drug interventions

Ganglion blockade with trimethaphan Both systolic and di-

astolic blood pressures were signi®cantly reduced

60 min following initiation of trimethaphan adminis-

tration (152 � 6 vs. 138 � 5 mmHg for systolic blood

pressure, P < 0.01, and 83 � 9 vs. 79 � 7 mmHg for

diastolic blood pressure, P < 0.05, Fig. 3). This re-

duction in blood pressure was accompanied by a

Figure 2 Relationship between the estimated turnover of

noradrenaline in the brain, as estimated from the combined internal

jugular venous over¯ows of noradrenaline and its principal lipophilic

metabolites, and muscle sympathetic nervous activity as assessed from

microneurographic nerve recording (y � 6.0x + 13.8; r � 0.64,

P � 0.02).

Figure 3 Mean arterial blood pressure, total body noradrenaline (NA)

spillover into plasma and estimated turnover of noradrenaline in the

brain, as estimated from the combined internal jugular venous

over¯ows of noradrenaline and its principal lipophilic metabolites, in

healthy subjects in response to the centrally acting inhibitor of

sympathetic nervous activity, clonidine, and the ganglion blocking drug,

trimethaphan. Values shown are means � standard error of the mean.

*P < 0.05 signi®cantly in¯uenced by pharmacological intervention.



non-signi®cant rise in heart rate (72 � 9 vs. 78 � 7

beats min)1). Partial ganglion blockade with trimet-

haphan resulted in an »30% reduction in the spillover

of noradrenaline for the body as a whole (4.40 � 0.87

vs. 2.95 � 0.38 nmol min)1, P < 0.05, Fig. 3).

The concomitant reductions in blood pressure and

total body noradrenaline spillover into plasma following

intravenous trimethaphan were accompanied by an

over 5-fold increase in the turnover of noradrenaline in

the brain, as indicated by the combined internal jugular

venous over¯ows of noradrenaline, MHPG and DHPG

(0.47 � 0.43 vs, 2.32 � 0.94 nmol L)1, P < 0.05,

Fig. 3). While internal jugular venous spillover of

adrenaline could not be detected at rest, following

trimethaphan adrenaline spillover into the internal

jugular veins was evident (Fig. 4).

Central sympathetic suppression with clonidine Both systolic

and mean arterial blood pressures were signi®cantly

reduced following 60 min of clonidine administration

(140 � 11 vs. 120 � 11 mmHg for systolic blood

pressure, P < 0.01, and 97 � 8 vs. 86 � 11 mmHg for

mean arterial blood pressure, P < 0.05, Fig. 3).

Clonidine resulted in greater than 50% reduction in

the rate of spillover of noradrenaline into plasma for

the body as a whole (2.85 � 0.62 vs. 1.24 �

0.17 nmol min)1, P < 0.05, Fig. 3). This reduction in

sympathetic nervous activity was accompanied by

a substantial fall in the secretion of adrenaline

(1.68 � 0.52 vs. 0.29 � 0.04 nmol min)1, P < 0.01).

The over¯ow of noradrenaline and its metabolites into

the internal jugular vein was reduced by »50% fol-

lowing clonidine treatment (P < 0.05, Fig. 3). No

consistent pattern in the internal jugular venous over-

¯ow of adrenaline emerged in response to clonidine.

Sympathetic nervous activity following adrenaline infusion Dur-

ing the infusion of adrenaline the level of sympathetic

activity, as indicated by the spillover of noradrenaline

into plasma for the body as a whole, was elevated

(3.4 � 0.7 vs. 5.6 � 0.4 nmol min)1, P < 0.01). This

re¯ex sympathoexcitation was accompanied by a sig-

ni®cant vasodilatation, manifested in a reduction in

diastolic blood pressure (73 � 3 vs. 64 � 4 mmHg,

P < 0.05). Muscle sympathetic nerve activity rose

progressively during the adrenaline infusion, increasing

from 28.8 � 3.9 to 37.5 � 7.4 bursts 100 heart beats)1

(Fig. 5). Following the termination of the adrenaline

infusion there occurred a progressive elevation in dia-

stolic blood pressure (66 � 3 vs. 77 � 4 mmHg,

P < 0.05). In line with this haemodynamic change there

occurred a concomitant elevation in the rate of sym-

pathetic nerve ®ring, with muscle sympathetic nerve

activity increasing to over 150% of preadrenaline

baseline within 5 min of stopping the adrenaline infu-

sion (Fig. 5). Central nervous system noradrenaline

turnover was signi®cantly elevated following cessation

of the adrenaline infusion (1.39 � 0.31 vs. 2.50 �

0.54 nmol min)1, P < 0.05, Fig. 5). Unilateral internal

Figure 4 (a) Internal jugular venous spillover of adrenaline in healthy

subjects in response to the ganglion blocking drug, trimethaphan.

Values shown are means plus standard error of the mean. * P < 0.05.

Figure 5 Microneurographic nerve recording of muscle sympathetic

nerve activity, burst frequency of muscle sympathetic nerve ®bres

(above) and estimated brain turnover of noradrenaline (bottom) prior

to and following an intravenous infusion of adrenaline (2±3 lg min)1

over 30±40 min). In top panel the arrows labelled `PRE' and `POST'

signify the time points at which internal jugular vein blood sampling

took place. * P < 0.05 signi®cantly different to control values.



jugular vein blood ¯ows were not signi®cantly in¯u-

enced by the adrenaline infusion (408 � 48 mL min)1

at rest and 594 � 83 mL min)1 following adrenaline

infusion).

DISCUSSION

A substantial research effort has focused on delineating

the central nervous system's processing of the afferent

circulatory information involved in the generation of

sympathetic nervous tonic activity and cardiovascular

control (Gebber 1990). Techniques involving the ret-

rograde transynaptic transport of live pseudorabies vi-

rus, a herpes virus endemic to swine, have identi®ed a

number of brain regions including the paraventricular

nucleus of the hypothalamus, the A5 noradrenergic cell

group, caudal raphe nuclei and the rostral ventrolateral

and ventromedial medulla, as innervating all levels of

sympathetic nervous out¯ow (Strack et al. 1989a,b,

Gebber 1990). While such studies provide valuable in-

formation on the localization of brain regions inner-

vating speci®c sympathetic ganglia, the functional

signi®cance of these neuronal pathways in human

subjects remains dif®cult to elucidate. In this paper we

have been able to document a relationship between

brain noradrenaline turnover and sympathetic nervous

activity. The observation that central nervous system

noradrenaline turnover is substantially increased in re-

sponse to blood pressure reduction following ganglion

blockade is reinforced by the parallel ®nding of a

centrally mediated reduction in blood pressure being

associated with diminished noradrenaline turnover in

the brain.

Although a previous report from our laboratory

demonstrated a signi®cant relationship between the

internal jugular venous spillover of noradrenaline and

the estimated central nervous system turnover of this

neurotransmitter (Lambert et al. 1995a,b), cerebral

noradrenaline ef¯ux per se may in fact underestimate the

degree of cerebral noradrenergic neuronal activation in

the clinical setting given that barriers to noradrenaline

over¯ow from the brain, whether they be meta-

bolic, structural or a combination of both, may exist

(Pardridge 1983, Glowinski et al. 1988). It is for this

reason that we chose to base our estimates of central

nervous system noradrenergic activity on the combined

internal jugular venous over¯ows of noradrenaline and

its metabolites, MHPG and DHPG. The primary

source of internal jugular noradrenaline is open to some

conjecture, with the well developed sympathetic in-

nervation of the cerebral arterial blood vessels being a

potential source that previously could not be excluded

with certainty. Our observation that patients with id-

iopathic peripheral autonomic insuf®ciency, in whom

there was biochemical evidence of almost complete

sympathetic nerve degeneration, have normal transce-

rebral over¯ows of noradrenaline and metabolites

supports the contention that the internal jugular venous

noradrenaline we measure emanates from the central

noradrenergic neurones and not from the cerebrovas-

cular sympathetic nerves. While in the present study the

patient's autonomic failure was not attributable to a

central de®cit it may well be instructive to examine

patients with central disorders, such as the Shy±Drager

syndrome, in whom one would anticipate, if our hy-

pothesis is correct, cerebral noradrenaline spillover to

be reduced despite a relatively normal degree of sym-

pathetic activation.

The largest group of noradrenaline containing neu-

rones, accounting for no less than 50% of noradrena-

line in the central nervous system, is the locus

coeruleus, or A6 region (Foote et al. 1983). The locus

coeruleus is involved in the integration and processing

of environmental stimuli (Foote et al. 1983), and plays

an important role in the innervation of both the auto-

nomic (Svensson 1987) and central (Kobayashi et al.

1975) nervous systems. Given that an acute reduction

in blood pressure has been shown to elicit a pro-

nounced increase in locus coeruleus neuronal activity

(Elam et al. 1985) and that trimethaphan acts periph-

erally, reducing blood pressure and sympathetic ner-

vous activity (Delius et al. 1972), the marked

compensatory elevation in central nervous noradrena-

line turnover we saw, following ganglion blockade or

for that matter in response to the reduction in central

venous pressure after infusion of adrenaline, was not

unexpected. Interestingly, in a study by Goldstein and

colleagues, canine cerebrospinal ¯uid noradrenaline

levels were substantially reduced following trimethap-

han administration (Goldstein et al. 1987). The author's

interpretation of this ®nding, although not totally

consistent with the observation that patients with

pheochromocytoma, in whom plasma sympathetic ne-

urotransmitter levels are elevated, exhibit normal cere-

brospinal ¯uid noradrenaline concentrations (Cubeddu

et al. 1984), was that cerebrospinal ¯uid noradrenaline is

derived, at least in part, from post-ganglionic sympa-

thetic nerves.

The central noradrenergic response to peripherally

induced blood pressure reduction contrasted with that

elicited by intravenously administered clonidine.

Clonidine is a centrally acting suppressant of sympa-

thetic nervous system activity known to inhibit the

®ring rate of locus coeruleus neurones (Svensson et al.

1975, Foote et al. 1983) and decrease the concentration

of MHPG in the brain (Braestrup 1974). Consistent

with the observations of this study, Maas and col-

leagues, using direct internal jugular vein blood sam-

pling techniques in the stump-tailed monkey, Macaca

arctoides, found a diminished rate of MHPG production



in the brain following clonidine administration (Maas

et al. 1977) and Cubeddu et al. (1984) demonstrated that

clonidine treatment reduced previously elevated cere-

brospinal ¯uid catecholamine levels in patients with

essential hypertension.

Pertinent to the ®ndings of the present study is the

observation that hydralazine-induced hypotension is

associated with increased MHPG concentrations in the

posterior hypothalamus (Kubo et al. 1988). Given its

extensive neuronal circuitry projecting to autonomic

premotor nuclei such as the A5 noradrenergic cell

group and the rostral ventrolateral medulla (Gebber

1990), the hypothalamus may play a pivotal role in the

regulation of autonomic re¯exes. Stimulation of the

paraventricular nucleus of the hypothalamus results in

sympathetic nervous activation (Kannan et al. 1989). In

agreement with the observations of the present study,

Qualy and Westfall demonstrated that a reciprocal re-

lationship between blood pressure and noradrenaline

over¯ow from the paraventricular nucleus of the hy-

pothalamus exists (Qualy & Westfall 1993).

The hypothalamus though is not unique in its ability

to respond to haemodynamical insults. For instance,

Singewald et al. have previously demonstrated that a

reduction in blood pressure, generated by bilateral

carotid occlusion, results in increased noradrenaline

release from the A6 region, while loading of barore-

ceptors by elevating blood pressure with phenylephrine

is accompanied by a decreased release of noradrenaline

in this region (Singewald et al. 1993). Elam and col-

leagues induced a reduction in both the rate of locus

coeruleus neuronal ®ring and splanchnic nerve activity

by blood volume load and by increasing blood pressure

via infusion of either noradrenaline or angiotensin

(Elam et al. 1984, 1985). Singewald and Philippu repli-

cated and advanced the ®ndings of Elam et al. (Elam

et al. 1984, 1985) by demonstrating that noradrenaline

release from the locus coeruleus is modi®ed by altera-

tions in blood pressure generated by vascular con-

striction, hypervolaemia and hypovolaemia, but not by

vasodilatation (Singewald & Philippu 1993). In view of

these ®ndings and keeping in mind that the locus co-

eruleus is such a rich source of noradrenaline it is not

unreasonable to postulate that alterations in A6 neu-

ronal activity is in some part responsible for the acute

variations in cerebral noradrenaline turnover docu-

mented in the present report.

In this paper we provide evidence implicating the

participation of brain noradrenergic cell groups in the

excitatory regulation of the sympathetic nervous system

and the maintenance of cardiovascular control. The

substantial increase in brain noradrenaline turnover

following ganglion blockade with trimethaphan pre-

sumably results from a compensatory response, in

pressor, sympathoexcitatory forebrain noradrenergic

neurones in the face of interruption in sympathetic

neural traf®c and reduction in arterial blood pressure.

The parallel ®nding of a reduction in central nervous

system noradrenaline turnover in response to intrave-

nous clonidine administration probably underlies the

blood pressure lowering action of the drug. While it is

impossible using our techniques to unequivocally elu-

cidate the central nervous system sites involved in the

responses described in this study, available evidence

implicates noradrenergic cell groups of the posterolat-

eral hypothalamus, amygdala, the A5 region and the

locus coeruleus as being intimately involved in the

regulation of sympathetic out¯ow and autonomic car-

diovascular control.

The authors wish to thank Elizabeth Dewar, Sister Leonie Johnston

and Kaye Varcoe for their patience and expert assistance in the

research catheter laboratory. This work was supported by a National

Health and Medical Research Council of Australia grant to the Baker

Medical Research Institute. Gavin Lambert is currently supported by a

National Health and Medical Research Council of Australia CJ Martin

Fellowship and is working in the Department of Physiology,

University of GoÈteborg, Sweden. Mario Vaz was a visiting scholar

to the Human Autonomic Function Laboratory from the Department

of Physiology, St John's Medical College, Bangalore, India.

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