Neurasr~nce Letters, 38 (1983) 125-130 Elsevier Scientific Publishers Ireland Ltd.

125

DISTRIBUTION OF SEROTONIN NERVE CELLS IN THE RABBIT BRAINSTEM

P.R.C. HOWE, E. MOON* and R.A.L. DAMPNEY

CSIRO Division of Human Nutrition, Kintore Avenue, Adelaide. SA 5000, and Department of Physiology, University of Sydney, NSW 2006 (Australia)

(Received March 27th, 1983, Accepted April I Ith, 1983)

Key words: 5-hydroxytryptaminc - immunofluorescence - ventrolateral medulla - raphe nuclei - cen- tral nervous system - rabbit

Serotonin (5-HTJ-containing nerve cells in the rabbit brain were visualized immunoh: ,tochemically and were mapped on photographs of coronal brain sections. The $-HT cells extended from the caudal medulla oblongata to the rostral midbrain and were mostly clustered in groups resembling those found in the rat brainstem. However. there were several notable differences between the rabbit and the rat in the distribution of 5-HT nerve cells. In particular, there were fewer pontine 5-HT cells in the rabbit and the major 5-HT cell groups of the midbrain and the ventral medulla oblongata were spread further laterally than in the rat brain,

The neuroanatomical distribution of central monoamine nerves has been studied extensively in a wide variety of vertebrate species using fluorescence histochemical, autoradiographic and, more recently, immunohistochemical techniques. These com- parative studies have revealed many common features in the distribution of catecholamine or serotonin ($-hydroxytryptamine, $-HT) nerve cell bodies, which are consistent with DahlstrOm and Fuxe's original description of monoamine nerve cell groups in the rat brain [3]. However, some subtle differences in distribution pat- terns do exist which may be of considerable importance when assigning specific physiological functions to nerves containing a particular monoamine neurotransmit- ter. For example, because of the apparent association of catecholamine nerves with brainstem nuclei which are implicated in central cardiovascular control mechanisms, detailed comparisons have been made of the precise locations of catecholamine nerve cells in species such as the cat [8] and rabbit [ I] which are frequently used for cardiovascular experimentation. 5-HT nerves are similarly involved in car- diovascular mechanisms in the rat brain [7] and the possibility that they might also account for the pressor activity obtained by stimulating nerve cells in the ven- trolateral medulla oblongata of the rabbit [4] has prompted the present study of the distribution of 5-HT nerve cells in this species. Apart from the identification of 5-HT cells by fluorescence histochemistry in a description of the raphe nuclei of the rabbit [5], no compJ ehensive map of 5-HT nerve cells in the rabbit brain is currently available. * Present address: Departme~ of Clinical Biochemistry, Flinders University of South Australia, Bedford

Park, SA 5042.

0304-3940/83/$ 03.00 © 1983 Elsevier Scientific Publishers Ireland Ltd.

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Figs. ! and 2. Distribution of 5-HT-containing nerve cells in the rabbit brain. Locations of individual immunored~.:;ve cells are shown as dots on drawings of actual coronal sections taken at ! mm intervals from a single r~,~bit brain. Caudo-rostral distances are relative to the obex. Abbreviations: VII, fac~ nerve: VIII, acousu¢ tl,.i've: A, aqueduct; AP, area postrema; CG, central grey matter; CP, cerebral peduncle; DPY, de~ussation of pyramidal tract; DR, dorsal raphe n; DSCP, decussalion of superior cerebellar peduncle; IC, inferior ¢olliculus; ICP, inferior cerebellar peduncle; IO, inferior olive; IP, in-

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terpcduncular n.; LC, locus cocrulcus; LRN, lateral reticular n.; MCP, middle ccre~llar peduncle; ML, medial lemniscus; MLF, medial longitudinal fasciculus; MR, median raphe n.; Nill, oculomotor n.; NIV, trochlear n.; NV, trigeminal n.; NVSp, spinal trigcminal n.; NVi, abducens n.; NVll, facial n.; NVIIR, retrofacial n.; NX, dorsal m o t o r vagus n.; NXil, h~l~glossal n.; NA, ambiguus n.; NDT, aorsal tegmental n.; NCV, ventral cochlear n.; NTS, solita~ tract n.; NP, pontine n.; NVLL, ventral lateral lemniscus n.; PF, pontine fibres; PY, pyramidal tract; RL, raph¢ linearis n.; RN, red n.; SC, superior colliculus; SCP, superior ccrebellar peduncle; SN, substantia nigra; SO, superior olive; (n. - nucleus).

In the presto expcrimems, central 5-HT nerve cells were visualized im- m,mohistochemically in 5 New Zealand White rabbits. After 2 h pretreatment with t-tr::ptophan (100 mg/kgo i.p.) and ~or a monoaminc oxidase inhibitor (parg vline, 100 r~g/k~ ,~r tranylcyprominc, 5 mg/kg, i.p.), the rabbits were anaesthetized with Ncmbuta!, perfused with forma'dehyde solution and the brain removed. Series of 40 -50 ~m thick coronal sections o f the brainstem were taken at 300 or 500 pm inter-

al, and processed for immunofluorescence using a monoclonal antibody to 5-HT, a~ described previously for rat brains [6]. The plane of sectioning was approximately perpendicular to the ventral ,~u~face of the hindbrain. Whole sections were p~otographcd under dark-field illumination and the positions of immunofluores- ¢¢nl cells ,,~¢r¢ mapped on the photographs.

immunorcacfi~ e ner~ e c¢11 bodies ~ere seen to extend through the brainstem from t t,¢ pyramidal d¢cussation to the rostral midbrain. None xsas found under the pre- .,¢nl con~tition,. ~n the hypothalamus. The distribution of 5-HT-containing nerve cells in ~ c rabbit brain.,tem is indicated in Figs. I and 2 by the dots seen on drawings o t a typical ..erie, of corovlal s~:ctions ,,paced at l ram intervals. The cells are largely confined to ventral and medial regions of the brainqem and tend to be clustered in ~toup,. r,.',¢,~tbling the BI B9 group,, and additional small 5-HT cell groups (in- cluding tho,.¢ ~ithin the area postretna and interpeduncular nucleus)described for the tat brain 1.1. gi. As its the rat. the 5-11 l ceils of the rabbit braill are not restricted :,~ ~h¢ saph¢ regions bm extend further laterally. I Io~e~ er. th~s lateral spread of ceils ~.. more i~fonotmced lit the rabbit, especially near ~h¢ ~entra! surface of the caudal medulla and it1 the midbrain. ~s here distinct I.'ltcral , 'olu,nns of fluorescent cells ap-

pc,tied to exfetld fronl the |sefiatluethietill grey statler t~l the interpeduneular tltJcletl.,, ih¢ higher proportion of lateJal c¢lb. in the rabbit brainsten~ is e~idem if Otl¢ ¢Ol1111a1¢.',, fl|¢ distribution ~ho~n m I'igs, ! and 2 ~ i lh maps of . ' ;-HI cells in fh¢ t;ll I,rain,,lCn~ i- ' ..1.91.

in the rabbil, the I i l cells of the ~.audal medulla (Fig. Ja. b)could be divided into 3 ,,ub-,.,roup,,: thong." concentrated in lhe raphe pallidus nucleus; those surrounding thu. p.~ ramidal t ract and ~ it hin ;rod vent romedial to the inferior oli~ e; Iho~¢ occurr- ing further laterally bet~ecn the inl~'rior olive and lateral reticular nucleus and ex- lending ,lot,ally Iron: the xemral surface of the medulla wx~'ard the retrofacial n~tcl,:u,, furti~er rostrall.x, l'hc 5 - t t1 cells in the ros~ral m,:duila also extend further

lateraltx than in the rat. xxith ceils occasionally seen within and lateral to the facial

nerve m~cleus and do,sai or lateral to the superior olive. Between the rostral medulla

a~d the medial lemni,~cus in the midbrain, there is a lack of 5-HT cell: in ventral region,,, i-cv, 5 - t t f cells were seen in the pontinc raphc nucleus of the rabbit; hence

a ,.¢i'~tr:u¢ group corresponding to the B5 group in the rat was not evident. Ceils ly- ing xsi[hin a~d medial to the dorsal [egmcmal nucleus correspond to the rat B6

group bu~. in the rabbit, there is a greater number of cells scattered lateral and ven- ~rol~e~at to [his group, in the sub-coeruleus area.

lhe highest densities of 5-HT cells occer in the vicinity of the dorsal and median

129

Fig. 3. Immunofluorescence micrographs showing the lateral spread of 5-HT nerve cells in transverse sec- tions of the ventral medulla o~longata, both caudally (a) and rostrally (b), and of the midbrain Ic) .Xlicroscope fields arc indicated on the accompanying dark-field micrographs of the whole, unstained sec- tions. Bar = 0.2 mm.

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raphe nuclei of the midbrain, in groups corresponding to the B7 and B8 groups of the rat. However, in the rabbit, these groups continue for some distance caudally and the distinction between IF/and B6 or B8 and 135 is less obvious than in the rat Main. The B7 group at its mnimum develolpmem is seen in Fis. 3¢ to contain a

fasciculus to connect with cells s-~rending lalerally from the B8 group throush the decussation of the superio~ ~ cemheBar peduncle. The 138 group is confined caudally to the median raphe auclens but diverges rostrally, with a higher proportion of cells occurring laterally in the region of the red nucleus than in the raphe region. However, the medial cells extend even further rostrally and, contrary to a previous report [:5], were also found in the linear raphe nucleus. In the rat, the 137 and B8 cells are predominantly medial; it is the 139 group of horizontally oriented 5-HT cells in the medial lenmiscus which extends out toward the lateral edge of the midbrain. By contrast, in the rostral midbrain of the rabbit, few cells are found in lateral regions of the medial lemniscus and the more medial cells appear as a continuation of the B8 cell group. Thus a distinct cell group equivalent to the B9 group of the rat was not seen. in the rabbit, however, there are more 5-HT cells within, surroun- ding, and even lining, the ventral surface of the interpeduncular nucleus.

The present immunohistochemicul study confirms the presence of 5-HT- containing nerve cells in the rabbit brainstem, with a general topography similar to that found in the rat brain. However the grouping of these cells in the rabbit brain differs considerably from that of the rat, particularly with respect to the higher pro- portion of 5-HT cells located in lateral regions of the midbrain and ventral medulla.

The authors wish to thank Mr. P.F. Rogers and Mr. D. Braybrook for assistance in preparing the figures. The study was supported by the National Health and Medical Research Council of Australia.

I Blessing. W.W.. Chalmers. J.P. and Howe. P.R.C.. i)islribulion of catecholamine-conlainin$ cell bodies in the rabbit central nervous system. J. comp, Neurol.. 1"79 (19'78) 407-424.

2 B~'ker. R.M,, Westlund. K.N. and Coulter. J.D., Origins of se,.)toner#c projections to the spinal cord in rat: an immum~:)lochemical-relro~ade transport smd.~. Brain Res.. 226 (1951) 187-199.

.~ Dahlstr0m. A., and Fuxe. K., Evidence for the existence of monoamine-containin$ neurons in the cen- tral nervous system. I. Demonstration of monoamines in the cell bodies of brainstem neurons. Acla physiol..~and. Suppl. 232 (1964)3-,~5.

4 l)ampney, R.A.L., Goodchild. A.K., Robenson. L.G. and Montgomery. W.. Role of ventrolateral medulla in vasomotor regulation: a correlative anatomical and physiological study. Brain Reg.. 2,19

Feltcn, D.L. and Cummings. J.P.. The raphe nuclei of the rabbit brain stem. J. comp, Neurol,. 187

6 Howe, P,R.C,, Cuello, A.C.. Costa. M. and Furness. J.B.. Improved immunohistochemical visualisa- don of central serotonin nerves after loading with .%?-dihydroxylryptumineo Neurosci. Lett.. 29 (1982) i -6 .

7 Ho~,:, P.R,C., Kuhn, D.M., Minson, J.. Stead. B.H. and Chalmers. J.P.. Evidence for a bulbosp'nal ~¢rotoner#c pn.~.~or pathway in the rat brain. Brain Res., in press. Poitras, D, and Parent, A,, Arias of the distribution of monoamine-containin$ nerve cell bodies in the brain stem of the cat, J. comp. Neurol.. I78 (1978) 699-'/18.

9 Steinbusch, H.W.M., Dimibution of serotonin-immunoreactivity in the ,:entral nervous system of the :at. Cell bodies and terminals, Neuroscience, 6 (1981) 557-618.