5-HT receptors and hyper- or hypothermia: Elucidation by catecholamine antagonists injected into the cat hypothalamus

Brain Research Bulletin,

Vol. 8, pp. 79-86, 1982. Printed in the U.S.A

5-HT Receptors and Hyper- or Hypothermia: Elucidation by Catecholamine Antagonists Injected into the Cat Hypothalamus W. D. RUWE’

AND

R. D. MYERS2

Departments of Psychiatry and Pharmacology and Center for Alcohol Studies University of North Carolina School of Medicine, Chap;1 Hill, NC 27514 Received RUWE, W. D. and R. D. MYERS.

5-HT

receptors

2 October

1981

and hyper- or hypothermia:

Elucidation

by catecholamine

antagonists

BRAIN RES. BULL. S(1) 7%86, 1982.-The receptor mechanisms in the hypothdamus and preoptic area which mediate thermoregulatory changes produced by serotonin (5-HT) were examined in the unrestrained cat. Stainless steel guide tubes were implanted just above the rostra1 diencephalon or preoptic area in each of twelve cats. S-HT in a dose of 1.5 to 10.5 pg was micro-injected in a volume of 0.75 to 1.5~1 into a total of 256 test sites in the hypothalamus. An individual site at which 5-HT evoked a rise or a fall in core temperature of 2 0.5”C within 30 min was considered to be reactive to the indoleamine. Within such a region of maximum sensitivity, i.e., the cat’s anterior hypothalamic, preoptic area (AHIPOA), either norepinephrine (NE) or dopamine (DA) micro-injected in a dose of 2.33 to 14.0 pg in similar volumes evoked only a dose-dependent decline in the body temperature of the cat. 5-HT was found to cause either hyper- or hypothermia. The direction of temperature change caused by 5-HT was dependent principally upon the neuroanatomical locus of injection. Each of the 5-HT reactive sites was characterized pharmacologically by their pre-treatment with phentolamine (PHT), d-butaclamol (BUTAC) or methysergide (METHY) in doses of 1.O to 10.0 pg. At the most rostral sites in POA, the catecholamine receptor antagonists selectively delayed the onset of the 5-HT hypothermia and limited the magnitude of the response. Within sites located in the AH in which 5-HT induced only a rise in body temperature, prior treatment of the site with the catecholamine receptor antagonists PHT or BUTAC failed to modify the response. These results indicate that 5-HT in producing hypothermia not only may saturate the 5-HT receptor sites but also may be taken up by catecholamine receptors which mediate the diencephalic heat loss pathway. injected

into the cut hypothalamus.

Serotonin receptors Dopamine antagonists

Catecholamine receptors Hypothermia Thermoregulation Norepinephrine antagonists Hyperthermia

ACCORDING to the monoamine theory of thermoregulation, serotonin (5-HT) and norepinephrine (NE) are released antagonistically within the anterior hypothalamic, preoptic area to control the heat-gain and heat-dissipation pathways, respectively [6,7]. Micro-injected into this thermosensitive zone of the hypothalamus, 5-HT ordinarily produces a rise and NE a fall in the body temperature of the cat, rat, monkey, rabbit and many other species [19,20]. The increase in temperature caused by intrahypothalamic S-HT is prevented by methysergide, a 5-HT receptor blocking agent, when both substances are micro-injected at the same site [24]. Similarly, the decline in temperature evoked by NE is prevented when the tissue site is pre-treated with phentolamine, an alpha-adrenergic antagonist [9]. Two factors complicate the functional interpretation of the local action of either monoamine on the thermosensitive region of the rostra1 hypothalamus: (1) the precise

Hypothalamus

neuroanatomical locus of micro-injection, and (2) the dose of each putative neurotransmitter used by different investigators. For example, a high dose of 5-HT given either in the cerebral ventricle or into the anterior hypothalamus can cause a sharp fall in temperature [4,22]. This could be due to saturation and consequent desensitization of the amine receptors, which results in a marked impairment in the animal’s capacity to thermoregulate. Moreover, since a high dose of 5-HT produces a characteristic malaise, inactivity, sleep and behavioral incapacitation, the decline in the animal’s body temperature thus could occur as a secondary side effect of the high dose [2]. An alternative possibility is that serotonergic neurons in the hypothalamus mediate a heat dissipating mechanism as an animal defends its temperature against heat stress. When 5-HT is infused into certain sites rostral to the cat’s anterior hypothalamus, i.e., the preoptic area, the animal’s tempera-

‘Present address: The Division of Medical Physiology, The University of Calgary, Alberta, Canada. *Please send reprint requests to R. D. Myers, Center for Alcohol Studies, Medical School Bldg. 207H, UNC School of Medicine, Chapel Hill, NC 27514.

80

RUWE AND MYERS

ture may fall [12]. Because 5-HT nerve terminals are not densely dist~buted in this ventral region of the forebrain 1251, the present experiments were undertaken to examine the pharmacological specificity of 5-IIT’s action on the preoptic area in inducing a change in body temperature. Blocking agents of serotonergic, noradrenergic and dopaminergic receptors were applied directly to tissue sites in the cat’s anterior hypothalamic, preoptic area (AHfPOA) which had already been identified as being reactive to 5-HT in terms of its evoking either hyper- or hypothermia. METHOD

Twelve cats of either sex and weighing from 2.5 to 5.0 kg were housed individually in a colony room maintained at an ambient temperature of 2224°C and illuminated on a IZhour light-dark cycle. Every morning the cats received fresh water and dried cat chow supplemented by canned cat food. Surgical Procedures

Each cat was anesthetized with 25-30 mg/kg sodium pentobarbital injected into the saphenous vein or one of its superficial branches. Using the stereotaxic coordinate system of Jasper and Ajmone-Marsan [3], an array of two to six 20 ga stainless steel guide tubes was implanted according to surgical procedures described previously [I6]. Each guide tube was fitted with an indwelling 23 ga stylet of corresponding length, both of which were beveled at a 45” angle. Craniotomy holes were drilled bilaterally in the cafvarium and enlarged with rongeurs to expose the dura. After the dura was incised, the guide tubes were lowered so that the tips rested 2.0 to 4.0 mm above the hypothalamus; thus, lesion of the intended sites of micro-injection was avoided. Following insertion of anchor screws into the calvarium, the cannulae were secured in place with cranioplastic cement. A pedestal was positioned over the guide tubes and fastened to the skull with additional screws and cement. Postoperatively, penicillin was administered for ten days. Micro-Injection

Procedures

To micro-inject .5-HT or other compounds directly into a selected anatomical site, a 28 ga tin-weed stainless steel injector needle was cut to 55 mm in length and its tip beveled at 45”. The needle was connected by a length of PE-20 tubing, filled with the solution to be injected, to a Hamilton 50 ~1 syringe which was mounted on a Harvard infusion pump. Between each micro-injection, the tubing, cannulae and syringes were stored in a 70% ethanol solution, and prior to use, flushed with the ethanol and an artificial CSF 1171. The micro-injection of a given agonist or antagonist was made in a volume ranging from 0.75-1.5 ~1. Following an injection, the needle was kept in place for an additional 30-60 set to assure a dispersion of the solution of 0.3-l. 1 mm [ 151, then replaced by the indwelling stylet. The following compounds were used: 5~hy~oxyt~ptamine creatinine sulfate (Sigma); 3-hy~oxytyra~ne HCI (Sigma); L-arterenol HCl (Sigma); methysergide maleate (Sandoz); phentolamine HCI (CIBA); practa.lol HCl and d-butaclamol HCl (Ayerst). Each solution of the compound injected was prepared in either the pyrogen-free artificial CSF or glass-distilled water and passed through a 0.2 Frn Swinnex millipore filter. In some experiments, the pH of the solution was adjusted to 3.8-4.0 by the edition of 0.1 mg/ml of ascorbic acid. Doses in the RESULTS are expressed as micrograms of the base.

Physiological

Measures

A YSI 401 thermistor probe was inserted 8 to 10 cm into the cat’s colon and held in place by surgical tape wrapped gently around the base of the tail. Skin temperature was measured by affixing a YSI 427 thermistor probe similarly to the base of the tail. Both colonic and skin temperatures were monitored on YSI tele-thermometers and recorded simuItaneously on an Esteriine-Angus multi-point recorder. The baseline temperature of each cat was obtained for at least 60 min before an experiment began, and at 5-15 min intervals food intake, ear temperature, respiratory rate. pupillary size, level of arousal, instances of panting, salivation, micturition, miaowing, licking, washing, motor movements, purring, piloerection, shivering, huddling, growling and postural adjustments were recorded. Histological Anaiysis of injection Sites

Upon completion of the experiments, each cat was given an overdose of sodium pentobarbital, its heart clamped and 0.9% saline followed by 1% neutral Forrnalin was perfused retrograde through the thoracic aorta. The brain was then blocked stereotaxically in the coronal plane and sectioned at 100 pm thickness on a sledge cryotome. Each section was stained following a modified Weil-Weigert hematoxylin or a buffered cresyl violet acetate method [28]. After the anatomical locus of each micro-injection was verified under light microscopy, the site was then mapped on standard histological ~~onst~ctions 1231.

RESULTS

Circumscribed sites in the cat’s hypothalamus and preoptic area were readily identified in which the micro-inj~tion of 5-HT evoked a change in the animal’s colonic temperature. Of the 256 total sites examined, 48 (1%) were categorized as reactive according to the criterion of an 0.5”C increase or decrease in core temperature, or greater, within 30 min after the micro-injection. The dose-response relationship between intrahypothalamic 5-HT and the magnitude of the change in temperature, is presented for 56 experiments in Table I. Following the micro-injection of 1.5 to 10.5 pg (0.008 M to 0.06 M of the free base) of 5-HT into a reactive site, the latency of the temperature response was usually 1.0 to 5.0 min, but nearly always less than 15.0 min. At sites in which 5-HT evoked a rise in the cat’s temperature, the lowest dose evoked the greatest increase, 1.3”C (Table 1, top). In contrast, a dose-response relationship was evident when sites at which 5-HT produced a fall in the cat’s colonic temperature (Table I, bottom) were tested. Not only is the dose of 5-HT critical but the anatomical site of the micro-injection of the indoleamine is of equal importance. As shown in Fig. 1 (top), 5-HT infused at AP 13.0 into the AH in a dose of I .5 lug evoked an almost immediate rise in body temperature of nearly 1.O”C. Given in a slightly higher dose of 2.0 pg, 5-HT injected in the same coronal plane but at a more ventral site caused a transitory hypothermia (Fig. 1, bottom) which was followed by the characteristic rise in the cat’s body temperature [IS]. The veterinary antipyretic, dipyrone, effectively reduced the efevated temperature folIowing 5-HT, as is shown by the deferveseent response in Fig. 1 (top).

THERMOREGULATION

81

AND 5-HT RECEPTORS

TABLE

1

EFFECT OF S-HT IN FOUR DOSES, ON THE TEMPERATURE OF CAT, CATEGORIZED ACCORDING TO DIRECTION OF CHANGE AND INDEPENDENT OF ANATOMICAL LOCUS OF MICRO-INJECTION

5-HT Hyperthermia Maximum Rise in Temperature (Mean k S.E.)

n

Dose (pg)

3 2 10 3

1.5

2.0 3.5 10.5

1.30 0.70 0.77 0.97

f 0.39 f 0.00 2 0.16 +- 0.32

n

1.5 2.0 3.5 10.5

6 8 13 11

Maximum Fall in Temperature (Mean f SE.) 0.65 0.74 0.88 0.98

& f f f

0.07 0.12 0.12 0.13

Antagonism

of 5-HT Temperature

The hyperthetmic response produced by 5-HT injected into an individual AH site at AP 13.0 (Fig. 2, histological inset) was abolished by 5.25 pg of the serotonergic antagonist, methysergide (METHY), which was infused at the same site 30 min prior to 5-HT. As seen in Fig. 2, a short-lived decline in the cat’s temperature persisted for 1.0 hr following the serotonin receptor blockade (Fig. 2, top). When 5-HT was micro-injected more rostrally into POA at AP 15.0 (Fig. 2, bottom) the same 10.5 pg dose evoked a fall in body temperature. Again, pre-treatment of the site 30 min earlier with 5.25 pg methysergide attenuated this hypothermic response (Fig. 2, bottom). Noradrenergic

5-HT Hypothermia Dose (cLg)

Pharmacological Changes

Antagonism

of 5-HT Hypothermia

A relatively low dose of 1.5 lug 5-HT infused unilaterally into the cat’s POA at AP 15.0 produced a fall in temperature of nearly 1.O”C as illustrated in a representative experiment (Fig. 3). This response of active heat loss was accompanied by a transitory vasodilation as reflected by the sharp rise in skin temperature. After the 5-HT sensitive site had been pre-treated with 6.2 pg phentolamine (PI-IT), the microinjection of 5-HT (Fig. 3, bottom) failed to evoke any change

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FIG. 1. Colonic temperature of two individual cats. (top) at 0 hr 1.5 pg WIT microinjected at site in AP 13.0 depicited by the dot in the histological inset. The asterisk denotes systemic injection of dipyrone. (bottom) 2.0 pg S-I-IT injected at site in AP 13.0 depicted in the histoloical inset. Time is in hours.

HOURS FIG. 2. Colonic temperature of two cats. Methysergide (METHY) in a dose of 5.25 pg injected at site in AP 13.0 (top) or at AP 15.0 (bottom) 30 min prior to 0 hr. At 0 hr, 5-HT in a dose of 10.5 yg was injected at the same site. In the control condition, only the 5-HT microinjection was given on a different experimental day. Time is in hours.

RUWE AND MYERS

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FIG. 3. Respirations per minute (top), skin temperature (mrddle) and

HOURS FIG. 4. Respirations per minute (top), skin temperature (middle) and

colonic temperature (bottoms of an individual cat. Phentolam~ne (PHT) in a dose of 6.2 pg was injected at AP 15.0, depicted by the dot in the histological inset, 30 min prior to 0 hr. 5-HT in a dose of I.5 pg was injected at the same site at 0 hr. On the control experimental day, only 5-HT was microinjected at the same site. Time is in hours.

colonic temperature (bottom) of an individual cat. Phentolamine (PHT) in a dose of 6.2 US was in&ted bilaterailv at AP 15.0. depicted by the dots in the h&tological inset, 30 min prior to 0 hr. j&T in a dose of I .Ogg was also injected bilaterally at the same sites at 0 hr. On the control experimental day, only 5-HT was microinjected bilaterally. Time is in hours.

in the animal’s body temperature; moreover, the vasodilatory status of the cat was similarly unaffected (Fig. 3, middle). When 5-HT was micro-injected bilaterally in a dose of 1.O pg into POA sites within AP 15.0, illustrated in Fig, 4 (bottom), the hypothermia was just as intense. Again, the pretreatment of the sites with 6.2 pg phentolamine 30 min before 5-HT virtually blocked the temperature decline. However, in less than 1.0 hr after the phentolamine-5-~T sequence of microinjections, a resultant hyperthermia ensued (Fig. 4, bottom). Since this cat in both the phentolamine and 5-HT control experiments was maximally vasodibted, with skin temperatures ranging between 3&3.X, there was thus no further evidence that a vasodilation occurred (Fig. 4, middle).

temperature of nearly 1.5”C, which was accompanied by a short-lived vasodilation (Fig. 5, middle). Pre-treatment of the same site with 5.25 pg butaclamol (BUTAC) 30 min before 5-HT not only abolished the hypothermia entirely but also caused an intense hyperthermia following the temperature decline. In both experiments, this elevation was attenuated by the anti-pyretic, dipyrone (Fig. 5, bottom). The marked vasodilatory response, which followed the S-HT injection, again was prevented by the b~taclamol pre-treatment of the AH site. Injected in a most rostra1 region of the POA at AP 16.0, 3.5 pg S-HT produced a brief fall in colonic temperature and in this case a minimal after-rise. As shown in Fig. 6, the fail was accompanied again by skin vasodilation (Fig. 6, middle). Butaclamol in a dose of 1.8 Fg not only attenuated the hypothermic response to 5-HT but, in fact, subsequently induced a slight rise in the cat’s body temperature over a 4.0 hr interval which was reversed by dipyrone (Fig. 6, bottom). When the same experiment was conducted in another cat in which the tip of the injection cannula had been positioned more caudaily, at AP 14.0, similar changes in body temperature and vasomotor responses occurred, as shown in Fig. 7

Dop:paminergic Antagonism

of S-HT Hypothermia

A dopaminergic receptor antagonist was also effective in preventing the hy~the~ic action of .5-HT. As shown in Fig. 5 (bottom) a high dose of 10.5 ,ug S-HT injected into a ventral hypothalamic site at AP 13.0 evoked a fall in the cat’s body

THERMGREGULATION

AND .5-HT RECEPTORS

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HOURS FIG. 5. Respirations per minute (top), skin temperature (middle) and colonic temperature (bottom) of an individual cat. Butaciamol (BUTAC) in a dose of 5.25 pg was injected at AP 13.0, depicted by the dot in the histological inset, 30 min prior to 0 hr. 5-HT in a dose of 10.5 gg was injected at the same site at 0 hr. On the control experimental day, only S-HT was microinjected at the same site. Time is in hours, dipyrone given at asterisk.

middle). However, in this case the blockade of dopaminergic receptors with 1.8 pg butaclamol not only prevented the hypothermia caused by 3.5 pg S-HT but also caused a very intense temperature increase of over 1.5”C. (bottom,

The specificity of the receptor analysis was demonst~ted by the injection of NE at a catecholamine-reactive site within the POA at AP 15.0. As shown in a representative experiment in Fig. 8 (bottom), NE alone induced its characteristic fall in body temperature of nearly l.O”C, accompanied by vasodilation (Fig. 8, middle). he-treatment of this sensitive site with 5.25 yg methysergide (METHY) 30 min prior to the microinjection of NE, markedly enhanced the fall of the cat’s temperature (Fig. 8, bottom) and similarly augmented the vas~ilatory response (Fig. 8, middle). The classification of temperature responses following the blockade of serotonergic, noradrenergic and dopaminergic classes of receptors within the AHfPOA is summa~~d in Table 2. Independent of anatomical locus, the 5-HT hy~the~ia is blocked equally as well by methysergide,

HOURS FIG. 6. Respirations per minute (top), skin temperature (middle) and

colonic temperature (bottom) of an individual cat. Butaclamol (BUTAC) in a dose of 1.8 fig was injected at AP 16.0, depicted by the dot in the histological inset, 30 min prior to 0 hr. 5-&M’in a dose of 3.5 pg was injected at the same site at 0 hr. On the control experimentat day, only S-HT was microinjected at the same site. Time is in hours, dipyrone given at asterisk.

phentolamine or butaclamol. In contrast, 5-HT hyperthermia is antagonized only by methysergide, whereas norepinephrine hypothermia is blocked only by phento~ine. The decline in temperature induced by dopamine is consistently antagonized by the dopaminergic antagonist butaclamol, but not always by phentol~ine. A representative histological section portraying the track of the guide cannula and the hypothetic sites of microinjection at successive depths is presented in Fig. 9, The point of deepest penetration is denoted by the arrow. DfSCUSSION Previous studies on the local hy~the~ic effect of 5-HT on body temperature have revealed a functional distinction between the anatomically differentiated AH and PGA. Essentially, 5-HT infused in a low dose within coronal planes AP 11.5-13.5 evokes a rise in temperature of the cat [19], whereas the indoleamine injected rostral to these planes may induce a fall [12]. A high dose of 5-HT infused into the AH may, however, produce a transient but intense decline in temperature which is always followed by hyperthermia [IS].

RUWE

AND MYERS

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FIG. 7. Respirations per minute (top), skin temperature (middle) and colonic temperature (bottom) of an individual cat. Butaclamol (BUTAC) in a dose of 1.8 pg was injected at AP 14.0, depicted by the dot in the histological inset, 30 min prior to 0 hr. 5-HT in a dose of 3.5 pg was injected at the same site at 0 hr. On the control experimental day, only S-HT was microinjected at the same site. Time is in hours.

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p-INJECTION I

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II

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FIG. 8. Respirations per minute (top), skin temperature (middle) and colonic temperature (bottom) of an individual cat. Methysergide (METHY) in a dose of 5.25 pg was injected at AP 15.0, depicted by the dot in the histological inset, 30 min prior to 0 hr. Norepinephrine (NE) in a dose of 3.5 fig was injected at the same site at 0 hr. On the control experimental day, only NE was microinjected at the same site. Time is in hours.

TABLE

2

CLASSIFICATION OF INDOLE- AND CATECHOLAMINE RECEFTOR BLOCKADE FOR S-HT, NE AND DA INDUCED CHANGES IN BODY TEMPERATURE OF THE CAT

5-HT Hyperthermia

5-HT Hypothermia

NE Hypothermia

DA Hypothermia

Yes No No

Yes* Yes? Yese

No Yes No

‘? Yes/No Yes

Methysergide Phentolamine Butaclamol *t(2)=3.60; ~~0.05. tc(4)=3.23; p
This fall is believed

to be due to a saturation

and conse-

quent desensitization of receptor sites [19,20] similar to that thought to occur when acetylcholine is applied in high doses to the autonomic ganglion to block ganglionic transmission (e.g., [8]). The present results confirm this morphological distinction, and correspond generally to the distribution of monoamines in the hypothalamus, as determined by his-

tochemical fluorescence and other anatomical methods. The areas of the anterior, dorsal and medial hypothalamus adjacent to the third cerebral ventricle are densely innervated by catecholaminergic nerve fibers [2,26]. These ascending fibers, as demonstrated by Cowchock et al. [3], project to the dorsomedial nucleus, the bed nucleus of the stria terminalis and the regions which include the infundibular complex, paraventricular and supraoptic nuclei. Whereas cell bodies

THERMOREGULATION

AND 5-HT RECEPTORS

FIG. 9. Histological section of a representative cat’s brain cut on a sledge cryotome at 100 pm and stained with cresyl violet according to the METHOD. The arrow depicts the point of deepest penetration of the microinjection cannula which is located in the lateral preoptic area in coronal plane AP 14.0.

in the caudal hypothalamic area exhibit only a moderate amount of catecholamine fluorescence, the preoptic area, anterior and dorsomedia1 hypothalamic areas of the feline diencephafon are rich in catecholaminergic neurons. The ventromedial hypoth~amus is almost entirely devoid of ~atecholamine-containing nerve terminals [3] and not unexpectedly is pharmacologically insensitive to catecholamines in terms of temperature effects 1193.The ventral region of the cat’s brain contains relatively high concentrations of 5-HT, particularly the hypothalamus, with less of the indoleamine present in the preoptic area [25]. Since methysergide applied to POA partially prevents the 5-HT induced hypothermia, the idea of an “over-load” of the serotonergic receptors is supported. Acting as a relatively specific blocking agent, methysergide presumably is taken up by 5-HT receptors on neurons comprising the heat production pathway, and thus the desensitizing action of 5-HT would be precluded. When micro-injected at the rostral edge of the anterior hypothalamus, a higher dose of 5-HT which often produces hy~thermia, nevertheless can evoke an intense rise in the animal’s body temperature. The reason for this is that the anatomical distance from the serotonergic cells that mediate heat pr~uction provides a region of local diffusion and dilution of the excess 5-HT moiety. In this case, one would once again predict that methysergide would

block the _5-HT-induced rise in temperature; and as shown in the results, this in fact does occur. The hypothe~ia produced by either a high or low dose of 5-HT injected in the AH or POA, respectively, is blocked by phentolamine or butaclamol. Thus, within the AH, the decline in the cat’s temperature caused by the high dose of S-HT may not be due entirely to an “over-load” of the receptor sites. As seen in Fig. 5, the 5-HT hypothermia was accompanied by the active heat loss response of vasodilation which would be caused by the occupation of catecholamine receptors by 5-HT. In either case, when the local action of 5-HT on AH neurons dissipates, then the functional responses for heat production are activated and hyperthermia, albeit delayed, then occurs. The likelihood that CatechoI~ine-cont~ning neurons are affected by 5-HT micro-injected locally is supported by many &dings. For example, Glowinski and co-workers find that S-NT is taken up at catecholaminergic synapses to influence a neuronal function normally served by catecholamines 1271. In addition, 5-HT can also interact with dopaminergic neurons as well as noradrenergic cells [S,13]. Drugs and other substances which cause significant changes in the serotonergic neurons affect either or both classes of neurons 1141.To illustrate, S-HTP in a dose that augments the content of 5-HT in the rat’s corpus striatum causes a marked change

86

RUWE AND MYERS

in the dopamine

concentration within this structure; in addition, the metabolites of dopamine, DOPAC and HVA, also increase sharply along with the elevated level of 5-HT in the striatum [ 11. In spite of the present pharmacological results, it is conceivable that serotonergic neurons in the hypothalamus constitute a part in the thermoregulatory pathway for heat loss. However, in order for endogenous 5-HT in the diencephalon to be considered as a mediator of synaptic transmission, the release of the indoleamine within the preoptic area in response to environmental heat is required. Thus far, anatomical studies of 5-HT release from both AH and POA during exposure of an animal to hot or cold are clear-cut. In the monkey, only cold, sufficiently intense to evoke heat production, causes the release of 5-HT from cells in AH/POA [21]. Similarly, the level of 5-HIAA in the AH is also elevated by cold exposure [IO]. Conversely, warming of

the animal, which activates heat dissipation, not only does not enhance 5-HT output from cells of the AH/POA but may inhibit the release of the indoleamine [21]. Taken together, these findings suggest that 5-HT in this thermoregulatory region is devoted to the physiological processes underlying heat production when an animal is subjected to a cold challenge. The fact that 5-HT can evoke a decline in body temperature thus could be explained solely on the basis of a locally induced pharmacological action rather than on the actual physiological mechanism for which the indoleamine is thought to play a vital role. ACKNOWLEDGEMENTS

This research was supported in part by National Science Foundation Grant BNS-78-24491 and Office of Naval Research Contract NOOO14-79-C-0078.The authors are indebted to Heidi Leverenz and Sue King for their histological and other technical assistance.

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