Function of serotonin in physiologic secretion of growth hormone and prolactin: action of 5,7-dihydroxytryptamine, fenfluramine and p-chlorophenylalanine

Chronic exposure to a predator or its scent does not inhibit male-male competition in male mice lacking brain serotonin

Although it is well-known that defective signaling of the 5-HT system in the brain and stressful stimuli can cause psychological disorders, their combined effects on male–male aggression and sexual attractiveness remain unknown. Our research aimed at examining such effects using tryptophan hydroxylase 2 (Tph2) knockout male mice vs. a rat- or rat scent-based chronic stress model. Tph2^+/+ and Tph2^−/− male mice were placed individually into the rat home cage (rat), a cage containing soiled rat bedding (rat scent) or a cage containing fresh bedding (control) for 5 h every other day for 56 consecutive days. In Tph2^+/+ male mice, rat-exposure decreased male–male aggression and sexual attractiveness of urine odor relative to either rat scent-exposure or control; and rat scent-exposure decreased aggression rather than sexual attractiveness of urine odor compared with control. However, such dose-dependent and long-lasting behavioral inhibitory effects vanished in Tph2^−/− male mice. RT-PCR assay further revealed that putative regulatory genes, such as AR, ERα and GluR4 in the prefrontal cortex, and TrkB-Tc and 5-HTR1A in the hippocampus, were down-regulated at the mRNA level in either rat- or rat scent-exposed Tph2^+/+ male mice, but partially in the Tph2^−/− ones. Hence, we suggest that the dose-dependent and long-lasting inhibitory effects of chronic predator exposure on male–male aggression, sexual attractiveness of urine odor, and mRNA expression of central regulatory genes might be mediated through the 5-HT system in the brain of male mice.

Reduction of serotonergic neurons in the dorsal raphe due to chronic prenatal administration of a tryptophan-free diet

Serotonin (5-HT) is a widely studied neurotransmitter which plays an important role in the development and proper functioning of the organism throughout life. The appearance of 5-HT system early in ontogeny suggests the hypothesis that 5-HT plays a regulatory role in neurodevelopment. This study investigated the effect of administration of a tryptophan deficient diet during prenatal development on the morphology and cell population of the dorsal raphe. The experimental diet, containing balanced amounts of carbohydrates, lipids and proteins, was provided to a time-mated group of rats from gestational day 5 until delivery. Control groups were fed with (i) the experimental diet formulation with 0.2% tryptophan added to the mixture, or (ii) a regular chow diet. At delivery, five pups per dam were euthanized. Body and brain weight was measured and brain sections were processed for immunohistochemistry for tryptophan hydroxylase (TrpH) and whole brain 5-HT analysis. Sections containing dorsal raphe were photographed with a light microscope and TrpH positive neurons quantified. Brain weights in the tryptophan deprived group showed no difference as compared with controls while body weights were reduced by 25%. Total numbers of serotonergic neurons at the dorsal raphe in the prenatal tryptophan deficient pups were reduced by 35%. A regional analysis of the dorsal raphe indicated a marked cellular reduction in the medial and caudal sections of the nucleus, which contains the majority of serotonergic neurons, in the tryptophan deprived condition. Quantitative 5-HT analysis showed that the brain concentration was similar among conditions. In conclusion, gestational tryptophan deprivation exerts adverse effects on the development of the 5-HT system, particularly in the dorsal raphe, manifested by decreased numbers of serotonergic neurons as well as altered topography in this important nucleus.

Control of prolactin secretion by excitatory amino acids

This article provides an overview of the increasing number of observations indicating that excitatory amino acids are involved in the control of prolactin secretion. The information available suggests that these amino acids exert a stimulatory action on hypophysial prolactin. Administration of a glutamate receptor agonist induces significant increase in prolactin release in rats, monkeys, and rams. In contrast, noncompetitive antagonists of N-methyl-D-aspartate receptors decrease plasma levels and attenuate the preovulatory surge of prolactin. It appears that the endogenous glutamatergic system participates not only in the regulation of basal secretion of prolactin, but also in the control of physiological prolactin responses induced by the suckling stimulus or by stress. Recent findings suggest that the glutamatergic innervation of the hypothalamic paraventricular nucleus is involved in the mediation of the neural signal of the suckling stimulus-induced prolactin release as well as in the mediation of the stress-induced release of prolactin.

Non-NMDA glutamate receptor antagonist injected into the hypothalamic paraventricular nucleus blocks the suckling stimulus-induced release of prolactin

The aim of the present investigations was to test the involvement of the glutamatergic innervation of the hypothalamic paraventricular nucleus in the prolactin response to the suckling stimulus. A non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-dione disodium (CNQX), or an NMDA receptor antagonist, dizocipine hydrogen malate (MK-801), was injected bilaterally into the hypothalamic paraventricular nucleus of lactating freely moving rats before the end of a 4-h separation of the dams from their pups. The litters were then returned. Blood samples for prolactin were taken at different time points. The effect of the non-NMDA receptor antagonist was also tested in animals receiving the drug bilaterally into the dorsomedial nucleus area or the arcuate nucleus. Bilateral injection of CNQX into the paraventricular nucleus blocked the elevation in plasma prolactin concentration induced by the suckling stimulus. In contrast, bilateral administration of the NMDA receptor antagonist MK-801 into the paraventricular nucleus or bilateral injection of CNQX into the dorsomedial nucleus area or the arcuate nucleus did not interfere with the prolactin response to the suckling stimulus. The findings indicate that the glutamatergic innervation of the paraventricular nucleus is involved in the mediation of the neural signal of the suckling stimulus inducing prolactin release.

Thyrotropin-releasing hormone mediates serotonin-induced secretion of GH in cattle

Serotonin stimulates secretion of growth hormone (GH) in cattle, but the mechanism is unknown. In rats, thyrotropin-releasing hormone (TRH) mediates serotonin-induced secretion of GH. We hypothesized that the same is true in cattle. Cattle were fed for 2h daily to synchronize secretion of GH, such that concentrations of GH were high before and low after feeding. Our first objective was to determine whether or not feeding suppresses serotonin receptor agonist (quipazine) induced secretion of GH. Holstein steers were injected with quipazine (0.2 mg/kg BW) either 1 h before or 1 h after feeding. Quipazine-induced secretion of GH which did not differ in magnitude before and after feeding. If TRH mediates serotonin-induced secretion of GH, then magnitude of TRH-induced secretion of GH should not be different before and after feeding (our second objective). Sixteen meal-fed Holstein steers were injected with 0.3 microg TRH/kg BW either 1 h before or 1 h after feeding. Indeed, magnitude of TRH-induced secretion of GH before and after feeding was not different. Our third objective was to inhibit endogenous TRH with 3,5,3'-triiodothyronine (T(3)) and examine basal, GH-releasing hormone (GHRH)-, TRH- and quipazine-induced secretion of GH. Sixteen Holstein steers were injected daily with either T(3) (3 or 6 microg/kg BW) or vehicle for 20 days and then challenged sequentially with vehicle or GHRH, TRH, or quipazine. T(3) did not affect basal, GHRH- or TRH-induced secretion of GH, but reduced basal secretion of thyroxine. T(3) reduced but did not completely block quipazine-induced secretion of GH. In conclusion, TRH mediates, in part, serotonin-induced secretion of GH in cattle.

Brain structures mediating the suckling stimulus-induced release of prolactin

Suckling-induced prolactin release is a widely studied neuroendocrine reflex, comprising a neural afferent and a humoral efferent component. The information on the brain structures involved in this reflex is fairly limited. The present studies focused on this question. The following hypothalamic interventions were made in lactating rats and the dams were tested for the suckling-induced prolactin response: (i) unilateral or (ii) bilateral frontal cuts at the level of the anterior and posterior hypothalamus; (iii) administration of 5,7-dihydroxytryptamine or (iv) 6-hydroxydopamine into the hypothalamic paraventricular nucleus (PVN) to destroy serotonergic and catecholaminergic innervation of the cell group, respectively; (v) lesion of the medial subdivision of the PVN; and (vi) horizontal knife cuts below the PVN. Bilateral posterior and bilateral or unilateral anterior frontal cuts caused blockade of the suckling-induced release of prolactin. Likewise, most dams receiving 5,7-dihydroxytryptamine in the PVN did not respond to the suckling stimulus. Immunocytochemistry revealed that, in those rats which did not show a rise in plasma prolactin, there were almost no serotonergic fibres and terminals in the PVN, while in dams which exhibited a response, numerous serotonergic elements were evident. 6-Hydroxydopamine treatment did not cause significant alteration in the prolactin response. Lesion of the medial, largely parvocellular subdivision of the PVN, or horizontal knife cuts below this cell group, blocked the hormone response. The findings demonstrate for the first time that: (i) interruption of the connections between the brain stem and the hypothalamus interferes with the prolactin response to the suckling stimulus; (ii) serotonergic fibres terminating in the hypothalamic PVN are involved in the mediation of the suckling stimulus; and (iii) within the PVN, neurones in the medial, largely parvocellular subdivision of the cell group take part in the transfer of the neural signal, eventually inducing prolactin release.

Behavioural and neuroendocrine responses to D-fenfluramine in rats treated with neurotoxic amphetamines

The amphetamine derivatives p-chloroamphetamine (pCA), 3,4-methylenedioxymethamphetamine (MDMA, 'Ecstasy') and D-fenfluramine can, if given repeatedly in high doses to rats, produce a degeneration of serotonergic nerve terminals which we have previously shown to result in a reduction in D-fenfluramine-evoked release of 5-HT in vivo. It is therefore possible that fenfluramine-evoked responses may have value as a probe of 5-HT neurodegeneration in man. The present study examined the effect of pre-treatment with these three agents (pCA 12 mg/kg×2; MDMA 20 mg/kg×8; D-fenfluramine 12.5 mg/kg×8, 14 days prior to testing) on behavioural (5-HT syndrome) and neuroendocrine [prolactin and adrenocorticotrophin (ACTH)] responses in rats to acute administration of D-fenfluramine and other serotonergic agonists. All three pre-treatments attenuated the D-fenfluramine-evoked behavioural syndrome, but did not affect the prolactin or ACTH responses to acute challenge with D-fenfluramine (apart from a small effect of pre-treatment with pCA on the ACTH response to D-fenfluramine). For comparison, the effect of pCA pre-treatment on the behavioural responses to acute administration of pCA and the 5-HT(1A) and 5-HT(2) receptor agonists 8-hydroxy-2-(di- n- propylamino)tetralin (8-OH-DPAT) and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), respectively, were also examined. pCA pre-treatment attenuated all components of the behavioural response to pCA but had little or no effect on the behavioural responses to 8-OH-DPAT or DOI, suggesting that there was no alteration in post-synaptic 5-HT(1A) or 5-HT(2) receptor function. While the loss of behavioural effect of D-fenfluramine on rats pre-treated with neurotoxic amphetamines can be understood in terms of the loss of D-fenfluramine's 5-HT-releasing action following 5-HT neurodegeneration, the lack of change in the neuroendocrine responses to D-fenfluramine is not easily explicable in this way. These results emphasise the need for further research into the actions of D-fenfluramine before carrying it forward as a probe of neurodegeneration in man.

Test-retest reliability of the prolactin and cortisol responses to D,L-fenfluramine challenge in disruptive behavior disorders

We examined the intraindividual stability of plasma prolactin (PRL) and cortisol responses to D,L-fenfluramine challenges (1.0 mg/kg, p.o.), at a 1-week interval, in boys with disruptive behavior disorders. Two acute administrations of fenfluramine produced consistent and predictable effects on net prolactin responses (peak delta PRL, area under the curve delta PRL), but variable and unpredictable effects on net cortisol responses. The time course and magnitude of fenfluramine blood levels, not nor-fenfluramine, paralleled net PRL responses to fenfluramine. These data indicate that the PRL response to fenfluramine shows continuity within individuals over the course of 1 week, providing a reliable index to reflect the overall function of the serotonin system in the limbic-hypothalamus.

Differential effects of antidepressant treatments on fenfluramine-induced increases in plasma prolactin and corticosterone in rats

Intravenous administration of 5-HT releasing agent, fenfluramine, to rats produced increases in plasma prolactin and corticosterone concentrations. Short-term or long-term treatment with either clorgyline or imipramine did not affect baseline levels of prolactin or corticosterone. On the other hand, short-term but not long-term lithium treatment significantly increased baseline levels of corticosterone but not of prolactin. Short-term treatment with lithium but not clorgyline or imipramine potentiated fenfluramine-induced increases in plasma prolactin but not corticosterone. On the other hand, long-term treatment with clorgyline but not imipramine or lithium attenuated fenfluramine's effect on plasma prolactin but not on corticosterone. These findings demonstrate differential effects of antidepressant treatments on fenfluramine-induced increases in plasma prolactin and corticosterone in rats and are consistent with several other clinical and animal studies demonstrating dissimilar actions of different antidepressant treatments on two different 5-HT-mediated neuroendocrine functions.

Hormonal responses to fenfluramine in depressive subtypes

In order to study putative differences in central neurotransmitter function in depressive subtypes, serum cortisol and prolactin responses to the putative serotonin agonist fenfluramine were examined in 30 subjects with major depression. Patients with endogenous depression (melancholia) as defined by each of ICD-9, DSM-III, RDC and Newcastle scale demonstrated a reduced prolactin response to 60 mg oral fenfluramine when compared with non-endogenous subjects. This was independent of either prolactin or cortisol baseline levels, and indicates that there are differences in brain neurotransmitter function in the endogenous and non-endogenous subtypes of depression. Basal prolactin levels were reduced in bipolar compared with unipolar subjects, and delusional compared with non-delusional patients, although there were no differences in the prolactin responses to fenfluramine between these subgroups. Basal cortisol levels and cortisol response to fenfluramine did not distinguish between any of the subtypes.

Depression and antidepressant therapy: receptor dynamics

1. The classical norepinephrine (NE) and serotonin (5-HT) theories of depression have been abandoned in light of recent chronic antidepressant drug studies. 2. The new NE and 5-HT theories of depression focus on the dynamics of receptor subtypes in depression and chronic antidepressant treatments. 3. Recent studies in molecular genetics suggest a reclassification of monoamine receptors based on receptor structural homologies in DNA and amino acid sequences rather than receptor affinity for ligands. 4. Electrophysiologic studies in rats suggest that 5-HT1 receptor function is facilitated by chronic antidepressant treatment. 5. Preclinical studies employing a range of 5-HT1 mediated behavioral models also suggest that chronic antidepressant treatment facilitates transmission at central 5-HT1 receptors. 6. Patient studies, employing a 5-HT1 mediated neuroendocrine model, suggest that depression is associated with decreased transmission at CNS 5-HT1 receptors; and that chronic antidepressant treatment facilitates 5-HT1 receptor responsiveness in depressed patients. 7. New 5-HT1 selective agonists have been developed and found to be clinically effective antidepressants. 8. The above clinical and preclinical data suggest that some forms of depression are related to a decreased responsiveness of 5-HT1 receptors which is reversed by chronic antidepressant treatment. 9. Beta adrenergic and NE-stimulated cyclic AMP studies suggest that chronic antidepressant treatment decreases the responsiveness of central beta-adrenergic receptors, particularly beta-1 receptors. 10. A novel approach to antidepressant drug development focuses on identifying centrally active beta-1 agonists, which like clinically proven antidepressants, decrease beta-1 receptor responsiveness with chronic treatment. 11. 5-HT2 receptor binding studies and initial studies of 5-HT2 receptor coupled PI turnover suggest that chronic antidepressant treatment decreases 5-HT2 receptor number and function. 12. The development of new atypical antidepressants with 5-HT2 receptor related mechanisms of action suggest that 5-HT2 receptors may be associated with certain types of depression and their clinical treatment.

Neuroendocrine aspects of the serotonergic hypothesis of depression

This review examines the role of serotonin (5-HT) in depression. Dysfunction of serotonergic neurons has been implicated as one of the causes of endogenous depression. Since serotonergic neurons innervate the hypothalamus and these neurons send collaterals to several other brain areas, it is possible that hypothalamic sites which control hormone secretion receive the same serotonergic afferents that innervate other limbic areas in the brain. Several investigators have devised neuroendocrine challenge tests measuring the effect of 5-HT agonists on plasma cortisol and prolactin in depressed patients. These tests help to identify dysfunctional 5-HT neurons, and are a "window into the brain." The secretion of cortisol and prolactin is increased predominantly by 5-HT1 receptors. However, changes in 5-HT2 receptors have also been implicated in depression. Results from our laboratory and by others suggest that brain serotonergic neurons stimulate renin and vasopressin secretion by activation of 5-HT2 receptors. Therefore, the renin and vasopressin response to 5-HT agonists should be included in neuroendocrine tests of serotonergic function in affective disorders. Since antidepressants produce a decrease in the density of 5-HT2 receptors, renin and vasopressin could be used to evaluate the antidepressant potential of new drugs.

Effects of individual housing on circadian rhythms of adult rats

Circadian rhythms of hypothalamic serotonin (5HT), its precursor tryptophan (TP) and its metabolite 5-hydroxy-indoleacetic acid (5HIAA), and of prolactin and corticosterone circulating hormones were determined in group-housed and in individually-housed male rats, adapted to a 12:12 light/dark cycle. After 5 weeks of individual housing, 5HT peaked later, TP and 5HIAA peaked earlier, and the mesor level of TP and 5HIAA decreased with respect to group-housed animals. Individual housing caused an increase in the corticosterone mesor level, but did not affect amplitude or acrophase. The circadian rhythm of prolactin was unchanged by individual housing.

Effects of trazodone treatment on serotonergic function in depressed patients

Changes in serotonergic (5HT) neurotransmission may mediate the therapeutic actions of some antidepressant drugs. In the present study, the 5HT precursor L-tryptophan (L-TRP) was administered intravenously to nine depressed patients before and during treatment with the triazolopyridine antidepressant trazodone (TRZ). Neuroendocrine, subjective mood, and cardiovascular responses to L-TRP were assessed. Unlike tricyclic antidepressants and monoamine oxidase inhibitors, TRZ did not enhance the prolactin response to L-TRP and had little effect on other measures. Since other studies indicate that the TRP-induced increase of prolactin in humans may reflect 5HT function, the present study suggests that TRZ treatment does not enhance net 5HT function in depressed patients.

Demonstration of serotoninergic axon terminals on somatostatin-immunoreactive neurons of the anterior periventricular nucleus of the rat hypothalamus

Using a combination of electron microscopic autoradiography and immunocytochemistry, the connections between serotoninergic axons and somatostatin neurons of the anterior periventricular nucleus of the rat hypothalamus were examined. The serotoninergic elements were identified after selective uptake of tritiated serotonin and the somatostatin neurons with immunocytochemistry. Synaptic connections between labeled serotoninergic nerve endings and somatostatin-immunoreactive neurons were observed. This finding provides morphological evidence for a direct influence of serotoninergic elements on somatostatin neurons of the anterior periventricular nucleus projecting to the median eminence of the hypothalamus.

Activation of serotonin receptors in the medial basal hypothalamus stimulates growth hormone secretion in the unanesthetized rat

In conscious rats, serotonin microinjected into the basal hypothalamus caused secretion of GH maximal within 10-25 min. The effects of serotonin on GH were blocked by the non-selective serotonin receptor blocker, metergoline 2.5 mg/kg, but not by the serotonin type 2 receptor blocker, ketanserine 0.2 mg/kg. Injections of serotonin in the preoptic/anterior hypothalamic area were without effect. It is concluded that activation of serotonin receptors, probably type I, on or near GH releasing factor neurons in the arcuate nucleus causes secretion of GH and that serotonin has no direct effect on or near somatostatin neurons in the preoptic anterior hypothalamic area.

Bi-directional, chemically specified neural connections between the subfornical organ and the midbrain raphe system

Reports of a serotonin-immunoreactive fiber plexus in the subfornical organ (SFO) and evidence that this input is relevant to the control of thirst and blood pressure by the SFO prompted an investigation of neural connections between the SFO and the serotonergic neurons in the raphe system. Serotonin-immunoreactive fibers were found to enter the SFO by a number of different routes and to form a plexus in the central part, which contains a bed of fenestrated capillaries. Injections of the anterogradely transported lectin, phaseolis vulgaris leucoagglutinin (PHA-L), into the dorsal raphe nucleus produced a pattern of labeled fibers in the SFO comparable to that revealed with the anti-serotonin staining. Injections into the SFO of the retrogradely transported dye, Fast Blue, labeled cell bodies in the dorsal and median raphe nuclei of the midbrain, and at least 60% of these cells also demonstrated serotonin immunoreactivity. Injections of PHA-L into the SFO identified a return projection to this part of the raphe system, where an angiotensin-immunoreactive plexus of fibers and varicosities is found. Injections of True Blue into various parts of the midbrain raphe nuclei reliably labeled neurons in the SFO, and some 56% of them could also be stained with anti-angiotensin. It is suggested that this circuitry is involved in the control of fluid balance by the SFO and that the serotonergic-raphe projection to the SFO may participate in the relay of visceral sensory information, perhaps related to blood pressure, from the nucleus of the solitary tract and the lateral parabrachial nucleus.

Effects of tranylcypromine treatment on neuroendocrine, behavioral, and autonomic responses to tryptophan in depressed patients

Effects of intravenous administration of the serotonin precursor tryptophan (TRP) on serum prolactin, neuromotor function, subjective mood, and blood pressure and pulse were determined in nine depressed patients before and during placebo-controlled treatment with the monoamine oxidase inhibitor (MAOI) tranylcypromine. Tranylcypromine significantly increased the prolactin response to TRP. Four patients developed a distinctive neuromotor syndrome following TRP during tranylcypromine, but not placebo, treatment. Symptoms included hyperreflexia, ankle clonus, nystagmus, incoordination, tremor, myoclonic jerks, and nausea. There were no differences in peak prolactin, mood, or autonomic responses between patients with and without the syndrome, but those with the syndrome had received active tranylcypromine for a significantly shorter duration. Tranylcypromine had little effect on TRP-induced changes in mood or autonomic function, except for a modest enhancement of the TRP-induced rise in diastolic blood pressure. These results suggest that tranylcypromine treatment may enhance serotonin function in depression.

Antidepressants and serotonergic neurotransmission: an integrative review

The effects of acute and chronic antidepressant treatment on various aspects of 5-HT neurotransmission are reviewed, in order to assess the net effect of antidepressants on transmission across 5-HT synapses. Events considered include presynaptic effects of antidepressants (on autoreceptor function, uptake and turnover) and effects on postsynaptic receptor function (assessed by electrophysiological, neuroendocrine, behavioural, and receptor binding methods). Acute antidepressant treatment has variable effects: transmission may be enhanced, unchanged or reduced, depending mainly upon the relative contributions of 5-HT uptake blockade and 5-HT receptor antagonism. However, on chronic administration, most antidepressants appear to enhance 5-HT transmission. This effect is clearest in the case of ECS, which has little effect on 5-HT turnover, but reduces uptake and increases postsynaptic receptor function. MAOIs may be an exception: there is little evidence that MAOIs enhance 5-HT transmission following chronic treatment. Most other antidepressant drugs, including some which are powerful receptor antagonists on acute administration, reduce 5-HT receptor function briefly, but enhance receptor function if several hours elapse between the final injection and testing. Zimelidine has little effect on postsynaptic receptor function, but enhances 5-HT transmission by its powerful blockade of 5-HT uptake. Chronic treatment with antidepressant drugs has usually been found to reduce binding to 5-HT2 receptors; it is difficult to reconcile these observations with the functional studies. In general, with the possible exception of MAOIs, chronic administration of antidepressants may enhance 5-HT transmission by both pre- and post-synaptic effects, and the relative contributions vary. This conclusion supports the classical "indoleamine hypothesis of depression" rather than the more recent "hypersensitive serotonin receptor" theory.

Distribution of 5-hydroxytryptamine (serotonin) in the brain of the teleost Gasterosteus aculeatus L

The distributions of serotoninergic neurons in the brain of the three-spined stickleback was demonstrated with the indirect peroxidase-antiperoxidase (PAP) immunohistochemical method with antibodies against serotonin. Serotoninergic perikarya were demonstrated in the brainstem reticular formation (nucleus raphe dorsalis, nucleus raphe medialis, and nucleus tegmenti dorsalis lateralis) and in the periventricular ventral thalamus and hypothalamus (nucleus ventromedialis thalami, nucleus posterioris periventricularis, nucleus recessus lateralis, and nucleus recessus posterioris). After pharmacological pretreatment of the animals with a monoamine oxidase inhibitor, serotoninergic perikarya were also visualized in area praetectalis and in the medial brainstem, caudal to nucleus raphe medialis. Whereas the cell groups of the brainstem give rise to both ascending and descending pathways, it was not possible to analyze the distribution of efferent projections from the diencephalic cell groups. Distribution of serotoninergic axons showed marked regional differences. Only scattered varicose fibers were demonstrated in the cerebellum, the facial lobes, and the lateral line lobes. In the mesencephalon, the dorsal periventricular tegmentum and the central gray receive only small numbers of serotoninergic axons, while torus semicircularis and the visual layers of tectum opticum are profusely innervated. In the diencephalon, the hypothalamus and ventral thalamus generally display the highest density of serotoninergic axons. Exceptions are found in nucleus glomerulosus and the ventromedial portion of lobus inferioris, where densities are low. In the telencephalon, the density of serotoninergic axons is very high in area dorsalis pars medialis and pars lateralis dorsalis, but low in area dorsalis pars dorsalis and pars lateralis ventralis, and intermediate in area ventralis.