Regulation of central corticosteroid receptors following short-term activation of serotonin transmission by 5-hydroxy-L-tryptophan or fluoxetine

Alterations of the hypothalamic-pituitary-adrenal (HPA) axis function characterized by a decreased negative feedback capacity are often associated with affective disorders and are corrected by treatment with antidepressant drugs. To gain a better understanding of the effects of the antidepressant drug fluoxetine, a specific serotonin (5-HT) reuptake inhibitor, on central corticosteroid receptors, the effects of short-term activation of serotonin transmission on central corticosteroid receptor expression were analysed in adrenalectomized (ADX) rats either supplemented or not with corticosterone. Serotonin transmission was stimulated either by a single injection of the 5-HT precursor, 5-hydroxy-L-tryptophan (5-HTP), or by a 2-day treatment with fluoxetine. In ADX rats, administration of 5-HTP decreased hippocampal mineralocorticoid (MR) and glucocorticoid (GR) receptor numbers 24 h later, while their respective mRNAs were unchanged and these effects of 5-HTP were mediated by 5-HT2 receptors. In the hypothalamus, GR mRNAs and binding sites decreased 3 h and 24 h after 5-HTP, respectively. By contrast, fluoxetine treatment increased hippocampal MR and GR mRNAs and MR binding sites while GR number remained unchanged. In ADX rats supplemented with corticosterone, 5-HTP and fluoxetine treatment had the same effects on corticosteroid receptors compared to those observed in non supplemented ADX rats: 5-HTP decreased hippocampal MR and GR and hypothalamic GR while fluoxetine treatment increased hippocampal MR. These results show that short-term stimulation of 5-HT transmission by 5-HTP decreases hippocampal and hypothalamic corticosteroid receptor numbers through a corticosterone-independent mechanism. It is hypothesized that the delayed maximal increase in extracellular 5-HT contents after fluoxetine treatment, due to negative feedback regulations induced by the activation of 5-HT1A and 5-HT1B autoreceptors, is not the primary cause for the delayed normalization of corticosteroid receptor numbers that regulates the HPA axis functioning.

Central 5-HT(1) and 5-HT(2) binding sites in transgenic mice with reduced glucocorticoid receptor number

Transgenic mice bearing a transgene coding for a glucocorticoid receptor antisense mRNA, which partially blocks glucocorticoid receptor expression, were used in order to clarify the role of glucocorticoid receptors in the regulation of 5-HT(1A), 5-HT(1nonA) and 5-HT(2) binding sites labelled by quantitative autoradiography in the frontal and prefrontal cortex, striatum, hypothalamus, amygdala and raphe nuclei. We found that 1 nM [3H]8-hydroxy-2-[di-N-propylamino]tetralin ([3H]8-OH-DPAT) binding to 5-HT(1A) sites was decreased in strata oriens (-15.1+/-3.5%) and radiatum-lacunosum-moleculare (-13.3+/-4.3%) of the hippocampal CA(3) area, and 2 nM [3H]5-hydroxytryptamine binding to 5-HT(1nonA) sites in the presence of 100 nM 8-OH-DPAT and mesulergine was decreased in the dorsal subiculum (-17.8+/-6.9%). By contrast, 5-HT(2) sites labelled by 0.5 nM of (+/-)-1-(2, 5-dimethoxy-4-[125I]iodophenyl)-2-aminopropane was increased in the dorsal subiculum (+35.2+/-11.5%) and CA(2) area (+29.2+/-11.3%). The observed differences in binding to 5-HT(1) and 5-HT(2) sites were all located in areas of the hippocampus that contain both gluco- and mineralo-corticoid receptors, and no difference was observed in anatomical structures which contain only glucocorticoid receptors. Therefore, it seems that the important factor for the regulation of these 5-HT receptors is the interaction between gluco- and mineralo-corticoid receptors rather than the absolute density of glucocorticoid receptors. These results suggest that some of the alterations of the serotonergic neurotransmission observed in depressed patients might be secondary to an altered glucocorticoid receptor function.

The effects of serotonin on glucocorticoid receptor binding in rat raphe nuclei and hippocampal cells in culture

The raphe-hippocampal serotonin (5-HT) system is involved in the regulation of the hypothalamus-pituitary-adrenal axis. The purpose of this study was to determine and compare the roles of 5-HT in the regulation of glucocorticoid receptor (GR) binding in the raphe nuclei and in the hippocampus. The effects of 5-HT, 5-HT agonists, and the 5-HT reuptake inhibitor citalopram on GR binding sites were studied in primary cultures of the fetal raphe nuclei and the hippocampus. Exposure of hippocampal cells to 5-HT, (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI; a 5-HT2 agonist), or citalopram resulted in an increase in number of GR binding sites. The effect of DOI was blocked by ketanserin (a 5-HT2 antagonist). Specific and saturable GR binding was found in raphe cells. Exposure of raphe cells to 5-HT, (+/-)-8 hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; a 5-HT1A agonist), or citalopram induced a significant decrease in number of GR binding sites. The effect of 8-OH-DPAT was reversed by WAY 100135 [N-tert-butyl-3-[1-[1-(2-methoxy)phenyl]piperazinyl]-1-phenylpropiona mide; a 5-HT1A antagonist]. These results show that the regulation of GRs during fetal life is structure-dependent and involves different 5-HT receptor subtypes. Moreover, the regulation of hippocampal GRs by citalopram suggests an action of antidepressants independent of their effects on monoamines.

Regional serotonin metabolism under basal and restraint stress conditions in the brain of transgenic mice with impaired glucocorticoid receptor function

Transgenic (TG) mice deficient in glucocorticoid receptors (GR) were used in order to study the effects of a reduced GR function on adrenocorticotropin hormone and corticosterone plasma levels and on serotonin metabolism in different brain areas under basal resting conditions, after a 30-min restraint stress and 60 min after the end of the restraint stress. There was no difference in basal or stress-induced levels of either adrenocorticotropin hormone or corticosterone in control and TG mice, but the return of adrenocorticotropin hormone to basal values after the end of the stress was delayed in TG mice. Under basal conditions, the ratio 5-hydroxyindoleacetic acid/5-hydroxytryptamine was decreased only in the hippocampus of TG mice compared to controls. In the brain stem, the ratio 5-hydroxyindoleacetic acid/5-hydroxytryptamine increased compared to basal values after a 30-min restraint stress and values were still high 60 min after the end of the restraint stress in both control and TG mice. In the hippocampus, the ratio 5-hydroxyindoleacetic acid/5-hydroxytryptamine increased at the end of the stress and returned to basal levels 60 min later in control mice, whereas there was no change at the end of the stress but an increase 60 min later in TG mice. Finally there was no change in serotonin metabolism in the cortex, striatum or hypothalamus in either group or situation. Our results support the hypothesis of a tonic activation of serotonin turnover by corticosterone through GR in the mouse hippocampus. Moreover, stress-induced stimulation of serotonin metabolism in the brain stem and hippocampus appears to be delayed in TG mice compared to control mice. These results are particularly relevant for mood disorders such as depression where alterations of serotoninergic transmission might be secondary to an impairment of GR functions.

Indirect evidence for an association of 5-HT(1B) binding sites with retinal and geniculate axon terminals in the rat suprachiasmatic nucleus

The purpose of the present study was to investigate the possible cellular location of 5-HT(1B) receptors on retinal and geniculate afferents in the rat suprachiasmatic nucleus (SCN). Biocular enucleation significantly decreased 5-HT(1B) binding site labeling (35%), specifically in the ventral part of the SCN, while monocular enucleation produced a decrease of smaller magnitude (12%), limited to the ventral part of the contralateral SCN, these results being consistent with the known distribution of retinal afferents in the nucleus. By contrast, bilateral geniculate lesion did not induce any significant variation of 5-HT(1B) binding site labeling in the SCN. Previously, we reported that serotonin (5-HT) synthesis inhibition by parachlorophenylalanine increases 5-HT(1B) binding site labeling in the SCN. Using saturation studies, we have now demonstrated that this upregulation reflected an increase in the total number of 5-HT(1B) binding sites (+41% in the dorsal and +67% in the ventral part of the SCN). Furthermore, we evaluated the effects of bilateral geniculate lesion after 5-HT stores depletion in order to overcome problems of technical resolution limits. The magnitude of upregulation was significantly decreased (27%) after bilateral geniculate lesion, suggesting that part of the 5-HT(1B) receptor population was located on geniculate axon terminals within the SCN. The possible involvement of 5-HT(1B) receptors, according to their cellular locations evidenced in the present study, in photic and nonphotic entrainment of the circadian clock is discussed.

Identification and role of serotonin 5-HT1A and 5-HT1B receptors in primary cultures of rat embryonic rostral raphe nucleus neurons

Autoregulatory mechanisms affecting serotonin [5-hydroxytryptamine (5-HT)] release and synthesis during the early period of development were investigated in dissociated cell cultures raised from embryonic rostral rat rhombencephalon. The presence of 5-HT1A and 5-HT1B receptors in serotoninergic neurons was assessed using binding assays. The involvement of 5-HT1A and 5-HT1B receptors in the control of the synthesis and release of [3H]5-HT was studied using biochemical approaches with several serotoninergic receptor ligands. A mean decrease of 30% in [3H]5-HT synthesis and release was observed in the presence of 5-HT (10(-8) M), the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), the 5HT1B/1A agonist 5-methoxy-3-(1,2,5,6-tetrahydro-4-pyridinyl)-1H-indole (RU 24969), the 5-HT1B agonist 3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one (CP-93,129), and the 5-HT(1D/1B) agonist sumatriptan. Inhibition of 5-HT synthesis and release induced by 8-OH-DPAT was blocked by chiral N-tert-butyl-3-[1-[1-(2-methoxy)phenyl]piperazinyl]-1-phenylpropionam ide dihydrochloride quaternary-hydrate (WAY 100135) (10(7) M) or methyl 4-[4-[4-(1,1,3-trioxo-2H-1,2-benzoisothiazol-2-yl)butyl]-1-p iperazinyl]-1Hindole-2-carboxylate (SDZ 216-525) (10(-7)M), and that of CP-93,129 was blocked by methiothepin (10(-7) M). Paradoxically, extracellular levels of [3H]5-HT increased in the presence of 8-OH-DPAT and RU 24969 at 10(-6) M. 5-HT uptake experiments showed that these two agonists interacted with the 5-HT transporter. 5-HT1 binding sites (620 fmol/mg of protein) and 5-HT1A (482 fmol/mg of protein) and 5-HT1B (127 fmol/mg of protein) receptors were detected in 12-day in vitro cell cultures. Experiments carried out with tetrodotoxin suggested that 5-HT1A receptors are located on nerve cell bodies, whereas 5-HT1B receptors are located on the nerve terminals. We concluded that autoregulatory mechanisms involving 5-HT1A and 5-HT1B autoreceptors are functionally mature in cells from rostral raphe nuclei during the early period of development.

Post-lesion up-regulation of 5-HT1B binding sites in the suprachiasmatic nucleus may be reversed after spontaneous or graft-induced serotonin reinnervation

We have previously reported that selective axotomy of serotoninergic neurons produced by an intraventricular injection of 5, 7-dihydroxytryptamine is followed by an increase in 5-HT1B binding sites in the suprachiasmatic nucleus of the hypothalamus. This post-lesion up-regulation is shown here to be spontaneously reversed after long-term survival in spite of an incomplete reinnervation of the nucleus. Recovery may be accelerated by fetal raphe transplants that produce more rapid reinnervation.

Effects of bilateral olfactory bulbectomy on circadian rhythms of ACTH, corticosterone, motor activity and body temperature in male rats

Bilateral olfactory bulbectomy (BOX) has major biochemical and behavioral effects, and is one of the most widely investigated of animal models of depression. We studied the consequences of BOX in male rats, on the organization of endogenous circadian rhythms for ACTH, corticosterone (Cort), motor activity (MA) and body temperature (BT). Mean levels were increased for Cort and MA, whereas no significant changes were observed for ACTH and BT. Significantly higher plasma Cort morning values were evidenced in BOX than sham-operated animals. In addition, compared with the single prominent power spectrum for the 24 hours period of control rats, the BOX animals displayed substantially lower 24 hours spectral power for the MA and BT circadian rhythms. These alterations suggest that olfactory bulbectomy, by disruption of the afferences and efferences, induced drastic changes in the function of the endogenous clock or of its regulating systems. From this point of view, bulbectomized rats may therefore be a valuable model to studying the etiology of psychiatric disorders with rhythm disturbance.

Serotonin directly stimulates luteinizing hormone-releasing hormone release from GT1 cells via 5-HT7 receptors

Luteinizing hormone-releasing hormone (LHRH release, which serves as the primary drive to the hypothalamic-pituitary gonadal axis, is controlled by many neuromediators. Serotonin has been implicated in this regulation. However, it is unclear whether the central effect of serotonin on LHRH secretion is exerted directly on LHRH neurosecretory neurons or indirectly via multisynaptic pathways. The present studies were undertaken in order to examine whether LHRH secretion from immortalized LHRH cell lines is directly regulated by serotonin and, if so, to identify the receptor subtype involved. 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A/7 receptor agonist, stimulated LHRH release from GT1-1 cells. This effect was blocked by ritanserin, a 5-HT2/7 receptor antagonist, but not by SDZ-216-525, a 5-HT1A antagonist. Basal LHRH release was not affected by the 5-HT2 agonist DOI. Reverse transcription and polymerase chain reaction technique (RT-PCR) was used in order to identify 5-HT1A and 5-HT7 receptor mRNA in immortalized LHRH cell lines. GT1-1 cells express mRNA for the 5-HT7, but not the 5-HT1A receptor subtypes. These results demonstrate a direct stimulatory effect of serotonin on LHRH release via 5-HT7 receptor.

8-OH-DPAT regulates the amplitude and the phase of LH surge in ovariectomized steroid-primed rats

Precise interactions between ovarian steroids and neurotransmitters are required for the secretion of phasic LH surge. Previous data suggested the existence of an interactive stimulatory effect of progesterone (P) and serotonin (5-HT) on LH release. In the present work the effects of 8-OH-DPAT, a selective 5-HT(1A) agonist, on phasic LH secretion were tested in ovariectomized rats implanted for 6 days with a pellet of 17 β estradiol (OVX-E(2)) and in OVX-E(2) treated with progesterone (OVX-E(2)-P). Intraperitoneal injection of 8-OH-DPAT at 11.00 h in the morning of the expected LH surge had no effect on circadian plasma levels of LH in OVX-E(2) rats, whereas it induced a phase advance and an increase in LH surge in OVX-E(2)-P rats. Administration of the antiprogestin RU 38486 in OVX-E(2)-P rat, totally abolished the combined effects of P and 8-OH-DPAT on phasic LH release. SDZ 216-525, a specific 5-HT(1A) antagonist administered 60 min before 8-OH-DPAT, inhibited the stimulatory effect of the 5-HT(1A) agonist on the amplitude of LH surge. The present data suggest that progesterone is required for the regulation of phasic LH release by 5-HT(1A) agonists and that under this hormonal condition the activation of 5-HT(1A) receptors induces a phase advance and an increase in LH surge.

Adrenergic control of alpha-melanocyte-stimulating hormone release in frog pituitary is mediated by both beta- and a nonconventional alpha 2-subtype of adrenoreceptors

Previous studies have shown that melanotrope cells of the pars intermedia of Rana ridibunda are inhibited by dopaminergic D2 agonists and stimulated by beta-adrenergic agonists. In the present study, we have examined the possible involvement of alpha-adrenoreceptors in the regulation of frog melanotrope cells. Reversed-phase HPLC analysis combined with electrochemical detection revealed the presence of both dopamine and noradrenaline in pars intermedia extracts (74.1 and 3.2 ng/mg protein, respectively), while adrenaline was undetectable. Administration of graded doses of noradrenaline and adrenaline (from 0.1 to 10 microM) to perifused frog neurointermediate lobes induced a dose-dependent inhibition of alpha-MSH release. The inhibitory effect of adrenaline was partially blocked by the D2-dopaminergic antagonist sulpiride and totally suppressed by concomitant administration of sulpiride and yohimbine (an alpha 2-adrenergic antagonist). Conversely, in the presence of sulpiride, noradrenaline provoked a strong stimulation of alpha-MSH secretion which was totally blocked by the beta-adrenergic antagonist propranolol. Taken together, our results indicate that endogenous catecholamines may exert a complex regulatory action on frog melanotrope cells through activation of dopaminergic D2, alpha 2- and beta-adrenergic receptors.

Stimulatory effects of 5HT1A receptor agonists on luteinizing hormone-releasing hormone release from cultured fetal rat hypothalamic cells: interactions with progesterone

Previous works have suggested an interactive stimulatory effect of progesterone (P) and serotonin (5-HT) on luteinizing hormone release. The purpose of the present study was to determine whether 5-HT via 5-HT1A receptors interacts with P in the process of luteinizing hormone-releasing hormone (LHRH) release. Using fetal hypothalamic neurons in primary cell cultures the first goal of this study was to determine the effects of 5-HT1A receptor agonists on LHRH secretion. 8-Hydroxy-2 (di-n-propylamino) tetralin (8-OH-DPAT) or ipsapirone (10(-5) M) significantly stimulated LHRH release. Pharmacological studies have allowed to rule out the possible involvement of alpha 2- or beta-adrenoreceptors, or 5-HT uptake sites, in the stimulatory effect of 8-OH-DPAT on LHRH release, thus demonstrating the specific involvement of 5-HT1A receptors in the stimulation of LHRH release. The second goal was to test the ability of P to stimulate LHRH release from fetal hypothalamic neurons. P (10(-6) M) applied for 30 or 120 min significantly stimulated LHRH secretion. The maintenance of the stimulation of LHRH release by P after a cycloheximide treatment or by an impermeable analog of P, P-3-BSA, has suggested a nongenomic effect of P on LHRH release. The effects of a pretreatment of cells by P on 8-OH-DPAT-induced LHRH release were tested. While 10(-7) M P alone did not stimulate LHRH release, this concentration of steroid potentiated the LHRH response to 10(-5) M 8-OH-DPAT. These findings led to the conclusion that P acting at the level of the plasma membrane potentiates the stimulatory effect of 5-HT1A receptor agonists on LHRH release.

Impairment of serotoninergic transmission is followed by adaptive changes in 5HT1B binding sites in the rat suprachiasmatic nucleus

Serotonin1B (5-HT1B) receptor binding in the suprachiasmatic nucleus (SCN) following impairment of serotoninergic transmission was studied by quantitative autoradiography. Serotonin (5-HT) denervation with 5,7-dihydroxytryptamine (5,7-DHT) caused a significant increase in the density of 5-HT1B receptors in both the ventral (62%) and dorsal (53%) parts of the SCN as early as 3 days after axotomy. The magnitude of this increase did not differ 3, 15 or 21 days post-lesion. An up-regulation of 5-HT1B receptors with similar magnitude was obtained in the two parts of the SCN after inhibition of 5-HT synthesis by chronic parachlorophenylalanine treatment. In this case, up-regulation was shown to be reversible after restoration of 5-HT synthesis with L-5-hydroxytryptophan. These results indicate that 5-HT1B receptor density in the SCN was inversely correlated with 5-HT levels. These plastic properties exhibited by 5-HT1B receptors in the SCN are discussed in relation to the mode of 5-HT transmission and possible localization of the receptors onto the main chemically defined cell populations of the nucleus.

Effect of tianeptine on the hypothalamic somatotropic axis in the conscious sheep

The action of serotonin on growth hormone (GH) secretion is controversial because of interspecies differences and lack of specificity of serotoninergic drugs. Serotonin (5-HT) appears to inhibit GH release in the sheep and in man. We have investigated the site of action of tianeptine, a 5-HT uptake enhancer, in sheep since it is possible to collect hypophysial portal blood for the simultaneous determination of growth hormone-releasing hormone (GHRH) and somatostatin in this species under conscious, unstressed conditions. Tianeptine injection (10 mg/kg i.v.) resulted in a significant, immediate and short-lasting (30 min) increase in peripheral GH (+750%; P < 0.01) and hypophysial portal GHRH (+180%; P < 0.01). No change in the secretion of somatostatin was recorded during the same time. These data suggest that serotoninergic inputs are inhibitory to GH secretion. Tianeptine acts centrally to stimulate GH secretion in the sheep and its effect is mediated through changes in GHRH but not somatostatin release into hypophysial portal blood.

N-methyl-D-aspartic acid/glycine interactions on the control of 5-hydroxytryptamine release in raphe primary cultures

Glutamic acid and glycine were quantified in cells and medium of cultured rostral rhombencephalic neurons derived from fetal rats. In the presence of 1 mM Mg2+, NMDA (50 microM) significantly stimulated (by 69%) release of newly synthesized 5-[3H]hydroxytryptamine ([3H]5-HT). D-2-Amino-5-phosphonopentanoate (AP-5; 50 microM) blocked the stimulatory effect of NMDA. AP-5 by itself inhibited [3H]5-HT release (by 25%), suggesting a tonic control of 5-HT by glutamate. In the absence of Mg2+, basal [3H]5-HT release was 60% higher as compared with release with Mg2+. AP-5 blocked the increased [3H]5-HT release observed without Mg2+, suggesting that this effect was due to the stimulation of NMDA receptors by endogenous glutamate. Glycine (100 microM) inhibited [3H]5-HT release in the absence of Mg2+. Strychnine (50 microM) blocked the inhibitory effect of glycine, indicating an action through strychnine-sensitive inhibitory glycine receptors. The [3H]5-HT release stimulated by NMDA was unaffected by glycine. In contrast, when tested in the presence of strychnine, glycine increased NMDA-evoked [3H]5-HT release (by 22%), and this effect was prevented by a selective antagonist of the NMDA-associated glycine receptor, 7-chlorokynurenate (100 microM). 7-Chlorokynurenate by itself induced a drastic decrease in [3H]5-HT release, indicating that under basal conditions these sites were stimulated by endogenous glycine. These results indicate that NMDA stimulated [3H]5-HT release in both the presence or absence of Mg2+. Use of selective antagonists allowed differentiation of a strychnine-sensitive glycine response (inhibition of [3H]5-HT release) from a 7-chlorokynurenate-sensitive response (potentiation of NMDA-evoked [3H]5-HT release).

Increase of central 5-HT1B binding sites following 5,7-dihydroxytryptamine axotomy in the adult rat

The effects of selective axotomy of serotoninergic neurons produced by an intracerebroventricular injection of 5,7-dihydroxytryptamine (200 micrograms free base) on 5-HT1B binding sites labeled with S-CM-G-[125I]TNH2 were investigated by quantitative autoradiography in the rat brain. Results show, 21 days after surgery, an upregulation of 5-HT1B receptors in the entorhinal cortex and the dorsomedial and suprachiasmatic nuclei of the hypothalamus. The cellular localization of those 5-HT1B receptors exhibiting post-lesion plastic properties is discussed.

Evidence for the presence of noradrenergic neurons and their inhibitory action on luteinizing hormone-releasing hormone release in cultured fetal rat hypothalamic cells

The control of LHRH release by catecholamine systems during fetal life (embryonic stages) was studied using hypothalamic neurons in primary cell cultures and an attempt was made to characterize the receptor type involved. Phenylephrine and clonidine, respectively alpha 1 an alpha 2 adrenoreceptor agonists, both inhibited LHRH release. These agonist inhibitory induced-effects were antagonized by the respective alpha 1 and alpha 2 adrenoreceptor antagonists (prazosin and rauwolscine). Both prazosin and rauwolscine applied alone induced a marked increase in LHRH release. Similarly, inhibition of catecholamine synthesis obtained by alpha-methyl-para-tyrosine (alpha-MT) led to a significant increase in LHRH release. The stimulatory effects induced by alpha 1 and alpha 2 adrenoreceptor antagonists or by alpha-MT on LHRH release suggest the presence of noradrenergic and/or adrenergic cells in fetal hypothalamic cultures. Therefore, catecholamine contents were measured in fetal hypothalamic cells in culture. Measurable amounts of norepinephrine and dopamine were found in cells, although epinephrine was undetectable. These results show: 1--noradrenergic cells are present in primary culture of fetal hypothalamic cells. 2--This intrinsic hypothalamic noradrenergic system exerts an inhibitory control on LHRH release at an early stage of development through alpha 1 and alpha 2 adrenoreceptors.

Population-specific modulation of 5-HT expression in cultures of embryonic rat rhombencephalon

This study aimed at analyzing the regulation of in vitro serotonin expression by neurons taken from different regions of the embryonic rat rhombencephalon. We studied the influence of co-culture with alarplate tissue using immunocytochemical and biochemical methods. Computer-assisted densitometry was used to estimate the co-culture effects on the serotonin content of the cell bodies. The more dynamic aspects of serotonin expression, such as synthesis and release, were studied by measuring (3H)serotonin newly synthesized from (3H)tryptophan. The density of the immunostaining was significantly decreased in B1,B2 cells by co-culture with both caudal and rostral alar-plate tissue. For B4-B9 cells, only co-culture with rostral alar-plate tissue produced a significant decrease. The de novo synthesis of serotonin was significantly decreased in B1,B2 neurons co-cultured with caudal alar-plate tissue only. Once again, the B4-B9 cells proved to be less influenced by the experimental conditions, as co-culture with both types of alar-plate tissue produced no significant effect. We concluded that the in vitro expression of serotonin can be modulated by environmental factors, but the relative influence of these factors is very different in rostral versus caudal serotonin expressing cell populations.

Effect of serotonin on alpha-melanocyte-stimulating hormone secretion from perifused frog neurointermediate lobe: evidence for the presence of serotonin-containing cells in the frog pars intermedia

We have examined the presence of 5-hydroxytryptamine (serotonin; 5-HT) in the intermediate lobe of the frog pituitary and investigated the effect of exogenous 5-HT on alpha-melanocyte-stimulating hormone (alpha-MSH) release from the perifused neurointermediate lobe (NIL). Using a specific antiserum against 5-HT, the indirect immunofluorescence technique revealed the presence of 5-HT-like immunoreactivity (5-HT-LI) in discrete cells, generally gathered in small clusters among parenchymal cells, and in numerous neurites surrounding melanotrophic cells. At the electron microscopic level, using a silver-gold intensification procedure, 5-HT-LI was localized in dense-core secretory vesicles within specific pituitary cells which appear to be different from pituitary melanotrophs. Dense accumulation of gold particles was also observed in nerve fibres running between parenchymal cells. A combination of high-performance liquid chromatography analysis and electrochemical detection showed the presence of both 5-HT and its metabolite 5-hydroxyindol acetic acid (5-HIAA) in frog NIL extracts (534 +/- 40 and 1245 +/- 65 (S.E.M.) pg/mg wet tissue respectively). Administration of graded doses of 5-HT (from 1 to 30 mumol/l) to perifused frog NIL induced a dose-dependent inhibition of alpha-MSH release. Repeated pulses of 5-HT (10 mumol/l each) induced a reproducible inhibition of alpha-MSH without any desensitization phenomena. The inhibitory effect of 5-HT was partially blocked by the serotonergic antagonists methysergide and ICS-205-930 (10 mumol/l each). Concomitant administration of methysergide and ICS-205-930 (10 mumol/l each) totally abolished 5-HT-evoked inhibition of alpha-MSH. Fenfluramine, a releaser of 5-HT, induced a slight but significant reduction of alpha-MSH secretion. While 5-HT caused a marked inhibition of alpha-MSH release from intact NIL, 5-HT was devoid of effect on acutely dispersed pars intermedia cells suggesting that 5-HT does not exert a direct action on pituitary melanotrophs. We have examined the effect of specific dopaminergic, GABAergic and alpha-adrenergic antagonists on 5-HT-induced alpha-MSH inhibition. We observed that sulpiride and SR 95531 (10 mumol/l each) did not affect the response of NIL to 5-HT while yohimbine (10 mumol/l) suppressed the inhibitory action of 5-HT. Taken together, our results indicate that discrete cells of the frog pars intermedia contain the neurotransmitter 5-HT which may act locally to inhibit alpha-MSH release.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunohistochemical and biochemical evidence for the presence of serotonin in amphibian adrenal chromaffin cells

The presence of serotonin (5-HT) in chromaffin cells of the frog adrenal (inter-renal) gland has been demonstrated both by immunocytochemical and biochemical techniques. Using antisera against 5-HT and tyrosine hydroxylase (TH) on consecutive sections, we found by means of the indirect immunofluorescence technique that a majority of chromaffin cells were also immunopositive for 5-HT. When antibodies to 5-HT and phenylethanolamine-N-methyltransferase (PNMT) were applied on consecutive sections, 5-HT-like immunoreactivity was observed in almost all epinephrine-producing cells which represented about 90% of the total chromaffin cells. No 5-HT-containing fibres could be detected. At the ultrastructural level, using a pre-embedding procedure associated with gold-silver intensification of the immunoperoxidase reaction, 5-HT-immunoreactivity was visualized in secretory vesicles essentially located in the periphery of epinephrine cells. Combination of high performance liquid chromatography and electrochemical detection showed the presence of both 5-HT and its metabolite 5-hydroxyindolacetic acid (5-HIAA) in frog adrenal extracts. Transection of the splanchnic nerve enhanced 5-HT immunoreactivity and augmented the amount of 5-HT in adrenal extracts. Taken together, these results indicate that epinephrine-producing cells of the frog adrenal contain significant amounts of serotonin. The observation of the storage of 5-HT in secretory vesicles of epinephrine cells suggests that serotonin may be released with catecholamines under stress conditions.

Daily variations in serotonin metabolism in the suprachiasmatic nucleus of the rat: influence of oestradiol impregnation

In order to determine the temporal relationships between variations in 5-hydroxy-tryptamine (5-HT, serotonin) metabolism in the suprachiasmatic nucleus (SCN) and the cyclic LH surge, and also to check whether implantation of oestradiol capsules might modulate 5-HT metabolism in the SCN, we carried out a parallel study of 5-HT content in the SCN and median eminence, and 5-HT metabolism in the SCN and supraoptic region in vitro. These experiments were performed on intact male rats, ovariectomized females and ovariectomized females implanted with oestradiol. It was only in ovariectomized rats implanted with oestradiol, in which we have described the existence of a clear-cut circadian rhythm of LH secretion, that we found fluctuations in the content, synthesis and utilization of 5-HT. The content and synthesis were characterized by a peak between 12.00 and 15.00 h, whereas utilization was 50% higher at 09.00 and 19.00 h than at 15.00 h. These fluctuations in 5-HT content and metabolism were specific to the SCN; the median eminence and the supraoptic region did not show such variations. They were also specific to ovariectomized rats implanted with oestradiol, since the patterns of 5-HT content and metabolism in the SCN were the same in males and ovariectomized females and did not differ from those in the median eminence, the supraoptic region or the whole hypothalamus. These results suggest that 5-HT terminals in the SCN play an important role in the control of cyclic LH secretion at a critical period. Moreover, oestradiol seems to be partly responsible for the fluctuations of 5-HT metabolism in the SCN of ovariectomized rats implanted with oestradiol.

Regulation of release processes in central serotoninergic neurons

Different technical, physiological and biochemical aspects concerning the study of the release of 5-HT are discussed herein. Isotopic methods are the most suitable techniques since these allow the release of 3H-5-HT to be measured after having determined the identity of the labelled compounds formed from 3H-tryptophan by co-chromatography. Under these conditions, the 3H-amine released in the superfusates comes from serotoninergic nerve endings, since tryptophan hydroxylase is exclusively localized in serotoninergic neurons. Moreover, it appears that newly synthesized 5-HT is preferentially released. The release of 5-HT is dependent on neuronal activity, but is not always linked to the synthesis of 5-HT. The increase in the firing rate of serotoninergic cell bodies by a local application of glutamate in the area of the nucleus raphe dorsalis induces a marked increase n the release of 5-HT in the caudate nucleus; an opposite effect is observed after cooling this region. The local depolarization of serotoninergic terminals located in the caudate nucleus increases the release of this amine. This effect is blocked by TTX. LSD reduces the stimulating effect of KCl, thus indicating that the release of 5-HT can be controlled at a presynaptic level. In addition, the release of the amine is dependent on the presence of calcium. Serotoninergic neuronal activity can be controlled at the preterminal or at the cell body levels by the activity of other neuronal systems. The effects of the release of dopamine from dendrites, and that of GABA in the substantia nigra are reported herein. Furthermore, changes in the activity of the dopaminergic, gabaergic and serotoninergic systems innervating the nucleus raphe dorsalis modulate the release of 5-HT, measured both in the caudate nucleus and in the nucleus raphe magnus. Finally, it has been reported that the release of 5-HT can be estimated in the raphe nuclei dorsalis and magnus. It has been shown that the amounts of 3H-5-HT continuously formed from 3H-TRP and released in the nucleus raphe dorsalis are much greater than those estimated in the caudate nucleus or in the substantia nigra. Although the quantities of endogenous 5-HT measured in the nucleus raphe dorsalis are the highest in the brain, this structure presents only a few serotoninergic nerve endings. This raises the question of the origin of the 5-HT released in serotoninergic nuclei. A possible dendritic release of 5-HT is discussed.

Effect of nerve activity on the in vivo release of [3H]serotonin continuously formed from L-[3H]tryptophan in the caudate nucleus of the cat

A new isotopic approach has been developed to study the in vivo release of serotonin (5-HT). 'Encéphale isolé' cats were implanted with a push-pull cannula in the ventrocaudal part of the head of the caudate nucleus to estimate the release of [3H]5-HT continuously synthesized from L-[3H]tryptophan. Both [3H]5-HT and [3H]tryptamine were found in superfusates. Resting steady state in the release of [3H]indoleamines was observed as soon as 20 min after the beginning of the superfusion with L-[3H]tryptophan; the levels of [3H]5-HT in superfusates were 2.5 times those of [3H]tryptamine and about 6 times the blank value. They were markedly enhanced in the presence of fluoxetine (5 x 10(-6)M), a blocker of the 5-HT uptake process. A marked increase in the release of [3H]5-HT was seen during the local depolarization of 5-HT terminals with potassium chloride (60 mM) or batrachotoxin (10(-6)M) or during the stimulation of 5-HT cell bodies in the nucleus raphe dorsalis with L-glutamic acid (5 x 10(-5)M). These treatments did not enhance the efflux of [3H]tryptamine. The potassium-evoked release of [3H]5-HT was reduced by LSD (10(-5)M). LSD added alone in the superfusing fluid was without effect. The batrachotoxin-evoked release of [3H]5-HT was inhibited in the presence of tetrodotoxin (9 x 10(-6)M). The spontaneous release of [3H]5-HT and [3H]tryptamine was markedly reduced in the presence of a calcium-free medium containing cobalt (10 mM). A transient slight reduction in the spontaneous release of [3H]5-HT was observed in the presence of tetrodotoxin (9 x 10(-6)M). The local cooling of 5-HT cell bodies with a cryoelectrode induced a slight reversible decrease in [3H]5-HT release. These last two treatments were without significant effect on [3H]tryptamine efflux in superfusates. These results indicate that the release of [3H]5-HT endogenously formed from [3H]tryptophan is dependent on nerve activity and that this is not the case for [3H]tryptamine. The advantages of the isotopic approach for in vivo studies on the release of 5-HT are discussed.

Characteristics of the activation by dithiothreitol and Fe(2+) of tryptophan hydroxylase from the rat brain

The preincubation of tryptophan hydroxylase extracted from various areas of the central nervous system of the rat with 30 mM dithiothreitol and 50 muM ferrous ammonium sulfate under nitrogen atmosphere resulted in a persistent increase of its activity. Studies on the enzyme characteristics indicated that this activation was associated with a doubling in its Vmax and a shift (from 7.6 to 7.2) of the optimal pH for its activity. In contrast, the molecular weight and the apparent affinities of tryptophan hydroxylase for its pterin cofactor and for tryptophan were not significantly altered by the preincubation with dithiothreitol and ferrous ammonium sulfate. Since this treatment did not prevent the stimulatory effects of various compounds (phosphatidylserine, ATP and MG(2+), Ca(2+)) on tryptophan hydroxylase activity, this might be a good procedure to activate this enzyme with only minor changes in its regulatory properties.

Paradoxical decrease of brain 5-HT turnover by metergoline, a central 5-HT receptor blocker

Since metergoline (1-methyl-8-beta-carbobenzyloxy-aminomethyl-10-alpha-ergoline) is a potent 5-HT antagonist in peripheral organs, its possible blocking effects on 5-HT receptors in the rat brain were investigated. In vitro, metergoline inhibited both the specific high affinity binding of 3-H-5-HT onto synaptosomal membranes (IC 50 = 18 nM) and the stimulating effect of 10 micron 5-HT on the adenylate cyclase activity in colliculi homogenates from newborn rats (IC 50 = 12 micron. In vivo, the administration of metergoline (10 mg/kg i.p., 60 min before death) resulted in a significant decrease in the 3-H-5-HT binding capacity of synaptosomal membranes from the forebrain of adult rats. Taken together, these data clearly indicated that metergoline is a potent blocker of some serotoninergic receptors in the rat brain. Surprisingly, the changes in 5-HT turnover occurring in the brainstem and in the forebrain 1 h after metergoline (2-10 mg/kg) treatment were similar to those normally induced by a central 5-HT agonist: both the rate of 5-HT utilisation and that of 5-HT synthesis were significantly decreased. These changes were in contrast to the acceleration of 5-HT turnover induced by the administration of another potent central 5-HT antagonist, methiothepin. These results are discussed in relation to the possible existence of several types of serotoninergic receptors in the rat brain. It is possible that the positive feedback regulation of 5-HT turnover is triggered by the blockade of serotoninergic receptors sensitive to methiothepin, but not to metergoline.

Midbrain raphe lesion in the newborn rat: II. Biochemical alterations in serotoninergic innervation

Electrolytic raphe lesion was performed in 4-6-day-old rats and the resulting changes of 5HT metabolism within the central nervous system were analyzed up to 9 months later. As soon as the 2nd day following the selective destruction of B7 and B8 nuclei, forebrain 5HT levels were decreased by more than 75%. This reduction persisted for at least 9 months with no sign of recovery. The time course of 5-HIAA decrease was parallel to that of the indoleamine so that the ratio of 5-HIAA over 5-HT levels in the forebrain of lesioned rats was similar to that estimated in controls, whatever their age. This result would suggest that the remaining serotoninergic neurons in the lesioned rats did not develop a compensatory hyperactivity. The raphe lesion induced no change in MAO activity and synaptosomal tryptophan uptake but a pronounce decrease in the Vmax of synaptosomal KHT uptake process in various forebrain areas occurred. The serotonin sensitive adenylate cyclase activity in colliculi homogenate was not altered by the lesion suggesting that this enzyme was probably located in postsynaptic membranes. In addition, this observation would indicate that 5-HT receptors which are linked to this adenylate cyclase did not become supersensitive following the selective degeneration of serotoninergic neurons. Animals without forebrain serotoninergic innervation might be of great interest to analyse the role of serotoninergic neurons in various functions (sleep, analgesia, thermoregulation).

The topographical distribution of serotoninergic terminals in the neostriatum of the rat and the caudate nucleus of the cat

The topographical distribution of serotoninergic terminals in the neostriatum of the rat and the caudate nucleus of the cat was established owing to the combined use of microdissection techniques and biochemical microassays. The density of 5-HT terminals in various areas of both structures was quantified first by measuring 5-HT levels in microdiscs of frozen tissue. Since the high affinity uptake process for 5-HT appeared undamaged in isotonic homogenates of previously frozen (--5 degrees C) tissues, it was possible to confirm the findings obtained with the measurement of 5-HT levels by also determining 5-HT uptake activity in these microdiscs. However, in the rat neostriatum, but not in the cat caudate nucleus, [3H]5-HT even at a very low extracellular concentration (4.4 -x 10(-8) M) was taken up not only by serotoninergic terminals but also to a significant extent by dopaminergic terminals. In presence of benztropine, this second component was suppressed and [3H]5-HT uptake activity could then be considered as a specific marker of serotoninergic terminals also in the neostriatum of the rat. In both species, 5-HT terminals were mainly localized in the ventrocaudal area of the structure. In this area, 5-HT levels were among the highest values found in the brain (17 ng/mg protein). The density of 5-HT terminals decreased progressively from the acudal to the rostral planes of the neostriatum in rats or the caudate nucleus in cats. The poorest area, i.e. the dorsorostral zone, contained about 4 times less 5-HT than the ventrocaudal zone of the structure. Electrolytic lesion of the dorsalis (B7) and centralis superior (B8) raphe nuclei during early life resulted in a large decrease of 5-HT levels (--90%) in various parts of the neostriatum of adult rats. The present findings might be of interest to further analyze the role of serotoninergic neurons in extrapyramidal functions.

Control of the release of newly synthetized 3H-5-hydroxytryptamine by nicotinic and muscarinic receptors in rat hypothalamic slices

The effects of various cholinergic agonists and antagonists on the spontaneous release of newly synthetized 3H-5-HT were examined in rat hypothalamic slices. 3H-5-HT was measured in incubating medium at the end of a 30 min incubation carried out with L-3H-tryptophan in the presence of the various drugs tested. ACh (10(-5) M) in the presence of eserine (2 X 10(-4) M), and carbachol (10(-5) M) stimulated the release of 3H-5-HT. In contrast, oxotremorine (10(-5) M) reduced the 3H-amine release. The effect of carbachol was blocked by two nicotinic blockers, mecamylamine (10(-6) M) and d-tubocurarine (10(-6) M). It was not reduced by the muscarinic antagonists, atropine (10(-6) M) and scopolamine (10(-6) M). In fact, each of two antagonists added alone to the incubating medium enhanced 3H-5-HT release. The scopolamine (10(-6) M) stimulating effect on 3H-5-HT release was suppressed by d-tubocurarine (10(-6) M). Finally, the inhibiting effect of oxotremorine on 3H-5-HT release was not prevented by d-tubocurarine (10(-6) M) but was in the presence of atropine (10(-6) M) or scopolamine (10(-6) M). In the concentrations used in the release study, the cholinergic agonists and antagonists had no effect on the total formation of 3H-5-HT and 3H-5-HIAA from L-3H-tryptophan and on the accumulation of L-3H-tryptophan in tissues. In these concentrations, except for eserine, they did not affect the uptake of exogenous 3H-5-HT in hypothalamic synaptosomes (P2 fraction). These results suggest that cholinergic receptors of the muscarinic and nicotinic type are involved in the control of 3H-5-HT release; since the stimulation of the muscarinic and nicotonic cholinergic receptors resulted in an inhibition and an activation of 3H-5-HT release, respectively. As in the case of peripheral noradrenergic and central dopaminergic neurons the cholinergic receptors could be localized on serotoninergic terminals.

Effects of halothane and nitrous oxide anaesthesia on 5-HT turn-over in the rat brain

The effects of anaesthesia with halothane and nitrous oxide on the turn-over of 5-HT in the rat brain were analysed. The rate of 5-HT synthesis was estimated on the basis of 5-HTP accumulation following 5-HTP decarboxylase inhibition. Inhalation anaesthetics reduced the rate of 5-HT synthesis in the whole brain, this effect being particularly marked in the forebrain. In addition, the reduction of the rate of 5-HT synthesis was still detected after a large tryptophan loading indicating that halothane and nitrous oxide anaesthesia reduced the activity of tryptophan hydroxylase in vivo. The utilization of 5-HT was also delayed during anaesthesia since the rate of 5-HT disappearance following the inhibition of its synthesis at the decarboxylation step was decreased in the whole brain. In spite of this reduced turnover of 5-HT in the brain of rats exposed to inhalation anaesthetics, the 5-HIAA levels were permanently elevated. This was the result of a partial inhibition of the active elimination of the acidic metabolite from the brain during this kind of anaesthesia.