Blockade of alpha 2-adrenoceptors stimulates basal and stress-induced adrenocorticotropin secretion in the developing rat through a central mechanism independent from corticotropin-releasing factor and arginine vasopressin.

In the neonatal rat, the response of the hypothalamo-pituitary-adrenal axis to stressful stimuli is markedly reduced during the first 2 weeks of life [stress-hyporesponsive period (SHRP)]. In this report, we studied the effect of idazoxan (an alpha 2-adrenergic antagonist) on plasma ACTH and corticosterone levels in 8-day-old rats. Indeed, it is known that in the adult rat, blockade of alpha 2-adrenoceptors increases ACTH secretion stimulated by CRF and arginine vasopressin (AVP) injection. Injection of 2.5 micrograms/g idazoxan induced a rapid (within 30 min) increase in basal plasma ACTH and corticosterone levels that lasted for 60 min. Injection of increasing doses of idazoxan led to a dose-dependent stimulation of ACTH and corticosterone levels. Administration of 2.5 micrograms/g idazoxan significantly increased the ACTH response to insulin-induced hypoglycemia, whereas it potentiated and accelerated the ACTH response to ether exposure stress. We next examined ACTH secretion from superfused anterior pituitary glands. Neither the alpha 2-adrenergic agonist clonidine nor idazoxan changed the basal ACTH secretory rate. Idazoxan had no effect on CRF- and AVP-stimulated ACTH secretion. This indicates that in vivo, idazoxan acts at a suprapituitary level. To investigate a possible effect of idazoxan on presynaptic alpha 2-adrenoceptors, we studied the effect of idazoxan on the concentrations of norepinephrine (NE) and L-DOPA in punches of locus coeruleus after dopa-decarboxylase blockade. Idazoxan injection induced a decrease in the NE content, without changing L-DOPA levels, indicating that idazoxan can act at the level of presynaptic alpha 2-adrenoceptors, inducing an increased release and/or degradation of NE without any effect on catecholamines synthesis. Finally, we investigated a possible involvement of CRF and AVP in mediating the effect of alpha 2-adrenoceptor blockade on ACTH secretion. Passive immunization against CRF or AVP did not change the ACTH response to idazoxan administration, whereas idazoxan pretreatment had simply an additive effect on CRF- and lysine vasopressin-stimulated ACTH secretion. In conclusion, our data demonstrate that during the SHRP, blockade of alpha 2-adrenoceptors induces an increase in both basal and stress-induced ACTH secretion. They suggest that a decreased catecholaminergic tone, consecutive to an increased occupation of alpha 2-adrenoceptors, acting through a central mechanism independent from CRF and AVP may be responsible for the SHRP in the developing rat.

The role of serotonin release and autoreceptors in the dorsalis raphe nucleus in the control of serotonin release in the cat caudate nucleus.

Using a push-pull cannula technique and an isotopic method for estimating [3H]serotonin continuously synthesized from [3H]tryptophan, the effects of changes in the release of serotonin in the dorsalis raphe nucleus on in vivo release of [3H]serotonin in the cat caudate nucleus were investigated. The increase in the release of serotonin in the dorsalis raphe nucleus caused by local application of parachlorophenylethylamine (10(-6) M) reduced striatal [3H]serotonin release. This inhibition in serotonin release in the striatum was blocked by the prior and continuous local superfusion of the dorsal raphe with methiothepin (10(-6) M), a serotonin autoreceptor antagonist. GABA (5 x 10(-5) M) applied to the dorsalis raphe reduced both local and striatal release of [3H]serotonin. However, picrotoxin (10(-5) M), a GABA A receptor antagonist applied locally in the dorsalis raphe nucleus increased [3H]serotonin release while decreasing striatal [3H]serotonin release. This decrease in serotonin release in the striatum was again blocked by continuous superfusion of the raphe with methiothepin. Furthermore, superfusion of serotonergic cell bodies of the dorsalis raphe nucleus with methiothepin alone never altered local release or striatal release of [3H]serotonin. These data strongly suggest that the release of serotonin from the cell body in the dorsalis raphe nucleus phasically controls release of the amine at the axonal nerve ending through serotonergic autoreceptors located on serotonergic nerve cell bodies in the dorsalis raphe nucleus. The origin of the serotonin released in the dorsalis raphe nucleus and the possibility that this type of regulation could be related to changes in nerve impulse conduction of the serotonergic raphe-striatal system are discussed.

Effect of thalamic parafascicularis nucleus stimulation in regulation of serotoninergic transmission in the cat caudate nucleus: involvement of autoreceptors in the dorsalis raphe nucleus.

The mechanisms involved in parafascicularis nucleus control on serotoninergic neurons projecting into the caudate nucleus were investigated in "encéphale-isole" cats. The effects of unilateral stimulation of the parafascicularis nucleus on the release of newly synthesized [3H]serotonin were simultaneously determined in the ipsilateral caudate nucleus and the dorsalis raphe nucleus using push-pull cannulae. The actions of various pharmacological treatments performed either in the caudate nucleus or in the dorsalis raphe nucleus were also examined. The electrical or chemical stimulation of the parafascicularis nucleus induced a decrease in striatal [3H]serotonin release and an increase in [3H]serotonin release in the dorsalis raphe nucleus. The blockade of cholinergic (mecamylamine) and glutamatergic (PK 26124) transmissions at the striatal level did not modify the thalamic stimulation-induced effect on serotonin release in the caudate nucleus or in the dorsalis raphe nucleus. However, a decrease induced by parafascicularis nucleus stimulation in serotonin release in the caudate nucleus could not be observed when the autoreceptors present on serotoninergic nerve cell bodies localized in the dorsalis raphe nucleus were blocked by a methiothepin perfusion within the nucleus. These results indicate that the parafascicularis nucleus controls striatal serotonin transmission by inducing changes in the nerve activity of serotoninergic neurons in the dorsalis raphe nucleus via somatodendritic serotonin release and autoreceptors.

In vivo evidence for acetylcholine control of serotonin release in the cat caudate nucleus: influence of halothane anaesthesia.

Using a push-pull cannula technique and an isotopic method for the estimation of [3H]serotonin continuously synthesized from [3H]tryptophan, the effects of acetylcholine were investigated on the in vivo release of [3H]serotonin in the cat basal ganglia and the dorsal raphe nucleus. The unilateral striatal application of acetylcholine (5 x 10(-5) M) reduced local release of [3H]serotonin. This effect was mimicked by nicotine (5 x 10(-5) M) and prevented by mecamylamine (10(-6) M. Oxotremorine (5 x 10(-5) M) had no effect on the local release of [3H]serotonin. All these treatments failed to modify [3H]serotonin release in the ipsilateral substantia nigra or in the dorsal raphe nucleus. The superfusion of serotonergic nerve terminals of the caudate nucleus with tetrodotoxin prevented the inhibitory acetylcholine-induced effect on serotonin release. Furthermore, bicuculline (5 x 10(-5) M) in the caudate nucleus blocked the effect of nicotine, while gamma-aminobutyric acid (10(-5) M) induced a decrease in local release of [3H]serotonin. These data strongly suggest that the inhibitory control exerted by acetylcholine on serotonergic transmission could involve gamma-aminobutyric acid interneurons. Acetylcholine-induced changes in [3H]serotonin release were only observed in non-anaesthetized "encéphale isolé" cats and not in halothane-anaesthetized animals. The possibility that such a regulation could be presynaptic (direct or through other neurotransmitters) or related to a change in the activity of the serotonergic raphe-striatal neuronal system is discussed.

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.

Circadian rhythm of luteinizing hormone secretion in the ovariectomized rat implanted with oestradiol.

Implantation of a solid source of oestradiol into ovariectomized rats produced constant plasma concentrations of the hormone over a long period of time. Under these conditions, LH is released in a circadian pattern with a very marked peak in the afternoon. This circadian rhythm is synchronized to the light--darkness cycle, since it follows exactly a shift in the nycthemeral cycle. The first peak appeared on day 3 after placement of the oestrogen implant; its amplitude was constant from days 3 to 9 after implantation, and decreased gradually during prolonged implantation. The afternoon peak was not correlated with changes in the pituitary sensitivity to exogenous LH releasing hormone (LH-RH), since the LH response to increasing doses of the peptide could be superimposed in the morning and in the afternoon. However, the decreased amplitude of the rhythm observed after more than 9 days of implantation seemed to depend upon a progressive desensitization of the pituitary gland to LH-RH. Pituitary LH content also decreased as a function of implantation time. It is concluded that, under conditions of constant plasma oestradiol concentrations and of constant pituitary sensitivity to LH-RH, a daily activation of the neural trigger releasing pituitary gonadotrophins occurs.

Analysis of the Inhibitory Effect of Oestradiol on Functional GABA/5-HT Relationship in the Rat Suprachiasmatic Area.

Abstract The purpose of this study was to test the capacity of oestradiol to modulate the stimulating effect of a-aminobutyric acid (GABA) on serotonin (5-HT) metabolism, previously described in the Suprachiasmatic area of the male rat. After an in vivo stimulation of GABA transmission by systemic administration of a GABA-transaminase inhibitor (amino-oxyacetic acid) or a GABA(B) agonist (RS-baclofen), the 5-HT metabolism was studied in the Suprachiasmatic area of ovariectomized, and ovariectomized oestradiol-treated rats. Amino-oxyacetic acid or RS-baclofen treatment increased the endogenous content of 5-HT in the Suprachiasmatic area of males and ovariectomized rats. These two treatments were without effect in ovariectomized oestradiol-treated rats. GABA transmission stimulation induced by amino-oxyacetic acid treatment failed to affect the release and synthesis of 5-HT in ovariectomized oestradiol-treated rats while it increased these two parameters of 5-HT metabolism in the Suprachiasmatic area of male and ovariectomized rats. To investigate the main target of oestradiol effect, comparative studies of the serotoninergic and GABAergic metabolism in the Suprachiasmatic area were performed in the three experimental groups. Under our experimental conditions the endogenous 5-HT metabolism was similar between ovariectomized and ovariectomized oestradiol-treated rats. Nevertheless, 5-HT metabolism was higher in the two female groups than in the male group. Neither GABA metabolism nor GABAergic response to GABA-related drug treatment differed between ovariectomized, and ovariectomized oestradiol-treated rats. However, the turnover of GABA was higher when compared to the two female groups. It is concluded that the lack of 5-HT responsiveness to GABA transmission stimulation in ovariectomized oestradiol-treated rats was not related to an effect of oestradiol on 5-HT metabolism or to an effect of the steroid on GABA turnover. Furthermore, our results suggest a sex difference in the activity of serotoninergic and GABAergic systems in the Suprachiasmatic area.

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.

Amines and choline acetyltransferase in rat intestine.

A biochemical study of the endogenous levels of serotonin (5-HT), noradrenaline (NA) and the activity of choline acetyltransferase (CAT) was carried out in the intestinal tract of the rat. High levels of 5-HT and NA were detected in the caecum and the colon. These anatomical regions also presented the highest activity of CAT. Similar activities of CAT were detected, after dissection, in the mucosa and the muscular layers containing the enteric plexuses. During the day-night cycle, 5-HT and NA amounts showed significant variations as a function of time. Treatment with pargyline (75 mg kg(?1)), a monoamine oxidase inhibitor, resulted in an increase in 5-HT content with parallel modifications in CAT activity. In spite of an important decrease in 5-HT endogenous level in the caecum of rats pretreated with parachlorophenylalanine (300 mg kg(?1)), no significant change in CAT activity was detected whatever was the duration of the treatment. ?-Methylparatyrosine (100 mg kg(?1)), known to block the synthesis of NA, did not affect the CAT activity in the caecum.

5-HT metabolism in the intestinal wall of the rat-II. The nerves plexuses-interactions between 5-HT containing cells.

The presence of serotonin (5-HT) in dissected intestinal muscular wall of the caecum was demonstrated by the determination of endogenous level of the amine by both spectrofluorimetric and radioenzymatic assays. Biosynthesis of [(3)H]5-HT from [(3)H]tryptophan in in vitro conditions revealed the presence of tryptophan hydroxylase in these muscular layers and therefore strongly suggest the presence of serotoninergic neurons. Following dissection of the mucosa from the muscular layers containing the nerve plexuses, endogenous 5-HT and 5-hydroxyindol acetic acid levels as well as amounts of [(3)H]5-HT synthesized from [(3)H]tryptophan were always higher than those found in intact fragments of the caecum. These results are discussed in terms of metabolic processes involved in the regulation between the two 5-HT containing compartments.

5-HT metabolism in the intestinal wall of the rat-I. The mucosa.

Serotonin (5-HT) metabolism was studied on isolated mucosa of the rat caecum in order to determine its characteristics in enterochromaffin cells. High levels of 5-HT were found in these cells and weak catabolic processes were demonstrated. Enterochromaffin cells of the mucosa are able to synthesize ((3)H)5-HT from ((3)H)tryptophan in vitro indicating that these cells might contain tryptophan hydroxylase. In addition, a high affinity uptake of exogenous ((3)H)5-HT is demonstrated. These results show that enterochromaffin cells of the mucosa present similar biochemical properties compared to those described for serotoninergic neurons of the central nervous system.

Serotonin release from the superior cervical ganglion of the cat.

Serotonin of the superior cervical ganglion is contained in a distinct and separate population of small intensely fluorescent (SIF) cells. We provide evidence, in in vitro experiments, that newly synthesized serotonin can be released in the cat superior cervical ganglion from the serotonin-containing SIF cells. Resting steady state in the release of [(3)H]serotonin was observed 30 min after the beginning of the superfusion with l-[(3)H]tryptophan. A marked increase was seen in the serotonin release either in the presence of fluoxetine, a potent reuptake blocker of serotonin, or during depolarization with potassium chloride or veratridine. Calcium-free medium led to a decrease of spontaneous and potassium-evoked release. The veratridine-stimulating response was abolished by tetrodotoxin. Paradoxically, a slight increase in the spontaneous release of serotonin was observed in the presence of tetrodotoxin. Serotonin released from serotonin-containing SIF cells could be involved in the modulation of ganglionic transmission.

Evidence for GABA control of serotonin metabolism in the rat suprachiasmatic area.

Changes in endogenous serotonin (5-HT) metabolism after in vivo stimulation of GABAergic transmission were investigated in the rat suprachiasmatic area (SCA). Activation of GABA transmission was performed by systemic administration of either amino-oxyacetic acid: AOAA, a GABA-transaminase inhibitor or RS baclofen, a GABA B agonist. After drugs administration, the amounts of endogenous 5-HT and 5-HIAA were measured. The release and synthesis of 5-HT were investigated in vitro, using a static incubation of tissue fragments. AOAA or RS baclofen induced an increase in endogenous 5-HT content but did not affect 5-hydroxyindole-acetic acid (5-HIAA). Both drugs induced an increase in the release and synthesis of 5-HT. Detailed study of the effects of AOAA over time on 5-HT metabolism showed that the increase in 5-HT release preceded the increase in amine synthesis. These results suggest that the in vivo stimulation of GABA transmission induces an increase in metabolic activity of the 5-HT neuronal system in the SCA. This effect may likely be mediated via activation of GABA B receptors.

The stimulation of GABA(B) receptors increases serotonin release in the rat suprachiasmatic area.

The action of ?-aminobutyric acid (GABA) and related compounds on the spontaneous release of newly synthesized [(3)H]5-hydroxytryptamine ([(3)H]5-HT) was studied in the suprachiasmatic area (SCA) using a superfusion system. GABA (10 ?M) increased [(3)H]5-HT release from SCA by up to 190%. Bicuculline or picrotoxin (10 ?M) failed to inhibit the stimulatory effect of GABA. Muscimol (10 ?M), a GABA(A) agonist, was ineffective, however ?-p-chlorophenyl GABA, R(?)baclofen, enhanced over 200% the release of the indoleamine; this latter effect was stereospecific. RS baclofen was twice less potent than the R(?)isomer in increasing the [(3)H]5-HT release. S(+)baclofen failed to affect the release of the indoleamine, whereas it attenuated the effect of its enantiomer. The effect of R(?)baclofen was Ca(2+) dependent and was abolished by tetrodotoxin (TTX). Taken together these results suggest that in the SCA, [(3)H]5-HT release is facilitated by the stimulation of GABA(B) receptors. The possible localization of these receptors is discussed in the light of morphological data recently reported by Bosler et al. (1985) and results obtained after TTX application.

Glutamate, GABA, glycine and taurine modulate serotonin synthesis and release in rostral and caudal rhombencephalic raphe cells in primary cultures.

Control of serotonin release and synthesis by amino acid neurotransmitters was investigated in rat rostral and caudal rhombencephalic raphe cells in primary cultures respectively. Endogenous amounts of taurine, glycine, GABA and glutamate were measured in both types of cultures. These amino acids were spontaneously released to the incubating medium. Exogenous taurine (10(-4) M) inhibited release and synthesis of newly formed [3H]serotonin [3H]5-HT from [3H]-tryptophan only in rostral raphe cells. Glycine (10(-3) M) decreased [3H]5-HT release in both types of cells, synthesis being diminished only in rostral raphe cells. Glycine inhibitory effect was totally blocked by strychnine (5 x 10(-5) M). GABA (10(-4) M) reduced [3H]5-HT metabolism in rostral as well as caudal raphe cells. This effect was totally antagonized in caudal and partially in rostral raphe cells by bicuculline (5 x 10(-5) M) a GABAA receptor antagonist. Baclofen (5 x 10(-5) M), a GABAB receptor agonist, induced a decrease of 5-HT release in rostral raphe cells. These observations suggest that monoamine release was entirely mediated by GABAA receptors in caudal raphe cells although GABAA and GABAB receptors were involved in control of 5-HT metabolism in rostral raphe cells. L-glutamate (10(-4) M) stimulated 5-HT metabolism in both types of cells, effect totally blocked by PK26124 (10(-6) M). N-methyl-D-aspartate (10(-4) M) enhanced 5-HT metabolism and the induced-effect was antagonized by the selective N-methyl-D-aspartate receptor antagonist D,L-2 amino-5-phosphonovaleric acid. Quisqualate (10(-5) M) stimulated [3H]5-HT release only in caudal raphe cells. This effect was mimicked by (RS)-a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, a quisqualate "ionotropic" receptor agonist, this increase being blocked by 6,7-dinitroquinoxaline 2,3-dione. These observations suggest that the glutamate stimulating-induced effect on serotonin metabolism is entirely mediated by N-methyl-D-aspartate receptor-type in rostral raphe cells and that quisqualate "ionotropic" receptors are also involved in caudal raphe cells. Taken together these results show that [3H]5-HT metabolism is controlled by taurine, glycine, GABA and glutamate in rhombencephalic raphe cells in primary cultures. However, some difference in amino acid receptor-types involved in the control of serotonin metabolism are observed according to the rostral or caudal origin of raphe cells.

Effect of vasoactive intestinal peptide on serotonin release in the suprachiasmatic area of the rat. Modulation by oestradiol.

The effect of vasoactive intestinal peptide (VIP) on spontaneous and induced release of newly synthesized 5-hydroxytryptamine (5-HT) was studied in the suprachiasmatic area (SCA) using a superfusion system. To test the possible modulation by E2 on the interaction VIP-5-HT, the experiments were conducted on male, ovariectomized (OVX) and ovariectomized oestradiol implanted rats (OVX-E2). VIP (10(-7)M) infused for 15 min caused an increase of 5-HT release from SCA of male and OVX. The positive effect of VIP on 5-HT release results partially from an inhibition of the reuptake of 5-HT: in male and OVX SCA, VIP inhibited the 3H-5-HT uptake by 40 to 50%. The infusion of VIP before a pulse of K+ (10-20-30-56 mM) leads to a potentialisation of the evoked release suggesting that VIP sensitized the presynaptic membrane to the process linking depolarization and release. When SCA taken from OVX-E2 were exposed to VIP, 5-HT uptake and consequently 5-HT release were unchanged. The present results suggest that the metabolism of 5-HT in the SCA is influenced by VIP and that this regulation may be modulated by E2. This interaction between E2, VIP and 5-HT at the SCA level may be involved in the regulation of phasic LH and prolactin surge.

Effect of vasoactive intestinal peptide on serotonin metabolism in the suprachiasmatic area of the rat: mechanism of action.

Vasoactive intestinal peptide (VIP) inhibits serotonin (5-HT) uptake in the suprachiasmatic area (SCA) of the rat. The present study investigates the possibility of a functional relationship between 5-HT uptake mechanisms and 5-HT autoreceptor activity in this effect of VIP in the SCA. The hypothesis of a linkage between these two mechanisms of 5-HT regulation has been recently proposed. We investigated the possibility of the presence of 5-HT autoreceptors in the SCA. Using superfusion system, exogenous 5-HT (500 and 50 nM) increased the release of newly synthesized 3H-5-HT. In contrast, 5 nM of exogenous 5-HT inhibited this release. This latter effect was antagonized by methiothepin (10(-7) M). In contrast, the concentration of methiothepin required to inhibit the VIP effect was 10(-6) or 10(-5) M, the same molarity found to decrease the 5-HT uptake. On the other hand, the increase of the 3H-5-HT in the synaptic cleft, induced by VIP, did not modify the inhibition of 3H-5-HT release induced by 5 nM of exogenous 5-HT. We conclude that the effect of VIP on 5-HT metabolism in the SCA is linked to the 5-HT uptake mechanism but not to the activity of 5-HT presynaptic autoreceptors. In our experimental conditions, the activity of 5-HT autoreceptors is independent of the 5-HT uptake processes.

In vivo evidence for an inhibitory glutamatergic control of serotonin release in the cat caudate nucleus: involvement of GABA neurons.

The local effect of L-glutamic acid (5 x 10(-5) M) on the release of [3H]serotonin continuously synthesized from [3H]tryptophan was examined in the caudate nucleus of 'encéphale isolé' unanaesthetized cats implanted with push-pull cannula. L-Glutamic acid (5 x 10(-5) M) decreased [3H]serotonin release from nerve terminals of the dorsalis raphe-striatal serotonergic neurons. The effect was antagonized by 2-amino-6-trifluoromethoxybenzothiazole (PK 26124) (10(-6) M), an antagonist of glutamatergic transmission. This effect was mimicked by N-methoxy-D-aspartic acid NMDA (5 x 10(-5) M) and prevented by DL-2-phosphono-valeric acid (APV) (5 x 10(-6) M), indicating that L-glutamic acid decreased serotonin release via a N-methoxy-D-aspartate type receptor. The superfusion of serotonergic nerve terminals in the caudate nucleus with tetrodotoxin prevented the inhibitory L-glutamic acid-induced effect on serotonin release. Furthermore, L-glutamic acid-induced inhibition of [3H]serotonin release was antagonized by bicuculline (5 x 10(-5) M). These data suggest that the glutamatergic receptors involved were not located directly on serotonin nerve terminals. The inhibitory control exerted by L-glutamic acid on serotonergic transmission could involve gamma-aminobutyric acid interneurons. Since no reduction of spontaneous [3H]serotonin release was observed in the presence of bicuculline, GABAergic neurons appeared to exert a phasic influence on serotonin release. Indirect inhibitory presynaptic control on serotonin release mediated by corticostriatal glutamatergic fibers is discussed in light of previous findings.

Release of serotonin from perikarya in cat nodose ganglia.

Newly synthesized serotonin (5-HT) can be released in the nodose ganglion from the nerve cell bodies of vago-aortic serotoninergic neurones. Free-calcium led to a decrease of spontaneous and potassium-evoked release. The veratridine-stimulating response was abolished by TTX. The concept that 5-HT released from perikarya in the extracellular space could be involved in the self-regulation of the activity of the vago-aortic pathway is discussed.

Effects of two estradiol antagonists upon the estradiol uptake in the rat brain and peripheral tissues.

Two estradiol antagonists, Parke-Davis CI 628 and CI 680, were studied for their inhibitory potency against labeled estradiol uptake within the hypothalamus, cerebral cortex, pituitary and uterus of the ovariectomized rat. Both drugs, tested from 15 min to 24 h prior to estradiol, induced a fast and long-lasting decrease of the hormone uptake. Three degrees of inhibition were observed depending on the tissue studied: (a) the most important inhibition concerned uterus and pituitary, where the estradiol uptake was prevented by up to 90% by the largest doses of antagonists used (6.3 and 6.1 mg, respectively, for CI 268 and CI 680); (b) in the hypothalamus, the estradiol uptake was counteracted to a lower degree; the inhibition being 39% for anterior hypothalamus and 22% for medial posterior hypothalamus; (c) cerebral cortex uptake was completely unaffected by pretreatments with the antagonists. After subcellular fractionation it was observed that the nuclear uptake of estradiol was completely abolished in both hypophyseal and hypothalamic tissues if the hormonal injection was preceded by the administration of 6.3 mg of CI 628.