The stimulation of GABAB receptors increases serotonin release in the rat suprachiasmatic area

The action of γ-aminobutyric acid (GABA) and related compounds on the spontaneous release of newly synthesized [³H]5-hydroxytryptamine ([³H]5-HT) was studied in the suprachiasmatic area (SCA) using a superfusion system. GABA (10 μM) increased [³H]5-HT release from SCA by up to 190%. Bicuculline or picrotoxin (10 μM) failed to inhibit the stimulatory effect of GABA. Muscimol (10 μM), a GABA_A agonist, was ineffective, however β-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 [³H]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 Ca²⁺ dependent and was abolished by tetrodotoxin (TTX).Taken together these results suggest that in the SCA, [³H]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.     

Inhibition by sodium bromide of acetylcholine release and synaptic transmission in the superior cervical ganglion of the rat

In the superior cervical ganglion (SCG) of rats, the interaction of sodium bromide (NaBr) with various drugs which interfere with the GABA system, such as 3-(4-chlorophenyl)-4-aminobutyrate [( + )baclofen, Bac], ( + )bicuculline (Bic), picrotoxin (Pic) and chlorpromazine (CPZ), and the effects of NaBr on the K⁺-induced release of [³H]acetylcholine ([³H]ACh) were studied in vitro. The effects on the evoked potentials induced by preganglionic stimulation were analysed in situ. The in vitro experiments revealed that 1 mM NaBr inhibits both the basal and the K⁺-induced release of [³H]ACh in a Ca²⁺-dependent manner. This NaBr effect was additive with the similar effect of the GABA agonist Bac, but it could not be blocked with any of the drugs applied. In vivo, 1 mM NaBr depressed the amplitude of the evoked potentials in the SCG. It is concluded that, in the SCG of rats, NaBr interacts with the presynaptic and postsynaptic membranes. The inhibitory effects of NaBr on both the [³H]ACh release and the potentials evoked by preganglionic stimulation cannot be attributed to a direct interference with GABA receptor complexes; some other binding site/s on the presynaptic and postsynaptic membranes might be responsible for the bromide-induced reduction of the synaptic transmission in the SCG of rats.     

CHANGES IN SEROTONIN LEVEL AND TURNOVER IN DISCRETE HYPOTHALAMIC NUCLEI AFTER PINEALECTOMY AND MELATONIN ADMINISTRATION TO RATS

The influence of the pineal gland on the hypothalamic serotonergic function was examined by studying the effects of long-term pinealectomy (1 month) and melatonin replacement (500 μg/kg; 10 days) on serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) content as well as on the in vivo 5-HT synthesis rate in discrete hypothalamic nuclei. Pinealectomy was followed by a significant decrease of 5-HT content in the anterior hypothalamic nuclei (AHN) and the ventromedial hypothalamic nuclei (VMHN), and also in 5-HIAA content in lateral (LPON) and medial preoptic nuclei (MPON). The 5-HT synthesis rate, estimated from the accumulation of 5-hydroxytryptophan after blockade of the 1-amino acid decarboxylase activity, were also decreased in the AHN and the paraventricular hypothalamic nuclei (PVHN) of pinealectomized rats. In contrast, an enhanced 5-HT synthesis rate and basal 5-HIAA content were found in the suprachiasmatic nuclei (SCN) after pinealectomy. Daily treatment with melatonin for 10 days reversed most of the effects induced by pinealectomy. Thus, melatonin increased the levels of 5-HT in the AHN and VMHN, and slightly increased the 5-HIAA content in preoptic nuclei. In addition, melatonin increased the 5-HT synthesis rate in the AHN and VMHN, but also in the MPON, VMHN and dorsomedial hypothalamic nuclei (DMHN) where pinealectomy had no effect. By contrast, melatonin treatment did not affect SCN 5-HT synthesis rate, although it decreased 5-HIAA levels. The results demonstrate that melatonin is able to stimulate 5-HT metabolism in most of the hypothalamic areas, but inhibits SCN 5-HT function. Some of the effects of melatonin seems to be exerted by modulating the synthesis of the amine, although melatonin likely also interacts with other regulatory processes of 5-HT function (i.e. release/uptake). The well defined presence of melatonin receptors in the rat SCN, and its absence in other hypothalamic structures, suggest that this may be the mechanism mediating the differential response to endogenous melatonin. Moreover, the larger effect of exogenous melatonin in relation to pinealectomy suggests the presence of melatonin unespecific effects possibly owing to supraphysiological doses. The present findings may be relevant for the mode of action of melatonin and its implication in several endocrine and behavioral functions mediated by serotonergic neurons. Copyright © 1996 Elsevier Science Ltd     

Pharmacological effects of GABA on serotonin metabolism in the rat brain

The pharmacological effects of GABA-related drugs were studied on the serotonin (5-HT) and 5-hydroxyindole-acetic acid (5-HIAA) contents of various regions of the rat brain. These effects were examined in the nuclei raphe dorsalis, magnus and centralis and in structures receiving a dense serotonin innervation such as the habenula complex and subcommissural organ. The GABA agonist, muscimol, increased the 5-HT contents and reduced 5-HIAA levels in structures containing serotoninergic terminals suggesting an inhibitory effect of GABA on the firing of serotoninergic neurons with concomitant reduction of 5-HT utilisation. In contrast, the GABA antagonist, bicuculline, probably stimulated 5-HT turnover since its intraperitoneally administration produced significant increase of 5-HT and/or 5-HIAA levels in the same brain regions. These data are in agreement with a transsynaptic inhibitory control of GABA on serotoninergic neurons. Drugs which inhibit the GABA catabolism such as amino-oxyacetic acid or gamma-vinyl-GABA and which should elevate GABA levels in the synaptic gap were capable of increasing or decreasing the 5-HT and the 5-HIAA levels depending on the experimental conditions. These results suggest that several processes are probably involved in the control of serotoninergic neurons by GABA in the rat brain. Among them, an intracellular effect of GABA on 5-HT metabolism might well occur in cells containing both GABA and 5-HT.     

DSIP affects adrenergic stimulation of rat pineal N-acetyltransferase in vivo and in vitro

The natural occurrence, sleep, and extra-sleep effects of delta sleep-inducing peptide (DSIP) have been shown by different laboratories. However, neither an in vitro assay system nor a probable mechanism of action of the peptide have been conclusively demonstrated so far. The recent finding that DSIP influences the nocturnal rise of N-acetyltransferase (NAT) activity in rat pineal led us to investigate a possible effect on pharmacologically induced NAT activity in vivo and in vitro. Stimulation of the enzyme with adrenergic drugs such as isoproterenol and phenylephrine was reduced by DSIP at doses of 150 and 300 μg/kg injected subcutaneously. In vitro, 6, 150 and 300 nM DSIP attenuated isoproterenol stimulation of the enzyme in cultured pineals, whereas 150 nM DSIP effectively reduced stimulation induced by a combination of the two drugs. The peptide alone did not influence NAT activity in vitro, but produced a slight stimulation in vivo. To our knowledge, these results represent the first report of a direct interaction of DSIP with adrenergic transmission. The in vitro system could prove useful for establishing possible mechanism(s) of action of the ‘sleep peptide.’     

Exogenous Tryptophan Increases Synthesis, Storage, and Intraneuronal Metabolism of 5-Hydroxytryptamine in the Rat Hypothalamus

The effects of tryptophan administration on neurochemical estimates of synthesis [5-hydroxytryptophan (5-HTP) accumulation following administration of a decarboxylase inhibitor], storage [5-hydroxytryptamine (5-HT) concentrations], and metabolism [5-hydroxyindoleacetic acid (5-HIAA) concentrations] of 5-HT in selected regions of the hypothalamus were determined using HPLC coupled to an electrochemical detector. Tryptophan methyl ester HCl (30-300 mg/kg i.p.) produced a dose-dependent increase in the rate of 5-HTP accumulation throughout the hypothalamus but had no effect on the rate of accumulation of 3,4-dihydroxyphenylalanine. Peak 5-HTP levels were attained by 30 min following administration of tryptophan (100 mg/kg i.p.) and were maintained for an additional 60 min. Tryptophan also produced concomitant dose-dependent increases in 5-HT and 5-HIAA concentrations in these same regions without changes in the 5-HIAA/5-HT ratio. These results indicate that exogenous tryptophan administration selectively increases the synthesis, storage, and metabolism of 5-HT in the hypothalamus without altering the synthesis of catecholamines. Inhibition of 5-HT uptake with chlorimipramine or fluoxetine produced modest (10-40%) reductions in 5-HIAA concentrations throughout the hypothalamus, revealing that only a minor portion of 5-HIAA is derived from released and recaptured 5-HT, whereas the major portion of this metabolite reflects intraneuronal metabolism of unreleased 5-HT. In both chlorimipramine- and fluoxetine-treated rats, 5-HIAA concentrations were significantly increased by tryptophan administration, indicating that the increase in synthesis of 5-HT following precursor loading is accompanied by an increase in the intraneuronal metabolism of 5-HT.     

EFFECTS OF AMINOOXYACETIC ACID AND l-GLUTAMIC ACID-γ-HYDRAZIDE ON GABA METABOLISM IN SPECIFIC BRAIN REGIONS

Abstract— Aminooxyacetic acid (AOAA) administration produced an increase in γ-aminobutyric acid (GABA) levels in regions of cerebral cortex, subcortex and cerebellum. In some cortical areas studied, the maximal effect was observed with 25 mg/kg AOAA; in other regions GABA levels were increased further with 50 and 75 mg/kg AOAA. Pretreatment with 25 mg/kg AOAA effectively inhibited GABA:2-oxoglutarate aminotransferase (GABA-T) and partially inhibited glutamic acid decarboxylase (GAD) activity in regions of cerebral cortex. However, this dose did not affect GAD activity in substantia nigra while GABA-T in the nigra and in the cerebellum was only partially inhibited. In both cortical and subcortical areas, the increase in GABA produced by 25 mg/kg of AOAA was linear. In contrast, l -glutamic acid-hydrazide (GAH) had no effect in the pyriform and cingulate cortex for the first 60 min after injection, and produced a biphasic GABA increase in caudate and substantia nigra over a 4 h period. Results suggest that GAH and AOAA affect regional GABA metabolism differentially and that there are several problems associated with estimating absolute GABA synthesis rates by measuring the rate or GABA accumulation after inhibition of GABA catabolism with these agents. This approach, however, may provide an easily obtainable indication of whether drugs or other manipulations are altering GABA synthesis in a given region.     

Effects of acute 17alpha-methyltestosterone,acute 17beta-estradiol,and chronic 17alpha-methyltestosterone on dopamine,norepinephrine and serotonin levels in the pituitary,hypothalamus and telencephalon of rainbow trout (Oncorhynchus mykiss)

This study investigated: (a) the effects of acute 17alpha-methyltestosterone (MT) or 17beta-estradiol (E(2)) administration on norepinephrine (NE), dopamine (DA), serotonin (5-HT), 3,4, dihydroxyphenylacetic acid (DOPAC), and 5-hydroxyindoleacetic acid (5-HIAA) contents in the hypothalamus, telencephalon and pituitary of previtellogenic female rainbow trout Oncorhynchus mykiss, and (b) the effects of chronic MT administration on the levels of these neurotransmitters in these brain regions in immature male rainbow trout. The acute administration of MT induced a significant decrease in pituitary levels of DOPAC as well as in the DOPAC/DA ratio. On the other hand, the acute administration of E(2) induced an increase in pituitary 5-HT levels as well as a decrease in the 5-HIAA/5-HT ratio. In a second experiment, 20 mg MT per kilogram body weight was implanted for 10, 20 or 40 days into sexually immature male rainbow trout. Implanted rainbow trout showed increased testosterone and decreased E(2) levels. In the pituitary, MT induced long-term decreases in NE, DA, DOPAC and 5-HT levels, as well as in the DOPAC/DA ratio. Hypothalamic and telencephalic DA, NE and 5-HT levels were not affected by MT implantation. However, 5-HIAA levels and the 5-HIAA/5-HT ratio were reduced by MT implantation in both brain regions. These results show that chronic treatment with MT exerts both long-term and region-specific effects on NE, DA, and 5-HT contents and metabolism, and thus that this androgen could inhibit pituitary catecholamine and 5-HT synthesis. A possible role for testosterone in the control of pituitary dopaminergic activity and gonadotropin II release is also discussed.     

Effects of muscimol and baclofen on levels of monoamines and their metabolites in the El mouse brain

We compared the changes in monoamines and their metabolites in the El mouse brain induced by GABA-A and GABA-B receptor agonists. Muscimol was used as a GABA-A receptor agonist, and baclofen as a GABA-B receptor agonist. Muscimol (3 mg/kg) significantly increased the DOPAC level in all parts of the mouse brain and the HVA level in the cortex, striatum, and midbrain. No significant change was observed in the dopamine (DA) level. These findings suggest that muscimol may accelerate both the synthesis and catabolism of DA. Baclofen (20 mg/kg) increased the DA level in the hippocampus and midbrain, and the DOPAC level in the hippocampus. Muscimol increased 5-HIAA levels and decreased 5-HT levels. This result suggests that 5-HT metabolism is accelerated by muscimol. No change in 5-HT or 5-HIAA levels was induced by baclofen. The GABA-A receptor system seems to have a potent effect not only on DA neurons, but on 5-HT neurons. However, the GABA-B receptor system appears to have almost no effect on 5-HT neurons, though it appears to have some effect on DA neurons.     

Phosphoramidon potentiates the endothelin-1-induced bronchopulmonary response in guinea-pigs

We investigated the effect of the endopeptidase-inhibitor, phosphoramidon, on the bronchopulmonary response induced by endothelin-1 in vivo or in isolated perfused lungs. In vivo aerosol administration of 1 or 3 μg/ml endothelin-1 for 2 min provoked no significant bronchopulmonary response. When awake animals were pretreated by an aerosol of phosphoramidon (0.1 mM, for 15 min), the bronchopulmonary response induced by 1 and 3 μg/ml endothelin-1 was markedly enhanced. In isolated guinea-pig lungs, aerosol administration of endothelin-1 (3 μg/ml, for 2 min) evoked a low increase in pulmonary inflation pressure. Treatment of awake animals with an aerosol of phosphoramidon before lung recollection led to a significant potentiation of the endothelin-1-induced increase in pulmonary inflation pressure. These results demonstrate that phosphoramidon potentiates the in vivo and in vitro bronchopulmonary response evoked by low doses of endothelin-1 and suggest that endopeptidase-like enzymes present in the airway tissue modulate the effect of the peptide.     

Arginine vasopressin induces periaqueductal gray release of enkephalin and endorphin relating to pain modulation in the rat

Previous study has proven that microinjection of arginine vasopressin (AVP) into periaqueductal gray (PAG) raises the pain threshold, in which the antinociceptive effect of AVP can be reversed by PAG pretreatment with V2 rather than V1 or opiate receptor antagonist. The present work investigated the AVP effect on endogenous opiate peptides, oxytocin (OXT) and classical neurotransmitters in the rat PAG. The results showed that AVP elevated the concentrations of leucine-enkephalin (L-Ek), methionine-enkephalin (M-Ek) and beta-endorphin (beta-Ep), but did not change the concentrations of dynorphinA(1-13) (DynA(1-13)), OXT, classical neurotransmitters including achetylcholine (Ach), choline (Ch), serotonin (5-HT), gamma-aminobutyric acid (GABA), glutamate (Glu), dopamine (DA), norepinephrine (NE) and epinephrine (E), and their metabolic products in PAG perfusion liquid. Pain stimulation increased the concentrations of AVP, L-EK, M-Ek, beta-Ep, 5-HT and 5-HIAA (5-HT metabolic product), but did not influence the concentrations of DynA(1-13), OXT, the other classical neurotransmitters and their metabolic products. PAG pretreatment with naloxone - an opiate receptor antagonist completely attenuated the pain threshold increase induced by PAG administration of AVP, but local pretreatment of OXT or classical neurotransmitter receptor antagonist did not influence the pain threshold increase induced by PAG administration of AVP. The data suggested that AVP in PAG could induce the local release of enkephalin and endorphin rather than dynophin, OXT and classical neurotransmitters to participate in pain modulation.     

Evidence for involvement of 5-hydroxytryptamine(1B) autoreceptors in the enhancement of serotonin turnover in the mouse brain following repeated treatment with fluoxetine

The effect of repeated treatment with the selective serotonin reuptake inhibitor fluoxetine on synthesis and turnover of 5-hydroxytryptamine (5-HT) was studied in the mouse brain in vivo. The concentration of 5-hydroxytryptophan (5-HTP), 5-hydroxyindoleacetic acid (5-HIAA) and 5-HT was measured in hypothalamus, hippocampus and frontal cortex after inhibition of the aromatic amino acid decarboxylase activity with m-hydroxybenzylhydrazine (NSD 1015). Fluoxetine 6.9 mg/kg s.c. was injected once daily for three weeks. Three days after the final daily injection of fluoxetine 5-HT synthesis (5-HTP accumulation) and turnover (5-HIAA/5-HT ratio) were significantly enhanced compared with saline-treated mice. The 5-HIAA/5-HT ratio was already significantly elevated after 3 days of fluoxetine treatment and continued to increase during treatment for 2-3 weeks. The increase in 5-HIAA/5-HT ratio was considerably larger (150-200% of controls) than the increase in 5-HTP accumulation (110-120%), which reached significance only after 3 weeks of treatment. The increase in 5-HT synthesis may be secondary to that of the turnover. The 5-HIAA/5-HT ratio returned to control values after a 14 days washout period. Simultaneous treatment with the long-acting 5-HT(1B)-receptor antagonist, SB 224289 for 14 days counteracted the fluoxetine-induced increase in 5-HIAA/5-HT ratio that indicates involvement of 5-HT(1B) autoreceptors in the development of this increase. It is proposed that the fluoxetine-induced enhancement of 5-HT turnover was evoked by the long-lasting stimulation of 5-HT(1B) autoreceptors that resulted in an intraneuronal compensatory adaptation of the basal 5-HT release.     

Involvement of the GABAA/benzodiazepine chloride ionophore receptor complex in the 5,7-DHT Induced anticonflict effect.

The effects of drugs interacting with the GABAA/benzodiazepine chloride ionophore receptor complex (GABAA/BDZ-RC) on the anticonflict and biochemical effects observed after intracerebroventricular (i.c.v.) administration of 5,7-dihydroxytryptamine (5,7-DHT; 450 micrograms -14 days) were investigated in the rat using a modified Vogel's drinking conflict test. The GABAergic antagonistic drugs bicuculline, picrotoxin and Ro 15-4513 all counteracted the 5,7-DHT induced anxiolytic-like action in doses that did not alter the behavior per se, whereas flumazenil was ineffective in this respect. Also i.c.v. administration of 5-HT antagonized the 5,7-DHT induced anticonflict effect. Furthermore, 5,7-DHT-lesioned animals appeared more sensitive to the anticonflict effects of diazepam than sham-lesioned controls. The 5,7-DHT treatment produced marked depletions of 5-HT in the limbic system (80-90%) and hippocampus (90-95%), and an increase in the 5-HIAA/5-HT quotient in hippocampus. The effects on the levels of noradrenaline were comparatively small. The doses of bicuculline and picrotoxin antagonizing the 5,7-DHT induced anticonflict effect did not uniformly influence 5-HT levels or 5-HIAA/5-HT quotients. It is suggested that the anxiolytic-like effect observed in 5,7-DHT-lesioned rats in Vogel's drinking conflict test involves enhanced transmission at the GABAA/BDZ-RC.     

Plasma free 5-hydroxytryptamine (5-HT): Evidence of an increase induced by LM 5008, a potent 5-HT uptake inhibitor

When 5-HT platelet uptake was inhibited in rats by single or repeated oral administration of 4-[2-(3-indolyl)ethyl]piperidine (LM 5008), 5-hydroxy-indole-acetic acid (5-HIAA) and 5-HT platelet concentration decreased. An oral administration of LM 5008 (10 mg/kg) to rats whose platelets were previously labeled with tritiated 5-HT provoked an increase in plasma free 5-HT and 5-HIAA. The maximum rise in 5-HT occured at 15 min while that of 5-HIAA appeared later (30 min). Concurrently urinary excretion of 5-HT was dramatically increased (about 5 times the control value) which indicates that 5-HT metabolism was not stimulated. According to the similarity between blood platelets and tryptaminergic neurons, plasma free 5-HT variations appeat to reflect changes of the neurotransmitter level into the synaptic cleft. Moreover, the excess of plasma free 5-HT induced by LM 5008 could improve 5-HT effects on vascular tone and pain.     

Role of Serotonin2A and Serotonin2B/2C Receptor Subtypes in the Control of Accumbal and Striatal Dopamine Release Elicited In Vivo by Dorsal Raphe Nucleus Electrical Stimulation

This study investigates, using in vivo microdialysis, the role of serotonin2A (5-HT2A) and 5-HT(2B/2C) receptors in the effect of dorsal raphe nucleus (DRN) electrical stimulation on dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), and 5-hydroxyindoleacetic acid (5-HIAA) extracellular levels monitored in the nucleus accumbens (NAC) and the striatum of halothane-anesthetized rats. Following DRN stimulation (300 microA, 1 ms, 20 Hz, 15 min) DA release was enhanced in the NAC and reduced in the striatum. The 5-HT2A antagonist SR 46349B (0.5 mg/kg) and the mixed 5-HT(2A/2B/2C) antagonist ritanserin (0.63 mg/kg) significantly reduced the effect of DRN stimulation on DA release in the NAC but not in the striatum. DA responses to DRN stimulation were not affected by the 5-HT(2B/2C) antagonist SB 206553 (5 mg/kg) in either region. None of these compounds was able to modify the enhancement of DOPAC and 5-HIAA outflow induced by DRN stimulation in either the NAC or the striatum. Finally, in both brain regions basal DA release was significantly increased only by SB 206553. These results indicate that 5-HT2A but not 5-HT(2B/2C) receptors participate in the facilitatory control exerted by endogenous 5-HT on accumbal DA release. Conversely, 5-HT(2B/2C) receptors tonically inhibit basal DA release in both brain regions.     

Pentylenetetrazol: Effect on carbaryl-induced changes of serotonin metabolism in pons-medulla of rat brain

Oral administration of carbaryl to adult male albino rats produced a dose dependent increase in the steady state level of 5-hydroxytryptamine (5-HT) at 1.00 h in pons-medulla (PM). 5-Hydroxyindole acetic acid (5-HIAA) concentration was significantly elevated only in response to a higher dose of this pesticide under similar conditions. A time course study with carbaryl and pentylenetetrazol (PTZ) showed a characteristic elevation of the steady state level of 5-HT in PM, but the 5-HIAA level was significantly elevated at 0.5 h only after carbaryl treatment. No significant change of the 5-HIAA level was evident after administration of PTZ alone or in combination with carbaryl. Tryptophan concentration was significantly elevated in PM at 0.5 h after carbaryl treatment and at 1.0 h after carbaryl + PTZ treatment. No significant change of tryptophan concentration was evident after the administration of PTZ alone under similar conditions. Measurement of (1) pargyline induced (a) accumulation of 5-HT and (b) depletion of 5-HIAA levels, and (2) probenecid-induced accumulation of 5-HIAA level in presence and absence of carbaryl and revealed that carbaryl accelerated the synthesis as well as the breakdown of 5-HT, whereas PTZ alone or in combination with carbaryl accelerated the synthesis of 5-HT without affecting its catabolism. The potency of this pesticide in elevating the pargyline-induced accumulation of 5-HT is in the order of carbaryl + PTZ greater than PTZ congruent to carbaryl. These results suggest that the carbaryl-induced increase in the synthesis of 5-HT is potentiated, and the turnover is reduced, in PM when PTZ is administered to the carbaryl-intoxicated rats.     

Inhibition of adrenal steroid metabolism by administration of 1-aminobenzotriazole to guinea pigs

Prior in vitro investigations demonstrated that the P450 suicide substrate, 1-aminobenzotriazole (ABT), was a potent inhibitor of xenobiotic metabolism but had no effect on steroidogenic enzymes in the guinea pig adrenal cortex. Studies were done to determine if ABT administration to guinea pigs in vivo also selectively inhibited adrenal xenobiotic metabolism. At single doses of 25 or 50 mg/kg, ABT effected rapid decreases in spectrally detectable adrenal P450 concentrations. The higher dose caused approx. 75% decreases in microsomal and mitochondrial P450 levels within 2 h. The decreases in P450 were sustained for 24 h but concentrations returned to control levels within 72 h. Accompanying the ABT-induced decreases in adrenal P450 content were proportionately similar decreases in P450-mediated xenobiotic and steroid metabolism. Microsomal benzo(a)pyrene hydroxylase, benzphetamine N-demethylase, 17-hydroxylase and 21-hydroxylase activities were decreased to 20–25% of control values by the higher dose of ABT. Mitochondrial 11β-hydroxylase and cholesterol sidechain cleavage activities were similarly diminished by ABT treatment. Adrenal 3β-hydroxysteroid dehydrogenase activity, by contrast, was not affected by ABT, indicating specificity for P450-catalyzed reactions. The results demonstrate that ABT in vivo is a non-selective inhibitor of adrenal steroid- and xenobiotic-metabolizing P450 isozymes. The absence of ABT effects on steroid metabolism in vitro suggests that an extra-adrenal metabolite may mediate the in vivo inhibition of steroidogenesis.     

Sertraline inhibits pre-synaptic Na⁺ channel-mediated responses in hippocampus-isolated nerve endings

In the present study, a possible sertraline action on cerebral pre-synaptic Na(+) channels was investigated. For this purpose, the effect of sertraline on responses induced by the Na(+) channel opener, veratridine, namely the increase in Na(+) and in neurotransmitter release in hippocampus-isolated nerve endings was investigated. Results show that sertraline in the low μM range (1.5-25?μM) progressively inhibits the rise in Na(+) and the release of pre-loaded [(3) H]Glu as well as the release of endogenous 5-HT, Glu and GABA (detected by HPLC) induced by veratridine depolarization either under external Ca(2+) -free conditions or in the presence of external Ca(2+) . In addition, under non-depolarized conditions, sertraline (25 μM) increased the external concentration of 5-HT at expense of its internal concentration, and unchanged the external and internal concentrations of the amino acid neurotransmitters and of the 5-HT main metabolite, 5-HIAA. This result is consistent with the sertraline inhibitory action of the serotonin transporter. However, sertraline is unlikely to inhibit pre-synaptic Na(+) channels permeability by increasing external 5-HT. Because 5-HT in a wide concentration range (1-1000 μM) did not change the veratridine-induced increase in Na(+) . In summary, present findings demonstrate that besides the inhibition of 5-HT reuptake, sertraline is an effective inhibitor of pre-synaptic Na(+) channels controlling neurotransmitter release.     

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.     

Multiple neurotransmitter systems influence the release of adenosine derivatives from the rabbit retina

Rabbit retinac preloaded with [³H]adenosine were superfused in vitro and the effect of neurotransmitter agonists and antagonists on the release of [³H]purines was studied. Glutamic acid, aspartic acid, kainic acid (KA), quisqualic acid (QUIS) and acid (NMDA) all stimulated the efflux of [³H] labelled and endogenous purines. Their effect was reduced in a Ca²⁺-free medium except when using a high concentration (100 μM) of KA, QUIS and NMDA. The effect of aspartic acid and of NMDA were blocked by 2-amino-7-phosphono-heptanoic acid (APH) and 2-amino-5-phosphono-valeric acid (APV). Carbachol also increased the release of adenosine-derived radioactivity and this effect was reduced by the removal of Ca²⁺ and by pretreatment with atropine. τ-Aminobutyric acid (GABA) and muscimol, induced a small increase in the release which was Ca²⁺-dependent and was blocked by bicuculline and picrotoxin. Dopamine elicited an increase in the release which was partially reduced in a Ca²⁺-free medium and was blocked by haloperidol. Glycine and 5-hydroxytryptamine (5-HT) also induced small but significant increases. The neurotransmitter antagonists had an effect of their own. Superfusion with APH and APV depressed the outflow of radioactivity whereas bicuculline, picrotoxin, strychnine and haloperidol enhanced it. The K⁺-evoked release of [³H]purines was reduced by haloperidol and by 5-HT. The observations indicate that stimulation of several important neurotransmitter receptors in the retina elicits the release of adenosine derivatives. The results with the antagonists also suggest that purines are continuously released as a result of a tonic activation of the respective membrane receptors.