Regulation of serotonin release from the in vitro rat hippocampus: effects of alterations in levels of depolarization and in rates of serotonin metabolism

L-Tryptophan: Basic Metabolic Functions, Behavioral Research and Therapeutic Indications

An essential component of the human diet, L-tryptophan is critical in a number of metabolic functions and has been widely used in numerous research and clinical trials. This review provides a brief overview of the role of L-tryptophan in protein synthesis and a number of other metabolic functions. With emphasis on L-tryptophan's role in synthesis of brain serotonin, details are provided on the research uses of L-tryptophan, particularly L-tryptophan depletion, and on clinical trials that have been conducted using L-tryptophan supplementation. The ability to change the rates of serotonin synthesis in the brain by manipulating concentrations of serum tryptophan is the foundation of much research. As the sole precursor of serotonin, experimental research has shown that L-tryptophan's role in brain serotonin synthesis is an important factor involved in mood, behavior, and cognition. Furthermore, clinical trials have provided some initial evidence of L-tryptophan's efficacy for treatment of psychiatric disorders, particularly when used in combination with other therapeutic agents.

Differential potentiation of L-tryptophan-induced head-twitch response in mice by cocaine and sertraline

Using selective monoamine uptake blockers and appropriate selective monoamine receptor antagonists, we have previously shown that cocaine enhances the frequency of 5-HT2A receptor-mediated 5-hydroxytryptophan (5-HTP)-induced head-twitch response (HTR) in mice via inhibition of serotonin uptake. Concomitantly, cocaine prevented the maximal producible HTR frequency via simultaneous indirect stimulation of the inhibitory presynaptic 5-HT1A and postsynaptic alpha 2 receptors. In the present study, we have investigated the effects of cocaine and the selective 5-HT (sertraline), norepinephrine (nisoxetine) and dopamine (GBR 12935) uptake inhibitors on the L-tryptophan-induced HTR in the presence of a nonselective monoamine oxidase inhibitor, tranylcypromine. We utilized two experimental protocols where cocaine or sertraline were administered either after (protocol 1) or prior to (protocol 2) L-tryptophan injection. Cocaine potentiated the ability of L-tryptophan to induce HTR to a greater extent in protocol 1, whereas sertraline induced a greater effect in protocol 2. However, in our earlier study cocaine (and also sertraline) up to 10 mg/kg produced a similar degree of potentiation in both experimental protocols on the 5-HTP-induced HTR. Furthermore, as in the latter study on the 5-HTP-induced HTR, in the present investigation nisoxetine potently attenuated whereas GBR 12935 did not modulate the induced HTR. The results show that the respective serotonergic and noradrenergic effects of cocaine also operate on the L-tryptophan-induced HTR. The differential effects of cocaine and sertraline in experimental protocols 1 and 2 on the L-tryptophan- versus 5-HTP-induced HTRs suggest that cocaine has additional effects on the conversion of L-tryptophan to 5-HT.

Overview of chemical sampling techniques

Although many of the ideas for sampling the chemical microenvironment of the brain were present, at least in nascent form, three decades ago or more, the last 10 years have witnessed a particularly spectacular surge of development, refinement, and use. We are now able to measure virtually any endogenous brain chemical in vivo at commendable levels of sensitivity, selectivity, and speed. The long-dreamt-of goal of being able to correlate neurochemical events with ongoing behavior and/or presentation of salient environmental cues and stimuli has already been largely achieved. Further refinements of existing techniques may well lead to levels of analysis inconceivable even a few years ago. The implications for theory-building and hypothesis-testing are enormous, particularly within such essentially virgin domains as behavioral neuroscience and biological psychiatry. These are truly exciting times.

Striatal dopamine release in vivo following neurotoxic doses of methamphetamine and effect of the neuroprotective drugs, chlormethiazole and dizocilpine

1. Administration to rats of methamphetamine (15 mg kg-1, i.p.) every 2 h to a total of 4 doses resulted in a neurotoxic loss of striatal dopamine of 36% and of 5-hydroxytryptamine (5-HT) in the cortex (43%) and hippocampus (47%) 3 days later. 2. Administration of chlormethiazole (50 mg kg-1, i.p.) 15 min before each dose of methamphetamine provided complete protection against the neurotoxic loss of monoamines while administration of dizocilpine (1 mg kg-1, i.p.) using the same dose schedule provided substantial protection. 3. Measurement of dopamine release in the striatum by in vivo microdialysis revealed that methamphetamine produced an approximate 7000% increase in dopamine release after the first injection. The enhanced release response was somewhat diminished after the third injection but still around 4000% above baseline. Dizocilpine (1 mg kg-1, i.p.) did not alter this response but chlormethiazole (50 mg kg-1, i.p.) attenuated the methamphetamine-induced release by approximately 40%. 4. Dizocilpine pretreatment did not influence the decrease in the dialysate concentration of the dopamine metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) produced by administration of methamphetamine while chlormethiazole pretreatment decreased the dialysate concentration of these metabolites still further. 5. The concentration of dopamine in the dialysate during basal conditions increased modestly during the course of the experiment. This increase did not occur in chlormethiazole-treated rats. HVA concentrations were unaltered by chlormethiazole administration. 6. Chlormethiazole (100-1000 microM) did not alter methamphetamine (100 microM) or K+ (35 mM)-evoked release of endogenous dopamine from striatal prisms in vitro. 7. Several NMDA antagonists prevent methamphetamine-induced neurotoxicity; however chlormethiazole is not an NMDA antagonist. Inhibition of striatal dopamine function prevents methamphetamine-induced toxicity of both dopamine and 5-HT pathways. Therefore the attenuation of the enhanced dopamine release which occurs in animals given chlormethiazole may be associated with the protective action of this drug against methamphetamine-induced neurotoxicity.

In Vivo Neurochemical Evaluation of Striatal Serotonergic Hyperinnervation in Rats Depleted of Dopamine at Infancy

Destruction of nigrostriatal dopamine (DA) neurons with 6-hydroxydopamine (6-OHDA) early in development results in hyperinnervation of striatum by the serotonergic afferents deriving from the dorsal raphe nucleus. We have used in vivo microdialysis to investigate the degree to which serotonergic neurotransmission in striatum is altered by this increase in the density of serotonin (5-HT) terminals. The effects of several manipulations known to influence 5-HT function on extracellular 5-HT and 5-hydroxyindoleacetic acid in striatum were compared in adult rats treated neonatally with 6-OHDA and in intact adult rats. Basal levels of 5-HT in extracellular fluid (ECF) of striatum were similar in neonatally DA-depleted rats and in intact rats. Perfusion with the 5-HT reuptake blocker, fluoxetine (100 microM), increased 5-HT in striatal ECF of neonatally DA-depleted rats to levels that were threefold greater than those achieved in intact rats. Likewise, K(+)-depolarization of the 5-HT terminals (100 mM in perfusate) or systemic administration of the 5-HT releaser, (+/-)-fenfluramine (10 mg/kg i.p.), increased the concentration of 5-HT in striatal ECF of neonatally DA-depleted rats to levels approximately threefold greater than those observed in striatum of intact rats. These findings indicate that the 5-HT hyperinnervation of striatum that takes place in rats depleted of DA at infancy is associated with an increased capacity for neurotransmitter release in this system. Concomitant increased in high-affinity 5-HT uptake may prevent the occurrence of any measurable changes in the resting concentration of 5-HT in striatal ECF.

Effect of acute administration of L-tryptophan on the release of 5-HT in rat hippocampus in relation to serotoninergic neuronal activity: an in vivo microdialysis study

Here we have used the brain microdialysis method to test the effect of the 5-HT precursor L-tryptophan on 5-HT release. The release of endogenous 5-HT was measured in ventral hippocampus of the anesthetized rat both under basal conditions and when serotoninergic neuronal activity was raised by electrical stimulation of the dorsal raphe nucleus (DRN). Low frequency electrical stimulation of the DRN evoked a frequency-dependent (2-10 Hz) release of hippocampal 5-HT. The electrically evoked release of 5-HT was markedly enhanced by pretreatment with L-tryptophan (50 and 100 mg/kg i.p.). The effect of L-tryptophan on evoked release of 5-HT was dose-related, detectable at low (2 Hz) stimulation frequencies, and became stronger as the stimulation frequency increased. L-Tryptophan (10, 50 and 100 mg/kg i.p.) had no effect on basal output of 5-HT. We conclude from these findings that elevation of 5-HT precursor availability increases 5-HT release in hippocampus in vivo under conditions of increased serotoninergic neuronal activity.

Studies on the enhancement of 5-hydroxytryptamine-mediated behaviour by chlormethiazole and phenytoin

Pre-treatment of rats with chlormethiazole (35 mg/kg) or diphenylhydantoin (phenytoin: 40 mg/kg) markedly enhanced the behavioural syndrome which is induced by injection of tranylcypromine (10 mg/kg) followed by L-tryptophan (50 mg/kg). Phenobarbitone (35 mg/kg) pre-treatment was without effect on the syndrome. This enhancement apparently involved a pre-synaptic mechanism since pre-treatment with chlormethiazole or phenytoin did not result in enhancement of the behavioural syndrome when it was induced by injection of the 5-HT(1A) agonist 8-OH-DPAT (0.75 mg/kg). Pre-treatment of rats with chlormethiazole did not alter the rate of 5-HT synthesis as measured by the accumulation of 5-HT following tranylcypromine. The K(+)-evoked release of endogenous 5-HT from brain slices was unaltered by addition of chlormethiazole (100 μM) to the medium while addition of phenytoin (100 μM) caused a small decrease. Administration of chlormethiazole or phenytoin failed to alter either the 5-HT(2) receptor-mediated head twitch behaviour in mice induced by 5-hydroxytryptophan or the hypothermic response induced in mice by injection of 8-OH-DPAT (0.5 mg/kg s.c.). These data extend the original observation of enhancement of the 5-HT(1A) receptor-mediated behavioural syndrome by phenytoin, using a lower dose of the drug, and show that chlormethiazole has a similar effect, apparently through a pre synaptic mechanism. Some similarities to the effect of administration of Ca(2+) antagonists and lithium are noted but no clear mechanism involving changes in ion flux have been identified to explain the mechanisms involved.

Enhanced aspartate release from hippocampal slices of epileptic (El) mice

The release of putative neurotransmitters [aspartate, glutamate, and gamma-aminobutyric acid (GABA)] was studied in hippocampal slices from adult normal C57BL/6J (B6) and El (epileptic) mice. The El mice, a genetic model of temporal lobe epilepsy, had an average of 86 seizures. Sets of B6 and El hippocampal slices (400 microns thick) were incubated in a series of normal and high potassium (60 mM) buffers in the presence or absence of calcium. The calcium-dependent and calcium-independent potassium-induced release of amino acids was compared in each mouse strain. Release of endogenous amino acids was measured using liquid chromatography with electrochemical detection and was expressed as picomoles of amino acid released per milliliter of incubation buffer per minute of incubation per slice +/- SEM. No significant differences were found between the El and B6 mice for the calcium-dependent potassium-evoked release of glutamate (18.20 +/- 2.62 and 15.41 +/- 3.56), or GABA (17.28 +/- 2.90 and 12.73 +/- 1.37), respectively. Aspartate release, however, was significantly higher in the El mice (6.62 +/- 0.69) than in the B6 mice (3.31 +/- 0.72). These findings suggest that enhanced aspartate release may be related to seizure expression in El mice.

TFMPP and RU24969 enhance serotonin release from rat hippocampus

Using a batch method for incubation of hippocampal slices, we have examined the effects of 5-methoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-indole (RU24969) and (m-trifluoromethylphenyl)piperazine (TFMPP) on release of endogenous 5-hydroxytryotamine (5-HT). Release of 5-HT from slices was enhanced by RU24969 and TFMPP at concentrations from 1 to 10 mumols. The 5-HT uptake inhibitors imipramine and fluoxetine, but not the autoreceptor antagonist methiothepin, blocked the enhancement in 5-HT. These results suggest that RU24969 and TFMPP, previously identified as potent agonists at the nerve terminal autoreceptor, also interact at higher concentrations with the reuptake carrier to enhance extracellular levels of 5-HT.

Serotonin release varies with brain tryptophan levels

This study examines directly the effects on serotonin release of varying brain tryptophan levels within the physiologic range. It also addresses possible interactions between tryptophan availability and frequency of membrane depolarization in controlling serotonin release. We demonstrate that reducing tryptophan levels in rat hypothalamic slices (by superfusing them with medium supplemented with 100 microM leucine) decreases tissue serotonin levels as well as both spontaneous and electrically-evoked serotonin release. Conversely, elevating tissue tryptophan levels (by superfusing slices with medium supplemented with 2 microM tryptophan) increases both tissue serotonin levels and serotonin release. Serotonin release was found to be affected independently by tryptophan availability and frequency of electrical field-stimulation (1-5 Hz), since increasing both variables produced nearly additive increases in release. These observations demonstrate for the first time that both precursor-dependent elevations and reductions in brain serotonin levels produce proportionate changes in serotonin release, and that the magnitude of the tryptophan effect is unrelated to neuronal firing frequency. The data support the hypothesis that serotonin release is proportionate to intracellular serotonin levels.

Measurement of the in vitro release of endogenous monoamine neurotransmitters as a means of identification of prejunctional receptors

In contrast to in vivo release methods, in vitro release techniques utilising brain slices or synaptosomes affords a simple and reproducible means of measuring both receptor affinity and efficacy of drugs acting at prejunctional receptors in the CNS. Most studies have used brain tissue loaded with radiolabelled neurotransmitter or its precursor via the high affinity uptake system for these substances which are present on nerve terminals. Depolarisation evoked release induced by either high K+ or electrical field stimulation increases the release of radioactivity and this overflow can be readily measured by liquid scintillation counting. Recent studies have started to emphasise the measurement of the release of endogenous neurotransmitters from brain tissue using similar depolarisation stimuli. Examples include the release of dopamine, noradrenaline, adrenaline and 5-HT and their control by presynaptic receptors. Most of these studies have used HPLC with ECD detection as a means of separating and analysing for the transmitter of interest. The relative strengths and weaknesses of the measurement of radiolabelled or endogenous neurotransmitter release in vitro as a means of identifying presynaptic receptors is discussed.

3,4-Methylenedioxymethamphetamine-induced release of serotonin and inhibition of dorsal raphe cell firing: potentiation by L-tryptophan

The effects of the serotonin (5-HT) precursor L-tryptophan on MDMA (3,4-methylenedioxymethamphetamine)-induced inhibition of dorsal raphe neuronal firing were characterized using extracellular single-unit recording and microdialysis techniques in the in vitro midbrain slice preparation. Pretreatment with L-tryptophan (100 microM) lowered the doses of MDMA required to inhibit unit activity. Based upon IC50 values, L-tryptophan increased the potency of MDMA by approximately 3-fold. In a parallel series of experiments, microdialysis probes resting on the brain slice surface provided a means to estimate 5-HT release from the dorsal raphe nucleus. Pretreatment with L-tryptophan increased MDMA-induced 5-HT release in a manner consistent with the suppression of dorsal raphe cell firing: compared to untreated preparations, peak 5-HT release, total release and the duration of release were all increased. Taken together, these data suggest that the enhancement by L-tryptophan of MDMA-induced 5-HT release and inhibition of dorsal raphe neuronal firing is due to an increase in the amount of 5-HT available for release. The question is raised as to what effect L-tryptophan may have on the psychotropic and neurotoxic actions of MDMA.

Chronic lithium administration enhances serotonin release in the lateral hypothalamus but not in the hippocampus in rats. A microdialysis study

Chronic administration of lithium displays therapeutic and prophylactic effects in bipolar affective disorders, but its mechanism of action remains unknown. Several studies in animals and humans strongly suggest that central serotonergic neurons might be involved in lithium effects. In the experiments reported here microdialysis with removable probes and high pressure liquid chromatography and electrochemical detection were used to assess the amphetamine-induced release of serotonin (5-HT) and the 5-hydroxy-indoleacetic acid (5-HIAA) levels in the perifornical hypothalamus (PFH) and hippocampus (HP) of freely moving rats before and after chronic lithium chloride administration (2 meq/kg, as intragastric daily injections for 14 days). The serum lithium levels were 0.66 +/- 0.08 meq/l. After lithium treatment, the amphetamine-induced 5-HT release was significantly enhanced in the PFH but not so in the HP. Basal levels of 5-HIAA in the control group decreased but remained unchanged in the lithium group in the PFH. No change of basal levels of 5-HIAA was observed in the HP. The effect of lithium on the PFH could be related to the improvement of the autonomic and cyclic symptoms of patients with manic depressive disorders undergoing lithium therapy.

The effects of Ca2+ antagonists and hydralazine on central 5-hydroxytryptamine biochemistry and function in rats and mice

1. The effects of calcium antagonists on behaviour mediated by 5-hydroxytryptamine (5-HT) have been studied in rats and mice together with an investigation of the effects of these drugs on 5-HT synthesis in rat brain and endogenous 5-HT release from brain slices. 2. Administration of felodipine (35 mg kg-1 i.p.) to rats pretreated with tranylcypromine (20 mg kg-1, i.p.) resulted in the animals displaying the complete 5-HT-mediated behavioural syndrome (including head weaving, reciprocal forepaw treading and hind limb abduction) 75 min later. No evidence was obtained for the rate of 5-HT synthesis in brain regions differing between control and felodipine-treated rats. 3. Pretreatment with felodipine (10 or 35 mg kg-1) enhanced the 5-HT-mediated behavioural syndrome induced by injection of tranylcypromine and L-tryptophan. The rate of 5-HT accumulation in the brain was similar in both groups. Administration of Bay K 8644 (1 mg kg-1, i.p.) did not prevent the enhanced behaviour induced by felodipine (10 mg kg-1). 4. The 5-HT behavioural syndrome induced by injection of the 5-HT1A agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) was unaltered by either acute injection of felodipine (35 mg kg-1) or administration of felodipine twice daily for 3 days. 5. Felodipine (10 microM), verapamil (10 microM) and Bay K 8644 (10 microM) did not alter either basal release of endogenous 5-HT from slices prepared from frontal cortex or hind brain, or release following addition of K+ at a concentration of 20 mM, or 35 mM. 6. Verapamil (25mgkg-', i.p.), nicardipine (25mgkg-1, i.p.) and nifedipine (20mgkg-1, i.p.) all markedly inhibited the 5-HT2 receptor-mediated head twitch response in mice produced by injection of 5- methoxy-N,N-dimethyl-tryptamine (5-MeODMT). Felodipine had the same effect with an ED50 of 2.6mgkg-'. Bay K 8644 did not reverse this effect. Both verapamil (IC50:2.5 microM) and nicardipine (IC50:8 microM) were 5-HT2 antagonists as indicated by inhibition of [3H]-ketanserin binding in mouse frontal cortex. However felodipine and nifedipine antagonized 5-HT2 receptor binding only in the millimolar range.7. Hydralazine (5mg kg 1, i.p.) induced the 5-HT behavioural syndrome in tranylcypromine pretreated rats, enhanced the tranylcypromine/L-tryptophan behavioural syndrome, inhibited 5-MeODMT-induced head twitch behaviour in mice and was not a 5-HT2 receptor antagonist. 8. These data indicate that at a high dose, Ca2+ antagonists produce complex changes in 5-HT function in rodents which are similar to those produced by lithium administration. The data with hydralazine suggest that the effects seen are not related to an action at Ca2 + channels.

Neurochemical effects of chronic haloperidol and lithium assessed by brain microdialysis in rats

1. Psychotropic drugs ameliorate psychotic symptoms only after repeated administration. 2. To assess the neurochemical effects of chronic haloperidol and lithium administration, microdialysis was performed simultaneously in the prefrontal cortex, the nucleus accumbens, and the striatum after haloperidol, and separately in the lateral hypothalamus and the hippocampus after lithium. 3. Chronic administration of haloperidol decreased dopamine turnover in the prefrontal cortex and the striatum. It did not affect the nucleus accumbens detectably. 4. No tolerance to haloperidol developed in any of the three regions. 5. Lithium enhanced the response of the serotonergic system to amphetamine in the lateral hypothalamus but not in the hippocampus. 6. The antipsychotic effect of haloperidol might be related to dopamine turnover decrease in the prefrontal cortex. 7. The antidepressant effect of lithium might be related to enhancement of serotonin responsiveness in the hypothalamus.

The mechanism of tetrahydroaminoacridine-evoked release of endogenous 5-hydroxytryptamine and dopamine from rat brain tissue prisms

1 Tetrahydroaminoacridine (THA) is an acetylcholinesterase (AChE) inhibitor which may have a greater therapeutic effect in Alzheimer-type dementia (ATD) than other cholinergic agents. This suggests possible non-cholinergic properties. We have therefore studied the effects of THA on the release of endogenous 5-hydroxytryptamine (5-HT) from rat cortical prisms and dopamine from striatal prisms. 2 In the presence of K+ (1 mM), THA stimulated release of both 5-HT and dopamine. THA (100 microM)-evoked monoamine release was comparable, but not additive with the release produced by K+ (35 mM). The effect was not maximal at 1 mM THA. THA-evoked release of 5-HT was independent of the presence of Ca2+ in the external medium. 3 Drugs acting on the cholinergic system, nicotine, mecamylamine, atropine, oxotremorine, physostigmine and neostigmine (all 10 microM) had no effect on 5-HT and dopamine-release. 4-Aminopyridine (4-AP), a potent acetylcholine-releasing agent, had no effect on 5-HT release and was approximately 100 fold less active than THA on dopamine release. 4 Both THA and reserpine enhanced the release of 5-HT in the presence of the monoamine oxidase inhibitor, pargyline. Reserpine- but not THA-evoked release was abolished in the absence of pargyline. Reserpine (5 mg kg-1, i.p.) markedly depleted brain monoamine concentrations 3 h after injection, while THA (15 mg kg-1, i.p.) had no effect. 5 Chloroamphetamine and fenfluramine both released 5-HT in a Ca2(+)-independent manner and with a similar potency to THA, while (+)-amphetamine released dopamine with a similar potency to THA. The effects of the amphetamines were not maximal at 1 mM. However, unlike THA, chloroamphetamine-evoked release of 5-HT was additive with release evoked by K+ (35 mM). 6 Clomipramine (IC50 = 0.036 microM) and THA (IC50 = 19.9 microM) all inhibited the uptake of [3H]-5-HT into a P2 membrane preparation. However, none of these compounds inhibited [3H]-5-HT uptake into tissue prisms during the release experiments in which the reuptake inhibitor fluoxetine (5 microM) was present. 7 We conclude that THA does not release endogenous 5-HT through a cholinergic, reserpine- or amphetamine-like mechanism or through inhibition of reuptake. The possibility exists that the release may occur via blockade of 4-AP-insensitive K+ channels.

Tryptophan availability modulates serotonin release from rat hypothalamic slices

Application of a novel in vitro experimental system has allowed us to describe the relationship between tryptophan availability and serotonin release from rat hypothalamic slices. Superfusing hypothalamic slices with a physiologic medium containing l-tryptophan (1, 2, 5, or 10 microM) caused dose-dependent elevations in tissue tryptophan levels; the magnitude of the elevations produced by supplementing the medium with less than 5 microM tryptophan was within the physiologic range for rat brain tryptophan levels. Slice serotonin levels rose biphasically as the tryptophan concentration in the medium was increased. Superfusing the slices with medium supplemented with a low tryptophan concentration (1 or 2 microM) caused proportionally greater incremental changes in serotonin levels than the increases caused by further elevating the tryptophan concentration (5 or 10 microM). The spontaneous release of serotonin from the slices exhibited a dose-dependent relationship with the tryptophan concentration of the superfusion medium. Electrically evoked serotonin release, which was calcium-dependent and tetrodotoxin-sensitive, also increased in proportion to the medium tryptophan concentration. These data suggest that the rate at which serotonin is released from hypothalamic nerve terminals is coupled to brain tryptophan levels. Accelerations in hypothalamic serotonin synthesis, caused by elevating brain tryptophan levels, result in proportionate increases in the rates of serotonin release during rest and with membrane depolarization.

Monosodium L-glutamate-induced convulsions: changes in uptake and release of catecholamines in cerebral cortex and caudate nucleus of adult rats

Adult rats (60 days old) were injected intraperitoneally with 5 mg/g monosodium L-glutamate (MSG). During the convulsive period (1 h after injection), uptake and release of [3H]norepinephrine (3H-NE) and [14C]dopamine (14C-DA) were measured in a crude synaptosomal fraction and in slices of cerebral cortex and caudate nucleus, respectively. A significant reduction of 3H-NE uptake was detected in cortical slices (by 42%) and in synaptosomal fraction (by 33%) of rats treated with MSG, whereas K+- stimulated 3H-NE release was decreased by 32% and 39% in brain slices and in a synaptosomal fraction of cerebral cortex, respectively, in comparison with animals injected with 0.9% NaCl aqueous solution (PSS). In the caudate nucleus, 14C-DA uptake was increased by 100% in brain slices and by 36% in the synaptosomal fraction following MSG administration, whereas K+- stimulated 14C-DA release was enhanced by 80% in slices and by 25% in synaptosomes as compared to PSS-injected rats. Data suggest that catecholaminergic neurotransmission may play an important role in the etiopathology of convulsions in the experimental model using MSG.

Neurochemical and immunocytochemical studies of serotonin in the Hermissenda central nervous system

In the preceding paper we showed that serotonin mimics the effects of associative conditioning on Type B photoreceptors of Hermissenda. Here we show that serotonin is present in the Hermissenda central nervous system, and that it is released in a calcium-dependent manner. A large number of serotonergic cell bodies are present in the cerebropleural ganglia (CPG). Cell bodies in the CPG are located in three small, symmetrical clusters. One cluster (2-3 cells) located at the anterior end of the CPG contains a large cell that projects to the buccal ganglion. A second cluster (2-3 cells) is located posterior to the first and near the midline of the nervous system. A third cluster (1-2 cells) was located slightly more posteriorly than the second. Type B photoreceptors and the S/E optic ganglion cell project to a region of the CPG neuropil which is heavily innervated by serotonin immunoreactive fine processes. Since serotonin mimicked the effects of associative conditioning on Type B cells, and is present in an appropriate region of the CPG neuropil, serotonin may mediate some effects of associative conditioning on Type B cells. Results supporting this hypothesis are presented in an accompanying paper.

GABAB-receptor mediated inhibition of potassium-evoked release of endogenous 5-hydroxytryptamine from mouse frontal cortex

The effect of baclofen, the GABAB-agent, on the potassium-evoked release of endogenous 5-hydroxytryptamine (5-HT) from slices of mouse frontal cortex has been investigated. The release of endogenous 5-HT evoked by addition of K+ (35 mM) was inhibited by (+/-)-baclofen in a dose-dependent manner with an IC50 of 0.1 microM. Inhibition of K+-evoked release of 5-HT was produced by (+/-)- and (-)-baclofen but not (+)-baclofen. This action of the (-)-enantiomer was not altered by the presence of the (+)-enantiomer. Addition of GABA (0.1-10 microM) also induced a dose-dependent inhibition of 5-HT release. This effect was neither enhanced by flurazepam (1 microM) nor antagonized by bicuculline (10 microM). The progabide metabolite, 4-[( (4-chlorophenyl) (5-fluoro-2-hydroxyphenyl)methylene]amino)butyric acid (SL75.102) (1 microM) inhibited the K+-evoked release of 5-HT by 61%. These data suggest that baclofen is a potent inhibitor of the K+-evoked release of endogenous 5-HT from the cortex and further indicate that the release of 5-HT may be controlled by a GABAB-receptor located presynaptically.

Stimulation of serotonergic neuronal maturation after fetal mesencephalic raphe transplantation into the 5,7-DHT-lesioned hippocampus of the adult rat

Neurotoxin lesioning of 5-HT fibers selectively induced the homotypic collateral sprouting of spared 5-HT fibers in the hippocampus. We have used this model to investigate the possibility that the neurotoxin-primed hippocampus will enhance the development of transplanted fetal serotonergic neurons in the brain. The neurotoxin 5,7-DHT, when microinjected into the FF, produced a specific and partial depletion of 5-HT in the hippocampus of adult rats. The ability of the 5,7-DHT-primed hippocampus to selectively support the neurochemical maturation of fetal serotonergic cells was tested by assaying the transplanted fetal raphe or LC 1 month after neuronal transplantation. The neurochemical maturation of fetal 5-HT and NE neurons was dramatically different when they were transplanted in the 5,7-DHT-FF-lesioned hippocampus as compared to the normal hippocampus. The transplanted 5-HT neurons had 480% more SHAU of [3H]5-HT and had a 250% greater content of 5-HT in the partially denervated hippocampus than in the normal hippocampus after 1 month. Furthermore, extracts obtained from lesioned hippocampus enhanced the 5-HT content of 5-HT neurons transplanted in the normal hippocampus, to a level similar to that seen in neurons transplanted in the lesioned hippocampus. In contrast, the implanted NE neurons of fetal LC had a lower NE level in the 5-HT partially denervated hippocampus than in normal hippocampus after 1 month in the host site. The growth of the NE transplants was not facilitated by the vacant postsynaptic space produced by the 5,7-DHT lesion. These results suggest that the 5-HT denervation triggered a trophic signal selectively enhancing the development of the 5-HT neurons but not the NE neurons. Our results are consistent with previous studies showing homotypic collateral sprouting in 5,7-DHT-primed hippocampus.

Denervation of serotonergic fibers in the hippocampus induced a trophic factor which enhances the maturation of transplanted serotonergic neurons but not norepinephrine neurons

The trophic effects of specific denervation on the growth and survival of fetal serotonergic (5-HT) or norepinephrinergic (NE) neurons grafted into the hippocampus were assessed by means of two transplantation paradigms. In the first, fetal raphe cells (containing 5-HT neurons) were transplanted into the control hippocampus. In the second, the transplantation was performed 2 weeks after the specific removal of 5-HT afferents to the hippocampus with 5,7-dihydroxytryptamine (5,7-DHT). We found that a month after transplantation, the number of 5-HT immunoreactive neurons was not significantly different between the two experimental paradigms. However, transplanted raphe neurons had 400% more 5-HT synaptosomal high-affinity uptake and 380% higher content of 5-HT in the hippocampus with prior 5,7-DHT lesion than in control hippocampus. Furthermore, immunocytochemistry showed that the transplanted 5-HT neurons had denser processes and varicosities in the hippocampus with lesion than in control hippocampus. The somatic area of the neurons with these denser processes and varicosities was 42% larger than that of control group. A greater 5-HT level could be achieved if transplanted neurons in the control hippocampus were treated with the supernatant extracted from the hippocampus with 5,7-DHT lesion. In contrast, the NE level of the implanted fetal locus ceruleus (containing NE neurons) was not significantly higher in the 5-HT denervated hippocampus than in control hippocampus a month after transplantation. These results suggest that 5-HT denervation in the hippocampus induces a trophic substance which promotes the maturation rather than survival of 5-HT neurons but not NE neurons.

Binding of psychoactive drugs to rat brain amine receptors, measured ex vivo, and their effects on the metabolism of biogenic amines

Ex vivo receptor binding experiments and measurements of the concentrations of the amine metabolites HVA, MOPEG-SO4 and 5-HIAA were carried out on rat brain tissues to analyse the effects of several neuroleptics (haloperidol, chlorpromazine, thioridazine, clozapine, fluperlapine), antidepressants (amitriptyline, mianserin, zimelidine) and other drugs (pizotifen, ketanserin, prazosin) on brain aminergic systems. It was found that in all cases where drugs reduced 3H-haloperidol binding to striatal D2-receptors ex vivo (haloperidol, chlorpromazine, thioridazine, clozapine, fluperlapine, pizotifen, zimelidine), the same drugs caused a pronounced increase, in vivo, in the striatal HVA-concentration, i.e. an increase in DA-turnover. The HVA-increase was directly proportional to the extent of the reduction of 3H-haloperidol binding. These findings suggested that in the 3H-haloperidol assay binding of drugs is to functional D2-receptors regulating the release of DA. In contrast, reduction of binding of 3H-prazosin to brain stem alpha 1-receptors ex vivo (all drugs except zimelidine) was only associated with an in vivo increase in the MOPEG-SO4 concentration in some cases (haloperidol, chlorpromazine, thioridazine, clozapine, fluperlapine, ketanserine). Similarly, reduction of 3H-spiperone binding to 5-HT2-receptors in the frontal cortex (all agents except prazosin and zimelidine) was only associated with an increase in the 5-HIAA concentration in certain cases (thioridazine, clozapine, fluperlapine, pizotifen, ketanserin). The present data indicate that in the brain alpha 1- and 5-HT2-receptors do not seem to be linked directly to the processes which govern the turnover of the neurotransmitters DA, NA or 5-HT.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonin turnover in raphe neurons transplanted into rat hippocampus

Injections of the neurotoxin, 5,7-dihydroxytryptamine, into rostral raphe nuclei in rats reduced serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) in hippocampus to 35-50% of normal levels but only reduced 5-HT synthesis and catabolism by 20-35%. The small reduction in 5-HT metabolism as compared to depletion of 5-HT suggests that 5-HT turnover was increased in nerve terminals that survived the neurotoxin lesion. Transplantation of rat fetal raphe cells into the 5-HT-denervated hippocampus restored 5-HT and 5-HIAA levels and 5-HT synthesis to 125-150% of normal. This demonstrates that transplantation of fetal raphe tissue can restore metabolism to normal levels in the 5-HT denervated hippocampus.