Characterization of 5-hydroxytryptaminergic autoreceptors in the rat hypothalamus

NAN-190, a possible specific antagonist for methamphetamine

Effect of NAN-190, a selective 5-HT(1A) receptor antagonist, on methamphetamine-induced locomotor activity, anorexia, analgesia, and hyperthermia was investigated in male mice. Methamphetamine (1.5 mg/kg, i.p) produced a significant increase in locomotor activity, which was significantly antagonized by NAN-190 at a dose of 4 mg/kg, i.p. NAN-190 did not alter the antinociceptive activity of mice when it was administered alone. Methamphetamine (2 mg/kg, i.p) produced a significant decrease in food intake of mice, which were deprived of food during the previous 24h. This anorectic activity of methamphetamine was significantly antagonized by NAN-190 at a dose of 2 mg/kg, i.p. NAN-190 did not alter the food intake of mice when it was administered alone. Methamphetamine (2 mg/kg, i.p) also produced a significant increase in body temperature of mice, which was significantly antagonized by NAN-190 at a dose of 0.5 mg/kg, i.p. NAN-190 did not alter the body temperature of mice when it was administered alone. In the writhing test, methamphetamine (1 mg/kg, i.p) produced a significant antinociceptive effect in mice. This was significantly antagonized by NAN-190 at a dose of 1 mg/kg, i.p. NAN-190 did not alter the antinociceptive activity of mice when it was administered alone. The results of the present study indicate a possible role for serotonergic mechanisms, in addition to the catecholaminergic systems, in the above-studied activities of methamphetamine in mice. This role is possibly mediated through direct stimulation of the 5-HT(1A) receptor subtype. All of the above-studied activities of methamphetamine were antagonized by NAN-190, which may indicate that NAN-190 is a possible antagonist for methamphetamine.

5-HT3 receptors which modulate [3H]5-HT release in the guinea pig hypothalamus are not autoreceptors

The 5-HT3 agonist 2-methyl-5-HT had previously been shown to enhance the electrically evoked release of [3H]5-HT from preloaded slices of the guinea pig brain. In the present study, 2-methyl-5-HT (1 microM) was also found to increase the K+ evoked release of [3H]5-HT from preloaded slices of the guinea pig hypothalamus and this effect was blocked by the selective 5-HT3 antagonist ondansetron. In the presence of tetrodotoxin, the enhancement of the K(+)-evoked release of [3H]5-HT by 2-methyl-5-HT in hypothalamus slices was blocked, thus suggesting that the 5-HT3 receptors mediating this effect are not located directly on 5-HT terminals. In agreement with this, 2-methyl-5-HT did not alter the K(+)-evoked release of [3H]5-HT in a synaptosomal preparation of the same brain structure, even at a concentration 10-fold greater than that used in the slices. Taken together, these data indicate that these facilitatory 5-HT3 receptors are not located on 5-HT terminals in the guinea pig hypothalamus and therefore are not autoreceptors.

Biochemical and behavioural effects of isamoltane, a beta-adrenoceptor antagonist with affinity for the 5-HT1B receptor of rat brain

The biochemical and behavioural effects of isamoltane, a beta-adrenoceptor and 5-HT1B receptor antagonist that has higher affinity for 5-HT1B receptors than for 5-HT1A receptors, on 5-HT neurotransmission in the rat brain were examined. In binding experiments isamoltane was found to be about five times more potent as a ligand for the 5-HT1B receptor than for the 5-HT1A receptor (Ki values 21 and 112 nmol/l, respectively). Isamoltane increased the K(+)-evoked overflow of 3H from 3H-5-HT loaded slices of rat occipital cortex at 0.1 mumol/l, consistent with inhibition of the terminal 5-HT autoreceptor. In vivo, isamoltane significantly increased the concentration of 5-hydroxyindoleacetic acid in hypothalamus and hippocampus indicating an increased 5-HT turnover with a maximal effect at 3 mg/kg s.c. A higher dose produced a less pronounced effect. This effect did not seem to be due to the beta-adrenoceptor blocking action of isamoltane since the beta-adrenoceptor antagonists. (-)-alprenolol, betaxolol or ICI 118.551 had no significant effects on 5-HT turnover at 5 mg/kg s.c. Isamoltane at 3 mg/kg s.c. induced the wet-dog shake response which was blocked by the tryptophan hydroxylase inhibitor p-chlorophenylalanine. In contrast, the same response induced by the 5-HT2 receptor agonist quipazine was not blocked by pretreatment with p-chlorophenylalanine. The wet-dog shakes evoked by isamoltane and quipazine were blocked by ritanserin, which indicates that 5-HT2 receptors are involved in their expression. These observations indicate that isamoltane, by inhibiting the terminal 5-HT autoreceptors, increased the synaptic concentration of 5-HT to a level that induced a behavioural response.

Presynaptic serotonin receptors in the central nervous system

Presynaptic 5-HT autoreceptors have been identified in any region of the mammalian CNS containing 5-HT nerve terminals that has been investigated for this purpose. They belong to the 5-HT1B receptor subclass in the rat and to the 5-HT1D subclass in the pig, guinea pig, and probably man. The presence and operation of presynaptic 5-HT autoreceptors have been proven by the ability of 5-HT receptor agonists to inhibit 5-HT release and of 5-HT receptor antagonists not only to competitively antagonize this effect but also to disclose the autoinhibitory effect of endogenous 5-HT by blocking the autoreceptor, thus interrupting the negative feedback loop. There is evidence that presynaptic 5-HT autoreceptors are operative in vivo. Presynaptic inhibitory 5-HT heteroreceptors have also been identified in various brain regions of the rat. DA nerve terminals in the striatum and nucleus accumbens as well as GLU nerve terminals in the cerebellum are endowed with such receptors, which were either not yet classified (DA neurone) or represent a not yet specified 5-HT1 subtype (GLU neurone). Release-inhibiting 5-HT receptors on the acetylcholine nerve terminals in the hippocampus are of the 5-HT1B subtype, and those in the striatum were not yet classified in detail. A 5-HT heteroreceptor mediating stimulation of release occurs on rat striatal DA nerve terminals; it belongs to the 5-HT3 class. Thus, presynaptic inhibitory 5-HT auto- and heteroreceptors as well as presynaptic excitatory 5-HT heteroreceptors are involved in the regulation of transmitter release in the brain.

MDMA (ecstasy) effects on cultured serotonergic neurons: evidence for Ca2(+)-dependent toxicity linked to release

Animal studies have established a correlation between release of 5-hydroxytryptamine (5-HT) and the long-term reduction of 5-HT (toxicity) by 3,4-methylenedioxymethamphetamine (MDMA) with the S(+) enantiomer being more active than the R(-). Using a microculture system of fetal raphe neurons, the enantiomers of MDMA were tested to determine if a similar difference in potency existed. The results showed that the development of the uptake capacity of [3H]5-HT in 4-day cultures was half-maximally inhibited by a single application at time of plating of 5 X 10(-6) M S(+)-MDMA and 5 X 10(-5) M R(-)-MDMA. In order to determine if the Ca2(+)-independent release (chemically induced through the transporter protein and inhibited by reuptake blockers) or the Ca2(+)-dependent release (K(+)-induced and inhibited by presynaptic receptors) contributed to the toxicity, fluoxetine and D1 and alpha 2 agonists were studied. The results showed that both forms of release were involved in the loss of [3H]5-HT uptake capacity, with the direct MDMA-induced Ca2(+)-independent (fluoxetine-sensitive) release being the first step. Evidence from binding studies indicates that MDMA has a micromolar affinity for the 5-HT2 receptor, and our studies in culture showed that ketanserin, a specific 5-HT2 antagonist, was effective at attenuating the effects of S(+)-MDMA on the development of the [3H]5-HT uptake capacity by the cultured raphe neurons. The 5-HT2 receptor is linked to increased intracellular Ca2+ through a second messenger phosphatidylinositol (PI)-hydrolysis mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Postmortem- and cryostability of the potassium-evoked release of [3H]5-hydroxytryptamine from rat cerebral cortical miniprisms

A prerequisite for the study of neurotransmitter release from human brain autopsy samples with histories of different diseases is that the cryo- and postmortem stability of the release process is good. In the present study, the effect of post-mortem delay and of storage at -70 degrees C by the "slow freeze--fast thaw" method of Hardy et al. [J Neurochem (1983) 40: 608-614] (which allows for the retention of metabolic activity of the tissue after the storage and thawing) of rat cerebral cortex samples upon the release of [3H]5-hydroxytryptamine ([3H]5-HT) from prelabelled miniprisms has been investigated. Storage of samples at -70 degrees C by this method resulted in samples that accumulated less [3H]5-HT but showed an increased sensitivity to the Ca2+-dependent releasing properties of K+ when compared with "fresh" samples. On the other hand, the sensitivity of the K+-evoked release to the inhibitory effects of the serotoninergic agonist 5-methoxy-N,N-dimethyltryptamine were reduced by storage. The effects on [3H]5-HT accumulation and on K+-evoked release were due mainly to the freeze-thaw procedure, the length of storage at -70 degrees C having only a minor influence on these parameters. A post-mortem interval of 5 hours at either +4 or +22 degrees C prior to storage of the tissue reduced the K+-evoked release of tritium, but did not affect the accumulation of [3H]5-HT or the inhibitory effects of 5-methoxy-N,N-dimethyltryptamine on the K+-evoked release over and above the effects produced by the storage per se.

Milacemide increases 5-hydroxytryptamine and dopamine levels in rat brain--possible mechanisms of milacemide antimyoclonic property in the p,p'-DDT-induced myoclonus

Milacemide, a glycine prodrug that is able to enter the brain readily, has been shown to have an antimyoclonic property in the p,p'-DDT-induced myoclonus syndrome. Milacemide increased regional 5-HT and dopamine and decreased 5-HIAA, DOPAC and HVA levels in naive rats. In p,p'-DDT-treated rats, 5-HT levels were unchanged at the time the rats experienced spontaneous myoclonus in all brain regions except in the striatum, where it increased. 5-HIAA levels increased but did not reach significant levels except in the striatum. Dopamine, DOPAC, HVA and norepinephrine were unchanged. When rats were treated concurrently with both p,p'-DDT and milacemide, regional 5-HT levels were increased and NE levels in the brainstem and cerebellum decreased. Depletion of brain serotonin by pretreatment with PCPA or with 5,7-DHT, or blocking 5-HT receptors with different 5-HT antagonists, failed to eliminate the antimyoclonic property of milacemide. This antimyoclonic effect of milacemide may be mediated through other mechanisms besides its ability to increase brain 5-HT levels. Possible mechanisms to be considered are its antiepileptic property, and its ability to increase brain glycine levels. Milacemide may have potential for therapeutic trials in patients with myoclonus.

Effect of chronic lithium treatment on 5-hydroxytryptamine autoreceptors and release of 5-[3H]hydroxytryptamine from rat brain cortical, hippocampal, and hypothalamic slices

The effect of acute and chronic lithium treatments on 5-hydroxytryptamine (5-HT, serotonin) release and on its regulation by presynaptic 5-HT autoreceptors was studied in [3H]5-HT preloaded superfused rat brain slices. The [3H]5-HT overflow evoked by a 30-s exposure to 65 mM K+ was increased after 3 weeks of ingestion of lithium-containing diet in the three brain areas examined. Acute injection of 4 mEq/kg lithium chloride did not affect 5-HT release. The K+-induced release observed in both control and chronically lithium-treated animals was Ca2+-dependent. Chronic lithium treatment was also found to be associated with a decrease in basal [3H]5-HT overflow in the cortex and hypothalamus but not in hippocampus [corrected]. The Ca2+-independent overflow induced by fenfluramine was also decreased in cortical slices from lithium-treated animals. The sensitivity of the inhibitory 5-HT autoreceptors was assessed by the response to the 5-HT agonist 5-methoxytryptamine. The results indicate a marked reduction in the maximal inhibition of [3H]5-HT release induced by 5-methoxytryptamine in slices obtained from animals which have been treated with lithium for 3 weeks. These data suggest that the functional down regulation of the prejunctional 5-HT sites may be responsible for the increase in K+-stimulated 5-HT overflow in brain slices of animals treated chronically with lithium.

Water intake induced in rats by serotonin and 5-hydroxytryptophan: different mechanisms?

The present studies were designed to investigate further the mechanism by which water intake is induced in rats by peripheral administration of either serotonin (5HT) or its precursor 5-hydroxytryptophan (5HTP). Consistent with previous studies that have implicated mediation by the renal renin-angiotensin system (RAS), we now report that bilateral nephrectomy completely abolishes the drinking response to various doses of 5HT. In contrast, nephrectomy had little effect on the drinking induced by 5HTP. Thus, 5HTP may induce drinking by mechanisms other than its peripheral conversion to 5HT and subsequent activation of the RAS. The drinking responses to both 5HT and 5HTP were blocked by peripheral administration of the 5HT receptor antagonist, metergoline, but the drug was at least ten-fold more potent against 5HTP than 5HT. Intracerebroventricular (ICV) administration of metergoline also prevented the drinking response to peripherally-administered 5HTP. The drinking responses to both 5HT and 5HTP were enhanced by peripheral administration of low doses of an angiotensin I converting enzyme inhibitor, captopril. Collectively, these findings support previous conclusions that 5HT-induced intake of water is mediated exclusively by the renal RAS. However, 5HTP-induced drinking may additionally involve a renin-independent, serotonin-mediated mechanism, possibly in the brain.

Release of endogenous monoamines into spinal cord superfusates following the microinjection of phentolamine into the nucleus raphe magnus

Previous studies have suggested that raphe-spinal neurons located in the nucleus raphe magnus (NRM) are tonically inhibited by noradrenergic neurons. Furthermore, blockade of the inhibitory noradrenergic input to the NRM induces antinociception which appears to be mediated by the release of both serotonin and norepinephrine in the spinal cord. The present experiments were designed to directly measure the release of endogenous serotonin and norepinephrine into spinal cord superfusates before and after the microinjection of the alpha-adrenergic antagonist phentolamine into the NRM. High-performance liquid chromatography with electrochemical detection was used to quantitate the monoamines. The injection of phentolamine into the NRM induced a significant increase in the amount of both norepinephrine and serotonin released in the spinal cord. This enhanced release was not observed following either the injection of phentolamine into sites outside the NRM or the injection of saline vehicle into the NRM. These results support the proposal that the antinociception induced by the blockade of the inhibitory noradrenergic input to the NRM is mediated by the activation of spinally-projecting serotonergic and noradrenergic neurons.

Differential effects of nialamide and clomipramine on serotonin efflux and autoreceptors

Serotonin (5-HT) activity in vivo and in vitro was evaluated in rats following acute and chronic administration of the antidepressants nialamide (NMD) and clomipramine (CMI). The 5-HT motor syndrome was used as an index of in vivo serotonergic function. In vitro, 3H-5-HT uptake, potassium-evoked 3H-5-HT release and 5-HT autoreceptor activity were evaluated as measures of presynaptic function. Repeated injections of NMD abolished the 5-methoxy-N, N-dimethyltryptamine (5-MeODMT)-induced motor syndrome and the ability of 5-methoxytryptamine (5-MEOT) to attenuate the potassium-evoked release of 3H-5HT. Autoreceptor subsensitivity was associated with a marked increase in basal and potassium-evoked 3H-5-HT release. In contrast, acute NMD, and acute and chronic CMI did not affect the expression of the motor syndrome or alter 3H-HT release or autoreceptor activity. Acute and chronic injections of NMD enhanced 3H-5-HT uptake. The results suggest that the antidepressant efficacy of monoamine oxidase inhibitor (MAOI) antidepressants may be related to their ability to increase endogenous levels of 5-HT and thereby produce a subsensitivity of 5-HT1 type receptors. This subsensitivity is reflected both by attenuation of the motor syndrome and enhanced 5-HT neurotransmission resulting in part from autoreceptor down-regulation.

Proposals for the classification and nomenclature of functional receptors for 5-hydroxytryptamine

As a result of controversy in the literature regarding the classification and nomenclature of functional receptors for 5-hydroxytryptamine (5-HT), a framework for classification is proposed. The formulation of these proposals has only been made possible by the recent advent of new drug tools. It is considered that there are three main types of 5-HT receptor, two of which have been well characterised pharmacologically, using selective antagonists, and which it is proposed to name 5-HT2 and 5-HT3. These two groups broadly encompass the "D" and "M" receptors, respectively, which Gaddum identified in the guinea-pig ileum (Gaddum and Picarelli, 1957). The 5-HT2 receptor, which mediates a variety of actions of 5-HT, has been definitively shown to correlate with the 5-HT2 binding site in the brain. No binding studies in brain tissue have yet been published with radiolabelled ligands specific for 5-HT3 receptors. A number of other actions of 5-HT appear to be mediated via receptors distinct from 5-HT2 or 5-HT3 receptors. Since selective antagonists are not yet available, these receptors cannot be definitively characterised, although in many cases they do have some similarities with 5-HT1 binding sites, which are a heterogeneous entity. Criteria are proposed for tentatively classifying these receptors as "5-HT1-like" (Table 1). Definitive characterisation of these receptors will await the identification of specific antagonists. This classification of 5-HT receptors into three main groups (Table 1) is based largely, but not exclusively, on data from studies in isolated peripheral tissues where definitive classification is possible. However, it is believed that this working classification will be relevant to functional responses to 5-HT in the central nervous system.

Stereoselective blockade at the 5-HT autoreceptor and inhibition of radioligand binding to central 5-HT recognition sites by the optical isomers of methiothepin

The enantiomers of the 5-HT autoreceptor antagonist methiothepin have been prepared and their activity as antagonists of the 5-HT autoreceptor and at the 5-HT recognition sites present in the frontal cortex of the rat have been evaluated. At the 5-HT autoreceptor, the order of potency as antagonists of 5-HT was (+)methiothepin (apparent pA2 5.95) less than (+/-)methiothepin (apparent pA2 6.62) less than or equal to (-)methiothepin (apparent pA2 6.81). At the 5-HT2 recognition site, the isomeric forms of methiothepin were potent (pIC50 approximately 8.2) and equiactive. At the subtypes of the 5-HT1 recognition sites, similar concentrations to those blocking the autoreceptor were effective and (+)methiothepin was less active than (-)methiothepin. It is concluded that the chiral association of methiothepin with the 5-HT autoreceptor provides further evidence for a pharmacological similarity between this receptor and the 5-HT1B subtype of the 5-HT1 recognition site.

Panuramine, a selective inhibitor of uptake of 5-hydroxytryptamine in the brain of the rat

The neurochemical profile of the novel inhibitor of uptake of 5-hydroxytryptamine (5-HT) panuramine (Wy 26002) has been investigated in the rat. In vitro, panuramine was found to be a potent and selective inhibitor of uptake of 5-HT with an IC50 of 22 +/- 4 nM. The IC50 for inhibition of uptake of noradrenaline was 848 nM and that for uptake of dopamine greater than 10 micron. Panuramine, in concentrations up to 10 micron did not displace the specific binding of either [3H]spiroperidol or [3H]5-HT and had no effect on the spontaneous or potassium-evoked release of 5-HT, suggesting that the compound had little effect on serotonergic transmission other than the inhibition of uptake of 5-HT. Panuramine also produced a dose-related antagonism of the depletion of 5-HT in brain induced by p-chloroamphetamine, confirming the ability of the drug to inhibit uptake of 5-HT in vivo.

Effects of methiothepin and lysergic acid diethylamide on serotonin release in vitro and serotonin synthesis in vivo: possible relation to serotonin autoreceptor function

An in vitro system characterizing the presynaptic serotonin (5-HT) autoreceptor which controls the release of 5-HT from rat brain slices is described. Using this system, methiothepin (1-10 microM) demonstrated 5-HT autoreceptor antagonist activity by enhancing 5-HT release, while several recognized postsynaptic 5-HT receptor antagonists were inactive: mianserin, cinanserin, cyproheptadine, methysergide. The activity of methiothepin was highest in hypothalamic slices and lowest in striatal slices and was inhibited by the autoreceptor agonists lysergic acid diethylamide (LSD) and 5-methoxytryptamine (5-MT). The reversal of the methiothepin-enhanced 5-HT release from hypothalamic slices by LSD was not influenced by 0.3 microM tetrodotoxin. The peripheral administration of LSD to rats has been shown to reduce 5-HT synthesis and release by a mechanism thought to involve, in part, an autoreceptor-mediated reduction in impulse flow of 5-HT neurons. In the present experiments, intraperitoneal injection of methiothepin antagonized the LSD-induced reduction in hypothalamic 5-HT synthesis (5-hydroxytryptophan accumulation) while exerting no influence by itself. Conversely, compounds which were not active as 5-HT autoreceptor antagonists in vitro (i.e., cyproheptadine, methysergide, cinanserin) did not influence the effect of LSD on 5-HT synthesis. Further, the reduction in 5-hydroxytryptophan (5-HTP) accumulation by LSD showed regional differences in inhibition by methiothepin (hypothalamus greater than cortex greater than striatum) which paralleled the autoreceptor antagonist activity of methiothepin in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Identification of presynaptic serotonin autoreceptors using a new ligand: 3H-PAT

Binding studies with appropriate labelled ligands have revealed the existence of two types of serotonin (5-HT) receptor, 5-HT1 and 5-HT2, in the central nervous system of mammals. The 5-HT1 type is characterized by a higher affinity for agonists than for antagonists, whereas the 5-HT2 type binds preferentially to antagonists. However, neither of these receptor types apparently corresponds to the presynaptic autoreceptor controlling 5-HT release. In an attempt to identify the presynaptic autoreceptor directly, we synthesized the tritiated derivative of 8-hydroxy-2-(di-n-propylamino) tetralin (PAT), a new tetralin derivative with potent 5-HT agonist properties and carried out binding studies with rat brain membranes. As we report here, in the hippocampus, the properties of 3H-PAT binding sites correspond closely to those of 5-HT1 sites. In contrast, in the striatum, 3H-PAT binding sites exhibit a subcellular distribution and pharmacological characteristics usually associated with presynaptic autoreceptors. Furthermore, a marked loss of 3H-PAT binding sites occurs in the striatum (but not in the hippocampus) after the selective degeneration of serotoninergic fibres in 5,7-hydroxytryptamine (5,7-HT)-treated rats. Conversely, the sprouting of additional 5-HT terminals in the brain stem of adult rats treated at birth with 5,7-HT is associated with an increased density of 3H-PAT binding sites in this region. 3H-PAT thus seems to be a useful ligand for studying the biochemical and pharmacological characteristics of presynaptic autoreceptors in selected regions of rat brain.