Behavioural pharmacology of 5-HT3 receptor ligands

Emerging Role of Neuron-Glia in Neurological Disorders: At a Glance

Based on the diverse physiological influence, the impact of glial cells has become much more evident on neurological illnesses, resulting in the origins of many diseases appearing to be more convoluted than previously happened. Since neurological disorders are often random and unknown, hence the construction of animal models is difficult to build, representing a small fraction of people with a gene mutation. As a result, an immediate necessity is grown to work within in vitro techniques for examining these illnesses. As the scientific community recognizes cell-autonomous contributions to a variety of central nervous system illnesses, therapeutic techniques involving stem cells for treating neurological diseases are gaining traction. The use of stem cells derived from a variety of sources is increasingly being used to replace both neuronal and glial tissue. The brain's energy demands necessitate the reliance of neurons on glial cells in order for it to function properly. Furthermore, glial cells have diverse functions in terms of regulating their own metabolic activities, as well as collaborating with neurons via secreted signaling or guidance molecules, forming a complex network of neuron-glial connections in health and sickness. Emerging data reveals that metabolic changes in glial cells can cause morphological and functional changes in conjunction with neuronal dysfunction under disease situations, highlighting the importance of neuron-glia interactions in the pathophysiology of neurological illnesses. In this context, it is required to improve our understanding of disease mechanisms and create potential novel therapeutics. According to research, synaptic malfunction is one of the features of various mental diseases, and glial cells are acting as key ingredients not only in synapse formation, growth, and plasticity but also in neuroinflammation and synaptic homeostasis which creates critical physiological capacity in the focused sensory system. The goal of this review article is to elaborate state-of-the-art information on a few glial cell types situated in the central nervous system (CNS) and highlight their role in the onset and progression of neurological disorders.

5-HT3 receptor within the amygdaloid complex modulates pain hypersensitivity induced by empathy model of cohabitation with a partner in chronic pain condition in mice

Cohabitation with a partner undergoing chronic pain induces pain hypersensitivity. Among a lot of other neurochemical pathways, the serotonin (5-HT) role, specifically the 5-HT₃ receptor (5-HT₃R), in the amygdala has never been evaluated in this model. Here we studied the effects of the amygdala's chemical inhibition, its neuronal activation pattern, and 5-HT, 5-HIAA, and 5-HT turnover within the amygdala. Furthermore, the systemic and intra-amygdala 5-HT₃R activation and blockade in mice that cohabited with a conspecific subjected to chronic constriction injury were investigated. Male Swiss mice were housed in partners for 28 days. The dyads were divided into two groups on the 14^th day: cagemate nerve constriction (CNC) and cagemate sham (CS). On the 24^th day, cagemates underwent a stereotaxic surgery (when necessary) and, on the 28^th day, they were evaluated on the writhing test. The amygdala inactivation promotes pain-hypersensitivity behaviors in groups and dyads; cohabitation with a partner with chronic pain did not change FosB-labeled cells in the amygdala's nucleus and increases 5-HT turnover in cagemates. Systemic and intra-amygdala 5-HT₃R activation attenuated and enhanced the number of writhes, respectively. In contrast, 5-HT₃R blockade reduced hypersensitivity pain response. Results suggest the involvement of amygdala serotonergic signaling via 5-HT₃R in empathy-like behavior.

Ethanol induces persistent potentiation of 5-HT3 receptor-stimulated GABA release at synapses on rat hippocampal CA1 neurons

Several studies have shown that ethanol (EtOH) can enhance the activity of GABAergic synapses via presynaptic mechanisms, including in hippocampal CA1 neurons. The serotonin type 3 receptor (5-HT3-R) has been implicated in the neural actions of ethanol (EtOH) and in modulation of GABA release from presynaptic terminals. In the present study, we investigated EtOH modulation of GABA release induced by 5-HT3-R activation using the mechanically isolated neuron/bouton preparation from the rat CA1 hippocampal subregion. EtOH application before and during exposure to the selective 5-HT3 receptor agonist, m-chlorophenylbiguanide (mCPBG) potentiated the mCPBG-induced increases in the peak frequency and charge transfer of spontaneous GABAergic inhibitory postsynaptic currents. Interestingly, the potentiation was maintained even after EtOH was removed from the preparation. A protein kinase A inhibitor reduced the magnitude of EtOH potentiation. Fluorescent Ca2+ imaging showed that Ca2+ transients in the presynaptic terminals increased during EtOH exposure. These findings indicate that EtOH produces long-lasting potentiation of 5-HT3-induced GABA release by modulating calcium levels, via a process involving cAMP-mediated signaling in presynaptic terminals.

Granisetron, a selective 5-HT3 antagonist, reduces L-3,4-dihydroxyphenylalanine-induced abnormal involuntary movements in the 6-hydroxydopamine-lesioned rat

Administration of L-3,4-dihydroxyphenylalanine (L-DOPA) provides Parkinson's disease patients with effective symptomatic relief. However, long-term L-DOPA therapy is often marred by complications such as dyskinesia. We have previously demonstrated that serotonin type 3 (5-HT3) receptor blockade with the clinically available and highly selective antagonist ondansetron alleviates dyskinesia in the 6-hydroxydopamine (6-OHDA)-lesioned rat. Here, we sought to explore the antidyskinetic efficacy of granisetron, another clinically available 5-HT3 receptor antagonist. Rats were rendered hemi-parkinsonian by 6-OHDA injection in the medial forebrain bundle. Following induction of stable abnormal involuntary movements (AIMs), granisetron (0.0001, 0.001, 0.01, 0.1 and 1 mg/kg) or vehicle was acutely administered in combination with L-DOPA and the severity of AIMs, both duration and amplitude, was determined. We also assessed the effect of granisetron on L-DOPA antiparkinsonian action by performing the cylinder test. Adding granisetron (0.0001, 0.001, 0.01, 0.1 and 1 mg/kg) to L-DOPA resulted in a significant reduction of AIMs duration and amplitude, with certain parameters being reduced by as much as 38 and 45% (P < 0.05 and P < 0.001, respectively). The antidyskinetic effect of granisetron was not accompanied by a reduction of L-DOPA antiparkinsonian action. These results suggest that 5-HT3 blockade may reduce L-DOPA-induced dyskinesia without impairing the therapeutic efficacy of L-DOPA. However, a U-shaped dose-response curve obtained with certain parameters may limit the therapeutic potential of this strategy and require further investigation.

2-(4-substituted piperazin-1-yl)-1,8-naphthyridine-3-carboxylic acids: Novel 5-HT3 receptor antagonists with anxiolytic-like activity in rodent behavioral models

The aim of this study was to investigate the anxiolytic potential of a series of novel carboxylic acid based 1,8 naphthyridines as 5-HT3 receptor antagonists. The pA2 values of all the compounds were determined against agonist 2-methyl-5-hydroxytryptamine in longitudinal muscle myenteric plexus preparations from guinea pig ileum. Compounds with higher pA2 values, particularly those greater than ondansetron, a standard 5-HT3 receptor antagonist, and optimal log P values were screened in mice by using behavioral tests such as a light–dark (L/D) aversion test, elevated plus maze (EPM) test, and an open field test (OFT). In the L/D test, compounds 7a, 7b, 7d, 7e, and 7i (2 mg/kg body mass, intraperitoneal) significantly (P < 0.05) increased the latency time to leave the light compartment, total time spent in the light compartment, and the number of transitions between the light and dark compartments. Compounds 7a, 7d, 7f, 7h, and 7i (2 mg/kg, i.p.) significantly (P < 0.05) increased the time spent in the open arms and the number of entries into the open arms in the EPM test. In addition, compounds 7a, 7d, 7e, 7f, and 7h (2 mg/kg, i.p.) significantly (P < 0.05) increased the ambulation scores and the frequency of rearing in the OFT.

Tardive dyskinesia: treatment with aripiprazole

Tardive dyskinesia is characterized by choreiform movements, or rhythmic abnormal involuntary movements of the face, mouth, tongue, trunk, and limbs. It is frequently associated with the use of neuroleptic medications. The choreiform movements are irreversible in some patients, even after the drug is withdrawn. Although no reliable treatment for tardive dyskinesia exists, atypical antipsychotics are associated with a significantly lower incidence of tardive dyskinesia than typical antipsychotics. Moreover, recent reports suggest that atypical antipsychotics may have a beneficial effect on tardive dyskinesia remission. Until recently, evidence for the effectiveness of aripiprazole on tardive dyskinesia has been mixed. Aripiprazole has a unique mechanism of action and has various effects in tardive dyskinesia. The drug acts as a partial D₂ receptor agonist that can stabilize D₂ up-regulation, and as a partial 5-HT_1A receptor agonist and a 5-HT_2A receptor antagonist, and can increase the release of dopamine in the striatum.

The Role of Ondansetron in the Treatment of Schizophrenia

Objective:

To evaluate the efficacy of ondansetron for the treatment of schizophrenia.

Data Sources:

Searches of MEDLINE (1950–March 2010) and Google Scholar were performed. Key search terms included ondansetron, Zofran, serotonin antagonists, 5-HT, serotonin receptor, and schizophrenia.

Study Selection and Data Extraction:

All articles published in English identified from the data sources were evaluated. All studies and case reports evaluating ondansetron for the treatment of schizophrenia were reviewed.

Data Synthesis:

Six clinical trials, including 3 double-blind, randomized trials, and 2 case reports pertinent to ondansetron use in schizophrenia, were identified. Ondansetron daily doses ranged from 4 to 16 mg, with doses administered once or twice daily. Ondansetron was used as monotherapy in 3 trials and as an adjunct to therapy with clozapine, haloperidol, or risperidone, respectively, in 3 trials. Studies were of varying durations, ranging from a single-dose study with a 3-hour follow-up to three 12-week studies. Most studies evaluated ondansetron's efficacy in treating schizophrenia as measured with changes in Positive and Negative Syndrome Scale (PANSS), Brief Psychiatric Rating Scale, and Clinical Global Impression scale scores. In the 2 largest trials, with a combined patient population of 151, treatment with adjunctive ondansetron resulted in statistically significant improvement in negative symptoms as assessed with PANSS. In all studies, ondansetron was well tolerated, with no severe adverse reactions reported.

Conclusions:

Ondansetron may be effective as an adjunct to antipsychotics for the treatment of schizophrenia, specifically negative symptoms, as assessed with PANSS. Due to the variation in concurrent therapies and dosing regimens, it is difficult to establish an optimal dose from the reviewed trials. Further large, randomized, double-blind, active-controlled studies would be helpful in determining the role of ondansetron in the treatment of schizophrenia.

Synthesis and biological evaluation of a novel structural type of serotonin 5-HT3 receptor antagonists

A series of novel 3-substituted quinoxalin-2-carboxamides were designed as per the pharmacophoric requirement for 5-HT(3) receptor antagonists and prepared by microwave irradiation and also by conventional method. The compounds were characterized by spectral data (IR, (1)H NMR, and MS) and the purity was ascertained by microanalysis. The synthesized compounds were evaluated for 5-HT(3) antagonisms in longitudinal muscle-myenteric plexus preparation from guinea pig ileum against 5-HT(3) agonist, 2-methyl-5-HT. Among the test compounds, N-{3-[(4-methylpiperazin-1-yl)methyl]-4-hydroxyphenyl}-3-methoxyquinoxalin-2-carboxamide 4e showed most favorable 5-HT(3) receptor antagonism.

The selective 5-HT6 receptor antagonist Ro4368554 restores memory performance in cholinergic and serotonergic models of memory deficiency in the rat

Antagonists at serotonin type 6 (5-HT(6)) receptors show activity in models of learning and memory. Although the underlying mechanism(s) are not well understood, these effects may involve an increase in acetylcholine (ACh) levels. The present study sought to characterize the cognitive-enhancing effects of the 5-HT(6) antagonist Ro4368554 (3-benzenesulfonyl-7-(4-methyl-piperazin-1-yl)1H-indole) in a rat object recognition task employing a cholinergic (scopolamine pretreatment) and a serotonergic- (tryptophan (TRP) depletion) deficient model, and compared its pattern of action with that of the acetylcholinesterase inhibitor metrifonate. Initial testing in a time-dependent forgetting task employing a 24-h delay between training and testing showed that metrifonate improved object recognition (at 10 and 30 mg/kg, p.o.), whereas Ro4368554 was inactive. Both, Ro4368554 (3 and 10 mg/kg, intraperitoneally (i.p.)) and metrifonate (10 mg/kg, p.o., respectively) reversed memory deficits induced by scopolamine and TRP depletion (10 mg/kg, i.p., and 3 mg/kg, p.o., respectively). In conclusion, although Ro4368554 did not improve a time-related retention deficit, it reversed a cholinergic and a serotonergic memory deficit, suggesting that both mechanisms may be involved in the facilitation of object memory by Ro4368554 and, possibly, other 5-HT(6) receptor antagonists.

Pharmacophore based synthesis of 3-chloroquinoxaline-2-carboxamides as serotonin3 (5-HT3) receptor antagonist

A series of 3-chloroquinoxaline-2-carboxamides were designed and prepared by the condensation of 3-chloro-2-quinoxaloylchloride with appropriate Mannich bases of the p-aminophenol in the microwave environment. The synthesized compounds were evaluated for serotonin(3) (5-HT(3)) receptor antagonistic activities in longitudinal muscle-myenteric plexus (LMMP) preparation from guinea pig ileum against the 5-HT(3) agonist, 2-methyl-5-HT. Compound 3g exhibited comparable 5-HT(3) antagonistic activity (pA(2) 6.4) to that of standard antagonist Ondansetron (pA(2) 6.9), while the other compounds exhibited mild to moderate 5-HT(3) antagonistic activities.

Lack of effect of 5HT3 antagonist in mediating subjective and behavioral responses to cotinine

Previous studies have shown that cotinine, a metabolite of nicotine, may antagonize some of the therapeutic effects of nicotine. The mechanisms underlying cotinine's effects are unclear, but cotinine has been observed to increase serotonin levels in the brain. Thus, it is possible that blocking serotonin effects may antagonize the actions of cotinine, thereby reducing its impact on responses to nicotine. This study determined whether granisetron, a 5HT(3) receptor antagonist, would enhance the efficacy of the nicotine patch. Subjects were randomly assigned to one of the three granisetron conditions (N=43 for 2 mg/day; N=43 for 1 mg/day; N=42 for 0 mg/day) and asked to take the assigned medication daily during 15 days of tobacco abstinence. Because we were interested in interactions between cotinine and serotonin, all groups were also treated with a 21-mg nicotine patch. Assessments of withdrawal symptoms were made for 1 week during baseline smoking and several times during the experimental period. There was a near but nonsignificant difference among groups on a measure of tobacco withdrawal and no significant differences on global measures of drug effects or physiological measures. The data do not strongly support the hypothesis that 5HT(3) agonism is the mechanism by which cotinine offsets the effects of nicotine.

Reversal of neuroleptic-induced orofacial dyskinesia by 5-HT3 receptor antagonists

Tardive dyskinesia, a syndrome of abnormal, involuntary hyperkinetic movements that occurs during long-term neuroleptic therapy is a major limitation of chronic neuroleptic therapy. The pathophysiology of tardive dyskinesia is still an enigma. The objective of the present study was to elucidate the role of 5-HT3 receptor involvement in neuroleptic-induced vacuous chewing movements in rats. Rats chronically (for 21 days) treated with haloperidol (1.5 mg/kg, i.p.) significantly developed vacuous chewing movements, as compared to vehicle-treated controls. Both ondansetron and tropisetron dose-dependently (0.25, 0.5 and 1.0 mg/kg, i.p.) reversed the haloperidol-induced vacuous chewing movements. Serotonin acting through 5-HT3 receptors might play a significant role in the pathophysiology of tardive dyskinesia, and 5-HT3 receptor ligands can be exploited as novel therapeutic agents for the treatment of tardive dyskinesia.

Distribution of [3H]GR65630 binding in human brain postmortem

We investigated the distribution of serotonin (5-HT) receptors of type 3 (5-HT3) in human brain areas, by means of the the specific binding of [3H]GR65630. The brains were obtained during autoptic sessions from 6 subjects. Human brain membranes and the binding of [3H]GR65630 were carried out according to standardized methods. The highest density (Bmax +/- SD, fmol/mg protein) of [3H]GR65630 binding sites was found in area postrema (13.1+/-9.7), followed at a statistically lower level, by nucleus tractus solitarius (6.7+/-3.4), nervus vagus (5.5+/-2.1), striatum (4.8+/-2.4) with a progressive decrease in amygdala, olivar nuclei, hippocampus, olfactory bulbus and prefrontal cortex, and then by the other cortical areas and the cerebellum, where no binding was detected. These observations extend previous findings on the distribution of 5-HT3 receptors and confirm interspecies variations that might explain the heterogeneous properties of 5-HT3 receptors in different animals.

Enhancement of hippocampal cholinergic neurotransmission through 5-HT1A receptor-mediated pathways by repeated lithium treatment in rats

Hippocampal cholinergic neuronal activity is reported to be regulated, at least partly, through serotonin1A (5-HT1A) receptors. Chronic lithium treatment has been shown to alter both behavioral and neurochemical responses mediated by postsynaptic 5-HT1A receptors. We investigated whether long-term lithium treatment affects central cholinergic neurotransmission through 5-HT1A receptor-mediated pathways. Changes in acetylcholine (ACh) release induced by 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A receptor agonist, in the rat hippocampus were measured using a microdialysis technique and a radioimmunoassay for ACh. Administration of lithium for 21 days resulted in a serum lithium concentration of 1.03 mM and caused little change in density or affinity of [3H]8-OH-DPAT binding sites in the hippocampus. The local application of 8-OH-DPAT into the hippocampus of lithium treated rats increased the ACh efflux in both the absence and the presence of physostigmine, a cholinesterase (ChE) inhibitor, in the perfusion fluid. The basal ACh efflux of lithium treated rats was not different from that of the control rats under normal conditions, but was significantly higher than that of the controls when ChE was inhibited. These results demonstrate that chronic lithium treatment increases spontaneous ACh release in the hippocampus under conditions of ChE inhibition, but not under normal conditions, and enhances cholinergic neurotransmission through 5-HT1A receptor-mediated pathways, and suggest that activation of 5-HT1A receptor function by lithium is related to the enhancement of hippocampal cholinergic neurotransmission.

Key words: Acetylcholine (ACh), hippocampus, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), lithium, serotonin1A (5-HT1A) receptor.

5-HT(3) receptor antagonists and anxiety; a preclinical and clinical review

The present paper reviews the evidence for anxiolytic activity of 5-HT(3) receptor antagonists in animal models of anxiety and in clinical trials in humans. Compared to the established anxiolytics (benzodiazepine receptor agonists and, to a lesser extent, 5-HT(1A) receptor agonists) 5-HT(3) receptor antagonists display a different anxiolytic profile. They are anxiolytic in a limited number of animal anxiety models. If active, they often are very potent and display bell-shaped dose response curves, whereas the ratio between therapeutic activity and side effects appears remarkably large. 5-HT(3) receptor antagonists remain active after chronic dosing and no indications for tolerance, dependence or rebound effects were found, which seems to make these drugs an attractive alternative to the benzodiazepines. However, the large body of animal data indicating a complete lack of psychotropic activity of 5-HT(3) receptor antagonists weakens the prediction of anxiolytic activity in these drugs. Human data are also controversial; some investigators have reported positive effects in anxiety disorders (panic disorder, GAD), others did not. It can be concluded that 5-HT(3) receptor antagonists do not represent a breakthrough in the treatment of various anxiety disorders, as initially suggested.

The pharmacological and functional characteristics of the serotonin 5-HT(3A) receptor are specifically modified by a 5-HT(3B) receptor subunit

While homomers containing 5-HT(3A) subunits form functional ligand-gated serotonin (5-HT) receptors in heterologous expression systems (Jackson, M. B., and Yakel, J. L. (1995) Annu. Rev. Physiol. 57, 447-468; Lambert, J. J., Peters, J. A., and Hope, A. G. (1995) in Ligand-Voltage-Gated Ion Channels (North, R., ed) pp. 177-211, CRC Press, Inc., Boca Raton, FL), it has been proposed that native receptors may exist as heteromers (Fletcher, S., and Barnes, N. M. (1998) Trends Pharmacol. Sci. 19, 212-215). We report the cloning of a subunit 5-HT(3B) with approximately 44% amino acid identity to 5-HT(3A) that specifically modified 5-HT(3A) receptor kinetics, voltage dependence, and pharmacology. Co-expression of 5-HT(3B) with 5-HT(3A) modified the duration of 5-HT(3) receptor agonist-induced responses, linearized the current-voltage relationship, increased agonist and antagonist affinity, and reduced cooperativity between subunits. Reverse transcriptase-polymerase chain reaction in situ hybridization revealed co-localization of both 5-HT(3B) and 5-HT(3A) in a population of neurons in the amygdala, telencephalon, and entorhinal cortex. Furthermore, 5-HT(3A) and 5-HT(3B) mRNAs were expressed in spleen and intestine. Our data suggest that 5-HT(3B) might contribute to tissue-specific functional changes in 5-HT(3)-mediated signaling and/or modulation.

An investigation into the effects of 5-HT agonists and receptor antagonists on ethanol self-administration in the rat

Pharmacological manipulation leading to altered 5-HT function has been widely demonstrated to reduce ethanol intake in free choice tests. The aim of the present study was to examine the effects of a range of compounds known to influence 5-HT neurotransmission, including selective 5-HT receptor agonists and antagonists, on ethanol ingestion and maintained behaviour in an operant self-administration paradigm. Female Sprague-Dawley rats were trained to respond for 8% ethanol (v/v) in a 60-min test by a previously described technique. The number of responses and ethanol reinforcers (dipper deliveries), ethanol consumption (g/kg of body weight), and locomotor activity (LMA) were measured following administration of 5-HT agonists (5-HT, d-fenfluramine, fluoxetine, buspirone, TFMPP, and DOI) and antagonists (metergoline, ritanserin, and ondansetron) 30 min prior to testing. d-Fenfluramine, fluoxetine, buspirone, TFMPP, and DOI all produced a reduction in ethanol ingestion and maintained behaviour at doses that failed to reduce LMA. Conversely, metergoline and ritanserin only reduced ethanol self-administration at doses that concomitantly reduced LMA. 5-HT and ondansetron were without effect on any measure. These results demonstrate that, under the present experimental conditions, activation of central 5-HT1A, 5-HT1B, and 5-HT2 receptors reduced ethanol intake and reinforced behaviour in an operant paradigm.

Phenylimidazolidin-2-one derivatives as selective 5-HT3 receptor antagonists and refinement of the pharmacophore model for 5-HT3 receptor binding

A possible bioisosterism between the benzamido and the phenylimidazolidin-2-one moieties has been suggested on the basis of the similarity between the molecular electrostatic potential (MEP) of metoclopramide, a D2 receptor antagonist with weak 5-HT3 receptor antagonist properties, and zetidoline, a D2 receptor antagonist. Starting from this premise, a series of phenylimidazolidin-2-one derivatives bearing a basic azabicycloalkyl or an imidazolylalkyl moiety were synthesized and evaluated for 5-HT3 receptor radioligand binding affinity ([3H]-GR 43,694). In vitro 5-HT3 receptor antagonist activity was tested in the guinea pig ileum assay (GPI). A number of high-affinity ligands were shown to be potent 5-HT3 receptor antagonists in vivo as determined by inhibition of the Bezold--Jarisch reflex in the anesthetized rat. In general, the imidazolylalkyl derivatives were found to be more active than azabicycloalkyls. 1-(3,5-Dichlorophenyl)-3-[(5-methyl-1H-imidazol-4-yl)methyl]imidazoli din-2-one (58), in particular, displayed very high affinity for the 5-HT3 receptor (Ki of 0.038 nM) with a Kb of 5.62 nM in the GPI assay, being more potent than the reference compounds (ondansetron, tropisetron, granisetron, and BRL 46,470) tested. 58 showed an ID50 comparable to that of ondansetron (2.2 micrograms/kg i.v.) in the Bezold--Jarisch reflex. A molecular modeling study based on this structurally novel series of compounds allowed the refinement of previously reported 5-HT3 receptor antagonist pharmacophore models.

Distribution of S(-)-zacopride-insensitive [125I]R(+)-zacopride binding sites in the rat brain and peripheral tissues

Increasing evidence indicates that the 5-HT3 receptor antagonist R(+)-zacopride labels an additional site in brain tissue that is not sensitive to 5-HT (non-5-HT R(+)-zacopride site, R(+)-site). Since the levels of R(+)-sites in the brain are relatively low, the present studies explored the use of [125I]R(+)-zacopride to label the R(+)-site; the incorporation of an [125I] atom considerably increasing the specific activity of the radioligand relative to [3H]R(+)-zacopride that has been utilised previously. Competition experiments with [125I]R(+)-zacopride (1.0 nM) binding to rat whole brain homogenates, in the presence of the 5-HT3 receptor antagonist granisetron (1.0 microM), identified that R(+)-zacopride and prazosin bound to two sites (pIC50: 7.59 and 5.28, respectively, for R(+)-zacopride; 6.75 and 4.42, respectively, for prazosin) whereas S(-)-zacopride and mianserin possessed relatively low affinity (pIC50: 4.37 and 3.80, respectively) while (-)sulpiride and 5-HT failed to compete for [125I]R(+)-zacopride binding at concentrations up to 10 microM. Autoradiographic radioligand binding studies using [125I]R(+)-zacopride (0.5 nM) identified a heterogeneous distribution of specific binding sites (defined by unlabelled R(+)-zacopride, 1.0 microM) throughout the rat brain. In the presence of a saturating concentration of granisetron (1.0 microM), highest levels of specific [125I]R(+)-zacopride, binding sites (defined by R(+)-zacopride, 1.0 microM; R(+)-site), were detected in the olfactory tubercle, thalamus, corpus callosum, colliculus, dorsal and median raphe nucleus, spinal cord and the pons (8.0-13.0 fmol/mg). Moderate densities of R(+)-sites were located in the striatum, nucleus accumbens, substantia nigra, ventral tegmental area, globus pallidus, septal nuclei, frontal cortex and cerebellum (2.0-7.9 fmol/mg). In the hippocampus, amygdala and cortical areas. R(+)-site levels were low but detectable (0.1-1.9 fmol/mg). [125I]R(+)-zacopride labelled R(+)-sites were also detected in some rat peripheral tissues, for instance kidney cortex, adrenal gland and liver (2.4-6.8 fmol/mg). The present results indicate that specific non-5-HT [125I]R(+)-zacopride sites are heterogeneously distributed throughout the rat brain and are expressed in various peripheral tissues.

Radioligand binding and photoaffinity labelling studies show a direct interaction of phenothiazines at 5-HT3 receptors

The effects of a range of phenothiazines were examined on 5-hydroxytryptamine3 (5-HT3) receptors in membranes from NIE-115 neuroblastoma cells using radioligand binding. Chlorpromazine, fluphenazine, perphenazine, trifluoperazine and prochlorperazine inhibited specific binding of both the 5-HT3 receptor antagonist [3H]GR65630 and agonist [3H]meta-chlorophenylbiguanide (mCPBG), with Ki values ranging from 0.4 to 3.9 microM. The mode of action of chlorpromazine was further examined using photoaffinity labelling in the presence and absence of 5-HT. Saturation radioligand binding data with both [3H]GR65630 and [3H]mCPBG showed that photoaffinity labelling with chlorpromazine (1 microM) caused a decrease in the maximum number of binding sites observed (35% and 28% for agonist and antagonist, respectively). This decrease was not observed when the membranes were incubated in the presence of 5-HT. The results demonstrate a direct interaction of a range of phenothiazines at the 5-HT3 receptor binding site.

Identification of 5-HT receptor sub-types in a homogeneous population of presumptive serotoninergic neurones

The expression of the known rat 5-HT receptor sub-types has been analyzed in a presumptive serotoninergic cell line derived from the rat raphé nuclei, using reverse transcription followed by polymerase chain reaction. By manipulating the activity of the oncogene (ts-SV40T) product used to immortalize the serotoninergic precursors, it has been possible to compare the expression of the 5-HT receptors in either replicative or differentiating cells. 5-HT1B, 5-HT1D, 5-HT3, 5-HT6 and 5-HT7 receptor gene expression were all observed in the replicating cells. However, under differentiation conditions, expression of all except the 5-HT1B receptor was lost. Only one novel amplification product appeared during early differentiation, in the 5-HT2B lane; its smaller than expected size was suggestive of a previously undescribed alternate splicing of the mRNA in brain. The curtailment of 5-HT receptor expression in differentiating neurones in vitro may reflect the normal ongoing restriction in the phenotypic potential during embryogenesis in vivo. The serotonin cells, therefore, constitute a pristine cell line in which to study the receptor pharmacology of one or more 5-HT receptor sub-types in isolation.

Interaction of S 21007 with 5-HT3 receptors. In vitro and in vivo characterization

The interaction of S 21007 [5-(4-benzyl piperazin-1-yl)4H pyrrolo [1,2-a]thieno[3,2-e]pyrazine] with serotonin 5-HT3 receptors was investigated using biochemical, electrophysiological and functional assays. Binding studies using membranes from N1E-115 neuroblastoma cells showed that S 21007 is a selective high affinity (IC50 = 2.8 nM) 5-HT3 receptor ligand. As expected of an agonist, S 21007 stimulated the uptake of [14C]guanidinium (EC50 approximately 10 nM) in NG 108-15 cells exposed to substance P, and this effect could be prevented by the potent 5-HT3 receptor antagonist ondansetron. In addition, like 5-HT and other 5-HT3 receptor agonists (phenylbiguanide and 3-chloro-phenylbiguanide), S 21007 (EC50 = 27 microM) produced a rapid inward current in N1E-115 cells. The 5-HT3 receptor agonist action of S 21007 was also demonstrated in urethane-anaesthetized rats as this drug (120 micrograms/kg i.v.) triggered the Bezold-Jarisch reflex (rapid fall in heart rate), and this action could be prevented by pretreatment with the potent 5-HT3 receptor antagonist zacopride. Finally, in line with its 5-HT3 receptor agonist properties, S 21007 also triggered emesis in the ferret. Evidence for 5-HT3 receptor antagonist-like properties of S 21007 was also obtained in some of these experiments since previous exposure to this compound prevented both the 5-HT-induced current in N1E-115 cells and the Bezold-Jarisch reflex elicited by an i.v. bolus of 5-HT (30 micrograms/kg) in urethane-anaesthetized rats. These data suggest that S 21007 is a selective 5-HT3 receptor agonist which can exhibit antagonist-like properties either by triggering a long lasting receptor desensitization or by a partial agonist activity at 5-HT3 receptors in some tissues.

5-HT1A receptor full and partial agonists and 5-HT2C (but not 5-HT3) receptor antagonists increase rates of punished responding in rats

Drugs with different intrinsic activity at 5-HT1A receptors and antagonists at 5-HT2A/2C and 5-HT3 receptors were studied for their ability to increase the rates of punished operant responding in rats. Like chlordiazepoxide (5 and 10 mg/kg) and diazepam (1.25 and 2.5 mg/kg), 0.125 mg/kg 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A receptor agonist, and 5 and 10 mg/kg ipsapirone, a partial agonist at these receptors, increased the rates of punished responding, whereas (S)-WAY 100135, a 5-HT1A receptor antagonist, had no effect at doses from 1 to 10 mg/kg. 8-OH-DPAT and ipsapirone, like benzodiazepines, significantly reduced unpunished responding. The 5-HT2A/2C receptor antagonists ritanserin (2 mg/kg), mianserin (8 mg/kg), and mesulergine (0.1 mg/kg) significantly increased the rates of punished responding, whereas 0.5-2 mg/kg ketanserin, that has higher affinity for 5-HT2A than 5-HT2C receptors, had no effect. Antagonists, at 5-HT3 receptors such as ondansetron (0.001-0.1 mg/kg) and tropisetron (0.001-0.1 mg/kg), had no effect on punished or unpunished responding. The results show that agents acting as full or partial agonists at 5-HT1A receptors and blockers of postsynaptic 5-HT2C receptors have anxiolytic-like effects in a model of punished operant responding, whereas antagonists at 5-HT1A and 5-HT3 receptors have no such effect.

Acetylcholine: a neurotransmitter for learning and memory?

The cholinergic hypothesis claims that the decline in cognitive functions in dementia is predominantly related to a decrease in cholinergic neurotransmission. This hypothesis has led to great interest in the putative involvement of the cholinergic neurotransmission in learning and memory processes. This review aims to assess the data of studies in which the role of acetylcholine (ACh) in cognitive functions was investigated. For this purpose, studies from three different fields of research, namely: (1) behavioral pharmacology (effects of drugs on behavior); (2) behavioral neuroscience (effects of brain lesions on behavior); and (3) dementia, are discussed separately. The experimental tools that have been used in pharmacological studies may appear to be inadequate to enable conclusions to be drawn about the involvement of ACh in learning and memory processes. Especially, the use of scopolamine as a pharmacological tool is criticized. In the field of behavioral neuroscience a highly specific cholinergic toxin has been developed. It appears that the greater and more specific the cholinergic damage, the fewer effects can be observed at the behavioral level. The correlation between the decrease in cholinergic markers and the cognitive decline in dementia may not be as clearcut as has been assumed. The involvement of other neurotransmitter systems in cognitive functions is briefly discussed. Taking into account the results of the different fields of research, the notion that ACh plays a pivotal role in learning and memory processes seems to be overstated. Even when the role of other neurotransmitter systems in learning and memory is taken into consideration, it is unlikely that ACh has a specific role in these processes. On basis of the available data, ACh seems to be more specifically involved in attentional processes than in learning and memory processes.

Evidence that the atypical 5-HT3 receptor ligand, [3H]-BRL46470, labels additional 5-HT3 binding sites compared to [3H]-granisetron

1. The radioligand binding characteristics of the 3H-derivative of the novel 5-HT3 receptor antagonist BRL46470 were investigated and directly compared to the well characterized 5-HT3 receptor radioligand [3H]-granisetron, in tissue homogenates prepared from rat cerebral cortex/hippocampus, rat ileum, NG108-15 cells, HEK-5-HT3As cells and human putamen. 2. In rat cerebral cortex/hippocampus, rat ileum, NG108-15 cell and HEK-5-HT3As cell homogenates, [3H]-BRL46470 bound with high affinity (Kd (nM): 1.57 +/- 0.18, 2.49 +/- 0.30, 1.84 +/- 0.27, 3.46 +/- 0.36, respectively; mean +/- s.e. mean, n = 3-4) to an apparently homogeneous saturable population of sites (Bmax (fmol mg-1 protein): 102 +/- 16, 44 +/- 4, 968 +/- 32 and 2055 +/- 105, respectively; mean +/- s.e. mean, n = 3-4) but failed to display specific binding in human putamen homogenates. 3. In the same homogenates of rat cerebral cortex/hippocampus, rat ileum, NG108-15 cells, HEK-5-HT3As cells and human putamen as used for the [3H]-BRL46470 studies, [3H]-granisetron also bound with high affinity (Kd (nM): 1.55 +/- 0.61, 2.31 +/- 0.44, 1.89 +/- 0.36, 2.03 +/- 0.42 and 6.46 +/- 2.58 respectively; mean +/- s.e. mean, n = 3-4) to an apparently homogeneous saturable population of sites (Bmax (fmol mg-1 protein): 39 +/- 4, 20 +/- 2, 521 +/- 47, 870 +/- 69 and 18 +/- 2, respectively; mean +/- s.e. mean, n = 3-4). 4. Competition studies with a range of structurally different 5-HT3 receptor ligands indicated that in both rat cerebral cortex/hippocampus and rat ileum homogenates, [3H]-BRL46470 binding exhibited a pharmacological profile consistent with the labelling the 5-HT3 receptor with compounds competing with Hill coefficients close to unity.5 In HEK-5-HT3As cell homogenates, [3H]-BRL46470 and [3H]-granisetron associated rapidly((3.84+/-0.4)106 M-1S-1 and (5.85+/-0.2)106 M-1S-1, respectively, mean+/-s.e.mean, n=3-4) in an apparently monophasic manner. Following the establishment of equilibrium, both [3H]-BRL46470 and [3H]-granisetron at a saturating concentration ([3H]-BRL46470 approximately 16 nM; [3H]-granisetron approximately 18 nM) and at a sub-Kd concentration (approximately 1 nm for both radioligands)dissociated biphasically in HEK-5-HT3As cell homogenates (saturating concentration; [3H]-BRL464704.05 x 10-3+/-2.53 x I0-3 s-1 and 5.83 x 10-5+0.91 x I0-5 s-1; [3H]-granisetron 3.20 x 10-3+ 1.70 x IO-3 s-1 and18.58 x 10-5 +/- 4.19 x I0-5 s-1: sub-Kd concentration; [3H]-BRL46470 2.47 x 10-3+/- 1.18 x 10-3 s-1 and 9.30x 10-5+/-2.59x 10-5 S-1; [3H]-granisetron 65.91 x 10-3+/-22.14x I0-3 s-1 and 49.96x 10-5+/-12.26x 10-5s- 1 mean+/- s.e.mean, n = 4-8) when induced by a 300 fold dilution in ice-cold Tris/Krebs.6 In conclusion, the present study provides evidence that [3H]-BRL46470 specifically labels the 5-HT3receptor in rat cerebral cortex/hippocampus, rat ileum, NG108-15 cell and HEK-5-HT3As cell homogenates, but fails to label the 5-HT3 receptor expressed in human putamen. Whilst the pharmacological profile of the site labelled by [3H]-BRL46470 is directly comparable to that labelled by [3H]-granisetron, [3H]-BRL46470 consistently labelled approximately twice the density of sites compared to [3H]-granisetron in the same tissue homogenates prepared from rat cortex/hippocampus, ratileum, NG108-15 cells and HEK-5-HT3As cells.

Involvement of 5-HT3 receptor in the pressure-induced increase in striatal and accumbens dopamine release and the occurrence of behavioral disorders in free-moving rats

Rats exposed to high pressure developed locomotor and motor activity (LMA) that correlated with an increase of DA release in both the nucleus accumbens and the caudate-putamen. We investigated the effects of the 5-HT3 receptor antagonist MDL 72222 on these pressure-induced neurochemical and behavioral disorders. MDL 72222 totally blocked the pressure-induced increase in accumbens DA release and the development of LMA, whereas it only reduced the increase in striatal DA release. This suggest that both LMA and the increase of DA release in the nucleus accumbens, but not in the caudate-putamen, could specifically result from a 5-HT3 receptor activation in rats exposed to high pressure.

Differential effect of buspirone and diazepam on negative contrast in one-way avoidance learning

The main aim of the present work was to investigate the effect of buspirone, a 5-HT1A receptor agonist, on successive negative contrast in one-way avoidance learning. Successive negative contrast was induced by shifting rats from a large reward (30 s spent in the safe compartment) to a small reward (1 s). Acute administration of buspirone (0.25, 0.5, 0.75 and 1.0 mg/kg i.p.) did not attenuate the contrast effect, as opposed to that observed for diazepam (1 mg/kg i.p.). The highest dose of buspirone used, however, did interfere with the learning of the avoidance response itself. Chronic buspirone (20 days, 0.5 and 0.75 mg/kg i.p.) did not have any effect on successive negative contrast either. Overall, these results could suggest that the 5-HT1A receptor is not involved in the negative contrast effect studied, quite different to that observed for the gamma-aminobutyric acid (GABA) system. The findings are compared to results obtained with animal models selectively sensitive to some anxiolytic drugs, as are the so-called 'conflict models'.

Evidence that the amygdala is involved in the inhibitory effects of 5-HT3 receptor antagonists on alcohol drinking in rats

Two 5-HT3 receptor antagonists, tropisetron (1 and 10 ng) and ondansetron (10 and 100 ng) were tested for effects on ethanol drinking in Wistar male rats after bilateral microinjection into the amygdala. The animals had limited access (2 h/day) to the 10% (v/v) ethanol solution, food and water were available ad lib during the scheduled access period. Both drugs caused a decrease in ethanol drinking. Tropisetron (1 and 10 ng) decreased ethanol intake during the first hour of access. The lower dose (10 ng) of ondansetron was more effective than the higher (100 ng) dose. The finding implicates amygdaloid 5-HT3 receptors in the mechanism of ethanol intake in Wistar rats.

The pharmacology of VA21B7: an atypical 5-HT3 receptor antagonist with anxiolytic-like properties in animal models

VA21B7 (3-[2-(4'-piperonylpiperazinyl) indolyl] carboxaldehyde) was synthesized as a potential 5-HT3 receptor antagonist. Even though VA21B7 showed a higher affinity towards 5-HT3 receptors as compared to other receptors studied, it was not a potent 5-HT3 receptor antagonist either in the periphery or in the brain. In a simple animal model of anxiety such as the two-compartment box in mice, a remarkable anxiolytic-like effect was found at doses of 2-500 micrograms/kg IP and also at low oral doses, in the microgram range. These drug doses did not produce any significant effect on spontaneous motor activity of mice. The anxiolytic profile of VA21B7 was further explored using other models of anxiety in rats such as the elevated plus-maze and punished-drinking. VA21B7 was compared with standard 5-HT3 receptor antagonists such as ondansetron, tropisetron and granisetron, with the 5-HT1A agent buspirone and with diazepam. In the plus-maze, VA21B7 showed an anxiolytic-like profile after doses of 0.25-0.5 mg/kg IP or 2-4 mg/kg PO which did not modify the number of total entries into the open and closed arms of the maze. Diazepam, granisetron and tropisetron were also effective in this test but not ondansetron and buspirone. VA21B7 was also able to release suppressed behaviour in the punished-drinking test. The dose-response curve was bell-shaped with a peak at 2-4 mg/kg. At variance with other studies, 5-HT3 receptor antagonists also increased the number of shocks taken in this test and the dose-response curve was also bell-shaped. VA21B7 was not anticonvulsant like diazepam, its anxiolytic action in the light/dark test was not flumazenil-sensitive and there was no rebound anxiogenic effect on withdrawal from chronic VA21B7 treatment for 15 consecutive days. Moreover, VA21B7 was not amnesic like the benzodiazepines but low doses of 2-4 mg/kg reduced the memory deficits induced in rats by scopolamine. Much higher doses were necessary to decrease spontaneous motor activity in rats. Since VA21B7 appears to be well tolerated in rodents at high doses, we think that it is of potential interest as an anxiolytic in humans.

5-HT1A and muscarinic acetylcholine receptors jointly regulate passive avoidance behavior

The present study was designed to investigate the effects of combined stimulation of 5-HT1A or 5-HT2 receptors and blockade of muscarinic acetylcholine receptors on passive avoidance behavior. Administration of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A receptor agonist, and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), a 5-HT2 receptor agonist, impaired passive avoidance acquisition (pre-training injections) and consolidation (post-training injections) performance. Ketanserin, a 5-HT2 receptor antagonist, blocked the performance-impairing effect of DOI on passive avoidance consolidation. Interestingly, 5-HT receptor agonists may affect passive avoidance consolidation only during the immediate post-training period, as passive avoidance testing performance was not modulated by 8-OH-DPAT or DOI injected 30 min after the training trial. Furthermore, passive avoidance retention (pre-testing injections) performance was impaired only by the highest dose of 8-OH-DPAT, and DOI had no effect on passive avoidance retention. Next, the effects of combined 5-HT and acetylcholine receptor manipulations on passive avoidance behavior were studied. The effects on passive avoidance behavior of a combination of subthreshold doses of scopolamine, a muscarinic acetylcholine receptor antagonist, and 8-OH-DPAT were compared to those of a single high dose of scopolamine. A combination of small doses of scopolamine and 8-OH-DPAT impaired acquisition and consolidation of passive avoidance performance, but a single high dose of scopolamine impaired only acquisition performance. The small dose of 8-OH-DPAT also aggravated medial septal lesion-induced passive avoidance acquisition and consolidation failure. The combination of small doses of scopolamine and DOI had no effect on passive avoidance behavior. Peripherally acting scopolamine methylbromide alone or in combination with 8-OH-DPAT had no effect on passive avoidance performance. Motor activity in a swimming pool was altered by single and combined drug treatments; high doses of 8-OH-DPAT and scopolamine, and the combination of small doses of 8-OH-DPAT + scopolamine increased speed of swimming. Medial septum-lesioning also increased speed of swimming but the speed was not increased further by 8-OH-DPAT. The present data suggest that behavioral defect caused by hypostimulation of muscarinic acetylcholine receptors is aggravated by concurrent 5-HT1A receptor stimulation.

Ondansetron, a selective 5-HT3 receptor antagonist, reduces palatable food consumption in the nondeprived rat

This study investigated the effects of ondansetron, a selective 5-HT3 receptor antagonist, on palatable food consumption in nondeprived male rats, under conditions of familiarity. The results showed that ondansetron (3.0-30 micrograms/kg, i.p.) significantly reduced food intake at each dose tested. The reduction in food intake was due not to a change in the rate of eating but to a reduction in the time spent eating. This, in turn, was due to a reduction in the mean duration of feeding bouts but not due to a change in the frequency of feeding bouts. Hence, the feeding-suppressant effect of ondansetron resulted from a quite specific alteration in the microstructural characteristics of feeding behaviour. In the 60-min observation period, ondansetron did not affect either locomotor activity or rearing, indicating that it did not have general excitatory or behavioural-suppressant effects. Following ondansetron, animals continued to show a typical decline in feeding over time, indicative of the development of within-meal satiety, but the level of feeding was reduced in such a way as to suggest that ondansetron enhances satiety. As a result, as feeding declined, the level of grooming which typically follows the end of feeding, was increased in ondansetron-treated animals. In a supplementary experiment, ondansetron had no effect on deprivation-induced feeding. Present evidence does not allow these data for a 5-HT3 receptor antagonist to be easily accommodated into the major current hypothesis dealing with serotonergic control of feeding responses. Therefore, the role of 5-HT3 receptor-mediated changes in ingestive behaviour requires further investigation.

Receptors for 5-hydroxytryptamine: current perspectives on classification and nomenclature

With the increasing number of 5-HT receptors recently identified, using molecular biology techniques, the classification of 5-HT receptors is under review. An integrated approach is proposed to include operational and transductional as well as structural criteria for definitive receptor characterization. On this basis the existence of as many as seven classes of 5-HT receptor are recognized although only the 5-HT1, 5-HT2 and 5-HT3 receptor classes are well defined.

Distribution and characterization of the [3H]granisetron-labelled 5-HT3 receptor in the human forebrain

The present study has demonstrated the distribution of [3H]granisetron-labelled 5-HT3 receptors in the human forebrain with relatively high levels of this receptor in homogenates of hippocampus, caudate nucleus, putamen, nucleus accumbens and amygdala. Lower levels of 5-HT3 receptors were found in other brain regions and the cervical vagus nerve. Pharmacological characterization of the labelled 5-HT3 receptor in human putamen homogenates identified a relatively low affinity for d-tubocurarine compared to the 5-HT3 receptor in NG108-15 neuroblastoma-glioma cell homogenates. In contrast, the affinities of 19 other 5-HT3 receptor ligands were not significantly different for the [3H]granisetron-labelled receptor in these two preparations. Such findings indicate that the human putamen 5-HT3 receptor displays a unique pharmacology which may have significance given the reported clinical potential of compounds active at this receptor when assessed in animal models of disease.

Labelling of 5-HT3 receptor recognition sites in the rat brain using the agonist radioligand [3H]meta-chlorophenylbiguanide

The binding of the tritiated derivative of the 5-HT3 receptor agonist meta-chlorophenylbiguanide ([3H]mCPBG) to rat cortical homogenates and whole rat brain sections was assessed in an attempt to further investigate the binding of agonists to the 5-HT3 receptor. In crude homogenates of rat cortex, no reproducible specific [3H]mCPBG (1.0 nM) binding (defined by either 10 microM granisetron, 100 microM 5-HT or 100 nM 'cold' mCPBG) was detected. Using autoradiographic techniques, in rat hindbrain sections, [3H]mCPBG (1.0 nM) labelled a differential distribution of specific binding sites (defined by the inclusion of granisetron, 1.0 microM). Specific binding was only detected within the dorsal vagal complex (nucleus tractus solitarius, area postrema and dorsal motor nucleus of the vagus nerve). An identical distribution of specific binding was detected in adjacent sections incubated with the selective 5-HT3 receptor radioligand, [3H](S)-zacopride (0.5 nM; non-specific binding defined by the inclusion of granisetron, 1.0 microM). No reproducible specific [3H]mCPBG (1.0 nM) binding (defined by the inclusion of granisetron, 1.0 microM) was detected within the rat forebrain. In contrast, [3H](S)-zacopride (0.5 nM) labelled specific sites (defined by the inclusion of granisetron, 1.0 microM) in some limbic brain structures (e.g. cerebral cortex, hippocampus, amygdala). These studies indicate that [3H]mCPBG labels the 5-HT3 receptor in rat brain tissue. However, the relatively high level of non-specific binding associated with this radioligand appears to mask the specific binding in regions which do not express relatively high densities of the 5-HT3 receptor.

Serotonergic innervation of the hippocampus and nucleus accumbens septi and the anxiolytic-like action of the 5-HT3 receptor antagonists

The roles of hippocampus (HP) and the nucleus accumbens septi (NAS) in the anxiolytic activity of two 5-HT3 receptor antagonists were studied in two animal models of anxiety, in rats. Injection of tropisetron (0.005 and 0.01 microgram) or ondansetron (1.0 and 2.5 micrograms) into the hippocampus increased punished consumption of water in the Vogel conflict test. In the open field test neither 5-HT3 receptor antagonists had anxiolytic-like effects. Tropisetron (0.01 and 0.025 microgram) injected into the NAS caused a marked increase in punished drinking, while ondansetron (0.01-15.0 micrograms) had no effect. In the open field test, tropisetron (0.001, 0.005 and 0.01 microgram) and ondansetron (1.0 and 2.5 micrograms) given to the NAS increased the number of entries into the central part of the open-field, and the time spent in the central sector of the arena. Depletion of 5-HT significantly enhanced the anxiolytic-like effect of intra-NAS-injected tropisetron in the open field, at the dose of 0.005 microgram. Moreover, 5,7-DHT lesions produced a tendency to increase motor activity in tropisetron-treated rats. Both hippocampal and accumbens 5-HT3 receptors seem to contribute to the anxiolytic-like effects of 5-HT3 antagonists in the Vogel test. It also appears that this effect of 5-HT3 receptor antagonists is related to their action on postsynaptic 5-HT3 receptors within the NAS, and depends on the functional state of the 5-HT innervation ascending from the raphe nuclei. Thus, the present data add more arguments for the more specific involvement of this limbic nucleus in emotional control.

Reduced levels of 5-HT3 receptor recognition sites in the putamen of patients with Huntington's disease

The present study assessed 5-HT3 receptor recognition site levels in homogenates of putamen derived from patients with clinically and neurochemically diagnosed Huntington's disease or Parkinson's disease and those from age-, sex- and post-mortem delay-matched neurologically and psychiatrically normal patients to investigate the cellular location of 5-HT3 receptors in the human putamen. Specific [3H]granisetron (0.91 nM) binding (defined by ondansetron, 10 microM) was significantly reduced in putamen homogenates from eight out of ten patients with Huntington's disease compared to similar homogenates from 'control' patients (72 +/- 6 and 39 +/- 8 fmol/g wet weight, mean +/- S.E.M., n = 10 and 8, tissue from 'control' and Huntington's disease patients, respectively, P = 0.004). In contrast, specific [3H]granisetron (1.04 nM) binding levels were similar in putamen homogenates from patients with Parkinson's disease when compared to homogenates from 'control' patients. The present results indicate that at least a proportion of the 5-HT3 receptor population in the human putamen is located on neurones that have their cell bodies within this brain region and that these receptors are not primarily located on dopamine neurone terminals in the human putamen.

The selective 5-HT3 receptor antagonist, ondansetron, augments the anorectic effect of d-amphetamine in nondeprived rats

Previous behavioural studies have shown that 5-hydroxytryptamine3 (5-HT3) receptor antagonists either block or have no effect on amphetamine-induced effects. The present experiments investigated whether or not the highly selective 5-HT3 receptor antagonist ondansetron would affect the anorectic effect of a small dose (1 mg/kg) of d-amphetamine. Nondeprived male rats were tested in two feeding paradigms: consumption of a palatable sweetened mash and ingestion of a 3% sucrose solution. Ondansetron (10-100 micrograms/kg) did not antagonize amphetamine-induced anorexia; instead, in both paradigms consumption was reduced still further when the 5-HT3 antagonist was given in conjunction with amphetamine. Ondansetron given alone had significant effects on consumption, but the direction of the effect differed according to the paradigm. Sweetened mash intake was significantly increased at 30 and 100 micrograms/kg, while sucrose ingestion was significantly reduced at 10 and 30 micrograms/kg ondansetron. It is suggested that ondansetron has two opposing effects on intake, one of which (hyperphagia) can be masked by d-amphetamine, leaving an anorectic effect that augments that of d-amphetamine.

Effects of d-fenfluramine, MK-212, and ondansetron on saline drinking in two-choice tests in the rehydrating rat

The aim of the present studies was to investigate the effects of serotonergic compounds on preference for isotonic saline and aversion to hypertonic saline, respectively. Twenty-two-hour water-deprived rats were divided into two groups: The first was given a choice between 0.9% saline and water in a 30-min test; the second was given a choice between 1.8% saline and water. Animals were tested following administration of d-fenfluramine, the 5-HT1C receptor agonist 6-chloro-2-(1-piperazinyl)pyrazine (MK-212), and the 5-HT3 receptor antagonist ondansetron. d-Fenfluramine (0.3-3.0 mg/kg) did not reduce 0.9% saline preference; instead, at 0.3 mg/kg there was a significant increase in saline drinking. In contrast, MK-212 (0.3-3.0 mg/kg) abolished the preference for isotonic saline whereas ondansetron (10-100 micrograms/kg) had no effect. d-Fenfluramine and MK-212 reduced hypertonic saline drinking, although at the highest dose for each drug water drinking was also reduced. These data add further to the evidence for an important serotonergic involvement in the control of saline drinking and preference in the rat.

The effects of tacrine and zacopride on the performance of adult rats in the working memory task

1. The present study investigated the effects of tacrine (an inhibitor of acetylcholinesterase) and zacopride (the antagonist of 5-HT3 receptors) on the performance of adult rats in a continuous operant delayed non-matching to position task assessing spatial working memory. 2. Adult rats had a decline in the percent correct responses at the longest delays (16 and 30 sec) in this task. Tacrine (1.0 mg/kg) or zacopride (0.0025, 0.05, 1.0 mg/kg) did not increase the percent correct responses at any time delays. The higher dose of tacrine reduced behavioural activity (e.g. the decreased number of trials completed and increased sample press latency) of rats during memory testing, and it slightly increased choice accuracy across all the delays. 3. The combination of zacopride (1.0 mg/kg) and tacrine (1.0 mg/kg) increased the percent correct responses at the shortest delays, but not at the longest delays. 4. These results indicate a non-mnemonic improvement in the accuracy performance of rats, and they suggest that the effects of acute, systemic administrations of zacopride (which is thought to increase the release of acetylcholine) or/and tacrine (which inhibits the breakdown of acetylcholine) do not improve spatial working/short-term memory in rats.

Lack of effects of 5-HT3 antagonists on normal and morphine-attenuated sexual behaviours in female and male rats

Although 5-HT1 and 5-HT2 receptor activity is known to influence copulation, the effects of 5-HT3 receptor-selective drugs on sexual activity have yet to be systematically studied. The following experiments investigated the effects of the 5-HT3-selective antagonists MDL 72222, ondansetron and ICS 205-930 on female sexual behaviour; male rats were studied using ondansetron and granisetron. These compounds influenced neither male nor female copulatory behaviours, suggesting that 5-HT3 receptors contribute little to the modulation of sexual activity. 5-HT3 receptor antagonists block certain opioid-induced behaviours and opioids selectively inhibit sexual behaviours; therefore, the ability of ondansetron and ICS 205-930 to modify morphine-attenuated copulatory activity was also tested. While morphine inhibited copulation, 5-HT3 antagonists failed to reverse the effects.

Nervous system distribution of the serotonin 5-HT3 receptor mRNA

The serotonin 5-HT3 receptor subtype has been implicated in many brain functions. Antagonists of this receptor have anxiolytic and antiemetic effects in humans and in animal models. To determine with cellular resolution the distribution of 5-HT3 receptor mRNA, in situ hybridization was performed in sections of mouse brain and dorsal root ganglia. Scattered labeled cells were observed throughout cortical regions, with highest densities in the piriform, cingulate, and entorhinal areas. Strong hybridization signals were seen in the hippocampal formation, where expression appeared primarily in interneurons. Labeled cells were most abundant in the posteroventral hippocampal region, particularly in the lacunosum moleculare layer of CA1. This distribution suggests that 5-HT3 receptors may mediate the known serotonergic inhibition of pyramidal cell populations via excitation of inhibitory interneurons. Labeled cells were also observed in the major subdivisions of the amygdaloid complex, the olfactory bulb, the trochlear nerve nucleus, the dorsal tegmental region, the facial nerve nucleus, the nucleus of the spinal tract of the trigeminal nerve, and the spinal cord dorsal horn. In the periphery, intense hybridization signals were seen in a subpopulation of cells in dorsal root ganglia. The data correlate generally with physiological, behavioral, and receptor autoradiographic studies, provide cellular resolution, and reveal regions of receptor expression not previously observed. The distribution of 5-HT3 receptor mRNA is consistent with roles for the receptor in cognition and affect and in the modulation of sensory input.

Effects of 5-hydroxytryptamine3 antagonist, ondansetron, on cigarette smoking, smoke exposure, and mood in humans

Recent studies in the animal laboratory indicate that 5-hydroxytryptamine3 (5-HT3) antagonists reduce the reinforcing effects of several psychoactive drugs, including nicotine. The purpose of our study was to determine if ondansetron, a selective 5-HT3 antagonist, affected tobacco cigarette consumption in smokers. In the first experiment, a prospective, crossover, placebo-controlled trial was used in which subjects (N = 7) were exposed in an inpatient research unit to 0, 0.15, 0.3, or 0.45 mg/kg ondansetron in three equally divided doses given 4 h apart. In the second experiment, seven different subjects were exposed to the same trial except the dose range was reduced to about 10%: 0, 0.01, 0.02, or 0.04 mg/kg. In each experiment, order of dosing conditions was determined by a Latin square design. Dependent measures included number of cigarettes smoked during the 24-h session, biologic exposure levels (carbon monoxide and plasma nicotine levels), and mood. Number of cigarettes smoked and biologic exposure levels did not differ across drug conditions in either experiment. There were also no effects of ondansetron on mood. From our study results, we conclude that acute administration of ondansetron, at doses appropriate for antiemesis or at markedly lower doses, does not alter tobacco consumption or smoke exposure in humans.

5-HT receptors as targets for the development of novel anxiolytic drugs: models, mechanisms and future directions

The introduction of buspirone for the treatment of anxiety, together with the eventual suggestion of a mode of action involving the serotonin (5-HT)1A receptor subtype, has generated considerable research activity and renewed interest in the potential role of 5-HT in anxiety. The further identification of multiple 5-HT1 receptors, coupled with the possibility that these subtypes potentially are involved in discrete biobehavioral regulation and pathophysiological conditions, has greatly expanded the search for tools capable of probing these receptors and has raised hopes for a new generation of more specific compounds to treat other disorders associated with the 5-HT system such as depression, aggression, and sleep and eating disturbances. The involvement of 5-HT in anxiety has prompted a careful reevaluation of several traditional areas of research. This has included those methods used in the in vivo evaluation of drugs in preclinical animal test procedures used to assess potential anxiolytic activity, as well as the mechanisms associated with adaptive changes occurring during long-term drug administration. The proliferation of various procedures for studying the anxiolytic effects of 5-HT drugs has not always been accompanied by systematic behavioral and pharmacological validation. At the present time, this area of research is characterized by numerous inconsistent findings. Procedures that are objective and impartial to the behavioral effects of drugs provide distinct advantages for addressing some of these issues, as will the results from carefully controlled clinical studies. The main objective of this article is to provide an overview of the recent developments in research involving the 5-HT system and anxiety. The emphasis will be on the 5-HT1 receptor system and a review of the results in the predominant animal models used to evaluate these drugs, as well as an overview of the mechanisms currently believed to be responsible for the therapeutic activity of this class of compounds. Studies with the pigeon are reviewed, since this species appears distinctly sensitive to the anxiolytic-like effects of 5-HT1A drugs in conflict procedures. Although chronic administration of 5-HT1A drugs appears necessary for clinical anxiolytic and antidepressant activity, the most noteworthy neuropharmacological effects in animals seem to occur in 5-HT2 and, perhaps, 5-HT3 receptors which are downregulated. Studies summarizing the activity of drugs interacting with 5-HT1C/2 and 5-HT3 receptor sites are also discussed as they too may be involved in anxiety or the actions of anxiolytic drugs.(ABSTRACT TRUNCATED AT 400 WORDS)

Differential modulation of extracellular levels of 5-hydroxytryptamine in the rat frontal cortex by (R)- and (S)-zacopride

1. The ability of various anxiolytic and potential anxiolytic agents to modify 5-hydroxytryptamine (5-HT) release in the frontal cortex of the rat was assessed by the microdialysis technique. 2. The benzodiazepine receptor agonist, diazepam (2.5 mg kg-1, i.p.), the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT, 0.32 mg kg-1, s.c.) and the 5-HT1A receptor partial agonist buspirone (4.0 mg kg-1, i.p.) maximally reduced extracellular levels of 5-HT in the rat frontal cortex by approximately 50-60%, 70-80% and 30-40%, respectively. 3. (R)-zacopride (1.0-100 micrograms kg-1, i.p.) dose-dependently reduced extracellular levels of 5-HT in the rat frontal cortex (approximately 80% maximal reduction) whereas the other 5-HT3 receptor antagonists ondansetron (10 micrograms kg-1, i.p.) and (S)-zacopride (10-100 micrograms kg-1, i.p.) were ineffective. 4. In contrast to (S)-zacopride (100 nM; administered via the microdialysis probe), (R)-zacopride (1.0-100 nM; administered via the microdialysis probe) induced a concentration-dependent reduction in extracellular levels of 5-HT in the rat frontal cortex (approximately 70% maximal reduction). 5. In contrast to ondansetron (100 micrograms kg-1, i.p.), (S)-zacopride (10-100 micrograms kg-1, i.p.) dose-dependently reversed the (R)-zacopride (10 micrograms kg-1, i.p.) induced reduction in extracellular levels of 5-HT in the rat frontal cortex. The highest dose of (S)-zacopride (100 micrograms kg-1, i.p.) completely prevented the (R)-zacopride response.In addition, (S)-zacopride (100 nM; administered via the microdialysis probe) attenuated the inhibitory action of (R)-zacopride (10 nM; administered via the microdialysis probe) on extracellular levels of 5-HT in the rat frontal cortex.6. In conclusion, the present study provides further evidence of the ability of diazepam, 8-OH-DPAT and buspirone to reduce the activity of the central 5-hydroxytryptaminergic system in vivo. Furthermore,the results indicate that the ability of (R)-zacopride to reduce the in vivo release of 5-HT in the rat frontal cortex does not correlate with its 5-HT3 receptor antagonism. However, the differential affinity of (R)- and (S)-zacopride for a (S)-zacopride-insensitive (R)-zacopride site in rat cerebral cortex mirrors the relative activity of the two zacopride stereoisomers to modify the in vivo release of 5-HT in the frontal cortex of the rat and their ability to release suppressed behaviour in animal models of anxiety.

Pharmacological comparison of the rat and guinea-pig cortical high affinity 5-hydroxytryptamine uptake system

The pharmacological characteristics of the high affinity [3H]5-hydroxytryptamine ([3H]5-HT) uptake system were investigated in the cerebral cortex of the rat and guinea-pig. In crude cortical synaptosomal preparations from the rat and guinea-pig, [3H]5-HT accumulated with high affinity (Km, 72 +/- 12 and 57 +/- 14 nM for rat and guinea-pig cortical synaptosomal preparation, respectively, mean +/- SEM, N = 5) and with a comparable maximum activity (Vmax, 1.22 +/- 0.21 and 0.90 +/- 0.19 pmol/min/mg protein for rat and guinea-pig cortical synaptosomal preparation, respectively, mean +/- SEM, N = 5). Competition studies employing a range of structurally diverse competing compounds showed that the [3H]5-HT uptake was pharmacologically similar in both preparations. However, citalopram possessed approximately 10-fold weaker affinity to prevent [3H]5-HT uptake in the guinea-pig preparation when compared to the rat and all of the tricyclic antidepressants assessed in the present studies (amitriptyline, nortriptyline, desipramine and imipramine) displayed higher affinity in the guinea-pig preparation when compared to the rat. It is concluded that the high affinity 5-HT uptake systems in the rat and guinea-pig cortex are similar but may not be identical.

Agonist interactions with 5-HT3 receptor recognition sites in the rat entorhinal cortex labelled by structurally diverse radioligands

1. The pharmacological properties of 5-HT3 receptor recognition sites labelled with [3H]-(S)-zacopride, [3H]-LY278,584, [3H]-granisetron and [3H]-GR67330 in membranes prepared from the rat entorhinal cortex were investigated to assess the presence of cooperativity within the 5-HT3 receptor complex. 2. In rat entorhinal cortex homogenates, [3H]-(S)-zacopride, [3H]-LY278,584, [3H]-granisetron and [3H]-GR67330 labelled homogeneous densities of recognition sites (defined by granisetron, 10 microM) with high affinity (Bmax = 75 +/- 5, 53 +/- 5, 92 +/- 6 and 79 +/- 6 fmol mg-1 protein, respectively; pKd = 9.41 +/- 0.04, 8.69 +/- 0.14, 8.81 +/- 0.06 and 10.14 +/- 0.04 for [3H]-(S)-zacopride, [3H]-LY278,584, [3H]-granisetron and [3H]-GR67330, respectively, n = 3-8). 3. Quipazine and granisetron competed for the binding of each of the radioligands in the rat entorhinal cortex preparation at low nanomolar concentrations (pIC50; quipazine 9.38-8.51, granisetron 8.62-8.03), whilst the agonists, 5-hydroxytryptamine (5-HT), phenylbiguanide (PBG) and 2-methyl-5-HT competed at sub-micromolar concentrations (pIC50; 5-HT 7.16-6.42, PBG 7.52-6.40, 2-methyl-5-HT 7.38-6.09). 4. Competition curves generated with increasing concentrations of quipazine, PBG, 5-HT and 2-methyl-5-HT displayed Hill coefficients greater than unity when the 5-HT3 receptor recognition sites in the entorhinal cortex preparation were labelled with [3H]-LY278,584, [3H]-granisetron and [3H]-GR67330. These competing compounds displayed Hill coefficients of around unity when the sites were labelled with [3H]-(S)-zacopride. Competition for the binding of [3H]-(S)-zacopride, [3H]-LY278,584, [3H]-granisetron and [3H]-GR67330 by granisetron generated Hill coefficients around unity.5. The nature of the interaction of competing compounds (quipazine, granisetron, PBG, 5-HT, 2-methyl-5-HT) for the [3H]-(S)-zacopride binding site in the rat entorhinal cortex preparation was not altered by the removal of the Krebs ions or the addition of the monoamine oxidase inhibitor, pargyline, to the HEPES/Krebs buffer.6. In conclusion, the present studies provide further evidence towards the presence of cooperativity within the 5-HT3 receptor macromolecule and indicate that either [3H]-(S)-zacopride labels a different site on the receptor complex from [3H]-LY278,584, [3H]-granisetron or [3H]-GR67330, or it binds in such a manner as to prevent the conformatory change in the receptor protein responsible for the cooperative binding of agonists (and quipazine).

Granisetron. A review of its pharmacological properties and therapeutic use as an antiemetic

Granisetron (BRL 43694) is a highly selective 5-HT3 receptor antagonist which possesses significant antiemetic activity, likely mediated through antagonism of 5-HT3 receptors on abdominal vagal afferents and possibly in or near the chemoreceptor trigger zone. Clinical trials in cancer patients demonstrate that, compared with placebo, granisetron significantly reduces the incidence of nausea and vomiting for 24 hours after administration of high-dose cisplatin. In large comparative trials, 70% of patients who received granisetron prior to cisplatin or other chemotherapy experienced complete inhibition of vomiting with little or no nausea for 24 hours after antineoplastic administration; these results were similar to those obtained with high-dose metoclopramide plus dexamethasone, and superior to a combination of chlorpromazine plus dexamethasone, or prochlorperazine plus dexamethasone, or methylprednisolone monotherapy. The most frequently reported adverse event associated with granisetron administration is headache which occurs in about 10 to 15% of patients while constipation, somnolence, diarrhoea and minor transient changes in blood pressure have been reported less frequently. Extrapyramidal effects, which can occur with high-dose metoclopramide and may be a limiting factor in its use, have not been noted with granisetron administration. Thus, granisetron is an effective, well tolerated and easily administered agent for the prophylaxis of nausea and vomiting induced by cancer chemotherapy which appears to be devoid of extrapyramidal side effects associated with metoclopramide. As a member of a new class of drugs, the selective 5-HT3 receptor antagonists, granisetron provides the medical oncologist with a new, potentially more acceptable antiemetic therapy.