Antagonism of [3H]zacopride binding to 5-HT3 recognition sites by its (R) and (S) enantiomers

Synthesis and evaluation of (S)-[(18)F]fesetron in the rat brain as a potential PET imaging agent for serotonin 5-HT3 receptors

Serotonin 5-HT3 receptors are involved in various brain functions including as an emesis target during cancer chemotherapy. We report here the development of (S)-2,3-dimethoxy-5-(3'-[(18)F]fluoropropyl)-N-(1-azabicyclo[2.2.2]oct-3-yl)benzamide ([(18)F]fesetron) as a potential PET imaging agent for serotonin 5-HT3 receptors. By radiolabeling((S)-2,3-dimethoxy-5-(3'-tosyloxypropyl)-N-(1-azabicyclo[2.2.2]oct-3-yl)benzamide) with fluorine-18, (S)-[(18)F]fesetron was obtained in 5 to 10% decay-corrected yields and with specific activities >74GBq/μmol at the end of radiosynthesis. PET imaging in rats showed low uptake of [(18)F]fesetron in the brain with retention of binding in the striatal and cerebellar regions. Using colliculi as a reference region, ratios were 3.4 for striata and 2.5 for cerebellum. Ex vivo brain PET analysis displayed binding of [(18)F]fesetron in the hippocampus, striatum and cerebellar regions. Cerebellar regions corresponded to area postrema and nucleus tract solitaris known to contain 5-HT3 receptors. Dorsal hippocampus showed the highest uptake with ratio of >17 with respect to colliculi, while area postrema and striata had ratios of >10. Thus, [(18)F]fesetron exhibited a unique binding profile to rat brain regions known to contain significant amounts of serotonin 5-HT3 receptors. However, the very low brain uptake limits its usefulness as a PET radiotracer in this animal model.

The neuronal 5-HT3 receptor network after 20 years of research — Evolving concepts in management of pain and inflammation

The 5-HT3 receptor is a pentameric ligand-gated cation channel which is found in the central and peripheral nervous system and on extraneuronal locations like lymphocytes, monocytes and fetal tissue. Five monomer subtypes, the 5-HT(3A-E) subunits, have been identified which show differences in the amino-terminal and the transmembrane region. The functional relevance of different receptor compositions is not yet clarified. 5-HT3 receptors are located predominantly in CNS regions that are involved in the integration of the vomiting reflex, pain processing, the reward system and anxiety control. The preferential localization on nerve endings is consistent with a physiological role of 5-HT3 receptors in the control of neurotransmitter release such as dopamine, cholecystokinin, glutamate, acetylcholine, GABA, substance P, or serotonin itself. 5-HT3-receptor agonists cause unpleasant effects like nausea and anxiety, and no clinical use has been considered. In contrast, the introduction of 5-HT3-receptor antagonists for chemotherapy-induced vomiting was extremely successful. After development of other gastrointestinal indications like postoperative vomiting and diarrhea-predominant irritable bowel syndrome recent research focuses on rheumatological indications such as fibromyalgia, rheumatoid arthritis and tendinopathies. Positive effects have also been observed for pain syndromes such as chronic neuropathic pain and migraine. These effects seem to be related to substance P-mediated inflammation and hyperalgesia. Furthermore, antiinflammatory and immunomodulatory properties have been observed for 5-HT3-receptor antagonists which might explain promising findings in systemic sclerosis and other immunological conditions. For all of these innovative indications the optimal dosing schedule is a crucial issue, since a bell-shaped dose-response curve has been observed repeatedly for 5-HT3-receptor antagonists, particularly in CNS effects.

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.

[3H]-Mesulergine labels 5-HT7 sites in rat brain and guinea-pig ileum but not rat jejunum

1. The primary aim of this investigation was to determine whether binding sites corresponding to the 5-HT7 receptor could be detected in smooth muscle of the rat jejunum. Binding studies in rat brain (whole brain minus cerebellum) and guinea-pig ileal longitudinal muscle were also undertaken in order to compare the binding characteristics of these tissues. Studies were performed using [3H]-mesulergine, as it has a high affinity for 5-HT7 receptors. 2. In the rat brain and guinea-pig ileum, pKD values for [3H]-mesulergine of 8.0 +/- 0.04 and 7.9 +/- 0.11 (n = 3) and Bmax values of 9.9 +/- 0.3 and 21.5 +/- 4.9 fmol mg(-1) protein were obtained respectively, but no binding was detected in the rat jejunum. [3H]-mesulergine binding in the rat brain and guinea-pig ileum was displaced with the agonists 5-carboxamidotryptamine (5-CT) > 5-hydroxytryptamine (5-HT) > or = 5-methoxytryptamine (5-MeOT) > sumatriptan and the antagonists risperidone > or = LSD > or = metergoline > ritanserin > > pindolol. 3. Despite the lack of [3H]-mesulergine binding in the rat jejunum, functional studies undertaken revealed a biphasic contractile response to 5-HT which was partly blocked by ondansetron (1 microM). The residual response was present in over 50% of tissues studied and was found to be inhibited by risperidone > LSD > metergoline > mesulergine = ritanserin > pindolol, but was unaffected by RS 102221 (3 microM), cinanserin (30 nM), yohimbine (0.1 microM) and GR 113808 (1 microM). In addition, the agonist order of potency was 5-CT > 5-HT > 5-MeOT > sumatriptan. 4. In conclusion, binding studies performed with [3H]-mesulergine were able to detect 5-HT7 sites in rat brain and guinea-pig ileum, but not in rat jejunum, where a functional 5-HT7-like receptor was present.

Signal transduction pathways for serotonin as an intestinal secretagogue

This review presents a signal transduction pathways for serotonin (5-hydroxytryptamine, 5-HT) as an intestinal secretagogue and some recently published related findings. 5-HT is a secretagogue in the small and large intestine of all studied species including pig and man. 5-HT mediates intestinal secretion through activation of at least the epithelial 5-HT2, and neuronal 5-HT3, and 5-HT4 receptors in the submucosal plexus, including a reflex arc. 5-HT activates both a cholinergic and a non-cholinergic pathway in its secretory response. Intracellular mediators include at least eicosanoids (prostaglandin E2), calcium, phosphoinositols (1,4,5-inositol trisphosphate) and maybe nitric oxide and cyclic nucleotides. Pig small intestine appears to be an appropriate model for the human small intestine with respect to the signal transduction pathways for 5-HT as an intestinal secretagogue. Species and segmental differences in the signal transduction pathways for 5-HT as an intestinal secretagogues are discussed together with related news on 5-HT receptors, 5-HT antagonists in clinical use, the enteric nervous system, and intracellular mediators.

Stereoselective effects of (R)- and (S)-zacopride on cognitive performance in a spatial navigation task in rats

In the present studies we investigated the actions of (R)- and (S)-zacopride, potent 5-HT3 receptor antagonists with 5-HT4 receptor agonists properties, on performance in a spatial learning and memory task in rats, the Morris water maze. A significant cognitive/performance deficit, as indicated by the increased escape latency across several trials, was produced by systemic administration of the muscarinic receptor antagonist atropine (30 mg/kg, IP). (R)-zacopride (0.001-1 microgram/kg, but not 10 or 100 micrograms/kg) significantly reduced escape latency in atropine-treated animals. (S)-Zacopride was inactive over the entire dose range examined (0.001-100 micrograms/kg, i.p.). Moreover, pretreatment with (S)-zacopride (1 or 100 micrograms/kg) did not alter the procognitive effects of (R)-zacopride (1 microgram/kg). These data demonstrate that the cognition enhancing properties of zacopride in this model of cholinergic hypofunction are exclusive to its (R)-enantiomer and imply that this action is unrelated to 5-HT, receptor antagonism or 5-HT4 receptor agonism. The possibility that the procognitive effects of (R)-zacopride may be related to actions at the novel "(R)-zacopride site" is discussed.

The enantiomers of zacopride: an intra-species comparison of their potencies in functional and anxiolytic models

1. The 5-HT3 receptor antagonist, zacopride, and its enantiomers, R(+)-zacopride and S(-)-zacopride, were examined in three pharmacological models: (i) 5-HT-induced depolarization of the mouse isolated vagus nerve preparation, (ii) the 5-HT-evoked von Bezold-Jarisch reflex in the mouse, and (iii) the mouse light:dark box model of anxiety. Other standard 5-HT3 receptor antagonists were also included for comparison in these studies. 2. Racemic zacopride, and both of the enantiomers, displayed potent 5-HT3 receptor antagonist activity in the isolated vagus nerve and in the von Bezold-Jarisch model. No 5-HT3 receptor agonist or partial agonist effects of these compounds were detected. 3. In the isolated vagus nerve, R(+)-zacopride and ondansetron were surmountable 5-HT3 receptor antagonists (pA2 values of 9.3 and 8.3, respectively), whereas racemic zacopride, S(-)-zacopride and tropisetron were insurmountable antagonists, markedly suppressing the maximum response to 5-HT. 4. In vivo, racemic zacopride, R(+)-zacopride, S(-)-zacopride and WAY100289 were potent antagonists of the 5-HT-evoked von Bezold-Jarisch reflex, with minimum effective doses (lowest dose required to reduce the reflex by > or = 85%; MED85) of 1.0, 3.0, 0.3 and 3.0 micrograms kg-1, s.c., respectively. 5. Racemic zacopride, R(+)-zacopride and S(-)-zacopride were active in the mouse light:dark box model of anxiety, with similar potencies (minimum effective dose 1 microgram kg-1, s.c.) and similar active dose-ranges (1-1000 micrograms kg-1, s.c.). 6. The doses of racemic zacopride, R( + )-zacopride and S(-)-zacopride required to block 5-HT3receptors in vivo correlated reasonably well with their potencies in an anxiety model within the same species. In these studies, there was no evidence of a marked difference between the anxiolytic potencies ofR( + )-zacopride and S(-)-zacopride.

Pharmacological characterization of 5-hydroxytryptamine3 receptors in murine brain and ileum using the novel radioligand [3H]RS-42358-197: evidence for receptor heterogeneity

Previous studies have demonstrated species-specific differences in 5-hydroxytryptamine3 (5-HT3) receptors, but unequivocal evidence of 5-HT3 receptor subtypes, within a species, has not yet been obtained. The purpose of the current study was to test for heterogeneity in 5-HT3 receptors in murine tissues. 5-HT3 receptors in membranes derived from brain cerebral cortex of CD-1, C57Bl/6, and Swiss Webster mice and ileum of CD-1 mice were labeled with the 5-HT3 receptor antagonist [3H]RS-42358-197. Structurally diverse competing ligands were then used to characterize the binding site. [3H]RS-42358-197 bound with similar affinity in each of the cortical tissues (mean KD = 0.14 nM; range, 0.06-0.32 nM) but bound with lower affinity in ileal tissue (2.5 nM). The density of sites labeled with [3H]RS-42358-197 ranged from 10.4 fmol/mg of protein in Swiss Webster mouse cortex to 44.2 fmol/mg of protein in Sprague-Dawley rat cortex. Displacing ligands produced a pharmacologic profile of the [3H]RS-42358-197 binding site consistent with it being a 5-HT3 receptor: (R)-YM060 > (S)-zacopride > (R)-zacopride > MDL 72222 > 2-methyl-5-HT. However, > or = 10-fold differences in the affinity of certain ligands were found when comparing 5-HT3 binding sites in membranes from cerebral cortex of the different strains of mice and when comparing 5-HT3 binding sites in brain and ileal membranes prepared from the CD-1 mouse strain.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterisation of the non-5-HT3 high-affinity 'R' binding site for (R)-zacopride in brain and other tissues

Previous studies showed that whereas the potent 5-HT3 receptor antagonist (S)-[3H]zacopride only labels 5-HT3 receptor binding sites, the (R)-enantiomer, (R)-[3H]zacopride, labels these receptors and another class of high-affinity binding sites, named the R sites, in membranes from the rat cerebral cortex and NG 108-15 clonal cells (Kidd et al., Eur. J. Pharmacol. 211, 133, 1992). Further studies of R sites revealed that they existed not only in the cerebral cortex but also in various other areas of the rat brain and spinal cord. In addition, R sites were also found in post-mortem human brain tissues. Both in the rat and in man, the regional distribution of central R sites was markedly different from that of 5-HT3 receptors specifically labelled with (S)-[3H]zacopride. Under appropriate conditions for the specific labelling of R sites (with (R)-[3H]zacopride in the presence of 1.0 microM ondansetron to saturate 5-HT3 receptor binding sites--and 0.1 mM mianserin for the determination of non-specific binding), these R sites were also found in rat peripheral tissues (intestine > spleen > kidney > testicles = liver > adrenals > lung > heart). At least in the kidney and the liver, the pharmacological profile of R sites corresponded exactly to that found in NG 108-15 cells. R sites were also detected in membranes from C6 glioma cells and glial cells cultured from the whole cortex of new born rats. In contrast, no specific binding of (R)-[3H]zacopride to R sites could be found in membranes from N1E-115 neuroblastoma cells. Conversely, 5-HT3 receptors could be labelled by (S)-[3H]zacopride in the latter cells but not in C6 glioma and cultured glial cells. As expected from their glial location, the density of R sites increased in the rat hippocampus lesioned with kainic or ibotenic acid to induce local gliosis. In contrast, the density of hippocampal 5-HT3 receptors was unchanged in lesioned rats. Finally, the determination of the apparent molecular size of R sites by radiation inactivation gave a value (approximately 30 kDa) which was significantly lower than that of 5-HT3 receptor binding sites in the rat entorhinal cortex (40 kDa) and NG 108-15 cells (57 kDa). All these data clearly showed that R sites and 5-HT3 receptors are different molecular species. Whether R sites mediate the 5-HT3 receptor-unrelated actions of (R)-zacopride deserves further investigations.

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)

Disposition kinetics of ML-1035 sulfoxide enantiomers and the prochiral sulfide in rats

ML-1035, 4-amino-5-chloro-2-[2-(methylsulfinyl)ethoxy]-N-[2- (diethyl-amino)ethyl]benzamide, is a sulfoxide compound and a racemic gastroprokinetic agent with a chiral center at the sulfur atom. We have investigated the disposition kinetics of (R)-ML-1035 sulfoxide (R) and (S)-ML-1035 sulfoxide (S) after the single enantiomers and the racemic mixture were administered to rats in separate experiments. There was no noticeable chiral inversion after either enantiomer dose. Both enantiomers were rapidly absorbed. After dosing with enantiomers or with the racemate, the resulting plasma concentration-time curve of R was closely parallel to that of S in both intravenous and oral experiments, suggesting that the two enantiomers have approximately the same disposition kinetics. After intravenous enantiomer doses, only S underwent conversion to sulfide, suggesting that sulfidation in the liver is enantioselective. However, the enantioselective sulfidation after intravenous dosing did not introduce a difference in the global plasma disposition profiles between R and S, since the reduction reaction is a minor metabolic process. Other metabolic reactions such as sulfonation and mono-N-desethylations were not enantioselective. After oral administration, conversion to sulfide was observed for both enationers, implicating the existence of a nonhepatic pathway in sulfidation. Administration of a prochiral sulfide dose was associated with an enantioselective sulfoxidation, in which the R/S concentration ratios increased as a function of time. In addition, enatiomeric interaction causing changes in pharmacokinetic parameters was observed after the oral racemate dose, while the interaction is negligible after an intravenous racemate dose, indicating a route dependency in enantiomeric interaction.

5-HT3 receptor antagonist effects of DAT-582, (R) enantiomer of AS-5370

The serotonin 5-HT3 receptor antagonist effects of DAT-582, the (R) enantiomer of AS-5370 ((+/-)-N-[1-methyl-4-(3-methyl-benzyl)hexahydro-1H-1,4-diazepin-6- yl]-1H- indazole-3-carboxamide dihydrochloride), and its antipode were compared with those of AS-5370 and existing 5-HT3 receptor antagonists. In anesthetized rats, DAT-582 antagonized 2-methyl-5-HT-induced bradycardia with an ED50 value of 0.25 microgram/kg i.v., whereas the (S) enantiomer was without effect even at 1000 micrograms/kg i.v. In antagonizing the bradycardia, DAT-582 was as potent as granisetron, slightly more potent than AS-5370, and 2, 5 and 18 times more potent than ondansetron, ICS 205-903 and renzapride, respectively, although it was less potent than zacopride. DAT-582 inhibited cisplatin (10 mg/kg i.v.)-induced emesis in ferrets with an ED50 value of 3.2 micrograms/kg i.v. twice. The antiemetic activity of DAT-582 was more potent than that of the existing 5-HT3 receptor antagonists examined, except zacopride. In contrast, the (S) enantiomer had little effect at 1000 micrograms/kg i.v. twice. In isolated guinea-pig ileum, DAT-582 inhibited 5-HT-induced contractions with an IC50 value of 91 nM, whereas the (S) enantiomer hardly inhibited them even at 1000 nM. These results suggest that DAT-582, the (R) enantiomer of AS-5370, potently and selectively blocks 5-HT3 receptors.

Zacopride and its optical isomers produce stereoselective antagonism of a 2-methylserotonin discriminative stimulus

The serotonin 5-HT3 receptor antagonist effects of zacopride and its optical isomers were evaluated in rats trained to discriminate 5.0 mg/kg of 2-methylserotonin (2-Me 5-HT) from saline in a 2-lever operant task. Zacopride and its enantiomers potently antagonized the 2-Me 5-HT stimulus; on the basis of 50% inhibition (ID50) of drug lever responding the order of potency is S(-)-zacopride (ID50 = 0.05 micrograms/kg) greater than (+/-)-zacopride (ID50 = 0.60 micrograms/kg) greater than R(+)-zacopride (ID50 = 1.6 micrograms/kg). The stereoselectivity and potency ratio of zacopride's isomers for antagonizing the 2-Me 5-HT stimulus parallels their previously reported results in binding studies. It is concluded that zacopride's isomers produce antagonism of a 2-Me 5-HT stimulus by stereoselective interaction at 5-HT3 receptors and that stereochemical factors are important in evaluating the stimulus properties of 5-HT3 agents.

The potent 5-HT3 receptor antagonist (R)-zacopride labels an additional high affinity site in the central nervous system

The binding characteristics of [3H](R)- and [3H](S)-zacopride were investigated in membranes from the rat entorhinal cortex and NG 108-15 clonal cells. In contrast to [3H](S)-zacopride which bound solely to 5-HT3 receptors, [3H](R)-zacopride recognized another class of binding sites, called the (R)-sites, in both membrane preparations. In addition to (R)-zacopride (Ki = 3-11 nM), only (R)-iodo-zacopride, (R)-dechloro-zacopride, prazosin and mianserin exhibited high to moderate affinity for the (R)-sites, whose possible functions remain to be established.

Agonist effects at putative central 5-HT4 receptors in rat hippocampus by R(+)- and S(-)-zacopride; no evidence for stereo-selectivity

The EEG of halothane anaesthetized rats was recorded from an electrode implanted into the hippocampus. In the present study the effect of R(+)- and S(-)-zacopride, administered intra-cerebroventricularly, on different frequency bands of the EEG was investigated. Both enantiomers induced similar dose-dependent (5-20 micrograms) increases in all frequency bands. The effects of R(+)- and S(-)-zacopride were inhibited by pretreatment with a high dose of ICS 205-930 (1 micrograms i.c.v.), which suggests the involvement of 5-HT4 receptors. The lack of stereo-selectivity of the zacopride enantiomers is in contrast to observations made in in vitro studies.

The resolution, isolation, and pharmacological characterization of the enantiomers of a benzamide containing a chiral sulfoxide

Rac-ML-1035 (MDL 201,035: 4-amino-5-chloro-2-[2-(methylsulfinyl)ethoxy]-N-[2-(diethylamino)ethyl] benzamide hydrochloride) is a racemic gastroprokinetic with serotonergic (5-hydroxytryptamine, 5-HT) activity and a novel chiral sulfoxide substituent. Chromatographic and chemical methods have been developed to resolve the enantiomers of rac-ML-1035, and the absolute configuration of the (R)-enantiomer has been determined. We also report pharmacological characterization of rac-ML-1035 and its respective isomers. Radioligand binding to rat cortical membranes revealed that (R)-ML-1035 (MDL 201,226) and (S)-ML-1035 (MDL 201,227) had equivalent activity at the 5-HT3 receptor. However, in isolated tissue studies including field-stimulated guinea pig ileum, field-stimulated rat fundic strip, and nonstimulated guinea pig ileum, (S)-ML-1035 was equally potent yet had greater maximal activity than (R)-ML-1035 in eliciting or facilitating cholinergic contractions. Thus, enantiomers of rac-ML-1035 can be resolved, and the relative configuration of these isomers influences their pharmacological activity.

Molecular modeling of 5-HT3 receptor ligands

Ligands of various chemical classes (e.g., indoles, indazoles, benzamides, carbazoles, and quinolines) have demonstrated high affinity for the 5-HT3 receptor in radiolabeled ligand-binding studies, and have shown 5-HT3 receptor antagonistic activity in functional assays which utilize the excitatory effects of 5-HT on enteric neurons and autonomic afferents. Several 5-HT3 antagonists are currently being evaluated for potential use in the treatment of migraine, schizophrenia, and anxiety, and a few have already demonstrated high efficacy as antiemetics in cancer chemotherapy. The purpose of this presentation is to highlight the significant structure-affinity relationships (SAFIR) and common geometrical features among 5-HT3 receptor ligands, and to describe the three-dimensional pharmacophore for the 5-HT3 recognition site derived from computational techniques. The chemical template containing the recognition elements (functional groups) for the 5-HT3 receptor are: an aromatic or heteroaromatic ring system, a coplanar carbonyl group, and a nitrogen center, interrelated by well-defined distances. Two "binding shapes" or "active shapes" for 5-HT3 ligands have been identified from detailed conformational analyses.

Anxiolytic-like activity of R(+)- and S(-)-zacopride in mice

The effects of the optical isomers of zacopride were assessed in mice in a fully automated two-compartment light/dark apparatus. A significant increase in time spent in the lit area was used as an indication of anxiolytic-like action. Intraperitoneal (i.p.) doses of R(+)-zacopride from 0.00001 to 10.0 mg/kg and S(-)-zacopride from 0.01 to 1.0 mg/kg produced significant anxiolytic-like activity. Oral (p.o.) doses of the R(+) isomer from 0.00001 to 10.0 mg/kg and S(-)isomer from 0.1 to 1.0 mg/kg also generated antianxiety-like action. In addition, R(+)-zacopride (0.0001 mg/kg) was evaluated for time course effects after i.p. and p.o. administration. By either route of injection, the onset to action of R(+)-zacopride was 0.5 h, while the duration of effect was greater than or equal to 16 h. It was concluded that R(+)-zacopride is a potent and long-acting drug and that it is principally responsible for the anxiolytic-like activity of racemic zacopride.

5-HT3-like receptors in the rat medial prefrontal cortex: an electrophysiological study

In this study, we have identified and characterized 5-HT3-like receptors in the rat medial prefrontal cortex (mPFc), an area with a moderate density of 5-HT3 binding sites, using the techniques of single unit recording and microiontophoresis. The microiontophoresis of the 5-HT3 receptor agonist 2-methylserotonin (2-Me-5HT), similar to the action of 5-HT, produced a current-dependent (10-80 nA) suppression of the firing rate of both spontaneously active and glutamate (GLU)-activated (quiescent) mPFc cells. Phenylbiguanide (PBG), another 5-HT3 receptor agonist, suppressed the firing rate of mPFc cells but was less effective compared to 2-Me-5HT. The continuous iontophoresis (10-20 min) of 1 M magnesium chloride markedly attenuated the suppressant effect produced by electrical stimulation of the ascending 5-HT pathway, but did not alter 2-Me-5HT's action, suggesting that the action of 2-Me-5HT is a direct one. The suppressant action of 2-Me-5HT on mPFc cells was blocked by a number of structurally diverse and selective 5-HT3 antagonists, with a rank order of effectiveness as follows: ICS 205930 = (+/-)-zacopride greater than granisetron = ondansetron = LY 278584 greater than MDL 72222. Furthermore, the intravenous administration of (+/-)-zacopride antagonized the action of 2-Me-5HT and PBG on mPFc cells. In contrast to the effects of the 5-HT3 receptors antagonists, other receptor antagonists such as metergoline (5-HT1A,1B,1C.2), (+/-)-pindolol (5-HT1A,1B, beta), SCH 23390 (5-HT1C.2, D1), l-sulpiride (D2) or SR 95103 (GABAA) failed to block 2-Me-5HT's action. These results combined suggest that 2-Me-5HT's suppressive action on mPFc cells is mediated directly by 5-HT3-like receptors.

Characteristics of 5-HT3 binding sites in NG108-15, NCB-20 neuroblastoma cells and rat cerebral cortex using [3H]-quipazine and [3H]-GR65630 binding

1. The biochemical and pharmacological properties of 5-HT3 receptors in homogenates of NG108-15 and NCB-20 neuroblastoma cells and rat cerebral cortex have been ascertained by the use of [3H]-quipazine and [3H]-GR65630 binding. 2. In NG108-15 and NCB-20 cell homogenates, [3H]-quipazine bound to a single class of high affinity (NG108-15: Kd = 6.2 +/- 1.1 nM, n = 4; NCB-20: Kd = 3.0 +/- 0.9 nM, n = 4; means +/- s.e.means) saturable (NG108-15: Bmax = 1340 +/- 220 fmol mg-1 protein; NCB-20: Bmax = 2300 +/- 200 fmol mg-1 protein) binding sites. In rat cortical homogenates, [3H]-quipazine bound to two populations of binding sites in the absence of the 5-hydroxytryptamine (5-HT) uptake inhibitor, paroxetine (Kd1 = 1.6 +/- 0.5 nM, Bmax1 = 75 +/- 14 fmol mg-1 protein; Kd2 = 500 +/- 300 nM, Bmax2 = 1840 +/- 1040 fmol mg-1 protein, n = 3), and to a single class of high affinity binding sites (Kd = 2.0 +/- 0.5 nM, n = 3; Bmax = 73 +/- 6 fmol mg-1 protein) in the presence of paroxetine. The high affinity (nanomolar) component probably represented 5-HT3 binding sites and the low affinity component represented 5-HT uptake sites. 3. [3H]-paroxetine bound with high affinity (Kd = 0.02 +/- 0.003 nM, n = 3) to a site in rat cortical homogenates in a saturable (Bmax = 323 +/- 45 fmol mg-1 protein, n = 3) and reversible manner. Binding to this site was potently inhibited by 5-HT uptake blockers such as paroxetine and fluoxetine (pKi s = 8.6-9.9), while 5-HT3 receptor ligands exhibited only low affinity (pK; < 7). No detectable specific [3H]-paroxetine binding was observed in NG108-15 or NCB-20 cell homogenates. 4. [3H]-quipazine binding to homogenates of NG108-15, NCB-20 cells and rat cortex (in the presence of 0.1 microM paroxetine) exhibited similar pharmacological characteristics. 5-HT3 receptor antagonists competed for [3H]-quipazine binding with high nanomolar affinities in the three preparations and the rank order of affinity was: (S)-zacopride > quarternized ICS 205-930 2 granisetron > ondansetron > ICS 205-209 (R)-zacopride > quipazine > renzapride > MDL-72222 > butanopride > metoclopramide. 5. [3H]-GR65630 labelled a site in NCB-20 cell homogenates with an affinity (Kd = 0.7 + 0.1 nms n = 4) and density (B__ = 1800 + 1000 fmol mg- protein) comparable to that observed with [3H]-quipazine. Competition studies also indicated a good correlation between the pharmacology of 5-HT3 binding sites when [3H]-GR65630 and [3H]-quipazine were used in these cells. 6. In conclusion, [3H]-quipazine labelled 5-HT3 receptor sites in homogenates of NG108-15 cells, NCB-20 cells and rat cerebral cortex. In rat cortical homogenates, [3H]-quipazine also bound to 5-HT uptake sites, which could be blocked by 0.1 microM paroxetine. The pharmacological specificity of the 5-HT3 receptor labelled by [3H]-quipazine was similar in the neuroblastoma cells and rat cortex and was substantiated in NCB-20 cells by the binding profile of the selective 5-HT3 receptor antagonist, [3H]-GR65630.

Studies on the emetic and antiemetic properties of zacopride and its enantiomers

In ferrets, the oral emetic activity of zacopride was compared with its R- and S-enantiomers. Increasing doses of 0.01, 0.1, 1.0, 10.0 and 31.6 mg/kg of zacopride or its 2 enantiomers were each administered at hourly intervals to separate groups of animals until emesis occurred. The emetic (100%) dose for zacopride and its S-enantiomer was 0.11 mg/kg p.o. (cumulative dose). The R-enantiomer at a cumulative dose of 42.71 mg/kg p.o. produced emesis in 25% of the animals. By the i.p. route zacopride and its S-enantiomer were more potent than the R-enantiomer in blocking the emetic activity of 0.1 mg/kg p.o. of zacopride. The involvement of 5-HT3 mechanisms is indicated by a correlation between zacopride and its enantiomers to cause and prevent emesis and their affinity at 5-HT3 binding sites. Further, the putative 5-HT3 agonists, 2-methyserotonin and phenylbiguanide, at 10 mg/kg p.o., produced emesis that was blocked by zacopride (0.1 mg/kg i.p.) or ICS 205-930 (1 mg/kg i.p.). The results suggest that in the ferret the S-enantiomer is predominantly responsible for both the emetic and antiemetic properties of zacopride and that 5-HT3 agonism and antagonism are involved in these actions.