Pharmacological properties of GR38032F, a novel antagonist at 5-HT3 receptors

The role of serotonin in schizophrenia: an overview of the nomenclature, distribution and alterations of serotonin receptors in the central nervous system

Serotonin is a neurotransmitter that is widely distributed throughout the central nervous system. The role of serotonin in schizophrenia is still unclear. Postmortem studies of serotonin receptor subtypes in schizophrenia have been inconclusive for the most part. The most promising findings involve a reduction in 5-HT2 receptors and 5-HT reuptake sites in the prefrontal cortex of schizophrenic patients. In this paper we review the function, distribution and pharmacological characteristics of serotonin receptors. Postmortem studies are also reviewed, focusing upon the role of these receptors in schizophrenia.

Ondansetron, an antagonist of 5-HT3 receptors, antagonizes the anti-exploratory effect of caerulein, an agonist of CCK receptors, in the elevated plus-maze

Systemic treatment with caerulein (0.25-5 micrograms/kg SC), non-selective agonist of cholecystokinin (CCK) receptors, dose-dependently suppressed the exploratory behaviour of rats in an elevated plus-maze without producing remarkable changes in the locomotor activity of animals in an open field test. Ondansetron, a selective antagonist of 5-HT3 receptors, increased the number of open arm entries in the plus-maze test only at a dose 10 micrograms/kg. The other doses of ondansetron (0.1, 1 and 100 micrograms/kg IP) did not significantly change either the locomotor activity or the exploratory behaviour of rats. Pretreatment of rats with ondansetron (at 10 micrograms/kg, but not at 0.1, 1 or 100 micrograms/kg) completely reversed the anti-exploratory effect of caerulein (5 micrograms/kg). The concomitant treatment with caerulein and ondansetron did not cause any major change in the locomotor activity of animals in open field. Consequently, we propose that 5-HT-ergic mechanisms are involved not only in the regulation of CCK release in the cerebral cortex and nucleus accumbens, but also in the modulation of the anti-exploratory effect of caerulein, a CCK agonist, in the elevated plus-maze.

Inhibition by 5-HT3 receptor antagonists of release of cholecystokinin-like immunoreactivity from the frontal cortex of freely moving rats

Effects of the 5-HT3 receptor antagonists, ondansetron and tropisetron, on the release of cholecystokinin-like immunoreactivity (CCK-LI) in rat frontal cortex were investigated in conscious, unrestrained rats using intracerebral microdialysis. The release of CCK-LI was augmented by perfusion with 100 micrograms/ml veratrine and was fully Ca(2+)-dependent and tetrodotoxin-sensitive. Ondansetron and tropisetron, each at 0.1-1 mumol/l, decreased concentration-dependently the veratrine-evoked efflux of CCK-LI. The reduction of CCK-LI output was approximately 30% when the antagonists were infused at 0.1 mumol/l. The data suggest that 5-HT3 receptor antagonists prevent the release of CCK evoked by endogenous 5-hydroxytryptamine. These drugs may thus represent a novel therapeutic approach in disease states, like anxiety, in which an inappropriately high release of brain CCK or 5-hydroxytryptamine seems to be involved.

The Psychopharmacology of 5-HT3Receptors

5-HT3 receptors have an exclusive neuronal location and evidence is presented of their involvement in behaviour. 5-HT3 receptor antagonists such as ondansetron, tropisetron and zacopride have provided the critical pharmacological tools to reveal a potent and efficacious ability to regulate disturbed behaviour. Thus the 5-HT3 receptor antagonists will restore to normal rodent and primate behaviour disturbed by increasing limbic dopamine function, aversive situations, cognitive impairments and drug abuse. The remarkable feature of their action is a failure to modify normal behaviour. This unique pharmacological signature has ensured a wide interest in the potential role of the 5-HT3 receptor antagonists in the treatment of schizophrenia, anxiety, age related memory impairment and the problems of withdrawal from drugs of abuse. The preclinical data and preliminary clinical observations are presented.

A component of 5-HT-evoked depolarization of the rat isolated vagus nerve is mediated by a putative 5-HT4 receptor

This study describes a component of 5-HT-evoked depolarization of the rat isolated vagus nerve which was unaffected by the 5-HT3 receptor antagonist ondansetron. A grease-gap extracellular recording technique was used. Ondansetron (10-100 nmol/l) displaced the 5-HT concentration-response curve to the right yielding a pA2 value of 8.6 (8.5-8.8), consistent with 5-HT3 receptor antagonism, and revealing a component of the 5-HT response which was resistant to ondansetron blockade. In the presence of ondansetron (100 nmol/l) the maximum depolarization in the resistant phase was 15.5 (12.6-19.2)% of the initial maximum response to 5-HT and the pEC50 value was 7.0 (6.7-7.3). The mechanism of the ondansetron-resistant component of the 5-HT response resembled a 5-HT4-receptor-effect in being absent in preparations equilibrated with 5-methoxytryptamine (10 mumol/l) and antagonised by ICS 205930 (tropisetron, pA2 6.4). 5-Methoxytryptamine alone was an agonist in the vagus nerve with a maximum response similar to that of the ondansetron resistant phase of the 5-HT response. Similarly renzapride alone evoked small depolarizations of this preparation but antagonized the ondansetron resistant phase of the 5-HT response (pA2 7.3-7.4). These effects of 5-methoxytryptamine and renzapride are also consistent with a 5-HT4 receptor mechanism. Ketanserin (1 mumol/l) and methysergide (1 mumol/l) had little effect on responses to 5-HT. The depolarization evoked by this putative 5-HT4 receptor mechanism was small but prolonged and appears to mask and after-hyperpolarizing phase of the 5-HT response in this tissue.

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.

The effects of intravenous administration of cholecystokinin on feeding behaviour and release of pituitary hormones in pigs are not mediated by serotonergic (5-HT3) receptors

Two experiments were carried out using the serotonergic (5-HT3) antagonist ondansetron (GR 38032F) to investigate whether the actions of peripherally-administered cholecystokinin octapeptide (CCK) in the prepubertal pig might involve 5-HT3 receptors. In Experiment 1, it was shown that ondansetron (10 or 30 mg, i.v.) did not affect the initial feeding response and did not modify the behavioural inhibition induced by a subsequent injection of CCK (1.3 microgram/kg, i.v.). In Experiment 2, it was demonstrated that the nauseogenic action of CCK, indicated by its stimulatory effect on release of vasopressin and cortisol, was not markedly altered by ondansetron (30 mg, i.v.). These results suggest that 5-HT3 receptors play a negligible part in mediating the behavioural and endocrine responses induced by bolus intravenous injections of CCK in this species.

AS-5370 potently antagonizes 5-HT3 receptor-mediated responses on NG108-15 cells and on the rat vagus

The action of a novel 5-HT3 receptor antagonist, AS-5370, has been studied on two electrophysiological models for 5-HT3 receptors: whole-cell patch-clamp recordings from mouse neuroblastoma-rat glioma (NG108-15) cells and grease-gap recordings from rat isolated vagus nerve. The 5-hydroxytryptamine (5-HT)-induced fast inward current of voltage-clamped NG108-15 cells was antagonized by 1 nM AS-5370 in an insurmountable manner. On the rat vagus, AS-5370 reduced the maximum depolarizing response to 5-HT in a concentration-dependent manner. The IC50 for AS-5370 on the rat vagus was 0.3-1.0 nM. The EC50 for 5-HT on the rat vagus was not appreciably affected by AS-5370. On the rat vagus, the (R) enantiomer of AS-5370 was about 30 times more potent than the (S) enantiomer. The antagonist action of AS-5370 on these two cell types was compared with that of (+)-tubocurarine. Unlike tubocurarine, the effect of AS-5370 on NG108-15 cells was not readily reversed during wash. On the rat vagus, tubocurarine antagonized in a competitive manner with an IC50 of 0.3-1.0 microM (pKb = 7.2). It is concluded that AS-5370 is a potent 5-HT3 receptor antagonist on both NG108-15 cells and the rat vagus, but it does not act in a competitive manner.

Cisplatin-induced conditioned taste aversion: attenuation by dexamethasone but not zacopride or GR38032F

The 5-HT3 receptor antagonists zacopride and GR38032F are highly effective inhibitors of emesis induced by ionizing radiation and chemotherapeutic drugs such as cisplatin. The present study evaluated zacopride and GR38032F for efficacy in inhibiting the formation of the conditioned taste aversion (CTA) induced by cisplatin or lithium chloride in rats. The glucocorticoid dexamethasone, which has been reported to be effective against both the emetic and CTA-inducing effects of cisplatin, was included as a reference compound. When administered alone by i.p. injection, zacopride (0.1-10 mg/kg), GR38032F (10 mg/kg) and cisplatin (0.32-1.8 mg/kg) induced a CTA to an 0.1% saccharin solution; lower doses of each compound were ineffective. When administered as a pretreatment, neither zacopride (0.001-0.1 mg/kg) nor GR38032F (0.01-10 mg/kg) attenuated the CTA induced by cisplatin (0.32 and 0.56 mg/kg) or lithium chloride (10 mg/kg). In contrast, dexamethasone (0.32 and 1.0 mg/kg) attenuated the CTA induced by 0.32 but not 0.56 mg/kg of cisplatin. In an attempt to evaluate higher doses of zacopride against cisplatin without the potentially confounding factor that these doses by themselves induce a CTA, rats were injected with zacopride on three separate days prior to the aversion conditioning session. This pre-exposure treatment blocked the formation of the zacopride-induced CTA, but did not improve the efficacy of zacopride in attenuating the cisplatin-induced CTA. These results suggest that neither the cisplatin- nor the lithium-induced CTA in rats are due to effects that are sensitive to 5-HT3 receptor blockade.

The neuropharmacology of loperamide-induced emesis in the ferret: the role of the area postrema, vagus, opiate and 5-HT3 receptors

Loperamide, an opiate receptor agonist, commonly used in the treatment of diarrhoea, reliably induced emesis in the ferret, when given subcutaneously. The response latency was short (less than 10 min) and the emesis lasted for approx 70 min. The dose-response curve for the emetic response was "bell-shaped" and all animals responded at 0.5 mg/kg but none at 5 mg/kg (s.c.). The response was unaffected by dopamine D2 receptor antagonism (domperidone 1.0 mg/kg, s.c.) or 5-HT3 receptor antagonism (granisetron or ondansetron 1.0 mg/kg, s.c.). The onset of the response was delayed for about 60 min by naloxone or naloxone methiodide (1.0 mg/kg, s.c.) and abolished by naloxanazine (1.0 mg/kg, s.c.), reported to be relatively selective for mu receptors. The results implicate mu receptors (possibly mu 1) in the induction of emesis by loperamide and provide some support for activation of opiate receptors also having anti-emetic effects, as suggested in previous studies. The emetic response to loperamide was unaffected by abdominal vagotomy but was abolished by ablation of the area postrema, indicating that loperamide-induced emesis may be used as a test for ablation of the area postrema in studies of the emetic mechanism in the ferret.

Antagonistic activity of Y-25130 on 5-HT3 receptors

This paper describes the 5-hydroxytryptamine3 (5-HT3) receptor antagonism of Y-25130 ((+-)-N-(1-azabicyclo[2.2.2]oct-3-yl)-6-chloro-4-methyl-3-oxo-3,4-dih yd ro- 2H-1,4-benzoxazine-8-carboxamide monohydrochloride) in the rat cerebral cortex, isolated rabbit heart and isolated guinea pig ileum. In an in vitro binding assay, Y-25130 inhibited the specific binding of [3H]quipazine to 5-HT3 receptors at the synaptic membranes of the rat cerebral cortex with a Ki value of 2.9 nM, the same as that of ondansetron. Metoclopramide, 5-HT and 2-methyl-5-HT also showed an inhibitory effect, but their affinities for 5-HT3 receptors were lower than that of Y-25130. Y-25130 showed low affinity for histamine H1 receptors (IC50 = 4.4 microM) but it could not reveal any affinities for the other receptors (5-HT1A, 5-HT2, dopamine D1, dopamine D2, alpha 1-adrenoceptor, alpha 2-adrenoceptor, muscarine and benzodiazepine) even at a 10 microM concentration. In the isolated rabbit heart, Y-25130 antagonized the indirect sympathomimetic responses to 5-HT (pA2 value = 10.06) and this effect was more potent than that of metoclopramide. In the isolated longitudinal smooth muscle of the guinea pig ileum, concentration-contraction effect curves for 5-HT were biphasic in the presence of ketanserin. Y-25130 shifted to the right only in the second phase of concentration-effect curves for 5-HT (pA2 value = 7.04) and its activity was more potent than that of metoclopramide. These results indicate that Y-25130 is a potent and selective 5-HT3 receptor antagonist.

Pharmacological characterization of 5-hydroxytryptamine-induced excitation of afferent cervical vagus nerve in anaesthetized rats

1. An excitatory response to 5-hydroxytryptamine (5-HT) was measured from the afferent vagus nerve of anaesthetized rats. Measurements were determined by an extracellular recording from the whole nerve. 2. Intravenous bolus injection of 5-HT (1.56-100 micrograms kg-1) evoked a dose-dependent excitation of afferent vagus nerve activity. This response was blocked not only by a selective 5-HT3 receptor antagonist, GR38032F (10 and 100 micrograms kg-1), but also by a 5-HT2 receptor antagonist, ketanserin (10 and 100 micrograms kg-1). 3. Both a 5-HT3 receptor agonist, 2-methyl-5-HT (3.12-100 micrograms kg-1), and a 5-HT2 receptor agonist, alpha-methyl-5-HT (3.12-50 micrograms kg-1), produced a dose-dependent excitation of afferent vagus nerve activity. These excitatory effects were antagonized by GR38032F (10 micrograms kg-1) and ketanserin (10 micrograms kg-1), respectively. 4. A 5-HT1 like receptor agonist, 5-carboxamidotryptamine (50 micrograms kg-1), and a putative 5-HT4 receptor agonist, 5-methoxytryptamine (100 micrograms kg-1), failed to produce excitatory effects on the afferent vagus nerve. 5. These results suggest that the 5-HT-induced excitatory response of the afferent vagus nerve might be mediated not only via 5-HT3 receptors but also via 5-HT2 receptors in anaesthetized rats. It is unlikely, however, that either 5-HT1-like or putative 5-HT4 receptors are involved in the excitatory response of the afferent vagus nerve to 5-HT.

Profiles of interaction of R(+)/S(-)-zacopride and anxiolytic agents in a mouse model

The mouse black and white test box was used to measure changes in behaviour in an aversive situation where the administration of R(+)-zacopride (but not S(-)-zacopride) alone decreased aversive responding to the white area. A similar anxiolytic profile of action was observed using parachlorophenylalanine (PCPA), whose effects were antagonised by a co-treatment with R(+)-zacopride and reversed by S(-)-zacopride to an exacerbation of the aversive response. An anxiolytic profile of action was also observed using ondansetron, granisetron, chlordiazepoxide, diazepam, ritanserin, 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin), E4424 (2-[4-[4-(4-chloro-l-pyrazoyl)butyl]-l-piperazinyl]-pyrimidine), umepsirone, DuP753 (2-n-butyl-4-chloro-5-hydroxy-methyl-1-[2(1H-tetrazol-5-yl) biphenyl-4-yl)methyl)]-imidazole), SQ29,852 ((S)-1-[6-amino-2[hydroxy)(4-phenyl-butyl)phosphinyl]-oxy)-1- nexy]-2-proline), devazepide and guanfacine, and this was retained following co-treatment with PCPA. The anxiolytic profile of action of PCPA was also retained following co-treatment with renzapride which when administered alone failed to modify behaviour. However, the ability of chlordiazepoxide, diazepam, ondansetron and E4424 (but not devazepide, DuP753 or SQ29,852) to reduce aversive responding was inhibited by co-treatment with R(+) and/or S(-)-zacopride. It is concluded that the reduction in aversive responding caused by pharmacological manipulation at the benzodiazepine, 5-HT receptor subtypes 5-HT1A, 5-HT1C/5-HT2 and 5-HT3 (but not at the cholecystokin CCKA or angiotensin receptors or inhibition of angiotensin converting enzyme) can be inhibited by R(+) and S(-)-zacopride. The data is discussed in terms of zacopride having an agonist or partial agonist effect at the 5-HT3 receptor.

Identification and distribution of 5-HT3 recognition sites in the rat gastrointestinal tract

1. Tritiated derivatives of the potent and selective 5-HT3 receptor antagonists GR65630 and LY278584 were used to identify 5-HT3 recognition sites in the rat gastrointestinal tract. 2. Binding studies were carried out in homogenates of the rat oesophagus, the cardia, fundus, body and antrum of the stomach, regions of the small intestine, caecum and large intestine. The specific binding of a single concentration of GR65630 (0.5 nM) defined by granisetron (10 microM) in these areas indicated that the density of 5-HT3 recognition sites varied from 2.4 +/- 1.0 to 10.1 +/- 1.0 fmol mg-1 protein. 3. Saturable binding of [3H]-GR65630 could only be demonstrated in the terminal regions of the small intestine (Bmax in the range of 13.83 +/- 4.54-21.19 +/- 0.89 fmol mg-1 protein; mean +/- s.e. mean) and of high affinity (Kd in the range of 0.42 +/- 0.18-0.79 +/- 0.24 nM). Use of [3H]-LY278584 revealed a similar binding density (Bmax 19.54 +/- 0.26 fmol mg-1 protein) and affinity (Kd 1.04 +/- 0.07 nM) in the terminal small intestine. 4. Binding of [3H]-GR65630 and [3H]-LY278584 to the terminal region of the small intestine was inhibited by 5-HT3 receptor ligands ondansetron and S-zacopride (and 5-hydroxytryptamine), but not by 5-HT1, 5-HT2, catecholamine, gamma-aminobutyric acid and opioid receptor ligands. 5. These data demonstrate that there are regional variations in the density of 5-HT3 recognition sites within the rat gastrointestinal tract. Such data are relevant to the potential use of 5-HT3 receptor ligands to modify secretory and contraction responses in the gastrointestinal system.

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.

Failure of ondansetron to block the discriminative or reinforcing stimulus effects of cocaine in the rat

Ondansetron (GR38032F), a serotonin 5HT3 antagonist, is active in numerous behavioral paradigms and neurochemical systems. Since 5HT3 antagonists have been suggested as therapeutic agents for the treatment of drug abuse, the action of ondansetron on cocaine drug discrimination and self-administration paradigms in rats was investigated. Doses of ondansetron (0.001 - 1.0 mg/kg) had no effect on the discriminative stimulus properties of 10 mg/kg cocaine. In contrast SCH23390, a dopamine D1 antagonist known to block cocaine discrimination, acted as previously reported. Ondansetron did not augment the effects of SCH23390, but at higher doses, combinations of ondansetron and SCH23390 produced disruption of lever pressing in the presence of cocaine. Ondansetron (0.001-1.0 mg/kg) had no effect on the self-administration of various doses of cocaine, nor did it have any effect on reacquisition of cocaine self-administration in animals with a history of active administration followed by a period of abstinence. As before, SCH23390, known to block cocaine self-administration, acted as previously reported. Although other 5HT antagonists may prove to be efficacious in cocaine abuse, ondansetron appears unlikely to alter the subjective or rewarding stimulus properties of cocaine.

5-Hydroxytryptamine3 receptors sited on cholinergic axon terminals of human cerebral cortex mediate inhibition of acetylcholine release

Synaptosomes prepared from freshly obtained human cerebral cortex and labeled with [3H]choline have been used to investigate the modulation of [3H]acetylcholine ([3H]ACh) release by 5-hydroxytryptamine (5-HT). The Ca(2+)-dependent release of [3H]-ACh occurring when synaptosomes were exposed in superfusion to 15 mM KCl was inhibited by 5-HT (0.01-1 microM) in a concentration-dependent manner. The effect of 5-HT was mimicked by 1-phenylbiguanide, a 5-HT3 receptor agonist, but not by 8-hydroxy-2-(di-n-propylamino)tetralin, a 5-HT1A receptor agonist. The 5-HT3 receptor antagonists tropisetron and ondansetron blocked the effect of 5-HT, whereas spiperone and ketanserin were ineffective. It is suggested that cholinergic axon terminals in the human cerebral cortex possess 5-HT receptors that mediate inhibition of ACh release and appear to belong to the 5-HT3 type.

Effect of sumatriptan, a selective 5-HT1-like receptor agonist, on pial vessel diameter in anaesthetised cats

The action of sumatriptan, a selective 5-HT1-like receptor agonist that is effective for the acute treatment of migraine, was compared on pial vessel diameter following perivascular or intravenous administration to anaesthetised cats. Sumatriptan (0.01-10 microM), when microinjected perivascularly, caused a decrease in pial artery diameter (maximum change of -19 +/- 9%; mean +/- SD) but had no effect on the diameter of pial veins. Sumatriptan (1 microM)-induced pial artery vasoconstriction was unaffected by coadministration of ketanserin (1 microM) or ondansetron (1 microM) but was significantly (p less than 0.01) attenuated by methiothepin (1 microM). Intravenous infusion of a clinically effective dose of sumatriptan (64 micrograms/kg/10 min) caused selective carotid vasoconstriction (22 +/- 6% increase in carotid vascular resistance with little or no change in blood pressure or heart rate) and no change in pial artery diameter, although sumatriptan (1 microM) administered perivascularly in these animals before and after the infusion caused pial artery vasoconstriction. These results demonstrate that perivascularly administered sumatriptan causes pial artery vasoconstriction via activation of 5-HT1-like receptors. However, intravenously administered sumatriptan does not cause pial artery vasoconstriction, which suggests that sumatriptan does not readily penetrate the cerebrovascular intima.

Membrane-bound and solubilized brain 5HT3 receptors: improved radioligand binding assays using bovine area postrema or rat cortex and the radioligands 3H-GR65630, 3H-BRL43694, and 3H-LY278584

Bovine area postrema tissue was used as a convenient source of tissue for studies of brain 5HT3 receptors. 5HT3 receptor density was determined to be 97 +/- 5 fmol/mg and 124 +/- 10 fmol/mg of protein using the commercially available 5HT3 radioligands, 3H-GR65630 and 3H-BRL43694, respectively. The equilibrium dissociation constants (KD) for 3H-GR65630 and 3H-BRL43694 were 0.5 +/- 0.1 nM and 1.7 +/- 0.3 nM, respectively. The affinities of a series of drugs for the 5HT3 receptor using the two radioligands were essentially identical; the Ki values and order of affinities of agonists and antagonists were very similar to data published in studies on radiolabeling of 5HT3 receptors in rat brain. 3H-LY278584 also labeled 5HT3 receptors in bovine area postrema homogenates with a KD of 3.1 +/- 0.1 nM and a Bmax of 84 +/- 6 fmol/mg. In rat cortical homogenates, 3H-LY278584 produced the most reliable specific signal of 72%, with a KD of 2.6 +/- 0.3 nM and a Bmax value of 10.5 +/- 1 fmol/mg. At 1 nM, 3H-GR65630 or 3H-BRL43694 specific binding represented 28 and 50% of total radioligand binding, respectively. These data in bovine and rat brain tissues indicate that bovine area postrema can be used with 3H-GR65630, 3H-BRL43694, or 3H-LY278584 for drug screening or molecular investigations of the brain 5HT3 receptor: only 3H-LY278584 can be used for studies on the regulation and/or the molecular properties of 5HT3 receptors in rat cortical homogenates. 5HT3 receptors were solubilized from bovine area postrema and characterized using 3H-GR65630.(ABSTRACT TRUNCATED AT 250 WORDS)

Behavioural pharmacology of 5-HT3 receptor ligands

Extensive studies have ascribed a role for the central 5-HT3 receptor in the modulation of behaviour. Much of the work stems from the actions of potent and selective 5-HT3 receptor antagonists; these agents reduce mesolimbic dopamine initiated hyperactivity, release suppressed behaviour, reduce the reinforcing properties and withdrawal symptoms of drugs of abuse, enhance cognitive performance and modulate appetite. This article reviews the preclinical and clinical evidence implicating the 5-HT3 receptor in these indications and discusses the potential neurochemical mechanisms underlying the behavioural changes.

5-hydroxytryptamine is a fast excitatory transmitter at 5-HT3 receptors in rat amygdala

A fast excitatory synaptic potential mediated by 5-hydroxytryptamine (5-HT) was recorded in rat lateral amygdala neurons in brain slices. The synaptic potential has brief duration (tens of milliseconds), is mimicked by 5-HT, is potentiated by a 5-HT uptake inhibitor, and is blocked by selective 5-HT3 receptor antagonists. The underlying synaptic current reversed polarity at about 0 mV. This is an example of fast neurotransmission in the mammalian brain mediated by an amine rather than an amino acid. The antiemetic, anxiolytic, and perhaps antipsychotic actions of 5-HT3 antagonists might result from blockade of such synapses.

5-HT3 agonist 2-methylserotonin as a training drug in discrimination studies

Using a standard two-lever operant procedure, rats were trained to discriminate 5 mg/kg of the 5-HT3 agonist 2-methylserotonin (2-Me 5-HT; ED50 = 2.6 mg/kg) from saline using a VI 15-s schedule of reinforcement. The 2-Me 5-HT stimulus did not generalize to the 5-HT1/5-HT2 agonist 5-methoxy-N,N-dimethyltryptamine, but did generalize to the new 5-HT3 agonist 1-(m-chlorophenyl)biguanide (ED50 = 1.6 mg kg). The 5-HT3 antagonist ICS 205-930 potently antagonized the 2-Me 5-HT stimulus (ID50 = 0.001 mg/kg), whereas its quaternary amine analog, which does not readily penetrate the blood-brain barrier, failed to completely antagonize the 2-Me 5-HT stimulus at a 10,000-fold higher dose. The results of the present investigation show that 2-Me 5-HT serves as a discriminative stimulus in rats when paired with saline and suggest that its stimulus properties are likely mediated via a central 5-HT3 mechanism. As such, this is the first demonstration that a 5-HT3 agonist can be used as a training drug in drug discrimination studies.

Effect of the 5-HT3 receptor antagonists, MDL72222 and ondansetron on morphine place conditioning

The purpose of the present study was to reassess the original findings of Carboni et al. (1988) who suggested that 5-HT3 receptor antagonists may block morphine-induced place conditioning in rats. These workers used a biased protocol with treatments allocated to compartments based on initial preference. In the present study we have adopted an unbiased approach with treatments randomly assigned to conditioning compartment in a counter-balanced fashion. Thus treatments were equally paired between distinct environmental cues. Using this protocol, morphine produced a dose-related place preference (0.3-3 mg/kg SC). Thirty-minute pretreatment with the selective 5-HT3 antagonists, MDL72222 (1 mg/kg SC) and ondansetron (0.01 mg/kg SC) before morphine (1.5 mg/kg SC), significantly antagonized the place conditioning to this treatment. However, with higher doses of ondansetron (0.1-1 mg/kg SC), the antagonism of morphine-induced place preference became variable and dependent on the conditioning compartment. This was probably a reflection of the fact that ondansetron when administered alone also appeared to produce an environmentally dependent place conditioning at these doses. Therefore it is concluded that at certain doses, 5-HT3 receptor antagonists may antagonize morphine place conditioning in a manner consistent with a blockade of the appetitive effects of this drug. However, at higher doses, at least with ondansetron, this antagonism became non-specific and dependent on the training environment. It is suggested that other animal models of opioid reinforcement (e.g., self-administration) are now needed to validate the hypothesis that 5-HT3 receptor antagonists may modify opioid reward.

The effect of ondansetron on radiation-induced emesis and diarrhoea

Fractionated radiotherapy of malignancies in the abdomen induces nausea and vomiting in approximately 50% of the patients. During abdominal irradiation the damaged gastrointestinal mucosa releases 5-HT with ensuing activation of 5-HT3 receptors which may explain the nausea and vomiting. Ondansetron is a new 5-HT3-antagonist with antiemetic properties. In this consecutive study, 33 patients receiving fractionated upper abdominal irradiation (> or = 100 cm2, 1,8-4 Gy daily dose for a mean of 13 days) were treated with ondansetron (8 mg t.d.s. p.o.). Emesis was completely controlled in 26/33 (79%) patients throughout their radiation course, which embraced 628 (94%) treatment days. Ondansetron was well tolerated. Eleven patients developed mild constipation. No patients experienced diarrhoea (a common distressing side-effect of abdominal irradiation). It is suggested that ondansetron can be of value in preventing emesis in patients receiving fractionated radiotherapy. The possible beneficial effect in preventing diarrhoea must be further evaluated.

Are all 5-HT3 receptor antagonists the same?

A number of 5-HT3 receptor antagonists are currently in clinical development as antiemetics. In this paper we focus on two of these antagonists, granisetron and ondansetron, and compare their antimetic activity against cisplatin (10 mg/kg i.v.)- or whole body X-irradiation (200 rads)-induced emesis in the conscious ferret. The results presented here have been discussed in the light of the recently published literature. Our data suggest that in comparison to ondansetron, granisetron is a more potent, longer acting and pharmacologically "cleaner" compound with a more conventional dose-response profile. The possible impact of these features upon the performance of these compounds in the clinic is discussed particularly with respect to dosing regimens and clinical efficacy. Differences appear to be emerging between granisetron and ondansetron in both these respects, although a direct head-to-head clinical comparison has yet to be carried out. This would involve studies monitoring a sufficiently high number of patients receiving severely emetogenic regimes to allow real clinical differences to be detected with the appropriate statistical power.

5-HT1-like receptors mediate potentiation of cholinergic nerve-mediated contraction of isolated mouse trachea

While it had no effect on the resting tension of mouse tracheal segments, 5-HT (10(-8)-10(-4) M) potentiated concentration dependently the contractions induced by electrical field stimulation (EFS). The maximal potentiation was 105 +/- 38% and the EC50 value was 1.4 +/- 0.6 x 10(-6) M (n = 6). The responsiveness of mouse trachea to acetylcholine was not altered by 5-HT (10(-5) M). The 5-HT1A,B antagonist pindolol (10(-6) M), the combined 5-HT2 and 5-HT1C receptor antagonist, ketanserin (10(-6) M), or the combined 5-HT1 and 5-HT2 receptor antagonist, methysergide (10(-6) M), all partially inhibited the effect of 5-HT on the twitch responses. Blockade of 5-HT3 receptors by GR 38032F (10(-6) M) did not affect the potentiation by 5-HT. Antagonism of 5-HT3 and 5-HT4 receptors by ICS 205,930 (3 x 10(-6) M) increased the potentiation of the twitch responses by 5-HT, this was probably due to a decrease of the baseline EFS-induced twitch response by ICS 205,930. Alkylation of the 5-HT2 receptor by phenoxybenzamine (3 x 10(-7) M) treatment did not significantly affect the potentiation of the twitch responses by 5-HT. The beta-adrenoceptor antagonist, timolol (10(-6) M), and the alpha-adrenoceptor antagonist, phentolamine (10(-6) M), did not influence the potentiation of the twitch responses by 5-HT, excluding the involvement of the adrenergic system.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological properties of a novel class of 5-HT3 receptor antagonists

The pharmacological profile of six representative members of a novel class of 5-HT3 receptor antagonists is described. The compounds are esters and amides of benzimidazolone-1-carboxylic acid with a basic azabicycloalkyl moiety (compounds 1-3) and their respective ethyl derivatives (compounds 4-6). In isolated preparations (rabbit heart and guinea pig ileum) all compounds antagonized the 5-HT3 receptor-mediated effects of serotonin, with potencies comparable with those of the reference compounds, ICS 205.930 and GR 38032F (-log IC50 9.30-11.9 and 6.8-8.20, in heart and ileum, respectively). In the anaesthetised rat, all agents potently inhibited the Bezold-Jarisch reflex whether given i.v. or i.d. I.v. administration of compounds prevented cisplatin-induced emesis in dogs (ID50 ranging from 3.7 to 147 micrograms/kg). All agents accelerated gastric emptying of solids in rats (ED50 about 10-160 micrograms/kg i.p.). In addition, compounds 4 and 5 were able to stimulate 5-HT4 receptors in the isolated guinea pig ileum, as well as enhance contractile activity in the Heidenhain gastric pouch of dogs, showing clearcut prokinetic properties.

5-HT3 receptor antagonists. An overview of their present status and future potential in cancer therapy-induced emesis

The serotonin (5-hydroxytryptamine, 5-HT) antagonists, which bind at the type 3 receptor (5-HT3 receptor), have been evaluated in several preclinical models and found to be effective in alleviating cancer therapy-related emesis. The antiemetic efficacy of ondansetron (GRF-38032F, odanserin), granisetron (BRL-43694), tropisetron (ICS-205930), MDL-72222 and MDL-73147EF, batanopride (BMY-25801-01) and several others is at various stages of investigation. Ondansetron is currently marketed in several countries and the same will soon be true for granisetron. At this stage it is not yet possible to evaluate the comparative efficacy of each of these compounds, although recent preclinical data reveal some differences in the affinity of these compounds, for other receptors. Side effects related to these agents have been minor, consisting mainly of slight headaches; possible rises in liver enzymes related to some compounds need further evaluation. Future studies will need to determine the exact role of 5-HT3 antagonists, although their cost may confine their use to patients at high risk for side effects from metoclopramide.

Evaluation of the effects of diazepam and an experimental anti-anxiety drug on regional cerebral blood flow

In the normal brain, cerebral blood flow (CBF) and function are closely coupled. Thus, changes in brain function associated with drug-induced anxiety reduction should be accompanied by parallel CBF changes. Benzodiazepines such as diazepam have been reported to reduce CBF. It is unclear, however, if this CBF decrease is specifically a function of the anti-anxiety property of the drug. To examine the relationship between drug-induced anxiety reduction and CBF changes more closely, i.v. injections of an established anxiolytic agent and an experimental anti-anxiety drug were given to patients with generalized anxiety disorder. CBF and anxiety levels were measured before and 30 min after i.v. administration of diazepam (0.12 mg/kg), ondansetron (0.24 mg/kg), and normal saline during separate visits to the laboratory. The order of drug administration was randomized, and the injections were given under double-blind conditions. Diazepam but not ondansetron or saline reduced anxiety. Global CBF reduction was seen after diazepam, but no changes were found following the other two experimental conditions. The CBF values were adjusted for test-retest changes in carbon dioxide levels. Postdiazepam decreases in CBF and anxiety levels did not correlate.

Cholecystokinin release mediated by 5-HT3 receptors in rat cerebral cortex and nucleus accumbens

1. The effects of 5-hydroxytryptamine (5-HT) on the release of cholexystokinin-like immunoreactivity (CCK-LI) were examined in synaptosomes prepared from rat cerebral cortex and nucleus accumbens and depolarized by superfusion with 15 mM KCl. 2. In both areas 5-HT, tested between 0.1 and 100 nM, increased the calcium-dependent, depolarization-evoked CCK-LI release in a concentration-related manner. The concentration-response curves did not differ significantly between the two brain areas (EC50: 0.4 +/- 0.045 nM and 0.48 +/- 0.053 nM, respectively, in cortical and n. accumbens synaptosomes; maximal effect: about 60% at 10 nM 5-HT). 3. The 5-HT1/5-HT2 receptor antagonist methiothepin (300 nM) did not affect the CCK-LI release elicited by 10 nM 5-HT. However, the effects of 10 nM 5-HT were antagonized in a concentration-dependent manner by the 5-HT3 receptor antagonists (3 alpha-tropanyl)-1H-indole-3-carboxylic acid ester (ICS 205-930; 0.1-100 nM; IC50: 3.56 +/- 0.42 nM in the cortex and 3.90 +/- 0.50 nM in the n. accumbens) and ondasetron (IC50: 8.15 +/- 0.73 nM in the cerebral cortex). 5-HT (10 nM) was also strongly antagonized by 100 nM 1 alpha H, 3 alpha 5 alpha H-tropan-3-yl-3,5-dichlorobenzoate (MDL 72222) another blocker of the 5-HT3 receptor. Moreover, the 5-HT3 receptor agonist 1-phenylbiguanide (tested in the cerebral cortex between 0.1 and 100 nM) enhanced CCK-LI release in a manner almost identical to that of 5-HT (EC50 = 0.64 +/- 0.071 nM). 4. It is concluded that 5-HT can act as a potent releaser of CCK-LI in rat cerebrocortex and nucleus accumbens through the activation of receptors of the 5-HT3 type situated on the CCK-releasing terminals. This interaction may provide a rationale for the clinical development of both 5-HT3 and CCK receptor antagonists as novel anxiolytic drugs.

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.

5-Hydroxytryptamine4 receptors mediate relaxation of the rat oesophageal tunica muscularis mucosae

The present study was designed to characterize an "atypical" 5-hydroxytryptamine (5-HT) receptor mediating relaxation of the rat oesophageal tunica muscularis mucosae. All experiments were performed under equilibrium conditions, using pargyline to inhibit the oxidative deamination of indoleamines, and cocaine and corticosterone to inhibit neuronal and extraneuronal uptake. Under these conditions 5-HT (0.3-1000 nmol/l) produced a concentration-dependent relaxation of carbachol-induced tension. The concentration-effect curve to 5-HT was unaffected by potent antagonists for 5-HT1, 5-HT2, 5-HT3 and so called 5-HT1P receptors (metergoline, methysergide, ketanserin, ondansetron, N-acetyl-5-hydroxytryptophyl-5-hydroxytryptophan amide), but was antagonized competitively by ICS 205-930 (pA2 = 6.7). Responses to 5-HT were mimicked by other indoleamines and substituted benzamides with the following order of potency: 5-HT greater than or equal to 5-methoxytryptamine greater than cisapride = alpha-methyl-5-HT = (S)-zacopride = renzapride greater than (RS)-zacopride greater than 5-carboxamido-tryptamine = metoclopramide = (R)-zacopride greater than tryptamine greater than 2-methyl-5-HT. ICS 205-930 afforded similar pA2 values (6.0-6.7) against each agonist, indicating a common site of action. Concentration-effect curves to 5-HT were not affected by tetrodotoxin or indomethacin, suggesting that 5-HT-induced relaxation of the tunica muscularis mucosae was mediated via a post-junctional receptor, independent of endogenous prostanoids. The pharmacological profile of the 5-HT receptor in the rat oesophageal tunica muscularis mucosae correlates well with the 5-HT4 receptor characterized recently in both the CNS and gastro-intestinal tract.

Investigation into the 5-hydroxytryptamine receptor mediating smooth muscle relaxation in the rat oesophagus

1. An investigation has been made into the 5-hydroxytryptamine (5-HT) receptor mediating relaxation of rat oesophagus in preparations precontracted with carbachol. 2. In tissues treated with pargyline (100 microM) and in the presence of corticosterone (30 microM) and cocaine (30 microM) the potency of 5-HT and 5-methoxytyramine (5-MeOT) was not changed but the maximum response to these agonists was reduced. Thus there was no evidence of metabolism and/or uptake through an amine depleting mechanism. 3. The relaxant concentration-effect curves to 5-HT were shifted to the left in a concentration-related manner by isobutylmethylxanthine (1 and 10 microM), suggesting the involvement of adenosine 3':5'-cyclic monophosphate in these responses. 4. 5-HT produced concentration-related relaxations of rat oesophagus with an EC50 value of 0.24 microM. Several indole agonists were tested and the following rank order of potency of key agonists obtained: 5-HT greater than alpha-methyl-5-hydroxytryptamine = 5-carboxamidotryptamine (5-CT) greater than 5-MeOT. In contrast, 2-methyl-5-hydroxytryptamine, sumatriptan and 8-hydroxy-2-(di-n-propylamino) tetralin were weak or inactive. 5. The substituted benzamides, metoclopramide, cisapride, renzapride and R,S-zacopride acted as partial agonists, producing 60-70% of the 5-HT maximum. 6. The relaxation responses to 5-HT were neither inhibited by antagonists selective for 5-HT1 or 5-HT2 receptors nor by the 5-HT3 receptor antagonists, ondansetron, granisetron or MDL 72222. 7. The relaxation responses induced by 5-HT, 5-CT, 5-MeOT and renzapride were selectively inhibited by high concentrations of ICS 205-930 with pKB values of approximately 6. 8. The 5-HT receptor mediating relaxation in rat oesophagus cannot be designated 5-HT1, 5-HT2 or 5-HT3 under the current 5-HT classification, but the observed effects are consistent with stimulation of the putative 5-HT4 receptor.

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.

Involvement of 5-hydroxytryptamine 3 receptors in regulation of interdigestive gastric contractions by motilin in the dog

The effects of IV injection of 5-hydroxytryptamine 3 receptor antagonists (BRL 43694 and GR38032F) on gastrointestinal contractile activity were studied in dogs with vagally denervated fundic pouch in the conscious state by means of chronically implanted force transducers in the gastrointestinal tract and the pouch. In the interdigestive state, 5-hydroxytryptamine 3 receptor antagonists (0.01-0.1 mg/kg), if given during phase III contractions, instantly and dose-dependently inhibited the spontaneous and motilin-induced phase III contractions in the intact stomach and altered the duodenal phase III contractions to a pattern of continuous contractions, the contractile force which was 65% of the spontaneous phase III contractions in the duodenum and caused immediate caudad migration of phase III contractions along the small intestine. However, the spontaneous and motilin-induced phase III-like contractions in the denervated pouch were not affected at all by 5-hydroxytryptamine 3 receptor antagonists. When 5-hydroxytryptamine 3 receptor antagonists (0.1-3.0 mg/kg) were given during the phase I period, they did not directly stimulate gastrointestinal contractions. The cyclic fluctuation of the plasma motilin concentration with phase III activity in the stomach was also not influenced by 5-hydroxytryptamine 3 receptor antagonists, but the next phase III contractions in the stomach were inhibited. During the digestive state, however, 5-hydroxytryptamine 3 receptor antagonists (0.1-3.0 mg/kg) did not influence contractile activity in the gastrointestinal tract and in the vagally denervated fundic pouch. On the basis of recent pharmacological studies showing that the distribution of 5-hydroxytryptamine 3 receptors is recognized in the area postrema, peripheral neurons of vagal afferents, and the enteric nervous system, the results of the current study provide a basis for a hypothesis that 5-hydroxytryptamine 3 receptor antagonists are most likely to block motilin-induced signals at 5-hydroxytryptamine 3 receptors on the vagal afferents. In conclusion, the present findings suggest the possible involvement of 5-hydroxytryptamine 3 receptors on vagal afferents especially in terms of endogenous release of acetylcholine in the control of interdigestive phase III activity in the stomach by motilin.

Ondansetron. Therapeutic use as an antiemetic

Ondansetron (GR 38032F) is a highly selective 5-HT3 receptor antagonist, one of a new class of compounds which may have several therapeutic applications. Animal and clinical studies show that ondansetron reduces the 24-hour incidence and severity of nausea and vomiting induced by cytotoxic drugs, including cisplatin, and by single exposure, high dose radiation. Ondansetron is more effective than high dose metoclopramide in the 24 hours following chemotherapy, and preliminary clinical evidence suggests that it is equally effective in the following 4 days. It is also more effective than the 'moderate' doses of metoclopramide used to suppress emesis following radiotherapy. The antiemetic efficacy of ondansetron is enhanced by dexamethasone in cisplatin-treated patients. Importantly, extrapyramidal effects have not been reported with ondansetron. Further comparisons are required with standard combination antiemetic therapy to complement the data presently available. Thus, ondansetron is a promising new agent for prophylaxis against nausea and vomiting in chemotherapy and radiotherapy. It may be particularly useful in young and elderly patients who are more susceptible to extrapyramidal symptoms induced by high dose metoclopramide. With its improved tolerability and clinical response profiles, ondansetron represents an important advance in a difficult area of therapeutics.

Ondansetron: a serotonin receptor (5-HT3) antagonist for antineoplastic chemotherapy-induced nausea and vomiting

Ondansetron represents a new class of drugs that exert their antiemetic activity by selective inhibition of a serotonin receptor subtype (5-HT3). Ondansetron has marked activity against emesis associated with cisplatin and other highly emetogenic drugs. Compared with high doses of metoclopramide, the antiemetic "gold standard," it demonstrates equal or superior efficacy. Although ondansetron is moderately well absorbed after oral administration, only a parenteral formulation will initially be available. Ondansetron is eliminated almost entirely by hepatic metabolism; less than five percent of an intravenously administered dose is recovered intact in urine. The half-life of ondansetron is approximately 3.5 hours; slightly shorter in children and prolonged in the elderly. Neither clinical efficacy nor adverse effects have correlated with serum concentrations. Ondansetron is generally well tolerated. Clinically relevant adverse effects include headache, diarrhea or constipation, sedation, and transient minor elevations of liver function tests. It is not associated with extrapyramidal reactions. Ondansetron is indicated as prophylaxis for nausea and vomiting associated with emetogenic chemotherapy. Studies to further evaluate and define its use are ongoing.