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

Roles of substance P and NK(1) receptor in the brainstem in the development of emesis

The emetic response is primarily a protective reflex occurring in a wide variety of vertebrates in response to the ingestion of toxic compounds. The role of the nuclei in the brainstem, including the area postrema, nucleus tractus solitarius, the dorsal motor nucleus of the vagus, and the central pattern generator for vomiting, as well as the involvement of the abdominal visceral innervation relevant to the emetic reflex, have all been discussed by many researchers. The introduction of serotonin 5-HT(3)-receptor antagonists into clinical practice allowed for a dramatic improvement in the management of vomiting. However, vomiting still remains a significant problem. The mechanism of the emetic response is even more complicated than was first thought. This review attempts to bring together some of the evidence suggesting the roles of substance P and its receptor, neurokinin NK(1) receptor, in the brainstem nuclei in the development of emesis. Accordingly, NK(1)-receptor antagonists might represent novel drugs for the management of major types of emesis.

Effects of alosetron on spontaneous migrating motor complexes in murine small and large bowel in vitro

Alosetron (Lotronex) is a serotonin subtype 3 (5-HT3) receptor antagonist that alleviates symptoms of irritable bowel syndrome (IBS) in female patients. Alosetron may act centrally, involve the alteration of ascending pain sensation, or modulate peristaltic, secretory, or sensory function. To investigate further the mechanisms underlying its action and gender selectivity we recorded the effect of increasing concentrations of alosetron or ondansetron on spontaneous migrating motor complexes (MMCs) from isolated terminal ileum or colon from C57BL/6 mice. Both antagonists inhibited MMC frequency before affects on duration or amplitude. The threshold of inhibition for alosetron was 100-fold less in small intestine from females (20 nM) than from males. The opposite effect of gender was observed with ondansetron in the colon. All MMCs were abolished by either drug at 10 microM. Our results demonstrate that alosetron selectively inhibits MMC frequency in isolated preparations of murine bowel. Because contractile events in the ileum correlate with symptoms of IBS in humans, the gender selectivity of alosetron may be caused by a direct action within the small intestine.

Ondansetron: a selective 5-HT(3) receptor antagonist and its applications in CNS-related disorders

Ondansetron is a selective 5-hydroxytryptamine(3) (5-HT(3)) receptor antagonist that has been introduced to clinical practice as an antiemetic for cancer treatment-induced and anesthesia-related nausea and vomiting. Its use under these circumstances is both prophylactic and therapeutic. It has a superior efficacy, safety and pharmacoeconomic profile compared with other groups of antiemetics, namely antidopaminergics, antihistamines and anticholinergics. However, its place in the management of anticipatory and delayed vomiting in cancer treatment and as a rescue antiemetic in surgical patients needs to be further explored. Furthermore, recent animal and human research also reflects its possible novel application in the treatment of other disease states, such as alcoholism, cocaine addiction, opioid withdrawal syndrome, anxiety disorders, gastrointestinal motility disorders, Tourette's syndrome and pruritus. This review revisits the widespread physiological and pathological effects of 5-HT and discusses both the basic science literature and the clinical developments responsible for the conventional and novel uses of ondansetron. In addition, new discoveries relating to the effects of ondansetron on other receptors/channels and their possible therapeutic applications are presented.

Association of two 3D QSAR analyses. application to the study of partial agonist serotonin-3 ligands

CATALYST and COMFA, two software packages for 3D QSAR studies, were associated to correlate the three-dimensional structures of 75 serotonin 5-HT3 ligands to their biological affinities. The conformational analysis and the influence of chemical function-based alignments (the basis of this association) on final results are discussed in this publication. These two analyses allow for precisely quantitating the weights of significant chemical groups or functions on the biological affinities.

R-zacopride, a 5-HT3 antagonist/5-HT4 agonist, reduces sleep apneas in rats

The effects of R-zacopride, a benzamide with potent 5-HT3 receptor antagonist and 5-HT4 receptor agonist properties, on spontaneous apneas were studied in 10 Sprague-Dawley rats by monitoring respiration and sleep for 6 h. R-zacopride (0.5, 1.0 and 10.0 mg/kg) suppressed spontaneous central apneas during non-rapid-eye-movement (NREM) sleep by 50% (P=.05 for 0.5 mg/kg, P=.02 for 1.0 mg/kg and P=.001 for 10.0 mg/kg dose vs. control), and during rapid-eye-movement (REM) sleep by 80% by all doses tested (P<.0007) for at least 2 h after intraperitoneal injection. We conclude that R-zacopride, over a 20-fold dose range, significantly reduces central apnea expression during NREM and REM sleep in the rat. The efficacy of this compound to suppress central apneas most probably arises from its antagonist actions at 5-HT3 receptors or from its mixed agonist/antagonist profile at 5-HT4/5-HT3 receptors.

The effects of propofol in the area postrema of rats

Propofol has an antiemetic effect that may be mediated by gamma-aminobutyric acid (GABA) influences on the serotonin system, the mechanism of which is not known. We used three techniques, immunohistochemistry, High Performance Liquid Chromatography, and electrophysiology, to define propofol's effects on the rat's brainstem. Paired male Wistar rats received propofol, 20 mg/kg/hr, or Intralipid for 6 h. The brains were then subjected to immunohistochemical analysis of serotonin. In a separate experiment after a propofol or Intralipid infusion, cerebrospinal fluid (CSF) was extracted from the fourth ventricle and analyzed for the amount of serotonin and 5-hydroxyindoleacetic acid. Electrophysiological neuronal recordings were made in the area postrema (AP) in response to propofol with and without a GABA or serotonin antagonist. Results showed that immunohistochemical staining for serotonin in the propofol rats was significantly increased (28 +/- 12%) in the dorsal raphe and decreased in the AP (17 +/- 6%) compared with control. There were no significant changes in the isoflurane-anesthetized animals. Both serotonin and 5-hydroxyindoleacetic acid in the CSF of the fourth ventricle at the level of the AP were significantly reduced by 63% and 36%, respectively. Both propofol and pentobarbital injections reduce AP neuronal activity, but only the propofol response was blocked by bicuculline, a GABA antagonist. We conclude that the reduced levels of serotonin in the AP and the CSF may explain the antiemetic property of propofol. Propofol may also directly act on AP neurons via a GABA(A) receptor to reduce their activity.

IMPLICATIONS

Propofol may produce its antiemetic effect by depleting the area postrema of serotonin as well as by a direct gamma-aminobutyric acid-mediated inhibition.

Feedback stimulation of somatodendritic serotonin release: a 5-HT3 receptor-mediated effect in the raphe nuclei of the rat

Slices from rat midbrain containing the raphe nuclei and from hippocampus were prepared, loaded with [3H]5-HT and superfused and the resting and the electrically stimulated [3H]5-HT release was measured. The 5-HT3 receptor agonist 2-methyl-5-HT (1 to 10 micromol/l) increased the resting tritium outflow in superfused raphe nuclei slices, EC50 5.3 micromol/l. The 2-methyl-5-HT-induced increase of tritium outflow was an external Ca2+-independent process and was not altered by reserpine pretreatment but it was reversed by addition of the 5-HT uptake inhibitor fluoxetine (1 micromol/l). The 5-HT3 receptor antagonists ondansetron and GYKI-46 903 (1 micromol/l) did not antagonize the stimulatory effect of 2-methyl-5-HT on resting tritium outflow. 2-Methyl-5-HT in lower concentration increased the electrically induced tritium overflow from raphe nuclei slices (EC50 0.56 micromol/l) and also from hippocampal slices preloaded with [3H]5-HT. These effects were reversed by 1 micromol/l of ondansetron and GYKI-46903. The 5-HT3 receptor antagonists (1 micromol/l) were without effects on depolarization-evoked [3H]5-HT release at 2 Hz stimulation, when 10 Hz stimulation was used, ondansetron and GYKI-46 903 reduced the tritium overflow from raphe nuclei slices. These data indicate that 5-HT3 receptors positively alter depolarization-induced somatodendritic 5-HT release in the raphe nuclei. They also show that 2-methyl-5-HT is able to evoke 5-HT release not only from vesicles but also from cytoplasmic stores via a transporter-dependent exchange process.

Pharmacological profile of YM-31636, a novel 5-HT3 receptor agonist, in vitro

We investigated the in vitro pharmacological profile of YM-31636 (2-(1H-imidazol-4-ylmethyl)-8H-indeno[1,2-d]thiazole monofumarate). In cloned human 5-HT3A receptors, YM-31636 had a pKi value of 9.67 vs. ramosetron and pKi values for other 5-HT3 receptor agonists were less than 7. YM-31636 showed very low affinities for other receptors. YM-31636 induced contraction of isolated guinea pig distal colon. The intrinsic activity was approximately 0.90 compared with 5-hydroxytryptamine's (5-HT) 1.0, and the potency was 26 times greater than that of 5-HT. YM-31636 increased short-circuit current (Isc) in the isolated guinea pig distal colon. In this case, the relative intrinsic activity was approximately 0.19. In isolated guinea pig right atrium, YM-31636 induced tachycardia with the relative intrinsic activity of approximately 0.23. All these effects of YM-31636 were antagonized by ramosetron, a selective 5-HT3 receptor antagonist. These results suggest that YM-31636 is a potent and selective 5-HT3 receptor agonist, preferentially acting on the contraction of the colon.

Pharmacological characterization of the 5-HT receptor-mediated contraction in the mouse isolated ileum

The pharmacological characterization of a 5-HT receptor-mediated contractile response in the mouse isolated ileum is described. In the presence of methysergide (1 microM), 5-hydroxytryptamine (5-HT, 0.3 - 100 microM) produced phasic concentration-dependent contractions of segments of the mouse isolated ileum with a pEC(50) value of 5.47+/-0.09. The 5-HT(3) receptor selective agonists m-chlorophenylbiguanide (0.3 - 100 microM, pEC(50) 5.81+/-0.04), 1-phenylbiguanide (3 - 100 microM, pEC(50) 5.05+/-0.06) and 2-methyl-5-HT (3 - 100 microM, pEC(50) 5.00+/-0.07) acted as full agonists to induce contractile responses. 5-methoxytryptamine (0.1 - 100 microM), RS 67506 (0.1 - 100 microM) and alpha-methyl-5-HT (0.1 - 100 microM) failed to mimic the 5-HT responses. The contractile response to 5-HT was not antagonized by either 5-HT(2) receptor antagonists ritanserin (0.1 microM) or ketanserin (1 microM) nor the 5-HT(4) receptor antagonist SB 204070 (0.1 microM). The 5-HT(3) receptor selective antagonists granisetron (0.3 - 1 nM), tropisetron (1 - 10 nM), ondansetron (10 nM - 1 microM) and MDL 72222 (10 nM - 1 microM) caused rightward displacement of the concentration-response curves to 5-HT. The lower concentrations of the antagonists caused approximate parallel rightward shifts of the concentration-response curves to 5-HT with apparent pK(B) values for granisetron (9.70+/-0. 39), tropisetron (9.18+/-0.20), ondansetron (8.84+/-0.24) and MDL 72222 (8.65+/-0.35). But higher concentrations of antagonists resulted in a progressive reduction in the maximum responses. The contractile response to 5-HT was abolished by tetrodotoxin (0.3 microM); atropine (0.1 and 1 microM) decreased the maximum response of the 5-HT concentration-response curve by approximately 65%. It is concluded that a neuronally located 5-HT(3) receptor mediates a contractile response to 5-HT in the mouse ileum. The 5-HT(3) receptor in the mouse ileum has a different pharmacological profile to that reported for the guinea-pig ileum.

Selective potentiating effect of RS14203 on a serotoninergic pathway in anesthetized rats

The usefulness of selective inhibitors of type 4 phosphodiesterase (PDE4) in the treatment of inflammation and pulmonary diseases is limited by their side effects: nausea and vomiting. We studied the effect of three structurally diverse PDE4 inhibitors on the vagal nerve afferent and efferent fibers in anesthetized rats. The effects of RS14203, (R)-rolipram, and CT-2450 were evaluated on the von Bezold-Jarisch reflex (vagal afferent fibers) and in a model of vagal electrical stimulation (vagal efferent fibers). All three PDE4 inhibitors were administered at 1, 10, or 100 µg/kg (iv) 15 min prior to the induction of bradycardia by an iv injection of 2-methyl-5-HT (von Bezold-Jarisch reflex) or by vagal electrical stimulation. At 100 µg/kg, RS14203 significantly potentiated the 2-methyl-5-HT response. No statistically significant effects were observed with (R)-rolipram or CT-2450 at the doses studied. RS14203, (R)-rolipram, or CT-2450 (1-100 µg/kg iv) did not affect the bradycardia induced by vagal electrical stimulation. Consequently, our results show that RS14203 selectively facilitates serotoninergic neurotransmission in vagal afferent fibers. The emetic action of RS14203 may be mediated by this mechanism.Key words: PDE4 inhibitors, von Bezold-Jarisch reflex, emesis, vagal afferent and efferent fibres, bradycardia.

Comparative review of 5-HT3 receptor antagonists in the treatment of acute chemotherapy-induced nausea and vomiting

Since their introduction, 5-HT3 receptor antagonists have become the agents of choice in the prevention of acute chemotherapy-induced nausea and vomiting and are generally superior to high-dose metoclopramide regimens. The availability of four different agents (ondansetron, granisetron, dolasetron, and tropisetron) within this class has prompted investigations into potential differences between the drugs, which appear to be few. More importantly, the results of recently conducted randomized comparative trials in patients receiving moderately or highly emetogenic chemotherapy have demonstrated similar efficacy. Although study designs and patient populations differed, seven large comparative trials in patients receiving highly emetogenic chemotherapy reported no significant differences in complete or complete plus major response rates among the agents. Similar results were generally reported in trials evaluating patients receiving moderately emetogenic chemotherapy. The safety and tolerability of these agents also appear to be similar. The most common adverse events include headache, gastrointestinal effects, lightheadedness, and sedation. All agents are available in both intravenous and oral dosage forms and may be administered as a single dose.

The novel NK1 receptor antagonist MK-0869 (L-754,030) and its water soluble phosphoryl prodrug, L-758,298, inhibit acute and delayed cisplatin-induced emesis in ferrets

The anti-emetic profile of the novel brain penetrant tachykinin NK1 receptor antagonist MK-0869 (L-754,030) 2-(R)-(1-(R)-(3,5-bis(trifluoromethyl)phenylethoxy)-3-(S)-(4-fluor o)phenyl-4-(3-oxo-1,2,4-triazol-5-yl)methylmorpholine and its water soluble prodrug, L-758,298, has been examined against emesis induced by cisplatin in ferrets. In a 4 h observation period, MK-0869 and L-758,298 (3 mg/kg i.v. or p.o.) inhibited the emetic response to cisplatin (10 mg/kg i.v.). The anti-emetic protection afforded by MK-0869 (0.1 mg/kg i.v.) was enhanced by combined treatment with either dexamethasone (20 mg/kg i.v.) or the 5-HT3 receptor antagonist ondansetron (0.1 mg/kg i.v.). In a model of acute and delayed emesis, ferrets were dosed with cisplatin (5 mg/kg i.p.) and the retching and vomiting response recorded for 72 h. Pretreatment with MK-0869 (4-16 mg/kg p.o.) dose-dependently inhibited the emetic response to cisplatin. Once daily treatment with MK-0869 (2 and 4 mg/kg p.o.) completely prevented retching and vomiting in all ferrets tested. Further when daily dosing began at 24 h after cisplatin injection, when the acute phase of emesis had already become established, MK-0869 (4 mg/kg p.o. at 24 and 48 h after cisplatin) prevented retching and vomiting in three out of four ferrets. These data show that MK-0869 and its prodrug, L-758,298, have good activity against cisplatin-induced emesis in ferrets and provided a basis for the clinical testing of these agents for the treatment of emesis associated with cancer chemotherapy.

5-HT(3) receptor antagonists

The 5-HT(3) receptor is a ligand-gated ion channel widely distributed in the central and peripheral nervous systems. Many selective 5-HT(3) receptor antagonists have been developed; animal studies with such compounds suggested their potential therapeutic value in combating emesis and a wide range of CNS diseases including anxiety, schizophrenia, drug dependence and Alzheimer's disease. Their successful introduction as anti-emetics, with irritable bowel syndrome emerging as a further indication have partially fulfilled this initial promise. However, the CNS area has been less productive and, to date, no selective 5-HT(3) receptor antagonist has been approved for use in a CNS disease.

Characterization of the 5-hydroxytryptamine receptors mediating contraction in the intestine of Suncus murinus

1. The effects of 5-HT and 5-HT agonists to induce contraction and the 5-HT receptors mediating these effects were investigated in the proximal, central and terminal intestinal segments of Suncus murinus. 2. The contraction curves to 5-HT (3 nM - 30 microM) were shifted to the right by methysergide (1 microM) and ritanserin (0.1 microM), without affecting the maximum response. 3. In the central and terminal segments (but not the proximal segments) ondansetron (1 microM) and atropine (1 microM) significantly attenuated the contractions to higher concentrations of 5-HT. The selective 5-HT4 receptor antagonist SB204070 (1 nM), failed to modify 5-HT induced contractions in any segment examined. 4. 5-carboxamidotryptamine, alpha-methyl-5-HT and 5-methoxytryptamine (0.003 - 3.0 microM) induced contractions but unlike 5-HT, higher concentrations of these three agents failed to increase the response or were associated with a decrease in response. 2-methyl-5-HT (0.03 - 1.0 microM) was ten times less potent than 5-HT to induce contraction but achieved the same maximum response. 5. The contractions induced by the lower concentrations of 2-methyl-5-HT (0.03 - 1.0 microM) in all segments were markedly reduced or abolished by methysergide (1.0 microM); the response to the higher concentrations of 2-methyl-5-HT (3 - 30.0 microM) were markedly reduced by atropine (1.0 microM) and ondansetron (1.0 microM). 6. In all segments examined, tetrodotoxin (1 microM) significantly reduced the 5-HT-induced contraction. 7. It is concluded that the 5-HT-induced contraction was mediated via 5-HT2 (ritanserin sensitive) receptors in all regions of the intestine, with 5-HT3 (ondansetron sensitive) receptors mediating an additional major component in the central and terminal regions.

Modulation of emesis by fentanyl and opioid receptor antagonists in Suncus murinus (house musk shrew)

The anti-emetic mechanism of action of fentanyl to inhibit nicotine (5 mg/kg, s.c.)-induced emesis was investigated in Suncus murinus. The anti-emetic action of fentanyl (40 microg/kg, s.c.) was antagonised by the opioid receptor antagonists naltrexone (1 mg/kg, s.c.), naloxone (1 mg/kg, s.c.), M8008 (16S-methylcyprenorphine; 1 mg/kg, s.c.) and MR 2266 (5,9-diethyl-2-(3-furylmethyl)2'-hydroxy-7,7-benzomorphan; 1 mg/kg) but not by naloxone methylbromide (1 mg/kg, s.c.), naloxone methyliodide (1 mg/kg, s.c.), naltrindole (1 mg/kg, s.c.), DIPPA (2-(3,4-dichlorophenyl)-N-methyl-N-[1S)-1-(3-isothiocyanatophenyl)-2-(1- pyrrolidinyl)-ethyl]acetamide; 3 mg/kg, i.p.) or naloxonazine (35 mg/kg, i.p.). This indicates an involvement of mu2-opioid receptors within the brain to mediate the anti-emetic effect of fentanyl. In other studies, naloxone 10-60 mg/kg, s.c. induced dose-related emesis but naltrexone was only emetic at 60 mg/kg, s.c. and naloxone methylbromide failed to induce emesis at doses up to 60 mg/kg, s.c. The emesis induced by a high dose of naloxone 60 mg/kg, s.c. was antagonized by CP-99,994 ((+)-(2S,3S)-3-(2-methoxybenzylamino)-2-phenylpiperidine; 3-30 mg/kg, i.p.), 8-OH-DPAT, ((+/-)-8-hydroxy-dipropylaminotetralin; 0.003-0.3 mg/kg, s.c.), buspirone (3 mg/kg, s.c.) and fluphenazine (1-3 mg/kg, i.p.) but not by naltrexone (1-30 mg/kg, s.c.), metoclopramide (0.3-3 mg/kg, i.p.), sulpiride (0.3-3 mg/kg, i.p.), domperidone (0.1-3 mg/kg, i.p.), ondansetron (0.3-3 mg/kg, i.p.), granisetron (0.3-3 mg/kg, i.p.), scopolamine (0.3-3 mg/kg, i.p.) or promethazine (0.3-3 mg/kg, i.p.). The data is discussed in relation to opioid receptor mechanisms moderating emesis and the identification of potential sites of drug action available to inhibit the emetic reflex.

Intragastric capsaicin stimulates motility of upper gut and proximal colon via distinct pathways in conscious dogs

The aim of the present study was to investigate the effect and mechanism of action of intragastric and intraduodenal capsaicin on gastrointestinal motility. Five mongrel dogs were equipped with eight strain gauge force transducers on the stomach, small intestine, and proximal colon. In the interdigestive state, capsaicin was administered into the gastric or duodenal lumen. The effects of atropine, hexamethonium, ondansetron, and FK888 on capsaicin-induced contractions were studied. Intragastric capsaicin induced contractions within 15 min in the gastric antrum, duodenum, proximal jejunum, and proximal colon. These stimulatory effects were inhibited by atropine at all sites; by hexamethonium in the small intestine and colon; by ondansetron in the antrum, duodenum, and colon; and by FK888 in the antrum and colon, respectively. Intraduodenal capsaicin had no effect on contractility. Stimulation of afferent fibers by capsaicin in the stomach but not in the duodenum augments contractile activity in local and distant regions of the gut via distinct pathways.

The pharmacological properties of the novel selective 5-HT3 receptor antagonist, alosetron, and its effects on normal and perturbed small intestinal transit in the fasted rat

The purpose of this study was to investigate the pharmacological properties of the novel, selective 5-HT3 receptor antagonist, alosetron, and its effects on transit time in both the normal and perturbed small intestine of the rat. Alosetron concentration-dependently inhibited radioligand binding in membranes containing rat and human 5-HT3 receptors with estimated pKi values of 9.8 (n = 3) and 9.4 (n = 6), respectively. In selectivity studies alosetron had little or no significant affinity for any of the many other receptors and ion channels studied. Alosetron potently antagonized the depolarization produced by 5-HT in the rat vagus nerve (estimated pKB value of 9.8, n = 25). In anaesthetized rats, i. v. administration of alosetron inhibited 2-methyl-5-HT induced bradycardia (Bezold Jarisch index) at 1 and 3 microg kg-1, with an agonist dose ratio of approximately 3.0 at 1.0 microg kg-1, = 3-5). Alosetron administered via the duodenum also inhibited this reflex, with duration of action that was significantly longer than that seen with ondansetron (120-60 min, respectively, n = 6). Alosetron had no significant effect on normal small intestinal propulsion in the rat, but fully reversed the increase in intestinal propulsion (96%, n = 3) produced by egg albumin challenge. Alosetron is a highly selective 5-HT3 antagonist which normalizes perturbed small intestinal propulsion. Previous clinical data in IBS patients together with the transit data provide a good rationale for further studies with alosetron in IBS patients.

5HT antagonists attenuate MK801-impaired radial arm maze performance in rats

Glutamatergic hypofunction occurs in Alzheimer's disease (AD). MK801, a noncompetitive blocker of glutamate N-methyl-D-aspartate receptors, was used to disrupt the cognitive performance of rats trained on a delayed nonmatching to sample radial maze task. Drugs which act by blocking serotonin (5-HT) receptors were evaluated for their ability to reduce the cognitive impairment produced by MK801. Specifically, WAY-100635, a selective 5-HT1A receptor antagonist, buspirone, a 5-HT1A partial agonist, ritanserin, a 5-HT2 antagonist, and ondansetron, a 5-HT3 antagonist, were assessed. In addition, the muscarinic agonist arecoline was evaluated for its potential cognitive benefit in this model. It was found that WAY-100635 significantly reduced the cognitive impairment induced by MK801. Treatment with single doses of ritanserin, ondansetron, or arecoline in combination with MK801 did not result in a cognitive impairment, indicating that these drugs attenuated the MK801 impairment. The combination of buspirone and MK801 resulted in an inability of the animals to complete the task. These results suggest that interactions between 5-HT and glutamate may mediate the beneficial effects of reducing cognitive impairment and that 5-HT antagonists, especially selective 5-HT1A antagonists, may be useful in treating AD. Further, it is indicated that the MK801 model of cognitive impairment may add to the armamentarium of tools available to predict treatment efficacy in AD.

Role of peripheral serotonin in the regulation of central sleep apneas in rats

STUDY OBJECTIVES

The aim of our study was to determine the effects of serotonin (5-HT), which does not penetrate the blood-brain barrier (BBB), and GR38032F, a 5-HT3 receptor antagonist that may cross the BBB, on spontaneous apneas in adult Sprague-Dawley rats.

MEASUREMENTS AND RESULTS

Rats were implanted with electrodes for EEG and electromyographic recording to monitor sleep, with a radiotelemetry transmitter for monitoring aortic BP and heart period (HP) and were placed inside a single chamber plethysmograph for monitoring respiration. Sleep, BP, HP, and respiration were monitored for 6 h following administration of drugs. Intraperitoneal injection of 5-HT (0.79 mg/kg) to rats increased spontaneous central apneas during rapid eye movement (REM) sleep by > 250% in comparison to control recording (p = 0.01). GR38032F (0.1 mg/kg), which produced no effect on apnea expression, completely blocked the 5-HT-induced increase in REM apneas. Administration of 5-HT did not affect apnea expression in non-REM sleep and had no effect on sleep or BP.

CONCLUSIONS

From these observations, we conclude that binding at 5-HT3 receptors in the peripheral nervous system promotes REM-related apnea genesis in rats. These findings further suggest that endogenous 5-HT, acting at least at peripheral 5-HT3 receptors, may play a baseline physiologic role in the expression of spontaneous central apneas in rats.

Selective 5-hydroxytryptamine3 (5-HT3) receptor blocking activity of KB-R6933, a novel benzimidazole derivative

5-Hydroxytryptamine3 (5-HT3)-receptor blocking activities of KB-R6933 (6-amino-5-chloro-1-isopropyl-2-(4-methyl-1-piperazinyl)-benzimidazole dimaleate) were assessed in vivo and in vitro. Intravenous administration of KB-R6933, granisetron, ondansetron and azasetron inhibited 5-HT-induced bradycardia (von Bezold-Jarisch reflex) in anesthetized rats, with ED50 values of 0.071, 0.71, 4.0 and 0.82 microg/kg, respectively. The inhibitory effect of KB-R6933 at a dose of 0.3 microg/kg lasted for at least 8 hr, whereas those of granisetron at 30 microg/kg, ondansetron at 100 microg/kg and azasetron at 30 microg/kg nearly disappeared within 2-4 hr. Oral administration of KB-R6933 and granisetron also inhibited the bradycardia, with ED50 values of 0.41 and 76.3 microg/kg, respectively. In guinea pig ileum, KB-R6933 concentration-dependently antagonized 5-HT-evoked contraction and reduced the maximal contraction (pK(B)=8.75). Granisetron, ondansetron and azasetron shifted the dose-response curve for 5-HT to higher concentrations with no reduction of maximal contraction, and their pK(B)s were 7.65, 7.00 and 6.29, respectively. In a radioligand receptor binding study, KB-R6933, granisetron, ondansetron and azasetron displaced [3H]GR65630 binding to rat entorhinal cortex membrane, with Ki values of 0.066, 0.99, 2.70 and 2.5 nM, respectively. On the other hand, KB-R6933 exhibited negligible affinities for other receptors or binding sites tested, except for a weak affinity for the cholinergic M1-receptor, even at concentrations up to 10 microM. These results suggest that KB-R6933 is a potent and selective 5-HT3-receptor antagonist with a longer duration of action than those of existing 5-HT3-receptor antagonists.

Definition of a pharmacophore for partial agonists of serotonin 5-HT3 receptors

A definition of a partial agonists serotonin 5-HT3 pharmacophore was carried out by considering a three-dimensional model which correlates the chemical structures of series of piperazinopyrrolothienopyrazines, piperazinopyridopyrrolopyrazines, piperazinopyrroloquinolaxines, piperazinopyridopyrroloquinoxalines, aminoalkyloximinopyrroloindoles, aminoalkyloximinothienopyrrolizines, and aminoalkyloximinopyrrolizines with the biological affinities. The model is formed by five features corresponding to two hydrogen bond acceptors, one aromatic ring, one hydrophobic group, and one positive ionizable site (quaternary ammonium ions). The nature of the features and the distances between them explain the partial agonist activities of these compounds.

Prophylactic anti-emetic efficacy of ondansetron in laparoscopic cholecystectomy under total intravenous anaesthesia. A randomised, double-blind comparison with droperidol, metoclopramide and placebo

The prophylactic anti-emetic efficacy and safety of pre-operative intravenous ondansetron was evaluated in a randomised, double-blind, comparison with droperidol, metoclopramide and placebo in 160 ASA grade 1 and 2 patients undergoing laparoscopic cholecystectomy under total intravenous anaesthesia. The patients were randomly allocated to receive ondansetron (4 mg), droperidol (1.25 mg), metoclopramide (10 mg) or placebo given as a single intravenous dose immediately before induction of a standardised general anaesthetic. There were no significant differences between the four study groups with regard to the demographic and anaesthetic data, postoperative analgesia, postoperative sedation scores, duration of postoperative hospital stay and incidence of adverse events. The incidence of nausea and vomiting was significantly lower (p < 0.05) between 1 h and 4 h after surgery in the ondansetron group compared with the droperidol, metoclopramide and placebo groups. The incidence of nausea was similar in the four groups in the other study periods: 0-1 h and 4-24 h. The incidence of vomiting was lower in the ondansetron, droperidol and metoclopramide groups than in the placebo group between 1 and 4 h but was the same between 4 and 24 h. As a result of the lower incidence of nausea and vomiting between 1 h and 4 h in the ondansetron group, the overall incidence of nausea and vomiting was lower during the first 24 h after surgery in this group than in the other three groups.

Guidelines for anti-emetic therapy: acute emesis

Anti-emetic therapy has become integral to the management of patients with cancer. Goals related to complete emesis control include providing treatment that reduces hospitalisation and time in the ambulatory setting, care that is convenient for the patient and therapy that enhances patients' quality of life. A panel of clinical, health economic and basic scientists with expertise in various oncology disciplines reviewed published literature to develop evidence-based consensus guidelines for the prevention and treatment of chemotherapy-induced emesis. Currently, serotonin receptor antagonists and corticosteroids are the two categories of anti-emetics that are most effective, have the fewest side-effects and are convenient to use. These agents are recommended in combination for highly emetogenic chemotherapy regimens and as single agents or in combination for moderately to highly emetogenic chemotherapy. When possible, these agents may be given orally in single doses; current evidence does not support dose escalation for either category of anti-emetics. In special situations, such as the use of high-dose chemotherapy combination regimens, the most emetogenic component of the regimen should dictate the choice of anti-emetic. Appropriate anti-emetic use described in these guidelines represents both good medical practice and a sensible economic approach to care.

Cisplatin impairs fluid and electrolyte absorption in rat small intestine: a role for 5-hydroxytryptamine

BACKGROUND

The antineoplastic drug cisplatin has been widely used for the treatment of cancer in humans but its use has been limited by vomiting and diarrhoea. Cisplatin releases 5-hydroxytryptamine into the gut which is thought to be the major mediator of cisplatin induced vomiting.

AIM

To determine whether cisplatin affects fluid and electrolyte transport in rat jejunum and whether this change can be modulated by the 5-hydroxytryptamine3 receptor antagonist, ondansetron.

METHODS

Jejunal perfusion in rats in vivo was performed one hour after intraperitoneal cisplatin (5 and 10 mg/kg) administration. The effect of pretreatment with subcutaneous ondansetron 300 microg/kg was investigated.

RESULTS

Median net fluid absorption after cisplatin 10 mg/kg (67 microl/min/g dry intestinal weight (interquartile range 46 to 100); n = 15) was reduced compared with controls (120 (107 to 151) microl/min/g; n = 13; p<0.001). Ondansetron reversed the impairment of jejunal fluid absorption produced by cisplatin to normal (161 (130 to 176) microl/min/g; n = 11; p<0.001). Electrolyte movement paralleled fluid movement. Jejunal histological examination of sections from cisplatin treated animals showed villus damage, which was not prevented by pretreatment with ondansetron.

CONCLUSION

These findings suggest that diarrhoea during cisplatin therapy may be due to altered fluid transport in the small bowel. The reversal of fluid transport to normal in the presence of a 5-hydroxytryptamine3 receptor antagonist suggests that 5-hydroxytryptamine is a local mediator in the small intestine.

Nitrogen-containing heteroalicycles with serotonin receptor binding affinity: development of gastroprokinetic and antiemetic agents

To obtain gastroprokinetic agents with more potent and selective activity than metoclopramide and cisapride, a series of N-(4-benzyl-2-morpholinylmethyl)benzamides were designed and prepared. Their synthesis and structure-activity relationships were described. As a result, mosapride was selected as a promising candidate for potent gastroprokinetic activity with selective 5-HT4 receptor agonistic activity. As an extension to this project, the novel benzamide and the carboxamide derivatives having 1-benzyl-4-methylhexahydro-1,4-diazepine ring in the amine moiety were prepared and evaluated for 5-HT3 receptor antagonistic activity. DAT-582 was identified as an antiemetic agent in cancer chemotherapy. The asymmetric synthesis of DAT-582 and the SAR studies were briefly reviewed. In further modifications of the N-(1-benzyl-4-methylhexahydro-1,4-diazepin-6-yl)benzamides, the novel nicotinamides with 1-ethyl-4-methylhexahydro-1,4-diazepin ring were found to have potent 5-HT3 and dopamine D2 and D3 receptor antagonistic activities and to show weak central nervous system depression and extrapyramidal syndrome. After extensive SARs, AS-8112 was selected as a broad antiemetic agent.

Benzoxazole derivatives as novel 5-HT3 receptor partial agonists in the gut

A series of benzoxazoles with a nitrogen-containing heterocyclic substituent at the 2-position was prepared and evaluated for 5-HT3 partial agonist activity on isolated guinea pig ileum. The nature of the substituent at the 5-position of the benzoxazole ring affected the potency for the 5-HT3 receptor, and the 5-chloro derivatives showed increased potency and lowered intrinsic activity. 5-Chloro-7-methyl-2-(4-methyl-1-homopiperazinyl)benzoxazole (6v) exhibited a high binding affinity in the same range as that of the 5-HT3 antagonist granisetron, and its intrinsic activity was 12% of that of 5-HT. Compound 6v inhibited 5-HT-evoked diarrhea but did not prolong the transition time of glass beads in the normal distal colon even at a dose of 100 times the ED50 for diarrhea inhibition in mice. Compounds of this type are expected to be effective for the treatment of irritable bowel syndrome without the side effect of constipation.

The response of rat colonic mucosa to 5-hydroxytryptamine in health and following restraint stress

The present study characterized the rat colonic secretory response to 5-hydroxytryptamine (5-HT) and determined alterations in this response following stress. 5-HT stimulated rat colonic short-circuit current in a concentration-dependent fashion (pD2 = 5.19). This response was subject to desensitization and was mimicked by the indolealkylamines with a rank order potency of 5-HT approximately alpha-methyl-5-HT > 5-carboxytryptamine approximately 5-methoxytryptamine. 2-Methyl-5-HT was a partial agonist. The colonic response to 5-HT was unaltered by methysergide (10 microM), ritanserin (0.1 microM), ondansetron (1 microM) or clozapine (10 microM), but was antagonized by the 5-HT4 receptor antagonists SB204070 (pD'2 = 9.32), GR113808 (pKb = 8.56), DAU6285 (pKb = 6.07) and SDZ205557 (pKb = 6.80). The response of colonic epithelial and oesophageal tunica muscularis mucosae to 5-HT is therefore mediated by a similar 5-HT4 receptor. Following wrap restraint stress, the colonic response to 5-HT became bimodal. Half of the preparations were hyper-responsive, while the rest were hypo-responsive to 5-HT. This 5-HT4 receptor may therefore be involved in stress related changes in fluid transport.

A double blind comparison of droperidol and ondansetron for prevention of emesis in children undergoing orthopaedic surgery

Emesis is common in the postoperative period following epidural opioid and general anaesthesia. Eighty patients ages two to 14 years scheduled for major orthopaedic surgery were enrolled in a randomized, double-blind study to compare the prophylactic effects of ondansetron, droperidol and a placebo for the prevention of postoperative emesis. Each child was assigned at random to one of the four treatment groups: ondansetron 100 micrograms.kg-1, ondansetron 50 micrograms.kg-1, droperidol 60 micrograms.kg-1 and saline control. Drugs were administered intravenously after the induction of anaesthesia. Anaesthesia was supplemented with epidural fentanyl, given as an infusion of 1 microgram.kg-1 and continued for postoperative pain control. The incidence of vomiting in the immediate postoperative period was 25% with ondansetron (100 micrograms.kg-1), 40% with ondansetron (50 micrograms.kg-1) and droperidol and 70% with the control group. In the next 48 h the incidence of emesis increased to 30% for ondansetron (100 micrograms.kg-1), 55% with ondansetron (50 micrograms.kg-1), 65% with droperidol and 85% for the control group. Those patients who had multiple emesis necessitating a second dose of the same drug treatment showed no difference in the incidence of emesis relative to the control group. Ondansetron (50 micrograms.kg-1) and droperidol groups had lower incidence of PONV compared to the control group. The ondansetron (100 micrograms.kg-1) group had a significant decrease in the incidence of emesis. We conclude that the prophylactic administration of ondansetron (100 micrograms.kg-1) is more effective than droperidol and ondansetron (50 micrograms.kg-1) and superior to saline (P < 0.02) for the prevention of emesis before epidural opioid and general anaesthesia.

5-HT3 receptors are not involved in conditioned taste aversions induced by 5-hydroxytryptamine, ipecacuanha or cisplatin

We have used the rat to examine the involvement of the 5-HT3 receptor in the mechanism(s) of conditioned taste aversion induced by 5-hydroxytryptamine (5-HT) and selected emetic drugs. 5-HT, ipecacuanha and cisplatin all induced conditioned taste aversion at doses known to induce emesis in other species but the responses were resistant to treatment with the 5-HT3 receptor antagonists ondansetron and granisetron. Further, m-chlorophenylbiguanide, a selective and potent 5-HT3 receptor agonist, failed to induce a conditioned taste aversion. The data provide strong evidence that the 5-HT3 receptor is not involved in conditioned taste aversion mechanisms in the rat. Results are discussed in terms of the usefulness of the rat conditioned taste aversion paradigm to anti-emetic research.

Arylpiperazines with serotonin-3 antagonist activity: a comparative molecular field analysis

Comparative molecular field analysis (CoMFA) is applied to antagonists of the 5-HT3 receptor. Analysis is done separately on three published sets of arylpiperazines and on a combination of the three sets. d-Tubocurarine, a conformationally restricted 5-HT3 ligand, is used as a template to assist in selecting the conformation of the antagonists for CoMFA alignment. Two forms of the arylpiperazines (neutral and protonated) and three different kinds of calculated charges (Gasteiger-Hückel, AM1, and AM1 with solvation effect included) are compared. Protonated structures give better statistical results than the neutral species. The way in which charges are calculated does not greatly affect the results. In terms of molecular fields, the behavior in each separate set of compounds cannot be extrapolated to the combined set of 47 compounds. The average value of r2cv from PLS cross-validation on the combined set is 0.70 and varies between 0.56 and 0.80 depending on the orientation of the molecules in the coordinate system. The CoMFA model is tested on four compounds not in the training set: quipazine, N-methylquipazine, 4-phenyl-N-methylquipazine, and KB-6933. Mean agreement of experimental and predicted pKi values of the antagonists is 0.7 log unit. Novel structural modifications are interpreted by the CoMFA model.

Acute intravenous administration of ondansetron and m-CPP, alone and in combination, in patients with obsessive-compulsive disorder (OCD): behavioral and biological results

Obsessive-compulsive disorder (OCD) has been linked to abnormal function of brain serotonin (5-HT) pathways. Since ondansetron is a highly selective 5-HT3 receptor antagonist, the present study was undertaken to investigate 5-HT3 function in OCD. We administered m-CPP (0.08 mg/kg i.v.) and the potent 5-HT3 antagonist, ondansetron (0.15 mg/kg i.v.), to 11 OCD patients. All of the subjects received four separate challenges (m-CPP + placebo, m-CPP + ondansetron, ondansetron + placebo and placebo + placebo). In comparison to placebo, administration of m-CPP was associated with significant behavioral effects, particularly self-rated measures of anxiety, altered self-reality, functional deficit and OCD symptoms. Pretreatment with ondansetron did not affect any of the self-rated behavioral symptoms. After administration of m-CPP relative to placebo, significant increases in plasma cortisol and prolactin were found. These changes were not affected by ondansetron. In conclusion, our results do not support the hypotheses that 5-HT3 receptor-mediated mechanisms modulate m-CPP's behavioral and neuroendocrine effects in patients with OCD.

Mechanisms of the cardiovascular deconditioning induced by tail suspension in the rat

The aim of the present work was to obtain insights into the pathophysiology of cardiovascular deconditioning (CVD) induced by tail suspension (TS) in the rat: during TS, when central venous pressure (CVP) has been normalized (E. Martel, P. Champéroux, P. Lacolley, S. Richard, M. Safar, and J. L. Cuche. J. Appl. Physiol. 80: 1390-1396, 1996), and during simulated orthostatism (SO), when transient episodes of hypotension and bradycardia are disclosed, bradycardia with SO represents a response that seems peculiar to the rat compared with humans. According to basic physiology, a reduced activity of the sympathetic system induced by increased CVP was suspected but was not supported by data obtained through spectral analysis of blood pressure (BP) and heart rate (HR) variability or measurements of plasma catecholamine concentration during TS. Nonetheless, indirect evidence was obtained. During SO, plasma catecholamine concentration was lower in TS rats than in controls, suggesting a reduced synthesis of catecholamines, itself secondary to reduced activity of the sympathetic system. Furthermore, after 48 h of TS, the number of binding sites and affinity of alpha-receptors in rat aorta were increased, compatible with a reduced level of neurotransmitter in the synaptic cleft. A second series of experiments was carried out to study hypotension and bradycardia in TS rats during SO. Hypersensitivity of serotonergic mechanisms was suspected. Two 5-HT3 receptor antagonists (ondansetron and MDL-72222) blocked hypotension and restored tachycardia, basic features of orthostatic adaptation of the circulatory system. Response to the 5-HT3 receptor agonist was measured through dose-response curves of BP and HR after injection of 2-methylserotonin. After low doses, hypotension (10 micrograms/kg) and bradycardia (3 and 10 micrograms/kg) were significantly greater in 48-h TS rats than in controls. Thus CVD in the rat induced by TS appears to implicate at least two mechanisms: reduced activity of the sympathetic system and hypersensitivity of serotonergic mechanisms.

The influence of ondansetron and m-chlorophenylpiperazine on scopolamine-induced cognitive, behavioral, and physiological responses in young healthy controls

BACKGROUND

There is evidence from animal and human experiments that learning and memory come under the separate influence of both cholinergic and serotonergic pathways. We were interested in learning whether serotonergic drugs could attenuate or exacerbate the memory-impairing effects of anticholinergic blockade in humans.

METHODS

The selective serotonin 5-HT3 receptor antagonist ondansetron (0.15 mg/kg i.v.) and the serotonergic agent m-chlorophenylpiperazine (m-CPP; 0.08 mg/kg i.v.) were administered in combination with the anticholinergic agent scopolamine (0.4 mg PO) and compared to scopolamine alone in 10 young, healthy volunteers. Testing occurred on three separate days.

RESULTS

As expected, i.v. administration of scopolamine induced significant impairments in episodic memory and processing speed; however, these scopolamine-induced cognitive deficits were not attenuated by pretreatment with i.v. ondansetron (0.15 mg/kg), nor were they exacerbated by administration of i.v. m-CPP (0.8 mg/kg) in addition to scopolamine; however, administration of i.v. m-CPP was followed by a significant increase of self-rated functional impairment, altered self-reality, and dysphoria ratings, and scopolamine's effect on pupil size was potentiated.

CONCLUSIONS

Together, these results suggest that in young, healthy volunteers scopolamine-induced changes of cognitive performance are only minimally modulated by the serotonergic effects on ondansetron and m-CPP. Further studies with older controls are needed to test whether these findings may be influenced by age.

Erythromycin gastrokinetic activity is partially vagally mediated

Erythromycin overcomes postvagotomy gastroparesia in patients without a distal stomach and functional pylorus. We investigate the role of the vagus in gastric emptying increased by erythromycin, using a model that preserves the physiology of the distal stomach and pylorus. The effects of erythromycin lactobionate (10 mg/kg) on transpyloric flow pattern and pyloric resistance were evaluated during repetitive bilateral vagal cooling in anesthetized pigs. Vagal cooling during erythromycin infusion produced a marked decreased of pyloric outflow (23 +/- 1.1 vs 50 +/- 2.6 ml/min) related to a reduced stroke volume of the flow pulses (7.8 +/- 3.31 vs. 14.1 +/- 2.44 ml). The amplitude and frequency of gastric and duodenal pressure events were unchanged or slightly reduced during vagal cooling. The smaller stroke volume of flow pulse was the consequence of increased pyloric resistance (6.2 +/- 1.98 vs. 2.3 +/- 0.21 mmHg.ml-1.s), which is associated with changes in the temporal relationship between a pyloric pressure event and flow pulse. In conclusion, erythromycin activity on the pylorus requires the integrity of vagal pathways. Enhancement of gastric outflow by erythromycin is also modulated by the vagus, since pyloric resistance was able to overcome increased gastric motility.

5-HT3 serotonergic receptor mediation of hypoglycemia-induced arginine-vasopressin but not oxytocin secretion in normal men

The present study was undertaken in order to establish the possible involvement of 5-HT3 serotonergic receptors in the control of basal and/or hypoglycemia-stimulated arginine vasopressin (AVP) and/or oxytocin (OT) secretion. For this purpose, 12 normal men were injected intravenously with a bolus of 4 mg ondansetron, a specific 5-HT3 receptor antagonist, under basal conditions (n = 6) or 30 min before insulin (0.15 IU/kg body weight) administration (n = 6) (insulin tolerance test (ITT)). Control experiments with normal saline instead of ondansetron treatment were performed. Furthermore, on a different occasion, the same subjects were tested in identical experimental conditions with 8 mg ondansetron. Our results showed that the hypoglycemic response to insulin was similar during the ITT and ondansetron plus ITT. Inhibition of 5-HT3 serotonergic receptors with ondansetron (4 or 8 mg) did not modify the basal secretion of AVP and OT and the OT response to insulin-induced hypoglycemia. In contrast, the administration of 4 or 8 mg ondansetron significantly reduced in a similar manner hypoglycemia-induced AVP rise. Mean peak level at 45 min after insulin injection was 2.25 times higher than baseline in the control ITT and 1.5 times higher than basal value in the ondansetron (4 or 8 mg) plus ITT. These data demonstrate that 5-HT3 serotonergic receptors at least partially mediate the AVP response to hypoglycemia, without modifying the simultaneous OT response. On the other hand, 5-HT3 receptors do not appear to be involved in the control of basal posterior pituitary hormone secretions.

Effects of ondansetron on electrically evoked contraction in rat stomach fundus: possible involvement of 5-HT2B receptors

This study examined the effects of ondansetron, an antagonist of the 5-hydroxytryptamine (5-HT3) receptor without 5-HT4 receptor agonistic activity. on electrically evoked contractions and acetylcholine release in rat stomach fundus strips. Ondansetron (10(-8)-10(-4) M) produced a concentration-dependent increase in the magnitude of the electrically evoked contraction, while it inhibited the release of acetylcholine induced by electrical field stimulation. The stimulatory effect of ondansetron (10(-6) M) on electrically evoked contractions was antagonized by yohimbine, a 5-HT2B receptor antagonist, but not by SDZ 205-557 (2-methoxy-4-amino-5-chloro-benzoic acid 2-[diethylamino] ethyl ester), a 5-HT4 receptor antagonist. In the presence of tetrodotoxin, ondansetron (10(-7)-10(-5) M) enhanced the contractions induced by acetylcholine. This stimulatory effect of ondansetron on acetylcholine-induced contractions was antagonized by yohimbine. These data suggest that ondansetron potentiates the contractile response to acetylcholine in the rat stomach fundus through the activation of 5-HT2B receptors.

Evidence for a 5-HT3 receptor involvement in the facilitation of peristalsis on mucosal application of 5-HT in the guinea pig isolated ileum

1. The 5-HT receptor involved in the effect of mucosal application of 5-HT to facilitate peristalsis was investigated in the isolated guinea pig ileum. 2. An application of 5-HT (3-100 microM) to the mucosal surface (by inclusion of 5-HT in the Krebs-Henseleit solution passing through the lumen of the ileum) caused a concentration related facilitation of peristalsis characterized by a reduction in the peristaltic threshold. 3. Peristalsis was not modified by methiothepine (0.1 microM), ritanserin (0.1 microM), ondansetron (5 microM), granisetron (1 microM) or SB 204070 (0.1 microM) administered alone to the mucosal surface. 4. The concentration-response curve to mucosally applied 5-HT was not altered by the mucosally applied 5-HT1/2 receptor antagonist methiothepine (0.1 microM), the 5-HT2 receptor antagonist ritanserin (0.1 microM) or the 5-HT4 receptor antagonist SB 204070 (0.1 microM). However, the mucosally applied 5-HT3 receptor antagonists ondansetron (5 microM) and granisetron (1 microM) shifted the response curves to mucosally applied 5-HT to the right in a parallel and surmountable manner. The pD2 values in the absence and presence of ondansetron were 5.42 +/- 0.07 and 4.12 +/- 0.10, respectively, (n = 6) and that of granisetron were 5.45 +/- 0.12 and 4.50 +/- 0.10 respectively, (n = 5). 5. Serosally applied ondansetron (5 microM) or granisetron (1 microM) had no effect on the concentration-response curve to mucosally applied 5-HT. However, the serosally applied ondansetron and granisetron antagonised the facilitatory effect of serosally applied 5-HT (10 microM) when administered in the presence of serosally applied SB 204070 (0.1 microM). 6. It is concluded that the facilitatory effect of mucosally applied 5-HT to reduce the peristaltic threshold in the guinea pig ileum is mediated via a 5-HT3 receptor located on the mucosal and not the serosal side of the ileum.

Comparative receptor mapping of serotoninergic 5-HT3 and 5-HT4 binding sites

The clinical use of currently available drugs acting at the 5-HT4 receptor has been hampered by their lack of selectivity over 5-HT3 binding sites. For this reason, there is considerable interest in the medicinal chemistry of these serotonin receptor subtypes, and significant effort has been made towards the discovery of potent and selective ligands. Computer-aided conformational analysis was used to characterize serotoninergic 5-HT3 and 5-HT4 receptor recognition. On the basis of the generally accepted model of the 5-HT3 antagonist pharmacophore, we have performed a receptor mapping of this receptor binding site, following the active analog approach (AAA) defined by Marshall. The receptor excluded volume was calculated as the union of the van der Waals density maps of nine active ligands (pKi > or = 8.9), superimposed in pharmacophoric conformations. Six inactive analogs (pKi < 7.0) were subsequently used to define the essential volume, which in its turn can be used to define the regions of steric intolerance of the 5-HT3 receptor. Five active ligands (pKi > or = 9.3) at 5-HT4 receptors were used to construct an antagonist pharmacophore for this receptor, and to determine its excluded volume by superimposition of pharmacophoric conformations. The volume defined by the superimposition of five inactive 5-HT4 receptor analogs that possess the pharmacophoric elements (pKi < or = 6.6) did not exceed the excluded volume calculated for this receptor. In this case, the inactivity may be due to the lack of positive interaction of the amino moiety with a hypothetical hydrophobic pocket, which would interact with the voluminous substituents of the basic nitrogen of active ligands. The difference between the excluded volumes of both receptors has confirmed that the main difference is indeed in the basic moiety. Thus, the 5-HT3 receptor can only accommodate small substituents in the position of the nitrogen atom, whereas the 5-HT4 receptor requires more voluminous groups. Also, the basic nitrogen is located at ca. 8.0 A from the aromatic moiety in the 5-HT4 antagonist pharmacophore, whereas this distance is ca. 7.5 A in the 5-HT3 antagonist model. The comparative mapping of both serotoninergic receptors has allowed us to confirm the three-component pharmacophore accepted for the 5-HT3 receptor, as well as to propose a steric model for the 5-HT4 receptor binding site. This study offers structural insights to aid the design of new selective ligands, and the resulting models have received some support from the synthesis of two new active and selective ligands: 24 (Ki(5-HT3) = 3.7 nM; Ki(5-HT4) > 1000 nM) and 25 (Ki(5-HT4) = 13.7 nM; Ki(5-HT3) > 10,000 nM).

Ondansetron modulates GABA(A) current of rat central nervous system neurons

We examined the effect of ondansetron, a 5-HT3 receptor antagonist, on the whole cell current response of freshly isolated hypothalamic and hippocampal neurons of rats to gamma-aminobutyric acid (GABA). The nystatin perforated patch technique was used to minimize run-down of the GABA current. While 1-150 microM ondansetron had no effect on membrane conductance, co-application with agonist reversibly depressed the maximal end GABA current. The concentration-response relation of GABA reveals a non-competitive mechanism. However, the inhibitory effect was more potent when ondansetron was co-applied with lower concentrations of GABA: i.e., the ondansetron concentration needed to depress the current induced by 5 microM GABA to half amplitude was 7 microM compared to 28 microM for the current induced by 10 microM GABA. Analysis of the current-voltage relationship with and without ondansetron indicated that the effect of ondansetron is not voltage dependent. Current-voltage relations also showed that the effect of ondansetron was not due to activation of a GABA-independent current because the reversal potentials were the same with and without ondansetron. The present data suggest that ondansetron's suppression of GABA-activated current may be the molecular basis of ondansetron-induced seizures observed in vivo.

Effect of ondansetron on Salmonella typhimurium-induced net fluid accumulation in the pig jejunum in vivo

Two major pathophysiological mechanisms explaining the diarrhoea induced by Salmonella typhimurium have been suggested to be: (a) invasion of the intestine by the bacteria, and (b) an enterotoxin resembling Vibrio cholerae toxin. Cholera toxin is a potent secretagogue in pig small intestine and induces secretion partly by activating 5-hydroxytryptamine receptors, following release of 5-hydroxytryptamine. Ondansetron is a selective 5-hydroxytryptamine-3 receptor antagonist, which reduces the cholera toxin-evoked fluid accumulation in pig jejunum. The aim of this study was to investigate the effect of ondansetron on Salmonella typhimurium-induced fluid accumulation in ligated loops of pig jejunum in vivo. 10(10) colony-forming units of the bacteria was injected into loops and incubated for 8 hr. 200 mg x kg-1 ondansetron given subcutaneously reduced the Salmonella typhimurium-induced fluid accumulation by about 40%. This results suggests the involvement of 5-hydroxytryptamine and 5-hydroxytryptamine-3 receptors in Salmonella typhimurium-induced diarrhoea.

High-resolution liquid chromatographic method using ultraviolet detection for determination of ondansetron in human plasma

This paper describes a simple technique for extraction and a sensitive high-performance liquid chromatographic method for separation and quantitation of ondansetron in human plasma. The procedure involved liquid-liquid extraction of ondansetron from plasma, reversed-phase HPLC separation and ultraviolet detection at 305 nm. The internal standard method was applied for quantitation. The recovery of ondansetron was >85%. Linearity was good throughout the concentration range anticipated in human plasma from investigations in panic disorder (0.5-15 ng/ml, r2 ranging from 0.9953 to 0.9988). This method was applied to the determination of plasma concentrations of ondansetron in humans.

Novel and selective partial agonists of 5-HT3 receptors. 2. Synthesis and biological evaluation of piperazinopyridopyrrolopyrazines, piperazinopyrroloquinoxalines, and piperazinopyridopyrroloquinoxalines

In continuation of our previous work on piperazinopyrrolothienopyrazine derivatives, three series of piperazinopyridopyrrolopyrazines, piperazinopyrroloquinoxalines, and piperazinopyridopyrroloquinoxalines were prepared and evaluated as 5-HT3 receptor ligands. The chemical modifications performed within these new series led to structure-activity relationships regarding both high affinity and selectivity for the 5-HT3 receptors that are in agreement with those established previously for the pyrrolothienopyrazine series. The best compound (8a) obtained in these new series is in the picomolar range of affinity for 5-HT3 receptors with a selectivity higher than 10(6). Four of the high-affinity 5-HT3 ligands (8a, 15a,b, and 16d) were selected in both the pyridopyrrolopyrazine and the pyrroloquinoxaline series and were characterized in vitro and in vivo as agonists or partial agonists. Compound 8a was also evaluated in the light/dark test where it showed potential anxiolytic-like activity at very low doses per os.

New 2-piperazinylbenzimidazole derivatives as 5-HT3 antagonists. Synthesis and pharmacological evaluation

A series of 2-piperazinylbenzimidazole derivatives were prepared and evaluated as 5-HT3 receptor antagonists. Their 5-HT3 receptor affinities were evaluated by radioligand binding assays, and their abilities to inhibit the 5-HT-induced Bezold-Jarisch reflex in anesthetized rats were determined. Compound 7e (lerisetron, pKi = 9.2) exhibited higher affinity for the 5-HT3 receptor than did tropisetron and granisetron, while compound 7q (pKi = 7.5) had very low affinity for this receptor, showing that substitution on the N1 atom of the benzimidazole ring is essential for affinity and activity. The effect of substitution on the aromatic ring of benzimidazole by several substituents in different positions is also discussed. A strong correlation between the 5-HT3 antagonistic activity of the studied compounds and the position of substitution on the aromatic ring was established. Thus, while the 4-methoxy derivative 7m showed weak affinity for the 5-HT3 receptor (pKi = 6.7), the 7-methoxy derivative 7n exhibited the highest affinity (pKi = 9.4). Compounds 7e and 7n are now under further investigation as drugs for the treatment of nausea and emesis evoked by cancer chemotherapy and radiation.

Physiology of chemotherapy-induced emesis and antiemetic therapy. Predictive models for evaluation of new compounds

The physiology of emesis has been studied for several hundred years, focusing on the different centres involved and the mechanics of expulsion. The vomiting centre receives inputs from various emetic detectors such as the gut, the vestibular labyrinths and the chemoreceptor trigger zone. Emesis is a common disabling effect in motion sickness, postoperative conditions and in radio- and chemotherapy. Our current understanding of the mechanisms has been provided mainly by the recent introduction of serotonin 5-HT3 receptor antagonists into therapeutic use. Nevertheless, despite the considerable advances made in the understanding of the different pathways involved in emesis, there are number of areas that still require experimental investigation. Different animal and human models are available to study the physiology of emesis and to evaluate the antiemetic activity of new compounds, but they need to be predictors of clinical situations.

VB20B7, a novel 5-HT-ergic agent with gastrokinetic activity. I. Interaction with 5-HT3 and 5-HT4 receptors

This study describes the in-vitro interaction of the gastrokinetic agent 2[1-(4-piperonyl)piperazinyl]benzothiazole (VB20B7) with the 5-hydroxytryptamine 5-HT3 and 5-HT4 receptor subtypes, using functional as well as radioligand binding studies. The benzamide derivative cisapride was used as a comparison. In radioligand binding assays VB20B7 showed, like cisapride, a weak affinity at 5-HT3 receptors from rat cerebral cortex. The new compound lacked any affinity at other 5-HT receptors or at dopaminergic D2 receptors, whereas cisapride showed high affinity for the 5-HT4 receptors from guinea-pig hippocampus and moderate affinity at dopaminergic D2 receptors. In the non-stimulated guinea-pig ileum, the concentration-response curves to the specific 5-HT3 agonist 2-Me-5-HT and to 5-HT were shifted to the right by VB20B7. In the rat oesophagus tunica muscularis mucosae preparation (TMM), VB20B7 was evaluated for its activity at 5-HT4 receptors. VB20B7 behaved as a 5-HT4 receptor agonist, inducing a concentration-dependent relaxation of the preparation precontracted with carbachol. In this preparation, VB20B7 and cisapride were able to stimulate adenylate cyclase activity, an effect probably mediated through activation of 5-HT4 receptors, as can be inferred from the blockade by the 5-HT4 antagonist, tropisetron, of the enhanced cAMP formation. However, consistent with the lack of affinity at central 5-HT4 receptors, VB20B7 did not stimulate cAMP formation in guinea-pig hippocampal slices. VB20B7 also caused an increase in the twitch response of the transmurally stimulated guinea-pig ileum, although at a concentration higher than cisapride. This effect was blocked by desensitization of the 5-HT4 receptor with 5-MeOT and also by the 5-HT4 receptor antagonist tropisetron. Both VB20B7 and cisapride increased the K(+)-evoked acetylcholine release in this preparation. The results show that VB20B7 possesses affinity for 5-HT4 receptors located in the rat TMM and guinea-pig ileum preparations, but is devoid of affinity at central 5-HT4 receptors. In addition, VB20B7 shows low to moderate affinity at both central and peripheral (enteric) 5-HT3 receptors. The interaction of VB20B7 with the peripheral 5-HT4 and 5-HT3 receptors may be relevant for the gastrokinetic effects of the new compound.

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.