5-HT3 receptors

Recent advances with 5-HT3 modulators for neuropsychiatric and gastrointestinal disorders

Serotonin (5-hydroxytryptophan [5-HT]) is a biologically active amine expressed in platelets, in gastrointestinal (GI) cells and, to a lesser extent, in the central nervous system (CNS). This biogenic compound acts through the activation of seven 5-HT receptors (5-HT_1-7 Rs). The 5-HT₃ R is a ligand-gated ion channel belonging to the Cys-loop receptor family. There is a wide variety of 5-HT₃ R modulators, but only receptor antagonists (known as setrons) have been used clinically for chemotherapy-induced nausea and vomiting and irritable bowel syndrome treatment. However, since the discovery of the setrons in the mid-1980s, a large number of studies have been published exploring new potential applications due their potency in the CNS and mild side effects. The results of these studies have revealed new potential applications, including the treatment of neuropsychiatric disorders such as schizophrenia, depression, anxiety, and drug abuse. In this review, we provide information related to therapeutic potential of 5-HT₃ R antagonists on GI and neuropsychiatric disorders. The major attention is paid to the structure, function, and pharmacology of novel 5-HT₃ R modulators developed over the past 10 years.

Therapeutics of 5-HT3 receptor antagonists: current uses and future directions

The 5-Hydroxytryptamine3 (5-HT3) receptor is a member of the cys-loop family of ligand gated ion channels, of which the nicotinic acetylcholine receptor is the prototype. All other 5-HT receptors identified to date are metabotropic receptors. The 5-HT3 receptor is present in the central and peripheral nervous systems, as well as a number of non-nervous tissues. As an ion channel that is permeable to the cations, Na(+), K(+), and Ca(2+), the 5-HT3 receptor mediates fast depolarizing responses in pre- and post-synaptic neurons. As such, 5-HT3 receptor antagonists that are used clinically block afferent and efferent synaptic transmission. The most well established physiological roles of the 5-HT3 receptor are to coordinate emesis and regulate gastrointestinal motility. Currently marketed 5-HT3 receptor antagonists are indicated for the treatment of chemotherapy, radiation, and anesthesia-induced nausea and vomiting, as well as irritable bowel syndrome. Other therapeutic uses that have been explored include pain and drug addiction. The 5-HT3 receptor is one of a number of receptors that play a role in mediating nausea and vomiting, and as such, 5-HT3 receptor antagonists demonstrate the greatest anti-emetic efficacy when administered in combination with other drug classes.

Gastrointestinal toxicity of chemotherapeutic agents

Chemotherapy-induced toxicities commonly occur in sites within the gastrointestinal (GI) tract and account for dose-limiting effects. These toxicities are major contributing factors to dose reduction, delays, and cessation of cancer treatment. Through intensive therapies including surgery, combination chemotherapy, hormonal therapy, and targeted therapy, an increasing number of patients with cancer are experiencing improved survival and long-term disease-free survival, as well as palliation of disease-related symptoms. Thus, GI toxicities should be predicted and appropriate interventions initiated to prevent them when possible and provide effective supportive measures and comprehensive follow-up care. This review will discuss the etiology, incidence, prevention, and treatment of GI toxicities of cancer chemotherapy.

Pharmacokinetics of granisetron in rat blood and brain by microdialysis

To characterize the pharmacokinetics of protein-free granisetron in blood and brain we implanted microdialysis probes into the jugular vein and cerebral frontal cortex of the rat. Granisetron (3 or 6 mg/kg, i.v., n=6) was then administered, and microdialysates from blood and brain were collected from both sites and assayed by a validated high-performance liquid chromatographic method. Pharmacokinetics parameters were calculated from the corrected dialysate concentrations of granisetron versus time data. The elimination half-lives of granisetron in blood and brain were 51.3+/-5.5 and 69.7+/-6.3 min for 6 mg/kg, and 50.7+/-4.3 and 74.3+/-12.5 min for 3 mg/kg, respectively. Granisetron rapidly entered the extracellular fluid of cerebral frontal cortex at Tmax of 24 min. The results suggest that simultaneous microdialysis in blood and brain can be usefully applied to study the pharmacokinetics of granisetron in the periphery and the central nervous system.

Review article: the therapeutic potential of 5-HT3 receptor antagonists in the treatment of irritable bowel syndrome

There is evidence from studies, in both animals and humans, that 5-HT3 receptor blockade has potential value in the treatment of irritable bowel syndrome, particularly in those patients with diarrhoea-predominant bowel habits. New findings suggest that 5-HT3 receptors exist on gut afferent neurones and that their activation by locally released 5-HT leads to visceral nociceptive stimulation, in addition to increased neuronally-mediated motor and secretory activity. If this concept is validated, it will provide a rationale for the use of 5-HT3 receptor antagonists in patients with increased gut motility, reduced fluid absorption and low nociceptive thresholds leading to abdominal pain. Alosetron is a highly selective, potent 5-HT3 receptor antagonist which is well absorbed with a long pharmacodynamic half-life. Its ability to provide long-lasting blockade of 5-HT3 receptors throughout the body make it an ideal candidate within its class to evaluate the clinical hypothesis that sustained and ubiquitous 5-HT3 receptor blockade is of value in the treatment of IBS.

[Treatment of chemotherapy induced emesis in children]

In recent years, the management of chemotherapy-induced emesis in children has been greatly modified by the introduction of a new therapeutic class: serotonin antagonists. Based on a better knowledge of mechanisms, the treatment now uses combinations of different drugs. These treatments need to be carefully adapted to the patient and to the emetic risk of the chemotherapy, also taking into account the minimal cost. A gradual treatment proposal in five steps is described.

Comparison of the effects of the 5-HT3 receptor antagonists WAY-100579 and ondansetron on spatial learning in the water maze in rats with excitotoxic lesions of the forebrain cholinergic projection system

The effects of the 5-HT3 receptor antagonists. WAY-100,579 and ondansetron (both at doses of 0.001, 0.01 and 0.1 mg/kg s.c.) and the muscarinic receptor agonist arecoline (1.0 mg/kg s.c.), on spatial learning and memory in the water maze were examined in rats after combined S-AMPA lesions to the nucleus basalis and medial septal brain regions. Lesioned rats showed substantially increased latency to find the submerged platform, and spent less time searching in the correct quadrant, and more time circling the periphery of the pool, relative to controls. Lesioned rats treated with WAY-100,579, ondansetron and arecoline exhibited marked improvement in these parameters of learning relative to lesioned animals, with arecoline-treated animals showing the most substantial recovery. Linear dose-related trends of improvement were seen with both of the 5-HT3 antagonists. In probe trials, testing retention of the platform position 24 and 72 h after the end of training, control rats exhibited substantial superiority relative to lesioned rats in accuracy of search in the training quadrant and former platform area, matched by rats treated with arecoline on the first, and by rats treated with the two higher doses of WAY-100,579 and ondansetron on the second probe trial. These results are consistent with our previous studies which demonstrated that another selective 5-HT3 receptor antagonist. WAY-100,289, significantly reversed the cognitive deficits in water maze performance induced by ibotenic acid lesions of forebrain cholinergic projection system. Therefore, selective 5-HT3 receptor antagonists may provide a novel effective therapy for treating cognitive deficits associated with degeneration of central cholinergic neurones, such as Alzheimer's disease or age-associated memory impairment.

The selective 5-HT3 receptor antagonist, WAY100289, enhances spatial memory in rats with ibotenate lesions of the forebrain cholinergic projection system

The effects of three doses (0.003, 0.03 and 1.0 mg/kg sc) of the 5-HT3 receptor antagonist, WAY 100289, on spatial learning and memory in the water maze were examined in rats before and after ibotenate lesions to the nucleus basalis and medial septal brain regions at the source of cholinergic projections to cortex and hippocampus. The representative cholinergic nicotinic and muscarinic receptor agonists nicotine (0.1 mg/kg) and arecoline (1.0 mg/kg) were also tested for comparison. Both arecoline and nicotine improved initial acquisition in rats before lesioning, in terms of latency to find a hidden platform and accuracy of search strategy. WAY100289 did not affect the performance of normal rats significantly, apart from some non-significant trends towards improvement with the highest dose. However, in animals showing transient navigational deficits in retention and relearning after lesioning, WAY100289 improved performance at all three doses, though ameliorative effects of nicotine and arecoline were more marked also in lesioned rats. These results show that WAY100289 improved spatial learning in animals impaired after lesions to cholinergic projection nuclei, which may reflect an interaction with cholinergic transmission to enhance cognitive function. However, in the present study, WAY100289 appeared to be less effective than direct cholinergic agonists.

Serotonin and alcohol intake, abuse, and dependence: clinical evidence

A large body of literature has emerged concerning the role of the neurotransmitter serotonin (5-hydroxytryptamine, or 5-HT) in the regulation of alcohol intake and the development of alcoholism. Despite the wealth of information, the functional significance of this neurotransmitter remains to be fully elucidated. This paper, part one of a two-part review, summarizes the available clinical research along two lines: the effects of alcohol on serotonergic functioning and the effects of pharmacological manipulation of serotonergic functioning on alcohol intake in normal (nonalcohol dependent) and alcohol-dependent individuals. It is concluded that considerable evidence exists to support the notion that some alcoholic individuals may have lowered central serotonin neurotransmission.

Facilitation of 5-hydroxytryptamine3 receptor desensitization by fluoxetine

Effect of fluoxetine on the desensitization of the inward current mediated by 5-hydroxytryptamine3 receptors in rat nodose ganglion neurons was investigated with whole cell patch-clamp recording. 5-Hydroxytryptamine3 current desensitization was best fitted in most experiments by a single exponential function and showed little dependence on membrane potential. Fluoxetine greatly facilitated the rate of 5-hydroxytryptamine3 current desensitization in a dose-dependent manner. The effect of fluoxetine was gradual, long-lasting, voltage-independent and the recovery was incomplete. The IC50 value for the decrease of the desensitization time-constant by fluoxetine was 0.171 microM and the Hill coefficient was 1.1. Fluoxetine also inhibited the peak and steady-state 5-hydroxytryptamine3 current with the latter being more sensitive to fluoxetine. The IC50 value for the effect of fluoxetine on peak current was 1.27 microM and that on steady-state current was 0.172 microM. There is a highly significant correlation between the two effects of fluoxetine on current desensitization and on current amplitudes: r-values for the correlation between the decrease in time-constant and the reduction in peak and steady-state current amplitudes were 0.82 and 0.88, respectively (P < 0.001). This action of fluoxetine on 5-hydroxytryptamine3 receptors may be involved in the behavioral effects of fluoxetine.

Effects of antidepressants on the inward current mediated by 5-HT3 receptors in rat nodose ganglion neurones

1. Effects of three different categories of antidepressants, imipramine (tricyclic), fluoxetine (selective 5-hydroxytryptamine (5-HT) uptake inhibitor), phenelzine and iproniazid (monoamine oxidase (MAO) inhibitor) on the inward current mediated by 5-HT3 receptors were investigated in rat nodose ganglion neurones. The whole-cell patch-clamp technique was used for recording the 5-HT current. 2. All the antidepressants tested inhibited the peak 5-HT current. The inhibition gradually reached a steady level and the recovery was incomplete when antidepressants were removed. IC50 values for imipramine, fluoxetine and phenelzine were 0.54 microM, 1.3 microM and 4.2 microM respectively. The correspondent Hill coefficients were 0.9, 0.87 and 0.92. 3. The antidepressants examined increased the rate of 5-HT current desensitization. IC50 values for imipramine, fluoxetine and phenelzine on the decrease in desensitization time constant were 0.11 microM, 0.18 microM and 2.4 microM respectively. The correspondent Hill coefficients were 0.9, 1.14 and 1.06. 4. Intracellular applications of the protein kinase inhibitor, H-7 (100 microM), GDP-beta-S (2 mM) and the calcium chelator BAPTA (20 mM) did not affect the 5-HT current and the actions of antidepressants on 5-HT current. 5. These results suggest that the 5-HT3 receptor is an acting site for the therapeutic use of antidepressants. The present observation is also helpful in explaining the analgesic effect of antidepressants seen in pain clinics.

Anorectic response to amino acid imbalance: a selective serotonin3 effect?

The anorectic responses to imbalanced amino acid diets (IMB) are ameliorated by pretreatment with large (mg/kg) doses of the serotonin antagonists, tropisetron [3-alpha-tropanyl-1H-indole-3-carboxylic acid ester, formerly known as ICS-205,930 (ICS)] and MDL 72,222 [1 alpha H,3 alpha,5 alpha-H-tropan-3-yl-3,5-dichlorobenzoate (MDL)], effects earlier attributed to the 5-hydroxytryptamine3 (5-HT3) receptor. Subsequent identification of the 5-HT4 receptor, and recognition that ICS and MDL also bind to 5-HT4 receptors, led us to question whether the results seen with these drugs were due to activity at the 5-HT3 or 5-HT4 receptor subtype. 1,2,3,9-Tetrahydro-9-methyl-3 [(2-methyl-1H-imidazol-1-yl) methyl] 4H-carbazol-4-one) [ondansetron (OND)], a reportedly 5-HT3-selective receptor antagonist, has been used to block 5-HT3 receptors in demonstrating the 5-HT4 receptor, and so seems securely selective for the 5-HT3 receptor type. Therefore, we tested the effects of OND on the rat's feeding responses to IMB. Pretreatment with 0.1 or 1 micrograms/kg OND fully restored intake of IMB to > 100% of control between 6 and 12 h after introduction of IMB. We conclude that the previous similar increases in IMB intake seen after ICS and MDL were due to their antagonist activity at the 5-HT3 receptor and that the 5-HT3 receptor may have an important role in mediating the rat's anorectic responses to IMB.

Control of chemotherapy-induced emesis

The development of antiemetic drugs has been one of the most rewarding areas of oncologic research, since therapeutic advances in this area can result in immediate improvement in the quality of life for patients undergoing chemotherapy. Antiemetic therapy has progressed dramatically during the past decade and a half. Fifteen years ago, patients receiving cisplatin for the first time had a median of 12 vomiting episodes within the first 24 hours, whereas now more than 50 percent of such patients have no vomiting episodes at all. Theoretical and clinical challenges remain, however, in the effort to control chemotherapy-induced emesis. The mechanisms of anticipatory vomiting and delayed vomiting are still not understood, and consistently effective therapeutic approaches to these problems have yet to be developed.

An immunocytochemical and autoradiographic investigation of the serotoninergic innervation of trigeminal mesencephalic and motor nuclei in the rabbit

The results of a previous experiment suggest that the cell bodies of many jaw closing muscle spindle afferents in the trigeminal mesencephalic nucleus of the rabbit are phasically inhibited during fictive mastication. The aim of this study was to investigate one possible neurotransmitter system that could be involved in this modulation, serotonin, by use of receptor autoradiography techniques and immunofluorescence combined with retrograde labelling of masseteric spindle afferents and motoneurons. A second objective was to compare the serotonin innervation of neurons in the trigeminal mesencephalic nucleus with that of masseteric motoneurons. Serotoninergic fibres were seen surrounding labelled masseteric spindle afferents, as well as unlabelled neurons, in the trigeminal mesencephalic nucleus. These fibres were close to the cell bodies and sometimes to the axon hillocks of the neurons. Although it has been reported that many neurons of the trigeminal nucleus are multipolar in some species, none of the labelled spindle afferent in this study had more than one process. Throughout the motor trigeminal nucleus, serotonin fibres were found in close proximity with cell bodies and with the proximal portions of axons and dendrites of labelled and unlabelled motoneurons. Serotonin fibres were also seen adjacent to cell bodies and processes of efferent neurons in cell group k. Autoradiography with several tritiated ligands was used to reveal the presence of receptors for serotonin as well as its uptake sites. Only serotonin2 receptors were found to be abundant in the trigeminal mesencephalic nucleus. The motor nucleus and cell group k contained serotonin2 and serotonin3 receptors, as well as serotonin uptake sites. Serotonin1A receptors appear to be absent from both nuclei. The findings suggest that release of serotonin from fibres in close proximity to trigeminal primary afferent somata could modify the transmission of action potentials from muscle spindle receptors during mastication through an action on serotonin2 receptors. In the motor nucleus and cell group k, serotonin may alter neuronal properties through actions on at least two receptor subtypes (serotonin2 and serotonin3).

Interactions between 5-HT3 receptors and cerebral dopamine function: implications for the treatment of schizophrenia and psychoactive substance abuse

This article reviews current knowledge on the interaction between 5-hydroxytryptamine (5-HT), acting at 5-HT3 receptors in the CNS, and cerebral dopamine systems. Since 1987, a growing body of behavioural, neurochemical and electrophysiological evidence from animal studies has demonstrated a clear role for 5-HT3 receptors in the modulation of activity of mesolimbic and mesocortical dopamine neurones. This evidence has led to the suggestion that 5-HT3 receptor antagonists have potential as novel antipsychotic agents and may also find use in the treatment of psychoactive substance abuse. Data emerging from clinical studies generally support this hypothesis and suggest that 5-HT3 antagonists may prove to be among the first agents available to treat schizophrenia which are not dopamine D2 antagonists and hence lack their side-effect problems.

Serotonin involvement in the discriminative stimulus effects of kappa opioids in pigeons

The purpose of the present study was to examine the role of serotonin (5HT) in the discriminative stimulus effects of kappa opioids. Pigeons were trained to discriminate 5.6 mg/kg of the kappa opioid, U50,488, from water. During substitution tests, both U50,488 and another kappa opioid, spiradoline, produced > 80% responding on the U50,488-appropriate key. In contrast, the non-opioid compound, phencyclidine and several serotonergic compounds failed to substitute for the U50,488 discriminative stimulus across a wide range of doses. During combination tests, the selective 5HT1A agonist, 8-OH-DPAT (0.001-3.2 mg/kg), dose-dependently attenuated the discriminative stimulus effects of 5.6 mg/kg U50,488 and 3.2 mg/kg spiradoline. This effect was reversed by the 5HT1A antagonist, NAN-190 (0.01-1 mg/kg), in a dose-dependent manner. Buspirone (0.01-10 mg/kg), a 5HT1A partial agonist, also attenuated the discriminative stimulus effects of the training dose of U50,488 but ipsapirone, another 5HT1A partial agonist, did not. Ketanserin, a 5HT2 antagonist, and MDL72222, a 5HT3 antagonist, attenuated the effects of U50,488, whereas the 5HT1B,1C agonist, mCPP, and the 5HT2 agonist, DOI, did not. Depletion of 5HT with p-CPA also attenuated U50,488's discriminative stimulus effects. Taken together, the results suggest that serotonin release is an important component in the discriminative stimulus effects produced by kappa opioids; however, the effects of DOI and mCPP alone suggest that activation of post-synaptic 5HT receptors is not sufficient to produce the full spectrum of kappa opioid discriminative stimulus effects.

The efficacy of various classes of anti-emetics in preventing deoxynivalenol-induced vomiting in swine

The Fusarium mycotoxin deoxynivalenol (DON) is a potent emetic agent. While the basic mechanisms which invoke and mediate emesis are still poorly understood, various neurotransmitters appear to be involved. The action of these transmitters can be blocked by various receptor-specific antagonists. The current study investigated the efficacy of several classes of receptor antagonists to block the emetic effect of DON. Following anti-emetic pretreatment, pigs were administered the toxin (i.v., 80 micrograms/kg, or oral, 300 micrograms/kg) and the onset of emesis was monitored. Certain specific serotonin (5HT3)-receptor antagonists (ICS 205-930, BRL 43694 A) were found to efficaciously prevent DON-induced vomiting. These observations support the hypothesis that serotonin plays an important role in chemically induced emesis. Also moderately effective, but requiring high doses, were the 5HT2-receptor antagonists, cyproheptadine and sulpiride. A variety of compounds possessing strong anticholinergic activity were also efficacious. These, however, apparently act directly at the emetic center and thus are capable of preventing emesis regardless of the cause, including chemically induced vomiting. Non-effective were the antihistaminic and antidopaminergic anti-emetics; except, those which also possessed considerable anticholinergic activity, and i.v. administered chlorpromazine which has been speculated to block specific receptors found in the brain's chemoreceptor trigger zone (CTZ) reportedly involved in initiating emesis.

Altered serotonergic activity in women with dysphoric premenstrual syndromes

OBJECTIVE

Dysphoric Premenstrual Syndromes (PMS) are quite prevalent and in some women they are severe enough to warrant treatment. Their pathophysiology is still unknown, despite increased interest and research. Here we review the possible role of serotonin in the multidimensional interactive pathophysiology of PMS.

METHOD

Over 170 articles are reviewed. An extensive library search has been conducted and articles are included because of their relevance to: 1) the phenomenology of PMS; 2) the putative association of serotonergic (5-HT) activity with syndromes that occur premenstrually; 3) changes in 5-HT activity along the menstrual cycle, especially the late luteal phase; 4) influence of gonadal hormones on serotonergic functions; 5) endocrine strategies for assessment of 5-HT abnormalities; and 6) treatment studies of PMS with serotonergic agonists.

RESULTS AND CONCLUSIONS

The data presented here suggest that post-synaptic serotonergic responsivity might be altered during the late-luteal-premenstrual phase of the menstrual cycle. Some serotonergic functions of women with PMS might be altered during the entire cycle and be associated with a vulnerability trait. It is hypothesized that gonadal hormones might cause changes in levels of activity of 5-HT systems as part of a multidimensional interactive system. Strategies to evaluate 5-HT activities in the context of the menstrual cycle are discussed--leading to the conclusion that the most promising approach is active stimulation with specific post-synaptic serotonin agonists. Treatment outcome studies of some imperfect compounds that are currently applied as a symptomatic treatment of PMS support the notion that 5-HT is involved in the pathophysiology of these syndromes.

The effects of 5-HT3 receptor antagonists on cisplatin-induced emesis in the pigeon

In the present study, the emetic effect of the anticancer drug cisplatin, and the protective effects of 5-HT3 receptor antagonists against cisplatin emesis were investigated in the pigeon. The experimental set-up involved the i.v. administration of drugs and subsequent observation of the percentage of vomiting animals and the number of emetic episodes per vomiting animal. It was observed that cisplatin induced dose-dependent emesis in the pigeon. 5-HT3 receptor antagonists afforded partial protection against cisplatin emesis, although some of them, i.e. indole, indole-like derivatives and zacopride, displayed intrinsic emetic activity. A serotonergic mechanism appears to be involved in both cisplatin- and 5-HT3 receptor antagonist-induced emesis, since pretreatment with an inhibitor of 5-HT synthesis, para-chlorophenylalanine (pCPA), prevented vomiting induced by either cisplatin or 5-HT3 receptor antagonists. It is concluded that the intrinsic emetic effects of 5-HT3 receptor antagonists provide pharmacological evidence of species differences in the properties of 5-HT3 receptors.

Naltrexone, serotonin receptor subtype antagonists, and glucoprivic intake: 2. Insulin

Opiate antagonist inhibition of deprivation-induced intake and 2-deoxy-D-glucose (2DG) hyperphagia is significantly enhanced by the 5-hydroxytryptamine3 (5-HT3) antagonist, ICS-205,930. Interactions between opiate antagonists and either 5-HT or 5-HT2 antagonists produced smaller effects. The present study evaluated whether insulin (5 U/kg) hyperphagia was affected by methysergide (0.5-5 mg/kg), ritanserin (0.25-2.5 mg/kg), and ICS-205,930 (0.5-5 mg/kg) alone or in combination with naltrexone (2.5-10 mg/kg). Whereas ICS-205,930 stimulated insulin hyperphagia across the 6-h time course, ritanserin and, to a lesser degree, methysergide reduced insulin hyperphagia. Naltrexone marginally (19-33%) reduced insulin hyperphagia. Pairing naltrexone with either ICS-205,930 or ritanserin significantly suppressed insulin hyperphagia after 2 h. Pairing naltrexone with each of the serotonin antagonists significantly enhanced insulin hyperphagia after 4 and 6 h. These data suggest that 5-HT2 and 5-HT3 receptor subtypes interact with opioid systems to modulate insulin hyperphagia. Given that central insulin reduces food intake and body weight, the interaction between serotonergic and opioid systems may occur peripherally.

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.

ICS 205-930 and feeding responses to amino acid imbalance: a peripheral effect?

Serotonin3 (5-HT3) receptor antagonists (ICS 205-930 and MDL 72222) have been shown to block or ameliorate the anorectic response of the rat to amino acid imbalanced (IMB) diets. Two experiments were conducted to determine whether the effects of these antagonists are mediated through central or peripheral 5-HT3 receptors. In Experiment One, ICS 205-930 (ICS) was injected centrally, into either the lateral ventricle (doses: 0.3 pmol to 10 nmol), or the cisterna magna (62 nmol). The intake of rats fed an isoleucine IMB diet was not affected by these injections. In Experiment Two, rats received an IP injection of either saline, ICS, or a quaternized derivative of ICS (Q-ICS) that should not cross the blood-brain barrier. Both ICS- and Q-ICS-injected rats ate significantly more (p less than 0.05) IMB diet than saline-injected rats. Intake of IMB diet was not different (p greater than 0.4) between ICS and Q-ICS groups. From these results, it appears that ICS restores intake of IMB through a peripheral component.