1 The 5-HT4 receptor antagonist, GRI13808, has been radiolabelled to a high specific activity with tritium. antagonists acting at the 5-HT4 receptor but not by compounds selective for other 5-HT receptors or other neurotransmitter receptors. 5 Autoradiographic analysis revealed a discrete localization in both guinea-pig and rat brain with high concentrations of binding in brain areas such as the striatum, substantia nigra and olfactory tubercle. 6 [3H]-GR113808 binding to homogenates of guinea-pig striatum meets the criteria for labelling of the 5-HT4 receptor and, as such, represents the first characterization of this receptor in a radioligand binding assay.

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1 The 5-HT4 receptor has only recently been identified but has yet to be cloned. This paper describes the pharmacology of a potent and selective 5-HT4 receptor antagonist, GR113808, which will be useful in the further characterization of this receptor. 2 On the guinea-pig ascending colon, GRI 13808 (1 nM-0.1 M) behaved as an antagonist of 5-hydroxytryptamine (5-HT)-induced contraction, producing rightward displacements of the concentration-effect curve to 5-HT and a concentration-related depression of the maximum effect. However, the compound had no effect on cholecystokinin (CCK-8)-induced contraction in concentrations up to 1 LM. 3 In the guinea-pig colon preparation, onset and offset of the antagonism by GRI13808 of 5-HTinduced contraction was examined. Incubation of the tissues for either 15 min, 30 min or 60 min produced similar rightward displacements of the concentration-effect curves to 5-HT, with no increase in the degree of depression of the maxima with increasing time of incubation. Experiments examining offset of antagonism (0.01 4M) demonstrated that washout for 30 min was required to reverse fully the effects of the antagonist. 4 Potency estimates in the colon for GRI13808 were made by determining approximate pA2 values (30 min) using the Gaddum equation. The values obtained were 9.2, 9.7 and 9.2 when tested against the agonists 5-HT, 5-methoxytryptamine and R,S-zacopride respectively. 5 On the carbachol-contracted tunica muscularis mucosae preparation of the rat thoracic oesophagus, GR 113808 behaved as an antagonist of 5-HT-induced relaxation, producing no reduction in maximum response. Analysis of these data yielded a pA2 of 9.3. GR1 13808 also antagonised the relaxant effects of 5-methoxytryptamine (pA2 = 9.0) and R,S-zacopride (pA2 = 9.4). The compound had no effect on isoprenaline-induced relaxation of the carbachol-contracted oesophagus at a concentration of 1 gM.6 In tests of selectivity, GR113808 had only low affinity for 5-HT3 receptors (pKi = 6.0) and had no functional activity at either 5-HT2 or 5-HT,-like receptors on vascular smooth muscle preparations. In a range of binding assays, GRi 13808 was shown to have no appreciable affinity for any other receptor type investigated. 7 In the anaesthetized piglet, GRI13808 was a potent antagonist of 5-methoxytryptamine-induced tachycardia (mean DRo = 97.2 gg kg-' h-'). The compound was ineffective against isoprenaline-induced tachycardia. 8 The present results are discussed in comparison with those for existing antagonists at the 5-HT4 receptor. The results of this study indicate that GRI13808 will be a valuable antagonist for studying 5-HT4 receptor mechanisms in vitro and in vivo and validate its use as a radioligand for determining 5-HT4 receptor distribution.

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