DEVELOPMENT OF A RADIOLIGAND BINDING ASSAY FOR 5-HT(4)-RECEPTORS IN GUINEA-PIG AND RAT-BRAIN

1 The 5-HT4 receptor antagonist, GR113808, has been radiolabelled to a high specific activity with tritium. 2 Characterization of specific [H-3]-GR113808 binding in homogenates of guinea-pig striatum and hippocampus revealed a single site of high affinity (K(d) values 0.20 and 0.13 nm respectively). 3 [H-3]-GR113808 binding was reversible and displayed rapid kinetics such that association and dissociation were complete within 3 min. 4 Specific [H-3]-GR113808 binding was potently and stereoselectively inhibited by agonists and 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 [H-3]-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.