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 hippoc ampus 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 displ ayed rapid kinetics such that association and dissociation were comple te within 3 min. 4 Specific [H-3]-GR113808 binding was potently and st ereoselectively inhibited by agonists and antagonists acting at the 5- HT4 receptor but not by compounds selective for other 5-HT receptors o r other neurotransmitter receptors. 5 Autoradiographic analysis reveal ed a discrete localization in both guinea-pig and rat brain with high concentrations of binding in brain areas such as the striatum, substan tia nigra and olfactory tubercle. 6 [H-3]-GR113808 binding to homogena tes of guinea-pig striatum meets the criteria for labelling of the 5-H T4 receptor and, as such, represents the first characterization of thi s receptor in a radioligand binding assay.