Development of a radioligand binding assay for 5‐HT4 receptors in guinea‐pig and rat brain

Abstract

The 5‐HT4 receptor antagonist, GR113808, has been radiolabelled to a high specific activity with tritium. Characterization of specific [3H]‐GR113808 binding in homogenates of guinea‐pig striatum and hippocampus revealed a single site of high affinity (Kd values 0.20 and 0.13 nm respectively). [3H]‐GR113808 binding was reversible and displayed rapid kinetics such that association and dissociation were complete within 3 min. Specific [3H]‐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. 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. [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. 1993 British Pharmacological Society