CHARACTERIZATION OF A SEROTONIN RECEPTOR IN THE CNIDARIAN RENILLA-KOELLIKERI - A RADIOBINDING ANALYSIS

Serotonin (5-HT) binding sites in membrane preparations from the sea p ansy Renilla koellikeri were identified using [H-3]5-HT as radioligand and unlabelled 5-HT as displacer of specific binding. Saturation and kinetic studies revealed a mixed population of [H-3]5-HT binding sites which as a whole displayed slow association kinetics, saturability, n egligible dissociation and low affinity. The dopaminergic antagonist t rifluoperazine specifically displaced the non-dissociated binding site s, allowing the characterization of a homogenous population of saturab le sites exhibiting faster association and full dissociation. Scatchar d analysis of this population revealed a K-D of 23-34 nM and binding s ite density (B-max) of 8.5-20.6 pmol/mg protein depending on body regi on. The pharmacological profile of the dissociable [H-3]5-HT binding s ites could not be categorized with that of any of the existing vertebr ate and invertebrate 5-HT receptor subtypes in the current nomenclatur e. Especially noteworthy was the absolute inability of LSD, a ligand o f choice for all hitherto identified invertebrate 5-HT receptors, to c ompete for the binding sites in sea pansy membrane preparations. It is proposed that these binding sites represent an evolutionary forerunne r of the primordial 5-HT receptor that diversified as multiple subtype s in higher invertebrates and vertebrates. (C) 1997 Elsevier Science L td.