Binding of super(125)I-iodohydroxybenzylpindolol to cerebral membranes: Association with 5-hydroxytryptamine recognition sites as well as beta-adrenoceptors

The binding characteristics of the radio-iodinated beta-adrenoceptor antagonist ligand super(125)I-iodohydroxybenzylpindolol ( super(125)I-IHYP) have been compared in membrane preparations derived from three different sources in a single mammalian species. In membrane preparations derived from rat lung and erythrocytes, super(125)I-IHYP binds solely to beta-adrenoceptors as defined by the stereoselectivity of the isomers of propranolol and the affinity of isoprenaline. However, in membranes prepared from rat cerebral cortex, although some 30% of super(125)I-HYP binding is to beta-adrenoceptors, a large proportion of the remainder is to sites that exhibit characteristics for 5-hydroxytryptamine (5HT) recognition. Thus, binding assays for super(125)I-IHYP in cerebral cortical membranes performed in the presence of 30 mu M(-)-isoprenaline (to eliminate the binding of the ligand to beta-adrenoceptors) show that isomers of propranolol still maintain a degree of stereoselectivity at these sites. Also, 5HT and its congeners exhibit binding activity similar to that seen at 5HT receptors identified by super(3)H-5HT.