Demonstration of alpha 1A- and alpha 1B-adrenoceptor binding sites in human brain tissue.

Radioligand binding studies suggest that alpha 1-adrenoceptor recognition sites are heterogeneous. Several adrenergic agents discriminate between two adrenoceptor binding sites designated alpha 1A and alpha 1B. In the present study we demonstrate for the first time that these two subtypes exist in the human brain. 5-Methyl-urapidil and (+)-niguldipine, which have previously been shown to be alpha 1A-selective, inhibited [3H]prazosin binding to cortical membranes in a biphasic manner. The irreversible alpha 1B-ligand, chloroethylclonidine, preferentially eliminated the binding sites with low affinity for (+)-niguldipine. In contrast, BE 2254 and unlabelled prazosin displaced the radioligand in a monophasic manner. The IC50 values for prazosin were not affected by pretreatment of the membranes with chloroethylclonidine. Our data on human brain membranes are in excellent agreement with recent findings in rat tissues and suggest that the alpha 1-adrenoceptor subtypes in human brain are similar to those in rat tissues.