ALPHA-ADRENOCEPTORS IN RAT-BRAIN - DIRECT IDENTIFICATION WITH PRAZOSIN

Tritiated prazosin was used to characterize high affinity binding sites with characteristics similar to α1 adrenoceptors in rat brain membranes. These sites were compared with α2 adrenoceptors labeled with tritiated clonidine. The prazosin sites had an association constant of 2 nM-1 and bound the ligand optimal around pH 7.0. The density of the sites was 300 fmoles per mg of protein; the half time of dissociation of prazosin was 7 min at 30° C. The order or potencies of agonists, determined from binding-inhibition experiments with labeled prazosin, was: naphazoline > clonidine > adrenaline > noradrenaline > phenylephrine > α-methylnoradrenaline > dophamine. The order of potencies of antagonists was: prazosin > phenoxybenzamine > phentolamine > clozapine > yohimbine. Sodium ions and divalent cations as well as guanyl nucleotides have little or no effect on the binding of the labeled antagonist. This is in contrast to the binding of the labeled agonist clonidine (Glossmann and Presek, 1979a, 1979b). Labeled prazosin may be a useful tool to characterize α1 adrenoceptors. © 1979 Springer-Verlag.