Binding characteristics of a radiolabeled agonist and antagonist at central nervous system alpha noradrenergic receptors

Binding of the alpha noradrenergic agonist [³H]clonidine and the alpha antagonist [³H]WB-4101 (2 ([2',6' dimethoxy]phenoxyethylamino)methylbenzodioxan) to rat brain membranes exhibits characteristics expected of alpha receptors for norepinephrine. Binding of both [³H]ligands is saturable, with KD values of 5.8 nM and 0.48 nM for [³H]clonidine and [³H]WB-4101, respectively. A series of catecholamines inhibits the binding of both ligands with the potency order epinephrine > norepinephrine >> isoproterenol, corresponding to the relative activities of these agents at alpha receptors in the periphery. Competition for binding is stereoselective, with (-) isomers of phenylethanolamines many times more potent than the corresponding (+) isomers. Classical alpha antagonists inhibit binding of both ligands at low concentrations, but beta antagonists are much weaker. Alpha agonists are more potent in displacing [³H]clonidine than [³H]WB-4101 binding, while alpha antagonists compete more avidly for [³H]WB-4101 sites. Partial agonist ergot alkaloids display similar affinities for the binding sites of both [³H]ligands. These findings may be explained by the existence of discrete agonist and antagonist states of the alpha receptor, which preferentially bind [³H]clonidine and [³H]WB-4101, respectively. Regional variations in the binding of both [³H]ligands in the brain are not pronounced, although levels tend to be highest in hypothalamus and cerebral cortex and lowest in cerebellum. Treatment with 6-hydroxydopamine fails to decrease the binding of either [³H]ligand, suggesting that binding occurs to postsynaptic sites.