Phenoxybenzamine treatment differentiates dopaminergic 3H-ligand binding sites in bovine caudate membranes

Phenoxybenzamine, the classic alpha-adrenergic receptor alkylating agent, also acts as an irreversible antagonist of the binding of [3H]spiroperidol, a D-2-selective dopaminergic ligand, to bovine caudate membranes. Doses completely eliminating the binding of this ligand leave the binding of [3H]dopamine to D-3 sites virtually unaffected. The binding sites for these two ligands thus represent distinct subtypes of dopamine receptors, not interconverting states of a single receptor. This phenoxybenzamine-mediated inhibition proceeds via a dose-dependent (pseudo-IC50 = 1 microM) decrease in Bmax with little or no change in affinity for 3H-ligands at the D-2 site. The effect is site-directed, as the dopaminergic agonists dopamine and apomorphine and the antagonist domperidone are able to protect against phenoxybenzamine-mediated attack in proportion to their affinities for D-2 sites. Epinephrine, norepinephrine, and serotonin are much less effective in protecting these sites. The sensitivity of [3H]apomorphine binding is intermediate to that of [3H]spiroperidol and [3H]dopamine. [3H]Apomorphine binding can be resolved into a phenoxybenzamine-labile population of binding sites which have equal phenoxybenzamine sensitivity, selectivity among protecting agents, and butyrophenone affinity to those of D-2 sites labeled by 3H-butyrophenones, and a separate phenoxybenzamine-stable population of sites which have an affinity for dopamine comparable to that of D-3 sites labeled by [3H]dopamine. [3H]Apomorphine therefore appears to label a portion of D-2 receptor sites in addition to D-3 receptors.