Binding of [3H]apomorphine to an aporphine binding site as well as to dopamine sites in tissue from bovine caudate nucleus.

Binding of the tritiated dopamine (DA) agonists, apomorphine (APO) and a dihydroxyaminotetralin (ADTN) to a membrane preparation from the caudate nucleus of calf brain was compared. Binding of [3H]dihydroxyaminotetralin at small (nM) concentrations followed simple, monophasic inhibition (over 80% at less than 500 nM) by concentrations of apomorphine between 50 pM and 1 mM. Inhibition of the binding of [3H]apomorphine by dihydroxyaminotetralin was more complex, and included in component with a low (microM) affinity for dihydroxyaminotetralin accounting for approx. 20% of total binding. The kinetics of binding of the ligands to high-affinity sites were virtually identical (apparent Kd = 0.81 nM; Bmax = 211 fmol/mg protein) and could not be distinguished by curve-fitting techniques adapted to analysis by microcomputer. In contrast, the binding of [3H]apomorphine with a "blank" defined by excess (10 microM) dihydroxyaminotetralin could be resolved into the same high-affinity component and a lower-affinity site (Kd = 124 nM; Bmax = 5740 fmol/mg). The pharmacology of the lower-affinity binding of [3H]apomorphine was evaluated by coincubating with 0.5 microM dihydroxyaminotetralin to "mask" high-affinity sites, and was compared to high-affinity binding of [3H]apomorphine and [3H]dihydroxyaminotetralin. The high-affinity binding was stereoselective for DA receptor agonists and antagonists. The pharmacology of the lower-affinity site resembled no known DA receptor type and showed highest affinities for aporphines but was not stereoselective and reacted weakly and nonspecifically with dihydroxyaminotetralin, DA, other catecholamines and neuroleptics. Thus, [3H]apomorphine, under certain conditions, may detect an aporphine binding site of uncertain pharmacological significance, as well as high-affinity DA agonist (D-3) sites.