Pharmacology of quinpirole-stimulated [35S]GTPgammaS binding: discrepancy with receptor binding profile

Functional consequences of receptor stimulation by quinpirole, a dopamine D(2)-like receptor agonist, were assessed using agonist-stimulated [35S]GTPgammaS binding in rat striatal membranes. Dopamine receptor antagonists inhibited quinpirole-stimulated [35SCH 23390 (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4, 5-tetrahydro-1H-3-benzazepine), consistent with a dopamine D(2)-like profile. In contrast, the monoamine oxidase inhibitors Ro 41-1049 (N-(2-aminoethyl)-5-(3-fluorophenyl)-4-thiazolecarboxemide), and (+)- and (-)-deprenyl, which inhibit [3H]quinpirole binding, had no effect on agonist-independent or quinpirole-stimulated [35S]GTPgammaS binding. Clorgyline inhibited [35S]GTPgammaS binding by a non-dopamine D(2) receptor-mediated mechanism. These findings demonstrate a notable discrepancy between the pharmacological profile of [3H]quinpirole binding and quinpirole-stimulated [35S]GTPgammaS binding.