DOPAMINE D-1 AND D-2 RECEPTORS AND THEIR SIGNAL SYSTEM PRESENT IN COATED VESICLES PREPARED FROM BOVINE STRIATAL TISSUE

Coated vesicles (CVs) isolated from bovine striatal tissue were examin ed to determine whether they are associated with dopamine signal syste ms consisting of dopamine D-1 and D-2 receptors, G proteins, and adeny late cyclase. Dopamine receptors in CVs were characterized by a dopami ne D-1 receptor antagonist, [H-3]SCH 23390, and a dopamine D-2 recepto r antagonist, [H-3]-spiroperidol. The bindings of both ligands were sp ecifically saturable and reversible with a dissociation constant (K-D) of 0.65 and 0.5 nM, respectively. Dopaminergic antagonists and agonis ts inhibited the specific bindings of [H-3]SCH 23390 and [H-3]spiroper idol in a stereoselective and concentration-dependent manner with an a ppropriate rank order potency for dopamine D-1 or D-2 receptors. The r egulations of the agonist binding by guanyl-5-ylimidodiphosphate were observed. ADP ribosylation of the CVs with [P-32]NAD demonstrated pred ominant labeling of bands of M(r) 47,000-52,000, 42,000-45,000, and 40 ,000-39,000, which corresponded to the known molecular weights of the alpha subunits of G(s) and G(i) proteins. The presence of alpha and be ta subunits of G proteins in the CVs was also confirmed by immunoblott ing assay. Adenylate cyclase activity, which was stimulated by SKF 383 93 and inhibited by dopamine D-2 receptor agonists, was present in the CVs. These findings suggest that the dopamine D-1 and D-2 receptors i n the CVs couple with adenylate cyclase via G(s)/G(i) protein.