Abstract

The interaction of dopaminergic antagonists with the D_1Adopamine receptor was assessed in PC2 cells that transiently express this receptor. The maximal binding and dissociation constants for the D_1A dopamine receptor, using the ligand [¹²⁵I]SCH23982 were 0.38 ± 0.09 nM and 1 to 4 pmol/mg, respectively, when assessed 48 h after transfection with cDNA encoding the rat D_1A receptor. Basal adenylyl cyclase activity increased 50 to 60% in membranes of transfected PC2 cells compared with control membranes. The dopaminergic antagonists clozapine, cis-flupenthixol, (+)-butaclamol, haloperidol, chlorpromazine, and fluphenazine inhibited constitutive adenylyl cyclase activity in membranes of cells expressing the D_1A receptor. SCH23390, a selective D₁ dopamine receptor antagonist, and (−)-butaclamol did not alter basal cyclase activity, whereas dopamine increased enzyme activity in membranes expressing the D_1A dopamine receptor. The coupling of D_1A receptors with G_s proteins was examined by immunoprecipitation of membrane G_sα followed by immunoblotting with a D_1A dopamine receptor monoclonal antibody. Clozapine, cis-flupenthixol, (+)-butaclamol, haloperidol, and fluphenazine but not SCH23390 or (−)-butaclamol decreased D_1A receptor-G_sα coupling by 70 to 80%, and SCH23390 was able to prevent the receptor-G_sαuncoupling induced by haloperidol or clozapine. These results indicate that some dopaminergic antagonists suppress basal signal transduction and behave as inverse agonists at the D_1A dopamine receptor. This action of the dopamine receptor antagonists may contribute to their antidopaminergic properties that seem to underlie their clinical actions as antipsychotic drugs.