Inverse agonist properties of dopaminergic antagonists at the D1A dopamine receptor:: Uncoupling of the D1A dopamine receptor from Gs protein

The interaction of dopaminergic antagonists with the D-1A dopamine 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 [I-125]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 D-1 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(s alpha) 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(s alpha) coupling by 70 to 80%, and SCH23390 was able to prevent the receptor-G(s alpha) uncoupling 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.