Inverse Agonist Properties of Dopaminergic Antagonists at the D1ADopamine Receptor: Uncoupling of the D1ADopamine Receptor from GsProtein

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Abstract
The interaction of dopaminergic antagonists with the D1Adopamine receptor was assessed in PC2 cells that transiently express this receptor. The maximal binding and dissociation constants for the D1A 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 D1A 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 D1A 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 D1A dopamine receptor. The coupling of D1A receptors with Gs proteins was examined by immunoprecipitation of membrane Gsα followed by immunoblotting with a D1A dopamine receptor monoclonal antibody. Clozapine, cis-flupenthixol, (+)-butaclamol, haloperidol, and fluphenazine but not SCH23390 or (−)-butaclamol decreased D1A receptor-Gsα coupling by 70 to 80%, and SCH23390 was able to prevent the receptor-Gsαuncoupling induced by haloperidol or clozapine. These results indicate that some dopaminergic antagonists suppress basal signal transduction and behave as inverse agonists at the D1A 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.