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Vol. 56, Issue 5, 989-996, November 1999
Laboratory of Molecular Pharmacology, Department of Pharmacology,
MCP Hahnemann School of Medicine, Philadelphia, Pennsylvania, (G.C.,
H.G., C.S., H.Y.W., E.F.); Department of Pharmacology, Medical Faculty
of Ankara University, Ankara, Turkey (H.G.)
The interaction of dopaminergic antagonists with the D1A
dopamine 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 Gs
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-Gs
coupling by 70 to
80%, and SCH23390 was able to prevent the receptor-Gs
uncoupling 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.
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