Constitutive Coupling of a Chimeric Dopamine D₂/α_1B Receptor to the Phospholipase C Pathway: Inverse Agonism to Silent Antagonism by Neuroleptic Drugs

Abstract

Neuroleptic drugs have been suggested to act as inverse agonists at the dopamine D₂ receptor, but no link between therapeutic efficacy and ligand's intrinsic activity could be determined. Since the resolving capacity to monitor inverse agonism at dopamine D₂ receptors is limited, we speculated that receptor constitutive activation could be enhanced by constructing chimeric D₂/α_1B receptors. Marked inverse agonist responses with a series of dopamine antagonists were obtained by: 1) exchange of the D_2short receptor's 3ICL by that of the α_1B-adrenoceptor, 2) incorporation of an activating mutation (Ala²⁷⁹Glu) in the distal portion of its 3ICL, and 3) coexpression with a G_α11 protein. This chimeric D₂/α_1B receptor construct displayed a ligand binding profile comparable to that of the wild-type (wt) D_2short receptor and an effector activation profile close to that of the wt α_1B-adrenoceptor. Most of the dopamine antagonists attenuated by −54 to −59% basal inositol phosphates (IP) formation, thus clearly acting as inverse agonists. Ziprasidone behaved as a silent antagonist (+5% versus basal IP level) and antagonized both dopamine-mediated (pK_B, 7.61) and tropapride-mediated (pK_B, 8.52) IP responses. Clozapine, olanzapine, and raclopride displayed partial inverse agonist properties (−31, −67, and −71% versus tropapride, respectively), whereas bromerguride (+63%) andcis-(+)-5-methoxy-1-methyl-2-(di-n-propylamino tetralin) [(+)-UH 232] (+88%) demonstrated positive agonism. In conclusion, analyses with the chimeric D₂/α_1B Ala²⁷⁹Glu 3ICL receptor construct suggest that neuroleptic drugs can be differentiated on the basis of their intrinsic activity, as they can either activate, inhibit, or be silent at this receptor construct.