¹Institute of Pharmacology, University of Vienna, A-1090 Wien, Austria

Correspondence: Dr M Freissmuth, Institute of Pharmacology, University of Vienna, Waehringer Str.13A, A-1090 Wien, Austria. Tel: +43 1 4277 64171; Fax: +43 1 4277 9641; E-mail: michael.freissmuth@univie.ac.at

Received 22 May 2002; Revised 10 February 2003; Accepted 20 February 2003; Published online 30 April 2003.

Abstract

The adenosine A_2A receptor and the dopamine D₂ receptor are prototypically coupled to G_s and G_i/G_o, respectively. In striatal intermediate spiny neurons, these receptors are colocalized in dendritic spines and act as mutual antagonists. This antagonism has been proposed to occur at the level of the receptors or of receptor–G protein coupling. We tested this model in PC12 cells which endogenously express A_2A receptors. The human D₂ receptor was introduced into PC12 cells by stable transfection. A_2A-agonist-mediated inhibition of D₂ agonist binding was absent in PC12 cell membranes but present in HEK293 cells transfected as a control. However, in the resulting PC12 cell lines, the action of the D₂ agonist quinpirole depended on the expression level of the D₂ receptor: at low and high receptor levels, the A_2A-agonist-induced elevation of cAMP was enhanced and inhibited, respectively. Forskolin-stimulated cAMP formation was invariably inhibited by quinpirole. The effects of quinpirole were abolished by pretreatment with pertussis toxin. A_2A-receptor-mediated cAMP formation was inhibited by other G_i/G_o-coupled receptors that were either endogenously present (P_2y12-like receptor for ADP) or stably expressed after transfection (A₁ adenosine, metabotropic glutamate receptor-7A). Similarly, voltage activated Ca²⁺ channels were inhibited by the endogenous P_2Y receptor and by the heterologously expressed A₁ receptor but not by the D₂ receptor. These data indicate functional segregation of signaling components. Our observations are thus compatible with the proposed model that D₂ and A_2A receptors are closely associated, but they highlight the fact that this interaction can also support synergism.