Abstract

Despite a great deal of research, the second messenger coupling of the dopamine D₃ receptor has not yet been clearly established. The closely related D₂ and D₄ receptors have been shown to inhibit adenylyl cyclase activity in a variety of cell types, but the D₃ receptor has little or no effect on this second messenger system. We now demonstrate that when the D₃ receptor and adenylyl cyclase type V are coexpressed in 293 cells, the agonist quinpirole causes 70% inhibition of forskolin-stimulated cAMP levels. This effect seems to be selective for this adenylyl cyclase isoform because the D₃ receptor does not inhibit adenylyl cyclase types I or VI and only weakly stimulates adenylyl cyclase type II. In contrast, the D₂ receptor inhibits cAMP accumulation in 293 cells in the absence of cotransfected adenylyl cyclases and stimulates adenylyl cyclase type II to a greater extent than the D₃ receptor. The inhibition of adenylyl cyclase type V by the D₃ receptor is sensitive to pertussis toxin, suggesting the involvement of G proteins of the G_ifamily. Guanosine-5′-O-(3-thio)triphosphate binding studies indicate that the D₃ receptor weakly activates all three G_iα subunits, whereas the D₂ receptor activates these G proteins to a substantially greater extent. However, despite its relative inability to promote G protein activation, the D₃ receptor is capable of substantial and consistent inhibition of adenylyl cyclase type V. The robust second messenger coupling of the D₃ receptor in a heterologous system with defined components provides a system for further studies of the function of this receptor and should facilitate the development and characterization of new D₃ receptor ligands.