Selective inhibition of adenylyl cyclase type V by the dopamine D3 receptor.

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 Gi family. Guanosine-5'-O-(3-thio)triphosphate binding studies indicate that the D3 receptor weakly activates all three Gialpha 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.