Dopamine D-2L receptor couples to G alpha(i2) and G alpha(i3) but not G alpha(i1), leading to the inhibition of adenylate cyclase in transfected cell lines

Previously, we showed that both D-2, and D-4 dopamine receptors inhibited adenylate cyclase in a pertussis toxin (Ptx)-sensitive manner in the dopamine-producing MN9D cell line, whereas only D-2 receptors did so in a fibroblast cell line CCL1.3. Of the known Ptx-sensitive G proteins, MN9D cells expressed G alpha(i2), G alpha(oA), and G alpha(oB), whereas CCL1.3 cells, expressed only G alpha(i2). Here we cotransfected MN9D and CCL1.3 cells with either the long form of the D-2 receptor (D-2L) or the D-4 receptor and a mutant Ptx-resistant G protein alpha-subunit. When cotransfected CCL1.3 cell lines were tested for the ability of Ptx to block receptor-mediated inhibition of cyclic AMP accumulation, D-2 receptors were found to couple to mutant G alpha(i2) and G alpha(i3) but not G alpha(i1) or G alpha(oA). D-2 also coupled to mutant G alpha(i2) but not G alpha(oA) in MN9D cells. In contrast, D-4 receptors did not couple to either mutant G alpha(i2) or G alpha(oA) subunits in MN9D cells. These data suggest that D-4 receptor-mediated inhibition of adenylate cyclase is not coupled via the same mechanisms used by D-2 receptors. D-2L receptors are capable of coupling to more than one G protein in the modulation of cyclic AMP.