Dopamine D_2L receptor couples to Gα_i2 and Gα_i3 but not Gα_i1, leading to the inhibition of adenylate cyclase in transfected cell lines

Previously, we showed that both D₂ and D₄ dopamine receptors inhibited adenylate cyclase in a pertussis toxin (Ptx)-sensitive manner in the dopamine-producing MN9D cell line, whereas only D₂ receptors did so in a fibroblast cell line, CCL1.3. Of the known Ptx-sensitive G proteins, MN9D cells expressed Gα_i2, Gα_oA and Gα_oB, whereas CCL1.3 cells expressed only Gα_i2. Here we cotransfected MN9D and CCL1.3 cells with either the long form of the D₂ receptor (D_2L) or the D₄ receptor and a mutant Ptx-resistant G protein α-subunit. When cotransfected CCL1.3 cell lines were tested for the ability of Ptx to block receptor-mediated inhibition of cyclic AMP accumulation, D₂ receptors were found to couple to mutant Gα_i2 and Gα_i3 but not Gα_i1 or Gα_oA. D₂ also coupled to mutant Gα₁₂ but not Gα_oA in MN9D cells. In contrast, D₄ receptors did not couple to either mutant Gα_i2 or Gα_oA subunits in MN9D cells. These data suggest that D₄ receptor-mediated inhibition of adenylate cyclase is not coupled via the same mechanisms used by D₂ receptors. D_2L receptors are capable of coupling to more than one G protein in the modulation of cyclic AMP.