Fig. 7. Treatment with dopamine agonists increases the D₂ receptor density at plasma membrane and decreases the D₂ level from sG_i2-D₂ complex. (A) Incubation of BHK cells expressing D_2S and sG_i2 protein with 10 µM dopamine (D) or 5 µM quinpirole (Q) for 30 minutes showed a significant increase (18%) in D₂ receptor localization at plasma membrane. By contrast, however, treatment with antagonist [raclopride (R), 5 µM] produced no effect. Controls are from C1 to C5 (C1, cells expressing D_2S receptors alone and treated with dopamine; C2, cells treated with quinpirole; C3, cells expressing G_i2 as well as D_2S receptor and treated with dopamine; C4, cells treated with quinpirole; C5, cells expressing sG_i2 and D_2S receptor but left untreated). (B) Extracts of same cells as used in A were passed through sG_i2 immunoaffinity columns and eluted to determine sG_i2-bound D_2S receptor in above conditions. Immunoblot assays show that agonist treatment resulted in loss of 75% (equals to 22% of the binding results) D_2S component (49 kDa) from sG_i2-D_2S complex, whereas the level of sG_i2 was the same in all conditions. (C) Average OD change in immunoblot experiments from B. Results show that only 28.6±2.48% and 25±2.78% of the D_2S receptor remained bound in the sG_i2-protein complex eluted from dopamine (D)-treated and quinpirole (Q)-treated cells, respectively. Values are representative of five different experiments.