Figure 3.

D₁ and D₅ receptors compete for binding to Gal₁ receptors. a, b, BRET experiments were performed with cells coexpressing D₁-Rluc (300 ng plasmid) and Gal₁-YFP receptors (4 μg plasmid) and increasing amounts of D₅ receptors (0 to 5.5 μg plasmid) (a) or D₁-Rluc (300 ng plasmid), Gal₁-YFP (0.3 to 5 μg plasmid), and D₅ receptors (1.5 μg plasmid) (b). In a, no significant variation in luminescence caused by D₁-Rluc receptors (about 150,000 luminescent units) or fluorescence caused by Gal₁-YFP receptors (about 10,000–70,000 fluorescent units) was observed by increasing D₅ receptor expression. In b, similar luminescence attributable to D₁-Rluc or fluorescence attributable to Gal₁-YFP receptors was obtained in the absence or presence of D₅ receptors. The relative amount of BRET is given as the ratio between the fluorescence of the acceptor minus the fluorescence detected in cells expressing only the donor and the luciferase activity of the donor. BRET data are expressed as the mean ± SD of 4–16 different experiments grouped as a function of the amount of BRET acceptor. The dashed curve represents BRET saturation curve obtained with the D₁-Rluc receptor and increasing amounts of cDNA for the Gal₁-YFP receptor in the absence of D₅ receptors shown in Figure 1.