Fig. 3.

Single-point competition measurements for displacement of [³H]MSP by various 1,4-DAPs. [³H]MSP-bound D₂ receptors were incubated with various 1,4-DAPs in the presence of different buffer conditions. A, the presence of 140 mM sodium enhances the affinity of D₂ receptor for 1,4-DAP ligands. B, the effect of sodium is greatly enhanced in the D₂ V2.61(91)F mutant. As a control for ionic strength and nonspecific charge effects, N-methyl-d-glucamine was tested and displayed no significant changes from binding buffer alone. ^* means significantly different (P < 0.05) from both binding buffer and NMDG. ++ denotes significantly different (P < 0.05) from only NMDG. +++ denotes significantly different from only binding buffer.