Sodium ions modulate the binding of sulpride to wild-type dopamine D<sub>2</sub> and D<sub>3</sub> receptors but not to mutant dopamine D<sub>2</sub><sup>D80A</sup> and D<sub>3</sub><sup>D75A</sup> receptors.

<p>Competition analyses of the inhibition of [<sup>3</sup>H]spiperone binding to membranes prepared from U-2 OS cells expressing (A) wild-type dopamine D<sub>2</sub> receptors (filled symbols)/dopamine D<sub>2</sub><sup>D80A</sup> mutant receptors (open symbols) or (B) wild-type dopamine D<sub>3</sub> receptors (filled symbols)/dopamine D<sub>3</sub><sup>D75A</sup> mutant receptors (open symbols), by sulpiride, were performed in the presence of 100 mM NaCl (■/□) or 100 mM NMDG (▲/Δ). Data are mean ± SEM of at least 3 independent experiments.</p>