Abstract

Recently, zinc has been shown to modulate antagonist drug interactions with the D₁ dopamine receptor (Schetz and Sibley, 1997) and the dopamine transporter (Norregaard et al., 1998). We now demonstrate that zinc also reversibly and dose-dependently modulates the specific binding of the butyrophenone antagonist [³H]methylspiperone to all D₂-like dopamine receptors: D_2L, D₃, and D₄. The molecular mechanisms of zinc regulation of these D₂-like receptor subtypes are distinct because zinc inhibition of [³H]methylspiperone binding to the D₄receptor is noncompetitive by both equilibrium and kinetic measures (lower B _max and essentially no change ink _off), whereas the corresponding inhibition of zinc at D_2L and D₃ receptors is primarily characterized by competitive allosterism (increases inK _D and k _off). Interestingly, thermodynamic measurements reveal that the macroscopic properties of zinc binding are entropy-driven for all receptor subtypes, despite their having distinct molecular mechanisms. Zinc also reduces the binding affinity of the D_2L receptor for [³H]raclopride, a structurally different antagonist of the substituted benzamide class. Sodium ions negatively modulate zinc inhibition of both sodium-insensitive [³H]methylspiperone binding and sodium-sensitive [³H]raclopride binding. In addition to its demonstrated effects on antagonist binding in membrane preparations, zinc also retards the functional effects of antagonist at the D_2L receptor in intact cells. These findings suggest that synaptic zinc may be a factor influencing the effectiveness of therapies that rely on dopamine receptor antagonists.