Zinc modulates antagonist interactions with D-2-like dopamine receptors through distinct molecular mechanisms

Recently, zinc has been shown to modulate antagonist drug interactions with the D-1 dopamine receptor (Schetz and Sibley, 1997) and the dopamine trans porter (Norregaard et al., 1998). We now demonstrate that zinc also reversi bly and dose-dependently modulates the specific binding of the butyrophenon e antagonist [H-3]methylspiperone to all D-2-like dopamine receptors: D-2L, D-3, and D-4. The molecular mechanisms of zinc regulation of these D-2-lik e receptor subtypes are distinct because zinc inhibition of [H-3]methylspip erone binding to the D-4 receptor is noncompetitive by both equilibrium and kinetic measures (lower B-max and essentially no change in k(off)), wherea s the corresponding inhibition of zinc at D-2L and D-3 receptors is primari ly characterized by competitive allosterism (increases in K-D and k(off)). Interestingly, thermodynamic measurements reveal that the macroscopic prope rties 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 [H-3]raclopride, a structurally different antagonist of the substituted benzamide class. Sodium ions negatively modu late zinc inhibition of both sodium-insensitive [H-3]methylspiperone bindin g and sodium-sensitive [H-3]raclopride binding. In addition to its demonstr ated effects on antagonist binding in membrane preparations, zinc also reta rds the functional effects of antagonist at the D-2L receptor in intact cel ls. These findings suggest that synaptic zinc may be a factor influencing t he effectiveness of therapies that rely on dopamine receptor antagonists.