COCAINE AND GBR-12783 RECOGNIZE NONIDENTICAL, OVERLAPPING BINDING DOMAINS ON THE DOPAMINE NEURONAL CARRIER

In incubation medium containing Na+ as the only cation, the specific b inding of [H-3]cocaine to a membrane preparation obtained from rat str iatum reached a maximal level for 10 mM Na+, whereas higher concentrat ions decreased its affinity. The specific binding of [H-3]cocaine was inhibited monophasically by GBR 12783, mazindol, nomifensine and subst rates of the transporter; in saturation experiments, GBR 12783 competi tively blocked the [H-3]cocaine specific binding and vice versa. Treat ment of the striatal membranes with N-ethylmaleimide resulted in a con centration-dependent reduction of the specific binding of [H-3]GBR 127 83 (1-[2-(diphenylmethoxy)ethyl]4-(3-phenyl [H-3]propenyl)-piperazine) which was significantly more marked than that of the specific binding of [H-3]cocaine, the nonspecific binding of [H-3]cocaine being measur ed with either cocaine or dopamine. Addition of substrates or pure upt ake inhibitors to the treatment medium afforded protection against the N-ethylmaleimide-induced reduction in both bindings. In particular, c ocaine offered protection for [H-3]GBR 12783 binding and vice versa. A ll results are consistent with a model in which pure uptake blockers a nd substrates recognize nonidentical but overlapping binding domains o n the neuronal carrier of dopamine.