Binding characteristics of the dopamine uptake inhibitor [³H]nomifensine to striatal membranes

Binding of the radiolabeled antidepressant [³H]nomifensine to rat and rabbit striatal membranes has been characterized. The specific binding of [³H]nomifensine to striatal membranes was stable, reversible and saturable. Saturation experiments indicated that [³H]nomifensine labeled a single site with an affinity (K_d) of 80 nM and a total number of binding sites (B_max) of 6.5 pmoles/mg protein both in rat and rabbit striatal membranes. The affinity constants obtained from kinetic analyses and competition experiments were in fairly good agreement with those obtained in saturation experiments. Compounds known to inhibit [³H]dopamine uptake in vitro, such as nomifensine, 4-hydroxy-nomifensine, mazindol, amfonelic acid and benztropine, were the most potent competitors of nomifensine binding. Additionally, the absolute potencies of various drugs in competing for [³H]nomifensine binding to rat and rabbit striatal membranes correlated closely with their potencies in inhibiting [³H]dopamine uptake into striatal synaptosomes. Specific [³H]nomifensine binding was dependent on the presence of NaCl which is also consistent with its association with the dopamine uptake pump. The number, but not the affinity, of striatal [³H]nomifensine binding sites was reduced significantly following in vivo lesions with 6-hydroxydopamine. The number of [³H]nomifensine binding sites was found to be highest in areas rich in dopamine nerve terminals such as the striatum and olfactory tubercle. These results suggest that [³H] nomifensine binds to a site on dopaminergic nerve terminals associated with the dopamine uptake pump.