Abstract:Although much is known about the effects of Na ϩ , K ϩ , and Cl Ϫ on the functional activity of the neuronal dopamine transporter, little information is available on their role in the initial event in dopamine uptake, i.e., the recognition step. This was addressed here by studying the inhibition by dopamine of the binding of at Ն140 mM decreasing the K i . At 290 mM Cl Ϫ and 300 mM Na ϩ the potency of K ϩ in inhibiting dopamine binding was enhanced as compared with the absence of Cl Ϫ in contrast to the lack of effect of Cl Ϫ up to 140 mM (Na ϩ up to 150 mM). The results indicate that Cl Ϫ at its extracellular level enhances dopamine binding through a mechanism not involving site 1. The observed correspondence between the WIN 35,428 and dopamine domains in their inclusion of the inhibitory cation site explains why many of the previously reported interrelated effects of Na ϩ and K ϩ on the binding site of radiolabeled blockers to the dopamine transporter are applicable to dopamine uptake in which dopamine recognition is the first step. Key Words: Dopamine transporter-WIN 35,428 binding-Dopamine binding-SodiumPotassium-Human.