Interactions of cations and anions with the binding of uptake blockers to the dopamine transporter.

Uptake blockers and substrates are likely to recognise a common binding domain on the dopamine neuronal transporter (DAT). Among cations that form ionic gradients at the level of the cellular plasma membrane, Na+ is the only one that can stimulate their binding. The binding stimulation appears over Na+ concentrations ranging from 0 to 10-60 mM; at higher Na+ concentrations, binding reaches a plateau or decreases, according to the uptake blocker that is studied. The majority of the other cations, including K+, Ca2+, Mg2+ and Tris+, inhibit the binding of uptake blockers. Several metals impair binding to the DAT and/or the dopamine transport, but, under specific conditions, some of them, and chiefly Zn2+, stimulate binding. The complex relationships between cations, uptake blockers and the DAT suggest that cations recognise at least three different sites: the first one, site 1, is for cation-induced binding inhibition; the second one, site 2, is for Na+-induced binding stimulation; and the third one, site 3, is for Zn2+-induced binding stimulation. Modelling of the interactions between Na+, K+ and radioligands allows a better understanding of the effects of cations at sites 1 and 2, and of uptake blockers at site 1. Some anions also facilitate the binding of uptake blockers to the DAT, as far as they are associated with Na+. The dependence of the binding of dopamine on ions could be involved in its preferential inward transport and used by uptake blockers for their own binding to the DAT.