Relationships between the catechol substrate binding site and amphetamine, cocaine, and mazindol binding sites in a kinetic model of the striatal transporter of dopamine in vitro

Abstract

Experiments were conducted to determine how (-)-cocaine and S(+)- amphetamine binding sites relate to each other and to the catechol substrate site on the striatal dopamine transporter (sDAT). In controls, m-tyramine and S(+)-amphetamine caused release of dopamine from intracellular stores at concentrations ≤ 12-fold those observed to inhibit inwardly directed sDAT activity for dopamine. In preparations from animals pretreated with reserpine, m-tyramine and S (+)-amphetamine caused release of preloaded dopamine at concentrations similar to those that inhibit inwardly directed sDAT activity. S (+)-Amphetamine and m-tyramine inhibited sDAT activity for dopamine by competing for a common binding site with dopamine and each other, suggesting that phenethylamines are substrate analogues at the plasmalemmal sDAT. (-)-Cocaine inhibited sDAT at a site separate from that for substrate analogues. This site is mutually interactive with the substrate site (K(int) = 583 nM). Mazindol competitively inhibited sDAT at the substrate analogue binding site. The results with (-)-cocaine suggest that the (-)-cocaine binding site on sDAT is distinct from that of hydroxyphenethylamine substrates, reinforcing the notion that an antagonist for (-)-cocaine binding may be developed to block (-)-cocaine binding with minimal effects on dopamine transporter activity. However, a strategy of how to antagonize drugs of abuse acting as substrate analogues is still elusive.