Mutation of Trp84 and Asp313 of the dopamine transporter reveals similar mode of binding interaction for GBR12909 and benztropine as opposed to cocaine

The different psychomotor-stimulant effects of cocaine, GBR12909, and benztropine may partially stem from their different molecular actions on the dopamine transporter (DAT). To explore this possibility, we examined binding of these inhibitors to mutated DATs with altered Na+ dependence of DAT activities and with enhanced binding of a cocaine analog, [H-3]2beta-carbomethoxy-3beta-(4-fluorophenyl)tropane (CFT). In [H-3]CFT competition assays with intact cells, the mutation-induced change in the ability of Na+ to enhance the apparent affinity of GIFT, cocaine, GBR12909, and benztropine was inhibitor-independent. Thus, for the four inhibitors, the curve of [Na+] versus apparent ligand affinity was steeper at W84L compared with wild type, shallower at D313N, and flat at W84LD313N. At each mutant, the apparent affinity of CFT and cocaine was enhanced regardless of whether Na+ was present. However, the apparent affinity of GBR12909 and benztropine for W84L was reduced in the absence of Na+ but near normal in the presence of 130 mm Na+, and that for D313N and W84LD313N was barely changed. At the single mutants, the alterations in Na+ dependence and apparent affinity of the four inhibitors were comparable between [H-3]CFT competition assays and [H-3]dopamine uptake inhibition assays. These results demonstrate that DAT inhibitors producing different behavioral profiles can respond in an opposite way when residues of the DAT protein are mutated. For GBR12909 and benztropine, their cocaine-like changes in Na+ dependence suggest that they prefer a DAT state similar to that for cocaine. However, their cocaine-unlike changes in apparent affinity argue that they, likely via their diphenylmethoxy moiety, share DAT binding epitopes that are different from those for cocaine.