[3H]WIN 35,065–2: A Ligand for Cocaine Receptors in Striatum

3H]WIN 35,065-2 binding to striatal membranes was characterized, primarily by centrifugation assay. Like 3H]cocaine, 3H]WIN 35,065-2 binds to both high- and low-affinity sites. 3H]WIN 35,065-2, however, exhibits consistently higher affinities than 3H]cocaine. Saturation experiments indicate a low-affinity binding site with an apparent KD of approximately 160 nM and a Bmax of 135 fmol/mg of tissue. A high-affinity site has also been identified with an apparent KD of 5.6 nM and a Bmax of 5.2 fmol/mg of tissue. The specific-to-nonspecific binding ratios with 3H]WIN 35,065-2 were higher than with 3H]cocaine in both centrifugation and filtration assays. Pharmacological characterization suggests that 3H]WIN 35,065-2 binds to the dopamine transporter. Mazindol, GBR 12909, nomifensine, and (-)-cocaine are potent inhibitors of 3H]WIN 35,065-2 binding. In contrast, the norepinephrine transporter ligand desipramine is a weak inhibitor, and the serotonin transporter ligand citalopram does not inhibit binding. The effect of sodium on binding was examined under conditions in which (a) the low-affinity site was primarily (87%) occupied and (b) approximately 50% of both sites were occupied. The results indicate that both sites are sodium dependent. Injection of 6-hydroxydopamine into the striatum results in a significant loss of both high- and low-affinity sites, a finding suggesting that both sites are on dopaminergic nerve terminals. Taken together, these data are consistent with the presence of multiple cocaine binding sites associated with the dopamine transporter.