[ 3 H]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 ∼ 160 nM and a Bmaxof 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) ∼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.