The binding of the selective dopamine uptake inhibitor 3H-GBR 12935 has been well characterized in the striatum but not in the frontal cortex. We have recently developed the assay conditions for quantitating specific 3H-GBR 12935 binding to the dopamine reuptake comples in the frontal cortex. Both in rats and humans, the specific binding is proportional to tissue, concentration of range from 5-10 mg. Optimal binding conditions correspond to 8 mg of issue with radioligand concentration ranging from 1-7 nM. The specificity of the binding was defined in displacement studies in which GBR 12903 benztropine and nomifensine were the most potent inhibitors with Ki 0.1-10 M. 3H-GBR 12935 was demonstrable on dopaminergic terminals in rats by administrating a 6-hydroxydopamine (6-OHDA) injection bilaterally into the medial prefrontal cortex 30 minutes after desmethylimipramine (10 mg/kg ip). One week later there was a 50% decrease in GBR 12935 binding sites, indicating that the sites had been on the presynaptic terminals. The kinetic parameters were the same in the human and the rat frontal cortex and are consistent with a single high affinity saturable site, having a Kd of lOnM and Bmax of 200 fmoles/mg tissue.
