In vivo binding of [125I]RTI-55 to dopamine transporters: pharmacology and regional distribution with autoradiography.

Previous studies have demonstrated that para-substituted WIN 35,065-2 analogs of cocaine show high binding affinity for dopamine uptake sites both in vitro and in vivo, and inhibit DA uptake in vitro. These analogs also produce potent cocaine-like behavioral effects in various procedures. The purpose of the present studies was to evaluate the iodinated WIN 35,065-2 analog [125I]RTI-55 as an in vivo ligand for the DA transporter. Following intravenous injection in mice, [125I]RTI-55 showed highest accumulation in areas with high densities of dopamine uptake sites. Light microscopic autoradiography was used to examine binding with higher resolution. Displacement studies demonstrated that [125I]RTI-55 binding in dopamine containing regions, striatum and olfactory tubercles, was saturable and inhibited by other cocaine analogs. GBR 12909 and WIN 35,428 significantly inhibited [125I]RTI-55 binding in striatum, while paroxetine significantly inhibited hypothalamic binding but had little effect in striatum. The latter finding suggests that [125I]RTI-55 also binds to the serotonin transporter. Haloperidol had no effect on [125I]RTI-55 binding in any brain region measured. In addition, treatment of animals with the dopamine neurotoxin MPTP caused significant reductions in striatal [125I]RTI-55 binding. The results of these studies indicate that [125I]RTI-55 binds primarily to the dopamine transporter in the mouse striatum in vivo.