Characterization of [125I]RTI-121 binding to dopamine transporter in vitro.

Aim: To characterize the binding of [125I]3 beta-(4-iodophenyl) tropane-2 beta-carboxylic acid isopropyl ester (RTI-121) to the dopamine transporter (DAT) under physiologically relevant conditions.

Methods: [125I]RTI-121 was used to label the DAT on fresh rat striatum synaptosomal membranes in artificial cerebrospinal fluid (ACSF) at 37 degrees C.

Results: [125I]RTI-121 binding reached equilibrium within 3 min and remained at its plateau value for at least 9 min. The data from kinetic, saturation, and competition studies supported a one-site model for the binding of [125I]RTI-121 to the DAT. Various DAT blockers (oocaine, GBR12935, and BTCP) and substrates (dopamine and d-amphetamine) competitively inhibited the binding of [125I]RTI-121. Compared with NaPhos-KCl-NaCl assay buffer, ACSF containing Ca2+ and Mg2+ markedly increased the IC50 of DAT blockers for inhibiting [125I]RTI-121 binding with less effect on that of substrates. Various D2 receptor ligands (pergolide, quinirole, sulpiride, and l-stepholidine) had no direct effect on the binding of [125I]RTI-121.

Conclusion: [125I]RTI-121 binding under physiologically relevant conditions fulfills the basic criteria for DAT binding assay.