Binding of the cocaine analog 2 beta-carbomethoxy-3 beta-(4-[125I]iodophenyl)tropane to serotonin and dopamine transporters: different ionic requirements for substrate and 2 beta-carbomethoxy-3 beta-(4-[125I]iodophenyl)tropane binding.

The iodinated cocaine analog 2 beta-carbomethoxy-3 beta-(4- [125I]iodophenyl)tropane (beta-[125I]CIT) binds with high affinity to the platelet plasma membrane serotonin transporter, as previously reported for dopamine transporters from rat brain [Eur. J. Pharmacol. 194:133-134 (1991)]. Unlabeled beta-CIT also inhibits serotonin transport by platelet membrane vesicles. In both rat striatal membranes and platelet plasma membranes, beta-[125I]CIT binding was found to be pH dependent, with a pKa of 6.4-6.9, and did not require the presence of Cl-. Na+ dramatically stimulated beta-[125I]CIT binding to both serotonin and dopamine transporters, although a small fraction of beta-[125I]CIT binding to the serotonin transporter was observed in the absence of Na+. The substrates serotonin and dopamine competed with beta-[125I]CIT for binding to their respective transporters. However, substrate affinity was enhanced by Cl-, whereas beta-[125I]CIT binding affinity was not. [3H]Imipramine binding to the platelet serotonin transporter and [3H]GBR-12935 binding to the dopamine transporter were not inhibited by decreasing the pH from 8 to 6.5. Likewise, the ability of serotonin to compete with [3H]imipramine binding and that of dopamine to inhibit [3H]GBR-12935 binding were equal at pH 6.5 or 8. Thus, beta-[125I]CIT binding to biogenic amine transporters is distinct from serotonin or dopamine binding by virtue of its inhibition by H+ and its insensitivity to Cl-.