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-.
Wall
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Department of Pharmacology, Yale University School of Medicine, New Haven, Connecticut 06510.
Innis
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Rudnick
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D000818:Animals; D001667:Binding, Competitive; D001792:Blood Platelets / Q000378:metabolism; D002352:Carrier Proteins / Q000378:metabolism*; D003042:Cocaine / Q000031:analogs & derivatives* / Q000378:metabolism; D003342:Corpus Striatum / Q000378:metabolism; D004298:Dopamine / Q000378:metabolism*; D050483:Dopamine Plasma Membrane Transport Proteins; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D007477:Ions; D008562:Membrane Glycoproteins / Q000378:metabolism*; D026901:Membrane Transport Proteins; D009419:Nerve Tissue Proteins; D009994:Osmolar Concentration; D011485:Protein Binding; D011869:Radioligand Assay; D051381:Rats; D012701:Serotonin / Q000378:metabolism*; D050486:Serotonin Plasma Membrane Transport Proteins
D002352:Carrier Proteins; D050483:Dopamine Plasma Membrane Transport Proteins; D007477:Ions; D008562:Membrane Glycoproteins; D026901:Membrane Transport Proteins; D009419:Nerve Tissue Proteins; D050486:Serotonin Plasma Membrane Transport Proteins; C493232:Slc6a4 protein, rat; D012701:Serotonin; C069723:2beta-carbomethoxy-3beta-(4-iodophenyl)tropane; D003042:Cocaine; D004298:Dopamine