Evidence for noncompetitive modulation of substrate-induced serotonin release.
Prior work indicated that serotonin transporter (SERT) inhibitors competitively inhibit substrate-induced [(3)H]5-HT release, producing rightward shifts in the substrate-dose response curve and increasing the EC(50) value without altering the E(max). We hypothesized that this finding would not generalize across a number of SERT inhibitors and substrates, and that the functional dissociation constant (Ke) of a given SERT inhibitor would not be the same for all tested substrates. To test this hypothesis, we utilized a well-characterized [(3)H]5-HT release assay that measures the ability of a SERT substrate to release preloaded [(3)H]5-HT from rat brain synaptosomes. Dose-response curves were generated for six substrates (PAL-287 [naphthylisopropylamine], (+)-fenfluramine, (+)-norfenfluramine, mCPP [meta-chlorophenylpiperazine], (±)-MDMA, 5-HT) in the absence and presence of a fixed concentration of three SERT inhibitors (indatraline, BW723C86, EG-1-149 [4-(2-(benzhydryloxy)ethyl)-1-(4-bromobenzyl)piperidine oxalate]). Consistent with simple competitive inhibition, all SERT inhibitors increased the EC(50) value of all substrates. However, in many cases a SERT inhibitor decreased the E(max) value as well, indicating that in the presence of the SERT inhibitor the substrate became a partial releaser. Moreover, the Ke values of a given SERT inhibitor differed among the six SERT substrates, indicating that each inhibitor/substrate combination had a unique interaction with the transporter. Viewed collectively, these findings suggest that it may be possible to design SERT inhibitors that differentially regulate SERT function.
Rothman
Richard B
RB
Clinical Psychopharmacology, IRP, NIDA, NIH, DHHS, Baltimore, Maryland, USA. RROTHMAN@intra.nida.nih.gov
Baumann
Michael H
MH
Blough
Bruce E
BE
Jacobson
Arthur E
AE
Rice
Kenner C
KC
Partilla
John S
JS
D000818:Animals; D001921:Brain / Q000187:drug effects / Q000648:ultrastructure; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D066298:In Vitro Techniques; D007211:Indoles / Q000494:pharmacology; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D012701:Serotonin / Q000378:metabolism*; D050486:Serotonin Plasma Membrane Transport Proteins / Q000378:metabolism*; D017366:Serotonin Receptor Agonists / Q000494:pharmacology; D017367:Selective Serotonin Reuptake Inhibitors / Q000494:pharmacology; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism; D013876:Thiophenes / Q000494:pharmacology; D013997:Time Factors; D014316:Tritium / Q000378:metabolism
D007211:Indoles; D012701:Serotonin; D050486:Serotonin Plasma Membrane Transport Proteins; D017366:Serotonin Receptor Agonists; D017367:Selective Serotonin Reuptake Inhibitors; D013876:Thiophenes; D014316:Tritium; C092430:1-(5-(2-thenyloxy)-1H-indol-3-yl)propan-2-amine