Evidence for Noncompetitive Modulation of Substrate-Induced Serotonin Release

Abstract

Prior work indicated that serotonin transporter (SERT) inhibitors competitively inhibit substrate-induced [H-3]5-HT release, producing rightward shifts in the substrate-dose response curve and increasing the EC50 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 [3H]5-HT release assay that measures the ability of a SERT substrate to release preloaded [H-3]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 EC50 value of all substrates. However, in many cases a SERT inhibitor decreased the En, x 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. Synapse 64:862-869, 2010. (C) 2010 Wiley-Liss, Inc