Evidence for noncompetitive modulation of substrate‐induced serotonin release

Prior work indicated that serotonin transporter (SERT) inhibitors competitively inhibit substrate‐induced [³H]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 [³H]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 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. Synapse 64:862–869, 2010. © 2010 Wiley‐Liss, Inc.