TY - JOUR T1 - Antidepressants Targeting the Serotonin Reuptake Transporter Act via a Competitive Mechanism JF - Journal of Pharmacology and Experimental Therapeutics JO - J Pharmacol Exp Ther SP - 982 LP - 990 DO - 10.1124/jpet.108.142315 VL - 327 IS - 3 AU - Subbu Apparsundaram AU - Daniel J. Stockdale AU - Robert A. Henningsen AU - Marcos E. Milla AU - Renee S. Martin Y1 - 2008/12/01 UR - http://jpet.aspetjournals.org/content/327/3/982.abstract N2 - Although several antidepressants (including fluoxetine, imipramine, citalopram, venlafaxine, and duloxetine) are known to inhibit the serotonin transporter (SERT), whether or not these molecules compete with 5-hydroxytryptamine (serotonin) (5-HT) for binding to SERT has remained controversial. We have performed radioligand competition binding experiments and found that all data can be fitted via a simple competitive interaction model, using Cheng-Prusoff analysis (Biochem Pharmacol 22:3099–3108, 1973). Two different SERT-selective radioligands, [3H]N,N-dimethyl-2-(2-amino-4-cyanophenyl thio)-benzylamine (DASB) and [3H]S-citalopram, were used to probe competitive binding to recombinantly expressed human SERT or native SERT in rat cortical membranes. All the SERT inhibitors that we tested were able to inhibit [3H]DASB and [3H]S-citalopram binding in a concentration-dependent manner, with unity Hill coefficient. In accordance with the Cheng-Prusoff relationship for a competitive interaction, we observed that test compound concentrations associated with 50% maximal inhibition of radiotracer binding (IC50) increased linearly with increasing radioligand concentration for all ligands: 5-HT, S-citalopram, R-citalopram, paroxetine, clomipramine, fluvoxamine, imipramine venlafaxine, duloxetine, indatraline, cocaine, and 2-β-carboxy-3-β-(4-iodophenyl)tropane. The equilibrium dissociation constant of 5-HT and SERT inhibitors were also derived using Scatchard analysis of the data set, and they were found to be comparable with the data obtained using the Cheng-Prusoff relationship. Our studies establish a reference framework that will contribute to ongoing efforts to understand ligand binding modes at SERT by demonstrating that 5-HT and the SERT inhibitors tested bind to the serotonin transporter in a competitive manner. The American Society for Pharmacology and Experimental Therapeutics ER -