Antidepressants targeting the serotonin reuptake transporter act via a competitive mechanism

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, [ 3 H] N , N -dimethyl-2-(2-amino-4-cyanophenyl thio)-benzylamine (DASB) and [ 3 H] 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 [ 3 H]DASB and [ 3 H] 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 (IC 50 ) 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.