Characterization of an allosteric citalopram-binding site at the serotonin transporter (original) (raw)

2005, Journal of Neurochemistry

The serotonin transporter (SERT), which belongs to a family of sodium/chloride-dependent transporters, is the major pharmacological target in the treatment of several clinical disorders, including depression and anxiety. In the present study we show that the dissociation rate, of [ 3 H]S-citalopram from human SERT, is retarded by the presence of serotonin, as well as by several antidepressants, when present in the dissociation buffer. Dissociation of [ 3 H]S-citalopram from SERT is most potently inhibited by S-citalopram followed by R-citalopram, sertraline, serotonin and paroxetine. EC 50 values for S-and R-citalopram are 3.6 ± 0.4 lM and 19.4 ± 2.3 lM, respectively. Fluoxetine, venlafaxine and duloxetine have no significant effect on the dissociation of [ 3 H]Scitalopram. Allosteric modulation of dissociation is independ-ent of temperature, or the presence of Na + in the dissociation buffer. Dissociation of [ 3 H]S-citalopram from a complex with the SERT double-mutant, N208Q/N217Q, which has been suggested to be unable to self-assemble into oligomeric complexes, is retarded to an extent similar to that found with the wild-type, raising the possibility that the allosteric mechanism is mediated within a single subunit. A species-scanning mutagenesis study comparing human and bovine SERT revealed that Met180, Tyr495 and Ser513 are important residues in mediating the allosteric effect, as well as contributing to high-affinity binding at the primary site.