Allosteric effects of R- and S-citalopram on the human 5-HT transporter: evidence for distinct high- and low-affinity binding sites

Eur J Pharmacol. 2007 Jul 12;567(1-2):1-9. doi: 10.1016/j.ejphar.2007.03.055. Epub 2007 Apr 14.


The human 5-HT transporter (hSERT) has two binding sites for 5-HT and 5-HT uptake inhibitors: the orthosteric high-affinity site and a low-affinity allosteric site. Activation of the allosteric site increases the dissociation half-life for some uptake inhibitors. The objectives of this study were 1) to identify hSERT mutations that inactivate the high-affinity site without affecting the allosteric site and 2) to observe allosteric effects in which hSERT binds R-citalopram with higher affinity than S-citalopram. Wild-type and mutant (Y95F, I172M, and Y95F/I172M) hSERTs were expressed in COS-7 cells, and their 5-HT uptake and uptake inhibitor-binding abilities were studied. The hSERT mutations did not alter affinities for 5-HT or paroxetine, but high-affinity binding of S-citalopram was severely affected, particularly by the I172M, and Y95F/I172M mutations - K(i) respectively 4 nM (wild-type), 35 nM, 1000 nM, and 17.100 nM (mutants). The allosteric site however, in wild-type hSERT and the three mutants was unaffected by the mutations as attenuation of the dissociation rate of the [(3)H]-paroxetine:hSERT complex in the presence of S-citalopram or paroxetine was the same for wild-type hSERT and the three mutants. Further, R-citalopram previously thought of as an inactive enantiomer strongly attenuated dissociation of the wild-type [(3)H]-imipramine:hSERT complex, whereas S-citalopram had almost no effect on this complex. These results suggest that 1: The allosteric site on hSERT is distinct from the site to which S-citalopram binds with high affinity. 2: The allosteric effects of R-citalopram on the dissociation of [(3)H]-imipramine from hSERT indicate that R-citalopram introduces a conformational change in hSERT.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Allosteric Regulation
  • Animals
  • Antidepressive Agents / chemistry
  • Antidepressive Agents / pharmacology*
  • Binding Sites
  • COS Cells
  • Chlorocebus aethiops
  • Citalopram / chemistry
  • Citalopram / pharmacology*
  • Humans
  • Mutagenesis, Site-Directed
  • Paroxetine / pharmacology
  • Radioligand Assay
  • Serotonin Plasma Membrane Transport Proteins / genetics
  • Serotonin Plasma Membrane Transport Proteins / metabolism*
  • Stereoisomerism


  • Antidepressive Agents
  • SLC6A4 protein, human
  • Serotonin Plasma Membrane Transport Proteins
  • Citalopram
  • Paroxetine