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Characterization of an allosteric citalopram-binding site at the serotonin transporter

Laboratory of Molecular Neurobiology, Department of Biological Psychiatry, Aarhus Psychiatric University Hospital, Risskov, Denmark.
Journal of Neurochemistry (Impact Factor: 4.24). 02/2005; 92(1):21-8. DOI: 10.1111/j.1471-4159.2004.02835.x
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ABSTRACT 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 [3H]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 [3H]S-citalopram from SERT is most potently inhibited by S-citalopram followed by R-citalopram, sertraline, serotonin and paroxetine. EC50 values for S- and R-citalopram are 3.6 +/- 0.4 microm and 19.4 +/- 2.3 microm, respectively. Fluoxetine, venlafaxine and duloxetine have no significant effect on the dissociation of [3H]S-citalopram. Allosteric modulation of dissociation is independent of temperature, or the presence of Na+ in the dissociation buffer. Dissociation of [3H]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.

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    • "*P <0.05 compared with Esc to counteract the action of escitalopram without causing pharmacokinetic interactions (Sanchez 2006; Mnie-Filali et al. 2007). Escitalopram interacts with both the orthosteric and allosteric sites of the SERT, whereas R-citalopram has much weaker binding to the orthosteric site, although its affinity for the allosteric site is comparable to that of escitalopram (Sanchez 2006; Chen et al. 2005). In our study, we found that compared with escitalopram treatment group, the co-culture with R-citalopram and escitalopram could partially increase the level of the phosphorylated tau at different epitopes. "
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    ABSTRACT: To investigate the effect of escitalopram (a widely used and highly efficacious antidepressant from the SSRI class) on tau hyperphosphorylation, HEK293/tau441 cells were pretreated with 4 μM of forskolin for 2 h. Then we treated the cells with different doses of escitalopram (0, 5, 10, 20, 40, 80 μM) for 22 h. We measured the phosphorylation level of tau by Western blotting. It was shown that escitalopram could protect tau from hyperphosphorylation induced by pharmacological activation of protein kinase A (PKA) at a dose of 20, 40, and 80 μM in vitro. Interestingly, the same dose of escitalopram could also increase the level of serine-9-phosphorylated GSK-3β (inactive form) and the phosphorylation level of Akt at Ser473 (active form) with no significant change in the level of total GSK-3β and Akt. Unexpectedly, 5-hydroxytryptamine 1A receptor (5-HT1A) agonist 8-OH-DPAT did not decrease forskolin-induced tau hyperphosphorylation. Our results suggest that escitalopram can ameliorate forskolin-induced tau hyperphosphorylation, which is not through the typical 5-HT1A pathway, and Akt/GSK-3β signaling pathway is involved. These findings may support an effective role of antidepressants in the prevention of dementia associated with depression in patients.
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    • "The long-term efficacy of escitalopram, in terms of improved symptom severity, has been demonstrated in a number of six-or 12-month open-label studies (Anders et al., 2008; Kasper et al., 2006a; Wade et al., 2006), in which the majority of patients (72% to 86%) achieved remission (MADRS 12). It would, therefore, appear that the mechanistic advantages of escitalopram over citalopram, in terms of increased potency (Hyttel et al., 1992) and its known effect on the SERT through an affinity-modulating allosteric site (Chen et al., 2005), translate into clinical benefits in patients with MDD, as demonstrated in the pooled analysis by Llorca et al. (2005), which showed significantly greater mean change from baseline in MADRS with escitalopram versus citalopram (56% vs 41%, respectively; p ¼ 0.007). "
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    Journal of Psychopharmacology 02/2010; 24(8):1143-52. DOI:10.1177/0269881109349835 · 2.81 Impact Factor
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    • "The mechanism through which R-citalopram exerts its inhibition on escitalopram is not yet established. However, it is proposed that there exist at least two binding sites on SERT for inhibitors and 5-HT, a primary , high-affinity binding site and a low-affinity allosteric site (Chen et al., 2005). The high-affinity site mediates the action of uptake inhibitors, whereas the low-affinity site modulates the binding of uptake inhibitor and 5-HT. "
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