title: Allosteric modulation of the effect of escitalopram, paroxetine and fluoxetine: in-vitro and in-vivo studies.
abstract: Clinical and preclinical studies have shown that the effect of citalopram on serotonin (5-HT) reuptake inhibition and its antidepressant activity resides in the S-enantiomer. In addition, using a variety of in-vivo and in-vitro paradigms, it was shown that R-citalopram counteracts the effect of escitalopram. This effect was suggested to occur via an allosteric modulation at the level of the 5-HT transporter. Using in-vitro binding assays at membranes from COS-1 cells expressing the human 5-HT transporter (hSERT) and in-vivo electrophysiological and microdialysis techniques in rats, the present study was directed at determining whether R-citalopram modifies the action of selective serotonin reuptake inhibitors (SSRIs) known to act on allosteric sites namely escitalopram, and to a lesser extent paroxetine, compared to fluoxetine, which has no affinity for these sites. In-vitro binding studies showed that R-citalopram attenuated the association rates of escitalopram and paroxetine to the 5-HT transporter, but had no effect on the association rates of fluoxetine, venlafaxine or sertraline. In the rat dorsal raphe nucleus, R-citalopram (250 microg/kg i.v.) blocked the suppressant effect on neuronal firing activity of both escitalopram (100 microg/kg i.v.) and paroxetine (500 microg/kg i.v.), but not fluoxetine (10 mg/kg i.v.). Interestingly, administration of R-citalopram (8 mg/kg i.p.) attenuated the increase of extracellular levels of 5-HT ([5-HT]ext) in the ventral hippocampus induced by both escitalopram (0.28 microM) and paroxetine (0.75 microM), but not fluoxetine (10 microM). In conclusion, the present in-vitro and in-vivo studies show that R-citalopram counteracts the activity of escitalopram and paroxetine, but not fluoxetine, by acting at the allosteric binding site of the 5-HT transporter, either located in the dorsal raphe nucleus or post-synaptically in the ventral hippocampus. This conclusion is strengthened by the observation that the inhibitory effect of fluoxetine, which has no stabilizing effect on the radioligand/hSERT complex, was not blocked by co-administration of R-citalopram.
authors.affiliation: Laboratory of Neuropharmacology and Neurochemistry, Faculty of Pharmacy, University of Claude Bernard Lyon I, INSERM EA-512, Lyon, France, and Department of Biological Psychiatry, Aarhus Psychiatric University Hospital, Risskov, Denmark.
authors.forename: Mostafa El
authors.initials: ME
authors.lastname: Mansari
authors.forename: Ove
authors.initials: O
authors.lastname: Wiborg
authors.forename: Ouissame
authors.initials: O
authors.lastname: Mnie-Filali
authors.forename: Nadia
authors.initials: N
authors.lastname: Benturquia
authors.forename: Connie
authors.initials: C
authors.lastname: Sánchez
authors.forename: Nasser
authors.initials: N
authors.lastname: Haddjeri
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000494:Allosteric Regulation; D000818:Animals; D018687:Antidepressive Agents, Second-Generation / Q000378:metabolism / Q000494:pharmacology*; D001921:Brain / Q000187:drug effects* / Q000378:metabolism; D019556:COS Cells; D002522:Chlorocebus aethiops; D015283:Citalopram / Q000378:metabolism / Q000494:pharmacology*; D004347:Drug Interactions; D005473:Fluoxetine / Q000378:metabolism / Q000494:pharmacology*; D006624:Hippocampus / Q000187:drug effects / Q000378:metabolism; D008297:Male; D017551:Microdialysis; D017374:Paroxetine / Q000378:metabolism / Q000494:pharmacology*; D011485:Protein Binding; D011903:Raphe Nuclei / Q000187:drug effects; D051381:Rats; D017207:Rats, Sprague-Dawley; D012701:Serotonin / Q000378:metabolism; D050486:Serotonin Plasma Membrane Transport Proteins / Q000187:drug effects / Q000378:metabolism; D017367:Selective Serotonin Reuptake Inhibitors / Q000378:metabolism / Q000494:pharmacology*; D013237:Stereoisomerism; D014162:Transfection
chemical_list: D018687:Antidepressive Agents, Second-Generation; D015283:Citalopram; D005473:Fluoxetine; D017374:Paroxetine; D012701:Serotonin; D050486:Serotonin Plasma Membrane Transport Proteins; D017367:Selective Serotonin Reuptake Inhibitors; C493230:SLC6A4 protein, human