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 μg/kg i.v.) blocked the suppressant effect on neuronal firing activity of both escitalopram (100 μg/kg i.v.) and paroxetine (500 μg/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 μM) and paroxetine (0.75 μM), but not fluoxetine (10 μM). 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.
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