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Int J Neuropsychopharmacol. 1999 Jun;2(2):95-99.

Differential effects of fluoxetine and citalopram treatments on serotonin 5-HT(2C) receptor occupancy in rat brain.

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  • 1Department of Pharmacology and Clinical Pharmacology, Institute of Biomedicine, University of Turku, Kiinamyllynkatu 10, 20520 Turku, Finland.


Ex vivo receptor occupancy measurements were performed in order to study the effects of the serotonin reuptake inhibitors fluoxetine and citalopram on serotonin 5-HT(2C) receptors. To determine the degree of 5-HT(2C) receptor occupancy, [(3)H]mesulergine binding in brain sections containing rat choroid plexus was measured at various time-points after drug injection. For comparison, [(3)H]ketanserin binding to frontal cortex 5-HT(2A) receptors was measured. Fluoxetine treatments (10 and 20 mg/kg) resulted in 5-HT(2C) receptor occupancy of up to 25 and 43%, respectively. Fluoxetine (20 mg/kg) caused a persistent effect: at the 24 h time-point, 23% of 5-HT(2C) receptors were still occupied. Citalopram treatment did not result in marked 5-HT(2C) receptor occupancy. Neither drug caused significant 5-HT(2A) receptor occupancy. In conclusion, the results demonstrate pharmacodynamic differences between fluoxetine and citalopram at the level of 5-HT(2C) receptors. These findings provide evidence that direct occupancy of 5-HT(2C) receptors may contribute to the mechanism of action of fluoxetine.

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