Selective serotonin reuptake inhibitors (SSRIs) are the most widely prescribed antidepressant class today and exert their effects by increasing synaptic concentrations of serotonin (5-HT).The forced swim test (FST) is the most widely used animal test predictive of antidepressant action. Rationale of the present study was to investigate the acute effects of citalopram on hepatic tryptophan metabolism and disposition in rats exposed to FST. Methods: We investigated the effects of acute citalopram

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... (20 mg/kg, ip) administration on rat's behavioral responses in FST paradigm, hepatic tryptophan 2,3-dioxygenase (TDO) activity, serum corticosterone levels and brain regional 5-HT metabolism. Results: Citalopram administered to swim-stressed rats showed a decrease in FST-induced increases in plasma corticosterone concentration and 5-HT turnover in hypothalamus, amygdala and hippocampus. The drug also decreases immobility and increases swimming during the FST. Citalopram administration to unstressed rats increases plasma corticosterone concentration but decreases 5-HT turnover in all three brain areas examined. Conclusions: Our findings support the hypothesis that acute citalopram administration increases tryptophan (by inhibiting TDO activity) availability for 5-HT synthesis and activates serotonergic neurotransmission in limbic brain areas in rats exposed to FST paradigm. The mechanism of action of citalopram in ameliorating social stress related depressive disorder in humans is discussed.