Serotonergic mediation effects of St John's wort in rats subjected to swim stress.

Present study shows the effects of St John's Wort (SJW) (20 mg/kg) on swim stress induced changes in tryptophan (TRP) metabolism and disposition in rats. The results show that after forced swim test (FST) hepatic tryptophan pyrrolase (holo and total) activities were significantly decreased (P<0.001). Liver TRP was increased (P<0.001) while serum TRP was decreased (P<0.01). Brain TRP, 5-hydroxytryptamine (5-HT, Serotonin) and 5-hydroxyindole acetic acid (5-HIAA) concentrations were increased (P<0.001), similarly immobility time during swim test was also increased. SJW pretreated FS group of rats showed reduced holo enzyme activity (P<0.001) while increase in total and apo enzyme activities (P<0.001). There was significant decrease in liver TRP (P<0.01), serum TRP (P<0.05), brain TRP (P<0.001), 5-HT (P<0.001) and 5-HIAA (P<0.001) concentrations with reduction in immobility time during swim test when compared with saline injected FS group. SJW injected group but when compared with untreated controls showed significant increase in total and apo enzyme activities (P<0.001) while holo enzyme activity was decreased (P<0.001), serum TRP, brain TRP and 5-HIAA levels were significantly decreased (P<0.001). Changes in 5-HT concentrations were not significant. It is concluded that SJW treatment alter stress induced augmented 5-HT levels by decreasing precursor availability to the brain and that serotonergic system is involve in the mechanism of action of the drug.