St. John's wort flavonoids and their metabolites show antidepressant activity and accumulate in brain after multiple oral doses.

ABSTRACT

Over the last few years many data have been published suggesting a participation of quercetin flavonoids in the antidepressive effect of St. John's wort (SJW) extract. To elucidate these data more deeply we performed two animal behavioural studies examining the antidepressant effects of SJW extract, rutin and, in addition, the quercetin metabolite isorhamnetin. The substances were in all cases compared to imipramine using Porsolt's forced swimming test (FST) after oral gavage of the substances over a 9 day period. All three compounds were found to be effective, with isorhamnetin exhibiting the strongest effect. In addition to this pharmacological study, we carried out two pharmacokinetic studies to examine the CNS level time-curve of the quercetin flavonoids after a single oral dose of SJW extract (1600 mg/kg) and isoquercitrin (100 mg/kg), respectively, and to observe the cumulative effects after daily repeated oral doses of SJW extract over 8 days. After a single dose the maximal CNS levels for quercetin (340 ng/g) and isorhamnetin/tamarixetin (50 ng/g) were found at 4 h. With repeated doses the maximal cumulation for quercetin (367 ng/g) occurred after 5 days whilst isorhamnetin/tamarixetin (640 ng/g) did not reach its maximal cumulation level within the 8 day test period.