Hydroalcoholic hypericum extract inhibits the synaptosomal uptake of serotonin, norepinephrine, and dopamine with about similar affinities and leads to a significant down-regulation of cortical beta-adrenoceptors and 5-HT2-receptors after subchronic treatment of rats. While neither hypericine nor kaempferol did show any reuptake inhibiting properties, hyperforin was identified as the unspecific reuptake inhibitor of hypericum extracts with half-maximal inhibitory concentrations for the three synaptosomal uptake systems mentioned above between 80 and 200 nmol/l. Moreover, a hyperforin-enriched (38%) CO2 extract also leads to a significant beta-receptor down-regulation after subchronic treatment. The data suggest hyperforin as the active principle of hypericum extracts in biochemical models of antidepressant activity.

Since the mechanism of the antidepressant activity of hypericum extract is not yet understood, we tested possible effects of standardized hypericum extract (LI 160) in several biochemical models relevant for the mechanism of action of other antidepressant drugs. While LI 160 was only a weak inhibitor of MAO-A and MAO-B activity, it inhibited the synaptosomal uptake of serotonin, dopamine and norepinephrine with about equal affinity-2 micrograms/ml). Moreover, subchronic treatment of rats with hypericum extract led to a significant down-regulation of beta-receptors and to a significant up-regulation of 5-HT2-receptors in the frontal cortex. The data might suggest that hypericum extract acts via similar biochemical mechanisms to other antidepressants (e.g. tricyclics).

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