Hypericum perforatum L. extract does not inhibit 5-HT transporter in rat brain cortex

The hydroalcoholic extract of Hypericum perforatum L. is an effective antid epressant. although its mechanism of action is still unknown. It inhibits t he synaptosomal uptake of serotonin (5-HT), dopamine and noradrenaline, sug gesting a biochemical mechanism similar to the synthetic standard antidepre ssants. In the present study, further investigating this hypothesis, we con firmed that a hydromethanolic extract of H. perforatum inhibited [H-3]5-HT accumulation in rat brain cortical synaptosomes with an IC50 value of 7.9 m u g/ml. The IC50 of pure hyperforin was 1.8 mu g/ml, so the activity of the total extract is not related only to its hyperforin content (<5%). This in hibitory effect, however, is not due to a direct interaction with, and bloc kade of, the 5-HT transporters since the extract, like hyperforin, did not inhibit [H-3]citalopram binding (IC50>100 mu g/ml and 10 mu g/ml, respectiv ely). We also found that 3-10 mu g/ml of the extract, or 0.3-1 mu g/ml hy p erforin, induced marked tritium release from superfused synaptosomes previo usly loaded with [H-3]S-HT. The re leasing effect of the extract resembles the releasing effect of a reserpine-like compound (Ro 04-1284), i.e. it was slightly delayed and was 5-HT carrier- and calcium-independent. These data suggest that the hydromethanolic extract of H. perforatum, similarly to Ro 04-1284, rapidly depletes storage vesicles, raising the cytoplasmic concen tration of 5-HT, and this increase is presumably responsible for the appare nt inhibition of [H-3]5-HT uptake. Therefore, our in vitro data do not conf irm that the hydromethanolic extract of H. perforatum acts as a classical 5 -HT uptake inhibitor but indicate reserpine-like properties. However, the c oncentrations of the active component(s) effective in vitro as reserpine-li ke agent(s) (i.e. corresponding to greater than or equal to 3 mu g/ml of th e hydromethanolic extract) do not seem to be achieved in the brain after ph armacologically effective doses of the extract, as indicated by the finding that there were no significant changes of rat brain 5-HT and 5-hydroxyindo leacetic acid levels after a schedule of treatment (3x300 mg/kg day, orally ) active in an animal model predictive of antidepressant-like activity. The se data also suggest that the antidepressant effect of H, perforatum extrac ts is unlikely to be associated with interaction with GABA, benzodiazepine and 5-HT1 receptors since, in receptor binding studies, we found IC50 value s higher than 5 mu g/ml. Therefore other, still unknown, mechanisms are pos sibly involved in Ii. perforatum antidepressant effects.