In Vitro Binding Studies with Two Hypericum Perforatum Extracts - Hyperforin, Hypericin and Biapigenin -
on 5-HT₆, 5-HT₇, GABA_A/Benzodiazepine, Sigma, NPY-Y₁ /Y₂
Receptors and Dopamine Transporters

Interactions between neurotransmitter receptors involved in the pathophysiology of depression, anxiety and ethanol consumption and two extracts (hydromethanolic and lipophilic extracts obtained with hypercritical CO₂) from Hypericum Perforatum L or St. John's wort (SJW) and three constituents (hyperforin, hypericin and biapigenin) were evaluated by in vitro binding assays. The two extracts, tested at 10 µg/ml, did not inhibit ligand binding at the following receptors: serotonin 5-HT₆ and 5-HT₇, benzodiazepine, sigma and neuropeptide Y (NPY) Y₁ and Y₂ receptors. The hydromethanolic extract, but not the lipophilic extract, interacted with GABA_A receptors (IC₅₀ 5.5 µg/ml), while both interacted with the dopamine (DA) transporters, albeit with high IC₅₀ values (24.5 and 12.9 µg/ml, respectively). Biapigenin (1 µg/ml, 2 µM) inhibited ligand binding at benzodiazepine receptors only (IC₅₀: 2 µM). Hyperforin (1 µg/ml, 2 µM) only inhibited [³H]WIN-35,428 binding to DA transporters, although the IC₅₀ (5 µM) was higher than the IC₅₀ found for inhibition of the synaptosomal DA reuptake (0.8 µM). This finding extended the same observation previously described for the 5-HTergic system to the DAergic system, confirming that the inhibition of monoamine reuptake is due to a different mechanism than that of synthetic antidepressants. Hypericin showed micromolar affinities for both NPY-Y₁ and Y₂ receptors and for sigma receptors (IC₅₀ 3 - 4 µM). These hypericin activities might be of interest because NPY and sigma receptors have been associated with anxiety disorders, depressive illnesses and ethanol consumption. However, they were present at relatively high hypericin concentrations, and were also light-dependent (i. e. the IC₅₀ values increased when binding assays were carried out in the dark). Thus, our in vitro binding results may suggest that either the pharmacological effects of SJW are due to other molecules than hypericin or hyperforin (other constituents or active metabolites), or that the mechanism of action is different from those that have been considered up to now.

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