Extracts of Hypericum perforatum (St. John's wort) have antidepressant properties in depressed patients and exert antidepressant-like action in laboratory animals. The phloroglucinol derivative hyperforin has become a topic of interest, as this Hypericum component is a potent inhibitor of monoamines reuptake. The molecular mechanism by which hyperforin inhibits monoamines uptake is yet unclear. In the present study we try to clarify the mechanism by which hyperforin inhibits the synaptic vesicle transport of monoamines. The pH gradient across the synaptic vesicle membrane, induced by vacuolar type H+-ATPase, is the major driving force for vesicular monoamines uptake and storage. We suggest that hyperforin, like the protonophore FCCP, dissipates an existing ΔpH generated by an efflux of inwardly pumped protons. Proton transport was measured by acridine orange fluorescence quenching. Adding Mg-ATP to a medium containing 130 mM KCl and synaptic vesicles caused an immediate decrease in fluorescence of acridine orange and the addition of 1 μM FCCP abolished this effect. H+-ATPase dependent proton pumping was inhibited by hyperforin in a dose dependent manner (IC50 = 1.9 × 10-7 M). Hyperforin acted similarly to the protonophore FCCP, abolishing the ATP induced fluorescence quenching (IC50 = 4.3 × 10-7 M). Hyperforin and FCCP had similar potencies for inhibiting rat brain synaptosomal uptake of [3H]monoamines as well as vesicular monoamine uptake. The efflux of [3H]5HT from synaptic vesicles was sensitive to both drugs, thus 50% of preloaded [3H]5HT was released in the presence of 2.1 × 10-7 M FCCP and 4 × 10-7 M hyperforin. The effect of hyperforin on the pH gradient in synaptic vesicle membrane may explain its inhibitory effect on monoamines uptake, but could only partially explain its antidepressant properties.
- Vesicular transport
- pH gradient