Hyperforin, a major antidepressant constituent of St. John's Wort, inhibits serotonin uptake by elevating free intracellular Na+

Extracts of Hypericum perforatum (St. John's Wort) are widely used for the treatment of depressive disorders and are unspecific inhibitors of the neur onal uptake of several neurotransmitters. Previous studies have shown that hyperforin represents the reuptake inhibiting constituent. To characterize the mechanism of serotonin reuptake inhibition, kinetic analyses in synapto somes of mouse brain were performed. Michaelis-Menten kinetics revealed tha t hyperforin (2 mu M) induces a decrease in V-max by more than 50% while on ly slightly decreasing K-m, indicating mainly noncompetitive inhibition. By contrast, citalopram (1 nM) leads to an elevation of K-m without changing V-max Monensin, a Na+/H+ exchanger, showed similar properties as hyperforin (decrease of V-max without changing K-m). Compared with classical antidepr essants, such as selective serotonin reuptake inhibitors and tricyclic anti depressants, hyperforin is only a weak inhibitor of [H-3]paroxetine binding relative to its effects on serotonin uptake. As monensin decreases seroton in uptake by increasing Na+/H+ exchange, we compared the effects of hyperfo rin and monensin on the free intracellular sodium concentration ([Na+](i)) in platelets by measuring 1,3-benzenedicarboxylic acid, 4,4'-[1,4,10-trioxa -7,13-diazacyclopentadecan-7, 13-diylbis(5-methoxy-6,2-benzofurandiyl)]bis- tetraammonium salt (SBFI/AM) fluorescence. Both drugs elevated [Na+](i) ov er basal levels, with a maximal [Na+](i) of 69 +/- 16.1 mM (50 mu M hyperfo rin) and 140 +/- 9.1 mM (10 mu M monensin). Citalopram at concentrations re levant for [H-3]serotonin uptake inhibition had no effect on [Na+](i). Alth ough the mode of action of hyperforin seems to be associated with elevated [Na+](i) similar to those levels found with monensin, the molecular mechani sm might be different, as even at high concentrations, hyperforin does not elevate free intracellular sodium concentration ([Na+](i)) up to the extrac ellular level, as monensin does. Hyperforin represents the first substance with a known preclinical antidepressant profile that inhibits serotonin upt ake by elevating [Na+](i).