Abstract

Extracts of Hypericum perforatum (St. John’s Wort) are widely used for the treatment of depressive disorders and are unspecific inhibitors of the neuronal 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 synaptosomes of mouse brain were performed. Michaelis-Menten kinetics revealed that hyperforin (2 μM) induces a decrease inV_max by more than 50% while only slightly decreasing K_m, indicating mainly noncompetitive inhibition. By contrast, citalopram (1 nM) leads to an elevation of K_m without changingV_max. Monensin, a Na⁺/H⁺ exchanger, showed similar properties as hyperforin (decrease of V_max without changing K_m). Compared with classical antidepressants, such as selective serotonin reuptake inhibitors and tricyclic antidepressants, hyperforin is only a weak inhibitor of [³H]paroxetine binding relative to its effects on serotonin uptake. As monensin decreases serotonin uptake by increasing Na⁺/H⁺ exchange, we compared the effects of hyperforin 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 over basal levels, with a maximal [Na⁺]_i of 69 ± 16.1 mM (50 μM hyperforin) and 140 ± 9.1 mM (10 μM monensin). Citalopram at concentrations relevant for [³H]serotonin uptake inhibition had no effect on [Na⁺]_i. Although the mode of action of hyperforin seems to be associated with elevated [Na⁺]_i similar to those levels found with monensin, the molecular mechanism might be different, as even at high concentrations, hyperforin does not elevate free intracellular sodium concentration ([Na⁺]_i) up to the extracellular level, as monensin does. Hyperforin represents the first substance with a known preclinical antidepressant profile that inhibits serotonin uptake by elevating [Na⁺]_i.