Inhibition of synaptosomal uptake of 3H-L-glutamate and 3H-GABA by hyperforin, a major constituent of St. John's Wort:: The role of amiloride sensitive sodium conductive pathways

Extracts of St. John's Wort are widely used for the treatment of depressive disorders. The active principles have not yet been finally elucidated. We have recently shown that hyperforin, a major active constituent of St. John's Wort, not only inhibits the neuronal uptake of serotonin, norepinephrine and dopamine, but also that of L-glutamate and GABA. No other antidepressant compound exhibits a similar broad uptake inhibiting profile. To investigate this unique kind of property, kinetic analyses were performed regarding the uptake of H-3-L-glutamate and H-3-GABA into Menten kinetics revealed a reduction of V-max (8.27 to 1.80 pmol/mg/min for H-3-L-glutamate, 2.76 to 0.77 pmol/mg/min for H-3-GABA) while K-m was nearly unchanged in both cases, suggesting non-competitive inhibition. The unselective uptake inhibition by hyperforin could be mimicked by the Na+-ionophore monensin and ny the Na+-K+-ATPase inhibitor ouabain. However, both mechanisms can be discarded for hyperforin. Several amiloride derivatives known to affect sodium conductance significantly enhance H-3-GABA and H-3-L-glutamate uptake and inhibit the uptake inhibition by hyperforin, while monensin or ouabain inhibition were not influenced. Selective concentrations of benzamil for amiloride sensitive Na+-channels and selective concentrations of 5'-ethylisopropylamiloride (EIPA) for the Na+-H+-exchangers both had an attenuating effect on the hyperforin inhibition of L-glutamate uptake, suggesting a possible role of amiloride sensitive Na+-channels and Na+-H+-exchangers in the mechanism of ation of hyperforin. (C) 2000 American College of Neuropsychopharmacology. Published by Elsevier Science Inc. All rights reserved.