title: 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.
abstract: 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 3H-L-glutamate and 3H-GABA into synaptosomal preparations of mouse brain. Michaelis-Menten kinetics revealed a reduction of Vmax (8.27 to 1.80 pmol/mg/min for 3H-L-glutamate, 2.76 to 0.77 pmol/mg/min for 3H-GABA) while Km 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 by the Na+-K+-ATPase inhibitor ouabain. However, both mechanisms can be discarded for hyperforin. Several amiloride derivatives known to affect sodium conductance significantly enhance 3H-GABA and 3H-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 action of hyperforin.
authors.affiliation: Department of Pharmacology, Biocenter Niederursel, University of Frankfurt, Frankfurt, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Wonnemann
authors.forename: A
authors.initials: A
authors.lastname: Singer
authors.forename: W E
authors.initials: WE
authors.lastname: Müller
mesh_terms: D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D001643:Bridged Bicyclo Compounds; D004305:Dose-Response Relationship, Drug; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D005260:Female; D018756:GABA Antagonists / Q000494:pharmacology; D018698:Glutamic Acid / Q000493:pharmacokinetics*; D020902:Hypericum; D007476:Ionophores / Q000494:pharmacology; D051379:Mice; D008815:Mice, Inbred Strains; D008985:Monensin / Q000494:pharmacology; D014179:Neurotransmitter Uptake Inhibitors / Q000494:pharmacology; D010042:Ouabain / Q000494:pharmacology; D010696:Phloroglucinol / Q000031:analogs & derivatives; D010946:Plants, Medicinal; D015222:Sodium Channels / Q000187:drug effects / Q000378:metabolism*; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism*; D013729:Terpenes / Q000494:pharmacology*; D005680:gamma-Aminobutyric Acid / Q000493:pharmacokinetics*
chemical_list: D001643:Bridged Bicyclo Compounds; D018691:Excitatory Amino Acid Antagonists; D018756:GABA Antagonists; D007476:Ionophores; D014179:Neurotransmitter Uptake Inhibitors; D015222:Sodium Channels; D013729:Terpenes; D018698:Glutamic Acid; D005680:gamma-Aminobutyric Acid; D010042:Ouabain; D008985:Monensin; D010696:Phloroglucinol; C001654:hyperforin