title: Inhibition of excitatory synaptic transmission in hippocampal neurons by levetiracetam involves Zn²⁺-dependent GABA type A receptor-mediated presynaptic modulation.
abstract: Levetiracetam (LEV) is an antiepileptic drug with a unique but as yet not fully resolved mechanism of action. Therefore, by use of a simplified rat-isolated nerve-bouton preparation, we have investigated how LEV modulates glutamatergic transmission from mossy fiber terminals to hippocampal CA3 neurons. Action potential-evoked excitatory postsynaptic currents (eEPSCs) were recorded using a conventional whole-cell patch-clamp recording configuration in voltage-clamp mode. The antiepileptic drug phenytoin decreased glutamatergic eEPSCs in a concentration-dependent fashion by inhibiting voltage-dependent Na⁺ and Ca²⁺ channel currents. In contrast, LEV had no effect on eEPSCs or voltage-dependent Na⁺ or Ca²⁺ channel currents. Activation of presynaptic GABA type A (GABA(A)) receptors by muscimol induced presynaptic inhibition of eEPSCs, resulting from depolarization block. Low concentrations of Zn²⁺, which had no effect on eEPSCs, voltage-dependent Na⁺ or Ca²⁺ channel currents, or glutamate receptor-mediated whole cell currents, reduced the muscimol-induced presynaptic inhibition. LEV applied in the continuous presence of 1 µM muscimol and 1 µM Zn²⁺ reversed this Zn²⁺ modulation on eEPSCs. The antagonizing effect of LEV on Zn²⁺-induced presynaptic GABA(A) receptor inhibition was also observed with the Zn²⁺ chelators Ca-EDTA and RhodZin-3. Our results clearly show that LEV removes the Zn²⁺-induced suppression of GABA(A)-mediated presynaptic inhibition, resulting in a presynaptic decrease in glutamate-mediated excitatory transmission. Our results provide a novel mechanism by which LEV may inhibit neuronal activity.
authors.affiliation: Research Division for Clinical Pharmacology, Medical Corporation, Jyuryokai, Kumamoto Kinoh Hospital, Kumamoto, Japan (M.W., N.A.); Research Division for Life Science, Kumamoto Health Science University, Kumamoto, Japan (M.W., N.A.); Research Division of Neurophysiology, Kitamoto Hospital, Koshigaya, Japan (N.K., N.A); and Kogure Medical Clinic, Chouseikai Medical Corporation, Fukaya City, Saitama, Japan (K.K.).
authors.forename: Masahito
authors.initials: M
authors.lastname: Wakita
authors.forename: Naoki
authors.initials: N
authors.lastname: Kotani
authors.forename: Kyuya
authors.initials: K
authors.lastname: Kogure
authors.forename: Norio
authors.initials: N
authors.lastname: Akaike
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D000927:Anticonvulsants / Q000494:pharmacology*; D056654:CA3 Region, Hippocampal / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D002478:Cells, Cultured; D002614:Chelating Agents / Q000494:pharmacology; D005071:Evoked Potentials / Q000187:drug effects; D005260:Female; D058785:GABA-A Receptor Agonists / Q000494:pharmacology; D058787:GABA-A Receptor Antagonists / Q000494:pharmacology; D059330:GABAergic Neurons / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D000077287:Levetiracetam; D008297:Male; D019599:Mossy Fibers, Hippocampal / Q000187:drug effects / Q000378:metabolism; D009419:Nerve Tissue Proteins / Q000819:agonists / Q000037:antagonists & inhibitors / Q000378:metabolism; D009433:Neural Inhibition / Q000187:drug effects*; D010889:Piracetam / Q000031:analogs & derivatives* / Q000494:pharmacology; D017729:Presynaptic Terminals / Q000187:drug effects / Q000378:metabolism; D051381:Rats; D011963:Receptors, GABA-A / Q000737:chemistry / Q000378:metabolism*; D015032:Zinc / Q000737:chemistry / Q000378:metabolism*
chemical_list: D000927:Anticonvulsants; D002614:Chelating Agents; D058785:GABA-A Receptor Agonists; D058787:GABA-A Receptor Antagonists; D009419:Nerve Tissue Proteins; D011963:Receptors, GABA-A; D000077287:Levetiracetam; D015032:Zinc; D010889:Piracetam