title: Prolonged exposure to levetiracetam reveals a presynaptic effect on neurotransmission.
abstract: OBJECTIVE
  The antiepileptic drug levetiracetam (LEV) is an enigma. Despite the fact that it specifically binds to the presynaptic vesicle protein, SV2A, no satisfactory mechanism of action has yet been identified. Using a combination of electrophysiological and cellular imaging techniques, we carefully tested the hypothesis that LEV directly interferes with neurotransmitter release.
  
  
  METHODS
  We measured extracellular evoked responses in the CA1 region of rat hippocampal slices after paired pulse stimulation and after application of up to 10 pulses applied at 5-80 Hz. In parallel experiments, we used quantitative 2-photon microscopy and the fluorescent vesicular marker FM1-43 to measure the effect of repetitive stimulation on presynaptic vesicle release.
  
  
  RESULTS
  Acute exposure to LEV (100 microM) had no effect on paired pulse synaptic responses. However, when slices were exposed to LEV for 3 h, there was a significant alteration in paired pulse responses and a more striking reduction in late synaptic potentials delivered in an 80 Hz train. LEV significantly reduced the rate of vesicle release assessed by FM1-43 destaining during 1 Hz stimulation.
  
  
  CONCLUSIONS
  LEV is unique among currently available antiepileptics, because it directly inhibits presynaptic neurotransmitter release in a use-dependent fashion. While there are alternate explanations for this observation, it is plausible that LEV exerts its effect by binding to a protein selectively expressed in presynaptic nerve terminals.
authors.affiliation: Department of Neurology, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
authors.forename: Xiao-Feng
authors.initials: XF
authors.lastname: Yang
authors.forename: Aryan
authors.initials: A
authors.lastname: Weisenfeld
authors.forename: Steven M
authors.initials: SM
authors.lastname: Rothman
mesh_terms: D000818:Animals; D000927:Anticonvulsants / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D005071:Evoked Potentials / Q000187:drug effects / Q000502:physiology; D019706:Excitatory Postsynaptic Potentials / Q000187:drug effects / Q000502:physiology; D006624:Hippocampus / Q000187:drug effects* / Q000502:physiology; D000077287:Levetiracetam; D008297:Male; D023421:Models, Animal; D010889:Piracetam / Q000031:analogs & derivatives* / Q000494:pharmacology; D017729:Presynaptic Terminals / Q000187:drug effects / Q000502:physiology; D011726:Pyridinium Compounds; D000644:Quaternary Ammonium Compounds; D051381:Rats; D017207:Rats, Sprague-Dawley; D017661:Receptors, Presynaptic / Q000187:drug effects; D009435:Synaptic Transmission / Q000187:drug effects* / Q000502:physiology
chemical_list: D000927:Anticonvulsants; C073804:FM1 43; D011726:Pyridinium Compounds; D000644:Quaternary Ammonium Compounds; D017661:Receptors, Presynaptic; D000077287:Levetiracetam; D010889:Piracetam