title: Enflurane directly depresses glutamate AMPA and NMDA currents in mouse spinal cord motor neurons independent of actions on GABAA or glycine receptors.
abstract: BACKGROUND
  The spinal cord is an important anatomic site at which volatile agents act to prevent movement in response to a noxious stimulus. This study was designed to test the hypothesis that enflurane acts directly on motor neurons to inhibit excitatory synaptic transmission at glutamate receptors.
  
  
  METHODS
  Whole-cell recordings were made in visually identified motor neurons in spinal cord slices from 1- to 4-day-old mice. Excitatory postsynaptic currents (EPSCs) or potentials (EPSPs) were evoked by electrical stimulation of the dorsal root entry area or dorsal horn. The EPSCs were isolated pharmacologically into glutamate N-methyl-d-aspartate (NMDA) receptor- and non-NMDA receptor-mediated components by using selective antagonists. Currents also were evoked by brief pulse pressure ejection of glutamate under various conditions of pharmacologic blockade. Enflurane was made up as a saturated stock solution and diluted in the superfusate; concentrations were measured using gas chromatography.
  
  
  RESULTS
  Excitatory postsynaptic currents and EPSPs recorded from motor neurons by stimulation in the dorsal horn were mediated by glutamate receptors of both non-NMDA and NMDA subtypes. Enflurane at a general anesthetic concentration (one minimum alveolar anesthetic concentration) reversibly depressed EPSCs and EPSPs. Enflurane also depressed glutamate-evoked currents in the presence of tetrodotoxin (300 nm), showing that its actions are postsynaptic. Block of inhibitory gamma-aminobutyric acid A and glycine receptors by bicuculline (20 micrometer) or strychnine (2 micrometer) or both did not significantly reduce the effects of enflurane on glutamate-evoked currents. Enflurane also depressed glutamate-evoked currents if the inhibitory receptors were blocked and if either D,L-2-amino-5-phosphonopentanoic acid (50 micrometer) or 6-cyano-7-nitroquinoxaline-2,3-dione disodium (10 micrometer) was applied to block NMDA or alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid-kainate receptors respectively.
  
  
  CONCLUSIONS
  Enflurane exerts direct depressant effects on both alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and NMDA glutamate currents in motor neurons. Enhancement of gamma-aminobutyric acid A and glycine inhibition is not needed for this effect. Direct depression of glutamatergic excitatory transmission by a postsynaptic action on motor neurons thus may contribute to general anesthesia as defined by immobility in response to a noxious stimulus.
authors.affiliation: Department of Anesthesia, Stanford University School of Medicine, Stanford, California, USA.
authors.forename: G
authors.initials: G
authors.lastname: Cheng
authors.forename: J J
authors.initials: JJ
authors.lastname: Kendig
mesh_terms: D015763:2-Amino-5-phosphonovalerate / Q000494:pharmacology; D018750:6-Cyano-7-nitroquinoxaline-2,3-dione / Q000494:pharmacology; D018685:Anesthetics, Inhalation / Q000494:pharmacology*; D000818:Animals; D018118:Chloride Channels / Q000037:antagonists & inhibitors / Q000502:physiology; D003864:Depression, Chemical; D004347:Drug Interactions; D004737:Enflurane / Q000494:pharmacology*; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D058787:GABA-A Receptor Antagonists; D018698:Glutamic Acid / Q000494:pharmacology / Q000502:physiology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D051379:Mice; D008810:Mice, Inbred C57BL; D009046:Motor Neurons / Q000187:drug effects* / Q000502:physiology; D018091:Receptors, AMPA / Q000037:antagonists & inhibitors / Q000502:physiology; D017459:Receptors, Amino Acid / Q000037:antagonists & inhibitors* / Q000502:physiology; D011963:Receptors, GABA-A / Q000502:physiology; D018009:Receptors, Glycine / Q000037:antagonists & inhibitors / Q000502:physiology; D016194:Receptors, N-Methyl-D-Aspartate / Q000037:antagonists & inhibitors / Q000502:physiology; D013116:Spinal Cord / Q000187:drug effects* / Q000502:physiology; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology; D013779:Tetrodotoxin / Q000494:pharmacology
chemical_list: D018685:Anesthetics, Inhalation; D018118:Chloride Channels; D018691:Excitatory Amino Acid Antagonists; D058787:GABA-A Receptor Antagonists; D018091:Receptors, AMPA; D017459:Receptors, Amino Acid; D011963:Receptors, GABA-A; D018009:Receptors, Glycine; D016194:Receptors, N-Methyl-D-Aspartate; D018698:Glutamic Acid; D013779:Tetrodotoxin; D018750:6-Cyano-7-nitroquinoxaline-2,3-dione; D015763:2-Amino-5-phosphonovalerate; D004737:Enflurane