title: Intrathecal picrotoxin minimally alters electro-encephalographic responses to noxious stimulation during halothane and isoflurane anesthesia.
abstract: BACKGROUND
  Isoflurane and halothane act in the spinal cord to blunt ascending transmission of impulses to the brain resulting from noxious stimulation. Because intrathecal picrotoxin (an antagonist at the gamma-aminobutyric acid-A receptor) partially reverses the immobilizing effect of isoflurane and halothane, we hypothesized that the electroencephalographic response to noxious stimulation would likewise be partially reversed by intrathecal picrotoxin.
  
  
  METHODS
  Rats were anesthetized with isoflurane (n = 8) or halothane (n = 8) and a laminectomy performed. Following determination of minimum alveolar concentration (MAC), the electroencephalogram (EEG) was recorded during separate applications of a hindpaw clamp, tail clamp and electrical current to the tail at 0.8 and 1.2 MAC. Picrotoxin was then applied to the exposed spinal cord and the EEG response to noxious stimulation again determined.
  
  
  RESULTS
  The EEG was more active during halothane anesthesia than isoflurane (spectral edge frequency for 95% power: 25.6 +/- 2.1 Hz vs. 23.1 +/- 1.6 Hz, P < 0.05). Noxious stimulation usually caused the EEG to shift to higher frequencies (e.g. for 0.8 MAC halothane, median edge frequency for 50% power: from 7.6 +/- 3.1 Hz to 10.7 +/- 2.6 Hz, P < 0.05). Picrotoxin minimally affected this response.
  
  
  CONCLUSIONS
  Noxious stimulation evokes an EEG response that is minimally altered by intrathecal picrotoxin. This suggests that isoflurane and halothane do not have GABAergic actions in the spinal cord that indirectly suppress the EEG response.
authors.affiliation: Department of Anesthesiology and Pain Medicine, University of California, Davis, CA 95616, USA.
authors.forename: C L
authors.initials: CL
authors.lastname: Dominguez
authors.forename: L S
authors.initials: LS
authors.lastname: Barter
authors.forename: J F
authors.initials: JF
authors.lastname: Antognini
mesh_terms: D018685:Anesthetics, Inhalation / Q000494:pharmacology*; D000818:Animals; D004558:Electric Stimulation; D004569:Electroencephalography / Q000187:drug effects*; D018756:GABA Antagonists / Q000008:administration & dosage / Q000494:pharmacology*; D006221:Halothane / Q000494:pharmacology*; D006439:Hemodynamics / Q000187:drug effects; D007278:Injections, Spinal; D007530:Isoflurane / Q000494:pharmacology*; D008297:Male; D009474:Neurons / Q000187:drug effects; D010812:Physical Stimulation; D010852:Picrotoxin / Q000008:administration & dosage / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013116:Spinal Cord / Q000187:drug effects
chemical_list: D018685:Anesthetics, Inhalation; D018756:GABA Antagonists; D006221:Halothane; D007530:Isoflurane; D010852:Picrotoxin