title: Differential effects of isoflurane on high-frequency and low-frequency γ oscillations in the cerebral cortex and hippocampus in freely moving rats.
abstract: BACKGROUND
  Cortical γ oscillations are thought to play a role in conscious cognitive functions. Suppression of 40-Hz γ activity was implicated in the loss of consciousness during general anesthesia. However, several experimental studies found that γ oscillations were preserved in anesthesia. The authors investigated the concentration-dependent effect of isoflurane on spontaneous γ oscillations in two frequency bands and three distinct brain regions in the rat.
  
  
  METHODS
  Adult Sprague-Dawley rats were chronically implanted with epidural and coaxial depth electrodes to record cortical field potentials in frontal cortex, visual cortex, and hippocampus in waking and at steady-state isoflurane concentrations of 0.4, 0.8, and 1.2%. The γ power was calculated for the frequency bands 30-50 and 70-140 Hz. Temporal variation and interregional synchrony of γ activity were analyzed using wavelet transform. Loss of consciousness was indexed by the loss of righting reflex.
  
  
  RESULTS
  Rats lost their righting reflex at 0.8 ± 0.1% isoflurane. High-frequency γ power was decreased by isoflurane in a concentration-dependent manner (P < 0.001, 50% decrease at 0.8% isoflurane) in all brain regions. Low-frequency γ power was unaffected by isoflurane. The duration and interregional synchrony of high-frequency γ bursts was also reduced (P l < 0.001, 40% decrease at 0.8% isoflurane).
  
  
  CONCLUSIONS
  Distinction between high- and low-frequency γ bands is important when evaluating the effect of general anesthetics on brain electrical activity. Spontaneous 40-Hz γ power does not indicate the state of consciousness. The attenuation and interregional desynchronization of high-frequency γ oscillations appear to correlate with the loss of consciousness.
authors.affiliation: Department of Anesthesiology, Medical College of Wisconsin, Milwaukee, Wisconsin 53226, USA. ahudetz@mcw.edu
authors.forename: Anthony G
authors.initials: AG
authors.lastname: Hudetz
authors.forename: Jeannette A
authors.initials: JA
authors.lastname: Vizuete
authors.forename: Siveshigan
authors.initials: S
authors.lastname: Pillay
mesh_terms: D018685:Anesthetics, Inhalation / Q000494:pharmacology*; D000818:Animals; D002540:Cerebral Cortex / Q000187:drug effects*; D003243:Consciousness / Q000187:drug effects; D003627:Data Interpretation, Statistical; D004305:Dose-Response Relationship, Drug; D004569:Electroencephalography / Q000187:drug effects*; D005625:Frontal Lobe / Q000187:drug effects; D006624:Hippocampus / Q000187:drug effects*; D007530:Isoflurane / Q000494:pharmacology*; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D012018:Reflex / Q000187:drug effects; D057897:Reflex, Righting; D014793:Visual Cortex / Q000187:drug effects; D058067:Wavelet Analysis
chemical_list: D018685:Anesthetics, Inhalation; D007530:Isoflurane