Differential Effects of Isoflurane on High-frequency and Low-frequency γ Oscillations in the Cerebral Cortex and Hippocampus in Freely Moving Rats

Background: Cortical gamma oscillations are thought to play a role in conscious cognitive functions. Suppression of 40-Hz gamma activity was implicated in the loss of consciousness during general anesthesia. However, several experimental studies found that gamma oscillations were preserved in anesthesia. The authors investigated the concentration-dependent effect of isoflurane on spontaneous gamma 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 gamma power was calculated for the frequency bands 30-50 and 70-140 Hz. Temporal variation and interregional synchrony of gamma 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 gamma 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 gamma power was unaffected by isoflurane. The duration and interregional synchrony of high-frequency gamma bursts was also reduced (P1 < 0.001, 40% decrease at 0.8% isoflurane). Conclusions: Distinction between high-and low-frequency gamma bands is important when evaluating the effect of general anesthetics on brain electrical activity. Spontaneous 40-Hz gamma power does not indicate the state of consciousness. The attenuation and interregional desynchronization of high-frequency gamma oscillations appear to correlate with the loss of consciousness.