Suppression of gamma activity in the human medial temporal lobe by sevoflurane anesthesia

We have reported the presence of continuous gamma (30-150 Hz) activity in t he human medial temporal lobe (MTL). Since the MTL is involved in learning and memory, we speculated that MTL gamma activity is related to such higher brain functions. It is thus of interest to learn how this activity changes during different states of consciousness. In this study, we recorded elect rocorticographic (ECoG) activity directly from the surface of the MTL after various doses of sevoflurane anesthesia. Five epileptic patients underwent electrode placement operations in which electrodes were attached to the su rfaces of the MTL and the basal temporal robe (BTL). Immediately following the operation ECoG was recorded from each patient under four concentrations of sevoflurane anesthesia (1.5, 2.0, 2.5 and 3.0%). Fast Fourier Transform (FFT) analysis was performed on the MTL ECoGs. Under the lowest sevofluran e concentration, MTL gamma activity was observed in all patients. However, gamma activity was progressively suppressed by increased concentrations of sevoflurane,in a dose-dependent manner. Sevoflurane has been known to reduc e neuronal excitability in the rat hippocampus in vitro, probably by changi ng GABAergic inhibition. The reduction of MTL gamma in the present study ma y be the result of such a mechanism. Although memory function was not teste d in this study, the amount of MTL gamma activity may be related to residua l memory function during anesthesia. NeuroReport 11:39-42 (C) 2000 Lippinco tt Williams & Wilkins.