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

Abstract

We have reported the presence of continuous gamma (30-150 Hz) activity in the 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 electrocorticographic (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 surfaces of the MTL and the basal temporal lobe (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 sevoflurane 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 reduce neuronal excitability in the rat hippocampus in vitro, probably by changing GABAergic inhibition. The reduction of MTL gamma in the present study may be the result of such a mechanism. Although memory function was not tested in this study, the amount of MTL gamma activity may be related to residual memory function during anesthesia.

Conjugate eye movements and gamma power modulation of the EEG in persistent vegetative state

“…Possibly this ongoing gamma may represent an idling search mechanism for a meaningful, task related binding of neuronal groups for a specific perceptual task. The presence of an ongoing gamma was also shown in animals (Imas et al [23]) and humans (Uchida et al [18]) undergoing deep anaesthesia.…”

“…Uchida and colleagues [18] showed that gamma frequencies are not only demonstrated during repeated intentional activities, but also appear spontaneously during different sleep phases and different degrees of anaesthesia.…”

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

“…However, the effect of anesthetics on γ oscillations has been controversial. Unlike human investigations 1-3 , most experimental studies found that both cortical and hippocampal γ oscillations were preserved or even augmented under general anesthesia 4-9 , thus questioning a presumed unitary correlation between γ activity and consciousness. A notable exception is the study by Ma et al 10 that found some correlation between hippocampal γ oscillations and consciousness in rats.…”

Limbic System Participates in Mediating the Effects of General Anesthetics

“…In addition to assessing behavioral response to pain and righting, we recorded neocortical and hippocampal EEGs. Neocortical slow waves, or delta waves of o4 Hz (Rampil, 1998;Steriade, 2003) and hippocampal theta (4-10 Hz) (MacIver et al, 1996;Vanderwolf, 1988) and gamma (30-100 Hz) rhythms (Leung et al, 1982;Uchida et al, 2000) are known to be indicators of sleep-wake behaviors and levels of consciousness. Some of the results have been presented in an abstract form (Ma et al, 2003).…”