Hypoglycemic and hypoxic modulation of cortical micro-EEG activity in rat brain slices

Electroencephalogram (EEG) recordings exhibit stereotypic alterations during transient ischemia in mammals. One disadvantage of using in vitro models for ischemia studies is the lack of a sensitive electrophysiological measure for the degree of ischemic damage to a large population of neurons. The present study examined effects of hypoglycemia, hypoxia or both on an in vitro micro-EEG model, to determine whether this model provides a sensitive measure.

Theta frequency (4-8 Hz) micro-EEG oscillations were evoked in rat neocortical brain slices using the cholinergic agonist carbachol (100 microM) and the GABA(A) antagonist bicuculline (10 microM). Extracellular field micro-EEG signals and whole cell patch clamp recordings were used to monitor electrical activity.

Upon removal of oxygen and/or glucose, theta oscillation amplitudes progressively declined to isoelectric levels. Low frequency delta oscillations (0.5-3.0 Hz) and burst suppression discharges were prominent during hypoglycemic episodes and upon recovery. Time to onset of isoelectric activity was faster in slices deprived of both glucose and oxygen (7.0 +/- 1.8 min) and oxygen alone (5.0 +/- 1.5 min) compared to hypoglycemia alone (25.6 +/- 3.8 min, P < 0.01, ANOVA). Hypoxia and hypoglycemia-induced isoelectric activity occurred prior to significant population spike depression from control levels (87.7 +/- 16.9% control amplitude, P > 0.35 (t test compared with control) for hypoglycemia; 93.6 +/- 27.0%, P > 0.72 for hypoxia). Spreading depression (SD) was observed in 11/12 (91.7%) slices deprived of both sugar and oxygen, but not in hypoxic (0/4) or hypoglycemic (0/5) slices. In all cases, SD occurred later than isoelectric activity. Theta oscillations recovered within 10 min in 12/13 (92.3%) slices that did not undergo SD, but slices that underwent SD failed to recover theta activity (0/4), though all (4/4) at least partially recovered the population spike (>40%).

These results suggest that synchronized micro-EEG activity may be a useful and sensitive indicator of early-onset and possibly reversible ischemic damage.