Correlation between electroencephalogram isoelectric time and hippocampal norepinephrine levels, measured by microdialysis, during ischemia in rats

It is suggested that norepinephrine (NE) plays a role during transient forebrain ischemia. NE may have a protective action against neuronal cell death in the hippocampus, or it may be one of the causes of injurious ischemic effects. We used the microdialysis technique to study extracellular NE levels in the rat hippocampus before, during, and after 30 min of transient incomplete forebrain ischemia (induced by four-vessel occlusion) to describe the time course of NE in this condition. There was a maximal increase (fivefold) in extracellular NE after 10 min of reflow only when the electroencephalogram was isoelectric. NE levels returned to baseline 40 min after release of the carotid clamps and remained constant for the next 80 min. Thus there appears to be a transient NE overflow in the hippocampus during ischemia, closely related to the complete loss of brain electrical activity.