Postischemic alterations of spontaneous activities in rat hippocampal CA1 neurons

Changes of spontaneous impulse discharges in rat hippocampal neurons during and after transient forebrain ischemia were investigated electrophysiologically. Spontaneous impulse frequencies of CA1 neurons before ischemia were varied from 0.4 to 20.0 impulses/s and its average was 5.8 +/- 1.2 (means +/- S.E., n = 36). These spontaneous discharges were completely suppressed during forebrain ischemia except for the transient hyperactivity observed just after the beginning of ischemia. Recovery of spontaneous discharges of CA1 neurons from suppression induced by 5 min ischemia started at 5 min, and neuronal activities were restored to pre-ischemic levels approximately 30 min after reperfusion. On the other hand, spontaneous impulse frequencies at all time points recorded after 20 min ischemia were less than 40% of the pre-ischemic levels. These continuous suppression of spontaneous activity after 20 min ischemia may suggest that neuronal function is impaired during and/or in the early stages of reperfusion, and functional disorders precede morphological degeneration.