Ischemia-induced changes in the electrical activity of the hippocampus

Consequences of transient (15-20 min) ischemia on the neuronal activity of the dentate gyrus and hippocampal CA 1 region were investigated in chronically implanted Sprague-Dawley rats. Forebrain ischemia was produced by occlusion of the carotids for 15 or 20 min, following cauterization of the vertebral arteries. Following the release of the carotids, both spontaneous and evoked activity showed a steady but partial recovery, reaching a maximum 12 to 24 h after the ischemic insult. From this plateau, both the power of rhythmic slow activity recorded during walking and the power of slow delta activity obtained during alert immobility decreased monotonically, with large changes occurring between postischemic days 2 and 4. The changes in spontaneous activity were accompanied by a decrease and eventual disappearance of the Schaffer collateral evoked responses in CA 1. Perforant path volleys were less efficient in activating the granule cells following ischemia compared to baseline levels. This decreased responsiveness was paralleled by a relative impairment of paired pulse depression. Neurophysiological signs of spontaneous or evoked neuronal hyperexcitability were not observed at any time point during the 8 postischemic days. Neuronal damage in the CA 1 region varied from moderate to complete loss of pyramidal cells. In addition, degenerating neurons were also observed in the hilus of the dentate gyrus. These findings do not support the "overwork" version of the excitoxic hypothesis of delayed neuronal damage and indicate that the cause of ischemic cell death should be sought in factors other than neuronal hyperactivity.