Electrophysiological recordings from rat hippocampus slices following in vivo brain ischemia

Pyramidal neurons in area CA1 of the septal hippocampus degenerate 2-3 days after an episode of transient global cerebral ischemia. The purpose of this study was to investigate synaptic transmission and passive neuronal properties in the post-ischemic period prior to neuronal death. Electrophysiological recordings were made from area CA1 in hippocampal slices prepared from rats which had survived a period of 20 min of ischemia for up to 5 days. In septal slices, field responses were in area CA1 unaltered up to 24 h after the ischemic insult. Forty-eight hours after ischemia, the mean amplitude of the population spike, but not the field-EPSP, was significantly reduced. In septal slices prepared more than 48 h after ischemia field potentials were absent or strongly attenuated, whereas they were intact in slices prepared from the temporal pole. No spontaneous discharges were detected in slices prepared at any time from post-ischemic rats. Intracellular recordings were obtained from slices up to 48 h after the ischemic episode. There was no significant difference in the resting membrane potential or input resistance between these neurons and those from control slices. Action potentials followed by a fast afterhyperpolarization and spike accommodation were preserved in all post-ischemic neurons. In all neurons investigated, orthodromic stimulation evoked an EPSP followed by a fast- and then a slow-IPSP. One hour after ischemia, the slow-IPSP was reduced. Forty-eight hours after ischemia, the fast-IPSP was significantly increased. The EPSP was markedly attenuated by the non N-methyl-D-aspartate receptor blocker 6-cyano-7-nitroquinoxaline-2,3-dione (10 microM). The residual depolarizing component was amplified by perfusing with Mg(2+)-free medium and blocked by the N-methyl-D-aspartate receptor antagonist DL-2-amino-5-phosphonovaleric acid. Paired-pulse facilitation of the EPSP was also preserved. As in control slices, the slow-IPSP and paired-pulse depression of the fast-IPSP were blocked by 1 microM baclofen. The present experiments provide no evidence that overt alteration of excitatory synaptic transmission or neuronal properties favouring hyperexcitability precede the ischemically induced death of CA1 pyramidal cells.