Postischemic synaptic excitation and N-methyl-D-aspartate receptor activation in gerbils

Transient forebrain ischemia leads to the delayed degeneration of CA1b hippocampal pyramidal cells. In previous studies using the gerbil carotid occlusion model, we demonstrated that CA1b pyramidal cell degeneration is preceded by a period of enhanced excitatory transmission. Experiments with hippocampal slices prepared after 5 minutes of bilateral carotid artery occlusion show that ischemia enhances excitatory synaptic transmission and reduces pyramidal cell excitability before it abolishes synaptic function. In the present study, we tested the hypothesis that these effects require the activation of N-methyl-D-aspartate receptors during the postischemic period. Hippocampal slices were prepared 20-30 minutes after carotid occlusion, and Schaffer collateral-commissural input-output curves were constructed from recordings made every 30-60 minutes for 11-14 hours. Inclusion of the selective, reversible N-methyl-D-aspartate receptor antagonist 3-((±)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid in the superfusion medium completely prevented the progressive loss of pyramidal cell excitability that normally follows this ischemic insult. This antagonist also prevented the postischemic increases in the duration and initial slope of the focally-recorded excitatory postsynaptic potential. The antagonist could still partially block the effects of transient forebrain ischemia when treatment was delayed for 4 hours. Our results confirm that the deleterious electrophysiologic changes in area CA1b depend on the continuing activation of N-methyl-D-aspartate receptors. Delayed ischemic neuronal death may result, in part, from excessive synaptic excitation during the postischemic period. However, other factors that are eliminated by preparing hippocampal slices appear to play an important role as well.