Hippocampal cell loss in transient global cerebral ischemia in rats: a critical assessment

The induction of transient global cerebral ischemia by permanent vertebral occlusion and temporary carotid ligation (four-vessel occlusion) is widely accepted as a valid tool for the study of pathogenesis and treatment of ischemia. The neural damage inflicted by this intervention is often assessed by measuring pyramidal cell loss in the CA1 hippocampal field. Nevertheless studies using this model in rats often fail to control variables that are relevant to the outcome, and/or apply biased methods to quantitate histological damage. We have applied unbiased stereological methods to estimate absolute numbers of surviving neurons in CA1 in Wistar rats subjected to either 10 or 20 min global ischemia using the Sugio et al. variant of the original four-vessel occlusion model. Animal mortality was high at both times, with neuron losses averaging 39% and 31%, respectively. Post-operative mortality was reduced substantially by using decompressive craniectomies and, even more effectively, by pre-treating the rats with low doses of phenytoin. Both maneuvers led to a severely increased CA1 neuron loss, which reached 50%, after an ischemia of 10 min. This finding strongly supports that mortality biases the sample. Other noteworthy findings that emerged from this study were a linear relationship between per-ischemic blood pressure increments and animal survival, and a negative correlation between cell survival and preferentially left-sided damage.