New variability in cerebrovascular anatomy determines severity of hippocampal injury following forebrain ischemia in the Mongolian gerbil.

Transient bilateral carotid artery occlusion in the Mongolian gerbil is a widely used model of forebrain ischemia due to species-specific absence of communicating arteries between the middle and posterior cerebral arteries. We have found that transient carotid occlusion induces a wide variation in histological injury of the hippocampus, suggesting that Mongolian gerbils currently available in the US have anomalous connections between the vertebral and carotid circulations. We subjected Mongolian gerbils from Harlan Sprague-Dawley and Charles River Laboratories to 5 min of bilateral carotid occlusion under continuous striatal temperature control and assessed hippocampal injury histologically 5 or 14 days later. The severity of occlusion-induced hippocampal injury depended on the presence and size of posterior cerebral communicating arteries. Injury was markedly attenuated in hippocampi having ipsilateral communicating arteries >50 microm in diameter. In contrast, severe ischemic injury occurred when either no posterior communicating artery was present, or when communicating arteries were smaller than 50 microm in diameter. Ischemic injury was independent of any communicating arteries on the contralateral side. Communicating arteries were present in 90% of gerbils from each vendor, ranging from 19 microm to 125 microm in diameter. The high prevalence of posterior communicating arteries and their profound effect on attenuating hippocampal injury indicates that an understanding of the presence and size of posterior communicating cerebral arteries in each animal is now required to interpret the extent of hippocampal injury following bilateral carotid artery occlusion in this species.