Level of ischemia and brain functions in the Mongolian gerbil in vivo

The Mongolian gerbil (Meriones unguiculatus) provides a very useful animal model to study the effects of ischemia on brain functions. In this model it is possible to induce two levels of ischemia in the same animal. Thus, monitoring the brain in vivo in real-time will provide meaningful information regarding the development of ischemic injury as well as the follow-up during the recirculation period. The aims of the study were as follows: (1) To elucidate the mechanism behind the development of ischemic depolarization under unilateral and bilateral carotid artery occlusion. (2) To exclude the possibility that removal of the dura mater will affect the results. (3) To correlate the kinetics of the recovery processes to the level of ischemia. We tested the correlation between energy depletion level (evaluated by intramitochondrial NADH redox state) and the development of ischemic depolarization (ID) and vasospasm (evaluated by extracellular K⁺, DC potential and 366 nm reflectance changes, respectively) under partial and complete ischemia (induced by unilateral or bilateral carotid artery occlusion) using the multiparametric monitoring system (MPA). In 12 out of 32 gerbils monitored by the MPA, the dura mater remained intact, while in the other 20, it was removed very gently before connecting the MPA to the brain. Two types of responses to unilateral carotid artery occlusion were recorded and the gerbils were divided into groups according to the development of the ID. In a third group of 5 gerbils we tested the effect of 1-5 min of bilateral occlusion on the various parameters monitored. The results could be summarized as follows: (1) Under bilateral occlusion, in all gerbils the ID was recorded within 1-2 min. (2) Under unilateral occlusion, the level of ischemia obtained was significantly smaller and led to the ID in about 60% of the gerbils. (3) The rate of K⁺ leakage in phase I (before the ID appearance) was significantly lower under unilateral occlusion. In group B where ID did not develop under unilateral occlusion, the level of ischemia was 50% of its maximum or less. (4) The rate of K⁺ leakage during phase I was energy dependent as evaluated in all subgroups of gerbils. (5) The K⁺ leakage during phase II (ID) had an 'all or none' nature in terms of maximal K⁺ levels and time to reach it and no significant variation was recorded between various groups. (6) The main effect of various lengths of bilateral occlusion was on the recovery time of extracellular K⁺ level, namely longer occlusion led to a slower recovery.