Background
Neurons acquire tolerance to ischemic stress when preconditioning ischemia occurs a few days beforehand. Because collateral circulation through leptomeningeal vessels may determine the severity of ischemic injury after occlusion of middle cerebral artery (MCA) in stroke patients, we focused on collateral development after mild reduction of perfusion pressure to find an endogenous response of the vascular system that contributes to development of ischemic tolerance.
Methods
After attachment of a probe with dental cement to the intact skull 3.5 mm lateral to the bregma, the left common carotid artery (CCA) of C57BL/6 mice was occluded with halothane anesthesia. The left middle cerebral artery (MCA) was subsequently occluded permanently with a microbipolar electrocoagulator just proximal to the point where the olfactory branch came off, on days 0, 1, 4, 14 and 28 (n=8 each). Body temperature was maintained at 37°C during surgery. The change in cortical perfusion during MCA occlusion was recorded. A sham group of mice received only exposure of the CCA and MCA occlusion 14 days later. In apoE-knockout mice, the MCA was occluded 14 days after CCA occlusion or sham CCA surgery. Four days after MCA occlusion, mice were examined for neurological deficits and then their brains were removed, fixed and embedded in paraffin. Tissue sections (5 um) were obtained every 1 mm, beginning at the frontal pole, and were examined after HE staining for measurement of infarct size.
Results
Unilateral occlusion of the left CCA resulted in 40–70% of baseline microperfusion over the MCA area in most mice. However, in 6 mice, cortical microperfusion was reduced to less than 35% of baseline. Only mice that had 45 to 65% of baseline perfusion after CCA occlusion were used in the subsequent experiments. Cortical perfusion after MCA occlusion was significantly preserved in day 14 (47+16%) and day 28 (46+7%) groups compared with day 0 (28+8%) and sham groups (32+9%). Infarct size and neurologic deficits were also attenuated in day 14 and day 28 groups compared with day 0 and sham groups. In apoE-knockout mice, there was no significant difference in perfusion (36.4+11.8% compared to 30.2+7.8%, P=0.23), neurologic deficits and infarction size (31.5+8.2 mm3 compared to 37.0+5.3 mm3, P=0.14) between groups with and without CCA occlusion.
Conclusion
Our results demonstrated that unilateral CCA occlusion treatment given 14 days before MCA occlusion preserved cortical perfusion and reduced infarct size after MCA occlusion. Chronic mild reduction of perfusion pressure may lead to collateral development and brain protection after focal cerebral ischemia. These responses of collaterals were impaired in apoE-knockout mice.