Spreading depolarization causes reperfusion failure after cerebral ischemia

Background Despite successful recanalization to treat acute ischemic stroke, reperfusion failure associated with poor functional outcomes develops in half of the patients. The cause of reperfusion failure remains the subject of intensive research. Here, we explore the possibility that spreading depolarization (SD), a potent ischemic injury mechanism is a significant contributor and reliable predictor of reperfusion failure. Methods Young adult male and female C57BL/6 mice (n=69) were anesthetized with isoflurane (0.6-0.9%) and prepared for transcranial optical imaging. After 10 min of baseline, incomplete global forebrain ischemia was induced by transient (45 min) bilateral common carotid artery (CCA) occlusion, followed by 75 min reperfusion. SD and cerebral blood flow (CBF) changes were visualized with intrinsic optical signal imaging and laser speckle contrast imaging. To block SD, the irreversible NMDA receptor antagonist MK801 was applied (0.3 mg/kg, i.p., n=29). Neurological deficit was evaluated at baseline and post-ischemia with a composite Garcia Neuroscore scale. Collaterals of the circle of Willis were examined after loading the vasculature with carbon black ink. Ischemic neuronal injury was evaluated in hematoxylin-eosin-stained brain sections. Results SD emerged after ischemia onset in one or both hemispheres under a perfusion threshold (CBF drop to 21.1±4.6 vs. 33.6±4.4 %, SD vs. no SD). The failure of later reperfusion (44.4±12.5 %) was invariably linked to previous ischemic SD. In contrast, reperfusion was adequate (98.9±7.4 %) in hemispheres devoid of SD during ischemia. CBF reduced below the perfusion threshold of SD, when the P1 segment was absent in the circle of Willis. SD occurrence and the linked reperfusion failure were associated with poor neurologic function, and greater neuronal necrosis. The inhibition of SD with MK801 significantly improved reperfusion. Conclusions SD occurrence during ischemia impairs later reperfusion, prognosticating poor functional outcomes. The increased likelihood of SD occurrence is predicted by inadequate collaterals.