Spreading Depolarization in the Early Post-Ischemic Brain
Stroke is one of the leading causes of death and long-term disability throughout developed countries. A propagating wave of mass depolarization of neurons and glia, referred to as spreading depolarization (SD), occurs within minutes after insufficient blood supply expands brain territory with depleted ATP and tissue damage. By harvesting live brain slices from mice that underwent focal stroke via middle cerebral artery occlusion (MCAo), we were able to image the ignition site of post-ischemic SD, assess the precise spatiotemporal propagation of the accompanying wave and record brain tissue changes in response to further metabolic stress. Post-MCAo brain slices superfused with elevated [K+]ext, [glu]ext or oxygen-and-glucose deprivation (OGD) display a decreased propensity to generate SD and tissue swelling in response to these extracellular conditions evident following stroke in vivo. Although this observation is counterintuitive to the high incidence of spontaneous SD observed in vivo by others two hours following ischemia, it is concordant with an ensuing period of decreased SD incidence. Furthermore, carbetapentane or dibucaine pre-treatment significantly delay SD onset in post-ischemic cerebral slices, similar to their effects in non-stroke brain slices. The healthy tissue observed immediately post-MCAo in the future ischemic core rapidly deteriorates during the ensuing 12 hours of infarct maturation. Although dramatic evidence of infarction occurs 12 hours following ischemia, a small and diffuse subset of pyramidal neurons in neocortex survives within the core. How they are protected and if they continue to survive post-MCAo are two intriguing issues for future study. Overall, this thesis specifically assesses the initiation, propagation and tissue changes in response to isolated mediators of swelling and SD in the post-ischemic brain. The observations of a delayed latency to SD onset and decreased susceptibility of swelling of the early post-ischemic brain support previous hypotheses of an adaptive mechanism of the post-ischemic brain to prevent or limit further depolarizations. The post-ischemic resistance to SD was over-ridden by chemical blockade of the Na+/K+ pump (and SD induction) by 100 µM ouabain. This finding resonates with an initial hypothesis of SD resistance by Na+/K+ pump hyperactivation postulated by early investigators of SD.