Spreading Depolarizations and Post-Ischemic Injury

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DESCRIPTION (provided by applicant): Current options for clinical treatment of ischemic strokes are currently very limited and there is a great need for interventions that can be safely administered during a period of several hours following the onset of a stroke and minimize neuronal loss. It has long been known that repetitive waves of spreading depolarizations (SD) (analogous to cortical spreading depression) occur in animal stroke models, but only recently has it been convincingly shown that SDs are very prevalent following human ischemic brain injuries. SDs produce massive ionic redistributions in neurons and glia, requiring the expenditure of metabolic energy to restore homeostasis. The repetitive SDs following ischemia place a severe additional metabolic demand on brain tissue that is already compromised by reductions in local blood flow. Thus approaches to prevent the onset and progression of these post-ischemic SD events, or even to limit their deleterious consequences, are likely to have substantial positive outcomes in clinical medicine. We have discovered that Zn2+ can play an important role in initiation of SD, and that Zn2+ release from synapses is substantial following each SD event. Zn2+ has previously been demonstrated to be toxic to both neurons and glia, and our overall hypothesis is that Zn2+ increases associated with SD make a significant contribution to injury following ischemia. We propose that this is due to Zn2+ accumulation in both neurons and astrocytes, which in turn 1) lowers the threshold for initiation of SD events and 2) serves as an upstream trigger for Ca2+ excitotoxicity. Studies in Aim 1 utilize hippocampal slice preparations from mice to evaluate the mechanisms of Zn2+ release and accumulation in single neurons and populations of astrocytes. Aim 2 examines mechanisms by which Zn2+ can facilitate the onset of SD in hippocampal slices, including inhibition of astrocyte uptake function and up-regulation of neuronal NMDA receptor function. Aim 3 tests the hypothesis that Zn2+ is upstream of Ca2+ deregulation following SD, and tests whether interventions that disrupt the processes identified in Aims 1&2 provide significant improvements in neuronal viability in brain slice and in vivo. Slice studies of synaptic structure and function will be complemented by in vivo studies of focal ischemia in mice. Each aim should independently provide significant new information for the field, and when taken together, these mechanistic studies should suggest novel approaches to limit the consolidation and spread of ischemic brain injury.