Brain Injury Depolarizations Impact on the Integrity of Synaptic Circuitry

Spreading depolarizations (SDs) are waves of sustained near-complete neuronal and glial depolarization that actively propagate a collapse of ion gradients through the brain with associated dramatic neuronal and glial swelling that entails cytotoxic edema. Recovery of ion gradients depends on the sufficient sodium pump activity which is energy-dependent. In stroke and brain trauma patients SDs are thought to exacerbate damage in the at-risk cortical territory supporting the view that SD may be an important mechanistic endpoint in clinical studies. We have shown that in the penumbra SDs result in rapid dendritic beading which is reversible, but signals leading to neuronal death could be initiated during this time. Hence, dendritic beading is the hallmark of neuronal injury. Once the energy demands for recovery of penumbral dendrites are no longer met by the diminishing blood flow, SD irreversibly injures dendrites and dendritic spines are lost signifying acute damage to synaptic circuitry. Importantly, we have shown that SD-induced dendritic injury in the penumbra could be stopped pharmacologically. We have also shown that persistent astroglial swelling is initiated and exacerbated during SD in brain tissue with moderate to severe energy deficits, likely disrupting astroglial maintenance of normal homeostatic function and thus their ability to support neurons. SD-induced cytotoxic edema contributes to stroke injury. Mammalian pyramidal neurons lack functional aquaporins, thus the molecular pathways by which they accumulate osmotically obligated water and rapidly swell during SD is unknown. Bulk water influx could occur through large-pore pannexin hemichannels opened by SD. Transporters may also be responsible for water accumulation as well as recovery. The specific aims are: 1) To test the hypothesis that SD-inflicted injury to dendrites and dendritic spines is exacerbated in tissue with selectively impaired glial metabolism. 2) To test the hypothesis that pannexin hemichannels and neuronal cotransporters participate in SD-induced dendritic beading. To achieve these aims we will combine in vivo 2-photon laser scanning microscopy of fluorescent neurons, astrocytes and blood flow in adult mouse somatosensory cortex with other sophisticated in vivo approaches such as laser speckle and intrinsic optical signal imaging while simultaneously monitoring occurrence of SD. The results will bring new insight to the development of acute cellular injury in stroke. (AHA Program: Grant-in-Aid)