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. In a healthy brain, neuronal activity can be present until the onset of SD which triggers spreading depression (silencing) of activity that translates into migraine aura. Recovery of ion gradients depends on the sufficient sodium pump activity which is energy-dependent. Yet, even in normal cortex SD could cause a shortage of energy supply suggesting that even normoxic SD might pose a threat in healthy cortex. Indeed, mild ischemia induced by normoxic SD could result in migrainous stroke. In stroke and trauma patients SDs are known to exacerbate tissue 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 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 dendritic structural rearrangements in naive healthy neocortex depend on the degree of the transient tissue hypoxia imposed by SD. 2) To test the hypothesis that SD-inflicted dendritic injury is aggravated in tissue with selectively impaired glial metabolism. 3) To examine possible routes of rapid water entry during SD-induced dendritic beading. 4) To test the hypothesis that SD is the mechanism implicated in rapid synapses disruption and loss. 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. Quantitative serial section electron microscopy analyses will be used to reveal SD-induced injury at the level of single synapses. The results will bring new insight to the development and recovery from acute injury of synaptic circuits in stroke.
The American Heart Association 2012 Heart Disease and Stroke Statistics Update reports that on average someone in the US has a stroke every 40 seconds and dies as a result every 4 minutes. Two of the major pathogenic events that increase injury in stroke patients are the depolarizing waves and the associated brain swelling that course across the cortex injuring brain cells. Detailed understanding of how this injury evolves early after insult will help to design treatments for brain recovery after stroke.
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