Impact of SAH on Parenchymal Arterioles and Neurovascular Coupling

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Subarachnoid hemorrhage (SAH) following cerebral aneurysm rupture is associated with substantial morbidity and mortality and existing therapeutic options have limited efficacy. For decades, the traditional view has been that blood-induced vasospasm of large diameter arteries on the brain surface is the major underiying cause of delayed neurological deficits in SAH patients. However, the emerging view is that additional factors that may include impaired function of the microcirculation within the brain contribute to poor outcome. To date, few studies have directiy examined the impact of SAH on parenchymal arterioles (PAs) in the brain. Our preliminary data demonstrate that isolated PAs from SAH model rats exhibit enhanced constriction. Further, in the context of brain slices where communication between neurons, astrocytes and PAs is intact (i.e., the intact neurovascular unit), we provide novel and exciting evidence that SAH causes a shift in neurovascular coupling from vasodilation to vasoconstriction. We propose that SAH-induced enhanced PA constriction and impaired neurovascular coupling are two distinct phenomena acting in concert to negatively impact blood flow to the brain. Our overarching objective is to understand the cellular mechanisms contributing to these events. The goal of Specific Aim 1 is to determine the cellular basis of enhanced pressure-induced constriction of PAs from SAH animals and to understand the impact that this enhanced constriction has on vasoactive influences implicated in neurovascular coupling. Our preliminary data suggest SAH, via a mechanism involving epidermal growth factor receptor (EGFR) activation, causes voltage-dependent K* channel (Kv) channel suppression, smooth muscle (SM) cell membrane potential (VM) depolarization and enhanced voltage-dependent Ca^* channel (Cav) activity. Specific Aim 2 will elucidate the role of astrocytic endfoot Ca^* and large-conductance Ca^*-activated K* (BK) channel activity in neurally and endfoot Ca^* uncaging evoked vasoconstriction in SAH animals. Here, we will also examine the effect of SAH on neurally evoked cortical cerebral blood flow changes in vivo. State-of-the-art techniques including two-photon Ca^* imaging and uncaging, patch clamp electrophysiology, quantitative real-time PCR and laser Doppler flowmetry are applied to a hierarchy of experimental approaches that range from the subcellular level to the intact brain slice, and the intact animal. This project will work closely with Project 1 (M. T. Nelson, brain slice imaging and neurovascular coupling; in vivo measurements of functional hyperemia) and Project 2 (J. E. Brayden, SM Cav and VM studies). Further, our K* channel studies in PA SM will complement Dr. Brayden's Transient Receptor Potential (TRP) channel studies. This project will interact with M. Cipolla (Project 3), as these projects use models of two distinct forms of stroke, which despite differences in etiology and proximate functional effects, may have similar consequences for neurovascular coupling and cognitive function. This work will greatiy add to current knowledge regarding the actions of SAH on PA function and neurovascular coupling. These studies will also provide fingerprints useful in identifying key mediators of these pathologies and are likely to identify novel therapeutic targets to help minimize the devastating consequences of cerebral aneurysm rupture.