The extent of brain injury following ischemic stroke is a time and space dependent function of the degree of oxygen deprivation. Although oxygen thresholds for ischemic damage have been investigated for many years, the detailed relationship between local oxygen levels and alterations in neuronal structure and function has been extremely challenging to study in vivo. Dendritic spines are of particular interest since they likely play a key role in cortical plasticity following brain injury and are a potential target for rehabilitative training strategies. However, the role of oxygen levels at the level of single dendrites is poorly understood, in large part due to a lack of methods capable of quantifying oxygenation with micron scale resolution in three dimensions. Although oxygen sensitive electrodes enable precise oxygen measurement at a single location, their invasive nature precludes them from chronic studies and for mapping oxygen distributions. Recent advances in oxygen sensitive phosphorescent dyes have opened up new possibilities for noninvasive oxygen mapping in rodent models when combined with multiphoton excitation. Despite the potential for this approach to yield new insight into microvascular oxygenation, low signals levels limit the speed and penetration depth of the oxygenation measurements. Therefore, the goal of this project is to develop an improved imaging method for three-dimensional determination of intra- and extra-vascular oxygenation levels simultaneously with imaging of dendritic morphology and to use this system to quantify the relationship between local oxygen levels and dendritic remodeling following cerebral ischemia. The technological advances in oxygen mapping will involve two components. First, we will improve the phosphorescence signal levels of the oxygen sensitive probe by developing a spatially multiplexed regenerative amplifier excitation source that will produce several spatially offset excitation spots. The minimal tradeoff in spatial resolution will be offset by the considerable gain in temporal resolution and imaging depth. Second, we will refine a method to transiently disrupt the blood brain barrier using microbubble-assisted focused ultrasound, which will permit delivery of the dye to the extravascular space. Together these advances will enable chronic measurement of oxygen levels with micron scale resolution. We will then use these tools to quantify the longitudinal oxygen gradients from surface and penetrating arterioles through capillaries and draining venules under baseline conditions and after occlusion of a single arteriole. Finally, we will quantify the relationship between oxygen levels surrounding individual dendrites and morphological changes in these dendrites during both acute and chronic ischemia.
Stroke is one of the leading causes chronic disability in the U.S. The disruptions in oxygen supply during ischemic stroke are responsible for a complex sequence of events that leads to cell death. However, the details of the role of oxygen deprivation on cell death and remodeling are not well understood. This project will develop new methods for high resolution imaging of oxygen levels in the brain that will be used to quantify the relationship between oxygen levels and cell death following stroke.
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