Investigation of the Modulators of Cerebrovascular Coupling

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We are interested in establishing the physiological role of well-established neurochemical pathways (such as NO and COX-2) in shaping the spatial specificity of CBF regulation. The working hypothesis is that vasoactive substances released by strategically located cells in the parenchyma act on the cerebral vessels and regulate their tone to mediate increases in CBF. The outstanding questions are: (i) which substances are released by what cells and under which circumstances? (ii) What is the spatial specificity of with respect to the cortical architecture? (iii) What is the physiological role of vasoactive agents in the context of underlying physiological and pathophysiological processes, such as hypertension and stroke? The main research approach that we have adopted has been to combine state-of-the-art neuroimaging techniques, able to probe the brain across different spatial and temporal scales, with pharmacological manipulations aimed at determining the relative contribution, spatial specificity and cellular basis of the different pathways to the HRF. The main neuroimaging techniques to be used are anatomical and functional MRI, which are able to provide non-invasive images of the brain with sub-millimeter spatial resolution and superior soft tissue contrast. However, at its present spatial resolution, MRI can barely visualize cortical cytoarchitecture and it is certainly not yet able to resolve individual cells and capillary vessels. To understand neurovascular coupling one must be able to resolve the neurovascular unit and study its signaling mechanisms at the level of its individual cellular constituents. Two-photon microscopy is an attractive technique that allows simultaneous measurements of the activity of individual neurons and astrocytes, along with the corresponding changes in diameter and red blood cell velocities in individual vessels. However, the excellent spatial resolution of 2-photon microscopy comes at a price in 3D spatial coverage. Not only the depth of penetration is limited, but also the limitation in field-of-view prevents one from observing the feeding arteries and draining veins to the capillary network of interest, and thus the impact that the supply and drainage of blood has on the capillary response cannot be evaluated. Our lab believes that a way to get around such limitations and make forward progress is to combine the advantages of MRI and 2-photon microscopy into simultaneous or parallel multi-modal recordings, so that neurovascular coupling can be studied in all relevant spatial and temporal scales. In addition, incorporation of modern electrophysiology techniques able to probe neural activity across different cortical layers will add crucial data about the flow of information within a functional cortical column, and allow better interpretation of the temporal evolution of the hemodynamic response within that column. The integration of the above multi-modal techniques constitutes a powerful and attractive experimental approach that will shed light on the intricate mechanisms of CBF control. Current and future experiments will continue to focus on understanding the relevance of these pathways to neurovascular coupling in the presence of pathophysiological states such as hypertension and ischemic stroke.