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. In the current review cycle, we have worked on understanding the influence of hypertension in stroke outcome. It is well known that hypertension is a major risk factor for ischemic stroke. However, the management of preexisting hypertension is still controversial in the treatment of acute stroke in hypertensive patients. So we sought out to evaluate the influence of preserving hypertension during focal cerebral ischemia on stroke outcome in a rat model of chronic hypertension, the spontaneously hypertensive rat (SHR). Focal cerebral ischemia was induced by transient (1h) occlusion of the middle cerebral artery, during which mean arterial blood pressure was maintained at normotension (110-120mm Hg, group 1, n=6) or hypertension (160-170mm Hg, group 2, n=6) using phenylephrine. T2-, diffusion- and perfusion-weighted MRI were performed serially at five different time points: before and during ischemia, and at 1, 4 and 7 days after ischemia. Lesion volume and brain edema were estimated from apparent diffusion coefficient maps and T2-weighted images. Regional cerebral blood flow (rCBF) was measured within and outside the perfusion deficient lesion and in the corresponding regions of the contralesional hemisphere. Neurological deficits were evaluated after reperfusion. Infarct volume, edema, and neurological deficits were significantly reduced in group 2 vs. group 1. In addition, higher values and rapid restoration of rCBF were observed in group 2, while rCBF in both hemispheres was significantly decreased in group 1. Maintaining preexisting hypertension alleviates ischemic brain injury in SHR by increasing collateral circulation to the ischemic region and allowing rapid restoration of rCBF. The data suggest that maintaining preexisting hypertension is a valuable approach to managing hypertensive patients suffering from acute ischemic stroke. This study was just published in Brain Research (Kang BT et al Brain Res 2012), and follows our previous study in Neuroimage (Leoni RF et al Neuroimagie 2011;58(1):75-81) demonstrating that hypertension, when combined with age, increases cerebrovascular resistance and reduces cerebrovascular compliance in a way that is hard to counteract acutely. Indeed, based on the above work, we have been investigating the correlation between temporal changes of regional cerebral blood flow (rCBF) and the severity of ischemic stroke in spontaneously hypertensive rats (SHR). T2-, diffusion- and perfusion-weighted magnetic resonance imaging were performed in Wistar-Kyoto rats (WKY) and SHR serially at six different time points;before and during 1 hour of middle cerebral artery occlusion (MCAO), 1 hour after reperfusion, and 1 day, 4 days and 7 days after MCAO. Regional CBF values were measured in both hemispheres. The area with three ranges of rCBF (0-6 (ischemic core), 6-15 (ischemic penumbra), and 15-23 (benign oligemia) mL/100 g/min) was measured. Most of the perfusion deficient lesion (33.953.68%) was progressed to ischemic lesion (33.025.41%) in SHR (P>0.05), but the final infarct volume of WKY (12.629.19%) was significantly smaller than the perfusion deficient lesion (32.524.08%) (P<0.01) and similar to the area with the lowest rCBF range (13.132.96%) (P>0.05). The regulation of CBF was lost in both hemispheres of SHR until 1 day after MCAO. There was no correlation between decreased blood supply and the final infarct volume (r=-0.443, P>0.05). Impaired CBF regulation and relatively high CBF threshold for infarction may greatly contribute to increased susceptibility of ischemic stroke in SHR. We are in final stages of submitting a manuscript to JCBFM reporting these findings.
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