We are interested in understanding the role of the different components of the vascular unit in the time course of the disease and to determine key molecular pathways that could be the target of therapeutical strategies. One focus has been on understanding the factors that contribute and lead to dysfunction of the neurovascular unit. We have used the spontaneously hypertensive rat (SHR) as an important experimental model in the study of stroke and other cerebrovascular diseases. In a first study, weve examined the effect of age and anesthesia on regional CBF, cerebrovascular resistance and CO2 reactivity in SHR rats. We found normal cerebral autoregulation in SHR and a linear relationship between CBF and arterial CO2 levels under chloralose. In contrast, 2% isoflurane impaired cerebral autoregulation and caused a clear saturation of CBF at arterial CO2 levels greater than 70 mm Hg, in both young and adult rats, regardless of absolute CBF values, suggesting that isoflurane interferes with the vasodilatory mechanisms of CO2. This behavior was observed for both cortical and subcortical structures. Under either anesthetic, CO2 reactivity values in adult SHR were decreased, confirming that hypertension, when combined with age, increases cerebrovascular resistance and reduces cerebrovascular compliance. In a second study, we evaluated the influence of preserving hypertension during focal cerebral ischemia on stroke outcome in SHR. Focal cerebral ischemia was induced by transient (1 h) occlusion of the middle cerebral artery, during which mean arterial blood pressure was maintained at normotension (110120 mm Hg, group 1, n = 6) or hypertension (160170 mm Hg, group 2, n = 6) using phenylephrine. We estimated lesion volume and brain edema from apparent diffusion coefficient maps and T2-weighted MRI, and regional CBF using arterial spin labeling MRI. We determined that infarct volume, edema, and neurological deficits were significantly reduced in the phenylephrine group compared to the normotensive group. In addition, the phenylephrine-treated group showed higher values and rapid restoration of rCBF, while rCBF in both hemispheres was significantly decreased in group 1. We concluded that maintaining preexisting hypertension alleviates ischemic brain injury in SHR by increasing collateral circulation to the ischemic region and allowing rapid restoration of rCBF. Our data suggest that maintaining preexisting hypertension is a valuable approach to managing hypertensive patients suffering from acute ischemic stroke. In a third study, we tried to understand better the underlying causes for the increased susceptibility of hypertensive patients to stroke. Again using the SHR animal model, we investigated the correlation between temporal changes of rCBF and the severity of transient ischemic stroke using T2-, diffusion- and perfusion-weighted MRI at six different time points: before and during 1 h of unilateral middle cerebral artery occlusion (MCAO), 1 h after reperfusion, and 1 day, 4 days and 7 days after MCAO. We measured rCBF values in both hemispheres, and the perfusion-deficient lesion (PDL) was defined as the area of the brain with a 57% or more reduction in basal CBF. Within the PDL, regions were further refined as ischemic core (rCBF = 06 mL/100 g/min), ischemic penumbra (rCBF = 615 mL/100 g/min) and benign oligemia (rCBF > 15 mL/100 g/min). We determined that while SHR and WKY had identical initial volume of the PDLs and identical rCBF maximum measured within those lesions, in SHR virtually all of the PDL progressed to become the final ischemic lesion, while in WKY the final ischemic lesion volume was significantly smaller than their original PDL, and quite similar to the ischemic core. We also determined that the region with the lowest range of rCBF was positively correlated with the final ischemic lesion volume. Both during ischemia and after reperfusion, and unlike in normotensive rats, rCBF in either ipsilesional and contralesional brain hemispheres of SHR could not be restored to pre-ischemic levels, and remained lower than in WKY until up to 4 days after MCAO. Thus our data suggest that impaired CBF regulation and relatively high CBF threshold for ischemia are strong contributors to the increased susceptibility of SHR to ischemic stroke. We want to understand how alterations in cortical functional domains induced by stroke lead to changes in the hemodynamic response to increased neural activity. This will allow us to refine the notion that the cerebrovasculature is built to support the functional organization of the cortex, and enable development of a better model of inferring about the flow of neuronal communication from careful analysis of the spatiotemporal features of the hemodynamic response. For this, we will develop focal ischemic models based on spatially targeted applications of vasoconstricting peptides such as endothelin-1 (ET-1) to test the relevance of different sub-regions of the cortex and, in particular, of individual functional areas (e.g. individual face patches or the representation of individual digits) in dictating the spatiotemporal characteristics of the hemodynamic response. We will use high-resolution MRI of the cortical cytoarchitecture to plan and chose the target sub-domains within the cortex to be made ischemic, and compare the post-ischemia functional data with those obtained from the same animals pre-ischemia. We will also compare cerebrovascular resistance and vascular territory maps obtained at both states. Because the stroke area will be made small, it is possible that these maps will not change, but task-induced hemodynamic responses will be significantly different due to the region-selective death of neuronal cells caused by stroke. These experiments will provide a better understanding of how the architecture of the vascular tree influences the spatiotemporal features of the hemodynamic response. We used the spontaneously hypertensive rat (SHR) and its normotensive control WKY to evaluate the effects of an intracortical injection of ET-1. ET-1 produces a larger infarct volume in SHR than in WKY. Both pre- and post-treatment of the animals with JZL184, a powerful and specific inhibitor of the enzyme monoacylglycerol lipase (MAGL) significantly reduces the infarct volume induced by ET-1, thus establishing that MAGL as an important therapeutic target for stroke. In addition, MAGL inhibition significantly improved neurological outcome post-ischemia. MAGL hydrolyzes 2-arachidonoyl glycerol (2-AG), the most abundant endogenous cannabinoid in the brain, into arachidonic acid (AA), an important precursor of pro-inflammatory prostaglandins and leukotrienes. 2-AG exhibits anti-inflammatory and neuroprotective properties not only through modulating the signaling of cannabinoid receptors, but also by controlling AA release. Thus we hypothesized that MAGL inhibition might be a novel anti-inflammatory and neuroprotective strategy for neurological disorders, including ischemic stroke. Inhibition of MAGL leads to suppressed neuroinflammation, as measured by a significant reduction in the number of activated microglia in the ischemic core. Thus, our results suggest that MAGL alone contributes to neuropathology of cerebral ischemia, and thus is a promising therapeutic target for the treatment of ischemic stroke. To validate the work in the primate brain, it will be exciting to reproduce the same above experiments in marmosets, and we intend to do so just as soon as we finish the rodent study.
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