Intracranial Collateral flow defines the boundaries of molecular penumbra in experimental ischemic stroke

Introduction. Intracranial collaterals are dynamically recruited after arterial occlusion and are emerging as a strong determinant of tissue outcome in both human and experimental ischemic stroke. The relationship between collateral flow and ischemic penumbra remains largely unexplored in pre-clinical studies. We investigated the relationship between intracranial collateral flow during transient MCA occlusion and the development of molecular penumbra and ischemic infarct after 24 hours. Material and Methods. MCA was transiently occluded (90 minutes) by intraluminal filament in adult male Wistar rats (n=25). Intracranial collateral flow was studied using multi-site laser Doppler with two probes. A first probe (Probe 1) was attached to the skull 1 mm posterior to the Bregma and 5 mm lateral to the midline (lateral probe, corresponding to the ischemic core of MCA territory). A second probe (Probe 2) was attached to the skull 2 mm anterior to the Bregma and 2 mm lateral to the midline (medial probe, corresponding to the borderzone territory between ACA and MCA territory). Cerebral perfusion monitoring was performed continuously during the entire period of anesthesia (approximately 140 minutes). Two hemodynamic parameters were considered for analysis: i) drop in cerebral perfusion in both probes during MCA occlusion following successful filament insertion. ii) biosignal fluctuation analysis in both probes during the pre-ischemic period and during MCA occlusion. Molecular penumbra was defined by topographical mapping and quantitative signal analysis of HSP70 immunohistochemistry. Functional deficit was assessed using a 18-points sensory-motor score and infarct volume was calculated on consecutive sections stained with Cresyl violet, performed 24 hours after ischemia induction. Results. The degree of functional performance of intracranial collaterals in the territory of leptomeningeal branches during MCA occlusion inversely correlated with HSP70 immunoreactive areas in both cortex and striatum, as well as with infarct size and functional deficit, and predicted the amount of intact tissue after MCA occlusion followed by reperfusion. Intracranial collateral flow is associated with reduced areas of both molecular penumbra and ischemic core and increased areas of intact tissue in rats subjected to MCA occlusion followed by reperfusion. Conclusions. The study has two main findings. First, molecular penumbra showed an irregular patchy topography, whose extent directly correlated with the extent of ischemic core. Second, good collateral status provides complete protection from ischemic injury, without increasing the amount of penumbral tissue, if reperfusion is achieved; conversely, poor collateral status is associated with a greater extent of both ischemic core and molecular penumbra. In conclusion, the degree of cerebral collateral perfusion is inversely correlated with both ischemic core and molecular penumbra during transient proximal MCA occlusion. Our findings prompt the development of collateral therapeutics to provide tissue-saving strategies in the hyper-acute phase of ischemic stroke prior to recanalization therapy.