Cerebral collateral circulation in experimental ischemic stroke: from molecular penumbra to collateral therapeutics

Cerebral collateral circulation is a subsidiary vascular network which is dynamically recruited after acute ischemic stroke and may provide residual blood flow to the affected areas, slowing down the progression of ischemic penumbra to irreversible ischemic damage. The anatomy and functional performance of the collateral circulation varies among individuals, both humans and rodents, and is emerging as a strong prognostic factor in patients. Nonetheless, collateral circulation in experimental ischemic stroke is frequently neglected. In the present work, ischemic stroke was modelled by 90-minutes transient occlusion of the middle cerebral artery (MCA) in Wistar rats, followed by 24 hours of reperfusion. During surgery, multi-site laser Doppler flowmetry (LDF) was used for real-time monitoring of cerebral blood flow both in the lateral MCA territory and in the borderzone between the MCA and anterior cerebral artery territories, where the collateral circulation is connecting these two different vascular territories. The relationship between collateral flow, evaluated by multi-site LDF in the collaterals territory, and the post-reperfusion molecular penumbra, defined by protein expression of Heat Shock Protein 70kDa (HSP70), was investigated. Good collateral flow was associated with reduced extent of both molecular penumbra and ischemic core and increased extent of intact tissue, suggesting a complete protection from ischemic injury in variable areas of the cortex and striatum, if reperfusion is achieved. Conversely, poor collateral status was associated with a greater extent of both ischemic core and molecular penumbra. High variability in infarct size is common in experimental stroke models and highly affects statistical power and validity of preclinical neuroprotection trials. Cerebral collateral flow was explored as a stratification factor for the prediction of ischemic outcome, using magnetic resonance imaging (MRI) as a reference. Multi-site LDF monitoring for collateral flow assessment was able to predict ischemic outcome and infarct typology and reflected perfusion deficit in good agreement with acute MRI. These results support the additional value of cerebral collateral flow monitoring for outcome prediction in experimental ischemic stroke, especially when acute MRI facilities are not available. Modulating collateral blood flow in order to augment or maintain perfusion to the ischemic penumbra could represent a new therapeutic strategy for the hyperacute (even pre-hospital) phase, particularly if applied before recanalization therapies. Four therapeutic strategies were administered 30 minutes after ischemia onset: acetazolamide (selective cerebral vasodilation), polygeline (intravascular volume load), phenylephrine (induced hypertension) and head-down tilt (cerebral flow diversion). Globally, the treated animals showed better outcomes at 24 hours post reperfusion and an augmentation of cerebral perfusion, especially in the collaterals territory, after treatment administration. These results prompt further study of strategies aiming at modulating the cerebral collateral circulation. In conclusion, the cerebral collateral circulation and its functional performance determined both ischemic outcome and penumbra in the experimental model considered. Monitoring collateral flow while testing therapeutic strategies provides insight about their mechanism of action.