Background
Ischemic preconditioning is a phenomenon by which a short duration of cerebral ischemia provides protection from a more severe, injurious ischemia. The exact mechanism for this neuroprotection is unknown, theories include: increased activity of heat shock proteins, decreased caspase activity and cell death, and upregulation of genes important for hibernation and metabolism. Hypothesis: Ischemic preconditioning causes tolerance to injurious cerebral ischemia due to an increase in regional cerebral blood flow (rCBF).
Materials and Methods
The effects of preconditioning were evaluated using laser doppler flowmetry and magnetic resonance imaging (MRI). Mice were assigned to one of five groups: 1) preconditioned with injurious ischemia 72 hours later, 2) injurious ischemia alone, 3) preconditioned alone, 4) preconditioned followed by injurious ischemia 2 weeks later or 5) control. These animals were temperature regulated.
Mouse surgical model
Mice had radio telemetry probes embedded into the peritoneal cavity one week prior to middle cerebral artery occlusion (MCAo), and at 24 hours prior to MCAo were placed on receivers to monitor temperature. Imaged mice were temperature regulated using a rectal temperature probe. The MCAo had a duration of either 15 minutes (pre-conditioning ischemia), or 45 minutes (injurious ischemia). Local blood flow velocity was measured using laser doppler flowmetry (LDF) throughout ischemia; another cohort of mice were imaged using MRI to observe perfusion deficits during occlusion.
Results
Mice were euthanized at 24 hours post-ischemia and the brain sectioned and affixed to slides. These sections were stained with hematoxylin and eosin and infarct volume analysis conducted. The blood flows were evaluated from LDF and perfusion/diffusion mismatch was calculated from the MRI data to quantify rCBF. Blood flow velocity was less than 20% in naïve mice, whereas induction of injurious ischemia in preconditioned mice had observed flows 35–40% of baseline. As well, there was a greater trace variability in the form of frequent spikes of low flow velocity in preconditioned mice as compared to naïve mice. Infarcts were significantly smaller in preconditioned mice; naïve mice had 16.75±3.6% of the brain infarcted, while preconditioned mice had 1.25±2.5% of the brain infarcted. The final infarct size was correlated to the rCBF calculated from the flow velocities of LDF. Flow drops of greater than 30% of baseline were necessary to cause a large infarction. Preconditioned mice consistently had flows of greater than 30% of baseline. Evaluation of the rCBF using 9.4 T MRI is currently underway. The mice will be imaged using T1, T2, arterial spin labelling (for perfusion) and apparent diffusion coefficient (ADC). The perfusion deficit will be observed during the injurious ischemia occlusion in both preconditioned and naïve mice. A perfusion/diffusion mismatch will be calculated to evaluate the difference in rCBF between preconditioned and naïve mice.
Conclusion
Mice that have been preconditioned showed smaller infarcts following injurious ischemia as compared to naïve mice, and preconditioned mice had better rCBF during injurious ischemia. A correlation between rCBF and infarct size was observed; preconditioned mice had blood flows of greater than 30% during injurious ischemia.