Repetitive hypoxia extends endogenous neurovascular protection for stroke

Abstract

OBJECTIVE Brief systemic hypoxia protects the rodent brain from subsequent ischemic injury, although the protection wanes within days. We hypothesized that the duration of ischemic tolerance could be extended from days to months by repeated intermittent hypoxia of varying magnitude and duration. METHODS Infarction volumes following a 60-min transient middle cerebral artery occlusion were determined in adult male mice 2 days through 8 wks after completion of a 2-week repetitive hypoxic preconditioning (RHP) protocol. Separate cohorts were studied for the protective effects of RHP on postischemic and cytokine-induced cerebrovascular inflammation, and for potential deleterious effects of the RHP stimulus itself. RESULTS RHP protection against transient focal stroke persisted for 8 weeks. Leukocyte adherence to cortical venules was attenuated in response to stroke, as well as following TNF-α administration, indicating that reductions in postischemic inflammation were not secondary to smaller infarct volumes. RHP reduced post-stroke leukocyte diapedesis concomitant with a long-lasting downregulation of endothelial adhesion molecule mRNAs, and also reduced postischemic blood-brain barrier permeability to endogenous IgG. RHP was without effect on hippocampal CA1 pyramidal cell viability, only transiently elevated hematocrit, and did not affect the magnitude of CBF during and after ischemia. INTERPRETATION Taken together, our findings reveal a novel form of epigenetic neurovascular plasticity characterized by a prominent anti-inflammatory phenotype that provides protection against stroke many weeks longer than previously established windows of preconditioning-induced tolerance. Translating these endogenous protective mechanisms into therapeutics could afford sustained periods of cerebroprotection in subpopulations of individuals at identified risk for stroke.

Hypoxic conditioning and the central nervous system: A new therapeutic opportunity for brain and spinal cord injuries?

“…Mice exposed to hypoxia showed an enhancement of cognitive functional recovery, in association with an increase of neuronal salvage. Interestingly, and in accordance with the results of preconditioning studies, 24 the authors suggested that IHC can accelerate cognitive function recovery by attenuating neuro-inflammation. Thus, lower levels of the inducible isoform of nitric oxide synthase (iNOS), an inflammation factor, were found in the hypoxia group in the border between infarct and normal tissues of the hippocampus.…”

Enhanced Retinal Ganglion Cell Survival in Glaucoma by Hypoxic Postconditioning After Disease Onset

“…In these settings, the adaptive response induced by the conditioning stimulus is short-lasting, abrogating the extent of an injury process that, while severe, typically 'matures' within hours or a couple of days. Our discovery that repetitive presentations of the conditioning stimulus could extend the duration over which both retina and brain could resist acute ischemic injury [8,26], a finding later validated by others in models of acute ischemic brain injury [27][28][29][30], carried the implicit assumption that the long-lasting, prosurvival phenotype induced epigenetically by repetitive conditioning might also protect against chronic disease. Indeed, we showed that six short bouts of systemic hypoxia over a 2-week period prior to the onset of experimental glaucoma reduced the resultant loss of RGC soma and axons that occurred over the subsequent 10 weeks of disease [9].…”

Quantification of Neurovascular Protection Following Repetitive Hypoxic Preconditioning and Transient Middle Cerebral Artery Occlusion in Mice

“…Repetitive presentations of the hypoxic preconditioning stimulus dramatically extend the duration of the neuroprotective phenotype. 6 Many studies have shown that the frequency, magnitude, and duration of the repetitive stimulus train are critical determinants of this response. For example, simply repeating the same intensity and duration of hypoxia (2 hr of 11% oxygen) 3 times per week (M-W-F) over 2 weeks did not extend the therapeutic window for neurovascular protection from tMCAo, 6 but was sufficient to induce long-term ischemic tolerance in the retina.…”

“…6 Many studies have shown that the frequency, magnitude, and duration of the repetitive stimulus train are critical determinants of this response. For example, simply repeating the same intensity and duration of hypoxia (2 hr of 11% oxygen) 3 times per week (M-W-F) over 2 weeks did not extend the therapeutic window for neurovascular protection from tMCAo, 6 but was sufficient to induce long-term ischemic tolerance in the retina. 30 Interestingly, although 5 exposures to systemic hypoxia (5 hr of 8% O 2 ) spaced 6 days apart protected against tMCAo induced 3 days after the last hypoxic exposure, neuroprotection was lost if the 5 hypoxic exposures were placed 3 days apart.…”

“…It should be noted that RHP-treated animals typically exhibited pink as opposed to pure white volumes. 6 In order to avoid washout of infarct in RHP-treated mice, we use a shorter staining period for TTC that results in more pink healthy tissue as opposed to dark red. Evans Blue processing may also darken the infarct area so pure white infarcts are unlikely with the dual quantification staining presented in this protocol.…”