Adaptation to intermittent hypoxia restricts nitric oxide overproduction and prevents beta-amyloid toxicity in rat brain

This study tested the hypothesis that adaptation to intermittent hypoxia (AIH) can prevent overproduction of nitric oxide (NO) in brain and neurodegeneration induced by beta-amyloid (Aβ) toxicity. Rats were injected with a Aβ protein fragment (25–35) into the nucleus basalis magnocellularis. AIH (simulated altitude of 4000 m, 14 days, 4 h daily) was produced prior to the Aβ injection. A passive, shock-avoidance, conditioned response test was used to evaluate memory function. Degenerating neurons were visualized in stained cortical sections. NO production was evaluated in brain tissue by the content of nitrite and nitrate. Expression of nNOS, iNOS, and eNOS was measured in the cortex and the hippocampus using Western blot analysis. 3-Nitrotyrosine formation, a marker of protein nitration, was quantified by slot blot analysis. Aβ injection impaired memory of rats; AIH significantly alleviated this disorder. Histological examination confirmed the protective effect of AIH. Degenerating neurons, which were numerous in the cortex of Aβ-injected, unadapted rats, were essentially absent in the brain of hypoxia-adapted rats. Injections of Aβ resulted in significant increases in NOx and in expression of all NOS isoforms in brain; AIH blunted these increases. NO overproduction was associated with increased amounts of 3-nitrotyrosine in the cortex and hippocampus. AIH alone did not significantly influence tissue 3-nitrotyrosine, but significantly restricted its increase after the Aβ injection. Therefore, AIH affords significant protection against experimental Alzheimer’s disease, and this protection correlates with restricted NO overproduction.