Effect of chronic continuous or intermittent hypoxia and reoxygenation on cerebral capillary density and myelination

Chronic hypoxia, whether continuous (CCH) or intermittent (CIH), occurs in many neonatal pathological conditions, such as bronchopulmonary dysplasia and obstructive sleep apnea. In this study, we explored the effect of CCH and CIH on cerebral capillary density and myelination. We subjected CD-1 mice starting at postnatal day 2 to either CCH 11% oxygen (O(2)), or CIH 11% O(2) (4-min cycles), for periods of 2 and 4 wk followed by reoxygenation for 4 wk. Mice were deeply anesthetized and perfused. Brains were removed to fixative for 24 h, then paraffin-embedded. Coronal brain sections were taken for analysis. Immunocytochemistry for glucose transporter 1 was used to assess angiogenesis, and Luxol fast blue and fluoromyelin stains were used to assess myelination. Capillary density increased after 2-wk exposure to CIH and CCH. By 4 wk, capillary density increased in both CIH and CCH by 25% and 47%, respectively, in cortex and by 29% and 44%, respectively, in hippocampus (P < 0.05). There was a decrease in myelination in the corpus callosum of mice exposed to CIH (75% of control) and CCH (50% of control) (P < 0.05). Reoxygenation reversed the increased capillary density seen in CCH to normoxic values. However, dysmyelination that occurred in CCH-exposed mice did not show any improvement upon reoxygenation. We conclude that neonatal chronic hypoxia 1) induces brain angiogenesis, which is reversible with reoxygenation, and 2) irreversibly reduces the extent of myelination in the corpus callosum. This potential irreversible effect on myelination in early life can, therefore, have long-term and devastating effects.