Oxygen resuscitation does not ameliorate neonatal hypoxia/ischemia-induced edema

Neonatal hypoxia/ischemia (HI) is the most common cause of developmental neurological, cognitive and behavioral deficits in children. Although cerebral edema is a common outcome after HI, the mechanisms leading to the excessive fluid accumulation are poorly understood. Hyperoxia treatment after HI (HHI) is the recommended clinical therapy for newborn resuscitation. Our objective was to evaluate edema development after HI and whether HHI treatment affected HI-induced edema. We induced HI by a permanent ligation of the left carotid artery followed by a systemic exposure to hypoxia (8% O2) in P7 rats; a cohort of these animals was immediately treated with hyperoxia (40% or 100% O2). Dry weight analyses and T2-MRI showed cerebral edema 1, 3, 7 and 21 days after HI in the ipsilateral cortex, and 3, 7 and 21 days in the contralateral cortex. Using a blood-brain barrier (BBB) assay we showed that HI induces BBB permeability 3 and 7 days after HI, leading to vasogenic edema in both cortices. HHI treatment failed to prevent BBB permeability and edema development. At the molecular level, we investigated the effect of HI on AQP4, the main water channel in the brain, which has been implicated in edema development after different neuropathological conditions. Our results showed that HI significantly increased AQP4 levels 3, 7 and 21 days after HI in the ipsilateral cortex, with no effect in the contralateral cortex. Furthermore, HHI treatment did not affect HI-induced changes in AQP4, consistent with the lack of effect of HHI on edema development. Given that developmental increases of AQP4 in the brain are accompanied by significant reduction in water content, we believe that HI-induced increase in AQP4 in the ipsilateral cortex is aimed at protecting the brain, and that the lack of increase in AQP4 levels in the contralateral cortex leads to the development of edema. In agreement with our hypothesis, we showed that HI induced impaired motor coordination 21 days after the insult and HHI did not ameliorate this behavioral outcome. We conclude that HHI treatment is effective as a resuscitating therapy, but does not ameliorate HI-induced cerebral edema and impaired motor coordination.