Hypoxic newborns have traditionally been given supplemental oxygen, and until recently, guidelines for neonatal resuscitation recommended that 100% oxygen be used. Exposure to 100% oxygen after hypoxic injury, however, may exacerbate oxidative stress. The current study evaluated the effect of exposure to 100, 40 or 21% oxygen after neonatal hypoxic-ischemic injury on the severity of brain injury after long-term survival. The severity of histological brain injury was not different in animals exposed to 100% oxygen versus room air. Male animals treated with 40% oxygen post-hypoxia had the lowest mean total histology scores, but this was not statistically significant due to the large variation in injury within each treatment group. These results support the growing number of studies in human infants and experimental animals that show no benefit of 100% oxygen over room air for neonatal resuscitation. Our results suggest that post-hypoxia treatment with 40% oxygen may be beneficial, particularly in males. Further studies of the effects of different concentrations of oxygen on brain injury are warranted and should have sufficient power to examine sex differences.

Poly(ADP-ribose) polymerase-1 (PARP-1) is a DNA repair-associated enzyme that has multiple roles in cell death. This study examined the involvement of PARP-1 in ischemic brain injury in the 7-day old rat, 0.5-48 h after unilateral carotid artery ligation and 2 h of 7.8% oxygen. This experimental paradigm produced a mild to moderate injury; 40-67% of animals in the ligated groups had histological evidence of neuronal death. Ipsilateral cortical injury was seen at all survival times, while mild contralateral cortical injury was seen only at the 1h survival time. Hippocampal injury was delayed relative to the cortex and did not show a biphasic pattern. Immunohistochemical staining for PARP showed bilateral increased staining as early as 1 h post-hypoxia. PARP staining at early time periods was most intense in layer V of cortex, but did not demonstrate a pattern of cell clusters or columns. Ipsilateral PARP-1 levels quantified by western blotting showed a biphasic pattern of elevation with peaks at 0.5 and 12 h post-hypoxia. Contralateral PARP-1 levels were also elevated at 0.5 and 24 h. PARP activity as determined by immunoreactivity for poly(ADP-ribose) (PAR) was increased ipsilaterally at 0.5, 2 and 12 h survival times. Cortical caspase 3-activity was increased ipsilaterally at 6, 12, and 24 h and contralaterally at 0.5, 1, 2 and 6 h post-hypoxia. There are three main findings in this study. First, changes in the distribution and amount of cell death correlate well with measured PARP-1 levels after hypoxia-ischemia, and both display biphasic characteristics. Second, there are significant early, transient morphological and biochemical changes in the contralateral cortex after neonatal hypoxia-ischemia due to unilateral permanent occlusion of a carotid artery followed by 2 h of systemic hypoxia. Third, variability in the responses of individual pups to hypoxia-ischemia suggests the presence of unidentified confounding factors.

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