Objective-Clinical MR studies show delayed and ongoing neurodegeneration following neonatal hypoxia-ischemia (HI), but the mechanisms and timing of this neurodegeneration remain unclear. We use ex vivo Diffusion Tensor Imaging (DTI) and brain neuropathology to determine if selective injury to white matter tracts occurs following neonatal HI in mice resulting in neural system associated attrition in remote regions and at delayed time points. Methods-TheRice Vannucci model (unilateral carotid ligation + 45 minutes of hypoxia FIO2=0.08) was used to cause brain injury in postnatal (p)7 C57BL6 mice and ex vivo DTI and correlative neuropathology were performed at p8,p11,p15,p21,p28, and p42.Results-DTI provides excellent contrast visualization of unmyelinated white matter in the immature mouse brain. Acute severe ipsilateral injury to the hippocampus is seen with both histopathology and diffusion-weighted MRI 24 hours post injury. Injury to axons is evident 24 hours after HI in the hippocampal alveus. By p11 and continuing until p28, the ipsilateral fimbria fornix degenerates. Beginning at p15, there is injury and loss of axons entering the ipsilateral septal nucleus followed by ipsilateral septal atrophy. Volume loss in the hippocampus is rapid and severe but is subacute and significantly slower in the ipsilateral septum. Neonatal HIE, also interrupts the normal developmental increase in fractional anisotropy in the ipsilateral fimbria but not in the contralateral fimbria from p8 to p42.Interpretation-In the neonatal brain there is a progressive systems-preferential injury following HI. DTI allows unparalleled visualization of this neural-network associated attrition so that it can be followed longitudinally in the developing brain.

Activation of Fas death receptor (Fas DR) signaling cascade is seen after neonatal hypoxia-ischemia (HI). Cell survival is favored when signaling through the death-inducing signaling complex and cleavage of caspase 8 to its active form is blocked by FLIP, a dominant negative of caspase 8. H₂O₂ quickly downregulates expression of FLIP. Neonatal mice overexpressing glutathione peroxidase (GPx) have less injury and less H₂O₂ accumulation compared with neonatal mice overexpressing superoxide dismutase (SOD) or wild-type (WT) littermates. Expression of both FLIP_L and FLIP_S is increased in GPx-oxerexpressing mice relative to WT mice at 24 h and relative to SOD-overexpressing mice at 2 and 24 h following neonatal HI (ANOVA, p < 0.05). There is an increase in Fas DR expression at 24 h in both WT and GPx-overexpressing mice and significant differences between WT and SOD-overexpressing mice (ANOVA, p < 0.01). There is no difference in FADD expression among the 3 groups 24 h after HI. At 24 h following HI, the ratio of FLIP to Fas DR expression supports a significant negative correlation with injury score (r² = 0.99, slope = –4.01), and expression of both the active fragment of caspase 8 and caspase 8 activity is increased in SOD overexpressors compared to GPx overexpressors at 24 h after HI (ANOVA, p < 0.05). The overall degree of injury previously seen in these 3 strains correlates well with changes in expression of Fas DR signaling proteins favoring neuroprotection in the GPx-overexpressing mice, i.e. increased FLIP expression and decreased caspase 8 activity compared to SODtg mice. The mechanism by which antioxidant status alters FLIP levels following neonatal HI may be related to the ability to detoxify H₂O₂ produced following neonatal HI.