ABSTRACT Spinal cord injury (SCI) interrupts blood flow, and the O2 partial pressure (PO2) in the injured spinal cord drops to near zero. This contributes to necrosis and secondary injury. Our central hypothesis is that increasing O2 delivery to the injured cervical spinal cord will attenuate inflammation and neuronal cell loss, thereby preserving breathing function. The vast majority of prior O2 therapy studies after SCI use hyperbaric O2 (HBO), and there is considerable support that HBO reduces inflammation and secondary neurodegeneration. However, HBO consists of 100% O2 (hyperoxia), which is easy to implement, but at elevated pressure (hyperbaria), which is challenging to implement. Preliminary data indicate that the more challenging hyperbaria may not be needed. Specifically, normobaric hyperoxia (i.e., 100% O2 at ambient pressure) rapidly restores spinal PO2 after acute SCI and triggers anti-inflammatory mechanisms with a specific impact on microglia. Neuroinflammation after SCI contributes to scarring and neuronal loss, and impairs plasticity in spinal respiratory motor pathways. Thus, Aim 1 will determine if normobaric O2 therapy, initiated acutely (i.e., hours-days) after cervical SCI (cSCI), increases spinal PO2, mitigates microglial-driven spinal neuroinflammation, and preserves breathing ability. Preliminary data also indicate that a 1-hour per day treatment with normobaric hyperoxia has only a modest impact on secondary neuronal loss after SCI (i.e., neuroprotection). However, more robust neuroprotection can be achieved with HBO therapy. Since the fundamental difference between normo- and hyperbaric therapy is total blood O2, we predict that supplementing O2 delivery through alternate means will enable normobaric therapy to achieve greater neuroprotection. To test this idea, we will study perfluorocarbons - molecules that increase plasma O2 solubility and delivery to the injured spinal cord. Preliminary data show that treatment with a “next generation” perfluorocarbon known as NanO2 is safe, well tolerated, and preserves spinal tissues post-SCI. In Aim 2 we will test the hypothesis that combining normobaric hyperoxia with NanO2 acutely after cervical SCI synergistically increases spinal PO2, and promotes neuroprotection in primary (acute) and secondary cSCI. The proposed work will utilize our established cervical SCI models in the rat, including mid-cervical contusion and C2 hemilesion. Outcome measures include 1) cell-specific molecular responses (e.g., neurons, astrocytes and microglia) via flow cytometry, 2) spinal immunohistochemistry and histological neuron counts, 3) in vivo magnetic resonance imaging (MRI) for visualizing lesion volume, and ex vivo MRI for evaluating neural tracts in high- resolution (tractography), 4) respiratory outcomes including diaphragm EMG and breathing in unanesthetized rats, and direct measure of phrenic nerve output in anesthetized rats, and 5) spinal O2 measurements (intraspinal optode).