Peroxynitrite contributes to the pathogenesis of various neurodegenerative disorders through multiple mechanisms and is thought to mediate secondary neuronal cell death after spinal cord injury (SCI). Here we establish that physiologically relevant levels of uric acid (UA), a selective inhibitor of certain peroxynitrite-mediated reactions, block the toxic effects of peroxynitrite on primary spinal cord neurons in vitro. Furthermore, administration of UA at the onset of SCI in a mouse model inhibits several pathological changes in the spinal cord including general tissue damage, nitrotyrosine formation, lipid peroxidation, activation of poly(ADP-ribose) polymerase, and neutrophil invasion. More importantly, UA treatment improves functional recovery from the injury. Taken together, our findings support the concept that peroxynitrite contributes to the pathophysiology of secondary damage after SCI. They also raise the possibility that elevating UA levels may provide a therapeutic approach for the treatment of SCI as well as other neurological diseases with a peroxynitrite-mediated pathological component.blood-brain barrier ͉ neutrophils ͉ peroxynitrite ͉ spinal cord neurons ͉ cytotoxicity A cascade of pathophysiological processes rapidly follows mechanical trauma to the spinal cord, resulting in secondary neuronal damage that can significantly exacerbate the original injury (1). An acute inflammatory response at the site of the initial lesion is at least partly responsible for this secondary spinal cord pathology (e.g., refs. 2-4). Among the inflammatory cells recruited to the injured area are macrophages͞microglia and neutrophils that can mediate tissue damage by producing a variety of cytotoxic factors including reactive nitrogen species (3-7). Several studies have implicated peroxynitrite, a molecule generated when nitric oxide and superoxide combine, in secondary neuronal damage after spinal cord injury (SCI) (5-7). Not only has evidence of peroxynitrite production been detected in spinal cord tissues from rats after traumatic injury (5-7), but administration of a peroxynitrite donor directly into the rat spinal cord has been shown to cause neuronal cell death and neurological deficit (8). In addition, a number of previous reports have demonstrated that peroxynitrite is toxic for neurons, including primary spinal cord neurons, and neuronal cell lines in vitro (9-13).Peroxynitrite is known to mediate several potentially destructive chemical reactions, including tyrosine nitration and lipid peroxidation (14). Mitochondrial respiration is directly inhibited by peroxynitrite and is an early marker of its cytotoxic effects (9,14). In addition, peroxynitrite causes DNA strand breakage, which activates the enzyme poly(ADP-ribose) polymerase (PARP), which in turn triggers a cascade of processes that often lead to cell death (14). Because increased nitrotyrosine formation, lipid peroxidation, and PARP activation have all been associated with secondary damage in spinal cord trauma (5-7, 14-20), it is possible that second...