Motor nerve terminals are especially vulnerable to ischemic stress. The proposed experiments will test the hypothesis that even at early ages motor terminals in mice that overexpress a mutant human superoxide dismutase I (SODIG93A, a model of familial amyotrophic lateral sclerosis) are more vulnerable to stress than terminals in mice that overexpress wild-type human superoxide dismutase (hSOD1). We will test three stresses that might sometimes be encountered by motor terminals in vivo: (1) hindlimb ischemia/reperfusion stress in vivo, (2) hypoxia/ reoxygenation stress in vitro, and (3) in vivo intense stimulation of a single motor nerve. Structural integrity of the stressed motor terminals will be assessed by a fluorescence endplate occupancy assay, testing the extent to which labeled skeletal muscle endplates in fast and slow muscles are occupied by an innervating motor nerve terminal. Preliminary results indicate that both the ischemic and stimulation stresses increase endplate denervation. The function of motor terminals will be assessed during and/or after the stress by measuring resting and stimulation-induced changes in cytosolic and mitochondrial [Ca2+] and mitochondrial membrane potential using fluorescent indicators, and by measuring quantal transmitter release using electrophysiological recording. Preliminary results show disruptions in all these functional parameters during the hypoxia/ reoxygenation stress. Other experiments will test whether stresses that damage motor terminals also produce immunohistochemical signs of damage in the parent motoneurons. We will also test whether agents shown to be neuroprotective for motoneurons (e.g. vascular endothelial growth factor, VEGF; insulin-like growth factor, IGF-1) can protect motor nerve terminals during these stresses.