Excessive activation of the N-methyl-D-aspartate receptor (NMDAR) is a key event in excitotoxicity and largely responsible for neuronal cell death after brain ischemia. Specifically, NMDAR-mediated Ca2+ influx and K+ efflux have been linked to necrosis and apoptosis. NMDAR activities are inhibited by NR3A, a newly identified NMDAR subunit that is abundantly expressed in neonatal neurons but diminished in older cells with increased neuronal vulnerability to excitotoxicity. Whether NR3A contributes to the neonatal brain tolerance is an important but undetermined question. We hypothesize that NR3A expression in the developing brain plays an important neuroprotective role in preventing neuronal cell death induced by excitotoxic insults. NMDAR-associated neuronal death will be studied in wild type (WT) and NR3A-deficient (NR3A"'") cultured mouse cortical neurons, in HEK293 cells transfected with NR subunits, and in neonatal and adult ischemic stroke models of WT and NR3A"'" mice. Specific Aim 1 will examine the inhibitory effects of NR3A expression on NMDA-induced membrane currents and excitotoxicity. Patch clamp experiments will examine a reduced voltage-dependent Mg2+ block of NMDAR by NR3A expression, and test the hypothesis that NR3A selectively attenuates the NMDA outward K+ (NMDA-K) current as a novel mechanism of NR3A-induced neuroprotection. Mild and severe NMDA insult-induced apoptotic and necrotic death will be examined in cultured cells expressing different levels of NR3A. Effects of NR3A on intracellular Ca2+ ([Ca2+]i), K+ ([K+]j), caspase activation, and their relationships to suppressed NMDA currents and cell death will be assessed. Specific Aim 2 will examine the neuroprotective effect of NR3A against hypoxia- and ischemia-induced cell death. The neonatal brain is more tolerant of hypoxic/ischemic injury than the adult brain, but the underlying mechanism is not well defined. The decline of NR3A expression during brain maturation parallels the weakening of tolerance. We will test the hypothesis that the high and low levels of NR3A expression correspond to the different ischemic tolerances of neonates and adults. The neuroprotective role of the NR3A subunit will be examined in hypoxia-induced cell death in vitro as well as in ischemic stroke models of WT and NR3A"'" mice. Based on the unique inhibitory effect of NR3A on NMDAR activities, its neuroprotective function in the developing brain is a logical but untested hypothesis. The investigation will provide novel evidence for an important endogenous self-defense mechanism which may underlie neonatal tolerance to brain ischemia. The findings from this investigation may also suggest a new strategy of receptor therapy in prevention and treatment of stroke.