AN OBLIGATE DOLE FOR OXYGEN IN THE EARLY STAGES OF GLUTAMATE-INDUCED,DELAYED NEURONAL DEATH

In vitro models of hypoxic/hypoglycemic injury reveal common mechanism s with glutamate excitotoxicity, but glutamate-induced toxicity in the absence of oxygen has never been directly addressed. Therefore, we as sessed neuronal survival and intracellular calcium concentrations ([Ca 2+](l)) in neonatal hippocampal cultures in response to glutamate in t he presence and absence of oxygen. Twenty-four hours of hypoxia alone killed 40% of the initial population, attributable to glutamate recept or-stimulated osmotic lysis. A 5 min glutamate exposure in ambient air killed 80% of the initial population by 24 hr later. When cultures we re deprived of oxygen during and for 2-24 hr after excitotoxin exposur e, glutamate did not cause additional neuronal death beyond that induc ed by oxygen depletion alone. Toxicities caused by activation of NMDA, AMPA, or kainate receptors were each ameliorated by oxygen depletion. In the absence of oxygen, glutamate evoked normal increases in [Ca2+] (l), indicating that glutamate receptors functioned normally. The glut amate-induced increases in [Ca2+](l) were not toxic in the absence of oxygen. In a similar manner, oxygen-depletion prevented neuronal killi ng by the calcium ionophore, ionomycin. Neuronal death produced by hyd rogen peroxide or iron sulfate was not ameliorated by oxygen removal. These oxidants maximally produced only a slow increase in [Ca2+](l) as the plasma membrane permeability increased nonspecifically. Therefore , oxygen-based reactions were an essential component of calcium-mediat ed, delayed neuronal death.