REMOVAL OF EXTRACELLULAR-SODIUM PREVENTS ANOXIA-INDUCED INJURY IN FRESHLY DISSOCIATED RAT CA1 HIPPOCAMPAL-NEURONS

Anoxia is believed to cause nerve injury and death in part, by inducing sustained, elevated levels of intracellular Ca2+. The increased concentration of intracellular Ca2+ is capable, by itself, of inducing nerve injury and death, even without the added stress of anoxia. However, we have recently shown that an increased level of intracellular Ca2+ is not necessary for anoxia-induced CA1 nerve injury. Since we have observed that extracellular Na+ decreases during anoxia, we studied the role of extracellular Na+ in anoxia-induced nerve injury. Removal of extracellular Na+ and its replacement with the impermeant cation N-methyl-D-glucamine (NMDG+) completely protected freshly dissociated CA1 neurons during and after severe anoxia, for up to 90 min. Intracellular Ca2+ decreased during anoxia, recovering during reoxygenation. Propidium iodide was excluded from the neurons for as long as Na+ was absent. Addition of Na+ (by replacing NMDG+) following anoxia resulted in rapid bleb formation, swelling and intracellular Ca2+ rise. Removal of Na+ before the rupture of blebs caused either shrinkage or pinching off of blebs so that the neuron apparently returned to its previous undisturbed state. We conclude that: (1) replacement of Na(o)+ with NMDG+ totally prevents anoxia-induced nerve injury as manifested by morphological changes, e.g., swelling, bleb formation and membrane injury; (2) upon re-exposure to Na(o)+ following anoxia in the absence of Na(o)+, neurons swell and bleb; (3) the appearance of blebs following anoxic exposure in the absence of Na(o)+ is reversible and Na(o)+ dependent. (4) Bleb formation itself is not necessarily immediately lethal to the neuron.