Anoxia-induced depolarization in CA1 hippocampal neurons: role of Na+-dependent mechanisms

We have previously shown that (1) removal of extracellular sodium (Na+) reduces the anoxia-induced depolarization in neurons in brain-slice preparations and (2) amiloride, which blocks Na+-dependent exchangers, prevents anoxic injury in cultured neocortical neurons. Since anoxia-induced depolarization has been linked to neuronal injury, we have examined in this study the role of Na+-dependent exchangers and voltage-gated Na+ channels in the maintenance of membrane properties of CA1 neurons at rest and during acute hypoxia. We recorded intracellularly from CA1 neurons in hippocampal slices, monitored V-m and measured input resistance (R-m) with periodic injections of negative current. We found that tetrodotoxin (TTX, 1 mu M) hyperpolarized CA1 neurons at rest and significantly attenuated both the rate of depolarization (Delta V-m/dt) and the rate of decline of R-m (Delta R-m/dt) by about 60% during the early phase of hypoxia. The effect of TTX was dose-dependent. Amiloride (1 mM) decreased V-m and increased R-m in the resting condition but changed little the effect of hypoxia on neuronal function. Benzamil and 5-(N-ethyl-N-isopropyl)-2',4'-amiloride (EIPA), two specific inhibitors of Na+ dependent exchangers, were similar to amiloride in their effect. We conclude that neuronal membrane properties are better maintained during anoxia by reducing the activity of TTX-sensitive channels and not by the action of Na+-dependent exchangers. (C) 1997 Elsevier Science B.V.