Mechanisms of anoxia-induced depolarization in brainstem neurons: in vitro current and voltage clamp studies in the adult rat.

To determine the mechanisms underlying the depolarization induced by anoxia in brainstem neurons, we studied single neurons in brainstem slices using conventional micro-electrodes and freshly dissociated hypoglossal and vagal cells using patch clamp techniques (whole-cell configuration). Since glutamate concentration increases in the extracellular space during O2 deprivation, we first tested whether N-methyl-D-aspartate (NMDA) and non-NMDA receptors are involved in this anoxia-induced depolarization. APV, MK-801, CNQX and KYN (NMDA and non-NMDA blockers), which bathed slices after control anoxia runs, did not affect the depolarization trajectory. Decreasing extracellular Na+ (Nao+) from 150 mM to 5 mM attenuated markedly and significantly the depolarization observed during anoxia (15-20% of control). The relation between intracellular adenosine triphosphate (ATP) and the anoxia-induced depolarization was also investigated in the slice and in dissociated single brainstem neurons. In the slice, iontophoresis of ATP did not give consistent results. Since we could not ascertain that ATP was actually iontophoresed through high resistance (50-80 M omega) microelectrodes, we patched single neurons and studied the effect of clamping intracellular ATP levels on the hyperpolarizing holding current (IH) in the voltage clamp mode. The increase in IH with anoxia (or cyanide) was markedly attenuated in cells patched with pipettes containing ATP. We conclude that in brainstem neurons, the anoxia-induced depolarization: (a) is not a function of an increase in extracellular glutamate concentration; and (b) depends on Na+ and ATP-mediated processes.