Ionic mechanisms underlying depolarizing responses of an identified insectmotor neuron to short periods of hypoxia

Hypoxia can dramatically disrupt neural processing because energy-dependent homeostatic mechanisms are necessary to support normal neuronal function. In a human context, the long-term effects of such disruption may become all too apparent after a "stroke," in which blood-flow to part of the brain is compromised. We used an insect preparation to investigate the effects of h ypoxia on neuron membrane properties. The preparation is particularly suita ble for such studies because insects respond rapidly to hypoxia, but can re cover when they are restored to normoxic conditions, whereas many of their neurons are large, identifiable, and robust. Experiments were performed on the ''fast'' coral depressor motoneuron (D-f) of cockroach (Periplaneta ame ricana). Five-minute periods of hypoxia caused reversible multiphasic depol arizations (10-25 mV; n = 88), consisting of an initial transient depolariz ation followed by a partial repolarization and then a slower phase of furth er depolarization. During the initial depolarizing phase, spontaneous plate au potentials normally occurred, and inhibitory postsynaptic potential freq uency increased considerably; 2-3 min after the onset of hypoxia all electr ical activity ceased and membrane resistance was depressed. On reoxygenatio n, the membrane potential began to repolarize almost immediately, becoming briefly more negative than the normal resting potential. All phases of the hypoxia response declined with repeated periods of hypoxia. Blockade of ATP -dependent Na/K pump by 30 mu M ouabain suppressed only the initial transie nt depolarization and the reoxygenation-induced hyperpolarization. Reductio n of aerobic metabolism between hypoxic periods (produced by bubbling air t hrough the chamber instead of oxygen) had a similar effect to that of ouaba in. Although the depolarization seen during hypoxia was not reduced by tetr odotoxin (TTX; 2 mu M), lowering extracellular Na+ concentration or additio n of 500 mu M Cd2+ greatly reduced all phases of the hypoxia-induced respon se, suggesting that Na influx occurs through a TTX-insensitive Cd2+-sensiti ve channel. Exposure to 20 mM tetraethylammonium and 1 mM 3,4-diaminopyridi ne increased the amplitude of the hypoxia-induced depolarization, suggestin g that activation of K channels may normally limit the amplitude of the hyp oxia response. In conclusion we suggest that the slow hypoxia-induced depol arization on motoneuron D-f is mainly carried by a TTX-resistant, Cd2+-sens itive sodium influx. Ca2+ entry may also make a direct or indirect contribu tion to the hypoxia response. The fast transient depolarization appears to result from block of the Na/K pump, whereas the reoxygenation-induced hyper polarization is largely caused by its subsequent reactivation.