The Journal of Neurophysiology Vol. 81 No. 1 January 1999, pp. 307-318
Copyright ©1999 The American Physiological Society

Ionic Mechanisms Underlying Depolarizing Responses of an Identified Insect Motor Neuron to Short Periods of Hypoxia

Hervé Le Corronc¹, Bernard Hue¹, and Robert M. Pitman²

¹ Laboratory of Neurophysiology, University of Angers, F-49045 Angers Cedex, France; and ² School of Biomedical Sciences, Gatty Marine Laboratory, University of St. Andrews, Fife KY16 8LB, Scotland

Le Corronc, Hervé, Bernard Hue, and Robert M. Pitman. Ionic mechanisms underlying depolarizing responses of an identified insect motor neuron to short periods of hypoxia. J. Neurophysiol. 81: 307-318, 1999. 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 hypoxia on neuron membrane properties. The preparation is particularly suitable for such studies because insects respond rapidly to hypoxia, but can recover when they are restored to normoxic conditions, whereas many of their neurons are large, identifiable, and robust. Experiments were performed on the "fast" coxal depressor motoneuron (D_f) of cockroach (Periplaneta americana). Five-minute periods of hypoxia caused reversible multiphasic depolarizations (10-25 mV; n = 88), consisting of an initial transient depolarization followed by a partial repolarization and then a slower phase of further depolarization. During the initial depolarizing phase, spontaneous plateau potentials normally occurred, and inhibitory postsynaptic potential frequency increased considerably; 2-3 min after the onset of hypoxia all electrical activity ceased and membrane resistance was depressed. On reoxygenation, 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 µM ouabain suppressed only the initial transient depolarization and the reoxygenation-induced hyperpolarization. Reduction of aerobic metabolism between hypoxic periods (produced by bubbling air through the chamber instead of oxygen) had a similar effect to that of ouabain. Although the depolarization seen during hypoxia was not reduced by tetrodotoxin (TTX; 2 µM), lowering extracellular Na⁺ concentration or addition of 500 µM Cd²⁺ greatly reduced all phases of the hypoxia-induced response, suggesting that Na influx occurs through a TTX-insensitive Cd²⁺-sensitive channel. Exposure to 20 mM tetraethylammonium and 1 mM 3,4-diaminopyridine increased the amplitude of the hypoxia-induced depolarization, suggesting that activation of K channels may normally limit the amplitude of the hypoxia response. In conclusion we suggest that the slow hypoxia-induced depolarization on motoneuron D_f is mainly carried by a TTX-resistant, Cd²⁺-sensitive sodium influx. Ca²⁺ entry may also make a direct or indirect contribution to the hypoxia response. The fast transient depolarization appears to result from block of the Na/K pump, whereas the reoxygenation-induced hyperpolarization is largely caused by its subsequent reactivation.

This article has been cited by other articles:

				F. Bergmann and B. U. Keller Impact of mitochondrial inhibition on excitability and cytosolic Ca2+ levels in brainstem motoneurones from mouse J. Physiol., February 15, 2004; 555(1): 45 - 59. [Abstract] [Full Text] [PDF]

				B. S. Wu, J. K. Lee, K. M. Thompson, V. K. Walker, C. D. Moyes, and R. M. Robertson Anoxia induces thermotolerance in the locust flight system J. Exp. Biol., March 15, 2002; 205(6): 815 - 827. [Abstract] [Full Text] [PDF]

				M. Haller, S. L. Mironov, and D. W. Richter Intrinsic Optical Signals in Respiratory Brain Stem Regions of Mice: Neurotransmitters, Neuromodulators, and Metabolic Stress J Neurophysiol, July 1, 2001; 86(1): 412 - 421. [Abstract] [Full Text] [PDF]

				J. W. Barclay and R. M. Robertson Enhancement of Short-Term Synaptic Plasticity by Prior Environmental Stress J Neurophysiol, March 1, 2001; 85(3): 1332 - 1335. [Abstract] [Full Text] [PDF]