Effects of oxygen deprivation on parapyramidal neurons of the ventrolateral medulla in the rat.

We characterized the electrophysiological properties and responses of neurons located in the parapyramidal region of the ventral aspect of the rat medulla oblongata (parapyramidal neurons, PP neurons) to oxygen deprivation, in order to understand the mechanisms involved in hypoxia induced respiratory depression. The responses of PP neurons to oxygen deprivation were compared to those of the functionally dissimilar neurons of the dentate gyrus (DG). Neurons from the PP region were found to fire spontaneously with a frequency of 3-3.5 spikes/sec in both adults and neonates and responded to an anoxic insult with a complete loss of spontaneous firing. Discrete metabolite analysis showed a small (about 17%) decrease in tissue adenosine triphosphate (ATP) levels of the PP neurons during an anoxic insult and the decrease was significantly smaller than in the DG cell region (28%). In contrast to the DG neurons, the PP neurons recovered from an anoxic insult lasting more than 30 min, indicating a greater survival capacity of the PP neurons during oxygen deprivation. The PP neurons were also capable of withstanding successive anoxic insults better than the DG cells as demonstrated by their complete recovery following reoxygenation. It is suggested that the PP neurons may depress their electrical activity as an energy conservation mechanism, and thereby survive anoxic insults longer than the dentate neurons, whereas the loss of cellular activity in the DG neurons may be a result of energy depletion.