Effects of cooling the ventrolateral medulla on diaphragm activity during NREM sleep

Dysfunction through cooling of neurons near the ventrolateral medullary (VLM) surface results in apnea in the anesthetized state, whereas similar neuronal dysfunction in the awake state only modestly decreases breathing. The purpose of this study was to investigate effects on breathing, as measured by diaphragm electromyogram (EMG(di)), of VLM neuronal dysfunction during NREM sleep, a naturally occurring change in state. In six goats, thermodes for cooling were chronically implanted between the first hypoglossal rootlet and the pontomedullary junction (area M and area S). During wakefulness and NREM sleep, bilateral VLM cooling (thermode temp = 20 degrees C) for 30 sec decreased EMG(di) mean activity and minute EMG(di) (p < 0.05) and lengthened the time between diaphragm contractions. During NREM sleep, reductions in mean and minute EMG(di) during cooling tended to be greater than during waking, but not significantly. However, following carotid body denervation, VLM cooling caused prolonged apnea during NREM sleep but only a brief apnea in the awake state. The data suggest that either intact VLM neuronal mechanisms or intact carotid afferents are necessary for sustained EMG(di) activity during NREM sleep.