Fetal respiratory neuronal activity during REM and NREM sleep

Chronically prepared near-term fetal lambs (129-133 days gestation) were exteriorized into a saline bath under maternal spinal anesthesia, and each head was rigidly connected to a stereotaxic frame. Multibarrel glass electrodes were inserted into the region of the nucleus tractus solitarius (NTS) during fetal breathing (FB) in rapid-eye-movement (REM) sleep. Of a total of 223 neurons, it was possible to record only 6 neurons for which firing amplitude did not change during the transition from REM to non-REM (NREM) sleep. The burst frequency, number of spikes per breath, and association with diaphragmatic activity were variable, with phasic activity preceding FB or disappearing and reappearing during FB. During the transition from REM to NREM sleep, phasic neuronal activity ceased, became tonic, and finally ceased altogether. L-Glutamate increased the number of spikes per breath and caused previous phasic activity to reappear but in NREM sleep produced only tonic activity. We conclude that during REM sleep the fetal respiratory neurons in the region of the NTS are to a large degree influenced by nonrespiratory REM sleep factors and that quiescence of respiratory neurons during NREM sleep is due to the lack of phasic input rather than to direct inhibition. Inhibition of FB during NREM sleep must occur upstream of the NTS neuron.