Breathing During Sleep in the Postnatal Period of Rats: The Contribution of Active Expiration.

Breathing is most vulnerable to apneas and other disturbances during sleep in both humans and rodents, especially in the newborn period. We recently demonstrated in adult rats that, in contrast to the atonia typical of skeletal muscles during rapid eye movement sleep, the normally passive expiratory muscles become active, and their activity is associated with stabilization of breathing and increased ventilation. In this study, we investigated the relationship between respiration and expiratory muscle recruitment across sleep states during the first 2 weeks of rat postnatal development. We instrumented rats with electromyography electrodes in neck and abdominal muscles while sleep states (active and quiet sleep) were classified based on nuchal muscle tone and overt behavior inside a whole-body plethysmograph. Our results indicate that breathing was most irregular in active sleep (AS) and that rats displayed frequent recruitment of expiratory muscle activity, which occurred in both active and quiet sleep states. While the occurrence of active expiration in quiet sleep did not affect ventilation and its frequency decreased with age, the recruitment of expiratory muscles during AS was present across development and it was associated with a reduction in respiratory variability across development and an increase in ventilation at most age groups considered. We conclude that the occurrence of active expiration is a common feature of respiration in the postnatal period, and it significantly contributes to ventilation, in particular in AS.