Importance of vagal afferents in determining ventilation in newborn rats.

We studied the effect of acute bilateral vagotomy on ventilation and ventilatory pattern in rats. In 1- to 6-day-old unanesthetized rats, vagotomy resulted in a substantial decrease (38%) in ventilation during air breathing. After vagotomy there was a threefold increase in tidal volume (VT), inspiratory time (TI) doubled, and expiratory time (TE) was six times longer. When studied under isoflurane anesthesia, newborn rats showed decreases in ventilation similar to that observed without anesthesia, whereas anesthetized adult rats had no consistent changes in ventilation. Adult and newborn rats had nearly identical proportionate increases in VT and TI after vagotomy, but TE lengthened to a greater extent in the newborns. Additionally, we demonstrated a significant decrease in ventilation when 100% O2 rather than air was supplied to nonvagotomized unanesthetized newborn rats. Ventilation decreased by 19% after vagotomy under hyperoxic conditions. We conclude that vagal afferent input, probably of pulmonary mechanoreceptor origin, provides positive feedback to respiration in newborn rats and that newborn rats greater than 24 h old also have a degree of peripheral chemoreceptor drive during air breathing.