Effects of vagotomy on the hypercapnic ventilatory response in the awake neonatal rat

We studied effects of vagotomy on the ventilatory response to hyperoxic hypercapnia in the awake neonatal rat at P0-3. The vagus nerves were cut bilaterally at mid-cervical level. Each rat was exposed successively to four levels of inspired CO ₂ (0, 3, 6, 9% CO ₂, mixed with 50% O ₂, balance N ₂). Tidal volume and respiratory frequency were determined by direct plethysmography. Under control conditions (room air), the breathing pattern, respiratory frequency and tidal volume were not strictly different between the vagus-intact and vagotomized neonatal rat. In the vagus-intact neonatal rat, the progressive hyperoxic hypercapnia increased both respiratory frequency and tidal volume. In the vagotomized neonatal rat, though the progressive hypercapnia increased tidal volume, this stimulus greatly decreased the respiratory frequency less than half. As a result, the increment of the minute ventilation evoked by hypercapnia was lower in the vagotomized neonatal rat than the vagus-intact neonatal rat. We also observed the vagotomized neonatal rat showed mouth-opening gasp-like breathing under these hypercapnic conditions. Thus the vagotomy affected not only respiratory frequency and volume control, but also breathing motor pattern at this neonatal period. We concluded that afferent feedback from the lung stretch receptor would be essential for the respiratory networks to organize proper respiratory motor pattern and keep proper lung ventilation in response to hypercapnia. [J Physiol Sci. 2008;58 Suppl:S119]