CHARACTERIZATION OF VENTILATORY RESPONSES TO HYPOXIA IN NEONATAL RATS

Newborn animals exhibit a biphasic response to hypoxia, with ventilati on increasing and then declining. Our purpose was to define if this re sponse could be supported by the pontile and medullary respiratory cen ters. Spontaneously breathing and paralyzed and ventilated decerebrate or anesthetized, vagotomized rats were studied from birth to 13 days thereafter. Peak integrated phrenic activity, or tidal volume, and fre quency initially increased and then declined after inspired oxygen was reduced from hyperoxic to hypoxic levels; most animals became apneic in hypoxia. Apnea occurred in a greater proportion of animals and more quickly with more severe hypoxia. Following sectioning of the carotid sinus nerves, ventilatory activity declined with a change from hypero xia to normoxia. We conclude that the biphasic ventilatory response to hypoxia represents a balance between synaptically-induced augmentatio ns and reductions of brainstem neuronal activities. The carotid chemor eceptors play a fundamental role in the augmentations, and reductions appear dependent upon actions of hypoxia upon brainstem mechanisms.