Augmentation of hypoxic respiration after brief hyperoxia in the anesthetized cat: Putative function of GABAA neurotransmission

Respiratory plasticity induced by chronic hyperoxia in juvenile and adult rats

Chronic hyperoxia during early postnatal development depresses breathing when neonatal rats are returned to room air and causes long-lasting attenuation of the hypoxic ventilatory response (HVR). In contrast, little is known about the control of breathing of juvenile or adult mammals after chronic exposure to moderate hyperoxia later in life. Therefore, Sprague-Dawley rats were exposed to 60 % O2 for 7 days (juveniles) or for 4 and 14 days (adults) and ventilation was measured by whole-body plethysmography immediately after the exposure or following a longer period of recovery in room air. Hyperoxia-treated juvenile rats appeared to hypoventilate when returned to room air (11-13 % lower ventilation and CO2 convection requirement relative to age-matched controls), but chronic hyperoxia did not alter normoxic ventilation in adult rats. In contrast, pre-treatment with chronic hyperoxia augmented the HVR in both juvenile rats (+41 %) and adult rats (+28-50 %). The hypercapnic ventilatory response (7 % CO2) also tended to be augmented in adult rats after 14 days of hyperoxia, but this effect was not significant after accounting for variation in metabolic rate (i.e, CO2 convection requirement). These findings confirm that chronic hyperoxia elicits age-specific respiratory plasticity in rats. These age-dependent differences are not caused by a lack of plasticity in adult-exposed rats; rather, there are qualitative differences in the plasticity that is expressed after chronic hyperoxia in neonates, juveniles, and adults as well as differences in its persistence.

Effects of Perinatal Hyperoxia on Breathing

Air-breathing animals do not experience hyperoxia (inspired O₂ > 21%) in nature, but preterm and full-term infants often experience hyperoxia/hyperoxemia in clinical settings. This article focuses on the effects of normobaric hyperoxia during the perinatal period on breathing in humans and other mammals, with an emphasis on the neural control of breathing during hyperoxia, after return to normoxia, and in response to subsequent hypoxic and hypercapnic challenges. Acute hyperoxia typically evokes an immediate ventilatory depression that is often, but not always, followed by hyperpnea. The hypoxic ventilatory response (HVR) is enhanced by brief periods of hyperoxia in adult mammals, but the limited data available suggest that this may not be the case for newborns. Chronic exposure to mild-to-moderate levels of hyperoxia (e.g., 30-60% O₂ for several days to a few weeks) elicits several changes in breathing in nonhuman animals, some of which are unique to perinatal exposures (i.e., developmental plasticity). Examples of this developmental plasticity include hypoventilation after return to normoxia and long-lasting attenuation of the HVR. Although both peripheral and CNS mechanisms are implicated in hyperoxia-induced plasticity, it is particularly clear that perinatal hyperoxia affects carotid body development. Some of these effects may be transient (e.g., decreased O₂ sensitivity of carotid body glomus cells) while others may be permanent (e.g., carotid body hypoplasia, loss of chemoafferent neurons). Whether the hyperoxic exposures routinely experienced by human infants in clinical settings are sufficient to alter respiratory control development remains an open question and requires further research. © 2020 American Physiological Society. Compr Physiol 10:597-636, 2020.