Integrated phrenic responses to carotid afferent stimulation in adult rats following perinatal hyperoxia

Chemoafferent degeneration and carotid body hypoplasia following chronic hyperoxia in newborn rats

1. To define the role of environmental oxygen in regulating postnatal maturation of the carotid body afferent pathway, light and electron microscopic methods were used to compare chemoafferent neurone survival and carotid body development in newborn rats reared from birth in normoxia (21 % O2) or chronic hyperoxia (60 % O2). 2. Four weeks of chronic hyperoxia resulted in a significant 41 % decrease in the number of unmyelinated axons in the carotid sinus nerve, compared with age-matched normoxic controls. In contrast, the number of myelinated axons was unaffected by hyperoxic exposure. 3. Chemoafferent neurones, located in the glossopharyngeal petrosal ganglion, already exhibited degenerative changes following 1 week of hyperoxia from birth, indicating that even a relatively short hyperoxic exposure was sufficient to derange normal chemoafferent development. In contrast, no such changes were observed in the vagal nodose ganglion, demonstrating that the effect of high oxygen levels was specific to sensory neurones in the carotid body afferent pathway. Moreover, petrosal ganglion neurones were sensitive to hyperoxic exposure only during the early postnatal period. 4. Chemoafferent degeneration in chronically hyperoxic animals was accompanied by marked hypoplasia of the carotid body. In view of previous findings from our laboratory that chemoafferent neurones require trophic support from the carotid body for survival after birth, we propose that chemoafferent degeneration following chronic hyperoxia is due specifically to the loss of target tissue in the carotid body.

Time domains of the hypoxic ventilatory response

The ventilatory response to hypoxia depends on the pattern and intensity of hypoxic exposure and involves several physiological mechanisms. These mechanisms differ in their effect (facilitation or depression) on different components of ventilation (tidal volume and frequency) and in their time course (seconds to years). Some mechanisms last long enough to affect future ventilatory responses to hypoxia, indicating 'memory' or functional plasticity in the ventilatory control system. A standard terminology is proposed to describe the different time domains of the hypoxic ventilatory response (HVR) and to promote integration of results from different experimental preparations and laboratories. In general, the neurophysiological and neurochemical basis for short time domains of the HVR (seconds and minutes) are understood better than longer time domains (days to years), primarily because short time domains are studied in the laboratory more easily. Understanding the mechanisms for different time domains of the HVR has important implications for both basic and clinical science.

Developmental plasticity of the hypoxic ventilatory response

This paper will describe recent studies concerning the existence of developmental plasticity in the hypoxic ventilatory control system and the locus of the functional impairment following perinatal sensory suppression. Suppression of peripheral arterial chemoreceptor activity was achieved by exposing rats to hyperoxia (60% O2) for the first month of life; all measurements were conducted 2-5 months after the exposure (perinatal treated rats). Hypoxic (but not hypercapnic) ventilatory responses were severely attenuated in awake perinatal treated rats, but not in rats exposed to hyperoxia as adults, indicating that the persistent effect is unique to development and is not the nonspecific result of O2 toxicity. Impairments of the hypoxic ventilatory response due to changes in pulmonary mechanics, gas exchange or central integration of carotid chemoafferent inputs were all ruled out as primary causal factors. However, a persistent impairment of carotid chemotransduction in perinatal treated rats was apparent. These studies suggest that the hypoxic ventilatory response is susceptible to developmental plasticity, and that a carotid chemoreceptor deficit is the primary cause. These findings may have important clinical implications for patients subjected to excessive O2 therapy during neonatal intensive care.