Carotid body growth during chronic postnatal hyperoxia

Blood and urine biomarkers associated with long-term respiratory dysfunction following neonatal hyperoxia exposure: Implications for prematurity and risk of SIDS

Former preterm infants, many of whom required supplemental O₂ support, exhibit sleep disordered breathing and attenuated ventilatory responses to acute hypoxia (HVR) beyond their NICU stay. There is an increasing awareness that early detection of biomarkers in biological fluids may be useful predictors/identifiers of short- and long-term morbidities. In the present study, we identified serotonin (5-HT), dopamine (DA) and hyaluronan (HA) as three potential biomarkers that may be increased by neonatal hyperoxia and tested whether they would be associated with an impaired HVR in a rat model of supplemental O₂ exposure. Neonatal rats (postnatal age (P) 6 days, P6) exposed to hyperoxia (40% FIO₂, 24 h/day between P1-P5 days of age) exhibited an attenuated early (1 min), but not the late (4-5 min) phase of the HVR compared to normoxia control rats; the attenuated early phase HVR was associated with increased levels of DA (urine and serum), 5-HT (platelet poor plasma only, PPP), and HA (serum only). At P21, both the early and late phases of the HVR were attenuated, but serum and urine levels of all 3 biomarkers were similar to age-matched control rats. These data indicate that changes in several serum and/or urine biomarkers (5-HT, DA, and HA) following short-term (days) neonatal hyperoxia can signify long-term (weeks) respiratory control dysfunction. Further studies are needed to determine whether early detection of similar biomarkers could be convenient predictors of increased risk of abnormalities in respiratory control including sleep disordered breathing in former preterm infants who had received prior supplemental O₂ and who might also be at increased risk of SIDS.

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.

Combined effects of intermittent hyperoxia and intermittent hypercapnic hypoxia on respiratory control in neonatal rats

Chronic exposure to intermittent hyperoxia causes abnormal carotid body development and attenuates the hypoxic ventilatory response (HVR) in neonatal rats. We hypothesized that concurrent exposure to intermittent hypercapnic hypoxia would influence this plasticity. Newborn rats were exposed to alternating bouts of hypercapnic hypoxia (10% O₂/6% CO₂) and hyperoxia (30-40% O₂) (5 cycles h^-1, 24 h d^-1) through 13-14 days of age; the experiment was run twice, once in a background of 21% O₂ and once in a background of 30% O₂ (i.e., "relative hyperoxia"). Hyperoxia had only small effects on carotid body development when combined with intermittent hypercapnic hypoxia: the carotid chemoafferent response to hypoxia was reduced, but this did not affect the HVR. In contrast, sustained exposure to 30% O₂ reduced carotid chemoafferent activity and carotid body size which resulted in a blunted HVR. When given alone, chronic intermittent hypercapnic hypoxia increased carotid body size and reduced the hypercapnic ventilatory response but did not affect the HVR. Overall, it appears that intermittent hypercapnic hypoxia counteracted the effects of hyperoxia on the carotid body and prevented developmental plasticity of the HVR.

Carotid body size measured by computed tomographic angiography in individuals born prematurely

PURPOSE

We tested the hypothesis that the carotid bodies would be smaller in individuals born prematurely or exposed to perinatal oxygen therapy when compared individuals born full term that did not receive oxygen therapy.

METHODS

A retrospective chart review was conducted on patients who underwent head/neck computed tomography angiography (CTA) at the Mayo Clinic between 10 and 40 years of age (n = 2503). Patients were identified as premature ( < 38 weeks) or receiving perinatal oxygen therapy by physician completion or billing codes (n = 16 premature and n = 7 receiving oxygen). Widest axial measurements of the carotid body images captured during the CTA were performed.

RESULTS

Carotid body visualization was possible in 43% of patients and 52% of age, sex, and body mass index (BMI)-matched controls but only 17% of juvenile preterm subjects (p = .07). Of the carotid bodies that could be visualized, widest axial measurements of the carotid bodies in individuals born prematurely (n = 7, 34 ± 4 weeks gestation, birth weight: 2460 ± 454 g; average size: 2.5 ± 0.2 cm) or individuals exposed to perinatal oxygen therapy (n = 3, 38 ± 2 weeks gestation, Average size: 2.2 ± 0.1 cm) were not different when compared to controls (2.3 ± 0.2 cm and 2.3 ± 0.2 cm, respectively, p > 0.05).

CONCLUSIONS

Carotid body size, as measured using CTA, is not smaller in adults born prematurely or exposed to perinatal oxygen therapy when compared to sex, age, and BMI-matched controls. However, carotid body visualization was lower in juvenile premature patients. The decreased ability to visualize the carotid bodies in these individuals may be a result of their prematurity.

Ventilatory and chemoreceptor responses to hypercapnia in neonatal rats chronically exposed to moderate hyperoxia

Rats reared in hyperoxia hypoventilate in normoxia and exhibit progressive blunting of the hypoxic ventilatory response, changes which are at least partially attributed to abnormal carotid body development. Since the carotid body also responds to changes in arterial CO₂/pH, we tested the hypothesis that developmental hyperoxia would attenuate the hypercapnic ventilatory response (HCVR) of neonatal rats by blunting peripheral and/or central chemoreceptor responses to hypercapnic challenges. Rats were reared in 21% O₂ (Control) or 60% O₂ (Hyperoxia) until studied at 4, 6-7, or 13-14days of age. Hyperoxia rats had significantly reduced single-unit carotid chemoafferent responses to 15% CO₂ at all ages; CO₂ sensitivity recovered within 7days after return to room air. Hypercapnic responses of CO₂-sensitive neurons of the caudal nucleus tractus solitarius (cNTS) were unaffected by chronic hyperoxia, but there was evidence for a small decrease in neuronal excitability. There was also evidence for augmented excitatory synaptic input to cNTS neurons within brainstem slices. Steady-state ventilatory responses to 4% and 8% CO₂ were unaffected by developmental hyperoxia in all three age groups, but ventilation increased more slowly during the normocapnia-to-hypercapnia transition in 4-day-old Hyperoxia rats. We conclude that developmental hyperoxia impairs carotid body chemosensitivity to hypercapnia, and this may compromise protective ventilatory reflexes during dynamic respiratory challenges in newborn rats. Impaired carotid body function has less of an impact on the HCVR in older rats, potentially reflecting compensatory plasticity within the CNS.

Cardiorespiratory events in preterm infants: etiology and monitoring technologies

Every year, an estimated 15 million infants are born prematurely (<37 weeks gestation) with premature birth rates ranging from 5 to 18% across 184 countries. Although there are a multitude of reasons for this high rate of preterm birth, once birth occurs, a major challenge of infant care includes the stabilization of respiration and oxygenation. Clinical care of this vulnerable infant population continues to improve, yet there are major areas that have yet to be resolved including the identification of optimal respiratory support modalities and oxygen saturation targets, and reduction of associated short- and long-term morbidities. As intermittent hypoxemia is a consequence of immature respiratory control and resultant apnea superimposed upon an immature lung, improvements in clinical care must include a thorough knowledge of premature lung development and pathophysiology that is unique to premature birth. In Part 1 of a two-part review, we summarize early lung development and diagnostic methods for cardiorespiratory monitoring.

Chronic intermittent hyperoxia alters the development of the hypoxic ventilatory response in neonatal rats

Chronic exposure to sustained hyperoxia alters the development of the respiratory control system, but the respiratory effects of chronic intermittent hyperoxia have rarely been investigated. We exposed newborn rats to short, repeated bouts of 30% O2 or 60% O2 (5 bouts h(-1)) for 4-15 days and then assessed their hypoxic ventilatory response (HVR; 10 min at 12% O2) by plethysmography. The HVR tended to be enhanced by intermittent hyperoxia at P4 (early phase of the HVR), but it was significantly reduced at P14-15 (primarily late phase of the HVR) compared to age-matched controls; the HVR recovered when individuals were returned to room air and re-studied as adults. To investigate the role of carotid body function in this plasticity, single-unit carotid chemoafferent activity was recorded in vitro. Intermittent hyperoxia tended to decrease spontaneous action potential frequency under normoxic conditions but, contrary to expectations, hypoxic responses were only reduced at P4 (not at P14) and only in rats exposed to higher O2 levels (i.e., intermittent 60% O2). Rats exposed to intermittent hyperoxia had smaller carotid bodies, and this morphological change may contribute to the blunted HVR. In contrast to rats exposed to intermittent hyperoxia beginning at birth, two weeks of intermittent 60% O2 had no effect on the HVR or carotid body size of rats exposed beginning at P28; therefore, intermittent hyperoxia-induced respiratory plasticity appears to be unique to development. Although both intermittent and sustained hyperoxia alter carotid body development and the HVR of rats, the specific effects and time course of this plasticity differs.

Role of TrkB during the postnatal development of the rat carotid body

Brain-derived neurotrophic factor (BDNF) supports innervation of the carotid body by neurons projecting from the petrosal ganglion. Although carotid body glomus cells also express TrkB, BDNF's high affinity receptor, the role of BDNF in carotid body growth and O2 sensitivity has not been studied. Neonatal rats were treated with the TrkB antagonist K252a (100 μg kg(-1), i.p., b.i.d.) or vehicle on postnatal days P0-P6 and studied on P7. Carotid body volume was decreased by 35% after chronic K252a (P<0.001); a reduction in carotid body size was also elicited using the more selective TrkB antagonist ANA-12 (500 μg kg(-1), i.p., b.i.d.). In contrast, single-unit chemoafferent responses to 5% O2, measured in vitro, were unaffected by chronic K252a administration. Normoxic and hypoxic ventilation, measured by head-body plethysmography, were also normal after chronic K252a administration, but acute K252a administration produced a slower, deeper breathing pattern during the transition into hypoxia. These data suggest that BDNF regulates postnatal carotid body growth but does not influence the development of glomus cell O2 sensitivity.

Challenges and opportunities in developmental integrative physiology

This review explores challenges and opportunities in developmental physiology outlined by a symposium at the 2014 American Physiological Society Intersociety Meeting: Comparative Approaches to Grand Challenges in Physiology. Across animal taxa, adverse embryonic/fetal environmental conditions can alter morphological and physiological phenotypes in juveniles or adults, and capacities for developmental plasticity are common phenomena. Human neonates with body sizes at the extremes of perinatal growth are at an increased risk of adult disease, particularly hypertension and cardiovascular disease. There are many rewarding areas of current and future research in comparative developmental physiology. We present key mechanisms, models, and experimental designs that can be used across taxa to investigate patterns in, and implications of, the development of animal phenotypes. Intraspecific variation in the timing of developmental events can be increased through developmental plasticity (heterokairy), and could provide the raw material for selection to produce heterochrony--an evolutionary change in the timing of developmental events. Epigenetics and critical windows research recognizes that in ovo or fetal development represent a vulnerable period in the life history of an animal, when the developing organism may be unable to actively mitigate environmental perturbations. 'Critical windows' are periods of susceptibility or vulnerability to environmental or maternal challenges, periods when recovery from challenge is possible, and periods when the phenotype or epigenome has been altered. Developmental plasticity may allow survival in an altered environment, but it also has possible long-term consequences for the animal. "Catch-up growth" in humans after the critical perinatal window has closed elicits adult obesity and exacerbates a programmed hypertensive phenotype (one of many examples of "fetal programing"). Grand challenges for developmental physiology include integrating variation in developmental timing within and across generations, applying multiple stressor dosages and stressor exposure at different developmental timepoints, assessment of epigenetic and parental influences, developing new animal models and techniques, and assessing and implementing these designs and models in human health and development.

Postnatal development of eupneic ventilation and metabolism in rats chronically exposed to moderate hyperoxia

Newborn rats chronically exposed to moderate hyperoxia (60% O2) exhibit abnormal respiratory control, including decreased eupneic ventilation. To further characterize this plasticity and explore its proximate mechanisms, rats were exposed to either 21% O2 (Control) or 60% O2 (Hyperoxia) from birth until studied at 3-14 days of age (P3-P14). Normoxic ventilation was reduced in Hyperoxia rats when studied at P3, P4, and P6-7 and this was reflected in diminished arterial O2 saturations; eupneic ventilation spontaneously recovered by P13-14 despite continuous hyperoxia, or within 24h when Hyperoxia rats were returned to room air. Normoxic metabolism was also reduced in Hyperoxia rats but could be increased by raising inspired O2 levels (to 60% O2) or by uncoupling oxidative phosphorylation within the mitochondrion (2,4-dinitrophenol). In contrast, moderate increases in inspired O2 had no effect on sustained ventilation which indicates that hypoventilation can be dissociated from hypometabolism. The ventilatory response to abrupt O2 inhalation was diminished in Hyperoxia rats at P4 and P6-7, consistent with smaller contributions of peripheral chemoreceptors to eupneic ventilation at these ages. Finally, the spontaneous respiratory rhythm generated in isolated brainstem-spinal cord preparations was significantly slower and more variable in P3-4 Hyperoxia rats than in age-matched Controls. We conclude that developmental hyperoxia impairs both peripheral and central components of eupneic ventilatory drive. Although developmental hyperoxia diminishes metabolism as well, this appears to be a regulated hypometabolism and contributes little to the observed changes in ventilation.

Perinatal hyperoxia exposure impairs hypoxia-induced depolarization in rat carotid body glomus cells

Chronic post-natal hyperoxia reduces the hypoxic ventilatory response by reducing the carotid body sensitivity to acute hypoxia as demonstrated by a reduced afferent nerve response, reduced calcium response of carotid body glomus cells and reduced catecholamine secretion in response to acute hypoxia. The present study examined whether hyperoxia alters the electrophysiological characteristics of glomus cells. Rats were treated with hyperoxia for 1 week starting at P1 or P7 and for 2 weeks starting at P1 followed by harvesting and dissociation of their carotid bodies for whole cell, perforated-patch recording. As compared to glomus cells from normoxia animals, hyperoxia treated cells showed a significant reduction in the magnitude of depolarization in response to hypoxia and anoxia, despite little change in the depolarizing response to 20 mM K(+). Resting cell membrane potential in glomus cells from rats exposed to hyperoxia from P1 to P15 and studied at P15 was slightly depolarized compared to other treatment groups and normoxia-treated cells, but conductance normalized to cell size was not different among groups. We conclude that postnatal hyperoxia impairs carotid chemoreceptor hypoxia transduction at a step between hypoxia sensing and membrane depolarization. This occurs without a major change in baseline electrophysiological characteristics, suggesting altered signaling or alterations in the relative abundance of different leak channel isoforms.

Chronic hyperoxia and the development of the carotid body

Preterm infants often experience hyperoxia while receiving supplemental oxygen. Prolonged exposure to hyperoxia during development is associated with pathologies such as bronchopulmonary dysplasia and retinopathy of prematurity. Over the last 25 years, however, experiments with animal models have revealed that moderate exposures to hyperoxia (e.g., 30-60% O(2) for days to weeks) can also have profound effects on the developing respiratory control system that may lead to hypoventilation and diminished responses to acute hypoxia. This plasticity, which is generally inducible only during critical periods of development, has a complex time course that includes both transient and permanent respiratory deficits. Although the molecular mechanisms of hyperoxia-induced plasticity are only beginning to be elucidated, it is clear that many of the respiratory effects are linked to abnormal morphological and functional development of the carotid body, the principal site of arterial O(2) chemoreception for respiratory control. Specifically, developmental hyperoxia reduces carotid body size, decreases the number of chemoafferent neurons, and (at least transiently) diminishes the O(2) sensitivity of individual carotid body glomus cells. Recent evidence suggests that hyperoxia may also directly or indirectly impact development of the central neural control of breathing. Collectively, these findings emphasize the vulnerability of the developing respiratory control system to environmental perturbations.

Carotid chemoreceptor development and neonatal apnea

The premature transition from fetal to neonatal life is accompanied by an immature respiratory neural control system. Most preterm infants exhibit recurrent apnea, resulting in repetitive oscillations in O(2) saturation (intermittent hypoxia, IH). Numerous factors are likely to play a role in the etiology of apnea including inputs from the carotid chemoreceptors. Despite major advances in our understanding of carotid chemoreceptor function in the early neonatal period, however, their contribution to the initiation of an apneic event and its eventual termination are still largely speculative. Recent findings have provided a detailed account of the postnatal changes in the incidence of hypoxemic events associated with apnea, and there is anecdotal evidence for a positive correlation with carotid chemoreceptor maturation. Furthermore, studies on non-human animal models have shown that chronic IH sensitizes the carotid chemoreceptors, which has been proposed to perpetuate the occurrence of apnea. An alternative hypothesis is that sensitization of the carotid chemoreceptors could represent an important protective mechanism to defend against severe hypoxemia. The purpose of this review, therefore, is to discuss how the carotid chemoreceptors may contribute to the initiation and termination of an apneic event in the neonate and the use of xanthine therapy in the prevention of apnea.

Carotid chemoreceptor development in mice

Mice are the most suitable species for understanding genetic aspects of postnatal developments of the carotid body due to the availability of many inbred strains and knockout mice. Our study has shown that the carotid body grows differentially in different mouse strains, indicating the involvement of genes. However, the small size hampers investigating functional development of the carotid body. Hypoxic and/or hyperoxic ventilatory responses have been investigated in newborn mice, but these responses are indirect assessment of the carotid body function. Therefore, we need to develop techniques of measuring carotid chemoreceptor neural activity from young mice. Many studies have taken advantage of the knockout mice to understand chemoreceptor function of the carotid body, but they are not always suitable for addressing postnatal development of the carotid body due to lethality during perinatal periods. Various inbred strains with well-designed experiments will provide useful information regarding genetic mechanisms of the postnatal carotid chemoreceptor development. Also, targeted gene deletion is a critical approach.