Chronic hyperoxia alters the early and late phases of the hypoxic ventilatory response in neonatal rats

Recovery of ventilatory and metabolic responses to hypoxia in neonatal rats after chronic hypoxia

Chronic hypoxia (CH) during postnatal development attenuates the hypoxic ventilatory response (HVR) in mammals, but there are conflicting reports on whether this plasticity is permanent or reversible. This study tested the hypothesis that CH-induced respiratory plasticity is reversible in neonatal rats and investigated whether the initial plasticity or recovery differs between sexes. Rat pups were exposed to 3 d of normobaric CH (12 % O2) beginning shortly after birth. Ventilation and metabolic CO2 production were then measured in normoxia and during an acute hypoxic challenge (12 % O2) immediately following CH and after 1, 4-5, and 7 d in room air. CH pups hyperventilated when returned to normoxia immediately following CH, but normoxic ventilation was similar to age-matched control rats within 7 d after return to room air. The early phase of the HVR (minute 1) was only blunted immediately following the CH exposure, while the late phase of the HVR (minute 15) remained blunted after 1 and 4-5 d in room air; recovery appeared complete by 7 d. However, when normalized to CO2 production, the late phase of the hypoxic response recovered within only 1 d. The initial blunting of the HVR and subsequent recovery were similar in female and male rats. Carotid body responses to hypoxia (in vitro) were also normal in CH pups after approximately one week in room air. Collectively, these data indicate that ventilatory and metabolic responses to hypoxia recover rapidly in both female and male neonatal rats once normoxia is restored following CH.

Neurochemical Plasticity of the Carotid Body

A striking feature of the carotid body (CB) is its remarkable degree of plasticity in a variety of neurotransmitter/modulator systems in response to environmental stimuli, particularly following hypoxic exposure of animals and during ascent to high altitude. Current evidence suggests that acetylcholine and adenosine triphosphate are two major excitatory neurotransmitter candidates in the hypoxic CB, and they may also be involved as co-transmitters in hypoxic signaling. Conversely, dopamine, histamine and nitric oxide have recently been considered inhibitory transmitters/modulators of hypoxic chemosensitivity. It has also been revealed that interactions between excitatory and inhibitory messenger molecules occur during hypoxia. On the other hand, alterations in purinergic neurotransmitter mechanisms have been implicated in ventilatory acclimatization to hypoxia. Chronic hypoxia also induces profound changes in other neurochemical systems within the CB such as the catecholaminergic, peptidergic and nitrergic, which in turn may contribute to increased ventilatory and chemoreceptor responsiveness to hypoxia at high altitude. Taken together, current data suggest that complex interactions among transmitters markedly influence hypoxia-induced transmitter release from the CB. In addition, the expression of a wide variety of growth factors, proinflammatory cytokines and their receptors have been identified in CB parenchymal cells in response to hypoxia and their upregulated expression could mediate the local inflammation and functional alteration of the CB under hypoxic conditions.

A homemade device for simultaneous measurement of pulmonary ventilation and metabolic rate in neonatal rodents

Various in vitro neonatal rodent models have been developed to study the control of breathing, but translation of the information requires a behavioral assay, which has led to the widespread use of plethysmography to measure breathing in awake neonatal rodents. Best practice requires correcting changes in ventilation to the corresponding change in metabolic rate, which is the main driver of pulmonary ventilation. Obtaining measures of both simultaneously is ideal, though technically difficult. Here we describe a simple, inexpensive home-made dual chamber approach for simultaneous measurement of pulmonary ventilation and metabolic rate. We found that the dual chamber provides values for pulmonary ventilation and metabolic rate that compare favorably with existing approaches.

The ventilatory response to hypoxia is blunted in some preterm infants during the second year of life

BACKGROUND

Preterm birth and subsequent neonatal ventilatory treatment disrupts development of the hypoxic ventilatory response (HVR). An attenuated HVR has been identified in preterm neonates, however it is unknown whether the attenuation persists into the second year of life. We investigated the HVR at 12-15 months corrected postnatal age and assessed predictors of a blunted HVR in those born very preterm (<32 weeks gestation).

METHODS

HVR was measured in infants born very preterm. Hypoxia was induced with a three-step reduction in their fraction of inspired oxygen (F_IO₂) from 0.21 to 0.14. Respiratory frequency (f), tidal volume (V _T), minute ventilation (V _E), inspiratory time (t _I), expiratory time (t _E), V _T/t _I, t_I/t _TOT, V _T/t _TOT, area under the low-volume loop and peak tidal expiratory flow (PTEF) were measured at the first and third minute of each F_IO₂. The change in respiratory variables over time was assessed using a repeated measures ANOVA with Greenhouse-Geisser correction. A blunted HVR was defined as a <10% rise in V _E, from normoxia. The relationship between neonatal factors and the magnitude of HVR was assessed using Spearman correlation.

RESULTS

Thirty nine infants born very preterm demonstrated a mean (SD) HVR of 11.4 (10.1)% (increase in V _E) in response to decreasing F_IO₂ from 0.21 to 0.14. However, 17 infants (44%) failed to increase V _E by ≥10% (range -14% to 9%) and were considered to have a blunted response to hypoxia. Males had a smaller HVR than females [ΔV _E (-9.1%; -15.4, -2.8; p = 0.007)].

CONCLUSION

Infants surviving very preterm birth have an attenuated ventilatory response to hypoxia that persists into the second year of life, especially in males.

Perinatal Hypoxemia and Oxygen Sensing

The development of the control of breathing begins in utero and continues postnatally. Fetal breathing movements are needed for establishing connectivity between the lungs and central mechanisms controlling breathing. Maturation of the control of breathing, including the increase of hypoxia chemosensitivity, continues postnatally. Insufficient oxygenation, or hypoxia, is a major stressor that can manifest for different reasons in the fetus and neonate. Though the fetus and neonate have different hypoxia sensing mechanisms and respond differently to acute hypoxia, both responses prevent deviations to respiratory and other developmental processes. Intermittent and chronic hypoxia pose much greater threats to the normal developmental respiratory processes. Gestational intermittent hypoxia, due to maternal sleep-disordered breathing and sleep apnea, increases eupneic breathing and decreases the hypoxic ventilatory response associated with impaired gasping and autoresuscitation postnatally. Chronic fetal hypoxia, due to biologic or environmental (i.e. high-altitude) factors, is implicated in fetal growth restriction and preterm birth causing a decrease in the postnatal hypoxic ventilatory responses with increases in irregular eupneic breathing. Mechanisms driving these changes include delayed chemoreceptor development, catecholaminergic activity, abnormal myelination, increased astrocyte proliferation in the dorsal respiratory group, among others. Long-term high-altitude residents demonstrate favorable adaptations to chronic hypoxia as do their offspring. Neonatal intermittent hypoxia is common among preterm infants due to immature respiratory systems and thus, display a reduced drive to breathe and apneas due to insufficient hypoxic sensitivity. However, ongoing intermittent hypoxia can enhance hypoxic sensitivity causing ventilatory overshoots followed by apnea; the number of apneas is positively correlated with degree of hypoxic sensitivity in preterm infants. Chronic neonatal hypoxia may arise from fetal complications like maternal smoking or from postnatal cardiovascular problems, causing blunting of the hypoxic ventilatory responses throughout at least adolescence due to attenuation of carotid body fibers responses to hypoxia with potential roles of brainstem serotonin, microglia, and inflammation, though these effects depend on the age in which chronic hypoxia initiates. Fetal and neonatal intermittent and chronic hypoxia are implicated in preterm birth and complicate the respiratory system through their direct effects on hypoxia sensing mechanisms and interruptions to the normal developmental processes. Thus, precise regulation of oxygen homeostasis is crucial for normal development of the respiratory control network. © 2021 American Physiological Society. Compr Physiol 11:1653-1677, 2021.

Influence of chronic hyperoxia on the developmental time course of the hypoxic ventilatory response relative to other traits in rats

Newborn mammals exhibit a biphasic hypoxic ventilatory response (HVR) in which an initial increase in ventilation is followed by a decline back toward baseline levels. The magnitude of the secondary decline diminishes with postnatal age, but this transition occurs earlier in rat pups reared in moderate hyperoxia. This pattern is consistent with heterokairy, a form of developmental plasticity in which environmental factors alter the timing of developmental events. The present study investigated whether this plasticity is specific to the HVR or if hyperoxia instead accelerates overall development. Rat pups reared in 60 % O₂ (Hyperoxia) exhibited a less biphasic ventilatory response to 12 % O₂ than pups reared in 21 % O₂ (Control) at 4 days of age (P4) and transitioned to a sustained HVR by P10-11; Control rats exhibited a biphasic HVR at both ages. However, the average ages at which pups attained other key developmental milestones (i.e., fur development at P5, incisor eruption at P9, and eye opening at P15) were similar between treatment groups. Moreover, growth rates and maturation of the metabolic response to cooling were not accelerated, and may have been delayed slightly, relative to Control rats. For example, the capacity for pups to increase their metabolic rate at low ambient temperatures increased with age, but this thermogenic capacity tended to be reduced in Hyperoxia pups at both P4 and P10-11 (i.e., lower CO₂ production rates below the lower critical temperature). Collectively, these data support the conclusion that hyperoxia specifically advances the age at which rat pups exhibit a sustained HVR, altering the relative timing of developmental events rather than compressing the entire period of development.

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.

Ventilatory and carotid body responses to acute hypoxia in rats exposed to chronic hypoxia during the first and second postnatal weeks

Chronic hypoxia (CH) during postnatal development causes a blunted hypoxic ventilatory response (HVR) in neonatal mammals. The magnitude of the HVR generally increases with age, so CH could blunt the HVR by delaying this process. Accordingly, we predicted that CH would have different effects on the respiratory control of neonatal rats if initiated at birth versus initiated later in postnatal development (i.e., after the HVR has had time to mature). Rats had blunted ventilatory and carotid body responses to hypoxia whether CH (12 % O₂) occurred for the first postnatal week (P0 to P7) or second postnatal week (P7 to P14). However, if initiated at P0, CH also caused the HVR to retain the "biphasic" shape characteristic of newborn mammals; CH during the second postnatal week did not result in a biphasic HVR. CH from birth delayed the transition from a biphasic HVR to a sustained HVR until at least P9-11, but the HVR attained a sustained (albeit blunted) phenotype by P13-15. Since delayed maturation of the HVR did not completely explain the blunted HVR, we tested the alternative hypothesis that the blunted HVR was caused by an inflammatory response to CH. Daily administration of the anti-inflammatory drug ibuprofen (4 mg kg^-1, i.p.) did not alter the effects of CH on the HVR. Collectively, these data suggest that CH blunts the HVR in neonatal rats by impairing carotid body responses to hypoxia and by delaying (but not preventing) postnatal maturation of the biphasic HVR. The mechanisms underlying this plasticity require further investigation.

Rodent models of respiratory control and respiratory system development-Clinical significance

The newborn infant's respiratory system must rapidly adapt to extra-uterine life. Neonatal rat and mouse models have been used to investigate early development of respiratory control and reactivity in both health and disease. This review highlights several rodent models of control of breathing and respiratory system development (including pulmonary function), discusses their translational strengths and limitations, and underscores the importance of creating clinically relevant models applicable to the human infant.

Acute and chronic changes in the control of breathing in a rat model of bronchopulmonary dysplasia

Infants born very prematurely (<28 wk gestation) have immature lungs and often require supplemental oxygen. However, long-term hyperoxia exposure can arrest lung development, leading to bronchopulmonary dysplasia (BPD), which increases acute and long-term respiratory morbidity and mortality. The neural mechanisms controlling breathing are highly plastic during development. Whether the ventilatory control system adapts to pulmonary disease associated with hyperoxia exposure in infancy remains unclear. Here, we assessed potential age-dependent adaptations in the control of breathing in an established rat model of BPD associated with hyperoxia. Hyperoxia exposure ( FI O 2 ; 0.9 from 0 to 10 days of life) led to a BPD-like lung phenotype, including sustained reductions in alveolar surface area and counts, and modest increases in airway resistance. Hyperoxia exposure also led to chronic increases in room air and acute hypoxic minute ventilation (V̇e) and age-dependent changes in breath-to-breath variability. Hyperoxia-exposed rats had normal oxygen saturation ( S p O 2 ) in room air but greater reductions in S p O 2 during acute hypoxia (12% O2) that were likely due to lung injury. Moreover, acute ventilatory sensitivity was reduced at P12 to P14. Perinatal hyperoxia led to greater glial fibrillary acidic protein expression and an increase in neuron counts within six of eight or one of eight key brainstem regions, respectively, controlling breathing, suggesting astrocytic expansion. In conclusion, perinatal hyperoxia in rats induced a BPD-like phenotype and age-dependent adaptations in V̇e that may be mediated through changes to the neural architecture of the ventilatory control system. Our results suggest chronically altered ventilatory control in BPD.

Effects of neonatal hyperoxia on the critical period of postnatal development of neurochemical expressions in brain stem respiratory-related nuclei in the rat

We have identified a critical period of respiratory development in rats at postnatal days P12‐13, when inhibitory influence dominates and when the response to hypoxia is at its weakest. This critical period has significant implications for Sudden Infant Death Syndrome (SIDS), the cause of which remains elusive. One of the known risk factors for SIDS is prematurity. A common intervention used in premature infants is hyperoxic therapy, which, if prolonged, can alter the ventilatory response to hypoxia and induce sustained inhibition of lung alveolar growth and pulmonary remodeling. The goal of this study was to test our hypothesis that neonatal hyperoxia from postnatal day (P) 0 to P10 in rat pups perturbs the critical period by altering the normal progression of neurochemical development in brain stem respiratory‐related nuclei. An in‐depth, semiquantitative immunohistochemical study was undertaken at P10 (immediately after hyperoxia and before the critical period), P12 (during the critical period), P14 (immediately after the critical period), and P17 (a week after the cessation of hyperoxia). In agreement with our previous findings, levels of cytochrome oxidase, brain‐derived neurotrophic factor (BDNF), TrkB (BDNF receptor), and several serotonergic proteins (5‐HT_1A and _2A receptors, 5‐HT synthesizing enzyme tryptophan hydroxylase [TPH], and serotonin transporter [SERT]) all fell in several brain stem respiratory‐related nuclei during the critical period (P12) in control animals. However, in hyperoxic animals, these neurochemicals exhibited a significant fall at P14 instead. Thus, neonatal hyperoxia delayed but did not eliminate the critical period of postnatal development in multiple brain stem respiratory‐related nuclei, with little effect on the nonrespiratory cuneate nucleus.

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.

Sensorimotor control of breathing in the mdx mouse model of Duchenne muscular dystrophy

KEY POINTS

Respiratory failure is a leading cause of mortality in Duchenne muscular dystrophy (DMD), but little is known about the control of breathing in DMD and animal models. We show that young (8 weeks of age) mdx mice hypoventilate during basal breathing due to reduced tidal volume. Basal CO₂ production is equivalent in wild-type and mdx mice. We show that carotid bodies from mdx mice have blunted responses to hyperoxia, revealing hypoactivity in normoxia. However, carotid body, ventilatory and metabolic responses to hypoxia are equivalent in wild-type and mdx mice. Our study revealed profound muscle weakness and muscle fibre remodelling in young mdx diaphragm, suggesting severe mechanical disadvantage in mdx mice at an early age. Our novel finding of potentiated neural motor drive to breathe in mdx mice during maximal chemoactivation suggests compensatory neuroplasticity enhancing respiratory motor output to the diaphragm and probably other accessory muscles.

ABSTRACT

Patients with Duchenne muscular dystrophy (DMD) hypoventilate with consequential arterial blood gas derangement relevant to disease progression. Whereas deficits in DMD diaphragm are recognized, there is a paucity of knowledge in respect of the neural control of breathing in dystrophinopathies. We sought to perform an analysis of respiratory control in a model of DMD, the mdx mouse. In 8-week-old male wild-type and mdx mice, ventilation and metabolism, carotid body afferent activity, diaphragm muscle force-generating capacity, and muscle fibre size, distribution and centronucleation were determined. Diaphragm EMG activity and responsiveness to chemostimulation was determined. During normoxia, mdx mice hypoventilated, owing to a reduction in tidal volume. Basal CO₂ production was not different between wild-type and mdx mice. Carotid sinus nerve responses to hyperoxia were blunted in mdx, suggesting hypoactivity. However, carotid body, ventilatory and metabolic responses to hypoxia were equivalent in wild-type and mdx mice. Diaphragm force was severely depressed in mdx mice, with evidence of fibre remodelling and damage. Diaphragm EMG responses to chemoactivation were enhanced in mdx mice. We conclude that there is evidence of chronic hypoventilation in young mdx mice. Diaphragm dysfunction confers mechanical deficiency in mdx resulting in impaired capacity to generate normal tidal volume at rest and decreased absolute ventilation during chemoactivation. Enhanced mdx diaphragm EMG responsiveness suggests compensatory neuroplasticity facilitating respiratory motor output, which may extend to accessory muscles of breathing. Our results may have relevance to emerging treatments for human DMD aiming to preserve ventilatory capacity.

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.

Breathing while altricial: the ontogeny of ventilatory chemosensitivity in red-winged blackbird (Agelaius phoeniceus) nestlings

Altricial bird species, like red-winged blackbirds, hatch at an immature state of functional maturity with limited aerobic capacity and no endothermic capacity. Over the next 10–12 days in the nest, red-winged blackbirds develop increased metabolic capacity before fledging. Although ontogeny of respiration has been described in precocial birds, ontogeny of ventilatory chemosensitivity is unknown in altricial species. Here we examined developmental changes in chemosensitivity of tidal volume (Vt), breathing frequency (ƒ), minute ventilation (V̇e), and whole animal oxygen consumption (V̇o2) from hatching to just before fledging in red-winged blackbirds on days 1, 3, 5, 7, and 9 posthatching (dph) in response to hypercapnia (2 and 4% CO2) and hypoxia (15 and 10% O2). Under control conditions, there was a developmental increase in V̇e with age due to increased Vt. Hypercapnic and hypoxic chemosensitivities were present as early as 1 dph. In response to hypoxia, 1, 3, and 9 dph nestlings increased V̇e at 10% O2, by increasing ƒ with some change in Vt in younger animals. In contrast to early neonatal altricial mammals, the hypoxic response of nestling red-winged blackbirds was not biphasic. In response to hypercapnia, 3 dph nestlings increased V̇e by increasing both ƒ and Vt. From 5 dph on, the hypercapnic increase in V̇e was accounted for by increased Vt and not ƒ. Chemosensitivity to O2 and CO2 matures early in nestling red-winged blackbirds, well before the ability to increase V̇o2 in response to cooling, and thus does not represent a limitation to the development of endothermy.

Early Life Exposure to Chronic Intermittent Hypoxia Primes Increased Susceptibility to Hypoxia-Induced Weakness in Rat Sternohyoid Muscle during Adulthood

Intermittent hypoxia is a feature of apnea of prematurity (AOP), chronic lung disease, and sleep apnea. Despite the clinical relevance, the long-term effects of hypoxic exposure in early life on respiratory control are not well defined. We recently reported that exposure to chronic intermittent hypoxia (CIH) during postnatal development (pCIH) causes upper airway muscle weakness in both sexes, which persists for several weeks. We sought to examine if there are persistent sex-dependent effects of pCIH on respiratory muscle function into adulthood and/or increased susceptibility to re-exposure to CIH in adulthood in animals previously exposed to CIH during postnatal development. We hypothesized that pCIH would cause long-lasting muscle impairment and increased susceptibility to subsequent hypoxia. Within 24 h of delivery, pups and their respective dams were exposed to CIH: 90 s of hypoxia reaching 5% O2 at nadir; once every 5 min, 8 h per day for 3 weeks. Sham groups were exposed to normoxia in parallel. Three groups were studied: sham; pCIH; and pCIH combined with adult CIH (p+aCIH), where a subset of the pCIH-exposed pups were re-exposed to the same CIH paradigm beginning at 13 weeks. Following gas exposures, sternohyoid and diaphragm muscle isometric contractile and endurance properties were examined ex vivo. There was no apparent lasting effect of pCIH on respiratory muscle function in adults. However, in both males and females, re-exposure to CIH in adulthood in pCIH-exposed animals caused sternohyoid (but not diaphragm) weakness. Exposure to this paradigm of CIH in adulthood alone had no effect on muscle function. Persistent susceptibility in pCIH-exposed airway dilator muscle to subsequent hypoxic insult may have implications for the control of airway patency in adult humans exposed to intermittent hypoxic stress during early life.

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.

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.

Hyperoxia-induced developmental plasticity of the hypoxic ventilatory response in neonatal rats: contributions of glutamate-dependent and PDGF-dependent mechanisms

Rats reared in hyperoxia exhibit a sustained (vs. biphasic) hypoxic ventilatory response (HVR) at an earlier age than untreated, Control rats. Given the similarity between the sustained HVR obtained after chronic exposure to developmental hyperoxia and the mature HVR, it was hypothesized that hyperoxia-induced plasticity and normal maturation share common mechanisms such as enhanced glutamate and nitric oxide signaling and diminished platelet-derived growth factor (PDGF) signaling. Rats reared in 21% O2 (Control) or 60% O2 (Hyperoxia) from birth until 4-5 days of age were studied after intraperitoneal injection of drugs targeting these pathways. Hyperoxia rats receiving saline showed a sustained HVR to 12% O2, but blockade of NMDA glutamate receptors (MK-801) restored the biphasic HVR typical of newborn rats. Blockade of PDGF-β receptors (imatinib) had no effect on the pattern of the HVR in Hyperoxia rats, although it attenuated ventilatory depression during the late phase of the HVR in Control rats. Neither nitric oxide synthase inhibitor used in this study (nNOS inhibitor I and l-NAME) altered the pattern of the HVR in Control or Hyperoxia rats. Drug-induced changes in the biphasic HVR were not correlated with changes in metabolic rate. Collectively, these results suggest that developmental hyperoxia hastens the transition from a biphasic to sustained HVR by upregulating glutamate-dependent mechanisms and downregulating PDGF-dependent mechanisms, similar to the changes underlying normal postnatal maturation of the biphasic HVR.

Perinatal hyperoxic exposure reconfigures the central respiratory network contributing to intolerance to anoxia in newborn rat pups

Perinatal exposure to hyperoxia (30-60% O2) alters the respiratory control system via modulation of peripheral arterial chemoreceptor development and function. Furthermore, hyperoxic exposure during the first two postnatal weeks of life can alternatively modulate the different phases of the hypoxic ventilatory response. Given the effects of perinatal hyperoxia, the aims of our study were 1) to determine the effect on survival time in response to lethal anoxic stimuli in rat pups and 2) to characterize the output of the isolated central respiratory network in response to acute hypoxic stimuli. We hypothesized that perinatal hyperoxic exposure would modify the neonatal rat ventilatory response to anoxia by affecting a central component of the respiratory network in addition to the maturation of the carotid body chemoreceptors. We found that animals continuously exposed to 60% oxygen up to age 5 days after parturition (P5) have reduced breathing frequency at baseline and within the first 10 min of a fatal anoxic challenge. Hyperoxic rat pups also have a shortened time to last gasp in response to anoxia that is not associated with lung injury or inflammation. This study is the first to demonstrate that these in vivo findings correlate with reduced phrenic burst frequency from the isolated brainstem ex vivo. Thus hyperoxic exposure reduced the phrenic burst frequency at baseline and in response to ex vivo anoxia. Importantly, our data suggest that perinatal hyperoxia alters ventilation and the response to anoxia at P5 in part by altering the frequency of phrenic bursts generated by the central respiratory network.

Developmental hyperoxia alters CNS mechanisms underlying hypoxic ventilatory depression in neonatal rats

Newborn mammals exhibit a biphasic hypoxic ventilatory response (HVR), but the relative contributions of carotid body-initiated CNS mechanisms versus central hypoxia on ventilatory depression during the late phase of the HVR are not well understood. Neonatal rats (P4-5 or P13-15) were treated with a nonselective P2 purinergic receptor antagonist (pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid, or PPADS; 125mgkg(-1), i.p.) to pharmacologically denervate the peripheral chemoreceptors. At P4-5, rats reared in normoxia showed a progressive decline in ventilation during a 10-min exposure to 12% O2 (21-28% decrease from baseline). No hypoxic ventilatory depression was observed in the older group of neonatal rats (i.e., P13-15), suggesting that the contribution of central hypoxia to hypoxic ventilatory depression diminishes with age. In contrast, rats reared in moderate hyperoxia (60% O2) from birth exhibited no hypoxic ventilatory depression at either age studied. Systemic PPADS had no effect on the ventilatory response to 7% CO2, suggesting that the drug did not cross the blood-brain barrier. These findings indicate that (1) CNS hypoxia depresses ventilation in young, neonatal rats independent of carotid body activation and (2) hyperoxia alters the development of CNS pathways that modulate the late phase of the hypoxic ventilatory response.

Gender considerations in ventilatory and metabolic development in rats: special emphasis on the critical period

In rats, a critical period exists around postnatal day (P) 12-13, when an imbalance between heightened inhibition and suppressed excitation led to a weakened ventilatory and metabolic response to acute hypoxia. An open question was whether the two genders follow the same or different developmental trends throughout the first 3 postnatal weeks and whether the critical period exists in one or both genders. The present large-scale, in-depth ventilatory and metabolic study was undertaken to address this question. Our data indicated that: (1) the ventilatory and metabolic rates in both normoxia and acute hypoxia were comparable between the two genders from P0 to P21; thus, gender was never significant as a main effect; and (2) the age effect was highly significant in all parameters studies for both genders, and both genders exhibited a significantly weakened response to acute hypoxia during the critical period. Thus, the two genders have comparable developmental trends, and the critical period exists in both genders in rats.

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.

Postnatal hyperoxia impairs acute oxygen sensing of rat glomus cells by reduced membrane depolarization

Previous work demonstrated that hyperoxia (30-60% O(2)) exposure in the post-natal period reduces the ventilatory response to acute hypoxia and this impairment may continue considerably beyond the period of hyperoxia exposure. Previous work from our laboratory demonstrated that 1-2 weeks of hyperoxia (60% O(2)) starting between P1 and P14: reduced the single chemoreceptor unit response to hypoxia, reduced the rise in glomus cell calcium caused by acute hypoxia and reduced hypoxia-induced catecholamine release (Donnelly 05, Donnelly 09). The present study asked whether the impairment extended to hypoxia-induced membrane depolarization, an earlier step in the transduction cascade. Perforated patch, whole-cell recordings were obtained from rat glomus cells exposed to hyperoxia from P0-P8 or P8-P15 and age-matched control groups. In both cases, hypoxia-induced membrane depolarization was significantly less in the hyperoxia treated groups compared to controls, while depolarization to 20 mM K(+) was not significantly affected. Resting membrane potential and input resistance were also not different in the hyperoxia treated groups. Whole carotid body quantitative real time PCR showed that TASK-1, TASK-3 and L-type Ca(2+) channel expression was significantly down-regulated at Hyper 8-15 compared to controls. We conclude that 1 week of postnatal hyperoxia during the early and late stage of CB maturation impairs organ function by affecting the coupling between hypoxia and glomus cell depolarization. This may be caused by altered expression of TASK1, TASK3 or L-type Ca(2+) channel gene expression. We speculate that an identification of cellular changes caused by hyperoxia may yield unique insights to the mechanism of oxygen sensing by the carotid bodies.

Carotid body growth during chronic postnatal hyperoxia

Rats reared in hyperoxia have smaller carotid bodies as adults. To study the time course and mechanisms underlying these changes, rats were reared in 60% O(2) from birth and their carotid bodies were harvested at various postnatal ages (P0-P7, P14). The carotid bodies of hyperoxia-reared rats were smaller than those of age-matched controls beginning at P4. In contrast, 7d of 60% O(2) had no effect on carotid body size in rats exposed to hyperoxia as adults. Bromodeoxyuridine (BrdU) and TdT-mediated dUTP nick end labeling (TUNEL) were used to assess cell proliferation and DNA fragmentation at P2, P4, and P6. Hyperoxia reduced the proportion of glomus cells undergoing cell division at P4; although a similar trend was evident at P2, hyperoxia no longer affected cell proliferation by P6. The proportion of TUNEL-positive glomus cells was modestly increased by hyperoxia. We did not detect changes in mRNA expression for proapoptotic (Bax) or antiapoptotic (Bcl-X(L)) genes or transcription factors that regulate cell cycle checkpoints (p53 or p21), although mRNA levels for cyclin B1 and cyclin B2 were reduced. Collectively, these data indicate that hyperoxia primarily attenuates postnatal growth of the carotid body by inhibiting glomus cell proliferation during the first few days of exposure.

Age and sex differences in the ventilatory response to hypoxia and hypercapnia in awake neonatal, pre-pubertal and young adult rats

There is evidence for a "sensitive period" in respiratory development in rats around postnatal age (P) 12-13d. Little is known about sex differences during that time. The purpose of this study was to assess the effect of sex on breathing development, specifically around the "sensitive period". We used whole-body plethysmography to study breathing in normoxic, hypoxic and hypercapnic gases in non-anesthetized male and female neonatal rats from P10 to P15, juvenile (P30) and young adult (P90) rats. Compared to other neonatal ages, P12-13 male rats had significantly lower ventilation during normoxia, hypoxia, and hypercapnia. Compared to age-matched females, P12-13 male rats had lower ventilation in normoxia and hypoxia and a lower O(2) saturation during hypoxia. Circulating estradiol was greater in P12-13 male vs. female rats. Estradiol and ventilatory responses to hypoxia and hypercapnia were negatively correlated in neonatal male, but not female rats. Our results suggest that P10-15 includes a critical developmental period in male but not female rats.

Adenosine A₂a receptors and O₂ sensing in development

Reduced mitochondrial oxidative phosphorylation, via activation of adenylate kinase and the resulting exponential rise in the cellular AMP/ATP ratio, appears to be a critical factor underlying O₂ sensing in many chemoreceptive tissues in mammals. The elevated AMP/ATP ratio, in turn, activates key enzymes that are involved in physiologic adjustments that tend to balance ATP supply and demand. An example is the conversion of AMP to adenosine via 5'-nucleotidase and the resulting activation of adenosine A(₂A) receptors, which are involved in acute oxygen sensing by both carotid bodies and the brain. In fetal sheep, A(₂A) receptors associated with carotid bodies trigger hypoxic cardiovascular chemoreflexes, while central A(₂A) receptors mediate hypoxic inhibition of breathing and rapid eye movements. A(₂A) receptors are also involved in hypoxic regulation of fetal endocrine systems, metabolism, and vascular tone. In developing lambs, A(₂A) receptors play virtually no role in O₂ sensing by the carotid bodies, but brain A(₂A) receptors remain critically involved in the roll-off ventilatory response to hypoxia. In adult mammals, A(₂A) receptors have been implicated in O₂ sensing by carotid glomus cells, while central A(₂A) receptors likely blunt hypoxic hyperventilation. In conclusion, A(₂A) receptors are crucially involved in the transduction mechanisms of O₂ sensing in fetal carotid bodies and brains. Postnatally, central A(₂A) receptors remain key mediators of hypoxic respiratory depression, but they are less critical for O₂ sensing in carotid chemoreceptors, particularly in developing lambs.

Recovery of carotid body O2 sensitivity following chronic postnatal hyperoxia in rats

Chronic postnatal hyperoxia blunts the hypoxic ventilatory response (HVR) in rats, an effect that persists for months after return to normoxia. To determine whether decreased carotid body O(2) sensitivity contributes to this lasting impairment, single-unit chemoafferent nerve and glomus cell calcium responses to hypoxia were recorded from rats reared in 60% O(2) through 7d of age (P7) and then returned to normoxia. Single-unit nerve responses were attenuated by P4 and remained low through P7. After return to normoxia, hypoxic responses were partially recovered within 3d and fully recovered within 7-8d (i.e., at P14-15). Glomus cell calcium responses recovered with a similar time course. Hyperoxia altered carotid body mRNA expression for O(2)-sensitive K(+) channels TASK-1, TASK-3, and BK(Ca), but only TASK-1 mRNA paralleled changes in chemosensitivity (i.e., downregulation by P7, partial recovery by P14). Collectively, these data do not support a role for reduced O(2) sensitivity of individual chemoreceptor cells in long-lasting reduction of the HVR after developmental hyperoxia.

Potentiation of the hypoxic ventilatory response by 1 day of hyperoxia in neonatal rats

The O(2) sensitivity of the neonatal rat carotid body is increased after 1 day in moderate hyperoxia (60% O(2)) (Donnelly et al., 2009). We investigated whether this enhanced peripheral chemosensitivity increases the hypoxic ventilatory response (HVR) and tested the hypothesis that this plasticity is mediated by the superoxide anion. Neonatal rats (7 d old) were injected with saline or MnTMPyP, a superoxide scavenger, and placed into 60% O(2) for 23-28h. Baseline ventilation was reduced and the acute HVR (12% O(2)) was enhanced in hyperoxia-treated rats relative to age-matched controls; MnTMPyP did not block these effects. An additional group of rats was studied after only 30min in 60% O(2). This shorter exposure had no effect on normoxic ventilation or the HVR. We conclude that 1 d, but not 30min, of 60% O(2) augments the HVR of neonatal rats and that production of the superoxide anion does not contribute to this plasticity.

Chronic hyperoxia alters the expression of neurotrophic factors in the carotid body of neonatal rats

Chronic exposure to hyperoxia alters the postnatal development and innervation of the rat carotid body. We hypothesized that this plasticity is related to changes in the expression of neurotrophic factors or related proteins. Rats were reared in 60% O(2) from 24 to 36h prior to birth until studied at 3d of age (P3). Protein levels for brain-derived neurotrophic factor (BDNF) were significantly reduced (-70%) in the P3 carotid body, while protein levels for its receptor, tyrosine kinase B, and for glial cell line-derived neurotrophic factor (GDNF) were unchanged. Transcript levels in the carotid body were downregulated for the GDNF receptor Ret (-34%) and the neuropeptide Vgf (-67%), upregulated for Cbln1 (+205%), and unchanged for Fgf2; protein levels were not quantified for these genes. Immunohistochemical analysis revealed that Vgf and Cbln1 proteins are expressed within the carotid body glomus cells. These data suggest that BDNF, and perhaps other neurotrophic factors, contribute to abnormal carotid body function following perinatal hyperoxia.