Mechanisms underlying a critical period of respiratory development in the rat

Adenosine and γ-aminobutyric acid partially regulate metabolic and ventilatory responses of Damaraland mole-rats to acute hypoxia

Fossorial Damaraland mole-rats (Fukomys damarensis) mount a robust hypoxic metabolic response (HMR) but a blunted hypoxic ventilatory response (HVR) to acute hypoxia. Although these reflex physiological responses have been described previously, the underlying signalling pathways are entirely unknown. Of particular interest are contributions from γ-aminobutyric acid (GABA), which is the primary inhibitory neurotransmitter in the nervous system of most adult mammals, and adenosine, the accumulation of which increases during hypoxia as a breakdown product of ATP. Therefore, we hypothesized that GABAergic and/or adenosinergic signalling contributes to the blunted HVR and robust HMR in Damaraland mole-rats. To test this hypothesis, we injected adult animals with saline alone (controls), or 100 mg kg-1 aminophylline or 1 mg kg-1 bicuculline, to block adenosine or GABAA receptors, respectively. We then used respirometry, plethysmography and thermal RFID probes to non-invasively measure metabolic, ventilator and thermoregulatory responses, respectively, to acute hypoxia (1 h in 5 or 7% O2) in awake and freely behaving animals. We found that bicuculline had relatively minor effects on metabolism and thermoregulation but sensitized ventilation such that the HVR became manifest at 7% instead of 5% O2 and was greater in magnitude. Aminophylline increased metabolic rate, ventilation and body temperature in normoxia, and augmented the HMR and HVR. Taken together, these findings indicate that adenosinergic and GABAergic signalling play important roles in mediating the robust HMR and blunted HVR in Damaraland mole-rats.

Gene regulatory changes underlie developmental plasticity in respiration and aerobic performance in highland deer mice

Phenotypic plasticity can play an important role in the ability of animals to tolerate environmental stress, but the nature and magnitude of plastic responses are often specific to the developmental timing of exposure. Here, we examine changes in gene expression in the diaphragm of highland deer mice (Peromyscus maniculatus) in response to hypoxia exposure at different stages of development. In highland deer mice, developmental plasticity in diaphragm function may mediate changes in several respiratory traits that influence aerobic metabolism and performance under hypoxia. We generated RNAseq data from diaphragm tissue of adult deer mice exposed to (1) life-long hypoxia (before conception to adulthood), (2) post-natal hypoxia (birth to adulthood), (3) adult hypoxia (6-8 weeks only during adulthood) or (4) normoxia. We found five suites of co-regulated genes that are differentially expressed in response to hypoxia, but the patterns of differential expression depend on the developmental timing of exposure. We also identified four transcriptional modules that are associated with important respiratory traits. Many of the genes in these transcriptional modules bear signatures of altitude-related selection, providing an indirect line of evidence that observed changes in gene expression may be adaptive in hypoxic environments. Our results demonstrate the importance of developmental stage in determining the phenotypic response to environmental stressors.

Maternal opioids age-dependently impair neonatal respiratory control networks

Infants exposed to opioids in utero are an increasing clinical population and these infants are often diagnosed with Neonatal Abstinence Syndrome (NAS). Infants with NAS have diverse negative health consequences, including respiratory distress. However, many factors contribute to NAS, confounding the ability to understand how maternal opioids directly impact the neonatal respiratory system. Breathing is controlled centrally by respiratory networks in the brainstem and spinal cord, but the impact of maternal opioids on developing perinatal respiratory networks has not been studied. Using progressively more isolated respiratory network circuitry, we tested the hypothesis that maternal opioids directly impair neonatal central respiratory control networks. Fictive respiratory-related motor activity from isolated central respiratory networks was age-dependently impaired in neonates after maternal opioids within more complete respiratory networks (brainstem and spinal cords), but unaffected in more isolated networks (medullary slices containing the preBötzinger Complex). These deficits were due, in part, to lingering opioids within neonatal respiratory control networks immediately after birth and involved lasting impairments to respiratory pattern. Since opioids are routinely given to infants with NAS to curb withdrawal symptoms and our previous work demonstrated acute blunting of opioid-induced respiratory depression in neonatal breathing, we further tested the responses of isolated networks to exogenous opioids. Isolated respiratory control networks also demonstrated age-dependent blunted responses to exogenous opioids that correlated with changes in opioid receptor expression within a primary respiratory rhythm generating region, the preBötzinger Complex. Thus, maternal opioids age-dependently impair neonatal central respiratory control and responses to exogenous opioids, suggesting central respiratory impairments contribute to neonatal breathing destabilization after maternal opioids and likely contribute to respiratory distress in infants with NAS. These studies represent a significant advancement of our understanding of the complex effects of maternal opioids, even late in gestation, contributing to neonatal breathing deficits, necessary first steps in developing novel therapeutics to support breathing in infants with NAS.

Patterns of Change in the Severity of Airway Obstruction with Robin Sequence in Early Infancy

Previous studies suggest that infants with Robin sequence show a pattern of steady improvement in the severity of airway obstruction, and of their treatment requirements, during infancy.

Methods

Three infants with Robin sequence and severe obstructive sleep apnea were managed with nasal continuous positive airways pressure (CPAP). Multiple measures of airway obstruction were made during infancy, including CPAP pressure evaluations and sleep studies (screening and polysomnography studies). Parameters reported include obstructive apnea-hypopnea index, oxygen desaturation parameters, and CPAP pressures required for effective airway management.

Results

CPAP pressure requirements increased in all three infants during their first weeks of life. Apnea indices on polysomnography did not track with the CPAP pressure requirements. Peak pressure requirements were at 5 and 7 weeks for two patients, with subsequent gradual decline and cessation of therapy CPAP at 39 and 74 weeks, respectively. The third patient had a complicated course, jaw distraction at 17 weeks, and biphasic CPAP pressure requirement (first peak at 3 weeks, but maximum pressure at 74 weeks), with cessation of CPAP at 75 weeks.

Conclusions

The observed pattern of early increases in CPAP pressure requirements for infants with Robin sequence adds to the complexities of managing this disorder. Factors that may lead to this pattern of change in airway obstruction are discussed.

Intra-uterine diazepam exposure decreases the number of catecholaminergic and serotoninergic neurons of neonate rats

Benzodiazepines, such as diazepam (DZP), are used to treat anxiety disorders, and are prescribed to pregnant woman for therapeutic purposes. Concerns regarding their consequences on postnatal development rise as they cross the placenta and interact with the embryo. Occurrence of malformation and behavioral syndromes have been reported for different ages, but little is known about their effects on the brain after exposure during intrauterine life. Thus, we sought to evaluate the effects of intrauterine exposure to DZP on the number of brainstem's catecholaminergic and serotonergic neurons, implicated in respiratory control, in male and female rats on postnatal (P) day 12-13, using immunofluorescence labeling for tyrosine-hydroxylase (TH) and serotonin (5-HT). We observed a reduction in the number of catecholaminergic neurons for males and females. Special attention is given to the reduction in the density of neurons in the A6 region, involved in ventilatory responses to CO₂. Interestingly, only males showed a reduction in the number of serotonergic neurons, while females were not affected. These findings suggest that in utero exposure to DZP results in deleterious neuroanatomical effects on P12-13 rats and raises a note of concern for women clinicians to make more informed choices about the use of anxiolytic treatments during gestation.

Sex- and age-specific respiratory alterations induced by prenatal exposure to the cannabinoid receptor agonist WIN 55,212-2 in rats

BACKGROUND AND PURPOSE

Cannabis legalization has risen in many countries, and its use during pregnancy has increased. The endocannabinoid system is present in the CNS at early stages of embryonic development, and regulates functional brain maturation including areas responsible for respiratory control, data on the influence of external cannabinoids on the development of the respiratory system and possible consequences during postnatal life are limited.

EXPERIMENTAL APPROACH

We evaluated the effects of prenatal exposure to synthetic cannabinoid (WIN 55,212-2 [WIN], 0.5 mg·kg^-1 ·day^-1 ) on the respiratory control system in neonatal (P0, P6-7 and P12-13) and juvenile (P27-28) male and female rats.

KEY RESULTS

WIN administration to pregnant rats interfered sex-specifically with breathing regulation of offspring, promoting a greater sensitivity to CO₂ at all ages in males (except P6-7) and in juvenile females. An altered hypoxic chemoreflex was observed in P0 (hyperventilation) and P6-7 (hypoventilation) males, which was absent in females. Along with breathing alterations, brainstem analysis showed an increase in the number of catecholaminergic neurons and cannabinoid receptor type 1 (CB₁ ) and changes in tissue respiration in the early males. A reduction in pulmonary compliance was observed in juvenile male rats. Preexposure to WIN enhanced spontaneous apnoea and reduced the number of serotoninergic (5-HT) neurons in the raphe magnus nucleus of P0 females.

CONCLUSIONS AND IMPLICATIONS

These data demonstrate that excess stimulation of the endocannabinoid system during gestation has prolonged and sex-specific consequences for the respiratory control system.

Prenatal fluoxetine has long lasting, differential effects on respiratory control in male and female rats

Serotonin (5-HT) is an important modulator of brain networks that control breathing. The selective serotonin reuptake inhibitor fluoxetine (FLX) is the first-line antidepressant drug prescribed during pregnancy. We investigated the effects of prenatal FLX on baseline breathing, ventilatory and metabolic responses to hypercapnia and hypoxia as well as number of brainstem 5-HT and tyrosine hydroxylase (TH) neurons of rats during postnatal development (P0-82). Prenatal FLX exposure of males showed a lower baseline that appeared in juveniles and remained in adulthood, with no sleep-wake state dependency. Prenatal FLX exposure of females did not affect baseline breathing. Juvenile male FLX rats showed increased CO₂ and hypoxic ventilatory responses, normalizing by adulthood. Alterations in juvenile-FLX treated males were associated with greater number of 5-HT neurons in the ROB and RMAG. Adult FLX-exposed males showed greater number of 5-HT neurons in the RPA and TH neurons in the A5, while reduced number of TH neurons in A7. Prenatal FLX exposure of female rats was associated with greater hyperventilation induced by hypercapnia at P0-2 and juveniles whereas P12-14 and adult FLX (NREM sleep) rats showed an attenuation of the hypercapnic hyperventilation.FLX-exposed females had fewer 5-HT neurons in the RPA and reduced TH A6 density at P0-2; and greater number of TH neurons in the A7 at P12-14. These data indicate that prenatal FLX exposure affects the number of neurons of some monoaminergic regions in the brain and results in long lasting, sex specific changes in baseline breathing pattern and ventilatory responses to respiratory challenges.

Adaptations to a hypoxic lifestyle in naked mole-rats

Hypoxia is one of the strongest environmental drivers of cellular and physiological adaptation. Although most mammals are largely intolerant of hypoxia, some specialized species have evolved mitigative strategies to tolerate hypoxic niches. Among the most hypoxia-tolerant mammals are naked mole-rats (Heterocephalus glaber), a eusocial species of subterranean rodent native to eastern Africa. In hypoxia, naked mole-rats maintain consciousness and remain active despite a robust and rapid suppression of metabolic rate, which is mediated by numerous behavioural, physiological and cellular strategies. Conversely, hypoxia-intolerant mammals and most other hypoxia-tolerant mammals cannot achieve the same degree of metabolic savings while staying active in hypoxia and must also increase oxygen supply to tissues, and/or enter torpor. Intriguingly, recent studies suggest that naked mole-rats share many cellular strategies with non-mammalian vertebrate champions of anoxia tolerance, including the use of alternative metabolic end-products and potent pH buffering mechanisms to mitigate cellular acidification due to upregulation of anaerobic metabolic pathways, rapid mitochondrial remodelling to favour increased respiratory efficiency, and systemic shifts in energy prioritization to maintain brain function over that of other tissues. Herein, I discuss what is known regarding adaptations of naked mole-rats to a hypoxic lifestyle, and contrast strategies employed by this species to those of hypoxia-intolerant mammals, closely related African mole-rats, other well-studied hypoxia-tolerant mammals, and non-mammalian vertebrate champions of anoxia tolerance. I also discuss the neotenic theory of hypoxia tolerance – a leading theory that may explain the evolutionary origins of hypoxia tolerance in mammals – and highlight promising but underexplored avenues of hypoxia-related research in this fascinating model organism.

Prenatal Hypoxia Induces Cl– Cotransporters KCC2 and NKCC1 Developmental Abnormality and Disturbs the Influence of GABAA and Glycine Receptors on Fictive Breathing in a Newborn Rat

Prenatal hypoxia is a recognised risk factor for neurodevelopmental disorders associated with both membrane proteins involved in neuron homeostasis, e.g., chloride (Cl–) cotransporters, and alterations in brain neurotransmitter systems, e.g., catecholamines, dopamine, and GABA. Our study aimed to determine whether prenatal hypoxia alters central respiratory drive by disrupting the development of Cl– cotransporters KCC2 and NKCC1. Cl– homeostasis seems critical for the strength and efficiency of inhibition mediated by GABAA and glycine receptors within the respiratory network, and we searched for alterations of GABAergic and glycinergic respiratory influences after prenatal hypoxia. We measured fictive breathing from brainstem in ex vivo preparations during pharmacological blockade of KCC2 and NKCC1 Cl– cotransporters, GABAA, and glycine receptors. We also evaluated the membrane expression of Cl– cotransporters in the brainstem by Western blot and the expression of Cl– cotransporter regulators brain-derived neurotrophic factor (BDNF) and calpain. First, pharmacological experiments showed that prenatal hypoxia altered the regulation of fictive breathing by NKCC1 and KCC2 Cl– cotransporters, GABA/GABAA, and glycin. NKCC1 inhibition decreased fictive breathing at birth in control mice while it decreased at 4 days after birth in pups exposed to prenatal hypoxia. On the other hand, inhibition of KCC2 decreased fictive breathing 4 days after birth in control mice without any change in prenatal hypoxia pups. The GABAergic system appeared to be more effective in prenatal hypoxic pups whereas the glycinergic system increased its effectiveness later. Second, we observed a decrease in the expression of the Cl– cotransporter KCC2, and a decrease with age in NKCC1, as well as an increase in the expression of BDNF and calpain after prenatal hypoxia exposure. Altogether, our data support the idea that prenatal hypoxia alters the functioning of GABAA and glycinergic systems in the respiratory network by disrupting maturation of Cl– homeostasis, thereby contributing to long-term effects by disrupting ventilation.

The critical period: neurochemical and synaptic mechanisms shared by the visual cortex and the brain stem respiratory system

The landmark studies of Wiesel and Hubel in the 1960's initiated a surge of investigations into the critical period of visual cortical development, when abnormal visual experience can alter cortical structures and functions. Most studies focused on the visual cortex, with relatively little attention to subcortical structures. The goal of the present review is to elucidate neurochemical and synaptic mechanisms common to the critical periods of the visual cortex and the brain stem respiratory system in the normal rat. In both regions, the critical period is a time of (i) heightened inhibition; (ii) reduced expression of brain-derived neurotrophic factor (BDNF); and (iii) synaptic imbalance, with heightened inhibition and suppressed excitation. The last two mechanisms are contrary to the conventional premise. Synaptic imbalance renders developing neurons more vulnerable to external stressors. However, the critical period is necessary to enable each system to strengthen its circuitry, adapt to its environment, and transition from immaturity to maturity, when a state of relative synaptic balance is attained. Failure to achieve such a balance leads to neurological disorders.

Distinct Mechanisms Underlie Developmental Plasticity and Adult Acclimation of Thermogenic Capacity in High-Altitude Deer Mice

Developmental plasticity can elicit phenotypic adjustments that help organisms cope with environmental change, but the relationship between developmental plasticity and plasticity in adult life (e.g., acclimation) remains unresolved. We sought to examine developmental plasticity and adult acclimation in response to hypoxia of aerobic capacity (V̇O_2max) for thermogenesis in deer mice (Peromyscus maniculatus) native to high altitude. Deer mice were bred in captivity and exposed to normoxia or one of four hypoxia treatments (12 kPa O₂) across life stages: adult hypoxia (6–8 weeks), post-natal hypoxia (birth to adulthood), life-long hypoxia (before conception to adulthood), and parental hypoxia (mice conceived and raised in normoxia, but parents previously exposed to hypoxia). Hypoxia during perinatal development increased V̇O_2max by a much greater magnitude than adult hypoxia. The amplified effect of developmental hypoxia resulted from physiological plasticity that did not occur with adult hypoxia – namely, increases in lung ventilation and volume. Evolved characteristics of deer mice enabled developmental plasticity, because white-footed mice (P. leucopus; a congener restricted to low altitudes) could not raise pups in hypoxia. Parental hypoxia had no persistent effects on V̇O_2max. Therefore, developmental plasticity can have much stronger phenotypic effects and can manifest from distinct physiological mechanisms from adult acclimation.

Adenosine A2a receptors modulate TrkB receptor-dependent respiratory plasticity in neonatal rats

Neuroplasticity is a fundamental property of the respiratory control system, enabling critical adaptations in breathing to meet the challenges, but little is known whether neonates express neuroplasticity similar to adults. We tested the hypothesis that, similar to adults, tyrosine receptor kinase B (TrkB) or adenosine A2a receptor activation in neonates are independently sufficient to elicit respiratory motor facilitation, and that co-induction of TrkB and A2a receptor-dependent plasticity undermines respiratory motor facilitation. TrkB receptor activation with 7,8-dihydroxyflavone (DHF) in neonatal brainstem-spinal cord preparations induced a long-lasting increase in respiratory motor output in 55 % of preparations, whereas adenosine A2a receptor activation with CGS21680 only sporadically induced respiratory motor plasticity. CGS21680 and DHF co-application prevented DHF-dependent respiratory motor facilitation, whereas co-application of MSX-3 (adenosine A2a receptor antagonist) and DHF more rapidly induced respiratory motor plasticity. Collectively, these data suggest that mechanisms underlying respiratory neuroplasticity may be only partially operational in early neonatal life, and that adenosine A2a receptor activation undermines TrkB-induced respiratory plasticity.

Mortality and ventilatory effects of central serotonin deficiency during postnatal development depend on age but not sex

Serotonin (5-HT) influences brain development and has predominantly excitatory neuromodulatory effects on the neural respiratory control circuitry. Infants that succumb to sudden infant death syndrome (SIDS) have reduced brainstem 5-HT levels and Tryptophan hydroxylase 2 (Tph2). Furthermore, there are age- and sex-dependent risk factors associated with SIDS. Here we utilized our established Dark Agouti transgenic rat lacking central serotonin KO to test the hypotheses that CNS 5-HT deficiency leads to: (1) high mortality in a sex-independent manner, (2) age-dependent alterations in other CNS aminergic systems, and (3) age-dependent impairment of chemoreflexes during post-natal development. KO rat pups showed high neonatal mortality but not in a sex-dependent manner and did not show altered hypoxic or hypercapnic ventilatory chemoreflexes. However, KO rat pups had increased apnea-related metrics during a specific developmental age (P12-16), which were preceded by transient increases in dopaminergic system activity (P7-8). These results support and extend the concept that 5-HT per se is a critical factor in supporting respiratory control during post-natal development.

Chronic, Episodic Nicotine Alters Hypoglossal Motor Neuron Function at a Critical Developmental Time Point in Neonatal Rats

Developmental nicotine exposure (DNE), alters brainstem neurons that control breathing, including hypoglossal motor neurons (XIIMNs), which innervate the tongue. Here, we tested the hypothesis that chronic, episodic DNE (eDNE), which mimics nicotine replacement therapies such as e-cigarettes or nicotine gum, alters the function of nicotinic acetylcholine receptors (nAChRs), XIIMN intrinsic properties, and tongue muscle function in vivo similar to what we have observed with a chronic, sustained exposure model. We delivered nicotine to pregnant Sprague Dawley rats through drinking water and studied pups of either sex in two age groups: postnatal day (P)1-P5 and P10-P12, which encompasses a critical period in brain development. At P1-P5, eDNE was associated with delayed recovery of nAChRs from desensitization; however, there were no changes in the magnitude of desensitization, XIIMN intrinsic properties, or tongue muscle function in vivo. By P10-P12, eDNE XIIMNs had lower peak firing frequencies in response to depolarizing current injection, larger delayed rectifier potassium currents, and continued to exhibit delayed nAChR recovery. Moreover, this age group exhibited a blunted and delayed tongue muscle response to nasal occlusion in vivo, indicating that changes to XIIMN intrinsic properties is an important mechanism behind this effect, as it is not produced by altered nAChR function alone. Together, these results show that eDNE alters XIIMNs and tongue muscle function during a critical period in brain development and that the specific effects of chronic nicotine exposure may be pattern dependent.

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.

Respiratory development in burrowing rodents: Effect of perinatal hypercapnia

Burrowing rodents have a blunted hypercapnic ventilatory response compared to non-burrowing rodents, but semi-fossorial ground squirrels and hamsters are not born with this blunted response when raised in room conditions. This study examined the hypercapnic ventilatory response of rats, hamsters, and ground squirrels raised in burrow-like hypercapnia (∼3 % CO₂) through development (embryonic day 16-18 to postnatal day 30) to determine if chronic hypercapnia exerts any effect on the developing and adult semi-fossorial response. Chronic hypercapnia attenuated the ventilatory response to 5 % CO₂ by 60 % (rats), 150 % (hamsters), and 70 % (squirrels) in newborns when compared to newborns raised in normal conditions. When raised in burrow conditions, squirrels and hamsters reached the blunted adult response ∼8-12 days sooner in development than their room air counterparts, while burrow-reared rats maintained a consistently blunted response until removal from chronic hypercapnia. Our study revealed no lasting effect of chronic hypercarbia on the ventilatory responses to CO₂ in burrowing rodents, but rather a change in the developmental profile such that the blunted adult response was reached earlier in development.

An overview of developmental dysregulation of autonomic control in infants

In this short review, we provide an overview of developmental disorders causing autonomic nervous system dysregulation. We briefly discuss perinatal conditions that adversely impact developmental outcomes including apnea of prematurity, sudden infant death syndrome, and Rett syndrome. We provide a brief clinical description, an overview of known or hypothesized mechanisms for the disorder, and current standard of practice for treatment of each condition. Additionally, we consider preventative measures and complications of these disorders to provide further insight into the pathogenesis of specific autonomic dysregulation in neonates. The goal of this short review is to provide an updated understanding of the impact of autonomic dysregulation on development of brainstem circuits and to briefly highlight promising future treatment options and controversies.

Life-long exposure to hypoxia affects metabolism and respiratory physiology across life stages in high-altitude deer mice (Peromyscus maniculatus)

Hypoxia exposure can have distinct physiological effects between early developmental and adult life stages, but it is unclear how the effects of hypoxia may progress during continuous exposure throughout life. We examined this issue in deer mice (Peromyscus maniculatus) from a population native to high altitude. Mice were bred in captivity in one of three treatment groups: normoxia (controls), life-long hypoxia (∼12 kPa O₂ from conception to adulthood) and parental hypoxia (normoxia from conception to adulthood, but parents previously exposed to hypoxia). Metabolic, thermoregulatory and ventilatory responses to progressive stepwise hypoxia and haematology were then measured at post-natal day (P) 14 and 30 and/or in adulthood. Life-long hypoxia had consistent effects across ages on metabolism, attenuating the declines in O₂ consumption rate (V̇ _O2 ) and body temperature during progressive hypoxia compared with control mice. However, life-long hypoxia had age-specific effects on breathing, blunting the hypoxia-induced increases in air convection requirement (quotient of total ventilation and V̇ _O2 ) at P14 and P30 only, but then shifting breathing pattern towards deeper and/or less frequent breaths at P30 and adulthood. Hypoxia exposure also increased blood-O₂ affinity at P14 and P30, in association with an increase in arterial O₂ saturation in hypoxia at P30. In contrast, parental hypoxia had no effects on metabolism or breathing, but it increased blood-O₂ affinity and decreased red cell haemoglobin content at P14 (but not P30). Therefore, hypoxia exposure has some consistent effects across early life and adulthood, and some other effects that are unique to specific life stages.

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.

Impact of inflammation on developing respiratory control networks: rhythm generation, chemoreception and plasticity

The respiratory control network in the central nervous system undergoes critical developmental events early in life to ensure adequate breathing at birth. There are at least three "critical windows" in development of respiratory control networks: 1) in utero, 2) newborn (postnatal day 0-4 in rodents), and 3) neonatal (P10-13 in rodents, 2-4 months in humans). During these critical windows, developmental processes required for normal maturation of the respiratory control network occur, thereby increasing vulnerability of the network to insults, such as inflammation. Early life inflammation (induced by LPS, chronic intermittent hypoxia, sustained hypoxia, or neonatal maternal separation) acutely impairs respiratory rhythm generation, chemoreception and increases neonatal risk of mortality. These early life impairments are also greater in young males, suggesting sex-specific impairments in respiratory control. Further, neonatal inflammation has a lasting impact on respiratory control by impairing adult respiratory plasticity. This review focuses on how inflammation alters respiratory rhythm generation, chemoreception and plasticity during each of the three critical windows. We also highlight the need for additional mechanistic studies and increased investigation into how glia (such as microglia and astrocytes) play a role in impaired respiratory control after inflammation. Understanding how inflammation during critical windows of development disrupt respiratory control networks is essential for developing better treatments for vulnerable neonates and preventing adult ventilatory control disorders.