Peripheral arterial chemoreceptors and sudden infant death syndrome

Vglut2-based glutamatergic signaling in central noradrenergic neurons is dispensable for normal breathing and chemosensory reflexes

Central noradrenergic (NA) neurons are key constituents of the respiratory homeostatic network. NA dysfunction is implicated in several developmental respiratory disorders including Congenital Central Hyperventilation Syndrome (CCHS), Sudden Infant Death Syndrome (SIDS), and Rett Syndrome. The current unchallenged paradigm in the field, supported by multiple studies, is that glutamate co-transmission in subsets of central NA neurons plays a role in breathing control. If true, NA-glutamate co-transmission may also be mechanistically important in respiratory disorders. However, the requirement of NA-derived glutamate in breathing has not been directly tested and the extent of glutamate co-transmission in the central NA system remains uncharacterized. Therefore, we fully characterized the cumulative fate maps and acute adult expression patterns of all three vesicular glutamate transporters (Slc17a7 (Vglut1), Slc17a6 (Vglut2), and Slc17a8 (Vglut3)) in NA neurons, identifying a novel, dynamic expression pattern for Vglut2 and an undescribed co-expression domain for Vglut3 in the NA system. In contrast to our initial hypothesis that NA-derived glutamate is required to breathing, our functional studies showed that loss of Vglut2 throughout the NA system failed to alter breathing or metabolism under room air, hypercapnia, or hypoxia in unrestrained and unanesthetized mice. These data demonstrate that Vglut2-based glutamatergic signaling within the central NA system is not required for normal baseline breathing and hypercapnic, hypoxic chemosensory reflexes. These outcomes challenge the current understanding of central NA neurons in the control of breathing and suggests that glutamate may not be a critical target to understand NA neuron dysfunction in respiratory diseases.

Vglut2-based glutamatergic signaling in central noradrenergic neurons is dispensable for normal breathing and chemosensory reflexes

Central noradrenergic (NA) neurons are key constituents of the respiratory homeostatic network. NA dysfunction is implicated in several developmental respiratory disorders including Congenital Central Hyperventilation Syndrome (CCHS), Sudden Infant Death Syndrome (SIDS) and Rett Syndrome. The current unchallenged paradigm in the field, supported by multiple studies, is that glutamate co-transmission in subsets of central NA neurons plays a role in breathing control. If true, NA-glutamate co-transmission may also be mechanistically important in respiratory disorders. However, the requirement of NA-derived glutamate in breathing has not been directly tested and the extent of glutamate co-transmission in the central NA system remains uncharacterized. Therefore, we fully characterized the cumulative fate maps and acute adult expression patterns of all three Vesicular Glutamate Transporters ( Slc17a7 (Vglut1), Slc17a6 (Vglut2), and Slc17a8 (Vglut3)) in NA neurons, identifying a novel, dynamic expression pattern for Vglut2 and an undescribed co-expression domain for Vglut3 in the NA system. In contrast to our initial hypothesis that NA derived glutamate is required to breathing, our functional studies showed that loss of Vglut2 throughout the NA system failed to alter breathing or metabolism under room air, hypercapnia, or hypoxia in unrestrained and unanesthetized mice. These data demonstrate that Vglut2-based glutamatergic signaling within the central NA system is not required for normal baseline breathing and hypercapnic, hypoxic chemosensory reflexes. These outcomes challenge the current understanding of central NA neurons in the control of breathing and suggests that glutamate may not be a critical target to understand NA neuron dysfunction in respiratory diseases.

Sociodemographic Factors Affecting Loss to Follow-Up After Newborn Hearing Screening: A Systematic Review and Meta-Analysis

OBJECTIVE

Universal newborn hearing screening (NBHS) has been widely implemented as a part of early hearing detection and intervention (EHDI) programs worldwide. Even with excellent provider knowledge and screening rates, many infants do not receive definitive hearing testing or intervention after initial screening. The objective of this study was to identify sociodemographic factors contributing to loss of follow-up.

DATA SOURCES

PubMed, Scopus, and CINAHL.

REVIEW METHODS

Per Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines, the databases were searched from the date of inception through December 28, 2021. Studies containing sociodemographic information on patients who were referred to NBHS were included. Meta-analysis of odds ratios (ORs) was performed comparing rates of sociodemographic variables between patients adherent and nonadherent to follow-up.

RESULTS

A total of 169,238 infants from 19 studies were included. Low birth weight (OR 1.6 [95% confidence interval, CI 1.2-2.2, p < .001), racial minority (OR 1.4 [95% CI 1.2-1.6], p < .001), rural residence (OR 1.5 [95% CI 1.1-1.9], p = .005), lack of insurance (OR 1 [95% CI 1.4-2.5], p < .001), and public or state insurance (OR 1.7 [95% CI 1.2-4.2], p = .008) were associated with missed follow-up after referred NBHS. Associated maternal factors included low maternal education (OR 1.8 [95% CI 1.6-2.0], p < .001), young maternal age (OR 1.5 [95% CI 1.5-1.6], p < .001), unmarried maternal status (OR 1.5 [95% CI 1.1-1.9], p = .003), and current or former maternal smoking status (OR 1.8 [95% CI 1.4-2.2], p < .001).

CONCLUSION

Both infant and maternal sociodemographic factors influence follow-up compliance after referred NBHS. Focused efforts should be made by medical providers and policymakers to address these factors to ensure appropriate newborn hearing care and interventions are achieved.

Carotid Body Dysfunction and Mechanisms of Disease

Emerging evidence shows that the carotid body (CB) dysfunction is implicated in various physiological and pathophysiological conditions. It has been revealed that the CB structure and neurochemical profile alter in certain human sympathetic-related and cardiometabolic diseases. Specifically, a tiny CB with a decrease of glomus cells and their dense-cored vesicles has been seen in subjects with sleep disordered breathing such as sudden infant death syndrome and obstructive sleep apnea patients and people with congenital central hypoventilation syndrome. Moreover, the CB degranulation is accompanied by significantly elevated levels of catecholamines and proinflammatory cytokines in such patients. The intermittent hypoxia stimulates the CB, eliciting augmented chemoreflex drive and enhanced cardiorespiratory and sympathetic responses. High CB excitability due to blood flow restrictions, oxidative stress, alterations in neurotransmitter gases and disruptions of local mediators is also observed in congestive heart failure conditions. On the other hand, the morpho-chemical changes in hypertension include an increase in the CB volume due to vasodilation, altered transmitter phenotype of chemoreceptor cells and elevated production of neurotrophic factors. Accordingly, in both humans and animal models CB denervation prevents the breathing instability and lowers blood pressure. Knowledge of the morphofunctional aspects of the CB, a better understanding of its role in disease and recent advances in human CB translational research would contribute to the development of new therapeutic strategies.

Apnea of prematurity and sudden infant death syndrome

Apnea is a frequent occurrence in prematurity and its prevalence in the most severely preterm population is indicative of an immature respiratory neural control system. Preterm infants are also at increased risk of Sudden Infant Death Syndrome (SIDS), which has been associated with similar respiratory neural control dysfunction seen in prematurity. Generally, abnormalities in both central and peripheral mechanisms of respiratory control are thought to be key underlying features of abnormal respiratory system development. Numerous factors contribute to the etiology of apnea and respiratory control dysfunction including the environment (e.g., substance use/misuse), sex, genetics, a vulnerable neonate, and various underlying comorbidities. However, there are major gaps in our understanding of both normal and abnormal respiratory control system development, which highlights the need for continued research using novel and innovative methods.

Respiratory disturbances and high risk of sudden death in the neonatal connexin-36 knockout mouse

Neural circuits at the brainstem involved in the central generation of the motor patterns of respiration and cardiorespiratory chemoreflexes organize as cell assemblies connected by chemical and electrical synapses. However, the role played by the electrical connectivity mainly mediated by connexin36 (Cx36), which expression reaches peak value during the postnatal period, is still unknown. To address this issue, we analyzed here the respiratory phenotype of a mouse strain devoid constitutively of Cx36 at P14. Male Cx36-knockout mice at rest showed respiratory instability of variable degree, including a periodic Cheyne-Stokes breathing. Moreover, mice lacking Cx36 exhibited exacerbated chemoreflexes to normoxic and hypoxic hypercapnia characterized by a stronger inspiratory/expiratory coupling due to an increased sensitivity to CO2 . Deletion of Cx36 also impaired the generation of the recurrent episodes of transient bradycardia (ETBs) evoked during hypercapnic chemoreflexes; these EBTs constituted a powerful mechanism of cardiorespiratory coupling capable of improving alveolar gaseous exchange under hypoxic hypercapnia conditions. Approximately half of the homo- and heterozygous Cx36KO, but none WT, mice succumbed by respiratory arrest when submitted to hypoxia-hypercapnia, the principal exogenous stressor causing sudden infant death syndrome (SIDS). The early suppression of EBTs, which worsened arterial O2  saturation, and the generation of a paroxysmal generalized clonic-tonic activity, which provoked the transition from eupneic to gasping respiration, were the critical events causing sudden death in the Cx36KO mice. These results indicate that Cx36 expression plays a pivotal role in respiratory control, cardiorespiratory coordination, and protection against SIDS at the postnatal period.

Laryngeal Chemoreflex in Health and Disease: A Review

The larynx plays a key role in airway protection via the laryngeal chemoreflex (LCR). This involuntary reflex can be evoked when hazardous substances activate mucosal receptors, which send signals to be processed within the brainstem. Although the LCR is meant to be protective, the reflex can become hyperstimulated, even to benign stimuli, which can result in pathological disorders, such as chronic cough and inducible laryngeal obstruction. In this review, we will outline the mechanism of the LCR and its associated pathological disorders.

Role of the Kölliker-Fuse nucleus in cardiovascular responses to hypoxia and baroreceptor activation in anesthetized rats

Introduction: Parabrachial Kölliker-Fuse (KF) complex, located in dorsolateral part of the pons, is involved in the respiratory control, however, its role in the baroreflex and chemoreflex responses has not been established yet. This study was performed to test the contribution of the KF to chemoreflex and baroreflex and the effect of microinjection of a reversible synaptic blocker (Cocl₂) into the KF in urethane anesthetized rats. Methods: Activation of chemoreflex was induced by systemic hypoxia caused by N2 breathing for 30 seconds "hypoxic- hypoxia methods" and baroreflex was evoked by intravenous injection (i.v.) of phenylephrine (Phe, 20 µg /kg/0.05-0.1 mL). N2 induced generalized vasodilatation followed by tachycardia reflex and Phe evoked vasoconstriction followed by bradycardia. Results: Microinjection of Cocl₂ (5 mM/100 nL/side) produced no significant changes in the Phe-induced hypertension and bradycardia, whereas the cardiovascular effect of N2 was significantly attenuated by the injection of CoCl₂ to the KF. Conclusion: The KF played no significant role in the baroreflex, but could account for cardiovascular chemoreflex in urethane anesthetized rats.

Premature birth, homeostatic plasticity and respiratory consequences of inflammation

Infants who are born premature can have persistent apnea beyond term gestation, reemergence of apnea associated with inflammation during infancy, increased risk of sudden unexplained death, and sleep disorder breathing during infancy and childhood. The autonomic nervous system, particularly the central neural networks that control breathing and peripheral and central chemoreceptors and mechanoreceptors that modulate the activity of the central respiratory network, are rapidly developing during the last trimester (22-37 weeks gestation) of fetal life. With advances in neonatology, in well-resourced, developed countries, infants born as young as 23 weeks gestation can survive. Thus, a substantial part of maturation of central and peripheral systems that control breathing occurs ex-utero in infants born at the limit of viability. The balance of excitatory and inhibitory influences dictates the ultimate output from the central respiratory network. We propose in this review that simply being born early in the last trimester can trigger homeostatic plasticity within the respiratory network tipping the balance toward inhibition that persists in infancy. We discuss the intersection of premature birth, homeostatic plasticity and biological mechanisms leading to respiratory depression during inflammation in former premature infants.

Respiratory rhythm generation, hypoxia, and oxidative stress-Implications for development

Encountered in a number of clinical conditions, repeated hypoxia/reoxygenation during the neonatal period can pose both a threat to immediate survival as well as a diminished quality of living later in life. This review focuses on our current understanding of central respiratory rhythm generation and the role that hypoxia and reoxygenation play in influencing rhythmogenesis. Here, we examine the stereotypical response of the inspiratory rhythm from the preBötzinger complex (preBötC), basic neuronal mechanisms that support rhythm generation during the peri-hypoxic interval, and the physiological consequences of inspiratory network responsivity to hypoxia and reoxygenation, acute and chronic intermittent hypoxia, and oxidative stress. These topics are examined in the context of Sudden Infant Death Syndrome, apneas of prematurity, and neonatal abstinence syndrome.

Central and peripheral chemoreceptors in sudden infant death syndrome

The pathogenesis of sudden infant death syndrome (SIDS) has been ascribed to an underlying biological vulnerability to stressors during a critical period of development. This paper reviews the main data in the literature supporting the role of central (e.g. retrotrapezoid nucleus, serotoninergic raphe nuclei, locus coeruleus, orexinergic neurons, ventral medullary surface, solitary tract nucleus) and peripheral (e.g. carotid body) chemoreceptors in the pathogenesis of SIDS. Clinical and experimental studies indicate that central and peripheral chemoreceptors undergo critical development during the initial postnatal period, consistent with the age range of SIDS (<1 year). Most of the risk factors for SIDS (gender, genetic factors, prematurity, hypoxic/hyperoxic stimuli, inflammation, perinatal exposure to cigarette smoke and/or substance abuse) may structurally and functionally affect the developmental plasticity of central and peripheral chemoreceptors, strongly suggesting the involvement of these structures in the pathogenesis of SIDS. Morphometric and neurochemical changes have been found in the carotid body and brainstem respiratory chemoreceptors of SIDS victims, together with functional signs of chemoreception impairment in some clinical studies. However, the methodological problems of SIDS research will have to be addressed in the future, requiring large and highly standardized case series. Up-to-date autopsy protocols should be produced, involving substantial, and exhaustive sampling of all potentially involved structures (including peripheral arterial chemoreceptors). Morphometric approaches should include unbiased stereological methods with three-dimensional probes. Prospective clinical studies addressing functional tests and risk factors (including genetic traits) would probably be the gold standard, allowing markers of intrinsic or acquired vulnerability to be properly identified.

α2 Receptors in the lateral parabrachial nucleus generates the pressor response of the cardiovascular chemoreflex, effects of GABAA receptor

The lateral parabrachial nucleus (LPBN) is a pontine area involved in cardiovascular chemoreflex. This study was performed to find the effects of reversible synaptic blockade of the LPBN on the chemoreflex responses, and to find the roles of GABA_A receptor and α₂-adenoreceptor (α₂-AR) in chemoreflex. It also aimed to seek possible interaction between GABA and noradrenergic systems of the LPBN in urethane-anesthetized male rats. Cardiovascular chemoreflex was activated by intravenous injection of potassium cyanide (KCN, 80 μg/kg). The cardiovascular responses of chemoreflex were evaluated before (control), 5 and 15 min after microinjection of each drug (100 nl) into the LPBN. Microinjections of cobalt chloride (5 mM), a reversible synaptic blocker, into the LPBN greatly attenuated the chemoreflex pressor and bradycardic responses indicating that the LPBN plays a main role in chemoreflex. Local injection of yohimbine (10 nmol), an α₂-AR antagonist, attenuated the pressor response with no effect on bradycardic response, suggesting that α₂-adrenoreceptors are involved in producing the pressor response of the chemoreflex. Microinjection of bicuculline methiodide (BMI, 100 pmol), a GABA_A antagonist, into the LPBN augmented the pressor response and attenuated the bradycardic response, indicating that GABA inhibits the sympathetic output to the heart and vasculature. Sequential injection of yohimbine and BMI had no significant effect on the pressor response but attenuated the bradycardia. In conclusion, the LPBN is essential for the chemoreflex responses. The pressor response of the chemoreflex, at least partly, is produced by α₂- adenoreceptors. GABA in the LPBN inhibits the cardiovascular system. Finally, there is no interaction between GABAergic and adrenergic neurons of the LPBN in producing the cardiovascular chemoreflex.

A hypothalamo-midbrain-medullary pathway involved in the inhibition of the respiratory chemoreflex response induced by potassium cyanide in rodents

Recent studies have demonstrated that a mild stimulation of the dorsomedian nucleus of the hypothalamus (DMH), a defense area, induces the inhibition of the carotid chemoreflex tachypnea. DMH activation reduces the cardiac chemoreflex response via the dorsolateral part of the periaqueductal grey matter (dlPAG) and serotonin receptors (5-HT₃ subtype) in the nucleus tractus solitarius (NTS). The objectives of this study were to assess whether dlPAG and subsequent NTS 5-HT₃ receptors are involved in chemoreflex tachypnea inhibition during mild activation of the DMH. For this purpose, peripheral chemoreflex was activated with potassium cyanide (KCN, 40 μg/rat, i.v.) during electrical and chemical minimal supra-threshold (mild) stimulation of the dlPAG or DMH. In both situations, changes in respiratory frequency (RF) following KCN administration were reduced. Moreover, pharmacological blockade of the dlPAG prevented DMH-induced KCN tachypnea inhibition. Activation of NTS 5-HT₃ receptors also reduced chemoreflex tachypnea in a dose-dependent manner. In addition, blockade of NTS 5-HT₃ receptors with granisetron (2.5 but not 1.25 mM), or the use of mice lacking the 5-HT_3a receptor (5-HT_3a KO), prevented dlPAG-induced KCN reductions in RF. A respiratory hypothalamo-midbrain-medullary pathway (HMM) therefore plays a crucial role in the inhibition of the hyperventilatory response to carotid chemoreflex.

Purinergic and Cholinergic Drugs Mediate Hyperventilation in Zebrafish: Evidence from a Novel Chemical Screen

A rapid test to identify drugs that affect autonomic responses to hypoxia holds therapeutic and ecologic value. The zebrafish (Danio rerio) is a convenient animal model for investigating peripheral O₂ chemoreceptors and respiratory reflexes in vertebrates; however, the neurotransmitters and receptors involved in this process are not adequately defined. The goals of the present study were to demonstrate purinergic and cholinergic control of the hyperventilatory response to hypoxia in zebrafish, and to develop a procedure for screening of neurochemicals that affect respiration. Zebrafish larvae were screened in multi-well plates for sensitivity to the cholinergic receptor agonist, nicotine, and antagonist, atropine; and to the purinergic receptor antagonists, suramin and A-317491. Nicotine increased ventilation frequency (f_V) maximally at 100 μM (EC₅₀ = 24.5 μM). Hypoxia elevated f_V from 93.8 to 145.3 breaths min^-1. Atropine reduced the hypoxic response only at 100 μM. Suramin and A-317491 maximally reduced f_V at 50 μM (EC₅₀ = 30.4 and 10.8 μM) and abolished the hyperventilatory response to hypoxia. Purinergic P2X3 receptors were identified in neurons and O₂-chemosensory neuroepithelial cells of the gills using immunohistochemistry and confocal microscopy. These studies suggest a role for purinergic and nicotinic receptors in O₂ sensing in fish and implicate ATP and acetylcholine in excitatory neurotransmission, as in the mammalian carotid body. We demonstrate a rapid approach for screening neuroactive chemicals in zebrafish with implications for respiratory medicine and carotid body disease in humans; as well as for preservation of aquatic ecosystems.

Chemoreception and asphyxia-induced arousal

Arousal protects against the adverse and potentially fatal effects of asphyxia during sleep. Asphyxia stimulates the carotid bodies and central chemoreceptors but the sequence of events leading to arousal is uncertain. In this review, the theoretical mechanisms leading to arousal from sleep are briefly summarized and the issue of whether central respiratory chemoreceptors (CRCs) or other types of CO2-responsive CNS neurons contribute to asphyxia-induced arousal is discussed. We focus on the role of the retrotrapezoid nucleus, the raphe and the locus coeruleus and emphasize the anatomical and neurophysiological evidence which suggests that these putative central chemoreceptors could contribute to arousal independently of their effects on breathing. Finally, we describe recent attempts to test the contribution of specific brainstem pathways to asphyxia-induced arousal using optogenetic and other tools and the possible contribution of a group of hypoxia-sensitive brainstem neurons (the C1 cells) to breathing and arousal.

Apnea of prematurity--perfect storm

With increased survival of preterm infants as young as 23 weeks gestation, maintaining adequate respiration and corresponding oxygenation represents a clinical challenge in this unique patient cohort. Respiratory instability characterized by apnea and periodic breathing occurs in premature infants because of immature development of the respiratory network. While short respiratory pauses and apnea may be of minimal consequence if oxygenation is maintained, they can be problematic if accompanied by chronic intermittent hypoxemia. Underdevelopment of the lung and the resultant lung injury that occurs in this population concurrent with respiratory instability creates the perfect storm leading to frequent episodes of profound and recurrent hypoxemia. Chronic intermittent hypoxemia contributes to the immediate and long term co-morbidities that occur in this population. In this review we discuss the pathophysiology leading to the perfect storm, diagnostic assessment of breathing instability in this unique population and therapeutic interventions that aim to stabilize breathing without contributing to tissue injury.

Inflammatory and immunomodulatory mechanisms in the carotid body

Evidence is available about the role of inflammatory/immunological factors in the physiology and plasticity of the carotid body, with potential clinical implications in obstructive sleep apnea syndrome and sudden infant death syndrome. In humans, lymphomonocytic aggregations (chronic carotid glomitis) have been reported in aging and opiate addiction. Glomus cells produce prostaglandin E2 and the cytokines interleukin 1β, interleukin 6 and TNF-α, with corresponding receptors. These factors modulate glomus cell excitability, catecholamine release and/or chemoreceptor discharge. The above cytokines are up-regulated in chronic sustained or intermittent hypoxia, and prevention of these changes, with ibuprofen or dexamethasone, may modulate hypoxia-induced changes in carotid body chemosensitivity. The main transcription factors considered to be involved are NF-kB and HIFs. Circulating immunogens (lipopolysaccharide) and cytokines may also affect peripheral arterial chemoreception, with the carotid body exerting an immunosensing function.

Perinatal nicotine/smoking exposure and carotid chemoreceptors during development

Tobacco smoking is still a common habit during pregnancy and is the most important preventable cause of many adverse perinatal outcomes. Prenatal smoking exposure can produce direct actions of nicotine in the fetus with the disruption of body and brain development, and actions on the maternal-fetal unit by causing repeated episodes of hypoxia and exposure to many toxic smoke products (such as carbon monoxide). Specifically, nicotine through binding to nicotinic acetylcholine receptors have ubiquitous effects and can affect carotid chemoreception development through structural, functional and neuroregulatory alterations of the neural circuits involved in the chemoafferent pathway, as well as by interfering with the postnatal resetting of the carotid bodies. Reduced carotid body chemosensitivity and tonic activity have thus been reported by the majority of the human and animal studies. This review focuses on the effects of perinatal exposure to tobacco smoke and nicotine on carotid chemoreceptor function during the developmental period. A description of the effects of smoking and nicotine on the control of breathing related to carotid body activity, and of the possible physiopathological mechanisms at the origin of these disturbances is presented.

Postnatal maturation of breathing stability and loop gain: the role of carotid chemoreceptor development

Any general model of respiratory control must explain a puzzling array of breathing patterns that are observed during the course of a lifetime. Particular challenges are to understand why periodic breathing is rarely seen in the first few days after birth, reaches a peak at 2-4 weeks postnatal age, and disappears by 6 months, why it is prevalent in preterm infants, and why it reappears in adults at altitude or with heart failure. In this review we use the concept of loop gain to obtain quantitative insight into the genesis of unstable breathing patterns with a particular focus on how changes in carotid body function could underlie the age-related dependence of periodic breathing.

The carotid body in Sudden Infant Death Syndrome

The aim of the present study is to provide a review of cytochemical, clinical and experimental data indicating disruption of perinatal carotid body maturation as one of the possible mechanisms underlying SIDS pathogenesis. SIDS victims have been reported to show alterations in respiratory regulation which may partly be ascribed to peripheral arterial chemoreceptors. Carotid body findings in SIDS victims, although not entirely confirmed by other authors, have included reductions in glomic tissue volume and cytoplamic granules of type I cells, changes in cytological composition (higher percentages of progenitor and type II cells) and increases in dopamine and noradrenaline contents. Prematurity and environmental factors, such as exposure to tobacco smoke, substances of abuse, hyperoxia and continuous or intermittent hypoxia, increase the risk of SIDS and are known to affect carotid body functional and structural maturation adversely, supporting a role for peripheral arterial chemoreceptors in SIDS.

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.

Development of ventilatory control in infants

Most abnormalities of ventilatory control in infants are due to immaturity or abnormal development of ventilatory control. This includes a broad range, from rare disorders like congenital central hypoventilation syndrome to common problems such as apnoea of prematurity. Development of the ventilatory control system, including central respiratory rhythmogenesis and central and peripheral chemoreception, begins early in gestation and continues for weeks or months after birth. Development of the neural components of central rhythmogenesis and their highly complex interconnectivity results from complex, timing-sensitive interactions between patterning and other genes, transcription factors and neurotrophic factors. At birth, nearly all aspects of ventilatory control remain immature, especially in preterm infants; and postnatal maturation can be altered by hypoxia, toxins and other stressors. Clinical care may be greatly enhanced by increased awareness of ventilatory control maturation and related disorders.

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

Chronic hyperoxia during the first 1-4 postnatal weeks attenuates the hypoxic ventilatory response (HVR) subsequently measured in adult rats. Rather than focusing on this long-lasting plasticity, the present study considered the influence of hyperoxia on respiratory control during the neonatal period. Sprague-Dawley rats were born and raised in 60% O2 until studied at postnatal ages (P) of 4, 6-7, or 13-14 days. Ventilation and metabolism were measured in normoxia (21% O2) and acute hypoxia (12% O2) using head-body plethysmography and respirometry, respectively. Compared with age-matched rats raised in room air, the major findings were 1) diminished pulmonary ventilation and metabolic O2 consumption in normoxia at P4 and P6-7; 2) decreased breathing stability during normoxia; 3) attenuation of the early phase of the HVR at P6-7 and P13-14; and 4) a sustained increase in ventilation during hypoxia (vs. the normal biphasic HVR) at all ages studied. Attenuation of the early HVR likely reflects progressive impairment of peripheral arterial chemoreceptors while expression of a sustained HVR in neonates before P7 suggests that hyperoxia also induces plasticity within the central nervous system. Together, these results suggest a complex interaction between inhibitory and excitatory effects of hyperoxia on the developing respiratory control system.

Kangaroo care with twins: a case study in which one infant did not respond as expected

PURPOSE

The purposes were to relate responses (including unacceptable ones) of twins to shared kangaroo care (KC), to provide explanations for the unexpected responses, and to offer suggestions for safe nursing practice.

DESIGN

A descriptive, evaluative case study was conducted. Both twins received KC while their vital signs and maternal breast temperatures were manually recorded every 30 seconds. Descriptive statistics were computed.

SAMPLE

Identical twins, born to a 19-year-old African-American primigravada, were 34 3/7 weeks postconceptional age with weights of 1,760 and 1,480 g, respectively, when tested. Preterm labor resulted in spontaneous vaginal birth at 30 weeks gestation.

MAIN OUTCOME VARIABLES

Infant heart and respiratory rates, oxygen saturations, abdominal temperatures, and maternal breast temperatures.

RESULTS

Infant A's vital signs exceeded acceptable clinical limits during shared KC; vital signs returned to normal range once Infant A was returned to the incubator. Infant B's vital signs approximated clinically acceptable ranges throughout the session. Breast temperatures did not differ. Individuality mandates vigilant assessment of infant responses to shared KC.

Time course of alterations in pre- and post-synaptic chemoreceptor function during developmental hyperoxia

Postnatal hyperoxia exposure reduces the carotid body response to acute hypoxia and produces a long-lasting impairment of the ventilatory response to hypoxia. The present work investigated the time course of pre- and post-synaptic alterations following exposure to hyperoxia (Fl(O2) = 0.6) for 1, 3, 5, 8 and 14 days (d) starting at postnatal day 7 (P7) as compared to age-matched controls. Hyperoxia exposure for 1d enhanced the nerve response and glomus cell calcium response to acute hypoxia, but exposure for 3-5d caused a significant reduction in both. Hypoxia-induced catecholamine release and nerve conduction velocity were significantly decreased by 5d hyperoxia. We conclude that hyperoxia exerts pre-synaptic (glomus cell calcium and secretory responses) and post-synaptic (afferent nerve excitability) actions to initially enhance and then reduce the chemoreceptor response to acute hypoxia. The parallel changes in glomus cell calcium response and nerve response suggest causality between the two and that environmental hyperoxia can affect the coupling between acute hypoxia and glomus cell calcium regulation.

Hypoxic-ischemic changes in SIDS brains as demonstrated by a reduction in MAP2-reactive neurons

Sudden infant death syndrome (SIDS) is characterized by a lack of any known morphological or functional organ changes that could explain the lethal process. In the present study we investigated the hypothesis of an association between hypoxic/ischemic injury and SIDS deaths. In a previous study, we could demonstrate by quantitative immunohistochemistry a distinct drop in microtubule-associated protein (MAP2) reactivity in neurons of adult, human brains secondary to acute hypoxic-ischemic injuries. Here we applied the same method on sections of the frontal cortex and hippocampus of 41 brains of infants younger than 1 year of age. For each brain area 100 selected neurons were evaluated for their MAP2 reactivity in the different layers of the frontal cortex and in the different segments of the hippocampus. Three groups were compared: (1) SIDS victims (n = 17), (2) infants with hypoxia/ischemia (control group one; n = 14), (3) infants without hypoxic/ischemic injury (control group two; n = 10). The SIDS group and hypoxic/ischemic group exhibited a general reduction in the number of MAP2 reactive neurons in comparison with the non-hypoxic/ischemic injury group. The SIDS group also had a significantly lower (P < 0.05) number of reactive neurons in the CA2 and CA3 areas of the hippocampus than did control group two. No difference was detected between the SIDS group and control group one. The SIDS brains were thus found to display hypoxic/ischemic features without however providing evidence as to the cause of the oxygen reduction.

Histaminergic and dopaminergic traits in the human carotid body

Carotid body (CB) chemoreceptors are the main sensors detecting systemic hypoxia. Studies in animals revealed that dopamine and histamine may serve as transmitters between the chemoreceptor cells and the afferent nerve. To gain insight whether histamine and dopamine could play a role in the human CB and thus be important for the understanding of breathing disorders, we have investigated the chemosensory traits in human CBs from nine subjects of different ages obtained at autopsy. Immunohistochemistry revealed expression of histidine decarboxylase, vesicular monoamine transporter 2, histamine receptors 1 and 3 in virtually all chemosensory cells within the glomeruli of different ages. By contrast, catecholaminergic traits (tyrosine hydroxylase and vesicular monoamine transporter 1) were only detected in a subset of CB chemosensory cells at each age group while dopamine D2 receptors were expressed in the great majority of them. Our data suggest that histamine along with catecholamines may serve as transmitters between chemoreceptor cells and the afferent nerve in humans as well.

Developmental hyperoxia attenuates the hypoxic ventilatory response in Japanese quail (Coturnix japonica)

Early life experiences can influence development of the respiratory control system. We hypothesized that chronic hyperoxia (60% O(2)) during development would attenuate the hypoxic ventilatory response (HVR) of Japanese quail (Coturnix japonica), similar to the effects of developmental hyperoxia in mammals. Quail were exposed to hyperoxia during prenatal development, during postnatal development, or during both prenatal and postnatal development (for approximately 2 or 4 weeks). HVR (11% O(2)) was subsequently assessed in adults (>6 weeks old) via barometric plethysmography and compared to quail raised in normoxia (i.e., control). The HVR of quail exposed to hyperoxia both prenatally and postnatally was reduced 50-60% compared to control quail whereas postnatally exposed quail exhibited normal HVR. The effects of prenatal hyperoxia on HVR were equivocal and depended on how HVR was expressed. We conclude that developmental exposure to 60% O(2) attenuates the HVR in quail and that the critical period for this plasticity encompasses the late prenatal and early postnatal periods.

Cardiovascular responses to peripheral chemoreflex activation and comparison of different methods to evaluate baroreflex gain in conscious mice using telemetry

Peripheral chemoreceptors located in the carotid bodies are the primary sensors of systemic hypoxia. Although the pattern of responses elicited by peripheral chemoreceptor activation is well established in rats, lambs, and rabbits, the cardiovascular responses to peripheral chemoreflex activation in conscious mice have not been delineated. Here we report that stimulation of peripheral chemoreceptors by potassium cyanide (KCN) in conscious mice elicits a unique biphasic response in blood pressure that is characterized by an initial and robust rise followed by a decrease in blood pressure, which is accompanied by a marked reduction in heart rate. The depressor and bradycardic responses to KCN were abolished by muscarinic receptor blockade with atropine, and the pressor response was abolished by alpha-adrenergic receptor blockade with prazosin, suggesting that vagal and sympathetic drive to the heart and sympathetic drive to the vasculature mediate these cardiovascular responses. These studies characterized the chemoreflex in conscious mice and established the reliability of using them for studying hypoxia-related diseases such as obstructive sleep apnea. In another series of experiments, two methods for analyzing baroreflex sensitivity were compared: the classical pharmacological approach using phenylephrine and sodium nitroprusside (i.e., the Oxford technique) or the sequence method for analyzing spontaneous baroreflex activity. Our findings indicate that both methods are reliable, and the sequence method certainly has its benefits as a predictive tool in the context of long-term noninvasive studies using telemetry. However, for absolute determination of baroreflex function, analysis of spontaneous baroreflex activity should be complemented by the classical pharmacological method.

Long-term effects of the perinatal environment on respiratory control

The respiratory control system exhibits considerable plasticity, similar to other regions of the nervous system. Plasticity is a persistent change in system behavior triggered by experiences such as changes in neural activity, hypoxia, and/or disease/injury. Although plasticity is observed in animals of all ages, some forms of plasticity appear to be unique to development (i.e., “developmental plasticity”). Developmental plasticity is an alteration in respiratory control induced by experiences during “critical” developmental periods; similar experiences outside the critical period will have little or no lasting effect. Thus complementary experiments on both mature and developing animals are generally needed to verify that the observed plasticity is unique to development. Frequently studied models of developmental plasticity in respiratory control include developmental manipulations of respiratory gas concentrations (O2and CO2). Environmental factors not specifically associated with breathing may also trigger developmental plasticity, however, including psychological stress or chemicals associated with maternal habits (e.g., nicotine, cocaine). Despite rapid advances in describing models of developmental plasticity in breathing, our understanding of fundamental mechanisms giving rise to such plasticity is poor; mechanistic studies of developmental plasticity are of considerable importance. Developmental plasticity may enable organisms to “fine tune” their phenotype to optimize the performance of this critical homeostatic regulatory system. On the other hand, developmental plasticity could also increase the risk of disease later in life. Future directions for studies concerning the mechanisms and functional implications of developmental plasticity in respiratory motor control are discussed.

Prenatal nicotine alters maturation of breathing and neural circuits regulating respiratory control

While perinatal nicotine effects on ventilation have been widely investigated, the prenatal impact of nicotine treatment during gestation on both breathing and neural circuits involved in respiratory control remains unknown. We examined the effects of nicotine, from embryonic day 5 (E5) to E20, on baseline ventilation, the two hypoxic ventilatory response components and in vivo tyrosine hydroxylase (TH) activity in carotid bodies and brainstem areas, assessed at postnatal day 7 (P7), P11 and P21. In pups prenatally exposed to nicotine, baseline ventilation and hypoxic ventilatory response were increased at P7 (+48%) and P11 (+46%), with increased tidal volume (p<0.05). Hypoxia blunted frequency response at P7 and revealed unstable ventilation at P11. In carotid bodies, TH activity increased by 20% at P7 and decreased by 48% at P11 (p<0.05). In most brainstem areas it was reduced by 20-33% until P11. Changes were resolved by P21. Prenatal nicotine led to postnatal ventilatory sequelae, partly resulting from impaired maturation of peripheral chemoreceptors and brainstem integrative sites.

Respiratory plasticity after perinatal hyperoxia is not prevented by antioxidant supplementation

Perinatal hyperoxia attenuates the hypoxic ventilatory response in rats by altering development of the carotid body and its chemoafferent neurons. In this study, we tested the hypothesis that hyperoxia elicits this plasticity through the increased production of reactive oxygen species (ROS). Rats were born and raised in 60% O(2) for the first two postnatal weeks while treated with one of two antioxidants: vitamin E (via milk from mothers whose diet was enriched with 1000 IU vitamin E kg(-1)) or a superoxide dismutase mimetic, manganese(III) tetrakis (1-methyl-4-pyridyl) porphyrin pentachloride (MnTMPyP; via daily intraperitoneal injection of 5-10 mg kg(-1)); rats were subsequently raised in room air until studied as adults. Peripheral chemoreflexes, assessed by carotid sinus nerve responses to cyanide, asphyxia, anoxia and isocapnic hypoxia (vitamin E experiments) or by hypoxic ventilatory responses (MnTMPyP experiments), were reduced after perinatal hyperoxia compared to those of normoxia-reared controls (all P<0.01); antioxidant treatment had no effect on these responses. Similarly, the carotid bodies of hyperoxia-reared rats were only one-third the volume of carotid bodies from normoxia-reared controls (P <0.001), regardless of antioxidant treatment. Protein carbonyl concentrations in the blood plasma, measured as an indicator of oxidative stress, were not increased in neonatal rats (2 and 8 days of age) exposed to 60% O(2) from birth. Collectively, these data do not support the hypothesis that perinatal hyperoxia impairs peripheral chemoreceptor development through ROS-mediated oxygen toxicity.

Ventilatory chemosensitivity of the 1-day-old chicken hatchling after embryonic hypoxia

We investigated the effects of sustained embryonic hypoxia on the neonatal ventilatory chemosensitivity. White Leghorn chicken eggs were incubated at 38°C either in 21% O2 throughout incubation (normoxia, Nx) or in 15% O2 from embryonic day 5 (hypoxia, Hx), hatching time included. Hx embryos hatched ∼11 h later than Nx, with similar body weights. Measurements of gaseous metabolism (oxygen consumption, V̇o2) and pulmonary ventilation (V̇e) were conducted either within the first 8 h (early) or later hours (late) of the first posthatching day. In resting conditions, Hx had similar V̇o2 and body temperature (Tb) and slightly higher V̇e and ventilatory equivalent (V̇e/V̇o2) than Nx. Ventilatory chemosensitivity was evaluated from the degree of hyperpnea (increase in V̇e) and of hyperventilation (increase in V̇e/V̇o2) during acute hypoxia (15 and 10% O2, 20 min each) and acute hypercapnia (2 and 4% CO2, 20 min each). The chemosensitivity differed between the early and late hours, and at either time the responses to hypoxia and hypercapnia were less in Hx than in Nx because of a lower increase in V̇e and a lower hypoxic hypometabolism. In a second group of Nx and Hx hatchlings, the V̇e response to 10% O2 was tested in the same hatchlings at the early and late hours. The results confirmed the lower hypoxic chemosensitivity of Hx. We conclude that hypoxic incubation affected the development of respiratory control, resulting in a blunted ventilatory chemosensitivity.