On the existence of a central respiratory oxygen sensor

The hypoxic respiratory response of the pre-Bötzinger complex

Since the discovery of the pre-Bötzinger Complex (preBötC) as a crucial region for generating the main respiratory rhythm, our understanding of its cellular and molecular aspects has rapidly increased within the last few decades. It is now apparent that preBötC is a highly flexible neuronal network that reconfigures state-dependently to produce the most appropriate respiratory output in response to various metabolic challenges, such as hypoxia. However, the responses of the preBötC to hypoxic conditions can be varied based on the intensity, pattern, and duration of the hypoxic challenge. This review discusses the preBötC response to hypoxic challenges at the cellular and network level. Particularly, the involvement of preBötC in the classical biphasic response of the respiratory network to acute hypoxia is illuminated. Furthermore, the article discusses the functional and structural changes of preBötC neurons following intermittent and sustained hypoxic challenges. Accumulating evidence shows that the preBötC neural circuits undergo substantial changes following hypoxia and contribute to several types of the respiratory system's hypoxic ventilatory responses.

Targeted TGF-βR2 Knockdown in the Retrotrapezoid Nucleus Mitigates Respiratory Dysfunction and Cognitive Decline in a Mouse Model of Cerebral Amyloid Angiopathy with and without Stroke

Introduction

Cerebral amyloid angiopathy (CAA) is characterized by the deposition of amyloid-beta peptides within cerebral blood vessels, leading to neurovascular complications. Ischemic strokes result from acute disruptions in cerebral blood flow, triggering metabolic disturbances and neurodegeneration. Both conditions often co-occur and are associated with respiratory dysfunctions. The retrotrapezoid nucleus (RTN), which is crucial for CO2 sensing and breathing regulation in the brainstem, may play a key role in breathing disorders seen in these conditions. This study aims to investigate the role of Transforming Growth Factor Beta (TGF-β) signaling in the RTN on respiratory and cognitive functions in CAA, both with and without concurrent ischemic stroke.

Methods

Adult male Tg-SwDI (CAA model) mice and C57BL/6 wild-type controls underwent stereotaxic injections of lentivirus targeting TGF-β2R2 in the RTN. Stroke was induced by middle cerebral artery occlusion using a monofilament. Respiratory functions were assessed using whole-body plethysmography, while cognitive functions were evaluated through the Barnes Maze and Novel Object Recognition Test (NORT). Immunohistochemical analysis was conducted to measure TGF-βR2 and GFAP expressions in the RTN.

Results

CAA mice exhibited significant respiratory dysfunctions, including reduced respiratory rates and increased apnea frequency, as well as impaired cognitive performance. TGF-βR2 knockdown in the RTN improved respiratory functions and cognitive outcomes in CAA mice. In CAA mice with concurrent stroke, TGF-βR2 knockdown similarly enhanced respiratory and cognitive functions. Immunohistochemistry confirmed reduced TGF-βR2 and GFAP expressions in the RTN following knockdown.

Conclusions

Our findings demonstrate that increased TGF-β signaling and gliosis in the RTN contribute to respiratory and cognitive dysfunctions in CAA and CAA with stroke. Targeting TGF-βR2 signaling in the RTN offers a promising therapeutic strategy to mitigate these impairments. This study is the first to report a causal link between brainstem gliosis and both respiratory and cognitive dysfunctions in CAA and stroke models.

Mechanisms underpinning sympathoexcitation in hypoxia

Sympathoexcitation is a hallmark of hypoxic exposure, occurring acutely, as well as persisting in acclimatised lowland populations and with generational exposure in highland native populations of the Andean and Tibetan plateaus. The mechanisms mediating altitude sympathoexcitation are multifactorial, involving alterations in both peripheral autonomic reflexes and central neural pathways, and are dependent on the duration of exposure. Initially, hypoxia-induced sympathoexcitation appears to be an adaptive response, primarily mediated by regulatory reflex mechanisms concerned with preserving systemic and cerebral tissue O2 delivery and maintaining arterial blood pressure. However, as exposure continues, sympathoexcitation is further augmented above that observed with acute exposure, despite acclimatisation processes that restore arterial oxygen content (C a O 2 ${C_{{\mathrm{a}}{{\mathrm{O}}_{\mathrm{2}}}}}$ ). Under these conditions, sympathoexcitation may become maladaptive, giving rise to reduced vascular reactivity and mildly elevated blood pressure. Importantly, current evidence indicates the peripheral chemoreflex does not play a significant role in the augmentation of sympathoexcitation during altitude acclimatisation, although methodological limitations may underestimate its true contribution. Instead, processes that provide no obvious survival benefit in hypoxia appear to contribute, including elevated pulmonary arterial pressure. Nocturnal periodic breathing is also a potential mechanism contributing to altitude sympathoexcitation, although experimental studies are required. Despite recent advancements within the field, several areas remain unexplored, including the mechanisms responsible for the apparent normalisation of muscle sympathetic nerve activity during intermediate hypoxic exposures, the mechanisms accounting for persistent sympathoexcitation following descent from altitude and consideration of whether there are sex-based differences in sympathetic regulation at altitude.

Hypoxia evokes a sequence of raphe-pontomedullary network operations for inspiratory drive amplification and gasping

Hypoxia can trigger a sequence of breathing-related behaviors, from tachypnea to apneusis to apnea and gasping, an autoresuscitative behavior that, via large tidal volumes and altered intrathoracic pressure, can enhance coronary perfusion, carotid blood flow, and sympathetic activity, and thereby coordinate cardiac and respiratory functions. We tested the hypothesis that hypoxia-evoked gasps are amplified through a disinhibitory microcircuit within the inspiratory neuron chain and a distributed efference copy mechanism that generates coordinated gasp-like discharges concurrently in other circuits of the raphe-pontomedullary respiratory network. Data were obtained from 6 decerebrate, vagotomized, neuromuscularly-blocked, and artificially ventilated adult cats. Arterial blood pressure, phrenic nerve activity, end-tidal CO2, and other parameters were monitored. Hypoxia was produced by ventilation with a gas mixture of 5% O2 in nitrogen (N2). Neuron spike trains were recorded at multiple pontomedullary sites simultaneously and evaluated for firing rate modulations and short-time scale correlations indicative of functional connectivity. Experimental perturbations evoked reconfiguration of raphe-pontomedullary circuits during tachypnea, apneusis and augmented bursts, apnea, and gasping. The functional connectivity, altered firing rates, efference copy of gasp drive, and coordinated step increments in blood pressure reported here support a distributed brain stem network model for amplification and broadcasting of inspiratory drive during autoresuscitative gasping that begins with a reduction in inhibition by expiratory neurons and an initial loss of inspiratory drive during hypoxic apnea.

Astrocytes produce nitric oxide via nitrite reduction in mitochondria to regulate cerebral blood flow during brain hypoxia

During hypoxia, increases in cerebral blood flow maintain brain oxygen delivery. Here, we describe a mechanism of brain oxygen sensing that mediates the dilation of intraparenchymal cerebral blood vessels in response to reductions in oxygen supply. In vitro and in vivo experiments conducted in rodent models show that during hypoxia, cortical astrocytes produce the potent vasodilator nitric oxide (NO) via nitrite reduction in mitochondria. Inhibition of mitochondrial respiration mimics, but also occludes, the effect of hypoxia on NO production in astrocytes. Astrocytes display high expression of the molybdenum-cofactor-containing mitochondrial enzyme sulfite oxidase, which can catalyze nitrite reduction in hypoxia. Replacement of molybdenum with tungsten or knockdown of sulfite oxidase expression in astrocytes blocks hypoxia-induced NO production by these glial cells and reduces the cerebrovascular response to hypoxia. These data identify astrocyte mitochondria as brain oxygen sensors that regulate cerebral blood flow during hypoxia via release of nitric oxide.

Commonalities and differences in carotid body dysfunction in hypertension and heart failure

Carotid body pathophysiology is associated with many cardiovascular-respiratory-metabolic diseases. This pathophysiology reflects both hyper-sensitivity and hyper-tonicity. From both animal models and human patients, evidence indicates that amelioration of this pathophysiological signalling improves disease states such as a lowering of blood pressure in hypertension, a reduction of breathing disturbances with improved cardiac function in heart failure (HF) and a re-balancing of autonomic activity with lowered sympathetic discharge. Given this, we have reviewed the mechanisms of carotid body hyper-sensitivity and hyper-tonicity across disease models asking whether there is uniqueness related to specific disease states. Our analysis indicates some commonalities and some potential differences, although not all mechanisms have been fully explored across all disease models. One potential commonality is that of hypoperfusion of the carotid body across hypertension and HF, where the excessive sympathetic drive may reduce blood flow in both models and, in addition, lowered cardiac output in HF may potentiate the hypoperfusion state of the carotid body. Other mechanisms are explored that focus on neurotransmitter and signalling pathways intrinsic to the carotid body (e.g. ATP, carbon monoxide) as well as extrinsic molecules carried in the blood (e.g. leptin); there are also transcription factors found in the carotid body endothelium that modulate its activity (Krüppel-like factor 2). The evidence to date fully supports that a better understanding of the mechanisms of carotid body pathophysiology is a fruitful strategy for informing potential new treatment strategies for many cardiovascular, respiratory and metabolic diseases, and this is highly relevant clinically.

Mechanisms underlying the health benefits of intermittent hypoxia conditioning

Intermittent hypoxia (IH) is commonly associated with pathological conditions, particularly obstructive sleep apnoea. However, IH is also increasingly used to enhance health and performance and is emerging as a potent non-pharmacological intervention against numerous diseases. Whether IH is detrimental or beneficial for health is largely determined by the intensity, duration, number and frequency of the hypoxic exposures and by the specific responses they engender. Adaptive responses to hypoxia protect from future hypoxic or ischaemic insults, improve cellular resilience and functions, and boost mental and physical performance. The cellular and systemic mechanisms producing these benefits are highly complex, and the failure of different components can shift long-term adaptation to maladaptation and the development of pathologies. Rather than discussing in detail the well-characterized individual responses and adaptations to IH, we here aim to summarize and integrate hypoxia-activated mechanisms into a holistic picture of the body's adaptive responses to hypoxia and specifically IH, and demonstrate how these mechanisms might be mobilized for their health benefits while minimizing the risks of hypoxia exposure.

The hypoxic ventilatory response and hypoxic burden are predictors of the magnitude of ventilatory long-term facilitation in humans

Mild intermittent hypoxia initiates progressive augmentation (PA) and ventilatory long-term facilitation (vLTF) in humans. The magnitude of these forms of plasticity might be influenced by anthropometric and physiological variables, as well as protocol elements. However, the impact of many of these variables on the magnitude of respiratory plasticity has not been established in humans. A meta-analysis was completed using anthropometric and physiological variables obtained from 124 participants that completed one of three intermittent hypoxia protocols. Simple correlations between the aggregate variables and the magnitude of PA and vLTF standardized to baseline was completed. Thereafter, the variables correlated to PA or vLTF were input into a multilinear regression equation. Baseline measures of the hypoxic ventilatory response was the sole predictor of PA (R = 0.370, P = 0.012). Similarly, this variable along with the hypoxic burden predicted the magnitude of vLTF (R = 0.546, P < 0.006 for both variables). In addition, the magnitude of PA was strongly correlated to vLTF (R = 0.617, P < 0.001). Anthropometric measures do not predict the magnitude of PA and vLTF in humans. Alternatively, the hypoxic ventilatory response was the sole predictor of PA, and in combination with the hypoxic burden, predicted the magnitude of vLTF. These influences should be considered in the design of mild intermittent hypoxia protocol studies in humans. Moreover, the strong correlation between PA and vLTF suggests that a common mechanistic pathway may have a role in the initiation of these forms of plasticity. KEY POINTS: Mild intermittent hypoxia initiates progressive augmentation (PA) and ventilatory long-term facilitation (vLTF) in humans. Many of the anthropometric and physiological variables that could impact the magnitude of these forms of plasticity are unknown. Anthropometric and physiological variables were measured from a total of 124 participants that completed one of three distinct intermittent hypoxia protocols. The variables correlated to PA or vLTF were input into a multilinear regression analysis. The hypoxic ventilatory response was the sole predictor of PA, while this variable in addition to the average hypoxic burden predicted the magnitude of vLTF. A strong correlation between PA and vLTF was also revealed. These influences should be considered in the design of mild intermittent hypoxia protocol studies in humans. Moreover, the strong correlation between PA and vLTF suggests that a common mechanistic pathway may have a role in the initiation of these forms of plasticity.

Control of Breathing

Substantial advances have been made recently into the discovery of fundamental mechanisms underlying the neural control of breathing and even some inroads into translating these findings to treating breathing disorders. Here, we review several of these advances, starting with an appreciation of the importance of V̇A:V̇CO2:PaCO2 relationships, then summarizing our current understanding of the mechanisms and neural pathways for central rhythm generation, chemoreception, exercise hyperpnea, plasticity, and sleep-state effects on ventilatory control. We apply these fundamental principles to consider the pathophysiology of ventilatory control attending hypersensitized chemoreception in select cardiorespiratory diseases, the pathogenesis of sleep-disordered breathing, and the exertional hyperventilation and dyspnea associated with aging and chronic diseases. These examples underscore the critical importance that many ventilatory control issues play in disease pathogenesis, diagnosis, and treatment.

A test of the interaction between central and peripheral respiratory chemoreflexes in humans

How central and peripheral chemoreceptor drives to breathe interact in humans remains contentious. We measured the peripheral chemoreflex sensitivity to hypoxia (PChS) at various isocapnic CO2 tensions (P C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ ) to determine the form of the relationship between PChS and centralP C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ . Twenty participants (10F) completed three repetitions of modified rebreathing tests with end-tidalP O 2 ${P_{{{\mathrm{O}}_{\mathrm{2}}}}}$ (P ET O 2 ${P_{{\mathrm{ET}}{{\mathrm{O}}_{\mathrm{2}}}}}$ ) clamped at 150, 70, 60 and 45 mmHg. End-tidalP C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ (P ETC O 2 ${P_{{\mathrm{ETC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ ),P ET O 2 ${P_{{\mathrm{ET}}{{\mathrm{O}}_{\mathrm{2}}}}}$ , ventilation (V ̇ $\dot{V}$ E ) and calculated oxygen saturation (SC O2 ) were measured breath-by-breath by gas-analyser and pneumotach. TheV ̇ $\dot{V}$ E -P ETC O 2 ${P_{{\mathrm{ETC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ relationship of repeat-trials were linear-interpolated, combined, averaged into 1 mmHg bins, and fitted with a double-linear function (V ̇ $\dot{V}$ E S, L min-1 mmHg-1 ). PChS was computed at intervals of 1 mmHg ofP ETC O 2 ${P_{{\mathrm{ETC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ as follows: the difference inV ̇ $\dot{V}$ E between the three hypoxic profiles and the hyperoxic profile (∆V ̇ $\dot{V}$ E ) was calculated; three ∆V ̇ $\dot{V}$ E values were plotted against corresponding SC O2 ; and linear regression determined PChS (Lmin-1 mmHg-1 %SC O2 -1 ). These processing steps were repeated at eachP ETC O 2 ${P_{{\mathrm{ETC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ to produce the PChS vs. isocapnicP C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ relationship. These were fitted with linear and polynomial functions, and Akaike information criterion identified the best-fit model. One-way repeated measures analysis of variance assessed between-condition differences.V ̇ $\dot{V}$ E S increased (P < 0.0001) with isoxicP ET O 2 ${P_{{\mathrm{ET}}{{\mathrm{O}}_{\mathrm{2}}}}}$ from 3.7 ± 1.5 L min-1 mmHg-1 at 150 mmHg to 4.4 ± 1.8, 5.0 ± 1.6 and 6.0 ± 2.2 Lmin-1 mmHg-1 at 70, 60 and 45 mmHg, respectively. Mean SC O2 fell progressively (99.3 ± 0%, 93.7 ± 0.1%, 90.4 ± 0.1% and 80.5 ± 0.1%; P < 0.0001). In all individuals, PChS increased withP ETC O 2 ${P_{{\mathrm{ETC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ , and this relationship was best described by a linear model in 75%. Despite increasing central chemoreflex activation, PChS increased linearly withP ETC O 2 ${P_{{\mathrm{ETC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ indicative of an additive central-peripheral chemoreflex response. KEY POINTS: How central and peripheral chemoreceptor drives to breathe interact in humans remains contentious. We measured peripheral chemoreflex sensitivity to hypoxia (PChS) at various isocapnic carbon dioxide tensions (P C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ ) to determine the form of the relationship between PChS and centralP C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ . Participants performed three repetitions of modified rebreathing with end-tidalP O 2 ${P_{{{\mathrm{O}}_{\mathrm{2}}}}}$ fixed at 150, 70, 60 and 45 mmHg. PChS was computed at intervals of 1 mmHg of end-tidalP C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ (P ETC O 2 ${P_{{\mathrm{ETC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ ) as follows: the difference inV ̇ $\dot{V}$ E between the three hypoxic profiles and the hyperoxic profile (∆V ̇ $\dot{V}$ E ) was calculated; three ∆V ̇ $\dot{V}$ E values were plotted against corresponding calculated oxygen saturation (SC O2 ); and linear regression determined PChS (Lmin-1 mmHg-1 %SC O2 -1 ). In all individuals, PChS increased withP ETC O 2 ${P_{{\mathrm{ETC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ , and this relationship was best described by a linear (rather than polynomial) model in 15 of 20. Most participants did not exhibit a hypo- or hyper-additive effect of central chemoreceptors on the peripheral chemoreflex indicating that the interaction was additive.

Rethinking O2 , CO2 and breathing during wakefulness and sleep

We have examined the importance of three long-standing questions concerning chemoreceptor influences on cardiorespiratory function which are currently experiencing a resurgence of study among physiologists and clinical investigators. Firstly, while carotid chemoreceptors (CB) are required for hypoxic stimulation of breathing, use of an isolated, extracorporeally perfused CB preparation in unanaesthetized animals with maintained tonic input from the CB, reveals that extra-CB hypoxaemia also provides dose-dependent ventilatory stimulation sufficient to account for 40-50% of the total ventilatory response to steady-state hypoxaemia. Extra-CB hyperoxia also provides a dose- and time-dependent hyperventilation. Extra-CB sites of O2 -driven ventilatory stimulation identified to date include the medulla, kidney and spinal cord. Secondly, using the isolated or denervated CB preparation in awake animals and humans has demonstrated a hyperadditive effect of CB sensory input on central CO2 sensitivity, so that tonic CB activity accounts for as much as 35-40% of the normal, air-breathing eupnoeic drive to breathe. Thirdly, we argue for a key role for CO2 chemoreception and the neural drive to breathe in the pathogenesis of upper airway obstruction during sleep (OSA), based on the following evidence: (1) removal of the wakefulness drive to breathe enhances the effects of transient CO2 changes on breathing instability; (2) oscillations in respiratory motor output precipitate pharyngeal obstruction in sleeping subjects with compliant, collapsible airways; and (3) in the majority of patients in a large OSA cohort, a reduced neural drive to breathe accompanied reductions in both airflow and pharyngeal airway muscle dilator activity, precipitating airway obstruction.

Cell Responses of the Ventrolateral Medulla to PAR1 Activation and Changes in Respiratory Rhythm in Newborn Rat En Bloc Brainstem-Spinal Cord Preparations

Proteinase-activated receptor-1 (PAR1) is expressed in astrocytes of various brain regions, and its activation is involved in the modulation of neuronal activity. Here, we report effects of PAR1 selective agonist TFLLR on respiratory rhythm generation in brainstem-spinal cord preparations. Preparations were isolated from newborn rats (P0-P4) under deep isoflurane anesthesia and were transversely cut at the rostral medulla. Preparations were superfused with artificial cerebrospinal fluid (25-26 °C), and inspiratory C4 ventral root activity was monitored. The responses to TFLLR of cells close to the cut surface were detected by calcium imaging or membrane potential recordings. Application of 10 μM TFLLR (4 min) induced a rapid and transient increase of calcium signal in cells of the ventrolateral respiratory regions of the medulla. More than 88% of responding cells (223/254 cells from 13 preparations) were also activated by low (0.2 mM) K+ solution, suggesting that they were astrocytes. Immunohistochemical examination demonstrated that PAR1 was expressed on many astrocytes. Respiratory-related neurons in the medulla were transiently hyperpolarized (-1.8 mV) during 10 μM TFLLR application, followed by weak membrane depolarization after washout. C4 burst rate decreased transiently in response to application of TFLLR, followed by a slight increase. The inhibitory effect was partially blocked by 50 μM theophylline. In conclusion, activation of astrocytes via PAR1 resulted in a decrease of inspiratory C4 burst rate in association with transient hyperpolarization of respiratory-related neurons. After washout, slow and weak excitatory responses appeared. Adenosine may be partially involved in the inhibitory effect of PAR1 activation.

Purinergic signaling mediates neuroglial interactions to modulate sighs

Sighs prevent the collapse of alveoli in the lungs, initiate arousal under hypoxic conditions, and are an expression of sadness and relief. Sighs are periodically superimposed on normal breaths, known as eupnea. Implicated in the generation of these rhythmic behaviors is the preBötzinger complex (preBötC). Our experimental evidence suggests that purinergic signaling is necessary to generate spontaneous and hypoxia-induced sighs in a mouse model. Our results demonstrate that driving calcium increases in astrocytes through pharmacological methods robustly increases sigh, but not eupnea, frequency. Calcium imaging of preBötC slices corroborates this finding with an increase in astrocytic calcium upon application of sigh modulators, increasing intracellular calcium through g-protein signaling. Moreover, photo-activation of preBötC astrocytes is sufficient to elicit sigh activity, and this response is blocked with purinergic antagonists. We conclude that sighs are modulated through neuron-glia coupling in the preBötC network, where the distinct modulatory responses of neurons and glia allow for both rhythms to be independently regulated.

The astrocytic Na+ -HCO3 - cotransporter, NBCe1, is dispensable for respiratory chemosensitivity

The interoceptive homeostatic mechanism that controls breathing, blood gases and acid-base balance in response to changes in CO2 /H+ is exquisitely sensitive, with convergent roles proposed for chemosensory brainstem neurons in the retrotrapezoid nucleus (RTN) and their supporting glial cells. For astrocytes, a central role for NBCe1, a Na+ -HCO3 - cotransporter encoded by Slc4a4, has been envisaged in multiple mechanistic models (i.e. underlying enhanced CO2 -induced local extracellular acidification or purinergic signalling). We tested these NBCe1-centric models by using conditional knockout mice in which Slc4a4 was deleted from astrocytes. In GFAP-Cre;Slc4a4fl/fl mice we found diminished expression of Slc4a4 in RTN astrocytes by comparison to control littermates, and a concomitant reduction in NBCe1-mediated current. Despite disrupted NBCe1 function in RTN-adjacent astrocytes from these conditional knockout mice, CO2 -induced activation of RTN neurons or astrocytes in vitro and in vivo, and CO2 -stimulated breathing, were indistinguishable from NBCe1-intact littermates; hypoxia-stimulated breathing and sighs were likewise unaffected. We obtained a more widespread deletion of NBCe1 in brainstem astrocytes by using tamoxifen-treated Aldh1l1-Cre/ERT2;Slc4a4fl/fl mice. Again, there was no difference in effects of CO2 or hypoxia on breathing or on neuron/astrocyte activation in NBCe1-deleted mice. These data indicate that astrocytic NBCe1 is not required for the respiratory responses to these chemoreceptor stimuli in mice, and that any physiologically relevant astrocytic contributions must involve NBCe1-independent mechanisms. KEY POINTS: The electrogenic NBCe1 transporter is proposed to mediate local astrocytic CO2 /H+ sensing that enables excitatory modulation of nearby retrotrapezoid nucleus (RTN) neurons to support chemosensory control of breathing. We used two different Cre mouse lines for cell-specific and/or temporally regulated deletion of the NBCe1 gene (Slc4a4) in astrocytes to test this hypothesis. In both mouse lines, Slc4a4 was depleted from RTN-associated astrocytes but CO2 -induced Fos expression (i.e. cell activation) in RTN neurons and local astrocytes was intact. Likewise, respiratory chemoreflexes evoked by changes in CO2 or O2 were unaffected by loss of astrocytic Slc4a4. These data do not support the previously proposed role for NBCe1 in respiratory chemosensitivity mediated by astrocytes.

Neuromedin B-Expressing Neurons in the Retrotrapezoid Nucleus Regulate Respiratory Homeostasis and Promote Stable Breathing in Adult Mice

Respiratory chemoreceptor activity encoding arterial Pco2 and Po2 is a critical determinant of ventilation. Currently, the relative importance of several putative chemoreceptor mechanisms for maintaining eupneic breathing and respiratory homeostasis is debated. Transcriptomic and anatomic evidence suggests that bombesin-related peptide Neuromedin-B (Nmb) expression identifies chemoreceptor neurons in the retrotrapezoid nucleus (RTN) that mediate the hypercapnic ventilatory response, but functional support is missing. In this study, we generated a transgenic Nmb-Cre mouse and used Cre-dependent cell ablation and optogenetics to test the hypothesis that RTN Nmb neurons are necessary for the CO2-dependent drive to breathe in adult male and female mice. Selective ablation of ∼95% of RTN Nmb neurons causes compensated respiratory acidosis because of alveolar hypoventilation, as well as profound breathing instability and respiratory-related sleep disruption. Following RTN Nmb lesion, mice were hypoxemic at rest and were prone to severe apneas during hyperoxia, suggesting that oxygen-sensitive mechanisms, presumably the peripheral chemoreceptors, compensate for the loss of RTN Nmb neurons. Interestingly, ventilation following RTN Nmb -lesion was unresponsive to hypercapnia, but behavioral responses to CO2 (freezing and avoidance) and the hypoxia ventilatory response were preserved. Neuroanatomical mapping shows that RTN Nmb neurons are highly collateralized and innervate the respiratory-related centers in the pons and medulla with a strong ipsilateral preference. Together, this evidence suggests that RTN Nmb neurons are dedicated to the respiratory effects of arterial Pco2/pH and maintain respiratory homeostasis in intact conditions and suggest that malfunction of these neurons could underlie the etiology of certain forms of sleep-disordered breathing in humans.SIGNIFICANCE STATEMENT Respiratory chemoreceptors stimulate neural respiratory motor output to regulate arterial Pco2 and Po2, thereby maintaining optimal gas exchange. Neurons in the retrotrapezoid nucleus (RTN) that express the bombesin-related peptide Neuromedin-B are proposed to be important in this process, but functional evidence has not been established. Here, we developed a transgenic mouse model and demonstrated that RTN neurons are fundamental for respiratory homeostasis and mediate the stimulatory effects of CO2 on breathing. Our functional and anatomic data indicate that Nmb-expressing RTN neurons are an integral component of the neural mechanisms that mediate CO2-dependent drive to breathe and maintain alveolar ventilation. This work highlights the importance of the interdependent and dynamic integration of CO2- and O2-sensing mechanisms in respiratory homeostasis of mammals.

Hypercapnic signaling influences hypoxic signaling in the control of breathing in C57BL6 mice

Interactions between hypoxic and hypercapnic signaling pathways, expressed as ventilatory changes occurring during and following a simultaneous hypoxic-hypercapnic gas challenge (HH-C) have not been determined systematically in mice. This study in unanesthetized male C57BL6 mice addressed the hypothesis that hypoxic (HX) and hypercapnic (HC) signaling events display an array of interactions indicative of coordination by peripheral and central respiratory mechanisms. We evaluated the ventilatory responses elicited by hypoxic (HX-C, 10%, O2, 90% N2), hypercapnic (HC-C, 5% CO2, 21%, O2, 90% N2), and HH-C (10% O2, 5%, CO2, 85% N2) challenges to determine whether ventilatory responses elicited by HH-C were simply additive of responses elicited by HX-C and HC-C, or whether other patterns of interactions existed. Responses elicited by HH-C were additive for tidal volume, minute ventilation and expiratory time, among others. Responses elicited by HH-C were hypoadditive of the HX-C and HC-C responses (i.e., HH-C responses were less than expected by simple addition of HX-C and HC-C responses) for frequency of breathing, inspiratory time and relaxation time, among others. In addition, end-expiratory pause increased during HX-C, but decreased during HC-C and HH-C, therefore showing that HC-C responses influenced the HX-C responses when given simultaneously. Return to room-air responses was additive for tidal volume and minute ventilation, among others, whereas they were hypoadditive for frequency of breathing, inspiratory time, peak inspiratory flow, apneic pause, inspiratory and expiratory drives, and rejection index. These data show that HX-C and HH-C signaling pathways interact with one another in additive and often hypoadditive processes.NEW & NOTEWORTHY We present data showing that the ventilatory responses elicited by a hypoxic gas challenge in male C57BL6 mice are markedly altered by coexposure to hypercapnic gas challenge with hypercapnic responses often dominating the hypoxic responses. These data suggest that hypercapnic signaling processes activated within brainstem regions, such as the retrotrapezoid nuclei, may directly modulate the signaling processes within the nuclei tractus solitarius resulting from hypoxic-induced increase in carotid body chemoreceptor input to these nuclei.

Persistent augmentation of fictive air breathing by hypoxia: An in vitro study of the role of GABAB signaling in pre-metamorphic tadpoles

Tadpole development is influenced by environmental cues and hypoxia can favor the emergence of the neural networks driving air breathing. Exposing isolated brainstems from pre-metamorphic tadpoles to acute hypoxia (~0% O₂; 15 min) leads to a progressive increase in fictive air breaths (~3 fold) in the hours that follow stimulation. Here, we first determined whether this effect persists over longer periods (<18 h); we then evaluated maturity of the motor output by comparing the breathing pattern of hypoxia-exposed brainstems to that of preparations from adult bullfrogs under basal conditions. Because progressive withdrawal of GABA_B-mediated inhibition contributes to the developmental increase in fictive lung ventilation, we then hypothesised that hypoxia reduces respiratory sensitivity to baclofen (selective GABA_B-agonist). Experiments were performed on isolated brainstem preparations from pre-metamorphic tadpoles (TK stages IV to XIV); respiratory-related neural activity was recorded from cranial nerves V/VII and X before and 18 h after exposure to hypoxia (0% O₂ + 2% CO₂; 25 min). Time-control experiments (no hypoxia) were performed. Exposing pre-metamorphic tadpoles to hypoxia did not affect gill burst frequency, but augmented the frequency of fictive lung bursts and the incidence of episodic breathing levels intermediate between pre-metamorphic and adult preparations. Addition of baclofen to the aCSF (0,2 μM - 20 min) reduced lung burst frequency, but the response of hypoxia-exposed brainstems was greater than controls. We conclude that acute hypoxia facilitates development and maturation of the motor command driving air breathing. We propose that a greater number of active rhythmogenic neurons expressing GABAb receptors contributes to this effect.

Noninvasive Respiratory Support Effects on Sighs in Preterm Infants by Electrical Impedance Tomography

OBJECTIVE

To evaluate differences regarding sigh frequency between noninvasive respiratory support types and to assess regional ventilation distribution, delta Z, and end-expiratory lung impedance differences before and after sighs.

METHODS

Very low-birth-weight infants with gestational ages less than 32 wk were included in the study. Participants were split into two groups: those receiving continuous positive airway pressure and infants receiving high-flow nasal cannula therapy.

RESULTS

The study enrolled 30 infants. The high-flow nasal cannula therapy group had more sighs per 10-min period than infants receiving continuous positive airway pressure (p = 0.016). Ventilation distribution was similar in the anterior and right ventilation distribution compartments pre- and post-sigh (46.30% vs. 45.68% and 54.27% vs. 55.26%, respectively). No statistically significant increase in end-expiratory lung impedance or delta Z was observed in global or separate lung regions (p > 0.05).

CONCLUSION

The study has demonstrated that sighs are more frequent in infants receiving high-flow nasal cannula respiratory support compared to continuous positive airway pressure. Spontaneously occurring sighs on noninvasive respiratory support due to respiratory distress syndrome (RDS) do not increase end-expiratory lung impedance or alter delta Z, and appear to have limited clinical significance.

TRIAL REGISTRATION

Prospectively registered at www.

CLINICALTRIALS

gov , reg. No. NCT04542096, reg. date 01/09/2020.

Role of microglia in blood pressure and respiratory responses to acute hypoxic exposure in rats

Microglia modulate cardiorespiratory activities during chronic hypoxia. It has not been clarified whether microglia are involved in the cardiorespiratory responses to acute hypoxia. Here we investigated this issue by comparing cardiorespiratory responses to two levels of acute hypoxia (13% O₂ for 4 min and 7% O₂ for 5 min) in conscious unrestrained rats before and after systemic injection of minocycline (MINO), an inhibitor of microglia activation. MINO increased blood pressure but not lung ventilation in the control normoxic condition. Acute hypoxia stimulated cardiorespiratory responses in MINO-untreated rats. MINO failed to significantly affect the magnitude of hypoxia-induced blood pressure elevation. In contrast, MINO tended to suppress the ventilatory responses to hypoxia. We conclude that microglia differentially affect cardiorespiratory regulation depending on the level of blood oxygenation. Microglia suppressively contribute to blood pressure regulation in normoxia but help maintain ventilatory augmentation in hypoxia, which underscores the dichotomy of central regulatory pathways for both systems.

fMRI studies evaluating central respiratory control in humans

A plethora of neural centers in the central nervous system control the fundamental respiratory pattern. This control is ensured by neurons that act as pacemakers, modulating activity through chemical control driven by changes in the O₂/CO₂ balance. Most of the respiratory neural centers are located in the brainstem, but difficult to localize on magnetic resonance imaging (MRI) due to their small size, lack of visually-detectable borders with neighboring areas, and significant physiological noise hampering detection of its activity with functional MRI (fMRI). Yet, several approaches make it possible to study the normal response to different abnormal stimuli or conditions such as CO₂ inhalation, induced hypercapnia, volitional apnea, induced hypoxia etc. This review provides a comprehensive overview of the majority of available studies on central respiratory control in humans.

Brain H+ /CO2 sensing and control by glial cells

Maintenance of constant brain pH is critically important to support the activity of individual neurons, effective communication within the neuronal circuits, and, thus, efficient processing of information by the brain. This review article focuses on how glial cells detect and respond to changes in brain tissue pH and concentration of CO2 , and then trigger systemic and local adaptive mechanisms that ensure a stable milieu for the operation of brain circuits. We give a detailed account of the cellular and molecular mechanisms underlying sensitivity of glial cells to H+ and CO2 and discuss the role of glial chemosensitivity and signaling in operation of three key mechanisms that work in concert to keep the brain pH constant. We discuss evidence suggesting that astrocytes and marginal glial cells of the brainstem are critically important for central respiratory CO2 chemoreception-a fundamental physiological mechanism that regulates breathing in accord with changes in blood and brain pH and partial pressure of CO2 in order to maintain systemic pH homeostasis. We review evidence suggesting that astrocytes are also responsible for the maintenance of local brain tissue extracellular pH in conditions of variable acid loads associated with changes in the neuronal activity and metabolism, and discuss potential role of these glial cells in mediating the effects of CO2 on cerebral vasculature.

Chemoreflex Control as the Cornerstone in Immersion Water Sports: Possible Role on Breath-Hold

Immersion water sports involve long-term apneas; therefore, athletes must physiologically adapt to maintain muscle oxygenation, despite not performing pulmonary ventilation. Breath-holding (i.e., apnea) is common in water sports, and it involves a decrease and increases PaO₂ and PaCO₂, respectively, as the primary signals that trigger the end of apnea. The principal physiological O₂ sensors are the carotid bodies, which are able to detect arterial gases and metabolic alterations before reaching the brain, which aids in adjusting the cardiorespiratory system. Moreover, the principal H⁺/CO₂ sensor is the retrotrapezoid nucleus, which is located at the brainstem level; this mechanism contributes to detecting respiratory and metabolic acidosis. Although these sensors have been characterized in pathophysiological states, current evidence shows a possible role for these mechanisms as physiological sensors during voluntary apnea. Divers and swimmer athletes have been found to displayed longer apnea times than land sports athletes, as well as decreased peripheral O₂ and central CO₂ chemoreflex control. However, although chemosensitivity at rest could be decreased, we recently found marked sympathoexcitation during maximum voluntary apnea in young swimmers, which could activate the spleen (which is a reservoir organ for oxygenated blood). Therefore, it is possible that the chemoreflex, autonomic function, and storage/delivery oxygen organ(s) are linked to apnea in immersion water sports. In this review, we summarized the available evidence related to chemoreflex control in immersion water sports. Subsequently, we propose a possible physiological mechanistic model that could contribute to providing new avenues for understanding the respiratory physiology of water sports.

Novel oxygen sensing mechanism in the spinal cord involved in cardiorespiratory responses to hypoxia

As blood oxygenation decreases (hypoxemia), mammals mount cardiorespiratory responses, increasing oxygen to vital organs. The carotid bodies are the primary oxygen chemoreceptors for breathing, but sympathetic-mediated cardiovascular responses to hypoxia persist in their absence, suggesting additional high-fidelity oxygen sensors. We show that spinal thoracic sympathetic preganglionic neurons are excited by hypoxia and silenced by hyperoxia, independent of surrounding astrocytes. These spinal oxygen sensors (SOS) enhance sympatho-respiratory activity induced by CNS asphyxia-like stimuli, suggesting they bestow a life-or-death advantage. Our data suggest the SOS use a mechanism involving neuronal nitric oxide synthase 1 (NOS1) and nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX). We propose NOS1 serves as an oxygen-dependent sink for NADPH in hyperoxia. In hypoxia, NADPH catabolism by NOS1 decreases, increasing availability of NADPH to NOX and launching reactive oxygen species-dependent processes, including transient receptor potential channel activation. Equipped with this mechanism, SOS are likely broadly important for physiological regulation in chronic disease, spinal cord injury, and cardiorespiratory crisis.

Amygdala-driven apnea and the chemoreceptive origin of anxiety

Although the amygdala plays an important part in the pathogenesis of anxiety and generation of exteroceptive fear, recent discoveries have challenged the directionality of this brain-behavior relationship with respect to interoceptive fear. Here we highlight several paradoxical findings including: (1) amygdala lesion patients who experience excessive fear and panic following inhalation of carbon dioxide (CO₂), (2) clinically anxious patients who have significantly smaller (rather than larger) amygdalae and a pronounced hypersensitivity toward CO₂, and (3) epilepsy patients who exhibit apnea immediately following stimulation of their amygdala yet have no awareness that their breathing has stopped. The above findings elucidate an entirely novel role for the amygdala in the induction of apnea and inhibition of CO₂-induced fear. Such a role is plausible given the strong inhibitory connections linking the central nucleus of the amygdala with respiratory and chemoreceptive centers in the brainstem. Based on this anatomical arrangement, we propose a model of Apnea-induced Anxiety (AiA) which predicts that recurring episodes of apnea are being unconsciously elicited by amygdala activation, resulting in transient spikes in CO₂ that provoke fear and anxiety, and lead to characteristic patterns of escape and avoidance behavior in patients spanning the spectrum of anxiety. If this new conception of AiA proves to be true, and activation of the amygdala can repeatedly trigger states of apnea outside of one's awareness, then it remains possible that the chronicity of anxiety disorders is being interoceptively driven by a chemoreceptive system struggling to maintain homeostasis in the midst of these breathless states.

Breathing during sleep

The depth, rate, and regularity of breathing change following transition from wakefulness to sleep. Interactions between sleep and breathing involve direct effects of the central mechanisms that generate sleep states exerted at multiple respiratory regulatory sites, such as the central respiratory pattern generator, respiratory premotor pathways, and motoneurons that innervate the respiratory pump and upper airway muscles, as well as effects secondary to sleep-related changes in metabolism. This chapter discusses respiratory effects of sleep as they occur under physiologic conditions. Breathing and central respiratory neuronal activities during nonrapid eye movement (NREM) sleep and REM sleep are characterized in relation to activity of central wake-active and sleep-active neurons. Consideration is given to the obstructive sleep apnea syndrome because in this common disorder, state-dependent control of upper airway patency by upper airway muscles attains high significance and recurrent arousals from sleep are triggered by hypercapnic and hypoxic episodes. Selected clinical trials are discussed in which pharmacological interventions targeted transmission in noradrenergic, serotonergic, cholinergic, and other state-dependent pathways identified as mediators of ventilatory changes during sleep. Central pathways for arousals elicited by chemical stimulation of breathing are given special attention for their important role in sleep loss and fragmentation in sleep-related respiratory disorders.

Neurobiology of the carotid body

The carotid body (CB) is a bilateral arterial chemoreceptor located in the carotid artery bifurcation with an essential role in cardiorespiratory homeostasis. It is composed of highly perfused cell clusters, or glomeruli, innervated by sensory fibers. Glomus cells, the most abundant in each glomerulus, are neuron-like multimodal sensory elements able to detect and integrate changes in several physical and chemical parameters of the blood, in particular O₂ tension, CO₂ and pH, as well as glucose, lactate, or blood flow. Activation of glomus cells (e.g., during hypoxia or hypercapnia) stimulates the afferent fibers which impinge on brainstem neurons to elicit rapid compensatory responses (hyperventilation and sympathetic activation). This chapter presents an updated view of the structural organization of the CB and the mechanisms underlying the chemosensory responses of glomus cells, with special emphasis on the molecular processes responsible for acute O₂ sensing. The properties of the glomus cell-sensory fiber synapse as well as the organization of CB output are discussed. The chapter includes the description of recently discovered CB stem cells and progenitor cells, and their role in CB growth during acclimatization to hypoxemia. Finally, the participation of the CB in the mechanisms of disease is briefly discussed.

Rostral ventrolateral medulla, retropontine region and autonomic regulations

The rostral half of the ventrolateral medulla (RVLM) and adjacent ventrolateral retropontine region (henceforth RVLMRP) have been divided into various sectors by neuroscientists interested in breathing or autonomic regulations. The RVLMRP regulates respiration, glycemia, vigilance and inflammation, in addition to blood pressure. It contains interoceptors that respond to acidification, hypoxia and intracranial pressure and its rostral end contains the retrotrapezoid nucleus (RTN) which is the main central respiratory chemoreceptor. Acid detection by the RTN is an intrinsic property of the principal neurons that is enhanced by paracrine influences from surrounding astrocytes and CO₂-dependent vascular constriction. RTN mediates the hypercapnic ventilatory response via complex projections to the respiratory pattern generator (CPG). The RVLM contributes to autonomic response patterns via differential recruitment of several subtypes of adrenergic (C1) and non-adrenergic neurons that directly innervate sympathetic and parasympathetic preganglionic neurons. The RVLM also innervates many brainstem and hypothalamic nuclei that contribute, albeit less directly, to autonomic responses. All lower brainstem noradrenergic clusters including the locus coeruleus are among these targets. Sympathetic tone to the circulatory system is regulated by subsets of presympathetic RVLM neurons whose activity is continuously restrained by the baroreceptors and modulated by the respiratory CPG. The inhibitory input from baroreceptors and the excitatory input from the respiratory CPG originate from neurons located in or close to the rhythm generating region of the respiratory CPG (preBötzinger complex).

Geoff Burnstock, purinergic signalling, and chemosensory control of breathing

This article is the authors' contribution to the tribute issue in honour of Geoffrey Burnstock, the founder of this journal and the field of purinergic signalling. We give a brief account of the results of experimental studies which at the beginning received valuable input from Geoff, who both directly and indirectly influenced our research undertaken over the last two decades. Research into the mechanisms controlling breathing identified ATP as the common mediator of the central and peripheral chemosensory transduction. Studies of the sources and mechanisms of chemosensory ATP release in the CNS suggested that this signalling pathway is universally engaged in conditions of increased metabolic demand by brain glial cells - astrocytes. Astrocytes appear to function as versatile CNS metabolic sensors that detect changes in brain tissue pH, CO2, oxygen, and cerebral perfusion pressure. Experimental studies on various aspects of astrocyte biology generated data indicating that the function of these omnipresent glial cells and communication between astrocytes and neurons are governed by purinergic signalling, - first discovered by Geoff Burnstock in the 70's and researched through his entire scientific career.

Activation of Astrocytes in the Persistence of Post-hypoxic Respiratory Augmentation

Acute hypoxia increases ventilation. After cessation of hypoxia loading, ventilation decreases but remains above the pre-exposure baseline level for a time. However, the mechanism of this post-hypoxic persistent respiratory augmentation (PHRA), which is a short-term potentiation of breathing, has not been elucidated. We aimed to test the hypothesis that astrocytes are involved in PHRA. To this end, we investigated hypoxic ventilatory responses by whole-body plethysmography in unanesthetized adult mice. The animals breathed room air, hypoxic gas mixture (7% O₂, 93% N₂) for 2min, and again room air for 10min before and after i.p. administration of low (100mg/kg) and high (300mg/kg) doses of arundic acid (AA), an astrocyte inhibitor. AA suppressed PHRA, with the high dose decreasing ventilation below the pre-hypoxic level. Further, we investigated the role of the astrocytic TRPA1 channel, a putative ventilatory hypoxia sensor, in PHRA using astrocyte-specific Trpa1 knockout (asTrpa1^−/−) and floxed Trpa1 (Trpa1^f/f) mice. In both Trpa1^f/f and asTrpa1^−/− mice, PHRA was noticeable, indicating that the astrocyte TRPA1 channel was not directly involved in PHRA. Taken together, these results indicate that astrocytes mediate the PHRA by mechanisms other than TRPA1 channels that are engaged in hypoxia sensing.

Acute hyperglycemia does not affect central respiratory chemoreflex responsiveness to CO2 in healthy humans

The central respiratory chemoreceptor complex (CCRC) is comprised of brainstem neurons and surrounding interoceptors, which collectively increase ventilation in response to elevated brainstem tissue CO₂/[H⁺] (i.e., central chemoreflex; CCR). The extent that the CCRC detects/responds to other metabolically related chemostimuli is unknown. We aimed to test the effects of acute oral glucose ingestion on CCR reactivity in heathy human participants (n = 38). We instrumented participants with a pneumotachometer (minute ventilation) and a gas sample line connected to a dual gas analyzer (pressure of end-tidal CO₂). Following a baseline (BL) period and capillary blood [glucose] (BG) sample, fasted (F) participants underwent a modified hyperoxic rebreathing test to assess CCR reactivity. Participants then consumed a 75 g standard glucose beverage (glucose loaded; GL). Following 30-min, they underwent a second BL, BG sample and hyperoxic rebreathing test. BG and metabolic rate were higher in GL, confirming the metabolic stimulus. However, the ventilatory recruitment threshold and the CCR responses were unchanged between F and GL states.

Role of astrocytes in rhythmic motor activity

Rhythmic motor activities such as breathing, locomotion, tremor, or mastication are organized by groups of interconnected neurons. Most synapses in the central nervous system are in close apposition with processes belonging to astrocytes. Neurotransmitters released from neurons bind to receptors expressed by astrocytes, activating a signaling pathway that leads to an increase in calcium concentration and the release of gliotransmitters that eventually modulate synaptic transmission. It is therefore likely that the activation of astrocytes impacts motor control. Here we review recent studies demonstrating that astrocytes inhibit, modulate, or trigger motor rhythmic behaviors.

Carotid body chemoreceptors: physiology, pathology, and implications for health and disease

The carotid body (CB) is the main peripheral chemoreceptor for arterial respiratory gases O2 and CO2 and pH, eliciting reflex ventilatory, cardiovascular, and humoral responses to maintain homeostasis. This review examines the fundamental biology underlying CB chemoreceptor function, its contribution to integrated physiological responses, and its role in maintaining health and potentiating disease. Emphasis is placed on 1) transduction mechanisms in chemoreceptor (type I) cells, highlighting the role played by the hypoxic inhibition of O2-dependent K+ channels and mitochondrial oxidative metabolism, and their modification by intracellular molecules and other ion channels; 2) synaptic mechanisms linking type I cells and petrosal nerve terminals, focusing on the role played by the main proposed transmitters and modulatory gases, and the participation of glial cells in regulation of the chemosensory process; 3) integrated reflex responses to CB activation, emphasizing that the responses differ dramatically depending on the nature of the physiological, pathological, or environmental challenges, and the interactions of the chemoreceptor reflex with other reflexes in optimizing oxygen delivery to the tissues; and 4) the contribution of enhanced CB chemosensory discharge to autonomic and cardiorespiratory pathophysiology in obstructive sleep apnea, congestive heart failure, resistant hypertension, and metabolic diseases and how modulation of enhanced CB reactivity in disease conditions may attenuate pathophysiology.

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.

Calcium Imaging Analysis of Cellular Responses to Hypercapnia and Hypoxia in the NTS of Newborn Rat Brainstem Preparation

It is supposed that the nucleus of the solitary tract (NTS) in the dorsal medulla includes gas sensor cells responsive to hypercapnia or hypoxia in the central nervous system. In the present study, we analyzed cellular responses to hypercapnia and hypoxia in the NTS region of newborn rat in vitro preparation. The brainstem and spinal cord were isolated from newborn rat (P0-P4) and were transversely cut at the level of the rostral area postrema. To detect cellular responses, calcium indicator Oregon Green was pressure-injected into the NTS just beneath the cut surface of either the caudal or rostral block of the medulla, and the preparation was superfused with artificial cerebrospinal fluid (25–26°C). We examined cellular responses initially to hypercapnic stimulation (to 8% CO₂ from 2% CO₂) and then to hypoxic stimulation (to 0% O₂ from 95% O₂ at 5% CO₂). We tested these responses in standard solution and in two different synapse blockade solutions: (1) cocktail blockers solution including bicuculline, strychnine, NBQX and MK-801 or (2) TTX solution. At the end of the experiments, the superfusate potassium concentration was lowered to 0.2 from 3 mM to classify recorded cells into neurons and astrocytes. Excitation of cells was detected as changes of fluorescence intensity with a confocal calcium imaging system. In the synaptic blockade solutions (cocktail or TTX solution), 7.6 and 8% of the NTS cells responded to hypercapnic and hypoxic stimulation, respectively, and approximately 2% of them responded to both stimulations. Some of these cells responded to low K⁺, and they were classified into astrocytes comprising 43% hypercapnia-sensitive cells, 56% hypoxia-sensitive cells and 54% of both stimulation-sensitive cells. Of note, 49% of the putative astrocytes identified by low K⁺ stimulation were sensitive to hypercapnia, hypoxia or both. In the presence of a glia preferential blocker, 5 mM fluoroacetate (plus 0.5 μM TTX), the percentage of hypoxia-sensitive cells was significantly reduced compared to those of all other conditions. This is the first study to reveal that the NTS includes hypercapnia and hypoxia dual-sensitive cells. These results suggest that astrocytes in the NTS region could act as a central gas sensor.

A Potential Role of the Renin-Angiotensin-System for Disturbances of Respiratory Chemosensitivity in Acute Respiratory Distress Syndrome and Severe Acute Respiratory Syndrome

Acute respiratory distress syndrome (ARDS) represents an acute diffuse inflammation of the lungs triggered by different causes, uniformly leading to a noncardiogenic pulmonary edema with inhomogeneous densities in lung X-ray and lung CT scan and acute hypoxemia. Edema formation results in "heavy" lungs, inducing loss of compliance and the need to spend more energy to "move" the lungs. Consequently, an ARDS patient, as long as the patient is breathing spontaneously, has an increased respiratory drive to ensure adequate oxygenation and CO2 removal. One would expect that, once the blood gases get back to "physiological" values, the respiratory drive would normalize and the breathing effort return to its initial status. However, in many ARDS patients, this is not the case; their respiratory drive appears to be upregulated and fully or at least partially detached from the blood gas status. Strikingly, similar alteration of the respiratory drive can be seen in patients suffering from SARS, especially SARS-Covid-19. We hypothesize that alterations of the renin-angiotensin-system (RAS) related to the pathophysiology of ARDS and SARS are involved in this dysregulation of chemosensitive control of breathing.

PO2 of the metathoracic ganglion in response to progressive hypoxia in an insect

Mammals regulate their brain tissue P_O2 tightly, and only small changes in brain P_O2 are required to elicit compensatory ventilation. However, unlike the flow-through cardiovascular system of vertebrates, insect tissues exchange gases through blind-ended tracheoles, which may involve a more prominent role for diffusive gas exchange. We tested the effect of progressive hypoxia on ventilation and the P_O2 of the metathoracic ganglion (neural site of control of ventilation) using microelectrodes in the American locust, Schistocerca americana. In normal air (21 kPa), P_O2 of the metathoracic ganglion was 12 kPa. The P_O2 of the ganglion dropped as air P_O2 dropped, with ventilatory responses occurring when ganglion P_O2 reached 3 kPa. Unlike vertebrates, insects tolerate relatively high resting tissue P_O2 levels and allow tissue P_O2 to drop during hypoxia, activity and discontinuous gas exchange before activating convective or spiracular gas exchange. Tracheated animals, and possibly pancrustaceans in general, seem likely to generally experience wide spatial and temporal variation in tissue P_O2 compared with vertebrates, with important implications for physiological function and the evolution of oxygen-using proteins.

Amelioration of sleep-disordered breathing with supplemental oxygen in older adults

Elderly adults demonstrate increased propensity for breathing instability during sleep compared with younger adults, and this may contribute to increased prevalence of sleep-disordered breathing (SDB) in this population. Hence, in older adults with SDB, we examined whether addition of supplemental oxygen (O₂) will stabilize breathing during sleep and alleviate SDB. We hypothesized that exposure to supplemental O₂ during non-rapid eye movement (NREM) sleep will stabilize breathing and will alleviate SDB by reducing ventilatory chemoresponsiveness and by widening the carbon dioxide (CO₂) reserve. We studied 10 older adults with mild-to-moderate SDB who were randomized to undergo noninvasive bilevel mechanical ventilation with exposure to room air or supplemental O₂ (Oxy) to determine the CO₂ reserve, apneic threshold (AT), and controller and plant gains. Supplemental O₂ was introduced during sleep to achieve a steady-state O₂ saturation ≥95% and fraction of inspired O₂ at 40%-50%. The CO₂ reserve increased significantly during Oxy versus room air (-4.2 ± 0.5 mmHg vs. -3.2 ± 0.5 mmHg, P = 0.03). Compared with room air, Oxy was associated with a significant decline in the controller gain (1.9 ± 0.4 L/min/mmHg vs. 2.5 ± 0.5 L/min/mmHg, P = 0.04), with reductions in the apnea-hypopnea index (11.8 ± 2.0/h vs. 24.4 ± 5.6/h, P = 0.006) and central apnea-hypopnea index (1.7 ± 0.6/h vs. 6.9 ± 3.9/h, P = 0.03). The AT and plant gain were unchanged. Thus, a reduced slope of CO₂ response resulted in an increased CO₂ reserve. In conclusion, supplemental O₂ reduced SDB in older adults during NREM sleep via reduction in chemoresponsiveness and central respiratory events.NEW & NOTEWORTHY This study demonstrates for the first time in elderly adults without heart disease that intervention with supplemental oxygen in the clinical range will ameliorate central apneas and hypopneas by decreasing the propensity to central apnea through decreased chemoreflex sensitivity, even in the absence of a reduction in the plant gain. Thus, the study provides physiological evidence for use of supplemental oxygen as therapy for mild-to-moderate SDB in this vulnerable population.

Physiology: New Insights into Central Oxygen Sensing

Astrocytes, the star-shaped brain cells, are known chemosensitive cells in the respiratory system. A new study shows that trafficking of TRPA1 channels in and out of the cell membrane in brainstem astrocytes contributes to their role as central respiratory oxygen sensors.

The need for specificity in quantifying neurocirculatory vs. respiratory effects of eucapnic hypoxia and transient hyperoxia

KEY POINTS

The carotid chemoreceptor mediates the ventilatory and muscle sympathetic nerve activity (MSNA) responses to hypoxia and contributes to tonic sympathetic and respiratory drives. It is often presumed that both excitatory and inhibitory tests of chemoreflex function show congruence in the end-organ responses. Ventilatory and neurocirculatory (MSNA, blood pressure and heart rate) responses to chemoreflex inhibition elicited by transient hyperoxia and to chemoreflex excitation produced by steady-state eucapnic hypoxia were measured in a cohort of 82 middle-aged individuals. Ventilatory and MSNA responsiveness to hyperoxia and hypoxia were not significantly correlated within individuals. It was concluded that ventilatory responses to hypoxia and hyperoxia do not predict MSNA responses and it is recommended that tests using the specific outcome of interest, i.e. MSNA or ventilation, are required. Transient hyperoxia is recommended as a sensitive and reliable means of quantifying tonic chemoreceptor-driven levels of sympathetic nervous system activity and respiratory drive.

ABSTRACT

Hypersensitivity of the carotid chemoreceptor leading to sympathetic nervous system activation and ventilatory instability has been implicated in the pathogenesis and consequences of several common clinical conditions. A variety of treatment approaches aimed at lessening chemoreceptor-driven sympathetic overactivity are now under investigation; thus, the ability to quantify this outcome variable with specificity and precision is crucial. Accordingly, we measured ventilatory and neurocirculatory responses to chemoreflex inhibition elicited by transient hyperoxia and chemoreflex excitation produced by exposure to graded, steady-state eucapnic hypoxia in middle-aged men and women (n = 82) with continuous positive airway pressure-treated obstructive sleep apnoea. Progressive, eucapnic hypoxia produced robust and highly variable increases in ventilation (+83 ± 59%) and muscle sympathetic nerve activity (MSNA) burst frequency (+55 ± 31%), whereas transient hyperoxia caused marked reductions in these variables (-35 ± 14% and -42 ± 16%, respectively). Coefficients of variation for ventilatory and MSNA burst frequency responses, indicating test-retest reproducibility, were respectively 9% and 24% for hyperoxia and 35% and 28% for hypoxia. Based on statistical measures of rank correlation or even comparisons across quartiles of corresponding ventilatory and MSNA responses, we found that the magnitudes of ventilatory inhibition with hyperoxia or excitation with eucapnic hypoxia were not correlated with corresponding MSNA responses within individuals. We conclude that, in conscious, behaving humans, ventilatory sensitivities to progressive, steady-state, eucapnic hypoxia and transient hyperoxia do not predict MSNA responsiveness. Our findings also support the use of transient hyperoxia as a reliable, sensitive, measure of the carotid chemoreceptor contribution to tonic sympathetic nervous system activity and respiratory drive.

Synuclein Deficiency Results in Age-Related Respiratory and Cardiovascular Dysfunctions in Mice

Synuclein (α, β, and γ) proteins are highly expressed in presynaptic terminals, and significant data exist supporting their role in regulating neurotransmitter release. Targeting the gene encoding α-synuclein is the basis of many animal models of Parkinson's disease (PD). However, the physiological role of this family of proteins in not well understood and could be especially relevant as interfering with accumulation of α-synuclein level has therapeutic potential in limiting PD progression. The long-term effects of their removal are unknown and given the complex pathophysiology of PD, could exacerbate other clinical features of the disease, for example dysautonomia. In the present study, we sought to characterize the autonomic phenotypes of mice lacking all synucleins (α, β, and γ; αβγ^-/-) in order to better understand the role of synuclein-family proteins in autonomic function. We probed respiratory and cardiovascular reflexes in conscious and anesthetized, young (4 months) and aged (18-20 months) αβγ^-/- male mice. Aged mice displayed impaired respiratory responses to both hypoxia and hypercapnia when breathing activities were recorded in conscious animals using whole-body plethysmography. These animals were also found to be hypertensive from conscious blood pressure recordings, to have reduced pressor baroreflex gain under anesthesia, and showed reduced termination of both pressor and depressor reflexes. The present data demonstrate the importance of synuclein in the normal function of respiratory and cardiovascular reflexes during aging.

Investigating Disturbances of Oxygen Homeostasis: From Cellular Mechanisms to the Clinical Practice

Soon after its discovery in the 18th century, oxygen was applied as a therapeutic agent to treat severely ill patients. Lack of oxygen, commonly termed as hypoxia, is frequently encountered in different disease states and is detrimental to human life. However, at the end of the 19th century, Paul Bert and James Lorrain Smith identified what is known as oxygen toxicity. The molecular basis of this phenomenon is oxygen's readiness to accept electrons and to form different variants of aggressive radicals that interfere with normal cell functions. The human body has evolved to maintain oxygen homeostasis by different molecular systems that are either activated in the case of oxygen under-supply, or to scavenge and to transform oxygen radicals when excess amounts are encountered. Research has provided insights into cellular mechanisms of oxygen homeostasis and is still called upon in order to better understand related diseases. Oxygen therapy is one of the prime clinical interventions, as it is life saving, readily available, easy to apply and economically affordable. However, the current state of research also implicates a reconsidering of the liberal application of oxygen causing hyperoxia. Increasing evidence from preclinical and clinical studies suggest detrimental outcomes as a consequence of liberal oxygen therapy. In this review, we summarize concepts of cellular mechanisms regarding different forms of disturbed cellular oxygen homeostasis that may help to better define safe clinical application of oxygen therapy.

AMPK and the Need to Breathe and Feed: What's the Matter with Oxygen?

We live and to do so we must breathe and eat, so are we a combination of what we eat and breathe? Here, we will consider this question, and the role in this respect of the AMP-activated protein kinase (AMPK). Emerging evidence suggests that AMPK facilitates central and peripheral reflexes that coordinate breathing and oxygen supply, and contributes to the central regulation of feeding and food choice. We propose, therefore, that oxygen supply to the body is aligned with not only the quantity we eat, but also nutrient-based diet selection, and that the cell-specific expression pattern of AMPK subunit isoforms is critical to appropriate system alignment in this respect. Currently available information on how oxygen supply may be aligned with feeding and food choice, or vice versa, through our motivation to breathe and select particular nutrients is sparse, fragmented and lacks any integrated understanding. By addressing this, we aim to provide the foundations for a clinical perspective that reveals untapped potential, by highlighting how aberrant cell-specific changes in the expression of AMPK subunit isoforms could give rise, in part, to known associations between metabolic disease, such as obesity and type 2 diabetes, sleep-disordered breathing, pulmonary hypertension and acute respiratory distress syndrome.

Continuous reduction in cerebral oxygenation during endurance exercise in patients with pulmonary arterial hypertension

BACKGROUND

Patients with pulmonary arterial hypertension (PAH) have lower cerebral blood flow (CBF) and oxygenation compared to healthy sedentary subjects, the latter negatively correlating with exercise capacity during incremental cycling exercise. We hypothesized that patients would also exhibit altered CBF and oxygenation during endurance exercise, which would correlate with endurance time.

METHODS

Resting and exercise cardiorespiratory parameters, blood velocity in the middle cerebral artery (MCAv; transcranial doppler) and cerebral oxygenation (relative changes in cerebral tissue oxygenation index (ΔcTOI) and cerebral deoxyhemoglobin (ΔcHHb); near-infrared spectroscopy) were continuously monitored in nine PAH patients and 10 healthy-matched controls throughout endurance exercise. Cardiac output (CO), systemic blood pressure (BP) and oxygen saturation (SpO2 ), ventilatory metrics and end-tidal CO2 pressure (PET CO2 ) were also assessed noninvasively.

RESULTS

Despite a lower workload and endurance oxygen consumption, similar CO and systemic BP, ΔcTOI was lower in PAH patients compared to controls (p < .01 for interaction). As expected during exercise, patients were characterized by an altered MCAv response to exercise, a lower PET CO2 and SpO2 , as wells as a higher minute-ventilation/CO2 production ratio ( V ˙ E / V ˙ CO 2 ratio). An uncoupling between changes in MCAv and PET CO2 during the cycling endurance exercise was also progressively apparent in PAH patients, but absent in healthy controls. Both cHHb and ΔcTOI correlated with V ˙ E / V ˙ CO 2 ratio (r = 0.50 and r = -0.52; both p < .05 respectively), but not with endurance time.

CONCLUSION

PAH patients present an abnormal cerebrovascular profile during endurance exercise with a lower cerebral oxygenation that correlate with hyperventilation but not endurance exercise time. These findings complement the physiological characterization of the cerebral vascular responses to exercise in PAH patients.

Update on Chemoreception: Influence on Cardiorespiratory Regulation and Pathophysiology

We examine recent findings that have revealed interdependence of function within the chemoreceptor pathway regulating breathing and sympathetic vasomotor activity and the hypersensitization of these reflexes in chronic disease states. Recommendations are made as to how these states of hyperreflexia in chemoreceptors and muscle afferents might be modified in treating sleep apnea, drug-resistant hypertension, chronic heart failure-induced sympathoexcitation, and the exertional dyspnea of chronic obstructive pulmonary disease.

Physiology of the Carotid Body: From Molecules to Disease

The carotid body (CB) is an arterial chemoreceptor organ located in the carotid bifurcation and has a well-recognized role in cardiorespiratory regulation. The CB contains neurosecretory sensory cells (glomus cells), which release transmitters in response to hypoxia, hypercapnia, and acidemia to activate afferent sensory fibers terminating in the respiratory and autonomic brainstem centers. Knowledge of the physiology of the CB has progressed enormously in recent years. Herein we review advances concerning the organization and function of the cellular elements of the CB, with emphasis on the molecular mechanisms of acute oxygen sensing by glomus cells. We introduce the modern view of the CB as a multimodal integrated metabolic sensor and describe the properties of the CB stem cell niche, which support CB growth during acclimatization to chronic hypoxia. Finally, we discuss the increasing medical relevance of CB dysfunction and its potential impact on the mechanisms of disease.

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.

Contribution of the Retrotrapezoid Nucleus and Carotid Bodies to Hypercapnia- and Hypoxia-induced Arousal from Sleep

The combination of hypoxia and hypercapnia during sleep produces arousal, which helps restore breathing and normalizes blood gases. Hypercapnia and hypoxia produce arousal in mammals by activating central (pH-sensitive) and peripheral (primarily O2-sensitive) chemoreceptors. The relevant chemoreceptors and the neuronal circuits responsible for arousal are largely unknown. Here we examined the contribution of two lower brainstem nuclei that could be implicated in CO2 and hypoxia-induced arousal: the retrotrapezoid nucleus (RTN), a CO2-responsive nucleus, which mediates the central respiratory chemoreflex; and the C1 neurons, which are hypoxia activated and produce arousal and blood pressure increases when directly stimulated. Additionally, we assessed the contribution of the carotid bodies (CBs), the main peripheral chemoreceptors in mammals, to hypoxia and CO2-induced arousal. In unanesthetized male rats, we tested whether ablation of the RTN, CBs, or C1 neurons affects arousal from sleep and respiratory responses to hypercapnia or hypoxia. The sleep-wake pattern was monitored by EEG and neck EMG recordings and breathing by whole-body plethysmography. The latency to arousal in response to hypoxia or hypercapnia was determined along with changes in ventilation coincident with the arousal. RTN lesions impaired CO2-induced arousal but had no effect on hypoxia-induced arousal. CB ablation impaired arousal to hypoxia and, to a lesser extent, hypercapnia. C1 neuron ablation had no effect on arousal. Thus, the RTN contributes to CO2-induced arousal, whereas the CBs contribute to both hypoxia and CO2-induced arousal. Asphyxia-induced arousal likely requires the combined activation of RTN, CBs and other central chemoreceptors.SIGNIFICANCE STATEMENT Hypercapnia and hypoxia during sleep elicit arousal, which facilitates airway clearing in the case of obstruction and reinstates normal breathing in the case of hypoventilation or apnea. Arousal can also be detrimental to health by interrupting sleep. We sought to clarify how CO2 and hypoxia cause arousal. We show that the retrotrapezoid nucleus, a brainstem nucleus that mediates the effect of brain acidification on breathing, also contributes to arousal elicited by CO2 but not hypoxia. We also show that the carotid bodies contribute predominantly to hypoxia-induced arousal. Lesions of the retrotrapezoid nucleus or carotid bodies attenuate, but do not eliminate, arousal to CO2 or hypoxia; therefore, we conclude that these structures are not the sole trigger of CO2 or hypoxia-induced arousal.