Redefining the components of central CO2 chemosensitivity--towards a better understanding of mechanism

Chloride Modulates Central pH Sensitivity and Plasticity of Brainstem Breathing-Related Biorhythms in Zebra Finch Embryos

All terrestrial vertebrate life must transition from aquatic gas exchange in the embryonic environment to aerial or pulmonary respiration at birth. In addition to being able to breathe air, neonates must possess functional sensory feedback systems for maintaining acid-base balance. Respiratory neurons in the brainstem act as pH sensors that can adjust breathing to regulate systemic pH. The central pH sensitivity of breathing-related motor output develops over the embryonic period in the zebra finch (Taeniopygia guttata). Due to the key role of chloride ions in electrochemical stability and developmental plasticity, we tested chloride's role in the development of central pH sensitivity. We blocked gamma-aminobutyric acid-A receptors and cation-chloride cotransport that subtly modulated the low-pH effects on early breathing biorhythms. Further, chloride-free artificial cerebrospinal fluid altered the pattern and timing of breathing biorhythms and blocked the stimulating effect of acidosis in E12-14 brainstems. Early and middle stage embryos exhibited rebound plasticity in brainstem motor outputs during low-pH treatment, which was eliminated by chloride-free solution. Results show that chloride modulates low-pH sensitivity and rebound plasticity in the zebra finch embryonic brainstem, but work is needed to determine the cellular and circuit mechanisms that control functional chloride balance during acid-base disturbances.

Influence of hypercapnia and hypercapnic hypoxia on the heart rate response to apnea

We aimed to determine the relative contribution of hypercapnia and hypoxia to the bradycardic response to apneas. We hypothesized that apneas with hypercapnia would cause greater bradycardia than normoxia, similar to the response seen with hypoxia, and that apneas with hypercapnic hypoxia would induce greater bradycardia than hypoxia or hypercapnia alone. Twenty-six healthy participants (12 females; 23 ± 2 years; BMI 24 ± 3 kg/m2) underwent three gas challenges: hypercapnia (+5 torr end tidal partial pressure of CO2 [PETCO2]), hypoxia (50 torr end tidal partial pressure of O2 [PETO2]), and hypercapnic hypoxia (combined hypercapnia and hypoxia), with each condition interspersed with normocapnic normoxia. Heart rate and rhythm, blood pressure, PETCO2, PETO2, and oxygen saturation were measured continuously. Hypercapnic hypoxic apneas induced larger bradycardia (-19 ± 16 bpm) than normocapnic normoxic apneas (-11 ± 15 bpm; p = 0.002), but had a comparable response to hypoxic (-19 ± 15 bpm; p = 0.999) and hypercapnic apneas (-14 ± 14 bpm; p = 0.059). Hypercapnic apneas were not different from normocapnic normoxic apneas (p = 0.134). After removal of the normocapnic normoxic heart rate response, the change in heart rate during hypercapnic hypoxia (-11 ± 16 bpm) was similar to the summed change during hypercapnia+hypoxia (-9 ± 10 bpm; p = 0.485). Only hypoxia contributed to this bradycardic response. Under apneic conditions, the cardiac response is driven by hypoxia.

Whole-brain analysis of CO2 chemosensitive regions and identification of the retrotrapezoid and medullary raphé nuclei in the common marmoset (Callithrix jacchus)

Respiratory chemosensitivity is an important mechanism by which the brain senses changes in blood partial pressure of CO2 (PCO2). It is proposed that special neurons (and astrocytes) in various brainstem regions play key roles as CO2 central respiratory chemosensors in rodents. Although common marmosets (Callithrix jacchus), New-World non-human primates, show similar respiratory responses to elevated inspired CO2 as rodents, the chemosensitive regions in marmoset brain have not been defined yet. Here, we used c-fos immunostainings to identify brain-wide CO2-activated brain regions in common marmosets. In addition, we mapped the location of the retrotrapezoid nucleus (RTN) and raphé nuclei in the marmoset brainstem based on colocalization of CO2-induced c-fos immunoreactivity with Phox2b, and TPH immunostaining, respectively. Our data also indicated that, similar to rodents, marmoset RTN astrocytes express Phox2b and have complex processes that create a meshwork structure at the ventral surface of medulla. Our data highlight some cellular and structural regional similarities in brainstem of the common marmosets and rodents.

Neuroimmune mechanisms in fear and panic pathophysiology

Panic disorder (PD) is unique among anxiety disorders in that the emotional symptoms (e.g., fear and anxiety) associated with panic are strongly linked to body sensations indicative of threats to physiological homeostasis. For example, panic attacks often present with feelings of suffocation that evoke hyperventilation, breathlessness, or air hunger. Due to the somatic underpinnings of PD, a major focus has been placed on interoceptive signaling and it is recognized that dysfunctional body-to-brain communication pathways promote the initiation and maintenance of PD symptomatology. While body-to-brain signaling can occur via several pathways, immune and humoral pathways play an important role in communicating bodily physiological state to the brain. Accumulating evidence suggests that neuroimmune mediators play a role in fear and panic-associated disorders, although this has not been systematically investigated. Currently, our understanding of the role of immune mechanisms in the etiology and maintenance of PD remains limited. In the current review, we attempt to summarize findings that support a role of immune dysregulation in PD symptomology. We compile evidence from human studies and panic-relevant rodent paradigms that indicate a role of systemic and brain immune signaling in the regulation of fear and panic-relevant behavior and physiology. Specifically, we discuss how immune signaling can contribute to maladaptive body-to-brain communication and conditioned fear that are relevant to spontaneous and conditioned symptoms of PD and identify putative avenues warranting future investigation.

Analyzing the brainstem circuits for respiratory chemosensitivity in freely moving mice

Regulation of systemic PCO2 is a life-preserving homeostatic mechanism. In the medulla oblongata, the retrotrapezoid nucleus (RTN) and rostral medullary Raphe are proposed as CO2 chemosensory nuclei mediating adaptive respiratory changes. Hypercapnia also induces active expiration, an adaptive change thought to be controlled by the lateral parafacial region (pFL). Here, we use GCaMP6 expression and head-mounted mini-microscopes to image Ca2+ activity in these nuclei in awake adult mice during hypercapnia. Activity in the pFL supports its role as a homogenous neuronal population that drives active expiration. Our data show that chemosensory responses in the RTN and Raphe differ in their temporal characteristics and sensitivity to CO2, raising the possibility these nuclei act in a coordinated way to generate adaptive ventilatory responses to hypercapnia. Our analysis revises the understanding of chemosensory control in awake adult mouse and paves the way to understanding how breathing is coordinated with complex non-ventilatory behaviours.

Kir5.1 channels: potential role in epilepsy and seizure disorders

Inwardly rectifying potassium (Kir) channels are broadly expressed in many mammalian organ systems, where they contribute to critical physiological functions. However, the importance and function of the Kir5.1 channel (encoded by the KCNJ16 gene) have not been fully recognized. This review focuses on the recent advances in understanding the expression patterns and functional roles of Kir5.1 channels in fundamental physiological systems vital to potassium homeostasis and neurological disorders. Recent studies have described the role of Kir5.1-forming Kir channels in mouse and rat lines with mutations in the Kcnj16 gene. The animal research reveals distinct renal and neurological phenotypes, including pH and electrolyte imbalances, blunted ventilatory responses to hypercapnia/hypoxia, and seizure disorders. Furthermore, it was confirmed that these phenotypes are reminiscent of those in patient cohorts in which mutations in the KCNJ16 gene have also been identified, further suggesting a critical role for Kir5.1 channels in homeostatic/neural systems health and disease. Future studies that focus on the many functional roles of these channels, expanded genetic screening in human patients, and the development of selective small-molecule inhibitors for Kir5.1 channels, will continue to increase our understanding of this unique Kir channel family member.

The subfornical organ regulates acidosis-evoked fear by engaging microglial acid-sensor TDAG8 and forebrain neurocircuits in male mice

An important role of pH homeostasis has been suggested in the physiology of panic disorder, with acidosis as an interoceptive trigger leading to fear and panic. Identification of novel mechanisms that can translate acidosis into fear will promote a better understanding of panic physiology. The current study explores a role of the subfornical organ (SFO), a blood-brain barrier compromised brain area, in translating acidosis to fear-relevant behaviors. We performed SFO-targeted acidification in male, wild-type mice and mice lacking microglial acid-sensing G protein-coupled receptor-T-cell death-associated gene 8 (TDAG8). Localized SFO acidification evoked significant freezing and reduced exploration that was dependent on the presence of acid-sensor TDAG8. Acidosis promoted the activation of SFO microglia and neurons that were absent in TDAG8-deficient mice. The assessment of regional neuronal activation in wild-type and TDAG8-deficient mice following SFO acidification revealed significant acidosis and genotype-dependent alterations in the hypothalamus, amygdala, prefrontal cortex, and periaqueductal gray nuclei. Furthermore, mapping of interregional co-activation patterns revealed that SFO acidosis promoted positive hypothalamic-cortex associations and desynchronized SFO-cortex and amygdala-cortex associations, suggesting an interplay of homeostatic and fear regulatory areas. Importantly, these alterations were not evident in TDAG8-deficient mice. Overall, our data support a regulatory role of subfornical organ microglial acid sensing in acidosis-evoked fear, highlighting a centralized role of blood-brain barrier compromised nodes in interoceptive sensing and behavioral regulation. Identification of pathways by which humoral information can modulate fear behavior is relevant to panic disorder, where aberrant interoceptive signaling has been reported.

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.

The interplay of hypoxic and mental stress: Implications for anxiety and depressive disorders

Adequate oxygen supply is essential for the human brain to meet its high energy demands. Therefore, elaborate molecular and systemic mechanism are in place to enable adaptation to low oxygen availability. Anxiety and depressive disorders are characterized by alterations in brain oxygen metabolism and of its components, such as mitochondria or hypoxia inducible factor (HIF)-pathways. Conversely, sensitivity and tolerance to hypoxia may depend on parameters of mental stress and the severity of anxiety and depressive disorders. Here we discuss relevant mechanisms of adaptations to hypoxia, as well as their involvement in mental stress and the etiopathogenesis of anxiety and depressive disorders. We suggest that mechanisms of adaptations to hypoxia (including metabolic responses, inflammation, and the activation of chemosensitive brain regions) modulate and are modulated by stress-related pathways and associated psychiatric diseases. While severe chronic hypoxia or dysfunctional hypoxia adaptations can contribute to the pathogenesis of anxiety and depressive disorders, harnessing controlled responses to hypoxia to increase cellular and psychological resilience emerges as a novel treatment strategy for these diseases.

Central respiratory chemoreception

Brain PCO2 is sensed primarily via changes in [H+]. Small pH changes are detected in the medulla oblongata and trigger breathing adjustments that help maintain arterial PCO2 constant. Larger perturbations of brain CO2/H+, possibly also sensed elsewhere in the CNS, elicit arousal, dyspnea, and stress, and cause additional breathing modifications. The retrotrapezoid nucleus (RTN), a rostral medullary cluster of glutamatergic neurons identified by coexpression of Phoxb and Nmb transcripts, is the lynchpin of the central respiratory chemoreflex. RTN regulates breathing frequency, inspiratory amplitude, and active expiration. It is exquisitely responsive to acidosis in vivo and maintains breathing autorhythmicity during quiet waking, slow-wave sleep, and anesthesia. The RTN response to [H+] is partly an intrinsic neuronal property mediated by proton sensors TASK-2 and GPR4 and partly a paracrine effect mediated by astrocytes and the vasculature. The RTN also receives myriad excitatory or inhibitory synaptic inputs including from [H+]-responsive neurons (e.g., serotonergic). RTN is silenced by moderate hypoxia. RTN inactivity (periodic or sustained) contributes to periodic breathing and, likely, to central sleep apnea. RTN development relies on transcription factors Egr2, Phox2b, Lbx1, and Atoh1. PHOX2B mutations cause congenital central hypoventilation syndrome; they impair RTN development and consequently the central respiratory chemoreflex.

CO2 sensing by connexin26 and its role in the control of breathing

Breathing is essential to provide the O2 required for metabolism and to remove its inevitable CO2 by-product. The rate and depth of breathing is controlled to regulate the excretion of CO2 to maintain the pH of arterial blood at physiological values. A widespread consensus is that chemosensory cells in the carotid body and brainstem measure blood and tissue pH and adjust the rate of breathing to ensure its homeostatic regulation. In this review, I shall consider the evidence that underlies this consensus and highlight historical data indicating that direct sensing of CO2 also plays a significant role in the regulation of breathing. I shall then review work from my laboratory that provides a molecular mechanism for the direct detection of CO2 via the gap junction protein connexin26 (Cx26) and demonstrates the contribution of this mechanism to the chemosensory regulation of breathing. As there are many pathological mutations of Cx26 in humans, I shall discuss which of these alter the CO2 sensitivity of Cx26 and the extent to which these mutations could affect human breathing. I finish by discussing the evolution of the CO2 sensitivity of Cx26 and its link to the evolution of amniotes.

Opposing modulation of Cx26 gap junctions and hemichannels by CO2

KEY POINTS

A moderate increase in P C O 2 (55 mmHg) closes Cx26 gap junctions. This effect of CO₂ is independent of changes in intra- or extracellular pH. The CO₂ -dependent closing effect depends on the same residues (K125 and R104) that are required for the CO₂ -dependent opening of Cx26 hemichannels. Pathological mutations of Cx26 abolish the CO₂ -dependent closing of the gap junction. Elastic network modelling suggests that the effect of CO₂ on Cx26 hemichannels and gap junctions is mediated through changes in the lowest entropy state of the protein.

ABSTRACT

Cx26 hemichannels open in response to moderate elevations of CO₂ ( P C O 2 55 mmHg) via a carbamylation reaction that depends on residues K125 and R104. Here we investigate the action of CO₂ on Cx26 gap junctions. Using a dye transfer assay, we found that an elevated P C O 2 of 55 mmHg greatly delayed the permeation of a fluorescent glucose analogue (NBDG) between HeLa cells coupled by Cx26 gap junctions. However, the mutations K125R or R104A abolished this effect of CO₂ . Whole cell recordings demonstrated that elevated CO₂ reduced the Cx26 gap junction conductance (median reduction 66.7%, 95% CI, 50.5-100.0%) but had no effect on Cx26^K125R or Cx31 gap junctions. CO₂ can cause intracellular acidification. Using 30 mm propionate, we found that acidification in the absence of a change in P C O 2 caused a median reduction in the gap junction conductance of 41.7% (95% CI, 26.6-53.7%). This effect of propionate was unaffected by the K125R mutation (median reduction 48.1%, 95% CI, 28.0-86.3%). pH-dependent and CO₂ -dependent closure of the gap junction are thus mechanistically independent. Mutations of Cx26 associated with the keratitis ichthyosis deafness syndrome (N14K, A40V and A88V), in combination with the mutation M151L, also abolished the CO₂ -dependent gap junction closure. Elastic network modelling suggests that the lowest entropy state when CO₂ is bound is the closed configuration for the gap junction but the open state for the hemichannel. The opposing actions of CO₂ on Cx26 gap junctions and hemichannels thus depend on the same residues and presumed carbamylation reaction.

Connexin26 mediates CO2-dependent regulation of breathing via glial cells of the medulla oblongata

Breathing is highly sensitive to the PCO2 of arterial blood. Although CO2 is detected via the proxy of pH, CO2 acting directly via Cx26 may also contribute to the regulation of breathing. Here we exploit our knowledge of the structural motif of CO2-binding to Cx26 to devise a dominant negative subunit (Cx26DN) that removes the CO2-sensitivity from endogenously expressed wild type Cx26. Expression of Cx26DN in glial cells of a circumscribed region of the mouse medulla - the caudal parapyramidal area - reduced the adaptive change in tidal volume and minute ventilation by approximately 30% at 6% inspired CO2. As central chemosensors mediate about 70% of the total response to hypercapnia, CO2-sensing via Cx26 in the caudal parapyramidal area contributed about 45% of the centrally-mediated ventilatory response to CO2. Our data unequivocally link the direct sensing of CO2 to the chemosensory control of breathing and demonstrates that CO2-binding to Cx26 is a key transduction step in this fundamental process.

Terminal Nerve GnRH3 Neurons Mediate Slow Avoidance of Carbon Dioxide in Larval Zebrafish

Escape responses to threatening stimuli are vital for survival in all animal species. Larval zebrafish display fast escape responses when exposed to tactile, acoustic, and visual stimuli. However, their behavioral responses to chemosensory stimuli remain unknown. In this study, we found that carbon dioxide (CO₂) induced a slow avoidance response, which was distinct from the touch-evoked fast escape response. We identified the gonadotropin-releasing hormone 3-expressing terminal nerve as the CO₂ sensor in the nose. Wide-field calcium imaging revealed downstream CO₂-activated ensembles of neurons along three distinct neural pathways, olfactory, trigeminal, and habenulo-interpeduncular, further reaching the reticulospinal neurons in the hindbrain. Ablation of the nose, terminal nerve, or trigeminal ganglion resulted in a dramatic decrease in CO₂-evoked avoidance responses. These findings demonstrate that the terminal nerve-trigeminal system plays a pivotal role in triggering a slow chemosensory avoidance behavior in the larval zebrafish.

A Physiological Overview of the Demands, Characteristics, and Adaptations of Highly Trained Artistic Swimmers: a Literature Review

Artistic swimming (AS) is a very unique sport consisting of difficult artistically choreographed routines ranging in the number of athletes (one to ten: solo, duet, team, combination, highlight routine) and with elements performed quickly and precisely above, below, and on the surface of the water. As a result, the physical and physiological demands placed on an athlete are unique to the sport with the most pronounced adaptation being the bradycardic response to long apneic periods spent underwater while performing strenuous movements. This indeed influences training prescription and the desired training outcomes. This review paper explores the physiological demands of AS, the physiological characteristics that influence AS performance, and innovative approaches to enhancing training and performance in elite performers.

The Role Of Parafacial Neurons In The Control Of Breathing During Exercise

Neuronal cell groups residing within the retrotrapezoid nucleus (RTN) and C1 area of the rostral ventrolateral medulla oblongata contribute to the maintenance of resting respiratory activity and arterial blood pressure, and play an important role in the development of cardiorespiratory responses to metabolic challenges (such as hypercapnia and hypoxia). In rats, acute silencing of neurons within the parafacial region which includes the RTN and the rostral aspect of the C1 circuit (pF_RTN/C1), transduced to express HM₄D (G_i-coupled) receptors, was found to dramatically reduce exercise capacity (by 60%), determined by an intensity controlled treadmill running test. In a model of simulated exercise (electrical stimulation of the sciatic or femoral nerve in urethane anaesthetised spontaneously breathing rats) silencing of the pF_RTN/C1 neurons had no effect on cardiovascular changes, but significantly reduced the respiratory response during steady state exercise. These results identify a neuronal cell group in the lower brainstem which is critically important for the development of the respiratory response to exercise and, determines exercise capacity.

Theoretical perspectives on central chemosensitivity: CO2/H+-sensitive neurons in the locus coeruleus

Central chemoreceptors are highly sensitive neurons that respond to changes in pH and CO₂ levels. An increase in CO₂/H⁺ typically reflects a rise in the firing rate of these neurons, which stimulates an increase in ventilation. Here, we present an ionic current model that reproduces the basic electrophysiological activity of individual CO₂/H⁺-sensitive neurons from the locus coeruleus (LC). We used this model to explore chemoreceptor discharge patterns in response to electrical and chemical stimuli. The modeled neurons showed both stimulus-evoked activity and spontaneous activity under physiological parameters. Neuronal responses to electrical and chemical stimulation showed specific firing patterns of spike frequency adaptation, postinhibitory rebound, and post-stimulation recovery. Conversely, the response to chemical stimulation alone (based on physiological CO₂/H⁺ changes), in the absence of external depolarizing stimulation, showed no signs of postinhibitory rebound or post-stimulation recovery, and no depolarizing sag. A sensitivity analysis for the firing-rate response to the different stimuli revealed that the contribution of an applied stimulus current exceeded that of the chemical signals. The firing-rate response increased indefinitely with injected depolarizing current, but reached saturation with chemical stimuli. Our computational model reproduced the regular pacemaker-like spiking pattern, action potential shape, and most of the membrane properties that characterize CO₂/H⁺-sensitive neurons from the locus coeruleus. This validates the model and highlights its potential as a tool for studying the cellular mechanisms underlying the altered central chemosensitivity present in a variety of disorders such as sudden infant death syndrome, depression, and anxiety. In addition, the model results suggest that small external electrical signals play a greater role in determining the chemosensitive response to changes in CO₂/H⁺ than previously thought. This highlights the importance of considering electrical synaptic transmission in studies of intrinsic chemosensitivity.

The sensory mechanism by which changes in CO₂ and H⁺ levels are detected in the brain is known as central chemoreception. Altered chemoreception is common to a wide variety of clinical conditions, including sleep apnea, sudden infant death syndrome, hyperventilation, depression, anxiety and asthma. In addition, CO₂/H⁺-sensitive neurons are present in some regions of the brain that have been identified as drug targets for the treatment of anxiety and panic disorders. We are interested in understanding the cellular mechanisms that determine and modulate the behavior of these neurons. We previously investigated possible mechanisms underlying their behavior in rats to elucidate whether they respond to changes in intracellular or extracellular pH, CO₂, or a combination of these stimuli. To study the roles that signals and ion channel targets play in individual neurons we develop mathematical models that simulate their electrochemical behavior and their responses to hypercapnic and/or acidotic stimuli. Nowadays, we are focused on using computational tools to explore the firing pattern of such neurons in response to chemical (CO₂/H⁺) and electrical (synaptic) stimulation. Our results reveal significant effects of electrical stimulation on the responses of brainstem neurons and highlight the importance of considering synaptic transmission in experimental studies of chemosensitivity.

Functional roles of three cues that provide nonsynaptic modes of communication in the brain: electromagnetic field, oxygen, and carbon dioxide

The integrative actions of the brain depend on the exchange of information among its computational elements. Hence, this phenomenon plays the key role in driving the complex dynamics of the central nervous system, in which true computations interact with noncomputational dynamical processes to generate brain representations of the body and of the body in the external world, and hence the finalistic behavior of the organism. In this context, it should be pointed out that, besides the intercellular interactions mediated by classical electrochemical signals, other types of interactions, namely, "cues" and "coercions," also appear to be exploited by the system to achieve its function. The present review focuses mainly on cues present in the environment and on those produced by cells of the body, which "pervade" the brain and contribute to its dynamics. These cues can also be metabolic substrates, and, in most cases, they are of fundamental importance to brain function and the survival of the entire organism. Three of these highly pervasive cues will be analyzed in greater detail, namely, oxygen, carbon dioxide, and electromagnetic fields (EMF). Special emphasis will be placed on EMF, since several authors have suggested that these highly pervasive energy fluctuations may play an important role in the global integrative actions of the brain; hence, EMF signaling may transcend classical connectionist models of brain function. Thus the new concept of "broadcasted neuroconnectomics" has been introduced, which transcends the current connectomics view of the brain.

How important is the CO2 chemoreflex for the control of breathing? Environmental and evolutionary considerations

Haldane and Priestley (1905) discovered that the ventilatory control system is highly sensitive to CO₂. This "CO₂ chemoreflex" has been interpreted to dominate control of resting arterial PCO₂/pH (P_aCO₂/pH_a) by monitoring P_aCO₂/pH_a and altering ventilation through negative feedback. However, P_aCO₂/pH_a varies little in mammals as ventilation tightly couples to metabolic demands, which may minimize chemoreflex control of P_aCO₂. The purpose of this synthesis is to (1) interpret data from experimental models with meager CO₂ chemoreflexes to infer their role in ventilatory control of steady-state P_aCO₂, and (2) identify physiological causes of respiratory acidosis occurring normally across vertebrate classes. Interestingly, multiple rodent and amphibian models with minimal/absent CO₂ chemoreflexes exhibit normal ventilation, gas exchange, and P_aCO₂/pH_a. The chemoreflex, therefore, plays at most a minor role in ventilatory control at rest; however, the chemoreflex may be critical for recovering P_aCO₂ following acute respiratory acidosis induced by breath-holding and activity in many ectothermic vertebrates. An apparently small role for CO₂ feedback in the genesis of normal breathing contradicts the prevailing view that central CO₂/pH chemoreceptors increased in importance throughout vertebrate evolution. Since the CO₂ chemoreflex contributes minimally to resting ventilation, these CO₂ chemoreceptors may have instead decreased importance throughout tetrapod evolution, particularly with the onset and refinement of neural innovations that improved the matching of ventilation to tissue metabolic demands. This distinct and elusive "metabolic ventilatory drive" likely underlies steady-state P_aCO₂ in air-breathers. Uncovering the mechanisms and evolution of the metabolic ventilatory drive presents a challenge to clinically-oriented and comparative respiratory physiologists alike.

Modulation of sensory information processing by a neuroglobin in Caenorhabditis elegans

Sensory receptor neurons match their dynamic range to ecologically relevant stimulus intensities. How this tuning is achieved is poorly understood in most receptors. The roundworm Caenorhabditis elegans avoids 21% O₂ and hypoxia and prefers intermediate O₂ concentrations. We show how this O₂ preference is sculpted by the antagonistic action of a neuroglobin and an O₂-binding soluble guanylate cyclase. These putative molecular O₂ sensors confer a sigmoidal O₂ response curve in the URX neurons that has highest slope between 15 and 19% O₂ and approaches saturation when O₂ reaches 21%. In the absence of the neuroglobin, the response curve is shifted to lower O₂ values and approaches saturation at 14% O₂ In behavioral terms, neuroglobin signaling broadens the O₂ preference of Caenorhabditis elegans while maintaining avoidance of 21% O₂ A computational model of aerotaxis suggests the relationship between GLB-5-modulated URX responses and reversal behavior is sufficient to broaden O₂ preference. In summary, we show that a neuroglobin can shift neural information coding leading to altered behavior. Antagonistically acting molecular sensors may represent a common mechanism to sharpen tuning of sensory neurons.

Differential behavioral sensitivity to carbon dioxide (CO2) inhalation in rats

Inhalation of carbon dioxide (CO₂) is frequently employed as a biological challenge to evoke intense fear and anxiety. In individuals with panic disorder, CO₂ reliably evokes panic attacks. Sensitivity to CO₂ is highly heterogeneous among individuals, and although a genetic component is implicated, underlying mechanisms are not clear. Preclinical models that can simulate differential responsivity to CO₂ are therefore relevant. In the current study we investigated CO₂-evoked behavioral responses in four different rat strains: Sprague-Dawley (SD), Wistar (W), Long Evans (LE) and Wistar-Kyoto, (WK) rats. We also assessed tryptophan hydroxylase 2 (TPH-2)-positive serotonergic neurons in anxiety/panic regulatory subdivisions of the dorsal raphe nucleus (DR), as well as dopamine β hydroxylase (DβH)-positive noradrenergic neurons in the locus coeruleus, implicated in central CO₂-chemosensitivity. Behavioral responsivity to CO₂ inhalation varied between strains. CO₂-evoked immobility was significantly higher in LE and WK rats as compared with W and SD cohorts. Differences were also observed in CO₂-evoked rearing and grooming behaviors. Exposure to CO₂ did not produce conditioned behavioral responses upon re-exposure to CO₂ context in any strain. Reduced TPH-2-positive cell counts were observed specifically in the panic-regulatory dorsal raphe ventrolateral (DRVL)-ventrolateral periaqueductal gray (VLPAG) subdivision in CO₂-sensitive strains. Conversely, DβH-positive cell counts within the LC were significantly higher in CO₂-sensitive strains. Collectively, our data provide evidence for strain dependent, differential CO₂-sensitivity and potential differences in monoaminergic systems regulating panic and anxiety. Comparative studies between CO₂-vulnerable and resistant strains may facilitate the mechanistic understanding of differential CO₂-sensitivity in the development of panic and anxiety disorders.

CO2-Induced ATP-Dependent Release of Acetylcholine on the Ventral Surface of the Medulla Oblongata

Complex mechanisms that detect changes in brainstem parenchymal PCO2/[H+] and trigger adaptive changes in lung ventilation are responsible for central respiratory CO2 chemosensitivity. Previous studies of chemosensory signalling pathways suggest that at the level of the ventral surface of the medulla oblongata (VMS), CO2-induced changes in ventilation are (at least in part) mediated by the release and actions of ATP and/or acetylcholine (ACh). Here we performed simultaneous real-time biosensor recordings of CO2-induced ATP and ACh release from the VMS in vivo and in vitro, to test the hypothesis that central respiratory CO2 chemosensory transduction involves simultaneous recruitment of purinergic and cholinergic signalling pathways. In anaesthetised and artificially ventilated rats, an increase in inspired CO2 triggered ACh release on the VMS with a peak amplitude of ~5 μM. Release of ACh was only detected after the onset of CO2-induced activation of the respiratory activity and was markedly reduced (by ~70%) by ATP receptor blockade. In horizontal slices of the VMS, CO2-induced release of ATP was reliably detected, whereas CO2 or bath application of ATP (100 μM) failed to trigger release of ACh. These results suggest that during hypercapnia locally produced ATP induces or potentiates the release of ACh (likely from the medullary projections of distal groups of cholinergic neurones), which may also contribute to the development and/or maintenance of the ventilatory response to CO2.

Effect of temperature on chemosensitive locus coeruleus neurons of savannah monitor lizards, Varanus exanthematicus

Savannah monitor lizards (Varanus exanthematicus) are unusual among ectothermic vertebrates in maintaining arterial pH nearly constant during changes in body temperature in contrast to the typical α-stat regulating strategy of most other ectotherms. Given the importance of pH in the control of ventilation, we examined the CO₂/H⁺ sensitivity of neurons from the locus coeruleus (LC) region of monitor lizard brainstems. Whole-cell patch-clamp electrophysiology was used to record membrane voltage in LC neurons in brainstem slices. Artificial cerebral spinal fluid equilibrated with 80% O₂, 0.0-10.0% CO₂, balance N₂, was superfused across brainstem slices. Changes in firing rate of LC neurons were calculated from action potential recordings to quantify the chemosensitive response to hypercapnic acidosis. Our results demonstrate that the LC brainstem region contains neurons that can be excited or inhibited by, and/or are not sensitive to CO₂ in V. exanthematicus While few LC neurons were activated by hypercapnic acidosis (15%), a higher proportion of the LC neurons responded by decreasing their firing rate during exposure to high CO₂ at 20°C (37%); this chemosensitive response was no longer exhibited when the temperature was increased to 30°C. Further, the proportion of chemosensitive LC neurons changed at 35°C with a reduction in CO₂-inhibited (11%) neurons and an increase in CO₂-activated (35%) neurons. Expressing a high proportion of inhibited neurons at low temperature may provide insights into mechanisms underlying the temperature-dependent pH-stat regulatory strategy of savannah monitor lizards.

CO2-evoked release of PGE2 modulates sighs and inspiration as demonstrated in brainstem organotypic culture

Inflammation-induced release of prostaglandin E₂ (PGE₂) changes breathing patterns and the response to CO₂ levels. This may have fatal consequences in newborn babies and result in sudden infant death. To elucidate the underlying mechanisms, we present a novel breathing brainstem organotypic culture that generates rhythmic neural network and motor activity for 3 weeks. We show that increased CO₂ elicits a gap junction-dependent release of PGE₂. This alters neural network activity in the preBötzinger rhythm-generating complex and in the chemosensitive brainstem respiratory regions, thereby increasing sigh frequency and the depth of inspiration. We used mice lacking eicosanoid prostanoid 3 receptors (EP3R), breathing brainstem organotypic slices and optogenetic inhibition of EP3R^+/+ cells to demonstrate that the EP3R is important for the ventilatory response to hypercapnia. Our study identifies a novel pathway linking the inflammatory and respiratory systems, with implications for inspiration and sighs throughout life, and the ability to autoresuscitate when breathing fails.

DOI:http://dx.doi.org/10.7554/eLife.14170.001

Humans and other mammals breathe air to absorb oxygen into the body and to remove carbon dioxide. We know that in a part of the brain called the brainstem, several regions work together to create breaths, but it is not clear precisely how this works. These regions adjust our breathing to the demands placed on the body by different activities, such as sleeping or exercising. Sometimes, especially in newborn babies, the brainstem’s monitoring of oxygen and carbon dioxide does not work properly, which can lead to abnormal breathing and possibly death.

In the brain, cells called neurons form networks that can rapidly transfer information via electrical signals. Here, Forsberg et al. investigated the neural networks in the brainstem that generate and control breathing in mice. They used slices of mouse brainstem that had been kept alive in a dish in the laboratory. The slice contained an arrangement of neurons and supporting cells that allowed it to continue to produce patterns of electrical activity that are associated with breathing. Over a three-week period, Forsberg et al. monitored the activity of the cells and calculated how they were connected to each other. The experiments show that the neurons responsible for breathing were organized in a “small-world” network, in which the neurons are connected to each other directly or via small numbers of other neurons.

Further experiments tested how various factors affect the behavior of the network. For example, carbon dioxide triggered the release of a small molecule called prostaglandin E2 from cells. This molecule is known to play a role in inflammation and fever. However, in the carbon dioxide sensing region of the brainstem it acted as a signaling molecule that increased activity. Therefore, inflammation could interfere with the body’s normal response to carbon dioxide and lead to potentially life-threatening breathing problems. Furthermore, prostaglandin E2 induced deeper breaths known as sighs, which may be vital for newborn babies to be able to take their first deep breaths of life. Future challenges include understanding how the brainstem neural networks generate breathing and translate this knowledge to improve the treatment of breathing difficulties in babies.

DOI:http://dx.doi.org/10.7554/eLife.14170.002

Bio-Inspired Controller on an FPGA Applied to Closed-Loop Diaphragmatic Stimulation

Cervical spinal cord injury can disrupt connections between the brain respiratory network and the respiratory muscles which can lead to partial or complete loss of ventilatory control and require ventilatory assistance. Unlike current open-loop technology, a closed-loop diaphragmatic pacing system could overcome the drawbacks of manual titration as well as respond to changing ventilation requirements. We present an original bio-inspired assistive technology for real-time ventilation assistance, implemented in a digital configurable Field Programmable Gate Array (FPGA). The bio-inspired controller, which is a spiking neural network (SNN) inspired by the medullary respiratory network, is as robust as a classic controller while having a flexible, low-power and low-cost hardware design. The system was simulated in MATLAB with FPGA-specific constraints and tested with a computational model of rat breathing; the model reproduced experimentally collected respiratory data in eupneic animals. The open-loop version of the bio-inspired controller was implemented on the FPGA. Electrical test bench characterizations confirmed the system functionality. Open and closed-loop paradigm simulations were simulated to test the FPGA system real-time behavior using the rat computational model. The closed-loop system monitors breathing and changes in respiratory demands to drive diaphragmatic stimulation. The simulated results inform future acute animal experiments and constitute the first step toward the development of a neuromorphic, adaptive, compact, low-power, implantable device. The bio-inspired hardware design optimizes the FPGA resource and time costs while harnessing the computational power of spike-based neuromorphic hardware. Its real-time feature makes it suitable for in vivo applications.

Time windows for postnatal changes in morphology and membrane excitability of genioglossal and oculomotor motoneurons

Time windows for postnatal changes in morphology and membrane excitability of genioglossal (GG) and oculomotor (OCM) motoneurons (MNs) are yet to be fully described. Analysis of data on brain slices in vitro of the 2 populations of MNs point to a well-defined developmental program that progresses with common age-related changes characterized by: (1) increase of dendritic surface along with length and reshaping of dendritic tree complexity; (2) disappearance of gap junctions early in development; (3) decrease of membrane passive properties, such as input resistance and time constant, together with an increase in the number of cells displaying sag, and modifications in rheobase; (4) action potential shortening and afterhyperpolarization; and (5) an increase in gain and maximum firing frequency. These modifications take place at different time windows for each motoneuronal population. In GG MNs, active membrane properties change mainly during the first postnatal week, passive membrane properties in the second week, and dendritic increasing length and size in the third week of development. In OCM MNs, changes in passive membrane properties and growth of dendritic size take place during the first postnatal week, while active membrane properties and rheobase change during the second and third weeks of development. The sequential order of changes is inverted between active and passive membrane properties, and growth in size does not temporally coincide for both motoneuron populations. These findings are discussed on the basis of environmental cues related to maturation of the respiratory and OCM systems.

Conditional depletion of methyl-CpG-binding protein 2 in astrocytes depresses the hypercapnic ventilatory response in mice

Mice that are deficient in the transcription factor methyl-CpG-binding protein 2 (MeCP2) have a depressed hypercapnic ventilatory response (HCVR). The expression of MeCP2 can be selectively removed from astrocytes or neurons, thus offering a tool to dissect the role of this transcription factor in astrocytes from that in neurons. Studies were carried out in the progeny of mice that were a cross between those harboring a tamoxifen (TAM)-inducible Cre recombinase transgene driven by the human astrocytic glial fibrillary acidic protein (hGFAP) promoter, or Cre recombinase under control of the synapsin promoter, with mice containing a Cre-excisable exon III in the Mecp2 gene. The TAM-conditional excision of the Mecp2 exon allowed the respiratory CO2 response to be studied in the same animals before and after selective depletion of MeCP2 in astrocytes. Immunohistochemistry showed that following TAM treatment only ∼20% of GFAP-labeled cells in the retrotrapazoid nucleus and in the raphé magnus were positive for MeCP2. The slope of the relative increase in minute ventilation as a function of 1, 3, and 5% inspired CO2 was depressed in mice with depleted astrocyte MeCP2 compared with wild-type littermates. In contrast, selective depletion of MeCP2 in neurons did not significantly affect slope. While neurons which constitute the respiratory network ultimately determine the ventilatory response to CO2, this study demonstrates that loss of MeCP2 in astrocytes alone is sufficient to result in a dramatic attenuation of the HCVR. We propose that the glial contribution to HCVR is under the control of the MeCP2 gene.

Facing the challenge of mammalian neural microcircuits: taking a few breaths may help

Breathing in mammals is a seemingly straightforward behaviour controlled by the brain. A brainstem nucleus called the preBötzinger Complex sits at the core of the neural circuit generating respiratory rhythm. Despite the discovery of this microcircuit almost 25 years ago, the mechanisms controlling breathing remain elusive. Given the apparent simplicity and well-defined nature of regulatory breathing behaviour, the identification of much of the circuitry, and the ability to study breathing in vitro as well as in vivo, many neuroscientists and physiologists are surprised that respiratory rhythm generation is still not well understood. Our view is that conventional rhythmogenic mechanisms involving pacemakers, inhibition or bursting are problematic and that simplifying assumptions commonly made for many vertebrate neural circuits ignore consequential detail. We propose that novel emergent mechanisms govern the generation of respiratory rhythm. That a mammalian function as basic as rhythm generation arises from complex and dynamic molecular, synaptic and neuronal interactions within a diverse neural microcircuit highlights the challenges in understanding neural control of mammalian behaviours, many (considerably) more elaborate than breathing. We suggest that the neural circuit controlling breathing is inimitably tractable and may inspire general strategies for elucidating other neural microcircuits.

Neural Control of Breathing and CO2 Homeostasis

Recent advances have clarified how the brain detects CO2 to regulate breathing (central respiratory chemoreception). These mechanisms are reviewed and their significance is presented in the general context of CO2/pH homeostasis through breathing. At rest, respiratory chemoreflexes initiated at peripheral and central sites mediate rapid stabilization of arterial PCO2 and pH. Specific brainstem neurons (e.g., retrotrapezoid nucleus, RTN; serotonergic) are activated by PCO2 and stimulate breathing. RTN neurons detect CO2 via intrinsic proton receptors (TASK-2, GPR4), synaptic input from peripheral chemoreceptors and signals from astrocytes. Respiratory chemoreflexes are arousal state dependent whereas chemoreceptor stimulation produces arousal. When abnormal, these interactions lead to sleep-disordered breathing. During exercise, central command and reflexes from exercising muscles produce the breathing stimulation required to maintain arterial PCO2 and pH despite elevated metabolic activity. The neural circuits underlying central command and muscle afferent control of breathing remain elusive and represent a fertile area for future investigation.

Environmental CO2 inhibits Caenorhabditis elegans egg-laying by modulating olfactory neurons and evokes widespread changes in neural activity

Carbon dioxide (CO2) gradients are ubiquitous and provide animals with information about their environment, such as the potential presence of prey or predators. The nematode Caenorhabditis elegans avoids elevated CO2, and previous work identified three neuron pairs called "BAG," "AFD," and "ASE" that respond to CO2 stimuli. Using in vivo Ca(2+) imaging and behavioral analysis, we show that C. elegans can detect CO2 independently of these sensory pathways. Many of the C. elegans sensory neurons we examined, including the AWC olfactory neurons, the ASJ and ASK gustatory neurons, and the ASH and ADL nociceptors, respond to a rise in CO2 with a rise in Ca(2+). In contrast, glial sheath cells harboring the sensory endings of C. elegans' major chemosensory neurons exhibit strong and sustained decreases in Ca(2+) in response to high CO2. Some of these CO2 responses appear to be cell intrinsic. Worms therefore may couple detection of CO2 to that of other cues at the earliest stages of sensory processing. We show that C. elegans persistently suppresses oviposition at high CO2. Hermaphrodite-specific neurons (HSNs), the executive neurons driving egg-laying, are tonically inhibited when CO2 is elevated. CO2 modulates the egg-laying system partly through the AWC olfactory neurons: High CO2 tonically activates AWC by a cGMP-dependent mechanism, and AWC output inhibits the HSNs. Our work shows that CO2 is a more complex sensory cue for C. elegans than previously thought, both in terms of behavior and neural circuitry.

State-dependent control of breathing by the retrotrapezoid nucleus

KEY POINTS

This study explores the state dependence of the hypercapnic ventilatory reflex (HCVR). We simulated an instantaneous increase or decrease of central chemoreceptor activity by activating or inhibiting the retrotrapezoid nucleus (RTN) by optogenetics in conscious rats. During quiet wake or non-REM sleep, hypercapnia increased both breathing frequency (fR ) and tidal volume (VT ) whereas, in REM sleep, hypercapnia increased VT exclusively. Optogenetic inhibition of RTN reduced VT in all sleep-wake states, but reduced fR only during quiet wake and non-REM sleep. RTN stimulation always increased VT but raised fR only in quiet wake and non-REM sleep. Phasic RTN stimulation produced active expiration and reduced early expiratory airflow (i.e. increased upper airway resistance) only during wake. We conclude that the HCVR is highly state-dependent. The HCVR is reduced during REM sleep because fR is no longer under chemoreceptor control and thus could explain why central sleep apnoea is less frequent in REM sleep.

ABSTRACT

Breathing has different characteristics during quiet wake, non-REM or REM sleep, including variable dependence on PCO2. We investigated whether the retrotrapezoid nucleus (RTN), a proton-sensitive structure that mediates a large portion of the hypercapnic ventilatory reflex, regulates breathing differently during sleep vs. wake. Electroencephalogram, neck electromyogram, blood pressure, respiratory frequency (fR ) and tidal volume (VT ) were recorded in 28 conscious adult male Sprague-Dawley rats. Optogenetic stimulation of RTN with channelrhodopsin-2, or inhibition with archaerhodopsin, simulated an instantaneous increase or decrease of central chemoreceptor activity. Both opsins were delivered with PRSX8-promoter-containing lentiviral vectors. RTN and catecholaminergic neurons were transduced. During quiet wake or non-REM sleep, hypercapnia (3 or 6% FI,CO2 ) increased both fR and VT whereas, in REM sleep, hypercapnia increased VT exclusively. RTN inhibition always reduced VT but reduced fR only during quiet wake and non-REM sleep. RTN stimulation always increased VT but raised fR only in quiet wake and non-REM sleep. Blood pressure was unaffected by either stimulation or inhibition. Except in REM sleep, phasic RTN stimulation entrained and shortened the breathing cycle by selectively shortening the post-inspiratory phase. Phasic stimulation also produced active expiration and reduced early expiratory airflow but only during wake. VT is always regulated by RTN and CO2 but fR is regulated by CO2 and RTN only when the brainstem pattern generator is in autorhythmic mode (anaesthesia, non-REM sleep, quiet wake). The reduced contribution of RTN to breathing during REM sleep could explain why certain central apnoeas are less frequent during this sleep stage.

Acid-base dysregulation and chemosensory mechanisms in panic disorder: a translational update

Panic disorder (PD), a complex anxiety disorder characterized by recurrent panic attacks, represents a poorly understood psychiatric condition which is associated with significant morbidity and an increased risk of suicide attempts and completed suicide. Recently however, neuroimaging and panic provocation challenge studies have provided insights into the pathoetiology of panic phenomena and have begun to elucidate potential neural mechanisms that may underlie panic attacks. In this regard, accumulating evidence suggests that acidosis may be a contributing factor in induction of panic. Challenge studies in patients with PD reveal that panic attacks may be reliably provoked by agents that lead to acid-base dysbalance such as CO2 inhalation and sodium lactate infusion. Chemosensory mechanisms that translate pH into panic-relevant fear, autonomic, and respiratory responses are therefore of high relevance to the understanding of panic pathophysiology. Herein, we provide a current update on clinical and preclinical studies supporting how acid-base imbalance and diverse chemosensory mechanisms may be associated with PD and discuss future implications of these findings.

Extracellular hemoglobin: the case of a friend turned foe

Hemoglobin (Hb) is a highly conserved molecule present in all life forms and functionally tied to the complexity of aerobic organisms on earth in utilizing oxygen from the atmosphere and delivering to cells and tissues. This primary function sustains the energy requirements of cells and maintains cellular homeostasis. Decades of intensive research has presented a paradigm shift that shows how the molecule also functions to facilitate smooth oxygen delivery through the cardiovascular system for cellular bioenergetic homeostasis and signaling for cell function and defense. These roles are particularly highlighted in the binding of Hb to gaseous molecules carbon dioxide (CO₂), nitric oxide (NO) and carbon monoxide (CO), while also serving indirectly or directly as sources of these signaling molecules. The functional activities impacted by Hb outside of bioenergetics homeostasis, include fertilization, signaling functions, modulation of inflammatory responses for defense and cell viability. These activities are efficiently executed while Hb is sequestered safely within the confines of the red blood cell (rbc). Outside of rbc confines, Hb disaggregates and becomes a danger molecule to cell survival. In these perpectives, Hb function is broadly dichotomous, either a friend in its natural environment providing and facilitating the means for cell function or foe when dislocated from its habitat under stress or pathological condition disrupting cell function. The review presents insights into how this dichotomy in function manifests.

The retrotrapezoid nucleus neurons expressing Atoh1 and Phox2b are essential for the respiratory response to CO₂

Maintaining constant CO₂ and H⁺ concentrations in the arterial blood is critical for life. The principal mechanism through which this is achieved in mammals is the respiratory chemoreflex whose circuitry is still elusive. A candidate element of this circuitry is the retrotrapezoid nucleus (RTN), a collection of neurons at the ventral medullary surface that are activated by increased CO₂ or low pH and project to the respiratory rhythm generator. Here, we use intersectional genetic strategies to lesion the RTN neurons defined by Atoh1 and Phox2b expression and to block or activate their synaptic output. Photostimulation of these neurons entrains the respiratory rhythm. Conversely, abrogating expression of Atoh1 or Phox2b or glutamatergic transmission in these cells curtails the phrenic nerve response to low pH in embryonic preparations and abolishes the respiratory chemoreflex in behaving animals. Thus, the RTN neurons expressing Atoh1 and Phox2b are a necessary component of the chemoreflex circuitry.

DOI:http://dx.doi.org/10.7554/eLife.07051.001

An adult at rest will typically breathe in and out up to 20 times per minute, inhaling oxygen and exhaling carbon dioxide in a process that, for the most part, occurs automatically. While we can choose to override this process and exert voluntary control over our breathing, we cannot suppress it indefinitely. Attempting to do so will ultimately trigger a reflex that forces us to start breathing again.

This reflex is mostly a response to the rise of carbon dioxide (CO₂) in the blood, which lowers the pH of the blood. This rise in CO₂ is toxic and triggers an increase in breathing so that the excess CO₂ is exhaled. The majority of the sensors that detect CO₂ are in the brainstem, which is at the junction of the brain and the spinal cord. However, the precise location of these sensors is not clear. Ruffault et al. now argue that the sensors are in a region called the ‘retrotrapezoid nucleus’, and that they can be identified by the presence of two proteins, Atoh1 and Phox2b.

In the brains of foetal mice, Ruffault et al. recorded cells in the retrotrapezoid nucleus and found that they fired in a rhythmic pattern, as would be expected for cells that control breathing. Moreover, the firing rate of these cells increased when the pH was lowered.

Ruffault et al. then created genetically modified mice with mutations in genes for Atoh1 or Phox2b. The retrotrapezoid nucleus was either absent or abnormal in these mutant mice. Moreover, new-born pups with these mutations were not able to increase their breathing when the level of CO₂ in their blood rose.

These results shed light on the respiratory distress experienced by patients with a rare disorder called congenital central hypoventilation syndrome (CCHS) that is caused by mutations in Phox2b. More commonly, unstable or irregular breathing is seen in human infants that are born prematurely, and sometimes in infants born at full term. In the light of the new findings by Ruffault et al., it is possible that abnormal development or immaturity of the retrotrapezoid nucleus is the cause.

DOI:http://dx.doi.org/10.7554/eLife.07051.002

Hypoxia silences retrotrapezoid nucleus respiratory chemoreceptors via alkalosis

In conscious mammals, hypoxia or hypercapnia stimulates breathing while theoretically exerting opposite effects on central respiratory chemoreceptors (CRCs). We tested this theory by examining how hypoxia and hypercapnia change the activity of the retrotrapezoid nucleus (RTN), a putative CRC and chemoreflex integrator. Archaerhodopsin-(Arch)-transduced RTN neurons were reversibly silenced by light in anesthetized rats. We bilaterally transduced RTN and nearby C1 neurons with Arch (PRSx8-ArchT-EYFP-LVV) and measured the cardiorespiratory consequences of Arch activation (10 s) in conscious rats during normoxia, hypoxia, or hyperoxia. RTN photoinhibition reduced breathing equally during non-REM sleep and quiet wake. Compared with normoxia, the breathing frequency reduction (Δf(R)) was larger in hyperoxia (65% FiO2), smaller in 15% FiO2, and absent in 12% FiO2. Tidal volume changes (ΔV(T)) followed the same trend. The effect of hypoxia on Δf(R) was not arousal-dependent but was reversed by reacidifying the blood (acetazolamide; 3% FiCO2). Δf(R) was highly correlated with arterial pH up to arterial pH (pHa) 7.5 with no frequency inhibition occurring above pHa 7.53. Blood pressure was minimally reduced suggesting that C1 neurons were very modestly inhibited. In conclusion, RTN neurons regulate eupneic breathing about equally during both sleep and wake. RTN neurons are the first putative CRCs demonstrably silenced by hypocapnic hypoxia in conscious mammals. RTN neurons are silent above pHa 7.5 and increasingly active below this value. During hyperoxia, RTN activation maintains breathing despite the inactivity of the carotid bodies. Finally, during hypocapnic hypoxia, carotid body stimulation increases breathing frequency via pathways that bypass RTN.

Role of parafacial nuclei in control of breathing in adult rats

Contiguous brain regions associated with a given behavior are increasingly being divided into subregions associated with distinct aspects of that behavior. Using recently developed neuronal hyperpolarizing technologies, we functionally dissect the parafacial region in the medulla, which contains key elements of the central pattern generator for breathing that are important in central CO2-chemoreception and for gating active expiration. By transfecting different populations of neighboring neurons with allatostatin or HM4D Gi/o-coupled receptors, we analyzed the effect of their hyperpolarization on respiration in spontaneously breathing vagotomized urethane-anesthetized rats. We identify two functionally separate parafacial nuclei: ventral (pFV) and lateral (pFL). Disinhibition of the pFL with bicuculline and strychnine led to active expiration. Hyperpolarizing pFL neurons had no effect on breathing at rest, or changes in inspiratory activity induced by hypoxia and hypercapnia; however, hyperpolarizing pFL neurons attenuated active expiration when it was induced by hypercapnia, hypoxia, or disinhibition of the pFL. In contrast, hyperpolarizing pFV neurons affected breathing at rest by decreasing inspiratory-related activity, attenuating the hypoxia- and hypercapnia-induced increase in inspiratory activity, and when present, reducing expiratory-related abdominal activity. Together with previous observations, we conclude that the pFV provides a generic excitatory drive to breathe, even at rest, whereas the pFL is a conditional oscillator quiet at rest that, when activated, e.g., during exercise, drives active expiration.

Carbon dioxide sensitivity and its role in multifunctional neurons in the mollusk Onchidium

Intrinsically photoresponsive neurons in the abdominal ganglion of the amphibious mollusk Onchidium named Ip-1 and Ip-2 (Ip-1/2) react to several different stimuli. These neurons respond to light with slow hyperpolarization and to CO2 stimulation with slow depolarization. In this study, increasing the concentration of CO2 in the air caused hyperventilation and enlargement of the pneumostome in the intact animal. In a semi-intact preparation, pouring artificial seawater (ASW) with dissolved CO2 onto the central ganglia caused the previously closed pneumostome to open. In an ASW environment, Ip-1/2 neurons depolarized even under conditions of constant pH (alkaline ASW) and after dissolution of CO2. This depolarization prolonged the firing of action potentials in Ip-1/2 neurons. Adding protons (H+) to ASW caused Ip-1/2 depolarization only when the neurons' membranes were depolarized to a potential above the resting potential. Furthermore, in the presence of the carbonic anhydrase inhibitor acetazolamide (AZ), CO2-induced excitation in Ip-1/2 neurons was increased in both normal and alkaline ASW. These results suggest that when dissolved in ASW, CO2 directly induced the depolarizing response in Ip-1/2 neurons. Since Ip-1/2 neurons participate in pneumostome opening, these results suggest that increased CO2 levels in ASW directly stimulate CO2-sensitive central neurons, promoting ventilation.

Regulation of breathing and autonomic outflows by chemoreceptors

Lung ventilation fluctuates widely with behavior but arterial PCO2 remains stable. Under normal conditions, the chemoreflexes contribute to PaCO2 stability by producing small corrective cardiorespiratory adjustments mediated by lower brainstem circuits. Carotid body (CB) information reaches the respiratory pattern generator (RPG) via nucleus solitarius (NTS) glutamatergic neurons which also target rostral ventrolateral medulla (RVLM) presympathetic neurons thereby raising sympathetic nerve activity (SNA). Chemoreceptors also regulate presympathetic neurons and cardiovagal preganglionic neurons indirectly via inputs from the RPG. Secondary effects of chemoreceptors on the autonomic outflows result from changes in lung stretch afferent and baroreceptor activity. Central respiratory chemosensitivity is caused by direct effects of acid on neurons and indirect effects of CO2 via astrocytes. Central respiratory chemoreceptors are not definitively identified but the retrotrapezoid nucleus (RTN) is a particularly strong candidate. The absence of RTN likely causes severe central apneas in congenital central hypoventilation syndrome. Like other stressors, intense chemosensory stimuli produce arousal and activate circuits that are wake- or attention-promoting. Such pathways (e.g., locus coeruleus, raphe, and orexin system) modulate the chemoreflexes in a state-dependent manner and their activation by strong chemosensory stimuli intensifies these reflexes. In essential hypertension, obstructive sleep apnea and congestive heart failure, chronically elevated CB afferent activity contributes to raising SNA but breathing is unchanged or becomes periodic (severe CHF). Extreme CNS hypoxia produces a stereotyped cardiorespiratory response (gasping, increased SNA). The effects of these various pathologies on brainstem cardiorespiratory networks are discussed, special consideration being given to the interactions between central and peripheral chemoreflexes.

Reprint of "Drawing breath without the command of effectors: the control of respiration following spinal cord injury"

The maintenance of blood gas and pH homeostasis is essential to life. As such breathing, and the mechanisms which control ventilation, must be tightly regulated yet highly plastic and dynamic. However, injury to the spinal cord prevents the medullary areas which control respiration from connecting to respiratory effectors and feedback mechanisms below the level of the lesion. This trauma typically leads to severe and permanent functional deficits in the respiratory motor system. However, endogenous mechanisms of plasticity occur following spinal cord injury to facilitate respiration and help recover pulmonary ventilation. These mechanisms include the activation of spared or latent pathways, endogenous sprouting or synaptogenesis, and the possible formation of new respiratory control centres. Acting in combination, these processes provide a means to facilitate respiratory support following spinal cord trauma. However, they are by no means sufficient to return pulmonary function to pre-injury levels. A major challenge in the study of spinal cord injury is to understand and enhance the systems of endogenous plasticity which arise to facilitate respiration to mediate effective treatments for pulmonary dysfunction.

Etiology, triggers and neurochemical circuits associated with unexpected, expected, and laboratory-induced panic attacks

Panic disorder (PD) is a severe anxiety disorder that is characterized by recurrent panic attacks (PA), which can be unexpected (uPA, i.e., no clear identifiable trigger) or expected (ePA). Panic typically involves an abrupt feeling of catastrophic fear or distress accompanied by physiological symptoms such as palpitations, racing heart, thermal sensations, and sweating. Recurrent uPA and ePA can also lead to agoraphobia, where subjects with PD avoid situations that were associated with PA. Here we will review recent developments in our understanding of PD, which includes discussions on: symptoms and signs associated with uPA and ePAs; Diagnosis of PD and the new DSM-V; biological etiology such as heritability and gene×environment and gene×hormonal development interactions; comparisons between laboratory and naturally occurring uPAs and ePAs; neurochemical systems that are associated with clinical PAs (e.g. gene associations; targets for triggering or treating PAs), adaptive fear and panic response concepts in the context of new NIH RDoc approach; and finally strengths and weaknesses of translational animal models of adaptive and pathological panic states.

Timing, sleep, and respiration in health and disease

Breathing is perhaps the physiological function that is most vital to human survival. Without breathing and adequate oxygenation of tissues, life ceases. As would be expected for such a vital function, breathing occurs automatically, without the requirement of conscious input. Breathing is subject to regulation by a variety of factors including circadian rhythms and vigilance state. Given the need for breathing to occur continuously with little tolerance for interruption, it is not surprising that breathing is subject to both circadian phase-dependent and vigilance-state-dependent regulation. Similarly, the information regarding respiratory state, including blood-gas concentrations, can affect circadian timing and sleep-wake state. The exact nature of the interactions between breathing, circadian phase, and vigilance state can vary depending upon the species studied and the methodologies employed. These interactions between breathing, circadian phase, and vigilance state may have important implications for a variety of human diseases, including sleep apnea, asthma, sudden unexpected death in epilepsy, and sudden infant death syndrome.

Purinergic signalling contributes to chemoreception in the retrotrapezoid nucleus but not the nucleus of the solitary tract or medullary raphe

Several brain regions are thought to function as important sites of chemoreception including the nucleus of the solitary tract (NTS), medullary raphe and retrotrapezoid nucleus (RTN). In the RTN, mechanisms of chemoreception involve direct H(+)-mediated activation of chemosensitive neurons and indirect modulation of chemosensitive neurons by purinergic signalling. Evidence suggests that RTN astrocytes are the source of CO2-evoked ATP release. However, it is not clear whether purinergic signalling also influences CO2/H(+) responsiveness of other putative chemoreceptors. The goals of this study are to determine if CO2/H(+)-sensitive neurons in the NTS and medullary raphe respond to ATP, and whether purinergic signalling in these regions influences CO2 responsiveness in vitro and in vivo. In brain slices, cell-attached recordings of membrane potential show that CO2/H(+)-sensitive NTS neurons are activated by focal ATP application; however, purinergic P2-receptor blockade did not affect their CO2/H(+) responsiveness. CO2/H(+)-sensitive raphe neurons were unaffected by ATP or P2-receptor blockade. In vivo, ATP injection into the NTS increased cardiorespiratory activity; however, injection of a P2-receptor blocker into this region had no effect on baseline breathing or CO2/H(+) responsiveness. Injections of ATP or a P2-receptor blocker into the medullary raphe had no effect on cardiorespiratory activity or the chemoreflex. As a positive control we confirmed that ATP injection into the RTN increased breathing and blood pressure by a P2-receptor-dependent mechanism. These results suggest that purinergic signalling is a unique feature of RTN chemoreception.

Temperature influences neuronal activity and CO2/pH sensitivity of locus coeruleus neurons in the bullfrog, Lithobates catesbeianus

The locus coeruleus (LC) is a chemoreceptive brain stem region in anuran amphibians and contains neurons sensitive to physiological changes in CO2/pH. The ventilatory and central sensitivity to CO2/pH is proportional to the temperature in amphibians, i.e., sensitivity increases with increasing temperature. We hypothesized that LC neurons from bullfrogs, Lithobates catesbeianus, would increase CO2/pH sensitivity with increasing temperature and decrease CO2/pH sensitivity with decreasing temperature. Further, we hypothesized that cooling would decrease, while warming would increase, normocapnic firing rates of LC neurons. To test these hypotheses, we used whole cell patch-clamp electrophysiology to measure firing rate, membrane potential ( Vm), and input resistance ( Rin) in LC neurons in brain stem slices from adult bullfrogs over a physiological range of temperatures during normocapnia and hypercapnia. We found that cooling reduced chemosensitive responses of LC neurons as temperature decreased until elimination of CO2/pH sensitivity at 10°C. Chemosensitive responses increased at elevated temperatures. Surprisingly, chemosensitive LC neurons increased normocapnic firing rate and underwent membrane depolarization when cooled and decreased normocapnic firing rate and underwent membrane hyperpolarization when warmed. These responses to temperature were not observed in nonchemosensitive LC neurons or neurons in a brain stem slice 500 μm rostral to the LC. Our results indicate that modulation of cellular chemosensitivity within the LC during temperature changes may influence temperature-dependent respiratory drive during acid-base disturbances in amphibians. Additionally, cold-activated/warm-inhibited LC neurons introduce paradoxical temperature sensitivity in respiratory control neurons of amphibians.

CO₂directly modulates connexin 26 by formation of carbamate bridges between subunits

Homeostatic regulation of the partial pressure of CO₂ (PCO₂) is vital for life. Sensing of pH has been proposed as a sufficient proxy for determination of PCO₂ and direct CO₂-sensing largely discounted. Here we show that connexin 26 (Cx26) hemichannels, causally linked to respiratory chemosensitivity, are directly modulated by CO₂. A ‘carbamylation motif’, present in CO₂-sensitive connexins (Cx26, Cx30, Cx32) but absent from a CO₂-insensitive connexin (Cx31), comprises Lys125 and four further amino acids that orient Lys125 towards Arg104 of the adjacent subunit of the connexin hexamer. Introducing the carbamylation motif into Cx31 created a mutant hemichannel (mCx31) that was opened by increases in PCO₂. Mutation of the carbamylation motif in Cx26 and mCx31 destroyed CO₂ sensitivity. Course-grained computational modelling of Cx26 demonstrated that the proposed carbamate bridge between Lys125 and Arg104 biases the hemichannel to the open state. Carbamylation of Cx26 introduces a new transduction principle for physiological sensing of CO₂.

DOI:http://dx.doi.org/10.7554/eLife.01213.001

A number of gaseous molecules, including nitric oxide and carbon monoxide, play important roles in many cellular processes by acting as signalling molecules. Surprisingly, however, it has long been assumed that carbon dioxide – a gaseous molecule that is produced during cellular metabolism – is not a signalling molecule.

Controlling the concentration of carbon dioxide (CO₂) in a biological system is essential to sustain life, and it was thought that the body used pH – which is the concentration of hydrogen ions – as a proxy for the level of CO₂. The concentration of CO₂ is related to pH because CO₂ reacts with water to form carbonic acid, which quickly breaks down to form hydrogen ions and bicarbonate ions. This close relationship has led many researchers to equate pH-sensing with CO₂-sensing, and to suggest that a physiological receptor for CO₂ does not exist.

Recent research into structures called connexin hemichannels has challenged this view. Researchers found that when pH levels were held constant, increasing the level of CO₂ caused the structures to open up, suggesting that CO₂ could be directly detected by the hemichannels. Each hemichannel contains six connexin subunits, but the details of how the CO₂ molecules interact with the individual connexin subunits to open up the hemichannels remained mysterious.

Now Meigh et al. show that CO₂ molecules bind to a specific amino acid (lysine) at a particular place (residue 125) in one of the connexin subunits to form a carbamate group. This group then interacts with the amino acid (arginine) at residue 104 in a neighbouring connexin subunit to form a carbamate bridge between the two subunits. This leads to structural changes that cause the gap junction hemichannels to open and release signals that can activate other cells. Since connexin hemichannels are found throughout the human body, these results suggest that CO₂ might act as a signalling molecule in processes as diverse as the control of blood flow, breathing, hearing and reproduction.

DOI:http://dx.doi.org/10.7554/eLife.01213.002

Short- and long-term effects of a maternal low-protein diet on ventilation, O₂/CO₂ chemoreception and arterial blood pressure in male rat offspring

Maternal undernutrition increases the risk of adult arterial hypertension. The present study investigated the short- and long-term effects of a maternal low-protein diet on respiratory rhythm, O₂/CO₂ chemosensitivity and arterial blood pressure (ABP) of the offspring. Male Wistar rats were divided into two groups according to their mothers' diets during gestation and lactation: control (NP, 17% of casein) and low-protein (LP, 8% of casein) groups. Direct measurements of ABP, respiratory frequency (RF), tidal volume (V T) and ventilation (VE), as well as hypercapnia (7% CO₂) and hypoxia (7% O₂) evoked respiratory responses were recorded from the awake male offspring at the 30th and 90th days of life. Blood samples were collected for the analyses of protein, creatinine and urea concentrations. The LP offspring had impaired body weight and length throughout the experiment. At 30 d of age, the LP rats showed a reduction in the concentrations of total serum protein (approximately 24%). ABP in the LP rats was similar to that in the NP rats at 30 d of age, but it was 20% higher at 90 d of age. With respect to ventilatory parameters, the LP rats showed enhanced RF (approximately 34%) and VE (approximately 34%) at 30 d of age, which was associated with increased ventilatory responses to hypercapnia (approximately 21% in VE) and hypoxia (approximately 82% in VE). At 90 d of age, the VE values and CO₂/O₂ chemosensitivity of the LP rats were restored to the control range, but the RF values remained elevated. The present data show that a perinatal LP diet alters respiratory rhythm and O₂/CO₂ chemosensitivity at early ages, which may be a predisposing factor for increased ABP at adulthood.

Carbon dioxide-sensing in organisms and its implications for human disease

The capacity of organisms to sense changes in the levels of internal and external gases and to respond accordingly is central to a range of physiologic and pathophysiologic processes. Carbon dioxide, a primary product of oxidative metabolism is one such gas that can be sensed by both prokaryotic and eukaryotic cells and in response to altered levels, elicit the activation of multiple adaptive pathways. The outcomes of activating CO2-sensitive pathways in various species include increased virulence of fungal and bacterial pathogens, prey-seeking behavior in insects as well as taste perception, lung function, and the control of immunity in mammals. In this review, we discuss what is known about the mechanisms underpinning CO2 sensing across a range of species and consider the implications of this for physiology, disease progression, and the possibility of developing new therapeutics for inflammatory and infectious disease.