A novel excitatory network for the control of breathing

Respiratory regulation & interactions with neuro-cognitive circuitry

It is increasingly being recognized that active control of breathing - a key aspect of ancient Vedic meditative practices, can relieve stress and anxiety and improve cognition. However, the underlying mechanisms of respiratory modulation of neurophysiology are just beginning to be elucidated. Research shows that brainstem circuits involved in the motor control of respiration receive input from and can directly modulate activity in subcortical circuits, affecting emotion and arousal. Meanwhile, brain regions involved in the sensory aspects of respiration, such as the olfactory bulb, are like-wise linked with wide-spread brain oscillations; and perturbing olfactory bulb activity can significantly affect both mood and cognition. Thus, via both motor and sensory pathways, there are clear mechanisms by which brain activity is entrained to the respiratory cycle. Here, we review evidence gathered across multiple species demonstrating the links between respiration, entrainment of brain activity and functional relevance for affecting mood and cognition. We also discuss further linkages with cardiac rhythms, and the potential translational implications for biorhythm monitoring and regulation in neuropsychiatric disorders.

Dual recombinase fate mapping reveals a transient cholinergic phenotype in multiple populations of developing glutamatergic neurons

Cholinergic transmission shapes the maturation of glutamatergic circuits, yet the developmental sources of acetylcholine have not been systematically explored. Here, we have used Cre-recombinase-mediated genetic labeling to identify and map both mature and developing CNS neurons that express choline acetyltransferase (ChAT). Correction of a significant problem with a widely used ChatCre transgenic line ensures that this map does not contain expression artifacts. ChatCre marks all known cholinergic systems in the adult brain, but also identifies several brain areas not usually regarded as cholinergic, including specific thalamic and hypothalamic neurons, the subiculum, the lateral parabrachial nucleus, the cuneate/gracilis nuclei, and the pontocerebellar system. This ChatCre fate map suggests transient developmental expression of a cholinergic phenotype in areas important for cognition, motor control, and respiration. We therefore examined expression of ChAT and the vesicular acetylcholine transporter in the embryonic and early postnatal brain to determine the developmental timing of this transient cholinergic phenotype, and found that it mirrored the establishment of relevant glutamatergic projection pathways. We then used an intersectional genetic strategy combining ChatCre with Vglut2Flp to show that these neurons adopt a glutamatergic fate in the adult brain. The transient cholinergic phenotype of these glutamatergic neurons suggests a homosynaptic source of acetylcholine for the maturation of developing glutamatergic synapses. These findings thus define critical windows during which specific glutamatergic circuits may be vulnerable to disruption by nicotine in utero, and suggest new mechanisms for pediatric disorders associated with maternal smoking, such as sudden infant death syndrome.

Kölliker-Fuse/Parabrachial complex mu opioid receptors contribute to fentanyl-induced apnea and respiratory rate depression

Overdoses caused by the opioid agonist fentanyl have increased exponentially in recent years. Identifying mechanisms to counter progression to fatal respiratory apnea during opioid overdose is desirable, but difficult to study in vivo. The pontine Kölliker-Fuse/Parabrachial complex (KF/PB) provides respiratory drive and contains opioid-sensitive neurons. The contribution of the KF/PB complex to fentanyl-induced apnea was investigated using the in situ arterially perfused preparation of rat. Systemic application of fentanyl resulted in concentration-dependent respiratory disturbances. At low concentrations, respiratory rate slowed and subsequently transitioned to an apneustic-like, 2-phase pattern. Higher concentrations caused prolonged apnea, interrupted by occasional apneustic-like bursts. Application of CTAP, a selective mu opioid receptor antagonist, directly into the KF/PB complex reversed and prevented fentanyl-induced apnea by increasing the frequency of apneustic-like bursting. These results demonstrate that countering opioid effects in the KF/PB complex is sufficient to restore phasic respiratory output at a rate similar to pre-fentanyl conditions, which could be beneficial in opioid overdose.

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.

National Institutes of Health (NIH) Executive Meeting Summary: Developing Medical Countermeasures to Rescue Opioid-Induced Respiratory Depression (a Trans-Agency Scientific Meeting)-August 6/7, 2019

On August 6th, 2019, a two-day trans-agency scientific meeting was convened by the United States (U.S.) National Institute of Allergy and Infectious Diseases (NIAID/NIH) on the research and development of medical countermeasures (MCMs) and treatment strategies to mitigate synthetic opioid-induced toxicities. This trans-agency meeting was an initiative of the Chemical Countermeasures Research Program (CCRP) and organized by the NIAID in collaboration with the National Institute of Drug Abuse (NIDA), the Biomedical Advanced Research and Development Authority (BARDA), the Food and Drug Administration (FDA), and the Defense Threat Reduction Agency (DTRA). The CCRP is part of the larger NIH biodefense research program coordinated by NIAID, which also includes MCM research and development programs against biological, radiological, and nuclear threats. Its overarching goal is to integrate cutting-edge research and technological advances in science and medicine to enhance the nation's medical response capabilities during and after a public health emergency involving the deliberate or accidental release of toxic chemicals. The potential of a mass casualty public health event involving synthetic opioids is a rapidly growing concern. As such, the overall goals of this trans-agency meeting are to better understand opioid-induced toxicities and advance the development of MCMs to mitigate and reverse opioid-induced respiratory depression (OIRD) to prevent consequential mortality. The primary objectives of the meeting were (1) highlight the latest research on mechanisms of OIRD and related toxicities, animal models, diagnostics, delivery technologies, and emerging new treatment options to prevent lethality; (2) identify current knowledge gaps to advance medical countermeasure development; (3) hear from the U.S. FDA on regulatory considerations to support new technology and treatment approaches; and (4) provide a forum for networking and collaborative partnerships. To accomplish this, a diverse group of almost 200 US domestic and international subject matter experts spanning fundamental and translational research from academia, industry, and government came together in-person to share their collective expertise and experience in this important field. This report briefly summarizes the information presented throughout the meeting, which was also webcast live in its entirety to registered remote attendees.

Development of the brainstem respiratory circuit

The respiratory circuit is comprised of over a dozen functionally and anatomically segregated brainstem nuclei that work together to control respiratory rhythms. These respiratory rhythms emerge prenatally but only acquire vital importance at birth, which is the first time the respiratory circuit faces the sole responsibility for O₂ /CO₂ homeostasis. Hence, the respiratory circuit has little room for trial-and-error-dependent fine tuning and relies on a detailed genetic blueprint for development. This blueprint is provided by transcription factors that have specific spatiotemporal expression patterns along the rostral-caudal or dorsal-ventral axis of the developing brainstem, in proliferating precursor cells and postmitotic neurons. Studying these transcription factors in mice has provided key insights into the functional segregation of respiratory control and the vital importance of specific respiratory nuclei. Many studies converge on just two respiratory nuclei that each have rhythmogenic properties during the prenatal period: the preBötzinger complex (preBötC) and retrotrapezoid nucleus/parafacial nucleus (RTN/pF). Here, we discuss the transcriptional regulation that guides the development of these nuclei. We also summarize evidence showing that normal preBötC development is necessary for neonatal survival, and that neither the preBötC nor the RTN/pF alone is sufficient to sustain normal postnatal respiratory rhythms. Last, we highlight several studies that use intersectional genetics to assess the necessity of transcription factors only in subregions of their expression domain. These studies independently demonstrate that lack of RTN/pF neurons weakens the respiratory circuit, yet these neurons are not necessary for neonatal survival because developmentally related populations can compensate for abnormal RTN/pF function at birth. This article is categorized under: Nervous System Development > Vertebrates: Regional Development.

Insights into the dynamic control of breathing revealed through cell-type-specific responses to substance P

The rhythm generating network for breathing must continuously adjust to changing metabolic and behavioral demands. Here, we examined network-based mechanisms in the mouse preBötzinger complex using substance P, a potent excitatory modulator of breathing frequency and stability, as a tool to dissect network properties that underlie dynamic breathing. We find that substance P does not alter the balance of excitation and inhibition during breaths or the duration of the resulting refractory period. Instead, mechanisms of recurrent excitation between breaths are enhanced such that the rate that excitation percolates through the network is increased. We propose a conceptual framework in which three distinct phases of inspiration, the burst phase, refractory phase, and percolation phase, can be differentially modulated to control breathing dynamics and stability. Unraveling mechanisms that support this dynamic control may improve our understanding of nervous system disorders that destabilize breathing, many of which involve changes in brainstem neuromodulatory systems.

Neurons in the Intermediate Reticular Nucleus Coordinate Postinspiratory Activity, Swallowing, and Respiratory-Sympathetic Coupling in the Rat

Breathing results from sequential recruitment of muscles in the expiratory, inspiratory, and postinspiratory (post-I) phases of the respiratory cycle. Here we investigate whether neurons in the medullary intermediate reticular nucleus (IRt) are components of a central pattern generator (CPG) that generates post-I activity in laryngeal adductors and vasomotor sympathetic nerves and interacts with other members of the central respiratory network to terminate inspiration. We first identified the region of the (male) rat IRt that contains the highest density of lightly cholinergic neurons, many of which are glutamatergic, which aligns well with the putative postinspiratory complex in the mouse (Anderson et al., 2016). Acute bilateral inhibition of this region reduced the amplitudes of post-I vagal and sympathetic nerve activities. However, although associated with reduced expiratory duration and increased respiratory frequency, IRt inhibition did not affect inspiratory duration or abolish the recruitment of post-I activity during acute hypoxemia as predicted. Rather than representing an independent CPG for post-I activity, we hypothesized that IRt neurons may instead function as a relay that distributes post-I activity generated elsewhere, and wondered whether they could be a site of integration for para-respiratory CPGs that drive the same outputs. Consistent with this idea, IRt inhibition blocked rhythmic motor and autonomic components of fictive swallow but not swallow-related apnea. Our data support a role for IRt neurons in the transmission of post-I and swallowing activity to motor and sympathetic outputs, but suggest that other mechanisms also contribute to the generation of post-I activity.SIGNIFICANCE STATEMENT Interactions between multiple coupled oscillators underlie a three-part respiratory cycle composed from inspiratory, postinspiratory (post-I), and late-expiratory phases. Central post-I activity terminates inspiration and activates laryngeal motoneurons. We investigate whether neurons in the intermediate reticular nucleus (IRt) form the central pattern generator (CPG) responsible for post-I activity. We confirm that IRt activity contributes to post-I motor and autonomic outputs, and find that IRt neurons are necessary for activation of the same outputs during swallow, but that they are not required for termination of inspiration or recruitment of post-I activity during hypoxemia. We conclude that this population may not represent a distinct CPG, but instead may function as a premotor relay that integrates activity generated by diverse respiratory and nonrespiratory CPGs.

Presynaptic Mechanisms and KCNQ Potassium Channels Modulate Opioid Depression of Respiratory Drive

Opioid-induced respiratory depression (OIRD) is the major cause of death associated with opioid analgesics and drugs of abuse, but the underlying cellular and molecular mechanisms remain poorly understood. We investigated opioid action in vivo in unanesthetized mice and in in vitro medullary slices containing the preBötzinger Complex (preBötC), a locus critical for breathing and inspiratory rhythm generation. Although hypothesized as a primary mechanism, we found that mu-opioid receptor (MOR1)-mediated GIRK activation contributed only modestly to OIRD. Instead, mEPSC recordings from genetically identified Dbx1-derived interneurons, essential for rhythmogenesis, revealed a prevalent presynaptic mode of action for OIRD. Consistent with MOR1-mediated suppression of presynaptic release as a major component of OIRD, Cacna1a KO slices lacking P/Q-type Ca2+ channels enhanced OIRD. Furthermore, OIRD was mimicked and reversed by KCNQ potassium channel activators and blockers, respectively. In vivo whole-body plethysmography combined with systemic delivery of GIRK- and KCNQ-specific potassium channel drugs largely recapitulated these in vitro results, and revealed state-dependent modulation of OIRD. We propose that respiratory failure from OIRD results from a general reduction of synaptic efficacy, leading to a state-dependent collapse of rhythmic network activity.

A6 neurons simultaneously modulate active expiration and upper airway resistance in rats

NEW FINDINGS

What is the central question of this study? Do A6 neurons modulate active expiratory and airway responses evoked by hypercapnia/acidosis? What is the main finding and its importance? Acute inhibition of A6 neurons reduced active expiratory, inspiratory and the associated oropharyngeal and laryngeal motor responses to hypercapnia/acidosis. A6 neurons provide excitatory synaptic drive contributing to the central generation of inspiratory and expiratory motor activity as well as the control of upper airway resistance.

ABSTRACT

During rest, inspiration is an active phenomenon, whereas expiration is passive. Under conditions of high chemical drive, such as hypercapnia/acidosis, there is an increase in inspiratory activity, expiration becomes active and upper airway resistance is reduced. The locus coeruleus noradrenergic neurons (A6 neurons) are activated when exposed to elevated CO₂ /[H⁺ ] levels and modulate respiratory brainstem neurons regulating ventilation. However, the role of A6 neurons in the control of upper airway resistance is not fully understood. We tested the hypothesis that A6 neurons contribute to the central generation of active inspiratory and expiratory responses and the associated changes in the motor nerves controlling upper airway resistance during hypercapnia/acidosis in rats. Using a perfused brainstem-spinal cord preparation, we inhibited A6 neurons using pharmacogenetics and evaluated the active expiratory (abdominal nerve), laryngeal (cervical vagus nerve), oropharyngeal (hypoglossal nerve) and inspiratory (phrenic nerve) motor nerve responses to hypercapnia/acidosis. Acute inhibition of A6 neurons did not produce significant changes in the respiratory pattern in normocapnia. However, the hypercapnia/acidosis-induced active expiratory response and the associated changes in the motor nerves responsible for control of oropharyngeal and laryngeal resistance, as well as the inspiratory response were all reduced after inhibition of A6 neurons. Our data demonstrate that A6 neurons exert an important excitatory synaptic drive to the central generation of both active inspiratory and expiratory activities and modulate the control of upper airway resistance during hypercapnia/acidosis.

Perinatal Breathing Patterns and Survival in Mice Born Prematurely and at Term

Infants born prematurely, often associated with maternal infection, frequently exhibit breathing instabilities that require resuscitation. We hypothesized that breathing patterns during the first hour of life would be predictive of survival in an animal model of prematurity. Using plethysmography, we measured breathing patterns during the first hour after birth in mice born at term (Term 19.5), delivered prematurely on gestational day 18.5 following administration of low-dose lipopolysaccharide (LPS; 0.14 mg/kg) to pregnant dams (LPS 18.5), or delivered on gestational day 18.7 or 17.5 by caesarian section (C-S 18.5 and C-S 17.5, respectively). Our experimental approach allowed us to dissociate effects caused by inflammation, from effects due to premature birth in the absence of an inflammatory response. C-S 17.5 mice did not survive, whereas mortality was not increased in C-S 18.5 mice. However, in premature pups born at the same gestational age (day 18.5) in response to maternal LPS injection, mortality was significantly increased. Overall, mice that survived had higher birth weights and showed eupneic or gasping activity that was able to transition to normal breathing. Some mice also exhibited a “saw tooth” breathing pattern that was able to transition into eupnea during the first hour of life. In contrast, mice that did not survive showed distinct, large amplitude, long-lasting breaths that occurred at low frequency and did not transition into eupnea. This breathing pattern was only observed during the first hour of life and was more prevalent in LPS 18.5 and C-S 18.5 mice. Indeed, breath tidal volumes were higher in inflammation-induced premature pups than in pups delivered via C-section at equivalent gestational ages, whereas breathing frequencies were low in both LPS-induced and C-section-induced premature pups. We conclude that a breathing pattern characterized by low frequency and large tidal volume is a predictor for the failure to survive, and that these characteristics are more often seen when prematurity occurs in the context of maternal inflammation. Further insights into the mechanisms that generate these breathing patterns and how they transition to normal breathing may facilitate development of novel strategies to manage premature birth in humans.

Central pattern generators in the brainstem and spinal cord: an overview of basic principles, similarities and differences

Central pattern generators (CPGs) are generally defined as networks of neurons capable of enabling the production of central commands, specifically controlling stereotyped, rhythmic motor behaviors. Several CPGs localized in brainstem and spinal cord areas have been shown to underlie the expression of complex behaviors such as deglutition, mastication, respiration, defecation, micturition, ejaculation, and locomotion. Their pivotal roles have clearly been demonstrated although their organization and cellular properties remain incompletely characterized. In recent years, insightful findings about CPGs have been made mainly because (1) several complementary animal models were developed; (2) these models enabled a wide variety of techniques to be used and, hence, a plethora of characteristics to be discovered; and (3) organizations, functions, and cell properties across all models and species studied thus far were generally found to be well-preserved phylogenetically. This article aims at providing an overview for non-experts of the most important findings made on CPGs in in vivo animal models, in vitro preparations from invertebrate and vertebrate species as well as in primates. Data about CPG functions, adaptation, organization, and cellular properties will be summarized with a special attention paid to the network for locomotion given its advanced level of characterization compared with some of the other CPGs. Similarities and differences between these networks will also be highlighted.

Robustness of respiratory rhythm generation across dynamic regimes

A central issue in the study of the neural generation of respiratory rhythms is the role of the intrinsic pacemaking capabilities that some respiratory neurons exhibit. The debate on this issue has occurred in parallel to investigations of interactions among respiratory network neurons and how these contribute to respiratory behavior. In this computational study, we demonstrate how these two issues are inextricably linked. We use simulations and dynamical systems analysis to show that once a conditional respiratory pacemaker, which can be tuned across oscillatory and non-oscillatory dynamic regimes in isolation, is embedded into a respiratory network, its dynamics become masked: the network exhibits similar dynamical properties regardless of the conditional pacemaker node's tuning, and that node's outputs are dominated by network influences. Furthermore, the outputs of the respiratory central pattern generator as a whole are invariant to these changes of dynamical properties, which ensures flexible and robust performance over a wide dynamic range.

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

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

Increasing Local Excitability of Brainstem Respiratory Nuclei Reveals a Distributed Network Underlying Respiratory Motor Pattern Formation

The core circuit of the respiratory central pattern generator (rCPG) is located in the ventrolateral medulla, especially in the pre-Bötzinger complex (pre-BötC) and the neighboring Bötzinger complex (BötC). To test the hypothesis that this core circuit is embedded within an anatomically distributed pattern-generating network, we investigated whether local disinhibition of the nucleus tractus solitarius (NTS), the Kölliker-Fuse nuclei (KFn), or the midbrain periaqueductal gray area (PAG) can similarly affect the respiratory pattern compared to disinhibition of the pre-BötC/BötC core. In arterially-perfused brainstem preparations of rats, we recorded the three-phase respiratory pattern (inspiration, post-inspiration and late-expiration) from phrenic and vagal nerves before and after bilateral microinjections of the GABA(A)R antagonist bicuculline (50 nl, 10 mM). Local disinhibition of either NTS, pre-BötC/BötC, or KFn, but not PAG, triggered qualitatively similar disruptions of the respiratory pattern resulting in a highly significant increase in the variability of the respiratory cycle length, including inspiratory and expiratory phase durations. To quantitatively analyze these motor pattern perturbations, we measured the strength of phase synchronization between phrenic and vagal motor outputs. This analysis showed that local disinhibition of all brainstem target nuclei, but not the midbrain PAG, significantly decreased the strength of phase synchronization. The convergent perturbations of the respiratory pattern suggest that the rCPG expands rostrally and dorsally from the designated core but does not include higher mid-brain structures. Our data also suggest that excitation-inhibition balance of respiratory network synaptic interactions critically determines the network dynamics that underlie vital respiratory rhythm and pattern formation.

Neuromodulation of central pattern generators and its role in the functional recovery of central pattern generator activity

Neuromodulators play an important role in how the nervous system organizes activity that results in behavior. Disruption of the normal patterns of neuromodulatory release or production is known to be related to the onset of severe pathologies such as Parkinson's disease, Rett syndrome, Alzheimer's disease, and affective disorders. Some of these pathologies involve neuronal structures that are called central pattern generators (CPGs), which are involved in the production of rhythmic activities throughout the nervous system. Here I discuss the interplay between CPGs and neuromodulatory activity, with particular emphasis on the potential role of neuromodulators in the recovery of disrupted neuronal activity. I refer to invertebrate and vertebrate model systems and some of the lessons we have learned from research on these systems and propose a few avenues for future research. I make one suggestion that may guide future research in the field: neuromodulators restrict the parameter landscape in which CPG components operate, and the removal of neuromodulators may enable a perturbed CPG in finding a new set of parameter values that can allow it to regain normal function.

Nicotinic receptor dependent regulation of cough and other airway defensive reflexes

Nicotinic receptor activation in the airways evokes airway defensive reflexes including cough. These reflexes are the direct result of bronchopulmonary afferent nerve activation, which may occur directly, through activation of nicotinic receptors expressed on the terminals of airway sensory nerves, or indirectly, secondary to the end organ effects associated with autonomic nerve stimulation. The irritating effects of nicotine delivered topically to the airways are counterbalanced by an inhibitory effect of nicotinic receptor activation in the central nervous system. We present evidence that these nicotinic receptors are components of essential transducing and encoding mechanisms regulating airway defense.

Regulation of breathing pattern by IL-10

Proinflammatory cytokines like interleukin-1β (IL-1β) affect the control of breathing. Our aim is to determine the effect of the anti-inflammatory cytokine IL-10 οn the control of breathing. IL-10 knockout mice (IL-10^-/-, n = 10) and wild-type mice (IL-10^+/+, n = 10) were exposed to the following test gases: hyperoxic hypercapnia 7% CO₂-93% O₂, normoxic hypercapnia 7% CO₂-21% O₂, hypoxic hypercapnia 7% CO₂-10% O₂, and hypoxic normocapnia 3% CO₂-10% O₂. The ventilatory function was assessed using whole body plethysmography. Recombinant mouse IL-10 (rIL-10; 10 μg/kg) was administered intraperitoneally to wild-type mice (n = 10) 30 min before the onset of gas challenge. IL-10 was administered in neonatal medullary slices (10-30 ng/ml, n = 8). We found that IL-10^-/- mice exhibited consistently increased frequency and reduced tidal volume compared with IL-10^+/+ mice during room air breathing and in all test gases (by 23.62 to 33.2%, P < 0.05 and -36.23 to -41.69%, P < 0.05, respectively). In all inspired gases, the minute ventilation of IL-10^-/- mice was lower than IL-10^+/+ (by -15.67 to -22.74%, P < 0.05). The rapid shallow breathing index was higher in IL-10^-/- mice compared with IL-10^+/+ mice in all inspired gases (by 50.25 to 57.5%, P < 0.05). The intraperitoneal injection of rIL-10 caused reduction of the respiratory rate and augmentation of the tidal volume in room air and also in all inspired gases (by -12.22 to -29.53 and 32.18 to 45.11%, P < 0.05, respectively). IL-10 administration in neonatal rat (n = 8) in vitro rhythmically active medullary slice preparations did not affect either rhythmicity or peak amplitude of hypoglossal nerve discharge. In conclusion, IL-10 may induce a slower and deeper pattern of breathing.

Reduced computational modelling of Kölliker-Fuse contributions to breathing patterns in Rett syndrome

KEY POINTS

Reduced computational models are used to test effects of loss of inhibition to the Kölliker-Fuse nucleus (KFn). Three reduced computational models that simulate eupnoeic and vagotomized respiratory rhythms are considered. All models exhibit the emergence of respiratory perturbations associated with Rett syndrome as inhibition to the KFn is diminished. Simulations suggest that application of 5-HT_1A agonists can mitigate the respiratory pathology. The three models can be distinguished and tested based on their predictions about connections and dynamics within the respiratory circuit and about effects of perturbations on certain respiratory neuron populations.

ABSTRACT

Rett syndrome (RTT) is a developmental disorder that can lead to respiratory disturbances featuring prolonged apnoeas of variable durations. Determining the mechanisms underlying these effects at the level of respiratory neural circuits would have significant implications for treatment efforts and would also enhance our understanding of respiratory rhythm generation and control. While experimental studies have suggested possible factors contributing to the respiratory patterns of RTT, we take a novel computational approach to the investigation of RTT, which allows for direct manipulation of selected system parameters and testing of specific hypotheses. Specifically, we present three reduced computational models, developed using an established framework, all of which successfully simulate respiratory outputs across eupnoeic and vagotomized conditions. All three models show that loss of inhibition to the Kölliker-Fuse nucleus reproduces the key respiratory alterations associated with RTT and, as suggested experimentally, that effects of 5-HT_1A agonists on the respiratory neural circuit suffice to alleviate this respiratory pathology. Each of the models makes distinct predictions regarding the neuronal populations and interactions underlying these effects, suggesting natural directions for future experimental testing.

A spatially dynamic network underlies the generation of inspiratory behaviors

Breathing is a vital rhythmic behavior that originates from neural networks within the brainstem. It is hypothesized that the breathing rhythm is generated by spatially distinct networks localized to discrete kernels or compartments. Here, we provide evidence that the functional boundaries of these compartments expand and contract dynamically based on behavioral or physiological demands. The ability of these rhythmic networks to change in size may allow the breathing rhythm to be very reliable, yet flexible enough to accommodate the large repertoire of mammalian behaviors that must be integrated with breathing.

The ability of neuronal networks to reconfigure is a key property underlying behavioral flexibility. Networks with recurrent topology are particularly prone to reconfiguration through changes in synaptic and intrinsic properties. Here, we explore spatial reconfiguration in the reticular networks of the medulla that generate breathing. Combined results from in vitro and in vivo approaches demonstrate that the network architecture underlying generation of the inspiratory phase of breathing is not static but can be spatially redistributed by shifts in the balance of excitatory and inhibitory network influences. These shifts in excitation/inhibition allow the size of the active network to expand and contract along a rostrocaudal medullary column during behavioral or metabolic challenges to breathing, such as changes in sensory feedback, sighing, and gasping. We postulate that the ability of this rhythm-generating network to spatially reconfigure contributes to the remarkable robustness and flexibility of breathing.

Cholinergic neurons in the pedunculopontine tegmental nucleus modulate breathing in rats by direct projections to the retrotrapezoid nucleus

KEY POINTS

Cholinergic projections from the pedunculopontine tegmental nucleus (PPTg) to the retrotrapezoid nucleus (RTN) are considered to be important for sleep-wake state-dependent control of breathing. The RTN also receives cholinergic input from the postinspiratory complex. Stimulation of the PPTg increases respiratory output under control conditions but not when muscarinic receptors in the RTN are blocked. The data obtained in the present study support the possibility that arousal-dependent modulation of breathing involves recruitment of cholinergic projections from the PPTg to the RTN.

ABSTRACT

The pedunculopontine tegmental nucleus (PPTg) in the mesopontine region has important physiological functions, including breathing control. The PPTg contains a variety of cell types, including cholinergic neurons that project to the rostral aspect of the ventrolateral medulla. In addition, cholinergic signalling in the retrotrapezoid nucleus (RTN), a region that contains neurons that regulate breathing in response to changes in CO₂ /H⁺ , has been shown to activate chemosensitive neurons and increase inspiratory activity. The present study aimed to identify the source of cholinergic input to the RTN and determine whether cholinergic signalling in this region influences baseline breathing or the ventilatory response to CO₂ in conscious male Wistar rats. Retrograde tracer Fluoro-Gold injected into the RTN labelled a subset of cholinergic PPTg neurons that presumably project directly to the chemosensitive region of the RTN. In unrestrained awake rats, unilateral injection of the glutamate (10 mm/100 nL) in the PPTg decreased tidal volume (V_T ) but otherwise increased respiratory rate (f_R ) and net respiratory output as indicated by an increase in ventilation (V_E ). All respiratory responses elicited by PPTg stimulation were blunted by prior injection of methyl-atropine (5 mm/50-75 nL) into the RTN. These results show that stimulation of the PPTg can increase respiratory activity in part by cholinergic activation of chemosensitive elements of the RTN. Based on previous evidence that cholinergic PPTg projections may simultaneously activate expiratory output from the pFRG, we speculate that cholinergic signalling at the level of RTN region could also be involved in breathing regulation.

Defining the Rhythmogenic Elements of Mammalian Breathing

Breathing's remarkable ability to adapt to changes in metabolic, environmental, and behavioral demands stems from a complex integration of its rhythm-generating network within the wider nervous system. Yet, this integration complicates identification of its specific rhythmogenic elements. Based on principles learned from smaller rhythmic networks of invertebrates, we define criteria that identify rhythmogenic elements of the mammalian breathing network and discuss how they interact to produce robust, dynamic breathing.

Increased central cholinergic drive contributes to the apneas of serotonin-deficient rat pups during active sleep

Infant rat pups lacking central nervous system (CNS) serotonin (5-hydroxytryptamine; 5-HT) have unstable breathing during prolonged periods of active sleep. Given that cholinergic neurons are drivers of active sleep and project to respiratory patterning regions in the brainstem, we hypothesized that 5-HT preserves respiratory stability in active sleep by dampening central cholinergic drive. We used whole-body plethysmography coupled with nuchal electromyography to monitor the breathing pattern of 2-wk-old tryptophan hydroxylase 2 (TPH2)+/+ and TPH2-deficient (TPH2-/-) pups in active sleep, before and after muscarinic blockade. For the group 1 experiment we injected methylatropine (Ap-M), a CNS-impermeant form of atropine, followed ~30 min later by an injection of atropine sulfate (Ap-S), the CNS-permeant form (both 1 mg/kg, 10 μl bolus iv); both injections occurred within an active sleep episode. We analyzed the effect of each drug on the coefficient of variation of the respiratory period (CV-P) during active sleep. For the group 2 experiment rats were cycled through several episodes of active and quiet sleep before administration of Ap-S (1 mg/kg, 200 μl ip) or vehicle. We assessed the effect of Ap-S on the apnea indices of both genotypes during quiet and active sleep. In group 1 Ap-S significantly reduced the CV-P of TPH2-/- pups (P = 0.03), an effect not observed in TPH2+/+ pups or following Ap-M. In group 2 the apnea index of TPH2-/- pups was significantly reduced following Ap-S injection (P = 0.04), whereas the apnea index of TPH2+/+ littermates was unaffected (P = 0.58). These findings suggest that central 5-HT reduces apnea and stabilizes breathing by reducing cholinergic signaling through muscarinic receptors. NEW & NOTEWORTHY Serotonin in the central nervous system (CNS) is necessary for maintaining the stability of breathing in the early postnatal period, particularly during active sleep. Here we show that the administration of atropine to the CNS selectively stabilizes the respiratory pattern of tryptophan hydroxylase 2-deficient rat pups and reduces their apneas. This suggests that CNS serotonin stabilizes breathing at least in part by reducing central cholinergic drive.

Trpm4 ion channels in pre-Bötzinger complex interneurons are essential for breathing motor pattern but not rhythm

Inspiratory breathing movements depend on pre-Bötzinger complex (preBötC) interneurons that express calcium (Ca2+)-activated nonselective cationic current (ICAN) to generate robust neural bursts. Hypothesized to be rhythmogenic, reducing ICAN is predicted to slow down or stop breathing; its contributions to motor pattern would be reflected in the magnitude of movements (output). We tested the role(s) of ICAN using reverse genetic techniques to diminish its putative ion channels Trpm4 or Trpc3 in preBötC neurons in vivo. Adult mice transduced with Trpm4-targeted short hairpin RNA (shRNA) progressively decreased the tidal volume of breaths yet surprisingly increased breathing frequency, often followed by gasping and fatal respiratory failure. Mice transduced with Trpc3-targeted shRNA survived with no changes in breathing. Patch-clamp and field recordings from the preBötC in mouse slices also showed an increase in the frequency and a decrease in the magnitude of preBötC neural bursts in the presence of Trpm4 antagonist 9-phenanthrol, whereas the Trpc3 antagonist pyrazole-3 (pyr-3) showed inconsistent effects on magnitude and no effect on frequency. These data suggest that Trpm4 mediates ICAN, whose influence on frequency contradicts a direct role in rhythm generation. We conclude that Trpm4-mediated ICAN is indispensable for motor output but not the rhythmogenic core mechanism of the breathing central pattern generator.

Functional brain stem circuits for control of nose motion

Rodents shift their nose from side to side when they actively explore and lateralize odors in the space. This motor action is driven by a pair of muscles, the deflector nasi. We studied the premotor control of this motion. We used replication-competent rabies virus to transsynaptically label inputs to the deflector nasi muscle and find putative premotor labeling throughout the parvocellular, intermediate, and gigantocellular reticular formations, as well as the trigeminal nuclei, pontine reticular formation, midbrain reticular formation, red nucleus, and superior colliculus. Two areas with extensive labeling were analyzed for their impact on nose movement. One area is in the reticular formation caudal to the facial motor nucleus and is denoted the nose retrofacial area. The second is in the caudal part of the intermediate reticular region near the oscillator for whisking (the nose IRt). Functionally, we find that optogenetic activation of glutamatergic cells in both areas drives deflection of the nose. Ablation of cells in the nose retrofacial area, but not the nose IRt, impairs movement of the nose in response to the presentation of odorants but otherwise leaves movement unaffected. These data suggest that the nose retrofacial area is a conduit for a sensory-driven orofacial motor action. Furthermore, we find labeling of neurons that are immediately upstream of premotor neurons in the preBötzinger complex that presumably synchronizes a small, rhythmic component of nose motion to breathing. NEW & NOTEWORTHY We identify two previously undescribed premotor areas in the medulla that control deflection of the nose. This includes a pathway for directed motion of the nose in response to an odorant.

The postnatal development of ultrasonic vocalization-associated breathing is altered in glycine transporter 2-deficient mice

KEY POINTS

Newborn mice produce ultrasonic vocalization to communicate with their mother. The neuronal glycine transporter (GlyT2) is required for efficient loading of synaptic vesicles in glycinergic neurons. Mice lacking GlyT2 develop a phenotype that resembles human hyperekplexia and the mice die in the second postnatal week. In the present study, we show that GlyT2-knockout mice do not acquire adult ultrasonic vocalization-associated breathing patterns. Despite the strong impairment of glycinergic inhibition, they can produce sufficient expiratory airflow to produce ultrasonic vocalization. Because mouse ultrasonic vocalization is a valuable read-out in translational research, these data are highly relevant for a broad range of research fields.

ABSTRACT

Mouse models are instrumental with respect to determining the genetic basis and neural foundations of breathing regulation. To test the hypothesis that glycinergic synaptic inhibition is required for normal breathing and proper post-inspiratory activity, we analysed breathing and ultrasonic vocalization (USV) patterns in neonatal mice lacking the neuronal glycine transporter (GlyT2). GlyT2-knockout (KO) mice have a profound reduction of glycinergic synaptic currents already at birth, develop a severe motor phenotype and survive only until the second postnatal week. At this stage, GlyT2-KO mice are smaller, have a reduced respiratory rate and still display a neonatal breathing pattern with active expiration for the production of USV. By contrast, wild-type mice acquire different USV-associated breathing patterns that depend on post-inspiratory control of air flow. Nonetheless, USVs per se remain largely indistinguishable between both genotypes. We conclude that GlyT2-KO mice, despite the strong impairment of glycinergic inhibition, can produce sufficient expiratory airflow to produce ultrasonic vocalization.

Developmental Maturation of Functional Coupling Between Ventilatory Oscillators in the American Bullfrog

Many vital motor behaviors - including locomotion, swallowing, and breathing - appear to be dependent upon the activity of and coordination between multiple endogenously rhythmogenic nuclei, or neural oscillators. Much as the functional development of sensory circuits is shaped during maturation, we hypothesized that coordination of oscillators involved in motor control may likewise be maturation-dependent, i.e., coupling and coordination between oscillators change over development. We tested this hypothesis using the bullfrog isolated brainstem preparation to study the metamorphic transition of ventilatory motor patterns from early rhythmic buccal (water) ventilation in the tadpole to the mature pattern of rhythmic buccal and lung (air) ventilation in the adult. Spatially distinct oscillators drive buccal and lung bursts in the isolated brainstem; we found these oscillators to be active but functionally uncoupled in the tadpole. Over the course of metamorphosis, the rhythms produced by the buccal and lung oscillators become increasingly tightly coordinated. These changes parallel the progression of structural and behavioral changes in the animal, with adult levels of coupling arising by the metamorphic stage (forelimb eruption). These findings suggest that oscillator coupling undergoes a maturation process similar to the refinement of sensory circuits over development.

Locus Coeruleus as a vigilance centre for active inspiration and expiration in rats

At rest, inspiration is an active process while expiration is passive. However, high chemical drive (hypercapnia or hypoxia) activates central and peripheral chemoreceptors triggering reflex increases in inspiration and active expiration. The Locus Coeruleus contains noradrenergic neurons (A6 neurons) that increase their firing frequency when exposed to hypercapnia and hypoxia. Using recently developed neuronal hyperpolarising technology in conscious rats, we tested the hypothesis that A6 neurons are a part of a vigilance centre for controlling breathing under high chemical drive and that this includes recruitment of active inspiration and expiration in readiness for flight or fight. Pharmacogenetic inhibition of A6 neurons was without effect on resting and on peripheral chemoreceptors-evoked inspiratory, expiratory and ventilatory responses. On the other hand, the number of sighs evoked by systemic hypoxia was reduced. In the absence of peripheral chemoreceptors, inhibition of A6 neurons during hypercapnia did not affect sighing, but reduced both the magnitude and incidence of active expiration, and the frequency and amplitude of inspiration. These changes reduced pulmonary ventilation. Our data indicated that A6 neurons exert a CO₂-dependent modulation of expiratory drive. The data also demonstrate that A6 neurons contribute to the CO₂-evoked increases in the inspiratory motor output and hypoxia-evoked sighing.

Development of a Novel FIJI-Based Method to Investigate Neuronal Circuitry in Neonatal Mice

The emergence of systems neuroscience tools requires parallel generation of objective analytical workflows for experimental neuropathology. We developed an objective analytical workflow that we used to determine how specific autonomic neural lineages change during postnatal development. While a wealth of knowledge exists regarding postnatal alterations in respiratory neural function, how these neural circuits change and develop in the weeks following birth remains less clear. In this study, we developed our workflow by combining genetic mouse modeling and quantitative immunofluorescent confocal microscopy and used this to examine the postnatal development of neural circuits derived from the transcription factors NKX2.2 and OLIG3 into three medullary respiratory nuclei. Our automated FIJI-based image analysis workflow rapidly and objectively quantified synaptic puncta in user-defined anatomic regions. Using our objective workflow, we found that the density and estimated total number of Nkx2.2-derived afferents into the pre-Bötzinger Complex significantly decreased with postnatal age during the first three weeks of postnatal life. These data indicate that Nkx2.2-derived structures differentially influence pre-Bötzinger Complex respiratory oscillations at different stages of postnatal development.

Classification of Tidal Breathing Airflow Profiles Using Statistical Hierarchal Cluster Analysis in Idiopathic Pulmonary Fibrosis

In idiopathic pulmonary fibrosis (IPF) breathing pattern changes with disease progress. This study aims to determine if unsupervised hierarchal cluster analysis (HCA) can be used to define airflow profile differences in people with and without IPF. This was tested using 31 patients with IPF and 17 matched healthy controls, all of whom had their lung function assessed using spirometry and carbon monoxide CO transfer. A resting tidal breathing (RTB) trace of two minutes duration was collected at the same time. A Euclidian distance technique was used to perform HCA on the airflow data. Four distinct clusters were found, with the majority (18 of 21, 86%) of the severest IPF participants (Stage 2 and 3) being in two clusters. The participants in these clusters exhibited a distinct minute ventilation (p < 0.05), compared to the other two clusters. The respiratory drive was greatest in Cluster 1, which contained many of the IPF participants. Unstructured HCA was successful in recognising different airflow profiles, clustering according to differences in flow rather than time. HCA showed that there is an overlap in tidal airflow profiles between healthy RTB and those with IPF. The further application of HCA in recognising other respiratory disease is discussed.

Neural mechanisms for sigh generation during prenatal development

The respiratory network of the preBötzinger complex (preBötC), which controls inspiratory behavior, can in normal conditions simultaneously produce two types of inspiration-related rhythmic activities: the eupneic rhythm composed of monophasic, low-amplitude, and relatively high-frequency bursts, interspersed with sigh rhythmic activity, composed of biphasic, high-amplitude, and lower frequency bursts. By combining electrophysiological recordings from transverse brainstem slices with computational modeling, new advances in the mechanisms underlying sigh production have been obtained during prenatal development. The present review summarizes recent findings that establish when sigh rhythmogenesis starts to be produced during embryonic development as well as the cellular, membrane, and synaptic properties required for its expression. Together, the results demonstrate that although generated by the same network, the eupnea and sigh rhythms have different developmental onset times and rely on distinct network properties. Because sighs (also known as augmented breaths) are important in maintaining lung function (by reopening collapsed alveoli), gaining insight into their underlying neural mechanisms at early developmental stages is likely to help in the treatment of prematurely born babies often suffering from breathing deficiencies.

Advances in cellular and integrative control of oxygen homeostasis within the central nervous system

Mammals must continuously regulate the levels of O2 and CO2 , which is particularly important for the brain. Failure to maintain adequate O2 /CO2 homeostasis has been associated with numerous disorders including sleep apnoea, Rett syndrome and sudden infant death syndrome. But, O2 /CO2 homeostasis poses major regulatory challenges, even in the healthy brain. Neuronal activities change in a differentiated, spatially and temporally complex manner, which is reflected in equally complex changes in O2 demand. This raises important questions: is oxygen sensing an emergent property, locally generated within all active neuronal networks, and/or the property of specialized O2 -sensitive CNS regions? Increasing evidence suggests that the regulation of the brain's redox state involves properties that are intrinsic to many networks, but that specialized regions in the brainstem orchestrate the integrated control of respiratory and cardiovascular functions. Although the levels of O2 in arterial blood and the CNS are very different, neuro-glial interactions and purinergic signalling are critical for both peripheral and CNS chemosensation. Indeed, the specificity of neuroglial interactions seems to determine the differential responses to O2 , CO2 and the changes in pH.

The Expression of Galanin in the Parafacial Respiratory Group and its Effects on Respiration in Neonatal Rats

The inhibitory peptide galanin is expressed within the retrotrapezoidal nucleus (RTN) - a key central chemoreceptor site that also contains the active expiratory oscillator. It was previously reported that microinjection of galanin into pre-Bötzinger complex - containing the inspiratory oscillator - exerts inhibitory effects on inspiratory motor output and respiratory rhythm. In neonatal rats, the present study aimed to investigate: (1) expression of galanin within the parafacial respiratory group (pFRG), which overlaps anatomically and functionally with the adult RTN, and; (2) effects of galanin on respiratory rhythm using the in vitro brainstem-spinal cord preparation. We showed that 14 ± 2% of Phox2b-immunoreactive (ir) neurons in the parafacial region were also galanin-ir. Galanin peptide expression was confirmed within 3/9 CO₂-sensitive, Phox2b-ir Pre-Inspiratory neurons (Pre-I) recorded in parafacial region. Bath application of galanin (0.1-0.2 µM): (1) decreased the duration of membrane depolarization in both Pre-I and inspiratory pFRG neurons, and; (2) decreased the number of C4 bursts that were associated with each burst in Pre-I neurons within the pFRG. In preparations showing episodic breathing at baseline, the respiratory patterning reverted to the 'normal' pattern of single, uniformly rhythmic C4 bursts (n = 10). In preparations with normal respiratory patterning at baseline, slowing of C4 rhythm (n = 7) resulted although rhythmic bursting in recorded Pre-I neurons remained unperturbed (n = 6). This study therefore demonstrates that galanin is expressed within the pFRG of neonatal rats, including neurons that are intrinsically chemosensitive. Overall the peptide has an inhibitory effect on inspiratory motor output, as previously shown in adults.

Fate mapping neurons and glia derived from Dbx1-expressing progenitors in mouse preBötzinger complex

The brainstem preBötzinger complex (preBötC) generates the inspiratory breathing rhythm, and its core rhythmogenic interneurons are derived from Dbx1‐expressing progenitors. To study the neural bases of breathing, tamoxifen‐inducible Cre‐driver mice and Cre‐dependent reporters are used to identify, record, and perturb Dbx1 preBötC neurons. However, the relationship between tamoxifen administration and reporter protein expression in preBötC neurons and glia has not been quantified. To address this problem, we crossed mice that express tamoxifen‐inducible Cre recombinase under the control of the Dbx1 gene (Dbx1^CreERT2) with Cre‐dependent fluorescent reporter mice (Rosa26^tdTomato), administered tamoxifen at different times during development, and analyzed tdTomato expression in the preBötC of their offspring. We also crossed Rosa26^tdTomato reporters with mice that constitutively express Cre driven by Dbx1 (Dbx1^Cre) and analyzed tdTomato expression in the preBötC of their offspring for comparison. We show that Dbx1‐expressing progenitors give rise to preBötC neurons and glia. Peak neuronal tdTomato expression occurs when tamoxifen is administered at embryonic day 9.5 (E9.5), whereas tdTomato expression in glia shows no clear relationship with tamoxifen timing. These results can be used to bias reporter protein expression in neurons (or glia). Tamoxifen administration at E9.5 labels 91% of Dbx1‐derived neurons in the preBötC, yet only 48% of Dbx1‐derived glia. By fate mapping Dbx1‐expressing progenitors, this study illustrates the developmental assemblage of Dbx1‐derived cells in preBötC, which can be used to design intersectional Cre/lox experiments that interrogate its cellular composition, structure, and function.

The Dynamic Basis of Respiratory Rhythm Generation: One Breath at a Time

Rhythmicity is a universal timing mechanism in the brain, and the rhythmogenic mechanisms are generally dynamic. This is illustrated for the neuronal control of breathing, a behavior that occurs as a one-, two-, or three-phase rhythm. Each breath is assembled stochastically, and increasing evidence suggests that each phase can be generated independently by a dedicated excitatory microcircuit. Within each microcircuit, rhythmicity emerges through three entangled mechanisms: ( a) glutamatergic transmission, which is amplified by ( b) intrinsic bursting and opposed by ( c) concurrent inhibition. This rhythmogenic triangle is dynamically tuned by neuromodulators and other network interactions. The ability of coupled oscillators to reconfigure and recombine may allow breathing to remain robust yet plastic enough to conform to nonventilatory behaviors such as vocalization, swallowing, and coughing. Lessons learned from the respiratory network may translate to other highly dynamic and integrated rhythmic systems, if approached one breath at a time.

The parafacial respiratory group and the control of active expiration

Breathing at rest is typically characterized by three phases: active inspiration, post-inspiration (or stage 1 expiration), and passive expiration (or stage 2 expiration). Breathing during periods of increased respiratory demand, on the other hand, engages active expiration through recruitment of abdominal muscles in order to increase ventilation. It is currently hypothesized that different phases of the respiratory rhythm are driven by three coupled oscillators: the preBötzinger Complex, driving inspiration, the parafacial respiratory group (pFRG), driving active expiration and the post-inspiratory Complex, driving post-inspiration. In this paper we review advances in the understanding of the pFRG and its role in the generation of active expiration across different developmental stages and vigilance states. Recent experiments suggest that the abdominal recruitment varies across development depending on the vigilance state, possibly following the maturation of the network responsible for the generation of active expiration and neuromodulatory systems that influence its activity. The activity of the pFRG is tonically inhibited by GABAergic inputs and strongly recruited by cholinergic systems. However, the sources of these modulatory inputs and the physiological conditions under which these mechanisms are used to recruit active expiration and increase ventilation need further investigation. Some evidence suggests that active expiration during hypercapnia is evoked through disinhibition, while during hypoxia it is elicited through activation of catecholaminergic C1 neurons. Finally, a discussion of experiments indicating that the pFRG is anatomically and functionally distinct from the adjacent and partially overlapping chemosensitive neurons of the retrotrapezoid nucleus is also presented.

Breathing matters

Breathing is a well-described, vital and surprisingly complex behaviour, with behavioural and physiological outputs that are easy to directly measure. Key neural elements for generating breathing pattern are distinct, compact and form a network amenable to detailed interrogation, promising the imminent discovery of molecular, cellular, synaptic and network mechanisms that give rise to the behaviour. Coupled oscillatory microcircuits make up the rhythmic core of the breathing network. Primary among these is the preBötzinger Complex (preBötC), which is composed of excitatory rhythmogenic interneurons and excitatory and inhibitory pattern-forming interneurons that together produce the essential periodic drive for inspiration. The preBötC coordinates all phases of the breathing cycle, coordinates breathing with orofacial behaviours and strongly influences, and is influenced by, emotion and cognition. Here, we review progress towards cracking the inner workings of this vital core.

Dbx1 Pre-Bötzinger Complex Interneurons Comprise the Core Inspiratory Oscillator for Breathing in Unanesthetized Adult Mice

The brainstem pre-Bötzinger complex (preBötC) generates inspiratory breathing rhythms, but which neurons comprise its rhythmogenic core? Dbx1-derived neurons may play the preeminent role in rhythm generation, an idea well founded at perinatal stages of development but incompletely evaluated in adulthood. We expressed archaerhodopsin or channelrhodopsin in Dbx1 preBötC neurons in intact adult mice to interrogate their function. Prolonged photoinhibition slowed down or stopped breathing, whereas prolonged photostimulation sped up breathing. Brief inspiratory-phase photoinhibition evoked the next breath earlier than expected, whereas brief expiratory-phase photoinhibition delayed the subsequent breath. Conversely, brief inspiratory-phase photostimulation increased inspiratory duration and delayed the subsequent breath, whereas brief expiratory-phase photostimulation evoked the next breath earlier than expected. Because they govern the frequency and precise timing of breaths in awake adult mice with sensorimotor feedback intact, Dbx1 preBötC neurons constitute an essential core component of the inspiratory oscillator, knowledge directly relevant to human health and physiology.

Organization of the core respiratory network: Insights from optogenetic and modeling studies

The circuit organization within the mammalian brainstem respiratory network, specifically within and between the pre-Bötzinger (pre-BötC) and Bötzinger (BötC) complexes, and the roles of these circuits in respiratory pattern generation are continuously debated. We address these issues with a combination of optogenetic experiments and modeling studies. We used transgenic mice expressing channelrhodopsin-2 under the VGAT-promoter to investigate perturbations of respiratory circuit activity by site-specific photostimulation of inhibitory neurons within the pre-BötC or BötC. The stimulation effects were dependent on the intensity and phase of the photostimulation. Specifically: (1) Low intensity (≤ 1.0 mW) pulses delivered to the pre-BötC during inspiration did not terminate activity, whereas stronger stimulations (≥ 2.0 mW) terminated inspiration. (2) When the pre-BötC stimulation ended in or was applied during expiration, rebound activation of inspiration occurred after a fixed latency. (3) Relatively weak sustained stimulation (20 Hz, 0.5-2.0 mW) of pre-BötC inhibitory neurons increased respiratory frequency, while a further increase of stimulus intensity (> 3.0 mW) reduced frequency and finally (≥ 5.0 mW) terminated respiratory oscillations. (4) Single pulses (0.2-5.0 s) applied to the BötC inhibited rhythmic activity for the duration of the stimulation. (5) Sustained stimulation (20 Hz, 0.5-3.0 mW) of the BötC reduced respiratory frequency and finally led to apnea. We have revised our computational model of pre-BötC and BötC microcircuits by incorporating an additional population of post-inspiratory inhibitory neurons in the pre-BötC that interacts with other neurons in the network. This model was able to reproduce the above experimental findings as well as previously published results of optogenetic activation of pre-BötC or BötC neurons obtained by other laboratories. The proposed organization of pre-BötC and BötC circuits leads to testable predictions about their specific roles in respiratory pattern generation and provides important insights into key circuit interactions operating within brainstem respiratory networks.

Efferent projections of excitatory and inhibitory preBötzinger Complex neurons

The preBötzinger Complex (preBötC), a compact medullary region essential for generating normal breathing rhythm and pattern, is the kernel of the breathing central pattern generator (CPG). Excitatory preBötC neurons in rats project to major breathing-related brainstem regions. Here, we provide a brainstem connectivity map in mice for both excitatory and inhibitory preBötC neurons. Using a genetic strategy to label preBötC neurons, we confirmed extensive projections of preBötC excitatory neurons within the brainstem breathing CPG including the contralateral preBötC, Bötzinger Complex (BötC), ventral respiratory group, nucleus of the solitary tract, parahypoglossal nucleus, parafacial region (RTN/pFRG or alternatively, pF_L /pF_V ), parabrachial and Kölliker-Füse nuclei, as well as major projections to the midbrain periaqueductal gray. Interestingly, preBötC inhibitory projections paralleled the excitatory projections. Moreover, we examined overlapping projections in the pons in detail and found that they targeted the same neurons. We further explored the direct anatomical link between the preBötC and suprapontine brain regions that may govern emotion and other complex behaviors that can affect or be affected by breathing. Forebrain efferent projections were sparse and restricted to specific nuclei within the thalamus and hypothalamus, with processes rarely observed in cortex, basal ganglia, or other limbic regions, e.g., amygdala or hippocampus. We conclude that the preBötC sends direct, presumably inspiratory-modulated, excitatory and inhibitory projections in parallel to distinct targets throughout the brain that generate and modulate breathing pattern and/or coordinate breathing with other behaviors, physiology, cognition, or emotional state.

The interdependence of excitation and inhibition for the control of dynamic breathing rhythms

The preBötzinger Complex (preBötC), a medullary network critical for breathing, relies on excitatory interneurons to generate the inspiratory rhythm. Yet, half of preBötC neurons are inhibitory, and the role of inhibition in rhythmogenesis remains controversial. Using optogenetics and electrophysiology in vitro and in vivo, we demonstrate that the intrinsic excitability of excitatory neurons is reduced following large depolarizing inspiratory bursts. This refractory period limits the preBötC to very slow breathing frequencies. Inhibition integrated within the network is required to prevent overexcitation of preBötC neurons, thereby regulating the refractory period and allowing rapid breathing. In vivo, sensory feedback inhibition also regulates the refractory period, and in slowly breathing mice with sensory feedback removed, activity of inhibitory, but not excitatory, neurons restores breathing to physiological frequencies. We conclude that excitation and inhibition are interdependent for the breathing rhythm, because inhibition permits physiological preBötC bursting by controlling refractory properties of excitatory neurons.

GABA-ergic neurotransmission in the nucleus of the solitary tract modulates cough in the cat

GABA, muscimol, and baclofen were microinjected into the rostral (rNTS) and caudal solitary tract nucleus (cNTS) in 24 anesthetized cats. Electromyograms (EMGs) of diaphragm (DIA) and abdominal muscles (ABD), blood pressure and esophageal pressure (EP) were recorded and analysed. Bilateral microinjections of 1 mM GABA (total 66 ± 4 nl), 1 mM baclofen (64 ± 4 nl) and unilateral microinjections of 0.5 mM muscimol (33 ± 1 nl) in the rNTS significantly reduced cough number (CN), amplitudes of ABD EMGs, expiratory EP, and prolonged the duration of the cough inspiratory phase. GABA microinjections decreased the amplitudes of cough-related DIA EMGs and inspiratory EP; muscimol microinjections decreased the cough DIA EMG on the contralateral side. Only microinjections of GABA into the cNTS suppressed CN. In some cases, microinjections prolonged the inspiratory phase, lowered respiratory rate, changed the depth of breathing, and increased blood pressure and heart rate. Our results confirm that GABA-ergic inhibitory mechanisms in the rNTS can regulate coughing in the anesthetized cat.

Vocal development through morphological computation

The vocal behavior of infants changes dramatically during early life. Whether or not such a change results from the growth of the body during development-as opposed to solely neural changes-has rarely been investigated. In this study of vocal development in marmoset monkeys, we tested the putative causal relationship between bodily growth and vocal development. During the first two months of life, the spontaneous vocalizations of marmosets undergo (1) a gradual disappearance of context-inappropriate call types and (2) an elongation in the duration of context-appropriate contact calls. We hypothesized that both changes are the natural consequences of lung growth and do not require any changes at the neural level. To test this idea, we first present a central pattern generator model of marmoset vocal production to demonstrate that lung growth can affect the temporal and oscillatory dynamics of neural circuits via sensory feedback from the lungs. Lung growth qualitatively shifted vocal behavior in the direction observed in real marmoset monkey vocal development. We then empirically tested this hypothesis by placing the marmoset infants in a helium-oxygen (heliox) environment in which air is much lighter. This simulated a reversal in development by decreasing the effort required to respire, thus increasing the respiration rate (as though the lungs were smaller). The heliox manipulation increased the proportions of inappropriate call types and decreased the duration of contact calls, consistent with a brief reversal of vocal development. These results suggest that bodily growth alone can play a major role in shaping the development of vocal behavior.

Circuits in the rodent brainstem that control whisking in concert with other orofacial motor actions

The world view of rodents is largely determined by sensation on two length scales. One is within the animal's peri-personal space; sensorimotor control on this scale involves active movements of the nose, tongue, head, and vibrissa, along with sniffing to determine olfactory clues. The second scale involves the detection of more distant space through vision and audition; these detection processes also impact repositioning of the head, eyes, and ears. Here we focus on orofacial motor actions, primarily vibrissa-based touch but including nose twitching, head bobbing, and licking, that control sensation at short, peri-personal distances. The orofacial nuclei for control of the motor plants, as well as primary and secondary sensory nuclei associated with these motor actions, lie within the hindbrain. The current data support three themes: First, the position of the sensors is determined by the summation of two drive signals, i.e., a fast rhythmic component and an evolving orienting component. Second, the rhythmic component is coordinated across all orofacial motor actions and is phase-locked to sniffing as the animal explores. Reverse engineering reveals that the preBötzinger inspiratory complex provides the reset to the relevant premotor oscillators. Third, direct feedback from somatosensory trigeminal nuclei can rapidly alter motion of the sensors. This feedback is disynaptic and can be tuned by high-level inputs. A holistic model for the coordination of orofacial motor actions into behaviors will encompass feedback pathways through the midbrain and forebrain, as well as hindbrain areas.

Current Perspectives for the use of Gonane Progesteronergic Drugs in the Treatment of Central Hypoventilation Syndromes

BACKGROUND

Central alveolar hypoventilation syndromes (CHS) encompass neurorespiratory diseases resulting from congenital or acquired neurological disorders. Hypercapnia, acidosis, and hypoxemia resulting from CHS negatively affect physiological functions and can be lifethreatening. To date, the absence of pharmacological treatment implies that the patients must receive assisted ventilation throughout their lives.

OBJECTIVE

To highlight the relevance of determining conditions in which using gonane synthetic progestins could be of potential clinical interest for the treatment of CHS.

METHODS

The mechanisms by which gonanes modulate the respiratory drive were put into the context of those established for natural progesterone and other synthetic progestins.

RESULTS

The clinical benefits of synthetic progestins to treat respiratory diseases are mixed with either positive outcomes or no improvement. A benefit for CHS patients has only recently been proposed. We incidentally observed restoration of CO2 chemosensitivity, the functional deficit of this disease, in two adult CHS women by desogestrel, a gonane progestin, used for contraception. This effect was not observed by another group, studying a single patient. These contradictory findings are probably due to the complex nature of the action of desogestrel on breathing and led us to carry out mechanistic studies in rodents. Our results show that desogestrel influences the respiratory command by modulating the GABAA and NMDA signaling in the respiratory network, medullary serotoninergic systems, and supramedullary areas.

CONCLUSION

Gonanes show promise for improving ventilation of CHS patients, although the conditions of their use need to be better understood.