GABA, in some cases together with glycine, is used as the inhibitory transmitter by pump cells in the Hering-Breuer reflex pathway of the rat

Neuroanatomical and neurochemical organization of brainstem and forebrain circuits involved in breathing regulation

Breathing regulation depends on a highly intricate and precise network within the brainstem, requiring the identification of all neuronal elements in the brainstem respiratory circuits and a comprehensive understanding of their organization into distinct functional compartments. These compartments play a pivotal role by providing essential input to three main targets: cranial motoneurons that regulate airway control, spinal motoneurons that activate the inspiratory and expiratory muscles, and higher brain structures that influence breathing behavior and integrate it with other physiological and behavioral processes. This review offers a comprehensive examination of the phenotypes, connections, and functional roles of the major compartments within the brainstem and forebrain respiratory circuits. In addition, it summarizes the diverse neurotransmitters used by neurons in these regions, highlighting their contributions to the coordination and modulation of respiratory activity.

Pulmonary stretch receptor modulation of synaptic inhibition shapes the discharge pattern of respiratory premotor neurons

Many studies focus on the mechanisms of respiratory rhythm generation through neuronal interactions in the preBötzinger and Bötzinger complex area. There is limited insight into how the varied discharge patterns of propriobulbar, rhythm generating neurons are integrated to generate the slowly augmenting and decrementing discharge patterns observed in respiratory premotor neurons. Neuronal discharge patterns were obtained, in vivo, from inspiratory (I) and expiratory (E) premotor neurons in the ventral respiratory group of adult, anesthetized and vagotomized canines. Electrical activation of vagal afferents was used to produce pulmonary stretch receptor (PSR), step-input patterns, throughout or within either the I- or E-phase. PSR inputs decreased the discharge pattern slopes of augmenting and decrementing E-neurons and increased the slopes of augmenting and decrementing I-neurons. PSR inputs that were applied only for part of the phase acutely changed the discharge pattern to the trajectory associated with those PSR throughout-phase inputs, but the pattern returned immediately to the original trajectory after the PSR input terminated. These types of responses can be reproduced with high fidelity by a mathematical model based on reciprocal inhibition between augmenting and decrementing neurons of the same respiratory phase. Best fit is achieved when PSR inputs solely modulate the strength of the synaptic inhibition of decrementing neurons by augmenting neurons at the presynaptic level. Leaky integrator functions are not necessary to generate the gradually augmenting and decrementing patterns. This model offers a novel and different mechanistic way to conceptualize the generation and PSR control of respiratory discharge patterns.

Molecular Ontology of the Nucleus of Solitary Tract

The nucleus of the solitary tract (NTS) receives visceral information and regulates appetitive, digestive, and cardiorespiratory systems. Within the NTS, diverse processes operate in parallel to sustain life, but our understanding of their cellular composition is incomplete. Here, we integrate histologic and transcriptomic analysis to identify and compare molecular features that distinguish neurons in this brain region. Most glutamatergic neurons in the NTS and area postrema co-express the transcription factors Lmx1b and Phox2b, except for a ventral band of neurons in the far-caudal NTS, which include the Gcg-expressing neurons that produce glucagon-like peptide 1 (GLP-1). GABAergic interneurons intermingle through the Lmx1b+Phox2b macropopulation, and dense clusters of GABAergic neurons surround the NTS. The Lmx1b+Phox2b macropopulation includes subpopulations with distinct distributions expressing Grp, Hsd11b2, Npff, Pdyn, Pou3f1, Sctr, Th, and other markers. These findings highlight Lmx1b-Phox2b co-expression as a common feature of glutamatergic neurons in the NTS and improve our understanding of the organization and distribution of neurons in this critical brain region.

Supraspinal glycinergic neurotransmission in pain: A scoping review of current literature

The neurotransmitter glycine is an agonist at the strychnine-sensitive glycine receptors. In addition, it has recently been discovered to act at two new receptors, the excitatory glycine receptor and metabotropic glycine receptor. Glycine's neurotransmitter roles have been most extensively investigated in the spinal cord, where it is known to play essential roles in pain, itch, and motor function. In contrast, less is known about supraspinal glycinergic functions, and their contributions to pain circuits are largely unrecognized. As glycinergic neurons are absent from cortical regions, a clearer understanding of how supraspinal glycine modulates pain could reveal new pharmacological targets. This review aims to synthesize the published research on glycine's role in the adult brain, highlighting regions where glycine signaling may modulate pain responses. This was achieved through a scoping review methodology identifying several key regions of supraspinal pain circuitry where glycine signaling is involved. Therefore, this review unveils critical research gaps for supraspinal glycine's potential roles in pain and pain-associated responses, encouraging researchers to consider glycinergic neurotransmission more widely when investigating neural mechanisms of pain.

Glutamatergic and purinergic transmitters and astrocyte modulation in the synaptic transmission in the NTS of rats exposed to short-term sustained hypoxia

There is evidence that astrocytes modulate synaptic transmission in the nucleus tractus solitarius (NTS) interacting with glutamatergic and purinergic mechanisms. Here, using in situ working heart-brainstem preparations, we evaluated the involvement of astrocyte and glutamatergic/purinergic neurotransmission in the processing of autonomic and respiratory pathways in the NTS of control and rats exposed to sustained hypoxia (SH). Baseline autonomic and respiratory activities and the responses to chemoreflex activation (KCN) were evaluated before and after microinjections of fluorocitrate (FCt, an astrocyte metabolic inhibitor), kynurenic acid, and pyridoxalphosphate-6-azophenyl-2',4'-disulfonate (PPADS) (nonselective antagonists of glutamatergic and purinergic receptors) into the rostral aspect of the caudal commissural NTS. FCt had no effects on the baseline parameters evaluated but reduced the bradycardic response to chemoreflex activation in SH rats. FCt combined with kynurenic acid and PPADS in control rats reduced the baseline duration of expiration, which was attenuated after SH. FCt produced a large increase in PN frequency discharge in control rats, which was reduced after SH, indicating a reduction in the astrocyte modulation after SH. The data show that 1) the bradycardic component of the peripheral chemoreflex is reduced in SH rats after astrocytes inhibition, 2) the inhibition of astrocytes in the presence of double antagonists in the NTS affects the modulation of baseline duration of expiration in control but not in SH rats, and 3) the autonomic and respiratory responses to chemoreflex activation are mediated by glutamatergic and purinergic receptors in the rostral aspect of the caudal commissural NTS.NEW & NOTEWORTHY Our findings indicate that the neurotransmission of autonomic and respiratory components of the peripheral chemoreflex in the nucleus tractus solitarius (NTS) is mediated by glutamatergic and purinergic mechanisms and reveal a selective involvement of NTS astrocytes in controlling the chemoreflex parasympathetic response in rats exposed to sustained hypoxia (SH) and the baseline duration of expiration mainly in control rats, indicating a selective role for astrocytes modulation in the NTS of control and SH rats.

Inhibitory synaptic transmission is impaired in the Kölliker-Fuse of male, but not female, Rett syndrome mice

Rett syndrome (RTT) is a severe neurodevelopmental disorder that mainly affects females due to silencing mutations in the X-linked MECP2 gene. One of the most troubling symptoms of RTT is breathing irregularity, including apneas, breath-holds, and hyperventilation. Mice with silencing mutations in Mecp2 exhibit breathing abnormalities similar to human patients and serve as useful models for studying mechanisms underlying breathing problems in RTT. Previous work implicated the pontine, respiratory-controlling Kölliker-Fuse (KF) in the breathing problems in RTT. The goal of this study was to test the hypothesis that inhibitory synaptic transmission is deficient in KF neurons from symptomatic male and female RTT mice. We performed whole cell voltage-clamp recordings from KF neurons in acute brain slices to examine spontaneous and electrically evoked inhibitory post-synaptic currents (IPSCs) in RTT mice and age- and sex-matched wild-type mice. The frequency of spontaneous IPSCs was reduced in KF neurons from male RTT mice but surprisingly not in female RTT mice. In addition, electrically evoked IPSCs were less reliable in KF neurons from male, but not female, RTT mice, which was positively correlated with paired-pulse facilitation, indicating decreased probability of release. KF neurons from male RTT mice were also more excitable and exhibited shorter-duration action potentials. Increased excitability of KF neurons from male mice was not explained by changes in axon initial segment length. These findings indicate impaired inhibitory neurotransmission and increased excitability of KF neurons in male but not female RTT mice and suggest that sex-dependent mechanisms contribute to breathing problems in RTT.NEW & NOTEWORTHY Kölliker-Fuse (KF) neurons in acute brain slices from male Rett syndrome (RTT) mice receive reduced inhibitory synaptic inputs compared with wild-type littermates. In female RTT mice, inhibitory transmission was not different in KF neurons compared with controls. The results from this study show that sex-specific alterations in synaptic transmission occur in the KF of RTT mice.

Inhibitory subpopulations in preBötzinger Complex play distinct roles in modulating inspiratory rhythm and pattern

Inhibitory neurons embedded within mammalian neural circuits shape breathing, walking, chewing, and other rhythmic motor behaviors. At the core of the neural circuit controlling breathing is the preBötzinger Complex (preBötC), a nucleus in the ventrolateral medulla necessary for generation of inspiratory rhythm. In the preBötC, a recurrently connected network of glutamatergic Dbx1-derived (Dbx1 + ) neurons generates rhythmic inspiratory drive. Functionally and anatomically intercalated among Dbx1 + preBötC neurons are GABAergic (GAD1/2 + ) and glycinergic (GlyT2 + ) neurons, whose roles in breathing remain unclear. To elucidate the inhibitory microcircuits within preBötC, we first characterized the spatial distribution of molecularly-defined inhibitory preBötC subpopulations in double reporter mice expressing either the red fluorescent protein tdTomato or EGFP in GlyT2 + , GAD1 + , or GAD2 + neurons. We found that, in postnatal mice, the majority of inhibitory preBötC neurons expressed a combination of GlyT2 and GAD2 while a much smaller subpopulation also expressed GAD1. To determine the functional role of these subpopulations, we used holographic photostimulation, a patterned illumination technique with high spatiotemporal resolution, in rhythmically active medullary slices from neonatal Dbx1 tdTomato ;GlyT2 EGFP and Dbx1 tdTomato ;GAD1 EGFP double reporter mice. Stimulation of 4 or 8 preBötC GlyT2 + neurons during endogenous rhythm prolonged the interburst interval in a phase-dependent manner and increased the latency to burst initiation when bursts were evoked by stimulation of Dbx1 + neurons. In contrast, stimulation of 4 or 8 preBötC GAD1 + neurons did not affect interburst interval or latency to burst initiation. Instead, photoactivation of GAD1 + neurons during the inspiratory burst prolonged endogenous and evoked burst duration and decreased evoked burst amplitude. We conclude that the majority of preBötC inhibitory neurons express both GlyT2 and GAD2 and modulate breathing rhythm by delaying burst initiation while a smaller GAD1 + subpopulation shapes inspiratory patterning by altering burst duration and amplitude.

Transformation of Our Understanding of Breathing Control by Molecular Tools

The rhythmicity of breath is vital for normal physiology. Even so, breathing is enriched with multifunctionality. External signals constantly change breathing, stopping it when under water or deepening it during exertion. Internal cues utilize breath to express emotions such as sighs of frustration and yawns of boredom. Breathing harmonizes with other actions that use our mouth and throat, including speech, chewing, and swallowing. In addition, our perception of breathing intensity can dictate how we feel, such as during the slow breathing of calming meditation and anxiety-inducing hyperventilation. Heartbeat originates from a peripheral pacemaker in the heart, but the automation of breathing arises from neural clusters within the brainstem, enabling interaction with other brain areas and thus multifunctionality. Here, we document how the recent transformation of cellular and molecular tools has contributed to our appreciation of the diversity of neuronal types in the breathing control circuit and how they confer the multifunctionality of breathing.

Regulation of breathing by cardiopulmonary afferents

This chapter broadly reviews cardiopulmonary sympathetic and vagal sensors and their reflex functions during physiologic and pathophysiologic processes. Mechanosensory operating mechanisms, including their central projections, are described under multiple sensor theory. In addition, ways to interpret evidence surrounding several controversial issues are provided, with detailed reasoning on how conclusions are derived. Cardiopulmonary sensory roles in breathing control and the development of symptoms and signs and pathophysiologic processes in cardiopulmonary diseases (such as cough and neuroimmune interaction) also are discussed.

Inspiratory Off-Switch Mediated by Optogenetic Activation of Inhibitory Neurons in the preBötzinger Complex In Vivo

The role of inhibitory neurons in the respiratory network is a matter of ongoing debate. Conflicting and contradicting results are manifold and the question whether inhibitory neurons are essential for the generation of the respiratory rhythm as such is controversial. Inhibitory neurons are required in pulmonary reflexes for adapting the activity of the central respiratory network to the status of the lung and it is hypothesized that glycinergic neurons mediate the inspiratory off-switch. Over the years, optogenetic tools have been developed that allow for cell-specific activation of subsets of neurons in vitro and in vivo. In this study, we aimed to identify the effect of activation of inhibitory neurons in vivo. Here, we used a conditional transgenic mouse line that expresses Channelrhodopsin 2 in inhibitory neurons. A 200 µm multimode optical fiber ferrule was implanted in adult mice using stereotaxic surgery, allowing us to stimulate inhibitory, respiratory neurons within the core excitatory network in the preBötzinger complex of the ventrolateral medulla. We show that, in anesthetized mice, activation of inhibitory neurons by blue light (470 nm) continuously or with stimulation frequencies above 10 Hz results in a significant reduction of the respiratory rate, in some cases leading to complete cessation of breathing. However, a lower stimulation frequency (4–5 Hz) could induce a significant increase in the respiratory rate. This phenomenon can be explained by the resetting of the respiratory cycle, since stimulation during inspiration shortened the associated breath and thereby increased the respiratory rate, while stimulation during the expiratory interval reduced the respiratory rate. Taken together, these results support the concept that activation of inhibitory neurons mediates phase-switching by inhibiting excitatory rhythmogenic neurons in the preBötzinger complex.

Mini Review: Central Organization of Airway Afferent Nerve Circuits

Airway afferents monitor the local chemical and physical micro-environments in the airway wall and lungs and send this information centrally to regulate neural circuits involved in setting autonomic tone, evoking reflex and volitional respiratory motor outflows, encoding perceivable sensations and contributing to higher order cognitive processing. In this mini-review we present a current overview of the central wiring of airway afferent circuits in the brainstem and brain, highlighting recent discoveries that augment our understanding of airway sensory processing. We additionally explore how advances in describing the molecular diversity of airway afferents may influence future research efforts aimed at defining central mesoscale connectivity of airway afferent pathways. A refined understanding of how functionally distinct airway afferent pathways are organized in the brain will provide deeper insight into the physiology of airway afferent-evoked responses and may foster opportunities for targeted modulation of specific pathways involved in disease.

Neurochemistry of the Kölliker-Fuse nucleus from a respiratory perspective

The Kölliker-Fuse nucleus (KF) is a functionally distinct component of the parabrachial complex, located in the dorsolateral pons of mammals. The KF has a major role in respiration and upper airway control. A comprehensive understanding of the KF and its contributions to respiratory function and dysfunction requires an appreciation for its neurochemical characteristics. The goal of this review is to summarize the diverse neurochemical composition of the KF, focusing on the neurotransmitters, neuromodulators, and neuropeptides present. We also include a description of the receptors expressed on KF neurons and transporters involved in each system, as well as their putative roles in respiratory physiology. Finally, we provide a short section reviewing the literature regarding neurochemical changes in the KF in the context of respiratory dysfunction observed in SIDS and Rett syndrome. By over-viewing the current literature on the neurochemical composition of the KF, this review will serve to aid a wide range of topics in the future research into the neural control of respiration in health and disease.

GABA-Glycine Cotransmitting Neurons in the Ventrolateral Medulla: Development and Functional Relevance for Breathing

Inhibitory neurons crucially contribute to shaping the breathing rhythm in the brain stem. These neurons use GABA or glycine as neurotransmitter; or co-release GABA and glycine. However, the developmental relationship between GABAergic, glycinergic and cotransmitting neurons, and the functional relevance of cotransmitting neurons has remained enigmatic. Transgenic mice expressing fluorescent markers or the split-Cre system in inhibitory neurons were developed to track the three different interneuron phenotypes. During late embryonic development, the majority of inhibitory neurons in the ventrolateral medulla are cotransmitting cells, most of which differentiate into GABAergic and glycinergic neurons around birth and around postnatal day 4, respectively. Functional inactivation of cotransmitting neurons revealed an increase of the number of respiratory pauses, the cycle-by-cycle variability, and the overall variability of breathing. In summary, the majority of cotransmitting neurons differentiate into GABAergic or glycinergic neurons within the first 2 weeks after birth and these neurons contribute to fine-tuning of the breathing pattern.

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.

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.

Vagal Afferent Innervation of the Airways in Health and Disease

Vagal sensory neurons constitute the major afferent supply to the airways and lungs. Subsets of afferents are defined by their embryological origin, molecular profile, neurochemistry, functionality, and anatomical organization, and collectively these nerves are essential for the regulation of respiratory physiology and pulmonary defense through local responses and centrally mediated neural pathways. Mechanical and chemical activation of airway afferents depends on a myriad of ionic and receptor-mediated signaling, much of which has yet to be fully explored. Alterations in the sensitivity and neurochemical phenotype of vagal afferent nerves and/or the neural pathways that they innervate occur in a wide variety of pulmonary diseases, and as such, understanding the mechanisms of vagal sensory function and dysfunction may reveal novel therapeutic targets. In this comprehensive review we discuss historical and state-of-the-art concepts in airway sensory neurobiology and explore mechanisms underlying how vagal sensory pathways become dysfunctional in pathological conditions.

GABAA- and glycine-mediated inhibitory modulation of the cough reflex in the caudal nucleus tractus solitarii of the rabbit

Cough-related sensory inputs from rapidly adapting receptors (RARs) and C fibers are processed by second-order neurons mainly located in the caudal nucleus tractus solitarii (NTS). Both GABAA and glycine receptors have been proven to be involved in the inhibitory control of second-order cells receiving RAR projections. We investigated the role of these receptors within the caudal NTS in the modulation of the cough reflex induced by either mechanical or chemical stimulation of the tracheobronchial tree in pentobarbital sodium-anesthetized, spontaneously breathing rabbits. Bilateral microinjections (30-50 nl) of the receptor antagonists bicuculline and strychnine as well as of the receptor agonists muscimol and glycine were performed. Bicuculline (0.1 mM) and strychnine (1 mM) caused decreases in peak abdominal activity and marked increases in respiratory frequency due to decreases in both inspiratory time (Ti) and expiratory time (Te), without concomitant changes in arterial blood pressure. Noticeably, these microinjections induced potentiation of the cough reflex consisting of increases in the cough number associated with decreases either in cough-related Ti after bicuculline or in both cough-related Ti and Te after strychnine. The effects caused by muscimol (0.1 mM) and glycine (10 mM) were in the opposite direction to those produced by the corresponding antagonists. The results show that both GABAA and glycine receptors within the caudal NTS mediate a potent inhibitory modulation of the pattern of breathing and cough reflex responses. They strongly suggest that disinhibition is one important mechanism underlying cough regulation and possibly provide new hints for novel effective antitussive strategies.

Presynaptic control of inhibitory neurotransmitter content in VIAAT containing synaptic vesicles

In mammals, fast inhibitory neurotransmission is carried out by two amino acid transmitters, γ-aminobutyric acid (GABA) and glycine. The higher brain uses only GABA, but in the spinal cord and brain stem both GABA and glycine act as inhibitory signals. In some cases GABA and glycine are co-released from the same neuron where they are co-packaged into synaptic vesicles by a shared vesicular inhibitory amino acid transporter, VIAAT (also called vGAT). The vesicular content of all other classical neurotransmitters (eg. glutamate, monoamines, acetylcholine) is determined by the presence of a specialized vesicular transporter. Because VIAAT is non-specific, the phenotype of inhibitory synaptic vesicles is instead predicted to be dependent on the relative concentration of GABA and glycine in the cytosol of the presynaptic terminal. This predicts that changes in GABA or glycine supply should be reflected in vesicle transmitter content but as yet, the mechanisms that control GABA versus glycine uptake into synaptic vesicles and their potential for modulation are not clearly understood. This review summarizes the most relevant experimental data that examines the link between GABA and glycine accumulation in the presynaptic cytosol and the inhibitory vesicle phenotype. The accumulated evidence challenges the hypothesis that vesicular phenotype is determined simply by the competition of inhibitory transmitter for VIAAT and instead suggest that the GABA/glycine balance in vesicles is dynamically regulated.

Inhibitory control of ascending glutamatergic projections to the lamprey respiratory rhythm generator

Neurons within the vagal motoneuron region of the lamprey have been shown to modulate respiratory activity via ascending excitatory projections to the paratrigeminal respiratory group (pTRG), the proposed respiratory rhythm generator. The present study was performed on in vitro brainstem preparations of the lamprey to provide a characterization of ascending projections within the whole respiratory motoneuron column with regard to the distribution of neurons projecting to the pTRG and related neurochemical markers. Injections of Neurobiotin were performed into the pTRG and the presence of glutamate, GABA and glycine immunoreactivity was investigated by double-labeling experiments. Interestingly, retrogradely labeled neurons were found not only in the vagal region, but also in the facial and glossopharyngeal motoneuron regions. They were also present within the sensory octavolateral area (OLA). The results show for the first time that neurons projecting to the pTRG are immunoreactive for glutamate, surrounded by GABA-immunoreactive structures and associated with the presence of glycinergic cells. Consistently, GABAA or glycine receptor blockade within the investigated regions increased the respiratory frequency. Furthermore, microinjections of agonists and antagonists of ionotropic glutamate receptors and of the GABAA receptor agonist muscimol showed that OLA neurons do not contribute to respiratory rhythm generation. The results provide evidence that glutamatergic ascending pathways to the pTRG are subject to a potent inhibitory control and suggest that disinhibition is one important mechanism subserving their function. The general characteristics of inhibitory control involved in rhythmic activities, such as respiration, appear to be highly conserved throughout vertebrate evolution.

Deficiency of GABAergic synaptic inhibition in the Kölliker-Fuse area underlies respiratory dysrhythmia in a mouse model of Rett syndrome

KEY POINTS

Life threatening breathing irregularity and central apnoeas are highly prevalent in children suffering from Rett syndrome. Abnormalities in inhibitory synaptic transmission have been associated with the physiopathology of this syndrome, and may underlie the respiratory disorder. In a mouse model of Rett syndrome, GABAergic terminal projections are markedly reduced in the Kölliker-Fuse nucleus (KF) in the dorsolateral pons, an important centre for control of respiratory rhythm regularity. Administration of a drug that augments endogenous GABA localized to this region of the pons reduced the incidence of apnoea and the respiratory irregularity of Rett female mice. Conversely, the respiratory disorder was recapitulated by blocking GABAergic transmission in the KF area of healthy rats. This study helps us understand the mechanism for generation of respiratory abnormality in Rett syndrome, pinpoints a brain site responsible and provides a clear anatomical target for the development of a translatable drug treatment. Central apnoeas and respiratory irregularity are a common feature in Rett syndrome (RTT), a neurodevelopmental disorder most often caused by mutations in the methyl-CpG-binding protein 2 gene (MECP2). We used a MECP2 deficient mouse model of RTT as a strategy to obtain insights into the neurobiology of the disease and into mechanisms essential for respiratory rhythmicity during normal breathing. Previously, we showed that, systemic administration of a GABA reuptake blocker in MECP2 deficient mice markedly reduced the occurrence of central apnoeas. Further, we found that, during central apnoeas, post-inspiratory drive (adductor motor) to the upper airways was enhanced in amplitude and duration in Mecp2 heterozygous female mice. Since the pontine Kölliker-Fuse area (KF) drives post-inspiration, suppresses inspiration, and can reset the respiratory oscillator phase, we hypothesized that synaptic inhibition in this area is essential for respiratory rhythm regularity. In this study, we found that: (i) Mecp2 heterozygous mice showed deficiency of GABA perisomatic bouton-like puncta and processes in the KF nucleus; (ii) blockade of GABA reuptake in the KF of RTT mice reduced breathing irregularity; (iii) conversely, blockade of GABAA receptors in the KF of healthy rats mimicked the RTT respiratory phenotype of recurrent central apnoeas and prolonged post-inspiratory activity. Our results show that reductions in synaptic inhibition within the KF induce rhythm irregularity whereas boosting GABA transmission reduces respiratory arrhythmia in a murine model of RTT. Our data suggest that manipulation of synaptic inhibition in KF may be a clinically important strategy for alleviating the life threatening respiratory disorders in RTT.

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.

The effect of tracheal occlusion on respiratory load compensation: changes in neurons containing inhibitory neurotransmitter in the nucleus of the solitary tract in conscious rats

Respiratory load compensation volume-time (Vt-T) relationships have been extensively studied in anesthetized animals. There are only a few studies in conscious animals although consciousness and behavior play a critical role in modulation of breathing. The aims of the study were to determine the effect of intermittent and transient tracheal occlusions (ITTO) elicited load compensation responses and the changes in activation of inhibitory glycinergic neurons in the nucleus of solitary tract (NTS) in conscious rats. The results showed that ITTO elicited an increase in expiratory time (T(e)) but did not affect inspiratory time (T(i)) and diaphragm activity (EMG(dia)). An increase in total breathing time (Ttot) was due exclusively to the increase in T(e). In addition, glycinergic neurons were activated in the intermediate NTS (iNTS) but not in the caudal NTS (cNTS). These results suggest that the activated glycinergic neurons in the iNTS may be important for the neurogenesis of load compensation responses in conscious animals.

Learning to breathe: habituation of Hering-Breuer inflation reflex emerges with postnatal brainstem maturation

The Hering-Breuer (HBR) reflex is considered a major regulatory feedback for the generation and patterning of respiratory activity. While HBR is important in neonates, its significance in adults is controversial. Previous experiments that investigated the plasticity of entrainment of the respiratory rhythm by vagal input demonstrated postnatal changes in HBR plasticity. Here we analyzed postnatal changes in the plasticity of HBR by mimicking the classic lung inflation tests with repetitive tonic vagal stimulation across different postnatal stages in an in situ perfused brainstem preparation of rat. The study shows that neonates stereotypically exhibit HBR stimulus-dependent prolongation of expiration while juvenile preparations (>postnatal day 16) showed significant habituation of HBR following repetitive stimulation. Subsequent experiments employing physiological lung inflation tests in situ confirmed HBR habituation in juveniles. We conclude that postnatal emergence of HBR habituation explains the weak contribution and high activation threshold of HBR in the regulation of eupnea.

Tracheal occlusion-evoked respiratory load compensation and inhibitory neurotransmitter expression in rats

Respiratory load compensation is a sensory-motor reflex generated in the brain stem respiratory neural network. The nucleus of the solitary tract (NTS) is thought to be the primary structure to process the respiratory load-related afferent activity and contribute to the modification of the breathing pattern by sending efferent projections to other structures in the brain stem respiratory neural network. The sensory pathway and motor responses of respiratory load compensation have been studied extensively; however, the mechanism of neurogenesis of load compensation is still unknown. A variety of studies has shown that inhibitory interconnections among the brain stem respiratory groups play critical roles for the genesis of respiratory rhythm and pattern. The purpose of this study was to examine whether inhibitory glycinergic neurons in the NTS were activated by external and transient tracheal occlusions (ETTO) in anesthetized animals. The results showed that ETTO produced load compensation responses with increased inspiratory, expiratory, and total breath time, as well as elevated activation of inhibitory glycinergic neurons in the caudal NTS (cNTS) and intermediate NTS (iNTS). Vagotomized animals receiving transient respiratory loads did not exhibit these load compensation responses. In addition, vagotomy significantly reduced the activation of inhibitory glycinergic neurons in the cNTS and iNTS. The results suggest that these activated inhibitory glycinergic neurons in the NTS might be essential for the neurogenesis of load compensation responses in anesthetized animals.

GABAergic and glycinergic inputs modulate rhythmogenic mechanisms in the lamprey respiratory network

We have previously shown that GABA and glycine modulate respiratory activity in the in vitro brainstem preparations of the lamprey and that blockade of GABAA and glycine receptors restores the respiratory rhythm during apnoea caused by blockade of ionotropic glutamate receptors. However, the neural substrates involved in these effects are unknown. To address this issue, the role of GABAA, GABAB and glycine receptors within the paratrigeminal respiratory group (pTRG), the proposed respiratory central pattern generator, and the vagal motoneuron region was investigated both during apnoea induced by blockade of glutamatergic transmission and under basal conditions through microinjections of specific antagonists. The removal of GABAergic, but not glycinergic transmission within the pTRG, causes the resumption of rhythmic respiratory activity during apnoea, and reveals the presence of a modulatory control of the pTRG under basal conditions. A blockade of GABAA and glycine receptors within the vagal region strongly increases the respiratory frequency through disinhibition of neurons projecting to the pTRG from the vagal region. These neurons were retrogradely labelled (neurobiotin) from the pTRG. Intense GABA immunoreactivity is observed both within the pTRG and the vagal area, which corroborates present findings. The results confirm the pTRG as a primary site of respiratory rhythm generation, and suggest that inhibition modulates the activity of rhythm-generating neurons, without any direct role in burst formation and termination mechanisms.

Postnatal development of glycine receptor subunits α1, α2, α3, and β immunoreactivity in multiple brain stem respiratory-related nuclear groups of the rat

The respiratory system is immature at birth and significant development occurs postnatally. A critical period of respiratory development occurs in rats around postnatal days 12-13, when enhanced inhibition dominates over suppressed excitation. The mechanisms underlying the heightened inhibition are not fully understood. The present study tested our hypothesis that the inhibition is marked by a switch in glycine receptor subunits from neonatal to adult form around the critical period. An in-depth immunohistochemical and single neuron optical densitometric study was undertaken on four respiratory-related nuclear groups (the pre-Bötzinger complex, nucleus ambiguus, hypoglossal nucleus, and ventrolateral subnucleus of solitary tract nucleus), and a non-respiratory cuneate nucleus in P2-21 rats. Our data revealed that in the respiratory-related nuclear groups: (1) the expressions of GlyRα2 and GlyRα3 were relatively high at P2, but declined after 1-1½ weeks to their lowest levels at P21; (2) the expression of GlyRα1 increased with age and reached significance at P12; and (3) the expression of GlyRβ rose from P2 to P12 followed by a slight decline until P21. No distinct increase in GlyRα1 at P12 was noted in the cuneate nucleus. Thus, there is a switch in dominance of expression from neonatal GlyRα2/α3 to the adult GlyRα1 and a heightened expression of GlyRα1 around the critical period in all respiratory-related nuclear groups, thereby supporting enhanced inhibition at that time. The rise in the expression of GlyRβ around P12 indicates that it plays an important role in forming the mature heteropentameric glycine receptors in these brain stem nuclear groups.

Role of inhibition in respiratory pattern generation

Postsynaptic inhibition is a key element of neural circuits underlying behavior, with 20-50% of all mammalian (nongranule) neurons considered inhibitory. For rhythmic movements in mammals, e.g., walking, swimming, suckling, chewing, and breathing, inhibition is often hypothesized to play an essential rhythmogenic role. Here we study the role of fast synaptic inhibitory neurotransmission in the generation of breathing pattern by blocking GABA(A) and glycine receptors in the preBötzinger complex (preBötC), a site essential for generation of normal breathing pattern, and in the neighboring Bötzinger complex (BötC). The breathing rhythm continued following this blockade, but the lung inflation-induced Breuer-Hering inspiratory inhibitory reflex was suppressed. The antagonists were efficacious, as this blockade abolished the profound effects of the exogenously applied GABA(A) receptor agonist muscimol or glycine, either of which under control conditions stopped breathing in vagus-intact or vagotomized, anesthetized, spontaneously breathing adult rats. In vagotomized rats, GABA(A)ergic and glycinergic antagonists had little, if any, effect on rhythm. The effect in vagus-intact rats was to slow the rhythm to a pace equivalent to that seen after suppression of the aforementioned Breuer-Hering inflation reflex. We conclude that postsynaptic inhibition within the preBötC and BötC is not essential for generation of normal respiratory rhythm in intact mammals. We suggest the primary role of inhibition is in shaping the pattern of respiratory motor output, assuring its stability, and in mediating reflex or volitional apnea, but not in the generation of rhythm per se.

Control of breathing by interacting pontine and pulmonary feedback loops

The medullary respiratory network generates respiratory rhythm via sequential phase switching, which in turn is controlled by multiple feedbacks including those from the pons and nucleus tractus solitarii; the latter mediates pulmonary afferent feedback to the medullary circuits. It is hypothesized that both pontine and pulmonary feedback pathways operate via activation of medullary respiratory neurons that are critically involved in phase switching. Moreover, the pontine and pulmonary control loops interact, so that pulmonary afferents control the gain of pontine influence of the respiratory pattern. We used an established computational model of the respiratory network (Smith et al., 2007) and extended it by incorporating pontine circuits and pulmonary feedback. In the extended model, the pontine neurons receive phasic excitatory activation from, and provide feedback to, medullary respiratory neurons responsible for the onset and termination of inspiration. The model was used to study the effects of: (1) "vagotomy" (removal of pulmonary feedback), (2) suppression of pontine activity attenuating pontine feedback, and (3) these perturbations applied together on the respiratory pattern and durations of inspiration (T(I)) and expiration (T(E)). In our model: (a) the simulated vagotomy resulted in increases of both T(I) and T(E), (b) the suppression of pontine-medullary interactions led to the prolongation of T(I) at relatively constant, but variable T(E), and (c) these perturbations applied together resulted in "apneusis," characterized by a significantly prolonged T(I). The results of modeling were compared with, and provided a reasonable explanation for, multiple experimental data. The characteristic changes in T(I) and T(E) demonstrated with the model may represent characteristic changes in the balance between the pontine and pulmonary feedback control mechanisms that may reflect specific cardio-respiratory disorders and diseases.

Influence of age, body temperature, GABAA receptor inhibition and caffeine on the Hering-Breuer inflation reflex in unanesthetized rat pups

We measured the duration of apnea induced by sustained end-inspiratory lung inflation (the Hering Breuer Reflex, HBR) in unanesthetized infant rat pups aged 4 days (P4) to P20 at body temperatures of 32°C and 36°C. The expiratory prolongation elicited by the HBR lasted longer in the younger pups and lasted longer at the higher body temperature. Blockade of adenosine receptors by caffeine following injection into the cisterna magna (ICM) significantly blunted the thermal prolongation of the HBR. Blockade of gama-amino-butyric acid A (GABAA) receptors by pre-treatment with ICM bicuculline had no effect on the HBR duration at either body temperature. To test the hypothesis that developmental maturation of GABAergic inhibition of breathing was modifying the response to bicuculline, we pretreated rat pups with systemically administered bumetanide to lower the intracellular chloride concentration, and repeated the bicuculline studies. Bicuculline still did not alter the HBR at either temperature after bumetanide treatment. We administered PSB-36, a selective adenosine A1 receptor antagonist, and this drug treatment did not modify the HBR. We conclude that caffeine blunts the thermal prolongation of the HBR, probably by blocking adenosine A2a receptors. The thermally sensitive adenosinergic prolongation of the HBR in these intact animals does not seem to depend on GABAA receptors.

Autonomic neural control of the airways

The airways and lungs are innervated by both sympathetic and parasympathetic nerves. Cholinergic parasympathetic innervation is well conserved in the airways while the distribution of noncholinergic parasympathetic and adrenergic sympathetic nerves varies considerably amongst species. Autonomic nerve function is regulated primarily through reflexes initiated upon bronchopulmonary vagal afferent nerves. Central regulation of autonomic tone is poorly described but some key elements have been defined.

Modulation of the cough reflex by GABA(A) receptors in the caudal ventral respiratory group of the rabbit

We have previously shown that the caudal ventral respiratory group (cVRG) is a possible site of action of some antitussive drugs and plays a crucial role in determining both the expiratory and inspiratory components of the cough motor pattern. In addition, it has been reported that medullary expiratory neurons of the cVRG are subject to potent GABAergic gain modulation. This study was devoted to investigate the role of cVRG GABA_A receptors in the control of baseline respiratory activity and cough responses to mechanical and chemical (citric acid) stimulation of the tracheobronchial tree. To this purpose, bilateral microinjections (30–50 nl) of bicuculline or muscimol were performed into the cVRG of pentobarbital sodium-anesthetized, spontaneously breathing rabbits. Bicuculline (1 mM) increased peak abdominal activity and respiratory frequency due to decreases in T_E. Cough responses were potentiated mainly owing to increases in the cough number. The recovery was observed within ~2 h. On the contrary, muscimol (0.3 mM) abolished abdominal activity and decreased respiratory frequency due to increases in T_E. In addition, cough responses were progressively reduced and completely suppressed within ~20 min. Partial recovery of cough responses was achieved after ~3 h or within ~5 min following bicuculline microinjections at the same locations. The sneeze reflex induced by mechanical stimulation of the nasal mucosa persisted following bicuculline and muscimol microinjections. However, the number and intensity of expiratory thrusts were enhanced by bicuculline and suppressed by muscimol. The results provide evidence that a potent GABA_A-mediated inhibitory modulation is exerted at the level of the cVRG not only on respiratory activity, but also on cough and sneeze reflex responses.

Caudal nuclei of the rat nucleus of the solitary tract differentially innervate respiratory compartments within the ventrolateral medulla

A substantial array of respiratory, cardiovascular, visceral and somatic afferents are relayed via the nucleus of the solitary tract (NTS) to the brainstem (and forebrain). Despite some degree of overlap within the NTS, specificity is maintained in central respiratory reflexes driven by second order afferent relay neurons in the NTS. While the topographic arrangement of respiratory-related afferents targeting the NTS has been extensively investigated, their higher order brainstem targets beyond the NTS has only rarely been defined with any precision. Nonetheless, the various brainstem circuits serving blood gas homeostasis and airway protective reflexes must clearly receive a differential innervation from the NTS in order to evoke stimulus appropriate behavioral responses. Accordingly, we have examined the question of which specific NTS nuclei project to particular compartments within the ventral respiratory column (VRC) of the ventrolateral medulla. Our analyses of NTS labeling after retrograde tracer injections in the VRC and the nearby neuronal groups controlling autonomic function indicate a significant distinction between projections to the Bötzinger complex and preBötzinger complex compared to the remainder of the VRC. Specifically, the caudomedial NTS, including caudal portions of the medial solitary nucleus and the commissural division of NTS project relatively densely to the region of the retrotrapezoid nucleus and rostral ventrolateral medullary nucleus as well as to the rostral ventral respiratory group while avoiding the intervening Bötzinger and preBötzinger complexes. Area postrema appears to demonstrate a pattern of projections similar to that of caudal medial and commissural NTS nuclei. Other, less pronounced differential projections of lateral NTS nuclei to the various VRC compartments are additionally noted.

Chemosensory control by commissural nucleus of the solitary tract in rats

The commissural nucleus of the solitary tract (commNTS) is a main area that receives afferent signals involved in the cardiovascular and respiratory control like those related to chemoreceptor activation, however, the importance of the commNTS for the cardiorespiratory responses to chemoreceptor activation is still controversial. In the present study, we investigated the cardiorespiratory responses to hypoxia or hypercapnia in anesthetized and conscious rats treated with injections of the GABA-A agonist muscimol into the caudal portion of the commNTS. Male Holtzman rats (280-300 g) were used. In conscious rats that had a stainless steel cannula previously implanted into the commNTS, the injection of muscimol (2 mM) into the commNTS reduced the pressor response (16±2 mmHg, vs. saline: 36±3 mmHg) and the increase in ventilation (250±17 ml/min/kg, vs. saline: 641±28 ml/min/kg) produced by hypoxia (8-10% O(2)). In urethane anesthetized rats, the injection of muscimol into the commNTS eliminated the pressor response (5±2 mmHg, vs. saline: 26±5 mmHg) and the increase in phrenic nerve discharge (PND) (20±6%, vs. saline: 149±15%) and reduced the increase in splanchnic sympathetic nerve discharge (sSND) (93±15%, vs. saline: 283±19% of baseline) produced by hypoxia. However, muscimol injected into the commNTS did not change hypercapnia (8-10% CO(2)) induced pressor response or the increase in the sSND or PND in urethane anesthetized rats or the increase in ventilation in conscious rats. The present results suggest that the cardiorespiratory responses to hypoxia are strongly dependent on the caudal portion of the commNTS, however, this area is not involved in the responses to hypercapnia.

Perinatal development of inhibitory synapses in the nucleus tractus solitarii of the rat

The nucleus tractus solitarii (NTS) plays a key role in the central control of the autonomic nervous system. In adult rats, both GABA and glycine are used as inhibitory neurotransmitter in the NTS. Using a quantitative morphological approach, we have investigated the perinatal development of inhibitory synapses in the NTS. The density of both inhibitory axon terminals and synapses increased from embryonic day 20 until the end of the second postnatal week (postnatal day 14). Before birth, only GABAergic axon terminals developed and their number increased during the first postnatal week. Mixed GABA/glycine axon terminals appeared at birth and their number increased during the first postnatal week. This suggests the development of a mixed GABA/glycine inhibition in parallel to pure GABA inhibition. However, whereas GABAergic axon terminals were distributed throughout the NTS, mixed GABA/glycine axon terminals were strictly located in the lateral part of the NTS. Established at birth, this specific topography remained in the adult rat. From birth, GABA(A) receptors, glycine receptors and gephyrin were clustered in inhibitory synapses throughout the NTS, revealing a neurotransmitter-receptor mismatch within the medial part of the NTS. Together these results suggest that NTS inhibitory networks develop and mature until postnatal day 14. Developmental changes in NTS synaptic inhibition may play an important role in shaping neural network activity during a time of maturation of autonomic functions. The first two postnatal weeks could represent a critical period where the impact of the environment influences the physiological phenotypes of adult rats.

Encoding of the cough reflex in anesthetized guinea pigs

We have previously described the physiological and morphological properties of the cough receptors and their sites of termination in the airways and centrally in the nucleus tractus solitarius (nTS). In the present study, we have addressed the hypothesis that the primary central synapses of the cough receptors subserve an essential role in the encoding of cough. We found that cough requires sustained, high-frequency (≥8-Hz) afferent nerve activation. We also found evidence for processes that both facilitate (summation, sensitization) and inhibit the initiation of cough. Sensitization of cough occurs with repetitive subthreshold activation of the cough receptors or by coincident activation of C-fibers and/or nTS neurokinin receptor activation. Desensitization of cough evoked by repetitive and/or continuous afferent nerve activation has a rapid onset (<60 s) and does not differentiate between tussive stimuli, suggesting a central nervous system-dependent process. The cough reflex can also be actively inhibited upon activation of other airway afferent nerve subtypes, including slowly adapting receptors and pulmonary C-fibers. The sensitization and desensitization of cough are likely attributable to the prominent, primary, and unique role of N-methyl-d-aspartate receptor-dependent signaling at the central synapses of the cough receptors. These attributes may have direct relevance to the presentation of cough in disease and for the effectiveness of antitussive therapies.

Co-localisation of markers for glycinergic and GABAergic neurones in rat nucleus of the solitary tract: implications for co-transmission

Immunoreactive structures visualised with antibodies to glycine were prominent in areas of the nucleus of the solitary tract (NTS) surrounding the tractus solitarius, but scarcer in medial and ventral areas of the nucleus. This contrasted with a higher density, more homogenous distribution of structures labelled for gamma-aminobutyric acid (GABA). Immunolabelling of adjacent semi-thin sections nonetheless indicated a close correspondence between cells and puncta labelled by glycine and GABA antisera in certain NTS areas. With post-embedding electron microscopic immunolabelling, synaptic terminals with high, presumed transmitter levels of glycine were discriminated from terminals containing low, metabolic levels by quantitative analysis of gold particle labelling densities. In a random sample of terminals, 28.5% qualified on this basis as glycinergic (compared to 44.4% GABAergic); these glycinergic terminals targeted mainly dendritic structures and contained pleomorphic vesicles and symmetrical synapses. Serial section analysis revealed few terminals (5.2%) immunoreactive for glycine alone, with 82% of glycinergic terminals also containing high levels of GABA immunoreactivity. No evidence for co-localisation of glycine and glutamate was found. Light, confocal and electron microscopic labelling with antibodies to proteins specific for glycine and GABA synthesis, release and uptake confirmed that glycinergic terminals also containing GABA are found predominantly in more lateral areas of NTS, despite glycine receptors and the 'glial' glycine transporter (GLYT1) being expressed throughout all areas of the nucleus. The data suggest that synaptic terminals in certain functionally distinct areas of NTS co-release both inhibitory amino acids, which may account for the previously reported differential inhibitory effects of glycine and GABA on NTS neurones.

Mixed GABA-glycine synapses delineate a specific topography in the nucleus tractus solitarii of adult rat

Using combined morphological and electrophysiological approaches, we have determined the composition of inhibitory synapses of the nucleus tractus solitarii (NTS), a brainstem structure that is a gateway for many visceral sensory afferent fibres. Immunohistochemical experiments demonstrate that, in adult rat, GABA axon terminals are present throughout the NTS while mixed GABA-glycine axon terminals are strictly located to the lateral part of the NTS within subnuclei surrounding the tractus solitarius. Purely glycine axon terminals are rare in the lateral part of the NTS and hardly detected in its medial part. Electrophysiological experiments confirm the predominance of GABA inhibition throughout the NTS and demonstrate the existence of a dual inhibition involving the co-release of GABA and glycine restricted to the lateral part of NTS. Since GABA(A) and glycine receptors are co-expressed postsynaptically in virtually all the inhibitory axon terminals throughout the NTS, it suggests that the inhibition phenotype relies on the characteristics of the axon terminals. Our results also demonstrate that glycine is mostly associated with GABA within axon terminals and raise the possibility of a dynamic regulation of GABA/glycine release at the presynaptic level. Our data provide new information for understanding the mechanisms involved in the processing of visceral information by the central nervous system in adult animals.

Early abnormalities of post-sigh breathing in a mouse model of Rett syndrome

Rett syndrome is a neurodevelopmental disease accompanied by complex, disabling symptoms, including breathing symptoms. Because Rett syndrome is caused by mutations in the transcriptional repressor methyl-CpG-binding protein 2 (MeCP2), Mecp2-deficient mice have been generated as experimental model. Males of Mecp2-deficient mice (Mecp2(-/y)) breathe normally at birth but show abnormal respiratory responses to hypoxia and hypercapnia from postnatal day 25 (P25). After P30, Mecp2(-/y) mice develop breathing symptoms reminiscent of Rett syndrome, aggravating until premature death at around P60. Using plethysmography, we analyzed the sighs and the post-sigh breathing pattern of unrestrained wild type male mice (WT) and Mecp2(-/y) mice from P15 to P60. Sighs are spontaneous large inspirations known to prevent lung atelectasis and to improve alveolar oxygenation. However, Mecp2(-/y) mice show early abnormalities of post-sigh breathing, with long-lasting post-sigh apnoeas, reduced tidal volume when eupnoea resumes and lack of post-sigh bradypnoea which develop from P15, aggravate with age and possibly contribute to breathing symptoms to come.

Use-dependent learning and memory of the Hering-Breuer inflation reflex in rats

The classic Hering-Breuer inflation reflex (HBIR) is a widely held tenet for understanding the lung volume-related vagal control of respiratory rhythm. Recent evidence, however, has revealed that the fictive HBIR elicited by electrical vagal stimulation in rats is not static but may be attenuated centrally by two forms of non-associative learning (habituation and desensitization) that continually mitigate the reflex effects with exponential adaptations like a differentiator or high-pass filter. Desensitization is analogous to habituation but exhibits an explicit short-term memory (STM) in the form of a rebound response with exponential decay during recovery from stimulation. To investigate whether such learning and memory effects are lung volume related and use-dependent (practice makes perfect), we compared the time-dependent changes in inspiratory and expiratory durations (t(I) and t(E)) during and after 1 or 8 min unilateral lung inflation or high-frequency, low-intensity vagal stimulation in anaesthetized, uni- or bi-vagotomized rats. Unilateral lung inflation and vagal stimulation both elicited abrupt shortening of t(I) and lengthening of t(E) (HBIR effects) and gradual habituation and desensitization throughout the 1 or 8 min test period, followed by rebound responses in t(I) and t(E) with exponential recovery (STM effects) in the post-test period. In both cases, the STM time constants for t(I) and t(E) were significantly longer with the 8 min test than with the 1 min test (17-45 versus 4-11 s, P < 0.01). We conclude that the HBIR and its central habituation and desensitization are mediated peripherally by lung volume-related vagal afferents, and that the STM of desensitization is use-dependent. The translational implications of these findings are discussed.

The chemical neuroanatomy of breathing

The chemical neuroanatomy of breathing must ultimately encompass all the various neuronal elements physiologically identified in brainstem respiratory circuits and their apparent aggregation into "compartments" within the medulla and pons. These functionally defined respiratory compartments in the brainstem provide the major source of input to cranial motoneurons controlling the airways, and to spinal motoneurons activating inspiratory and expiratory pump muscles. This review provides an overview of the neuroanatomy of the major compartments comprising brainstem respiratory circuits, and a synopsis of the transmitters used by their constituent respiratory neurons.

Identification of neurotransmitters and co-localization of transmitters in brainstem respiratory neurons

Identifying the major ionotropic neurotransmitter in a respiratory neuron is of critical importance in determining how the neuron fits into the respiratory system, whether in producing or modifying respiratory drive and rhythm. There are now several groups of respiratory neurons whose major neurotransmitters have been identified and in some of these cases, more than one transmitter has been identified in particular neurons. This review will describe the physiologically identified neurons in major respiratory areas that have been phenotyped for major ionotropic transmitters as well as those where more than one transmitter has been identified. Although the purpose of the additional transmitter has not been elucidated for any of the respiratory neurons, some examples from other systems will be discussed.

Functional connectivity in the pontomedullary respiratory network

Current models propose that a neuronal network in the ventrolateral medulla generates the basic respiratory rhythm and that this ventrolateral respiratory column (VRC) is profoundly influenced by the neurons of the pontine respiratory group (PRG). However, functional connectivity among PRG and VRC neurons is poorly understood. This study addressed four model-based hypotheses: 1) the respiratory modulation of PRG neuron populations reflects paucisynaptic actions of multiple VRC populations; 2) functional connections among PRG neurons shape and coordinate their respiratory-modulated activities; 3) the PRG acts on multiple VRC populations, contributing to phase-switching; and 4) neurons with no respiratory modulation located in close proximity to the VRC and PRG have widely distributed actions on respiratory-modulated cells. Two arrays of microelectrodes with individual depth adjustment were used to record sets of spike trains from a total of 145 PRG and 282 VRC neurons in 10 decerebrate, vagotomized, neuromuscularly blocked, ventilated cats. Data were evaluated for respiratory modulation with respect to efferent phrenic motoneuron activity and short-timescale correlations indicative of paucisynaptic functional connectivity using cross-correlation analysis and the "gravity" method. Correlogram features were found for 109 (3%) of the 3,218 pairs composed of a PRG and a VRC neuron, 126 (12%) of the 1,043 PRG-PRG pairs, and 319 (7%) of the 4,340 VRC-VRC neuron pairs evaluated. Correlation linkage maps generated for the data support our four motivating hypotheses and suggest network mechanisms for proposed modulatory functions of the PRG.

Reconfiguration of the pontomedullary respiratory network: a computational modeling study with coordinated in vivo experiments

A large body of data suggests that the pontine respiratory group (PRG) is involved in respiratory phase-switching and the reconfiguration of the brain stem respiratory network. However, connectivity between the PRG and ventral respiratory column (VRC) in computational models has been largely ad hoc. We developed a network model with PRG-VRC connectivity inferred from coordinated in vivo experiments. Neurons were modeled in the "integrate-and-fire" style; some neurons had pacemaker properties derived from the model of Breen et al. We recapitulated earlier modeling results, including reproduction of activity profiles of different respiratory neurons and motor outputs, and their changes under different conditions (vagotomy, pontine lesions, etc.). The model also reproduced characteristic changes in neuronal and motor patterns observed in vivo during fictive cough and during hypoxia in non-rapid eye movement sleep. Our simulations suggested possible mechanisms for respiratory pattern reorganization during these behaviors. The model predicted that network- and pacemaker-generated rhythms could be co-expressed during the transition from gasping to eupnea, producing a combined "burst-ramp" pattern of phrenic discharges. To test this prediction, phrenic activity and multiple single neuron spike trains were monitored in vagotomized, decerebrate, immobilized, thoracotomized, and artificially ventilated cats during hypoxia and recovery. In most experiments, phrenic discharge patterns during recovery from hypoxia were similar to those predicted by the model. We conclude that under certain conditions, e.g., during recovery from severe brain hypoxia, components of a distributed network activity present during eupnea can be co-expressed with gasp patterns generated by a distinct, functionally "simplified" mechanism.

GABAergic neurons in the ventrolateral subnucleus of the nucleus tractus solitarius are in contact with Kölliker-Fuse nucleus neurons projecting to the rostral ventral respiratory group and phrenic nucleus in the rat

After ipsilateral injections of biotinylated dextran amine (BDA) into the ventrolateral subnucleus of the nucleus tractus solitarius (vlNTS) and Fluoro-gold (FG) into the rostral ventral respiratory group (rVRG) region or into the phrenic nucleus (PhN) region in the rat, an overlapping distribution of BDA-labeled axon terminals and FG-labeled neurons was found in the Kölliker-Fuse (KF) nucleus ipsilateral to the injection sites. Using retrograde tracing combined with immunohistochemistry for glutamic acid decarboxylase isoform 67 (GAD67), we indicated that as many as 40% of the vlNTS neurons projecting to the KF were immunoreactive for GAD67. Using a combination of anterograde and retrograde tracing techniques, and immunohistochemistry for GAD67, we further demonstrated that the vlNTS axon terminals with GAD67 immunoreactivity established close contact to the rVRG- or PhN-projecting KF neurons. The present results suggest that GABAergic vlNTS fibers may exert inhibitory influences on the rVRG- as well as PhN-projecting KF neurons and these circuits may be involved in the respiratory reflexes such as the Hering-Breuer reflex.

Transcription factors and the genetic organization of brain stem respiratory neurons

Breathing is a genetically determined behavior generated by neurons in the brain stem. Transcription factors, in part, determine the basic developmental identity of neurons, but the relationships between these genes and the neural populations generating and modulating respiration are unclear. The diversity of brain stem populations has been proposed to result from a combinatorial code of transcription factor expression corresponding to the anterior-posterior (A-P) and dorsal-ventral (D-V) location of a neuron's birth. I provide a schematic of transcription factor coding identifying at least 15 genetically distinct D-V subdivisions of brain stem neurons that, combined with A-P patterning, may provide a genetic organization of the brain stem in general, with the eventual goal of describing respiratory populations in particular. Using a combination of fate mapping in transgenic mouse lines and immunohistochemistry, we confirm the parabrachial nuclei are derived from a subset of Atoh1 expression progenitor neurons. I hypothesize the Kölliker-Fuse nucleus can be uniquely defined in the neonate mouse by the coexpression of the transcription factor FoxP2 in Atoh1-derived neurons of rhombomere 1.

Elevated body temperature exaggerates laryngeal chemoreflex apnea in decerebrate piglets

We investigated the interaction between body temperature and the duration of the laryngeal chemoreflex (LCR) in decerebrate piglets. Elevating body temperature by approximately 2 degrees C prolongs the duration of the LCR and the length of apnea associated with the reflex. This thermal prolongation seems to arise within the nucleus of the solitary tract in the brainstem, and we believe the thermal effect is mediated by enhanced GABAergic neurotransmission.

Central nervous system distribution of the transcription factor Phox2b in the adult rat

Phox2b is required for development of the peripheral autonomic nervous system and a subset of cranial nerves and lower brainstem nuclei. Phox2b mutations in man cause diffuse autonomic dysfunction and deficits in the automatic control of breathing. Here we study the distribution of Phox2b in the adult rat hindbrain to determine whether this protein is selectively expressed by neurons involved in respiratory and autonomic control. In the medulla oblongata, Phox2b-immunoreactive nuclei were present in the dorsal vagal complex, intermediate reticular nucleus, dorsomedial spinal trigeminal nucleus, nucleus ambiguus, catecholaminergic neurons, and retrotrapezoid nucleus (RTN). Phox2b was expressed by both central excitatory relays of the sympathetic baroreflex (nucleus of the solitary tract and C1 neurons) but not by the inhibitory relay of this reflex. Phox2b was absent from the ventral respiratory column (VRC) caudal to RTN and rare within the parabrachial nuclei. In the pons, Phox2b was confined to cholinergic efferent neurons (salivary, vestibulocochlear) and noncholinergic peritrigeminal neurons. Rostral to the pons, Phox2b was detected only in the oculomotor complex. In adult rats, Phox2b is neither a comprehensive nor a selective marker of hindbrain autonomic pathways. This marker identifies a subset of hindbrain neurons that control orofacial movements (dorsomedial spinal trigeminal nucleus, pontine peritrigeminal neurons), balance and auditory function (vestibulocochlear efferents), the eyes, and both divisions of the autonomic efferent system. Phox2b is virtually absent from the respiratory rhythm and pattern generator (VRC and dorsolateral pons) but is highly expressed by neurons involved in the chemical drive and reflex regulation of this oscillator.

Unilateral microdialysis of gabazine in the dorsal medulla reverses thermal prolongation of the laryngeal chemoreflex in decerebrate piglets

The laryngeal chemoreflex (LCR) is elicited by water in the larynx and leads to apnea and respiratory disruption in immature animals. The LCR is exaggerated by the elevation of brain temperature within or near the nucleus of the solitary tract (NTS) in decerebrate piglets. Thermal prolongation of reflex apnea elicited by superior laryngeal nerve stimulation is reduced by systemic administration of GABA(A) receptor antagonists. Therefore, we tested the hypothesis that microdialysis within or near the NTS of gabazine, a GABA(A) receptor antagonist, would reverse thermal prolongation of the LCR. We examined this hypothesis in 21 decerebrate piglets (age 3-13 days). We elicited the LCR by injecting 0.1 ml of water into the larynx before and after each piglet's body temperature was elevated by approximately 2.5 degrees C and before and after 2-5 mM gabazine was dialyzed unilaterally and focally in the medulla. Elevated body temperature failed to prolong the LCR in one piglet, which was excluded from analysis. Elevated body temperature prolonged the LCR in all the remaining animals, and dialysis of gabazine into the region near the NTS (n = 10) reversed the thermal prolongation of the LCR even though body temperature remained elevated. Dialysis of gabazine in other medullary sites (n = 10) did not reverse thermal prolongation of the LCR. Gabazine had no consistent effect on baseline respiratory activity during hyperthermia. These findings are consistent with the hypothesis that hyperthermia activates GABAergic mechanisms in or near the NTS that are necessary for the thermal prolongation of the LCR.