PreBötzinger complex and pacemaker neurons: hypothesized site and kernel for respiratory rhythm generation

A novel mechanism for ramping bursts based on slow negative feedback in model respiratory neurons

Recordings from pre-Bötzinger complex neurons responsible for the inspiratory phase of the respiratory rhythm reveal a ramping burst pattern, starting around the time that the transition from expiration to inspiration begins, in which the spike rate gradually rises until a transition into a high-frequency burst occurs. The spike rate increase along the burst is accompanied by a gradual depolarization of the plateau potential that underlies the spikes. These effects may be functionally important for inducing the onset of inspiration and hence maintaining effective respiration; however, most mathematical models for inspiratory bursting do not capture this activity pattern. Here, we study how the modulation of spike height and afterhyperpolarization via the slow inactivation of an inward current can support various activity patterns including ramping bursts. We use dynamical systems methods designed for multiple timescale systems, such as bifurcation analysis based on timescale decomposition and averaging over fast oscillations, to generate an understanding of and predictions about the specific dynamic effects that lead to ramping bursts. We also analyze how transitions between ramping and other activity patterns may occur with parameter changes, which could be associated with experimental manipulations, environmental conditions, and/or development.

Galaninergic and hypercapnia-activated neuronal projections to the ventral respiratory column

In mammals, the ventral respiratory column (VRC) plays a pivotal role in integrating neurochemically diverse inputs from brainstem and forebrain regions to generate respiratory motor patterns. VRC microinjection of the neuropeptide galanin has been reported to dampen carbon dioxide (CO2)-mediated chemoreflex responses. Additionally, we previously demonstrated that galaninergic neurons in the retrotrapezoid nucleus (RTN) are implicated in the adaptive response to hypercapnic stimuli, suggesting a link between RTN neuroplasticity and increased neuronal drive to the VRC. VRC neurons express galanin receptor 1, suggesting potential regulatory action by galanin, however, the precise galaninergic chemoreceptor-VRC circuitry remains to be determined. This study aimed to identify sources of galaninergic input to the VRC that contribute to central respiratory chemoreception. We employed a combination of retrograde neuronal tracing, in situ hybridisation and immunohistochemistry to investigate VRC-projecting neurons that synthesise galanin mRNA. In an additional series of experiments, we used acute hypercapnia exposure (10% CO2, 1 h) and c-Fos immunohistochemistry to ascertain which galaninergic nuclei projecting to the VRC are activated. Our findings reveal that a total of 30 brain nuclei and 51 subnuclei project to the VRC, with 12 of these containing galaninergic neurons, including the RTN. Among these galaninergic populations, only a subset of the RTN neurons (approximately 55%) exhibited activation in response to acute hypercapnia. Our findings highlight that the RTN is the likely source of galaninergic transmission to the VRC in response to hypercapnic stimuli.

Interdependence of cellular and network properties in respiratory rhythm generation

How breathing is generated by the preBötzinger complex (preBötC) remains divided between two ideological frameworks, and a persistent sodium current (INaP) lies at the heart of this debate. Although INaP is widely expressed, the pacemaker hypothesis considers it essential because it endows a small subset of neurons with intrinsic bursting or "pacemaker" activity. In contrast, burstlet theory considers INaP dispensable because rhythm emerges from "preinspiratory" spiking activity driven by feed-forward network interactions. Using computational modeling, we find that small changes in spike shape can dissociate INaP from intrinsic bursting. Consistent with many experimental benchmarks, conditional effects on spike shape during simulated changes in oxygenation, development, extracellular potassium, and temperature alter the prevalence of intrinsic bursting and preinspiratory spiking without altering the role of INaP. Our results support a unifying hypothesis where INaP and excitatory network interactions, but not intrinsic bursting or preinspiratory spiking, are critical interdependent features of preBötC rhythmogenesis.

Role of the basal ganglia in innate and learned behavioural sequences

Integrating individual actions into coherent, organised behavioural units, a process called chunking, is a fundamental, evolutionarily conserved process that renders actions automatic. In vertebrates, evidence points to the basal ganglia - a complex network believed to be involved in action selection - as a key component of action sequence encoding, although the underlying mechanisms are only just beginning to be understood. Central pattern generators control many innate automatic behavioural sequences that form some of the most basic behaviours in an animal's repertoire, and in vertebrates, brainstem and spinal pattern generators are under the control of higher order structures such as the basal ganglia. Evidence suggests that the basal ganglia play a crucial role in the concatenation of simpler behaviours into more complex chunks, in the context of innate behavioural sequences such as chain grooming in rats, as well as sequences in which innate capabilities and learning interact such as birdsong, and sequences that are learned from scratch, such as lever press sequences in operant behaviour. It has been proposed that the role of the striatum, the largest input structure of the basal ganglia, might lie in selecting and allowing the relevant central pattern generators to gain access to the motor system in the correct order, while inhibiting other behaviours. As behaviours become more complex and flexible, the pattern generators seem to become more dependent on descending signals. Indeed, during learning, the striatum itself may adopt the functional characteristics of a higher order pattern generator, facilitated at the microcircuit level by striatal neuropeptides.

Revisiting the two rhythm generators for respiration in lampreys

In lampreys, respiration consists of a fast and a slow rhythm. This study was aimed at characterizing both anatomically and physiologically the brainstem regions involved in generating the two rhythms. The fast rhythm generator has been located by us and others in the rostral hindbrain, rostro-lateral to the trigeminal motor nucleus. More recently, this was challenged by researchers reporting that the fast rhythm generator was located more rostrally and dorsomedially, in a region corresponding to the mesencephalic locomotor region. These contradictory observations made us re-examine the location of the fast rhythm generator using anatomical lesions and physiological recordings. We now confirm that the fast respiratory rhythm generator is in the rostro-lateral hindbrain as originally described. The slow rhythm generator has received less attention. Previous studies suggested that it was composed of bilateral, interconnected rhythm generating regions located in the caudal hindbrain, with ascending projections to the fast rhythm generator. We used anatomical and physiological approaches to locate neurons that could be part of this slow rhythm generator. Combinations of unilateral injections of anatomical tracers, one in the fast rhythm generator area and another in the lateral tegmentum of the caudal hindbrain, were performed to label candidate neurons on the non-injected side of the lateral tegmentum. We found a population of neurons extending from the facial to the caudal vagal motor nuclei, with no clear clustering in the cell distribution. We examined the effects of stimulating different portions of the labeled population on the respiratory activity. The rostro-caudal extent of the population was arbitrarily divided in three portions that were each stimulated electrically or chemically. Stimulation of either of the three sites triggered bursts of discharge characteristic of the slow rhythm, whereas inactivating any of them stopped the slow rhythm. Substance P injected locally in the lateral tegmentum accelerated the slow respiratory rhythm in a caudal hindbrain preparation. Our results show that the fast respiratory rhythm generator consists mostly of a population of neurons rostro-lateral to the trigeminal motor nucleus, whereas the slow rhythm generator is distributed in the lateral tegmentum of the caudal hindbrain.

Interdependence of cellular and network properties in respiratory rhythmogenesis

How breathing is generated by the preBötzinger Complex (preBötC) remains divided between two ideological frameworks, and the persistent sodium current (INaP) lies at the heart of this debate. Although INaP is widely expressed, the pacemaker hypothesis considers it essential because it endows a small subset of neurons with intrinsic bursting or "pacemaker" activity. In contrast, burstlet theory considers INaP dispensable because rhythm emerges from "pre-inspiratory" spiking activity driven by feed-forward network interactions. Using computational modeling, we discover that changes in spike shape can dissociate INaP from intrinsic bursting. Consistent with many experimental benchmarks, conditional effects on spike shape during simulated changes in oxygenation, development, extracellular potassium, and temperature alter the prevalence of intrinsic bursting and pre-inspiratory spiking without altering the role of INaP. Our results support a unifying hypothesis where INaP and excitatory network interactions, but not intrinsic bursting or pre-inspiratory spiking, are critical interdependent features of preBötC rhythmogenesis.

The history of Danish neuroscience

The history of Danish neuroscience starts with an account of impressive contributions made at the 17th century. Thomas Bartholin was the first Danish neuroscientist, and his disciple Nicolaus Steno became internationally one of the most prominent neuroscientists in this period. From the start, Danish neuroscience was linked to clinical disciplines. This continued in the 19th and first half of the 20th centuries with new initiatives linking basic neuroscience to clinical neurology and psychiatry in the same scientific environment. Subsequently, from the middle of the 20th century, basic neuroscience was developing rapidly within the preclinical university sector. Clinical neuroscience continued and was even reinforced during this period with important translational research and a close co-operation between basic and clinical neuroscience. To distinguish 'history' from 'present time' is not easy, as many historical events continue in present time. Therefore, we decided to consider 'History' as new major scientific developments in Denmark, which were launched before the end of the 20th century. With this aim, scientists mentioned will have been born, with a few exceptions, no later than the early 1960s. However, we often refer to more recent publications in documenting the developments of initiatives launched before the end of the last century. In addition, several scientists have moved to Denmark after the beginning of the present century, and they certainly are contributing to the present status of Danish neuroscience-but, again, this is not the History of Danish neuroscience.

Chronic maternal exposure to titanium dioxide nanoparticles alters breathing in newborn offspring

Background

Over the last two decades, nanotechnologies and the use of nanoparticles represent one of the greatest technological advances in many fields of human activity. Particles of titanium dioxide (TiO₂) are one of the nanomaterials most frequently found in everyday consumer products. But, due in particular to their extremely small size, TiO₂ nanoparticles (NPs) are prone to cross biological barriers and potentially lead to adverse health effects. The presence of TiO₂ NPs found in human placentae and in the infant meconium has indicated unequivocally the capacity for a materno-fetal transfer of this nanomaterial. Although chronic exposure to TiO₂ NPs during pregnancy is known to induce offspring cognitive deficits associated with neurotoxicity, the impact of a gestational exposure on a vital motor function such as respiration, whose functional emergence occurs during fetal development, remains unknown.

Results

Using in vivo whole-body plethysmographic recordings from neonatal mice, we show that a chronic exposure to TiO₂ NPs during pregnancy alters the respiratory activity of offspring, characterized by an abnormally elevated rate of breathing. Correspondingly, using ex vivo electrophysiological recordings performed on isolated brainstem-spinal cord preparations of newborn mice and medullary slice preparations containing specific nuclei controlling breathing frequency, we show that the spontaneously generated respiratory-related rhythm is significantly and abnormally accelerated in animals prenatally exposed to TiO₂ NPs. Moreover, such a chronic prenatal exposure was found to impair the capacity of respiratory neural circuitry to effectively adjust breathing rates in response to excitatory environmental stimuli such as an increase in ambient temperature.

Conclusions

Our findings thus demonstrate that a maternal exposure to TiO₂ NPs during pregnancy affects the normal development and operation of the respiratory centers in progeny.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12989-022-00497-4.

Predictions and experimental tests of a new biophysical model of the mammalian respiratory oscillator

Previously our computational modeling studies (Phillips et al., 2019) proposed that neuronal persistent sodium current (INaP) and calcium-activated non-selective cation current (ICAN) are key biophysical factors that, respectively, generate inspiratory rhythm and burst pattern in the mammalian preBötzinger complex (preBötC) respiratory oscillator isolated in vitro. Here, we experimentally tested and confirmed three predictions of the model from new simulations concerning the roles of INaP and ICAN: (1) INaP and ICAN blockade have opposite effects on the relationship between network excitability and preBötC rhythmic activity; (2) INaP is essential for preBötC rhythmogenesis; and (3) ICAN is essential for generating the amplitude of rhythmic output but not rhythm generation. These predictions were confirmed via optogenetic manipulations of preBötC network excitability during graded INaP or ICAN blockade by pharmacological manipulations in slices in vitro containing the rhythmically active preBötC from the medulla oblongata of neonatal mice. Our results support and advance the hypothesis that INaP and ICAN mechanistically underlie rhythm and inspiratory burst pattern generation, respectively, in the isolated preBötC.

Mechanisms of opioid-induced respiratory depression

Opioid-induced respiratory depression (OIRD), the primary cause of opioid-induced death, is the neural depression of respiratory drive which, together with a decreased level of consciousness and obstructive sleep apnea, cause ventilatory insufficiency. Variability of responses to opioids and individual differences in physiological and neurological states (e.g., anesthesia, sleep-disordered breathing, concurrent drug administration) add to the risk. Multiple sites can independently exert a depressive effect on breathing, making it unclear which sites are necessary for the induction of OIRD. The generator of inspiratory rhythm is the preBötzinger complex (preBötC) in the ventrolateral medulla. Other important brainstem respiratory centres include the pontine Kölliker-Fuse and adjacent parabrachial nuclei (KF/PBN) in the dorsal lateral pons, and the dorsal respiratory group in the medulla. Deletion of μ opioid receptors from neurons showed that the preBötC and KF/PBN contribute to OIRD with the KF as a respiratory modulator and the preBötC as inspiratory rhythm generator. Glutamatergic neurons expressing NK-1R and somatostatin involved in the autonomic function of breathing, and modulatory signal pathways involving GIRK and KCNQ potassium channels, remain poorly understood. Reversal of OIRD has relied heavily on naloxone which also reverses analgesia but mismatches between the half-lives of naloxone and opioids can make it difficult to clinically safely avoid OIRD. Maternal opioid use, which is rising, increases apneas and destabilizes neonatal breathing but opioid effects on maternal and neonatal respiratory circuits in neonatal abstinence syndrome (NAS) are not well understood. Methadone, administered to alleviate symptoms of NAS in humans, desensitizes rats to RD.

Putting the theory into 'burstlet theory' with a biophysical model of burstlets and bursts in the respiratory preBötzinger complex

Inspiratory breathing rhythms arise from synchronized neuronal activity in a bilaterally distributed brainstem structure known as the preBötzinger complex (preBötC). In in vitro slice preparations containing the preBötC, extracellular potassium must be elevated above physiological levels (to 7–9 mM) to observe regular rhythmic respiratory motor output in the hypoglossal nerve to which the preBötC projects. Reexamination of how extracellular K⁺ affects preBötC neuronal activity has revealed that low-amplitude oscillations persist at physiological levels. These oscillatory events are subthreshold from the standpoint of transmission to motor output and are dubbed burstlets. Burstlets arise from synchronized neural activity in a rhythmogenic neuronal subpopulation within the preBötC that in some instances may fail to recruit the larger network events, or bursts, required to generate motor output. The fraction of subthreshold preBötC oscillatory events (burstlet fraction) decreases sigmoidally with increasing extracellular potassium. These observations underlie the burstlet theory of respiratory rhythm generation. Experimental and computational studies have suggested that recruitment of the non-rhythmogenic component of the preBötC population requires intracellular Ca²⁺ dynamics and activation of a calcium-activated nonselective cationic current. In this computational study, we show how intracellular calcium dynamics driven by synaptically triggered Ca²⁺ influx as well as Ca²⁺ release/uptake by the endoplasmic reticulum in conjunction with a calcium-activated nonselective cationic current can reproduce and offer an explanation for many of the key properties associated with the burstlet theory of respiratory rhythm generation. Altogether, our modeling work provides a mechanistic basis that can unify a wide range of experimental findings on rhythm generation and motor output recruitment in the preBötC.

Respiratory rhythm and pattern generation: Brainstem cellular and circuit mechanisms

Breathing movements in mammals are driven by rhythmic neural activity automatically generated within spatially and functionally organized brainstem neural circuits comprising the respiratory central pattern generator (CPG). This chapter reviews up-to-date experimental information and theoretical studies of the cellular and circuit mechanisms of respiratory rhythm and pattern generation operating within critical components of this CPG in the lower brainstem. Over the past several decades, there have been substantial advances in delineating the spatial architecture of essential medullary regions and their regional cellular and circuit properties required to understand rhythm and pattern generation mechanisms. A fundamental concept is that the circuits in these regions have rhythm-generating capabilities at multiple cellular and circuit organization levels. The regional cellular properties, circuit organization, and control mechanisms allow flexible expression of neural activity patterns for a repertoire of respiratory behaviors under various physiologic conditions that are dictated by requirements for homeostatic regulation and behavioral integration. Many mechanistic insights have been provided by computational modeling studies driven by experimental results and have advanced understanding in the field. These conceptual and theoretical developments are discussed.

Lysophosphatidic Acid and Several Neurotransmitters Converge on Rho-Kinase 2 Signaling to Manage Motoneuron Excitability

Intrinsic membrane excitability (IME) sets up neuronal responsiveness to synaptic drive. Several neurotransmitters and neuromodulators, acting through G-protein-coupled receptors (GPCRs), fine-tune motoneuron (MN) IME by modulating background K⁺ channels TASK1. However, intracellular partners linking GPCRs to TASK1 modulation are not yet well-known. We hypothesized that isoform 2 of rho-kinase (ROCK2), acting as downstream GPCRs, mediates adjustment of MN IME via TASK1. Electrophysiological recordings were performed in hypoglossal MNs (HMNs) obtained from adult and neonatal rats, neonatal knockout mice for TASK1 (task1^–/–) and TASK3 (task3^–/–, the another highly expressed TASK subunit in MNs), and primary cultures of embryonic spinal cord MNs (SMNs). Small-interfering RNA (siRNA) technology was also used to knockdown either ROCK1 or ROCK2. Furthermore, ROCK activity assays were performed to evaluate the ability of various physiological GPCR ligands to stimulate ROCK. Microiontophoretically applied H1152, a ROCK inhibitor, and siRNA-induced ROCK2 knockdown both depressed AMPAergic, inspiratory-related discharge activity of adult HMNs in vivo, which mainly express the ROCK2 isoform. In brainstem slices, intracellular constitutively active ROCK2 (aROCK2) led to H1152-sensitive HMN hyper-excitability. The aROCK2 inhibited pH-sensitive and TASK1-mediated currents in SMNs. Conclusively, aROCK2 increased IME in task3^–/–, but not in task1^–/– HMNs. MN IME was also augmented by the physiological neuromodulator lysophosphatidic acid (LPA) through a mechanism entailing G_αi/o-protein stimulation, ROCK2, but not ROCK1, activity and TASK1 inhibition. Finally, two neurotransmitters, TRH, and 5-HT, which are both known to increase MN IME by TASK1 inhibition, stimulated ROCK2, and depressed background resting currents via G_αq/ROCK2 signaling. These outcomes suggest that LPA and several neurotransmitters impact MN IME via G_αi/o/G_αq-protein-coupled receptors, downstream ROCK2 activation, and subsequent inhibition of TASK1 channels.

Dynamic Rhythmogenic Network States Drive Differential Opioid Responses in the In Vitro Respiratory Network

Death from opioid overdose is typically caused by opioid-induced respiratory depression (OIRD). A particularly dangerous characteristic of OIRD is its apparent unpredictability. The respiratory consequences of opioids can be surprisingly inconsistent, even within the same individual. Despite significant clinical implications, most studies have focused on average dose-r esponses rather than individual variation, and there remains little insight into the etiology of this apparent unpredictability. The preBötzinger complex (preBötC) in the ventral medulla is an important site for generating the respiratory rhythm and OIRD. Here, using male and female C57-Bl6 mice in vitro, we demonstrate that the preBötC can assume different network states depending on the excitability of the preBötC and the intrinsic membrane properties of preBötC neurons. These network states predict the functional consequences of opioids in the preBötC, and depending on network state, respiratory rhythmogenesis can be either stabilized or suppressed by opioids. We hypothesize that the dynamic nature of preBötC rhythmogenic properties, required to endow breathing with remarkable flexibility, also plays a key role in the dangerous unpredictability of OIRD.SIGNIFICANCE STATEMENT Opioids can cause unpredictable, life-threatening suppression of breathing. This apparent unpredictability makes clinical management of opioids difficult while also making it challenging to define the underlying mechanisms of OIRD. Here, we find in brainstem slices that the preBötC, an opioid-sensitive subregion of the brainstem, has an optimal configuration of cellular and network properties that results in a maximally stable breathing rhythm. These properties are dynamic, and the state of each individual preBötC network relative to the optimal configuration of the network predicts how vulnerable rhythmogenesis is to the effects of opioids. These insights establish a framework for understanding how endogenous and exogenous modulation of the rhythmogenic state of the preBötC can increase or decrease the risk of OIRD.

Dynamics of ramping bursts in a respiratory neuron model

Intensive computational and theoretical work has led to the development of multiple mathematical models for bursting in respiratory neurons in the pre-Bötzinger Complex (pre-BötC) of the mammalian brainstem. Nonetheless, these previous models have not captured the pre-inspiratory ramping aspects of these neurons' activity patterns, in which relatively slow tonic spiking gradually progresses to faster spiking and a full-blown burst, with a corresponding gradual development of an underlying plateau potential. In this work, we show that the incorporation of the dynamics of the extracellular potassium ion concentration into an existing model for pre-BötC neuron bursting, along with some parameter adjustments, suffices to induce this ramping behavior. Using fast-slow decomposition, we show that this activity can be considered as a form of parabolic bursting, but with burst termination at a homoclinic bifurcation rather than as a SNIC bifurcation. We also investigate the parameter-dependence of these solutions and show that the proposed model yields a greater dynamic range of burst frequencies, durations, and duty cycles than those produced by other models in the literature.

Apnoea of Prematurity and Neurodevelopmental Outcomes: Current Understanding and Future Prospects for Research

Infants who are born prematurely are at significant risk of apnoea. In addition to the short-term consequences such as hypoxia, apnoea of prematurity has been associated with long-term morbidity, including poor neurodevelopmental outcomes. Clinical trials have illustrated the importance of methylxanthine drugs, in particular caffeine, in reducing the risk of long term adverse neurodevelopmental outcomes. However, the extent to which apnoea is causative of this secondary neurodevelopmental delay or is just associated in a background of other sequelae of prematurity remains unclear. In this review, we first discuss the pathophysiology of apnoea of prematurity, previous studies investigating the relationship between apnoea and neurodevelopmental delay, and treatment of apnoea with caffeine therapy. We propose a need for better methods of measuring apnoea, along with improved understanding of the neonatal brain's response to consequent hypoxia. Only then can we start to disentangle the effects of apnoea on neurodevelopment in preterm infants. Moreover, by better identifying those infants who are at risk of apnoea, and neurodevelopmental delay, we can work toward a risk stratification system for these infants that is clinically actionable, for example, with doses of caffeine tailored to the individual. Optimising treatment of apnoea for individual infants will improve neonatal care and long-term outcomes for this population.

The lamprey respiratory network: Some evolutionary aspects

Breathing is a complex behaviour that involves rhythm generating networks. In this review, we examine the main characteristics of respiratory rhythm generation in vertebrates and, in particular, we describe the main results of our studies on the role of neural mechanisms involved in the neuromodulation of the lamprey respiration. The lamprey respiratory rhythm generator is located in the paratrigeminal respiratory group (pTRG) and shows similarities with the mammalian preBötzinger complex. In fact, within the pTRG a major role is played by glutamate, but also GABA and glycine display important contributions. In addition, neuromodulatory influences are exerted by opioids, substance P, acetylcholine and serotonin. Both structures respond to exogenous ATP with a biphasic response and astrocytes there located strongly contribute to the modulation of the respiratory pattern. The results emphasize that some important characteristics of the respiratory rhythm generating network are, to a great extent, maintained throughout evolution.

Neural mechanisms underlying respiratory regulation within the preBötzinger complex of the rabbit

The preBötzinger complex (preBötC) is a medullary area essential for normal breathing and widely recognized as necessary and sufficient to generate the inspiratory phase of respiration. It has been studied mainly in rodents. Here we report the main results of our studies revealing the characteristics of the rabbit preBötC identified by means of neuronal recordings, D,L-homocysteic acid microinjections and histological controls. A crucial role in the respiratory rhythmogenesis within this neural substrate is played by excitatory amino acids, but also GABA and glycine display important contributions. Increases in respiratory frequency are induced by microinjections of neurokinins, somatostatin as well by serotonin (5-HT) through an action on 5-HT_1A and 5-HT₃ receptors or the disinhibition of a GABAergic circuit. Respiratory depression is observed in response to microinjections of the μ-opioid receptor agonist DAMGO. Our results show similarities and differences with the rodent preBötC and emphasize the importance of comparative studies on the mechanisms underlying respiratory rhythmogenesis in different animal species.

Respiratory dysfunction in a mouse model of spinocerebellar ataxia type 7

Spinocerebellar ataxia type 7 (SCA7) is an autosomal-dominant neurodegenerative disorder caused by a CAG repeat expansion in the coding region of the ataxin-7 gene. Infantile-onset SCA7 patients display extremely large repeat expansions (>200 CAGs) and exhibit progressive ataxia, dysarthria, dysphagia and retinal degeneration. Severe hypotonia, aspiration pneumonia and respiratory failure often contribute to death in affected infants. To better understand the features of respiratory and upper airway dysfunction in SCA7, we examined breathing and putative phrenic and hypoglossal neuropathology in a knock-in mouse model of early-onset SCA7 carrying an expanded allele with 266 CAG repeats. Whole-body plethysmography was used to measure awake spontaneously breathing SCA7-266Q knock-in mice at baseline in normoxia and during a hypercapnic/hypoxic respiratory challenge at 4 and 8 weeks, before and after the onset of disease. Postmortem studies included quantification of putative phrenic and hypoglossal motor neurons and microglia, and analysis of ataxin-7 aggregation at end stage. SCA7-266Q mice had profound breathing deficits during a respiratory challenge, exhibiting reduced respiratory output and a greater percentage of time in apnea. Histologically, putative phrenic and hypoglossal motor neurons of SCA7 mice exhibited a reduction in number accompanied by increased microglial activation, indicating neurodegeneration and neuroinflammation. Furthermore, intranuclear ataxin-7 accumulation was observed in cells neighboring putative phrenic and hypoglossal motor neurons in SCA7 mice. These findings reveal the importance of phrenic and hypoglossal motor neuron pathology associated with respiratory failure and upper airway dysfunction, which are observed in infantile-onset SCA7 patients and likely contribute to their early death.

Influence of Electric Current and Magnetic Flow on Firing Patterns of Pre-Bötzinger Complex Model

The dynamics of neuronal firing activity is vital for understanding the pathological respiratory rhythm. Studies on electrophysiology show that the magnetic flow is an essential factor that modulates the firing activities of neurons. By adding the magnetic flow to Butera's neuron model, we investigate how the electric current and magnetic flow influence neuronal activities under certain parametric restrictions. Using fast-slow decomposition and bifurcation analysis, we show that the variation of external electric current and magnetic flow leads to the change of the bistable structure of the system and hence results in the switch of neuronal firing pattern from one type to another.

The Mammalian Diving Response: Inroads to Its Neural Control

The mammalian diving response (DR) is a remarkable behavior that was first formally studied by Laurence Irving and Per Scholander in the late 1930s. The DR is called such because it is most prominent in marine mammals such as seals, whales, and dolphins, but nevertheless is found in all mammals studied. It consists generally of breathing cessation (apnea), a dramatic slowing of heart rate (bradycardia), and an increase in peripheral vasoconstriction. The DR is thought to conserve vital oxygen stores and thus maintain life by directing perfusion to the two organs most essential for life-the heart and the brain. The DR is important, not only for its dramatic power over autonomic function, but also because it alters normal homeostatic reflexes such as the baroreceptor reflex and respiratory chemoreceptor reflex. The neurons driving the reflex circuits for the DR are contained within the medulla and spinal cord since the response remains after the brainstem transection at the pontomedullary junction. Neuroanatomical and physiological data suggesting brainstem areas important for the apnea, bradycardia, and peripheral vasoconstriction induced by underwater submersion are reviewed. Defining the brainstem circuit for the DR may open broad avenues for understanding the mechanisms of suprabulbar control of autonomic function in general, as well as implicate its role in some clinical states. Knowledge of the proposed diving circuit should facilitate studies on elite human divers performing breath-holding dives as well as investigations on sudden infant death syndrome (SIDS), stroke, migraine headache, and arrhythmias. We have speculated that the DR is the most powerful autonomic reflex known.

Emergent Elements of Inspiratory Rhythmogenesis: Network Synchronization and Synchrony Propagation

We assessed the mechanism of mammalian breathing rhythmogenesis in the preBötzinger complex (preBötC) in vitro, where experimental tests remain inconsistent with hypotheses of canonical rhythmogenic cellular or synaptic mechanisms, i.e., pacemaker neurons or inhibition. Under rhythmic conditions, in each cycle, an inspiratory burst emerges as (presumptive) preBötC rhythmogenic neurons transition from aperiodic uncorrelated population spike activity to become increasingly synchronized during preinspiration (for ∼50-500 ms), which can trigger inspiratory bursts that propagate to motoneurons. In nonrhythmic conditions, antagonizing GABAA receptors can initiate this synchronization while inducing a higher conductance state in nonrhythmogenic preBötC output neurons. Our analyses uncover salient features of preBötC network dynamics where inspiratory bursts arise when and only when the preBötC rhythmogenic subpopulation strongly synchronizes to drive output neurons. Furthermore, downstream propagation of preBötC network activity, ultimately to motoneurons, is dependent on the strength of input synchrony onto preBötC output neurons exemplifying synchronous propagation of network activity.

Complex bursting dynamics in an embryonic respiratory neuron model

Pre-Bötzinger complex (pre-BötC) network activity within the mammalian brainstem controls the inspiratory phase of the respiratory rhythm. While bursting in pre-BötC neurons during the postnatal period has been extensively studied, less is known regarding inspiratory pacemaker neuron behavior at embryonic stages. Recent data in mouse embryo brainstem slices have revealed the existence of a variety of bursting activity patterns depending on distinct combinations of burst-generating I_NaP and I_CAN conductances. In this work, we consider a model of an isolated embryonic pre-BötC neuron featuring two distinct bursting mechanisms. We use methods of dynamical systems theory, such as phase plane analysis, fast-slow decomposition, and bifurcation analysis, to uncover mechanisms underlying several different types of intrinsic bursting dynamics observed experimentally including several forms of plateau bursts, bursts involving depolarization block, and various combinations of these patterns. Our analysis also yields predictions about how changes in the balance of the two bursting mechanisms contribute to alterations in an inspiratory pacemaker neuron activity during prenatal development.

Respiratory drive in the acute respiratory distress syndrome: pathophysiology, monitoring, and therapeutic interventions

Neural respiratory drive, i.e., the activity of respiratory centres controlling breathing, is an overlooked physiologic variable which affects the pathophysiology and the clinical outcome of acute respiratory distress syndrome (ARDS). Spontaneous breathing may offer multiple physiologic benefits in these patients, including decreased need for sedation, preserved diaphragm activity and improved cardiovascular function. However, excessive effort to breathe due to high respiratory drive may lead to patient self-inflicted lung injury (P-SILI), even in the absence of mechanical ventilation. In the present review, we focus on the physiological and clinical implications of control of respiratory drive in ARDS patients. We summarize the main determinants of neural respiratory drive and the mechanisms involved in its potentiation, in health and ARDS. We also describe potential and pitfalls of the available bedside methods for drive assessment and explore classical and more “futuristic” interventions to control drive in ARDS patients.

Muscarinic Modulation of Morphologically Identified Glycinergic Neurons in the Mouse PreBötzinger Complex

The cholinergic system plays an essential role in central respiratory control, but the underlying mechanisms remain elusive. We used whole-cell recordings in brainstem slices from juvenile mice expressing enhanced green fluorescent protein (EGFP) under the control of the glycine transporter type 2 (GlyT₂) promoter, to examine muscarinic modulation of morphologically identified glycinergic neurons in the preBötzinger complex (preBötC), an area critical for central inspiratory rhythm generation. Biocytin-filled reconstruction of glycinergic neurons revealed that the majority of them had few primary dendrites and had axons arborized within their own dendritic field. Few glycinergic neurons had axon collaterals extended towards the premotor/motor areas or ran towards the contralateral preBötC, and had more primary dendrites and more compact dendritic trees. Spontaneously active glycinergic neurons fired regular spikes, or less frequently in a "burst-like" pattern at physiological potassium concentration. Muscarine suppressed firing in the majority of regular spiking neurons via M₂ receptor activation while enhancing the remaining neurons through M₁ receptors. Interestingly, rhythmic bursting was augmented by muscarine in a small group of glycinergic neurons. In contrast to its heterogeneous modulation of glycinergic neuronal excitability, muscarine generally depressed inhibitory and excitatory synaptic inputs onto both glycinergic and non-glycinergic preBötC neurons, with a stronger effect on inhibitory input. Notably, presynaptic muscarinic attenuation of excitatory synaptic input was dependent on M₁ receptors in glycinergic neurons and on M₂ receptors in non-glycinergic neurons. Additional field potential recordings of excitatory synaptic potentials in the M₂ receptor knockout mice indicate that glycinergic and non-glycinergic neurons contribute equally to the general suppression by muscarine of excitatory activity in preBötC circuits. In conclusion, our data show that preBötC glycinergic neurons are morphologically heterogeneous, and differ in the properties of synaptic transmission and muscarinic modulation in comparison to non-glycinergic neurons. The dominant and cell-type-specific muscarinic inhibition of synaptic neurotransmission and spiking may contribute to central respiratory disturbances in high cholinergic states.

Multiple Rhythm-Generating Circuits Act in Tandem with Pacemaker Properties to Control the Start and Speed of Locomotion

In vertebrates, specific command centers in the brain can selectively drive slow-explorative or fast-speed locomotion. However, it remains unclear how the locomotor central pattern generator (CPG) processes descending drive into coordinated locomotion. Here, we reveal, in adult zebrafish, a logic of the V2a interneuron rhythm-generating circuits involving recurrent and hierarchical connectivity that acts in tandem with pacemaker properties to provide an ignition and gear-shift mechanism to start locomotion and change speed. A comprehensive mapping of synaptic connections reveals three recurrent circuit modules engaged sequentially to increase locomotor speed. The connectivity between V2a interneurons of different modules displayed a clear asymmetry in favor of connections from faster to slower modules. The interplay between V2a interneuron pacemaker properties and their organized connectivity provides a mechanism for locomotor initiation and speed control. Thus, our results provide mechanistic insights into how the spinal CPG transforms descending drive into locomotion and align its speed with the initial intention.

Evaluating the Burstlet Theory of Inspiratory Rhythm and Pattern Generation

The preBötzinger complex (preBötC) generates the rhythm and rudimentary motor pattern for inspiratory breathing movements. Here, we test “burstlet” theory (Kam et al., 2013a), which posits that low amplitude burstlets, subthreshold from the standpoint of inspiratory bursts, reflect the fundamental oscillator of the preBötC. In turn, a discrete suprathreshold process transforms burstlets into full amplitude inspiratory bursts that drive motor output, measurable via hypoglossal nerve (XII) discharge in vitro. We recap observations by Kam and Feldman in neonatal mouse slice preparations: field recordings from preBötC demonstrate bursts and concurrent XII motor output intermingled with lower amplitude burstlets that do not produce XII motor output. Manipulations of excitability affect the relative prevalence of bursts and burstlets and modulate their frequency. Whole-cell and photonic recordings of preBötC neurons suggest that burstlets involve inconstant subsets of rhythmogenic interneurons. We conclude that discrete rhythm- and pattern-generating mechanisms coexist in the preBötC and that burstlets reflect its fundamental rhythmogenic nature.

Time and dose-dependent impairment of neonatal respiratory motor activity after systemic inflammation

Neonatal respiratory impairment during infection is common, yet its effects on respiratory neural circuitry are not fully understood. We hypothesized that the timing and severity of systemic inflammation is positively correlated with impairment in neonatal respiratory activity. To test this, we evaluated time- and dose-dependent impairment of in vitro fictive respiratory activity. Systemic inflammation (induced by lipopolysaccharide, LPS, 5 mg/kg, i.p.) impaired burst amplitude during the early (1 h) inflammatory response. The greatest impairment in respiratory activity (decreased amplitude, frequency, and increased rhythm disturbances) occurred during the peak (3 h) inflammatory response in brainstem-spinal cord preparations. Surprisingly, isolated medullary respiratory circuitry within rhythmic slices showed decreased baseline frequency and delayed onset of rhythm only after higher systemic inflammation (LPS 10 mg/kg) early in the inflammatory response (1 h), with no impairments at the peak inflammatory response (3 h). Thus, different components of neonatal respiratory circuitry have differential temporal and dose sensitivities to systemic inflammation, creating multiple windows of vulnerability for neonates after systemic inflammation.

Averting Opioid-induced Respiratory Depression without Affecting Analgesia

The ventilatory control system is highly vulnerable to exogenous administered opioid analgesics. Particularly respiratory depression is a potentially lethal complication that may occur when opioids are overdosed or consumed in combination with other depressants such as sleep medication or alcohol. Fatalities occur in acute and chronic pain patients on opioid therapy and individuals that abuse prescription or illicit opioids for their hedonistic pleasure. One important strategy to mitigate opioid-induced respiratory depression is cotreatment with nonopioid respiratory stimulants. Effective stimulants prevent respiratory depression without affecting the analgesic opioid response. Several pharmaceutical classes of nonopioid respiratory stimulants are currently under investigation. The majority acts at sites within the brainstem respiratory network including drugs that act at α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (ampakines), 5-hydroxytryptamine receptor agonists, phospodiesterase-4 inhibitors, D1-dopamine receptor agonists, the endogenous peptide glycyl-glutamine, and thyrotropin-releasing hormone. Others act peripherally at potassium channels expressed on oxygen-sensing cells of the carotid bodies, such as doxapram and GAL021 (Galleon Pharmaceuticals Corp., USA). In this review we critically appraise the efficacy of these agents. We conclude that none of the experimental drugs are adequate for therapeutic use in opioid-induced respiratory depression and all need further study of efficacy and toxicity. All discussed drugs, however, do highlight potential mechanisms of action and possible templates for further study and development.

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.

The rhythm section: An update on spinal interneurons setting the beat for mammalian locomotion

To initiate and support locomotion, rhythm generating neurons in the spinal central pattern generator convert descending input into a rhythmic signal which is conveyed to downstream neurons, leading to the recruitment of motor neurons and activation of muscles. Although two genetically-defined neuronal populations have been linked to rhythm generation, a single all-inclusive rhythm generating population has yet to be identified. Here, we consolidate recent work aimed at identifying rhythm generating neurons, summarize the evidence for the involvement of two neuronal populations in rhythm generation, describe the challenges in identifying a marker for rhythm generating neurons, and discuss potential directions to take in integrating spinal rhythm generating neurons into recently identified speed-dependent locomotor circuits.

Biophysical mechanisms in the mammalian respiratory oscillator re-examined with a new data-driven computational model

An autorhythmic population of excitatory neurons in the brainstem pre-Bötzinger complex is a critical component of the mammalian respiratory oscillator. Two intrinsic neuronal biophysical mechanisms—a persistent sodium current (INaP) and a calcium-activated non-selective cationic current (ICAN)—were proposed to individually or in combination generate cellular- and circuit-level oscillations, but their roles are debated without resolution. We re-examined these roles in a model of a synaptically connected population of excitatory neurons with ICAN and INaP. This model robustly reproduces experimental data showing that rhythm generation can be independent of ICAN activation, which determines population activity amplitude. This occurs when ICAN is primarily activated by neuronal calcium fluxes driven by synaptic mechanisms. Rhythm depends critically on INaP in a subpopulation forming the rhythmogenic kernel. The model explains how the rhythm and amplitude of respiratory oscillations involve distinct biophysical mechanisms.

Lethal avian influenza A (H5N1) virus replicates in pontomedullary chemosensitive neurons and depresses hypercapnic ventilatory response in mice

The highly pathogenic H5N1 (HK483) viral infection causes a depressed hypercapnic ventilatory response (dHCVR, 20%↓) at 2 days postinfection (dpi) and death at 7 dpi in mice, but the relevant mechanisms are not fully understood. Glomus cells in the carotid body and catecholaminergic neurons in locus coeruleus (LC), neurokinin 1 receptor (NK1R)-expressing neurons in the retrotrapezoid nucleus (RTN), and serotonergic neurons in the raphe are chemosensitive and responsible for HCVR. We asked whether the dHCVR became worse over the infection period with viral replication in these cells/neurons. Mice intranasally inoculated with saline or the HK483 virus were exposed to hypercapnia for 5 min at 0, 2, 4, or 6 dpi, followed by immunohistochemistry to determine the expression of nucleoprotein of H5N1 influenza A (NP) alone and coupled with 1) tyrosine hydroxylase (TH) in the carotid body and LC, 2) NK1R in the RTN, and 3) tryptophan hydroxylase (TPH) in the raphe. HK483 viral infection blunted HCVR by ∼20, 50, and 65% at 2, 4, and 6 dpi. The NP was observed in the pontomedullary respiratory-related nuclei (but not in the carotid body) at 4 and 6 dpi, especially in 20% of RTN NK1R, 35% of LC TH, and ∼10% raphe TPH neurons. The infection significantly reduced the local NK1R or TPH immunoreactivity and population of neurons expressing NK1R or TPH. We conclude that the HK483 virus infects the pontomedullary respiratory nuclei, particularly chemosensitive neurons in the RTN, LC, and raphe, contributing to the severe depression of HCVR and respiratory failure at 6 dpi. NEW & NOTEWORTHY The H5N1 virus infection is lethal due to respiratory failure, but the relevant mechanisms remain unclear. In this study, we demonstrated a gradual diminution of hypercapnic ventilatory response to a degree, leading to respiratory failure over a 6-day infection. Death was associated with viral replication in the pontomedullary respiratory-related nuclei, especially the central chemosensitive neurons. These results not only provide insight into the mechanisms of the lethality of H5N1 viral infection but also offer clues in the development of corresponding treatments to minimize and prevent respiratory failure.

Effects of neonatal hyperoxia on the critical period of postnatal development of neurochemical expressions in brain stem respiratory-related nuclei in the rat

We have identified a critical period of respiratory development in rats at postnatal days P12‐13, when inhibitory influence dominates and when the response to hypoxia is at its weakest. This critical period has significant implications for Sudden Infant Death Syndrome (SIDS), the cause of which remains elusive. One of the known risk factors for SIDS is prematurity. A common intervention used in premature infants is hyperoxic therapy, which, if prolonged, can alter the ventilatory response to hypoxia and induce sustained inhibition of lung alveolar growth and pulmonary remodeling. The goal of this study was to test our hypothesis that neonatal hyperoxia from postnatal day (P) 0 to P10 in rat pups perturbs the critical period by altering the normal progression of neurochemical development in brain stem respiratory‐related nuclei. An in‐depth, semiquantitative immunohistochemical study was undertaken at P10 (immediately after hyperoxia and before the critical period), P12 (during the critical period), P14 (immediately after the critical period), and P17 (a week after the cessation of hyperoxia). In agreement with our previous findings, levels of cytochrome oxidase, brain‐derived neurotrophic factor (BDNF), TrkB (BDNF receptor), and several serotonergic proteins (5‐HT_1A and _2A receptors, 5‐HT synthesizing enzyme tryptophan hydroxylase [TPH], and serotonin transporter [SERT]) all fell in several brain stem respiratory‐related nuclei during the critical period (P12) in control animals. However, in hyperoxic animals, these neurochemicals exhibited a significant fall at P14 instead. Thus, neonatal hyperoxia delayed but did not eliminate the critical period of postnatal development in multiple brain stem respiratory‐related nuclei, with little effect on the nonrespiratory cuneate nucleus.

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.

Clinical neurophysiology of apnea

Understanding the clinical neurophysiology of apnea generation encompasses discussion of the neuroanatomic aspects of central respiratory rhythm and pattern generation, including the central respiratory control networks, central and peripheral chemoreceptors, mechanisms of respiratory muscles, and sleep state dependent differences. Anatomical and functional links to apnea also involve central respiratory motor output recruited from the hypoglossal nerve, which has led to novel treatments for obstructive sleep apnea. Autonomic fluctuations occur in relation to sleep-wake and sleep states (i.e., REM vs NREM sleep), with both parasympathetic and sympathetic contributions. Finally, our understanding of the pathophysiology of obstructive sleep apnea now includes concepts of critical closing pressure of the upper airway, increased loop gain as reflected by high responsiveness to external perturbations, inadequate responsiveness of upper airway muscle recruitment, and reductions in arousal threshold leading to ventilatory instability. In turn, these concepts have led to the development of novel therapies such as hypoglossal nerve stimulation and targeting key culprit physiologic mechanisms specific to the individual.

Mutation in LBX1/Lbx1 precludes transcription factor cooperativity and causes congenital hypoventilation in humans and mice

Maintaining low CO₂ levels in our bodies is critical for life and depends on neurons that generate the respiratory rhythm and monitor tissue gas levels. Inadequate response to increasing levels of CO₂ is common in congenital hypoventilation diseases. Here, we identified a mutation in LBX1, a homeodomain transcription factor, that causes congenital hypoventilation in humans. The mutation alters the C terminus of the protein without disturbing its DNA-binding domain. Mouse models carrying an analogous mutation recapitulate the disease. The mutation spares most Lbx1 functions, but selectively affects development of a small group of neurons central in respiration. Our work reveals a very unusual pathomechanism, a mutation that hampers a small subset of functions carried out by a transcription factor.

The respiratory rhythm is generated by the preBötzinger complex in the medulla oblongata, and is modulated by neurons in the retrotrapezoid nucleus (RTN), which are essential for accelerating respiration in response to high CO₂. Here we identify a LBX1 frameshift (LBX1^FS) mutation in patients with congenital central hypoventilation. The mutation alters the C-terminal but not the DNA-binding domain of LBX1. Mice with the analogous mutation recapitulate the breathing deficits found in humans. Furthermore, the mutation only interferes with a small subset of Lbx1 functions, and in particular with development of RTN neurons that coexpress Lbx1 and Phox2b. Genome-wide analyses in a cell culture model show that Lbx1^FS and wild-type Lbx1 proteins are mostly bound to similar sites, but that Lbx1^FS is unable to cooperate with Phox2b. Thus, our analyses on Lbx1^FS (dys)function reveals an unusual pathomechanism; that is, a mutation that selectively interferes with the ability of Lbx1 to cooperate with Phox2b, and thus impairs the development of a small subpopulation of neurons essential for respiratory control.

Probing the function of glycinergic neurons in the mouse respiratory network using optogenetics

Glycine is a primary inhibitory transmitter in the ventral medullary respiratory network, but the functional role of glycinergic neurons for breathing remains a matter of debate. We applied optogenetics to selectively modulate glycinergic neuron activity within regions of the rostral ventral respiratory column (VRC). Responses of the phrenic nerve activity to the light-driven stimulation were studied in the working heart-brainstem preparation from adult glycine transporter 2 Cre mice (GlyT2-Cre), which received a unilateral injection of a Cre-dependent AAV virus into Bötzinger and preBötzinger Complex. Sustained light stimulation from the ventral medullary surface resulted in a substantial depression of the phrenic nerve (PN) frequency, which in most cases was compensated by an increase in PN amplitude. Periodic, burst stimulation with variable intervals could alter and reset respiratory rhythm. We conclude that unilateral activation of the rostral VRC glycinergic neurons can significantly affect respiratory pattern by lengthening the expiratory interval and modulating phase transition.

Variability of the medullary arcuate nucleus in humans

INTRODUCTION

The arcuate nucleus is a component of the ventral medullary surface involved in chemoreception and breathing control. The hypoplasia of this nucleus is a very frequent finding in victims of sudden unexplained fetal and infant death (from the last weeks of pregnancy to the first year of life). On the contrary, this developmental alteration is rarely present in age-matched controls who died of defined causes. These observations lead to hypothesize that a well-developed and functional arcuate nucleus is generally required to sustain life. The aim of this study was to investigate whether the arcuate nucleus maintains the same supposed function throughout life.

METHODS

We carried out neuropathological examinations of brainstems obtained from 25 adult subjects, 18 males and 7 females, aged between 34 and 89 years, who died from various causes.

RESULTS

For almost half of the cases (44%) microscopic examinations of serial histological sections of medulla oblongata showed a normal cytoarchitecture of the arcuate nucleus, extending along the pyramids. For the remaining 56% of cases, various degrees of hypodevelopment of this nucleus were observed, validated through the application of quantitative morphometric investigations, from decreased area, neuron number and volume, to full aplasia.

CONCLUSIONS

These unexpected findings indicate that the involvement of the arcuate nucleus in chemoreception in adulthood is questionable, given the possibility of living until late age without this nucleus. This opens new perspectives for researchers on the role and function of the arcuate nucleus in humans from birth to old age.

Maturation of Breathing-Related Inhibitory Neurotransmission in the Medulla Oblongata of the Embryonic and Perinatal Zebra Finch (Taeniopygia guttata)

The medullary portion of the embryonic zebra finch hindbrain was isolated and superfused with physiologically relevant artificial cerebral spinal fluid. This in vitro preparation produced uninterrupted rhythmic episodes of neural activity via cranial nerve IX (glossopharyngeal) from embryonic day 4 (E4) through hatching on E14. Cranial nerve IX carries motor activity to the glottis during the inspiratory phase of breathing, and we focused on the role of synaptic inhibition during the embryonic and perinatal maturation of this branchiomotor outflow. We show that spontaneous neural activity (SNA) is first observed on E4 and temporally transforms as the embryo ages. To start, SNA is dependent on the excitatory actions of GABAA and glycine. As the embryo continues to develop, GABAergic and glycinergic neurotransmission take on a modulatory role, albeit an excitatory one, through E10. After that, data show that GABAergic and glycinergic neurotransmission switches to a phenotype consistent with inhibition, coincident with the onset of functional breathing. We also report that the inhibitory action of GABAergic and glycinergic receptor gating is not necessary for the spontaneous generation of branchiomotor motor rhythms in these birds near hatching. This is the first report focusing on the development of central breathing-related inhibitory neurotransmission in birds during the entire period of embryogenesis.

Pituitary adenylate cyclase-activating polypeptide: Postnatal development in multiple brain stem respiratory-related nuclei in the rat

The pituitary adenylate cyclase-activating polypeptide (PACAP) plays an important role in anterior pituitary hormone secretion, neurotransmission, and the control of breathing. Mice lacking PACAP die suddenly mainly in the 2^nd postnatal week, coinciding temporally with a critical period of respiratory development uncovered by our laboratory in the rat. The goal of the current study was to test our hypothesis that PACAP expression is reduced during the critical period in normal rats. We undertook immunohistochemistry and optical densitometry of PACAP (specifically PACAP38) in several brain stem respiratory-related nuclei of postnatal days P2-21 rats, and found that PACAP immunoreactivity was significantly reduced at P12 in the pre-Bötzinger complex, nucleus ambiguus, hypoglossal nucleus, and the ventrolateral subnucleus of the nucleus tractus solitarius. No changes were observed in the control, non-respiratory cuneate nucleus at P12. Results imply that the down-regulation of PACAP during normal postnatal development may contribute to the critical period of vulnerability, when the animals' response to hypoxia is at its weakest.

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.

Acute exposure to zinc oxide nanoparticles critically disrupts operation of the respiratory neural network in neonatal rat

Due to their extremely small size that gives them unique physicochemical properties, nanoparticles (NPs) are used in the production of everyday materials. However, NPs can accumulate in body organs and could cause various diseases. Moreover, NPs that cross biological membranes such as the blood-brain barrier can aggregate in the brain and potentially produce neuronal damage. Although studies have reported the effects of diverse NPs on the bioelectrical properties of individual neurons, their potential influences on the operation of whole neuronal networks have not been documented. Here, we aimed to evaluate the effects of an acute exposure to zinc oxide (ZnO) NPs on the central neural networks responsible for mammalian respiratory rhythm generation. Using an isolated ex vivo brainstem-spinal cord preparation from neonatal rat in which the circuitry for the central respiratory command remained intact, we show that ZnO NPs accelerate, then profoundly disrupt respiratory-related activity produced by the pre-Bötzinger complex (preBötC) responsible for inspiratory rhythm generation. Consequently, a sudden and definitive cessation of respiratory-related activity occurs in ZnO NPs-exposed preparations. Part of these effects is related to zinc ions released from NPs. Using brainstem slice preparations containing the preBötC network, whole-cell patch-clamp recordings revealed that ZnO NPs depolarize preBötC inspiratory neurons and affect their bioelectrical properties by reducing the amplitude of action potentials, thereby leading to a depression of intra-network activity and the ultimate termination of respiratory rhythmogenesis. These findings support the conclusion that ZnO NPs may have deleterious effects on the central respiratory centers of newborn mammals.

Progress in understanding mechanisms of opioid-induced gastrointestinal adverse effects and respiratory depression

Opioids evoke analgesia through activation of opioid receptors (predominantly the μ opioid receptor) in the central nervous system. Opioid receptors are abundant in multiple regions of the central nervous system and the peripheral nervous system including enteric neurons. Opioid-related adverse effects such as constipation, nausea, and vomiting pose challenges for compliance and continuation of the therapy for chronic pain management. In the post-operative setting opioid-induced depression of respiration can be fatal. These critical limitations warrant a better understanding of their underpinning cellular and molecular mechanisms to inform the design of novel opioid analgesic molecules that are devoid of these unwanted side-effects. Research efforts on opioid receptor signalling in the past decade suggest that differential signalling pathways and downstream molecules preferentially mediate distinct pharmacological effects. Additionally, interaction among opioid receptors and, between opioid receptor and non-opioid receptors to form signalling complexes shows that opioid-induced receptor signalling is potentially more complicated than previously thought. This complexity provides an opportunity to identify and probe relationships between selective signalling pathway specificity and in vivo production of opioid-related adverse effects. In this review, we focus on current knowledge of the mechanisms thought to transduce opioid-induced gastrointestinal adverse effects (constipation, nausea, vomiting) and respiratory depression.

Neuropeptides and breathing in health and disease

Regulatory neuropeptides control and regulate breathing in physiological and pathophysiological conditions. While they have been identified in the neurons of major respiratory areas, they can be active not only at the central level, but also at the periphery via chemoreceptors, vagal afferents, or locally within lungs and airways. Some neuropeptides, such as leptin or substance P, are respiratory stimulants; others, such as neurotensin, produce variable effects on respiration depending on the site of application. Some neuropeptides have been implicated in pathological states, such as obstructive sleep apnea or asthma. This article provides a concise review of the possible role and functions of several selected neuropeptides in the process of breathing in health and disease and in lung pathologies.

Lethal avian influenza A (H5N1) virus induces ataxic breathing in mice with apoptosis of pre-Botzinger complex neurons expressing neurokinin-1 receptor

Lethal influenza A (H5N1) induces respiratory failure in humans. Although it also causes death at 7 days postinfection (dpi) in mice, the development of the respiratory failure and the viral impact on pre-Botzinger complex (PBC) neurons expressing neurokinin 1 receptor (NK₁R), which is the respiratory rhythm generator, have not been explored. Body temperature, weight, ventilation, and arterial blood pH and gases were measured at 0, 2, 4, and 6 dpi in control, lethal HK483, and nonlethal HK486 viral-infected mice. Immunoreactivities (IR) of PBC NK₁R, H5N1 viral nucleoprotein (NP), and active caspase-3 (CASP3; a marker for apoptosis) were detected at 6 dpi. HK483, but not HK486, mice showed the following abnormalities: 1) gradual body weight loss and hypothermia; 2) tachypnea at 2-4 dpi and ataxic breathing with long-lasting apneas and hypercapnic hypoxemia at 6 dpi; and 3) viral replication in PBC NK₁R neurons with NK₁R-IR reduced by 75% and CASP3-IR colabeled at 6 dpi. Lethal H5N1 viral infection causes tachypnea at the early stage and ataxic breathing and apneas (hypercapnic hypoxemia) leading to death at the late stage. Its replication in the PBC induces apoptosis of local NK₁R neurons, contributing to ataxic breathing and respiratory failure.

Computational study on neuronal activities arising in the pre-Bötzinger complex

Experimental investigations have shown that the pre-Bötzinger complex (pre-BötC) within the mammalian brainstem generates the inspiratory phase of respiratory rhythm. Based on a single-compartment model of a pre-BötC inspiratory neuron, we, in this paper, use semi-analytical, numerical as well as fast-slow dynamical methods to investigate the effects of sodium conductance ([Formula: see text]) and potassium conductance ([Formula: see text]) on the firing activities of pre-BötC and try to reveal the dynamical mechanisms behind them. We show how [Formula: see text] and [Formula: see text] affect the bifurcations of the fast-subsystem and how the the firing patterns of pre-BötC transit according to the bifurcations.