Looking for inspiration: new perspectives on respiratory rhythm

Chronic Opioid Use and Central Sleep Apnea, Where Are We Now and Where To Go? A State of the Art Review

Opioids are commonly used for pain management, perioperative procedures, and addiction treatment. There is a current opioid epidemic in North America that is paralleled by a marked increase in related deaths. Since 2000, chronic opioid users have been recognized to have significant central sleep apnea (CSA). After heart failure-related Cheyne-Stokes breathing (CSB), opioid-induced CSA is now the second most commonly seen CSA. It occurs in around 24% of chronic opioid users, typically after opioids have been used for more than 2 months, and usually corresponds in magnitude to opioid dose/plasma concentration. Opioid-induced CSA events often mix with episodes of ataxic breathing. The pathophysiology of opioid-induced CSA is based on dysfunction in respiratory rhythm generation and ventilatory chemoreflexes. Opioids have a paradoxical effect on different brain regions, which result in irregular respiratory rhythm. Regarding ventilatory chemoreflexes, chronic opioid use induces hypoxia that appears to stimulate an augmented hypoxic ventilatory response (high loop gain) and cause a narrow CO2 reserve, a combination that promotes respiratory instability. To date, no direct evidence has shown any major clinical consequence from CSA in chronic opioid users. A line of evidence suggested increased morbidity and mortality in overall chronic opioid users. CSA in chronic opioid users is likely to be a compensatory mechanism to avoid opioid injury and is potentially beneficial. The current treatments of CSA in chronic opioid users mainly focus on continuous positive airway pressure (CPAP) and adaptive servo-ventilation (ASV) or adding oxygen. ASV is more effective in reducing CSA events than CPAP. However, a recent ASV trial suggested an increased all-cause and cardiovascular mortality with the removal of CSA/CSB in cardiac failure patients. A major reason could be counteracting of a compensatory mechanism. No similar trial has been conducted for chronic opioid-related CSA. Future studies should focus on (1) investigating the phenotypes and genotypes of opioid-induced CSA that may have different clinical outcomes; (2) determining if CSA in chronic opioid users is beneficial or detrimental; and (3) assessing clinical consequences on different treatment options on opioid-induced CSA.

Breathing-driven prefrontal oscillations regulate maintenance of conditioned-fear evoked freezing independently of initiation

Brain-body interactions are thought to be essential in emotions but their physiological basis remains poorly understood. In mice, regular 4 Hz breathing appears during freezing after cue-fear conditioning. Here we show that the olfactory bulb (OB) transmits this rhythm to the dorsomedial prefrontal cortex (dmPFC) where it organizes neural activity. Reduction of the respiratory-related 4 Hz oscillation, via bulbectomy or optogenetic perturbation of the OB, reduces freezing. Behavioural modelling shows that this is due to a specific reduction in freezing maintenance without impacting its initiation, thus dissociating these two phenomena. dmPFC LFP and firing patterns support the region's specific function in freezing maintenance. In particular, population analysis reveals that network activity tracks 4 Hz power dynamics during freezing and reaches a stable state at 4 Hz peak that lasts until freezing termination. These results provide a potential mechanism and a functional role for bodily feedback in emotions and therefore shed light on the historical James-Cannon debate.

Reciprocal connectivity of the periaqueductal gray with the ponto-medullary respiratory network in rat

Synaptic activities of the periaqueductal gray (PAG) can modulate or appropriate the respiratory motor activities in the context of behavior and emotion via descending projections to nucleus retroambiguus. However, alternative anatomical pathways for the mediation of PAG-evoked respiratory modulation via core nuclei of the brainstem respiratory network remains only partially described. We injected the retrograde tracer Cholera toxin subunit B (CT-B) in the pontine Kölliker-Fuse nucleus (KFn, n = 5), medullary Bötzinger (BötC, n = 3) and pre-Bötzinger complexes (pre-BötC; n = 3), and the caudal raphé nuclei (n = 3), and quantified the descending connectivity of the PAG targeting these brainstem respiratory regions. CT-B injections in the KFn, pre-BötC, and caudal raphé, but not in the BötC, resulted in CT-B-labeled neurons that were predominantly located in the lateral and ventrolateral PAG columns. In turn, CT-B injections in the lateral and ventrolateral PAG columns (n = 4) produced the highest numbers of CT-B-labeled neurons in the KFn and far fewer numbers of labeled neurons in the pre-BötC, BötC, and caudal raphé. Analysis of the relative projection strength revealed that the KFn shares the densest reciprocal connectivity with the PAG (ventrolateral and lateral columns, in particular). Overall, our data imply that the PAG may engage a distributed respiratory rhythm and pattern generating network beyond the nucleus retroambiguus to mediate downstream modulation of breathing. However, the reciprocal connectivity of the KFn and PAG suggests specific roles for synaptic interaction between these two nuclei that are most likely related to the regulation of upper airway patency during vocalization or other volitional orofacial behaviors.

KCNQ Current Contributes to Inspiratory Burst Termination in the Pre-Bötzinger Complex of Neonatal Rats in vitro

The pre-Bötzinger complex (preBötC) of the ventral medulla generates the mammalian inspiratory breathing rhythm. When isolated in explants and deprived of synaptic inhibition, the preBötC continues to generate inspiratory-related rhythm. Mechanisms underlying burst generation have been investigated for decades, but cellular and synaptic mechanisms responsible for burst termination have received less attention. KCNQ-mediated K+ currents contribute to burst termination in other systems, and their transcripts are expressed in preBötC neurons. Therefore, we tested the hypothesis that KCNQ channels also contribute to burst termination in the preBötC. We recorded KCNQ-like currents in preBötC inspiratory neurons in neonatal rat slices that retain respiratory rhythmicity. Blocking KCNQ channels with XE991 or linopirdine (applied via superfusion or locally) increased inspiratory burst duration by 2- to 3-fold. By contrast, activation of KCNQ with retigabine decreased inspiratory burst duration by ~35%. These data from reduced preparations suggest that the KCNQ current in preBötC neurons contributes to inspiratory burst termination.

Excitatory and inhibitory modulation of parafacial respiratory neurons in the control of active expiration

In order to increase ventilation, the respiratory system engages active expiration through recruitment of abdominal muscles. Here, we reviewed the new advances in the modulation of parafacial respiratory (pF) region to trigger active expiration. In addition, we also made a comprehensive discussion of experiments indicating that the lateral aspect of the pF (pFL) is anatomically and functionally distinct from the adjacent and partially overlapping chemosensitive neurons of the ventral aspect of the pF (pFV) also named the retrotrapezoid nucleus. Recent evidence suggest a complex network responsible for the generation of active expiration and neuromodulatory systems that influence its activity. The activity of the pFL is tonically inhibited by inhibitory inputs and also receives excitatory inputs from chemoreceptors (central x peripheral) as well as from catecholaminergic C1 neurons. Therefore, the modulatory inputs and the physiological conditions under which these mechanisms are used to recruit active expiration and increase ventilation need further investigation.

Maternal Methadone Destabilizes Neonatal Breathing and Desensitizes Neonates to Opioid-Induced Respiratory Frequency Depression

Pregnant women and developing infants are understudied populations in the opioid crisis, despite the rise in opioid use during pregnancy. Maternal opioid use results in diverse negative outcomes for the fetus/newborn, including death; however, the effects of perinatal (maternal and neonatal) opioids on developing respiratory circuitry are not well understood. Given the profound depressive effects of opioids on central respiratory networks controlling breathing, we tested the hypothesis that perinatal opioid exposure impairs respiratory neural circuitry, creating breathing instability. Our data demonstrate maternal opioids increase apneas and destabilize neonatal breathing. Maternal opioids also blunted opioid-induced respiratory frequency depression acutely in neonates; a unique finding since adult respiratory circuity does not desensitize to opioids. This desensitization normalized rapidly between postnatal days 1 and 2 (P1 and P2), the same age quantal slowing emerged in respiratory rhythm. These data suggest significant reorganization of respiratory rhythm generating circuits at P1-2, the same time as the preBötzinger Complex (key site of respiratory rhythm generation) becomes the dominant respiratory rhythm generator. Thus, these studies provide critical insight relevant to the normal developmental trajectory of respiratory circuits and suggest changes to mutual coupling between respiratory oscillators, while also highlighting how maternal opioids alter these developing circuits. In conclusion, the results presented demonstrate neurorespiratory disruption by maternal opioids and blunted opioid-induced respiratory frequency depression with neonatal opioids, which will be important for understanding and treating the increasing population of neonates exposed to gestational opioids.

Breath-hold task induces temporal heterogeneity in electroencephalographic regional field power in healthy subjects

We demonstrated that changes in CO₂ values cause oscillations in the cortical activity in δ-and α-bands. The analysis of the regional field power (RFP) showed evidence that different cortical areas respond with different time delays to CO₂ challenges. An opposite behavior was found for the end-tidal O₂. We suppose that the different cortical time delays likely expresse specific ascending pathways to the cortex, generated by chemoreceptor nuclei in the brain stem. Although the brain stem is in charge of the automatic control of ventilation, the cortex is involved in the voluntary control of breathing but also receives inputs from the brain stem, which influences the perception of breathing, the arousal state and sleep architecture in conditions of hypoxia/hypercapnia. We evaluated in 11 healthy subjects the effects of breath hold (BH; 30 s of apneas and 30 s of normal breathing) and BH-related CO₂/O₂ changes on electroencephalogram (EEG) global field power (GFP) and RFP in nine different areas (3 rostrocaudal sections: anterior, central, and posterior; and 3 sagittal sections: left, middle, and right) in the δ- and α-bands by cross correlation analysis. No significant differences were observed in GFP or RFP when comparing free breathing (FB) with the BH task. Within the BH task, the shift from apnea to normal ventilation was accompanied by an increase in the δ-power and a decrease in the α-power. The end-tidal pressure of CO₂ ([Formula: see text]) was positively correlated with the δ-band and negatively with the α- band with a positive time shift, whereas an opposite behavior was found for the end-tidal pressure of O₂ ([Formula: see text]). Notably, the time shift between [Formula: see text] / [Formula: see text] signals and cortical activity at RFP was heterogenous and seemed to follow a hierarchical activation, with the δ-band responding earlier than the α-band. Overall, these findings suggest that the effect of BH on the cortex may follow specific ascending pathways from the brain stem and be related to chemoreflex stimulation.NEW & NOTEWORTHY We demonstrated that the end tidal CO₂ oscillation causes oscillations of delta and alpha bands. The analysis of the regional field power showed that different cortical areas respond with different time delays to CO₂ challenges. An opposite behavior was found for the end-tidal O₂. We can suppose that the different cortical time delay response likely expresses specific ascending pathways to the cortex generated by chemoreceptor nuclei in the brainstem.

Precision Medicine for Breath-Focused Mind-Body Therapies for Stress and Anxiety: Are We Ready Yet?

In this viewpoint, we present an argument for transdisciplinary "precision medicine" approaches that combine studies of basic neurobiology of breathing in animal and human models of stress that can help characterize physiological and neural biomarkers and mechanisms of breathing control and emotion regulation in humans. Such mechanistic research is fundamental for the development of more effective and mechanism-based mind-body therapies. The potential for this research to positively impact public health is high, as breathing techniques are inexpensive, accessible, and cross-culturally accepted, with fewer complications then observed with other standard therapies for stress-related disorders.

The role of glycinergic inhibition in respiratory pattern formation and cardio-respiratory coupling in rats

Cardio-respiratory coupling is reflected as respiratory sinus arrhythmia (RSA) and inspiratory-related bursting of sympathetic nerve activity. Inspiratory-related inhibitory and/or postinspiratory-related excitatory drive of cardiac vagal motoneurons (CVMs) can generate RSA. Since respiratory oscillations may depend on synaptic inhibition, we investigated the effects of blocking glycinergic neurotransmission (systemic and local application of the glycine receptor (GlyR) antagonist, strychnine) on the expression of the respiratory motor pattern, RSA and sympatho-respiratory coupling. We recorded heart-rate, phrenic, recurrent laryngeal and thoracic sympathetic nerve activities (PNA, RLNA, t-SNA) in a working-heart-brainstem preparation of rats, and show that systemic strychnine (50–200 ​nM) abolished RSA and triggered a shift of postinspiratory RLNA into inspiration, while t-SNA remained unchanged. Bilateral strychnine microinjection into the ventrolateral medullary area containing CVMs and laryngeal motoneurons (LMNs) of the nucleus ambiguus (NA/CVLM), the nucleus tractus solitarii, pre-Bötzinger Complex, Bötzinger Complex or Kölliker-Fuse nuclei revealed that only NA/CVLM strychnine microinjections mimicked the effects of systemic application. In all other target nuclei, except the Bötzinger Complex, GlyR-blockade attenuated the inspiratory-tachycardia of the RSA to a similar degree while evoking only a modest change in respiratory motor patterning, without changing the timing of postinspiratory-RLNA, or t-SNA. Thus, glycinergic inhibition at the motoneuronal level is involved in the generation of RSA and the separation of inspiratory and postinspiratory bursting of LMNs. Within the distributed ponto-medullary respiratory pre-motor network, local glycinergic inhibition contribute to the modulation of RSA tachycardia, respiratory frequency and phase duration but, surprisingly it had no major role in the mediation of respiratory-sympathetic coupling.

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Glycinergic inhibition controls inspiratory tachycardia via inhibition of cardiac vagal motoneurons.

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Glycinergic inhibition controls the discharge pattern of expiratory laryngeal motoneurons.

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Glycinergic neurotransmission has no major role in pattern formation at the pre-motor level.

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Glycinergic inhibition has no role in sympatho-respiratory coupling.

Forebrain projection neurons target functionally diverse respiratory control areas in the midbrain, pons, and medulla oblongata

Eupnea is generated by neural circuits located in the ponto-medullary brainstem, but can be modulated by higher brain inputs which contribute to volitional control of breathing and the expression of orofacial behaviors, such as vocalization, sniffing, coughing, and swallowing. Surprisingly, the anatomical organization of descending inputs that connect the forebrain with the brainstem respiratory network remains poorly defined. We hypothesized that descending forebrain projections target multiple distributed respiratory control nuclei across the neuroaxis. To test our hypothesis, we made discrete unilateral microinjections of the retrograde tracer cholera toxin subunit B in the midbrain periaqueductal gray (PAG), the pontine Kölliker-Fuse nucleus (KFn), the medullary Bötzinger complex (BötC), pre-BötC, or caudal midline raphé nuclei. We quantified the regional distribution of retrogradely labeled neurons in the forebrain 12-14 days postinjection. Overall, our data reveal that descending inputs from cortical areas predominantly target the PAG and KFn. Differential forebrain regions innervating the PAG (prefrontal, cingulate cortices, and lateral septum) and KFn (rhinal, piriform, and somatosensory cortices) imply that volitional motor commands for vocalization are specifically relayed via the PAG, while the KFn may receive commands to coordinate breathing with other orofacial behaviors (e.g., sniffing, swallowing). Additionally, we observed that the limbic or autonomic (interoceptive) systems are connected to broadly distributed downstream bulbar respiratory networks. Collectively, these data provide a neural substrate to explain how volitional, state-dependent, and emotional modulation of breathing is regulated by the forebrain.

Supraspinal fatigue in human inspiratory muscles with repeated sustained maximal efforts

To investigate the involvement of supraspinal fatigue in the loss of maximal inspiratory pressure (Pi_max), we fatigued the inspiratory muscles. Six participants performed 5 sustained maximal isometric inspiratory efforts (15-s contractions, duty cycle ∼75%) which reduced Pi_max, as measured from esophageal and mouth pressure, to around half of their initial maximums. Transcranial magnetic stimulation (TMS) delivered over the motor cortex near the beginning and end of each maximal effort evoked superimposed twitch-like increments in the ongoing Pi_max, increasing from ∼1.0% of Pi_max in the unfatigued contractions to ≥40% of ongoing Pi_max for esophageal and mouth pressures. The rate of increase in the superimposed twitch as Pi_max decreased with fatigue was not significantly different between the esophageal and mouth pressure measures. The inverse relationship between superimposed twitch pressure and Pi_max indicates a progressive decline in the ability of motor cortical output to drive the inspiratory muscles maximally, leading to the development of supraspinal fatigue. TMS also evoked silent periods in the electromyographic recordings of diaphragm, scalenes, and parasternal intercostal. The duration of the silent period increased with fatigue in all three muscles, which suggests greater intracortical inhibition, with the largest change observed in the diaphragm. The peak rate of relaxation in pressure during the silent period slowed as fatigue developed, indicating peripheral contractile changes in the active inspiratory muscles. These changes in the markers of fatigue show that both central and peripheral fatigue contribute to the loss in Pi_max when inspiratory muscles are fatigued with repeated sustained maximal efforts.NEW & NOTEWORTHY When the inspiratory muscles are fatigued with repeated sustained maximal efforts, supraspinal fatigue, a component of central fatigue, contributes to the loss in maximal inspiratory pressure. The presence of supraspinal fatigue was confirmed by the increase in amplitude of twitch-like increments in pressure evoked by motor cortical stimulation during maximal efforts, indicating that motor cortical output was not maximal as extra muscle force could be generated to increase inspiratory pressure.

Brainstem lesions are associated with sleep apnea in multiple sclerosis

Background

Studies linking MRI findings in MS patients with obstructive sleep apnea severity are limited.

Objective

We conducted a retrospective study to assess MRI abnormalities associated with obstructive sleep apnea (OSA) in patients with multiple sclerosis (MS).

Methods

We performed retrospective chart review of 65 patients with multiple sclerosis who had undergone polysomnography (PSG) for fatigue as well as brain MRI. We measured the number of lesions in the brainstem and calculated the standardized third ventricular width (sTVW) as a measure of brain atrophy, and subsequently performed correlation analyses of the apnea-hypopnea index (AHI) with brainstem lesion location, sTVW, and Expanded Disability Status Scale (EDSS).

Results

MS Patients with OSA were significantly older and had a higher body mass index (BMI) and higher AHI measures than patients without OSA. After adjustment for covariates, significant associations were found between AHI and lesion burden in the midbrain (p < 0.01) and pons (p = 0.05), but not medulla.

Conclusions

Midbrain and pontine lesions burden correlated with AHI, suggesting MS lesion location could contribute to development of OSA.

The pontine Kölliker-Fuse nucleus gates facial, hypoglossal, and vagal upper airway related motor activity

The pontine Kölliker-Fuse nucleus (KFn) is a core nucleus of respiratory network that mediates the inspiratory-expiratory phase transition and gates eupneic motor discharges in the vagal and hypoglossal nerves. In the present study, we investigated whether the same KFn circuit may also gate motor activities that control the resistance of the nasal airway, which is of particular importance in rodents. To do so, we simultaneously recorded phrenic, facial, vagal and hypoglossal cranial nerve activity in an in situ perfused brainstem preparation before and after bilateral injection of the GABA-receptor agonist isoguvacine (50-70 nl, 10 mM) into the KFn (n = 11). Our results show that bilateral inhibition of the KFn triggers apneusis (prolonged inspiration) and abolished pre-inspiratory discharge of facial, vagal and hypoglossal nerves as well as post-inspiratory discharge in the vagus. We conclude that the KFn plays a critical role for the eupneic regulation of naso-pharyngeal airway patency and the potential functions of the KFn in regulating airway patency and orofacial behavior is discussed.

Machine learning-based data analytic approaches for evaluating post-natal mouse respiratory physiological evolution

Respiratory parameters change during post-natal development, but the nature of their changes have not been well-described. The advent of commercially available plethysmographic instruments provided improved repeatability of measurements and standardization of measured breathing in mice across laboratories. These technologies thus allowed for exploration of more precise respiratory pattern changes during the post-natal developmental epoch. Current methods to analyze respiratory behavior utilize plethysmography to acquire standing values of frequency, volume and flow at specific time points in murine maturation. These metrics have historically been independently analyzed as a function of time with no further analysis examining the interplay these variables have with each other and in the context of postnatal maturation or during blood gas homeostasis. We posit that machine learning workflows can provide deeper physiological understanding into the postnatal development of respiration. In this manuscript, we delineate a machine learning workflow based on the R-statistical programming language to examine how variation and relationships of frequency (f) and tidal volume (TV) change with respect to inspiratory and expiratory parameters. Our analytical workflows could successfully predict age and found that the variation and relationships between respiratory metrics are dynamically shifting with age and during hypercapnic breathing. Thus, our work demonstrates the utility of high dimensional analyses to provide reliable class label predictions using non-invasive respiratory metrics. These approaches may be useful in large-scale phenotyping across development and in disease.

MRI and pathology correlations in the medulla in sudden unexpected death in epilepsy (SUDEP): a postmortem study

AIMS

Sudden unexpected death in epilepsy (SUDEP) likely arises as a result of autonomic dysfunction around the time of a seizure. In vivo MRI studies report volume reduction in the medulla and other brainstem autonomic regions. Our aim, in a pathology series, is to correlate regional quantitative features on 9.4T MRI with pathology measures in medullary regions.

METHODS

Forty-seven medullae from 18 SUDEP, 18 nonepilepsy controls and 11 epilepsy controls were studied. In 16 cases, representing all three groups, ex vivo 9.4T MRI of the brainstem was carried out. Five regions of interest (ROI) were delineated, including the reticular formation zone (RtZ), and actual and relative volumes (RV), as well as T1, T2, T2* and magnetization transfer ratio (MTR) measurements were evaluated on MRI. On serial sections, actual and RV estimates using Cavalieri stereological method and immunolabelling indices for myelin basic protein, synaptophysin and Microtubule associated protein 2 (MAP2) were carried out in similar ROI.

RESULTS

Lower relative RtZ volumes in the rostral medulla but higher actual volumes in the caudal medulla were observed in SUDEP (P < 0.05). No differences between groups for T1, T2, T2* and MTR values in any region was seen but a positive correlation between T1 values and MAP2 labelling index in RtZ (P < 0.05). Significantly lower MAP2 LI were noted in the rostral medulla RtZ in epilepsy cases (P < 0.05).

CONCLUSIONS

Rostro-caudal alterations of medullary volume in SUDEP localize with regions containing respiratory regulatory nuclei. They may represent seizure-related alterations, relevant to the pathophysiology of SUDEP.

Dynamical phase separation on rhythmogenic neuronal networks

We explore the firing-rate model of excitatory neurons with dendritic adaptation (the Feldman-Del Negro model [J. L. Feldman and C. A. Del Negro, Nat. Rev. Neurosci. 7, 232 (2006)10.1038/nrn1871; D. J. Schwab et al., Phys. Rev. E 82, 051911 (2010)10.1103/PhysRevE.82.051911] interacting on a fixed, directed Erdős-Rényi network. This model is applied to the dynamics of the pre-Bötzinger complex, the mammalian central pattern generator with N∼10^{3} neurons, which produces a collective metronomic signal that times inspiration. In the all-to-all coupled variant of the model, there is spontaneous symmetry breaking in which some fraction of the neurons becomes stuck in a high-firing-rate state, while others become quiescent. This separation into firing and nonfiring clusters persists into more sparsely connected networks. In these sparser networks, the clustering is influenced by k cores of the underlying network. The model has a number of features of the dynamical phase diagram that violate the predictions of mean-field analysis. In particular, we observe in the simulated networks that stable oscillations do not persist in the high-sensitivity limit, in contradiction to the predictions of mean-field theory. Moreover, we observe that the oscillations in these sparse networks are remarkably robust in response to killing neurons, surviving until only approximately 20% of the network remains. This robustness is consistent with experiment.

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.

A Neurologic Examination for Anesthesiologists: Assessing Arousal Level during Induction, Maintenance, and Emergence

Anesthetics have profound effects on the brain and central nervous system. Vital signs, along with the electroencephalogram and electroencephalogram-based indices, are commonly used to assess the brain states of patients receiving general anesthesia and sedation. Important information about the patient's arousal state during general anesthesia can also be obtained through use of the neurologic examination. This article reviews the main components of the neurologic examination focusing primarily on the brainstem examination. It details the components of the brainstem examination that are most relevant for patient management during induction, maintenance, and emergence from general anesthesia. The examination is easy to apply and provides important complementary information about the patient's arousal level that cannot be discerned from vital signs and electroencephalogram measures.

Physiology of the Respiratory Drive in ICU Patients: Implications for Diagnosis and Treatment

This article is one of ten reviews selected from the Annual Update in Intensive Care and Emergency Medicine 2020. Other selected articles can be found online at https://www.biomedcentral.com/collections/annualupdate2020. Further information about the Annual Update in Intensive Care and Emergency Medicine is available from http://www.springer.com/series/8901.

Opioid-Related Sleep-Disordered Breathing: An Update for Clinicians

Opioids are an effective treatment for patients with intractable pain. Long-term administration of opioids for pain relief is being delivered by an increasing number of medical providers in the United States including primary care physicians and nonspecialists. One common complication of chronic opioid use is sleep-disordered breathing which can result in various morbidities as well as an increase in all-cause mortality. It is important for providers to understand the relationship between opioids and sleep-disordered breathing as well as methods to improve diagnosis and strategies for treatment. This review aims to update clinicians on the mechanism, diagnosis, and treatment of opioid-related sleep-disordered breathing in order to improve the quality of care for patients with chronic pain.

Effects of inflammation on the developing respiratory system: Focus on hypoglossal (XII) neuron morphology, brainstem neurochemistry, and control of breathing

Breathing is fundamental to life and any adverse change in respiratory function can endanger the health of an organism or even be fatal. Perinatal inflammation is known to adversely affect breathing in preterm babies, but lung infection/inflammation impacts all stages of life from birth to death. Little is known about the role of inflammation in respiratory control, neuronal morphology, or neural function during development. Animal models of inflammation can provide understanding and insight into respiratory development and how inflammatory processes alter developmental phenotype in addition to providing insight into new treatment modalities. In this review, we focus on recent work concerning the development of neurons, models of perinatal inflammation with an emphasis on two common LPS-based models, inflammation and its impact on development, and current and potential treatments for inflammation within the respiratory control circuitry of the mammalian brainstem. We have also discussed models of inflammation in adults and have specifically focused on hypoglossal motoneurons (XII) and neurons of the nucleus tractus solitarii (nTS) as these nuclei have been studied more extensively than other brainstem nuclei participating in breathing and airway control. Understanding the impact of inflammation on the developmental aspects of respiratory control and breathing pattern is critical to addressing problems of cardiorespiratory dysregulation in disease and this overview points out many gaps in our current knowledge.

Is there a common drive for buccal movements associated with buccal and lung 'breath' in Lithobates catesbeianus?

In amphibians, there is some evidence that (1) anatomically separate brainstem respiratory oscillators are involved in rhythm generation, one for the buccal rhythm and another for the lung rhythm and (2) they become functionally coupled during metamorphosis. The present analysis, performed on neurograms recorded using brainstem preparations from Lithobates catesbeianus, aims to investigate the temporal organisation of lung and buccal burst types. Continuous Wavelet Transfom applied to the separated buccal and lung signals of a neurogram revealed that both buccal and lung frequency profiles exhibited the same low frequency peak around 1 Hz. This suggests that a common 'clock' organises both rhythms within an animal. A cross-correlation analysis applied to the buccal and lung burst signals revealed their similar intrinsic oscillation features, occurring at approximately 25 Hz. These observations suggest that a coupling between the lung and buccal oscillators emerges at metamorphosis. This coupling may be related to inter-connectivity between the two oscillators, and to a putative common drive.

Brainstem dysfunction in critically ill patients

The brainstem conveys sensory and motor inputs between the spinal cord and the brain, and contains nuclei of the cranial nerves. It controls the sleep-wake cycle and vital functions via the ascending reticular activating system and the autonomic nuclei, respectively. Brainstem dysfunction may lead to sensory and motor deficits, cranial nerve palsies, impairment of consciousness, dysautonomia, and respiratory failure. The brainstem is prone to various primary and secondary insults, resulting in acute or chronic dysfunction. Of particular importance for characterizing brainstem dysfunction and identifying the underlying etiology are a detailed clinical examination, MRI, neurophysiologic tests such as brainstem auditory evoked potentials, and an analysis of the cerebrospinal fluid. Detection of brainstem dysfunction is challenging but of utmost importance in comatose and deeply sedated patients both to guide therapy and to support outcome prediction. In the present review, we summarize the neuroanatomy, clinical syndromes, and diagnostic techniques of critical illness-associated brainstem dysfunction for the critical care setting.

Industrial perspectives on brain-computer interface technology

Neuromodulation therapies offer a unique opportunity for translating brain-computer interface (BCI) technologies into a clinical setting. Several diseases such as Parkinson's disease are effectively treated by invasive device stimulation therapies, and the addition of sensing and algorithm technology is an obvious evolutionary expansion of capabilities. In addition, this infrastructure might enable a roadmap of novel BCI technologies. While the initial applications are focused on epilepsy and movement disorders, the technology is potentially transferable to a broader base of disorders, including stroke and rehabilitation. The ultimate potential of BCI technology will be determined by forthcoming chronic evaluation in multiple neurologic disorders.

ERS International Congress, Madrid, 2019: highlights from the Respiratory Intensive Care Assembly

The Respiratory Intensive Care Assembly of the European Respiratory Society is delighted to present the highlights from the 2019 International Congress in Madrid, Spain. We have selected four sessions that discussed recent advances in a wide range of topics: from acute respiratory failure to cough augmentation in neuromuscular disorders and from extra-corporeal life support to difficult ventilator weaning. The subjects are summarised by early career members in close collaboration with the Assembly leadership. We aim to give the reader an update on the most important developments discussed at the conference. Each session is further summarised into a short list of take-home messages.

The effect of acetazolamide on the improvement of central apnea caused by abusing opioid drugs in the clinical trial

PURPOSE

Acetazolamide is utilized as a treatment which falls effective in treating some type of CSA. Hence, it might be effective as far as opium addicts who suffer from CSA are concerned.

MATERIALS AND METHOD

The current study was a double-blind, placebo-controlled, cross-over study ( clinicalTrials.gov ID: NCT02371473). The whole procedures were identical for both placebo and acetazolamide phases of clinical research. There were 14 CSA more than 5/h and more than 50% of apnea-hypopnea index (AHI). Out of these 14 patients, 10 volunteered to participate in the study. Fast Fourier transformation was used to separate heart rate variability (HRV) into its component VLF (very low frequency band), LF (low frequency band), and HF (high frequency band) rhythms that operate within different frequency ranges.

RESULT

There are significant results in terms of decreased mix apnea and central apnea together due to acetazolamide compared with placebo (P < 0.023). Time of SatO₂ < 90% is decreased as well (P < 0.1). There is also decrease of SDNN and NN50 after treatment with acetazolamide respectively (P < 0.001). Regarding fast Fourier transformation, there is increase of pHF and decrease of pLF after acetazolamide treatment (P < 0.001).

CONCLUSION

Acetazolamide seems to be effective in improving oxygenation and a decrease of mixed and central apnea events together. In HRV analysis section, LF power has decreased significantly, which may more likely improve prognosis of the patients.

Respiratory muscle activity in voluntary breathing tracking tasks: Implications for the assessment of respiratory motor control

How the involuntary (bulbospinal) and voluntary (corticospinal) pathways interact in respiratory muscle control is not established. To determine the role of excitatory corticobulbar pathways in humans, studies typically compare electromyographic activity (EMG) or evoked responses in respiratory muscles during hypercapnic and voluntary tasks. Although ventilation is matched between tasks by having participants track signals of ventilation, these tasks may not result in matched respiratory muscle activity. The aim of this study was to describe respiratory muscle activity and ribcage and abdominal excursions during two different voluntary conditions, compared to hypercapnic hyperventilation. Ventilation was matched in the voluntary conditions via (i) a simple target of lung volume ('volume tracking') or (ii) targets of both ribcage and abdominal excursions, adjusted to end-expiratory lung volume in hypercapnic hyperventilation ('bands tracking'). Compared to hypercapnic hyperventilation, respiratory parameters such as tidal volume were similar, but the ratio of ribcage to abdominal excursion was higher for both voluntary tasks. Inspiratory scalene and parasternal intercostal muscle activity was higher in volume tracking, but diaphragm and abdominal muscle activity showed little to no change. There were no differences in muscle activity in bands tracking for any muscle, compared to hypercapnic hyperventilation. An elevated ratio of ribcage to abdominal excursion in the bands tracking task indicates that participants could not accurately match the targets in this condition. Inspiratory muscle activity is altered in some muscles in some voluntary tasks, compared to hypercapnia. Therefore, differences in muscle activity should be considered in interpretation of studies that use these protocols to investigate respiratory muscle control.

Heart rate variability in the tegu lizard, Salvator merianae, its neuroanatomical basis and role in the assessment of recovery from experimental manipulation

Using long-term, remote recordings of heart rate (f_H) on fully recovered, undisturbed lizards, we identified several components of heart rate variability (HRV) associated with respiratory sinus arrhythmia (RSA): 1.) A peak in the spectral representation of HRV at the frequency range of ventilation. 2.) These cardiorespiratory interactions were shown to be dependent on the parasympathetic arm of the autonomic nervous system. 3.) Vagal preganglionic neurons are located in discrete groups located in the dorsal motor nucleus of the vagus and also, in a ventro-lateral group, homologous to the nucleus ambiguus of mammals. 4.) Myelinated nerve fibers in the cardiac vagus enabling rapid communication between the central nervous system and the heart. Furthermore, the study of the progressive recovery of f_H in tegu following anesthesia and instrumentation revealed that 'resting' levels of mean f_H and reestablishment of HRV occurred over different time courses. Accordingly, we suggest that, when an experiment is designed to study a physiological variable reliant on autonomic modulation at its normal, resting level, then postsurgical reestablishment of HRV should be considered as the index of full recovery, rather than mean f_H.

Apnea Associated with Brainstem Seizures in Cacna1a S218L Mice Is Caused by Medullary Spreading Depolarization

Seizure-related apnea is common and can be lethal. Its mechanisms however remain unclear and preventive strategies are lacking. We postulate that brainstem spreading depolarization (SD), previously associated with lethal seizures in animal models, initiates apnea upon invasion of brainstem respiratory centers. To study this, we assessed effects of brainstem seizures on brainstem function and respiration in male and female mice carrying a homozygous S218L missense mutation that leads to gain-of-function of voltage-gated Ca_V2.1 Ca²⁺ channels and high risk for fatal seizures. Recordings of brainstem DC potential and neuronal activity, cardiorespiratory activity and local tissue oxygen were performed in freely behaving animals. Brainstem SD occurred during all spontaneous fatal seizures and, unexpectedly, during a subset of nonfatal seizures. Seizure-related SDs in the ventrolateral medulla correlated with respiratory suppression. Seizures induced by stimulation of the inferior colliculus could evoke SD that spread in a rostrocaudal direction, preceding local tissue hypoxia and apnea, indicating that invasion of SD into medullary respiratory centers initiated apnea and hypoxia rather than vice versa Fatal outcome was prevented by timely resuscitation. Moreover, NMDA receptor antagonists MK-801 and memantine prevented seizure-related SD and apnea, which supports brainstem SD as a prerequisite for brainstem seizure-related apnea in this animal model and has translational value for developing strategies that prevent fatal ictal apnea.SIGNIFICANCE STATEMENT Apnea during and following seizures is common, but also likely implicated in sudden unexpected death in epilepsy (SUDEP). This underlines the need to understand mechanisms for potentially lethal seizure-related apnea. In the present work we show, in freely behaving SUDEP-prone transgenic mice, that apnea is induced when spontaneous brainstem seizure-related spreading depolarization (SD) reaches respiratory nuclei in the ventrolateral medulla. We show that brainstem seizure-related medullary SD is followed by local hypoxia and recovers during nonfatal seizures, but not during fatal events. NMDA receptor antagonists prevented medullary SD and apnea, which may be of translational value.

Laryngeal afferent modulation of swallowing interneurons in the dorsal medulla in perfused rats

OBJECTIVES

The purpose of this study was to investigate the influence of laryngeal afferent inputs on brainstem circuits that mediate and transmit swallowing activity to the orofacial musculature.

METHODS

Experiments were performed on 19 arterially perfused juvenile rats. The activities of swallowing interneurons in relation to their respective motor outputs in the hypoglossal and vagus nerves were assessed during fictive swallowing with or without concurrent laryngeal sensory stimulation at intensities of 20, 40, and 60 μA.

RESULTS

The hypoglossal nerve activity was gradually enhanced with increasing intensity of the sensory stimulation, while the vagus nerve activity was not altered. The activities of various interneurons were modulated by the laryngeal stimulation, but more than 50% of the recorded neurons were inhibited by the stimulation. Some interneurons demonstrated no obvious change in their discharge rates with laryngeal sensory stimulation during fictive swallowing.

CONCLUSION

Laryngeal afferent inputs partially modulated the swallowing motor activity via enhanced or suppressed activities of the swallowing interneurons, while the essential motor pattern underlying the pharyngeal stage of swallowing remained basically unchanged. Thus, the output patterns of the complex sequential movements of swallowing could be basically predetermined and further adjusted according to sensory information related to the properties of the ingested food by a swallowing central pattern generator.

LEVEL OF EVIDENCE

NA Laryngoscope, 130: 1885-1893, 2020.

Postictal serotonin levels are associated with peri-ictal apnea

OBJECTIVE

To determine the relationship between serum serotonin (5-HT) levels, ictal central apnea (ICA), and postconvulsive central apnea (PCCA) in epileptic seizures.

METHODS

We prospectively evaluated video EEG, plethysmography, capillary oxygen saturation (SpO₂), and ECG for 49 patients (49 seizures) enrolled in a multicenter study of sudden unexpected death in epilepsy (SUDEP). Postictal and interictal venous blood samples were collected after a clinical seizure for measurement of serum 5-HT levels. Seizures were classified according to the International League Against Epilepsy 2017 seizure classification. We analyzed seizures with and without ICA (n = 49) and generalized convulsive seizures (GCS) with and without PCCA (n = 27).

RESULTS

Postictal serum 5-HT levels were increased over interictal levels for seizures without ICA (p = 0.01), compared to seizures with ICA (p = 0.21). In patients with GCS without PCCA, serum 5-HT levels were increased postictally compared to interictal levels (p < 0.001), but not in patients with seizures with PCCA (p = 0.22). Postictal minus interictal 5-HT levels also differed between the 2 groups with and without PCCA (p = 0.03). Increased heart rate was accompanied by increased serum 5-HT levels (postictal minus interictal) after seizures without PCCA (p = 0.03) compared to those with PCCA (p = 0.42).

CONCLUSIONS

The data suggest that significant seizure-related increases in serum 5-HT levels are associated with a lower incidence of seizure-related breathing dysfunction, and may reflect physiologic changes that confer a protective effect against deleterious phenomena leading to SUDEP. These results need to be confirmed with a larger sample size study.

Effects of persistent sodium current blockade in respiratory circuits depend on the pharmacological mechanism of action and network dynamics

The mechanism(s) of action of most commonly used pharmacological blockers of voltage-gated ion channels are well understood; however, this knowledge is rarely considered when interpreting experimental data. Effects of blockade are often assumed to be equivalent, regardless of the mechanism of the blocker involved. Using computer simulations, we demonstrate that this assumption may not always be correct. We simulate the blockade of a persistent sodium current (I_NaP), proposed to underlie rhythm generation in pre-Bötzinger complex (pre-BötC) respiratory neurons, via two distinct pharmacological mechanisms: (1) pore obstruction mediated by tetrodotoxin and (2) altered inactivation dynamics mediated by riluzole. The reported effects of experimental application of tetrodotoxin and riluzole in respiratory circuits are diverse and seemingly contradictory and have led to considerable debate within the field as to the specific role of I_NaP in respiratory circuits. The results of our simulations match a wide array of experimental data spanning from the level of isolated pre-BötC neurons to the level of the intact respiratory network and also generate a series of experimentally testable predictions. Specifically, in this study we: (1) provide a mechanistic explanation for seemingly contradictory experimental results from in vitro studies of I_NaP block, (2) show that the effects of I_NaP block in in vitro preparations are not necessarily equivalent to those in more intact preparations, (3) demonstrate and explain why riluzole application may fail to effectively block I_NaP in the intact respiratory network, and (4) derive the prediction that effective block of I_NaP by low concentration tetrodotoxin will stop respiratory rhythm generation in the intact respiratory network. These simulations support a critical role for I_NaP in respiratory rhythmogenesis in vivo and illustrate the importance of considering mechanism when interpreting and simulating data relating to pharmacological blockade.

The application of pharmacological agents that affect transmembrane ionic currents in neurons is a commonly used experimental technique. A simplistic interpretation of experiments involving these agents suggests that antagonist application removes the impacted current and that subsequently observed changes in activity are attributable to the loss of that current’s effects. The more complex reality, however, is that different drugs may have distinct mechanisms of action, some corresponding not to a removal of a current but rather to a changing of its properties. We use computational modeling to explore the implications of the distinct mechanisms associated with two drugs, riluzole and tetrodotoxin, that are often characterized as sodium channel blockers. Through this approach, we offer potential explanations for disparate findings observed in experiments on neural respiratory circuits and show that the experimental results are consistent with a key role for the persistent sodium current in respiratory rhythm generation.

Role of fast inhibitory synaptic transmission in neonatal respiratory rhythmogenesis and pattern formation

Several studies have investigated the general role of chloride-based neurotransmission (GABA_A and glycinergic signaling) in respiratory rhythmogenesis and pattern formation. In several brain regions, developmental alterations in these signaling pathways have been shown to be mediated by changes in cation-chloride cotransporter (CC) expression. For instance, CC expression changes during the course of neonatal development in medullary respiratory nuclei and other brain/spinal cord regions in a manner which decreases the cellular import, and increases the export, of chloride ions, shifting reversal potentials for chloride to progressively more negative values with maturation. In slice preparations of the same, this is related to an excitatory-to-inhibitory shift of GABA_A- and glycinergic signaling. In medullary slices, GABA_A-/glycinergic signaling in the early neonatal period is excitatory, becoming inhibitory over time. Additionally, blockade of the Na⁺/K⁺/2Cl^- cotransporter, which imports these ions via secondary active transport, converts excitatory response to inhibitory ones. These effects have not yet been demonstrated at the individual respiratory-related neuron level to occur in intact (in vivo or in situ) animal preparations, which in contrast to slices, possess normal network connectivity and natural sources of tonic drive. Developmental changes in respiratory rhythm generating and pattern forming pontomedullary respiratory circuitry may contribute to critical periods, during which there exist increased risk for perinatal respiratory disturbances of central, obstructive, or hypoxia/hypercapnia-induced origin, including the sudden infant death syndrome. Thus, better characterizing the neurochemical maturation of the central respiratory network will enhance our understanding of these conditions, which will facilitate development of targeted therapies for respiratory disturbances in neonates and infants.

The Purinome and the preBötzinger Complex - A Ménage of Unexplored Mechanisms That May Modulate/Shape the Hypoxic Ventilatory Response

Exploration of purinergic signaling in brainstem homeostatic control processes is challenging the traditional view that the biphasic hypoxic ventilatory response, which comprises a rapid initial increase in breathing followed by a slower secondary depression, reflects the interaction between peripheral chemoreceptor-mediated excitation and central inhibition. While controversial, accumulating evidence supports that in addition to peripheral excitation, interactions between central excitatory and inhibitory purinergic mechanisms shape this key homeostatic reflex. The objective of this review is to present our working model of how purinergic signaling modulates the glutamatergic inspiratory synapse in the preBötzinger Complex (key site of inspiratory rhythm generation) to shape the hypoxic ventilatory response. It is based on the perspective that has emerged from decades of analysis of glutamatergic synapses in the hippocampus, where the actions of extracellular ATP are determined by a complex signaling system, the purinome. The purinome involves not only the actions of ATP and adenosine at P2 and P1 receptors, respectively, but diverse families of enzymes and transporters that collectively determine the rate of ATP degradation, adenosine accumulation and adenosine clearance. We summarize current knowledge of the roles played by these different purinergic elements in the hypoxic ventilatory response, often drawing on examples from other brain regions, and look ahead to many unanswered questions and remaining challenges.

The Neural Basis of Timing: Distributed Mechanisms for Diverse Functions

Timing is critical to most forms of learning, behavior, and sensory-motor processing. Converging evidence supports the notion that, precisely because of its importance across a wide range of brain functions, timing relies on intrinsic and general properties of neurons and neural circuits; that is, the brain uses its natural cellular and network dynamics to solve a diversity of temporal computations. Many circuits have been shown to encode elapsed time in dynamically changing patterns of neural activity-so-called population clocks. But temporal processing encompasses a wide range of different computations, and just as there are different circuits and mechanisms underlying computations about space, there are a multitude of circuits and mechanisms underlying the ability to tell time and generate temporal patterns.

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.

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

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

Structural and functional defects of the respiratory neural system in the medulla and spinal cord of Pax6 mutant rats

Pax6 is an important transcription factor expressed in several discrete domains of the developing central nervous system. It has been reported that Pax6 is involved in the specification of subtypes of hindbrain motor neurons. Pax6 homozygous mutant (rSey²/rSey²) rats die soon after birth, probably due to impaired respiratory movement. To determine whether the respiratory center in the medulla functions normally, we analyzed the histological and neurophysiological properties of the medulla and spinal cord in fetal rats with this mutation. First, the medulla of rSey²/rSey² at embryonic (E) 21.5-E22.5 tended to be smaller than those from heterozygous mutant (rSey²/+) and wild-type (+/+) littermates. Through immunohistochemical analysis, we confirmed normal distribution of Phox2b-expressing cells in the parafacial respiratory group (pFRG) of rSey²/rSey² rats. Expression of neurokinin-1 receptor (NK-1R) was weak and dispersed in rSey²/rSey² rats. In addition, rSey²/rSey² rats have a defect of the hypoglossal nerve root. Electrophysiological analysis using brainstem-spinal cord preparations (E21.5-E22.5) revealed that rSey²/rSey² rats showed larger fluctuation of the amplitude of inspiratory activity monitored from the fourth cervical root although there was no significant difference in the respiratory rate among rSey²/rSey², rSey²/+, and +/+ littermates. The response of respiratory rhythm to high CO₂ was similar among all genotypes. Optical recordings of neuronal activity revealed that the activity of the pFRG tended to be weaker and inspiratory activity appeared in more scattered areas in the caudal ventral medulla in the rSey²/rSey² rats. These results suggest that the basal activity of the respiratory system was preserved with mild impairment of the inspiratory activity in the rSey²/rSey² rats and that the Pax6 gene is involved in the functional development of the neuronal system producing effective inspiratory motor outputs for survival.

The ventrolateral medulla and medullary raphe in sudden unexpected death in epilepsy

Sudden unexpected death in epilepsy (SUDEP) is a leading cause of premature death in patients with epilepsy. One hypothesis proposes that sudden death is mediated by post-ictal central respiratory depression, which could relate to underlying pathology in key respiratory nuclei and/or their neuromodulators. Our aim was to investigate neuronal populations in the ventrolateral medulla (which includes the putative human pre-Bötzinger complex) and the medullary raphe. Forty brainstems were studied comprising four groups: 14 SUDEP, six epilepsy controls, seven Dravet syndrome cases and 13 non-epilepsy controls. Serial sections through the medulla (from obex 1 to 10 mm) were stained for Nissl, somatostatin, neurokinin 1 receptor (for pre-Bötzinger complex neurons) and galanin, tryptophan hydroxylase and serotonin transporter (neuromodulatory systems). Using stereology total neuronal number and densities, with respect to obex level, were measured. Whole slide scanning image analysis was used to quantify immunolabelling indices as well as co-localization between markers. Significant findings included reduction in somatostatin neurons and neurokinin 1 receptor labelling in the ventrolateral medulla in sudden death in epilepsy compared to controls (P < 0.05). Galanin and tryptophan hydroxylase labelling was also reduced in sudden death cases and more significantly in the ventrolateral medulla region than the raphe (P < 0.005 and P < 0.05). With serotonin transporter, reduction in labelling in cases of sudden death in epilepsy was noted only in the raphe (P ≤ 0.01); however, co-localization with tryptophan hydroxylase was significantly reduced in the ventrolateral medulla. Epilepsy controls and cases with Dravet syndrome showed less significant alterations with differences from non-epilepsy controls noted only for somatostatin in the ventrolateral medulla (P < 0.05). Variations in labelling with respect to obex level were noted of potential relevance to the rostro-caudal organization of respiratory nuclear groups, including tryptophan hydroxylase, where the greatest statistical difference noted between all epilepsy cases and controls was at obex 9-10 mm (P = 0.034), the putative level of the pre-Bötzinger complex. Furthermore, there was evidence for variation with duration of epilepsy for somatostatin and neurokinin 1 receptor. Our findings suggest alteration to neuronal populations in the medulla in SUDEP with evidence for greater reduction in neuromodulatory neuropeptidergic and mono-aminergic systems, including for galanin, and serotonin. Other nuclei need to be investigated to evaluate if this is part of more widespread brainstem pathology. Our findings could be a result of previous seizures and may represent a pathological risk factor for SUDEP through impaired respiratory homeostasis during a seizure.

An aromatic substance, eugenol induces distinct depressant effects on respiratory activity in different postnatal developmental stages of the rat

Eugenol modulates neuronal activity through actions on voltage-gated ionic channels and/or transient receptor potential channels. We previously suggested that eugenol inhibited cellular (and/or network) mechanisms essential for the maintenance of the respiratory burst activity in a brainstem-spinal cord preparation from newborn rat (postnatal day 0-3). Study of the distinct effects of eugenol in neonatal and later developmental stage rats may offer new information about postnatal developmental changes of respiratory neuron networks. In the present study, therefore, we compared effects of eugenol in an in vitro newborn rat preparation with those in an arterially perfused in situ preparation from juvenile rat (postnatal day 12-15). In the former preparation, application of 1 mM eugenol decreased respiratory rate and inspiratory burst duration. In contrast, in the latter preparation, 1 mM eugenol induced a gradual decrease in the amplitude of integrated phrenic nerve activity. Phrenic nerve activity gradually recovered at 25-30 min after washout with a burst duration similar to control values. We hypothesized that the depressant effects of eugenol were caused by inhibition of cell excitability in the neonatal rat in vitro preparation but by a reduction of synaptic interactions in the juvenile rat in situ preparation.

Neural network model of an amphibian ventilatory central pattern generator

The neuronal multiunit model presented here is a formal model of the central pattern generator (CPG) of the amphibian ventilatory neural network, inspired by experimental data from Pelophylax ridibundus. The kernel of the CPG consists of three pacemakers and two follower neurons (buccal and lung respectively). This kernel is connected to a chain of excitatory and inhibitory neurons organized in loops. Simulations are performed with Izhikevich-type neurons. When driven by the buccal follower, the excitatory neurons transmit and reorganize the follower activity pattern along the chain, and when driven by the lung follower, the excitatory and inhibitory neurons of the chain fire in synchrony. The additive effects of synaptic inputs from the pacemakers on the buccal follower account for (1) the low frequency buccal rhythm, (2) the intra-burst high frequency oscillations, and (3) the episodic lung activity. Chemosensitivity to acidosis is implemented by an increase in the firing frequency of one of the pacemakers. This frequency increase leads to both a decrease in the buccal burst frequency and an increase in the lung episode frequency. The rhythmogenic properties of the model are robust against synaptic noise and pacemaker jitter. To validate the rhythm and pattern genesis of this formal CPG, neurograms were built from simulated motoneuron activity, and compared with experimental neurograms. The basic principles of our model account for several experimental observations, and we suggest that these principles may be generic for amphibian ventilation.

Enhancement of excitatory transmission in NTS neurons projecting to ventral medulla of rats exposed to sustained hypoxia is blunted by minocycline

KEY POINTS

Rats subjected to sustained hypoxia (SH) present increases in arterial pressure (AP) and in glutamatergic transmission in the nucleus tractus solitarius (NTS) neurons sending projections to ventrolateral medulla (VLM). Treatment with minocycline, a microglial inhibitor, attenuated the increase in AP in response to SH. The increase in the amplitude of glutamatergic postsynaptic currents in the NTS-VLM neurons, induced by postsynaptic mechanisms, was blunted by minocycline treatment. The number of microglial cells was increased in the NTS of vehicle-treated SH rats but not in the NTS of minocycline-treated rats. The data show that microglial recruitment/proliferation induced by SH is associated with the enhancement of excitatory neurotransmission in NTS-VLM neurons, which may contribute to the observed increase in AP.

ABSTRACT

Short-term sustained hypoxia (SH) produces significant autonomic and respiratory adjustments and triggers activation of microglia, the resident immune cells in the brain. SH also enhances glutamatergic neurotransmission in the NTS. Here we evaluated the role of microglial activation induced by SH on the cardiovascular changes and mainly on glutamatergic neurotransmission in NTS neurons sending projections to the ventrolateral medulla (NTS-VLM), using a microglia inhibitor (minocycline). Direct measurement of arterial pressure (AP) in freely moving rats showed that SH (24 h, fraction of inspired oxygen ( F I , O 2 ) 0.1) in vehicle and minocycline (30 mg/kg i.p. for 3 days)-treated groups produced a significant increase in AP in relation to control groups under normoxic conditions, but this increase was significantly lower in minocycline-treated rats. Whole-cell patch-clamp recordings revealed that the active properties of the membrane were comparable among the groups. Nevertheless, the amplitudes of glutamatergic postsynaptic currents, evoked by tractus solitarius stimulation, were increased in NTS-VLM neurons of SH rats. Changes in asynchronous glutamatergic currents indicated that the observed increase in amplitude was due to postsynaptic mechanisms. These changes were blunted in the SH group previously treated with minocycline. Using immunofluorescence, we found that the number of microglial cells was increased in the NTS of vehicle-treated SH rats but not in the NTS neurons of minocycline-treated rats. Our data support the concept that microglial activation induced by SH is associated with the enhancement of excitatory neurotransmission in NTS-VLM neurons, which may contribute to the increase in AP observed in this experimental model.

Medullary Respiratory Circuit Is Reorganized by a Seasonally-Induced Program in Preparation for Hibernation

Deep hibernators go through several cycles of profound drops in body temperature during the winter season, with core temperatures sometimes reaching near freezing. Yet unlike non-hibernating mammals, they can sustain breathing rhythms. The physiological processes that make this possible are still not understood. In this study, we focused on the medullary Ventral Respiratory Column of a facultative hibernator, the Syrian hamster. Using shortened day-lengths, we induced a "winter-adapted" physiological state, which is a prerequisite for hibernation. When recording electrophysiological signals from acute slices in the winter-adapted pre-Bötzinger complex (preBötC), spike trains showed higher spike rates, amplitudes, complexity, as well as higher temperature sensitivity, suggesting an increase in connectivity and/or synaptic strength during the winter season. We further examined action potential waveforms and found that the depolarization integral, as measured by the area under the curve, is selectively enhanced in winter-adapted animals. This suggests that a shift in the ion handling kinetics is also being induced by the winter-adaptation program. RNA sequencing of respiratory pre-motor neurons, followed by gene set enrichment analysis, revealed differential regulation and splicing in structural, synaptic, and ion handling genes. Splice junction analysis suggested that differential exon usage is occurring in a select subset of ion handling subunits (ATP1A3, KCNC3, SCN1B), and synaptic structure genes (SNCB, SNCG, RAB3A). Our findings show that the hamster respiratory center undergoes a seasonally-cued alteration in electrophysiological properties, likely protecting against respiratory failure at low temperatures.