Endogenous glutamatergic inputs to the Parabrachial Nucleus/Kölliker-Fuse Complex determine respiratory rate

CB1 cannabinoid receptor agonists induce acute respiratory depression in awake mice

Recreational use of synthetic cannabinoid agonists (i.e., "spice compounds") that target the cannabinoid type 1 receptor (CB1) can cause acute respiratory failure in humans. However, Δ9-tetrahydrocannabinol (Δ9-THC), the major psychoactive phytocannabinoid in cannabis, is not traditionally thought to interact with the brain respiratory system, based largely upon sparse labeling of CB1 receptors in the medulla and relative safety suggested by widespread human use. Here we used whole body plethysmography and RNAscope in situ hybridization in mice to reconcile this conflict between conventional wisdom and human data. We examined the respiratory effects of the synthetic CB1 full agonist CP55,940 and Δ9-THC in male and female mice. CP55,940 and Δ9-THC potently and dose-dependently suppressed minute ventilation and tidal volume, decreasing measures of respiratory effort (i.e., peak inspiratory and expiratory flow). Both cannabinoids reduced respiratory frequency, decreasing inspiratory and expiratory time while markedly increasing inspiratory and expiratory pause. Respiratory suppressive effects were fully blocked by the CB1 antagonist AM251, were minimally impacted by the peripherally-restricted CB1 antagonist AM6545, and occurred at doses lower than those that produce cardinal behavioral signs of CB1 activation. Using RNAscope in situ hybridization, we also demonstrated extensive coexpression of Cnr1 (encoding the CB1 receptor) and Oprm1 (encoding the µ-opioid receptor) mRNA in respiratory cells in the medullary pre-Bötzinger complex, a critical nucleus of respiratory control. Our results show that mRNA for CB1 is present in respiratory cells in a medullary brain region essential for breathing and demonstrate that cannabinoids produce respiratory suppression via activation of central CB1 receptors.

Multiple Neural Networks Originating from the Lateral Parabrachial Nucleus Modulate Cough-like Behavior and Coordinate Cough with Pain

It has been reported that experimental pain can diminish cough sensitivity and that the lateral parabrachial nucleus (LPBN) coordinates pain with breathing, but whether the LPBN regulates cough-like behaviors and pain-induced changes in cough sensitivity remains elusive. We investigated the roles of LPBN γ-aminobutyric acidergic (GABAergic) and glutamatergic neurons in the regulation of cough sensitivity and its relationship with pain in mice via chemogenetic approaches. Adenovirus-associated virus tracing combined with chemogenetics was used to map the projections of LPBN GABAergic and glutamatergic neurons to the periaqueductal gray. LPBN neurons were activated by cough challenge, and nonspecific inhibition of LPBN neurons suppressed cough-like behavior. Chemogenetic suppression of LPBN GABAergic neurons reduced cough sensitivity in mice, whereas suppression of LPBN glutamatergic neurons counteracted the pain-driven decrease in cough sensitivity, and so did silencing LPBN glutamatergic neurons projecting to the periaqueductal gray. Our data suggest that GABAergic and glutamatergic neurons in the LPBN critically are involved in cough sensitivity and coordinate pain with cough through inhibitory or activating mechanisms at the midbrain level.

Muscles and Central Neural Networks Involved in Breathing: State of the Art

Breathing is a systemic act, which involves not only the lungs, but the entire body system. To have a comprehensive clinical picture, it is necessary to have all the patient's data; from this assumption, we can affirm that it is necessary to know all the muscles involved in breathing to understand how to obtain a comprehensive approach for the care and treatment of the patient to improve respiratory capacity. The text reviews the efferent connections of the respiratory centers and cites all the muscles that are involved in the mechanism of breathing and that are controlled and managed by the respiratory centers, starting from the muscular description of the cranial area, the bucco-cervical area, the cervicothoracic area, and the thoracic area. Knowing the function of the respiratory accessory muscles allows us to obtain, in some clinical cases, valuable data that can prove predictive of the diagnostic path of the pathology. This is the first article in the literature, to the authors' knowledge, that attempts to list and include in a single text all the muscles directly or indirectly involved in breathing. The goal of this narrative review article is to remind clinicians and researchers involved in the study of different muscular respiratory responses that we need to analyze and work all the skeletal musculature involved in breathing to better understand what happens in the pathological or physiological phases during breathing. This step will allow us to better individualize the therapeutic and training approach for healthy subjects.

Pharmacological modulation of respiratory control: Ampakines as a therapeutic strategy

Ampakines are a class of compounds that are positive allosteric modulators of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors and enhance glutamatergic neurotransmission. Glutamatergic synaptic transmission and AMPA receptor activation are fundamentally important to the genesis and propagation of the neural impulses driving breathing, including respiratory motoneuron depolarization. Ampakines therefore have the potential to modulate the neural control of breathing. In this paper, we describe the influence of ampakines on respiratory motor output in health and disease. We dissect the molecular mechanisms underlying ampakine action, delineate the diverse targets of ampakines along the respiratory neuraxis, survey the spectrum of respiratory disorders in which ampakines have been tested, and culminate with an examination of how ampakines modulate respiratory function after spinal cord injury. Collectively, the studies reviewed here indicate that ampakines may be a useful adjunctive strategy to pair with conventional respiratory rehabilitation approaches in conditions with impaired neural activation of the respiratory muscles.

Role of the pontine respiratory group in the suppression of cough by codeine in cats

Codeine was microinjected into the area of the Kölliker-Fuse nucleus and the adjacent lateral parabrachial nucleus, within the pontine respiratory group in 8 anesthetized cats. Electromyograms (EMGs) of the diaphragm (DIA) and abdominal muscles (ABD), esophageal pressures (EP), and blood pressure were recorded and analyzed during mechanically induced tracheobronchial cough. Unilateral microinjections of 3.3 mM codeine (3 injections, each 37 ± 1.2 nl) had no significant effect on the cough number. However, the amplitudes of the cough ABD EMG, expiratory EP and, to a lesser extent, DIA EMG were significantly reduced. There were no significant changes in the temporal parameters of the cough. Control microinjections of artificial cerebrospinal fluid in 6 cats did not show a significant effect on cough data compared to those after codeine microinjections. Codeine-sensitive neurons in the rostral dorsolateral pons contribute to controlling cough motor output, likely through the central pattern generator of cough.

Sodium Leak Channel in Glutamatergic Neurons of the Lateral Parabrachial Nucleus Helps to Maintain Respiratory Frequency Under Sevoflurane Anesthesia

The lateral parabrachial nucleus (PBL) is implicated in the regulation of respiratory activity. Sodium leak channel (NALCN) mutations disrupt the respiratory rhythm and influence anesthetic sensitivity in both rodents and humans. Here, we investigated whether the NALCN in PBL glutamatergic neurons maintains respiratory function under general anesthesia. Our results showed that chemogenetic activation of PBL glutamatergic neurons increased the respiratory frequency (RF) in mice; whereas chemogenetic inhibition suppressed RF. NALCN knockdown in PBL glutamatergic neurons but not GABAergic neurons significantly reduced RF under physiological conditions and caused more respiratory suppression under sevoflurane anesthesia. NALCN knockdown in PBL glutamatergic neurons did not further exacerbate the respiratory suppression induced by propofol or morphine. Under sevoflurane anesthesia, painful stimuli rapidly increased the RF, which was not affected by NALCN knockdown in PBL glutamatergic neurons. This study suggested that the NALCN is a key ion channel in PBL glutamatergic neurons that maintains respiratory frequency under volatile anesthetic sevoflurane but not intravenous anesthetic propofol.

Advances in attenuating opioid-induced respiratory depression: A narrative review

Opioids exert analgesic effects by agonizing opioid receptors and activating signaling pathways coupled to receptors such as G-protein and/or β-arrestin. Concomitant respiratory depression (RD) is a common clinical problem, and improvement of RD is usually achieved with specific antagonists such as naloxone; however, naloxone antagonizes opioid analgesia and may produce more unknown adverse effects. In recent years, researchers have used various methods to isolate opioid receptor-mediated analgesia and RD, with the aim of preserving opioid analgesia while attenuating RD. At present, the focus is mainly on the development of new opioids with weak respiratory inhibition or the use of non-opioid drugs to stimulate breathing. This review reports recent advances in novel opioid agents, such as mixed opioid receptor agonists, peripheral selective opioid receptor agonists, opioid receptor splice variant agonists, biased opioid receptor agonists, and allosteric modulators of opioid receptors, as well as in non-opioid agents, such as AMPA receptor modulators, 5-hydroxytryptamine receptor agonists, phosphodiesterase-4 inhibitors, and nicotinic acetylcholine receptor agonists.

Anatomical distribution of µ-opioid receptors, neurokinin-1 receptors, and vesicular glutamate transporter 2 in the mouse brainstem respiratory network

µ-Opioid receptors (MORs) are responsible for mediating both the analgesic and respiratory effects of opioid drugs. By binding to MORs in brainstem regions involved in controlling breathing, opioids produce respiratory depressive effects characterized by slow and shallow breathing, with potential cardiorespiratory arrest and death during overdose. To better understand the mechanisms underlying opioid-induced respiratory depression, thorough knowledge of the regions and cellular subpopulations that may be vulnerable to modulation by opioid drugs is needed. Using in situ hybridization, we determined the distribution and coexpression of Oprm1 (gene encoding MORs) mRNA with glutamatergic (Vglut2) and neurokinin-1 receptor (Tacr1) mRNA in medullary and pontine regions involved in breathing control and modulation. We found that >50% of cells expressed Oprm1 mRNA in the preBötzinger complex (preBötC), nucleus tractus solitarius (NTS), nucleus ambiguus (NA), postinspiratory complex (PiCo), locus coeruleus (LC), Kölliker-Fuse nucleus (KF), and the lateral and medial parabrachial nuclei (LBPN and MPBN, respectively). Among Tacr1 mRNA-expressing cells, >50% coexpressed Oprm1 mRNA in the preBötC, NTS, NA, Bötzinger complex (BötC), PiCo, LC, raphe magnus nucleus, KF, LPBN, and MPBN, whereas among Vglut2 mRNA-expressing cells, >50% coexpressed Oprm1 mRNA in the preBötC, NTS, NA, BötC, PiCo, LC, KF, LPBN, and MPBN. Taken together, our study provides a comprehensive map of the distribution and coexpression of Oprm1, Tacr1, and Vglut2 mRNA in brainstem regions that control and modulate breathing and identifies Tacr1 and Vglut2 mRNA-expressing cells as subpopulations with potential vulnerability to modulation by opioid drugs.NEW & NOTEWORTHY Opioid drugs can cause serious respiratory side-effects by binding to µ-opioid receptors (MORs) in brainstem regions that control breathing. To better understand the regions and their cellular subpopulations that may be vulnerable to modulation by opioids, we provide a comprehensive map of Oprm1 (gene encoding MORs) mRNA expression throughout brainstem regions that control and modulate breathing. Notably, we identify glutamatergic and neurokinin-1 receptor-expressing cells as potentially vulnerable to modulation by opioid drugs and worthy of further investigation using targeted approaches.

Evaluating the Risk Index for Serious Prescription Opioid-Induced Respiratory Depression or Overdose in Patients with Cancer

The Commercially Insured health Plan Risk Index for Overdose or Serious Opioid-induced Respiratory Depression (CIP-RIOSORD) is an evidence-based tool to determine serious opioid-induced respiratory depression (OIRD) or overdose risk. The CIP-RIOSORD total score determines a risk class and estimates the probability for an OIRD event within the next 6 months. We performed a single-center, retrospective analysis to determine CIP-RIOSORD baseline scores and the most common predictive factors in patients with cancer. Patients (n = 160) were split into new consultations (n = 83, Group 1) versus the first documented follow-up consultation (n = 77, Group 2). Most patients were Caucasian women with metastatic gastrointestinal cancer. CIP-RIOSORD scores for Group 1 patients were 14.8 ± 15.2 (mean ± SD, risk class 4). Group 2 patients had higher CIP-RIOSORD scores (16.6 ± 14.9, risk class 4). For Group 1, the most common CIP-RIOSORD predictive factors were use of a long-acting opioid formulation (n = 24, 29%) and daily oral morphine equivalent (OME) ≥100 (n = 20, 24%); for Group 2, predictive factors were use of an antidepressant (n = 34, 44%) and a long-acting opioid formulation (n = 27, 35%). Based on the CIP-RIOSORD, there is a 15% probability of experiencing a serious OIRD event or overdose within the next 6 months.

CB 1 Cannabinoid Receptor Agonists Induce Acute Respiratory Depression in Awake Mice

Recreational use of synthetic cannabinoid agonists (i.e., "Spice" compounds) that target the Cannabinoid Type 1 receptor (CB 1 ) can cause respiratory depression in humans. However, Δ 9 -tetrahydrocannabinol (THC), the major psychoactive phytocannabinoid in cannabis, is not traditionally thought to interact with CNS control of respiration, based largely upon sparse labeling of CB1 receptors in the medulla and few reports of clinically significant respiratory depression following cannabis overdose. The respiratory effects of CB 1 agonists have rarely been studied in vivo , suggesting that additional inquiry is required to reconcile the conflict between conventional wisdom and human data. Here we used whole body plethysmography to examine the respiratory effects of the synthetic high efficacy CB 1 agonist CP55,940, and the low efficacy CB 1 agonist Δ 9 -tetrahydrocannabinol in male and female mice. CP55,940 and THC, administered systemically, both robustly suppressed minute ventilation. Both cannabinoids also produced sizable reductions in tidal volume, decreasing both peak inspiratory and expiratory flow - measures of respiratory effort. Similarly, both drugs reduced respiratory frequency, decreasing both inspiratory and expiratory time while markedly increasing expiratory pause, and to a lesser extent, inspiratory pause. Respiratory suppressive effects occurred at lower doses in females than in males, and at many of the same doses shown to produce cardinal behavioral signs of CB 1 activation. We next used RNAscope in situ hybridization to localize CB 1 mRNA to glutamatergic neurons in the medullary pre-Bötzinger Complex, a critical nucleus in controlling respiration. Our results show that, contrary to previous conventional wisdom, CB 1 mRNA is expressed in glutamatergic neurons in a brain region essential for breathing and CB 1 agonists can cause significant respiratory depression.

Influence of Brainstem's Area A5 on Sympathetic Outflow and Cardiorespiratory Dynamics

Area A5 is a noradrenergic cell group in the brain stem characterised by its important role in triggering sympathetic activity, exerting a profound influence on the sympathetic outflow, which is instrumental in the modulation of cardiovascular functions, stress responses and various other physiological processes that are crucial for adaptation and survival mechanisms. Understanding the role of area A5, therefore, not only provides insights into the basic functioning of the sympathetic nervous system but also sheds light on the neuronal basis of a number of autonomic responses. In this review, we look deeper into the specifics of area A5, exploring its anatomical connections, its neurochemical properties and the mechanisms by which it influences sympathetic nervous system activity and cardiorespiratory regulation and, thus, contributes to the overall dynamics of the autonomic function in regulating body homeostasis.

Central Autonomic Mechanisms Involved in the Control of Laryngeal Activity and Vocalization

In humans, speech is a complex process that requires the coordinated involvement of various components of the phonatory system, which are monitored by the central nervous system. The larynx in particular plays a crucial role, as it enables the vocal folds to meet and converts the exhaled air from our lungs into audible sounds. Voice production requires precise and sustained exhalation, which generates an air pressure/flow that creates the pressure in the glottis required for voice production. Voluntary vocal production begins in the laryngeal motor cortex (LMC), a structure found in all mammals, although the specific location in the cortex varies in humans. The LMC interfaces with various structures of the central autonomic network associated with cardiorespiratory regulation to allow the perfect coordination between breathing and vocalization. The main subcortical structure involved in this relationship is the mesencephalic periaqueductal grey matter (PAG). The PAG is the perfect link to the autonomic pontomedullary structures such as the parabrachial complex (PBc), the Kölliker-Fuse nucleus (KF), the nucleus tractus solitarius (NTS), and the nucleus retroambiguus (nRA), which modulate cardiovascular autonomic function activity in the vasomotor centers and respiratory activity at the level of the generators of the laryngeal-respiratory motor patterns that are essential for vocalization. These cores of autonomic structures are not only involved in the generation and modulation of cardiorespiratory responses to various stressors but also help to shape the cardiorespiratory motor patterns that are important for vocal production. Clinical studies show increased activity in the central circuits responsible for vocalization in certain speech disorders, such as spasmodic dysphonia because of laryngeal dystonia.

Interaction between Kölliker-Fuse/A7 and the parafacial respiratory region on the control of respiratory regulation

Respiration is regulated by various types of neurons located in the pontine-medullary regions. The Kölliker-Fuse (KF)/A7 noradrenergic neurons play a role in modulating the inspiratory cycle by influencing the respiratory output. These neurons are interconnected and may also project to brainstem and spinal cord, potentially involved in regulating the post-inspiratory phase. In the present study, we hypothesize that the parafacial (pF) neurons, in conjunction with adrenergic mechanisms originating from the KF/A7 region, may provide the neurophysiological basis for breathing modulation. We conducted experiments using urethane-anesthetized, vagotomized, and artificially ventilated male Wistar rats. Injection of L-glutamate into the KF/A7 region resulted in inhibition of inspiratory activity, and a prolonged and high-amplitude genioglossal activity (GGEMG). Blockade of the α1 adrenergic receptors (α1-AR) or the ionotropic glutamatergic receptors in the pF region decrease the activity of the GGEMG without affecting inspiratory cessation. In contrast, blockade of α2-AR in the pF region extended the duration of GG activity. Notably, the inspiratory and GGEMG activities induced by KF/A7 stimulation were completely blocked by bilateral blockade of glutamatergic receptors in the Bötzinger complex (BötC). While our study found a limited role for α1 and α2 adrenergic receptors at the pF level in modulating the breathing response to KF/A7 stimulation, it became evident that BötC neurons are responsible for the respiratory effects induced by KF/A7 stimulation.

Changes in pontine and preBötzinger/Bötzinger complex neuronal activity during remifentanil-induced respiratory depression in decerebrate dogs

Introduction: In vivo studies using selective, localized opioid antagonist injections or localized opioid receptor deletion have identified that systemic opioids dose-dependently depress respiratory output through effects in multiple respiratory-related brainstem areas. Methods: With approval of the subcommittee on animal studies of the Zablocki VA Medical Center, experiments were performed in 53 decerebrate, vagotomized, mechanically ventilated dogs of either sex during isocapnic hyperoxia. We performed single neuron recordings in the Pontine Respiratory Group (PRG, n = 432) and preBötzinger/Bötzinger complex region (preBötC/BötC, n = 213) before and during intravenous remifentanil infusion (0.1-1 mcg/kg/min) and then until complete recovery of phrenic nerve activity. A generalized linear mixed model was used to determine changes in Fn with remifentanil and the statistical association between remifentanil-induced changes in Fn and changes in inspiratory and expiratory duration and peak phrenic activity. Analysis was controlled via random effects for animal, run, and neuron type. Results: Remifentanil decreased Fn in most neuron subtypes in the preBötC/BötC as well as in inspiratory (I), inspiratory-expiratory, expiratory (E) decrementing and non-respiratory modulated neurons in the PRG. The decrease in PRG inspiratory and non-respiratory modulated neuronal activity was associated with an increase in inspiratory duration. In the preBötC, the decrease in I-decrementing neuron activity was associated with an increase in expiratory and of E-decrementing activity with an increase in inspiratory duration. In contrast, decreased activity of I-augmenting neurons was associated with a decrease in inspiratory duration. Discussion: While statistical associations do not necessarily imply a causal relationship, our data suggest mechanisms for the opioid-induced increase in expiratory duration in the PRG and preBötC/BötC and how inspiratory failure at high opioid doses may result from a decrease in activity and decrease in slope of the pre-inspiratory ramp-like activity in preBötC/BötC pre-inspiratory neurons combined with a depression of preBötC/BötC I-augmenting neurons. Additional studies must clarify whether the observed changes in neuronal activity are due to direct neuronal inhibition or decreased excitatory inputs.

Understanding and countering opioid-induced respiratory depression

Respiratory depression is the proximal cause of death in opioid overdose, yet the mechanisms underlying this potentially fatal outcome are not well understood. The goal of this review is to provide a comprehensive understanding of the pharmacological mechanisms of opioid-induced respiratory depression, which could lead to improved therapeutic options to counter opioid overdose, as well as other detrimental effects of opioids on breathing. The development of tolerance in the respiratory system is also discussed, as are differences in the degree of respiratory depression caused by various opioid agonists. Finally, potential future therapeutic agents aimed at reversing or avoiding opioid-induced respiratory depression through non-opioid receptor targets are in development and could provide certain advantages over naloxone. By providing an overview of mechanisms and effects of opioids in the respiratory network, this review will benefit future research on countering opioid-induced respiratory depression. LINKED ARTICLES: This article is part of a themed issue on Advances in Opioid Pharmacology at the Time of the Opioid Epidemic. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v180.7/issuetoc.

The integrated brain network that controls respiration

Respiration is a brain function on which our lives essentially depend. Control of respiration ensures that the frequency and depth of breathing adapt continuously to metabolic needs. In addition, the respiratory control network of the brain has to organize muscular synergies that integrate ventilation with posture and body movement. Finally, respiration is coupled to cardiovascular function and emotion. Here, we argue that the brain can handle this all by integrating a brainstem central pattern generator circuit in a larger network that also comprises the cerebellum. Although currently not generally recognized as a respiratory control center, the cerebellum is well known for its coordinating and modulating role in motor behavior, as well as for its role in the autonomic nervous system. In this review, we discuss the role of brain regions involved in the control of respiration, and their anatomical and functional interactions. We discuss how sensory feedback can result in adaptation of respiration, and how these mechanisms can be compromised by various neurological and psychological disorders. Finally, we demonstrate how the respiratory pattern generators are part of a larger and integrated network of respiratory brain regions.

Fentanyl effects on respiratory neuron activity in the dorsolateral pons

Opioids suppress breathing through actions in the brainstem, including respiratory-related areas of the dorsolateral pons, which contain multiple phenotypes of respiratory patterned neurons. The discharge identity of dorsolateral pontine neurons that are impacted by opioids is unknown. To address this, single neuronal units were recorded in the dorsolateral pons of arterially perfused in situ rat preparations that were perfused with an apneic concentration of the opioid agonist fentanyl, followed by the opioid antagonist naloxone (NLX). Dorsolateral pontine neurons were categorized based on respiratory-associated discharge patterns, which were differentially affected by fentanyl. Inspiratory neurons and a subset of inspiratory/expiratory phase-spanning neurons were either silenced or had reduced firing frequency during fentanyl-induced apnea, which was reversed upon administration of naloxone. In contrast, the majority of expiratory neurons continued to fire tonically during fentanyl-induced apnea, albeit with reduced firing frequency. In addition, pontine late-inspiratory and postinspiratory neuronal activity were absent from apneustic-like breaths during the transition to fentanyl-induced apnea and the naloxone-mediated transition to recovery. Thus, opioid-induced deficits in respiratory patterning may occur due to reduced activity of pontine inspiratory neurons, whereas apnea occurs with loss of all phasic pontine activity and sustained tonic expiratory neuron activity.NEW & NOTEWORTHY Opioids can suppress breathing via actions throughout the brainstem, including the dorsolateral pons. The respiratory phenotype of dorsolateral pontine neurons inhibited by opioids is unknown. Here, we describe the effect of the highly potent opioid fentanyl on the firing activity of these dorsolateral pontine neurons. Inspiratory neurons were largely silenced by fentanyl, whereas expiratory neurons were not. We provide a framework whereby this differential sensitivity to fentanyl can contribute to respiratory pattern deficits and apnea.

Current research in pathophysiology of opioid-induced respiratory depression, neonatal opioid withdrawal syndrome, and neonatal antidepressant exposure syndrome

Respiratory depression (RD) is the primary cause of death due to opioids. Opioids bind to mu (µ)-opioid receptors (MORs) encoded by the MOR gene Oprm1, widely expressed in the central and peripheral nervous systems including centers that modulate breathing. Respiratory centers are located throughout the brainstem. Experiments with Oprm1-deleted knockout (KO) mice undertaken to determine which sites are necessary for the induction of opioid-induced respiratory depression (OIRD) showed that the pre-Bötzinger complex (preBötC) and the pontine Kölliker-Fuse nucleus (KF) contribute equally to OIRD but RD was not totally eliminated. Morphine showed a differential influence on preBötC and KF neurons - low doses attenuated RD following deletion of MORs from preBötC neurons and an increase in apneas after high doses whereas deletion of MORs from KF neurons but not the preBötC attenuated RD at both high and low doses. In other KO mice studies, morphine administration after deletion of Oprm1 from both the preBötC and the KF/PBN neurons, led to the conclusion that both respiratory centres contribute to OIRD but the preBötC predominates. MOR-mediated post-synaptic activation of GIRK potassium channels has been implicated as a cause of OIRD. A complementary mechanism in the preBötC involving KCNQ potassium channels independent of MOR signaling has been described. Recent experiments in rats showing that morphine depresses normal, but not gasping breathing, cast doubt on the belief that eupnea, sighs, and gasps, are under the control of preBötC neurons. Methadone, administered to alleviate symptoms of neonatal opioid withdrawal syndrome (NOWES), desensitized rats to OIRD. Protection lost between postnatal days 1 and 2 coincides with the preBötC becoming the dominant generator of respiratory rhythm. Neonatal antidepressant exposure syndrome (NADES) and serotonin toxicity (ST) show similarities including RD. Enzyme CYP2D6 involved in opioid detoxification is polymorphic. Individuals of different CYP2D6 genotype may show increased, decreased, or no enzyme activity, contributing to the variability of patient responses to different opioids and OIRD.

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.

Taking a deep breath: How a brainstem pathway integrates pain and breathing

In this issue of Neuron, Liu et al. (2022) shed light on the neural circuits supporting pain- and anxiety-induced elevated breathing rhythms. They reveal PBL core-Oprm1 neurons projecting onto the CeA and shell-Oprm1 neurons projecting onto the preBötC as differential regulators of these behaviors.

Divergent brainstem opioidergic pathways that coordinate breathing with pain and emotions

Breathing can be heavily influenced by pain or internal emotional states, but the neural circuitry underlying this tight coordination is unknown. Here we report that Oprm1 (μ-opioid receptor)-expressing neurons in the lateral parabrachial nucleus (PBL) are crucial for coordinating breathing with affective pain in mice. Individual PBL^Oprm1 neuronal activity synchronizes with breathing rhythm and responds to noxious stimuli. Manipulating PBL^Oprm1 activity directly changes breathing rate, affective pain perception, and anxiety. Furthermore, PBL^Oprm1 neurons constitute two distinct subpopulations in a "core-shell" configuration that divergently projects to the forebrain and hindbrain. Through non-overlapping projections to the central amygdala and pre-Bötzinger complex, these two subpopulations differentially regulate breathing, affective pain, and negative emotions. Moreover, these subsets form recurrent excitatory networks through reciprocal glutamatergic projections. Together, our data define the divergent parabrachial opioidergic circuits as a common neural substrate that coordinates breathing with various sensations and behaviors such as pain and emotional processing.

Contribution of the caudal medullary raphe to opioid induced respiratory depression

BACKGROUND

Opioid-induced respiratory depression can be partially antagonized in the preBötzinger Complex and Parabrachial Nucleus/Kölliker-Fuse Complex. We hypothesized that additional opioid antagonism in the caudal medullary raphe completely reverses the opioid effect.

METHODS

In adult ventilated, vagotomized, decerebrate rabbits, we administrated remifentanil intravenously at "analgesic", "apneic", and "very high" doses and determined the reversal with sequential naloxone microinjections into the bilateral Parabrachial Nucleus/Kölliker-Fuse Complex, preBötzinger Complex, and caudal medullary raphe. In separate animals, we injected opioid antagonists into the raphe without intravenous remifentanil.

RESULTS

Sequential naloxone microinjections completely reversed respiratory rate depression from "analgesic" and "apneic" remifentanil, but not "very high" remifentanil concentrations. Antagonist injection into the caudal medullary raphe without remifentanil independently increased respiratory rate.

CONCLUSIONS

Opioid-induced respiratory depression results from a combined effect on the respiratory rhythm generator and respiratory drive. The effect in the caudal medullary raphe is complex as we also observed local antagonism of endogenous opioid receptor activation, which has not been described before.

Multi-Level Regulation of Opioid-Induced Respiratory Depression

Opioids depress minute ventilation primarily by reducing respiratory rate. This results from direct effects on the preBötzinger Complex as well as from depression of the Parabrachial/Kölliker-Fuse Complex, which provides excitatory drive to preBötzinger Complex neurons mediating respiratory phase-switch. Opioids also depress awake drive from the forebrain and chemodrive.

Dose-dependent Respiratory Depression by Remifentanil in the Rabbit Parabrachial Nucleus/Kölliker-Fuse Complex and Pre-Bötzinger Complex

BACKGROUND

Recent studies showed partial reversal of opioid-induced respiratory depression in the pre-Bötzinger complex and the parabrachial nucleus/Kölliker-Fuse complex. The hypothesis for this study was that opioid antagonism in the parabrachial nucleus/Kölliker-Fuse complex plus pre-Bötzinger complex completely reverses respiratory depression from clinically relevant opioid concentrations.

METHODS

Experiments were performed in 48 adult, artificially ventilated, decerebrate rabbits. The authors decreased baseline respiratory rate ~50% with intravenous, "analgesic" remifentanil infusion or produced apnea with remifentanil boluses and investigated the reversal with naloxone microinjections (1 mM, 700 nl) into the Kölliker-Fuse nucleus, parabrachial nucleus, and pre-Bötzinger complex. In another group of animals, naloxone was injected only into the pre-Bötzinger complex to determine whether prior parabrachial nucleus/Kölliker-Fuse complex injection impacted the naloxone effect. Last, the µ-opioid receptor agonist [d-Ala,2N-MePhe,4Gly-ol]-enkephalin (100 μM, 700 nl) was injected into the parabrachial nucleus/Kölliker-Fuse complex. The data are presented as medians (25 to 75%).

RESULTS

Remifentanil infusion reduced the respiratory rate from 36 (31 to 40) to 16 (15 to 21) breaths/min. Naloxone microinjections into the bilateral Kölliker-Fuse nucleus, parabrachial nucleus, and pre-Bötzinger complex increased the rate to 17 (16 to 22, n = 19, P = 0.005), 23 (19 to 29, n = 19, P < 0.001), and 25 (22 to 28) breaths/min (n = 11, P < 0.001), respectively. Naloxone injection into the parabrachial nucleus/Kölliker-Fuse complex prevented apnea in 12 of 17 animals, increasing the respiratory rate to 10 (0 to 12) breaths/min (P < 0.001); subsequent pre-Bötzinger complex injection prevented apnea in all animals (13 [10 to 19] breaths/min, n = 12, P = 0.002). Naloxone injection into the pre-Bötzinger complex alone increased the respiratory rate to 21 (15 to 26) breaths/min during analgesic concentrations (n = 10, P = 0.008) but not during apnea (0 [0 to 0] breaths/min, n = 9, P = 0.500). [d-Ala,2N-MePhe,4Gly-ol]-enkephalin injection into the parabrachial nucleus/Kölliker-Fuse complex decreased respiratory rate to 3 (2 to 6) breaths/min.

CONCLUSIONS

Opioid reversal in the parabrachial nucleus/Kölliker-Fuse complex plus pre-Bötzinger complex only partially reversed respiratory depression from analgesic and even less from "apneic" opioid doses. The lack of recovery pointed to opioid-induced depression of respiratory drive that determines the activity of these areas.

EDITOR’S PERSPECTIVE

Interaction between the pulmonary stretch receptor and pontine control of expiratory duration

Medial parabrachial nucleus (mPBN) neuronal activity plays a key role in controlling expiratory (E)-duration (TE). Pulmonary stretch receptor (PSR) activity during the E-phase prolongs TE. The aims of this study were to characterize the interaction between the PSR and mPBN control of TE and underlying mechanisms. Decerebrated mechanically ventilated dogs were studied. The mPBN subregion was activated by electrical stimulation via bipolar microelectrode. PSR afferents were activated by low-level currents applied to the transected central vagus nerve. Both stimulus-frequency patterns during the E-phase were synchronized to the phrenic neurogram; TE was measured. A functional mathematical model for the control of TE and extracellular recordings from neurons in the preBötzinger/Bötzinger complex (preBC/BC) were used to understand mechanisms. Findings show that the mPBN gain-modulates, via attenuation, the PSR-mediated reflex. The model suggested functional sites for attenuation and neuronal data suggested correlates. The PSR- and PB-inputs appear to interact on E-decrementing neurons, which synaptically inhibit pre-I neurons, delaying the onset of the next I-phase.

Neural basis of opioid-induced respiratory depression and its rescue

Opioid-induced respiratory depression (OIRD) causes death following an opioid overdose, yet the neurobiological mechanisms of this process are not well understood. Here, we show that neurons within the lateral parabrachial nucleus that express the µ-opioid receptor (PBL Oprm1 neurons) are involved in OIRD pathogenesis. PBL Oprm1 neuronal activity is tightly correlated with respiratory rate, and this correlation is abolished following morphine injection. Chemogenetic inactivation of PBL Oprm1 neurons mimics OIRD in mice, whereas their chemogenetic activation following morphine injection rescues respiratory rhythms to baseline levels. We identified several excitatory G protein-coupled receptors expressed by PBL Oprm1 neurons and show that agonists for these receptors restore breathing rates in mice experiencing OIRD. Thus, PBL Oprm1 neurons are critical for OIRD pathogenesis, providing a promising therapeutic target for treating OIRD in patients.

Enhanced Synaptic Transmission in the Extended Amygdala and Altered Excitability in an Extended Amygdala to Brainstem Circuit in a Dravet Syndrome Mouse Model

Dravet syndrome (DS) is a developmental and epileptic encephalopathy with an increased incidence of sudden death. Evidence of interictal breathing deficits in DS suggests that alterations in subcortical projections to brainstem nuclei may exist, which might be driving comorbidities in DS. The aim of this study was to determine whether a subcortical structure, the bed nucleus of the stria terminalis (BNST) in the extended amygdala, is activated by seizures, exhibits changes in excitability, and expresses any alterations in neurons projecting to a brainstem nucleus associated with respiration, stress response, and homeostasis. Experiments were conducted using F1 mice generated by breeding 129.Scn1a+/- mice with wild-type C57BL/6J mice. Immunohistochemistry was performed to quantify neuronal c-fos activation in DS mice after observed spontaneous seizures. Whole-cell patch-clamp and current-clamp electrophysiology recordings were conducted to evaluate changes in intrinsic and synaptic excitability in the BNST. Spontaneous seizures in DS mice significantly enhanced neuronal c-fos expression in the BNST. Further, the BNST had altered AMPA/NMDA postsynaptic receptor composition and showed changes in spontaneous neurotransmission, with greater excitation and decreased inhibition. BNST to parabrachial nucleus (PBN) projection neurons exhibited intrinsic excitability in wild-type mice, while these projection neurons were hypoexcitable in DS mice. The findings suggest that there is altered excitability in neurons of the BNST, including BNST-to-PBN projection neurons, in DS mice. These alterations could potentially be driving comorbid aspects of DS outside of seizures, including respiratory dysfunction and sudden death.

Neuronal mechanisms underlying opioid-induced respiratory depression: our current understanding

Opioid-induced respiratory depression (OIRD) represents the primary cause of death associated with therapeutic and recreational opioid use. Within the United States, the rate of death from opioid abuse since the early 1990s has grown disproportionally, prompting the classification as a nationwide "epidemic." Since this time, we have begun to unravel many fundamental cellular and systems-level mechanisms associated with opioid-related death. However, factors such as individual vulnerability, neuromodulatory compensation, and redundancy of opioid effects across central and peripheral nervous systems have created a barrier to a concise, integrative view of OIRD. Within this review, we bring together multiple perspectives in the field of OIRD to create an overarching viewpoint of what we know, and where we view this essential topic of research going forward into the future.

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.

Neurochemistry of the Kölliker-Fuse nucleus from a respiratory perspective

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