Airway Chemosensitive Receptors in Vagus Nerve Perform Neuro-Immune Interaction for Lung-Brain Communication

Research journey into multiple-sensor theory

In 1998, I was asked by the American Physiological Society to review a book written by Dr. Michael de Burgh Daly, Peripheral Arterial Chemoreceptors and Respiratory-Cardiovascular Integration. Inspired by this work, I came to appreciate how researchers in the later stages of their careers and who provide a detailed review of their experimental approach might effectively contribute to science, especially to the benefit of young scientists (Yu J. The Physiologist 41: 231, 1998.). This article is written in that vein. Over several decades of intensive investigation of cardiopulmonary reflexes, focused on the sensory receptors, my colleagues and I advanced a novel multiple-sensor theory (MST) to explain the role of the vagal mechanosensory system. Described here is our research journey through various stages of developing MST and the process of how the problem was identified, approached, and tackled. MST redefines conventional mechanosensor doctrines and is supported by new studies that clarify a century of research data. It entails reinterpretation of many established findings. Hopefully, this article will benefit young scientists, such as graduate and postdoctoral students in the cardiopulmonary sensory research field.

Regulation of breathing by cardiopulmonary afferents

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

Lung-brain axis

The appreciation of human microbiome is gaining strong grounds in biomedical research. In addition to gut-brain axis, is the lung-brain axis, which is hypothesised to link pulmonary microbes to neurodegenerative disorders and behavioural changes. There is a need for analysis based on emerging studies to map out the prospects for lung-brain axis. In this review, relevant English literature and researches in the field of 'lung-brain axis' is reported. We recommend all the highlighted prospective studies to be integrated with an interdisciplinary approach. This might require conceptual research approaches based on physiology and pathophysiology. Multimodal aspects should include experimental animal units, while exploring the research gaps and making reference to the already existing human data. The overall microbiome medicine is gaining more ground. Aetiological paths and experimental recommendations as per prospective studies in this review will be an important guideline to develop effective treatments for any lung induced neurodegenerative diseases. An in-depth knowledge of the bi-directional communication between host and microbiome in the lung could help treatment to respiratory infections, alleviate stress, anxiety and enhanced neurological effects. The timely prevention and treatment of neurodegenerative diseases requires paradigm shift of the aetiology and more innovative experimentation.Impact statementThe overall microbiome medicine is gaining more ground. An in-depth knowledge of the bi-directional communication between host and microbiome in the lung could confer treatment to respiratory infections, alleviate stress, anxiety and enhanced neurological effects. Based on this review, we recommend all the highlighted prospective studies to be integrated and be given an interdisciplinary approach. This might require conceptual research approaches based on physiology and pathophysiology. Multimodal aspects should include experimental animal units; while exploring the research gaps and making reference to the already existing human data.

Cyclooxygenase and nitric oxide synthase pathways mediate the respiratory effects of TNF-α in rats

TNF-α is the key inflammatory cytokine. TNF-α receptors are expressed in brain stem regions involved in respiratory control and also in the carotid bodies, which are the sensory organs monitoring arterial blood O₂. We hypothesised that the circulating tumour necrosis factor (TNF)-α may affect the lung ventilation and modulate the hypoxic ventilatory response via activation of cyclooxygenase (COX) and nitric oxide synthase (NOS) pathways. The aim of the current study was to compare the respiratory effects of TNF-α before and after pretreatment with diclofenac or L-NG-nitro arginine methyl ester (L-NAME) nonspecific inhibitors of COX and NOS, respectively. The hypoxic ventilatory response was measured in anaesthetised rats using rebreathing techniques. We found that TNF-α increased the lung ventilation in normoxia but decreased the ventilatory response to hypoxia. Pretreatment with each of these inhibitors reduced respiratory effects of TNF-α. We believe that activation of COX and NOS-related pathways and also "cross-talk" between them mediates the TNF-α respiratory effects and underlies the impact of inflammation on the respiratory function.

Nicotinic Cholinergic System and COVID-19: In Silico Identification of an Interaction between SARS-CoV-2 and Nicotinic Receptors with Potential Therapeutic Targeting Implications

While SARS-CoV-2 uses angiotensin converting enzyme 2 (ACE2) as the receptor for cell entry, it is important to examine other potential interactions between the virus and other cell receptors. Based on the clinical observation of low prevalence of smoking among hospitalized COVID-19 patients, we examined and identified a “toxin-like” amino acid (aa) sequence in the Receptor Binding Domain of the Spike Glycoprotein of SARS-CoV-2 (aa 375–390), which is homologous to a sequence of the Neurotoxin homolog NL1, one of the many snake venom toxins that are known to interact with nicotinic acetylcholine receptors (nAChRs). We present the 3D structural location of this “toxin-like” sequence on the Spike Glycoprotein and the superposition of the modelled structure of the Neurotoxin homolog NL1 and the SARS-CoV-2 Spike Glycoprotein. We also performed computational molecular modelling and docking experiments using 3D structures of the SARS-CoV-2 Spike Glycoprotein and the extracellular domain of the nAChR α9 subunit. We identified a main interaction between the aa 381–386 of the SARS-CoV-2 Spike Glycoprotein and the aa 189–192 of the extracellular domain of the nAChR α9 subunit, a region which forms the core of the “toxin-binding site” of the nAChRs. The mode of interaction is very similar to the interaction between the α9 nAChR and α-bungarotoxin. A similar interaction was observed between the pentameric α7 AChR chimera and SARS-CoV-2 Spike Glycoprotein. The findings raise the possibility that SARS-CoV-2 may interact with nAChRs, supporting the hypothesis of dysregulation of the nicotinic cholinergic system being implicated in the pathophysiology of COVID-19. Nicotine and other nicotinic cholinergic agonists may protect nAChRs and thus have therapeutic value in COVID-19 patients.

Regulation of breathing pattern by IL-10

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

Sensory nerves in lung and airways

Sensory nerves innervating the lung and airways play an important role in regulating various cardiopulmonary functions and maintaining homeostasis under both healthy and disease conditions. Their activities conducted by both vagal and sympathetic afferents are also responsible for eliciting important defense reflexes that protect the lung and body from potential health-hazardous effects of airborne particulates and chemical irritants. This article reviews the morphology, transduction properties, reflex functions, and respiratory sensations of these receptors, focusing primarily on recent findings derived from using new technologies such as neural immunochemistry, isolated airway-nerve preparation, cultured airway neurons, patch-clamp electrophysiology, transgenic mice, and other cellular and molecular approaches. Studies of the signal transduction of mechanosensitive afferents have revealed a new concept of sensory unit and cellular mechanism of activation, and identified additional types of sensory receptors in the lung. Chemosensitive properties of these lung afferents are further characterized by the expression of specific ligand-gated ion channels on nerve terminals, ganglion origin, and responses to the action of various inflammatory cells, mediators, and cytokines during acute and chronic airway inflammation and injuries. Increasing interest and extensive investigations have been focused on uncovering the mechanisms underlying hypersensitivity of these airway afferents, and their role in the manifestation of various symptoms under pathophysiological conditions. Several important and challenging questions regarding these sensory nerves are discussed. Searching for these answers will be a critical step in developing the translational research and effective treatments of airway diseases.

Tumor necrosis factor-α plays an initiating role in extracorporeal circulation-induced acute lung injury

BACKGROUND

Despite advances in critical care, the mortality rate for patients with acute lung injury (ALI) remains high. The aim of this study was to test the hypothesis that tumor necrosis factor-α (TNF-α) plays an initiating role in the onset of extracorporeal circulation (ECC)-induced ALI.

METHODS

Eight New Zealand rabbits subjected to 1 h of ECC and 40 min of observation after termination of ECC were used for monitoring pulmonary nociceptor activity. Fifty Sprague-Dawley (SD) rats that received 2 h of ECC and 4 h of rest were used to measure the pulmonary function and inflammatory cytokines release, including total cells, neutrophils, and TNF-α in bronchoalveolar lavage (BAL) and white blood cell (WBC) and neutrophils in blood. An additional 40 SD rats were randomized to pretreatment with inhalation of phosphate buffer solution (control group), IgG (IgG inh group), or TNF-α antibody (anti-TNF-α inh group) and venous injection of TNF-α antibody (anti-TNF-α iv group). After 2 h of ECC and 4 h of rest, the arterial blood and BAL fluid were collected for measurement of arterial oxygen pressure (PaO2) and inflammatory cytokines release. The left-lower-lung tissues of animals were stained with hematoxylin & eosin (H&E).

RESULTS

The results demonstrated that the activities of airway nociceptor and TNF-α release were similarly upregulated at the early stage and in a time-related manner in ECC-induced ALI. Pretreatment with TNF-α antibody inhalation, but not venous injection, improved pulmonary function, inhibited pulmonary inflammation, and attenuated pulmonary histopathological changes after ECC.

CONCLUSION

We concluded that TNF-α played an important role in the pathogenesis of ALI and acted as an initiating cytokine at the early stage of ECC-induced ALI.

Activation of mouse bronchopulmonary C-fibres by serotonin and allergen-ovalbumin challenge

Abstract  The effect of serotonin on capsaicin-sensitive vagal C-fibre afferent nerves was evaluated in an ex vivo vagally innervated mouse lung preparation. Action potentials arising from receptive fields in the lungs were recorded with an extracellular electrode positioned in the nodose/jugular ganglion. Among the 62 capsaicin-sensitive C-fibres studied (conduction velocity ∼0.5 m s(-1)), 71% were of the nodose phenotype and 29% of the jugular phenotype. The nodose C-fibres responded strongly to serotonin and this effect was blocked with the 5-HT3-receptor antagonist ondansetron. Using single cell RT-PCR, we noted that the vast majority of nodose neurons retrogradely labelled from the lung, expressed 5-HT3 receptor mRNA. The jugular C-fibres also responded strongly to serotonin with action potential discharge, but this effect was not inhibited by ondansetron. Lung-specific jugular neurons did not express 5-HT3 receptor mRNA but frequently expressed 5-HT1 or 5-HT4 receptor mRNA. Mast cells are the major source of serotonin in healthy murine airways. Ovalbumin-induced mast cell activation in actively sensitized lungs caused action potential discharge in jugular but not nodose C-fibres. The data show that vagal C-fibres in the respiratory tract of the mouse are strongly activated by serotonin. Depending on the C-fibre subtype both 5-HT3 and non-5-HT3 mechanisms are involved.

The cytokines interleukin-1β and tumor necrosis factor-α stimulate CFTR-mediated fluid secretion by swine airway submucosal glands

The airway is kept sterile by an efficient innate defense mechanism. The cornerstone of airway defense is mucus containing diverse antimicrobial factors that kill or inactivate pathogens. Most of the mucus in the upper airways is secreted by airway submucosal glands. In patients with cystic fibrosis (CF), airway defense fails and the lungs are colonized by bacteria, usually Pseudomonas aeruginosa. Accumulating evidence suggests that airway submucosal glands contribute to CF pathogenesis by failing to respond appropriately to inhalation of bacteria. However, the regulation of submucosal glands by the innate immune system remains poorly understood. We studied the response of submucosal glands to the proinflammatory cytokines interleukin-1β and tumor necrosis factor-α. These are released into the airway submucosa in response to infection with the bacterium P. aeruginosa and are elevated in CF airways. Stimulation with IL-1β and TNF-α increased submucosal gland secretion in a concentration-dependent manner with a maximal secretion rate of 240 ± 20 and 190 ± 40 pl/min, respectively. The half maximal effective concentrations were 11 and 20 ng/ml, respectively. The cytokine effect was dependent on cAMP but was independent of cGMP, nitric oxide, Ca(2+), or p38 MAP kinase. Most importantly, IL-1β- and TNF-α-stimulated secretion was blocked by the CF transmembrane conductance regulator (CFTR) blocker, CFTRinh172 (100 μmol/l) but was not affected by the Ca(2+)-activated Cl(-) channel blocker, niflumic acid (1 μmol/l). The data suggest, that during bacterial infections and resulting release of proinflammatory cytokines, the glands are stimulated to secrete fluid, and this response is mediated by cAMP-activated CFTR, a process that would fail in patients with CF.

Activation of nicotinic cholinergic receptors prevents ventilator-induced lung injury in rats

Respiratory distress syndrome is responsible for 40 to 60 percent mortality. An over mortality of about 10 percent could result from additional lung injury and inflammation due to the life-support mechanical ventilation, which stretches the lung. It has been recently demonstrated, in vitro, that pharmacological activation of the alpha 7 nicotinic receptors (α7-nAChR) could down regulate intracellular mediators involved in lung cell inflammatory response to stretch. Our aim was to test in vivo the protective effect of the pharmacological activation of the α7-nAChR against ventilator-induced lung injury (VILI). Anesthetized rats were ventilated for two hours with a high stretch ventilation mode delivering a stroke volume large enough to generate 25-cmH₂O airway pressure, and randomly assigned to four groups: pretreated with parenteral injection of saline or specific agonist of the α7-nAChR (PNU-282987), or submitted to bilateral vagus nerve electrostimulation while pre-treated or not with the α7-nAChR antagonist methyllycaconitine (MLA). Controls ventilated with a conventional stroke volume of 10 mL/kg gave reference data. Physiological indices (compliance of the respiratory system, lung weight, blood oxygenation, arterial blood pressure) and lung contents of inflammatory mediators (IL-6 measured by ELISA, substance P assessed using HPLC) were severely impaired after two hours of high stretch ventilation (sham group). Vagal stimulation was able to maintain the respiratory parameters close to those obtained in Controls and reduced lung inflammation except when associated to nicotinic receptor blockade (MLA), suggesting the involvement of α7-nAChR in vagally-mediated protection against VILI. Pharmacological pre-treatment with PNU-282987 strongly decreased lung injury and lung IL-6 and substance P contents, and nearly abolished the increase in plasmatic IL-6 levels. Pathological examination of the lungs confirmed the physiological differences observed between the groups. In conclusion, these data suggest that the stimulation of α7-nAChR is able to attenuate VILI in rats.