Acute activation of bronchopulmonary vagal nociceptors by type I interferons

Widespread hyperexcitability of nociceptor somata outlasts enhanced avoidance behavior after incision injury

ABSTRACT

Nociceptors with somata in dorsal root ganglia (DRGs) readily switch from an electrically silent state to a hyperactive state of tonic, nonaccommodating, low-frequency, irregular discharge of action potentials (APs). Spontaneous activity (SA) during this state is present in vivo in rats months after spinal cord injury (SCI) and has been causally linked to SCI pain. Intrinsically generated SA and, more generally, ongoing activity (OA) are induced by various neuropathic conditions in rats, mice, and humans and are retained in nociceptor somata after dissociation and culturing, providing a powerful tool for investigating its mechanisms and functions. The present study shows that long-lasting hyperexcitability that can generate OA during modest depolarization in probable nociceptors dissociated from DRGs of male and female rats is induced by plantar incision injury. OA occurred when the soma was artificially depolarized to a level within the normal range of membrane potentials where large, transient depolarizing spontaneous fluctuations (DSFs) can approach AP threshold. This hyperexcitability persisted for at least 3 weeks, whereas behavioral indicators of affective pain-hind paw guarding and increased avoidance of a noxious substrate in an operant conflict test-persisted for 1 week or less. The most consistent electrophysiological alteration associated with OA was enhancement of DSFs. An unexpected discovery after plantar incisions was hyperexcitability in neurons from thoracic DRGs that innervate dermatomes distant from the injured tissue. Potential in vivo functions of widespread, low-frequency nociceptor OA consistent with these and other findings are to contribute to hyperalgesic priming and to drive anxiety-related hypervigilance.

Induction of long-term hyperexcitability by memory-related cAMP signaling in isolated nociceptor cell bodies

Persistent hyperactivity of nociceptors is known to contribute significantly to long-lasting sensitization and ongoing pain in many clinical conditions. It is often assumed that nociceptor hyperactivity is mainly driven by continuing stimulation from inflammatory mediators. We have tested an additional possibility: that persistent increases in excitability promoting hyperactivity can be induced by a prototypical cellular signaling pathway long known to induce late-phase long-term potentiation (LTP) of synapses in brain regions involved in memory formation. This cAMP-PKA-CREB-gene transcription-protein synthesis pathway was tested using whole-cell current clamp methods on small dissociated sensory neurons (primarily nociceptors) from dorsal root ganglia (DRGs) excised from previously uninjured ("naïve") male rats. Six-hour treatment with the specific Gαs-coupled 5-HT4 receptor agonist, prucalopride, or with the adenylyl cyclase activator forskolin induced long-term hyperexcitability (LTH) in DRG neurons that manifested 12-24 h later as action potential (AP) discharge (ongoing activity, OA) during artificial depolarization to -45 mV, a membrane potential that is normally subthreshold for AP generation. Prucalopride treatment also induced significant long-lasting depolarization of resting membrane potential (from -69 to -66 mV), enhanced depolarizing spontaneous fluctuations (DSFs) of membrane potential, and produced trends for reduced AP threshold and rheobase. LTH was prevented by co-treatment of prucalopride with inhibitors of PKA, CREB, gene transcription, or protein synthesis. As in the induction of synaptic memory, many other cellular signals are likely to be involved. However, the discovery that this prototypical memory induction pathway can induce nociceptor LTH, along with reports that cAMP signaling and CREB activity in DRGs can induce hyperalgesic priming, suggest that early, temporary, cAMP-induced transcriptional and translational mechanisms can induce nociceptor LTH that might last for long periods. The present results also raise the question of whether reactivation of primed signaling mechanisms by re-exposure to inflammatory mediators linked to cAMP synthesis during subsequent challenges to bodily integrity can "reconsolidate" nociceptor memory, extending the duration of persistent hyperexcitability.

Inflammation of some visceral sensory systems and autonomic dysfunction in cardiovascular disease

The sensitization and hypertonicity of visceral afferents are highly relevant to the development and progression of cardiovascular and respiratory disease states. In this review, we described the evidence that the inflammatory process regulates visceral afferent sensitivity and tonicity, affecting the control of the cardiovascular and respiratory system. Some inflammatory mediators like nitric oxide, angiotensin II, endothelin-1, and arginine vasopressin may inhibit baroreceptor afferents and contribute to the baroreflex impairment observed in cardiovascular diseases. Cytokines may act directly on peripheral afferent terminals that transmit information to the central nervous system (CNS). TLR-4 receptors, which recognize lipopolysaccharide, were identified in the nodose and petrosal ganglion and have been implicated in disrupting the blood-brain barrier, which can potentiate the inflammatory process. For example, cytokines may cross the blood-brain barrier to access the CNS. Additionally, pro-inflammatory cytokines such as IL-1β, IL-6, TNF-α and some of their receptors have been identified in the nodose ganglion and carotid body. These pro-inflammatory cytokines also sensitize the dorsal root ganglion or are released in the nucleus of the solitary tract. In cardiovascular disease, pro-inflammatory mediators increase in the brain, heart, vessels, and plasma and may act locally or systemically to activate/sensitize afferent nervous terminals. Recent evidence demonstrated that the carotid body chemoreceptor cells might sense systemic pro-inflammatory molecules, supporting the novel proposal that the carotid body is part of the afferent pathway in the central anti-inflammatory reflexes. The exact mechanisms of how pro-inflammatory mediators affects visceral afferent signals and contribute to the pathophysiology of cardiovascular diseases awaits future research.

Xingbei antitussive granules ameliorate cough hypersensitivity in post-infectious cough guinea pigs by regulating tryptase/PAR2/TRPV1 pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Xingbei antitussive granules (XB) is a classic Chinese Medicine prescription for treating post-infectious cough(PIC), based on the Sanao Decoction from Formularies of the Bureau of People's Welfare Pharmacies in the Song Dynasty and Jiegeng decoction from Essentials of the Golden Chamber in the Han Dynasty. However, the therapeutic effects and pharmacological mechanisms are still ambiguous. In the present study, we endeavored to elucidate these underlying mechanisms.

AIMS OF THE STUDY

This study aimed to explore the potential impact and mechanism of XB on PIC, and provide a scientific basis for its clinical application.

MATERIALS AND METHODS

Cigarette smoking (CS) combined with lipopolysaccharide (LPS) nasal drops were administered to induce the PIC guinea pig with cough hypersensitivity status. Subsequently, the model guinea pigs were treated with XB and the cough frequency was observed by the capsaicin cough provocation test. The pathological changes of lung tissue were assessed by HE staining, and the levels of inflammatory mediators, mast cell degranulating substances, and neuropeptides were detected. The protein and mRNA expression of transient receptor potential vanilloid type 1(TRPV1), proteinase-activated receptor2(PAR2), and protein kinase C (PKC) were measured by Immunohistochemical staining, Western blot, and RT-qPCR. Changes in the abundance and composition of respiratory bacterial microbiota were determined by 16S rRNA sequencing.

RESULTS

After XB treatment, the model guinea pigs showed a dose-dependent decrease in cough frequency, along with a significant alleviation in inflammatory infiltration of lung tissue and a reduction in inflammatory mediators. In addition, XB high-dose treatment significantly decreased the levels of mast cell Tryptase as well as β-hexosaminidase (β-Hex) and downregulated the expression of TRPV1, PAR2, and p-PKC. Simultaneously, levels of neuropeptides like substance P (SP), calcitonin gene-related peptide (CGRP), neurokinin A (NKA), and nerve growth factor (NGF) were improved. Besides, XB also can modulate the structure of respiratory bacterial microbiota and restore homeostasis.

CONCLUSION

XB treatment alleviates cough hypersensitivity and inflammatory responses, inhibits the degranulation of mast cells, and ameliorates neurogenic inflammation in PIC guinea pigs whose mechanism may be associated with the inhibition of Tryptase/PAR2/PKC/TRPV1 and the recovery of respiratory bacterial microbiota.

SARS-CoV-2 papain-like protease activates nociceptors to drive sneeze and pain

SARS-CoV-2, the virus responsible for COVID-19, triggers symptoms such as sneezing, aches and pain.1 These symptoms are mediated by a subset of sensory neurons, known as nociceptors, that detect noxious stimuli, densely innervate the airway epithelium, and interact with airway resident epithelial and immune cells.2-6 However, the mechanisms by which viral infection activates these neurons to trigger pain and airway reflexes are unknown. Here, we show that the coronavirus papain-like protease (PLpro) directly activates airway-innervating trigeminal and vagal nociceptors in mice and human iPSC-derived nociceptors. PLpro elicits sneezing and acute pain in mice and triggers the release of neuropeptide calcitonin gene-related peptide (CGRP) from airway afferents. We find that PLpro-induced sneeze and pain requires the host TRPA1 ion channel that has been previously demonstrated to mediate pain, cough, and airway inflammation.7-9 Our findings are the first demonstration of a viral product that directly activates sensory neurons to trigger pain and airway reflexes and highlight a new role for PLpro and nociceptors in COVID-19.

Direct activation of airway sensory C-fibers by SARS-CoV-2 S1 spike protein

Respiratory viral infection can lead to activation of sensory afferent nerves as indicated by the consequential sore throat, sneezing, coughing, and reflex secretions. In addition to causing troubling symptoms, sensory nerve activation likely accelerates viral spreading. The mechanism how viruses activate sensory nerve terminals during infection is unknown. In this study, we investigate whether coronavirus spike protein activates sensory nerves terminating in the airways. We used isolated vagally-innervated mouse trachea-lung preparation for two-photon microscopy and extracellular electrophysiological recordings. Using two-photon Ca2+ imaging, we evaluated a total number of 786 vagal bronchopulmonary nerves in six experiments. Approximately 49% of the sensory fibers were activated by S1 protein (4 μg/mL intratracheally). Extracellular nerve recording showed the S1 protein evoked action potential discharge in sensory C-fibers; of 39 airway C-fibers (one fiber per mouse), 17 were activated. Additionally, Fura-2 Ca2+ imaging was performed on neurons dissociated from vagal sensory ganglia (n = 254 from 22 mice). The result showed that 63% of neurons responded to S1 protein. SARS-CoV-2 S1 protein can lead to direct activation of sensory C-fiber nerve terminals in the bronchopulmonary tract. Direct activation of C-fibers may contribute to coronavirus symptoms, and amplify viral spreading in a population.

Neuroimmune pathways regulating airway inflammation

Airways diseases are typically accompanied by inflammation, which has long been known to contribute to obstruction, mucus hypersecretion, dyspnea, cough, and other characteristic symptoms displayed in patients. Clinical interventions, therefore, often target inflammation to reverse lung pathology and reduce morbidity. The airways and lungs are densely innervated by subsets of nerve fibers, which are not only impacted by pulmonary inflammation but, in addition, likely serve as important regulators of immune cell function. This bidirectional neuroimmune crosstalk is supported by close spatial relationships between immune cells and airway nerve fibers, complementary neural and immune signaling pathways, local specialized airway chemosensory cells, and dedicated reflex circuits. In this article, we review the recent literature on this topic and present state-of-the-art evidence supporting the role of neuroimmune interactions in airway inflammation. In addition, we extend this evidence to synthesize considerations for the clinical translation of these discoveries to improve the management of patients with airway disease.

Intrapulmonary IFN-γ instillation causes chronic lymphocytic inflammation in the spleen and lung through the CXCR3 pathway

Some patients with chronic refractory cough have high levels of pulmonary IFN-γ and IFN-γ-producing T lymphocytes. Pulmonary IFN-γ administration causes acute airway lymphocytic inflammation and cough hypersensitivity by increasing the number of pulmonary IFN-γ-producing T lymphocytes, but these lymphocytes may be recruited from other organs. Intraperitoneal IFN-γ injection can increase the spleen weight of mice. It remains elusive whether pulmonary IFN-γ can induce chronic airway lymphocytic inflammation and cough hypersensitivity by stimulating the proliferation of IFN-γ -producing T lymphocytes in the spleen. Here, we found that pulmonary IFN-γ administration induced chronic airway inflammation and chronic cough hypersensitivity with an increased number of IFN-γ-producing T lymphocytes in the spleen, blood and lung. Pulmonary IFN-γ administration also increased 1) the proliferation of spleen lymphocytes in vivo and 2) the IP-10 level and CXCR3+ T lymphocyte numbers in the spleen and lung of mice. IP-10 could promote the proliferation of spleen lymphocytes in vitro but not blood lymphocytes or lung-resident lymphocytes. AMG487, a potent inhibitor of binding between IP-10 and CXCR3, could block pulmonary IFN-γ instillation-induced chronic airway lymphocytic inflammation and the proliferation of IFN-γ-producing T lymphocytes in mouse spleens. In conclusion, intrapulmonary IFN-γ instillation may induce the proliferation of splenic IFN-γ-producing T lymphocytes through IP-10 and the CXCR3 pathway. The IFN-γ-producing T lymphocytes in blood, partly released from the mouse spleen, may be partly attracted to the lung by pulmonary IP-10 through the CXCR3 pathway. IFN-γ-producing T lymphocytes and IFN-γ in the lung may cause chronic airway lymphocytic inflammation and chronic cough hypersensitivity.

The bidirectional roles of the cGAS-STING pathway in pain processing: Cellular and molecular mechanisms

Pain is a common clinical condition. However, the mechanisms underlying pain are not yet fully understood. It is known that the neuroimmune system plays a critical role in the pathogenesis of pain. Recent studies indicated that the cyclic-GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) pathway can activate the innate immune system by sensing both extrinsic and intrinsic double-stranded DNA in the cytoplasm, which is involved in pain processing. In this review, we summarise (1) the roles of the cGAS-STING pathway in different pain models, (2) the effect of the cGAS-STING pathway in different cells during pain regulation, and (3) the downstream molecular mechanisms of the cGAS-STING pathway in pain regulation. This review provides evidence that the cGAS-STING pathway has pro- and anti-nociceptive effects in pain models. It has different functions in neuron, microglia, macrophage, and T cells. Its downstream molecules include IFN-I, NF-κB, NLRP3, and eIF2α. The bidirectional roles of the cGAS-STING pathway in pain processing are mediated by regulating nociceptive neuronal sensitivity and neuroinflammatory responses. However, their effects in special brain regions, activation of astrocytes, and the different phases of pain require further exploration.

Role of NaV 1.9 in inflammatory mediator-induced activation of mouse airway vagal C-fibres

The influence of NaV 1.9 on inflammatory mediator-induced activation of airway vagal nodose C-fibres was evaluated by comparing responses in wild-type versus NaV 1.9-/- mice. A single-cell RT-PCR analysis indicated that virtually all nodose C-fibre neurons expressed NaV 1.9 (SCN11A) mRNA. Using extracellular electrophysiological recordings in an isolated vagally innervated mouse trachea-lung preparation, it was noted that mediators acting via G protein-coupled receptors (PAR2), or ionotropic receptors (P2×3) were 70-85% less effective in evoking action potential discharge in the absence of NaV 1.9. However, there was no difference in action potential discharge between wild-type and NaV 1.9-/- when the stimulus was a rapid punctate mechanical stimulus. An analysis of the passive and active properties of isolated nodose neurons revealed no difference between neurons from wild-type and NaV 1.9-/- mice, with the exception of a modest difference in the duration of the afterhyperpolarization. There was also no difference in the amount of current required to evoke action potentials (rheobase) or the action potential voltage threshold. The inward current evoked by the chemical mediator by a P2×3 agonist was the same in wild-type versus NaV 1.9-/- neurons. However, the current was sufficient to evoke action potential only in the wild-type neurons. The data support the speculation that NaV 1.9 could be an attractive therapeutic target for inflammatory airway disease by selectively inhibiting inflammatory mediator-associated vagal C-fibre activation. KEY POINTS: Inflammatory mediators were much less effective in activating the terminals of vagal airway C-fibres in mice lacking NaV 1.9. The active and passive properties of nodose neurons were the same between wild-type neurons and NaV 1.9-/- neurons. Nerves lacking NaV 1.9 responded, normally, with action potential discharge to rapid punctate mechanical stimulation of the terminals or the rapid stimulation of the cell bodies with inward current injections. NaV 1.9 channels could be an attractive target to selectively inhibit vagal nociceptive C-fibre activation evoked by inflammatory mediators without blocking the nerves' responses to the potentially hazardous stimuli associated with aspiration.

Infectious and Inflammatory Pathways to Cough

Coughing is a dynamic physiological process resulting from input of vagal sensory neurons innervating the airways and perceived airway irritation. Although cough serves to protect and clear the airways, it can also be exploited by respiratory pathogens to facilitate disease transmission. Microbial components or infection-induced inflammatory mediators can directly interact with sensory nerve receptors to induce a cough response. Analysis of cough-generated aerosols and transmission studies have further demonstrated how infectious disease is spread through coughing. This review summarizes the neurophysiology of cough, cough induction by respiratory pathogens and inflammation, and cough-mediated disease transmission.

Pulmonary IFN-γ Causes Lymphocytic Inflammation and Cough Hypersensitivity by Increasing the Number of IFN-γ-Secreting T Lymphocytes

PURPOSE

Respiratory viral infection increases the number of lung-resident T lymphocytes, which enhance cough sensitivity by producing interferon-γ (IFN-γ). It is poorly understood why IFN-γ-secreting T lymphocytes persist for a long time when the respiratory viruses have been removed.

METHODS

Repeated pulmonary administration of IFN-γ and intraperitoneal injection with different inhibitors were used to study the effects of pulmonary IFN-γ in mice and guinea pigs.

RESULTS

IFN-γ administration caused the increasing of IFN-γ-secreting T lymphocytes in both lung and blood, followed by the elevated physiological level of IFN-γ in the lung, the airway inflammation and the airway epithelial damage. IFN-γ administration also enhanced the cough sensitivity of guinea pigs. IFN-γ activated the STAT1 and extracellular signal-regulated kinase (ERK) pathways in lung tissues, released IFN-γ-inducible protein 10 (IP-10), and resulted in F-actin accumulation in lung-resident lymphocytes. The CXC chemokine receptor 3 (CXCR3) inhibitor potently suppressed all the IFN-γ-induced inflammatory changes. The STAT1 inhibitor mitigated IFN-γ-secreting T lymphocytes infiltration by inhibiting T lymphocytes proliferation. F-actin accumulation and the ERK1/2 pathway contributed to pulmonary IFN-γ-induced augmentation of the airway inflammation and increasing of IFN-γ-secreting T lymphocytes in blood.

CONCLUSIONS

High physiological levels of IFN-γ in the lung may cause pulmonary lymphocytic inflammation and cough hypersensitivity by increasing the number of IFN-γ-secreting T lymphocytes through the IP-10 and CXCR3 pathways.

Ultrasound Neuromodulation of the Spleen Has Time-Dependent Anti-Inflammatory Effect in a Pneumonia Model

Interfaces between the nervous and immune systems have been shown essential for the coordination and regulation of immune responses. Non-invasive ultrasound stimulation targeted to the spleen has recently been shown capable of activating one such interface, the splenic cholinergic anti-inflammatory pathway (CAP). Over the past decade, CAP and other neuroimmune pathways have been activated using implanted nerve stimulators and tested to prevent cytokine release and inflammation. However, CAP studies have typically been performed in models of severe, systemic (e.g., endotoxemia) or chronic inflammation (e.g., collagen-induced arthritis or DSS-induced colitis). Herein, we examined the effects of activation of the splenic CAP with ultrasound in a model of local bacterial infection by lung instillation of 10⁵ CFU of Streptococcus pneumoniae. We demonstrate a time-dependent effect of CAP activation on the cytokine response assay during infection progression. CAP activation-induced cytokine suppression is absent at intermediate times post-infection (16 hours following inoculation), but present during the early (4 hours) and later phases (48 hours). These results indicate that cytokine inhibition associated with splenic CAP activation is not observed at all timepoints following bacterial infection and highlights the importance of further studying neuroimmune interfaces within the context of different immune system and inflammatory states.

K V 1/D‐type potassium channels inhibit the excitability of bronchopulmonary vagal afferent nerves

Abstract

The K_V1/D‐type potassium current (I_D) is an important determinant of neuronal excitability. This study explored whether and how I_D channels regulate the activation of bronchopulmonary vagal afferent nerves. The single‐neuron RT‐PCR assay revealed that nearly all mouse bronchopulmonary nodose neurons expressed the transcripts of α‐dendrotoxin (α‐DTX)‐sensitive, I_D channel‐forming K_V1.1, K_V1.2 and/or K_V1.6 α‐subunits, with the expression of K_V1.6 being most prevalent. Patch‐clamp recordings showed that I_D, defined as the α‐DTX‐sensitive K⁺ current, activated at voltages slightly more negative than the resting membrane potential in lung‐specific nodose neurons and displayed little inactivation at subthreshold voltages. Inhibition of I_D channels by α‐DTX depolarized the lung‐specific nodose neurons and caused an increase in input resistance, decrease in rheobase, as well as increase in action potential number and firing frequency in response to suprathreshold current steps. Application of α‐DTX to the lungs via trachea in the mouse ex vivo vagally innervated trachea–lungs preparation led to action potential discharges in nearly half of bronchopulmonary nodose afferent nerve fibres, including nodose C‐fibres, as detected by the two‐photon microscopic Ca²⁺ imaging technique and extracellular electrophysiological recordings. In conclusion, I_D channels act as a critical brake on the activation of bronchopulmonary vagal afferent nerves by stabilizing the membrane potential, counterbalancing the subthreshold depolarization and promoting the adaptation of action potential firings. Down‐regulation of I_D channels, as occurs in various inflammatory diseases, may contribute to the enhanced C‐fibre activity in airway diseases that are associated with excessive coughing, dyspnoea, and reflex bronchospasm and secretions.

Key points

The α‐dendrotoxin (α‐DTX)‐sensitive D‐type K⁺ current (I_D) is an important determinant of neuronal excitability.

Nearly all bronchopulmonary nodose afferent neurons in the mouse express I_D and the transcripts of α‐DTX‐sensitive, I_D channel‐forming K_V1.1, K_V1.2 and/or K_V1.6 α‐subunits.

Inhibition of I_D channels by α‐DTX depolarizes the bronchopulmonary nodose neurons, reduces the minimal depolarizing current needed to evoke an action potential (AP) and increases AP number and AP firing frequency in response to suprathreshold stimulations.

Application of α‐DTX to the lungs ex vivo elicits AP discharges in about half of bronchopulmonary nodose C‐fibre terminals.

Our novel finding that I_D channels act as a critical brake on the activation of bronchopulmonary vagal afferent nerves suggests that their down‐regulation, as occurs in various inflammatory diseases, may contribute to the enhanced C‐fibre activity in airway inflammation associated with excessive respiratory symptoms.

Neural control of the lower airways: Role in cough and airway inflammatory disease

Airway function is under constant neurophysiological control, in order to maximize airflow and gas exchange and to protect the airways from aspiration, damage, and infection. There are multiple sensory nerve subtypes, whose disparate functions provide a wide array of sensory information into the CNS. Activation of these subtypes triggers specific reflexes, including cough and alterations in autonomic efferent control of airway smooth muscle, secretory cells, and vasculature. Importantly, every aspect of these reflex arcs can be impacted and altered by local inflammation caused by chronic lung disease such as asthma, bronchitis, and infections. Excessive and inappropriate activity in sensory and autonomic nerves within the airways is thought to contribute to the morbidity and symptoms associated with lung disease.

Innate Receptors Expression by Lung Nociceptors: Impact on COVID-19 and Aging

The lungs are constantly exposed to non-sterile air which carries harmful threats, such as particles and pathogens. Nonetheless, this organ is equipped with fast and efficient mechanisms to eliminate these threats from the airways as well as prevent pathogen invasion. The respiratory tract is densely innervated by sensory neurons, also known as nociceptors, which are responsible for the detection of external stimuli and initiation of physiological and immunological responses. Furthermore, expression of functional innate receptors by nociceptors have been reported; however, the influence of these receptors to the lung function and local immune response is poorly described. The COVID-19 pandemic has shown the importance of coordinated and competent pulmonary immunity for the prevention of pathogen spread as well as prevention of excessive tissue injury. New findings suggest that lung nociceptors can be a target of SARS-CoV-2 infection; what remains unclear is whether innate receptor trigger sensory neuron activation during SARS-CoV-2 infection and what is the relevance for the outcomes. Moreover, elderly individuals often present with respiratory, neurological and immunological dysfunction. Whether aging in the context of sensory nerve function and innate receptors contributes to the disorders of these systems is currently unknown. Here we discuss the expression of innate receptors by nociceptors, particularly in the lungs, and the possible impact of their activation on pulmonary immunity. We then demonstrate recent evidence that suggests lung sensory neurons as reservoirs for SARS-CoV-2 and possible viral recognition via innate receptors. Lastly, we explore the mechanisms by which lung nociceptors might contribute to disturbance in respiratory and immunological responses during the aging process.

Interferons in Pain and Infections: Emerging Roles in Neuro-Immune and Neuro-Glial Interactions

Interferons (IFNs) are cytokines that possess antiviral, antiproliferative, and immunomodulatory actions. IFN-α and IFN-β are two major family members of type-I IFNs and are used to treat diseases, including hepatitis and multiple sclerosis. Emerging evidence suggests that type-I IFN receptors (IFNARs) are also expressed by microglia, astrocytes, and neurons in the central and peripheral nervous systems. Apart from canonical transcriptional regulations, IFN-α and IFN-β can rapidly suppress neuronal activity and synaptic transmission via non-genomic regulation, leading to potent analgesia. IFN-γ is the only member of the type-II IFN family and induces central sensitization and microglia activation in persistent pain. We discuss how type-I and type-II IFNs regulate pain and infection via neuro-immune modulations, with special focus on neuroinflammation and neuro-glial interactions. We also highlight distinct roles of type-I IFNs in the peripheral and central nervous system. Insights into IFN signaling in nociceptors and their distinct actions in physiological vs. pathological and acute vs. chronic conditions will improve our treatments of pain after surgeries, traumas, and infections.

Contribution of tetrodotoxin-sensitive, voltage-gated sodium channels (NaV1) to action potential discharge from mouse esophageal tension mechanoreceptors

Action potentials depend on voltage-gated sodium channels (NaV1s), which have nine α subtypes. NaV1 inhibition is a target for pathologies involving excitable cells such as pain. However, because NaV1 subtypes are widely expressed, inhibitors may inhibit regulatory sensory systems. Here, we investigated specific NaV1s and their inhibition in mouse esophageal mechanoreceptors-non-nociceptive vagal sensory afferents that are stimulated by low threshold mechanical distension, which regulate esophageal motility. Using single fiber electrophysiology, we found mechanoreceptor responses to esophageal distension were abolished by tetrodotoxin. Single-cell RT-PCR revealed that esophageal-labeled TRPV1-negative vagal neurons expressed multiple tetrodotoxin-sensitive NaV1s: NaV1.7 (almost all neurons) and NaV1.1, NaV1.2, and NaV1.6 (in ∼50% of neurons). Inhibition of NaV1.7, using PF-05089771, had a small inhibitory effect on mechanoreceptor responses to distension. Inhibition of NaV1.1 and NaV1.6, using ICA-121341, had a similar small inhibitory effect. The combination of PF-05089771 and ICA-121341 inhibited but did not eliminate mechanoreceptor responses. Inhibition of NaV1.2, NaV1.6, and NaV1.7 using LSN-3049227 inhibited but did not eliminate mechanoreceptor responses. Thus, all four tetrodotoxin-sensitive NaV1s contribute to action potential initiation from esophageal mechanoreceptors terminals. This is different to those NaV1s necessary for vagal action potential conduction, as demonstrated using GCaMP6s imaging of esophageal vagal neurons during electrical stimulation. Tetrodotoxin-sensitive conduction was abolished in many esophageal neurons by PF-05089771 alone, indicating a critical role of NaV1.7. In summary, multiple NaV1 subtypes contribute to electrical signaling in esophageal mechanoreceptors. Thus, inhibition of individual NaV1s would likely have minimal effect on afferent regulation of esophageal motility.

Palpation of the Respiratory System in Osteopathic Manual Medicine: From the Trachea to the Lungs

There is a lack of published literature in osteopathic manual medicine on how to perform palpation of the lower respiratory tree such as the trachea, main bronchi, and lungs. Several authors have studied the osteopathic effect and respiratory response of palpation but have failed to demonstrate how to perform palpation of the visceral areas involved in breathing, either in the context of a clinical trial or as a case report. This paper reviews the innervation of these anatomical areas, the mechano-metabolic weight of the passage of fluids and air in the respiratory tract, the anatomical topography, and the movements involved in respiration. Drawing from current knowledge, this article illustrates, for the first time, how to place the hands for an effective osteopathic assessment of the tracheal, bronchial, and pulmonary structures. Understanding how to perform palpation of the lower areas is a fundamental tool in the clinic and potential therapy in osteopathic manual medicine.

The impact of influenza pulmonary infection and inflammation on vagal bronchopulmonary sensory neurons

Influenza A virus (IAV) is rapidly detected in the airways by the immune system, with resident parenchymal cells and leukocytes orchestrating viral sensing and the induction of antiviral inflammatory responses. The airways are innervated by heterogeneous populations of vagal sensory neurons which also play an important role in pulmonary defense. How these neurons respond to IAV respiratory infection remains unclear. Here, we use a murine model to provide the first evidence that vagal sensory neurons undergo significant transcriptional changes following a respiratory IAV infection. RNA sequencing on vagal sensory ganglia showed that IAV infection induced the expression of many genes associated with an antiviral and pro-inflammatory response and this was accompanied by a significant increase in inflammatory cell recruitment into the vagal ganglia. Assessment of gene expression in single-vagal sensory neurons confirmed that IAV infection induced a neuronal inflammatory phenotype, which was most prominent in bronchopulmonary neurons, and also evident in some neurons innervating other organs. The altered transcriptome could be mimicked by intranasal treatment with cytokines and the lung homogenates of infected mice, in the absence of infectious virus. These data argue that IAV pulmonary infection and subsequent inflammation induces vagal sensory ganglia neuroinflammation and this may have important implications for IAV-induced morbidity.

Confronting COVID-19-associated cough and the post-COVID syndrome: role of viral neurotropism, neuroinflammation, and neuroimmune responses

Cough is one of the most common presenting symptoms of COVID-19, along with fever and loss of taste and smell. Cough can persist for weeks or months after SARS-CoV-2 infection, often accompanied by chronic fatigue, cognitive impairment, dyspnoea, or pain-a collection of long-term effects referred to as the post-COVID syndrome or long COVID. We hypothesise that the pathways of neurotropism, neuroinflammation, and neuroimmunomodulation through the vagal sensory nerves, which are implicated in SARS-CoV-2 infection, lead to a cough hypersensitivity state. The post-COVID syndrome might also result from neuroinflammatory events in the brain. We highlight gaps in understanding of the mechanisms of acute and chronic COVID-19-associated cough and post-COVID syndrome, consider potential ways to reduce the effect of COVID-19 by controlling cough, and suggest future directions for research and clinical practice. Although neuromodulators such as gabapentin or opioids might be considered for acute and chronic COVID-19 cough, we discuss the possible mechanisms of COVID-19-associated cough and the promise of new anti-inflammatories or neuromodulators that might successfully target both the cough of COVID-19 and the post-COVID syndrome.