Synergistic interactions between airway afferent nerve subtypes regulating the cough reflex in guinea-pigs

Pathophysiology of Laryngopharyngeal Reflux Disease: The Reflux vs Reflex Theory

Laryngopharyngeal reflux (LPR) involves the reflux of stomach contents into the upper aerodigestive tract, causing symptoms through both direct injury and reflex-mediated mechanisms. While traditional reflux and reflex theories provide insights into LPR pathogenesis, they do not fully encompass its clinical spectrum. Emerging evidence highlights the role of hypersensitivity-both peripheral and central-in LPR, with significant implications for treatment responses and the impact of psychological factors on symptom severity. Understanding these complex interactions is crucial for developing more effective management strategies for LPR.

The Potential Role of Oxidative Stress in Modulating Airway Defensive Reflexes

Airway defensive reflexes, such as pharyngeal swallowing, coughing, and sneezing, play a pivotal role in maintaining airway homeostasis. These reflexes are controlled by complex mechanisms primarily governed by specific neuronal circuitry in the brainstem, referred to as central pattern generators. These behaviors also require optimal conditions for the peripheral organs within the airway and alimentary tracts, including the nose, pharynx, larynx, and trachea, which are vital for ensuring appropriate responsiveness and motor outputs. Oxidative stress is linked to the development and progress of impaired functions of those behaviors. Dysphagia caused by central or peripheral impairments, such as neurodegeneration of related neuronal networks and laryngeal desensitization, is likely associated with an increased level of oxidative stress. Chronic inflammation and allergic airway sensitization in the lower airways, including asthma, elevate oxidative stress levels and diminish the activity of antioxidant defense enzymes, which exacerbate the severity of respiratory conditions. Antioxidant supplements offer promising therapeutic benefits by facilitating the recovery of distorted airway defensive reflexes, although limited information has been provided concerning therapeutic strategies. Further studies are necessary to enhance our understanding of the pathophysiology of dysphagia and airway diseases related to oxidative stress, as well as to develop new treatment strategies for these disorders.

Complex respiratory effects of nebulised citric acid and capsaicin as tussive agents: A comparative study in conventional and specific pathogen-free guinea pigs

BACKGROUND

Citric acid and capsaicin are commonly used in cough research due to their reproducible effects on animal models and humans. However, these extend beyond cough provocation.

OBJECTIVE

To characterize and describe the respiratory responses to citric acid and capsaicin exposure using whole-body plethysmography in conventional (CON) and specific pathogen-free (SPF) guinea pigs.

METHODS

Male and female guinea pigs were exposed to aerosols of saline, citric acid (0.4 M), and capsaicin (25 µM). Cough and respiratory parameters (inspiratory and expiratory time, respiratory rate, tidal volume, enhanced pause and mid-expiratory flow) were recorded.

RESULTS

Both tussive agents induced upper and lower airway responses besides cough, with significant differences in respiratory parameters between CON and SPF animals. Citric acid elicited a stronger upper airway response compared to capsaicin.

CONCLUSION

Citric acid and capsaicin trigger complex respiratory responses (bronchoconstriction, braking, breathing pattern changes). These findings highlight the need to consider broader respiratory responses in translational cough research.

A vagal-brainstem interoceptive circuit for cough-like defensive behaviors in mice

Coughing is a respiratory behavior that plays a crucial role in protecting the respiratory system. Here we show that the nucleus of the solitary tract (NTS) in mice contains heterogenous neuronal populations that differentially control breathing. Within these subtypes, activation of tachykinin 1 (Tac1)-expressing neurons triggers specific respiratory behaviors that, as revealed by our detailed characterization, are cough-like behaviors. Chemogenetic silencing or genetic ablation of Tac1 neurons inhibits cough-like behaviors induced by tussive challenges. These Tac1 neurons receive synaptic inputs from the bronchopulmonary chemosensory and mechanosensory neurons in the vagal ganglion and coordinate medullary regions to control distinct aspects of cough-like defensive behaviors. We propose that these Tac1 neurons in the NTS are a key component of the airway-vagal-brain neural circuit that controls cough-like defensive behaviors in mice and that they coordinate the downstream modular circuits to elicit the sequential motor pattern of forceful expiratory responses.

Efficacy of behavioral cough suppression therapy for refractory chronic cough or unexplained chronic cough: a meta-analysis of randomized controlled trials

BACKGROUND

The efficacy of behavioral cough suppression therapy (BCST) for refractory chronic cough (RCC) and unexplained chronic cough (UCC) remains unclear due to limited evidence from small-scale single-center studies.

OBJECTIVE

To compile and assess the quality of evidence from randomized controlled trials to evaluate the effectiveness of BCST.

DESIGN

This study included randomized controlled studies and self-controlled studies related to BCST involving adult patients with RCC or UCC.

DATA SOURCES AND METHODS

We conducted an extensive search of various English and Chinese databases (e.g., PubMed, CNKI, CBM, VIP, and Wanfang Data Journal Full-text Database) and the Clinical Trial Registration website up to April 2024. The selected studies underwent meta-analysis to investigate the impact of BCST on the patient's quality of life and cough frequency.

RESULTS

The included 12 studies showed that BCST significantly improved the Leicester Cough Questionnaire scores of the patients (MD = 4.50, 95% CI (4.03, 4.97), p < 0.001) compared to the simple verbal education group. In addition, a significant reduction in objective cough frequency was observed in patients compared to before BCST, with a statistically significant difference (MD = -8.06, 95% CI (-9.71, -6.41), p < 0.001). Other measures of cough symptoms, such as symptom scores, Visual Analog Scale (VAS) scores, and Cough Severity Index (CSI) also showed improvement.

CONCLUSION

This meta-analysis revealed positive therapeutic effects of BCST in patients with RCC/UCC, potentially advancing its application in broader clinical settings.

TRIAL REGISTRATION

This study was registered on PROSPERO with the registration number CRD42024530746.

Practical multidisciplinary framework for the assessment and management of patients with unexplained chronic aerodigestive symptoms

OBJECTIVE

Patients experiencing unexplained chronic throat symptoms (UCTS) are frequently referred to gastroenterology and otolaryngology outpatient departments for investigation. Often despite extensive investigations, an identifiable structural abnormality to account for the symptoms is not found. The objective of this article is to provide a concise appraisal of the evidence-base for current approaches to the assessment and management of UCTS, their clinical outcomes, and related healthcare utilisation.

DESIGN

This multidisciplinary review critically examines the current understanding of aetiological theories and pathophysiological drivers in UCTS and summarises the evidence base underpinning various diagnostic and management approaches.

RESULTS

The evidence gathered from the review suggests that single-specialty approaches to UCTS inadequately capture the substantial heterogeneity and pervasive overlaps among clinical features and biopsychosocial factors and suggests a more unified approach is needed.

CONCLUSION

Drawing on contemporary insights from the gastrointestinal literature for disorders of gut-brain interaction, this article proposes a refreshed interdisciplinary approach characterised by a positive diagnosis framework and patient-centred therapeutic model. The overarching aim of this approach is to improve patient outcomes and foster collaborative research efforts.

Cough and cough hypersensitivity as treatable traits of asthma

Cough is a common and troublesome symptom in people with asthma and is often associated with poorer asthma control and exacerbations. Apart from asthma, other causes or comorbidities might underlie cough in asthma, such as rhinosinusitis and bronchiectasis. Eosinophilic inflammation and bronchoconstriction can lead to an acute episode of cough or worsen chronic cough. Cough hypersensitivity with laryngeal paraesthesia, allotussia, and hypertussia might underlie the cough of asthma through augmented sensory nerve excitability of upper-airway vagal sensory nerves. Cough associated with bronchoconstriction and type 2 inflammation should respond to inhaled corticosteroids and long-acting β-adrenoceptor agonist therapy. For cough hypersensitivity in adults, speech and language therapy and neuromodulators (eg, gabapentin) could be considered. In children, there is no consistent association of asthma with cough sensitivity or between cough and asthma severity. Further research is needed to realise the potential of cough as a measure of asthma control, to understand the mechanisms of cough in asthma, and to develop safe, effective treatments and a precision-medicine approach to the management of cough in asthma in children and adults.

Gabapentin for chronic refractory cough: A system review and meta-analysis

Objective

To evaluate the efficacy and safety of gabapentin in the treatment of chronic refractory cough by Meta-Analysis.

Methods

Literatures were retrieved from PubMed, Embase (OvidIP), Cochrane Library, CNKI, VIP, Wanfang Database and China Biomedical Management System and eligible prospective studies were screened. Data were extracted and analyzed by using RevMan 5.4.1 software.

Results

Six articles (2 RCTs and 4 prospective studies) with 536 participants were finally included. Meta-analysis showed that gabapentin was better than placebo in cough-specific quality of life (LCQ score, MD = 4.02, 95%CI [3.26,4,78], Z = 10.34, P < 0.00001), cough severity (VAS score, MD = -29.36, 95% CI (-39.46, -19.26), Z = 5.7, P < 0.00001), cough frequency (MD = -29.87, 95% CI [- 43.84, -15.91], Z = 4.19, P < 0.0001) and therapeutic efficacy (RR = 1.37,95%CI [1.13,1.65], Z = 3.27, P = 0.001), and equal in safety (RR = 1.32,95%CI [0.47,3.7], Z = 0.53, P = 0.59). Gabapentin was similar to other neuromodulators in therapeutic efficacy (RR = 1.07,95%CI [0.87,1.32], Z = 0.64, P = 0.52), but its safety was better.

Conclusion

Gabapentin is effective in the treatment of chronic refractory cough in both subjective and objective evaluations, and its safety is better than other neuromodulators.

Pathogenesis and management of gastroesophageal reflux disease-associated cough: a narrative review

Background and Objective

Gastroesophageal reflux disease (GERD)-associated cough is defined as a special GERD with a predominant cough symptom and is a common cause of chronic cough. This review summarizes our current understanding on the pathogenesis and management of GERD-associated cough.

Methods

Main literatures on the pathogenesis and management of GERD-associated cough were reviewed and our understandings derived from the published studies were showed then.

Key Content and Findings

Although esophageal-tracheobronchial reflex mainly underlies the pathogenesis of GERD-associated cough, its counterpart-tracheobronchial-esophageal reflex might exist and initiate the cough due to reflux induced by upper respiratory tract infection through the signaling of transient receptor potential vanilloid 1 linking airway and esophagus. The presence of reflux-associated symptoms such as regurgitation and heartburn along with coughing suggests an association between cough and GERD, which is supported by the objective evidence of abnormal reflux as detected by reflux monitoring. Although there is no general consensus, esophageal reflux monitoring provides the main diagnostic criteria for GERD-associated cough. Despite that acid exposure time and symptom associated probability are useful and mostly employed reflux diagnostic criteria, they are imperfect and far from being the gold standard. Acid suppressive therapy has long been recommended as the first choice for GERD-associated cough. However, the overall benefits of proton pump inhibitors have been controversial and need to be further assessed, especially in patients with cough due to non-acid reflux. Neuromodulators have demonstrated potential therapeutic effects for refractory GERD-associated cough, for which anti-reflux surgery may also be a promising treatment option.

Conclusions

Tracheobronchial-esophageal reflex might initiate reflux-induced cough provoked by the upper respiratory tract infection. It is necessary to optimize the current standards and to explore new criteria with higher diagnostic potency. Acid suppressive therapy is the first choice for GERD-associated cough, followed by neuromodulators and anti-reflux surgery for refractory GERD-associated cough.

Somatically evoked cough responses help to identify patients with difficult-to-treat chronic cough: a six-month observational cohort study

BACKGROUND

Recently we identified in patients with chronic cough a sensory dysregulation via which the urge-to-cough (UTC) or coughing are evoked mechanically from "somatic points for cough" (SPCs) in the neck and upper trunk. We investigated the prevalence and the clinical relevance of SPCs in an unselected population of patients with chronic cough.

METHODS

From 2018 to 2021, symptoms of 317 consecutive patients with chronic cough (233 females) were collected on four visits (V1-V4) 2 months apart at the Cough Clinic of the University Hospital in Florence (I). Participants rated the disturbance caused by the cough (0-9 modified Borg Scale). We attempted to evoke coughing and/or UTC using mechanical actions in all participants who were subsequently categorised as responsive (somatic point for cough positive, SPC+) or unresponsive (SPC-) to these actions. An association was established between chronic cough and its commonest causes; treatments were administered accordingly.

FINDINGS

169 patients were SPC+ and had a higher baseline cough score (p < 0.01). In most of the patients, the treatments reduced (p < 0.01) cough-associated symptoms. All patients reported a decrease (p < 0.01) in cough score at V2 (from 5.70 ± 1.4 to 3.43 ± 1.9 and from 5.01 ± 1.5 to 2.74 ± 1.7 for SPC+ and SPC- patients respectively). However, whilst in SPC- patients the cough score continued to decrease indicating virtually complete cough disappearance at V4 (0.97 ± 0.8), in SPC+ patients this variable remained close to V2 values during the entire follow-up.

INTERPRETATION

Our study suggests that the assessment of SPCs may identify patients whose cough is unresponsive and are eligible for specific treatments.

FUNDING

This work was funded by an unrestricted grant from Merck (Italy).

Comprehensive Approaches to Aspiration Pneumonia and Dysphagia in the Elderly on the Disease Time-Axis

Pneumonia in the elderly has been increasing on an annual basis. To a greater or lesser extent, aspiration is a major contributor to the development of pneumonia in the elderly. Antimicrobials alone are not sufficient for the treatment of pneumonia, and the condition may become intractable or even recur repeatedly. In addition, some patients with pneumonia may have no problems with eating, while others are unable to receive the necessary nutrition due to severe dysphagia. It has recently been found that pneumonia decreases both the muscle mass and strength of the swallowing and respiratory muscles, a condition named pneumonia-associated sarcopenia. This contributes to a pathophysiological time-axis of aspiration pneumonia and dysphagia in the elderly, in which silent aspiration leads to the development of pneumonia, and further to dysphagia, malnutrition, and low immunity. Therefore, it is recommended that the treatment and prevention of developing pneumonia should also differ according to an individual’s placement in the disease time-axis. In particular, approaches for preventing aspiration based on scientific findings are able to be implemented at home.

The effect of stimulation and unloading of baroreceptors on cough in experimental conditions

Cough, the main airway defensive process, is modulated by multiple sensory inputs from the respiratory system and outside of it. This modulation is one of the mechanisms that contributes to the sensitization of cough pathways at the peripheral and/or central level via neuroplasticity and it manifests most often as augmented coughing. Cardiorespiratory coupling is an important mechanism responsible for a match between oxygenation and cardiac output and bidirectional relationships exist between respiration and cardiovascular function. While the impact of cough with the robust swings of the intrathoracic pressure on haemodynamic parameters and heart electrophysiology are well characterized, little is known about the modulation of cough by haemodynamic parameters - mainly the blood pressure. Some circumstantial findings from older animal studies and more recent sophisticated analysis confirm that baroreceptor stimulation and unloading alters coughing evoked in experiments. Clinical relevance of such findings is not presently known.

Tracheobronchial-esophageal reflex initiates esophageal hypersensitivity and aggravates cough hyperreactivity in guinea pigs with esophageal acid infusion

Esophageal-tracheobronchial reflex is considered the main mechanism underlying cough due to gastroesophageal reflux, and is associated with esophageal hypersensitivity. We hypothesized that tracheobronchial-esophageal reflex may also exist, and may be related to esophageal hypersensitivity. To test this hypothesis, conscious and ether-anesthetized guinea pigs were subjected to repetitive capsaicin inhalation to establish models of cough (conscious) and cough-free (anesthetized) airway injury, respectively, followed by esophageal acid infusion. Recurrent capsaicin inhalation induced similar cough hyperreactivity to inhaled capsaicin after esophageal acid infusion in guinea pigs with cough and guinea pigs with cough-free airway injury during recurrent capsaicin inhalation. Cough hyperreactivity, along with overexpression of transient receptor potential vanilloid 1 (TRPV1) receptors in esophageal mucosa and in nerve fibers of tracheal mucosa of guinea pigs were blocked by pretreatment with esophageal infusion of capsazepine, but not atropine. Thus, recurrent airway nociceptive stimuli induce esophageal hyperreactivity via a tracheobronchial-esophageal reflex mediated by vagal C afferents expressing TRPV1, and enhance cough due to reflux.

Modeling and simulation of vagal afferent input of the cough reflex

We employed computational modeling to investigate previously conducted experiments of the effect of vagal afferent modulation on the cough reflex in an anesthetized cat animal model. Specifically, we simulated unilateral cooling of the vagus nerve and analyzed characteristics of coughs produced by a computational model of brainstem cough/respiratory neuronal network. Unilateral vagal cooling was simulated by a reduction of cough afferent input (corresponding to unilateral vagal cooling) to the cough network. All these attempts resulted in only mild decreases in investigated cough characteristics such as cough number, amplitudes of inspiratory and expiratory cough efforts in comparison with experimental data. Multifactorial alterations of model characteristics during cough simulations were required to approximate cough motor patterns that were observed during unilateral vagal cooling in vivo. The results support the plausibility of a more complex NTS processing system for cough afferent information than has been proposed.

Cough hypersensitivity and chronic cough

Chronic cough is globally prevalent across all age groups. This disorder is challenging to treat because many pulmonary and extrapulmonary conditions can present with chronic cough, and cough can also be present without any identifiable underlying cause or be refractory to therapies that improve associated conditions. Most patients with chronic cough have cough hypersensitivity, which is characterized by increased neural responsivity to a range of stimuli that affect the airways and lungs, and other tissues innervated by common nerve supplies. Cough hypersensitivity presents as excessive coughing often in response to relatively innocuous stimuli, causing significant psychophysical morbidity and affecting patients' quality of life. Understanding of the mechanisms that contribute to cough hypersensitivity and excessive coughing in different patient populations and across the lifespan is advancing and has contributed to the development of new therapies for chronic cough in adults. Owing to differences in the pathology, the organs involved and individual patient factors, treatment of chronic cough is progressing towards a personalized approach, and, in the future, novel ways to endotype patients with cough may prove valuable in management.

Neuro-allergology: Mast cell-nerve cross-talk

Mast cells (MCs) are derived from hematopoietic stem cells in the bone marrow, and their maturation is regulated by the tissue environment, such as the skin, lung and gut, leading to host defense. Peripheral nerve fibers located in various tissues are involved in diverse physiological and pathological processes. Anatomical relationships between MCs and nerve fibers were reported to have been observed in various organs. Moreover, MCs are positive for a large number of receptors for classical neurotransmitters (e.g., acetylcholine and corticotropin-releasing hormone) and neuropeptides (e.g., substance P, calcitonin gene-related peptides and hemokinin), and MC's functions are regulated by those nerve-derived factors. Also, histamine and proteases produced and released by MCs modulate nerve fiber functions. This functional cross-talk between MCs and nerve fibers can play physiological and pathological roles. MCs are key effector cells of allergic inflammation, such as atopic dermatitis, airway inflammation and food allergy. Here, we summarize and discuss the molecular mechanisms underlying the functional and anatomical cross-talk between MCs and nerve fibers in allergic inflamed tissues.

Mapping of the Sensory Innervation of the Mouse Lung by Specific Vagal and Dorsal Root Ganglion Neuronal Subsets

The airways are densely innervated by sensory afferent nerves, whose activation regulates respiration and triggers defensive reflexes (e.g., cough, bronchospasm). Airway innervation is heterogeneous, and distinct afferent subsets have distinct functional responses. However, little is known of the innervation patterns of subsets within the lung. A neuroanatomical map is critical for understanding afferent activation under physiological and pathophysiological conditions. Here, we quantified the innervation of the mouse lung by vagal and dorsal root ganglion (DRG) sensory subsets defined by the expression of Pirt (all afferents), 5HT3 (vagal nodose afferents), Tac1 (tachykinergic afferents), and transient receptor potential vanilloid 1 channel (TRPV1; defensive/nociceptive afferents) using Cre-mediated reporter expression. We found that vagal afferents innervate almost all conducting airways and project into the alveolar region, whereas DRG afferents only innervate large airways. Of the two vagal ganglia, only nodose afferents project into the alveolar region, but both nodose and jugular afferents innervate conducting airways throughout the lung. Many afferents that project into the alveolar region express TRPV1. Few DRG afferents expressed TRPV1. Approximately 25% of blood vessels were innervated by vagal afferents (many were Tac1+). Approximately 10% of blood vessels had DRG afferents (some were Tac1+), but this was restricted to large vessels. Lastly, innervation of neuroepithelial bodies (NEBs) correlated with the cell number within the bodies. In conclusion, functionally distinct sensory subsets have distinct innervation patterns within the conducting airways, alveoli and blood vessels. Physiologic (e.g., stretch) and pathophysiological (e.g., inflammation, edema) stimuli likely vary throughout these regions. Our data provide a neuroanatomical basis for understanding afferent responses in vivo.

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.

Prostaglandin E2 sensitizes the cough reflex centrally via EP3 receptor-dependent activation of NaV 1.8 channels

Background

Cough hypersensitivity is a major characteristic feature associated with several types of cough, including chronic cough, but its underlying mechanisms remain to be fully understood. Inflammatory mediators, such as prostaglandin E₂ (PGE₂), have been implicated in both peripheral induction and sensitization of the cough reflex. In this study, using a conscious guinea pig model of cough, we investigated whether PGE₂ can sensitize the cough reflex via central actions and, if so, via which mechanisms.

Methods

All drugs were administered by intracerebroventricular (i.c.v.) route and whole-body plethysmograph set-up was used for both induction, using aerosolized citric acid (0.2 M), and recording of cough. Immunohistochemistry was performed to confirm the expression of NaV 1.8 channels in the nucleus tractus solitarius (nTS).

Results

We show that both PGE₂ and the non-selective EP1/EP3 agonist, sulprostone, dose-dependently enhanced the citric acid-induced cough (P ≤ 0.001, P ≤ 0.01, respectively). Pretreatment with the EP1 antagonist, ONO-8130, did not affect the sulprostone-induced cough sensitization, whilst the EP3 antagonist, L-798,106, dose-dependently inhibited this effect (P ≤ 0.05). Furthermore, treatment with either the EP2 agonist, butaprost or the EP4 agonist, L-902,688, had no effect on cough sensitization. Additionally, pretreatment with either the TRPV1 antagonist, JNJ-17203212 or the TRPA1 antagonist, HC-030031, alone or in combination, nor with the NaV 1.1, 1.2, 1.3, 1.4, 1.6 and 1.7 channel blocker, tetrodotoxin, had any effect on the cough. In contrast, pretreatment with the NaV 1.8 antagonist, A-803467, dose-dependently inhibited this effect (P ≤ 0.05). Furthermore, NaV 1.8 channels were shown to be expressed in the nTS.

Conclusion

Collectively, our findings show that PGE₂ sensitizes the cough reflex centrally via EP3 receptor-dependent activation of NaV 1.8 but independently of TRPV1,TRPA1 and TTX-sensitive sodium channel activation. These results indicate that PGE₂ plays an important role in central sensitization of the cough reflex and suggest that central EP3 receptors and/or NaVv 1.8 channels may represent novel antitussive molecular targets.

Graphical Abstract

Antitussive effects of NaV 1.7 blockade in Guinea pigs

Our previous studies implicated the voltage-gated sodium channel subtype NaV 1.7 in the transmission of action potentials by the vagal afferent nerves regulating cough and thus identified this channel as a rational therapeutic target for antitussive therapy. But it is presently unclear whether a systemically administered small molecule inhibitor of NaV 1.7 conductance can achieve therapeutic benefit in the absence of side effects on cardiovascular function, gastrointestinal motility or respiration. To this end, we have evaluated the antitussive effects of the NaV 1.7 selective blocker Compound 801 administered systemically in awake guinea pigs or administered topically in anesthetized guinea pigs. We also evaluated the antitussive effects of ambroxol, a low affinity NaV blocker modestly selective for tetrodotoxin resistant NaV subtypes. Both Compound 801 and ambroxol dose-dependently inhibited action potential conduction in guinea pig vagus nerves (assessed by compound potential), with ambroxol nearly 100-fold less potent than the NaV 1.7 selective Compound 801 in this and other NaV 1.7-dependent guinea pig and human tissue-based assays. Both drugs also inhibited citric acid evoked coughing in awake or anesthetized guinea pigs, with potencies supportive of an NaV 1.7-dependent mechanism. Notably, however, the antitussive effects of systemically administered Compound 801 were accompanied by hypotension and respiratory depression. Given the antitussive effects of topically administered Compound 801, we speculate that the likely insurmountable side effects on blood pressure and respiratory drive associated with systemic dosing make topical formulations a viable and perhaps unavoidable therapeutic strategy for targeting NaV 1.7 in cough.

Symptom severity is associated with signs of central sensitization in patients with asthma

INTRODUCTION

Asthma is characterized by recurrent episodes of wheezing, dyspnoea, chest tightness and cough. In addition to respiratory symptoms, previous studies have reported the presence of pain.

OBJECTIVE

To analyse the nociceptive processing of young adults with well-controlled asthma.

METHODS

A cross-sectional case-control study was performed. Patients diagnosed with persistent well-controlled asthma were recruited from the 'Complejo Hospitalario Universitario' (Granada). Main outcomes included pain processing, measured by the pressure-pain thresholds (PPTs) and temporal summation and latency of pain; symptoms, including cough (Leicester Cough Questionnaire) and dyspnoea (Borg scale); and catastrophic cognitions about breathlessness, assessed by the Breathlessness Catastrophizing Scale (BCS).

RESULTS

Seven-two participants were finally recruited in our study. Patients with asthma presented lower pressure thresholds (p < 0.05) and significant differences in latency and summation tests. These patients also presented a greater cough level, with significant differences in all subscales (p < 0.05). Significant differences were also found in the BCS between groups (p < 0.001).

CONCLUSION

Our results show a decrease of PPTs and a greater pain intensity in latency and summation tests, suggesting an abnormal pain processing in patients with asthma.

Jugular vagal ganglia neurons and airway nociception: A target for treating chronic cough

The airways receive a dense supply of sensory nerve fibers that are responsive to damaging or potentially injurious stimuli. These airway nociceptors are mainly derived from the jugular and nodose vagal ganglia, and when activated they induce a range of reflexes and sensations that play an essential role in airway protection. Jugular nociceptors differ from nodose nociceptors in their embryonic origins, molecular profile and termination patterns in the airways and the brain, and recent discoveries suggest that excessive activity in jugular nociceptors may be central to the development of chronic cough. For these reasons, targeting jugular airway nociceptor signaling processes at different levels of the neuraxis may be a promising target for therapeutic development. In this focused review, we present the current understanding of jugular ganglia nociceptors, how they may contribute to chronic cough and mechanisms that could be targeted to bring about cough suppression.

A direct injection technique for investigation of lung sensory properties and reflex functions

NEW FINDINGS

This article reviews a unique direct injection technique that complements the more conventional right atrial injection and aerosol delivery methods to study sensory and reflex effects of the lung sensors. Used in combination with other methods, this technique should contribute to the pulmonary sensory research.

ABSTRACT

The lungs house sensory receptors (sensors) that mediate a variety of sensory and reflex responses to mechanical or chemical changes. These reflexes are mainly carried through pulmonary sympathetic and vagal afferent pathways. The chemosensors in the lung periphery are especially important in pulmonary diseases and their reflex responses have traditionally been studied either by aerosol delivery, which also activates receptors in the central airways, or by right atrial injection, which also activates receptors lying outside the lung. Thus, these techniques may confound the interpretation of sensory function. Our laboratory has developed a direct injection technique to deliver agents into the lung parenchyma, which complements the conventional techniques with some important advantages. This article reviews the technique.

Aprepitant for Cough in Lung Cancer. A Randomized Placebo-controlled Trial and Mechanistic Insights

Rationale: Effective cough treatments are a significant unmet need in patients with lung cancer. Aprepitant is a licensed treatment for nausea and vomiting, which blocks substance P activation of NK-1 (neurokinin 1) receptors, a mechanism also implicated in cough.Objectives: To assess aprepitant in patients with lung cancer with cough and evaluate mechanisms in vagal nerve tissue.Methods: Randomized double-blind crossover trial of patients with lung cancer and bothersome cough. They received 3 days of aprepitant or matched placebo; after a 3-day washout, patients crossed to the alternative treatment. The primary endpoint was awake cough frequency measured at screening and Day 3 of each treatment; secondary endpoints included patient-reported outcomes. In vitro, the depolarization of isolated guinea pig and human vagus nerve sections in grease-gap recording chambers, indicative of sensory nerve activation, was measured to evaluate the mechanism.Measurements and Main Results: Twenty patients with lung cancer enrolled, with a mean age 66 years (±7.7); 60% were female and 80% had non-small cell cancer, 50% had advanced stage, and 55% had World Health Organization performance status 1. Cough frequency improved with aprepitant, reducing by 22.2% (95% confidence interval [CI], 2.8-37.7%) over placebo while awake (P = 0.03), 30.3% (95% CI, 12.7-44.3) over 24 hours (P = 0.002), and 59.8% (95% CI, 15.1-86.0) during sleep (P = 0.081). Patient-reported outcomes all significantly improved. Substance P depolarized both guinea pig and human vagus nerve. Aprepitant significantly inhibited substance P-induced depolarization by 78% in guinea pig (P = 0.0145) and 94% in human vagus (P = 0.0145).Conclusions: Substance P activation of NK-1 receptors appears to be an important mechanism driving cough in lung cancer, and NK-1 antagonists show promise as antitussive therapies.

Molecular identity, anatomy, gene expression and function of neural crest vs. placode-derived nociceptors in the lower airways

The lower airways (larynx to alveoli) are protected by a complex array of neural networks that regulate respiration and airway function. Harmful stimuli trigger defensive responses such as apnea, cough and bronchospasm by activating a subpopulation of sensory afferent nerves (termed nociceptors) which are found throughout the airways. Airway nociceptive fibers are projected from the nodose vagal ganglia, the jugular vagal ganglia and the dorsal root ganglia, which are derived from distinct embryological sources: the former from the epibranchial placodes, the latter two from the neural crest. Embryological source determines nociceptive gene expression of receptors and neurotransmitters and recent evidence suggests that placode- and neural crest-derived nociceptors have distinct stimuli sensitivity, innervation patterns and functions. Improved understanding of the function of each subset in specific reflexes has substantial implications for therapeutic targeting of the neuronal components of airway disease such as asthma, viral infections and chronic obstructive pulmonary disease.

Mechanisms and Rationale for Targeted Therapies in Refractory and Unexplained Chronic Cough

Chronic cough, defined as a cough lasting > 8 weeks, is a common medical condition that exerts a substantial physical, mental, and social burden on patients. A subset of patients with chronic cough are troubled with a cough that persists despite optimal treatment of presumed associated common and uncommon conditions (refractory chronic cough; RCC) or in which no diagnosable cause for cough can be identified despite extensive assessment (unexplained chronic cough; UCC). Many of these patients exhibit clinical features of cough hypersensitivity, including laryngeal paresthesia, hypertussia, and allotussia. Over-the-counter cough remedies are ineffective and can lead to intolerable side effects when used for RCC/UCC, and the lack of approved treatments indicated for these conditions reflects a major unmet need. An increased understanding of the anatomy and neurophysiology of protective and pathologic cough has fostered a robust clinical development pipeline of several targeted therapies for RCC/UCC. This manuscript reviews the mechanisms presumed to underly RCC/UCC together with the rationale and clinical evidence for several targeted therapies currently under clinical investigation, including transient receptor potential channel antagonists, P2X3-receptor antagonists, voltage-gated sodium channel blockers, neuromodulators, and neurokinin-1-receptor antagonists. Finally, we provide an overview of targets that have been investigated in preclinical models of cough and other airway diseases that may hold future promise for clinical studies in RCC/UCC. Development of targeted therapies with different sites of action may foster a precision medicine approach to treat this heterogeneous, underserved patient population.

A role for neurokinin 1 receptor expressing neurons in the paratrigeminal nucleus in bradykinin-evoked cough in guinea-pigs

KEY POINTS

Airway projecting sensory neurons arising from the jugular vagal ganglia terminate centrally in the brainstem paratrigeminal nucleus, synapsing upon neurons expressing the neurokinin 1 receptor. This study aimed to assess the involvement of paratrigeminal neurokinin 1 receptor neurons in the regulation of cough, breathing and airway defensive responses. Lesioning neurokinin 1 receptor expressing paratrigeminal neurons significantly reduced cough evoked by inhaled bradykinin but not inhaled ATP or tracheal mechanical stimulation. The reduction in bradykinin-evoked cough was not accompanied by changes in baseline or evoked respiratory variables (e.g. frequency, volume or timing), animal avoidance behaviours or the laryngeal apnoea reflex. These findings warrant further investigations into targeting the jugular ganglia and paratrigeminal nucleus as a therapy for treating cough in disease.

ABSTRACT

Jugular vagal ganglia sensory neurons innervate the large airways and are thought to mediate cough and associated perceptions of airway irritations to a range of chemical irritants. The central terminals of jugular sensory neurons lie within the brainstem paratrigeminal nucleus, where postsynaptic neurons can be differentiated based on the absence or presence of the neurokinin 1 (NK1) receptor. Therefore, in the present study, we set out to test the hypothesis that NK1 receptor expressing paratrigeminal neurons play a role in cough evoked by inhaled chemical irritants. To test this, we performed selective neurotoxin lesions of NK1 receptor expressing neurons in the paratrigeminal nucleus in guinea-pigs using substance P conjugated to saporin (SSP-SAP). Sham lesion control or SSP-SAP lesion guinea-pigs received nebulised challenges, with the pan-nociceptor stimulant bradykinin or the nodose ganglia specific stimulant adenosine 5'-triphosphate (ATP), in conscious whole-body plethysmography to study cough and associated behaviours. Laryngeal apnoea reflexes and cough evoked by mechanical stimulation of the trachea were additionally investigated in anaesthetised guinea-pigs. SSP-SAP significantly and selectively reduced the number of NK1 receptor expressing neurons in the paratrigeminal nucleus. This was associated with a significant reduction in bradykinin-evoked cough, but not ATP-evoked cough, mechanical cough or laryngeal apnoeic responses. These data provide further evidence for a role of jugular vagal pathways in cough, and additionally suggest an involvement of NK1 receptor expressing neurons in the paratrigeminal nucleus. Therefore, this neural pathway may provide novel therapeutic opportunities to treat conditions of chronic cough.

Mapping of Sensory Nerve Subsets within the Vagal Ganglia and the Brainstem Using Reporter Mice for Pirt, TRPV1, 5-HT3, and Tac1 Expression

Vagal afferent sensory nerves, originating in jugular and nodose ganglia, are composed of functionally distinct subsets whose activation evokes distinct thoracic and abdominal reflex responses. We used Cre-expressing mouse strains to identify specific vagal afferent populations and map their central projections within the brainstem. We show that Pirt is expressed in virtually all vagal afferents; whereas, 5-HT3 is expressed only in nodose neurons, with little expression in jugular neurons. Transient receptor potential vanilloid 1 (TRPV1), the capsaicin receptor, is expressed in a subset of small nodose and jugular neurons. Tac1, the gene for tachykinins, is expressed predominantly in jugular neurons, some of which also express TRPV1. Vagal fibers project centrally to the nucleus tractus solitarius (nTS), paratrigeminal complex, area postrema, and to a limited extent the dorsal motor nucleus of the vagus. nTS subnuclei preferentially receive projections by specific afferent subsets, with TRPV1⁺ fibers terminating in medial and dorsal regions predominantly caudal of obex, whereas TRPV1⁻ fibers terminate in ventral and lateral regions throughout the rostral–caudal aspect of the medulla. Many vagal Tac1⁺ afferents (mostly derived from the jugular ganglion) terminate in the nTS. The paratrigeminal complex was the target of multiple vagal afferent subsets. Importantly, lung-specific TRPV1⁺ and Tac1⁺ afferent terminations were restricted to the caudal medial nTS, with no innervation of other medulla regions. In summary, this study identifies the specific medulla regions innervated by vagal afferent subsets. The distinct terminations provide a neuroanatomic substrate for the diverse range of reflexes initiated by vagal afferent activation.

Exploring the role of nerves in asthma; insights from the study of cough

Cough in asthma predicts disease severity, prognosis, and is a common and troublesome symptom. Cough is the archetypal airway neuronal reflex, yet little is understood about the underlying neuronal mechanisms. It is generally assumed that symptoms arise because of airway hyper-responsiveness and/or airway inflammation, but despite using inhaled corticosteroids and bronchodilators targeting these pathologies, a large proportion of patients have persistent coughing. This review focuses on the prevalence and impact of cough in asthma and explores data from pre-clinical and clinical studies which have explored neuronal mechanisms of cough and asthma. We present evidence to suggest patients with asthma have evidence of neuronal dysfunction, which is further heightened and exaggerated by both bronchoconstriction and airway eosinophilia. Identifying patients with excessive coughing with asthma may represent a neuro-phenotype and hence developing treatment for this symptom is important for reducing the burden of disease on patients' lives and currently represents a major unmet clinical need.

Molecular/Ionic Basis of Vagal Bronchopulmonary C-Fiber Activation by Inflammatory Mediators

Stimulation of bronchopulmonary vagal afferent C fibers by inflammatory mediators can lead to coughing, chest tightness, and changes in breathing pattern, as well as reflex bronchoconstriction and secretions. These responses serve a defensive function in healthy lungs but likely contribute to many of the signs and symptoms of inflammatory airway diseases. A better understanding of the mechanisms underlying the activation of bronchopulmonary C-fiber terminals may lead to novel therapeutics that would work in an additive or synergic manner with existing anti-inflammatory strategies.

Cough Suppression during Flexible Bronchoscopy Using Transcutaneous Electric Acupoint Stimulation: A Randomized Controlled Study

Background and Objective. Transcutaneous electric acupoint stimulation (TEAS) is recommended for its sedative and analgesic effects. We sought to evaluate the effect of TEAS on cough suppression during flexible bronchoscopy (FB) and explore the underlying mechanism. Methods. In this single-center, randomized, single-blind, parallel-controlled study, we randomized 100 patients scheduled for FB into two equal groups treated with or without TEAS (TEAS group and control group). Patients in the TEAS group received 30 min of stimulation at the Hegu (LI4), Neiguan (PC6), and Lieque (LU7) points before FB. The control group underwent the same procedure, but without stimulation. The primary outcome was the intraoperative cough score determined by the bronchoscopist. The secondary outcomes were patient-reported discomfort scores and other procedural parameters. Results. Compared with the controls, patients who received TEAS preconditioning had lower cough scores (P=0.0027) and requirement of lidocaine and fentanyl (P < 0.05) and significantly higher postprocedural plasma β-endorphin levels (P=0.0367). There were no intergroup differences in discomfort scores, midazolam dosage, rate of premature termination, oxygen requirement, sedation level, airway assistance, oxygen saturation, lowest oxygen saturation level, heart rate, plasma substance-P levels, and rate of complications after 24 h. The total procedure duration, time for passage of the bronchoscope through the vocal cords, and systolic and diastolic blood pressure levels were less in the TEAS group than in the control group (P=0.033, 0.039 and <0.05, respectively). Conclusion. The combination of midazolam and TEAS was superior to midazolam alone for cough suppression during FB, probably due to increased plasma β-endorphin levels. This trial is registered with ChiCTR1800016612 at chictr.org.cn/index.aspx.

Emerging targets for cough therapies; NK1 receptor antagonists

Cough is mediated by vagal afferent fibres innervating the larynx and proximal airways. Pre-clinical studies suggest that vagal C fibres produce Substance P, one of the tachykinin family of neuropeptides, which has been shown to enhance cough via the neurokinin-1 (NK-1) receptor and studies in animal models have also shown that NK-1 antagonists are effective at blocking induced cough. In the past, tachykinin receptor antagonists have yielded disappointing results in treating asthma and cough, however most of the activity of the agents tested was restricted to the peripheral nervous system and also the outcomes measures evaluating cough not optimal. More recently a small proof of concept study has suggest that aprepitant, an NK-1 antagonist licensed for the prevention of chemotherapy induced nausea and vomiting, might have beneficial effects on cough frequency in patients with lung cancer. In this review we investigate the current evidence for the anti-tussive effect of these therapies and the clinical trials in progress.

Vagal Afferent Processing by the Paratrigeminal Nucleus

The paratrigeminal nucleus is an obscure region in the dorsal lateral medulla, which has been best characterized as a collection of interstitial cells located in the dorsal tip of the spinal trigeminal tract. The paratrigeminal nucleus receives afferent input from the vagus, trigeminal, spinal, and glossopharyngeal nerves, which contribute to its long-known roles in the baroreceptor reflex and nociceptive processing. More recently, studies have shown that this region is also involved in the processing of airway-derived sensory information. Notably, these studies highlight an underappreciated complexity in the neuronal content and circuit connectivity of the paratrigeminal nucleus. However, much remains to be understood about how paratrigeminal processing of vagal afferents is altered in disease. The aim of the present review is to provide an update of the current understanding of vagal afferent processing in the paratrigeminal nucleus and to explore how dysregulation at this site may contribute to vagal sensory neural dysfunction during disease.

The Neurokinin-1 Receptor Antagonist Orvepitant Is a Novel Antitussive Therapy for Chronic Refractory Cough: Results From a Phase 2 Pilot Study (VOLCANO-1)

BACKGROUND

Substance P and the neurokinin-1 (NK-1) receptor are implicated in chronic refractory cough pathophysiology. We assessed the efficacy and safety of orvepitant, a brain-penetrant NK-1 antagonist, in an open-label study in CRC patients with chronic refractory cough.

METHODS

Thirteen patients with daytime cough frequency >3 to <250 coughs/h took orvepitant 30 mg once daily for 4 weeks. Objective cough frequency was measured over 24 h at baseline and weeks 1, 4, and 8. The primary end point was change from Baseline in daytime cough frequency at week 4. Secondary end points included cough severity visual analog scale (VAS) score, global ratings of change for cough frequency and severity, and Cough-specific Quality of Life Questionnaire score.

RESULTS

All patients completed the study. Mean baseline cough frequency was 71.4/h. A statistically and clinically significant improvement in objective daytime cough frequency was observed at week 4: reduction from baseline of 18.9 (26%) coughs/h (95% CI, 9.6-28.3; P < .001). This effect was apparent at week 1 (reduction from baseline of 27.0 [38%] coughs/h [95% CI, 11.4-42.7; P = .001]) and sustained after drug discontinuation at week 8 (reduction from baseline of 20.4 [29%] coughs/h [95% CI, 3.2-37.5; P = .020]). Statistically significant improvements were seen for severity VAS and quality of life. Orvepitant was safe and well-tolerated.

CONCLUSIONS

Orvepitant resulted in a significant and sustained improvement in objective cough frequency, severity VAS, and quality of life; appeared safe; and merits further clinical investigation.

TRIAL REGISTRY

EU Clinical Trials Register; No.: 2014-003947-36; URL: www.clinicaltrialsregister.eu.

Airway Innervation and Plasticity in Asthma

Airway nerves represent a mechanistically and therapeutically important aspect that requires better highlighting in the context of diseases such as asthma. Altered structure and function (plasticity) of afferent and efferent airway innervation can contribute to airway diseases. We describe established anatomy, current understanding of how plasticity occurs, and contributions of plasticity to asthma, focusing on target-derived growth factors (neurotrophins). Perspectives toward novel treatment strategies and future research are provided.

Increased expression of lung TRPV1/TRPA1 in a cough model of bleomycin-induced pulmonary fibrosis in Guinea pigs

Background

Chronic cough is a difficult-to-treat comorbidity of idiopathic pulmonary fibrosis (IPF), and significantly impacts on the quality of life of patients with IPF. Transient receptor potential (TRP) channel proteins may play an important role in chronic cough. However, expression of these proteins in lung of IPF is largely unknown.

Methods

Guinea pig model of pulmonary fibrosis was established by single intratracheal delivery of bleomycin. Respiratory ungated micro-CT scans were performed on days 7, 14, 21 and 28 to assess progression of pulmonary fibrosis. Cough sensitivity to capsaicin was evaluated in conscious animals on days 13 and 27. Real-time PCR (qPCR) and immunohistochemistry were employed to measure expression of TRPV1 and TRPA1 in lung tissue.

Results

Micro-CT showed that lung consolidation was detectable from day 7 distributing mainly in the middle and lower lung fields, which was significantly correlated to Ashcroft fibrosis score (r = 0.7993, p < 0.001). Cough sensitivity to capsaicin in bleomycin-treated animals was significantly increased on days 13 and 27. qPCR showed that expression of TRPV1 and TRPA1 was positively correlated each other and significantly upregulated in lung tissues of model group compared with that of controls, which was further supported by immunohistochemistry. Furthermore, immunoreactivity for TRPV1 and TRPA1 was negatively correlated with Ashcroft fibrosis score.

Conclusion

Expression of TRPV1/TRPA1 was upregulated in the chronic cough related to bleomycin induced pulmonary fibrosis in guinea pigs, which provided new insights into the mechanism of IPF-associated cough hypersensitivity. Micro-CT is very helpful methodology to access pulmonary fibrosis progression in small animal models.

Trigger Reduction Prior to Systemic Drugs for Neurogenic Chronic Cough

OBJECTIVE/HYPOTHESIS

Neurogenic chronic cough typically presents as a postviral chronic cough, often with paroxysms of coughing preceded by a tickle sensation with multiple triggers and often recalcitrant to multiple treatments for reflux disease, sinus disease, and asthma. Current treatment uses neuromodulating agents with moderate success. Post nasal drainage and laryngopharyngeal reflux can be triggers in the setting of laryngopharyngeal hypersensitivity. Treatment will focus on trigger reduction using nasal toilet and a dietary regimen for laryngopharyngeal reflux.

STUDY DESIGN

Systematic review of retrospective cohort studies METHODS: One-year retrospective review of new patients with cough (R05.0) excluding asthma, proton pump inhibitor response, and sinus or pulmonary disease. Cough severity index (CSI) and reflux symptom index (RSI) were evaluated initially and 6 weeks after trigger-reduction treatment using nasal saline irrigation, nasal steroids, nasal antihistamines, and a plant-based diet with alkaline water.

RESULTS

Of 119 patients, 29 met the criteria. Using the six-point reduction (improvement) in RSI as an accepted response, 20 of 29 patients (68.9%, P = .0014) experienced a clinical response. Using reduction in RSI and CSI as a continuous variable to assess response, patients experienced a 10 (95% confidence interval [CI]: 6.75-13.2) and 10.9 (95% CI: 7.4-14.3) mean point reduction, respectively. The mean percent reduction in RSI following 6 weeks of treatment was 54.7% (95% CI: 41.5-68.0; P = .0001). These patients experienced a 59.8% (95% CI: 43.4-76.2; P = .0001) reduction in CSI.

CONCLUSIONS

A trigger-reduction approach using nasal toilet and a plant-based diet in patients with neurogenic chronic cough prior to the initiation of systemic neuromodulating medications should be considered.

LEVEL OF EVIDENCE

4 Laryngoscope, 129:198-202, 2019.

Central adenosine A1 receptors inhibit cough via suppression of excitatory glutamatergic and tachykininergic neurotransmission

BACKGROUND AND PURPOSE

The adenosine A1 receptor is reported to mediate several excitatory effects in the airways and has inhibitory effects in the CNS. In this study, we investigated the role of peripheral and central A1 receptors in regulating cough and airway obstruction.

EXPERIMENTAL APPROACH

Drugs were administered to guinea pigs via inhalation or i.c.v. infusion. Following the administration of different drugs, cough was induced by exposing guinea pigs to aerosolized 0.4 M citric acid. An automated analyser recorded both cough and airway obstruction simultaneously using whole-body plethysmography.

KEY RESULTS

The A1 receptor agonist, cyclopentyladenosine (CPA, administered by inhalation), dose-dependently inhibited cough and also inhibited airway obstruction. Similarly, CPA, administered i.c.v., inhibited both the citric acid-induced cough and airway obstruction; this was prevented by pretreatment with the A1 receptor antagonist DPCPX (i.c.v.). Treatment with DPCPX alone dose-dependently enhanced the citric acid-induced cough and airway obstruction. This effect was reversed following treatment with either the glutamate GluN1 receptor antagonist D-AP5 or the neurokinin NK1 receptor antagonist FK-888.

CONCLUSIONS AND IMPLICATIONS

These findings suggest that activation of either peripheral or central adenosine A1 receptors inhibits citric acid-induced cough and airway obstruction. The data also suggest that tonic activation of central adenosine A1 receptors serves as a negative regulator of cough and airway obstruction, secondary to inhibition of excitatory glutamatergic and tachykininergic neurotransmission.

The research of the possible mechanism and the treatment for capsaicin-induced cough

Unexplained chronic cough (UCC) affects millions of patients worldwide. New therapeutic approaches to this condition are urgently needed, since current treatment options provide only symptomatic relief. Cough reflex hypersensitivity has been shown to play an important role in the pathogenesis of UCC. The transient receptor potential vanilloid type 1 (TRPV1) is present on peripheral terminals of airway sensory nerves and modulation of its activity represents a potential target for the pharmacological treatment of UCC. The aim of this study was to explore the efficacy and the possible mechanism of SB705498, a TRPV1 antagonist, for cough in a capsaicin-induced cough animal model (i.e. guinea pigs). Induction of cough by capsaicin was successfully implemented in the guinea pigs, and the animals that met the inclusion criteria were randomly divided into four treatment groups: (1) Saline inhalation group (NSInh group, N = 10, negative control group), (2) Codeine phosphate intraperitoneal injection group (CPInp group, N = 10, positive control group), (3) SB705498 inhalation group (SBInh group, N = 10), (4) SB705498 intragastric administration group (SBIng group, N = 10). After treatment with above compounds, the capsaicin-induced cough experiment was performed again. The cough numbers and the cough incubation periods were recorded to evaluate the antitussive effect of SB705498. Enzyme-linked immunosorbent assay (ELISA) testing and Immunohistochemistry (IHC) staining for substance P (SP), calcitonin gene related peptide (CGRP) and neurokinin A (NKA) expression in lung and brain tissues were performed as an indication of neurogenic inflammation. Hematoxylin-Eosin (H&E) staining was used to observe the pathology morphology of lung and brain tissues. When the CPInp, SBInh and SBIng groups were compared to the NSInh group, the cough numbers were significantly reduced (p < .001), while the cough incubation periods were significantly prolonged (P < .001). In addition, the expression of SP, CGRP and NKA in lung and brain tissue was reduced (P < .05). None of the animals in the four groups exhibited lung and brain parenchymal inflammation. The results from this study showed that SB705498 had a significant antitussive effect, could reduce the neurogenic inflammation by reducing the expression of SP, CGRP and NKA in a capsaicin-induced cough model of guinea pigs. The results further indicated that TRPV1 played an important role in UCC and SB705498 might be a promising therapeutic agent for UCC.

Opposing effects of bronchopulmonary C-fiber subtypes on cough in guinea pigs

We have addressed the hypothesis that the opposing effects of bronchopulmonary C-fiber activation on cough are attributable to the activation of C-fiber subtypes. Coughing was evoked in anesthetized guinea pigs by citric acid (0.001-2 M) applied topically in 100-µl aliquots to the tracheal mucosa. In control preparations, citric acid evoked 10 ± 1 coughs cumulatively. Selective activation of the pulmonary C fibers arising from the nodose ganglia with either aerosols or continuous intravenous infusion of adenosine or the 5-HT₃ receptor-selective agonist 2-methyl-5-HT nearly abolished coughing evoked subsequently by topical citric acid challenge. Delivering adenosine or 2-methyl-5-HT directly to the tracheal mucosa (where few if any nodose C fibers terminate) was without effect on citric acid-evoked cough. These actions of pulmonary administration of adenosine and 2-methyl-5-HT were accompanied by an increase in respiratory rate, but it is unlikely that the change in respiratory pattern caused the decrease in coughing, as the rapidly adapting receptor stimulant histamine also produced a marked tachypnea but was without effect on cough. In awake guinea pigs, adenosine failed to evoke coughing but reduced coughing induced by the nonselective C-fiber stimulant capsaicin. We conclude that bronchopulmonary C-fiber subtypes in guinea pigs have opposing effects on cough, with airway C fibers arising from the jugular ganglia initiating and/or sensitizing the cough reflex and the intrapulmonary C fibers arising from the nodose ganglia actively inhibiting cough upon activation.

Airway expression of Transient Receptor Potential (TRP) Vanniloid-1 and Ankyrin-1 channels is not increased in patients with Idiopathic Pulmonary Fibrosis

Dry cough is a common symptom described in patients with Idiopathic Pulmonary Fibrosis (IPF) and impairs quality of life. The exact mechanisms causing cough in IPF remain unclear, however evidence suggests altered cough neurophysiology and sensitisation plays a role; IPF patients have an enhanced cough reflex sensitivity to inhaled capsaicin. The Transient Receptor Potential Vanniloid-1 channel (TRPV-1) has a role in the cough reflex and airway expression is increased in patients with chronic cough. The Ankyrin-1 receptor (TRPA-1) is often co-expressed. It was hypothesised that, like chronic cough patients, IPF patients have increased airway TRP receptor expression. Bronchial biopsies were obtained from 16 patients with IPF, 11 patients with idiopathic chronic cough and 8 controls without cough. All other causes of cough were rigorously excluded. Real-time quantitative Polymerase Chain Reaction was used to detect TRPV-1 and TRPA-1 mRNA expression with Immunohistochemistry demonstrating protein expression. Mean TRPV-1 and TRPA-1 gene expression was higher in IPF patients compared with controls, but the difference did not reach statistical significance. Immunostaining supported these findings. This study suggests that structural up-regulation of central airway TRP receptors is not the key mechanism for cough in IPF patients. It is probable that IPF cough results from altered neuronal sensitivity at multiple levels of the cough pathway.

Sense and Immunity: Context-Dependent Neuro-Immune Interplay

The sensory nervous and immune systems, historically considered autonomous, actually work in concert to promote host defense and tissue homeostasis. These systems interact with each other through a common language of cell surface G protein-coupled receptors and receptor tyrosine kinases as well as cytokines, growth factors, and neuropeptides. While this bidirectional communication is adaptive in many settings, helping protect from danger, it can also become maladaptive and contribute to disease pathophysiology. The fundamental logic of how, where, and when sensory neurons and immune cells contribute to either health or disease remains, however, unclear. Our lab and others’ have begun to explore how this neuro-immune reciprocal dialog contributes to physiological and pathological immune responses and sensory disorders. The cumulative results collected so far indicate that there is an important role for nociceptors (noxious stimulus detecting sensory neurons) in driving immune responses, but that this is highly context dependent. To illustrate this concept, we present our findings in a model of airway inflammation, in which nociceptors seem to have major involvement in type 2 but not type 1 adaptive immunity.