Cough and expiration reflexes elicited by inhaled irritant gases are intensified in ovalbumin-sensitized mice

Divergent sensory pathways of sneezing and coughing

Sneezing and coughing are primary symptoms of many respiratory viral infections and allergies. It is generally assumed that sneezing and coughing involve common sensory receptors and molecular neurotransmission mechanisms. Here, we show that the nasal mucosa is innervated by several discrete populations of sensory neurons, but only one population (MrgprC11+MrgprA3-) mediates sneezing responses to a multitude of nasal irritants, allergens, and viruses. Although this population also innervates the trachea, it does not mediate coughing, as revealed by our newly established cough model. Instead, a distinct sensory population (somatostatin [SST+]) mediates coughing but not sneezing, unraveling an unforeseen sensory difference between sneezing and coughing. At the circuit level, sneeze and cough signals are transmitted and modulated by divergent neuropathways. Together, our study reveals the difference in sensory receptors and neurotransmission/modulation mechanisms between sneezing and coughing, offering neuronal drug targets for symptom management in respiratory viral infections and allergies.

Release of ATP in the lung evoked by inhalation of irritant gases in rats

Adenosine triphosphate (ATP) can be released into the extracellular milieu from various types of cells in response to a wide range of physical or chemical stresses. In the respiratory tract, extracellular ATP is recognized as an important signal molecule and trigger of airway inflammation. Chlorine (Cl2), sulfur dioxide (SO2), and ammonia (NH3) are potent irritant gases and common industrial air pollutants due to their widespread uses as chemical agents. This study was carried out to determine if acute inhalation challenges of these irritant gases, at the concentration and duration simulating the accidental exposures to these chemical gases in industrial operations, triggered the release of ATP in the rat respiratory tract; and if so, whether the level of ATP in bronchoalveolar lavage fluid (BALF) evoked by inhalation challenge of a given irritant gas was elevated by chronic allergic airway inflammation. Our results showed: 1) inhalation of these irritant gases caused significant increases in the ATP level in BALF, and the magnitude of evoked ATP release was in the order of Cl2 > SO2 > NH3. 2) Chronic airway inflammation induced by ovalbumin-sensitization markedly elevated the ATP level in BALF during baseline (breathing room air) but did not potentiate the release of ATP in the lung triggered by inhalation challenges of these irritant gases. These findings suggested a possible involvement of the ATP release in the lung in the regulation of overall airway responses to acute inhalation of irritant gases and the pathogenesis of chronic allergic airway inflammation.NEW & NOTEWORTHY Extracellular adenosine triphosphate (ATP) is a contributing factor and signaling molecule of airway inflammation. This study demonstrated for the first time that the ATP release in the lung was markedly elevated after acute inhalation challenges of three common industrial air pollutants; the order of the response magnitude was chlorine > sulfur dioxide > ammonia. These findings provided new information and improved our understanding of the adverse pulmonary effects caused by accidental inhalation exposures to these irritant gases.

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.

Identifying vagal bronchopulmonary afferents mediating cough response to inhaled sulfur dioxide in mice

Sulfur dioxide (SO2), a common environmental and industrial air pollutant, possesses a potent effect in eliciting cough reflex, but the primary type of airway sensory receptors involved in its tussive action has not been clearly identified. This study was carried out to determine the relative roles of three major types of vagal bronchopulmonary afferents [slowly adapting receptors (SARs), rapidly adapting receptors (RARs), and C-fibers] in regulating the cough response to inhaled SO2. Our results showed that inhalation of SO2 (300 or 600 ppm for 8 min) evoked an abrupt and intense stimulatory effect on bronchopulmonary C-fibers, which continued for the entire duration of inhalation challenge and returned toward the baseline in 1-2 min after resuming room air-breathing in anesthetized and mechanically ventilated mice. In stark contrast, the same SO2 inhalation challenge generated a distinct and consistent inhibitory effect on both SARs and phasic RARs; their phasic discharges synchronized with respiratory cycles during the baseline (breathing room air) began to decline progressively within 1-3 min after the onset of SO2 inhalation, ceased completely before termination of the 8-min inhalation challenge, and then slowly returned toward the baseline after >40 min. In a parallel study in awake mice, inhalation of SO2 at the same concentration and duration as that in the nerve recording experiments evoked cough responses in a pattern and time course similar to that observed in the C-fiber responses. Based on these results, we concluded that stimulation of vagal bronchopulmonary C-fibers is primarily responsible for triggering the cough response to inhaled SO2.NEW & NOTEWORTHY This study demonstrated that inhalation of a high concentration of sulfur dioxide, an irritant gas and common air pollutant, completely and reversibly inhibited the neural activities of both slowly adapting receptor and rapidly adapting receptor, two major types of mechanoreceptors in the lungs with their activities conducted by myelinated fibers. Furthermore, the results of this study suggested that stimulation of vagal bronchopulmonary C-fibers is primarily responsible for triggering the cough reflex responses to inhaled sulfur dioxide.

Current understanding of Bordetella-induced cough

Typical pathogenic bacteria of the genus Bordetella cause respiratory diseases, many of which are characterized by severe coughing in host animals. In human infections with these bacteria, such as whooping cough, coughing imposes a heavy burden on patients. The pathophysiology of this severe coughing had long been uncharacterized because convenient animal models that reproduce Bordetella-induced cough have not been available. However, rat and mouse models were recently shown as useful for understanding, at least partially, the causative factors and the mechanism of Bordetella-induced cough. Many types of coughs are induced under various physiological conditions, and the neurophysiological pathways of coughing are considered to vary among animal species, including humans. However, the neurophysiological mechanisms of the coughs in different animal species have not been entirely understood, and, accordingly, the current understanding of Bordetella-induced cough is still incomplete. Nevertheless, recent research findings may open the way for the development of prophylaxis and therapeutic measures against Bordetella-induced cough.

Chronic cough relief by allosteric modulation of P2X3 without taste disturbance

P2X receptors are cation channels that sense extracellular ATP. Many therapeutic candidates targeting P2X receptors have begun clinical trials or acquired approval for the treatment of refractory chronic cough (RCC) and other disorders. However, the present negative allosteric modulation of P2X receptors is primarily limited to the central pocket or the site below the left flipper domain. Here, we uncover a mechanism of allosteric regulation of P2X3 in the inner pocket of the head domain (IP-HD), and show that the antitussive effects of quercetin and PSFL2915 (our nM-affinity P2X3 inhibitor optimized based on quercetin) on male mice and guinea pigs were achieved by preventing allosteric changes of IP-HD in P2X3. While being therapeutically comparable to the newly licensed P2X3 RCC drug gefapixant, quercetin and PSFL2915 do not have an adverse effect on taste as gefapixant does. Thus, allosteric modulation of P2X3 via IP-HD may be a druggable strategy to alleviate RCC.

What Causes the Cough in Whooping Cough?

What causes the cough in whooping cough (pertussis) has been a longstanding question in the field but has been difficult to answer because of the perceived lack of convenient small animal models. Y. Hiramatsu, K. Suzuki, T. Nishida, N. Onoda, et al. (mBio 13:e01397-21, 2022, https://doi.org/10.1128/mbio.03197-21) used a mouse model and cellular studies to investigate bacterial and host factors that contribute to cough production during Bordetella pertussis infection. In elegant studies, they found that the bacterial factors pertussis toxin, lipooligosaccharide, and Vag8 function cooperatively to produce cough. These factors induce production of host bradykinin, a known cough inducer that sensitizes the ion channel TRPV1 on neurons, and they investigated host signaling pathways altered by the bacterial factors that exacerbate cough responses. This is a highly significant and important finding that not only elucidates mechanisms underlying the pathophysiology of the severe cough, but also may reveal potential novel therapeutic approaches to treat individuals suffering from the debilitating effects of cough in pertussis.

The Mechanism of Pertussis Cough Revealed by the Mouse-Coughing Model

Pertussis, also known as whooping cough, is a contagious respiratory disease caused by the Gram-negative bacterium Bordetella pertussis. This disease is characterized by severe and uncontrollable coughing, which imposes a significant burden on patients. However, its etiological agent and the mechanism are totally unknown because of a lack of versatile animal models that reproduce the cough. Here, we present a mouse model that reproduces coughing after intranasal inoculation with the bacterium or its components and demonstrate that lipooligosaccharide (LOS), pertussis toxin (PTx), and Vag8 of the bacterium cooperatively function to cause coughing. Bradykinin induced by LOS sensitized a transient receptor potential ion channel, TRPV1, which acts as a sensor to evoke the cough reflex. Vag8 further increased bradykinin levels by inhibiting the C1 esterase inhibitor, the major downregulator of the contact system, which generates bradykinin. PTx inhibits intrinsic negative regulation systems for TRPV1 through the inactivation of G_i GTPases. Our findings provide a basis to answer long-standing questions on the pathophysiology of pertussis cough.

Non-primate animal models for pertussis: back to the drawing board?

Despite considerable progress in the understanding of clinical pertussis, the contemporary emergence of antimicrobial resistance for Bordetella pertussis and an evolution of concerns with acellular component vaccination have both sparked a renewed interest. Although simian models of infection best correlate with the observed attributes of human infection, several animal models have been used for decades and have positively contributed in many ways to the related science. Nevertheless, there is yet the lack of a reliable small animal model system that mimics the combination of infection genesis, variable upper and lower respiratory infection, systemic effects, infection resolution, and vaccine responses. This narrative review examines the history and attributes of non-primate animal models for pertussis and places context with the current use and needs. Emerging from the latter is the necessity for further such study to better create the optimal model of infection and vaccination with use of current molecular tools and a broader range of animal systems. KEY POINTS: • Currently used and past non-primate animal models of B. pertussis infection often have unique and focused applications. • A non-primate animal model that consistently mimics human pertussis for the majority of key infection characteristics is lacking. • There remains ample opportunity for an improved non-primate animal model of pertussis with the use of current molecular biology tools and with further exploration of species not previously considered.

A descending pathway emanating from the periaqueductal gray mediates the development of cough-like hypersensitivity

Chronic cough is a common refractory symptom of various respiratory diseases. However, the neural mechanisms that modulate the cough sensitivity and mediate chronic cough remain elusive. Here, we report that GABAergic neurons in the lateral/ventrolateral periaqueductal gray (l/vlPAG) suppress cough processing via a descending pathway. We found that l/vlPAG neurons are activated by coughing-like behaviors and that tussive agent-evoked coughing-like behaviors are impaired after activation of l/vlPAG neurons. In addition, we showed that l/vlPAG neurons form inhibitory synapses with the nucleus of the solitary tract (NTS) neurons. The synaptic strength of these inhibitory projections is weaker in cough hypersensitivity model mice than in naïve mice. Important, activation of l/vlPAG GABAergic neurons projecting to the NTS decreases coughing-like behaviors. In contrast, suppressing these neurons enhances cough sensitivity. These results support the notion that l/vlPAG GABAergic neurons play important roles in cough hypersensitivity and chronic cough through disinhibition of cough processing at the medullary level.

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GABAergic neurons in the l/vlPAG inhibit coughing-like behaviors

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The l/vlPAG sends predominately inhibitory projections to the NTS

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l/vlPAG GABAergic neurons modulate coughing-like behaviors via descending projections

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l/vlPAG-NTS projections mediate cough hypersensitivity via disinhibitory mechanisms

Animal models of cough

Cough is a vital airway reflex that keeps the respiratory tract wisely protected. It is also a sign of many diseases of the respiratory system and it may become a disease in its own right. Even though the efficacy of antitussive compounds is extensively studied in animal models with promising results, the treatment of pathological cough in humans is insufficient at the moment. The limited translational potential of animal models used to study cough causes, mechanisms and possible therapeutic targets stems from multiple sources. First of all, cough induced in the laboratory by mechanical or chemical stimuli is far from natural cough present in human disease. The main objective of this review is to provide a comprehensive summary of animal models currently used in cough research and to address their advantages and disadvantages. We also want to encourage cough researchers to call for precision is research by addressing the sex bias which has existed in basic cough research for decades and discuss the role of specific pathogen-free (SPF) animals.

Mechanisms underlying the stimulatory effect of inhaled sulfur dioxide on vagal bronchopulmonary C-fibres

KEY POINTS

Brief inhalation of SO₂ of concentration >500 p.p.m. triggered a pronounced stimulatory effect on vagal bronchopulmonary C-fibres in anaesthetized rats. This stimulatory effect was drastically diminished by a pretreatment with NaHCO₃ that raised the baseline arterial pH, suggesting a possible involvement of acidification of airway fluid and/or tissue generated by inhaled SO₂ . The stimulation was completely abolished by pretreatment with antagonists of both acid-sensing ion channels and transient receptor potential vanilloid type-1 receptors, indicating that this effect was caused by acid activation of these cation channels expressed in airway sensory nerves. This conclusion was further supported by the results obtained from studies in isolated rat vagal bronchopulmonary sensory neurones and also in the cough response to SO₂ inhalation challenge in awake mice. These results provide new insight into the underlying mechanism of harmful irritant effects in the respiratory tract caused by accidental exposure to a high concentration of SO₂ .

ABSTRACT

Inhalation of sulfur dioxide (SO₂ ) triggers coughs and reflex bronchoconstriction, and stimulation of vagal bronchopulmonary C-fibres is primarily responsible. However, the mechanism underlying this stimulatory effect is not yet fully understood. In this study, we tested the hypothesis that the C-fibre stimulation was caused by SO₂ -induced local tissue acidosis in the lung and airways. Single-unit activities of bronchopulmonary C-fibres in response to inhalation challenges of SO₂ (500-1500 p.p.m., 10 breaths) were measured in anaesthetized rats. Inhalation of SO₂ reproducibly induced a pronounced and sustained stimulation (lasting for 15-60 s) of pulmonary C-fibres in a concentration-dependent manner. This stimulatory effect was significantly attenuated by an increase in arterial pH generated by infusion of sodium bicarbonate (NaHCO₃ ), and completely abrogated by a combined pretreatment with amiloride (an antagonist of acid-sensing ion channels, ASICs) and AMG8910 (a selective antagonist of the transient receptor potential vanilloid type-1 receptor, TRPV1). Furthermore, in isolated rat vagal pulmonary sensory neurones, perfusion of an aqueous solution of SO₂ evoked a transient increase in the intracellular Ca²⁺ concentration; this response was also markedly diminished by a pretreatment with amiloride and AMG8910. In addition, inhalation of SO₂ consistently evoked coughs in awake mice; responses were significantly smaller in TRPV1^-/- mice than in wild-type mice, and almost completely abolished after a pretreatment with amiloride in TRPV1^-/- mice. These results suggested that the stimulatory effect of inhaled SO₂ on bronchopulmonary C-fibres was generated by acidification of fluid and/or tissue in the lung and airways, which activated both ASICs and TRPV1 expressed in these sensory nerves.

Airway hypersensitivity induced by eosinophil granule-derived cationic proteins

Vagal bronchopulmonary C-fiber sensory nerves play an important role in the manifestation of airway hypersensitivity, a common and prominent pathophysiological feature of airway inflammatory diseases. Eosinophil granule-derived cationic proteins are known to be involved in the mucosal damage and development of bronchial hyperresponsiveness during allergic airway inflammation. In view of these background information, we have carried out a series of studies to investigate the effect of cationic proteins on these C-fiber afferents and the mechanism(s) possibly involved; a summary of these studies is presented in this mini-review. Intra-tracheal instillation of either eosinophil granule-derived (e.g., major basic protein, MBP) or synthetic cationic proteins (e.g., poly-l-lysine) induced a sporadic, but intense and lingering discharge of pulmonary C-fibers, and greatly enhanced the chemical and mechanical sensitivities of these afferents in anesthetized rats. The stimulatory and sensitizing effects of these proteins were completely nullified when their cationic charges were neutralized or removed. Furthermore, in isolated rat bronchopulmonary capsaicin-sensitive neurons, eosinophil granule cationic proteins induced a direct and long-lasting (>60 min) but reversible sensitizing effect on their responses to chemical and electrical stimulations. More importantly, our study showed that these cationic proteins exerted an inhibitory effect on the sustained delayed-rectifier voltage-gated K⁺ current and the A-type, fast-inactivating K⁺ current; these actions were at least in part responsible for the sensitizing effect in these neurons. In awake mice, intra-tracheal instillation of MBP also induced a slowly developing (peaking in 2-3 days), progressive and sustained (lasting for 3-7 days) elevation of the cough responses to inhaled irritant gases. Taken together, these findings suggest that the enhanced sensitivity of bronchopulmonary C-fibers induced by the eosinophil granule cationic proteins may be a contributing factor in the pathogenesis of bronchial hyperresponsiveness and chronic cough associated with eosinophilic infiltration of the airways.

Cough responses to inhaled irritants are enhanced by eosinophil major basic protein in awake mice

A distinct association between airway eosinophilia and chronic cough is well documented. Eosinophil granule-derived cationic proteins, such as major basic protein (MBP), have been shown to activate and enhance the excitability of bronchopulmonary C-fiber sensory nerves, which may then lead to an increase in cough sensitivity. This study was carried out to determine whether cough responses to inhaled irritant gases were altered by delivery of MBP into the airways. An awake mouse moved freely in a recording chamber that was ventilated with a constant flow of air or irritant gas mixture. Cough responses to separate inhalation challenges of sulfur dioxide (SO₂; 300 and 600 ppm) and ammonia (NH₃; 0.1 and 0.2%), each for 5-min duration, were measured daily for 3 days before and for up to 8 days after MBP (10-20 µg) instillation into the trachea. During control, inhalations of SO₂ and NH₃ consistently elicited cough responses in a dose-dependent manner. After MBP treatment, cough responses to both SO₂ and NH₃ increased significantly and progressively and reached peaks 2-3 days after the treatment before returning to control level in 3-7 days. In sharp contrast, cough responses to these irritant gases were not affected by the treatment with the vehicle of MBP. These results suggest that the MBP-induced lingering elevation of cough responsiveness may be a contributing factor in the pathogenesis of chronic cough associated with eosinophilic infiltration of the airways.

KCNQ/M-channels regulate mouse vagal bronchopulmonary C-fiber excitability and cough sensitivity

Increased airway vagal sensory C-fiber activity contributes to the symptoms of inflammatory airway diseases. The KCNQ/Kv7/M-channel is a well-known determinant of neuronal excitability, yet whether it regulates the activity of vagal bronchopulmonary C-fibers and airway reflex sensitivity remains unknown. Here we addressed this issue using single-cell RT-PCR, patch clamp technique, extracellular recording of single vagal nerve fibers innervating the mouse lungs, and telemetric recording of cough in free-moving mice. Single-cell mRNA analysis and biophysical properties of M-current (IM) suggest that KCNQ3/Kv7.3 is the major M-channel subunit in mouse nodose neurons. The M-channel opener retigabine negatively shifted the voltage-dependent activation of IM, leading to membrane hyperpolarization, increased rheobase, and suppression of both evoked and spontaneous action potential (AP) firing in nodose neurons in an M-channel inhibitor XE991-sensitive manner. Retigabine also markedly suppressed the α,β-methylene ATP-induced AP firing in nodose C-fiber terminals innervating the mouse lungs, and coughing evoked by irritant gases in awake mice. In conclusion, KCNQ/M-channels play a role in regulating the excitability of vagal airway C-fibers at both the cell soma and nerve terminals. Drugs that open M-channels in airway sensory afferents may relieve the sufferings associated with pulmonary inflammatory diseases such as chronic coughing.

Structure of vagal afferent nerve terminal fibers in the mouse trachea

The structure of primary afferent nerve terminals profoundly influences their function. While the complex vagal airway nerve terminals (stretch receptors, cough receptors and neuroepithelial bodies) were thoroughly characterized, much less is known about the structure of airway nerves that do not form distinct complex terminals (often termed free nerve fibers). We selectively induced expression of GFP in vagal afferent nerves in the mouse by transfection with AAV-GFP virus vector and visualized nerve terminals in the trachea by whole organ confocal imaging. Based on structural characteristics we identified four types of vagal afferent nerve fiber terminals in the trachea. Importantly, we found that distinct compartments of tracheal tissue are innervated by distinct nerve fiber terminal types in a non-overlapping manner. Thus, separate terminal types innervate tracheal epithelium vs. anterolateral tracheal wall containing cartilaginous rings and ligaments vs. dorsal wall containing smooth muscle. Our results will aid the study of structure-function relationships in vagal airway afferent nerves and regulation of respiratory reflexes.

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.

Sustained sensitizing effects of tumor necrosis factor alpha on sensory nerves in lung and airways

Tumor necrosis factor alpha (TNFα) plays a significant role in the pathogenesis of airway inflammatory diseases. Inhalation of aerosolized TNFα induced airway hyperresponsiveness accompanied by airway inflammation in healthy human subjects, but the underlying mechanism is not fully understood. We recently reported a series of studies aimed to investigate if TNFα elevates the sensitivity of vagal bronchopulmonary sensory nerves in a mouse model; these studies are summarized in this mini-review. Our results showed that intratracheal instillation of TNFα induced pronounced airway inflammation 24 h later, as illustrated by infiltration of eosinophils and neutrophils and the release of inflammatory mediators and cytokines in the lung and airways. Accompanying these inflammatory reactions, the sensitivity of vagal pulmonary C-fibers and silent rapidly adapting receptors to capsaicin, a selective agonist of transient receptor potential vanilloid type 1 receptor, was markedly elevated after the TNFα treatment. A distinct increase in the sensitivity to capsaicin induced by TNFα was also observed in isolated pulmonary sensory neurons, suggesting that the sensitizing effect is mediated primarily through a direct action of TNFα on these neurons. Furthermore, the same TNFα treatment also induced a lingering (>7days) cough hyperresponsiveness to inhalation challenge of NH₃ in awake mice. Both the airway inflammation and the sensitizing effect on pulmonary sensory neurons caused by the TNFα treatment were abolished in the TNF-receptor double homozygous mutant mice, indicating the involvement of TNF-receptor activation. These findings suggest that the TNFα-induced hypersensitivity of vagal bronchopulmonary afferents may be responsible for, at least in part, the airway hyperresponsiveness caused by inhaled TNFα in healthy individuals.