Acid-sensing by airway afferent nerves

Peripheral Neuroplasticity of Respiratory Chemoreflexes, Induced by Prenatal Nicotinic Exposure: Implication for SIDS

Sudden Infant Death Syndrome (SIDS) occurs during sleep in seemingly healthy infants. Maternal cigarette smoking and hypoxemia during sleep are assumed to be the major causal factors. Depressed hypoxic ventilatory response (dHVR) is observed in infants with high risk of SIDS, and apneas (lethal ventilatory arrest) appear during the fatal episode of SIDS. Disturbance of the respiratory center has been proposed to be involved, but the pathogenesis of SIDS is still not fully understood. Peripherally, the carotid body is critical to generate HVR, and bronchopulmonary and superior laryngeal C-fibers (PCFs and SLCFs) are important for triggering central apneas; however, their roles in the pathogenesis of SIDS have not been explored until recently. There are three lines of recently accumulated evidence to show the disorders of peripheral sensory afferent-mediated respiratory chemoreflexes in rat pups with prenatal nicotinic exposure (a SIDS model) in which acute severe hypoxia leads to dHVR followed by lethal apneas. (1) The carotid body-mediated HVR is suppressed with a reduction of the number and sensitivity of glomus cells. (2) PCF-mediated apneic response is largely prolonged via increased PCF density, pulmonary IL-1β and serotonin (5-hydroxytryptamine, 5-HT) release, along with the enhanced expression of TRPV1, NK₁R, IL1RI and 5-HT₃R in pulmonary C-neurons to strengthen these neural responses to capsaicin, a selective stimulant to C-fibers. (3) SLCF-mediated apnea and capsaicin-induced currents in superior laryngeal C-neurons are augmented by upregulation of TRPV1 expression in these neurons. These results, along with hypoxic sensitization/stimulation of PCFs, gain insight into the mechanisms of prenatal nicotinic exposure-induced peripheral neuroplasticity responsible for dHVR and long-lasting apnea during hypoxia in rat pups. Therefore, in addition to the disturbance in the respiratory center, the disorders of peripheral sensory afferent-mediated chemoreflexes may also be involved in respiratory failure and death denoted in SIDS victims.

Neurokinin 1 and 2 Receptors Are Involved in PEG2- and Citric Acid-Induced Cough and Ventilatory Responses

Exposure to aerosolized citric acid (CA, 150mM) and prostaglandin E₂ (PGE₂, 0.43mM) for 10min in guinea pigs reportedly produces the distinct cough patterns (Type I vs. II) and ventilatory responses (long-lasting hyperventilation vs. brief tachypnea) even though triggering the same cough numbers. Type I and II coughs are primarily mediated by activation of TRPV1 and EP3 receptors (a PGE₂ receptor) of vagal C-fibers respectively. Substance P (SP) and neurokinin A (NKA) released by vagal pulmonary sensory fibers peripherally are capable of affecting CA-induced cough and ventilation via preferentially activating neurokinin 1 and 2 receptors (NK₁R and NK₂R) respectively. This study aimed to define the impacts of CA- and PGE₂-exposure on pulmonary SP and NKA levels and the roles of NK₁R and NK₂R in modulating CA- and PGE₂-evoked cough and ventilatory responses. In unanesthetized guinea pigs, we determined: 1) pulmonary SP and NKA contents induced by the CA- or PGE₂-exposure; 2) effects of CP-99994 and SR-48968 (a NK₁R and a NK₂R antagonist respectively) given by intraperitoneal injection (IP) or aerosol inhalation (IH) on the CA- and PGE₂-evoked cough and ventilatory responses; and 3) immunocytochemical expressions of NK₁R/NK₂R in vagal C-neurons labeled by TRPV1 or EP3 receptors. We found that CA- and PGE₂-exposure evoked Type I and II cough respectively associated with different degrees of increases in pulmonary SP and NKA. Applications of CP-99994 and SR-48968 via IP and IH efficiently suppressed the cough responses to CA with less impact on the cough response to PGE₂. These antagonists inhibited or blocked the ventilatory response to CA and caused hypoventilation in response to PGE₂. Moreover, NK₁R and NK₂R were always co-expressed in vagal C-neurons labeled by TRPV1 or EP3 receptors. These results suggest that SP and NKA endogenously released by CA- and PGE₂-exposure play important roles in generating the cough and ventilatory responses to CA and PGE₂, at least in part, via activation of NK₁R and NK₂R expressed in vagal C-neurons (pulmonary C-neurons).

Cross-effect of TRPV1 and EP3 receptor on coughs and bronchopulmonary C-neural activities

Prostaglandin E₂ (PGE₂)-induced coughs in vivo and vagal nerve depolarization in vitro are inhibited by systemic and local administration of prostaglandin EP3 receptor (L-798106) and TRPV1 antagonists (JNJ 17203212). These results indicate a modulating effect of TRPV1 on the EP3 receptor-mediated cough responses to PGE₂ likely through the vagal sensory nerve. This study aimed to determine whether 1) inhalation of aerosolized JNJ 17203212 and L-798106 affected cough responses to citric acid (CA, mainly stimulating TRPV1) and PGE₂; 2) TRPV1 and EP3 receptor morphologically are co-expressed and electrophysiologically functioned in the individual of vagal pulmonary C-neurons (cell bodies of bronchopulmonary C-fibers in the nodose/jugular ganglia); and 3) there was a cross-effect of TRPV1 and EP3 receptor on these neural excitations. To this end, aerosolized CA or PGE₂ was inhaled by unanesthetized guinea pigs pretreated without or with each antagonist given in aerosol form. Immunofluorescence was applied to identify the co-expression of TRPV1 and EP3 receptor in vagal pulmonary C-neurons (retrogradely traced by DiI). Whole-cell voltage patch clamp approach was used to detect capsaicin (CAP)- and PGE₂-induced currents in individual vagal pulmonary C-neurons and determine the effects of the TRPV1 and EP3 receptor antagonists on the evoked currents. We found that PGE₂-induced cough was attenuated by JNJ 17203212 or L-798106 and CA-evoked cough greatly suppressed only by JNJ 17203212. Approximately 1/4 of vagal pulmonary C-neurons co-expressed EP3 with a cell size < 20 μm. Both CAP- and PGE₂-induced currents could be recorded in the individuals of some vagal pulmonary C-neurons. The former was largely inhibited only by JNJ 17203212, while the latter was suppressed by JNJ 17203212 or L-798106. The similarity of the cross-effect of both antagonists on cough and vagal pulmonary C-neural activity suggests that a subgroup of vagal pulmonary C-neurons co-expressing TRPV1 and EP3 receptor is, at least in part, responsible for the cough response to PGE₂.

Acid exposure disrupts mucus secretion and impairs mucociliary transport in neonatal piglet airways

Tenacious mucus produced by tracheal and bronchial submucosal glands is a defining feature of several airway diseases, including cystic fibrosis (CF). Airway acidification as a driving force of CF airway pathology has been controversial. Here we tested the hypothesis that transient airway acidification produces pathologic mucus and impairs mucociliary transport. We studied pigs challenged with intra-airway acid. Acid had a minimal effect on mucus properties under basal conditions. However, cholinergic stimulation in acid-challenged pigs revealed retention of mucin 5B (MUC5B) in the submucosal glands, decreased concentrations of MUC5B in the lung lavage fluid, and airway obstruction. To more closely mimic a CF-like environment, we also examined mucus secretion and transport following cholinergic stimulation under diminished bicarbonate and chloride transport conditions ex vivo. Under these conditions, airways from acid-challenged pigs displayed extensive mucus films and decreased mucociliary transport. Pretreatment with diminazene aceturate, a small molecule with ability to inhibit acid detection through blockade of the acid-sensing ion channel (ASIC) at the doses provided, did not prevent acid-induced pathologic mucus or transport defects but did mitigate airway obstruction. These findings suggest that transient airway acidification early in life has significant impacts on mucus secretion and transport properties. Furthermore, they highlight diminazene aceturate as an agent that might be beneficial in alleviating airway obstruction.

Prolongation of bronchopulmonary C-fiber-mediated apnea by prenatal nicotinic exposure in rat pups: role of 5-HT3 receptors

Prenatal nicotinic exposure (PNE) reportedly sensitizes bronchopulmonary C-fibers (PCFs) and prolongs PCF-mediated apnea in rat pups, contributing to the pathogenesis of sudden infant death syndrome. Serotonin, or 5-hydroxytryptamine (5-HT), induces apnea via acting on 5-HT receptor 3 (5-HT3R) in PCFs, and among the 5-HT3R subunits, 5-HT3B is responsible for shortening the decay time of 5-HT3R-mediated currents. We examined whether PNE would promote pulmonary 5-HT secretion and prolong the apnea mediated by 5-HT3Rs in PCFs via affecting the 5-HT3B subunit. To this end, the following variables were compared between the control and PNE rat pups: 1) the 5-HT content in bronchoalveolar lavage fluid, 2) the apneic response to the right atrial bolus injection of phenylbiguanide (a 5-HT3R agonist) before and after PCF inactivation, 3) 5-HT3R currents and the stimulus threshold of the action currents of vagal pulmonary C-neurons, and 4) the immunoreactivity (IR) and mRNA expression of 5-HT3A and 5-HT3B in these neurons. Our results showed that PNE up-regulated the pulmonary 5-HT concentration and strengthened the PCF 5-HT3R-mediated apnea. PNE significantly facilitated neural excitability by shortening the decay time of 5-HT3R currents, lowering the stimulus threshold, and increasing 5-HT3B IR. In summary, PNE prolongs the apnea mediated by 5-HT3Rs in PCFs, likely by increasing 5-HT3B subunits to enhance the excitability of 5-HT3 channels.-Zhao, L., Gao, X., Zhuang, J., Wallen, M., Leng, S., Xu, F. Prolongation of bronchopulmonary C-fiber-mediated apnea by prenatal nicotinic exposure in rat pups: role of 5-HT3 receptors.

An advanced recording and analysis system for the differentiation of guinea pig cough responses to citric acid and prostaglandin E2 in real time

Cough number and/or sound have been used to assess cough sensitivity/intensity and to discriminate cough patterns in clinical settings. However, to date, only manual counting of cough number in an offline manner is applied in animal cough studies, which diminishes the efficiency of cough identification and hinders the diagnostic discrimination of cough patterns, especially in animals with pulmonary diseases. This study aims to validate a novel recording/analysis system by which cough numbers are automatically counted and cough patterns are comprehensively differentiated in real time. The experiment was carried out in conscious guinea pigs exposed to aerosolized citric acid (CA, 150 mM) and prostaglandin E2 (PGE2, 0.43 mM). Animal body posture (video), respiratory flow, and cough acoustics (audio) were simultaneously monitored and recorded. Cough number was counted automatically, and cough sound parameters including waveform, duration, power spectral density, spectrogram, and intensity, were analyzed in real time. Our results showed that CA- and PGE2-evoked coughs had the same cough numbers but completely different patterns [individual coughs vs. bout(s) of coughs]. Compared to CA-evoked coughs, PGE2-evoked coughs possess a longer latency, higher cough rate (coughs/min), shorter cough sound duration, lower cough sound intensity, and distinct cough waveforms and spectrograms. A few mucus- and wheeze-like coughs were noted in response to CA but not to PGE2. In conclusion, our recording/analysis system is capable of automatically counting the cough number and successfully differentiating the cough pattern by using valuable cough sound indexes in real time. Our system enhances the objectivity, accuracy, and efficiency of cough identification and count, improves the intensity evaluation, and offers ability for pattern discrimination compared to traditional types of cough identification. Importantly, this approach is beneficial for assessing the efficacy of putative antitussive drugs in animals without or with pulmonary diseases, particularly in cases without significant change in cough number.

Sex-specific airway hyperreactivity and sex-specific transcriptome remodeling in neonatal piglets challenged with intra-airway acid

Acute airway acidification is a potent stimulus of sensory nerves and occurs commonly with gastroesophageal reflux disease, cystic fibrosis, and asthma. In infants and adults, airway acidification can acutely precipitate asthma-like symptoms, and treatment-resistant asthma can be associated with gastroesophageal reflux disease. Airway protective behaviors, such as mucus secretion and airway smooth muscle contraction, are often exaggerated in asthma. These behaviors are manifested through activation of neural circuits. In some populations, the neural response to acid might be particularly important. For example, the immune response in infants is relatively immature compared with adults. Infants also have a high frequency of gastroesophageal reflux. Thus, in the current study, we compared the transcriptomes of an airway-nervous system circuit (e.g., tracheal epithelia, nodose ganglia, and brain stem) in neonatal piglets challenged with intra-airway acid. We hypothesized that the identification of parallel changes in the transcriptomes of two neutrally connected tissues might reveal the circuit response, and, hence, molecules important for the manifestation of asthma-like features. Intra-airway acid induced airway hyperreactivity and airway obstruction in male piglets. In contrast, female piglets displayed airway obstruction without airway hyperreactivity. Pairwise comparisons revealed parallel changes in genes directly implicated in airway hyperreactivity ( scn10a) in male acid-challenged piglets, whereas acid-challenged females exhibited parallel changes in genes associated with mild asthma ( stat 1 and isg15). These findings reveal sex-specific responses to acute airway acidification and highlight distinct molecules within a neural circuit that might be critical for the manifestation of asthma-like symptoms in pediatric populations.

Prenatal nicotinic exposure prolongs superior laryngeal C-fiber-mediated apnea and bradycardia through enhancing neuronal TRPV1 expression and excitation

Maternal cigarette smoke, including prenatal nicotinic exposure (PNE), is responsible for sudden infant death syndrome (SIDS). The fatal events of SIDS are characterized by severe bradycardia and life-threatening apneas. Although activation of transient receptor potential vanilloid 1 (TRPV1) of superior laryngeal C fibers (SLCFs) could induce bradycardia and apnea and has been implicated in SIDS pathogenesis, how PNE affects the SLCF-mediated cardiorespiratory responses remains unexplored. Here, we tested the hypothesis that PNE would aggravate the SLCF-mediated apnea and bradycardia via up-regulating TRPV1 expression and excitation of laryngeal C neurons in the nodose/jugular (N/J) ganglia. To this end, we compared the following outcomes between control and PNE rat pups at postnatal days 11-14: 1) the cardiorespiratory responses to intralaryngeal application of capsaicin (10 µg/ml, 50 µl), a selective stimulant for TRPV1 receptors, in anesthetized preparation; 2) immunoreactivity and mRNA of TRPV1 receptors of laryngeal sensory C neurons in the N/J ganglia retrogradely traced by 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate; and 3) TRPV1 currents and electrophysiological characteristics of these neurons by using whole-cell patch-clamp technique in vitro Our results showed that PNE markedly prolonged the apneic response and exacerbated the bradycardic response to intralaryngeal perfusion of capsaicin, which was associated with up-regulation of TRPV1 expression in laryngeal C neurons. In addition, PNE increased the TRPV1 currents, depressed the slow delayed rectifier potassium currents, and increased the resting membrane potential of these neurons. Our results suggest that PNE is capable of aggravating the SLCF-mediated apnea and bradycardia through TRPV1 sensitization and neuronal excitation, which may contribute to the pathogenesis of SIDS.-Gao, X., Zhao, L., Zhuang, J., Zang, N., Xu, F. Prenatal nicotinic exposure prolongs superior laryngeal C-fiber-mediated apnea and bradycardia through enhancing neuronal TRPV1 expression and excitation.

Acid-Sensing Ion Channel 1a Contributes to Airway Hyperreactivity in Mice

Neurons innervating the airways contribute to airway hyperreactivity (AHR), a hallmark feature of asthma. Several observations suggested that acid-sensing ion channels (ASICs), neuronal cation channels activated by protons, might contribute to AHR. For example, ASICs are found in vagal sensory neurons that innervate airways, and asthmatic airways can become acidic. Moreover, airway acidification activates ASIC currents and depolarizes neurons innervating airways. We found ASIC1a protein in vagal ganglia neurons, but not airway epithelium or smooth muscle. We induced AHR by sensitizing mice to ovalbumin and found that ASIC1a^-/- mice failed to exhibit AHR despite a robust inflammatory response. Loss of ASIC1a also decreased bronchoalveolar lavage fluid levels of substance P, a sensory neuropeptide secreted from vagal sensory neurons that contributes to AHR. These findings suggest that ASIC1a is an important mediator of AHR and raise the possibility that inhibiting ASIC channels might be beneficial in asthma.

Menthol suppresses laryngeal C-fiber hypersensitivity to cigarette smoke in a rat model of gastroesophageal reflux disease: the role of TRPM8

Patients with gastroesophageal reflux disease (GERD) display enhanced laryngeal reflex reactivity to stimuli that may be due to sensitization of the laryngeal C-fibers by acid and pepsin. Menthol, a ligand of transient receptor potential melastatin-8 (TRPM8), relieves throat irritation. However, the possibility that GERD induces laryngeal C-fiber hypersensitivity to cigarette smoke (CS) and that menthol suppresses this event has not been investigated. We delivered CS into functionally isolated larynxes of 160 anesthetized rats. Laryngeal pH 5-pepsin treatment, but not pH 5-denatured pepsin, augmented the apneic response to CS, which was blocked by denervation or perineural capsaicin treatment (a procedure that blocks the conduction of C fibers) of the superior laryngeal nerves. This augmented apnea was partially attenuated by capsazepine [an transient receptor potential vanilloid 1 (TRPV1) antagonist], SB-366791 (a TRPV1 antagonist), and HC030031 [a transient receptor potential ankyrin 1 (TRPA1) antagonist] and was completely prevented by a combination of TRPV1 and TRPA1 antagonists. Local application of menthol significantly suppressed the augmented apnea and this effect was reversed by pretreatment with AMTB (a TRPM8 antagonist). Our electrophysiological studies consistently revealed that laryngeal pH 5-pepsin treatment increased the sensitivity of laryngeal C-fibers to CS. Likewise, menthol suppressed this laryngeal C-fiber hypersensitivity and its effect could be reversed by pretreatment with AMTB. Our results suggest that laryngeal pH 5-pepsin treatment increases sensitivity to CS of both TRPV1 and TRPA1, which are presumably located at the terminals of laryngeal C-fibers. This sensory sensitization leads to enhanced laryngeal reflex reactivity and augmentation of the laryngeal C-fiber responses to CS, which can be suppressed by menthol acting via TRPM8.

Ionotropic and metabotropic proton-sensing receptors involved in airway inflammation in allergic asthma

An acidic microenvironment has been shown to evoke a variety of airway responses, including cough, bronchoconstriction, airway hyperresponsiveness (AHR), infiltration of inflammatory cells in the lung, and stimulation of mucus hyperproduction. Except for the participation of transient receptor potential vanilloid-1 (TRPV1) and acid-sensing ion channels (ASICs) in severe acidic pH (of less than 6.0)-induced cough and bronchoconstriction through sensory neurons, the molecular mechanisms underlying extracellular acidic pH-induced actions in the airways have not been fully understood. Recent studies have revealed that ovarian cancer G protein-coupled receptor 1 (OGR1)-family G protein-coupled receptors, which sense pH of more than 6.0, are expressed in structural cells, such as airway smooth muscle cells and epithelial cells, and in inflammatory and immune cells, such as eosinophils and dendritic cells. They function in a variety of airway responses related to the pathophysiology of inflammatory diseases, including allergic asthma. In the present review, we discuss the roles of ionotropic TRPV1 and ASICs and metabotropic OGR1-family G protein-coupled receptors in the airway inflammation and AHR in asthma and respiratory diseases.

Role of extracellular proton-sensing OGR1 in regulation of insulin secretion and pancreatic β-cell functions

Insulin secretion with respect to pH environments has been investigated for a long time but its mechanism remains largely unknown. Extracellular pH is usually maintained at around 7.4 and, its change has been thought to occur in non-physiological situations. Acidification takes place under ischemic and inflammatory microenvironments, where stimulation of anaerobic glycolysis results in the production of lactic acid. In addition to ionotropic ion channels, such as transient receptor potential V1 (TRPV1) and acid-sensing ion channels (ASICs), metabotropic proton-sensing G protein-coupled receptors (GPCRs) have also been identified recently as proton-sensing machineries. While ionotropic ion channels usually sense strong acidic pH, proton-sensing GPCRs sense pH of 7.6 to 6.0 and have been shown to mediate a variety of biological actions in neutral and mildly acidic pH environments. Studies with receptor knockout mice have revealed that proton-sensing receptors, including ovarian cancer G protein-coupled receptor 1 (OGR1), a proton-sensing GPCRs, play a role in the regulation of insulin secretion and glucose metabolism under physiological conditions. Small molecule 3,5-disubstituted isoxazoles have recently been identified as OGR1 agonists working at neutral pH and have been shown to stimulate pancreatic β-cell differentiation and insulin synthesis. Thus, proton-sensing OGR1 may be an important player for insulin secretion and a potential target for improving β-cell function.

Airway smooth muscle in airway reactivity and remodeling: what have we learned?

It is now established that airway smooth muscle (ASM) has roles in determining airway structure and function, well beyond that as the major contractile element. Indeed, changes in ASM function are central to the manifestation of allergic, inflammatory, and fibrotic airway diseases in both children and adults, as well as to airway responses to local and environmental exposures. Emerging evidence points to novel signaling mechanisms within ASM cells of different species that serve to control diverse features, including 1) [Ca(2+)]i contractility and relaxation, 2) cell proliferation and apoptosis, 3) production and modulation of extracellular components, and 4) release of pro- vs. anti-inflammatory mediators and factors that regulate immunity as well as the function of other airway cell types, such as epithelium, fibroblasts, and nerves. These diverse effects of ASM "activity" result in modulation of bronchoconstriction vs. bronchodilation relevant to airway hyperresponsiveness, airway thickening, and fibrosis that influence compliance. This perspective highlights recent discoveries that reveal the central role of ASM in this regard and helps set the stage for future research toward understanding the pathways regulating ASM and, in turn, the influence of ASM on airway structure and function. Such exploration is key to development of novel therapeutic strategies that influence the pathophysiology of diseases such as asthma, chronic obstructive pulmonary disease, and pulmonary fibrosis.