Induction of tachykinin gene and peptide expression in guinea pig nodose primary afferent neurons by allergic airway inflammation

House Dust Mite and Cat Dander Extract Induce Asthma-Like Histopathology with an Increase of Mucosal Mast Cells in a Guinea Pig Model

Background

Asthma is a chronic inflammatory disease with structural changes in the lungs defined as airway remodelling. Mast cell responses are important in asthma as they, upon activation, release mediators inducing bronchoconstriction, inflammatory cell recruitment, and often remodelling of the airways. As guinea pigs exhibit anatomical, physiological, and pharmacological features resembling human airways, including mast cell distribution and mediator release, we evaluated the effect of extracts from two common allergens, house dust mite (HDM) and cat dander (CDE), on histopathological changes and the composition of tryptase- and chymase-positive mast cells in the guinea pig lungs.

Methods

Guinea pigs were exposed intranasally to HDM or CDE for 4, 8, and 12 weeks, and airway histology was examined at each time point. Hematoxylin and eosin, Picro-Sirius Red, and Periodic Acid-Schiff staining were performed to evaluate airway inflammation, collagen deposition, and mucus-producing cells. In addition, Astra blue and immunostaining against tryptase and chymase were used to visualize mast cells.

Results

Repetitive administration of HDM or CDE led to the accumulation of inflammatory cells into the proximal and distal airways as well as increased airway smooth muscle mass. HDM exposure caused subepithelial collagen deposition and mucus cell hyperplasia at all three time points, whereas CDE exposure only caused these effects at 8 and 12 weeks. Both HDM and CDE induced a substantial increase in mast cells after 8 and 12 weeks of challenges. This increase was primarily due to mast cells expressing tryptase, but not chymase, thus indicating mucosal mast cells.

Conclusions

We here show that exposure to HDM and CDE elicits asthma-like histopathology in guinea pigs with infiltration of inflammatory cells, airway remodelling, and accumulation of primarily mucosal mast cells. The results together encourage the use of HDM and CDE allergens for the stimulation of a clinically relevant asthma model in guinea pigs.

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.

Multicolor labeling of airway neurons and analysis of parasympathetic heterogeneity

We report subpopulations of airway parasympathetic neurons expressing substance P, neuronal nitric oxide synthase, and tyrosine hydroxylase, highlighting unexplored heterogeneity in this population. These neurotransmitter-specific subpopulations did not form intraganglionic interneurons, but rather, extended outside the ganglia, into the airways, to distant innervation targets. Our experiments demonstrate the utility of multicolor labeling to characterize airway innervation, allowing us to confirm the extensive heterogeneity of postganglionic parasympathetic neurons. These methods will facilitate future investigations of neurophysiology and neural contributions to airway disease.

Airway Sensory Nerve Plasticity in Asthma and Chronic Cough

Airway sensory nerves detect a wide variety of chemical and mechanical stimuli, and relay signals to circuits within the brainstem that regulate breathing, cough, and bronchoconstriction. Recent advances in histological methods, single cell PCR analysis and transgenic mouse models have illuminated a remarkable degree of sensory nerve heterogeneity and have enabled an unprecedented ability to test the functional role of specific neuronal populations in healthy and diseased lungs. This review focuses on how neuronal plasticity contributes to development of two of the most common airway diseases, asthma and chronic cough, and discusses the therapeutic implications of emerging treatments that target airway sensory nerves.

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

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

Mini review: Neural mechanisms underlying airway hyperresponsiveness

Neural changes underly hyperresponsiveness in asthma and other airway diseases. Afferent sensory nerves, nerves within the brainstem, and efferent parasympathetic nerves all contribute to airway hyperresponsiveness. Inflammation plays a critical role in these nerve changes. Chronic inflammation and pre-natal exposures lead to increased airway innervation and structural changes. Acute inflammation leads to shifts in neurotransmitter expression of afferent nerves and dysfunction of M₂ muscarinic receptors on efferent nerve endings. Eosinophils and macrophages drive these changes through release of inflammatory mediators. Novel tools, including optogenetics, two photon microscopy, and optical clearing and whole mount microscopy, allow for improved studies of the structure and function of airway nerves and airway hyperresponsiveness.

A TRPA1 inhibitor suppresses neurogenic inflammation and airway contraction for asthma treatment

Despite the development of effective therapies, a substantial proportion of asthmatics continue to have uncontrolled symptoms, airflow limitation, and exacerbations. Transient receptor potential cation channel member A1 (TRPA1) agonists are elevated in human asthmatic airways, and in rodents, TRPA1 is involved in the induction of airway inflammation and hyperreactivity. Here, the discovery and early clinical development of GDC-0334, a highly potent, selective, and orally bioavailable TRPA1 antagonist, is described. GDC-0334 inhibited TRPA1 function on airway smooth muscle and sensory neurons, decreasing edema, dermal blood flow (DBF), cough, and allergic airway inflammation in several preclinical species. In a healthy volunteer Phase 1 study, treatment with GDC-0334 reduced TRPA1 agonist-induced DBF, pain, and itch, demonstrating GDC-0334 target engagement in humans. These data provide therapeutic rationale for evaluating TRPA1 inhibition as a clinical therapy for asthma.

Neurotrophic factors and nerve growth factor in ocular allergy

PURPOSE OF REVIEW

During allergic reaction, nervous and immune systems mutually interact through release of mediators, including neurotrophic factors and nerve growth factor (NGF). These mediators modulate allergic reaction through binding their receptors expressed by immune and structural cells and by stimulating neuropeptide release by nerves. The role of neuropeptides and NGF has been demonstrated in allergic asthma and rhinitis, and, to a lesser extent, in allergic conjunctivitis. The aim of this review are to elucidate the evidence of the role of NGF and neuropeptides in the pathogenesis of allergic conjunctivitis.

RECENT FINDINGS

NGF modulates allergic reaction by stimulating release of cytokines, inflammatory mediators and neuropeptides by immune and structural cells and nerve endings at the site of inflammation. Evidence showed that local and systemic NGF levels increase in patients with allergic conjunctivitis, including allergic rhinoconjuncivitis, vernal keratoconjunctivitis and atopic keratoconjunctivitis. We recently described an increase of conjunctival p75NTR expression in patients with allergic rhinoconjuncivitis, and an increase of tear levels of NGF after conjunctival provocation test with allergen.

SUMMARY

NGF modulates ocular allergic reaction. Increasing understanding of the role of neuropeptides in allergic conjunctivitis may pave the way to the development of novel therapeutic approaches and improvement of patients' management.

Modulation of protective reflex cough by acute immune driven inflammation of lower airways in anesthetized rabbits

Chronic irritating cough in patients with allergic disorders may reflect behavioral or reflex response that is inappropriately matched to the stimulus present in the respiratory tract. Such dysregulated response is likely caused by sensory nerve damage driven by allergic mediators leading to cough hypersensitivity. Some indirect findings suggest that even acid-sensitive, capsaicin-insensitive A-δ fibers called “cough receptors” that are likely responsible for protective reflex cough may be modulated through immune driven inflammation. The aim of this study was to find out whether protective reflex cough is altered during acute allergic airway inflammation in rabbits sensitized to ovalbumin. In order to evaluate the effect of such inflammation exclusively on protective reflex cough, C-fiber mediated cough was silenced using general anesthesia. Cough provocation using citric acid inhalation and mechanical stimulation of trachea was realized in 16 ovalbumin (OVA) sensitized, anesthetized and tracheotomised rabbits 24h after OVA (OVA group, n = 9) or saline challenge (control group, n = 7). Number of coughs provoked by citric acid inhalation did not differ between OVA and control group (12,2 ±6,1 vs. 17,9 ± 6,9; p = 0.5). Allergic airway inflammation induced significant modulation of cough threshold (CT) to mechanical stimulus. Mechanically induced cough reflex in OVA group was either up-regulated (subgroup named “responders” CT: 50 msec (50–50); n = 5 p = 0.003) or down-regulated (subgroup named “non responders”, CT: 1200 msec (1200–1200); n = 4 p = 0.001) when compared to control group (CT: 150 msec (75–525)). These results advocate that allergen may induce longer lasting changes of reflex cough pathway, leading to its up- or down-regulation. These findings may be of interest as they suggest that effective therapies for chronic cough in allergic patients should target sensitized component of both, reflex and behavioral cough.

New aspects of neuroinflammation and neuroimmune crosstalk in the airways

Neuroimmune interaction has long been discussed in the pathogenesis of allergic airway diseases, such as allergic asthma. Mediators released during inflammation can alter the function of both sensory and parasympathetic neurons innervating the airways. Evidence has been provided that the inflammatory response can be altered by various mediators that are released by sensory and parasympathetic neurons and vice versa. Our aim is to demonstrate recent developments in the reciprocal neuroimmune interaction and to include, if available, data from in vivo and clinical studies.

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.

Eosinophil and airway nerve interactions in asthma

Airway eosinophils are increased in asthma and are especially abundant around airway nerves. Nerves control bronchoconstiction and in asthma, airway hyperreactivity (where airways contract excessively to inhaled stimuli) develops when eosinophils alter both parasympathetic and sensory nerve function. Eosinophils release major basic protein, which is an antagonist of inhibitory M₂ muscarinic receptors on parasympathetic nerves. Loss of M₂ receptor inhibition potentiates parasympathetic nerve-mediated bronchoconstriction. Eosinophils also increase sensory nerve responsiveness by lowering neurons' activation threshold, stimulating nerve growth, and altering neuropeptide expression. Since sensory nerves activate parasympathetic nerves via a central neuronal reflex, eosinophils' effects on both sensory and parasympathetic nerves potentiate bronchoconstriction. This review explores recent insights into mechanisms and effects of eosinophil and airway nerve interactions in asthma.

NGF and Its Receptors in the Regulation of Inflammatory Response

There is growing interest in the complex relationship between the nervous and immune systems and how its alteration can affect homeostasis and result in the development of inflammatory diseases. A key mediator in cross-talk between the two systems is nerve growth factor (NGF), which can influence both neuronal cell function and immune cell activity. The up-regulation of NGF described in inflamed tissues of many diseases can regulate innervation and neuronal activity of peripheral neurons, inducing the release of immune-active neuropeptides and neurotransmitters, but can also directly influence innate and adaptive immune responses. Expression of the NGF receptors tropomyosin receptor kinase A (TrkA) and p75 neurotrophin receptor (p75NTR) is dynamically regulated in immune cells, suggesting a varying requirement for NGF depending on their state of differentiation and functional activity. NGF has a variety of effects that can be either pro-inflammatory or anti-inflammatory. This apparent contradiction can be explained by considering NGF as part of an endogenous mechanism that, while activating immune responses, also activates pathways necessary to dampen the inflammatory response and limit tissue damage. Decreases in TrkA expression, such as that recently demonstrated in immune cells of arthritis patients, might prevent the activation by NGF of regulatory feed-back mechanisms, thus contributing to the development and maintenance of chronic inflammation.

Vagal Afferent Innervation of the Airways in Health and Disease

Vagal sensory neurons constitute the major afferent supply to the airways and lungs. Subsets of afferents are defined by their embryological origin, molecular profile, neurochemistry, functionality, and anatomical organization, and collectively these nerves are essential for the regulation of respiratory physiology and pulmonary defense through local responses and centrally mediated neural pathways. Mechanical and chemical activation of airway afferents depends on a myriad of ionic and receptor-mediated signaling, much of which has yet to be fully explored. Alterations in the sensitivity and neurochemical phenotype of vagal afferent nerves and/or the neural pathways that they innervate occur in a wide variety of pulmonary diseases, and as such, understanding the mechanisms of vagal sensory function and dysfunction may reveal novel therapeutic targets. In this comprehensive review we discuss historical and state-of-the-art concepts in airway sensory neurobiology and explore mechanisms underlying how vagal sensory pathways become dysfunctional in pathological conditions.

Substance P/Neurokinin 1 and Trigeminal System: A Possible Link to the Pathogenesis in Sudden Perinatal Deaths

Sudden demise of a healthy fetus or a neonate is a very tragic episode in the life of parents. These deaths have been a mystery since ages but still remain unexplained. This review proposes the involvement of trigeminal nerve, neurotransmitter substance P (SP), and its receptor neurokinin 1 (NK-1R) in regulation of cardiorespiratory control in fetuses and newborns. Anomalies and immaturity of neuroregulatory systems such as trigeminal system in medulla oblongata of brainstem may provide a possible mechanism of sudden perinatal deaths. Vulnerable infants are born with respiratory center immaturity which in combination with any stressor such as cold, hypoxia, and smoking may lead to cessation of breathing and ventilatory response. SP/NK-1R may be involved in regulating the ventilatory control in neonates while it is decreased in fetal and adult life in humans, and any alterations from these may lead to irreversible sleep apnea and fatal breathing, ultimately sudden death. This review summarizes the studies performed to highlight the expression of SP or NK-1R in sudden perinatal deaths and proposes the involvement of trigeminal ganglion along with its nerve and SP/NK-1R expression alteration as one of the possible pathophysiological underlying mechanism. However, further studies are required to explore the role of SP, NK-1R, and trigeminal system in the pathogenesis of sudden infant deaths, sudden intrauterine deaths, stillbirths, and sudden deaths later in human life.

The nervous system of airways and its remodeling in inflammatory lung diseases

Inflammatory lung diseases are associated with bronchospasm, cough, dyspnea and airway hyperreactivity. The majority of these symptoms cannot be primarily explained by immune cell infiltration. Evidence has been provided that vagal efferent and afferent neurons play a pivotal role in this regard. Their functions can be altered by inflammatory mediators that induce long-lasting changes in vagal nerve activity and gene expression in both peripheral and central neurons, providing new targets for treatment of pulmonary inflammatory diseases.

Role of reactive oxygen species and TRP channels in the cough reflex

The cough reflex is evoked by noxious stimuli in the airways. Although this reflex is essential for health, it can be triggered chronically in inflammatory and infectious airway disease. Neuronal transient receptor potential (TRP) channels such as ankyrin 1 (TRPA1) and vanilloid 1 (TRPV1) are polymodal receptors expressed on airway nociceptive afferent nerves. Reactive oxygen species (ROS) and other reactive compounds are associated with inflammation, from either NADPH oxidase or mitochondria. These reactive compounds cause activation and hyperexcitability of nociceptive afferents innervating the airways, and evidence suggests key contributions of TRPA1 and TRPV1.

Side-stream tobacco smoke-induced airway hyperresponsiveness in early postnatal period is involved nerve growth factor

Epidemiological studies have shown that children are more susceptible to adverse respiratory effects of passive smoking than adults. The goal of this study is to elucidate the possible neural mechanism induced by exposure to passive smoking during early life. Postnatal day (PD) 2 and PD 21 mice were exposed to side-stream tobacco smoke (SS), a surrogate to secondhand smoke, or filtered air (FA) for 10 consecutive days. Pulmonary function, substance P (SP) airway innervation, neurotrophin gene expression in lung and nerve growth factor (NGF) release in bronchoalveolar lavage (BAL) fluid were measured at different times after the last SS or FA exposure. Exposure to SS significantly altered pulmonary function in PD2, accompanied with an enhanced SP innervation in airway. However, exposure to SS during the later developmental period (PD21) did not appear to affect pulmonary function and SP innervation of the airways. Interestingly, SS exposure in PD2 group significantly induced an increased gene expression on NGF, and decreased NGF receptor P75 in lung; parallel with high levels of NGF protein in BAL. Furthermore, pretreatment with NGF antibody significantly diminished SS-induced airway hyperresponsivenss and the increased SP airway innervation in the PD2 group. These findings suggest that enhanced NGF released in the lung contributes to SS-enhanced SP tracheal innervation and airway responsiveness in early life.

Steroid Treatment Reduces Allergic Airway Inflammation and Does Not Alter the Increased Numbers of Dendritic Cells and Calcitonin Gene-Related Peptide-Expressing Neurons in Airway Sensory Ganglia

OBJECTIVES

Our previous data demonstrated that allergic airway inflammation induces migration of dendritic cells (DC) into airway sensory jugular and nodose ganglia (jugular-nodose ganglion complex; JNC). Here we investigated the effects of steroid treatment regarding the expression and migration of DC and calcitonin gene-related peptide (CGRP)-immunoreactive neurons of vagal sensory ganglia during allergic airway inflammation.

METHODS

A house dust mite (HDM) model for allergic airway inflammation was used. The mice received 0.3 mg fluticasone propionate per kilogram of body weight in the last 9 days. JNC slices were analyzed on MHC II, the neuronal marker PGP9.5, and the neuropeptide CGRP.

RESULTS

Allergic airway inflammation increased the numbers of DC and CGRP-expressing neurons in the JNC significantly in comparison to the controls (DC/neurons: HDM 44.58 ± 1.6% vs. saline 33.29 ± 1.6%, p < 0.05; CGRP-positive neurons/total neurons: HDM 30.65 ± 1.9% vs. saline 19.49 ± 2.3%, p < 0.05). Steroid treatment did not have any effect on the numbers of DC and CGRP-expressing neurons in the JNC compared to HDM-treated mice.

CONCLUSIONS

The present findings indicate an important role of DC and CGRP-containing neurons in the pathogenesis of allergic airway inflammation. However, steroid treatment did not have an effect on the population of DC and neurons displaying CGRP in the JNC, whereas steroid treatment was found to suppress allergic airway inflammation.

Airway Vagal Neuroplasticity Associated with Respiratory Viral Infections

Respiratory virus infections leads to coughing, sneezing, and increases in reflex parasympathetic bronchoconstriction and secretions. These responses to viral infection are exclusively or largely secondary to changes in the function of the nervous system. For many with underlying airway pathologies such as asthma and COPD, this neuroplasticity can lead to disease exacerbations and hospitalization. Relatively little is understood about the cellular and molecular mechanisms that underlie the changes in neuronal control of the respiratory tract during viral infection, but the evidence supports the idea that changes occur in the physiology of both the sensory and autonomic innervation. Virus infection can lead to acute increases in the activity of sensory nerves as well as to genetic changes causing alterations in sensory nerve phenotype. In addition, respiratory viral infections are associated with changes in the control of neurotransmitter release from cholinergic nerve endings terminating at the level of the airway smooth muscle.

Nasal allergen deposition leads to conjunctival mast cell degranulation in allergic rhinoconjunctivitis

BACKGROUND

The naso-ocular interaction in allergic rhinoconjunctivitis is well recognized from epidemiological, clinical, and experimental observations. The precise mechanisms remain incompletely understood. A new mouse model of allergic rhinoconjunctivitis was used to investigate the contribution of mast cells and trigeminal ganglia activation to conjunctival (conj.) inflammation after nasal allergen provocation.

METHODS

Sensitized mice were exposed to ovalbumin (OVA) via the nose and/or conjunctiva, and conj. homogenates were analyzed for histamine and substance P (using ELISA) and by eosinophil peroxidase (EPO) and beta-hexosaminidase assays. The conj. effects of nasal allergen deposition were compared with those induced by the mast cell activator C48/80 and with pretreatment of the mast cell stabilizer ketotifen or the transient receptor potential channel receptor (TRP) agonist capsaicin. Protachykinin 1 (TAC1) expression was quantified in the trigeminal ganglia using real time polymerase chain reaction.

RESULTS

At 1 hour after nasal application of OVA, increased conj. levels of beta-hexosaminidase (0.68 ± 0.03 nm versus 0.56 ± 0.02 nm; p = 0.02), histamine (751.1 ± 52.17 ng/mL versus 546.3 ± 76.91 ng/mL; p = 0.05), and EPO (0.66 ± 0.09 nm versus 0.37 ± 0.03 nm; p = 0.02) were detected compared with saline. Higher levels of TAC1 expression were found in the trigeminal ganglia at 24 hours after OVA application (1326 ± 255 versus 687.5 ± 90.77 TAC1/beta-actin; p = 0.04). Nasal challenge with C48/80 increased substance P and beta-hexosaminidase levels in the conjunctiva, as well as TAC1 expression. Pretreatment with ketotifen resulted in lower levels of substance P as well as TAC1 expression. Destruction of sensory nerves in the nose by capsaicin reduced the OVA-induced conj. levels of substance P, histamine, and beta-hexosaminidase.

CONCLUSION

Nasal allergen deposition in sensitized mice induced trigeminal TAC1 expression and conj. mast cell degranulation. These data represent a significant step forward in understanding the close interaction between nasal and conj. inflammation in allergy.

Airway extravasation induced by increasing airway temperature in ovalbumin-sensitized rats

This study was carried out to determine whether hyperventilation of humidified warm air (HWA) induced airway extravasation in ovalbumin (Ova)-sensitized rats. Our results showed: (1) After isocapnic hyperventilation with HWA for 2 min, tracheal temperature (Ttr) was increased to 40.3°C, and the Evans blue contents in major airways and lung tissue were elevated to 651% and 707%, respectively, of that after hyperventilation with humidified room air in Ova-sensitized rats; this striking effect of HWA was absent in control rats. (2) The HWA-induced increase in Evans blue content in sensitized rats was completely prevented by a pretreatment with either L-732138, a selective antagonist of neurokinin type 1 (NK-1) receptor, or formoterol, a selective agonist of β2 adrenoceptor. This study demonstrated that an increase in airway temperature induced protein extravasation in the major airways and lung tissue of sensitized rats, and an activation of the NK-1 receptor by tachykinins released from bronchopulmonary C-fiber nerve endings was primarily responsible.

Peripheral neural circuitry in cough

Cough is a reflex that serves to protect the airways. Excessive or chronic coughing is a major health issue that is poorly controlled by current therapeutics. Significant effort has been made to understand the mechanisms underlying the cough reflex. The focus of this review is the evidence supporting the role of specific airway sensory nerve (afferent) populations in the initiation and modulation of the cough reflex in health and disease.

Ozone exposure initiates a sequential signaling cascade in airways involving interleukin-1beta release, nerve growth factor secretion, and substance P upregulation

Previous studies demonstrated that interleukin-1β (IL-1β) and nerve growth factor (NGF) increase synthesis of substance P (SP) in airway neurons both after ozone (O3) exposure and by direct application. It was postulated that NGF mediates O3-induced IL-1β effects on SP. The current study specifically focused on the influence of O3 on IL-1β, NGF, and SP levels in mice bronchoalveolar lavage fluid (BALF) and whether these mediators may be linked in an inflammatory-neuronal cascade in vivo. The findings showed that in vivo O3 exposure induced an increase of all three proteins in mouse BALF and that O3-induced elevations in both NGF and SP are mediated by the inflammatory cytokine IL-1β. Further, inhibition of NGF reduced O3 induced increases of SP in both the lung BALF and lung tissue, demonstrating NGF serves as a mediator of IL-1β effects on SP. These data indicate that IL-1β is an early mediator of O3-induced rise in NGF and subsequent SP release in mice in vivo.

Sensory nerves in lung and airways

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

Correlation of c-fos protein expression with neuropeptide content in the lung of bronchial asthmatic rat

OBJECTIVE

To investigate and analyze the correlation between the c-fos protein expression and neuropeptide content in the lung of bronchial asthmatic rats.

METHODS

Thirty-two (32) SD rats were randomly allocated into 4 groups of the normal control, the non-acute asthma, the acute asthma and the dexamethasone intervention. Immunohistochemistry was performed for histological observation, and substance P (SP) and vasoactive intestinal peptide (VIP) concentrations in the bronchoalveolar lavage fluid were measured by enzyme-linked immunosorbent assay (ELISA).

RESULTS

SP concentration in the alveolar lavage of asthmatic rat was significantly higher than that in the normal control group (P < 0.0001), whereas VIP concentration was significantly lower (P < 0.0001). The optical density of c-fos protein in the lung tissues of groups of the non-acute asthma, the acute asthma and the dexamethasone intervention was positively correlated with SP concentration in the bronchoalveolar lavage fluid (r = 0.908, r = 0.967, r = 0.865), and negatively correlated with the VIP concentration in the alveolar lavage (r = -0.974, r = -0.949, r = -0.962).

CONCLUSION

The c-fos protein expression and neuropeptide content in the lungs of asthmatic rats are related with asthma attacks.

Anatomy and neurophysiology of cough: CHEST Guideline and Expert Panel report

Bronchopulmonary C-fibers and a subset of mechanically sensitive, acid-sensitive myelinated sensory nerves play essential roles in regulating cough. These vagal sensory nerves terminate primarily in the larynx, trachea, carina, and large intrapulmonary bronchi. Other bronchopulmonary sensory nerves, sensory nerves innervating other viscera, as well as somatosensory nerves innervating the chest wall, diaphragm, and abdominal musculature regulate cough patterning and cough sensitivity. The responsiveness and morphology of the airway vagal sensory nerve subtypes and the extrapulmonary sensory nerves that regulate coughing are described. The brainstem and higher brain control systems that process this sensory information are complex, but our current understanding of them is considerable and increasing. The relevance of these neural systems to clinical phenomena, such as urge to cough and psychologic methods for treatment of dystussia, is high, and modern imaging methods have revealed potential neural substrates for some features of cough in the human.

Allergic airway inflammation induces the migration of dendritic cells into airway sensory ganglia

BACKGROUND

A neuroimmune crosstalk between dendritic cells (DCs) and airway nerves in the lung has recently been reported. However, the presence of DCs in airway sensory ganglia under normal and allergic conditions has not been explored so far. Therefore, this study aims to investigate the localisation, distribution and proliferation of DCs in airway sensory ganglia under allergic airway inflammation.

METHODS

Using the house dust mite (HDM) model for allergic airway inflammation BALB/c mice were exposed to HDM extract intranasally (25 μg/50 μl) for 5 consecutive days a week over 7 weeks. With the help of the immunohistochemistry, vagal jugular-nodose ganglia complex (JNC) sections were analysed regarding their expression of DC-markers (MHC II, CD11c, CD103), the neuronal marker PGP 9.5 and the neuropeptide calcitonin gene-related peptide (CGRP) and glutamine synthetase (GS) as a marker for satellite glia cells (SGCs). To address the original source of DCs in sensory ganglia, a proliferation experiment was also carried in this study.

RESULTS

Immune cells with characteristic DC-phenotype were found to be closely located to SGCs and vagal sensory neurons under physiological conditions. The percentage of DCs in relation to neurons was significantly increased by allergic airway inflammation in comparison to the controls (HDM 51.38 ± 2.38% vs. control 28.16 ± 2.86%, p < 0.001). The present study also demonstrated that DCs were shown to proliferate in jugular-nodose ganglia, however, the proliferation rate of DCs is not significantly changed in the two treated animal groups (proliferating DCs/ total DCs: HDM 0.89 ± 0.38%, vs. control 1.19 ± 0.54%, p = 0.68). Also, increased number of CGRP-positive neurons was found in JNC after allergic sensitisation and challenge (HDM 31.16 ± 5.41% vs. control 7.16 ± 1.53%, p < 0.001).

CONCLUSION

The present findings suggest that DCs may migrate from outside into the ganglia to interact with sensory neurons enhancing or protecting the allergic airway inflammation. The increase of DCs as well as CGRP-positive neurons in airway ganglia by allergic airway inflammation indicate that intraganglionic DCs and neurons expressing CGRP may contribute to the pathogenesis of bronchial asthma. To understand this neuroimmune interaction in allergic airway inflammation further functional experiments should be carried out in future studies.

Lung diffusion capacity in children with respiratory symptoms and untreated GERD

BACKGROUND

Gastroesophageal reflux disease (GERD) is associated with many respiratory disorders, among which, chronic cough, laryngitis, and asthma are among the most common. We investigated lung function, including gas diffusion capacity, in children with poor asthma control or chronic laryngitis with untreated GERD.

MATERIAL AND METHODS

A total of 71 children, aged 6-17 years, with chronic respiratory and other symptoms suggestive for GERD, were enrolled and divided into 2 groups: chronic laryngitis and asthma. Participants underwent 24-hour pH monitoring and lung function assessment, measurement of single-breath diffusing capacity of the lung for carbon monoxide (DLCO), and fraction of exhaled nitric oxide (FENO) measurement.

RESULTS

24-hour pH monitoring was positive for GERD in 92.1% of preselected children with asthma and 90.1% of children with chronic recurrent laryngitis. All flows (PEF, MEF75, MEF50, and MEF25) were significantly lower in the asthma group, while FENO and DLCO were significantly lower in the laryngitis group. A significant inverse relationship was found between DLCO and all reflux indexes in the laryngitis group. Each unit change of Johnson-DeMeester score and Boix-Ochoa score increased the odds for significantly lower DLCO in laryngitis patients by 3.9% and 5.5%, respectively.

CONCLUSIONS

In children with uncontrolled asthma and chronic laryngitis, the regurgitation of gastric contents due to GERD contributes to poor asthma control and aggravation of chronic laryngitis. Despite having normal lung function, the gas diffusion capacity should be controlled in patients with GERD and chronic laryngitis, and it might be the very first abnormality in distal airways.

Putative roles of neuropeptides in vagal afferent signaling

The vagus nerve is a major pathway by which information is communicated between the brain and peripheral organs. Sensory neurons of the vagus are located in the nodose ganglia. These vagal afferent neurons innervate the heart, the lung and the gastrointestinal tract, and convey information about peripheral signals to the brain important in the control of cardiovascular tone, respiratory tone, and satiation, respectively. Glutamate is thought to be the primary neurotransmitter involved in conveying all of this information to the brain. It remains unclear how a single neurotransmitter can regulate such an extensive list of physiological functions from a wide range of visceral sites. Many neurotransmitters have been identified in vagal afferent neurons and have been suggested to modulate the physiological functions of glutamate. Specifically, the anorectic peptide transmitters, cocaine and amphetamine regulated transcript (CART) and the orexigenic peptide transmitters, melanin concentrating hormone (MCH) are differentially regulated in vagal afferent neurons and have opposing effects on food intake. Using these two peptides as a model, this review will discuss the potential role of peptide transmitters in providing a more precise and refined modulatory control of the broad physiological functions of glutamate, especially in relation to the control of feeding.

An NT4/TrkB-dependent increase in innervation links early-life allergen exposure to persistent airway hyperreactivity

Children who are exposed to environmental respiratory insults often develop asthma that persists into adulthood. In this study, we used a neonatal mouse model of ovalbumin (OVA)-induced allergic airway inflammation to understand the long-term effects of early childhood insults on airway structure and function. We showed that OVA sensitization and challenge in early life led to a 2-fold increase in airway smooth muscle (ASM) innervation (P<0.05) and persistent airway hyperreactivity (AHR). In contrast, OVA exposure in adult life elicited short-term AHR without affecting innervation levels. We found that postnatal ASM innervation required neurotrophin (NT)-4 signaling through the TrkB receptor and that early-life OVA exposure significantly elevated NT4 levels and TrkB signaling by 5- and 2-fold, respectively, to increase innervation. Notably, blockade of NT4/TrkB signaling in OVA-exposed pups prevented both acute and persistent AHR without affecting baseline airway function or inflammation. Furthermore, biophysical assays using lung slices and isolated cells demonstrated that NT4 was necessary for hyperreactivity of ASM induced by early-life OVA exposure. Together, our findings show that the NT4/TrkB-dependent increase in innervation plays a critical role in the alteration of the ASM phenotype during postnatal growth, thereby linking early-life allergen exposure to persistent airway dysfunction.

Mechanisms underlying the neuronal-based symptoms of allergy

Persons with allergies present with symptoms that often are the result of alterations in the nervous system. Neuronally based symptoms depend on the organ in which the allergic reaction occurs but can include red itchy eyes, sneezing, nasal congestion, rhinorrhea, coughing, bronchoconstriction, airway mucus secretion, dysphagia, altered gastrointestinal motility, and itchy swollen skin. These symptoms occur because mediators released during an allergic reaction can interact with sensory nerves, change processing in the central nervous system, and alter transmission in sympathetic, parasympathetic, and enteric autonomic nerves. In addition, evidence supports the idea that in some subjects this neuromodulation is, for reasons poorly understood, upregulated such that the same degree of nerve stimulus causes a larger effect than seen in healthy subjects. There are distinctions in the mechanisms and nerve types involved in allergen-induced neuromodulation among different organ systems, but general principles have emerged. The products of activated mast cells, other inflammatory cells, and resident cells can overtly stimulate nerve endings, cause long-lasting changes in neuronal excitability, increase synaptic efficacy, and also change gene expression in nerves, resulting in phenotypically altered neurons. A better understanding of these processes might lead to novel therapeutic strategies aimed at limiting the suffering of those with allergies.

Bronchoconstriction induced by increasing airway temperature in ovalbumin-sensitized rats: role of tachykinins

This study was carried out to determine the effect of allergic inflammation on the airway response to increasing airway temperature. Our results showed the following: 1) In Brown-Norway rats actively sensitized by ovalbumin (Ova), isocapnic hyperventilation with humidified warm air (HWA) for 2 min raised tracheal temperature (Ttr) from 33.4 ± 0.6°C to 40.6 ± 0.1°C, which induced an immediate and sustained (>10 min) increase in total pulmonary resistance (Rl) from 0.128 ± 0.004 to 0.212 ± 0.013 cmH2O·ml(-1)·s (n = 6, P < 0.01). In sharp contrast, the HWA challenge caused the same increase in Ttr but did not generate any increase in Rl in control rats. 2) The increase in Rl in sensitized rats was reproducible when the same HWA challenge was repeated 60-90 min later. 3) This bronchoconstrictive effect was temperature dependent: a slightly smaller increase in peak Ttr (39.6 ± 0.2°C) generated a significant but smaller increase in Rl in sensitized rats. 4) The HWA-induced bronchoconstriction was not generated by the humidity delivered by the HWA challenge alone, because the same water content delivered by saline aerosol at room temperature had no effect. 5) The HWA-evoked increase in Rl in sensitized rats was not blocked by atropine but was completely prevented by pretreatment either with a combination of neurokinin (NK)-1 and NK-2 antagonists or with formoterol, a β2 agonist, before the HWA challenge. This study showed that increasing airway temperature evoked a pronounced and reversible increase in airway resistance in sensitized rats and that tachykinins released from the vagal bronchopulmonary C-fiber endings were primarily responsible.

Nociceptin/orphanin FQ receptor activation decreases the airway hyperresponsiveness induced by allergen in sensitized mice

Several studies suggest that the N/OFQ (nociceptin/orphanin FQ)-NOP (N/OFQ peptide) receptor pathway is involved in airway physiology. We previously demonstrated a modulation of the endogenous N/OFQ levels in allergen-sensitized mice. Here, we investigated the effects of NOP receptor activation in allergen sensitization using a murine model of allergen-induced airway hyperresponsiveness (AHR). BALB/c mice were intraperitoneally treated with the NOP receptor agonist UFP-112, either during the sensitization phase (30 min before ovalbumin administration) or at the end of sensitization process (15 min before bronchopulmonary reactivity evaluation). At day 21 from the first allergen exposure, bronchopulmonary reactivity and total and differential cell count in bronchoalveolar lavage fluid were evaluated. In a separate set of experiments cell proliferation in lymphocytes, cytokine levels, IgE serum levels, and the effect of UFP-112 on IL-13-induced AHR were evaluated. Pretreatment with UFP-112, during the sensitization phase, caused a significant reduction in allergen-induced AHR and total cell lung infiltration. No effect on allergen-induced AHR was observed when the treatment was performed at the end of sensitization process, on tissues harvested from OVA-sensitized mice and on IL-13-induced AHR. The in vitro proliferative response of lymphocytes was significantly reduced by pretreatment during the sensitization phase with UFP-112. This effect was paralleled by a significant modulation of cytokine secretion in pulmonary tissues and lymphocytes. In conclusion, we demonstrated a role for the NOP receptor and N/OFQ pathway in the AHR induced by allergen, probably through a modulation of the immune response that triggers the development of AHR that involves pro- and anti-inflammatory cytokines.

The effects of nociceptin peptide (N/OFQ)-receptor (NOP) system activation in the airways

The heptadecapeptide nociceptin/orphanin FQ (N/OFQ) is the endogenous ligand for the N/OFQ peptide (NOP) receptor. It is cleaved from a larger precursor identified as prepronociceptin (ppN/OFQ). NOP is a member of the seven transmembrane-spanning G-protein coupled receptor (GPCR) family. ppN/OFQ and NOP receptors are widely distributed in different human tissues. Asthma is a complex heterogeneous disease characterized by variable airflow obstruction, bronchial hyper-responsiveness and chronic airway inflammation. Limited therapeutic effectiveness of currently available asthma therapies warrants identification of new drug compounds. Evidence from animal studies suggests that N/OFQ modulates airway contraction and inflammation. Interestingly up regulation of the N/OFQ-NOP system reduces airway hyper-responsiveness. In contrast, inflammatory cells central to the inflammatory response in asthma may be both sources of N/OFQ and respond to NOP activation. Hence paradoxical dysregulation of the N/OFQ-NOP system may potentially play an important role in regulating airway inflammation and airway tone. To date there is no data on N/OFQ-NOP expression in the human airways. Therefore, the potential role of N/OFQ-NOP system in asthma is unknown. This review focuses on its physiological effects within airways and potential value as a novel asthma therapy.

Neuropeptides control the dynamic behavior of airway mucosal dendritic cells

The airway mucosal epithelium is permanently exposed to airborne particles. A network of immune cells patrols at this interface to the environment. The interplay of immune cells is orchestrated by different mediators. In the current study we investigated the impact of neuronal signals on key functions of dendritic cells (DC). Using two-photon microscopic time-lapse analysis of living lung sections from CD11c-EYFP transgenic mice we studied the influence of neuropeptides on airway DC motility. Additionally, using a confocal microscopic approach, the phagocytotic capacity of CD11c(+) cells after neuropeptide stimulation was determined. Electrical field stimulation (EFS) leads to an unspecific release of neuropeptides from nerves. After EFS and treatment with the neuropeptides vasoactive intestinal peptide (VIP) or calcitonin gene-related peptide (CGRP), airway DC in living lung slices showed an altered motility. Furthermore, the EFS-mediated effect could partially be blocked by pre-treatment with the receptor antagonist CGRP(8-37). Additionally, the phagocytotic capacity of bone marrow-derived and whole lung CD11c(+) cells could be inhibited by neuropeptides CGRP, VIP, and Substance P. We then cross-linked these data with the in vivo situation by analyzing DC motility in two different OVA asthma models. Both in the acute and prolonged OVA asthma model altered neuropeptide amounts and DC motility in the airways could be measured. In summary, our data suggest that neuropeptides modulate key features motility and phagocytosis of mouse airway DC. Therefore altered neuropeptide levels in airways during allergic inflammation have impact on regulation of airway immune mechanisms and therefore might contribute to the pathophysiology of asthma.

Substance p regulates environmental tobacco smoke-enhanced tracheal smooth muscle responsiveness in mice

Environmental tobacco smoke (ETS) is an environmental trigger that leads to airway inflammation and airway hyperresponsiveness (AHR) in susceptible individuals and animals, but the underlying mechanism is not fully understood. Substance P (SP) release from sensory nerve fibers has been linked to AHR. The present experiments characterize the role of SP in tracheal smooth muscle on ETS-increased airway responses. The mice were exposed to either sidestream tobacco smoke (SS), a surrogate to ETS, or filtered air (FA) for 1 day or 5 consecutive days. Contractions of tracheal smooth muscle to SP and electrical field stimulation (EFS) were not significantly altered in 1 of day SS-exposed mice. However, 5 of days SS exposure significantly increased airway smooth muscle contractions to SP and EFS. Administration of CP-99994, an antagonist of the neurokinin (NK)1 receptor, attenuates the SS exposure-enhanced tracheal smooth muscle responses to EFS. Furthermore, the immunohistochemistry showed that SP nerve fibers were increased in tracheal smooth muscle after 5 of days SS exposure. These results suggest that the increased SP production may contribute to SS-enhanced smooth muscle responsiveness in mice trachea.

Effect of airway inflammation on short-latency reflex inhibition to inspiratory loading in human scalene muscles

The short-latency reflex inhibition of human inspiratory muscles produced by loading is prolonged in asthma and obstructive sleep apnoea, both diseases involving airway and systemic inflammation. Both diseases also involve repetitive inspiratory loading. Although airway mucosal afferents are not critical components of the normal reflex arc, during airway inflammation, prolongation of the reflex may be caused by altered mucosal afferent sensitivity, or altered central processing of their inputs. We hypothesised that acute viral airway inflammation would replicate the reflex abnormality. The reflex was tested in 9 subjects with a "common cold" during both the acute infection and when well. Surface electrodes recorded electromyographic (EMG) activity bilaterally from scalene muscles. Latencies of the inhibitory response (IR) did not differ significantly (IR peak 67 vs 70 ms (p=0.12), and IR offset 87 vs 90 ms (p=0.23), between the inflamed and well conditions, respectively). There was no difference in any measure of the size of the reflex inhibition.

The neuropeptide calcitonin gene-related peptide affects allergic airway inflammation by modulating dendritic cell function

BACKGROUND

The neuropeptide calcitonin gene-related peptide (CGRP) is released in the lung by sensory nerves during allergic airway responses. Pulmonary dendritic cells (DC) orchestrating the allergic inflammation could be affected by CGRP.

OBJECTIVE

To determine the immunomodulatory effects of CGRP on DC function and its impact on the induction of allergic airway inflammation.

METHODS

CGRP receptor expression on lung DC was determined by RT-PCR and immunofluorescence staining. The functional consequences of CGRP receptor triggering were evaluated in vitro using bone marrow-derived DC. DC maturation and the induction of ovalbumin (OVA)-specific T cell responses were analysed by flow cytometry. The in vivo relevance of the observed DC modulation was assessed in a DC-transfer model of experimental asthma. Mice were sensitized by an intrapharyngeal transfer of OVA-pulsed DC and challenged with OVA aerosol. The impact of CGRP pretreatment of DC on airway inflammation was characterized by analysing differential cell counts and cytokines in bronchoalveolar lavage fluid (BALF), lung histology and cytokine responses in mediastinal lymph nodes.

RESULTS

RT-PCR, immunofluorescence and cAMP assay demonstrated the expression of functionally active CGRP receptors in lung DC. RT-PCR revealed a transcriptional CGRP receptor down-regulation during airway inflammation. CGRP specifically inhibited the maturation of in vitro generated DC. Maturation was restored by blocking with the specific antagonist CGRP(8-37) . Consequently, CGRP-pretreated DC reduced the activation and proliferation of antigen-specific T cells and induced increased the numbers of T regulatory cells. The transfer of CGRP-pretreated DC diminished allergic airway inflammation in vivo, shown by reduced eosinophil numbers and increased levels of IL-10 in BALF.

CONCLUSIONS AND CLINICAL RELEVANCE

CGRP inhibits DC maturation and allergen-specific T cell responses, which affects the outcome of the allergic airway inflammation in vivo. This suggests an additional mechanism by which nerve-derived mediators interfere with local immune responses. Thus, CGRP as an anti-inflammatory mediator could represent a new therapeutic tool in asthma therapy.

Degeneration of sensory afferent nerves enhances pulmonary inflammatory alterations in acute myocardial infarction in rats

BACKGROUND

Evidence suggests proinflammatory changes in the lungs during acute myocardial infarction and a participation of neural mechanisms and substance P in the pathology. This study was undertaken to investigate the role and the mechanisms by which sensory afferent degeneration at neonatal stages exacerbates the pulmonary inflammatory responses to acute myocardial infarction in the adult rats.

METHODS

The degeneration of capsaicin-sensitive afferent nerves was induced by administration of capsaicin to neonatal rats. The pulmonary inflammatory changes following coronary artery occlusion (CAO) were assessed by the analysis of the infiltration of neutrophils and tissue morphology in the lungs.

RESULTS

Significant increases in the pulmonary infiltration of neutrophils, up to 240% and 218% of the sham controls at 3 and 6 h, respectively, after CAO (P<.05) and marked pulmonary edema were observed. Degeneration of capsaicin-sensitive afferent nerves or antagonism of endogenous neurokinin (NK)-1 receptor exacerbated the pulmonary infiltration of neutrophils (up to 214% and 254% of the controls, respectively) and pulmonary tissue edema following the CAO.

CONCLUSION

The findings indicate that degeneration of sensory afferent nerves enhances the pulmonary inflammatory changes in acute myocardial infarction, in which the endogenous NK may play a role.

Airway inflammation and hypersensitivity induced by chronic smoking

Airway hypersensitivity, characterized by enhanced excitability of airway sensory nerves, is a prominent pathophysiological feature in patients with airway inflammatory diseases. Although the underlying pathogenic mechanism is not fully understood, chronic airway inflammation is believed to be primarily responsible. Cigarette smoking is known to cause chronic airway inflammation, accompanied by airway hyperresponsiveness. Experimental evidence indicates that enhanced excitability of vagal bronchopulmonary sensory nerves and increased tachykinin synthesis in these nerves resulting from chronic inflammation are important contributing factors to the airway hyperresponsiveness. Multiple inflammatory mediators released from various types of structural and inflammatory cells are involved in the smoking-induced airway inflammation, which is mainly regulated by redox-sensitive signaling pathways and transcription factors. Furthermore, recent studies have reported potent sensitizing and stimulatory effects of these inflammatory mediators such as prostanoids and reactive oxygen species on these sensory nerves. In summary, these studies using cigarette smoking as an experimental approach have identified certain potentially important cell signaling pathways and underlying mechanisms of the airway hypersensitivity induced by chronic airway inflammation.

Neurotrophin and GDNF family ligand receptor expression in vagal sensory nerve subtypes innervating the adult guinea pig respiratory tract

We combined retrograde tracing techniques with single-neuron RT-PCR to compare the expression of neurotrophic factor receptors in nodose vs. jugular vagal sensory neurons. The neurons were further categorized based on location of their terminals (tracheal or lungs) and based on expression of the ionotropic capsaicin receptor TRPV1. Consistent with functional studies, nearly all jugular neurons innervating the trachea and lungs expressed TRPV1. With respect to the neurotrophin receptors, the TRPV1-expressing jugular C-fiber neurons innervating both the trachea and lung compartments preferentially expressed tropomyosin-receptor kinase A (TrkA), with only a minority of neurons expressing TrkB or TrkC. The nodose neurons that express TRPV1 (presumed nodose C-fibers) innervate mainly intrapulmonary structures. These neurons preferentially expressed TrkB, with only a minority expressing TrkA or TrkC. The expression pattern in tracheal TRPV1-negative neurons, nodose tracheal presumed Aδ-fiber neurons as well as the intrapulmonary TRPV1-negative presumed Aβ-fiber neurons, was similar to that observed in the nodose C-fiber neurons. We also evaluated the expression of GFRα receptors and RET (receptors for the GDNF family ligands). Virtually all vagal sensory neurons innervating the respiratory tract expressed RET and GFRα1. The jugular neurons also categorically expressed GFRα3, as well as ∼50% of the nodose neurons. GFRα2 was expressed in ∼50% of the neurons irrespective of subtype. The results reveal that Trk receptor expression in vagal afferent neurons innervating the adult respiratory tract depends more on the location of the cell bodies (jugular vs. nodose ganglion) than either the location of the terminals or the functional phenotype of the nerve. The data also reveal that in addition to neurotrophins, the GDNF family ligands may be important neuromodulators of vagal afferent nerves innervating the adult respiratory tract.

Early Postnatal Ozone Exposure Alters Rat Nodose and Jugular Sensory Neuron Development

Sensory neurons originating in nodose and jugular ganglia that innervate airway epithelium (airway neurons) play a role in inflammation observed following exposure to inhaled environmental irritants such as ozone (O(3)). Airway neurons can mediate airway inflammation through release of the neuropeptide substance P (SP). While susceptibility to airway irritants is increased in early life, the developmental dynamics of afferent airway neurons are not well characterized. The hypothesis of this study was that airway neuron number might increase with increasing age, and that an acute, early postnatal O(3) exposure might increase both the number of sensory airway neurons as well as the number SP-containing airway neurons. Studies using Fischer 344 rat pups were conducted to determine if age or acute O(3) exposure might alter airway neuron number. Airway neurons in nodose and jugular ganglia were retrogradely labeled, removed, dissociated, and counted by means of a novel technique employing flow cytometry. In Study 1, neuron counts were conducted on postnatal days (PD) 6, 10, 15, 21, and 28. Numbers of total and airway neurons increased significantly between PD6 and PD10, then generally stabilized. In Study 2, animals were exposed to O(3) (2 ppm) or filtered air (FA) on PD5 and neurons were counted on PD10, 15, 21, and 28. O(3) exposed animals displayed significantly less total neurons on PD21 than FA controls. This study shows that age-related changes in neuron number occur, and that an acute, early postnatal O(3) exposure significantly alters sensory neuron development.

Role of nerve growth factor in ozone-induced neural responses in early postnatal airway development

Airway neural plasticity contributes to the process of airway remodeling in response to airway irritants. However, the mechanisms of neural remodeling in the airways during the early postnatal period, when responses to airway irritation may be most sensitive, have not been characterized. This study used a rat model to examine a possible mechanism of ozone (O(3))-induced neural hyperresponsiveness during a critical period of developmental, postnatal day (PD) 6, that may be mediated by the neurotrophin nerve growth factor (NGF), resulting in an enhanced release of inflammatory neuropeptide substance P (SP) from airway nerves. Rat pups between PD6-PD28 were killed 24 hours after exposure to O(3) (2 ppm, 3 hours) or filtered air (FA), to establish a timeline of NGF synthesis, or else they were exposed to O(3) or NGF on PD6 or PD21 and re-exposed to O(3) on PD28, and killed on PD29. Measurement endpoints included NGF mRNA in tracheal epithelial cells, NGF protein in bronchoalveolar lavage fluid, airway SP-nerve fiber density (NFD), and SP-positive airway neurons in vagal ganglia. Acute exposure to O(3) increased NGF in bronchoalveolar lavage fluid on PD10 and PD15, and mRNA expression in epithelial cells on PD6, compared with FA controls. NGF protein and mRNA expression in the O(3)-PD6/O(3)-PD28 groups were significantly higher than in the O(3)-PD21/O(3)-PD28 and O(3)-PD6/FA-PD28 groups. NGF-PD6/O(3)-PD28 increased the SP innervation of airway smooth muscle and SP-positive sensory neurons, compared with the NGF-PD21/O(3)-PD28 or NGF-PD6/FA-PD28 groups. NGF enhanced sensory innervation, which may mediate acute responses or prolong sensitivity to O(3) during early life. The model may be relevant in O(3) responses during early childhood.

Nociceptin modulates bronchoconstriction induced by sensory nerve activation in mouse lung

Nociceptin/orphanin FQ (N/OFQ), the endogenous ligand for the N/OFQ peptide receptor (NOP), inhibits tachykinin release in the airway of several animal models. The aim of this study was to investigate the role of the N/OFQ-NOP receptor system in bronchoconstriction induced by sensory nerve activation in the isolated mouse lung. We used C57BL/6J NOP(+/+), NOP(-/-), and Balb/C mice sensitized (or not) to ovalbumin. Bronchopulmonary function coupled with measurements of endogenous N/OFQ levels before and after capsaicin-induced bronchoconstriction in the presence or absence of NOP-selective agonists/antagonists are presented. N/OFQ significantly inhibited capsaicin-induced bronchoconstriction in both naive and sensitized mice, these latter animals displaying airway hyperresponsiveness to capsaicin. The inhibitory effect of N/OFQ were not observed in NOP(-/-) mice, and were mimicked/abolished by the selective NOP agonist/antagonist University of Ferrara Peptide (UFP)-112/UFP-101 in NOP(+/+) mice. UFP-101 alone potentiated the effect of capsaicin in naive mice, but not in sensitized mice. Endogenous N/OFQ levels significantly decreased in sensitized mice relative to naive mice. We have demonstrated that a reduction in endogenous N/OFQ, or the lack of its receptor, causes an increase in capsaicin-induced bronchoconstriction, implying a role for the N/OFQ-NOP receptor system in the modulation of capsaicin effects. Moreover, for the first time, we document differential airway responsiveness to capsaicin between naive and sensitized mice due, at least in part, to decreased endogenous N/OFQ levels in sensitized mice.

Sensory neural responses to ozone exposure during early postnatal development in rat airways

Airway infections or irritant exposures during early postnatal periods may contribute to the onset of childhood asthma. The purpose of this study was to examine critical periods of postnatal airway development during which ozone (O(3)) exposure leads to heightened neural responses. Rats were exposed to O(3) (2 ppm) or filtered air for 1 hour on specific postnatal days (PDs) between PD1 and PD29, and killed 24 hours after exposure. In a second experiment, rats were exposed to O(3) on PD2-PD6, inside a proposed critical period of development, or on PD19-PD23, outside the critical period. Both groups were re-exposed to O(3) on PD28, and killed 24 hours later. Airways were removed, fixed, and prepared for substance P (SP) immunocytochemistry. SP nerve fiber density (NFD) in control extrapulmonary (EXP) epithelium/lamina propria (EPLP) increased threefold, from 1% to 3.3% from PD1-PD3 through PD13-PD15, and maintained through PD29. Upon O(3) exposure, SP-NFD in EXP-smooth muscle (SM) and intrapulmonary (INT)-SM increased at least twofold at PD1-PD3 through PD13-PD15 in comparison to air exposure. No change was observed at PD21-PD22 or PD28-PD29. In critical period studies, SP-NFD in the INT-SM and EXP-SM of the PD2-PD6 O(3) group re-exposed to O(3) on PD28 was significantly higher than that of the group exposed at PD19-PD23 and re-exposed at PD28. These findings suggest that O(3)-mediated changes in sensory innervation of SM are more responsive during earlier postnatal development. Enhanced responsiveness of airway sensory nerves may be a contributing mechanism of increased susceptibility to environmental exposures observed in human infants and children.

Neural control of airway inflammation

Abnormal neural function contributes to the pathogenesis of airway disease. In addition to affecting airway physiology, the nerves produce and release inflammatory mediators, contributing to the recruitment and activation of leukocytes. Activated inflammatory cells in turn affect the function of airway nerves, changing the production and release of neurotransmitters. Cross-talk between airway nerves and leukocytes helps to maintain chronic inflammation and accentuates neural control of the airways.