The role of neuroeffector mechanisms in the pathogenesis of asthma

The airway neuro-immune axis as a therapeutic target in allergic airway diseases

Recent evidence has increasingly underscored the importance of the neuro-immune axis in mediating allergic airway diseases, such as allergic asthma and allergic rhinitis. The intimate spatial relationship between neurons and immune cells suggests that their interactions play a pivotal role in regulating allergic airway inflammation. Upon direct activation by allergens, neurons and immune cells engage in interactions, during which neurotransmitters and neuropeptides released by neurons modulate immune cell activity. Meanwhile, immune cells release inflammatory mediators such as histamine and cytokines, stimulating neurons and amplifying neuropeptide production, thereby exacerbating allergic inflammation. The dynamic interplay between the nervous and immune systems suggests that targeting the neuro-immune axis in the airway could represent a novel approach to treating allergic airway diseases. This review summarized recent evidence on the nervous system's regulatory mechanisms in immune responses and identified potential therapeutic targets along the peripheral nerve-immune axis for allergic asthma and allergic rhinitis. The findings will provide novel perspectives on the management of allergic airway diseases in the future.

Short-term exposure to ozone and asthma exacerbation in adults: A longitudinal study in China

Background

The relationships between short-term ozone exposure and the acute exacerbations of asthma in adults have not been fully studied. Existing studies commonly ignored the effects of ozone on mild or early asthma exacerbations.

Objective

To investigate the associations between short-term ozone exposure and asthma exacerbations in Chinese adults.

Methods

We administered health management for adult asthma patients through the Respiratory Home Platform and required them to monitor their lung function every morning and evening by themselves. Finally, a total of 4,467 patients in 18 Chinese cities were included in the current analyses, with 79,217 pairs of lung function records. The maximum daily 8-h average ozone concentrations were collected from fixed-site air quality monitoring stations. We calculated diurnal peak expiratory flow (PEF) variation using morning and evening measurements of PEF and then defined different severity of asthma exacerbations with diurnal PEF variations >10, 15, and 20%, respectively. A binomial distributed generalized additive mixture model combined with distribution non-linear models was applied to examine the association of ozone with asthma exacerbations. We further conducted stratified analyses by sex, age, season of lung function tests, and region.

Measurements and results

We found that short-term ozone exposure was independently associated with an elevated risk of asthma exacerbations defined by lung function and the effects could last for about 2 days. At lag 0-2 days, each 10 μg/m3 increment in ozone concentration was associated with odds ratios of 1.010 [95% confidence interval (CI): 1.003, 1.017], 1.014 (95% CI: 1.005, 1.023), and 1.017 (95% CI: 1.006, 1.028) for asthma exacerbations that were defined by diurnal PEF variation over 10, 15, and 20%, respectively. The associations remained significant after adjusting for other pollutants, and became unstable when using 24-h average ozone concentration. We also found that the associations were relatively stronger in males, those aged 45 years and older, and in the warm season.

Conclusions

Our results suggest that short-term ozone exposure can increase the risk of asthma exacerbations, even in the early stage of exacerbation. Male and older asthma patients may be more vulnerable to ozone air pollution, especially in the warm season.

Sex, Cells, and Asthma

There are marked sex differences in asthma prevalence and severity. Sex hormones play a central role in these sex biases and directly interact with multiple key cells involved in the pathogenesis of asthma. Here we review the known effects of estrogen, progesterone, and testosterone on airway epithelial cells, airway smooth muscle cells, the mononuclear phagocyte system, innate lymphoid cells, eosinophils, mast cells, T cells, and B cells, all in the context of asthma. Furthermore, we explore unresolved clinical questions, such as the role of sex hormones in the link between asthma and obesity.

Asthma: New Integrative Treatment Strategies for the Next Decades

Asthma is a chronic disease whose main anatomical-functional alterations are grouped into obstruction, nonspecific bronchial hyperreactivity, inflammation and airway remodeling. Currently, the Global Initiative of Asthma 2020 (GINA 2020) suggests classifying it into intermittent cases, slightly persistent, moderately persistent and severely persistent, thus determining the correct guidelines for its therapy. In general, the drugs used for its management are divided into two groups, those with a potential bronchodilator and the controlling agents of inflammation. However, asthmatic treatments continue to evolve, and notable advances have been made possible in biological therapy with monoclonal antibodies and in the relationship between this disease and oxidative stress. This opens a new path to dietary and herbal strategies and the use of antioxidants as a possible therapy that supports conventional pharmacological treatments and reduces their doses and/or adverse effects. This review compiles information from different published research on risk factors, pathophysiology, classification, diagnosis and the main treatments; likewise, it synthesizes the current evidence of herbal medicine for its control. Studies on integrative medicine (IM) therapies for asthmatic control are critically reviewed. An integrative approach to the prevention and management of asthma warrants consideration in clinical practice. The intention is to encourage health professionals and scientists to expand the horizons of basic and clinical research (preclinical, clinical and integrative medicine) on asthma control.

Mechanisms of Stress-Mediated Modulation of Upper and Lower Respiratory Tract Infections

Stress is an external factor known to be a potent exacerbator of respiratory infections. Most explanations of how stress affects susceptibility to airway infections focus on the immune system. However, evidence is increasing that respiratory pathogens are equally responsive to the hormonal output of stress. This chapter considers the bacterial and mucosal determinants of respiratory tract infections and their interrelationship during stressful conditions.

Sex steroid signaling: implications for lung diseases

There is increasing recognition that sex hormones (estrogen, progesterone, and testosterone) have biological and pathophysiological actions in peripheral, non-reproductive organs, including the lung. Clinically, sex differences in the incidence, morbidity and mortality of lung diseases such as asthma, chronic obstructive pulmonary disease (COPD), pulmonary fibrosis, lung cancer and pulmonary hypertension have been noted, although intrinsic sex differences vs. the roles of sex steroids are still not well-understood. Accordingly, it becomes important to ask the following questions: 1) Which sex steroids are involved? 2) How do they affect different components of the lung under normal circumstances? 3) How does sex steroid signaling change in or contribute to lung disease, and in this regard, are sex steroids detrimental or beneficial? As our understanding of sex steroid signaling in the lung improves, it is important to consider whether such information can be used to develop new therapeutic strategies to target lung diseases, perhaps in both sexes or in a sex-specific manner. In this review, we focus on the basics of sex steroid signaling, and the current state of knowledge regarding how they influence structure and function of specific lung components across the life span and in the context of some important lung diseases. We then summarize the potential for sex steroids as useful biomarkers and therapeutic targets in these lung diseases as a basis for future translational research in the area of gender and individualized medicine.

International consensus on (ICON) pediatric asthma

Asthma is the most common chronic lower respiratory disease in childhood throughout the world. Several guidelines and/or consensus documents are available to support medical decisions on pediatric asthma. Although there is no doubt that the use of common systematic approaches for management can considerably improve outcomes, dissemination and implementation of these are still major challenges. Consequently, the International Collaboration in Asthma, Allergy and Immunology (iCAALL), recently formed by the EAACI, AAAAI, ACAAI, and WAO, has decided to propose an International Consensus on (ICON) Pediatric Asthma. The purpose of this document is to highlight the key messages that are common to many of the existing guidelines, while critically reviewing and commenting on any differences, thus providing a concise reference. The principles of pediatric asthma management are generally accepted. Overall, the treatment goal is disease control. To achieve this, patients and their parents should be educated to optimally manage the disease, in collaboration with healthcare professionals. Identification and avoidance of triggers is also of significant importance. Assessment and monitoring should be performed regularly to re-evaluate and fine-tune treatment. Pharmacotherapy is the cornerstone of treatment. The optimal use of medication can, in most cases, help patients control symptoms and reduce the risk for future morbidity. The management of exacerbations is a major consideration, independent of chronic treatment. There is a trend toward considering phenotype-specific treatment choices; however, this goal has not yet been achieved.

A Shh/miR-206/BDNF cascade coordinates innervation and formation of airway smooth muscle

Dysfunctional neural control of airway smooth muscle (ASM) is involved in inflammatory diseases, such as asthma. However, neurogenesis in the lung is poorly understood. This study uses mouse models to investigate developmental mechanisms of ASM innervation, a process that is highly coordinated with ASM formation during lung branching morphogenesis. We show that brain-derived neurotrophic factor (BDNF) is an essential ASM-derived signal for innervation. Although BDNF mRNA expression is temporally dissociated with ASM formation and innervation, BDNF protein is coordinately produced through post-transcriptional suppression by miR-206. Using a combination of chemical and genetic approaches to modulate sonic hedgehog (Shh) signaling, a pathway essential for lung branching and ASM formation, we show that Shh signaling blocks miR-206 expression, which in turn increases BDNF protein expression. Together, our work uncovers a functional cascade that involves Shh, miR-206 and BDNF to coordinate ASM formation and innervation.

A Shh/miR-206/BDNF cascade coordinates innervation and formation of airway smooth muscle

Dysfunctional neural control of airway smooth muscle (ASM) is involved in inflammatory diseases, such as asthma. However, neurogenesis in the lung is poorly understood. This study uses mouse models to investigate developmental mechanisms of ASM innervation, a process that is highly coordinated with ASM formation during lung branching morphogenesis. We show that brain-derived neurotrophic factor (BDNF) is an essential ASM-derived signal for innervation. Although BDNF mRNA expression is temporally dissociated with ASM formation and innervation, BDNF protein is coordinately produced through post-transcriptional suppression by miR-206. Using a combination of chemical and genetic approaches to modulate sonic hedgehog (Shh) signaling, a pathway essential for lung branching and ASM formation, we show that Shh signaling blocks miR-206 expression, which in turn increases BDNF protein expression. Together, our work uncovers a functional cascade that involves Shh, miR-206 and BDNF to coordinate ASM formation and innervation.

The absence of VPAC2 leads to aberrant antibody production in Aspergillus fumigatus sensitized and challenged mice

Vasoactive intestinal peptide (VIP) facilitates a "pro-allergy" phenotype when signaling through its G protein-coupled receptor, VPAC(2). We have shown that VPAC(2) knock-out (KO) mice developed an allergic phenotype marked by eosinophilia and elevated serum IgE. Therefore, we hypothesized that the humoral response to allergen challenge in these mice was T(H)2 dominant similar to wild-type (WT) C57BL/6 mice. Antibody responses in WT and KO mice were measured after Aspergillus fumigatus conidia inhalation. In contrast to previous reports, basal levels of serum IgG(2a) and IgA were significantly higher in naïve VPAC(2) KO animals. Antibody availability in the serum as well as the bronchoalveolar lavage fluid after fungal challenge was dominated by the pro-inflammatory isotype IgG(2a) and the mucosal isotype, IgA. IgA localizing cells dominated in the peribronchovascular areas of allergic KO mice while IgE immune complexes were found in WT allergic lungs. This research shows for the first time that VPAC(2) has a significant effect on antibody regulation, in the context of allergy.

New insight into the genes susceptible to asthma

There is considerable worldwide interest in identifying genes related to susceptibility to asthma. Progress has been slow in part because of the complexity and heterogeneity of the disease. Although at least 170 genes located on 10 chromosomes have been associated with or in linkage with asthma and asthma-related phenotypes, the majority of the reports have either been preliminary or the results have been controversial. In order to overcome the problems with the inherent complexity of asthma and methodological issues, the authors propose a strategy for identification of asthma susceptibility genes based on theories of systems biology and bioinformatics and candidate gene approach.

Increased parasympathetic tone as the underlying cause of asthma: a hypothesis

Asthma is a chronic inflammatory disease of airways that is characterized by increased responsiveness of the tracheo-bronchial tree to multiple number of stimuli. Immunological theory does not explain all features in asthma, for example hyper-reactivity of the airways. Neurogenic theory also fails to explain the pathogenesis of asthma comprehensively. Higher parasympathetic tone has been reported in asthmatics but has never been suggested as a major underlying cause of asthma. This article attempts to explain the occurrence of hyper-responsiveness, inflammatory/allergic reactions and broncho-constriction in asthma on a common basis of inherent higher parasympathetic tone in asthmatics. The higher background parasympathetic firing leads to increased nitric oxide (NO) production owing to its co-localization with acetylcholine (ACh) in inhibitory non-adrenergic and non-cholinergic (i-NANC) nerves. NO is a neurotransmitter of i-NANC system and it mediates bronchodilation. Increased NO release has been found to be responsible for hyper-responsiveness and increased inflammation in the airways. The authors suggest that an inherently higher background parasympathetic tone in concert with inflammation or a specific genetic background could modify the effects of NO on lung homeostasis in humans leading to increased susceptibility to an asthmatic state.

Pulmonary peptidergic innervation remodeling and development of airway hyperresponsiveness induced by RSV persistent infection

Respiratory syncytial virus (RSV) infection causes bronchiolitis in infants and children, which is an important risk factor for the development of chronic asthma. To probe the underlying mechanisms that RSV infection increases the susceptibility of asthma, this present study was designed to establish a RSV persistent infection animal model by cyclophosphamide (CYP) pretreatment that more closely mimic human RSV infection. CYP is an immunosuppressant, which induced deficiency in cellular and humoral immunity. Pulmonary RSV titers, airway function and peptidergic innervation were measured on 7d, 28 d, 42 d and 60 d postinfection. The results showed that during RSV persistent infection, the lungs of RSV-inoculated animals pretreated with CYP showed higher RSV titers and exhibited obvious chronic inflammation. The results also showed that protein gene product 9.5 (PGP9.5), substance P (SP) and calcitonin gene-related peptide (CGRP)-immunoreactive fibers increased and vasoactive intestinal peptide (VIP)-immunoreactive fibers decreased during RSV persistent infection. These results demonstrate that RSV persistent infection induces significant alterations in the peptidergic innervation in the airways, which may be associated with the development of altered airway function.

Diagnosis and treatment of asthma in childhood: a PRACTALL consensus report

Asthma is the leading chronic disease among children in most industrialized countries. However, the evidence base on specific aspects of pediatric asthma, including therapeutic strategies, is limited and no recent international guidelines have focused exclusively on pediatric asthma. As a result, the European Academy of Allergy and Clinical Immunology and the American Academy of Allergy, Asthma and Immunology nominated expert teams to find a consensus to serve as a guideline for clinical practice in Europe as well as in North America. This consensus report recommends strategies that include pharmacological treatment, allergen and trigger avoidance and asthma education. The report is part of the PRACTALL initiative, which is endorsed by both academies.

Allergic lung inflammation affects central noradrenergic control of cholinergic outflow to the airways in ferrets

Brain stem noradrenergic cell groups mediating autonomic responses to stress project to airway-related vagal preganglionic neurons (AVPNs). In ferrets, their activation produces withdrawal of cholinergic outflow to the airways via release of norepinephrine and activation of alpha(2A)-adrenergic receptors (alpha(2A)-AR) expressed by AVPNs. In these studies, we examined the effects of allergen exposure of the airway (AE) with ovalbumin on noradrenergic transmission regulating the activity of AVPNs and, consequently, airway smooth muscle tone. Experiments were performed in vehicle control (Con) and AE ferrets. Microperfusion of an alpha(2A)-AR agonist (guanabenz) in close proximity to AVPNs elicited more pronounced effects in Con than AE ferrets, including a decrease in unit activity and reflexly evoked responses of putative AVPN neurons with a corresponding decrease in cholinergic outflow to the airways. Although no differences were found in the extent of noradrenergic innervation of the AVPNs, RT-PCR and Western blot studies demonstrated that AE and repeated exposure to antigen significantly reduced expression of alpha(2A)-ARs at message and protein levels. These findings indicate that, in an animal model of allergic asthma, sensitization and repeated challenges with a specific allergen diminish central inhibitory noradrenergic modulation of AVPNs, possibly via downregulation of alpha(2A)-AR expression by these neurons.

The effect of the leukotriene receptor antagonist zafirlukast on neurokinin A-induced bronchoconstriction in patients with asthma--A comparison with leukotriene D4 induced broncoconstriction

The bronchoconstriction caused by inhaled neurokinin A (NKA) in patients with asthma is indirect. The mediators involved in NKA-induced bronchoconstriction are unknown. Studies with various H1 receptor antagonists were negative, making an important contribution of histamine unlikely. To study the role of cysteinyl leukotrienes in neurokinin A-induced bronchoconstriction, we performed a randomised, double-blind, cross-over, placebo controlled trial in 12 patients with mild to moderate asthma. Zafirlukast and matching placebo were given orally, 40 mg the evening before and 40 mg the morning of assessment. In one period NKA was administered, in the other period leukotriene D4 (LTD4). Increasing concentrations of NKA and LTD4 were inhaled from a 30 L bag, after nebulization via a Mallinckrodt nebuliser. The difference between log10PC20LTD4 after treatment with placebo or zafirlukast was highly significant (p<0.0001). A trend was observed towards a difference between log10PC20 neurokinin A after treatment with placebo or zafirlukast (p=0.0741). The dose ratio for the neurokinin A provocation was 4.4 and for the LTD4 provocation 67.7. In conclusion, zafirlukast had a large inhibitory effect on LTD4-induced bronchoconstriction, but offered only limited protective effect against neurokinin A-induced bronchoconstriction. We suggest that leukotrienes play a limited role in the bronchoconstrictor effect of neurokinin A in patients with asthma.

Hydrogen sulfide causes vanilloid receptor 1-mediated neurogenic inflammation in the airways

Hydrogen sulfide (H(2)S) is described as a mediator of diverse biological effects, and is known to produce irritation and injury in the lung following inhalation. Recently, H(2)S has been found to cause contraction in the rat urinary bladder via a neurogenic mechanism. Here, we studied whether sodium hydrogen sulfide (NaHS), used as donor of H(2)S, produces responses mediated by sensory nerve activation in the guinea-pig airways. NaHS evoked an increase in neuropeptide release in the airways that was significantly attenuated by capsaicin desensitization and by the transient receptor potential vanilloid 1 (TRPV1) antagonist capsazepine. In addition, NaHS caused an atropine-resistant contraction of isolated airways, which was completely prevented by capsaicin desensitization. Furthermore, NaHS-induced contraction was reduced by TRPV1 antagonism (ruthenium red, capsazepine and SB366791), and was abolished by pretreatment with the combination of tachykinin NK(1) (SR140333) and NK(2) (SR48968) receptor antagonists. In anesthetized guinea-pigs, intratracheal instillation of NaHS increased the total lung resistance and airway plasma protein extravasation. These two effects were reduced by TRPV1 antagonism (capsazepine) and tachykinin receptors (SR140333 and SR48968) blockade. Our results provide the first pharmacological evidence that H(2)S provokes tachykinin-mediated neurogenic inflammatory responses in guinea-pig airways, and that this effect is mediated by stimulation of TRPV1 receptors on sensory nerves endings. This novel mechanism may contribute to the irritative action of H(2)S in the respiratory system.

Evidence for a restricted rather than generalized stimulatory response of skin-derived human mast cells to substance P

To resolve the controversy regarding substance P (SP) mediated stimulation of mast cells (MC), we demonstrate that SP triggers histamine release from purified human skin MC (sMC), but contrast to stimulation via FcepsilonRI, does not effect the production of TNF-alpha or IL-8. Conversely, both anti-IgE and SP are suppressive in terms of IL-6. By quantitative RT-PCR, the amount of templates at baseline (per 25 ng total RNA) is 2178 (IL-6), 2,665 (IL-8) and 94 (TNF-alpha), and remains unaltered by SP. Contrast to sMC, LAD2 MC respond to SP with stronger histamine release and robust TNF-alpha production in an only partially neurokinin-1R mediated manner, while histamine release of sMC is chiefly mediated by this receptor. We conclude that human sMC are responsive to SP in a selective manner by eliciting degranulation without the induction of cytokines and that SP-triggered cytokine production varies among MC subtypes, likely through differences in signaling mechanisms.

Increased relaxation of immature airways to β2-adrenoceptor agonists is related to attenuated expression of postjunctional smooth muscle muscarinic M2 receptors

Spontaneous or agonist-induced contraction of airway smooth muscle can be observed very early in fetal life, thus explaining the possible occurrence of bronchospasm in very low birth weight infants within the first days of life. In an attempt to better manage such bronchospasms, the aim of the present study was to investigate the age-specific modifications in airway smooth muscle relaxation to β2-agonists and muscarinic antagonists using a combination of functional and molecular techniques. In the rat, isometric relaxation to the β2-agonist salbutamol was examined in tracheae; we also examined muscarinic receptor expression (M2R and M3R mRNA levels) in airway smooth muscle by immunochemistry, Western blotting, and real-time PCR. Compared with adults, salbutamol-induced relaxation was twofold greater in immature rat isolated tracheae preconstricted by carbachol. This effect was associated with a lower expression of M2R in the smooth muscle of immature animals (sixfold and almost twofold as assessed by immunochemistry and Western blotting, respectively). Real-time PCR data indicate that changes in M2R expression according to age occurred at a posttranscriptional level. In adult airways, there was a significantly greater functional efficacy of M2R blockade by methoctramine compared with that shown in immature rats. Because of the limited availability of human neonate lung tissue, only the molecular part of the study was performed, and we observed a qualitatively similar effect, i.e., a lower M2R expression in the neonatal airway smooth muscle, although this was quantitatively smaller. We conclude that β2-agonist-induced relaxation is enhanced in immature compared with adult airways as a result of greater postjunctional M2R expression in adult airway smooth muscle. This finding may be of importance in the clinical management of bronchoconstriction in neonates.

Relationship between airway responsiveness to neurokinin A and methacholine in asthma

Non-adrenergic non-cholinergic (NANC) nerves release bronchoactive tachykinins such as substance P (SP) and neurokinin A (NKA) that can induce features of asthma. The airway response to NKA in humans closely resembles that of methacholine (M). Hence, we investigated the relationship between airway responsiveness to NKA and M in subjects with asthma. To this end, we analyzed baseline data of 27 subjects with mild persistent asthma (20F/7M) 19-46 y; FEV1 81-136% pred.; PC20FEV1 (M)<80 micromol/mL) participating in a proof-of-concept study. All subjects were non-smokers and asthma was controlled by on demand short-acting beta2-agonists only. Dose-response curves to M (0.15-80 micromol/mL) and NKA (3.4 (10(-3))-0.88 micromol/mL) were performed on two separate days, and airway response was measured by FEV1 until a > or = 20% fall from baseline (PC20FEV1). Twenty-two subjects reached a PC20FEV1 on both occasions. The PC20FEV1 values of both agonists correlated significantly (Spearman's r=-0.721; p=0.0002), and the relationship was given by 10log(PC20FEV1(NKA))= -1.36 + (0.60 x 10log(PC20FEV1(M)). We have demonstrated a significant relationship between airway responsiveness to NKA and methacholine in asthma. This suggests that both agonists may share common final pathways in causing bronchoconstriction in patients with mild persistent asthma. Based on our data and previous studies in asthma, it can be hypothesized that this direct NKA-induced bronchoconstrictor response may be mediated by predominant stimulation of the tachykinin NK-2 receptors on airway smooth muscle cells.

The role of substance P in inflammatory disease

The diffuse neuroendocrine system consists of specialised endocrine cells and peptidergic nerves and is present in all organs of the body. Substance P (SP) is secreted by nerves and inflammatory cells such as macrophages, eosinophils, lymphocytes, and dendritic cells and acts by binding to the neurokinin-1 receptor (NK-1R). SP has proinflammatory effects in immune and epithelial cells and participates in inflammatory diseases of the respiratory, gastrointestinal, and musculoskeletal systems. Many substances induce neuropeptide release from sensory nerves in the lung, including allergen, histamine, prostaglandins, and leukotrienes. Patients with asthma are hyperresponsive to SP and NK-1R expression is increased in their bronchi. Neurogenic inflammation also participates in virus-associated respiratory infection, non-productive cough, allergic rhinitis, and sarcoidosis. SP regulates smooth muscle contractility, epithelial ion transport, vascular permeability, and immune function in the gastrointestinal tract. Elevated levels of SP and upregulated NK-1R expression have been reported in the rectum and colon of patients with inflammatory bowel disease (IBD), and correlate with disease activity. Increased levels of SP are found in the synovial fluid and serum of patients with rheumatoid arthritis (RA) and NK-1R mRNA is upregulated in RA synoviocytes. Glucocorticoids may attenuate neurogenic inflammation by decreasing NK-1R expression in epithelial and inflammatory cells and increasing production of neutral endopeptidase (NEP), an enzyme that degrades SP. Preventing the proinflammatory effects of SP using tachykinin receptor antagonists may have therapeutic potential in inflammatory diseases such as asthma, sarcoidosis, chronic bronchitis, IBD, and RA. In this paper, we review the role that SP plays in inflammatory disease.

Catecholaminergic microcircuitry controlling the output of airway-related vagal preganglionic neurons

In this study, we have investigated the ultrastructure and function of the catecholaminergic circuitry modulating the output of airway-related vagal preganglionic neurons (AVPNs) in ferrets. Immunoelectron microscopy was employed to characterize the nature of catecholaminergic innervation of AVPN at the ultrastructural level. In addition, immunofluorescence was used to examine the expression of the alpha(2A)-adrenergic receptor (alpha(2A)-AR) on AVPNs, and norepinephrine release within the rostral nucleus ambiguous (rNA) was measured by using microdialysis. Physiological experiments were performed to determine the effects of stimulation of the noradrenergic locus coeruleus (LC) cell group on airway smooth muscle tone. The results showed that 1) catecholaminergic nerve endings terminate in the vicinity of identified AVPNs but very rarely form axosomatic or axodendritic synapses with the AVPNs that innervate the extrathoracic trachea; 2) AVPNs express the alpha(2A)-AR; 3) LC stimulation-induced norepinephrine release within the rNA region was associated with airway smooth muscle relaxation; and 4) blockade of alpha(2A)-AR on AVPNs diminished the inhibitory effects of LC stimulation on airway smooth muscle tone. It is concluded that a noradrenergic circuit originating within the LC is involved in the regulation of AVPN activity within the rNA, and stimulation of the LC dilates the airways by the release of norepinephrine and activation of alpha(2A)-AR expressed by AVPNs, mainly via volume transmission.