Innervation of tracheal epithelium and smooth muscle by neurons in airway ganglia

Illuminating Airway Nerve Structure and Function in Chronic Cough

Airway nerves regulate vital airway functions including bronchoconstriction, cough, and control of respiration. Dysregulation of airway nerves underlies the development and manifestations of airway diseases such as chronic cough, where sensitization of neural pathways leads to excessive cough triggering. Nerves are heterogeneous in both expression and function. Recent advances in confocal imaging and in targeted genetic manipulation of airway nerves have expanded our ability to visualize neural organization, study neuro-immune interactions, and selectively modulate nerve activation. As a result, we have an unprecedented ability to quantitatively assess neural remodeling and its role in the development of airway disease. This review highlights our existing understanding of neural heterogeneity and how advances in methodology have illuminated airway nerve morphology and function in health and disease.

Neural control of the lower airways: Role in cough and airway inflammatory disease

Airway function is under constant neurophysiological control, in order to maximize airflow and gas exchange and to protect the airways from aspiration, damage, and infection. There are multiple sensory nerve subtypes, whose disparate functions provide a wide array of sensory information into the CNS. Activation of these subtypes triggers specific reflexes, including cough and alterations in autonomic efferent control of airway smooth muscle, secretory cells, and vasculature. Importantly, every aspect of these reflex arcs can be impacted and altered by local inflammation caused by chronic lung disease such as asthma, bronchitis, and infections. Excessive and inappropriate activity in sensory and autonomic nerves within the airways is thought to contribute to the morbidity and symptoms associated with lung disease.

Physiologic and histopathologic effects of targeted lung denervation in an animal model

Parasympathetic efferent innervation of the lung is the primary source of lung acetylcholine. Inhaled long-acting anticholinergics improve lung function and symptoms in patients with chronic obstructive pulmonary disease. Targeted lung denervation (TLD), a bronchoscopic procedure intended to disrupt pulmonary parasympathetic inputs, is an experimental treatment for chronic obstructive pulmonary disease. The physiologic and histologic effects of TLD have not previously been assessed. Eleven sheep and two dogs underwent circumferential ablation of the main bronchi with simultaneous balloon surface cooling using a lung denervation system (Nuvaira, Inc., Minneapolis, MN). Changes in pulmonary air flow resistance were monitored before and following TLD. Four animals were assessed for the presence or abolishment of the sensory axon-mediated Hering-Breuer reflex before and following TLD. Six sheep were histologically evaluated 30 days post-TLD for the extent of lung denervation (axonal staining) and effect on peribronchial structures near the treatment site. No adverse clinical effects were seen in any treated animals. TLD produced a ~30% reduction in pulmonary resistance and abolished the sensory-mediated Hering-Breuer reflex. Axonal staining was consistently decreased 60% at 30 days after TLD. All treated airways exhibited 100% epithelial integrity. Damage to other peribronchial structures was minimal. Tissue 1 cm proximal and distal to the treatment was normal, and the esophagus and periesophageal vagus nerve branches were unaffected. TLD treatment effectively denervates the lung while protecting the bronchial epithelium and minimizing effects on peribronchial structures.

NEW & NOTEWORTHY The feasibility of targeted lung denervation, a new minimally invasive therapy for obstructive lung disease, has been demonstrated in humans with preliminary clinical studies demonstrating improvement in symptoms, pulmonary function, and exercise capacity in patients with chronic obstructive pulmonary disease. This preclinical animal study demonstrates the ability of targeted lung denervation to disrupt vagal inputs to the lung and details its physiologic and histopathologic effects.

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.

Airway Gland Structure and Function

Submucosal glands contribute to airway surface liquid (ASL), a film that protects all airway surfaces. Glandular mucus comprises electrolytes, water, the gel-forming mucin MUC5B, and hundreds of different proteins with diverse protective functions. Gland volume per unit area of mucosal surface correlates positively with impaction rate of inhaled particles. In human main bronchi, the volume of the glands is ∼ 50 times that of surface goblet cells, but the glands diminish in size and frequency distally. ASL and its trapped particles are removed from the airways by mucociliary transport. Airway glands have a tubuloacinar structure, with a single terminal duct, a nonciliated collecting duct, then branching secretory tubules lined with mucous cells and ending in serous acini. They allow for a massive increase in numbers of mucus-producing cells without replacing surface ciliated cells. Active secretion of Cl(-) and HCO3 (-) by serous cells produces most of the fluid of gland secretions. Glands are densely innervated by tonically active, mutually excitatory airway intrinsic neurons. Most gland mucus is secreted constitutively in vivo, with large, transient increases produced by emergency reflex drive from the vagus. Elevations of [cAMP]i and [Ca(2+)]i coordinate electrolyte and macromolecular secretion and probably occur together for baseline activity in vivo, with cholinergic elevation of [Ca(2+)]i being mainly responsive for transient increases in secretion. Altered submucosal gland function contributes to the pathology of all obstructive diseases, but is an early stage of pathogenesis only in cystic fibrosis.

Neural control of submucosal gland and apical membrane secretions in airways

The mechanisms that lay behind the low-level secretions from airway submucosal glands and the surface epithelium in the absence of external innervation have been investigated in small areas (1.0-1.5 cm(2)) of mucosa from sheep tracheas, freshly collected from a local abattoir. Glandular secretion was measured by an optical method while short circuit current was used as a measure of surface secretion. Activation of neurones in the intrinsic nerve net by veratrine alkaloids caused an immediate increase in both glandular secretion and short circuit current, both effects being blocked by the addition of tetrodotoxin. However, agents known to be acting directly on the glands, such as muscarinic agonists (e.g., carbachol) or adenylate cyclase activators (e.g., forskolin) were not influenced by tetrodotoxin. The toxin alone had no discernable effect on the low-level basal secretion shown by unstimulated glands. Calu-3 cell monolayers, generally agreed to be a surrogate for the secretory cells of submucosal glands, showed no sensitivity to veratrine alkaloids, strengthening the view that the veratrine-like drugs acted exclusively on the intrinsic nerve net. The data are discussed in relation way in which transplanted lungs can maintain mucociliary clearance and hence a sterile environment in the absence of external innervation, as in transplanted lungs.

Diacetyl increases sensory innervation and substance P production in rat trachea

Inhalation of diacetyl, a butter flavoring, causes airway responses potentially mediated by sensory nerves. This study examines diacetyl-induced changes in sensory nerves of tracheal epithelium. Rats (n = 6/group) inhaled 0-, 25-, 249-, or 346-ppm diacetyl for 6 hr. Tracheas and vagal ganglia were removed 1-day postexposure and labeled for substance P (SP) or protein gene product 9.5 (PGP9.5). Vagal ganglia neurons projecting to airway epithelium were identified by axonal transport of fluorescent microspheres intratracheally instilled 14 days before diacetyl inhalation. End points were SP and PGP9.5 nerve fiber density (NFD) in tracheal epithelium and SP-positive neurons projecting to the trachea. PGP9.5-immunoreactive NFD decreased in foci with denuded epithelium, suggesting loss of airway sensory innervation. However, in the intact epithelium adjacent to denuded foci, SP-immunoreactive NFD increased from 0.01 ± 0.002 in controls to 0.05 ± 0.01 after exposure to 346-ppm diacetyl. In vagal ganglia, SP-positive airway neurons increased from 3.3 ± 3.0% in controls to 25.5 ± 6.6% after inhaling 346-ppm diacetyl. Thus, diacetyl inhalation increases SP levels in sensory nerves of airway epithelium. Because SP release in airways promotes inflammation and activation of sensory nerves mediates reflexes, neural changes may contribute to flavorings-related lung disease pathogenesis.

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.

Early postnatal exposure of mice to side-steam tobacco smoke increases neuropeptide Y in lung

Our recent study showed that prenatal and early postnatal exposure of mice to side-steam tobacco smoke (SS), a surrogate to environmental tobacco smoke (ETS), leads to increased airway responsiveness and sensory innervation later in life. However, the underlying mechanism initiated in early life that affects airway responses later in life remains undefined. The concomitant increase in nerve growth factor (NGF) after exposures suggests that NGF may be involved the regulation of airway innervation. Since NGF regulates sympathetic nerve responses, as well as sensory nerves, we extended previous studies by examining neuropeptide Y (NPY), a neuropeptide associated with sympathetic nerves. Different age groups of mice, postnatal day (PD) 2 and PD21, were exposed to either SS or filtered air (FA) for 10 consecutive days. The level of NPY protein in lung and the density of NPY nerve fibers in tracheal smooth muscle were significantly increased in the PD2-11SS exposure group compared with PD2-11FA exposure. At the same time, the level of NGF in lung tissue was significantly elevated in the PD2-11SS exposure groups. However, neither NPY (protein or nerves) nor NGF levels were significantly altered in PD21-30SS exposure group compared with the PD21-30FA exposure group. Furthermore, pretreatment with NGF antibody or K252a, which inhibits a key enzyme (tyrosine kinase) in the transduction pathway for NGF receptor binding, significantly diminished SS-enhanced NPY tracheal smooth muscle innervation and the increase in methacholine-induced airway resistance. These findings show that SS exposure in early life increases NPY tracheal innervation and alters pulmonary function and that these changes are mediated through the NGF.

Prenatal and early, but not late, postnatal exposure of mice to sidestream tobacco smoke increases airway hyperresponsiveness later in life

BACKGROUND

Cigarette smoke exposure in utero and during early postnatal development increases the incidence of asthma and airway hyperresponsiveness (AHR) later in life, suggesting that a possible critical period of developmental sensitivity exists in the prenatal and early postnatal periods.

OBJECTIVE

We investigated mechanisms of susceptibility during critical developmental periods to sidestream smoke (SS) exposure and evaluated the possible effects of SS on neural responses.

METHODS

We exposed three different age groups of mice to either SS or filtered air (FA) for 10 consecutive days beginning on gestation day (GD) 7 by maternal exposure or beginning on postnatal day (PND) 2 or PND21 by direct inhalation. Lung function, airway substance P (SP) innervation, and nerve growth factor (NGF) levels in broncho alveolar lavage fluid were measured after a single SS exposure on PND59.

RESULTS

Methacholine (MCh) dose response for lung resistance (R(L)) was significantly elevated, and dynamic pulmonary compliance (C(dyn)) was significantly decreased, in the GD7 and PND2 SS exposure groups compared with the FA groups after SS exposure on PND59. At the same time points, the percent area of SP nerve fibers in tracheal smooth muscle and the levels of NGF were significantly elevated. MCh dose-response curves for R(L) and C(dyn), SP nerve fiber density, and the level of NGF were not significantly changed in the PND21 exposure group after SS exposure on PND59.

CONCLUSIONS

These results suggest that a critical period of susceptibility to SS exposure exists in the prenatal and early postnatal period of development in mice that results in increased SP innervation, increased NGF levels in the airway, and enhanced MCh AHR later in life.

Airway receptors and their reflex function--invited article

Sensory information in the lung is generated by airway receptors located throughout the respiratory tract. This information is mainly carried by the vagus nerves and yields multiple reflex responses in disease states (cough, bronchoconstriction and mucus secretion). Airway receptors are also essential for breathing control and lung defense. A single sensory unit contains homogeneous or heterogeneous types of receptors, providing varied and mixed behavior. Thus, the sensory units are not only transducers, but also processors that integrate information in different modes.

Interleukin (IL)-1 regulates ozone-enhanced tracheal smooth muscle responsiveness by increasing substance P (SP) production in intrinsic airway neurons of ferret

Exposure to ozone induces airway hyperresponsiveness (AHR) mediated partly by substance P (SP) released from nerve terminals of intrinsic airway neurons. Our recent studies showed that interleukin (IL)-1, an important multifunctional proinflammatory cytokine, increases synthesis and release of SP from intrinsic airway neurons. The purpose of this study is to investigate the possible involvement of endogenous IL-1 in modulating neural responses associated with ozone-enhanced airway responsiveness. Ferrets were exposed to 2ppm ozone or filtered air for 3h. IL-1 in the bronchoalveolar lavage (BAL) fluid was significantly increased in ozone-exposed animals and responses of tracheal smooth muscle to methacholine (MCh) and electrical field stimulation (EFS) were elevated significantly. Both the SP nerve fiber density in tracheal smooth muscle and the number of SP-containing neurons in airway ganglia were significantly increased following ozone exposure. Pretreatment with IL-1 receptor antagonist (IL-1 Ra) significantly diminished ozone-enhanced airway responses to EFS as well as ozone-increased SP in the airway. To selectively investigate intrinsic airway neurons, segments of ferret trachea were maintained in culture conditions for 24h to eliminate extrinsic contributions from sensory nerves. The segments were then exposed to 2ppm ozone in vitro for 3h. The changes of ozone-induced airway responses to MCh and EFS, and the SP levels in airway neurons paralleled those observed with in vivo ozone exposure. The ozone-enhanced airway responses and neuronal SP levels were inhibited by pretreatment with IL-1 Ra. These findings show that IL-1 is released during ozone exposure enhances airway responsiveness by modulating SP expression in airway neurons.

Synergistic airway gland mucus secretion in response to vasoactive intestinal peptide and carbachol is lost in cystic fibrosis

Cystic fibrosis (CF) is caused by dysfunction of the CF transmembrane conductance regulator (CFTR), an anion channel whose dysfunction leads to chronic bacterial and fungal airway infections via a pathophysiological cascade that is incompletely understood. Airway glands, which produce most airway mucus, do so in response to both acetylcholine (ACh) and vasoactive intestinal peptide (VIP). CF glands fail to secrete mucus in response to VIP, but do so in response to ACh. Because vagal cholinergic pathways still elicit strong gland mucus secretion in CF subjects, it is unclear whether VIP-stimulated, CFTR-dependent gland secretion participates in innate defense. It was recently hypothesized that airway intrinsic neurons, which express abundant VIP and ACh, are normally active and stimulate low-level gland mucus secretion that is a component of innate mucosal defenses. Here we show that low levels of VIP and ACh produced significant mucus secretion in human glands via strong synergistic interactions; synergy was lost in glands of CF patients. VIP/ACh synergy also existed in pig glands, where it was CFTR dependent, mediated by both Cl(-) and HCO(3) (-), and clotrimazole sensitive. Loss of "housekeeping" gland mucus secretion in CF, in combination with demonstrated defects in surface epithelia, may play a role in the vulnerability of CF airways to bacterial infections.

Parasympathetic control of airway submucosal glands: central reflexes and the airway intrinsic nervous system

Airway submucosal glands produce the mucus that lines the upper airways to protect them against insults. This review summarizes evidence for two forms of gland secretion, and hypothesizes that each is mediated by different but partially overlapping neural pathways. Airway innate defense comprises low level gland secretion, mucociliary clearance and surveillance by airway-resident phagocytes to keep the airways sterile in spite of nearly continuous inhalation of low levels of pathogens. Gland secretion serving innate defense is hypothesized to be under the control of intrinsic (peripheral) airway neurons and local reflexes, and these may depend disproportionately on non-cholinergic mechanisms, with most secretion being produced by VIP and tachykinins. In the genetic disease cystic fibrosis, airway glands no longer secrete in response to VIP alone and fail to show the synergy between VIP, tachykinins and ACh that is observed in normal glands. The consequent crippling of the submucosal gland contribution to innate defense may be one reason that cystic fibrosis airways are infected by mucus-resident bacteria and fungi that are routinely cleared from normal airways. By contrast, the acute (emergency) airway defense reflex is centrally mediated by vagal pathways, is primarily cholinergic, and stimulates copious volumes of gland mucus in response to acute, intense challenges to the airways, such as those produced by very vigorous exercise or aspiration of foreign material. In cystic fibrosis, the acute airway defense reflex can still stimulate the glands to secrete large amounts of mucus, although its properties are altered. Importantly, treatments that recruit components of the acute reflex, such as inhalation of hypertonic saline, are beneficial in treating cystic fibrosis airway disease. The situation for recipients of lung transplants is the reverse; transplanted airways retain the airway intrinsic nervous system but lose centrally mediated reflexes. The consequences of this for gland secretion and airway defense are poorly understood, but it is possible that interventions to modify submucosal gland secretion in transplanted lungs might have therapeutic consequences.

Nerve growth factor-enhanced airway responsiveness involves substance P in ferret intrinsic airway neurons

Nerve growth factor (NGF), a member of the neurotrophin family, enhances synthesis of neuropeptides in sensory and sympathetic neurons. The aim of this study was to examine the effect of NGF on airway responsiveness and determine whether these effects are mediated through synthesis and release of substance P (SP) from the intrinsic airway neurons. Ferrets were instilled intratracheally with NGF or saline. Tracheal smooth muscle contractility to methacholine and electrical field stimulation (EFS) was assessed in vitro. Contractions of isolated tracheal smooth muscle to EFS at 10 and 30 Hz were significantly increased in the NGF treatment group (10 Hz: 33.57 ± 2.44%; 30 Hz: 40.12 ± 2.78%) compared with the control group (10 Hz: 27.24 ± 2.14%; 30 Hz: 33.33 ± 2.31%). However, constrictive response to cholinergic agonist was not significantly altered between the NGF treatment group and the control group. The NGF-induced modulation of airway smooth muscle to EFS was maintained in tracheal segments cultured for 24 h, a procedure that causes a significant anatomic and functional loss of SP-containing sensory fibers while maintaining viability of intrinsic airway neurons. The number of SP-containing neurons in longitudinal trunk and superficial muscular plexus and SP nerve fiber density in tracheal smooth muscle all increased significantly in cultured trachea treated with NGF. Pretreatment with CP-99994, an antagonist of neurokinin 1 receptor, attenuated the NGF-induced increased contraction to EFS in cultured segments but had no effect in saline controls. These results show that the NGF-enhanced airway smooth muscle contractile responses to EFS are mediated by the actions of SP released from intrinsic airway neurons.

The release of nerve growth factor from the nasal mucosa following toluene diisocyanate

Toluene diisocyanate (TDI) produces rhinitis, nasal irritation, and increased synthesis and release of substance P (SP) from airway sensory nerves. Nerve growth factor (NGF) secretion in the nasal cavity is believed to mediate the irritant-induced upregulation of SP, but the cellular source of NGF in the nasal mucosa remains unclear. Studies to localize a source of NGF within the nasal mucosa are complicated by inflammatory-cell influx into the nasal mucosa following TDI, which obscures immunocytochemical identification of endogenous NGF sources. The purpose of this study was to determine the cellular source of NGF within the nasal mucosa following irritant exposure using a combined in vivo and ex vivo approach to reduce or eliminate contribution from inflammatory cells. Both nasal cavities of adult, male Sprague-Dawley rats were instilled with 5 microl of 10% TDI or control vehicle. After 15 min, nasal lavages were performed and the nasal mucosa was removed and placed into culture for 3 or 24 h. NGF was measured in the lavage supernatant and the culture media. Fifteen minutes after TDI exposure, NGF was significantly increased in the nasal lavage fluid. NGF levels in the culture medium of nasal mucosa from rats exposed to TDI ex vivo were significantly increased compared to controls following a 3-h culture. NGF levels in media after 24 h in culture was higher than at the 3-h point, but there was no difference between control and TDI groups. Since the nasal mucosa was removed prior to inflammatory cell influx, these findings suggest that cells in the nasal mucosa release NGF following exposure to TDI.

Innervation of the rat trachea by bilateral cholinergic projections from the nucleus ambiguus and direct motor fibers from the cervical spinal cord: a retrograde and anterograde tracer study

A tract-tracer method was employed to examine the innervation of the rat trachea. Cholera toxin beta subunit (CTB) was injected into the following locations in separate groups of rats: (1) ventral trachea, (2) lateral trachea, (3) ventral trachea after the excision of the nodose ganglion, and (4) ventral trachea after the transection of C1-C2 spinal nerves. CTB injection in the ventral trachea showed bilateral labeling of neurons in the nucleus ambiguus (NA), medial subnucleus of the nucleus of the solitary nucleus, dorsal motor nucleus of the vagus (DMV), and lamina IX of C1-C6. CTB injection in the lateral trachea showed significant ipsilateral predominance of neuronal labeling in the NA and lamina IX of C1-C2 segments. CTB injection in rats after the excision of the nodose ganglion revealed no labeling in the ipsilateral DMV and NA and a significant reduction of neuronal labeling in C1. CTB injection in rats after the transection of C1-C2 spinal nerves showed a significant decrease in the number of labeled neurons in ipsilateral NA, C1, and C2 and no labeling of fibers in C1-C2. The combination of retrograde fluorogold labeling and choline acetyltransferase (ChAT) immunostaining revealed that all fluorogold-labeled neurons in the NA and lamina IX of C1-C2 colocalized with ChAT. The injection of biotinylated dextran amine in NA produced labeling in axonal terminals on postganglionic neurons, but not in other regions of the trachea. Our findings indicate that the rat trachea is innervated bilaterally by cholinergic motor neurons in NA and C1-C2, while those traveling through the spinal nerves project directly to the trachea.

Nerve plexuses in the trachea and extrapulmonary bronchi of the rat

Intrinsic nerve plexuses of the rat trachea and extrapulmonary bronchi were examined by immunohistochemistry. Three nerve plexuses--peritracheal and peribronchial, intramuscular, and submucosal--were found in the wall of the trachea and bronchi. Nerve cell bodies were located in the peritracheal and peribronchial nerve plexuses. They occurred singly or formed ganglia in the plexus, and regional differences in cell numbers were found in the cervical and thoracic portions of the trachea and in the extrapulmonary bronchia. In total, 83.5 +/- 28.3 ganglia (mean +/- SD, 57-131, n=5) and 749.8 +/- 221.1 nerve cell bodies (540-1,080, n=5) were found in the nerve plexus. The mean densities of ganglia were 0.31, 0.97 and 1.15/mm2, and the mean densities of the nerve cell bodies were 1.82, 9.26 and 11.54/mm2 in the cervical region, thoracic region of trachea, and extrapulmonary bronchi, respectively. Almost all nerve cell bodies in ganglia were positive for choline acetyltransferase and neuropeptide Y (NPY), and a few cells were positive for vasoactive intestinal peptide (VIP). In addition, in cholinergic nerves, a few nerve fibers in the smooth muscles were positive for substance P (SP), calcitonin gene-related peptide (CGRP), and VIP, and a moderate number of fibers were positive for NPY. Tyrosine hydroxylase-immunoreactive nerve fibers were observed around blood vessels and within nerve bundles in the tunica adventitia. In the epithelium, nerve fibers were positive for SP and CGRP. Our results indicate that postganglionic neurons form three layers of cholinergic plexuses in the rat trachea and extrapulmonary bronchi, and that all of these possess intrinsic and extrinsic peptidergic innervation.

Substance P released from intrinsic airway neurons contributes to ozone-enhanced airway hyperresponsiveness in ferret trachea

Exposure to ozone (O3) induces airway hyperresponsiveness mediated partly through the release of substance P (SP) from nerve terminals in the airway wall. Although substantial evidence suggests that SP is released by sensory nerves, SP is also present in neurons of airway ganglia. The purpose of this study was to investigate the role of intrinsic airway neurons in O3-enhanced airway responsiveness in ferret trachea. To remove the effects of sensory innervation, segments of ferret trachea were maintained in culture conditions for 24 h before in vitro exposure to 2 parts/million of O3 or air for 1 h. Sensory nerve depletion was confirmed by showing that capsaicin did not affect tracheal smooth muscle responsiveness to cholinergic agonist or contractility responses to electrical field stimulation (EFS). Contractions of isolated tracheal smooth muscle to EFS were significantly increased after in vitro O3 exposure, but the constrictor response to cholinergic agonist was not altered. Pretreatment with CP-99994, an antagonist of the neurokinin 1 receptor, attenuated the increased contraction to EFS after O3 exposure but had no effect in the air exposure group. The number of SP-positive neurons in longitudinal trunk ganglia, the extent of SP innervation to superficial muscular plexus nerve cell bodies, and SP nerve fiber density in tracheal smooth muscle all increased significantly after O3 exposure. The results show that release of SP from intrinsic airway neurons contributes to O3-enhanced tracheal smooth muscle responsiveness by facilitating acetylcholine release from cholinergic nerve terminals.

Structure of slowly adapting pulmonary stretch receptors in the lung periphery

Pulmonary sensory receptors are the initiating sites for lung reflexes; however, little is known about their structure, especially the relationship between the structure and function of these receptors. Using a novel approach (combining electrophysiological and morphological techniques), we examined the structures of the typical slowly adapting pulmonary stretch receptors (SARs) located in the lung periphery. We recorded SAR activities in the cervical vagus nerve, identified the receptive field, dissected the SARs in blocks, fixed and processed these blocks for immunohistochemical staining using anti-Na+/K+-ATPase, and examined the blocks under a confocal microscope. These SAR structures have multiple endings that have terminal knobs. Some structures that are located in the airway walls have terminal knobs buried in smooth muscle. Others are in the most peripheral part of the lung, and their terminal knobs have no obvious relation to smooth muscle, suggesting that muscle contraction may not be a direct factor for SAR activation.

Interleukin-1beta-induced airway hyperresponsiveness enhances substance P in intrinsic neurons of ferret airway

Interleukin (IL)-1beta causes airway inflammation, enhances airway smooth muscle responsiveness, and alters neurotransmitter expression in sensory, sympathetic, and myenteric neurons. This study examines the role of intrinsic airway neurons in airway hyperresponsiveness (AHR) induced by IL-1beta. Ferrets were instilled intratracheally with IL-1beta (0.3 microg/0.3 ml) or saline (0.3 ml) once daily for 5 days. Tracheal smooth muscle contractility in vitro and substance P (SP) expression in tracheal neurons were assessed. Tracheal smooth muscle reactivity to acetylcholine (ACh) and methacholine (MCh) and smooth muscle contractions to electric field stimulation (EFS) both increased after IL-1beta. The IL-1beta-induced AHR was maintained in tracheal segments cultured for 24 h, a procedure that depletes SP from sensory nerves while maintaining viability of intrinsic airway neurons. Pretreatment with CP-99994, an antagonist of neurokinin 1 receptor, attenuated the IL-1beta-induced hyperreactivity to ACh and MCh and to EFS in cultured tracheal segments. SP-containing neurons in longitudinal trunk, SP innervation of superficial muscular plexus neurons, and SP nerve fiber density in tracheal smooth muscle all increased after treatment with IL-1beta. These results show that IL-1beta-enhanced cholinergic airway smooth muscle contractile responses are mediated by the actions of SP released from intrinsic airway neurons.

On lung nerves and neurogenic injury

Information accumulated in recent years has begun to unveil a previously unsuspected complexity in the innervation of the lungs. We know now that the conducting airways receive a highly redundant supply of vagal motor and sensory fibers; that many of these fibers cross over from the contralateral side of the brain to reach distant portions of the lung, thereby assuring the symmetry and simultaneity of the bronchomotor responses; and that, perhaps in recognition of the different functions and properties of proximal and distal airways, vagal motor fibers have a distinctive segmental distribution. Both sensory and motor neurons serve as the input and output elements of a complex brain stem neuronal network, which integrates the regulation of airway smooth muscle tone into the control of ventilation. This network has a local counterpart in the airway walls, where a heterogeneous population of intrinsic neurons may act not only as a relay for cholinergic stimuli, but also as a local mechanism of inflammatory modulation. The interruption of the nerve supply to the lungs (for instance after lung transplantation) abolishes the integration of bronchomotor and ventilatory activities, and, by increasing airway deformation, may initiate fibroproliferative responses in the airway walls. In addition, the destruction of vagal motor and sensory fibers leaves behind a surviving population of denervated intrinsic neurons, which may act as a disregulated mechanism of inflammatory amplification.

Role of intrinsic airway neurons in ozone-induced airway hyperresponsiveness in ferret trachea

Exposure to ozone (O(3)) enhances airway responsiveness, which is mediated partly by the release of substance P (SP) from airway neurons. In this study, the role of intrinsic airway neurons in O(3)-induced airway responses was examined. Ferrets were exposed to 2 ppm O(3) or air for 1 h. Reactivity of isolated tracheal smooth muscle to cholinergic agonists was significantly increased after O(3) exposure, as were contractions to electrical field stimulation at 10 Hz. Pretreatment with CP-99994, a neurokinin type 1 receptor antagonist, partially abolished the O(3)-induced reactivity to cholinergic agonists and electrical field stimulation. The O(3)-enhanced airway responses were present in tracheal segments cultured for 24 h, a procedure shown to deplete sensory nerves while maintaining viability of intrinsic airway neurons, and all the enhanced smooth muscle responses were also diminished by CP-99994. Immunocytochemistry showed that the percentage of SP-containing neurons in longitudinal trunk and the percentage of neurons innervated by SP-positive nerve fibers in superficial muscular plexus were significantly increased at 1 h after exposure to O(3). These results suggest that enhanced SP levels in airway ganglia contribute to O(3)-induced airway hyperresponsiveness.

Projections and pathways of VIP- and nNOS-containing airway neurons in ferret trachea

Airway ganglia in ferret trachea are organized into two identifiable plexuses, the longitudinal trunk (LT) ganglia and the superficial muscular plexus (SMP) ganglia. LT ganglia contain cholinergic neurons and neurons in SMP ganglia synthesize vasoactive intestinal peptide (VIP) and nitric oxide (NO). Both plexuses of ganglia project to airway smooth muscle, but communications between LT and SMP ganglia have not been extensively studied. Ganglia of the LT were injected with neural tracers, either rhodamine- or biotin-labeled dextran amines, and colocalized with VIP or neuronal NO synthase (nNOS) by immunocytochemistry. Anterograde tracers were found in nerve fibers that formed basket-like complexes associated with neurons of LT and SMP ganglia and were observed in the nerve fibers of smooth muscle and tracheal plexus. Some VIP or nNOS positive neurons in the SMP ganglia contained retrogradely transported tracer. The findings suggest that neurons in the SMP and LT ganglia communicate in a reciprocal fashion and that VIP and nNOS neurons both receive and send neural inputs to neurons of LT, presumably cholinergic, ganglia. These connections probably contribute to the integration of neural control of airway and vascular smooth muscle tone and glandular secretion in the airways.

Substance P and neurokinin-1 receptor expression by intrinsic airway neurons in the rat

Tachykinins and their receptors are involved in the amplification of inflammation in the airways. We analyzed the expression of preprotachykinin-A (PPT-A) and neurokinin-1 (NK-1) receptor genes by intrinsic airway neurons in the rat. We also tested the hypothesis that PPT-A-encoded peptides released by these neurons fulfill the requisite role of substance P in immune complex injury of the lungs. We found that ganglion neurons in intact and denervated airways or in primary culture coexpress PPT-A and NK-1 receptor mRNAs and their protein products. Denervated ganglia from tracheal xenografts (nu/nu mice) or syngeneic lung grafts had increased PPT-A mRNA contents, suggesting preganglionic regulation. Formation of immune complexes in the airways induced comparable inflammatory injuries in syngeneic lung grafts, which lack peptidergic sensory fibers, and control lungs. The injury was attenuated in both cases by pretreatment with the NK-1 receptor antagonist LY-306740. We conclude that tachykinins released by ganglia act as a paracrine or autocrine signal in the airways and may contribute to NK-1 receptor-mediated amplification of immune injury in the lungs.