Endogenous neurokinins facilitate synaptic transmission in guinea pig airway parasympathetic ganglia

Molecular identity, anatomy, gene expression and function of neural crest vs. placode-derived nociceptors in the lower airways

The lower airways (larynx to alveoli) are protected by a complex array of neural networks that regulate respiration and airway function. Harmful stimuli trigger defensive responses such as apnea, cough and bronchospasm by activating a subpopulation of sensory afferent nerves (termed nociceptors) which are found throughout the airways. Airway nociceptive fibers are projected from the nodose vagal ganglia, the jugular vagal ganglia and the dorsal root ganglia, which are derived from distinct embryological sources: the former from the epibranchial placodes, the latter two from the neural crest. Embryological source determines nociceptive gene expression of receptors and neurotransmitters and recent evidence suggests that placode- and neural crest-derived nociceptors have distinct stimuli sensitivity, innervation patterns and functions. Improved understanding of the function of each subset in specific reflexes has substantial implications for therapeutic targeting of the neuronal components of airway disease such as asthma, viral infections and chronic obstructive pulmonary disease.

Mechanisms of organophosphorus pesticide toxicity in the context of airway hyperreactivity and asthma

Numerous epidemiologic studies have identified an association between occupational exposures to organophosphorus pesticides (OPs) and asthma or asthmatic symptoms in adults. Emerging epidemiologic data suggest that environmentally relevant levels of OPs may also be linked to respiratory dysfunction in the general population and that in utero and/or early life exposures to environmental OPs may increase risk for childhood asthma. In support of a causal link between OPs and asthma, experimental evidence demonstrates that occupationally and environmentally relevant OP exposures induce bronchospasm and airway hyperreactivity in preclinical models. Mechanistic studies have identified blockade of autoinhibitory M2 muscarinic receptors on parasympathetic nerves that innervate airway smooth muscle as one mechanism by which OPs induce airway hyperreactivity, but significant questions remain regarding the mechanism(s) by which OPs cause neuronal M2 receptor dysfunction and, more generally, how OPs cause persistent asthma, especially after developmental exposures. The goals of this review are to 1) summarize current understanding of OPs in asthma; 2) discuss mechanisms of OP neurotoxicity and immunotoxicity that warrant consideration in the context of OP-induced airway hyperreactivity and asthma, specifically, inflammatory responses, oxidative stress, neural plasticity, and neurogenic inflammation; and 3) identify critical data gaps that need to be addressed in order to better protect adults and children against the harmful respiratory effects of low-level OP exposures.

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.

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.

The effects of suplatast tosilate on acutely dissociated sensory and paratracheal ganglia neurons

In this study, we investigated the effects of suplatast on acutely dissociated single neurons of sensory and paratracheal ganglia using a patch-clamp technique. Suplatast had little effect on various responses caused by capsaicin, acid, bradykinin, serotonin and adenosine 5'-triphosphate in rat sensory neurons. Suplatast, even at 10-3 M, also did not induce any current at various membrane potentials in rat and guinea pig paratracheal ganglia neurons. Further, acetylcholine- and bradykinin-induced depolarizations were not affected by suplatast. On the other hand, in rat paratracheal ganglia neurons, 10-5 M nicotine-induced current were inhibited by suplatast in a concentration-dependent manner with a 50% inhibitory concentration of 9.86x10-5 M. The effect was noncompetitive and voltage-dependent. Furthermore, the effect was use-independent and not affected by the pretreatment time of suplatast. The results suggested that suplatast may inhibit neurotransmission at the paratracheal ganglia via the inhibition of nicotinic current. Thus, suplatast may attenuate cough production through the improvement of pathological conditions of the lower airway via suppressed acetylcholine release from the postganglionic nerve terminal.

The effect of hyperpolarization-activated cyclic nucleotide-gated ion channel inhibitors on the vagal control of guinea pig airway smooth muscle tone

BACKGROUND AND PURPOSE

Subtypes of the hyperpolarization-activated cyclic nucleotide-gated (HCN) family of cation channels are widely expressed on nerves and smooth muscle cells in many organ systems, where they serve to regulate membrane excitability. Here we have assessed whether HCN channel inhibitors alter the function of airway smooth muscle or the neurons that regulate airway smooth muscle tone.

EXPERIMENTAL APPROACH

The effects of the HCN channel inhibitors ZD7288, zatebradine and Cs(+) were assessed on agonist and nerve stimulation-evoked changes in guinea pig airway smooth muscle tone using tracheal strips in vitro, an innervated tracheal tube preparation ex vivo or in anaesthetized mechanically ventilated guinea pigs in vivo. HCN channel expression in airway nerves was assessed using immunohistochemistry, PCR and in situ hybridization.

KEY RESULTS

HCN channel inhibition did not alter airway smooth muscle reactivity in vitro to exogenously administered smooth muscle spasmogens, but significantly potentiated smooth muscle contraction evoked by the sensory nerve stimulant capsaicin and electrical field stimulation of parasympathetic cholinergic postganglionic neurons. Sensory nerve hyperresponsiveness was also evident in in vivo following HCN channel blockade. Cs(+) , but not ZD7288, potentiated preganglionic nerve-dependent airway contractions and over time induced autorhythmic preganglionic nerve activity, which was not mimicked by inhibitors of potassium channels. HCN channel expression was most evident in vagal sensory ganglia and airway nerve fibres.

CONCLUSIONS AND IMPLICATIONS

HCN channel inhibitors had a previously unrecognized effect on the neural regulation of airway smooth muscle tone, which may have implications for some patients receiving HCN channel inhibitors for therapeutic purposes.

Cholinergic EPSCs and their potentiation by bradykinin in single paratracheal ganglion neurons attached with presynaptic boutons

We have found that bradykinin (BK) potentiates the nicotine-induced currents in airway paratracheal/parabronchial ganglia (PTG) neurons. In this study, we investigated if BK affects the cholinergic synaptic transmission in rat PTG neurons attached with synaptic buttons. Excitatory postsynaptic currents (EPSCs) were recorded in acutely dissociated PTG neurons attached with presynaptic boutons. EPSC frequency was increased in the high-K(+) external solution without affecting their amplitude. Activation and deactivation kinetics also did not change in the high-K(+) solution. Cd(2+) inhibited the EPSC frequency at 10(-7) M and also amplitude at higher concentrations without changing the kinetics. Mecamylamine inhibited both the amplitude and frequency of EPSCs and reduced the activation and deactivation kinetics. 10(-8) M BK potentiated the EPSC amplitude to 1.37 ± 0.19 times of preapplication control. In addition, its frequency was increased to 2.04 ± 0.41 times. BK did not affect the activation and deactivation kinetics. The effects of BK were mimicked by [Hyp(3)]-BK, a B2 kinin receptor agonist, whereas HOE 140, a B2 kinin receptor antagonist, abolished the effects of BK. In conclusion, BK potentiates the cholinergic synaptic transmission via B2 kinin receptors in the PTG. Since predominant control of airway function is thought to be exerted by cholinergic nerves arising from the PTG, the present findings might underlie at least partly the inflammatory pathological conditions of the lower airway.

Autonomic neural control of intrathoracic airways

Autonomic neural control of the intrathoracic airways aids in optimizing air flow and gas exchange. In addition, and perhaps more importantly, the autonomic nervous system contributes to host defense of the respiratory tract. These functions are accomplished by tightly regulating airway caliber, blood flow, and secretions. Although both the sympathetic and parasympathetic branches of the autonomic nervous system innervate the airways, it is the later that dominates, especially with respect to control of airway smooth muscle and secretions. Parasympathetic tone in the airways is regulated by reflex activity often initiated by activation of airway stretch receptors and polymodal nociceptors. This review discusses the preganglionic, ganglionic, and postganglionic mechanisms of airway autonomic innervation. Additionally, it provides a brief overview of how dysregulation of the airway autonomic nervous system may contribute to respiratory diseases.

Electrophysiology of airway nerves

Several different electrophysiological approaches have been used to study the pharmacology of the afferent, central, and efferent nervous systems in airways. This unit describes electrophysiological methods used to study nerves in these pathways and includes: (1) extracellular recording of afferent nerve activity in vivo and from the isolated airway in vitro, (2) intracellular and patch clamp recording of identified airway sensory neurons, (3) patch clamp recording of secondary afferent central nervous system neurons, (4) in vitro and in vivo intracellular recording of intact parasympathetic ganglionic neurons, and (5) patch recordings of dissociated parasympathetic ganglionic neurons.

Three days after a single exposure to ozone, the mechanism of airway hyperreactivity is dependent on substance P and nerve growth factor

Ozone causes persistent airway hyperreactivity in humans and animals. One day after ozone exposure, airway hyperreactivity is mediated by release of eosinophil major basic protein that inhibits neuronal M(2) muscarinic receptors, resulting in increased acetylcholine release and increased smooth muscle contraction in guinea pigs. Three days after ozone, IL-1β, not eosinophils, mediates ozone-induced airway hyperreactivity, but the mechanism at this time point is largely unknown. IL-1β increases NGF and the tachykinin substance P, both of which are involved in neural plasticity. These experiments were designed to test whether there is a role for NGF and tachykinins in sustained airway hyperreactivity following a single ozone exposure. Guinea pigs were exposed to filtered air or ozone (2 parts per million, 4 h). In anesthetized and vagotomized animals, ozone potentiated vagally mediated airway hyperreactivity 24 h later, an effect that was sustained over 3 days. Pretreatment with antibody to NGF completely prevented ozone-induced airway hyperreactivity 3 days, but not 1 day, after ozone and significantly reduced the number of substance P-positive airway nerve bundles. Three days after ozone, NK(1) and NK(2) receptor antagonists also blocked this sustained hyperreactivity. Although the effect of inhibiting NK(2) receptors was independent of ozone, the NK(1) receptor antagonist selectively blocked vagal hyperreactivity 3 days after ozone. These data confirm mechanisms of ozone-induced airway hyperreactivity change over time and demonstrate 3 days after ozone that there is an NGF-mediated role for substance P, or another NK(1) receptor agonist, that enhances acetylcholine release and was not present 1 day after ozone.

Innervation of tracheal parasympathetic ganglia by esophageal cholinergic neurons: evidence from anatomic and functional studies in guinea pigs

In the present study, we describe a subset of nerve fibers, characterized by their immunoreactivity for the calcium-binding protein calretinin, that are densely and selectively associated with cholinergic postganglionic neurons in the guinea pig tracheal ganglia. Retrograde neuronal tracing with cholera toxin B, combined with immunohistochemical analyses, showed that these nerve fibers do not originate from sensory neurons in the nodose, jugular, or dorsal root ganglia or from motor neurons in the nucleus ambiguus, dorsal motor nucleus of the vagus nerve, spinal cord, stellate ganglia, or superior cervical ganglia. Calretinin-immunoreactive nerve fibers disappeared from tracheal segments after 48 h in organotypic culture, indicating that the fibers were of extrinsic origin. However, calretinin-positive nerve fibers persisted in tracheal ganglia when tracheae were cocultured with the adjacent esophagus intact. Immunohistochemical analysis of the esophagus revealed a population of cholinergic neurons in the esophageal myenteric plexus that coexpressed calretinin. In functional studies, electrical stimulation of the esophagus in vitro evoked measurable contractions of the trachea. These contractions were not altered by prior organotypic culture of the trachea and esophagus to remove the extrinsic innervation to the airways but were significantly (P < 0.05) inhibited by the ganglionic blocker hexamethonium or by physical disruption of the tissue connecting the trachea and esophagus. These data suggest that a subset of esophageal neurons, characterized by the expression of calretinin and acetylcholine, provide a previously unrecognized excitatory input to tracheal cholinergic ganglia in guinea pigs.

The role of neuro-immune cross-talk in the regulation of inflammation and remodelling in asthma

Despite recent advances in the development of anti-asthmatic medication, asthma continues to be a major health problem worldwide. The symptoms of asthmatic patients include wheezing, chest tightness, cough and shortness of breath, which, together with airway hyperresponiveness, previously have been attributed to a dysfunction of airway nerves. However, research in the last two decades identified Th2-sensitization and the subsequent allergic reaction to innocuous environmental antigens as a basic immunological mechanism leading to chronic airway inflammation. Recent evidence suggests that the development of allergic asthma is influenced by events and circumstances in early childhood and even in utero. Allergen, ozone or stress exposure, as well as RSV infection in early life could be able to induce irreversible changes in the developing epithelial-mesenchymal trophic unit of the airways. The co-existence of chronic inflammation and neural dysfunction have recently drawn attention to the involvement of interaction pathways between the nervous and the immune system in the airways. Intensive basic research has accumulated morphological as well as functional evidence for the interaction between nerves and immune cells. Neuropeptides and neurotrophins have come into focus of attention as the key mediators of neuro-immune interactions, which lead to the development of several pharmacological compounds specifically targeting these molecules. This review will integrate our current knowledge on the involvement of neuro-immune pathways in asthma on the cellular and molecular level. It will summarize the results of pharmacological studies addressing the potential of neuropeptides and neurotrophins as novel therapeutic targets in asthma.

IL-1 receptors mediate persistent, but not acute, airway hyperreactivity to ozone in guinea pigs

Ozone exposure in the lab and environment causes airway hyperreactivity lasting at least 3 days in humans and animals. In guinea pigs 1 day after ozone exposure, airway hyperreactivity is mediated by eosinophils that block neuronal M(2) muscarinic receptor function, thus increasing acetylcholine release from airway parasympathetic nerves. However, mechanisms of ozone-induced airway hyperreactivity change over time, so that depleting eosinophils 3 days after ozone makes airway hyperreactivity worse rather than better. Ozone exposure increases IL-1beta in bone marrow, which may contribute to acute and chronic airway hyperreactivity. To test whether IL-1beta mediates ozone-induced airway hyperreactivity 1 and 3 days after ozone exposure, guinea pigs were pretreated with an IL-1 receptor antagonist (anakinra, 30 mg/kg, intraperitoneally) 30 minutes before exposure to filtered air or to ozone (2 ppm, 4 h). One or three days after exposure, airway reactivity was measured in anesthetized guinea pigs. The IL-1 receptor antagonist prevented ozone-induced airway hyperreactivity 3 days, but not 1 day, after ozone exposure. Ozone-induced airway hyperreactivity was vagally mediated, since bronchoconstriction induced by intravenous acetylcholine was not changed by ozone. The IL-1 receptor antagonist selectively prevented ozone-induced reduction of eosinophils around nerves and prevented ozone-induced deposition of extracellular eosinophil major basic protein in airways. These data demonstrate that IL-1 mediates ozone-induced airway hyperreactivity at 3 days, but not 1 day, after ozone exposure. Furthermore, preventing hyperreactivity was accompanied by decreased eosinophil major basic protein deposition within the lung, suggesting that IL-1 affects eosinophil activation 3 days after ozone exposure.

Applicability of DPI formulations for novel neurokinin receptor antagonist

A novel triple neurokinin receptor antagonist (TNRA) could have pharmaceutical efficacy for asthma and/or chronic obstructive pulmonary disease. TNRA is potentially developed as inhalation medicine. The aim of this investigation was to evaluate the applicability of dry powder inhaler (DPI) formulation for TNRA. DPI formulation containing lactose was used for this feasibility study. Mechanofusion process for surface modification was applied on lactose particles to prepare four different DPI formulations. The mixture of TNRA and lactose was administered to rats intratracheally using an insufflator. The deposition pattern and blood concentration profile of TNRA were evaluated. Although there was no significant difference in deposition on deep lungs between the four formulations, DPI formulations containing mechanofusion-processed lactose showed longer T(max) and t(1/2) and higher AUC(0-infinity) and MRT compared to that containing intact lactose. On the other hand, the contact angle measurement showed that the mechanofusion process decreased the polar part of the surface energy of the lactose. Therefore, the prolongation of the wetting of the formulated powder mixture seemed to delay the dissolution of TNRA deposited in respiratory tract. It was concluded that DPI formulation containing mechanofusion-processed lactose could be suitable for inhalation of TNRA.

Calcitonin gene-related peptide affects synaptic and membrane properties of bronchial parasympathetic neurons

Calcitonin gene-related peptide (CGRP) is located with substance P in nerve varicosities in close apposition to principal neurons in airway parasympathetic ganglia. Substance P has multiple effects on airway parasympathetic neurons but the role of CGRP is unknown. Using intracellular current clamp recording of ganglionic neurons, stimulation of vagal afferent nerves in the presence of neurokinin receptor antagonists evoked hyperpolarization of the membrane potential which was blocked by the CGRP-1 receptor antagonist, CGRP(8-37). Exogenous application of alpha-CGRP (0.001-0.1 microM) hyperpolarized the membrane potential, which was either blocked or reversed to depolarization in the presence of CGRP(8-37), whereas higher concentrations of alpha-CGRP (1.0-10.0 microM) caused depolarization. Action potential accommodation in phasic-type neurons decreased in the presence of alpha-CGRP (0.1-10 microM). The co-localization of substance P- and CGRP-immunoreactivity was observed in nerve varicosities within ganglia; prolonged exposure to capsaicin in vitro depleted substance P and CGRP immunostaining in nerve varicosities. These results demonstrate that CGRP has multiple effects on the excitability of airway parasympathetic neurons and may alter their activity, ultimately affecting parasympathetic tone in the lower airways.

Central and peripheral mechanisms of narcotic antitussives: codeine-sensitive and -resistant coughs

Narcotic antitussives such as codeine reveal the antitussive effect primarily via the mu-opioid receptor in the central nervous system (CNS). The kappa-opioid receptor also seems to contribute partly to the production of the antitussive effect of the drugs. There is controversy as to whether delta-receptors are involved in promoting an antitussive effect. Peripheral opioid receptors seem to have certain limited roles. Although narcotic antitussives are the most potent antitussives at present, certain types of coughs, such as chronic cough, are particularly difficult to suppress even with codeine. In guinea pigs, coughs elicited by mechanical stimulation of the bifurcation of the trachea were not able to be suppressed by codeine. In gupigs with sub-acute bronchitis caused by SO2 gas exposure, coughing is difficult to inhibit with centrally acting antitussives such as codeine. Some studies suggest that neurokinins are involved in the development of codeine-resistant coughs. However, evidence supporting this claim is still insufficient. It is very important to characterize opiate-resistant coughs in experimental animals, and to determine which experimentally induced coughs correspond to which types of cough in humans. In this review, we describe the mechanisms of antitussive effects of narcotic antitussives, addressing codeine-sensitive and -resistant coughs, and including our own results.

Motor and sensory re-innervation of the lung and heart after re-anastomosis of the cervical vagus nerve in rats

There is no study in the literature dealing with re-innervation of the cardiopulmonary vagus nerve after its transection followed by re-anastomosis. In the present study, we explored the bronchomotor, heart rate and respiratory responses in rats at 2, 3 and 6 months after re-anastomosis of one cervical vagus trunk. The conduction velocity of A, B and C waves was calculated in the compound vagal action potential. We searched for afferent vagal activities in phase with pulmonary inflation to assess the persistence of pulmonary stretch receptor (PSR) discharge in re-innervated lungs. In each animal, data from the stimulation or recording of one re-anastomosed vagus nerve were compared with those obtained in the contra-lateral intact one. Two and three months after surgery, the conduction velocities of A and B waves decreased, but recovery of conduction velocity was complete at 6 months. By contrast, the conduction velocity of the C wave did not change until 6 months, when it was doubled. The PSR activity was present in 50% of re-anastomosed vagus nerves at 2 and 3 months and in 75% at 6 months. Respiratory inhibition evoked by vagal stimulation was significantly weaker from the re-anastomosed than intact nerve at 2 but not 3 months. Vagal stimulation did not elicit cardiac slowing or bronchoconstriction 6 months after re-anastomosis. Our study demonstrates the capacity of pulmonary vagal sensory neurones to regenerate after axotomy followed by re-anastomosis, and the failure of the vagal efferents to re-innervate both the lungs and heart.

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.

Bilateral vagotomy or atropine pre-treatment reduces experimental diesel-soot induced lung inflammation

To investigate the role of the vagus nerve in acute inflammatory and cardiorespiratory responses to diesel particulate (DP) in the rat airway, we measured changes in respiration, blood pressure and neutrophils in lungs of urethane anesthetized Wistar rats 6-h post-instillation of DP (500 microg) and studied the effect of mid-cervical vagotomy or atropine (1 mg kg(-1)) pre-treatment. In conscious rats, we investigated DP, with and without atropine pre-treatment. DP increased neutrophil level in BAL (bronchoalveolar lavage) fluid from intact anesthetized rats to 2.5+/-0.7x10(6) cells (n=8), compared with saline instillation (0.3+/-0.1x10(6), n=7; P<0.05). Vagotomy reduced DP neutrophilia to 0.8+/-0.2x10(6) cells (n=8; P<0.05 vs. intact); atropine reduced DP-induced neutrophilia to 0.3+/-0.2x10(6) (n=4; P<0.05). In conscious rats, DP neutrophilia of 8.5+/-1.8x10(6), n=4, was reduced by pre-treatment with atropine to 2.2+/-1.2x10(6) cells, n=3. Hyperventilation occurred 6 h after DP in anesthetized rats with intact vagi, but not in bilaterally vagotomized or atropine pre-treated animals and was abolished by vagotomy (P<0.05, paired test). There were no significant differences in the other variables (mean blood pressure, heart rate and heart rate variability) measured before and 360 min after DP. In conclusion, DP activates a pro-inflammatory vago-vagal reflex which is reduced by atropine. Muscarinic ACh receptors in the rat lung are involved in DP-induced neutrophilia, and hence muscarinic antagonists may reduce airway and/or cardiovascular inflammation evoked by inhaled atmospheric DP in susceptible individuals.

Nerve growth factor acutely potentiates synaptic transmission in vitro and induces dendritic growth in vivo on adult neurons in airway parasympathetic ganglia

Elevated levels of nerve growth factor (NGF) and NGF-mediated neural plasticity may have a role in airway diseases such as asthma and chronic obstructive pulmonary disease (COPD). Although NGF is known to affect sensory and sympathetic nerves, especially during development, little is known regarding its effect on parasympathetic nerves, especially on adult neurons. The purpose of this study was to analyze the acute and chronic effects of NGF on the electrophysiological and anatomical properties of neurons in airway parasympathetic ganglia from adult guinea pigs. Using single cell recording, direct application of NGF caused a lasting decrease in the cumulative action potential afterhyperpolarization (AHP) and increased the amplitude of vagus nerve-stimulated nicotinic fast excitatory postsynaptic potentials. Neuronal responsiveness to nicotinic receptor stimulation was increased by NGF, which was blocked by the tyrosine kinase inhibitor, K-252a, implicating neurotrophin-specific (Trk) receptors. Neurotrophin-3 and brain-derived neurotrophic factor had no effect on the synaptic potentials, AHP, or nicotinic response; inhibition of cyclooxygenase with indomethacin inhibited the effect of NGF on the cumulative AHP. Forty-eight hours after in vivo application of NGF to the trachealis muscle caused an increase in dendritic length on innervating neurons. These results are the first to demonstrate that NGF increases the excitability of lower airway parasympathetic neurons, primarily through enhanced synaptic efficacy and changes to intrinsic neuron properties. NGF also had dramatic effects on the growth of dendrites in vivo. Such effects may indicate a new role for NGF in the regulation of parasympathetic tone in the diseased or inflamed lower airways.

Neurokinin3 receptor regulation of the airways

Neurokinin(3) (NK(3)) receptors may regulate the airways primarily through actions on the nerves. In the periphery, airway parasympathetic ganglia neurons are depolarized following NK(3) receptor activation resulting subsequently in the facilitation of synaptic transmission. Such an effect may account for the excessive parasympathetic reflex effects (e.g. airway smooth muscle contraction, vascular engorgement, mucus secretion) associated with asthma and chronic obstructive pulmonary disease (COPD). In the central nervous system (CNS), NK(3) receptor activation may regulate airway vagal afferent relay neurons, rendering them hyperresponsive to parallel inputs from glutamate containing afferent nerves. This process is analogous to the process of central sensitization regulating hyperalgesia and pain in somatic tissues. In both the CNS and in the airways, NK(3) receptors are likely activated by either substance P and/or neurokinin A (NKA), both of which are full agonists at NK(3) receptors, as there is little evidence that airway nerves express neurokinin B (NKB). Evidence for other potential sites of regulation by NK(3) receptors in the airways (e.g. vasculature, airway smooth muscle, epithelium, mucus glands) is either inconclusive or conflicting.

Vagal afferent nerves regulating the cough reflex

Coughing is initiated by activation of mechanically and chemically sensitive vagal afferent nerves innervating the airways. All afferent nerve subtypes innervating the airways can modulate the cough reflex. Rapidly adapting and slowly adapting stretch receptors (RARs and SARs, respectively) innervating the intrapulmonary airways and lung may enhance and facilitate coughing. Activation of intrapulmonary C-fibers has been shown to inhibit coughing in anesthetized animals. Extrapulmonary C-fibers and RARs can initiate coughing upon activation. C-fiber-dependent coughing is uniquely sensitive to anesthesia. Tracheal and bronchial C-fibers may also interact with other afferents to enhance coughing. Recent studies in anesthetized guinea pigs have identified a myelinated afferent nerve subtype that can be differentiated from intrapulmonary RARs and SARs and play an essential role in initiating cough. Whether these "cough receptors" are the guinea pig equivalent of the irritant receptors described in the extrapulmonary airways of other species is unclear.

Potentiation of nicotinic currents by bradykinin in the paratracheal ganglia neurons of rats

The effects of bradykinin on nicotine-induced responses were investigated in neurons dissociated from rat paratracheal ganglia using the nystatin-perforated patch-clamp recording technique. When bradykinin (10(-9) to 10(-8) M) was pretreated and then simultaneously applied with 10(-5) M nicotine, bradykinin potentiated the nicotine-induced currents. The potentiation was mimicked by [Hyp3]-bradykinin and inhibited by HOE-140, pertussis toxin, neomycin and U-73122, but not U-73433. These results suggest that bradykinin potentiates nicotinic currents via bradykinin B2 receptor, pertussis toxin-sensitive G-protein and phospholipase C. Since bradykinin inhibits the M-current via bradykinin B2 receptor and pertussis toxin-insensitive G-protein [Mochidome, T., Ishibashi, H., Takahama, K., 2001. Bradykinin activates airway parasympathetic ganglion neurons by inhibiting M-currents. Neuroscience 105, 785-791.], it seemed that bradykinin B2 receptor activated two distinct signal transduction pathways in the paratracheal ganglia neurons. This effect of bradykinin might cause enhanced synaptic transmission in paratracheal ganglia neurons and contribute to the aggravation of pathological conditions of the lower airway via enhanced acetylcholine release from the postganglionic nerve terminals.

Anatomy and Neurophysiology of the Cough Reflex

OBJECTIVES

To describe the anatomy and neurophysiology of the cough reflex.

METHODS

A review of the literature was carried out using PubMed and the ISI Web of Knowledge from 1951 to 2004. Most of the referenced studies were carried out in animals

CONCLUSIONS

Studies carried out in animals provide suggestive but inconclusive evidence that C-fibers and rapidly adapting receptors (RARs) arising from the vagus nerves mediate coughing. Recent studies also have suggested that a vagal afferent nerve subtype that is not readily classified as a RAR or a C-fiber may play an important role in regulating cough. Afferent nerves innervating other viscera, as well as somatosensory nerves innervating the chest wall, diaphragm, and abdominal musculature also likely play a less essential but important accessory role in regulating cough. The responsiveness and morphology of the airway vagal afferent nerve subtypes and the extrapulmonary afferent nerves that regulate coughing are described.

The triple neurokinin-receptor antagonist CS-003 inhibits neurokinin A-induced bronchoconstriction in patients with asthma

Neurokinin A (NKA) causes bronchoconstriction in asthmatic patients. In vitro both NK1 and NK2 receptors can mediate airway contraction. Moreover in guinea pigs, NK3 receptors facilitate cholinergic neurotransmission. Dual tachykinin NK1/NK2 receptor antagonism results in prevention of NKA-induced bronchoconstriction. We have now examined the effect of a single dose of the triple tachykinin receptor antagonist CS-003 on NKA-induced bronchoconstriction in asthmatics. A double blind, crossover, placebo-controlled trial in 16 mild asthmatics was performed. One single dose of CS-003 (200 mg, solution in distilled water) or matched placebo was given orally on the assessment days. NKA-provocation tests were performed pre-dose and 1, 8 and 24 h after dosing. There was a significant shift to the right of the dose-response curve at 1 and 8 h after intake of CS-003. PC20 was not reached in 12/16 patients at 1h post-dose and in 5/16 patients at 8 h post-dose. This did not occur under placebo treatment. A single dose of 200 mg CS-003 protected significantly against NKA-induced bronchoconstriction at 1 and 8h post-dose in mild asthmatics.

CNS determinants of sleep-related worsening of airway functions: implications for nocturnal asthma

This review summarizes the recent neuroanatomical and physiological studies that form the neural basis for the state-dependent changes in airway resistance. Here, we review only the interactions between the brain regions generating quiet (non-rapid eye movement, NREM) and active (rapid eye movement, REM) sleep stages and CNS pathways controlling cholinergic outflow to the airways. During NREM and REM sleep, bronchoconstrictive responses are heightened and conductivity of the airways is lower as compared to the waking state. The decrease in conductivity of the lower airways parallels the sleep-induced decline in the discharge of brainstem monoaminergic cell groups and GABAergic neurons of the ventrolateral periaqueductal midbrain region, all of which provide inhibitory inputs to airway-related vagal preganglionic neurons (AVPNs). Withdrawal of central inhibitory influences to AVPNs results in a shift from inhibitory to excitatory transmission that leads to an increase in airway responsiveness, cholinergic outflow to the lower airways and consequently, bronchoconstriction. In healthy subjects, these changes are clinically unnoticed. However, in patients with bronchial asthma, sleep-related alterations in lung functions are troublesome, causing intensified bronchopulmonary symptoms (nocturnal asthma), frequent arousals, decreased quality of life, and increased mortality. Unquestionably, the studies revealing neural mechanisms that underlie sleep-related alterations of airway function will provide new directions in the treatment and prevention of sleep-induced worsening of airway diseases.

Brain stem excitatory and inhibitory signaling pathways regulating bronchoconstrictive responses

This review summarizes recent work on two basic processes of central nervous system (CNS) control of cholinergic outflow to the airways: 1) transmission of bronchoconstrictive signals from the airways to the airway-related vagal preganglionic neurons (AVPNs) and 2) regulation of AVPN responses to excitatory inputs by central GABAergic inhibitory pathways. In addition, the autocrine-paracrine modulation of AVPNs is briefly discussed. CNS influences on the tracheobronchopulmonary system are transmitted via AVPNs, whose discharge depends on the balance between excitatory and inhibitory impulses that they receive. Alterations in this equilibrium may lead to dramatic functional changes. Recent findings indicate that excitatory signals arising from bronchopulmonary afferents and/or the peripheral chemosensory system activate second-order neurons within the nucleus of the solitary tract (NTS), via a glutamate-AMPA signaling pathway. These neurons, using the same neurotransmitter-receptor unit, transmit information to the AVPNs, which in turn convey the central command to airway effector organs: smooth muscle, submucosal secretory glands, and the vasculature, through intramural ganglionic neurons. The strength and duration of reflex-induced bronchoconstriction is modulated by GABAergic-inhibitory inputs and autocrine-paracrine controlling mechanisms. Downregulation of GABAergic inhibitory influences may result in a shift from inhibitory to excitatory drive that may lead to increased excitability of AVPNs, heightened airway responsiveness, and sustained narrowing of the airways. Hence a better understanding of these normal and altered central neural circuits and mechanisms could potentially improve the design of therapeutic interventions and the treatment of airway obstructive diseases.

Post-stimulus potentiation of transmission in pelvic ganglia enhances sympathetic dilatation of guinea-pig uterine artery in vitro

Vasodilatation produced by stimulation of preganglionic neurones in lumbar and sacral pathways to pelvic ganglia was studied using an in vitro preparation of guinea-pig uterine artery and associated nerves in a partitioned bath allowing selective drug application to the ganglia or artery. Arterial diameter was monitored using real time video imaging. Vasodilatations produced by hypogastric nerve stimulation (HN; 300 pulses, 10 Hz) were significantly larger and longer in duration than with pelvic nerve stimulation (N = 18). Stimulation of ipsilateral lumbar splanchnic nerves or ipsilateral third lumbar ventral roots also produced prolonged vasodilatations. Blockade of ganglionic nicotinic receptors (0.1-1 mM hexamethonium) delayed the onset and sometimes reduced the peak amplitude of dilatations, but slow dilatations persisted in 16 of 18 preparations. These dilatations were not reduced further by 3 microM capsaicin applied to the artery and ganglia, or ganglionic application of 1 microM hyoscine, 30-100 microM suramin or 10 microM CNQX. Dilatations were reduced slightly by ganglionic application of NK1 and NK3 receptor antagonists (SR140333, SR142801; 1 microM), but were reduced significantly by bathing the ganglia in 0.5 mM Ca2+ and 10 mM Mg2+. Intracellular recordings of paracervical ganglion neurones revealed fast excitatory postsynaptic potentials (EPSPs) in all neurones on HN stimulation (300 pulses, 10 Hz), and slow EPSPs (3-12 mV amplitude) in 25 of 37 neurones. Post-stimulus action potential discharge associated with slow EPSPs occurred in 16 of 37 neurones (firing rate 9.4 +/- 1.5 Hz). Hexamethonium (0.1-1 mM) abolished fast EPSPs. Hexamethonium and hyoscine (1 microM) did not reduce slow EPSPs and associated post-stimulus firing in identified vasodilator neurones (with VIP immunoreactivity) or non-vasodilator paracervical neurones. These results demonstrate a predominantly sympathetic origin of autonomic pathways producing pelvic vasodilatation in females. Non-cholinergic mediators of slow transmission in pelvic ganglia produce prolonged firing of postganglionic neurones and long-lasting dilatations of the uterine artery. This mechanism would facilitate maintenance of pelvic vasodilatation on stimulation of preganglionic neurones during sexual activity.

Involvement of enhanced neurokinin NK3 receptor expression in the severe asthma guinea pig model

In this study, we investigated the involvement of neurokinin NK3 receptors in a severe asthma model prepared by administering ovalbumin via inhalation three times to systemically sensitized guinea pigs. [3H]senktide, a neurokinin NK3 receptor ligand, showed significant specific binding to the lungs from the model animals, but not to those from negative control animals. The airway responsiveness to intravenous neurokinin B, a neurokinin NK3 receptor agonist, was increased in the model, indicating an increase in functional NK3 receptors. Furthermore, SB 223956 ((-)-3-methoxy-2-phenyl-N-[(1S)-phenylpropyl]quinoline-4-carboxamide), a selective neurokinin NK3 receptor antagonist, significantly inhibited the ovalbumin-induced airway hyperresponsiveness to inhaled methacholine, but it did not show significant effects on the ovalbumin-induced airway narrowing and eosinophil accumulation. These results suggest that the expressed neurokinin NK3 receptors in the severe asthma model are involved in the development of airway hyperresponsiveness.

A Novel Role for Tachykinin Neurokinin-3 Receptors in Regulation of Human Bronchial Ganglia Neurons

The neuropeptide tachykinins and their receptors have been implicated in the pathogenesis of lung disease, although the role of the tachykinin neurokinin-3 receptor has not been elucidated. Using confocal microscopy, we identified tachykinin neurokinin-3 receptors on human bronchial parasympathetic ganglion neurons. Electrophysiologic recordings demonstrated that activation of sensory nerve fibers, either by antidromic stimulation or capsaicin, depolarized these neurons. This response was mimicked by exogenously applied tachykinin neurokinin-3 receptor-selective agonist, senktide analogue, but not significantly by tachykinin neurokinin-1 or neurokinin-2 receptor-selective agonists. Responses to endogenous tachykinins or exogenous selective tachykinin neurokinin-3 receptor activation with senktide analogue were inhibited by the selective tachykinin neurokinin-3 receptor antagonists, SB 223412 or SB 235375. We provide the first evidence that tachykinin neurokinin-3 receptors regulate human bronchial parasympathetic ganglion neurotransmission by activation of a peripheral reflex. This pathway may play a significant role in controlling bronchomotor tone and air flow to the lung.

Chemical profile of vagal preganglionic motor cells innervating the airways in ferrets: the absence of noncholinergic neurons

In ferrets, we investigated the presence of choline acetyltransferase (ChAT), vasoactive intestinal peptide (VIP), and markers for nitric oxide synthase (NOS) in preganglionic parasympathetic neurons innervating extrathoracic trachea and intrapulmonary airways. Cholera toxin β-subunit, a retrograde axonal transganglionic tracer, was used to identify airway-related vagal preganglionic neurons. Double-labeling immunohistochemistry and confocal microscopy were employed to characterize the chemical nature of identified airway-related vagal preganglionic neurons at a single cell level. Physiological experiments were performed to determine whether activation of the VIP and ChAT coexpressing vagal preganglionic neurons plays a role in relaxation of precontracted airway smooth muscle tone after muscarinic receptor blockade. The results showed that 1) all identified vagal preganglionic neurons innervating extrathoracic and intrapulmonary airways are acetylcholine-producing cells, 2) cholinergic neurons innervating the airways coexpress ChAT and VIP but do not contain NOS, and 3) chemical stimulation of the rostral nucleus ambiguus had no significant effect on precontracted airway smooth muscle tone after muscarinic receptor blockade. These studies indicate that vagal preganglionic neurons are cholinergic in nature and coexpress VIP but do not contain NOS; their stimulation increases cholinergic outflow, without activation of inhibitory nonadrenergic, noncholinergic ganglionic neurons, stimulation of which induces airway smooth muscle relaxation. Furthermore, these studies do not support the possibility of direct inhibitory innervation of airway smooth muscle by vagal preganglionic fibers that contain VIP.

Influence of maturation on constrictive response to stimulation of C-fiber afferents in isolated guinea pig airways

We investigated whether the airway constrictive response to stimulation of bronchopulmonary C-fiber afferents is altered during the maturation process. Isometric tension was measured in airway rings isolated from three tracheobronchial locations (intrathoracic trachea and main and hilar bronchi) and compared in mature [M, 407 ± 10 (SE) g body wt, n = 36] and immature (IM, 161 ± 5 g body wt, n = 35) guinea pigs. Our results showed no difference in the ACh (10−5 M)- or KCl (40 mM)-induced contraction between M and IM groups, regardless of the airway location. In sharp contrast, the concentration-response curves of 10−8–10−6 M capsaicin were distinctly lower in IM hilar bronchi; for example, response to the same concentration of capsaicin (10−6 M) was 89.2 ± 15.3% of the response to 10−5 M ACh in IM and 284.7 ± 43.2% in M animals. Similar, but smaller, differences in the bronchoconstrictive response to capsaicin between IM and M groups were also observed in the trachea and main bronchus. Electrical field stimulation induced airway constriction in all three locations in M and IM groups. However, after administration of 10−6 M atropine and 10−6 M propranolol, electrical field stimulation-induced contraction was significantly smaller in the hilar bronchus of IM than M animals, and this difference was not prevented by pretreatment with 5 × 10−5 M indomethacin. Although radioimmunoassay showed no difference in the tissue content of substance P between M and IM airways, the constrictive responses to exogenous substance P and neurokinin A were markedly greater in M airways at all three locations. In conclusion, the constriction of isolated airways evoked by C-fiber stimulation was significantly weaker in the IM guinea pigs, probably because of a less potent effect of tachykinins on the airway smooth muscle.

Bronchopulmonary afferent nerves

Vagal afferent nerves are the primary communication pathways between the bronchopulmonary system and the central nervous system. Input from airway afferent nerves to the CNS is integrated in the brainstem and ultimately leads to sensations and various reflex outputs. Afferent nerves innervating the airways can be classified into various distinct phenotypes. However, there is no single classification scheme that takes all features, including conduction velocity, cell body diameter, ganglionic origin, and stimuli to which they respond (modality) into account. At present, bronchopulmonary afferent nerves are typically considered to belong to one of three general categories, namely C-fibres, rapidly adapting stretch receptors (RARs), and slowly adapting stretch receptors (SARs). As our understanding of bronchopulmonary afferent nerves continues to deepen, we are likely to see more sophisticated classification schemes emerge. It is clear that the function of afferent fibres can be substantively influenced by airway inflammation and remodelling. The perturbations and perversions of afferent nerve function that occur during these states almost certainly contributes to many of the signs and symptoms of inflammatory airway disease. A more lucid characterization of bronchopulmonary afferent nerves, and a better understanding of the mechanisms by which these nerves influence pulmonary physiology during health and disease anticipates future research.

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.

Peripheral tachykinin receptors as potential therapeutic targets in visceral diseases

More than 10 years of intensive preclinical investigation of selective tachykinin (TK) receptor antagonists has provided a rationale to the speculation that peripheral neurokinin (NK)-1, -2 and -3 receptors may be involved in the pathophysiology of various human diseases at the visceral level. In the airways, despite promising effects in animal models of asthma, pilot clinical trials with selective NK-1 or -2 receptor antagonists in asthmatics have been ambiguous, whereas the potential antitussive effects of NK-1, -2 or -3 antagonists have not yet been verified in humans. In the gastrointestinal (GI) tract, irritable bowel syndrome (IBS) and pancreatitis are appealing targets for peripherally-acting NK-1 and -2 antagonists, respectively. In the genito-urinary tract, NK-1 receptor antagonists could offer some protection against nephrotoxicity and cytotoxicity induced by chemotherapeutic agents, whereas NK-2 receptor antagonists appear to be promising new agents for the treatment of neurogenic bladder hyperreflexia. Finally, there is preclinical evidence for hypothesising an effect of NK-3 receptor antagonists on the cardiovascular disturbance that characterises pre-eclampsia. Other more speculative applications are also mentioned.

The role of neural inflammation in asthma and chronic obstructive pulmonary disease

The tachykinins substance P and neurokinin A are found within airway nerves and immune cells. They have various effects on the airways that can contribute to the changes observed in asthma and chronic obstructive pulmonary disease. Both tachykinin NK(1) and NK(2) receptors have been involved in the bronchoconstriction and the proinflammatory changes induced by substance P and neurokinin A. Tachykinin NK(1) and NK(2) receptor antagonists have activity in various animal models of allergic asthma and chronic bronchitis. It is suggested that dual NK(1)/NK(2) and triple NK(1)/NK(2)/NK(3) tachykinin receptor antagonists have potential in the treatment of obstructive airway diseases.