Effects of tachykinin NK1 receptor antagonists on vagal hyperreactivity and neuronal M2 muscarinic receptor function in antigen challenged guinea-pigs

From bench to bedside: The role of cough hypersensitivity in chronic cough

BACKGROUND

Chronic cough is a burdensome condition characterized by persistent cough lasting longer than 8 weeks. Chronic cough can significantly affect quality of life, physical function and productivity, with many people troubled with a cough that lasts for months or even years. People with chronic cough commonly report a persistent urge to cough with frequent bouts of coughing triggered by innocuous stimuli, which has led to the concept of cough hypersensitivity.

MAIN BODY

Both central and peripheral neural pathways regulate cough, and although mechanisms driving development of cough hypersensitivity are not fully known, sensitization of these neural pathways contributes to excessive cough triggering in cough hypersensitivity. Effective therapies that control chronic cough are currently lacking. Recent therapeutic development has focused on several ion channels and receptors involved in peripheral activation of cough (e.g., transient receptor potential channels, P2 × 3 receptors and voltage-gated sodium channels) or central cough processing (e.g., neurokinin-1 [NK-1] receptors and nicotinic acetylcholine receptors).

CONCLUSION

These targeted therapies provide novel insights into mechanisms underlying cough hypersensitivity and may offer new treatment options for people with chronic cough. In this review, we explore preclinical and clinical studies that have improved our understanding of the mechanisms responsible for chronic cough and discuss the most promising targeted approaches to date, including trials of P2 × 3-receptor antagonists and NK-1-receptor antagonists.

Inflammatory mechanisms linking maternal and childhood asthma

Asthma is a chronic inflammatory airway disease characterized by airway hyperresponsiveness, inflammation, and remodeling. Asthma often develops during childhood and causes lifelong decrements in lung function and quality of life. Risk factors for childhood asthma are numerous and include genetic, epigenetic, developmental, and environmental factors. Uncontrolled maternal asthma during pregnancy exposes the developing fetus to inflammatory insults, which further increase the risk of childhood asthma independent of genetic predisposition. This review focuses on the role of maternal asthma in the development of asthma in offspring. We will present maternal asthma as a targetable and modifiable risk factor for childhood asthma and discuss the mechanisms by which maternal inflammation increases childhood asthma risk. Topics include how exposure to maternal asthma in utero shapes structural lung development with a special emphasis on airway nerves, how maternal type-2 cytokines such as IL-5 activate the fetal immune system, and how changes in lung and immune cell development inform responses to aero-allergens later in life. Finally, we highlight emerging evidence that maternal asthma establishes a unique "asthma signature" in the airways of children, leading to novel mechanisms of airway hyperreactivity and inflammatory cell responses.

Eosinophil and airway nerve interactions in asthma

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

Neuromodulation mediated by the tachykinin NK3-receptor agonist [MePhe7]-neurokinin B in the isolated perfused lung of nonsensitized nonchallenged and ovalbumin-sensitized and -challenged guinea pig

The neuromodulatory action of the tachykinin NK(3)-receptor agonist [MePhe(7)]-neurokinin B ([MePhe(7)]-NKB) was evaluated on vagal stimulation-induced bronchoconstriction in nonsensitized nonchallenged and ovalbumin (OVA)-sensitized and -challenged guinea pig using the isolated perfused lung preparation. Lungs were placed inside a warmed (37°C) glass chamber and suspended from a force displacement transducer (Grass FT-03) with both vagi connected to a stimulating electrode. Isolated lungs were stimulated at a constant voltage (20 V) and pulse duration (5 ms) with electrical stimulation frequencies ranging from 1 to 128 Hz. The authors demonstrated that vagal stimulation produced frequency-dependent bronchoconstriction and [MePhe(7)]-NKB, at a dose (0.1 μM) that does not produce bronchoconstriction by itself, potentiated the vagally induced bronchoconstriction at all frequencies in nonsensitized nonchallenged animals and to a greater extent in OVA-sensitized and -challenged guinea pigs; the potentiations were totally inhibited by the tachykinin NK(3)-receptor antagonist SR 142801 (1 μM). In a second set of experiments, [MePhe(7)]-NKB produced bronchoconstriction in a dose-dependent (1 to 300 μg/mL) manner with similar potencies and maximum responses in nonsensitized nonchallenged (EC(50) = 8.6 ± 1.1 μM; E(Max) = 61.1 ± 3.5 mm Hg) and OVA-sensitized and -challenged (EC(50) = 8.5 ± 1.3 μM; E(Max) = 63.5 ± 3.7 mm Hg) animals. In conclusion, these results demonstrated that [MePhe(7)]-NKB potentiated vagal stimulation-induced bronchoconstriction via the tachykinin NK(3)-receptors and OVA sensitization caused development of airway hyperresponsiveness in these potentiations. However, OVA sensitization had no effect on airway responsiveness of vagal stimulation-and [MePhe(7)]-NKB-induced bronchoconstrictions.

Prescription pattern of oxybutynin ER in patients with overactive bladder in real life practice: a multicentre, open-label, prospective observational study

AIMS OF STUDY

To investigate the prescription pattern and dose distribution of the antimuscarinic agent oxybutynin extended release (ER) in patients with overactive bladder (OAB) in actual clinical practice.

MATERIALS AND METHODS

In this multicentre, prospective, observational, flexible-dosing study, the dosage of oxybutynin ER for each patient was adjusted after discussions of efficacy and tolerability between doctor and patient, over a 12 week treatment period. Efficacy was measured by administering the Primary OAB Symptom Questionnaire (POSQ) before and after treatment. Patients were also administered, the patient perception of treatment benefit (PPTB) questionnaire at the end of the study. Adverse events (AE) were documented at each study visit.

RESULTS

Of the 809 patients enrolled, 590 (73.2%) continued to take study medication for 12 weeks. Most patients were prescribed 5 (24.2%) or 10 (68.8%) mg/day oxybutynin ER at the start of treatment. Most were also prescribed 5 (19.1%) or 10 (67.4%) mg/day at the end of treatment, with a dose escalation rate of 14.9%. All OAB symptoms evaluated by the POSQ were improved; 94.1% of patients reported benefits from treatment and 89.3% were satisfied. Independent predictive factors for discontinuation were female, younger age (age ≤ 65), obesity (BMI ≥ 25), severe symptoms (USS, Gr. 5) and larger numbers of other co-medications.

CONCLUSIONS

Most patients were prescribed 5-10 mg/day oxybutynin ER as both starting and maintenance doses, with a dose escalation rate of only 14.9%. Prescription of > 10 mg/day oxybutynin ER was not frequent in real life practice.

Neural control of airway inflammation

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

Pulmonary inflammation and airway hyperresponsiveness in a mouse model of asthma complicated by acid aspiration

Several studies have indicated a strong association between asthma and aspiration of stomach contents. However, the complex association between these inflammatory processes has not been studied extensively in animal models. In the present study, we developed an animal model to evaluate the inflammatory cell, chemokine, and airway responses to asthma complicated by aspiration. The model was produced by sensitizing mice to cockroach allergens from house-dust extracts. Mice with asthma-like airway responses then were inoculated intratracheally with either an acidic solution or saline. Acid aspiration increased airway hyperresponsiveness in mice with asthma for at least 8 h. After 6 h, the combined injury caused an additive, not synergistic, increase in airway hyperresponsiveness and neutrophil recruitment to the airways. Although cysteinyl leukotrienes in bronchoalveolar lavage fluid were higher after acid aspiration, treatment with a receptor antagonist before aspiration did not diminish airway hyperresponsiveness. Vagal mechanisms reportedly mediate airway responses in acid aspiration; however, pretreatment with an anticholinergic agent did not reduce airway responses to acid. These results are consistent with an effective model of the acute effects of aspiration on the allergic lung. Further studies could examine how various forms of aspiration influence the severity of asthma.

Mechanisms underlying the inhibitory effects of tachykinin receptor antagonists on eosinophil recruitment in an allergic pleurisy model in mice

The activation of tachykinin NK receptors by neuropeptides may induce the recruitment of eosinophils in vivo. The aim of the present study was to investigate the effects and underlying mechanism(s) of the action of tachykinin receptor antagonists on eosinophil recruitment in a model of allergic pleurisy in mice. Pretreatment of immunized mice with capsaicin partially prevented the recruitment of eosinophils after antigen challenge, suggesting the potential contribution of sensory nerves for the recruitment of eosinophils Local (10-50 nmol per pleural cavity) or systemic (100-300 nmol per animal) pretreatment with the tachykinin NK1 receptor antagonist SR140333 prevented the recruitment of eosinophils induced by antigen challenge of immunized mice. Neither tachykinin NK2 nor NK3 receptor antagonists suppressed eosinophil recruitment. Pretreatment with SR140333 failed to prevent the antigen-induced increase of interleukin-5 concentrations in the pleural cavity. Similarly, SR140333 failed to affect the bone marrow eosinophilia observed at 48 h after antigen challenge of immunized mice. SR140333 induced a significant increase in the concentrations of antigen-induced eotaxin at 6 h after challenge. Antigen challenge of immunized mice induced a significant increase of Leucotriene B4 (LTB4) concentrations at 6 h after challenge. Pretreatment with SR140333 prevented the antigen-induced increase of LTB4 concentrations. Our data suggest an important role for NK1 receptor activation with consequent LTB4 release and eosinophil recruitment in a model of allergic pleurisy in the mouse. Tachykinins appear to be released mainly from peripheral endings of capsaicin-sensitive sensory neurons and may act on mast cells to facilitate antigen-driven release of LTB4.

Neurokinin-1 receptor mediates stress-exacerbated allergic airway inflammation and airway hyperresponsiveness in mice

BACKGROUND

A wealth of clinical observation has suggested that stress and asthma morbidity are associated. We have previously established a mouse model of stress-exacerbated allergic airway inflammation, which reflects major clinical findings.

OBJECTIVE

The aim of the current study was to investigate the role of the neurokinin- (NK-)1 receptor in the mediation of stress effects in allergic airway inflammation.

METHODS

BALB/c mice were systemically sensitized with ovalbumin (OVA) on assay days 1, 14, and 21 and repeatedly challenged with OVA aerosol on days 26 and 27. Sound stress was applied to the animals for 24 hours, starting with the first airway challenge. Additionally, one group of stressed and one group of nonstressed mice received the highly specific NK-1 receptor antagonist RP 67580. Bronchoalveolar lavage fluid was obtained, and cell numbers and differentiation were determined. Airway hyperreactivity was measured in vitro by electrical field stimulation of tracheal smooth-muscle elements.

RESULTS

Application of stress in sensitized and challenged animals resulted in a significant increase in leukocyte number in the bronchoalveolar lavage fluid. Furthermore, stressed animals showed enhanced airway reactivity. The increase of inflammatory cells and airway reactivity was blocked by treatment of animals with the NK-1 receptor antagonist.

CONCLUSION

These data indicate that the NK-1 receptor plays an important role in mediating stress effects in allergen-induced airway inflammation.

Neurotrophins and neurotrophin receptors in allergic asthma

The neurotrophins nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3 (NT-3) and NT-4 play a pivotal role in the development of the nervous system. Despite their well-known effects on neurons, elevated neurotrophin concentrations have been observed under pathological conditions in sera of patients with inflammatory disorders. Patients with asthma feature both airway inflammation and an abnormal airway reactivity to many unspecific stimuli, referred to as airway hyperresponsiveness, which is, at least partly, neuronally controlled. Interestingly, these patients show increased levels of neurotrophins in the blood as well as locally in the lung. It has been demonstrated that neurotrophin release from immune cells is triggered by allergen contact. The presence of neurotrophins and the neurotrophin receptors p75 (p75NTR), tyrosine kinase A (TrkA), TrkB and TrkC have been described in several immune cells. There is strong evidence for an involvement of neurotrophins in regulation of hematopoiesis and, in addition, in modulation of immune cell function in mature cells circulating in blood or resting in lymphatic organs and peripheral tissues. The aim of this review is to demonstrate possible roles of neurotrophins during an allergic reaction in consideration of the temporospatial compartimentalization.

Neurokinin-1 Receptor Antagonists Protect Mice from CD95- and Tumor Necrosis Factor-α-Mediated Apoptotic Liver Damage

Previously, we have shown that primary afferent neurons are necessary for disease activity in immune-mediated liver injury in mice. These nerve fibers are detectable by substance P (SP) immunocytochemistry in the portal tract of rodent liver. Antagonists of the neurokinin-1 receptor (NK-1R), which is the prime receptor of SP, prevented liver damage by suppressing the synthesis of proinflammatory cytokines. Here, we investigated the influence of primary afferent nerve fibers, SP, and NK-1 receptor antagonists on hepatocyte apoptosis in vivo induced by administration of activating anti-CD95 monoclonal antibody (mAb) to mice. Depletion of primary afferent nerve fibers by neonatal capsaicin treatment prevented CD95-mediated activation of caspase-3, measured as enzymatic activity in liver homogenates or by demonstration of hepatocellular immunoreactivity for active caspase-3 in liver slices, and liver damage. This effect was reversed by administration of SP to anti-CD95 mAb-treated mice depleted from primary afferent neurons. The presence of the NK-1R on mouse hepatocytes was demonstrated by immunocytochemistry and flow cytometry. Intraperitoneal pretreatment with the NK-1 receptor antagonists (2S,3S)-cis-2-(diphenylmethyl)-N-([2-methoxyphenyl]-methyl)-1-azabicyclo(2.2.2.)-octan-3-amine (CP-96,345) or (2S,3S)3-([3,5-bis(trifluoromethyl)phenyl]methoxy)-2-phenylpiperadine (L-733,060) dose dependently protected mice from CD95-mediated liver injury. Similar results were obtained when apoptotic liver damage was induced by administration of tumor necrosis factor-alpha to d-galactosamine-sensitized mice. In conclusion, SP, probably by binding to its receptor on hepatocytes, might aggravate apoptotic signals in these cells. Because NK-1 receptor antagonists not only suppress the proinflammatory cytokine response in the liver but also prevent liver cell apoptosis in vivo, they might be suitable drugs for treatment of immune-mediated liver disease.

Stress enhances airway reactivity and airway inflammation in an animal model of allergic bronchial asthma

OBJECTIVE

Despite the long-standing clinical assumption that stress and asthma morbidity are associated, convincing experimental evidence on mechanisms has been unavailable. A wide range of immunological, endocrinological, and neuronal pathways are known to mediate and modulate a systemic stress response. Interestingly, most of these mediators play a crucial role in initiating and perpetuating symptoms associated with bronchial asthma. To explore potential mechanisms linking stress to asthma exacerbation we developed an animal model that combines allergic airway inflammation and exposure to stress.

METHODS

CBA/J mice were sensitized by intraperitoneal injection of ovalbumin (OVA) and challenged with OVA aerosol via the airways. Additionally, some mice were stressed by exposure to an ultrasonic stressor. Airway hyperreactivity (AHR) was measured in vitro by electric field stimulation (EFS) of tracheal smooth muscle elements. Bronchoalveolar lavage fluid (BAL) was obtained and cell numbers determined. Cytokine levels of IL-4, IL-5, and IFN-gamma in BAL were determined by ELISA.

RESULTS

Our findings demonstrate that exogenously applied stress dramatically enhances airway reactivity in OVA-sensitized and challenged mice. Further, stress significantly increases allergen-induced airway inflammation identified by increased leukocyte (ie, eosinophil) numbers in bronchoalveolar lavage fluids.

CONCLUSIONS

We found further evidence that stress can indeed exacerbate airway hyperreactivity and airway inflammation in an animal model of allergic bronchial asthma and now introduce a novel murine model to identify stress-triggered pathways, including mediators as neurohormones, neuropeptides, and markers of inflammation.

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.

Neurokinin-1 receptor antagonists CP-96,345 and L-733,060 protect mice from cytokine-mediated liver injury

Previously, we have shown that primary afferent sensory neurons are necessary for disease activity in T cell-mediated immune hepatitis in mice. In the present study, we analyzed the possible role of substance P (SP), an important proinflammatory neuropeptide of these nerve fibers, in an in vivo mouse model of liver inflammation. Liver injury was induced by bacterial lipopolysaccharide (LPS) in D-galactosamine (GalN)-sensitized mice. Depletion of primary afferent nerve fibers by neonatal capsaicin treatment down-regulated circulating levels of the proinflammatory cytokines tumor necrosis factor-alpha (TNFalpha) and interferon-gamma (IFNgamma) and protected mice from GalN/LPS-induced liver injury. Likewise, pretreatment of mice with antagonists of the SP-specific neurokinin-1 receptor (NK-1R), i.e., (2S,3S)-cis-2-(diphenylmethyl)-N-((2-methoxyphenyl)-methyl)-1-azabicyclo(2.2.2.)-octan-3-amine (CP-96,345) and (2S,3S)3-([3,5-bis(trifluoromethyl)phenyl]methoxy)-2-phenylpiperidine (L-733,060), dose dependently protected mice from GalN/LPS-induced liver injury. The presence of the NK-1R in the murine liver was demonstrated by reverse transcription-polymerase chain reaction, sequence analysis, and immunocytochemistry. NK-1R blockade reduced inflammatory liver damage, i.e., edema formation, neutrophil infiltration, hepatocyte apoptosis, and necrosis. To get further insight into the mechanism by which receptor blockade attenuated GalN/LPS-induced liver damage, we analyzed plasma levels and intrahepatic expression of TNFalpha, IFNgamma, interleukin (IL)-6, and IL-10. NK-1R blockade clearly inhibited GalN/LPS-induced production of TNFalpha and IFNgamma, whereas synthesis of the hepatoprotective cytokines IL-6 and IL-10 was increased. NK-1 receptor antagonists might be potent drugs for treatment of inflammatory liver disease, most likely by inhibiting SP effects.

Acid-induced mesenteric hyperemia in rats: role of CGRP, substance P, prostaglandin, adenosine, and histamine

Intraduodenal acidification produces a mesenteric hyperemia that is mediated in part by mucosal capsaicin-sensitive afferent nerves and the bradykinin B2 receptor in anesthetized rats. We hypothesized that novel mechanisms mediated by substance P, adenosine, and histamine1 receptors are involved. Confirmation of a role for calcitonin gene-related peptide (CGRP) but not endogenous prostaglandin was also sought. In study 1, vehicle or antagonists (CGRP(8-37), CP 96345) was administered intravenously. Capsaicin or acid was administered intraduodenally, followed by intravenous CGRP or substance P. In study 2, pretreatments included indomethacin, 8-phenyltheophylline, pyrilamine, or the respective vehicles. Acid was then administered intraduodenally. In both studies, superior mesenteric artery blood flow was monitored. In study 1, the antagonists significantly attenuated capsaicin- and acid-induced mesenteric hyperemia. In study 2, the pretreatments did not alter acid-induced hyperemia. The data confirmed the role of CGRP and indicated for the first time an involvement for substance P in acid-induced mesenteric hyperemia.

Pulmonary responses to tracheal or esophageal acidification in guinea pigs with airway inflammation

The association between asthma and gastroesophageal reflux has been attributed to microaspiration of gastric contents and/or vagally mediated reflex bronchoconstriction. In previous experimental studies concerning the pulmonary effects of tracheal or esophageal acid infusion, only animals without airway inflammation have been studied. We assessed the effects of esophageal and tracheal administration of hydrochloric acid (HCl) on normal guinea pigs (GP) and GP with airway inflammation induced by repeated ovalbumin exposures. These GP were anesthetized (pentobarbital sodium) and received 1) 20 microl of either 0.2 N HCl or saline into the trachea, or 2) 1 ml of either 1 N HCl or saline into the esophagus. Intratracheal HCl resulted in a significant increase in both respiratory system elastance and resistance (P < 0.001). There were no significant changes in respiratory mechanics when HCl was infused into the esophagus. In conclusion, we observed that infusion of large volumes of HCl into the esophagus did not change pulmonary mechanics significantly, even in guinea pigs with chronic allergen-induced airway inflammation. In contrast, intratracheal administration of small amounts of acid had substantial effects in normal GP and GP with airway inflammation.

The involvement of sensory neuropeptides in airway hyper-responsiveness in rabbits sensitized and challenged to Parietaria judaica

BACKGROUND

C-fibres have received considerable attention in the context of airway hyper-responsiveness (AHR), in fact several lines of evidence suggest that tachykinins might be involved in the pathogenesis of AHR.

OBJECTIVE

The aim of this study was to investigate the role of capsaicin-sensitive sensory C-fibres and tachykinins in rabbits sensitized to the major allergen of Parietaria judaica pollen (Par j1).

METHODS

Airway responsiveness was determined by exposing sensitized rabbits to cumulative concentrations of aerosolized histamine before and after an allergic challenge and after a pre-treatment with either vehicle or capsaicin or tachykinin receptor antagonists. Bronchoalveolar lavage was performed following histamine challenge and total and differential cell counts were performed.

RESULTS

In sensitized rabbits, an AHR to inhaled histamine was observed 24 h after a Par j1 challenge. Capsaicin pre-treatment inhibited the AHR achieved 24 h following antigen exposure (P < 0.01). Pre-treatment with the tachykinin NK2 receptor antagonist, SR 48968, significantly reduced the antigen-induced AHR (P < 0.05), while pre-treatment with tachykinin NK1 (SR 140333) and NK3 (SR 142801) receptor antagonists did not significantly modify it. Bronchoalveolar lavage fluid obtained from vehicle and capsaicin-treated rabbits challenged with Par j1 exhibited no significant differences in total and differential cell counts.

CONCLUSIONS

Parietaria judaica-induced AHR in immunized rabbits was shown to be inhibited by pre-treatment with capsaicin, an effect that is not related to an action on the associated pulmonary infiltration of eosinophils. The involvement of NK2 receptor stimulation in this phenomenon also suggests that NK2 receptor antagonists may be useful for investigating mechanisms of bronchopulmonary alterations in asthmatic patients.

Substance P in the descending cholinergic projection to REM sleep-induction regions of the rat pontine reticular formation: anatomical and electrophysiological analyses

Release of acetylcholine within the pontine reticular formation (PRF) from the axon terminals of mesopontine cholinergic neurons has long been hypothesized to play an important role in rapid eye movement (REM) sleep generation. As some of these cholinergic neurons are known to contain substance P (SP), we used anatomical, electrophysiological and pharmacological techniques to characterize this projection in the rat. Double immunofluorescence demonstrated that 16% of all cholinergic neurons within the mesopontine tegmentum contained SP; this percentage increased to 27% in its caudal regions. When double immunofluorescence was combined with retrograde tracing techniques, it was observed that up to 11% of all SP-containing cholinergic neurons project to the PRF. Whole-cell patch-clamp recordings from in vitro brainstem slices revealed that SP administration depolarized or evoked an inward current in a dose-dependent manner in all PRF neurons examined, and that these effects were antagonized by a SP antagonist. The amplitude of the SP-induced inward current varied with changes in the Na+ concentration, did not reverse at the calculated K+ or Cl- equilibrium potentials, and was not attenuated in the presence of tetrodotoxin, low Ca2+ concentration or caesium ions. These data suggest that activation of a tetrodotoxin-insensitive cation channel(s) permeable to Na+ is responsible for a SP-induced inward current at resting membrane potentials. The depolarizing actions of SP appeared to be primarily due to activation of the adenylate cyclase pathway, and were additive with cholinergic receptor activation even at maximal concentrations. These data indicate that SP is colocalized in a subpopulation of mesopontine tegmental cholinergic neurons projecting to REM sleep-induction regions of the PRF, and that actions of these two neuroactive substances on PRF neurons are additive. If SP is coreleased with acetylcholine, the additive actions of the two neurotransmitters might heighten the excitability of postsynaptic PRF neurons and ensure the initiation and maintenance of REM sleep.

Effects of human eosinophil granule-derived cationic proteins on C-fiber afferents in the rat lung

Experiments were performed to test the hypothesis that human eosinophil granule-derived cationic proteins stimulate vagal C-fiber afferents in the lungs and elicit pulmonary chemoreflex responses in anesthetized Sprague-Dawley rats. Intratracheal instillation of eosinophil cationic protein (ECP; 1-2 mg/ml, 0.1 ml) consistently induced an irregular breathing pattern, characterized by tachypnea (change in breathing frequency of 44.7%) and small unstable tidal volume (VT). The tachypnea, accompanied by decreased heart rate and arterial blood pressure, started within 30 s after the delivery of ECP and lasted for >30 min. These ECP-induced cardiorespiratory responses were completely prevented by perineural capsaicin treatment of both cervical vagi, which selectively blocked C-fiber conduction, suggesting the involvement of these afferents. Indeed, direct recording of single-unit activities of pulmonary C-fibers further demonstrated that the same dose of ECP evoked a pronounced and sustained (>30-min) stimulatory effect on pulmonary C-fibers. Furthermore, the sensitivity of these afferents to lung inflation was also markedly elevated after the ECP instillation, whereas the vehicle of ECP administered in the same manner had no effect. Other types of eosinophil granule cationic proteins, such as major basic protein and eosinophil peroxidase, induced very similar respiratory and cardiovascular reflex responses. In conclusion, these results show that eosinophil granule-derived cationic proteins induce a distinct stimulatory effect on vagal pulmonary C-fiber endings, which may play an important role in the airway hyperresponsiveness associated with eosinophil infiltration in the airways.

Effects of dexamethasone on antigen-induced airway eosinophilia and M(2) receptor dysfunction

In antigen-challenged guinea pigs, airway hyperreactivity is due to recruitment of eosinophils to the airway nerves and dysfunction of M(2) muscarinic receptors. M(2) receptor dysfunction is caused by eosinophil major basic protein, which is an allosteric antagonist at the receptor. Because glucocorticoids inhibit airway hyperreactivity in humans and in animal models of asthma, we tested whether dexamethasone treatment (6 microg. kg(-)(1). d(-)(1) for 3 d, intraperitoneal) before antigen challenge prevents M(2) receptor dysfunction and airway hyperreactivity. Guinea pigs were sensitized to ovalbumin via intraperitoneal injections, and were challenged with ovalbumin via inhalation. Twenty-four hours later, hyperreactivity and M(2) receptor function were tested. Antigen-challenged animals were hyperreactive to vagal stimulation, and demonstrated loss of M(2) receptor function. Dexamethasone pretreatment prevented hyperreactivity and M(2) receptor dysfunction in antigen-challenged guinea pigs. Antigen challenge resulted in recruitment of eosinophils to the airways and to the airway nerves. Dexamethasone prevented recruitment of eosinophils to the airway nerves but did not affect total eosinophil influx into the airways. These results demonstrate that dexamethasone prevents antigen-induced hyperreactivity by protecting neuronal M(2) muscarinic receptors from antagonism by eosinophil major basic protein, and this protective mechanism appears to be by specifically inhibiting eosinophil recruitment to the airway nerves.

Tachykinin receptor antagonists: potential in airways diseases

Several lines of evidence indicate a role for the tachykinin peptides in airways diseases. For instance, elevated levels of tachykinins have been recovered from the airways of patients with asthma and chronic obstructive pulmonary disease (COPD), and airway inflammation leads to an upregulation of the tachykinin NK1 and NK2 receptors. Recent advances in tachykinin receptor pharmacology have allowed a more detailed analysis of this system and preclinical animal studies have indicated a role for the NK1 and NK2 receptors in bronchoconstriction, airway hyperresponsiveness and airway inflammation caused by allergic and nonallergic stimuli. In the past three years, work has entered the clinic and selective or dual-selective NK1/NK2 receptor antagonists appear to have the potential to affect the different aspects of asthma and COPD.

Eosinophil recruitment to the airway nerves

Increased vagal reflexes contribute to bronchoconstriction in asthma. Antigen challenge of sensitized animals induces vagal hyperresponsiveness. This review will discuss the evidence that eosinophils increase release of acetylcholine from the parasympathetic nerves. After antigen challenge, eosinophils are actively recruited to the airway nerves, possibly through expression of chemotactic substances and adhesion molecules by the nerves. Tachykinins acting on neurokinin 1 receptors activate the eosinophils. Activated eosinophils release eosinophil major basic protein (MBP), which is an endogenous antagonist for M2 muscarinic receptors. The M2 muscarinic receptors on the parasympathetic nerves in the lungs normally inhibit release of acetylcholine. When M2 receptors are blocked by MBP, acetylcholine release is increased, resulting in hyperresponsiveness. Neutralization of MBP with polyanionic substances restores M2 receptor function and eliminates hyperresponsiveness. Antibodies to MBP prevent M2 receptor dysfunction and hyperresponsiveness, as do antibodies to the adhesion molecule very late antigen 4, which prevent eosinophil migration. A low dose of dexamethasone, which does not affect total eosinophil influx into the lungs and airways, prevents eosinophils from clustering around the nerves and prevents antigen-induced M2 dysfunction and hyperresponsiveness. Furthermore, animal studies show that viral infections, which are important precipitants of asthma attacks, and exposure to air pollutants such as ozone can also activate airway eosinophils, leading to a chain of events similar to that seen after antigen challenge. Finally, a similar clustering of eosinophils around airway nerves, as well as release of MBP onto the nerves, is seen in fatal asthma, suggesting that similar mechanisms may be involved in human airway hyperresponsiveness.

Neural integration and allergic disease

Changes in neural activity play a key role in many symptoms of allergic disease, including sneezing, coughing, itching, and ocular irritation, among others. The mechanisms underlying allergen-induced changes in neural activity (reflexes) are largely unknown and under active investigation. Allergic inflammation can affect neural activity on a variety of levels, including at the primary afferent sensory nerve, integrative centers of the central nervous system, autonomic ganglia, and autonomic neuroeffector junction. At the level of the afferent sensory nerve, mediators released after allergen exposure either directly or indirectly increase neuronal firing. At the level of sensory ganglia, which contain cell bodies that innervate a variety of organs, changes in neuronal excitability may lead to a generalization of allergic symptoms. In the central nervous system, where afferent inputs from throughout the body converge, allergic inflammation may be associated with central sensitization, leading to the modulation of the neural reflexes. Finally, at the autonomic ganglia and neuroeffector junction, allergic inflammation appears to be associated with enhanced ganglionic transmission and neurotransmitter release, respectively. Mechanisms by which allergen challenge affects neuronal activity at various levels of the nervous system are reviewed, with a primary emphasis on studies of airway physiologic factors.

Substance P-induced airway hyperreactivity is mediated by neuronal M(2) receptor dysfunction

Neuronal muscarinic (M(2)) receptors inhibit release of acetylcholine from the vagus nerves. Hyperreactivity in antigen-challenged guinea pigs is due to blockade of these M(2) autoreceptors by eosinophil major basic protein (MBP) increasing the release of acetylcholine. In vivo, substance P-induced hyperactivity is vagally mediated. Because substance P induces eosinophil degranulation, we tested whether substance P-induced hyperreactivity is mediated by release of MBP and neuronal M(2) receptor dysfunction. Pathogen-free guinea pigs were anesthetized and ventilated. Thirty minutes after intravenous administration of [Sar(9),Met(O(2))(11)]- substance P, guinea pigs were hyperreactive to vagal stimulation and M(2) receptors were dysfunctional. The depletion of inflammatory cells with cyclophosphamide or the administration of an MBP antibody or a neurokinin-1 (NK(1)) receptor antagonist (SR-140333) all prevented substance P-induced M(2) dysfunction and hyperreactivity. Intravenous heparin acutely reversed M(2) receptor dysfunction and hyperreactivity. Thus substance P releases MBP from eosinophils resident in the lungs by stimulating NK(1) receptors. Substance P-induced hyperreactivity is mediated by blockade of inhibitory neuronal M(2) receptors by MBP, resulting in increased release of acetylcholine.

Effects of neurokinin receptor antagonists in virus-infected airways

We investigated the effects of a neurokinin-1 (NK(1)) receptor antagonist (SR-140333) and a NK(2) receptor antagonist (SR-48968) on airway responsiveness and on the function of neuronal M(2) muscarinic receptors, which normally inhibit vagal acetylcholine release, in guinea pigs infected with parainfluenza virus. Antagonists were given 1 h before infection and daily thereafter. Four days later, bronchoconstriction induced by either intravenous histamine (which is partly vagally mediated) or electrical stimulation of the vagus nerves was increased by viral infection compared with control. In addition, the ability of the muscarinic agonist pilocarpine to inhibit vagally induced bronchoconstriction was lost in virus-infected animals, demonstrating loss of neuronal M(2) receptor function. Macrophage influx into the lungs was inhibited by pretreatment with both antagonists. However, only the NK(1) receptor antagonist prevented M(2) receptor dysfunction and inhibited hyperresponsiveness (measured as an increase in either vagally induced or histamine-induced bronchoconstriction). Thus virus-induced M(2) receptor dysfunction and hyperresponsiveness are prevented by a NK(1) receptor antagonist, but not by a NK(2) receptor antagonist, whereas both antagonists had similar anti-inflammatory effects.

Antigen-induced bronchial hyperresponsiveness in the rabbit is not dependent on M(2)-receptor dysfunction

We have assessed the effect of sensitization to allergen on airway smooth muscle responsiveness and acetylcholine (ACh) release from cholinergic nerves in tracheal preparations from rabbits immunized at birth to Alternaria tenuis and littermate control rabbits injected with saline. ACh release induced by EFS was significantly greater in tracheal preparations obtained from immunized rabbits compared with littermate controls. The ability of the muscarinic-receptor agonist, oxotremorine, to inhibit ACh release to EFS (4 Hz) was not altered by immunization. The contractile response evoked by electrical field stimulation (EFS), ACh and 5-hydroxytryptamine (5-HT) was not significantly altered in tracheal preparations from antigen immunized rabbits compared with littermate controls. Antigen challenge of immunized rabbits did not affect the release of ACh from isolated trachea following EFS, or the ability of oxotremorine to inhibit ACh release. Furthermore, antigen challenge of immunized rabbits failed to alter the contractile response to EFS or ACh, but reduced the contractile potency of 5-HT. These results demonstrate increased ACh release in tracheal preparations following immunization which had no functional consequence on airway smooth muscle responsiveness. Moreover, the increased release in ACh was not associated with an alteration in M(2)-receptor function. Thus, antigen-induced bronchial hyperresponsiveness in the rabbit does not appear to depend upon M(2)-receptor dysfunction.

Role of tachykinin NK2-receptor activation in the allergen-induced late asthmatic reaction, airway hyperreactivity and airway inflammatory cell influx in conscious, unrestrained guinea-pigs

1. In a guinea-pig model of allergic asthma, we investigated the involvement of the tachykinin NK2 receptors in allergen-induced early (EAR) and late (LAR) asthmatic reactions, airway hyperreactivity (AHR) after these reactions and inflammatory cell influx in the airways, using the selective non-peptide NK2 receptor antagonist SR48968. 2. On two different occasions, separated by a 1 week interval, ovalbumin (OA)-sensitized guinea-pigs inhaled either vehicle (3 min) or SR48968 (100 nM, 3 min) at 30 min before as well as at 5.5 h after OA provocation (between the EAR and LAR) in a random crossover design. 3. SR48968 had no significant effect on the EAR, but significantly attenuated the LAR by 44.2+/-16.4% (P<0.05) compared to saline control. 4. The NK2 receptor antagonist did not affect the OA-induced AHR to histamine after the EAR at 5 h after OA challenge (3.59+/-0.59 fold increase in histamine reactivity vs 3.79+/-0.61 fold increase in the controls, NS), but significantly reduced the AHR after the LAR at 23 h after OA challenge (1.59+/-0.24 fold increase vs 1.93+/-0.15 fold increase, respectively, P<0.05). 5. Bronchoalveolar lavage studies performed at 25 h after the second OA provocation showed that SR48968 significantly inhibited the allergen-induced infiltration of neutrophils (P<0.05) and lymphocytes (P<0.01) in the airways. 6. These results indicate that NK2 receptor activation is importantly involved in the development of the allergen-induced late (but not early) asthmatic reaction and late (but not early) AHR to histamine, and that NK2 receptor-mediated infiltration of neutrophils and lymphocytes in the airways may contribute to these effects.

Effects of inflammatory cells on neuronal M2 muscarinic receptor function in the lung

In the lungs, acetylcholine released from the parasympathetic nerves stimulates M3 muscarinic receptors on airway smooth muscle inducing contraction and bronchoconstriction. The amount of acetylcholine released from these nerves is limited locally by neuronal M2 muscarinic receptors. These neuronal receptors are dysfunctional in asthma and in animal models of asthma. Decreased M2 muscarinic receptor function results in increased release of acetylcholine and in airway hyperreactivity. Inflammation has long been associated with hyperreactivity and the role of inflammatory cells in loss of neuronal M2 receptor function has been examined. There are several different mechanisms for loss of neuronal M2 receptor function. These include blockade by endogenous antagonists such as eosinophil major basic protein, decreased expression of M2 receptors following infection with viruses or exposure to pro inflammatory cytokines such as gamma interferon. Finally, the affinity of acetylcholine for these receptors can be decreased by exposure to neuraminidase.