Effects of airway inflammation on cough response in the guinea pig

Advancing cough research: Methodological insights into cough challenge in guinea pig models using double chamber vs whole-body plethysmography

OBJECTIVE

This study compares two methods of citric acid-induced cough in guinea pigs in whole-body plethysmography (WBP) and double chamber plethysmography (DCP) to evaluate their efficacy.

METHODS

Sixteen specific pathogen-free (SPF) and sixteen conventionally-bred (CON) animals were exposed to 0.4 M citric acid aerosol. They underwent cough provocation using both DCP and WBP methods. The number of coughs and latency to the first cough were recorded and analysed using statistical methods to determine significant differences between the two techniques.

RESULTS

WBP resulted in significantly higher cough counts (WBP vs. DCP: 13±9 vs 2±3 for SPF; 14±8 vs 5±5 for CON; p<0.0001) and shorter latency (WBP vs. DCP: 59±6 s vs 159±14 s for SPF; 77±4 s vs 112±12 s for CON; p<0.0001) compared to DCP in both groups.

CONCLUSION

Methodological differences substantially impact cough responses. WBP provides a more reliable and physiologically relevant methodology for cough assessment, suggesting the need for standardized protocols in cough research to enhance translational relevance.

A vagal-brainstem interoceptive circuit for cough-like defensive behaviors in mice

Coughing is a respiratory behavior that plays a crucial role in protecting the respiratory system. Here we show that the nucleus of the solitary tract (NTS) in mice contains heterogenous neuronal populations that differentially control breathing. Within these subtypes, activation of tachykinin 1 (Tac1)-expressing neurons triggers specific respiratory behaviors that, as revealed by our detailed characterization, are cough-like behaviors. Chemogenetic silencing or genetic ablation of Tac1 neurons inhibits cough-like behaviors induced by tussive challenges. These Tac1 neurons receive synaptic inputs from the bronchopulmonary chemosensory and mechanosensory neurons in the vagal ganglion and coordinate medullary regions to control distinct aspects of cough-like defensive behaviors. We propose that these Tac1 neurons in the NTS are a key component of the airway-vagal-brain neural circuit that controls cough-like defensive behaviors in mice and that they coordinate the downstream modular circuits to elicit the sequential motor pattern of forceful expiratory responses.

An advanced recording and analysis system for the differentiation of guinea pig cough responses to citric acid and prostaglandin E2 in real time

Cough number and/or sound have been used to assess cough sensitivity/intensity and to discriminate cough patterns in clinical settings. However, to date, only manual counting of cough number in an offline manner is applied in animal cough studies, which diminishes the efficiency of cough identification and hinders the diagnostic discrimination of cough patterns, especially in animals with pulmonary diseases. This study aims to validate a novel recording/analysis system by which cough numbers are automatically counted and cough patterns are comprehensively differentiated in real time. The experiment was carried out in conscious guinea pigs exposed to aerosolized citric acid (CA, 150 mM) and prostaglandin E2 (PGE2, 0.43 mM). Animal body posture (video), respiratory flow, and cough acoustics (audio) were simultaneously monitored and recorded. Cough number was counted automatically, and cough sound parameters including waveform, duration, power spectral density, spectrogram, and intensity, were analyzed in real time. Our results showed that CA- and PGE2-evoked coughs had the same cough numbers but completely different patterns [individual coughs vs. bout(s) of coughs]. Compared to CA-evoked coughs, PGE2-evoked coughs possess a longer latency, higher cough rate (coughs/min), shorter cough sound duration, lower cough sound intensity, and distinct cough waveforms and spectrograms. A few mucus- and wheeze-like coughs were noted in response to CA but not to PGE2. In conclusion, our recording/analysis system is capable of automatically counting the cough number and successfully differentiating the cough pattern by using valuable cough sound indexes in real time. Our system enhances the objectivity, accuracy, and efficiency of cough identification and count, improves the intensity evaluation, and offers ability for pattern discrimination compared to traditional types of cough identification. Importantly, this approach is beneficial for assessing the efficacy of putative antitussive drugs in animals without or with pulmonary diseases, particularly in cases without significant change in cough number.

Neurophenotypes in Airway Diseases. Insights from Translational Cough Studies

RATIONALE

Most airway diseases, including chronic obstructive pulmonary disease (COPD), are associated with excessive coughing. The extent to which this may be a consequence of increased activation of vagal afferents by pathology in the airways (e.g., inflammatory mediators, excessive mucus) or an altered neuronal phenotype is unknown. Understanding whether respiratory diseases are associated with dysfunction of airway sensory nerves has the potential to identify novel therapeutic targets.

OBJECTIVES

To assess the changes in cough responses to a range of inhaled irritants in COPD and model these in animals to investigate the underlying mechanisms.

METHODS

Cough responses to inhaled stimuli in patients with COPD, healthy smokers, refractory chronic cough, asthma, and healthy volunteers were assessed and compared with vagus/airway nerve and cough responses in a cigarette smoke (CS) exposure guinea pig model.

MEASUREMENTS AND MAIN RESULTS

Patients with COPD had heightened cough responses to capsaicin but reduced responses to prostaglandin E2 compared with healthy volunteers. Furthermore, the different patient groups all exhibited different patterns of modulation of cough responses. Consistent with these findings, capsaicin caused a greater number of coughs in CS-exposed guinea pigs than in control animals; similar increased responses were observed in ex vivo vagus nerve and neuron cell bodies in the vagal ganglia. However, responses to prostaglandin E2 were decreased by CS exposure.

CONCLUSIONS

CS exposure is capable of inducing responses consistent with phenotypic switching in airway sensory nerves comparable with the cough responses observed in patients with COPD. Moreover, the differing profiles of cough responses support the concept of disease-specific neurophenotypes in airway disease. Clinical trial registered with www.clinicaltrials.gov (NCT 01297790).

Therapeutic effects of naringin in a guinea pig model of ovalbumin-induced cough-variant asthma

Naringin, a well known component isolated from Exocarpium Citri Grandis, has significant antitussive effects. Recently, Naringin exhibited novel anti-inflammatory effect in chronic inflammatory diseases. In this work, we firstly evaluated the effects of naringin on enhanced cough, airway hyper-responsiveness (AHR), and airway inflammation in an ovalbumin-induced experimental cough-variant asthma (CVA) model in guinea pigs. We investigated the effect of naringin (18.4 mg/kg, per os, single dose or consecutively) on cough to inhaled capsaicin after challenge with an aerosolized antigen in actively sensitized guinea pigs. The effect of naringin on AHR to inhaled methacholine was evaluated 24 h after cough determination. Airway inflammation was assessed via bronchoalveolar lavage fluid (BALF) cytology and lung histopathology. Naringin, given consecutively, significantly reduced ovalbumin-induced enhanced cough and AHR, inhibited the increases in the leukocytes, interleukin-4 (IL-4), IL-5, and IL-13 in BALF compared with the model group. Moreover, the pathologic changes in lung tissues were clearly ameliorated by naringin treatment. These results suggest that naringin may be a beneficial agent for CVA treatment.

Chronic cough and pain: Janus faces in sensory neurobiology?

Both chronic cough and chronic pain are critical clinical issues in which a large number of patients remain unsatisfied with available treatments. These conditions have considerable effects on sufferers' quality of life, who often show co-morbidities such as anxiety and depression. There is therefore a pressing need to find new effective therapies. The basic neurobiological mechanisms and pathologies of these two conditions show substantial homologies. However, whilst chronic pain has received a great deal of attention over the last few decades, the same cannot be said for the neurological underpinnings of chronic cough. There is a substantial literature around mechanisms of chronic pain which is likely to be useful in advancing knowledge about the pathologies of chronic cough. Here we compare the basic pain and cough pathways, in addition to the clinical features and possible pathophysiologies of each; including mechanisms of peripheral and central sensitisation which may underlie symptoms such as hyperalgesia and allodynia, and hypertussitvity and allotussivity. Due to the substantial overlap that emerges, it is likely that therapies may be effective over both areas.

An inexpensive system for evaluating the tussive and anti-tussive properties of chemicals in conscious, unrestrained guinea pigs

INTRODUCTION

Commercial whole-body plethysmography systems used to evaluate the anti-tussive potential of drugs employ sophisticated technology, but these systems may be cost prohibitive for some laboratories. The present study describes an alternative, inexpensive system for evaluating the tussive and anti-tussive potential of drugs in conscious, unrestrained guinea pigs.

METHODS

The system is composed of a transparent small animal anesthesia induction box fitted with a microphone, a camera and a pneumotachometer to simultaneously capture audio, video, air flow and air pressure in real time. Data acquisition and analysis was performed using free software for audio and video, and a research pneumotach system for flow and pressure. System suitability testing was performed by exposing conscious, unrestrained guinea pigs to nebulized aqueous solutions of a selective agonist for TRPV1 (citric acid) or a selective agonist for TRPA1 (AITC), with or without pre-treatment with a selective antagonist for TRPV1 (BCTC) or a selective antagonist for TRPA1 (HC-030031).

RESULTS

The system easily discerned coughs from other respiratory events like sneezes. System suitability test results are as follows: AITC caused 10.7 (SEM=1.4592) coughs vs. 5.8 (SEM=1.6553) when pre-treated with HC-030031 (P<0.05). Citric acid caused 12.4 (SEM=1.4697) coughs vs. 3.2 (SEM=1.3928) when pre-treated with BCTC (P<0.002).

DISCUSSION

We have described in detail an inexpensive system for evaluating the tussive and anti-tussive potential of chemicals in conscious, unrestrained guinea pigs. Suitability testing indicates that the system is comparable to a commercial whole-body plethysmography system for detecting and differentiating between coughs and sneezes. This system may provide some investigators a cost-conscious alternative to more expensive commercial whole-body plethysmography systems.

Cardiocrinum seeds as a replacement for Aristolochia fruits in treating cough

AIM OF THE STUDY

The antitussive Chinese herb Madouling derived from Aristolochia species is banned due to aristolochic acid-induced nephropathy. A substitute is found dispensed as Madouling in Taiwan. This study aims to determine the source plant and verify the antitussive properties of the Madouling substitute used in Taiwan.

MATERIALS AND METHODS

Forensically informative nucleotide sequencing (FINS) approach based on the trnL-trnF and psbA-trnH regions was applied to facilitate identification of the genuine species and substitute. The antitussive effect of both genuine Madouling and the substitute were evaluated in guinea pigs.

RESULTS

FINS approach based on the trnL-trnF and psbA-trnH regions readily identified the sample of Madouling in Taiwan to the seeds of Cardiocrinum giganteum var. yunnanense. Ethanol extracts of the substitute showed significant antitussive properties in guinea pigs.

CONCLUSION

Cardiocrinum seeds may have potential as a replacement of Aristolochia fruits.

Modulation of sensory nerve function and the cough reflex: understanding disease pathogenesis

To cough is a protective defence mechanism that is vital to remove foreign material and secretions from the airways and which in the normal state serves its function appropriately. Modulation of the cough reflex pathway in disease can lead to inappropriate chronic coughing and an augmented cough response. Chronic cough is a symptom that can present in conjunction with a number of diseases including chronic obstructive pulmonary disease (COPD) and asthma, although often the cause of chronic cough may be unknown. As current treatments for cough have proved to exhibit little efficacy and are largely ineffective, there is a need to develop novel, efficacious and safe antitussive therapies. The underlying mechanisms of the cough reflex are complex and involve a network of events, which are not fully understood. It is accepted that the cough reflex is initiated following activation of airway sensory nerves. Therefore, in the hope of identifying novel antitussives, much research has focused on understanding the neural mechanisms of cough provocation. Experimentally this has been undertaken using chemical or mechanical tussive stimuli in conjunction with animal models of cough and clinical cough assessments. This review will discuss the neural mechanisms involved in the cough, changes that occur under pathophysiological conditions and and how current research may lead to novel therapeutic opportunities for the treatment of cough.

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.

Effect of pressure stress applied to the airway on cough-reflex sensitivity in Guinea pigs

RATIONALE

We hypothesized that cough stress of the airway wall results in a self-perpetuating cough-reflex cycle in which antigen-induced increase in cough-reflex sensitivity results in pathologic cough, and the cough in turn further amplifies cough-reflex sensitivity.

OBJECTIVES

To examine cough-reflex sensitivity in an experimental animal model.

METHODS

We developed an experimental guinea pig model in which airway collapse similar to that in cough was induced by rapid negative pressure applied to the airway of artificially ventilated animals. We examined the influence of this stimulus on cough-reflex sensitivity to inhaled capsaicin and bronchoalveolar lavage (BAL) cell components. After the termination of artificial ventilation, the number of coughs due to capsaicin was measured, and BAL was performed.

MEASUREMENTS AND MAIN RESULTS

Capsaicin cough-reflex sensitivity and the number of BAL neutrophils were increased 6 hours after stimulus application, decreasing to control levels by 24 hours. Cough-reflex sensitivity or BAL cell components were not changed in the absence of stimulus application. The number of BAL neutrophils correlated significantly with the number of coughs. Hydroxyurea inhibited the stimulus-induced increase in the number of coughs and airway neutrophil accumulation.

CONCLUSIONS

Our findings suggest that cough itself is a traumatic mechanical stress to the airway wall that induces neutrophilic airway inflammation and cough-reflex hypersensitivity. Cough stress to the airway wall results in a self-perpetuating cough-reflex cycle.

The cough reflex in animals: relevance to human cough research

All mammalian species studied cough or display some similar respiratory reflex upon aerosol challenge with tussigenic stimuli such as citric acid or capsaicin. Animals cough to the same stimuli that evoke coughing in humans, and therapeutic agents that display antitussive effects in human studies also prevent coughing in animals. The many invasive procedures and complementary in vitro studies possible in animals but not readily reproduced in human subjects, along with the proven predictive value of cough studies in animals, provide the rationale for animal modeling of human cough. The advantages and disadvantages of studying cough in animals are discussed.

Effect of montelukast in a guinea pig model of cough variant asthma

Cough variant asthma is known as a major cause of chronic cough. Fundamental features of cough variant asthma are prolonged non-productive cough responding to bronchodilator therapy, no history of wheezing or dyspnea attack, normal cough sensitivity and slightly increased bronchial responsiveness. Recently, we reported the animal model of cough variant asthma. The aim of this study was to clarify the involvement of cysteinyl leukotrienes (cysLTs) in this model by using a specific leukotriene receptor antagonist, montelukast. Cough number and specific airway resistance (sRaw) were measured during the antigen inhalation (1.5 min) and following 18.5 min, which was carried out 72 h after the first antigen inhalation in actively sensitized guinea pigs, and then total cell number and cell differentials in bronchoalveolar lavage fluid (BALF) were measured. Montelukast significantly reduced the antigen re-inhalation-induced cough, increase in sRaw, and increase in total cell number in BALF. In conclusion, cysLTs may play an important part in antigen-induced cough associated with bronchoconstriction and airway inflammation in cough variant asthma.

Antitussive effects of Stemona tuberosa with different chemical profiles

The root tubers of Stemona tuberosa, Stemona japonica and Stemona sessilifolia are recognized by the Pharmacopoeia of the People's Republic of China as authentic sources of the herb Radix Stemonae (Baibu). Careful anatomical analyses of these three species, whose identities were confirmed by flowering and fruiting samples, revealed that the root tubers of Stemona tuberosa could be distinguished from those of the other two species by the presence of scattered fibers in the cortex and pith and by the absence of thickened striations on the surface of velamen cells. HPLC analyses demonstrated that the total alkaloid profiles could be grouped into four types represented as the major component by stenine-type Stemona alkaloids such as tuberostemonine (4) and neotuberostemonine (3), or by non-stenine types such as croomine (1) and stemoninine (2). Nevertheless, all these samples demonstrated different degrees of antitussive properties in guinea pigs. These results suggested that non-stenine-type of Stemona alkaloids also contributed to the antitussive properties. The variations in chemical profiles among herb samples add difficulty in ensuring quality control in botanical products.

Effects of eppikahangeto, a Kampo formula, and Ephedrae herba against citric acid-induced laryngeal cough in guinea pigs

To evaluate the efficacy of three common antitussive Kampo formulas, eppikahangeto (EPP), bakumondoto (BAK), and shoseiryutogomakyokansekito (SGM), a new cough model of guinea pig was used, which could specifically induce a laryngeal cough by microinjection of citric acid solution into the larynx. Kampo extract was dissolved in water and the animals were given access ad libitum for 3 days, and then the number of coughs during 10 min was counted. EPP extract decreased the number of coughs dose-dependently (0.3% extract, -22.9 +/- 6.6%, P<0.01; 1.0% extract, -32.4 +/- 5.5%, P<0.01). BAK extract and SGM extract had no significant effect. Intraperitoneal injection of codeine (60 mg/kg) also decreased the number of coughs (-36.1 +/- 9.1%, P<0.05). Furthermore, Ephedrae herba (EH) extract reduced the number of coughs (-18.3 +/- 6.0%, P<0.05), but the extract of EPP without EH did not. These results suggest that EPP has an antitussive effect against laryngeally-induced cough in guinea pigs, and the crucial herbal medicine is EH.

Reflex regulation of airway sympathetic nerves in guinea-pigs

Sympathetic nerves innervate the airways of most species but their reflex regulation has been essentially unstudied. Here we demonstrate sympathetic nerve-mediated reflex relaxation of airway smooth muscle measured in situ in the guinea-pig trachea. Retrograde tracing, immunohistochemistry and electrophysiological analysis identified a population of substance P-containing capsaicin-sensitive spinal afferent neurones in the upper thoracic (T1-T4) dorsal root ganglia (DRG) that innervate the airways and lung. After bilateral vagotomy, atropine pretreatment and pre-contraction of the trachealis with histamine, nebulized capsaicin (10-60 microm) evoked a 63+/-7% reversal of the histamine-induced contraction of the trachealis. Either the beta-adrenoceptor antagonist propranolol (2 microm, administered directly to the trachea) or bilateral sympathetic nerve denervation of the trachea essentially abolished these reflexes (10+/-9% and 6+/-4% relaxations, respectively), suggesting that they were mediated primarily, if not exclusively, by sympathetic adrenergic nerve activation. Cutting the upper thoracic dorsal roots carrying the central processes of airway spinal afferents also markedly blocked the relaxations (9+/-5% relaxation). Comparable inhibitory effects were observed following intravenous pretreatment with neurokinin receptor antagonists (3+/-7% relaxations). These reflexes were not accompanied by consistent changes in heart rate or blood pressure. By contrast, stimulating the rostral cut ends of the cervical vagus nerves also evoked a sympathetic adrenergic nerve-mediated relaxation that were accompanied by marked alterations in blood pressure. The results indicate that the capsaicin-induced reflex-mediated relaxation of airway smooth muscle following vagotomy is mediated by sequential activation of tachykinin-containing spinal afferent and sympathetic efferent nerves innervating airways. This sympathetic nerve-mediated response may serve to oppose airway contraction induced by parasympathetic nerve activation in the airways.

Modulation of citric acid-induced cough following lipopolysaccharide-mediated neutrophilia in the guinea pig

This investigation examined a possible correlation between lipopolysaccharide (LPS)-induced pulmonary neutrophilia and cough. Conscious male guinea pigs were acutely exposed to aerosolized LPS and thereafter at various times challenged with citric acid aerosol (CA; 250mM) to induce cough followed by bronchoalveolar lavage (BAL) to quantitate inflammatory cell accumulation. LPS caused a hyporesponsive cough at 24h post-LPS with neutrophilia apparent from 2h post-LPS. By 96h post-LPS both cough and neutrophilia had returned towards normal. Dexamethasone (DEX, 2mgkg(-1)/day for 3 days prior) did not affect the cough hyporesponsiveness at 24h; however it attenuated LPS-induced BAL fluid neutrophilia. Since LPS can stimulate inducible nitric oxide synthase (iNOS) we hypothesized that the cough hyporesponsiveness may involve nitric oxide. To investigate this we treated animals with an aerosolized iNOS inhibitor 1400W (1mM) immediately prior to LPS. 1400W had no significant effect on either cough hyporesponsiveness or BAL fluid neutrophilia at 24h post-LPS. Despite differing effects on neutrophilia, these findings clearly indicate that neither DEX nor iNOS inhibition had any direct effect on LPS-induced cough hyporesponsiveness. The mechanism underlying the LPS-induced cough hyporesponsiveness does not appear to be directly linked to LPS-induced neutrophilic inflammation.

Mechanisms of capsaicin- and citric-acid-induced cough reflexes in guinea pigs

Previously, we developed a novel coughing model that evoked coughs via citric acid microinjection into the larynx of unanesthetized, unrestrained guinea pigs. Here, we compared the effects of capsaicin and citric acid administration into the larynx using this model. Inhalation of capsaicin (30 microM) or citric acid (0.4 M) for 5 min induced cough mimetic responses over a 10-min observation period (mean +/- S.E.M.: 8.85 +/- 1.60 and 10.40 +/- 1.17 coughs, respectively, n = 18). Microinjection of 0.4 M citric acid into the larynx (20 microl in total in 10 aliquots of 2 microl at 30-s intervals) induced 27.29 +/- 2.69 coughs over a 10-min observation period, whereas microinjection of 3 mM capsaicin evoked a maximum of 8.32 +/- 1.82 coughs (n = 18). Desensitization of C-fibers by capsaicin (100 mg/kg, s.c.) abolished the coughs evoked upon inhalation or microinjection of capsaicin and inhalation of citric acid, but had no significant effect on those induced by microinjection of citric acid. These results indicate that there were fewer non-myelinated C-fiber afferents in the larynx area than in other regions (bronchi or alveoli). Consequently, citric acid microinjected into the larynx may induce the cough reflex by stimulating other fibers (for example, Adelta-fibers), rather than C-fibers. In contrast, inhaled citric acid predominantly stimulated C-fibers, and hardly affected Adelta-fibers.

Reduction of antigen-induced respiratory abnormalities and airway inflammation in sensitized guinea pigs by a superoxide dismutase mimetic

Reactive oxygen species have been implicated in the pathogenesis of asthma and, in atopic asthmatics, endogenous superoxide dismutase (SOD) enzyme levels are known to decrease. This suggests that replacing a failed endogenous SOD enzyme system with a mimetic of the endogenous enzyme would be beneficial and protective. In this study we demonstrate that removal of superoxide by the SOD mimetic (SODm) M40403 reduces the respiratory and histopathological lung abnormalities due to ovalbumin (OA) aerosol in a model of allergic asthma-like reaction in sensitized guinea pigs. Both respiratory abnormalities and bronchoconstriction in response to OA challenge are nearly absent in naïve animals, while they sharply became severe in sensitized animals. In addition, OA aerosol induced a reduction of MnSOD activity which was paralleled with bronchiolar lumen reduction, pulmonary air space hyperinflation, mast cell degranulation, eosinophil infiltration, bronchial epithelial cell apoptosis, increase in myeloperoxidase activity, malonyldialdehyde production and 8-hydroxy-2'-deoxyguanosine formation in the lung tissue, as well as elevation of PGD2 in the bronchoalveolar lavage fluid. Treatment with M40403 restored the levels of MnSOD activity and significantly reduced all the above parameters. In summary, our findings support the potential therapeutic use of SOD mimetics in asthma and anaphylactic reactions and account for a critical role for superoxide in acute allergic asthma-like reaction in actively sensitized guinea pig.

Effects of antitussive agents administered before bronchoalveolar lavage in horses

OBJECTIVE

To determine whether treatment of horses with antitussive agents before bronchoalveolar lavage (BAL) reduces the frequency and intensity of the cough reflex during BAL.

ANIMALS

8 healthy horses.

PROCEDURE

Standard BAL was performed on each horse weekly for 6 weeks. Detomidine was used as a general sedative, and various antitussive agents were evaluated for their suitability to suppress undesirable coughing. Treatments administered prior to BAL consisted of saline (0.9% NaCl) solution (control treatment), codeine, butorphanol tartrate, glycopyrrolate, lidocaine hydrochloride (final concentration, 0.33%), and lidocaine hydrochloride at a final concentration of 0.66% (lidocaine 0.66%). Frequency and intensity of coughing were digitally recorded throughout the BAL procedure. The volume of BAL fluid collected was measured, and the fluid was cytologically examined to assess potential effects of the medications on composition.

RESULTS

Coughing frequency was significantly reduced after intratracheal administration of lidocaine 0.66%. Moreover, intratracheal administration of lidocaine 0.66% or IV administration of butorphanol resulted in a significant reduction in the intensity of coughing episodes. All other treatments failed to significantly suppress coughing frequency and intensity, compared with results for the saline treatment. Glycopyrrolate caused obvious adverse clinical effects. Treatments did not influence the volume of BAL fluid collected nor composition of the fluid.

CONCLUSIONS AND CLINICAL RELEVANCE

Intratracheal administration of lidocaine (final concentration, 0.66%) proved to be the most reliable method to reduce frequency and intensity of coughing in horses during BAL.

Cough reflex in allergic dogs

This study investigated the effects of antigen challenge on the cough reflex in dogs that were neonatally sensitized to ragweed. Tidal volume (V(T)), respiratory rate (f), pulmonary resistance (R(L)), dynamic lung compliance (C(Dyn)) and the number and amplitude (increase in mean peak expiratory pressure) of coughs induced by mechanical stimulation of the intrathoracic trachea were measured in propofol-anesthetized dogs. Aerosolized ragweed challenge had no effect to induce spontaneous cough but increased f and R(L) and reduced V(T) and C(Dyn). Mechanical stimulation of the intrathoracic trachea at this time produced 19+/-5 coughs with an average increase in cough amplitude of 11+/-1 cm H(2)O which differed significantly from the number (9+/-2 coughs) and amplitude (30+/-5.5 cm H(2)O) of mechanically induced coughs after treatment with aerosolized saline. Both the number and amplitude of mechanically induced coughs returned to baseline values by 24-48 h after the ragweed challenge. Similar results were obtained after challenge with aerosolized histamine (0.3-1% histamine) that did not induce spontaneous coughs but increased f, reduced V(T) and decreased C(Dyn) and increased the number but reduced the amplitude of the mechanically induced coughs. In conclusion, both antigen and histamine bronchoprovocation changed the characteristics of the mechanically induced cough in dogs to a response of increased cough number but reduced mean expiratory cough amplitude.

The role of substance P and bradykinin in the cough reflex and bronchoconstriction in guinea-pigs

In this study we investigated the ability of aerosolized substance P to induce either cough or bronchoconstriction in guinea-pigs. We have also examined whether pre-treatment, by the inhaled route, of animals with a combination of the neutral endopeptidase inhibitor, phosphoramidon (10(-3) M), and the diaminopeptidase IV inhibitor, diprotin A (10(-3) M), enhances the airway response to substance P. Moreover, we also assessed whether aerosol pre-treatment of guinea-pigs with either substance P or bradykinin, at 10(-4) M, affects the citric acid-induced cough and/or bronchoconstriction. Challenge of guinea-pigs with substance P only at 10(-3) M resulted in significant bronchconstriction but only a weak and variable cough response (1.1+/-0.6; P>0.05). Pre-treatment of guinea-pigs with both phosphoramidon and diprotin A resulted in a small non-significant increase in the cough response (2.8+/-0.9 vs. 1.1+/-0.6; P>0.05) but significantly enhanced substance P-induced bronchoconstriction (P<0.05). Moreover, exposure of guinea-pigs to substance P (10(-4) M) prior to citric acid challenge (0.6 M) resulted in a significant (P<0.05) enhancement of the citric acid-induced bronchoconstriction but not the citric acid-induced cough (11.7+/-1.8 vs. 12.8+/-1.5; P>0.05). In contrast, exposure of guinea-pigs to bradykinin (10(-4) M) prior to the citric acid challenge resulted in a significant enhancement of the cough response (9.2+/-1.9 vs. 25.8+/-2.5; P<0.05) but not the bronchoconstriction (P>0.05). These data do not support a major peripheral role for substance P in the cough reflex, although bradykinin is able to sensitize the cough reflex. Furthermore, these data suggest that bronchoconstriction, induced by citric acid, is not responsible for the cough associated with this irritant.

Experimental models and mechanisms of enhanced coughing

Enhanced coughing can be produced in a variety of animal models, including the guinea pig, cat, dog and pig. Typically, airway inflammation has been produced by sensitization, exposure to cigarette smoke, sulphur dioxide or angiotensin-converting enzyme inhibitors. In some of these models, inflammatory mediators such as bradykinin and tachykinins have been shown to contribute to the enhanced coughing. While most of these studies have focussed on peripheral mechanisms, increases in central excitability of the cough reflex have been shown to occur as a result of airway inflammation. As such, we propose that enhanced coughing in pathological conditions is the result of plastic changes in both peripheral and central neural elements. Furthermore, we present a modified model of the neurogenesis of cough that takes into account peripheral and central plasticity induced by mediators of inflammation.

Phrenic and iliohypogastric nerve discharges during tussigenic stimulation in paralyzed and decerebrate guinea pigs and rats

Although effects of antitussive drugs have been examined in inbred small animals using a whole body plethysmography, neuronal mechanisms underlying the cough reflex are not fully understood. The present study analyzed the reflex discharge patterns of the phrenic (PN) and iliohypogastric nerves (IHN) evoked in decerebrate and paralyzed guinea pigs and rats. In guinea pigs, electrical stimulation of the superior laryngeal nerve, chemical stimulation with capsaicin and mechanical stimulation to the intratracheal mucosa equally produced a serial PN-IHN response. This response was characterized by an increased PN discharge and following spindle-shaped burst of the IHN. The evoked discharges overlapped for 20 ms. In rats, by contrast, mechanical stimulation was without effect while capsaicin and electrical stimulation produced two types of responses, both of which differed from that observed in guinea pigs. The first type consisted of an augmented burst of the IHN that was immediately followed by an increased PN discharge. The second type was a large spindle-shaped burst of the IHN that occurred 80 ms after the end of the preceding PN discharge. Codeine (3 mg/kg i.v.) depressed all types of responses evoked in guinea pigs and rats. The present study demonstrated that the fictive cough comparable with those induced in other experimental animals was produced consistently in guinea pigs, but not in rats. Therefore, guinea pigs are suitable for investigation of the neuronal mechanisms underlying the cough reflex and assessment of antitussive drugs.

Inhibition of poly(ADP-ribose) polymerase prevents allergen-induced asthma-like reaction in sensitized Guinea pigs

Poly(ADP-ribose) polymerase (PARP) plays an important role in tissue injury in conditions associated with oxidative stress and inflammation. Because asthma is a chronic inflammatory disorder of the airways, we designed the present experimental study to evaluate the effects of PARP inhibition on allergen-induced asthma-like reaction in ovalbumin-sensitized guinea pigs. Cough and dyspnea in response to ovalbumin aerosol were absent in naive guinea pigs, whereas they became severe in the sensitized animals. In the latter ones, ovalbumin aerosol also induced a rapid increase in PARP activity, bronchiolar constriction, pulmonary air space inflation, mast cell degranulation, poly(ADP-ribose) and nitrotyrosine immunostaining, myeloperoxidase activity, and malondialdehyde in lung tissue, as well as a rise in the amounts of nitrites and tumor necrosis factor-alpha in bronchoalveolar lavage fluid. Pretreatment with the PARP inhibitors 3-aminobenzamide (10 mg/kg b.wt.) or 5-aminoisoquinolinone (0.5 mg/kg b.wt.) given i.p. 3 h before ovalbumin challenge significantly reduced the severity of cough and the occurrence of dyspnea and delayed the onset of respiratory abnormalities. Both PARP inhibitors were also able to prevent the above morphological and biochemical changes of lung tissue or bronchoalveolar lavage fluid induced by ovalbumin challenge. Conversely, p-aminobenzoic acid, the inactive analog of 3-aminobenzamide, had no effects.

Inhibition of guinea-pig and human sensory nerve activity and the cough reflex in guinea-pigs by cannabinoid (CB2) receptor activation

1. There is considerable interest in novel therapies for cough, since currently used agents such as codeine have limited beneficial value due to the associated side effects. Sensory nerves in the airways mediate the cough reflex via activation of C-fibres and RARs. Evidence suggests that cannabinoids may inhibit sensory nerve-mediated responses. 2. We have investigated the inhibitory actions of cannabinoids on sensory nerve depolarisation mediated by capsaicin, hypertonic saline and PGE2 on isolated guinea-pig and human vagus nerve preparations, and the cough reflex in conscious guinea-pigs. 3. The non-selective cannabinoid (CB) receptor agonist, CP 55940, and the selective CB2 agonist, JWH 133 inhibited sensory nerve depolarisations of the guinea-pig vagus nerve induced by hypertonic saline, capsaicin and PGE2. These responses were abolished by the CB2 receptor antagonist SR144528, and unaffected by the CB1 antagonist SR141716A. Similarly, JWH 133 inhibited capsaicin-evoked nerve depolarisations in the human vagus nerve, and was prevented by SR144528. 4. Using a guinea-pig in vivo model of cough, JWH 133 (10 mg kg-1, i.p., 20 min) significantly reduced citric acid-induced cough in conscious guinea pigs compared to those treated with the vehicle control. 5. These data show that activation of the CB2 receptor subtype inhibits sensory nerve activation of guinea-pig and human vagus nerve, and the cough reflex in guinea-pigs, suggesting that the development of CB2 agonists, devoid of CB1-mediated central effects, will provide a new and safe antitussive treatment for chronic cough.

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.

Coughing, mucus accumulation, airway obstruction, and airway inflammation in control horses and horses affected with recurrent airway obstruction

OBJECTIVE

To investigate relationships between cough frequency and mucus accumulation, airway obstruction, and airway inflammation and to determine effects of dexamethasone on coughing and mucus score.

ANIMALS

13 horses with recurrent airway obstruction (RAO) and 6 control horses.

PROCEDURE

6 RAO-affected and 6 control horses were stabled for 3 days. Coughing was counted for 4 hours before and on each day horses were stabled. Before and on day 3 of stabling, tracheal mucus accumulation was scored, airway obstruction was assessed via maximal change in pleural pressure (deltaPpl(max)), and airway inflammation was evaluated by use of cytologic examination of bronchoalveolar lavage fluid (BALF). Effects of dexamethasone (0.1 mg/kg, IV, q 24 h for 7 days) were determined in 12 RAO-affected horses.

RESULTS

To assess frequency, coughing had to be counted for 1 hour. In RAO-affected horses, stabling was associated with increases in cough frequency, mucus score, and deltaPpl(max). Control horses coughed transiently when first stabled. In RAO-affected horses, coughing was correlated with deltaPpl(max), mucus score, and airway inflammation and was a sensitive and specific indicator of deltaPpl(max) > 6 cm H2O, mucus score > 1.0, and > 100 neutrophils/microL and > 20% neutrophils in BALF Dexamethasone reduced cough frequency, mucus score, and deltaPpl(max), but BALF neutrophil count remained increased.

CONCLUSIONS AND CLINICAL RELEVANCE

Because of its sporadic nature, coughing cannot be assessed accurately by counting during brief periods. In RAO-affected horses, coughing is an indicator of airway inflammation and obstruction. Corticosteroid treatment reduces cough frequency concurrently with reductions in deltaPpl(max) and mucus accumulation in RAO-affected horses.

Cough Reflex Induced by Microinjection of Citric Acid Into the Larynx of Guinea Pigs: New Coughing Model

We developed a new coughing model that evoked coughs by microinjection of citric acid into the larynx in unanesthetized unrestrained guinea pigs; additionally, we recorded synchronous sounds and waveforms of coughing utilizing built-in microphones and a whole body plethysmograph. The coughing model was able to distinguish a coughing response from other expiratory responses, such as an expiratory reflex or a sigh, by examining the waveform of the expiratory response and the existence of sound. It was not necessary to distinguish a cough from a sneeze, since the administration site was restricted to the larynx. Microinjection of 0.4 M citric acid, total of 20 microl (10 times, 2 microl at 30-s intervals), induced coughs (27.03 +/- 4.03 coughs in 10-min observation) that were stable and independent of the inhalation volume. In the inhalation studies, animals were exposed to citric acid only once, because the number of coughs remarkably decreased with repeated administration at intervals of 24 h (tachyphylaxis). However our coughing model was able to repeatedly challenge the microinjection of citric acid at an interval of 24 h. These results indicated that this coughing model was highly sensitive and correctly assessed the cough response.

Sensory nerves and airway inflammation: role of A delta and C-fibres

It is generally accepted that stimulation of primary afferent sensory neurons, that innervate the airways, by chemical and mechanical stimuli leads to a range of homeostatic and defensive reflexes such as cough. However, there is still much debate regarding the exact type of sensory fibre involved in evoking these reflex events. The current dogma suggests that the major fibre types implicated in participating in reflex events of a protective nature are the A delta fibres and those stimulated in response to inflammation by noxious stimuli and mediators associated with tissue damage are the unmyelinated C-fibres. Furthermore, the C-fibre afferents are also believed to be responsible for mediating local axon reflexes, the release of neuropeptides and neurogenic inflammation. This review will concentrate on describing the characteristics of these sensory fibres and their proposed role in airway defensive reflexes and their possible exaggerated function in response to the inflammatory process.

[Coughing model by microinjection of citric acid into the larynx in guinea pig]

Many studies of cough were performed under the restrained or anesthetized condition, and coughs were evoked by inhalation of capsaicin or citric acid. Inhalation of irritants induced by "diving response" with apnea and coughs, and these responses induced a change of tidal volume. As a result, respiratory responses are dependent on the inhalation volume. Therefore we developed a new coughing model, and coughs were evoked by microinjection of citric acid into the larynx in the unanesthetized unrestrained guinea pig. Microinjection of 7.5% citric acid (2 microliters/30 s, 5 min) induced coughs (27.03 +/- 4.03 coughs/10 min), and citric acid-induced responses were stable independent of the inhalation volume. In the inhalation studies, animals were exposed to citric acid only once because induced-responses were remarkably decreased by repeated administration at an interval of 24 h. However in our coughing model it was possible to repeatedly challenge the animals by microinjection of citric acid at intervals of 24 h. Microinjection of citric acid into the larynx induced coughs in Sprague-Dawley rats, but inhalation of citric acid did not induce cough. These results indicate that this coughing model is highly sensitive and correctly assessed cough responses.

Capsaicin cough sensitivity in allergic asthmatic patients increases during the birch pollen season

BACKGROUND

A change in neural responsiveness may occur as the result of allergic inflammation in the lower airways as well as in the upper airways. In the lower airways, capsaicin cough sensitivity is known to reflect sensory neural reactivity.

OBJECTIVE

The aim of this study was to establish whether allergic inflammation changes airway neural sensory reactivity during prolonged allergen exposure.

METHODS

Ten nonsmoking patients with birch pollen-allergic asthma performed a capsaicin inhalation challenge twice, once in the off-pollen season and once during the pollen season. The number of coughs and symptoms induced by capsaicin were recorded and compared with those of healthy control subjects.

RESULTS

The response to capsaicin, expressed as number of coughs, increased in a dose-dependent manner during both tests. Before the season, the response was similar to that of healthy control subjects, but during the pollen season, the reactivity was significantly increased. Variations in forced expiratory volume in 1 second were not significant before and after each challenge, and values did not change during the pollen season as compared with the winter season.

CONCLUSIONS

Sensory reactivity in allergic asthmatic patients may be increased during prolonged allergen exposure as during the pollen season. This finding suggests that allergic inflammation in the lower and/or upper airways may trigger neurogenic mechanisms of significant clinical importance.

Antitussive effect of the leukotriene receptor antagonist zafirlukast in subjects with cough-variant asthma

Cough-variant asthma (CVA) occurs in a subgroup of asthmatics whose sole or predominant respiratory symptom is cough. Although bronchodilators are often sufficient to treat CVA, refractory cough may require therapy with inhaled or systemic corticosteroids. In a randomized, double-blind, placebo-controlled, crossover study, we examined the effect of a 14-day course of the leukotriene receptor antagonist zafirlukast on subjective cough score and cough-reflex sensitivity to inhaled capsaicin in eight subjects with CVA refractory to inhaled beta agonists, and in five subjects refractory to inhaled corticosteroids. Seven of eight subjects experienced significant subjective and objective improvement in cough after treatment with zafirlukast. Mean (+/- SEM) cough score improved from 7.75 +/- 0.56 to 3.25 +/- 0.84 (p = 0.0006). Cough sensitivity to capsaicin was suppressed by zafirlukast in all subjects. Patients with CVA may represent a distinct subgroup of asthmatics whose afferent cough receptors within the respiratory epithelium are hypersensitive relative to those of patients with the typical form of asthma. Zafirlukast appears to be particularly effective in treating CVA by inhibiting the sensitivity of these receptors. Leukotriene receptor antagonists may offer an alternative to corticosteroids for the treatment of CVA refractory to inhaled bronchodilators.

Involvement of thromboxane A(2) in airway mucous cells in asthma-related cough

The aim of this study was to elucidate the role of thromboxane A(2) (TxA(2)) on asthma-related cough in guinea pigs. Animals were immunosensitized and repeatedly challenged with ovalbumin as an antigen. Coughs were induced by the inhalation of 10(-5) M capsaicin solution for 10 min. Thromboxane synthetase (TxS) inhibitor OKY-046 and thromboxane-receptor antagonist AA-2414 significantly inhibited cough responses in repeatedly challenged animals. Inhalation of TxA(2) mimic STA-2- potentiated cough responses in normal and immunosensitized animals but not in repeatedly challenged ones. Moreover, STA-2-potentiated coughs were inhibited by administration of neurokinin-receptor antagonist FK-224. In repeatedly challenged animals, concentration of TxB(2) in airway lavage fluid, expression of TxS mRNA in tracheal epithelia, and the immunostaining intensity against TxS in mucous cells of the epithelium significantly increased compared with normal and sensitized animals. These results suggest that TxA(2) derived from mucous cells potentiated cough responses to capsaicin in allergic airway inflammation.

[Physiopathology of the cough]

Cough is initiated from organs innervated by the vagus nerve. The larynx and the lower airways are the main sites involved in cough initiation. Cough is mainly triggered by mechanical stimuli, especially at the laryngeal level, and also by inflammatory stimuli and chemical irritants. Two receptor types are involved in the cough reflex: the irritant receptors (IR) and the C fibers. IR are directly involved in cough initiation whereas stimulated C fibers act essentially through tachykinin release, which in turn activates IR. Cough in asthma was initially thought to be related to the bronchospasm reflex, but seems in fact mainly due to airway inflammation.

Characterization of increased cough sensitivity after antigen challenge in guinea pigs

Increased sensitivity of cough reflex is a fundamental feature of bronchodilator resistant non-productive cough associated with eosinophilic tracheobronchitis. Our hypothesis is that cough sensitivity is increased by airway allergic reaction characterized by airway eosinophilic inflammation. The aim of this study was to elucidate the hypothesis and clarify the characteristics of the increased cough sensitivity. Number of coughs elicited by inhalation of increasing concentrations of capsaicin (10-8, 10-6 and 10-4 M) was counted 24 h after an aerosolized antigen or saline in actively sensitized or non-sensitized (naive) conscious guinea pigs and then bronchoalveolar lavage was performed. The cough response was also measured 1 day before and 1, 2, 3, 5 and 7 days after an aerosolized antigen challenge in sensitized or naive animals. In addition, effect of procaterol (0.1 mg/kg), atropine (1 or 10 mg/kg), phosphoramidon (2.5 mg/kg) given intraperitoneally 30 min before the capsaicin challenge or capsaicin desensitization on the cough response was examined. Furthermore, the thromboxane A2 (TXA2) receptor antagonist S-1452 in a dose of 0.01 or 0.1 mg/kg or vehicle (saline) was given intraperitoneally at 24 and 1 h before the measurement of cough response. Number of coughs caused by capsaicin was extremely increased 24 h after an antigen challenge in sensitized guinea pigs compared with a saline or an antigen challenge in naive animals or a saline challenge in sensitized animals. The increased cough response disappeared at 3-7 days after the antigen challenge. Eosinophils in bronchoalveolar lavage fluid obtained after the measurement of capsaicin-induced coughs, which was performed 24 h after the antigen challenge, were significantly increased in sensitized guinea pigs. The eosinophil count was significantly correlated to the number of capsaicin-induced coughs. Procaterol or atropine did not alter the antigen-induced increase of cough sensitivity, whereas atropine did reduce the cough response in naive animals. Phosphoramidon increased the number of capsaicin-induced coughs in naive guinea pigs but not in sensitized and antigen-challenged animals. Capsaicin desensitization decreased the cough response in both antigen-challenged sensitized guinea pigs and naive animals. S-1452 reduced the antigen-induced increase of cough response in sensitized guinea pigs, but not in naive animals. Airway allergy accompanied with airway eosinophilia induces transient increase in cough sensitivity, which is not mediated by bronchoconstriction. The increased cough sensitivity may result in part from inactivation of neutral endopeptidase and TXA2, one of the inflammatory mediators.

Role of bradykinin and tachykinins in the potentiation by enalapril of coughing induced by citric acid in pigs

Angiotensin-converting enzyme (ACE) inhibitors are among the first-choice drugs for treating hypertension and congestive heart disease. It has been reported, however, that these drugs could induce chronic cough and airway hyperresponsiveness. The aim of this work was to assess in pigs the effects of bradykinin and tachykinins on citric-acid-induced coughing after ACE inhibitor pretreatment. Coughing was induced by challenging pigs with an aerosol of 0.8 M citric acid over 15 min. Coughs were counted by a trained observer for 30 min. The animals underwent two cough induction tests two days apart (days 1 and 3), the first being taken as a control. All drugs were injected intravenously 30 min before the second challenge. In the control group, no difference was observed between days 1 and 3. The ACE inhibitor enalapril (7.5 and 15 microg/kg) caused the cough frequency to increase significantly. In contrast, a dose-related decrease was observed with Hoe140 (icatibant), a bradykinin B2 receptor antagonist (0.5 and 1 mg/kg). When both drugs were administered simultaneously (15 microg/kg for enalapril and 1 mg/kg for Hoe140), a significant increase was observed as compared with the control value obtained on day 1. When enalapril was combined with the three tachykinin receptor antagonists SR 140333 (NK1 receptor antagonist), SR 48968 (NK2 receptor antagonist) and SR 142801 (NK3 receptor antagonist), a significant decrease was observed as compared with control value obtained on day 1; the percentage of variation was also significantly different as compared with those observed in enalapril groups at both doses. These data suggest that ACE-inhibitor-induced enhancement of the cough reflex is mainly due to tachykinins and not to bradykinin in our pig model. Bradykinin, however, plays a major role in coughing induced by citric acid alone.

Role of substance P and tachykinin receptor antagonists in citric acid-induced cough in pigs

The purpose of this work was to investigate the role of tachykinins in cough induced by citric acid (0.8 M) in pigs. With this object, we have studied the effect of citric acid on substance P content in the tracheo-bronchial tree and the effects of substance P and of tachykinin receptor antagonists on citric acid-induced cough. Citric acid exposure significantly increased substance P concentration in both broncho-alveolar and tracheal lavage fluids, while it decreased significantly the substance P content in tracheal mucosa. Substance P did not elicit cough, but significantly potentiated the citric acid-induced cough frequency. Tachykinin NK(1), NK(2) or NK(3) receptor antagonists, SR 140333 (nolpitantium), SR 48968 (saredutant) and SR 142801 (osanetant), respectively, significantly inhibited citric acid-induced cough. The same inhibitory effect of tachykinin receptor antagonists was observed, when substance P was nebulised before citric acid challenge. We conclude that citric acid induces in pigs a release of substance P in the tracheo-bronchial tree, which plays a sensitising role on the cough reflex. The involvement of tachykinin NK(1), NK(2), NK(3) receptors are also demonstrated in this reflex.