Sensory receptor activation by mediators of defense reflexes in guinea-pig lungs

Baroreceptor Modulation of the Cardiovascular System, Pain, Consciousness, and Cognition

Baroreceptors are mechanosensitive elements of the peripheral nervous system that maintain cardiovascular homeostasis by coordinating the responses to external and internal environmental stressors. While it is well known that carotid and cardiopulmonary baroreceptors modulate sympathetic vasomotor and parasympathetic cardiac neural autonomic drive, to avoid excessive fluctuations in vascular tone and maintain intravascular volume, there is increasing recognition that baroreceptors also modulate a wide range of non-cardiovascular physiological responses via projections from the nucleus of the solitary tract to regions of the central nervous system, including the spinal cord. These projections regulate pain perception, sleep, consciousness, and cognition. In this article, we summarize the physiology of baroreceptor pathways and responses to baroreceptor activation with an emphasis on the mechanisms influencing cardiovascular function, pain perception, consciousness, and cognition. Understanding baroreceptor-mediated effects on cardiac and extra-cardiac autonomic activities will further our understanding of the pathophysiology of multiple common clinical conditions, such as chronic pain, disorders of consciousness (e.g., abnormalities in sleep-wake), and cognitive impairment, which may result in the identification and implementation of novel treatment modalities. © 2021 American Physiological Society. Compr Physiol 11:1373-1423, 2021.

Vagal Afferent Innervation of the Airways in Health and Disease

Vagal sensory neurons constitute the major afferent supply to the airways and lungs. Subsets of afferents are defined by their embryological origin, molecular profile, neurochemistry, functionality, and anatomical organization, and collectively these nerves are essential for the regulation of respiratory physiology and pulmonary defense through local responses and centrally mediated neural pathways. Mechanical and chemical activation of airway afferents depends on a myriad of ionic and receptor-mediated signaling, much of which has yet to be fully explored. Alterations in the sensitivity and neurochemical phenotype of vagal afferent nerves and/or the neural pathways that they innervate occur in a wide variety of pulmonary diseases, and as such, understanding the mechanisms of vagal sensory function and dysfunction may reveal novel therapeutic targets. In this comprehensive review we discuss historical and state-of-the-art concepts in airway sensory neurobiology and explore mechanisms underlying how vagal sensory pathways become dysfunctional in pathological conditions.

Pharmacology of Bradykinin-Evoked Coughing in Guinea Pigs

Bradykinin has been implicated as a mediator of the acute pathophysiological and inflammatory consequences of respiratory tract infections and in exacerbations of chronic diseases such as asthma. Bradykinin may also be a trigger for the coughing associated with these and other conditions. We have thus set out to evaluate the pharmacology of bradykinin-evoked coughing in guinea pigs. When inhaled, bradykinin induced paroxysmal coughing that was abolished by the bradykinin B2 receptor antagonist HOE 140. These cough responses rapidly desensitized, consistent with reports of B2 receptor desensitization. Bradykinin-evoked cough was potentiated by inhibition of both neutral endopeptidase and angiotensin-converting enzyme (with thiorphan and captopril, respectively), but was largely unaffected by muscarinic or thromboxane receptor blockade (atropine and ICI 192605), cyclooxygenase, or nitric oxide synthase inhibition (meclofenamic acid and N(G)-nitro-L-arginine). Calcium influx studies in bronchopulmonary vagal afferent neurons dissociated from vagal sensory ganglia indicated that the tachykinin-containing C-fibers arising from the jugular ganglia mediate bradykinin-evoked coughing. Also implicating the jugular C-fibers was the observation that simultaneous blockade of neurokinin2 (NK2; SR48968) and NK3 (SR142801 or SB223412) receptors nearly abolished the bradykinin-evoked cough responses. The data suggest that bradykinin induces coughing in guinea pigs by activating B2 receptors on bronchopulmonary C-fibers. We speculate that therapeutics targeting the actions of bradykinin may prove useful in the treatment of cough.

Exposure to Allergen Causes Changes in NTS Neural Activities after Intratracheal Capsaicin Application, in Endocannabinoid Levels and in the Glia Morphology of NTS

Allergen exposure may induce changes in the brainstem secondary neurons, with neural sensitization of the nucleus solitary tract (NTS), which in turn can be considered one of the causes of the airway hyperresponsiveness, a characteristic feature of asthma. We evaluated neurofunctional, morphological, and biochemical changes in the NTS of naive or sensitized rats. To evaluate the cell firing activity of NTS, in vivo electrophysiological experiments were performed before and after capsaicin challenge in sensitized or naive rats. Immunohistochemical studies, endocannabinoid, and palmitoylethanolamide quantification in the NTS were also performed. This study provides evidence that allergen sensitization in the NTS induced: (1) increase in the neural firing response to intratracheal capsaicin application, (2) increase of endocannabinoid anandamide and palmitoylethanolamide, a reduction of 2-arachidonoylglycerol levels in the NTS, (3) glial cell activation, and (4) prevention by a Group III metabotropic glutamate receptor activation of neural firing response to intratracheal application of capsaicin in both naïve and sensitized rats. Therefore, normalization of ovalbumin-induced NTS neural sensitization could open up the prospect of new treatments based on the recovery of specific brain nuclei function and for extensive studies on acute or long-term efficacy of selective mGlu ligand, in models of bronchial hyperreactivity.

Sensory nerves in lung and airways

Sensory nerves innervating the lung and airways play an important role in regulating various cardiopulmonary functions and maintaining homeostasis under both healthy and disease conditions. Their activities conducted by both vagal and sympathetic afferents are also responsible for eliciting important defense reflexes that protect the lung and body from potential health-hazardous effects of airborne particulates and chemical irritants. This article reviews the morphology, transduction properties, reflex functions, and respiratory sensations of these receptors, focusing primarily on recent findings derived from using new technologies such as neural immunochemistry, isolated airway-nerve preparation, cultured airway neurons, patch-clamp electrophysiology, transgenic mice, and other cellular and molecular approaches. Studies of the signal transduction of mechanosensitive afferents have revealed a new concept of sensory unit and cellular mechanism of activation, and identified additional types of sensory receptors in the lung. Chemosensitive properties of these lung afferents are further characterized by the expression of specific ligand-gated ion channels on nerve terminals, ganglion origin, and responses to the action of various inflammatory cells, mediators, and cytokines during acute and chronic airway inflammation and injuries. Increasing interest and extensive investigations have been focused on uncovering the mechanisms underlying hypersensitivity of these airway afferents, and their role in the manifestation of various symptoms under pathophysiological conditions. Several important and challenging questions regarding these sensory nerves are discussed. Searching for these answers will be a critical step in developing the translational research and effective treatments of airway diseases.

Rapidly adapting receptor activity during oxidative stress induced airway hyperresponsiveness

The responses of airway rapidly adapting receptors (RARs) to ovalbumin challenge and histamine were investigated in guinea pigs which were sensitized with ovalbumin. Sensitization alone increased the basal RAR activity. Antigen challenge stimulated them. Histamine doses which caused a 50% increase in airway resistance (ED50) were reduced immediately and 24h after antigen challenge indicating respectively early and late onset airway hyperresponsiveness. At these doses, there was a greater stimulation of the RARs compared to controls. An increase in lipid peroxidation and a decrease in glutathione peroxidase were observed also. With oral intake of vitamins C and E, attenuations in the basal RAR activity, the responses of RARs to antigen challenge and the oxidative stress were observed. With an increase in ED50, the RAR response to histamine became similar as in control. It is concluded that by decreasing the RAR responses to allergen and histamine, antioxidants may reduce reflex bronchoconstriction occurring in asthmatics.

Pre-clinical studies in cough research: role of Transient Receptor Potential (TRP) channels

Cough is a protective reflex and defence mechanism in healthy individuals, which helps clear excessive secretions and foreign material from the lungs. Cough often presents as the first and most persistent symptom of many respiratory diseases and some non-respiratory disorders, but can also be idiopathic, and is a common respiratory complaint for which medical attention is sought. Chronic cough of various aetiologies is a regular presentation to specialist respiratory clinics, and is reported as a troublesome symptom by a significant proportion of the population. Despite this, the treatment options for cough are limited. The lack of effective anti-tussives likely stems from our incomplete understanding of how the tussive reflex is mediated. However, research over the last decade has begun to shed some light on the mechanisms which provoke cough, and may ultimately provide us with better anti-tussive therapies. This review will focus on the in vitro and in vivo models that are currently used to further our understanding of the sensory innervation of the respiratory tract, and how these nerves are involved in controlling the cough response. Central to this are the Transient Receptor Potential (TRP) ion channels, a family of polymodal receptors that can be activated by such diverse stimuli as chemicals, temperature, osmotic stress, and mechanical perturbation. These ion channels are thought to be molecular pain integrators and targets for novel analgesic agents for the treatment of various pain disorders but some are also being developed as anti-tussives.

A role for ATP in bronchoconstriction-induced activation of guinea pig vagal intrapulmonary C-fibres

Activation of vagal afferent sensory C-fibres in the lungs leads to reflex responses that produce many of the symptoms associated with airway allergy. There are two subtypes of respiratory C-fibres whose cell bodies reside within two distinct ganglia, the nodose and jugular, and whose properties allow for differing responses to stimuli. We here used extracellular recording of action potentials in an ex vivo isolated, perfused lung-nerve preparation to study the electrical activity of nodose C-fibres in response to bronchoconstriction. We found that treatment with both histamine and methacholine caused strong increases in tracheal perfusion pressure that were accompanied by action potential discharge in nodose, but not in jugular C-fibres. Both the increase in tracheal perfusion pressure and action potential discharge in response to histamine were significantly reduced by functionally antagonizing the smooth muscle contraction with isoproterenol, or by blocking myosin light chain kinase with ML-7. We further found that pretreatment with AF-353 or 2',3'-O-(2,4,6-Trinitrophenyl)-adenosine-5'-triphosphate (TNP-ATP), structurally distinct P2X3 and P2X2/3 purinoceptor antagonists, blocked the bronchoconstriction-induced nodose C-fibre discharge. Likewise, treatment with the ATPase apyrase, in the presence of the adenosine A1 and A2 receptor antagonists 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and SCH 58261, blocked the C-fibre response to histamine, without inhibiting the bronchoconstriction. These results suggest that ATP released within the tissues in response to bronchoconstriction plays a pivotal role in the mechanical activation of nodose C-fibres.

Sensing pulmonary oxidative stress by lung vagal afferents

Oxidative stress in the bronchopulmonary airways can occur through a variety of inflammatory mechanisms and also following the inhalation of environmental pollutants. Oxidative stress causes cellular dysfunction and thus mammals (including humans) have developed mechanisms for detecting oxidative stress, such that defensive behavior and defensive biological mechanisms can be induced to lessen its potential damage. Vagal sensory nerves innervating the airways play a critical role in the detection of the microenvironment in the airways. Oxidative stress and associated compounds activate unmyelinated bronchopulmonary C-fibers, initiating action potentials in these nerves that conduct centrally to evoke unpleasant sensations (e.g. urge to cough, dyspnea, chest-tightness) and to stimulate/modulate reflexes (e.g. cough, bronchoconstriction, respiratory rate, inspiratory drive). This review will summarize the published evidence regarding the mechanisms by which oxidative stress, reactive oxygen species, environmental pollutants and lipid products of peroxidation activate bronchopulmonary C-fibers. Evidence suggests a key role for transient receptor potential ankyrin 1 (TRPA1), although transient receptor potential vanilloid 1 (TRPV1) and purinergic P2X channels may also play a role. Knowledge of these pathways greatly aids our understanding of the role of oxidative stress in health and disease and represents novel therapeutic targets for diseases of the airways.

Sensory nerves and airway irritability

The lung, like many other organs, is innervated by a variety of sensory nerves and by nerves of the parasympathetic and sympathetic nervous systems that regulate the function of cells within the respiratory tract. Activation of sensory nerves by both mechanical and chemical stimuli elicits a number of defensive reflexes, including cough, altered breathing pattern, and altered autonomic drive, which are important for normal lung homeostasis. However, diseases that afflict the lung are associated with altered reflexes, resulting in a variety of symptoms, including increased cough, dyspnea, airways obstruction, and bronchial hyperresponsiveness. This review summarizes the current knowledge concerning the physiological role of different sensory nerve subtypes that innervate the lung, the factors which lead to their activation, and pharmacological approaches that have been used to interrogate the function of these nerves. This information may potentially facilitate the identification of novel drug targets for the treatment of respiratory disorders such as cough, asthma, and chronic obstructive pulmonary disease.

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.

Vagal afferent nerves with the properties of nociceptors

Vagal afferent nerves are essential for optimal neural regulation of visceral organs, but are not often considered important for their defense. However, there are well-defined subsets of vagal afferent nerves that have activation properties indicative of specialization to detect potentially harmful stimuli (nociceptors). This is clearly exemplified by the vagal bronchopulmonary C-fibers that are quiescent in healthy lungs but are readily activated by noxious chemicals and inflammatory molecules. Vagal afferent nerves with similar activation properties have been also identified in the esophagus and probably exist in other visceral tissues. In addition, these putative vagal nociceptors often initiate defensive reflexes, can be sensitized, and have the capacity to induce central sensitization. This set of properties is a characteristic of nociceptors in somatic tissues.

Clinical cough VI: the need for new therapies for cough: disease-specific and symptom-related antitussives

Cough is a common symptom that can be self-limiting or persistent. Ideally, treatment of the underlying cause(s) of cough with specific treatments should eliminate cough. This approach may not be successful if no cause can be established or if the treatment of the cause fails. Suppression of cough may be disease-specific or symptom-related. There has been a long tradition in acute cough usually due to upper respiratory tract infections to use symptom-related antitussives. In chronic cough, suppression of cough may be achieved by disease-specific therapies, but in many patients it may be necessary to use symptomatic antitussives. The efficacy of some over-the-counter symptomatic antitussives is often no better than that of a placebo. Currently available cough suppressants include the centrally acting opioids such as morphine, codeine, pholcodeine, and dextromethorphan. Early studies reported success in reducing cough in patients with chronic bronchitis or chronic obstructive pulmonary disease (COPD); however, a carefully conducted blinded controlled study showed no effect of codeine on cough of COPD. Success with these cough suppressants may be achieved at high doses that are associated with side effects. A slow-release preparation of morphine has been shown to have some degree of efficacy, but this should be reserved for the most severe chronic cough patient, and for patients with terminal cancer who may also benefit from its analgesic effects. There are case reports of the success of centrally acting drugs such as amitriptyline, paroxetine, gabapentin, and carbamezepine in chronic cough. New agents derived from basic research such as new opioids such as nociceptin or antagonists of transient receptor potential vanniloid-1 may turn out to have antitussive effects. Efficacy of symptomatic cough suppressants must be tested in double-blind randomized trials using validated measures of cough in patients with chronic cough not responding to specific treatments. Patients with chronic cough need effective antitussives that could be used either on demand or on a long-term basis.

Cough sensors. V. Pharmacological modulation of cough sensors

Several airway afferent nerve subtypes have been implicated in coughing. These include bronchopulmonary C-fibers, rapidly adapting airway mechanoreceptors and touch-sensitive tracheal Adelta-fibers (also called cough receptors). Although the last two afferent nerve subtypes are primarily sensitive to mechanical stimuli, all can be acted upon by one or more different chemical stimuli. In this review we catalogue the chemical agents that stimulate and/or modulate the activity of the airway afferent nerves involved in cough, and describe the specific mechanisms involved in these effects. In addition, we describe the mechanisms of action of a number of chemical inhibitors of these afferent nerve subtypes, and attempt to relate this information to the regulation of coughing in health and disease.

Airway nerves and dyspnea associated with inflammatory airway disease

The neurobiology of dyspnea is varied and complex, but there is little doubt that vagal nerves within the airways are capable of causing or modulating some dyspneic sensations, especially those associated with inflammatory airway diseases. A major contributor to the dyspnea associated with inflammatory airway disease is explained by airway narrowing and increases in the resistance to airflow. The autonomic (parasympathetic) airway nerves directly contribute to this by regulating bronchial smooth muscle tone and mucus secretion. In addition, a component of the information reaching the brainstem via airway mechanosensing and nociceptive afferent nerves likely contributes to the overall sensations of breathing. The airway narrowing can lead to activation of low threshold mechanosensitive stretch receptors, and vagal and spinal C-fibers as well as some rapidly adapting stretch receptor in the airways that are directly activated by various aspects of the inflammatory response. Inflammatory mediators can induce long lasting changes in afferent nerve activity by modulating the expression of key genes. The net effect of the increase in afferent traffic to the brainstem modulates synaptic efficacy at the second-order neurons via various mechanisms collectively referred to as central sensitization. Many studies have shown that stimuli that activate bronchopulmonary afferent nerves can lead to dyspnea in healthy subjects. A logical extension of the basic research on inflammation and sensory nerve function is that the role of vagal sensory nerve in causing or shaping dyspneic sensations will be exaggerated in those suffering from inflammatory airway disease.

Mechanisms of the cough associated with rhinosinusitis

The diseases of the nose and paranasal sinuses (rhinosinusitis) often in combination with asthma and gastroesophageal reflux are common causes of chronic cough in patients with normal chest radiograph. The relationships between rhinosinusitis and cough are incompletely understood. We investigated modulation of the cough reflex by the inputs from the nose. We demonstrate that the cough reflex is sensitized by the intranasal administration of sensory nerve activators in animal models and in humans. Cough reflex is also sensitized in the guinea pig model of allergic nasal inflammation and in patients with allergic rhinitis. In patients with allergic rhinitis the cough sensitization is augmented during the allergen season. We conclude that the cough reflex can be sensitized from the nose. Our data indicate that this sensitization is mediated by nasal sensory nerves. We speculate that by inducing the cough reflex sensitization rhinosinusitis contributes to chronic cough. If combined with environmental or endogenous cough triggers, the cough reflex sensitization is predicted to cause excessive coughing. The potential endogenous cough triggers may be associated with rhinosinusitis (postnasal drip, aspiration of nasal secrets) or secondary to a coexistent disease such as asthma or gastroesophageal reflux.

Cough sensors. I. Physiological and pharmacological properties of the afferent nerves regulating cough

The afferent nerves regulating cough have been reasonably well defined. The selective effects of general anesthesia on C-fiber-dependent cough and the opposing effects of C-fiber subtypes in cough have led to some uncertainty about their regulation of this defensive reflex. But a role for C-fibers in cough seems almost certain, given the unique pharmacological properties of these unmyelinated vagal afferent nerves and the ability of many C-fiber-selective stimulants to evoke cough. The role of myelinated laryngeal, tracheal, and bronchial afferent nerve subtypes that can be activated by punctate mechanical stimuli, inhaled particulates, accumulated secretions, and acid has also been demonstrated. These "cough receptors" are distinct from the slowly and rapidly adapting intrapulmonary stretch receptors responding to lung inflation. Indeed, intrapulmonary rapidly and slowly adapting receptors and pulmonary C-fibers may play no role or a nonessential role in cough, or might even actively inhibit cough upon activation. A critical review of the studies of the afferent nerve subtypes most often implicated in cough is provided.

Differential effects of airway afferent nerve subtypes on cough and respiration in anesthetized guinea pigs

The hypothesis that respiratory reflexes, such as cough, reflect the net and often opposing effects of activation of multiple afferent nerve subpopulations throughout the airways was evaluated. Laryngeal and tracheal mucosal challenge with either citric acid or mechanical probing reliably evoked coughing in anesthetized guinea pigs. No other stimulus reliably evoked coughing in these animals, regardless of route of administration and despite some profound effects on respiration. Selectively activating vagal C-fibers arising from the nodose ganglia with either adenosine or 2-methyl-5-HT evoked only tachypnea. Selectively activating vagal afferents arising from the jugular ganglia induced respiratory slowing and apnea. Nasal afferent nerve activation by capsaicin, citric acid, hypertonic saline, or histamine evoked only respiratory slowing. Histamine, which activates intrapulmonary rapidly adapting receptors but not airway or lung C-fibers or tracheal bronchial cough receptors induced bronchospasm and tachypnea, but no coughing. The results indicate that the reflexes initiated by stimuli thought to be selective for some afferent nerve subtypes will likely depend on the net and potentially opposing effects of multiple afferent nerve subpopulations throughout the airways. The data also provide further evidence that the afferent nerves regulating cough in anesthetized guinea pigs are distinct from either C-fibers or intrapulmonary rapidly adapting receptors.

Effect of TRFK-5 on airway responsiveness in ovalbumin-treated guinea pigs exposed to tobacco smoke

Tobacco smoke (TS) exposure can induce airway hyperresponsiveness, especially in asthma. A feature of asthma is eosinophilia. We hypothesized that tobacco smoke exposure enhances eosinophil responsiveness in sensitized guinea pigs. Tobacco smoke-exposed, ovalbumin (OA)-sensitized guinea pigs were treated with TRFK-5 (1.0 mg/kg, intraperitoneal), an anti-interleukin (IL)-5 agent, or its vehicle. Guinea pigs were challenged with aerosols of OA, capsaicin, histamine, and methacholine. TRFK-5 attenuated airway responsiveness to OA but not to capsaicin, histamine, or methacholine. Bronchial alveolar lavage fluid analysis confirmed TRFK-5 attenuated airway eosinophilia in OA-treated guinea pigs. Therefore, airway responsiveness to OA is enhanced by eosinophils or IL-5 itself.

Tobacco smoke is an adjuvant for maintained airway sensitization in guinea pigs

Tobacco smoke (TS) exposure exacerbates asthma and may induce airway hyperresponsiveness in asymptomatic individuals. We hypothesized that TS exposure is an adjuvant to airway responsiveness. Ovalbumin (OA) sensitized guinea pigs were TS or air exposed. At 30 exposure days OA airway responsiveness was demonstrable in OA-treated animals exposed to either TS or air. After 130 exposure days only TS-exposed guinea pigs demonstrated OA airway responsiveness. Capsaicin airway responsiveness developed in non-sensitized and OA-sensitized guinea pigs exposed to TS. Therefore TS-exposure acts as an adjuvant to antigenic and neurogenic airway responsiveness. Combined antigen and adjuvant avoidance may attenuate or reverse airway responsiveness.

Na+-K+-2Cl− cotransporters and Cl− channels regulate citric acid cough in guinea pigs

Loop diuretics have been shown to inhibit cough and other airway defensive reflexes via poorly defined mechanisms. We test the hypothesis that the furosemide-sensitive Na+-K+-2Cl− cotransporter (NKCC1) is expressed by sensory nerve fibers innervating the airways where it plays an important role in regulating sensory neural activity. NKCC1 immunoreactivity was present on the cell membranes of most nodose and jugular ganglia neurons projecting to the trachea, and it was present on the peripheral terminals of putative mechanosensory nerve fibers in the airways. In urethane-anesthetized, spontaneously breathing guinea pigs, bolus application of citric acid (1 mM to 2 M) to an isolated and perfused segment of the tracheal mucosa evoked coughing and respiratory slowing. Removal of Cl− from the tracheal perfusate evoked spontaneous coughing and significantly potentiated cough and respiratory slowing reflexes evoked by citric acid. The NKCC1 inhibitor furosemide (10–100 μM) significantly reduced both the number of coughs evoked by citric acid and the degree of acid-evoked respiratory slowing ( P < 0.05). Localized tracheal pretreatment with the Cl− channel inhibitors DIDS or niflumic acid (100 μM) also significantly reduced cough, whereas the GABAA receptor agonist muscimol potentiated acid-evoked responses. These data suggest that vagal sensory neurons may accumulate Cl− due to the expression of the furosemide-sensitive Cl− transporter, NKCC1. Efflux of intracellular Cl−, in part through calcium-activated Cl− channels, may play an important role in regulating airway afferent neuron activity.

Vagal afferent nerves regulating the cough reflex

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

Plasticity of peripheral mechanisms of cough

The cough reflex pathway is characterized by a remarkable plasticity often resulting in a persistent and uncontrollable urge to cough during airway inflammation. In many instances cough becomes up regulated to the extent that ceases to fulfill its defensive role in protecting the airways. The exact mechanisms underlying this plasticity are unknown and likely involves a variety of factors influencing the function of the peripheral and central nervous system. This review outlines the evidence of increased cough sensitivity during airway disease. This is followed by a discussion of the peripheral mechanisms involved including the potential role of inflammatory mediators, neutrophins and changes in the airway mucosal structure. A greater understanding of the mechanisms leading to enhanced cough should lead to the development of more effective therapeutic strategies.

Pulmonary sensory and reflex responses in the mouse

Mouse model research is proliferating because of its readiness for genetic manipulation. Little is known about pulmonary vagal afferents in mice, however. The purpose of this study was to determine whether their pulmonary afferents are similar to those in large animals. Single-unit activity was recorded in the cervical vagus nerve of anesthetized, open-chest, and mechanically ventilated mice. We evaluated airway sensory activity in 153 single units; 141 were mechanosensitive, with 134 inflation receptors and 7 deflation receptors. The remaining 12 receptors were chemosensitive and mechanically insensitive, showing low basal firing frequency and behaving like C-fiber or high-threshold Adelta-receptors. In separate studies, phrenic activity was recorded as an index of respiratory drive to assess pulmonary reflexes. Lung inflation produced a typical Hering-Breuer reflex, and intravenous injection of phenylbiguanide produced the typical chemoreflex resulting in apnea, bradycardia, and hypotension. These reflexes were blocked by bilateral vagotomy. We conclude that mice possess a similar set of airway sensors and pulmonary reflexes as typically found in larger animals.

Transduction mechanisms in airway sensory nerves

The induction of action potentials in airway sensory nerves relies on events leading to the opening of cation channels in the nerve terminal membrane and subsequent membrane depolarization. If the membrane depolarization is of sufficient rate and amplitude, action potential initiation will occur. The action potentials are then conducted to the central nervous system, leading to the initiation of various sensations and cardiorespiratory reflexes. Triggering events in airway sensory nerves include mechanical perturbation, inflammatory mediators, pH, temperature, and osmolarity acting through a variety of ionotropic and metabotropic receptors. Action potential initiation can be modulated (positively or negatively) through independent mechanisms caused mainly by autacoids and other metabotropic receptor ligands. Finally, gene expression of sensory nerves can be altered in adult mammals. This neuroplasticity can change the function of sensory nerves and likely involve both neurotrophin and use-dependent mechanisms. Here we provide a brief overview of some of the transduction mechanisms underlying these events.

Bradykinin B2 receptors mediate pulmonary sympathetic afferents induced reflexes in rabbits

Endogenous bradykinin (BK) is an established mediator of pulmonary inflammation, yet its role in lung disease is unclear. In the rabbit, injecting BK into the lung parenchyma elicits reflex hyperpnea, tachypnea, hypotension, and bradycardia by stimulating pulmonary sympathetic afferents. To further explore bradykinin effects, breathing pattern (phrenic nerve and abdominal muscle activities) and hemodynamics (blood pressure and heart rate) were examined in anesthetized, open-chest, and mechanically ventilated rabbits. Three receptor agonists [bradykinin, selective B(1) (des-Arg(9)-BK), and selective B(2) (Tyr(8)-BK)], as well as three B(2) receptor antagonists, B6029 (N alpha-Adamantaneacetyl)-Bradykinin, B(1)650 (D-Arg-[Hyp(3), Thi(5,8), D-Phe(7)]-Bradykinin, or Hoe-140 (D-Arg-[Hyp(3), Thi(5), D-Tic(7), Oic(8)] bradykinin), were used to identify the responsible receptor subtype. In both intact and vagotomized rabbits, injecting BK or a selective B(2) agonist into the lung elicited similar cardiopulmonary responses. These reflex responses were greatly attenuated or blocked by pre-injecting B(2) antagonists into the right atrium or into the lung parenchyma. In contrast, the B(1) agonist elicited fewer cardiopulmonary effects in intact rabbits and had no effect in vagotomized rabbits. We conclude that BK stimulates pulmonary sympathetic afferents [Soukhova, G., Wang, Y., Ahmed, M., Walker, J., Yu, J., 2003. Bradykinin stimulates respiratory drive by activating pulmonary sympathetic afferents in the rabbit. J. Appl. Physiol. 95, 241-249.; Wang, Y., Soukhova, G., Proctor, M., Walker, J., Yu, J., 2003. Bradykinin causes hypotension by activating pulmonary sympathetic afferents in the rabbit. J. Appl. Physiol. 95, 233-240.], eliciting a characteristic cardiopulmonary reflex via B(2) receptors.

Dissociation in the effect of nedocromil on mannitol-induced cough or bronchoconstriction in asthmatic subjects

OBJECTIVE

Inhaled mannitol induces both bronchoconstriction and cough. Nedocromil sodium greatly attenuates mannitol-induced bronchoconstriction. Knowledge about the effect of nedocromil on mannitol-provoked cough might, therefore, clarify the mechanisms of this response.

METHODOLOGY

Inhalation challenges with mannitol powder were performed after inhalation of 8 mg of nedocromil or its placebo in 24 subjects with asthma. The study was double-blind, randomised, and placebo-controlled. The mannitol-provoked coughs were manually recorded and the mannitol-induced bronchoconstriction was measured with a spirometer.

RESULTS

The cumulative dose of mannitol that provoked at least two coughs tended to be higher on the nedocromil day than on the placebo day (34 (22--53) mg vs 26 (18--37) mg, P=0.051). The cumulative number of coughs per dose of mannitol was slightly, but significantly, lower on the nedocromil than on the placebo day (4.2 (2.8--6.3) coughs/100 mg vs 6.1 (4.0--9.4) coughs/100 mg, P=0.037). However, when analysed on a constant-dose basis, nedocromil provided no protection for coughing (-1% protection), whereas the protection for bronchoconstriction was clear (55% protection).

CONCLUSIONS

Nedocromil strongly attenuates mannitol-induced bronchoconstriction but has a negligible effect on mannitol-provoked cough. Therefore, these responses seem to have different pathways in asthma. Recording of both provoked coughs and induced bronchoconstriction during mannitol challenge may provide supplementary information about a patient's disease.

Prostaglandin involvement in lung C-fiber activation by substance P in guinea pigs

Airway hyperresponsiveness is a cardinal feature of asthma. Lung C-fiber activation induces central and local defense reflexes that may contribute to airway hyperresponsiveness. Initial studies show that substance P (SP) activates C fibers even though it is produced and released by these same C fibers. SP may induce release of other endogenous mediators. Bradykinin (BK) is an endogenous mediator that activates C fibers. The hypothesis was tested that SP activates C fibers via BK release. Guinea pigs were anesthetized, and C-fiber activity (FA), pulmonary insufflation pressure (PIP), heart rate, and arterial blood pressure were monitored before and after intravenous injection of capsaicin (Cap), SP, and BK. Identical agonist challenges were repeated after infusion of an antagonist cocktail of des-Arg9-[Leu8]-BK (10(-3) M, B1 antagonist), and HOE-140 (10(-4) M, B2 antagonist). After antagonist administration, BK increased neither PIP nor FA. Increases in neither PIP nor FA were attenuated after Cap or SP challenge. In a second series of experiments, Cap and SP were injected before and after infusion of indomethacin (1 mg/kg iv) to determine whether either agent activates C fibers through release of arachidonic acid metabolites. Indomethacin administration decreased the effect of SP challenge on FA but not PIP. The effect of Cap on FA or PIP was not altered by indomethacin. In subsequent experiments, C fibers were activated by prostaglandin E2 and F2alpha. Therefore, exogenously applied SP stimulates an indomethacin-sensitive pathway leading to C-fiber activation.

Sensory transduction in cough-associated nerves

Before a tussive stimulus in the airways can evoke a cough reflex it must first cause action potential discharge in cough-associated vagal sensory nerves. This is initiated by the stimulus first interacting with the receptors and ion channels in the terminal membrane of the sensory fiber in a manner that leads to membrane depolarization. If the stimulus-induced membrane depolarization, referred to as a generator potential, is of sufficient magnitude, action potentials are elicited that are then conducted to the central nervous system. If the action potentials are of sufficient number and frequency, a cough is evoked. The most common tussive stimuli include mechanical perturbations, anosmotic solutions, acidic solutions, and various chemical agents. The mechanisms underlying the transduction of most of these tussive stimuli into a generator potential are only partially understood. In general terms, chemical stimuli interact directly with receptors that are classified as either ligand gated ion channels or metabotropic receptors (e.g. G-protein coupled receptors). Ligand gated receptors are those in which the receptor protein also serves as the ion channel. The metabotropic receptors indirectly modulate the ion channels activity via various signal transduction schemes. Mechanical stimuli are thought to interact with mechanically gated ion channels, and acid can interact with acid sensing ion channels in addition to the capsaicin receptor TRPV1. In this overview some of the specific receptors and ion channels involved in the tussive stimulus-induced generator potentials in vagal afferent nerve terminals are discussed.

Anatomy and Neurophysiology of the Cough Reflex

OBJECTIVES

To describe the anatomy and neurophysiology of the cough reflex.

METHODS

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

CONCLUSIONS

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

Interpretation of cough provoked by airway challenges

STUDY OBJECTIVE

To analyze the cough response to three airway challenges in order to clarify whether the recording of the provoked coughs would be beneficial in the management of asthma.

DESIGN

A prospective study.

SETTING

University hospital.

PARTICIPANTS

Fifteen healthy subjects, 16 steroid-naïve subjects with asthma, and 16 subjects with steroid-treated asthma.

INTERVENTIONS

Inhalation challenges with isotonic histamine, hypertonic saline solution, and hypertonic histamine, using an ultrasonic nebulizer and 2-min tidal breathing method.

MEASUREMENTS

Airflow parameters were measured with a spirometer, and the coughs were recorded manually.

RESULTS

Coughing during the isotonic histamine challenge was associated with the degree of the bronchoconstriction induced. When this was taken into account, the healthy subjects coughed as frequently as the asthmatic subjects. During the two hypertonic challenges, the asthmatic subjects coughed more frequently than did the healthy subjects when the induced bronchoconstriction had not yet developed. At that stage of the hypertonic saline solution challenge, the mean coughing frequency was 0.7 coughs per minute (95% confidence interval [CI], 0.03 to 1.3 coughs per minute) for the healthy subjects, 2.7 coughs per minute (95% CI, 0.8 to 4.5 coughs per minute) for the steroid-naïve asthmatic subjects, and 1.3 coughs per minute (95% CI, 0.6 to 1.9 coughs per minute) for the steroid-treated asthmatic subjects (p = 0.018). For the hypertonic histamine challenge, the respective values were 0.8 coughs per minute (95% CI, 0.4 to 1.2 coughs per minute), 3.6 coughs per minute (95% CI, 2.4 to 4.9 coughs per minute), and 2.1 coughs per minute (95% CI, 1.0 to 3.1 coughs per minute; p = 0.001). This cough did not correlate with airway hyperresponsiveness.

CONCLUSIONS

Coughing during isotonic histamine challenge seems to be a manifestation of bronchoconstriction, and recording of the coughs may not provide additional information to airflow measurements. Frequent coughing during hypertonic saline solution and hypertonic histamine challenges in the absence of bronchoconstriction is a pathologic phenomenon. Sensitivity to the cough-provoking effect of hypertonic challenges seems to be enhanced in patients with asthma but unrelated to airway hyperresponsiveness. Therefore, the recording of the provoked coughs during these challenges may add to the information obtained from airflow measurements.

An overview of the sensory receptors regulating cough

The cough reflex represents a primary defensive mechanism for airway protection in a variety of mammalian species. However, excessive and inappropriate coughing can emerge as a primary presenting symptom of many airway diseases. Cough disorders are characterized by a reduction in the threshold for reflex initiation and, as a consequence, the occurrence of cough in response to stimuli that are normally innocuous in nature. The current therapeutic strategies for the treatment of cough disorders are only moderately effective. This undoubtedly relates in part to limitations in our understanding of the neural components comprising the cough reflex pathway. The aim of this review is to provide an overview of current concepts relating to the sensory innervation to the mammalian airways, focusing particularly on the sensory receptors that regulate cough. In addition, the review will highlight particular areas and issues relating to cough neurobiology that are creating controversy in the field.

Synergistic interactions between airway afferent nerve subtypes regulating the cough reflex in guinea-pigs

Cough initiated from the trachea and larynx in anaesthetized guinea-pigs is mediated by capsaicin-insensitive, mechanically sensitive vagal afferent neurones. Tachykinin-containing, capsaicin-sensitive C-fibres also innervate the airways and have been implicated in the cough reflex. Capsaicin-sensitive nerves act centrally and synergistically to modify reflex bronchospasm initiated by airway mechanoreceptor stimulation. The hypothesis that polymodal mechanoreceptors and capsaicin-sensitive afferent nerves similarly interact centrally to regulate coughing was addressed in this study. Cough was evoked from the tracheal mucosa either electrically (16 Hz, 10 s trains, 1-10 V) or by citric acid (0.001-2 m). Neither capsaicin nor bradykinin evoked a cough when applied to the trachea of anaesthetized guinea-pigs, but they substantially reduced the electrical threshold for initiating the cough reflex. The TRPV1 receptor antagonist capsazepine prevented the increased cough sensitivity induced by capsaicin. These effects of topically applied capsaicin and bradykinin were not due to interactions between afferent nerve subtypes within the tracheal wall or a direct effect on the cough receptors, as they were mimicked by nebulizing 1 mg ml(-1) bradykinin into the lower airways and by microinjecting 0.5 nmol capsaicin into nucleus of the solitary tract (nTS). Citric acid-induced coughing was also potentiated by inhalation of bradykinin. The effects of tracheal capsaicin challenge on cough were mimicked by microinjecting substance P (0.5-5 nmol) into the nTS and prevented by intracerebroventricular administration (20 nmol h(-1)) of the neurokinin receptor antagonists CP99994 or SB223412. Tracheal application of these antagonists was without effect. C-fibre activation may thus sensitize the cough reflex via central mechanisms.

Antitussive effect of NS-398, a selective cyclooxygenase-2 inhibitor, in guinea pigs

Several reports have demonstrated that the number of capsaicin-induced coughs is increased in the presence of prostaglandins in the airway. Moreover, it has been reported that the expression of cyclooxygenase-2, which converts arachidonic acid to prostaglandins, was found in cultured human airway epithelial cells in the absence of inflammatory cytokine stimulation. Thus, it is possible that cyclooxygenase-2 inhibitor may produce an antitussive effect. To test this hypothesis, we investigated the effects of N-[2-(cyclohexyloxy)-4-nitrofenyl]-methane sulfonamide (NS-398), a selective cyclooxygenase-2 inhibitor, and 5-(4-chlorophenyl)-1-(4-methoxyphenyl)-3-trifluoromethyl-pyrazole (SC-560), a selective cyclooxygenase-1 inhibitor, on capsaicin-induced coughs in guinea pigs. NS-398 (1-10 mg/kg, p.o.) dose-dependently and significantly reduced the number of capsaicin-induced coughs. In contrast, SC-560 (10 mg/kg, p.o.) did not reduce the number of capsaicin-induced coughs. The antitussive effect of NS-398 (10 mg/kg, p.o.) was not antagonized by pretreatment with methysergide (3 mg/kg, i.p.), a non-selective serotonin (5-HT) receptor antagonist, or glibenclamide (10 mg/kg, i.p.), an ATP-sensitive K(+) channel blocker. Furthermore, although NS-398 did not significantly affect the cough reflex induced by substance P (10(-16) M), it significantly reduced the capsaicin-induced release of substance P in bronchoalveolar lavage fluid (BALF). The present findings clearly show that cyclooxygenase-2 inhibitor, but not cyclooxygenasez-1 inhibitor, has a potent antitussive effect. Furthermore, it is possible that the antitussive action of NS-398 does not depend on centrally acting mechanisms, since 5-HT receptors play an important role in the cough-depressant activities of centrally acting antitussive drugs. NS-398 may exert peripheral antitussive effects by inhibiting the release of substance P from capsaicin-sensitive afferent C-fibers in the airways. These results suggest that cyclooxygenase-2 inhibitors may have a therapeutic benefit in reducing coughs.

Prostaglandin E2 potentiates a TTX-resistant sodium current in rat capsaicin-sensitive vagal pulmonary sensory neurones

Capsaicin-sensitive vagal pulmonary neurones (pulmonary C neurones) play an important role in regulating airway function. During airway inflammation, the level of prostaglandin E(2) (PGE(2)) increases in the lungs and airways. PGE(2) has been shown to sensitize isolated pulmonary C neurones. The somatosensory correlate of the pulmonary C neurone, the small-diameter nociceptive neurone of the dorsal root ganglion, contains a high percentage of tetrodotoxin-resistant sodium currents (TTX-R I(Na)). Therefore, this study was carried out to determine whether these channel currents are involved in the PGE(2)-induced sensitization of pulmonary C neurones. We used the perforated patch-clamp technique to study the effects of PGE(2) on the TTX-R I(Na) in acutely cultured capsaicin-sensitive pulmonary neurones that were identified by retrograde labelling with a fluorescent tracer, DiI. We found that the pulmonary neurones sensitive to capsaicin had a higher percentage of TTX-R I(Na) than that of capsaicin-insensitive pulmonary neurones. PGE(2) exposure increased the evoked TTX-R I(Na) when experiments were performed at both room temperature and at 37 degrees C. Furthermore, stimulation of the adenylyl cyclase/protein kinase A pathway with either forskolin or Sp-5,6-DCl-cBiMPS potentiated the TTX-R I(Na) in a manner similar to that of PGE(2). We conclude that these modulatory effects of PGE(2) on TTX-R I(Na) play an important role in the sensitization of pulmonary C neurones.

Sensory regulation of the cough reflex

Coughing is a highly coordinated reflex that serves to protect the airways from a variety of potentially harmful stimuli. However, in airways disease the cough reflex threshold is lowered and coughing can become exaggerated and inappropriate. Excessive coughing not only affects an individual's quality of life, but may contribute to the pathology of the disease. Understanding the neural components of the cough reflex is essential for establishing new treatments for cough disorders. This review will summarize the current understanding of the afferent neural pathways mediating cough, including how interactions between airway afferent nerve fibre subtypes may modulate the cough reflex pathway and underlie the manifestation of cough disorders.

Depressed ventilatory reflexes after capsaicin challenge in streptozotocin-treated rats

Diabetic sensory neuropathy is an affliction that decreases sensory perception in a number of organ systems. Although little is known of its pulmonary effects certain diabetic patients have reduced airway reactivity to cold air and elevated cough threshold to irritant inhalation, reflexes reported to be mediated by pulmonary C-fibers. Therefore we studied the effects the selective C-fiber activator capsaicin (0.01% aerosol, 30 s) on variables of ventilation using a whole-body plethysmograph in age-matched rats treated with streptozotocin (STZ) or its vehicle at 6 and 12 weeks after treatment. Body weight increased and plasma glucose and glycosylated hemoglobin were stable in vehicle-treated rats. In STZ-treated rats body weight decreased and plasma glucose and glycosylated hemoglobin increased. Capsaicin challenge decreased tidal volume, respiratory rate and therefore minute ventilation in non-treated and vehicle-treated rats. However capsaicin challenge increased tidal volume thereby altering minute ventilation in STZ-treated rats. Specific airway resistance increased in both groups after capsaicin challenge. Changes in ventilation in response to capsaicin challenge in STZ-treated rats may involve C-fiber sensory neuropathy.

Identification of the tracheal and laryngeal afferent neurones mediating cough in anaesthetized guinea-pigs

We have identified the tracheal and laryngeal afferent nerves regulating cough in anaesthetized guinea-pigs. Cough was evoked by electrical or mechanical stimulation of the tracheal or laryngeal mucosa, or by citric acid applied topically to the trachea or larynx. By contrast, neither capsaicin nor bradykinin challenges to the trachea or larynx evoked cough. Bradykinin and histamine administered intravenously also failed to evoke cough. Electrophysiological studies revealed that the majority of capsaicin-sensitive afferent neurones (both Adelta- and C-fibres) innervating the rostral trachea and larynx have their cell bodies in the jugular ganglia and project to the airways via the superior laryngeal nerves. Capsaicin-insensitive afferent neurones with cell bodies in the nodose ganglia projected to the rostral trachea and larynx via the recurrent laryngeal nerves. Severing the recurrent nerves abolished coughing evoked from the trachea and larynx whereas severing the superior laryngeal nerves was without effect on coughing. The data indicate that the tracheal and laryngeal afferent neurones regulating cough are polymodal Adelta-fibres that arise from the nodose ganglia. These afferent neurones are activated by punctate mechanical stimulation and acid but are unresponsive to capsaicin, bradykinin, smooth muscle contraction, longitudinal or transverse stretching of the airways, or distension. Comparing these physiological properties with those of intrapulmonary mechanoreceptors indicates that the afferent neurones mediating cough are quite distinct from the well-defined rapidly and slowly adapting stretch receptors innervating the airways and lungs. We propose that these airway afferent neurones represent a distinct subtype and that their primary function is regulation of the cough reflex.

Capsaicin-sensitive and -insensitive vagal bronchopulmonary C-fibres in the mouse

We developed an isolated tracheally perfused (35-37 degrees C) nerve-lung preparation for the study of bronchopulmonary afferent nerve activity in the mouse. Extracellular recordings were made from the vagal sensory neurons located in the jugular-nodose ganglia complex (JNC) with identified receptive fields in the lungs. Analysis of the vagal compound action potential revealed that the mouse vagal C-fibre conduction velocities range from 0.3 to 1.5 m s(-1). A total of 83 bronchopulmonary C-fibres were studied. The sensitivity of the bronchopulmonary C-fibres to the vanilloid receptor 1 (VR1) agonist capsaicin was dependent on conduction velocity. Thus C-fibres with conduction velocities between 0.3 and 0.7 m s(-1) responded to capsaicin (1 microM) while C-fibres with conduction velocities between 0.7 and 1.5 m s(-1) were capsaicin insensitive. Similarly, bradykinin (1 microM) excited only those C-fibres with conduction velocities < 0.7 m s(-1). The response to bradykinin was not mimicked by the B1 receptor agonist [des-Arg9]bradykinin (1 microM) and was abolished by the bradykinin B2 receptor antagonist HOE 140 (1 microM). Adenosine 5'-triphosphate (ATP, 30 microM) activated the C-fibres irrespective of the conduction velocities. This response was mimicked by the selective P2X agonist alpha,beta-methylene-adenosine 5'-triphosphate (30 microM). Consistent with the electrophysiology, morphological analysis revealed that only approximately 40% of the lung-specific small diameter (< 20 microm) JNC neurons consistent with the C-fibre cell bodies display VR1 immunoreactivity. This study describes a convenient in vitro method for the study of mouse bronchopulmonary C-fibres. The results indicate that C-fibres in the mouse lungs are not homogeneous, but can be subclassified into capsaicin-sensitive and capsaicin-insensitive phenotypes.

Bradykinin stimulates respiratory drive by activating pulmonary sympathetic afferents in the rabbit

We recently identified a vagally mediated excitatory lung reflex by injecting hypertonic saline into the lung parenchyma (Yu J, Zhang JF, and Fletcher EC. J Appl Physiol 85: 1485-1492, 1998). This reflex increased amplitude and burst rate of phrenic (inspiratory) nerve activity and suppressed external oblique abdominal (expiratory) muscle activity. In the present study, we tested the hypothesis that bradykinin may activate extravagal pathways to stimulate breathing by assessing its reflex effects on respiratory drive. Bradykinin (1 microg/kg in 0.1 ml) was injected into the lung parenchyma of anesthetized, open-chest and artificially ventilated rabbits. In most cases, bradykinin increased phrenic amplitude, phrenic burst rate, and expiratory muscle activity. However, a variety of breathing patterns resulted, ranging from hyperpnea and tachypnea to rapid shallow breathing and apnea. Bradykinin acts like hypertonic saline in producing hyperpnea and tachypnea, yet the two agents clearly differ. Bradykinin produced a higher ratio of phrenic amplitude to inspiratory time and had longer latency than hypertonic saline. Although attenuated, bradykinin-induced respiratory responses persisted after vagotomy. We conclude that bradykinin activates multiple afferent pathways in the lung; portions of its respiratory reflexes are extravagal and arise from sympathetic afferents.

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.

Functional morphology and physiology of pulmonary rapidly adapting receptors (RARs)

Rapidly adapting receptors (RARs) in the airway mucosa are found from the nasopharynx to the bronchi. They have thin (Adelta) vagal afferent fibres and lie in and under the epithelium, but their morphology has not been defined. They are very sensitive to mechanical stimuli, and have a rapidly adapting irregular discharge. However, with in vitro preparations they are rather insensitive to chemical stimuli, apart from acid and nonisosmolar solutions. Their pattern of response varies with site. RARs in the nasopharynx, larynx, and trachea usually respond only during the onset of stimuli, while those in the trachea often have an off-response as well. Those in the bronchi are less rapidly adapting and more chemosensitive. Their membranes have mechanosensitive and acid-sensitive ion channels, but no vanilloid receptors. In vivo RARs are sensitive to a wide range of chemical irritants and mediators, and presumably are excited secondarily to mechanical changes in the mucosa and airway smooth muscle. In the central nervous system (CNS) they interact with other vagal afferent pathways. The reflexes they cause vary with site (inspiratory efforts from the nasopharynx, cough or expiratory efforts from the larynx and trachea, and deep breaths or tachypnoea from the bronchi). Pathways from RARs and other vagal reflexes show plasticity at the peripheral, ganglionic, and CNS levels.

Physiology and plasticity of putative cough fibres in the Guinea pig

Cough is initiated by activation of afferent nerve fibers with rapidly adapting receptors (RAR) that conduct action potentials in the Adelta range. In addition, various stimuli that activate airway unmylenated C-fibres evoke cough reflexes. We have used a vagally innervated, larynx-trachea-bronchus preparation, isolated from guinea pigs, to study the pharmacology of RARs and C-fibres in vitro. In this preparation afferent fibres with the RAR phenotype are exquisitely sensitive to mechanical perturbation of their receptive fields, but are unaffected by a variety of mediators (e.g. prostaglandins, histamine, bradykinin, serotonin) and by capsaicin. By contrast, C-fibres are much less sensitive to mechanical stimulation, but can be activated by capsaicin and bradykinin. Preliminary evidence supports the hypothesis that bradykinin activate C-fibre by stimulating the capsaicin (vanilloid) receptor VR1. Acids activate both C-fibres and RARs. Acids stimulate RAR fibres by a mechanism that is rapidly inactivated. C-fibres are stimulated by both a rapidly inactivating mechanism, as well as a slowly inactivating mechanism. Drugs that block VR1 inhibit the latter mechanism. Airway inflammation substantially increases the mechanical sensitivity of RAR fibres without affecting their adaptive properties. Airway inflammation also causes a phenotypic switch in neuropeptide innervation of the airways that RAR neurons begin to synthesis neurokinins and calcitonin gene related peptide. In non-inflamed animals these peptides are expressed only in C-fibre neurons. Thus, airway inflammation may not only increase the sensitivity of cough fibres, but may also qualitatively change the role played by sensory neuropeptides in cough reflexes.

Interactions between vagal afferent nerve subtypes mediating cough

The cough reflex is initiated through activation of vagal afferent nerves. Rapidly adapting receptors fulfill all criteria for the afferents subserving the cough reflex. Bronchopulmonary C-fibres may also initiate cough when activated. C-fibre-mediated cough may depend upon ongoing or initiated activity in rapidly adapting receptors. The interaction between airways C-fibres and rapidly adapting receptors may occur at sites in the periphery or in the brainstem. C-fibre mediated cough must also overcome a coincident inhibitory effect of C-fibre activation on cough, an inhibitory effect that becomes prominent under general anesthaesia.

Asthma -- a need for a rethink?

Asthma is a major medical problem but, despite decades of research, the mechanisms that underlie this condition remain elusive. Although the eosinophil has been regarded as a cell that is central to the pathogenesis of asthma, the failure to abrogate asthma symptoms by novel treatments that are designed to suppress the recruitment of eosinophils to the airways challenges this dogma. Our approach to understanding bronchial asthma needs to be broadened to include alterations in the function of afferent nerves that supply airways. Changes in the activity of these nerves offer a possible mechanism by which asthmatic subjects are uniquely responsive to a wide range of physiological and chemical stimuli. Here, we review the current status of asthma research.

Cough: potential pharmacological developments

Cough is an important defensive reflex of the upper airway and is also a very common symptom of respiratory disease. Cough following an upper respiratory viral infection is transient, and persistent cough is associated with a whole range of conditions, such as asthma, rhino-sinusitis and gastro-oesophageal reflux. Treatment directed at these conditions may improve the associated cough. There is often a need, however, to control cough itself whatever the cause. The most effective drugs in this class are the opioids, such as morphine, codeine or pholcodeine, but at effective doses they have side effects including drowsiness, nausea, constipation and physical dependence. Investigations into the cough reflex and into the potential mechanisms of sensitised cough reflex have uncovered several potential targets for novel drugs. New opioids apart from mu-agonists such as kappa- and delta -receptor agonists, have been developed, in addition to non-opioids such as nociceptin. Neurokinin receptor antagonists, bradykinin receptor antagonists, vanniloid receptor VR-1 antagonists may be beneficial by blocking effects of tachykinins and sensory nerve activation. Local anaesthetics, blockers of sodium-dependent channels and maxi-K Ca2+-dependent channel activators of afferent nerves are inhibitors of the cough reflex. Some of these novel agents may act centrally or peripherally or at both sites as antitussives. Large scale trials of these novel compounds have not been carried out in cough in man but there is a serious need for more effective antitussives devoid of side effects.

Synergistic interactions between airway afferent nerve subtypes mediating reflex bronchospasm in guinea pigs

The hypothesis that airway afferent nerve subtypes act synergistically to initiate reflex bronchospasm in guinea pigs was addressed. Laryngeal mucosal application of capsaicin or bradykinin or the epithelial lipoxygenase metabolite 15(S)-hydroxyeicosatetraenoic acid evoked slowly developing but pronounced and sustained increases in tracheal cholinergic tone in situ. These reflexes were reversed by atropine and prevented by vagotomy, trimethaphan, or laryngeal denervation. Central nervous system-acting neurokinin receptor antagonists also abolished the reflexes without altering baseline cholinergic tone. Baseline tone was, however, reversed by disrupting pulmonary afferent innervation while preserving the innervation of the trachea and larynx. Surprisingly, selective pulmonary denervation also prevented the laryngeal capsaicin-induced tracheal reflexes, suggesting that laryngeal C-fibers act synergistically with continuously active intrapulmonary mechanoreceptors to initiate reflex bronchospasm. Indeed, reflex bronchospasm evoked by histamine was markedly potentiated by bradykinin, an effect mimicked by intracerebroventricular, but not intravenous, substance P. These data, as well as anatomic evidence for afferent nerve subtype convergence in the commissural nucleus of the solitary tract, suggest that airway nociceptors and mechanoreceptors may act synergistically to regulate airway tone.

Pharmacology of airway afferent nerve activity

Afferent nerves in the airways serve to regulate breathing pattern, cough, and airway autonomic neural tone. Pharmacologic agents that influence afferent nerve activity can be subclassified into compounds that modulate activity by indirect means (e.g. bronchial smooth muscle spasmogens) and those that act directly on the nerves. Directly acting agents affect afferent nerve activity by interacting with various ion channels and receptors within the membrane of the afferent terminals. Whether by direct or indirect means, most compounds that enter the airspace will modify afferent nerve activity, and through this action alter airway physiology.