Stuart B. Mazzone

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 Aδ‐ 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 Aδ‐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.

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.

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.

Reflex mechanisms in gastroesophageal reflux disease and asthma.

This article presents a brief description of the reflex mechanisms responsible for cough and bronchospasm, and identifies several potential mechanisms by which gastroesophageal reflux (GER) may precipitate these reflexes. Airway and esophageal reflexes related to various mechanoreceptors and chemoreceptors have been elucidated, primarily in animal studies. Central nervous system (CNS) reflex pathways as well as local axon reflexes may each contribute to the pathogenesis of both asthma and GER disease (GERD). When activated, airway nociceptors precipitate defensive reflexes such as cough, bronchospasm, and mucus secretion. Nociceptors innervating both the airways and the esophagus respond to similar stimuli with defensive manuevers. The pathways of some esophageal and airway sensory nerves terminate in the same regions of the CNS. It appears possible that synergistic interactions between esophageal nociceptors and airway sensory nerves may precipitate the asthma-like symptoms associated with GERD.

Chronic cough as a neuropathic disorder

Chronic cough is a common symptom that can be a daunting challenge for clinicians since treatment of the underlying cause does not always provide adequate relief, an obvious cause can remain elusive, and current antitussives have fairly poor efficacy and undesirable side-effects. Patients with chronic cough typically describe a range of sensory symptoms suggestive of upper-airway and laryngeal neural dysfunction. Additionally, patients often report cough triggered by low-level physical and chemical stimuli, which is suggestive of cough-reflex hyperresponsiveness. Pathophysiological mechanisms underlying peripheral and central augmentation of the afferent cough pathways have been identified, and compelling evidence exists for a neuropathy of vagal sensory nerves after upper-respiratory viral infections or exposure to allergic and non-allergic irritants. In this Personal View, we argue that chronic cough is a neuropathic disorder that arises from neural damage caused by a range of inflammatory, infective, and allergic factors. In support of this idea, we discuss evidence of successful treatment of chronic cough with agents used for treatment of neuropathic pain, such as gabapentin and amitriptyline. Regarding cough as a neuropathic disorder could lead to new, more effective antitussives.

Investigation of the neural control of cough and cough suppression in humans using functional brain imaging.

Excessive coughing is one of the most common reasons for seeking medical advice, yet the available therapies for treating cough disorders are inadequate. Humans can voluntarily cough, choose to suppress their cough, and are acutely aware of an irritation that is present in their airways. This indicates a significant level of behavioral and conscious control over the basic cough reflex pathway. However, very little is known about the neural basis for higher brain regulation of coughing. The aim of the present study was to use functional brain imaging in healthy humans to describe the supramedullary control of cough and cough suppression. Our data show that the brain circuitry activated during coughing in response to capsaicin-evoked airways irritation is not simply a function of voluntarily initiated coughing and the perception of airways irritation. Rather, activations in several brain regions, including the posterior insula and posterior cingulate cortex, define the unique attributes of an evoked cough. Furthermore, the active suppression of irritant-evoked coughing is also associated with a unique pattern of brain activity, including an involvement of the anterior insula, anterior mid-cingulate cortex, and inferior frontal gyrus. These data demonstrate for the first time that evoked cough is not solely a brainstem-mediated reflex response to irritation of the airways, but rather requires active facilitation by cortical regions, and is further regulated by distinct higher order inhibitory processes.

Central nervous system control of the airways: Pharmacological implications

Autonomic innervation of the airways is derived primarily from the parasympathetic nervous system. Preganglionic fibers originating in the brainstem project to parasympathetic ganglion neurons, which regulate airway smooth-muscle tone, glandular secretion and blood-vessel diameter. Airway preganglionic nerve activity is regulated by subsets of pulmonary and extrapulmonary afferent nerve fibers, which continuously provide polysynaptic input to brainstem preganglionic nuclei. Each of these synapses in the central nervous system is a potential site for therapeutic intervention. Potential targets include increasing opioid, GABAergic and serotonergic controls on central neurons, and blockade of tachykinin and glutamate receptors. Unfortunately, much is still unknown about the control of airway nerves at the level of the central nervous system. Recently, however, interaction between vagal afferent nerve subtypes regulating airway function has been described. This interaction, made possible by their convergence at key sites of integration in the brainstem, may lead to central sensitization analogous to that described in somatic pathways regulating pain sensation. Improved understanding of the central pharmacology of airway reflexes may provide novel therapeutics for treating symptoms associated with respiratory disorders such as chronic obstructive pulmonary disease, asthma and sleep-disordered breathing.

Selective Expression of a Sodium Pump Isozyme by Cough Receptors and Evidence for Its Essential Role in Regulating Cough

We have identified a distinct subtype of airway vagal afferent nerve that plays an essential role in regulating the cough reflex. These afferents are exquisitely sensitive to punctate mechanical stimuli, acid, and decreases in extracellular chloride concentrations, but are insensitive to capsaicin, bradykinin, histamine, adenosine, serotonin, or changes in airway intraluminal pressures. In this study we used intravital imaging, retrograde neuronal tracing, and electrophysiological analyses to characterize the structural basis for their peculiar mechanical sensitivity and to further characterize the regulation of their excitability. In completing these experiments, we uncovered evidence for an essential role of an isozyme of Na+-K+ ATPase in regulating cough. These vagal sensory neurons arise bilaterally from the nodose ganglia and are selectively and brilliantly stained intravitally with the styryl dye FM2-10. Cough receptor terminations are confined and adherent to the extracellular matrix separating the airway epithelium and smooth muscle layers, a site of extensive remodeling in asthma and chronic obstructive pulmonary disease. The cough receptor terminals uniquely express the α3 subunit of Na+-K+ ATPase. Intravital staining of cough receptors by FM2-10, cough receptor excitability in vitro, and coughing in vivo are potently and selectively inhibited by the sodium pump inhibitor ouabain. These data provide the first detailed morphological description of the peripheral terminals of the sensory nerves regulating cough and identify a selective molecular target for their modulation.

Respiratory action of capsaicin microinjected into the nucleus of the solitary tract: involvement of vanilloid and tachykinin receptors

The respiratory response to microinjection of capsaicin into the commissural nucleus of the solitary tract (cNTS) of urethane‐anaesthetized rats was investigated in the absence and presence of the competitive vanilloid (capsaicin) antagonist, capsazepine, and selective tachykinin NK1, NK2 and NK3 antagonists (RP 67580, SR 48968 and SR 142801, respectively). Microinjection of capsaicin reduced respiratory frequency but not tidal volume (VT), leading to an overall reduction in minute ventilation (V{dot above}E). The effect was dose‐dependent between 0.5 and 2 nmol capsaicin. Doses greater than 2 nmol produced apnoea. Tachyphylaxis was observed following repeated injection of capsaicin (1 nmol, 30 min apart). Capsazepine (1 nmol) had no effect on frequency or VT when injected alone but completely blocked the respiratory response to capsaicin (1 nmol). RP 67580 (1 but not 5 nmol) alone depressed frequency and VT slightly. Moreover, RP 67580 appeared to potentiate the bradypnoeic effect of capsaicin. In contrast, SR 48968 and SR 142801 (1 and 5 nmol) alone had no significant effect on respiration. However, both agents significantly attenuated the reduction in frequency produced by capsaicin. In conclusion, microinjection of capsaicin into the cNTS decreases overall ventilation, primarily by reducing frequency. The action of capsaicin appears from the data to be mediated by vanilloid receptors since it is blocked by the competitive vanilloid antagonist capsazepine and is subject to tachyphylaxis. However, since NK2 (SR 48968) and NK3 (SR 142801) receptor antagonists block the actions of capsaicin, we propose that capsaicin acts also by releasing tachykinins from central afferent terminals in the cNTS.

Respiratory actions of tachykinins in the nucleus of the solitary tract: characterization of receptors using selective agonists and antagonists

The respiratory response to microinjection of tachykinins and analogues into the commissural nucleus of the solitary tract (cNTS) of urethane‐anaesthetized rats was investigated in the presence and absence of selective tachykinin NK1, NK2 and NK3 antagonists (RP 67580, SR 48968 and SR 142801, respectively). All tachykinins, except for the selective NK2 agonist, [Nle10]‐NKA(4‐10), increased tidal volume (VT). The rank potency order of naturally‐occurring tachykinins was neurokinin A (NKA)substance P (SP)>>NKB, whereas the rank order for selective analogues was senktideseptide>> [Sar9,Met(O2)11]‐SP>>[Nle10]‐NKA(4‐10). Septide (NK1‐selective) and senktide (NK3‐selective) were 22 fold more potent (pD2∼12) at stimulating VT than SP (pD2∼10.5). Tachykinin agonists produced varying degrees of respiratory slowing, independent of changes in VT. At doses producing maximum stimulation of VT, agonists induced either a mild (<10 breaths min−1 decrease; SP and septide), moderate (10–25 breaths min−1 decrease; NKA, NKB and [Sar9,Met(O2]‐SP) or severe (∼40 breaths min−1 decrease; senktide) bradypnoea. [Nle10]‐NKA(4‐10) produced a dose‐dependent bradypnoea without affecting VT. RP 67580 significantly attenuated the VT response to SP (33 pmol) and NKA (10 pmol) but not NKB (100 pmol). In the presence of RP 67580, the mild bradypnoeic response to NKB was significantly enhanced whereas SP and NKA induced a bradypnoea which was not observed in the absence of RP 67580. SR 48968 had no effect on the VT response to SP or NKB, markedly enhanced the VT response to NKA and completely blocked the bradypnoeic response to [Nle10]‐NKA(4‐10). Only SR142801 attenuated the VT response to NKB. The present data suggest that all three tachykinin receptors (NK1, NK2 and NK3) are present in the cNTS and are involved in the central control of respiration.