Neurophenotypes in Airway Diseases. Insights from Translational Cough Studies

Abstract

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).

Trial registration: ClinicalTrials.gov NCT01297790.

Keywords: COPD; asthma; cough; guinea pig; vagus nerve.

Figure 1.

(A–C) Cough reflex sensitivity to tussive agents in airway diseases compared with healthy control subjects. Concentrations of tussive agents causing at least five coughs for all patient groups (C5). The lower the C5, the greater the sensitivity of the cough reflex. Note the logarithmic scale on the y-axes. Horizontal lines and error bars represent mean values and 95% confidence intervals. Asterisks denote significant differences compared with healthy control group (P < 0.05, analysis of variance). COPD = chronic obstructive pulmonary disease; PGE₂ = prostaglandin E₂.

Figure 2.

The numbers of coughs evoked by nebulized solutions of (A) capsaicin (5 min; n = 12) or (B) prostaglandin E₂ (PGE₂; 10 min; n = 10) in guinea pigs exposed to either air (open squares) or cigarette smoke (CS; solid squares) for 8 days. Coughs were recorded for 10 minutes from the start of nebulization. Data are presented as mean ± SE. *P < 0.05 as determined by Mann-Whitney U test.

Figure 3.

Depolarization of guinea pig and human vagus nerve. (A and B) Depolarization induced by capsaicin (n = 9) (A) or prostaglandin E₂ (PGE₂; n = 4) (B) of isolated vagus nerve taken from guinea pigs exposed to either air (open triangles) or cigarette smoke (CS; solid triangles) for 8 days. (C and D) Depolarization induced by capsaicin (n = 7 nonsmoker, 13 smoker) (C) or PGE₂ (open squares, nonsmoker, n = 9; solid squares, smoker, n = 10) (D) of isolated vagus nerve taken from human smokers and nonsmokers (with no other respiratory disease). Data are presented as mean ± SE. *P < 0.05 as determined by Mann-Whitney U test.

Figure 4.

[Ca²⁺]i flux in guinea pig airway-terminating vagal ganglia neurons. Intracellular calcium increases induced by (A and C) capsaicin or (B and D) prostaglandin E₂ (PGE₂) in isolated airway-terminating (A and B) jugular or (C and D) nodose ganglia neurons taken from guinea pigs exposed to either air (open symbols) or cigarette smoke (CS; solid symbols) for 8 days. The percentage displayed denotes the proportion of neurons recorded from where the responses of the Δ[Ca2⁺]i was at least 10% of the internal control response. Data are presented as the mean ± SE; note different y-axis scales for [Ca²⁺]i flux with capsaicin application (A and C). *P < 0.05 as determined by Mann-Whitney U test. Representative traces (F340/F380 ratio, y-axis scale) of the data in each panel are shown in the respective inset graphs. These representative traces show [Ca²⁺]i recorded with Fura2 for airway neurons from air- (black line) or CS-exposed (red line) guinea pigs, where application of either capsaicin (1 μM) or PGE₂ (10 μM) is indicated by the black bar. AUC = area under the curve.