Cough and bronchoconstriction mediated by capsaicin-sensitive sensory neurons in the guinea-pig

Neural pathways involved in cough and reflex bronchoconstriction and the effects of drugs on these airway reflexes have been studied in unanaesthetised guinea-pigs exposed to aerosols of citric acid (0.13-0.78 M), capsaicin (30 microM), nicotine (9.2 mM) and histamine (0.9 mM). The number of coughs was counted during the first 3 min of exposure and the time to onset of signs of dyspnea, as an indication of bronchoconstriction, was measured. Citric acid produced bronchoconstriction and dose-dependently increased the number of coughs. Capsaicin produced both cough and bronchoconstriction. Nicotine mainly produced cough and histamine bronchoconstriction. Pretreatment of adult guinea-pigs with capsaicin (50 mg kg-1 s.c.) produced a long-lasting (greater than or equal to 10 weeks) depletion of substance P- and calcitonin gene related peptide-like immunoreactivities in the sensory nerves of the larynx, tracheobronchial tree and lung. In capsaicin-treated animals, citric acid (0.39 M) and capsaicin (30 microM) caused neither cough nor bronchoconstriction. Nicotine (9.2 mM) and mechanical stimulation still produced cough, and histamine (0.9 mM) bronchoconstriction. It is concluded that in guinea-pigs both capsaicin-sensitive (probably C-fibre endings) and capsaicin-resistant (probably rapidly adapting stretch receptors) afferent neurons may be involved in cough and reflex bronchoconstriction.