Cholinergic influence on the sensitivity of cough reflex in awake guinea-pigs

1. We studied the effects of an intraperitoneal (i.p.) administration of atropine on bronchomotor tone induced by carbachol aerosols in anaesthetized and ventilated guinea-pigs and on the sensitivity of cough reflex to capsaicin aerosols in awake guinea-pigs. 2. The intraperitoneal administration of atropine (10^-8-10^-6 M kg^-1) reduced carbachol (8 mg ml^-1; 1 min)-induced increases in pulmonary insufflation pressure and a complete inhibition was achieved at a dose of 10^-6 M kg^-1 in anaesthetized guinea-pigs. 3. In awake guinea pigs, capsaicin aerosols (10^-5-10^-3 M) caused dose-dependent increases in the number of coughs. Atropine (10^-7-10^-6 M kg^-1 i.p.) shifted the dose-response curves to capsaicin to the right in a dose-dependent fashion. Carbachol aerosols, at the maximal concentration (2 mg ml^-1) that did not cause bronchoconstriction, shifted them to the left compared with the control. 4. Atropine (10^-6 M kg^-1 i.p.) significantly increased the provocative concentration of capsaicin that caused five coughs (P < 0.01) and carbachol aerosols (2 mg ml^-1) significantly decreased it (P < 0.01). 5. We conclude that cholinergically mediated airway tone regulates cough sensitivity in awake guinea pigs.