Reflex activation of the nonadrenergic noncholinergic inhibitory nervous system in feline airways

Experiments were undertaken to learn if the nonadrenergic noncholinergic inhibitory nervous system (NANCIS) in feline airways could be activated reflexly either by mechanically stimulating the laryngeal mucosa or by inducing acute bronchospasm with boluses of 5-hydroxytryptamine (5HT) injected intravenously. Mechanical stimulation of the dorsal laryngeal mucosa evoked a biphasic bronchomotor response in anesthetized cats that had received intravenously an infusion of 5HT to raise their basal airway smooth muscle tone. The response consisted of a transient augmentation of bronchoconstriction followed by a prolonged bronchodilation. After muscarinic cholinergic receptor blockade with atropine, the constriction phase of the response disappeared, but the relaxation phase persisted. Bronchodilation elicited by laryngeal stimulation was resistant to beta-adrenergic receptor blockade with propranolol but was abolished by autonomic ganglionic blockade with hexamethonium and blocked reversibly by vagal cooling. The latter interventions, when imposed between successive dose-response curves generated by intravenous 5HT in animals pretreated with atropine and propranolol, did not alter the positions or slopes of the curves. These findings support the conclusion that mechanical stimulation of the larynx reflexly activates not only the well-known vagal cholinergic excitatory pathway to the airways but also the more recently described vagal, nonadrenergic noncholinergic inhibitory pathway. The results further indicate that bronchoconstriction is neither a prerequisite for the bronchodilator component of the laryngeal bronchomotor reflex nor an independent initiating stimulus for NANCIS-mediated reflex bronchodilation.