Laryngeal and tracheobronchial cough in anesthetized dogs

Abstract

Tussigenic sensitivity of laryngeal and tracheobronchial regions to mechanical and chemical stimuli was compared in 22 urethan-alpha-chloralose-anesthetized dogs. In addition, the contribution of myelinated and unmyelinated vagal fibers in mediating laryngeal and tracheobronchial cough was investigated. The intensity of cough was evaluated from changes in esophageal pressure. Whereas all mechanical stimulations and citric acid inhalations into tracheobronchial region elicited cough, only 56.7% of mechanical stimulation and 33.3% of citric acid challenges to larynx were effective. The intensity of tracheobronchial cough was significantly higher than that of laryngeal cough. When mechanical stimulation was conducted under visual control (bronchofiberscope), cough elicitability was found to be higher from tracheal bifurcation and main stem bronchi (62.5-87.5%) than from any laryngeal structure (0-42.9%). During partial block of vagal conduction (cooling to 6 degrees C), mechanical and citric acid tracheobronchial stimulations failed to elicit cough and mechanical laryngeal stimulation was effective only in 1 of 10 dogs. Intensity of cough was strongly decreased when mechanical stimulation followed capsaicin administration into trachea (0.3 ml; 100 micrograms/ml) or intravenously (10 micrograms/kg). We conclude that, in anesthetized dogs, stimulation of tracheobronchial region is more effective and prompt in eliciting cough than stimulation of larynx, myelinated vagal afferent fibers play an important role in mediating mechanically and citric acid-induced tracheobronchial cough and mechanically induced laryngeal cough, and stimulation of tracheobronchial and pulmonary capsaicin-sensitive receptors strongly inhibits mechanically induced cough.

Modulation of the cough reflex by antitussive agents within the caudal aspect of the nucleus tractus solitarii in the rabbit

“…However, RARs are far more mechanically sensitive to epithelial pressure than are C-fiber receptors, indicating that if relatively gentle stimuli are used they should cause cough via activation of RARs (e.g., 24, 49). In any case, stimulation of pulmonary and bronchial C-fiber receptors in anesthetized and unanesthetized animals by selective stimuli has never been shown to cause cough, rather it has been reported to inhibit this defensive reflex (58,59: for a review, see Ref. 49).…”

Neurophysiology of the cough reflex

“…Their site is appropriate and they respond to the same mediators and irritants as do the RARs and pulmonary C-fibre receptors [28]. There is, however, evidence that bronchial C-fibre receptors, like those at the pulmonary level, inhibit cough [55,56]. As shown in table 3, virtually all the same stimuli activate all three groups of receptor.…”

Inhibiting Effect of Ammonia on Citric Acid‐Induced Cough in Pigs: A Possible Involvement of Substance P

“…On this basis, it is possible to explain the antitussive effect of ammonia by different mechanisms. Firstly, knowing that ammonia can stimulate the electrophysiological activity of C-fibers in the rabbit (Matsumoto 1988), and in the dog (Tatar et al 1994), stimulation of pulmonary C-fiber receptors can inhibit, through a central gating mechanism, the cough reflex induced by mechanical stimulation of the larynx or trachea. A similar neurophysiological mechanism might apply to citric acid-induced cough in pigs.…”

“…These receptors induce coughing through a central reflex when activated by several other exogenous physical and chemical stimuli (Widdicombe 1995). However, by enhancing the electrophysiological activity of the afferent amyelinated C-fibers, ammonia could also modulate cough via an inhibitory control gating mechanism (Matsumoto 1989;Tatar et al 1994). Moreover, these C-fibers can also release neurokinins, as substance P, which has been shown to be involved in the citric acid-induced cough reflex in pigs (Moreaux et al 2000).…”