Action of serotonin on the laryngeal airway in anaesthetized cats.

The pulmonary chemoreflex induced by an intravenous injection of serotonin (5-hydroxytryptamine) in cats consists of prompt apnoea, bradycardia and hypotension, followed by rapid, shallow breathing. The present study had two purposes (1) to compare the effect of 5HT on ventilation and laryngeal resistance in cats and (2) to assess the role of laryngeal afferents in these responses. The effects of an intravenous injection of serotonin at a dose of 0.05 mg per kg of body weight were studied in eighteen anaesthetized cats, breathing spontaneously via a tracheal cannula. In eleven cats the larynx was isolated in situ to measure laryngeal resistance. In post-serotonin apnoea, the expiratory laryngeal resistance rose four-fold. This coincided with the increased afferent activity of the superior laryngeal nerve. In the initial phase of resumed shallow breathing, the increase in the expiratory laryngeal resistance was coupled with reduced tidal volume. Bilateral section of the superior laryngeal nerve failed to affect laryngeal constriction and the ventilatory response to serotonin. Thus laryngeal afferents running within the superior laryngeal nerve are not essential for the respiratory phenomena induced by serotonin.