Reflex apneic response evoked by laryngeal exposure to wood smoke in rats: Neural and chemical mechanisms

We investigated the neural and chemical mechanisms contributing to the immediate ventilatory responses to laryngeal exposure to wood smoke in anesthetized Sprague-Dawley rats. Five milliliters of wood smoke were delivered into a functionally isolated larynx at a constant flow rate of 1.4 ml/s while the animals breathed spontaneously. Within 1 s after exposure, laryngeal wood smoke consistently triggered an apnea in each of the 42 rats tested. The apneic duration reached 1,636.4 ± 105.4 (SE) % (n = 42) of the baseline expiratory duration. This apneic response was not affected by denervation of recurrent laryngeal nerves (n = 6) or by removal of smoke particulates (n = 14), but it was totally eliminated by topical application of an anesthetic (n = 8; lidocaine hydrochloride, 8%) to the laryngeal mucosa or by sectioning of the superior laryngeal nerves (n = 42). Furthermore, laryngeal application of a hydroxyl radical scavenger (dimethylthiourea; 500 mg/ml; n = 8) greatly diminished or abolished the smoke-induced apneic response, but it did not affect the apneic response evoked by laryngeal exposure to air saturated with 6% ammonia. These results suggest that the immediate apneic response to laryngeal wood smoke is a reflex resulting from the stimulation of the superior laryngeal afferents by the gas phase of wood smoke and that the stimulation is mediated through a hydroxyl radical mechanism.