Capsaicin administration inhibits the abducent branch but excites the thyroarytenoid branch of the recurrent laryngeal nerves in the rat

Abstract

Our recent study showed that both inspiratory and expiratory activities of the recurrent laryngeal nerve (RLN) were enhanced by capsaicin administration in the rat (Lu IJ, Ku LC, Lin JT, Lee KZ, and Hwang JC. Chin J Physiol 45: 143-154, 2002). There are two intralaryngeal branches of the RLN: one innervates the thyroarytenoid (TA) muscle and the other innervates the abductor (Abd) muscles. To examine whether these two intralaryngeal branches respond similarly to capsaicin administration, their discharges as well as activities of the phrenic nerve (PNA) and the superior laryngeal nerve (SLNA) were monitored in anesthetized and ventilated rats at normocapnia in hyperoxia. The low dose of capsaicin (0.625 microg/kg) produced a cardiopulmonary chemoreflex, showing apnea, a decrease in PNA, hypotension, and bradycardia, and significant decreases in SLNA and the activity of the Abd branch. Concurrently, there was an increase in the intralaryngeal TA activity during both apnea and the recovery from apnea. The high dose of capsaicin (1.25 microg/kg) evoked larger chemoreflexive responses and laryngeal nerve activities. In addition, both doses of capsaicin initiated a similar delay in the onset of Abd activity and SLNA but an earlier onset for the TA branch to commence during inspiration. A bilateral vagotomy abolished the laryngeal responses to capsaicin administration. However, PNA and blood pressure were enhanced with capsaicin administration after the vagotomy. These results suggest that laryngeal adduction in response to capsaicin administration is vagal afferent dependent and that it may also represent reflexive protection for the airway and lungs.

Uncoupling of upper airway motor activity from phrenic bursting by positive end-expired pressure in the rat

“…The HN was exposed by a ventral approach and cut proximal to the bifurcation of this nerve into distinct medial and lateral branches (13,23). The SLN was isolated near the larynx and cut distally (25). The FN was dissected with a lateral approach similar to that in our previous report (16) and Fig.…”

“…Similar to the HN, the onset of FN, SLN, and RLN I bursting occurs before the onset of phrenic bursting (16,25).…”

“…This preinspiratory (Pre-I) activity has been suggested to benefit UAW patency by dilating and stiffening the UAW before the onset of I airflow (30). Similar to the HN, the onset of FN, SLN, and RLN I bursting occurs before the onset of phrenic bursting (16,25).The respiratory-related discharge of the UAW muscles is influenced by various chemical and mechanical stimuli. For example, activation of both pulmonary stretch receptors and vagal C fiber receptors inhibits respiratory bursting in UAW muscles and their corresponding motor nerves (11,23,25,40,42).…”

“…This preinspiratory (Pre-I) activity has been suggested to benefit UAW patency by dilating and stiffening the UAW before the onset of I airflow (30). Similar to the HN, the onset of FN, SLN, and RLN I bursting occurs before the onset of phrenic bursting (16,25).…”

“…For example, activation of both pulmonary stretch receptors and vagal C fiber receptors inhibits respiratory bursting in UAW muscles and their corresponding motor nerves (11,23,25,40,42). This inhibition is associated with a delay in the onset of the UAW muscle I activity relative to the phrenic burst (23,25).…”