Cardiopulmonary Responses to Capsaicin Instillation to the Laryngeal Lumen and Their Reflex Mechanisms in Rats

Abstract

Cardiorespiratory effects of capsaicin (CAPS) solution instilled into the larynx and the reflex mechanisms were investigated in rats with spontaneous breathing or under artificial ventilation. The first challenge with CAPS (100 μg/ml, 20 μl) markedly inhibited spontaneous breathing due to a considerable prolongation of expiration time (T_E) (1785% of control) in all rats. Circulatory changes such as hypertension (mean systolic blood pressure=210 mmHg) and bradycardia (10.5% decrease in heart rate) were also elicited by the 1st challenge with CAPS. These changes were largely reduced by the second challenge of CAPS; but not abolished in 3 of 5 rats by bilateral section of the superior laryngeal nerves (SLNs) and recurrent laryngeal nerves (RLNs). The bradycardia and hypertension after the CAPS-instillation were able to be elicited to the same extent in rats in the absence of apnea under artificial ventilation. The bradycardia was entirely abolished by pretreatment with atropine injection, while the hypertension was largely inhibited by phentolamine and propranolol. These results demonstrated that the laryngeal and/or pharyngeal noxious stimulus could induce marked cardiorespiratory reflexes, where the circulatory changes could be elicited in the absence of apnea, suggesting the presence of eliciting mechanisms of circulatory changes independent on the occurrence of apnea. In addition, it was also suggested that those cardiorespiratory responses were mediated by the afferents such as unmyelinated C-fiber endings and thin myelinated fibers and by the efferents, both the parasympathetic and sympathetic nervous systems.