CARDIOPULMONARY RESPONSES TO CAPSAICIN INSTILLATION TO THE LARYNGEAL LUMEN AND THEIR REFLEX MECHANISMS IN RATS

Cardiorespiratory effects of capsaicin (CAPS) solution instilled into the larynx and the reflex mechanisms were investigated in rats with sp ontaneous breathing or under artificial ventilation. The first challen ge with CAPS (100 mu g/ml, 20 mu l) markedly inhibited spontaneous bre athing due to a considerable prolongation of expiration time (T-E) (17 85% of control) in all rats. Circulatory changes such as hypertension (mean systolic blood pressure=210 mmHg) and bradycardia (10.5% decreas e in heart rate) were also elicited by the ist challenge with CAPS. Th ese changes were largely reduced by the second challenge of CAPS; but not abolished in 3 of 5 rats by bilateral section of the superior lary ngeal nerves (SLNs) and recurrent laryngeal nerves (RLNs). The bradyca rdia and hypertension after the CAPS-instillation were able to be elic ited to the same extent in rats in the absence of apnea under artifici al 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 cardi orespiratory reflexes, where the circulatory changes could be elicited in the absence of apnea, suggesting the presence of eliciting mechani sms 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 a nd thin myelinated fibers and by the efferents, both the parasympathet ic and sympathetic nervous systems.