APNEIC RESPONSES TO PULMONARY AND SYSTEMIC CHALLENGE OF CAPSAICIN IN CATS

The contribution of sensory laryngeal and pulmonary inputs to expirato ry apnoea and post-apnoeic breathing induced by capsaicin given to pul monary circulation and to aortic arch was studied in the anaesthetized , spontaneously breathing, normoxic cats. Breathing was ria tracheosto my. Capsaicin (10 mu g(kg body wt)(-1)) was injected intravenously and to the aortic arch in the intact animal, then after section of the su perior laryngeal nerves, and finally after midcervical vagotomy. Capsa icin, injected as a bolus, induced expiratory arrest of breathing on b oth ways of injection, larger and vagally dependent (P < 0.05) on intr avenous route, and apparently disparate in ventilatory sequence from t he systemic challenge. Tidal volume was affected in the opposite direc tion on either route and the respiratory rate increased significantly more with an intravenous administration (P < 0.01). Bilateral section of the cervical vagi virtually abolished the effects of capsaicin on t he breathing pattern independent of the site of challenge.