Apnoeic responses to pulmonary and systemic challenge of capsaicin in cats.

The contribution of sensory laryngeal and pulmonary inputs to expiratory apnoea and post-apnoeic breathing induced by capsaicin given to pulmonary circulation and to aortic arch was studied in the anaesthetized, spontaneously breathing, normoxic cats. Breathing was via tracheostomy. Capsaicin (10 micrograms (kg body wt)-1) was injected intravenously and to the aortic arch in the intact animal, then after section of the superior laryngeal nerves, and finally after midcervical vagotomy. Capsaicin, injected as a bolus, induced expiratory arrest of breathing on both ways of injection, larger and vagally dependent (P < 0.05) on intravenous route, and apparently disparate in ventilatory sequence from the systemic challenge. Tidal volume was affected in the opposite direction 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 the breathing pattern independent of the site of challenge.