The effects of hypercapnia and cooling the ventral medullary surface on capsaicin induced respiratory reflexes

Abstract

The effect of right atrial (RA) injection of 3 μg/kg capsaicin on phrenic, hypoglossal and recurrent laryngeal activities was studied in chloralose anesthetized, paralyzed and artificially ventilated cats. Within 2 sec following capsaicin injection, the phrenic and hypoglossal activities completely disappeared (apnea), while the recurrent laryngeal activity markedly increased. Similar responses were also obtained with RA injection of phenyldiguinide (PDG), suggesting that the respiratory responses of both drugs are essentially similar. Sino-aortic denervation did not affect the capsaicin induced respiratory responses. Bilateral vagotomy abolished the responses, suggesting that vagal sensory receptors are responsible for the reflex effects. Hyperoxic hypercapnia (3 and 7% CO 2 in O 2) reduced the apneic duration of phrenic and hypoglossal nerves. The magnitude of the recurrent laryngeal excitation was decreased during CO 2 breathing. Graded focal cooling of the intermediate area (I S area) of the ventral medullary surface (to inhibit central chemoreceptor activity) significantly prolonged capsaicin induced apneic duration of hypoglossal nerves more than the phrenic. The recurrent laryngeal responses, however, were unaffected by cooling of the ventral medullary surface. The results show that capsaicin and PDG, presumably by stimulating C fibers, affect cranial nerves as well as the phrenic. The reflex responses to C fiber stimulation seem to be altered by intervention which stimulate (hypercapnia) or depress (I S cooling) ‘central chemoreceptors’.