Effects of centrally administered naloxone on phrenic nerve activity

The present study examined the effect on respiration of naloxone administered centrally. Experiments were performed on anesthetized, paralyzed cats whose vagi and carotid sinus nerves had been cut. The animals' end-tidal pCO2 and body temperature were kept constant with servocontrollers. Naloxone administered into the third cerebral ventricle in doses of 40 micrograms/kg and 400 micrograms/kg resulted in significant (P less than 0.05) increases in phrenic neural activity. The response to the 400 micrograms/kg dose was significantly greater (P less than 0.05) than the response to the 40 micrograms/kg dose. A dose of 40 micrograms/kg naltrexone administered to each of two animals produced similar increases in respiratory activity. These data indicate that in the preparation studied endogenous opiates exert a tonic inhibitory effect on respiration through their action on central opiate receptors.