[Effects of naloxone on the cough depressant activities of antitussive drugs].

Effects of naloxone on the cough depressant action of antitussive drugs were investigated in pentobarbital anesthetized male and female cats. Respiration and cough reflex were measured using a pneumotachograph via a cannula inserted into the trachea. The cough reflex was elicited by electrical stimuli to the superior laryngeal nerve. An i.v. administration of morphine (1 mg/kg) significantly inhibited the cough reflex for at least 60 min. The antitussive effect of morphine was antagonized by naloxone (400 micrograms/kg). The inhibitory effect of morphine on the respiratory frequency was also antagonized by naloxone. The cough reflex was significantly inhibited by fominoben (5 mg/kg, i.v.) and dextromethorphan (3 mg/kg, i.v.). However, the respiratory frequency was increased by these two drugs. An i.v. administration of naloxone prevented the antitussive effects and the excitatory effects in respiratory frequency of fominoben and dextromethorphan. The present study suggests that the cough depressant actions of antitussive drugs may be mediated by endogenous opiates and/or by neurotransmitters which are modified by endogenous opiates.