[Effects of antitussive drugs on the central respiratory mechanisms during the cough reflex]

Effects of antitussive drugs on the phrenic nerve activities during the cough reflex were investigated by means of a power spectrum analysis as a quantitative evaluation of each of the frequency band components of the phrenic nerve activities in anesthetized dogs. The efferent activities of the phrenic nerve were recorded from the central cut end of the phrenic nerve. Each fraction of the phrenic nerve activity was fractionated into bands spanning a range of 100 Hz each by a variable filter and analyzed using a program for the power spectrum analysis. The increase in power of each of the frequency band components was observed during the cough reflex induced by mechanical stimulation of the tracheal mucosa. Particularly, the power of the 2 approximately 100 Hz band components increased significantly as compared with the other frequency band components. An i.v. administration of codeine (3 mg/kg) significantly inhibited the increase in power of all frequency band components during the cough reflex. After administration of dextromethorphan (10 mg/kg, i.v.) or fominoben (8 mg/kg, i.v.), the increase in power of the 2 approximately 100 Hz band components of the phrenic nerve activity during the cough reflex was decreased significantly for 5 approximately 10 min; however, the other frequency band components were not affected. These results provide some evidence for a difference between these three antitussive drugs with respect to the mechanisms of action in the process of the central integration for the cough reflex.