Antitussive and expectorant activities of licorice and its major compounds.

Licorice has been used as an antitussive and expectorant herbal medicine for a long history. This work evaluated the activities of 14 major compounds and crude extracts of licorice, using the classical ammonia-induced cough model and phenol red secretion model in mice. Liquiritin apioside (1), liquiritin (2), and liquiritigenin (3) at 50 mg/kg (i.g.) could significantly decrease cough frequency by 30-78% (p < .01). The antitussive effects could be partially antagonized by the pretreatment of methysergide or glibenclamide, but not naloxone. Moreover, compounds 1-3 showed potent expectorant activities after 3 days treatment (p < .05). The water and ethanol extracts of licorice, which contain abundant 1 and 2, could decrease cough frequency at 200 mg/kg by 25-59% (p < .05), and enhance the phenol red secretion (p < .05), while the ethyl acetate extract showed little effect. These results indicate liquiritin apioside and liquiritin are the major antitussive and expectorant compounds of licorice. Their antitussive effects depend on both peripheral and central mechanisms.