Effect of d-pseudoephedrine on cough reflex and its mode of action in guinea pigs.

d-Pseudoephedrine (PSE) is one of the main ingredients of Ephedrae herba. Although PSE is widely applied for patients with a common cold and upper respiratory inflammation as a decongestant, the effects of PSE on cough have never been reported. In this study, we investigated the antitussive effects of intraperitoneal injection of PSE on the cough reflex induced by microinjection of citric acid into the larynx of guinea pigs. PSE decreased the number of cough reflexes dose-dependently (-18.3 +/- 5.0% at 20 mg/kg, P<0.05; -41.1 +/- 7.2% at 60 mg/kg, P<0.01). Furthermore, PSE (60 mg/kg) increased the threshold intensity for inducing fictive cough by electrical micro-stimulation of the nucleus tractus solitarius (+72.7 +/- 8.4%, P<0.01). On the afferent discharge of the superior laryngeal nerve, PSE suppressed the increases of amplitude and frequency when stimulated by citric acid at laryngeal mucosa. These results demonstrate that PSE possesses an antitussive effect that might be derived from both central and peripheral actions.