Effects of rosemary, plantago, and tea tree oil on the capsaicin-induced coughs in guinea pigs

Rosemary, plantago, and tea tree oil have been customarily used for the treatment of the common cold, but the detailed mechanisms of these antitussive effects are not well known. Thus, the aim of this study was to assess the antitussive properties of these herbs using a capsaicin-induced cough model in guinea pigs. Rosemary (10, 30 μL/ kg, p.o.), plantago (10, 30 mg/kg, p.o.), and tea tree oil (10, 30 μL/kg, p.o.) produced dose-dependent and significant antitussive effects. Methysergide (a serotonin receptor antagonist), but not naloxone (a opioid receptor antagonist), antagonize the antitussive effects of plantago and tea tree oil. On the other hand, neither methysergid nor naloxone had a significant effect on the antitussive effect of rosemary. Furthermore, γ-terpinene, terpinene-4-ol, cineole, and α-pinene, main components of tea tree oil and rosemary, significantly inhibited the number of capsaicin-induced coughs in guinea pigs. These results suggest that the antitussive effects of plantago and tea tree oil may depend on central mechanisms (modulation of serotonergic systems). On the other hand, the antitussive effects of rosemary may depend on peripheral mechanisms. Furthermore, it is suggested that γ-terpinene, terpinene-4-ol, cineole, and α-pinene may play an important role in the antitussive effects of tea tree oil and rosemary.