Inhibitory effect of Zataria multiflora Boiss and carvacrol on histamine (H1) receptors of guinea-pig tracheal chains

The inhibitory effect of aqueous-ethanolic extract of Zataria multiflora Boiss (Labiatae) and carvacrol on histamine (H1) receptors was examined on tracheal chains of guinea-pigs. The effects of three concentrations of aqueous-ethanolic extract, carvacrol, 10 nm chlorpheniramine, and saline on histamine (H1) receptors were tested on three groups of guinea-pig tracheal chains as follows: incubated trachea with (i) indomethacin (n = 9), (ii) indomethacin, propranolol, and atropine (n = 7), and (iii) indomethacin and propranolol (n = 6). The EC50 (effective concentration of histamine causing 50% of maximum response) obtained in the presence of chlorpheniramine for all concentrations of the extract and carvacrol in all three groups was significantly higher than that of saline (P < 0.001 for all cases). The EC50 obtained in the presence of all concentrations of extract in groups 2 and 3 was lower than group 1 and in group 3 lower than group 2 (P < 0.01 to P < 0.001). However, EC50 obtained in the presence of all concentrations of carvacrol in group 3 and two higher concentrations in group 2 was higher than that of group 1 (P < 0.01 to P < 0.001). There was no significant difference in the maximum response obtained in the presence of different concentrations of extract and carvacrol between three groups. There was a parallel rightward shift in concentrationresponse curves obtained in the presence of all concentrations of the extract and carvacrol in all three groups. These results indicated an inhibitory effect of Z similar to multiflora and its constituent carvacrol on histamine H1 receptors.