Analgesic and Antiinflammatory Effects of Two Chemotypes of Lippia alba: a Comparative Study

The antinociceptive and antiedematogenic effects of essential oils (EO, types I and II) from the leaves of two chemotypes of Lippia alba were studied with mice using the hot plate test, acetic acid-induced writhing, and the formalin test, and with rats using paw edema induced by carrageenan or dextran. The results showed dose-dependent inhibition of writhing with doses of 0.5 and 1 mg/kg, i.p., and 1 and 2 mg/kg, p.o., with chemotypes I and II, respectively. A similar but less intense effect was detected in the formalin test, where the two chemotypes (0.5 and 1 mg/kg, i.p.) predominantly inhibited the 2nd phase of the response, and only the effect of the EO I was reversed by the opioid antagonist, naloxone. Latency time to the thermic stimulus as detected by the hot plate test was increased with I but not with II, at doses higher than 10 mg/kg, p.o. A significant antiedematogenic effect was seen at 2 h with 10 and 50 mg/kg, p.o., of I, in paw edema induced by carr ageenan or dextran. However, in the same dose range, II was more effective against dextran-induced edema, but no effect was seen with the carrageenan model. The essential oils of the two types of L. alba are chemically distinct, with I containing a high content of citral and II a high content of carvone with no citral, which could explain the observed differences in their pharmacological actions.