Antinociceptive Properties of 1,8-Cineole and β-Pinene, from the Essential Oil of Eucalyptus camaldulensis Leaves, in Rodents

1,8-Cineole (cineole) and β-pinene, two monoterpenes isolated from the essential oil obtained from Eucalyptus camaldulensis Dehn leaves were tested for antinociceptive properties. Tail-flick and hot-plate methods, reflecting the spinal and supraspinal levels, respectively, were used in mice and/or rats using morphine and naloxone for comparison. Cineole exhibited an antinociceptive activity comparable to that of morphine, in both algesic stimuli. A significant synergism between cineole and morphine was observed, but naloxone failed to antagonize the effect of cineole. /3-Pinene exerted supraspinal antinociceptive actions in rats only and it reversed the antinociceptive effect of morphine in a degree equivalent to naloxone, probably acting as a partial agonist through the μ opioid receptors. From structure-activity relationships of the pairs morphine + cineole and naloxone + β-pinene, it was shown that similarities exist in the stereochemistry and in the respective atomic charges of these molecules. Further studies are in progress in order to elucidate the mechanism of action of the two terpenoids.