Antinoceptive effect of triterpenoid alpha,beta-amyrin in rats on orofacial pain induced by formalin and capsaicin.

The effects of alpha,beta-amyrin, a pentacyclic triterpene isolated from Protium heptaphylum was investigated on rat model of orofacial pain induced by formalin or capsaicin. Rats were pretreated with alpha,beta-amyrin (10, 30, and 100mg/kg, i.p.), morphine (5mg/kg, s.c.) or vehicle (3% Tween 80), before formalin (20 microl, 1.5%) or capsaicin (20 microl, 1.5 microg) injection into the right vibrissa. In vehicle-treated controls, formalin induced a biphasic nociceptive face-rubbing behavioral response with an early first phase (0-5 min) and a late second phase (10-20 min) appearance, whereas capsaicin produced an immediate face-rubbing (grooming) behavior that was maximal at 10-20 min. Treatment with alpha,beta-amyrin or morphine significantly inhibited the face-rubbing response in both test models. While morphine produced significant antinociception in both phases of formalin test, alpha,beta-amyrin inhibited only the second phase response, more prominently at 30 mg/kg, in a naloxone-sensitive manner. In contrast, alpha,beta-amyrin produced much greater antinociceptive effect at 100mg/kg in the capsaicin test, which was also naloxone-sensitive. These results provide first time evidence to show that alpha,beta-amyrin attenuates orofacial pain at least, in part, through a peripheral opioid mechanism but warrants further detailed study for its utility in painful orofacial pathologies.