Attenuation of capsaicin-induced acute and visceral nociceptive pain by a - and h -amyrin, a triterpene mixture isolated from Protium heptaphyllum resin in mice

Abstract

The triterpene mixture, a - and h -amyrin, isolated from Protium heptaphyllum resin was evaluated on capsaicin- evoked nociception in mice. Orally administered a - and h -amyrin (3 to 100 mg/kg) significantly suppressed the nociceptive behaviors—evoked by either subplantar (1.6 A g) or intracolonic (149 A g) application of capsaicin. The antinociception produced by a - and h -amyrin against subplantar capsaicin-induced paw-licking behavior was neither potentiated nor attenuated by ruthenium red (1.5 mg/kg, s.c.), a non-specific antagonist of vanilloid receptor (TRPV1), but was greatly abolished in animals pretreated with naloxone (2 mg/kg, s.c.), suggesting an opioid mechanism. However, participation of a 2 -adrenoceptor involvement was unlikely since yohimbine (2 mg/ kg, i.p.) pretreatment failed to block the antinociceptive effect of a - and h -amyrin in the experimental model of visceral nociception evoked by intracolonic capsaicin. The triterpene mixture (3 to 30 mg/kg, p.o.) neither altered significantly the pentobarbital sleeping time, nor impaired the ambulation or motor coordination in open-field and rota-rod tests, respectively, indicating the absence of sedative or motor abnormality that could account for its antinociception. Nevertheless, a - and h -amyrin could significantly block the capsaicin (10 mg/kg, s.c.)-induced