Further analyses of mechanisms underlying the antinociceptive effect of the triterpene 3β, 6β, 16β-trihydroxylup-20(29)-ene in mice

The present study investigated the mechanisms involved in the antinociception produced by the triterpene 3β, 6β, 16β-trihydroxylup-20(29)-ene (TTHL) in mice. TTHL administered by intra-gastric (i.g.) gavage inhibited glutamate-induced nociception with an ID50 of 19.0 (13.2-27.5) mg/kg. This action started 60 min (inhibition of: 59 ± 6%) after i.g. administration and remained significant up to 6 h (inhibition of 37 ± 6%). Moreover, TTHL inhibited both phases of formalin induced pain. The antinociception of TTHL was reversed by the pre-administration of naloxone (1 mg/kg; non-selective opioid receptor antagonist), CTOP (1 mg/kg; selective μ-opioid receptor antagonist), nor-binaltorphimine (1 mg/kg; selective κ-opioid receptor antagonist), naltrindol (3 mg/kg; selective δ-opioid receptor antagonist), p-chlorophenylalanine methyl ester (100 mg/kg for 4 consecutive days; inhibitor of serotonin synthesis), WAY100635 (0.5 mg/kg; selective 5-HT1A receptor antagonist) and ketanserin (0.3 mg/kg; selective 5-HT2A receptor antagonist) but not by L-arginine (600 mg/kg; nitric oxide precursor) or ondansetron (0.5 mg/kg; 5-HT3 receptor antagonist). Furthermore, the TTHL antinociception was prevented by intrathecal (i.t.) pre-treatment with pertussis toxin (0.5 μg/site; inactivator of Gi/o protein), charybdotoxin (250 pg/site; blocker of large-conductance calcium-gated K+ channels), tetraethylammonium (1 μg/site; blocker of voltage-gated K+ channels) and glibenclamide (80 μg/site; blocker of ATP-gated K+ channels) but not by apamin (50 ng/site; blocker of small-conductance calcium-gated K+ channels). The antinociception of TTHL was not it associated with locomotor impairment or sedation. These results showed that TTHL presented a pronounced antinociceptive effect, which is dependent on opioid and serotonergic systems, Gi/o protein activation and the opening of specific K+ channels.