Comparison of the analgesic actions of THIP and morphine

The antinociceptive actions of intraperitoneally-administered 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) and morphine were compared using three strains of mice. With the hot-plate assay, ED50 values for the action of THIP were about 4 mg/kg in OF1, CD1 and NMRI strains, whereas ED50 values for morphine varied among strains, being 6.8 mg/kg for OF1, 16.9 mg/kg for CD1, and about 29 mg/kg for NMRI mice; thus, the genetic control of the analgesic action of THIP appears to differ from that of morphine. The analgesic action of THIP in the hot-plate test was not blocked by naloxone, bicuculline, phentolamine or methysergide, but was partially reversed by a high dose of atropine, indicating that classic opiate-receptors, bicuculline-sensitive GABA-receptors, alpha-adrenoceptors and serotonin-receptors do not appear to mediate the action of THIP but that cholinergic receptors might be indirectly involved. THIP was about equipotent or more potent than morphine in the phenylbenzoquinone writhing test, evasion test, and traction test. Since the ED50 values for THIP in OF1 mice were similar for hot-plate, evasion and traction tests, the analgesic action of THIP might not be readily dissociated from its sedative or myorelaxant action.