Objective To investigate the analgesia induced by receptin (REC), a chemically modified cobratoxin (CTX, a long-chain postsynaptic alpha -neurotoxin from Thailand cobra venom), and the effects of atropine and naloxone on antinociceptive activity of REC in rodent pain models. Methods REC was administered intraperitoneally (5 mg/kg, 7.07 mg/kg, or 10 mg/kg, i.p.) or intra-cerebral venticularly (62.5 mu g/kg, i.c.v.). The antinociceptive action was determined using the hot-plate test, the acetic acid writhing test and tail flick assay in mice and rats. The involvement of cholinergic and the opioid peptidergic systems in REC-induced analgesia were examined by pretreatment of animals with atropine (Atr; 0.5 mg/kg, i.m. or 10 mg/kg, i.p.) or naloxone (Nal; 3 mg/kg, i.p.). The effect of REC on motor activity was tested using the Animex test in mice. Results REC (5 mg/kg, 7.07 mg/kg or 10 mg/kg, i.p.) exhibited a dose-dependent analgesic action in mice as determined with hot-plate test and acetic acid writhing test. The significant analgesia of REC was seen 2 h to 3 h after its administration. In the rat-tail flick assay, the administration of REC at 62.5 mu g/kg (1/160 of systemic dose; i.c.v.) produced marked analgesic effects. Atropine at 0.5 mg/kg (i.m.), 10 mg/kg (i.p.) or naloxone at 3 mg/kg (i.p.) failed to block the analgesic effects of REC. REC at the highest effective dose of 10 mg/kg did not change the spontaneous mobility of mice. Conclusion These results demonstrate that REC has analgesic effect. This activity appears to be mediated through the peripheral nervous system though central nervous system may contribute to REC' s analgesic effects. The central cholinergic system and opioid peptidergic system appear not to be involved in the antinociceptive action of REC.

Objective To investigate the analgesia induced by cobrotoxin (CT) from venom of Naja naja atra, and the effects of atropine and naloxone on the antinociceptive activity of CT in rodent pain models. Methods CT was administered intraperitoneally (33.3, 50, 75 mu g/kg), intra-cerebral venticularly (2.4 mu g/kg) or microinjected into periaqueductal gray (PAG, 1.2 mu g/kg). The antinoCiceptive action was tested using the hot-plate test and the acetic acid writhing test in mice and rats. The involvement of cholinergic system and the opioid system in CT-induced analgesia was examined by pretreatment of animals with atropine (0.5 mg/kg, im or 10 mg/kg, ip) or naloxone (3 mg/kg, ip). The effect of CT on motor activity was tested using the Animex test. Results CT (33.3, 50 and 75 mu g/kg, ip) exhibited a dosedependent analgesic action in mice as determined with hot-plate test and acetic acid writhing test. In the mouse acetic acid writhing test, the intra-cerebral ventricle administration of CT 2.4 mu g/kg (1/23th of a systemic dose) produced marked analgesic effects. Microinjection of CT 1.2 mu g/kg (1/46th of systemic dose) into the PAG also elicited a robust analgesic action in the hot-plate test in rats. Atropine at 0.5 mg/kg (im) or naloxone at 3 mg/kg (ip) failed to block the analgesic effects of CT, but atropine at 10 mg/kg (ip) did antagonize the analgesia mediated by CT in the mouse acetic acid writhing test. At the highest effective dose of antinociception (75 mu g/kg), CT did not change the spontaneous mobility of mice. Conclusion These results suggest that CT from Naja naja atra venom has analgesic effects. Central nervous system may be involved in CT's analgesic effects and the PAG may be the primary central site where CT exerts its effects. The central cholinergic system but not opioid system appears to be involved in the antinociceptive action of CT.