Aim:In light of the antinociceptive activity of the short-chain neurotoxin,cobrotoxin,and other acetylcholine antagonists,the antinociceptive activity andmechanisms of cobratoxin(CTX),a long-chain postsynaptic α-neurotoxin,wasinvestigated in rodent pain models.Methods:CTX was administered intraperito-neally(30,45,68 μg/kg),intra-cerebral ventricularly(4.5 μg/kg)or microinjectedinto periaqueductal gray(PAG;4.5 μg/kg).The antinociceptive action was testedusing the hot-plate and acetic acid writhing tests m mice and rats.The involve-ment of the cholinergic system and opioid system in CTX-induced analgesia wasexamined by pretreatment of animals with atropine(0.5 mg/kg,im;or 10 mg/kg,ip)or naloxone(1 and 5 mg/kg,ip).The effect of CTX on motor activity was testedusing the Animex test.Results:CTX exhibited a dose-dependent analgesic ac-tion in mice as determined by both the hot-plate and acetic acid writhing tests.The peak effect of analgesia was seen 3 h after administration.In the mouse aceticacid writhing test,the intra-cerebral ventricular administration of CTX at 4.5 μg/kg(1/12th of a systemic dose)produced marked analgesic effects.Microinjection ofCTX(4.5 μgkg)into the PAG region did not elicit an analgesic action in rats in thehot-plate test.Atropine at 0.5 mg/kg(im)and naloxone at 1 and 5 mg/kg(ip)bothfailed to block the analgesic effects of CTX,but atropine at 10 mg/kg(ip)didantagonize the analgesia mediated by CTX in the mouse acetic acid writhing test.Acetylsalicylic acid(300 mg/kg)did not enhance the analgesic effects of CTX.Atthe highest effective dose of 68 μg/kg the neurotoxin did not change the sponta-neous mobility of mice.Conclusion:CTX has analgesic effects,which are medi-ated in the central nervous system though not through the PAG.The centralcholinergic system but not opioid system appears to be involved in theantinociceptive action of CTX. Aim: In light of the antinociceptive activity of the short-chain neurotoxin, cobrotoxin, and other acetylcholine antagonists, the antinociceptive activity and mechanisms of cobratoxin (CTX), a long-chain postsynaptic alpha-neurotoxin, was investigated in rodent pain models. Methods: CTX The antinociceptive action was tested using the hot-plate and acetic (PAG; 4.5 μg / kg). Administration of intra-cerebral ventricularly (4.5 μg / kg) or microinjection of periaqueductal gray acid writhing tests m mice and rats. Involving the cholinergic system and opioid system in CTX-induced analgesia wasexamined by pretreatment of animals with atropine (0.5 mg / kg, im; or 10 mg / kg, ip) or naloxone 1 and 5 mg / kg, ip). The effect of CTX on motor activity was tested using the Animex test. Results: CTX exhibited a dose-dependent analgesic ac- tion in mice as determined by both the hot-plate and acetic acid writhing tests The peak effect of analgesia was seen 3 h after administration. In the mouse aceticacid writhing test, the intra-cerebral ventricular administration of CTX at 4.5 μg / kg (1/12 th of a systemic dose) produced marked analgesic effects. Microinjection of CTX (4.5 μg kg) into the PAG region did not elicit an analgesic action in rats in thehot-plate test. Atropine at 0.5 mg / kg (im) and naloxone at 1 and 5 mg / kg (ip) both failed to block the analgesic effects of CTX, but atropine at 10 mg / kg (ip) didantagonize the analgesia mediated by CTX in the mouse acetic acid writhing test. Acetylsalicylic acid (300 mg / kg) did not enhance the analgesic effects of CTX. The highest effective dose of 68 μg / kg of the neurotoxin did not change the sponta- neous mobility of mice. Conlusion: CTX has analgesic effects, which are medi-ated in the central nervous system though not through the PAG. Central nervous system but not opioid system appears to be involved in the actinociceptive action of CTX.