A long-form alpha-neurotoxin from cobra venom produces potent opioid-independent analgesia.
OBJECTIVE
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 was administered intraperitoneally (30, 45, 68 microg/kg), intra-cerebral ventricularly (4.5 microg/kg) or microinjected into periaqueductal gray (PAG; 4.5 microg/kg). The antinociceptive action was tested using the hot-plate and acetic acid writhing tests in mice and rats. The involvement of the cholinergic system and opioid system in CTX-induced analgesia was examined 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 action 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 acetic acid writhing test, the intra-cerebral ventricular administration of CTX at 4.5 microg/kg (1/12th of a systemic dose) produced marked analgesic effects. Microinjection of CTX (4.5 microg/kg) into the PAG region did not elicit an analgesic action in rats in the hot-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) did antagonize 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. At the highest effective dose of 68 microg/kg the neurotoxin did not change the spontaneous mobility of mice.


CONCLUSIONS
CTX has analgesic effects, which are mediated in the central nervous system though not through the PAG. The central cholinergic system but not opioid system appears to be involved in the antinociceptive action of CTX.
Department of Pharmacology, Soochow University School of Medicine, Suzhou 215007, China.
Zhi-Xin
ZX
Chen
Hui-Ling
HL
Zhang
Zhen-Lun
ZL
Gu
Bo-Wen
BW
Chen
Rong
R
Han
Paul F
PF
Reid
Laurence N
LN
Raymond
Zheng-Hong
ZH
Qin
D000698:Analgesia; D000700:Analgesics / Q000008:administration & dosage / Q000494:pharmacology*; D000818:Animals; D001285:Atropine / Q000037:antagonists & inhibitors; D003039:Cobra Neurotoxin Proteins / Q000008:administration & dosage / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D005260:Female; D007276:Injections, Intraventricular; D008297:Male; D051379:Mice; D008845:Microinjections; D009270:Naloxone / Q000037:antagonists & inhibitors; D010147:Pain Measurement / Q000187:drug effects*; D010487:Periaqueductal Gray / Q000187:drug effects*; D051381:Rats; D017207:Rats, Sprague-Dawley
D000700:Analgesics; D003039:Cobra Neurotoxin Proteins; D009270:Naloxone; D001285:Atropine