Study On The Antinociceptive Effect Of Cobratoxin And Its Mechanisms

Objective: To investigate the antinociceptive effect of cobratoxin, a long-chain postsynaptic neurotoxin isolated from Thailand cobra (Naja naja kaouthia) venom and analyze its mechanisms.Methods: Behavioral tests (hot plate test, von Frey test, formalin test and acetic acid-writhing test) were used to define the antinociceptive effect of cobratoxin for acute pain in mice. Neuropathic pain model was established by partial sciatic nerve ligature (PSNL) in rats. The mechanical pain threshold and thermal pain threshold were measured using mechanical pressure-paw withdrawal and radiant heat-paw withdrawal methods, respectively. The neuronal activation in the dorsal horn of spinal cord of PSNL rats was evaluated by c-fos expression. Intrathecal or intravenous administration of tropicamide, a selective M4 muscarinic acetylcholine receptor (mAChR) antagonist or intrathecal administration of methyllycaconitine (MLA), a selectiveα7 nicotinic acetylcholine receptor (nAChR) antagonist were conducted to study the reversal effect on the antinociceptive effect of cobratoxin.Results: (1)Intrathecal administration of cobratoxin (5μg/kg) produced antinociceptive effects in thermal, mechanical, chemical acute pain models in mice. These cobratoxin-induced antinociception could be reversed by intravenous injection of tropicamide, a selective M4 mAChR antagonist. (2)The mechanical and thermal hypersalgesia were observed and the number of Fos-positive neurons in the dorsal horn of spinal cord was up-regulated significantly (P<0.01) seven days after PSNL. (3) Intrathecal injection of cobratoxin at different doses (0.56, 1.12 and 4.5μg/kg) elicited a dose-dependent inhibition of mechanical hyperalgesia. The Fos expression in PSNL rats was also inhibited significantly (P<0.001) after cobratoxin injection. (4)The antinociceptive effect and the decreased Fos expression induced by cobratoxin in the PSNL rats were significantly reversed by intrathecal administration of M4 mAChR antagonist tropicamide and byα7 nAChR antagonist MLA.Conclusions: Cobratoxin produced a significant antinociceptive effect in acute and chronic pain models. The antinociceptive effect of cobratoxin was proved to be mediated by M4 mAChR subtype, as well as byα7 nAChR subtype.