Antinociceptive properties of Micrurus lemniscatus venom.

The therapeutic potential of snake venoms for pain control has been previously demonstrated. In the present study, the antinociceptive effects of Micrurus lemniscatus venom (MlV) were investigated in experimental models of pain. The antinociceptive activity of MIV was evaluated using the writhing, formalin, and tail flick tests. Mice motor performance was assessed in the rota rod and open field tests. In a screening test for new antinociceptive substances--the writhing test--oral administration of MlV (19.7-1600 μg/kg) produced significant antinociceptive effect. The venom (1600 μg/kg) also inhibited both phases of the formalin test, confirming the antinociceptive activity. The administration of MlV (1600 μg/kg) did not cause motor impairment in the rota rod and open field tests, which excluded possible non-specific muscle relaxant or sedative effects of the venom. The MIV (177-1600 μg/kg) also increases the tail flick latency response, indicating a central antinociceptive effect for the venom. In this test, the MlV-induced antinociceptive effect was long-lasting and higher than that of morphine, an analgesic considered the gold standard. In another set of experiments, the mechanisms involved in the venom-induced antinociception were investigated through the use of pharmacological antagonists. The MlV (1600 μg/kg) antinociceptive effect was prevented by naloxone (5 mg/kg), a non-selective opioid receptor antagonist, suggesting that this effect is mediated by activation of opioid receptors. In addition, the pre-treatment with the μ-opioid receptor antagonist CTOP (1 mg/kg) blocked the venom antinociceptive effect, while the k-opioid receptor antagonist nor-BNI (0.5 mg/kg) or the δ-opioid receptor antagonist naltrindole (3 mg/kg) only partially reduced the venom-induced antinociception. The present study demonstrates, for the first time, that oral administration of M. lemniscatus venom, at doses that did not induce any motor performance alteration, produced potent and long-lasting antinociceptive effect mediated by activation of opioid receptors.