Analgesic effects of Naja kaouthia snake venom and its fractions on inflammatory pain are mediated by peripheral opioid receptors

ABSTRACT

Venom of cobras of genus Naja, including Naja kaouthia, can relieve pain in acute and chronic conditions. We investigated the effects of oral and intraplantar administration of the Naja kaouthia venom and its fractions on painrelated responses in an inflammatory pain model in rats. Male Wistar rats received a hind paw injection of prostaglandin E2 (PGE2) to induce inflammatory pain and either oral or intraplantar administration of Naja kaouthia venom and its fractions (fractions 1 to 5). In addition, separate groups of rats with oral administration of fraction 3 of the Naja kaouthia venom also received either μ-, κ- or δ-opioid receptor antagonists, which were injected into the hind paw by intraplantar route. Mechanical thresholds were assessed on the hind paw before and after treatments. Fractionation of Naja kaouthia venom was performed using size exclusion chromatography. Naja kaouthia venom reduced pain-related responses in the inflammatory pain model when administered by oral and intraplantar routes. Fractions 1, 3, 4 and 5 of the Naja kaouthia venom administered by oral route decreased PGE2-induced pain sensitivity, while fraction 2 did not modify pain-related responses. Hind paw injection of naloxone, a non-specific opioid receptor antagonist, abolished the analgesic effects of the Naja kaouthia venom as well of that for fraction 3. Additionally, hind paw injection of either μ-, κ- or δ-opioid receptor antagonists blocked the pain relief induced by fraction 3. This study indicates that the Naja kaouthia venom and its fractionated forms, particularly fraction 3, may be potential therapeutic targets for pain management and peripheral opioid receptors mediate the pain relief induced by fraction 3.