TRPV1 channel inhibition contributes to the antinociceptive effects of Croton macrostachyus extract in mice

Previous study showed that extracts from Croton macrostachyus (Euphorbiaceae) exhibit analgesic effects in acute pain models. The present study evaluates the antinociceptive properties of the methanol/methylene chloride extract (MECM) of the stem bark of this plant using mice models of persistent inflammatory and neuropathic pain, and assesses its mechanism of action. MECM was tested on Complete Freund adjuvant (CFA)-induced persistent thermal and mechanical pain, neuropathic pain induced by partial sciatic nerve ligation (PSNL), prostaglandin E2 (PGE2)-induced acute mechanical hyperalgesia, as well as on nociception induced by capsaicin in mice. Mechanical hyperalgesia was assessed using von Frey hair in awake mice. The mechanism of action of MECM was evaluated by using glibenclamide on PGE2-induced hyperalgesia or rimonabant on capsaicin-induced pain. MECM administered orally at the doses of 250 and 500 mg/kg, induced long lasting and significant antihyperalgesic effects on CFA-inflammatory and PSNL-induced neuropathic pain. MECM significantly reduced the mechanical hyperalgesia induced by PGE2 either when administered preventively or therapeutically. MECM also significantly and time dependently inhibited the capsaicin-induced nociception. These effects were not affected by glibenclamide or by rimonabant. The present results demonstrate that the oral administration of MECM to mice resulted in long lasting antihyperalgesic activity in inflammatory and neuropathic pain as well as in acute and persistent pain. The mechanism underlying the long lasting MECM antihyperalgesic effect is currently unknown, but might be mediated, at least partially, through the modulation of TRPV1 receptors.