Analysis of the mechanisms underlying the antinociceptive effect of the extracts of plants from the genus Phyllanthus

• 1.1. We examine some of the mechanisms underlying the analgesic effects of the hydroalcoholic extracts (HE) of Phyllanthus urinaria and P. niruri against formalin-induced nociception in mice. In addition, we also investigate the action of both HEs against capsaicin-mediated pain.
• 2.2. Both prazosin and yohimbine (0.15 mg/kg, i.p.) induced a marked inhibition of the analgesic effect caused by phenylephrine (10 mg/kg, i.p.) and clonidine (0.1 mg/kg, i.p.), respectively, but had no effect on the antinociceptive action caused by HE of P. urinaria (10 mg/kg, i.p.) or P. niruri (30 mg/kg, i.p.).
• 3.3. N^G-nitro-l-arginine (l-NOARG, 75 mg/kg, i.p.) caused marked analgesic effect against the second phase of formalin-induced pain. Treatment of animals with l-arginine (600 mg/kg) completely antagonized the antinociceptive effect of l-NOARG but had no significant effect against the HE of P. urinaria (10 mg/kg, i.p.) or P. niruri (30 mg/kg, i.p.) analgesic properties.
• 4.4. The antinociceptive effects caused by the HEs of P. urinaria (10 mg/kg, i.p.) and P. niruri (30 mg/kg, i.p.) were unaffected by methysergide (5 mg/kg, i.p.), p-chloro-phenylalanine-methyl-ester (100 mg/kg, i.p., once a day for 4 consecutive days) or after previous adrenalectomy of animals.
• 5.5. The HE of P. urinaria and P. niruri given either intraperitoneally (1–30 mg/kg) or orally (25–200 mg/kg) caused marked and dose-related inhibition of capsaicin-induced pain with ID₅₀ of 2.1 and 6.1 mg/kg given intraperitoneally and 39 and 35 mg/kg given orally, respectively.