ANALYSIS OF THE MECHANISMS UNDERLYING THE ANTINOCICEPTIVE EFFECT OF THE EXTRACTS OF PLANTS FROM THE GENUS PHYLLANTHUS

1. We examine some of the mechanisms underlying the analgesic effects of the hydroalcoholic extracts (HE) of Phyllanthus urinaria and P. nir uri against formalin-induced nociception in mice. In addition, we also investigate the action of both HEs against capsaicin-mediated pain. 2 . Both prazosin and yohimbine (0.15 mg/kg, i.p.) induced a marked inhi bition 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. 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/k g) completely antagonized the antinociceptive effect of L-NOARG but ha d 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. The antinocice ptive 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 f or 4 consecutive days) or after previous adrenalectomy of animals. 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 inhi bition 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.