Background and Aim: Nowadays, We have focused our attention to herbal medicines because of their side effects and expensiveness of chemical drugs . In this research, we evaluated antinociceptive effects of hydroalcoholic extract of L actuca sativa longifolia leaves using tail-flick and formalin test in male mice.

  Materials and Methods: We evaluated antinociceptive effects of hydroalcoholic exract of L actuca sativa longifolia leaves at doses of 85, 165,330,660 and 1300 mg/kg. All injections were administered intraperitoneally. Sham group received normal saline and Morphine group (1 mg/ kg i.p.) was used as positive control. The possible mechanisms of antinociceptive action of the extract were also investigated by pretreatment with Naloxane (1mg/kg i.p.) .

  Results: Hydroalcoholic extract of L actuca sativa longifolia leaves exhibited significant antinociceptive activity at doses of 165,330,660 and 1300 mg/kg in tail flick test, and maximal inhibitions were observed at doses of 660 and 1300 mg/kg (P<0.01). This extract showed antinociceptive effect at doses of 660 and 1300 mg/kg in the first phase of formalin test. The maximal effect was observed at dose of 660mg/kg, and lactuca sativa longifolia leaves extract showed significant decrease in Licking time at doses of 330,660 and 1300 mg/kg in the second phase of formalin test (P<0.001). Antinociceptive effect of hydroalcoholic extract of LS was inhibited by a Naloxane pretreatment .

  Conclusion: This study presents a remarkable antinociceptive effect which supported the traditional use of lactuca sativa longifolia. Furthermore, it seems that opioid pathway is one of the possible mechanisms of antinociceptive effect of the extract.