Anti-nociceptive mechanisms of Melilotus officinalis Linn. ethanoic extract in mice: Involvement of opioidergic, nitrergic and muscarinic receptors

Background and Objective: Pain is a physiologic protective function with regard to external or internal harmful stimulus or tissue damage. The aim of the current study was to determine antinociceptive activity of Melilotus officinalis (Linn.) extract on formalininduced pain in mice. Materials and Methods: In experiment 1, adult male mice were injected (i.p) with saline, ethanoic extract of Melilotus officinalis (Linn.) (EEMO) (100, 200 and 400 mg/kg) or morphine (5 mg/kg). In experiment 2, mice were injected with saline, EEMO (400 mg/kg), naloxone (2 mg/kg) and coinjection of EEMO (400 mg/kg) + naloxone (2 mg/kg). In experiment 3, animal received i.p injection of saline, EEMO (400 mg/kg), LNAME (10 mg/kg) and EEMO (400 mg/kg) + LNAME (10 mg/kg). In experiment 4, mice were injected (i.p) with saline, EEMO (400 mg/kg), atropine (1 mg/kg) and coadministration of EEMO (400 mg/kg) + atropine (1 mg/kg). Then, the time spent for paw licking was determined the in first and second phase after formalin injection. Then, licking and biting time of the injected paw was recorded after formalin injection in first and second phases.    Results: According to the results, EEMO in a dose dependent manner significantly diminished licking and biting time of injected paw (pain response) in comparison with the control group (P 0.05). Coinjection of the naloxone + EEMO significantly amplified pain response compared to the EEMO group (P 0.05). Coinjection of the LNAME + EEMO significantly decreased pain response in comparison with the EEMO group (P 0.05). Pretreatment with atropine significantly enhanced pain response in comparison with the EEMO group (P 0.05). Conclusion: These findings suggest that antinociceptive activity of the EEMO is mediated via opioidergic, nitrergic and muscarinergic systems in mice.