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The Antinociceptive Action of Aqueous Extract from Muntingia calabura Leaves: The Role of Opioid Receptors

Objective: The present study was carried out to investigate the antinociceptive activity of the aqueous extract of Muntingia calabura (MCAE) leaves and to determine the effect of temperature and the involvement of the opioid receptor on the said activity using the abdominal constriction test (ACT) and hot-plate test (HPT) in mice. Materials and Methods: The extract was prepared by soaking the dried powdered leaves of M. calabura in distilled water (dH2O) overnight, and the supernatant obtained was considered as a stock solution with 100% concentration. The stock solution was diluted to 1, 5, 10, 50 and 100% and used to determine the antinociceptive activity of MCAE. A further experiment was done with 50% concentration to determine the effect of temperature and naloxone involvement of the opioid receptor system in MCAE antinociceptive activity. Results: At the various concentrations MCAE showed significant antinociceptive activity in both tests. However, the concentration-dependent activity was observed only in the ACT but not in the HPT. The 50% concentration of MCAEs were also stable against the effect of various temperatures as indicated by the presence of activity in both tests. The temperatures (40, 60 and 100°C) also showed an enhanced extract activity only in the HPT. Pre-treatment with naloxone (2 and 10 mg/kg) blocked the extract activity in both tests, indicating the involvement of the opioid receptor system in MCAE antinociceptive activity. Conclusion: Our data indicate that M. calabura leaves possess antinociceptive activity against chemically and thermally induced noxious stimuli. The bioactive compound(s) responsible for its antinociceptive activity is/are heat-stable and work partly via the opioid receptor system.