Evaluation of Analgesia Induced by Mitragynine, Morphine and Paracetamol on Mice

The leaves of Mitragyna speciosa were chewed as a substitute to opium in Thailand and Malaysia. A study was therefore undertaken to compare the antinociceptive activity of morphine and paracetamol to that of mitragynine, a major constituent of fresh leaves of M. speciosa. The tests employed were acetic acid induced writhings, hot tail-flick and cold tail flick. All test drugs were administered orally to mice. Results indicated that mitragynine (200 mg/kg) and morphine (5 mg/kg) reduced writhings from 17.5 ± 2.8 per 5 min to 9.6 ± 0.6 and 7.3 ± 0.6, respectively. Paracetamol (100 mg/kg) did not significantly reduce writhings in mice. All three drugs produced significant analgesia when tested by the hot tail-flick producing peak maximum possible analgesia (MPA) of 35.1 ± 4.8% (morphine), 27.8 ± 2.1% (paracetamol) and 42.8 ± 7.2% (mitragynine). Morphine produced significant and marked analgesia when tested by the cold tail-flick technique achieving a peak MPA of 66.2 ± 2.4% at 45 min following oral administration. Mitragynine produced a peak MPA of 49.0 ± 5.9% at 30 min whilst paracetamol did not appear to be active. Mitragynine may be a potential new analgesic and requires further study.