interaction between insulin and opioid system in the formalin test of mice

effects of insulin (1, 2 and 3 iu/kg, i.p.), morphine (5mg/kg, i.p.), naloxone (5mg/kg, s.c.), insulin before morphine and after naloxone were studied in the formalin test of mice. formalin test was induced by intraplantar injection of formalin (20m, 5%). morphine alone and insulin with morphine decreased the first phase (0-5 min) of pain. naloxone increased the interphase (6-20 min) of niciception. insulin and morphine decreased the second phase (21-40 min) of pain. insulin enhanced the morphine - induced antinociception. naloxone attenuated the suppressive effect of insulin. it seems that the antinociceptive effect of insulin in the formalin test may be dependent on the activation of opioid system.