INSULIN ATTENUATES FORMALIN-INDUCED NOCICEPTIVE RESPONSE IN MICE THROUGH A MECHANISM THAT IS DERANGED BY DIABETES-MELLITUS

Although hypoglycemic doses of insulin (0.24-7.5 U/kg s.c.) did not si gnificantly change acetic acid-induced writhing in mice, they dose-dep endently attenuated formalin-induced nociceptive responses, and their effects were more potent on the second phase (ID50 = .62 U/kg) than on the first (ID50 > 7.5 U/kg). Intracerebroventricular doses of insulin (250-1000 mu U/animal) mimicked the effects of the s.c. dose, but cau sed little change in blood glucose levels. The antinociceptive activit y of insulin (0.75 U/kg, s.c.) in the formalin test was significantly inhibited by naloxone (10 mg/kg i.p., an opiate receptor antagonist), sulpiride (10 mg/kg i.p., a dopamine 2 receptor antagonist), pindolol (1 mg/kg i.p., a 5-hydroxytryptamine 1 receptor antagonist) and ketans erin (1 mg/kg i.p., a 5-hydroxytryptamine 2 receptor antagonist), but not by 3-tropanyl-indole-3-carboxylate (1 mg/kg i.p., a 5-hydroxytrypt amine 3 receptor antagonist). Insulin also exerted antinociception in streptozotocin-induced diabetic mice and genetically diabetic db/db mi ce which, however, were less sensitive (ID(50)s around 7.5 U/kg) to th e of insulin effect than normal mice. The results suggest that insulin attenuates chronic rather than acute pains through a mechanism mediat ed by dopamine, 5-hydroxytryptamine and opioids. The antinociceptive p athway appears to be deranged by diabetes mellitus.