Effects of opioid receptor antagonists on Phe-Phe amide (2 pmol, i.t.)-induced antinociception in non-diabetic (A) and diabetic (B) mice. Naloxone (NLX, 1 mg/kg i.p) and naltrindole (NTI, 3 mg/kg, s.c.) were administered 30 min before the injection of Phe-Phe amide. β-Funaltrexamine (β-FNA, 20 mg/kg, s.c.) and nor-binaltorphimine (BNI, 20 mg/kg, s.c.) were administered 24 h before injection of the Phe-Phe amide. Each column represents the mean with S.E.M. for 6-11 mice. ***P < 0.001 vs. respective before Phe-Phe amide-treatment group; ###P < 0.001 vs. saline-pretreated Phe-Phe amide group (Bonferroni test).