Reversal effects of single subcutaneous injection of d-amino acid oxidase (DAAO) inhibitors on established morphine antinociceptive tolerance in the mouse tail-flick test (A–C) and hot-plate test (D–F). Mice received subcutaneous injections of saline (10 ml/kg) or morphine (10 mg/kg) twice daily for 7 days. On day 8, mice received subcutaneous administration of 10 ml/kg saline, 5-chloro-benzo[d]isoxazol-3-ol (CBIO; 1, 3, or 10 mg/kg, A and D), AS057278 (3, 10, or 30 mg/kg, B and E), or sodium benzoate (10, 30, 100, or 300 mg/kg, C and F) 20 min before saline (10 ml/kg) or morphine (5 mg/kg) challenge. Dose–response analysis in the mouse tail-flick test (G) and hot-plate test (H). The correlation of DAAO inhibitors between their reversal of established morphine tolerance and DAAO enzymatic inhibition (I and J). Effects of the noncompetitive antagonist MK-801 of N-methyl-d-aspartic acid receptors on morphine antinociceptive tolerance in the mouse tail-flick test (K) and hot-plate test (L), measured by areas under curve (AUC) over 2 h, respectively. Mice received subcutaneous injections of saline (10 ml/kg), morphine (10 mg/kg), MK-801 (0.1 mg/kg) + morphine (10 mg/kg), or morphine (10 mg/kg), twice daily at 12-h intervals for 7 days. On day 8, mice received a single-bolus subcutaneous administration of saline or 0.1 mg/kg MK-801 20 min before saline or morphine (5 mg/kg) challenge. Data are presented as means ± SEM (n = 6 in each group). Correlation coefficient was calculated and statistically tested by unpaired and two-tailed Student t test. *Denotes statistically significant difference from morphine-tolerant mice (P < 0.05 by repeated measures two-way ANOVA followed by post hoc Student–Newman–Keuls test).