Influence of continuously delivered i.c.v. vehicle (VEH), 1.0 µg/day naltrexone (NTX) and 1.0 µg/day L-NAME on the HBO2-induced late-phase antinociceptive response 14 days after the fourth HBO2 treatment. Osmotic minipumps containing VEH, NTX or L-NAME were activated prior to the first HBO2 treatment and removed immediately after the fourth HBO2 treatment. The data are expressed as the mean ± S.E.M. of 8–12 mice per group. Significance of difference: **, p < 0.01 and ***, p < 0.001, compared to HBO2 control group (one-way ANOVA and post-hoc Bonferroni’s multiple comparison test).