Figure 4.

Absence of tolerance development after naltrexone pellet implantation. Homozygous mutant mice were implanted with a naltrexone pellet for 72 h, and naltrexone antinociceptive effects were measured by the tail-withdrawal test 10 h after pellet removal. The ED₅₀ values for naltrexone were determined to be 0.27 ± 0.35 and 0.31 ± 0.26 mg/kg in placebo pellet-treated and naltrexone pellet-treated mice, respectively. *, P < 0.05; Student's t test as compared between the two treatments within the same dose group, n = 8–12.