Rats (n=6) were administrated intrathecally (twice daily) with vehicle, morphine (MS, 15 nmol), morphine plus MG-132 (1, 2.5, 5 or 10 nmol) or MG132 alone (5 nmol) for seven consecutive days. A, Morphine antinociceptive tolerance was developed on day 5 after the repeated treatment with morphine but not with morphine plus MG132 (5 nmol). B, Morphine antinociceptive tolerance was prevented in a dose-dependent manner by co-administration of morphine and MG-132 (day 7). C, The rightward shift of the dose-response curve from morphine exposure was prevented in these rats treated with morphine plus MG-132 (5 nmol). D, MG132 alone did not change the baseline latency. One-way ANOVA followed by Tukey-Kramer Test. * P< 0.05.