The anti-allodynia effects of naltrexone (10 mg·kg−1, s.c.) or morphine (10 mg·kg−1) on the ipsilateral (injured) or contralateral (uninjured) hind paw, determined by the von Frey test after sham or SNL surgery in mice which had been transfected with MOR-S196ACSTA in the spinal cord. Time course of (A-1) sham operation with the chronic saline treatment group, (B-1) SNL surgery with the chronic saline treatment group, (C-1) SNL surgery with the chronic naltrexone treatment group, and (D-1) SNL surgery with the chronic morphine treatment group. (A-2), (B-2), (C-2) and (D-2) are the corresponding AUC values (days × g) from days 7 to 19 of each group. Data shown are means ± SEM. **P < 0.01, ***P < 0.001, significantly different from basal level of contralateral site (basal-contra); ##P < 0.01, ###P < 0.001, significant effect of treatment; one-way anova and Newman–Keuls test.