Induction of mechanical allodynia in 129 WT, 129 HINT1−/−, CD1 WT, and CD1 σ1R−/− mice. Chronic constriction injury (CCI) of the sciatic nerve causes neuropathic pain in mice. For each mouse strain, sham (n = 6) and CCI (n = 6) operated mice were examined. The paw withdrawal thresholds after CCI, corresponding to contralateral and ipsilateral paws were measured before (indicated as 0) and 3, 7, and 14 days after surgery. The force (in grams) at which the mice withdrew their paws in response to von Frey hair stimulation was determined as an index of mechanical allodynia. The data are shown as the mean ± SD of six mice: * significantly different at the corresponding time interval relative to the nociceptive threshold on day 0; θ indicates a significant difference relative to the contralateral paw. All the data were analyzed by pairwise Holm-Sidak multiple comparison tests following ANOVA: p < 0.05, 1 − β > 0.80.