Fig. 1. Paw withdrawal frequencies to mechanical stimuli of wild-type and μ-opioid receptor knockout mice before and after spinal nerve injury. (A ) Before injury; (B ) 1 day after nerve injury; (C ) 1 week after nerve injury. Whereas preinjury mechanical withdrawal frequencies were similar in the two groups of mice, paw withdrawal frequencies increased significantly after nerve injury in both wild-type (+/+) and homozygous knockout (−/−) mice. The increase in mechanical withdrawal responses on the side of nerve injury was enhanced on day 7 in the knockout mice compared with the wild-type mice (#P < 0.05, ##P < 0.01). Knockout mice also expressed contralateral allodynia at all time points studied. Wild-type, n = 9; homozygous, n = 9. *P < 0.05, ipsilateral versus contralateral paw. Error bars = SDs.