Fig. 2. Time course of allodynia to mechanical stimuli after spinal nerve injury. Preinjury withdrawal frequencies to the 0.52 g–force von Frey hair were similar in homozygous μ-opioid receptor knockout (+/+) and wild-type (−/−) mice. Mechanical withdrawal frequency increased significantly in both genotypes after nerve injury compared with preinjury values. Mechanical hyperalgesia was enhanced in the homozygous mice, which also expressed a contralateral increase in paw withdrawal frequencies. Wild type, n = 9; homozygous, n = 9. ##P < 0.01, homozygous versus  wild type. *P < 0.05, ipsilateral versus  contralateral paw. Error bars = SDs.