Fig. 7. (A ) Effects of systemic morphine administration on paw withdrawal latencies to radiant heat in wild-type (+/+) mice. Morphine increased the paw withdrawal latencies to heat ipsilateral to the nerve injury at the 10-mg/kg subcutaneous dose; the effects were naloxone (1 mg/kg) reversible. Morphine, however, had no effects on heat hyperalgesia in homozygous and heterozygous μ-opioid receptor knockout mice (data not shown). n = 9. **P < 0.01 compared with values after normal saline. (B ) Effects of systemic morphine administration on paw withdrawal frequencies to acetone in homozygous and heterozygous μ-opioid receptor knockout and wild-type mice. Morphine had no effect on the response to acetone in homozygous mice; in contrast, morphine significantly decreased the withdrawal frequencies of the ipsilateral paw to acetone in heterozygous and wild-type mice. The effects were naloxone (1 mg/kg) reversible. n = 9 or 10. #P < 0.05, ##P < 0.01 compared with normal saline. ** P < 0.01 compared with pre-SNL values. Error bars = SDs; Nal = naloxone, 1 mg/kg subcutaneous; sal = saline; SNL = spinal nerve ligation and section.