Fig. 8. Effects of systemic administration of U50488H on paw withdrawal frequency to a 0.52 g–force von Frey hair in (A ) homozygous (−/−) and (B ) heterozygous (+/−) μ-opioid receptor knockout and (C ) wild-type (+/+) mice. Test protocols were similar to that described in figure 6. The κ-opioid agonist U50488H attenuated mechanical withdrawal frequencies dose dependently in all three genotypes; the effects were naloxone reversible. Wild type, n = 9; homozygous, n = 9; heterozygous, n = 7. *P < 0.05, **P < 0.01 compared with values after saline. Error bars = SDs; NAL = naloxone, 1 mg/kg subcutaneous (s.c.); sal = saline.