Figure 5.

(A) Five days of chronic escalating treatment with morphine (10–50 mg·kg⁻¹ i.p., twice daily) in δ‐KO mice (−/−) and their wild‐type littermates (+/+) produces no shift in the dose–effect curve in δ‐KO mice but produces a threefold rightward shift in wild‐type mice. (B) Wild‐type (+/+) and δ KO (−/−) mice were treated for 5 days with escalating doses of morphine (10–50 mg·kg⁻¹ i.p., twice daily). Withdrawal was precipitated with 10 mg·kg⁻¹ naltrexone. There was no significant difference in the number of jumps observed across genotype. * P<0.05, significantly different from data from day 1. Data shown are means ± SEM for all groups (n = 6 for each group).