Figure 3.

Reduced NK activity in MOR +/+ but not in MOR −/− mutant mice after chronic morphine administration. Wild-type and MOR-deleted mice were chronically treated with saline or morphine (see Materials and Methods). Splenic NK cell activity was measured in the presence of YAC target cells using a ⁵¹Cr assay (see Materials and Methods). Data are expressed as mean percentages of YAC cell lysis (± SEM) at four different effector/target ratios. The number of mice per group ranged from 14 to 15. Significant differences are obtained between saline- and morphine-treated wild-type animals, and are indicated as follows: ★ for P < 0.05 and ★★ for P < 0.01.