Figure 1.

Lymphoid organ atrophy in MOR +/+ but not in MOR −/− mutant mice after chronic morphine administration. Mice were treated with increasing doses of morphine for 6 days (see Materials and Methods). The total number of cells in spleen and thymus were determined from each individual animal (13–15 mice per group) and results are expressed as means ± SEM. For both organs there is a significant difference between saline and morphine-treated wild-type animals, as indicated by ★★ for P < 0.01. No significant effect of morphine treatment is observed in mutant mice. In the thymus there is a significant difference between morphine-treated wild-type and mutant animals, as shown by II for P < 0.01 (thymus).