Fig. 6. Effects of systemic morphine treatment on the nociceptive scores in tumor-bearing (circles ), caerulein-injected (squares ), or complete Freund's adjuvant (CFA)-injected mice (triangles ). Six hours after the last injection of caerulein (six injections, 50 μg/kg) or 3 days after CFA-injection, groups of mice were treated with subcutaneous morphine (1, 3, and 5 mg/kg). Twenty-eight days after the inoculation of tumor cells, groups of mice were treated with subcutaneous morphine (3, 5, and 10 mg/kg). The behavioral test was performed 30 min after the injection of morphine. Hypersensitivity to mechanical stimulation was quantified by counting the number of withdrawal behaviors in response to mechanical stimuli. The results are presented as the means ± SD. #P < 0.05, caerulein-saline group versus  caerulein-morphine group. *P < 0.05, CFA-saline group versus  CFA-morphine group. *P < 0.05, tumor-saline group versus  tumor-morphine group, one-way ANOVA followed by the Bonferroni multiple comparisons test.