Chronic mild stress facilitates melanoma tumor growth in mouse lines selected for high and low stress-induced analgesia.

Both chronic stress conditions and hyperergic reaction to environmental stress are known to enhance cancer susceptibility. We described two mouse lines that displayed high (HA) and low (LA) swim stress-induced analgesia (SSIA) to investigate the relationship between inherited differences in sensitivity to stress and proneness to an increased growth rate of subcutaneously inoculated melanoma. These lines display several genetic and physiological differences, among which distinct sensitivity to mutagens and susceptibility to cancer are especially noticeable. High analgesic mice display high proneness both to stress and a rapid local spread of B16F0 melanoma. However, stress-resistant LA mice do not develop melanoma tumors after inoculation, or if so, tumors regress spontaneously. We found that the chronic mild stress (CMS) procedure leads to enhanced interlinear differences in melanoma susceptibility. Tumors developed faster in stress conditions in both lines. However, LA mice still displayed a tendency for spontaneous regression, and 50% of LA mice did not develop a tumor, even under stressed conditions. Moreover, we showed that chronic stress, but not tumor progression, induces depressive behavior, which may be an important clue in cancer therapy. Our results clearly indicate how the interaction between genetic susceptibility to stress and environmental stress determine the risk and progression of melanoma. To our knowledge, HA/LA mouse lines are the first animal models of distinct melanoma progression mediated by inherited differences in stress reactivity.

Distinct susceptibility to inoculated melanoma and sensitivity to cancer pain in mouse lines with high and low sensitivity to stress.

Approximately 30 years ago, we developed 2 mouse lines with enhanced and decreased opioid system activity using bidirectional selection for high (high analgesia [HA] line) and low (low analgesia [LA] line) swim stress-induced analgesia. These mouse lines differ substantially in pain sensitivity, measured as hind paw withdrawal latency in a hot plate test. Moreover, compared with the LA mice, the HA mice exhibited reduced energy expenditure under stress and different depression-like behavior as well as higher sensitivity to mutagens and the high frequency of spontaneous and carcinogen-induced tumors. In the current study, we observed distinct differences in the growth rate of orthotopically implanted melanoma and the onset of cancer pain. Whereas the HA line was prone to tumors and carcinogenesis was rapid in all specimens, the LA mice either did not develop tumors (70%) or developed tumors that often regressed spontaneously (30%). Animals from both lines developed robust thermal hypersensitivity in the tumor-bearing paw compared with animals that were injected with saline. However, we found that hyperalgesia in tumor-bearing mice persists for a much shorter time in the HA than in LA mice. Naltrexone, given subcutaneously, restored hyperalgesia in the HA mice, whereas it was ineffective in the LA mice. The results suggest that activity of the opioid system may influence carcinogenesis and the intensity of cancer pain and indicates that HA and LA mice are good models for such studies.