Facilitation of dimethylbenz[a]anthracene-induced rat mammary tumorigenesis by restraint stress: role of beta-endorphin, prolactin and naltrexone.

In order to investigate the involvement of opioid peptides and prolactin in stress-facilitated mammary cancer, we studied the effect of chronic restraint stress on dimethylbenz[a]-anthracene (DMBA)-induced mammary tumorigenesis and the effect of an opiate antagonist, naltrexone, on this process. Female Fischer-344 rats (n = 160) were administered 15 mg DMBA/ml of sesame oil/rat by intragastric intubation. Eighty rats were subjected to daily 30 min restraint stress in a plastic cylinder, and 80 rats served as control not subjected to the stressor. Half of the rats from each group received naltrexone (1 mg/kg, i.p. daily). Five rats from each group (restraint stress +/- naltrexone and control +/- naltrexone) were killed every 2-3 weeks. Rats subjected to restraint stress developed a greater number of tumors earlier. Naltrexone decreased the tumor incidence in the stressed animals from 32 to 12% (P less than 0.001) and in unstressed rats from 27 to 15% (P less than 0.001) at the end of 18 weeks. Stressed rats showed a decrease of 48% (P less than 0.001) in the level of hypothalamic beta-endorphin. Plasma prolactin increased from 4-13 ng/ml in the control rats to 109-396 ng/ml (P less than 0.001) in the stressed rats throughout the 18 week period. The beneficial effect of naltrexone was associated with 42% (P less than 0.01) increase in T cell proliferation, but greater than 90% (P less than 0.001) decrease in plasma prolactin level was observed in naltrexone-treated rats compared to the untreated animals. Rats subjected to restraint stress showed a 15% (P less than 0.001) decrease in weight gain at the end of the experiment (18 weeks). Neither restraint stress nor naltrexone administration affected the caloric intake of rats during this period. Thus, we believe that restraint stress facilitates DMBA-induced mammary tumors by releasing beta-endorphin and prolactin, and naltrexone shows a beneficial effect by opposing the effect of beta-endorphin on prolactin release in the stressed animals.