Neonatal chronic stress induces subsensitivity to chronic stress in adult rats: II. Effects on estrous cycle in females.

The purpose of the present report was to determine the effect of exposure of females rats to the unpredictable chronic stress model and two models of predictable chronic stress (cold and handling), from day 2-15 of life, on the estrous cycle alterations caused by the unpredictable chronic stress in adulthood. Adult control and neonatally stressed rats were submitted to estrous cycle analysis for 8 days through microscopic observations of vaginal smears. They were then exposed to chronic aleatory stress, and vaginal smears were analyzed daily throughout the stress period (17 days) up to day 5 after completion of the treatment. It was found that this treatment caused constant diestrus in a majority of control females. Such diestrus started at day 5.75 +/- 0.96 of stress administration and was maintained up to day 20.0 +/- 0.49 (i.e., about 3 days after interruption of stress). This effect was prevented by the neonatal aleatory stress and the neonatal cold stress. Neonatal handling only attenuated the estrous cycle alterations; this group showed a period of diestrus no longer than 4 days during the 17-day exposure to stress. The increased resistance of neonatally stressed rats to the estrous cycle effects of chronic aleatory stress in adulthood supports the speculation that neonatal manipulation can increase resistance of rats to stress-induced reactions throughout life.