It is known that over-exercise or forced running interrupts the regular ovulatory (estrous) cycle in female mammals, including women. The serotonin content of the brain changes under stress conditions. In this experiment, radiofrequency lesions were made in the dorsal (DRL) or median (MRL) raphe nuclei of the midbrain, in which serotonergic neurons are abundant, and changes in the estrous cycle with forced running using an electric-motor running wheel were examined in female rats. Through the tests, the estrous cycle was checked by taking vaginal smears. Female rats with a regular 4-day estrous cycle were forced to run in the wheel for 30. min daily over 15 days. As a result, 27.3% of the control and 30.0% of the sham-operated rats showed an irregular estrous cycle. In contrast, 100% of the DRL and 87.5% of the MRL rats showed an irregular cycle (P< 0.05 vs. control and sham). Statistical analysis revealed that the median onset day of an irregular cycle was in excess of 15 days in both the control and sham groups. In the DRL and MRL groups, the median onset days of the irregular cycle were day 5 and 3, respectively, being shorter than those in control and sham groups (P< 0.01). These results indicate that the dorsal and median raphe nuclei play an important role in preventing the effect of stress conditions in the ovulatory system in female rats.
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