Effect of an oral contraceptive on spontaneous running activity of female rats

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Abstract
Dietary administration of Enovid® at 7.5 mg/kg of food for 11 days to intact female rats reduced their cyclic running activity and induced changes in vaginal cytology characteristic of estrus. There was a linear relationship between the logarithm of the dose of Enovid® ingested and the percent decrease in running activity from the average observed before drug administration. Dietary administration of norethynodrel, one of the two components of Enovid®, at 7.5 mg/kg of food for 21 days decreased the magnitude of the running activity slightly but failed to affect its cyclic nature. In contrast, dietary administration of mestranol at either 0.125 or 0.54 μg/kg of food for 24 and 29 days respectively reduced running activity and increased the length of the estrus cycle. Hence, the effect of Enovid® on the running activity of intact rats is probably attributable to the mestranol contained in it. An apparent escape from the effects of chronic administration of mestranol on spontaneous running activity was observed from 20 to 30 days after beginning treatment with either dose used. The escape was characterized by a reinitiation of cyclic running activity during chronic drug administration. Elucidation of the mechanism of the escape phenomenon will require further study.

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