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Biology of Reproduction, Vol 11, 117-124, Copyright © 1974 by Society for the Study of Reproduction

Photoperiodic Regulation of the Estrous Cycle of the Rat: Role of the Pineal Gland

EDWARD P. WALLEN 1, and JEROME M. YOCHIM 1

1 Department of Physiology and Cell Biology, University of Kansas, Lawrence, Kansas 66045


A 20-h daily photoperiod was utilized in an attempt to predictably modify the estrous cycles of rats exposed to this environment for 20 days. Predicted changes in the cycle were based on the theoretical phase relationships of two component rhythms which interact to generate a characteristic pineal HIOMT pattern during the estrous cycle. Whereas control rats from a 14/10 L:D photoperiod normally show vaginal cornification about 50% of the time, rats on the exotic, 20-h schedule (L:D, 1.4/1) showed variations in cornification from 46 to 68% of the time, depending on the phase relationship between the two component rhythms. Rats pinealectomized and placed into this environment at selected times showed cornification about 57% of the time but no longer exhibited the relationship between vaginal cornification and phase. These experiments indicated that the pineal gland may affect the regularity of 4- and 5-day estrous cycles of the rat by modulating the magnitude and duration of action of the hypothalamic-hypophyseal axis, but not its timing.

Accepted on January 2, 1974






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Copyright © 1974 by the Society for the Study of Reproduction.