The interaction of estradiol and daylength in modifying serum prolactin secretion in female hamsters.

Changes in serum levels of PRL have been implicated in the development of seasonal reproductive quiescence in male hamsters, yet the role of serum PRL in seasonal anestrus in the female has not been examined. The present studies were designed to examine the secretory patterns of serum PRL and the impact of estradiol on PRL secretion in female hamsters exposed to stimulatory and nonstimulatory photoperiods. In the first study, anestrous short day [6 h of light, 18 h of darkness (LD 6:18)] hamsters or metestrous cycling long day (LD 16:8) hamsters were decapitated at one of six timepoints over the 24-day. PRL was never detectable in the serum of anestrous females in short days; in cycling metestrous females, PRL was elevated during the light phase of the 24-h day. In a second experiment, LD 16:8 females were ovariectomized and implanted with 10-mm Silastic capsules that were either empty or filled with estradiol benzoate. Animals were then maintained in long days or transferred to short days and decapitated 4 weeks later. Estradiol dramatically enhanced serum PRL in both long and short day ovariectomized females, but levels were lower in short day females. Ovariectomy without estradiol treatment led to low levels of serum PRL, but levels were consistently lower in short day females than in long day females. The results suggest that low levels of serum PRL in the short day anestrous hamster result both from a loss of estradiol facilitation and from extraovarian effects of the short day photoperiod and may play a critical role in the loss of estrous cyclicity. (Endocrinology 108: 371, 1981)