Reproductive activity of sheep varies with season. Female sheep in temperate latitudes become anestrus during late winter or early spring and resume reproductive activity in late summer or early fall. Seasonal fluctuation in reproductive activity is most likely due to changes in photoperiod because similar changes in reproductive function are manifest in sheep exposed to differing artificial photoperiods in an otherwise constant environment. Photo-periodic cues modulate reproductive function in sheep through steroid-dependent and steroid-independent mechanisms.
The steroid-dependent effect of photoperiod is clearly evident in sheep chronically treated with estradiol. In gonadectomized sheep, regardless of sex, the negative feedback of exogenous estradiol is most profound during the anestrous season. The effect of photoperiod on secretion of gonadotropins may reflect a seasonal shift in the sensitivity of the hypothalamus or other neural centers to estradiol. Indeed, secretion of GnRH is attenuated during the anestrous season in ovariectomized ewes treated with estradiol. But estradiol, in addition to its action on the hypothalamus, has a direct effect on the anterior pituitary gland. Estradiol increases tissue concentration of GnRH receptor in ovine pituitary cells in culture and hypothalamic-pituitary disconnected ewes. On the other hand, estradiol may exert an inhibitory action on gonadotropin secretion directly at the pituitary gland. Thus, it appears likely that estradiol acts on gonadotropin secretion at two different levels, hypothalamic and pituitary, probably through different molecular mechanisms.