Effects of day length on sheep neuroendocrine estrogen and progestin receptors

Gonadal steroid hormone receptors were studied in anterior pituitary, preoptic area, and hypothalamus of ovariectomized ewes. We established that systemic administration of estradiol benzoate elevates levels of nuclear estrogen receptor concentrations and results in the appearance of cytosolic progestin receptor the binding characteristics of which are comparable with that reported in rats and guinea pigs [0.1 nM Kd, binding site density (Bmax) of 111, 22, and 2 fmol/mg protein in pituitary, medial basal hypothalamus, and preoptic area, respectively]. Brain tissues also contained a second, low-affinity progestin binding site. To examine possible effects of photoperiod on neuroendocrine steroid receptors, two groups of sheep were exposed to artificial photoperiods. Ewes experienced alternating periods of long (LD 16:8) or short (LD 8:16) days for 290 days to induce breeding season or anestrous states. Animals were then ovariectomized and given steroid treatments designed to mimic a luteal phase followed by either follicular phase levels of estradiol or withdrawal of all exogenous hormones before determination of neuroendocrine steroid receptor concentrations. Despite the ability of long photoperiods to arrest ovarian cyclicity, magnify negative feedback actions of estradiol upon luteinizing hormone, and reduce the ability of estrogen to trigger behavioral estrus, photoperiod did not influence nuclear estrogen receptor concentrations or the induction of cytosolic progestin receptors in the pituitary, hypothalamus, or preoptic area.