Estrogen receptors in the neuroendocrine tissues of the ewe in relation to breed, season, and stage of the estrous cycle

Estrogen receptors (ER) in the neuroendocrine tissues of female sheep have been studied with respect to breed, season, and stage of the estrous cycle. In the first study, cytoplasmic ER measured by Scatchard analysis in neuroendocrine tissues from seasonally breeding Merino X Border Leicester (Crossbred) ewes demonstrated no seasonal variation in the pineal, pituitary, and cerebral cortex; in contrast, total cytoplasmic ER levels in the hypothalamus were almost twofold higher (31.5 ± 2.7 fmoles/mg protein; mean ± SEM) in anestrous ewes than in cycling ewes (18.5 ± 2.7 fmoles/mg protein). In a second study, cytoplasmic and nuclear ER levels were measured in pituitary glands from Crossbred ewes in anestrus and Merino and Crossbred ewes during the luteal and follicular phases of the estrous cycle. Cytoplasmic ER levels during the luteal phase were significantly (P<0.005) higher in Crossbred ewes (9717 ± 952 fmoles/g wet wt; mean ± SEM) than in Merino ewes (5477 ± 858 fmoles/g wet wt). In both breeds, nuclear ER levels were significantly (P<0.001) higher during the follicular phase (Crossbred, 1052 ± 147; Merino, 841 ± 156) than during the luteal phase (Crossbred, 292 ± 31; Merino, 394 ± 47). In Crossbred ewes nuclear ER levels during anestrus (347 ± 55) were similar to luteal phase levels. These data indicate that hypothalamic control of the circannual breeding pattern in the sheep may be related to ER levels. In the short term, during the estrous cycle, pituitary nuclear ER levels reflect estrogen secretion. A breed difference in ER levels may have a fundamental genotypic basis that explains breed disparities in responsiveness to estrogen.