Effects of Season and Sex on the Distribution of Cytosolic Estrogen Receptors Within the Brain and the Anterior Pituitary Gland of Sheep 1

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Abstract
This study was undertaken to evaluate the effects of season and sex on the distribution of cytosolic estradiol-17 beta receptors (E2-receptors) within multiple regions of the ovine brain and the anterior pituitary gland. Our data confirm the existence of a saturable, high-affinity receptor specific for estradiol-17 beta (E2) in the hypothalamus and anterior pituitary gland. The concentrations of E2-receptors in tissues from castrated ewes and rams killed in December (breeding season) and May ( nonbreeding season) were determined by incubating cytosols with a saturating concentration of [3H] E2. For both sexes, the anterior pituitary gland contained the highest concentration of E2-receptor. Within the brain, the highest concentrations of E2-receptor were in the hypothalamus and preoptic area (POA), and within the hypothalamus, the median eminence-infundibular stalk (ME) contained a higher concentration of E2-receptor than the anterior and posterior hypothalamus. The numbers of E2-receptors in the anterior hypothalamus and ME of ewes were 2- 3X those for rams. In ewes, there was little seasonal difference in the numbers of E2-receptors in the POA, anterior hypothalamus and ME. In rams, there was little seasonal difference in the numbers of E2-receptors in the POA and anterior hypothalamus. For both sexes, there were fewer E2-receptors in the anterior pituitary gland and posterior hypothalamus in May than in December. Thus, during the nonbreeding season, the major neuroendocrine site of action of E2 probably is within the brain, specifically in the POA, anterior hypothalamus and ME of the ewe, and in the POA and the anterior hypothalamus of the ram. These data do not support a mechanism whereby the increased sensitivity to sex steroids during the nonbreeding season is due to enhanced numbers of E2-receptors.