Size heterogeneity of affinity-labeled estrogen receptor in the ram hypothalamo-pituitary axis

Absence of photoperiodic modulation of gonadotrophin secretion in HPD rams following chronic pulsatile infusion of GnRH

Hypothalamo-pituitary disconnected Soay rams (HPD rams) were treated chronically with a pulsatile infusion of GnRH (250 ng/2 h) for 10 weeks to reactivate the pituitary-gonadal axis under long days (16L:8D; LD group n=8) and short days (8L:16D, SD group n=8). The aim was to establish whether photoperiod modulates the secretion of gonadotrophins (presumably through the daily melatonin signal) by an action at the level of the pituitary gland. The treatment of the HPD rams with GnRH induced a marked, long-term increase in the peripheral blood concentrations of FSH, LH and testosterone, growth of the testes and reappearance of the androgen-dependent sexual skin colouration. There were parallel changes in the short-term, pulsatile fluctuations in the blood concentrations of LH and FSH, the pituitary responsiveness to GnRH and the testicular responsiveness to a standardized injection of oLH, as observed previously in normal intact rams during reactivation of the reproductive axis induced by a switch from long to short days. There were no significant differences in any of the reproductive parameters between the LD and SD HPD groups, although the blood concentrations of prolactin were significantly higher in the LD group, and GnRH provoked a chronic increase in the blood concentrations of prolactin, particularly in the SD group. Overall, the results support the conclusion that (i) melatonin does not act within the pituitary gland to mediate effects of photoperiod on GnRH-induced gonadotrophin secretion; and (ii) changes in prolactin secretion which result from the local action of melatonin in the pituitary gland do not affect gonadotrophin secretion in the ram.

Differential regulation of AP-1 DNA binding activity in rat hypothalamus and pituitary by estrogen

Ap-1 proteins such as Fos and Jun are nuclear transcription factors that have been postulated to function as third messengers in signal transduction pathways to regulate target gene expression. Using electrophoretic mobility shift assays (EMSA), we have studied estrogen (E) effects on regulation of AP-1 DNA binding activity in the rat hypothalamus and pituitary. AP-1 binding is defined herein as the specific association with a consensus AP-1 site during EMSA. Specific AP-1 binding activity was observed in nuclear extracts from the hypothalamus and pituitary of ovariectomized (OVX) female and castrated (CAS) male rats. Treatment with E increased the levels of AP-1 binding activity in the pituitary and uterus, whereas E decreased the levels of AP-1 binding in the hypothalamus, of OVX females. These effects were observed within 60 min and maintained for at least 72 h after a single dose of estrogen. Estrogen-induced changes in AP-1 binding were much more prominent in OVX females than in CAS males. Treatment with progesterone in OVX females had no significant effects on AP-1 binding activity in either pituitary or hypothalamus. Analysis of AP-1 binding activity in both hypothalamus and pituitary by supershift, immunodepletion and shift-Western blot indicated that part of the AP-1 binding was due to the presence of Fos and Jun proteins. However, Western blot analysis shows that the levels of Fos and Jun proteins in the hypothalamic nuclear extracts were not altered by E treatment. We conclude that E produced tissue and sex-differentiated alterations in AP-1 DNA binding activity in the hypothalamus and pituitary of female rats, which may be related to differential estrogenic actions on gene regulation.

Estrogen receptor protein and mRNA expression in the ovary of sheep

Using immunohistochemistry and in situ hybridization, we attempted to identify the estrogen receptor (ER) protein and messenger RNA (mRNA) in sheep ovaries during the follicular phase of the estrous cycle. Monoclonal anti-ER antibodies H222 and 1D5 were used for localizing estrogen receptor on ovarian cryo-sections. Labeling for ER was found over the nuclei of surface epithelium, interstitial tissue, and granulosa cells of small as well as large ovarian follicles. In the preantral and small antral follicles, intense nuclear ER labeling was observed in mural granulosa cells and particularly in cumulus/granulosa cells surrounding the oocyte. In the large healthy looking follicles, greater diversity in labeling for ER was observed, which is characterized by mixed populations of granulosa cells expressing positive and more or less negative nuclear labeling. Such a pattern of labeling was particularly evident in follicles showing the signs of atresia. Generally, more intense nuclear staining was localized in granulosa cells proximal to basal membrane. In situ hybridization studies revealed the presence of ER mRNA in ovarian tissue. Autoradiographic visualization localized ER mRNA expression over the granulosa cells of healthy follicles of all sizes. Level of hybridization signal was comparable in mural and cumulus granulosa cells. In atretic follicles, the level of hybridization signal in granulosa cells was comparable to that of healthy follicles. A relatively weaker level of labeling was observed in granulosa cells dispersed in follicular antrum in follicles with advanced atretic lesions. Theca cells expressed a lower level of labeling than granulosa cells. Specificity of labeling for both ER protein and mRNA in ovary was proved by parallel probing the ovine uterus. Ovine ER recognition by both H222 and 1D5 antibodies was also proved by immunoblotting. These studies demonstrate the presence of the estrogen receptor and its messenger RNA in the sheep ovary and suggest an autocrine/paracrine role of estradiol and its receptor in the regulation of ovarian follicle development in sheep.

Specific modulation of estrogen receptor mRNA isoforms in rat pituitary throughout the estrous cycle and in response to steroid hormones

We have identified several estrogen receptor (ER) mRNA isoforms in rat pituitary and characterized their regulation by gonadal steroids. The ER mRNAs correspond to splice variants in which either exon 4, exons 3 and 4, or exons 5 and 6 are deleted. A previously isolated pituitary-specific truncated mRNA, TERP-1, containing a unique 5'-end and exons 5 through 8 of the full-length ER, was also studied. The exon deletion variants were expressed in males and females, in pituitary, uterus, testes, heart, hypothalamus, and liver. An antibody to the ER C-terminus bound to full-length (64 kDa) and smaller (50 55 kDa and 40-45 kDa) ER proteins in uterus and pituitary and a pituitary-specific ER of 20-24 kDa corresponding to TERP-1. Estrogen (E) treatment in vivo stimulated full-length ER 2-3-fold, and TERP-1 7-10-fold, but had no effect on any exon deletion variant. Progesterone treatment, alone or with E, had no consistent effect on any ER mRNA form. TERP-1 mRNA was also dramatically and specifically modulated during the estrous cycle, increasing approximately 500-fold between the morning of diestrous and the afternoon of proestrus. Thus, ER mRNA variants exist in estrogen-responsive tissues; the pituitary contains at least one tissue-specific ER which is regulated by steroids and which may contribute to changes in regulated biological activity.

Role and site of estrogen action in follicular atresia

Subsequent to an initial understanding that estrogen was only stimulatory to folliculogenesis, we have come full circle to the present recognition that many actions of estrogen are inhibitory to follicular function. The development of this interpretation has frequently been associated with the controversial issue o f the likely site o f estrogen action, especially in primates, where much of the evidence has been amassed. The accumulated findings in a variety of species seem to demonstrate clearly that at least part of the atretogenic effect of estrogen is exerted directly on the ovary, apparently by interaction with the nuclear estrogen receptor. Recent observations include identification of messenger RNAs for the estrogen receptor and for creatine kinase in the macaque ovary, and a current focus is to localize messages within specific compartments of the ovary.