Interactions between 17 beta-estradiol and the hypothalamo-pituitary beta-endorphin system in the regulation of the cyclic LH secretion

This paper further substantiates the physiological role of beta-endorphin (beta-END) in the control of the cyclic LH secretion and provides new data on the interactions between 17 beta-estradiol (17 beta-E2) and beta-END at both the hypothalamic and pituitary levels. At the hypothalamic level, during the estrous cycle in rats, beta-END concentrations were highest on diestrus I in the arcuate nucleus, median preoptic area and median eminence and lowest at the time of the preovulatory 17 beta-E2 surge on proestrus, before the subsequent preovulatory hypothalamic GnRH and plasma LH surges. Data obtained in ovariectomized 17 beta-E2-treated ewes support the direct involvement of 17 beta-E2 in changes in beta-END and GnRH concentrations in these hypothalamic areas. At the anterior pituitary level, in vitro results obtained using anterior pituitaries from the proestrus morning cycling female rat have shown that 17 beta-E2 strongly suppresses beta-END secretion and that GnRH stimulates the release of beta-END. Furthermore, marked fluctuations were observed for plasma beta-END throughout the menstrual cycle in the woman. Low beta-END concentrations were observed in the period preceding the LH preovulatory surge. Taken together, these results show that: (1) decreases in hypothalamic beta-END concentrations, which are controlled at least by circulating levels of 17 beta-E2, modulate GnRH synthesis and/or release and contribute to the mechanisms which initiate the LH surge; (2) anterior pituitary beta-END might be involved in the mechanisms which terminate the LH surge.     

Responses of luteinizing hormone (LH) and follicle-stimulating hormone levels to exogenous gonadotropin-releasing hormone during the estrogen-induced LH surge in the ovariectomized rhesus monkey

Experiments were performed to study the responsiveness of the pituitary to gonadotropin-releasing hormone (GnRH) during the dynamic changes in gonadotropin secretion associated with the estrogen-induced luteinizing hormone (LH) surge in the ovariectomized (OVX) rhesus monkey. Silastic capsules filled with estradiol-17-beta were implanted subcutaneously in ovariectomized rhesus monkeys, resulting in an initial lowering of circulating LH and follicle-stimulating hormone (FSH) concentrations followed by an LH-FSH surge. GnRH was injected intravenously just before estrogen implantation, during the negative feedback response and during the rising, the peak, and the declining phases of the LH surge. The LH and FSH responses during the negative feedback phase were as large as those before estrogen treatment (control responses). During the rising phase of the LH surge, the acute response to GnRH injection did not differ significantly from the control response, but the responses 60 and 120 min after injection were somewhat increased. During the declining phase of the LH surge, the pituitary was not responsive to exogenous GnRH, although LH probably continued to be secreted at this time since the LH surge decreased more slowly than predicted by the normal rate of disappearance of LH in the monkey. We conclude that an increased duration of response to GnRH may be an important part of the mechanism by which estrogen induces the LH surge, but we do not see evidence of increased sensitivity of the pituitary to GnRH as an acute releasing factor at that time.     

Binding studies of substance P anterior pituitary binding sites: changes in substance P binding sites during the rat estrous cycle

Previous studies have shown that substance P (SP), an undecapeptide widely distributed in the gastrointestinal tract and in the peripheral and central nervous system, is a putative regulatory peptide involved in the control of reproductive function. Specifically, SP inhibited, at the anterior pituitary (AP) level, the stimulatory action of a physiological concentration (10(-8) M) of Gonadotropin Releasing Hormone (GnRH) on the release of the luteinizing hormone (LH). In the present work, we have demonstrated the presence of specific SP binding sites in the AP and related changes in the number of these sites to GnRH receptor number, hypothalamic SP and GnRH content and LH secretion during the rat estrous cycle. High affinity saturable SP binding sites (Kd, 1.5 approximately equal to 10 nM) were demonstrated in AP membranes using [3H]-SP or a novel analog, [125I]-(D-Tyr0, NorLeu11)SP. The binding affinity of SP fragments decreased with progressive removal of amino acid residues from N or C termini of the molecule. Other neuropeptides had low affinity for the SP binding sites. During the rat estrous cycle, SP and GnRH binding capacity of the anterior pituitary were inversely related. At the time of the proestrous LH surge, the AP binding capacity was low for GnRH but high for SP. The highest content of SP in the hypothalamus were recorded during the afternoon of proestrus when hypothalamic GnRH levels were lowest and the preovulatory surge occurred. These studies have established the presence of high affinity specific binding sites for SP in the AP which alter during the estrous cycle in a manner appropriate for mediating the direct inhibitory effects of SP on LH release in vitro.     

Attenuation of the estradiol-induced surge release of luteinizing hormone by antihistamine in ovariectomized ewes

Ovariectomized ewes received intramuscular (i.m.) injections of an H₁-histamine receptor antagonist, diphenhydramine, or saline during the anestrous and breeding seasons to determine if histamine may regulate the estradiol-induced surge release of LH in ewes. In addition, concentrations of histamine and GnRH in hypothalamic regions and histamine and LH in the pituitary gland were determined during the estradiol-induced surge of LH. Pretreatment mean, basal, and estradiol-induced secretion of LH did not differ (P > 0.05) among seasons. However, the quantity of LH (ng) measured during the estradiol-induced surge of LH was less (P < 0.05) in ewes treated with diphenhydramine (411 ± 104) than saline (747 ± 133). Treatment with diphenhydramine did not (P > 0.05) influence steady-state concentrations of histamine in hypothalamic or pituitary gland tissues, hypothalamic concentrations of GnRH, or anterior pituitary concentrations of LH during the estradiol-induced surge of LH. It is concluded that histamine may modulate the estradiol-induced surge release of LH in ewes by affecting the secretion of GnRH.     

Correlation between LH and estrogen receptor turnover in pituitary and hypothalamus of castrate rats following estrogen agonists and antagonists

A series of studies was undertaken to correlate the short-term dynamics of LH secretion and depletion-replenishment patterns of estrogen receptors (ER) in hypothalamic and pituitary cytosols of ovariectomized rats. Animals castrated for 2 weeks were administered various test compounds and analyzed at 1, 3, 5, 10 and 15 h post-treatment. A single injection of 10 micrograms 17 beta-estradiol (E2) to ovariectomized rats elicited a rapid depletion of ER in both pituitary and hypothalamus and a dramatic, though delayed, fall in serum LH. ER replenishment occurred in both tissues through 15 h and LH recovered in a similar manner. When cycloheximide was administered along with E2, ER replenishment was completely inhibited in both tissues; serum LH fell and failed to recover. Actinomycin D injected with E2 blocked replenishment in pituitary but not hypothalamus; serum LH recovered in parallel with the hypothalamic ER pattern. 17 alpha-E2 elicited only slight changes in ER and LH was suppressed 10-20% through 15 h. CI-628 caused a near total depletion of pituitary ER with no subsequent replenishment, whereas hypothalamic ER content was virtually unaltered; serum LH was suppressed and later recovered. Orchidectomized rats given 5 micrograms E2 demonstrated a less complete ER depletion in hypothalamus, and an earlier replenishment than that seen in pituitary or hypothalamus of similarly treated ovariectomized females. Serum LH rebounded to 157% of control levels at 15 h. The results indicate that the acute feedback suppression of LH by exposure to estrogens correlates with binding to ER and nuclear translocation. Replenishment and/or retention of cytoplasmic ER in hypothalamus appears to be required for full resumption of LH secretion, following acute suppression.     

Hirsutism, virilism, polycystic ovarian disease, and the steroid-gonadotropin-feedback system: a career retrospective

This career retrospective describes how the initial work on the mechanism of hormone action provided the tools for the study of hirsutism, virilism, and polycystic ovarian disease. After excessive ovarian and or adrenal androgen secretion in polycystic ovarian disease had been established, the question whether the disease was genetic or acquired, methods to manage hirsutism and methods for the induction of ovulation were addressed. Recognizing that steroid gonadotropin feedback was an important regulatory factor, initial studies were done on the secretion of LH and FSH in the ovulatory cycle. This was followed by the study of basic mechanisms of steroid-gonadotropin feedback system, using castration and steroid replacement and the events surrounding the natural onset of puberty. Studies in ovariectomized rats showed that progesterone was a pivotal enhancer of estrogen-induced gonadotropin release, thus accounting for the preovulatory gonadotropin surge. The effects of progesterone were manifested by depletion of the occupied estrogen receptors of the anterior pituitary, release of hypothalamic LHRH, and inhibition of enzymes that degrade LHRH. Progesterone also promoted the synthesis of FSH in the pituitary. The 3α,5α-reduced metabolite of progesterone brought about selective LH release and acted using the GABA(A) receptor system. The 5α-reduced metabolite of progesterone brought about selective FSH release; the ability of progesterone to bring about FSH release was dependent on its 5α-reduction. The GnRH neuron does not have steroid receptors; the steroid effect was shown to be mediated through the excitatory amino acid glutamate, which in turn stimulated nitric oxide. These observations led to the replacement of the long-accepted belief that ovarian steroids acted directly on the GnRH neuron by the novel concept that the steroid feedback effect was exerted at the glutamatergic neuron, which in turn regulated the GnRH neuron. The neuroprotective effects of estrogens on brain neurons are of considerable interest.     

Steroid control of gonadotropin secretion

Current knowledge about the mechanism and site of action of estradiol (E2) and progesterone (P) during the menstrual cycle and the physiological role of androgens is reviewed. In normal women, the positive feedback effect of E2 at the pituitary level is the principal event of the follicular phase inducing the LH surge. P, by its negative feedback at the hypothalamic level and by its positive feedback at the pituitary level regulates GnRH and LH secretion during the luteal phase. Androgens do not directly play a role in gonadotropin regulation.     

A potential role of modulating inositol 1,4,5-trisphosphate receptor desensitization and recovery rates in regulating ovulation

Regulation of the human menstrual cycle is a frequency dependent process controlled in part by the pulsatile release of gonadotropin releasing hormone (GnRH) from the hypothalamus. The binding of GnRH to gonadotroph cells in the pituitary stimulates inositol 1,4,5-trisphosphate (IP3) mediated release of calcium from the endoplasmic reticulum, resulting in calcium oscillations and the secretion of luteinizing hormone (LH). A sudden increase in serum LH concentrations known as the LH surge triggers ovulation. Here we model the intracellular calcium dynamics of gonadotroph cells by adapting the model of Li and Rinzel (J. Theor. Biol. 166 (1994) 461) to include the desensitization of IP3 receptors to IP3. Allowing the resensitization rate of these receptors to vary over the course of the cycle suffices to explain the LH surge in both the normal menstrual cycle, and in the treatment of Kallmann's syndrome (a condition where endogenous production of GnRH is absent).     

Alterations in the GnRH-LH system in relation to gonadal stage and Aroclor 1254 exposure in Atlantic croaker

Exposure of Atlantic croaker (Micropogonias undulatus) to the polychlorinated biphenyl mixture (Aroclor 1254, PCB; 1 mg/kg body wt/day for 30 days) during the early-recrudescence phase of the gonadal cycle results in the impairment of LH secretion and gonadal growth. In order to determine whether impairment was due to disruption of the stimulatory GnRH neuroendocrine pathway, we compared various parameters of the GnRH-LH system in early recrudescing vs. spermiating (mature) fish. Seabream GnRH (GnRH) content in the preoptic anterior hypothalamic area (POAH) and pituitary, pituitary GnRH receptor concentrations, and basal and GnRH analog (GnRHa)-induced LH secretion were significantly higher in gonadally mature croaker compared to early-recrudescing fish. In a subsequent experiment, the effects of PCB on the same neuroendocrine indices were investigated during the gonadal recrudescence phase of croaker. PCB exposure during the period of testicular maturation prevented the natural increase in GnRH content in the POAH but not in the pituitary. This finding suggests that PCB may impair GnRH synthesis in the POAH. The number of pituitary GnRH receptors also remained significantly lower in the PCB-exposed group, which was likely due to an impairment of GnRH release. The GnRH content in the POAH, number of pituitary GnRH receptors, and LH secretion in the PCB-exposed group were comparable to those in early-recrudescing fish, suggesting an impairment of normal maturation of the GnRH-LH system during the gonadal recrudescence phase. This impairment may be due to a direct action of PCB on GnRH neurons and/or indirectly via interference with other neurotransmitter pathways that modulate GnRH function.     

Estradiol modulation of growth hormone secretion in the ewe: no growth hormone-releasing hormone neurons and few somatotropes express estradiol receptor alpha

Evidence suggests that estrogen modulates growth hormone (GH) release and that GH plays an important role in follicular and ovulatory processes. How estradiol affects GH secretion is unclear. Having verified that there is a coincident surge of GH at the time of the preovulatory LH surge, immunocytochemical studies incorporating high-temperature antigen retrieval were used to determine whether GH-releasing hormone (GHRH) neurons, somatotropes, or both, expressed estrogen receptor alpha (ER), in the ewe. Although GHRH neurons were surrounded by many ER cells, they did not express immunocytochemically detectable ERs. In contrast to gonadotropes, in which the majority expressed ERs, few somatotropes were estrogen receptive. These data suggest that estrogen does not act directly on GHRH neurons to influence GH secretion, and any direct effect on pituitary GH release, through the ERalpha, may be small.     

Differential modulation of gonadotropin secretion by selective estrogen receptor 1 and estrogen receptor 2 agonists in ovariectomized ewes

The objectives of this study were to determine whether activation of estrogen receptor 1 (ESR1; also known as ERalpha), or estrogen receptor 2 (ESR2; also known as ERbeta), or both are required to: 1) acutely inhibit secretion of LH, 2) induce the preovulatory-like surge of LH, and 3) inhibit secretion of FSH in ovariectomized (OVX) ewes. OVX ewes (n = 6) were administered intramuscularly 25 micrograms estradiol (E2), 12 mg propylpyrazoletriol (PPT; a subtype-selective ESR1 agonist), 21 mg diaprylpropionitrile (DPN; a subtype-selective ESR2 agonist), or PPT + DPN. Like E2, administration of PPT, DPN, or combination of the two rapidly decreased (P < 0.05) secretion of LH. Each agonist induced a gradual, prolonged rise in secretion of LH after the initial inhibition, but neither agonist alone nor the combined agonists was able to induce a "normal" preovulatory-like surge of LH similar to that induced by E2. Compared with E2-treated ewes, the beginning of the increase in secretion of LH occurred earlier (P < 0.01) in DPN-treated ewes, later (P < 0.05) in PPT-treated ewes, and at a similar interval in ewes receiving the combined agonist treatment. Like E2, PPT decreased (P < 0.05) secretion of FSH, but the duration of suppression was much longer in PPT-treated ewes. DPN did not alter secretion of FSH in this study. Modulation of the number of GnRH receptors by PPT and DPN was examined in primary cultures of ovine pituitary cells. In our hands, both PPT and DPN increased the number of GnRH receptors, but the dose of DPN required to stimulate synthesis of GnRH receptors was 10 times higher than that of PPT. We conclude that in OVX ewes: 1) ESR1 and ESR2 mediate the negative feedback of E2 on secretion of LH at the level of the pituitary gland, 2) ESR1 and ESR2 do not synergize or antagonize the effects of each other; however, they do interact to synchronize the beginning of the stimulatory effect of E2 on secretion of LH, 3) ESR1 and ESR2 may mediate at least partially the positive feedback of E2 on LH secretion by increasing the number of GnRH receptors, and 4) only ESR1 appears to be involved in the negative feedback of E2 on secretion of FSH.     

Centrally applied somatostatin inhibits the estrogen-induced luteinizing hormone surge via hypothalamic gonadotropin-releasing hormone cell activation in female rats

Overexpression of growth hormone (GH) as well as GH-deficiency dramatically impairs reproductive function. Decreased reproductive function as a result of altered GH release is, at least partially, due to changes at the hypothalamic-pituitary level. We hypothesize that hypothalamic somatostatin (SOM), the inhibiting factor of GH release from the pituitary, may play a central role in the "crosstalk" between the somatotropic and gonadotropic axes. In the present study we investigated the possible effects of a centrally applied SOM analog on the LH surge and the concurrent activation of hypothalamic GnRH neurons in female rats. To this end, female rats were treated with estradiol 2 wk after ovariectomy and were given a single central injection with either the SOM analog, octreotide, or saline just prior to surge onset, after which hourly blood samples were taken to measure LH. Two weeks later, the experimental setup was randomly repeated to collect brains during the anticipated ascending phase of the LH surge. Vibratome sections were subsequently double-stained for GnRH and cFos peptide. Following octreotide treatment, LH surges were significantly attenuated compared to those in saline-treated control females. Also, octreotide treatment significantly decreased the activation of hypothalamic GnRH neurons. These results clearly demonstrate that SOM is able to inhibit LH release, at least in part by decreasing the activation of GnRH neurons. Based on these results, we hypothesize that hypothalamic SOM may be critically involved in the physiological regulation of the proestrus LH surge.     

Pituitary regulation of preovulatory estrogen secretion in the rat

The factors stimulating estrogen secretion in the preovulatory phase and an attempt to explain the mechanism of termination of estrogen secretion are discussed. Female Wistar rats, hypophysectomized at 1 p.m. in proestrus, were injected with rat pituitary extracts. Ovarian venous blood was collected and the estrogen activity of the plasma was measured. The estrogen secretion was minimized within 3 hours after hypophysectomy. The rat pituitary extract caused an 11-fold increase of estrogen concentration in the ovarian venous blood within 1 hour. Either LH or FSH alone was able to restore the estrogen secretion: LH took 1 hour to reach maximal response, FSH 2 hours. In the 1-hour test, the minimal effective dose for LH appeared to be less than .25 mcg per rat, for FSH, 2.5 mcg per rat. The total ability of the two preparations to produce estrogen appeared to be the same. 10 I.U. of prolactin slightly stimulated estrogen secretion, but 20 mU of ACTH was quite negative. These results demonstrate the pituitary gonadotropin dependency of estrogen secretion from the ovary having ripened follicles. It also showed that the ovary, after completion of ovulatory surge of LH, abolished its reactivity to the pituitary extract containing sufficient amount of substances in promoting estrogen secretion. Either LH or FSH was able to terminate estrogen secretion even at minute doses as small as 10 mcg. This shows that both FSH and LH provide a dual effect on ovarian estrogen secretion at the preovulatory stage, promotion and suppression. Promotion is an acute and direct action of hormones on steroidogenesis and suppression probably a delayed and indirect action of ovulation-inducing hormone, the release of which initiates the differentiation of estrogen-forming cells towards ovulation unfavorable to estrogen synthesis.     

Pituitary and gonadal function in hypogonadotrophic hypogonadal (hpg) mice bearing hypothalamic implants

GnRH receptor values are 30-50% of normal in pituitaries of hpg male mice, and testicular LH receptors only 8% of normal (160.4 +/- 17.6 and 2013 +/- 208.1 fmol/testis respectively). In male hpg mice bearing fetal preoptic area (POA) hypothalamic implants for 10 days there was no change in pituitary GnRH receptors, pituitary gonadotrophin content, or seminal vesicle weight. However, testicular weights and LH receptors were doubled in 4/10 mice and 2 had increased serum FSH levels. Between 26 and 40 days after implantation pituitary GnRH receptors and pituitary LH increased to normal male levels, although at 40 days serum and pituitary FSH concentrations had reached only 50% of normal values. Testicular and seminal vesicle weights increased more than 10-fold by 40 days after implantation and LH receptors to 70% of normal. In hpg female mice bearing hypothalamic implants for 30-256 days pituitary gonadotrophin concentrations were normal, even though GnRH receptors reached only 60% of normal values (6.18 +/- 0.4 and 9.8 +/- 0.4 fmol/pituitary respectively). Serum FSH was substantially increased from values of less than 30 ng/ml in hpg mice to within the normal female range in hypothalamic implant recipients. Ovarian and uterine weights increased after hypothalamic grafting from only 4-5% to over 74% of normal values. LH receptors increased from 6.5 +/- 1.3 fmol/ovary for hpg mice to 566.9 +/- 39.2 fmol/ovary for implant recipients. Vaginal opening occurred about 23 days after implantation and these animals displayed prolonged periods of oestrus.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pituitary gonadotropin-releasing hormone (GnRH) receptor: structure, distribution and regulation of expression

Reproduction in mammals is controlled by interactions between the hypothalamus, anterior pituitary and gonads. Interaction of GnRH with its cognate receptor is essential to regulating reproduction. Characterization of the structure, distribution and expression of GnRH receptors (GnRH-R) has furthered our understanding of the physiological consequences of GnRH stimulation of pituitary gonadotropes. Based on the putative topology of the amino acid sequence of the GnRH-R and point mutation studies, key elements of the GnRH-R have been identified to play a role in ligand recognition and binding, G-protein activation and internalization. Normally, reproductive function is mediated by GnRH-R expressed only on the membranes of pituitary gonadotropes. The density of GnRH-R on gonadotropes determines their ability to respond to GnRH. This density is highest just prior to ovulation and likely is important for complete expression of the pre-ovulatory surge of LH. Therefore, knowledge regarding what regulates the density of GnRH-R is essential to understanding changes in pituitary sensitivity to GnRH and ultimately, to expression of the LH surge. Regulation of GnRH-R gene expression is influenced by a multitude of factors including gonadal steroid hormones, inhibin, activin and perhaps most importantly GnRH itself.     

Progesterone blockade of the positive feedback effects of 17β oestradiol in the ewe

Ovariectomized ewes (n = 24) were treated with implants that resulted in circulating concentrations of progesterone and 17β-oestradiol similar to those seen in intact ewes in the luteal phase of an oestrous cycle. Progesterone implants were left in for 10 days, and 17β-oestradiol implants for 14 days. Twelve of these ewes received daily injections of 17β-oestradiol in oil (i.m.) at doses sufficient to cause a surge release of luteinizing hormone (LH) in the absence of progesterone. The other 12 ewes were treated daily with vehicle (oil). Following progesterone withdrawal on Day 10, each group of 12 ewes was divided into three subgroups. Ewes in each subgroup of the groups treated daily with 17β-oestradiol or vehicle, received an injection of either 17β-oestradiol (oil i.m.), gonadotrophin-releasing hormone (GnRH) (saline, i.v.) or vehicle, 24 h after progesterone withdrawal. Following progesterone withdrawal, no LH surge was detected in ewes treated with vehicle. Surge secretion of LH was detected in ewes of all other groups. The data suggested that in progesterone-treated ewes, daily exposure to stimulatory doses of 17β-oestradiol did not desensitize the hypothalamic pituitary axis to the positive feedback effects of 17β-oestradiol. Daily exposure to 17β-oestradiol did not suppress pituitary responsiveness to GnRH. It was concluded that circulating concentrations of progesterone, similar to those seen during the luteal phase of an oestrous cycle in intact ewes, may prevent all necessary components of the LH surge secretory mechanism from responding to 17β-oestradiol.     

Effects of GABA(A) receptor modulation on the expression of GnRH gene and GnRH receptor (GnRH-R) gene in the hypothalamus and GnRH-R gene in the anterior pituitary gland of follicular-phase ewes

The effect of prolonged, intermittent infusion of GABA(A) receptor agonist (muscimol) or GABA(A) receptor antagonist (bicuculline) into the third cerebral ventricle on the expression of GnRH gene and GnRH-R gene in the hypothalamus and GnRH-R gene in the anterior pituitary gland was examined in follicular-phase ewes by real-time PCR. The activation or inhibition of GABA(A) receptors in the hypothalamus decreased or increased the expression of GnRH and GnRH-R genes and LH secretion, respectively. The present results indicate that the GABAergic system in the hypothalamus of follicular-phase ewes may suppress, via hypothalamic GABA(A) receptors, the expression of GnRH and GnRH-R genes in this structure. The decrease or increase of GnRH-R mRNA in the anterior pituitary gland and LH secretion in the muscimol- or bicuculline-treated ewes, respectively, is probably a consequence of parallel changes in the release of GnRH from the hypothalamus activating GnRH-R gene expression. It is suggested that GABA acting through the GABA(A) receptor mechanism on the expression of GnRH gene and GnRH-R gene in the hypothalamus may be involved in two processes: the biosynthesis of GnRH and the release of this neurohormone in the hypothalamus.     

Pituitary desensitization and the regulation of pituitary gonadotropin-releasing hormone (GnRH) receptors following chronic administration of a superactive GnRH analog and testosterone

We recently demonstrated that chronic daily administration of a superactive GnRH analog to intact rats resulted in an initial stimulation of serum LH levels with a subsequent return of LH levels to baseline at a time when testosterone levels were marked decreased. These data demonstrated pituatary desensitization following chronic GnRH analog treatment. Administration of GnRH analog with a dose of testosterone which did not markedly lower serum LH levels when administered alone prevented the stimulation of LH secretion by analog. The present studies were undertaken to determine the effects of GnRH analog and testosterone administration on the regulation of pituitary GnRH receptors. Pituitary GnRH receptor binding was increased by analog treatment alone at 20 days and returned to control levels at 40 and 60 days of treatment in parallel to the observed changes in serum LH, demonstrating that one mechanism by which chronic GnRH analog treatment leads to pituitary desensitization is down-regulation of pituitary GnRH receptors. Testosterone administration alone decreased pituitary GnRH receptor binding. Combined GnRH analog and testosterone administration prevented the increase in pituitary GnRH receptors observed with analog administration alone. These studies demonstrate that changes in pituitary GnRH receptor binding correlate with changes in serum LH and that the stimulatory effects of analog administration on LH are sensitive to inhibition by small doses of testosterone.     

Membrane-initiated actions of estradiol (E2) in the regulation of LH secretion in ovariectomized (OVX) ewes

Background  

We demonstrated that E2 conjugated to BSA (E2BSA) induces a rapid membrane-initiated inhibition of LH secretion followed hours later by a slight increase in LH secretion. Whether these actions of E2BSA are restricted to the pituitary gland and whether the membrane-initiated pathway of E2BSA contributes to the up-regulation of the number of GnRH receptors during the positive feedback effect of E2 were evaluated here. We have shown that the suppression of LH secretion induced by E2 and E2BSA is the result of a decreased responsiveness of the pituitary gland to GnRH. In this study we further tested the ability of E2BSA to decrease the responsiveness of the pituitary gland to GnRH under the paradigm of the preovulatory surge of LH induced by E2.