Progesterone treatment that either blocks or augments the estradiol-induced gonadotropin-releasing hormone surge is associated with different patterns of hypothalamic neural activation

Progesterone-modified natural cycle preparation for frozen embryo transfer

RESEARCH QUESTION

Is there any difference in clinical outcomes between the progesterone-modified natural cycle (P4mNC) and hormone replacement therapy (HRT) endometrial preparation protocols after single euploid blastocyst frozen embryo transfer (FET) cycles?

DESIGN

A retrospective cohort study was performed at a single, private, high-volume fertility centre. Patients who underwent single euploid blastocyst FET between January 2017 and December 2019 were included. A total of 1933 FET cycles were reviewed, and 723 FET cycles from 548 patients met the inclusion criteria. Two groups were compared according to endometrial preparation: 327 P4mNC-FET and 396 HRT-FET cycles. The primary outcome was the live birth rate. The secondary outcomes included the clinical pregnancy rate and the miscarriage rate.

RESULTS

There were no differences in the clinical pregnancy rate (50.2% versus 47.0%, P = 0.688), miscarriage rate (9.8% versus 14.5%, P = 0.115) and live birth rate (45.0% versus 39.6%, P = 0.331) between the P4mNC-FET and HRT-FET groups after covariate adjustments.

CONCLUSIONS

There were no differences in the clinical outcomes between the P4mNC-FET and HRT-FET cycles. These results indicate that P4mNC-FET cycles produce clinical outcomes comparable to those of more traditional HRT-FET while allowing greater flexibility in the timing of embryo transfer.

Comparison of fixed and flexible progestin-primed ovarian stimulation in women classified in patient-oriented strategies encompassing individualized oocyte number (POSEIDON) group 4

PURPOSE

This study aimed to compare the fixed and flexible protocols for progestin-primed ovarian stimulation (PPOS) in poor ovarian responders.

METHODS

This retrospective study included 95 poor ovarian responders classified using the Patient-Oriented Strategies Encompassing Individualized Oocyte Number group 4 criteria. Treatment involved assisted reproductive medicine using fixed and flexible PPOS protocols at Shiga University of Medical Science between July 2019 and August 2023. PPOS cycles were assigned to the fixed and flexible groups at the discretion of attending physicians. The results of assisted reproductive medicine were compared between groups.

RESULTS

The fixed and flexible groups included 68 and 27 patients, respectively. The flexible group obtained more retrieved oocytes and two pro-nuclei than the fixed group, without an early luteinizing hormone surge. Multiple linear regression analysis demonstrated that differences in protocols and anti-müllerian hormone (AMH) levels were related to the number of retrieved oocytes. The differences in protocols were more strongly correlated with the number of oocytes than with the AMH levels.

CONCLUSION

Among poor ovarian responders, the flexible PPOS protocol provided more retrieved oocytes than the fixed PPOS protocol, possibly because the total dosage of progestins was lower in the flexible group and progestins were not administered at the time when ovarian stimulation was initiated.

Letrozole cotreatment progestin-primed ovarian stimulation in women undergoing controlled ovarian stimulation for in vitro fertilization

AIM

To investigate the impact of letrozole cotreatment progestin-primed ovarian stimulation (PPOS) (Le PPOS) in controlled ovarian stimulation (COS) and the pregnancy outcomes in frozen-thawed embryo transfer cycles.

METHODS

This retrospective cohort study included women who underwent in vitro fertilization (IVF)/intracytoplasmic sperm injection (ICSI). A total of 2575 cycles were included (1675 in the Le PPOS group and 900 in the PPOS group). The primary outcome was the clinical pregnancy rates. The secondary outcome was the live birth rates.

RESULTS

In this study, propensity score matching (PSM) was performed to create a perfect match of 379 patients in each group. After matching, the numbers of oocytes retrieved, mature oocytes, fertilization, and clinical pregnancy rates were more favorable in the Le PPOS group than in the PPOS group (all p < 0.05). The multivariable analysis showed that the clinical pregnancy rate was higher in the Le PPOS than in the PPOS group (odds ratio = 1.46, 95% confidence interval: 1.05-2.04, p = 0.024) after adjusting for potentially confounding factors (age, anti-Müllerian hormone levels, antral follicular count, the type of embryo transferred, number of transferred embryos, body mass index, and follicular stimulating hormone and estradiol levels on starting day).

CONCLUSIONS

This retrospective study with a limited sample size suggests that the Le PPOS protocol might be an alternative to the PPOS protocol in women undergoing COS and could lead to better pregnancy outcomes. The results should be confirmed using a formal randomized controlled trial.

Fixed Gonadotropin-Releasing Hormone Antagonist Protocol Versus Flexible Progestin-Primed Ovarian Stimulation Protocol in Patients With Asynchronous Follicular Development During Controlled Ovulation Stimulation: A Retrospective Study

Protocols utilizing gonadotropin-releasing hormone (GnRH) antagonists have emerged as mainstream procedures for ovarian stimulation; however, GnRH increases the risk for periodic cancellation of embryos. Therefore, this study aimed to compare the pregnancy outcomes of a fixed GnRH antagonist protocol and a flexible progestin-primed ovarian stimulation (fPPOS) protocol in patients with asynchronous follicular development during controlled ovulation stimulation and to explore the feasibility of converting patients undergoing a fixed GnRH antagonist protocol to an fPPOS protocol. This was the first retrospective study exploring the fPPOS protocol in patients with asynchronous follicular development, and it was conducted in a public reproductive medicine center from January to December 2020. We included infertile women. All participants were scheduled to undergo administration of a GnRH antagonist on the fifth day of controlled ovulation stimulation. The study group included 129 women who were converted from the fixed GnRH antagonist protocol to the fPPOS protocol for their asynchronous follicular development, while the antagonist group consisted of 258 women (ratio 1:2) who proceeded with a fixed GnRH antagonist protocol. On the second or third day of the menstrual period, 100-300 IU/day gonadotropin injections were administered. For patients who were converted to the fPPOS protocol, medroxyprogesterone acetate tablets at 10 mg/day were started on the fifth day of stimulation or when only one leading follicle reached 14 mm and the other follicles were ≤10 mm in diameter, whichever came first. The rates of embryo implantation, clinical pregnancy, and early pregnancy loss were obtained. The number of oocytes retrieved and the number of high-quality embryos in the antagonist group were significantly higher than those in the fPPOS group (P = 0.039 and P = 0.025, respectively). No significant differences in the rates of embryo implantation, clinical pregnancy, and early pregnancy loss were observed between the two groups. Our study found that in patients who were scheduled for administration of GnRH antagonists but presented with asynchronous follicular development on the fifth stimulation day, it was feasible to switch to the fPPOS protocol.

Evidence That the LH Surge in Ewes Involves Both Neurokinin B-Dependent and -Independent Actions of Kisspeptin

Recent evidence has implicated neurokinin B (NKB) signaling in the retrochiasmatic area (RCh) of the ewe in the LH surge. To test this hypothesis, we first lesioned NK3R neurons in this area by using a saporin conjugate (NK3-SAP). Three weeks after bilateral injection of NK3-SAP or a blank control (BLK-SAP) into the RCh, an LH surge was induced by using an artificial follicular-phase model in ovariectomized ewes. NK3-SAP lesioned approximately 88% of RCh NK3R-containing neurons and reduced the amplitude of the estrogen-induced LH surge by 58%, an inhibition similar to that seen previously with intracerebroventricular (icv) infusion of a KISS1R antagonist (p271). We next tested the hypothesis that NKB signaling in the RCh acts via kisspeptin by determining whether the combined effects of NK3R-SAP lesions and icv infusion of p271 were additive. Experiment 1 was replicated except that ewes received two sequential artificial follicular phases with infusions of p271 or vehicle using a crossover design. The combination of the two treatments decreased the peak of the LH surge by 59%, which was similar to that seen with NK3-SAP (52%) or p271 (54%) alone. In contrast, p271 infusion delayed the onset and peak of the LH surge in both NK3-SAP- and BLK-SAP-injected ewes. Based on these data, we propose that NKB signaling in the RCh increases kisspeptin levels critical for the full amplitude of the LH surge in the ewe but that kisspeptin release occurs independently of RCh input at the onset of the surge to initiate GnRH secretion.

Comparison of a novel flexible progestin primed ovarian stimulation protocol and the flexible gonadotropin-releasing hormone antagonist protocol for assisted reproductive technology

OBJECTIVE

To determine whether a flexible progestin primed ovarian stimulation (fPPOS) protocol is effective for preventing premature ovulation.

DESIGN

Retrospective cohort study.

SETTING

Private assisted reproduction center.

PATIENT(S)

Eighty-seven oocyte donors and 191 recipients of fresh oocytes.

INTERVENTION(S)

Each donor was stimulated with a flexible gonadotropin-releasing hormone (GnRH) antagonist protocol in one cycle and with the new fPPOS protocol in the other, within a period of 6 months. FSH was started on cycle day 2-3, and 0.25 mg/day GnRH antagonist or 10 mg/day medroxyprogesterone acetate (MPA) was started on stimulation day 7 or when the leading follicle reached 14 mm, whichever came first.

MAIN OUTCOME MEASURE(S)

Duration of stimulation, gonadotropin consumption, duration of GnRH antagonist or MPA administration, number of metaphase II oocytes, and pregnancy rates in fresh oocyte recipients.

RESULTS

Duration of stimulation was 11 (10-11) days in both groups. Total gonadotropin consumption was similar. Pituitary suppression was started on day 7 and lasted for 5 days in each group. There were no premature ovulations in any group. The fPPOS yielded a significantly higher number of cumulus oocyte complexes than GnRH antagonist cycles (33 [21-39] vs. 26 [18-36], respectively). Likewise, the fPPOS generated significantly more metaphase II oocytes than GnRH antagonist cycles (24 [17-34] vs. 21 [15-28], respectively). Recipients of fresh oocytes from fPPOS and GnRH antagonist cycles had similar cleavage, blastulation, implantation, and live birth/ongoing pregnancy rates (50% vs. 48.6%).

CONCLUSION(S)

FPPOS with MPA seems to be an effective choice for preventing premature ovulation in women undergoing ovarian stimulation without compromising oocyte quality.

Use of medroxyprogesterone acetate in women with ovarian endometriosis undergoing controlled ovarian hyperstimulation for in vitro fertilization

This study investigated the use of medroxyprogesterone acetate (MPA) or a short protocol for controlled ovarian hyperstimulation (COH) in patients with advanced endometriosis who have normal ovarian function, and to compare cycle characteristics and pregnancy outcomes after frozen-thawed embryo transfer (FET). This was a retrospective case-control study of 244 patients with advanced endometriosis undering COH. The patients were allocated to three groups: the surgery group with MPA COH (62 patients, 71 IVF/ICSI cycles, 78 FET cycles); the aspiration group with MPA COH (85 patients had ovarian “chocolate” cysts (>3 cm) aspirated, 90 IVF/ICSI cycles, 76 FET cycles); and the short protocol group (97 patients, 101 IVF/ICSI cycles, 51 FET cycles). The results showed that higher rates of mature oocyte, D3 high quality embryo, hMG dose were observed in the two study groups using MPA compared with the short protocol. The number of >10–14 mm follicles on the trigger day, D3 top-quality embryos, viable embryos, rates of cancellation, fertilization, implantation, pregnancy outcomes were similar among the three groups. The oocytes, embryos, and pregnancy outcomes were not influenced by endometrioma surgery or presence of endometrioma. MPA COH could be effective for women with ovarian advanced endometriosis who had normal ovarian function.

The effect of human chorionic gonadotrophin contained in human menopausal gonadotropin on the clinical outcomes during progestin-primed ovarian stimulation

Progestin-primed ovarian stimulation (PPOS) protocol has recently been demonstrated to be an novel regimen for preventing premature LH surges during controlled ovarian hyperstimulation (COH) in combination with frozen-thawed embryo transfer (FET). Our prospective controlled study was to explore the effect of human chorionic gonadotropin (hCG) contained in human menopausal gonadotropin (hMG) on the clinical outcomes in normalovulatory women undergoing COH with PPOS. A total of 180 patients were allocated into three groups according to the gonadotropin (Gn) used: group A (human menopausal gonadotropin, hMG-A), group B (hMG-B) or group C (follicle stimulating hormone, FSH). The primary outcome measured was the number of oocytes retrieved. The number of oocytes retrieved in group A B C was 10.72±5.78 11.33±5.19and13.38±8.97, respectively, with no statistic significance (p>0.05). Other embryological indicators were also similar (p>0.05). The concentration of serum and urinary β-hCG on the trigger day in group A and B were not associated with embryo results (p>0.05). There was no significant differences in the clinical pregnancy rate (41.67% vs. 51.56% vs. 39.51%, p>0.05) and implantation rate (31.58%vs. 34.75%vs.25.33%) after FET among the three groups. Thus the clinical characteristics were not affected by the hCG contained in hMG in normalovulatory women treated with PPOS.

Controlled Ovarian Stimulation Using Medroxyprogesterone Acetate and hMG in Patients With Polycystic Ovary Syndrome Treated for IVF: A Double-Blind Randomized Crossover Clinical Trial

Ovarian hyperstimulation syndrome (OHSS) during ovarian stimulation is a current challenge for patients with polycystic ovarian syndrome (PCOS). Our previous studies indicated that progestin can prevent premature luteinizing hormone (LH) surge or moderate/severe OHSS in the general subfertile population, both in the follicular-phase and luteal-phase ovarian stimulation but it is unclear if this is true for patients with PCOS. The aim of the article was to analyze cycle characteristics and endocrinological profiles using human menopausal gonadotropin (hMG) in combination with medroxyprogesterone acetate (MPA) for PCOS patients who are undergoing IVF/intracytoplasmic sperm injection (ICSI) treatments and investigate the subsequently pregnancy outcomes of frozen embryo transfer (FET). In the randomized prospective controlled study, 120 PCOS patients undergoing IVF/ICSI were recruited and randomly classified into 2 groups according to the ovarian stimulation protocols: hMG and MPA (group A, n = 60) or short protocol (group B, n = 60). In the study group, hMG (150-225IU) and MPA (10 mg/d) were administered simultaneously beginning on cycle day 3. Ovulation was cotriggered by a gonadotropinreleasing hormone (GnRH) agonist (0.1 mg) and hCG (1000IU) when dominant follicles matured. A short protocol was used as a control. The primary end-point was the ongoing pregnancy rate per transfer and incidence of OHSS. Doses of hMG administrated in group A are significantly higher than those in the controls. LH suppression persisted during ovarian stimulation and no incidence of premature LH surge was seen in both groups. The fertilization rate and the ongoing pregnant rate in the study group were higher than that in the control. The number of oocytes retrieved, mature oocytes, clinical pregnancy rates per transfer, implantation rates, and cumulative pregnancy rates per patient were comparable between the 2 groups. The incidence of OHSS was low between the 2 groups, with no significant difference. The study showed that MPA has the advantages of an oral administration route, easy access, more control over LH levels. A possible reduction in the incidence of moderate or severe OHSS with the MPA protocol should be viewed with caution as the data is small. Large randomized trials with adequate sample size remain necessary.

Medroxyprogesterone acetate is an effective oral alternative for preventing premature luteinizing hormone surges in women undergoing controlled ovarian hyperstimulation for in vitro fertilization

OBJECTIVE

To investigate the use of medroxyprogesterone acetate (MPA) to prevent LH surge during controlled ovarian hyperstimulation (COH) and to compare cycle characteristics and pregnancy outcomes in subsequently frozen-thawed ET (FET) cycles.

DESIGN

A prospective controlled study.

SETTING

Tertiary-care academic medical center.

PATIENT(S)

Three hundred patients undergoing IVF/intracytoplasmic sperm injection treatment.

INTERVENTION(S)

In the study group, hMG and MPA were administered simultaneously beginning on cycle day 3. Ovulation was induced with a GnRH agonist or cotriggered by a GnRH agonist and hCG when dominant follicles matured. A short protocol was used in the control group. Viable embryos were cryopreserved for later transfer in both protocols.

MAIN OUTCOME MEASURE(S)

The primary outcome measure was the number of oocytes retrieved. Secondary outcomes included the number of mature oocytes, the incidence of premature LH surge, and clinical pregnancy outcomes from FETs.

RESULT(S)

The number of oocytes retrieved in the study group was similar to those in the controls (9.9 ± 6.7 vs. 9.0 ± 6.0), and higher doses of hMG were administered. In the study group, LH suppression persisted during ovarian stimulation, and the incidence of premature LH surge was 0.7% (1/150). No statistically significant differences were found in the clinical pregnancy rates (47.8% vs. 43.3%), implantation rates (31.9% vs. 27.7%), and live-birth rates (42.6% vs. 35.5%) in the study group and controls.

CONCLUSION(S)

The results show that MPA is an effective oral alternative for the prevention of premature LH surge in woman undergoing COH. This finding will help establish a new regimen for ovarian stimulation in combination with embryo cryopreservation.

CLINICAL TRIAL REGISTRATION NUMBER

ChiCTR-ONRC-14004419.

Utrogestan as an effective oral alternative for preventing premature luteinizing hormone surges in women undergoing controlled ovarian hyperstimulation for in vitro fertilization

A major cause of cycle cancellation during controlled ovarian hyperstimulation (COH) in women undergoing in vitro fertilization (IVF) is the occurrence of premature luteinizing hormone (LH) surges. Steroidal preparations can modulate the secretion of gonadotropins (Gn); however, few studies using progesterone to inhibit the premature LH surges in COH have been published. The purpose of the study was to evaluate the oral delivery of progesterone soft capsules (Utrogestan) to prevent LH surges from the follicular phase and to compare cycle characteristics as well as to evaluate pregnancy outcomes in subsequent frozen-thawed embryo transfer (FET) cycles. A total of 374 patients were enrolled in this retrospective study, among which 187 patients were simultaneously administered Utrogestan and human menopausal gonadotrophin (hMG) from cycle day 3 until the trigger day. A short protocol including 187 controls with comparable age, body mass index (BMI), infertility duration, and antral follicle count was also used. GnRH agonist (0.1 mg) or hCG (3000 IU) was used for a trigger when the dominant follicles matured. Viable embryos were cryopreserved for later transfer in both groups. The primary outcome was the number of oocytes retrieved. The secondary outcomes included the number of mature oocytes, incidence of premature LH surge, and clinical pregnancy outcomes from FET cycles. Consistent LH suppression was achieved during COH, with a range of 0.07 to 8.9 IU/L, and no premature LH surge was detected. The number of oocytes retrieved in the Utrogestan and hMG protocol was comparable with that in the short protocol (10.92 ± 5.74 vs 10.6 ± 6.22, P > 0.05), and the dose of hMG was higher than that used in the short protocol (1884.22 ± 439.47 IU vs 1446.26 ± 550.48 IU, P < 0.05). No significant between-group difference was observed in the mature oocyte rate (88.88% vs 90.12%), cleavage rate (96.58% vs 96.58%), clinical pregnancy rate (54.27% vs 51.65%), or implantation rate (33.59% vs 34.02%). The study shows that Utrogestan is an effective oral alternative for preventing premature LH surges in women undergoing COH, which will help to establish a convenient user regimen in combination with FET.

Estradiol upregulates progesterone receptor and orphanin FQ colocalization in arcuate nucleus neurons and opioid receptor-like receptor-1 expression in proopiomelanocortin neurons that project to the medial preoptic nucleus in the female rat

BACKGROUND

Ovarian steroids regulate sexual receptivity in the female rat by acting on neurons that converge on proopiomelanocortin (POMC) neurons in the arcuate nucleus of the hypothalamus (ARH) that project to the medial preoptic nucleus (MPN). Estradiol rapidly activates these neurons to release β-endorphin that activates MPN μ-opioid receptors (MOP) to inhibit lordosis. Lordosis is facilitated by the subsequent action of progesterone that deactivates the estradiol-induced MPN MOP activation. Orphanin FQ (OFQ/N; also known as nociceptin) infusions into the ARH, like progesterone, deactivate MPN MOP and facilitate lordosis in estradiol-primed rats. OFQ/N reduces the activity of ARH β-endorphin neurons through post- and presynaptic mechanisms via its cognate receptor, ORL-1.

METHODS

We tested the hypotheses that progesterone receptors (PR) are expressed in ARH OFQ/N neurons by immunohistochemistry and ORL-1 is expressed in POMC neurons that project to the MPN by combining Fluoro-Gold injection into the MPN and double-label fluorescent in situ hybridization (FISH). We also hypothesized that estradiol increases coexpression of PR-OFQ/N and ORL-1-POMC in ARH neurons of ovariectomized rats.

RESULTS

The number of PR- and OFQ/N-immunopositive ARH neurons was increased as was their colocalization by estradiol treatment. FISH for ORL-1 and POMC mRNA revealed a subpopulation of ARH neurons that was triple labeled, indicating these neurons project to the MPN and coexpress ORL-1 and POMC mRNA. Estradiol was shown to upregulate ORL-1 and POMC expression in MPN-projecting ARH neurons.

CONCLUSION

Estradiol upregulates the ARH OFQ/N-ORL-1 system projecting to the MPN that regulates lordosis.

Neuroendocrine regulation of GnRH release and expression of GnRH and GnRH receptor genes in the hypothalamus-pituitary unit in different physiological states

This review is focused on the relationship between neuroendocrine regulation of GnRH/LH secretion and the expression of GnRH and GnRH receptor (GnRHR) genes in the hypothalamic-pituitary unit during different physiological states of animals and under stress. Moreover, the involvement of hypothalamic GABA-ergic, Beta-endorphinergic, CRH-ergic, noradrenergic, dopaminergic and GnRH-ergic systems in the regulation of expression of the GnRH and GnRHR genes as well as secretion of GnRH/LH is analyzed. It appears that the neural mechanisms controlling GnRH gene expression in different physiological states may be distinct from those regulating GnRH/LH release. The hypothalamic GnRHR gene is probably located in different neural systems and may act in a specific way on GnRH gene expression and GnRH release.

Progesterone can block the preovulatory gonadotropin-releasing hormone/luteinising hormone surge in the ewe by a direct inhibitory action on oestradiol-responsive cells within the hypothalamus

Elevated oestradiol concentrations during the follicular phase stimulate a surge in gonadotropin-releasing hormone (GnRH) and luteinising hormone (LH) concentrations, which leads to ovulation. Progesterone can block the oestradiol-induced GnRH/LH surge, but the mechanism that is involved is unclear. We examined the effect of progesterone on oestradiol-induced activation of cells within the ovine hypothalamus/preoptic area (POA) to determine: (i) in which regions progesterone acts to block the GnRH/LH surge and (ii) whether progesterone directly or indirectly prevents activation of oestradiol-responsive cells. Cellular activation was assessed by measuring the number of cells that expressed Fos (an immediate early gene). Exposure to increased oestradiol concentrations in the absence of progesterone (which normally stimulates a LH surge) did not cause any region-specific changes in hypothalamic Fos expression during the activation stage of the LH surge-induction process (Experiment 1). The same treatment significantly increased cellular activation within the POA, lateral septum (LS), and arcuate nucleus at the time of surge onset (Experiment 2). Concurrent exposure to increased oestradiol and progesterone concentrations during the activation stage of the surge-induction process (which normally blocks the LH surge) was associated with significantly reduced cellular activation within the ventromedial hypothalamus and anterior hypothalamic area, relative to the positive controls (oestradiol increment alone) and arcuate nucleus relative to the negative controls (no increment in oestradiol) during the activation stage (Experiment 1). At the time of surge onset (Experiment 2), exposure to progesterone during the activation period prevented the oestradiol-induced increase in cellular activation that occurred in the POA, LS and arcuate nucleus of the positive controls. These results demonstrated that oestradiol and progesterone induced differential region- and time-specific effects on cellular activation within the regions of the ovine brain that generate the preovulatory GnRH/LH surge. Moreover, the lack of cellular activation within the POA, LS and arcuate nucleus at the time of surge onset in animals exposed to progesterone during the activation stage is consistent with the hypothesis that progesterone can block the preovulatory surge by direct inhibition of oestradiol-induced cellular activation in these areas.

Somatostatin-14 neurons in the ovine hypothalamus: colocalization with estrogen receptor alpha and somatostatin-28(1-12) immunoreactivity, and activation in response to estradiol

Pituitary gland growth hormone (GH) secretion is influenced by two hypothalamic neuropeptides: growth hormone-releasing hormone (GHRH) and somatostatin. Recent data also suggest that estrogen modulates GH release, particularly at the time of the preovulatory luteinizing hormone surge, when a coincident surge of GH is observed in sheep. The GHRH neurons do not possess estrogen receptor alpha (ERalpha), suggesting that estrogen does not act directly on GHRH neurons. Similarly, few somatotropes express ERalpha, suggesting a weak pituitary effect of estradiol on GH. It was hypothesized, therefore, that estradiol may affect somatostatin neurons to modulate GH release from the pituitary. Using immunocytochemical approaches, the present study revealed that although somatostatin neurons were located in several hypothalamic sites, only those in the arcuate nucleus (13% +/- 2%) and ventromedial nucleus (VMN; 29% +/- 1%) expressed ERalpha. In addition, we found that all neurons immunoreactive for somatostatin-14 were also immunoreactive for somatostatin-28(1-12). To determine whether increased GH secretion in response to estradiol is through modulation of GHRH and/or somatostatin neuronal activity, a final study investigated whether c-fos expression increased in somatostatin- and GHRH-immunoreactive cells at the time of the estradiol-induced LH surge in intact anestrous ewes. Estradiol significantly (P < 0.05) increased the percentage of GHRH (estradiol, 75% +/- 3%; no estradiol, 19% +/- 2%) neurons expressing c-fos in the hypothalamus. The percentage of somatostatin-immunoreactive neurons coexpressing c-fos in the estradiol-treated animals was significantly (P < 0.05) higher (periventricular, 44% +/- 3%; arcuate, 72% +/- 5%; VMN, 81% +/- 5%) than in the control animals (periventricular, 22% +/- 1%; arcuate, 29% +/- 3%; VMN, 31% +/- 3%). The present study suggests that estradiol modulates the activity of GHRH and somatostatin neurons but that this effect is most likely mediated through an indirect interneuronal pathway.

Progesterone blocks the estradiol-stimulated luteinizing hormone surge by disrupting activation in response to a stimulatory estradiol signal in the ewe

The preovulatory surges of GnRH and LH are activated by increased concentrations of circulating estradiol, but ovulation is blocked when progesterone concentrations are elevated. Although it is has been shown that this action of progesterone is due to a central inhibition of the GnRH surge, the mechanisms that underlie the blockade of the GnRH surge are poorly understood. In this study we investigated whether progesterone can block the estradiol-dependent activation stage of the GnRH surge induction process, and thus prevent expression of the LH surge. The results demonstrated that exposure to progesterone for half or the full duration of the activation stage can prevent the stimulation of LH surges by estradiol (experiment 1), whereas exposure to progesterone midway though a period of estradiol exposure, which in itself is sufficient to activate the surge, did not block the LH surge (experiment 2). These results suggest that progesterone 1) disrupts activation of the surge induction system in response to a stimulatory estradiol signal and 2) does not compromise the ability of animals to respond to a stimulatory estradiol signal applied immediately after progesterone exposure. Because the disruptive effects of activated progesterone in response to estradiol are rapid but transient, it may be that progesterone directly interferes with the activation of estradiol-responsive neural systems to block the GnRH/LH surge.

Role of endogenous opioid peptides in mediating progesterone-induced disruption of the activation and transmission stages of the GnRH surge induction process

How progesterone blocks the E2-induced GnRH surge in females is not known. In this study we assessed whether the endogenous opioid peptides (EOPs) that mediate progesterone negative feedback on pulsatile GnRH secretion also mediate the blockade of the GnRH surge. We treated ovariectomized ewes with physiological levels of E2 and progesterone to stimulate and block the GnRH surge, respectively, using LH secretion as an index of GnRH release. A pilot study confirmed that blocking opioidergic neurotransmission with the opioid receptor antagonist, naloxone (NAL; 1 mg/kg.h, i.v.), could prevent the suppression of pulsatile LH secretion by progesterone in our model. By contrast, antagonizing EOP receptors with NAL did not restore LH surges in ewes in which the E2-induced GnRH surge was blocked by progesterone treatment during the E2-dependent activation stage (Exp 1) of the GnRH surge induction process. However, in ewes treated with progesterone during the E2-independent transmission stage (Exp 2), NAL partially restored blocked LH surges, as indicated by increased fluctuations in LH that, in some cases, resembled LH surges. We conclude, therefore, that the EOPs that mediate progesterone negative feedback on pulsatile GnRH secretion are not involved in blockade of activation of the E2-induced GnRH surge by progesterone, but do appear to be part of the mechanism by which progesterone disrupts the transmission stage.