Effect of luteinizing hormone releasing hormone pulse characteristics on comparative luteinizing hormone and follicle stimulating hormone secretion from superfused rat anterior pituitary cell cultures

We have shown that 4 ng luteinizing hormone releasing hormone (LHRH) pulses induced significantly greater luteinizing hormone (LH) release from proestrous rat superfused anterior pituitary cells with no cycle related differences in follicle stimulating hormone (FSH). Current studies gave 8 ng LHRH in various pulse regimens to study amplitude, duration and frequency effects on LH and FSH secretion from estrous 0800, proestrous 1500 and proestrous 1900 cells. Regimen 1 gave 8 ng LHRH as a single bolus once/h; regimen 2 divided the 8 ng into 3 equal 'minipulses' given at 4 min intervals to extend duration; regimen 3 gave the 3 'minipulses' at 10 min intervals, thereby further extending duration: regimen 4 was the same as regimen 2, except that the 3 'minipulses' were given at a pulse frequency of 2 h rather than 1 h. In experiment 1, all four regimens were employed at proestrus 1900. FSH was significantly elevated by all 8 ng regimens as compared to 4 ng pulses; further, 8 ng divided into 3 equal 'minipulses' separated by 4 min at 1 and 3 h frequencies (regimens 2 and 4) resulted in FSH secretion that was significantly greater than with either a single 8 ng bolus (regimen 1) or when the 'minipulses' were separated by 10 min (regimen 3). In experiment 2, at proestrus 1500, FSH response to the second pulse of regimen 4 was significantly greater than in regimen 2; LH release was significantly suppressed at pulse 2 compared to regimen 2 accentuating divergent FSH secretion. At estrus 0800, FSH response to the second pulse of regimen 4 was significantly stimulated FSH at proestrus 1900, 1500 and estrus 0800, FSH divergence was most marked at proestrus 1500. These data indicate a potential role for hypothalamic LHRH secretory pattern in inducing divergent gonadotropin secretion in the rat.     

Superfused pituitary cell cultures: comparative responsiveness of cells derived from various stages of the estrous cycle to LHRH stimulation administered as short duration pulses

We have reinvestigated the question of maintenance of differential LHRH sensitivity in culture and further investigated the role of pulsatile LHRH in the in vitro release of pulsatile LH and FSH at different stages of the estrous cycle. Pituitaries were collected on each day of the 4 day cycle at 0800. In addition, pituitaries were also collected at 1500 and 1900 on proestrous. The cells were dispersed and exposed 48 hrs later to short duration 4 ng LHRH pulses; this dose was optimized for LH release and was applied at a frequency of 1 pulse/60 min. In terms of absolute magnitude of LH response, observed responsiveness was ranked in the following order: proestrous 1900 greater than estrous 0800 greater than diestrous 1 0800 greater than proestrous 1500 greater than diestrous 2 0800. Responsiveness was significantly greater at proestrous 1900 (p greater than 0.01), estrous 0800 (p greater than 0.05) and diestrous 1 0800 (p greater than 0.05) when compared to either of the other stages tested. The heightened LHRH sensitivity of proestrous was therefore maintained in cell culture indicating that the system should be valid for conducting studies on the control of gonadotropin secretion during this period. FSH did not respond in pulsatile manner to the LHRH levels employed further substantiating recent evidence that LHRH seems to function somehow less directly in FSH as compared to LH secretion.     

Pituitary and ovarian luteinizing hormone releasing hormone receptors during the estrous cycle, pregnancy and lactation in the rat

Female Sprague-Dawley rats were decapitated at various stages of the estrous cycle, pregnancy, lactation and following ovariectomy. Anterior pituitary and ovarian tissues were collected and assayed to quantify luteinizing hormone releasing hormone (LHRH) receptors. No changes were noted in receptor affinity either between tissues or physiological stages studied. Pituitary LHRH receptor concentrations and content were greater (P less than 0.05) during diestrus II and proestrus than during estrus. Pituitary LHRH receptor concentrations and content during pregnancy were not different from those during estrus, however, a significant decrease was noted in pituitary LHRH receptor content and concentrations during lactation compared to estrus. Ovarian LHRH receptor content did not change with stage of reproduction (P less than 0.05). There was, however, a decrease (P less than 0.05) in ovarian LHRH receptor concentrations at Week 3 of pregnancy and Week 1 of lactation which was possibly due to the increase ovarian weight noted at both these physiological stages. There was no correlation (P less than 0.1) between ovarian and pituitary LHRH receptor numbers (r = 0.096). These findings suggest that the internal mechanisms which control changes in pituitary LHRH receptor numbers do not control ovarian LHRH receptor numbers.     

A Mathematical Model for the Actions of Activin,Inhibin, and Follistatin on Pituitary Gonadotrophs

The timed secretion of the luteinizing hormone (LH) and follicle stimulating hormone (FSH) from pituitary gonadotrophs during the estrous cycle is crucial for normal reproductive functioning. The release of LH and FSH is stimulated by gonadotropin releasing hormone (GnRH) secreted by hypothalamic GnRH neurons. It is controlled by the frequency of the GnRH signal that varies during the estrous cycle. Curiously, the secretion of LH and FSH is differentially regulated by the frequency of GnRH pulses. LH secretion increases as the frequency increases within a physiological range, and FSH secretion shows a biphasic response, with a peak at a lower frequency. There is considerable experimental evidence that one key factor in these differential responses is the autocrine/paracrine actions of the pituitary polypeptides activin and follistatin. Based on these data, we develop a mathematical model that incorporates the dynamics of these polypeptides. We show that a model that incorporates the actions of activin and follistatin is sufficient to generate the differential responses of LH and FSH secretion to changes in the frequency of GnRH pulses. In addition, it shows that the actions of these polypeptides, along with the ovarian polypeptide inhibin and the estrogen-mediated variations in the frequency of GnRH pulses, are sufficient to account for the time courses of LH and FSH plasma levels during the rat estrous cycle. That is, a single peak of LH on the afternoon of proestrus and a double peak of FSH on proestrus and early estrus. We also use the model to identify which regulation pathways are indispensable for the differential regulation of LH and FSH and their time courses during the estrous cycle. We conclude that the actions of activin, inhibin, and follistatin are consistent with LH/FSH secretion patterns, and likely complement other factors in the production of the characteristic secretion patterns in female rats.     

Hypersecretion of follicle-stimulating hormone (FSH) on estrous afternoon in rats treated with RU486 in proestrus

Summary 1. Intact or ovariectomized (OVX) cyclic rats injected or not with RU486 (4 mg/0.2 ml oil) from proestrus onwards were bled at 0800 and 1800h on proestrus, estrus and metestrus. Additional RU486-treated rats were injected with: LHRH antagonist (LHRHa), estradiol benzoate (EB) or bovine follicular fluid (bFF) and sacrified at 1800 h in estrous afternoon. LH and FSH serum levels were determined by RIA.2. RU486-treated intact or OVX rats had decreased preovulatory surges of LH and FSH, abolished secondary secretion of FSH and hypersecretion of FSH in estrous afternoon. The latter was decreased by LHRHa and abolished by EB or bFF. In contrast, EB induced an hypersecretion of LH in RU486-treated rats at 1800h in estrus.3. It can be concluded that in the absence of the proestrous progesterone actions, the absence of the inhibitory effect of the ovary in estrus evoked a LHRH independent secretion of FSH.     

Effects of thymulin and GnRH on the release of gonadotropins by in vitro pituitary cells obtained from rats in each day of estrous cycle

The effects of thymulin and GnRH on FSH and LH release were studied in suspension cultures of anterior pituitary cells from female adult rats sacrificed on each day of the estrous cycle. The spontaneous release of gonadotropins by pituitaries, as well as their response to GnRH or thymulin addition, fluctuated during the estrous cycle. Adding thymulin to pituitary cells from rats in diestrus 1 increased the concentration of FSH; while in cells from rats in estrus, FSH level decreased. Thymulin had a stimulatory effect on the basal concentration of LH during most days of the estrous cycle. Adding GnRH increased FSH release in cells from rats in diestrus 1, diestrus 2, or proestrus, and resulted in higher LH levels in cells obtained from rats in all days of the estrous cycle. Compared to the GnRH treatment, the simultaneous addition of thymulin and GnRH to cells from rats in diestrus 1, diestrus 2, or proestrus resulted in lower FSH concentrations. Similar results were observed in the LH release by cells from rats in diestrus 1, while in cells from rats in proestrus or estrus, LH concentrations increased. A directly proportional relation between progesterone serum levels and the effects of thymulin on FSH release was observed. These data suggest that thymulin plays a dual role in the release of gonadotropins, and that its effects depend on the hormonal status of the donor's pituitary.     

Pituitary luteinizing hormone (LH) and follicle-stimulating hormone (FSH) responses to gonadotropin-releasing hormone during the rat estrous cycle: an increased ratio of FSH to LH is secreted during the secondary FSH surge

The hormonal interactions required for the generation of a secondary surge of FSH on the evening of proestrus have not been clearly defined. The role of GnRH in driving a surge of FSH has been questioned by findings in previous studies. In the current study, gonadotropin secretion was measured from pituitary fragments obtained from rats at 0900 and 2400 h on each day of the estrous cycle. Pituitary fragments were perifused in basal (unstimulated) conditions or in the presence of GnRH pulses to determine whether a selective increase in basal release of FSH and/or an increase in the responsiveness to GnRH occurs during the secondary FSH surge. Each anterior pituitary was cut into eighths and placed into a microchamber for perifusion. Seven pulses of GnRH (peak amplitude = 50 ng/ml; duration = approximately 2 min) were administered at a rate of one per hour starting at 30 min. Fractions of perfusate were collected every 5 min and frozen until RIA for LH and FSH. The mean total amount of LH or FSH secreted during the hour interval following each of the last six pulses of GnRH (or the corresponding basal hour) was calculated. Analysis of variance with repeated measures indicated that the evening secretion of LH on proestrus (2400 h) dropped significantly (p less than 0.05) from a maximum on the morning of proestrus (0900 h), whereas the FSH secretion remained elevated at this time. Therefore, the ratio of FSH to LH secreted in response to GnRH pulses was highest during the secondary FSH surge and lowest on the morning of proestrus.(ABSTRACT TRUNCATED AT 250 WORDS)     

In-vitro pulsatile luteinizing hormone-releasing hormone output is dependent on hypothalamic region and the stage of the estrous cycle

In the following experiments, the role of the preoptic-suprachiasmatic area (POA-SCN) in the control of luteinizing hormone-releasing (LHRH) release was examined by in vitro superfusion of either mediobasal hypothalamus (MBH) or MBH-POA-SCN fragments obtained from cycling rats killed on various days of the estrous cycle. The lowest level of LHRH output occurred during estrus, highest levels during diestrus, and intermediate levels on proestrus in the MBH-POA-SCN preparation. As expected, significant decreases in LHRH output from the MBH alone occurred during both days of diestrus and on proestrus, as compared to output from the MBH-POA-SCN tissue, since this structure contains most of the LHRH perikarya. However, similar LHRH secretion patterns were detected in estrus from both preparations. The average period of the LHRH pulses for the estrous cycle in the MBH-POA-SCN was 30.9 +/- 1.2 min compared to 97.7 +/- 25.1 min in the MBH, with significant differences occurring on diestrus 2. The increase and extreme variability of the period of LHRH pulses in the MBH region, compared to the MBH-POA-SCN region, suggests that it is the latter region that contains the neural circuits that control the LHRH pulse generator. The LHRH pulse amplitudes from both hypothalamic regions were similar during all phases of the estrous cycle, except diestrus 2, when the LHRH pulse amplitude from the MBH region was significantly lower than the LHRH pulse amplitude from MBH-POA-SCN. The percentage of LHRH released in the MBH did not vary with the estrous cycle, however, in the MBH-POA-SCN significant changes were observed.(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibin secretion during the rat estrous cycle: relationships to FSH secretion and FSH beta subunit mRNA concentrations

Serum inhibin and FSH and FSH beta subunit mRNA levels were measured at 3h intervals throughout the 4 day estrous cycle in female rats and hourly between 1000 and 2400 h of proestrus. On proestrus, serum inhibin concentrations fell during the late morning-early afternoon, then increased transiently during the late afternoon gonadotropin surges. Inhibin levels decreased during the late evening of proestrus, coincident with the FSH surge-related rise in FSH beta mRNA levels. Serum inhibin remained relatively stable during estrus and early metestrus, but rose during the late evening of metestrus and remained elevated until early diestrus. FSH beta mRNA levels were elevated on late estrus and early metestrus and declined during the evening of metestrus as serum inhibin levels increased. These data show that concentrations of serum inhibin change during the estrous cycle and that a general inverse relationship exists between serum inhibin and FSH levels and FSH beta mRNA concentrations in the pituitary. This suggests that inhibin may inhibit FSH beta gene expression and FSH secretion during the 4 day cycle in female rats.     

Specific binding of 125I-LHRH agonist to hippocampal membranes: fluctuations during the estrous cycle.

Specific binding of 125I-[D-Ala6-CH3-Leu7-Pro9,NHET]LHRH, a LHRH agonist, to hippocampal membranes prepared from ovariectomized female rates was examined. One high affinity binding site was observed with a Kd of 0.12 +/- 0.01 nM and an apparent Bmax of 13.0 +/- 3.8 fmol/mg. Luteinizing hormone-releasing hormone and the behaviorally active Ac-LHRH(5-10) were able to compete for the agonist binding site. Native LHRH had an apparent Ki of 1.73 nM, while AC-LHRH(5-10) was 30 times less potent. Competition studies examined over the rat estrous cycle revealed an eighteenfold decrease in apparent affinity during diestrus I and estrus compared with ovariectomized animals. Tissue from animals in proestrus had a Ki of 5.0 nM. Specific binding studies indicate that receptor concentration is highest in proestrus (6.11 +/- 0.90 fmol/mg) and significantly lower during estrus (2.4 +/- 0.29 fmol/mg). These data suggest that at least one fragment of native LHRH can interact with neuronal LHRH receptors and that these receptors, like those in the pituitary, can be modulated by circulating steroid hormones.     

Follicle-stimulating hormone within and secreted from anterior pituitaries of female golden hamsters during the estrous cycle and after ovariectomy

Anterior pituitary (AP) glands were removed from groups of female golden hamsters at 0900 h on estrus (E), diestrus I (DI), and diestrus II (DII) and at 1200 h and 1500 h on proestrus (P12 and P15), as well as at 0900 h from ovariectomized hamsters (OVX). Hemipituitaries were incubated in culture medium with or without 10(-8) M luteinizing hormone-releasing hormone (LHRH) for 3 h at 37 degrees C to determine the magnitude of basal and LHRH-stimulated follicle-stimulating hormone (FSH) release. All samples were assessed for FSH activity by radioimmunoassay and radioreceptor assay. In a second set of experiments, AP were removed from E, DII, and OVX hamsters and bisected. One hemipituitary was homogenized in 10 mM Tris-HCl and the other half was incubated for 3 h. Follicle-stimulating hormone forms present within pituitary extracts or secreted into medium were separated by an isoelectric focusing technique, chromatofocusing. Basal FSH release was lowest in AP collected on DII and P12, higher in AP collected on E and DI, and highest in AP from OVX. Luteinizing hormone-releasing hormone-stimulated release of FSH was highest in AP obtained on DII and P12, lower in AP collected on E and DI, and lowest in AP from OVX. Radio-receptor-to-radioimmunoassay ratio of secreted FSH was greatest when basal FSH secretion was low and LHRH sensitivity was high (DII and P12) and least when basal FSH secretion was high and LHRH sensitivity low (E and after OVX).(ABSTRACT TRUNCATED AT 250 WORDS)     

Peptidase activity in the hypothalamus and pituitary of the rat: fluctuations and possible regulatory role of luteinizing hormone releasing hormone-degrading activity during the estrous cycle

Peptidase activity capable of inactivating luteinizing hormone (LHRH) may have a physiological role in partially determining hypothalamic LHRH levels as well as LHRH levels at the gonadotrope. In our previous work ( Lapp and O' Conner , 1984, companion paper), use of the synthetic substrate leucine-p-nitroanilide (Leu-p-NA) to assay LHRH-degradative activity was validated by several methods. The current studies were conducted in order to monitor peptidase activity in the hypothalamus and pituitary throughout the rat 4-day estrous cycle. Activity in both tissues was significantly decreased during proestrus and diestrus I. It seems possible that the proestrous reduction in peptidase activity represents a permissive period necessary for the induction of the LHRH and LH surges. The decreased degradative activity in the pituitary on diestrus I may be involved in inducing the pituitary LHRH receptors which are reportedly synthesized prior to proestrus. The peptidase exhibits positive cooperativity with Leu-p-NA, and the degree of this cooperativity also fluctuates during the estrous cycle. Estradiol and progesterone given alone or in combination to prepubertal castrate animals increased the activity of the hypothalamic peptidase in vitro. The degree of positive cooperativity with which the enzyme functioned was also apparently altered by these gonadal steroids.     

Increases in the basal secretion rate of follicle-stimulating hormone (FSH) accompany age-associated changes in serum FSH levels on estrus

This laboratory has recently reported that by 5-6 months of age, alterations in the secretion and production of follicle-stimulating hormone (FSH) occur in virgin female rats which precedes the age-related disruption of estrous cycles and attenuation of preovulatory gonadotropin surges. Specifically, circulating immunoreactive FSH levels are higher on estrus in rats 5 months and older compared to levels measured in 2- to 3-month-old rats. Therefore, the present study was conducted to explore a possible mechanism for this age-related increase in FSH levels. At 1400 hr on proestrus, estrus and diestrus-1, groups (n = 6-12 rats/group) of 3- and 7-month-old, cyclic rats were decapitated, trunk blood was collected, and anterior pituitary glands were bisected and placed in incubation flasks containing 1 ml media (medium 199). Following a 30-min preincubation period, hemipituitary fragments were incubated for an additional 2 hr. Media and serum FSH levels were quantified by RIA. Levels of FSH were twofold higher in the serum of 7-month-old rats than 3-month-old rats on estrus. Similarly, the basal secretion rate (BSR) of FSH (expressed as ng FSH/ml/2 hr) was significantly (P less than 0.05) higher from incubated hemipituitary fragments of 7-month-old estrous rats than from fragments obtained from younger estrous rats (7 month: 1637 ng/ml/2 hr vs 3 months: 1253 ng/ml/2 hr). Neither the serum FSH levels nor the BSR of FSH differed between age groups on proestrus or diestrus-1. These results show that age-associated increases in circulating FSH levels on estrus may be attributed to an enhanced basal secretion of FSH from the pituitary gland.     

Prolactin secretion and its response to stress during the estrous cycle of the rats

Serum and pituitary prolactin (PRL) concentrations were measured during the estrous cycle of the rat with particular attention to the afternoons of the days of proestrus and estrus. Homogenizing machines, a Polytron and Sonifier, were used to extract PRL from the pituitary gland. The effects of ether anesthesia and restraint were also examined on the afternoons of both proestrus and estrus. The occurrence of a surge in PRL secretion during proestrus was confirmed with a peak at 1500 h, and this was accompanied by a decline in pituitary PRL content. A relatively high level of serum PRL was observed in the afternoon of estrus, during which time pituitary PRL content increased progressively. Ether anesthesia had no effect on the proestrus PRL surge, while restraint enhanced it. On the afternoon of estrus, restraint completely suppressed the rise in serum PRL, but ether anesthesia failed to suppress it completely. From these results, the following conclusions were drawn: 1) the PRL surge on the afternoon of proestrus occurs without synthesis of the hormone in the pituitary; 2) PRL secretion on the afternoon of estrus is accompanied by its synthesis in the gland; 3) the PRL response is distinct for each type of stress applied; and 4) PRL secretion is thus regulated by different mechanisms in proestrus and estrus.     

Restoration of the periovulatory follicle-stimulating hormone surges in sera by luteinizing hormone releasing hormone in phenobarbital-blocked rats.

The periovulatory increases of follicle-stimulating hormone (FSH) in rat sera can be divided into two phases. The first phase consists of a rise and fall during proestrus and the second phase consists of a rise and fall during estrus. The second phase was not blocked by phenobarbital (100 mg/kg BW) injected i.p. between the first and second phases. In contrast, phenobarbital administered prior to the onset of the first phase blocked both phases of increased serum FSH. In phenobarbital-blocked rats, administration of luteinizing hormone releasing hormone (LHRH) during proestrus, either by s.c. injection (10 μg) or by a 3 hr constant-rate i.v. infusion (50 ng/hr), simulated both the proestrous and estrous phases of increased serum FSH. These results indicate that 1) the second phase of the serum FSH rise is itself not susceptible to phenobarbital blockade, 2) a proestrous mechanism susceptible to phenobarbital alteration is necessary for both phases of increased serum FSH to occur, and 3) administration of LHRH to phenobarbital-blocked rats during proestrus restores both phases of FSH release.     

Mechanism of the stimulatory action of antisteroid RU486 on follicle-stimulating hormone secretion in the cyclic Rat

These experiments explored the mechanism underlying FSH hypersecretion on estrous afternoon in rats injected with RU486 (RU) on proestrus. Four-day cyclic rats were injected with RU at 12:00 h on proestrus (1 or 4 mg/0.2 ml oil; s.c.), and its effects on LH and FSH secretion at 18:30 h on estrus were compared with those of antiprogestagens ZK299 (ZK) (1 or 4 mg/0.2 ml oil; s.c.) and Org31806 (OR) (2 or 8 mg/0.2 ml oil; s.c.). Additionally, rats treated with RU or nembutal (PB) (60 mg/kg; i.p. at 13:00 h on proestrus) were injected with an LHRH antagonist (LHRHa) at 10:00 h on estrus (1 mg/0.2 ml saline; s.c.) or progesterone (P) (7.7, 15.5 or 30.9 mg/0.2 ml oil; s.c.) on proestrus at 10:00 h in RU-injected rats and at 14:00 h in PB-injected rats. Animals were killed by decapitation at 18:30 h on estrus and serum LH and FSH concentrations were determined. Rats treated with 1 or 4 mg of RU or Org or 4 mg of ZK recorded increased serum FSH on estrous afternoon, while 1 mg ZK had no effect. PB increased mainly serum LH levels and, to a lesser extent, FSH levels. P decreased serum FSH concentrations in both RU- and PB-injected rats. LHRHa reversed the effects of PB on FSH secretions, but reduced FSH hypersecretion induced by RU only. These results are interpreted to mean that, in the absence of proestrous afternoon P-inhibitory action of the neural stimulus controlling LHRH release, FSH secretion on estrous afternoon involves two components: one is LHRH dependent while, in contrast to LH secretion, the other is LHRH independent, and only expressed in a low estrogen background.     

Effects of complete hypothalamic deafferentation on the estrous phase of follicle-stimulating hormone release in the cyclic rat

We investigated whether neural afferents to the medial basal hypothalamus play an acute role in the estrous phase of FSH release in the 4-day cyclic rat. A cannula was inserted into the right atrium of the heart under brief ether anesthesia during the early afternoon of proestrus for subsequent blood collections and injection of LHRH. In some of the rats, the medial basal hypothalamus was surgically isolated from the rest of the brain with a small knife under brief ether anesthesia between 2000 h and 2130 h of proestrus. Control groups consisted of naive rats which were not treated during the night of proestrus and sham-operated animals in which the knife was lowered to the corpus callosum between 2000 h and 2130 h or proestrus. Rats were bled at 2200 h of proestrus and at 0200 h, 0600 h and 1000 h of estrus for radioimmunoassay of plasma FSH and LH. The plasma FSH levels in all 3 groups between 2200 h of proestrus and 1000 h of estrus were elevated above levels observed in other cannulated rats bled to the onset of the proestrous phase of FSH release at 1400 h of proestrus. There were no statistically significant differences in plasma FSH or LH concentrations at any of the time periods between the 3 groups of serially bled rats. The deafferentation procedure did not appear to impair the pituitary gland's ability to secret gonadotrophins as injection of 50 ng of LHRH after the bleeding at 1000 h of estrus caused substantial elevations in plasma FSH and LH concentrations which were not different between the 3 groups. The results suggest that neural afferents to the medial basal hypothalamus play no acute role in the estrous phase of FSH release in the cyclic rat.     

Effect of alterations in pulsatile luteinizing hormone release on ovarian follicular atresia and steroid secretion on diestrus 1 in the rat estrous cycle

This study examined the importance of pulsatile luteinizing hormone (LH) release on diestrus 1 (D1; metestrus) in the rat estrous cycle to ovarian follicular development and estradiol (E2) secretion. Single injections of a luteinizing hormone-releasing hormone (LHRH) antagonist given at -7.5 h prior to the onset of a 3-h blood sampling period on D1 reduced mean blood LH levels by decreasing LH pulse amplitude, while frequency was not altered. Sequential injections at -7.5 and -3.5 h completely eliminated pulsatile LH secretion. Neither treatment altered the total number of follicles/ovary greater than 150 mu in diameter, the number of follicles in any size group between 150 and 551 mu, or plasma E2, progesterone, or follicle-stimulating hormone (FSH) levels. However, both treatments with LHRH antagonist significantly increased the percentage of atretic follicles in the ovary. These data indicate that: 1) pulsatile LH release is an important factor in determining the rate at which follicles undergo atresia on D1; 2) reductions in LH pulse amplitude alone are sufficient to increase the rate of follicular atresia on D1; 3) an absence of pulsatile LH release for a period of up to 10 h on D1 is not sufficient to produce a decline in ovarian E2 secretion, most likely because the atretic process was in its early stages and had not yet affected a sufficient number of E2-secreting granulosa cells to reduce the follicle's capacity to secrete E2; and 4) suppression or elimination of pulsatile LH release on D1 is not associated with diminished FSH secretion.     

Inhibitory effects of a luteinizing-hormone-releasing hormone agonist implant on ovine fetal gonadotrophin secretion and pituitary sensitivity to luteinizing-hormone-releasing hormone.

Sheep fetuses at day 70 of gestation (term = 145 days) were implanted subcutaneously with a biodegradable implant containing a luteinizing-hormone-releasing hormone (LHRH) agonist (buserelin) to investigate whether treatment with LHRH agonist would induce a state of desensitization of the fetal gonadotrophs and thus influence fetal gonadal development. Treatment with the LHRH agonist for 35-40 days caused a significant reduction in mean fetal plasma concentrations of LH and follicle-stimulating hormone (FSH) compared with control fetuses. LH pulses were evident in control fetuses but were completely abolished by buserelin treatment. Furthermore, the pituitary content of LH and FSH was significantly depleted in fetuses implanted with LHRH agonist. A bolus intravenous injection of 500 ng LHRH given to control fetuses caused a rapid and significant increase in plasma LH and FSH concentrations which was sustained for at least 60 min after injection. Pretreatment with buserelin completely abolished the LH and FSH responses to a bolus injection of LHRH. There were no differences between the sexes in fetal gonadotrophin concentrations or pituitary sensitivity to LHRH in control or agonist-treated fetuses. Furthermore, buserelin treatment for 35-40 days had no effect on the morphological appearance of the fetal gonads when compared with control fetuses, at least to day 110 of pregnancy. These results provide evidence for the induction of a state of desensitization of the LHRH receptors of the fetal pituitary gonadotrophs following long-term treatment with an LHRH agonist, but provide no evidence for a role for gonadotrophin secretion in gonadal development at this stage in fetal life.     

Preovulatory secretion of progesterone, luteinizing hormone, and prolactin in 4-day and 5-day cycling rats

Timing of ovulation and changes in plasma progesterone, luteinizing hormone (LH), and prolactin (PRL) during periovulatory stages were determined in Holtzman rats exhibiting regular 4- or 5-day cycles under a daily artificial illumination from 0500 to 1900 h. The 5-day cycling rats ovulated between 0130 and 0930 h on estrus, whereas some of the 4-day cycling animals ovulated as early as about 0130 h and others as late as 1130 h on estrus. Onset time of preovulatory LH and progesterone surges was about 1500 h on proestrus in both the 4- and the 5-day cycling rats. Peak levels of plasma LH and progesterone were measured at 1700 to 1900 h on proestrus, while the first rises and peak values of plasma PRL were evident a few hours earlier than those of plasma LH in the rats with two cycle lengths. Plasma LH levels at 1900 h on proestrus as well as plasma progesterone levels at 1600 and 2300 h on proestrus and at 0130 and 0330 h on estrus were significantly lower in the 5-day cycling rats than in the 4-day cycling animals (p less than 0.05). In contrast, PRL levels from 1500 through 2300 h on proestrus remained consistently higher in 5-day cycling rats than in 4-day cycling rats, and significant differences in PRL levels between these rats were apparent at 1500, 1600, and 2100 h (p less than 0.05-0.01). Thus, these results demonstrate that the 5-day cycling rats exhibit the attenuated magnitude of LH surge accompanied by the augmented preovulatory PRL release, and that plasma progesterone levels reflect the magnitude of LH surge. A tentative working hypothesis concerning the etiology of the 5-day cycle has been proposed.