Preovulatory biosynthesis and granulosa cell secretion of immunoreactive oxytocin by goat ovaries

Oestrous cycles of goats were synchronized hormonally. Immunoreactive oxytocin was undetectable (less than 0.1 ng/mg protein) in media from granulosa cells isolated before the LH surge for small (1-2 mm), medium (3-5 mm) and large (greater than 5 mm diameter) follicles when cultured for 24 h without or with added hormones. Granulosa cells from large and medium, but not small, follicles isolated 6-12 h after spontaneous preovulatory LH surges secreted high concentrations of oxytocin (4-12 ng/mg protein). Addition of PGE-2 (1 microgram/ml) caused a further significant (P less than 0.05) increase in oxytocin secretion by cultured granulosa cells, whereas PGF-2 alpha, FSH and LH were ineffective when added to culture media. Ovarian venous blood and granulosa cells were collected at 0, 6, 12 or 18 h after GnRH injection in hormonally synchronized goats. Peripheral serum LH values were increased significantly in all but 2 of 22 goats within 2 h of GnRH injection. At the earliest sampling time after GnRH (6 h), ovarian venous levels of oxytocin were increased significantly from basal levels of 0.4 pg/ml to 2.4 pg/ml. Oxytocin concentrations in follicular fluid increased from a basal value of 67 pg/ml to 155 pg/ml by 6 h and to 372 pg/ml by 18 h after GnRH injection. Oxytocin secretion by cultured granulosa cells was not increased significantly by 6 h (0.1 ng/mg protein) but rose to 1.4 and 3.5 ng/mg protein at 12 and 18 h, respectively. Approximately parallel increases occurred in progesterone in ovarian venous blood and granulosa cell culture media over the same time period. (ABSTRACT TRUNCATED AT 250 WORDS)     

Exogenous GnRH induces ovulation in seasonally anoestrous lactating goats (Capra hircus)

A specific sheep LH radioimmunoassay was validated for the measurement of goat LH, and used to monitor luteal-phase LH episodes and the preavulatory LH surge in progestagen sponge-synchronized cycling goats. No luteal-phase LH episodes were detected during 12 h of frequent (15-min) blood sampling in 2 goats. A preovulatory LH surge was recorded in 5/5 goats, with a mean amplitude of 45.4 +/- 7.2 ng/ml and a mean time of onset of 38.4 +/- 1.2 h after removal of a progestagen-impregnated sponge. In anoestrous goats, single i.v. injections of 1000 and 2000 ng GnRH induced LH episodes with a mean amplitude of 2.04 +/- 0.11 and 3.67 +/- 0.06 ng/ml respectively, but injections of 250 or 500 ng did not consistently elevate LH concentrations. Progestagen-primed, seasonally anoestrous lactating goats were treated with repeated injections of 1500 ng GnRH (every 2 h for 52 or 78 h) in May 1985 or 1986. All 10 had kidded in March of the same year, and were consequently at peak lactation at the time of GnRH treatment. A preovulatory LH surge was detected in 9 goats with a mean time of onset of 59.5 +/- 2.9 h (1985) or 39.6 +/- 3.3 h (1986) after vaginal sponge removal. All animals displayed oestrus and ovulated, and 9 of the goats were mated: in 5 of these animals pregnancies were successfully carried to term. The results show episodic LH release in response to GnRH and indicate that ovulation can be induced in seasonally anoestrous goats, even at peak lactation, and normal pregnancies may result.     

Estradiol induces and progesterone inhibits the preovulatory surges of luteinizing hormone and follicle-stimulating hormone in heifers

Objectives were to determine: 1) whether estradiol, given via implants in amounts to stimulate a proestrus increase, induces preovulatory-like luteinizing hormone (LH) and follicle-stimulating hormone (FSH) surges; and 2) whether progesterone, given via infusion in amounts to simulate concentrations found in blood during the luteal phase of the estrous cycle, inhibits gonadotropin surges. All heifers were in the luteal phase of an estrous cycle when ovariectomized. Replacement therapy with estradiol and progesterone was started immediately after ovariectomy to mimic luteal phase concentrations of these steroids. Average estradiol (pg/ml) and progesterone (ng/ml) resulting from this replacement were 2.5 and 6.2 respectively; these values were similar (P greater than 0.05) to those on the day before ovariectomy (2.3 and 7.2, respectively). Nevertheless, basal concentrations of LH and FSH increased from 0.7 and 43 ng/ml before ovariectomy to 2.6 and 96 ng/ml, respectively, 24 h after ovariectomy. This may indicate that other ovarian factors are required to maintain low baselines of LH and FSH. Beginning 24 h after ovariectomy, replacement of steroids were adjusted as follows: 1) progesterone infusion was terminated and 2 additional estradiol implants were given every 12 h for 36 h (n = 5); 2) progesterone infusion was maintained and 2 additional estradiol implants were given every 12 h for 36 h (n = 3); or 3) progesterone infusion was terminated and 2 additional empty implants were given every 12 h for 36 h (n = 6). When estradiol implants were given every 12 h for 36 h, estradiol levels increased in plasma to 5 to 7 pg/ml, which resembles the increase in estradiol that occurs at proestrus. After ending progesterone infusion, levels of progesterone in plasma decreased to less than 1 ng/ml by 8 h. Preovulatory-like LH and FSH surges were induced only when progesterone infusion was stopped and additional estradiol implants were given. These surges were synchronous, occurring 61.8 +/- 0.4 h (mean +/- SE) after ending infusion of progesterone. We conclude that estradiol, at concentrations which simulate those found during proestrus, induces preovulatory-like LH and FSH surges in heifers and that progesterone, at concentrations found during the luteal phase of the estrous cycle, inhibits estradiol-induced gonadotropin surges. Furthermore, ovarian factors other than estradiol and progesterone may be required to maintain basal concentrations of LH and FSH in heifers.     

Oestradiol and the preovulatory surges of luteinising hormone and follicle stimulating hormone in ewes during the breeding season and transition into anoestrus

This study was designed to see if giving exogenous oestradiol, during the follicular phase of the oestrous cycle of intact ewes, during the breeding season or transition into anoestrus, would alter the occurrence, timing or magnitude of the preovulatory surge of secretion of luteinising hormone (LH) or follicle stimulating hormone (FSH). During the breeding season and the time of transition, separate groups of ewes were infused (intravenously) with either saline (30 ml h⁻¹; n = 6) or oestradiol in saline (n = 6) for 30 h. Infusion started 12 h after removal of progestin-containing intravaginal sponges that had been in place for 12 days. The initial dose of oestradiol was 0.02 μg h⁻¹; this was doubled every 4 h for 20 h, followed by every 5 h up to 30 h, to reach a maximum of 1.5 μg h⁻¹. Following progestin removal during the breeding season, peak serum concentrations of oestradiol in control ewes were 10.31 ± 1.04 pg ml⁻¹, at 49.60 ± 3.40 h after progestin removal. There was no obvious peak during transition, but at a time after progestin removal equivalent to the time of the oestradiol peak in ewes at mid breeding season, oestradiol concentrations were 6.70 ± 1.14 pg ml⁻¹ in ewes in transition (P < 0.05). In oestradiol treated ewes, peak serum oestradiol concentrations (24.8 ± 2.1 pg ml⁻¹) and time to peak (41.00 ± 0.05 h) did not differ between seasons (P > 0.05). During the breeding season, all six control ewes and four of six ewes given oestradiol showed oestrus with LH and FSH surges. The two ewes not showing oestrus did not respond to oestrus synchronisation and had persistently high serum concentrations of progesterone. During transition, three of six control ewes showed oestrus but only two had LH and FSH surges; all oestradiol treated ewes showed oestrus and gonadotrophin surges (P < 0.05). The timing and magnitude of LH and FSH surges did not vary with treatment or season. In blood samples collected every 12 min for 6 h, from 12 h after the start of oestradiol infusion, mean serum concentrations of LH and LH pulse frequency were lower in control ewes during transition than during mid breeding season (P < 0.05). Oestradiol treatment resulted in lower mean serum concentrations of LH in season and lower LH pulse frequency in transition (P < 0.05). We concluded that enhancing the height of the preovulatory peak in serum concentrations of oestradiol during the breeding season did not alter the timing or the magnitude of the preovulatory surge of LH and FSH secretion and that at transition into anoestrus, oestradiol can induce oestrus and the surge release of LH and FSH as effectively as during the breeding season.     

Oestradiol-17beta, progesterone, FSH and LH in prepubertal calves induced to superovulate

Fluorogestone acetate (vaginal sponge for 4 days) and PMSG (i.m. injection at the time of sponge insertion) treatment was administered to seven 3-month-old calves to induce superovulation. Samples of peripheral plasma were taken every 4 h during treatment (4 days) and then every 2 h for 7 days. FSH, LH, oestradiol and progesterone were measured by radioimmunoassays. In all calves oestradiol concentrations increased 24 h after PMSG injection and reached the highest levels (41-502 pg/ml) during the preovulatory surge of both gonadotropins. The surge of LH and FSH occurred from 12 to 22 h after cessation of treatment. The maximum levels of LH and FSH were 11-72 ng/ml and 23-40 ng/ml respectively and occurred within 4 h of each other. Between 40 and 68 h after the LH peak the concentrations of progesterone began to increase from basal values, reaching 24.0-101.7 ng/ml when the animals were killed. A quantitative relationship was found between plasma oestradiol concentration and the numbers of ovulating follicles. Progesterone levels seemed to be related to the numbers of corpora lutea and also to the numbers of unovulated follicles. Gonadotrophin output was not quantitatively related to ovarian activity or to steroid secretion.     

Increased blood LH and testosterone after administration of prostaglandin F2α in bulls

Two types of experiments were conducted to determine the relationship of changes in blood luteinizing hormone (LH) and testosterone in bulls given prostaglandin F_2α (PGF_2α). Episodic surges of LH and testosterone occurred in tandem, apparently at random intervals, on the average once during the 8-hr period after bulls were given saline. In contrast, after sc injection of 20 mg PGF_2α, blood serum testosterone increased synchronously to a peak within 90 minutes four-fold greater than pre-injection values, and the testosterone surges were prolonged about three-fold compared to those in controls. Each of the PGF_2α-induced surges of testosterone was preceded by a surge of blood serum LH which persisted for about 45 minutes and peaked at about 3 ng/ml. In a second experiment, PGF_2α was infused (iv, 0.2 mg/min) for 20 hr; blood plasma testosterone increased from 7.0 ± 0.6 to 16.0±1.5 ng/ml within 2.5 hr and remained near this peak for 10 hr. Then testosterone gradually declined to about 9 ng/ml at the conclusion of the 20-hr infusion. These changes in testosterone were paralleled by similar changes in blood plasma LH, although LH declined 3 hr earlier than testosterone. Random episodic peaks of blood plasma LH and testosterone typical of untreated bulls resumed within 8 hr after conclusion of PGF_2α infusion. In both experiments, the surge of testosterone after PGF_2α was preceded by increased blood LH. We conclude that increased LH after administration of PGF_2α probably caused the increased testosterone. However the mechanisms of these actions of PGF_2α remain to be determined.     

Effects of removal in late pregnancy of the corpus luteum, graafian follicle or ovaries on plasma progesterone, oestradiol, LH, parturition and post-partum oestrus in the tammar wallaby, Macropus eugenii

Concentrations of oestradiol-17 beta, progesterone, and luteinizing hormone (LH) were measured in plasma collected at 6- to 12-h intervals from tammars around the time of parturition and post-partum oestrus. Parturition occurred on Day 26 or 27 after reactivation of lactation-delayed pregnancy and coincided with a precipitous decline in progesterone levels. A sharp rise in oestradiol, from basal concentrations of less than 10 pg/ml to a peak of 13 to 32 pg/ml, as well as oestrus, followed the drop in progesterone by 8.3 and 9.8 h, respectively. The LH surge was dependent on the oestradiol rise and followed it by 7 h. Ovulation followed mating by about 30 h and the LH surge by 24 h. Removal of the ovary with the large Graafian follicle prevented the oestradiol rise, oestrus and the LH surge, but not parturition. Peripartum changes in peripheral oestradiol do not appear to be involved in initiation of parturition but the oestradiol rise and associated change in the oestradiol:progesterone ratio are important signals for post-partum oestrus and the LH surge.     

Gonadotropin-releasing hormone at estrus: luteinizing hormone, estradiol, and progesterone during the periestrual and postinsemination periods in dairy cattle

Administering gonadotropin-releasing hormone (GnRH) improved conception rates in our previous studies. Our objective was to determine if the effect of GnRH was mediated through serum luteinizing hormone (LH) and/or by altered secretion of serum progesterone (P) and estradiol-17 beta (E) during the periestrual and post-insemination periods. Cattle were given either GnRH (n = 54) or saline (n = 55) at 72 h and inseminated artificially (AI) 80 h after the second of two injections of either prostaglandin F2 alpha or its analog, cloprostenol. Progesterone and E were measured in blood serum collected during 3 wk after AI (estrus) from 60 females. Blood was collected for LH determinations via indwelling jugular cannulae from 14 cows and 11 heifers. Collections were taken every 4 h from 32 to 108 h after the second PGF injection (PGF-2) (periestrual period) and at more frequent intervals during 240 min after administration of GnRH (n = 18) or saline (n = 7). Ten females had a spontaneous preovulatory LH surge before GnRH treatment (GnRH-spontaneous), whereas GnRH induced the preovulatory LH surge in six females. A spontaneous LH surge appeared to be initiated in two heifers at or near the time of GnRH treatment (spontaneous and/or induced). The remaining seven cows had spontaneous LH surges with no subsequent change in LH after saline treatment. Serum P during the 21 days after estrus was lower (p less than 0.05) in both pregnant and nonpregnant (open) cattle treated previously with GnRH compared with saline. Serum P during the first week after estrus was greater (p less than 0.01) and increased (p less than 0.05) more rapidly in saline controls and in GnRH-spontaneous cattle than in those exhibiting GnRH-induced or GnRH-spontaneous and/or-induced surges of LH. Conception rate of cattle receiving GnRH was higher (p = 0.06) than that of saline-treated controls. These data suggest that GnRH treatment at insemination initiated the preovulatory LH surge in some cattle, but serum P in both pregnant and open cows was compromised during the luteal phase after GnRH treatment. Improved fertility may be associated with delayed or slowly rising concentrations of serum progesterone after ovulation.     

Plasma LH and FSH responses and ovarian activity in prepubertal heifers treated with repeated injections of low doses of GnRH for 72 h

Twelve 5-month-old Hereford X Friesian heifers were injected i.v. with 2.0 micrograms GnRH at 2-h intervals for 72 h. Blood samples were collected at 15-min intervals from 24 h before the start until 8 h after the end of the GnRH treatment period. Over the 24-h pretreatment period, mean LH concentrations ranged from 0.4 to 2.2 ng/ml and FSH concentrations from 14.1 to 157.4 ng/ml; LH episodes (2-6 episodes/24 h) were evident in all animals. Each injection of GnRH resulted in a distinct episode-like response in LH, but not FSH. Mean LH, but not FSH, concentrations were significantly increased by GnRH treatment. The GnRH-induced LH episodes were of greater magnitude than naturally-occurring episodes (mean maximum concentration 6.7 +/- 0.5 and 4.9 +/- 0.6 ng/ml respectively). Preovulatory LH surges occurred between 17.0 and 58.8 h after the start of treatment in 9/12 heifers, with a coincident FSH surge in 8 of these animals. This was not followed by normal luteal function. There were no apparent correlations between pretreatment hormone concentrations, and either the pituitary response to GnRH or the occurrence of preovulatory gonadotrophin release.     

No evidence for pituitary priming to gonadotropin-releasing hormone in relation to luteinizing hormone (LH) secretion prior to the preovulatory LH surge in ewes

The purpose of this study was to determine the occurrence of and the regulatory mechanisms involved in priming of the pituitary to GnRH before the preovulatory LH surge in sheep. Experiment 1: Forty-two ewes had progestagen devices removed after 14 days and were assigned to luteal (Lut) or follicular (Foll) groups. Fifteen days later, blood sampling was initiated either immediately or 36 h after induced luteolysis in groups Lut and Foll, respectively. After 4 h, ewes were administered either saline (n = 5) or 250 ng (n = 8) or 10 microg (n = 8) of GnRH. Five ewes per treatment group were killed 1 h later, while remaining animals were blood sampled for a further 7 h. Experiment 2: Eighteen ewes were allocated to Lut and Foll groups (described above). Blood samples were collected from 2 h before GnRH (10 microg) treatment until 7 h after. Despite up-regulated GnRH-R mRNA levels in Foll ewes, pituitary content and plasma levels of LH and LHbeta mRNA levels were similar between groups. Mean FSHbeta mRNA and plasma FSH levels were elevated in Lut ewes but declined after GnRH treatment. Inversely, plasma estradiol and inhibin-A concentrations were higher in Foll ewes and declined after GnRH treatment. Fewer LH(+ve)/secretogranin II(-ve) (SgII(-ve)) granules were present in gonadotropes of Foll ewes, coincident with increased basal LH levels. Fewer smaller sized granules were present after GnRH treatment. In conclusion, there was no evidence of self-priming before onset of the preovulatory LH surge. Constitutive release of LH(+ve)/SgII(-ve) granules may maintain basal LH levels while smaller sized, presumably mature granules may be preferentially released after GnRH stimulation.     

Site of action for endotoxin-induced cortisol release in the suppression of preovulatory luteinizing hormone surges

The study was conducted to identify the mechanisms of endotoxin/cortisol action in the suppression of preovulatory LH surges in heifers infused with Escherichia coli (E. coli ) endotoxin. The hypotheses tested were that 1) endotoxin stimulates the release of progesterone, possibly from the adrenal leading to the LH blockade; 2) cortisol released in response to endotoxin infusion blocks the synthesis of estradiol at the ovarian level, culminating in a failure of the LH surge. Eight Holstein heifers were given two injections of prostaglandin F(2alpha) (PG), 11 d apart, to synchronize estrus. Starting from 25 h after the second injection of PG (PG-2), the uterus of each heifer was infused either with 5 ml of pyrogen-free water (control, n = 3) or with E. coli endotoxin (5 mug/kg of body weight) in 5 ml of pyrogen-free water (treated, n = 5), once every 6 h for 10 treatments. Blood samples were obtained every 15 min for 1 h before infusion and again 2 h after each infusion, then hourly until 1 h before the next infusion. After the tenth infusion, blood was collected daily until estrus. Serum progesterone concentrations remained at baseline values (< 1 ng/ml) in control and treated heifers. The total amount of progesterone measured starting 24 to 84 h after PG-2 injection was not different between control and treated heifers (P 0.05). In the control heifers, serum estradiol concentrations remained basal (< 10 pg/ml) until 4 h before the LH surge. Serum estradiol concentrations increased to 20 +/- 5.6 pg/ml, 4 h before the LH surge in control heifers (LH surge occurred 60 to 66 h after the PG-2 injection). There were no changes in serum estradiol concentrations in treated heifers during the sampling period, and the concentrations remained < 10 pg/ml. The total amount of estradiol measured in control heifers was higher (P < 0.05) than in treated heifers. The results if this study suggest that increases in cortisol concentrations after the infusion of endotoxin might block the synthesis of estradiol at the ovarian level, resulting in the failure of a preovulatory LH surge to occur.     

Plasma gonadotrophin responses in beef cows two progesterone plus prostaglandin treatment

Progesterone Releasing Intravaginal Devices (Prids) were inserted into six post-partum beef cows for nine days and 0.5 mg cloprostenol was injected i m on day eight. Blood samples were taken via jugular venous catheters at frequent intervals for seven days after Prid removal and assayed for LH, FSH and progesterone. The induced pre-ovulatory type LH and FSH surges occurred between 35 and 123h after Prid withdrawal in five of the cows. In four cows which underwent surges during the time of most intensive sampling, LH levels were significantly higher during the 30h period prior to the LH surge than during the 30h period after the surge. FSH values were low for the 30h period preceding and the 14h period following the time of maximum FSH/LH concentrations. 16 - 30h after the FSH and LH surges, FSH values were again significantly raised compared with the period immediately after the surge. Despite the success of this Prid/PG regime in inducing ovulation, the variability in time between progestagen withdrawal and the LH surge and ovulation is such that the use of fixed time artificial insemination may give poor results.     

Effect of suckling on basal and GnRH-induced LH release in post-partum dairy buffaloes

Suckling, a common practice in smallholder dairy-farming systems in the developing world, delays the onset of post-partum ovarian activity in dairy buffalo. The present study was designed to assess the effect of suckling on pituitary function in lactating buffaloes 25-35 days post-partum. Six suckled and nine non-suckled buffaloes were challenged intravenously with a bolus injection of GnRH (20microg buserelin acetate; Receptal). Heparinized venous blood samples were collected at 15min intervals for 2h before and up to 4h after GnRH for luteinizing hormone (LH) estimation. Pretreatment basal LH concentrations were similar in the suckled (0.6+/-0.2ng/ml) and the non-suckled (0.5+/-0.1ng/ml) buffaloes. All but one suckled buffaloes released a LH surge, starting 15-60min post-GnRH treatment, which lasted for 180-225min. While one suckled buffalo did not respond to GnRH, the LH response in the remaining suckled buffaloes was significantly less than in the non-suckled buffaloes in terms of peak LH concentrations (14.3+/-2.7ng/ml versus 26.2+/-4.3ng/ml) and area under the LH curve (1575.6+/-197.4mm(2) versus 2108.9+/-323.9mm(2)). The LH response was least in suckled buffaloes challenged with GnRH while in the luteal phase of an oestrus cycle and with plasma progesterone concentration >1ng/ml. In conclusion, suckling suppressed pituitary responsiveness to exogenous GnRH challenge in post-partum buffaloes.     

Oxytocin levels during breast-feeding in established lactation

Serial blood samples were taken from eight lactating women while they were nursing their babies 1–4 months postpartum, and from four lactating controls while they were not nursing. The plasma was assayed for oxytocin by radioimmunoassay after extraction with activated Vycor glass powder. In the suckling mothers mean plasma oxytocin rose from 5.4 pg/ml before nursing to 13.0 pg/ml during nursing. Oxytocin levels changed rapidly from minute to minute, with individual peaks as high as 54 pg/ml. Oxytocin levels in the control mothers averaged 4.4 pg/ml.     

Fluctuations in the plasma levels of the follicle-stimulating hormone during estrous cycle, anestrus, gestation and lactation in the ewe: evidence for an endogenous rhythm of FSH release

Two experiments were carried out to analyse FSH secretion in the ewe. The first was a long-term study during which four ewes under controlled photoperiods were checked for plasma concentrations of FSH twice daily for a period of 16 months. They were successively anestrous, cycling, gestating and lactating. The results suggested that an endogenous secretion rhythm of FSH persisted throughout each of the physiological states of the ewes. The periodic cycles of FSH production lasted about 5 days during anestrus and gestation but extended to about 6 days during estrus. One of the three waves of secretion we noted during one cycle was represented by the two periovulatory surges, the first coincident with the LH peak, the second occuring 30-40 h later. Plasma levels of FSH were similar during estrous cycles and anestrus, whereas the FSH secretion decreased gradually throughoug gestation. During lactation, large differences were observed among animals before the recovery of cyclic ovarian activity. The second experiment consisted of frequent blood sampling (every ten minutes) of eight ewes for 6 hours during anestrus. FSH was secreted differently compared to LH. No pulsatile production of FSH was demonstrated and no increase in FSH levels was seen at the time of the episodic LH surge.     

The Sulawesi Crested Black Macaque (Macaca nigra) menstrual cycle: changes in perineal tumescence and serum estradiol, progesterone, follicle-stimulating hormone, and luteinizing hormone levels.

Events in the normal menstrual cycle of the endangered Sulawesi Crested Black Macaque (Macaca nigra) were characterized. Daily blood samples were obtained during 10 menstrual cycles from five M. nigra demonstrating regular cycles. The amount of perineal tumescence was scored daily. Serum levels of estradiol and progesterone were determined by RIA, serum LH levels were determined by the mouse Leydig cell bioassay, and serum FSH levels were determined by the rat granulosa cell aromatase bioassay. Cycle length was 39.8 +/- 1.0 days (mean +/- SEM) with an LH surge occurring 25 +/- 1.5 days from the onset of menses. After menses, both LH and estradiol were initially depressed, with estradiol first exceeding 50 pg/ml 8 days before the LH surge. In five cycles, peak estradiol levels (340 +/- 44 pg/ml) occurred on the day of the LH surge (637 +/- 58 ng/ml) and in the other five cycles, on the day before the LH surge. There was a broad increase of FSH in midcycle without a well-defined surge corresponding to the LH surge. Progesterone began increasing on the day of the LH surge and reached peak levels (6.8 +/- 0.96 ng/ml) 8 days later. Maximal perineal tumescence was generally associated with the time of the LH surge, but variation between animals made it impossible to predict accurately the day of the LH surge by perineal tumescence scores alone.     

Compensation by pulsatile GnRH infusions for incompetence for oestradiol-induced LH surges in long-term ovariectomized gilts and castrated male pigs.

The aim of this study was to investigate incompetence for oestradiol-induced LH surges in long-term ovariectomized gilts and male pigs. Gilts (250 days old; n = 36), which had been ovariectomized 30 (OVX 30) or 100 days (OVX 100) before the start of treatment, were challenged i.m. with oestradiol benzoate and were either given no further treatment, fed methallibure to inhibit endogenous GnRH release or fed methallibure and given i.v. pulses of 100 or 200 ng GnRH agonist at 1 h intervals during the LH surge (48-96 h after oestradiol benzoate). The same treatments were applied to long-term orchidectomized male pigs (ORC, n = 23). In addition, one ORC group was not injected with oestradiol benzoate but was fed methallibure and given pulses of 200 ng GnRH agonist. Oestradiol benzoate alone induced an LH surge in the OVX 30 group only (5/6 gilts), methallibure suppressed (P < 0.05) oestradiol benzoate-induced LH secretion, while pulses of 100 ng GnRH agonist in animals fed methallibure produced LH surges in four of six OVX 30 and four of six OVX 100 gilts. The induced LH surges were similar to those produced by oestradiol benzoate alone in OVX 30 gilts. Pulses of 200 ng GnRH agonist produced LH surges in OVX 30 (6/6) and OVX 100 (6/6) gilts and increased the magnitude of the induced LH surge in OVX 100 gilts (P < 0.05 compared with 100 ng GnRH agonist or OVX 30 control). Pulses of 200 ng GnRH agonist also induced LH surge release in ORC male pigs (5/6), but were unable to increase LH concentrations in a surge-like manner in ORC animals that had not been given oestradiol benzoate, indicating that oestradiol increases pituitary responsiveness to GnRH. These results support the hypothesis that oestradiol must inhibit secretion of LH before an LH surge can occur. It is concluded that incompetence for oestradiol-induced LH surges in long-term ovarian secretion-deprived gilts and in male pigs is due to the failure of oestradiol to promote a sufficient increase in the release of GnRH.     

Gonadotropin-releasing hormone (GnRH) inhibits ovulation induced with luteinizing hormone (LH) in proestrous hypophysectomized rats

In this paper we present evidence that a single low dose of the natural synthetic gonadotropin-releasing hormone (GnRH), inhibits ovulation induced by LH in proestrous-hypophysectomized rats. Rats hypophysectomized by the parapharyngeal route in the morning of proestrus received an intravenous injection of 100 or 300 ng GnRH at 1400 h immediately followed by 1.0 microgram LH per 100 g bw. In control groups, either one or both hormones were replaced with 0.9% NaCl. Ovulation was assessed the following morning by counting the ova present in oviductal flushings. All the rats treated with LH alone ovulated, and the addition of GnRH reduced significantly the number of ovulating rats and the number of ova per ovulating rat. In other groups of rats hypophysectomized in the morning of proestrus and treated in the same way, ovarian or adrenal secretory rates of estradiol and/or progesterone were measured after cannulation of the corresponding vein, in the afternoon of proestrus. In these animals, GnRH failed to inhibit either the ovarian progesterone surge observed 2 h after LH administration, or the adrenal progesterone secretion. All hypophysectomized rats showed lower ovarian secretory rate of estradiol than intact rats; this rate was not affected by treatment with LH or LH plus GnRH. The systemic estradiol levels in plasma of hypophysectomized rats were distributed within a range of 20 pg/ml to 50 pg/ml. The number of rats whose levels were above 21 pg/ml on estrus day was significantly higher in rats receiving 300 ng GnRH as compared to those receiving 100 ng GnRH, reaching values that surpassed the concentration found in intact, untreated animals at the same time of estrus. This effect did not depend on LH administration.     

Suppression of pulsatile luteinizing hormone secretion by gonadotrophin-releasing hormone antagonist does not affect episodic progesterone secretion or corpus luteum function in ewes.

Progesterone secretion has been observed to be episodic in the late luteal phase of the oestrous cycle of ewes and is apparently independent of luteinizing hormone (LH). This study investigated the effects of suppressing the pulsatile release of LH in the early or late luteal phase on the episodic secretion of progesterone. Six Scottish Blackface ewes were treated i.m. with 1 mg kg-1 body weight of a potent gonadotrophin-releasing hormone (GnRH) antagonist on either day 4 or day 11 of the luteal phase. Six ewes received saline at each time and acted as controls. Serial blood samples were collected at 10 or 15 min intervals between 0 and 8 h, 24 and 32 h, and 48 and 56 h after GnRH antagonist treatment and daily from oestrus (day 0) of the treatment cycle for 22 days. Oestrous behaviour was determined using a vasectomized ram present throughout the experiment. Progesterone secretion was episodic in both the early and late luteal phase with a frequency of between 1.6 and 3.2 pulses in 8 h. The GnRH antagonist abolished the pulsatile secretion and suppressed the basal concentrations of LH for at least 3 days after treatment. This suppression of LH, in either the early or late luteal phase, did not affect the episodic release of progesterone. Daily concentrations of progesterone in plasma showed a minimal reduction on days 11 to 14 after GnRH antagonist treatment on day 4, although this was significant (P < 0.05) only on days 11 and 13. There was no effect of treatment on day 11 on daily progesterone concentration, and the timing of luteolysis and the duration of corpus luteum function was unaffected by GnRH antagonist treatment on either day 4 or day 11. These results indicate that the episodic secretion of progesterone during the luteal phase of the oestrous cycle in ewes is independent of LH pulses and normal progesterone secretion by the corpus luteum can be maintained with minimal basal concentrations of LH.     

Continuous infusion of GnRH reduces the LH response to an intravenous GnRH injection but does not inhibit endogenous LH secretion in cows

Plasma LH concentrations were monitored in 6 Hereford X Friesian suckled cows at about 80 days post partum, before and during a 14-day period of continuous s.c. infusion of GnRH (20 micrograms/h). Blood samples were collected at 10-min intervals on Days -2, -1, 1, 2, 3, 4, 7, 10, 13 and 14 (Day 1 = start of infusion). Plasma LH concentrations rose from mean pretreatment levels of 1.3 +/- 0.20 ng/ml to a maximum of 17.1 +/- 3.09 ng/ml within the first 8 h of GnRH infusion, but returned to pretreatment levels by Day 2 or 3. In 4/6 animals, the initial increase was of a magnitude characteristic of the preovulatory LH surge. In all animals, an i.v. injection of 10 micrograms GnRH, given before the start and again on the 14th day of continuous infusion, induced an increase in LH concentrations but the increase to the second injection was significantly (P less than 0.01) less (mean max. conc. 6.4 +/- 0.76 and 2.3 +/- 0.19 ng/ml). Mean LH concentrations (1.0 +/- 0.08, 1.1 +/- 0.08 and 0.9 +/- 0.06 ng/ml) and LH episode frequencies (3.3,4.3 and 3.2 episodes/6 h) did not differ significantly on Days -2,7 and 13. However, the mean amplitude of LH episodes was significantly lower (P less than 0.05) on Day 13 (1.3 +/- 0.10 ng/ml) than on Day -2 (1.8 +/- 0.16 ng/ml). Therefore, although the elevation in plasma LH concentrations that occurs in response to continuous administration of GnRH is short-lived and LH levels return to pre-infusion values within 48 h of the start of infusion, these results show that the pituitary is still capable of responding to exogenous GnRH, although the LH response to an i.v. bolus injection of GnRH is reduced. In addition, this change in pituitary sensitivity is not fully reflected in endogenous patterns of episodic LH secretion.