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A. S. McNeilly

Summary. Treatment of Damline ewes with i.v. injections of various doses (2, 5 or 10 ml) of bovine follicular fluid for 72 h after prostaglandin-induced luteal regression resulted in a significant decrease in plasma concentrations of FSH after a 1·5–2 h delay but did not affect LH. The half life of this decrease in plasma FSH levels (156 min) after injection of follicular fluid was similar to that for clearance (159 min) of ovine FSH after infusion. A significant rebound increase in plasma FSH levels occurred by 13 h after all follicular fluid injections, and the magnitude of this rebound was inversely related to the dose of follicular fluid injected. A significant delay in the onset of oestrus occurred only with 5 and 10 ml bovine follicular fluid. There was no significant effect on ovulation rate or subsequent corpus luteum function as measured by plasma concentrations of progesterone. Infusion of ovine FSH (50 μg/h for 48 h) during the period of follicular fluid treatment prevented the delay in onset of oestrus and resulted in a substantial (2–10-fold) increase in ovulation rate. Corpus luteum function in terms of progesterone secretion was also enhanced. These results show that (1) intermittent suppression of FSH during the preovulatory period in the ewe does not affect subsequent ovulation rate or corpus luteum function and (2) the delay in the onset of oestrus induced by bovine follicular fluid can be prevented by exogenous FSH.

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A. S. McNeilly

Summary. Treatment of Welsh Mountain ewes with i.v. injections of untreated or charcoal-treated bovine follicular fluid (BFF) after prostaglandin-induced luteal regression resulted in a significant delay in the onset of oestrous behaviour. Injections of BFF caused a significant decrease in plasma concentrations of FSH but levels were only suppressed below those in control ewes during the first 24 h of the 48-h period of treatment. At the end of BFF treatment there was a substantial increase in plasma FSH levels. In contrast, there were no significant effects of BFF treatment on basal plasma concentrations of LH or pulse frequency or amplitude of LH secretion. Corpus luteum function, as measured by plasma concentrations of progesterone, in BFF-treated ewes was normal. These results suggest that the delay in oestrus caused by treatment of ewes with BFF is due to a specific suppression of plasma levels of FSH while LH secretion remains unaffected.

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P. A. Fowler and A. S. McNeilly

To investigate the mechanism controlling the fall in maternal pituitary responsiveness to GnRH, LH synthesis and pituitary GnRH receptor content during pregnancy, maternal pituitaries were collected from sheep on days 35, 45, 60, 90, 110, 125 and 135 of pregnancy. Circulating steroids and gonadotrophins were determined in blood samples collected from these ewes immediately before death. Pituitary blocks from each ewe were perifused with either medium alone (control) or medium supplemented with oestradiol, oestradiol plus progesterone or oestradiol plus RU486, for 150 min before administration of two 15 s GnRH pulses 90 min apart. The amounts of mRNA encoding LHβ and GnRH receptor were determined in pituitary tissue fragments snap-frozen in liquid N2 at the time of collection from the ewes. While basal LH secretion fell during pregnancy, pituitary responsiveness to GnRH remained high (up to seven times basal LH concentrations). After day 90, the first GnRH pulse elicited LH peaks equivalent to the LH peaks produced by the second GnRH pulse. Therefore, GnRH self-priming was not evident possibly because the pituitaries were constantly primed by increased concentrations of maternal oestradiol. Around day 90, circulating concentrations of progesterone rose from 7.8 ± 1.5 to 12.2 ± 3.8 ng ml−1. Up to day 60, oestradiol in the perifusion buffer had stimulatory effects on LH secretion although this was reduced by RU486. By day 125, the content of mRNA encoding LHβ had declined during pregnancy to 7% of the content on day 35, although the content of mRNA encoding GnRH receptor remained unchanged. From these data, there appears to be a transitional period at around day 90 of gestation when pituitary sensitivity to steroids in vitro is lost together with detectable GnRH self-priming. In conclusion, the marked decline in pituitary amounts of mRNA encoding LHβ, but not in GnRH responsiveness or expression of GnRH receptor, after day 45 of pregnancy suggests that the principal effect of pregnancy on gonadotroph function is mediated via a mechanism other than reduced pituitary amounts of GnRH receptors. Two possible mechanisms are (1) a reduction in GnRH output leading to lowered LH synthesis, or (2) the presence of an inhibitory factor with a short half-life in the maternal circulation.

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A. N. Brooks and A. S. McNeilly

Summary. 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 densensitization 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.

Keywords: fetus; sheep; gonadotrophin; LHRH; agonist

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A. S. McNeilly and R. B. Land

Summary. Nineteen Scottish Blackface ewes were given LH-RH (3 × 30 μg i.v., 90-min intervals) during anoestrus when prolactin levels were elevated. Plasma levels of prolactin were suppressed with CB 154 (twice daily, i.m.) on Days −5 to 0 (N = 5), 0 to +5 (N = 5) or −5 to +5 ( N = 5) around the day of LH-RH treatment (Day 0). Control animals (N = 4) received saline on Days −5 to +5. Nine animals ovulated forming corpora lutea as judged by laparoscopy on Day +7. No difference in FSH or LH levels was found between treatments and ovulations occurred equally in all treatment groups. Progesterone levels were < 1 ng/ml in all animals up to Day 14. It is concluded that short-term suppression of prolactin does not affect the incidence of ovulation or corpus luteum progesterone production in LH-RH-treated anoestrous ewes.

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H. M. Fraser and A. S. McNeilly

Summary. Neutralization of LH-RH by injection of an ovine antiserum to LH-RH in ewes during the late follicular phase of the oestrous cycle resulted in an immediate blockade of pulsatile secretion of LH. Plasma concentrations of FSH gradually rose in the antiserum-treated ewes during the 36-h study period but levels declined in control ewes. These results show that, in the ewe, pulsatile LH secretion is dependent on LH-RH from the hypothalamus, while FSH is largely unresponsive to short-term reduction of LH-RH stimulation. Since reduction in LH secretion is likely to reduce ovarian function, the changes in FSH secretion may be attributed to the removal of a negative feedback influence of an ovarian factor, perhaps oestradiol, on FSH secretion.

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Jacqueline M. Wallace and A. S. McNeilly

Summary. Administration of charcoal-treated bovine follicular fluid to Damline ewes twice daily (i.v.) from Days 1 to 11 of the luteal phase (Day 0 = oestrus) resulted in a delay in the onset of oestrous behaviour and a significant increase in ovulation rate following cloprostenol-induced luteolysis on Day 12. During follicular fluid treatment plasma levels of FSH in samples withdrawn just before injection of follicular fluid at 09:00 h (i.e. 16 h after previous injection of follicular fluid) were initially suppressed, but by Day 8 of treatment had returned to those of controls. However, the injection of follicular fluid at 09:00 h on Day 8 still caused a significant suppression of FSH as measured during a 6-h sampling period. Basal LH levels were higher throughout treatment due to a significant increase in amplitude and frequency of pulsatile secretion. After cloprostenol-induced luteal regression at the end of treatment on Day 12, plasma levels of FSH increased 4-fold over those of controls and remained higher until the preovulatory LH surge. While LH concentrations were initially higher relative to those of controls, there was no significant difference in the amount of LH released immediately before or during the preovulatory surge. These results suggest that the increase in ovulation rate observed during treatment with bovine follicular fluid is associated with the change in the pattern of gonadotrophin secretion in the luteal and follicular phases of the cycle.

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H. M. Picton and A. S. McNeilly

Summary. The mean and peak concentrations of follicle-stimulating hormone (FSH) during the luteal phase of a normal cycle were measured in 8 Welsh Mountain ewes. Gonadotrophin secretion and follicle growth were then suppressed by the chronic administration of the GnRH agonist buserelin for 5 weeks. During the 6th week of agonist treatment, each ewe was given a continuous infusion of FSH to produce a peripheral concentration of FSH equal to either the mean or peak of the gonadotrophin measured for that individual in the cycle preceding agonist treatment. Treatment had no effect on the total number of follicles, the number of follicles ⩽ 2·5 mm in diameter or the in-vitro production of oestradiol by the small follicles when compared with control animals. None of the animals infused with the mean luteal-phase FSH equivalent developed large follicles > 2·5 mm diameter which could be classified as preovulatory follicles (oestradiol > 1000 pg/follicle/h). All of the animals infused with the peak luteal-phase FSH equivalent developed large follicles, some of which were preovulatory. The results suggest that an individual threshold concentration exists for FSH above which the later stages of preovulatory follicular development are stimulated.

Keywords: FSH; follicle selection; GnRH agonist; sheep

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A. S. McNeilly and D. T. Baird

Summary. The effect of sustained high plasma levels of prolactin, induced by repeated 2-h i.v. injections of thyrotrophin-releasing hormone (TRH; 20 μg), on ovarian oestradiol secretion and plasma levels of LH and FSH was investigated during the preovulatory period in the ewe. Plasma levels of progesterone declined at the same rate after prostaglandin-induced luteal regression in control and TRH-treated ewes. However, TRH treatment resulted in a significant increase in plasma levels of LH and FSH compared to controls from 12 h after luteal regression until 5 to 6 h before the start of the preovulatory surge of LH. In spite of this, and a similar increase in pulse frequency of LH in control and TRH-treated ewes, ovarian oestradiol secretion was significantly suppressed in TRH-treated ewes compared to that in control ewes. The preovulatory surge of LH and FSH, the second FSH peak and subsequent luteal function in terms of plasma levels of progesterone were not significantly different between control and TRH-treated ewes.

These results show that TRH treatment, presumably by maintaining elevated plasma levels of prolactin, results in suppression of oestradiol secretion by a direct effect on the ovary in the ewe.

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B. J. McLeod and A. S. McNeilly

Summary. The specific requirement for FSH in the final stages of preovulatory follicle development was assessed in seasonally anoestrous ewes given 2-h injections of GnRH (250 ng/injection), with (N = 10) or without (N = 10) concurrent treatment with bovine follicular fluid (bFF: 2 ml given i.v. at 8-h intervals). Treatment with bFF significantly (P < 0·01) suppressed plasma FSH concentrations, but, at least for the first 30 h of treatment, did not influence the magnitude of GnRH-induced LH episodes (mean max. conc. 3·00 ± 0·39 and 3·63 ± 0·51 ng/ml for bFF-treated and control ewes, respectively). Of 10 animals treated with GnRH for 72 h, 5/5 control ewes showed oestrus and ovulated whereas 0/5 bFF-treated ewes showed oestrus or ovulated in response to GnRH treatment. There was, however, a transient (13·2 ± 1·0 h) increase in plasma LH concentrations in the ewes given bFF (mean max. conc. 4·64 ± 1·57 ng/ml), which was coincident with the preovulatory LH surge recorded in animals given GnRH alone. In 10 GnRH-treated ewes slaughtered after 32 h of treatment, the mean diameter of the largest antral follicle was significantly (P < 0·001) greater in control ewes (5·92 ± 0·17 mm) than in animals that were also given bFF (3·94 ± 0·14 mm). In addition, the incidence of atresia in the 3 largest antral follicles present at this time was greater in bFF-treated ewes. These results show that, when plasma FSH concentrations are suppressed by administration of bFF, although the magnitude of GnRH-induced LH episodes is unchanged, preovulatory follicular development is impaired and ovulation does not occur. This may be indicative of a specific requirement for FSH in the final stages of preovulatory follicle development, or due to direct inhibitory effects of bovine follicular fluid.