Plasma follicle stimulating hormone in Merino ewes immunized with an inhibin-enriched fraction from bovine follicular fluid

Plasma FSH concentrations were measured in Merino ewes immunized with either an inhibin-enriched preparation from bovine follicular fluid (bFFI) or bovine serum albumin. When compared during the normal oestrous cycle, ewes reimmunized three times with bFFI and which showed increased ovulation rates before the experiment had significantly elevated plasma FSH concentrations on Day 13–14 and at Day 2 of the subsequent cycle. There was a positive correlation (P < 0.05) between plasma FSH concentration and the ovulation rate of the ewes in previous cycles (during the period of immunization) and in the cycle under investigation. In a larger group of ewes immunized against bFFI, which showed a variable increase in ovulation rate, there was no comparable increase in plasma FSH concentration when compared with control ewes in the follicular phase of the cycle.By contrast, when luteolysis was induced by a prostaglandin analogue the bFFI-immunized ewes had lower plasma FSH concentrations than control ewes immediately before and after the preovulatory LH surge. This decrease was significant in the period 9–21 h after the LH surge (P < 0.05–0.01) so that the onset of the second FSH peak was delayed.When the ewes were ovariectomized, the post-castration rise in plasma FSH concentration (but not LH) was delayed for a period of 24 h in bFFI-immunized ewes relative to controls.These experiments show that immunization of ewes with an inhibin-like fraction of bFF does not lead to consistently elevated plasma FSH. However, such ewes have altered feedback regulation leading to differential responses of FSH to prostaglandin-induced luteolysis and to castration.     

Increase in ovulation rate after treatment of ewes with bovine follicular fluid in the luteal phase of the oestrous cycle

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 During follicular fluid treatment plasma levels of FSH in samples withdrawn just before injection of follicular fluid at 09:00 h (i.e. 16 h after previous injection of follicular fluid) were initially suppressed, but by Day 8 of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.     

Ovarian function in ewes at the onset of the breeding season

Transrectal ultrasonography of ovaries was performed each day, during the expected transition from anoestrus to the breeding season (mid-August to early October), in six Western white-faced cross-bred ewes, to record ovarian antral follicles > or = 3 mm in size and luteal structures. Jugular blood samples were collected daily for radioimmunoassay (RIA) of follicle-stimulating hormone (FSH), oestradiol and progesterone. The first ovulation of the breeding season was followed by the full-length oestrous cycle in all ewes studied. Prior to the ovulation, all ewes exhibited a distinct increase in circulating concentrations of progesterone, yet no corpora lutea (CL) were detected and luteinized unovulated follicles were detected in only three ewes. Secretion of FSH was not affected by the cessation of anoestrus and peaks of episodic FSH fluctuations were associated with the emergence of ovarian follicular waves (follicles growing from 3 to > or = 5 mm). During the 17 days prior to the first ovulation of the breeding season, there were no apparent changes in the pattern of emergence of follicular waves. Mean daily numbers of small antral follicles (not growing beyond 3 mm in diameter) declined (P < 0.05) after the first ovulation. The ovulation rate, maximal total and mean luteal volumes and maximal serum progesterone concentrations, but not mean diameters of ovulatory follicles, were ostensibly lower during the first oestrous cycle of the breeding season compared with the mid-breeding season of Western white-faced ewes. Oestradiol secretion by ovarian follicles appeared to be fully restored, compared with anoestrous ewes, but it was not synchronized with the growth of the largest antral follicles of waves until after the beginning of the first oestrous cycle. An increase in progesterone secretion preceding the first ovulation of the breeding season does not result, as previously suggested, from the ovulation of immature ovarian follicles and short-lived CL, but progesterone may be produced by luteinized unovulated follicles and/or interstitial tissue of unknown origin. This increase in serum concentrations of progesterone does not alter the pattern of follicular wave development, hence it seems to be important mainly for inducing oestrous behaviour, synchronizing it with the preovulatory surge of luteinizing hormone (LH), and preventing premature luteolysis during the ensuing luteal phase. Progesterone may also enhance ovarian follicular responsiveness to circulating gonadotropins through a local mechanism.     

Ovarian antral follicular dynamics and their relationships with endocrine variables throughout the oestrous cycle in breeds of sheep differing in prolificacy.

Transrectal ultrasonography of ovaries was performed each day in non-prolific Western white-faced (n = 12) and prolific Finn ewes (n = 7), during one oestrous cycle in the middle portion of the breeding season (October-December), to record the number and size of all follicles > or = 3 mm in diameter. Blood samples collected once a day were analysed by radioimmunoassay for concentrations of LH, FSH and oestradiol. A cycle-detection computer program was used to identify transient increases in concentrations of FSH and oestradiol in individual ewes. Follicular and hormonal data were then analysed for associations between different stages of the lifespan of the largest follicles of follicular waves, and detected fluctuations in serum concentrations of FSH and oestradiol. A follicular wave was defined as a follicle or a group of follicles that began to grow from 3 to > or = 5 mm in diameter within a 48 h period. An average of four follicular waves per ewe emerged during the interovulatory interval in both breeds of sheep studied. The last follicular wave of the oestrous cycle contained ovulatory follicles in all ewes, and the penultimate wave contained ovulatory follicles in 10% of white-faced ewes but in 57% of Finn ewes. Transient increases in serum concentrations of FSH were detected in all animals and concentrations reached peak values on days that approximated to follicle wave emergence. Follicular wave emergence was associated with the onset of transient increases in serum concentrations of oestradiol, and the end of the growth phase of the largest follicles (> or = 5 mm in diameter) was associated with peak serum concentrations of oestradiol. Serum FSH concentrations were higher in Finn than in Western white-faced ewes during the follicular phase of the cycle (P < 0.05). There were no significant differences in serum concentrations of LH between Western white-faced and Finn ewes (P > 0.05). Mean serum concentrations of oestradiol were higher in Finn compared with Western white-faced ewes (P < 0.01). It was concluded that follicular waves (follicles growing from 3 to > or = 5 mm in diameter) occurred in both prolific and non-prolific genotypes of ewes and were closely associated with increased secretion of FSH and oestradiol. The increased ovulation rate in prolific Finn ewes appeared to be due primarily to an extended period of ovulatory follicle recruitment.     

Increasing ovulation rate and lambing rate in sheep by treatment with a steroid enzyme inhibitor

Treatment of ewes with a 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD) inhibitor (Epostane) resulted in a significant increase in both ovulation rate and in the mean number of lambs per ewe lambing. The progestagen sponge plus 3 beta-HSD inhibitor treatment also caused a significant increase in oestrous cycle duration of approximately 1.5 days. Treatment of ewes with the 3 beta-HSD inhibitor caused a significant decrease in peripheral progesterone concentrations, which were reduced even further when 3 beta-HSD inhibitor treatment was given to ewes after insertion of a progestagen sponge. However, mean oestradiol concentrations were significantly higher in the two treatment groups, both at the end of the luteal phase and during the follicular phase of the oestrous cycle. These results demonstrate that ovulation rate and the production of lambs per ewe lambing can be significantly increased by 3 beta-HSD inhibitor treatment.     

Consequences of increasing or decreasing plasma FSH concentrations during the preovulatory period in Romney ewes

Romney ewes were infused with ovine FSH (NIADDK-oFSH-16) for 48 h from the initiation of luteolysis with cloprostenol. Doses of 2.5 or 5 micrograms/h which partly or completely prevented the normal preovulatory decline in plasma FSH concentrations caused a significant increase in mean ovulation rates. Ovulation rates were not increased significantly if the FSH (5 micrograms/h) was infused for only 20 h starting from the initiation of luteolysis or 24 h later. Infusion of a less potent and relatively impure preparation of FSH (i.e. FSH-P) at 0.5 mg/h for 48 h after cloprostenol treatment also increased the mean ovulation rate significantly. However, if the FSH-P was given for only the first 24 h, or if the start of the infusion was delayed for more than 12 h, mean ovulation rates were not increased significantly. Infusion of LH (NIADDK-oLH-25, 5 micrograms/h) for 48 h from the initiation of luteolysis decreased the mean ovulation rate significantly. Administration of bovine follicular fluid to suppress plasma FSH concentrations below normal during the first 24 h after cloprostenol injection did not delay oestrus. However, oestrus was delayed by approximately 2 days if plasma FSH concentrations were reduced by bovine follicular fluid 24 h after the initiation of luteolysis. As ovulation rate increased, the mean weight of individual corpora lutea of each ewe decreased. In ewes with a single ovulation, most corpora lutea weighed greater than 600 mg, but as the ovulation rate increased the proportion of corpora lutea present weighing less than 400 mg rose steadily.(ABSTRACT TRUNCATED AT 250 WORDS)     

Melanin concentrating hormone (MCH) in the cerebrospinal fluid of ewes during spontaneous oestrous cycles and ram effect induced follicular phases

The melanin-concentrating hormone (MCH) is a neuropeptide synthesized by neurons of the lateral hypothalamus and incerto-hypothalamic area that project throughout the central nervous system. The aims of the present report were: (1) to determine if MCH levels in cerebrospinal fluid (CSF) of ewes vary between the mid-luteal and the oestrous phase of spontaneous oestrous cycles; and (2) to study if MCH levels in CSF of ewes vary acutely during the follicular phase induced with the ram effect in anoestrous ewes. In the first experiment, CSF was collected from 8 adult ewes during spontaneous oestrous and during the mid-luteal phase (8-10 days after natural oestrus). In the second experiment, performed during the mid non-breeding season, a follicular phase was induced with the ram effect. After isolating a group of 16 ewes from rams, CSF was obtained from 5 of such ewes (control group). Three rams were joined with the ewes, and samples were obtained 12h (n=6) and 24h (n=5) later. In Experiment 1, there were no differences in MCH concentrations in CSF measured during the mid-luteal phase and spontaneous oestrus (0.14 ± 0.04 vs. 0.16 ± 0.05 ng/mL respectively). In Experiment 2, MCH concentrations tended to increase 12h after rams introduction (0.15 ± 0.08 vs. 0.35 ± 0.21 ng/mL, P=0.08), and increased significantly 24h after rams introduction (0.37 ± 0.15 ng/mL, P=0.02). We concluded that MCH concentration measured in the CSF from ewes increased markedly during the response to the ram effect but not during the natural oestrous cycle of ewes.     

Comparative expression of luteinizing hormone and follicle-stimulating hormone receptors in ovarian follicles from high and low prolific sheep breeds

Expression of gonadotropin receptors and granulosa cell sensitivity to gonadotropin hormones by small (1-3 mm) and large (3.5-7 mm) follicles were compared in Romanov (ROM, ovulation rate = 3) and Ile-de-France (IF, ovulation rate = 1) ewes in the early and late follicular phase. In healthy follicles, LH receptor levels in granulosa cells increased with increasing follicular size (p < 0. 001) while FSH receptor levels decreased (p < 0.05). In granulosa cells of large follicles, LH receptor (LHR) mRNA levels were greater in the late than in the early follicular phase (p < 0.001, p < 0.05, for ROM and IF, respectively). In the early follicular phase, LHR levels in granulosa (p < 0.001) and theca cells (p < 0.05) of small follicles were greater in ROM than in IF ewes. FSH receptor mRNA levels in granulosa cells of small and large ROM follicles were greater than in the corresponding IF follicles (p < 0.05). Finally, a greater responsiveness (increase in cAMP secretion) to both FSH and hCG was observed by granulosa cells collected during the early follicular phase from ROM vs. IF ewes. Data provide evidence that the greater ovulation rate in the ROM as compared to the IF breed is associated with a greater gonadotropin responsiveness during the early follicular phase.     

Ovarian morphology and the concentration of steroids, and of gonadotrophins during the breeding season in ewes actively immunized against oestradiol-17 beta or oestrone

Ewes were actively immunized against oestrone-6-(O-carboxymethyl)-oxime-bovine serum albumin, 17 beta-oestradiol-6-(O-carboxymethyl)oxime-bovine serum albumin or bovine serum albumin (controls). All 4 control ewes, 1 of 5 oestradiol-immunized ewes and 1 of 5 oestrone-immunized ewes had regular oestrous cycles. The other animals displayed oestrus irregularly or remained anoestrous. The plasma concentrations of LH and, to a lesser degree, FSH were increased relative to those in control ewes on Days 11-12 after oestrus or a similar total period after progestagen treatment in ewes not showing oestrus. The ovaries were examined and jugular venous blood, ovarian venous blood and follicular fluid were collected at laparotomy on Days 9-10 of the oestrous cycle. The ovaries of immunized ewes were heavier than those of control ewes. There were no CL in 5 of the immunized ewes but in the other 5 there were more CL than in the control ewes. Ovaries from 4 of 5 oestrone-immunized ewes contained luteinized follicles, while ovaries from 4 of 5 oestradiol-immunized ewes contained very large follicles with a degenerated granulosa and a hyperplastic theca interna. Both types of follicles produced progesterone, detectable in ovarian venous plasma and production of other steroids, particularly androstenedione, was also increased. The steroid-binding capacity of plasma was increased in the immunized ewes. The binding capacity of follicular fluid for oestradiol-17 beta and oestrone was similar to that of jugular venous plasma from the same ewes. These results suggest that immunization against oestrogens disrupts reproductive function by interfering with the feedback mechanisms controlling gonadotrophin secretion.     

Use of bovine follicular fluid to increase ovulation rate or prevent ovulation in sheep

Romney ewes were injected intramuscularly once or twice daily for 3 days with 0, 0.1, 0.5, 1 or 5 ml of bovine follicular fluid (bFF) treated with dextran-coated charcoal, starting immediately after injection of cloprostenol to initiate luteolysis on Day 10 of the oestrous cycle. There was a dose-related suppression of plasma concentrations of FSH, but not LH, during the treatment period. On stopping the bFF treatment, plasma FSH concentrations 'rebounded' to levels up to 3-fold higher than pretreatment values. The mean time to the onset of oestrus was also increased in a dose-related manner by up to 11 days. The mean ovulation rates of ewes receiving 1.0 ml bFF twice daily (1.9 +/- 0.2 ovulations/ewe, mean +/- s.e.m. for N = 34) or 5.0 ml once daily (2.0 +/- 0.2 ovulations/ewe, N = 25) were significantly higher than that of control ewes (1.4 +/- 0.1 ovulations/ewe, N = 35). Comparison of the ovaries of ewes treated with bFF for 24 or 48 h with the ovaries of control ewes revealed no differences in the number or size distribution of antral follicles. However, the large follicles (greater than or equal to 5 mm diam.) of bFF-treated ewes had lower concentrations of oestradiol-17 beta in follicular fluid, contained fewer granulosa cells and the granulosa cells had a reduced capacity to aromatize testosterone to oestradiol-17 beta and produce cyclic AMP when challenged with FSH or LH. No significant effects of bFF treatment were observed in small (1-2.5 mm diam.) or medium (3-4.5 mm diam.) sized follicles. Ewes receiving 5 ml bFF once daily for 27 days, from the onset of luteolysis, were rendered infertile during this treatment period. Oestrus was not observed and ovulation did not occur. Median concentrations of plasma FSH fell to 20% of pretreatment values within 2 days. Thereafter they gradually rose over the next 8 days to reach 60% of pretreatment values where they remained for the rest of the 27-day treatment period. Median concentrations of plasma LH increased during the treatment period to levels up to 6-fold higher than pretreatment values. When bFF treatment was stopped, plasma concentrations of FSH and LH quickly returned to control levels, and oestrus was observed within 2 weeks. The ewes were mated at this first oestrus and each subsequently delivered a single lamb.     

Ovulation rate and embryo survival in Damline ewes after treatment with bovine follicular fluid in the luteal phase of the oestrous cycle

Treatment of Damline ewes with twice daily i.v. injections of bovine follicular fluid during the luteal phase for 10, 6 or 2 days before prostaglandin-induced luteolysis resulted in an increase in ovulation rate. This was associated with a large rebound increase in plasma concentrations of FSH after the last injection of bovine follicular fluid. While conception rate was not affected by bovine follicular fluid treatment, a higher percentage embryonic loss was observed between Days 3 and 34 of pregnancy in the 10-day treatment group only compared to controls. This reflected the increase in ovulation rate above the optimum for embryonic survival in this breed. The present results suggest that the increase in ovulation rate induced by bovine follicular fluid treatment in the luteal phase of the cycle before mating would result in a significant increase in the number of lambs born.     

Progestagen synchronisation in the absence of a corpus luteum results in the ovulation of a persistent follicle in cyclic ewe lambs

Progestagens are widely used to synchronise oestrous in sheep but the effects on follicular dynamics are not clear. We tested the hypothesis that when luteolysis occurs early during progestagen synchronisation prolonged growth of the ovulatory follicle will occur. Cyclic ewe lambs (40.0+/-0.3 kg) were divided into three groups: eight ewes (Long group) received a progestagen sponge (60 mg medroxyprogesterone acetate) from Days 5 to 19 after oestrous and eight ewes (Short group) received a progestagen sponge on Day 5 which was replaced on Day 10 and again on Day 15, and removed on Day 19 after oestrous. On Days 6 and 7, ewes in both groups received prostaglandin. A third group (n=5, Control) did not receive any treatment. The growth and development of follicles > or =2 mm in diameter were characterised using daily transrectal ultrasonography. On Day 18, blood samples were collected every 12 min for 8 h from five ewes in the Long and Short groups. Data were analysed by ANOVA. The maximum diameter and age (emergence to ovulation) of the ovulatory follicle was greater (P<0.01) in ewes in the Long group (7. 4+/-0.2 mm and 12.1+/-0.6 days) than in ewes in the Short group (6. 3+/-0.2 mm and 5.1+/-0.5 days) and Control group (6.3+/-0.4 mm and 6. 8+/-0.6 days). On Day 18 of the cycle, LH pulse frequency and oestradiol concentrations were greater (P<0.05) in ewes in the Long group (3.2+/-1.1 pulse per 8 h and 1.15+/-0.09 pg ml(-1)) than the Short group (0.8+/-0.4 pulses per 8 h and 0.54+/-0.08 pg ml(-1)).We suggest that the negative feedback efficacy of a long-term progestagen sponge decreased with time and led to an increase in LH pulse frequency and prolonged growth of the ovulatory follicle. We conclude that, in the absence of luteal progesterone, synchronisation with a single progestagen sponge for 14 days resulted in higher LH pulse frequency and ovulation of a persistent follicle with a larger maximum diameter, compared with controls.     

Effect of mouse epidermal growth factor on plasma concentrations of FSH, LH and progesterone and on oestrus, ovulation and ovulation rate in merino ewes

The 24 h i.v. infusion of Merino ewes with 60 or 100 microgram mouse epidermal growth factor (EGF)/kg body weight on Days 4, 9 or 14 of the oestrous cycle decreased the strength of wool attachment and caused marked changes in subsequent reproductive performance. In ovaries removed 2 days after EGF treatment all follicles greater than or equal to 0.6 mm diameter were atretic. After 7 days either a normal pattern of atresia or no atresia was evident while after 12 days the pattern of follicular atresia was similar to that in controls. Irrespective of stage of cycle EGF caused dose-dependent increases in plasma FSH concentrations that persisted for up to 14 days. Changes in plasma LH concentrations were generally similar after infusion on Days 4 and 14, but were smaller and shorter-lived after infusion on Day 9. Irrespective of dose, the infusion of EGF on Days 4 and 14 caused immediate luteolysis then the formation of a luteinized follicle in many ewes. Most ewes treated on Day 4 returned to oestrus between Days 17 and 21 with the same ovulation rate (1.3) as the controls. Of those infused on Day 14 oestrus occurred about a cycle length later than expected and their ovulation rate then (1.9) was also similar to that of the controls (1.7). Luteal function was not affected in ewes infused on Day 9, and most returned to oestrus between Days 17 and 20 with an ovulation rate of 3.2. Fertile rams were not placed with the ewes until after the differences in ovulation rate had been observed. Mating occurred generally 2-4 weeks after treatment, and there were no differences between EGF-treated and control ewes in fertility or fecundity. The results are interpreted as indicating that mouse EGF induces ovarian follicular atresia but has differential effects on luteal function according to the stage of the oestrous cycle at which it is given. As a consequence of these two effects, which lead to differential changes in gonadotrophin secretion, ovarian function may be temporarily impaired, little affected or improved.     

Pre-ovulatory follicular characteristics and ovulation rates in different breed crosses, carriers or non-carriers of the Booroola or Cambridge fecundity gene

Terminal follicular dynamics and ovulation rates (OR) were compared in different local breeds after introducing fecundity genes of different origin. Crossbred ewes which were carriers (F+) or non-carriers (++) of Booroola (BFec) or Cambridge genes (CFec) were included: CambridgexCambridge (CC), CambridgexSuffolk (CS), CambridgexTexel (CT), BooroolaxTexel (BT) and BooroolaxGerman Mutton Merino (BGM). The numbers of small (diameter 2-3.5 mm), medium (diameter >3.5-5.0 mm) and large (diameter >5.0 mm) growing follicles, the maximum diameter before ovulation and the regression and artesia rates of ovarian follicles >/=2 mm in diameter were studied laparoscopically and repeatedly during the last 5 days of an induced oestrous cycle. The ORs were determined one cycle before and two cycles after the repeated laparoscopy. BFec and CFec significantly enhanced the OR of all crossbreeds. Carriers of BFec or CFec did not have significantly different ORs due to any crossbreeding effect. The same observation was made for non-carriers of both Fec gene types. Whatever the crossbreed, the number of small, medium and large growing follicles were similar between carriers and non-carriers in spite of a higher number of ovulating follicles in carriers of both Fec gene types. The diameter of ovulatory follicles did not differ among crossbreds, or between carriers and non-carriers except in the BT (5.2+/-0.2 vs. 6.5+/-0.8 mm, respectively) and CC (6.6+/-0.2 vs. 5.6+/-0.3 mm) ewes.The higher OR in the presence of the Booroola gene was associated with a low atresia rate of large follicles in all crossbreeds (BT: 52+/-8% (F+) vs. 61+/-7% (++); BGM: 51+/-6% vs. 75+/-5%). The high OR of the carriers of the CFec gene seemed to be associated with a lower number of large growing follicles with a lower (P<0.05) atresia rate as compared with Booroola crossbreeds.In conclusion, follicular features were similar between purebred Cambridge and its crossbred CS and CT. In ewes carrying the BFec or CFec gene, the reduction in follicular atresia seemed to be one of the main follicular features implicated in the higher OR.     

Changes in gonadotrophin secretion and ovarian antral follicular activity in seasonally breeding sheep throughout the year

Overall, significantly more antral follicles greater than or equal to 1 mm diameter were present in Romney ewes during anoestrus than in the breeding season (anoestrus, 35 +/- 3 (mean +/- s.e.m.) follicles per ewe, 23 sheep; Day 9-10 of oestrous cycle, 24 +/- 1 follicles per ewe, 22 sheep; P less than 0.01), although the mean numbers of preovulatory-sized follicles (greater than or equal to 5 mm diam.) were similar (anoestrus, 1.3 +/- 0.2 per ewe; oestrous cycle, 1.0 +/- 0.1 per ewe). The ability of ovarian follicles to synthesize oestradiol did not differ between anoestrus and the breeding season as assessed from the levels of extant aromatase enzyme activity in granulosa cells and steroid concentrations in follicular fluid. Although the mean plasma concentration of LH did not differ between anoestrus and the luteal phase of the breeding season, the pattern of LH secretion differed markedly; on Day 9-10 of the oestrous cycle there were significantly more (P less than 0.001) high-amplitude LH peaks (i.e. greater than or equal to 1 ng/ml) in plasma and significantly fewer (P less than 0.001) low amplitude peaks (less than 1 ng/ml) than in anoestrous ewes. Moreover, the mean concentrations of FSH and prolactin were significantly lower during the luteal phase of the cycle than during anoestrus (FSH, P less than 0.05, prolactin, P less than 0.001). It is concluded that, in Romney ewes, the levels of antral follicular activity change throughout the year in synchrony with the circannual patterns of prolactin and day-length. Also, these data support the notion that anovulation during seasonal anoestrus is due to a reduced frequency of high-amplitude LH discharges from the pituitary gland.     

Effects of oestradiol implants on the ovulation rate of the ewe

In a series of 5 experiments, ewes were treated with implants releasing oestradiol-17 beta and the effects on ovulation rate were observed. Large doses of oestradiol-17 beta (greater than 20 micrograms/day) produced anovulation while smaller amounts only reduced the proportion of twin ovulations. Amounts of exogenous oestradiol comparable to ovarian production rate in the luteal phase (less than 1 microgram/day) produced a significant (P less than 0.01) suppression in ovulation rate. Treatment during the follicular phase of the oestrous cycle was most effective, but treatment during the luteal phase alone also appeared to suppress ovulation rate. Furthermore, in 2 of 3 experiments ewes treated with low amounts of oestradiol during the first half of the luteal phase were less likely to have multiple ovulations at the subsequent oestrous period. The results support the hypothesis that oestrogen is involved in the physiological control of ovulation rate in the ewe, but this action is probably not restricted to the assertion of dominance by a maturing follicle during the follicular phase.     

Effects of bromocriptine administration during the follicular phase of the oestrous cycle on prolactin and gonadotrophin secretion and follicular dynamics in merino monovular ewes

Two experiments using Spanish Merino ewes were conducted to investigate whether the secretion of prolactin during the follicular phase of the sheep oestrous cycle was involved in the patterns of growth and regression of follicle populations. In both experiments, oestrus was synchronized with two cloprostenol injections which were administered 10 days apart. Concurrent with the second injection (time 0), ewes (n = 6 per group) received one of the following treatments every 12 h from time 0 to 72 h: group 1: vehicle injection (control); group 2: 0.6 mg bromocriptine (0.03 mg per kg per day); and group 3: 1.2 mg bromocriptine (0.06 mg per kg per day). In Expt 1, blood samples were collected every 3 h from 0 to 72 h, and also every 20 min from 38 to 54 h to measure prolactin, LH and FSH concentrations. In Expt 2, transrectal ultrasonography was carried out every 12 h from time 0 until oestrus, and blood samples were collected every 4 h to measure prolactin, LH and FSH concentrations. Ovulation rates were determined by laparoscopy on day 4 after oestrus. Bromocriptine markedly decreased prolactin secretion, but did not affect FSH concentrations, the mean time of the LH preovulatory surge or LH concentrations in the preovulatory surge. Both doses of bromocriptine caused a similar decrease in LH pulse frequency before the preovulatory surge. The highest bromocriptine dose led to a reduction (P < 0.01) in the number of 2-3 mm follicles detected in the ovaries at each time point. However, bromocriptine did not modify the total number or the number of newly detected 4-5 mm follicles at each time point, the number of follicles > 5 mm or the ovulation rate. In conclusion, the effects of bromocriptine on gonadotrophin and prolactin secretion and on the follicular dynamics during the follicular phase of the sheep oestrous cycle indicate that prolactin may influence the viability of gonadotrophin-responsive follicles shortly after luteolysis.     

Effect of immunizing prepuberal lambs of low and high ovulation rate genotypes with inhibin partially purified from bovine follicular fluid

Active immunization of prepuberal lambs with a partially purified inhibin preparation, isolated from bovine follicular fluid, increased the ovulation rate. In ewe lambs of a low fecundity breed (Suffolk x Galway), the ovulation rate rose from 1.15 to 1.95 (P<0.05) compared with that of the controls. An ovulation rate of 3.38 was recorded for immunized ewe lambs of a high fecundity breed (Finn x Dorset Horn), while the rate for mature ewes from the same flock was 2.29. Immunization did not affect the time of onset of puberty or estrous cycle length. Following immunization, antibodies were produced that bound to a pure preparation of 68kDa bovine inhibin. This report demonstrates the production of antibody to a 68kDa preparation of inhibin following active immunization of sheep using a partially purified preparation. It was concluded that the increased ovulation rate was due to the production of antibodies to inhibin, which may have reduced its negative feedback effect of FSH secretion.     

Changes in the feedback control of gonadotrophin secretion in ewes from lines selected for testis size in the ram lamb

The dynamics of FSH and LH secretion were studied in sheep genetically selected for High (H) and Low (L) rates of testis growth. Gonadotrophin secretion had previously been shown to be affected in the ram lamb with H-line lambs more sensitive to steroid feedback than L. While there were significant differences in mean LH concentrations during the luteal and follicular phases of the oestrous cycle, mean LH values were essentially similar in the two lines in response to ovariectomy, the effect of oestradiol implants on the response to ovariectomy and the response to LHRH. However, the frequency of LH pulses in the H line was similar during both phases of the oestrous cycle, showing a surprising insensitivity to steroid feedback. By contrast, LH pulse frequency was markedly lower in the L-line ewes in the luteal than the follicular phase (0.6 vs 1.1 pulses/h) as expected from the literature. Mean FSH concentrations were significantly higher in the L-line ewes during the follicular phase of the oestrous cycle and after ovariectomy but no significant differences were detected at the other sampling periods. There were no differences in ovulation rate between the lines. It was concluded that selection for testis size had affected the feedback control of gonadotrophin release in the ewe, as in the ram, and hence the expression of the genes controlling this is not sex limited.     

Increased ovulation rate in androstenedione-immune ewes is not due to elevated plasma concentrations of FSH

Two experiments were undertaken to determine the hormonal response of Merino ewes to immunization against androstenedione (Fecundin). In Exp. 1 peripheral concentrations of LH, FSH and progesterone were monitored in spontaneously cycling ewes (20 immunized and 21 controls). In Exp. 2 (10 immunized and 10 controls) the same hormones were measured in ewes before and after prostaglandin (PG)-induced luteolysis and, in addition, the pattern of pulsatile LH secretion was determined during the luteal (PG + 12 days), early follicular (PG + 24 h) and late follicular (PG + 40 h) phase of the oestrous cycle. Ovulation rates were measured in both experiments. The results of these experiments indicate that androstenedione-immune animals have elevated ovulation rates (0.6-0.7 greater than control animals; P less than 0.05) associated with elevated plasma concentrations of LH and progesterone. The magnitude of the increase in plasma progesterone was correlated with androstenedione antibody titre (r = 0.6, P less than 0.001). LH pulse frequency of androstenedione-immune ewes tended to be higher at all stages of the oestrous cycle, but this difference was only significant (P less than 0.05) during the luteal phase. Mean plasma concentrations of FSH did not differ significantly between immunized and control ewes at any stage of the cycle. Analysis of periodic fluctuations in FSH during the luteal phase revealed that androstenedione-immune animals had a similar number of fluctuations of a similar amplitude to those of control animals, but the nadir of these fluctuations was lower (P less than 0.05) in immunized animals. A significant (P less than 0.05) negative correlation existed between androstenedione antibody titre and the interval between FSH peaks (r = -0.49) and androstenedione antibody titre and FSH nadir concentrations (r = -0.46). It is concluded that plasma FSH concentrations are not a determinant of ovulation rate in androstenedione-immune ewes and that increased LH concentrations, or perturbation of normal intraovarian mechanisms, may be responsible for the increase in ovulation rate observed in ewes immunized against androstenedione.