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.     

Luteinizing hormone, oestrogen and progesterone levels in peripheral serum of anoestrous and cyclic ewes as determined by radioimmunoassay.

Jugular vein blood was collected daily from four mature ewes throughout anoestrus and the first oestrous cycle of the breeding season until 4 days after the second oestrus. The levels of oestrogen, progesterone and LH were determined by radioimmunoassay. There were fluctuations in the LH level throughout most of the observed anoestrous period with a mean plus or minus S.E. value of 2-3 plus or minus 0-9 ng/ml. High LH values of 20-0, 41-2 and 137-5 ng/ml were observed in three ewes on Day - 24 of anoestrus. A brief minor rise in progesterone level was also observed around this period. Progesterone levels were consistently low (0.11 plus or minus 0-01 ng/ml) before Day - 25 of anoestrus. A major rise occurred on Day - 12 of anoestrous and this was followed by patterns similar to those that have been previously reported for the oestrous cycle of the ewe. Random fluctuations of oestrogens deviating from a mean level of 4-40 plus or minus 0-1 pg/ml were observed during anoestrus and the mean level during the period from the first to the second oestrus was 5-2 plus or minus 0-3 pg/ml. A well-defined peak of 13-3 plus or minus 0-7 pg/ml was seen in all ewes on the day of the second oestrus. Results of the present study suggest that episodic releases of LH occur during anoestrus and periods of low luteal activity. The fluctuations in LH levels, as observed during the period of low luteal activity, i.e. before Day - 25 of anoestrus, were less pronounced during the periods of high luteal activity. The view that luteal activity precedes the first behavioural oestrus of the breeding season is supported.     

Peripheral plasma prolactin concentrations during oestrous cycles in different types of primitive gilt

Plasma prolactin concentrations were determined by radioimmunoassay during oestrous cycles and around the time of oestrus in different types of primitive gilts: Vietnamese, Zlotnicka and wild-boar X domestic pig hybrids. The animals were bled without stress from an indwelling arterial catheter. The following results were obtained: (1) in all gilts the main prolactin peak was observed at Day 15 or 16 of the oestrous cycle; (2) Vietnamese and hybrid gilts showed a second smaller prolactin surge after (Day 2) or before (Day 17) oestrus; (3) base levels of prolactin during the oestrous cycle were 14.8 +/- 0.93 ng/ml (Vietnamese gilts), 13.2 +/- 1.05 ng/ml (Zlotnicka gilts) and 15.6 +/- 2.01 ng/ml (hybrid gilts). The 15-16-day prolactin peaks reached maximum values of 36.4, 43.4 and 56.5 ng/ml respectively.     

Effect of intrauterine infusion of Escherichia coli on hormonal patterns in gilts during the oestrous cycle

The aim of this study was to determine the effect of intrauterine Escherichia coli infusion on the patterns of plasma LH, prolactin, progesterone, androstenedione, testosterone, oestrone, oestradiol-17beta, cortisol and 13,14-dihydro-15-keto-prostaglandin F2alpha (PGFM) in gilts during the oestrous cycle. On day 4 of the oestrous cycle (day 0), 25 mL of saline or 25 mL of Escherichia coli suspension, containing 10(7) colony forming units x mL(-1), was infused once into the each uterine horn in group I or II respectively. The control gilts developed a new oestrous cycle at the expected time but not bacteria-treated. Endometritis and vaginal discharge developed in all gilts after Escherichia coli infusion. The administration of Escherichia coli resulted in a reduction of plasma levels of LH, prolactin, oestrone and oestradiol-17beta (P < 0.05-0.001), mainly on days 15-18 after treatment (expected perioestrous period). During this time, the plasma androstenedione level was elevated (P < 0.05-0.001) after bacteria infusion. In the gilts receiving bacteria, progesterone concentration decreased from day 8 after treatment and was low until the end of the study (P < 0.05-0.001). On days 8-12 after bacteria administration, the level of PGFM was higher (P < 0.001) than that found in the control group. These results suggest that the developing inflammatory process of the endometrium in gilts following Escherichia coli infusion significantly affects the pituitary-ovarian axis function as well as prostaglandin production leading to anoestrus.     

Bovine Steroid Hormone and SHBG Concentrations Postpartum and during the Oestrous Cycle

Changes in consecutive estimates of milk progesterone concentrations and serum steroid hormone and sex hormone-binding globulin (SHBG) concentrations in the postpartum period were examined in Finnish Ayrshire and Friesian dairy cows which were divided according to feeding into a hay group and a silage group. Milk progesterone concentrations rose above 10 nmol/1, indicating the start of ovarian luteal activity, slightly earlier in the silage group (28.4 ± 8.7 (S.D.) days, n = 19) than in the hay group (33.4 ± 10.3, n = 28) after calving. Likewise, the first normal oestrous cycles began slightly earlier in cows fed with silage. On the other hand, no differences in the beginning of ovarian luteal activity were observed between the breeds. Serum oestradiol-17β, oestrone, testosterone, 5α-dihydrotestosterone (5α-DHT), pregnenolone and progesterone concentrations were fairly unchanged during postpartum anoestrus after uterine involution and before ovarian cyclic activity. After first ovulation, considerable increases in milk and serum progesterone concentrations were observed. The increase was accompanied by elevations in serum pregnenolone and 5α-DHT concentrations. In the late luteal phase, progesterone, 5α-DHT and pregnenolone concentrations rapidly declined, leading to low hormone levels in pro-oestrus. Thereafter, serum pregenolone and 5α-DHT concentrations slightly increased during the follicular phase. On the other hand, oestradiol-17β concentrations were elevated in pro-oestrus and decreased after that, being lowest at met-oestrous. Serum testosterone concentrations appeared to be unchanged during postpartum anoestrus and over the oestrous cycle. Serum SHBG concentrations were unchanged during postpartum anoestrus and over the oestrous cycle, as well as in pregnant animals. The serum SHBG concentrations were about double those found in women with normal menstrual cycles, whereas oestradiol concentrations were much lower. At present, it cannot be explained how the biological effects of oestradiol become evident under such conditions.     

Plasma prolactin concentrations during the oestrous cycle of sows

Plasma prolactin levels were measured in 7 sows during the oestrous cycle after the 1st farrow. Blood samples were taken 4 times during the day (07:00, 11:00, 14:00 and 19:00 h). The prolactin concentration was determined by a double-antibody radioimmunoassay method. There was an increase in plasma prolactin at the second oestrus after weaning, with a peak of prolactin 4 days before oestrus, regardless of the length of the cycle.     

Prolactin administration during the luteal and follicular phase of the oestrous cycle of gilts

In Exp. I infusions of prolactin (0.5 mg in 2 ml sterile saline) were repeated every 2 h for 36 h on Days 12-13 of the cycle. In Exp. II infusions of prolactin were administered from Days 17 to 19 (60 h) at 2-h intervals. Control gilts were given 2 ml sterile saline at similar intervals during the same period. Basal prolactin concentrations before initiation of infusions ranged from 1.3 +/- 0.1 to 5.6 +/- 2.2 ng/ml in both experiments. By 5 min after a prolactin infusion, mean plasma prolactin concentration ranged from 74.9 +/- 5.8 to 113.0 +/- 9.5 ng/ml, but then declined to approximately equal to 10 ng/ml just before the next infusion of prolactin. Administration of prolactin during the luteal phase of the oestrous cycle of the gilts had no effect on basal levels of progesterone, oestradiol or LH. During the follicular phase there were no differences (P greater than 0.05) between control and prolactin-treated gilt progesterone and LH concentrations, but oestradiol plasma values were decreased (P less than 0.05) on the 2nd and 3rd day of prolactin treatment. Our results would indicate that prolactin does not play a major role in the regulation of the oestrous cycle of the pig.     

Season influences FSH concentration in ovariectomized Ile-de-France ewes

Ile-de-France ewes were ovariectomized during anoestrus or the mid-luteal phase of an oestrous cycle (day of ovariectomy = Day 0). In a short-term study, FSH concentrations were measured in blood samples collected hourly the day before and on Days 1, 3, 7 and 15 after ovariectomy (10 ewes per group). FSH concentrations increased significantly from 6.1 to 16.5 ng/ml within 1 day of ovariectomy and increased further to 47.1 ng/ml by Day 15. Differences between seasons of ovariectomy were not significant. In a long-term study, FSH concentrations were measured in blood samples collected hourly on Days 7, 15, 30, 60, 90, 120, 150 and 180 after ovariectomy in anoestrus or the breeding season (10 ewes per group). Further samples were taken (5 ewes/group) at 240 and 365 days after ovariectomy. The pattern of change in FSH after ovariectomy differed between the two seasons and the interaction between season and sampling day was significant. For ewes ovariectomized during anoestrus, FSH concentrations increased to a maximum by Day 180 and remained high thereafter. In contrast FSH increased more slowly in ewes ovariectomized in the breeding season and differences between the groups were significant from Day 90 to Day 270. However, both groups had similar FSH concentrations at Day 365. These results show that FSH concentrations increase rapidly after ovariectomy. There are seasonal differences in FSH concentrations in the absence of ovarian feedback with increases in FSH concentration around the time of the onset of the breeding season. Once FSH concentrations had reached a maximum, major seasonal changes were no longer apparent.(ABSTRACT TRUNCATED AT 250 WORDS)     

Vaginal oestrus during the reproductive and non-reproductive period in European ground squirrels

Most hibernating animals are thought to be monoestrous because reproductive activity is subject to strong time constraints. In previous studies, female European ground squirrels (Spermophilus citellus) turned out to have elevated oestradiol levels during late lactation and after weaning, indicating ovarian activity during summer. Therefore, we monitored vaginal cytology, endocrine changes, and vulval development in semi-free-living female European ground squirrels throughout one active season. Vaginal oestrus, defined by the predominance of cornified cells in smear samples, was found during the mating period shortly after vernal emergence. This phase was followed by metoestrus, characterised by the appearance of leukocytes, and a subsequent anoestrous phase. During weaning or postlactation, a second vaginal oestrus was documented in all experimental females, again followed by a metoestrous and an anoestrous phase lasting until hibernation. In line with the second vaginal oestrus, plasma oestradiol concentrations peaked during postlactation. Progesterone levels were elevated from gestation to postlactation, and titres were marginally higher during vaginal oestrus in summer than in spring. Vulval swelling was more pronounced during the first than the second vaginal oestrus. The second oestrous cycle was non-reproductive, as males were sexually inactive with regressed testes during summer. We assume that the second oestrous cycle and the accompanying endocrine changes have beneficial effects on prehibernatory fattening and reproductive performance in the subsequent season. This might allow females to become oestrous immediately after emergence from hibernation in spring.     

Concentrations of somatotropin and prolactin in the follicular fluid and blood serum of cows in different phases of the estrous cycle

Comparative investigations of somatotropin and prolactin contents in the fluid of antral follicles and blood serum of cows in different phases of the oestrous cycle were performed. The somatotropin concentration in the fluid was shown to rise with increasing the follicle diameters from 3-5 to 6-10 mm in the follicular phase and to decrease in follicles of diameter 11-20 mm in the luteal phase. The prolactin concentration was higher in the fluid of follicles 11-20 mm in diameter than in those of 3-5 mm in diameter in the follicular phase and did not depend on the follicle size in the luteal phase. Concurrently, the prolactin content in follicles 3-5 mm in diameter was higher in the luteal than follicular phase of the cycle. As compared to the follicular phase, an increase in the prolactin concentration in the bovine blood serum during the luteal phase was also found. The data obtained indicate that changes in the somatotropin and prolactin contents in the follicular fluid are related to processes regulating growth and development of antral follicles depending on the phase of oestrous cycle and to changes in the blood hormone concentrations as well.     

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.     

The effect of prolonged stress on the oestrous cycles and prolactin secretion in sheep

The effect of repeated and prolonged stimuli on the release of luteinizing hormone (LH) and the course of oestrous cycles was studied in sheep. Weak electric footshocks were administered in different phases of the cycle in a programmed schedule for 9 h daily during 3–4 days. The enduring and repetitive character of the stimuli was supposed to induce some emotional state which approximated to the so-called management stress. Plasma prolactin concentration was also determined in the pro-oestrous phase of the cycle to follow the interrelationship between the pre-ovulatory release of LH and this hormone.Five out of 26 ewes stimulated in different phases of the oestrous cycle showed inhibition in the release of LH and disturbances in the function of the ovaries (cystic or inactive ovaries). The disturbances of the oestrous cycles appeared not only in the course of the current cycle (in which stimulation was applied), but also in the subsequent ones.Increased plasma prolactin levels after stimulation seem not to have an inhibitory action on the pre-ovulatory LH release. The other cause of the observed disturbances in the course of the oestrous cycle, i.e. the impairment of neuro-hormonal regulation, is discussed.     

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.     

Plasma hormonal and electrolyte alterations in cycling buffaloes (Bubalus bubalis) during hot summer months

Plasma levels of progesterone, prolactin, luteinizing hormone, and electrolytes were monitored by radioimmunoassay in ten cycling buffaloes maintained at Punjab Agricultural University, Ludhiana during the hot summer months of June–July. The plasma progesterone concentration ranged from 0.28±0.04 to 3.09±0.03 ng/ml at various stages of the oestrous cycle. Prolactin values ranged from 319±23 to 371±25 ng/ml and LH levels from 0.95±0.05 to 1.35±0.08 ng/ml. Concentrations differed significantly (P0.05) at various stages of the cycle. Levels of electrolytes, viz. Ca^{+ +}, Na⁺ and K⁺, were well within the normal range. The high levels of prolactin, progesterone and LH during the hot summer were assessed in relation to poor reproductive efficiency in buffaloes.     

Seasonal variations in prolactin, growth hormone and thyroid hormones and the prolactin surge at ovulation do not affect litter size of ewes during pregnancy in the oestrous or the anoestrous season

Injection of bromocriptine from 5 days before until 5 days after mating clearly suppressed the periovulatory prolactin surge in ewes in the anoestrous and oestrous season but did not change the litter size significantly. Progesterone, GH, TSH or thyroid hormone concentrations were not influenced by the bromocriptine treatment. The progesterone concentrations were lower during the first weeks after mating in the anoestrous season compared to the oestrous season, while there was no difference between pregnant and non-pregnant ewes. During later gestation this seasonal difference was only observed in the non-pregnant ewes. At the same time there was a clear difference between pregnancy and non-pregnancy in both seasons. The prolactin, GH and thyroid hormone values also varied significantly during gestation. Since these patterns are identical in pregnant and non-pregnant ewes, the fluctuations are due to environmental factors and not to pregnancy or altered progesterone concentrations. In the anoestrous season prolactin, GH, T4 and T3 levels were higher than in the breeding season, while rT3 showed the opposite pattern. The TSH concentration did not differ between the two seasons. These results suggest that seasonal variations in prolactin, GH and thyroid hormones or the periovulatory prolactin surge do not affect litter size of ewes during pregnancy in the oestrous or the anoestrous season.     

Control of ovulation in farm animals

Examination of hormonal changes occurring in farm species at the onset of puberty, during the follicular phase of the oestrous cycle, and at those times when ovarian activity is re-established after periods of seasonal or lactational anoestrus, provides circumstantial evidence that the final phases of follicular development are dependent on a pattern of tonic (episodic) LH secretion. A suppression of episodic LH secretion is associated with periods of anovulation. Stimulation of tonic LH secretion by repeated injections of small doses of synthetic Gn-RH or purified LH restores normal reproductive function in all but deeply anoestrous animals. Continuous infusion of Gn-RH is as effective as repeated injections. It is suggested that an additional inadequacy, possibly endocrine, contributes to the anovulatory state in deep anoestrus.     

Luteinizing hormone (LH) release after single injections of a synthetic LH-releasing hormone (LH-RH) in the ewe at three different reproductive stages and comparison with natural LH release at oestrus

The possibility was investigated of using single i.v. injections of a synthetic luteinizing hormone-releasing hormone (LH-RH) to manipulate the reproductive pattern of the ewe.Single i.v. injections of 150 μg synthetic LH-RH were given on Day 12 of the oestrous cycle, during seasonal anoestrus and on Day 16 $\text{post}$-$\text{partum}$ in ewes which lambed during the breeding season. Blood samples were obtained at 5-, 10- or 15-minute intervals for 1 hour before and for 3 hours after treatment. Plasma LH concentrations were measured using a specific double antibody radioimmunoassay, the development of which is described. Laparotomy was performed on each animal 2–3 days after treatment.The treatment induced LH peaks in all animals and ovulation in the majority. There was no significant difference between the groups in the LH response. The LH release was, however, much less than that found in untreated ewes sampled every 15 minutes for 18 hours during oestrus.     

Hormonal basis of variation in oestrous cyclicity in selected strains of mice

Reproductive hormone secretion and ovarian LH receptor content were studied during the oestrous cycle of mice that differed in fertility after genetic selection. Strain variation in the secretory pattern of progesterone was observed along with differences in the timing and magnitude of prolactin release. Scatchard analysis showed similar affinities of the LH receptor for hCG in strains with increased or decreased reproductive performance, with a single order of binding sites during both pro-oestrus and dioestrus. The number of unoccupied LH receptors during pro-oestrus was greatest in mice with increased reproductive performance. These results provide evidence that trait selection can change gonadotrophin receptor concentration and the dynamics of hormone secretion during the oestrous cycle of the mouse.     

Scanning electron microscope studies of the endometrium of the cyclic mare.

Endometrial biopsies obtained from mares at different stages of the oestrous cycle, during anoestrus and in various abnormal conditions were examined with the scanning electron microscope. Preliminary observations suggest that the patterns of secretory and ciliary activity in the uterine epithelium are similar to those observed by electron microscopical techniques in laboratory and other large domestic animals. The response of the epithelial cells to hormonal variations and infections is compared with that of the endometrium as seen with the light microscope.     

Plasma relaxin immunoactivity during the oestrous cycle of the ewe.

Plasma relaxin immunoactivity was measured every 2 hr during 4-day periods in a series of sheep to cover the 17-day period of the ovine oestrous cycle. The immunoactivity fluctuated considerably throughout eacn 4-day period, and a large betwee-animal variation was found. A marked episodic release, occurring at approximately 12.00 and 24.00 hours, was identified and shown to occur more regularly either at certain times of the cycle or in certain animals. Relaxin immunoactivity was high throughout the late pro-oestrous phase of the cycle (Days 15 and 16), and at 24 hr after the onset of the LH peak, conincidnet with the approximate time when ovulation occurs. Bursts of relaxin activity were found on Days 8 to 9 in one ewe, and Days 10 and 11 and 13 to 14 in another. There was no significant correlation between prolactin levels and relaxin immunoactivity in one ewe studied throughout the oestrous period.