Ovarian function in ewes during the transition from breeding season to anoestrus

Transrectal ovarian ultrasonography was conducted in six Western white-faced ewes for 35 days from the last oestrus of the breeding season, to record the number and size of all ovarian follicles > or = 3 mm in diameter and luteal structures. Blood samples were collected once a day for estimation of serum concentrations of follicle-stimulating hormone (FSH), oestradiol and progesterone. Each ewe had five follicular waves (follicles growing from 3 to > or = 5 mm in diameter) over the scanning period. The duration of the growth phase of the largest ovarian follicles did not differ (P > 0.05) between waves, but follicular static and regressing phases decreased significantly (P < 0.05) after the decline in serum progesterone concentrations at the end of the last luteal phase of the breeding season. The intervals between the five follicular waves were: 9.2+/-0.4, 5.2+/-0.7, 8.3+/-0.8 and 5.8+/-0.7 days; the two shorter intervals differed (P < 0.05) from the two longer intervals. Using the cycle-detection program, rhythmic increases in serum FSH concentrations were detected in all ewes; the amplitude, duration and periodicity of FSH fluctuations did not vary (P > 0.05) throughout the period of study. The number of identified FSH peaks (7.8+/-0.5 peaks per ewe, per scanning period) was greater (P < 0.05) than the number of emerging follicular waves. Serum concentrations of oestradiol remained low (< or = 1 pg/ml) on most days, in five out of the six ewes studied, and sporadic elevations in oestradiol secretion above the non-detectable level were not associated with the emergence of follicular waves. The ovulation rate was lower than that seen during the middle portion of the breeding season (November-December) in white-faced ewes but the transitional ewes had larger corpora lutea (CL). Maximal serum concentrations of progesterone appeared to be lower and the plateau phase of progesterone secretion appeared to be shorter during the last luteal phase of the ovulatory season in comparison to the mid-breeding season of Western white-faced ewes. During the transition into anoestrus in ewes, the endogenous rhythm of FSH release is remarkably robust but the pattern of emergence of sequential follicular waves is dissociated from FSH and oestradiol secretion. Luteal progesterone secretion is suppressed because of fewer ovulations and diminished total luteal volume, but it may also result from diminished gonadotropic support. These season-related alterations in the normal pattern of ovine ovarian cycles appear to be due to reduction in ovarian responsiveness to gonadotropins and/or attenuation in secretion of luteinizing hormone (LH) occurring at the onset of the anovulatory season in ewes.     

Effects of FGA and PMSG on follicular growth and LH secretion in Suffolk ewes

The effects of fluorogestone acetate (FGA) and/or pregnant mare serum gonadotrophin (PMSG) on follicular growth and LH secretion in cyclic ewes were determined. Suffolk ewes (n = 40), previously synchronized with cloprostenol were divided into 4 experimental groups (n = 10 ewes per group). Group I served as the control, while groups II, III and IV received FGA, PMSG, FGA and PMSG respectively. Four ewes of each group underwent daily laparascopy for 17 d. All the ovarian follicles >/= 2 mm were measured, and their relative locations were recorded on an ovarian map in order to follow the sequential development of each individual follicle. Comparisons were made of the mean day of emergence and the mean number of small, medium and large follicles, the atresia rate and the ovulation rate. For each group, 3 waves of follicular growth and atresia were observed during the cycle. During luteal phase, FGA treatment accelerated the mechanisms of follicular growth but reduced the number of large follicles and increased the atresia rate. In the follicular phase, FGA treatment was detrimental to both the number of large follicles and the ovulation rate. By contrast, PMSG enhanced recruitment of small follicles and the ovulation rate. Serial blood samples were collected during the luteal and follicular phases to study LH secretion. None of the treatments had any effect on LH secretion patterns.     

Induction of reproductive function in anestrous mares using a dopamine antagonist

We investigated the role of dopamine in the regulation of seasonal reproductive activity in mares. Nine seasonal anestrous mares, maintained under a natural photoperiod, were treated daily with a dopamine D2 antagonist, [-]-sulpiride (200 mg/mare, im), beginning February 5 (day of year = 36) until the first ovulation of the year or for a maximum of 58. Nine untreated anestrous mares were maintained under the same conditions. The ovaries were examined by ultrasonography twice a week, and blood was collected three times a week for progesterone, LH, FSH and prolactin determinations. Mean day of first ovulation was significantly advanced for [-]-sulpiride-treated mares than control mares (mean day of year +/- SEM = 77.3 +/- 7.9 and 110.0 +/- 6.8, respectively; P < 0.01). Eight mares ovulated during [-]-sulpiride treatment while one mare failed to ovulate. Ovulation occurred 91 d after the start of treatment or on Day 127. All mares continued to have normal estrous cycles after the first ovulation. First cycle length and luteal progesterone concentrations did not differ between [-]-sulpiride-treated and control mares. Plasma prolactin concentrations were significantly increased at 2 and 9 h after [-]-sulpiride administration (P < 0.05), and had returned to basal levels by 24 h. At the time of the LH surge associated with the first ovulation, mean LH and FSH secretion was significantly higher in [-]-sulpiride-treated mares than in control mares (P < 0.05). These results suggest that dopamine plays a role in the control of reproductive seasonality in mares and exerts a tonic inhibition on reproductive activity during the anovulatory season.     

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.     

Episodic LH secretion patterns in the mare during the oestrous cycle.

Jugular blood samples were obtained from 8 mares at 5- and/or 20-min intervals for 2 to 5 days during various phases of the oestrous cycle for plasma LH determination. An episodic release pattern was observed in 1 of 3 mares sampled during the ovulatory period. One mare had one secretory burst and the other mare had several periods of fluctuating plasma LH concentration. During dioestrus, episodic secretions were observed in 2 mares sampled 11 to 13 days before and, in 1 mare, 9 days after ovulation. During the 2 to 5-day period before ovulation, episodic secretion was not observed (3 mares) but plasma LH concentrations fluctuated as much as 6 ng/ml during a period of 3--4 h. Daily plasma samples were obtained form 10 mares (1--8 oestrous cycles/mare) during which 22 single, 18 double and 2 luteal-phase ovulations occurred. Dioestrous ovulations were accompanied by small increases in plasma LH (1--4 ng/ml), but many similar increases in LH were not accompanied by ovulation. No significant differences in secretory patterns were observed between single and multiple ovulations. In one mare, 4 ovulations occurred in the presence of a prolonged luteal phase; 3 were accompanied by increasing LH concentrations and the other occurred when LH was at a low concentration.     

Reproductive cycle of the elephant

The combination of a few factors, including poor captive reproduction, secession of importation from the wild and advances in hormone detection and ultrasonography, has contributed to the current knowledge on the elephant reproductive cycle. Several reproductive features in elephants differ markedly from other mammals. These include the urogenital tract anatomy, length and structure of the reproductive cycle, the formation of multiple corpora lutea and the type and secretion pattern of reproductive hormones. Being 13-18 weeks in length, the elephant estrous cycle is the longest amongst all studied non-seasonal mammals to date. Progesterone increases 1-3 days after ovulation, indicating the start of the luteal phase, which lasts 6-12 weeks. This is followed by a 4- to 6-week follicular phase that is concluded by two, precisely spaced and timed, LH surges. In general, the first, anovulatory LH surge occurs exactly 19-21 days before the second, ovulatory surge. Normally, a single follicle is ovulated. However, beside a corpus luteum (CL) forming on the site of ovulation, multiple accessory CLs can be found on the ovaries. Unlike many other species, the predominant progestagen secreted by luteal tissues is not progesterone, but rather its 5-alpha-reduced metabolites. The currently known aspects of the unique estrous cycle in Asian and African elephants, covering estrous behavior, circulating hormones, ultrasonography and anatomy of the reproductive organs as well as hormonal manipulation treatment possibilities, will be reviewed here.     

Folliculogenesis during the transitional period and early ovulatory season in mares

Individual follicles were monitored by ultrasonography in 15 mares during the transitional period preceding the first ovulation of the year and in 9 mares during the first interovulatory interval. During the transitional period, 7 mares developed 1-3 anovulatory follicular waves characterized by a dominant follicle (maximum diameter greater than or equal to 38 mm) that had growing, static, and regressing phases. The emergence of a subsequent wave (anovulatory or ovulatory) did not occur until the dominant follicle of the previous wave was in the static phase. After the emergence of the subsequent wave, the previous dominant follicle regressed. The mean (+/- s.d.) length of the interval between successive waves was 10.8 +/- 2.2 days. Before the emergence of waves (identified by a dominant follicle), follicular activity seemed erratic and follicles did not reach greater than 35 mm. During the interovulatory interval, 6 mares developed 2 waves (an anovulatory wave and a subsequent ovulatory wave) and 3 mares developed only 1 detected wave (the ovulatory wave). The ovulatory follicle at the end of the transitional period reached 20 mm earlier (Day - 15), grew slower (2.6 +/- 0.1 mm/day; mean +/- s.e.m.) but reached a larger diameter on Day - 1 (50.5 +/- 1.1 mm) than for the ovulatory follicle at the end of the interovulatory interval (Day - 10, 3.6 +/- 0.2 mm/day, 44.4 +/- 1.0 mm, respectively; P less than 0.05 for each end point). The interval from cessation of growth of the largest subordinate follicle to the occurrence of ovulation was longer (P less than 0.05) for end of the transitional period (9.5 +/- 0.7 days) than for the end of the interovulatory interval (6.8 +/- 0.6 days). Results demonstrated the occurrence of rhythmic follicular waves during some transitional periods and the occurrence of 2 waves during some of the first oestrous cycles of the year.     

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.     

Concentrations of inhibin,progesterone and oestradiol in fluid from dominant and subordinate follicles from mares during spring transition and the breeding season

Dominant and subordinate follicles were collected from mares on the day after the dominant follicle reached 30 mm in diameter, to investigate regulation of folliculogenesis during spring transition and the breeding season. Concentrations of oestradiol-17beta, progesterone and inhibin A, but not inhibin isoforms with pro- and alpha C-immunoreactivity, were significantly higher in preovulatory follicles than in dominant anovulatory transitional follicles. Steroidogenic activity was regained gradually in the dominant follicles of successive anovulatory waves through spring transition. The dominant follicles, during both spring transition and cyclicity, contained higher concentrations of oestradiol, progesterone and inhibin A, but not inhibin pro- and alpha C-isoforms, than subordinate follicles. The results indicate that high follicular levels of oestradiol, progesterone and inhibin A are associated with continued follicle growth and ovulation. The low concentrations of oestradiol and progesterone in transitional follicles indicate that the deficiency in steroidogenesis exists early in the steroidogenic pathway. The similarity in patterns of follicular hormones in spring transition and during cyclicity strongly suggests that the mechanism of dominance is the same in both types of follicle.     

Reproductive hormone profiles in mares during the autumn transition as determined by collection of jugular blood at 6 h intervals throughout ovulatory and anovulatory cycles

The aim was to define precisely the FSH secretion pattern in mares during the two ovulatory cycles before, and for 24 days after, the last ovulation of the season and to compare this with the profiles of other reproductive hormones and follicular growth to identify changes which may lead to the termination of follicular cycles. Jugular blood was collected every 6 h from ten light horse mares for 6 weeks in autumn. Samples were assayed for FSH, LH, prolactin, inhibin, oestrone conjugates and progesterone. Luteolysis occurred earlier and periovulatory oestrone, but not inhibin, concentrations were significantly lower in the last than in the second to last cycles. In ovulatory and anovulatory cycles, daily mean FSH concentrations were low at the expected time of ovulation and high between days 9 and 11 (day 0 = ovulation), which were usually after luteolysis. However, the periovulatory FSH nadir was prolonged in the last compared with the second to last cycles, and the difference between peak and trough values was not significant in anovulatory cycles. Between day 5 and day 8, the FSH interpulse interval was approximately 2 days, and did not vary in successive cycles. The LH profile also showed progressive changes as mares entered acyclicity; the surge terminated sooner in the last than in the second to last cycles, and failed to occur when expected in acyclicity. Sporadic prolactin pulses occurred at luteolysis in a similar proportion of ovulatory and anovulatory cycles. These results indicate that inadequate gonadotrophin stimulation in early dioestrus may be a critical event leading to suboptimal follicular and luteal development, and eventually acyclicity. Moreover, the time relationships amongst changes in pituitary and ovarian hormones and follicular growth become increasingly disrupted during the autumn transition, which may contribute to the cessation of cyclicity.     

The introduction of rams induces an increase in pulsatile LH secretion in cyclic ewes during the breeding season

Application of the ram effect during the breeding season has been previously disregarded because the ewe reproductive axis is powerfully inhibited by luteal phase progesterone concentrations. However, anovulatory ewes treated with exogenous progestagens respond to ram introduction with an increase in LH concentrations. We therefore tested whether cyclic ewes would respond to ram introduction with an increase in pulsatile LH secretion at all stages of the estrous cycle. We did two experiments using genotypes native to temperate or Mediterranean regions. In Experiment 1 (UK), 12 randomly cycling, North of England Mule ewes were introduced to rams midway through a frequent blood-sampling regime. Ewes in the early (EL; n=3) [corrected] and late luteal (LL; n=6) phase responded to ram introduction with an increase in LH pulse frequency and mean and basal concentration [corrected] of LH (at least P<0.05). In Experiment 2 (Australia), the cycles of 32 Merino ewes were synchronised using intravaginal progestagen pessaries. Pessary insertion was staggered to produce eight ewes at each stage of the estrous cycle: follicular (F), early luteal (EL), mid-luteal (ML) and late luteal (LL). In all stages of the cycle, ewes responded to ram introduction with an increase in LH pulse frequency (P<0.01); EL, ML and LL ewes also had an increase in mean LH concentration (P<0.05). In conclusion, ram introduction to cyclic ewes stimulated an increase in pulsatile LH secretion, independent of ewe genotype or stage of the estrous cycle.     

Follicle populations,ovulation rates and plasma profiles of LH,FSH and prolactin in Scottish Blackface ewes in high and low levels of body condition

Scottish Blackface ewes in high body condition (mean score = 2.86) had a higher mean ovulation rate (1.8 v. 0.9; P < 0.05) and more large (⪖ 4 mm diameter) follicles (4.6 v 2.2; P < 0.05) than ewes in low condition (mean score = 1.84) but similar numbers of small (1–4 mm diameter) follicles (6.3 v 6.0; NS). There was little difference in LH profiles with body condition but FSH and prolactin concentrations were significantly greater, during both luteal and follicular phases of the cycle, in ewes in high condition.Despite the relationships between body condition and ovulation rate and between condition and hormone concentrations, within the high condition groups, there was no significant difference in FSH levels with ovulation rate. Prolactin levels were higher in ewes with a single ovulation than in ewes with two or three ovulations. There was a trend towards a higher mean LH pulse frequency in the luteal phase and a higher mean LH pulse amplitude in the follicular phase in ewes with multiple ovulations compared with ewes with a single ovulation. During oestrus, only circulating prolactin concentrations differed with body condition, being significantly higher in ewes in high condition, but mean LH concentrations were higher and FSH concentrations lower in ewes with multiple ovulations. Subsequent luteal function, as measured by circulating progesterone concentrations, was normal in all ewes. It is concluded that body condition affected the size of the large follicle (⪖ 4 mm diameter) population through changes in FSH and possibly pulsatile LH secretion and prolactin secretion during the luteal and follicular phases of the cycle and that the number of follicles that were potentially ovulatory was probably determined during the luteal phase of the cycle. However, their ability to undergo the final stages of development and to ovulate may be related to the amount of LH secreted during the follicular phase.     

Effects of melatonin implants in pony mares. 2. Long-term effects

The effects of melatonin implant treatment over a 4 wk period at the summer solstice on the transition into and out of the following anovulatory season were evaluated in ovary-intact and ovariectomized mares. Melatonin implants tended to delay the timing of the final ovulation of the breeding season (P = 0.0797) in the ovary-intact mares. Although the decline in LH secretion associated with the end of the breeding season was parallel between treatments and ovarian statuses, the rate of LH secretion, as expressed by its mathematical accumulation, was lower in ovariectomized, melatonin-treated mares than in ovariectomized, control mares suggesting that melatonin administration advanced the offset of the breeding season in ovariectomized mares (P = 0.0001). The first ovulation of the subsequent breeding season was significantly delayed in the melatonin-treated mares as compared with that of control mares (P = 0.0031). During reproductive recrudescence, the time of the onset of the increase in LH secretion was similar among all 4 groups but the patterns of LH secretion were different for each treatment and ovarian status combination (P = 0.0112). Mares with melatonin implants had a slower rate of increase in LH secretion than control mares (P = 0.0001), and ovariectomized mares had a faster rate of LH increase than intact mares (P = 0.0001). These results suggest that melatonin implants during the summer solstice can alter the annual reproductive rhythm in mares and support the concept that endocrine patterns of reproductive recrudescence are not entirely independent of the ovary.     

Reproductive cycles in sheep

During the last three decades, there has been remarkable progress in many aspects of ovarian biology due to advances in real-time ultrasonography, which permits non-invasive, repeated monitoring of ovarian structures in conscious and non-anaesthetised animals. This review is primarily concerned with ovarian activity, as determined by transrectal ultrasonography, and measurements of circulating concentrations of gonadotrophins and ovarian steroids during reproductive cycles in sheep. The growth of antral follicles reaching ostensibly ovulatory sizes occurs in a wave-like pattern throughout the breeding season in both prolific and non-prolific breeds of sheep. There are typically 3 or 4 waves of follicle development during the interovulatory interval. Follicular wave emergence is primarily controlled by changes in circulating concentrations of follicle-stimulating hormone (FSH) but diminished ovarian responsiveness to gonadotrophic signals may result in reduced numbers of follicular waves. In cyclic ewes, the largest ovarian follicles acquire the ability to secrete oestradiol from the day of emergence with peak oestradiol secretion occurring about the time they reach maximum diameter. The high ovulation rate in some prolific breeds may be achieved by the ovulation of follicles from the last two waves of the interovulatory interval. Prolific ewes tend to produce more but smaller corpora lutea (CL) and have lower serum concentrations of progesterone during the luteal phase of the oestrous cycle as compared to less prolific genotypes. Lastly, recent studies of the endocrine influences on ovarian function have brought into question the existence of strong follicular dominance, as seen in cattle, and provided new insights into the effects of luteal progesterone on antral follicular development in ewes.     

Role of the double luteinizing hormone peak, luteinizing follicles, and the secretion of inhibin for dominant follicle selection in Asian elephants (Elephas maximus)

Elephants express two luteinizing hormone (LH) peaks timed 3 wk apart during the follicular phase. This is in marked contrast with the classic mammalian estrous cycle model with its single, ovulation-inducing LH peak. It is not clear why ovulation and a rise in progesterone only occur after the second LH peak in elephants. However, by combining ovarian ultrasound and hormone measurements in five Asian elephants (Elephas maximus), we have found a novel strategy for dominant follicle selection and luteal tissue accumulation. Two distinct waves of follicles develop during the follicular phase, each of which is terminated by an LH peak. At the first (anovulatory) LH surge, the largest follicles measure between 10 and 19.0 mm. At 7 ± 2.4 days before the second (ovulatory) LH surge, luteinization of these large follicles occurs. Simultaneously with luteinized follicle (LUF) formation, immunoreactive (ir) inhibin concentrations rise and stay elevated for 41.8 ± 5.8 days after ovulation and the subsequent rise in progesterone. We have found a significant relationship between LUF diameter and serum ir-inhibin level (r(2) = 0.82, P < 0.001). The results indicate that circulating ir-inhibin concentrations are derived from the luteinized granulosa cells of LUFs. Therefore, it appears that the development of LUFs is a precondition for inhibin secretion, which in turn impacts the selection of the ovulatory follicle. Only now, a single dominant follicle may deviate from the second follicular wave and ovulate after the second LH peak. Thus, elephants have evolved a different strategy for corpus luteum formation and selection of the ovulatory follicle as compared with other mammals.     

The temporal relationship between patterns of LH and FSH secretion, and development of ovulatory-sized follicles during the mid- to late-luteal phase of sheep

The aim of the present study was to investigate the temporal relationship between the secretory pattern of serum LH and FSH concentrations and waves of ovarian antral follicles during the luteal phase of the estrous cycle in sheep. The growth pattern of ovarian antral follicles and CL were monitored by transrectal ultrasonography and gonadotropin concentrations were measured in blood samples collected every 12 min for 6 h/d from 7 to 14 d after ovulation. There were two follicular waves (penultimate and final waves of the cycle) emerging and growing during the period of intensive blood sampling. Mean and basal LH concentrations and LH pulse frequency increased (P < 0.001) with decreasing progesterone concentration at the end of the cycle. Mean and basal FSH concentrations reached a peak (P < 0.01) on the day of follicular wave emergence before declining to a nadir by 2 d after emergence. None of the parameters of pulsatile LH secretion varied significantly with either the emergence of the final follicular wave or with the end of the growth phase of the largest follicle of the penultimate wave of the cycle. However, mean and basal LH concentrations did increase (P < 0.05) after the end of the growth phase of the largest follicle of the final follicular wave of the cycle. Furthermore, the end of the growth phase of the largest follicle of the final wave coincided with functional luteolysis. In summary, there was no abrupt or short-term change in pulsatile LH secretion in association with the emergence or growth of the largest follicle of a wave. We concluded that the emergence and growth of ovarian antral follicles in follicular waves do not require changes in LH secretion, but may involve changes in sensitivity of ovarian follicles to serum LH concentrations.     

Seasonal changes of gonadotropin-releasing hormone secretion in the ewe.

A sustained volley of high-frequency pulses of GnRH secretion is a fundamental step in the sequence of neuroendocrine events leading to ovulation during the breeding season of sheep. In the present study, the pattern of GnRH secretion into pituitary portal blood was examined in ewes during both the breeding and anestrous seasons, with a focus on determining whether the absence of ovulation during the nonbreeding season is associated with the lack of a sustained increase in pulsatile GnRH release. During the breeding season, separate groups (n = 5) of ovary-intact ewes were sampled during the midluteal phase of the estrous cycle and following the withdrawal of progesterone (removal of progesterone implants) to synchronize onset of the follicular phase. During the nonbreeding season, another two groups (n = 5) were sampled either in the absence of hormonal treatments or following withdrawal of progesterone. Pituitary portal and jugular blood for measurement of GnRH and LH, respectively, were sampled every 10 min for 6 h during the breeding season or for 12 h in anestrus. During the breeding season, mean frequency of episodic GnRH release was 1.4 pulses/6 h in luteal-phase ewes; frequency increased to 7.8 pulses/6 h during the follicular phase (following progesterone withdrawal). In marked contrast, GnRH pulse frequency was low (mean less than 1 pulse/6 h) in both groups of anestrous ewes (untreated and following progesterone withdrawal), but GnRH pulse amplitude exceeded that in both luteal and follicular phases of the estrous cycle.(ABSTRACT TRUNCATED AT 250 WORDS)     

Seasonal effects on the endocrine pattern of semi-captive female Asian elephants (Elephas maximus): timing of the anovulatory luteinizing hormone surge determines the length of the estrous cycle

Better breeding strategies for captive Asian elephants in range countries are needed to increase populations; this requires a thorough understanding of their reproductive physiology and factors affecting ovarian activity. Weekly blood samples were collected for 3.9 years from 22 semi-captive female Asian elephants in Thai elephant camps to characterize LH and progestin patterns throughout the estrous cycle. The duration of the estrous cycle was 14.6+/-0.2 weeks (mean+/-S.E.M.; n=71), with follicular and luteal phases of 6.1+/-0.2 and 8.5+/-0.2 weeks, respectively. Season had no significant effect on the overall length of the estrous cycle. However, follicular and luteal phase lengths varied among seasons and were negatively correlated (r=-0.658; P<0.01). During the follicular phase, the interval between the decrease in progestin concentrations to baseline and the anovulatory LH (anLH) surge varied in duration (average 25.9+/-2.0 days, range 7-41, n=23), and was longer in the rainy season (33.4+/-1.8 days, n=10) than in both the winter (22.2+/-4.5 days, n=5; P<0.05) and summer (18.9+/-2.6 days, n=8; P<0.05). By contrast, the interval between the anLH and ovulatory LH (ovLH) surge was more consistent (19.0+/-0.1 days, range 18-20, n=14). Thus, seasonal variation in estrous cycle characteristics were mediated by endocrine events during the early follicular phase, specifically related to timing of the anLH surge. Overall reproductive hormone patterns in Thai camp elephants were not markedly different from those in western zoos. However, this study was the first to more closely examine how timing of the LH surges impacted estrous cycle length in Asian elephants. These findings, and the ability to monitor reproductive hormones in range countries (and potentially in the field), should improve breeding management of captive and semi-wild elephants.     

Postpartum acyclicity in suckled beef cows: a review

Prolonged postpartum acyclicity in suckled beef cows is a source of economic loss to beef cattle producers. Duration of postpartum acyclicity is influenced by suckling status, nutritional status, calving season, age, and several other factors. Although uterine involution begins and ovarian follicular waves resume soon after parturition, dominant follicles of these waves fail to ovulate, due to a failure to undergo terminal maturation. As a result, postpartum anovulatory dominant follicles are smaller than the ovulatory follicles in cyclic cows. Failure of postpartum dominant follicles to undergo terminal maturation is due to absence of appropriate LH pulses, a prerequisite for follicular terminal maturation prior to ovulation. Absence of LH pulses early post partum is primarily due to depletion of anterior pituitary LH stores, although GnRH pulses are also absent during this period due to suckling. Following replenishment of LH stores between Days 15 and 30 post partum, absence of LH pulses is due to continued sensitivity of the hypothalamic GnRH pulse-generator to the negative feedback effect of ovarian estradiol-17beta, which results in absence of GnRH pulses. This negative feedback effect of estradiol-17beta is modulated by suckling which stimulates release of endogenous opioid peptides from the hypothalamus. As the postpartum interval increases, sensitivity of the GnRH pulse-generator to the negative feedback effect of ovarian estradiol-17beta decreases. This is followed by an increasing frequency of GnRH discharges and LH pulses, terminal follicular maturation, ovulation, and continued cyclicity. The first ovulation post partum is usually followed by a short cycle due to premature luteolysis because of premature release of PGF2alpha from the uterine endometrium, which is possibly intensified by the suckling-induced oxytocin release from the posterior pituitary. A model for the postpartum ovulatory acyclicity and for the resumption of cyclicity is presented.     

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.