Effects of gonadotrophin deprivation on follicular growth in gilts

Application of Exogenous GnRH in Food Animal Production

Over several decades, exogenous GnRH and agonists have been employed for controlling reproductive cascades in animals, and treating some reproductive morbidities. The administration of GnRH is used in animals to counter ovarian dysfunction, induce ovulation, and to increase conception and pregnancy rates. GnRH and its agonists are used in the treatment of cystic ovarian degeneration and repeat breeder syndrome. The development of protocols for GnRH administration by intramuscular injection, intramuscular or subcutaneous implants, and intravaginal deposition has empowered their clinical use worldwide. Currently, exogenous GnRH products are a central part of several pre- and post-breeding programs for the enhancement of fertility, including the control of estrous cycles and timing of ovulation, development of fixed-time artificial insemination protocols, improved embryo survival, and the treatment of reproductive morbidity. The aim of the present review is to summarize the application of exogenous GnRH agonists in food animal production.

The effect of fixed-time artificial insemination protocol initiated at different stages of the estrous cycle on follicle development and ovulation in gilts

Hormonal products have been developed for fixed-time artificial insemination (FTAI) to improve the efficiency of swine production. Here, we evaluated the effect of an FTAI protocol initiated during different phases of the estrous cycle on follicle development and ovulation in gilts. A total of 36 gilts were equally divided into three groups designated as the luteal (L), follicular (F), and post-ovulation (O) groups and fed with 20 mg of altrenogest for 18 days, followed by intramuscular injection of 1000 IU PMSG at 42 h after withdrawal of altrenogest, and 100 μg of GnRH after an 80-h interval. The L group had the highest number of follicles 4–6 mm in diameter, as well as corpora hemorrhagica. The mRNA expression of caspase-9 in the L group were significantly lower than those in the O and F groups (P < 0.05), while CYP11A1 and VEGF mRNA expression levels were significantly higher (P < 0.05). Moreover, FSHR mRNA levels were significantly higher in the O group than in the L, F, and control groups (P < 0.05). LHCGR and CYP19A1 mRNA levels were the highest in the F group (P < 0.05). Thus, the changes in the expression of genes associated with follicular development, maturation, and ovulation identified in this study indicated that initiation of the FTAI protocol during the luteal phase induced a better environment for follicle development and ovulation in gilts.

Ovulatory Response of Weaned Sows to an Altered Ratio of Exogenous Gonadotrophins

Simple Summary

Efficient pork production relies on a consistent supply of market pigs. To achieve breeding targets, gonadotrophins can be administered at weaning to stimulate estrus onset. The present study examined the impact of supplemental human chorionic gonadotrophin activity (i.e., hCG), during a follicular phase induced by a standard gonotrophin protocol (i.e., PG600), in both ovarian follicular development and fertility in multiparous sows. The results confirmed that supplemental hCG at 24 h after PG600 increased follicle growth and reduced the interval to ovulation, but also increased the incidence of follicle cysts and reduced pregnancy success.

Abstract

At weaning, 33 mixed parity Hypor sows received either an injection of 400 IU equine chorionic gonadotrophin and 200 IU human chorionic gonadotrophin (hCG) (PG600; n = 13), PG600 with an additional 200 IU hCG 24 h later (Gn800; n = 11), or served as non-injected controls (n = 9). All gonadotrophin treated sows received an injection of 750 IU hCG at 80 h after weaning to induce ovulation (designated as time 0 h). At 0, 24, 36, 40, 44, 48, and 60 h, all sows were subject to transrectal ultrasonography to determine numbers and sizes of large (>6 mm) follicles and time of ovulation. The interval from injection of 750 IU hCG to ovulation was shorter in Gn800 compared to PG600 sows (p = 0.02), and more Gn800 sows had ≥9 preovulatory follicles compared to PG600 and controls (p = 0.02 and 0.003, respectively). Follicular cysts were evident in both PG600 and Gn800 sows.

Effect of altering the ratio of exogenous gonadotropins on reproductive performance of primiparous sows during the seasonal infertility period

At weaning, 150 primiparous sows were assigned sequentially to receive 400 IU equine chorionic gonadotropin (eCG) with 200 IU human chorionic gonadotropin (hCG) (Gn600), or 400 IU eCG with 400 IU hCG (Gn800), or served as untreated controls. Compared with control and Gn600, the Gn800 had increased estrus response by day 7 and reduced wean-to-estrus intervals (P < 0.05). Compared with controls, both Gn600 and Gn800 increased numbers of large follicles (≥6 mm) at estrus detection (P < 0.05) and the subsequent farrowing rates (P < 0.05). Litter size was unaffected. These data demonstrate the efficacy of supplemental hCG for alleviating effects of seasonal infertility in primiparous sows.

Effect of duration of altrenogest treatment on farrowing rate and litter size of gilts

The objective of the present study was to compare two common durations of altrenogest (ALT) feeding during different periods of the year on the fertility of gilts after ALT withdrawal. During a 12-month period gilt replacements that were assumed to be cyclic were subjected to oestrus synchronisation with 15 mg/day ALT administered for 18 days (ALT-18; n = 268) or 14 days (ALT-14; n = 153) whereas 275 non-treated gilts served as controls. Fewer ALT-14 than ALT-18 gilts expressed oestrus by 7 days after last ALT treatment (79.1% vs 88.8%; P < 0.05). Farrowing rate was lower (P < 0.05) for ALT-14 than for the other groups (81%, 91% and 92% for ALT-14, ALT-18, and Control, respectively) but farrowing rates were not affected by time of year. Control litter sizes were not different from ALT-14 except during September to November when Control litter sizes were larger than either ALT treatments (13.6 ± 0.33, 12.3 ± 0.65 and 12.7 ± 0.39 for Control, ALT-14 and ALT-18, respectively; P < 0.05). The ALT-18 gilts had larger litter sizes during January to August. The present data suggest that the appropriate duration of ALT feeding to synchronise oestrus in gilts is 18 days.

Controlling lactation oestrus: The final frontier for breeding herd management

Lactation anoestrus limits the flexibility of modern pig production systems such that any increase in lactation length reduces farrowing frequency, and thus profit. This review focuses on post-partum development of the sow's reproductive system, the physiology of lactation anoestrus and how it can be overcome, as well as the fertility of sows mated while lactating. The propensity for sows to ovulate spontaneously while lactating is high (24-31%), and a high proportion of sows will ovulate rapidly and synchronously in response to combinations of altered suckling (split weaning, interrupted suckling), daily boar contact, exogenous gonadotrophins, and group housing. The apparent ease with which lactation anoestrus can be overcome represents an opportunity to uncouple sow mating from weaning, thus reducing the impact of lactation length on productivity. This is especially true when considering the benefits of the described stimulation methods on the reproductive performance (i.e., shorter weaning to oestrus intervals and higher litter sizes) of the low proportion of sows that maintain lactation anoestrus.

Seasonal infertility in gilts and sows: Aetiology, clinical implications and treatments

In gilts and sows, the summer-autumn period often is characterized by reduced fertility. Heat stress and long photoperiods during the warm season can cause a reduction in feed intake and an imbalance of the hypothalamic-hypophysial-ovarian axis. The increased variability in the interval between oestrus onset and ovulation results in an increased number of poorly timed inseminations. The altered endocrine activity compromises follicular and corpora luteal development, reduces oocyte quality and increases embryo mortality. This paper reviews current knowledge on the metabolic and endocrine mechanisms associated with seasonal infertility in gilts and sows and describes some pharmacological approaches that can be utilized to counter this infertility.

Human chorionic gonadotrophin in early gestation induces growth of estrogenic ovarian follicles and improves primiparous sow fertility during summer

Reduced summer farrowing rates may be due to inadequate corpora luteal (CL) support. Porcine CL become dependent on LH from 12 d of pregnancy and the embryonic estrogen signal for maternal recognition of pregnancy (MRP) is initiated at about 11-12 d after insemination. We hypothesised that injection of the LH analogue human chorionic gonadotropin (hCG) would induce growth of estrogenic follicles and, by mimicking the signal for MRP and stimulating progesterone secretion, increase primiparous sow fertility. In Experiment 1, during a 28 d lactation 53 mixed parity sows were full-fed either throughout lactation (n=16) or until 18 d and then feed restricted during the last 10 d of lactation (n=36). At 12 d after mating restrict-fed sows were injected with 1000IU hCG (n=17) or were not injected (n=19); the full-fed sows acted as non-treated positive controls. Transrectal ovarian ultrasound exams were performed on days 12, 16, 20, 24, and 28; blood samples were obtained on days 12, 14, and 15 for estradiol and progesterone assay. For Experiment 2, during the summer months primiparous sows received 1000IU hCG 12 d after mating (n=28) or were non-injected controls (n=27). Pregnancy status was determined at 28 d and sows allowed to go to term to determine farrowing rates and litter sizes. In Experiment 1, injection of hCG increased (P<0.001) follicle diameter and serum concentrations of estradiol (P<0.01) and progesterone (P<0.05). There were no effects of lactation feeding level on wean-estrus interval, farrowing rate or subsequent litter size. In Experiment 2, hCG injection was associated with a higher pregnancy rate (P<0.05) and farrowing rate (P<0.08). There was no effect on litter size. These data confirm that hCG stimulates growth of estrogenic follicles and CL function, and improves primiparous sow fertility during the summer months.

An essential role for the intra-oocyte MAPK activity in the NSN-to-SN transition of germinal vesicle chromatin configuration in porcine oocytes

The mechanisms for the transition from non-surrounded nucleolus (NSN) to surrounded nucleolus (SN) chromatin configuration during oocyte growth/maturation are unclear. By manipulating enzyme activities and measuring important molecules using small-follicle pig oocytes with a high proportion of NSN configuration and an extended germinal vesicle stage in vitro, this study has the first time up-to-date established the essential role for intra-oocyte mitogen-activated protein kinase (MAPK) in the NSN-to-SN transition. Within the oocyte in 1–2 mm follicles, a cAMP decline activates MAPK, which prevents the NSN-to-SN transition by activating nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) while inhibiting histone deacetylase (HDAC). In cumulus cells of 1–2 mm follicles, a lower level of estradiol and oocyte-derived paracrine factor (ODPF) reduces natriuretic peptide receptor 2 (NPR2) while enhancing FSH and cAMP actions. FSH elevates cAMP levels, which decreases NPR2 while activating MAPK. MAPK closes the gap junctions, which, together with the NPR2 decrease, reduces cyclic guanosine monophosphate (cGMP) delivery leading to the cAMP decline within oocytes. In 3–6 mm follicles, a higher level of estradiol and ODPF and a FSH shortage initiate a reversion of the above events leading to MAPK inactivation and NSN-to-SN transition within oocytes.

Effect of a GnRH analogue (Maprelin) on the reproductive performance of gilts and sows

The ability of peforelin (l-GnRH-III) to stimulate follicular growth, FSH release, and estrus in gilts after altrenogest treatment and in sows after weaning was investigated. In three farrow-to-wean herds, with at least 600 sows and average production performance, 216 gilts, 335 primiparous, and 1299 pluriparous sows were randomly allocated to three treatments: peforelin (M group: Maprelin), eCG (F group: Folligon), and physiological saline solution (C group). Animals were treated 48 hours after their last altrenogest treatment (gilts) or 24 hours after weaning (sows). The weaning-to-estrus interval, estrus duration, estrus rate (ER), pregnancy rate, and total born (TB), live born, and stillborn (SB) numbers were recorded and compared between treatments for the different parity groups (gilts and primiparous and pluriparous sows). Follicle sizes were measured in representative animals from each group on the occasion of their last altrenogest treatment or at weaning, and also on the occasions of their first (FS1) and second (FS2) attempted inseminations. Blood samples were taken to determine FSH concentrations at weaning and 2 hours after injection, and progesterone concentrations 10 days after the first insemination attempt. The relative change in FSH concentrations was calculated. Significant differences were found for ER within 7 days of weaning in pluriparous sows (95%, 91%, and 90% for the M, F, and C groups, respectively, P = 0.005). Gilts in the F-group had high TB numbers, and pluriparous sows in the M group had high SB numbers (TB gilts = 13.6, 15.4, and 14.9 [P = 0.02] and SB pluriparous sows = 1.8, 1.4, and 1.7 [P = 0.05] for the M, F, and C groups, respectively). The M group had the highest FS1 (for gilts) and FS2 (for pluriparous sows) values: FS1 = 5.4, 4.9, and 4.9 mm [P = 0.02] and FS2 = 6.8, 5.3, and 6.3 mm [P = 0.03] for the M, F, and C groups, respectively. There were no significant differences between the different treatments within each parity group with respect to any of the other variables. Overall, peforelin treatment had small but positive effects on the ER and follicle growth in certain parity groups but did not seem to affect litter sizes or FSH and progesterone levels in sows on the occasions of the corresponding examinations.

Effect of gonadotropin treatment on estrus, ovulation, and litter size in weaned and anestrous sows

In the first of 2 experiments, we evaluated the effects on anestrous sows of pretreatment with FSH to stimulate the growth of small follicles, followed by eCG to stimulate the growth of medium follicles, estrus, and ovulation. In Exp. 2, we examined the effect of sows receiving 400 IU of eCG plus 200 IU of hCG (PG 600, Intervet/Schering Plough Animal Health, Boxmeer, the Netherlands) at weaning and then different doses and timing of supplemental hCG. In Exp. 1, a total of 87 multiparous Hypor sows deemed anestrus 7 d after weaning were assigned to intramuscular (i.m.) injection of 1) PG 600, 2) eCG (600 IU), 3) pretreatment with 87.5 IU of FSH on d 7 and 8 plus eCG on d 9, or were 4) noninjected controls. Sows had daily boar contact for 15 d after weaning for estrus detection. Blood samples were obtained on d 9 and 19 and assayed for progesterone to determine ovulation status. The weaning-to-estrus interval, number of sows in estrus and ovulating, farrowing rate, and litter size were not different (P > 0.1) in treated groups compared with controls. In Exp. 2, a total of 247 Hypor sows were assigned at weaning by parity (1 and 2 or > or = 3) to receive 1) an i.m. injection of PG 600, 2) PG 600 supplemented with 100 IU of hCG injected either concurrently or after 24 h, 3) 200 IU of hCG after 24 h, or 4) no injection (controls). Sows were exposed to boars daily for 7 d. After treatment of parity 1 and 2 sows, all gonadotropin-treated groups had an increased (P < 0.05) number of sows in estrus compared with the control group; weaning-to-estrus interval, farrowing rates, and litter size were unaffected (P > 0.1). After treatment of parity > or = 3 sows, there was no treatment effect on the estrous response and weaning-to-estrus interval; compared with control and PG 600-treated sows, farrowing rate was decreased (P < 0.05) for sows receiving 200 IU of hCG after 24 h. There was no effect (P > 0.1) of treatment on litter size. We conclude that gonadotropins can be used to increase estrus response in weaned sows, but that hCG treatment subsequent to PG 600 may be detrimental to sow fertility in parity > or = 3 sows.

Effect of hCG on early luteal serum progesterone concentrations in PG600-treated gilts

Gilt oestrus and ovulation responses to injection of a combination of equine chorionic gonadotrophin (eCG) and human chorionic gonadotrophin (hCG) (PG600) can be unpredictable, possibly reflecting inadequate circulating LH activity. The objective of this study was to determine the effect of PG600 followed by supplemental hCG on gilt ovarian responses. In experiment 1, 212 Hypor gilts (160 day of age) housed on two farms in Spain received intramuscular (i.m.) injections of PG600 (n = 47), or PG600 with an additional 200 IU hCG injected either concurrently (hCG-0; n = 39), or at 24 h (hCG-24; n = 41) or 48 h (hCG-48; n = 45) after PG600. A further 40 gilts served as non-injected controls. Ovulation responses were determined on the basis of initial blood progesterone concentrations being <1 ng/ml and achieving >5 ng / ml 10 d after the PG600 injection. The incidence of ovulating gilts having progesterone concentrations >30 ng/ml were recorded. During the study period, 10% of control gilts ovulated whereas 85-100% of hormone-treated gilts ovulated. There were no significant differences among hormone groups for proportions of gilts ovulating. The proportions of gilts having circulating progesterone concentrations >30 ng/ml were increased (p < or = 0.02) in all hCG treated groups compared with the PG600 group. In experiment 2, a total of 76 Hypor gilts at either 150 or 200 days of age were injected with PG600 (n = 18), 400 IU eCG followed by 200 IU hCG 24 h later (n = 20), PG600 followed by 100 IU hCG 24 h later (n = 17), or 400 IU eCG followed by 300 IU hCG 24 h later (n = 21). Blood samples were obtained 10 days later for progesterone assay. There were no effects of treatment or age on incidence of ovulation, but fewer 150-day-old gilts treated with PG600 or 400 IU eCG followed by 200 IU hCG had progesterone concentrations >30 ng / ml. We conclude that hCG treatment subsequent to PG600 treatment will generate a higher circulating progesterone concentration, although the effect is not evident in older, presumably peripubertal, gilts. The mechanism involved and implications for fertility remain to be determined.

Effect of hCG treatment on the oestrous and ovulation responses to FSH in prepubertal gilts

To ensure sufficient numbers of pregnant females, particularly at hotter times of the year, hormonal induction of gilt oestrus may be necessary. However, the gilt oestrus and ovulation responses to gonadotrophin treatment have often proven unpredictable. The objective of this study was to examine possible reasons for this unpredictability. Prepubertal gilts (approximately 150 days of age, n = 63) were assigned to one of three treatments: injection of 300 IU hCG (n = 15); pre-treatment with 100 mg FSH in polyvinylpyrrolidinone administered as 2 x 50 mg injections 24 h apart, followed by 600 IU eCG at 24 h after the second FSH injection (n = 23); or FSH pre-treatment as above followed by 300 IU hCG at 24 h after the second FSH injection (n = 25). To facilitate oestrus detection, gilts were exposed to a mature boar for 15 min daily for 7 days. Blood samples were obtained on the day of eCG or hCG injection and again 10 days later and gilt ovulation responses determined based on elevated progesterone concentrations. The oestrus responses by 7 days were 6.7%, 17.5% and 64.0% for gilts treated with hCG, FSH + eCG and FSH + hCG, respectively (p < 0.001). The oestrous gilt receiving hCG alone and one oestrous FSH + hCG gilt did not ovulate, all other oestrous gilts ovulated. A further two anoestrous FSH + eCG-treated gilts ovulated. These data suggest that FSH pre-treatment facilitated the development of ovarian follicles to the point where they became responsive to hCG, but had little effect on the response to eCG.

Physiological mechanisms of ovarian follicular growth in pigs--a review

Follicular growth after antrum formation is determined by follicle-stimulating hormone (FSH). Only two ways are possible for recruited follicles, continuing development or atresia. In gilts, intensive ovarian follicular growth begins between 60 and 100 days of age, and fluctuations of the ovarian morphological status last about 20 days; however, at that time there are no really large follicles. Final follicular development is under luteinising hormone (LH) control; this is why the attainment of puberty is related to an increase in serum oestradiol to a level that causes a preovulatory surge of this gonadotropin. The pool of follicles at the beginning of the oestrous cycle is about 30-40, most of which are small (< 3 mm) and growing. Then, the pool of follicles increases to about 80 in the mid-luteal phase but about 50 of them are small and 30 are medium sized (3-6.9 mm). Some of these follicles are in the growing phase, but some are atretic. Between days 7 and 15 of the oestrous cycle the percentage of atretic follicles fluctuates between 12 and 73%. At that time there are no large (> 7 mm) follicles because of the suppressing effect of progesterone. The number of small follicles declines after luteolysis. From the pool of medium follicles, large follicles are selected under the influence of LH, but about 70% of the medium-sized follicles become atretic. Because of the long-lasting selection process there is a significant heterogeneity in the diameter of large follicles in oestrus. However, the number of follicles correlates with the number of corpora lutea after ovulation. Individual follicular development and the relationship between follicles are still poorly known. The use of ultrasonography may give a closer insight into these phenomena.

Effect of prior FSH treatment on the estrus and ovulation responses to eCG in prepubertal gilts

The objective of this study was to determine the effect of pre-treatment of prepubertal gilts with FSH on the estrus and ovulatory responses to eCG injection at two ages. A total of 149 prepubertal Hypor gilts were selected at 150 days (n=76) or 180 days (n=73) of age and assigned to injection of 400 IU eCG plus 200 IU hCG (PG600), 600IU eCG alone (Folligon), pre-treatment with 72 mg FSH (Folltropin) administered as 6 x 12 mg injections at 12 h intervals with 600 IU Folligon 12h after last FSH injection, or non-injected controls. To facilitate detection of estrus, gilts were exposed to a mature boar for 15 min daily for 7 days. To determine ovulatory responses, blood samples were obtained on the day of injection and 10 days later and assayed for progesterone content. Following treatment at 150 days, one control gilt (5.3%) was deemed estrus but ovulation did not occur. Compared to treatment with Folligon alone, PG600 injection tended (P=0.1) to increase the estrus response (52.6% compared with. 26.3%) and increased (P<0.01) the ovulatory response (89.5% compared with. 47.4%). The estrous response in gilts pretreated with Folltropin was intermediate (42.1%) but the ovulatory response (47.4%) was the same as for Folligon alone. Following treatment at 180 days, two control gilts (10.5%) were deemed estrus and ovulation did occur in these gilts. There was no difference between hormone-treated groups for estrus or ovulatory responses, although the ovulatory response of PG600-treated gilts tended (P=0.1) to be greater than for the Folligon-treated group (89.5% compared with 66.7%), with Folltropin-pretreated gilts being intermediate (76.5%). These data demonstrate that the estrus and ovulatory responses of gilts were greater for PG600 than for Folligon and that while responses to PG600 were not affected by gilt age, for the combined Folligon groups, estrous response (P<0.02) and ovulatory response (P<0.05) improved with increased gilt age.

A growth-maturation system that enhances the meiotic and developmental competence of porcine oocytes isolated from small follicles

In livestock, most of the follicles on the ovarian surface are small follicles. A procedure that supports the in vitro growth and maturation of these small follicle-derived oocytes may offer a new source of useable oocytes for both biotechnological and fundamental research purposes. The objective of the current study was to test the hypothesis that providing a more growth-supporting and less maturation-promoting environment during the first phase of small follicle-derived oocyte maturation may improve oocyte competence for meiosis and embryo development upon activation. In our small follicle-derived oocyte growth-maturation system (SGM group), cumulus-oocyte complexes (COCs) from small follicles (1-3 mm) were first cultured in oocyte growth medium for 24 h, then in oocyte maturation medium for 20 h. As controls, COCs from small (SM group) and large (LM group) follicles were cultured using a conventional in vitro maturation (IVM) approach in which they were directly cultured in oocyte maturation medium. At 24 h of culture, the percentage of small follicle-derived oocytes that underwent germinal vesicle breakdown (GVBD) in the SGM group was comparable to that of large follicle-derived oocytes (LM group) but was significantly higher than that of the SM group (P < 0.05). At 44 h of culture, compared to 36% in the SM group, 55% of the SGM group oocytes reached metaphase II (MII; P < 0.05). In addition, the level of cyclin B in oocytes of the SGM group was comparable to that of oocytes from LM group and was significantly higher than that of oocytes from the SM group (P < 0.05). When activated and in vitro fertilized (IVF), 7.3 and 9.0 times more parthenogenetic and IVF embryos developed to blastocyst stage in the SGM group than in the SM group (P < 0.05). The mRNA expression levels of three developmentally important genes--DNA-methyltransferase 1, Pou domain class 5 transcription factor 1, and Fibroblast growth factor receptor 2--in embryos of the SGM group were comparable to those of embryos developed from the LM group, whereas they were significantly lower in those of the SM group (P < 0.05). Our data suggest that the oocyte growth-maturation system facilitates the final stage of oocyte growth and thus resulted in better oocyte nuclear, cytoplasmic maturation, and developmental competency compared with the conventional direct oocyte maturation system.

Gonadotropin-releasing hormone (GnRH) and its natural analogues: a review

The pivotal role of gonadotropin-releasing hormone (GnRH) during the hormonal regulation of reproductive processes is indisputable. Likewise, many factors are known to affect reproductive function by influencing either GnRH release from hypothalamus or pituitary gland responsiveness to GnRH. In veterinary medicine, GnRH and its agonists (GnRHa) are widely used to overcome reduced fertility by ovarian dysfunction, to induce ovulation, and to improve conception rate. GnRHa are, moreover, integrative part of other pro-fertility treatments, e.g. for synchronization of the estrous cycle or stimulation for embryo transfer. Additionally, continuous GnRH which shows desensitizing effects of the pituitary-ovarian axis has been recommended for implementation in anti-fertility treatments like inhibition of ovulation or reversible blockade of the estrous cycle. Just as much, another group of GnRH analogues, antagonists, are now in principle disposable for use. For a few decades, GnRH was thought to be a unique structure with a primary role in regulation gonadotropins. However, it became apparent that other homologous ligands of the GnRH receptor (GnRHR) exist. In the meantime, more than 20 natural variants of the mammalian GnRH have been identified in different species which may compete for binding and/or have their own receptors. These GnRH forms (GnRHs) have apparently common and divergent functions. More studies on GnRHs should contribute to a better understanding of reproductive processes in mammals and interactions between reproduction and other physiological functions. Increased information on GnRHs might raise expectations in the application of these peptides in veterinary practice. It is the aim of this review to discuss latest results from evolutionarily based studies as well as first experimental tests and to answer the question how realistic might be the efforts to develop effective and animal friendly practical applications for endogenous GnRHs and synthetic analogues.

Lamprey GnRH-III Stimulates FSH Secretion in Barrows

Although studies have indicated that follicle-stimulating hormone (FSH) and luteinizing hormone (LH) release can be dissociated in the pig, the underlying mechanisms are still to be answered. Since it was demonstrated that lamprey gonadotropin-releasing hormone (l-GnRH-III) has preferential FSH-releasing potency in several mammalian species, we have investigated the gonadotropin-releasing activity of l-GnRH-III in barrows. Each of nine barrows (body weight: 85-90 kg; age: 207 days) received 2 ml saline (S-barrow), followed by 150 microg l-GnRH-III (1.6-1.7 microg/kg body weight) dissolved in 2 ml saline intramuscularly 7 days later. Three pre-treatment and 13 post-treatment blood samples were taken at intervals of 30 min to 8 h to assess basal and treatment-associated concentrations of FSH and LH, respectively, by radioimmunoassay. Animals were defined as having responded to treatment if, 2 h post-treatment, plasma FSH and/or LH levels were >3 SD of the respective basal concentrations. There was no treatment-associated FSH response after saline treatment, but a clear FSH response in all l-GnRH-III-injected barrows. On average, the maximum FSH level (205% of the basal concentration) was observed at 1 h post-treatment. Mean FSH values were elevated until 10 h post-treatment. There was no LH response either to saline or to l-GnRH-III. In conclusion, this study demonstrates a selective FSH-releasing activity of 150 microg l-GnRH-III in barrows. Further studies are needed to investigate whether this effect is ubiquitous in the pig and what the physiological relevance is.

Recruitment and selection of ovarian follicles for determination of ovulation rate in the pig

Gonadotropins determine the follicle selection and ovulation rate. Follicle growth is independent of gonadotropins until antrum formation, at which time recruitment occurs. Once recruited, follicles will continue to grow or degenerate. In gilts, visible surface follicles are classified as small (<3mm), medium (3-6.9 mm) and large (> or =7.0mm). At estrus (day 0), there are approximately 15 small and medium follicles, and approximately 15 large follicles. By day 3, there may be approximately 30 small, 5 medium and no large follicles. During the remainder of the luteal phase, the pool of follicles increases and peaks at day 11-13 with approximately 50 small, and 30 medium, but with no large follicles observed. By the start of the follicular phase at day 15, numbers of small and medium follicles rapidly decline, while a pool of medium follicles is selected for the ovulation. The size of large follicles at estrus is heterogeneous (6.5-10.0 mm) but their number is reflective of the subsequent number of corpora lutea found following the ovulation. However, the time of medium follicle selection for ovulation is variable during the late luteal and early follicular phases. Suppression of FSH before and at the time of luteolysis reduces medium and large follicles but does not reduce the ovulation rate. In contrast, suppression of FSH for 3 days or unilateral ovariectomy after 3 days of the follicular phase prevents full ovulatory compensation. Therefore, FSH appears to be involved in the maintenance of a pool of medium follicles that can be selected by LH to mature and ovulate.

Endocrine and paracrine control of follicular development and ovulation rate in farm species

Productivity in farm species is controlled by many factors, including ovulation rate. In cattle, single ovulations occur most frequently and in sheep (and goats) the number of ova released can range from one to many depending upon the breed, whilst the pig is polyovular. The processes of recruitment and selection determine the number of ovulatory follicles in all these species with FSH and subsequently LH playing major roles. GnRH-agonist models in which endogenous gonadotrophin secretion is suppressed and exogenous LH and/or FSH are administered at specific concentrations in defined patterns, are useful in all three species for elucidating the precise roles of specific hormones in stimulating follicular development. Differences in the hypothalamic-pituitary-ovarian feedback response lead to the differences in the number of ovulatory follicles, as does the pool of antral follicles from which the ovulatory ones are selected. Precocious development of follicles is also associated with more ovulations, as is the case with the Booroola due to the single gene acting through bone morphogenetic proteins (BMPs). It is well established that ovulation rate can also be influenced by exogenous hormone administration and by environmental factors such as nutrition. It has become apparent that these nuritional effects are mediated by a direct action at the level of the ovary, involving insulin, insulin-like growth factors (IGF) I and II and their binding proteins among other factors. These factors can also affect the quality of the oocyte and consequently embryo development and survival. Recently, the regulation of follicular angiogenesis has been shown to be important for the development of ovulatory follicles, particularly vascular endothelial growth factor (VEGF) which is produced primarily by the granulosa cells within the ovary and can be stimulated by gonadotrophins. Administration of VEGF has been shown to stimulate pre-antral follicular growth and increase the number of pre-ovulatory follicles. In summary both extra- and intra-ovarian factors are involved in the control of ovulation rate. Manipulation of the angiogenic process may also provide new opportunities for regulating the quality and number of follicles that ovulate.

Ovarian follicular wave synchronization and pregnancy rate after fixed-time natural mating in llamas

The study was designed to compare the efficacy of treatments intended to induce follicular wave synchronization among llamas (Experiment 1), and to determine the effect of these treatments on pregnancy rates after fixed-time natural mating (Experiment 2). In Experiment 1, llamas were treated with: (1) saline (control, n=20); (2) estradiol and progesterone (E/P, n=20); (3) LH (LH, n=20); or (4) transvaginal ultrasound-guided follicle ablation (FA, n=20). The ovarian response was monitored daily by transrectal ultrasonography. The intervals from treatment to follicular wave emergence and to the day on which the new dominant follicle reached >/=7 mm, respectively, did not differ between the LH (2.1+/-0.3 days and 5.2+/-0.5 days, respectively) and FA groups (2.3+/-0.3 days and 5.0+/-0.5 days), but both were shorter (P<0.05) and less variable (P<0.01) than in the control group (5.5+/-1.0 days and 8.4+/-2.0 days), while the E/P group (4.5+/-0.8 days and 7.7+/-0.5 days) was intermediate. In Experiment 2, llamas at unknown stages of follicular development were assigned randomly to control, E/P, and LH groups (n=30 per group). A single, fixed-time natural mating was permitted 10-12 days after treatment. Ovulation rates did not differ among groups (control, 93%; E/P, 90%; LH, 90%; P=0.99), but the pregnancy rate was higher (P<0.05) for synchronized llamas (LH and E/P groups combined, 41/54) than for non-synchronized llamas (control group, 15/28). In conclusion, LH and FA treatments were most effective for inducing follicular wave synchronization, while E/P treatment was intermediate. Synchronization treatments did not influence ovulation rate subsequent to fixed-time natural mating, but a higher pregnancy rate in synchronized than non-synchronized llamas warrants critical evaluation of the effects of follicular status on the developmental competence of the contained oocyte.

Effect of a gonadotrophin-releasing hormone antagonist on luteinising hormone secretion and early pregnancy in gilts

The aims of the present study were: (1) to determine the duration of suppression of luteinising hormone (LH) following a single treatment with a gonadotrophin-releasing hormone (GnRH) antagonist (BIM-21009; Biomeasure) at a dose of 100 μg kg−1; (2) to block LH pulses only for certain days of pregnancy; and (3) to determine the period of early pregnancy most susceptible to suppression of LH. Three groups of gilts were injected with 100 μg kg−1 on Day 16 (n = 5), 14 (n = 6) or 19 (n = 4) of pregnancy. Blood for LH analysis was collected at 20-min intervals for 12 h on the day before treatment and during varying stages of early pregnancy. Blood for progesterone analysis was collected daily and development of pregnancy was followed using real-time ultrasound. Prior to treatment, gilts had 2.6 ± 0.7 LH pulses per 12 h. The GnRH antagonist abolished LH pulses for a period of 2.7 ± 1.8 days and, thereafter, suppressed the resumed LH pulses (P < 0.05). Pregnancy was disrupted in three pigs (20%) with a mean treatment-to-abortion period of 4.7 days concurrent with a mean treatment-to-progesterone decline interval of 4.3 days. In a proportion of pigs, short-term LH suppression may cause early disruption of pregnancy.

Effects of Culture Medium, Serum Type, and Various Concentrations of Follicle-Stimulating Hormone on Porcine Preantral Follicular Development and Antrum Formation In Vitro1

Developing a culture system for preantral follicles has important biotechnological implications due to the potential to produce large number of oocytes for embryo production and transfer. As an initial step toward accomplishing this long-term goal, a study was conducted to determine the effects of culture medium, serum type, and different concentrations of FSH on preantral follicular development in vitro. Specific endpoints included follicular growth rate, antrum formation, recovery rate of cumulus cell-oocyte complexes (COCs) from follicles, and oocyte meiotic competence. Compared with the North Carolina State University medium 23 (NCSU23), preantral follicles cultured in TCM199 medium for 4 days grew faster (P < 0.02). However, more follicles cultured in NCSU23 differentiated to form an antrum than in TCM199 (P < 0.01). For this reason, NCSU23 was chosen to investigate the role of FSH and serum type in regulating preantral follicular growth. Compared with the 0 mIU/ml FSH control, addition of 2 mIU/ml FSH to the medium stimulated follicular growth and antrum formation and suppressed apoptosis of granulosa cells (P < 0.05), supporting the essential role of FSH in preantral follicular growth and development. Another experiment compared fetal calf serum (FCS) with prepubertal gilt serum (PGS) and studied different concentrations of FSH in the culture medium (0.5, 1, and 2 mIU/ml). The best follicular growth rate was obtained with 2 mIU/ml compared with 0.5 or 1 mIU/ml FSH. Compared with PGS, FCS supplementation increased the cumulative percentage of antral follicles and COC recovery rate (P < 0.04). None of the oocytes recovered from any of these experiments reached metaphase II stage after maturation in vitro. In summary, culture medium, serum type, and FSH concentration in the medium interacted to affect follicular growth and antrum formation in vitro. These results suggest that a longer term culture of preantral follicles (>4 days) may be needed to produce oocytes capable of undergoing meiosis in vitro.

Effect of GnRH and its antagonist (Antarelix) on LH release from cultured bovine anterior pituitary cells

In the following investigations, the LH secretion of cells from pituitaries in heifers on days 16-18 of their oestrous cycle (n = 14) was analysed. Cells were dissociated with trypsin and collagenase and maintained in a static culture system. For the estimation of LH release, the cells were incubated with various concentrations of mammalian GnRH (Lutrelef) for 6 h. To determine the action of Antarelix (GnRH antagonist), the cells were preincubated for 1 h with concentrations of 10(-5) or 10(-4) M Antarelix followed by 10(-6) M GnRH coincubation for a further 6 h. At the end of each incubation, the medium was collected for LH analysis. Parallel, intracellular LH was qualitatively detected by immunocytochemistry. Changes in the intensity of LH staining within the cells in dependence of different GnRH concentrations were not observed, but a significant increase LH secretion in pituitary cells was measured at 10(-6) M GnRH. Antarelix had no effect on basal LH secretion at concentrations of 10(-4) and 10(-5) M. After coincubation of pituitary cells with Antarelix and GnRH, Antarelix blocked the GnRH-stimulated LH secretion with a maximal effect of 10(-4) M, but the staining of immunoreactive intracellular LH was detected at approximately the same level compared to the pituitary cells treated with exogenous GnRH alone. These data demonstrate that Antarelix is effective in influencing the GnRH-stimulated LH secretion of pituitary cells in vitro. After administration of Antarelix in vivo, the GnRH-stimulated LH secretion of cultured pituitary cells was not inhibited.

Alteration of gonadotrophin and steroid hormone release, and of ovarian function by a GnRH antagonist in gilts

This study examined the impact of the gonadotrophin-releasing hormone (GnRH) antagonist Antarelix on LH, FSH, ovarian steroid hormone secretion, follicular development and pituitary response to LHRH in cycling gilts. Oestrous cycle of 24 Landrace gilts was synchronised with Regumate (for 15 days) followed by 800 IU PMSG 24h later. In experiment 1, Antarelix (n=6 gilts) was injected i.v. (0.5mg per injection) twice daily on four consecutive days from day 3 to 6 (day 0=last day of Regumate feeding). Control gilts (n=6) received saline. Blood was sampled daily, and every 20 min for 6h on days 2, 4, 6, 8 and 10. In experiment 2, gilts (n=12) were assigned to the following treatments: Antarelix; Antarelix + 50 microg LHRH on day 4; Antarelix + 150 microg LHRH on day 4 or control, 50 microg LHRH only on day 4. Blood samples were collected daily and every 20 min for 6h on days 2, 4 and 6 to assess LH pulsatility. Ovarian follicular development was evaluated at slaughter. Antarelix suppressed (P<0.05) serum LH concentrations. The amount of LH released on days 4-9 (experiment 1) was 8.80 versus 36.54 ngml(-1) (S.E.M.=6.54). The pattern of FSH, and the preovulatory oestradiol rise was not affected by GnRH antagonist. Suppression of LH resulted in a failure (P<0.05) of postovulatory progesterone secretion. Exogenous LHRH (experiment 2) induced a preovulatory-like LH peak, however in Antarelix treated gilts the LH surge started earlier and its duration was less compared to controls (P<0.01). Furthermore, the amount of LH released from day 4 to 5 was lower (P<0.01) in Antarelix, Antarelix + 50 and Antarelix + 150 treated animals compared to controls. No differences were estimated in the number of LH pulses between days and treatment. Pulsatile FSH was not affected by treatment. Mean basal LH levels were lower (P<0.05) after antagonist treatment compared to controls. Antarelix blocked the preovulatory LH surge and ovulation, but the effects of Antarelix were reduced by exogenous LHRH treatment. The development of follicles larger than 4mm was suppressed (P<0.05) by antagonist treatment. In conclusion, Antarelix treatment during the follicular phase blocked preovulatory LH surge, while FSH and oestradiol secretion were not affected. Antarelix failed to alter pulsatile LH and FSH secretor or pituitary responsiveness to LHRH during the preovulatory period.

Regulation of ovarian follicular dynamics in farm animals. Implications for manipulation of reproduction

In this review, the main features of folliculogenesis are summarized and compared among species. In the past few years, ultrasonography has clarified follicle growth patterns, and our understanding of follicle maturation has improved considerably. As the follicles develop towards the ovulatory stage, three features appear to be highly conserved across all species: 1) the sequence of events (recruitment, selection and dominance); 2) the sequential need for gonadotropins (FSH for recruitment, LH for dominance) and 3) the large variability of numerical parameters (number of waves per cycle, number of follicles per wave) as well as temporal requirements (time of selection, duration of dominance). In addition, specific follicles may also have variable gonadotropin requirements (thresholds). When patterns of follicle development at different physiological states are compared across species, follicular waves were detected in cattle, sheep and horses and during the prepubertal period in swine, suggesting that ovaries of all species operate on a wave basis unless they are prevented from doing so. Efficient estrus control treatments should have the ability to affect 1) the wave pattern by preventing the development of persistent dominant follicles containing aging oocytes, and 2) the recruitment of the future ovulatory follicle whatever the stage of the wave at the time of treatment. This would allow synchronous ovulation of a growing dominant follicle. Manipulation of the luteal phase follicular waves after mating or AI may also optimize fertility. Superovulation is still an efficient technique to obtain progeny from genetically valuable females. Administration of exogenous gonadotropins acts to reveal the underlying ovarian variability. Ovarian response of each female depends on the number of gonado-sensitive follicles present at the time when treatment is initiated. Identification of the number of such follicles for each female would improve efficacy of superovulation, by allocating potential nonresponders to other techniques (OPU/FIV). One of the main components of the within female response to superovulation is the stage of the wave when gonadotropins are injected. Treatment in the absence of a dominant follicle ensures a response close to the female's specific maximum. The development of practical approaches to achieve this still requires further research.

Effect of boar contact on follicular development and on estrus expression after weaning in primiparous sows

Boar contact can induce ovarian activity, advance estrus and stimulate estrous behavior in sows. High amounts of boar contact can, however, suppress estrous behaviour. The present study with primiparous sows was designed to compare sows that had contact with a teaser boar during detection of estrus, with sows that had no boar contact at all. Number of sows detected in estrus within 9 d after weaning, onset and duration of estrus, follicular dynamics and timing of ovulation were studied. Boar contact increased the number of sows that ovulated and showed estrus from 14 of 47 to 24 of 47 (P < 0.05). Average timing of ovulation was later for sows with boar contact (165 h vs 150 h after weaning). Duration of estrus, detected without a boar, was similar in the two groups. For the sows with boar contact, duration of estrus detected with a boar was longer than estrus detected without a boar (56 vs 38 h; P < .01). Follicular dynamics were not affected by boar contact; boar contact only increased the number of sows with ovulation. Ovulatory sows showed a larger increase in follicular diameter (P < 0.01) from weaning to Day 4 after weaning (from 2.3 to 5.4 mm) than anovulatory sows (from 2.5 to 4 mm). Anovulatory sows did not show follicular growth after Day 4. It is concluded that boar contact can increase the number of sows that ovulate and show estrus after weaning. Estrous behavior does not seem to be suppressed by contact with a teaser boar, compared to sows without boar contact.

Growth and the initiation of steroidogenesis in porcine follicles are associated with unique patterns of gene expression for individual componentsof the ovarian insulin-like growth factor system

Ovarian follicular growth and steroidogenesis are controlled by the interaction of insulin-like growth factors (IGFs) and gonadotropins. The objective was to determine the temporal and spatial relationships for gonadotropin receptor, steroidogenic enzyme, and IGF system gene expression during the development of preovulatory porcine follicles. Sows (n = 18) were weaned and follicles were monitored by transrectal ultrasonography. Ovaries were collected from sows when the mean diameter of the preovulatory follicular cohort was approximately 2, 4, 6, or 8 mm. mRNA were measured by in situ hybridization for individual follicles within the preovulatory cohort (3 to 5 follicles per sow). Patterns of gene expression detected by in situ hybridization were confirmed by RNase protection analyses of pooled RNA samples. The amount of LH receptor mRNA and steroidogenic enzyme mRNA (17alpha-hydroxylase and aromatase) increased as the mean diameter of the follicular cohort increased from 2 to 6 mm, but then decreased abruptly for 8-mm follicles. Estradiol concentrations in follicular fluid closely followed the expression patterns of steroidogenic enzymes and LH receptor mRNA. FSH receptor mRNA was present in cohorts of 2-mm follicles but declined in 4-mm follicles and was undetectable in 6- and 8-mm follicles. The expression of IGF-I and type I IGF receptor mRNA were similar for follicles of 2, 4, 6, and 8 mm. In contrast, IGF-II mRNA progressively increased in follicles collected from 2-, 4-, and 6-mm cohorts, and then decreased slightly at 8 mm. Type II IGF receptor mRNA was greatest in 8-mm follicles. IGF binding protein-2 (BP-2) mRNA decreased as follicles achieved progressively larger sizes during the preovulatory period (2 to 8 mm), whereas the IGFBP-4 mRNA remained relatively low for follicles in 2- to 6-mm cohorts but then increased markedly in 8-mm follicles. In summary, temporal and spatial patterns of gene expression for gonadotropin receptor, steroidogenic enzyme, and IGF system genes were characterized in preovulatory porcine follicles by using in situ hybridization and RNase protection analyses. The unique patterns of gene expression suggest interdependence among specific genes that may be essential for preovulatory follicular development.

Suppression of ovarian activity in the gilt and reversal by exogenous gonadotrophin administration

The aim of the current experiment was to study the regulation of follicle development in the pig using a potent GnRH agonist (GnRH-A) to initially suppress follicle development. Large-White hybrid gilts (n = 8) were treated during the luteal phase with GnRH-A. Four of these GnRH-A treated gilts and four control gilts were given a GnRH bolus on days 14 and 28 after GnRH-A administration or during the luteal phase in control gilts. Blood samples were collected for 10 h for FSH and LH, after which 1500 IU PMSG were administered and the ovaries and uteri recovered 72 h later. A further four GnRH-A treated gilts and four control gilts were slaughtered either 28 days after GnRH-A administration or during the luteal phase respectively, and all follicles > or = 1 mm diameter were dissected. The mean basal plasma FSH level was lower (P < 0.01) in GnRH-A treated than control gilts and showed no response to the GnRH challenge although levels increased (P < 0.01) in control gilts. The mean basal plasma LH levels were similar (P > 0.1) in GnRH-A treated and control gilts. Whilst in GnRH-A treated gilts plasma LH levels showed no response to the GnRH challenge, plasma LH levels were increased (P < 0.01) in control gilts. Pulsatile LH secretion was abolished in GnRH-A treated but not in control gilts. Plasma oestradiol levels were lower (P < 0.001) in GnRH-A treated gilts than in control gilts, but nevertheless both GnRH-A treated and control gilts responded to PMSG with increased plasma oestradiol levels. Treatment with GnRH-A reduced both the mean (2.1 vs. 2.7 mm; P < 0.01) and the maximal follicle diameter (4 vs. 6 mm) and reduced (P < 0.01) the total number of follicles > or = 2 mm diameter compared with control gilts. Administration of PMSG increased both mean follicle diameter (5.1 vs. 4.4 mm; P < 0.01) and maximal follicle diameter (7 vs. 9 mm) and caused a reduction (P < 0.001) in the total number of follicles > or = 2 mm diameter in both GnRH-A treated and control gilts. In summary, this study has demonstrated, for the first time in the pig, that the inhibition of follicle development as a result of pituitary down regulation/desensitisation can be reversed by exogenous gonadotrophin treatment. This model will be a powerful tool with which to investigate the precise regulation of follicle development in the pig.