Gonadotropin-Releasing Hormone (GnRH) is a neuropeptide hormone produced in the hypothalamus of horses. It is involved in the regulation of reproductive processes by stimulating the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the anterior pituitary gland. These hormones are key regulators of gonadal function, influencing processes such as estrus, ovulation, and spermatogenesis. The study of GnRH in horses includes understanding its physiological roles, mechanisms of action, and its application in managing reproductive health. This page compiles peer-reviewed research studies and scholarly articles that explore the function, regulation, and clinical applications of Gonadotropin-Releasing Hormone in equine reproduction.
Gentry LR, Thompson DL, Gentry GT, Davis KA, Godke RA.Mares that had previously been fed to attain body condition scores (BCS) of 7.5 to 8.5 (high) or 3.0 to 3.5 (low) were used to determine the interaction of BCS with the responses to 1) administration of equine somatotropin (eST) daily for 14 d beginning January 20 followed by administration of GnRH analog (GnRHa) daily for 21 d and 2) 4-d treatment with dexamethasone later in the spring when mares in low BCS had begun to ovulate. The majority of mares with high BCS continued to cycle throughout the winter, as evidenced by larger ovaries (P < 0.002), more corpora lutea (P < 0.05), greater proge...
Johnson CA, Thompson DL, Cartmill JA.The present experiment characterized the pituitary responsiveness to exogenous GnRH in the first 10 d after ovulation following commercially available deslorelin acetate implantation at the normal dosage for hastening ovulation in mares. Twelve mature, cyclic mares were assessed daily for estrus and three times weekly for ovarian activity starting May 1. Mares achieving a follicle at least 25 mm in diameter or showing signs of estrus were checked daily thereafter for ovarian characteristics. When a follicle >30 mm was detected, mares were administered either a single deslorelin acetate impl...
Guillaume D, Bruneau B, Briant C.The effects of two GnRH antagonists were tested in order to delay and/or synchronise ovulation in mares. Five mares received Antarelix (0.01 mg.kg(-1)), 5 mares received Cetrorelix (the same dose), 5 mares (control mares) received the vehicle intravenously, twice daily, for 8 days from the day the largest follicle reached 22 mm following prostaglandin administration. Ovulation was postponed in all mares injected with Antarelix (19.4 +/- 1.2 days after the beginning of the treatment) and in 2/5 mares injected with Cetrorelix (20 +/- 1 days) vs. 6.2 +/- 0.4 days in control mares. During the trea...
Evans MJ, Kitson NE, Alexander SL, Irvine CH, Turner JE, Perkins NR, Livesey JH.We wish to use a gonadotrophin-releasing hormone (GnRH) antagonist in the mare as a tool for investigating the control of the oestrous cycle. The aim of this study was to test the effectiveness of the antagonist cetrorelix by testing both in vitro, using perifused equine anterior pituitary cells, and in vivo in seasonally acyclic mares. Pituitary cells were prepared and after 3-4 days incubation, loaded onto columns and given four pulses of GnRH (at 0, 30, 60 and 90 min; dose-response study). After the second GnRH pulse, infusion of cetrorelix began (0, 100, 1000 and 2000 pmol/l) and continued...
McCue PM, Farquhar VJ, Carnevale EM, Squires EL.Deslorelin implants, approved for use in inducing ovulation in mares, have been associated with prolonged interovulatory intervals in some mares. Administration of prostaglandins in the diestrous period, following a deslorelin-induced ovulation, has been reported to increase the incidence of delayed ovulations. The goals of the present study were: (1) to determine the percentage of mares given deslorelin that experience delayed ovulations with or without subsequent prostaglandin treatment, and (2) to determine if removal of the implant 48 h after administration would effect the interval to sub...
Blanchar TL, Brinsko SP, Rigby SL.A tendency for deslorelin implants to suppress subsequent follicular growth and delay return to estrus following induced ovulation has been documented in nonlactating mares. To investigate this phenomenon in lactating mares, 22 broodmares in southeast Texas were administered either deslorelin or hCG to induce ovulation in the first postpartum estrus during February and March 2001. Mares were teased daily and examined twice weekly (Tuesdays and Thursdays) by transrectal ultrasonography. When a follicle >35 mm diameter was detected on Tuesday, mares were treated with either 2,500 U hCG admini...
Scoggin CF, Meira C, McCue PM, Carnevale EM, Nett TM, Squires EL.Equine pituitary extract (EPE) has been reported to induce heightened follicular development in mares, but the response is inconsistent and lower than results obtained in ruminants undergoing standard superovulatory protocols. Three separate experiments were conducted to improve the ovarian response to EPE by evaluating: (1) effect of increasing the frequency or dose of EPE treatment; (2) use of a potent gonadotropin-releasing hormone agonist (GnRH-a) prior to EPE stimulation; (3) administration of EPE twice daily in successively decreasing doses. In the first experiment, 50 mares were randoml...
Chopin JB, Chopin LK, Knott LM, de Kretser DM, Dowsett KF.An 8-year-old mare, with a foal at foot, was inseminated on foal heat with frozen semen, with the resultant pregnancy lost between days 34 and 41. The right ovary developed a large anovulatory follicle that was non-responsive to multiple doses of ovulating agents. The follicle eventually appeared to luteinise, although plasma progesterone concentrations did not reflect this. Another follicle developed, responded to GnRH and resulted in a pregnancy from frozen semen that went to term with a healthy foal. When the mare was examined after foaling, the structure on the right ovary appeared to be a...
Farquhar VJ, McCue PM, Carnevale EM, Nett TM, Squires EL.Following induction of ovulation with deslorelin acetate (Ovuplant), gonadotrophin concentrations are reduced in the subsequent cycle, leading to increased interovulatory intervals in some mares. This study determined whether implant removal after 2 days prevented the decrease in gonadotrophin concentrations and follicular growth during the ensuing cycle. Twenty-four mares were randomised equally into 3 groups. Group 1 ovulated spontaneously, Groups 2 and 3 received the deslorelin implant to induce ovulation. Two days after treatment, the implant was removed from Group 3. On Day 10 postovulati...
Call GB, Wolfe MW.Activation of the luteinizing hormone beta (LHbeta) promoter by gonadotropin-releasing hormone (GnRH) via the transcription factor early growth response protein-1 (Egr1) has been well characterized. To determine the mechanisms affecting Egr1 regulation of LHbeta, we analyzed five different species of LHbeta promoters (equine, mouse, rat, bovine and human). Electrophoretic mobility shift assays (EMSAs) identified multiple transcription factors binding to the Egr regions on the LHbeta promoter. Species-specific differences existed in the binding affinity for Sp1, Sp3, steroidogenic factor-1 (SF-...
Dalin AM, Andresen O, Malmgren L.The aim of the present study was to investigate the effect of active immunization against GnRH in mature Standardbred mares (three experimental and one control mare) on antibody titres, ovarian function, hormonal levels and oestrous behaviour. The mares were individually teased with a stallion once each day. During the first part of the experiment (period I: late April until November), blood was sampled every third day during the first 3 months, thereafter once per week. In the second part of the experiment (period II: December until August), sampling was carried out every second week. Progest...
Donadeu FX, Thompson DL.Sixteen seasonally anovulatory mares were randomly allotted to two groups and injected daily with either sulpiride (1 mg/kg body weight) or vehicle from 14 January to 14 February. Sulpiride administration increased daily plasma prolactin concentrations (P < 0.05), although the prolactin response during the 6 h following sulpiride injections decreased markedly from the 1st to the 6th day of treatment (treatment by day, P < 0.0001). Plasma concentrations of LH and FSH were not affected by treatment (P > 0.1). Injection of GnRH and TRH on 15 February showed that the response of plasma prolactin t...
Tortonese DJ, Gregory SJ, Eagle RC, Sneddon CL, Young CL, Townsend J.The intrahypophysial mechanisms involved in the control of gonadotrophin secretion remain unclear. In the horse, a divergent pattern of gonadotrophins is observed at different stages of the reproductive cycle in response to a single secretagogue (gonadotrophin-releasing hormone), and dramatic changes in fertility take place throughout the year in response to photoperiod. This species thus provides a useful model to investigate the regulation of fertility directly at the level of the hypophysis. A series of studies were undertaken to examine the cytological arrangements and heterogeneity of gon...
Kennedy SR, Thompson DL, Pruett HE, Burns PJ, Deghenghi R.A series of experiments was performed to determine the factor(s) responsible for an apparent inhibition of GH secretion in mares administered the GH secretagogue EP51389 in combination with GnRH, thyrotropin-releasing hormone (TRH), and sulpiride. Experiment 1 tested the repeatability of the original observation: 10 mares received EP51389 at 10 microg/kg BW; five received TRH (10 microg/kg BW), GnRH (1 microg/kg BW), and sulpiride (100 microg/kg BW) immediately before EP51389, and five received saline. The mixture of TRH, GnRH, and sulpiride reduced (P = 0.0034) the GH response to EP51389, con...
Gentry LR, Thompson DL, Stelzer AM.Seventeen seasonally anovulatory light horse mares were treated daily, starting January 5 (d 1), for 28 d with GnRH analog (GnRH-A; 50 ng/kg BW) and(or) thyrotropin-releasing hormone (TRH; 5 microg/kg BW) in a 2 x 2 factorial arrangement of treatments to test the hypothesis that combined treatment may stimulate follicular growth and development. Ovaries were examined via ultrasonography and jugular blood samples were collected every 3 d. Frequent blood samples were collected after treatment injections on d 1, 2, 4, 7, 11, 16, and 22; on d 29, all mares received an i.v. mixture of GnRH, TRH, su...
Hinojosa AM, Bloeser JR, Thomson SR, Watson ED.Relatively little is known about endocrine control of reproduction in the stallion, but gonadotropins are thought to be central in regulating spermatogenesis and libido. The ability to effectively antagonise GnRH, and thereby gonadotropins, is therefore important both in further investigations of hormonal control of reproduction in stallions, and for clinical applications. In the present study four pony stallions were treated with a potent GnRH antagonist, Antarelix. Their libido, seminal parameters, and hormonal profiles were compared with those recorded before administration of the antagonis...
Newcombe JR, Martinez TA, Peters AR.We conducted a series of trials over a four-year period on a total of 2,346 mares, to determine the effect of a single dose of the GnRH analog buserelin (20 to 40 microg i.m. or s.c.) on pregnancy rates when given between 8 and 12 days after service. Although there were some statistically significant improvements in pregnancy rates in individual trials, meta-analysis of the data overall showed significant improvements at all times examined, i.e. 13 to 16, 19 to 23, 28 to 31 and 38 to 42 days after service. These results indicate that treatment of mares with 20 to 40 microg buserelin between Da...
Barrier-Battut I, Le Poutre N, Trocherie E, Hecht S, Grandchamp des Raux A, Nicaise JL, Vérin X, Bertrand J, Fiéni F, Hoier R, Renault A, Egron L....Inducing ovulation in a cyclic mare is often necessary. For this purpose, hCG has been used commonly, but the response can be reduced after successive administrations. The aims of this study were to test the effectiveness of buserelin in hastening ovulation in estrus mares, and its influence on fertility; and to investigate the effect of treatment on LH secretion. Five crossover trials were designed to compare the effect of two treatments: buserelin (40 microg in 4 doses i.v. at 12 h intervals) vs placebo (Experiments 1 and 2); buserelin 40 microg (in 4 doses i.v.) vs 20 microg (Experiment 3);...
Aurich C, Aurich JE, Parvizi N.In equine species, luteinising hormone (LH) and prolactin (PRL) release are reduced throughout pregnancy but increase at foaling. The present experiments were designed to study a possible opioidergic regulation of LH and PRL release in pregnant Shetland mares (n=6). At various stages of pregnancy (days 26.4+/-0.6, 75.4+/-5.4, 171.8+/-2.4, 226.2+/-4.8, 282.7+/-3.4 and 319.8+/-2.1), mares were injected with the opioid antagonist naloxone (0.5 mg/kg body weight) and saline. The two treatments were always separated by 2 days, and mares served as their own controls. Two hours after being given nalo...
Parlevliet JM, Bevers MM, van de Broek J, Colenbrander B.Gonadotrophin-releasing hormone (GnRH) (a single intravenous injection with 0.042 mg busereline acetate) was administered to control stallions (n=5), aged stallions (n=5) and stallions with lack of libido (n=5). Jugular blood samples were taken at -10, 0, 10, 20, 40 and 80 minutes after treatment and measured for luteinizing hormone (LH) and testosterone concentrations. A single intravenous injection of hCG (3000 IE) was given 1 day later. Venous blood samples were taken at -60, 0, 15, 30, 60, 120, and 240 minutes after treatment and measured for the testosterone concentration. The experiment ...
Driancourt MA.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, numbe...
Greaves HE, Kalariotes V, Cleaver BD, Porter MB, Sharp DC.The potential involvement of ovarian factors in regulating GnRH and LH postovulation was studied in ovarian intact (Group 1; n=3) and ovariectomized (OVX; Group 2; n=3) mares (OVX within 12 hr of ovulation). Blood samples were collected every 10 min for 6 hr from jugular vein (JV) and intercavernous sinus (ICS) during estrus and on Day 8 postovulation for LH and GnRH analysis. Additionally, JV samples were collected twice daily (12-hr intervals) for 30 days for LH and progesterone (P4) analysis. A significant treatment x day effect (P<0.0001) describes declining plasma LH concentrations in ...
Farquhar VJ, McCue PM, Nett TM, Squires EL.To evaluate gonadotropin secretion and ovarian function after administration of deslorelin acetate to induce ovulation in mares. Methods: Randomized controlled trial. Methods: 16 healthy mares with normal estrous cycles. Methods: 8 control mares were allowed to ovulate spontaneously, whereas 8 study mares received deslorelin to induce ovulation when an ovarian follicle > 35 mm in diameter was detected. Follicle development and serum concentrations of gonadotropins were monitored daily during 1 estrous cycle. Pituitary responsiveness to administration of gonadotropin-releasing hormone (GnRH)...
Malmgren L, Andresen O, Dalin AM.The aim of this study was to investigate the effect of gonadotrophin-releasing hormone (GnRH) immunisation on mature stallions that had been used for breeding. Four Standardbred stallions were used in the study: 3 experimental animals and 1 control animal. Semen was collected regularly, i.e. twice/week, during the 4 months prior to the experimental period. The stallions were immunised against GnRH with a GnRH-BSA conjugate. Equimune was used as the adjuvant. The stallions were immunised on 5 occasions, 4 at 2 week intervals, and the fifth 4 weeks after the fourth. Blood samples were taken once...
Carnevale EM, Squires EL, Maclellan LJ, Alvarenga MA, Scott TJ.In some mares with lesions of the reproductive tract, embryo collection and survival rates are low, or collection of embryos is not feasible. For these mares, oocyte transfer has been proposed as a method to induce pregnancies. In this report, a method for oocyte transfer in mares and results of oocyte transfer performed over 2 breeding seasons, using mares with long histories of subfertility and various reproductive lesions, are described. Human chorionic gonadotropin or an implant containing a gonadotropin-releasing hormone analog was used to initiate follicular and oocyte maturation. Oocyte...
Watson ED, Pedersen HG, Thomson SR, Fraser HM.Control of the equine estrous cycle was studied by suppressing gonadotropin secretion by administration of a GnRH antagonist to cyclic pony mares. Four mares received vehicle (control cycle) or a GnRH antagonist, Antarelix (100 microg/kg) on Day 8 of diestrus, and blood samples were collected at 15-min intervals from 0 to 16 h, 24 to 36 h, and daily until the next ovulation. Ovarian activity was monitored by transrectal ultrasonography, and measurement of plasma concentrations of progesterone and estradiol. Antagonist treatment eliminated large diestrous pulses of LH. Progesterone concentratio...
Gerlach T, Aurich JE.This review considers seasonal reproduction in male animals with emphasis on the stallion, ram and hamster. The pineal hormone melatonin is the common link between photoperiod and reproduction. An increase in the daily diurnal period of melatonin secretion is associated with a decrease in GnRH release in long-day breeders, but an increase in GnRH release in short-day breeders. Melatonin influences GnRH release within or close to the mediobasal hypothalamus in rams; whereas melatonin receptors have not been found in the hypothalamus of horses. Prolactin release is positively correlated with day...
Roser JF, Tarleton M, Belanger JM.Recent studies in our laboratory investigating plasma gonadotrophin concentrations before and after castration indicate that the hypothalamic-pituitary axis is not the original site of dysfunction in stallions with idiopathic infertility. In the present study, fertile, subfertile and infertile stallions were subjected to replacement therapy with oestradiol and testosterone 1 year after castration to investigate hypothalamic-pituitary axis function further. Fertile (n=5), subfertile (n=3) and infertile (n=2) castrated stallions of Light horse breeds, aged 11-23 years, were given either oestradi...
Daels PF, Fatone S, Hansen BS, Concannon PW.The effect of the dopamine antagonist sulpiride on FSH secretion and onset of reproductive activity in anoestrous mares under different environmental conditions was investigated. In Expt 1, sulpiride (0.5 mg (-)-sulpiride kg(-1) twice a day) had no affect on FSH pulse frequency, mean FSH concentration, basal FSH concentration or FSH pulse amplitude in anoestrous mares. These data do not support the hypothesis that dopamine inhibits reproductive activity by suppressing GnRH secretion, as it does in other species. In Expt 2, the interval to first ovulation (14.8 +/- 1.9 days; range 12-22 days) i...
Greaves HE, Porter MB, Sharp DC.Long-term ovariectomized Pony mares were treated with oestradiol (0.2-5.0 mg; i.m.) at 12 h intervals for 10 days. Blood samples were collected by jugular venepuncture three times a day throughout the experiment and additional blood samples were collected at 15 min intervals for 12 h on days 0 and 10 (sampling periods 1 and 2, respectively). There were significant effects of oestradiol treatment (P < 0.05) and oestradiol treatment x day (P < 0.0001) on the mean LH concentrations each day. Regression analysis of LH time trends each day indicates that there is heterogeneity (P < 0.001) ...
Daels PF, Fatone S, Hansen BS, Concannon PW.The effect of the dopamine antagonist sulpiride on FSH secretion and onset of reproductive activity in anoestrous mares under different environmental conditions was investigated. In Expt 1, sulpiride (0.5 mg (-)-sulpiride kg(-1) twice a day) had no affect on FSH pulse frequency, mean FSH concentration, basal FSH concentration or FSH pulse amplitude in anoestrous mares. These data do not support the hypothesis that dopamine inhibits reproductive activity by suppressing GnRH secretion, as it does in other species. In Expt 2, the interval to first ovulation (14.8 +/- 1.9 days; range 12-22 days) i...
Sieme H, Troedsson MH, Weinrich S, Klug E.Twelve fertile stallions were divided into two groups, either receiving gonadotropin-releasing hormone (GnRH) (n = 6) or Placebo (n = 6). Based on the history of frozen/thawed semen characteristics three stallions within each group were assigned as being "good freezers" [GnRH (+); Placebo (+)] and three stallions were assigned as being "poor freezers" [GnRH (-); Placebo (-)]. The study was performed as a "blinded" investigation and stallions were treated twice daily by an intramuscular injection of 1 ml GnRH (Buserelin), 50 microg) or Placebo. The experiment was divided into three time periods...
Fanelli D, Tesi M, Rota A, Beltramo M, Conte G, Giorgi M, Barsotti G, Camillo F, Panzani D.Human Chorionic Gonadotropin (hCG) and Gonadotropin Releasing Hormone agonists (GnRHa) are routinely used to induce ovulation in mares. However, GnRHa efficacy in transitional mares has been suggested to be low. Objective: The aims of this study were as follows: (a) to compare the efficacy of hCG and GnRHa in inducing the first ovulation of the breeding season and (b) to evaluate the correlation between ovulatory response, uterine oedema and teasing score at the time of treatment during the early or late transitional phase. Methods: Randomised controlled superiority trial. Methods: Mares in wi...
Cleaver BD, Grubaugh WR, Davis SD, Sheerin PC, Franklin KJ, Sharp DC.Melatonin is thought to play a role in relaying photic information to the central nervous system as part of the seasonal reproductive cycle of the mare. However, the mechanisms by which melatonin may act are unknown. Therefore, this study was designed to determine whether exposure to constant light would, by reducing circulating melatonin concentrations, have any effect on hypothalamic gonadotrophin-releasing hormone (GnRH) content and circulating levels of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Blood samples were collected for 12 h at 15-min intervals from 8 ovariect...
Briant C, Ottogalli M, Morel M, Guillaume D.The GnRH antagonist antarelix (Teverelix) was administered to mares (0.01 mg/kg, i.v., twice a day) during the periovulatory period. In Experiment 1, 20 mares were divided into a treated (A3d-) and a control (Control-) group. A3d- mares received antarelix for 3 days from the day when the dominant follicle (F1) reached 32 mm (D0). In Experiment 2, 10 mares were divided into a treated (A6d+) and a control (Control+) group. A6d+ mares received antarelix for 6 days from D0 and hCG was injected in all animals (1600 IU, i.v.) on D1. Pregnancies were determined 13 days after ovulation. In both experi...
Aurich C, Kaps M.The behaviour of mares is often detrimental to their performance resulting in frequent demand for methods to suppress gonadal function. In addition, prevention of unintended reproduction especially in feral horse populations may require methods for suppression of gonadal function. Surgical ovariectomy is a safe method but not an acceptable approach in feral mares and undesired in mares where future breeding is considered. There are different approaches for artificial prolongation of the luteal phase resulting in transient inhibition of oestrus and ovulation. Among those, treatment with natural...
Joonè CJ, Schulman ML, Fosgate GT, Claes ANJ, Gupta SK, Botha AE, Human A, Bertschinger HJ.Circulating anti-Müllerian hormone concentration (AMH) is positively correlated to the number of small growing follicles in the mare and may reflect ovarian function. Dynamics of AMH during immunocontraception have not previously been investigated. This study aimed to compare serum AMH in mares following treatment with native porcine zona pellucida (pZP), recombinant pZP3 and pZP4 (reZP) or gonadotrophin releasing hormone (GnRH) vaccines, and saline-treated controls. Stored sera collected during two previous studies examining ovarian activity in mares during zona pellucida (ZP) or GnRH immuno...
Fitzgerald BP, Davison LA.This study investigated the hypothesis that for a subpopulation of horse mares continuation of estrous cycles during the nonbreeding season may be attributed to continued stimulatory glutamatergic activity on GnRH-secreting neurons. The gonadotropin response to the glutamatergic agonist N-methyl-DL-aspartic acid (NMA) was compared in cycling and anestrous mares during the nonbreeding season. It was anticipated that the gonadotropin response to NMA in cycling mares would be attenuated, compared with that of anestrous mares. The experiment used 16 anestrous mares and 15 mares that cycled during ...
Farquhar VJ, McCue PM, Nett TM, Squires EL.To evaluate gonadotropin secretion and ovarian function after administration of deslorelin acetate to induce ovulation in mares. Methods: Randomized controlled trial. Methods: 16 healthy mares with normal estrous cycles. Methods: 8 control mares were allowed to ovulate spontaneously, whereas 8 study mares received deslorelin to induce ovulation when an ovarian follicle > 35 mm in diameter was detected. Follicle development and serum concentrations of gonadotropins were monitored daily during 1 estrous cycle. Pituitary responsiveness to administration of gonadotropin-releasing hormone (GnRH)...
Guillaume D, Bruneau B, Briant C.The effects of two GnRH antagonists were tested in order to delay and/or synchronise ovulation in mares. Five mares received Antarelix (0.01 mg.kg(-1)), 5 mares received Cetrorelix (the same dose), 5 mares (control mares) received the vehicle intravenously, twice daily, for 8 days from the day the largest follicle reached 22 mm following prostaglandin administration. Ovulation was postponed in all mares injected with Antarelix (19.4 +/- 1.2 days after the beginning of the treatment) and in 2/5 mares injected with Cetrorelix (20 +/- 1 days) vs. 6.2 +/- 0.4 days in control mares. During the trea...
Irvine CH, Alexander SL.To study the mechanisms responsible for the unusually slow decline of the ovulatory LH surge in mares, secretion patterns of GnRH, LH and FSH were monitored in pituitary venous blood collected every 2 or 5 min for 10.5-18.0 h from five mares on the third (n = 4) or fifth day after ovulation (first sampling period). To determine the effectiveness of progesterone negative feedback, mares were then given a luteolytic dose of a prostaglandin analogue (PGF2 alpha) and pituitary venous sampling (every 2 or 5 min for 16 h) recommenced 20-22 h later (second sampling period). During the declining arm o...
Dowsett KF, Knott LM, Tshewang U, Jackson AE, Bodero DA, Trigg TE.To investigate the effect of two dose rates (200 and 400 ng) of a gonadotrophin releasing hormone (GnRH) vaccine on testicular function. Methods: A vaccination dose rate experiment. Methods: Two injections were administered 4 weeks apart to six colts in each treatment group. To maintain immunosuppression until the end of the breeding season, a third injection was given if antibody titres fell below 1000. Results: Effective antibody titres were present for 12 to 27 weeks. Testosterone concentrations decreased from 2.22 to 0.31 nmol/L 6 weeks after primary vaccination. Androstenedione concentrat...
Kaps M, Okada CTC, Gautier C, Aurich J, Scarlet D, Kuhl J, Aurich C.Behavior during the estrous cycle of mares can affect their performance and therefore inhibition of cyclical ovarian activity is indicated. We hypothesized that implants containing the GnRH analog deslorelin downregulate GnRH receptors and inhibit ovulation in mares. The estrous cycles of Shetland mares were synchronized with 2 injections of a PGF analog. One day after the second injection (day 0), mares received 9.4 (group D1, n = 6) and 4.7 mg deslorelin (D2, n = 5) as slow-release implants or 1.25 mg short-acting deslorelin as a control (C, n = 5). Ultrasonography of the reproductive trac...
Ware VC.In an attempt to understand more fully processes that control the selection or recruitment of follicles for ovulation, the superovulation paradigm in combination with the androgen, 5 alpha-dihydrotestosterone, or the antiandrogens, cyproterone or cyproterone acetate, was used in the immature mouse to alter the developmental potential of follicles destined to ovulate or to become atretic. Quantitative analysis of the numbers of eggs ovulated after one or more rounds of stimulation by pregnant mare's serum gonadotrophin followed by human chorionic gonadotrophin (PMSG-hCG), revealed a dose-depend...
Irvine DS, Downey BR, Parker WG, Sullivan JJ.Synthetic Gn-RH, administered during oestrus, stimulates the release of pituitary LH in the cyclic mare. Duration of oestrus was significantly reduced by 1 mg Gn-RH given on Day 2 of oestrus; the time of ovulation, measured in days from the onset of oestrus, also tended to be reduced. An injection of 2 mg Gn-RH had nor further effect, but daily injections from Day 2 until ovulation significantly shortened the duration of oestrus and the time to ovulation. The profiles of LH were found to be variable from mare to mare, but in all mares, treated and control, elevation of LH was detected close to...
Mumford EL, Squires EL, Peterson KD, Nett TM, Jasko DJ.One hundred anestrous mares (early February) were injected s.c. with implants containing 0, .9, 1.8, 3.6, or 5.4 mg of a GnRH analogue (goserelin acetate) in an attempt to induce ovarian cyclicity. Follicular activity and concentrations of progesterone and LH were determined every 3 d, or daily during estrus. In treated mares that ovulated, the interval to the second ovulation of the season was compared to that for an additional group receiving 16 h/d of light beginning December 16 (positive controls). Of the mares that did not ovulate in 30 d, eight from each dose group were challenged on d 3...
Newcombe JR, Martinez TA, Peters AR.We conducted a series of trials over a four-year period on a total of 2,346 mares, to determine the effect of a single dose of the GnRH analog buserelin (20 to 40 microg i.m. or s.c.) on pregnancy rates when given between 8 and 12 days after service. Although there were some statistically significant improvements in pregnancy rates in individual trials, meta-analysis of the data overall showed significant improvements at all times examined, i.e. 13 to 16, 19 to 23, 28 to 31 and 38 to 42 days after service. These results indicate that treatment of mares with 20 to 40 microg buserelin between Da...
Thorson JF, Allen CC, Amstalden M, Williams GL.The continuous, subcutaneous infusion of native GnRH into seasonally anovulatory mares stimulates the synthesis and secretion of LH without pituitary refractoriness, offering opportunities to markedly accelerate the timing of ovulation within the operational breeding season. Herein, we tested the hypothesis that ovarian cycles induced in winter anovulatory mares using continuous administration of native GnRH for 28 days, beginning in either early February or early March (North America) would not revert to an anovulatory state after treatment withdrawal. Anovulatory mares received sham pumps (c...
Clay CM, Squires EL, Amann RP, Nett TM.Effects of season and photoperiod on the anterior pituitary gland and testes were studied by responses to exogenous GnRH. Stallions were assigned to one of three treatments: 1) control, exposed to natural day length; 2) S-L, 8 h of light and 16 h dark (8:16) for 20 wk beginning July 16, 1982 then 16:8 from December 2, 1982 until March 5, 1984; or 3) S-S, 8:16 from July 16, 1982 until March 5, 1984. Approximately every 8 wk, stallions were administered GnRH (2 micrograms/kg BW) and blood was sampled at 20-min intervals for 2 h before and 8 h after GnRH administration. Concentrations of LH, FSH ...
Hinojosa AM, Bloeser JR, Thomson SR, Watson ED.Relatively little is known about endocrine control of reproduction in the stallion, but gonadotropins are thought to be central in regulating spermatogenesis and libido. The ability to effectively antagonise GnRH, and thereby gonadotropins, is therefore important both in further investigations of hormonal control of reproduction in stallions, and for clinical applications. In the present study four pony stallions were treated with a potent GnRH antagonist, Antarelix. Their libido, seminal parameters, and hormonal profiles were compared with those recorded before administration of the antagonis...
Fitzgerald BP, Meyer SL, Affleck KJ, Silvia PJ.The potential of a gonadotropin-releasing hormone (GnRH) agonist (goserelin acetate), delivered constantly for 28 days via a subcutaneous depot, to induce ovulation in seasonally anestrous mares, was investigated. Two experiments were conducted, in which a range of doses (30 to 240 micrograms/mare/d) was examined. Mares were selected on the basis of lack of substantial follicular development (follicle diameter < 20 mm determined ultrasonically) and low serum concentrations of luteinizing hormone (LH) and progesterone. Constant administration of the GnRH agonist-induced ovulation in anestrou...
Collins SM, Zieba DA, Williams GL.We tested the hypothesis that continuous subcutaneous treatment with low-dose GnRH, administered to mares from late September/early October through March, would prevent the development of seasonal anovulation. Quarter Horse mares (n=20) were stratified by age and body condition score and assigned randomly to either a saline control (n=9) or a GnRH (n=11) treatment group. Gonadotropin-releasing hormone was delivered continuously via osmotic minipumps, with sham pumps placed in control mares. Initial pumps were inserted on Day 3 following ovulation or during the follicular phase if the next anti...
Joonè CJ, Nolan MB, Bertschinger HJ, Schulman ML.A sequence of studies is reviewed that reported the domestic horse (Equus caballus) mare as an appropriate and accessible research platform for recording clinical and laboratory data post-immunisation with anti- GnRH and -zona pellucida (ZP) immunocontraceptive vaccines. Experience with a native porcine ZP (pZP) vaccine in African elephant (Loxodonta africana) cows highlighted needs for improving vaccine formulations and more clearly defining associated ovarian effects and safety profiles. Initially, the efficacy, reversibility and safety of the GnRH vaccine Improvac® in mares was demonstrate...
Kwong GPS, Klein C.Horses are long-day breeders and commence ovarian follicular activity during the spring. Evidence suggests that there is an endogenous circannual rhythm in mares, and it is uncertain whether hormonal manipulation during or immediately following the fall transition induces follicular development. The current study was designed to test the hypothesis that both deslorelin and naltrexone induce follicular development in late fall transitioning or anestrous mares. Five of six mares treated with deslorelin, and 4 of 6 mares treated with naltrexone, developed a pre-ovulatory-sized follicle and were i...
Roser JF, Tarleton M, Belanger JM.Recent studies in our laboratory investigating plasma gonadotrophin concentrations before and after castration indicate that the hypothalamic-pituitary axis is not the original site of dysfunction in stallions with idiopathic infertility. In the present study, fertile, subfertile and infertile stallions were subjected to replacement therapy with oestradiol and testosterone 1 year after castration to investigate hypothalamic-pituitary axis function further. Fertile (n=5), subfertile (n=3) and infertile (n=2) castrated stallions of Light horse breeds, aged 11-23 years, were given either oestradi...
Townsend J, Westcott K, Tortonese DJ.In the horse, pronounced changes in fertility occur annually in response to photoperiod. However, the mechanisms regulating gonadotrophin synthesis and release in this species remain unclear. Here, we investigated the expression of gonadotrophin subunits and GnRH receptor (GnRH-R) mRNA in the pituitary glands of Thoroughbred horses during the breeding (BS) and non-breeding (NBS) season. Seasonal effects on the prevalence of gonadotrophs in the pars distalis were also examined. GnRH-R and common alpha-, LHbeta- and FSHbeta-subunit mRNA contents were determined by Northern analysis and the preva...
