Gonadal and pituitary responsiveness of stallions is not down-regulated by prolonged pulsatile administration of GnRH (original) (raw)
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The effect of a potent GnRH agonist on gonadal and sexual activity in the horse
Theriogenology, 1990
A potent agonist of gonadotropin releasing hormone (GnRHa) was administered to stallions (n=3) and mares (n=4) during the physiologic breeding season in order to downregulate gonadal activity. Stallions received an increasing regimen of 12.5 mg (5 d), 25 mg (10 d), and 50 mg (15 d) GnRHa per day, for a total of 30 d. Libido and semen quality were not affected by the treatment. The only treatment effect observed was an initial rise in luteinizing hormone (LH) concentration and a slight decline in testosterone concentration during treatment in two of three stallions. These data suggest that daily GnRHa therapy is not a practical method for the suppression of sexual behavior in the stallion at the dosage and interval utilized. Mares received 25 mg i.m. GnRHa per day, for a total of 35 d (n=2), or 40 d (n=2). While estrus and ovulation occurred in two mares during the first half of treatment, follicular development was suppressed in all mares by the end of treatment. Following treatment, ovulation occurred in all mares between 12 and 16 d after treatment. While gonadal activity and estrous behavior can be suppressed in mares with GnRHa therapy, animals require up to 30 d of treatment before suppression of ovarian activity is achieved.
Reproduction, 2003
Administration of GnRH agonist for an extended period inhibits pulsatile LH release but enhances testicular function of bulls. The mechanism whereby long-term administration of GnRH agonist enhances testosterone concentration in the blood of bulls has not been determined. The aim of this study was to determine whether chronic treatment with the GnRH agonist, azagly-nafarelin, increases blood concentrations of LH and FSH in prepubertal bulls. Two different doses of the GnRH agonist were administered via Alzet mini-osmotic pumps for 28 days. Blood samples were collected at 20 min intervals for 24 h at days 2, 13 and 25 of treatment. Agonist-treated groups had reduced testosterone pulse frequency (P < 0.05) and increased mean and basal concentrations of testosterone (P < 0.05) compared with untreated control bulls. Basal LH concentrations were higher in agonist-treated bulls during all three periods (P < 0.05) and overall (1 ng ml −1 higher, compared with control bulls; P < 0.001). Frequency of LH pulses in the agonist-treated groups was reduced to less than one pulse in 24 h. Agonist-treated bulls tended to have (P < 0.10) or had (P < 0.05) a slight but significant increase in blood FSH concentration. In conclusion, the higher blood testosterone concentration in bulls after prolonged treatment with GnRH agonist may result, at least in part, from changes in the testes induced by enhanced basal concentration of LH.
Reproduction, Fertility and Development, 2007
The present study tested whether exogenous gonadotrophin-releasing hormone (GnRH) and luteinising hormone (LH) can stimulate LH and testosterone secretion in dogs chronically treated with a GnRH superagonist. Twenty male adult dogs were assigned to a completely randomised design comprising five groups of four animals. Each dog in the control group received a blank implant (placebo) and each dog in the other four groups received a 6-mg implant containing a slow-release formulation of deslorelin (d-Trp 6 -Pro 9 -des-Gly 10 -LH-releasing hormone ethylamide). The same four control dogs were used for all hormonal challenges, whereas a different deslorelin-implanted group was used for each challenge. Native GnRH (5 µg kg −1 bodyweight, i.v.) was injected on Days 15, 25, 40 and 100 after implantation, whereas bovine LH (0.5 µg kg −1 bodyweight, i.v.) was injected on Days 16, 26, 41 and 101. On all occasions after Day 25-26 postimplantation, exogenous GnRH and LH elicited higher plasma concentrations of LH and testosterone in control than deslorelin-treated animals (P < 0.05). It was concluded that, in male dogs, implantation of a GnRH superagonist desensitised the pituitary gonadotrophs to GnRH and also led to a desensitisation of the Leydig cells to LH. This explains, at least in part, the profound reduction in the production of androgen and spermatozoa in deslorelin-treated male dogs.
Journal of Animal Science, 2000
Considerable variation exists in the serum levels of gonadotropins in boars; this results in differential testicular function. Boars (Chinese Meishan, European White composite, and crosses of the two breeds) selected for high and low circulating FSH concentrations were used to define possible differences in pituitary sensitivity to GnRH and GnRH antagonist and gonadal and adrenal responses. After a 2-h pretreatment sampling period, boars were injected with GnRH or GnRH antagonist and repetitively sampled via jugular cannula for changes in serum concentrations of FSH, LH, testosterone, and cortisol. In response to varying doses of GnRH or GnRH antagonist, FSH, LH, or testosterone changes were not different in highor low-FSH boars. Declines in LH after GnRH stimulation were consistently faster in boars selected for high
The effects of growth hormone or melatonin on the reproductive axis of stallions
Two experiments were conducted to determine the effects of growth hormone (GH) or melatonin on the reproductive axis of the stallion. In Experiment 1, nine stallions were treated with GH (20 µg/kg BW) or saline for 21 d starting in January. During the last week of treatment, stallions were subjected to low and high dose injections of luteinizing hormone (LH), as well as low and high dose combined injections of gonadotropin releasing hormone (GnRH) and thyrotropin releasing hormone (TRH). Two months after the onset of GH treatment, semen was collected from all stallions every other day for 2 weeks. Treatment with recombinant equine GH increased (P < 0.001) daily IGF-I concentrations, but had no effect (P > 0.1) on concentrations of LH, follicle stimulating hormone (FSH), or testosterone. The testosterone responses to injections of LH were similar (P > 0.1) between treatments. Likewise, the LH, FSH, prolactin, and testosterone responses to the injections of GnRH/TRH were similar (P > 0.1) between groups. Stallions treated with GH exhibited greater volumes of gel-free semen (P < 0.01) and gel (P < 0.05) and had decreased time until ejaculation (P < 0.05). In Experiment 2, nine stallions were given corn syrup containing either melatonin (0.06 mg/kg BW) or nothing for 90 d starting in July. Between d 68 and 75 of treatment, stallions were given injections of LH and combined injections of GnRH and TRH, similar to Experiment I. Semen was collected from all stallions for three days during the last week of treatment. Treated stallions exhibited decreased daily concentrations of prolactin (P < 0.01) and FSH (P < 0.05), and tended to have lower (P = 0.07) LH concentrations for the first 30 d. Testosterone concentrations were similar between groups. In treated stallions, the low dose administration of GnRH/TRH was not as effective (P < 0.01) at increasing plasma concentrations of FSH and testosterone, and the response in plasma prolactin concentrations to a high dose administration of GnRH/TRH was decreased (P < 0.01). Melatonin treatment did not alter seminal characteristics or libido. In conclusion, GH may alter the long-term accessory gland contribution to seminal volume, but does not appear to interact with other constituents of the reproductive axis in the stallion. Longterm melatonin administration decreases plasma concentrations of gonadotropins and prolactin, vi but the role of melatonin in perturbation of hypothalamic interaction with the pituitary deserves further study.
Endocrine Regulation of Reproductive Function in Fertile, Subfertile and Infertile Stallions
Reproduction in Domestic Animals, 1995
The specific nature and relative contribution of the various factors involved in the endocrine/paracrine/autocrine control of reproductive function in normal stallions are not well defined nor have they been elucidated in the idiopathic subfertilelinfertile stallion. Over the last 9 years, our laboratory has been engaged in characterizing the hypothalamic-pituitary-testicular axis (HIT) in fertile, subfertile (idiopathic oligospermia) and infertile (idiopathic azw spermia) stallions. Our studes have not only identlfied endocrine factors and mechanisms important for normal reproductive function, but they have demonstrated specific hormonal alterations in pituitary and testicular function between fertile, subfertile and infertile stallions. Recent evidence suggests that the primary defect is at the level of the testes. The nature of the dysfunction does not appear to involve changes in LH receptor binding kinetics but may be related to post-receptor mechanisms.
Research in Veterinary Science, 2015
GnRH treatment has been suggested to increase testosterone levels temporarily and to stimulate libido in stallions, but its use has not fully ascertained in dromedary camels. The aim of this work was to study the effects of administering 100 µg of GnRH on testosterone profile, libido and semen parameters in dromedary camels. The same bulls were used as self-controls and experimental group. Blood samples were collected every 20 minutes (T0-T12) for 4 hours, and semen collections were performed over a 2-hour period after T12. GnRH was administered immediately after T0. In GnRH-treated bulls, testosterone levels showed an upward trend, peaking after 140 minutes, and then slowly decreasing. GnRH administration also led to a decrease in mating time and an increase in spermatozoa concentration. Overall, it seems that administration of 100 µg GnRH might increase testosterone levels temporarily and enhance camel reproduction performance.
The present study aims to investigate the Luteinizing hormone (LH), testosterone and total estrogens response to exogenous gonadotropin-releasing hormone (GnRH) in adult crossbred bulls with differing semen quality. Fourteen adult crossbred bulls of differing semen quality were selected and treated with 10 mg of GnRH (Buserelin acetate) intramuscularly. Blood samples of the bulls were collected at an interval of 30 min commencing 1 h prior to GnRH treatment until 4 h post-GnRH treatment and thereafter, at an interval of 1 h for the next 3 h. The endocrine response in terms of peak values, area under the curve, and the time taken to attain peak values for LH, testosterone, and total estrogens were evaluated in all the bulls. The mean 7 SEM peak levels of LH, testosterone, and total estrogens were found to be 150 7 24.1, 5.3 7 0.69, and 0.077 0.01 ng/mL, respectively. The mean 7 SEM area under the curve of LH, testosterone, and total estrogens were found to be 392 7 51.3, 23.57 3.4, and 0.32 7 0.04 ng/mL Â h, respectively. The relationships between hormonal responses (LH, testosterone, and total estrogens) and semen quality were analyzed using the linear regression method, which provided nonsignificant (P 40.05) results. This study indicated that the gonadal and pituitary hormonal response to single exogenous GnRH treatment may have no relationship with the semen quality of crossbred bulls.
Biology of Reproduction, 1995
The objectives of this study were to evaluate and compare endocrine profiles in fertile, subfertile, and infertile stallions and to assess differential endocrine responses to hCG administration. Fourteen stallions were allotted to one of three groups on the basis of their 3-wk semen evaluation and conception rates. Fertile stallions were characterized as having normal seminal parameters and a a 75% conception rate. Subfertile stallions were characterized as having low motility, abnormal morphology, and a £ 10% conception rate. Infertile stallions were characterized as having low motility, abnormal morphology, low sperm concentrations, and a 0% conception rate. Seven fertile, four subfertile, and three infertile stallions were treated once with sterile saline and then with 10,000 IU of hCG 14 days later. Blood samples were collected periodically before and after treatment and analyzed for LH, FSH, inhibin, estrogen conjugates-(EC), estradiol-17(S (E 2), and testosterone (T) by RIA. Mean basal plasma concentrations of LH, FSH, inhibin, and E 2 were similar between the fertile and subfertile groups, but LH and FSH values were higher (p < 0.05) and inhibin and E 2 values were lower (p < 0.05) in the infertile group. Plasma T concentrations were similar between all three groups. The infertile group had a poor T response (p < 0.05) to hCG compared to the fertile and subfertile groups. An hCG by group interaction was not observed among the other hormones. In conclusion, routine measurement of baseline concentrations of LH, FSH, E 2 , and inhibin along with an hCG stimulation test could provide valuable information for identification of area(s) of endocrine dysfunction and indicate the type of endocrine therapy possible in stallions with poor fertility. While low basal plasma concentrations of gonadotropins may reflect a dysfunctional hypothalamic-pituitary condition that may respond to GnRH therapy, a poor steroid response to hCG could reflect a dysfunctional testicular condition that may respond to steroid replacement therapy.
Journal of Reproduction and Development, 2011
The present study investigated the basal levels and GnRH-induced responses of peripheral testosterone and estrogen in Holstein bulls with poor semen quality. On the basis of semen parameters, bulls (n=5) having poor semen quality were selected as experimental bulls, and good semen quality bulls (n=4) were used as control bulls. Both groups were treated intramuscularly once with GnRH (250 μg of fertirelin acetate). Blood samples were collected at-1 day (d),-30 min and 0 h (treatment) followed by every 30 min for 5 h and 1, 3 and 5 d post-GnRH treatment (PGT), and LH, testosterone and estradiol-17β (E2) concentrations were measured. The pretreatment concentrations were used as basal levels. The percentage increments based on the 0-h levels were calculated per bull for each sampling time until 5 h PGT, and differences were compared between the experimental and control groups. The PGT concentrations of testosterone and basal and PGT concentrations of E2 were significantly lower in the experimental group. The testosterone increment in the experimental group was delayed and significantly lower from 1 to 5 h PGT than those in the control group. It can be suggested that bulls with poor semen quality have delayed and lower GnRH-induced testosterone response and may also have lower estrogen levels.