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Effects of deslorelin acetate implants in horses: Single implants in stallions and steroid-treated geldings and multiple implants in mares¹

Department of Animal Science, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge 70803-4210

² Correspondence:
phone: 225-578-3445; fax: 225-578-3279; E-mail:
dthompson{at}agctr.lsu.edu.

	Abstract

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Three experiments were performed to test the following hypotheses: 1) stallions and/or progesterone-estradiol-treated geldings could serve as models for the effects of a single implant of the GnRH analog, deslorelin acetate, on LH and FSH secretion by mares; and 2) multiple implants of deslorelin acetate could be used as a means of inducing ovarian atrophy in mares for future study of the mechanisms involved in the atrophy observed in some mares after a single implant. In Exp. 1, nine light horse stallions received either a single deslorelin implant (n = 5) or a sham injection (n = 4) on d 0. In Exp. 2, 12 geldings received daily injections of progesterone on d -20 through -4, followed by twice-daily injections of estradiol on d -2 to 0. On the morning of d 0, geldings received either a single deslorelin implant (n = 6) or a sham injection (n = 6). Daily injections of progesterone were resumed on d 2 through 15. In Exp. 1, plasma LH and FSH were elevated (P < 0.05) in the treatment group relative to controls at 4, 8, and 12 h after implant insertion. In the treated stallions, FSH was decreased (P < 0.05) on d 3 to 13, and LH was decreased on d 6 to 13. In Exp. 2, plasma LH and FSH were elevated (P < 0.05) at 4, 8, and 12 h after deslorelin implant insertion. Plasma LH was suppressed (P < 0.05) below controls on d 2 to 7, 9, and 11 to 15; plasma FSH was suppressed (P < 0.05) on d 4 to 15. In Exp. 3, 21 mares were used to determine whether multiple doses of deslorelin would cause ovarian atrophy. Mares received one of three treatments: 1) sham injections; 2) three implants on the first day; or 3) one implant per day for 3 d (n = 7 per group). Treatment with multiple implants increased (P < 0.05) the interovulatory interval by 14.8 d and suppressed (P < 0.01) LH and FSH concentrations for approximately 25 d; no mare exhibited ovarian atrophy. In conclusion, after an initial short-term increase in LH and FSH secretion, deslorelin implants caused long-term suppression of both gonadotropins in stallions as well as in geldings treated with progesterone and estradiol to mimic the estrous cycle. It is likely that either of these models may be useful for further study of this suppression in horses. Although multiple implants in mares suppressed gonadotropin secretion longer than a single implant, the lack of ovarian atrophy indicates that the atrophy observed after a single implant in previous experiments was likely due to the susceptibility of individual mares.

Key Words: Analogs • Gonadotropin-Releasing Hormone • Gonadotropins • Mares • Stallions

	Introduction

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The use of deslorelin acetate implants to hasten ovulation in mares has been associated with extended interovulatory intervals (Johnson et al., 2000; Morehead et al., 2000), suppression of LH and FSH secretion for 10 d after ovulation (Johnson et al., 2000), and an insensitivity of the pituitary to exogenous GnRH (Farquhar et al., 2001; Johnson et al., 2002). Given that many of the LH and FSH responses to GnRH and to steroidal feedback in horses are similar for both sexes (Thompson et al., 1979; 1991; Reville-Moroz et al., 1984), male horses may be useful models for studying the effects of deslorelin implant insertion.

In addition to gonadotropin suppression and extended interovulatory interval after deslorelin implant insertion, a small percentage of mares exhibit ovarian atrophy following treatment (Johnson et al., 2000). This atrophy can last for months, resulting in mares being unavailable for breeding during that breeding season. During this ovarian atrophy, LH and FSH rebound and reach castrate-like levels (Johnson et al., 2000), even though the ovaries remain quiescent.

Only a small percentage of mares exhibit ovarian atrophy in response to deslorelin implantation, and it has been hypothesized (Johnson et al., 2000) that such mares are more sensitive to the effects of deslorelin than the average mare. Thus, multiple implants may induce ovarian atrophy in a greater percentage of mares. A consistent means of producing ovarian atrophy is needed for future study of the mechanism(s) responsible for its occurrence.

The experiments described herein were designed to test the hypotheses that stallions and/or progesterone-estradiol-treated geldings could serve as models for the effects of a single implant of the GnRH analog, deslorelin acetate, on LH and FSH secretion reported for mares, and that multiple implants of deslorelin acetate could be used as a means of inducing ovarian atrophy in mares.

	Materials and Methods

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Experiment 1

Nine light horse stallions between 4 and 20 yr of age were used during June and July. They were housed in individual outdoor lots and were fed a commercially available, nutritionally balanced, pelleted ration plus free-choice grass hay to maintain BCS between 5 and 7 (Henneke et al., 1983).

The day of treatment was d 0. On that day, stallions were allotted to two groups such that age and breed type were distributed approximately evenly in the groups. One group was then randomly selected to receive a single deslorelin implant (n = 5) and the other group a sham injection (n = 4). All treatments were administered s.c. in the side of the neck. Samples of jugular blood were collected daily each morning from d -5 through 13. On d 0, additional blood samples were collected at 0, 4, 8, and 12 h relative to treatment. All blood samples were collected via jugular venipuncture into heparinized tubes and the plasma was harvested via centrifugation at 1,200 x g and stored at -15°C.

Experiment 2

Twelve light horse geldings between 6 and 16 yr of age were used during May and June. They were maintained as a group on native grass pasture and had BCS of 6 to 8 (Henneke et al., 1983).

All geldings were treated with progesterone and estradiol to mimic the normal changes in these steroid hormones during the estrous cycle of mares (Ginther, 1992). Starting on d -20, all geldings received daily i.m. injections of progesterone (500 µg/kg of BW; Sigma Chemical Co., St. Louis) in corn oil to mimic the diestrous period; these injections were given in the morning for a total of 17 d (through d -4). No injections were given on d -3. On d -2, -1, and 0, all geldings received twice daily i.m. injections (morning and evening) of estradiol (17.5 µg/kg of BW; Sigma) to mimic estrus. On the morning of d 0, the geldings were randomly and equally allotted to groups and administered either a single deslorelin implant (n = 6) or a sham injection (n = 6) given s.c. in the neck. Daily (morning) injections of progesterone were again given on d 2 (125 µg/kg), 3 (250 µg/kg), and 4 through 15 (500 µg/kg). A sample of jugular blood was collected immediately before any injections on each day from d -20 through d 15; additional blood samples were collected at 0, 4, 8, and 12 h after deslorelin or sham injections on d 0.

Experiment 3

Twenty-one light horse mares with BCS of 5 to 7 (Henneke et al., 1983) were used beginning in April. Mares were maintained on native grass pastures. All mares were determined to be through the transition period if progesterone concentrations were >1 ng/mL for at least 12 d. Mares were teased daily with a vigorous stallion for signs of behavioral estrus. Ultrasound examinations were performed three times weekly for determination of follicular activity. Ultrasound exams were performed daily once a mare developed a follicle >25 mm or exhibited behavioral signs of estrus.

Upon achieving a follicle >30 mm, each mare was randomly assigned to one of three treatment groups: 1) three s.c. deslorelin implants; 2) one deslorelin implant per day for three consecutive days; or 3) a single sham injection (control). Treatment assignments and administration were performed by personnel other than those performing the data collection and were unknown to those involved in data collection. Follicular activity was assessed on a daily basis from the first treatment through two ovulations or for 30 d. If a mare had not ovulated for the second time by d 30 relative to the first ovulation, ultrasound exams were performed twice weekly until she ovulated or until 90 d after the first ovulation.

Blood samples were collected via jugular venipuncture into heparinized tubes on a daily basis beginning when a follicle >25 mm was detected or when the mare showed behavioral signs of estrus; this sampling was continued until 4 d after the second ovulation or 30 d after the first ovulation. If the mare had not ovulated by 30 d following the first ovulation, blood samples were collected twice weekly until the second ovulation or for 90 d from the first ovulation. Additional samples of blood were collected at 0, 2, 4, 8, 12, 24, 26, 28, 32, 36, 48, 50, 52, 56, and 60 h after the first treatment.

Sample and Statistical Analyses

All plasma samples were analyzed for LH and FSH with RIA previously validated for horse samples (Thompson et al., 1983a,b). Plasma samples from stallions in Exp. 1 were analyzed for testosterone, and plasma samples from mares in Exp. 3 were analyzed for progesterone, with commercially available RIA kits (Diagnostic Systems Laboratory, Webster TX). Intra- and interassay CV and assay sensitivities were 6%, 9%, and 0.2 ng/mL for LH; 7%, 11%, and 1.4 ng/mL for FSH; 5%, 8%, and 0.02 ng/mL for testosterone; and 5%, 9%, and 0.05 ng/mL for progesterone.

Data were analyzed in each experiment by the GLM procedure of SAS (SAS Inst., Inc., Cary, NC). Data from repetitive sampling over time were analyzed in a split-plot ANOVA (Gill and Hafs, 1971) for a completely random design (Steel and Torrie, 1980). The main effect of treatment was tested with the horse within treatment term; the main effect of time and its interaction with treatment were tested with the residual error term. Differences between or among groups for each time period in the treatment x time interaction(s) were assessed by the LSD test (Steel and Torrie, 1980) only when the interaction was significant (P < 0.05).

In Exp. 1, there was a large variation in gonadotropin concentrations among stallions before the onset of treatment, and in addition, their responses to treatment were multiplicative, not additive (varied proportionally with their pretreatment levels). To adjust for this, the first five data points were averaged for each stallion, and then all his subsequent data points (d 0 through 13) were expressed as a percentage of that mean (Steel and Torrie, 1980). Subsequent ANOVA, as described above, were performed on these data to determine treatment differences. In Exp. 2, gonadotropin concentrations differed (P < 0.05) between the two treatment groups during the first progesterone treatment phase (d -20 through -4), when all geldings were treated the same. To adjust for this, the first 6 d of data were averaged for each gelding and subtracted from each of his subsequent data points; the resulting residuals were analyzed in a split-plot ANOVA to determine treatment differences (Steel and Torrie, 1980).

	Results

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Experiment 1

Plasma concentrations of LH and FSH (Figure 1a,b), expressed as percentage of pretreatment, were elevated (P < 0.05) in the treated stallions at 4, 8, and 12 h after deslorelin implant administration; in addition, plasma LH concentrations remained elevated (P < 0.05) at 24 h. After that, LH concentrations decreased (P < 0.05) to below control concentrations by d 3 and remained lower (P < 0.05) through d 13. Plasma FSH concentrations in the treated stallions also decreased (P < 0.05) below those of control stallions on d 6 through 13. Plasma testosterone concentrations (Figure 1c) in the treated stallions followed a pattern similar to that of the gonadotropins in response to deslorelin implant administration: elevated (P < 0.05) above controls at 4, 8, 12, 24 and 48 h. In contrast to the gonadotropins, testosterone concentrations were lower (P < 0.05) in deslorelin-treated stallions relative to controls only on d 11 and 13.

View larger version (22K): [in this window] [in a new window]   Figure 1. Plasma concentrations of LH (a), FSH (b), and testosterone (c), expressed as a percentage of pretreatment values, in deslorelin-treated and control stallions relative to the day of treatment (d 0). The vertical bar in each graph indicates the LSD value (P < 0.05) for comparisons between groups for each time period. The pooled SEM from the analyses of variance were 14, 12, and 16% for LH, FSH, and testosterone, respectively.

On d -2, when all geldings began receiving estradiol injections, plasma LH concentrations (Figure 2a) began to rise. Above that increase, plasma LH concentrations in treated geldings increased (P < 0.05) rapidly above control concentrations at 4, 8, and 12 h after administration of the deslorelin-containing implant. Thereafter, plasma LH concentrations decreased and were below (P < 0.05) control concentrations on d 2 through 7, 9, and 11 through 15. During the mimicked diestrus (d 2 through 15), when all geldings again received progesterone, plasma LH concentrations gradually decreased in control geldings back to pre-estradiol concentrations.

View larger version (22K): [in this window] [in a new window]   Figure 2. Plasma concentrations of LH (a) and FSH (b) in deslorelin-treated and control geldings relative to the day of treatment (d 0). Due to pretreatment differences, data are expressed as the net change relative to the average of the first 6 d of data collection. The vertical bar in each graph indicates the LSD value (P < 0.05) for comparisons between groups for each time period. The pooled SEM from the analyses of variance were 0.63 and 2.3 ng/mL for LH and FSH, respectively.

Experiment 3

The interovulatory interval was extended (P = 0.0004) in both deslorelin-treated groups relative to control mares (Table 1), but the two treated groups did not differ from each other. The size of the largest follicle (Table 1) did not differ on the day of treatment, the day prior to ovulation (d -1), the day of ovulation (d 0), or on d 1 to 7 after the first ovulation. The size of the largest follicle in both groups of mares receiving deslorelin (Figure 3) was smaller (P < 0.05) than in control mares on d 8, 9, 11, and 13 through 23 after the first ovulation, but not on d 24 and 25. Mares receiving three implants at treatment also had a smaller (P < 0.05) largest follicle than control mares on d 10 and 12. The size of the largest follicle was similar (P > 0.05) for the two treated groups throughout the experiment. There was no difference (P > 0.05) among groups in the length of estrus for the first or second ovulation (Table 1).

View larger version (16K): [in this window] [in a new window]   Figure 3. The diameter of the largest follicle in mares administered either sham injections (Control), three deslorelin implants in 1 d (Des 3-1d), or a single deslorelin implant each day for three consecutive days (Des 1-3d) during the first estrus. Treatment of a given mare was started on the day she developed a follicle >30 mm. Data are expressed relative to the day of ovulation on the first estrus (d 0). The vertical bar indicates the LSD value (P < 0.05) for comparisons among groups for each time period. The pooled SEM from the analysis of variance was 2.2 mm.

View larger version (20K): [in this window] [in a new window]   Figure 4. Plasma concentrations of LH (a) and FSH (b) in mares administered either sham injections (Control), three deslorelin implants in 1 d (Des 3-1d), or a single deslorelin implant each day for three consecutive days (Des 1-3d) during the first estrus. Data are expressed relative to the day of treatment on the first estrus (d 0) for all groups. The vertical bar in each graph indicates the LSD value (P < 0.05) for comparisons among groups for each time period. The pooled SEM from the analyses of variance were 2.4 and 2.8 ng/mL for LH and FSH, respectively.

View larger version (25K): [in this window] [in a new window]   Figure 5. Plasma concentrations of LH (a) and FSH (b) in mares administered either sham injections (Control), three deslorelin implants in 1 d (Des 3-1d), or a single deslorelin implant each day for three consecutive days (Des 1-3d) during the first estrus. Data are expressed relative to the day of ovulation on the first estrus (d 0) for all groups. The vertical bar in each graph indicates the LSD value (P < 0.05) for comparisons among groups for each time period. The pooled SEM from the analyses of variance were 1.70 and 2.37 ng/mL for LH and FSH, respectively.

View larger version (21K): [in this window] [in a new window]   Figure 6. Plasma concentrations of progesterone in mares administered either sham injections (Control), three deslorelin implants in 1 d (Des 3-1d), or a single deslorelin implant each day for three consecutive days (Des 1-3d) during the first estrus. Data are expressed relative to the day of ovulation on the first estrus (d 0) for all groups. The vertical bar indicates the LSD value (P < 0.05) for comparisons among groups for each time period. The pooled SEM from the analysis of variance was 1.99 ng/mL for progesterone.

	Discussion

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In mares administered a single Ovuplant (Johnson et al., 2000), gonadotropin concentrations eventually return to normal, albeit delayed by 5 to 6 d. In addition, sensitivity to exogenous GnRH returns to normal by 10 d after Ovuplant administration (Johnson et al., 2002). Concentrations of LH and FSH were still suppressed in these stallions 13 d after implant insertion, and showed no tendency of increasing at that time. Thus, stallions may be more sensitive to the inhibitory effects of Ovuplant, and further research is needed to characterize the full recovery of the stallion pituitary after deslorelin implant insertion.

In Exp. 2, progesterone-estradiol treatment of geldings resulted in changes in gonadotropin concentrations similar to those that occur during the estrous cycle in mares. Pretreatment with progesterone allowed both gonadotropins to stabilize (LH relatively low and FSH relatively high). Cessation of progesterone treatment followed by estradiol treatment then resulted in increasing LH concentrations and decreasing FSH concentrations in both groups, similar to the follicular phase in mares (Ginther, 1992; Johnson et al., 2000) immediately before implant insertion with deslorelin. Once estradiol treatment stopped and progesterone treatment resumed, LH concentrations decreased and FSH concentrations increased in the control geldings, similar to the postovulation changes in mares (Ginther, 1992; Johnson et al., 2000).

Following insertion of the deslorelin implant on the morning of d 0, concentrations of both LH and FSH increased in a manner similar to the stallions in Exp. 1 and mares (Donadeu, 1997). After the initial increase, both gonadotropins steadily declined to below control values and remained suppressed through d 15. The changes in LH and FSH concentrations observed in these geldings, both treated and control, after implant insertion were virtually identical to the changes previously reported for deslorelin-treated and control mares (Johnson et al., 2000; 2002). Unlike mares receiving one implant (Johnson et al., 2000), concentrations of LH and FSH in these geldings showed no signs of recovery out to d 15, which again may indicate that these males are more sensitive to this dose of deslorelin than mares.

As in previous experiments (Johnson et al., 2000; Morehead et al., 2000; Vanderwall et al., 2001), administration of Ovuplant to estrous mares in Exp. 3 resulted in an extension of the interovulatory interval and a suppression of follicular activity and gonadotropin concentrations. In previous experiments (Johnson et al., 2000; 2002), a single deslorelin implant caused a consistent suppression in daily concentrations of both LH and FSH for 10 to 14 d, which was likely the cause of the prolonged interovulatory interval seen in those mares (Johnson et al., 2000; 2002). The interovulatory interval for mares receiving a total of three deslorelin implants was considerably longer than that observed in mares receiving only one implant (36.8 vs. 22 d, respectively) and coincided with a longer suppression of the gonadotropins (20 to 25 d) seen in mares receiving three implants.

The reduced size of the largest follicle in the deslorelin-treated mares on d 8 through 23 is further evidence of the suppression of the gonadotropins, although the size of the largest follicle did not differ on d 24 or 25. This latter fact is due to the control mares already having ovulated the dominant follicle of the second estrus by this time. The size of the largest follicle for the two treated groups was similar throughout the study, indicating that there was no difference in treating mares with three deslorelin implants at once or one deslorelin implant per day for three consecutive days. This was somewhat unexpected, due to the fact that the timing of the clearance of deslorelin from the blood should have differed by 2 d, and that the lingering plasma deslorelin beyond 48 h after implant insertion supposedly is responsible for its suppressive effects (McCue et al., 2002).

Although treatment with a total of three deslorelin implants resulted in a longer suppression of follicular activity and gonadotropin secretion, it did not induce long-term ovarian atrophy, as seen in mares in our previous experiment (Johnson et al., 2000). One mare, which experienced total ovarian atrophy for the entire breeding season, was followed through her subsequent breeding season. After an apparently normal first ovulation in May, treatment with a single Ovuplant again resulted in complete ovarian atrophy for the second breeding season. Thus, the question arises as to why certain mares exhibit complete ovarian atrophy in response to a single deslorelin implant whereas none of the mares in Exp. 3 shut down after three implants. It is possible that the percentage of mares that are exceptionally sensitive to deslorelin is small, and by chance none were in the treatment groups in Exp. 3. This is reassuring for horse breeders, because the chance of having a mare shut down in response to a single deslorelin implant is minimal. However, some means of detecting the sensitive mares prior to treatment with deslorelin is needed in order to avoid treating those mares.

In conclusion, after an initial short-term (24 h) stimulation of LH and FSH secretion, deslorelin implant insertion caused a long-term suppression of both gonadotropins in stallions as well as steroid-treated geldings. The suppression was very similar to that observed in deslorelin-treated mares (Johnson et al., 2000; 2002), and either of these male models may prove useful for studying the apparent down regulation of pituitary function in horses administered a slow-releasing deslorelin implant. In contrast, three implants administered to mares did not induce ovarian atrophy, regardless of the timing of administration, and may indicate that the atrophy observed after a single implant in previous experiments was due to the individual mare’s susceptibility under those conditions.

	Implications

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Hastening ovulation with deslorelin acetate implants has been associated with extended interovulatory intervals and, occasionally, complete anestrus in mares. The current research indicates that intact male horses and/or castrated male horses treated with progesterone and estradiol may be useful models for future studies on the underlying mechanisms of these effects on the hypothalamic-pituitary axis. An attempt to develop a model for studying the possible causes of the anestrus in mares (three implants rather than one) was not successful in that no mares experienced complete ovarian atrophy, even though all mares displayed a greater degree of suppression in hormonal concentrations and ovarian activity relative to mares receiving single implants in previous experiments.

	Footnotes

 
¹ Approved for publication by the Director of the Louisiana Agric. Exp. Stn. as manuscript No. 02-11-0443. We thank A. F. Parlow and the NIDDKD, National Hormone and Pituitary Program, Harbor-UCLA Medical Center, Torrance, CA, for reagents.

Received for publication October 9, 2002. Accepted for publication February 14, 2003.

	Literature Cited

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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Johnson, C. A. Articles by Cartmill, J. A. Search for Related Content PubMed PubMed Citation Articles by Johnson, C. A. Articles by Cartmill, J. A.