**3.6 Induction of ovulation in mares**

A reliable ovulation-inducing drug is one that can trigger ovulation within a certain "fixed" period of time. This pharmacological action can provide enormous advantages in anticipating the right time for artificial insemination. Several pharmacological agents such as GnRH and GnRH agonist, hCG, recombinant equine LH, and equine pituitary extracts, prostaglandins and kisspeptin have been used to determine their efficacy in ovulation induction [68].

#### *3.6.1 GnRH*

*Animal Reproduction in Veterinary Medicine*

they show regular post-treatment cycles [56].

**3.4 Recombinant equine FSH (reFSH) and LH (reLH)**

the start of the breeding season.

tropin (hCG) administration [60].

used for the experiment [61].

**3.5 Dopamine antagonists and prolactin**

domperidone, and perphenazine have been studied [63].

the transition period and first ovulation in mares with deep anestrous.

The administration of P4 suppresses the release of LH from the anterior pituitary

Different studies indicate that mares in deep anestrous or early transition do not anticipate the first ovulation of the year with P4 treatments [30, 55]. However, it has been shown that, if treatment is carried out at the end of the transition period and the mares have at least one follicle of more than 20 mm in diameter in the ovaries,

Intravaginal devices containing P4 (CIDR, PRID, and intravaginal sponges) have been used in mares. Indeed, Hanlon and Firth [57] examined the effect of intravaginal devices placed during 10 days in transitional Thoroughbred mares. The results of the experiment showed that the use of P4 has a positive effect in bringing forward the first estral cycle of the breeding season. Compared to control mares, in the first 21 days of the season, 95.2% treated mares were served and conceived sooner after

Regumate is the most commonly used orally administered progestogen. Its active ingredient is allyl trenbolone, also called Altrenogest. Allen et al. [55] evaluated the effect of oral P4 treatment in mares with seasonal anestrous. Within 8 days, 88% of the treated mares showed estrous behavior and within 18 days of treatment interruption, 84% had ovulated. Based on these figures, the treatment gave a positive result in the acceleration of cyclicity in mares, but its response depends on the depth of the anestrus.

The use of recombinant equine FSH (reFSH) has been reported to induce follicular growth in cyclic mares [58, 59]. A study reviewed in 2013 however determined the efficacy of it in deep anestrous mares to be very successful with ovulation rate of 76.7% in response to FSH treatment followed by human chorionic gonado-

Mares in deep anestrous treated with reFSH alone or reFSH and reLH in combination under natural photoperiod showed a significant increase in follicular development within 6 days on average and all of them ovulated within 10 days. In comparison, the control group needed a significantly longer time for follicular growth and only 30% of the control mares had ovulated at the end of the 14 days

Studies in sheep found that dopamine antagonists are effective in increasing LH secretion during estrus by inhibiting the release of dopamine in the brain [62]. In mares, the increased release of dopamine during winter anestrous has been confirmed in studies measuring a higher concentration of dopamine in the cerebrospinal fluid during deep anestrous. It has also been shown that inhibition of dopamine D2 receptors may accelerate the onset of the ovulatory season in mares. Sulpiride,

Mari et al. [64] compared the efficacy of sulpiride and domperidone, two longacting dopamine antagonists, to induce ovarian activity in mares with deep anestrous. The results showed that sulpiride administration was effective in accelerating

gland. Once P4 supplementation ceases, the so-called "rebound effect" induces follicular maturation and ovulation. Its use in equine reproduction is a common practice and the available protocols include progestogens administered orally or parenterally. However, its use in mares with seasonal anestrous is questionable.

**3.3 Progesterone and progestins**

**10**

The frequency of GnRH pulses is the main regulator of LH secretion by the adenohypophysis [69]. Because of this stimulation, they can be used as an ovulatory agent and therefore can be used to induce ovulation in mares. On the other hand, due to its natural origin, it does not cause an immune response after being administered in several sessions. There is also little risk of contamination as GnRH is a synthetic product. In the 1990s, several experiments were conducted to evaluate the efficacy of GnRH in ovulation induction in cyclic mares [70, 71]. In one of them, the effect of a single administration of 2 mg of synthetic GnRH was tested but did not affect ovulation induction. However, daily injections of the same compound from day 2 of heat to ovulation resulted in a shortening of the duration of heat and the time for ovulation [72]. Likewise, Duchamp et al. [73] conducted a study to try to identify a more suitable ovulatory agent. To do that, they compared the effect of an intramuscular injection of 2.500 i.u. hCG and 2 mg GnRH (not synthetic). The use of hCG, injected when the follicle reached 35 mm in diameter, induced ovulation in 24 or 48 h. However, GnRH was not effective in shortening ovulation time compared to the control group.

On the other hand, the pulsatile infusion of endogenous GnRH was effective in advancing ovulation time in cyclic mares [70]. Treatments with low doses of endogenous GnRH (2.5 μg) continuous infusion for 14 days demonstrated increased LH and ovulation in all treated mares compared to controls [74].

#### *3.6.2 GnRH-agonist*

#### *3.6.2.1 Deslorelin (ovuplant and other products)*

Deslorelin is a potent GnRH agonist and is marketed as a controlled-release subcutaneous implant under the trade name Ovuplant™. In the past, several authors have investigated the efficacy of Deslorelin in inducing ovulation in mares [29, 75, 76].

It has been shown that between 84 and 93% of mares ovulate after 2 or 3 days of treatment, respectively [77]. However, adverse effects have been reported for this drug. Mares treated with Ovuplant™ showed a prolonged interovulatory interval and estrual cycles of 3–7 days longer than controls [78]. In this sense, it was suggested that the GnRH agonist may cause a decrease in the regulation of the pituitary gonadotropic cells [79]. Besides, additional studies reported suppression of follicular growth and decreased FSH levels in mares treated with Ovuplant™ [80]. A study conducted by McCue et al. [81] showed that the extraction of Ovuplant™ after 48 h prevented a prolonged interovulatory interval. These authors also observed an alteration in ovulation rates. However, Ovupant™ is currently not commercially available.

A short-term release product of deslorelin was developed in a biocompatible liquid vehicle called BioRelease™ [82]. This product releases deslorelin for approximately 6–36 h. An increase in the number of ovulations within 48 h has been demonstrated (75% vs. 7% for controls). There was also no effect of fertility and the number of coverages per conception decreased in treated mares (1.6 vs. 2.9).

Subsequently, a greater number of injectable deslorelin products have been developed. Many of them are suspensions in saline or sterile water and do not contain any slow-release mechanism. McCue et al. [83] compared several deslorelin formulations and reported that all of the formulations tested in their study resulted in a shortening of the follicular phase, acceleration of ovulation and a similar response to human chorionic gonadotropin (hCG). It is important to note that these studies were conducted in the middle of the breeding season.

### *3.6.2.2 Buserelin*

Different works have also tested Buserelin for its effect of inducing ovulation in mares [84]. Treatment with 40 μg de buserelin (4 doses/12 h) caused ovulation without altering fertility in mares [84, 85]. Also, the effect of treatments with 20 μg or 13.3 μg of buserelin (4 doses/12 h; or 3 doses/6 h respectively) was comparable with treatment with 2.500 IU of hCG (iv).

However, some problems with Buserelin to induce ovulation were also reported [86]. Mares treated with 40 μg iv. of Buserelin (2 times daily), 2.500 IU of hCG (single dose iv) and 2 ml of water distilled as placebo (iv) were compared. The highest ovulation rate was found in hCG treatments where 88% of the mares ovulated between 36 and 48 h. However, Buserelin treatment caused only 22.7% ovulation within 48 h.

Buserelin has also been given during early diestrus to pregnant mares as a means of improving pregnancy rates [87, 88]. These studies used doses of 20–40 mg of Buserelin between days 8 and 12. The results showed that pregnancy rates after ovulation increased by approximately 10%. The exact mechanism of how GnRH increases pregnancy rates is unclear since P4 does not appear to be increased.

#### *3.6.2.3 Human chorionic gonadotropin*

hCG is a glycoprotein hormone and has a biological function like LH. It is composed of two subunits (α-subunit and β-subunit). The biological activity of hCG is determined by β-subunit, which is composed of 145 amino acids [89]. Several experiments have been conducted to test the efficacy of hCG in ovulation induction [73, 90, 91]. The results of these studies showed that administration of 1.500– 3.300 IU of hCG to mares with a follicle in the ovary 35 mm in diameter, or after estrus day 2, induced ovulation within 48 h. The administration of hCG to mares

**13**

*Physiological and Clinical Aspects of the Endocrinology of the Estrous Cycle and Pregnancy…*

with a follicle in the ovary 35 mm in diameter, or after estrus day 2, induced ovulation within 48 h. The administration of hCG to mares with a follicle in the ovary 35 mm in diameter, or after estrus day 2, induced ovulation within 48 h. However, the adverse effect of consecutive administration of hCG has been reported. The results demonstrate a null effect from the second administration of hCG.

On the other hand, significant levels of antibodies to hCG were also observed after repeated injections [91, 92]. However, there is much conflicting evidence as to

The recombinant equine LH (reLH) was successfully developed and tested for both in vitro and in vivo efficacy [94, 95]. To test the efficacy of reLH in ovulation induction, a study was performed in mares with 35 mm follicles that were treated with 0.3, 0.6, 0.75, 0.9 mg reLH, 2.500 IU hCG and the number of ovulations within 48 h of injection was monitored. With a total of 84 mares of various breeds 28.6, 50, 90, and 80% ovulated within 48 h in response to 0.3, 0.6, 0.75, and 0.9 mg reLH, respectively. Changes in hormonal profiles (LH, FSH, P4, E2) in response to 5, 0.65, or 10 mg reLH were similar to those of mares of the control group, except for the early increase in LH after reLH injection. The result of this study indicates that reLH is a drug that induces ovulation in mares with a follicle size of 35 mm in 48 h. It is important to point out that as a synthetic

The raw extract of equine gonadotropin (CEG) from the pituitary, contains FSH and LH. These extracts have been tested to determine if they can be used as agents to control the estrual cycles of mares. Also, due to their LH content, the effect of CEG for ovulation induction has been tested. Duchamp et al. [73] showed that 80% of ponies and 57% of mares ovulated 2 days after the administration of 50 mg and 25 mg of CEG, respectively. However, there is one major obstacle to these results; the FSH and LH relationship in cEG is not always consistent. Another important factor to keep in mind is that CEG may be contaminated with other pituitary hormones. Also, the potential transmission of certain associated diseases between

Savage and Liptrap [99], reported on the use of PGF2α was able to induce ovulation in mares. By administering 250 μg PGF2α synthetic (Fenprostalene) 60 h after the onset of estrus, the interval between treatment and ovulation and the duration

Despite these good results, no other PGF2α could be found that could give similar results [100]. It is therefore believed that the prolonged action of Fenprostalene was responsible for these results. Another PGF2α (Luprostiol), has also been shown to

Kisspeptin is a neuropeptide that induces the secretion of gonadotropins through the stimulation of GnRH secretion and has also been described as having a role in

product it offers good potential by having, for example, a low production cost.

whether antibody formation affects the efficacy of hCG [93].

*DOI: http://dx.doi.org/10.5772/intechopen.90387*

*3.6.2.4 Equine recombinant LH*

*3.6.2.5 Equine pituitary extracts*

*3.6.2.6 Prostaglandins*

*3.6.2.7 Kisspeptin*

of estrus were significantly reduced.

animals or between animals and humans [96–98].

induce a release of LH from the anterior pituitary gland [101].

*Physiological and Clinical Aspects of the Endocrinology of the Estrous Cycle and Pregnancy… DOI: http://dx.doi.org/10.5772/intechopen.90387*

with a follicle in the ovary 35 mm in diameter, or after estrus day 2, induced ovulation within 48 h. The administration of hCG to mares with a follicle in the ovary 35 mm in diameter, or after estrus day 2, induced ovulation within 48 h. However, the adverse effect of consecutive administration of hCG has been reported. The results demonstrate a null effect from the second administration of hCG.

On the other hand, significant levels of antibodies to hCG were also observed after repeated injections [91, 92]. However, there is much conflicting evidence as to whether antibody formation affects the efficacy of hCG [93].
