*3.6.2.4 Equine recombinant LH*

*Animal Reproduction in Veterinary Medicine*

available.

*3.6.2.2 Buserelin*

within 48 h.

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

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

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

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

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.

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

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.

with treatment with 2.500 IU of hCG (iv).

*3.6.2.3 Human chorionic gonadotropin*

**12**

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

#### *3.6.2.5 Equine pituitary extracts*

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

### *3.6.2.6 Prostaglandins*

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 of estrus were significantly reduced.

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

#### *3.6.2.7 Kisspeptin*

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

triggering the onset of puberty [102, 103]. A study in pony mares demonstrated the anticipated ovulation when treated with 10 mg of kisspeptin. Another report identified that the administration of 500 μg and 1.0 mg of kisspeptin induces indistinguishable LH and FSH responses to 25 μg GnRH. However, a single injection of 1.0 mg of kisspeptin (iv) was insufficient to induce ovulation in the mare in heat [104].
