Increased ovulation rates in mares after immunisation against recombinant bovine inhibin alpha-subunit

Granulosa cell tumors of the equine ovary

The granulosa cell tumor is the most common ovarian tumor in mares. A clinical diagnosis can be made based on the presence ofa unilaterally enlarged ovary and a small inactive contralateral ovary. Endocrine testing may be beneficial to confirm a diagnosis. Surgical removal of the tumor eliminates the adverse effect on pituitary function and results in resumption of follicular development and ovulation in the opposite ovary over time.

Superovulation in mares

Embryo recovery from single ovulating mares is approximately 50 per cent per estrous cycle. Superovulation could be used to increase embryo recovery and provide extra embryos for embryo freezing. This review addresses some historical approaches to superovulation, as well as examines factors that affect the response of mares to equine FSH. eCG, GnRH and inhibin vaccines have been of limited success in stimulating multiple ovulation. Numerous studies have shown that injection of equine pituitary extract (EPE) will result in three to four ovulations per estrous cycle and two embryos. A purified, standardized EPE preparation (eFSH) also results in a similar response to EPE. Factors affecting the response to EPE and eFSH include day of initial treatment, size of largest follicle at initial treatment and frequency of injection. Embryos from single ovulating, untreated mares and eFSH-treated mares provide similar pregnancy rates upon nonsurgical transfer. Five to 7 days of eFSH treatment also has been shown to hasten the first ovulation of the breeding season. Potential problems after eFSH injections include anovulatory or luteinized follicles and overstimulation. Studies are needed to further evaluate the criteria for initiation of treatment and to determine how to increase ovulation rate without decreasing embryo recovery per ovulation.

Superovulation in Mares

Recently, a commercial product has been made available (equine follicle-stimulating hormone [eFSH]) for superovulating mares. This has provided the practitioner with a hormonal product that is readily available for enhancing multiple ovulations. Additional benefits of stimulating multiple follicles include an increased number of follicles available for oocyte collection, availability of extra embryos for embryo freezing, enhancement of fertility in subfertile mares, and advancement of the first ovulation of the year. This article provides a short historical background, but most of it centers on the use of eFSH for stimulation of follicular development and ovulation in mares.

Effect of Active Immunization of Pony Mares against Recombinant Porcine Inhibin .ALPHA. Subunit on Ovarian Follicular Development and Plasma Steroids and Gonadotropins

Two pony mares were immunized against recombinant porcine inhibin alpha subunit three times with 39 day intervals. Clinical findings and endocrinological changes before immunization were taken as the control. The first significant rise in the anti-inhibin titre (P<0.05) in the circulation was found 27 days after the first injection. Maximum binding activity was reached by the 12th day after the second booster dose. The number of small, medium and large sized follicles had increased significantly compared to before immunization (11.75 +/- 4.30, 2.75 +/- 0.69 and 2.51 +/- 0.63 vs 6.50 +/- 1.43, 1.83 +/- 0.44 and 1.33 +/- 0.38, respectively), but the ovulation rate remained unchanged after immunization. The average plasma concentration of FSH and estradiol-17beta during the estrous cycle increased significantly (P<0.05) after immunization. These results suggest that immunization against inhibin is a useful tool to increase the number of ovarian follicles during the estrous cycle of pony mares. Moreover, the present study supported the concept that inhibin plays a major role in the control of follicular growth through its inhibitory effect on FSH secretion synergistically with steroid hormones.

Mechanisms responsible for increase in circulating inhibin levels at the time of ovulation in mares

In female mammals, inhibin is secreted by the granulosa cells and selectively inhibits secretion of FSH. Although circulating immunoreactive (ir)-inhibin levels decrease after ovulation as a result of the disappearance of its main source, they abruptly increase at the time of ovulation in mares. To investigate the mechanisms responsible for this increase, 50 ml of equine follicular fluid (eFF) was administered into the abdominal cavity of mares during the luteal phase (eFF, n = 4). One hour after treatment, plasma levels of ir-inhibin and inhibin pro-alphaC (but not estradiol-17beta) were significantly higher in eFF-treated mares than in control mares (n = 4). The hormone profiles in eFF-treated mares were similar to those in mares with the spontaneous or hCG induced ovulations. The present study demonstrates that the release of follicular fluid into the abdominal cavity when the follicle ruptures is responsible for the ovulatory inhibin surge in the mare. These findings also suggest that circulating inhibin pro-alphaC may be useful for determining the time of ovulation in the mare.

Selection of the dominant follicle in cattle and horses

The nature of selection of the dominant follicle is reviewed by comparing research results between cattle and horses. In both species, emergence of a follicular wave is stimulated by an FSH surge. The surge reaches a peak by the time the follicles attain 4 mm in diameter in cattle and 13 mm in mares. In cattle, all of the growing follicles >/=5 mm contribute to the decline in FSH concentrations. However, the declining FSH concentrations are still needed by the growing follicles. Several days after the peak of the FSH surge and emergence of the wave, the two largest follicles reach means of 8.5 and 7.7 mm in cattle and 22 and 19 mm in horses. At this approximate time, the follicles begin to undergo deviation in follicle diameters, which is characterized by continued growth of the largest follicle to become the dominant follicle and reduced or terminated growth of the remaining follicles to become subordinate follicles. In both species, on average, the future dominant follicle emerges before the future largest subordinate follicle, and the two follicles grow in parallel until deviation. The difference in diameter between the two largest follicles at the beginning of deviation is equivalent in growth to approximately 8 h in cattle and 24 h in mares. Apparently, this is adequate time for the largest follicle to establish the deviation process before the second-largest follicle reaches a similar diameter. During this time, the largest follicle plays the primary role in further suppressing circulating FSH concentrations to below the requirements of the smaller follicles, which causes their regression. The follicle-produced FSH suppressants appear to be estradiol and inhibin. In addition to enhancing its FSH-suppressing ability, the largest follicle also develops the ability to utilize the reduced concentrations of FSH for its continued growth. It is therefore postulated that the essence of selection of a dominant follicle in these two species is a close two-way functional coupling between changing FSH concentrations and follicle growth and development. Elevated concentrations of circulating LH encompass deviation in both species and may play a role in continued growth of the largest follicle. It is not known if LH begins to be utilized by the largest follicle before, at, or after the beginning of diameter deviation. However, results of studies in mares suggested that LH does not influence growth of the dominant follicle until after the beginning of deviation.

The current status of equine embryo transfer

The use of embryo transfer in the horse has increased steadily over the past two decades. However, several unique biological features as well as technical problems have limited its widespread use in the horse as compared with that in the cattle industry. Factors that affect embryo recovery include the day of recovery, number of ovulations, age of the donor and the quality of sire's semen. Generally, embryo recoveries are performed 7 or 8 d after ovulation unless the embryos are to be frozen, in which case recovery is performed 6 d after ovulation. Most embryos are recovered from single-ovulating mares. Because there is no commercially available hormonal preparation for inducing multiple ovulation in the horse, equine pituitary extract has been used to increase the number of ovulations in treated mares, but FSH of ovine or porcine origin is relatively ineffective in inducing multiple ovulation in the mare. Factors shown to affect pregnancy rates after embryo transfer include method of transfer, synchrony of the donor and recipient, embryo quality, and management of the recipient. One of the major improvements in equine embryo transfer over the last several years is the ability to store embryos at 5 degrees C and thus ship them to a centralized station for transfer into recipient mares. Embryos are collected by practitioners on the farm, cooled to 5 degrees C in a passive cooling unit and shipped to an embryo transfer station without a major decrease in fertility. However, progress in developing techniques for freezing equine embryos has been slow. Currently, only small, Day-6 equine embryos can be frozen with reasonable success. Additional studies are needed to refine the techniques for freezing embryos collected from mares 7 or 8 d after ovulation. Demand for the development of assisted reproductive techniques in the horse has increased dramatically. Collection of equine oocytes by transvaginal, ultrasound-guided puncture and the transfer of these oocytes into recipients is now being used to produce pregnancies from donors that had previously been unable to provide embryos. In vitro fertilization, however, has been essentially unsuccessful in the horse. One alternative to in vitro fertilization that has shown promise is intracytoplasmic sperm injection. However, culture conditions for in vitro-produced embryos appear to be inadequate. The continued demand for assisted reproductive technology will likely result in the further development of techniques that are suitable for use in the horse.

Effect of passive immunization against inhibin on FSH secretion, folliculogenesis and ovulation rate during the follicular phase of the estrous cycle in mares

Physiological roles of inhibin in mares were investigated by means of passive immunization using an antiserum to inhibin that had been raised in a castrated goat. Eight mares were given an intravenous injection of either 100 mL (n = 4) or 200 mL (n = 4) of inhibin antiserum 4 d after a single intramuscular injection of PGF2 alpha on Day 8 after ovulation, 4 control mares were treated with 100 mL castrated goat serum in the same manner. Jugular vein blood samples were collected after treatment with the serum until 192 h post treatment. Follicular growth and ovulations were monitored by ultrasound examination at 24-h intervals. The ability of the inhibin antiserum to neutralize the bioactivity of equine inhibin was examined in vitro using a rat pituitary cell culture system. Suppression of secretion of FSH from cultured rat pituitary cells by equine follicular fluid was reversed by the addition of increasing doses of the inhibin antiserum, thereby indicating its bioactivity. Plasma levels of FSH and estradiol-17 beta were higher in mares treated with the inhibin antiserum. The ovulation rate was significantly higher in mares treated with antiserum (100 mL = 3.75 +/- 0.63; 200 mL = 4.50 +/- 0.65) than in control mares (1.25 +/- 0.25). These results demonstrate that inhibin is important in regulating FSH secretion and folliculogenesis in mares. They also show that neutralization of the bioactivity of inhibin may become a new method for the control of folliculogenesis and ovulation rate in mares.

Production of embryos by assisted reproduction in the horse

In vitro embryo production is not yet successful in the horse, largely due to low rates of fertilization in vitro. However, methods to produce embryos from isolated oocytes have been developed. Oocytes may be recovered from living mares by aspiration of the dominant preovulatory follicle by trans-abdominal puncture, and from both preovulatory and immature follicles by trans-vaginal ultrasound-guided puncture. Transfer of in vivo-matured oocytes to the oviducts of bred recipient mares has resulted in good pregnancy rates (75-85%). Little work has been done on transfer of horse oocytes matured in vitro. Recovery rates of immature oocytes from mares in vivo are lower than those for cattle. In addition, work on oocytes recovered from horse ovaries post-mortem has shown that horse oocytes from smaller (< 20 mm diameter) viable follicles may not yet be meiotically competent. Methods for in vitro fertilization and for obtaining adequate numbers of competent immature oocytes from the mare must be developed before in vitro embryo production can become a useful clinical and research procedure in the horse.

Production of a genetically engineered inhibin vaccine

The goal of this research was to synthesize a chimeric ovalbumin/inhibin antigen using recombinant techniques. Antigenic epitopes of the translated product from an ovalbumin cDNA were mapped using computer modeling techniques. The corresponding nucleotide sequences of ovalbumin epitopes were examined for unique restriction sites to allow insertion of a synthetic bovine inhibin alpha gene fragment which encoded the first 26 amino acids. A plasmid (pETOI-8) was synthesized which contained the chimeric ovalbumin/inhibin alpha 1-26 gene. A 46 kDa recombinant protein (ovalin) was produced from BL21 (DE3) Escherichia coli cells containing pETOI-8 that was identified with anti-ovalbumin and anti-inhibin alpha 1-26 antisera. Rabbits were immunized subcutaneously in four sites along the back against ovalbumin (n = 3), crude ovalin (n = 3), or gel-purified ovalin (n = 3) at week 0, 4, 7, and 18. Primary and booster immunizations contained ca 300 micrograms of antigen emulsified in complete Freund's adjuvant and incomplete Freund's adjuvant, respectively. Blood samples collected at week 0, 6, 8, and 19 were evaluated for their ability to bind ovalbumin, 32 kDa bovine inhibin and bovine inhibin alpha 1-26 using ELISA. Mean anti-ovalbumin titers at week 19 were 1:100000 in rabbits immunized against ovalbumin or crude ovalin, and 1:23333 in rabbits immunized against gel-purified ovalin. Anti-inhibin and anti-inhibin alpha 1-26 titers were nonexistent in antisera from pre-immunized rabbits and rabbits immunized against ovalbumin. Mean anti-inhibin titers at week 19 were 1:833 and 1:10000 in rabbits immunized against crude ovalin or gel-purified ovalin, respectively. Mean anti-inhibin alpha 1-26 titers at week 19 were 1:3333 and 1:6666 in rabbits immunized against crude or gel-purified ovalin, respectively. We conclude that genetic engineering of inhibin alpha 1-26 into the antigenic epitopes of ovalbumin provides potential for the development of an anti-inhibin vaccine.

Superovulation

Development of a superovulation technique that is successful, safe, and commercially available would revolutionize the equine breeding industry. However, the reality is that ovulation rates for mares following existing superovulatory treatment are much lower than for cattle. This dichotomy has been attributed to the relatively limited area available in the ovulation fossa for ovulation to occur, combined with the large size of the equine preovulatory follicle. In addition, the number of ovulations in the mare may be limited physiologically by the size of the follicular cohort that may be rescued by administration of gonadotropins. Clearly, additional research effort is necessary to optimize superovulation treatment regimens in the mare.

Inhibin activity in the mare and stallion

An overnight double antibody RIA, employing a rabbit antiserum raised to bovine 31 kDa inhibin (rAs-#1989, NICHD) and purified bovine 31 kDa inhibin (bINH-I-90/1, NICHD) as trace and standard, was validated to measure immunoreactive inhibin (iINH) concentrations in equine peripheral plasma, follicular fluid (FF), ovarian vein (OV) plasma, testicular tissue extracts (TTE) and testicular vein (TV) plasma. The dynamic relationship of iINH and follicle stimulating hormone (FSH) was investigated during the estrous cycle of the mare and the annual reproductive cycle of the stallion. In the RIA, parallel dose-response curves were observed between the bovine inhibin standard and serial dilutions of equine FF, OV, TTE, TV and plasma. The average recovery of a known amount of purified bovine inhibin added to gelding plasma was approximately 100%. In the inhibin bioassay, serial dilution of equine FF and TTE were observed to be parallel to the bovine inhibin standard. A five-fold difference (p < 0.05) between jugular and gonadal vein plasma iINH concentrations was observed in the mare and an eight-fold difference (p < 0.05) was observed in the stallion. Plasma levels of iINH in ovariectomized mares or geldings were undetectable in the RIA. Concentrations of FSH, estradiol and iINH changed significantly in the mare during the estrous cycle (p < 0.05). Immunoreactive inhibin levels were highest (0.54 +/- 0.06 ng/ml) on the day of ovulation, declined rapidly following ovulation and reached a nadir (0.21 +/- 0.03 ng/ml) on day 7 post-ovulation. Plasma iINH and estradiol concentrations followed a similar profile and were found to be positively correlated (r = 0.7064; p < 0.01), whereas iINH and FSH levels demonstrated an inverse relationship (r = -0.7359, p < 0.01) throughout the estrous cycle. Concentrations of FSH were also inversely related (-0.8498, p < 0.01) with estradiol during the cycle. In the stallion, plasma iINH and FSH levels changed significantly during the year (p < 0.05). The iINH profile reflected seasonal changes in testicular activity, with highest concentrations in late spring (3.37 +/- 0.44 ng/ml) and lowest concentrations in the fall (2.21 +/- 0.33 ng/ml). Plasma concentrations of iINH were positively correlated (r = 0.7691, p < 0.01) with FSH concentrations throughout the year. In conclusion, a specific and sensitive RIA for iINH has been validated for plasma and biological fluids in the horse. Furthermore, the gonads appear to be the source of bioactive and immunoreactive inhibin as observed in other species.(ABSTRACT TRUNCATED AT 400 WORDS)