Managing the Mare for Optimal Fertility

Transcription

1 REVIEW Managing the Mare for Optimal Fertility C.H.G. IRVINE 1 * and Susan L. ALEXANDER 2 1 Animal & Veterinary Sciences Group, Lincoln University, Canterbury, and 2 Department of Endocrinology, Christchurch Public Hospital, Christchurch, New Zealand The horse has evolved over the millennia to use environmental cues to maximize reproductive efficiency. In domesticating the horse, man has removed or altered some of these cues, often to the detriment of the horse s fertility. This review will describe research of ourselves and others on the hormones controlling reproduction in the mare, and particularly the effect of environmental factors on these hormones and thus upon fertility. Key words: environment, fertility, gonadotropin, ovulation, pituitary J. Equine Sci. Vol. 9, No. 3 pp , 1998 Gonadotropin-Releasing Hormone (GnRH) In the mare as in other species, the primary hormone controlling reproductive cycles is GnRH which is secreted by neurons in the hypothalamus. These neurons comprise a functional group called the GnRH pulse generator [22] which secretes GnRH in rhythmic pulses. These GnRH pulses stimulate receptors on the surface of the pituitary gonadotropes causing them to release simultaneous pulses of follicle stimulating hormone (FSH) and luteinizing hormone (LH) which then act at the ovary to induce follicular development, steroid and peptide hormone secretion and ovulation. Studying the secretion patterns of GnRH is difficult, because GnRH is secreted into the local hypothalamohypophysial portal vessels which are inaccessible without major surgery. Unfortunately, portal blood is so diluted (approximately fold) in the closest accessible drainage vein, the jugular, that GnRH is immeasurably low there. However, about 15 years ago we devised a novel atraumatic method of collecting blood in mobile horses for long periods from the intercavernous sinus in which pituitary blood is diluted only 2 10 fold so that hypothalamic hormones are readily measurable [16]. Because the degree of dilution is readily calculated this has enabled us to monitor GnRH secretion and to investigate factors which regulate it in normal horses and in response to various environmental signals. In the absence of modulating inputs, the GnRH pulse generator discharges spontaneously approximately once per hour. During diestrus, this fundamental frequency is slowed to 1 3 pulses per day by the inhibitory inputs of the endogenous opioids and progesterone [18]. On the other hand, during estrus, GnRH pulse frequency rises to once every min [19], possibly due to stimulation by estradiol. The changing frequency of GnRH pulses during the ovulatory cycle is an important part of the signal to the pituitary: high frequency pulses of GnRH (as in estrus) preferentially stimulate LH secretion and thus ovulation. Low frequency GnRH pulses (as in diestrus) preferentially stimulate FSH secretion and thus follicle development [29]. Continuous GnRH administration can stimulate follicular development and ovulation in mares [7, 29]. This is different to nearly all other species in which the stimulation of the receptor by GnRH must be intermittent otherwise the receptor becomes refractory or downregulated [22]. In fact continuous GnRH stimulation shuts down the reproductive axis and thus acts as a contraceptive. This does not happen in the mare unless the doses of GnRH analogue used are astronomic [12]. Although the mare s pituitary responds reasonably well to continuous administration of GnRH, there is no evidence that a significant proportion of GnRH is actually secreted continuously in mares at any cycle stage [18, 19]. *corresponding author.

2 84 C.H.G. IRVINE AND S.L. ALEXANDER Environmental Inputs The activity of the hypothalamus, particularly the GnRH pulse generator, is strongly influenced by the environment. This is vitally important since, for grazing animals, the key to successful reproduction is to give birth when temperature, rainfall and food availability are most favorable for the survival and growth of the young. Several environmental factors are important in the mare. Daylength In the horse the most powerful environmental regulator of the reproductive axis is daylength. Short photoperiods inhibit the GnRH pulse generator and probably act through increasing the duration of melatonin secretion by the pineal. In the depth of winter, GnRH pulses occur less than once a day [2]; this no longer supports folliculogenesis. The pulse generator is inhibited by endogenous opioids during the non-breeding season, as shown by the buoyant GnRH and gonadotropin responses to a small dose of the opioid antagonist, naloxone [9, 20]. Indeed, the degree of opioid inhibition during the non-breeding season may exceed that in mid-diestrus [30]. Changes in other neurotransmitters, eg dopamine, α2-adrenergic agonists, glutamate, may also be involved in the seasonal suppression of the GnRH pulse generator. Presence of the Stallion It is a discouraging fact for those involved in equine reproduction that the first-cycle pregnancy rates in freerunning or pasture bred horses are typically well above the 75% level considered to be excellent at horsebreeding farms [24]. A major difference between the pasture and the breedingfarm is the time that mares and stallions spend interacting. Left to their own devices, horses spend several days in courtship without mating [24]. In early estrus, mares are often found in the vicinity of the stallion and display the behavioral signs of estrus, sometimes many times per hour, to attract his attention. The stallion may respond vocally, or by sudden, prancing approaches to the mare after which he nips, sniffs and nuzzles her and may mount her without an erection. Eventually, when the time of ovulation nears and the mare is in full estrus, the stallion mates the mare. We have found that the interplay between mare and stallion alters reproductive hormones in both sexes. Sexual arousal, without copulation, acts as an environmental signal, and stimulates GnRH, LH and FSH secretion in both the stallion and the mare [4, 17]. More striking, though, is the effect of sexual arousal on the release of another hypothalamic hormone, oxytocin. In both sexes, sexual arousal stimulates the secretion of sufficiently large quantities of oxytocin to raise jugular concentrations [4, 23, 28]. For example in estrous mares, insemination generates approximately a thousand-fold rise in oxytocin concentrations in pituitary blood [23] and a five-fold rise in jugular blood. The increase in oxytocin secretion is likely to perform several fertilityfavoring functions in the mare. Acting peripherally, oxytocin stimulates peristaltic contractions of the uterus and oviduct. This increases the clearance of inflammatory products from the uterus, thus playing a useful part in resisting the establishment of uterine infections [26]. An increase in reproductive tract motility may also improve the chances of union of sperm and ovum in the ampulla. Acting at the pituitary, oxytocin is a weak stimulus for LH release in horses [4] as in other species [14]. It is possible that repeated oxytocin bursts caused by frequent interaction between the mare and stallion could induce sufficient LH secretion to advance ovulation. Finally, it has been shown in other species, but not yet in the horse, that oxytocin acts within the brain to help forge emotional bonds between individuals [15]. It may be then that repeated bursts of oxytocin secretion during the prolonged equine courtship serve to strengthen the social structure of the harem. The stallion conveys his presence to the mare by auditory, olfactory, tactile, and visual signals. We and others have investigated which sensory pathways are important in stimulating responses in mares [23]. 1) Auditory signals. Sexually active stallions use a wide range of vocalizations to communicate with mares and this sweet-talking is important to mares. When given a choice, mares favor noisy stallions and prefer an equine Pavarotti to a Mr. Bean. Just the sound of a stallion is adequate to cause estrous mares to release oxytocin [4]. 2) Tactile signals. Artificial insemination provides a purely tactile signal, stimulating the vagina, clitoris and cervix. This doubles jugular oxytocin concentrations and increases pituitary venous oxytocin a thousand-fold [4, 23]. Tactile stimulation alone therefore appears to be as arousing as the presence of the stallion himself. 3) Olfactory cues or pheromones are among the most widely used signals in reproduction, but their role in equine reproductive physiology has not been systematically investigated. It is obvious that stallions can be sexually aroused by the scent of a cloth soaked in estrous mare s urine [13]. However there don t appear to be any data showing that mares respond to male pheromones.

3 MANAGING THE MARE FOR OPTIMUM FERTILITY 85 4) Visual signals have been shown to raise LH and testosterone levels in male army recruits but the responses of the human female to erotic visual stimuli from the male are disappointing. In the stallion the sight of the phantom used for semen collection can be highly arousing although this may be a conditioned reflex rather than a true visual response. Nutrition Both the quantity and quality of feed affect fertility. The appearance of spring growth of pasture is concurrent with resumption of estrous cycles. It is also concurrent with increasing photoperiod. However, experiments in England [6] and U.S.A. [13] suggest that spring pasture plays a specific role in increasing fertility. That is: mares grazing lush pasture ovulate earlier in the season than control mares fed on hay. The factor that stimulates reproduction in spring pasture is unknown. One candidate, vitamin A or its precursor, beta-carotene has been added to the diet but no significant improvement in fertility has resulted. It appears that vitamins improve fertility only if the animal has previously been on a vitamin deficient diet. Several trace elements have been claimed to increase fertility. There is evidence that deficiencies in manganese, selenium, iodine, copper or zinc are detrimental to fertility and fetal development. Regional deficiencies in these elements are relatively common in many horsebreeding areas and the rather spectacular responses which have sometimes been obtained when they have been added to the diet has led to their indiscriminate use to increase fertility. Attempts to increase fertility by providing an excess of one or more dietary components in already adequately-fed animals have had little success. Inhibitory signals The environmental effects described thus far have a positive action on reproduction. However many effects are negative, such as pain, excess heat or cold, social disruptions, and inadequate diet. It is not surprising that a system has evolved for ensuring that animals which are already stressed don t take on the extra stress of pregnancy. These negative signals activate the stress axis, which has been discussed by Dr. Alexander in a companion paper. In horses as in other species, stress causes the hypothalamus to secrete corticotropin-releasing hormone and arginine vasopressin [3]. These induce the pituitary to synthesize and/or secrete pro-opiomelanocortin, which is cleaved into several biologically-active peptides before release into blood. One of these peptides is ACTH which stimulates the adrenal glands to secrete cortisol. Another cleavage product is the opioid, β-endorphin. Several of these stress hormones are able to inhibit the reproductive axis. In some species, CRH inhibits GnRH secretion either directly or by inducing opioid release [27]. In normal unstressed horses, opioid inhibition of GnRH secretion can be demonstrated at all stages of the ovulatory and annual cycles [5, 9, 30]. Moreover, the normal horse appears to be opioiddependent as indicated by the withdrawal symptoms of excitement, diarrhea, tremor, and hypersensitivity to pain, induced by opioid antagonists given at dose rates which induce no responses in other species [21]. With chronic stress, this opioid suppression may increase sufficiently to prevent the rise in GnRH pulse frequency needed to generate an ovulatory LH surge. Cortisol may also reduce GnRH secretion or lower pituitary responsiveness to GnRH. In mares, long-term administration of the synthetic corticosteroid, dexamethasone, blocks estrous behavior and ovulation [8]. Chronic stresses, like crowding or social instability, cause prolonged elevations in plasma cortisol in horses as in other species, but the effect of this on reproduction in horses has not been studied. It is often claimed that mares may be more resistant to stress-induced infertility since most racing mares cycle normally despite hard training, racing and in some situations extensive travel. Under the same circumstances, women athletes often cease menstrual cycles. However, we have found that most racehorses cope well with these stresses and do not suffer from chronic hypercortisolemia. Strenuous training also reduces the body s fat: lean mass ratio which may be the factor that disrupts the menstrual cycle in women athletes. Although this doesn t happen in racemares, nutritional stress does damp fertility in horses, since mares kept on a low plane of nutrition after conception resorb their embryo more often than do well-fed controls [31]. Why this happens is unknown. It may be a non-specific starvation-type effect. Summary: Practical relevance Our research has shown that environmental factors can alter reproductive hormones in mares and thus affect fertility. Several of these environmental factors can be controlled by the stud manager. Management can therefore influence fertility more directly and profoundly than has previously been appreciated. First and foremost, there should be adequate contact between the sexes. If a mare isn t cycling, keeping her close to a teaser stallion or a testosterone treated gelding can help. Good stud managers intuitively know this and we have shown the

4 86 C.H.G. IRVINE AND S.L. ALEXANDER basis for its action. However, there is an increasing tendency to segregate mares and stallions, even to keep them on separate farms. This would be expected to reduce fertility. Obviously, stresses, like rough handling, crowding, painful states, inadequate feed, should be avoided. Treating the Problem Mare A small percentage of mares may exhibit reproductive inactivity during the normal breeding season [11]. When faced with a mare with small hard ovaries in the summer, the first step in treatment should be to identify and eliminate any unfavorable environmental factors. If this fails to induce cyclicity, GnRH replacement therapy can be considered. However, you need to determine that the site of the problem is the hypothalamus and not the pituitary or ovaries. To do this, you should: Collect jugular blood every 30 min for 4 hr Give 10 µg GnRH iv Collect jugular blood 10, 20, 30, 45 and 60 min later. Measure FSH and LH. Several blood samples should be collected before GnRH administration since LH and FSH secretion is pulsatile and results could be misleading if only one sample were collected. For example, the peak of a oncedaily pulse could be sampled leading to the conclusion that LH and FSH levels were high. If FSH and LH levels are high and stable throughout the four-hour period before GnRH administration, then ovarian failure would be indicated, since the hypothalamus and pituitary are responding normally to the positive environmental inputs. Low FSH and LH levels before GnRH would place the problem in the hypothalamus or the pituitary. A normal FSH/LH response to GnRH, ie a 2-fold increase in LH [1] and a 3 5 fold increase in FSH [10] would exonerate the pituitary and incriminate the hypothalamus. If the mare does appear to be GnRH deficient and can respond to GnRH, then GnRH replacement therapy is appropriate. We would recommend a pulsatile administration regimen which simulates the physiological stimulation pattern as being the most effective in inducing follicular development and ovulation. For example, administer 5 µg buserelin twice daily for days. When a preovulatory follicle develops, give either 10 µg gonadorelin hourly for 48 hr (any route of administration is acceptable) or one 2.2 mg deslorelin implant. However, we must emphasize again that the first step in correcting hormonal problems which affect fertility is by improving management and eliminating all the negative inputs described in this review. Hormone administration should be a last resort. Acknowledgments We would like to thank Julie Turner and Natalie Shand for technical assistance and the New Zealand Equine Research Foundation for financial support. References 1. Alexander, S.L. and Irvine, C.H.G Effect of graded doses of gonadotrophin-releasing hormone on serum LH concentrations in mares in various reproductive states: comparison with endogenously generated LH pulses. J. Endocrinol. 110: Alexander, S.L. and Irvine, C.H.G Control of the onset of the breeding season in the mare and its artificial regulation by progesterone treatment. J. Reprod. Fertil. Suppl. 44: Alexander, S.L., Irvine, C.H.G. and Donald, R.A Dynamics of the regulation of the hypothalamo-pituitary-adrenal (HPA) axis determined using a nonsurgical method for collecting pituitary venous blood from horses. Front. Neuroendocrinol. 17: Alexander, S.L., Irvine, C.H.G., Shand, N.and Evans, M.J Is LH secretion modulated by endogenous oxytocin in the mare? Biol. Reprod. Monograph 1: Alexander, S.L., Irvine, C.H.G., Shand, N. and Turner, J.E Are endogenous opioids involved in slowing the rate of rise of the mare s ovulatory LH surge? J. Reprod. Fertil. Suppl. (in press). 6. Allen, W. R Control of oestrus and ovulation in the mare. pp In: Control of Ovulation, eds. (Crighton, D.B., Haynes, N.B., Foxcroft, G.R. and Lamming, G.E. eds.), Butterworths, Oxford. 7. Allen, W.R., Sanderson, M.W., Greenwood, R.E.S., Ellis, D.R., Crowhurst, J.S., Simpson, D.J. and Rossdale, P.D Induction of ovulation in anoestrous mares with a slow-release implant of GnRH analogue (ICI ). J. Reprod. Fertil. Suppl. 35: Asa, C.S. and Ginther, O.J Glucocorticoid suppression of oestrus, follicles, LH and ovulation in the mare. J. Reprod. Fertil. Suppl. 32:

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