Hormones in horses are chemical messengers produced by various glands and tissues, regulating numerous physiological processes essential for maintaining homeostasis. These hormones influence a wide range of functions, including growth, metabolism, reproduction, and stress responses. Key hormones in equine physiology include cortisol, estrogen, testosterone, and insulin, among others. The levels and effects of these hormones can vary based on factors such as age, sex, and environmental conditions, impacting overall health and performance. This page compiles peer-reviewed research studies and scholarly articles that explore the production, regulation, and physiological roles of hormones in equine biology.
Oh R, Tamaoki BI.The microsomal fraction (10 000–105 000 × g precipitate) of equine testes was fractionated into the smooth- and the rough-surfaced microsomal subfractions by a sucrose density-gradient centrifugation in the presence of CsCl. The validity of this fractionating procedure was confirmed by electron microscopic examination and also by chemical analysis of the RNA contents in these subfractions. The aromatizing enzyme system (19-hydroxylase and aromatase) which was concentrated in the microsomal fractions among the organellae was found to be localized in the smoothsurfaced microsomal fraction. Th...
Setchell BP, Duggan MC, Evans RW.Single intravenous injections of ovine luteinizing hormone (LH) in adult hamsters and rats had no effect on fluid secretion by the testes, as measured by the gain in weight or water content during a 10-h period after ligation of the efferent ducts (EDL). Neither was there any obvious effect on the liberation of spermatozoa, as judged by the total number of sperm in the unligated and EDL testes and from the concentration of spermatozoa in the secreted fluid, calculated from the difference between the number of sperm in the EDL and unligated testes divided by the difference in weight.
In adul...
Brück I, Anderson GA, Hyland JH.The influence of different maternal plasma progesterone concentrations on embryonic glucose metabolism was studied. Uterine flushes were obtained after treating ovariectomized mares (n = 3) with 0 (control), 100 or 200 mg progesterone daily for 7 d. A group of progesterone-induced proteins (PIP) of Mr approximately 20,000 were identified in flushes from progesterone treatments by SDS-PAGE but were not observed in control flushes. Progesterone-induced proteins were removed from half the pooled flush in each treatment group by Sepharose blue CL-6B. In a 3 x 2 factorial (progesterone treatments, ...
Martinuk SD, Bristol F, Murphy BD.Breeding trials were designed to determine the influence of the mare on serum concentrations of equine chorionic gonadotropin (eCG) from Day 39 to Day 104 of gestation. Sires were ranked according to mean eCG concentrations found in the groups of randomly selected mares to which they were mated in 1983. Mares were ranked according to their mean eCG concentrations on Days 55, 71 and 85 of gestation (Day 0 = mating), in 1983 and 1985. In the 1986 breeding season, mares that had pregnancies characterized by high eCG levels were mated to sires previously associated with low eCG concentration pregn...
Van Leeuwen W.Two trotter stud farms were visited on Mondays, Wednesdays, and Fridays during 1972 and 1973. A total of 356 mares were bored on these farms during these breeding seasons. There were 194 nursing, 113 barren, and 49 maiden mares. Of the 356 mares, 78 (21.9%) had functional disorders of the ovaries at one time or another. The functional disorders consisted of: 27 anoestrous mares after early embryonic death, 20 with low ovarian activity, 19 with multifollicular ovaries, and 12 mares with persistent corpora lutea. The following therapies were used: (a) 1000 I.U. PMSG i.v. + 10 mg stilboestrol i.m...
Schönert S, Reher M, Gruber AD, Carstanjen B.This case report describes the use of a subcutaneously applied 4.7-mg deslorelin acetate implant in a three-year-old Arabian crossbred stallion showing unwanted strong male behaviour. Following deslorelin acetate implantation the stallion showed a short transitional increase in male behaviour. A 'gelding-like' behaviour was noted 15 days (D15) after treatment. The horse was surgically castrated at the owners request at D52 after treatment. Serum testosterone, oestradiol-17β and oestrone sulphate values decreased after deslorelin acetate implantation, but serum LH and FSH levels remained uncha...
Retamal C, Urzúa J, Alves EW, López ML.It has been suggested that proteins produced in specific regions of the epididymis, mostly androgen dependent glycoproteins, are involved in the sperm maturation process. In the present work, the glycoconjugated distribution pattern and the electrophoretic characteristics of the stallion epididymal proteins were examined using lectin probes. The identification in the luminal fluid of some new proteins, probably synthesized and secreted by the epididymis, is an important initial step to investigate their interaction with the stallion sperm membrane. The binding of FITC-lectins (ConA, WGA, LPA, ...
Greaves HE, Kalariotes V, Cleaver BD, Porter MB, Sharp DC.The potential involvement of ovarian factors in regulating GnRH and LH postovulation was studied in ovarian intact (Group 1; n=3) and ovariectomized (OVX; Group 2; n=3) mares (OVX within 12 hr of ovulation). Blood samples were collected every 10 min for 6 hr from jugular vein (JV) and intercavernous sinus (ICS) during estrus and on Day 8 postovulation for LH and GnRH analysis. Additionally, JV samples were collected twice daily (12-hr intervals) for 30 days for LH and progesterone (P4) analysis. A significant treatment x day effect (P<0.0001) describes declining plasma LH concentrations in ...
Ellerbrock RE, Podico G, Scoggin KE, Ball BA, Carossino M, Canisso IF.The expression pattern and distribution of sex steroid receptors and steroidogenic enzymes during development of the equine accessory sex glands has not previously been described. We hypothesized that equine steroidogenic enzyme and sex steroid receptor expression is dependent on reproductive status. Accessory sex glands were harvested from mature stallions, pre-pubertal colts, geldings, and fetuses. Expression of mRNA for estrogen receptor 1 (ESR1), estrogen receptor 2 (ESR2), androgen receptor (AR), 3β-Hydroxysteroid dehydrogenase/Δ5-4 isomerase (3βHSD), P450,17α hydroxylase, 17-20 lyase...
Ginther OJ, Meira C, Beg MA, Bergfelt DR.Deviation during a follicular wave in mares begins when the largest follicle (F1) reaches a mean diameter of 22.5 mm and is characterized by continued growth of F1 to become the dominant follicle and regression of F2 to become the largest subordinate follicle. In the present study, F1 was ablated at the expected beginning of deviation (Hour 0) to provide a reference point for characterizing the intrafollicular changes preceding experimental deviation between F2 and F3. Diameters and concentrations of follicular fluid factors in F2 and F3 were determined in F1-ablated mares at Hours 0, 12, 24, ...
Davis SD, Sharp DC.This experiment investigated steroid production by ovarian tissues, in vitro, of pony mares during vernal transition from anoestrus to the breeding season. Follicular dynamics were monitored to detect the first, second, third or fourth transition follicle, greater than or equal to 30 mm diameter or the first large post luteal follicle of the breeding season. Twenty-four hours after a large follicle was detected, theca (T) and granulosa (G) tissues were harvested. Separate and co-incubations of these tissues were conducted to determine steroid production in early transition (ET), late transitio...
Hines KK, Fitzgerald BP, Loy RG.Changes in the pattern of LH and FSH in serum were studied in 6 mares foaling during the summer. Samples were collected frequently (every 15 min) for 24 h twice before foaling, -33 +/- 2 and -12 +/- 2 days, and for 12 h after foaling, on Day 0 and Day 4. Simultaneous pulses of FSH and LH were observed before foaling (r2 = 0.99). Before foaling, gonadotrophin pulses were infrequent (6 in 264 h of observation). On the day of foaling, LH and FSH pulse frequency increased (P less than 0.005) with 2-4 pulses per mare. The amplitudes of pulses of LH and FSH were higher before parturition than for th...
Rabb MH, Thompson DL, Barry BE, Colborn DR, Hehnke KE, Garza F.Six pony geldings were actively immunized against GnRH conjugated to bovine serum albumin (BSA) to study 1) the relative dependency of LH and FSH storage, secretion and response to GnRH analog on GnRH bioavailability and 2) the effects of reduced GnRH bioavailability on GnRH storage in the hypothalamus. Five geldings were immunized against BSA. Geldings were immunized in December and 4, 8, 14, 20, 26 and 32 wk later. Ponies immunized against GnRH had increased (P less than .01) GnRH binding in plasma within 6 wk. By June, plasma concentrations of LH and FSH in ponies immunized against GnRH had...
Ousey JC, Kölling M, Kindahl H, Allen WR.The foal requires an active hypothalamo-pituitary-adrenal (HPA) axis for organ maturation and post natal survival. Prenatal administration of synthetic glucocorticoids may provide an effective method for inducing fetal maturation safely in the mare. Objective: To determine whether dexamethasone administered to late pregnant mares: 1) will induce fetal maturation and precocious delivery; 2) is safe to use and 3) to identify endocrine responses in the mare and foal. Methods: Pregnant Thoroughbred mares received either 100 mg dexamethasone i.m. (treated n = 5) or 50 ml saline i.m. (control n = 5)...
Logan NL, McCue PM, Alonso MA, Squires EL.Superovulation could potentially increase embryo recovery for immediate transfer or cryopreservation. The objectives were to evaluate the effect of pretreatment with progesterone and estradiol (P+E) on follicular response to eFSH and compare doses of eFSH and ovulatory agents on follicular development and ovulation in mares. In Experiment 1, 40 mares were assigned to one of four treatment groups. Group 1 consisted of untreated controls. Group 2 mares were administered eFSH without pretreatment with P+E. Group 3 mares were administered P+E for 10 days starting in mid-diestrus followed by eFSH t...
Martínez-Boví R, Plaza-Dávila M, Cuervo-Arango J.The use of flunixin-meglumine (a potent non-steroidal anti-inflammatory drug) during the critical period of intrafollicular prostaglandin production before ovulation (24 and 36 h after hCG treatment) results in a high rate of ovulatory failure and formation of haemorrhagic anovulatory follicles (HAF) in the mare. Dexamethasone is commonly used to prevent persistent mating-induced endometritis in susceptible mares, but the effect on ovulation blockage within the pre-ovulatory critical window of intrafollicular prostaglandins production following hCG administration has not been determined. Six ...
Johnson AL.Four seasonally anestrous mares (Standardbred), housed under a nonstimulatory photoperiod of 8 hours light:16 hours dark, were administered gonadotropin-releasing hormone (GnRH) in a pulsatile pattern (50 or 250 micrograms of GnRH/hour) for 8 to 18 days during February and March 1985. Treatment with GnRH, irrespective of dose or month, induced an increase in serum luteinizing hormone from a mean pretreatment value typical of anestrus (0.58 +/- 0.02 ng/ml +/- SE) to 10.84 +/- 1.27 ng/ml on day 8 of GnRH treatment. Ovulation in the 4 mares occurred 8.8 +/- 0.7 days after the initiation of pulsat...
Epper P, Glüge S, Vidondo B, Wróbel A, Ott T, Sieme H, Kaeser R, Burger D.To successfully inseminate mares, precise detection of ovulation time is crucial, especially when using frozen-thawed semen. Monitoring body temperature, as has been described in women, could be a noninvasive way to detect ovulation. The objective of this study was to investigate the relationship between the time of ovulation and the variation of body temperature in mares based on automatic continuous measurements during estrus. The experimental group included 21 mares for 70 analyzed estrous cycles. When the mares showed estrous behavior, they were administered intramuscular deslorelin acet...
Sharp DC, Garcia MC, Ginther OJ.Luteinizing hormone (LH) was quantified in pony mares during artificially induced sexual maturation. Ovarian follicular development was also assessed by rectal palpation of the ovaries. With the exception of large periovulatory LH concentrations in two mares that ovulated, LH concentrations were not significantly different in mares undergoing sexual maturation and thus having marked follicular development when compared with mares that were not undergoing sexual maturation and thus did not have marked follicular development. These results indicate a dissociation in time between the onset of fol...
Hinrichs K, Sertich PL, Solorzano NM, Caldwell LA.An immediate, qualitative enzyme-linked immunosorbent assay (ELISA) for progesterone was evaluated for use in determining the day of ovulation in an equine embryo transfer program. Plasma samples were collected from 27 mares from the third day of estrus to the second day of diestrus for 50 cycles. Ovulation was detected by ultrasound examination per rectum. Plasma progesterone concentrations were estimated using the qualitative assay to detect the time of the rise in progesterone after ovulation. Qualitative scores were compared to progesterone concentrations for the same samples as measured b...
Hensel B, Jakop U, Schmicke M, Schröter F, Jung M, Schulze M.Artificial insemination (AI) is commonly used in the equine industry to enhance the genetic value in breeding programs and to effectively utilize ejaculates. Many stallions are used as breeding stallions as well as in high-level sports competitions to improve their market value. The goal of the present study was to investigate whether this dual use of stallions influences the animals´ stress levels and/or the quality of their ejaculates. For this purpose, 18 stallions were grouped into two categories: breeding stallions with (BSC = breeding stallion competition), and breeding stallions with...
Bergfelt DR, Gastal EL, Ginther OJ.The increase in LH concentrations at the time of the decrease in FSH concentrations during follicle deviation in mares was studied to determine the role of LH in the production of estradiol and immunoreactive inhibin (ir-inhibin). Ten days after ovulation, all follicles > or =6 mm were ablated, prostaglandin F(2 alpha) was given, and either 0 mg (control group, n = 15) or 100 mg of progesterone in safflower oil (treated group, n = 16) was given daily for 14 days, encompassing the day of diameter deviation. The follicular and hormonal data were normalized to the expected day of the beginning...
Hay MF, Allen WR, Lewis IM.Graafian follicles of various sizes obtained from mares at different stages of the oestrous cycle were examined histologically and histochemically for delta5-3beta-hydroxysteroid dehydrogenase (3beta-HSD) activity and related enzymes. The 3beta-HSD activity was not found in the theca interna of any follicles but was present in the membrana granulosa of well-vascularized large follicles in the late luteal phase of the cycle and at oestrus. These findings indicate that pregnenolone cannot be converted into progesterone in the theca interna. It is suggested that this conversion occurs in the memb...
Munro CD, Renton JP, Butcher R.Using a range of positive and negative sexual behaviour components, proceptivity of cycling, non-lactating mares and postpartum, lactating Pony mares was quantified around ovulation. Behavioural observations were compared to plasms concentrations of progesterone, total oestrogens and androstenedione. In addition, in cycling mares, comparison with plasma testosterone concentrations was carried out. Overall rejection behaviour by the mare was apparent both during dioestrus and during periods of basal plasma progesterone concentrations. Within cycling, non-lactating mares, and between postpartum ...
Thompson DL, Pickett BW, Squires EL, Nett TM.Sixteen stallions were castrated and 30 days later assigned to one of four treatments: (1) testosterone propionate (175 microgram/kg body weight), (2) 17 beta-estradiol-3-benzoate (44 micrograms/kg body weight), (3) a combination of both steroids or, (4) vehicle only. These dosage were administered every other day for 18 days. The dosages were then doubled and continued for 20 days. Concentrations of testosterone and estradiol in serum decreased rapidly after castration and stabilized within about 6 hours. Mean concentrations of testosterone and estradiol maintained by the steroids were 1.4 an...
Dierks C, Sieme H, Piechotta M, Lehner S, Merkt JC, Uphaus H, Klug E, Distl O.A female thoroughbred successful in horse racing was positively tested for high testosterone values. This horse neither showed stallion-like-behaviour nor signs of ambiguous external genitalia. The karyotype of this horse was 2n = 64,XY and the sex-determining region of Y (SRY) PCR was positive. Hair samples tested for naturally testosterone revealed values normal for stallions, and tests for eight synthetic testosterone esters remained negative. The phenotype, ultrasonographic examination, hormone status, cytogenetic evaluation and molecular diagnostics lead to the diagnosis of an XY testicul...
Wesson JA, Ginther OJ.Thirteen 6-8 month-old fillies were assigned to 3 treatment groups: Group 1 had a 16-h fixed daily photoperiod (16L:8D, N = 4), Group 2 a daily photoperiod equivalent to ambient daylength (control, N = 5), and Group 3 a 9-h fixed daily photoperiod (9L:15D, N = 4). The light treatments extended from 17 December to 9 August. Hair shedding occurred first in Group 1 followed by Group 2, then Group 3. The proportions of fillies with 1 or more ovulations (puberty) by the end of the project were 2/4, 5/5, 2/4 in Groups 1, 2 and 3, respectively. The hypothesis that a fixed daily photoperiod which corr...
Sobiraj A, Bostedt H.This review article deals with a critical comparison between the direct clinical diagnosis for the pregnancy of the mare (rectal and in some cases also vaginal exploration) and indirect methods. Both methods are discussed whether they can be seen as a mutual completion or are suitable for their own. The indirect methods for the pregnancy diagnosis include the hormone-analytic tests as progesterone concentration in serum or milk, the biological and immunological measurements for PMSG in the serum, finally the biological and chemical methods for estrogen contents in the urine of the mare. Furthe...
Roser JF, Dudan F, Little TV, Livesay-Wilkins P, Hillman RB.Studies were undertaken to investigate the effects of oxytocin induction on prolactin release in term (Group II) and preterm (Group III) mares and to compare these effects to spontaneously foaling mares (Group I). Since physiological concentrations of prolactin in blood have not been measured in the neonatal foal, experiments were designed to monitor prolactin in the cord artery and jugular blood of the foals from all groups of mares. Although prolactin levels varied in term mares (Group I and II) during the last 11 days of pregnancy, an increase was observed between Day -6 and Day 0 (2.7 and ...
Sönmez C, Eroglu A.The present study was carried out to investigate the pregnancy rate after covering in the foal heat (Group I), in the subsequent spontaneous heat (Group II), and in the induced heat (Group III) after administration of 7.5 mg of the prostaglandin F2 alpha analogue Luprostiol (Reprodin, Bayer) between the 20th and 22nd day post partum. Breeding during foal heat resulted in a pregnancy rate of 43.9% compared to 48.6% in the subsequent spontaneous heat post partum. Of 18 mares in group III, 14 mares had a foal heat, whereas 4 others had not shown a foal heat. 1-8 days after treatment, 14 mares (77...
Sarnecky BA, Vanderwall DK, Mason HM, Kirschner SM, Ambrose B, Parker TL.Prolonging function of the corpus luteum (CL) is a method of suppressing estrus that relies on continued secretion of endogenous progesterone to keep mares out of heat naturally. The use of oxytocin treatment to prolong CL function is gaining increasing use, and the most common treatment protocol involves administration of 60 units of oxytocin intramuscularly (IM) once daily on days 7-14 after ovulation (eight daily treatments). Although that protocol induces prolonged CL function in ≥70% of treated mares, the need for daily administration is a drawback to its use. Therefore, the objective o...
Sneddon JC, Van Der Walt J, Mitchell G, Hammer S, Taljaard JJ.We have investigated the change in plasma vasopressin and aldosterone concentrations in Namib (desert-adapted) and in control horses from a subtropical region, during an acute 12% dehydration and during rehydration, while food was available. During dehydration, vasopressin concentrations increased significantly in both groups of horses, but the increase was significantly greater in Namib horses than in control horses. During rehydration, vasopressin levels fell, but fell significantly less in Namib horses. The change in vasopressin concentration correlated significantly with plasma osmolality ...
