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Topic:Endocrine System
The endocrine system in horses comprises a network of glands and hormones that regulate various physiological processes, including metabolism, growth, reproduction, and stress response. Key components of the equine endocrine system include the pituitary gland, thyroid gland, adrenal glands, and pancreas. Hormones such as insulin, cortisol, and thyroid hormones are produced and released into the bloodstream to maintain homeostasis and respond to internal and external stimuli. Dysregulation of the endocrine system can lead to conditions such as Equine Cushing's Disease (Pituitary Pars Intermedia Dysfunction) and Equine Metabolic Syndrome. This page compiles peer-reviewed research studies and scholarly articles that explore the structure, function, and disorders of the endocrine system in horses, providing insights into its impact on equine health and management.
Glucocorticoid suppression of oestrus, follicles, LH and ovulation in the mare. Dexamethasone, a synthetic glucocorticoid, was administered (30 mg/day) from Day 10 after ovulation to assess the involvement of the adrenal glands in the ovulatory cycle. Only 1 of 8 mares treated in this way exhibited behavioural oestrus, compared to 7 of 8 control mares. Mean maximum LH concentration and follicle size were significantly reduced. Ovulation occurred in 1 and possibly in 2 other treated mares, compared to all 8 control mares. The results demonstrated that dexamethasone can interfere with ovulation and associated events in the mare, but the mechanism of action is uncertain.
Effects of exogenous steroids on serum FSH and LH, and on follicular development in cyclic mares. Cyclic mares were given daily i.m. injections of 150 mg progesterone (Group P, N = 4), 150 mg progesterone and 10 mg oestradiol-17 beta (Group PE, N = 3), 10 mg oestradiol-17 beta (Group E, N = 3) or cottonseed oil vehicle (Group C, N = 4), from the day after ovulation (Day 1) to Day 28. Blood samples were collected daily, and the ovaries were palpated every 1-2 days. Serum FSH and LH concentrations were measured in all samples, and means determined for 7 consecutive 4-day periods throughout treatment. Comparisons within each steroid treatment group between time periods and comparisons between...
Effect of prolonged administration of anabolic and androgenic steroids on reproductive function in the mare. Administration to mares of the anabolic steroid, methandriol, at the maximum recommended dose (300 mg every 3 weeks) for 1 1/2 years had no effect on reproductive characteristics except for suppression of GnRH-induced LH release and a tendency to suppress basal LH levels and the height of the ovulatory LH surge. A 4-fold increase in dosage caused marked suppression of basal LH, the LH surge, and GnRH-induced LH release. Other reproductive responses were minimally affected. There were no behavioural effects, and no changes in weight occurred when mares were compared with matched controls. Small...
Radioimmunoassay and in-vitro bioassay of serum LH throughout the equine oestrous cycle. Mares were bled once daily throughout a cycle, or 3 times daily from the first day of oestrus to the 2nd day after ovulation. LH was measured by heterologous radioimmunoassay and by an in-vitro bioassay based on LH-stimulated testosterone production by mouse Leydig cells. The patterns of bio- and immuno-active LH during the oestrous cycle were similar but not identical, so that in both groups of mares the ratio of biological: immunological (B:I) activity during the LH surge was significantly higher before than after ovulation (P less than 0 . 001). Considerable individual variation in cycle me...
Importance of testicular hormones in maintaining the annual pattern of LH secretion in the male horse. Serum LH was measured by radioimmunoassay in 5 long-term (greater than or equal to 3 years) castrated male horses bled at 30-min intervals for 2 h twice a month from March to February. There was no significant effect of month on LH levels; however, mean levels were significantly lower in spring than autumn (P less than 0.05). By contrast, stallions in the same environment showed a markedly seasonal pattern of LH secretion, with LH rising at the onset of the breeding season to reach levels in late spring 3-4 times those in early winter. Despite differences in seasonal patterns of secretion, ann...
Changes in equine follicular aromatase activity during transition from winter anoestrus. Follicular aromatase activity during sexual resurgence after the winter anoestrus was investigated in 3 groups of 5 Pony mares. Group ET was studied during the early transition period, Group LT in late transition and Group C in full breeding condition. Granulosa and theca cells were incubated for 3 h with 3H-labelled androstenedione or progesterone. Analysis of the free oestrogenic products was by high performance liquid chromatography (HPLC) and recrystallization revealed highly variable oestrogen production in both cell types from mares in all 3 groups. Only oestrone and oestradiol peaks wer...
Pancreatic beta cell function in the neonatal foal. Plasma concentrations of insulin and glucose were measured in Pony and Thoroughbred foals at birth and at intervals thereafter for up to 7 days. The plasma concentrations of insulin in foals of both breeds at birth were 11.2 +/- 2.5 microU/ml (N = 6) and 13.5 +/- 1.5 microU/ml (N = 16) respectively. These values were similar to those obtained for foals in utero but were significantly less than those in adult animals. There was little variation in the plasma concentrations of insulin and glucose during the first 2 h of life and both concentrations tended to rise in the next 24-48 h. Although th...
Effects of pituitary stalk transection on endocrine function in Pony mares. The pituitary stalk was transected in 10 Pony mares by a surgical approach that involved dorsal reflection of the brain and micro-dissection from the ventro-lateral aspect of the pituitary. Diabetes insipidus was the most immediate and marked result, requiring extensive electrolyte and antidiuretic therapy for approximately 48 h after operation. Fluid stasis then developed and no further supportive measures were necessary. Endocrine challenge tests with GnRH and TRH before and after stalk transection indicated a loss of responsiveness (GnRH) or suppressed responsiveness (TRH) after the operati...
Secretion of free and conjugated steroids by the horse testis into lymph and venous blood. In 3 testes of 2 adult Pony stallions under halothane anaesthesia, catheters were inserted into a vein and a lymphatic vessel in the spermatic cord and into a vein on the surface of the testis. Lymph and venous blood were collected from the catheters in the cord and p-aminohippurate (2% w/v, 0 . 1 ml/min) was infused into the vein on the testis to determine blood flow by dilution. After 1 h, 6000 i.u. hCG was injected i.v. and collections continued for 45 min. The testes weighed 126-176 g. Lymph flow was 20-150 microliter/min before hCG and 100-270 microliter/min after hCG; the range of blood ...
Seasonal and individual effects on ovarian and endocrine responses of mares to a synchronization treatment with progestagen-impregnated vaginal sponges. The 8 saddle-type mares were treated with progestagen-impregnated vaginal sponges for 8 days and prostaglandin on the day of sponge removal. The treatment was given at random days of the cycle in April, May, July and September. Sponge insertion induced a sharp decrease in LH levels; at sponge removal, there was an immediate increase in LH and a large FSH rebound. Ovulation was synchronized 11 +/- 2 . 8 days (s.d.) after the end of treatment. In April the interval from sponge removal to ovulation was longer (14 days compared with 10 . 1, 10 . 7 and 10 . 2 days), the basal and peak LH levels low...
Plasma oxytocin concentrations in cyclic mares and sexually aroused stallions. An experiment was conducted to measure plasma oxytocin concentrations at 4 different stages of the estrous cycle in 11 pony mares. Plasma oxytocin concentrations (muU/ml +/- SE) were found to be higher (P<.01) on day 2 of estrous (39.8 +/- 12.5) and day 5 post-ovulation (33.1 +/- 12.0) than on day 10 (2.3 +/- 1.6) and day 15 post-ovulation (6.8 +/- 4.1). A second experiment was conducted to measure jugular plasma oxytocin concentrations before and after sexual arousal in six pony stallions. Oxytocin concentrations (muU/ml +/- SE) were higher (P<0.06) after sexual arousal (50.5 +/- 8.9) than be...
Clinical, morphological and endocrinological aspects of cryptorchidism in the horse. The authors analysed clinical, histological and hormonal data obtained from 205 cryptorchid horses. The majority of the unilaterally and bilaterally retained testes were located in the inguinal canal; however, the ratio of inguinal vs abdominal retention appeared to decrease with advancing age. In unilateral cryptorchidism, a pronounced preference was noted for left abdominal retention, whereas for inguinal cryptorchids, the retained testes occurred equally on both sides. Right inguinal retention was found to decrease with advancing age. Histology of cryptorchid testes revealed apparently norm...
Behavioral, follicular and gonadotropin changes during the estrous cycle in donkeys. Sexual behavior, follicular development and ovulation, and concentrations of circulating gonadotropins during the estrous cycle were studied during the summer in 7 jennies. Mean behavioral estrous length was 6.4 +/- 0.6 days (mean +/- SEM, n=19; 5.6 +/- 0.5 days preovulatory and 0.8 +/- 0.2 days post-ovulatory). Mean diestrous length was 19.3 +/- 0.6 days (n=14). Females in estrus typically showed posturing, mouth clapping, clitoral winking, urinating and tail raising. Mouth clapping began approximately one day sooner and lasted approximately one day longer than winking and tail raising, so th...
Puberty in the female pony: reproductive behavior, ovulation, and plasma gonadotropin concentrations. Reproductive behavior and gonadotropin concentrations were studied in 14 female ponies during the period from 10 to 21 months of age (February 1978 to January 1979). Nine fillies were born during April of 1977 (spring-born) and five were born during the summer and fall of 1977 (late-born). Three of the spring-born fillies had been ovariectomized (OVX) at 4 months of age. All intact spring-born fillies ovulated during late spring when they were 12–15 months old. Two of five late-born fillies did not ovulate, and there tended to be fewer ovulations and a shorter breeding season in late-born th...
Effect of manipulating central catecholamines on puberty and the surge of luteinizing hormone and gonadotropin releasing hormone induced by pregnant mare serum gonadotropin in female rats. We have investigated the effect of manipulating central catecholamines on the timing of puberty (as assessed by vaginal opening) in female rats and the surge of luteinizing hormone (LH) and gonadotropin releasing hormone (GnRH) induced by pregnant mare serum gonadotropin (PMSG) in immature female rats. Manipulation of the catecholamines was carried out with either 6-hydroxydopamine (6-OHDA) administered with or without either desipramine (DMI) or pargyline, or alpha-methyl-p-tyrosine (alpha-MPT). The neonatal administration of 6-OHDA delayed puberty, an effect which was potentiated by pretreat...
[Study, after stopping treatment, of the consequences of the injection of male hormones in mares on their social behavior and hierarchical position]. The androgenization of a more belonging to a social group where it held a stable hierarchic rank, or a mare recently admitted to this group, increases their status in the hierarchic order and the position acquired is subsequently maintained; in some cases, they continue to rise in the hierarchic order long after the injections of male hormones has been stopped. Some elements of the social behaviour of a stallion appear during the treatment; these elements may persist long after the mares have regained their female hormone balance, corroborated by the establishment of a standard pregnancy.
Development of the adrenal cortex in the fetal foal: an ultrastructural study. The adrenal cortex from twelve fetal foals (gestational ages from 61 to 300 days) was examined by light and electron microscopy. Adrenal glands from three newborn foals were also examined by light microscopy. Between 61 and 100 days of gestation the adrenal cortex became organised into two distinct regions, the zona glomerulosa and zona fasciculata, which grew steadily in thickness until the 300th day. Between 300 days and birth there was a dramatic increase in the width of the zona fasciculata. From 200 days a narrow band of compact cells marked the cortico-medullary border. Though these cell...
Pancreatic beta-cell function in the fetal foal and mare. Insulin secretion and the factors influencing beta-cell function were investigated in the chronically catheterized fetal foal and mare during the second half of gestation. The response of the fetal beta cells to exogenous glucose was also examined. The mean concentration of insulin in the fetal foal was 7.5 +/- 0.5 (S.E.M.) microunit./ml (n = 20) which was significantly less than the corresponding maternal value of 49.0 +/- 5.0 microunit./ml (n = 20, P < 0.01). The insulin concentration in non-pregnant horses was 24.5 +/- 1.5 microunit./ml (n = 5) which was significantly less than the value...
Regulation of the estrous cycle in domestic animals– a review. Neuroendocrine and endocrine factors involved in the regulation of reproductive cycles in domestic animals are discussed. Although research data from many species are considered, emphasis is placed on their relevance for the cow, sow, ewe and, to a lesser extent, the mare. Literature cited is not designed to be complete, but rather to be representative of the large volume of material which has been written on the subject.Gonadotropin-releasing hormone is synthesized and secreted in response to various exteroceptive stimuli, but both its release and its effects on the anterior pituitary are mod...
The effect of season on the ovulatory response of Merino ewes to serum from pregnant mares. Groups of 50 ewes were injected with serum from pregnant mares on 5 occasions, 3 months apart, and ovulation rate was estimated at laparoscopy. In Exp. 1, new ewes were used at each time and 3 dose rates (identical to 400, 630 and 1000 i.u. PMSG) were studied. In Exp. 2, the same ewes were used throughout and only 1 dose was given (1000 i.u.). In both experiments the response in ovulation rate of ewes varied significantly throughout the year, with the lowest response in spring (September) and the highest in autumn (March). The ewes in Exp. 2 were as responsive at the end as at the beginning of...
Response of plasma LH and FSH to gonadotropin-releasing hormone in pony foals and ovariectomized pony mares. Plasma FSH and LH response to a synthetic GnRH analog was measured in adult ovariectomized pony mares (OVX) and in pony foals (<70 days of age) during late spring (May-June). FSH and LH responded in a similar fashion (200% increase) in the OVX mare, which is different from other reports for intact mares. There was a greater mean response to a comparable dose of GnRH in the prepubertal foal for both FSH (500%) and LH (900%) than in the OVX mare. There was a positive correlation between age and the maximum FSH response to GnRH in male and female foals. The LH response was positively correlate...
