The Nonpregnant Mare: A Review of Some Current Research and of the Last 25 Years of Endocrinology

Irvine, C. H. G.

doi:10.1093/biolreprod/52.monograph_series1.343

Cited by 11 publications

(5 citation statements)

References 73 publications

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“…To overcome the first problem, we applied our unique nonsurgical method for collecting pituitary venous blood from horses (7). This blood contains readily measurable concentrations of hypothalamic and pituitary hormones, and samples large enough to allow many different hormones to be assayed can be collected every 0.5–1 min for many hours (8, 9). Sampling pituitary venous blood has given much insight into the regulation of the gonadal (9) and adrenal (8) axes.…”

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confidence: 99%

“…This blood contains readily measurable concentrations of hypothalamic and pituitary hormones, and samples large enough to allow many different hormones to be assayed can be collected every 0.5–1 min for many hours (8, 9). Sampling pituitary venous blood has given much insight into the regulation of the gonadal (9) and adrenal (8) axes. To overcome the second problem, we validated a highly specific radioimmunoassay for TRH for use in the horse, and assessed our model rigorously for sites of TRH loss.…”

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confidence: 99%

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Inter‐Relationships Between the Secretory Dynamics of Thyrotrophin‐Releasing Hormone, Thyrotrophin and Prolactin in Periovulatory Mares: Effect of Hypothyroidism

Alexander

Irvine

Evans

2004

J Neuroendocrinology

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We used our nonsurgical technique for collecting pituitary venous blood to relate the dynamics of thyrotrophin-releasing hormone (TRH) secretion to the secretion patterns of both prolactin and thyrotrophin in periovulatory mares, either euthyroid (n = 5) or made hypothyroid by treatment with propyl-thiouracil (n = 5). Pituitary venous blood was collected continuously and divided into 1-min aliquots for 4 h. To test the effect of dopamine on the relationship between secretion patterns, sulpiride, a selective D2 receptor antagonist, was given i.m. after 2 h of sampling. Thorough testing of the model and blood collection procedure revealed no sites of TRH loss. Hypothyroidism increased the mean secretion rates of TRH (P = 0.04) and thyrotrophin (P < 0.0001) but not prolactin. Sulpiride increased prolactin secretion rates in hypothyroid (P < 0.0001) and control (P = 0.007) mares, but did not alter TRH or thyrotrophin secretion rates. In both groups of mares, all three hormones were secreted episodically but not rhythmically. In both groups, the secretion pattern of TRH was almost always significantly related to that of thyrotrophin, as assessed by cross correlation and cross approximate entropy (ApEn) analysis. However, the degree of linear correlation was weak, with only 14% (hypothyroid) or 8% (controls) of the variation in thyrotrophin secretion rates attributable to TRH. Prolactin and TRH secretion patterns before sulpiride were coupled on cross ApEn analysis in both groups, and the minute-to-minute secretion rates of the two hormones were correlated in four hypothyroid and three euthyroid mares. Overall, the small, but significant, degree of association between TRH and prolactin was similar to that between TRH and thyrotrophin. In hypothyroid mares, sulpiride increased (P = 0.02) the synchrony between TRH and prolactin patterns. We conclude that in horses: (i) little TRH degradation occurs during passage through the pituitary or in blood after 1 h at 37 degrees C; (ii) TRH is not the major factor controlling minute-to-minute fluctuations in either thyrotrophin or prolactin; and (iii) reducing two strongly inhibitory inputs (i.e. dopamine and thyroid hormones) may magnify the stimulatory effect of TRH on prolactin secretion.

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Inter‐Relationships Between the Secretory Dynamics of Thyrotrophin‐Releasing Hormone, Thyrotrophin and Prolactin in Periovulatory Mares: Effect of Hypothyroidism

Alexander

Irvine

Evans

2004

J Neuroendocrinology

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“…During anestrus, pituitary gland LH content is significantly lower than in the breeding season, whereas pituitary FSH concentrations do not change [17,45,46]. Most likely a reduced secretion of GnRH during anestrus underlies these changes in gonadotropins (see for review [47]). It has been demonstrated that this seasonal variation in pituitary gland content is also accompanied by a change in responsiveness to GnRH, which is highest during the breeding season [48].…”

Section: Discussionmentioning

confidence: 99%

Comparison of the Effects of N-Methyl-DL-Aspartic Acid on Gonadotropin and Prolactin Secretion in Anestrous Mares and Mares Exhibiting Estrous Cycles during Anestrus1

Fitzgerald¹,

Davison²

1997

Biology of Reproduction

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This study investigated the hypothesis that for a subpopulation of horse mares continuation of estrous cycles during the nonbreeding season may be attributed to continued stimulatory glutamatergic activity on GnRH-secreting neurons. The gonadotropin response to the glutamatergic agonist N-methyl-DL-aspartic acid (NMA) was compared in cycling and anestrous mares during the nonbreeding season. It was anticipated that the gonadotropin response to NMA in cycling mares would be attenuated, compared with that of anestrous mares. The experiment used 16 anestrous mares and 15 mares that cycled during the nonbreeding season. The effect of NMA on prolactin secretion was also evaluated. In addition, the seasonal rhythm of prolactin secretion was compared in anestrous and cycling mares during October-April. In cycling mares, the response to NMA was dependent on the stage of the cycle, and a significantly (p < 0.05) larger proportion responded during the luteal phase (6 of 8), compared with the follicular phase (1 of 7 mares). The proportion of anestrous mares that responded to NMA was similar to that of cycling mares during the luteal phase, but larger than during the follicular phase. In anestrous and cycling mares, NMA suppressed prolactin secretion, and in both groups prolactin secretion decreased during the nonbreeding season. Thus, we conclude that differences in reproductive activity in mares during the nonbreeding season are unlikely to reflect a change in glutamatergic activity.

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“…Mares are seasonally polyestrous, with onset of cyclicity beginning in late winter in the Northern Hemisphere (see Irvine, 1995). Ovarian activity is inhibited by decreasing day length (photoperiod), but often lags several months after the solstice.…”

Section: Estrous Cycle and Luteolysismentioning

confidence: 99%

Pregnancy recognition signals in mammals: the roles of interferons and estrogens

Bazer

Johnson

2017

Anim Reprod

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Reproduction is a highly complex biological process requiring a dialogue between the developing conceptus (embryo-fetus and associated placental membranes) and maternal uterus which must be established during the peri-implantation period for pregnancy recognition signaling and regulation of gene expression by uterine epithelial and stromal cells. The uterus provide a microenvironment in which molecules secreted by uterine epithelia or transported into the uterine lumen represent histotroph or the secretome required for growth and development of the conceptus and receptivity of the uterus to implantation by the conceptus. Pregnancy recognition signaling as related to sustaining the functional lifespan of the corpora lutea (CL) which produce progesterone; the hormone of pregnancy essential for uterine functions that support implantation and placentation required for successful outcomes of pregnancy. It is within the periimplantation period that most embryonic deaths occur in mammals due to deficiencies attributed to uterine functions or failure of the conceptus to develop appropriately, signal pregnancy recognition and/or undergo implantation and placentation. The endocrine status of the pregnant female and her nutritional status are critical for successful establishment and maintenance of pregnancy. The challenge is to understand the complexity of key mechanisms that are characteristic of successful reproduction and to use that knowledge to enhance fertility and reproductive health of animals including nonhuman primates. It is important to translate knowledge gained from studies of animals to address issues of fertility and reproductive health in humans.

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The Nonpregnant Mare: A Review of Some Current Research and of the Last 25 Years of Endocrinology

Cited by 11 publications

References 73 publications

Inter‐Relationships Between the Secretory Dynamics of Thyrotrophin‐Releasing Hormone, Thyrotrophin and Prolactin in Periovulatory Mares: Effect of Hypothyroidism

Inter‐Relationships Between the Secretory Dynamics of Thyrotrophin‐Releasing Hormone, Thyrotrophin and Prolactin in Periovulatory Mares: Effect of Hypothyroidism

Comparison of the Effects of N-Methyl-DL-Aspartic Acid on Gonadotropin and Prolactin Secretion in Anestrous Mares and Mares Exhibiting Estrous Cycles during Anestrus1

Pregnancy recognition signals in mammals: the roles of interferons and estrogens

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