Determinants of attenuated LH-release associated with the first ovulation of the equine breeding season.
Abstract: An attenuated ovulatory rise in circulating concentrations of LH is characteristically associated with the first seasonal reproductive cycle of horse mares. Unlike ovulations (OV) of subsequent estrous cycles, the first OV of the breeding season (OV1) is not preceded by elevated concentrations of progesterone (PROG). Hence, the ability of pretreatment with PROG to abolish attenuation of LH-secretion associated with OV1 was investigated. Ten nonpregnant anestrous mares were randomly divided into 2 groups; control (C) and treated (T). Per individual, when diameter of the largest follicle was consistently greater than or equal to 35 mm, C mares received 3 times daily injections of cottonseed oil (IM), for 15 d or until OV, while T mares received exogenous PROG (IM) for 15 d, in a manner designed to mimic a diestrous pattern of release. Jugular blood samples were collected daily from onset of treatment through 10 d following the third OV (OV3). Repeated measures analyses of area under the ovulatory LH-rise (AUC) and the maximum concentration of LH associated with OV (MAX) revealed a significant main effect of OV (P less than .005) but no main effect of group or OV by group interaction (P greater than or equal to .5). When groups were combined, a significant increase in mean AUC and MAX from OV1 to OV3 was observed (P less than .01). To evaluate the influence of hypothalamic-hypophyseal recrudescence on ovulatory LH-release at OV1, 16 mares were bred to foal, and subsequently initiate reproductive activity, early (E; mid April; n = 8) or late (L; mid July; n = 8) in the year. A significant OV (1, 2 or 3) by group (E or L) interaction was observed for AUC (P less than .06) and MAX (P less than .04). Mean AUC and MAX increased progressively from OV1 to OV3 in E mares (P less than .05). In L mares, neither AUC or MAX changed from OV1 to OV3 (P greater than .4). Based on these data, we suggest that attenuation of the LH-rise at OV1 in the mare, is a consequence of incomplete recrudescence of the hypothalamic-pituitary axis.
Publication Date: 1991-04-01 PubMed ID: 2070600DOI: 10.1016/0739-7240(91)90061-nGoogle Scholar: Lookup
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- Journal Article
- Research Support
- Non-U.S. Gov't
Summary
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The research investigates the attenuated (reduced) release of Luteinising Hormone (LH) during the first ovulation of the horse breeding season, specifically studying the effect of progesterone treatment and the role of the hypothalamic-pituitary axis in this process.
Study Design
- The researchers undertook a study involving ten non-pregnant anestrous mares that were randomly divided into two groups: Control and Treated.
- When the diameter of the largest follicle exceeded or equaled 35 mm, control mares received three daily injections of cottonseed oil, for 15 days or until ovulation. Treated mares received exogenous progesterone for the same duration, designed to mimic a diestrous pattern of release.
- Jugular blood samples were collected from the onset of the treatment through ten days following the third ovulation.
- For further examination of the influence of hypothalamic-pituitary axis on ovulatory LH-release, 16 mares were bred to foal and initiate reproductive activities either early or late in the year.
Results & Interpretation
- Analysis of ‘area under the curve'(AUC) and the maximum concentration of LH associated with ovulation found a significant increase from first to third ovulation. However, there was no significant difference between the control and treated groups, implying that the pretreatment with progesterone did not have a significant effect on attenuating LH-secretion.
- The research also found that hypothalamic-pituitary axis recrudescence (process of becoming active again after a period of dormancy) significantly influenced LH-release during the first ovulation of the season.
- Specifically, mares that started breeding early in the year showed a progressive increase in mean AUC and LH concentration from the first to third ovulation, while mares that started breeding late in the year showed no significant changes. This finding suggests that early initiation of breeding results in better preparedness and higher performance of the reproductive system in mares.
- Based on the data collected, researchers concluded that the attenuated LH-rise at the first ovulation of the breeding season is a result of incomplete recrudescence of the hypothalamic-pituitary axis, rather than the lack of elevated concentrations of progesterone.
Significance of the Study
- This research provides insights into hormonal regulation during the onset of the breeding season in mares, by elucidating the role of the hypothalamic-pituitary axis and its recrudescence.
- The findings help clarify that progesterone pretreatment does not significantly influence LH-secretion around ovulation. This conclusion has important implications for breeding strategies and monitoring reproductive health in mares.
Cite This Article
APA
Silvia PJ, Fitzgerald BP.
(1991).
Determinants of attenuated LH-release associated with the first ovulation of the equine breeding season.
Domest Anim Endocrinol, 8(2), 255-264.
https://doi.org/10.1016/0739-7240(91)90061-n Publication
Researcher Affiliations
- University of Kentucky, Department of Veterinary Science, Lexington 40546-0099.
MeSH Terms
- Animals
- Estrus / metabolism
- Female
- Horses / metabolism
- Hypothalamo-Hypophyseal System / physiology
- Luteinizing Hormone / metabolism
- Ovarian Follicle / physiology
- Ovulation / physiology
- Progesterone / blood
- Progesterone / pharmacology
- Random Allocation
Citations
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