A negative feedback role for the ovaries of the mare on tonic LH secretion before the first ovulation of the breeding season.
Abstract: Eleven anoestrous mares were assigned randomly to Group A (intact, n = 6) or Group B (ovariectomized in January, n = 5). Jugular blood samples were collected during February to April. Ovarian activity was assessed by ultrasound and ovulation was confirmed by progesterone analysis. Intact mares ovulated between 2 and 28 April. Mean diameter of the largest follicle was less than 20 mm on 17 or 18 March (Period 3), but increased to 29 mm by 31 March or 1 April (Period 4). During Periods 1 and 2 (14 or 15 February and 3 or 4 March) mean luteinizing hormone (LH) concentrations were similar in both groups. In intact mares, circulating concentrations remained low during Periods 3 and 4. In contrast, in ovariectomized mares concentrations tended to increase during Period 3, and were significantly increased by Period 4, at which time concentrations were significantly (P less than 0.05) greater in ovariectomized as opposed to intact mares. Increased concentrations of LH in ovariectomized mares during Periods 3 and 4 were associated with well defined pulsatile profiles. In contrast, pulses in intact mares remained infrequent and low in amplitude during the same time periods. The absence of LH pulses in intact mares during Periods 3 and 4 may reflect a low pulse frequency or a decrease in amplitude such that pulses escape detection. These results support the hypothesis that in intact mares, low circulating concentrations of LH during the transition into the breeding season, in part, may reflect inhibition by a factor of ovarian origin. It remains to be determined whether this apparent inhibition of LH secretion undergoes a seasonal modification in effectiveness and plays a role in regulating the annual breeding cycle of the mare.
Publication Date: 1991-01-01 PubMed ID: 1795266
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- Journal Article
- Research Support
- Non-U.S. Gov't
Summary
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The research examines a possible feedback mechanism in mare ovaries that influences LH (luteinizing hormone) secretion prior to the first round of breeding. It suggests that one or more factors originating from the ovaries restrain the levels of circulating LH during the transition into the breeding phase.
Methodology
- Eleven anoestrous (not in heat) mares were arbitrarily grouped into two: Group A – intact (6 mares) and Group B – ovariectomized (ovaries removed) in January (5 mares).
- During the months of February to April, blood samples were drawn from the jugular vein.
- Ovarian activity was gauged through ultrasound and ovulation was affirmed through progesterone analysis.
- Intact mares (Group A) ovulated anywhere between 2 and 28 April.
Results
- During periods 1 and 2 (14 or 15 February and 3 or 4 March), the LH concentrations were similar in both groups.
- In intact mares (Group A), the concentration of LH in the bloodstream remained low during periods 3 and 4.
- While in the ovariectomized group (Group B), LH concentrations started to heighten in period 3 and drastically increased by period 4.
- LH levels in Group B during this period were significantly higher than in Group A.
- Increased LH levels in the ovariectomized mares during periods 3 and 4 were related to well-defined pulsatile profiles.
- Mares in Group A showcased infrequent and low-amplitude pulses during the same timeframe.
- The absence of LH pulses in intact mares during periods 3 and 4 might have been due to low pulse frequency or a decrease in amplitude leading to pulses going undetected.
Interpretation and Hypotheses
- The study indicates that in intact mares, low LH concentrations during the transition into the breeding season may hence, be influenced by a factor of ovarian origin.
- It remains unknown whether this presumed hindrance of LH production encounters a seasonal change in effectiveness and contributes to regulating the annual breeding cycle of the mare.
Cite This Article
APA
Affleck KJ, Conboy HS, Fitzgerald BP.
(1991).
A negative feedback role for the ovaries of the mare on tonic LH secretion before the first ovulation of the breeding season.
J Reprod Fertil Suppl, 44, 241-247.
Publication
Researcher Affiliations
- Department of Veterinary Science, Maxwell Gluck Equine Research Center, University of Kentucky, Lexington 40546-0099.
MeSH Terms
- Anestrus / physiology
- Animals
- Feedback / physiology
- Female
- Gonadotropins, Equine / blood
- Gonadotropins, Equine / metabolism
- Horses / physiology
- Luteinizing Hormone / blood
- Luteinizing Hormone / metabolism
- Ovariectomy
- Ovary / physiology
- Ovulation / physiology
- Seasons
Citations
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