Gonadotropin response to naloxone in the mare: effect of time of year and reproductive status.
Abstract: In the mare, endogenous opioids have been implicated in the suppression of gonadotropin secretion during seasonal anestrus (AN). The present study tested whether continuation of reproductive activity during the nonbreeding season (NBS) reflects the absence of a seasonal shift in opioid tone compared to what occurs in AN mares. During the NBS, 11 AN and 8 luteal-phase mares received 0.1, 0.05, 0. 025 mg/kg naloxone (NAL) or vehicle on alternate days. Whereas cycling mares responded to all dosages of NAL, AN mares responded only to the higher dosages for FSH, and LH failed to increase at any dosage employed. During the breeding season (BS), the response to these dosages of NAL was reevaluated in 12 mares in the luteal phase of a synchronized estrous cycle. Although there was no difference between cycling mares during the breeding and nonbreeding seasons in FSH response, those mares that had cycled during the NBS showed a greater LH response to 0.05 mg/kg NAL than mares during the BS. From these data, we conclude that opioid tone is lower during the BS than during AN and that this shift in inhibitory tone does not occur in mares that cycle during the NBS. Thus, reduced opioid tone may play a role in the mechanisms controlling the nonseasonal exhibition of estrous cycles in the mare.
Publication Date: 1998-10-22 PubMed ID: 9780327DOI: 10.1095/biolreprod59.5.1195Google Scholar: Lookup
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- Journal Article
- Research Support
- Non-U.S. Gov't
Summary
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The research studied the role of endogenous opioids in the reproduction of mares, specifically their effect on gonadotropin secretion. The study found that opioid tone is lower during the breeding season than during anestrus and that this shift does not occur in mares that cycle during the nonbreeding season.
Objective and Methodology
- The research aimed to unravel the role of endogenous opioids in gonadotropin secretion during the seasonal anestrus and nonbreeding season in mares.
- The endogenous opioids have been previously suggested to suppress gonadotropin secretion during the nonbreeding season.
- The researchers set out to confirm whether the continuity of reproductive activity during the nonbreeding season reflects an absence of a seasonal shift in opioid tone when compared to anestrus mares.
- For the experiment, they used varying doses of Naloxone, a medication used for opioid overdose, on 11 anestrus and 8 luteal-phase mares during the nonbreeding season.
Results
- During the nonbreeding season, the cycling mares responded to all dosages of Naloxone. In contrast, the anestrus ones responded only to the higher dosages for Follicle Stimulating Hormone (FSH), and Luteinizing Hormone (LH) failed to increase at any of the doses used.
- The response to Naloxone dosages was reevaluated in 12 mares in the luteal phase of a synchronized estrous cycle during the breeding season.
- While there was no difference in FSH response between the cycling mares during the breeding and nonbreeding seasons, the mares that had cycled during the nonbreeding season showed a greater LH response to 0.05 mg/kg Naloxone than mares during the breeding season.
Conclusion
- The data suggest that opioid tone is lower during the breeding season than during anestrus. Moreover, this shift in inhibitory tone does not occur in mares that cycle during the nonbreeding season.
- The findings propose that reduced opioid tone could be a key factor in the mechanisms controlling the nonseasonal exhibition of estrous cycles in mares.
Cite This Article
APA
Davison LA, McManus CJ, Fitzgerald BP.
(1998).
Gonadotropin response to naloxone in the mare: effect of time of year and reproductive status.
Biol Reprod, 59(5), 1195-1199.
https://doi.org/10.1095/biolreprod59.5.1195 Publication
Researcher Affiliations
- Department of Veterinary Science, The Maxwell Gluck Equine Research Center, University of Kentucky, Lexington, Kentucky 40546-0099, USA. ldavison@pop.uky.edu
MeSH Terms
- Anestrus
- Animals
- Breeding
- Female
- Follicle Stimulating Hormone / metabolism
- Gonadotropins, Pituitary / metabolism
- Horses / physiology
- Hypothalamus / physiology
- Luteal Phase
- Luteinizing Hormone / metabolism
- Naloxone / administration & dosage
- Naloxone / pharmacology
- Narcotic Antagonists / pharmacology
- Pituitary Gland / physiology
- Reproduction
- Seasons
Citations
This article has been cited 1 times.- Kwong GPS, Klein C. Deslorelin and naltrexone stimulate follicular development in mares during autumn transition and early anestrus. Can Vet J 2019 Aug;60(8):855-858.
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