Differential Regulation of Gonadotropins in Response to Continuous Infusion of Native Gonadotropin-Releasing Hormone in the Winter Anovulatory Mare and Effects of Treatment With Estradiol-17β.
Authors: O'Neil Meaghan M, Korthanke Curtis M, Scarpa José O, Welsh Thomas H, Cardoso Rodolfo C, Williams Gary L
Journal: Journal of equine veterinary science
Summary
# Editorial Summary Winter anovulation in mares presents a significant management challenge, yet the neuroendocrine mechanisms underlying seasonal infertility remain incompletely understood. Researchers at Texas A&M University investigated how the pituitary gland responds to continuous GnRH stimulation during winter dormancy, and whether oestradiol supplementation could enhance this response using two experimental designs: the first compared injectable oestradiol combined with GnRH infusion against controls over 14 days, whilst the second evaluated commercial cattle oestradiol implants (at high and low doses) combined with continuous GnRH over up to 28 days in groups of 20 and 24 anovulatory mares respectively. Injectable oestradiol (5 mg twice daily) significantly amplified luteinising hormone (LH) secretion from day 3 onwards in GnRH-treated mares, and by day 28, the injectable oestradiol group achieved follicle development and ovulation in five of six mares; however, the commercial implants paradoxically failed to enhance gonadotropin secretion despite producing marked uterine oedema, with only two of six high-dose implant mares developing 35 mm follicles. Critically, GnRH infusion produced distinctly different hormonal profiles—follicle-stimulating hormone (FSH) spiked within 24 hours then returned to baseline by day 4, whilst LH showed sustained elevation throughout the treatment period—yet these varying gonadotropin patterns did not explain the differences in ovarian responsiveness between treatment groups. For practitioners, these findings suggest that injectable oestradiol protocols warrant further investigation as adjunctive therapies for winter anovulation, whilst highlighting that ovarian response to GnRH stimulation is influenced by factors beyond
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Practical Takeaways
- •For winter anovulatory mares, twice-daily estradiol injections combined with continuous GnRH infusion significantly improve the likelihood of follicle development and ovulation compared to GnRH alone—practical for breeding programs requiring specific breeding windows
- •Commercial cattle estradiol implants are not appropriate for stimulating reproductive cycling in mares and should not be substituted for injectable estradiol when treating winter anestrus
- •Continuous GnRH infusion alone can successfully induce follicular development and ovulation in anovulatory mares without additional estradiol treatment in over 50% of cases, offering a single-modality alternative approach
Key Findings
- •Twice-daily injections of 5 mg estradiol-17β enhanced pituitary LH secretion in response to continuous GnRH infusion (100 μg/hr) beginning on Day 3, with significantly greater LH levels than GnRH alone (P < 0.001)
- •Commercial estradiol-17β cattle implants (Compudose) failed to increase plasma estradiol or estrone sulfate and did not enhance LH response despite inducing marked uterine edema
- •Continuous GnRH infusion produced a differential gonadotropin secretion pattern: FSH showed marked but transient increase within 24 hours returning to baseline by Day 4, while LH showed sustained elevation (P < 0.0001)
- •By Day 28, GnRH treatment alone achieved 6/6 mares developing 35-mm preovulatory follicles, while GnRH + high-dose E2 implant achieved only 2/6, suggesting injected estradiol superior to implant formulation for reproductive stimulation