Prolonged pulsatile administration of gonadotrophin-releasing hormone (GnRH) to fertile stallions.

The research investigates the hormonal reactions in fertile male horses to extended administration of gonadotrophin-releasing hormone (GnRH). Results show that the hormone affects luteinizing hormone (LH), follicle stimulating hormone (FSH) and testosterone levels depending on the season, with greater effects observed in winter.

Background and Methodology

• The study was carried out on 7 fertile stallions during both the winter and summer seasons.
• The research focused on the effects of prolonged administration of gonadotrophin-releasing hormone (GnRH), a hormone crucial to the reproduction process.
• The stallions were split into 4 groups. Each group received varying amounts of GnRH: one stallion received 0.625 micrograms, while two animals each received 1.25, 2.5, or 5.0 micrograms, every 30 minutes for 5 days.
• Blood samples were collected daily from 5 days before to 5 days after the treatment. These were analyzed for plasma concentrations of various hormones: luteinizing hormone (LH), follicle-stimulating hormone (FSH), testosterone (T), and estrogen conjugates (EC).

Hormonal Effects and Seasonal Differences

• Concentrations of LH and testosterone were found to be higher during summer before treatment, whereas FSH and EC concentrations remained consistent between the two seasons.
• Increasing the dose of GnRH seemed to elevate the plasma concentrations of LH, FSH, and testosterone above the baseline in winter. However, a significant effect was not observed during summer. Plasma EC concentrations remained unaffected by treatment in either season.

Correlations in Hormone Concentrations

• In the treated stallions, there was a positive correlation between plasma LH and testosterone concentrations in winter – the hormones’ profiles seemingly changed in a similar direction over time. This correlation wasn’t observed in summer.
• FSH and EC concentrations weren’t correlated in either season. A positive correlation between LH and FSH concentrations was observed in both seasons.
• But these changes in LH, testosterone, FSH, and EC concentrations were not correlated over time in the control stallions. Instead, a positive correlation between LH and FSH was notably observed in summer, but not winter.

Effects of GnRH Administration on LH Pulse Patterns

• Before treatment, the pattern of LH pulses was similar in both seasons. However, post GnRH administration, several changes were noted in the winter: an increase in pulse frequency, a decrease in pulse duration, resulting in a lower total amount of LH released per pulse.
• The treatment had no effect on the amplitude and interval of the pulses in either season.

Research Conclusion

• The study concludes that the pituitary gland and testes of fertile stallions respond more to the administration of prolonged pulsatile GnRH in winter than summer.