Effect of graded doses of gonadotrophin-releasing hormone on serum LH concentrations in mares in various reproductive states: comparison with endogenously generated LH pulses.

The research conducted an experiment to determine the effects of reduced doses of Gonadotropin-releasing hormone (GnRH) on Luteinizing hormone (LH) levels in mares that were acyclic, in a luteal phase, and in a follicular phase. The study suggests a smaller-than-usual GnRH dose can stimulate a similar response as an endogenous LH surge during the luteal and follicular phases.

Research Methodology

• The research measured LH release in response to both small (3.3-33 micrograms) and large (300-500 micrograms) doses of GnRH in a sample of mares in different reproductive states: acyclic (n = 4), luteal phase (n = 3), and follicular phase (n = 5).
• The observed LH pulses were then compared with endogenously generated LH pulses in the same reproductive states.
• Analysis of log-linear dose-response curves was undertaken to compare the reactions to the injections of GnRH.

Key Findings

• The researchers started by extrapolating from log-linear dose-response curves. They found that to simulate an LH pulse comparable to a naturally occurring (endogenous) one, much smaller doses of GnRH were required: 7.0 (n = 4) micrograms for luteal phase mares and 4.1 (n = 6) micrograms for follicular phase mares, compared to the usual 500 micrograms clinically given. However, for acyclic mares, LH pulses were not frequent enough to characterize.
• The study also discovered that small doses of GnRH induced an LH release that was similar across all three reproductive states, regardless of the differences in the surrounding steroid hormone environment.
• The observed mean variations in serum LH concentrations between states (acyclic, luteal, and follicular) were suggested to have been produced by differences in GnRH pulse frequency and/or amplitude rather than by changes in pituitary response to GnRH induced by steroids.
• LH pulse frequency was significantly lower in acyclic and luteal mares compared to those in the follicular phase, indicating the key role pulse frequency plays in determining average LH concentration.