Species differences in the sensitivity to a GnRH antagonist

Area of Science:

• Reproductive Endocrinology
• Pharmacology

Background:

• Gonadotropin-releasing hormone (GnRH) plays a crucial role in regulating reproductive functions.
• GnRH antagonists offer a potential therapeutic strategy for controlling reproductive processes.

Purpose of the Study:

• To investigate the efficacy of a potent GnRH antagonist ([Ac-D-NAL(2)1, 4F-D-Phe2, D-Trp3, D-Arg6]-GnRH, GnRH-A) in inhibiting ovulation.
• To assess the effects of GnRH-A on serum luteinizing hormone (LH) and follicle-stimulating hormone (FSH) levels across different species.

Main Methods:

• Dose-response studies were conducted in cycling rats and rabbits to determine ovulation inhibition thresholds.
• Serum LH and FSH levels were measured in ovariectomized rats, rabbits, and mice following GnRH-A administration.

Main Results:

• A low dose of GnRH-A (1 microgram) completely inhibited ovulation in rats, while higher doses were needed for rabbits.
• Rabbits that ovulated despite GnRH-A treatment exhibited delayed and reduced LH surges.
• Rats demonstrated the highest sensitivity to GnRH-A's pituitary inhibitory effects, with slight FSH suppression.
• FSH levels remained largely unaffected in rabbits and mice.

Conclusions:

• GnRH-A is a potent inhibitor of ovulation in rats and rabbits, with dose-dependent effects.
• Species-specific differences in GnRH receptor affinity to GnRH-A likely account for the observed variations in efficacy and pituitary response.