Administration of the GnRH antagonist cetrorelix during the luteal phase of the mare

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The GnRH antagonist cetrorelix was given during the early (Days 1–5), mid (Days 6–10 or 5–12) or for the entire (Days 1–16) luteal phase of mares to inhibit the secretion of FSH and LH (Day 0 = ovulation). Frequent blood sampling from Day 6 to Day 14 was used to determine the precise time-course of the suppression (cetrorelix given Days 6–10). Cetrorelix treatment caused a decrease in FSH and LH concentrations by 8 and 16 h, respectively, and an obliteration of the response to exogenous GnRH given 24 h after treatment onset. Treatment never suppressed gonadotropin concentrations to undetectable levels; e.g. frequent sampling showed that the nadirs reached in FSH and LH were 46.2 ± 6% and 33.1 ± 11%, respectively, of pre-treatment concentrations. Daily FSH concentrations were decreased in all treatment groups but daily LH concentrations were lower only when treatment commenced at the beginning of the luteal phase; progesterone concentrations depended on the time of cetrorelix administration, but the changes suggested a role for LH in corpus luteum function. The inter-ovulatory interval was longer than controls when cetrorelix was given in the mid- or for the entire luteal phase, but was unaffected by treatment in the early phase. Nevertheless, in all groups, FSH concentrations were higher (P < 0.05 when compared to Day 0, subsequent ovulation) approximately 6–10 days before this next ovulation. This consistent relationship suggests a stringent requirement for a GnRH-induced elevation of FSH above a threshold at, but only at, this time; i.e. approximately 6–10 days before ovulation.