title: Administration of a gonadotropin-releasing hormone antagonist to mares at different times during the luteal phase of the estrous cycle.
abstract: 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 24h 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.
authors: Endolab, Canterbury Health Laboratories, Christchurch Hospital, Christchurch, New Zealand. margaret.evans@cdhb.govt.nz
authors: Margaret J
authors: MJ
authors: Evans
authors: Susan L
authors: SL
authors: Alexander
authors: Clifford H G
authors: CH
authors: Irvine
authors: Niere E
authors: NE
authors: Kitson
authors: T Bruce
authors: TB
authors: Taylor
mesh_terms: D000818:Animals; D000953:Antigens, Protozoan; D004334:Drug Administration Schedule; D030762:Estrous Cycle / Q000097:blood / Q000187:drug effects* / Q000502:physiology; D005260:Female; D005640:Follicle Stimulating Hormone / Q000097:blood; D007987:Gonadotropin-Releasing Hormone / Q000008:administration & dosage / Q000031:analogs & derivatives / Q000037:antagonists & inhibitors* / Q000494:pharmacology; D006727:Hormone Antagonists / Q000008:administration & dosage* / Q000494:pharmacology; D006736:Horses / Q000502:physiology; D008183:Luteal Phase / Q000187:drug effects* / Q000502:physiology; D007986:Luteinizing Hormone / Q000097:blood; D011374:Progesterone / Q000097:blood; D013997:Time Factors; D028761:Withholding Treatment
chemical_list: D000953:Antigens, Protozoan; D006727:Hormone Antagonists; C063362:QF116 antigen, Plasmodium falciparum; D007987:Gonadotropin-Releasing Hormone; D011374:Progesterone; D007986:Luteinizing Hormone; D005640:Follicle Stimulating Hormone; C062876:cetrorelix