Pulsatile gonadotropin-releasing hormone (GnRH) increases concentrations of GnRH receptor messenger ribonucleic acid and numbers of GnRH receptors during luteolysis in the ewe.

As circulating concentrations of progesterone decrease during the early preovulatory period, concentrations of mRNA encoding ovine GnRH receptor in the anterior pituitary gland increase. The purpose of this study was to determine whether removal of progesterone affects amounts of GnRH receptor mRNA directly or whether withdrawal of progesterone affects GnRH receptor gene expression indirectly by permitting secretion of GnRH to increase. Ovulation was induced in seasonally anestrous ewes, and luteolysis was initiated with prostaglandin F2 alpha (PGF2 alpha) 11 or 12 days later. Anterior pituitary glands were collected 0 h, 4 h, 12 h, or 24 h after treatment with PGF2 alpha, and 24 h after injection of saline (n = 3 or 4 animals/group). Two groups of ewes (n = 3) received infusions of GnRH (250 ng infused over 6 min) hourly for 12 h; luteolysis was induced in one of these groups at the time that treatment with GnRH was initiated, and anterior pituitary glands were collected at the end of the 12-h infusion period. Blood samples were collected at 15-min intervals for 12 h from all ewes treated with GnRH and from animals administered PGF2 alpha and killed 12 h later. No differences in concentrations of GnRH receptor mRNA, numbers of GnRH receptors, or circulating concentrations of progesterone or estradiol were detected between groups of animals at 0 h and 24 h after treatment with saline; therefore, data from these control groups were combined. Concentrations of progesterone in serum decreased in PGF2 alpha-treated ewes and were lower (p < 0.05) than those in controls 24 h after treatment with PGF2 alpha.(ABSTRACT TRUNCATED AT 250 WORDS)