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 F(2α) (PGF(2α)) 11 or 12 days later. Anterior pituitary glands were collected 0 h, 4 h, 12 h, or 24 h after treatment with PGF(2α), 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 PGF(2α) 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 PGF(2α)-treated ewes and were lower (p < 0.05) than those in controls 24 h after treatment with PGF(2α). Concentrations of progesterone in sera of ewes treated only with GnRH were similar to those in controls. Circulating concentrations of estradiol were below the limit of detection in all animals. Ewes that received infusions of GnRH had more LH pulses during the 12-h sampling period than did animals that received only PGF(2α) (9.7 ± 1.0 vs. 1.7 ± 0.3 pulses, respectively; p < 0.001). Concentrations of GnRH receptor mRNA were similar among controls and ewes killed 4 h, 12 h, and 24 h after treatment with PGF(2α). In ewes treated with PGF(2α) plus GnRH, as well as in ewes treated with GnRH only, infusions of GnRH increased mean concentrations of GnRH receptor mRNA (p < 0.05) and concentrations of GnRH receptors (p < 0.05). Mean concentrations of GnRH receptors 4 h and 12 h after treatment with PGF(2α) were not different from those in controls, and concentrations of GnRH receptors 24 h after treatment with PGF(2α) were intermediate to those in controls and GnRH-treated ewes. Results of this study provide evidence that after luteolysis, concentrations of mRNA encoding ovine GnRH receptor are increased by pulsatile secretion of GnRH. In the naturally cycling ewe, this stimulatory effect is probably due to increased pulsatile secretion of GnRH following functional demise of the CL and removal of negative feedback by progesterone. Withdrawal of progesterone and increased pulsatile secretion of GnRH during the early preovulatory period appear to be important events leading to maximal expression of GnRH receptors prior to the LH surge in the ewe.
All Science Journal Classification (ASJC) codes
- Reproductive Medicine
- Cell Biology