REGULATION OF GONADOTROPIN-RELEASING-HORMONE RECEPTOR MESSENGER-RNA EXPRESSION IN THE SHEEP

To investigate the regulation of the sheep gonadotrophin-releasing hormone receptor (GnRH-R) gene expression, two different treatment regimes were used. Experiment 1 examined the effects of twice daily injections of ovine follicular fluid (oFF, 1.5 ml s.c.) as a source of inhibin, and daily GnRH antagonist injections (Nal-Glu.HOAc, 2 mg s.c.) on days 9-12 of the oestrous cycle. Luteolysis was induced on day 12 with prostaglandin (PG) and the ewes killed at two different stages; day 12 (luteal) and 18 h after PG injection. Experiment 2 examined the effect of a single injection of oestradiol benzoate (100 mu g i.m.) 18 h before death in luteal phase ewes and ewes chronically implanted with the GnRH agonist, buserelin. In both experiments, pituitaries were removed at death for determination of pituitary GnRH binding, LH content and levels of GnRH-R and LH beta mRNA. In addition in experiment 1, follicles greater than or equal to 2.5 mm were dissected from the ovaries for determination of oestradiol content. In experiment 1, oFF treatment during the luteal phase completely inhibited follicle oestradiol production but was without effect on the other parameters measured. After cessation of oFF treatment and induction of luteolysis, a significant (P < 0.05) increase in plasma LH occurred but the normal follicular increase in both GnRH-R mRNA levels and GnRH binding seen in control ewes was prevented. GnRH antagonist treatment alone or in combination with oFF also inhibited follicle oestradiol production, prevented the increase in GnRH-R mRNA, completely inhibited GnRH binding and significantly decreased LH beta mRNA levels. Pituitary LH content was unaffected by any treatment. In experiment 2, oestradiol treatment did not affect GnRH-R mRNA levels, GnRH binding, LH beta mRNA or pituitary LH content in luteal phase ewes, whilst chronic GnRH agonist treatment acted to decrease these parameters dramatically. A single injection of oestradiol in the GnRH agonist treated ewes significantly (P < 0.05) increased GnRH-R mRNA levels and completely restored GnRH binding to luteal levels, without any effect on LH beta mRNA or pituitary LH content. These results suggest that the control of GnRH receptor expression in the sheep is directly related to oestradiol and not to the action of GnRH itself.