Regulation of gonadotrophin-releasing hormone receptor expression in the ewe

The interaction of gonadotrophin-releasing hormone (GnRH) with its receptor is of crucial importance to gonadotroph function, although until recently, little was known of the structure of the GnRH receptor (GnRH-R). This report has addressed the steps leading to the cloning of the sheep GnRH-R cDNA and the endocrine mechanisms associated with the regulation of GnRH-R mRNA expression. Differences between species and between in vivo and in vitro studies are apparent. The expression of GnRH-R mRNA was shown to increase prior to the gonadotrophin surge and decrease thereafter; these changes were correlated with GnRH binding in the pituitary and follicle oestradiol production. In the early follicular phase of the sheep oestrous cycle, oestradiol appears to be the primary regulator of GnRH-R mRNA, whereas in the periovulatory period, the action of GnRH itself is more important.