Regulation of GnRH receptor gene expression in sheep and cattle.

The GnRH receptor plays a pivotal role in reproduction. This review summarizes current knowledge of the regulation of GnRH receptor gene expression by endocrine factors in sheep and cattle. Expression of the GnRH receptor gene, measured by steady-state amounts of GnRH receptor messenger RNA (mRNA), is maximal during the preovulatory period. The molecular events leading to maximal GnRH receptor gene expression are probably triggered by decreased circulating concentrations of progesterone at luteolysis. Because GnRH is a positive homologous regulator of its own receptor, increased pulsatile GnRH after removal of negative feedback effects of progesterone stimulates expression of the GnRH receptor gene early in the preovulatory period. Oestradiol is also a positive regulator of GnRH receptor gene expression, and increased serum concentrations of oestradiol from developing follicles probably maintain high abundance of GnRH receptor mRNA later in the preovulatory period. Since increased amount of GnRH receptor mRNA precedes maximal numbers of GnRH receptors before the LH surge, increased expression of the GnRH receptor gene is an important mechanism by which maximal sensitivity of gonadotrophs to GnRH is achieved. Future efforts should be directed towards elucidating the molecular mechanisms underlying transcriptional regulation of the GnRH receptor gene in ruminants by endocrine factors.