Regulation of GnRH receptor gene expression in sheep and cattle

The GnRH receptor plays a pivotal role in reproduction. This review summari zes current knowledge of the regulation of GnRH receptor gene expression by endocrine factors in sheep and cattle. Expression of the GnRH receptor gen e, 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 c irculating 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 expr ession of the GnRH receptor gene early in the preovulatory period. Oestradi ol is also a positive regulator of GnRH receptor gene expression, and incre ased serum concentrations of oestradiol from developing follicles probably maintain high abundance of GnRH receptor mRNA later in the preovulatory per iod. Since increased amount of GnRH receptor mRNA precedes maximal numbers of GnRH receptors before the LH surge, increased expression of the GnRH rec eptor gene is an important mechanism by which maximal sensitivity of gonado trophs to GnRH is achieved. Future efforts should be directed towards eluci dating the molecular mechanisms underlying transcriptional regulation of th e GnRH receptor gene in ruminants by endocrine factors.