Pituitary and extrapituitary actions of gonadotrophin-releasing hormone and its analogues.

The hypothalamic decapeptide gonadotrophin-releasing hormone (GnRH) binds to high affinity receptors on pituitary gonadotrophs. These receptors mediate the effects of GnRH on secretion and synthesis of gonadotrophins. The GnRH receptor is coupled to Gq/G11, which activates phospholipase C. This enzyme leads to the generation of several second messenger molecules. Among these, diacylglycerol (DG) and inositol 1,4,5-tris-phosphate (IP3) are critically important. DG leads to activation of protein kinase C and IP3 releases Ca2+ from intracellular pools. Both events result in secretion and synthesis of luteinizing hormone (LH) and follicle stimulating hormone (FSH). In addition, other components of the GnRH signal transduction pathway are involved in cellular responses to GnRH. GnRH receptors and their functions are regulated by GnRH itself or other hormones such as ovarian steroids. The prolonged exposure of pituitary gonadotrophs to GnRH leads to desensitization and consequently to suppressed LH and FSH secretion. This mechanism is employed for the clinical use of GnRH agonists. GnRH antagonists act by competitive binding to the pituitary GnRH receptors. Apart from the well-established pituitary actions of GnRH, receptors for the decapeptide have been demonstrated in a variety of extrapituitary tissues. Here we report on the ovarian actions of GnRH which are predominantly inhibitory in the rat ovary. In the human ovary the existence of GnRH receptors is controversial. Recent reports have demonstrated the mRNA for the GnRH receptor in the human ovary. However, to date there is no consensus on the ovarian actions of GnRH or its analogues.