Intracellular mechanisms triggering gonadotrophin secretion

The recently cloned GnRH receptor, a G-protein coupled receptor that spans the membrane seven times, plays a central role in the maintenance of normal reproductive events. In pituitary gonadotrophs, activation of the GnRH receptor stimulates a concert of intracellular signalling pathways. Phospholipase C stimulation generates inositol 1,4,5 trisphosphate and diacylglycerol, which release calcium and activate protein kinase C, respectively. After these primary signals, prolonged activation of protein kinase C arises from the continued production of diacylglycerol from additional signal transduction pathways. While characteristic calcium responses, involving specific calcium pools, are instrumental in triggering exocytosis, the precise role of protein kinase C activation is unclear. Key players within the exocytotic machinery are also elusive but may include a range of membrane, guanine nucleotide and calcium-binding proteins, inositol 1,4,5 trisphosphate receptors and the cell cytoskeleton. Cellular signalling is also important in determining pituitary responsiveness to GnRH, involving intracellular cross-talk between the GnRH, oestradiol and progesterone receptors.