Gonadotropin releasing hormone (GnRH) binds to and activates pituitary gonadotrophs to release luteinizing hormone (LH) and follicle stimulating hormone (FSH). We have recently characterized functional GnRH receptors in cultured pituitary cells using superogonist analogs of GnRH as the radio-iodinated ligands. The availability of a radio-receptor assay for GnRH should enable us to study homologous regulation of GnRH receptor in vitro and the role of up- and down-regulation of GnRH receptors in the peptide action using cultured rat pituitary cells. Following its binding, GnRH mobilizes calcium presumably from extracellular and intracellular pools and cytoplasmic calcium probably binds to calmodulin released from the plasma membrane. We assume that GnRH initiates a calcium calmodulin-dependent chain of biochemical events leading to pituitary hormone release. We have recently demonstrated that neither prostaglandins nor cAMP or cGMP are involved in GnRH stimulation of gonadotropin release. Therefore, we will try to isolate the """"""""second messenger"""""""", and characterize the biochemical pathway involved in GnRH action. Preliminary data suggest that phospholipid turnover and the lipoxygenase pathway may be involved in GnRH action. Studies are therefore designed to evaluate the contribution of phospholipid turnover and leukotrienes production to gonadotropin release, and to identify the active arachidonic acid product involved in the peptide action using enriched populations of gonadotrophs. GnRH-induced phospholipid turnover might also be involved in receptor regulation thus providing a feed-back mechanism to regulate target-cell sensitivity. The above studies should provide insight into GnRH action in particular and peptide-hormone action in general, and indicate alternative mechanisms of action for hormonal systems in which cAMP is not the second messenger.
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