Physiological Regulation of Pituitary GnRH Receptors
In common with other oligo- and poly-peptide hormones, gonadotrophin releasing hormone (GnRH) initiates the processes leading to release and synthesis of LH and FSH following interaction with specific receptors in gonadotroph cell membranes. Initial attempts at measurement of pituitary GnRH-R employed (125I)-GnRH as the radio-ligand in receptor assays (RRA) and were unsuccessful. However the availability of iodinated non-degradable agonist analogs of GnRH (GnRH-A) resolved the major problem viz: 125I-GnRH binding was largely to low-affinity sites. Thus, a reliable GnRH receptor assay was validated, which measured only high-affinity GnRH binding sites, is highly specific for a whole series of GnRH agonist and antagonist analogs, and ligand binding is restricted to gonadotrophs (for review see 1). Assays based upon 125I-GnRH-A ligands are now widely accepted as the method of choice for receptor quantitation. By analogy with other ligand cell surface membrane receptor systems, it seemed likely that changes at the first site of cellular interaction could be a site for regulation of the target cell’ponses. Thus, with a reliable RRA it was possible to directly measure the GnRH-R content of individual rat pituitaries in different physiological circumstances to establish the relationship, if any, between GnRH-R, pituitary gonadotrophin content, and serum LH and FSH concentrations. The present review is concerned largely with these relationships in vivo in laboratory animals, although some more recent in vitro studies will also be briefly discussed.
KeywordsGnRH Agonist Physiological Regulation GnRH Receptor Pituitary Gonadotrophin GnRH Secretion
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