Nucleotide sequence analyses predict that human pituitary and human placental gonadotropin-releasing hormone receptors have identical primary structures

Gonadotropin-releasing hormone (GnRH) interacts with a putative receptor in human placenta to cause the dose-dependent release of human chorionic gonadotropin (hCG) in a manner analogous to hypothalamic GnRH stimulation of luteinizing hormone (LH), and follicle-stimulating hormone (FSH) by the pituitary gland. However, GnRH agonists bind a placental binding site at a lower affinity than they bind the pituitary GnRH receptor, suggesting the two sites differ. To address this issue, human placental GnRH receptor mRNA from an 8-wk placental sample was amplified using primers based on the sequence of the human pituitary receptor, cloned, and sequenced. The sequence of the placental transcript was found to be identical to its pituitary counterpart. Thus, as with the pituitary receptor, this placental receptor transcript appears to encode a single polypeptide chain with seven intramembranous domains and two glycosylation sites. However, it is possible these receptors differ in their posttranslational processing or there is more than one placental GnRH receptor. In addition, subsequent gene amplification studies, modified to increase the sensitivity of the method, revealed a band of the predicted size (similar to 1051 bp) in a 6-wk and two 8-wk placental samples, but not in 5-wk and 7-wk samples. The failure to amplify GnRH receptor mRNA at these gestational ages did not appear to be an issue of cell health or sample degradation, since GnRH, alpha hCG, and beta hCG mRNA were amplified from each of these tissues. Instead these data suggest GnRH receptor mRNA may have a short half-life or the mRNA may be translated as rapidly as it is transcribed, with the result that it has not accumulated in the cell.