Comparative Sequence Analysis and Functional Characterization of the Cloned Sheep Gonadotropin-Releasing Hormone Receptor Reveals Differences in Primary Structure and Ligand Specificity among Mammalian Receptors

The cloned sheep gonadotropin-releasing hormone (GnRH) receptor was analysed for sequence homology/differences among mammalian receptors and its pharmacology characterized in COS-1 cells. Transmembrane domains TM2, TM3, TM5, TM6 and TM7, and extracellular loop 1 are most highly conserved (>90%) in this G-protein coupled receptor. The K_d of the sheep receptor in binding assays (4.9 nM) was similar to the human and rat receptors, but lower than the mouse receptor. The rank order of potency of a series of GnRH analogues for binding and inositol phosphate stimulation in transfected COS-1 cells was identical to that of the receptor characterized in sheep pituitary gonadotropes. Northern blot analysis identified four transcripts sized 5.4 kb, 3.6 kb, 2.3 kb and 1.3 kb in sheep pituitanes which were upregulated by castration.