title: The chicken type III GnRH receptor homologue is predominantly expressed in the pituitary, and exhibits similar ligand selectivity to the type I receptor.
abstract: Two GnRH isoforms (cGnRH-I and GnRH-II) and two GnRH receptor subtypes (cGnRH-R-I and cGnRH-R-III) occur in chickens. Differential roles for these molecules in regulating gonadotrophin secretion or other functions are unclear. To investigate this we cloned cGnRH-R-III from a broiler chicken and compared its structure, expression and pharmacological properties with cGnRH-R-I. The broiler cGnRH-R-III cDNA was 100% identical to the sequence reported in the red jungle fowl and white leghorn breed. Pituitary cGnRH-R-III mRNA was approximately 1400-fold more abundant than cGnRH-R-I mRNA. Northern analysis indicated a single cGnRH-R-III transcript. A pronounced sex and age difference existed, with higher pituitary transcript levels in sexually mature females versus juvenile females. In contrast, higher expression levels occurred in juvenile males versus sexually mature males. Functional studies in COS-7 cells indicated that cGnRH-R-III has a higher binding affinity for GnRH-II than cGnRH-I (K(d): 0.57 vs 19.8 nM) with more potent stimulation of inositol phosphate production (ED(50): 0.8 vs 4.38 nM). Similar results were found for cGnRH-R-I, (K(d): 0.51 vs 10.8 nM) and (ED(50): 0.7 vs 2.8 nM). The initial rate of internalisation was faster for cGnRH-R-III than cGnRH-R-I (26 vs 15.8%/min). Effects of GnRH antagonists were compared at the two receptors. Antagonist #27 distinguished between cGnRH-R-I and cGnRH-R-III (IC(50): 2.3 vs 351 nM). These results suggest that cGnRH-R-III is probably the major mediator of pituitary gonadotroph function, that antagonist #27 may allow delineation of receptor subtype function in vitro and in vivo and that tissue-specific recruitment of cGnRH-R isoforms has occurred during evolution.
authors.affiliation: Royal (Dick) School of Veterinary Studies, The Roslin Institute, University of Edinburgh, Roslin, Midlothian, Edinburgh, UK. nerine.joseph@bbsrc.ac.uk
authors.forename: Nerine T
authors.initials: NT
authors.lastname: Joseph
authors.forename: Kevin
authors.initials: K
authors.lastname: Morgan
authors.forename: Robin
authors.initials: R
authors.lastname: Sellar
authors.forename: Derek
authors.initials: D
authors.lastname: McBride
authors.forename: Robert P
authors.initials: RP
authors.lastname: Millar
authors.forename: Ian C
authors.initials: IC
authors.lastname: Dunn
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D019556:COS Cells; D002645:Chickens / Q000235:genetics* / Q000378:metabolism; D002522:Chlorocebus aethiops; D005260:Female; D007987:Gonadotropin-Releasing Hormone / Q000378:metabolism*; D008024:Ligands; D008297:Male; D008969:Molecular Sequence Data; D010802:Phylogeny; D010902:Pituitary Gland / Q000378:metabolism*; D011485:Protein Binding; D020033:Protein Isoforms / Q000235:genetics / Q000378:metabolism; D011966:Receptors, LHRH / Q000235:genetics* / Q000378:metabolism*; D017385:Sequence Homology; D013379:Substrate Specificity; D014162:Transfection
chemical_list: C434455:GnRH receptor 3; D008024:Ligands; D020033:Protein Isoforms; D011966:Receptors, LHRH; D007987:Gonadotropin-Releasing Hormone