GONADOTROPIN-RELEASING-HORMONE RECEPTORS WITH INTRACELLULAR CARBOXYL-TERMINAL TAILS UNDERGO ACUTE DESENSITIZATION OF TOTAL INOSITOL PHOSPHATE PRODUCTION AND EXHIBIT ACCELERATED INTERNALIZATION KINETICS

The mammalian gonadotropin-releasing hormone receptor (GnRH-R) is the only G-protein-coupled receptor (GPCR) in which the intracellular C-te rminal tail is completely absent. In contrast to other GPCRs, the GnRH -R does not show rapid desensitization of total inositol (IP) producti on, and the rates of internalization are exceptionally slow. We invest igated whether the incorporation of a cytoplasmic tail into the C term inus of the GnRH-R affects desensitization events and receptor interna lization rates. A GnRH-R/TRH-R chimera was created where the intracell ular tail of the rat thyrotropin-releasing hormone receptor (TRH-R) wa s engineered into the C terminus of the rat GnRH-R, Three different ra t GnRH-R cDNA stop codon mutations tone for each reading frame) were a lso made. The GnRH-stimulated IP production of the wild-type rat GnRH- R expressed in either COS-7 or HEK 293 cells did not desensitize even after prolonged stimulation with GnRH., In contrast, the catfish GnRH- R (which does possess an intracellular tail) and the TRH-R rapidly (<1 0 min) desensitized following agonist stimulation. The GnRH-R/TRH-R ch imera also desensitized following treatment with GnRH, resembling the pattern shown by the TRH-R and the catfish GnRH-R, Two of the stop cod on mutants did not show desensitization of IP production, and the thir d mutant with the longest tail was not functional. Internalization exp eriments showed that the rat GnRH-R had the slowest endocytosis and re cycling rates compared with the TRH-R, the catfish GnRH-R, and the chi meric GnRH/TRH-R. This study demonstrates that the addition of a funct ional intracellular C-terminal tail to the GnRH-R produces rapid desen sitization of IP production and significantly increases internalizatio n rates.