Functional expression of a novel human neurokinin-3 receptor homolog that binds [H-3]senktide and [(125)-I-MePhe(7)]neurokinin B, and is responsive to tachykinin peptide agonists

In 1992, Xie et al. identified a cDNA sequence in the expression cloning search for the kappa opioid receptor. When the cDNA was expressed in Cos-7 cells, binding of opioid compounds was observed to he of low affinity and without kappa, mu, or delta selectivity [Xie, G.-X., Miyajima, A. and Goldstein, A. (1992) Proc. Natl. Acad. Sci. USA 89, 4124-4128]. This cDNA was highly homologous to the human neurokinin-3 (NK-3) receptor sequence, and displayed lower homology to NK-1 and NK-2 sequences. This sequence was stably expressed in Chinese hamster ovary cells, which do not express neurokinin receptors naturally, and ligand binding and second messenger characteristics were compared with a human NK-3 receptor. The NK-3 receptor homolog bound [H-3] senktide with a K-d of 39 nM, similar to that of the NK-3 receptor. The rank order of tachykinin peptides competing for [H-3]senktide binding at the NK-3 receptor homolog was [MePhe(7)]neurokinin B > senktide > substance P = neurokinin A > neurokinin B. This cell line also bound [I-125-MePhe(7)] neurokinin B; however, neurokinin B was an effective competitor, Tachykinin peptides stimulated both inositol phospholipid hydrolysis and arachidonic acid release at NK-3 and NK-3 receptor homolog cell lines, with similar rank orders of potency of [MePhe(7)] neurokinin B = neurokinin B = senktide > NKA = substance P. These results indicate that expression of the NK-3 receptor homolog cDNA in the Chinese hamster ovary cell system induces the expression of a receptor site with many similarities but certain key differences from that of the human NK-3 receptor. The results are discussed with reference to the existence of a novel human tachykinin receptor.