Vol. 58, Issue 3, 552-559, September 2000

Evidence That the Proposed Novel Human "Neurokinin-4" Receptor Is Pharmacologically Similar to the Human Neurokinin-3 Receptor but Is Not of Human Origin

Henry M. Sarau, Jeffrey L. Mooney, Dulcie B. Schmidt, James J. Foley, Peter T. Buckley, Giuseppe A. M. Giardina, Da Y. Wang, Jonathan A. Lee,¹ and Douglas W. P. Hay

Departments of Pulmonary Biology (H.M.S., D.B.S., J.J.F., P.T.B., D.W.P.H.), Gene Expression Sciences (D.Y.W., J.A.L.), and Molecular Biology (J.L.M.), SmithKline Beecham Pharmaceuticals, King of Prussia, Pennsylvania; and Department of Medicinal Chemistry, Via Zambeletti, Milan, Italy (G.A.M.G.)

There have been proposals that the tachykinin receptor classification should be extended to include a novel receptor, the "neurokinin-4" receptor (NK-4R), which has a close homology with the human NK-3 receptor (hNK-3R). We compared the pharmacological and molecular biological characteristics of the hNK-3R and NK-4R. Binding experiments, with ¹²⁵I-[MePhe⁷]-NKB binding to HEK 293 cell membranes transiently expressing the hNK-3R (HEK 293-hNK-3R) or NK-4R (HEK 293-NK-4R), and functional studies (Ca²⁺ mobilization in the same cells) revealed a similar profile of sensitivity to tachykinin agonists and antagonists for both receptors; i.e., in binding studies with the hNK-3R, MePhe⁷-NKB > NKB > senktide NKA = Substance P; with the NK-4R, MePhe⁷-NKB > NKB = senktide Substance P = NKA; and with antagonists, SB 223412 = SR 142801 > SB 222200  SR 48968  CP 99994 for both hNK-3R and NK-4R. Thus, the pharmacology of the two receptors was nearly identical. However, attempts to isolate or identify the NK-4R gene by using various molecular biological techniques were unsuccessful. Procedures, including nested polymerase chain reaction studies, that used products with restriction endonuclease sites specific for either hNK-3R or NK-4R, failed to demonstrate the presence of NK-4R in genomic DNA from human, monkey, mouse, rat, hamster, or guinea pig, and in cDNA libraries from human lung, brain, or heart, whereas the hNK-3R was detectable in the latter libraries. In view of the failure to demonstrate the presence of the putative NK-4R it is thought to be premature to extend the current tachykinin receptor classification.