EVIDENCE FOR A COMMON MOLECULAR-MODE OF ACTION FOR CHEMICALLY DISTINCT NONPEPTIDE ANTAGONISTS AT THE NEUROKININ-1 (SUBSTANCE-P) RECEPTOR

The molecular mechanism of action of three chemically distinct nonpept ide antagonists, SR 140,333, FK 888, and RP 67,580, was compared with that of the previously characterized compound CP 96,345, using a serie s of chimeric constructs between their common target, the rat neurokin in (NK)-1 (substance P) receptor, and the homologous nonresponsive NK- 3 (NKB) receptor. The ability of all four nonpeptide compounds to disp lace radiolabeled substance P from the NK-1 receptor and their ability to inhibit the peptide-induced increase in inositol phosphate turnove r were critically dependent on structural elements located in an area from the middle of the second extracellular loop through transmembrane segments V and VI to the middle of the third extracellular loop of th e NK-1 receptor. Dissection of the domain around the outer part of tra nsmembrane segments V and VI into smaller segments demonstrated that t he individual nonpeptide antagonists, in agreement with their distinct chemical structures, were dependent on different subepitopes within t he common putative binding domain. Full NK-1-like susceptibility to SR 140,333, FK 888, and CP 96,345 could be transferred to the NK-3 recep tor by exchange of transmembrane segments V and VI and adjacent parts with corresponding segments from the NK-1 receptor. For SR 140,333 and CP 96,345, almost the same effect could be achieved by transfer of tw o discontinuous segments around the top of transmembrane segments V an d VI. RP 67,580 shared interaction sites with the other compounds arou nd the top of transmembrane segment VI but appeared also to be depende nt on transmembrane segment VII. It is concluded that four nonpeptide antagonists, despite overt chemical differences, appear to block NK-1 receptor function by interacting in distinct ways with a common site l ocated spatially around the outer part of transmembrane segment VI.