Tachykinin NK-1 receptor probed with constrained analogues of substance P

Article abstract of DOI:10.1016/S0968-0896(96)00230-1

The action of rotameric probes introduced either in position 7 or 8 in the sequence of substance P (SP) was investigated, i.e. L-tetrahydroisoquinoleic acid (Tic) L-fluorenylglycine (Flg), L-diphenylalanine (Dip), the diastereoisomers of L-1-indanylglycine (Ing) and L-benz[f]indanylglycine (Bfi), the Z- and E-isomers of dehydrophenylalanine and dehydronaphthylananine (Δ(Z)Phe, Δ(E)Phe, Δ(Z)Nal, Δ(E)Nal) and L-o,o'-dimethylphenylalanine (Dmp). The aim of this study was the topographical characterization of the binding subsites of human NK-1 receptor expressed in CHO cells, especially the S₇ and 8, subsites, corresponding to residues Phe⁷ and Phe⁸ of substance P. According to the binding potencies of these substituted-SP analogues, the S₇ binding subsite is smaller than the S₈ subsite: the S₇ subsite accepts only one aromatic nucleus, while the S₈ can accommodate three coplanar nuclei altogether. These findings are compatible with the idea that the S₈ binding subsite may reside in the extracellular loops of the hNK-1 receptor. NK-1 agonists bind to human NK-1 receptor and activate the production of both inositol phosphates and cyclic AMP. As already quoted for septide, [pGlu⁶, Pro⁹]]SP(6-11), discrepancies are observed between affinity (K(i)) and activity (EC₅₀) values for IPs production. While a weak correlation between K(i) and EC₅₀ values for IPs production could be found (r=0.70), an excellent correlation could be demonstrated between their affinities (K(i)) and their potencies (EC₅₀) for cAMP production (r=0.97). The high potency (EC₅₀) observed for 'septide-like' molecules on PI hydrolysis, compared to their affinity is not an artefact related to the high level of NK-1 receptors expressed on CHO cells since a good correlation was found between EC₅₀ values obtained for PI hydrolysis and those measured for spasmogenic activity in guinea pig ileum bioassay (r=0.94).