Structure−Activity Relationship Studies of Spinorphin as a Potent and Selective Human P2X<sub>3</sub> Receptor Antagonist JungKwan-Young MoonHyoun Duk LeeGa Eun LimHyun-Ho ParkChul-Seung KimYong-Chul 2007 Spinorphin, an endogenous antinociceptive peptide (LVVYPWT), showed potent and non-competitive antagonism at the ATP-activated human P2X<sub>3</sub> receptor (IC<sub>50</sub> = 8.3 pM) in a two-electrode voltage clamp assay with recombinant human P2X<sub>3</sub> receptors expressed in <i>Xenopus</i> oocytes. Single alanine substitutions from 1st to 4th amino acids and the cyclic form of LVVYPWT sustained antagonistic properties at the human P2X<sub>3</sub> receptors, whereas the threonine to alanine substitution resulted in an enhancing effect of the agonistic activity.