Endomorphinsarerecentlydiscoveredendogenousopioidpeptidescomposedoffouraminoacids:Endomorphin-1(EM-1)isTyr-Pro-Trp-Phe-NH2,andEndomorphin-2(EM-2)isTyr-Pro-Phe-Phe-NH2.Thesepeptideshaveaveryhighaffinityandspecificityformu-opioidreceptorsasithasbeenreportedinmanypharmacologicalstudies.EM-1isdistributedinmanybrainregions,whereasEM-2isprevalentinbrainstemandspinalcord.EM-1andEM-2exerttheirantinociceptiveactivitybyinteractingwithmu-opioidreceptors.EM-1anfEM-2bindwithmu-opioidreceptorwithhighaffinityandselectivityincomparisonwithbindingtoopioiddeltaandkappareceptors.EM-1andEM-2tetrapeptidesactivateG-proteinsandinhibitadenylylcyclaseactivitywhenbindtomu-opioidreceptors.