Morphine, but not diacetyl morphine (heroin), possesses opiate antagonist activity in the mouse vas deferens

Abstract In an attempt to determine whether or not it is possible to differentiate between the efficacy of μ and δ agonists, activities of morphine, nomorphine, diamorphine, methionine enkephalin (ME) and the μ selective peptides RX 783006 (D-Ala 2 MePhe 4 Gly (ol) 5 - Enkephalin) and RX 783016 (Tyr -D-AlaGlyMePheNH(CH 2 ) 2 NMe 2 ) have been investigated in mouse vas deferens preparations stimulated by trains of high frequency (10 Hz) pulses. In the present study both the μ and δ selective peptides were found to possess equal efficacy (producing 80–90% inhibition of the contractions of the MVD). However, morphine which is accepted as the prototype pure μ agonist, was found to possess very low efficacy (reducing contractions by