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 < 30%) and to act as an antagonist of both μ and δ peptides, being 35–50 times more effective against the μ agonists. The antagonist potency of morphine was approximately 1% that of naloxone at the μ receptor and 0.5% that of naloxone at the δ receptor. Diamorphine, in contrast to morphine, appears to be a relatively pure agonist in this preparation.
Published Version
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