Abstract
The synthetic enkephalins especially methionine enkephalin are more potent in inhibiting the stereospecific binding of 3H-dihydromorphine than that of 3H-naloxone in mouse brain homogenates. Methionine enkephalin is a more potent inhibitor of 3H-dihydromorphine binding in whole mouse brain homogenates than in washed mouse brain membranes. No difference was observed with regard to the inhibitory effect of methionine enkephalin on the binding of 3H-dihydromorphine in whole rat brain homogenates or washed rat brain membranes. The use of different radiolabelled drugs (agonist versus antagonist), different species (mouse versus rat) and/or the variation in the preparation (brain homogenates versus washed membranes) may account for the difference between the IC 50 of methionine enkephalin versus 3H-dihydromorphine and versus 3H-naloxone stereospecific binding. The increased inhibitory effect of methionine enkephalin when the supernatant was added to the washed brain membranes supports the hypothesis that methionine enkephalin may be one part of the real endogenous morphine ligand.
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