Interactions between the weakly analgesic enkephalins and morphine on morphine-induced analgesia were studied. Met-enkephalin exhibited morphine analgesia whereas Leu-enkephalin potentiated it. Both Met- and Leu-enkephalin, when tested alone, were not analgesic. The strongly analgesic FK33824 (Sandoz) compound, like Leu-enkephalin, also potentiated morphine analgesia. Tolerance developed to morphine analgesia but not to Met-enkephalin inhibition of morphine analgesia.
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