THE identification in brain of opiate receptors1 and morphine-like pentapeptides (methionine-enkephalin and leucine-enkephalin)2 has led to suggestions that enkephalin receptors may be sites at which opiate drugs induce analgesia and euphoria, and that the enkephalins themselves may be transmitters or modulators in brain systems for the regulation of pain and pleasure3–6. Observations in animals reveal that appropriate central administration of enkephalin produces morphine-like analgesia7–9 and physical dependence10. We report here that enkephalin also may share the euphorigenic or rewarding properties of morphine in that rats will work for enkephalin injections delivered directly into the ventricles of their own brains11. Of particular interest, in the light of the generally superior potency of the methionine peptide2,8,9, is the observation that leucine-enkephalin is self-administered with greater avidity than methionine-enkephalin.
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BELLUZZI, J., STEIN, L. Enkephalin may mediate euphoria and drive-reduction reward. Nature 266, 556–558 (1977). https://doi.org/10.1038/266556a0
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