A pair of pentapeptides, Met- and Leu-enkephalin were recently isolated from brain tissue1,2. The two peptides seem to represent endogenous opiate receptor ligands and have by immunocytochemical and radioimmunoassay studies been shown to occur in an extensive system of cerebral and peripheral nerves3-8. The relative proportions between Met- and Leu-enkephalin varies between different brain regions and also between different species, suggesting the existence of separate populations of Met- and Leu-enkephalin nerves9-12. Until now, however, immunocytochemistry has given no support for this notion. We report here evidence of separate populations of Met-and Leu-enkephalin nerves.
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