The peroxidase‐antiperoxidase (PAP) technique is used to examine the localization of methionine [Met5]‐enkephalin and substance P in the dorsal tegmental nuclei of the human fetal brain. Specific antiserum to both peptides is immunocytochemically detectable in neuronal perikarya and varicose processes in the human dorsal tegmental nuclei at 17–21 weeks of gestation. However the two peptides show a differential distribution in the central versus peripheral portion of the nucleus. Enkephalin‐like immunoreactivity is primarily localized in perikarya along the peripheral borders of the tegmental nuclei, whereas substance P is present primarily in varicose terminals and a few perikarya in the more central portions. These findings suggest that substance P‐containing terminals in dorsal tegmental nuclei are probably derived from afferent axons and that enkephalin is present in intrinsic neurons.
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