Proenkephalin A associated peptides in the autonomic nervous system of the human infant gastrointestinal tract.

The digestive tract of neonates and infants were examined by immunohistochemistry using specific antisera raised against proenkephalin A related peptides. Proenkephalin A, methionine-enkephalin and leucine-enkephalin are observed in nerve fibres in the smooth muscles in the myenteric and submucosal plexuses or in neuronal cell bodies of the myenteric plexus. In these structures synenkephalin has general distribution as methionine-enkephalin but not the same as leucine-enkephalin. Co-localization of synenkephalin and methionine-enkephalin is found in several neurones. These results suggest that proenkephalin A is the precursor-protein in some enkephalinergic neurones of the human gut. A gradient in the density of immunoreactivity is observed and is maximal in the distal small bowel. This gradient contrasts with observations made in rodents where major enkephalin immunoreactivity is observed in the proximal digestive tract. These findings give evidence that proenkephalin A-derived peptides could have effects in the motility of the human gut.