Immunohistochemical studies on the gastrointestinal tract using antisera to Met-enkephalin and Met-enkephalin Arg6Phe7.

Antisera specific for the C-terminus of Met-enkephalin and two of its variants isolated from adrenal medulla and brain, namely Met-enk Arg6 and Met-enk Arg6Phe7, have been used in immunohistochemical studies of the gastrointestinal tract in rat, mouse and guinea pig. Met-enk and Met-enk Arg6Phe7-like immunoreactivities were found with similar distribution in nerve cell bodies of the myenteric plexus, and in fibers that were particularly dense in the myenteric plexus and circular smooth muscle. The Met-enk Arg6 antiserum showed weak staining of nerve cells and fibers. In rat and mouse, the antiserum to Met-enk Arg6Phe7, but not those to Met-enk or Met-enk Arg6, also stained numerous endocrine-like cells of the antral mucosa. These were identified as gastrin cells by elution and re-staining experiments with C-terminal gastrin antisera. The rat and mouse gastrin cells might conceivably express the enkephalin gene, but fail to process it to yield Met-enkephalin; alternatively, the Met-enk Arg6Phe7-like immunoreactivity in gastrin cells could be due to a cross-reacting peptide that does not belong to the opioid series.