Co-existence of the enkephalinergic system and the melanotropinergic system in the rat duodenum shown by immunohistochemistry.

The immunohistochemical distribution of alpha-melanotropin (alpha-MSH) and Met-enkephalin (Met-ENK) immunoreactivities in the rat duodenum was examined by using immunofluorescence microscopy. Alternately staining of adjacent frozen serial sections with specific antisera directed to alpha-MSH or Met-ENK revealed that within a subpopulation of myenteric plexus perikarya alpha-MSH immunostaining co-exists with that of Met-ENK. Some myenteric plexus nerve fibres also contain both Met-ENK and alpha-MSH immunoreactivity. These findings might indicate that the genes encoding for the precursors of melanotropins (the pro-opiomelanocortin precursor) and enkephalin (the pro-enkephalin precursor) are generated from a common, large and single ancestor gene which remained conserved during evolution in the rat enteric nervous system.