Localization of Met-enkephalin-Arg6-Gly7-Leu8-like immunoreactivity in the gastrointestinal tract of rat and pig.

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Abstract
One of the opioid precursor molecules, pre-pro-enkephalin A, contains within it, in addition to Leu-enkephalin (Leu-Enk) and Met-enkephalin (Met-Enk), Met-enkephalin-Arg6-Gly7-Leu8 (Met-Enk-8), which is specific to this precursor. This study deals with the localization of Met-Enk-8-like immunoreactivity in the gastrointestinal tract of rat and pig. Immunoreactivity was identified in intramural nerve elements of rat and pig, and in gut endocrine cells of pig. Immunoreactive (IR) nerve fibers were seen mainly in the myenteric plexus of rat and in both the myenteric and submucosal plexuses of pig. Some IR fibers were dispersed throughout the lamina propria mucosae of rat. Porcine IR endocrine cells were dispersed in the epithelium from the pyloric antrum to the ileum, existing concomitantly with enterochromaffin (EC) cells. Specificity tests revealed that immunoreactivity to Met-Enk-8 antiserum was not influenced by preincubation of the antiserum with Leu-Enk and Met-Enk. This suggests the possibility that pre-pro-enkephalin A is contained in the gastroenteric nerves of rat and pig and in a population of porcine EC cells.