A qualitative and quantitative study on the enkephalinergic innervation of the pig gastrointestinal tract

Enkephalins are involved in neural control of digestive functions such as motility, secretion, and absorption. To better understand their role in pigs, we analyzed the qualitative and quantitative distribution of enkephalin immunoreactivity (ENK-IR) in components of the intestinal wall from the esophagus to the anal sphincter. Immunohistochemical labelings were analyzed using conventional fluorescence and confocal microscopy. ENK-IR was compared with the synaptophysin immunoreactivity (SYN-IR). The results show that maximal ENK-IR levels in the entire digestive tract are reached in the myenteric plexuses and, to a lesser extent, in the external submucous plexus and the circular muscle layer. In the longitudinal muscle layer, ENK-IR was present in the esophagus, stomach, rectum, and anal sphincter, whereas it was absent from the duodenum to the distal colon. In the ENK-IR plexuses and muscle layers, more than 60% of the nerve fibers identified by SYN-IR expressed ENK-IR. No ENK-IR was observed in the internal submucous plexus and the mucosa; the latter was found to contain ENK-IR endocrine cells. These results strongly suggest that, in pigs, enkephalins play a major role in the regulatory mechanisms that underlie the neural control of digestive motility.