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

Enkephalins are involved in neural control of digestive functions such as m otility, secretion, and absorption. To better understand their role in pigs , we analyzed the qualitative and quantitative distribution of enkephalin i mmunoreactivity (ENK-IR) in components of the intestinal wall from the esop hagus to the anal sphincter. Immunohistochemical labelings were analyzed us ing conventional fluorescence and confocal microscopy. ENK-IR was compared with the synaptophysin immunoreactivity (SYN-IR). The results show that max imal ENK-IR levels in the entire digestive tract are reached in the myenter ic plexuses and, to a lesser extent, in the external submucous plexus and t he circular muscle layer. In the longitudinal muscle layer. ENK-IR was pres ent in the esophagus, stomach, rectum, and anal sphincter, whereas it was a bsent from the duodenum to the distal colon. In the ENK-IR plexuses and mus cle layers, more than 60% of the nerve fibers identified by SYN-IR expresse d ENK-IR. No ENK-IR was observed in the internal submucous plexus and the m ucosa; the latter was found to contain ENK-IR endocrine cells. These result s strongly suggest that, in pigs, enkephalins play a major role in the regu latory mechanisms that underlie the neural control of digestive motility.