Development of enkephalinergic neurons in the gut of the chick.

The content and distribution of Met-enkephalin immunoreactivity in the developing chick gut was studied by radioimmunoassay and immunocytochemistry. Met-enkephalin was detected by radioimmunoassay in the duodenum of the 5-day chick embryo. The concentration in this region increased 4-fold by 13 days of incubation and declined thereafter to the levels found in the 4-week chicken. The concentration of enkephalin in the midgut increased about 2-fold between 9 and 13 days of incubation and remained constant until hatching. In the 7-day duodenum, metenkephalin immunoreactivity was found in a network of darkly stained nodes (accumulations of ganglion cells) faintly stained internodal nerve bundles; this network of immunoreactivity was localized to the myenteric plexus. By 9 days of incubation, the network was more extensive and the intensity of staining was increased. At 13 days of incubation, varicosities were found in the region of the ganglion cells and in internodal nerve bundles. At this time, immunoreactivity was clearly visualized in the submucosal plexus. In the newly hatched chicken, met-enkephalin was found in nerves within the circular smooth muscle, as well as the myenteric and submucosal plexuses. The early appearance of met-enkephalin in the developing chick enteric nervous system suggests this peptide may act as a primary neurotransmitter in the organization and control of gut motility.