Adrenocorticotropin and alpha-melanotropin in the myenteric plexus of the rat duodenum: an electron microscopic study.

Adrenocorticotropin (ACTH) immunoreactivity was localized at the ultrastructural level as positive 'cores' within large dense-cored vesicles (LDVs) of axons and dendrites of the rat duodenum. The immunostained vesicle 'cores' were 35-50 nm in mean diameter, corresponding to 'cores' of LDVs with a mean diameter of 80-90 nm. alpha-melanotropin (alpha-MSH) was detected also within LDVs, expressing the same mean diameter as ACTH-stained vesicles. alpha-MSH and ACTH were localized only within structures belonging to the enteric nervous system of the rat duodenum. alpha-MSH and ACTH, as detected by immunostaining, were absent in endocrine cells of the rat duodenum. These findings suggest the possibility that these peptides may have important physiological roles in the rat duodenum.