VIP immunoreactivity in enteric nerves and endocrine cells of the vertebrate gut

The gastrointestinal tracts of various species chosen as representatives of the seven vertebrate classes were investigated for the occurrence of VIP immunoreactivity. The study was carried out with identically fixed and embedded tissue using the peroxidase-antiperoxidase (PAP) technique after primary incubation with a C-terminally directed antiserum (R 501). In the mammalian and avian species the antiserum failed to demonstrate VIP immunoreactive endocrine cells, while in the gut of all representatives of the lower vertebrate classes such cells were seen. The VIP immunoreactive cells were scattered among the epithelial cells of the entire intestine with a slightly higher density in the duodenum of bony fish, amphibians and reptiles. In contrast, VIP immunoreactivity in enteric nerve fibers and perikarya was found in all species studied except for the cyclostomes where the presence of VIP immunoreactive enteric fibers is questionable. The results may support, at least for mammals and birds, the hypothesis that true VIP is of nervous origin, while the endocrine cells contain VIP-related peptides.