Relations between neuropeptides and gut hormones

Certain peptides have a dual distribution in brain and in neurons or endocrine cells, or both, in the gut. Substance P, neurotensin and somatostatin have been isolated from brain and gut (or pancreas); cholecystokinin has been isolated from intestine as peptides of 33 and 39 residues and their active C-terminal octapeptide has been isolated from brain, which suggests differences between biosynthetic processing pathways for cholecystokinin in neurons and those in endocrine cells. Identification of other peptides (vasoactive intestinal peptide, bombesin, enkephalin etc.) rests in part on immunochemical evidence and needs to be confirmed by isolation. The combined evidence of immunochemical and physiological studies suggests that gut target cells can receive peptides in at last four ways: (1) as hormones in the blood; (2) from intrinsic neurons in the enteric plexuses; (3) from extrinsic nerves like the vagus; (4) by local (paracrine) release from mucosal endocrine-like cells, possibly by basal axon-like processes.