Numerous peptides appear to be neurotransmitter candidates in the brain. Some, such as the opioid peptide enkephalins, neurotensin, and substance P, were first isolated from the brain. Peptides, such as cholecystokinin and vasoactive intestinal polypeptide, were known as intestinal hormones and later recognized as brain constituents. Certain hypothalamic-reteasing hormones, pituitary peptides, and blood-derived peptides like angiotensin II and bradykinin, may also be central neurotransmitters. The diversity of localization of these peptides throughout the brain implies a multiplicity of potential roles.
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