Any of a group of polypeptides formed in the brain tissue and pituitary gland of higher animals that are thought to control the transfer of signals at nerve junctions, thus ensuring that behavior patterns in the individual remain normal. Imbalance or malfunction of these polypeptides has been reported to be a factor in irrational and violent actions and other emotional disorders, as well as in epilepsy and memory processes. This belongs to a developing field of medicinal chemistry called neuropharmacology.