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neuropeptide Any of numerous peptides that influence the activity of neurons. Examples include the hypothalamic releasing hormones, antidiuretic hormone, and the gastric peptides (e.g. VIP) released from cells in the duodenal wall. Neuropeptides may act as neurotransmitters, as cotransmitters to modify the action of neurotransmitters, or as neurohormones, and some play all three roles depending on their location.