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 (e.g. substance P), as cotransmitters to modify the action of neurotransmitters, as neuromodulators, or as neurohormones; some play several roles depending on their location.