n. any of several peptides that are released by neurons as neurotransmitters or neurohormones. They include the endogenous opioids (e.g., enkephalin, endorphin); peptides found in both the brain and the peripheral nervous system (e.g., substance P, neurotensin); hypothalamic releasing hormones (e.g., thyrotropin-releasing hormone); pituitary hormones (e.g., growth hormone, prolactin); and other circulating peptides (e.g., atrial natriuretic peptide, bradykinin).