A neuropeptide is any of a family of molecules formed by a short chain of amino acids and which plays a role in humanoid brain activity. An increase or decrease in neuropeptide levels may be a cause or symptom of abnormal brain activity. In 2372, the neuropeptide levels in Tuvok's limbic systems decreased by 15 per cent, after a mind meld with Lon Suder. (VOY: "Meld") Also in 2373, The Doctor noted that Kathryn Janeway's neuropeptides had risen to an abnormal level during her Nechani endurance ritual. (VOY: "Sacred Ground")