Norepinephrine (abbreviated norep) or noradrenaline was a hormone and neurotransmitter in the humanoid central nervous system. As a stress hormone, it affected the part of the brain that controlled alertness as well as the fight-or-flight response. Along with epinephrine or adrenaline, it increased heart rate and blood pressure, thus increasing blood flow to muscles in addition to triggering the release of glucose from the body's energy stores.
In 2372, The Doctor found elevated levels of norepinephrine in Lon Suder, which suggested increased aggressive tendencies. However, he also noted that Suder's levels were not significantly different from any of the other former Maquis crew. (VOY: "Meld")
The Clown, a manifestation of fear in Viorsa's virtual reality stasis system, metaphorically chopped his victims' heads off by raising the amount of norepinephrine in their bodies to such extreme levels that they suffered massive heart attacks and died. (VOY: "The Thaw")
In 2375, when Sarina Douglas began responding to Doctor Julian Bashir's treatment for her catalepsy, he discovered that her norephinephrine levels were a little high, which Douglas herself explained was because of limbic overcompensation for the new thalamic connections. (DS9: "Chrysalis")
In 2376, The Doctor found that Neelix's norepinephrine levels were three times what they should have been, after his exposure to neurogenic pulses from the Tarakis memorial monument. (VOY: "Memorial")