Norepinephrine, also called noradrenaline, is an organic chemical in the catecholamine family that functions in the human brain and body as a hormone and neurotransmitter.n the most basic terms, the function of norepinephrine is to mobilize the brain and body for action. Norepinephrine release is lowest during sleep, rises during wakefulness, and reaches much higher levels during situations of stress or danger, in what has been called the fight-or-flight response.