Norepinephrine (chemical name C8H11NO3)is a catecholamine present in the human brain, sometimes referred to as noradrenaline. Norepinephrine acts as a hormone when it is released from the adrenal glands into the bloodstream. In concert with epinephrine, norepinephrine influences the fight or flight response by increasing heart rate and muscle readiness, and increasing energy by causing glucose release, thereby heightening readiness to respond (e.g., to fight or flee) to a stressful event.
Norepinephrine also acts as a neurotransmitter in the central nervous system and sympathetic nervous system. It influences alertness and arousal, and lower levels are associated with depressed mood. Many antidepressant medications cause increased norepinephrine levels in the brain. Drugs that increase norepinephrine are also used in the treatment of Attention Deficit Hyperactivity Disorder and hypotension (low blood pressure).