Norepinephrine (INN)(abbr. norepi) or noradrenaline (BAN) is a catecholamine and a phenethylamine with chemical formula C8H11NO3. The natural stereoisomer is L-(−)-(R)-norepinephrine. The prefix nor- stands for the German abbreviation "Nitrogenium Ohne Rest" meaning "Nitrogen without rest" referring to the absence of a methyl group at the N-atom present in epinephrine/adrenaline. It is released from the medulla of the adrenal glands as a hormone into the blood, but it is also a neurotransmitter in the central nervous system and sympathetic nervous system where it is released from noradrenergic neurons during synaptic transmission. As a stress hormone, it affects parts of the human brain where attention and responding actions are controlled. Along with epinephrine, norepinephrine underlies the fight-or-flight response, directly increasing heart rate, triggering the release of glucose from energy stores, and increasing skeletal muscle readiness.
However, when norepinepherine acts as a drug, it will increase systolic and diastolic pressure, causing compensatory vagal reflex, which overcomes its direct cardioaccelerator action. Therefore, it will cause reflex bradycardia in patient.
Norepinephrine is released when a host of physiological changes are activated by a stressful event. This is caused in part by activation of an area of the brain stem called the locus ceruleus. This nucleus is the origin of most norepinephrine pathways in the brain. Neurons that are activated by norepinephrine project bilaterally (send signals to both sides of the brain) from the locus ceruleus along distinct pathways to many locations, including the cerebral cortex, limbic system, and the spinal cord. The locus ceruleus (or "LC") is located within the dorsal wall of the rostral pons in the lateral floor of the fourth ventricle (see picture).