Norepinephrine is released from the medulla of the adrenal glands as a hormone into the blood, but it is also a neurotransmitter in the 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, this compound affects the fight-or-flight response, activating the sympathetic nervous system to directly increase heart rate, release energy from glucose and glycogen, and increase muscle readiness.

Norepinephrine, along with dopamine, has come to be recognized as playing a large role in attention and focus. For people with ADD/ADHD, psychostimulant medications such as Ritalin (methylphenidate), Dexedrine (dextroamphetamine), and Adderall are prescribed to help increase levels of norepinephrine and dopamine. Norepinephrine is also used as a vasopressor medication for patients with critical hypotension to cause vasoconstriction and beta-1 adrenergic receptors to increase heart rate, stroke volume and cardiac output. Norepinephrine is mainly used to treat patients in septic shock and has shown a survival benefit over dopamine.

Formal Chemical Name (IUPAC)
4-(2-amino-1-hydroxyethyl)benzene-1,2-diol

References

http://en.wikipedia.org/wiki/Norepinephrine