Norepinephrine is both a hormone and a neurotransmitter. As a hormone, secreted by the adrenal gland, it works alongside epinephrine / adrenaline to give the body sudden energy in times of stress, known as the "fight or flight" response. As a neurotransmitter, it passes nerve impulses from one neuron to the next.
Medications that inhibit the reuptake of norepinephrine and serotonin may be effective to treat depression. In addition, some studies have found elevated norepinephrine levels in patients experiencing mania.
Schatzburg, A.F., and Schildkraut, J.J. (2000). "Recent Studies on Norepinephrine Systems in Mood Disorders." Retrieved October 8, 2006 from http://www.acnp.org/g4/GN401000092/CH090.html.