Norepinephrine, a neurotransmitter as well as a hormone, is related to adrenaline (also referred to as epinephrine), known for its stimulating effects on the body. Norepinephrine is responsible for, among other things, arousal and alertness. For many years it was thought to be the primary neurotransmitter responsible for depression. Reduced levels of norepinephrine may partially explain the apathy seen in individuals with this and other mental disorders. Elevated levels of norepinephrine are strongly associated with anxiety disorders, which frequently coexist with depression.
The adrenergic receptors (or adrenoceptors) are a class of G protein-coupled receptors that are targets of the catecholamines, especially norepinephrine (noradrenaline) and epinephrine (adrenaline).
Many cells possess these receptors, and the binding of a catecholamine to the receptor will generally stimulate the sympathetic nervous system. The sympathetic nervous system is responsible for the fight-or-flight response, which includes widening the pupils of the eye, mobilizing energy, and diverting blood flow from non-essential organs to skeletal muscle.
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