Noradrenaline (BAN) (abbreviated NA or NAd) or norepinephrine (INN) (abbreviated norepi or NE) is a catecholamine with dual roles as a hormone and a neurotransmitter.
As a stress hormone, norepinephrine affects parts of the brain where attention and responding actions are controlled. Along with epinephrine, norepinephrine also underlies the fight-or-flight response, directly increasing heart rate, triggering the release of glucose from energy stores, and increasing blood flow to skeletal muscle.
However, when norepinephrine acts as a drug it will increase blood pressure by its prominent increasing effects on the vascular tone from α-adrenergic receptor activation. The resulting increase in vascular resistance triggers a compensatory reflex that overcomes its direct stimulatory effects on the heart, called the baroreceptor reflex, which results in a drop in heart rate called reflex bradycardia.
Mechanism
Norepinephrine is synthesized from tyrosine as a precursor, and packed into synaptic vesicles. It performs its action by being released into the synaptic cleft, where it acts on adrenergic receptors, followed by the signal termination, either by degradation of norepinephrine, or by uptake by surrounding cells.
Biosynthesis
Norepinephrine is synthesized by a series of enzymatic steps in the adrenal medulla and postganglionic neurons of the sympathetic nervous system from the amino acid tyrosine:
The first reaction is the hydroxylation into dihydroxyphenylalanine (L-DOPA) (DOPA = 3,4-DiHydroxy-L-Phenylalanine), catalyzed by tyrosine hydroxylase. This is the rate-limiting step.
This is followed by decarboxylation into the neurotransmitter dopamine, catalyzed by pyridoxal phosphate & DOPA decarboxylase.
Last is the final β-oxidation into norepinephrine by dopamine beta hydroxylase, requiring ascorbate as a cofactor (electron donor).
Norepinephrine. (2009, December 11). In Wikipedia, The Free Encyclopedia. Retrieved 12:38, December 16, 2009, from http://en.wikipedia.org/w/index.php?title=Norepinephrine&oldid=330987829
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