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Norepinephrine, also known as noradrenaline, is a monoamine neurotransmitter, a term that
refers its chemical structure and the fact that it’s derived from an amino acid.
It is also a catecholamine, a term that again refers to its chemical structure and the fact
that it contains a catechol nucleus.
Norepinephrine also functions as a hormone.
It is synthesized from dopamine in a reaction catalyzed by the enzyme dopamine beta-hydroxylase.
Norepinephrine-producing neurons in the central nervous system are primarily concentrated
in the pons and medulla. The most prominent of these groups of neurons is a nucleus called
the locus coeruleus, which is the main site of norepinephrine production for the central
nervous system. Norepinephrine is also the primary neurotransmitter used by the sympathetic
nervous system, and is found in clusters of sympathetic neurons located near the spinal
cord known as sympathetic ganglia.
It is also released from the adrenal glands as a hormone.
Norepinephrine acts on g-protein coupled receptors referred to as adrenergic receptors or adrenoceptors.
There are thought to be at least three main types of adrenergic receptors, alpha-1, alpha-2
and beta-adrenergic receptors, each of which has multiple subtypes.
Norepinephrine is removed from the synaptic cleft by a transport protein called the norepinephrine
transporter.
Like any neurotransmitter, the actions of norepinephrine depend on the type of receptor
it activates, and where that receptor is located.
Thus, although norepinephrine in the central nervous system is frequently associated with
arousal, alertness, and attention, the full extent of its actions are more complex.
Its release in the sympathetic nervous system is typically associated with responses linked
to increased activity, like elevated heart rate and blood pressure.