Norepinephrine (noradrenaline) belongs to a family of biological compounds called catecholamines. These compounds are synthesized in sympathetic neurons and in the adrenal glands. Norepinephrine is produced from the catecholamine dopamine by the action of the enzyme dopamine β -hydroxylase. This enzyme is responsible for the addition of a hydroxyl (–OH) group at the β carbon. (See Figure 1.) In certain cells of the adrenal glands, norepinephrine is chemically transformed into epinephrine (adrenaline), the hormone responsible for the fight-or-flight response. Epinephrine differs from norepinephrine by the presence of a methyl (–CH 3 ) group on the nitrogen atom.
Norepinephrine functions biologically as a neurotransmitter, transmitting a signal from one neuron to another neuron or muscle cell. After release from a neuron, norepinephrine diffuses through the tiny space between the cells (the synapse), where it can bind to a receptor protein on the surface of a nearby cell. Nerve impulses are typically short-lasting because the neurotransmitter dissociates from its receptor. Once this happens, the neurotransmitter can quickly be chemically altered or transported into another cell, either of these terminating the nerve impulse.
The family of receptors that responds to norepinephrine and related compounds are called adrenergic receptors. Adrenergic receptors in the peripheral nervous system are important in the activity of smooth muscle and cardiac muscle and in metabolism . The effect on most smooth muscle is relaxation, whereas the effect on cardiac muscle is to increase the force and rate of contraction . Drugs that mimic the action of norepinephrine are often used to treat asthma because they relax bronchial smooth muscle, helping the asthma patient to breathe more easily. Drugs called β -blockers bind to adrenergic receptors but block activation. Because this results in a decrease in blood pressure, β -blockers are commonly prescribed to treat high blood pressure. Adrenergic receptor activity is also important within the central nervous system. Some drugs used to treat depression prolong the adrenergic nerve impulse by allowing norepinephrine to remain in synapses for longer periods.
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