norepinephrine
Meaning of norepinephrine: Most of what circulates leaks from sympathetic nerve endings rather than pouring out of the adrenal glands, unlike its better known relative.
Definition of norepinephrine
Norepinephrine is a signaling chemical that acts in two ways: as a neurotransmitter carrying messages between nerve cells, and as a hormone traveling in the blood to distant tissues. It is released mainly by the nerve endings of the sympathetic nervous system and in smaller amounts by the adrenal glands above the kidneys. Its characteristic effects are narrowed blood vessels, higher blood pressure and heightened alertness.
Where does it come from?
Two sources, of very unequal size. The bulk is made and released at the endings of sympathetic nerves, the branch of the nervous system that prepares the body for exertion or threat. Those endings release it directly onto the tissue they serve, and only the surplus that escapes reuptake spills into the blood, which is where measured levels come from. The adrenal glands supply the rest as a true hormone, though their inner core secretes mostly epinephrine and comparatively little norepinephrine. A separate population of nerve cells in the brainstem uses it inside the brain, where it supports arousal, attention and the response to stress.
How does it differ from epinephrine?
| Norepinephrine | Epinephrine | |
|---|---|---|
| Main source | Sympathetic nerve endings | The inner core of the adrenal gland |
| Chemical difference | Lacks a methyl group on its nitrogen | Carries that extra methyl group |
| Dominant effect | Narrows vessels strongly, raising pressure | Raises heart rate and output more, and widens the airways |
| Other name | Noradrenaline | Adrenaline |
The prefix nor- marks that missing methyl group, and epinephrine is made from norepinephrine by adding it. That one small change shifts which receptors each favors, and the different receptor preferences are what produce their different effects.
How does it matter in medicine?
A laboratory-made form is given by continuous infusion to raise dangerously low blood pressure in shock, usually through a large central vein because leakage into surrounding tissue can injure it. Its receptors are also drug targets in the other direction: beta blockers and alpha blockers work by preventing it from acting. Medicines that increase its availability in the brain are used in depression, attention disorders and some pain conditions. Levels also carry diagnostic weight, since a tumor of the adrenal core or of nearby nerve tissue can pour out norepinephrine and epinephrine, causing episodes of pounding heartbeat, headache, sweating and severe pressure spikes.
Also known as
Used in a sentence
Norepinephrine was administered to support the patient's blood pressure while in the intensive care unit.
Good to know
Norepinephrine and epinephrine are closely related chemicals with overlapping but distinct effects, and norepinephrine's effect on narrowing blood vessels tends to be more pronounced relative to its effect on heart rate.
Word origin
Where “norepinephrine” comes from
The epinephrine portion derives from Greek epi-, meaning upon, and nephros, meaning kidney, reflecting the location of the adrenal glands; the prefix nor- indicates a chemically demethylated form of that molecule.