Adrenaline, noradrenaline and dopamine are built on one production line
Your fight-or-flight chemicals are siblings made in sequence. Cells turn the amino acid tyrosine into L-DOPA, then dopamine; add one enzyme and dopamine becomes noradrenaline, add another and it becomes adrenaline. A neuron's identity depends simply on how far down that assembly line its enzymes can go.
Catecholamines are small signalling molecules defined by their shape: a benzene ring carrying two neighbouring hydroxyl groups, called a catechol, joined by a short ethyl chain to an amine. The best known are adrenaline, noradrenaline and dopamine. They dissolve in water, and about half of what circulates in the blood rides attached to plasma proteins.
The raw material, tyrosine, comes either straight from dietary protein or from another amino acid, phenylalanine, converted mainly in the liver; people who lack enough of that converting enzyme develop phenylketonuria. The slowest, rate-limiting step is turning tyrosine into L-DOPA. After that things move so fast that L-DOPA is hard to detect in the brain unless the next enzyme is blocked. A dopamine neuron needs two enzymes, a noradrenaline neuron three, and an adrenaline-making cell all four.
Most production happens in the chromaffin cells of the adrenal medulla and in sympathetic nerve fibres. In the brain, dopamine comes largely from two brainstem regions, the ventral tegmental area and the substantia nigra, while the pigmented locus coeruleus makes noradrenaline. Released into the blood during a perceived threat, these chemicals raise heart rate, blood pressure and blood sugar to ready the body for action. They last only a few minutes in circulation before enzymes called COMT and MAO break them down; drugs known as MAOIs work by jamming the second of these.
Levels climb with stress, including loud noise, bright light and low blood sugar. Extreme surges can follow brainstem injury, known in emergency medicine as a catecholamine dump, or come from adrenal tumours called pheochromocytomas, which urine tests help detect. Loss of noradrenaline-producing cells with age is being studied as a possible factor in Alzheimer's disease, though that research is preliminary. Plants make catecholamines too: they turn up in 44 plant families, where they seem to influence growth and flowering and may help defend against insects.
Source: Catecholamine
