Stimulating compounds made from the amino acid dopa, which is in turn derived from tyrosine. The catecholamines form part of a larger family of substances known as monoamines. The best known catecholamines found in the human body are adrenaline (known in the United States as epinephrine) and noradrenaline (norepinephrine), both produced by the adrenal medulla in response to stress. Dopamine, found mainly in the brain, is also an important catecholamine. Catecholamines may also be formed when the amino acid tyrosine is taken up by the terminals of nerve cells. The enzymes present in these terminals determine which catecholamine will be formed.
The main functions of these compounds include the peripheral excitation or inhibition of certain muscles, cardiac excitation, metabolic actions, endocrine actions, and central nervous system actions. Catecholamines bind to receptors in nerve terminals, and their effects depend on the receptors.
Adrenaline and noradrenaline
Also known as the ‘fight or flight’ hormone, adrenaline prepares the body for dealing with stress situations. Like noradrenaline, when it combines with alpha receptors on the muscles of blood vessel walls, it causes them to contract. However the small arteries which supply skeletal muscle also have beta receptors, and when adrenaline combines with these, there is an opposite, relaxing effect. The net result is that more blood is made available to the skeletal muscles, which are then able to respond to extra physical demands such as running, self-defence or competitive sports. Both adrenaline and noradrenaline also increase the force of contraction of the heart muscle and raise blood pressure, and adrenaline speeds up the heart rate.
Low blood sugar stimulates the release of adrenaline for the metabolic tasks of inhibiting insulin secretion, stimulating glucagon secretion, stimulating the breakdown of glycogen (stored carbohydrate), mobilizing fat stores, and promoting the conversion of amino acids, lactate and glycerol into glucose. The effect of these actions is to raise the blood sugar. Other effects of adrenaline include an increase of 30 per cent or more in the body’s heat production, and a raised metabolic rate.
The catecholamines are thought to be involved in the mechanisms of clinical depression and mania. Individuals with depression excrete reduced amounts of catecholamines and other monoamines in their urine. Individuals with mania, on the other hand, excrete increased amounts. Monoamine oxidase inhibitors, which are drugs used to treat depression, work by inhibiting an enzyme which breaks down catecholamines, thus allowing higher concentrations of them to circulate in the body. The drug lithium, which is used to treat mania, is thought to work by reducing the release of noradrenaline from nerve terminals and enhancing its uptake.
Dopamine is primarily known for its role as a neurotransmitter, and as a precursor of adrenaline and noradrenaline. Virtually all drugs which improve or worsen the mental illness schizophrenia have some effect on levels of dopamine in the body. In particular the antipsychotic drugs work by blocking dopamine receptors. Dopamine is also involved in Parkinson’s disease, where the dopamine-releasing nerve cells in the substantia nigra, a portion of the brain, start to degenerate, thus reducing the availability of dopamine to other cells in the brain. (This explains why dopamine-inhibiting drugs given to schizophrenics can cause a parkinsonian-like syndrome known as tardive dyskinesia.)
Another important effect of dopamine is its ability to inhibit the secretion of prolactin, in which role it is known as prolactin release-inhibiting hormone, or PIH.