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Catecholamines
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
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.
Adapted from the Nutritional Health Bible by Linda Lazarides Download the whole database |
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