dopamine

Dopamine is a catecholamine, from which other important catecholamines (adrenaline and noradrenaline) are derived, but it is also an important neurotransmitter in its own right, especially in the brain. Of particular importance are central nervous pathways involved with the co-ordination of movement and with behaviour and emotion. As with the other catecholamines, dopamine is released from nerve endings where it acts upon receptors on other nerve cells to produce its effects. There are two sorts of receptors for dopamine, namely D₁ and D₂, of which the second are of greater importance. After release the dopamine is rapidly destroyed or taken up back into the nerve fibres for reuse. A number of medical conditions are associated with under-activity or over-activity of dopamine pathways in the brain. The rigidity and tremor, together with the hypokinesia (relative lack of voluntary movement), of Parkinson's disease are associated with lack of dopaminergic function in the nigrostriatal pathway in the brain. One form of treatment for this disease is to give large amounts of L-DOPA, the precurser of dopamine, so that some reaches the brain, is converted to dopamine, and restores some lost functions. Another approach is to transplant fetal dopamine-secreting cells into the relevant brain nuclei to take over the function of the diseased nerve cells. Over-activity of dopaminergic nerves, especially in the limbic system, is associated with schizophrenia. It has also been claimed that individuals with mutations affecting the dopamine D₂ receptors show enhanced risk-taking behaviour. Some drugs, like the amphetamines, release dopamine from nerve endings in the brain, leading to hyperactivity and manic behaviour. Dopamine pathways are also associated with the vomiting centre in the brain, so drugs which block the dopamine receptors, such as the phenothiazines, have a calming effect on schizophrenics and are also useful anti-emetic drugs. There are a few dopamine pathways in other parts of the body, for example the kidney, where activation causes vasodilation. Dopamine increases the force of contraction of the heart and an infusion is sometimes used to treat shock resulting from blood loss.

— Alan W. Cuthbert

See also basal ganglia; drug abuse.

A sympathomimetic catecholamine used in the treatment of shock, hypotension, and low cardiac output.

A metabolic neurotransmitter belonging to the biogenic amines and is classified as a stimulant, but it is not on the World Anti-Doping Agency's Prohibited list. Dopamine is secreted by some sympathetic ganglia in the hypothalamus and by some neurones in the midbrain. It is the main neurotransmitter in extrapyramidal tracts. Its release is enhanced by amphetamines. High dopamine levels have been linked to aggression. Anti-psychotic drugs such as thorazine, have been used to block dopamine receptors and reduce aggression. However, overuse of these antipsychotics may cause motor problems because dopamine levels may be reduced in parts of the brain controlling skeletal muscle.

Dopamine