What Is a Dopamine Marker?

A biological marker, or biomarker, is an indirect measurement that helps to diagnose illness. For example, an increase of white blood cells represents a biomarker for infection. The cause of the infection is not directly measured; rather the marker used is the white blood cell count. Other biomarkers, such as dopamine markers, may be used to evaluate treatment or the progression of a disease.

Function

• Dopamine is a neurotransmitter produced by approximately 0.3 percent of the neurons in the brain. Dopamine is involved in kidney, hormonal, cardiovascular and central nervous system regulation. The use of dopamine as a pharmaceutical helps patients with dangerously low blood pressure, such as those suffering from septic shock. In such cases, dopamine aids by directly dilating the kidney, abdominal wall, brain and heart blood vessels.

Metabolism

• Dopamine cannot cross the blood-brain barrier. Consequently, synthesis of dopamine occurs directly in the brain from the amino acid tyrosine. Tyrosine is synthesized first into L-dopa, and then into dopamine. Dietary sources of tyrosine include eggs, cheese, poultry and beef. Products of the metabolism of dopamine, such as homovanillic acid, provide dopamine researchers with markers to study.

Parkinson's Disease

• Brain production of dopamine is via the dopaminergic neurons. In Parkinson's disease, which can affect middle-age and elderly people, there is an irreversible loss of these neurons and of dopamine. Progressive muscle tremors characterize Parkinson's, as the overall effect of dopamine is to inhibit muscle contraction. Treatment for this loss of dopamine includes providing the patient with the dopamine precursor L-dopa. Given that it is impossible to measure the loss of dopaminergic neurons directly, researchers make use of dopamine markers to follow Parkinson's disease treatment and progression.

Addiction

• Dopamine plays a role in addiction because it is part of the brain's pleasure and reward circuit. Drugs that imitate or stimulate production of dopamine include cocaine, heroin, nicotine and alcohol. With use, the delicate balance of the brain becomes altered and the brain produces less natural dopamine. When the drug is withdrawn, feelings of depression follow. The brain learns to seek out these drugs to maintain the new balance, and this creates an addiction. Researcher Stephen J. Kish, in an article titled "Striatal Dopaminergic and Serotonergic Markers in Human Heroin User" and published in the journal Neuropsychopharmacology, noted a 33 percent reduction in the dopamine marker homovanillic acid among long-term heroin users. This reduction reflected a possible drop in dopamine synthesis in response to the overstimulation caused by heroin use.

The Future

• New biomarkers for dopamine are on the horizon. Diagnostic methods like nuclear magnetic resonance imaging may help diagnose Parkinson's disease while symptoms are still minor. Advances in DNA analysis may allow the discovery of markers for genetic dopamine disorders like schizophrenia. The perfect marker would be one that doctors can test for inexpensively and easily. In the future, research may make that perfect marker available.