Dopamine

Dopamine is a neurotransmitter (a chemical used to send signals between nerve cells) in the same family as epinephrine (adrenaline). Dopamine is one of the primary neurotransmitters and it affects motor functions (movement), emotions, learning , and behavior . It was originally identified as the brain chemical associated with pleasure. A decrease in the amount of dopamine in specific sections of the brain has been implicated as a possible cause of Parkinson's disease , while an excess of dopamine in some regions of the brain has been suggested as a possible cause of schizophrenia . Dopamine is also thought to play a role in depression , attention deficit hyperactivity disorder, high blood pressure , and drug addiction . Recently, dopamine has been used as a treatment for victims of heart attacks.

Basic definitions and chemical information

Dopamine is one of a group of chemicals known as catecholamine neurotransmitters. Catecholamines are a group of chemicals that include epinephrine (adrenalin); histamine , which is responsible for many of the symptoms of allergies; and serotonin, a molecule that has been suggested as aiding in sleep . This group of compounds is sometimes collectively known as the biogenic amines. Neurotransmitters are chemicals used by the body to signal or send information between nerve cells or nerve and muscle cells. The chemical structure of dopamine is shown below. The NH2 group on the molecule is the amine group in the term biogenic amines. This entire group of chemicals has been implicated in depression and general moods.

Dopamine and Parkinsons disease

Parkinson disease is a disorder of the nervous system that is characterized by slow movements and difficulty initiating movements, a shuffle when walking, and increased muscular rigidity. It is estimated to affect as many as one million Americans and is far more prevalent in the elderly. The main cause of Parkinson disease is thought to be a lack of dopamine in a region of the brain known as the substantia nigra. Whether the cells in that area do not produce enough dopamine or whether there are too few of the dopamine-producing cells is a matter of debate and active research. A chemical known as Levodopa or L-dopa, which our bodies rapidly metabolize to dopamine, is the main treatment. Levodopa reduces the symptoms of the disease, but does not stop the progression of the disease. A lack of dopamine in some areas of the brain also has been implicated in depression.

Dopamine and schizophrenia

Schizophrenia is a form of psychosis or loss of contact with reality. It is estimated to affect about 1% of the population, or over 2.5 million Americans. A great deal of research is being done on the origins of schizophrenia. One widely accepted theory is that it is caused by an excess of dopamine or dopamine receptors. Receptors are proteins on the surfaces of cells that act as signal acceptors for the cells. They allow cells to send information, usually through neurotransmitter molecules. This hypersensitivity to dopamine (the prefix "hyper" means over or excessive) is treated by using chemicals that block (or inactivate) the receptors for the dopamine signals. However, there are a number of different types of dopamine receptors and there are many differences among individuals in the structures of these receptors. Drugs usually block all of the receptors, not just the ones related to schizophrenic symptoms, resulting in many side effects. Other approaches to the treatment of schizophrenia have focused on decreasing the amounts of dopamine in the brain. In doing so, however, symptoms of Parkinson disease often result, since less dopamine (or the ability to respond to dopamine) is present. The origins of schizophrenia are unclear; dopamine excess is probably not the sole cause of the disease as strong evidence for genetic and environmental factors exists as well. To date, treatments that focus on excess dopamine sensitivity have been the most successful.

Dopamine as heart medicine

Since dopamine can increase blood pressure, it is used as a treatment for shock (low blood pressure throughout the body) which carries the risk of damage to major organs in patients who have suffered serious heart attacks. Dopamine raises the blood pressure and causes small blood vessels to constrict, thus raising the blood pressure throughout the body. Chemically related molecules such as adrenaline act similarly and both are often used to help patients.

Dopamine and attention deficit hyperactivity disorder

Attention deficit hyperactivity disorder (ADHD), a syndrome that affects as many as 3.5 million American children, and many adults as well, is characterized by an inability to pay attention, over-activity, and impulsive behaviors. ADHD has been associated with certain forms of the dopamine D4 receptor, and with individual differences in the gene that encodes the dopamine transporter, a molecule that binds and carries dopamine. ADHD often is treated with stimulatory drugs such as Ritalin, which increase the availability of dopamine in the brain.

Dopamine and drug addiction

Alcohol , nicotine , and a variety of other drugs including marijuana , cocaine , amphetamines , and heroin all appear to raise the level or the availability of dopamine in different parts of the brain. Pathways of nerve cells that produce dopamine and contain dopamine receptors are affected by all of these drugs. There is evidence that certain forms of the dopamine D4 receptor may predispose a person to drug addiction. Based on this information, researchers are attempting to develop drugs to treat addictions.

Dopamine and aging

Although individuals vary greatly in the amount of dopamine activity in their brains, in general dopamine appears to decline with age in those parts of the brain responsible for thinking. In particular, as people age, the number of dopamine D2 receptors decreases significantly. Thus, dopamine may be involved with the age-related loss of intellectual skills.

Resources

books

ackerman, s. discovering the brain. washington, dc: national academy press, 1992.

restak, richard m. receptors. new york: bantam press, 1994.

periodicals

bower, bruce. "the birth of schizophrenia: a debilitating mental illness may take root in the fetal brain." science news (may 29, 1993): 346.

Concur, Bruce. "A Dangerous Pathway." New Scientist (July 5, 1997).

Miller, Susan. "Picking up Parkinson's Pieces." Discover (May 1991): 22.

Zamula, Evelyn. "Drugs Help People with Parkinson's Disease." FDA Consumer (January-February 1992): 28.

Louis J. Gotlib

KEY TERMS

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Neurotransmitter

—A chemical used to send information between nerve cells or nerve and muscle cells.

Psychosis

—A loss of contact with reality. It may be caused by drugs, chemical imbalances, or even severe stress or depression.

Receptors

—Protein molecules on a cells surface that acts as a "signal receiver" and allows communication between cells.