Dopamine, chemical known as a neurotransmitter essential to the functioning of the central nervous system. In the process of neurotransmission, dopamine is transferred from one nerve cell, or neuron, to another, playing a key role in brain function and human behavior.
Dopamine forms from a precursor molecule called dopa, which is manufactured in the liver from the amino acid tyrosine. Dopa is then transported by the circulatory system to neurons in the brain, where the conversion to dopamine takes place.
Dopamine is a versatile neurotransmitter. Among its many functions, it plays a major role in two activities of the central nervous system: one that helps control movement, and a second that is strongly associated with emotion-based behaviors.
The pathway involved in movement control is called the nigrostriatal pathway. Dopamine is released by neurons that originate from an area of the brain called the substantia nigra and connect to the part of the brain known as the corpora striata, an area known to be important in controlling the musculoskeletal system.
The second brain pathway in which dopamine plays a major role is called the mesocorticolimbic pathway. Neurons in an area of the brain called the ventral tegmentalarea transmit dopamine to other neurons connected to various parts of the limbic system, which is responsible for regulating emotion, motivation, behavior, the sense of smell, and various autonomic, or involuntary, functions like heartbeat and breathing.
A growing body of evidence suggests that dopamine is involved in several major brain disorders. Narcolepsy, a disorder characterized by brief, recurring episodes of sudden, deep sleep, is associated with abnormally high levels of both dopamine and a second neurotransmitter, acetylcholine. Huntington's chorea, an inherited, fatal illness in which neurons in the base of the brain are progressively destroyed, is also linked to an excess of dopamine.
Commonly known as shaking palsy, Parkinson's disease is another brain disorder in which dopamine is involved. Besides tremors of the limbs, Parkinson's patients suffer from muscular rigidity, which leads to difficulties in walking, writing, and speaking. This disorder results from the degeneration and death of neurons in the nigrostriatal pathway, resulting in low levels of dopamine. The symptoms of Parkinson's disease can be minimized by treatment with a drug called levodopa, or L-dopa, which causes the surviving neurons in the nigrostriatal pathway to increase production of dopamine.
Schizophrenia is a psychiatric disorder characterized by loss of contact with reality and major changes in personality. Schizophrenics have normal levels of dopamine in the brain, but because they are highly sensitive to this neurotransmitter, these normal levels of dopamine trigger unusual behaviors. Drugs such as thorazine that block the action of dopamine have been found to decrease the symptoms of schizophrenia.
Studies indicate that people who are addicted to alcohol and other drugs like cocaine and nicotine have less dopamine in the mesocorticolimbic pathway. These drugs appear to increase dopamine levels, resulting in the pleasurable feelings associated with the drugs.

Contributed By: 
Ernest P. Noble