Dopamine

Dopamine belongs to a family of biological compounds called catecholamines (see Figure 1). Dopamine is synthesized from the compound L-dihydroxyphenylalanine (L-dopa) via the enzyme dopa decarboxylase. In noradrenergic neurons and in the adrenal glands, dopamine is the precursor for the neurotransmitter norepinephrine. In dopaminergic neurons, dopamine itself acts as a neurotransmitter. Although dopaminergic neurons are not as widely distributed in the brain as noradrenergic neurons, they act to coordinate movement, to control the secretion of some hormones, and to regulate mood and emotional stability.

Dopamine's role in the coordination of movement can be partially understood by examining Parkinson's disease. This illness is associated with low levels of dopamine in the brain and is characterized by spastic motion of the eyelids as well as rhythmic tremors of the hands and other parts of the body. One method of treating Parkinson's disease is to increase the concentration of dopamine in the brain. This is most effectively accomplished by administering the precursor of dopamine, L-dopa. In order to prevent concentrations of norepinephrine from increasing as well, L-dopa is given in conjunction with a drug that inhibits norepinephrine synthesis .

The role that dopamine plays in regulating mood and emotional stability can be at least partially grasped by examining dopamine's role in schizophrenia and drug addiction. Schizophrenia is a disorder characterized by delusions, hallucinations, withdrawal from external reality, and emotional unresponsiveness. The dopamine theory of schizophrenia, proposed in 1965, attributes the disorder to elevated brain concentrations of dopamine or to a hypersensitivity of dopaminergic receptors , especially the D ₂ and D ₄ receptor subtypes. Several drugs used to treat schizophrenic patients bind to D ₂ and D ₄ receptors and block the dopaminergic response.

Dopamine is also an important component of the brain's "reward system" and is believed to play a role in drug addiction. Increased levels of dopamine have been associated with cocaine, amphetamine , and marijuana use, as well as alcohol and nicotine addiction.

Figure 1. The structure of dopamine as it exists in solution.

SEE ALSO Neurotransmitters .

Jennifer L. Powers

Bibliography

Balter, Michael (1996). "New Clues to Brain Dopamine Control, Cocaine Addiction." Science 271:909.

Internet Resources

Indiana University School of Medicine, Terre Haute Center for Medical Education. The Medical Biochemistry Page. "Biochemistry of Nerve Transmission." Available from http://web.indstate.edu/thcme/mwking/nerves.html .

Northeastern University, Physical Therapy Department. Neuroanatomy Cyberlectures. "Pharmacology: The Chemistry of the Nervous System." Available from http://www.ptd.neu.edu/neuroanatomy/cyberclass/Pharmacology .