Dopamine is a type of natural chemical messenger in the brain called a neurotransmitter. It relays messages from one nerve cell to the next, passing through a small space between the cells called a synapse and binding to specialized receptors in the nerve cell membrane on the other side. As a critical player in healthy function of the body and mind, normally mechanisms tightly regulate the levels of dopamine available in the nerve cells.
Role
The Merck Manual classifies dopamine as a "major" neurotransmitter, one of the 18 most important among approximately one hundred other chemicals with similar roles. Dopamine operates in many systems, affecting diverse processes like muscle movement, thinking, emotion, motivation, and the sensation of pleasure. Although the broad definition of hormone as "a chemical substance...which has a specific regulatory effect on cells" cited by The Free Dictionary could encompass dopamine and other neurotransmitters, dopamine is generally not considered a hormone under the more traditional and strict definitions because it is not produced by an endocrine gland and it does not circulate in the blood.
Receptors
Dopamine can interact with any one of five different receptors in nerve cell membranes. The receptors are named D1 through D5. Each receptor type has a slightly different structure, distribution and role.
Brain Areas
Dopamine does act on some peripheral nerves, but the bulk of its actions occur in the central nervous system. Dopamine acts primarily in the hypothalamus, midbrain, and substantia nigra.
Production
The Merck Manual describes how the body makes dopamine from an amino acid precursor called tyrosine. Special nerve cells in the central nervous system, called dopaminergic neurons, actively take in tyrosine. The enzyme tyrosine hydroxylase converts tyrosine to an intermediate product, which another enzyme converts to dopamine. Dopamine molecules are packaged into little granules called synaptic vesicles until their release into the synaspse. After dopamine has performed its role in neurotransmission, the nerve cell that released it actively pumps it back in. An enzyme called monoamine oxidase or MAO degrades unpackaged dopamine back down to an inactive form.
Dopamine and Disease
In Parkinson's disease, a progressive neurological disorder, the nerves fail to communicate normally with muscles. People with Parkinson's disease do not have enough natural dopamine, and the most effective treatment, according to the Mayo Clinic, is the drug levodopa or other drugs that mimic dopamine. These drugs can cross the blood-brain barrier, unlike dopamine itself, and once in the brain act like natural dopamine, reducing the symptoms of Parkinson's disease. Problems with regulation of dopamine levels also contribute to schizophrenia, says the Mayo Clinic. The Merck Manual says that stimulating the D4 sub-type of dopamine receptors helps control the symptoms of schizophrenia.
Dopamine and Addiction
The National Institute of Drug Abuse (NIDA) describes the important role of the dopamine system in the development of addiction. According to NIDA, most drugs of abuse artificially trigger the dopamine system in the brain, but far more powerfully than normally rewarding sensations like sex or eating, creating an intense feeling of pleasure. Over time though, the dopamine system wears down; the abuser is unable to feel pleasure in response to normal sensations and develops tolerance to effects of the drugs.
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