Dopamine is one of a class of neurotransmitters known as catecholamines. A neurotransmitter is a chemical messenger released from a presynaptic nerve cell or neuron at its terminal into a synapse. A synapse is a narrow gap between the terminal of a presynaptic neuron's axon and a signal- receiving portion (dendrite or cell body) of another neuron or effector cell. The neurotransmitter is released from synaptic vescicles in the presynaptic neuron, diffuses across the synaptic cleft, binds to the postsynaptic cell and stimulates it. After stimulation, the neurotransmitter is removed from the synapse by enzyme action or cell uptake. There are different types of neurotransmitters including acetylcholine, biogenic amines, amino acids, and neuropeptides. Dopamine is one of the biogenic amines, neurotransmitters that are derived from amino acids, and is a member of the group called catecholamines that are derived from the amino acid, tyrosine. Dopamine is produced mainly in the brain, but also in the peripheral nervous system. Neurons clustered in the brain stem in the substantia nigra, a pigmented area, produce dopamine in their cell bodies and transmit it from their axon terminals. Some axons from the substantia nigra project into the cerebral cortex where dopamine may involve emotions. Other axons project to the basal ganglia of the cerebrum. Basal ganglia regulate subconscious contractions of skeletal muscles. These skeletal muscles are also consciously regulated by motor areas of the cerebral cortex. Dopamine usually stimulates postsynaptic cells but at some synapses its action is inhibitory.
Disorders occur when dopamine is not in proper balance in the nervous system, A lack of dopamine in the brain results in Parkinson's disease, a chronic progressive nervous disorder. Researchers have found that sufferers of Parkinson's disease lose more than 80% of dopamine-producing cells in the substantia nigra portion of the brain.
Scientists do not know the cause of the deficiency. Lacking dopamine, these cells cannot communicate with cells in the basal ganglia. The basal ganglia in turn are no longer capable of sending out certain messages and Parkinson's disease results. About one million Americans suffer from Parkinson's disease, and more than 50,000 new patients are diagnosed each year. The symptoms include rigid muscles, tremors, strange involuntary movements, and shuffling gait. Severe immobility and intellectual decline may eventually result. Scientists are seeking ways to supply dopamine to the needy areas of the brain. Physicians prescribe the drug levodopa which enters the brain where certain converter cells change it to dopamine. This helps temporarily but the disease often reappears in three to five years. Some researchers are working on drugs called dopamine agonists that act like dopamine but do not require the action of converter cells. Other researchers are attempting to transplant dopamine-secreting cells into Parkinson patients. In addition to Parkinson's disease, a dopamine imbalance is also associated with other disorders. An excess of dopamine is associated with the mental illness, schizophrenia, and the drug chlorpromazine often used to treat patients works by blocking a dopamine receptor in the brain. Psychotic depression also involves the activity of dopamine, while Tourette syndrome, a chronic tic disorder, seems to entail a greater sensitivity on the part of dopamine receptors. Researchers have found that drug addiction including nicotine use affects the production of dopamine in the brain. Scientists are looking for the genetic link to dopamine production in the hope of helping conditions that the neurotransmitter affects.
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