Dopamine is an essential neurotransmitter. Like other neurotransmitters, it's responsible for transmitting electrical signals from the brain to other parts of the body. The University of Texas says “dopamine affects brain processes that control movement, emotional response, and ability to experience pleasure and pain.” When there is a lack of dopamine, either due to a disorder or drug induced, problems occur with the associated functions.
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The National Institutes of Health (NIH) says the brain cells that produce dopamine, mainly in the substantia nigra, are destroyed in the case of Parkinson's disease. Without these cells, dopamine can't be produced, resulting in a complete lack of dopamine. Parkinson's patients have muscular problems because dopamine is no longer sending messages between the brain and muscles. Symptoms include bradykinesia (slowed movement), tremors, pain and rigid movements.
Science Daily reports that researchers at the Duke University Medical Center found a lack of dopamine in mice leads to sleep disturbances. The researchers genetically engineered mice to have fewer dopamine-producing cells to mimic Parkinson's disease. Although they found the muscular symptoms of Parkinson's disease appearing when 60 percent to 70 percent of the dopamine-producing cells were damaged, sleeping problems started when there were less dysfunctional dopamine cells. By tracking the brain's activity, researchers found that the mice were unable to sleep or dream, and were rigid and immobile.
Certain drugs can cause a decrease of dopamine in the brain. The University of Texas says dopamine antagonists prevent dopamine from binding to its receptors. If dopamine can't bind to the receptors, less is utilized by the brain. Dopamine antagonists are used to treat disorders such as schizophrenia, in which the brain produces too much dopamine. However, if a patient takes too much of a dopamine antagonist, the significant drop in dopamine can result in Parkinson's-like symptoms. Nevertheless, dopamine antagonists temporarily decrease the level of dopamine. Dopamine will return to its previous level if the antagonists are discontinued.