Dopamine is a chemical that is used by both the central and peripheral nervous systems, where it performs diverse functions that range from controlling movement and coding reward in the brain to regulating blood flow in the periphery. It is synthesized from the amino acid L-tyrosine through a series of enzymatic steps, the last of which is conversion of L-dopa into dopamine. Dopamine cannot cross the blood brain barrier, which means that in order for it to work in the brain, it must be either directly injected into the brain or ingested as L-dopa, which readily crosses the blood brain barrier.
Peripheral Effects
Increased dopamine levels can cause enhanced blood flow to the kidneys via vasodilation. This is accompanied by increased urine output. It can similarly affect blood flow to the stomach and intestines. Dopamine can also affect the cardiac system by increasing the contraction strength of the heart muscle. Finally, dopamine can induce tissue necrosis if leakage into the surrounding tissue occurs during intravenous administration.
Central Effects
In the brain, dopamine is a neurotransmitter that is active in reward circuitry. It also contributes to impulsive and compulsive behavior. In a study recently published in the Archives of Neurology, Daniel Weintraub, M.D., suggests that increased dopamine levels via prolonged treatment with Levodopa is associated with 2 to 3.5 times increased likelihood of developing an impulse control disorder, or ICD. ICDs include uncontrollable behaviors that can be related to gambling, eating and sexual activity. Another side effect of prolonged Levodopa treatment is the development of Levodopa-induced dyskinesias. These include involuntary movements that can be dance-like or violent, as well as involuntary sustained muscle contractions.
Drug Interactions
Other drugs can impact the effects of dopamine. For example, monoamine oxidase inhibitors are prescribed for depression. They work by inhibiting the enzyme MAO that breaks down serotonin, which is a monoamine that has been implicated in depressive symptoms. Dopamine is also a monoamine that is broken down by MAO. Therefore, metabolism of dopamine is decreased in the presence of MAO inhibitors.
References
- "Experimental Neurology"; Dopamine Synthesis, Uptake, Metabolism and Receptors: Relevance to Gene Therapy of Parkinson's Disease; John D. Elsworth and Robert H. Roth; March 1, 1997
- "Archives of Neurology"; Impulse Control Disorders in Parkinson's Disease: A Cross-Sectional Study of 3,090 Patients; Daniel Weintraub, M.D., et al.; May 12, 2010
- "Postgraduate Medical Journal"; Levodopa-Induced Dyskinesia in Parkinson's Disease: Clinical Features, Pathogenesis, Prevention and Treatment; Dr. Bhomraj Thanvi et al.; June 7, 2007
- DynaMed Resource Database: Dopamine
