Dopamine is a critical neurotransmitter, a chemical that relays nerve impulses through the brain to control diverse processes such as muscle movement, the sensation of pleasure and emotions, and motivation. Conditions such as Parkinson's disease, schizophrenia, depression and drug addiction can result if dopamine levels fluctuate outside the normal limits. Dopamine-like drugs can help treat Parkinson's disease.
Tyrosine
The Merck Manual describes how the amino acid tyrosine forms the basis for the body's production of dopamine. Dopaminergic nerve cells in the brain actively import tyrosine and convert it to 3,4-dihydroxyphenylalanine, also called dopa, using the enzyme tyrosine hydroxylase. With another enzyme, decarboxylated dopa, the result is the active form of the neurotransmitter dopamine.
Levodopa
The Mayo Clinic states that levodopa is the most effective drug treatment for Parkinson's disease. Levodopa occurs naturally in the body as a precursor that is decarboxylated to produce dopamine. Unlike dopamine replacement, levodpa pills are effective as a treatment for Parkinson's disease because levodopa can cross the blood-brain barrier to reach the dopaminergic neurons. According to the Merck Manual, levodopa can produce some unpleasant side effects, including hallucinations and involuntary muscle movement called dyskinesia. Its effects gradually wear off, so many doctors prefer to save its use for the later stages of Parkinson's disease.
Ropinirole
Ropinirole is a dopamine agonist, a substance so chemically similar to dopamine that it tricks dopamine receptors into thinking it is dopamine. Based on its remarkable similarity to real, natural dopamine, ropinirole can effectively treat Parkinson's disease. Although it binds with highest affinity to the D3 dopamine receptor sub-type, scientists believe that ropinirole relieves the symptoms of Parkinson's by binding to and stimulating the D2 receptor sub-type in the caudate putamen of the brain.
Pramipexole
Like ropinirole, pramipexole treats Parkinson's disease because of its chemical similarity to dopamine. Pramipexole binds to the D2 receptors in the nerves of the striatum and substantia nigra to quiet symptoms of Parkinson's, but it can also bind to D3 and D4 receptors. Like levodopa, both pramipexole and ropinirole can cause hallucinations and dyskinesia. Unlike levodopa, the use of dopamine agonists can cause problems with "impulse control," leading to gambling addiction, over-eating or inappropriate sexual behaviors.
Apomorphine
Patients taking levodopa and dopamine agonists can experience periods when symptoms worsen despite treatment, a phenomenon known as an on-off effect. The Merck Manual outlines the use of apomorphine, a fast-acting but short-lived injectable dopamine agonist, for "rescue" treatment of Parkinson's symptoms during off-periods.
Other Agonists
The Merck Manual lists some other dopamine agonists once frequently used to treat Parkinson's disease that have now fallen out of favor. Pergolide was recalled because it caused hardening of the valves of the heart, known as cardiac valvular fibrosis. The dopamine agonist bromocriptine can cause cardiac valvular fibrosis or fibrosis of lungs.
