Parkinson's disease is a progressive disorder of the central nervous system, characterized by degeneration of neurons that use the neurotransmitter dopamine to transmit signals in specific parts of the brain such as the substantia niagra. This neurological degeneration leads to impairment of movement and speech functions. Symptoms include muscle rigidity, slowing of physical movement, tremor and slurred speech, according to an article published in "J Neurol Neurosurg Psychiatry" in April 2008.
Levidopa
For more than 30 years, the drug levodopa has been widely prescribed to treat Parkinson's disease. Levodopa enters the blood stream and is taken up by neurons where it is converted by the enzyme dopa-decarboxylase into dopamine. Movement problems caused by lack of dopamine in specific brain regions can be temporarily counteracted by administration of levodopa, according to research published in "Parkinsonism and Related Disorders" in November 2008.
Short Term Side Effects
Only 5 to 10 percent of levodopa enters the brain and the remaining drug is converted into dopamine in other regions of the body, leading to widespread side effects including nausea, lack of voluntary movement and stiffness. For this reason levodopa is administered in combination with other drugs such as carbidopa and benserazide, which prevent conversion of the drug into dopamine before it reaches the brain, thereby reducing side effects, according to an article published in "Parkinsonism Relat Disord" in November 2008.
Chronic Side Effects
Use of levodopa for extended periods frequently leads to complications including involuntary moments, and fluctuations in response to the drug, eventually resulting in drug insensitivity. Levodopa is routinely administered in the lowest dose possible to alleviate symptoms of the patient, to extend the time period throughout which the drug remains effective, according to a May 2009 study published in "Neurology."
Dopamine Agonists
Administration of dopamine agonists such as Mirapexin and ReQui can alleviate some of the symptoms of Parkinson's disease through a similar mechanism to levodopa. These drugs are often prescribed prior to levodopa in the early stages of the disease, to delay problems that occur with chronic levodopa use. However, after 1 to 3 years use dopamine agonists alone often become inadequate to treat the progressive symptoms of Parkinson's disease and levodopa must be prescribed, according to Parkinson's UK.
MAO-B inhibitors
An increase in dopamine levels in the brain can be achieved via inhibition of enzymes that break this neurotransmitter down, and this is how MAO-B inhibitors such as selegiline and rasagiline exert their actions. As with dopamine agonists, these drugs are often prescribed in the early stages of Parkinson's disease to delay the need for taking levidopa, but become inadequate for treatment of more advanced stages of the condition, according to the British National Health Service.
References
- PubMed; "Journal of Neurology Neurosurgery and Psychiatry"; Parkinson's Disease: Clinical Features and Diagnosis"; J. Janovik; April 2008
- PubMed; "Parkinsonism and Related Disorders" Dopamine Dysregulation Syndrome, Addiction and Behavioral Changes in Parkinson's Disease; D. Merims; November 2008
- PubMed; "Neurology"; The Scientific and Clinical Basis for the Treatment of Parkinson disease (2009); C. Onolow et al; May 2009
- Parkinson's UK: Dopamine Agonists - Parkinson's drugs
- National Health Service: Parkinson's Disease
