Parkinson's disease (PD) is a slowly progressive disorder that affects movement, muscle control, and balance. Parkinson's disease is referred to as idiopathic, which means that the cause is unknown. This term distinguishes the primary disease from parkinsonism, which are the symptoms occurring from a known cause.In addition to its effects on motor control, Parkinson's disease is now recognized as a broader condition that can include cognitive and behavioral disturbances, sleep disorders, speech difficulties, and other problems.
Parkinson's Disease and Dopamine Loss
Parkinson's disease occurs from the following process in the brain:
- PD develops as cells are destroyed in certain parts of the brain stem, particularly the crescent-shaped cell mass known as the substantia nigra.
- Nerve cells in the substantia nigra send out fibers to the corpus stratia, gray and white bands of tissue located in both sides of the brain.
- There the cells release dopamine, an essential neurotransmitter (a chemical messenger in the brain). Loss of dopamine in the corpus stratia is the primary defect in Parkinson's disease.
Dopamine. Dopamine deficiency is the hallmark feature in PD. It is one of three major neurotransmitters known as catecholamines, which help the body respond to stress and prepare it for the fight-or-flight response. Loss of dopamine negatively affects the nerves and muscles controlling movement and coordination, resulting in the major symptoms characteristic of Parkinson's disease. Dopamine also appears to be important for efficient information processing, and deficiencies may also be responsible for problems in memory and concentration that occur in many patients.
Although it is clear that dopamine deficiency is the primary defect in Parkinson's disease, it is not clear what causes dopamine loss. The culprit is less likely to be a single cause than a combination of genetic and biologic factors, which are triggered by some environmental assault.
Other Changes. The PD disease process also appears to impair nerve endings in the heart to cause dysautonomia--changes in the autonomic (also called sympathetic) nervous system. Such changes may impair the release of norepinephrine, a hormone that regulates blood pressure, pulse rate, perspiration, and other automatic responses to stress. Evidence suggests this may be responsible for the abrupt drops in blood pressure when standing that occur in PD. Further research is underway to determine if the loss of nerve terminals is confined to the heart or if it affects other organs as well.
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Biologic Factors
Apoptosis and Alpha Synuclein. Important research now suggests that three molecules are critical in the development of inherited PD: alpha synuclein, parkin, and ubiquitin, which all interact in the normal brain. Abnormally high levels of alpha synuclein, which is produced in dopamine-rich nerve cells, may play a central role. Normally, two other molecules, parkin and ubiquitin, are involved in the natural self-destruction of synuclein -- a natural process of programmed cell death called apoptosis. If this process goes awry, for instance with a defective parkin gene, then apoptosis fails to occur. If synuclein is not eliminated in these cells, it builds up and becomes toxic to dopamine. In such cases, synuclein accumulates in Lewy bodies, the deposits of fibrous tissue found in all patients with PD.
Another protein, beta amyloid, also increases the build-up of synuclein. Beta amyloid is a known factor in Alzheimer's disease, and may help explain the co-existence between Alzheimer's and Parkinson's disease in many patients.
Lewy Bodies. The fibrous deposits known as Lewy bodies are the hallmark signs of Parkinson's disease. They are found in the substantia nigra, the place in the brain where dopamine is first released. It is not clear whether Lewy bodies are the major killers of the nerve cells or whether they are simply a byproduct of the degenerative process. They are found not only in the brains of patients with Parkinson's disease but, in rare cases, may show up in cells in other parts of the body (the heart, intestine), causing severe disabling symptoms. These substances are also present in other diseases that cause dementia, such as Alzheimer's, and can occur in people without neurologic symptoms.
The Mitochondria and Oxygen-Free Radicals. Some research has observed that certain patients with PD have significantly low levels of complex I, an enzyme found in the mitochondria, sausage-like structures that are the primary source of energy within cells. Some theories suggest that low amounts of complex I may make nerve cells vulnerable to the assault of oxygen free radicals (also called oxidants). Oxidants are unstable molecules that bind to other molecules in the body. They are normally produced by the natural chemical processes in the body. If the body is subjected to environmental stresses, however, they can be over-produced. And, in excess, they can damage any cell, including nerve cells in the brain, and even interferes with their DNA.
NMDA Receptors. Also of interest in PD are processes that occur in an area of the brain called the subthalamic nucleus. Here, receptors known as glutamatergic N-methyl-D-aspartate (NMDA) become persistently overexcited and produce high levels of calcium ions within brain cells. This in turn leads to a cascade of events that trigger oxygen-free radicals and cell damage.
Immune Factors and the Inflammatory Response. An over-responsive immune system triggered by initial damage may also play a role in perpetuating Parkinson's disease. When the immune system becomes overactive, it produces excessive numbers of potent factors called cytokines, which cause inflammation and further injury in brain cells. Important cytokines under investigation are interleukin-1 and tumor necrosis factor.
Genetic Factors
Specific genetic factors appear to play a strong role in early-onset Parkinson's disease, an uncommon form of the disease. Research from the last several years suggests that multiple genetic factors may also be involved in late-onset Parkinson’s disease. Several important studies, published in 2005, lay the groundwork for potential genetic screening for this disease. Researchers identified the leukine-rich repeat kinase 2 (LRRK2) gene, located on a region of chromosome 12 known as PARK8, as a key gene involved in inherited forms of Parkinson’s. The researchers estimate that a single gene mutation in LRRK2 may be responsible for 5% of inherited Parkinson’s cases and approximately 2% of isolated cases.
Early Onset PD. The cases of genetic early-onset Parkinson's disease have most often been detected in specific family groups.
- Defective genes that regulate the molecules alpha synuclein and parkin, which are important in the PD disease process, may be responsible for a number of early-onset cases. For example, genetic abnormalities the alpha synuclein protein have been detected in some early-onset Parkinson's patients of European descent.
- The parkin gene may be the cause of many cases of early-onset Parkinson's in young adults. (Parkinson's cases associated with this mutation tend to progress slowly and respond well to treatment, even after years of symptoms. Dementia is also rare with this form.)
Late Onset PD. Two landmark studies published in the Journal of the American Medical Association provided the first evidence of a genetic link to late-onset Parkinson’s disease. In these 2001 studies, researchers found that regions on chromosomes 5, 6, 8, 9, and 17 were implicated with Parkinson’s. The parkin gene (located on chromosome 6) and the tau gene (located on chromosome 17) were both found in families that had late onset Parkinson’s. Parkin was previously thought to be responsible only for early-onset Parkinson’s, but this research identified it in families that had both early- and late-onset disease forms. These studies also bolstered the theory that Parkinson’s does have a genetic component and is not caused solely by environmental factors. A 2005 study found that a G2019S mutation in the LRRK gene, located on the PARK8 region of chromosome 12, was definitively associated with late-onset Parkinson’s disease in North American and European families.
Environmental Assaults and Oxygen-Free Radicals
Environmental toxins, infections, and other triggers can provoke excessive production in the body of oxygen free-radicals, damaging particles that may play a major role in the deterioration of nerve cells that lead to Parkinson's.
Infectious Organisms. Some research has identified immune factors that suggest a viral presence in the Lewy bodies and swollen nerve pathways of Parkinson's brains. Influenza and other potent viruses have long been known to be a cause of parkinsonism. In one well-known example, a major flu epidemic causing encephalitis in the early twentieth century left many of its victims with parkinsonism.
Environmental and Industrial Chemicals. Intense exposure to certain environmental and industrial chemicals is also being studied.
- Pesticides and Herbicides. Some evidence implicates pesticides and herbicides as important factors in many cases of Parkinson's disease. A higher incidence of parkinsonism has long been noted in people who live in rural areas, particularly those who drink private well water or are agricultural workers. A large 2000 study found a strong link between high exposure to insecticides and herbicides at home and a 50% to 70% increase in risk of Parkinson's.
- Other Chemicals. Intense exposure to other industrial chemicals and metals (manganese, copper, lead, iron, mercury, zinc, aluminum, and others) has also been linked with parkinsonism, which is often reversible. The role of long-term exposure in the development of Parkinson's disease is unclear. High levels of iron content observed in critical parts of the brain in PD are under particular scrutiny.
Aging Process
Most, but not all, Parkinson's victims are elderly. Some studies indicate that the very elderly are not susceptible to the disease, indicating that the aging process itself is not the major player in the disease. Aging does appear to reduce the concentration of dopamine in structures called dopamine transporters, which carry the neurotransmitter back and forth between nerve cells. Some researchers posit that any excessive stress on these transporters might trigger Parkinson's disease in the aging, and more vulnerable, brain.
