Parkinsonís disease is one of the most common neurodegenerative diseases, affecting about 1% of all people over the age of 65. It affects the Pope, Michael J. Fox and Muhammad Ali. The main symptoms include:
] Tremor at rest, usually starting in the hands that tends to diminish during voluntary activity,
] Muscle rigidity,
] Suppression of voluntary movements, movement is difficult to stop as well as start.
] Speech difficulties,
] Muscle weakness,
Parkinsonís sufferers tend to walk in a fast shuffle. They find it hard to start and once moving cannot quickly stop or change direction. People with Parkinsonís disease can also develop a blank, mask-like facial expression and a soft, monotonous voice. These symptoms are caused by the progressive degeneration of dopamine-containing neurones in a part of the brain called the substantia nigra. It is often associated with dementia, probably because the degenerative process also occurs in other parts of the brain. Another characteristic feature of Parkinsonís disease is that microscopic protein deposits, known as Lewy bodies are found in the brain.
Parkinsonís disease often occurs with no obvious cause, but it may be the result of pathological damage, and some cases have been linked to genetic risk factors. The main risk factor is age, but symptoms can also be drug-induced, the main drugs involved being those that either reduce the amount of dopamine in the brain or block dopamine receptors. Many antipsychotic drugs block D2 (dopamine) receptors and their main side-effect is to cause movement disorders. These disorders are reversible on stopping drug treatment. Genetically engineered mice lacking D2-receptors show greatly reduced spontaneous movement, resembling Parkinsonís disease. Unlike schizophrenia and many other neurological disorders, Parkinsonís disease shows no hereditary tendency, and therefore an environmental cause seems more likely.
One of the few benefits of smoking includes a significant reduction in the risk of contracting Parkinsonís disease. The incidence of Parkinsonís among smokers is approximately half that of non-smokers. This effect is due to the activation of nicotine receptors, which benefits Parkinsonís disease by causing dopamine release.
The understanding of Parkinsonís disease was advanced by a chance event. In 1982 a group of young drug addicts suddenly developed an exceptionally severe form of Parkinsonís. The cause was traced to the compound 1-methyl 4-phenyl 1,2,3,6-tetrahydropyridine (MPTP) which was a contaminant in a preparation used as a heroin substitute. MPTP produces a Parkinsonís disease-like state in primates by causing irreversible destruction to dopamine neurones in a particular part of the brain. This means that it is a very useful experimental tool for testing possible therapies for Parkinsonís disease.