Parkinson’s 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,
Parkinson’s 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 Parkinson’s 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 Parkinson’s disease is that microscopic protein deposits, known as Lewy
bodies are found in the brain.
Parkinson’s 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 Parkinson’s disease. Unlike schizophrenia and many other neurological disorders, Parkinson’s 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 Parkinson’s disease.
The incidence of Parkinson’s among smokers is approximately half that of
non-smokers. This effect is due to the activation of nicotine receptors,
which benefits Parkinson’s disease by causing dopamine release.
The understanding of Parkinson’s disease was
advanced by a chance event. In 1982 a group of young drug addicts
suddenly develo
ped an exceptionally severe form of Parkinson’s. 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 Parkinson’s 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 Parkinson’s disease.