Parkinson’s disease is one of the most common neuro-degenerative diseases, affecting about 1% of all people over the age of 65, but its incidence in a majority of sufferers has remained unknown, until now. Tim Greenamyre and colleagues at Emory University in Atlanta, USA, now believe that excessive exposure to environmental pesticides may contribute to the development of this commonplace disease in humans.

In experiments on rats, exposure to rotenone, a commonly used organic pesticide, induced the major features of Parkinson’s disease, the forthcoming issue of the journal Nature Neuroscience will report. The scientists administered rotenone intravenously to rats over a period of several weeks, and observed gradual degeneration of the dopamine neurons, which caused the characteristic rigidity, reduced movement and tremors, associated with Parkinsonism. The rats’ brains were also found to have developed a “formation of structures that closely resemble Lewy bodies”, which are microscopic protein deposits found in the brains of Parkinson’s sufferers.

The likely explanation, but which is as yet untested, is that rotenone acts by causing the mitochondria, intracellular organelles that provide each braincell with energy, to produce free radicals, reactive chemicals that produce oxidative damage in a variety of contexts and have been implicated in many human degenerative diseases.

Rotenone is a naturally occurring pesticide, and it is widely used both as an insecticide and as a method for killing fish, as part of water management programmes. It is considered relatively benign compared to many other pesticides. The researchers say that, although the new study does not prove that rotenone causes Parkinsonism in humans, it is likely to raise new questions about its safety. More generally, it lends credence to the idea that chronic exposure to environmental toxins, including pesticides, may contribute to the incidence of the disease.

The main risk factor for Parkinson’s disease is age, and it has also been claimed, more controversially, that the disease is associated with living in rural environments. Determining to what extent pesticide exposure can account for Parkinsonism will require a great deal of further work. The present findings, however, are consistent with the idea that chronic exposure to low levels of environmental toxin may cause cumulative damage to the brain’s dopamine system, eventually leading to the clinical symptoms of the disease.

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