Although it is not really known how Parkinson's disease develops, environmental and genetic factors have been associated with PD. Although PD has not been linked to a single environmental factor (no epidemics of PD have been recorded), it does not rule out that these environmental factors play a role in the development and progression of this disease. Genetic factors account for just a small sample of the population of PD patients. In many cases, the problem may be multifactoral, and to separate the different factors that contribute to PD is the challenge before researchers today.
If you think about it, there are many things that could contribute to the absence or minimization of dopamine in the brain. Dopamine production could be stopped altogether, stunted, blocked, or could be eliminated in a natural or accelerated way. Read more about dopamine elimination here.
Factors contributing to the advance of PD that have been identified include toxins, nutritional deficiency, stress, physical makeup, and genetic susceptibility.
Before World War I, it was thought that PD was inherited (a genetic disorder). Then, during World War I, a number of people were affected by a disease called Encephalitis lethargica and exhibited PD-like symptoms (this was dramatized in the film Awakenings).
In the 1980s, Northern California drug users were using a synthetically designed heroin called MPTP, and this was then found to cause PD in a small but young population of drug addicts. However, because the use of this drug was limited to a select portion of the population, this could not account for all PD cases. Then, in the 1990s, Dr. Lawrence Golbe investigated a familial form of PD, and this spurred an investigation into the genetic theory of the disease once again.
Either way, there appear to be many ways to destroy or weaken the operation of the dopaminergic neurons of the Substantia Nigra (SN). Genetics could account for the deterioration or short lives of these cells, as could toxic factors, or the brain's response to nutritional deficiencies. Stress affects the immune system. Stress also causes the body to release cortisol. I don't know yet if there exist any studies between levels of cortisol and PD (assuming cortisol can penetrate the blood-brain barrier). I suspect such a study would be equivalent to any of these other studies that have identified a small population of patients with PD attributable to any single one of these factors.
