Age, environments, and the number of substantia nigra neurons.

We examined forty-eight midbrain sections from normal brains containing SN at the level of the exiting fibers of the IIIrd cranial nerve. The cases were classified into two groups based on at least 80% life exposure to either rural or urban provincial environments, respectively. Linear regression analysis revealed a lower SN neuron count in the rural group by age 20, although this difference was not statistically significant. There was a 34% decline in urban and a 17% decline in rural residents' neuron counts between ages 20 and 80. These results are consistent with our previous studies, which indicate a higher risk of PD in the Saskatchewan rural population. We believe that studies of environmental factors in early life will provide the most rewarding clues to the etiology of PD.