Low numbers and no loss of melanized nigral neurons with increasing age in normal human brains from India.

The prevalence of Parkinson's disease (PD) is higher in whites than in nonwhites and it increases with advancing age. The pathological hallmarks of PD are loss of pigmented neurons in the substantia nigra pars compacta (SNpc) and presence of Lewy bodies. With increasing age, a similar loss of pigmented neurons in the SNpc has been reported. Hence, age and race possibly play a role in the pathogenesis of PD. The objectives of this study were to count the number of melanized neurons in the SNpc in normal human brains from India and study the change in neuronal count with advancing age and to compare the neuronal counts from this Indian population with counts reported in normal brains from the United Kingdom. Melanized neurons in the SNpc were counted in 84 normal human brains (age range, 5-84 years) in a single 7-microm section at the level of emergence of the oculomotor nerve. In the brains from India, there was no loss of melanized nigral neurons with advancing age. The absolute number of these melanized neurons was about 40% lower than the brains from UK. Despite a low number of melanized nigral neurons in the brains from India, individuals function normally and have dopamine levels comparable with their Western counterparts, suggesting that it is not the absolute number of melanized nigral neurons but the percent loss of nigral neurons that results in dopaminergic deficiency in PD. There is no significant loss of pigmented nigral neurons with age, suggesting that the loss seen in PD is exclusively due to the disease process itself. Indians have a lower prevalence of PD despite having a low count of melanized nigral neurons, suggesting that better protective mechanisms may be present in the Indians to prevent the loss of nigral neurons.