Many cellular mechanisms contributing to impaired neuronal function during aging are also present in PD, including mitochondrial dysfunction, inflammation, oxidative stress, and impaired DA metabolism.83–85 More specifically for brain DA in aging, there is a decrease in the synthesis of DA, DA receptors and transporters, as well as tyrosine hydroxylase positive neurons.83,85–87 The age-related decrease in brain DA activity is associated with a decline in cognitive and motor functions for both men and women.88–90 Changes during aging could render DA neurons more vulnerable to insults. This evidence concerns the gene TH and Parkinson disease.