TH and Parkinson disease: Cellular mechanisms contributing to impaired neuronal function during aging are also present in PD, such as mitochondrial dysfunction, inflammation, oxidative stress, and impaired DAergic metabolism (Collier et al., 2011, 2017), more specifically with a decrease in TH+ neurons, dopamine (DA) transporters, and the synthesis of DA during aging (Irwin et al., 1992; Rollo, 2009; Hindle, 2010; Collier et al., 2011).