TH and Parkinson disease: The results of this study demonstrate a significant reduction in the optical density of TH immunoreactivity in the striatum of the nigrostriatal pathway within the PD group compared to the NC and HP groups, as illustrated in Figure 4d (PD group 0.11 ± 0.019, NC 0.37 ± 0.029, HP 0.35 ± 0.029, ***p < .001, respectively).