Aging of the Nigrostriatal Tract in the Human Brain: A Diffusion Tensor Imaging Study

Background and Objectives: The loss of dopamine neurons in the nigrostriatal tract (NST) is one of the main pathological features of Parkinson's disease (PD), and degeneration of the NST leads to the motor symptoms observed in PD, which include hypokinesia, tremors, rigidity, and postural imbalance. In this study, we used diffusion tensor tractography (DTT) to investigate the aging of the NST in normal human subjects to elucidate human brain structures. Materials and Methods: Fifty-nine healthy subjects were recruited for this study and allocated to three groups, that is, a 20 to ≤39 year old group (the young group), a 40 to ≤59 year old group (the middle-aged group), and a ≥60 year old group (the old group). DTT scanning was performed, and NSTs were reconstructed using the probabilistic tractography method. NSTs were defined by selecting fibers passing through seed and target regions of interest placed on the substantia nigra and the striatum. Results: A significant negative correlation was observed between age and fractional anisotropy and tract volume (TV) of the NST. Mean TV values of the NST were significantly lower in the old group than in the young and middle-aged groups (p < 0.05). The TV values of the NST were significantly reduced with age for men and women (p < 0.05). Conclusion: We found that aging of the NST began in the 3rd decile and progressed steadily throughout life until old age, when it exhibited significant degeneration. We suspect these results are related to the correlation between the incidence of PD and age.