GDNF and Parkinson disease: Some studies have shown that neurotrophic factors such as glial cell-derived neurotrophic factor (GDNF) and brain-derived neurotrophic factor (BDNF) upregulate tyrosine hydroxylase (TH), improve movement disorders and provide neuroprotection in animal models of Parkinson’s disease (Gash et al., 2005; Lindholm and Saarma, 2021).