GDNF and Parkinson disease: More interestingly, in PD patients, there is a decrease of butyrate, a histone deacetylase inhibitor which protects dopaminergic neurons from degeneration by upregulating the neurotrophic factors, such as BDNF (Brain Derived Neurotrophic Factor) and GDNF (Glial Cell Line-derived Neurotrophic Factor) [101].