GDNF and Parkinson disease: Among the HDAC inhibitors, valproate (VPA) was considered to be the most promising drug for the treatment of PD, because VPA not only increases histone acetylation and reduces the expression of inflammatory factors, but also induces expression of BDNF and glial cell-derived neurotrophic factor (GDNF) for neuroprotection (Harrison and Dexter, 2013).