GDNF and major depressive disorder: Moreover, clinical studies have demonstrated that patients with MDD and several pharmacological and non-pharmacological treatments are associated with altered blood/serum levels of growth factors such as brain-derived neurotrophic factor (BDNF) [5,6], insulin-like growth factor-1 (IGF-1), vascular endothelial growth factor (VEGF) [7], glial cell line derived neurotrophic factor (GDNF) and fibroblast growth factor-2 (FGF-2) [4].