GDNF and attention deficit-hyperactivity disorder: In this regard, there are studies addressing the participation of fibroblast growth factor (FGF), vascular endothelial growth factor, insulin-like growth factor (IGF2) [47], glial-derived neurotrophic factor (GDNF), nerve growth factor (NGF), and neurotrophin-3 (NTF-3) [47,53] in ADHD pathophysiology.