GDNF and major depressive disorder: The central regulatory role of neurotrophic factors including brain-derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF), glial cell line-derived neurotrophic factor (GDNF), and nerve growth factor (NGF) in neuroplasticity highlights their involvement in the pathogenesis of brain diseases including epilepsy and depression [6,7].