BDNF and major depressive disorder: The commonly reported biomarkers of depression include serum corticosterone levels (cortisol levels in humans) to assess the involvement of the HPA axis, pro-inflammatory cytokines (TNF-alpha, IL-1, and IL-6), brain-derived neurotrophic factor (BDNF), and neurotransmitters (5-HT, DA, NE, and GABA).