BDNF and major depressive disorder: Due to its role in brain signaling and synaptic plasticity, coupled with its involvement in emotional comorbidities like memory, decision making, and depression, cerebral BDNF in brain areas including the hippocampus, prefrontal cortex, and reward centers including the mesocorticolimbic system (Mitsi & Zachariou, 2016; Navratilova et al., 2012; H. Zhang et al., 2017) has been proposed as a marker of nociception in chronic pain.