BDNF and neurodegenerative disease: Previous studies have demonstrated that decreased regulation of BDNF was involved in the pathogenesis of POCD (Qiu et al., 2016a), and accumulating evidence established that dysregulation of BDNF/TrkB signaling contributed to many neurodegenerative diseases (Plotkin et al., 2014; Jerónimo-Santos et al., 2015).