BDNF and depressive symptom measurement: Tropomyosin-related kinase B (TrkB), a specific BDNF receptor, has been pointed to activate BDNF-TrkB signaling to exert the antidepressant action (Rantamäki et al., 2007), and ketamine improves postoperative depression symptoms by upregulating BDNF-TrkB signaling as well.