BDNF and major depressive disorder: On a clinical study by detecting BDNF, proBDNF and receptors in the sera and lymphocytes of depression patients, the levels of proBDNF, sortilin and p75NTR are higher while mature BDNF and TrkB levels are lower compared with that of healthy controls, which implies that the balance between the proBDNF/p75NTR/sortilin and mBDNF/TrkB signaling pathways is dysregulated32.