BDNF and major depressive disorder: As shown in Figure 3A, patients with mild depression had a significantly higher level of serum BDNF (45.283 ± 4.213 pg/ml) than those with moderate (33.178 ± 4.289 pg/ml, P < 0.001) or severe depression (28.190 ± 3.143 pg/ml, P < 0.001).