Second, BDNF may be an important biomarker as antidepressant medications up-regulate BDNF [95]. BDNF also enhances the effect of antidepressants in animal models, suggesting that antidepressants and BDNF may operate through similar neurochemical pathways [96]. BDNF mimics the effect of antipsychotic drugs on the expression of dopamine receptors in the brain, suggesting that it may reduce manic or elevated mood [97]. MDD patients exhibit lower levels of BDNF than their healthy counterparts, and higher levels of BDNF have been associated with fewer depressive symptoms as well as better cognitive functioning [98–100]. BDNF has also increased following acute and longer programs of exercise; however, some studies report no difference [101–103]. A meta-analysis noted a moderate effect size for increases in BDNF following a single session of exercise and a more pronounced pre- to post-exercise effect following a longer program of regular exercise [104]. Finally, exercise, similar to antidepressants, upregulates BDNF mRNA levels in the hippocampus [105]. BDNF has shown the most robust changes as a result of exercise manipulations [88, 106].
