BDNF and major depressive disorder: Exercise-mediated alleviation of CUMS-induced depression-like behaviors, and reversal of CUMS-induced decrease in spine density in the hippocampus, may result partly from reversal of the CUMS-induced decrease in the BDNF level and increase in the pro-BDNF level by exercise (Sigwalt et al., 2011; Park et al., 2017; Shafia et al., 2017).