BDNF and major depressive disorder: Exercise can upregulate BDNF expression by stimulating neuronal regeneration and activating the AMPK and Calcium/Calmodulin-dependent protein kinase II (CaMKII) signaling pathways, promoting the production of cytokines and proteins involved in synaptic remodeling, and improving mitochondrial function, thereby enhancing neural plasticity and alleviating depression [124].