MTOR and major depressive disorder: Regarding the association of miRNAs and pathogenic mechanisms in major depressive disorder (inflammation, HPA axis, neurogenesis, neuroplasticity, neurotransmitter regulation, and neuroplasticity), it is important to note that miRNA-let7 shows a particularly important set of actions; for example, Let-7d targets D3R in the hippocampus, Let7b and let7c regulate the PI3k-Akt-mTOR pathway, as proved by studies in the literature [44,45,47,48,49,50,51,52,53].