FNDC5 and major depressive disorder: With the aim to evaluate the effect of irisin treatment on the brain FNDC5/irisin system, the gene expression of irisin precursor, Fndc5, and the transcriptional regulator that increases its expression (PGC-1α) were analyzed in the cerebral areas that are the most implicated in MDD, i.e., the hippocampus and the Prefrontal Cortex (PFC) [29,30].