AbstractMajor depressive disorder (MDD) is a major cause of disability in adults. MDD is both a comorbidity and a risk factor for Alzheimer’s disease (AD), and regular physical exercise has been associated with reduced incidence and severity of MDD and AD. Irisin is an exercise-induced myokine derived from proteolytic processing of fibronectin type III domain-containing protein 5 (FNDC5). FNDC5/irisin is reduced in the brains of AD patients and mouse models. However, whether brain FNDC5/irisin expression is altered in depression remains elusive. Here, we investigate changes infndc5expression in postmortem brain tissue from MDD individuals and mouse models of depression. We found decreasedfndc5expression in the MDD prefrontal cortex, both with and without psychotic traits. We further demonstrate that the induction of depressive-like behavior in male mice by lipopolysaccharide decreasedfndc5expression in the frontal cortex, but not in the hippocampus. Conversely, chronic corticosterone administration increasedfndc5expression in the frontal cortex, but not in the hippocampus. Social isolation in mice did not result in alteredfndc5expression in either frontal cortex or hippocampus. Finally, fluoxetine, but not other antidepressants, increasedfndc5gene expression in the mouse frontal cortex. Results indicate a region-specific modulation offndc5in depressive-like behavior and by antidepressant in mice. Our finding of decreased prefrontal cortexfndc5expression in MDD individuals differs from results in mice, highlighting the importance of carefully interpreting observations in mice. The reduction infndc5mRNA suggests that decreased central FNDC5/irisin could comprise a shared pathologic mechanism between MDD and AD.