FNDC5 and major depressive disorder: In line with these findings, in this study, with the aim of assessing the impact of short-term irisin administration on the expression of the most well-known neutrophic/growth factors associated with depression [30,31,32,33], we evaluated the gene expression of Bdnf, Igf-1, Ngf, and Fgf-2 in hippocampal and PFC tissues.