Fisher's r-to-z transformations were applied to these correlations for each group and then compared; results indicated that the relationship between serum leptin and gain versus non-gain BOLD signal change was significantly more negative in MDD than HC, within the left insula (z =  − 3.88, p < 0.001) and the left dorsolateral putamen (z =  − 3.31, p < 0.001). Here, LEP is linked to major depressive disorder.