LINGO3 and major depressive disorder: OL density ↓ in WM of cingulate cortex in MDD+CA, not in MDD w/o CAAxonal diameter↓, myelin thickness↓, g‐ratio ↑ in MDD+CAMethylation of LINGO3, POU3F1 ↓ in MDD+CA, not in MDD w/o CAITGB1 mRNA ↓ in MDD+CAStrong correlation between myelin gene expression changesCorrelation between myelin gene expression in LLG rats and CA humans