Sex differences in myelin content of white matter tracts in adolescents with depression.

Depression is a chronic and debilitating condition that often emerges during adolescence, a period of significant brain maturation. Few studies, however, have examined how mechanisms of neuroplasticity, including myelination, are affected by adolescent-onset depression. Here, we used multimodal MR imaging to characterize myelin, indexed by R1, in white matter tracts previously associated with depression and compare 48 adolescents with lifetime depression (45 with current depression, 3 remitted) and 35 healthy controls in R1. Compared to healthy controls, R1 was higher in adolescents with lifetime depression in the uncinate fasciculus and corpus callosum genu (all βs > 0.42; all ps < 0.037). Sex significantly moderated the association between depression and R1 in the left uncinate fasciculus and corpus callosum genu (all βs > 0.86; all ps < 0.02), such that depressed female adolescents had significantly higher R1 in these tracts than did healthy female adolescents (all βs > 0.82; all ps < 0.0012). In contrast, depressed and non-depressed male adolescents did not differ in R1 in these tracts (all ps > 0.32). While fractional anisotropy (FA), a commonly examined measure of white matter organization based on diffusion-weighted MRI, in the left uncinate was positively associated with lifetime depression in our sample (β = 0.56; p = 0.016), we found no evidence of sex-specific effects of depression in FA. Our results suggest that R1 is more sensitive to sex-specific effects of depression than FA, particularly in female adolescents. Given evidence that myelin inhibits synapse formation and reduces brain plasticity, our findings implicate experience-driven regional myelination as a mechanism underlying depression during periods of significant neural maturation such as adolescence.