Correlation of proinflammatory cytokines levels and reduced gray matter volumes between patients with bipolar disorder and unipolar depression

Background: Gray matter volume reduction in specific brain regions, such as the prefrontal cortex, was found in patients with bipolar disorder and those with unipolar depression. However, few studies have directly compared gray matter volumes between bipolar disorder and unipolar depression. In addition, it is unknown whether proinflammatory cytokines play a role in the gray matter volume difference between bipolar disorder and unipolar depression. Methods: Twenty-three patients with bipolar disorder and 23 with unipolar depression in a mildly ill state (Clinical Global Impression-Severity <= 3) were enrolled in our study. Each participant underwent structural magnetic resonance imaging and proinflammatory cytokines examination. Voxel-based morphometry was performed to investigate the gray matter volume difference between bipolar disorder and unipolar depression. Correlations of the proinflammatory cytokines and the gray matter volume difference were analyzed. Results: Several brain regions, including the orbitofrontal cortex, lingual gyrus, inferior frontal cortex, middle frontal cortex, and planum polare, were significantly smaller in patients with bipolar disorder than in those with unipolar depression. These gray matter volume differences between bipolar disorder and unipolar depression were negatively correlated with soluble IL-6 receptor levels. Discussion: Proinflammatory cytokines, especially IL-6, were associated with the gray matter volumes in bipolar disorder and unipolar depression. However, the complicated associations between proinflammatory cytokines, neurocognitive function, and gray matter volumes require further investigation.