Results from a Norwegian study suggest that the dentate gyrus (DG) and fimbria may play an important role in the pathophysiology of bipolar II disorder (BD II).
Torbjørn Elvsåshagen (Oslo University Hospital) and team found that volumes of the DG-cornu ammonis 4 (DG-CA4), a subfield of the hippocampus, and fimbria, the main pathway between the hippocampus and subcortical structures, were significantly smaller in BD II patients compared with controls.
The findings are important, say the researchers, as "DG-dependent inhibition of the stress response might play an important role in mood disorders."
They add that "during stress, hippocampal projections traversing the fimbria, a white matter bundle on the hippocampal surface, inhibit the hypothalamic-pituitary-adrenal (HPA) axis."
In total, 37 patients (mean age 33.4 years) with BD-II and 42 mentally healthy controls (mean age 31.0 years) underwent 3-Tesla magnetic resonance imaging of the brain, with a focus on the DG-CA4 and fimbria.
After accounting for demographic and clinical variables, the team found that left DG-CA4 volume was a significant 5.4% smaller in patients compared with controls. The volume of the right DG-CA4 was 3.0% smaller in BD II patients compared to controls, but the difference was not significant.
Left fimbria volume was a significant 12.0% smaller in BD II patients compared with controls. But although the left fimbria volume was 7.7% smaller in BD II patients compared with controls, the difference was not significant.
Overall, total DG-CA4 and fimbria volumes were significantly smaller in BD II patients compared with controls.
Explorative analyses also indicated a smaller left CA2-3 volume in BD II patients compared with controls, and a smaller left fimbria volume in unmedicated compared with medicated BD II patients.
Elvsåshagen et al conclude in Bipolar Disorders: "Our results provide evidence for the involvement of the DG and fimbria in BD II.
"Longitudinal studies of the DG and fimbria with assessments of the HPA axis in BD-II are warranted."
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