• bipolar II disorder;
  • dentate gyrus;
  • fimbria;
  • hippocampus;
  • neurogenesis

Objectives:  Dentate gyrus (DG)-dependent inhibition of the stress response might play an important role in mood disorders. During stress, hippocampal projections traversing the fimbria, a white matter bundle on the hippocampal surface, inhibit the hypothalamic–pituitary–adrenal (HPA) axis. The aim of the present study was to measure the volumes of the DG–cornu ammonis 4 (DG–CA4) and fimbria in patients with bipolar II disorder (BD-II) and healthy controls using a recently developed magnetic resonance imaging (MRI)-based technique.

Methods:  Thirty-seven individuals with a DSM-IV diagnosis of BD-II and 42 healthy controls underwent 3-Tesla MRI. Hippocampal subfield volumes were estimated using a novel segmentation algorithm implemented in FreeSurfer.

Results:  In patients with BD-II there was a significant reduction in the volume of the left [analysis of covariance (ANCOVA), = 7.84, p = 0.006] and total (left + right) (= 4.01, p = 0.047) DG–CA4 and left (= 4.38, p = 0.040) and total (= 4.15, p = 0.045) fimbria compared to healthy controls. Explorative analyses indicated a smaller left CA2–3 volume in subjects with BD-II compared to healthy controls, and a reduced left fimbria volume in unmedicated patients compared to medicated patients and controls.

Conclusions:  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.