An investigation of the influence of photoperiod on subcortical brain volumes and their mediation effects on the seasonality of depressive symptoms

Abstract

Seasonal differences in depressive symptoms affect a large percentage of the general population. Females show greater seasonal mood variations than males. The hippocampus, amygdala, and brainstem have been shown to be affected by photoperiodic differences, and that longer photoperiods are associated with increased hippocampal, amygdala, and brainstem volumes as well as decreased depressive-like behaviours. It has also been shown that environment affects brain volumes. Little is known about the morphological differences of these brain regions with seasons nor the effect of environment in humans. Therefore, I hypothesised that seasonal differences in photoperiod would predict differences in both hippocampal, amygdala, and brainstem substructure volumes and depressive scores, and that seasonal differences in hippocampal, amygdala or brainstem substructure volumes would mediate the seasonality of depressive symptoms. I also hypothesised that place of residence affects seasonal variations in brain volumes, and that rural dwellers show greater seasonal variations than urban dwellers. A total of 10,033 participants were scanned by MRI in a single location as part of the UK Biobank project. Hippocampal, amygdala, and brainstem substructure volumes were obtained using automated and segmentation algorithm. Photoperiod was defined as the number of hours between sunrise and sunset on the day of scan. Photoperiod was found to be negatively correlated with low mood and anhedonia in females while photoperiod was found to be positively correlated with hippocampal, amygdala and brainstem substructure volumes. There were significant correlations between brainstem substructure volumes and depressive symptoms, while hippocampal and amygdala subnuclei volumes were not significantly associated with iv depressive symptoms. Interestingly, in females, whole brainstem, pons, and medulla volumes individually fully mediated the relationship between photoperiod and both anhedonia and low mood, while midbrain volume fully mediated the relationship between photoperiod and anhedonia. No mediation effects for any individual depressive symptom score were seen in males. Rural dwellers showed greater seasonal variations in hippocampal and amygdala subnuclei volumes compared to urban dwellers. These results offer preliminary evidence that hippocampal, amygdala, and brainstem plasticity could be associated with photoperiod, and that seasonal mood variations might be mediated by brainstem volume, indicating a need for longitudinal studies.