Discussion
neuronal atrophy, mood disorders, depressive, depressive symptoms, depression
REMISSION, PATHOLOGY
Our randomized study demonstrated that time and group had significant effects on the volumes of the left DG-head and left DG-total. These effects remained even after adjusting for confounding factors, such as age, sex, log-transformed environmental light, and diagnoses. Furthermore, a significant increase in the volumes of the left DG-head and left DG-total in the BL group, whereas no significant changes were detected in the volume of the DG in the DL group. Moreover, the %Δ in the volume of the left DG-total was positively associated with the %Δ in the VAS of mood scores. To the best of our knowledge, this is the first study to investigate the relationship between BLT and longitudinal volumetric changes in the hippocampal DG as well as examine the correlation between alterations in DG volume and amelioration of depressive symptoms in patients with mood disorders.The DG of the hippocampus is a key brain region involved in stress response, depression pathology, and antidepressant response, making it a useful target for interventional therapy for mood disorders, particularly via the modulation of adult hippocampal neurogenesis [With regard to the impact of light on clinical symptoms, both the BL and DL groups showed efficacy in ameliorating depressive symptom. However, no significant differences were detected between time and group and the HAM-D and VAS of mood scores. This could be because the patients in this study exhibited mild depression, which could explain the modest decrease in scores and the absence of a significant difference. In a randomized controlled trial by Sit et al. that used light therapy for BP, the DL group demonstrated reduced HAM-D scores after 6 weeks, with 20% of the patients achieving remission [We posited that the increase in the volume of the left hippocampal DG (expressed as %Δ) would correspond with a reduction in the depression scores on the questionnaires. However, we did not observe a correlation with the HAM-D or BDI scores. This may be attributed to the fact that the mild depressive symptoms exhibited by the patients in this study did not generate a significant difference in scores and the %Δ detected in the HAM-D and BDI scores was relatively small. Nevertheless, a positive correlation was observed between the %Δ in the volume of the left DG-total and the %Δ in the VAS of mood scores in the BL group. This finding suggests that an increase in the volume of the left DG is linked to a significant improvement in mood. The DL group also showed a significant positive correlation between the %Δ in the volume of the left DG-body and the %Δ in the HAM-D scores. However, the DL group did not demonstrate an increase in the volume of the left-DG body after the intervention, which suggests that this significant correlation was coincidental.Our current study has some limitations. First, the sample size was relatively small, which may limit the generalisability of our findings. Second, our results were based on volumetric changes in the DG, which are not a direct indicator of neurogenesis. This is because changes in the DG volume may arise not only from neurogenesis but also from enhancements in neuronal atrophy and synaptogenesis via dendritic reshaping. Third, although participants were instructed to maintain their lifestyle unchanged throughout the study’s duration at the inception of the study, it was difficult to completely exclude the effects of daily behaviors such as diet, exercise, and sleep on the hippocampal DG.In conclusion, our study results suggest that BLT exhibits a dose-response effect, wherein light exposure of a higher intensity is more efficacious than that of a lower intensity in terms of increasing the volume of the left DG and alleviating depression symptoms. Thus, BLT may trigger neurogenesis in the left DG of patients with mood disorders, indicating a novel mechanism of action of BLT. Further studies with large sample sizes are needed to confirm these findings.