Abstract: BACKGROUND: Studies have found that patients with depression have lower levels of norepinephrine in their peripheral blood, and patients with insomnia have norepinephrine metabolism disorders, suggesting that abnormalities in the function of the locus coeruleus-norepinephrine system may be the neurobiological basis for the comorbidity of depression and insomnia.
OBJECTIVE: Using functional magnetic resonance imaging (fMRI) to observe the functional connectivity of the brainstem locus coeruleus in patients with comorbid depression and insomnia under resting-state conditions and to investigate the potential mechanisms underlying the comorbidity by examining the relationship with peripheral norepinephrine levels. 
METHODS: From March 2023 to September 2024, 60 patients with comorbid depression and insomnia were recruited from Bao’an Traditional Chinese Medicine Hospital in Shenzhen and the general public (patient group), and 30 healthy controls were recruited during the same period (healthy control group). At baseline, the Hamilton Depression Rating Scale (HAMD-17), Self-Rating Depression Scale (SDS), Pittsburgh Sleep Quality Index (PSQI), and Insomnia Severity Index (ISI) were used to assess the participants' depression status and sleep quality. Resting-state fMRI was used to examine the functional connectivity of the brainstem locus coeruleus, and peripheral serum norepinephrine levels were measured using the enzyme-linked immunosorbent assay. Intergroup comparisons were made for all indicators, and a Pearson correlation analysis was conducted between significantly different functional connectivity regions, peripheral serum norepinephrine levels, and clinical scale scores. 
RESULTS AND CONCLUSION: (1) The HAMD-17, SDS, PSQI, and ISI scores were all higher in the patient group than in the control group (P < 0.05). The functional connectivity values between the left locus coeruleus and left precuneus, as well as between the left locus coeruleus and left inferior parietal lobule, were significantly lower in the patient group than the control group (P < 0.05). Peripheral serum norepinephrine levels were also significantly lower in the patient group than the control group (P < 0.05). Pearson correlation analysis revealed a positive correlation between peripheral serum norepinephrine levels and the functional connectivity values of the left locus coeruleus with the left precuneus and left inferior parietal lobule (r=0.40, P < 0.01; r=0.36, P < 0.01), as well as a negative correlation between peripheral serum norepinephrine levels and HAMD-17 (r=-0.42, P < 0.01) and PSQI scores (r=-0.46, P < 0.01). Moreover, the functional connectivity values of the left locus coeruleus with the left precuneus were negatively correlated with HAMD-17 (r=-0.41, P < 0.01) and PSQI scores (r=-0.44, P < 0.01), and the functional connectivity values of the left locus coeruleus with the left inferior parietal lobule were also negatively correlated with HAMD-17 (r=-0.29, P < 0.01) and PSQI scores (r=-0.36, P < 0.01). To conclude, the reduction in functional connectivity of the left locus coeruleus with the left precuneus and left inferior parietal lobule, as well as decreased peripheral serum norepinephrine levels, is closely associated with the exacerbation of depression and insomnia symptoms. This suggests that dysregulation of the locus coeruleus-norepinephrine system may mediate the mechanisms underlying comorbid depression and insomnia by affecting the functional connectivity of the brain's default mode network, including the left precuneus and left inferior parietal lobule.

Key words: depression, insomnia, locus coeruleus, norepinephrine, functional magnetic resonance imaging, functional connectivity, mechanism study