Abstract
Type 2 diabetes mellitus and depression are mutually reinforcing, yet combined pharmacotherapy is limited by side effects and poor tolerability. Transcutaneous auricular vagus nerve stimulation (taVNS) is a non-invasive neuromodulatory approach with emerging metabolic and antidepressant potential, although its central mechanisms remain unclear. Here, we investigated whether hypothalamic serotonergic signaling is involved in the dual effects of taVNS in a mouse model of type 2 diabetes with depression (T2DD). T2DD was induced in db/db mice by chronic unpredictable mild stress, followed by taVNS intervention (2/15 Hz, 1 mA, 30 min/day for 3 weeks). Behavioral performance, body weight, and fasting blood glucose were assessed, while brain-wide neuronal activity was mapped using fluorescence micro-optical sectioning tomography (fMOST). Hypothalamic 5-HT release was monitored by fiber photometry, and 5-HT content and 5-HT₁A receptor expression were quantified by ELISA and Western blotting. TaVNS significantly reduced body weight and fasting blood glucose, alleviated depression-like behaviors, and improved locomotor activity. Brain-wide mapping revealed state-dependent modulation of neuronal activity, characterized by widespread activation under physiological conditions and suppression of stress-induced hyperactivation in T2DD. Notably, taVNS induced robust hypothalamic 5-HT release, increased long-term 5-HT levels, and upregulated 5-HT₁A receptor expression. Correlation analyses further linked enhanced hypothalamic serotonergic signaling to improved glycemic control and behavioral outcomes. These findings suggest that hypothalamic 5-HT signaling may contribute to the dual metabolic and antidepressant effects of taVNS and highlight its potential as an integrative neuromodulatory intervention for metabolic-psychiatric comorbidity.