Effects of transcutaneous auricular vagus nerve stimulation on the hippocampal TLR4/MyD88/NF-κB signaling pathway in rats exposed to chronic unpredictable mild stress

Abstract

Background

Transcutaneous auricular vagus nerve stimulation (taVNS), a non-invasive neuromodulation technique, has been demonstrated to effectively treat depression and is associated with modulating stress-induced neuroinflammation, while its specific antidepressant mechanism remains unclear.

Objective

To investigate whether taVNS can exert antidepressant effects by regulating the hippocampal TLR4/NF-κB signaling pathway and inhibiting neuroinflammation.

Methods

A total of 36 SD rats were randomized into 4 groups, a control group, a model group, a taVNS group and a transcutaneous auricular non-vagus nerve stimulation (tnVNS) group, 9 rats in each one. Except the control group, all other groups were subjected to a chronic unpredictable mild stress (CUMS) with single-cage housing to formulate depression model, lasting for 35 days. After the successful modeling, 1 h prior to the stress test, the rats in the taVNS group and tnVNS group were subjected to electrical stimulation at the bilateral auricular concha/auricular margin rim for 30 min daily at an intensity of 2 mA and a frequency of 2/15 Hz, for 21 days. The body weight gain and the immobility time in forced swimming test (FST) were recorded before and after the intervention. After experiment, the samples were collected and the relative protein expression of Toll-like receptor 4 (TLR4), myeloid differentiation factor 88 (MyD88), nuclear factor κB p65 (NF-κB p65), interleukin-1β (IL-1β), and interleukin-18 (IL-18) in the hippocampus of rats were detected by using Western blot. The expression and co-localization of TLR4 in hippocampal microglia (Iba1) were examined by using immunofluorescence.

Results

CUMS could induce depression-like behavior and hippocampal neuroinflammation in rats. Compared with the control group, the model group exhibited a prolonged forced swimming immobility time (P < 0.01) and increased protein expression of TLR4, MyD88, NF-κB p65, IL-1β and IL-18 in the hippocampus (P < 0.01). On Day 35 (21 days after intervention), compared with the model group, the taVNS group exhibited an increased body weight (P < 0.01) and shortened forced swimming immobility time (P < 0.01). Additionally, the expression of TLR4, MyD88, NF-κB p65, IL-1β and IL-18 in the hippocampus was decreased (P < 0.01), and the expression of TLR4 also decreased in microglia.

Conclusion

TaVNS could alleviate depression-like behaviors in CUMS rats, and its mechanism may be associated with the inhibition of the hippocampal TLR4/NF-κB signaling pathway and the suppression of microglia-mediated neuroinflammation.