Stress-induced vagal activity influences anxiety-relevant prefrontal and amygdala neuronal oscillations in male mice

The vagus nerve (VN) plays a critical role in communication between peripheral organs and the brain, transmitting interoceptive signals that significantly influence emotional states. Alterations in vagal signals, such as those resulting from changes in gut microbiota or vagotomy, are linked to increased anxiety and depression-like behaviors. Conversely, vagus nerve stimulation (VNS) has shown anxiolytic and antidepressant effects in both humans and rodents. Despite the established importance of vagal-brain interactions in emotional functions, the dynamic changes in VN activity during anxiety states and the underlying neurophysiological mechanisms remain largely unknown. The prefrontal cortex (PFC) and amygdala (AMY) are key brain regions involved in anxiety, and their neuronal oscillations are thought to modulate anxiogenic behavior. This study aimed to elucidate the dynamic relationship between VN activity and anxiety-related PFC-AMY neuronal oscillations, focusing on how this relationship is pathologically altered in stress-susceptible mice. Furthermore, the study explored the physiological mechanisms of VNS in ameliorating stress-induced anxiety.

Existing literature highlights the crucial role of the vagus nerve in emotional regulation and psychiatric disorders. Studies have shown that alterations in vagal signaling, including vagotomy and changes in gut microbiota, can lead to increased anxiety and depression-like behaviors. Conversely, VNS has demonstrated efficacy in treating treatment-resistant depression in humans and inducing anxiolytic and antidepressant effects in rodents. While these studies support the importance of vagal interoceptive signals in maintaining emotional states, the precise neurophysiological mechanisms underlying VN-related anxiety and mental disorders remain largely unclear. Research also points to the PFC and AMY as central brain regions involved in anxiety, with their interregional coordination of neuronal oscillations modulating anxiogenic behavior. This study builds upon this existing knowledge by investigating the dynamic interplay between VN activity and anxiety-related brain activity patterns in the PFC and AMY.

The study utilized male C57BL/6J mice, subjecting them to 10-day social defeat (SD) stress to induce stress susceptibility. Mice were then classified as stress-susceptible or stress-resilient based on their performance in a social interaction (SI) test. Electrode implants were placed on the left cervical VN, dorsal neck muscle (EMG), and in the PFC and AMY for simultaneous recordings of VN activity and local field potential (LFP) signals. VNS was delivered using a cuff-shaped electrode. Behavioral tests, including the elevated plus maze (EPM), were conducted to assess anxiety-related behavior. c-Fos immunostaining was used to assess neuronal activation in the nucleus tractus solitarius (NTS), PFC, and AMY following VNS. Data analysis included VN spike rate calculation, power spectral density analysis of LFP signals, and calculation of correlation coefficients between VN power and LFP power. Statistical analyses included ANOVAs, t-tests, and correlation analyses.

Stress-susceptible mice exhibited reduced VN spike rates during quiescent periods compared to naïve and stress-resilient mice. In naïve and stress-resilient mice, VN spike power dynamically changed depending on the behavioral state in the EPM test (open vs. closed arms, move vs. stop states). This dynamic change was absent in stress-susceptible mice. Analysis revealed negative correlations between VN spike power and 2–4 Hz PFC-AMY LFP power, and positive correlations between VN spike power and 20–30 Hz PFC-AMY LFP power. A subset of PFC neurons showed phase locking to 2–4 Hz and 20–30 Hz oscillations. Behavior-dependent changes in 2–4 Hz and 20–30 Hz LFP power were observed in naïve and resilient mice but not in stress-susceptible mice. Vagotomy replicated the behavioral and LFP pattern changes observed in stress-susceptible mice. Finally, chronic VNS in stress-susceptible mice restored both anxiety-related behavior and behavior-relevant PFC-AMY LFP patterns.

The findings indicate that reduced VN activity is a key factor in the altered anxiety-related behavior observed in stress-susceptible mice. The correlations between VN activity and specific frequency bands (2–4 Hz and 20–30 Hz) in the PFC and AMY suggest these oscillations are crucial for mediating the effects of vagal signaling on anxiety. The disruption of these oscillations in both stress-susceptible and vagotomized mice underscores the vital role of VN activity in regulating anxiety-related brain circuits. The successful restoration of behavior and LFP patterns through VNS supports the therapeutic potential of targeting the VN for treating anxiety disorders. Further research is needed to clarify the precise mechanisms by which VN activity influences specific neuronal populations and oscillatory patterns within the PFC and AMY.

This study demonstrates that stress-induced attenuation of VN activity is linked to altered anxiety behavior and disrupted PFC-AMY neuronal oscillations in mice. The restoration of these patterns via VNS highlights the potential of this therapeutic approach for mood disorders. Future research could focus on identifying specific subtypes of VN afferents and their target neurons in the PFC and AMY to further elucidate the mechanisms underlying vagal-brain communication in anxiety.

The study primarily used male mice, limiting the generalizability of the findings to females. The specific mechanisms by which SD stress reduces VN activity require further investigation. The relatively small sample size in some analyses could influence statistical power. While the study established correlations, further research is needed to definitively determine causal relationships between VN activity, brain oscillations, and anxiety behavior.