Our increasing knowledge about gut-brain interaction is revolutionising the understanding of the links between digestion, mood, health, and even decision making in our everyday lives. In support of this interaction, the vagus nerve is a crucial pathway transmitting diverse gut-derived signals to the brain to monitor of metabolic status, digestive processes, or immune control to adapt behavioural and autonomic responses. Hence, neuromodulation methods targeting the vagus nerve are currently explored as a treatment option in a number of clinical disorders, including diabetes, chronic pain, and depression. The non-invasive variant of vagus nerve stimulation (VNS), transcutaneous auricular VNS (taVNS), has been implicated in both acute and long-lasting effects by modulating afferent vagus nerve target areas in the brain. The physiology of neither of those effects is, however, well understood, and evidence for neuronal response upon taVNS in vagal afferent projection regions in the brainstem and its downstream targets remain to be established.

Therefore, to examine time-dependent effects of taVNS on brainstem neuronal responses in healthy human subjects, we applied taVNS during task-free fMRI in a single-blinded crossover design. During fMRI data acquisition, we either stimulated the left earlobe (sham), or the target zone of the auricular branch of the vagus nerve in the outer ear (cymba conchae, verum) for several minutes, both followed by a short ‘stimulation OFF’ period. Time-dependent effects were assessed by averaging the BOLD response for consecutive 1-minute periods in an ROI-based analysis of the brainstem.

We found a significant response to acute taVNS stimulation, relative to the control condition, in downstream targets of vagal afferents, including the nucleus of the solitary tract, the substantia nigra, and the subthalamic nucleus. Most of these brainstem regions remarkably showed increased activity in response to taVNS, and these effect sustained during the post-stimulation period. These data demonstrate that taVNS activates key brainstem regions, and highlight the potential of this approach to modulate vagal afferent signalling. Furthermore, we show that carry-over effects need to be considered when interpreting fMRI data in the context of general vagal neurophysiology and its modulation by taVNS.

我们对肠脑相互作用的了解不断增加，正在彻底改变对消化、情绪、健康甚至日常生活中决策之间联系的理解。为了支持这种相互作用，迷走神经是将各种肠道衍生信号传输到大脑以监测代谢状态、消化过程或免疫控制以适应行为和自主反应的关键途径。因此，目前正在探索针对迷走神经的神经调节方法作为许多临床疾病的治疗选择，包括糖尿病、慢性疼痛和抑郁症。迷走神经刺激 (VNS) 的非侵入性变体，经皮耳廓 VNS (taVNS)，通过调节大脑中的传入迷走神经目标区域，与急性和长期影响有关。

因此，为了检查 taVNS 对健康人类受试者脑干神经元反应的时间依赖性影响，我们在单盲交叉设计的无任务 fMRI 期间应用了 taVNS。在 fMRI 数据采集过程中，我们要么刺激左耳垂（假），要么刺激外耳中迷走神经耳支的目标区域（cymba conchae，verum）几分钟，然后都是短暂的“刺激关闭”时期。在基于 ROI 的脑干分析中，通过对连续 1 分钟的 BOLD 响应进行平均来评估时间依赖性效应。

我们发现，相对于对照条件，迷走神经传入神经下游靶区（包括孤束核、黑质和丘脑底核）对急性 taVNS 刺激有显着反应。这些脑干区域中的大多数显着表现出对 taVNS 的响应活动增加，并且这些效应在刺激后期间持续存在。这些数据表明 taVNS 激活了关键的脑干区域，并突出了这种方法调节迷走神经传入信号的潜力。此外，我们表明，在一般迷走神经生理学及其通过 taVNS 调节的背景下解释 fMRI 数据时，需要考虑结转效应。