**3.3 Vagal nerve stimulation**

The vagus nerve is a large tract originating at the brainstem and is known for its widespread innervation, targeting every major thoracic and abdominal organ [52, 53]. Vagal nerve stimulation (VNS) has similarly been shown to provide multi-systems effects, and thus useful for a wide range of disease interventions. The recent development of non-invasive vagal nerve stimulation; via transcutaneous auricular vagus nerve stimulation (ta-VNS); rather than the traditional cervical implantation; increases therapeutic potential of the intervention as it removes the need for surgery [54]. Due to the novelty of ta-VNS there is currently a lack of consensus over the optimal stimulation protocol [55]. Stimulation is typically of low amplitude current (~5 mA) with pulses of 250–500 μs with a frequency of between 10 and 25 Hz [54]. Recently a number of studies have made efforts to individualize the stimulation level based on perceptual threshold using sequential testing protocols.

There is increasing evidence that VNS has anti-nociceptive effects [56, 57]. Analgesia is thought to occur through both the inhibition of spinal nociceptive reflexes and ascending transmission. There is evidence VNS and ta-VNS also modulates ascending inputs in the brain by altered activity in pain processing regions as observed via fMRI [55, 58]. Further a recent study examined the brainstem fMRI response to a respiratory gated ta-VNS protocol (known as RAVANS). Interestingly this study found that stimulation led to greater blood oxygen level dependent (BOLD) responses in the PAG [59]. Further this study explored the use of different stimulation frequencies, with a frequency of 100 Hz showing increased responsiveness of PAG. This alteration to a key site for endogenous pain modulation provides additional support for the potential of VNS for pain relief. Opioid receptor antagonists are found to reduce the efficacy of VNS, indicating that there is an opioid based mechanism for analgesia. Further VNS is also widely thought to have anti-inflammatory effects [56]. These anti-inflammatory effects are proposed to be due to neural-immune interaction at the peripheral nerves [60], with electric stimulation of the vagus nerve triggering a neural-immune reflex via cholinergic anti-inflammatory pathways that dampen the inflammatory response to infection or tissue injury and suppress the release of pro-inflammatory cytokines.
