Auricular neuromodulation may act through interoception modulation

Abstract

Auricular neuromodulation (ANM) is a non-invasive method of stimulating the trigeminal, spinal, and vagal nerves that innervate the auricle, the outer part of the ear. ANM, and the vagal nerve stimulation in particular, has been shown to be effective in treating a number of symptoms, including pain and anxiety. However, the mechanisms by which ANM exerts its beneficial effects are still under investigation. We hypothesized that ANM produces its effects by modulating interoception, a physiological function involved in homeostatic regulation and awareness of internal physical and emotional sensations. Interoception, activated by visceral and sensory afferences, in turn modulates body function by regulating the autonomic nervous system (ANS). Therefore, we reviewed the anatomical, physiological, and pathophysiological evidence linking ANM to interoception, ultimately defining an auricle-brain axis. We propose two functional pathways that are activated directly or indirectly by the auricular nerves. The first pathway processes the sensory information coming from the parabrachial and solitary tract nuclei. The second pathway activates the monoaminergic and cholinergic pathways, thereby regulating the interoceptive response. ANM modulates the two outputs of interoception — that is, the ANS and the stress response — and changes the body response. We hypothesized that ANM provides new sensory information for interoception and thereby improves body-brain interactions previously compromised by ongoing pathological processes. Therefore, we assessed the effects of ANM on interoception-related pathologies. Finally, we evaluated whether empirical features of ANM can be explained by interoceptive mechanisms.