Low-frequency galvanic vestibular stimulation evokes two peaks of modulation in skin sympathetic nerve activity

We have previously shown that sinusoidal galvanic vestibular stimulation (sGVS), delivered bilaterally at 0.2-2.0 Hz, evokes a potent entrainment of sympathetic outflow to muscle and skin. Most recently, we showed that stimulation at 0.08-0.18 Hz generates two bursts of modulation of muscle sympathetic nerve activity (MSNA), more pronounced at 0.08 Hz, which we interpreted as reflecting bilateral projections from the vestibular nuclei to the medullary nuclei responsible for the generation of MSNA. Here, we test the hypothesis that these very low frequencies of sGVS modulate skin sympathetic nerve activity (SSNA) in a similar fashion. SSNA was recorded via tungsten microelectrodes inserted into the left common peroneal nerve in 11 awake-seated subjects. Bipolar binaural sGVS (±2 mA, 100 cycles) was applied to the mastoid processes at 0.08, 0.13 and 0.18 Hz. As with MSNA, cross-correlation analysis revealed two bursts of modulation of SSNA for each cycle of stimulation but, unlike MSNA, this modulation was equally pronounced at all frequencies. These results further support our conclusion that bilateral sGVS causes cyclical modulation of the left and right vestibular nerves and a resultant modulation of sympathetic outflow that reflects the summed activity of bilateral projections from the vestibular nuclei onto, in this case, the primary output nuclei responsible for SSNA-the medullary raphé. Furthermore, these findings emphasise the role of the vestibular system in the control of skin sympathetic outflow, and the cutaneous expression of motion sickness: pallor and sweat release. Indeed, vestibular modulation of SSNA was higher in those subjects reporting nausea than in those who did not report nausea during this low-frequency sGVS.