Superentrainment of muscle sympathetic nerve activity during sinusoidal galvanic vestibular stimulation

Sinusoidal galvanic vestibular stimulation (sGVS), delivered at frequencies ranging from 0.08 to 2.0 Hz, induces vestibular illusions of side-to-side motion and robust modulation of muscle sympathetic nerve activity (MSNA) to the lower legs. We have previously documented, in seated subjects, de novo synthesis of bursts of MSNA that are temporally locked to the sinusoidal stimulus rather than to the cardiac-related rhythm. Here we tested the hypothesis that this vestibular entrainment of MSNA is higher in the upright than in the supine position. MSNA was recorded from the common peroneal nerve in 10 subjects lying on a tilt table. Bipolar binaural sGVS (+/- 2 mA, 200 cycles) was applied to the mastoid processes at 0.2, 0.8, and 1.4 Hz in the supine and upright (75 degrees) positions. In four subjects, superentrainment of MSNA occurred during sGVS, with strong bursts locked to one phase of the sinusoidal stimulus. This occurred more prominently in the upright position. On average, cross-correlation analysis revealed comparable vestibular modulation of MSNA in both positions at 0.2 Hz (84.9 +/- 3.6% and 78.7 +/- 5.7%), 0.8 Hz (77.4 +/- 3.9% and 74.4 +/- 8.9%), and 1.4 Hz (69.8 +/- 4.6% and 80.2 +/- 7.4%). However, in the supine position there was a significant linear fall in the magnitude of vestibular modulation with increasing frequency, whereas this was not present in the upright position. We conclude that vestibular contributions to the control of blood pressure are higher in the upright position.