Sinusoidal galvanic vestibular stimulation (sGVS) induces a vasovagal response in the rat

Blood pressure (BP) and heart rate (HR) were studied in isoflurane-anesthetized Long-Evans rats during sinusoidal galvanic vestibular stimulation (sGVS) and sinusoidal oscillation in pitch to characterize vestibular influences on autonomic control of BP and HR. sGVS was delivered binaurally via Ag/AgCl needle electrodes inserted over the mastoids at stimulus frequencies 0.008-0.4 Hz. Two processes affecting BP and HR were induced by sGVS: 1) a transient drop in BP (a parts per thousand 15-20 mmHg) and HR (a parts per thousand 3 beat*s(-1)), followed by a slow recovery over 1-6 min; and 2) inhibitory modulations in BP (a parts per thousand 4.5 mmHg/g) and HR (a parts per thousand 0.15 beats*s(-1)/g) twice in each stimulus cycle. The BP and HR modulations were approximately in-phase with each other and were best evoked by low stimulus frequencies. A wavelet analysis indicated significant energies in BP and HR at scales related to twice and four times the stimulus frequency bands. BP and HR were also modulated by oscillation in pitch at frequencies 0.025-0.5 Hz. Sensitivities at 0.025 Hz were a parts per thousand 4.5 mmHg/g (BP) and a parts per thousand 0.17 beat*s(-1)/g (HR) for pitches of 20-90A degrees. The tilt-induced BP and HR modulations were out-of-phase, but the frequencies at which responses were elicited by tilt and sGVS were the same. The results show that the sGVS-induced responses, which likely originate in the otolith organs, can exert a powerful inhibitory effect on both BP and HR at low frequencies. These responses have a striking resemblance to human vasovagal responses. Thus, sGVS-activated rats can potentially serve as a useful experimental model of the vasovagal response in humans.