Vestibular Activation Habituates the Vasovagal Response in the Rat

Abstract

Vasovagal syncope is a significant medical problem without effective therapy, postulated to be related to a collapse of baroreflex function. While some studies have shown that repeated static tilts can block vasovagal syncope, this was not found in other studies. Using anesthetized, male Long-Evans rats that were highly susceptible to generation of vasovagal responses, we found that repeated activation of the vestibulosympathetic reflex (VSR) with ±2 and ±3 mA, 0.025 Hz sinusoidal galvanic vestibular stimulation (sGVS) caused incremental changes in blood pressure (BP) and heart rate (HR) that blocked further generation of vasovagal responses. Initially, BP and HR fell ≈20-50 mmHg and ≈20-50 beats/min (bpm) into a vasovagal response when stimulated with Sgv\S in susceptible rats. As the rats were continually stimulated, HR initially rose to counteract the fall in BP; then the increase in HR became more substantial and long lasting, effectively opposing the fall in BP. Finally, the vestibular stimuli simply caused an increase in BP, the normal sequence following activation of the VSR. Concurrently, habituation caused disappearance of the low-frequency (0.025 and 0.05 Hz) oscillations in BP and HR that must be present when vasovagal responses are induced. Habituation also produced significant increases in baroreflex sensitivity (p < 0.001). Thus, repeated low-frequency activation of the VSR resulted in a reduction and loss of susceptibility to development of vasovagal responses in rats that were previously highly susceptible. We posit that reactivation of the baroreflex, which is depressed by anesthesia and the disappearance of low-frequency oscillations in BP and HR are likely to be critically involved in producing resistance to the development of vasovagal responses. SGVS has been widely used to activate muscle sympathetic nerve activity in humans and is safe and well tolerated. Potentially, it could be used to produce similar habituation of vasovagal syncope in humans.

Keywords: baroreflex sensitivity; head-up tilt; sinusoidal galvanic vestibular stimulation; vasovagal syncope; vestibulosympathetic reflex.

Figures

Figure 1

Progressive loss of susceptibility to generation of vasovagal responses. (A) Incidence of generation of vasovagal responses on repeated tests for R012 with ±3 mA, 0.025 Hz sinusoidal galvanic vestibular stimulation (sGVS) on the ordinate and sequential test days on the abscissa. The rat progressively lost its susceptibility until vasovagal responses were no longer induced by repeated testing on days 8, 10, 11, and 12. (B) Similar loss of susceptibility to generation of vasovagal responses for R020 induced by repeated stimulation with ±3 mA, 0.025 Hz sGVS. The incidence of vasovagal responses fell rapidly after day 2 and they could not be induced, despite repeated testing on days 8, 9, 11, and 12. (C) Loss of susceptibility in the 11 rats that form the basis of this report. Of the 11, 7 rats were repeatedly stimulated with the various forms of vestibular (otolith) stimulation over extended periods in previous studies. These seven rats lost their susceptibility progressively (see text for description). Also included are four rats (R009, R011, R012, and R020) that were habituated with ±2 and ±3 mA, 0.025 Hz sGVS over a 2-week period. The percentage of positive responses to the multiple test stimuli of the rats in the tested sample for each day of testing is shown on the ordinate and sequential test days on the abscissa. If the original group of rats did not respond to the multiple tests for 3 consecutive test days, they were removed from the sample after the third day. Despite the different time sequences in the original and later habituation procedures, the rats successively lost their susceptibility to the generation of vasovagal responses after approximately the same number of test sessions. As in (A,B), the number of susceptible rats declined steadily until none were left by the 11th day.

Figure 2

Changes in blood pressure (BP) and heart rate (HR) during habituation in R009. (A–C) Changes in BP (top traces) and HR (middle traces) in response to 70° nose-up tilt (bottom traces) at three stages during habituation. (A) Initially, both BP and HR fell in response to the 70° nose-up tilt. (B) As habituation progressed, the tilt induced a fall in BP, but a rise in HR of ≈45–50 bpm that persisted for ≈250 s. (C) At the end of habituation, the nose-up tilt now induced only small increases in both BP and HR. (D–F) Changes in BP (top traces) and HR (middle traces) in response to stimulation with ±2 mA, 0.025 Hz sGVS (bottom traces). (D) Initially, both BP and HR fell together. The fall in HR was slower and persisted for over 10 min. The maximum fall in HR was ≈50 bpm. (E) Later, during habituation, the fall in BP was initially countered by a ≈25 bpm increase in HR that lasted for ≈100 s before HR also fell. (F) Finally, there was only a small rise in both BP and HR in response to the sGVS. BP increased by ≈10 mmHg and persisted for over 10 min. The rise in HR was also small and lasted for more than 10 min. The data for the tilt and sGVS were taken on the same days during the initial tests (A,D), during the middle stages (B,E), and at the end of the habituation process (C,F). In comparison, the changes produced in the intermediate sessions (B,E) were more profound with the ±2 mA sGVS than with the 70° (0.91 g) nose-up tilt.

Figure 3

Changes in blood pressure (BP) and heart rate (HR) during habituation in R011. Changes in BP (top traces) and HR (middle traces) in response to ±3 mA, 0.025 Hz sGVS (bottom traces). (A) There was an initial fall in both BP and HR that was larger and lasted longer for the HR. (B) During the habituation process, initially, there were several, small transient increases in BP before BP fell in response to the sGVS. There was also a ≈20 bpm initial increase in HR (first upwards arrow) that persisted for ≈150 s before HR also fell (second upwards arrow). (C) At the same time as in B, nose-up tilt caused a drop in BP, but a concomitant rise in HR of ≈30 bpm (first upwards arrow) that persisted for nearly 300 s (second upwards arrow). The difference in the response of the HR to ± 3 mA sGVS and the 70° nose-up tilt suggests that the sGVS was a more powerful stimulus to the otolith system than the 70° nose-up tilt.

Figure 4

Progressive decline in susceptibility to generation of vasovagal responses and coincident increase in heart rate (HR) during habituation in R009. The days of habituation are represented sequentially on the abscissae (see Materials and Methods). (A) Susceptibility fell rapidly over the first 2 days. (B) HR increased dramatically on the fourth day of testing, peaked on the sixth test day, and then fell rapidly over the next 2 test days. Finally, there was only a minimal response in blood pressure and HR by the eighth day, when the animal had become habituated. These data demonstrate that the fall in susceptibility was associated with a transient increase in HR that then fell back as the habituation proceeded.

Figure 5

Sequential changes in blood pressure (BP) and heart rate (HR) during habituation in R009. (A–C) Graphs of changes in BP (top traces) and HR (bottom traces) induced by activation of the vestibulosympathetic reflex with 400 ms trains of ±2 mA, 0.025 Hz sGVS at the beginning, during, and at the end of habituation. The trains of sGVS caused different slopes of the combined responses in BP and HR at the different times during the habituation process. (A) At the beginning of habituation, both BP and HR fell together. The decline in HR persisted for more than 100 s. (B) During habituation, BP fell, but there was an increase in HR that persisted for >100 s. (C) At the end of habituation, BP was essentially unaffected, but there was a small rise in HR. (D–F) The slopes of the combined changes in BP and HR from (A–C) were derived from a least squares regression of these recordings. The vectors were originally in the 180–240° quadrant (D), then in the 240–360° quadrant during habituation (E), and finally in the 0–90° quadrant after habituation had taken place (F).

Figure 6

Consecutive changes in blood pressure (BP) and heart rate (HR) during habituation in R009. (A) Plots of the vectors from Figures 5D–F are given in polar coordinates. This graph demonstrates the progressive changes in BP and HR during habituation. (B) Changes in the slopes of the BP/HR relationship as R009 became habituated plotted from the first to the last tests (bottom to top). The angle of tilt of the vector formed from BP and HR is shown on the ordinate, and the sequential test numbers are shown on the abscissa. There were 30 tests during the habituation procedure and 3 tests after the animal was habituated. The relationship between BP and HR systematically and gradually changed during habituation. There was a continual increase in the angle of the vector formed from BP and HR. The dashed line at 180° was achieved by the 27th test and was maintained through the 33rd test. Thus, there was a steady change in BP and HR from the beginning to the end of habituation.

Figure 7

Pulse stimulation of the vestibulosympathetic reflex before, during, and after habituation in R012. Increases in blood pressure (BP) (top traces) from +3 mA (A–C) and −3 mA (D–F) 1 s pulses of Galvanic Vestibular Stimulation. The stimuli induced increases in BP of ≈10–25 mmHg and were somewhat larger when the initial BP was lower. The BP rose rapidly at the onset of stimulation before, during and after habituation. These data demonstrate that the response of the cardiovascular system to vestibular stimulation was not significantly affected by the habituation process. There were no changes in HR (not shown).

Figure 8

Changes in low-frequency oscillations of blood pressure (BP) and heart rate (HR) during habituation in R009. The four traces show the low-frequency oscillations in BP in Power Bands 11 (0.025 Hz; A), and 10 (0.05 Hz; B), and HR for Power Bands 11 (0.025 Hz, C), and 10 (0.05 Hz, D) before, during, and after habituation. The circles above the abscissa reflect the power of the low-frequency components in the BP and HR signals for the duration of the stimulation, and the dashed, horizontal line reflects the upper limit of oscillation in the normal rat when no stimulation was applied. The ordinate reflects the power (energy), which is calculated by computing the total energy of a particular band, divided by the length of the signal. The six dates below the bottom trace indicate some of the dates on which the data were taken. The circles on each graph indicate the sequential changes in BP and HR over the period of testing and habituation. The initial testing to determine that the rat was susceptible to the generation of vasovagal responses took ≈10 weeks. The actual habituation in R009 occurred over 2 weeks from 6/6/14 to 6/20/14. The low-frequency oscillations disappeared promptly after the onset of habituation and remained at or close to normal levels for both bands 11 and 10 in BP and HR. The increases in HR in Bands 11 and 10 came close to the normal levels upon habituation, but did not fall below it. There were no significant low-frequency oscillations in HR toward the end of habituation. These data demonstrate a general loss of low-frequency oscillations when R009 had reached habituation.

Figure 9

Changes in oscillation of blood pressure (BP) and heart rate (HR) during habituation in R011. The four traces show the low-frequency oscillations in BP for Power Bands 11 (0.025 Hz, A) and 10 (0.05 Hz, B) and HR for Power Bands 11 (0.025 Hz, C) and 10 (0.05 Hz, D) before, during, and after habituation. Habituation began after initial testing to determine that the rat was susceptible to generation of vasovagal responses. In this segment, the total time of intense habituation occurred over 2 weeks from 6/5/14 to 6/19/14. The five dates below the bottom trace indicate some of the dates on which the data were taken. The circles above the abscissa reflect the power of the low-frequency components in the BP and HR signals for the duration of the stimulation, and the dashed, horizontal line reflects the upper limit of oscillation in the normal rat when no stimulation was applied. The ordinate reflects the power (energy), which is calculated by computing the total energy of a particular band, divided by the length of the signal. The circles on each graph indicate the sequential changes in BP and HR over the period of testing and habituation. Initially, there were significant increases in the low-frequency oscillations in both BP and HR in Bands 11 and 10 that finally fell to normal or close to normal levels in the last nine tests. The increases in BP fell below the normal level for bands 11 and 10, toward the end of the habituation. The increases in HR in Bands 11 and 10 came close to the normal levels upon habituation, but did not fall below it. There were no significant low-frequency oscillations in HR toward the end of habituation. These data demonstrate a general loss of low-frequency oscillations when R011 had reached the habituated state.

Figure 10

Comparison of the increase in baroreflex sensitivity and blood pressure (BP) Power Band 11 responses (0.025 Hz) for R009 and R011. The four traces show the increase in baroreflex sensitivity (A and C) and low-frequency oscillations in BP for Power Band 11 (B and D) for R009 and R011. The increases in baroreflex sensitivity were significant in both animals (R009: p = 0.006, R011: p < 0.001). The R2 value was higher in R011 than in R009, indicating that there was a higher linear correlation for R011. There was no dramatic change in baroreflex sensitivity that was associated with the loss of low-frequency oscillations in BP and HR in both animals. The traces of the low-frequency oscillations in BP were repeated to demonstrate that changes in these oscillations were not reflected specifically in the changes in baroreflex sensitivity. Regardless, these data demonstrate that baroreflex sensitivity could significantly increase as a result of the vestibular stimulation and the habituation process.