Vestibular system plays a significant role in arterial pressure control during head-up tilt in young subjects

Relationship between orthostatic hypotension and recurrence of benign paroxysmal positional vertigo

Blood pressure is maintained by a combined mechanism of the baroreceptor reflex and the vestibulosympathetic reflex. This study is intended to verify the hypothesis that the orthostatic hypotension (OH) seen when benign paroxysmal positional vertigo (BPPV) occurred may act as a factor that affects the recurrence of BPPV. The subjects of present study were selected from among 239 patients diagnosed with idiopathic BPPV. The average age of the group with OH was 59.3 years, and the age of the group without OH was 50.3 years, with a statistically significant difference (P = 0.013). It was shown that drug-taking increased the risk of OH occurrence by 4.08 times (C.I for exp(B): 1.20-13.77) compared to the group that did not take drugs. It was shown that the risk of recurrence of BPPV was significantly reduced in the no recurrence group compared to the multiple recurrence group when there was no OH (p = 0.000; aOR 0.0000002). Also, the risk of recurrence was significantly reduced in the no recurrence group compared to the multiple recurrence group when there was no drug-taking (p = 0.000 aOR 0.0000001). This study is the first study that studied the effect of OH on the recurrence of BPPV and showed the possibility that OH could partially influence the recurrence of BPPV.

Autonomic Tone and Baroreflex Sensitivity during 70° Head-up Tilt in Yoga Practitioners

Introduction:

The intervention of yoga was shown to improve the autonomic conditioning in humans evident from the enhancement of parasympathetic activity and baroreflex sensitivity (BRS). From the documented health benefits of yoga, we hypothesized that the experience of yoga may result in adaptation to the orthostatic stress due to enhanced BRS.

Aim:

To decipher the effects of yoga in the modulation of autonomic function during orthostatic challenge.

Materials and Methods:

This was a comparative study design conducted in autonomic function test lab, of the Department of Physiology, All India Institute of Medical Sciences, New Delhi, India. Heart rate variability (HRV), blood pressure variability, and BRS were analyzed on forty naïve to yoga (NY) subjects and forty yoga practitioners with an average age of 31.08 ± 7.31 years and 29.93 ± 7.57 years, respectively. All participants were healthy. Seventy degrees head up tilt (HUT) was used as an intervention to evaluate the cardiovascular variability during orthostatic challenge.

Results:

During HUT, the R-R interval (P = 0.042), root mean square of succesive R-R interval differences (RMSSD) (P = 0.039), standard deviation of instantaneous beat-to-beat R-R interval variability (SD1) (P = 0.039) of HRV, and sequence BRS (P = 0.017) and α low frequency of spectral BRS (P = 0.002) were higher in the yoga group. The delta decrease in RRI (P = 0.033) and BRS (P < 0.01) was higher in the yoga group than the NY group.

Conclusion:

The efferent vagal activity and BRS were higher in yoga practitioners. The delta change (decrease) in parasympathetic activity and BRS was higher, with relatively stable systolic blood pressure indicating an adaptive response to orthostatic challenge by the yoga practitioners compared to the NY group.

Impacts of Microgravity Analogs to Spaceflight on Cerebral Autoregulation

It is well known that exposure to microgravity in astronauts leads to a plethora physiological responses such as headward fluid shift, body unloading, and cardiovascular deconditioning. When astronauts return to Earth, some encounter problems related to orthostatic intolerance. An impaired cerebral autoregulation (CA), which could be compromised by the effects of microgravity, has been proposed as one of the mechanisms responsible for orthostatic intolerance. CA is a homeostatic mechanism that maintains cerebral blood flow for any variations in cerebral perfusion pressure by adapting the vascular tone and cerebral vessel diameter. The ground-based models of microgravity are useful tools for determining the gravitational impact of spaceflight on human body. The head-down tilt bed rest (HDTBR), where the subject remains in supine position at -6 degrees for periods ranging from few days to several weeks is the most commonly used ground-based model of microgravity for cardiovascular deconditioning. head-down bed rest (HDBR) is able to replicate cephalic fluid shift, immobilization, confinement, and inactivity. Dry immersion (DI) model is another approach where the subject remains immersed in thermoneutral water covered with an elastic waterproof fabric separating the subject from the water. Regarding DI, this analog imitates absence of any supporting structure for the body, centralization of body fluids, immobilization and hypokinesia observed during spaceflight. However, little is known about the impact of microgravity on CA. Here, we review the fundamental principles and the different mechanisms involved in CA. We also consider the different approaches in order to assess CA. Finally, we focus on the effects of short- and long-term spaceflight on CA and compare these findings with two specific analogs to microgravity: HDBR and DI.

Understanding vestibular-related physiological functions could provide clues on adapting to a new gravitational environment

The peripheral vestibular organs are sensors for linear acceleration (gravity and head tilt) and rotation. Further, they regulate various body functions, including body stability, ocular movement, autonomic nerve activity, arterial pressure, body temperature, and muscle and bone metabolism. The gravitational environment influences these functions given the highly plastic responsiveness of the vestibular system. This review demonstrates that hypergravity or microgravity induces changes in vestibular-related physiological functions, including arterial pressure, muscle and bone metabolism, feeding behavior, and body temperature. Hopefully, this review contributes to understanding how human beings can adapt to a new gravitational environment, including the moon and Mars, in future.

Hypergravity-induced plastic alteration of the vestibulo-sympathetic reflex involves decrease in responsiveness of CAMK2-expressing neurons in the vestibular nuclear complex

The vestibular system contributes to not only eye movement and posture but also the sympathetic response. Plastic alteration of the vestibulo-sympathetic reflex is induced by hypergravity load; however, the mechanism remains unknown. Here, we examined 2 g-induced changing in responsiveness of CAMK2-expressing neurons in the vestibular nucleus complex using optogenetic tools. The excitatory photostimulation of the CAMK2-expressing neurons in the unilateral vestibular nuclear complex induced body tilt to the contralateral side, while inhibitory photostimulation showed the opposite response. Photoactivation of either cell body or the axonal terminal in the rostral ventrolateral medulla showed sympathoexcitation followed by the pressor response. Furthermore, this response was significantly attenuated (49.8 ± 4%) after the 1st day of 2 g loading, and this value was further reduced by the 5th day (22.4 ± 3%), suggesting that 2 g-induced attenuation of the vestibulo-sympathetic reflex involves at least decrease in responsiveness of CAMK2-expressing neurons in the vestibular nuclear complex.

Vestibular modulation of muscle sympathetic nerve activity assessed over a 100-fold frequency range of sinusoidal galvanic vestibular stimulation

We have previously shown that sinusoidal galvanic vestibular stimulation (sGVS), delivered at 0.2-2.0 Hz, evokes a partial entrainment of muscle sympathetic nerve activity (MSNA). Moreover, at lower frequencies of stimulation (0.08-0.18 Hz) sGVS produces two peaks of modulation: one (primary) peak associated with the positive peak of the sinusoidal stimulus and a smaller (secondary) peak associated with the trough. Here we assessed whether sGVS delivered at 0.05 Hz causes a more marked modulation of MSNA than at higher frequencies and tested the hypothesis that the primary and secondary peaks are of identical amplitude because of the longer cycle length. MSNA was recorded via tungsten microelectrodes inserted into the left peroneal nerve in 11 seated subjects. Bipolar binaural sGVS (±2 mA, 100 cycles) was applied to the mastoid processes at 0.05, 0.5, and 5.0 Hz (500 cycles). Cross-correlation analysis revealed two bursts of modulation of MSNA for each cycle at 0.05 and 0.5 Hz but only one at 5 Hz. There was a significant inverse linear relationship between vestibular modulation (primary peak) and frequency (P < 0.0001), with the amplitudes of the peaks being highest at 0.05 Hz. Moreover, the secondary peak at this frequency was not significantly different from the primary peak. These results indicate that vestibular modulation of MSNA operates over a large range of frequencies but is greater at lower frequencies of sGVS. We conclude that the vestibular apparatus, through its influence on muscle sympathetic outflow, preferentially contributes to the control of blood pressure at low frequencies. NEW & NOTEWORTHY Vestibulosympathetic reflexes have been documented in experimental animals and humans. Here we show that sinusoidal galvanic vestibular stimulation, a means of selectively exciting vestibular afferents in humans, induces greater modulation of muscle sympathetic nerve activity when delivered at a very low frequency (0.05 Hz) than at 0.5 or 5.0 Hz.

Interaction between graviception and carotid baroreflex function in humans during parabolic flight-induced microgravity

The aim of the present study was to assess carotid baroreflex (CBR) function during acute changes in otolithic activity in humans. To address this question, we designed a set of experiments to identify the modulatory effects of microgravity on CBR function at a tilt angle of -2°, which was identified to minimize changes in central blood volume during parabolic flight. During parabolic flight at 0 and 1 g, CBR function curves were modeled from the heart rate (HR) and mean arterial pressure (MAP) responses to rapid pulse trains of neck pressure and neck suction ranging from +40 to -80 Torr; CBR control of HR (carotid-HR) and MAP (carotid-MAP) function curves, respectively. The maximal gain of both carotid-HR and carotid-MAP baroreflex function curves were augmented during microgravity compared with 1 g (carotid-HR, -0.53 to -0.80 beats·min^-1·mmHg^-1, P < 0.05; carotid-MAP, -0.24 to -0.30 mmHg/mmHg, P < 0.05). These findings suggest that parabolic flight-induced acute change of otolithic activity may modify CBR function and identifies that the vestibular system contributes to blood pressure regulation under fluctuations in gravitational forces. NEW & NOTEWORTHY The effect of acute changes in vestibular activity on arterial baroreflex function remains unclear. In the present study, we assessed carotid baroreflex function without changes in central blood volume during parabolic flight, which causes acute changes in otolithic activity. The sensitivity of both carotid heart rate and carotid mean arterial pressure baroreflex function was augmented in microgravity compared with 1 g, suggesting that the vestibular system contributes to blood pressure regulation in humans on Earth.

The impaired subjective perception of verticality independent of peripheral vestibular function in dizzy elderly with orthostatic hypotension

BACKGROUND

The elderly often complain of faintness, lightheadedness, dizziness, and unsteadiness, which may be associated with the orthostatic hypotension (OH), but most of them are subclinical. The pathogenic mechanism of subjective symptoms in the elderly with OH is controversial.

AIMS

The aim of this study was to assess the involvement of the irregularity of the vertical perception in the dizzy elderly with OH.

METHODS

This study consisted of 403 patients seeking treatment for dizziness in our hospital. The neurotological examinations include the subjective visual vertical (SVV) test and the bithermal caloric test. The self-perceived handicapping effects imposed by dizziness were assessed by the dizziness handicap inventory (DHI). The variability (standard deviation) and the average deviation of eight trials (four trials in each direction) were calculated. In addition, they underwent the orthostatic Schellong test for the diagnosis of the OH. They were separated into three groups, including patients aged ≥65 years, patients aged 50-64 years, and patients aged <50 years.

RESULTS

The variability of the SVV in the patients aged ≥65 years with OH was significantly larger than that in the patients aged ≥65 years without OH. There was no significant difference in the average deviation of the SVV, the canal paresis % by the bithermal caloric test, and the DHI score among groups.

CONCLUSIONS

The result suggests that the elderly patients with OH have subclinical impairment in the perception of the verticality independent of the peripheral vestibular function.

Adaptation to microgravity, deconditioning, and countermeasures

Humans are generally in standing or sitting positions on Earth during the day. The musculoskeletal system supports these positions and also allows motion. Gravity acting in the longitudinal direction of the body generates a hydrostatic pressure difference and induces footward fluid shift. The vestibular system senses the gravity of the body and reflexively controls the organs. During spaceflight or exposure to microgravity, the load on the musculoskeletal system and hydrostatic pressure difference is diminished. Thus, the skeletal muscle, particularly in the lower limbs, is atrophied, and bone minerals are lost via urinary excretion. In addition, the heart is atrophied, and the plasma volume is decreased, which may induce orthostatic intolerance. Vestibular-related control also declines; in particular, the otolith organs are more susceptible to exposure to microgravity than the semicircular canals. Using an advanced resistive exercise device with administration of bisphosphonate is an effective countermeasure against bone deconditioning. However, atrophy of skeletal muscle and the heart has not been completely prevented. Further ingenuity is needed in designing countermeasures for muscular, cardiovascular, and vestibular dysfunctions.

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°) 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.

Long-term exposure to microgravity impairs vestibulo-cardiovascular reflex

The vestibular system is known to have an important role in controlling blood pressure upon posture transition (vestibulo-cardiovascular reflex, VCR). However, under a different gravitational environment, the sensitivity of the vestibular system may be altered. Thus, the VCR may become less sensitive after spaceflight because of orthostatic intolerance potentially induced by long-term exposure to microgravity. To test this hypothesis in humans, we investigated the ability of the VCR to maintain blood pressure upon head-up tilt before and after a 4–6 months stay on the International Space Station. To detect the functional state of the VCR, galvanic vestibular stimulation (GVS) was applied. As GVS transiently interrupts the vestibular-mediated pressor response, impaired VCR is detected when the head-up tilt-induced blood pressure response does not depend on GVS. During the first 20 s of head-up tilt, a transient blood pressure increase (11.9 ± 1.6 mmHg) was observed at pre-spaceflight but not at 1–4 days after return from spaceflight. The magnitude of VCR recovered to the pre-spaceflight levels within 2 months after return. These results indicate that long-term exposure to microgravity induces VCR impairment, which may be involved in a mechanism of spaceflight-induced orthostatic intolerance.

Body height and arterial pressure in seated and supine young males during +2 G centrifugation

It is known that arterial pressure correlates positively with body height in males, and it has been suggested that this is due to the increasing vertical hydrostatic gradient from the heart to the carotid baroreceptors. Therefore, we tested the hypothesis that a higher gravito-inertial stress induced by the use of a human centrifuge would increase mean arterial pressure (MAP) more in tall than in short males in the seated position. In short (162–171 cm; n = 8) and tall (194–203 cm; n = 10) healthy males (18–41 yr), brachial arterial pressure, heart rate (HR), and cardiac output were measured during +2G centrifugation, while they were seated upright with the legs kept horizontal (+2Gz). In a separate experiment, the same measurements were done with the subjects supine (+2Gx). During +2Gz MAP increased in the short (22 ± 2 mmHg, P < 0.0001) and tall (23 ± 2 mmHg, P < 0.0001) males, with no significant difference between the groups. HR increased more ( P < 0.05) in the tall than in the short group (14 ± 2 vs. 7 ± 2 bpm). Stroke volume (SV) decreased in the short group (26 ± 4 ml, P = 0.001) and more so in the tall group (39 ± 5 ml, P < 0.0001; short vs. tall, P = 0.047). During +2Gx, systolic arterial pressure increased ( P < 0.001) and SV ( P = 0.012) decreased in the tall group only. In conclusion, during +2Gz, MAP increased in both short and tall males, with no difference between the groups. However, in the tall group, HR increased more during +2Gz, which could be caused by a larger hydrostatic pressure gradient from heart to head, leading to greater inhibition of the carotid baroreceptors.

Projection neurons of the vestibulo-sympathetic reflex pathway

Changes in head position and posture are detected by the vestibular system and are normally followed by rapid modifications in blood pressure. These compensatory adjustments, which allow humans to stand up without fainting, are mediated by integration of vestibular system pathways with blood pressure control centers in the ventrolateral medulla. Orthostatic hypotension can reflect altered activity of this neural circuitry. Vestibular sensory input to the vestibulo-sympathetic pathway terminates on cells in the vestibular nuclear complex, which in turn project to brainstem sites involved in the regulation of cardiovascular activity, including the rostral and caudal ventrolateral medullary regions (RVLM and CVLM, respectively). In the present study, sinusoidal galvanic vestibular stimulation was used to activate this pathway, and activated neurons were identified through detection of c-Fos protein. The retrograde tracer Fluoro-Gold was injected into the RVLM or CVLM of these animals, and immunofluorescence studies of vestibular neurons were conducted to visualize c-Fos protein and Fluoro-Gold concomitantly. We observed activated projection neurons of the vestibulo-sympathetic reflex pathway in the caudal half of the spinal, medial, and parvocellular medial vestibular nuclei. Approximately two-thirds of the cells were ipsilateral to Fluoro-Gold injection sites in both the RVLM and CVLM, and the remainder were contralateral. As a group, cells projecting to the RVLM were located slightly rostral to those with terminals in the CVLM. Individual activated projection neurons were multipolar, globular, or fusiform in shape. This study provides the first direct demonstration of the central vestibular neurons that mediate the vestibulo-sympathetic reflex.

RR interval variability during galvanic vestibular stimulation correlates with arterial pressure upon head-up tilt

RR interval variability (RRIV) in the supine position without and with galvanic vestibular stimulation (GVS (off) and GVS (on), respectively), changes in mean arterial pressure (MAP) at the onset of 60° head-up tilt (HUT) during GVS (off), and their relationship were analyzed in 25 healthy young subjects. MAP decreased by less than 5mmHg or increased upon HUT in 12 subjects (UP), but MAP decreased by more than 5mmHg in 13 subjects (DOWN). Applying sinusoidal GVS of 2mA at a random frequency of 0.2 to 10.0Hz did not change the RR intervals or MAP. However, the high frequency component (HF) of RRIV increased in both UP and DOWN subjects. The increase in DOWN subjects was larger than that in UP subjects. The ratio of the low frequency component to HF (L/H) increased in UP subjects during GVS (on), but did not reach a significant level in DOWN subjects. The changes in the HF were significantly correlated with changes in MAP at the onset of HUT; i.e., the subjects with larger increases in the HF during GVS (on) showed larger decreases in MAP. Thus, GVS or vestibular input during HUT possibly activates the vagal nerves, and the dominance of excitation in sympathetic or vagal nerves during vestibular stimulation is important for controlling MAP at the onset of HUT.

Additive role of the vestibular end organ and baroreceptors on the regulation of blood pressure in rats

Contribution of the vestibular end organ to regulation of arterial pressure was quantitatively compared with the role of baroreceptors in terms of baroreflex sensitivity and c-Fos protein expression in the rostral ventrolateral medulla (RVLM). Baroreflex sensitivity and c-Fos protein expression in the RVLM were measured in conscious rats that had undergone bilateral labyrinthectomy (BL) and/or baroreceptor unloading. BL attenuated baroreflex sensitivity during intravenous infusion of sodium nitroprusside (SNP), but did not significantly affect the sensitivity following infusion of phenylephrine (PE). Baroreflex sensitivity became positive following sinoaortic denervation (SAD) during infusion of PE and attenuated sensitivity during infusion of SNP. Baroreflex sensitivity also became positive following double ablation (BL+SAD) during infusion of PE, and attenuated sensitivity during infusion of SNP. c-Fos protein expression increased significantly in the RVLM in the sham group after SNP administration. However, the BL, SAD, and SAD+BL groups showed significant decreases in c-Fos protein expression compared with that in the sham group. The SAD group showed more reduced c-Fos protein expression than that in the BL group, and the SAD+BL group showed less expression than that in the SAD group. These results suggest that the vestibular system cooperates with baroreceptors to maintain arterial pressure during hypotension but that baroreceptors regulate arterial pressure during both hypotension and hypertension. Additionally, afferent signals for maintaining blood pressure from the vestibular end organs and the baroreceptors may be integrated in the RVLM.

Pain and anxiety mediate the relationship between dizziness and falls in older people

OBJECTIVES

To identify medical, psychological, and physiological mediators of the relationship between dizziness and falls in older adults.

DESIGN

Secondary analysis of a prospective cohort study.

SETTING

Community.

PARTICIPANTS

Five hundred sixteen community-dwelling adults aged 73 to 92.

MEASUREMENTS

Participants completed questionnaires related to health and psychological well-being and underwent a tilt table blood pressure test, the Physiological Profile Assessment (PPA; vision, reaction time, proprioception, postural sway, and quadriceps strength), and leaning balance tests. Prospective falls data were collected using monthly calendars for 12 months. Participants were categorized into dizzy and nondizzy groups based on self-report of dizziness, vertigo, and light-headedness.

RESULTS

Two hundred seventeen (42%) participants reported vertigo or dizziness (10%), light-headedness (16%), or both (16%). The dizzy participants were significantly more likely to report neck and back pain, past transient ischemic attacks, and feeling dizzy upon upright tilting. They also had poorer balance and less strength and scored higher on measures of depression and anxiety (P < .05). There were no blood pressure measurement-related differences between the groups. Dizziness increased the risk of multiple falls in an unadjusted analysis (relative risk (RR) = 1.55, 95% confidence interval = 1.08-2.23). After controlling for PPA scores, neck and back pain and anxiety were mediators that reduced the RR of the relationship between dizziness and faller status the most (14%) in a modified Poisson regression model.

CONCLUSION

Suffering from neck and back pain and anxiety were mediators of the relationship between dizziness and falls after controlling for poor sensorimotor function and balance. Older people with dizziness might benefit from interventions targeting these mediators such as pain management and cognitive behavioral therapy.

Evidence for vestibular dysfunction in orthostatic hypotension

There is little definitive evidence of the clinical significance of the vestibular-cardiovascular reflex in humans, despite the fact that the vestibular system is known to contribute to cardiovascular control in animals. The present study involved 248 dizzy patients (127 male patients and 121 female patients) aged 65 years and younger. We classified all participants into three groups based on their vestibular evoked myogenic potential (VEMP) responses; absent VEMP, asymmetry VEMP and normal VEMP. To investigate the effect of the otolith disorder, which was estimated by the VEMP, on the orthostatic blood pressure responses, the subjects' systolic blood pressure (SBP), diastolic blood pressure (DBP) and heart rate were monitored during the orthostatic test after they actively stood up. The male patients in the absent VEMP group had a significant drop in their DBP at 1 min after active standing up (P < 0.05) without any change in their SBP. Conversely, male patients in the asymmetry VEMP and normal VEMP groups showed a significant increase in the SBP at 1 min after active standing up (P < 0.05). Female patients in the absent VEMP group did not show any significant drop in their blood pressure after standing up (P > 0.05). In the entire group of participants, a total of 19.6% of the patients in the absent VEMP group fulfilled the criteria for orthostatic hypotension (OH), which was significantly > the 8.6% of patients in the normal VEMP group and the 7.2% in the asymmetry VEMP group (P < 0.05). Our results suggest that vestibular disorders due to the dysfunction of otolith organs provoke OH.

Subsensory galvanic vestibular stimulation augments arterial pressure control upon head-up tilt in human subjects

The vestibular system plays an important role in control of arterial pressure (AP) upon head-up tilt (HUT). To examine this role in human subjects, we previously compared changes in AP with and without high-amplitude galvanic vestibular stimulation (GVS), which is considered to obscure vestibular input. In contrast, regarding sensory function in skin and muscle, it has been documented that low-amplitude electrical stimulation improves both sensitivity and response. In the present study, we examined whether GVS of smaller amplitude improves AP control upon HUT. GVS was applied at the amplitude of the somatosensory threshold (0.3-0.8 mA), 0.1 mA over the threshold, and 0.1 and 0.2 mA below the threshold during HUT. AP decreased at the onset of HUT compared with that in the supine position in 15 of 25 subjects without GVS (-12±2 mmHg), but applying GVS at 0.1 mA below the somatosensory threshold diminished the decrease (0.3±0.7 mmHg). The APs of another 10 subjects were maintained or decreased by less than 5 mmHg without GVS at the onset of HUT (4±2 mmHg), but applying GVS at the amplitude of 0.1 mA below the somatosensory threshold further increased the AP (12±2 mmHg). GVS at the other amplitudes did not result in AP changes in either group. Thus, subsensory weak GVS enhances AP control at the onset of HUT.

Interaction between vestibulo-cardiovascular reflex and arterial baroreflex during postural change in rats

To examine a cooperative role for the baroreflex and the vestibular system in controlling arterial pressure (AP) during voluntary postural change, AP was measured in freely moving conscious rats, with or without sinoaortic baroreceptor denervation (SAD) and/or peripheral vestibular lesion (VL). Voluntary rear-up induced a slight decrease in AP (-5.6 ± 0.8 mmHg), which was significantly augmented by SAD (-14.7 ± 1.0 mmHg) and further augmented by a combination of VL and SAD (-21 ± 1.0 mmHg). Thus we hypothesized that the vestibular system sensitizes the baroreflex during postural change. To test this hypothesis, open-loop baroreflex analysis was conducted on anesthetized sham-treated and VL rats. The isolated carotid sinus pressure was increased stepwise from 60 to 180 mmHg while rats were placed horizontal prone or in a 60° head-up tilt (HUT) position. HUT shifted the carotid sinus pressure-sympathetic nerve activity (SNA) relationship (neural arc) to a higher SNA, shifted the SNA-AP relationship (peripheral arc) to a lower AP, and, consequently, moved the operating point to a higher SNA while maintaining AP (from 113 ± 5 to 114 ± 5 mmHg). The HUT-induced neural arc shift was completely abolished in VL rats, whereas the peripheral arc shifted to a lower AP and the operating point moved to a lower AP (from 116 ± 3 to 84 ± 5 mmHg). These results indicate that the vestibular system elicits sympathoexcitation, shifting the baroreflex neural arc to a higher SNA and maintaining AP during HUT.

Role of the vestibular system in the arterial pressure response to parabolic-flight-induced gravitational changes in human subjects

Arterial pressure (AP) is known to fluctuate during parabolic-flight-induced gravitational changes in human subjects, increasing during hypergravity and decreasing during microgravity. In this study, we examined whether the vestibular system participates in the AP response to the gravitational changes induced by parabolic flight in human subjects. Eight subjects performed parabolic flights in a supine position as their AP was measured. Their vestibular inputs during the gravitational changes were reversibly masked by artificial electrical stimulation (galvanic vestibular stimulation, GVS). The AP responses during the parabolas were then compared between the GVS-off and GVS-on conditions. AP increased during hypergravity and decreased during microgravity. The AP responses at the onset of hypergravity and microgravity were abolished by GVS. These results indicate that the vestibular system elicits pressor and depressor responses during parabolic-flight-induced hypergravity and microgravity, respectively.

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 (≈15-20 mmHg) and HR (≈3 beat*s(-1)), followed by a slow recovery over 1-6 min; and 2) inhibitory modulations in BP (≈4.5 mmHg/g) and HR (≈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 ≈4.5 mmHg/g (BP) and ≈0.17 beat*s(-1)/g (HR) for pitches of 20-90°. 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.

Restriction of rear-up-behavior-induced attenuation of vestibulo-cardiovascular reflex in rats

Previously, we have demonstrated that the vestibulo-cardiovascular reflex was attenuated in rats reared in a 3G environment for 14 days. Because continuous galvanic vestibular stimulation preserved the vestibulo-cardiovascular reflex in rats at 3G, this attenuation might be attributable to a reduction in the phasic input to the vestibular system. The present study shows that the head movements of rats were significantly suppressed in the 3G environment. Therefore, we hypothesized that the attenuation of the vestibulo-cardiovascular reflex is induced by the reduced vestibular phasic input caused by the restriction of rear-up behavior. To examine this hypothesis, the pressor responses to linear acceleration were measured in rats reared in a low-roof cage. The linear-acceleration-induced pressor response was significantly suppressed in these rats. The suppressive effect of the low-roof cage was similar to that of 3G. There was no difference in the air-jet-induced pressor response among three groups (rats reared in a usual 1G environment, rats reared in the low-roof cage, and rats reared in the 3G environment), suggesting that the sensitivity of the vestibulo-cardiovascular reflex was selectively suppressed. These results indicate that a reduction in the vestibular phasic input acts to attenuate the vestibulo-cardiovascular reflex.

Effects of postural changes and removal of vestibular inputs on blood flow to and from the hindlimb of conscious felines

Considerable data show that the vestibular system contributes to blood pressure regulation. Prior studies reported that lesions that eliminate inputs from the inner ears attenuate the vasoconstriction that ordinarily occurs in the hindlimbs of conscious cats during head-up rotations. These data led to the hypothesis that labyrinthine-deficient animals would experience considerable lower body blood pooling during head-up postural alterations. The present study tested this hypothesis by comparing blood flow though the femoral artery and vein of conscious cats during 20-60 degrees head-up tilts from the prone position before and after removal of vestibular inputs. In vestibular-intact animals, venous return from the hindlimb dropped considerably at the onset of head-up tilts and, at 5 s after the initiation of 60 degrees rotations, was 66% lower than when the animals were prone. However, after the animals were maintained in the head-up position for another 15 s, venous return was just 33% lower than before the tilt commenced. At the same time point, arterial inflow to the limb had decreased 32% from baseline, such that the decrease in blood flow out of the limb due to the force of gravity was precisely matched by a reduction in blood reaching the limb. After vestibular lesions, the decline in femoral artery blood flow that ordinarily occurs during head-up tilts was attenuated, such that more blood flowed into the leg. Contrary to expectations, in most animals, venous return was facilitated, such that no more blood accumulated in the hindlimb than when labyrinthine signals were present. These data show that peripheral blood pooling is unlikely to account for the fluctuations in blood pressure that can occur during postural changes of animals lacking inputs from the inner ear. Instead, alterations in total peripheral resistance following vestibular dysfunction could affect the regulation of blood pressure.

Galvanic vestibular stimulation counteracts hypergravity-induced plastic alteration of vestibulo-cardiovascular reflex in rats

Recent data from our laboratory demonstrated that, when rats are raised in a hypergravity environment, the sensitivity of the vestibulo-cardiovascular reflex decreases. In a hypergravity environment, static input to the vestibular system is increased; however, because of decreased daily activity, phasic input to the vestibular system may decrease. This decrease may induce use-dependent plasticity of the vestibulo-cardiovascular reflex. Accordingly, we hypothesized that galvanic vestibular stimulation (GVS) may compensate the decrease in phasic input to the vestibular system, thereby preserving the vestibulo-cardiovascular reflex. To examine this hypothesis, we measured horizontal and vertical movements of rats under 1-G or 3-G environments as an index of the phasic input to the vestibular system. We then raised rats in a 3-G environment with or without GVS for 6 days and measured the pressor response to linear acceleration to examine the sensitivity of the vestibulo-cardiovascular reflex. The horizontal and vertical movement of 3-G rats was significantly less than that of 1-G rats. The pressor response to forward acceleration was also significantly lower in 3-G rats (23 +/- 1 mmHg in 1-G rats vs. 12 +/- 1 mmHg in 3-G rats). The pressor response was preserved in 3-G rats with GVS (20 +/- 1 mmHg). GVS stimulated Fos expression in the medial vestibular nucleus. These results suggest that GVS stimulated vestibular primary neurons and prevent hypergravity-induced decrease in sensitivity of the vestibulo-cardiovascular reflex.