Vestibulosympathetic reflex during orthostatic challenge in aging humans

Impaired vestibular function associated with orthostatic hypotension in patients with multiple system atrophy

BACKGROUND

Orthostatic hypotension (OH) is one of the most common symptoms in patients with multiple system atrophy (MSA). Vestibular system plays an important role in blood pressure regulation during orthostatic challenges through vestibular-sympathetic reflex. The current study aimed to investigate the relationship between vestibular function and OH in patients with MSA.

METHODS

Participants with MSA, including 20 with OH (mean age, 57.55 ± 8.44 years; 7 females) and 15 without OH (mean age, 59.00 ± 8.12 years; 2 females) and 18 healthy controls (mean age, 59.03 ± 6.44 years; 8 females) were enrolled. Cervical and ocular vestibular evoked myogenic potentials (cVEMPs and oVEMPs) tests were conducted to evaluate vestibular function.

RESULTS

Patients with MSA presented with significantly higher rate of absent cVEMPs (57.1% vs 11.1%, p = 0.001) and oVEMPs (25.7% vs 0, p = 0.021) than controls. MSA patients with OH showed more absent cVEMPs (75.0% vs 11.1%, Bonferroni corrected p < 0.001) and oVEMPs (40.0% vs 0, Bonferroni corrected p = 0.003) than controls. Patients with OH also showed higher rate of absent cVEMPs than those without OH (33.3%, Bonferroni corrected p = 0.014).

CONCLUSIONS

Our results demonstrated that impairment of vestibular function was associated with MSA, particularly in those with OH. Absent VEMPs may be a potential marker for MSA severity. Our findings suggest that impaired vestibular function is involved in OH development and may serve as an intervention target.

Nocturnal vestibular stimulation using a rocking bed improves a severe sleep disorder in a patient with mitochondrial disease

Mitochondrial diseases are rare genetic disorders often accompanied by severe sleep disorders. We present the case of a 12-year-old boy diagnosed with a severe primary mitochondrial disease, exhibiting ataxia, spasticity, progressive external ophthalmoplegia, cardiomyopathy and severely disrupted sleep, but no cognitive impairment. Interestingly, his parents reported improved sleep during night train rides. Based on this observation, we installed a rocking bed in the patient's bedroom and performed different interventions, including immersive multimodal vestibular, kinesthetic and auditory stimuli, reminiscent of the sensory experiences encountered during train rides. Over a 5-month period, we conducted four 2-week nocturnal interventions, separated by 1-week washout phases, to determine the subjectively best-perceived stimulation parameters, followed by a final 4-week intervention using the optimal parameters. We assessed sleep duration and quality using the Mini Sleep Questionnaire, monitored pulse rate changes and used videography to document nocturnal interactions between the patient and caregivers. Patient-reported outcome measures, clinical examinations and personal outcomes of specific interests were used to document daytime sleepiness, restlessness, anxiety, fatigue, cognitive performance and physical posture. In the final 4-week intervention, sleep duration increased by 25%, required caregiver interactions reduced by 75%, and caregiving time decreased by 40%. Subjective fatigue, assessed by the Checklist Individual Strength, decreased by 40%, falling below the threshold of severe fatigue. Our study suggests that rocking beds could provide a promising treatment regime for selected patients with persistent severe sleep disorders. Further research is required to validate these findings in larger patient populations with sleep disorders and other conditions.

Blood Pressure Response to the Head-Up Tilt Test in Benign Paroxysmal Positional Vertigo

The vestibular organ is involved in controlling blood pressure through vestibulosympathetic reflexes of the autonomic nervous system. This study aimed to investigate the effect of benign paroxysmal positional vertigo (BPPV) on blood pressure control by the autonomic nervous system by observing changes in blood pressure before and after BPPV treatment using the head-up tilt test (HUTT). A total of 278 patients who underwent the HUTT before and after treatment were included. The HUTT measured blood pressure repeatedly on the day of diagnosis and the day of complete recovery, and the results were analyzed using repeated measures analysis of variance. Regarding the difference in the systolic blood pressure of patients with BPPV, the blood pressure at 1, 2, and 3 min in the upright position after complete recovery was significantly lower than before treatment (p = 0.001, p = 0.001, and p = 0.012, respectively). Blood pressure at 1 and 2 min in the diastolic blood pressure of patients with BPPV in the upright position after complete recovery was significantly lower than before treatment (p = 0.001 and p = 0.034, respectively). This study shows that BPPV increases blood pressure during the initial response to standing in the HUTT.

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.

Vestibular-autonomic interactions: beyond orthostatic dizziness

PURPOSE OF REVIEW

This review aims to summarize the current literature describing vestibular-autonomic interactions and to describe their putative role in various disorders' clinical presentations, including orthostatic dizziness and motion sensitivity.

RECENT FINDINGS

The vestibular-autonomic reflexes have long been described as they relate to cardiovascular and respiratory function. Although orthostatic dizziness may be in part related to impaired vestibulo-sympathetic reflex (orthostatic hypotension), there are various conditions that may present similarly. A recent clinical classification aims to improve identification of individuals with hemodynamic orthostatic dizziness so that appropriate recommendations and management can be efficiently addressed. Researchers continue to improve understanding of the underlying vestibular-autonomic reflexes with recent studies noting the insular cortex as a cortical site for vestibular sensation and autonomic integration and modulation. Work has further expanded our understanding of the clinical presentation of abnormal vestibular-autonomic interactions that may occur in various conditions, such as aging, peripheral vestibular hypofunction, traumatic brain injury, and motion sensitivity.

SUMMARY

The vestibular-autonomic reflexes affect various sympathetic and parasympathetic functions. Understanding these relationships will provide improved identification of underlying etiology and drive improved patient management.

Lower Body Negative Pressure: Physiological Effects, Applications, and Implementation

This review presents lower body negative pressure (LBNP) as a unique tool to investigate the physiology of integrated systemic compensatory responses to altered hemodynamic patterns during conditions of central hypovolemia in humans. An early review published in Physiological Reviews over 40 yr ago (Wolthuis et al. Physiol Rev 54: 566-595, 1974) focused on the use of LBNP as a tool to study effects of central hypovolemia, while more than a decade ago a review appeared that focused on LBNP as a model of hemorrhagic shock (Cooke et al. J Appl Physiol (1985) 96: 1249-1261, 2004). Since then there has been a great deal of new research that has applied LBNP to investigate complex physiological responses to a variety of challenges including orthostasis, hemorrhage, and other important stressors seen in humans such as microgravity encountered during spaceflight. The LBNP stimulus has provided novel insights into the physiology underlying areas such as intolerance to reduced central blood volume, sex differences concerning blood pressure regulation, autonomic dysfunctions, adaptations to exercise training, and effects of space flight. Furthermore, approaching cardiovascular assessment using prediction models for orthostatic capacity in healthy populations, derived from LBNP tolerance protocols, has provided important insights into the mechanisms of orthostatic hypotension and central hypovolemia, especially in some patient populations as well as in healthy subjects. This review also presents a concise discussion of mathematical modeling regarding compensatory responses induced by LBNP. Given the diverse applications of LBNP, it is to be expected that new and innovative applications of LBNP will be developed to explore the complex physiological mechanisms that underline health and disease.

Vestibular Modulation of Sympathetic Nerve Activity to Muscle and Skin in Humans

We review the existence of vestibulosympathetic reflexes in humans. While several methods to activate the human vestibular apparatus have been used, galvanic vestibular stimulation (GVS) is a means of selectively modulating vestibular afferent activity via electrodes over the mastoid processes, causing robust vestibular illusions of side-to-side movement. Sinusoidal GVS (sGVS) causes partial entrainment of sympathetic outflow to muscle and skin. Modulation of muscle sympathetic nerve activity (MSNA) from vestibular inputs competes with baroreceptor inputs, with stronger temporal coupling to the vestibular stimulus being observed at frequencies remote from the cardiac frequency; “super entrainment” was observed in some individuals. Low-frequency (<0.2 Hz) sGVS revealed two peaks of modulation per cycle, with bilateral recordings of MSNA or skin sympathetic nerve activity, providing evidence of lateralization of sympathetic outflow during vestibular stimulation. However, it should be noted that GVS influences the firing of afferents from the entire vestibular apparatus, including the semicircular canals. To identify the specific source of vestibular input responsible for the generation of vestibulosympathetic reflexes, we used low-frequency (<0.2 Hz) sinusoidal linear acceleration of seated or supine subjects to, respectively, target the utricular or saccular components of the otoliths. While others had discounted the semicircular canals, we showed that the contributions of the utricle and saccule to the vestibular modulation of MSNA are very similar. Moreover, that modulation of MSNA occurs at accelerations well below levels at which subjects are able to perceive any motion indicates that, like vestibulospinal control of posture, the vestibular system contributes to the control of blood pressure through potent reflexes in humans.

Aging augments renal vasoconstrictor response to orthostatic stress in humans

The ability of the human body to maintain arterial blood pressure (BP) during orthostatic stress is determined by several reflex neural mechanisms. Renal vasoconstriction progressively increases during graded elevations in lower body negative pressure (LBNP). This sympathetically mediated response redistributes blood flow to the systemic circulation to maintain BP. However, how healthy aging affects the renal vasoconstrictor response to LBNP is unknown. Therefore, 10 young (25 ± 1 yr; means ± SE) and 10 older (66 ± 2 yr) subjects underwent graded LBNP (-15 and -30 mmHg) while beat-to-beat renal blood flow velocity (RBFV; Doppler ultrasound), arterial BP (Finometer), and heart rate (HR; electrocardiogram) were recorded. Renal vascular resistance (RVR), an index of renal vasoconstriction, was calculated as mean BP/RBFV. All baseline cardiovascular variables were similar between groups, except diastolic BP was higher in older subjects (P < 0.05). Increases in RVR during LBNP were greater in the older group compared with the young group (older: -15 mmHg Δ10 ± 3%, -30 mmHg Δ20 ± 5%; young: -15 mmHg Δ2 ± 2%, -30 mmHg Δ6 ± 2%; P < 0.05). RBFV tended to decrease more (P = 0.10) and mean BP tended to decrease less (P = 0.09) during LBNP in the older group compared with the young group. Systolic and diastolic BP, pulse pressure, and HR responses to LBNP were similar between groups. These findings suggest that aging augments the renal vasoconstrictor response to orthostatic stress in humans.

The function of the autonomic nervous system during spaceflight

INTRODUCTION

Despite decades of study, a clear understanding of autonomic nervous system activity in space remains elusive. Differential interpretation of fundamental data has driven divergent theories of sympathetic activation and vasorelaxation.

METHODS

This paper will review the available in-flight autonomic and hemodynamic data in an effort to resolve these discrepancies. The NASA NEUROLAB mission, the most comprehensive assessment of autonomic function in microgravity to date, will be highlighted. The mechanisms responsible for altered autonomic activity during spaceflight, which include the effects of hypovolemia, cardiovascular deconditioning, and altered central processing, will be presented.

RESULTS

The NEUROLAB experiments demonstrated increased sympathetic activity and impairment of vagal baroreflex function during short-duration spaceflight. Subsequent non-invasive studies of autonomic function during spaceflight have largely reinforced these findings, and provide strong evidence that sympathetic activity is increased in space relative to the supine position on Earth. Others have suggested that microgravity induces a state of relative vasorelaxation and increased vagal activity when compared to upright posture on Earth. These ostensibly disparate theories are not mutually exclusive, but rather directly reflect different pre-flight postural controls.

CONCLUSION

When these results are taken together, they demonstrate that the effectual autonomic challenge of spaceflight is small, and represents an orthostatic stress less than that of upright posture on Earth. In-flight countermeasures, including aerobic and resistance exercise, as well short-arm centrifugation, have been successfully deployed to counteract these mechanisms. Despite subtle changes in autonomic activity during spaceflight, underlying neurohumoral mechanisms of the autonomic nervous system remain intact and cardiovascular function remains stable during long-duration flight.

Vestibulo-sympathetic responses

Evidence accumulated over 30 years, from experiments on animals and human subjects, has conclusively demonstrated that inputs from the vestibular otolith organs contribute to the control of blood pressure during movement and changes in posture. This review considers the effects of gravity on the body axis, and the consequences of postural changes on blood distribution in the body. It then separately considers findings collected in experiments on animals and human subjects demonstrating that the vestibular system regulates blood distribution in the body during movement. Vestibulosympathetic reflexes differ from responses triggered by unloading of cardiovascular receptors such as baroreceptors and cardiopulmonary receptors, as they can be elicited before a change in blood distribution occurs in the body. Dissimilarities in the expression of vestibulosympathetic reflexes in humans and animals are also described. In particular, there is evidence from experiments in animals, but not humans, that vestibulosympathetic reflexes are patterned, and differ between body regions. Results from neurophysiological and neuroanatomical studies in animals are discussed that identify the neurons that mediate vestibulosympathetic responses, which include cells in the caudal aspect of the vestibular nucleus complex, interneurons in the lateral medullary reticular formation, and bulbospinal neurons in the rostral ventrolateral medulla. Recent findings showing that cognition can modify the gain of vestibulosympathetic responses are also presented, and neural pathways that could mediate adaptive plasticity in the responses are proposed, including connections of the posterior cerebellar vermis with the vestibular nuclei and brainstem nuclei that regulate blood pressure.

Anatomical observations of the caudal vestibulo-sympathetic pathway

The vestibular system senses the movement and position of the head in space and uses this information to stabilize vision, control posture, perceive head orientation and self-motion in three-dimensional space, and modulate autonomic and limbic activity in response to locomotion and changes in posture. Most vestibular signals are not consciously perceived and are usually appreciated through effector pathways classically described as the vestibulo-ocular, vestibulo-spinal, vestibulo-collic and vestibulo-autonomic reflexes. The present study reviews some of the recent data concerning the connectivity and chemical anatomy of vestibular projections to autonomic sites that are important in the sympathetic control of blood pressure.

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.

Melatonin attenuates the vestibulosympathetic but not vestibulocollic reflexes in humans: selective impairment of the utricles

Melatonin has been reported to decrease nerve activity of medial vestibular nuclei in the rat and is associated with attenuated muscle sympathetic nerve activity (MSNA) responses to baroreceptor unloading in humans. The purpose of this study was to determine if melatonin alters the vestibulosympathetic reflex (VSR) and vestibulocollic reflex (VCR) in humans. In study 1, MSNA, arterial blood pressure, and heart rate were measured in 12 healthy subjects (28 ± 1 yr; 6 men, 6 women) during head-down rotation (HDR) before and 45 min after ingestion of either melatonin (3 mg) or placebo (sucrose). Subjects returned at least 2 days later at the same time of day to repeat the trial after ingesting the opposite treatment (melatonin or placebo). Melatonin significantly attenuated MSNA responses during HDR compared with placebo (burst frequency Δ 4 ± 1 vs. Δ 7 ± 1 bursts/min, and total MSNA Δ 51 ± 20 and Δ 96 ± 15%, respectively; P < 0.02). In study 2, vestibular evoked myogenic potentials (VEMP) were measured in 10 healthy subjects (26 ± 1 yr; 4 men and 6 women) before and after ingestion of 3 mg melatonin. Melatonin did not alter the timing of the p13 and n23 peaks (pre-melatonin 13.2 ± 0.4 and 21.3 ± 0.6 ms vs. post-melatonin 13.5 ± 0.4 and 21.4 ± 0.7 ms, respectively) or the p13-n23 interpeak amplitudes [pre-melatonin 22.5 ± 4.6 arbitrary units (au) and post-melatonin 22.7 ± 4.6 au]. In summary, melatonin attenuates the VSR and supports the concept that melatonin negatively affects orthostatic tolerance. However, melatonin does not alter the VCR in humans suggesting melatonin's effect on the VSR appears to be mediated by the utricles.

Animal aging and regulation of sympathetic nerve discharge

Studies completed in human subjects have made seminal contributions to understanding the effects of age on sympathetic nervous system (SNS) regulation. Numerous experimental constraints limit the design of studies involving human subjects; therefore, completion of studies in animal models of aging would be expected to provide additional insight regarding mechanisms mediating age-related changes in sympathetic nerve discharge (SND) regulation. The present review assesses the current state of the literature regarding contributions from animal studies on the effects of advancing age on SND regulation, focusing primarily on studies that have used direct recordings of sympathetic nerve outflow. Few studies using direct SND recordings have been completed in animal models of aging, regardless of the fundamental component of SND regulation reviewed (basal levels, acute responsiveness, relationships between the discharges in sympathetic nerves, central neural regulation). SNS responsiveness to various acute stressors is altered in aged compared with young animals; however, mechanisms remain virtually unexplored. There is a marked dearth of studies that have used central neural microinjection techniques in conjunction with SND recordings in aged animals, making it difficult to develop an evidence-based framework regarding potential age-associated effects on central regulation of SND. Determination of age-related changes in mechanisms regulating SND is important for understanding relationships between chronic disease development and changes in SNS function; however, this can only be achieved by substantially extending the current knowledge base regarding the effects of age on SND regulation in animal studies.

Heart rate and blood pressure effects during caloric vestibular testing

OBJECTIVES

To determine whether the caloric vestibular test causes significant changes in heart rate and mean arterial blood pressure.

MATERIALS AND METHODS

Changes in heart rate and mean arterial blood pressure before and after caloric irrigation were compared with the degree of nystagmus (as measured by maximum slow phase velocity) and the patient's subjective dizziness (scored from 0 to 10). A cardiologist reviewed each patient's heart rate and mean arterial blood pressure changes. Patients' anxiety levels were also assessed.

RESULTS

Eighteen patients were recruited. There were no adverse events in any patient. There were no overall significant differences between the heart rate and mean arterial pressure before and after each irrigation. There was a significant correlation between the maximum slow phase velocity and patients' subjective dizziness scores.

CONCLUSION

Heart rate and mean arterial blood pressure are not significantly influenced by the caloric vestibular test. This preliminary study will enable patients with stable cardiovascular disease to be recruited for further risk determination.

Menstrual cycle elicits divergent forearm vascular responses to vestibular activation in humans

The menstrual cycle has been reported to alter mean arterial pressure (MAP), but not muscle sympathetic nerve activity (MSNA), during vestibular activation. Specifically, MAP responses to head-down rotation (HDR) are augmented during the mid-luteal (ML) phase compared to the early follicular (EF) phase in young, eumenorrheic women. The purpose of the present study was to determine if the menstrual cycle influences vestibular-mediated changes in limb blood flow. MSNA, MAP, heart rate, and limb blood flow responses to HDR were measured in 12 healthy women. Resting MSNA, MAP, heart rate, forearm blood flow and calf blood flow were not altered by the menstrual cycle. HDR elicited similar increases in MSNA during the EF (Delta3+/-1 bursts/min; P<0.05) and ML (Delta2+/-1 bursts/min; P<0.05) phase, but only increased MAP during the ML phase (Delta4+/-2 mmHg; P<0.05). HDR did not change heart rate during either the EF or ML phase. HDR elicited similar increases in calf vascular resistance during the EF (Delta6+/-2 mmHg/mL/100 mL/min; P<0.05) and ML (Delta7+/-2 mmHg/mL/100mL/min; P<0.05) phases of the menstrual cycle. In contrast, HDR increased forearm vascular resistance during the ML phase (Delta4+/-2 mmHg/mL/100mL/min; P<0.05), but not the EF phase (Delta0+/-2 mmHg/mL/100mL/min). These findings suggest an increased transduction of sympathetic nerve activity into forearm vascular resistance during the ML phase, and reveal the first recorded divergent vascular response to vestibular excitation in human limbs.

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.

Otolithic activation on visceral circulation in humans: effect of aging

Engagement of the otolith organs elicits differential activation of sympathetic nerve activity and vascular responses to muscle and skin in humans. Additionally, aging attenuates the otolith organ-mediated increases in muscle sympathetic nerve activity in older adults. In this study, we hypothesized that 1) the vestibulosympathetic reflex (VSR) would elicit visceral vascular vasoconstriction and 2) visceral vascular response to the VSR would be attenuated in older subjects compared with young. To test these hypotheses, heart rate, mean arterial blood pressure, and renal, celiac trunk, and superior mesenteric arterial blood velocity (Doppler ultrasound) were measured in 22 young (25+/-1 yr) and 18 older (65+/-2 yr) healthy subjects during head-down rotation (HDR), which selectively activates the otolith organs. Mean arterial pressure and heart rate did not change from baseline during HDR in young or older subjects. Renal blood velocity (Delta -2+/-1 cm/s) and vascular conductance (Delta -0.03+/-0.01 cm.s(-1).mmHg(-1)) significantly decreased from baseline during HDR (P<0.05) in young subjects. In contrast, renal blood velocity and conductance did not change in older subjects (Delta -0.2+/-1 cm/s and Delta0.02+/-0.08 mmHg.cm(-1).s(-1), respectively) during HDR. Superior mesenteric and celiac blood velocity and vascular conductance did not change in response to HDR in either the young or older subjects. These data suggest that renal vasoconstriction occurs during otolith organ activation in young but not older humans. Together with our previous studies, we conclude that the VSR elicits a diverse patterning of sympathetic outflow that results in heterogeneous vascular responses in humans and that these responses are significantly attenuated in older humans.

Sympathetic responses to vestibular activation in humans

Activation of sympathetic neural traffic via the vestibular system is referred to as the vestibulosympathetic reflex. Investigations of the vestibulosympathetic reflex in humans have been limited to the past decade, and the importance of this reflex in arterial blood pressure regulation is still being determined. This review provides a summary of sympathetic neural responses to various techniques used to engage the vestibulosympathetic reflex. Studies suggest that activation of the semicircular canals using caloric stimulation and yaw rotation do not modulate muscle sympathetic nerve activity (MSNA) or skin sympathetic nerve activity (SSNA). In contrast, activation of the otolith organs appear to alter MSNA, but not SSNA. Specifically, head-down rotation and off-vertical axis rotation increase MSNA, while sinusoidal linear accelerations decrease MSNA. Galvanic stimulation, which results in a nonspecific activation of the vestibule, appears to increase MSNA if the mode of delivery is pulse trained. In conclusion, evidence strongly supports the existence of a vestibulosympathetic reflex in humans. Furthermore, attenuation of the vestibulosympathetic reflex is coupled with a drop in arterial blood pressure in the elderly, suggesting this reflex may be important in human blood pressure regulation.

Effect of dimenhydrinate on autonomic activity in humans

The purpose of this study was to examine the effect of dimenhydrinate on resting muscle sympathetic nerve activity (MSNA), the vestibulosympathetic reflex, and the baroreflexes. Sixteen subjects participated in two double-blinded studies that measured mean arterial pressure (MAP), heart rate (HR), and MSNA responses before and after oral administration of dimenhydrinate (100 mg) or a placebo. In study one, 3 min of head-down rotation (HDR) was performed to engage the otolith organs. Dimenhydrinate (n = 10) did not alter resting MSNA, MAP, or HR. HDR increased MSNA before (Delta5 +/- 1 bursts/min; P < 0.01) and after (Delta4 +/- 1 bursts/min; P < 0.01) drug administration, but these responses were not different from the placebo (n = 6). In study two, 4 min of lower body negative pressure (LBNP) at -30 mmHg was performed. During the third min of LBNP, HDR was performed. MSNA increased during the first 2 min of LBNP before (Delta13 +/- 2 bursts/min; P < 0.01) and after (Delta14 +/- 2 bursts/min; P < 0.01) dimenhydrinate. HDR combined with LBNP increased MSNA further during the third min of LBNP (Delta18 +/- 2 bursts/min before and Delta17 +/- 2 bursts/min after dimenhydrinate; P < 0.01). These responses were not significantly different from the placebo. In contrast, HR responses to LBNP during the dimenhydrinate trial were increased when compared to all other trials (Delta5 +/- 1 beats/min; P < 0.01). These results indicate that dimenhydrinate augments heart rate responses to baroreceptor unloading, but does not alter resting MSNA, the sympathetic baroreflexes, or the vestibulosympathetic reflex.

Long-term hypergravity induces plastic alterations in vestibulo-cardiovascular reflex in conscious rats

To test the hypothesis that an altered gravitational environment induces plastic changes in the vestibulo-cardiovascular reflex, arterial pressure (AP) and hypothalamic glutamate concentration were examined in 2 groups of conscious rats, i.e., a 3-G group and a 1-G group, in which rats were maintained under a 3-G and 1-G environment for 2 weeks, respectively. The vestibulo-cardiovascular reflex was stimulated by a gravitational change induced by a parabolic flight that consisted of 3 phases: "pull-up", during which the G load gradually increased to 2G; a 20s "push-over" into microgravity; and "pull-out", during which the G load increased to 1.8. In the 1-G group, the AP increased by 11.9+/-1.2 mmHg during the pull-up hypergravity period. The AP response was significantly attenuated in the 3-G group (4.0+/-0.8 mmHg). During the push-over microgravity period, the AP decreased from the peak level in the pull-up period and recovered to the pre-parabolic control level (-1.8+/-2.4 mmHg). In rats of the 3-G group, the AP was not altered by push-over microgravity. These AP responses were associated with a significant increase in the glutamate concentration in the hypothalamus (4.4+/-0.7%). The glutamate response was also significantly attenuated in the 3-G group compared with that in the 1-G group. These results indicate that an altered gravitational environment induces plastic alterations in the vestibulo-cardiovascular reflex.

Resetting of the arterial baroreflex increases orthostatic sympathetic activation and prevents postural hypotension in rabbits

Since humans are under ceaseless orthostatic stress, the mechanism to maintain arterial pressure (AP) under orthostatic stress against gravitational fluid shift is of great importance. We hypothesized that (1) orthostatic stress resets the arterial baroreflex control of sympathetic nerve activity (SNA) to a higher SNA, and (2) resetting of the arterial baroreflex contributes to preventing postural hypotension. Renal SNA and AP were recorded in eight anaesthetized, vagotomized and aortic-denervated rabbits. Isolated intracarotid sinus pressure (CSP) was increased stepwise from 40 to 160 mmHg with increments of 20 mmHg (60 s for each CSP level) while the animal was placed supine and at 60 deg upright tilt. Upright tilt shifted the CSP-SNA relationship (the baroreflex neural arc) to a higher SNA, shifted the SNA-AP relationship (the baroreflex peripheral arc) to a lower AP, and consequently moved the operating point to marked high SNA while maintaining AP. A simulation study suggests that resetting in the neural arc would double the orthostatic activation of SNA and increase the operating AP in upright tilt by 10 mmHg, compared with the absence of resetting. In addition, upright tilt did not change the CSP-AP relationship (the baroreflex total arc). A simulation study suggests that although a downward shift of the peripheral arc could shift the total arc downward, resetting in the neural arc would compensate this fall and prevent the total arc from shifting downward to a lower AP. In conclusion, upright tilt increases SNA by resetting the baroreflex neural arc. This resetting may compensate for the reduced pressor responses to SNA in the peripheral cardiovascular system and contribute to preventing postural hypotension.

Effects of bilateral vestibular nucleus lesions on cardiovascular regulation in conscious cats

The vestibular system participates in cardiovascular regulation during postural changes. In prior studies (Holmes MJ, Cotter LA, Arendt HE, Cas SP, and Yates BJ. Brain Res 938: 62-72, 2002, and Jian BJ, Cotter LA, Emanuel BA, Cass SP, and Yates BJ. J Appl Physiol 86: 1552-1560, 1999), transection of the vestibular nerves resulted in instability in blood pressure during nose-up body tilts, particularly when no visual information reflecting body position in space was available. However, recovery of orthostatic tolerance occurred within 1 wk, presumably because the vestibular nuclei integrate a variety of sensory inputs reflecting body location. The present study tested the hypothesis that lesions of the vestibular nuclei result in persistent cardiovascular deficits during orthostatic challenges. Blood pressure and heart rate were monitored in five conscious cats during nose-up tilts of varying amplitude, both before and after chemical lesions of the vestibular nuclei. Before lesions, blood pressure remained relatively stable during tilts. In all animals, the blood pressure responses to nose-up tilts were altered by damage to the medial and inferior vestibular nuclei; these effects were noted both when animals were tested in the presence and absence of visual feedback. In four of the five animals, the lesions also resulted in augmented heart rate increases from baseline values during 60 degrees nose-up tilts. These effects persisted for longer than 1 wk, but they gradually resolved over time, except in the animal with the worst deficits. These observations suggest that recovery of compensatory cardiovascular responses after loss of vestibular inputs is accomplished at least in part through plastic changes in the vestibular nuclei and the enhancement of the ability of vestibular nucleus neurons to discriminate body position in space by employing nonlabyrinthine signals.

Aging, opioid-receptor agonists and antagonists, and the vestibulosympathetic reflex in humans

Animal studies indicate that opioids inhibit the firing rate of vestibular neurons, which are important in mediating the vestibulosympathetic reflex. Furthermore, this inhibition appears to be greater in more mature rats. In the present study, we tested the hypotheses that opioids inhibit the vestibulosympathetic reflex in humans and that endogenous opioids contribute to the age-related impairment of the vestibulosympathetic reflex. These hypotheses were tested by measuring muscle sympathetic nerve activity (MSNA), arterial blood pressure, and heart rate responses to otolith organ engagement during head-down rotation (HDR) in young (24 ± 2 yr old) and older (63 ± 2 yr) subjects before and after administration of either an opioid-receptor antagonist (16 mg naloxone in 9 young and 8 older subjects) or an opioid-receptor agonist (60 mg codeine in 7 young and 7 older subjects). Naloxone did not augment the reflex increase in MSNA during HDR in young (Δ7 ± 2 vs. Δ4 ± 2 bursts/min and Δ81 ± 23 vs. Δ60 ± 24% change in burst frequency and total MSNA before and after naloxone, respectively) or older subjects (Δ2 ± 2 vs. Δ1 ± 2 burst/min and Δ8 ± 7 vs. Δ8 ± 9% before and after naloxone). Similarly, codeine did not attenuate the increase in MSNA during HDR in young (Δ8 ± 1 vs. Δ7 ± 2 bursts/min and Δ53 ± 4 vs. Δ64 ± 16% before and after codeine) or older subjects (Δ6 ± 4 vs. Δ3 ± 3 bursts/min and Δ38 ± 21 vs. Δ33 ± 20%). Mean arterial blood pressure and heart rate responses to HDR were not altered by either naloxone or codeine. These data do not provide experimental support for the concept that opioids modulate the vestibulosympathetic reflex in humans. Moreover, endogenous opioids do not appear to contribute the age-associated impairment of the vestibulosympathetic reflex.

Roles of baroreflex and vestibulosympathetic reflex in controlling arterial blood pressure during gravitational stress in conscious rats

Gravity acts on the circulatory system to decrease arterial blood pressure (AP) by causing blood redistribution and reduced venous return. To evaluate roles of the baroreflex and vestibulosympathetic reflex (VSR) in maintaining AP during gravitational stress, we measured AP, heart rate (HR), and renal sympathetic nerve activity (RSNA) in four groups of conscious rats, which were either intact or had vestibular lesions (VL), sinoaortic denervation (SAD), or VL plus SAD (VL + SAD). The rats were exposed to 3 G in dorsoventral axis by centrifugation for 3 min. In rats in which neither reflex was functional (VL + SAD group), RSNA did not change, but the AP showed a significant decrease (-8 +/- 1 mmHg vs. baseline). In rats with a functional baroreflex, but no VSR (VL group), the AP did not change and there was a slight increase in RSNA (25 +/- 10% vs. baseline). In rats with a functional VSR, but no baroreflex (SAD group), marked increases in both AP and RSNA were observed (AP 31 +/- 6 mmHg and RSNA 87 +/- 10% vs. baseline), showing that the VSR causes an increase in AP in response to gravitational stress; these marked increases were significantly attenuated by the baroreflex in the intact group (AP 9 +/- 2 mmHg and RSNA 38 +/- 7% vs. baseline). In conclusion, AP is controlled by the combination of the baroreflex and VSR. The VSR elicits a huge pressor response during gravitational stress, preventing hypotension due to blood redistribution. In intact rats, this AP increase is compensated by the baroreflex, resulting in only a slight increase in AP.

Aging attenuates the vestibulorespiratory reflex in humans

Activation of the vestibular system changes ventilation in humans. The purpose of the present study was to investigate whether aging alters the vestibulorespiratory reflex in humans. Because aging attenuates the vestibulosympathetic reflex, it was hypothesized that aging would attenuate the vestibulorespiratory reflex. Changes in ventilation during engagement of the semicircular canals and/or the otolith organs were measured in fourteen young (26 +/- 1 years) and twelve older subjects (66 +/- 1 years). In young subjects, natural engagement of the semicircular canals and the otolith organs by head rotation increased breathing frequency during dynamic upright pitch at 0.25 Hz (15 cycles min-1) and 0.5 Hz (30 cycles min-1) (delta2 +/- 1 and delta4 +/- 1 breaths min-1, respectively; P < 0.05) and during dynamic upright roll (delta2 +/- 1 and delta4 +/- 1, respectively; P < 0.05). In older subjects, the only significant changes in breathing frequency occurred during dynamic pitch and roll at 0.5 Hz (delta2 +/- 1 and delta2 +/- 1 for pitch and roll, respectively). Stimulation of the horizontal semicircular canals by yaw rotation increased minute ventilation in young but not older subjects. Selective engagement of the otolith organs during static head-down rotation did not alter breathing frequency in either the young or older subjects. The results of this study indicate that the vestibulorespiratory reflex is attenuated in older humans, with greater vestibular stimulation needed to activate the reflex.