Vestibular stimulation leads to distinct hemodynamic patterning

Effect of the early diastolic blood pressure response to the head-up tilt test on the recurrence of benign paroxysmal positional vertigo

BACKGROUND

Otolith organ acts complementarily with the autonomic nervous system to maintain blood pressure. However, the effect of blood pressure variability in the autonomic nervous system on otolith organ has not yet been determined. This study aimed to verify the hypothesis that blood pressure variability in the autonomic nervous system affects the recurrence of benign paroxysmal positional vertigo (BPPV), which is the most common disease of the vestibular organs, by using the head-up tilt test (HUTT).

METHODS

This study included 432 patients diagnosed with idiopathic BPPV. The follow-up period for all patients was 12 months. Age, sex, hypertension, diabetes and recurrence were analyzed. The HUTT parameters were divided into a group of patients whose average diastolic blood pressure increased in the upright position compared to supine position during the HUTT (DBP1) and a group of patients whose average diastolic blood pressure decreased in the upright position compared to supine position during the HUTT (DBP2). Model selection, general loglinear analysis, and logit loglinear analysis were performed using a hierarchically progressing loglinear analysis.

RESULTS

In summary, the group with increased average diastolic blood pressure (DBP1) showed a higher tendency for BPPV recurrence compared to the group with decreased diastolic blood pressure (DBP2) in the upright position during the HUTT, although the difference was not statistically significant (p = 0.080). However, in males, the DBP1 group demonstrated a significantly higher recurrence rate of BPPV than the DBP2 group during the HUTT (95% CI, -20.021 to -16.200; p < 0.001).

CONCLUSIONS

It is presumed that poor autonomic nervous system response through vestibulosympathetic reflex maintains elevated diastolic blood pressure in the upright position during the HUTT. This variability is assumed to affect the recurrence of BPPV.

Can Dyssynergia of Vestibulosympathetic and Baroreflexes Cause Vestibular Syncope? The Hypothesis Based on the Velocity-Storage Function

The mechanism of vestibular syncope, the syncope occurring during the vertigo attacks, remains uncertain. This study aims to clarify the mechanism of vestibular syncope by pursuing the function of vestibular system in cardiovascular autonomic control and by defining neuro-hemodynamic changes in vestibular syncope. By integrating the velocity-storage (VS) circuit in the brainstem and cerebellum, we propose that the vestibular syncope develops as a result of dyssynergia of the vestibulosympathetic and baroreflexes in which centrally estimated downward inertial acceleration during the vertigo attacks acts as a trigger. Recognition of the vestibular disorders as a possible cause of syncope would allow proper managements for prevention of further syncope and related complications in patients with vestibular disorders.

Responses of neurons in the rostral ventrolateral medulla of conscious cats to anticipated and passive movements

The vestibular system contributes to regulating sympathetic nerve activity and blood pressure. Initial studies in decerebrate animals showed that neurons in the rostral ventrolateral medulla (RVLM) respond to small-amplitude (<10°) rotations of the body, as in other brain areas that process vestibular signals, although such movements do not affect blood distribution in the body. However, a subsequent experiment in conscious animals showed that few RVLM neurons respond to small-amplitude movements. This study tested the hypothesis that RVLM neurons in conscious animals respond to signals from the vestibular otolith organs elicited by large-amplitude static tilts. The activity of approximately one-third of RVLM neurons whose firing rate was related to the cardiac cycle, and thus likely received baroreceptor inputs, was modulated by vestibular inputs elicited by 40° head-up tilts in conscious cats, but not during 10° sinusoidal rotations in the pitch plane that affected the activity of neurons in brain regions providing inputs to the RVLM. These data suggest the existence of brain circuitry that suppresses vestibular influences on the activity of RVLM neurons and the sympathetic nervous system unless these inputs are physiologically warranted. We also determined that RVLM neurons failed to respond to a light cue signaling the movement, suggesting that feedforward cardiovascular responses do not occur before passive movements that require cardiovascular adjustments.

Distinct activation of the sympathetic adreno-medullar system and hypothalamus pituitary adrenal axis following the caloric vestibular test in healthy subjects

INTRODUCTION

The vestibular acute stress induces reversible alert-like reactions that involve the sympathetic adrenal-medullar system and hypothalamic-pituitary-adrenal axis responses. The present study aimed to evaluate salivary α-amylase and salivary cortisol production in relation with cardiovascular reactivity induced by acute stress in healthy subjects.

MATERIAL AND METHODS

Forty-eight young healthy male volunteers were examined under basal conditions and at various times after reaching the maximal nystagmic reaction following air caloric vestibular test. Heart rate, systolic blood pressure, diastolic blood pressure and mean arterial pressure were recorded at the same time as measurement of the salivary α-amylase and salivary cortisol. At the end of the caloric vestibular test session, perceived stress scale questionnaires were administered to measure the self-perceived stress impact induced by the task, and individual scores were compared with those measured on the enrollment day.

RESULTS

Following caloric vestibular test-evoked vertigo, salivary α-amylase and cortisol showed distinct trends in their production after acute stress: Student's t-test was used to compare the α-amylase vs cortisol slopes of the respective interpolated regression lines, and the difference was significant (t = -3.283; p<0.001); an increase in salivary cortisol production corresponded with a decrease in the salivary α-amylase concentration. In addition, salivary biomarker modifications were associated with consistent changes in the heart rate, systolic blood pressure and mean arterial pressure.

CONCLUSIONS

Using the air caloric vestibular test task as a stressor, the present study demonstrated a connection between the acute hormonal stress response to vestibular stimulation and cardiovascular output. However, further research is needed before we can define the potential importance of the consistent cardiovascular activity changes evoked by vestibular stimulation and the possible functional consequences for cardiovascular regulation and orthostatic tolerance in humans.

Cardiovascular adjustments during anticipated postural changes

It is well‐documented that feedforward cardiovascular responses occur at the onset of exercise, but it is unclear if such responses are associated with other types of movements. In this study, we tested the hypothesis that feedforward cardiovascular responses occur when a passive (imposed) 60° head‐up tilt is anticipated, such that changes in heart rate and carotid artery blood flow (CBF) commence prior to the onset of the rotation. A light cue preceded head‐up tilts by 10 sec, and heart rate and CBF were determined for 5‐sec time periods prior to and during tilts. Even after these stimuli were provided for thousands of trials spanning several months, no systematic changes in CBF and heart rate occurred prior to tilts, and variability in cardiovascular adjustments during tilt remained substantial over time. We also hypothesized that substitution of 20° for 60° tilts in a subset of trials would result in exaggerated cardiovascular responses (as animals expected 60° tilts), which were not observed. These data suggest that cardiovascular adjustments during passive changes in posture are mainly elicited by feedback mechanisms, and that anticipation of passive head‐up tilts does not diminish the likelihood that a decrease in carotid blood flow will occur during the movements.

Distinct activation of the sympathetic adreno-medullar system and hypothalamus pituitary adrenal axis following the caloric vestibular test in healthy subjects

INTRODUCTION

The vestibular acute stress induces reversible alert-like reactions that involve the sympathetic adrenal-medullar system and hypothalamic-pituitary-adrenal axis responses. The present study aimed to evaluate salivary α-amylase and salivary cortisol production in relation with cardiovascular reactivity induced by acute stress in healthy subjects.

MATERIAL AND METHODS

Forty-eight young healthy male volunteers were examined under basal conditions and at various times after reaching the maximal nystagmic reaction following air caloric vestibular test. Heart rate, systolic blood pressure, diastolic blood pressure and mean arterial pressure were recorded at the same time as measurement of the salivary α-amylase and salivary cortisol. At the end of the caloric vestibular test session, perceived stress scale questionnaires were administered to measure the self-perceived stress impact induced by the task, and individual scores were compared with those measured on the enrollment day.

RESULTS

Following caloric vestibular test-evoked vertigo, salivary α-amylase and cortisol showed distinct trends in their production after acute stress: Student's t-test was used to compare the α-amylase vs cortisol slopes of the respective interpolated regression lines, and the difference was significant (t = -3.283; p<0.001); an increase in salivary cortisol production corresponded with a decrease in the salivary α-amylase concentration. In addition, salivary biomarker modifications were associated with consistent changes in the heart rate, systolic blood pressure and mean arterial pressure.

CONCLUSIONS

Using the air caloric vestibular test task as a stressor, the present study demonstrated a connection between the acute hormonal stress response to vestibular stimulation and cardiovascular output. However, further research is needed before we can define the potential importance of the consistent cardiovascular activity changes evoked by vestibular stimulation and the possible functional consequences for cardiovascular regulation and orthostatic tolerance in humans.

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.

Glutamate and GABA in Vestibulo-Sympathetic Pathway Neurons

The vestibulo-sympathetic reflex (VSR) actively modulates blood pressure during changes in posture. This reflex allows humans to stand up and quadrupeds to rear or climb without a precipitous decline in cerebral perfusion. The VSR pathway conveys signals from the vestibular end organs to the caudal vestibular nuclei. These cells, in turn, project to pre-sympathetic neurons in the rostral and caudal ventrolateral medulla (RVLM and CVLM, respectively). The present study assessed glutamate- and GABA-related immunofluorescence associated with central vestibular neurons of the VSR pathway in rats. Retrograde FluoroGold tract tracing was used to label vestibular neurons with projections to RVLM or CVLM, and sinusoidal galvanic vestibular stimulation (GVS) was employed to activate these pathways. Central vestibular neurons of the VSR were identified by co-localization of FluoroGold and cFos protein, which accumulates in some vestibular neurons following galvanic stimulation. Triple-label immunofluorescence was used to co-localize glutamate- or GABA- labeling in the identified VSR pathway neurons. Most activated projection neurons displayed intense glutamate immunofluorescence, suggestive of glutamatergic neurotransmission. To support this, anterograde tracer was injected into the caudal vestibular nuclei. Vestibular axons and terminals in RVLM and CVLM co-localized the anterograde tracer and vesicular glutamate transporter-2 signals. Other retrogradely-labeled cFos-positive neurons displayed intense GABA immunofluorescence. VSR pathway neurons of both phenotypes were present in the caudal medial and spinal vestibular nuclei, and projected to both RVLM and CVLM. As a group, however, triple-labeled vestibular cells with intense glutamate immunofluorescence were located more rostrally in the vestibular nuclei than the GABAergic neurons. Only the GABAergic VSR pathway neurons showed a target preference, projecting predominantly to CVLM. These data provide the first demonstration of two disparate chemoanatomic VSR pathways.

The Vestibular System: A Newly Identified Regulator of Bone Homeostasis Acting Through the Sympathetic Nervous System

The vestibular system is a small bilateral structure located in the inner ear, known as the organ of balance and spatial orientation. It senses head orientation and motion, as well as body motion in the three dimensions of our environment. It is also involved in non-motor functions such as postural control of blood pressure. These regulations are mediated via anatomical projections from vestibular nuclei to brainstem autonomic centers and are involved in the maintenance of cardiovascular function via sympathetic nerves. Age-associated dysfunction of the vestibular organ contributes to an increased incidence of falls, whereas muscle atrophy, reduced physical activity, cellular aging, and gonadal deficiency contribute to bone loss. Recent studies in rodents suggest that vestibular dysfunction might also alter bone remodeling and mass more directly, by affecting the outflow of sympathetic nervous signals to the skeleton and other tissues. This review will summarize the findings supporting the influence of vestibular signals on bone homeostasis, and the potential clinical relevance of these findings.

Inner Ear Vestibular Signals Regulate Bone Remodeling via the Sympathetic Nervous System

The inner ear vestibular system has numerous projections on central brain centers that regulate sympathetic outflow, and skeletal sympathetic projections affect bone remodeling by inhibiting bone formation by osteoblasts and promoting bone resorption by osteoclasts. In this study, we show that bilateral vestibular lesions in mice cause a low bone mass phenotype associated with decreased bone formation and increased bone resorption. This reduction in bone mass is most pronounced in lower limbs, is not associated with reduced locomotor activity or chronic inflammation, and could be prevented by the administration of the β-blocker propranolol and by genetic deletion of the β2-adrenergic receptor, globally or specifically in osteoblasts. These results provide novel experimental evidence supporting a functional autonomic link between central proprioceptive vestibular structures and the skeleton. Because vestibular dysfunction often affects the elderly, these results also suggest that age-related bone loss might have a vestibular component and that patients with inner ear pathologies might be at risk for fracture. Lastly, these data might have relevance to the bone loss observed in microgravity, as vestibular function is altered in this condition as well. © 2015 American Society for Bone and Mineral Research.

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.

Bone remodeling is regulated by inner ear vestibular signals

Bone remodeling allows the conservation of normal bone mass despite constant changes in internal and external environments. The adaptation of the skeleton to these various stimuli leads credence to the notion that bone remodeling is a true homeostatic function, and as such is under the control of specific centers in the central nervous system (CNS). Hypothalamic and brainstem centers, as well as the sympathetic nervous system (SNS), have been identified as regulators of bone remodeling. However, the nature of the afferent CNS stimuli that may modulate CNS centers involved in the control of bone remodeling, with the exception of leptin, remains unclear. Based on the partial efficacy of exercise and mechanical stimulation regimens to prevent microgravity-induced bone loss and the known alterations in vestibular functions associated with space flights, we hypothesized that inner ear vestibular signals may contribute to the regulation of bone remodeling. Using an established model of bilateral vestibular lesions and microtomographic and histomorphometric bone analyses, we show here that induction of bilateral vestibular lesion in rats generates significant bone loss, which is restricted to weight-bearing bones and associated with a significant reduction in bone formation, as observed in rats under microgravity conditions. Importantly, this bone loss was not associated with reduced locomotor activity or metabolic abnormalities, was accompanied with molecular signs of increased sympathetic outflow, and could be prevented by the β-blocker propranolol. Collectively, these data suggest that the homeostatic process of bone remodeling has a vestibulosympathetic regulatory component and that vestibular system pathologies might be accompanied by bone fragility.

Collateralization of projections from the rostral ventrolateral medulla to the rostral and caudal thoracic spinal cord in felines

Stimulation of vestibular receptors elicits distinct changes in blood flow to the forelimb and hindlimb, showing that the nervous system has the capacity to produce changes in sympathetic outflow which are specific for a particular region of the body. However, it is unclear whether the rostral ventrolateral medulla (RVLM), the primary region of the brainstem that regulates sympathetic outflow to vascular smooth muscle, has the appropriate connectivity with sympathetic preganglionic neurons to generate anatomically patterned responses. To make this determination, the retrograde fluorescent tracer Fast Blue was injected into the T(4) spinal cord segment of cats, which regulates upper body blood flow, whereas Fluoro-Ruby was injected into the T(10) segment to label projections to a region of the spinal cord that regulates lower body blood flow. More neurons were single-labeled by a particular tracer (92 %) than were double labeled by both tracers (8 %), supporting the notion that the RVLM can regulate sympathetic outflow from a limited number of spinal cord segments. Since a large fraction of RVLM neurons that control sympathetic outflow in rodents contain epinephrine, we additionally determined whether the tracer-labeled cells were immunopositive for the enzyme tyrosine hydroxylase (TH), which participates in the synthesis of catecholamines. Double labeling by the two tracers injected into the spinal cord was more common for TH-immunopositive neurons than for the general population of RVLM neurons: 19 % of the TH-positive cells contained both Fast Blue and Fluoro-Ruby, 30 % contained one of the tracers, and 51 % were not labeled by either tracer. Furthermore, many spinally projecting neurons in close proximity to the RVLM catecholaminergic neurons (41 % of the population) were not immunopositive for TH, suggesting that feline RVLM is neurochemically heterogeneous.

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.

Role of the rostral ventrolateral medulla (RVLM) in the patterning of vestibular system influences on sympathetic nervous system outflow to the upper and lower body

Research on animal models as well as human subjects has demonstrated that the vestibular system contributes to regulating the distribution of blood in the body through effects on the sympathetic nervous system. Elimination of vestibular inputs results in increased blood flow to the hindlimbs during vestibular stimulation, because it attenuates the increase in vascular resistance that ordinarily occurs in the lower body during head-up tilts. Additionally, the changes in vascular resistance produced by vestibular stimulation differ between body regions. Electrical stimulation of vestibular afferents produces an inhibition of most hindlimb vasoconstrictor fibers and a decrease in hindlimb vascular resistance, but an initial excitation of most upper body vasoconstrictor fibers accompanied by an increase in upper body vascular resistance. The present study tested the hypothesis that neurons in the principal vasomotor region of the brainstem, the rostral ventrolateral medulla (RVLM), whose projections extended past the T10 segment, to spinal levels containing sympathetic preganglionic neurons regulating lower body blood flow, respond differently to electrical stimulation of the vestibular nerve than RVLM neurons whose axons terminate rostral to T10. Contrary to our hypothesis, the majority of RVLM neurons were excited by vestibular stimulation, despite their level of projection in the spinal cord. These findings indicate that the RVLM is not solely responsible for establishing the patterning of vestibular-sympathetic responses. This patterning apparently requires the integration by spinal circuitry of labyrinthine signals transmitted from the brainstem, likely from regions in addition to the RVLM.

Innervation of skeletal muscle by leptin receptor-containing neurons

In addition to suppressing food intake, leptin reduces body adiposity by altering metabolism within peripheral tissues such as adipose tissue and muscle. Recent work indicates that leptin action within the brain is sufficient to promote glucose uptake and increase fat oxidation within skeletal muscle, and that these effects are dependent on the sympathetic nervous system. To identify neuronal circuits through which leptin impacts skeletal muscle metabolism, we used LepRb-GFP reporter mice in combination with muscle-specific injection of an mRFP-expressing pseudorabies virus (PRV), which acts as a transsynaptic retrograde tracer. Consistent with previous observations in the rat, muscle-specific PRV injection lead to labeling within multiple areas of the hypothalamus and brainstem. However, the only areas in which PRV and LepRb colocalization was detected were within the brainstem nucleus of the solitary tract (NTS) and the hypothalamic retrochiasmatic area. Within the NTS 28.5+/-9.4% of PRV-positive neurons contained LepRb-GFP, while in the RCH 37+/-1.7% of PRV neurons also contained LepRb. In summary, these data clearly implicate the NTS and RCH as key sites through which brain leptin impacts skeletal muscle, and as such provide an anatomical framework within which to interpret physiological data indicating that leptin acts in the brain to influence metabolism within skeletal muscle.

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.

Excitatory pathways from the vestibular nuclei to the NTS and the PBN and indirect vestibulo-cardiovascular pathway from the vestibular nuclei to the RVLM relayed by the NTS

Previous studies have confirmed the existence of vestibulo-sympathetic pathways in the central nervous system. However, the exact pathways and neurotransmitters underlying this reflex are unclear. The present study was undertaken to investigate whether the vestibulo-cardiovascular responses are a result of activated glutamate receptors in the caudal vestibular nucleus. We also attempt to verify the indirect excitatory pathways from the vestibular nucleus (VN) to the rostral ventrolateral medulla (RVLM) using a tracing method combined with a vesicular glutamate transporter (VGluTs) immunofluorescence. In anesthetized rats, unilateral injection of l-glutamate (5 nmol) into the medial vestibular nucleus (MVe) and spinal vestibular nucleus (SpVe) slightly increased the mean arterial pressure (MVe: 93.29+/-11.58 to 96.30+/-11.66, SpVe: 91.72+/-15.20 to 95.48+/-17.16). Local pretreatment with the N-methyl-D-aspartate (NMDA)-receptor antagonist MK-801 (2 nmol) significantly attenuated the pressor effect of L-glutamate injected into the MVe compared to the contralateral self-control. After injection of biotinylated dextran amine (BDA) into the MVe and SpVe, and fluorogold (FG) into the RVLM, some BDA-labeled fibres and terminals in the nucleus of solitary tract (NTS) and the parabrachial nucleus (PBN) were immunoreactive for VGluT1 and VGluT2. Several BDA-labeled fibres were closely apposed to FG-labeled neurons in the NTS. These results suggested that activation of caudal vestibular nucleus neurons could induce pressor response and NMDA receptors might contribute to this response in the MVe. The glutamatergic VN-NTS and VN-PBN pathways might exist, and the projections from the VN to the RVLM relayed by the NTS comprise an indirect vestibulo-cardiovascular pathway in the brain stem.

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.

Vestibulosympathetic reflex during the early follicular and midluteal phases of the menstrual cycle

Evidence suggests that both the arterial baroreflex and vestibulosympathetic reflex contribute to blood pressure regulation, and both autonomic reflexes integrate centrally in the medulla cardiovascular center. A previous report indicated increased sympathetic baroreflex sensitivity during the midluteal (ML) phase of the menstrual cycle compared with the early follicular (EF) phase. On the basis of this finding, we hypothesize an augmented vestibulosympathetic reflex during the ML phase of the menstrual cycle. Muscle sympathetic nerve activity (MSNA), mean arterial pressure (MAP), and heart rate responses to head-down rotation (HDR) were measured in 10 healthy females during the EF and ML phases of the menstrual cycle. Plasma estradiol (Delta72 +/- 13 pg/ml, P < 0.01) and progesterone (Delta8 +/- 2 ng/ml, P < 0.01) were significantly greater during the ML phase compared with the EF phase. The menstrual cycle did not alter resting MSNA, MAP, and heart rate (EF: 13 +/- 3 bursts/min, 80 +/- 2 mmHg, 65 +/- 2 beats/min vs. ML: 14 +/- 3 bursts/min, 81 +/- 3 mmHg, 64 +/- 3 beats/min). During the EF phase, HDR increased MSNA (Delta3 +/- 1 bursts/min, P < 0.02) but did not change MAP or heart rate (Delta0 +/- 1 mmHg and Delta1 +/- 1 beats/min). During the ML phase, HDR increased both MSNA and MAP (Delta4 +/- 1 bursts/min and Delta3 +/- 1 mmHg, P < 0.04) with no change in heart rate (Delta0 +/- 1 beats/min). MSNA and heart rate responses to HDR were not different between the EF and ML phases, but MAP responses to HDR were augmented during the ML phase (P < 0.03). Our results demonstrate that the menstrual cycle does not influence the vestibulosympathetic reflex but appears to alter MAP responses to HDR during the ML phase.

Linear acceleration-evoked cardiovascular responses in awake rats

It has been well documented that vestibular-mediated cardiovascular regulation plays an important role in maintaining stable blood pressure (BP) during postural changes. But the underlying neural mechanisms remain to be elucidated. In particular, because the vestibular stimulation employed in previous animal studies activated both semicircular canals and otolith organs, the contributions of the otolith system has not been studied selectively. The goal of the present study was to characterize cardiovascular responses to natural otolith stimulation in awake rats that were subjected to pure linear motion. In any of the four directions tested, transient linear motion produced a short-latency (∼520 ms) increase in mean BP with a peak of 8.27 ± 0.66 mmHg and was followed by a decrease in BP. There was an initial small biphasic response in heart rate (HR) that was followed by a longer duration increase. The short-latency increase in BP was absent in rats that were pentobarbital sodium anesthetized or that were labyrinthectomized bilaterally, but it was unaffected by baroreceptor denervation, indicating that it was of otolith origin. The increase in BP was linear acceleration intensity dependent and was not affected by absence of visual cues. Furthermore, the BP response was attenuated by inactivation of the medial and inferior vestibular nuclei by microinjections of muscimol, indicating that the otolith-driven cardiovascular responses are mediated by the neurons in these areas. These results not only demonstrate the otolith specific influences on the cardiovascular system but also they establish the first rodent model for examining the neural mechanisms underlying the otolith-mediated cardiovascular regulation.

Transneuronal tracing of neural pathways that regulate hindlimb muscle blood flow

Despite considerable interest in the neural mechanisms that regulate muscle blood flow, the descending pathways that control sympathetic outflow to skeletal muscles are not adequately understood. The present study mapped these pathways through the transneuronal transport of two recombinant strains of pseudorabies virus (PRV) injected into the gastrocnemius muscles in the left and right hindlimbs of rats: PRV-152 and PRV-BaBlu. To prevent PRV from being transmitted to the brain stem via motor circuitry, a spinal transection was performed just below the L2 level. Infected neurons were observed bilaterally in all of the areas of the brain that have previously been shown to contribute to regulating sympathetic outflow: the medullary raphe nuclei, rostral ventrolateral medulla (RVLM), rostral ventromedial medulla, A5 adrenergic cell group region, locus coeruleus, nucleus subcoeruleus, and the paraventricular nucleus of the hypothalamus. The RVLM, the brain stem region typically considered to play the largest role in regulating muscle blood flow, contained neurons infected following the shortest postinoculation survival times. Approximately half of the infected RVLM neurons were immunopositive for tyrosine hydroxylase, indicating that they were catecholaminergic. Many (47%) of the RVLM neurons were dually infected by the recombinants of PRV injected into the left and right hindlimb, suggesting that the central nervous system has a limited capacity to independently regulate blood flow to left and right hindlimb muscles.

Cerebral hemodynamics during orthostatic stress assessed by nonlinear modeling

The effects of orthostatic stress, induced by lower body negative pressure (LBNP), on cerebral hemodynamics were examined in a nonlinear context. Spontaneous fluctuations of beat-to-beat mean arterial blood pressure (MABP) in the finger, mean cerebral blood flow velocity (MCBFV) in the middle cerebral artery, as well as breath-by-breath end-tidal CO2 concentration (PetCO2) were measured continuously in 10 healthy subjects under resting conditions and during graded LBNP to presyncope. A two-input nonlinear Laguerre-Volterra network model was employed to study the dynamic effects of MABP and PetCO2 changes, as well as their nonlinear interactions, on MCBFV variations in the very low (VLF; below 0.04 Hz), low (LF; 0.04–0.15 Hz), and high frequency (HF; 0.15–0.30 Hz) ranges. Dynamic cerebral autoregulation was described by the model terms corresponding to MABP, whereas cerebral vasomotor reactivity was described by the model PetCO2 terms. The nonlinear model terms reduced the output prediction normalized mean square error substantially (by 15–20%) and had a prominent effect in the VLF range, both under resting conditions and during LBNP. Whereas MABP fluctuations dominated in the HF range and played a significant role in the VLF and LF ranges, changes in PetCO2 accounted for a considerable fraction of the VLF and LF MCBFV variations, especially at high LBNP levels. The magnitude of the linear and nonlinear MABP-MCBFV Volterra kernels increased substantially above −30 mmHg LBNP in the VLF range, implying impaired dynamic autoregulation. In contrast, the magnitude of the PetCO2-MCBFV kernels reduced during LBNP at all frequencies, suggesting attenuated cerebral vasomotor reactivity under dynamic conditions. We speculate that these changes may reflect a progressively reduced cerebrovascular reserve to compensate for the increasingly unstable systemic circulation during orthostatic stress that could ultimately lead to cerebral hypoperfusion and syncope.

Vestibular inputs elicit patterned changes in limb blood flow in conscious cats

Previous experiments have demonstrated that the vestibular system contributes to regulating sympathetic nervous system activity, particularly the discharges of vasoconstrictor fibres. In the present study, we examined the physiological significance of vestibulosympathetic responses by comparing blood flow and vascular resistance in the forelimb and hindlimb during head-up tilt from the prone position before and after the removal of vestibular inputs through a bilateral vestibular neurectomy. Experiments were performed on conscious cats that were trained to remain sedentary on a tilt table during rotations up to 60 deg in amplitude. Blood flow through the femoral and brachial arteries was recorded during whole-body tilt using perivascular probes; blood pressure was recorded using a telemetry system and vascular resistance was calculated from blood pressure and blood flow measurements. In vestibular-intact animals, 60 deg head-up tilt produced approximately 20% decrease in femoral blood flow and approximately 37% increase in femoral vascular resistance relative to baseline levels before tilt; similar effects were also observed for the brachial artery ( approximately 25% decrease in blood flow and approximately 38% increase in resistance). Following the removal of vestibular inputs, brachial blood flow and vascular resistance during head-up tilt were almost unchanged. In contrast, femoral vascular resistance increased only approximately 6% from baseline during 60 deg head-up rotation delivered in the first week after elimination of vestibular signals and approximately 16% in the subsequent 3-week period (as opposed to the approximately 37% increase in resistance that occurred before lesion). These data demonstrate that vestibular inputs associated with postural alterations elicit regionally specific increases in vascular resistance that direct blood flow away from the region of the body where blood pooling may occur. Thus, the data support the hypothesis that vestibular influences on the cardiovascular system serve to protect against the occurrence of orthostatic hypotension.

Rostral elements of sympatho-motor circuitry: a virally mediated transsynaptic tracing study

Numerous physiological and emotionally motivated behaviors, including locomotion, exercise, escape, and attack behaviors as well as passive coping responses, require concomitant activation of motor and sympathetic efferents. Such functional heterogeneity suggests the existence of dual function neurons that can simultaneously coordinate motor and sympathetic output. Because previous physiological investigations have implicated a number of mesencephalic and telencephalic regions in mediating these behaviors, we hypothesized the presence of dual function sympatho-motor neurons in these neural structures. To test this hypothesis, we used recombinant strains of the pseudorabies virus (PRV) for transsynaptic tract-tracing. PRV-152, a strain that expresses enhanced green fluorescent protein, was injected into sympathectomized gastrocnemius muscle, whereas PRV-BaBlu, which expresses beta-galactosidase, was injected into the adrenal gland in the same animals. Although coinfected neurons were detected in a number of mesencephalic and telencephalic regions, >50% of such neurons were located within specific subdivisions of two general areas: the hypothalamus and periaqueductal gray. These subdivisions included the ventrolateral periaqueductal gray, dorsomedial hypothalamus, dorsolateral lateral hypothalamus, and ventral portion of the medial parvocellular subdivision of the paraventricular nucleus of the hypothalamus (PVN). A subset of the sympatho-motor neurons within the PVN also contained either arginine vasopressin or oxytocin. This sympatho-motor circuitry likely plays an important role in mediating different aspects of stress responses and emotionally motivated behaviors.

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

Prior studies have shown that removal of vestibular inputs produces lability in blood pressure during orthostatic challenges (Holmes MJ, Cotter LA, Arendt HE, Cass SP, and Yates BJ. Brain Res 938: 62-72, 2002; Jian BJ, Cotter LA, Emanuel BA, Cass SP, and Yates BJ. J Appl Physiol 86: 1552-1560, 1999). Furthermore, these studies led to the prediction that the blood pressure instability results in susceptibility for orthostatic intolerance. The present experiments tested this hypothesis by recording common carotid blood flow (CCBF) in conscious cats during head-up tilts of 20, 40, and 60 degrees amplitudes, before and after the surgical elimination of labyrinthine inputs through a bilateral vestibular neurectomy. Before vestibular lesions in most animals, CCBF remained stable during head-up rotations. Unexpectedly, in five of six animals, the vestibular neurectomy resulted in a significant increase in baseline CCBF, particularly when the laboratory was illuminated; on average, basal blood flow measured when the animals were in the prone position was 41 +/- 17 (SE) % higher after the first week after the lesions. As a result, even when posturally related lability in CCBF occurred after removal of vestibular inputs, blood supply to the head was not lower than when labyrinthine inputs were present. These data suggest that vestibular influences on cardiovascular regulation are more complex than previously appreciated, because labyrinthine signals appear to participate in setting basal rates of blood flow to the head in addition to triggering dynamic changes in the circulation to compensate for orthostatic challenges.

Relation between motion sickness susceptibility and vasovagal syncope susceptibility

Motion sickness is common in the population, especially in children, but its physiopathology is only partially understood and the true nature of the particular susceptibility of certain subjects remains completely unknown. Some symptoms of motion sickness, like pallor and cold sweating, are of an autonomic nature and the role of the autonomic nervous system in vasovagal syncope is well known. Our aim was therefore to study the relationship between motion sickness susceptibility and vasovagal syncope susceptibility. Questionnaires about susceptibility to motion sickness and to vasovagal syncope or presyncope in adulthood and childhood, filled in by 899 students (20.4 +/- 2.1 years, 405 men), were analysed. Motion sickness susceptibility in childhood was 31.1% and in adulthood 7.9% (p < 0.001). Vasovagal syncope susceptibility in childhood was 36.4% and in adulthood 33.9% (NS). A relationship between motion sickness susceptibility in adulthood and vasovagal syncope susceptibility in childhood and adulthood (p = 0.004 and 0.005, respectively) was found. Despite the limitations of a retrospective study this relationship between motion sickness susceptibility and vasovagal syncope susceptibility may indicate that a common mechanism exists, explaining the particular susceptibility of some subjects to both disorders. This paradigm may prove useful in better understanding the true nature of motion sickness and vasovagal syncope.

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.

Effect of thermal stress on the vestibulosympathetic reflexes in humans

Both heat stress and vestibular activation alter autonomic responses; however, the interaction of these two sympathetic activators is unknown. To determine the effect of heat stress on the vestibulosympathetic reflex, eight subjects performed static head-down rotation (HDR) during normothermia and whole body heating. Muscle sympathetic nerve activity (MSNA; peroneal microneurography), mean arterial blood pressure (MAP), heart rate (HR), and internal temperature were measured during the experimental trials. HDR during normothermia caused a significant increase in MSNA (Δ5 ± 1 bursts/min; Δ53 ± 14 arbitrary units/min), whereas no change was observed in MAP, HR, or internal temperature. Whole body heating significantly increased internal temperature (Δ0.9 ± 0.1°C), MSNA (Δ10 ± 3 bursts/min; Δ152 ± 44 arbitrary units/min), and HR (Δ25 ± 6 beats/min), but it did not alter MAP. HDR during whole body heating increased MSNA (Δ16 ± 4 bursts/min; Δ233 ± 90 arbitrary units/min from normothermic baseline), which was not significantly different from the algebraic sum of HDR during normothermia and whole body heating (Δ15 ± 4 bursts/min; Δ205 ± 55 arbitrary units/min). These data suggest that heat stress does not modify the vestibulosympathetic reflex and that both the vestibulosympathetic and thermal reflexes are robust, independent sympathetic nervous system activators.

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.

Vestibular activation of sympathetic nerve activity

AIM

The vestibulosympathetic reflex refers to sympathetic nerve activation by the vestibular system. Animal studies indicate that the vestibular system assists in blood pressure regulation during orthostasis. Although human studies clearly demonstrate activation of muscle sympathetic nerve activity (MSNA) during engagement of the otolith organs, the role of the vestibulosympathetic reflex in maintaining blood pressure during orthostasis is not well-established. Examination of the vestibulosympathetic reflex with other cardiovascular reflexes indicates that it is a powerful and independent reflex. Ageing, which is associated with an increased risk for orthostatic hypotension, attenuates the vestibulosympathetic reflex. The attenuated reflex is associated with a reduction in arterial pressure.

CONCLUSION

These findings suggest that the vestibulosympathetic reflex assists in blood pressure regulation in humans, but future studies examining this reflex in other orthostatically intolerant populations are necessary to address this hypothesis.

Head position modifies cerebrovascular response to orthostatic stress

Previous experiments have shown that the vestibular system participates in cardiovascular control. However, the effects of vestibular activation on cerebrovascular regulation are not known. Therefore, the present experiment tested the hypothesis that specific vestibular activations may be beneficial to cerebral circulation during simulated orthostatic stress. Middle cerebral artery blood flow velocity (CBV; Doppler ultrasound) was measured to examine the effects of head-down neck flexion (HDNF) compared to head-down neck extension (HDNE) with and without lower body negative pressure (LBNP; -40 mmHg) (n=9). The change in CBV (DeltaCBV) during HDNF and HDNE were not different during baseline conditions, however, during LBNP, DeltaCBV was greater in HDNE compared to HDNF (-5.5+/-3.2 cm/s, -11+/-4.6%) vs. (-0.7+/-1.0 cm/s, -1.9+/-1.9%), respectively (P<0.05). Concomitantly, the change in cerebrovascular resistance (DeltaCVR) between rest and LBNP was also greater during HDNE (0.48+/-0.08 mmHg/cm per s, 42.8+/-10.8%) compared with HDNF (0.26+/-0.05 mmHg/cm per s, 22+/-4.1%) (P<0.05). P(ET)CO(2) was greater in HDNE (45+/-2 mmHg) compared to HDNF (42+/-2 mmHg; P<0.05) during LBNP. These results suggest that the vestibular system may affect cerebrovascular tone during simulated postural stress by either constriction or dilation, depending on the vestibular stimulus.

Vestibulosympathetic reflex during orthostatic challenge in aging humans

Aging attenuates the increase in muscle sympathetic nerve activity (MSNA) and elicits hypotension during otolith organ engagement in humans. The purpose of the present study was to determine the neural and cardiovascular responses to otolithic engagement during orthostatic stress in older adults. We hypothesized that age-related impairments in the vestibulosympathetic reflex would persist during orthostatic challenge in older subjects and might compromise arterial blood pressure regulation. MSNA, arterial blood pressure, and heart rate responses to head-down rotation (HDR) performed with and without lower body negative pressure (LBNP) in prone subjects were measured. Ten young (27 +/- 1 yr) and 11 older subjects (64 +/- 1 yr) were studied prospectively. HDR performed alone elicited an attenuated increase in MSNA in older subjects (Delta106 +/- 28 vs. Delta20 +/- 7% for young and older subjects). HDR performed during simultaneous orthostatic stress increased total MSNA further in young (Delta53 +/- 15%; P < 0.05) but not older subjects (Delta-5 +/- 4%). Older subjects demonstrated consistent significant hypotension during HDR performed both alone (Delta-6 +/- 2 mmHg) and during LBNP (Delta-7 +/- 2 mmHg). These data provide experimental support for the concept that age-related impairments in the vestibulosympathetic reflex persist during orthostatic challenge in older adults. Furthermore, these findings are consistent with the concept that age-related alterations in vestibular function might contribute to altered orthostatic blood pressure regulation with age in humans.

Asymmetric integration recorded from vestibular-only cells in response to position transients

Angular and translational accelerations excite the semicircular canals and otolith organs, respectively. While canal afferents approximately encode head angular velocity due to the biomechanical integration performed by the canals, otolith signals have been found to approximate head translational acceleration. Because central vestibular pathways require velocity and position signals for their operation, the question has been raised as to how the integration of the otolith signals is accomplished. We recorded responses from 62 vestibular-only neurons in the vestibular nucleus of two monkeys to position transients in the naso-occipital and interaural orientations and varying directions in between. Responses to the transients were directionally asymmetric; one direction elicited a response that approximated the integral of the acceleration of the stimulus. In the opposite direction, the cells simply encoded the acceleration of the motion. We present a model that suggests that a neural integrator is not needed. Instead a neuron with a long membrane time constant and an excitatory postsynaptic potential duration that increases with the firing rate of the presynaptic cell can emulate the observed behavior.

Dynamic time course of hemodynamic responses after passive head-up tilt and tilt back to supine position

Mechanisms involved in the control of arterial pressure during postural changes were studied by analysis of the dynamic time course of cardiovascular changes during head-up tilt (HUT) and tilt back to supine position (TB). Beat-to-beat values of cardiovascular variables were recorded continuously before, during, and after passive HUT to 30 degrees in seven healthy humans. Left cardiac stroke volume (SV, Doppler ultrasound), mean arterial blood pressure (MAP), heart rate (HR), cardiac output (CO), and total peripheral conductance (TPC) were recorded. During HUT, MAP at the level of the carotid baroreceptors decreased by approximately 5 mmHg. There was a striking asymmetry between the time courses of cardiovascular changes on HUT and on TB. Adjustments generally took up to 30 s after HUT, whereas most changes were completed during the first 10 s after TB. Cardiovascular reflex adjustments of HR and TPC were more symmetrical. After HUT, SV was maintained during the first 4-6 s and then decreased steadily during the next 30 s to a stable level approximately 25% below its pretilt value. However, after TB, SV increased rapidly to its pretilt value in <10 s. This asymmetry in SV dynamics may be explained in part by a more rapid change in left cardiac filling after TB than after HUT. On TB, there must be a rapid inflow of stagnant blood from the legs, whereas venous valves will impede backward filling of veins in the lower body on HUT. In conclusion, we have revealed a characteristic asymmetry in cardiovascular responses to inverse variations in gravity forces in humans. This asymmetry can be explained in part by nonlinear, hydrodynamic factors, such as the one-way effect of venous valves in the lower part of the body.

Influence of vestibular activation on respiration in humans

The purpose of this study was to determine the effects of the semicircular canals and otolith organs on respiration in humans. On the basis of animal studies, we hypothesized that vestibular activation would elicit a vestibulorespiratory reflex. To test this hypothesis, respiratory measures, arterial blood pressure, and heart rate were measured during engagement of semicircular canals and/or otolith organs. Dynamic upright pitch and roll (15 cycles/min), which activate the otolith organs and semicircular canals, increased respiratory rate (Delta2 +/- 1 and Delta3 +/- 1 breaths/min, respectively; P < 0.05). Dynamic yaw and lateral pitch (15 cycles/min), which activate the semicircular canals, increased respiration similarly (Delta3 +/- 1 and Delta2 +/- 1, respectively; P < 0.05). Dynamic chair rotation (15 cycles/min), which mimics dynamic yaw but eliminates neck muscle afferent, increased respiration (Delta3 +/- 1; P < 0.05) comparable to dynamic yaw (15 cycles/min). Increases in respiratory rate were graded as greater responses occurred during upright (Delta5 +/- 2 breaths/min) and lateral pitch (Delta4 +/- 1) and roll (Delta5 +/- 1) performed at 30 cycles/min. Increases in breathing frequency resulted in increases in minute ventilation during most interventions. Static head-down rotation, which activates otolith organs, did not alter respiratory rate (Delta1 +/- 1 breaths/min). Collectively, these data indicate that semicircular canals, but not otolith organs or neck muscle afferents, mediate increased ventilation in humans and support the concept that vestibular activation alters respiration in humans.

Aging attenuates the vestibulosympathetic reflex in humans

BACKGROUND

The vestibular system contributes to sympathetic activation by engagement of the otolith organs. However, there is a significant loss of vestibular function with aging. Therefore, the purpose of the present study was to determine if young and older individuals differ in their cardiovascular and sympathetic responses to otolithic stimulation (ie, head-down rotation, HDR). We hypothesized that responses to otolithic stimulation would be attenuated in older adults because of morphological and physiological alterations that occur in the vestibular system with aging.

METHODS AND RESULTS

Arterial blood pressure, heart rate, muscle sympathetic nerve activity (MSNA), and head rotation were measured during HDR in 11 young (26 +/- 1 years) and 11 older (64 +/- 1 years) subjects in the prone posture. Five older subjects performed head rotation (chin to chest) in the lateral decubitus position, which simulates HDR but does not alter afferent inputs from the vestibular system. MSNA responses to HDR were significantly attenuated in older as compared with young subjects (P<0.01). MSNA increased in the older subjects by only 12 +/- 5% as compared with 85 +/- 16% in the young. Furthermore, HDR elicited significant reductions in mean arterial blood pressure in older (Delta-6 +/- 1 mm Hg; P<0.01) but not young subjects (Delta1 +/- 1 mm Hg). In contrast to HDR, head rotation performed in the lateral decubitus position did not elicit hypotension. MSNA responses to baroreceptor unloading and the cold pressor test were not different between the age groups.

CONCLUSIONS

These data indicate that aging attenuates the vestibulosympathetic reflex in humans and may contribute to the increased prevalence of orthostatic hypotension with age.

Limb neurovascular control during altered otolithic input in humans

Head-down rotation (HDR), which activates the vestibulosympathetic reflex, increases leg muscle sympathetic nerve activity (MSNA) and produces calf vasoconstriction with no change in either cardiac output or arterial blood pressure. Based on animal studies, it was hypothesized that differential control of arm and leg MSNA explains why HDR does not alter arterial blood pressure. Fifteen healthy subjects were studied. Heart rate, arterial blood pressure, forearm and calf blood flow, and leg MSNA responses were measured during HDR in these subjects. Simultaneous recordings of arm and leg MSNA were obtained from five of the subjects. Forearm and calf blood flow, vascular conductances, and vascular resistances were similar before HDR, as were arm and leg MSNA. HDR elicited similar significant increases in leg (Delta 6 +/- 1 bursts min(-1); 59 +/- 16 % from baseline) and arm MSNA (Delta 5 +/- 1 bursts min(-1); 80 +/- 28 % from baseline). HDR significantly decreased calf (-19 +/- 2 %) and forearm vascular conductance (-12 +/- 2 %) and significantly increased calf (25 +/- 4 %) and forearm vascular resistance (15 +/- 2 %), with ~60 % greater vasoconstriction in the calf than in the forearm. Arterial blood pressure and heart rate were not altered by HDR. These results indicate that there is no differential control of MSNA in the arm and leg during altered feedback from the otolith organs in humans, but that greater vasoconstriction occurs in the calf than in the forearm. These findings indicate that vasodilatation occurs in other vascular bed(s) to account for the lack of increase in arterial blood pressure during HDR.

Anatomic patterning in the expression of vestibulosympathetic reflexes

To investigate the possibility that expression of vestibulosympathetic reflexes (VSR) is related to a nerve's anatomic location rather than its target organ, we compared VSR recorded from the same type of postganglionic fiber [muscle vasoconstrictor (MVC)] located at three different rostrocaudal levels: hindlimb, forelimb, and face. Experiments were performed on chloralose-anesthetized cats, and vestibular afferents were stimulated electrically. Single MVC unit activity was extracted by spike shape analysis of few-fiber recordings, and unit discrimination was confirmed by autocorrelation. Poststimulus time histogram analysis revealed that about half of the neurons were initially inhibited by vestibular stimulation (type 1 response), whereas the other MVC fibers were initially strongly excited (type 2 response). MVC units with types 1 and 2 responses were present in the same nerve fascicle. Barosensitivity was equivalent in the two groups, but fibers showing type 1 responses fired significantly faster than those giving type 2 responses (0.29 +/- 0.04 vs. 0.20 +/- 0.02 Hz). Nerve fibers with type 1 responses were most common in the hindlimb (21 of 29 units) and least common in the face (2 of 11 units), the difference in relative proportion being significant (P < 0.05, chi(2) test). These results support the hypothesis that VSR are anatomically patterned.