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

Changes in sleep performance and chronotype behaviour after vestibular rehabilitation in unilateral vestibular hypofunction

OBJECTIVE

This study aimed to investigate changes in sleep parameters and self-perceived sleep quality in unilateral vestibular hypofunction participants after vestibular rehabilitation.

METHOD

Forty-six unilateral vestibular hypofunction participants (before and after vestibular rehabilitation) along with a control group of 60 healthy patients underwent otoneurological examination, a one-week actigraphy sleep analysis and a series of self-report and performance measures.

RESULTS

After vestibular rehabilitation, unilateral vestibular hypofunction participants showed a significant score decrease in the Pittsburgh Sleep Quality Index, a self-rated reliable questionnaire depicting sleep quality during the last month, as well as a reduction in sleep onset latency and an increase in total sleep time, indicating an objective improvement in sleep quality as measured by actigraphy analysis. However, after vestibular rehabilitation, unilateral vestibular hypofunction participants still showed statistically significant differences with respect to the control group in both self-rated and objective measurements of sleep quality.

CONCLUSION

Vestibular rehabilitation may impact on sleep performance and chronotype behaviour, possibly by opposing long-term structural changes along neural pathways entangled in sleep activity because of the deafferentation of the vestibular nuclei.

Sleep Performance and Chronotype Behavior in Unilateral Vestibular Hypofunction

OBJECTIVE

To evaluate sleep behavior and its relation to otoneurological parameters in a group of patients with chronic unilateral vestibular hypofunction (UVH) without self-reported sleep disturbances when compared with healthy subjects serving as a control group (CG).

METHODS

Fifty-one patients affected by UVH underwent a retrospective clinical and instrumental otoneurological examination, a 1-week actigraphy sleep analysis, and a series of self-report and performance measures (SRM/PM). A CG of 60 gender- and age-matched healthy subjects was also enrolled. A between-group analysis of variance was performed for each variable, while correlation analysis was performed in UVH patients between otoneurological, SRM/PM, and actigraphy measure scores.

RESULTS

When compared with CG subjects, UVH patients were found to be spending less time sleeping and taking more time to go from being fully awake to asleep, based on actigraphy-based sleep analysis. Also, SRM/PM depicted UVH patients to have poor sleep quality and to be more prone to an evening-type behavior. Correlations were found between vestibular-related functionality indexes and subjective sleep quality, as well as between longer disease duration and reduced sleep time.

CONCLUSION

For the first time, a multiparametric sleep analysis was performed on a large population-based sample of chronic UVH patients. While a different pattern in sleep behavior was found, the cause is still unclear. Further research is needed to expand the extent of knowledge about sleep disruption in vestibular disorders.

LEVEL OF EVIDENCE

Level 3 Laryngoscope, 2021.

Changes in body composition in unilateral vestibular hypofunction: relationships between bioelectrical impedance analysis and neuro-otological parameters

PURPOSE

Experimental works have indicated the potential of the vestibular system to affect body composition to be mediated by its extensive connections to brainstem nuclei involved in regulating metabolism and feeding behavior. The aim of this study was to evaluate-by means of bioelectrical impedance analysis (BIA)-the body composition in a group of chronic UVH normal-weighted patients when compared with an equally balanced group of healthy participants, serving as a control group (CG).

METHODS

Forty-six chronic UVH and 60 CG participants underwent otoneurological (including video Head Impulse Test [vHIT] and static posturography testing [SPT]), BIA measurements and self-report (SRM) and performance measures (PM).

RESULTS

Beyond significant (p < 0.001) changes in SPT variables (surface and length) and SRM/PM (including Dizziness Handicap Inventory, Dynamic Gait Index and Activity Balance Confidence scales), UVH participants demonstrated significant (p < 0.001) higher values of fat mass and visceral fat and lower values of muscle mass (p = 0.004), when compared to CG. Significant correlations were found in UVH participants between otoneurological and BIA measurements.

CONCLUSION

These study findings represent the first clinical in-field attempt at depicting, with the use of BIA parameters, changes in body composition related to chronic UVH. Since such alterations in metabolic parameters could be considered both the consequences and/or the cause of vestibular-related quality of life deficit, BIA parameters could be considered as cheap, easy to use, noninvasive assessments in case of chronic UVH.

VGLUT2-expressing neurons in the vestibular nuclear complex mediate gravitational stress-induced hypothermia in mice

The vestibular system, which is essential for maintaining balance, contributes to the sympathetic response. Although this response is involved in hypergravity load-induced hypothermia in mice, the underlying mechanism remains unknown. This study showed that hypergravity (2g) decreased plasma catecholamines, which resulted in hypoactivity of the interscapular brown adipose tissue (iBAT). Hypothermia induced by 2g load was significantly suppressed by administration of beta-adrenergic receptor agonists, suggesting the involvement of decrease in iBAT activity through sympathoinhibition. Bilateral chemogenetic activation of vesicular glutamate transporter 2 (VGLUT2)-expressing neurons in the vestibular nuclear complex (VNC) induced hypothermia. The VGLUT2-expressing neurons contributed to 2g load-induced hypothermia, since their deletion suppressed hypothermia. Although activation of vesicular gamma-aminobutyric acid transporter-expressing neurons in the VNC induced slight hypothermia instead of hyperthermia, their deletion did not affect 2g load-induced hypothermia. Thus, we concluded that 2g load-induced hypothermia resulted from sympathoinhibition via the activation of VGLUT2-expressing neurons in the VNC.

Chikara Abe, Yusuke Yamaoka et al. show that chemogenetic activation of VGLUT2-expressing neurons in the vestibular nuclear complex induces hypothermia, while their deletion suppresses hypergravity load-induced hypothermia in mice. These findings suggest an important role for these glutamatergic neurons in thermoregulation.

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

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

Nondestructive and objective assessment of the vestibular function in rodent models: A review

The normal function of the vestibular system is crucial for the sense of balance. The techniques used to assess the vestibular function plays a vital role in the research of the vestibular system. In this article, we have systematically reviewed some popular methods employing vestibular reflexes and vestibular evoked potentials for assessing the vestibular function in rodent models. These vestibular reflexes and vestibular evoked potentials to effective stimuli have been used as nondestructive and objective functional measures. The main types of vestibular reflexes include the vestibulo-ocular reflex (VOR), vestibulocollic reflex (VCR), and vestibulo-sympathetic reflex (VSR). They are all capable of indicating the functions of the semicircular canals and otoliths. However, the VOR assessment is much more prevalently used because of the relatively stereotypical inputoutput relationship and simple motion pattern of the ocular response. In contrast, the complicated motion pattern and small gain of the VCR response, as well as the undesired component possibly contributed from the acceleration receptors outside the labyrinths in the VSR response, restrict the widespread applications of VCR and VSR in the assessment of the vestibular system. The vestibular evoked myogenic potentials (VEMPs) and vestibular sensory evoked potentials (VsEPs) are the two typical evoked potentials that have been also employed for evaluating the vestibular function. Through exploiting different types of the VEMPs, the saccular and utricular functions can be evaluated separately. The sound-induced VEMPs, moreover, are capable of noninvasively assessing the unilateral vestibular function. The VsEPs, via the morphology of their signal waveforms, enable the access to the location-specific information that indicates the functional statuses of different components within the vestibular neural pathway.

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

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

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

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

Comparison among ultrasonic, electrical apparatus, and toxic chemicals for vestibular lesion in mice

BACKGROUND

The vestibular lesion (VL) is required to examine the physiological function of the vestibular system in animals. Toxic chemicals or electrical apparatus have been used for the VL, however, they are not ideal as they have low specificity, and can result in unintended damage, and systemic toxic effect.

NEW METHOD

Localized vibration-induced VL, using an ultrasonicator, is expected to overcome the problems associated with chemical and electrical lesions. Thus, we examined the effect of the ultrasonication on the VL from the aspects of both the physiological function and histology in the present study.

RESULTS

and Comparison with Existing Method(s) Complete VL, which was evaluated by deterioration of swimming skills, righting reflex, and body stability, was induced using an ultrasonicator or electrical apparatus. Histological evaluation shows that hair cell layers in the saccule and utricle were completely destroyed in both methods Furthermore, significant drop in body mass was observed in VL. However, abscess at the cranial base was observed in VL induced by the electrical apparatus in ICR mice. Complete chemically-induced VL was observed in C57BL/6J but not ICR mice, and systemic leakage of the toxic chemicals (arsenic) was not detectable even 1day after surgery.

CONCLUSIONS

Compared to the electrical apparatus, the ultrasonicator is useful for inducing VL in ICR and C57BL/6J mice, as it results in less non-specific damage. Toxic chemicals can be used for inducing VL in C57BL/6J mice; however, this method does not ensure complete disruption of the hair cells in the saccule and utricle.

Changes in blood pressure, blood flow towards the head and heart rate during 90 deg head-up tilting for 30 min in anaesthetized male rats

NEW FINDINGS

What is the central question of this study? How does the baroreceptor reflex contribute to systemic blood pressure (BP) control during prolonged 90 deg head-up tilting (HUT) in unconscious rats? What is the main finding and its importance? In intact rats, heart rate (HR) increased after the transition to upright by HUT, and BP was maintained in this posture throughout the 30 min experimental period. After sinoaortic denervation, which results in lack of discharge of the baroreflex afferents, HR and BP decreased gradually during 30 min of 90 deg HUT, suggesting that the baroreceptor reflex is an important factor for maintenance of BP during long-term upright posture. Changes in cardiovascular parameters during prolonged 90 deg head-up tilting (HUT) in animals have not been elucidated in detail. We clarified changes in systemic blood pressure (BP), blood flow towards the head (BF) and heart rate (HR) and the role of the baroreceptor reflex after a transition from the supine posture to 90 deg HUT for 30 min in anaesthetized rats (n = 13). In control rats with the baroreceptor reflex afferents intact, mean BP and BF after the onset of 90 deg HUT decreased significantly by -15.4 ± 5.9 and -26.2 ± 11.5% at 2.9 ± 1.1 s (mean ± SD, n = 12), respectively, compared with control values and then immediately increased and steadied at 30.7 ± 13.1 s (plateau; -2.8 ± 8.5% in BP and -17.5 ± 17.4% in BF compared with control values; BP was maintained during 90 deg HUT). After acute sinoaortic denervation in seven rats, initial reductions in BP and BF after 90 deg HUT were observed at 3.9 ± 1.0 s, similar to the reductions in the nerve-intact rats; the percentage changes from control were -19.2 ± 3.7 and -32.3 ± 8.4%, respectively. These parameters reached a plateau at 22.4 ± 5.8 s at -8.6 ± 7.7 and -29.5 ± 15.0%, respectively, and then BP decreased gradually throughout 90 deg HUT. Heart rate increased slightly after 90 deg HUT in nerve-intact rats, but after sinoaortic denervation this increase disappeared and HR decreased gradually during 90 deg HUT. These results suggest that the baroreceptor reflex contributes to the maintenance of adequate BP during long-term 90 deg HUT.

Long-term exposure to microgravity impairs vestibulo-cardiovascular reflex

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

Open-loop static and dynamic characteristics of the arterial baroreflex system in rabbits and rats

The arterial baroreflex system is the most important negative feedback system for stabilizing arterial pressure (AP). This system serves as a key link between the autonomic nervous system and the cardiovascular system, and is thus essential for understanding the pathophysiology of cardiovascular diseases and accompanying autonomic abnormalities. This article focuses on an open-loop systems analysis using a baroreceptor isolation preparation to identify the characteristics of two principal subsystems of the arterial baroreflex system, namely, the neural arc from pressure input to efferent sympathetic nerve activity (SNA) and the peripheral arc from SNA to AP. Studies on the static and dynamic characteristics of the two arcs under normal physiological conditions and also under various interventions including diseased conditions are to be reviewed. Quantitative understanding of the arterial baroreflex function under diseased conditions would help develop new treatment strategies such as electrical activation of the carotid sinus baroreflex for drug-resistant hypertension.

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.

Effects of tempol on baroreflex neural arc versus peripheral arc in normotensive and spontaneously hypertensive rats

Although oxidative redox signaling affects arterial pressure (AP) regulation via modulation of vascular tone and sympathetic nerve activity (SNA), it remains unknown which effect plays a dominant role in the determination of AP in vivo. Open-loop systems analysis of the carotid sinus baroreflex was conducted to separately quantify characteristics of the neural arc from baroreceptor pressure input to SNA and the peripheral arc from SNA to AP in normotensive Wistar-Kyoto (WKY; n = 8) and spontaneously hypertensive rats (SHR; n = 8). Responses in SNA and AP to a staircase-wise increase in carotid sinus pressure were examined before and during intravenous administration of the membrane-permeable superoxide dismutase mimetic tempol (30 mg/kg bolus followed by 30 mg·kg−1·h−1). Two-way ANOVA indicated that tempol significantly decreased the response range of SNA (from 89.1 ± 2.4% to 60.7 ± 2.5% in WKY and from 77.5 ± 3.2% to 56.9 ± 7.3% in SHR, P < 0.001) without affecting the lower plateau of SNA (from 12.5 ± 2.4% to 9.5 ± 2.5% in WKY, and from 28.8 ± 2.8% to 30.4 ± 5.7% in SHR, P = 0.800) in the neural arc. While tempol did not affect the peripheral arc characteristics in WKY, it yielded a downward change in the regression line of AP vs. SNA in SHR. In conclusion, oxidative redox signaling plays an important role, not only in the pathological AP elevation, but also in the baroreflex-mediated physiological AP regulation. The effect of modulating oxidative redox signaling on the peripheral arc contributed to the determination of AP in SHR but not in WKY.

Calibration of baroreflex equilibrium diagram based on exogenous pressor agents in chronic heart failure rats

A baroreflex equilibrium diagram describes the relation between input pressure and sympathetic nerve activity (SNA) and that between SNA and arterial pressure (AP). To calibrate the SNA axis (abscissa) of the baroreflex equilibrium diagram, the AP response to intravenous bolus injections of phenylephrine (0.2-50 μg/kg) or norepinephrine (NE, 0.02-5 μg/kg) was examined in normal control rats (NC, n = 9) and rats with chronic heart failure (CHF, n = 6). The maximum slope of the dose-effect curve was significantly smaller in the CHF group than in the NC group (57.3 ± 5.2 vs 80.9 ± 6.3 mmHg/decade for phenylephrine, 60.2 ± 7.8 vs 80.4 ± 5.9 mmHg/decade for NE; P < 0.01). The CHF/NC ratio of the maximum slope was used to calibrate SNA. While the calibrated baroreflex equilibrium diagram showed increased maximum SNA and operating-point SNA in CHF rats compared with NC rats, the magnitude of increase was smaller than that expected from the excess plasma NE concentration in CHF rats. Plasma NE concentration in the CHF group could be disproportionally high relative to SNA.

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.

Water drinking-related muscle contraction induces the pressor response via mechanoreceptors in conscious rats

Water drinking is known to induce the pressor response. The efferent pathway in this response involves sympathoexcitation, because the pressor response was completely abolished by ganglionic blockade or an α(1)-adrenergic antagonist. However, the afferent pathway in this response has not been identified. In the present study, we hypothesized that water itself stimulates the upper digestive tract to induce the pressor response, and/or drinking-related muscle contraction induces the pressor response via mechanoreceptors. To examine this hypothesis, we evaluated the pressor response induced by spontaneous or passive water drinking in conscious rats. Since the baroreflex modulates and obscures the pressor response, the experiments were conducted using rats with sinoaortic denervation. The pressor response was not suppressed by 1) transient oral surface anesthesia using lidocaine, 2) bilateral denervation of the glossopharyngeal nerve and sensory branch of the superior laryngeal nerve, or 3) denervation of the tunica adventitia in the esophagus. However, the pressor response was significantly suppressed (by -52%) by intravenous gadolinium chloride administration. Electrical stimulation of the hypoglossal nerve induced the pressor response, which was significantly suppressed (by -57%) by intravenous gadolinium chloride administration and completely abolished by severing the distal end of this nerve. These results indicate that afferent signals from mechanoreceptors in drinking-related muscles are involved in the water drinking-induced pressor response.