Neurovestibular modulation of circadian and homeostatic regulation: vestibulohypothalamic connection?

Exploration of sleep quality and rest-activity rhythms characteristics in Bilateral Vestibulopathy patients

Sleep and circadian timing systems are constantly regulated by both photic and non-photic signals. Connections between the vestibular nuclei and the biological clock raise the question of the effect of peripheral vestibular loss on daily rhythms, such as the sleep-wake cycle and circadian rhythm. To answer this question, we compared the sleep and rest-activity rhythm parameters of 15 patients with bilateral vestibulopathy (BVP) to those of 15 healthy controls. Sleep and rest-activity cycle were recorded by a device coupling actimetry with the heart rate and actigraphy at home over 7 days. Subjective sleep quality was assessed by the Pittsburgh Sleep Quality Index (PSQI). Sleep efficiency and subjective sleep quality were significantly reduced, and sleep fragmentation was increased in BVP patients compared to controls. BVP patients displayed a damped amplitude of the rest-activity rhythm and higher sleep fragmentation, reflected by a higher nocturnal activity compared to controls. These results suggest that both rest-activity and sleep cycles are impaired in BVP patients compared to healthy controls. BVP patients seem to have greater difficulty maintaining good sleep at night compared to controls. BVP pathology appears to affect the sleep-wake cycle and disturb the circadian rhythm synchronization. Nevertheless, these results need further investigation to be confirmed, particularly with larger sample sizes.

Exploration of effects of galvanic vestibular stimulation on circadian rhythms and its associations with sleep and spatial memory in patients with breast cancer: The ICANSLEEP-2 protocol

BACKGROUND

Patients with breast cancer (BC) exhibit circadian rhythm disruptions, mainly of rest-activity rhythm (RAR), of which sleep is an essential component, and cortisol rhythm. Sleep complaints such as insomnia and cognitive impairments are prevalent in BC. In general population, sleep is known to contribute greatly to cognition. Thus, improving RAR (and particularly sleep) could help limiting cognitive impairments in BC patients. It has recently been suggested that, in addition to its essential role in spatial memory, the vestibular system contributes to RAR synchronization. Its stimulation could therefore limit both sleep disturbances and spatial memory deficits in BC.

OBJECTIVES

The main aim of the ICANSLEEP-2 study is to assess the effects of galvanic vestibular stimulation (GVS) on circadian rhythms. The secondary aim is to assess whether GVS improves sleep and spatial memory in BC patients.

METHODS

Two groups with insomnia complaints (Insomnia Severity Index > 7) will be included: a patients' group with BC (n = 50) and a healthy control group without history of cancer (n = 25). There will be two assessment sessions, before and after 2 weeks of GVS. Patients will be randomly assigned to either a GVS group or a sham group (noneffective stimulation). Controls will receive GVS. GVS effects will be quantified and compared between groups. Assessments will include actigraphy, salivary cortisol, polysomnography, a cognitive test battery (including a computer-based task for spatial memory) and validated questionnaires (for psychological functioning and sleep complaints).

DISCUSSION

Current methods for improving sleep in BC have had controversial outcomes regarding sleep structure. We expect GVS to offer a new mean of directly targeting RAR disruptions in BC patients, with beneficial effects on sleep structure. Given the crucial impact of sleep on cognitive functioning, notably spatial memory, improving sleep of BC patients should enhance their cognitive functioning.

ETHICS AND DISSEMINATION

This study received ethical approval from the Ile de France IV institutional review board on 19 April 2022 (no. ID-RCB: 2022-A00437-36). The findings yielded by this protocol will be presented at various conferences and in peer-reviewed journals.

CLINICALTRIALS.GOV REGISTRATION NUMBER

NCT05414357.

Does sleep quality affect balance? The perspective from the somatosensory, vestibular, and visual systems

OBJECTIVE

Previous studies have focused on the balance system's involvement in sleep deprivation or disorders. This study investigated how daily routine sleep quality affects the balance system of people without sleep deprivation or diagnosed sleep disorders.

METHODS

The study included 45 participants with a BMI score of <25. The PSQI was used to determine sleep quality. The SOT, HS-SOT, and ADT evaluated the vestibular system's functionality.

RESULTS

In SOT, condition 3, 4, 5, and 6 composite scores, VIS and VEST composite balance scores, and HS-SOT 5 scores were lower in the HPSQI group. At the same time, there is a statistically significant negative correlation between these scores and PSQI scores.

CONCLUSION

Poor sleep quality may be a factor influencing the balance system. Sleep quality affects the visual and vestibular systems rather than the somatosensory system. The population should be made aware of this issue, and clinicians should consider the potential impact of sleep quality when evaluating the balance system.

The Effect of Time-Restricted Feeding on Postural Balance: From a Vestibular Perspective

INTRODUCTION

The aims of the present study were to evaluate postural balance performance of the subjects on the time-restricted feeding (TRF) and reveal the effect of TRF on the vestibular system by comparing the results to those of traditional daily dietary (DD) condition.

METHODS

Sixteen adults (3 males, 13 females; mean age: 25.4 ± 4 years) who had experienced at least 1 month of TRF were included in the study. The Sensory Organization Test (SOT) and Head-Shake SOT (HS-SOT) - which evaluate proprioceptive, visual, and vestibular systems - were performed on TRF and DD conditions via the Computerized Dynamic Posturography system.

RESULTS

Significant differences were obtained between TRF and DD situations in SOT-5 (p = 0.008), SOT-6 (p = 0.01), and HS-SOT5 (p = 0.007) conditions in which the vestibular system dominated.

CONCLUSION

We revealed that TRF has an effect on postural balance in the absence of proprioceptive and visual systems. This feeding model is a negative stressor that has a substantial effect on the vestibular system, but this impact is minimal once the proprioceptive and visual systems are intact. To the best of our knowledge, it is the first study to evaluate postural balance utilizing vestibular parameters in TRF.

Hypergravity stimulates mechanical behavior and micro-architecture of tibia in rats

INTRODUCTION

The bone tissue is susceptible to hypergravity (+ G) environment. It is necessary to discuss the extent to which specific + G values are beneficial or detrimental to bone tissue. The objective of this study was to characterize the effects of high + G values on mechanical properties, microstructures, and cellular metabolism of bone.

MATERIALS AND METHODS

30 male Wistar rats aged 12 weeks were randomly divided into 5 groups, and bore different + G (namely + 1G, + 4G, + 8G, + 10G and + 12G) environments respectively for 4 weeks, 5 days each week, and 3 minutes each day. The macro-mechanical parameters, microstructure parameters, and mRNA transcription levels of the tibia were determined through the three-point bending method, micro-CT detection, and q-PCR analysis, respectively.

RESULTS

As the + G value increases, hypergravity becomes increasingly detrimental to the macro-mechanical performance of rat tibia. Concerning the microstructure of cancellous bone, there appears to be a favorable trend at + 4G, followed by a progressively detrimental trend at higher G values. In addition, the mRNA transcription levels of OPG and RANKL show an initial tendency of enhanced bone absorption at +4G, followed by an increase in bone remodeling capacity as G value increases.

CONCLUSION

The higher G values correspond to poorer macro-mechanical properties of the tibia, and a + 4G environment benefits the microstructure of the tibia. At the cellular level, bone resorption is enhanced in the + 4G group, but the bone remodeling capability gradually increases with further increments in G values.

Vestibular CCK signaling drives motion sickness-like behavior in mice

Travel can induce motion sickness (MS) in susceptible individuals. MS is an evolutionary conserved mechanism caused by mismatches between motion-related sensory information and past visual and motion memory, triggering a malaise accompanied by hypolocomotion, hypothermia, hypophagia, and nausea. Vestibular nuclei (VN) are critical for the processing of movement input from the inner ear. Motion-induced activation of VN neurons recapitulates MS-related signs. However, the genetic identity of VN neurons mediating MS-related autonomic and aversive responses remains unknown. Here, we identify a central role of cholecystokinin (CCK)-expressing VN neurons in motion-induced malaise. Moreover, we show that CCK VN inputs onto the parabrachial nucleus activate Calca-expressing neurons and are sufficient to establish avoidance to novel food, which is prevented by CCK-A receptor antagonism. These observations provide greater insight into the neurobiological regulation of MS by identifying the neural substrates of MS and providing potential targets for treatment.

Chronic Vestibular Hypofunction Is Associated with Impaired Sleep: Results from the DizzyReg Patient Registry

Temporary or permanent vestibular hypofunction has been hypothesized to affect circadian rhythm, sleep, and thermoregulation. Chronic or long-term vestibular disorders such as unilateral vestibular hypofunction may have an even greater negative impact on sleep quality than acute vestibular problems. This study examines self-reported sleep quality, as assessed by the Pittsburgh Sleep Quality Index (PSQI), and its association with vestibular symptom duration in a group of patients with vestibular disorders. We used data from the cross-sectional DizzyReg patient registry of the German Center for Vertigo and Balance Disorders outpatient clinic. Vestibular diagnoses were ascertained based on the International Classification of Vestibular Disorders. A total of 137 patients were included (60% female, mean age 55.4 years, standard deviation, SD, 16.7). The mean PSQI total score was 6.3 (SD = 3.2), with 51% reporting overall poor sleep quality. Patients who had vertigo for two years or longer reported significantly poorer global sleep quality (63% vs. 37%, p = 0.021) and significantly more difficulties with sleep latency (79% vs. 56%, p = 0.013) and sleep efficiency (56% vs. 34%, p = 0.022). The association of poor sleep quality with a longer duration of vertigo remained significant after multivariable adjustment. Further research should investigate the interaction of vestibular disorders, sleep, and their potential mechanisms.

Tackling Insomnia Symptoms through Vestibular Stimulation in Patients with Breast Cancer: A Perspective Paper

Insomnia symptoms are common among patients with breast cancer (BC; 20-70%) and are predictors of cancer progression and quality of life. Studies have highlighted sleep structure modifications, including increased awakenings and reduced sleep efficiency and total sleep time. Such modifications may result from circadian rhythm alterations consistently reported in this pathology and known as carcinogenic factors, including lower melatonin levels, a flattened diurnal cortisol pattern, and lower rest-activity rhythm amplitude and robustness. Cognitive behavioral therapy and physical activity are the most commonly used non-pharmacological interventions to counter insomnia difficulties in patients with BC. However, their effects on sleep structure remain unclear. Moreover, such approaches may be difficult to implement shortly after chemotherapy. Innovatively, vestibular stimulation would be particularly suited to tackling insomnia symptoms. Indeed, recent reports have shown that vestibular stimulation could resynchronize circadian rhythms and improve deep sleep in healthy volunteers. Moreover, vestibular dysfunction has been reported following chemotherapy. This perspective paper aims to support the evidence of using galvanic vestibular stimulation to resynchronize circadian rhythms and reduce insomnia symptoms in patients with BC, with beneficial effects on quality of life and, potentially, survival.

Taste Function in Adult Humans from Lean Condition to Stage II Obesity: Interactions with Biochemical Regulators, Dietary Habits, and Clinical Aspects

Differences in gustatory sensitivity, nutritional habits, circulating levels of modulators, anthropometric measures, and metabolic assays may be involved in overweight (OW) development. The present study aimed at evaluating the differences in these aspects between 39 OW (19 female; mean age = 53.51 ± 11.17), 18 stage I (11 female; mean age = 54.3 ± 13.1 years), and 20 II (10 female; mean age = 54.5 ± 11.9) obesity participants when compared with 60 lean subjects (LS; 29 female; mean age = 54.04 ± 10.27). Participants were evaluated based on taste function scores, nutritional habits, levels of modulators (leptin, insulin, ghrelin, and glucose), and bioelectrical impedance analysis measurements. Significant reductions in total and subtests taste scores were found between LS and stage I and II obesity participants. Significant reductions in total and all subtests taste scores were found between OW and stage II obesity participants. Together with the progressive increase in plasmatic leptin levels, insulin, and serum glucose, decrease in plasmatic ghrelin levels, and changes in anthropometric measures and nutritional habits along with body mass index, these data for the first time demonstrated that taste sensitivity, biochemical regulators, and food habits play a parallel, concurring role along the stages evolving to obesity.

Vestibular Disorders and Hormonal Dysregulations: State of the Art and Clinical Perspectives

The interaction between endocrine and vestibular systems remains poorly documented so far, despite numerous observations in humans and animals revealing direct links between the two systems. For example, dizziness or vestibular instabilities often accompany the menstrual cycle and are highly associated with the pre-menopause period, while sex hormones, together with their specific receptors, are expressed at key places of the vestibular sensory network. Similarly, other hormones may be associated with vestibular disorders either as causal/inductive factors or as correlates of the pathology. This review was carried out according to the PRISMA method, covering the last two decades and using the MEDLINE and COCHRANE databases in order to identify studies associating the terms vestibular system and/or vestibular pathologies and hormones. Our literature search identified 646 articles, 67 of which referred directly to vestibular dysfunction associated with hormonal variations. While we noted specific hormonal profiles depending on the pathology considered, very few clinical studies attempted to establish a direct link between the expression of the vestibular syndrome and the level of circulating hormones. This review also proposes different approaches to shed new light on the link between hormones and vestibular disorders, and to improve both the diagnosis and the therapeutic management of dizzy patients.

Changes in metabolism and vestibular function depend on gravitational load in mice

The vestibular system is known to participate in controlling posture and metabolism. Different gravitational environments, including microgravity or hypergravity, cause plastic alteration of the vestibular system, and plasticity is important for adaptation to a novel gravitational environment. However, it is unclear whether the degree of change in vestibular-related physiological function depends on gravitational loading. To examine this, we used a hypergravity environment including 1.33 G, 1.67 G, and 2 G for 29 days. We found that a gravitational threshold induces physiological changes, including vestibular-related posture control and metabolism in mice. Body mass did not return to the preloading level in 1.67 G and 2 G mice. A significant drop in food intake, observed on the first day of hypergravity load, disappeared in all mice after longer exposure. However, a reduction in water intake was sustained in 2 G mice but not 1.33 G and 1.67 G mice. Body temperature did not return to the preloading level in 2 G mice by the final day. A decrease in the skill of the righting reflex was observed in 2 G mice but not 1.33 G and 1.67 G mice. In conclusion, this study showed that hypergravity-induced changes in metabolism and vestibular function depended on the amount of gravitational loading. The 2 G load affected vestibular-related posture control and metabolism considerably, compared with 1.33 G and 1.67 G loads.NEW & NOTEWORTHY It is unclear whether the degree of change in vestibular-related physiological function depends on gravitational loading. Present study showed that exposure to hypergravity-induced degrees of change in metabolism and vestibular function depended on the gravitational loading. The response of body mass depended on the gravitational loading size. Especially in 2 G environment, water intake, body temperature, and vestibular function were influenced. These changes could involve plastic alteration of vestibular-related autonomic and motor functions.

Effects of vestibular rehabilitation on body composition and daily-living physical activity in chronic unilateral vestibular hypofunction

BACKGROUND

Unilateral vestibular hypofunction (UVH) may lead to modifications on metabolism and body composition. Vestibular rehabilitation (VR) demonstrated its effectiveness in ameliorating balance function and several other daily-living aspects.

OBJECTIVES

The aim of this study was to evaluate metabolic composition, by means of bioelectrical impedance analysis (BIA) and daily activity, with the use of a wrist-worn movement tracker, in UVH participants before and after VR, and to compare data with a healthy control group (CG) of adults.

METHODS

46 UVH and 60 CG participants underwent otoneurological testing, self-report and performance questionnaires, BIA, and wore a device tracking daily movement and energy expenditure for one full day; this was performed before and after VR.

RESULTS

UVH participants demonstrated a significant (p = 0.008) increase in muscle mass after VR, and, when compared to CG, no differences were present with respect to visceral fat and muscle mass. UVH adults reported a significant increase in energy expenditure spent in movement (p = 0.008) and during the day (p = 0.009), daily number of strides (p = 0.009) and calories spent in sweeping (p = 0.009) and stairing (p = 0.008).

CONCLUSIONS

Results from this study show that VR provided an improvement of metabolic function and body composition of people with UVH, possibly by contrasting structural modifications in neural pathways stemming from the vestibular nuclei and connected to autonomous function.

The Role of the Cerebellar and Vestibular Networks in Anxiety Disorders and Depression: the Internal Model Hypothesis

Clinical data and animal studies confirmed that the cerebellum and the vestibular system are involved in emotions. Nowadays, no real consensus has really emerged to explain the clinical symptoms in humans and behavioral deficits in the animal models. We envisage here that the cerebellum and the vestibular system play complementary roles in emotional reactivity. The cerebellum integrates a large variety of exteroceptive and proprioceptive information necessary to elaborate and to update the internal model: in emotion, as in motor processes, it helps our body and self to adapt to the environment, and to anticipate any changes in such environment in order to produce a time-adapted response. The vestibular system provides relevant environmental stimuli (i.e., gravity, self-position, and movement) and is involved in self-perception. Consequently, cerebellar or vestibular disorders could generate « internal fake news» (due to lack or false sensory information and/or integration) that could, in turn, generate potential internal model deficiencies. In this case, the alterations provoke false anticipation of motor command and external sensory feedback, associated with unsuited behaviors. As a result, the individual becomes progressively unable to cope with the environmental solicitation. We postulate that chronically unsuited, and potentially inefficient, behavioral and visceral responses to environmental solicitations lead to stressful situations. Furthermore, this inability to adapt to the context of the situation generates chronic anxiety which could precede depressive states.

The application of artificial gravity in medicine and space

Gravity plays a crucial role in physiology. The lack of gravity, like in long duration spaceflight missions, cause pathologies in e.g., the musculoskeletal system, cardiovascular deconditioning, immune system deprivation or brain abnormalities, to just mention a few. The application of artificial gravity through short-arm human centrifugation (SAHC) has been studied as a possible countermeasure to treat spaceflight deconditioning. However, hypergravity protocols applied by using SAHC have also been used to treat different, ground-based pathologies. Such gravitational therapies have been applied in Uruguay for more than four decades now. The aim of this overview is to summarize the most important findings about the effects of gravitational therapy in different, mainly vascular based pathologies according to the experience in the Gravitational Therapy Center and to discuss the current research in the field of hypergravity applications in medicine but also as multisystem countermeasure for near weightlessness pathologies. New insight is needed on the use of hypergravity in medicine and space research and application.

Whole-brain monosynaptic outputs and presynaptic inputs of GABAergic neurons in the vestibular nuclei complex of mice

GABAergic neurons in the vestibular nuclei (VN) participate in multiple vital vestibular sensory processing allowing for the maintenance and rehabilitation of vestibular functions. However, although the important role of GABA in the central vestibular system has been widely reported, the underlying neural circuits between VN GABAergic neurons and other brain functional regions remain elusive, which limits the further study of the underlying mechanism. Hence, it is necessary to elucidate neural connectivity based on outputs and inputs of GABAergic neurons in the VN. This study employed a modified rabies virus retrograde tracing vector and cre-dependent adeno-associated viruses (AAVs) anterograde tracing vector, combined with a transgenic VGAT-IRES-Cre mice, to map the inputs and outputs of VN GABAergic neurons in the whole brain. We found that 51 discrete brain regions received projections from VN GABAergic neurons in the whole brain, and there were 77 upstream nuclei innervating GABAergic neurons in the VN. These nuclei were mainly located in four brain regions, including the medulla, pons, midbrain, and cerebellum. Among them, VN GABAergic neurons established neural circuits with some functional nuclei in the whole brain, especially regulating balance maintenance, emotion control, pain processing, sleep and circadian rhythm regulation, and fluid homeostasis. Therefore, this study deepens a comprehensive understanding of the whole-brain neural connectivity of VN, providing the neuroanatomical information for further research on the neural mechanism of the co-morbidities with vestibular dysfunction.

Hypergravity load-induced hyperglycemia occurs due to hypothermia and increased plasma corticosterone level in mice

Hypothermia has been observed during hypergravity load in mice and rats. This response is beneficial for maintaining blood glucose level, although food intake decreases. However, saving glucose is not enough to maintain blood glucose level during hypergravity load. In this study, we examined the contribution of humoral factors related to glycolysis in maintaining blood glucose level in a 2 G environment. Increased plasma corticosterone levels were observed in mice with intact peripheral vestibular organs, but not in mice with vestibular lesions. Plasma glucagon levels did not change, and decrease in plasma adrenaline levels was observed in mice with intact peripheral vestibular organs. Accordingly, it is possible that increase in plasma corticosterone level and hypothermia contribute to prevent hypoglycemia in a 2 G environment.

Smell Impairment in Stage I-II Obesity: Correlation with Biochemical Regulators and Clinical Aspects

OBJECTIVE

To evaluate the differences in olfactory sensitivity, nutritional habits, levels of modulators of feeding and smell, bioelectrical impedance analysis (BIA) measures and metabolic assays between two groups of participants with stage I and II obesity and reciprocal relationships between these parameters.

METHODS

Eighteen participants with stage I (11 female; mean age = 54.3 ± 13.1 years) and 20 participants with stage II (10 female; mean age = 54.5 ± 11.9) obesity underwent a food frequency questionnaire and Sniffin' Sticks® test battery, anthropometric parameters, and BIA measurements as well as metabolic assays (including plasma levels of leptin, insulin, ghrelin, glucose, insulin-like growth factor-1 [IGF-1] and usual laboratory parameters).

RESULTS

The stage II obesity participants demonstrated significant higher levels of insulin and leptin and lower levels of ghrelin and IGF-1, a reduction in odor identification (OI) and in total olfactory score, and an increase in visceral and total fat percentage. Among a mosaic of multiple correlations, ghrelin was found to positively correlate with OI and leptin negatively with odor discrimination.

CONCLUSION

The present study expands the notions positing the olfactory perception - and its connections with metabolic cues, foods habits and BIA measures - changes across the two most important obesity stages. This could ameliorate clinical and research deepening of obesity-related olfactory behavior with possible consequences on diagnosis, treatment and prevention of onset and development of obesity, thus opening possible future strategies involving multidisciplinary contributions.

LEVEL OF EVIDENCE

Level 3 Laryngoscope, 2022.

Effects of Vestibular Damage on the Sleep and Expression Level of Orexin in the Hypothalamus of Rats and Its Correlation with Autophagy and Akt Tumor Signal Pathway

The aim of this study was to investigate the effect of vestibular disruption on autophagy-related proteins and the tumour-associated pathway P13K/Akt in rat sleep and its hypothalamus tissue and to examine whether catechins trigger tumour autophagy. Healthy adult male rats were randomly selected and divided into the vestibular damage group, the sham operation group, and the control group, with 8 rats in each group. A vestibular damage model was established through penetrating the tympanic membrane of the external auditory canal by injecting sodium p-aminophenylarsonate. The electroencephalogram (EGG) activity was used to record the sleep-wakefulness cycle of rats, and the expression levels of hypothalamic orexin (orexin) mRNA and autophagy proteins were detected. Primary hippocampal neurons were intervened with orexin at different concentrations and at different times to detect cell viability and the expression of autophagy protein and P13K/Akt signal pathway protein. The results showed that compared with the control group and the sham operation group, NREM duration in the vestibular damage group decreased significantly (P < 0.05), while its  W  time increased significantly (P < 0.05). The expression level of orexin mRNA in the hypothalamus of the vestibular damage group was significantly higher than that of the other two groups (P < 0.05), the expression of autophagy microtubule-related proteins LC3B and Beclin-1 increased significantly (P < 0.05), and the protein expression level of p62 decreased significantly (P < 0.05). After orexin intervention, compared with the control group, the expression of Beclin-1 protein that positively correlated with autophagy decreased significantly (P < 0.05) and the expression of mTOR, PDK1, and Akt protein increased significantly (P < 0.05). Compared with the orexin intervention group, the expression of Beclin-1 and LC3B proteins in cells of the orexin receptor inhibitor (Almorexant) group, the autophagy activator (Rapamycin) group, the orexin + Almorexant group, and the orexin + Rapamycin group increased significantly (P < 0.05), and the expression of mTOR, PDK1, and Akt proteins decreased significantly (P < 0.05). Catechins trigger autophagy in part by regulating the p-Akt/p-mTOR and P13K pathways and by stimulating the MAPK pathway. Catechins initiate apoptosis in common tumour types of hepatocellular carcinoma cells by activating autophagy-related pathways. The conclusion is that vestibular damage can affect the sleep-wakefulness cycle of rats; the level of autophagy in hypothalamic tissue is upregulated and may affect cell proliferation and activity through mTOR-P13K/Akt, which has a certain reference value for tumor formation and provides a basis for the research of insomnia or sleep disorders caused by tumors. Autophagy activation is a key process by which catechins promote apoptosis in tumour cells, providing an avenue for more research on the use of catechins-rich diets for cardiovascular protection in the treatment of tumours.

Hormones and Vestibular Disorders: The Quest for Biomarkers

The vestibular system exerts control over various functions through neural pathways that are not yet fully mapped. Functional dysregulations or tissue lesions at different levels of the peripheral and the central vestibular networks can alter these different functions, causing a wide variety of symptoms, ranging from posturo-locomotor alterations to psychiatric syndromes such as PPPD, including the deregulation of the main biological functions. These different symptoms differ by their expression kinetics (they each appear and regress with their own kinetics) by the targets affected (muscles, organs, and brain areas) and by the sensitivity specific to each individual. Vestibular pathologies thus cover a mosaic of distinct effects, and they involve various effectors—which constitute the many markers of their different types and stages. It is therefore crucial, to predict the onset of a vertigo syndrome, to follow its temporal course, or to monitor the impact of therapeutic approaches, and to have specific and reliable biomarkers. Hormonal variations are among the possible sources of biomarkers for neurotology. We know that specific hormonal profiles can promote the appearance of vestibular disorders. We also know that the expression of vertigo syndrome is accompanied by measurable hormonal variations. The link between endocrine deregulation and vestibular alterations therefore no longer needs to be proven. However, there are still few data on their precise correlations with the vertigo syndrome. This study was undertaken with the aim to deliver an extensive review of the hormonal alterations linked to vestibular disorders. A review of the literature covering the last two decades was carried out using the MEDLINE and COCHRANE databases in order to identify studies associating the terms vestibular system or vestibular pathologies and hormones. Bibliographic data provides several outcomes in terms of therapeutic innovation in the diagnosis and therapeutic follow-up of vestibular pathologies.

Vestibular-autonomic interactions: beyond orthostatic dizziness

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

Neuro-consequences of the spaceflight environment

As human space exploration advances to establish a permanent presence beyond the Low Earth Orbit (LEO) with NASA's Artemis mission, researchers are striving to understand and address the health challenges of living and working in the spaceflight environment. Exposure to ionizing radiation, microgravity, isolation and other spaceflight hazards pose significant risks to astronauts. Determining neurobiological and neurobehavioral responses, understanding physiological responses under Central Nervous System (CNS) control, and identifying putative mechanisms to inform countermeasure development are critically important to ensuring brain and behavioral health of crew on long duration missions. Here we provide a detailed and comprehensive review of the effects of spaceflight and of ground-based spaceflight analogs, including simulated weightlessness, social isolation, and ionizing radiation on humans and animals. Further, we discuss dietary and non-dietary countermeasures including artificial gravity and antioxidants, among others. Significant future work is needed to ensure that neural, sensorimotor, cognitive and other physiological functions are maintained during extended deep space missions to avoid potentially catastrophic health and safety outcomes.

Vestibular Illusions and Alterations in Aerospace Environment

As the aerospace industry has grown rapidly over the years, aviators and astronauts have been exposed to some abnormal physiological changes arising from the dynamics of the aerospace environment. The vestibular system, encoding linear and angular movements of the head, is one of the main affected systems in which those abnormal changes can occur during flight. Despite the intricate and solid organization, vestibular units are such delicate structures that they can easily be deceived by aerial dynamics and gravity changes. Therefore, it is of vital importance for the continuity of flight safety to be aware of the detrimental alterations and impairments regarding the vestibular system and its reflex pathways. The aim of this paper was to present a review about how a healthy vestibular system is negatively affected within the aerospace environment and how some vestibular disorders become exaggerated or impaired during aviation and space activities.

Changes in daily energy expenditure and movement behavior in unilateral vestibular hypofunction: Relationships with neuro-otological parameters

The vestibular system has been found to affect energy homeostasis and body composition, due to its extensive connections to the brainstem and melanocortin nuclei involved in regulating the metabolism and feeding behavior. The aim of this study was to evaluate - by means of a wrist-worn physical activity tracker and bioelectrical impedance analysis (BIA) - the energy expenditure (EE) in resting (REE) and free-living conditions and movement behavior in a group of chronic unilateral vestibular hypofunction (UVH) patients when compared with a control group (CG) of healthy participants. Forty-six chronic UVH and 60 CG participants underwent otoneurological (including video-Head Impulse Test [vHIT] for studying vestibulo-ocular reflex [VOR] and static posturography testing [SPT]), and EE and movement measurements and self-report (SRM) andperformance measures (PM). As well as significant (p < 0.001) changes in SPT variables (area and path length) and SRM/PM, UVH participants also demonstrated significantly (p < 0.001) lower values in REE, movement EE, hours/day spent upright, number of strides and distance covered and total daily EE (p = 0.007) compared to the CG. UVH patients consumed significantly lower Kcal/min in sweeping (p = 0.001) and walking upstairs and downstairs (p < 0.001) compared to the CG. Multiple correlations were found between free-living and resting EE and neuro-otological parameters in UVH participants. Since the melanocortin system could be affected along the central vestibular pathways as a consequence of chronic vestibular deafferentation, data collected by reliable wearables could reflect the phenomena that constitute an increased risk of falls and sedentary lifestyle for patients affected by UVH, and could improve rehabilitation stages.

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.

Twenty-four-hour variation of vestibular function in young and elderly adults

Animal and human studies demonstrate anatomical and functional links between the vestibular nuclei and the circadian timing system. This promotes the hypothesis of a circadian rhythm of vestibular function. The objective of this study was to evaluate the vestibular function through the vestibulo-ocular reflex using a rotatory chair at different times of the day to assess circadian rhythmicity of vestibular function. Two identical studies evaluating temporal variation of the vestibulo-ocular reflex (VOR) were performed, the first in young adults (age: 22.4 ± 1.5 y), and the second in older adults (70.7 ± 4.7 y). The slow phase velocity and time constant of the VOR were evaluated in six separate test sessions, i.e., 02:00, 06:00, 10:00, 14:00, 18:00, and 22:00 h. In both studies, markers of circadian rhythmicity (temperature, fatigue, and sleepiness) displayed expected usual temporal variation. In young adults, the time constant of the VOR showed variation throughout the day (p < .005), being maximum 12:25 h (06:00 h test session) before the acrophase of temperature circadian rhythm. In older adults, the slow phase velocity and time constant also displayed temporal variation (p < .05). Maximum values were recorded at 10:35 h (06:00 h test session) before the acrophase of temperature circadian rhythm. The present study demonstrates that vestibular function is not constant throughout the day. The implication of the temporal variation in vestibular system in equilibrium potentially exposes the elderly, in particular, to differential risk during the 24 h of losing balance and falling.

Suprachiasmatic VIP neurons are required for normal circadian rhythmicity and comprised of molecularly distinct subpopulations

The hypothalamic suprachiasmatic (SCN) clock contains several neurochemically defined cell groups that contribute to the genesis of circadian rhythms. Using cell-specific and genetically targeted approaches we have confirmed an indispensable role for vasoactive intestinal polypeptide-expressing SCN (SCN^VIP) neurons, including their molecular clock, in generating the mammalian locomotor activity (LMA) circadian rhythm. Optogenetic-assisted circuit mapping revealed functional, di-synaptic connectivity between SCN^VIP neurons and dorsomedial hypothalamic neurons, providing a circuit substrate by which SCN^VIP neurons may regulate LMA rhythms. In vivo photometry revealed that while SCN^VIP neurons are acutely responsive to light, their activity is otherwise behavioral state invariant. Single-nuclei RNA-sequencing revealed that SCN^VIP neurons comprise two transcriptionally distinct subtypes, including putative pacemaker and non-pacemaker populations. Altogether, our work establishes necessity of SCN^VIP neurons for the LMA circadian rhythm, elucidates organization of circadian outflow from and modulatory input to SCN^VIP cells, and demonstrates a subpopulation-level molecular heterogeneity that suggests distinct functions for specific SCN^VIP subtypes.

Cell groups in the hypothalamic suprachiasmatic clock contribute to the genesis of circadian rhythms. The authors identified two populations of vasoactive intestinal polypeptide-expressing neurons in the suprachiasmatic nucleus which regulate locomotor circadian rhythm in mice.

Effect of vestibular stimulation using a rotatory chair in human rest/activity rhythm

The vestibular system is responsible for sensing every angular and linear head acceleration, mainly during periods of motor activity. Previous animal and human experiments have shown biological rhythm disruptions in small rodents exposed to a hypergravity environment, but also in patients with bilateral vestibular loss compared to a control population. This raised the hypothesis of the vestibular afferent influence on circadian rhythm synchronization. The present study aimed to test the impact of vestibular stimulation induced by a rotatory chair on the rest/activity rhythm in human subjects. Thirty-four healthy adults underwent both sham (SHAM) and vestibular stimulation (STIM) sessions scheduled at 18:00 h. An off-vertical axis rotation on a rotatory chair was used to ecologically stimulate the vestibular system by head accelerations. The rest/activity rhythm was continuously registered by actigraphy. The recording started one week before the first session (BASELINE), continued in the week between the two sessions and one week after the second session. Vestibular stimulation caused a significant decrease in the average activity level in the evening following the vestibular stimulation. A significant phase advance in the rest/activity rhythm occurred two days after the 18:00 h vestibular stimulation session. Moreover, the level of motion sickness symptoms increased significantly after vestibular stimulation. The present study confirms previous results on the effect of vestibular stimulation and the role of vestibular afferents on circadian biological rhythmicity. Our results support the hypothesis of the implication of vestibular afferents as non-photic stimuli acting on circadian rhythms.

Anti-diabetic effect of vestibular stimulation is mediated via AMP-activated protein kinase

There is abundant animal evidence that vestibular stimulation, particularly of the otolith organs, can trigger a shift in body mass composition towards a leaner physique. One way of non-invasively stimulating the otolith organs is via a small electrical current applied to the skin behind the ears. This technique is called vestibular nerve stimulation, or VeNS, and is believed to have a good safety profile. Thus, it has previously been argued that VeNS could be used in human health as a means of treating the complications of metabolic syndrome, such as obesity and type 2 diabetes mellitus. Weight loss itself is known to improve diabetic control, however, tantalizing evidence is now emerging that the improvements seen in the glycemic control of type 2 diabetics who undergo repeated vestibular stimulation are significantly better than would be expected on the basis of weight loss alone. As vestibular stimulation has been shown to increase levels of an anti-inflammatory protein, called sirtuin 1, we hypothesize here that VeNS will increase levels of an associated enzyme called AMP-activated protein kinase (AMPK). AMPK plays an important role in glucose and fat metabolism and is activated by exercise, as well as being a known target for certain anti-diabetic drugs. This hypothesis is readily amenable to clinical testing as specific assays for testing human AMPK are available. If substantiated, then this hypothesis could prove an important clinical insight and potentially offer a new treatment avenue for patients with type 2 diabetes; a condition which remains a major cause of morbidity and premature mortality worldwide.

Vestibular stimulation by 2G hypergravity modifies resynchronization in temperature rhythm in rats

Input from the light/dark (LD) cycle constitutes the primary synchronizing stimulus for the suprachiasmatic nucleus (SCN) circadian clock. However, the SCN can also be synchronized by non-photic inputs. Here, we hypothesized that the vestibular system, which detects head motion and orientation relative to gravity, may provide sensory inputs to synchronize circadian rhythmicity. We investigated the resynchronization of core temperature (Tc) circadian rhythm to a six-hour phase advance of the LD cycle (LD + 6) using hypergravity (2 G) as a vestibular stimulation in control and bilateral vestibular loss (BVL) rats. Three conditions were tested: an LD + 6 exposure alone, a series of seven 2 G pulses without LD + 6, and a series of seven one-hour 2 G pulses (once a day) following LD + 6. First, following LD + 6, sham rats exposed to 2 G pulses resynchronized earlier than BVL rats (p = 0.01), and earlier than sham rats exposed to LD + 6 alone (p = 0.002). Each 2 G pulse caused an acute drop of Tc in sham rats (-2.8 ± 0.3 °C; p < 0.001), while BVL rats remained unaffected. This confirms that the vestibular system influences chronobiological regulation and supports the hypothesis that vestibular input, like physical activity, should be considered as a potent time cue for biological rhythm synchronization, acting in synergy with the visual system.

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.

Roles of the vestibular system in obesity and impaired glucose metabolism in high-fat diet-fed mice

The vestibular system controls balance, posture, blood pressure, and gaze. However, the roles of the vestibular system in energy and glucose metabolism remain unknown. We herein examined the roles of the vestibular system in obesity and impaired glucose metabolism using mice with vestibular lesions (VL) fed a high-sucrose/high-fat diet (HSHFD). VL was induced by surgery or arsenic. VL significantly suppressed body fat enhanced by HSHFD in mice. Glucose intolerance was improved by VL in mice fed HSHFD. VL blunted the levels of adipogenic factors and pro-inflammatory adipokines elevated by HSHFD in the epididymal white adipose tissue of mice. A β-blocker antagonized body fat and glucose intolerance enhanced by HSHFD in mice. The results of an RNA sequencing analysis showed that HSHFD induced alterations in genes, such as insulin-like growth factor-2 and glial fibrillary acidic protein, in the vestibular nuclei of mice through the vestibular system. In conclusion, we herein demonstrated that the dysregulation of the vestibular system influences an obese state and impaired glucose metabolism induced by HSHFD in mice. The vestibular system may contribute to the regulation of set points under excess energy conditions.

The influence of vestibular stimulation on metabolism and body composition

Obesity, diabetes and metabolic disease represent an ongoing and rapidly worsening public health issue in both the developed, and much of the developing world. Although there are many factors that influence fat storage, it has been clearly demonstrated that the homeostatic cornerstone of metabolism lies within the hypothalamus. Moreover, neuronal damage to vital areas of the hypothalamus can drive reregulation or dysregulation of endocrine function, energy expenditure and appetite, thereby promoting a shift in overall metabolic function towards a state of obesity. Therefore, identification of treatments that influence the hypothalamus to improve obesity and associated metabolic diseases has long been a medical goal. Interestingly, evidence from animal studies suggests that activating the vestibular system, specifically the macular gravity receptor, influences the hypothalamus in a way that decreases body fat storage and causes a metabolic shift towards a leaner state. Given that the macular element of the vestibular system has been shown to activate with transdermal electrical stimulation applied to the mastoids, this may be a potential therapeutic approach for obesity, diabetes or related metabolic diseases, whereby repetitive stimulation of the vestibular system influences hypothalamic control of metabolic homeostasis, thereby encouraging decreased fat storage. Here, we present an up-to-date review of the current literature surrounding the vestibular influence of the hypothalamus and associated homeostatic sites in the context of current and novel therapeutic approaches for improved clinical management of obesity and diabetes.

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.

Cooperation of the vestibular and cerebellar networks in anxiety disorders and depression

The discipline of affective neuroscience is concerned with the neural bases of emotion and mood. The past decades have witnessed an explosion of research in affective neuroscience, increasing our knowledge of the brain areas involved in fear and anxiety. Besides the brain areas that are classically associated with emotional reactivity, accumulating evidence indicates that both the vestibular and cerebellar systems are involved not only in motor coordination but also influence both cognition and emotional regulation in humans and animal models. The cerebellar and the vestibular systems show the reciprocal connection with a myriad of anxiety and fear brain areas. Perception anticipation and action are also major centers of interest in cognitive neurosciences. The cerebellum is crucial for the development of an internal model of action and the vestibular system is relevant for perception, gravity-related balance, navigation and motor decision-making. Furthermore, there are close relationships between these two systems. With regard to the cooperation between the vestibular and cerebellar systems for the elaboration and the coordination of emotional cognitive and visceral responses, we propose that altering the function of one of the systems could provoke internal model disturbances and, as a result, anxiety disorders followed potentially with depressive states.

Effects of hypergravity on the gene expression of the hypothalamic feeding-related neuropeptides in mice via vestibular inputs

The effects of hypergravity on the gene expression of the hypothalamic feeding-related neuropeptides in sham-operated (Sham) and vestibular-lesioned (VL) mice were examined by in situ hybridization histochemistry. Corticotrophin-releasing hormone (CRH) in the paraventricular nucleus was increased significantly in Sham but not in VL mice after 3 days of exposure to a 2 g environment compared with a 1 g environment. Significant decreases in pro-opiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript and significant increases in neuropeptide Y, agouti-related protein in the arcuate nucleus and orexin in the lateral hypothalamic area were observed in both Sham and VL mice. After 2 weeks of exposure, CRH and POMC were increased significantly in Sham but not in VL mice. After 8 weeks of exposure, the hypothalamic feeding-related neuropeptides were comparable between Sham and VL mice. These results suggest that the hypothalamic feeding-related neuropeptides may be affected during the exposed duration of hypergravity via vestibular inputs.

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.

International roadmap for artificial gravity research

In this paper, we summarize the current and future research activities that will determine the requirements for implementing artificial gravity (AG) to mitigate the effects of long duration exposure to microgravity on board exploration class space vehicles. NASA and its international partners have developed an AG roadmap that contains a common set of goals, objectives, and milestones. This roadmap includes both ground-based and space-based projects, and involves human subjects as well as animal and cell models. It provides a framework that facilitates opportunities for collaboration using the full range of AG facilities that are available worldwide, and a forum for space physiologists, crew surgeons, astronauts, vehicle designers, and mission planners to review, evaluate, and discuss the issues of incorporating AG technologies into the vehicle design.

Protein expression changes caused by spaceflight as measured for 18 Russian cosmonauts

The effects of spaceflight on human physiology is an increasingly studied field, yet the molecular mechanisms driving physiological changes remain unknown. With that in mind, this study was performed to obtain a deeper understanding of changes to the human proteome during space travel, by quantitating a panel of 125 proteins in the blood plasma of 18 Russian cosmonauts who had conducted long-duration missions to the International Space Station. The panel of labeled prototypic tryptic peptides from these proteins covered a concentration range of more than 5 orders of magnitude in human plasma. Quantitation was achieved by a well-established and highly-regarded targeted mass spectrometry approach involving multiple reaction monitoring in conjunction with stable isotope-labeled standards. Linear discriminant function analysis of the quantitative results revealed three distinct groups of proteins: 1) proteins with post-flight protein concentrations remaining stable, 2) proteins whose concentrations recovered slowly, or 3) proteins whose concentrations recovered rapidly to their pre-flight levels. Using a systems biology approach, nearly all of the reacting proteins could be linked to pathways that regulate the activities of proteases, natural immunity, lipid metabolism, coagulation cascades, or extracellular matrix metabolism.

Evidence for a Role of Orexin/Hypocretin System in Vestibular Lesion-Induced Locomotor Abnormalities in Rats

Vestibular damage can induce locomotor abnormalities in both animals and humans. Rodents with bilateral vestibular loss showed vestibular deficits syndrome such as circling, opisthotonus as well as locomotor and exploratory hyperactivity. Previous studies have investigated the changes in the dopamine system after vestibular loss, but the results are inconsistent and inconclusive. Numerous evidences indicate that the orexin system is implicated in central motor control. We hypothesized that orexin may be potentially involved in vestibular loss-induced motor disorders. In this study, we examined the effects of arsanilate- or 3,3′-iminodipropionitrile (IDPN)-induced vestibular lesion (AVL or IVL) on the orexin-A (OXA) labeling in rat hypothalamus using immunohistochemistry. The vestibular lesion-induced locomotor abnormalities were recorded and verified using a histamine H4 receptor antagonist JNJ7777120 (20 mg/kg, i.p.). The effects of the orexin receptor type 1 antagonist SB334867 (16 μg, i.c.v.) on these behavior responses were also investigated. At 72 h post-AVL and IVL, animals exhibited vestibular deficit syndrome and locomotor hyperactivity in the home cages. These responses were significantly alleviated by JNJ7777120 which also eliminated AVL-induced increases in exploratory behavior in an open field. The numbers of OXA-labeled neurons in the hypothalamus were significantly increased in the AVL animals at 72 h post-AVL and in the IVL animals at 24, 48, and 72 h post-IVL. SB334867 significantly attenuated the vestibular deficit syndrome and locomotor hyperactivity at 72 h post-AVL and IVL. It also decreased exploratory behavior in the AVL animals. These results suggested that the alteration of OXA expression might contribute to locomotor abnormalities after acute vestibular lesion. The orexin receptors might be the potential therapeutic targets for vestibular disorders.

Genetic Evidence for a Neurovestibular Influence on the Mammalian Circadian Pacemaker

The mammalian circadian timing system (CTS) exerts endogenous temporal control over virtually every biochemical, physiological, and neurobiological process. Recent studies have suggested an interrelationship between the neurovestibular system, specifically the macular otoconial gravity receptors, and the CTS. To test for a functional relationship between these 2 seemingly disparate neuronal systems, the authors performed a study to evaluate the influence of the vestibular system on 3 fundamental properties of the CTS: entrainment, photic modulation, and period. The present study used a nonrecombinant mutant mouse, the head-tilt mouse (abbr. het), which lacks otoconia and hence gravity reception, to evaluate CTS function in mice lacking vestibular inputs. Circadian rhythms of body temperature (Tb) and locomotor activity (ACT) were recorded continuously by biotelemetry in het mice as well as wild-type (PWT) controls during exposure to 4 photic regimens: 12:12 LD, DD (0 μmoles s-1m-2), constant bright light (LLB; 0.5 μmoles s-1m-2), and constant dim light (LLD; 0.02 μmoles s-1m-2). In DD, the circadian period of the Tband ACT rhythms was significantly longer ( p < 0.001) in het than in PWT mice. In addition, the circadian period of Tband ACT was significantly longer ( p < 0.01) in LLBthan in DD for both the het and PWT groups, although increasing ambient illuminance (i.e., DD to LLB) had a significantly greater ( p < 0.01) period-lengthening effect in the PWT group than in the het group. The results of the present study demonstrate for the first time that the vestibular macular gravity receptors influence 2 fundamental properties of the mammalian CTS: (1) the intrinsic circadian pacemaker period and (2) the period-altering response to changes in tonic light intensity. The results of the present study thus provide the first neurobehavioral evidence for a vestibular-circadian interrelationship as well as suggest a novel mechanism underlying the signaling of activity-based nonphotic stimuli to the CTS.

Exploration of Circadian Rhythms in Patients with Bilateral Vestibular Loss

BACKGROUND

New insights have expanded the influence of the vestibular system to the regulation of circadian rhythmicity. Indeed, hypergravity or bilateral vestibular loss (BVL) in rodents causes a disruption in their daily rhythmicity for several days. The vestibular system thus influences hypothalamic regulation of circadian rhythms on Earth, which raises the question of whether daily rhythms might be altered due to vestibular pathology in humans. The aim of this study was to evaluate human circadian rhythmicity in people presenting a total bilateral vestibular loss (BVL) in comparison with control participants.

METHODOLOGY AND PRINCIPAL FINDINGS

Nine patients presenting a total idiopathic BVL and 8 healthy participants were compared. Their rest-activity cycle was recorded by actigraphy at home over 2 weeks. The daily rhythm of temperature was continuously recorded using a telemetric device and salivary cortisol was recorded every 3 hours from 6:00AM to 9:00PM over 24 hours. BVL patients displayed a similar rest activity cycle during the day to control participants but had higher nocturnal actigraphy, mainly during weekdays. Sleep efficiency was reduced in patients compared to control participants. Patients had a marked temperature rhythm but with a significant phase advance (73 min) and a higher variability of the acrophase (from 2:24 PM to 9:25 PM) with no correlation to rest-activity cycle, contrary to healthy participants. Salivary cortisol levels were higher in patients compared to healthy people at any time of day.

CONCLUSION

We observed a marked circadian rhythmicity of temperature in patients with BVL, probably due to the influence of the light dark cycle. However, the lack of synchronization between the temperature and rest-activity cycle supports the hypothesis that the vestibular inputs are salient input to the circadian clock that enhance the stabilization and precision of both external and internal entrainment.

The impact of time of day on the gait and balance control of Alzheimer's patients

Alzheimer's patients suffer from circadian dysregulation. The aim of this study was to examine the evolution of balance control and gait at different times of the day (11:00, 14:00, 18:00) in order to identify whether Alzheimer's patients were more likely to fall at certain periods of the day. Spatio-temporal parameters of centre of foot pressure displacements were measured with a force platform and spatio-temporal parameters of walking were evaluated with a gait analysis device. The results highlighted that balance control was worse in the evening and the afternoon than in the morning. Furthermore, the walking speed was faster and support duration, swing duration and cycle duration were shorter in the evening than in the morning and afternoon. The combined analysis of balance control and gait parameters revealed that balance control and walking are concomitantly altered in the evening which increases the fall risk in the evening, in comparison with the morning, for Alzheimer's patients.

Motion Sickness: Current Knowledge and Recent Advance

Motion sickness (MS) is a common physiological response to real or virtual motion. Numerous studies have investigated the neurobiological mechanism and the control measures of MS. This review summarizes the current knowledge about pathogenesis and pathophysiology, prediction, evaluation, and countermeasures of MS. The sensory conflict hypothesis is the most widely accepted theory for MS. Both the hippocampus and vestibular cortex might play a role in forming internal model. The pathophysiology focuses on the visceral afference, thermoregulation and MS-related neuroendocrine. Single-nucleotide polymorphisms (SNPs) in some genes and epigenetic modulation might contribute to MS susceptibility and habituation. Questionnaires, heart rate variability (HRV) and electrogastrogram (EGG) are useful for diagnosing and evaluating MS. We also list MS medications to guide clinical practice. Repeated real motion exposure and combined visual-vestibular interaction training accelerate the progress of habituation. Behavioral and dietary countermeasures, as well as physiotherapy, are also effective in alleviating MS symptoms.

Self-motion direction discrimination in the visually impaired

Despite the close interrelation between vestibular and visual processing (e.g., vestibulo-ocular reflex), surprisingly little is known about vestibular function in visually impaired people. In this study, we investigated thresholds of passive whole-body motion discrimination (leftward vs. rightward) in nine visually impaired participants and nine age-matched sighted controls. Participants were rotated in yaw, tilted in roll, and translated along the interaural axis at two different frequencies (0.33 and 2 Hz) by means of a motion platform. Superior performance of visually impaired participants was found in the 0.33 Hz roll tilt condition. No differences were observed in the other motion conditions. Roll tilts stimulate the semicircular canals and otoliths simultaneously. The results could thus reflect a specific improvement in canal-otolith integration in the visually impaired and are consistent with the compensatory hypothesis, which implies that the visually impaired are able to compensate the absence of visual input.

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.