Unilateral inner ear damage results in lasting changes in hippocampal CA1 field potentials in vitro

Long-lasting spatial memory deficits and impaired hippocampal plasticity following unilateral vestibular loss

Unilateral vestibular loss (UVL) induces a characteristic vestibular syndrome composed of various posturo-locomotor, oculomotor, vegetative and perceptivo-cognitive symptoms. Functional deficits are progressively recovered over time during vestibular compensation, that is supported by the expression of multiscale plasticity mechanisms. While the dynamic of post-UVL posturo-locomotor and oculomotor deficits is well characterized, the expression over time of the cognitive deficits, and in particular spatial memory deficits, is still debated. In this study we aimed at investigating spatial memory deficits and their recovery in a rat model of unilateral vestibular neurectomy (UVN), using a wide spectrum of behavioral tasks. In parallel, we analyzed markers of hippocampal plasticity involved in learning and memory. Our results indicate the UVN affects all domains of spatial memory, from working memory to reference memory and object-in-place recognition. These deficits are associated with long-lasting impaired plasticity in the ipsilesional hippocampus. These results highlight the crucial role of symmetrical vestibular information in spatial memory and contribute to a better understanding of the cognitive disorders observed in vestibular patients.

Evidence for cognitive impairment in patients with vestibular disorders

OBJECTIVE

 Extensive animal research has shown that vestibular damage can be associated with cognitive deficits. More recently, new evidence has emerged linking vestibular disorders to cognitive impairment in humans. Herein, we review contemporary research on the pathophysiology of cognitive-vestibular interactions and discuss its emerging clinical relevance.

DATA SOURCES

PubMed, Embase, and Cochrane databases.

REVIEW METHODS

A systematic literature search was performed with combinations of search terms: "cognition," "cognitive impairment," "chronic fatigue," "brain fog," "spatial navigation," "attention," "memory," "executive function," "processing speed," and "vestibular hypofunction." Relevant articles were considered for inclusion, including basic and clinical studies, case series, and major reviews.

CONCLUSIONS

Patients with vestibular disorders can demonstrate long-term deficits in both spatial and nonspatial cognitive domains. The underlying mechanism(s) linking the vestibular system to cognitive function is not well characterized, but several neuro-biologic correlates have been identified. Additional screening tools are required to identify individuals at risk for cognitive impairment, and further research is needed to determine whether treatment of vestibular dysfunction has the capacity to improve cognitive function.

IMPLICATIONS FOR PRACTICE

Physicians should be aware of emerging data supporting the presence of cognitive deficits in patients with vestibular disorders. Prevention and treatment of long-term cognitive deficits may be possible through screening and rehabilitation.

The Differential Effects of Acute Right- vs. Left-Sided Vestibular Deafferentation on Spatial Cognition in Unilateral Labyrinthectomized Mice

This study aimed to investigate the disparity in locomotor and spatial memory deficits caused by left- or right-sided unilateral vestibular deafferentation (UVD) using a mouse model of unilateral labyrinthectomy (UL) and to examine the effects of galvanic vestibular stimulation (GVS) on the deficits over 14 days. Five experimental groups were established: the left-sided and right-sided UL (Lt.-UL and Rt.-UL) groups, left-sided and right-sided UL with bipolar GVS with the cathode on the lesion side (Lt.-GVS and Rt.-GVS) groups, and a control group with sham surgery. We assessed the locomotor and cognitive-behavioral functions using the open field (OF), Y maze, and Morris water maze (MWM) tests before (baseline) and 3, 7, and 14 days after surgical UL in each group. On postoperative day (POD) 3, locomotion and spatial working memory were more impaired in the Lt.-UL group compared with the Rt.-UL group (p < 0.01, Tamhane test). On POD 7, there was a substantial difference between the groups; the locomotion and spatial navigation of the Lt.-UL group recovered significantly more slowly compared with those of the Rt.-UL group. Although the differences in the short-term spatial cognition and motor coordination were resolved by POD 14, the long-term spatial navigation deficits assessed by the MWM were significantly worse in the Lt.-UL group compared with the Rt.-UL group. GVS intervention accelerated the vestibular compensation in both the Lt.-GVS and Rt.-GVS groups in terms of improvement of locomotion and spatial cognition. The current data imply that right- and left-sided UVD impair spatial cognition and locomotion differently and result in different compensatory patterns. Sequential bipolar GVS when the cathode (stimulating) was assigned to the lesion side accelerated recovery for UVD-induced spatial cognition, which may have implications for managing the patients with spatial cognitive impairment, especially that induced by unilateral peripheral vestibular damage on the dominant side.

Galvanic Vestibular Stimulation Improves Spatial Cognition After Unilateral Labyrinthectomy in Mice

Objectives: To investigate the deficits of spatial memory and navigation from unilateral vestibular deafferentation (UVD) and to determine the efficacy of galvanic vestibular stimulation (GVS) for recovery from these deficits using a mouse model of unilateral labyrinthectomy (UL).

Methods: Thirty-six male C57BL/6 mice were allocated into three groups that comprise a control group and two experimental groups, UVD with (GVS group) and without GVS intervention (non-GVS group). In the experimental groups, we assessed the locomotor and cognitive behavioral function before (baseline) and 3, 7, and 14 days after surgical UL, using the open field (OF), Y maze, and Morris water maze (MWM) tests. In the GVS group, the stimulations were applied for 30 min daily from postoperative day (POD) 0–4 via the electrodes inserted subcutaneously close to both bony labyrinths.

Results: Locomotion and spatial cognition were significantly impaired in the mice with UVD non-GVS group compared to the control group. GVS significantly accelerated recovery of locomotion compared to the control and non-GVS groups on PODs 3 (p < 0.001) and 7 (p < 0.05, Kruskal–Wallis and Mann–Whitney U tests) in the OF and Y maze tests. The mice in the GVS group were better in spatial working memory assessed with spontaneous alternation performance and spatial reference memory assessed with place recognition during the Y maze test than those in the non-GVS group on POD 3 (p < 0.001). In addition, the recovery of long-term spatial navigation deficits during the MWM, as indicated by the escape latency and the probe trial, was significantly better in the GVS group than in the non-GVS group 2 weeks after UVD (p < 0.01).

Conclusions: UVD impairs spatial memory, navigation, and motor coordination. GVS accelerated recoveries in short- and long-term spatial memory and navigation, as well as locomotor function in mice with UVD, and may be applied to the patients with acute unilateral vestibular failure.

Stratification of hippocampal electrophysiological activation evoked by selective electrical stimulation of different angular and linear acceleration sensors in the rat peripheral vestibular system

It has become well established that vestibular information is important for hippocampal function and spatial memory. However, as yet, relatively little is known about how different kinds of vestibular information are 'represented' in different parts of the hippocampus. This study used selective electrical stimulation of each of the 5 vestibular sensors (the horizontal (HC), anterior (AC) and posterior (PC) semi-circular canals, and the utricle and saccule) in the rat and recorded local field potentials (LFPs) across the hippocampus, using a 16 electrode microarray. We found that stimulation of any vestibular sensor in the left labyrinth evoked triphasic LFPs in both hippocampi, although it was clear that, in general, the amplitudes were greater for the right, contralateral side. This was particularly true for Phase 1 for the HC, AC, utricle and saccule, Phase 2 for the HC, PC, utricle and saccule, and Phase 3 for the AC, PC and saccule. Overall, our results suggest that vestibular input to the hippocampus is bilateral, preferentially contralateral, but highly stratified in that stimulation of the same vestibular sensor results in activation of different specific areas of the hippocampus, with different LFP amplitudes and latencies. This suggests the possibility that different regions of the hippocampus use different kinds of vestibular information for different purposes and that there may be a high degree of redundancy in the representation of vestibular input, perhaps ensuring that the hippocampus is more robust to the partial loss of vestibular information.

Vestibular Modulation of Long-Term Potentiation and NMDA Receptor Expression in the Hippocampus

Loss of vestibular function is known to cause spatial memory deficits and hippocampal dysfunction, in terms of impaired place cell firing and abnormal theta rhythm. Based on these results, it has been of interest to determine whether vestibular loss also affects the development and maintenance of long-term potentiation (LTP) in the hippocampus. This article summarizes and critically reviews the studies of hippocampal LTP following a vestibular loss and its relationship to NMDA receptor expression, that have been published to date. Although the available in vitro studies indicate that unilateral vestibular loss (UVL) results in reduced hippocampal field potentials in CA1 and the dentate gyrus (DG), the in vivo studies involving bilateral vestibular loss (BVL) do not. This may be due to the differences between UVL and BVL or it could be a result of in vitro/in vivo differences. One in vitro study reported a decrease in LTP in hippocampal slices following UVL; however, the two available in vivo studies have reported different results: either no effect or an increase in EPSP/Population Spike (ES) potentiation. This discrepancy may be due to the different high-frequency stimulation (HFS) paradigms used to induce LTP. The increased ES potentiation following BVL may be related to an increase in synaptic NMDA receptors, possibly increasing the flow of vestibular input coming into CA1, with a loss of selectivity. This might cause increased excitability and synaptic noise, which might lead to a degradation of spatial learning and memory.

Reduction of long-term potentiation at Schaffer collateral-CA1 synapses in the rat hippocampus at the acute stage of vestibular compensation

Vestibular compensation is a recovery process from vestibular symptoms over time after unilateral loss of peripheral vestibular end organs. The aim of the present study was to observe time-dependent changes in long-term potentiation (LTP) at Schaffer collateral-CA1 synapses in the CA1 area of the hippocampus during vestibular compensation. The input-output (I/O) relationships of fEPSP amplitudes and LTP induced by theta burst stimulation to Schaffer's collateral commissural fibers were evaluated from the CA1 area of hippocampal slices at 1 day, 1 week, and 1 month after unilateral labyrinthectomy (UL). The I/O relationships of fEPSPs in the CA1 area was significantly reduced within 1 week post-op and then showed a non-significant reduction at 1 month after UL. Compared with sham-operated animals, there was a significant reduction of LTP induction in the hippocampus at 1 day and 1 week after UL. However, LTP induction levels in the CA1 area of the hippocampus also returned to those of sham-operated animals 1 month following UL. These data suggest that unilateral injury of the peripheral vestibular end organs results in a transient deficit in synaptic plasticity in the CA1 hippocampal area at acute stages of vestibular compensation.

Hippocampal gray matter volume in bilateral vestibular failure

Bilateral vestibular failure (BVF) is a severe chronic disorder of the labyrinth or the eighth cranial nerve characterized by unsteadiness of gait and disabling oscillopsia during head movements. According to animal data, vestibular input to the hippocampus is proposed to contribute to spatial memory and spatial navigation. Except for one seminal study showing the association of impaired spatial navigation and hippocampal atrophy, patient data in BVF are lacking. Therefore, we performed a voxel-wise comparison of the hippocampal gray matter volume (GMV) in a clinically representative sample of 27 patients with incomplete BVF and 29 age- and gender-matched healthy controls to test the hypothesis of hippocampal atrophy in BVF. Although the two groups did not generally differ in their hippocampal GMV, a reduction of GMV in the bilateral hippocampal CA3 region was significantly correlated with increased vestibulopathy-related clinical impairment. We propose that GMV reduction in the hippocampus of BVF patients is related to the severity of vestibular-induced disability which is in line with combined hippocampal atrophy and disorders of spatial navigation in complete vestibular deafferentation due to bilateral nerve section. Clinically, however, the most frequent etiologies of BVF cause incomplete lesions. Accordingly, hippocampus atrophy and deficits in spatial navigation occur possibly less frequently than previously suspected. Hum Brain Mapp 37:1998-2006, 2016. © 2016 Wiley Periodicals, Inc.

The Effects of Complete Vestibular Deafferentation on Spatial Memory and the Hippocampus in the Rat: The Dunedin Experience

Our studies conducted over the last 14 years have demonstrated that a complete bilateral vestibular deafferentation (BVD) in rats results in spatial memory deficits in a variety of behavioural tasks, such as the radial arm maze, the foraging task and the spatial T maze, as well as deficits in other tasks such as the five-choice serial reaction time task (5-CSRT task) and object recognition memory task. These deficits persist long after the BVD, and are not simply attributable to ataxia, anxiety, hearing loss or hyperactivity. In tasks such as the foraging task, the spatial memory deficits are evident in darkness when vision is not required to perform the task. The deficits in the radial arm maze, the foraging task and the spatial T maze, in particular, suggest hippocampal dysfunction following BVD, and this is supported by the finding that both hippocampal place cells and theta rhythm are dysfunctional in BVD rats. Now that it is clear that the hippocampus is adversely affected by BVD, the next challenge is to determine what vestibular information is transmitted to it and how that information is used by the hippocampus and the other brain structures with which it interacts.

Sequential [(18)F]FDG µPET whole-brain imaging of central vestibular compensation: a model of deafferentation-induced brain plasticity

Unilateral inner ear damage is followed by a rapid behavioural recovery due to central vestibular compensation. In this study, we utilized serial [(18)F]Fluoro-deoxyglucose ([(18)F]FDG)-µPET imaging in the rat to visualize changes in brain glucose metabolism during behavioural recovery after surgical and chemical unilateral labyrinthectomy, to determine the extent and time-course of the involvement of different brain regions in vestibular compensation and test previously described hypotheses of underlying mechanisms. Systematic patterns of relative changes of glucose metabolism (rCGM) were observed during vestibular compensation. A significant asymmetry of rCGM appeared in the vestibular nuclei, vestibulocerebellum, thalamus, multisensory vestibular cortex, hippocampus and amygdala in the acute phase of vestibular imbalance (4 h). This was followed by early vestibular compensation over 1-2 days where rCGM re-balanced between the vestibular nuclei, thalami and temporoparietal cortices and bilateral rCGM increase appeared in the hippocampus and amygdala. Subsequently over 2-7 days, rCGM increased in the ipsilesional spinal trigeminal nucleus and later (7-9 days) rCGM increased in the vestibulocerebellum bilaterally and the hypothalamus and persisted in the hippocampus. These systematic dynamic rCGM patterns during vestibular compensation, were confirmed in a second rat model of chemical unilateral labyrinthectomy by serial [(18)F]FDG-µPET. These findings show that deafferentation-induced plasticity after unilateral labyrinthectomy involves early mechanisms of re-balancing predominantly in the brainstem vestibular nuclei but also in thalamo-cortical and limbic areas, and indicate the contribution of spinocerebellar sensory inputs and vestibulocerebellar adaptation at the later stages of behavioural recovery.

Glutamate receptor subunit and calmodulin kinase II expression, with and without T maze training, in the rat hippocampus following bilateral vestibular deafferentation

Many previous studies have shown that lesions of the peripheral vestibular system result in spatial memory deficits and electrophysiological dysfunction in the hippocampus. Given the importance of glutamate as a neurotransmitter in the hippocampus, it was predicted that bilateral vestibular deafferentation (BVD) would alter the expression of NMDA and AMPA receptors in this area of the brain.

Spatial separation of visual and vestibular processing in the human hippocampal formation

The hippocampal formation, that is, the hippocampus proper and the parahippocampal region, is essential for various aspects of memory and plays an important role in human navigation. Navigational cues can be provided by both the visual system (e.g., landmarks, optic flow) and the vestibular system (e.g., estimation of direction during path integration). This study reviews anatomical, electrophysiological, and imaging data that support the view that vestibular input is primarily processed in the anterior part of the hippocampal formation, whereas visual cues are primarily integrated in the posterior part. In cases of reduced vestibular or visual input or excessive sensory stimulation, this hippocampal navigational network is reorganized. The separation of vestibular and visual information in the hippocampal formation has a twofold functional consequence: missing input from either system may be partially substituted for, and the task-dependent sensorial weight can be shifted to, the more reliable modality for navigation.

Vestibular information is necessary for maintaining metric properties of representational space: evidence from mental imagery

The vestibular system contributes to a wide range of functions, from postural and oculomotor reflexes to spatial representation and cognition. Vestibular signals are important to maintain an internal, updated representation of the body position and movement in space. However, it is not clear to what extent they are also necessary to mentally simulate movement in situations that do not involve displacements of the body, as in mental imagery. The present study assessed how vestibular loss can affect object-based mental transformations (OMTs), i.e., imagined rotations or translations of objects relative to the environment. Participants performed one task of mental rotation of 3D-objects and two mental scanning tasks dealing with the ability to build and manipulate mental images that have metric properties. Menière's disease patients were tested before unilateral vestibular neurotomy and during the recovery period (1 week and 1 month). They were compared to healthy participants tested at similar time intervals and to bilateral vestibular-defective patients tested after the recovery period. Vestibular loss impaired all mental imagery tasks. Performance varied according to the extent of vestibular loss (bilateral patients were frequently the most impaired) and according to the time elapsed after unilateral vestibular neurotomy (deficits were stronger at the early stage after neurotomy and then gradually compensated). These findings indicate that vestibular signals are necessary to perform OMTs and provide the first demonstration of the critical role of vestibular signals in processing metric properties of mental representations. They suggest that vestibular loss disorganizes brain structures commonly involved in mental imagery, and more generally in mental representation.

Evidence that spatial memory deficits following bilateral vestibular deafferentation in rats are probably permanent

Previous studies of rats with bilateral vestibular deafferentation (BVD) have demonstrated spatial memory deficits, suggesting adverse effects on the hippocampus. However, the longest post-operative time interval that has been studied was approx. 5-7 months post-surgery. In this study, we investigated whether rats exhibited spatial memory deficits at 14 months following BVD and whether these deficits could be exacerbated by administration of cannabinoid (CB) drugs. Twenty-eight adult rats were divided into four groups: (1) sham surgery+vehicle; (2) sham surgery+the CB1/CB(2) receptor agonist WIN55,212-2 ('WIN'); (3) BVD+vehicle; and (4) BVD+WIN. WIN (1.0 or 2.0 mg/kg/day) or vehicle, was administered (s.c.) on days 1-10 and 11-20 (respectively), 30 min before the rats performed in a foraging task. On day 21, the CB receptor inverse agonist, AM251 (3.0 mg/kg, s.c.), was administered before WIN or vehicle. To our surprise, BVD animals were impaired in using the visual cues during the probe test in light. In the dark trials, when visual cues were unavailable, BVD animals were unable to use self-movement cues in homing. However, WIN at 2 mg/kg, significantly improved BVD animals' homing time and number of errors in the dark through strategies other than the improvement in using self-movement cues. Furthermore, AM251 significantly improved heading angle in vehicle-treated animals and the first home choice in WIN-treated animals. These results suggest that at 14 months post-BVD, the animals are not only impaired in path integration, but also piloting and that the spatial memory deficits may be permanent. The involvement of the cannabinoid system is more complicated than expected.

Hippocampal synaptic transmission and LTP in vivo are intact following bilateral vestibular deafferentation in the rat

Numerous studies in animals and humans have shown that damage to the vestibular system in the inner ear results in spatial memory deficits, presumably because areas of the brain such as the hippocampus require vestibular input to accurately represent the spatial environment. Consistent with this hypothesis, studies in animals have demonstrated that complete bilateral vestibular deafferentation (BVD) causes a disruption of place cell firing as well as theta activity. The aim of this study was to investigate whether BVD in rats affects baseline field potentials (field excitatory postsynaptic potentials and population spikes) and long-term potentiation (LTP) in CA1 and the dentate gyrus (DG) of awake freely moving rats up to 43 days post-BVD and of anesthetized rats at 7 months post-BVD. Compared to sham controls, BVD had no significant effect on either baseline field potentials or LTP in either condition. These results suggest that although BVD interferes with the encoding, consolidation, and/or retrieval of spatial memories and the function of place cells, these changes are not related to detectable in vivo decrements in basal synaptic transmission or LTP, at least in the investigated pathways.

Move it or lose it--is stimulation of the vestibular system necessary for normal spatial memory?

Studies in both experimental animals and human patients have demonstrated that peripheral vestibular lesions, especially bilateral lesions, are associated with spatial memory impairment that is long-lasting and may even be permanent. Electrophysiological evidence from animals indicates that bilateral vestibular loss causes place cells and theta activity to become dysfunctional; the most recent human evidence suggests that the hippocampus may cause atrophy in patients with bilateral vestibular lesions. Taken together, these studies suggest that self-motion information provided by the vestibular system is important for the development of spatial memory by areas of the brain such as the hippocampus, and when it is lost, spatial memory is impaired. This naturally suggests the converse possibility that activation of the vestibular system may enhance memory. Surprisingly, there is some human evidence that this may be the case. This review considers the relationship between the vestibular system and memory and suggests that the evolutionary age of this primitive sensory system as well as how it detects self-motion (i.e., detection of acceleration vs. velocity) may be the reasons for its unique contribution to spatial memory.

Balance before reason in rats and humans

Considerable clinical and experimental evidence indicates that loss of vestibular function results in cognitive deficits, especially deficits in spatial memory. These studies demonstrate the importance of balance for the most fundamental of cognitive processes and suggest that information about head acceleration and orientation must have been critical to the evolution of brain structures such as the hippocampus. Studies of animals with bilateral vestibular lesions have shown that theta rhythm and the activity of hippocampal place cells are severely disrupted; recent human studies show that bilateral vestibular loss is even associated with hippocampal atrophy. While it is conceivable that the effects of vestibular lesions on the hippocampus are due to chronic stress and increased glucocorticoid levels, at present there is little evidence to support this hypothesis. It is also possible that the hippocampal changes are due to a reduction in exploration and active behavior; however, in rats, at least, bilateral vestibular lesions cause hyperactivity rather than hypoactivity. Alternatively, the hippocampus may have developed a special dependence upon the vestibular system during evolution, since it was the first sensory system to reliably indicate gravitational vertical.

Long-term deficits on a foraging task after bilateral vestibular deafferentation in rats

Animal studies have shown that bilateral vestibular deafferentation (BVD) causes deficits in spatial memory that may be related to electrophysiological and neurochemical changes in the hippocampus. Recently, human studies have also indicated that human patients can exhibit spatial memory impairment and hippocampal atrophy even 8-10 yr following BVD. Our previous studies have shown that rats with unilateral vestibular deafferentation (UVD) showed an impairment at 3 months after the surgery on a food foraging task that relies on hippocampal integration of egocentric cues, such as vestibular information; however, by 6 months postop, they showed a recovery of function. By contrast, the long-term effects of BVD on spatial navigation have never been well studied. In this study, we tested BVD or sham rats on a food foraging task at 5 months postop. Under light conditions, BVD rats were able to use visual cues to guide themselves home, but did so with a significantly longer homing time. However, in darkness, BVD rats were severely impaired in the foraging task, as indicated by a significantly longer homing distance and homing time, with more errors and larger heading angles when compared with sham rats. These results suggest that, unlike UVD, BVD causes long-term deficits in spatial navigation that are unlikely to recover, even with repeated T-maze training.

Neurodevelopment milestone abnormalities in rats exposed to stress in early life

Manipulation of the corticosteroid milieu by interfering with the mother-newborn relationship has received much attention because of its potential bearing on psychopathology later in life. In the present study, infant rats that were deprived of maternal contact between the 2nd and the 15th postnatal days (MS2-15) for 6 h/day were subjected to a systematic assessment of neurodevelopmental milestones between postnatal days 2 and 21. The analyses included measurements of physical growth and maturation and evaluation of neurological reflexes. Although some somatic milestones (e.g. eye opening) were anticipated, MS2-15 animals showed retardation in the acquisition of postural reflex, air righting and surface righting reflexes, and in the wire suspension test; the latter two abnormalities were only found in males. A gender effect was also observed in negative geotaxis, with retardation being observed in females but not males. To better understand the delay of neurological maturation in MS2-15 rats, we determined the levels of various monoamines in different regions of the brain stem, including the vestibular area, the substantia nigra, ventral tegmental area and dorsal raphe nuclei. In the vestibular region of MS2-15 rats the levels of 5-HT were reduced, while 5-HT turnover was increased. There was also a significant increase of the 5-HT turnover in MS2-15 animals in the raphe nuclei, mainly due to increased 5-hydroxyindoleacetic acid (5-HIAA) levels, and an increase of 3,4-dihydroxyphenylacetic acid (DOPAC) levels in the ventral tegmental area (VTA) of stressed females. No significant differences were found in the immunohistochemical sections for tyrosine and tryptophan hydroxylase in these regions of the brain stem. In conclusion, the present results show that postnatal stress induces signs of neurological pathology that may contribute to the genesis of behavioral abnormalities later in life.

Bilateral vestibular deafferentation impairs performance in a spatial forced alternation task in rats

Converging behavioral, electrophysiological, and neurochemical data suggest that lesions to the peripheral vestibular system result in impairment of the hippocampus. Nonetheless, relatively few studies have examined the hippocampus or behavior related to it, over a long period of time following the lesion, to determine if any recovery takes place. Here we used the spatial forced alternation task in a T maze, which is sensitive to the integrity of the hippocampus, to evaluate learning and memory in rats at 3 weeks, 3 months, and 5 months following bilateral vestibular deafferentation (BVD) or sham surgery. BVD rats made significantly fewer correct choices at all time points when compared with the sham controls. However, the percentage correct choice for BVD rats was at chance level at 3 weeks postop, and was significantly above chance at 5 months postop. These results add to the evidence that BVD causes a long-term impairment of hippocampal function and spatial learning and memory, but suggest that some recovery of function might take place over the long term.

Spatial memory and hippocampal volume in humans with unilateral vestibular deafferentation

Patients with acquired chronic bilateral vestibular loss were recently found to have a significant impairment in spatial memory and navigation when tested with a virtual Morris water task. These deficits were associated with selective and bilateral atrophy of the hippocampus, which suggests that spatial memory and navigation also rely on vestibular input. In the present study 16 patients with unilateral vestibular deafferentation due to acoustic neurinoma were examined 5- to 13-yrs post-surgery. Volumetry of the hippocampus was performed in patients and age- and sex-matched healthy controls by manually tracing the structure and by an evaluator-independent voxel-based morphometry. Spatial memory and navigation were assessed with a virtual Morris water task. No significant deficits in spatial memory and navigation could be demonstrated in the patients with left vestibular failure, whereas patients with right vestibular loss showed a tendency to perform worse on the respective tests. Impairment was significant only for one computed measure (heading error). The subtle deficiencies with right vestibular loss are compatible with the recently described dominance of the right labyrinth and the vestibular cortex in the right hemisphere. Volumetry did not reveal any atrophy of the hippocampus in either patient group.

Impairment and recovery on a food foraging task following unilateral vestibular deafferentation in rats

It has been suggested that the vestibular system may contribute to the development of higher cognitive function, especially spatial learning and memory that uses idiothetic cues (e.g., dead reckoning). However, few studies have been done using behavioral tasks that could potentially separate the animals' ability for dead reckoning from piloting. The food foraging task requires the animal to continuously monitor and integrate self-movement cues and generate an accurate return path. It has been shown that bilateral vestibular-lesioned rats were impaired on this task. The present study used the same task to further examine the contribution of vestibular information to spatial navigation by comparing unilateral and bilateral lesions and by testing the animals at different time points following the lesion. The results demonstrated that animals with unilateral vestibular deafferentation were impaired in performing the task in the dark at 3 months after the lesion, and this impairment disappeared at 6 months after the lesion. This supports the notion that vestibular information contributes to dead reckoning and suggests possible recovery of function over time after the lesion. Animals with bilateral vestibular deafferentation were not able to be tested on the foraging task because they exhibited behavior distinct from the unilateral-lesioned animals, with significant hesitation in leaving their home cage for as long as 6 months after the lesion.

The effects of vestibular lesions on hippocampal function in rats

Interest in interaction between the vestibular system and the hippocampus was stimulated by evidence that peripheral vestibular lesions could impair performance in learning and memory tasks requiring spatial information processing. By the 1990s, electrophysiological data were emerging that the brainstem vestibular nucleus complex (VNC) and the hippocampus were connected polysynaptically and that hippocampal place cells could respond to vestibular stimulation. The aim of this review is to summarise and critically evaluate research published in the last 5 years that has seen major progress in understanding the effects of vestibular damage on the hippocampus. In addition to new behavioural studies demonstrating that animals with vestibular lesions exhibit impairments in spatial memory tasks, electrophysiological studies have confirmed long-latency, polysynaptic pathways between the VNC and the hippocampus. Peripheral vestibular lesions have been shown to cause long-term changes in place cell function, hippocampal EEG activity and even CA1 field potentials in brain slices maintained in vitro. During the same period, neurochemical investigations have shown that some hippocampal subregions exhibit long-term changes in the expression of neuronal nitric oxide synthase, arginase I and II, and the NR1 and NR2A N-methyl-D-aspartate (NMDA) receptor subunits following peripheral vestibular damage. Despite the progress, a number of important issues remain to be resolved, such as the possible contribution of auditory damage associated with vestibular lesions, to the hippocampal effects observed. Furthermore, although these studies demonstrate that damage to the vestibular system does have a long-term impact on the electrophysiological and neurochemical function of the hippocampus, they do not indicate precisely how vestibular information might be used in hippocampal functions such as developing spatial representations of the environment. Understanding this will require detailed electrical stimulation and lesion studies to elucidate the way in which different kinds of vestibular information are transmitted to various hippocampal subregions.

Vestibular loss causes hippocampal atrophy and impaired spatial memory in humans

The human hippocampal formation plays a crucial role in various aspects of memory processing. Most literature on the human hippocampus stresses its non-spatial memory functions, but older work in rodents and some other species emphasized the role of the hippocampus in spatial learning and memory as well. A few human studies also point to a direct relation between hippocampal size, navigation and spatial memory. Conversely, the importance of the vestibular system for navigation and spatial memory was until now convincingly demonstrated only in animals. Using magnetic resonance imaging volumetry, we found that patients (n = 10) with acquired chronic bilateral vestibular loss (BVL) develop a significant selective atrophy of the hippocampus (16.9% decrease relative to controls). When tested with a virtual variant (on a PC) of the Morris water task these patients exhibited significant spatial memory and navigation deficits that closely matched the pattern of hippocampal atrophy. These spatial memory deficits were not associated with general memory deficits. The current data on BVL patients and bilateral hippocampal atrophy revive the idea that a major--and probably phylogenetically ancient--function of the archicortical hippocampal tissue is still evident in spatial aspects of memory processing for navigation. Furthermore, these data demonstrate for the first time in humans that spatial navigation critically depends on preserved vestibular function, even when the subjects are stationary, e.g. without any actual vestibular or somatosensory stimulation.

Clomethiazole: mechanisms underlying lasting neuroprotection following hypoxia-ischemia

Damage after hypoxia-ischemia (HI) is observed in both cortical and subcortical regions. In this study, we employed a "Levine" rat model of HI (left carotid ligation + 1 h global hypoxia on PND-26) and used histological and electrophysiological paradigms to assess the long-term neuroprotective properties of clomethiazole (CMZ; a GABA(A) receptor modulator). Key enzymes involved in inflammation, namely nitric oxide synthase (NOS) and arginase, were also examined to assess potential CMZ mechanisms not involving GABA-R activation. Assessments were carried out 3 and 90 days post-HI. Extensive CNS lesions were evident after HI ipsilaterally at both short- and long-term intervals. CMZ significantly decreased the lesion size at 3 and 90 days (P<0.01; P<0.05). Evoked field potential analyses were used to assess hippocampal CA1 neuronal activity ex vivo. Electrophysiological measurements contralateral to the occlusion revealed impaired neuronal function after HI relative to short- and long-term controls (P<0.001, 3 and 14 days; P<0.01, 90 days), with CMZ treatment providing near complete protection (P<0.001 at 3 and 14 days; P<0.01 at 90 days). Both NOS and arginase activities were significantly increased at 3 days (P<0.01), with arginase remaining elevated at 90 days post-HI (P<0.05) ipsilaterally. CMZ suppressed the HI-induced increase in iNOS and arginase activities (P<0.001; P<0.05). These data provide evidence of long-term functional neuroprotection by CMZ in a model of HI. We further conclude that under conditions of HI, functional deficits are not restricted to the ipsilateral hemisphere and are due, at least in part, to changes in the activity of NOS and arginase.

Cytosolic glucocorticoid receptor expression in the rat vestibular nucleus and hippocampus following unilateral vestibular deafferentation

It has been suggested that vestibular compensation, the process of behavioural recovery that occurs following peripheral vestibular damage, might be partially dependent on the release of glucocorticoids (GC) during the early stages of recovery from the lesion. One possibility is that glucocorticoid receptors (GRs) in the vestibular nucleus complex (VNC) might change following the lesion, altering their response to GCs. We sought to test this hypothesis by quantifying the expression of cytosolic GRs in the bilateral VNCs at 10 h, 58 h and 2 weeks following unilateral vestibular deafferentation (UVD) in rat, using western blotting. We also examined GR expression in the CA1, CA2/3 and dentate gyrus (DG) subregions of the hippocampus and measured serum corticosterone levels. Compared with sham surgery and anaesthetic controls, we found no significant changes in GR expression in the ipsilateral or contralateral VNCs at any time post-UVD. However, we did find a significant decrease in GR expression in the ipsilateral CA1 at 2 weeks post-UVD. Serum corticosterone levels were significantly lower in all groups at 58 h post-op. compared to 10 h and 2 weeks; however, there were no significant differences between the UVD and control groups at any time point. These results suggest that changes in GR expression in the VNC are unlikely to contribute to the development of vestibular compensation. However, long-term changes in GR expression in CA1 might be related to chronic deficits in hippocampal function and spatial cognition following vestibular damage.

Bilateral labyrinthectomy causes long-term deficit in object recognition in rat

It has been reported that patients with vestibular disorders experience a wide range of cognitive disorders, including memory loss. However, to our knowledge, no study has investigated the contribution of vestibular information to episodic memory in experimental animals using vestibular deafferentation. In the present study, the effects of a complete unilateral or bilateral surgical lesion of the vestibular labyrinths in a spontaneous object recognition task were evaluated in Wistar rats 3 and 6 months following the surgery. We found that rats with bilateral vestibular deafferentation, but not those with unilateral vestibular deafferentation were impaired on the task at both time points. These results suggest for the first time that vestibular information may contribute to non-spatial memory to some extent.