Effects of bilateral vestibular deafferentation on anxiety-related behaviours in Wistar rats

Vertigoheel improves central vestibular compensation after unilateral peripheral vestibulopathy in rats

The aim of this study was to assess the effect of Vertigoheel on central vestibular compensation and cognitive deficits in rats subjected to peripheral vestibular loss. Young adult male Long Evans rats were subjected to bilateral vestibular insults through irreversible sequential ototoxic destructions of the vestibular sensory organs. Vestibular syndrome characteristics were monitored at several time points over days and weeks following the sequential insults, using a combination of behavioral assessment paradigms allowing appreciation of patterns of change in static and dynamic deficits, together with spatial navigation, learning, and memory processes. Vertigoheel administered intraperitoneally significantly improved maximum body velocity and not moving time relative to its vehicle control on days 2 and 3 and on day 2, respectively, after unilateral vestibular lesion (UVL). It also significantly improved postural control relative to its vehicle 1 day after UVL. Conversely, Vertigoheel did not display any significant effect vs. vehicle on the severity of the syndrome, nor on the time course of other examined parameters, such as distance moved, mean body velocity, meander, and rearing. Spatial cognition testing using Y- and T-maze and eight-radial arm maze did not show any statistically significant difference between Vertigoheel and vehicle groups. However, Vertigoheel potentially enhanced the speed of learning in sham animals. Evaluating Vertigoheel's effect on thigmotaxis during the open-field video tracking test revealed no significant difference between Vertigoheel and its vehicle control groups suggesting that Vertigoheel does not seem to induce sedative or anxiolytic effects that could negatively affect vestibular and memory function. Present observations reveal that Vertigoheel improves central vestibular compensation following the unilateral peripheral vestibular loss as demonstrated by improvement of specific symptoms.

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.

The modulation of hippocampal theta rhythm by the vestibular system

The vestibular system is a sensory system that has evolved over millions of years to detect acceleration of the head, both rotational and translational, in three dimensions. One of its most important functions is to stabilize gaze during unexpected head movement; however, it is also important in the control of posture and autonomic reflexes. Theta rhythm is a 3- to 12-Hz oscillating EEG signal that is intimately linked to self-motion and is also known to be important in learning and memory. Many studies over the last two decades have shown that selective activation of the vestibular system, using either natural rotational or translational stimulation, or electrical stimulation of the peripheral vestibular system, can induce and modulate theta activity. Furthermore, inactivation of the vestibular system has been shown to significantly reduce theta in freely moving animals, which may be linked to its impairment of place cell function as well as spatial learning and memory. The pathways through which vestibular information modulate theta rhythm remain debatable. However, vestibular responses have been found in the pedunculopontine tegmental nucleus (PPTg) and activation of the vestibular system causes an increase in acetylcholine release into the hippocampus, probably from the medial septum. Therefore, a pathway from the vestibular nucleus complex and/or cerebellum to the PPTg, supramammillary nucleus, posterior hypothalamic nucleus, and septum to the hippocampus is likely. The modulation of theta by the vestibular system may have implications for vestibular effects on cognitive function and the contribution of vestibular impairment to the risk of dementia.

From ear to uncertainty: vestibular contributions to cognitive function

In addition to the deficits in the vestibulo-ocular and vestibulo-spinal reflexes that occur following vestibular dysfunction, there is substantial evidence that vestibular loss also causes cognitive disorders, some of which may be due to the reflexive deficits and some of which are related to the role that ascending vestibular pathways to the limbic system and neocortex play in spatial orientation. In this review we summarize the evidence that vestibular loss causes cognitive disorders, especially spatial memory deficits, in animals and humans and critically evaluate the evidence that these deficits are not due to hearing loss, problems with motor control, oscillopsia or anxiety and depression. We review the evidence that vestibular lesions affect head direction and place cells as well as the emerging evidence that artificial activation of the vestibular system, using galvanic vestibular stimulation (GVS), can modulate cognitive function.

A multivariate statistical and data mining analysis of spatial memory-related behaviour following bilateral vestibular loss in the rat

Vestibular dysfunction in animals and humans is associated with a variety of cognitive and anxiety disorders, and it has been difficult to determine how the different symptoms may be related to one another. The aim of this study was to determine the extent to which the spatial memory deficits that occur following bilateral vestibular deafferentation (BVD) in rats can be attributed to other behavioural symptoms. Spatial memory was measured using a spatial T maze alternation task (STM), while locomotor activity and anxiety were measured using open field, elevated plus and T mazes, respectively. Using multiple linear and random forest regression, we determined that the best predictors of performance in the STM were whether the animals had received a BVD or sham lesion, and the duration of rearing. Using linear discriminant analysis, random forest classification, support vector machines and cluster analysis, we found that BVD animals could be clearly distinguished from sham controls by their behavioural syndrome, in particular their decreased duration of rearing in the open field maze (suggesting reduced exploration), decreased time spent in the outer zone of the open field maze ('reduced thigmotaxis', suggesting increased risk taking), and spatial memory deficits in the STM. These results suggest that the poor performance of rats with BVD in spatial memory tasks is largely due to spatial memory deficits themselves rather than a result of other changes in locomotor activity or anxiety.

Interactions between Stress and Vestibular Compensation - A Review

Elevated levels of stress and anxiety often accompany vestibular dysfunction, while conversely complaints of dizziness and loss of balance are common in patients with panic and other anxiety disorders. The interactions between stress and vestibular function have been investigated both in animal models and in clinical studies. Evidence from animal studies indicates that vestibular symptoms are effective in activating the stress axis, and that the acute stress response is important in promoting compensatory synaptic and neuronal plasticity in the vestibular system and cerebellum. The role of stress in human vestibular disorders is complex, and definitive evidence is lacking. This article reviews the evidence from animal and clinical studies with a focus on the effects of stress on the central vestibular pathways and their role in the pathogenesis and management of human vestibular disorders.

Performance in anxiety and spatial memory tests following bilateral vestibular loss in the rat and effects of anxiolytic and anxiogenic drugs

Vestibular dysfunction in humans is associated with anxiety and cognitive disorders. However, various animal studies of the effects of vestibular loss have yielded conflicting results, from reduced anxiety to increased anxiety, depending on the particular model of vestibular dysfunction and the anxiety test used. In this study we revisited the question of whether rats with surgical bilateral vestibular deafferentation (BVD) exhibit changes in anxiety-related behaviour by testing them in the open field maze (OFM), elevated plus maze (EPM) and elevated T maze (ETM) in the presence of a non-sedating anxiolytic drug, buspirone, or an anxiogenic drug, FG-7142. We also tested the animals in a spatial T maze (STM) in order to evaluate their cognitive function under the same set of conditions. We found that BVD animals exhibited increased locomotor activity (P≤0.003), reduced supported and unsupported rearing (P≤0.02 and P≤0.000, respectively) and reduced thigmotaxis (P≤0.000) in the OFM, which for the most part the drugs did not modify. By contrast, there were no significant differences between BVD and sham control animals in the EPM and the BVD animals exhibited a marginally longer escape latency in the ETM (P≤0.03), with no change in avoidance latency. In the STM, the BVD animals demonstrated a large and significant decrease in accuracy compared to the sham control animals (P≤0.000), which was not affected by drug treatment. These results have replicated previous findings regarding increased locomotor activity, reduced rearing and thigmotaxis in the OFM, and impaired performance in the STM. However, they failed to replicate some previous results obtained using the EPM and ETM. Overall, they do not support the hypothesis that BVD animals exhibit increased anxiety-like behaviour and suggest that the cognitive deficits may be independent of the emotional effects of vestibular loss.

Influence of anxiety in spatial memory impairments related to the loss of vestibular function in rat

It is now well established that vestibular information plays an important role in spatial memory processes. Although vestibular lesions induce anxiety in humans, this finding remains controversial in rodents. However, it is possible that anxiety-related behavior is associated with spatial memory impairments after vestibular lesions. We aimed to evaluate anxiety-like behavior and the effect of an anxiolytic treatment during a complex spatial memory task in a rat model of compensated bilateral vestibular lesions. Adult rats were divided into four groups, with or without vestibular lesions and, treated or untreated by diazepam. The vestibular lesion was performed by transtympanic injection of arsanilate and compared to transtympanic saline injection. Diazepam or saline was administered 1h before each test or learning session. Vestibular-lesioned rats exhibited anxiety-like behavior which was decreased with diazepam. Spatial memory performance was similar in control-treated and untreated groups, suggesting no effect on memory at the dose of diazepam used. Spatial memory performances were not modified by anxiolytic drug treatment in vestibular-lesioned rats compared to vestibular-lesioned rats without drug treatment. We conclude that bilateral vestibular lesions in rats induced anxiety-like behavior which was unrelated to spatial memory impairment and was probably specifically related to the loss of vestibular information.

The effects of bilateral vestibular loss on hippocampal volume, neuronal number, and cell proliferation in rats

Previous studies in humans have shown that bilateral loss of vestibular function is associated with a significant bilateral atrophy of the hippocampus, which correlated with the patients’ spatial memory deficits. More recently, patients who had recovered from unilateral vestibular neuritis have been reported to exhibit a significant atrophy of the left posterior hippocampus. Therefore, we investigated whether bilateral vestibular deafferentation (BVD) would result in a decrease in neuronal number or volume in the rat hippocampus, using stereological methods. At 16 months post-BVD, we found no significant differences in hippocampal neuronal number or volume compared to sham controls, despite the fact that these animals exhibited severe spatial memory deficits. By contrast, using bromodeoxyuridine (BrdU) as a marker of cell proliferation, we found that the number of BrdU-labeled cells significantly increased in the dentate gyrus of the hippocampus between 48 h and 1 week following BVD. Although a substantial proportion of these cells survived for up to 1 month, the survival rate was significantly lower in BVD animals when compared with that in sham animals. These results suggest a dissociation between the effects of BVD on spatial memory and hippocampal structure in rats and humans, which cannot be explained by an injury-induced increase in cell proliferation.

Septal elicitation of hippocampal theta rhythm did not repair cognitive and emotional deficits resulting from vestibular lesions

Bilateral vestibular lesions cause atrophy of the hippocampus in humans and subsequent deficits in spatial memory and the processing of emotional stimuli in both rats and humans. Vestibular lesions also impair hippocampal theta rhythm in rats. The aim of the present study was to investigate whether restoring theta rhythm to the hippocampus of a rat, via stimulation of the medial septum, would repair the deficits caused by vestibular lesions. It was hypothesized that the restoration of theta would repair the deficits and the vestibular rats would exhibit behavior and EEG similar to that of the sham rats. Rats were given either sham surgery or bilateral vestibular deafferentation (BVD) followed in a later operation by electrode implants. Half of the lesioned rats received stimulation. Subjects were tested in open field, elevated T-maze and spatial nonmatching to sample tests. BVD caused a deficit in hippocampal theta rhythm. Stimulation restored theta power at a higher frequency in the vestibular-lesioned rats, however, the stimulation did not repair the cognitive and emotional deficits caused by the lesions. It was concluded that stimulation, at least in the form used here, would not be a viable treatment option for vestibular damaged humans.

Modulation of memory by vestibular lesions and galvanic vestibular stimulation

For decades it has been speculated that there is a close association between the vestibular system and spatial memories constructed by areas of the brain such as the hippocampus. While many animal studies have been conducted which support this relationship, only in the last 10 years have detailed quantitative studies been carried out in patients with vestibular disorders. The majority of these studies suggest that complete bilateral vestibular loss results in spatial memory deficits that are not simply due to vestibular reflex dysfunction, while the effects of unilateral vestibular damage are more complex and subtle. Very recently, reports have emerged that sub-threshold, noisy galvanic vestibular stimulation can enhance memory in humans, although this has not been investigated for spatial memory as yet. These studies add to the increasing evidence that suggests a connection between vestibular sensory information and memory in humans.

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.

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.