Effects of bilateral vestibular deafferentation in rat on hippocampal theta response to somatosensory stimulation, acetylcholine release, and cholinergic neurons in the pedunculopontine tegmental nucleus

Advanced progress of vestibular compensation in vestibular neural networks

Vestibular compensation is the natural process of recovery that occurs with acute peripheral vestibular lesion. Here, we summarize the current understanding of the mechanisms underlying vestibular compensation, focusing on the role of the medial vestibular nucleus (MVN), the central hub of the vestibular system, and its associated neural networks. The disruption of neural activity balance between the bilateral MVNs underlies the vestibular symptoms after unilateral vestibular damage, and this balance disruption can be partially reversed by the mutual inhibitory projections between the bilateral MVNs, and their top-down regulation by other brain regions via different neurotransmitters. However, the detailed mechanism of how MVN is involved in vestibular compensation and regulated remains largely unknown. A deeper understanding of the vestibular neural network and the neurotransmitter systems involved in vestibular compensation holds promise for improving treatment outcomes and developing more effective interventions for vestibular disorders.

Vestibular dysfunction and its association with cognitive impairment and dementia

The vestibular system plays an important role in maintaining balance and posture. It also contributes to vertical perception, body awareness and spatial navigation. In addition to its sensory function, the vestibular system has direct connections to key areas responsible for higher cognitive functions, such as the prefrontal cortex, insula and hippocampus. Several studies have reported that vestibular dysfunction, in particular bilateral vestibulopathy, is associated with an increased risk of cognitive impairment and the development of dementias such as Alzheimer's disease. However, it is still controversial whether there is a causal relationship between vestibular damage and cognitive dysfunction. In this mini-review, we will explore the relationship between the vestibular system, cognitive dysfunction and dementia, hypotheses about the hypothesis and causes that may explain this phenomenon and also some potential confounders that may also lead to cognitive impairment. We will also review multimodal neuroimaging approaches that have investigated structural and functional effects on the cortico-vestibular network and finally, describe some approaches to the management of patients with vestibular damage who have shown some cognitive impairment.

Older adults and individuals with Parkinson's disease control posture along suborthogonal directions that deviate from the traditional anteroposterior and mediolateral directions

A rich and complex temporal structure of variability in postural sway characterizes healthy and adaptable postural control. However, neurodegenerative disorders such as Parkinson's disease, which often manifest as tremors, rigidity, and bradykinesia, disrupt this healthy variability. This study examined postural sway in young and older adults, including individuals with Parkinson's disease, under different upright standing conditions to investigate the potential connection between the temporal structure of variability in postural sway and Parkinsonism. A novel and innovative method called oriented fractal scaling component analysis was employed. This method involves decomposing the two-dimensional center of pressure (CoP) planar trajectories to pinpoint the directions associated with minimal and maximal temporal correlations in postural sway. As a result, it facilitates a comprehensive assessment of the directional characteristics within the temporal structure of sway variability. The results demonstrated that healthy young adults control posture along two orthogonal directions closely aligned with the traditional anatomical anteroposterior (AP) and mediolateral (ML) axes. In contrast, older adults and individuals with Parkinson's disease controlled posture along suborthogonal directions that significantly deviate from the AP and ML axes. These findings suggest that the altered temporal structure of sway variability is evident in individuals with Parkinson's disease and underlies postural deficits, surpassing what can be explained solely by the natural aging process.

Recent developments in the understanding of the interactions between the vestibular system, memory, the hippocampus, and the striatum

Over the last two decades, evidence has accumulated to demonstrate that the vestibular system has extensive connections with areas of the brain related to spatial memory, such as the hippocampus, and also that it has significant interactions with areas associated with voluntary motor control, such as the striatum in the basal ganglia. In fact, these functions are far from separate and it is believed that interactions between the striatum and hippocampus are important for memory processing. The data relating to vestibular-hippocampal-striatal interactions have considerable implications for the understanding and treatment of Alzheimer's Disease and Parkinson's Disease, in addition to other neurological disorders. However, evidence is accumulating rapidly, and it is difficult to keep up with the latest developments in these and related areas. The aim of this review is to summarize and critically evaluate the relevant evidence that has been published over the last 2 years (i.e., since 2021), in order to identify emerging themes in this research area.

A Direct Comparison of Theta Power and Frequency to Speed and Acceleration

Decades of hippocampal neurophysiology research have linked the hippocampal theta rhythm to voluntary movement. A consistent observation has been a robust correlation between the amplitude (or power) and frequency of hippocampal theta and running speed. Recently, however, it has been suggested that acceleration, not running speed, is the dominating influence on theta frequency. There is an inherent interdependence among these two variables, as acceleration is the rate of change in velocity. Therefore, we investigated theta frequency and amplitude of the local-field potential recorded from the stratum pyramidale, stratum radiatum, and stratum lacunosum moleculare of the CA1 subregion, considering both speed and acceleration in tandem as animals traversed a circular task or performed continuous alternation. In male and female rats volitionally controlling their own running characteristics, we found that running speed carries nearly all of the variability in theta frequency and power, with a minute contribution from acceleration. These results contradicted a recent publication using a speed-clamping task, where acceleration and movement are compelled through the use of a bottomless car (Kropff et al., 2021a). Therefore, we reanalyzed the speed-clamping data replicating a transient increase in theta frequency during acceleration. Compared with track running rats, the speed-clamped animals exhibited lower velocities and acceleration values but still showed a stronger influence of speed on theta frequency relative to acceleration. As navigation is the integration of many sensory inputs that are not necessarily linearly related, we offer caution in making absolute claims regarding hippocampal physiology from correlates garnered from a single behavioral repertoire.SIGNIFICANCE STATEMENT A long-standing, replicable observation has been the increase of hippocampal theta power and frequency with increasing running speed. Recently, however, an experimental approach that clamps the running speed of an animal has suggested that acceleration is the dominant influence. Therefore, we analyzed data from freely behaving rats as well as data from the speed-clamping experiment. In unrestrained behavior, speed remains the dominant behavioral correlate to theta amplitude and frequency. Positive acceleration in the speed-clamp experiment induced a transient increase in theta frequency and power. However, speed retained the dominant influence over theta frequency, changing with velocity in both acceleration and deceleration conditions.

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.

Melamine Disrupts Acetylcholine-Mediated Neural Information Flow in the Hippocampal CA3-CA1 Pathway

Considering the cognitive and synaptic deficits following intragastric administration of melamine, the aim of the current investigation was to test whether the hippocampal oscillations were affected. The local field potential (LFP) was recorded in the hippocampal CA3–CA1 pathway of Wistar rats during a spatial-dependent Y-maze task. The general partial directed coherence (gPDC) method was used to assess the directionality of neural information flow (NIF) between the CA3 and CA1 regions. The levels of acetylcholine (ACh) and its esterolytic protease, acetylcholinesterase (AChE), were detected in the hippocampus (HPC) following the behavioral test. The values of phase synchronization between the CA3 and CA1 regions in delta, low theta, and high theta oscillations were reduced significantly in the melamine-treated group. Moreover, the coupling directional index and the strength of CA3 driving CA1 were critically decreased in the above three frequency bands as well. Meanwhile, a reduction in ACh expression and an enhancement in AChE activity were found in the HPC of melamine-treated rats. Intrahippocampal infusion with ACh could mitigate the weakened neural coupling and directional NIF in parallel with spatial learning improvements. However, infusion of scopolamine, an acetylcholine receptor antagonist, could block the mitigative effects of ACh treatment in melamine rats. These findings provide first evidence that ACh-mediated neuronal coupling and NIF in the CA3–CA1 pathway are involved in spatial learning deficits induced by chronic melamine exposure.

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.

Stimulation or lesion of the medial vestibular nucleus increases the number of choline acetyltransferase-positive efferent vestibular neurons in the brainstem

The vestibular center of the brainstem contains afferent and efferent vestibular neurons, which play an important role in information perception, processing, and sensory integration. Vestibular efferent neurons (VENs) can receive changes in vestibular afferent information and regulate peripheral vestibular function; however, it remains unclear how VENs change after vestibular afferent information increases or weakens. In this study, we used animal models with altered vestibular afferent information by electrically stimulating or destroying the vestibular medial nucleus (MVe). We confirmed the location of VENs in the brainstem by injecting five adult male Wistar rats in the vestibular region with a retrograde tracer. Following this, the MVe was stimulated electrically for 30 min in 20 naive rats. Rats were anesthetized and euthanized 1, 3, 6, and 12 h after stimulation. The MVe was electrolytically lesioned in another group (n=20); then, the rats were anesthetized and euthanized 1, 3, 5, and 7 days after lesioning. VENs were clearly identified dorsolateral to the genu of the facial nerve (g7) in coronal brainstem sections using choline acetyltransferase (ChAT) staining. The number of ChAT-positive VENs dorsolateral to g7 increased significantly on both sides compared with the control group 3 and 6 h after electrical stimulation. The number of ChAT-positive VENs dorsolateral to g7 was significantly greater on both sides compared with controls 3 and 5 days after electrolytic lesion. In summary, we found that the number of ChAT-positive VENs was significantly increased following a change in the excitability of MVe neurons. This suggests that VENs can respond to changes in afferent vestibular information and feedback, and regulate the peripheral vestibule. In addition, this shows that acetylcholine is an important neurotransmitter that plays an important role in the perception and fine regulation of the vestibular system.

Vestibular Functions and Parkinson's Disease

For decades it has been speculated that Parkinson's Disease (PD) is associated with dysfunction of the vestibular system, especially given that postural instability is one of the major symptoms of the disorder. Nonetheless, clear evidence of such a connection has been slow to emerge. There are still relatively few studies of the vestibulo-ocular reflexes (VORs) in PD. However, substantial evidence of vestibulo-spinal reflex deficits, in the form of abnormal vestibular-evoked myogenic potentials (VEMPs), now exists. The evidence for abnormalities in the subjective visual vertical is less consistent. However, some studies suggest that the integration of visual and vestibular information may be abnormal in PD. In the last few years, a number of studies have been published which demonstrate that the neuropathology associated with PD, such as Lewy bodies, is present in the central vestibular system. Increasingly, stochastic or noisy galvanic vestibular stimulation (nGVS) is being investigated as a potential treatment for PD, and a number of studies have presented evidence in support of this idea. The aim of this review is to summarize and critically evaluate the human and animal evidence relating to the connection between the vestibular system and PD.

The effects of electrical stimulation of the peripheral vestibular system on neurochemical release in the rat striatum

For over a century, it has been speculated that the vestibular system transmits information about self-motion to the striatum. There have been inconsistent reports of such a connection, and interest in the subject has been increased by the experimental use of galvanic vestibular stimulation in the treatment of Parkinson's Disease patients. Nonetheless, there are few data available on the effects of vestibular stimulation on neurochemical changes in the striatum. We used in vivo microdialysis to analyse changes in the extracellular levels of amino acids and monoamines in the rat striatum, following electrical vestibular stimulation. Stimulation caused a significant decrease in serine and threonine, compared to the no-stimulation controls (P ≤ 0.005 and P ≤ 0.01, respectively). The ratio of DOPAC:dopamine, decreased on the ipsilateral side following stimulation (P ≤ 0.005). There was a significant treatment x side x intensity interaction for taurine levels (P ≤ 0.002), due to a decrease on the contralateral side in stimulated animals, which varied as a function of current. These results show that peripheral vestibular stimulation causes some neurochemical changes in the striatum and support the view that activaton of the vestibular system exerts effects on the function of the striatum.

Vestibular Function and Hippocampal Volume in the Baltimore Longitudinal Study of Aging (BLSA)

OBJECTIVE

This study evaluated whether reduced vestibular function in aging adults is associated with lower hippocampal volume.

STUDY DESIGN

Cross-sectional study.

SETTING

Baltimore Longitudinal Study of Aging, a long-running longitudinal cohort study of healthy aging.

PATIENTS

Eligible participants were aged ≥ 60 years and had both vestibular physiological testing and brain magnetic resonance imaging at the same visit.

INTERVENTION

Vestibular function testing consisted of the cervical vestibular-evoked myogenic potential (cVEMP) to assess saccular function, ocular VEMP to assess utricular function, and video head-impulse testing to assess the horizontal semicircular canal vestibulo-ocular reflex.

MAIN OUTCOME MEASURE

Hippocampal volume calculated using diffeomorphometry.

RESULTS

The study sample included 103 participants (range of 35-90 participants in subanalyses) with mean (±SD) age 77.2 years (±8.71). Multivariate linear models including age, intracranial volume, sex, and race showed that 1 μV amplitude increase of cVEMP was associated with an increase of 319.1 mm (p = 0.003) in mean hippocampal volume. We did not observe a significant relationship between ocular VEMP amplitude or vestibulo-ocular reflex gain and mean hippocampal volume.

CONCLUSIONS

Lower cVEMP amplitude (i.e., reduced saccular function) was significantly associated with lower mean hippocampal volume. This is in line with previous work demonstrating a link between saccular function and spatial cognition. Hippocampal atrophy may be a mechanism by which vestibular loss contributes to impaired spatial cognition in older adults. Future work using longitudinal data will be needed to evaluate the causal nature of the association between vestibular loss and hippocampal atrophy.

Differential regulation of NMDA receptor-expressing neurons in the rat hippocampus and striatum following bilateral vestibular loss demonstrated using flow cytometry

There is substantial evidence that loss of vestibular function impairs spatial learning and memory related to hippocampal (HPC) function, as well as increasing evidence that striatal (Str) plasticity is also implicated. Since the N-methyl-d-aspartate (NMDA) subtype of glutamate receptor is considered essential to spatial memory, previous studies have investigated whether the expression of HPC NMDA receptors changes following vestibular loss; however, the results have been contradictory. Here we used a novel flow cytometric method to quantify the number of neurons expressing NMDA receptors in the HPC and Str following bilateral vestibular loss (BVL) in rats. At 7 and 30 days post-op., there was a significant increase in the number of HPC neurons expressing NMDA receptors in the BVL animals, compared to sham controls (P ≤ 0.004 and P ≤ 0.0001, respectively). By contrast, in the Str, at 7 days there was a significant reduction in the number of neurons expressing NMDA receptors in the BVL group (P ≤ 0.05); however, this difference had disappeared by 30 days post-op. These results suggest that BVL causes differential changes in the number of neurons expressing NMDA receptors in the HPC and Str, which may be related to its long-term impairment of spatial memory.

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.