Neuronal activity in the contralateral medial vestibular nucleus of the guinea pig following unilateral labyrinthectomy

Histaminergic System and Vestibular Function in Normal and Pathological Conditions

Most neurotransmitter systems are represented in the central and peripheral vestibular system and are thereby involved both in normal vestibular signal processing and the pathophysiology of vestibular disorders. However, there is a special relationship between the vestibular system and the histaminergic system. The purpose of this review is to document how the histaminergic system interferes with normal and pathological vestibular function. In particular, we will discuss neurobiological mechanisms such as neuroinflammation that involve histamine to modulate and allow restoration of balance function in the situation of a vestibular insult. These adaptive mechanisms represent targets of histaminergic pharmacological compounds capable of restoring vestibular function in pathological situations. The clinical use of drugs targeting the histaminergic system in various vestibular disorders is critically discussed.

Vestibular rehabilitation improves spontaneous nystagmus normalization in patients with acute unilateral vestibulopathy

Introduction

Spontaneous nystagmus (SN) can be observed after acute unilateral vestibulopathy (AUVP). The slow phase eye velocity of the SN progressively decreases in darkness as the result of rebalanced neurophysiological activity between both vestibular nuclei, a process that can take several months. Although this compensatory process can occur spontaneously, there is poor evidence that vestibular rehabilitation (VR) can facilitate the process.

Methods

We documented the natural time course of SN reduction in patients with AUVP, as well as the effects of VR by means of a unilateral rotation paradigm. In a retrospective study (Study 1: n = 126 AUVP patients), we compared the time course of the SN reduction in patients with VR (n = 33) and without VR (n = 93). In a prospective study (Study 2: n = 42 AUVP patients), we compared the effects of early VR (n = 22; initiated within the first two weeks of symptoms onset) or late VR (n = 20; initiated after the second week of symptoms onset) on the time course of the SN reduction.

Results

Study 1 showed shorter median time of SN normalization in patients with VR compared to patients without VR (14 days and 90 days, respectively). Study 2 showed that AUVP patients with early and late VR had a similar median time of SN normalization. The SN slow phase eye velocity was significantly decreased as early as the end of the first VR session in both groups, and kept decreasing at each subsequent VR session. In the early VR group, 38% of the patients had slow phase eye velocity below 2°/s after the first VR session, 100% after the fifth session. Similar findings were observed in the late VR group.

Discussion

Taken together, these results indicate that VR with a unidirectional rotation paradigm speeds up the normalization of SN. This effect seems independent of the time between symptoms onset and commencement of VR, but early intervention is recommended to speed up the SN reduction.

The Neural Basis of Skull Vibration Induced Nystagmus (SVIN)

I list a summary of the major clinical observations of SVIN in patients with total unilateral vestibular loss (TUVL) and show how basic results from neurophysiology can explain these clinical observations. The account integrates results from single neuron recordings of identified semicircular canal and otolith afferent neurons in guinea pigs in response to low frequency skull vibration with evidence of the eye movement response in cats to selective semicircular canal stimulation (both individual and combined) and a simple model of nystagmus generation to show how these results explain most of the major characteristics of SVIN.

Unilateral vestibular neurectomy induces a remodeling of somatosensory cortical maps

Unilateral Vestibular Neurectomy (UVN) induces a postural syndrome whose compensation over time is underpinned by multimodal sensory substitution processes. However, at a chronic stage of compensation, UVN rats exhibit an enduring postural asymmetry expressed by an increase in the body weight on the ipsilesional paws. Given the anatomo-functional links between the vestibular nuclei and the primary somatosensory cortex (S1), we explored the interplay of vestibular and somatosensory cortical inputs following acute and chronic UVN. We determined whether the enduring imbalance in tactilo-plantar inputs impacts response properties of S1 cortical neurons and organizational features of somatotopic maps. We performed electrophysiological mapping of the hindpaw cutaneous representations in S1, immediately and one month after UVN. In parallel, we assessed the posturo-locomotor imbalance during the compensation process. UVN immediately induces an expansion of the cortical neuron cutaneous receptive fields (RFs) leading to a partial dedifferentiation of somatotopic maps. This effect was demonstrated for the ventral skin surface representations and was greater on the contralesional hindpaw for which the neuronal threshold to skin pressure strongly decreased. The RF enlargement was amplified for the representation of the ipsilesional hindpaw in relation to persistent postural asymmetries, but was transitory for the contralesional one. Our study shows, for the first time, that vestibular inputs exert a modulatory influence on S1 neuron's cutaneous responses. The lesion-induced cortical malleability highlights the influence of vestibular inputs on tactile processing related to postural control.

Detection of VOR dysfunction during the gaze stabilization test: Does target size matter?

BACKGROUND

The Gaze Stabilization Test (GST) identifies vestibulo-ocular reflex (VOR) dysfunction using a decline in target recognition with increasing head velocity, but there is no consensus on target (optotype) size above static visual acuity.

OBJECTIVE

To determine the optimal initial optotype size above static visual acuity to be used during the GST in subjects with unilateral vestibular dysfunction and healthy individuals.

METHODS

Eight subjects with unilateral vestibular dysfunction (UVD) and 19 age-matched, healthy control subjects were studied with the standard GST protocol using two optotype sizes, 0.2 and 0.3 logMAR above static visual acuity (ΔlogMAR). Maximal head velocity achieved while maintaining fixation on both optotypes was measured. Sensitivity, specificity and receiver-operator characteristic area under the curve (ROC AUC) analyses were performed to determine the optimal head velocity cut off point for each optotype, based on ability to identify the lesioned side of the UVD group from the control group.

RESULTS

There was a significant difference in maximal head velocity between the UVD group and control group using 0.2 ΔlogMAR (p = 0.032) but not 0.3 ΔlogMAR (p = 0.061). While both targets produced similar specificities (90%) for distinguishing normal from subjects with UVD, 0.2 ΔlogMAR targets yielded higher sensitivity (75%) than 0.3 logMAR (63%) and accuracy (86% vs 80%, respectively) in detecting the lesioned side in subjects with UVD versus controls with maximal head velocities≤105 deg/s (p = 0.017). Furthermore, positive likelihood ratios were nearly twice as high when using 0.2 ΔlogMAR targets (+ LR 10) compared to 0.3 ΔlogMAR (+ LR 6.3).

CONCLUSION

The 0.2 ΔlogMAR optotype demonstrated significantly superior identification of subjects with UVD, better sensitivity and positive likelihood ratios than 0.3 ΔlogMAR for detection of VOR dysfunction. Using a target size 0.2logMAR above static visual acuity (ΔlogMAR) during GST may yield better detection of VOR dysfunction to serve as a baseline for gaze stabilization rehabilitation therapy.

The Anatomical and Physiological Basis of Clinical Tests of Otolith Function. A Tribute to Yoshio Uchino

Otolithic receptors are stimulated by gravitoinertial force (GIF) acting on the otoconia resulting in deflections of the hair bundles of otolithic receptor hair cells. The GIF is the sum of gravitational force and the inertial force due to linear acceleration. The usual clinical and experimental tests of otolith function have used GIFs (roll tilts re gravity or linear accelerations) as test stimuli. However, the opposite polarization of receptors across each otolithic macula is puzzling since a GIF directed across the otolith macula will excite receptors on one side of the line of polarity reversal (LPR at the striola) and simultaneously act to silence receptors on the opposite side of the LPR. It would seem the two neural signals from the one otolith macula should cancel. In fact, Uchino showed that instead of canceling, the simultaneous stimulation of the oppositely polarized hair cells enhances the otolithic response to GIF—both in the saccular macula and the utricular macula. For the utricular system there is also commissural inhibitory interaction between the utricular maculae in each ear. The results are that the one GIF stimulus will cause direct excitation of utricular receptors in the activated sector in one ear as well as indirect excitation resulting from the disfacilitation of utricular receptors in the corresponding sector on the opposite labyrinth. There are effectively two complementary parallel otolithic afferent systems—the sustained system concerned with signaling low frequency GIF stimuli such as roll head tilts and the transient system which is activated by sound and vibration. Clinical tests of the sustained otolith system—such as ocular counterrolling to roll-tilt or tests using linear translation—do not show unilateral otolithic loss reliably, whereas tests of transient otolith function [vestibular evoked myogenic potentials (VEMPs) to brief sound and vibration stimuli] do show unilateral otolithic loss. The opposing sectors of the maculae also explain the results of galvanic vestibular stimulation (GVS) where bilateral mastoid galvanic stimulation causes ocular torsion position similar to the otolithic response to GIF. However, GVS stimulates canal afferents as well as otolithic afferents so the eye movement response is complex.

Why the cerebellar shutdown/clampdown hypothesis of vestibular compensation is inconsistent with neurophysiological evidence

BACKGROUND

Vestibular compensation is the process by which the central nervous system (CNS) attempts to adapt to the loss of vestibular sensory inputs. As such, the compensation process is critically involved in the vestibular rehabilitation programs that are implemented by physical therapists for patients with vestibular disorders. One hypothesis regarding vestibular compensation, which has persisted in some of the published vestibular compensation literature and particularly on some vestibular and physical therapy websites, is the 'cerebellar shutdown' or 'cerebellar clampdown' hypothesis proposed by McCabe and Ryu in 1969. This hypothesis proposes that the cerebellum inhibits neuronal activity in the bilateral vestibular nuclei (VN) following unilateral vestibular loss (UVL), causing the VN contralateral to the UVL to be electrically silent during the early phases of vestibular compensation. Despite a wealth of evidence against this idea, it has gained traction amongst some physical therapists and has implications for vestibular rehabilitation early in the compensation process.

CONCLUSIONS

In this paper it is argued that the 'cerebellar shutdown' or 'clampdown' hypothesis is inconsistent with well accepted neurophysiological and imaging evidence and that it is also logically flawed.

Identification of New Biomarkers of Posturo-Locomotor Instability in a Rodent Model of Vestibular Pathology

The vestibular system plays a crucial role in maintaining postural balance. Unilateral vestibular lesions result in a typical syndrome characterized by postural imbalance, altered locomotor patterns and gaze stabilization, as well as cognitive and neurovegetative disorders. One of the main difficulties encountered in the development of new anti-vertigo drugs is the lack of sensitivity in the evaluation of this syndrome. Qualitative assessments of the vestibular syndrome have been developed, but methods of conducting quantitative evaluations are critically lacking. Recently, assessments with a dynamic weight-bearing device (DWB®, Bioseb) revealed postural alterations in rats subjected to unilateral vestibular neurectomy (UVN). Our team is evaluating a new version of this device capable of quantifying additional parameters of postural and locomotor equilibrium. The objective of this study was to use this device to assess these new posturo-locomotor parameters in a rat model of a vestibular pathology. The biomarkers measured by this device are as follows: the barycenter, the support surface and the weight distribution of the rats when they were moving or stationary. Before UVN, the rats showed a symmetric distribution of their weight along the lateral axis. In the acute phase after UVN on the left side, the rats distributed more weight on the right side than on the left side and then distributed more weight on the left side. These results corroborate those presented in our previous study. The support surface of the rats increased between 1 day and 30 days after UVN, and the barycenter distribution reflected the weight distribution. In addition, our results show smaller changes in the weight distributions when the animals are moving compared with when they are stationary in the acute phase after UVN. This study provides new information on the static and dynamic postural balance patterns observed after unilateral vestibular loss in rats. These data are relevant because they objectively quantify the posturo-locomotor component of vestibular syndrome as well as the compensatory strategies used after vestibular loss. These results may guide the development of rehabilitation protocols for vestibular patients and the validation of pharmacological compounds favoring the restoration of equilibrium.

Single neuron activity and c-Fos expression in the rat striatum following electrical stimulation of the peripheral vestibular system

Connections between the vestibular system and the basal ganglia have been postulated since the early 20th century. However, the results of electrophysiological studies investigating neuronal responses to electrical stimulation of the vestibular system have been inconsistent. The aim of this study was to investigate the effects of electrical stimulation of the vestibular labyrinth on single neuron activity and c-Fos expression in the rat striatum. We used electrical stimulation of the vestibular labyrinth (various intensities delivered to the round window) to examine the electrophysiological response of striatal neurons and c-Fos expression. From 507 single neurons recorded (n = 20 rats), no vestibular-responsive neuron was found at 1× and 2× the nystagmus threshold; however, 6 neurons were found at 3× the threshold. These neurons were found bilaterally, with a response latency of ~50 msec from the end of the stimulus. For the c-Fos study, the number of neurons expressing c-Fos was quantified using stereological methods. Stimulation at 2× the threshold for nystagmus (n = 5 rats) resulted in a significant decrease in the number of neurons expressing c-Fos in the bilateral striatum compared to both the sham control group (n = 5) and the lower stimulus intensity group (n = 5) (P ≤ 0.0001 for both). The results of this study demonstrate that: (1) some single striatal neurons respond to electrical vestibular stimulation, however, these responses are circumscribed and infrequent; (2) electrical stimulation of the vestibular labyrinth results in a decrease in the number of striatal neurons expressing c-Fos, in a current-dependent manner.

The mammalian efferent vestibular system plays a crucial role in vestibulo-ocular reflex compensation after unilateral labyrinthectomy

The α9-nicotinic acetylcholine receptor (α9-nAChR) subunit is expressed in the vestibular and auditory periphery, and its loss of function could compromise peripheral input from the predominantly cholinergic efferent vestibular system (EVS). A recent study has shown that α9-nAChRs play an important role in short-term vestibulo-ocular reflex (VOR) adaptation. We hypothesize that α9-nAChRs could also be important for other forms of vestibular plasticity, such as that needed for VOR recovery after vestibular organ injury. We measured the efficacy of VOR compensation in α9 knockout mice. These mice have deletion of most of the gene (chrna9) encoding the nAChR and thereby lack α9-nAChRs. We measured the VOR gain (eye velocity/head velocity) in 20 α9 knockout mice and 16 cba129 controls. We measured the sinusoidal (0.2-10 Hz, 20-100°/s) and transient (1,500-6,000°/s2) VOR in complete darkness before (baseline) unilateral labyrinthectomy (UL) and then 1, 5, and 28 days after UL. On day 1 after UL, cba129 mice retained ~50% of their initial function for contralesional rotations, whereas α9 knockout mice only retained ~20%. After 28 days, α9 knockout mice had ~50% lower gain for both ipsilesional and contralesional rotations compared with cba129 mice. Cba129 mice regained ~75% of their baseline function for ipsilesional and ~90% for contralesional rotations. In contrast, α9 knockout mice only regained ~30% and ~50% function, respectively, leaving the VOR severely impaired for rotations in both directions. Our results show that loss of α9-nAChRs severely affects VOR compensation, suggesting that complimentary central and peripheral EVS-mediated adaptive mechanisms might be affected by this loss.NEW & NOTEWORTHY Loss of the α9-nicotinic acetylcholine receptor (α9-nAChR) subunit utilized by the efferent vestibular system (EVS) has been shown to significantly affect vestibulo-ocular reflex (VOR) adaptation. In our present study we have shown that loss of α9-nAChRs also affects VOR compensation, suggesting that the mammalian EVS plays an important role in vestibular plasticity, in general, and that VOR compensation is a more distributed process than previously thought, relying on both central and peripheral changes.

Responses of non-eye-movement central vestibular neurons to sinusoidal yaw rotation in compensated macaques after unilateral semicircular canal plugging

After vestibular labyrinth injury, behavioral measures of vestibular performance recover to variable degrees (vestibular compensation). Central neuronal responses after unilateral labyrinthectomy (UL), which eliminates both afferent resting activity and sensitivity to movement, have been well-studied. However, unilateral semicircular canal plugging (UCP), which attenuates angular-velocity detection while leaving afferent resting activity intact, has not been extensively studied. The current study reports response properties of yaw-sensitive non-eye-movement rhesus macaque vestibular neurons after compensation from UCP. The responses at a series of frequencies (0.1-2 Hz) and peak velocities (15-210°/s) were compared between neurons recorded before and at least 6 wk after UCP. The gain (sp/s/°/s) of central type I neurons (responding to ipsilateral yaw rotation) on the side of UCP was reduced relative to normal controls at 0.5 Hz, ±60°/s [0.48 ± 0.30 (SD) normal, 0.32 ± 0.15 ipsilesion; 0.44 ± 0.2 contralesion]. Type II neurons (responding to contralateral yaw rotation) after UCP have reduced gain (0.40 ± 0.27 normal, 0.35 ± 0.25 ipsilesion; 0.25 ± 0.18 contralesion). The difference between responses after UCP and after UL is primarily the distribution of type I and type II neurons in the vestibular nuclei (type I neurons comprise 66% in vestibular nuclei normally; 51% ipsilesion UCP; 59% contralesion UCP; 38% ipsilesion UL; 65% contralesion UL) and the magnitude of the responses of type II neurons ipsilateral to the lesion. These differences suggest that the need to compensate for unilateral loss of resting vestibular nerve activity after UL necessitates a different strategy for recovery of dynamic vestibular responses compared to after UCP.

Visual dependency and dizziness after vestibular neuritis

Symptomatic recovery after acute vestibular neuritis (VN) is variable, with around 50% of patients reporting long term vestibular symptoms; hence, it is essential to identify factors related to poor clinical outcome. Here we investigated whether excessive reliance on visual input for spatial orientation (visual dependence) was associated with long term vestibular symptoms following acute VN. Twenty-eight patients with VN and 25 normal control subjects were included. Patients were enrolled at least 6 months after acute illness. Recovery status was not a criterion for study entry, allowing recruitment of patients with a full range of persistent symptoms. We measured visual dependence with a laptop-based Rod-and-Disk Test and severity of symptoms with the Dizziness Handicap Inventory (DHI). The third of patients showing the worst clinical outcomes (mean DHI score 36-80) had significantly greater visual dependence than normal subjects (6.35° error vs. 3.39° respectively, p = 0.03). Asymptomatic patients and those with minor residual symptoms did not differ from controls. Visual dependence was associated with high levels of persistent vestibular symptoms after acute VN. Over-reliance on visual information for spatial orientation is one characteristic of poorly recovered vestibular neuritis patients. The finding may be clinically useful given that visual dependence may be modified through rehabilitation desensitization techniques.

Long-term deficits in motion detection thresholds and spike count variability after unilateral vestibular lesion

The vestibular system operates in a push-pull fashion using signals from both labyrinths and an intricate bilateral organization. Unilateral vestibular lesions cause well-characterized motor deficits that are partially compensated over time and whose neural correlates have been traced in the mean response modulation of vestibular nuclei cells. Here we compare both response gains and neural detection thresholds of vestibular nuclei and semicircular canal afferent neurons in intact vs. unilateral-lesioned macaques using three-dimensional rotation and translation stimuli. We found increased stimulus-driven spike count variability and detection thresholds in semicircular canal afferents, although mean responses were unchanged, after contralateral labyrinth lesion. Analysis of trial-by-trial spike count correlations of a limited number of simultaneously recorded pairs of canal afferents suggests increased noise correlations after lesion. In addition, we also found persistent, chronic deficits in rotation detection thresholds of vestibular nuclei neurons, which were larger in the ipsilesional than the contralesional brain stem. These deficits, which persisted several months after lesion, were due to lower rotational response gains, whereas spike count variability was similar in intact and lesioned animals. In contrast to persistent deficits in rotation threshold, translation detection thresholds were not different from those in intact animals. These findings suggest that, after compensation, a single labyrinth is sufficient to recover motion sensitivity and normal thresholds for the otolith, but not the semicircular canal, system.

The changes in mGluR2 and mGluR7 expression in rat medial vestibular nucleus and flocculus following unilateral labyrinthectomy

It is known that the medial vestibular nucleus (MVN) and the cerebellar flocculus are the key areas, which contribute to the behavioral recovery ("vestibular compensation") after unilateral labyrinthectomy (UL). In these areas, how the genetic activities of the metabotropic glutamate receptors mGluR2 and mGluR7 performance after UL is unknown. With the means of quantitative real-time PCR, Western blotting, and immunohistochemistry, we analyzed the expression of mGluR2 and mGluR7 in the bilateral MVN and the flocculus of rats in different stages after UL (the 1st, 3rd, and 7th day). Our results show that in the MVN, the mRNA, and protein expressions of mGluR7 were ipsilaterally decreased at the 1st day following UL. However, in the MVN, no change was observed in the mRNA and protein expressions of mGluR2. On the other hand, the mRNA and protein expression of mGluR2 were enhanced in the ipsilateral flocculus at the 1st day following UL, while in the flocculus no change was shown in mGluR7 mRNA and protein expressions. Our results suggest that mGluR2 and mGluR7 may contribute to the early rebalancing of spontaneous resting activity in the MVN.

Responses of central vestibular neurons to sinusoidal yaw rotation in compensated macaques after unilateral labyrinthectomy

After vestibular labyrinth injury, behavioral measures of vestibular function partially recover through the process of vestibular compensation. The present study was performed to improve our understanding of the physiology of macaque vestibular nucleus neurons in the compensated state (>6 wk) after unilateral labyrinthectomy (UL). The responses of neurons to sinusoidal yaw rotation at a series of frequencies (0.1-2.0 Hz) and peak velocities (7.5-210°/s) were examined to determine how the behavior of these cells differed from those in animals with intact labyrinths. The sensitivity of neurons responding to ipsilateral rotation (type I) did not differ between the intact and injured sides after UL, although this sensitivity was lower bilaterally after lesion than before lesion. The sensitivity of neurons that increase firing with contralateral rotation (type II) was higher ipsilateral to the UL than before lesion or in the nucleus contralateral to the UL. UL did not increase asymmetry in the responses of individual type I or II neurons to ipsilateral vs. contralateral rotation, nor does it change the power law relationship between neuronal firing and level of stimulation. Increased sensitivities of contralesional type I neurons to the remaining vestibular nerve input and increased efficacy of inhibitory vestibular commissures projecting to the ipsilesional vestibular nucleus appear to be responsible for recovery of dynamic function of central vestibular neurons in compensated animals. The portion of type I neurons on the ipsilesional side is reduced in compensated animals, which likely accounts for the asymmetries in vestibular reflexes and perception that characterize vestibular function after UL.

Neurobiological mechanisms of acute vertigo

The vestibular system provides us with reflexive responses of eye movements and balance control, as well as with perceptual estimates of self-motion and gravity direction. Crucial to its proper functioning is a bilaterally balanced vestibular signal originating from the vestibular end organs in the inner ears and projecting via vestibular nerve afferents to the brainstem vestibular nuclei. Disturbances of the bilateral vestibular interplay become evident in cases of acute unilateral peripheral vestibular deafferentation. The resultant sudden imbalance of vestibular afferent tone at the level of the vestibular nuclei leads to pronounced ocular–motor and postural impairment, as well as to intensive vertigo and/or dizziness, accompanied by autonomic symptoms, such as nausea and vomiting. Subsequent compensatory mechanisms efficiently diminish these static symptoms (such as spontaneous nystagmus) within days and allow functional recovery of dynamic symptoms (such as blurred vision during fast head turns) to such a degree that most patients return to their normal daily activities within weeks. This article aims to provide an understanding about the pathophysiological changes after unilateral vestibular deafferentation and the current knowledge on the compensatory mechanisms.

Vestibular perception following acute unilateral vestibular lesions

Little is known about the vestibulo-perceptual (VP) system, particularly after a unilateral vestibular lesion. We investigated vestibulo-ocular (VO) and VP function in 25 patients with vestibular neuritis (VN) acutely (2 days after onset) and after compensation (recovery phase, 10 weeks). Since the effect of VN on reflex and perceptual function may differ at threshold and supra-threshold acceleration levels, we used two stimulus intensities, acceleration steps of 0.5°/s(2) and velocity steps of 90°/s (acceleration 180°/s(2)). We hypothesised that the vestibular lesion or the compensatory processes could dissociate VO and VP function, particularly if the acute vertiginous sensation interferes with the perceptual tasks. Both in acute and recovery phases, VO and VP thresholds increased, particularly during ipsilesional rotations. In signal detection theory this indicates that signals from the healthy and affected side are still fused, but result in asymmetric thresholds due to a lesion-induced bias. The normal pattern whereby VP thresholds are higher than VO thresholds was preserved, indicating that any 'perceptual noise' added by the vertigo does not disrupt the cognitive decision-making processes inherent to the perceptual task. Overall, the parallel findings in VO and VP thresholds imply little or no additional cortical processing and suggest that vestibular thresholds essentially reflect the sensitivity of the fused peripheral receptors. In contrast, a significant VO-VP dissociation for supra-threshold stimuli was found. Acutely, time constants and duration of the VO and VP responses were reduced - asymmetrically for VO, as expected, but surprisingly symmetrical for perception. At recovery, VP responses normalised but VO responses remained shortened and asymmetric. Thus, unlike threshold data, supra-threshold responses show considerable VO-VP dissociation indicative of additional, higher-order processing of vestibular signals. We provide evidence of perceptual processes (ultimately cortical) participating in vestibular compensation, suppressing asymmetry acutely in unilateral vestibular lesions.

Can imagined whole-body rotations improve vestibular compensation?

Unilateral damage to the labyrinth and the vestibular nerve cause rotational vertigo, postural imbalance, oculomotor disorders and spatial disorientation. Electrophysiological investigations in animals revealed that such deficits are partly due to imbalanced spontaneous activity and sensitivity to motion in neurons located in the ipsilesional and contralesional vestibular nuclei. Neurophysiological reorganizations taking place in the vestibular nuclei are the basis of the decline of the symptoms over time, a phenomenon known as vestibular compensation. Vestibular compensation is facilitated by motor activity and sensory experience, and current rehabilitation programs favor physical activity during the acute stage of a unilateral vestibular loss. Unfortunately, vestibular-defective patients tend to develop strategies in order to avoid movements causing imbalance and nausea (in particular body movements towards the lesioned side), which impedes vestibular compensation. Neuroanatomical evidence suggests a cortical control of postural and oculomotor reflexes based on corticofugal projections to the vestibular nuclei and, therefore, the possibility to manipulate vestibular functions through top-down mechanisms. Based on evidence from neuroimaging studies showing that imagined whole-body movements can activate part of the vestibular cortex, we propose that mental imagery of whole-body rotations to the lesioned and to the healthy side will help rebalancing the activity in the ipsilesional and contralesional vestibular nuclei. Whether imagined whole-body rotations can improve vestibular compensation could be tested in a randomized controlled study in such patients beneficiating, or not, from a mental imagery training. If validated, this hypothesis will help developing a method contributing to reduce postural instability and falls in vestibular-defective patients. Imagined whole-body rotations thus could provide a simple, safe, home-based and self-administered therapeutic method with the potential to overcome the inconvenience related to physical movements.

Multimodal integration after unilateral labyrinthine lesion: single vestibular nuclei neuron responses and implications for postural compensation

Plasticity in neuronal responses is necessary for compensation following brain lesions and adaptation to new conditions and motor learning. In a previous study, we showed that compensatory changes in the vestibuloocular reflex (VOR) following unilateral vestibular loss were characterized by dynamic reweighting of inputs from vestibular and extravestibular modalities at the level of single neurons that constitute the first central stage of VOR signal processing. Here, we studied another class of neurons, i.e., the vestibular-only neurons, in the vestibular nuclei that mediate vestibulospinal reflexes and provide information for higher brain areas. We investigated changes in the relative contribution of vestibular, neck proprioceptive, and efference copy signals in the response of these neurons during compensation after contralateral vestibular loss in Macaca mulata monkeys. We show that the time course of recovery of vestibular sensitivity of neurons corresponds with that of lower extremity muscle and tendon reflexes reported in previous studies. More important, we found that information from neck proprioceptors, which did not influence neuronal responses before the lesion, were unmasked after lesion. Such inputs influenced the early stages of the compensation process evidenced by faster and more substantial recovery of the resting discharge in proprioceptive-sensitive neurons. Interestingly, unlike our previous study of VOR interneurons, the improvement in the sensitivity of the two groups of neurons did not show any difference in the early or late stages after lesion. Finally, neuronal responses during active head movements were not different before and after lesion and were attenuated relative to passive movements over the course of recovery, similar to that observed in control conditions. Comparison of compensatory changes observed in the vestibuloocular and vestibulospinal pathways provides evidence for similarities and differences between the two classes of neurons that mediate these pathways at the functional and cellular levels.

The relationship of head movements to semicircular canal size in cetaceans

The semicircular canals measure head rotations, providing information critical for maintaining equilibrium. The canals of cetaceans (including whales, dolphins and porpoises) are extraordinarily small, making them unique exceptions to the allometric relationship shared by all other vertebrates between canal size and animal mass. Most modern cetaceans have shorter and less flexible necks than those of their ancestors, an adaptation hypothesized to have led to exaggerated head movements during locomotion. These movements are thought to have necessitated a decrease in the size and sensitivity of the canals, increasing their operating range to accommodate increased head motion. We tested whether the size of the semicircular canals in cetaceans is related to their head movements by comparing the rotational head velocities, frequencies and accelerations of the bottlenose dolphin (Tursiops truncatus) and a terrestrial relative, cattle (Bos taurus), using an array of three orthogonal head-fixed miniaturized rotational ratemeters. We collected data during typical locomotion (swimming; trotting) and during behaviors with enhanced head movements (rapid spiraling underwater; bucking). Cattle head movements always exceeded those of dolphins. Maximum head velocities were 528 deg. s(-1) in dolphins and 534 deg. s(-1) in cattle; maximum head frequencies were 2.86 Hz in dolphins and 3.45 Hz in cattle; and maximum head accelerations were 5253 deg. s(-2) in dolphins and 10,880 deg. s(-2) in cattle. These results indicate that accentuated head movements cannot explain the reduced size and sensitivity of cetacean semicircular canals. The evolutionary cause for their reduced canal size remains uncertain.

An increase in glycinergic quantal amplitude and frequency during early vestibular compensation in mouse

The process of vestibular compensation includes both behavioral and neuronal recovery after unilateral loss of peripheral vestibular organs. The mechanisms that underlie this process are poorly understood. Previous research has shown the presence of both gamma-aminobutyric acid type A (GABA(A)) and glycine receptors in the medial vestibular nuclei (MVN). It has been suggested that inhibitory transmission mediated by these receptors may have a role in recovery during vestibular compensation. This study investigated changes in fast inhibitory synaptic transmission of GABA(A)ergic and glycinergic quantal events after unilateral labyrinthectomy (UL) at three different time points. Mice were anesthetized and peripheral vestibular organs were removed from one side of the head. After recovery, transverse brain stem sections (300 mum) were prepared from mice that had undergone UL either 4 hours, 2 days, or 7 days earlier. Our experiments do not show evidence for alterations in synaptic GABA(A) receptor properties in MVN neurons after UL at any time point investigated. In contrast, during early vestibular compensation (4 hours post UL) there is a significant increase in the glycinergic quantal current amplitude in contralesional MVN neurons compared with control. Our results also show an increase in the frequency of glycinergic quantal events of both ipsi- and contralesional MVN neurons during this early period. We suggest that changes in both pre- and postsynaptic glycine receptor mediated inhibitory synaptic transmission after sensory loss is an important mechanism by which neuronal discharge patterns can be modulated.

Adaptation of chicken vestibular nucleus neurons to unilateral vestibular ganglionectomy

Vestibular compensation refers to the behavioral recovery after a unilateral peripheral vestibular lesion. In chickens, posture and balance deficits are present immediately following unilateral vestibular ganglionectomy (UVG). After three days, most operated chickens begin to recover, but severe deficits persist in others. The tangential nucleus is a major avian vestibular nucleus whose principal cells are vestibular reflex projection neurons. From patch-clamp recordings on brain slices, the percentage of spontaneous spike firing principal cells, spike discharge rate, ionic conductances, and spontaneous excitatory postsynaptic currents (sEPSCs) were investigated one and three days after UVG. Already by one day after UVG, sEPSC frequency increased significantly on the lesion side, although no differences were detected in the percentage of spontaneous spike firing cells or discharge rate. In compensated chickens three days after UVG, the percentage of spontaneous spike firing cells increased on the lesion side and the discharge rate increased bilaterally. In uncompensated chickens three days after UVG, principal cells on the lesion side showed increased discharge rate and increased sEPSC frequency, whereas principal cells on the intact side were silent. Typically, silent principal cells exhibited smaller persistent sodium conductances and higher activation thresholds for the fast sodium channel than spiking cells. In addition, silent principal cells on the intact side of uncompensated chickens had larger dendrotoxin-sensitive potassium conductance, with a higher ratio of Kv1.1 surface/cytoplasmic expression. Increased sEPSC frequency in principal cells on the lesion side of uncompensated chickens was accompanied by decreased Kv1.2 immunolabeling of presynaptic terminals on principal cell bodies. Thus, both intrinsic ionic conductances and excitatory synaptic inputs play crucial roles at early stages after lesions. Unlike the principal cells in compensated chickens which showed similar percentages of spontaneous spike firing cells, discharge rates, and sEPSC frequencies bilaterally, principal cells in uncompensated chickens displayed gross asymmetry in these properties bilaterally.

Anatomical and Physiological Considerations in Vestibular Dysfunction and Compensation

Sensory information from the vestibular, visual, and somatosensory/proprioceptive systems are integrated in the brain in complex ways to produce a final motor output to muscle groups for maintaining gaze, head and body posture, and controlling static and dynamic balance. The balance system is complex, which can make differential diagnosis of dizziness quite challenging. On the other hand, this complex system is organized anatomically in a variety of pathways and some of these pathways have been well studied. The vestibulo-ocular reflex (VOR) is one such pathway. Understanding the anatomy and physiology of the VOR facilitates our understanding of normal and abnormal eye movements and research is advancing our understanding of the plasticity of the vestibular system. This review highlights anatomical and physiological features of the normal vestibular system, applies these concepts to explain some clinical findings in some common peripheral vestibular disorders, and discusses some of the research investigating the anatomical and physiological basis for vestibular compensation.

Asymmetric recovery in cerebellar-deficient mice following unilateral labyrinthectomy

The term "vestibular compensation" refers to the resolution of motor deficits resulting from a peripheral vestibular lesion. We investigated the role of the cerebellum in the compensation process by characterizing the vestibuloocular reflex (VOR) evoked by head rotations at frequencies and velocities similar to those in natural behaviors in wild-type (WT) versus cerebellar-deficient Lurcher (Lc/+) mice. We found that during exploratory activity, normal mice produce head rotations largely consisting of frequencies < or =4 Hz and velocities and accelerations as large as 400 degrees/s and 5,000 degrees/s2, respectively. Accordingly, the VOR was characterized using sinusoidal rotations (0.2-4 Hz) as well as transient impulses (approximately 400 degrees/s; approximately 2,000 degrees/s2). Before lesions, WT and Lc/+ mice produced similar VOR responses to sinusoidal rotation. Lc/+ mice, however, had significantly reduced gains for transient stimuli. After unilateral labyrinthectomy, VOR recovery followed a similar course for WT and Lc/+ groups during the first week: gain was reduced by 80% for ipsilesionally directed head rotations on day 1 and improved for both strains to values of approximately 0.4 by day 5. Moreover, responses evoked by contralesionally directed rotations returned to prelesion in both strains within this period. However, unlike WT, which showed improving responses to ipsilesionally directed rotations, recovery plateaued after first week for Lc/+ mice. Our results show that despite nearly normal recovery in the acute phase, long-term compensation is compromised in Lc/+. We conclude that cerebellar pathways are critical for long-term restoration of VOR during head rotation toward the lesioned side, while noncerebellar pathways are sufficient to restore proper gaze stabilization during contralesionally directed movements.

Disynaptic and polysynaptic statocyst pathways to an identified set of premotor nonspiking interneurons in the crayfish brain

Nonspiking giant interneurons (NGIs) in the crayfish brain occupy a key position in the neuronal circuit for eyestalk motor control and, hence, play a crucial role in the central compensation process following unilateral deprivation of the statocyst that functions as an equilibrium sensory system. The synaptic organization of their input pathways, however, remains unknown. In the present study we identified 11 local interneurons that were polysynaptically connected to NGIs by making simultaneous intracellular recordings. We also identified three other local interneurons that connected to NGIs monosynaptically. PLNI-2 was a nonspiking interneuron making an excitatory synaptic connection to an NGI that had its cell body on the opposite side. PLSI was a spiking interneuron that made an inhibitory connection to an ipsilateral NGI. These cells were entirely confined to the protocerebrum. Another local spiking interneuron termed UGLI-1 was found to make an excitatory connection with a contralateral NGI, extending dendrites in the anterior and posterior medial protocerebral neuropils and the lateral antenna I neuropil in the deutocerebrum where statocyst afferents project. When a depolarizing current was injected into the UGLI-1, the frequency of discrete excitatory PSPs increased remarkably in the postsynaptic NGI, each PSP following the UGLI-1 spike in one-to-one correspondence. The UGLI was previously reported to be activated monosynaptically by statocyst afferents. The present study thus finally demonstrates the tri-synaptic organization of the statocyst-to-eyestalk motor neuron pathway in its simplest form, suggesting the critical role of the UGLI-1 in the central compensation.

Adaptive rescaling of central sensorimotor signals is preserved after unilateral vestibular damage

Adaptive rescaling is a widespread phenomenon that dynamically adjusts the input-output relationship of a sensory system in response to changes in the ambient stimulus conditions. Rescaling has been described in the central vestibular neurons of normal cats. After recovery from unilateral vestibular damage, the vestibulo-ocular reflex (VOR) remains nonlinear for rotation toward the damaged side. Therefore, rescaling in the VOR pathway may be especially important after damage. Here, we demonstrate that central vestibular neurons adjust their input-output relationships depending on the input velocity range, suggesting that adaptive rescaling is preserved after vestibular damage and can contribute to the performance of the VOR. We recorded from isolated vestibular neurons in alert cats that had recovered from unilateral vestibular damage. The peak velocity of 1-Hz sinusoidal rotation was varied from 10 to 120 degrees/s and the sensitivities and dynamic ranges of vestibular neurons were measured. Most neuronal responses showed significant nonlinearities even at the lowest peak velocity that we tested. Significant rescaling was seen in the responses of neurons both ipsilateral and contralateral to chronic unilateral damage. On the average, when the peak rotational velocity increased by a factor of 8, the average sensitivity to rotation decreased by roughly a factor of 2. Rescaling did not depend on eye movement signals. Our results suggest that the dynamic ranges of central neurons are extended by rescaling and that, after vestibular damage, adaptive rescaling may act to reduce nonlinearities in the response of the VOR to rotation at high speeds.

Physiological changes of premotor nonspiking interneurons in the central compensation of eyestalk posture following unilateral sensory ablation in crayfish

We investigated how the physiological characteristics and synaptic activities of nonspiking giant interneurons (NGIs), which integrate sensory inputs in the brain and send synaptic outputs to oculomotor neurons innervating eyestalk muscles, changed after unilateral ablation of the statocyst in order to clarify neuronal mechanisms underlying the central compensation process in crayfish. The input resistance and membrane time constant in recovered animals that restored the original symmetrical eyestalk posture 2 weeks after operation were significantly greater than those immediately after operation on the operated side whereas in non-recovered animals only the membrane time constant showed a significant increase. On the intact side, both recovered and non-recovered animals showed no difference. The frequency of synaptic activity showed a complex pattern of change on both sides depending on the polarity of the synaptic potential. The synaptic activity returned to the bilaterally symmetrical level in recovered animals while bilateral asymmetry remained in non-recovered ones. These results suggest that the central compensation of eyestalk posture following unilateral impairment of the statocyst is subserved by not only changes in the physiological characteristics of the NGI membrane but also the activity of neuronal circuits presynaptic to NGIs.

Dynamics of the horizontal vestibuloocular reflex after unilateral labyrinthectomy: response to high frequency, high acceleration, and high velocity rotations

Loss of vestibular information from one labyrinth results in a marked asymmetry in the horizontal vestibuloocular reflex (VOR). The results of prior studies suggest that long-term deficits in VOR are more severe in response to rapid impulses than to sinusoidal head movements. The goal of the present study was to investigate the VOR following unilateral labyrinthectomy in response to different stimuli covering the full range of physiologically relevant head movements in macaque monkeys. The VOR was studied 1-39 days post-lesion using transient head perturbations (up to 12,000 degrees/s(2)), rapid rotations (up to 500 degrees/s), and sinusoidal rotations (up to 15 Hz). In response to rotations with high acceleration or velocity, both contra- and ipsilesional gains remained subnormal. VOR gains decreased as a function of increasing stimulus acceleration or velocity, reaching minimal values of 0.7-0.8 and 0.3-0.4 for contra and ipsilesional rotations, respectively. For sinusoidal rotations with low frequencies and velocities, responses to contralesional stimulation recovered within approximately 4 days. With increasing velocities and frequencies of rotation, however, the gains of contra- and ipsilesional responses remained subnormal. For each of the most challenging stimuli tested (i.e., 12,000 degrees/s(2 )transient head perturbations, 500 degrees/s fast whole-body rotations and 15 Hz stimulation) no significant compensation was observed in contra- or ipsilesional responses over time. Moreover, we found that gain of the cervico-ocular reflex (COR) remained negligible following unilateral loss indicating that neck reflexes did not contribute to the observed compensation. VOR responses elicited by both sinusoidal and transient rotations following unilateral labyrinthectomy could be described by the same mathematical model. We conclude that the compensated VOR has comparable response dynamics for impulses and sinusoidal head movements.

Intrinsic membrane properties of vertebrate vestibular neurons: function, development and plasticity

Central vestibular neurons play an important role in the processing of body motion-related multisensory signals and their transformation into motor commands for gaze and posture control. Over recent years, medial vestibular nucleus (MVN) neurons and to a lesser extent other vestibular neurons have been extensively studied in vivo and in vitro, in a range of species. These studies have begun to reveal how their intrinsic electrophysiological properties may relate to their response patterns, discharge dynamics and computational capabilities. In vitro studies indicate that MVN neurons are of two major subtypes (A and B), which differ in their spike shape and after-hyperpolarizations. This reflects differences in particular K(+) conductances present in the two subtypes, which also affect their response dynamics with type A cells having relatively low-frequency dynamics (resembling "tonic" MVN cells in vivo) and type B cells having relatively high-frequency dynamics (resembling "kinetic" cells in vivo). The presence of more than one functional subtype of vestibular neuron seems to be a ubiquitous feature since vestibular neurons in the chick and frog also subdivide into populations with different, analogous electrophysiological properties. The ratio of type A to type B neurons appears to be plastic, and may be determined by the signal processing requirements of the vestibular system, which are species-variant. The membrane properties and discharge pattern of type A and type B MVN neurons develop largely post-natally, through the expression of the underlying ion channel conductances. The membrane properties of MVN neurons show rapid and long-lasting plastic changes after deafferentation (unilateral labyrinthectomy), which may serve to maintain their level of activity and excitability after the loss of afferent inputs.

Fos responses to short-term adaptation of the horizontal vestibuloocular reflex before and after vestibular compensation in the Mongolian gerbil

Fos expression in vestibular brainstem and cerebellar regions was evaluated during vestibular adaptation in the Mongolian gerbil. In addition, vestibular adaptation was evaluated in both normal and compensated animals, as vestibular compensation reorganizes the vestibular pathway constraining adaptive processes. Behaviorally, discordant optokinetic and vestibular input induced appropriate high and low gain in horizontal angular vestibuloocular reflex responses. In normal animals, low gain adaptation was more complete than high gain. However, in compensated animals, only low gain adaptation produced adaptive responses both toward and away from the lesion with appropriate gain shifts. High gain adaptation in compensated animals failed to result in gain adaptation for head movements toward the side of the lesion. Fos expression during acute vestibular adaptation in normal animals was found in the flocculus/paraflocculus, the dorsal cap of the inferior olive (IOK), and the prepositus hypoglossi (PrH). Floccular Fos labeling was increased under both high and low gain conditions. IOK and PrH labeling was increased and correlated during low gain conditions, but was reduced and uncorrelated during high gain conditions. The pattern of Fos labeling in compensated animals was asymmetric-favoring the ipsilesional flocculus and contralesional vestibular brainstem. Both compensated high and low gain adaptation groups displayed increased floccular and IOK Fos labeling, but only compensated high gain adaptation produced increased Fos labeling in the medial vestibular nucleus. The behavioral and Fos labeling results are consistent with visual-vestibular adaptation requiring direct vestibular input.

Torsional optokinetic nystagmus after unilateral vestibular loss: asymmetry and compensation

The aim of this study was to analyse torsional optokinetic nystagmus (tOKN) in 17 patients with Menière's disease before and after (1 week, 1 month and 3 months) a curative unilateral vestibular neurotomy (UVN). The tOKN was investigated during optokinetic stimulations around the line of sight directed towards either the lesioned or the healthy side, at various constant angular velocities. Dynamic properties of tOKN and static ocular cyclotorsion were analysed using videonystagmography. Patients' performances were compared with those of 10 healthy subjects. The results indicate that, in the acute stage after UVN, patients exhibited drastic impairment of tOKN velocity that depended on the direction of stimulation: tOKN velocity increased for ipsilesional stimulations and decreased for contralesional stimulations. These changes were responsible for a dramatic tOKN asymmetry, with ipsilesional directional preponderance of torsional slow-phase eye velocity. The changes were associated with static ocular cyclotorsion towards the operated side. Despite progressive compensation of tOKN deficits over time, tOKN velocity still differed from that recorded preoperatively, and tOKN asymmetry remained uncompensated 3 months after UVN. A static ocular cyclotorsion remained up to 3 months after lesion. These results are the first description of tOKN deficits and recovery after unilateral vestibular loss. They show that vestibular cues contribute to gaze stabilization during optokinetic stimulation around the line of sight. They also strongly suggest that tOKN impairment could be part of the long-term asymmetrical functions reported after unilateral loss of vestibular functions.

VOR and Fos response during acute vestibular compensation in the Mongolian gerbil in darkness and in light

We measured binocular horizontal eye movements in the gerbil following unilateral labyrinthectomy during the acute phase (1-24 h) of vestibular compensation. Regardless of whether the animals compensated in the light or the dark, VOR gain progressively reduced following the lesion, and normal oculomotor symmetry was disrupted. Initially, the VOR was comparable at 1 h post-lesion for both visual conditions. However, by 3 h post-lesion the VOR response for head turns away from the lesion continued to drop in animals compensating in the dark. By 24 h, both groups displayed reduced VOR gains, but animals compensating in the light had improved frequency response characteristics. Optokinetic responses became unstable but were generally elevated compared to pre-lesion levels. Animals with vision had reduced optokinetic gains by 24 h, while the OKR response for animals in the dark remained elevated. Brainstem Fos labeling generally increased from 1 to 3 h, then decreased by 24 h. However, at 1 h, Fos labeling in the inferior olivary dorsal cap and prepositus contralateral to the lesion was significantly increased in animals compensating in the light. In both visual conditions, flocculus and paraflocculus Purkinje cell labeling was also observed, and some of the Fos-labeled cells in the medial vestibular nucleus were commissural. Fos in the dorsal cap and prepositus could be attributed to the presence of visual input. While the visually related prepositus Fos labeling preceded improved VOR performance, the dorsal cap appeared to be involved in resolving visual and motor deficits from spontaneous nystagmus.

Relationship of static and dynamic mechanisms in vestibuloocular reflex compensation

OBJECTIVES/HYPOTHESIS

We tested the hypothesis that recovery of dynamic oculomotor behavior (specifically the vestibuloocular reflex [VOR]) after a unilateral labyrinthectomy (UL) is independent from static deficit recovery (e.g., spontaneous nystagmus). This hypothesis predicts that VOR recovery from peripheral vestibular lesions that do not cause static symptoms, such as unilateral semicircular canal plugging (UCP), would have a similar time course and magnitude as recovery from a lesion that creates both a static and dynamic imbalance, such as UL. Furthermore, animals compensated after UCP would be expected to retain their compensated VOR response after the additional insult of ipsilateral labyrinthectomy.

STUDY DESIGN

An experimental study in the Mongolian gerbil animal model.

METHODS

The horizontal VOR was measured from both eyes using infrared video-oculography on gerbils before and after UCP, UL, or ipsilateral labyrinthectomy after a previous UCP. Eye movements were recorded during yaw rotation in the dark.

RESULTS

UL resulted in a more severe acute deficit than the UCP. Over several weeks, the UCP animals compensated their horizontal VOR, particularly on rotation toward the intact side, quicker and more completely than the UL animals. Animals that underwent ipsilateral labyrinthectomy 8 to 11 weeks after UCP demonstrated preservation of the improved gain, particularly on rotation toward the intact labyrinth. However, the difference between the UL groups with or without precedent UCP was not retained after 72 hours, and long-term compensation was poorest in the UL after UCP group.

CONCLUSIONS

Plasticity in dynamic vestibular reflexes induced by UCP is preserved after a subsequent UL. However, neurologic events during the first and second day after UL appear to limit, change, or suppress the long-term dynamic compensation of the VOR, regardless of whether the animal had a previous UCP.

An in situ hybridization and immunofluorescence study of GABAA and GABAB receptors in the vestibular nuclei of the intact and unilaterally labyrinthectomized rat

We investigated whether the production of the sixteen subunits of the GABA(A) receptors and of the different variants of GABA Breceptors are modulated in rat medial vestibular nuclei (MVN) following unilateral labyrinthectomy. Specific alpha1-6, beta1-3, gamma1-3 and delta GABA(A) and GABA(B) B1 and B2receptor radioactive oligonucleotides were used for in situ hybridization to probe sections of rat vestibular nuclei. Specific antibodies against alpha1, beta2, beta3 and gamma2 subunits of GABA(A) receptors and against GABA( B)receptors were also used to detect a potential protein expression modulation. No asymmetry was observed by autoradiography in the intact and deafferented MVN at any time (5 h to 8 days) following the lesion and for any of the oligonucleotide probes used. Also, no difference in the alpha1, beta2, beta3 and gamma2 of the GABA(A) and in the GABA(B) receptor immunohistochemical signal could be detected between the intact and deafferented vestibular nuclei at any time following the lesion. Our data suggest that GABA(A) and GABA Breceptor density changes most probably were not involved in the early stage of the vestibular compensation process, i.e., in the restoration of a normal resting discharge of the deafferented vestibular neurons and consequently in the recovery of a normal posture and eye position.

Glutamate release in the rat medial vestibular nucleus following unilateral labyrinthectomy using in vivo microdialysis

We investigated the changes in glutamate release from the ipsi- and contra-lesional medial vestibular nucleus (MVN) following unilateral labyrinthectomy (UL) by in vivo microdialysis study. The concentration of glutamate in the ipsi-lesional MVN was decreased until 4 h. Twelve hours after UL, the concentration of glutamate was restored back to the basal level, after which the release did not show any change between 24 and 48 h post-UL. In contrast, the concentration of glutamate in the contra-lesional MVN, which increased immediately after UL, decreased gradually to the basal level until 3-4 h post-UL, followed by no further change. The difference in the glutamate concentration between ipsi- and contra-lesional MVN increased immediately after UL and gradually decreased accompanied by a reduction in the frequency of nystagmus, although spontaneous nystagmus had not disappeared by the time the imbalance of glutamate release diminished. These results suggest that the imbalance of glutamate release between bilateral nuclei induced the nystagmus, and the change in release is concerned with the rapid development of vestibular compensation.

Central vestibular system: vestibular nuclei and posterior cerebellum

The vestibular nuclei and posterior cerebellum are the destination of vestibular primary afferents and the subject of this review. The vestibular nuclei include four major nuclei (medial, descending, superior and lateral). In addition, smaller vestibular nuclei include: Y-group, parasolitary nucleus, and nucleus intercalatus. Each of the major nuclei can be subdivided further based primarily on cytological and immunohistochemical histological criteria or differences in afferent and/or efferent projections. The primary afferent projections of vestibular end organs are distributed to several ipsilateral vestibular nuclei. Vestibular nuclei communicate bilaterally through a commissural system that is predominantly inhibitory. Secondary vestibular neurons also receive convergent sensory information from optokinetic circuitry, central visual system and neck proprioceptive systems. Secondary vestibular neurons cannot distinguish between sources of afferent activity. However, the discharge of secondary vestibular neurons can distinguish between "active" and "passive" movements. The posterior cerebellum has extensive afferent and efferent connections with vestibular nuclei. Vestibular primary afferents are distributed to the ipsilateral uvula-nodulus as mossy fibers. Vestibular secondary afferents are distributed bilaterally. Climbing fibers to the cerebellum originate from two subnuclei of the contralateral inferior olive; the dorsomedial cell column and beta-nucleus. Vestibular climbing fibers carry information only from the vertical semicircular canals and otoliths. They establish a coordinate map, arrayed in sagittal zones on the surface of the uvula-nodulus. Purkinje cells respond to vestibular stimulation with antiphasic modulation of climbing fiber responses (CFRs) and simple spikes (SSs). The modulation of SSs is out of phase with the modulation of vestibular primary afferents. Modulation of SSs persists, even after vestibular primary afferents are destroyed by a unilateral labyrinthectomy, suggesting that an interneuronal network, triggered by CFRs is responsible for SS modulation. The vestibulo-cerebellum, imposes a vestibular coordinate system on postural responses and permits adaptive guidance of movement.

The vestibulo ocular reflex (VOR) in otoconia deficient head tilt (het) mutant mice versus wild type C57BL/6 mice

The horizontal and vertical vestibulo ocular reflex (VOR) of head tilted (het) mutant mice was compared to C57BL/6 controls. Eye movements were recorded in darkness using a temporarily attached search coil. Contributions of semicircular canal versus otolith organ signals were investigated by providing a canal only (vertical axis) or canal plus otolith organ (horizontal axis) stimulus. In controls, rotations that stimulated only the canals (upright yaw and on tail roll) produced accurate VOR timing during middle frequency rotations at 0.5 Hz (gain 0.27, phase error 6 degrees), and a phase advanced VOR during low frequency rotations at 0.05 Hz (0.05, 115 degrees). In het mutant mice, these rotations produced a highly attenuated VOR response and phase errors at both 0.5 Hz (0.11, 42 degrees) and 0.05 Hz (0.01, 36 degrees). In controls, rotations that stimulated both the otolith organs and semicircular canals (upright roll and on tail yaw) produced higher VOR gains overall than were elicited during vertical axis rotations, with phase accurate VOR at both 0.5 Hz (0.52, 4 degrees) and 0.05 Hz (0.34, 9 degrees). In het mutant mice, these rotations produced a highly attenuated VOR response and phase errors at both 0.5 Hz (0.14, 51 degrees) and 0.05 Hz (0.01, 43 degrees). During constant velocity rotations about an earth horizontal axis, eye velocity bias and modulation were virtually absent in het mutant mice, while robust in controls. Control mice produced compensatory ocular deviations in response to static head tilt, but responses in het mice were weak and inconsistent. These results show that het mice not only lack all aspects of otolith mediated VOR, but also are deficient in canal mediated VOR. Because semicircular canals are normal in het mice, we conclude that central neurons of the canal VOR are dependent on otolith organ signals for normal performance.

A realistic neural-network simulation of both slow and quick phase components of the guinea pig VOR

A realistic neural-network model was constructed to simulate production of both the slow-phase and quick-phase components of vestibular nystagmus by incorporating a quick-phase pathway into a previous model of the slow phase. The neurons in the network were modelled by multicompartmental Hodgkin-Huxley-style spiking neurons based on known responses and projections of physiologically identified vestibular neurons. The modelling used the GENESIS software package. The slow-phase network consisted of ganglion and medial vestibular nucleus (MVN) neurons; the latter were constructed using biophysical models of MVN type A and B neurons. The quick-phase network contained several types of bursting cells which have been shown to have major roles in the generation of the quick phase: burster-driver neurons, long-lead burst neurons, pause neurons, excitatory burst neurons and inhibitory burst neurons. Comparison of the output neural responses from the model with guinea pig behavioural responses from the companion paper showed consistency between model and animal data for neuron firing patterns, maximal firing rates, and timing, duration and number of quick phases. Comparisons were made for stable head input and for sinusoidal angular stimuli at a range of frequencies from 0.1 to 2 Hz. Except for data at 0.1 Hz, where the simulation produced one more quick phase per half cycle than the animal data, the number of quick phases was consistent between the model and the animal data. The model was also used to simulate the effects both of unilateral vestibular deafferentation (UVD) and of vestibular compensation after UVD, and the responses in the modelled MVN neurons were affected in a way similar to those measured in guinea pig MVN neurons: the number of quick phases and their timing changed in a similar fashion to that observed in behavioural data.

The response of vestibulo-ocular reflex pathways to electrical stimulation after canal plugging

The vestibulo-ocular reflex (VOR) allows clear vision during head movements by generating compensatory eye movements. Its response to horizontal rotation is reduced after one horizontal semicircular canal is plugged, but recovers partially over time. The majority of VOR interneurons contribute to the shortest VOR pathway, the so-called three-neuron arc, which includes only two synapses in the brainstem. After a semicircular canal is plugged, transmission of signals by the three-neuron arc originating from the undamaged side may be altered during recovery. We measured the oculomotor response to single current pulses delivered to the vestibular labyrinth of alert cats between 9 h and 1 month after plugging the contralateral horizontal canal. The same response was also measured after motor learning induced by continuously-worn telescopes (optically induced motor learning). Optically induced learning did not change the peak velocity of the evoked eye movement (PEEV) significantly but, after a canal plug, the PEEV increased significantly, reaching a maximum during the first few post-plug days and then decreasing. VOR gain also showed transient changes during recovery. Because the PEEV occurred early in the eye movement evoked by a current pulse, we think the observed increase in PEEV represented changes in transmission by the three-neuron arc. Sham surgery did not result in significant changes in the response to electrical stimulation or in VOR gain. Our data suggest that different pathways and processes may underlie optically induced motor learning and recovery from plugging of the semicircular canals.

Changes in fos expression in the rat brainstem after bilateral labyrinthectomy

In order to elucidate the role of the vestibulocerebellar neural circuits during two-stage bilateral labyrinthectomy (BL) we examined Fos-like immunoreactive (-LIR) neurons as a marker of neural activation in the rat brainstem after BL and the projections of these neurons into the vestibulocerebellum using retrograde tracing and immunohistochemical techniques. Simultaneous BL did not show any Fos expression in the medial vestibular nucleus (MVe). However, with an interval of > 6 h between the two stages of BL, Fos-LIR neurons were induced in the ipsilateral (ipsi-) MVe of the side operated on second, partially projecting into the ipsi-vestibulocerebellum. A previous study demonstrated that some of the unilateral labyrinthectomy-induced Fos-LIR neurons in the ipsi-MVe projected into the ipsi-vestibulocerebellum and inhibited the contra-MVe neurons, resulting in the restoration of the right-left balance. Taken together with our present data, it is suggested that some of the labyrinthectomy-induced Fos expression in the ipsi-MVe is representative of a switch-on signal for the vestibulo-cerebello-vestibular inhibitory neural circuits, which remain turned off after symmetrical lesions such as simultaneous BL but are turned on after severe asymmetrical lesions in order to restore the balance.

Spontaneous activity in rat vestibular nuclei in brain slices and effects of acetylcholine agonists and antagonists

Extracellular recording was used to investigate spontaneously active neurons in all four major nuclei of the rat vestibular nuclear complex (VNC) in brainstem slices. The density of spontaneously active neurons was highest in the medial vestibular nucleus (MVN), slightly lower in the superior (SuVN) and spinal (SpVN) nuclei, and lowest in the lateral vestibular nucleus (LVN). We compared the effects of acetylcholine agonists and antagonists on spontaneously discharging neurons in MVN, SuVN, and SpVN with those in the nearby dorsal cochlear nucleus (DCN). The proportion of neurons responding to carbachol was greatest in DCN and smallest in SpVN. Unlike in DCN, some neurons in MVN, SuVN, and SpVN showed decreased firing during carbachol or muscarine. Magnitudes of responses to carbachol and muscarine were closely correlated (P<0.01). MVN neurons possessed nicotinic as well as muscarinic receptors. Activation of either type was unaffected by blocking synaptic transmission. The IC(50) values for the muscarinic subtype-preferential antagonists were compared, and tropicamide, preferential for M(4), was the most potent. Our results suggest that: (1) the relative numbers of spontaneously active neurons in rat VNC differ among nuclei; (2) acetylcholine agonists elicit changes in mean firing rates of neurons in MVN, SuVN and SpVN, but fewer neurons respond, and responses are smaller than in DCN; (3) both muscarinic and nicotinic acetylcholine receptors are present on MVN neurons, but muscarinic receptors may be more prominent.

Modifications of vestibular responses of individual reticulospinal neurons in lamprey caused by unilateral labyrinthectomy

A postural control system in the lamprey is driven by vestibular input and maintains the dorsal-side-up orientation of the animal during swimming. After a unilateral labyrinthectomy (UL), the lamprey continuously rolls toward the damaged side. Normally, a recovery of postural equilibrium ("vestibular compensation") takes about 1 mo. However, illumination of the eye contralateral to UL results in an immediate and reversible restoration of equilibrium. Here we used eye illumination as a tool to examine a functional recovery of the postural network. Important elements of this network are the reticulospinal (RS) neurons, which are driven by vestibular input and transmit commands for postural corrections to the spinal cord. In this study, we characterized modifications of the vestibular responses in individual RS neurons caused by UL and the effect exerted on these responses by eye illumination. The activity of RS neurons was recorded from their axons in the spinal cord by chronically implanted electrodes, and spikes in individual axons were extracted from the population activity signals. The same neurons were recorded both before and after UL. Vestibular stimulation (rotation in the roll plane through 360 degrees ) and eye illumination were performed in quiescent animals. It was found that the vestibular responses on the UL-side changed only slightly, whereas the responses on the opposite side disappeared almost completely. This asymmetry in the bilateral activity of RS neurons is the most likely cause for the loss of equilibrium in UL animals. Illumination of the eye contralateral to UL resulted, first, in a restoration of vestibular responses in the neurons inactivated by UL and in an appearance of vestibular responses in some other neurons that did not respond to vestibular input before UL. These responses had directional sensitivity and zones of spatial sensitivity similar to those observed before UL. However, their magnitude was smaller than before UL. Second, the eye illumination caused a reduction of the magnitude of vestibular responses on the UL side. These two factors tend to restore symmetry in bilateral activity of RS neurons, which is the most likely cause for the recovery of equilibrium in the swimming UL lamprey. Results of this study are discussed in relation to the model of the roll control system proposed in our previous studies as well as in relation to the vestibular compensation.

Influence of canal paresis and compensation on gain and time constant of nystagmus slow-phase velocity to yaw-axis rotation

In order to assess the influence of canal paresis and compensation on the yaw-axis rotatory chair test parameters gain and time constant, a study was conducted in 435 patients diagnosed with unilateral peripheral vestibulopathy. Patients were grouped according to the amount of canal paresis found in the caloric test and by the result of a bedside examination of the vestibulo-ocular reflex (VOR). The time constant of the VOR diminishes as canal paresis increases and, when using a high-velocity sinusoidal test, there is a similar reduction in gain. Mean time constant was reduced when rotations were to the side of the lesion (ipsilesional) and to the normal (contralesional) and differences between both rotations were found for 21-80%, canal paresis. Ipsilesional gain was significantly less than contralesional gain, which had a normal result independent of the amount of canal paresis; differences between them were significant when canal paresis was > 41%. Spontaneous nystagmus significantly influenced the appearance of asymmetries in gain and time constant in the tests performed while, with high-velocity sinusoidal rotation, gain was found to be significantly different for ipsilesional and contralesional rotations in non-acute symptomatic patients.

Synaptic plasticity in the medial vestibular nuclei: role of glutamate receptors and retrograde messengers in rat brainstem slices

The analysis of cellular-molecular events mediating synaptic plasticity within vestibular nuclei is an attempt to explain the mechanisms underlying vestibular plasticity phenomena. The present review is meant to illustrate the main results, obtained in vitro, on the mechanisms underlying long-term changes in synaptic strength within the medial vestibular nuclei. The synaptic plasticity phenomena taking place at the level of vestibular nuclei could be useful for adapting and consolidating the efficacy of vestibular neuron responsiveness to environmental requirements, as during visuo-vestibular recalibration and vestibular compensation. Following a general introduction on the most salient features of vestibular compensation and visuo-vestibular adaptation, which are two plastic events involving neuronal circuitry within the medial vestibular nuclei, the second and third sections describe the results from rat brainstem slice studies, demonstrating the possibility to induce long-term potentiation and depression in the medial vestibular nuclei, following high frequency stimulation of the primary vestibular afferents. In particular the mechanisms sustaining the induction and expression of vestibular long-term potentiation and depression, such as the role of various glutamate receptors and retrograde messengers have been described. The relevant role of the interaction between the platelet-activating factor, acting as a retrograde messenger, and the presynaptic metabotropic glutamate receptors, in determining the full expression of vestibular long-term potentiation is also underlined. In addition, the mechanisms involved in vestibular long-term potentiation have been compared with those leading to long-term potentiation in the hippocampus to emphasize the most significant differences emerging from vestibular studies. The fourth part, describes recent results demonstrating the essential role of nitric oxide, another retrograde messenger, in the induction of vestibular potentiation. Finally the fifth part suggests the possible functional significance of different action times of the two retrograde messengers and metabotropic glutamate receptors, which are involved in mediating the presynaptic mechanism sustaining vestibular long-term potentiation.

Intrinsic excitability changes in vestibular nucleus neurons after unilateral deafferentation

Two synergistic plastic mechanisms have recently been identified in rat medial vestibular nucleus (MVN) neurons during 'vestibular compensation', the behavioral recovery that follows damage to the vestibular receptors or nerve of one inner ear. Ipsi-lesional MVN neurons develop a significant increase in their intrinsic excitability, and a marked decrease in the functional efficacy of GABA(A) and GABA(B) receptors, within 4 h of unilateral vestibular deafferentation. These mechanisms presumably counteract the disfacilitation and excessive commissural inhibition of the ipsi-lesional cells after deafferentation, and thus promote the recovery of resting activity. In this study, we investigated the intrinsic membrane properties and spike firing characteristics of rostral ipsi-lesional MVN neurons in slices from animals that underwent vestibular compensation for either 24-72 h or 7-10 days. Significant changes were observed in the spontaneous in vitro discharge rate, resting membrane potentials and voltage-activated membrane conductances of type B cells, but not type A cells. There was a significant increase in the number of type B(LTS) cells compared to normal. These findings indicate that during vestibular compensation marked changes occur in ion channel expression and function selectively in type B MVN neurons. These changes are appropriate to increase the responsiveness of type B cells both to their own intrinsic pacemaker-like membrane conductances and excitatory synaptic inputs. Together with the downregulation of inhibitory receptor efficacy, this increased intrinsic excitability may be sufficient to restore the resting discharge of the deafferented neurons in vivo. These results therefore provide further evidence for synaptic and neuronal plasticity in ipsi-lesional MVN neurons during vestibular compensation.

Rapid compensatory changes in GABA receptor efficacy in rat vestibular neurones after unilateral labyrinthectomy

1. The inhibitory effects of the GABAA agonist muscimol and the GABAB agonist baclofen on tonically active medial vestibular nucleus (MVN) neurones were recorded in slices of the rat dorsal brainstem in vitro, to determine whether any changes occurred in the functional efficacy of GABAergic inhibition in these cells during the initial rapid stage of 'vestibular compensation', the behavioural recovery that takes place after unilateral labyrinthectomy (UL). These experiments were carried out in preparations where the midline was cut, severing all commissural connections between the two vestibular nuclei. 2. Slices of the MVN were prepared from normal animals and animals that had been unilaterally labyrinthectomised 4 h earlier. The mean in vitro discharge rate of MVN neurones in the rostral region of the ipsi-lesional nucleus after UL was significantly higher than that in control slices, confirming our earlier reports of an increase in intrinsic excitability of these cells in the early stage of vestibular compensation. The in vitro discharge rates of caudal ipsi-lesional MVN cells, and rostral and caudal contra-lesional MVN cells, were not different from controls. 3. Muscimol and baclofen caused reversible, dose-related inhibition of the tonic discharge rate of MVN cells in control slices. In slices prepared from UL animals, MVN cells in the rostral region of the ipsi-lesional nucleus showed a marked downregulation of their response to both muscimol and baclofen, seen as a rightward shift and a decrease in slope of the dose-response relationships for the two agonists. In the contra-lesional nucleus, there was a small but significant upregulation of the responsiveness of both rostral and caudal MVN cells to baclofen, and a marked upregulation of the responsiveness of caudal MVN cells to muscimol. 4. In slices from animals that had undergone bilateral labyrinthectomy 4 h earlier, the downregulation of the functional efficacy of GABA receptors in the rostral MVN cells did not occur. The changes in GABA receptor efficacy after UL are therefore not due to the vestibular de-afferentation itself, but are instead due to the imbalance in excitability of the vestibular nuclei of the lesioned and intact sides, and the enhanced commissural inhibition of the ipsi-lesional MVN cells that follows UL. 5. The downregulation of GABA receptor efficacy in the ipsi-lesional MVN neurones is functionally compensatory, in that their response to commissural and cerebellar inhibitory drive will be significantly reduced after UL. Their intrinsic membrane conductances, and their remaining excitatory synaptic inputs, will consequently be more effective in causing depolarisation and the restoration of resting activity. Simultaneously the upregulation of GABAergic efficacy in the contra-lesional MVN will tend to reduce the hyperactivity on the contralateral side. These adaptive changes therefore represent a plausible cellular mechanism for the recovery of resting discharge in the ipsi-lesional MVN neurones, and the 're-balancing' of the excitability of the vestibular neurones of the lesioned and intact sides, as occurs after UL in vivo. 6. We propose that the adaptive regulation of the functional efficacy of GABA receptors in the MVN neurones may be an important cellular mechanism for the 'homeostasis of bilateral excitability' of the vestibular nuclei of the two sides.