Survival of function in the deafferentated vestibular nerve

How Peripheral Vestibular Damage Affects Velocity Storage: a Causative Explanation

Velocity storage is a centrally-mediated mechanism that processes peripheral vestibular inputs. One prominent aspect of velocity storage is its effect on dynamic responses to yaw rotation. Specifically, when normal human subjects are accelerated to constant angular yaw velocity, horizontal eye movements and perceived angular velocity decay exponentially with a time constant circa 15-30 s, even though the input from the vestibular periphery decays much faster (~ 6 s). Peripheral vestibular damage causes a time constant reduction, which is useful for clinical diagnoses, but a mechanistic explanation for the relationship between vestibular damage and changes in these behavioral dynamics is lacking. It has been hypothesized that Bayesian optimization determines ideal velocity storage dynamics based on statistics of vestibular noise and experienced motion. Specifically, while a longer time constant would make the central estimate of angular head velocity closer to actual head motion, it may also result in the accumulation of neural noise which simultaneously degrades precision. Thus, the brain may balance these two effects by determining the time constant that optimizes behavior. We applied a Bayesian optimal Kalman filter to determine the ideal velocity storage time constant for unilateral damage. Predicted time constants were substantially lower than normal and similar to patients. Building on our past work showing that Bayesian optimization explains age-related changes in velocity storage, we also modeled interactions between age-related hair cell loss and peripheral damage. These results provide a plausible mechanistic explanation for changes in velocity storage after peripheral damage. Results also suggested that even after peripheral damage, noise originating in the periphery or early central processing may remain relevant in neurocomputations. Overall, our findings support the hypothesis that the brain optimizes velocity storage based on the vestibular signal-to-noise ratio.

Increased Glial Fibrillary Acidic Protein Immunoreactivity in Astrocytes within the Lateral Vestibular Nucleus of the Cat following Labyrinthectomy and Vestibular Neurectomy

Unilateral vestibular injury evokes a characteristic pattern of acute disorganization of posture, locomotion, and eye movements. Following this acute stage, functional recovery occurs. In the present study, unilateral labyrinthectomy and vestibular neurectomy were performed in cats. The lateral vestibular nucleus (LVN) and vestibular nerve root entry zone on both sides of the brain stem were examined 24 hours 3 days and 8 weeks after operation by use of an immunochemical astrocyte marker, glial fibrillary acidic protein (GFAP). The results demonstrate extensive GFAP immunoreactivity within the ipsilateral nerve root following neurectomy, but not after labyrinthectomy Prominent GFAP-immunoreactive astrocytic processes were detected in the LVN both ipsilateral and contralateral to neurectomy and labyrinthectomy Within the ipsilateral LVN, the intensity of GFAP immunoreactivity was greater following neurectomy than after labyrinthectomy but the pattern of GFAP reactivity remained similar. In the contralateral LVN, GFAP reactivity was noted exclusively in the dorsal-rostral region corresponding to the zone of cerebellar afferents to the LVN. The results of the present study suggest that reactive astroglia may play an important role in the mechanism that leads to vestibular compensation.

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.

Vestibular damage in chronic ototoxicity: a mini-review

Ototoxicity is a major cause of the loss of hearing and balance in humans. Ototoxic compounds include pharmaceuticals such as aminoglycoside antibiotics, anti-malarial drugs, loop diuretics and chemotherapeutic platinum agents, and industrial chemicals including several solvents and nitriles. Human and rodent data indicate that the main target of toxicity is hair cells (HCs), which are the mechanosensory cells responsible for sensory transduction in both the auditory and the vestibular system. Nevertheless, the compounds may also affect the auditory and vestibular ganglion neurons. Exposure to ototoxic compounds has been found to cause HC apoptosis, HC necrosis, and damage to the afferent terminals, of differing severity depending on the ototoxicity model. One major pathway frequently involved in HC apoptosis is the c-jun N-terminal kinase (JNK) signaling pathway activated by reactive oxygen species, but other apoptotic pathways can also play a role in ototoxicity. Moreover, little is known about the effects of chronic low-dose exposure. In rodent vestibular epithelia, extrusion of live HCs from the sensory epithelium may be the predominant form of cell demise during chronic ototoxicity. In addition, greater involvement of the afferent terminals may occur, particularly the calyx units contacting type I vestibular HCs. As glutamate is the neurotransmitter in this synapse, excitotoxic phenomena may participate in afferent and ganglion neuron damage. Better knowledge of the events that take place in chronic ototoxicity is of great interest, as it will increase understanding of the sensory loss associated with chronic exposure and aging.

Basic Concepts in Understanding Recovery of Function in Vestibular Reflex Networks during Vestibular Compensation

Unilateral peripheral vestibular lesions produce a syndrome of oculomotor and postural deficits with the symptoms at rest, the static symptoms, partially or completely normalizing shortly after the lesion due to a process known as vestibular compensation. The symptoms are thought to result from changes in the activity of vestibular sensorimotor reflexes. Since the vestibular nuclei must be intact for recovery to occur, many investigations have focused on studying these neurons after lesions. At present, the neuronal plasticity underlying early recovery from the static symptoms is not fully understood. Here we propose that knowledge of the reflex identity and input–output connections of the recorded neurons is essential to link the responses to animal behavior. We further propose that the cellular mechanisms underlying vestibular compensation can be sorted out by characterizing the synaptic responses and time course for change in morphologically defined subsets of vestibular reflex projection neurons. Accordingly, this review focuses on the perspective gained by performing electrophysiological and immunolabeling studies on a specific subset of morphologically defined, glutamatergic vestibular reflex projection neurons, the principal cells of the chick tangential nucleus. Reference is made to pertinent findings from other studies on vestibular nuclei neurons, but no comprehensive review of the literature is intended since broad reviews already exist. From recording excitatory and inhibitory spontaneous synaptic activity in principal cells, we find that the rebalancing of excitatory synaptic drive bilaterally is essential for vestibular compensation to proceed. This work is important for it defines for the first time the excitatory and inhibitory nature of the changing synaptic inputs and the time course for changes in a morphologically defined subset of vestibular reflex projection neurons during early stages of 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.

New neurons in the vestibular nuclei complex after unilateral vestibular neurectomy in the adult cat

Recent findings revealed a reactive neurogenesis after lesions and in several models of disease. After unilateral vestibular neurectomy (UVN), we previously reported gamma-aminobutyric acid (GABA)ergic neurons are upregulated in the vestibular nuclei (VN) in the adult cat. Here, we ask whether this upregulation of GABAergic neurons resulted from a reactive neurogenesis. To determine the time course of cell proliferation in response to UVN, 5-bromo-2'-deoxyuridine (BrdU) was injected 3 h, 1, 3, 7, 15 and 30 days after UVN. We investigated the survival and differentiation in UVN cats injected with BrdU at 3 days and perfused 30 days after UVN. Results show a high number of BrdU-immunoreactive nuclei in the deafferented VN with a peak at 3 days after UVN and a decrease at 30 days. Most of the newly generated cells survived up to 1 month after UVN and gave rise to a variety of cell types. Confocal analysis revealed three cell lineages: microglial cells (OX 42/BrdU-immunoreactive cells); astrocytes [glial fibrillary acidic protein (GFAP)/BrdU-immunoreactive cells]; and neurons (NeuN/BrdU-immunoreactive cells). That UVN induced new neurons was confirmed by an additional marker (nestin) expressed by neural precursor cells. We show that most of the newly generated neurons have a GABAergic phenotype [glutamate decarboxylase (GAD)-67/BrdU-immunoreactive cells]. Morphological analysis showed two subtypes of GABAergic neurons: medium and small (30 vs. 10 microm, respectively). This is the first report of reactive neurogenesis in the deafferented VN in the adult mammalian CNS.

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

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

The effects of vestibular lesions on hippocampal function in rats

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

Effects of Intratympanic Gentamicin on Vestibular Afferents and Hair Cells in the Chinchilla

Gentamicin is toxic to vestibular hair cells, but its effects on vestibular afferents have not been defined. We treated anesthetized chinchillas with one injection of gentamicin (26.7 mg/ml) into the middle ear and made extracellular recordings from afferents after 5–25 (early) or 90–115 days (late). The relative proportions of regular, intermediate, and irregular afferents did not change after treatment. The spontaneous firing rate of regular afferents was lower ( P < 0.001) on the treated side (early: 44.3 ± 16.3; late: 33.9 ± 13.2 spikes·s−1) than on the untreated side (54.9 ± 16.8 spikes·s−1). Spontaneous rates of irregular and intermediate afferents did not change. The majority of treated afferents did not measurably respond to tilt or rotation (82% in the early group, 76% in the late group). Those that did respond had abnormally low sensitivities ( P < 0.001). Treated canal units that responded to rotation had mean sensitivities only 5–7% of the values for untreated canal afferents. Treated otolith afferents had mean sensitivities 23–28% of the values for untreated otolith units. Sensitivity to externally applied galvanic currents was unaffected for all afferents. Intratympanic gentamicin treatment reduced the histological density of all hair cells by 57% ( P = 0.04). The density of hair cells with calyx endings was reduced by 99% ( P = 0.03), although some remaining hair cells had other features suggestive of type I morphology. Type II hair cell density was not significantly reduced. These findings suggest that a single intratympanic gentamicin injection causes partial damage and loss of vestibular hair cells, particularly type I hair cells or their calyceal afferent endings, does not damage the afferent spike initiation zones, and preserves enough hair cell synaptic activity to drive the spontaneous activity of vestibular afferents.

The differential response of astrocytes within the vestibular and cochlear nuclei following unilateral labyrinthectomy or vestibular afferent activity blockade by transtympanic tetrodotoxin injection in the rat

In this study, we investigated whether changes in the vestibular neuronal activity per se influence the pattern of astrocytes morphology, glial fibrillary acidic protein (GFAP) expression and ultimately their activation within the vestibular nuclei after unilateral transtympanic tetrodotoxin (TTX) injections and after unilateral inner ear lesion. The rationale was that, theoretically the noninvasive pharmacological functional blockade of peripheral vestibular inputs with TTX, allowed us to dissociate the signals exclusively related to the shutdown of the resting activity of the first-order vestibular neurons and from neuronal signals associated with trans-ganglionic changes in first order vestibular neurons induced by unilateral labyrinthectomy (UL). Since the cochlea was removed during the surgical procedure, we also studied the astrocytic reaction within the deafferented cochlear nuclei. No significant changes in the distribution or relative levels of GFAP mRNA expression, relative levels of GFAP protein or immunoreactivity for GFAP were found in the ipsilateral vestibular nuclei at any post-TTX injection times studied. In addition, no sign of microglia activation was observed. In contrast, a robust increase of the distribution and relative levels of GFAP mRNA expression, protein levels and immunoreactivity was observed in the deafferented vestibular and cochlear nuclei beginning at 1 day after inner ear lesion. GFAP mRNA expression and immunoreactivity in the cochlear nucleus was qualitatively stronger than in the ipsilateral vestibular nuclei. The results suggest that astrocyte activation in the vestibular nuclei is not related to drastic changes of vestibular nuclei neuronal activity per se. Early trans-ganglionic changes due to vestibular nerve dendrites lesion provoked by the mechanical destruction of vestibular receptors, most probably induced the glial reaction. Its functional role in the vestibular compensation process remains to be elucidated.

Effects of unilateral and bilateral labyrinthectomy on rat postural muscle properties: the soleus

The aim of our study was to determine whether the suppression of the vestibular inputs could have effects on the soleus muscle properties similar to the modifications observed after an episode of microgravity. The inner ear lesion was performed by surgical labyrinthectomy. Twenty-nine male Wistar rats were used for this study and were divided into three experimental groups: control (CONT, n=7), unilateral labyrinthectomized (UL, n=14) and bilateral labyrinthectomized (BL, n=8). Mechanical, histochemical and electrophoretic parameters were determined 17 days after the operation. Furthermore, electromyographic (EMG) activity of the soleus muscle was examined at 1 h, 1 day and 17 days. Our results showed that UL and BL groups did not present any sign of muscle atrophy when compared to CONT group. However, the contractile and phenotypical characteristics of UL and BL soleus muscles revealed that the muscle evolved from slow toward a slower type. This transition was correlated with a more tonic EMG activity pattern. To conclude, our data demonstrated that soleus muscle transformations observed after microgravity (muscle atrophy, slow-to-fast transition, phasic EMG activity) were not directly the consequence of a vestibular silence.

Long-term effects of permanent vestibular lesions on hippocampal spatial firing

The hippocampus is thought to be important for spatial representation processes that depend on the integration of both self-movement and allocentric cues. The vestibular system is a particularly important source of self-movement information that may contribute to this spatial representation. To test the hypothesis that the vestibular system provides self-movement information to the hippocampus, rats were given either a bilateral labyrinthectomy (n = 6) or a sham surgery (n = 6), and at least 60 d after surgery hippocampal CA1 neurons were recorded extracellularly while the animals foraged freely in an open arena. Recorded cells were classified as complex spiking (n = 80) or noncomplex spiking (n = 33) neurons, and their spatial firing fields (place fields) were examined. The most striking effect of the lesion was that it appeared to completely abolish location-related firing. The results of this and previous studies provide converging evidence demonstrating that vestibular information is processed by the hippocampus. The disruption of the vestibular input to the hippocampus may interfere with the reconciliation of internal self-movement signals with the changes to the external sensory inputs that occur as a result of that movement. This would disrupt the ability of the animal to integrate allocentric and egocentric information into a coherent representation of space.

Labyrinthectomy changes T-type calcium channels in vestibular neurones of the guinea pig

In the vestibular nuclei of the awake guinea pig, all neurones are spontaneously active. After unilateral labyrinthectomy, this activity virtually disappears on the ipsilateral side, but is completely restored one week later. In a recent study, we observed that the restoration of spontaneous activity was correlated with an increase in pacemaker activity. In the current study, we found that the ratio of medial vestibular nucleus (MVN) neurones endowed with one of the currents known to play a role in pacemaker activity (i.e. low-threshold calcium current; LTCC) increased from 29% in control guinea pigs to 65% in animals labyrinthectomised on the ipsilateral side one week earlier. Yet this change was not correlated with a modification of the ratio of neurones expressing any of the three related protein-channels (alpha1G, alpha1H and alpha1I).

Effect of labyrinthectomy on the spike generator of vestibular neurons in the guinea pig

In the guinea pig, in the absence of any stimulation, all the neurons of the vestibular nuclei are tonically firing. After an ipsilateral labyrinthectomy, these neurons first cease to fire but recover their previous discharge in 7 days. Here, we tested whether a modification of the spike generator, the process transforming synaptic currents into spike patterns, could be a factor underlying this restoration. For this purpose, we studied the firing rate responses of neurons of the medial vestibular nucleus in brain stem slices to intracellularly injected currents. We conclude that although labyrinthectomy induces some plastic changes in the excitability of the neurons of the medial vestibular nucleus, these changes do not underlie the restoration of activity which occurs in these neurons when they are deprived of their labyrinthine input.

Temporal changes of cFos-like protein expression in medial vestibular nuclei following arsanilate-induced unilateral labyrinthectomy in rats

The compensation of spontaneous nystagmus and head deviation following chemical unilateral labyrinthectomy (UL) induced by arsanilate was delayed compared with surgical UL. Surgical UL produced two phases of significant asymmetry of cFos-like (cFL) protein expression between the bilateral medial vestibular nuclei, with more expression in the contralateral medial vestibular nuclei to the injured side than in the ipsilateral medial vestibular nuclei 2 h after UL; the pattern reversed after 6 h and expression disappeared after 72 h. Chemical UL produced three phases of asymmetric expression, with more cFL protein expression in the contralateral medial vestibular nuclei than in the ipsilateral medial vestibular nuclei 6 h after UL and a reversed pattern after 12 h. Asymmetric expression 72 h after UL followed increased expression in the contralateral medial vestibular nuclei. These results suggest that the course of vestibular compensation and the temporal expression of cFL protein in the medial vestibular nuclei following UL differed between surgical and chemical labyrinthectomy.

Modification of the pacemaker activity of vestibular neurons in brainstem slices during vestibular compensation in the guinea pig

In the guinea pig, unilateral labyrinthectomy causes an immediate and severe depression of the spontaneous activity of the ipsilateral central vestibular neurons, which subsequently recovers completely within one week. A possible underlying mechanism could be an increase in the endogenous activity of the neurons deprived of their labyrinthine input. Here, we addressed this hypothesis. The endogenous activity of the neurons was assessed by their spontaneous activity recorded extracellularly in brainstem slices in the presence of a cocktail of neurotransmitter blockers (CNQX, D-APV, bicuculline and strychnine) which freed them from their main synaptic influences. The left medial vestibular nucleus (MVN) was explored in a very systematic way and strict methodological precautions were taken in order to validate comparisons between the numbers of spontaneously active neurons recorded in the MVN of distinct slices. In the presence of neurotransmitter antagonists, the mean number of spontaneously active neurons detected in a single MVN increased dramatically from 9.5 in slices from control guinea pigs to 26.3 in slices from animals labyrinthectomized on the left side one week beforehand. The mean firing rate of the recorded neurons also increased from 7.5 +/- 5.6 spikes/s in slices from control animals to 12.3 +/- 7.6 spikes/s in slices from guinea pigs labyrinthectomized one week beforehand. These results show that deprivation of the vestibular neurons of their labyrinthine input caused a change in the deprived neurons themselves. They suggest that an increase in pacemaker activity might be a factor responsible for the restoration of spontaneous activity in the vestibular neurons after labyrinthectomy.

Plastic changes underlying vestibular compensation in the guinea-pig persist in isolated, in vitro whole brain preparations

Vestibular compensation for the postural and oculomotor deficits induced by unilateral labyrinthectomy is a model of post-lesional plasticity in the central nervous system. Just after the removal of one labyrinth, the deafferented, ipsilateral vestibular nucleus neurons are almost silent, and the discharge of the contralateral vestibular nucleus neurons is increased. The associated static disorders disappear in a few days, as normal activity is restored in both vestibular nuclei. In this study, we searched for traces of vestibular compensation in isolated whole brains taken from adult guinea-pigs. The electrophysiological responses evoked in control brains were compared to those evoked in brains taken from animals that had previously been labyrinthectomized. Guinea-pigs compensated for an initial labyrinthectomy within three days. In vivo, subsequent deafferentation of vestibular nucleus neurons on the intact side triggered "Bechterew's phenomenon": a new postural and oculomotor syndrome appeared, similar to the one induced by the first lesion, but directed to the newly deafferented side. These disturbances would be caused by the new imbalance between the discharges of neurons in the two vestibular nuclei triggered by the second deafferentation. Experiments were designed to search for a similar imbalance in vitro in brains taken from labyrinthectomized animals, where the intact vestibular nerve is cut during the dissection. Isolated whole brains were obtained from young guinea-pigs at various times (one to seven days) following an initial labyrinthectomy. An imbalance between the resting activities of medial vestibular nucleus neurons on both sides of the brainstem was revealed in brains taken more than three days after the lesion: their discharge was higher on the compensated, initially lesioned side than on the newly deafferented side. In some cases, an oscillatory pattern of discharge, reminiscent of the spontaneous nystagmus associated in vivo with Bechterew's syndrome, appeared in both abducens nerves. These data demonstrate that most of the changes underlying vestibular compensation persist, and can thus be investigated in the isolated whole brain preparation. Brains removed only one day after the lesion displayed normal commissural responses and symmetric spinal inputs to vestibular nucleus neurons. However, an unusually large proportion of the neurons recorded on both sides of the preparation had very irregular spontaneous discharge rates. These data suggest that the first stages of vestibular compensation might be associated with transient changes in the membrane properties of vestibular nucleus neurons. Brains taken from compensated animals displayed a significant, bilateral decrease of the inhibitory commissural responses evoked in the medial vestibular nucleus by single-shock stimulation of the contralateral vestibular nerve. The sensitivity of abducens motoneurons on the initially lesioned, compensated side to synaptic activation from the contralesional vestibular nucleus neurons was also decreased. Both changes may explain the long-term, bilateral decrease of vestibular-related reflexes observed following unilateral labyrinthectomy. Spinal inputs to vestibular nucleus neurons became progressively asymmetric: their efficacy was increased on the lesioned side and decreased on the intact one. This last modification may support a functional substitution of the deficient, vestibular-related synergies involved in gaze and posture stabilization by neck-related reflexes.

Evidence for a microglial reaction within the vestibular and cochlear nuclei following inner ear lesion in the rat

Following unilateral inner ear lesion, astrocytes undergo hypertrophy in the deafferented vestibular and cochlear nuclei as shown by an increase in the level of glial fibrillary acid. The present study extends our understanding of vestibular and cochlear system plasticity by examining microglial changes in these deafferented nuclei. The microglial reaction was studied 1, 2, 4, 8, 14, 21, 28 and 42 days following the lesion with a monoclonal OX-42 antibody and lectins (Griffonia simplicifolia, B4 isolectin) labelled with horseradish peroxidase or fluorescein. The deafferented nuclei were also examined for apoptotic cells by terminal transferase-mediated nick end labelling of nuclear DNA fragments. In control and sham-operated rats, the distribution of the resting microglial cells was uniform in both the vestibular and cochlear nuclei. In the deafferented vestibular complex, the microglial cells increased in number, became hypertrophied and were distributed in the medial, lateral, superior and inferior vestibular nuclei. Reactive microglial cells were also detected in the ipsilateral cochlear nuclei. Some of the immunostained cells were hypertrophic whereas others presented an ameboid morphology with few short and stout processes. The microglial reaction was confined to the antero- and posteroventral cochlear nuclei. Finally, reactive microglia was also observed in the prepositus hypoglossi ipsilateral to the lesion. The microglial reactions within the prepositus hypoglossi, the vestibular and the cochlear nuclei were detectable as early as one day after the lesion and persisted several weeks in both the vestibular and cochlear nuclei. Apoptotic cells were not detected in the vestibular nuclei at any stage following the lesion. In contrast, terminal deoxynucleotidyl transferase-mediated digoxygenin-11-dUTP nick end labelling-positive cells were first detected in the deafferented cochlear nuclei on the 3rd day following the lesion. They reached an apparent maximum by day 8 and then declined until day 24. Double labelling experiments demonstrate that these cochlear terminal deoxynucleotidyl transferase-mediated digoxygenin-11-dUTP nick end labelling-positive cells were also lectin-positive suggesting that reactive cochlear lectin-positive microglia cells were eliminated by a programmed cell death. Our results establish the two experimental models as reliable tools to understand the role of microglia in adult brain plasticity. The cochlear microglial reaction was probably induced by the degeneration of the acoustic nerve which follows the acoustic ganglion destruction. Interestingly, the same reasoning cannot apply to the vestibular microglial reaction following unilateral labyrinthectomy: the vestibular ganglion was spared and the primary vestibular neurons did not degenerate, at least during the first week following the lesion.

The vestibular system as a model of sensorimotor transformations. A combined in vivo and in vitro approach to study the cellular mechanisms of gaze and posture stabilization in mammals

To understand the cellular mechanisms underlying behaviours in mammals, the respective contributions of the individual properties characterizing each neuron, as opposed to the properties emerging from the organization of these neurons in functional networks, have to be evaluated. This requires the use, in the same species, of various in vivo and in vitro experimental preparations. The present review is meant to illustrate how such a combined in vivo in vitro approach can be used to investigate the vestibular-related neuronal networks involved in gaze and posture stabilization, together with their plasticity, in the adult guinea-pig. Following first a general introduction on the vestibular system, the second section describes various in vivo experiments aimed at characterizing gaze and posture stabilization in that species. The third and fourth parts of the review deal with the combined in vivo-in vitro investigations undertaken to unravel the physiological and pharmacological properties of vestibulo-ocular and vestibulo-spinal networks, together with their functional implications. In particular, we have tried to use the central vestibular neurons as examples to illustrate how the preparation of isolated whole brain can be used to bridge the gap between the results obtained through in vitro, intracellular recordings on slices and those collected in vivo, in the behaving animal.

Role of the flocculus in the development of vestibular compensation: immunohistochemical studies with retrograde tracing and flocculectomy using Fos expression as a marker in the rat brainstem

After unilateral labyrinthectomy in rats, Fos-like immunoreactive neurons appeared in the ipsilateral medial vestibular nucleus, contralateral prepositus hypoglossal nucleus and contralateral inferior olive beta subnucleus. and thereafter gradually disappeared in accordance with the development of vestibular compensation. This finding indicated that the activation of these nuclei is the initial event of vestibular compensation. In the present study, retrograde tracing experiments revealed that these Fos-like immunoreactive neurons project a proportion of their axons to the vestibulocerebellum (uvula-nodulus, flocculus). Before vestibular compensation was accomplished, right, left or bilateral flocculectomy was performed in right-labyrinthectomized rats. All these treatments caused reappearance of unilateral labyrinthectomy-induced behavioral deficits and Fos expression in the left medial vestibular nucleus and right prepositus hypoglossal nucleus. Since floccular efferents are GABAergic, these results indicate that the neurons in which Fos expression was detected by flocculectomy had been inhibited after unilateral labyrinthectomy by floccular Purkinje neurons and that disinhibition of these neurons induced by flocculectomy caused decompensation. Based on our present findings, we propose a hypothesis that the bilateral flocculus serves the restoration of balance between intervestibular nuclear activities to induce vestibular compensation after unilateral labyrinthectomy.

Evidence for reactive astrocytes in rat vestibular and cochlear nuclei following unilateral inner ear lesion

We investigated whether unilateral removal of the labyrinthine and cochlear receptors induces a macroglial reaction in rat vestibular and cochlear nuclei using vimentin and glial fibrillary acidic protein (GFAP) immunochemical markers. Antibody binding was visualized using the avidin-biotin method and 3,3'-diaminobenzidine as the peroxidase substrate. In addition, double-labelling experiments were performed using specific secondary fluorescent antibodies. Potentially degenerating axon terminals were also studied using a silver impregnation method. In normal adult rats, vimentin was found only in ependymal cells, tanicytes around the fourth ventricle, endothelial cells in the blood vessels and Bergmann glia in the molecular layer of the cerebellum. In lesioned rats, all deafferented vestibular and ventral cochlear nuclei showed strong vimentin immunoreactivity. Furthermore, double-labelling experiments demonstrated that these vimentin-positive cells were also GFAP-positive. The reaction became evident on the second day after the lesion, was intense for 3-8 days and then declined until day 21. No vimentin immunoreactivity could be detected at the level of the ipsilateral dorsal cochlear nucleus. Therefore, unilateral inner ear lesion induced an astroglial reaction within the deafferented vestibular and cochlear nuclei. The decrease in the resting discharge of the primary vestibular afferents and/or in the deafferented central vestibular neurons may induce the glial reaction in the vestibular complex, whereas both degeneration and silence of the cochlear nerve and central cochlear neurons are most probably responsible for the cochlear vimentin-immunoreactive staining. The role of the reactive astrocytes in the vestibular compensation process remains to be determined.

Comparison of labyrinthectomy and vestibular neurectomy in the control of vertigo

One hundred twenty-six patients who were treated with labyrinthectomy (81 patients) or vestibular neurectomy (45) between the years 1979 and 1994 were reviewed. The cause for vertigo in 124 of the 126 patients was Meniere's disease (89 patients), labyrinthitis (15), delayed endolymphatic hydrops (8), vestibular neuritis (7), and failed labyrinthectomy (5). In the remaining 2 patients, a normal labyrinth was sacrificed to fistulize a petrous apex cyst. Both procedures were equally effective in relieving vertigo (labyrinthectomy 98.8%; neurectomy 97.8%), but the length of hospitalization, length of disability before return to work, and cost were twice as great with vestibular neurectomy than with labyrinthectomy. More patients exhibited prolonged ataxia following neurectomy (5 patients) than after labyrinthectomy (2). Vestibular neurectomy was associated with several complications: reversible facial paresis (15 patients), meningitis (1), cerebrospinal fluid leak (1), and epidural hematoma (1). Labyrinthectomy was complicated by postoperative hyponatremia in 1 patient. Selective vestibular neurectomy preserved hearing in 32 (82%) of 39 patients. Criteria for recommending either ablation procedure are discussed. The incidence of sequential involvement of the contralateral ear was 1.5%.

Effects of MK801 on Fos expression in the rat brainstem after unilateral labyrinthectomy

Unilateral labyrinthectomy (UL) causes ocular and postural asymmetries, which disappear over time in the processes of equilibrium recovery known as vestibular compensation. It has been reported that N-methyl-D-aspartate (NMDA) receptors are involved in vestibular compensation. In the present study, in order to elucidate the NMDA receptor-mediated neural circuit responsible for the development of vestibular compensation, we used Fos expression as a marker of neural activation and examined the effects of MK801, a specific antagonist of NMDA receptors, on UL-induced Fos expression in the rat brainstem. After UL, Fos-like immunoreactive (-LIR) neurons were observed in the ipsilateral medial vestibular nucleus (ipsi-MVe), the contralateral prepositus hypoglossal nucleus (contra-PrH) and the contralateral inferior olive beta subnucleus (contra-IOb). Fos-LIR neurons gradually disappeared in the processes of vestibular compensation. It is suggested that the activation of the ipsi-MVe, the contra-PrH and the contra-IOb neurons after UL are the initial event of vestibular compensation. Intraperitoneal injection of MK801 in the processes of vestibular compensation caused reappearance of UL-induced behavioral deficits. During the decompensation induced by MK801, Fos-LIR neurons appeared in the contra-MVe, the ipsi-PrH and the bilateral-IOB. It is suggested that the contra-MVe, the ipsi-PrH and the bilateral-IOb neurons are inhibited by glutamatergic synapses driving inhibitory neurons via NMDA receptors in the processes of vestibular compensation and that disinhibition of these nuclei induced by MK801 causes decompensation. However, MK801 caused neither Fos expression nor behavioral decompensation after vestibular compensation is accomplished. All these findings that the NMDA receptor-mediated inhibitory modulation in the central vestibular system plays an important role for the initial processes of the development of vestibular compensation.

Comparison of surgeries for removal of primary vestibular inputs: a combined anatomical and behavioral study in rats

Unilateral removal of Scarpa's ganglion and neurectomy of the peripheral vestibular nerve branches were compared in rats as methods to eliminate primary vestibular input. Ocular nystagmus was consistently observed after both types of lesion, but it completely disappeared within 4 to 7 days. Imbalance and rotation were more serious and prolonged after ganglionectomy than after peripheral neurectomy. Corresponding with these differences in symptoms were differences in terminal degeneration. After ganglionectomy, degenerated axons and terminals were distributed throughout all terminal regions of primary vestibular fibers on the lesioned side, while after peripheral neurectomy, the degeneration was more limited. The results of this study suggest that vestibular ganglionectomy is a more successful approach than peripheral vestibular neurectomy for removing the primary vestibular input.

Cytochrome oxidase histochemistry in Scarpa's ganglion after hemilabyrinthectomy

Cytochrome oxidase histochemistry was studied in neurons in the vestibular ganglion in gerbils two weeks after hemilabyrinthectomy. This study measured the staining density in ganglion cells on both the lesioned and non-lesioned side of the brainstem. Cytochrome oxidase staining was significantly reduced in ganglion cells ipsilateral to the lesion. This decrease may have been related to the concomitant loss of spontaneous discharge and reduced energy demand for oxidative metabolism.

Distribution of glutamatergic receptors and GAD mRNA-containing neurons in the vestibular nuclei of normal and hemilabyrinthectomized rats

Vestibular compensation is an attractive model for investigations of cellular mechanisms underlying post-lesional plasticity in the adult central nervous system. Immediately after hemilabyrinthectomy, the spontaneous activity in the deafferented second-order vestibular neurons falls to zero, resulting in a strong asymmetry between the resting discharge of the vestibular complexes on the lesioned and intact sides. This asymmetry most probably causes the static and dynamic vestibular deficits observed in the acute stage. After approximately 50 h, the deafferented vestibular neurons recover a quasi-normal resting activity which is thought to be the key of the compensation of the static vestibular syndromes. However, the molecular mechanisms underlying this recovery are unknown. In this study, we investigate possible changes in the distribution of glutamatergic N-methyl-D-aspartate (NMDA) and glutamate metabotropic receptors and of glutamate decarboxylase 67k (GAD 67k) mRNAs in the deafferented vestibular neurons induced by the labyrinthine lesion. Specific radioactive oligonucleotides were used to probe sections of rat vestibular nuclei according to in situ hybridization methods. Animals were killed at different times (5 h, 3 days and 3 weeks) following the lesion. Signal was detected by means of film or emulsion autoradiography. In the normal animals, several brainstem regions including the medial, lateral, inferior and superior vestibular nuclei were densely labelled by the antisense oligonucleotide NMDAR1 probe. However, the vestibular nuclei were not labelled by the glutamate metabotropic oligonucleotide antisense probe (mGluR 1). The GAD 67k antisense oligonucleotide probe labelled numerous small- to medium-sized central vestibular neurons but not the larger cell bodies in the lateral vestibular nucleus. This agrees with previous studies. In the hemilabyrinthectomized rats, no asymmetry could be detected, at either the autoradiographic or cellular levels, between the two medial vestibular nuclei whatever the probe used and whatever the delay following the lesion. However, for the NMDAR1 probe, the mean density of silver grains in both the deafferented and intact medial vestibular neurons was 20% lower 5 h after the lesion. Three days and 3 weeks later, the intensity of labelling over all cells was the same as in the control group. Further studies are necessary to confirm the relatively weak modification of the NMDAR1 mRNAs expression and to exclude a change of GAD 65 and of other NMDA subunit mRNAs during the vestibular compensation process.

Morphologic changes in superior vestibulo-ocular neurons and vestibular nerve following labyrinthectomy in the cat

Morphologic changes in ipsilateral superior vestibulo-ocular neurons (SVON) and the vestibular nerve were measured in 4 cats 8 weeks after labyrinthectomy and 4 cats 1 year after labyrinthectomy. There is a 20% decrease in SVON size and 30% decrease in rough endoplasmic reticulum and ribosomes with no change in the volume fractions of Golgi apparatus or mitochondria. In the central nervous system degeneration of the vestibular nerve terminals ipsilateral to the labyrinthectomy was represented by a 25% loss of synaptic profiles (SP) on SVON at 8 weeks and 57% loss of SP at 1 year after labyrinthectomy. There was no significant loss of fiber number in the vestibular nerve at 8 weeks post lesion but a 35% loss of fibers primarily of the large size at 1 year post lesion.

Molecular mechanisms of brainstem plasticity. The vestibular compensation model

Vestibular compensation is the process of behavioral recovery that occurs following unilateral deafferentation of the vestibular nerve fibers (unilateral labyrinthectomy, UL). Since UL results in a permanent loss of vestibular input from the ipsilateral vestibular (VIIIth) nerve, vestibular compensation is attributed to CNS plasticity and has been used as a general model of lesion-induced CNS plasticity. Behavioral recovery from the ocular motor and postural symptoms of UL is correlated with a partial return of resting activity to neurons in the vestibular nucleus (VN) on the deafferented side (the "deafferented VN"), and lesions to the deafferented VN prevent compensation; therefore, the regeneration of resting activity within the deafferented VN is believed to have a causal role in vestibular compensation. The biochemical mechanisms responsible for the adaptive neuronal changes within the deafferented VN are poorly understood. Neuropeptide hormone fragments, such as adrenocorticotrophic hormone (ACTH)-4-10, have been shown to accelerate vestibular compensation and can act directly on some VN neurons in vitro. Antagonists for the N-methyl-D-aspartate (NMDA) receptor have been shown to inhibit vestibular compensation if administered early in the compensation process. Biochemical studies in frog indicate marked alterations in the phosphorylation patterns of several proteins during compensation, and the in vitro phosphorylation of some of these proteins is modulated by ACTH-(1-24), calcium (Ca2+), and calmodulin or protein kinase C. It is therefore possible that ACTH fragments and NMDA antagonists (via their effects on NMDA receptor-mediated Ca2+ channels) modulate vestibular compensation through their action on Ca(2+)-dependent pathways within VN neurons. Recent studies have shown that some Ca2+ channel antagonists and the Ca(2+)-dependent enzyme inhibitor calmidazolium chloride facilitate vestibular compensation. How the regulation of Ca2+ may be related to the neuronal changes responsible for vestibular compensation is unclear at present.

Neurochemical mechanisms of recovery from peripheral vestibular lesions (vestibular compensation)

This paper reviews the literature relating to the neurochemical basis of vestibular compensation, a process of behavioral recovery which occurs following the removal of afferent input from one labyrinth (unilateral labyrinthectomy, UL). Although vestibular compensation is known to be correlated with a return of resting activity to the vestibular nucleus (VN) ipsilateral to the UL (the deafferented VN), the neurochemical mechanisms by which this neuronal recovery occurs, are unknown. At present, there is little evidence to support the hypothesis that denervation supersensitivity of excitatory amino acid, dopamine, norepinephrine or acetylcholine receptors in the deafferented VN, is responsible for vestibular compensation: binding studies for glutamate or acetylcholine do not support an upregulation of these receptor types. However, changes in the affinity or efficacy of these receptor complexes cannot be ruled out. There are still many neurotransmitter systems, such as serotonergic and histaminergic systems, which have not been investigated in relation to vestibular compensation. In several species it has been shown that treatment with adrenocorticotropic hormone, fragment 4-10 (ACTH-(4-10], can accelerate vestibular compensation. It is not clear how these drugs exert their effects. In vitro electrophysiological studies have shown that VN neurons are capable of generating resting activity in the absence of their normal afferent inputs and it is possible that these neurons have pacemaker-like membrane characteristics which contribute to the regeneration of activity following UL. Recent biochemical studies have revealed changes in the phosphorylation patterns of a number of proteins during compensation. The possible relationship between these phosphorproteins and the synaptic or membrane changes which are responsible for vestibular compensation remains to be determined.

Vestibular compensation after labyrinthectomy and vestibular neurectomy in cats

Labyrinthectomy and vestibular neurectomy are two ablative procedures used for definitive control of disabling vertigo. It is not known if vestibular compensation after labyrinthectomy and vestibular neurectomy differs. We have addressed this question by examining the pattern of recovery of the vestibular ocular reflex in cats after either labyrinthectomy or vestibular neurectomy. Temporal bone histologic examination confirmed the surgical lesion. Our results demonstrate a reduction of the long time constant of the vestibular ocular reflex in both groups of animals. Although gain of the vestibular ocular reflex recovered substantially, it never returned to control levels in either group. In general, animals that had undergone vestibular neurectomy demonstrated greater vestibular ocular reflex asymmetries than did labyrinthectomized animals. The recovery pattern of the vestibular ocular reflex indicates vestibular compensation is more rapid after labyrinthectomy than after vestibular neurectomy. We believe this result is related to survival of the vestibular nerve after labyrinthectomy, but not after vestibular neurectomy, suggesting that the vestibular nerve can contribute to the adaptive response after labyrinthectomy.

Compensation of horizontal canal related activity in the medial vestibular nucleus following unilateral labyrinth ablation in the decerebrate gerbil. I. Type I neurons

The spontaneous activity and dynamic responses to two frequencies (1.3 and 0.13 Hz) of sinusoidal angular horizontal head acceleration of type I neurons in the medial vestibular nucleus were recorded bilaterally in decerebrate Mongolian gerbils (Meriones unguiculatus) under three experimental conditions; normal labyrinth intact, acutely following unilateral labyrinthine lesion, and four to seven weeks following labyrinthine lesion. The mean spontaneous activity and number of detected type I neurons decreased immediately ipsilateral to the lesion but recovered significantly with time. In contrast, spontaneous activity on the contralateral side increased during compensation following hemilabyrinthectomy. The mean response gains at both frequencies of head oscillation were depressed bilaterally and asymmetrically acutely following the lesion such that the response gain of cells on the intact side exceeded that of the neurons recorded on the injured side. After compensation the number of detected type I neurons on the side ipsilateral to the injury increased but remained below normal levels. The mean gains remained depressed but became symmetric with compensation as a result of improvement in the response of ipsilateral neurons. The phase of responses were significantly advanced in the compensated animals. Although response gain is not fully restored, the linearity of the dynamic modulation in compensated animals is improved as evidenced by a continuous modulation of the increased spontaneous activity of neurons contralateral to the hemilabyrinthectomy. It is proposed that this effect is related to the concurrent improvement in the linearity of the horizontal vestibulo-ocular response. Electrical cathodal polarization of the vestibular nerve ipsilateral to the ablated labyrinth was utilized to investigate the relationship between recovery of spontaneous activity and dynamic function. Acutely following hemilabyrinthectomy, cathodal polarization restored activity in second-order type I neurons to near normal levels but their response gain to head rotation remained depressed. Similar galvanic stimulation in compensated animals also elevated ipsilateral spontaneous activity. As in the acute preparation, such stimulation did not modify the response gain or phase. Thus, the improvement in response of type I neurons in the compensated gerbil was not a direct consequence of restoration of spontaneous activity on the side of the injury.

Experimental reorganization in the alar plate of the clawed toad, Xenopus laevis. I. Quantitative and qualitative effects of embryonic otocyst extirpation

The area octavolateralis and the lateral line projections were examined in larval and postmetamorphic clawed toads, which had one otic vesicle removed at embryonic stage 38 (ref. 20). Premetamorphic tadpoles show a smaller octavolateral area on the operated side as compared to the contralateral control side. This area is in postmetamorphic toadlets significantly reduced as compared to the control side. The largest cells of the magnocellular vestibular nucleus show no significant difference in size to the contralateral side. A dorsolateral auditory nucleus develops prior to metamorphosis and shows in tadpoles no differences in cell size. Cell of this nucleus are in small toads only about 60% the size of cells in the contralateral nucleus. Countings in toadlets indicate a reduction of 40% in cell number on the operated side. Both tadpoles and small toads show virtually no differences in the lateral line projection compared to controls. Only rare collaterals of lateral line fibres can be traced into the neighbouring vestibular and auditory nuclei. The data show no preferential innervation of inner ear afferent deprived auditory and vestibular nuclei by lateral line afferents. In contrast, some collaterals of somatosensory fibres reach into the area deprived of octaval afferents.

Neuronal activity in the guinea pig medial vestibular nucleus in vitro following chronic unilateral labyrinthectomy

Unilateral labyrinthectomy (UL) causes ocular motor and postural disorders which disappear over time in a process of recovery known as vestibular compensation. Vestibular compensation is due to CNS plasticity which generates a partial recovery of resting activity in the vestibular nucleus ipsilateral to the UL, however the mechanism of this neural recovery is unknown. It has been suggested that other areas of the CNS may substitute non-vestibular sensory inputs for the missing labyrinthine input, thereby causing vestibular compensation. The present results show that resting activity can be recorded from medial vestibular nucleus (MVN) neurons in vitro, in brainstem slices from guinea pigs which have compensated for an ipsilateral UL. This result suggests that MVN neurons are capable of generating resting activity without inputs from many other CNS areas. Perfusion with high Mg2+ solution did not abolish resting activity in most cases, suggesting that part of the resting activity may be generated spontaneously by the neurons, possibly through changes in the electrical excitability of the cell membrane.

Survival of the vestibular nerve after labyrinthectomy in the cat

Temporal bone studies in cat, monkey, and man demonstrate that the cell bodies of the primary vestibular neurons located in Scarpa's ganglion persist after labyrinthectomy. However, it is not known whether the centrally directed axon process of deafferented vestibular neurons survive or degenerate after labyrinthectomy. If the central axon were to persist, then the primary vestibular neuron could influence vestibular compensation or produce symptoms of vestibular dysfunction. In the present study the temporal bones and brain stem of four cats were prepared for light microscopic examination with hematoxylin-eosin, silver, and trichrome connective tissue stains. Cell counts within Scarpa's ganglion were performed. After labyrinthectomy, many intact axons were demonstrated in the brain stem, a finding that correlated with survival of neurons in Scarpa's ganglion. This study provides anatomic evidence that primary vestibular neurons that survive labyrinthectomy may retain their central axon processes. The persistence of this neural pathway and data from behavioral studies in the cat suggest that vestibular neurons may affect vestibular compensation after labyrinthectomy. Deafferented vestibular neurons may play a role in human vestibular compensation and dysfunction.

Mechanisms of recovery following unilateral labyrinthectomy: a review

This paper reviews the literature on the mechanisms responsible for the behavioural recovery which occurs following unilateral labyrinthectomy (UL), UL causes a syndrome of ocular motor and postural disorders, which diminish over time in a process of behavioural recovery known as vestibular compensation. Electrophysiological studies show that the VIIIth nerve does not undergo a functional recovery, therefore vestibular compensation has been attributed to CNS plasticity. However, the nature of the plasticity responsible for vestibular compensation is not understood. Single-neuron studies have demonstrated that a significant recovery of resting activity has occurred in the vestibular nuclei (VN) ipsilateral to the UL by the time symptoms such as spontaneous nystagmus and roll head tilt (static symptoms) have largely disappeared. However, many of the deficits in the response of VN neurons to head acceleration persist and may be permanent. This lack of recovery in the response of neurons to head acceleration correlates with the incomplete and sometimes poor recovery of the vestibulo-ocular and vestibulo-spinal reflex responses to head movement (dynamic symptoms). The major neuronal change in the VN during vestibular compensation appears to be the recovery of resting activity in the VN ipsilateral to the UL, although this recovery is more pronounced in the medial VN than in the lateral VN. The mechanism responsible for the regeneration of resting activity in VN neurons is unknown. In frogs, there is evidence to suggest that transcommissural synaptic input to the VN, from the contralateral (intact) labyrinth, increases in efficacy.(ABSTRACT TRUNCATED AT 250 WORDS)

The NMDA antagonists MK801 and CPP disrupt compensation for unilateral labyrinthectomy in the guinea pig

Unilateral labyrinthectomy results in eye movement and postural disorders which diminish over time in a process of behavioral recovery called vestibular compensation. This compensation process is due to CNS plasticity which generates a renewal of spontaneous activity in vestibular nucleus neurons ipsilateral to the labyrinthectomy. However, the mechanisms responsible for the induction and maintenance of this neural recovery are unknown. The present results show that i.p. injections of the N-methyl-D-aspartate (NMDA) receptor antagonists MK801 [( (+)-5-methyl-10,11-dihydro-5H- dibenzo[a,d]-cyclohepten-5,10-imine maleate]) and CPP (3-[+/- )-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid) disrupt the maintenance of compensation following unilateral labyrinthectomy, producing a reappearance of eye movement symptoms after compensation has been attained. These results suggest that NMDA receptors may be involved in the maintenance of the neural changes responsible for vestibular compensation.

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

The recovery of normal ocular motor and postural behavior following unilateral labyrinthectomy is termed vestibular compensation and it has been attributed to the return of normal resting activity to neurons in the bilateral vestibular nuclei. Previous studies in the cat have reported that approximately normal resting activity returns to type I neurons in the medial vestibular nucleus (MVN) contralateral to the deafferentiation by 4-8 weeks post-operation (post-op.), while the gain of the response of these neurons to horizontal angular acceleration remains lower than normal. The present data demonstrate that, in the guinea pig, normal resting activity returns to the contralateral MVN type I neurons by only 52-60 h post-op., while the gain and phase of the response of these neurons to horizontal sinusoidal acceleration remains abnormal up to 8-12 months post-op. By contrast with previous studies, the present data show that type II neurons in the contralateral MVN exhibit some increase in their frequency of occurrence and gain toward normal values during vestibular compensation (at 52-60 h and 8-12 months post-op.). The rapid recovery of normal type I resting activity correlates with the disappearance of spontaneous nystagmus and postural asymmetries in the guinea pig by 52 h post-op.

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

The recovery of normal ocular motor and postural behavior following unilateral labyrinthectomy (vestibular compensation) has been attributed to the return of normal resting activity to neurons in the bilateral vestibular nuclei. However, previous studies in the cat have reported that average resting activity recovers to no more than 50% of the normal value in neurons in the vestibular nucleus ipsilateral to the labyrinthectomy even after 4 months post-operation (post-op.), despite the fact that, for some symptoms, vestibular compensation is complete by this time. The present data demonstrate that in the guinea pig, normal average resting activity is restored to type I neurons in the ipsilateral medial vestibular nucleus (MVN) by 52-60 h post-op., although type I neurons remain scarce compared to normal. This recovery of resting activity correlates with the compensation of spontaneous nystagmus and postural asymmetries by 52 h post-op. which we have previously reported. In addition, the present data further confirm that the recovery of type I resting activity in the ipsilateral MVN is not due to recovery of resting activity in ipsilateral Scarpa's ganglion neurons or to input from the dorsal brainstem commissures.

Regional differences of brain glucose metabolic compensation after unilateral labyrinthectomy in rats: a [14C]2-deoxyglucose study

A unilateral labyrinthectomy was performed on anesthetized adult albino rats. Brain [14C]2-deoxyglucose (2DG) uptake was measured autoradiographically 3.5 h to 20 days later and compared to sham-operated controls. In the vestibular nuclei (nn.) of labyrinthectomized subjects, large left-right differences of 2DG uptake occurred, which decreased over time. The equalization of vestibular nuclear 2DG uptake paralleled behavioral compensation of body, neck and head postural abnormalities, and known equalization of vestibular nuclear cell firing rates during compensation. There was a small difference of 2DG uptake in medial and lateral vestibular nn. 20 days after lesions when animals had a residual head tilt and tonic eye deviation. In the oculomotor nn., trochlear nn. and interstitial n. of Cajal, large left-right differences of 2DG uptake occurred, which did not change over time. The higher 2DG uptake in these nn. occurred ipsilateral to the labyrinthine lesion and did not correlate with the onset and cessation of nystagmus. The persistent asymmetry did appear to correlate with ipsilateral downward and contralateral upward eye deviation which continued for long periods after the lesion. We hypothesize that the non-compensating metabolic asymmetry in the oculomotor and trochlear nn. could be due to lesioned otolithic input to the vestibular nn. which relays to trochlear and oculomotor nn.

Initial, rapid phase of recovery from unilateral vestibular lesion in rat not dependent on survival of central portion of vestibular nerve

The behavioral effects of vestibular endorgan lesions were compared with those of vestibular ganglion lesions in the albino rat. No differences in head tilt angle or spontaneous eye nystagmus beat frequency were noted between the two groups during the first 36 h after the lesion was made. Of rats studied beyond 36 h, 2/7 with lesions restricted to the endorgans and 2/3 with ganglion lesions showed pronounced secondary increases in head tilt and tonic eye deviation, but not eye nystagmus. Single units were recorded in the ganglion acutely, as well as 1,2, and 14 days after an endorgan lesion was made. Practically no resting activity could be recorded in the ganglion acutely (2-7 h) after endorgan damage, and the resting activity at subsequent times was slight. It is concluded that an intact vestibular ganglion isolated from the sensory periphery is of no functional significance during the first 36 h, when the largest decreases in magnitude of the behavioral signs of unilateral labyrinthectomy occur in the rat. A slight significance at later times is not ruled out.