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

Neuronal response sensitivity to bidirectional off-vertical axis rotations: a dimension of imbalance in the bilateral vestibular nuclei of cats after unilateral labyrinthectomy

In decerebrate cats after acute hemilabyrinthectomy, the response sensitivity of extracellularly recorded vestibular nuclear neurons on the lesioned and labyrinth-intact sides were examined quantitatively during constant velocity off-vertical axis rotations with an aim to elucidate the functional contribution of otolithic inputs to the ipsilateral and contralateral vestibular nuclei. The bidirectional response sensitivity, delta, was determined as the ratio of the gain during clockwise to that during counterclockwise rotations. A continuum of response sensitivity was identified: one-dimensional neurons showed symmetrically bidirectional response patterns, while two-dimensional neurons showed asymmetrically bidirectional patterns that in some cases approached unidirectional patterns with change in velocity. The proportion of two-dimensional neurons was significantly increased after acute hemilabyrinthectomy. Two-dimensional neurons that responded only to one direction of rotation in at least one of the velocities tested were described as unidirectional neurons. This unidirectional response pattern was observed in one-third of the entire neuronal population studied, but not in cats with both labyrinths intact, thus suggesting that such prominent broadly tuned responses are normally masked by converging otolithic inputs from the contralateral side. These neurons were found in higher proportion on the lesioned side than on the labyrinth-intact side. Among the 70% of unidirectional neurons that exhibited bidirectional response at some velocities and unidirectional response at others, prominent shifts in delta values (i.e. between 0/infinity and finite values) with velocity can be computed for each neuron. The shifts in delta values correlated with large shifts in the response dynamics and spatial orientation as the response pattern changed with velocity. The response orientations of the unidirectional neurons pointed in all directions on the horizontal plane. When all the two-dimensional neurons (i.e. both the unidirectionally and bidirectionally responsive ones) were pooled, imbalances in the distribution of the response orientations and in response gain were found between the ipsilateral-side-down/head-down half-circle and the contralateral-side-down/head-up half-circle on the labyrinth-intact side, but not on the lesioned side. These results, derived from spatiotemporal processing of gravitational signals, reveal a novel dimension of imbalance between neuronal populations in the two vestibular nuclear complexes after acute lesion of one labyrinth. This feature would provide, on the one hand, deranged cues of spatial orientation and direction during slow head excursions and, on the other, a framework for the dynamic behavioral deficits associated with hemilabyrinthectomy.

Lesion-induced plasticity in rat vestibular nucleus neurones dependent on glucocorticoid receptor activation

1. We have recently shown that neurones in the rostral region of the medial vestibular nucleus (MVN) develop a sustained increase in their intrinsic excitability within 4 h of a lesion of the vestibular receptors of the ipsilateral inner ear. This increased excitability may be important in the rapid recovery of resting activity in these neurones during 'vestibular compensation', the behavioural recovery that follows unilateral vestibular deafferentation. In this study we investigated the role of the acute stress that normally accompanies the symptoms of unilateral labyrinthectomy (UL), and in particular the role of glucocorticoid receptors (GRs), in the development of the increase in excitability in the rostral MVN cells after UL in the rat. 2. The compensatory increase in intrinsic excitability (CIE) of MVN neurones failed to occur in animals that were labyrinthectomized under urethane anaesthesia and kept at a stable level of anaesthesia for either 4 or 6 h after UL, so that they did not experience the stress normally associated with the vestibular deafferentation syndrome. In these animals, 'mimicking' the stress response by administration of the synthetic GR agonist dexamethasone at the time of UL, restored and somewhat potentiated CIE in the MVN cells. Administration of dexamethasone in itself had no effect on the intrinsic excitability of MVN cells in sham-operated animals. 3. In animals that awoke after labyrinthectomy, and which therefore experienced the full range of oculomotor and postural symptoms of UL, there was a high level of Fos-like immunoreactivity in the paraventricular nucleus of the hypothalamus over 1.5-3 h post-UL, indicating a strong activation of the stress axis. 4. The GR antagonist RU38486 administered at the time of UL abolished CIE in the rostral MVN cells, and significantly delayed behavioural recovery as indicated by the persistence of circular walking. The mineralocorticoid receptor (MR) antagonist spironolactone administered at the time of UL had no effect. 5. Vestibular compensation thus involves a novel form of 'metaplasticity' in the adult brain, in which the increase in intrinsic excitability of rostral MVN cells and the initial behavioural recovery are dependent both on the vestibular deafferentation and on the activation of glucocorticoid receptors, during the acute behavioural stress response that follows UL. These findings help elucidate the beneficial effects of neuroactive steroids on vestibular plasticity in various species including man, while the lack of such an effect in the guinea-pig may be due to the significant differences in the physiology of the stress axis in that species.

Responses of hypothalamic neurons to stimulation of the vestibular nerve and lateral vestibular nucleus in the rabbit

Acute experiments were performed on rabbits to study the responses of neurons in the anterior, ventromedial, and posterior nuclei of the hypothalamus to single, paired, and rhythmic stimulation of the vestibular nerve and lateral vestibular nucleus of Deiters. The data obtained showed that neurons of the posterior nucleus of the hypothalamus were the most sensitive. Three types of response were seen from hypothalamic neurons, with short, long, and intermediate latent periods. This provides evidence that ascending afferent spike activity from the lateral vestibular nucleus of Deiters to the hypothalamus is mediated by mono-, oligo-, and polysynaptic pathways.

Increased transmission by direct vestibulo-ocular reflex pathways after peripheral vestibular damage: a preliminary report

The vestibulo-ocular reflex (VOR) allows clear vision during head movements by generating compensatory eye movements. Its response is reduced following damage to the vestibular endorgan, but recovers over time. The VOR is mediated by both direct and indirect anatomical pathways; most direct pathways include only two central synapses, both located in the brainstem. To investigate the possibility that a direct pathway is modified during the recovery of VOR gain, we measured the oculomotor response to single current pulses delivered to the vestibular labyrinth of two alert cats after plugging the contralateral horizontal canal. The response was also measured after motor learning induced by continuously worn lenses (optically induced motor learning) in two cats. The gain of the VOR was monitored concurrently. The eye movement evoked by a current pulse increased more than 100% during recovery from a plug. The electrically evoked eye movement did not change during optically induced motor learning either before the plug or after recovery. The gain of the VOR was modified in both situations. We conclude that direct VOR pathways are modified significantly during recovery after a plug, but not during optically induced learning. Our results suggest that significant modification of direct pathways may require a change in vestibular sensory input.

Neuronal activity in the vestibular nuclei after contralateral or bilateral labyrinthectomy in the alert guinea pig

In the guinea pig, a unilateral labyrinthectomy is followed by an initial depression and a subsequent restoration of the spontaneous activity in the neurons of the ipsilateral vestibular nuclei. In two previous works, we have established the time course of these changes in the alert guinea pig using electrical stimulation as a search stimulus to select the analyzed neurons. The latter criterion was important to capture the many ipsilateral neurons that are silent at rest during the immediate postlabyrinthectomy stage. Because it is known that a pathway originating from the vestibular nuclei on one side crosses the midline and functionally inhibits the activity of the vestibular nuclei on the other side, we investigated in the first part of this study the spiking behavior of the neurons in the vestibular nuclei contralateral to the labyrinthectomy using the same procedure as that used for the ipsilateral neurons. The spiking behavior of 976 neurons was studied during 4-h recording sessions in intact animals and 1 h, 1 day, 2 days, or 1 wk postlabyrinthectomy. Neurons selected according to the electrical activation criterion were classified further as type I (their firing rate increased during ipsilateral rotation), type II (their firing rate increased during contralateral rotation), or unresponsive. The resting activity of type I neurons, which was 38.1 +/- 20.9 spikes/s (mean +/- SD) in the control state, increased statistically significantly 1 h after the lesion (53.3 +/- 29.1 spikes/s) and remained at this level 1 wk later (56.0 +/- 20.3 spikes/s). The sensitivity of type I units, which was 0.80 +/- 0.46 spikes/s per deg/s in the control population, decreased to 0.49 +/- 0.26 spikes/s per deg/s 1 h after the lesion and remained at this level 1 wk later (0.50 +/- 0.39 spikes/s per deg/s). When all monosynaptically activated neurons (type I, type II, unresponsive) were pooled, the sensitivity to horizontal rotation fell from 0.58 +/- 0.51 spikes/s per deg/s in the control state to 0. 15 +/- 0.25 spikes/s per deg/s 1 h after the lesion and to 0.20 +/- 0.32 spikes/s per deg/s 1 wk later. The major findings of the first part of this study in the alert guinea pig are thus in accord with those of Curthoys et al. and Smith and Curthoys in anesthetized guinea pigs. In the second part of this work, we studied the spiking behavior of the neurons in the vestibular nuclei after bilateral labyrinthectomy. After unilateral labyrinthectomy, the resting discharge of the ipsilateral monosynaptically activated vestibular neurons fell from 36.9 +/- 21 spikes/s (basal activity) to 6.7 +/- 17.0 spikes/s 1 h after the lesion and then recovered, reaching 17.4 +/- 18.9 and 40.8 +/- 23.7 spikes/s 1 day and 1 wk after the lesion, respectively. These observations raise the two following questions. What are the relative contributions of the loss of the excitatory influence from the ipsilateral labyrinth (destroyed) and of the persistence of the inhibitory influence from the contralateral labyrinth (intact) in the labyrinthectomy-induced depression of activity? And are the left-right asymmetries caused by a unilateral labyrinthectomy the driving force for restoration of activity? Here, we addressed these two questions by studying the spiking behavior of 473 second-order vestibular neurons in the alert guinea pig after a bilateral labyrinthectomy. In the acute stage, 1 h after bilateral labyrinthectomy, the resting discharge of the second-order vestibular neurons was 16.2 +/- 22.4 spikes/s. From comparison with the results obtained in the acute stage after a unilateral labyrinthectomy, we inferred that the ipsilateral excitatory influence was between two and three times more powerful than the contralateral inhibitory influence. (ABSTRACT TRUNCATED)

Time course of eye nystagmus and body movement after peripheral vestibular lesions in guinea pigs

Body movement of guinea pigs was measured using a force platform at various times before and after unilateral end organ ablation and before and after sham surgery. Both spontaneous and drop-evoked movement patterns differed in the same animal after vestibular ablation and from control animals that received sham lesions. Whereas measures of eye nystagmus disappeared by 48 h postablation, measures of body movement indicated persistent differences even at 72 h. We conclude that the force platform can differentiate between movement patterns of normal and vestibular-lesioned animals and, in fact, measures a vestibular deficiency that is independent of eye nystagmus. The force platform appears to be a useful addition to evaluate vestibular deficits as well as to detect any benefits of pharmacological or surgical therapies.

Central vestibular networks in the guinea-pig: functional characterization in the isolated whole brain in vitro

The isolated, in vitro whole brain of guinea-pig was used to assess some of the main physiological and pharmacological properties of the vestibulo-ocular pathways in this species. Extracellular and intracellular recordings were obtained from the vestibular, abducens and oculomotor nuclei, as well as from the abducens and oculomotor nerves, while inputs from the vestibular afferents, the visual pathways and the spinal cord were activated. The three main types of medial vestibular nucleus neurons (A, B and B+LTS), previously described on slices, were also identified in the isolated brain. They had similar membrane properties in both preparations. Eighty-five per cent of cells recorded in the vestibular nucleus responded with monosynaptic, excitatory postsynaptic potentials (latency 1.05-1.9 ms) to stimulation of the ipsilateral vestibular nerve, and were thus identified as second-order vestibular neurons. In addition, stimulation of the contralateral vestibular afferents revealed in most cases a disynaptic or trisynaptic, commissural inhibition. Second-order vestibular neurons displayed in the isolated brain a high degree of variability of their spontaneous activity, as in alert guinea-pigs. Type A neurons always exhibited a regular firing, while type B and B+LTS cells could have very irregular patterns of spontaneous discharge. Thus, type A and type B neurons might correspond, respectively, to the tonic and phasic vestibular neurons described in vivo. The regularity of spontaneous discharge was positively correlated with the amplitude of spike after hyperpolarization, and there was a trend for irregular neurons to be excited from ipsilateral vestibular afferents at shorter latencies than regular units. Synaptic activation could trigger subthreshold plateau potentials and low-threshold spikes in some of the second-order vestibular neurons. As a second step, the pharmacology of the synaptic transmission between primary vestibular afferents and second-order neurons was assessed using specific antagonists of the glutamatergic receptors. Both the synaptic field potentials and excitatory postsynaptic potentials elicited in the medial vestibular nucleus by single shock stimulation of the ipsilateral vestibular nerve were largely or, sometimes, totally blocked by 6-cyano-7-nitroquinoxaline-2,3-dione, indicating a dominating role of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor-mediated glutamatergic transmission. The remaining component of the responses was completely or partially suppressed by DL-2-amino-5-phosphonovaleric acid in 35% of the cases, suggesting a concomitant, moderate involvement of N-methyl-D-asparate receptors. In addition, a synaptic response resistant to both antagonists, but sensitive to a zero Ca2+/high Mg(2+)-containing solution, was often observed. Finally, recordings from abducens and oculomotor complexes confirmed the existence in the guinea-pig of strong bilateral, disynaptic excitatory and inhibitory inputs from vestibular afferents to motoneurons of extraocular muscles, which contribute to generation of the vestibulo-ocular reflex. The functional integrity of vestibular-related pathways in the isolated brain was additionally checked by stimulation of the spinal cord and optic tract. Stimulation of the spinal cord evoked, in addition to antidromic responses in the vestibular nucleus, short-latency synaptic responses in both the vestibular nucleus and abducens motoneurons, suggesting possible recruitment of spinal afferents. Activation of visual pathways at the level of the optic chiasm often induced long latency responses in the various structures under study. These results demonstrate that the in vitro isolated brain can be readily used for detailed, functional studies of the neuronal networks underlying gaze and posture control.

Cellular basis of vestibular compensation: changes in intrinsic excitability of MVN neurones

A systematic survey of the intrinsically generated in vitro discharge rates of rat medial vestibular nucleus (MVN) neurones was carried out in slices from normal animals and animals undergoing vestibular compensation over 48 h after unilateral labyrinthectomy (UL). Isolation of the individual MVN in vitro revealed that the tonic discharge rates of neurones in the rostral MVN ipsilateral to the lesion were not different from control 2 h post-UL, but increased significantly at 4 h post-UL and remained significantly higher until 24 h post-UL. There were no significant changes in the in vitro discharge rates of MVN cells in the contralateral nucleus. The increase in excitability of the ipsilateral MVN cells after UL may be accounted for by a down-regulation of GABA receptors on these cells, following their sustained exposure to excessive commissural inhibition after labyrinthectomy. We suggest that the increased intrinsic excitability of the ipsilateral MVN cells is responsible for the restoration of the resting discharge in these cells after UL and the consequent recovery of static vestibular function.

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.

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.

NMDA receptors of the vestibular nuclei neurones

Cloning and pharmacological studies have shown that glutamatergic receptors can be divided in two classes (refer to Table 1): ionotropic receptors including N-methyl-D-aspartate (NMDA) and non-NMDA subtypes, and the G-protein-coupled metabotropic receptors (glutamate metabotropic receptor). There are two types of non-NMDA receptors: the AMPA/low-affinity kainate receptor type (the AMPA receptors) activated by a specific agonist, the alpha-amino-3-hydroxy-5-methyl-4-iso-xalone propionate (AMPA), and the high affinity kainate receptors. The vestibular nuclei neurones are endowed with all these types of glutamatergic receptors, which fits well with the fact that various afferents, including the primary vestibular afferents, most probably use glutamate or aspartate as a neurotransmitter. This article is aimed at summarising several past studies of our group and some more recent data obtained in the in vitro whole-brain preparation concerning the NMDA receptors of the central vestibular neurones. In that process, we will detail also many valuable studies of other groups that had been devoted to the same topic.

Ginkgolide B accelerates vestibular compensation of spontaneous ocular nystagmus in guinea pig following unilateral labyrinthectomy

The aim of this study was to investigate the effects of ginkgolide B on the behavioral recovery process (vestibular compensation) which occurs following surgical removal of the vestibular receptor cells in one labyrinth (unilateral labyrinthectomy, UL). Guinea pigs received a single ip injection of ginkgolide B at the time of the UL (25, 50, or 100 mg/kg) and the effects on the compensation of the UL symptoms, spontaneous ocular nystagmus (SN), yaw head tilt (YHT), and roll head tilt (RHT), were evaluated. The effects of a single vehicle injection or a similar injection of ginkgolide A were used for comparison. Twenty-five mg/kg ginkgolide B significantly increased the rate of SN compensation compared to the vehicle control group (P < 0.02). However, 50 mg/kg ginkgolide B had no significant effect on either SN frequency or the rate of SN compensation. Ginkgolide B (100 mg/kg) significantly altered the rate of SN compensation (P < 0.02); however, SN frequency values were higher at most measurement times. YHT and RHT were not significantly affected by ginkgolide B at any of the doses used. Twenty-five mg/kg ginkgolide A had no significant effect on any of the UL symptoms. These results suggest that, at the optimal dose of 25 mg/kg, a single ip injection of ginkgolide B at the time of the UL can produce an acceleration of SN compensation.

Distribution of calretinin mRNA in the vestibular nuclei of rat and guinea pig and the effects of unilateral labyrinthectomy: a non-radioactive in situ hybridization study

We examined the localization of neurons expressing mRNA for calretinin, a cytosolic EF hand calcium-binding protein, throughout the vestibular nuclei of rat and guinea pig by non-radioactive in situ hybridization, using an alkaline phosphatase labeled oligonucleotide probe. Labeled cells were particularly numerous in the medial vestibular nucleus (mVN) and their distribution was similar in rat and guinea pig, and presented a characteristic rostrocaudal and mediolateral pattern. The effects of hemilabyrinthectomy were assessed at various times post lesion from 10 h to 30 days by comparison of the pattern of labeling in the ipsi- and contra-lateral vestibular nuclei of guinea pig. After up to 48 h no modification in the calretinin mRNA distribution was detected. After 3 to 30 days of survival, there was a decrease (about 30%) of the calretinin expressing neurons in the nucleus on the side of the lesion. The unilateral sensory deprivation seemed to induce a permanent asymmetry in the expression of calretinin which was not abolished after vestibular compensation. These results suggested that the calretinin expression in these neurons depends upon the integrity and activity of sensorineuronal peripheral vestibular influences.

Role of glucocorticoid in vestibular compensation in relation to activation of vestibular nucleus neurons

It is still not established whether or not glucocorticoids are effective in the treatment of vestibular disorders such as dizziness and imbalance, although these drugs in combination with several others are used to treat dizziness and imbalance in some diseases. This study was undertaken to investigate the effects of a glucocorticoid, dexamethasone, on vestibular disorder following unilateral labyrinthectomy in pigmented rabbits. Neuronal activities of the medial vestibular nucleus (MVN) in alpha-chloralose-anesthetized cats were also investigated. Systemic injection of dexamethasone decreased the frequency of nystagmus and head deviation dose-dependently following hemilabyrinthectomy, and the rate of decrease was faster than that obtained by saline. In contrast, RU38486 (a glucocorticoid receptor antagonist) delayed the reduction of nystagmus and head deviation. Micro-iontophoretic application of dexamethasone rapidly enhanced the spontaneous firing of MVN neurons in a dose-dependent manner. These increases were blocked by RU38486, but not by GDEE (a glutamate receptor antagonist) or Co2+ (a Ca2+ channel blocker). These results suggest that dexamethasone directly activates the MVN neurons, thereby accelerating vestibular compensation.

Effects of Ginkgo biloba extract (EGb 761) on the guinea pig vestibular system

Previous studies have demonstrated that the administration of Ginkgo biloba extract (EGb 761) improves the compensation of the vestibular syndrome induced by transection of the VIIIth nerve. To investigate the mechanisms at play, the vestibular nuclei of alert guinea pigs were perfused with EGb 761. This perfusion always induced a stereotyped reversible postural syndrome that was the mirror image of the syndrome provoked by the unilateral lesion of the otolithical receptors. This result supports the hypothesis that EGb 761 has a direct excitatory effect on the lateral vestibular nuclei (LVN) neurons. In a second step, we quantified the horizontal vestibuloocular reflex (HVOR) of the normal guinea pig following IP injection of EGb 761. In normal guinea pig, IP administration of EGb 761 led to a reversible, dose-dependent decrease of the HVOR gain without affecting the phase of the reflex. These data help to explain the therapeutic effects of EGb 761 during vestibular syndromes and strongly suggest an impact at the neuronal level.

Vestibulo-ocular compensation following unilateral vestibular deafferentation

Vestibulo-ocular compensation following vestibular deafferentation was investigated in 26 acoustic neuroma patients following tumor removal and in 5 Menière's disease patients following vestibular nerve section by using sinusoidal harmonic acceleration testing. All three test parameters (phase lead, gain, and asymmetry), when averaged, shifted significantly on the first postoperative test (average 0.4 months after operation). A marked progression in central compensation with gain returning to the range of normal control values was seen in the tests performed 1 to 6 months (average 2.6 months) postoperatively. However, phase lead and asymmetry (especially at 0.01, 0.02, 0.04, and 0.08 Hz) remained outside the range of normal control values, and all three parameters failed to return to their preoperative level even when tested at more than 12 months (average 20.2 months) after operation. This finding indicates that the deficits of the vestibulo-ocular reflex to head acceleration are of long duration if not permanent.

Medial vestibular nucleus in the guinea-pig: NMDA-induced oscillations

We have recently shown in vivo that N-Methyl-D-Aspartate (NMDA) receptors are present in the guinea-pig vestibular complex and demonstrated that they are involved in the regulation of the resting discharge of vestibular neurones. A parallel in vitro study has identified in the guinea-pig medial vestibular nuclei (MVN) two main neuronal cell types, A and B MVNn, differing by their intrinsic membrane properties. One subtype of B MVNn was further characterized by the presence of a low threshold calcium spike (LTS). The present study investigated in vitro the responses of these different cell types to NMDA. Both A and B MVNn were depolarized by NMDA, which also induced a decrease in membrane resistance and an increase in the spontaneous firing rate. These effects could be blocked by D-AP5, a specific antagonist of NMDA receptors. Following a 10-30 mV hyperpolarization, a long-lasting oscillatory behavior could be induced in presence of NMDA. These oscillations were however restricted to the subtype of B MVNn without LTS. The NMDA-induced oscillations were tetrodotoxine-resistant, but could be eliminated by D-AP5 or by replacing sodium with choline. Functional implications of this oscillatory behavior are discussed.

Plasticity mechanisms in vestibular compensation in the cat are improved by an extract of Ginkgo biloba (EGb 761)

The effects of administration of an extract of Ginkgo biloba (EGb 761) on vestibular compensation was studied in unilateral vestibular neurectomized cats. This experimental model of CNS plasticity was investigated by using behavioral tests (postural disorders compensation, locomotor balance recovery), electrophysiological (spontaneous and evoked neck muscle activity) and neurophysiological (spontaneous firing rate recovery of deafferented vestibular cells) recordings, and immunocytochemical methods (synaptic loss and synaptic reoccupation within the deafferented vestibular nuclei). In all experiments, EGb 761 was administered over 30 days at daily doses of 50 mg/kg IP. The results showed a faster recovery in the EGb-treated group of cats as compared to an untreated control group. EGb administration strongly accelerated postural and locomotor balance recovery. Concomitantly, spontaneous neck muscle activity, vestibulo-collic reflexes and spontaneous firing rate of vestibular units located on the lesioned side were restored earlier. Morphological correlates characterized by a more rapid synaptic reoccupation were found in the deafferented medial vestibular nucleus by means of immunoreactive labelling using an antibody against a synaptic vesicle-associated protein (synaptophysin), but they displayed a longer time-constant in comparison with the behavioral and neurophysiological data. These results clearly demonstrate that EGb 761 acts on recovery mechanisms considered as key processes in vestibular compensation. They suggest that this substance would possess neurotrophic and/or neuritogenic properties improving functional recovery after CNS injury.

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.

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.

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

The spontaneous activity and dynamic responses to sinusoidal horizontal head angular acceleration of type II horizontal semicircular canal related neurons in the medial vestibular nucleus (MVN) were recorded bilaterally in decerebrate Mongolian gerbils (Meriones unguiculatus) under three experimental conditions: normal labyrinths intact, acutely following unilateral labyrinthine lesion, and four to seven weeks following labyrinthine lesion. The number of type II neurons detected contralateral to the lesion was greatly reduced both in the acutely hemilabyrinthectomized animals and following compensation. The gain of the responses was depressed bilaterally acutely following the lesion. A greater reduction in response gain was noted in cells contralateral to the lesion. The gain of the contralateral type II responses increased with time such that in the compensated animal bilaterally symmetric gains were recorded. While the significant changes which occur in the gain of type II neurons with recovery from peripheral vestibular lesions can largely be attributed to type I neurons on the other side of the midline, changes in type I neurons were not entirely reflected in the type II population. The spontaneous activity of type II neurons did not undergo any significant changes following the labyrinthine lesion. We present a model utilizing the dynamic responses to estimate the functional recovery of commissural connections in compensated animals. The overall gain of the contralateral type I to ipsilateral type I commissural polysynaptic pathway appears to improve, while the efficacy in the reverse direction remains depressed, suggesting that modifications in commissural connections, particularly involving the type II to type I connections within the MVN on the injured side, mediate aspects of behavioral recovery.

The human horizontal vestibulo-ocular reflex in response to high-acceleration stimulation before and after unilateral vestibular neurectomy

The normal horizontal vestibulo-ocular reflex (HVOR) is largely generated by simultaneous stimulation of the two horizontal semicircular canals (HSCCs). To determine the dynamics of the HVOR when it is generated by only one HSCC, compensatory eye movements in response to a novel vestibular stimulus were measured using magnetic search coils. The vestibular stimulus consisted of low-amplitude, high-acceleration, passive, unpredictable, horizontal rotations of the head with respect to the trunk. While these so called head "impuses" had amplitudes of only 15-20 degrees with peak velocities up to 250 deg/s, they had peak accelerations up to 3000 deg/s/s. Fourteen humans were studied in this way before and after therapeutic unilateral vestibular neurectomy; 10 were studied 1 week or 1 year afterwards; 4 were studied 1 week and 1 year afterwards. The results from these 14 patients were compared with the results from 30 normal control subjects and with the results from one subject with absent vestibular function following bilateral vestibular neurectomy. Compensatory eye rotation in normal subjects closely mirrored head rotation. In contrast there was no compensatory eye rotation in the first 170 ms after the onset of head rotation in the subject without vestibular function. Before unilateral vestibular neurectomy all the patients' eye movement responses were within the normal control range. One week after unilateral vestibular neurectomy however there was a asymmetrical bilateral HVOR deficit. The asymmetry was much more profound than has been shown in any previous studies. The HVOR generated in response to head impulses directed away from the intact side largely by ampullofugal disfacilitation from the single intact HSCC (ignoring for the moment the small contribution to the HVOR from stimulation of the vertical SCCs), was severely deficient with an average gain (eye velocity/head velocity) of 0.25 at 122.5 deg/sec head velocity (normal gain = 0.94 +/- 0.08). In contrast the HVOR generated in response to head impulses directed toward the intact side, largely by ampullopetal excitation from the single intact HSCC, was only mildly (but nonetheless significantly) deficient, with an average gain of 0.80 at 122.5 deg/sec head velocity. At these accelerations there was no significant improvement in the average HVOR velocity gain in either direction over the following year. These results indicate that ampullopetal excitation from one HSCC can, even in the absence of ampullofugal disfacilitation from the opposite HSCC, generate a near normal HVOR in response to high-acceleration stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Low threshold calcium spikes in medial vestibular nuclei neurones in vitro: a role in the generation of the vestibular nystagmus quick phase in vivo?

Intracellular recordings were obtained from medial vestibular nuclei neurones in guinea-pig brainstem slices. A subpopulation of neurones in this nucleus was found to have burst firing properties. Using ionic channel blockers the underlying mechanism was shown to be a low threshold calcium spike. It is speculated that this property could be implicated in the generation of the quick phase of the vestibular nystagmus in the behaving guinea-pig.

Linear acceleration perception in the roll plane before and after unilateral vestibular neurectomy

The ability of 33 patients to perceive the direction, relative to the body long axis, of a linear acceleration vector acting in the coronal plane, roll-tilt perception, was studied at various times, before and from 1 week to 6 months after unilateral, selective vestibular neurectomy for Meniere's disease, acoustic neuroma or intractable paroxysmal vertigo. The results of these patients were compared with the results of 31 normal subjects and two control patients who had both vestibular nerves surgically sectioned. Rotating on a fixed-chair centrifuge in an otherwise darkened room, each observer was required to indicate his perception of the direction of the resultant gravito-inertial vector by setting a small, motor-driven, illuminated bar, attached to the chair but rotatable in the fronto-parallel plane, to the perceived gravitational horizontal. Normal subjects accurately align the bar with respect to the gravito-inertial resultant vector which, in the dark, they assume to be the gravitational vertical. This percept has been called the oculogravic illusion. Accurate roll-tilt perception is due to vestibular (probably mainly otolithic) sensory information since patients with bilateral vestibular neurectomies do not perceive the resultant vector accurately. Whereas normal subjects perceive resultant vectors directed to the right and to the left equally accurately, roll-tilt perception was invariably asymmetrical one week after unilateral vestibular neurectomy. Even at rest there was an asymmetry in the baseline settings, so that patients set the bar down on the side of the operated ear, in order for it to appear gravitationally horizontal: if a patient had a right vestibular nerve section then he set the bar clockwise (from the patient's view) below the true gravitational horizontal. With increasing gravito-inertial resultant angles there was an increasing asymmetry of roll-tilt perception due both to decreased sensitivity to roll-tilt stimuli directed towards the operated ear and to transiently increased sensitivity to roll-tilt stimuli directed towards the intact ear. A progressive decrease in both perceptual asymmetries followed, rapidly in the first 3 weeks, more slowly in the next 6 months.(ABSTRACT TRUNCATED AT 400 WORDS)

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)

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.