NMDA and AMPA receptor subunit protein expression in the rat vestibular nucleus following unilateral labyrinthectomy

Further studies of the effects of aging on arginine metabolites in the rat vestibular nucleus and cerebellum

Some studies have demonstrated that aging is associated with impaired vestibular reflexes, especially otolithic reflexes, resulting in postural instability. However, the neurochemical basis of these age-related changes is still poorly understood. The l-arginine metabolic system has been implicated in changes in the brain associated with aging. In the current study, we examined the levels of l-arginine and its metabolizing enzymes and downstream metabolites in the vestibular nucleus complex (VNC) and cerebellum (CE) of rats with and without behavioral testing which were young (4months old), middle-aged (12months old) or aged (24months old). We found that aging was associated with lower nitric oxide synthase activity in the CE of animals with testing and increased arginase in the VNC and CE of animals with testing. l-citrulline and l-ornithine were lower in the VNC of aged animals irrespective of testing, while l-arginine and l-citrulline were lower in the CE with and without testing, respectively. In the VNC and CE, aging was associated with lower levels of glutamate in the VNC, irrespective of testing. In the VNC it was associated with higher levels of agmatine and putrescine, irrespective of testing. In the CE, aging was associated with higher levels of putrescine in animals without testing and with higher levels of spermine in animals with testing, and spermidine, irrespective of testing. Multivariate analyses indicated significant predictive relationships between the different variables, and there were correlations between some of the neurochemical variables and behavioral measurements. Cluster analyses revealed that aging altered the relationships between l-arginine and its metabolites. The results of this study demonstrate that there are major changes occurring in l-arginine metabolism in the VNC and CE as a result of age, as well as behavioral activity.

Neuronal detection thresholds during vestibular compensation: contributions of response variability and sensory substitution

The vestibular system is responsible for processing self-motion, allowing normal subjects to discriminate the direction of rotational movements as slow as 1-2 deg s(-1). After unilateral vestibular injury patients' direction-discrimination thresholds worsen to ∼20 deg s(-1), and despite some improvement thresholds remain substantially elevated following compensation. To date, however, the underlying neural mechanisms of this recovery have not been addressed. Here, we recorded from first-order central neurons in the macaque monkey that provide vestibular information to higher brain areas for self-motion perception. Immediately following unilateral labyrinthectomy, neuronal detection thresholds increased by more than two-fold (from 14 to 30 deg s(-1)). While thresholds showed slight improvement by week 3 (25 deg s(-1)), they never recovered to control values - a trend mirroring the time course of perceptual thresholds in patients. We further discovered that changes in neuronal response variability paralleled changes in sensitivity for vestibular stimulation during compensation, thereby causing detection thresholds to remain elevated over time. However, we found that in a subset of neurons, the emergence of neck proprioceptive responses combined with residual vestibular modulation during head-on-body motion led to better neuronal detection thresholds. Taken together, our results emphasize that increases in response variability to vestibular inputs ultimately constrain neural thresholds and provide evidence that sensory substitution with extravestibular (i.e. proprioceptive) inputs at the first central stage of vestibular processing is a neural substrate for improvements in self-motion perception following vestibular loss. Thus, our results provide a neural correlate for the patient benefits provided by rehabilitative strategies that take advantage of the convergence of these multisensory cues.

Restoration of descending inputs fails to rescue activity following deafferentation of a motor network

Motor networks such as the pyloric network of the stomatogastric ganglion often require descending neuromodulatory inputs to initiate, regulate, and modulate their activity and their synaptic connectivity to manifest physiologically appropriate output. Prolonged removal of these descending inputs often results in a compensatory response that alters the inputs themselves, their targets, or both. Using the pyloric network of the crab, Cancer borealis, we investigated whether isolation of motor networks would result in alterations that change the responses of these networks to restored modulatory input. We used a reversible block with isotonic sucrose to transiently alter descending inputs into the pyloric network of the crab stomatogastric ganglion. Using this method, we found that blocking neuromodulatory inputs caused a reduced ability for subsequently restored modulatory projections to appropriately generate network output. Our results suggest that this could be due to changes in activity of descending projection neurons as well as changes in sensitivity to neuromodulators of the target neurons that develop over the time course of the blockade. These findings suggest that although homeostatic plasticity may play a critical role in recovery of functional output in a deafferented motor network, the results of these compensatory changes may alter the network such that restored inputs no longer function appropriately.

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

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

Neural correlates of motor learning in the vestibulo-ocular reflex: dynamic regulation of multimodal integration in the macaque vestibular system

Motor learning is required for the reacquisition of skills that have been compromised as a result of brain lesion or disease, as well as for the acquisition of new skills. Behaviors with well characterized anatomy and physiology are required to yield significant insight into changes that occur in the brain during motor learning. The vestibulo-ocular reflex (VOR) is well suited to establish connections between neurons, neural circuits, and motor performance during learning. Here, we examined the linkage between neuronal and behavioral VOR responses in alert behaving monkeys (Macaca mulatta) during the impressive recovery that occurs after unilateral vestibular loss. We show, for the first time, that motor learning is characterized by the dynamic reweighting of inputs from different modalities (i.e., vestibular vs extravestibular) at the level of the single neurons that constitute the first central stage of vestibular processing. Specifically, two types of information, which did not influence neuronal responses before the lesion, had an important role during compensation. First, unmasked neck proprioceptive inputs played a critical role in the early stages of this process demonstrated by faster and more substantial recovery of vestibular responses in proprioceptive sensitive neurons. Second, neuronal and VOR responses were significantly enhanced during active relative to passive head motion later in the compensation process (>3 weeks). Together, our findings provide evidence linking the dynamic regulation of multimodal integration at the level of single neurons and behavioral recovery, suggesting a role for homeostatic mechanisms in VOR motor learning.

Amino acid transporter (VIAAT, VGLUT2) and chloride cotransporter (KCC1, KCC2 and NKCC1) expression in the vestibular nuclei of intact and labyrinthectomized rat

We report the first investigation of whether unilateral labyrinthectomy in adult rats affects the expression of two amino acid transporters, vesicular glutamate transporter 2 (VGLUT2) and vesicular inhibitory amino acid transporter (VIAAT) and of chloride cotransporters (KCC1, KCC2 and NKCC1) in the intact and deafferented medial vestibular nuclei (MVN). In situ hybridization with specific radioactive oligonucleotide probes and immunofluorescent methods were used in normal and unilaterally labyrinthectomized rats at various times following the lesion: 5 h, and 1, 3 and 8 days. In normal animals, several brainstem regions including the lateral, medial, superior and inferior vestibular nuclei contained VGLUT2, VIAAT and KCC2 mRNA. In contrast, no or a very faint labeling was observed with KCC1 and NKCC1 probes. In unilaterally lesioned rats, there was no asymmetry between the two MVN with any of the oligonucleotide probes at any time after the lesion. Similarly, there were no differences in the intensity of MVN labeling between controls and lesioned animals. Finally, no over-expression of the cotransporter KCC1 and NKCC1 was found in ipsilateral or controlateral MVN in lesioned rats at any time. Immunohistochemical experiments gave similar conclusions. Our findings suggest that the recovery of the resting discharge of the deafferented MVN neurons, and consequently the functional compensation of the deficits, are not dependent on changes in the expression of amino acid transporters (VIAAT, VGLUT2), and chloride cotransporters (KCC1, KCC2 and NKCC1) or on their mRNAs.

Auditory nerve input is not an absolute requirement for the expression, distribution and calcium permeability of AMPA receptors in the adult rat ventral cochlear nucleus

In order to understand whether glutamatergic excitatory presynaptic input is an absolute requirement for the adult regulation of postsynaptic glutamate receptors we analyzed if a period of 11 days of excitatory deprivation affects the expression, distribution and Ca(2+) permeability of AMPA receptor subunits in the ventral cochlear nucleus of the rat. Bilateral cochlear ablations were performed in 30-day-old rats. After 11 days of survival, immunohistochemistry for GluR1, GluR2/3 and GluR4 AMPA receptor subunits showed no changes in the normal pattern of distribution, with GluR2/3 and GluR4 immunoreactivity predominating, and little GluR1. No changes in the amount of these AMPA receptor subunits were found between normal and cochleotomized rats in Western blots. AMPA receptors lacking the GluR2 subunit are Ca(2+) permeable. Kainate-induced Co(2+) uptake histochemistry, which labels AMPA Ca(2+) permeable receptors, demonstrated no changes in somatic labeling intensity for Co(2+), 11 days after cochleotomy. Therefore, our data indicate that excitatory input is not an absolute requirement to maintain AMPA receptor subunit expression, distribution and functional properties such as Ca(2+) permeability in VCN neurons. Nevertheless, subtle changes in AMPA receptors through regulatory post-transductional mechanisms cannot be ruled out.

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.

Spatial coding capacity of central otolith neurons

This review focuses on recent approaches to unravel the capacity of otolith-related brainstem neurons for coding head orientations. In the first section, the spatiotemporal features of central vestibular neurons in response to natural otolithic stimulation are reviewed. Experiments that reveal convergent inputs from bilateral vestibular end organs bear important implications on the processing of spatiotemporal signals and integration of head orientational signals within central otolith neurons. Another section covers the maturation profile of central otolith neurons in the recognition of spatial information. Postnatal changes in the distribution pattern of neuronal subpopulations that subserve the horizontal and vertical otolith systems are highlighted. Lastly, the expression pattern of glutamate receptor subunits and neurotrophin receptors in otolith-related neurons within the vestibular nuclear complex are reviewed in relation to the potential roles of these receptors in the development of vestibular function.

Cytosolic glucocorticoid receptor expression in the rat vestibular nucleus and hippocampus following unilateral vestibular deafferentation

It has been suggested that vestibular compensation, the process of behavioural recovery that occurs following peripheral vestibular damage, might be partially dependent on the release of glucocorticoids (GC) during the early stages of recovery from the lesion. One possibility is that glucocorticoid receptors (GRs) in the vestibular nucleus complex (VNC) might change following the lesion, altering their response to GCs. We sought to test this hypothesis by quantifying the expression of cytosolic GRs in the bilateral VNCs at 10 h, 58 h and 2 weeks following unilateral vestibular deafferentation (UVD) in rat, using western blotting. We also examined GR expression in the CA1, CA2/3 and dentate gyrus (DG) subregions of the hippocampus and measured serum corticosterone levels. Compared with sham surgery and anaesthetic controls, we found no significant changes in GR expression in the ipsilateral or contralateral VNCs at any time post-UVD. However, we did find a significant decrease in GR expression in the ipsilateral CA1 at 2 weeks post-UVD. Serum corticosterone levels were significantly lower in all groups at 58 h post-op. compared to 10 h and 2 weeks; however, there were no significant differences between the UVD and control groups at any time point. These results suggest that changes in GR expression in the VNC are unlikely to contribute to the development of vestibular compensation. However, long-term changes in GR expression in CA1 might be related to chronic deficits in hippocampal function and spatial cognition following vestibular damage.

Effects of unilateral vestibular ganglionectomy on glutaminase activity in the vestibular nerve root and vestibular nuclear complex of the rat

The metabolism of glutamate, the most likely neurotransmitter of vestibular ganglion cells, includes synthesis from glutamine by the enzyme glutaminase. We used microdissection combined with a fluorometric assay to measure glutaminase activity in the vestibular nerve root and nuclei of rats with unilateral vestibular ganglionectomy. Glutaminase activity in the lesioned-side vestibular nerve root decreased by 62% at 4 days after ganglionectomy and remained at similar values through 30 days. No change occurred in the contralateral vestibular nerve root. Glutaminase activity changes in the vestibular nuclei were lesser in magnitude and more complex, including contralateral increases as well as ipsilateral decreases. At 4 days after ganglionectomy, glutaminase activity was 10-20% lower in individual lesioned-side nuclei compared with their contralateral counterparts. By 14 and 30 days after ganglionectomy, there were no statistically significant differences between the nuclei on the two sides. This transient asymmetry of glutaminase activities in the vestibular nuclei contrasts with the sustained asymmetry in the vestibular nerve root and suggests that intrinsic, commissural, or descending pathways are involved in the recovery of chemical symmetry. This recovery resembles our previous finding for glutamate concentrations in the vestibular nuclei and may partially underlie central vestibular compensation after peripheral lesions.

Unilateral inner ear damage results in lasting changes in hippocampal CA1 field potentials in vitro

We investigated the effects of a surgical lesion of one vestibular inner ear (unilateral vestibular damage [UVD]) on the field potential responses of CA1 neurons in vitro. Hippocampal slices were removed from rats at 4-6 weeks or 5-6 months post-UVD, and the field responses of CA1 neurons to electrical stimulation of the Schaffer collateral commissural pathway were analyzed. Compared with slices from sham and naive control animals, slices from UVD animals at 5-6 months post-UVD exhibited decreases in the population spike amplitude, the somal field excitatory postsynaptic potential (sfEPSP) slope, and the field EPSP (fEPSP) slope. For the population spike amplitude and fEPSP slope, this effect was observed in both CA1 ipsilateral and contralateral to the UVD. On both the ipsilateral and contralateral sides, paired-pulse testing showed increases in paired-pulse inhibition at the shortest interstimulus intervals (ISIs), with increases in paired-pulse facilitation at longer ISIs. This study provides the first evidence that peripheral vestibular damage can produce long-term changes in hippocampal electrophysiological activity in vitro.

Long-term changes in hippocampal n-methyl-D-aspartate receptor subunits following unilateral vestibular damage in rat

Previous studies have indicated that damage to the peripheral vestibular system results in dysfunction of hippocampal place cells and an impairment of spatial learning and memory. The aim of this study was to determine whether lesions of one vestibular labyrinth (unilateral vestibular deafferentation, UVD) result in changes in the expression of the NR1 and NR2A subunits of the N-methyl-D-aspartate (NMDA) receptor, and the GluR2 subunit of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptor, in subregions of the rat hippocampus (CA1, CA2/3 and the dentate gyrus) at 10 h or 2 weeks following UVD. Compared with sham surgery controls and anaesthetic controls, the expression of the NR1 subunit was significantly reduced in the ipsilateral CA2/3 region at 2 weeks post-UVD. The expression of the NR2A subunit was also significantly reduced in the ipsilateral CA2/3 and, to a smaller extent, in the contralateral CA2/3 region, at 2 weeks post-UVD. The only other change in NR2A expression was an increase in the ipsilateral CA1 at 10 h post-UVD. No other changes in NR1, NR2A or GluR2 expression were observed in any hippocampal subregion, at any time point, or in cortical tissue at any time point. These results suggest that UVD may result in long-term changes in NMDA receptor subunit expression in the rat hippocampus.

Nitric oxide synthase and arginase expression in the vestibular nucleus and hippocampus following unilateral vestibular deafferentation in the rat

The aim of this study was to investigate the possible relationship between changes in neuronal and endothelial nitric oxide synthase (nNOS and eNOS) and arginase expression in the vestibular nucleus complex and the hippocampus (CA1, CA2/3 and the dentate gyrus (DG) at 10 h or 2 weeks following a unilateral vestibular deafferentation (UVD) in rats. There were no significant differences in nNOS or arginase II expression in the ipsilateral or contralateral VNC at either 10 h or 2 weeks post-UVD. For eNOS, there was only a significant decrease in expression in the ipsilateral VNC at 2 weeks post-UVD (P<0.01). In the hippocampus, the only significant difference in nNOS expression was a decrease in the ipsilateral DG at 2 weeks post-UVD (P<0.05). There was a significant decrease in eNOS expression in the contralateral CA2/3 region at 10 h post-UVD (P<0.01). The only other significant change in eNOS was an increase in the contralateral DG at 10 h post-UVD (P<0.01). Although arginase II was expressed in all regions of the hippocampus, there were no significant differences in arginase II expression at any time point following UVD. These results suggest that the changes in NOS expression that occur in the VNC and hippocampus following UVD are not correlated with one another or with changes in arginase II.