Comparison of the effects of NMDA antagonists on medial vestibular nucleus neurons in brainstem slices from labyrinthine-intact and chronically labyrinthectomized guinea pigs

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

We examined 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 (AMPA) receptor, in the ipsilateral and contralateral vestibular nucleus complexes (VNCs) at 10 h and 2 weeks following unilateral vestibular deafferentation (UVD) in rats, in order to directly test the hypothesis that the behavioural recovery following UVD ('vestibular compensation') is associated with an up-regulation of NMDA receptors. We found no significant changes in NR1 or NR2A expression at 10 hs or 2 weeks post-op. compared to sham and anesthetic controls. We did find a significant (p < 0.01 and p < 0.05) increase in GluR2 expression in both VNCs at 10 h but not 2 weeks post-op. compared to sham and anesthetic controls; however, comparison over time post-UVD failed to detect a significant difference, suggesting that it was small and transient at best. These results add further evidence to the conclusion that NMDA receptors do not undergo up-regulation in the ipsilateral VNC during vestibular compensation.

The contribution of the intrinsic excitability of vestibular nucleus neurons to recovery from vestibular damage

Damage to the peripheral vestibular system results in a syndrome of ocular motor and postural abnormalities that partially and gradually abate over time in a process known as 'vestibular compensation'. The first, rapid, phase of compensation has been associated with a recovery of spontaneous resting activity in the ipsilateral vestibular nucleus complex (VNC), as a consequence of neuronal and synaptic plasticity. Increasing evidence suggests that normal VNC neurons in labyrinthine-intact animals, as well as ipsilateral VNC neurons following unilateral vestibular deafferentation (UVD), rely to some extent on intrinsic pacemaker activity provided by voltage-dependent conductances for their resting activity. Modification of this intrinsic pacemaker activity may underlie the recovery of resting activity that occurs in ipsilateral VNC neurons following UVD. This review summarizes and critically evaluates the 'intrinsic mechanism hypothesis', identifying discrepancies amongst the current evidence and suggesting experiments that may test it further.

Molecular mechanisms of recovery from vestibular damage in mammals: recent advances

The aim of this review is to summarise and critically evaluate studies of vestibular compensation published over the last 2 years, with emphasis on those concerned with the molecular mechanisms of this process of lesion-induced plasticity. Recent studies of vestibular compensation have confirmed and extended the previous findings that: (i) compensation of the static ocular motor and postural symptoms occurs relatively rapidly and completely compared to the dynamic symptoms, many of which either do not compensate substantially or else compensate variably due to sensory substitution and the development of sensori-motor strategies which suppress or minimize symptoms; (ii) static compensation is associated with, and may be at least partially caused by a substantial recovery of resting activity in the ipsilateral vestibular nucleus complex (VNC), which starts to develop very quickly following the unilateral vestibular deafferentation (UVD) but does not correlate perfectly with the development of some aspects of static compensation (e.g., postural compensation); and (iii) many complex biochemical changes are occurring in the VNC, cerebellum and even areas of the central nervous system like the hippocampus, following UVD. However, despite many recent studies which suggest the importance of excitatory amino acid receptors such as the N-methyl-D-aspartate receptor, expression of immediate early gene proteins, glucocorticoids, neurotrophins and nitric oxide in the vestibular compensation process, how these various factors are linked and which of them may have a causal relationship with the physiological changes underlying compensation, remains to be determined.

Short-term reorganization of the rat somatosensory cortex following hypodynamia-hypokinesia

This study was performed to determine if hypodynamia-hypokinesia (HH) could induce a reorganization of the rat somatosensory cortex. The cortical hindpaw representation was determined by stimulating the limb and recording multi-unit cortical activity. The size of the cutaneous receptive fields was also measured. After 14 days of HH, the size of the cortical hindpaw representation was decreased. The proportion of small cutaneous receptive fields decreased while the large ones increased. After 7 days of HH, no change in the two studied parameters was noticed in five animals. In the other rats, a number of sites unresponsive to cutaneous stimulation or with high thresholds was observed. This study provides evidence of a plasticity of the somatosensory cortex induced by a situation that reduces both sensory and motor functions. The cortical reorganization occurs in two stages.

Neurotrophin 3, not brain-derived neurotrophic factor or neurotrophin 4, knockout mice have delay in vestibular compensation after unilateral labyrinthectomy

On the basis that neurotrophins (NTs) affect neuronal synaptic plasticity, are expressed in various cell types of the vestibular system, and exert a trophic influence on statoacoustic neurons, the authors hypothesized a role for NTs in vestibular compensation. To test this hypothesis, they performed unilateral surgical labyrinthectomy in 11 heterozygous (+/-) neurotrophin 3 (NT3) and brain-derived neurotrophic factor (BDNF) knockout mice and in two neurotrophin 4 (NT4) homozygous (-/-) knockout mice, each with a control (+/+) sibling, for a total of 26 mice. Four BDNF(+/-) and four NT3(+/-) mice with their (+/+) controls each were allowed to recover in a normal lighted room for 3, 7, 14, and 30 days following labyrinthectomy. Two BDNF(+/-) and two NT4(-/-) mice with controls were kept in total darkness for 1- and 16-day survival periods. One NT3(+/-) mouse without a control (which died in surgery) was sacrificed after 16 days in darkness. The behavior of all mice was videorecorded to monitor their recovery. Compared with normal (+/+) littermate controls, NT3(+/-) mice demonstrated a delay in compensation (8 to 10 days) in light surround, whereas NT4(-/-) mice showed only a minor delay in dark surround. Despite a 40% lower vestibular ganglion cell population in BDNF(+/-) mice compared with (+/+) controls, BDNF(+/-) mice did not reveal a detectable delay in recovery following labyrinthectomy. These findings suggest that a 50% loss of NT3 protein significantly affects vestibular recovery in adult mice. Perhaps variations in achieving vestibular compensation in humans may be partly secondary to genetically different NT3 levels in vestibular pathways.

The contribution of N-methyl-D-aspartate receptors to lesion-induced plasticity in the vestibular nucleus

The aim of this paper is to: i) review the behavioural, electrophysiological, pharmacological and biochemical evidence relating to the involvement of N-methyl-D-aspartate (NMDA) receptors in the vestibular compensation process which follows unilateral peripheral vestibular deafferentation (UVD); and ii) suggest a unifying hypothesis based on this literature and recent studies of long-term depression (LTD)-like phenomena in the brainstem vestibular nucleus complex (VNC). It is suggested that NMDA receptors may induce a form of heterosynaptic LTD in the ipsilateral VNC, which is partly responsible for the extent of the hypoactivity which occurs immediately following UVD, and the severity of the associated vestibular syndrome. It is also suggested that vestibular compensation may develop as this LTD dissipates, allowing remaining synaptic inputs and the intrinsic properties of ipsilateral VNC neurons to re-establish the resting activity which is responsible for static vestibular compensation. It is argued that this hypothesis accounts for the majority of the available data on NMDA receptors in relation to vestibular compensation, and may serve as a useful working hypothesis, in order to formulate further experiments to investigate the contribution of NMDA receptors to the compensation process.

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.

Quantitative autoradiography of 5-[3H]6-cyano-7-nitro-quinoxaline-2,3-dione and (+)-3-[3H]dizocilpine maleate binding in rat vestibular nuclear complex after unilateral deafferentation, with comparison to cochlear nucleus

The distributions of non-N-methyl-D-aspartate and N-methyl-D-aspartate receptors in the rat vestibular nuclear complex were estimated by quantitative autoradiography of 5-[3H]6-cyano-7-nitro-quinoxaline-2,3-dione and (+)-3-[3H]dizocilpine maleate binding, respectively. The binding of 5-[3H]6-cyano-7-nitro-quinoxaline-2,3-dione in the vestibular nuclear complex was also compared with that in the cerebellar cortex and cochlear nucleus. Measurements were made in control rats and in rats with unilateral destruction of the inner ear and removal of the vestibular ganglion. Compared to the unlesioned side, 5-[3H]6-cyano-7-nitro-quinoxaline-2,3-dione binding in the lesioned-side vestibular nuclear complex was decreased significantly in all regions at two to four postoperative days. However, the bilateral asymmetry disappeared in most regions by 30 days. 5-[3H]6-Cyano-7-nitro-quinoxaline-2,3-dione binding increased in the molecular layer of the cerebellar cortex at 30 days after lesion, although there were no clear changes at two to seven days. 5-[3H]6-Cyano-7-nitro-quinoxaline-2,3-dione binding in the cochlear nucleus decreased on the lesioned side, compared to the unlesioned side, in regions receiving significant auditory nerve innervation, but increased in the molecular layer of the dorsal cochlear nucleus. (+)-3-[3H]Dizocilpine maleate binding in regions of the vestibular nuclear complex was reduced on the lesioned side, compared to the unlesioned side, after deafferentation, with the largest reductions usually at 30 postoperative days. It is suggested that: (i) non-N-methyl-D-aspartate receptors are involved in synaptic transmission for both vestibular and auditory nerve fibers, while the involvement of N-methyl-D-aspartate receptors is less certain; (ii) unilateral deafferentation of the vestibular nuclear complex can result in bilateral asymmetries for non-N-methyl-D-aspartate and N-methyl-D-aspartate receptors, which are most prominent at earlier and later survival times, respectively; and (iii) vestibular compensation may involve regulation of both non-N-methyl-D-aspartate and N-methyl-D-aspartate receptors in the vestibular nuclear complex and activation of non-N-methyl-D-aspartate receptor-related processes in cerebellar cortex.

Role of vestibulocerebellar N-methyl-D-aspartate receptors for behavioral recovery following unilateral labyrinthectomy in rats

The purpose of current study was to elucidate whether vestibulocerebellar N-methyl-D-aspartate (NMDA) receptors are implicated in MK801 induced vestibular decompensation. Sprague-Dawley rats were unilaterally labyrinthectomized (ULX) and some of them were uvulonodullectomized before ULX (UNL + ULX). Number of spontaneous nystagmus (SN) and degree of head deviation (HD) were used as a parameter of behavioral recovery. MK801 treatment 6 h after ULX produced significant increases in SN and decreased HD in ULX rats, indicating decompensation. In marked contrast, however, MK801 treatment resulted in a great reduction of SN and HD in UNL + ULX rats; suggesting involvement of vestibulocerebellar NMDA receptors in MK801 induced decompensation during early stage of vestibular compensation.

Changes in Fos and Jun expression in the rat brainstem in the process of vestibular compensation

By means of immunohistochemical technique, we examined the changes in Fos and Jun expression after unilateral labyrinthectomy (UL) in the rat brainstem. We observed Fos-like immunoreactivity (-LIR) in the ipsilateral medial vestibular nucleus (ipsi-MVe), the contralateral prepositus hypoglossal nucleus (contra-PrH) and the contralateral inferior olive beta subnucleus (contra-IOb) 1 h after UL. Then, Fos expression in the contra-PrH and the contra-IOb disappeared 3 days after surgery. However, we still found the residual expression in the ipsi-MVe, which disappeared within 7 days. On the other hand, no Jun-LIR was detected in the vestibular or vestibular-related nuclei before or after the operation. Fos expression in the MVe, PrH and IOb was induced immediately after UL. Then, the Fos expression disappeared in accordance with the development of the vestibular compensation. These findings suggest that the transient Fos expression in the vestibular and vestibular related nuclei is a trigger of vestibular compensation.

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.

Evidence that L-type calcium channels do not contribute to static vestibular function in the guinea pig vestibular nucleus

Labyrinthine-intact guinea pigs received unilateral, brainstem cannula injections of (1) 2.5 micrograms of the selective dihydropyridine L-type Ca2+ channel agonist, Bay K 8644 (n = 4 animals); (2) 10 micrograms Bay K 8644 (n = 4); 12.5 micrograms of the selective dihydropyridine L-type Ca2+ channel antagonist, nifedipine (n = 4); or 40 micrograms nifedipine (n = 4). In 11/16 cases, the lesion associated with the cannula tip was located within or near the border of the right vestibular nucleus (VN) complex. All cannula injections were delivered in a 1 microliter volume of artificial cerebrospinal fluid (ACSF) and dimethylsulphoxide (DMSO) (70% DMSO, 30% ACSF for Bay K 8644; 80% DMSO, 20% ACSF for nifedipine), adjusted to a pH of approx. 7.0. The effects of these injections were compared with control injections of ACSF/DMSO in our previous studies. Animals were observed for signs of a labyrinthine syndrome (i.e. spontaneous ocular nystagmus, yaw and roll head tilt) directed to the contralateral or ipsilateral side. In no case did Bay K 8644 or nifedipine cause ocular motor or postural symptoms similar to those produced by a unilateral labyrinthectomy. These results suggest that L-type Ca2+ channels do not contribute significantly to the resting activity of VN neurons and therefore do not contribute to static vestibular function at the level of the VN.

Comparison of the effects of ACTH-(4-10) on medial vestibular nucleus neurons in brainstem slices from labyrinthine-intact and compensated guinea pigs

The effects of adrenocorticotrophic hormone fragment 4-10 (ACTH-(4-10)) on single medial vestibular nucleus (MVN) neurons, in brainstem slices from guinea pigs which had undergone vestibular compensation for a previous ipsilateral surgical unilateral labyrinthectomy, were compared with those on MVN neurons in slices from labyrinthine-intact guinea pigs observed in a previous study. Although the average resting discharge of MVN neurons in slices from compensated animals was significantly higher than that for MVN neurons from labyrinthine-intact animals, the responses of the two groups of MVN neurons to ACTH-(4-10) were very similar. These results suggest that ACTH-(4-10) treatment is unlikely to accelerate behavioral recovery following unilateral labyrinthectomy (vestibular compensation) by acting on a receptor within the MVN for which sensitivity to ACTH-(4-10) changes during the compensation process.