Quantitative autoradiographic characterization of L-[3H] glutamate binding sites in rat vestibular nuclei

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.

Influence of serotonin on the glutamate-induced excitations of secondary vestibular neurons in the rat

The excitatory responses evoked by glutamate and its agonists in secondary vestibular neurons of the rat were studied during microiontophoretic application of 5-hydroxytryptamine (5-HT). Ejection of 5-HT modified neuronal responsiveness to glutamate in 86% of the studied units, the effect being a depression of the excitatory responses in two-thirds of cases and an enhancement in the remaining third. 5-HT was also effective in modifying 94% of the responses evoked by N-methyl-d-aspartate (NMDA), inducing a depressive effect in 76% of cases and an enhancement in the remaining ones. Quisqualate-evoked effects were depressed and enhanced by 5-HT in about the same number of cases; in contrast, kainate-evoked responses were enhanced. The depressive action of 5-HT was mimicked by application of alpha-methyl-5-hydroxytryptamine (alpha-Me-5-HT), a 5-HT(2) receptor agonist, whereas the enhancing effect could be evoked by application of 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), a selective 5-HT(1A) receptor agonist. The 5-HT(2) receptor antagonist ketanserin was able to reduce, but not to block totally, the depressive action of 5-HT on glutamate- or NMDA-evoked responses. No significant difference was detected between neuronal responses in the lateral and the superior vestibular nucleus. These results indicate that 5-HT is able to modulate the responsiveness of secondary vestibular neurons to excitatory amino acids. Its action is mostly depressive, involves 5-HT(2) receptors, and is exerted on NMDA receptors. A minor involvement of other 5-HT receptors (at least 5-HT(1A)) and other glutamate receptors (for quisqualate and kainate) in the modulatory action of 5-HT is plausible.

Monoclonal L-citrulline immunostaining reveals nitric oxide-producing vestibular neurons

Nitric oxide is an unstable free radical that serves as a novel messenger molecule in the central nervous system (CNS). In order to understand the interplay between classic and novel chemical communication systems in vestibular pathways, the staining obtained using a monoclonal antibody directed against L-citrulline was compared with the labeling observed using more traditional markers for the presence of nitric oxide. Brainstem tissue from adult rats was processed for immunocytochemistry employing a monoclonal antibody directed against L-citrulline, a polyclonal antiserum against neuronal nitric oxide synthase, and/or NADPH-diaphorase histochemistry. Our findings demonstrate that L-citrulline can be fixed in situ by vascular perfusion, and can be visualized in fixed CNS tissue sections by immunocytochemistry. Further, the same vestibular regions and cell types are labeled by NADPH-diaphorase histochemistry, by the neuronal nitric oxide synthase antiserum, and by our anti-L-citrulline antibody. Clusters of L-citrulline-immunoreactive neurons are present in subregions of the vestibular nuclei, including the caudal portion of the inferior vestibular nucleus, the magnocellular portion of the medial vestibular nucleus, and the large cells in the ventral tier of the lateral vestibular nucleus. NADPH-diaphorase histochemical staining of these neurons clearly demonstrated their multipolar, fusiform and globular somata and long varicose dendritic processes. These results provide support for the suggestion that nitric oxide serves key roles in both vestibulo-autonomic and vestibulo-spinal pathways.

Cholinergic and glutamatergic transmission in medial vestibular nucleus neurons responding to lateral roll tilt in rats

The responses of the medial vestibular nucleus (MVN) neurons to lateral tilt and the neurotransmitters mediating otolith information to MVN neurons were investigated using rats. A computer-operated goniometer was tilted 20 degrees clockwise and counterclockwise at an angular speed of 5 degrees /s and paused in the inclined positions for 10 s to record neuronal responses in the static phase. The 185 MVN neurons recorded were classified into eight types according to their responses to tilt (alpha, beta, gamma, delta, epsilon, zeta, eta and theta). A majority showed increased firing in response to ipsilateral tilting and decreased firing in response to contralateral tilting (alpha type: 31.4%) or exhibited the reverse pattern (beta type: 36.8%). Further, other groups of neurons increased (gamma type) or decreased (delta type) firing rates to either side tilting and increased (epsilon and zeta type) or decreased (eta and theta type) firing only on one side. Atropine or L-glutamic acid diethyl ester hydrochloride (GDEE) applied microiontophoretically antagonized tilt-induced firing of alpha type neurons in 58.8% or 60.0%, respectively, and of beta type neurons in 66.7% or 58.3%, respectively. When the effects of atropine and GDEE were examined in the same neurons, antagonizing effects of both drugs on tilt-induced firing were obtained in 28.6% and 40.0% of alpha and beta type neurons, respectively. These results suggest that both acetylcholine and glutamate act as neurotransmitters in the transmission of otolith information to most MVN neurons.

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.

Regulation of NMDA receptor subunit mRNA expression in the guinea pig vestibular nuclei following unilateral labyrinthectomy

The localization of neurons expressing mRNAs for the NR1 and NR2A-D subunits of the glutamatergic NMDA receptor was examined by non-radioactive in situ hybridization throughout the guinea pig vestibular nuclei. After deafferentation of the vestibular nuclei by unilateral labyrinthectomy, modifications of the mRNA distributions were followed for 30 days. A quantitative analysis was performed in the medial vestibular nucleus by comparison of the labelled neurons in the ipsi- and contra-lateral nuclei. In vestibular nuclei, the NR1 subunit mRNA was found in various populations of neurons. The NR2A and NR2C subunit mRNAs were less widely distributed, whereas little NR2D mRNA was detected and only rare cells contained NR2B mRNA. NR1 and NR2A-D mRNAs were colocalized in some but not other neuronal types. Twenty hours after the lesion, there was a transient ipsilateral increase of NR1 mRNA level in the medial vestibular nucleus, followed by a decrease 48 h after the lesion and, at 3 days, by recovery to the control level. An ipsilateral increase in the mRNA level of NR2C subunit was detected 20 h after lesion and maintained at 48 h. No significant changes were apparent in NR2A, NR2B and NR2D mRNA levels. The distributions and the differential signal intensities of NR2A-D mRNAs suggest various subunit organizations of the NMDA receptors in different neurons of the vestibular nuclei. Neuronal plasticity reorganizations in the vestibular nuclei following unilateral labyrinthectomy appear to include only changes in NR1 and NR2C mRNA levels modifying the functional diversity of the NMDA receptor in the ipsilateral medial vestibular nucleus neurons. The transient changes in NR1 and the NR2C subunit mRNA expressions in response to sensory deprivation are consistent with an active role for NMDA receptors in the appearance and development of the vestibular compensatory process.

Glutamate as a primary afferent neurotransmitter in the medial vestibular nucleus as detected by in vivo microdialysis

An in vivo microdialysis study using alpha-chloralose-anesthetized cats was performed to elucidate whether glutamate is actually released from the vestibular nerve terminals in the medial vestibular nucleus (MVN) with electrical stimulation of the vestibular nerve. When repetitive stimuli composed of rectangular pulses (200 micros in duration, 0.5 mA, and 0.1-50 Hz) were applied to the vestibular nerve for 10 min, a significant frequency-dependent increase in the release of glutamate was observed in the MVN. However, the levels of other amino acids such as aspartate, glycine and GABA remained unaltered with the stimuli. These findings indicate that glutamate is the primary afferent neurotransmitter from the vestibular nerve to the MVN neurons.

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.

Expression of the AMPA-selective receptor subunits in the vestibular nuclei of the chinchilla

The distribution of the AMPA type glutamate receptor has been investigated throughout the central nervous system; however, no detailed description of its distribution is available in the vestibular nuclei. In the present study, in situ hybridization histochemistry and immunohistochemistry were used to localize the messenger RNAs and proteins of the AMPA-selective receptor subunits GluR1, GluR2, GluR3 and GluR4 in the vestibular nuclei of the chinchilla. Immunohistochemistry with subunits specific antisera showed differential distribution of the subunits in the vestibular nuclei. GluR2/3 antiserum labeled the most neurons, suggesting that many if not all vestibular neurons receive glutamatergic input. GluR1-positive neurons were fewer than GluR2/3 immunoreactive neurons and GluR4 immunoreactivity was found in the fewest number of neurons. GluR1 and GluR4 immunoreactivity was also found in astrocyte-like structures. In situ hybridization with 35S-labeled complementary RNA probes confirmed the distribution of the AMPA receptor subunits obtained by immunohistochemistry. Quantitative analysis of the levels of hybridization showed a high degree of diversity in the levels of expression of the GluR2 subunit mRNA, with the highest levels of expression in the giant Deiter's cells of the lateral vestibular nuclei and the lowest levels in the small neurons throughout the vestibular nuclei. The subunit compositions of the AMPA receptors determine their physiological properties. Differential distribution and levels of expression of the receptor subunits in the vestibular nuclei may be related to the characteristics of information processing through the vestibular system.

Immunohistochemical localization of neurotransmitters utilized by neurons in the rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) that project to the oculomotor and trochlear nuclei in the cat

The rostral interstitial nucleus of the medial longitudinal fasciculus (riMLF) contains excitatory and inhibitory burst neurons that are related to the control of vertical and torsional eye movements. In the present study, light microscopic examination of the immunohistochemical localization of amino acid neurotransmitters demonstrated that the riMLF in the cat contains overlapping populations of neurons that are immunoreactive to the putative inhibitory neurotransmitter gamma-aminobutyric acid (GABA) and the excitatory neurotransmitters glutamate and aspartate. By using a double-labelling paradigm, GABA-, glutamate-, and aspartate-immunoreactive neurons in the riMLF were retrogradely labelled by transport of horseradish peroxidase (HRP) from the oculomotor and trochlear nuclei. Electron microscopy showed that the oculomotor and trochlear nuclei contain synaptic endings that are immunoreactive to GABA, glutamate, or aspartate. Each neurotransmitter-specific population of synaptic endings has distinctive ultrastructural and synaptic features. Synaptic endings in the oculomotor and trochlear nuclei that are anterogradely labelled by transport of biocytin from the riMLF are immunoreactive to GABA, glutamate, or aspartate. Taken together, the findings from these complimentary retrograde and anterograde double-labelling studies provide rather conclusive evidence that GABA is the inhibitory neurotransmitter, and glutamate and aspartate are the excitatory neurotransmitters, utilized by premotor neurons in the riMLF that are related to the control of vertical saccadic eye movements.

The role of GABA in NMDA-dependent long term depression (LTD) of rat medial vestibular nuclei

The role of GABA in NMDA-dependent long term depression (LTD) in the medial vestibular nuclei (MVN) was studied on rat brainstem slices. High frequency stimulation (HFS) of the primary vestibular afferents induces a long lasting reduction of the polysynaptic (N2) component of the field potentials recorded in the dorsal portion of the MVN. The induction but not the maintenance of this depression was abolished by AP5, a specific blocking agent for glutamate NMDA receptors. The involvement of GABA in mediating the depression was checked by applying the GABAA and GABAB receptor antagonists, bicuculline and saclofen, before and after HFS. Under bicuculline and saclofen perfusion, HFS provoked a slight potentiation of the N2 wave, while the N2 depression clearly emerged after drug wash-out. This indicates that GABA is not involved in inducing the long term effect, but it is necessary for its expression. Similarly, the LTD reversed and a slight potentiation appeared when both drugs were administered after its induction. Most of these effects were due to the bicuculline, suggesting that GABAA receptors contribute to LTD more than GABAB do. According to our results, it is unlikely that the long lasting vestibular depression is the result of a homosynaptic LTD. On the contrary, our findings suggest that the depression is due to an enhancement of the GABA inhibitory effect, caused by an HFS dependent increase in gabaergic interneuron activity, which resets vestibular neuron excitability at a lower level.

Cellular and subcellular localization of AMPA-selective glutamate receptors in the mammalian peripheral vestibular system

The cellular and subcellular distribution of AMPA-selective glutamate receptors in the mammalian peripheral vestibular system was examined using antibodies against peptides corresponding to the C-terminal portions of AMPA receptor subunits: GluR1, GluR2/R3 and GluR4. The light and electron microscopic immunocytochemical studies were carried out on Vibratome sections of rat and guinea pig vestibular sensory epithelial and ganglia. In the epithelium, GluR1 subunit immunoreactivity appeared as accumulations of patches outlining the baso-lateral periphery of the type I sensory cells. The GluR1-immunoreactive microareas were postsynaptically distributed on the membranes of calyceal afferent fibers. GluR2/R3 immunoreactivity was present in the sensory cells. GluR4 was not detected. In the vestibular ganglion, the neurons were densely stained with antibodies to GluR2/R3 and GluR4. The fibroblasts and the Schwann cells were also intensely stained with antibodies to GluR2/R3 and GluR4. In the sensory cells, the AMPA receptors, GluR2/R3, may function as (1) autoreceptors controlling afferent neurotransmitter release or (2) 'postsynaptic' receptors activated by the neurotransmitter release of the afferent calyx. The detection of GluR1 at postsynaptic sites in the afferent fibers provides anatomical evidence for the role of glutamate as a neurotransmitter of sensory cells. In the ganglion neurons, GluR2/R3 and GluR4 may represent reserve intracytoplasmic pools of receptor subunits in transit to the postsynaptic sites. In the Schwann cells, GluR2/R3 and GluR4 may be involved in neuronal-glial signalling at the nodes of Ranvier.

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.

Synaptic input-induced increase in intraneuronal Ca2+ in the medial vestibular nucleus of young rats

In the medial vestibular nucleus (MVN), an input-dependent influx of Ca2+ into neurons through N-methyl-D-aspartate (NMDA) receptor-linked channels and/or voltage-dependent Ca2+ channels is suggested as underlying certain mechanisms of plasticity of the vestibular system. To see whether there is an increase in intracellular Ca2+ induced by afferent synaptic inputs to MVN neurons, we measured changes in [Ca2+]i with microfluorometry using a Ca2+ indicator, rhod-2, following electrical stimulation of ipsilateral vestibular afferents and commissural fibers in slice preparations of the brainstem of young rats (4-7 days postnatal). Single shock stimulation of ipsilateral afferents or commissural fibers induced an increase in fluorescence intensity lasting for several seconds. An application of 2-amino-5-phosphonovaleric acid (APV), an antagonist of NMDA receptors, almost completely blocked this stimulus-induced rise in fluorescence intensity. Nifedipine, an L-type Ca2+ channel blocker, also reduced the stimulus-induced rise in fluorescence intensity to 44-51% of the control value. These results suggest that synaptic inputs from the afferent and commissural pathways induce an influx of Ca2+ into MVN neurons due, at least in part, to the activation of NMDA receptors and the subsequent operation of L-type Ca2+ channels in young rats.

N-methyl-D-aspartate receptors contribute to afferent synaptic transmission in the medial vestibular nucleus of young rats

In the medial vestibular nucleus (MVN), the non-N-methyl-D-aspartate (NMDA) subtype of ionotropic glutamate receptors has been reported as operating at synapses between ipsilateral vestibular afferents and neurons. In the present study, we addressed the question of whether or not NMDA receptors contribute to afferent synaptic transmission in the MVN and if so, to what degree. Using nystatin-perforated or conventional whole-cell patch clamp methods in brainstem slices of young rats (postnatal day 4-6), we found that NMDA receptors contribute to a substantial extent to afferent synaptic transmission in the MVN of young rats.

The first developing "mixed" synapses between vestibular sensory neurons mediate glutamate chemical transmission

In the present study, the nature of the synaptic transmission responsible for a monophasic potential generated by vestibular nerve stimulation of the principal cells in the chick tangential nucleus was established. This work was performed in slice preparations at the critical embryonic age of 15-16 days, the time of first observation of morphologically mixed (chemical and electrical) synapses at the axosomatic endings called spoon endings. The spoon endings are formed by the primary vestibular fibers with the largest diameters, the colossal vestibular fibers. This monophasic potential fits the criteria for chemical rather than electrical transmission due to the following responses in most cases: (i) the absence of collision between a direct spike initiated by depolarization in the principal cell and a vestibular-evoked action potential; (ii) failure to follow high frequency stimulation (up to 50 Hz); (iii) sensitivity to low calcium solution (0.1 mM). These tests indicate that strong electrical coupling between spoon endings and principal cells does not prevail at this stage. The recordings were obtained from principal cells injected intracellularly with biocytin, allowing their identification by morphological criteria. The lack of tracer coupling between the stained principal cells and their innervating vestibular fibers (n = 17) is consistent with the absence of electrical coupling. Identification of the neurotransmitter involved in this vestibular response was achieved by bath application of glutamate receptor antagonists, DL-2-amino-5-phosphonovaleric acid (40 microM) and 6-cyano-7-nitro-quinoxaline-2,3-dione (10 microM), which blocked transmission reversibly. These results suggest that at the onset of formation of these "mixed" vestibular synapses, the gap junctions identified morphologically are likely not functional, and that the main response of the principal cells to vestibular nerve stimulation is mediated by glutamate.

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.

Excitatory amino acid receptors in normal and abnormal vestibular function

Although excitatory amino acid (EAA) receptors have been investigated extensively in the limbic system and neocortex, less is known of the function of EAA receptors in the brainstem. A number of biochemical and electrophysiological studies suggest that the synapse between the ipsilateral vestibular (VIIIth) nerve and the brainstem vestibular nucleus (VN) is mediated by an EAA acting predominantly on kainate or alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors. In addition, there is electrophysiological evidence that input from the contralateral vestibular nerve via the contralateral VN is partially mediated by N-methyl-D-aspartate (NMDA) receptors. Input to the VN from the spinal cord may also be partially mediated by NMDA receptors. All of the electrophysiological studies conducted so far have used in vitro preparations, and it is possible that denervation of the VN during the preparation of an explant or slice causes changes in EAA receptor function. Nonetheless, these results suggest that EAA receptors may be important in many different parts of the vestibular reflex pathways. Studies of the peripheral vestibular system have also shown that EAAs are involved in transmission between the receptor hair cells and the vestibular nerve fibers. A number of recent studies in the area of vestibular plasticity have reported that antagonists for the NMDA receptor subtype disrupt the behavioral recovery that occurs following unilateral deafferentation of the vestibular nerve fibers (vestibular compensation). It has been suggested that vestibular compensation may be owing to an upregulation or increased affinity of NMDA receptors in the VN ipsilateral to the peripheral deafferentation; however; at present, there is no clear evidence to support this hypothesis.

Evidence that NMDA receptors contribute to synaptic function in the guinea pig medial vestibular nucleus

Single medial vestibular nucleus neurons were recorded from guinea pig brainstem slices in vitro while superfusing with the selective N-methyl-D-aspartate (NMDA) antagonists, MK801 and CPP. The majority of neurons tested showed a decrease in firing rate in response to these NMDA antagonists, suggesting that NMDA receptors may contribute to the resting activity of MVN neurons.

The effects of N-methyl-D-aspartate antagonists on the development of vestibular compensation in the guinea pig

In order to investigate the possible role of the N-methyl-D-aspartate (NMDA) receptor in the development of the behavioural recovery which occurs following unilateral labyrinthectomy (vestibular compensation) in the guinea pig, we administered systemically the specific NMDA receptor/channel antagonists MK801 and CPP (3-((+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid) during the compensation process. MK801 disrupted the development of ocular motor compensation when administered at 18-22 h post-op (1.0 mg/kg i.p.). CPP had a smaller but still significant disruptive effect when injected at this time (1.0 or 5.0 mg/kg i.p.). We conclude that NMDA receptors may contribute to the development of ocular motor compensation in the guinea pig.