Rat Medial Vestibular Nucleus Research Articles

Spontaneous firing properties of rat medial vestibular nucleus neurons in brain slices by infrared visual patch clamp technique

Domestic application of infrared patch clamp techniques on brain slices is limited. The key of the technique is to prepare high-quality brain slices. The present paper describes the preparation procedure of brainstem slices and the spontaneous firing properties of rat medial vestibular nucleus (MVN) neurons. By infrared differential interference contrast technique, neurons of rat MVN were visualized directly at the depth of 50–100 μm underneath the surface of slices. Firing activities of MVN neurons were recorded by the whole-cell patch clamp technique in artificial cerebrospinal fluid (ACSF) and low Ca2+-high Mg2+ fluid. The firing mode was more irregular and depressive in low Ca2+-high Mg2+ fluid than in ACSF. According to the averaged waveform of action potentials, cells were classified as the neurons with monophasic after-hyperpolarization potential (AHP), and the neurons with biphasic AHP. The resting membrane potential (RMP), input resistance (Rin) and membrane capacitance (Cm) of neurons were recorded and compared between groups. With infrared videomicroscopy, patch clamp recordings could be made under direct observation in freshly prepared brainstem slices. The discharge activities of MVN neurons were spontaneous and the firing mode was modulated by extracellular calcium concentration. The basic membrane properties of two types of neurons were not significantly different, while the differences in waveform might play a role in the segregation between tonic and kinetic cells.

Relationship between afterhyperpolarization profiles and the regularity of spontaneous firings in rat medial vestibular nucleus neurons

Our previous in vivo and in vitro whole-cell patch-clamp recording studies demonstrated that neurons in the medial vestibular nucleus (MVN) could be characterized on the basis of three electrophysiological properties: afterhyperpolarization (AHP) profile; firing pattern; and response pattern to hyperpolarizing current pulses. In the present study, to clarify which types of the classified MVN neurons correspond to neurons with regular or irregular firing, we investigated their spike discharge patterns using whole-cell patch-clamp recording in both in vivo and in vitro preparations. The discharge regularity was related to AHP profiles, and we found that: (i) the coefficient of variation (CV) of interspike intervals during spike discharges was smaller in neurons exhibiting AHP with a slow component [AHP(s+)] than in those without a slow component [AHP(s-)], or with a slow AHP component preceded by afterdepolarization (ADP) [AHP(s+) with ADP]; (ii) the blockade of Ca(2+)-dependent K(+) channels by 100 nm apamin abolished the slow component and increased the CV in neurons exhibiting AHP(s+); and (iii) the modulation of firing (firing gain) in response to ramp current was larger in neurons exhibiting AHP(s-) than in the other two neuronal types. These results suggest that neurons exhibiting AHP(s+) are regularly discharging neurons with small firing gains to stimulus, neurons exhibiting AHP(s+) with ADP are irregularly discharging neurons with small firing gains, and neurons exhibiting AHP(s-) are irregularly discharging neurons with large firing gains. The regular firing of neurons exhibiting AHP(s+) is attributed to the activation of apamin-sensitive Ca(2+)-dependent K(+) channels.

Functional role of cyclic nucleotide‐gated channels in rat medial vestibular nucleus neurons

Although cyclic nucleotide-gated (CNG) channels are expressed in numerous brain areas, little information is available on their functions in CNS neurons. The aim of the present study was to define the distribution of CNG channels in the rat medial vestibular nucleus (MVN) and their possible involvement in regulating MVN neuron (MVNn) excitability. The majority of MVNn expressed both CNG1 and CNG2 A subunits. In whole-cell current-clamp experiments carried out on brainstem slices containing the MVNn, the membrane-permeant analogues of cyclic nucleotides, 8-Br-cGMP and 8-Br-cAMP (1 mM), induced membrane depolarizations (8.9 +/- 0.8 and 9.2 +/- 1.0 mV, respectively) that were protein kinase independent. The cGMP-induced depolarization was associated with a significant decrease in the membrane input resistance. The effects of cGMP on membrane potential were almost completely abolished by the CNG channel blockers, Cd(2+) and L-cis-diltiazem, but they were unaffected by blockade of hyperpolarization-activated cyclic nucleotide-gated channels. In voltage-clamp experiments, 8-Br-cGMP induced non-inactivating inward currents (-22.2 +/- 3.9 pA) with an estimated reversal potential near 0 mV, which were markedly inhibited by reduction of extracellular Na(+) and Ca(2+) concentrations. Membrane depolarization induced by CNG channel activation increased the firing rate of MVNn without changing the action potential shape. Collectively, these findings provide novel evidence that CNG channels affect membrane potential and excitability of MVNn. Such action should have a significant impact on the function of these neurons in sensory-motor integration processes. More generally, it might represent a broad mechanism for regulating the excitability of different CNS neurons.

Neurosteroid modulation of neuronal excitability and synaptic transmission in the rat medial vestibular nuclei

In rat brainstem slices, we investigated the influence of the neurosteroids tetrahydrodeoxycorticosterone (THDOC) and allopregnanolone (ALLO) on the synaptically driven and spontaneous activity of vestibular neurons, by analysing their effects on the amplitude of the field potentials evoked in the medial vestibular nuclei (MVN) by vestibular afferent stimulation and on the spontaneous firing rate of MVN neurons. Furthermore, the interaction with gamma-aminobutyric acid (GABA) and glutamate receptors was analysed by using specific antagonists for GABA(A) (bicuculline), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/ kainate [2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo(f)quinoxaline-7-sulphonamide disodium salt (NBQX)], N-methyl-D-aspartate (NMDA) [D-(-)-2-amino-5-phosphonopentanoic acid (AP-5)] and group I metabotropic glutamate receptors (mGlu-I) [(R,S)-1-aminoindan-1,5-dicarboxylic acid (AIDA)] receptors. THDOC and ALLO evoked two opposite long-lasting effects, consisting of either a potentiation or a reduction of field potential and firing rate, which showed early and late components, occurring in conjunction or separately after neurosteroid application. The depressions depended on GABA(A) receptors, as they were abolished by bicuculline, while early potentiation involved glutamate AMPA/kainate receptors, as NBQX markedly reduced the incidence of early firing rate enhancement and, in the case of ALLO, even provoked depression. This suggests that THDOC and ALLO enhance the GABA(A) inhibitory influence on the MVN neurons and facilitate the AMPA/kainate facilitatory one. Conversely, a late potentiation effect, which was still induced after glutamate and GABA(A) receptor blockade, might involve a different mechanism. We conclude that the modulation of neuronal activity in the MVN by THDOC and ALLO, through their actions on GABA(A) and AMPA/kainate receptors, may have a physiological role in regulating the vestibular system function under normal conditions and during the stress response that accompanies many forms of vestibular dysfunction.

Unilateral labyrinthectomy induced changes on GDNF receptor complex proteins in the rat medial vestibular nuclei

In the auditory periphery, GDNF has been shown to have a trophic effect to spiral ganglion neurons, both during development and in adult animals. We have studied the effect of unilateral labyrinthectomy (UL) on protein levels and expression of GDNF multicomponent receptor complex: the ret tyrosine kinase and coreceptor GFRalpha-1 in the medial vestibular nucleus of the adult rat. GFRalpha-1 protein levels display an increasing trend in ipsilateral medial vestibular nucleus culminating at 48 h post UL. On the other hand, GFRalpha-1 mRNA expression levels in ipsi- and contralateral medial vestibular nucleus show a steadily decreasing trend that is significant at 1 week post-lesion. Protein levels for c-Ret isoforms also show an initial bilateral decreasing trend that ceases at 48 h in ipsilateral medial vestibular nucleus but persists on the contralateral side. c-Ret mRNA expression levels show a significant decrease at 4 h post UL followed by another significant decrease 1 week post UL. Our data would suggest that neurotrophins belonging to the GDNF family are involved in this model of post-lesional CNS plasticity.

Endothelin-1-induced Barrel Rotation: No Direct Modulation of Rat Medial Vestibular Nucleus Neurone Activity by Endothelin-1

Fourth ventricular injections of endothelin-1 (30-100 ng) induced rotation along the long axis of the body (barrel rotation) in rats. N-methyl-D-aspartate (NMDA, 1-10 μg), an excitatory amino acid, and muscimol (1-10 μg), a GABAA receptor agonist, also evoked barrel rotation. Bath application of NMDA and muscimol on the spontaneous tonic discharge of medial vestibular nucleus (MVN) neurones in rats, showed excitatory and inhibitory responses, respectively. Interestingly, endothelin-1 did not affect the spontaneous discharge rate of MVN neurones when applied either as a 60 s pulse or when the drug was continuously perfused for a period of 5 min. Reciprocal relationship between excitatory and inhibitory inputs, such as glutamate and GABA receptor agonists, for MVN neurones may be crucial in inducing barrel rotation. The results also suggest that the barrel rotation induced by endothelin-1 is mediated by its modulatory effects on indirect pathways to the vestibular nucleus.

Changes in protein expression in the rat medial vestibular nuclei during vestibular compensation

The molecular mechanisms of neural and synaptic plasticity in the vestibular nuclei during 'vestibular compensation', the behavioural recovery that follows deafferentation of one inner ear, are largely unknown. In this study we have used differential proteomics techniques to determine changes in protein expression in ipsi-lesional and contra-lesional medial vestibular nuclei (MVN) of rats, 1 week after either sham surgery or unilateral labyrinthectomy (UL). A systematic comparison of 634 protein spots in two-dimensional electrophoresis gels across five experimental conditions revealed 54 spots, containing 26 proteins whose level was significantly altered 1 week post-UL. The axon-guidance-associated proteins neuropilin-2 and dehydropyriminidase-related protein-2 were upregulated in the MVN after UL. Changes in levels of further specific proteins indicate a coordinated upregulation of mitochondrial function, ATP biosynthesis and phosphate metabolism in the vestibular nuclei 1 week post-UL. These may reflect the metabolic energy demands of processes such as gliosis, neuronal outgrowth and synaptic remodelling that occur after UL. Our findings suggest novel roles for axon elaboration and guidance molecules, as well as mitochondrial and metabolic regulatory proteins, in the post-lesional physiology of the MVN during vestibular system plasticity.

Expression of cyclic nucleotide-gated channels in the rat medial vestibular nucleus

The role of cyclic nucleotide-gated (CNG) channels in sensory signal transduction in retinal and olfactory cells is widely recognized, but there is increasing evidence that they also play more general functions in the central nervous system as downstream effectors of cyclic nucleotides. Here, we demonstrate the expression of the alpha-subunit of rod- and olfactory-type CNG channels (CNG1 and CNG2, respectively) in the rat medial vestibular nucleus (MVN). Nested polymerase chain reaction revealed CNG channel mRNA in the MVN, and CNG1 and CNG2 proteins were also detected by Western blotting and immunohistochemistry. Finally, electrophysiological evidence is provided suggesting that CNG channels play a functional role in the MVN.

Role of group II metabotropic glutamate receptors 2/3 and group I metabotropic glutamate receptor 5 in developing rat medial vestibular nuclei

In brainstem slices from developing rats, metabotropic glutamate receptors mGluR2/3 and mGluR5 play different inhibitory roles in synaptic transmission and plasticity of the medial vestibular nuclei. The mGluR2/3 block (LY341495) reduces the occurrence of long-term depression after vestibular afferent high frequency stimulation at P8-P10, and increases that of long-term potentiation, while the mGluR5 block prevents high frequency stimulation long-term depression. Later on, the receptor block does not influence high frequency stimulation effects. In addition, while mGluR2/3 agonist (APDC) always provokes a transient reduction of synaptic responses, that of mGluR5 (CHPG) induces long-term depression per se at P8-P10. These results show a key role of mGluR5 in inducing high frequency stimulation long-term depression in developing medial vestibular nuclei, while mGluR2/3 modulate synaptic transmission, probably through presynaptic control of glutamate release.

Immunocytochemical and stereological study of glucocorticoid receptors in rat medial vestibular nucleus neurons and the effects of unilateral vestibular deafferentation

Conclusion. The results of this study suggest that neither the number of medial vestibular nucleus (MVN) neurons expressing cytosolic glucocorticoid receptors nor blood corticosterone levels change significantly during the development of vestibular compensation. Objective. Vestibular compensation is a process of partial behavioral recovery that occurs following damage to the vestibular labyrinth. It has been suggested that this compensation process might be dependent on the release of glucocorticoids such as corticosterone at the time of unilateral vestibular deafferentation (UVD) and that changes in glucocorticoid receptors in the MVN might contribute to the initiation of the compensation process. Material and methods. We compared the number of MVN neurons expressing cytosolic glucocorticoid receptors in rats at 10 h and 2 weeks following UVD, and in sham and anesthetic control animals; we also measured blood corticosterone levels. Results. Using immunocytochemistry and stereology, we found that the majority of MVN neurons expressed glucocorticoid receptors, but there were no significant differences in the number of glucocorticoid receptor-expressing neurons in the ipsilateral or contralateral MVNs at 10 h or 2 weeks post-UVD; furthermore, corticosterone levels did not vary significantly between the UVD and control groups.

An in situ hybridization and immunofluorescence study of GABA(A) and GABA(B) receptors in the vestibular nuclei of the intact and unilaterally labyrinthectomized rat.

We investigated whether the production of the sixteen subunits of the GABA(A) receptors and of the different variants of GABA Breceptors are modulated in rat medial vestibular nuclei (MVN) following unilateral labyrinthectomy. Specific alpha1-6, beta1-3, gamma1-3 and delta GABA(A) and GABA(B) B1 and B2receptor radioactive oligonucleotides were used for in situ hybridization to probe sections of rat vestibular nuclei. Specific antibodies against alpha1, beta2, beta3 and gamma2 subunits of GABA(A) receptors and against GABA( B)receptors were also used to detect a potential protein expression modulation. No asymmetry was observed by autoradiography in the intact and deafferented MVN at any time (5 h to 8 days) following the lesion and for any of the oligonucleotide probes used. Also, no difference in the alpha1, beta2, beta3 and gamma2 of the GABA(A) and in the GABA(B) receptor immunohistochemical signal could be detected between the intact and deafferented vestibular nuclei at any time following the lesion. Our data suggest that GABA(A) and GABA Breceptor density changes most probably were not involved in the early stage of the vestibular compensation process, i.e., in the restoration of a normal resting discharge of the deafferented vestibular neurons and consequently in the recovery of a normal posture and eye position.

Nitric oxide increases the spontaneous firing rate of rat medial vestibular nucleus neurons in vitro via a cyclic GMP-mediated PKG-independent mechanism.

The effects of nitric oxide (NO) on the discharge rate of medial vestibular nucleus neurons (MVNn) were investigated in rat brainstem slices. The NO-donor sodium nitroprusside (SNP, 200 microM) caused a marked enhancement (+36.7%) of MVNn spontaneous firing rate, which was prevented by the NO-scavenger, carboxy-PTIO (300 microM). The SNP effects were not modified (+37.4%) by synaptic uncoupling, suggesting that NO influences intrinsic membrane properties of MVNn rather than the synaptic input they receive. The excitatory action of SNP was virtually abolished by slice pretreatment with the soluble guanylyl cyclase inhibitor, ODQ (10 microM), and it was mimicked (+33.1%) by the cGMP analogue 8-Br-cGMP (400 microM). Protein kinase G (PKG) and cAMP/protein kinase A (PKA) were both excluded as downstream effectors of the NO/cGMP-induced excitation. However, the cyclic nucleotide-gated (CNG) channel blockers, L-cis-diltiazem (LCD, 100 microM) and Sp-8-Br-PET-cGMPS (100 microM), significantly reduced the firing rate increase produced by 8-Br-cGMP. Moreover, LCD alone decreased spontaneous MVNn firing (-19.7%), suggesting that putative CNG channels may contribute to the tonic control of resting MVNn discharge. 8-Br-cAMP (1 mM) also elicited excitatory effects in MVNn (+40.8%), which occluded those induced by 8-Br-cGMP, indicating that the two nucleotides share a common target. Finally, nested-polymerase chain reaction assay revealed the expression of CNG channel alpha subunit transcript in MVNn. Our data provide the first demonstration that NO/cGMP signalling modulates MVNn spontaneous firing through a mechanism that is independent of PKG or PKA and probably involves activation of CNG channels.

Membrane and firing properties of glutamatergic and GABAergic neurons in the rat medial vestibular nucleus.

In previous studies, neurons in the medial vestibular nucleus (MVN) were classified mainly into 2 types according to their intrinsic membrane properties in in vitro slice preparations. However, it has not been determined whether the classified neurons are excitatory or inhibitory ones. In the present study, to clarify the relationship between the chemical and electrophysiological properties of MVN neurons, we explored mRNAs of cellular markers for GABAergic (glutamic acid decarboxylase 65, 67, and neuronal GABA transporter), glutamatergic (vesicular glutamate transporter 1 and 2), glycinergic (glycine transporter 2), and cholinergic neurons (choline acetyltransferase and vesicular acetylcholine transporter) expressed in electrophysiologically characterized MVN neurons in rat brain stem slice preparations. For this purpose, we combined whole cell patch-clamp recording analysis with single-cell reverse transcription-polymerase chain reaction (RT-PCR) analysis. We examined the membrane properties such as afterhyperpolarization (AHP), firing pattern, and response to hyperpolarizing current pulse to classify MVN neurons. From the single-cell RT-PCR analysis, we found that GABAergic neurons consisted of heterogeneous populations with different membrane properties. Comparison of the membrane properties of GABAergic neurons with those of other neurons revealed that AHPs without slow components and a firing pattern with delayed spike generation (late spiking) were preferential properties of GABAergic neurons. On the other hand, most glutamatergic neurons formed a homogeneous subclass of neurons exhibiting AHPs with slow components, repetitive firings with constant interspike intervals (continuous spiking), and time-dependent inward rectification in response to hyperpolarizing current pulses. We also found a small number of cholinergic neurons with various membrane properties. These findings clarify the electrophysiological properties of excitatory and inhibitory neurons in the MVN, and the information about the preferential membrane properties may be useful for identifying GABAergic and glutamatergic MVN neurons electrophysiologically.

Plasticity of histamine H3 receptor expression and binding in the vestibular nuclei after labyrinthectomy in rat

BackgroundIn rat, deafferentation of one labyrinth (unilateral labyrinthectomy) results in a characteristic syndrome of ocular and motor postural disorders (e.g., barrel rotation, circling behavior, and spontaneous nystagmus). Behavioral recovery (e.g., diminished symptoms), encompassing 1 week after unilateral labyrinthectomy, has been termed vestibular compensation. Evidence suggesting that the histamine H3 receptor plays a key role in vestibular compensation comes from studies indicating that betahistine, a histamine-like drug that acts as both a partial histamine H1 receptor agonist and an H3 receptor antagonist, can accelerate the process of vestibular compensation.ResultsExpression levels for histamine H3 receptor (total) as well as three isoforms which display variable lengths of the third intracellular loop of the receptor were analyzed using in situ hybridization on brain sections containing the rat medial vestibular nucleus after unilateral labyrinthectomy. We compared these expression levels to H3 receptor binding densities.Total H3 receptor mRNA levels (detected by oligo probe H3X) as well as mRNA levels of the three receptor isoforms studied (detected by oligo probes H3A, H3B, and H3C) showed a pattern of increase, which was bilaterally significant at 24 h post-lesion for both H3X and H3C, followed by significant bilateral decreases in medial vestibular nuclei occurring 48 h (H3X and H3B) and 1 week post-lesion (H3A, H3B, and H3C). Expression levels of H3B was an exception to the forementioned pattern with significant decreases already detected at 24 h post-lesion. Coinciding with the decreasing trends in H3 receptor mRNA levels was an observed increase in H3 receptor binding densities occurring in the ipsilateral medial vestibular nuclei 48 h post-lesion.ConclusionProgressive recovery of the resting discharge of the deafferentated medial vestibular nuclei neurons results in functional restoration of the static postural and occulomotor deficits, usually occurring within a time frame of 48 hours in rats. Our data suggests that the H3 receptor may be an essential part of pre-synaptic mechanisms required for reestablishing resting activities 48 h after unilateral labyrinthectomy.

Developmental shift from long-term depression to long-term potentiation in the rat medial vestibular nuclei: role of group I metabotropic glutamate receptors.

The effects of high frequency stimulation (HFS) of the primary vestibular afferents on synaptic transmission in the ventral part of the medial vestibular nuclei (vMVN) were studied during postnatal development and compared with the changes in the expression of the group I metabotropic glutamate receptor (mGluR) subtypes, mGluR1 and mGluR5. During the first stages of development, HFS always induced a mGluR5- and GABAA-dependent long-term depression (LTD) which did not require NMDA receptor and mGluR1 activation. The probability of inducing LTD decreased progressively throughout the development and it was zero at about the end of the second postnatal week. Conversely, long-term potentiation (LTP) appeared at the beginning of the second week and its occurrence increased to reach the adult value at the end of the third week. Of interest, the sudden change in the LTP frequency occurred at the time of eye opening, about the end of the second postnatal week. LTP depended on NMDA receptor and mGluR1 activation. In parallel with the modifications in synaptic plasticity, we observed that the expression patterns and localizations of mGluR5 and mGluR1 in the medial vestibular nuclei (MVN) changed during postnatal development. At the earlier stages the mGluR1 expression was minimal, then increased progressively. In contrast, mGluR5 expression was initially high, then decreased. While mGluR1 was exclusively localized in neuronal compartments and concentrated at the postsynaptic sites at all stages observed, mGluR5 was found mainly in neuronal compartments at immature stages, then preferentially in glial compartments at mature stages. These results provide the first evidence for a progressive change from LTD to LTP accompanied by a distinct maturation expression of mGluR1 and mGluR5 during the development of the MVN.

Glutamate release in the rat medial vestibular nucleus following unilateral labyrinthectomy using in vivo microdialysis

We investigated the changes in glutamate release from the ipsi- and contra-lesional medial vestibular nucleus (MVN) following unilateral labyrinthectomy (UL) by in vivo microdialysis study. The concentration of glutamate in the ipsi-lesional MVN was decreased until 4 h. Twelve hours after UL, the concentration of glutamate was restored back to the basal level, after which the release did not show any change between 24 and 48 h post-UL. In contrast, the concentration of glutamate in the contra-lesional MVN, which increased immediately after UL, decreased gradually to the basal level until 3–4 h post-UL, followed by no further change. The difference in the glutamate concentration between ipsi- and contra-lesional MVN increased immediately after UL and gradually decreased accompanied by a reduction in the frequency of nystagmus, although spontaneous nystagmus had not disappeared by the time the imbalance of glutamate release diminished. These results suggest that the imbalance of glutamate release between bilateral nuclei induced the nystagmus, and the change in release is concerned with the rapid development of vestibular compensation.

Characterization of spontaneous synaptic transmission in rat medial vestibular nucleus.

Spontaneous synaptic currents of medial vestibular nucleus (MVN) neurons were studied using whole cell recording in slices prepared from 13- to 17-day-old rats. The spontaneous inhibitory postsynaptic currents (sIPSCs) were significantly reduced by the GABAA antagonist bicuculline (20 microM), but were not affected by the glycine antagonist strychnine (1 microM). The frequency, amplitude, and decay time constant of sIPSCs were 4.3 +/- 0.9 Hz, 18.1 +/- 2.0 pA and 8.9 +/- 0.4 ms, respectively. The specific AMPA receptor antagonist GYKI-52466 (50 microM) completely blocked the non-NMDA-mediated spontaneous excitatory postsynaptic currents (sEPSCs), indicating that they are mediated by an AMPA-preferring receptor. The AMPA-mediated sEPSCs were characterized by low frequency (1.5 +/- 0.4 Hz), small amplitude (13.9 +/- 1.9 pA) and rapid decay kinetics (2.8 +/- 0.2 ms). The majority (15/21) displayed linear I-V relationships, suggesting the presence of GluR2-containing AMPA receptors.

Long-term Potentiation and Depression after Unilateral Labyrinthectomy in the Medial Vestibular Nucleus of Rats

We previously demonstrated in rat brainstem slices that high-frequency stimulation (HFS) of the vestibular afferents induces long-term potentiation (LTP) in the ventral part (Vp) of the medial vestibular nucleus (MVN) and long-term depression (LTD) in the dorsal part (Dp). Both LTP and LTD depend on N-methyl-D-aspartate receptor activation, which increases synaptic efficacy; however, in the Dp, LTP reverses to LTD because of the activation of γ-aminobutyric acid-ergic neurons. Here we show that the probability of inducing long-term effects in the MVN of rat brainstem slices is altered after unilateral labyrinthectomy (UL). In fact, LTP occurs less frequently in the ventral contra-lesional side compared with sham-operated rats. In the dorsal ipsi-lesional side, LTD is reduced and LTP enhanced, while the opposite occurs in the dorsal contra-lesional side. These changes in synaptic plasticity may be useful for re-balancing the tonic discharge of the MVN of the two sides during vestibular compensation, and for enhancing the dynamic responses of the deafferented MVN neurons in the long term.

Properties of neurons from the rat medial vestibular nucleus in microexplant culture

This study is a first step in an attempt to identify the factors which determine and maintain the electrophysiological phenotype(s) of mature neurons of the medial vestibular nucleus (MVN). We cultured MVN microexplants obtained from slices of the brainstem of newborn rats, using a hollow punching needle. The electrophysiological maturation of the neurons was followed by analyzing their responses to 1 s steps of current of increasing amplitude. The maximal number of spikes that was generated in response to such stimuli increased dramatically over time in vitro. However, even after 28 days in vitro, it did not exceed about 60 spikes/s. At this stage of culture, the input–output properties of the spike generator of the MVN neurons were similar to those observed in brainstem slices of newborn rats, but clearly inferior to those of adult neurons which can generate sustained firing up to 150–200 spikes/s.

Different metabotropic glutamate receptors play opposite roles in synaptic plasticity of the rat medial vestibular nuclei.

In the medial vestibular nuclei (MVN) of rat brainstem slices, the role of group II and III metabotropic glutamate receptors (mGluRs) and of the subtypes of group I mGluRs: mGluR1, mGluR5, was investigated in basal synaptic transmission and in the induction and maintenance of long-term potentiation (LTP). We used selective antagonists and agonists for mGluRs and we analysed the field potentials evoked by vestibular afferent stimulation before and after high-frequency stimulation (HFS) to induce LTP. The group II and III mGluR antagonist, (R,S)-alpha-2-methyl-4sulphonophenylglycine (MSPG), induced LTP per se and caused a reduction of the paired-pulse facilitation (PPF) ratio indicating an enhancement of glutamate release. This suggests that group II and III mGluRs are activated under basal conditions to limit glutamate release. Both the group II and III mGluR selective antagonists, 2S-2-amino-2-(1S,2S-2-carboxycycloprop-1-yl)-3-(xanth-9-yl)propanoate (LY341495) and (R,S)-alpha-methylserine-O-phosphate (MSOP), induced LTP, and the selective agonists, (2R,4R)-4-aminopyrrolidine-2,4-dicarboxylate (APDC) and L(+)-2-amino-4-phosphonobutyric acid (L-AP4) depressed the field potentials and prevented HFS-LTP, with a prevailing contribution of group II mGluRs over that of group III mGluRs. The mGluR1 antagonist, 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester (CPCCOEt) prevented the full development and maintenance of HFS-LTP. By contrast, the mGluR5 antagonist, 2-methyl-6-phenylethynylpyridine (MPEP) induced LTP per se, which was impeded by CPCCOEt, and it had no effect on LTP once induced by HFS. The PPF analysis showed an enhancement of glutamate release during MPEP potentiation. The group I mGluR agonist, (R,S)-3,5-dihydroxyphenylglycine (DHPG) induced LTP per se, which was blocked by CPCCOEt. By contrast the mGluR5 agonist, (R,S)-2-chloro-5-hydroxypheylglycine (CHPG) prevented LTP elicited by HFS and DHPG as well. In conclusion vestibular LTP is inhibited by group II and III mGluRs during the early induction phase while it is facilitated by mGluR1 for achieving its full expression and consolidation. An additional inhibitory control is exerted by mGluR5 at the level of this facilitatory phase.