Neurochemical mechanisms of recovery from peripheral vestibular lesions (vestibular compensation)

The effects of continuous administration of diazepam on the recovery of lesion-induced nystagmus in unilaterally labyrinthectomised rats

BACKGROUND

Diazepam, a gamma-aminobutyric acid type A receptor agonist, is classified as a vestibular suppressant and is effective in treating acute vertigo. However, its effects on vestibular compensation (VC) remain unclear.

OBJECTIVES

We examined the effects of continuous administration of diazepam on the frequency of spontaneous nystagmus (SN) after unilateral labyrinthectomy (UL) as an index of the initial process of VC in rats.

MATERIALS AND METHODS

Diazepam was continuously administered at doses of 3.5 and 7.0 mg/kg/day, intraperitoneally, via an osmotic minipump. The frequency of SN beating against the lesion side after UL was measured. Potassium chloride (KCl) solution (1 M) was injected intratympanically to induce SN beating to the injection side.

RESULTS

Continuous administration of diazepam significantly and dose-dependently decreased the frequency of SN after UL, and also reduced the x intercept of the nonlinear regression curve of the decline in UL-induced SN with time in rats. However, the continuous administration of diazepam did not affect the frequency of intratympanic KCl-induced SN in the rats.

CONCLUSION

These findings suggested that continuous administration of diazepam accelerates the initial process of VC; however, it does not suppress the nystagmus-driving mechanisms in rats.

Effects of Betahistine on the Development of Vestibular Compensation after Unilateral Labyrinthectomy in Rats

Background: Vestibular compensation (VC) after unilateral labyrinthectomy (UL) consists of the initial and late processes. These processes can be evaluated based on the decline in the frequency of spontaneous nystagmus (SN) and the number of MK801-induced Fos-positive neurons in the contralateral medial vestibular nucleus (contra-MVe) in rats. Histamine H3 receptors (H3R) are reported to be involved in the development of VC. Objective: We examined the effects of betahistine, an H3R antagonist, on the initial and late processes of VC in UL rats. Methods: Betahistine dihydrochloride was continuously administered to the UL rats at doses of 100 and 200 mg/kg/day using an osmotic minipump. MK801 (1.0 mg/kg) was intraperitoneally administered on days 7, 10, 12, and 14 after UL, while Fos-positive neurons were immunohistochemically stained in the contra-MVe. Results: The SN disappeared after 42 h, and continuous infusion of betahistine did not change the decline in the frequency of SN. The number of MK801-induced Fos-positive neurons in contra-MVe significantly decreased on days 7, 10, and 12 after UL in a dose-dependent manner in the betahistine-treated rats, more so than in the saline-treated rats. Conclusion: These findings suggest that betahistine facilitated the late, but not the initial, process of VC in UL rats.

Quantitative Evaluation of a New Posturo-Locomotor Phenotype in a Rodent Model of Acute Unilateral Vestibulopathy

Vestibular pathologies are difficult to diagnose. Existing devices make it possible to quantify and follow the evolution of posturo-locomotor symptoms following vestibular loss in static conditions. However, today, there are no diagnostic tools allowing the quantitative and spontaneous analysis of these symptoms in dynamic situations. With this in mind, we used an open-field video tracking test aiming at identifying specific posturo-locomotor markers in a rodent model of vestibular pathology. Using Ethovision XT 14 software (Noldus), we identified and quantified several behavioral parameters typical of unilateral vestibular lesions in a rat model of vestibular pathology. The unilateral vestibular neurectomy (UVN) rat model reproduces the symptoms of acute unilateral peripheral vestibulopathy in humans. Our data show deficits in locomotion velocity, distance traveled and animal mobility in the first day after the injury. We also highlighted alterations in several parameters, such as head and body acceleration, locomotor pattern, and position of the body, as well as “circling” behavior after vestibular loss. Here, we provide an enriched posturo-locomotor phenotype specific to full and irreversible unilateral vestibular loss. This test helps to strengthen the quantitative evaluation of vestibular disorders in unilateral vestibular lesion rat model. It may also be useful for testing pharmacological compounds promoting the restoration of balance. Transfer of these novel evaluation parameters to human pathology may improve the diagnosis of acute unilateral vestibulopathies and could better follow the evolution of the symptoms upon pharmacological and physical rehabilitation.

Modeling Vestibular Compensation: Neural Plasticity Upon Thalamic Lesion

The present study in rats was conducted to identify brain regions affected by the interruption of vestibular transmission and to explore selected aspects of their functional connections. We analyzed, by positron emission tomography (PET), the regional cerebral glucose metabolism (rCGM) of cortical, and subcortical cerebral regions processing vestibular signals after an experimental lesion of the left laterodorsal thalamic nucleus, a relay station for vestibular input en route to the cortical circuitry. PET scans upon galvanic vestibular stimulation (GVS) were conducted in each animal prior to lesion and at post-lesion days (PLD) 1, 3, 7, and 20, and voxel-wise statistical analysis of rCGM at each PLD compared to pre-lesion status were performed. After lesion, augmented metabolic activation by GVS was detected in cerebellum, mainly contralateral, and in contralateral subcortical structures such as superior colliculus, while diminished activation was observed in ipsilateral visual, entorhinal, and somatosensory cortices, indicating compensatory processes in the non-affected sensory systems of the unlesioned side. The changes in rCGM observed after lesion resembled alterations observed in patients suffering from unilateral thalamic infarction and may be interpreted as brain plasticity mechanisms associated with vestibular compensation and substitution. The second set of experiments aimed at the connections between cortical and subcortical vestibular regions and their neurotransmitter systems. Neuronal tracers were injected in regions processing vestibular and somatosensory information. Injections into the anterior cingulate cortex (ACC) or the primary somatosensory cortex (S1) retrogradely labeled neuronal somata in ventral posteromedial (VPM), posterolateral (VPL), ventrolateral (VL), posterior (Po), and laterodorsal nucleus, dorsomedial part (LDDM), locus coeruleus, and contralateral S1 area. Injections into the parafascicular nucleus (PaF), VPM/VPL, or LDDM anterogradely labeled terminal fields in S1, ACC, insular cortex, hippocampal CA1 region, and amygdala. Immunohistochemistry showed tracer-labeled terminal fields contacting cortical neurons expressing the μ-opioid receptor. Antibodies to tyrosine hydroxylase, serotonin, substance P, or neuronal nitric oxide-synthase did not label any of the traced structures. These findings provide evidence for opioidergic transmission in thalamo-cortical transduction.

A new immunohistochemical method to evaluate the development of vestibular compensation after unilateral labyrinthectomy in rats

BACKGROUND

Unilateral labyrinthectomy (UL) causes the disappearance of ipsilateral medial vestibular nuclear (ipsi-MVe) activity and induces spontaneous nystagmus (SN), which disappears during the initial process of vestibular compensation (VC). Ipsi-MVe-activity restores in the late process of VC.

OBJECTIVE

We evaluated the late process of VC after UL in rats and examined the effects of thioperamide (H3 antagonist) on VC.

MATERIALS AND METHODS

MK801 (NMDA antagonist)-induced Fos-like immunoreactive (-LIR) neurons in contra-MVe, which had been suppressed by NMDA-mediated cerebellar inhibition in UL rats was used as an index.

RESULTS

The number of MK801-induced Fos-LIR neurons in contra-MVe gradually decreased to the same level as that of sham-operated rats 14 days after UL. Thioperamide moved the disappearance of the MK801-induced Fos-LIR neurons 2 days earlier. The number of MK801-induced Fos-LIR neurons in thioperamide-treated rats was significantly decreased, compared with that of vehicle rats on days 7 and 12 after UL. But, thioperamide did not influence the decline of SN frequency in UL rats.

CONCLUSION

These findings suggested that the number of MK801-induced Fos-LIR neurons in contra-MVe was decreased in concordance with the restoration of ipsi-MVe-activity during the late process of VC after UL and that thioperamide accelerated the late, but not the initial process of VC.

Reduced Balance Restoration Capacities Following Unilateral Vestibular Insult in Elderly Mice

Acute vestibular syndrome (AVS) is characterized by severe posturo-locomotor and vestibulo-oculomotor impairment and accompanies several types of peripheral vestibulopathies (PVP). We know very little about its etiology, how its various symptoms are expressed and how it evolves with age. Robust repair capabilities of primary vestibular synapses have recently been shown to restore behavioral functionality. In this study, we used a mouse model of an excitotoxically induced unilateral vestibular lesion to compare the ability to restore balance and posture between old and young adult mice. We compared the temporal evolution of the evoked vestibular syndrome using a battery of behavioral tests to follow the evolution of postural-locomotor alterations and equilibrium. For the first time, we show that young adult (3 months) and elderly (22 months) mice are together able to restore normal postural-locomotor function following transient unilateral excitotoxic vestibular insult, though with different time courses. This animal study paves way for future, more detailed studies of how the early postural and locomotor disturbances following a unilateral insult are compensated for by various plasticity mechanisms, and in particular how age influences these mechanisms.

Betahistine Treatment in a Cat Model of Vestibular Pathology: Pharmacokinetic and Pharmacodynamic Approaches

This study is a pharmacokinetic (PK) and pharmacodynamics (PD) approach using betahistine doses levels in unilateral vestibular neurectomized cats (UVN) comparable to those used in humans for treating patients with Menière's disease. The aim is to investigate for the first time oral betahistine administration (0.2 and 2 mg/kg/day) with plasma concentrations of betahistine and its major metabolite 2-pyridylacetic acid (2-PAA) (N = 9 cats), the time course of posture recovery (N = 13 cats), and the regulation of the enzyme synthesizing histamine (histidine decarboxylase: HDC) in the tuberomammillary nuclei (TMN) of UVN treated animals (N = the same 13 cats plus 4 negative control cats). In addition the effect of co-administration of the lower betahistine dose (0.2 mg/kg/day) and selegiline (1 mg/kg/day), an inhibitor of the monamine oxidase B (MAOBi) implicated in betahistine catabolism was investigated. The PK parameters were the peak concentration (C_max), the time when the maximum concentration is reached (T_max) for both betahistine and 2-PAA and the area under the curve (AUC). The PD approach consisted at quantifying the surface support area, which is a good estimation of posture recovery. The plasma concentration-time-profiles of betahistine and 2-PAA in cats were characterized by early C_max-values followed by a phase of rapid decrease of plasma concentrations and a final long lasting low level of plasma concentrations. Co administration of selegiline and betahistine increased values of C_max and AUC up to 146- and 180-fold, respectively. The lowest dose of betahistine (0.2 mg/kg) has no effects on postural function recovery but induced an acute symptomatic effect characterized by a fast balance improvement (4–6 days). The higher dose (2 mg/kg) and the co-administration treatment induced both this acute effect plus a significant acceleration of the recovery process. The histaminergic activity of the neurons in the TMN was significantly increased under treatment with the 2 mg/kg betahistine daily dose, but not with the lower dose alone or in combination with selegiline. The results show for the first time that faster balance recovery in UVN treated cats is accompanied with high plasma concentrations of betahistine and 2-PAA, and upregulation of HDC immunopositive neurons in the TMN. The higher betahistine dose gives results similar to those obtained with the lower dose when co-administrated with an inhibitor of betahistine metabolism, selegiline. From a clinical point of view, the study provides new perspectives for Menière's disease treatment, regarding the daily betahistine dose that should be necessary for fast and slow metabolizers.

BDNF Signaling Promotes Vestibular Compensation by Increasing Neurogenesis and Remodeling the Expression of Potassium-Chloride Cotransporter KCC2 and GABAA Receptor in the Vestibular Nuclei

Reactive cell proliferation occurs rapidly in the cat vestibular nuclei (VN) after unilateral vestibular neurectomy (UVN) and has been reported to facilitate the recovery of posturo-locomotor functions. Interestingly, whereas animals experience impairments for several weeks, extraordinary plasticity mechanisms take place in the local microenvironment of the VN: newborn cells survive and acquire different phenotypes, such as microglia, astrocytes, or GABAergic neurons, whereas animals eventually recover completely from their lesion-induced deficits. Because brain-derived neurotrophic factor (BDNF) can modulate vestibular functional recovery and neurogenesis in mammals, in this study, we examined the effect of BDNF chronic intracerebroventricular infusion versus K252a (a Trk receptor antagonist) in our UVN model. Results showed that long-term intracerebroventricular infusion of BDNF accelerated the restoration of vestibular functions and significantly increased UVN-induced neurogenesis, whereas K252a blocked that effect and drastically delayed and prevented the complete restoration of vestibular functions. Further, because the level of excitability in the deafferented VN is correlated with behavioral recovery, we examined the state of neuronal excitability using two specific markers: the cation-chloride cotransporter KCC2 (which determines the hyperpolarizing action of GABA) and GABAA receptors. We report for the first time that, during an early time window after UVN, significant BDNF-dependent remodeling of excitability markers occurs in the brainstem. These data suggest that GABA acquires a transient depolarizing action during recovery from UVN, which potentiates the observed reactive neurogenesis and accelerates vestibular functional recovery. These findings suggest that BDNF and/or KCC2 could represent novel treatment strategies for vestibular pathologies.

SIGNIFICANCE STATEMENT

In this study, we report for the first time that brain-derived neurotrophic factor potentiates vestibular neurogenesis and significantly accelerates functional recovery after unilateral vestibular injury. We also show that specific markers of excitability, the potassium-chloride cotransporter KCC2 and GABAA receptors, undergo remarkable fluctuations within vestibular nuclei (VN), strongly suggesting that GABA acquires a transient depolarizing action in the VN during the recovery period. This novel plasticity mechanism could explain in part how the system returns to electrophysiological homeostasis between the deafferented and intact VN, considered in the literature to be a key parameter of vestibular compensation. In this context, our results open new perspectives for the development of therapeutic approaches to alleviate the vestibular symptoms and favor vestibular function recovery.

The changes in mGluR2 and mGluR7 expression in rat medial vestibular nucleus and flocculus following unilateral labyrinthectomy

It is known that the medial vestibular nucleus (MVN) and the cerebellar flocculus are the key areas, which contribute to the behavioral recovery ("vestibular compensation") after unilateral labyrinthectomy (UL). In these areas, how the genetic activities of the metabotropic glutamate receptors mGluR2 and mGluR7 performance after UL is unknown. With the means of quantitative real-time PCR, Western blotting, and immunohistochemistry, we analyzed the expression of mGluR2 and mGluR7 in the bilateral MVN and the flocculus of rats in different stages after UL (the 1st, 3rd, and 7th day). Our results show that in the MVN, the mRNA, and protein expressions of mGluR7 were ipsilaterally decreased at the 1st day following UL. However, in the MVN, no change was observed in the mRNA and protein expressions of mGluR2. On the other hand, the mRNA and protein expression of mGluR2 were enhanced in the ipsilateral flocculus at the 1st day following UL, while in the flocculus no change was shown in mGluR7 mRNA and protein expressions. Our results suggest that mGluR2 and mGluR7 may contribute to the early rebalancing of spontaneous resting activity in the MVN.

Effects of cyclic nucleotide-gated channels in vestibular nuclear neurons

This study was designed to investigate the effects an 8-Br-cGMP on the neuronal activity of rat vestibular nuclear cells. Sprague-Dawley rats aged 14 to 16 days were decapitated under ether anesthesia. After treatment with pronase and thermolysin, the dissociated vestibular nuclear cells were transferred into a chamber on an inverted microscope. Spontaneous action potentials and potassium currents were recorded by standard patch-clamp techniques under current and voltage-clamp modes. Twelve vestibular nuclear cells revealed excitatory responses to 1-5 µM of 8-Br-cGMP, and 3 neurons did not respond to 8-Br-cGMP. Whole potassium currents of vestibular nuclear cells were decreased by 8-Br-cGMP (n=12). After calcium-dependent potassium currents were blocked by tetraethylammonium, the potassium currents were not decreased by 8-Br-cGMP. These experimental results suggest that 8-Br-cGMP changes the neuronal activity of vestibular nuclear cells by blocking the calcium-dependent potassium currents that underlie the afterhyperpolarization.

Vestibular function and quality of life in vestibular schwannoma: does size matter?

Objectives: Patients with vestibular schwannoma (VS) frequently suffer from disabling vestibular symptoms. This prospective follow-up study evaluates vestibular and auditory function and impairment of quality of life due to vertigo, dizziness, and imbalance in patients with unilateral VS of different sizes before/after microsurgical or radiosurgical treatment. Methods: Thirty-eight patients with unilateral VS were included. Twenty-two received microsurgery, 16 CyberKnife radiosurgery. Two follow-ups took place after a median of 50 and 186.5 days. Patients received a standardized neuro-ophthalmological examination, electronystagmography with bithermal caloric testing, and pure-tone audiometry. Quality of life was evaluated with the Dizziness Handicap Inventory (DHI). Patient data was grouped and analyzed according to the size of the VS (group 1: <20 mm vs group 2: ≥20 mm). Results: In group 1, the median loss of vestibular function was +10.5% as calculated by Jongkees Formula (range −43 to +52; group 2: median +36%, range −56 to +90). The median change of DHI scores was −9 in group 1 (range −68 to 30) and +2 in group 2 (−54;+20). Median loss of hearing was 4 dB (−42; 93) in group 1 and 12 dB in group 2 (5; 42). Conclusion: Loss of vestibular function in VS clearly correlates with tumor size. However, loss of vestibular function was not strictly associated with a long-term deterioration of quality of life. This may be due to central compensation of vestibular deficits in long-standing large tumors. Loss of hearing before treatment was significantly influenced by the age of the patient but not by tumor size. At follow-up 1 and 2, hearing was significantly influenced by the size of the VS and the manner of treatment.

Changes of some amino acid concentrations in the medial vestibular nucleus of conscious rats following acute hypotension

Microdialysis and high performance liquid chromatography (HPLC) were used to measure the changes of certain amino acids in the medial vestibular nucleus (MVN) of conscious rats in order to understand whether those amino acids are involved in the regulation of blood pressure. Acute hypotension was induced by infusing sodium nitroprusside (SNP) into the femoral vein. In the control group, glutamate (Glu) release increased, though gamma-aminobutyric acid (GABA) and taurine (Tau) release decreased in the MVN following acute hypotension. In the unilateral labyrinthectomy group, the levels of Glu, GABA, and Tau were unchanged in the ipsilateral MVN to the lesion following acute hypotension. Furthermore, in the contralateral MVN to the lesion, Glu release increased, and GABA and Tau release decreased following acute hypotension. These results suggest that SNP-induced acute hypotension can influence the activity of neurons in the MVN through afferent signals from peripheral vestibular receptors, and that certain amino acid transmitters in the MVN are involved in this process.

Cell proliferation and survival in the vestibular nucleus following bilateral vestibular deafferentation in the adult rat

Cell proliferation and neurogenesis in the brainstem vestibular nucleus complex (VNC) has previously been reported following unilateral vestibular neurectomy in the cat. In this study, we examined the rate of cell proliferation and survival in the adult rat VNC following bilateral vestibular deafferentation (BVD), using injections of bromodeoxyuridine (BrdU) and stereological cell counting. We measured cell proliferation at 24, 48, 72 h and 1 week following BVD and found that it was significantly greater than in sham controls (P=0.002) and that it varied significantly over time (P=0.01), peaking at 48 h in the BVD group. Of note was that sham surgery was also associated with an increase in cell proliferation, which changed over time. When we compared the survival of new cells at 1 month after BrdU injection, there was no significant difference in survival between the sham and BVD groups. These results raise questions about the potential functional significance of cell proliferation in the VNC following vestibular lesions.

Subjective visual vertical during eccentric rotation in patients with vestibular neuritis

The subjective visual vertical (SVV) is a useful tool to evaluate clinical manifestations of vestibular loss, but there have been normal variations of the SVV within 1 degrees -3 degrees , and over time, the absolute deviated degrees of SVV tilts decreases. We investigated SVV values in patients with vestibular neuritis (VN) during eccentric rotation, the method used to assess utricular function during stimulation of one labyrinth. And we performed SVV in the resting state and during eccentric rotation to the lesion side and the healthy side of 15 patients with VN and 20 normal subjects. No difference in the resting state SVV values was observed between the VN patients and the control group, but there were significant differences in SVV values between these two groups during eccentric rotation. Therefore, SVV during eccentric rotation allowed us to obtain information about unilateral vestibular loss that could not be found by conventional SVV in patients with VN. Thus, SVV during eccentric rotation might be a good tool to diagnose unilateral vestibular loss.

Changes of amino acid concentrations in the rat vestibular nuclei after inferior cerebellar peduncle transection

Although there is a close relationship between the vestibular nuclear complex (VNC) and the cerebellum, little is known about the contribution of cerebellar inputs to amino acid neurotransmission in the VNC. Microdissection of freeze-dried brain sections and high-performance liquid chromatography (HPLC) were combined to measure changes of amino acid concentrations within the VNC of rats following transection of the cerebellovestibular connections in the inferior cerebellar peduncle. Distributions of 12 amino acids within the VNC at 2, 4, 7, and 30 days after surgery were compared with those for control and sham-lesioned rats. Concentrations of gamma-aminobutyric acid (GABA) decreased by 2 days after unilateral peduncle transection in nearly all VNC regions on the lesioned side and to lesser extents on the unlesioned side and showed partial recovery up to 30 days postsurgery. Asymmetries between the two sides of the VNC were maintained through 30 days. Glutamate concentrations were reduced bilaterally in virtually all regions of the VNC by 2 days and showed complete recovery in most VNC regions by 30 days. Glutamine concentrations increased, starting 2 days after surgery, especially on the lesioned side, so that there was asymmetry generally opposite that of glutamate. Concentrations of taurine, aspartate, and glycine also underwent partially reversible changes after peduncle transection. The results suggest that GABA and glutamate are prominent neurotransmitters in bilateral projections from the cerebellum to the VNC and that amino acid metabolism in the VNC is strongly influenced by its cerebellar connections.

The systemic application of diazepam facilitates the reacquisition of a well-balanced vestibular function in a unilateral vestibular re-input model with intracochlear tetrodotoxin infusion using an osmotic pump

Diazepam is a popular medicine used in the treatment of acute vertigo. In the past, many studies investigating the effect of diazepam in peripheral vestibular destruction have been reported. However, no previous study has yet investigated the effect of diazepam on a model with a transient and reversible vestibular function similar to recurrent vertigo as seen in Meniere's disease. We thus made a peripheral vestibular re-input model by the unilateral intracochlear administration of tetrodotoxin (TTX) using an osmotic pump and then examined the influence of diazepam on the vestibular system in this model. Hartley white guinea pigs were intracochlearly administered with TTX on the right side for 3 days by an osmotic pump. Animals were divided into three groups, TTX alone (control group (n = 7)), TTX and an intraperitoneal diazepam injection once a day for 3 days (diazepam group (n = 6)) and vehicle injection (vehicle group (n = 6)). A caloric response and vestibuloocular reflex (VOR) were observed at 7 and 14 days after completing 3 days of TTX administration. Seven days after vestibular re-input, a directional preponderance of the nystagmus (DP) to the TTX-treated side was observed in the control and vehicle groups on VOR examination. DP was not observed in the diazepam group on any examined day. The R/L time ratio of caloric response showed no statistical difference between three groups on any examined day. These results suggest that diazepam may thus be useful for patients in an acute stage of peripheral vestibular vertigo by decreasing their vertiginous symptoms.

Modulation of neuronal activity by EGCG

This study aims to investigate whether (-)-epigallocatechin-3-gallate (EGCG) affects neuronal activity of acutely isolated rat medial vestibular nuclear neurons in whole-cell configuration patch-clamp experiments. EGCG (0.5 and 1 muM) lowered the spontaneous firing rate and hyperpolarized the membrane potential of medial vestibular nuclear neurons. However, it did not change the amplitude of afterhyperpolarization or the spike width of the action potential. A second application of EGCG with the same concentration elicited lesser responses. These results suggest that EGCG decreases neuronal activity by affecting potassium currents which are responsible for membrane potentials.

Effects of CGS-12066A on medial vestibular nuclear neurons

This study aims to explore the effects of a selective 5-HT1B receptor agonist, CGS-12066A, on the neuronal excitability of the rat medial vestibular nuclear neurons. The spontaneous firing rate was decreased, and the membrane potential was hyperpolarized by CGS-12066A. The whole potassium currents were inhibited by CGS-12066A. After the calcium-dependent potassium, currents were blocked, however, CGS-12066A did not inhibit the potassium currents, suggesting that the 5-HT action site is calcium-dependent potassium currents.

The differential response of astrocytes within the vestibular and cochlear nuclei following unilateral labyrinthectomy or vestibular afferent activity blockade by transtympanic tetrodotoxin injection in the rat

In this study, we investigated whether changes in the vestibular neuronal activity per se influence the pattern of astrocytes morphology, glial fibrillary acidic protein (GFAP) expression and ultimately their activation within the vestibular nuclei after unilateral transtympanic tetrodotoxin (TTX) injections and after unilateral inner ear lesion. The rationale was that, theoretically the noninvasive pharmacological functional blockade of peripheral vestibular inputs with TTX, allowed us to dissociate the signals exclusively related to the shutdown of the resting activity of the first-order vestibular neurons and from neuronal signals associated with trans-ganglionic changes in first order vestibular neurons induced by unilateral labyrinthectomy (UL). Since the cochlea was removed during the surgical procedure, we also studied the astrocytic reaction within the deafferented cochlear nuclei. No significant changes in the distribution or relative levels of GFAP mRNA expression, relative levels of GFAP protein or immunoreactivity for GFAP were found in the ipsilateral vestibular nuclei at any post-TTX injection times studied. In addition, no sign of microglia activation was observed. In contrast, a robust increase of the distribution and relative levels of GFAP mRNA expression, protein levels and immunoreactivity was observed in the deafferented vestibular and cochlear nuclei beginning at 1 day after inner ear lesion. GFAP mRNA expression and immunoreactivity in the cochlear nucleus was qualitatively stronger than in the ipsilateral vestibular nuclei. The results suggest that astrocyte activation in the vestibular nuclei is not related to drastic changes of vestibular nuclei neuronal activity per se. Early trans-ganglionic changes due to vestibular nerve dendrites lesion provoked by the mechanical destruction of vestibular receptors, most probably induced the glial reaction. Its functional role in the vestibular compensation process remains to be elucidated.

An in situ hybridization and immunofluorescence study of GABAA and GABAB 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.

Central vestibular system: vestibular nuclei and posterior cerebellum

The vestibular nuclei and posterior cerebellum are the destination of vestibular primary afferents and the subject of this review. The vestibular nuclei include four major nuclei (medial, descending, superior and lateral). In addition, smaller vestibular nuclei include: Y-group, parasolitary nucleus, and nucleus intercalatus. Each of the major nuclei can be subdivided further based primarily on cytological and immunohistochemical histological criteria or differences in afferent and/or efferent projections. The primary afferent projections of vestibular end organs are distributed to several ipsilateral vestibular nuclei. Vestibular nuclei communicate bilaterally through a commissural system that is predominantly inhibitory. Secondary vestibular neurons also receive convergent sensory information from optokinetic circuitry, central visual system and neck proprioceptive systems. Secondary vestibular neurons cannot distinguish between sources of afferent activity. However, the discharge of secondary vestibular neurons can distinguish between "active" and "passive" movements. The posterior cerebellum has extensive afferent and efferent connections with vestibular nuclei. Vestibular primary afferents are distributed to the ipsilateral uvula-nodulus as mossy fibers. Vestibular secondary afferents are distributed bilaterally. Climbing fibers to the cerebellum originate from two subnuclei of the contralateral inferior olive; the dorsomedial cell column and beta-nucleus. Vestibular climbing fibers carry information only from the vertical semicircular canals and otoliths. They establish a coordinate map, arrayed in sagittal zones on the surface of the uvula-nodulus. Purkinje cells respond to vestibular stimulation with antiphasic modulation of climbing fiber responses (CFRs) and simple spikes (SSs). The modulation of SSs is out of phase with the modulation of vestibular primary afferents. Modulation of SSs persists, even after vestibular primary afferents are destroyed by a unilateral labyrinthectomy, suggesting that an interneuronal network, triggered by CFRs is responsible for SS modulation. The vestibulo-cerebellum, imposes a vestibular coordinate system on postural responses and permits adaptive guidance of movement.

Plasticity of GABA(a) system during ageing: focus on vestibular compensation and possible pharmacological intervention

The lesion of the vestibular end organ evokes static and dynamic symptoms, which spontaneously regress during a complex process known as 'vestibular compensation'. Vestibular compensation is age-dependent and involves several transmitter-identified pathways in the central nervous system. In this paper we studied the time course of vestibular compensation in adult (3 months) and old (24 months) rats and correlated behavioral recovery with modifications of glutamic acid decarboxylase (GAD) mRNA expression and benzodiazepine receptor density in different brain areas. Compensation in adult rats was complete 28 days after hemilabyrinthectomy, whereas old rats still showed significant behavioral impairment. A higher GABAergic tone was found in old rats, as indicated by higher benzodiazepine receptor density in lateral vestibular nucleus and higher mRNA level for glutamic acid decarboxylase in cerebral cortex and medial vestibular nucleus. In adult, compensated rats, benzodiazepine receptor density in the vestibular nuclei was normal 28 days after lesion, whereas GAD mRNA level was higher in anterior cingulate cortex, only. On the contrary, these parameters were still altered in anterior cingulate and somatosensory cortex, basal ganglia, vestibular nuclei and cerebellum in old rats 28 days after vestibular lesion. We also evaluated the effect of the ergoline derivative nicergoline on behavioral and neurochemical correlates of vestibular compensation in old rats. Nicergoline treatment attenuated the severity of oculomotor and postural symptoms after vestibular lesion and reversed most of these age- and lesion-induced alterations in GAD mRNA expression. Thus, lesion-related alterations of the GABAergic transmission and behavioral profile after vestibular lesion are age-dependent.

Histamine and betahistine in the treatment of vertigo: elucidation of mechanisms of action

The aim of this review is to provide clinicians with a picture of the mechanisms by which: histamine and histaminergic agonists act on the vestibular system both peripherally and centrally; and histaminergic agonists and antagonists interfere with the recovery process after peripheral vestibular lesion. We have focused on betahistine, a structural analogue of histamine with weak histamine H(1) receptor agonist and more potent H(3) receptor antagonist properties, to review the currently available data on the role of the histaminergic system in the recovery process after peripheral vestibular deficits and the effects of histamine analogues in the clinical treatment of vertigo. This review provides new insights into the basic mechanisms by which betahistine improves vestibular compensation in animal models of unilateral vestibular dysfunction, and elucidates particularly the mechanisms of action of this substance at the level of the CNS. It is proposed that betahistine may reduce peripherally the asymmetric functioning of the sensory vestibular organs in addition to increasing vestibulocochlear blood flow by antagonising local H(3) heteroreceptors. Betahistine acts centrally by enhancing histamine synthesis within tuberomammillary nuclei of the posterior hypothalamus and histamine release within vestibular nuclei through antagonism of H(3) autoreceptors. This mechanism, together with less specific effects of betahistine on alertness regulation through cerebral H(1) receptors, should promote and facilitate central vestibular compensation. Elucidation of the mechanisms of action of betahistine is of particular interest for the treatment of vestibular and cochlear disorders and vertigo.

Vestibular compensation after ganglionectomy: ultrastructural study of the tangential vestibular nucleus and behavioral study of the hatchling chick

The tangential nucleus is a major part of the avian vestibular nuclear complex, and its principal cells are structurally distinctive neurons participating in the vestibuloocular and vestibulocollic reflexes. After unilateral peripheral vestibular lesion, a behavioral recovery of function defined as vestibular compensation is observed. Because sprouting and hypertrophy of synapses have been reported in other regions of immature animals after central nervous system injury, we investigated whether this also occurs in the vestibular nuclei during compensation. To test this hypothesis, unilateral vestibular ganglionectomy was performed on 4-6-day-old hatchlings and vestibular function was tested during the next 2 months. Degeneration and evidence for regeneration of synapses were studied in the tangential nucleus at 1, 3, 7, and 56 days after surgery. Spoon endings, large vestibular terminals on the principal somata, degenerated 1-3 days after surgery. However, the small synaptic terminals showed no significant change in the percentage or number covering the soma or in mean terminal lengths in the deafferented or contralateral tangential nucleus. Furthermore, there was no evidence of neuron death in the tangential nucleus. Vestibular compensation occurred in three stages: 0-3 days, when vestibular synapses degenerated and severe behavioral deficits were seen; 4-9 days, when primary vestibular fibers degenerated centrally and marked improvement in both the static and the dynamic symptoms were observed; and 10-56 days, when changes in neuronal morphology were not detected but the dynamic symptoms gradually improved. Accordingly, after unilateral vestibular ganglionectomy, vestibular compensation proceeded without ultrastructural evidence of sprouting or hypertrophy of axosomatic synapses in the hatchling tangential nucleus. This rapid behavioral recovery of function distinguishes the vestibular system from other sensory systems, which, in general, exhibit much less robust recovery after injury to their peripheral receptors.

Synaptic plasticity in the medial vestibular nuclei: role of glutamate receptors and retrograde messengers in rat brainstem slices

The analysis of cellular-molecular events mediating synaptic plasticity within vestibular nuclei is an attempt to explain the mechanisms underlying vestibular plasticity phenomena. The present review is meant to illustrate the main results, obtained in vitro, on the mechanisms underlying long-term changes in synaptic strength within the medial vestibular nuclei. The synaptic plasticity phenomena taking place at the level of vestibular nuclei could be useful for adapting and consolidating the efficacy of vestibular neuron responsiveness to environmental requirements, as during visuo-vestibular recalibration and vestibular compensation. Following a general introduction on the most salient features of vestibular compensation and visuo-vestibular adaptation, which are two plastic events involving neuronal circuitry within the medial vestibular nuclei, the second and third sections describe the results from rat brainstem slice studies, demonstrating the possibility to induce long-term potentiation and depression in the medial vestibular nuclei, following high frequency stimulation of the primary vestibular afferents. In particular the mechanisms sustaining the induction and expression of vestibular long-term potentiation and depression, such as the role of various glutamate receptors and retrograde messengers have been described. The relevant role of the interaction between the platelet-activating factor, acting as a retrograde messenger, and the presynaptic metabotropic glutamate receptors, in determining the full expression of vestibular long-term potentiation is also underlined. In addition, the mechanisms involved in vestibular long-term potentiation have been compared with those leading to long-term potentiation in the hippocampus to emphasize the most significant differences emerging from vestibular studies. The fourth part, describes recent results demonstrating the essential role of nitric oxide, another retrograde messenger, in the induction of vestibular potentiation. Finally the fifth part suggests the possible functional significance of different action times of the two retrograde messengers and metabotropic glutamate receptors, which are involved in mediating the presynaptic mechanism sustaining vestibular long-term potentiation.

Effects of postural changes and vestibular lesions on diaphragm and rectus abdominis activity in awake cats

Changes in posture can affect the resting length of the diaphragm, requiring alterations in the activity of both the abdominal muscles and the diaphragm to maintain stable ventilation. To determine the role of the vestibular system in regulating respiratory muscle discharges during postural changes, spontaneous diaphragm and rectus abdominis activity and modulation of the firing of these muscles during nose-up and ear-down tilt were compared before and after removal of labyrinthine inputs in awake cats. In vestibular-intact animals, nose-up and ear-down tilts from the prone position altered rectus abdominis firing, whereas the effects of body rotation on diaphragm activity were not statistically significant. After peripheral vestibular lesions, spontaneous diaphragm and rectus abdominis discharges increased significantly (by approximately 170%), and augmentation of rectus abdominis activity during nose-up body rotation was diminished. However, spontaneous muscle activity and responses to tilt began to recover after a few days after the lesions, presumably because of plasticity in the central vestibular system. These data suggest that the vestibular system provides tonic inhibitory influences on rectus abdominis and the diaphragm and in addition contributes to eliciting increases in abdominal muscle activity during some changes in body orientation.

Gamma amino butyric acid (GABA) immunoreactivity in the vestibular nuclei of normal and unilateral vestibular neurectomized cats

Recent neurochemical investigations of the central vestibular pathways have demonstrated that several neurotransmitters are involved in various operations required for stabilizing posture and gaze. Neurons of the vestibular nuclei (VN) receive GABAergic inhibitory afferents, and GABAergic neurons distributed throughout the vestibular complex are implicated in inhibitory vestibulo-ocular and vestibulo-spinal pathways. The aim of this study was to analyse the modifications of GABA immunoreactivity (GABA-ir) in the cat VN after unilateral vestibular neurectomy (UVN). Indeed, compensation of vestibular deficits is a good model for studying adult central nervous system (CNS) plasticity and the GABAergic system is involved in CNS plasticity. We studied GABA-ir by using a purified polyclonal antibody raised against GABA. Light microscopic preparations of thin (20 microm) sections of cat VN were used to quantify GABA-ir by an image analysing system measuring GABA-positive punctate structures and the number of GABA-positive neurons. Both the lesioned and intact sides were analysed in three populations of UVN cats killed at different times after injury (1 week, 3 weeks and 1 year). These data were compared to those collected in normal unlesioned and sham-operated cats. Results showed a spatial distribution of GABA-ir in the control cats that confirmed previous studies. GABA-ir neurons, fibres and nerve terminals were scattered in all parts of the VN. A higher concentration of GABA-positive neurons (small cells) was detected in the medial and inferior VN (MVN and IVN) and in the dorsal part of the lateral VN (LVNd). A higher level of GABA-positive punctate structures was observed in the MVN and in the prepositus hypoglossi (PH) nucleus. Lesion-induced changes were found at each survival time. One week after injury the number of GABA-positive neurons was significantly increased in the MVN, the IVN and the dorsal part of the LVN on the lesioned side and in the ventral part of the LVN on the intact side. One year later a bilateral increase in GABA-positive neurons was detected in the MVN whilst a bilateral decrease was observed in both the SVN and the ventral part of the LVN. Changes in the GABA-staining varicosities did not strictly coincide with the distribution of GABA-ir cells, suggesting that GABA-ir fibres and nerve terminals were also modified. One week and later after injury, higher GABA-staining varicosities were seen unilaterally in the ipsilateral MVN. In contrast, bilateral increases (in PH) and bilateral decreases (in SVN and the ventral part of the LVN) were recorded in the nearly (3 weeks) or fully (1 year) compensated cats. At this stage GABA-staining varicosities were significantly increased in the lesioned side of the MVN. These findings demonstrate the reorganization of the GABAergic system in the VN and its possible role in recovery process after UVN in the cat. The changes seen during the acute stage could be causally related to the VN neuron deafferentation, contributing to the static vestibular deficits. Those found in the compensated cats would be more functionally implicated in the dynamic aspects of vestibular compensation.

Roles of glutamate receptor subtypes in the development of vestibular compensation after unilateral labyrinthectomy in the guinea pig

We investigated the roles of ionotropic glutamate receptor subtypes in the development and recovery of spontaneous nystagmus (SN) after unilateral labyrinthectomy (UL) in guinea pigs. When administered at 3 h after UL, N-methyl-D-aspartate (NMDA) and kainate (KA), which are NMDA and non-NMDA receptor agonists, respectively, increased the frequency of SN. The effect of KA was more potent than that of NMDA. In contrast to these agonists, MK-801 and CNQX decreased the frequency of SN. Although the administration of KA at 48 h after UL increased the frequency of SN, it did not exhibit any effects at 72 h after UL. MK-801 caused a recurrence of SN following administration at 48 and 72 h after UL. Neither NMDA nor CNQX exhibited any effects after administration at 48 or 72 h after UL. A newly synthesized compound, NC-1200, which has inhibitory action on the glutamate response, decreased the frequency of SN in a dose-dependent manner following administration at 3 h after UL, but did not exhibit any effects when administered at 48 and 72 h after UL. From these results, it was found that NMDA and non-NMDA receptors play important roles in the development of SN after UL, and that the NMDA receptor contributes to the development of ocular motor compensation.

Effects of nitric oxide on the vestibular functional recovery after unilateral labyrinthectomy

The effects of nitric oxide on the vestibular function recovery following unilateral labyrinthectomy were studied. Male Sprague-Dawley rats treated with N-omega-nitro-L-arginine methyl ester (L-NAME), a nitric oxide synthase (NOS) inhibitor, were subjected to destruction of the unilateral vestibular apparatus and spontaneous nystagmus was observed. To explore the role of nitric oxide on the potassium current, the whole cell patch clamp technique was applied on isolated medial vestibular nuclear neurons. The frequency of spontaneous nystagmus that appeared in L-NAME-treated rats was higher and maintained longer than in control animals. Potassium currents in the isolated medial vestibular nucleus were inhibited by nitric oxide liberating agents, sodium nitroprusside and S-nitroso-N-acetylpenicillamine. After blockade of calcium dependent potassium currents by high EGTA (11 mM)-containing pipette solution, sodium nitroprusside did not inhibit the outward potassium currents. 8-Bromoguanosine 3,5-cyclic monophosphate, a membrane-permeable cGMP analogue, produced similar effects to inhibit the outward potassium currents as sodium nitroprusside. These results suggest that nitric oxide production after unilateral labyrinthectomy would help to facilitate vestibular compensation by inhibiting calcium-dependent potassium currents through increasing intracellular cyclic GMP, thereby increasing excitability in ipsilateral vestibular nuclear neurons.

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.

Distribution of choline acetyltransferase immunoreactivity in the vestibular nuclei of normal and unilateral vestibular neurectomized cats

Post-lesion recovery of vestibular functions is a suitable model for studying adult central nervous system plasticity. The vestibular nuclei complex (VN) plays a major role in the recovery process and neurochemical reorganizations have been described at this brainstem level. The cholinergic system should be involved because administration of cholinergic agonists and antagonists modify the recovery time course. This study was aimed at analysing the postlesion changes in choline acetyltransferase immunoreactivity (ChAT-Ir) in the VN of cats killed 1 week, 3 weeks or 1 year following unilateral vestibular neurectomy. ChAT-positive neurons and varicosities were immunohistochemically labelled and quantified (cell count and surface measurement, respectively) by means of an image analysing system. The spatial distribution of ChAT-Ir within the VN of control cats showed darkly stained neurons and varicosities mainly located in the caudal parts of the medial (MVN) and inferior (IVN) VN, the nucleus prepositus hypoglossi (PH) and, to a lesser extent, in the medial part of the superior vestibular nucleus (SVN). Lesion-induced changes consisted in a significant increase in both the number of ChAT-positive neurons (IVN, SVN) and the surface of ChAT-positive varicosities (IVN, SVN, PH). They were observed bilaterally in the acute (1 year and 3 weeks) and compensated (1 year) cats for the SVN and PH, while they persisted only in the IVN on the lesioned side in the compensated cats. These findings demonstrate vestibular lesion-induced reorganization of the cholinergic system in the IVN, SVN and PH which could contribute to postural and oculomotor function recovery.

Output-to-input approach to neural plasticity in vestibular pathways

Some thoughts on current interpretations of available data regarding vestibular compensation at functional, network, and neural levels are presented. Basic concepts related to neural plasticity (or elasticity) underlying motor learning and regeneration also are discussed briefly. Modifiability in vestibular pathways, at both the functional and structural levels, after peripheral and central axotomy, and subsequent to transient or permanent chemical target removal, is presented as an experimental ground to explain similarities and differences between regenerative, compensatory, and adaptive mechanisms in the mammal central nervous system.

Effects of the GABA agonists baclofen and THIP on long-term compensation in hemilabyrinthectomised rats

Horizontal eye movements, elicited by sinusoidal rotation in darkness, were recorded with a magnetic search coil technique in pigmented rats, hemilabyrinthectomised 8-12 weeks before the investigation. Separate gains during rotation towards the lesioned side (LS) and the intact side (IS) were calculated by a computer program, demonstrating an asymmetry. Systemic single administration of the GABAB agonist baclofen caused a dose-related temporary rebalancing of the compensatory eye movements to the LS and the IS. At an optimal dose of 14 micromol/kg b.wt symmetry was achieved by excitation of eye movements during rotation to the LS and depression during rotation to the IS. Administration of the GABAA agonist THIP did not obviously reduce the asymmetry. It is suggested that stimulation of GABAB receptors modifies the tonic imbalance between the bilateral vestibular nuclei and/or the central processing of the input from the peripheral sensory organs.

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

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

In vitro phosphorylation of medial vestibular nucleus and prepositus hypoglossi proteins during behavioural recovery from unilateral vestibular deafferentation in the guinea pig

Unilateral removal of vestibular nerve input to the vestibular nuclei (e.g. by unilateral labyrinthectomy, UL) results in severe ocular motor and postural disorders which disappear over time (vestibular compensation). We investigated whether recovery of ocular motor function is temporally correlated with changes in protein phosphorylation in the medial vestibular nucleus (MVN) and prepositus hypoglossi (PH; MVN/PH) in vitro. Bilateral MVN/PH were dissected from 48 guinea pigs following decapitation at 10 h, 53 h or 2 weeks post-UL, or -sham operation and frozen. Tissue extracts were incubated with [gamma-32P]ATP +/- Ca2+ plus phorbol 12,13-dibutyrate and phosphatidylserine. UL resulted in a significant bilateral increase in the 32P-incorporation into a 65-85 kDa band (probably the myristoylated alanine-rich C kinase substrate, MARCKS) in compensated animals (53 h post-UL) under conditions which favoured the activation of protein kinase C. Under identical conditions, the labelling of a 42-49 kDa protein (P46) was increased significantly in the bilateral MVN/PH between either 10 h or 53 h and 2 weeks post-UL; there were no significant changes over time in sham controls. These results show that later stages of vestibular compensation are accompanied by changes in the phosphorylation of several likely protein kinase C substrates in the MVN/PH in vitro.

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.

Effect of MK801 on cFos-like protein expression in the medial vestibular nucleus at early stage of vestibular compensation in uvulonodullectomized rats

The purpose of this study was to evaluate the effect of uvulonodullectomy (UNL) on the expression of cFos-like protein (FLP) in the medial vestibular nucleus (MVe) during vestibular compensation and effect of MK801, an N-methyl-D-aspartate (NMDA) antagonist, on FLP expression in the brain stem nuclei at 6 h after unilateral labyrinthectomy (ULX) with UNL in Sprague-Dawley rats. Immunohistochemical staining was performed to visualize FLP in the brain stem nuclei and FLP-positive cells were counted by image analyzer. Lesion-induced asymmetric expression of FLP in the bilateral MVe was observed and maintained up to for 72 h in the ULX group, and 120 h in the UNL + ULX group. Moreover, spatial pattern of FLP expression in the bilateral MVe exhibited the marked difference between the ULX and UNL + ULX groups. MK801 treatment 6 h after ULX showed significant increase in the number of FLP in contralateral MVe (cMVe) of the ULX group, but decrease in cMVe of the UNL + ULX group. These results suggest that the lesion of vestibulocerebellum delays the temporal recovery of FLP expression in MVe and the vestibulocerebellar NMDA receptors relate to FLP expression in MVe.

Histamine immunoreactivity changes in vestibular-lesioned and histaminergic-treated cats

Histamine is likely involved in vestibular function recovery since histaminergic medications are effective in vestibular-related syndromes. We investigated the histamine immunoreactivity changes after unilateral vestibular neurectomy and the effects of betahistine (a partial histamine H1 receptor agonist and an histamine H3 receptor antagonist) and thioperamide (a pure histamine H3 receptor antagonist) treatment in cats. Histamine staining was analyzed in the tuberomammillary and vestibular nuclei through immunohistochemical methods and quantification techniques in light microscopy. Unilateral vestibular neurectomy induced a strong bilateral decrease in histamine immunoreactivity in the vestibular nuclei and a smaller reduction in the tuberomammillary nuclei in both acute (1 week) and compensated (3 weeks, 1 year) cats. One-week thioperamide or betahistine treatment led to a near-total lack of staining in these structures in both lesioned and control cats. One-month betahistine treatment had weaker effects in the compensated cats. We conclude that vestibular lesions reduce histamine staining because of an increase in histamine release in the vestibular and tuberomammillary nuclei, promoting vestibular functions recovery, and betahistine could contribute to this process by acting on both the presynaptic histamine H3 and postsynaptic histamine H1 receptors.

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.

DA1 and DA2 receptor regulation in the striatum of young and old rats after peripheral vestibular lesion

Anatomical, lesion and functional studies have indicated that the mesostriatal dopaminergic (DAergic) system may serve as supravestibular center in posture and locomotion control. Nevertheless, no data are available on the involvement of DAergic systems during vestibular compensation. This study was designed for the analysis of DA1 and DA2 receptors in the striatum by means of quantitative receptor autoradiography 28 days after unilateral or bilateral lesion of the labyrinth in 3-month-old rats. Considering the severe decline of DA content and receptors in striatum and the difference in behavioral recovery after vestibular lesions in old age, we also analyzed 24-month-old, lesioned and unlesioned rats. In young rats, hemilabyrinthectomy caused a bilateral increase (20-30%) of DA1 receptors and a two-fold increase of DA2 receptors. In old-rats, we observed a similar modification of DA2 receptors, and a 50% increase in DA1 receptors. Bilabyrinthectomy did not modify DA1 receptor density and decreased DA2 receptor density in young animals, whereas it produced an increase in both DA1 and DA2 in old rats. This study provides evidence for the involvement of the DAergic system during vestibular compensation. Our results also indicate great biochemical plasticity of the remaining DA receptors in the striatum of old rats.

The ACTH(4-9) analog ORG 2766 and recovery after brain damage in animal models--a review

Treatment with adrenocorticotrophic hormone (ACTH), as well as with ACTH fragments and analogues, can influence behaviour of animals and humans. Furthermore it facilitates recovery of damaged peripheral nervous tissue. The question whether ACTH/MSH peptides affect recovery processes after injury to the central nervous system as well is addressed in the present review. The effects of administration of the ACTH(4-9) analog ORG 2766 after brain lesions has been studied frequently. However, the interpretation of the available data is confused by the variability of the results. Several factors can be identified which influence the efficacy of the peptide: (i) not all behavioural tests are equally suitable to reveal a peptide effect on behavioural recovery; (ii) the affected brain area; (iii) whether cell bodies or terminals are affected; (iv) the post-operative housing conditions; and (v) the onset and duration of peptide administration. Two possible explanations of peptide efficacy on functional recovery are considered: first, the peptide may accelerate spontaneously occurring recovery processes and second, the peptide may induce compensatory mechanisms underlying functional recovery without recuperation of the damaged neurons. These compensatory mechanisms seem to rely mainly on enhanced non-selective attention by activation of limbic structures. It is as yet unknown to which receptor system ORG 2766 binds; the analog lacks affinity for the known melanocortin (MC) receptors in brain, yet ORG 2766 is able to modulate the activity of endogenous opioids and the NMDA-receptor. A modulating influence of the peptide on NMDA-receptor activity might indirectly account for both enhanced attention--with ensuing behavioural recovery--and the acceleration of spontaneous recovery.

c-fos Expression in the rat brain after unilateral labyrinthectomy and its relation to the uncompensated and compensated stages

The expression of the immediate early gene c-fos has been studied in the entire brain of rats 3, 6 and 24 h after surgical unilateral labyrinthectomy. We combined in situ hybridization for c-fos messenger RNA with immunocytochemistry for Fos protein to document very early changes in c-fos expression and to identify with cellular resolution neuronal populations activated by unilateral labyrinthectomy. Three hours after unilateral labyrinthectomy a bilateral increase in both c-fos messenger RNA and protein levels was seen in the superior, medial and spinal vestibular nuclei, nucleus Y, and prepositus hypoglossal nucleus. These changes were asymmetric in the medial vestibular nucleus, being most prominent in the dorsal part of the contralateral nucleus (where second order vestibular neurons are located) and in the ventral part of the ipsilateral nucleus (where commissural neurons acting on the medial vestibular nucleus of the intact side are located). An increase in c-fos messenger RNA expression was seen bilaterally, but with an ipsilateral predominance, in the vermal and paravermal areas of the cerebellar cortex, flocculus and paraflocculus, as well as in the precerebellar lateral and paramedian reticular nuclei. c-fos messenger RNA and protein levels increased in a few regions of the contralateral inferior olive. A predominantly ipsilateral increase in c-fos expression also occurred in the caudate-putamen. A bilateral but not exactly symmetric increase in both c-fos messenger RNA and protein levels was present in several nuclei of the dorsal pontine tegmentum (parabrachial nucleus, locus coeruleus and laterodorsal tegmental nucleus), mesencephalic periaqueductal gray, and several hypothalamic, thalamic and cerebrocortical regions. No change was seen in the cerebellar nuclei, lateral vestibular nucleus and red nucleus. The increased expression of c-fos observed 3 h after unilateral labyrinthectomy, in conjunction with the sudden occurrence of postural and motor deficits, usually declined 6-24 h after the lesion, i.e. during the development of vestibular compensation. In the dorsal part of the medial vestibular nucleus, however, the pattern of c-fos expression observed 3 h after unilateral labyrinthectomy was reversed 6-24 h after the lesion: both c-fos messenger RNA and protein levels increased on the ipsilateral side, but greatly decreased on the contralateral side. In conclusion, asymmetric changes in c-fos expression occurred within 3 h after unilateral labyrinthectomy, but gradually declined or reversed 6 and 24 h after the lesion, thus being temporally related to the appearance and development of vestibular compensation.

Effects of MK801 on Fos expression in the rat brainstem after unilateral labyrinthectomy

Unilateral labyrinthectomy (UL) causes ocular and postural asymmetries, which disappear over time in the processes of equilibrium recovery known as vestibular compensation. It has been reported that N-methyl-D-aspartate (NMDA) receptors are involved in vestibular compensation. In the present study, in order to elucidate the NMDA receptor-mediated neural circuit responsible for the development of vestibular compensation, we used Fos expression as a marker of neural activation and examined the effects of MK801, a specific antagonist of NMDA receptors, on UL-induced Fos expression in the rat brainstem. After UL, Fos-like immunoreactive (-LIR) neurons were observed in the ipsilateral medial vestibular nucleus (ipsi-MVe), the contralateral prepositus hypoglossal nucleus (contra-PrH) and the contralateral inferior olive beta subnucleus (contra-IOb). Fos-LIR neurons gradually disappeared in the processes of vestibular compensation. It is suggested that the activation of the ipsi-MVe, the contra-PrH and the contra-IOb neurons after UL are the initial event of vestibular compensation. Intraperitoneal injection of MK801 in the processes of vestibular compensation caused reappearance of UL-induced behavioral deficits. During the decompensation induced by MK801, Fos-LIR neurons appeared in the contra-MVe, the ipsi-PrH and the bilateral-IOB. It is suggested that the contra-MVe, the ipsi-PrH and the bilateral-IOb neurons are inhibited by glutamatergic synapses driving inhibitory neurons via NMDA receptors in the processes of vestibular compensation and that disinhibition of these nuclei induced by MK801 causes decompensation. However, MK801 caused neither Fos expression nor behavioral decompensation after vestibular compensation is accomplished. All these findings that the NMDA receptor-mediated inhibitory modulation in the central vestibular system plays an important role for the initial processes of the development of vestibular compensation.