Role of glucocorticoid in vestibular compensation in relation to activation of vestibular nucleus neurons

Attenuated cortisol reactivity to psychosocial stress is associated with greater visual dependency in postural control

Despite known anatomical links between the hypothalamic-pituitary-adrenal (HPA) axis and the vestibular system, there are no studies on the relationship between postural control and HPA axis function. Visual dependence in postural control, often measured by increased postural sway on exposure to visual motion, is an indication of altered visual-vestibular integration with greater weighting towards visual cues for balance. Visual dependence is more common in older age and a range of vestibular and non-vestibular health conditions. The relationship between visual dependence in postural control was investigated in relation to cortisol reactivity to psychosocial stress (using the Trier Social Stress Test for groups: TSST-G), as an index of HPA axis function, in healthy young females. In those who exhibited a cortisol response (>2 nmol/l), a negative relationship between stress-induced cortisol reactivity and visual dependence in postural control was observed, since those with the largest cortisol response showed less visual motion induced postural sway (measured by force platform). This finding in healthy females indicates that subtle non-clinical differences in vestibular function are associated with dysregulated HPA axis activity as indicated by lower cortisol reactivity to psychosocial stress. It adds to the growing body of evidence linking blunted cortisol reactivity to stress to poor homeostatic regulation and potential negative health and behavioural outcomes.

Dexamethasone protects against arsanilic acid‑induced rat vestibular dysfunction through the BDNF and JNK 1/2 signaling pathways

The brain‑derived neurotrophic factor (BDNF) and c‑Jun NH 2‑terminal kinase (JNK) signaling pathways are therapeutic targets to prevent degeneration in the central nervous system. Dexamethasone (DXMS), a glucocorticoid, protects against vestibular brain injury, however, the molecular mechanisms have yet to be fully elucidated. To investigate whether the BDNF and JNK signaling pathways are involved in the protective effects of DXMS in rats with vestibular dysfunction, a rat model of severe vestibular deficits was established by middle ear injection of arsanilic acid (AA; 100 mg/ml; 0.05 ml). After 3 days, rat symptoms and behavior scores with vestibular disorders were detected. In brain tissues, histopathological alterations, cell apoptosis, expression levels and patterns of BDNF signaling pathway‑associated BDNF, tyrosine receptor kinase B (TrKB) and K+/Cl‑ cotransporter isoform 2 (KCC2), and the expression of apoptosis‑related cleaved‑caspase 3 and the JNK signaling pathway were detected. It was identified that DXMS relieved AA‑induced vestibular dysfunction, leading to improvement in rat behavior scores to normal levels, minimizing brain damage at the histopatholojnnkngical level, reducing cell apoptosis, enhancing the expression of BDNF, TrKB and KCC2, and downregulating cleaved‑caspase 3 and phosphorylated‑JNK1/2 in brain tissues. Together, these findings indicated the protective effect of DXMS on AA‑induced rat vestibular dysfunction, and that activating BDNF and inhibiting JNK singling pathways were the underlying mechanisms. In addition, with additional treatment of mifepristone (RU486), a specific glucocorticoid agonist, all the events elicited by DXMS mentioned above in the AA‑treated rat rats were reversed. In conclusion, DXMS was identified as a therapeutic agent targeting the BDNF and JNK singling pathways for AA‑induced rat vestibular dysfunction.

Detection of intralabyrinthine abnormalities using post-contrast delayed 3D-FLAIR MRI sequences in patients with acute vestibular syndrome

PURPOSE

3D-FLAIR sequences with delayed acquisition after contrast medium injection have demonstrated new insights into blood-labyrinthine barrier (BLB) abnormalities in various diseases. The aim of this study was to assess the BLB in patients referred with unilateral acute vestibular syndrome (UAVS).

MATERIALS AND METHODS

In this retrospective multicenter imaging study, we performed 3D-FLAIR and steady-state free precession (SSFP) sequences 4 h after contrast medium administration in 26 healthy volunteers and in 30 patients with UAVS. Two radiologists, blinded to the clinical data, independently assessed the asymmetrical enhancement of the labyrinthine structures and the vestibular nerve on 3D-FLAIR sequences, and the signal of the labyrinthine structures on SSFP sequences. Inter-reader agreement tests were performed.

RESULTS

An asymmetrical enhancement of the semicircular canals was observed in 26 out of 30 ears (86.6%, p < 0.001) and never observed in healthy subjects. An asymmetrical enhancement of the vestibular nerve was never observed in either patients or healthy subjects. An asymmetrical enhancement of the cochlea was observed on the 3D-FLAIR sequence in 6 out of 30 ears only in the patients' group (20%, p = 0.03) and always associated with an enhancement of at least one semicircular canal. A low signal on SSFP sequences was observed only in 11 out of 30 symptomatic ears (36.7%, p < 0.001), involving the utricle in 7 ears and the superior semicircular canal in 4 ears.

CONCLUSION

Patients with typical UAVS presented with semicircular canal enhancement on MRI, while an asymmetrical enhancement of the vestibular nerve was not displayed.

TRIAL REGISTRATION

NCT02529475 KEY POINTS: • Patients with typical vestibular neuronitis presented with semicircular canal enhancement on MRI in 87% of cases. • An enhancement of the vestibular nerve was never displayed.

Interactions between Stress and Vestibular Compensation - A Review

Elevated levels of stress and anxiety often accompany vestibular dysfunction, while conversely complaints of dizziness and loss of balance are common in patients with panic and other anxiety disorders. The interactions between stress and vestibular function have been investigated both in animal models and in clinical studies. Evidence from animal studies indicates that vestibular symptoms are effective in activating the stress axis, and that the acute stress response is important in promoting compensatory synaptic and neuronal plasticity in the vestibular system and cerebellum. The role of stress in human vestibular disorders is complex, and definitive evidence is lacking. This article reviews the evidence from animal and clinical studies with a focus on the effects of stress on the central vestibular pathways and their role in the pathogenesis and management of human vestibular disorders.

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.

Corticosteroids for the treatment of idiopathic acute vestibular dysfunction (vestibular neuritis)

BACKGROUND

Idiopathic acute vestibular dysfunction (vestibular neuritis) is the second most common cause of peripheral vertigo after benign paroxysmal positional vertigo (BPPV) and accounts for 7% of the patients who present at outpatient clinics specialising in the treatment of dizziness. The exact aetiology of the condition is unknown and the effects of corticosteroids on the condition and its recovery are uncertain.

OBJECTIVES

To assess the effectiveness of corticosteroids in the management of patients with idiopathic acute vestibular dysfunction (vestibular neuritis).

SEARCH STRATEGY

We searched the Cochrane ENT Group Trials Register; CENTRAL; PubMed; EMBASE; CINAHL; Web of Science; BIOSIS Previews; Cambridge Scientific Abstracts; ICTRP and additional sources for published and unpublished trials. The date of the most recent search was 28 December 2010.

SELECTION CRITERIA

Randomised controlled trials comparing corticosteroids with placebo, no treatment or other active treatments, for adults diagnosed with idiopathic acute vestibular dysfunction.

DATA COLLECTION AND ANALYSIS

Two authors independently selected studies from the search results and extracted data. Three authors independently assessed risk of bias.

MAIN RESULTS

Four trials, involving a total of 149 participants, compared the effectiveness of oral corticosteroids against placebo. All the trials were small and of low methodological quality. Although there was an overall significant effect of corticosteroids compared with placebo medication on complete caloric recovery at one month (risk ratio (RR) of 2.81; 95% confidence interval (CI) 1.32 to 6.00, P = 0.007), no significant effect was seen on complete caloric recovery at 12 months (RR 1.58; 95% CI 0.45 to 5.62, P = 0.48), or on the extent of caloric recovery at either one month (mean difference (MD) 9.60%; 95% CI -20.66 to 39.86, P = 0.53) or at 12 months (MD 6.83%; 95% CI -27.69 to 41.36, P = 0.70). In addition, there was no significant difference between corticosteroids and placebo medication in the symptomatic recovery of vestibular function following idiopathic acute vestibular dysfunction with respect to vertigo at 24 hours (RR 0.39; 95% CI 0.04 to 3.57, P = 0.40) and use of the Dizziness Handicap Inventory score at one, three, six and 12 months.

AUTHORS' CONCLUSIONS

Overall, there is currently insufficient evidence from these trials to support the administration of corticosteroids to patients with idiopathic acute vestibular dysfunction. We found no trials with a low risk of methodological bias that used the highest level of diagnostic criteria and outcome measures. We recommend that future studies should include health-related quality of life and symptom-based outcome measures, in addition to objective measures of vestibular improvement, such as caloric testing and electronystagmography.

Corticosteroids in the treatment of vestibular neuritis: a systematic review and meta-analysis

OBJECTIVE

To systematically review and meta-analyze the results of all randomized controlled trials comparing corticosteroids with placebo for the treatment of patients with vestibular neuritis.

DATA SOURCES

An electronic search was performed in MEDLINE, EMBASE, Cochrane Library, and CENTRAL databases, and then extensive hand-searching was performed for the identification of relevant studies. No time and language limitations were applied.

STUDY SELECTION

Prospective randomized controlled trials comparing corticosteroids with placebo for the treatment of patients with vestibular neuritis.

DATA EXTRACTION

Odds ratios (ORs), weighted mean differences (WMD), 95% confidence intervals (CIs), and tests for heterogeneity were reported.

DATA SYNTHESIS

Four studies were eventually identified and systematically reviewed. Meta-analysis was feasible for 3 studies. Regarding the recovery of clinical symptoms, the proportion of patients with clinical recovery at 1 month after the initiation of therapy did not differ significantly between the corticosteroids and placebo groups (OR, 1.45; 95% CI, 0.26-8.01; p = 0.67). The proportion of patients with caloric complete recovery was significantly different between the corticosteroids and placebo groups both at 1 (OR, 12.64; 95% CI, 2.6-61.52; p = 0.002; heterogeneity, p = 0.53; fixed effects model) and 12 months (OR, 3.35; 95% CI, 1.45-7.76; p = 0.005; heterogeneity, p = 0.03; random effects model) after the initiation of therapy. The caloric extent of canal paresis at 12 months after the initiation of therapy seemed to differ significantly between patients who received corticosteroids and those who received placebo (WMD, -12.15; 95% CI, -19.85 to -4.46; p < 0.05; heterogeneity, p < 0.05; random effects model).

CONCLUSION

The present systematic review and meta-analysis, based on the currently available evidence, suggests that corticosteroids improve only the caloric extent and recovery of canal paresis of patients with vestibular neuritis. At present, clinical recovery does not seem be better in patients receiving corticosteroids.

Stress axis plasticity during vestibular compensation in the adult cat

The postural, ocular motor, perceptive and neurovegetative syndromes resulting from unilateral vestibular neurectomy (UVN) symptoms could generate a stress and thereby activate the hypothalamo-pituitary-adrenal (HPA) axis. This study was aimed at determining whether UVN causes changes in the activity of the HPA axis, and if so, evaluating the time course of changes associated with UVN syndrome. At the cellular level, corticotropin-releasing factor (CRF) and arginine vasopressin (AVP) immunoreactivity (Ir) were analyzed and quantified in the paraventricular nucleus (PVN) and the vestibular nuclei (VN) complex of cats killed early (1 and 7 days) or late (30 and 90 days) after UVN. Dopamine-beta-hydroxylase (DbetaH), the enzyme synthesizing noradrenaline was examined in the locus coeruleus (LC) in these same cats. At the behavioral level, the time course of recovery of the postural and locomotor functions was quantified at the same postoperative delays in another group of UVN cats. Results showed a significant bilateral increase in the number of both AVP-Ir and CRF-Ir neurons in the PVN and an increase of DbetaH-Ir neurons in the LC at 1, 7 and 30 days after UVN. This increased number of neurons was no longer observed at 90 days. Conversely, a significant bilateral decrease of CRF-Ir neurons was observed in the VN at these same postlesion times, with a similar return to control values at 90 days. Our behavioral observations showed strong posturo-locomotor functional deficits early after UVN (1 and 7 days), which had recovered partially at 30 days and completely by 90 days postlesion. We demonstrate a long-lasting activation of the HPA axis, which likely reflects a chronic stress, experienced by the animals, which corresponds to the time course of full vestibular compensation, and which is no longer present when the animals are completely free of posturo-locomotor symptoms at 90 days.

Projections from the vestibular nuclei to the hypothalamic paraventricular nucleus: morphological evidence for the existence of a vestibular stress pathway in the rat brain

Although it has been reported by several laboratories that vestibular stress activates the hypothalamo-pituitary-adrenocortical axis (HPA), the existence of neuronal connections between vestibular and hypothalamic paraventricular neurons has not yet been demonstrated. By the use of a virus-based retrograde trans-synaptic tracing technique in the rat, here we demonstrate vestibular projections to the paraventricular nucleus (PVN). Pseudorabies virus (Bartha strain, type BDR62) was injected into the PVN, and the progression of the infection along synaptically connected neurons was followed in the pons and the medulla, 3 and 4 days post-inoculation. Virus-infected neurons were revealed mainly in the medial vestibular nucleus. Labeled cells were scattered in the spinal, and very rarely in the superior nuclei, but none of them in the lateral vestibular nucleus. Injections of cholera toxin B subunit, a monosynaptic retrograde tracer into the PVN failed to label any cells in the vestibular nuclei. These results provide anatomical evidence for the existence of a vestibulo-paraventricular polysynaptic pathway and support the view that the HPA axis is modulated by vestibular stress.

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.

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.

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

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

11Beta-hydroxysteroid dehydrogenase type 1 activity in medial vestibular nucleus and cerebellum after unilateral vestibular deafferentation in the rat

In the early stages of vestibular compensation (VC) (the behavioural recovery that follows unilateral vestibular deafferentation), neurons in the medial vestibular nucleus (MVN) on the lesioned side develop a sustained up-regulation of their intrinsic excitability. This plasticity is dependent on the activation of glucocorticoid receptors, which presumably occurs during the acute stress response that accompanies the vestibular deafferentation symptoms. Recent studies have established that the access of glucocorticoids to their intracellular receptors in brain is potently modulated by 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1), which catalyses the generation of active glucocorticoids from their inert 11-keto forms. In this study, we investigated the presence of 11beta-HSD1 bioactivity, and possible changes in activity in the early stage after vestibular deafferentation, in the cerebellar nodulus and uvula, the flocculus/paraflocculus (F/PF) complex and the MVN of the rat. 11beta-HSD1 activity was found in each of these brain areas, with especially high levels of activity in the F/PF complex. No differences were found in the level of 11beta-HSD1 activity in these brain areas between control rats, sham-operated rats and rats that underwent VC for 4 h after unilateral vestibular deafferentation. These findings demonstrate 11beta-HSD1 bioactivity in the MVN and vestibulocerebellum, but exclude the possibility that changes in 11beta-HSD1 activity occur in the early period after deafferentation, over the time when changes in MVN neuronal properties take place.

Interaction between the hypothalamic-pituitary-adrenal axis and behavioural compensation following unilateral vestibular deafferentation

Vestibular compensation is defined as the process of behavioural recovery that occurs following the loss of sensory input from one or both vestibular labyrinths. The visual and postural instability resulting from the vestibular damage must alter the homeostasis of the subject; however, very little research has been conducted that investigates the interaction between vestibular compensation and the adaptive stress response of the body, i.e. the hypothalamic-pituitary-adrenal (HPA) axis. The aim of this review is to describe and evaluate the experimental evidence indicating a link between vestibular compensation and the body's response to stress, via the HPA axis.

Steroid effects on vestibular compensation in human

Vestibular neuritis (VN) rapidly damages unilateral vestibular periphery, inducing severe balance disorders. In most cases, such vestibular imbalance is gradually restored to within the normal level after clinical therapies. This successive clinical recovery occurs due to regeneration of vestibular periphery and/or accomplishment of central vestibular compensation. We experienced 36 patients with VN treated at our hospital, including cases in our previous preliminary report. To elucidate effects of steroid therapy both on the recovery of peripheral function and on the adaptation of central vestibular compensation, we examined caloric test and several questionnaires with two randomly divided groups, 18 steroid-treated and 18 nonsteroid-treated patients, over two years after the onset. These examinations revealed that steroid-treated patients had a tendency of better canal improvements (13/18, 72%) than nonsteroid-treated ones (10/18, 55.6%). However, there was no significant difference between these two groups. In cases with persistent canal paresis, steroid-treated patients (n = 5) reduced handicaps in their everyday life due to the dizziness induced by head and/or body movements and the disturbance of their mood, more effectively than those with nonsteroid therapy (n = 8). These findings suggest that steroid therapy with VN could be effective on not only vestibular periphery but central vestibular system, to restore the balance.

Role of the flocculus in mediating vestibular nucleus neuron plasticity during vestibular compensation in the rat

We investigated the role of the cerebellar flocculus in mediating the adaptive changes that occur in the intrinsic properties of brainstem medial vestibular nucleus (MVN) neurons during vestibular compensation. Ipsi-lesional, but not contra-lesional, flocculectomy prevented the compensatory increase in intrinsic excitability (CIE) that normally occurs in the de-afferented MVN neurons within 4 h after unilateral labyrinthectomy (UL). Flocculectomy did not, however, prevent the down-regulation of efficacy of GABA receptors that also occurs in these neurons after UL, indicating that these responses of the MVN neurons to deafferentation are discrete, parallel processes. CIE was also abolished by intra-floccular microinjection of the metabotropic glutamate receptor (mGluR) antagonist AIDA, and the protein kinase C inhibitor bisindolymaleimide I (BIS-I). The serene-threonine kinase inhibitor H-7 had no effect when microinjected at the time of de-afferentation, but abolished CIE if microinjected 2 h later. These cellular effects are in line with the recently reported retardatory effects of BIS-I and H-7 on behavioural recovery after UL. They demonstrate that the increase in intrinsic excitability in MVN neurons during vestibular compensation is cerebellum dependent, and requires mGluR activation and protein phosphorylation in cerebellar cortex. Furthermore, microinjection of the glucocorticoid receptor (GR) antagonist RU38486 into the ipsi-lesional flocculus also abolished CIE in MVN neurons. Thus an important site for glucocorticoids in facilitating vestibular compensation is within the cerebellar cortex. These observations ascribe functional significance to the high levels of GR and 11-beta-HSD Type 1 expression in cerebellum.

Dexamethasone attenuates by colchicine induced Fos expression in the rat deep cerebellar and vestibular nuclei

1. The intent of the present study was to find out whether dexamethasone pretreatment may affect the induction of Fos protein in cell nuclei of the cerebellar vestibular neuronal complex (CVNC) elicited by central administration of colchicine. Specifically, the rate of the dexamethasone-sensitive cell population was analyzed and compared at different levels of the CVNC using a light microscopic avidin-biotin peroxidase immunohistochemistry. 2. Male Wistar rats were pretreated with dexamethasone 3 days prior (2.5 mg/kg/day, s.c.) and 24 h after an intracerebroventricular delivery of colchicine (60 microg/10 microL). Animals were sacrificed 48 h after colchicine treatment by a transcardial perfusion with fixative. 3. Dexamethasone in itself had no effect on the activity of cells of the CVNC. However, in colchicine treated animals, which exhibited a large number of Fos-positive cells over the entire CVNC, the dexamethasone elicited a substantial reduction in the number of the Fos-immunoreactive cells over the CVNC. Distinct dexamethasone dependent reduction (50-90%) of Fos-immunoreactivity was observed in each of the deep cerebellar nuclei. On the other hand, less number of dexamethasone-sensitive cells were recognized in the vestibular structures. From these, maximal Fos-inhibition by dexamethasone was recognized in the medial vestibular nucleus, however, even in this case the number of suppressed cells did not exceed 50%. 4. The results provide for the first time evidence about the dexamethasone dependent reduction of Fos-immunoreactivity in the cells of the CVNC in response to stimulation elicited by colchicine. The data also indicate that the glucocorticoids might be involved in the regulation of some functions of the CVNC under stress conditions.

Exercise and drug therapy alter recovery from labyrinth lesion in humans

Acute unilateral vestibular failure is characterized by rotatory vertigo, horizontal-rotatory nystagmus, and postural imbalance, all of which last from days to weeks. These signs and symptoms are caused by a vestibular tone imbalance between the two labyrinths. Recovery results from a combination of peripheral restoration of labyrinthine function (usually incomplete) and central vestibular compensation (CVC) of the vestibular tone imbalance. Acute unilateral failure is most often caused by vestibular neuritis, which is most likely due to the reactivation of a latent HSV-1 infection. Therefore, therapeutic strategies to improve the outcome of VN are theoretically based on two principles: (a) vestibular exercises and drugs to improve CVC and (b) drug treatment of the assumed viral inflammation. The following conclusions can be drawn from studies in animals and/or humans: (1) There is strong evidence that vestibular exercises may improve vestibulo-spinal compensation. These exercises should begin as early as possible after symptom onset. Moreover, slower exercises are likely to be more effective than faster exercises because slower ones seem to depend more on the vestibular system. (2) Despite extensive data from animal experiments indicating that drugs have a favorable effect on CVC, this has not been clinically proven and thus cannot be recommended yet. (3) Preliminary results of an interim analysis from an ongoing randomized, prospective study showed that methylprednisolone (plus an antiviral agent?) may be useful for improving peripheral vestibular function in vestibular neuritis.

Inhibitory effects of tandospirone, a 5-HT1A agonist, on medial vestibular nucleus neurons responding to lateral roll tilt stimulation in rats

An electrophysiological study was performed using chloral hydrate-anesthetized rats to determine whether tandospirone, a 5-HT1A agonist, affects neuronal activities of the medial vestibular nucleus (MVN), since serotonergic innervation and 5-HT1A receptors are present in this nucleus. Tandospirone applied microiontophoretically at a current of 20-60 nA caused an inhibition of tilt-induced firing of alpha-type neurons, which showed increased and decreased firing with lateral tilt ipsilateral and contralateral to the recording site, respectively, along with that of beta-type neurons which exhibited the reverse responses to ipsilateral and contralateral tilt stimulation. The inhibition was antagonized during simultaneous, iontophoretic application of WAY-100635 (20-60 nA), a 5-HT1A receptor antagonist, although WAY-100635 alone rarely affected spontaneous or tilt-induced firing in either type of neurons. These results suggest that tandospirone acts on a 5-HT1A receptor to inhibit transmission of otolith information to alpha- and beta-type MVN neurons.

Lesion-induced plasticity in rat vestibular nucleus neurones dependent on glucocorticoid receptor activation

1. We have recently shown that neurones in the rostral region of the medial vestibular nucleus (MVN) develop a sustained increase in their intrinsic excitability within 4 h of a lesion of the vestibular receptors of the ipsilateral inner ear. This increased excitability may be important in the rapid recovery of resting activity in these neurones during 'vestibular compensation', the behavioural recovery that follows unilateral vestibular deafferentation. In this study we investigated the role of the acute stress that normally accompanies the symptoms of unilateral labyrinthectomy (UL), and in particular the role of glucocorticoid receptors (GRs), in the development of the increase in excitability in the rostral MVN cells after UL in the rat. 2. The compensatory increase in intrinsic excitability (CIE) of MVN neurones failed to occur in animals that were labyrinthectomized under urethane anaesthesia and kept at a stable level of anaesthesia for either 4 or 6 h after UL, so that they did not experience the stress normally associated with the vestibular deafferentation syndrome. In these animals, 'mimicking' the stress response by administration of the synthetic GR agonist dexamethasone at the time of UL, restored and somewhat potentiated CIE in the MVN cells. Administration of dexamethasone in itself had no effect on the intrinsic excitability of MVN cells in sham-operated animals. 3. In animals that awoke after labyrinthectomy, and which therefore experienced the full range of oculomotor and postural symptoms of UL, there was a high level of Fos-like immunoreactivity in the paraventricular nucleus of the hypothalamus over 1.5-3 h post-UL, indicating a strong activation of the stress axis. 4. The GR antagonist RU38486 administered at the time of UL abolished CIE in the rostral MVN cells, and significantly delayed behavioural recovery as indicated by the persistence of circular walking. The mineralocorticoid receptor (MR) antagonist spironolactone administered at the time of UL had no effect. 5. Vestibular compensation thus involves a novel form of 'metaplasticity' in the adult brain, in which the increase in intrinsic excitability of rostral MVN cells and the initial behavioural recovery are dependent both on the vestibular deafferentation and on the activation of glucocorticoid receptors, during the acute behavioural stress response that follows UL. These findings help elucidate the beneficial effects of neuroactive steroids on vestibular plasticity in various species including man, while the lack of such an effect in the guinea-pig may be due to the significant differences in the physiology of the stress axis in that species.

Synaptic mechanisms for coding timing in auditory neurons

Neurons in the cochlear ganglion and auditory brain stem nuclei preserve the relative timing of action potentials passed through sequential synaptic levels. To accomplish this task, these neurons have unique morphological and biophysical specializations in axons, dendrites, and nerve terminals. At the membrane level, these adaptations include low-threshold, voltage-gated potassium channels and unusually rapid-acting transmitter-gated channels, which govern how quickly and reliably action potential threshold is reached during a synaptic response. Some nerve terminals are remarkably large and release large amounts of excitatory neurotransmitter. The high output of transmitter at these terminals can lead to synaptic depression, which may itself be regulated by presynaptic transmitter receptors. The way in which these different cellular mechanisms are employed varies in different cell types and circuits and reflects refinements suited to different aspects of acoustic processing.