Effects of flunarizine on ocular motor and postural compensation following peripheral vestibular deafferentation in the guinea pig

Unilateral vestibular deafferentation-induced changes in calcium signaling-related molecules in the rat vestibular nuclear complex

Inquiries into the neurochemical mechanisms of vestibular compensation, a model of lesion-induced neuronal plasticity, reveal the involvement of both voltage-gated Ca(2+) channels (VGCC) and intracellular Ca(2+) signaling. Indeed, our previous microarray analysis showed an up-regulation of some calcium signaling-related genes such as the alpha2 subunit of L-type calcium channels, calcineurin, and plasma membrane Ca(2+) ATPase 1 (PMCA1) in the ipsilateral vestibular nuclear complex (VNC) following unilateral vestibular deafferentation (UVD). To further elucidate the role of calcium signaling-related molecules in vestibular compensation, we used a quantitative real-time polymerase chain reaction (PCR) method to confirm the microarray results and investigated changes in expression of these molecules at various stages of compensation (6 h to 2 weeks after UVD). We also investigated the changes in gene expression during Bechterew's phenomenon and the effects of a calcineurin inhibitor on vestibular compensation. Real-time PCR showed that genes for the alpha2 subunit of VGCC, PMCA2, and calcineurin were transiently up-regulated 6 h after UVD in ipsilateral VNC. A subsequent UVD, which induced Bechterew's phenomenon, reproduced a complete mirror image of the changes in gene expressions of PMCA2 and calcineurin seen in the initial UVD, while the alpha2 subunit of VGCC gene had a trend to increase in VNC ipsilateral to the second lesion. Pre-treatment by FK506, a calcineurin inhibitor, decelerated the vestibular compensation in a dose-dependent manner. Although it is still uncertain whether these changes in gene expression are causally related to the molecular mechanisms of vestibular compensation, this observation suggests that after increasing the Ca(2+) influx into the ipsilateral VNC neurons via up-regulated VGCC, calcineurin may be involved in their synaptic plasticity. Conversely, an up-regulation of PMCA2, a brain-specific Ca(2+) pump, would increase an efflux of Ca(2+) from those neurons and perhaps prevent cell damage following UVD.

Effects of Chronic Infusion of a GABAA Receptor Agonist or Antagonist into the Vestibular Nuclear Complex on Vestibular Compensation in the Guinea Pig

The aim of this study was to determine the effects of chronic infusion of a GABA(A) receptor agonist/antagonist into the ipsilateral or contralateral vestibular nuclear complex (VNC) on vestibular compensation, the process of behavioral recovery that occurs after unilateral vestibular deafferentation (UVD). This was achieved by a mini-osmotic pump that infused, over 30 h, muscimol or gabazine into the ipsilateral or contralateral VNC. Spontaneous nystagmus (SN), yaw head tilt (YHT), and roll head tilt (RHT) were measured. Infusion of muscimol or gabazine into either the ipsilateral or the contralateral VNC had little effect on SN compensation. In contrast, infusion of muscimol (250, 500, and 750 ng) into the contralateral VNC and gabazine (31.25, 62.5, and 125 ng) into the ipsilateral VNC significantly affected YHT and RHT (p < 0.05), but not their rate of compensation (p > 0.05). Interestingly, the effects of muscimol and gabazine on YHT and RHT were consistent throughout the first 30 h post-UVD. Infusion of muscimol (62.5, 125, and 250 ng) into the ipsilateral VNC and gabazine (125, 375, and 750 ng) into the contralateral VNC had little effect on YHT and RHT or their rate of compensation. These results suggest that the ipsilateral gabazine and contralateral muscimol infusions are modifying the expression of the symptoms without altering the mechanism of compensation. Furthermore, the neurochemical mechanism responsible for vestibular compensation can cope with the both the GABA(A) receptor-mediated and the UVD-induced decrease in resting activity.

Rapid vestibular compensation in guinea pig even with prolonged anesthesia

The results of previous studies have suggested that prolonged anesthesia following unilateral labyrinthectomy (UL) results in a retardation of vestibular compensation, the process of behavioral recovery that occurs following the lesion. In this study we investigated the effects of short-term (25 min) and long-term (4 h) anesthesia with isoflurane on the time course of vestibular compensation following UL in guinea pig. Although there were significant differences in the frequency of spontaneous nystagmus (SN) (p < 0.05) and its rate of compensation (p < 0.05) between the 25 min and 4h isoflurane groups, these differences appeared to be due largely to the 5, 9 and 13 h time points. There was also a significant difference in the rate of yaw head tilt (YHT) compensation, largely due to the 5 h time point. When exponential regression analysis was performed to evaluate the overall pattern of compensation, there was no significant difference in the time required to reach 100% SN or YHT compensation between the 25 min and 4 h isoflurane groups. Furthermore, there were no significant differences in roll head tilt (RHT) compensation between the two groups. These results suggest that the time course of vestibular compensation is largely independent of the duration of the anesthesia used for UL surgery.

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.

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.

alpha-Difluoromethylornithine delays behavioral recovery and induces decompensation after unilateral labyrinthectomy

Biochemical and pharmacologic studies suggest a role for the ornithine decarboxylase-polyamine system as a modulator of behavioral changes during vestibular compensation. alpha-Difluoromethylornithine specifically blocks the rate-limiting step of polyamine biosynthesis. To assess the effects of alpha-difluoromethylornithine on the acute phase of postural compensation, guinea pigs were divided into groups subjected to either unilateral labyrinthectomy only (n = 7), alpha-difluoromethylornithine (500 mg/kg/day) for 4 days before labyrinthectomy (n = 10), equivalent volumes of saline for 4 days before labyrinthectomy (n = 8), and sham operations (n = 5). Yaw head tilt and roll head tilt, trunk curvature, and air-righting reflex were measured at baseline and at regular intervals up to 4 weeks. alpha-Difluoromethylornithine significantly delayed recovery of normal air-righting but had no effect on yaw head tilt, roll head tilt, and trunk curvature. We also evaluated effects of alpha-difluoromethylornithine in compensated guinea pigs. Fully compensated animals from phase 1 were randomly assigned to receive alpha-difluoromethylornithine (500 mg/kg/day) or saline once daily for 4 days. Only 33% of alpha-difluoromethylornithine animals maintained air-righting, compared with 100% of saline-treated animals (p = 0.003). Maximum trunk curvature was greater in the alpha-difluoromethylornithine group (p = 0.02). Thus alpha-difluoromethylornithine not only delayed the time course for postural recovery after unilateral labyrinthectomy, it also transiently disrupted the maintenance of the compensated state.

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

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

Comparison of the effects of pretreatment with competitive or noncompetitive NMDA antagonists on vestibular compensation

Unilateral labyrinthectomy (UL) results in a syndrome of ocular motor and postural disorders which abates over time in a process of behavioural recovery known as vestibular compensation. We have previously reported that a single systemic pre-UL injection of the organic Ca2+ channel antagonist verapamil or the noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 reduces the behavioural effects of UL in guinea pigs. The present study was conducted to determine if similar effects would be obtained with single injections of the competitive NMDA receptor antagonists 3-[(+/-)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid (CPP) or cis-4-(phosphonomethyl)-piperidine-2-carboxylic acid (CGS 19755). Guinea pigs received an IP injection of 5 mg/kg CPP 2.5 h pre-UL, 5 or 10 mg/kg CPP 1 h pre-UL, 10 or 20 mg/kg CGS 19755 1 h pre-UL, or 1 ml/kg vehicle (saline) 1 h pre-UL, and the effects on the compensation of spontaneous nystagmus were measured over the following 52 h post-UL. Pretreatment with CPP had no significant effect on spontaneous nystagmus frequency or its compensation over 52 h post-UL. However, pretreatment with CGS 19755 resulted in a significant decrease in spontaneous nystagmus frequency without any acceleration of the rate of compensation.