Effect of electric stimulation on vestibular compensation in guinea pigs

Modulated discharge of Purkinje and stellate cells persists after unilateral loss of vestibular primary afferent mossy fibers in mice

Cerebellar Purkinje cells are excited by two afferent pathways: climbing and mossy fibers. Climbing fibers evoke large "complex spikes" (CSs) that discharge at low frequencies. Mossy fibers synapse on granule cells whose parallel fibers excite Purkinje cells and may contribute to the genesis of "simple spikes" (SSs). Both afferent systems convey vestibular information to folia 9c-10. After making a unilateral labyrinthectomy (UL) in mice, we tested how the discharge of CSs and SSs was changed by the loss of primary vestibular afferent mossy fibers during sinusoidal roll tilt. We recorded from cells identified by juxtacellular neurobiotin labeling. The UL preferentially reduced vestibular modulation of CSs and SSs in folia 8-10 contralateral to the UL. The effects of a UL on Purkinje cell discharge were similar in folia 9c-10, to which vestibular primary afferents project, and in folia 8-9a, to which they do not project, suggesting that vestibular primary afferent mossy fibers were not responsible for the UL-induced alteration of SS discharge. UL also induced reduced vestibular modulation of stellate cell discharge contralateral to the UL. We attribute the decreased modulation to reduced vestibular modulation of climbing fibers. In summary, climbing fibers modulate CSs directly and SSs indirectly through activation of stellate cells. Whereas vestibular primary afferent mossy fibers cannot account for the modulated discharge of SSs or stellate cells, the nonspecific excitation of Purkinje cells by parallel fibers may set an operating point about which the discharges of SSs are sculpted by climbing fibers.

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.

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.

Effects of galvanic stimulation of the mastoid process on the gastric motility induced by caloric stimulation

The effects of galvanic stimulation to the mastoid portion on the vestibuloautonomic symptoms induced by caloric stimulation, such as nausea, vomiting, and vertigo, were evaluated in this study. Gastric motility was measured by electrogastrography (EGG) in 20 healthy volunteers (11 male and nine female) aged 20-30 (average: 25.4) years. Electrical stimulation of the mastoid process with 1.0-3.0 mA, 1.0 ms, 100 Hz was applied using a bipolar-biaural method during caloric stimulation of the external auditory canal. The dominant frequency and power of EGG were determined using running spectral frequency analysis and the time-course of EGG was evaluated in a pseudo three dimensional graphic. Frequency of EGG was classified into normogastria with 3 cpm, bradygastria with lower than 3 cpm, and tachygastria with higher than 3 cpm. At quiescent period, normogastria was 78.7 +/- 3.7%, bradygastria 5.0 +/- 1.1%, and tachygastria 16.4 +/- 3.7%. Caloric stimulation with warm water in the unilateral ear and cold water in the contralateral ear elicited vestibuloautonomic symptoms, with accompanied decreases in normogastria (57.7 +/-4.6%, P < 0.01) and increases in tachygastria (34.8 +/- 4.8%, P < 0.01). Cathodal stimulation to the mastoid process ipsilateral to cold water irrigation during caloric stimulation restored normal pattern of gastric motility: normogastria in 77.1 + 5.3% and tachygastria in 19.3 +/- 4.7%, and relieved vestibuloautonomic symptoms. Cathodal stimulation to the inhibited vestibular system ameliorates the vestibular symptoms induced by caloric stimulation.

Vestibular compensation in guinea pigs given intravenous lidocaine after unilateral labyrinthectomy

Intravenous lidocaine has been reported to alleviate vertigo in Meniere's disease and suggested as a possible antivertigo agent after unilateral labyrinthectomy in a study of cats. To further evaluate the effects of intravenous lidocaine on the acute phase of postural compensation, we subjected 13 pigmented guinea pigs to unilateral labyrinthectomy. Seven received intravenous lidocaine (4 mg/kg) immediately after labyrinthectomy. The other six served as controls and received an equivalent-volume injection of normal saline solution. Total body curvature, trunk curvature, yaw head tilt, and roll head tilt were measured at frequent intervals for up to 30 hours after surgery. Both groups had immediate difficulties with posturing that gradually improved. The lidocaine group tended to exhibit delayed postural compensation, but this was only statistically significant for roll head tilt. These results do not show improvement in postural compensation from unilateral labyrinthectomy after the administration of intravenous lidocaine. A species-specific effect on the vestibular pathways is suggested, and we conclude that further evaluation of lidocaine and the vestibular system is warranted.

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.

Dissociations between behavioural recovery and restoration of vestibular activity in the unilabyrinthectomized guinea-pig

1. In the guinea-pig, a unilateral labyrinthectomy induces postural disturbances and an ocular nystagmus which abate or disappear over time. These behavioural changes are accompanied by an initial collapse and a subsequent restoration of the spontaneous activity in the neurones of the ipsilateral vestibular nuclei. Recently, it has been shown that the vestibular neuronal activity remained collapsed over at least 10 h whereas its restoration was complete 1 week after the lesion. The aims of this study were to determine when restoration of spontaneous activity in the partially deafferented vestibular neurones started and to compare the time courses of the behavioural and neuronal recoveries in guinea-pigs that had undergone a unilateral labyrinthectomy. 2. Neuronal discharge measurements were made using chronic extracellular recording of single unit activity. After a left labyrinthectomy, electrodes, were placed on the site of the destroyed labyrinth to enable stimulation of the left vestibular nerve. Behavioural measurements included chronic recording of eye movements by the scleral search coli technique. After a left labyrinthectomy, lateral deviation of the head, twisting of the head, and eye velocity of the slow phases of the nystagmus were measured. 3. The neuronal activity of the rostral part of the vestibular nuclear complex on the lesioned side was recorded in alert guinea-pigs over 4 h recording sessions between 12 and 72 h after the lesion. 4. The criterion used to select vestibular neurones for analysis was their recruitment by an electric shock on the vestibular nerve. In addition, in order to explore a uniform population, we focused on neurones recruited at monosynaptic latencies (0.85-1.15 ms). 5. For each recording period, the mean resting rate was calculated animal by animal and the grand mean of these individual resting rate means was calculated. Previously, a decline in the grand mean resting rate from 35.8 +/- 6.0 spikes s-1 (control state) to 7.1 +/- 4.2 spikes s-1 during the first 4 h after labyrinthectomy has been shown. In the present study, the first sign of recovery was observed during the 12-16 h recording period when the resting rate grand mean increased to 16.3 +/- 3.9 spikes s-1. This grand mean activity did not change significantly during the following 12 h. Thereafter, restoration of neuronal activity improved and was complete 1 week after the lesion. 6. Although the abatement of the vestibular symptoms roughly paralleled the restoration of neuronal activity in the vestibular nuclei, some discrepancies between the time courses of both phenomena emerged. An important step in postural recovery (the animals managed to stand up) and a major part of the abatement of the nystagmus occurred before the recovery of vestibular neuronal activity. In addition, lateral deviation of the head disappeared while restoration of the neuronal activity was incomplete, but significant head twisting was still evident when vestibular resting rates had recovered completely. 7. We conclude that restoration of neuronal activity in the ipsilateral vestibular nuclei starts 12 h after the lesion and that restoration of neuronal activity in the ipsilateral vestibular nuclei is not the only mechanism underlying behavioural vestibular compensation.

Effect of sensory deprivation or electrical stimulation on acute vestibular symptoms following unilateral labyrinthectomy in rabbit

The effect of sensory deprivation or electrical stimulation on vestibular compensation was investigated for 7 days after unilateral labyrinthectomy (ULX) in 50 rabbits. Eye movements induced by sinusoidal rotation of the whole body and spontaneous nystagmus were measured for vestibulo-ocular compensation, and head deviation was measured for vestibulospinal compensation. The rabbits were divided into 4 groups: one with ULX only (LX only), ULX with bilateral tarsorrhaphy (LX+TX), ULX with cervical dorsal root ganglionectomy (LX+GX), and a group stimulated electrically on the lesion-sided vestibular system with square pulse for 8 h/day (LX+ES). In LX only, the frequency of spontaneous nystagmus was 2.93 +/- 0.19 beats/sec just after ULX, declining to zero in 4 days. In eye movements induced by sinusoidal rotation on the 7th day after ULX, directional preponderance was 24-43% and gain decreased to 32-48% by rotation toward the intact side and 17-29% by rotation toward the lesion side, compared with that before ULX. Roll head tilt and yaw head tilt were 82 +/- 9 degrees, 45 +/- 6 degrees, respectively, on the 7th day. Recovery of vestibulo-ocular reflex and head deviation was delayed in LX+TX and LX+GX compared with that in ULX only, but recovery was enhanced in LX+ES. Therefore, in this study, electrical stimulation of the lesion side seemed to have a favorable effect on suppression of acute vestibular symptoms induced by unilateral vestibular lesion.