Neuronal response sensitivity to bidirectional off-vertical axis rotations: a dimension of imbalance in the bilateral vestibular nuclei of cats after unilateral labyrinthectomy

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.

Responses of non-eye movement central vestibular neurons to sinusoidal horizontal translation in compensated macaques after unilateral labyrinthectomy

After vestibular labyrinth injury, behavioral deficits partially recover through the process of vestibular compensation. The present study was performed to improve our understanding of the physiology of the macaque vestibular system in the compensated state (>7 wk) after unilateral labyrinthectomy (UL). Three groups of vestibular nucleus neurons were included: pre-UL control neurons, neurons ipsilateral to the lesion, and neurons contralateral to the lesion. The firing responses of neurons sensitive to linear acceleration in the horizontal plane were recorded during sinusoidal horizontal translation directed along six different orientations (30° apart) at 0.5 Hz and 0.2 g peak acceleration (196 cm/s(2)). This data defined the vector of best response for each neuron in the horizontal plane, along which sensitivity, symmetry, detection threshold, and variability of firing were determined. Additionally, the responses of the same cells to translation over a series of frequencies (0.25-5.0 Hz) either in the interaural or naso-occipital orientation were obtained to define the frequency response characteristics in each group. We found a decrease in sensitivity, increase in threshold, and alteration in orientation of best responses in the vestibular nuclei after UL. Additionally, the phase relationship of the best neural response to translational stimulation changed with UL. The symmetry of individual neuron responses in the excitatory and inhibitory directions was unchanged by UL. Bilateral central utricular neurons still demonstrated two-dimension tuning after UL, consistent with spatio-temporal convergence from a single vestibular end-organ. These neuronal data correlate with known behavioral deficits after unilateral vestibular compromise.

Long-term deficits in motion detection thresholds and spike count variability after unilateral vestibular lesion

The vestibular system operates in a push-pull fashion using signals from both labyrinths and an intricate bilateral organization. Unilateral vestibular lesions cause well-characterized motor deficits that are partially compensated over time and whose neural correlates have been traced in the mean response modulation of vestibular nuclei cells. Here we compare both response gains and neural detection thresholds of vestibular nuclei and semicircular canal afferent neurons in intact vs. unilateral-lesioned macaques using three-dimensional rotation and translation stimuli. We found increased stimulus-driven spike count variability and detection thresholds in semicircular canal afferents, although mean responses were unchanged, after contralateral labyrinth lesion. Analysis of trial-by-trial spike count correlations of a limited number of simultaneously recorded pairs of canal afferents suggests increased noise correlations after lesion. In addition, we also found persistent, chronic deficits in rotation detection thresholds of vestibular nuclei neurons, which were larger in the ipsilesional than the contralesional brain stem. These deficits, which persisted several months after lesion, were due to lower rotational response gains, whereas spike count variability was similar in intact and lesioned animals. In contrast to persistent deficits in rotation threshold, translation detection thresholds were not different from those in intact animals. These findings suggest that, after compensation, a single labyrinth is sufficient to recover motion sensitivity and normal thresholds for the otolith, but not the semicircular canal, system.

Reduced choice-related activity and correlated noise accompany perceptual deficits following unilateral vestibular lesion

Signals from the bilateral vestibular labyrinths work in tandem to generate robust estimates of our motion and orientation in the world. The relative contributions of each labyrinth to behavior, as well as how the brain recovers after unilateral peripheral damage, have been characterized for motor reflexes, but never for perceptual functions. Here we measure perceptual deficits in a heading discrimination task following surgical ablation of the neurosensory epithelium in one labyrinth. We found large increases in heading discrimination thresholds and large perceptual biases at 1 wk postlesion. Repeated testing thereafter improved heading perception, but vestibular discrimination thresholds remained elevated 3 mo postlesion. Electrophysiological recordings from the contralateral vestibular and cerebellar nuclei revealed elevated neuronal discrimination thresholds, elevated neurometric-to-psychometric threshold ratios, and reduced trial-by-trial correlations with perceptual decisions ["choice probabilities" (CPs)]. The relationship between CP and neuronal threshold was shallower, but not significantly altered, suggesting that smaller CPs in lesioned animals could be largely attributable to greater neuronal thresholds. Simultaneous recordings from pairs of neurons revealed that correlated noise among neurons was also reduced following the lesion. Simulations of a simple pooling model, which takes into account the observed changes in tuning slope and correlated noise, qualitatively accounts for the elevated psychophysical thresholds and neurometric-to-psychometric ratios, as well as the decreased CPs. Thus, cross-labyrinthine interactions appear to play important roles in enhancing neuronal and perceptual sensitivity, strengthening interneuronal correlations, and facilitating correlations between neural activity and perceptual decisions.

Frequency-dependent spatiotemporal tuning properties of non-eye movement related vestibular neurons to three-dimensional translations in squirrel monkeys

Responses of vestibular-only translation sensitive (VOTS) neurons in vestibular nuclei of two squirrel monkeys were studied at multiple frequencies to three-dimensional translations and rotations. A novel frequency-dependent spatiotemporal analysis examined in each neuron whether complex models, with unrestricted response dynamics in three-dimensional (3D) space, provided significantly better fits than restricted models following simple, cosine rule. Subsequently, the statistically selected optimal model was used to predict the maximum translation direction, expressed as a unitary vector, Vt(max), and its associated sensitivity and phase across frequencies. Simple models were sufficient to quantify the 3D translational responses of 66% of neurons. Most VOTS neurons, complex or simple, exhibited flat-gain or low-pass response dynamics. The Vt(max) of simple neurons was fixed, whereas that of complex neurons changed with frequency. The spatial distribution of Vt(max) in simple neurons, which fell within 30 degrees of either the horizontal plane or/and the sagittal plane, was closely aligned with Vt(max) of vestibular afferents. In contrast, the frequency-dependent Vt(max) of most complex neurons migrated from the dorsoventral axis at higher frequency toward the horizontal plane, especially the interaural axis, at lower frequency. When the maximum rotation direction was estimated from responses of the same VOTS neurons to 1.2 Hz yaw, pitch, and roll rotations, complex neurons were more likely to respond to rotations activating vertical canals. Responses to 0.15-0.3 Hz linear accelerations produced by inertial or gravitational forces were indistinguishable in most complex neurons but significantly different in most simple neurons. These observations suggest that simple and complex VOTS neurons constitute distinctive vestibular pathways where complex neurons, exhibiting a novel spatiotemporal filtering mechanism in processing otolith-related signals, are well suited to drive tilt-related responses, whereas simple neurons probably mediate pure translation related responses.

Developmental expression of NMDA and AMPA receptor subunits in vestibular nuclear neurons that encode gravity-related horizontal orientations

We examined the expression profile of subunits of ionotropic glutamate receptors [N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole-proprionate (AMPA)] during postnatal development of connectivity in the rat vestibular nucleus. Vestibular nuclear neurons were functionally activated by constant velocity off-vertical axis rotation, a strategy to stimulate otolith organs in the inner ear. These neurons indicated Fos expression as a result. By immunodetection for Fos, otolith-related neurons that expressed NMDA/AMPA receptor subunits were identified as early as P7, and these neurons were found to increase progressively up to adulthood. Although there was developmental invariance in the percentage of Fos-immunoreactive neurons expressing the NR1, NR2A, GluR1, or GluR2/3 subunits, those expressing the NR2B subunit decreased from P14 onward, and those expressing the GluR4 subunit decreased in adults. These double-immunohistochemical data were corroborated by combined immuno-/hybridization histochemical data obtained from Fos-immunoreactive neurons expressing NR2B mRNA or GluR4 mRNA. The staining of both NR2B and GluR4 in the cytoplasm of these neurons decreased upon maturation. The percentage of Fos-immunoreactive neurons expressing the other ionotropic glutamate receptor subunits (viz. NR1, NR2A, GluR1, and GluR2/3) remained relatively constant throughout postnatal maturation. Triple immunofluorescence further demonstrated coexpression of NR1 and NR2 subunits in Fos-immunoreactive neurons. Coexpression of NR1 subunit with each of the GluR subunits was also observed among the Fos-immunoreactive neurons. Taken together, the different expression profiles of ionotropic glutamate receptor subunits constitute the histological basis for glutamatergic neurotransmission in the maturation of central vestibular connectivity for the coding of gravity-related horizontal head movements.

Spatial coding capacity of central otolith neurons

This review focuses on recent approaches to unravel the capacity of otolith-related brainstem neurons for coding head orientations. In the first section, the spatiotemporal features of central vestibular neurons in response to natural otolithic stimulation are reviewed. Experiments that reveal convergent inputs from bilateral vestibular end organs bear important implications on the processing of spatiotemporal signals and integration of head orientational signals within central otolith neurons. Another section covers the maturation profile of central otolith neurons in the recognition of spatial information. Postnatal changes in the distribution pattern of neuronal subpopulations that subserve the horizontal and vertical otolith systems are highlighted. Lastly, the expression pattern of glutamate receptor subunits and neurotrophin receptors in otolith-related neurons within the vestibular nuclear complex are reviewed in relation to the potential roles of these receptors in the development of vestibular function.

Expression of Trk receptors in otolith-related neurons in the vestibular nucleus of rats

The expression of the three Trk receptors (TrkA, TrkB, and TrkC) in otolith-related neurons within the vestibular nuclei of adult Sprague-Dawley rats was examined immunohistochemically. Conscious animals were subjected to sinusoidal linear acceleration along either the anterior-posterior (AP) or interaural (IA) axis on the horizontal plane. Neuronal activation was defined by Fos expression in cell nuclei. Control animals, viz labyrinthectomized rats subjected to stimulation and normal rats that remained stationary, showed only a few sporadically scattered Fos-labeled neurons. Among experimental rats, the number of Fos-labeled neurons and their distribution pattern in each vestibular subnucleus in animals stimulated along the antero-posterior axis were similar to those along the interaural axis. No apparent topography was observed among neurons activated along these two directions. Only about one-third of the Trk-immunoreactive neurons in the vestibular nucleus expressed Fos. Double-labeled Fos/TrkA, Fos/TrkB and Fos/TrkC neurons constituted 85-98% of the total number of Fos-labeled neurons in vestibular nuclear complex and its subgroups x and y. Our findings suggest that Trk receptors and their cognate neurotrophins in central otolith neurons may contribute to the modulation of gravity-related spatial information during horizontal head movements.

Three dimensional spatial-temporal convergence of otolith related signals in vestibular only neurons in squirrel monkeys

Spatio-temporal convergence (STC) properties of 19 vestibular only neurons' responses to translational head movements were examined in an alert, behaving squirrel monkey. In addition to standard tests that included 1.2 Hz yaw/pitch/roll rotations and inter-aural/naso-occipital/dorso-ventral translations, we also observed responses to translations along multiple directions in several orthogonal planes. Neural responses were fitted first by a model that permitted STC in all planes, characterized by a non-zero minimum and a gradual shift of phase. We then evaluated statistically whether models with fewer independent dynamic parameters yielded equally satisfactory results. The responses of 13 neurons were adequately fit by simple cosine models (1-D) as well as models that allowed complex STC behavior. Of the six neurons exhibiting STC, five could be modeled with two independent phase parameters (2-D) while the remaining neuron required a model with three independent phase parameters (3-D). The maximum translation sensitivity and phase, Smax and varphimax, and minimum translation sensitivity and phase, Smin and varphimin, were estimated from the reconstructed sensitivity and phase surfaces. The tuning ratio, Smin/Smax, in STC neurons was>0.40 while in 1-D neurons it was <0.25. Furthermore, the maximum response vectors of most 1-D neurons lay within 20 degrees of either the horizontal or sagittal plane while those of STC neurons lay >20 degrees from both planes. No difference in other response properties, such as varphimax or rotational responses, was found between neurons exhibiting STC and the simple cosine tuning. Our results suggest that the STC behavior observed in otolith-related vestibular neurons probably arises from summing inputs from afferents, with diverse response dynamics, innervating different otolith macula.

Fos expression in otolith-related brainstem neurons of postnatal rats following off-vertical axis rotation

To determine the critical time of responsiveness of developing otolith organ-related brainstem neurons and their distribution, Fos protein expression in response to off-vertical axis rotations (OVAR) was mapped in conscious Sprague Dawley rats from P5 to adulthood. OVAR was used to activate sequentially all utricular hair cells per 360 degrees revolution. We detected the coding of horizontal head positions in otolith organ-related neurons within the vestibular nucleus as early as P7. In the vestibular nuclear complex and its subgroups, the density of Fos-immunoreactive (Fos-ir) neurons increased steadily with age and reached the adult level by P21. In both labyrinthectomized rats subjected to OVAR and normal rats kept stationary, labeled neurons were found sporadically in the aforementioned brain regions in each age group, confirming that Fos labeling observed in neurons of normal experimental rats subjected to OVAR was due to otolith organ stimulation. Whereas OVAR-induced Fos-ir neurons were also first observed in vestibular-related brain areas, such as the prepositus hypoglossal nucleus, gigantocellular reticular nucleus, and locus coeruleus, of normal experimental rats at P7, those in the inferior olive were observed only from P14 onward. This indicates the unique maturation time of inferior olivary neurons in gravity-related spatial coding. In general, age-dependent increase in OVAR-induced Fos-ir neurons was observed in brain areas that received otolith inputs. The locus coeruleus was exceptional in that prominent OVAR-induced Fos-ir neuronal number did not change with maturation, and this was well above the low but significant number of Fos-ir neurons in control preparations. Taken together, our results suggest that neuronal subpopulations within the developing network of the horizontal otolith system provide an anatomical basis for the postnatal development of otolith organ-related sensorimotor functions. J. Comp. Neurol. 470:282-296, 2004.

Response properties of Y group neurons to crossed otolith inputs in the cat

The response properties of extracellularly recorded Y group neurons on the lesioned side were examined in decerebrate cats after acute hemilabyrinthectomy, with the use of constant velocity off-vertical axis rotations (OVAR) to stimulate the remaining intact otolith receptors. During rotation in the clockwise or counterclockwise direction, Y group neurons displayed a spectrum of position-dependent bidirectional response sensitivities, ranging from one- to two-dimensional. Some two-dimensional neurons even exhibited unidirectional responses with change in OVAR velocity. These findings indicate that Y group neurons have the capacity to code spatiotemporal signals arising from the contralateral otolith. The best response orientations of one-dimensional and two-dimensional neurons were found predominantly along the antero-posterior direction, thus providing a spatial framework for the otolithic reflexes.

Modifications of vestibular responses of individual reticulospinal neurons in lamprey caused by unilateral labyrinthectomy

A postural control system in the lamprey is driven by vestibular input and maintains the dorsal-side-up orientation of the animal during swimming. After a unilateral labyrinthectomy (UL), the lamprey continuously rolls toward the damaged side. Normally, a recovery of postural equilibrium ("vestibular compensation") takes about 1 mo. However, illumination of the eye contralateral to UL results in an immediate and reversible restoration of equilibrium. Here we used eye illumination as a tool to examine a functional recovery of the postural network. Important elements of this network are the reticulospinal (RS) neurons, which are driven by vestibular input and transmit commands for postural corrections to the spinal cord. In this study, we characterized modifications of the vestibular responses in individual RS neurons caused by UL and the effect exerted on these responses by eye illumination. The activity of RS neurons was recorded from their axons in the spinal cord by chronically implanted electrodes, and spikes in individual axons were extracted from the population activity signals. The same neurons were recorded both before and after UL. Vestibular stimulation (rotation in the roll plane through 360 degrees ) and eye illumination were performed in quiescent animals. It was found that the vestibular responses on the UL-side changed only slightly, whereas the responses on the opposite side disappeared almost completely. This asymmetry in the bilateral activity of RS neurons is the most likely cause for the loss of equilibrium in UL animals. Illumination of the eye contralateral to UL resulted, first, in a restoration of vestibular responses in the neurons inactivated by UL and in an appearance of vestibular responses in some other neurons that did not respond to vestibular input before UL. These responses had directional sensitivity and zones of spatial sensitivity similar to those observed before UL. However, their magnitude was smaller than before UL. Second, the eye illumination caused a reduction of the magnitude of vestibular responses on the UL side. These two factors tend to restore symmetry in bilateral activity of RS neurons, which is the most likely cause for the recovery of equilibrium in the swimming UL lamprey. Results of this study are discussed in relation to the model of the roll control system proposed in our previous studies as well as in relation to the vestibular compensation.

Spontaneous discharge and response characteristics of central otolith neurons of rats during postnatal development

To study the developmental profile of otolith-related vestibular nuclear neurons, their spontaneous activities and response dynamics were examined in decerebrate rats aged seven, 14, 21 and 84 (adult) days. Extracellular recordings were performed in the lateral and descending vestibular nucleus of animals held at the stationary position in the earth-horizontal or subjected to constant velocity off-vertical axis rotation, which selectively stimulates the otolith receptors. All neurons displayed sinusoidal position-dependent modulation in discharge rate, indicating their capability in coding spatial information during low-frequency head movement. Some neurons showed a full-cycle response to off-vertical axis rotation (non-clipped), while other neurons were silenced in discharge during parts of each rotary cycle (clipped). In seven-day-old rats, three-quarters of the responsive neurons sampled were clipped and the proportion progressively decreased to less than one-quarter in adult rats. In each age group, the clipped neurons discharged in approximately 60% of the stimulus cycle. Response gains of the neurons increased with age, reaching a plateau from 21 days of age for clipped neurons and 14 days for non-clipped neurons. The clipped neurons demonstrated higher response gains than the non-clipped neurons at or beyond 21 days of age. Spontaneous activities of the neurons at the stationary and earth-horizontal positions were analysed in relation to their response gains; a positive correlation was observed from 14 days of age onwards. Both types of neurons showed progressive increase in spontaneous activity as the rats matured, though the clipped neurons exhibited significantly lower resting rates than the non-clipped neurons at each of the age groups studied. Some neurons that responded to off-vertical axis rotation were not spontaneously active at the stationary position, but the proportion of these decreased significantly with age. The coefficient of variation of each age group showed a bimodal distribution, thereby allowing spontaneously active neurons to be assigned as regular or irregular. Though the vast majority of both the clipped and non-clipped neurons showed irregular discharge patterns at seven days of age, the overall population became more regular as the rats matured. Irregular neurons of young rats exhibited phase-stable and phase-shift responses, while those of older rats showed only the phase-stable response. This distinction was not observed amongst regular neurons over the ages studied. Our results reveal features of central otolith neurons that can be taken as signs of maturation during the first three postnatal weeks. These neuronal features provide the framework for the analysis of behaviours mediated by the otolith system during postnatal maturation.