Primary Vestibular Afferents Research Articles

Latency and Initiation of the Human Vestibuloocular Reflex to Pulsed Galvanic Stimulation

Cathodal galvanic currents activate primary vestibular afferents, whereas anodal currents inhibit them. Pulsed galvanic vestibular stimulation (GVS) was used to determine the latency and initiation of the human vestibuloocular reflex. Three-dimensional galvanic vestibuloocular reflex (g-VOR) was recorded with binocular dual-search coils in response to a bilateral bipolar 100-ms rectangular pulse of current at 0.9 (near-threshold), 2.5, 5.0, 7.5, and 10.0 mA in 11 normal subjects. The g-VOR consisted of three components: conjugate torsional eye rotation away from cathode toward anode; vertical divergence (skew deviation) with hypertropia of the eye on the cathodal and hypotropia of the eye on the anodal sides; and conjugate horizontal eye rotation away from cathode toward anode. The g-VOR was repeatable across all subjects, its magnitude a linear function of the current intensity, its latency about 9.0 ms with GVS of >or=2.5 mA, and was not suppressed by visual fixation. At 10-mA stimulation, the g-VOR [x, y, z] on the cathodal side was [0.77 +/- 0.10, -0.05 +/- 0.05, -0.18 +/- 0.06 degrees ] (mean +/- 95% confidence intervals) and on the anodal side was [0.79 +/- 0.10, 0.16 +/- 0.05, -0.19 +/- 0.06 degrees ], with a vertical divergence of 0.20 degrees . Although the horizontal g-VOR could have arisen from activation of the horizontal semicircular canal afferents, the vertical-torsional g-VOR resembled the vestibuloocular reflex in response to roll-plane head rotation about an Earth-horizontal axis and might be a result of both vertical semicircular canal and otolith afferent activations. Pulsed GVS is a promising technique to investigate latency and initiation of the human vestibuloocular reflex because it does not require a large mechanical apparatus nor does it pose problems of head inertia or slippage.

Muscarinic ACh Receptor Activation Causes Transmitter Release From Isolated Frog Vestibular Hair Cells

In the frog, vestibular efferent fibers innervate only type-II vestibular hair cells. Through this direct contact with hair cells, efferent neurons are capable of modifying transmitter release from hair cells onto primary vestibular afferents. The major efferent transmitter, acetylcholine (ACh), is known to produce distinct pharmacological actions involving several ACh receptors. Previous studies have implicated the presence of muscarinic ACh receptors on vestibular hair cells, although, surprisingly, a muscarinic-mediated electrical response has not been demonstrated in solitary vestibular hair cells. This study demonstrates that muscarinic receptors can evoke transmitter release from vestibular hair cells. Detection of this release was obtained through patch-clamp recordings from catfish cone horizontal cells, serving as glutamate detectors after pairing them with isolated frog semicircular canal hair cells in a two-cell preparation. Although horizontal cells alone failed to respond to carbachol, application of 20 microM carbachol to the two-cell preparation resulted in a horizontal cell response that could be mimicked by exogenous application of glutamate. All of the horizontal cells in the two-cell preparation responded to 20 microM CCh. Furthermore, this presumed transmitter release persisted in the presence of d-tubocurarine at concentrations that block all known hair cell nicotinic ACh receptors. The effect on the detector cell, imparted by the carbachol application to the hair cell-horizontal cell preparation, was blocked both by 2-amino-5-phosphonopentanoic acid, a selective N-methyl-D-aspartate antagonist, and the muscarinic antagonist, atropine. Thus vestibular hair cells from the frog semicircular canal can be stimulated to release transmitter by activating their muscarinic receptors.

Vestibular afferent innervation in the vestibular efferent nucleus of rats

To delineate the vestibular afferent innervation in the vestibular efferent nucleus in the brainstem, neurobiotin or biotinylated dextran amine was injected into the superior Scarpa's ganglion of Sprague–Dawley rats. The locations of vestibular efferent neurons in the brainstem were identified by neutral red or choline acetyltransferase staining. Of the three pairs of vestibular efferent nuclei, labeled fibers and bouton-like endings were found only within the dorsolateral vestibular efferent nucleus on the ipsilateral side. Labeled afferent terminals with bouton-like varicosities were observed in the vicinity of cell bodies or dendrites of these efferent neurons. Our findings suggest that vestibular primary afferents may exert direct influence on vestibular efferent neurons, constituting an ipsilateral close-loop arrangement in the central vestibular system.

Vestibular primary afferent activity in an in vitro preparation of the mouse inner ear

Most information on the properties of mammalian vestibular primary afferents has been obtained in deeply anesthetized animals, in vivo. Generally, non-human primates and larger rodents have been the species of choice. Investigations using smaller rodents, such as the laboratory mouse, have been limited despite the increasing availability of naturally occurring or engineered mutants that result in balance disorders. Furthermore, in vitro preparations of the intact peripheral vestibular apparatus are only available for non-mammalian vertebrates. To take advantage of the genetic/molecular advances available in mice and the utility of in vitro preparations that permit manipulations of the extracellular milieu, we developed an isolated mouse inner ear preparation with the attached eighth cranial nerve for electrophysiological recording. Intra-axonal recordings of background activity in vestibular primary afferents were obtained in a modified Ringer's solution (0.25 mM Ca 2+; 3.25 mM Mg 2+) at 22 °C. We also recorded afferent activity in the presence of neuroactive drugs known to affect various stages of the transduction cascade. These results, together with responses to sinusoidal mechanical deformation of the membranous ducts, showed that transduction mechanisms remain viable. Where possible, we also obtained results in vivo for comparison. In future, the in vitro mouse preparation will allow investigation of the effects of genetic manipulations and pharmacological agents on the intact peripheral vestibular apparatus.

Responses of primary vestibular neurons to galvanic vestibular stimulation (GVS) in the anaesthetised guinea pig

Previous studies in humans and animals which have shown that DC galvanic vestibular stimulation (GVS) induces horizontal and torsional eye movements have been interpreted as being due to a preferential activation of primary vestibular afferents innervating the horizontal semicircular canals and otoliths by GVS. The present study sought to determine in guinea pigs whether GVS does indeed selectively activate primary horizontal canal and otolith afferents. Constant-current GVS was passed between electrodes implanted in the tensor-tympani muscle of each middle ear or between electrodes on the skin over the mastoid. During this stimulation, responses from single primary vestibular neurons were recorded extracellularly by glass microelectrodes in Scarpa's ganglion. Afferents from all vestibular sensory regions were activated by both surface and tensor-tympani galvanic stimulation. Tensor tympani GVS was approximately 10 times more effective than surface GVS. At larger current intensities irregularly discharging afferents showed an asymmetrical response: cathodal stimulation resulted in a larger change in firing (increase) than anodal stimulation (decrease), whereas regularly discharging afferents responded symmetrically to the two polarities of GVS. Across all afferents tuned for different types of natural vestibular stimulation, neuronal sensitivity for GVS was found to increase with discharge variability (as indexed by CV*). Anterior canal afferents showed a slightly higher sensitivity than afferents from other vestibular sensory regions. Hence, the present study concluded that GVS activates primary vestibular afferents innervating all sensory regions in a uniform fashion. Therefore, the specific pattern of GVS-induced eye movements reported in previous studies are not due to differential sensitivity between different vestibular sensory regions, but are likely to reflect an involvement of central processing.

Vestibulo-reticular projections in adult lamprey: Their role in locomotion

This study describes the anatomical projections from vestibular secondary neurons to reticulospinal neurons in the adult lamprey and the modulation of vestibular inputs during fictive locomotion. Anatomical tracers were applied in the posterior (PRRN) and middle rhombencephalic reticular nuclei as well as to the proximal stumps of cut vestibular nerve branches to identify the neurons projecting to the reticular nuclei that were in close proximity with vestibular primary afferents. Labeled neurons were found in the intermediate (ION) and posterior (PON) octavomotor nuclei, and were more numerous on the side of the injection (around 56–87 and 101–107 for the ION and the PON, respectively). Morphologies varied but cells were mostly round or oval. Axonal projections from the PON formed a dense bundle, whereas those from the ION were less densely packed. Based on their morphology and the distribution of their projections, most vestibulo-reticular neurons were presumed to be vestibulospinal cells. Reticulospinal cells from the PRRN were recorded intracellularly in the in vitro brainstem–spinal cord preparation and large excitatory post-synaptic potentials (EPSPs) were evoked following stimulation of the ipsilateral anterior and the contralateral posterior branches of the vestibular nerves, whereas inhibitory post-synaptic potentials (IPSPs) or smaller EPSPs were elicited by stimulation of the ipsilateral posterior or of the contralateral anterior branches. During fictive locomotion, both the excitatory and the inhibitory responses displayed phasic changes in amplitude such that the amplitude of the EPSPs was minimal when the spinal cord activity switched from the ipsilateral to the contralateral side of the recorded reticulospinal cell. The IPSPs were then of maximal amplitude. We propose that this modulation could serve to reduce the influence of vestibular inputs in response to head movements during locomotion.

Developmental shift from long-term depression to long-term potentiation in the rat medial vestibular nuclei: role of group I metabotropic glutamate receptors.

The effects of high frequency stimulation (HFS) of the primary vestibular afferents on synaptic transmission in the ventral part of the medial vestibular nuclei (vMVN) were studied during postnatal development and compared with the changes in the expression of the group I metabotropic glutamate receptor (mGluR) subtypes, mGluR1 and mGluR5. During the first stages of development, HFS always induced a mGluR5- and GABAA-dependent long-term depression (LTD) which did not require NMDA receptor and mGluR1 activation. The probability of inducing LTD decreased progressively throughout the development and it was zero at about the end of the second postnatal week. Conversely, long-term potentiation (LTP) appeared at the beginning of the second week and its occurrence increased to reach the adult value at the end of the third week. Of interest, the sudden change in the LTP frequency occurred at the time of eye opening, about the end of the second postnatal week. LTP depended on NMDA receptor and mGluR1 activation. In parallel with the modifications in synaptic plasticity, we observed that the expression patterns and localizations of mGluR5 and mGluR1 in the medial vestibular nuclei (MVN) changed during postnatal development. At the earlier stages the mGluR1 expression was minimal, then increased progressively. In contrast, mGluR5 expression was initially high, then decreased. While mGluR1 was exclusively localized in neuronal compartments and concentrated at the postsynaptic sites at all stages observed, mGluR5 was found mainly in neuronal compartments at immature stages, then preferentially in glial compartments at mature stages. These results provide the first evidence for a progressive change from LTD to LTP accompanied by a distinct maturation expression of mGluR1 and mGluR5 during the development of the MVN.

Cerebellar Climbing Fibers Modulate Simple Spikes in Purkinje Cells

Purkinje cells have two action potentials: Climbing fiber responses (CFRs) and simple spikes (SSs). CFRs reflect the discharge of a single climbing fiber at multiple synaptic sites on the proximal dendrite of the Purkinje cell. SSs reflect the summed action of a subset of parallel fiber synapses on Purkinje cell dendritic spines. Because mossy fiber afferents terminate on granule cells, the ascending axons of which bifurcate, giving rise to parallel fibers, the modulation of SSs has been attributed to mossy fiber afferent signals. This inference has never been tested. Conversely, the low discharge frequency of CFRs has led many to conclude that they have a unique and intermittent role in cerebellar signal processing. We examine the relative potency of vestibularly modulated mossy fiber and climbing fiber signals in evoking CFRs and SSs in Purkinje cells of the uvula-nodulus in chloralose-urethane-anesthetized rabbits. Vestibular primary afferents were blocked by unilateral labyrinthectomy (UL). A UL destroys the vestibular primary afferent signal to the ipsilateral uvula-nodulus, while leaving intact the vestibular climbing fiber signal from the contralateral inferior olive. After UL, vestibular stimulation modulated CFRs and SSs in ipsilateral uvula-nodular Purkinje cells, demonstrating that the primary vestibular afferent mossy fiber input to the ipsilateral uvula-nodulus was not necessary for SS modulation. Unilateral microlesions of the caudal half of the beta-nucleus of the inferior olive reduced a modulated climbing fiber signal to the contralateral uvula-nodulus, causing loss of both vestibularly modulated CFRs and SSs in contralateral Purkinje cells. Vestibular climbing fibers not only evoke low-frequency CFRs, but also indirectly modulate higher-frequency SSs. This modulation must be attributed to cerebellar interneurons. Golgi cell inhibition of granule cells may provide the interneuronal mechanism for CFR-induced SS modulation.

Vesicular glutamate transporters VGLUT1 and VGLUT2 define two subsets of unipolar brush cells in organotypic cultures of mouse vestibulocerebellum

Different isoforms of a vesicular glutamate transporter (VGLUT) mediate glutamate uptake into synaptic vesicles of excitatory neurons. There is agreement that the VGLUTs are differentially expressed in brain, and that two isoforms, VGLUT1 and VGLUT2, are localized to excitatory axon terminals in the cerebellar cortex. While granule cells express solely VGLUT1, there is no report about the VGLUT(s) of the unipolar brush cell (UBC), the second type of glutamatergic interneuron residing in the cerebellar granular layer. In the mouse, UBCs are particularly numerous in the uvula (lobule IX) and nodulus (lobule X). These folia contain two distinct subsets of UBCs: one kind expresses the calcium-binding protein calretinin (CR), and the other kind expresses the metabotropic glutamate receptor (mGluR) 1α. UBCs give rise to an extensive system of intrinsic mossy fibers (MF), whose terminals innervate granule cells and other UBCs, altogether similar to those formed by the extrinsic MFs. The presence of both extrinsic and intrinsic MFs in the vestibulocerebellum makes it difficult to determine which type of VGLUT is contained in MFs formed by the UBC axons. Hence, the nodulus was isolated from sagittal cerebellar slices from postnatal day 10 mice, and cultured for 15–20 days in vitro. Double immunofluorescence and confocal microscopy showed that mossy terminals of CR-positive (CR +) UBCs were immunoreactive for VGLUT1 and VGLUT2, while mossy terminals of mGluR1α-positive (mGluR1α +) UBCs were provided with VGLUT1 only. Moreover, CR + dendritic brushes were contacted by mossy terminals provided with both transporters, while mGluR1α + dendritic brushes were contacted by mossy terminals immunopositive for VGLUT1 and immunonegative for VGLUT2. These data indicate that the two UBC subsets use different modalities of vesicular glutamate storage and form separate networks. We consider it possible that expressions of CR with VGLUT1/VGLUT2 and mGluR1α + with VGLUT1 in the two subsets of vestibulocerebellar UBCs are determined by specific vestibular inputs, carried by groups of primary and/or secondary vestibular afferents.

Vestibularly evoked climbing-fiber responses modulate simple spikes in rabbit cerebellar Purkinje neurons.

The nodulus receives a primary vestibular afferent input from the ipsilateral labyrinth and a vestibularly related climbing-fiber input originating from the contralateral labyrinth. Previously we demonstrated that increased discharge of vestibularly evoked climbing-fiber responses (CFRs) in nodular Purkinje cells was correlated with decreased discharge of simple spikes (SSs). This left unresolved the question of whether vestibularly evoked antiphasic behavior of CFRs and SSs reflects a common neural mechanism or the activation of two separate parallel pathways. We answered this question using natural vestibular stimulation to modulate the discharge of uvula-nodular Purkinje cells recorded extracellularly in unilaterally labyrinthectomized, chloralose urethane-anesthetized rabbits. In such animals, vestibular primary afferents projecting to the uvula-nodulus as mossy fibers remained intact on the side contralateral to the unilateral labyrinthectomy. The discharge of CFRs recorded in ipsilateral nodular Purkinje cells was increased by ipsilateral roll-tilt while the discharge of SSs was increased by contralateral roll-tilt. These polarities were reversed for Purkinje cells recorded in the contralateral uvula-nodulus. The polarity of SS discharge recorded from Purkinje cells on both sides of the nodulus was opposite to that of the vestibular primary mossy-fiber afferents. SSs continued to respond to contralateral roll-tilt even when the primary vestibular afferent mossy-fiber pathway was destroyed by the unilateral labyrinthectomy. Although the discharge of SSs recorded in the contralateral uvula-nodulus was increased by contralateral roll-tilt, this modulation was reduced relative to that observed in Purkinje cells recorded in the ipsilateral uvula-nodulus. We conclude that vestibularly evoked CFRs caused the modulation of SS discharge.

Vestibular projections in the human cortex.

There is considerable evidence from studies on cats and monkeys that several cortical areas such as area 2v at the tip of the intraparietal sulcus, area 3av in the sulcus centralis, the parietoinsular vestibular cortex adjacent to the posterior insula (PIVC) and area 7 in the inferior parietal lobule are involved in the processing of vestibular information. Microelectrode recordings from these areas have shown that: (1) most of these cortical neurons are connected trisynaptically to the labyrinthine endorgans and (2) they receive converging vestibular, visual and somatosensory inputs. These data suggest that a multimodal cortical system is involved in postural and gaze control. In humans, recent positron emission tomography (PET) scans and functional magnetic resonance imaging (fMRI) studies have largely confirmed these data. However, because of the limited temporal resolution of these two methods, the minimum time of arrival of labyrinthine inputs from the vestibular hair cells to these cortical areas has not yet been determined. In this study, we used the evoked potential method to attempt to answer this question. Due to its excellent temporal resolution, this method is ideal for the investigation of the tri- or polysynaptic nature of the vestibulocortical pathways. Eleven volunteer patients, who underwent a vestibular neurectomy due to intractable Meniere's disease (MD) or acoustic neurinoma resection, were included in this experiment. Patients were anesthetized and the vestibular nerve was electrically stimulated. The evoked potentials were recorded by 30 subcutaneous active electrodes located on the scalp. The brain electrical source imaging (BESA) program (version 2.0, 1995) was used to calculate dipole sources. The latency period for the activation of five distinct cortical zones, including the prefrontal and/or the frontal lobe, the ipsilateral temporoparietal cortex, the anterior portion of the supplementary motor area (SMA) and the contralateral parietal cortex, was 6 ms. The short latency period recorded for each of these areas indicates that several trisynaptic pathways, passing through the vestibular nuclei and the thalamic neurons, link the primary vestibular afferents to the cortex. We suggest that all these areas, including the prefrontal area, process egomotion information and may be involved in planning motor synergies to counteract loss of equilibrium.

Vestibulospinal reflexes: quantitative effects of sensory feedback and postural task.

Vestibulospinal and vestibulocollic reflexes evoked by galvanic stimulation were studied in 20 normal volunteers. In an initial "baseline" study, subjects stood unsupported on a flat surface and a narrow base with their eyes shut and with their heads rotated to the left. The effects of vision, external support and increasing stance width were examined both individually and in combination. In a second series, the effects of the same factors were examined while subjects stood on a compliant surface. Short latency (SL) and medium latency (ML) vestibulospinal reflex responses were evoked using 4 mA/20 ms galvanic vestibular stimulation (GVS) and measured from the right soleus muscle. Vestibulocollic reflexes to short duration galvanic stimulation (4 mA/2 ms) were also measured under similar conditions. Both SL and ML vestibulospinal reflexes decreased significantly and to a similar degree with vision, external support and increasing stance width on a flat surface. On the compliant surface, stance width did not result in a decrease in the ML reflex. Reflex amplitudes further decreased in a non-linear fashion with each additional sensory modality ("factor") that was made available; the degree of attenuation due to the addition of a second and third factor closely approximated the product of the effect of each factor in isolation. Standing on a compliant surface resulted in enhancement of average SL and ML reflexes under all conditions. Vestibulocollic reflexes in contrast showed no significant modulation with vision, external support and stance width. The dissociation between vestibulocollic and vestibulospinal reflexes indicates that the modulation of vestibulospinal reflexes with task occurs proximal to the primary vestibular afferents. Vestibulospinal reflexes were largest when subjects stood on a narrow base, on a compliant surface, deprived of vision and external support, consistent with the importance of vestibular function under these conditions. Although attenuated, vestibulospinal reflexes were preserved in most subjects even when vision and external support were available and a wider stance width was adopted. The combination of different factors caused a multiplicative attenuation of the initial response.

Cerebellar unipolar brush cells are targets of primary vestibular afferents: an experimental study in the gerbil.

The unipolar brush cell (UBC) is an excitatory glutamatergic interneuron, situated in the cerebellar granular layer, that itself receives excitatory synaptic input on its dendritic brush from a single mossy fiber terminal in the form of a giant glutamatergic synapse. The UBC axon branches within the granular layer, giving rise to large terminals that synapse with both granule cell and UBC dendrites within glomeruli and resemble in morphological and functional terms those formed by extrinsic mossy fibers. So far, the only demonstrated extrinsic afferents to the UBC are the choline acetyltransferase (ChAT)-positive mossy fibers, some of which originate from the medial and descending vestibular nuclei. To ascertain whether UBCs are innervated by primary vestibular fibers, we performed a tract-tracing light and electron microscopic study of the vestibulocerebellum in gerbils. Macular and canal vestibular end-organs were individually labeled by injection of biotinylated dextran amine. After an appropriate survival time, gerbils were then processed for light and electron microscopic analysis of central vestibular projections. In the nodulus and uvula, labeled primary vestibular fibers formed mossy terminals synapsing with both granule cells and UBCs in all of the injected gerbils. Thus, innervation of UBCs by extrinsic mossy fibers carrying static and dynamic vestibular signals represents the first synapse of networks that contribute a powerful form of distributed excitation in the granular layer.

Cellular basis of vestibular compensation: analysis and modelling of the role of the commissural inhibitory system

In this study we used a cellular network model of the brainstem vestibulo-ocular reflex (VOR) pathways to investigate the role of the vestibular commissural system in "vestibular compensation", the behavioural recovery that takes place after unilateral labyrinthectomy (UL). The network was initialized on the basis of mathematical analysis and trial simulations to generate a VOR response with a physiologically realistic gain and time constant. The effects of a selective decrease in the strength of commissural inhibitory input to the ipsi-lesional medial vestibular nucleus (MVN) neurones, without changes in other parts of the network, were investigated. Thus we simulated the marked down-regulation of GABA receptor efficacy that our recent experimental results have demonstrated in these cells after UL. The main outcome of this study is the delineation, for the first time, of a specific region of parameter space within which an adaptive change in commissural inhibitory gain is appropriate and sufficient to bring about a re-balancing of bilateral vestibular nucleus activity after UL. For this to be achieved, the relative contribution of the intrinsic, pacemaker-like membrane properties of the ipsi-lesional MVN cells must be equal to or greater than the synaptic input from the primary vestibular afferents in determining the in vivo resting discharge rate of these cells. Recent experimental evidence supports the view that the intrinsic properties of the MVN cells do contribute substantially to their resting discharge in vivo. Previous modelling studies that have excluded a role for the commissural system in vestibular compensation have arrived at this conclusion, because their models operated outside this region of parameter space. A second finding of this study is that, in a network that compensates through a selective change in commissural gain, the time constant of the VOR response is significantly reduced, mimicking the loss of velocity storage after UL in vivo. By contrast, the time constant is unchanged in a network that compensates through changes involving other nonvestibular inputs. These findings indicate that adaptive changes in commissural gain, through the dynamic regulation of GABA receptor efficacy in the vestibular nucleus neurones, may play an important role in vestibular plasticity.

Origin of Vestibular Dysfunction in Usher Syndrome Type 1B

It is still debated to what extent the vestibular deficits in Usher patients are due to either central vestibulocerebellar or peripheral vestibular problems. Here, we determined the origin of the vestibular symptoms in Usher 1B patients by subjecting them to compensatory eye movement tests and by investigating the shaker-1 mouse model, which is known to have the same mutation in the myosin-VIIa gene as Usher 1B patients. We show that myosin-VIIa is not expressed in the human or mouse cerebellum and that the vestibulocerebellum of both Usher 1B patients and shaker-1 mice is functionally intact in that the gain and phase values of their optokinetic reflex are normal. In addition, Usher 1B patients and shaker-1 mice do not show an angular vestibuloocular reflex even though eye movement responses evoked by electrical stimulation of the vestibular nerve appear intact. Finally, we show histological abnormalities in the vestibular hair cells of shaker-1 mice at the ultrastructural level, while the distribution of the primary vestibular afferents and the vestibular brainstem circuitries are unaffected. We conclude that the vestibular dysfunction of Usher 1B patients and shaker-1 mice is peripheral in origin.

Different Contributions of Platelet-activating Factor and Nitric Oxide in Long-term Potentiation of the Rat Medial Vestibular Nuclei

In rat brainstem slices, we investigated the differential role of nitric oxide (NO) and platelet-activating factor (PAF) in long-term potentiation (LTP) induced in the ventral portion of the medial vestibular nuclei (MVN) by high-frequency stimulation (HFS) of the primary vestibular afferents. The NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (carboxy-PTIO) and the PAF receptor antagonist ginkgolide B (BN-52021) were administered before and after induction of potentiation. The effect of carboxy-PTIO was to completely prevent LTP. By contrast, BN-52021 only reduced the amplitude of HFS potentiation, which could develop fully at the drug washout or decline to zero, becoming a short-term phenomenon, in the case of long-lasting PAF receptor block. Both drugs, when given after HFS, had no effect on the already established potentiation, but whilst BN-52021 showed an influence within 5 min of the LTP induction, carboxy-PTIO did not affect the response once HFS was delivered. Moreover, we showed that the NO donor, sodium nitroprusside, and methylcarbamyl PAF (mc-PAF) induced LTP which was associated with an increase in glutamate release as shown by reduction in the paired-pulse facilitation ratio. The mc-PAF LTP was prevented by the NO scavenger, while NO LTP was only reduced by BN-52021. We suggest that NO and PAF are implicated as retrograde messengers in two different phases of vestibular LTP: NO in the induction phase; and PAF in the full expression phase.

Role of nitric oxide in long-term potentiation of the rat medial vestibular nuclei

In rat brainstem slices, we investigated the role of nitric oxide in long-term potentiation induced in the ventral portion of the medial vestibular nuclei by high-frequency stimulation of the primary vestibular afferents. The nitric oxide scavenger [2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide] and the nitric oxide synthase inhibitor N G-nitro- l-arginine methyl ester were administered before and after induction of potentiation. Both drugs completely prevented long-term potentiation, whereas they did not impede the potentiation build-up, or affect the already established potentiation. These results demonstrate that the induction, but not the maintenance of vestibular long-term potentiation, depends on the synthesis and release into the extracellular medium of nitric oxide. In addition, we analysed the effect of the nitric oxide donor sodium nitroprusside on vestibular responses. Sodium nitroprusside induced long-term potentiation, as evidenced through the field potential enhancement and unit peak latency decrease. This potentiation was impeded by d, l-2-amino-5-phosphonopentanoic acid, and was reduced under blockade of synaptosomal platelet-activating factor receptors by ginkgolide B and group I metabotropic glutamate receptors by ( R,S)-1-aminoindan-1,5-dicarboxylic acid. When reduced, potentiation fully developed following the washout of antagonist, demonstrating an involvement of platelet-activating factor and group I metabotropic glutamate receptors in its full development. Potentiation induced by sodium nitroprusside was also associated with a decrease in the paired-pulse facilitation ratio, which persisted under ginkgolide B, indicating that nitric oxide increases glutamate release independently of platelet-activating factor-mediated presynaptic events. We suggest that nitric oxide, released after the activation of N-methyl- d-aspartate receptors, acts as a retrograde messenger leading to an enhancement of glutamate release to a sufficient level for triggering potentiation. Once the synaptic efficacy has changed, it becomes a long-lasting phenomenon only through a subsequent action of platelet-activating factor.

Vestibular Signals in the Parasolitary Nucleus

Vestibular primary afferents project to secondary vestibular neurons located in the vestibular complex. Vestibular primary afferents also project to the uvula-nodulus of the cerebellum where they terminate on granule cells. In this report we describe the physiological properties of neurons in a "new" vestibular nucleus, the parasolitary nucleus (Psol). This nucleus consists of 2,300 GABAergic neurons that project onto the ipsilateral inferior olive (beta-nucleus and dorsomedial cell column) as well as the nucleus reticularis gigantocellularis. These olivary neurons are the exclusive source of vestibularly modulated climbing fiber inputs to the cerebellum. We recorded the activity of Psol neurons during natural vestibular stimulation in anesthetized rabbits. The rabbits were placed in a three-axis rate table at the center of a large sphere, permitting vestibular and optokinetic stimulation. We recorded from 74 neurons in the Psol and from 23 neurons in the regions bordering Psol. The activity of 72/74 Psol neurons and 4/23 non-Psol neurons was modulated by vestibular stimulation in either the pitch or roll planes but not the horizontal plane. Psol neurons responded in phase with ipsilateral side-down head position or velocity during sinusoidal stimulation. Approximately 80% of the recorded Psol neurons responded to static roll-tilt. The optimal response planes of evoked vestibular responses were inferred from measurement of null planes. Optimal response planes usually were aligned with the anatomical orientation of one of the two ipsilateral vertical semicircular canals. The frequency dependence of null plane measurements indicated a convergence of vestibular information from otoliths and semicircular canals. None of the recorded neurons evinced optokinetic sensitivity. These results are consistent with the view that Psol neurons provide the vestibular signals to the inferior olive that eventually reached the cerebellum in the form of modulated climbing fiber discharges. These signals provide information about spatial orientation about the longitudinal axis.

Relationship between vestibular primary afferents and vestibulospinal neurons in lampreys.

The distribution of vestibular primary afferents as well as their relationship with vestibulospinal and other brainstem neurons were studied in lampreys using anatomical tracers. Afferents from the anterior (aVIIIn) and the posterior (pVIIIn) branches of the vestibular nerve were located mainly in the ventral nucleus of the octavolateral area. The relationship between afferents and vestibulospinal neurons was studied by applying one fluorescent tracer to the whole vestibular nerve or one of its branches and applying another tracer to the spinal cord. Some afferents showed large, bulb-like enlargements (bulbs) and about 20 of these were found in the anterior and the intermediate octavomotor nucleus, whereas about 40 were found in the posterior octavomotor nucleus. Some of the bulbs made apparent contact with vestibulospinal neurons in the intermediate octavomotor nucleus and originated mostly from the aVIIIn, whereas bulbs in the posterior octavomotor nucleus originated from the pVIIIn. Applications of biocytin to hemisegments of rostral spinal cord labeled vestibulospinal neurons located in the ipsilateral intermediate octavomotor nucleus and the contralateral posterior octavomotor nucleus. In addition, vestibular primary afferents with bulbs in apparent contact with vestibulospinal neurons were transneuronally labeled by biocytin. They were observed in the ipsilateral aVIIIn and the contralateral pVIIIn and could be followed in the labyrinths, where they innervated the vertical and horizontal arms of the semicircular canal crests. Taken together, these results indicate that vestibular primary afferents from the aVIIIn innervate predominantly vestibulospinal neurons of the intermediate octavomotor nucleus, whereas afferents from the pVIIIn innervate vestibulospinal neurons in the posterior octavomotor nucleus. This anatomical organization suggests that afferents carrying bulbs convey dynamic information to vestibulospinal neurons, which, in turn, project to the spinal cord networks.

Linear spatio-temporal convergence in vestibular neurons of the primate nucleus fastigii.

Vestibular responses in the primate fastigial nucleus (FN) do often not follow the simple cosine tuning observed in primary vestibular afferents. The present report demonstrates that these more complex patterns can mostly be attributed to simple linear summation of spatially and temporally diverse cosine-tuned input signals (linear spatio-temporal convergence, STC). Analyses following from this elementary finding, however, reveal frequency-dependent properties in many FN neurons, which are difficult to reconcile with existing concepts of possible functions of STC in central vestibular areas. The demonstration that STC linearity holds for FN responses is thus of both theoretical and practical relevance, allowing shortening of future experimental protocols and facilitating comparison of the observed spatio-temporal response dynamics with those at other stages of vestibular signal processing.