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PubMed1984

[Antidromal activation and synaptic processes of Deiters nucleus induced by stimulation of the inferior olive and reticular nucleus of the pontine tegmentum].

Effects of stimulation of the inferior olive and nucleus reticularis tegmenti pontis on the activity of neurons of the lateral vestibular nucleus of Deiters were studied in cats by means of intracellular recording technique. The stimulation of the rostral region of inferior olive evoked mainly antidromic responses in the Deiters' neurons whereas during the stimulation of the caudal region of inferior olive synaptic responses were prevailing. Stimulation of the rostral and, particularly, the caudal regions of inferior olive as well as nucleus reticularis tegmenti pontis produced mono- and/or oligosynaptic and polysynaptic IPSPs. Mono- and/or oligosynaptic and polysynaptic EPSPs occurred in the Deiters' neurons during stimulation of the caudal region of inferior olive. Convergence of influences from the stimulated structures on the vestibular neurons under study was shown. Peculiarities and functional significance of these influences are discussed.

Animals↗
PubMed1992

Vestibular inputs to brain stem neurons that participate in motor learning in the primate vestibuloocular reflex.

1. Previous studies have described a subpopulation of interneurons in the vestibuloocular reflex (VOR) pathways that express large changes in their responses to head turns in conjunction with motor learning in the VOR. These neurons are called flocculus target neurons (FTNs) because they are inhibited at monosynaptic latencies by stimulation of the flocculus and ventral paraflocculus. 2. Electrical stimulation of the vestibular labyrinth revealed that FTNs receive excitatory monosynaptic inputs from the ipsilateral vestibular labyrinth and longer-latency, excitatory inputs from the contralateral labyrinth. 3. Our data show that commissural inhibition, which has been thought to be an important feature of vestibular processing, does not provide the dominant inputs from the contralateral labyrinth to FTNs. Instead, the inputs from both labyrinths are excitatory and may be functionally antagonistic. Changes in the balance of excitatory inputs from the two horizontal canals to FTNs could contribute to motor learning in the VOR.

Animals↗
PubMed1999

Fastigial nucleus activity during different frequencies and orientations of vertical vestibular stimulation in the monkey.

Neurons in the rostral part of the fastigial nucleus (FN) respond to vestibular stimulation but are not related to eye movements. To understand the precise role of these vestibular-only neurons in the central processing of vestibular signals, unit activity in the FN of alert monkeys (Macaca mulatta) was recorded. To induce vestibular stimulation, the monkey was rotated sinusoidally around an earth-fixed horizontal axis at stimulus frequencies between 0.06 (+/-15 degrees) and 1.4 Hz (+/-7.5 degrees). During stimulation head orientation was changed continuously, allowing for roll, pitch, and intermediate planes of orientation. At a frequency of 0.6 Hz, 59% of the neurons had an optimal response orientation (ORO) and a null response (i.e., no modulation) 90 degrees apart. The phase of neuronal response was constant except for a steep shift of 180 degrees around the null response. This group I response is compatible with a semicircular canal input, canal convergence, or a single otolith input. Several other features indicated more complex responses, including spatiotemporal convergence (STC). 1) For 35% of the responses at 0.6 Hz, phase changes were gradual with different orientations. Fifteen percent of these had a null response (group II), and 20% showed only a minimal response but no null response (group III). The remaining responses (6%), classified as group IV, were characterized by a constant sensitivity at different orientations in most instances. 2) For the vast majority of neurons, the stimulus frequency determined the response group, i.e., an individual neuron could show a group I response at one frequency and a group II (III or IV) response at another frequency. 3) ORO changed with frequency by >45 degrees for 44% of the neurons. 4) Although phase changes at different frequencies were close to head velocity (+/-45 degrees ) or head position (+/-45 degrees ) for most neurons, they exceeded 90 degrees for 29% of the neurons between 0.1 and 1.0 Hz. In most cases, this was a phase advance. The change in sensitivity with change in frequency showed a similar pattern for all neurons; the average sensitivity increased from 1.24 imp. s-1. deg-1 at 0.1 Hz to 2.97 imp. s-1. deg-1 at 1.0 Hz. These data demonstrate that only an analysis based on measurements at different frequencies and orientations reveals a number of complex features. They moreover suggest that for the vast majority of neurons several sources of canal and otolith information interact at this central stage of vestibular information processing.

Acoustic Stimulation↗
PubMedDate not supplied

Head impulses after unilateral vestibular deafferentation validate Ewald's second law.

To determine the relative contributions of ampullofugal (AF) and ampullopetal (AP) stimulation of the horizontal semicircular canal (HSCC) to the horizontal vestibulo-ocular reflex (HVOR), 12 patients were studied 1 year after total unilateral vestibular deafferentation (UVD). Compensatory eye movement responses to impulses of horizontal head rotation were studied using magnetic search coils. The head impulses were rapid (up to 3000 deg/sec/sec) passive, unpredictable, step displacements of horizontal angular head position with respect to the trunk. The results from these 12 patients were compared with results from 30 normal subjects. An HVOR deficit was found to each side. The HVOR in response to head impulses toward the deafferented side, a response generated exclusively by ampullofugal stimulation of the single functioning HSCC, was severely deficient with an average gain of 0.25; the HVOR in response to head impulses toward the intact side, a response generated exclusively by ampullopetal stimulation of the single functioning HSCC, was mildly but significantly deficient compared with normal subjects. These results show that rapid, unpredictable head movements, unlike slow, predictable head movements, do demonstrate the AP-AF HVOR asymmetry, which could be expected from consideration of the behavior of single vestibular afferent neurons, an asymmetry that is expressed by Ewald's 2nd Law.

Eye Movements↗
PubMed1980

Inhibitory connections of nystagmus-related reticular burst neurons with neurons in the abducens, prepositus hypoglossi and vestibular nuclei in the cat.

Action potentials of inhibitory burst neurons (IBNs) were extracellularly recorded in the pontomedullary reticular formation in the cat. These neurons were identified by their burst activity coincident with the quick inhibitory phase of the contralateral abducens nerve during vestibular nystagmus and by their antidromic activation from the contralateral abducens nucleus. During extracellular recording from the soma of single IBNs, another electrode for microstimulation was systematically tracked throughout the brain stem. For each IBN investigated, the effective sites for antidromic activation were invariably found in the contralateral abducens, prepositus hypoglossi, medial vestibular nuclei and the area ventral to the prepositus hypoglossi nucleus. Stimulation of neither the ipsilateral brain stem nor the oculomotor nuclei evoked antidromic responses in IBNs. Extracellular spikes of single IBNs and neurons in the overlying projection area were recorded simultaneously. Their correlation was examined by using peri-spike time histograms. Shortly after the spikes of single IBNs, the activity of motoneurons and internuclear interneurons in the abducens nucleus, and of type II neurons in the prepositus hypoglossi and vestibular nuclei, was depressed. Connections of IBNs with ipsilateral medial rectus motoneurons were studied by spike-triggered averaging of membrane potentials of the motoneurons and action potentials of the medial rectus nerve. Single IBN spikes induced a di- or polysynaptic disfacilitation in the motoneurons. This disfacilitation was concluded to be mediated by some of the above-described interneurons which were directly inhibited by IBNs. Their depressant effect on medial rectus motoneuronal spike activity was comparable to that on the spike activity of contralateral abducens motoneurons.

Abducens Nerve↗
PubMed1995

Long-term potentiation of glutamatergic pathways in the lamprey brainstem.

Plasticity of synaptic transmission has been investigated in the lamprey brainstem. In this preparation, neurons of the vestibular nuclei make monosynaptic connections to reticulospinal neurons of the posterior (PRRN) and middle (MRRN) rhombencephalic reticular nuclei. This glutamatergic projection shows a long-lasting enhancement in efficacy following a brief high frequency (50 Hz) train of stimuli (tetanus). Enhancement was found on the inputs from the octavomotorius intermediate (nOMI) and octavomotorius posterior (nOMP) nuclei to the reticulospinal neurons of the PRRN and MRRN, and persisted for the entire duration of the recordings. The enhancement was limited to the pathway that received tetanic stimulation for the inputs to any given reticulospinal neuron. It depended upon the activation of postsynaptic processes at least in part; dialysis of the reticulospinal neuron by recording with a whole-cell patch pipette prevented its induction. This dialysis-dependent abolition of enhancement was shown to be dependent on a change in Ca2+ concentration in the postsynaptic neuron. The enhancement was not affected by blockade of NMDA receptors with D,L-2-amino-5-phosphonopentanoate (AP5) but was prevented by the metabotropic glutamate receptor (mGluR) antagonist (R,S)-alpha-methyl-4-carboxyphenylglycine (MCPG). In conclusion, this study demonstrates that vestibular inputs to reticulospinal neurons are capable of undergoing long-term potentiation (LTP) and that this LTP shows synapse specificity. Furthermore, this LTP is activated by synaptic stimulation of a subtype of the m-GluR family and its induction is mediated by changing Ca2+ concentrations in the postsynaptic neuron.

Animals↗
PubMed1982

Efferent neurons of the lateral-line system and the VIII cranial nerve in the brainstem of anurans. A comparative study using retrograde tracer methods.

The octavo-lateral efferent system of several anuran species was studied by means of retrograde transport of horseradish peroxidase. This system is organized similarly in all larval anurans and in all adult aglossids. All have two groups of efferent neurons in the nucleus reticularis medialis between the VIIIth and the IXth motor nucleus. The caudal group consists of efferent neurons that supply the posterior lateral-line nerve (NLLp) and a considerably smaller group of neurons supplying both the NLLp and the anterior lateral-line nerve (NLLa). The rostral group is composed of efferent neurons supplying the NLLa, neurons projecting to the inner ear and neurons supplying both the inner ear and the NLLa. Efferent neurons of the VIIIth cranial nerve exhibit a rostrocaudal cytoarchitectonic differentiation. Caudal perikarya, which are rounder in shape than those of the rostral part, have a dendritic projection to the superior olive. It is suggested that this differentiation reflects a functional differentiation of acoustic and vestibular efferent neurons. Labeled neurons were ipsilateral to the site of application of HRP. None were found in the vestibular nuclei or in the cerebellum. Efferent axons projecting to neuromasts of the NLLa leave the medulla with the VIIth nerve, axons projecting to neuromasts of the NLLp exit via the IXth nerve. Cell counts and the observation of axonal branching revealed that efferent units of both the lateral-line and the VIIIth-nerve system supply more than one receptor organ. In contrast to the lateral-line system, dendrites of efferent neurons of the VIIIth nerve project dorsally onto its nuclei, and afferents of the VIIIth nerve project onto efferent neurons. These structures most probably represent a feedback loop between the afferent and efferent systems of the VIIIth cranial nerve.

Animals↗
PubMed2001

Transcription factor GATA-3 alters pathway selection of olivocochlear neurons and affects morphogenesis of the ear.

Patterning the vertebrate ear requires the coordinated expression of genes that are involved in morphogenesis, neurogenesis, and hair cell formation. The zinc finger gene GATA-3 is expressed both in the inner ear and in afferent and efferent auditory neurons. Specifically, GATA-3 is expressed in a population of neurons in rhombomere 4 that extend their axons across the floor plate of rhombomere 4 (r4) at embryonic day 10 (E10) and reach the sensory epithelia of the ear by E13.5. The distribution of their cell bodies corresponds to that of the cell bodies of the cochlear and vestibular efferent neurons as revealed by labeling with tracers. Both GATA-3 heterozygous and GATA-3 null mutant mice show unusual axonal projections, such as misrouted crossing fibers and fibers in the facial nerve, that are absent in wild-type littermates. This suggests that GATA-3 is involved in the pathfinding of efferent neuron axons that navigate to the ear. In the ear, GATA-3 is expressed inside the otocyst and the surrounding periotic mesenchyme. The latter expression is in areas of branching of the developing ear leading to the formation of semicircular canals. Ears of GATA-3 null mutants remain cystic, with a single extension of the endolymphatic duct and no formation of semicircular canals or saccular and utricular recesses. Thus, both the distribution of GATA-3 and the effects of null mutations on the ear suggest involvement of GATA-3 in morphogenesis of the ear. This study shows for the first time that a zinc finger factor is involved in axonal navigation of the inner ear efferent neurons and, simultaneously, in the morphogenesis of the inner ear.

Animals↗
PubMed1983

The dual origins of the olivocochlear bundle in the albino rat.

Recent studies of the origins and terminations of the olivocochlear bundle (OCB) in the cat provide evidence that separate efferent systems differentially innervate the two types of hair cells in the organ of Corti. To begin to test the generality of these separate olivocochlear systems, the cells of origin of the OCB were determined in the albino rat by using axonal transport of horseradish peroxidase. Our findings are that, as in the cat, two classes of olivocochlear (OC) neurons project to the cochlea. These neurons could be dichotomized according to their location in the superior olivary complex (lateral or medial), their size (small or large), and their preferred side of projection (ipsilateral or contralateral). All labeled OC neurons also exhibited a positive reaction for acetylcholinesterase. In the rat, however, lateral and medial OC neurons are each restricted to a single nucleus, and, furthermore, the lateral OC neurons project only ipsilaterally. The rat also has significantly fewer mean totals of both lateral (240 vs. 710) and medial (240 vs. 520) OC neurons than the cat. The present methods also demonstrated the course of axons of the OCB and axon collaterals entering the cochlear nuclear complex. Vestibular efferent neurons were also labeled bilaterally medial and lateral to the facial genu. Our results suggest that the general organizational plan of OC neurons in the rat may offer advantages over the situation in the cat for studies of connectional neuroanatomy, neurophysiology, and behavioral functions of the two olivocochlear systems.

Animals↗
PubMed1990

Localization of GABAA-receptor alpha 1 subunit mRNA-containing neurons in the lower brainstem of the rat.

The localization of gamma-aminobutyric acid-A (GABAA) receptors (GABAA-R) in the lower brainstem of the rat was examined by means of in situ hybridization histochemistry using an oligonucleotide probe to the sequence of the alpha 1 subunit (GABAA-R alpha 1). Strongly labeled neurons were found in the cranial motor nuclei, the dorsal motor nucleus of the vagus, reticular formation (large neurons), lateral vestibular nucleus, dorsal nucleus of the lateral lemniscus, central nucleus of the inferior colliculus, intermediate and white layers of the superior colliculus, red nucleus and substantia nigra. In addition, moderately labeled cells were abundant in the nucleus of the solitary tract, medial and inferior vestibular nuclei, parabrachial area, dorsal and ventral tegmental nuclei of Gudden, central gray matter, ventral nucleus of the lateral lemniscus, and reticular formation (small neurons). This study has therefore revealed some of the target neurons of GABA-containing fibers in the lower brainstem.

Animals↗
PubMed2004

Plasticity of histamine H3 receptor expression and binding in the vestibular nuclei after labyrinthectomy in rat.

BACKGROUND: In rat, deafferentation of one labyrinth (unilateral labyrinthectomy) results in a characteristic syndrome of ocular and motor postural disorders (e.g., barrel rotation, circling behavior, and spontaneous nystagmus). Behavioral recovery (e.g., diminished symptoms), encompassing 1 week after unilateral labyrinthectomy, has been termed vestibular compensation. Evidence suggesting that the histamine H3 receptor plays a key role in vestibular compensation comes from studies indicating that betahistine, a histamine-like drug that acts as both a partial histamine H1 receptor agonist and an H3 receptor antagonist, can accelerate the process of vestibular compensation. RESULTS: Expression levels for histamine H3 receptor (total) as well as three isoforms which display variable lengths of the third intracellular loop of the receptor were analyzed using in situ hybridization on brain sections containing the rat medial vestibular nucleus after unilateral labyrinthectomy. We compared these expression levels to H3 receptor binding densities. Total H3 receptor mRNA levels (detected by oligo probe H3X) as well as mRNA levels of the three receptor isoforms studied (detected by oligo probes H3A, H3B, and H3C) showed a pattern of increase, which was bilaterally significant at 24 h post-lesion for both H3X and H3C, followed by significant bilateral decreases in medial vestibular nuclei occurring 48 h (H3X and H3B) and 1 week post-lesion (H3A, H3B, and H3C). Expression levels of H3B was an exception to the forementioned pattern with significant decreases already detected at 24 h post-lesion. Coinciding with the decreasing trends in H3 receptor mRNA levels was an observed increase in H3 receptor binding densities occurring in the ipsilateral medial vestibular nuclei 48 h post-lesion. CONCLUSION: Progressive recovery of the resting discharge of the deafferentated medial vestibular nuclei neurons results in functional restoration of the static postural and occulomotor deficits, usually occurring within a time frame of 48 hours in rats. Our data suggests that the H3 receptor may be an essential part of pre-synaptic mechanisms required for reestablishing resting activities 48 h after unilateral labyrinthectomy.

Animals↗