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PubMed1977

Vestibular responses in the rhesus monkey ventroposterior thalamus. II. Vestibulo-proprioceptive convergence at thalamic neurons.

The vestibular thalamic relay in the Rhesus ventrobasal complex, identified in a previous field potential study (part I, Deecke et al., 1974), has now been investigated with neuronal recordings in the thalamus in order to clarify its functional role. In part I, short latency responses (2.5 msec) were found in the corner between VPL, VPM and VPI nuclei, largely including dorsal portions of the VPI nucleus. Field potentials of somewhat longer latency (4-5 msec) were recorded in VPL and in other thalamic nuclei, including the posterior nuclear group. Neuronal responses were recorded in thalamic nuclei of awake flaxedilized Rhesus monkeys. Cells not responding to vestibular stimulation (round window polarisation of either labyrinth) were ignored. The great majority (80%) of those neurons responding to labyrinth polarisation showed convergence with deep somatic (proprioceptive) input from joints and muscles of vertebral column and limbs. 60% of these bimodal neurons responded to movement of cervical joints. Very few vestibularly responsive cells received cutaneous (6.6%), non-optokinetic visual or auditory (2.6% each) input. Proprioceptive fields tended to be large, frequently involving more than one joint, and could be even bilateral. For a few cells the pattern of vestibulo-proprioceptive convergence could be fitted to a coordinated body position that might occur during normal locomotion. 78% of the cells responded to polarisation of both labyrinths, indicating strong bilateral projection.

Animals↗
PubMed1999

Ultrastructural features of non-commissural GABAergic neurons in the medial vestibular nucleus of the monkey.

The ultrastructural characteristics of non-degenerating GABAergic neurons in rostrolateral medial vestibular nucleus were identified in monkeys following midline transection of vestibular commissural fibers. In the previous papers, we reported that most degenerated cells and terminals in this tissue were located in rostrolateral medial vestibular nucleus, and that many of these neurons were GABA-immunoreactive. In the present study, we examined the ultrastructural features of the remaining neuronal elements in this medial vestibular nucleus region, in order to identify and characterize the GABAergic cells that are not directly involved in the vestibular commissural pathway related to the velocity storage mechanism. Such cells are primarily small, with centrally-placed nuclei. Axosomatic synapses are concentrated on polar regions of the somata. The proximal dendrites of GABAergic cells are surrounded by boutons, although distal dendrites receive only occasional synaptic contacts. Two types of non-degenerated GABAergic boutons are distinguished. Type A terminals are large, with very densely-packed spherical synaptic vesicles and clusters of large, irregularly-shaped mitochondria with wide matrix spaces. Such boutons form symmetric synapses, primarily with small GABAergic and non-GABAergic dendrites. Type B terminals are smaller and contain a moderate density of round/pleomorphic vesicles, numerous small round or tubular mitochondria, cisterns and vacuoles. These boutons serve both pre- and postsynaptic roles in symmetric contacts with non-GABAergic axon terminals. On the basis of ultrastructural observations of immunostained tissue, we conclude that at least two types of GABAergic neurons are present in the rostrolateral portion of the monkey medial vestibular nucleus: neurons related to the velocity storage pathway, and a class of vestibular interneurons. A multiplicity of GABAergic bouton types are also observed, and categorized on the basis of subcellular morphology. We hypothesize that "Type A" boutons correspond to Purkinje cell afferents in rostrolateral medial vestibular nucleus, "Type B" terminals represent the axons of GABAergic medial vestibular nucleus interneurons, and "Type C" boutons take origin from vestibular commissural neurons of the velocity storage pathway.

Animals↗
PubMed1984

Neuron activity in monkey vestibular nuclei during vertical vestibular stimulation and eye movements.

To elucidate how information is processed in the vestibuloocular reflex (VOR) pathways subserving vertical eye movements, extracellular single-unit recordings were obtained from the vestibular nuclei of alert monkeys trained to track a visual target with their eyes while undergoing sinusoidal pitch oscillations (0.2-1.0 Hz). Units with activity related to vertical vestibular stimulation and/or eye movements were classified as either vestibular units (n = 53), vestibular plus eye-position units (n = 30), pursuit units (n = 10), or miscellaneous units (n = 5), which had various combinations of head- and eye-movement sensitivities. Vestibular units discharged in relation to head rotation, but not to smooth eye movements. On average, these units fired approximately in phase with head velocity; however, a broad range of phase shifts was observed. The activities of 8% of the vestibular units were related to saccades. Vestibular plus eye-position units fired in relation to head velocity and eye position and, in addition, usually to eye velocity. Their discharge rates increased for eye and head movements in opposite directions. During combined head and eye movements, the modulation in unit activity was not significantly different from the sum of the modulations during each alone. For saccades, the unit firing rate either decreased to zero or was unaffected. Pursuit units discharged in relation to eye position, eye velocity, or both, but not to head movements alone. For saccades, unit activity usually either paused or was unaffected. The eye-movement-related activities of the vestibular plus eye-position and pursuit units were not significantly different. A quantitative comparison of their firing patterns suggests that vestibular, vestibular plus eye-position, and pursuit neurons in the vestibular nucleus could provide mossy fiber inputs to the flocculus. In addition, the vertical vestibular plus eye-position neurons have discharge patterns similar to those of fibers recorded rostrally in the medial longitudinal fasciculus. Therefore, our data support the view that vertical vestibular plus eye-position neurons are interneurons of the VOR.

Animals↗
PubMed1999

Plastic changes underlying vestibular compensation in the guinea-pig persist in isolated, in vitro whole brain preparations.

Vestibular compensation for the postural and oculomotor deficits induced by unilateral labyrinthectomy is a model of post-lesional plasticity in the central nervous system. Just after the removal of one labyrinth, the deafferented, ipsilateral vestibular nucleus neurons are almost silent, and the discharge of the contralateral vestibular nucleus neurons is increased. The associated static disorders disappear in a few days, as normal activity is restored in both vestibular nuclei. In this study, we searched for traces of vestibular compensation in isolated whole brains taken from adult guinea-pigs. The electrophysiological responses evoked in control brains were compared to those evoked in brains taken from animals that had previously been labyrinthectomized. Guinea-pigs compensated for an initial labyrinthectomy within three days. In vivo, subsequent deafferentation of vestibular nucleus neurons on the intact side triggered "Bechterew's phenomenon": a new postural and oculomotor syndrome appeared, similar to the one induced by the first lesion, but directed to the newly deafferented side. These disturbances would be caused by the new imbalance between the discharges of neurons in the two vestibular nuclei triggered by the second deafferentation. Experiments were designed to search for a similar imbalance in vitro in brains taken from labyrinthectomized animals, where the intact vestibular nerve is cut during the dissection. Isolated whole brains were obtained from young guinea-pigs at various times (one to seven days) following an initial labyrinthectomy. An imbalance between the resting activities of medial vestibular nucleus neurons on both sides of the brainstem was revealed in brains taken more than three days after the lesion: their discharge was higher on the compensated, initially lesioned side than on the newly deafferented side. In some cases, an oscillatory pattern of discharge, reminiscent of the spontaneous nystagmus associated in vivo with Bechterew's syndrome, appeared in both abducens nerves. These data demonstrate that most of the changes underlying vestibular compensation persist, and can thus be investigated in the isolated whole brain preparation. Brains removed only one day after the lesion displayed normal commissural responses and symmetric spinal inputs to vestibular nucleus neurons. However, an unusually large proportion of the neurons recorded on both sides of the preparation had very irregular spontaneous discharge rates. These data suggest that the first stages of vestibular compensation might be associated with transient changes in the membrane properties of vestibular nucleus neurons. Brains taken from compensated animals displayed a significant, bilateral decrease of the inhibitory commissural responses evoked in the medial vestibular nucleus by single-shock stimulation of the contralateral vestibular nerve. The sensitivity of abducens motoneurons on the initially lesioned, compensated side to synaptic activation from the contralesional vestibular nucleus neurons was also decreased. Both changes may explain the long-term, bilateral decrease of vestibular-related reflexes observed following unilateral labyrinthectomy. Spinal inputs to vestibular nucleus neurons became progressively asymmetric: their efficacy was increased on the lesioned side and decreased on the intact one. This last modification may support a functional substitution of the deficient, vestibular-related synergies involved in gaze and posture stabilization by neck-related reflexes.

Animals↗
PubMed1996

Effects of caloric vestibular stimulation on prepositus hypoglossi neurons in rats.

In our previous study, we found that the majority of locus coeruleus (LC) neurons were inhibited by caloric stimulation and that the inhibition was mediated by gamma aminobutyric acid (GABAA) receptors. It has been reported that the nucleus prepositus hypoglossi (PrH) sends GABAergic inputs to the LC. In the present study, an attempt was made to determine whether PrH neurons contribute to the LC neuronal inhibition caused by caloric stimulation. We characterized the neuronal response in the PrH to caloric stimulation in rats. About three-fourths of PrH neurons showed various responses to both ipsi- and contralateral caloric stimulation, suggesting that PrH neurons receive bilateral vestibular inputs. However, these neurons were not activated by antidromic stimulation of the LC. The remaining one-fourth of the PrH neurons that were antidromically activated did not respond to caloric stimulation. Lesions in the PrH did not attenuate the LC neuronal inhibition caused by caloric stimulation. These findings suggest that the inhibition of LC neurons caused by caloric stimulation is not mediated by the PrH.

Animals↗
PubMed1999

Effects of the antihistaminergic drugs diphenhydramine and zolantidine on vestibular-induced hypothalamic neuronal activity in the guinea pig.

The mode of action of diphenhydramine in treating motion sickness is unknown. Using an electrophysiologic technique, we investigated the effects of intravenous diphenhydramine and zolantidine on the changes in neuronal activity produced by caloric stimulation in the hypothalamic paraventricular nucleus (PVN) in the guinea pig. Changes in neuronal activity were modulated by the administration of diphenhydramine in a high percentage of the neurons tested (71%), while zolantidine affected only a small number (29%). This finding reinforces the involvement of a histaminergic system in vestibular autonomic responses. The modulatory effect of diphenhydramine on PVN neuron activity may explain in part this drug's efficacy in treating motion sickness.

Animals↗
PubMed1977

Iontophoretic studies of histamine and histamine antagonists in the feline vestibular nuclei.

The activity of single neurons in the vestibular neuronal complex of midcollicular decerebrate, decerebellectomized cats were recorded and their responsiveness to iontophoretically applied histamine and other agents determined. The majority of the cells tested were inhibited by iontophoresis of histamine while 24% were excited by this agent. Neurons exhibiting inhibitory responses were widely distributed throughout the four vestibular nuclei and adjacent reticular formation whereas excitatory responses to histamine were obtained mainly in the region of the lateral vestibular nucleus. The H2-receptor blocking agents metiamide and cimetidine were examined as to their actions on spontaneously firing cells and cells affected by histamine. Metiamide was selective in blocking histamine-induced inhibition but not excitation while cimetidine was ineffective in blocking either response. These results suggest that histamine has both inhibitory and excitatory actions on brain stem neurons and metiamide is an effective antagonist of histamine-induced inhibition.

Action Potentials↗
PubMed1999

[The neurons of the medial vestibular nucleus initiating the relaxation of the rat's stomach wall].

In acute rat experiments the technique of retrograde axonal transport of horseradish peroxidase in the medial vestibular nucleus allowed to identify a group of neurones sending axons to the "stomach" region of a single tract nucleus. These neurones and, accordingly, the descending vestibular/solitary links can be viewed as the morphologic basis for vestibular influences on the gastric motor activity. As was shown, local irritation of the neurones initiates relaxation of the stomach wall. Activation of the anterior limbic cortex modulates the vestibular/autonomous responses of the intragastric pressure reduction. Irritation of the infralimbic cortex of the rat's brain may have a preventive effect on the stomach wall relaxation stimulated by the vestibular neurones projecting on the "stomach" region of single tract nucleus.

Animals↗
PubMed2001

Responses of rostral fastigial neurons to linear acceleration in an alert monkey.

Vestibular-only neuronal responses to angular acceleration have been systematically characterized in the rostral fastigial nucleus (FN) by several studies. However, responses of these neurons to linear acceleration have not been examined. In this study, we recorded single-unit activity of vestibular-only neurons in an alert monkey during pure sinusoidal linear acceleration along different directions in the horizontal plane. Spatiotemporal response properties were quantified by computing two-dimensional response ellipses in the horizontal plane. Based on this analysis, neurons were classified as narrowly or broadly tuned. About 29% (5/17) of neurons were broadly tuned. The other 71% (12/17) were narrowly tuned. Unlike vestibular nuclei neurons, all recorded FN neurons exhibited irregular resting discharge rates (CV*0.2). Based on studies of linear motion-sensitive neurons in the vestibular nuclei, the data suggest that irregular neurons in the rostral FN and the vestibular nuclei have similar responses to linear acceleration in behaving monkeys.

Acceleration↗
PubMed1997

Morphometric analysis of the human vestibular nuclei.

BACKGROUND: Cytoarchitectural investigations of the vestibular nuclei have been undertaken in different species of mammals. These data provide a description of the general architecture of the nuclei but limited information about quantitative characteristics of their cell population. We have recently obtained data about the morphometric parameters of the vestibular nuclei neurons in some species. The application of quantitative image analysis techniques to the research of the cellular morphology in the vestibular area of humans might provide basic information to compare with data from animal studies, taking into account the observed correlation between physiological and morphological properties of vestibular neurons. METHODS: The characteristics of the major vestibular nuclei in humans have been studied with light microscopic techniques in serially cut sections. Camera lucida drawings of the vestibular nuclei and their neurons were made and subjected to computerized image analysis. For each vestibular nucleus, information was obtained about topography, morphological characteristics (i.e., location, volume, and length), and the number and morphometric parameters of their neurons (cross-sectional areas, maximum and minimum diameters). Morphometric data about cell parameters were statistically analyzed by comparing the populations within different parts of each nucleus and from different nuclei. RESULTS: Among the vestibular nuclei, the medial, which is the largest, has the greatest number of neurons, and the interstitial, the least. The lateral and interstitial nuclei contain the largest cells, and the descending nucleus has the smallest cells. The superior nucleus contains cells of intermediate size. The size of cells decreases in a rostrocaudal direction in the medial, lateral, and descending nuclei, the opposite trend being observed in the superior nucleus. Within the superior and medial nuclei, there are discrete areas with cells with distinctive characteristics. CONCLUSIONS: These results suggest that, just as most of the anatomical characteristics of the second-order neurons found in animals have been preserved in humans, so the physiological mechanisms observed in the vestibular system of animals should apply to humans.

Adult↗