Efferent activity in the goldfish vestibular nerve and its influence on afferent activity

Out of 326 fibres in the horizontal semicircular canal branch of the goldfish vestibular nerve, 7 fibres could be identified as efferents. They showed irregular spontaneous activity and responded to rotatory stimuli with double frequency. Additionally in the central stump of the dissected nerve, efferent fibres were found, the spontaneous and stimulus modulated activity of which could not be differentiated from afferents.Efferents could be driven by a number of stimuli (vestibular, visual, somatosensory). Disruption of the efferent influence upon the receptors by dissection of the nerve or by pharmacological means (Gallamine) led to an increase of spontaneous afferent activity by 50%, showing that there is tonic efferent inhibition. Transfer functions of afferents were not changed after release from efferent influence. Electrical stimulation of efferents in 41% of the fibres led to an increase of afferent activity instead of the expected inhibition, which was seen in another 32%.     

Tonic influence from one labyrinth onto the contralateral one

Summary In the frog's isolated head preparations, spontaneous activity was recorded from the whole nerve of the left horizontal semicircular canal (HC) for 6 min before and 16 min after destruction of the right labyrinth by heating or administration of d-tubocurarine (0.5 l, 5.10^–6M) into the perilymph of the right labyrinth. Just after destruction of the right labyrinth, spontaneous activity of the left HC nerve abruptly increased by 20–400% in 24 preparations out of the 40 studied; activity then increased slowly and regularly and in most cases reached a steady level. In the 16 other preparations such destruction had no effect (15 preparations) or elicited a slight decrease of the HC nerve activity (one preparation). After curare administration, the spontaneous activity of the left HC nerve decreased by 20–100% in 27 preparations out of the 40 studied; in most cases such a decrease was reversed 25–80 min after administration of the drug. The spontaneous discharges were unaffected in 11 preparations and slightly increased in the two others. Destruction of the right labyrinth or administration of curare never modified spontaneous activity recorded from the left HC nerve when the connections between the two labyrinths had been interrupted either by sagittal section of the medulla oblongata or section of the right vestibular nerve close to the brain stem. These results demonstrate that one labyrinth has a tonic inhibitory influence on the contralateral one.Supported by grants from I.N.S.E.R.M. (ASR No. 6, contrat No. 005) and C.N.R.S. (aide individuelle No. 031865)     

Discharge properties of afferent fibres of the goldfish semicircular canal with high frequency stimulation

The horizontal semicircular canals of goldfish were sinusoidally stimulated between 0.07 and 63 Hz (about 3 decades). Single afferent fibre recordings showed sinusoidal modulation of discharges. Above 4 Hz the discharges became phase-locked to the stimulus. With increasing frequency the number of spikes per period decreased so that finally only one spike per period remained. At 63 Hz a stimulus of as little as 0.005^o was sufficient to drive the units to far above their spontaneous activity. As cupular deflection is less than the angle of body movement, the cupular deflection threshold for modulation of afferent discharges must be much less than 0.005^o. Transfer functions of afferent activity were determined. The simple pendulum model does not fit the data. Additional introduction of a third time constant and the low pass properties of the receptor cell membrane, the synaptic delay and the leaky integrator of the post synaptic afferent terminal improve the fit.     

Spatial tuning and dynamics of vestibular semicircular canal afferents in rhesus monkeys

Rotational head motion in vertebrates is detected by the three semicircular canals of the vestibular system whose innervating primary afferent fibers encode movement information in specific head planes. In order to further investigate the nature of vestibular central processing of rotational motion in rhesus monkeys, it was first necessary to quantify afferent information coding in this species. Extracellular recordings were performed to determine the spatial and dynamic properties of semicircular canal afferents to rotational motion in awake rhesus monkeys. We found that the afferents innervating specific semicircular canals had maximum sensitivity vectors that were mutually orthogonal. Similar to other species, afferent response dynamics varied, with regular firing afferents having increased long time constants (t ₁), decreased cupula velocity time constants (t _v), and decreased fractional order dynamic operator values (s ^k) as compared to irregular firing afferents.     

Behavior of horizontal semicircular canal afferents in alert monkey during vestibular and optokinetic stimulation

Summary The behavior of single vestibular nerve fibers from the lateral semicircular canal was recorded during sinusoidal oscillations of the head, during optokinetic stimulation with the head stationary, and during spontaneous oculomotor behavior in the alert monkey. The response of similar fibers to adequate vestibular stimulation was also studied in some of the animals under deeply anesthetized conditions. In the alert animals all units were spontaneously active and their discharge was modulated only by adequate vestibular stimulation. Ipsilateral horizontal rotations of the head were excitatory for all units. No modification of this basic vestibular response by visual stimulation including full-field striped drum rotation was observed. Furthermore no correlation of unit activity with oculomotor function including voluntary saccadic and pursuit eye movements was found in any of the units. The regularity of spontaneous discharge was the most consistent characteristic that differentiated the unit response into types. Most units were very regular in discharge, but a few were very irregular. The averaging of unit discharge over several cycles of oscillatory head rotation showed that the irregular type units were also consistently modulated by adequate vestibular stimulation. Both regular and irregular type units were found in the anesthetized animals. Unimodal distributions of the quantitative values for unit resting discharge rate, sensitivity, and phase relationship were found. The distributions for these three parameters were similar in the units recorded in the anesthetized animals. Thus at least these characteristics of semicircular canal response seem not to be affected by the vestibular efferent system which should be altered or eliminated in the case of the anesthetized animals.Research supported by NIH Grant EY0995-04.     

Differences between otolith- and semicircular canal-activated neural circuitry in the vestibular system

In the last two decades, we have focused on establishing a reliable technique for focal stimulation of vestibular receptors to evaluate neural connectivity. Here, we summarize the vestibular-related neuronal circuits for the vestibulo-ocular reflex, vestibulocollic reflex, and vestibulospinal reflex arcs. The focal stimulating technique also uncovered some hidden neural mechanisms. In the otolith system, we identified two hidden neural mechanisms that enhance otolith receptor sensitivity. The first is commissural inhibition, which boosts sensitivity by incorporating inputs from bilateral otolith receptors, the existence of which was in contradiction to the classical understanding of the otolith system but was observed in the utricular system. The second mechanism, cross-striolar inhibition, intensifies the sensitivity of inputs from both sides of receptive cells across the striola in a single otolith sensor. This was an entirely novel finding and is typically observed in the saccular system. We discuss the possible functional meaning of commissural and cross-striolar inhibition. Finally, our focal stimulating technique was applied to elucidate the different constructions of axonal projections from each vestibular receptor to the spinal cord. We also discuss the possible function of the unique neural connectivity observed in each vestibular receptor system.     

Effects of cromakalim (BRL 34915) on resting and evoked activity in frog semicircular canals

The effects of endolymphatic administration of cromakalim and glibenclamide were tested in isolated semicircular canals of the frog. The actions of the drugs were evaluated by recording: (1) the transepithelial potential between the endolymphatic and the perilymphatic sides of the crista ampullaris; (2) the slow nerve potential from the ampullar nerve; (3) the action potential discharge in afferent ampullar nerve fibres; and (4) the perilympathatic potassium concentration in the fluid bathing the outer surface of the crista ampullaris. The above mentioned parameters were recorded both at rest and during mechanical stimulation of the sensory organ. The results demonstrated that the endolymphatic administration of cromakalim (10^-4M) produced an increase in both ampullar receptor resting activity and in perilymphatic K⁺resting levels. By contrast all the parameters related to the mechanically evoked responses were practically unaffected. Glibenclamide (10^-4M) proved able to cancel or to prevent cromakalim effects. These data suggest that the membrane of the hair cells is endowed with K⁺channels regulated by internal ATP whose activation is mainly involved in the processes sustaining ampullar receptors' resting firing rate.     

The effect of vestibular nerve section on the expression of the hyaluronan in the frog, Rana esculenta

Following postganglionic lesion of the eighth cranial nerve, the changes in the expression of hyaluronan (HA), one of the extracellular matrix macromolecules, were examined in the medial (MVN) and lateral (LVN) vestibular nuclei and in the entry or transitional zone (TZ) of the nerve in the frog. HA was detected in different survival times by using a specific biotinylated hyaluronan-binding probe. HA expression was defined by the area-integrated optical density (AIOD), calculated from pixel intensities of digitally captured images. During the first postoperative days the perineuronal net (PN), a HA-rich area around the neurons, was not distinguishable from the surrounding neuropil in the MVN and LVN, characterized by a bilateral drop of AIOD specifically on the operated side. From postoperative day 14 onwards AIOD increased whilst the PN reorganized. In contrast, the AIOD wobbled up and down bilaterally without any trend in the TZ. Statistical analysis indicated that AIOD changes in the structures studied ran parallel bilaterally presumably because of the operation. Our results demonstrated for the first time that (1) the lesion of the eighth cranial nerve is accompanied by the modification of AIOD reflected HA expression in the MVN, LVN and TZ, (2) different tendencies exist in the time course of AIOD in the structures studied and (3) these tendencies are similar on the intact and operated sides. Our findings may suggest an area dependent molecular mechanism of HA in the restoration of vestibular function.     

Tonic activity of rat medial vestibular nucleus neurones in vitro and its inhibition by GABA

Summary The spontaneous discharge of 48 medial vestibular nucleus (MVN) neurones was recorded extracellularly in horizontal slices of the rat brainstem in vitro. The mean tonic rate of discharge was 17.1±8.2 imp/s, similar to that observed by others in transverse (coronal) slices of the rat and guinea pig MVN. The tonic rate of discharge of individual MVN cells either increased or decreased after synaptic blockade in low Ca²⁺ media, suggesting that ongoing synaptic activity has an important influence on the spontaneous activity of MVN cells in vitro. However the persistence of tonic activity after synaptic blockade indicates that an intrinsic, pacemaker-like mechanism is involved in the generation of the tonic activity. GABA, muscimol, baclofen and 3-APA inhibited the tonic activity of all MVN cells tested. Bicuculline antagonised, and picrotoxin blocked, the inhibitory responses to muscimol, but the effects of GABA were only partially blocked in 50 M picrotoxin. The effects of baclofen and 3-APA persisted in low Ca²⁺ media, and were antagonised by saclofen and phaclofen. Picrotoxin-resistant responses to GABA persisted in low Ca²⁺ media, and were also antagonised by saclofen. These results suggest that the inhibitory control of MVN neurones by GABA involves both the GABA_A and GABA_B subtypes of GABA receptor. GABA_B receptors appear to be distributed both pre- and post-synaptically in the rat MVN. The possible significance of the intrinsic, tonic activity of MVN cells in normal vestibular function and in vestibular compensation, and the effects of GABA, are discussed.     

Visually mediated inhibition of lateral line primary afferent activity by the octavolateralis efferent system during predation in the free-swimming toadfish,Opsanus tau

Summary The activity of single lateral line afferent neurons was chronically recorded in free-swimming toadfish. CNS efferent neurons, known to be inhibitory upon peripheral lateral line mechanoreceptors, were activated by stroboscopic and natural visual stimuli. Discharges from irregular-type afferents caused by water movement relative to lateral line neuromasts decreased following stroboscopic stimulation of unrestrained and behaving fish. Visual presentation of natural prey also decreased mechanically evoked afferent firing rates. We show that visual stimuli can activate the efferent system and function in the peripheral processing of mechanical stimuli to the lateral line in biologically relevant contexts.     

Unilateral vestibular deafferentation causes permanent impairment of the human vertical vestibulo-ocular reflex in the pitch plane

Rapid, passive, unpredictable, low-amplitude (10–20°), high-acceleration (3000–000°/s²) head rota tions were used to study the vertical vestibulo-ocular reflex in the pitch plane (pitch-vVOR) after unilateral vestibular deafferentation. The results from 23 human subjects who had undergone therapeutic unilateral vestibular deafferentation were compared with those from 19 normals. All subjects were tested while seated in the upright position. Group means and two-tailed 95% confidence intervals are reported for the pitch-vVOR gains in normal and unilateral vestibular deafferented subjects. In normal subjects, at a head velocity of 125°/s the pitch-vVOR gains were: upward 0.89±0.06, down ward 0.91±0.04. At a head velocity of 200°/s, the pitchvVOR gains were: upward 0.92±0.06, downward 0.96±0.04. There was no significant up-down asymme try. In the 15 unilateral vestibular deafferented subjects who were studied more than 1 year after unilateral vestibular deafferentation, the pitch-vVOR was signifi cantly impaired. At a head velocity of 125°/s the pitchvVOR gains were: upward 0.67±0.11, downward 0.63 ± 0.07. At a head velocity of 200°/s, the pitch-vVOR gains were: upward 0.67±0.07, downward 0.58±0.06. There was no significant up-down asymmetry. The pitch-vVOR gain in unilateral vestibular deafferented subjects was significantly lower (P<0.05) than the pitch-vVOR gain in normal subjects at the same head velocities. These results show that total, permanent uni lateral loss of vestibular function produces a permanent symmetrical 30% (approximately) decrease in pitchv-VOR gain. This pitch-vVOR deficit is still present more than 1 year after deafferentation despite retinal slip velocities greater than 30°/s in response to head accelerations in the physiological range, indicating that compensation of pitch-vVOR function following unilat eral vestibular deafferention remains incomplete.     

Efferent influence on the vestibular organ during active movements of the body

Summary In the goldfish one can induce optokinetic turning motions with a stimulus consisting of a pattern of moving stripes. These movements are active movements, performed freely by the fish. In the experiments reported here, recordings were made from the primary afferent fibers of the horizontal semicircular canal of relaxed (Flaxedil) goldfish. Although the fish remained completely motionless because of the relaxation, the activity of the nerve fibers under investigation changed when a moving pattern of stripes was shown. Depending on the direction of the moving pattern the activity decreased or increased. To explain this it is assumed that the optokinetic stimulus induces an intention in the fish to move, which the fish is not able to perform when relaxed. In conjunction with the command to move, it is further assumed that the sensitivity of the receptors in the vestibular organ is shifted by means of efferent innervation. This shift would keep the afferent activity constant during the actual performance of active body motions. Since the body movement does not occur during relaxation, one can observe a change in the afferent activity. The physical stimulus on the vestibular organ during active body movements would be compensated for by this mechanism, so that the organ would still be completely capable of responding to any additional stimulus (passive body movements, disturbances of equilibrium).Preliminary report of this work was given at the 35th meeting of the Deutsche Physiologische Gesellschaft Mainz, March 26–28, 1969.     

Morphohistochemical changes of the blood cells in the hibernating frog (Rana esculenta L.)

Some morphological features (degree of nuclear segmentation in neutrophils and eosinophils), histochemical patterns (DHFR, LDH, G6PDH) and, in addition, the nucleic acid distribution (DNA, RNA with acridine orange) in the peripheral blood cells of Rana esculenta, during the hibernation phase were investigated. All the observed parameters varied significantly in the frog during hibernation in comparison with the active period. The most evident changes were an increase in the nuclear segmentation of the neutrophils and a lower activity of the histochemically demonstrable DHFR and LDH, probably due to cell ageing. These findings suggest that, in hibernating frogs, there is an increase in the life span of the peripheral blood cells as a consequence of a reduced metabolic activity and a slowing of haematopoiesis.     

Acetylcholinesterase activity is associated with efferent endings in the sensory epithelia of the utricle and semicircular canals of the rainbow trout inner ear

The mechanosensory hair cells of the utricle and semicircular canals of the trout inner ear are morphologically similar to type II hair cells of the avain and mammalian vestibular end organs. These cells are innervated by two types of nerve terminals. The nonvesiculated terminals are considered to be afferent, and the vesiculated endings are presumed to be efferent. The presumptive efferent endings contain numerous clear, round vesicles and a few that are dense-cored. Histocytochemical, electron microscopic analysis has localized acetylcholinesterase activity to plasma membranes of vesiculated, presumptive efferent nerve terminals in sensory epithelia of the utricle and semicircular canals. No reaction product was observed at the receptoneural synapse or found in nonvesiculated, presumptive afferent endings. Control specimens incubated in the presence of eserine sulfate, an inhibitor of acetylcholinesterase, were devoid of reaction product. These results support the tenet that vesiculated nerve endings in the sensory epithelia of the utricle and semicircular canals of the trout are cholinergic. © 1993 Wiley-Liss, Inc.     

Effects of activation of the divergent efferent fibers on the spontaneous activity of vestibular afferent fibers in the toad

In anesthetized toads, spontaneous activities were recorded from single afferent fibers of three semicircular canals and three otolith organs. Since some efferent fibers ramify within the eighth nerve and innervate two or more vestibular organs, a single branchlet of the eighth nerve was disconnected from its end organ, and was electrically stimulated to activate divergent efferent collaterals leading to other vestibular organs. The stimulation elicited an inhibitory effect on spontaneous activities of about one third of the afferent population, and a facilitatory effect on those of another one third. The remaining one third was unaffected. Whether or not the inhibitory or facilitatory effect was observed in an individual unit seemed to be related to its pattern and its rate of spontaneous activity. Most of the units showing relatively high and regular spontaneous firing were insensitive to the electrical stimulation, and units with a low firing rate and an irregular pattern of activity tended to be affected by the electrical stimulation. The activation of divergent efferent fibers elicited both inhibition and facilitation on the spontaneous afferent activities in all vestibular nerve branchlets, except in the saccular branchlet, where only inhibition was elicited. Electrical stimulation of the central stump of the saccular nerve branchlet, however, could produce both inhibitory and facilitatory effects in other vestibular nerve branchlets.     

Analysis of pre- and postsynaptic activity in the frog semicircular canal following ototoxic insult: differential recovery of background and evoked afferent activity

Frogs were treated with a single dose of gentamicin administered intraotically to produce severe degeneration of posterior semicircular canal hair cells and to evaluate the time course of functional damage and recovery both at pre- and postsynaptic level. In isolated canal preparations the endoampullar potential, which reflects the summed receptor potentials of crista hair cells, was progressively reduced in amplitude and completely abolished 6 days after gentamicin treatment. At this time the crista epithelium was devoid of hair cells. The recovery of the endoampullar potential began around 9 days after the ototoxic insult and its amplitude progressively increased to reach, after 20 days, values close to those observed in control experiments. The endoampullar potential amplitude was related to the degree of hair cell regeneration in the crista epithelium. Consistent with the presynaptic damage, the slow generator potential (representing the summed miniature excitatory postsynaptic potential [mEPSP] activity of all posterior nerve fibres) and the resting and evoked spike discharge recorded from the whole ampullar nerve were abolished 6 days after gentamicin treatment. The recovery of the background and evoked afferent activity showed different behaviours. Background spike activity became detectable around 8 days after the ototoxic insult, but was not modulated by canal stimulation at this time, and no generator potential was detected. Moreover, the resting spike frequency fully recovered and reached control values around 15 days after gentamicin treatment, whereas the evoked activity attained normal values only 20 days after the ototoxic insult. These results were confirmed by intracellular recordings from single afferent fibres of the ampullar nerve in intact labyrinth preparations. Absence of any resting and evoked discharge was the most common pattern observed in the early period from 7 to 8 days after gentamicin treatment. Fifty-five percent of impaled afferents were silent while the others showed low resting frequencies of mEPSPs and spikes, and were unresponsive to canal rotation. In the intermediate period from 14 to 15 days after gentamicin treatment, background mEPSP and spike frequencies approached those evaluated in control experiments, but the frequencies of the evoked mEPSPs and spikes were clearly lower than in controls. In the late period, from 18 to 20 days after the ototoxic insult, the impaled afferents showed normal evoked mEPSP and spike frequencies. The present data indicate that the frog semicircular canal completely recovers its pre- and postsynaptic activity following severe ototoxic insult. During the regeneration process, the cytoneural junction regains function and the resting discharge reappears before recovery of mechanoelectrical transduction.     

Timing of bilateral cerebellar output evoked by unilateral vestibular stimulation in the frog

Electrical stimulation of one VIIIth nerve evoked simple spike activity in Purkinje cells located on either side of the cerebellum. This cerebellar output was delayed by ca. 10 ms with respect to its mossy fiberparallel fiber input. The onset of the cerebellar output occurs on the average simultaneously on either side of the corpus cerebelli. The delay is explained by slowly rising EPSPs in PC induced by primary afferent and by second and higher order vestibular fibers. The latter inputs are stronger and terminate ipsi- and contralaterally in the granular layer.Supported by Deutsche Forschungsgemeinschaft (Pr. 158/1)     

Premitotic DNA synthesis in the brain of the adult frog (Rana esculenta L.): An autoradiographic 3H-thymidine study

Replicative synthesis of DNA in the brain of the adult frog was studied by light microscope autoradiography. Animals collected during the active period (May–June) and in hibernation (January) were used. In active frogs, 3H-thymidine labelling occurred mainly in the ependymal cells which line the ventricles. The mean labelling index (LI%) was higher in the ependyma of the lateral and fourth ventricles than in the ependyma of the lateral diencephalon and tectal parts of the mesencephalon. In the recessus infundibularis and preopticus the number of labelled cells (LCs) was several times greater than in the lateral parts of the third ventricle. LCs were seen subependymally only occasionally. The incidence of LCs in the parenchyma of the brain was much lower in most regions than in the ventricular ependyma; LCs were mainly small and, from their nuclear morphology, they were glial cells. The LI% reached the highest value in the septum hippocampi and in the nucleus entopeduncularis. In these locations, LCs were larger and closer in size to the nerve cells of these regions. From comparison with data obtained earlier in the brain of mammals, it is evident that the distribution of proliferating cells in the olfactory and limbic system is phylogenetically conservative. The occurrence of pyknotic cells in the same areas which contain LCs, suggests that cell division reflects in part the process of cell renewal observed in mammals. However, proliferating cells could also be linked to the continuous growth observed in non-mammalian vertebrates. In hibernating frogs, LCs and pyknoses were not seen or were found occasionally, which further indicates the functional significance of both processes.