Bilateral communication between vestibular labyrinths in pigeons

How Peripheral Vestibular Damage Affects Velocity Storage: a Causative Explanation

Velocity storage is a centrally-mediated mechanism that processes peripheral vestibular inputs. One prominent aspect of velocity storage is its effect on dynamic responses to yaw rotation. Specifically, when normal human subjects are accelerated to constant angular yaw velocity, horizontal eye movements and perceived angular velocity decay exponentially with a time constant circa 15-30 s, even though the input from the vestibular periphery decays much faster (~ 6 s). Peripheral vestibular damage causes a time constant reduction, which is useful for clinical diagnoses, but a mechanistic explanation for the relationship between vestibular damage and changes in these behavioral dynamics is lacking. It has been hypothesized that Bayesian optimization determines ideal velocity storage dynamics based on statistics of vestibular noise and experienced motion. Specifically, while a longer time constant would make the central estimate of angular head velocity closer to actual head motion, it may also result in the accumulation of neural noise which simultaneously degrades precision. Thus, the brain may balance these two effects by determining the time constant that optimizes behavior. We applied a Bayesian optimal Kalman filter to determine the ideal velocity storage time constant for unilateral damage. Predicted time constants were substantially lower than normal and similar to patients. Building on our past work showing that Bayesian optimization explains age-related changes in velocity storage, we also modeled interactions between age-related hair cell loss and peripheral damage. These results provide a plausible mechanistic explanation for changes in velocity storage after peripheral damage. Results also suggested that even after peripheral damage, noise originating in the periphery or early central processing may remain relevant in neurocomputations. Overall, our findings support the hypothesis that the brain optimizes velocity storage based on the vestibular signal-to-noise ratio.

Synaptic transmission at the vestibular hair cells of amniotes

A harmonized interplay between the central nervous system and the five peripheral end organs is how the vestibular system helps organisms feel a sense of balance and motion in three-dimensional space. The receptor cells of this system, much like their cochlear equivalents, are the specialized hair cells. However, research over the years has shown that the vestibular endorgans and hair cells evolved very differently from their cochlear counterparts. The structurally unique calyceal synapse, which appeared much later in the evolutionary time scale, and continues to intrigue researchers, is now known to support several forms of synaptic neurotransmission. The conventional quantal transmission is believed to employ the ribbon structures, which carry several tethered vesicles filled with neurotransmitters. However, the field of vestibular hair cell synaptic molecular anatomy is still at a nascent stage and needs further work. In this review, we will touch upon the basic structure and function of the peripheral vestibular system, with the focus on the various modes of neurotransmission at the type I vestibular hair cells. We will also shed light on the current knowledge about the molecular anatomy of the vestibular hair cell synapses and vestibular synaptopathy.

Differences in the Structure and Function of the Vestibular Efferent System Among Vertebrates

The role of the mammalian vestibular efferent system in everyday life has been a long-standing mystery. In contrast to what has been reported in lower vertebrate classes, the mammalian vestibular efferent system does not appear to relay inputs from other sensory modalities to the vestibular periphery. Furthermore, to date, the available evidence indicates that the mammalian vestibular efferent system does not relay motor-related signals to the vestibular periphery to modulate sensory coding of the voluntary self-motion generated during natural behaviors. Indeed, our recent neurophysiological studies have provided insight into how the peripheral vestibular system transmits head movement-related information to the brain in a context independent manner. The integration of vestibular and extra-vestibular information instead only occurs at next stage of the mammalian vestibular system, at the level of the vestibular nuclei. The question thus arises: what is the physiological role of the vestibular efferent system in mammals? We suggest that the mammalian vestibular efferent system does not play a significant role in short-term modulation of afferent coding, but instead plays a vital role over a longer time course, for example in calibrating and protecting the functional efficacy of vestibular circuits during development and aging in a role analogous the auditory efferent system.

A review of efferent cholinergic synaptic transmission in the vestibular periphery and its functional implications

It has been over 60 years since peripheral efferent vestibular terminals were first identified in mammals, and yet the function of the efferent vestibular system remains obscure. One reason for the lack of progress may be due to our deficient understanding of the peripheral efferent synapse. Although vestibular efferent terminals were identified as cholinergic less than a decade after their anatomical characterization, the cellular mechanisms that underlie the properties of these synapses have had to be inferred. In this review we examine how recent mammalian studies have begun to reveal both nicotinic and muscarinic effects at these terminals and therefore provide a context for fast and slow responses observed in classic electrophysiological studies of the mammalian efferent vestibular system, nearly 40 years ago. Although incomplete, these new results together with those of recent behavioral studies are helping to unravel the mysterious and perplexing action of the efferent vestibular system. Armed with this information, we may finally appreciate the behavioral framework in which the efferent vestibular system operates.

Confirming a Role for α9nAChRs and SK Potassium Channels in Type II Hair Cells of the Turtle Posterior Crista

In turtle posterior cristae, cholinergic vestibular efferent neurons (VENs) synapse on type II hair cells, bouton afferents innervating type II hair cells, and afferent calyces innervating type I hair cells. Electrical stimulation of VENs releases acetylcholine (ACh) at these synapses to exert diverse effects on afferent background discharge including rapid inhibition of bouton afferents and excitation of calyx-bearing afferents. Efferent-mediated inhibition is most pronounced in bouton afferents innervating type II hair cells near the torus, but becomes progressively smaller and briefer when moving longitudinally through the crista toward afferents innervating the planum. Sharp-electrode recordings have inferred that efferent-mediated inhibition of bouton afferents requires the sequential activation of alpha9-containing nicotinic ACh receptors (α9^*nAChRs) and small-conductance, calcium-dependent potassium channels (SK) in type II hair cells. Gradations in the strength of efferent-mediated inhibition across the crista likely reflect variations in α9^*nAChRs and/or SK activation in type II hair cells from those different regions. However, in turtle cristae, neither inference has been confirmed with direct recordings from type II hair cells. To address these gaps, we performed whole-cell, patch-clamp recordings from type II hair cells within a split-epithelial preparation of the turtle posterior crista. Here, we can easily visualize and record hair cells while maintaining their native location within the neuroepithelium. Consistent with α9^*nAChR/SK activation, ACh-sensitive currents in type II hair cells were inward at hyperpolarizing potentials but reversed near −90 mV to produce outward currents that typically peaked around −20 mV. ACh-sensitive currents were largest in torus hair cells but absent from hair cells near the planum. In current clamp recordings under zero-current conditions, ACh robustly hyperpolarized type II hair cells. ACh-sensitive responses were reversibly blocked by the α9nAChR antagonists ICS, strychnine, and methyllycaconitine as well as the SK antagonists apamin and UCL1684. Intact efferent terminals in the split-epithelial preparation spontaneously released ACh that also activated α9^*nAChRs/SK in type II hair cells. These release events were accelerated with high-potassium external solution and all events were blocked by strychnine, ICS, methyllycaconitine, and apamin. These findings provide direct evidence that activation of α9^*nAChR/SK in turtle type II hair cells underlies efferent-mediated inhibition of bouton afferents.

Reviewing the Role of the Efferent Vestibular System in Motor and Vestibular Circuits

Efferent circuits within the nervous system carry nerve impulses from the central nervous system to sensory end organs. Vestibular efferents originate in the brainstem and terminate on hair cells and primary afferent fibers in the semicircular canals and otolith organs within the inner ear. The function of this efferent vestibular system (EVS) in vestibular and motor coordination though, has proven difficult to determine, and remains under debate. We consider current literature that implicate corollary discharge from the spinal cord through the efferent vestibular nucleus (EVN), and hint at a potential role in overall vestibular plasticity and compensation. Hypotheses range from differentiating between passive and active movements at the level of vestibular afferents, to EVS activation under specific behavioral and environmental contexts such as arousal, predation, and locomotion. In this review, we summarize current knowledge of EVS circuitry, its effects on vestibular hair cell and primary afferent activity, and discuss its potential functional roles.

Efferent innervation of turtle semicircular canal cristae: comparisons with bird and mouse

In the vestibular periphery of nearly every vertebrate, cholinergic vestibular efferent neurons give rise to numerous presynaptic varicosities that target hair cells and afferent processes in the sensory neuroepithelium. Although pharmacological studies have described the postsynaptic actions of vestibular efferent stimulation in several species, characterization of efferent innervation patterns and the relative distribution of efferent varicosities among hair cells and afferents are also integral to understanding how efferent synapses operate. Vestibular efferent markers, however, have not been well characterized in the turtle, one of the animal models used by our laboratory. Here we sought to identify reliable efferent neuronal markers in the vestibular periphery of turtle, to use these markers to understand how efferent synapses are organized, and to compare efferent neuronal labeling patterns in turtle with two other amniotes using some of the same markers. Efferent fibers and varicosities were visualized in the semicircular canal of red-eared turtles (Trachemys scripta elegans), zebra finches (Taeniopygia guttata), and mice (Mus musculus) utilizing fluorescent immunohistochemistry with antibodies against choline acetyltransferase (ChAT). Vestibular hair cells and afferents were counterstained using antibodies to myosin VIIa and calretinin. In all species, ChAT labeled a population of small diameter fibers giving rise to numerous spherical varicosities abutting type II hair cells and afferent processes. That these ChAT-positive varicosities represent presynaptic release sites were demonstrated by colabeling with antibodies against the synaptic vesicle proteins synapsin I, SV2, or syntaxin and the neuropeptide calcitonin gene-related peptide. Comparisons of efferent innervation patterns among the three species are discussed.

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

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

Internal models of self-motion: computations that suppress vestibular reafference in early vestibular processing

In everyday life, vestibular sensors are activated by both self-generated and externally applied head movements. The ability to distinguish inputs that are a consequence of our own actions (i.e., active motion) from those that result from changes in the external world (i.e., passive or unexpected motion) is essential for perceptual stability and accurate motor control. Recent work has made progress toward understanding how the brain distinguishes between these two kinds of sensory inputs. We have performed a series of experiments in which single-unit recordings were made from vestibular afferents and central neurons in alert macaque monkeys during rotation and translation. Vestibular afferents showed no differences in firing variability or sensitivity during active movements when compared to passive movements. In contrast, the analyses of neuronal firing rates revealed that neurons at the first central stage of vestibular processing (i.e., in the vestibular nuclei) were effectively less sensitive to active motion. Notably, however, this ability to distinguish between active and passive motion was not a general feature of early central processing, but rather was a characteristic of a distinct group of neurons known to contribute to postural control and spatial orientation. Our most recent studies have addressed how vestibular and proprioceptive inputs are integrated in the vestibular cerebellum, a region likely to be involved in generating an internal model of self-motion. We propose that this multimodal integration within the vestibular cerebellum is required for eliminating self-generated vestibular information from the subsequent computation of orientation and posture control at the first central stage of processing.

Responses of pigeon vestibular hair cells to cholinergic agonists and antagonists

Acetylcholine (ACh) is the major neurotransmitter released from vestibular efferent terminals onto hair cells and afferents. Previous studies indicate that the two classes of acetylcholine receptors, nicotinic (nAChRs) and muscarinic receptors (mAChRs), are expressed by vestibular hair cells (VHCs). To identify if both classes of receptors are present in VHCs, whole cell, voltage-clamp- and current-clamp-patch recordings were performed on isolated pigeon vestibular type I and type II HCs during the application of the cholinergic agonists, acetylcholine and carbachol, and the cholinergic antagonists, D-tubocurarine and atropine. By applying in different combinations, these compounds were used to selectively activate either nAChRs or mAChRs. The effects of nAChR and mAChR activation on HC currents and zero electrode current potential (V(z)) were monitored. It was found that presumed mAChR activation decreased both inward and outward currents in both type I and type II HCs, resulting in either a depolarization or hyperpolarization. Conversely, nAChR activation mainly increased both inward and outward currents in type II HCs, resulting in a hyperpolarization of their V(z). nAChR activation also increased outward currents in type I HCs resulting in either a depolarization or hyperpolarization of their V(z). The decrease of inward and outward currents and the depolarization of the V(z) in type I pigeon HCs by activation of mAChRs represents a new finding. Ion channel candidates in pigeon vestibular HCs that might underlie the modulation of the macroscopic ionic currents and V(z) by different AChR activation are discussed.

Efferent-mediated responses in vestibular nerve afferents of the alert macaque

The peripheral vestibular organs have long been known to receive a bilateral efferent innervation from the brain stem. However, the functional role of the efferent vestibular system has remained elusive. In this study, we investigated efferent-mediated responses in vestibular afferents of alert behaving primates (macaque monkey). We found that efferent-mediated rotational responses could be obtained from vestibular nerve fibers innervating the semicircular canals after conventional afferent responses were nulled by placing the corresponding canal plane orthogonal to the plane of motion. Responses were type III, i.e., excitatory for rotational velocity trapezoids (peak velocity, 320 degrees/s) in both directions of rotation, consistent with those previously reported in the decerebrate chinchilla. Responses consisted of both fast and slow components and were larger in irregular (approximately 10 spikes/s) than in regular afferents (approximately 2 spikes/s). Following unilateral labyrinthectomy (UL) on the side opposite the recording site, similar responses were obtained. To confirm the vestibular source of the efferent-mediated responses, the ipsilateral horizontal and posterior canals were plugged following the UL. Responses to high-velocity rotations were drastically reduced when the superior canal (SC), the only intact canal, was in its null position, compared with when the SC was pitched 50 degrees upward from the null position. Our findings show that vestibular afferents in alert primates show efferent-mediated responses that are related to the discharge regularity of the afferent, are of vestibular origin, and can be the result of both afferent excitation and inhibition.

Response of vestibular nerve afferents innervating utricle and saccule during passive and active translations

The distinction between sensory inputs that are a consequence of our own actions from those that result from changes in the external world is essential for perceptual stability and accurate motor control. In this study, we investigated whether linear translations are encoded similarly during active and passive translations by the otolith system. Vestibular nerve afferents innervating the saccule or utricle were recorded in alert macaques. Single unit responses were compared during passive whole body, passive head-on-body, and active head-on-body translations (vertical, fore-aft, or lateral) to assess the relative influence of neck proprioceptive and efference copy-related signals on translational coding. The response dynamics of utricular and saccular afferents were comparable and similarly encoded head translation during passive whole body versus head-on-body translations. Furthermore, when monkeys produced active head-on-body translations with comparable dynamics, the responses of both regular and irregular afferents remained comparable to those recorded during passive movements. Our findings refute the proposal that neck proprioceptive and/or efference copy inputs coded by the efferent system function to modulate the responses of the otolith afferents during active movements. We conclude that the vestibular periphery provides faithful information about linear movements of the head in the space coordinates, regardless of whether they are self- or externally generated.

Mechanisms of efferent-mediated responses in the turtle posterior crista

To study the cellular mechanisms of efferent actions, we recorded from vestibular-nerve afferents close to the turtle posterior crista while efferent fibers were electrically stimulated. Efferent-mediated responses were obtained from calyx-bearing (CD, calyx and dimorphic) afferents and from bouton (B) afferents distinguished by their neuroepithelial locations into BT units near the torus and BM units at intermediate sites. The spike discharge of CD units is strongly excited by efferent stimulation, whereas BT and BM units are inhibited, with BM units also showing a postinhibitory excitation. Synaptic activity was recorded intracellularly after spikes were blocked. Responses of BT/BM units to single efferent shocks consist of a brief depolarization followed by a prolonged hyperpolarization. Both components reflect variations in hair-cell quantal release rates and are eliminated by pharmacological antagonists of alpha9/alpha10 nicotinic receptors. Blocking calcium-dependent SK potassium channels converts the biphasic response into a prolonged depolarization. Results can be explained, as in other hair-cell systems, by the sequential activation of alpha9/alpha10 and SK channels. In BM units, the postinhibitory excitation is based on an increased rate of hair-cell quanta and depends on the preceding inhibition. There is, in addition, an efferent-mediated, direct depolarization of BT/BM and CD fibers. In CD units, it is the exclusive efferent response. Nicotinic antagonists have different effects on hair-cell efferent actions and on the direct depolarization of CD and BT/BM units. Ultrastructural studies, besides confirming the efferent innervation of type II hair cells and calyx endings, show that turtle efferents commonly contact afferent boutons terminating on type II hair cells.

Efferent-mediated fluctuations in vestibular nerve discharge: a novel, positive-feedback mechanism of efferent control

We compared the background discharge of vestibular nerve afferents in barbiturate-anesthetized and unanesthetized, decerebrate chinchillas. Based on their interspike-interval statistics, units were categorized as regular, intermediate, or irregular. Background discharge rates were higher in irregular units from decerebrates compared to anesthetized preparations; no such difference was observed for regular or intermediate units. Large fluctuations in discharge rate were confined to intermediate and irregular units in decerebrates, but were not seen at all in anesthetized animals. The most prominent examples of fluctuations consisted of oscillations with periods exceeding 500 s and peak-to-peak amplitudes as large as 300 spikes/s. Several observations show that the fluctuations are mediated by the efferent vestibular system (EVS): (1) they are abolished when the vestibular nerve is cut proximal to the recording electrode; (2) their amplitude is correlated with the size of efferent-mediated rotational responses in individual units; and (3) they occur even when vital signs are stable. Previous studies had provided evidence that the EVS involves positive feedback: vestibular nerve afferents and EVS neurons excite one another. To study how oscillations could be produced, we developed a nonlinear model of positive feedback in which afferent feed-forward discharge was nonlinearly related to its inputs from hair cells and the EVS, while these inputs declined (adapted) as discharge was prolonged. Provided that the gain of the efferent feedback loop was sufficiently large, the model showed oscillations similar to those observed experimentally. Although large fluctuations in afferent discharge are unlikely to occur under physiological circumstances, positive feedback may be a normal feature that can amplify the influence of the EVS.

Polysynaptic inputs to vestibular efferent neurons as revealed by viral transneuronal tracing

The Bartha strain of the alpha-herpes pseudorabies virus (PrV) was used as a retrograde transneuronal tracer to map synaptic inputs to the vestibular efferent neurons of the Mongolian gerbil, Meriones unguiculatus. Although previous experiments have shown that vestibular efferent neurons respond to visual motion and somatosensory stimuli, the anatomic connections mediating those responses are unknown. PrV was injected unilaterally into the horizontal semicircular canal neuroepithelium of gerbils, where it was taken up by efferent axon terminals. The virus was then retrogradely transported to efferent cell bodies, replicated, and transported into synaptic endings projecting onto the efferent cells. Thirty animals were sacrificed at approximately 5-h increments between 75 and 105 h post-infection after determining that shorter time points had no central infection. Infected cells were visualized immunohistochemically. Temporal progression of neuronal infection was used to determine the nature of primary and higher order projections to the vestibular efferent neurons. Animals sacrificed at 80-94 h post-inoculation exhibited immunostaining in the dorsal and ventral group of vestibular efferent neurons, predominately on the contralateral side. Neurons within the medial, gigantocellular, and lateral reticular formations were among the first cells infected thereafter. At 95 h, additional virus-labeled cell groups included the solitary, area postrema, pontine reticular, prepositus, dorsal raphe, tegmental, the subcoeruleus nuclei, the nucleus of Darkschewitsch, and the inferior olivary beta and ventrolateral subnuclei. Analysis beyond 95 h revealed virus-infected neurons located in the vestibulo-cerebellar and motor cortices. Paraventricular, lateral, and posterior hypothalamic cells, as well as central amygdala cells, were also labeled. Spinal cord tissue exhibited no labeling in the intermediolateral cell column, but scattered cells were found in the central cervical nucleus. The results suggest functional associations among efferent feedback regulation of labyrinthine sensory input and both behavioral and autonomic systems, and support a closed-looped vestibular feedback model with additional open-loop polysynaptic inputs.

Na+ currents in vestibular type I and type II hair cells of the embryo and adult chicken

In birds, type I and type II hair cells differentiate before birth. Here we describe that chick hair cells, from the semicircular canals, begin expressing a voltage-dependent Na current (INa) from embryonic day 14 (E14) and continue to express the current up to hatching (E21). During this period, INa was present in most (31/43) type I hair cells irrespective of their position in the crista, in most type II hair cells located far from the planum semilunatum (48/63), but only occasionally in type II hair cells close to the planum semilunatum (2/35). INa activated close to -60 mV, showed fast time- and voltage-dependent activation and inactivation, and was completely, and reversibly, blocked by submicromolar concentrations of tetrodotoxin (Kd = 17 nM). One peculiar property of INa concerns its steady-state inactivation, which is complete at -60 mV (half-inactivating voltage = -96 mV). INa was found in type I and type II hair cells from the adult chicken as well, where it had similar, although possibly not identical, properties and regional distribution. Current-clamp experiments showed that INa could contribute to the voltage response provided that the cell membrane was depolarized from holding potentials more negative than -80 mV. When recruited, INa produced a significant acceleration of the cell membrane depolarization, which occasionally elicited a large rapid depolarization followed by a rapid repolarization (action-potential-like response). Possible physiological roles for INa in the embryo and adult chicken are discussed.

Calcitonin gene-related Peptide and choline acetyltransferase colocalization in the human vestibular periphery

Within the vestibular system, calcitonin gene-related peptide (CGRP) has been localized in the efferent terminals and their brainstem neuronal cell bodies in several animal models. Presently, very few studies have verified these findings in the vestibular system in adult primates or humans. CGRP immunoreactivity (CGRPi) and its colocalization with choline acetyltransferase immunoreactivity (ChATi) in human vestibular end organs and Scarpa's ganglion were studied using polyclonal antibodies against CGRP and ChAT, at the light-microscopic level. The CGRPi axons ramified to produce numerous CGRPi terminals throughout the neurosensory epithelium of the maculae and cristae, primarily in the basal and midbasal areas. Numerous CGRPi efferent terminals made contact with both type II vestibular hair cells and the afferent chalices surrounding type I vestibular hair cells. All CGRP immunoreactive fibers also exhibited ChATi. As in the animal models, no CGRPi was found within Scarpa's ganglion. This study provides evidence for CGRPi in the human vestibular periphery and validates the biomedical relevance of the current animal models.

Reflections of efferent activity in rotational responses of chinchilla vestibular afferents

To study presumed efferent-mediated responses, we determined if afferents responded to head rotations that stimulated semicircular canals other than the organ being innervated. To minimize stimulation of an afferent's own canal, its plane was placed nearly orthogonal to the rotation plane. Otolith units were tested in a horizontal head position with the ear placed near the rotation axis to minimize linear forces. Under these circumstances, angular-velocity trapezoids (2-s ramps, 2-s plateau) evoked excitatory responses for both rotation directions. These type III responses were considerably larger in decerebrate than in anesthetized preparations. In addition to their being exclusively excitatory, the responses resembled those obtained with electrical stimulation of efferent pathways in including per-stimulus and more prolonged post-stimulus components and in being larger in irregularly discharging than in regularly discharging units. Responses, which were not seen for rotations <80 degrees/s, grew as velocity increased between 80 and 500 degrees/s but were seldom larger than 20 spikes/s. Complete section of the VIIIth nerve abolished type III responses, leaving conventional afferent responses intact. To study the separate contributions of canals on the two sides, responses were compared when the labyrinths were intact and when the ipsilateral or contralateral horizontal canal was mechanically inactivated. Both sides contributed to the efferent-mediated responses. That afferents could be influenced from the contralateral labyrinth was confirmed with the use of unilateral galvanic currents. Following inactivation, excitatory responses were produced by rotations exciting or inhibiting the intact horizontal canal with the responses resulting from excitatory rotations being much larger. Such a response asymmetry is consistent with a semicircular-canal origin for the type III responses. A similar asymmetry was seen in the post-stimulus responses to contralateral cathodal (excitatory) and anodal (inhibitory) galvanic currents. We conclude that the efferent system receives a sufficiently powerful vestibular input from both the ipsilateral and contralateral labyrinths to affect afferent discharge.

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

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

Ligand-gated purinergic receptors are differentially expressed in the adult rat vestibular periphery

To further characterize the pattern of expression of the ligand-gated purinergic P2X receptors in the peripheral vestibular system, we conducted reverse transcription-polymerase chain reaction amplification of P2X1 and P2X2 messenger RNA extracted from adult rat vestibular ganglia (Scarpa's ganglia) and vestibular end organs. Transcripts encoding P2X1 were found in both Scarpa's ganglia and the end organs, but transcripts encoding P2X2 were found only in the vestibular end organs. These results support previous electrophysiological data, and they provide a more complete understanding of the specific role of purinergic (adenosine-5'-triphosphate) transmission in the vestibular periphery.

Responses to efferent activation and excitatory response-intensity relations of turtle posterior-crista afferents

Multivariate statistical formulas were used to infer the morphological type and longitudinal position of extracellularly recorded afferents. Efferent fibers were stimulated electrically in the nerve branch interconnecting the anterior and posterior VIIIth nerves. Responses of bouton (B) units depended on their inferred position: BP units (near the planum semilunatum) showed small excitatory responses; BT units (near the torus) were inhibited; BM units (in an intermediate position) had a mixed response, including an initial inhibition and a delayed excitation. Calyx-bearing (CD-high) units with an appreciable background discharge showed large per-train excitatory responses followed by smaller post-train responses that could outlast the shock train by 100 s. Excitatory responses were smaller in calyx-bearing (CD-low) units having little or no background activity than in CD-high units. Excitatory response-intensity functions, derived from the discharge during 2-s angular-velocity ramps varying in intensity, were fit by empirical functions that gave estimates of the maximal response (r(MAX)), a threshold velocity (v(T)), and the velocity producing a half-maximal response (v(1/2)). Linear gain is equal to r(MAX)/v(S), v(S) = v(1/2) - v(T). v(S) provides a measure of the velocity range over which the response is nearly linear. For B units, r(MAX) declines by as much as twofold over the 2-s ramp, whereas for CD units, r(MAX) increases by 15% during the same time period. At the end of the ramp, r(MAX) is on average twice as high in CD as in B units. Thresholds are negligible in most spontaneously active units, including almost all B and CD-high units. Silent CD-low units typically have thresholds of 10-100 deg/s. BT units have very high linear gains and v(S) < 10 deg/s. Linear gains are considerably lower in BP units and v(S) > 150 deg/s. CD-high units have intermediate gains and near 100 deg/s v(S) values. CD-low units have low gains and v(S) values ranging from 150 to more than 300 deg/s. The results suggest that BT units are designed to measure the small head movements involved in postural control, whereas BP and CD units are more appropriate for monitoring large volitional head movements. The former units are silenced by efferent activation, whereas the latter units are excited. This suggests that the efferent system switches the turtle posterior crista from a "postural" to a "volitional" mode.

Topographic distribution of nicotinic acetylcholine receptors in the cristae of a turtle

The neurochemical basis of cholinergic efferent modulation of afferent function in the vestibular periphery remains incompletely understood; however, there is cellular, biochemical and molecular biological evidence for both muscarinic and nicotinic acetylcholine (ACh) receptors (nAChRs) in this system. This study examined the topographic distribution of alpha-bungarotoxin (alpha-BTX) nAChRs in the cristae of a turtle species. Cristae were perfusion-fixed, cut at 20 micrometer on a cryostat and incubated with alpha-BTX or polyclonal antibodies raised against Torpedo nAChR. Light microscopy showed abundant specific labeling of nAChR in the central zone of each hemicrista on the calyx-bearing afferents surrounding type I hair cells and on the base of the type II hair cells. Within the peripheral zone, dense labeling of type II hair cells near the torus and sparse or no label was observed on type II hair cells near the planum. The alpha-BTX binding showed a similar pattern within the cristae. The similarity between the topographic distribution of alpha-BTX binding nAChR and of efferent inhibition of afferents supports the notion that the inhibitory effect of afferents is mediated by nAChR.

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

In decerebrate cats after acute hemilabyrinthectomy, the response sensitivity of extracellularly recorded vestibular nuclear neurons on the lesioned and labyrinth-intact sides were examined quantitatively during constant velocity off-vertical axis rotations with an aim to elucidate the functional contribution of otolithic inputs to the ipsilateral and contralateral vestibular nuclei. The bidirectional response sensitivity, delta, was determined as the ratio of the gain during clockwise to that during counterclockwise rotations. A continuum of response sensitivity was identified: one-dimensional neurons showed symmetrically bidirectional response patterns, while two-dimensional neurons showed asymmetrically bidirectional patterns that in some cases approached unidirectional patterns with change in velocity. The proportion of two-dimensional neurons was significantly increased after acute hemilabyrinthectomy. Two-dimensional neurons that responded only to one direction of rotation in at least one of the velocities tested were described as unidirectional neurons. This unidirectional response pattern was observed in one-third of the entire neuronal population studied, but not in cats with both labyrinths intact, thus suggesting that such prominent broadly tuned responses are normally masked by converging otolithic inputs from the contralateral side. These neurons were found in higher proportion on the lesioned side than on the labyrinth-intact side. Among the 70% of unidirectional neurons that exhibited bidirectional response at some velocities and unidirectional response at others, prominent shifts in delta values (i.e. between 0/infinity and finite values) with velocity can be computed for each neuron. The shifts in delta values correlated with large shifts in the response dynamics and spatial orientation as the response pattern changed with velocity. The response orientations of the unidirectional neurons pointed in all directions on the horizontal plane. When all the two-dimensional neurons (i.e. both the unidirectionally and bidirectionally responsive ones) were pooled, imbalances in the distribution of the response orientations and in response gain were found between the ipsilateral-side-down/head-down half-circle and the contralateral-side-down/head-up half-circle on the labyrinth-intact side, but not on the lesioned side. These results, derived from spatiotemporal processing of gravitational signals, reveal a novel dimension of imbalance between neuronal populations in the two vestibular nuclear complexes after acute lesion of one labyrinth. This feature would provide, on the one hand, deranged cues of spatial orientation and direction during slow head excursions and, on the other, a framework for the dynamic behavioral deficits associated with hemilabyrinthectomy.

Recovery of semicircular canal primary afferent activity in the pigeon after streptomycin ototoxicity

Recovery of semicircular canal primary afferent activity in the pigeon after streptomycin ototoxicity. J. Neurophysiol. 80: 3297-3311, 1998. The electrophysiological activity of horizontal semicircular canal primary afferents (HSCPA) was investigated in vivo in the barbiturate-anesthetized pigeon by means of extracellular single-fiber vestibular nerve action potential recordings. The spontaneous and driven discharges to pulse (step/trapezoid waveform, peak velocity = 120 degrees/s) and sum-of-sines (0.03, 0.09, 0.21, 0.39, 0.93, 1.83 Hz, peak velocity = 30 degrees/s for each frequency) rotations were measured both in normal control animals and a group of animals at 30, 40, 50, 60, 71, and 150 days postinjection sequence (PIS) of streptomycin sulfate. Prior to 30 days PIS, the activity in the nerve was not appropriately modulated during and after rotation. At 30 days PIS and thereafter, the responses resembled those observed in control animals but with systematic changes in parameters of fitted pulse responses and fitted Bode plots as days PIS increased. The return of parameters characterizing the neural dynamics of the semicircular canals were monotonic and could be best described by either linear or exponential functions. After 30 days PIS, the mechanical cupula-endolymph system, the function of which can be inferred from the cupula long time constant (tauL) following step velocity, did not change systematically (tauL = 6.92 +/- 3.96, 8.64 +/- 5.52, 8.35 +/- 4.21, 10.00 +/- 2.79, 9.05 +/- 3.67, 7.05 +/- 2.72; means +/- SD). However, the mean gain (G) of the HSCPA response to pulse rotation nearly doubled between 30 and 150 days PIS (from 1.31 +/- 0. 39 to 2.40 +/- 1.04) and returned linearly to control values (G = 2. 39 +/- 0.77) over this time period [G = 1.33 + 0.009(PIS-30), R2 = 0. 92, P < 0.05]. Meanwhile, neural adaptation as quantitated using a fractional operator, k, decayed exponentially (single exponential) to an asymptote. The time constant of this exponential was approximately 55 days [k = 0.034 + 0.33e-(PIS-30)/55.4, R2 = 0.99, P < 0.01]. Features of the spontaneous discharge previously shown to be correlated with k changed appropriately. That is, the coefficient of variation (CV) and frequency of firing (FF) decayed and grew asymptotically, respectively. These parameters also exhibited an exponential time course of return to control values from 30 to 150 days PIS [CV = 0.44 + 0.65e-(PIS-30)/21.5, R2 = 0.96, P < 0.01, and FF = 39.97 + 101.42(1 - e-(PIS-30)/32.6), R2 = 0.97, P < 0.01]. The trends of recovery for G, k, and tauL derived from analysis of the pulse response were confirmed by strong positive correlations with best fitted parameters obtained from analysis of the sum-of-sines frequency domain response of HSCPAs. There were statistically significant correlations (r = 0.90, P < 0.05 and r = 0.93, P < 0.05) between parameters (G, k) derived from pulse responses and those (G', k') from sum-of-sines responses, respectively. The cupula time constant based on sum-of-sines' data (tau'L) showed no statistically significant change between 30 and 150 days PIS (P > 0.05, analysis of variance). Thus the results in present study indicate that both the spontaneous discharge and the driven response to rotation of pigeon HSCPAs recovered their normal physiological status between 30 and 150 days PIS after hair cell death due to aminoglycoside ototoxicity. The recovery was systematic for the parameters chosen to be tested with the exception of the cupula long time constant, tauL. The mechanisms (changes in ciliary dynamics, changes in hair cell ionic currents, changes in bouton terminals, etc.) underlying these changes await further morphophysiological studies.

Evidence for three additional P2X2 purinoceptor isoforms produced by alternative splicing in the adult rat vestibular end-organs

P2X2 receptors are ligand-gated ion channels that are activated by extracellular ATP. To characterize the expression of P2X2 purinoceptor in the adult rat vestibular periphery, reverse transcription-polymerase chain reaction (RT-PCR) was used. No transcript for P2X2 receptor was found in the vestibular primary afferent neurons (Scarpa's ganglia); however, partial cDNAs encoding four splice variants of the P2X2 receptor were isolated from vestibular end-organs. In all four cDNAs, the deletions were of different lengths but started at the same position on the P2X2 gene (Val-370 codon) located toward the intracellular carboxyl terminus. One of these receptor isoforms was identical in sequence to the recently published P2X2(b) receptor (Simon et al., 1997, Mol. Pharmacol. 52, 237-248) (also known as P2X2-2, in the nomenclature of Brändle et al., 1997, FEBS Lett. 404, 294-298). The remaining three novel splice variants of the P2X2 receptor were designated P2X2(e), P2X2(f) and P2X2(g) (GenBank accession numbers AF028603, AF028604 and AF028605, respectively). The functional significance of these three splice variants remains to be determined. Pituitary and cerebellum were used as survey tissues and only the P2X2(b) receptor cDNA was found.

Evolutionary trends in the organization of the vertebrate crista ampullaris

Intraaxonal labeling studies in the toadfish, frog, turtle, and chinchilla suggest broad evolutionary trends in the vertebrate crista ampullaris. The crista of anamniotes (fish, amphibians) contains type II hair cells innervated by bouton afferents and is longitudinally organized. Type I hair cells are first seen in reptiles and birds, where they are confined to a central zone and are innervated by calyx and dimorphic afferents. The central zone is surrounded by a peripheral zone containing only type II hair cells innervated by bouton afferents. Results in the turtle suggest that the peripheral zone in reptiles and birds is organized similarly to the entire anamniote crista. The turtle central zone finds no parallel in anamniotes but resembles the mammalian central zone in its structure and afferent physiology. With the advent of a central zone in reptiles, a concentric organization is superimposed on a linearly organized peripheral zone. The mammalian crista, in contrast, has an entirely concentric organization. This may be related to the extension of the neuroepithelium further down the slopes of the crista in mammals than in other vertebrates and to the distribution of type I hair cells throughout the mammalian neuroepithelium.

Differential expression of alpha2-7, alpha9 and beta2-4 nicotinic acetylcholine receptor subunit mRNA in the vestibular end-organs and Scarpa's ganglia of the rat

To further characterize the pattern of expression of the nicotinic acetylcholine receptor (nAChR) subunits in the peripheral vestibular system, we conducted RT-PCR of all known mammalian nAChR alpha and beta subunits in mRNA extracted from adult rat vestibular primary afferent neurons (Scarpa's ganglia) and vestibular end-organs. Transcripts encoding the alpha2-7 and beta2-4 nAChR subunits were found in the vestibular ganglia, while alpha3, alpha5-7, alpha9 and beta2-4 nAChR subunits were expressed in the vestibular end-organs. These results support previous electrophysiological, immunocytochemical and molecular biological data, and also provide a more complete understanding of the role of nAChRs in the neurochemical transmission subserving the efferent-afferent interaction in the vestibular periphery.

Efferent neurons and vestibular cross talk in the frog

A galvanic stimulus (30- to 120-s, 0.3-mA constant current pulse) was used to depolarize the spike-generating region of horizontal and anterior canal afferent neurons. The galvanically induced spike activity from these neurons served as a driving input to the efferent vestibular system in the bullfrog. Efferent-mediated effects were assessed by intracellular recordings of posterior canal afferent spike activity, either ipsilateral or contralateral to the driving stimulus. Ipsilateral to the driving stimulus, efferent-mediated spike rate changes occurred in 62 (39%) of 158 posterior canal afferent neurons. Ipsilateral efferent-mediated effects were overwhelmingly excitatory (92%). Of responding units, 3% were inhibited during stimulus application and 5% showed mixed responses involving 3-20 s of inhibition followed by facilitation. Contralateral to the driving stimulus, efferent-mediated spike rate changes occurred in 18 (23%) of 77 posterior canal afferent neurons. Contralateral efferent-mediated effects were overwhelmingly inhibitory (95%). Only one unit was facilitated during stimulation and no mixed responses to contralateral stimulation were observed. Analysis of the coefficient of variation in interspike intervals (CV) before and during stimulation showed no significant efferent-mediated effects on spike train noise. Comparisons of resting spike rates between units showing efferent-mediated effects and those that did not were in general agreement with previous studies. Responding units had a lower mean spike rate (6.8 +/- 0.70 spikes/s, mean +/- SE) than did nonresponding units (10.7 +/- 0.42 spikes/s, mean +/- SE; P < 0.001; 2-tailed t-test of log-normalized data). Comparison between groups in the regularity of their resting spike rates, as quantified by CV, showed considerable overlap. When responding and nonresponding units with similar resting spike rates were compared, responding units had more irregular resting spike rates than did nonresponding units (P < 0.004; 2-tailed, paired t-test). In most cases (77%) the temporal pattern and general shapes of efferent-mediated responses mirrored the driving input of the galvanically activated afferent neurons. The other 23% of efferent-mediated responses exhibited a marked adaptation of the response. Adapting and nonadapting units were not significantly different in their mean resting spike rates or in the regularity of their resting spike rates.

Distribution of calcitonin gene-related peptide immunoreactivity in vestibular efferent neurons of the chinchilla

The distribution of calcitonin gene-related peptide immunoreactivity (CGRPi) within efferent vestibular neurons in the chinchilla was investigated using fluorescent retrograde labeling combined with immunohistochemistry. Efferent vestibular neurons were found bilaterally in clusters: dorsolateral (group E1) and medial (group E2) to the genu of CN VII, as well as ventromedial to the descending CN VII fibers in the parvicellular reticular formation (PCR). The percentage of retrogradely labeled cells containing CGRPi was 77.1 +/- 5.7 for group E1 neurons, 90.3 +/- 3.8 in the E2 region. Among the PCR efferents more then half of the neurons (61.4 +/- 19.9%) expressed CGRP peptide or message. The wide distribution of CGRP among vestibular efferent neurons suggests that CGRP plays an important role in vestibular efferent function. In addition, the differential distribution among the groups of vestibular efferent neurons suggests that efferent modulation of vestibular function is different between the E cell group efferent neurons and the PCR efferent neurons.

Differential central projections of vestibular afferents in pigeons

The question of whether a differential distribution of vestibular afferent information to central nuclear neurons is present in pigeons was studied using neural tracer compounds. Discrete tracing of afferent fibers innervating the individual semicircular canal and otolith organs was produced by sectioning individual branches of the vestibular nerve that innervate the different receptor organs and applying crystals of horseradish peroxidase, or a horseradish peroxidase/cholera toxin mixture, or a biocytin compound for neuronal uptake and transport. Afferent fibers and their terminal distributions within the brainstem and cerebellum were visualized subsequently. Discrete areas in the pigeon central nervous system that receive primary vestibular input include the superior, dorsal lateral, ventral lateral, medial, descending, and tangential vestibular nuclei; the A and B groups; the intermediate, medial, and lateral cerebellar nuclei; and the nodulus, the uvula, and the paraflocculus. Generally, the vertical canal afferents projected heavily to medial regions in the superior and descending vestibular nuclei as well as the A group. Vertical canal projections to the medial and lateral vestibular nuclei were observed but were less prominent. Horizontal canal projections to the superior and descending vestibular nuclei were much more centrally located than those of the vertical canals. A more substantial projection to the medial and lateral vestibular nuclei was seen with horizontal canal afferents compared to vertical canal fibers. Afferents innervating the utricle and saccule terminated generally in the lateral regions of all vestibular nuclei in areas that were separate from the projections of the semicircular canals. In addition, utricular fibers projected to regions in the vestibular nuclei that overlapped with the horizontal semicircular canal terminal fields, whereas saccular afferents projected to regions that received vertical canal fiber terminations. Lagenar afferents projected throughout the cochlear nuclei, to the dorsolateral regions of the cerebellar nuclei, and to lateral regions of the superior, lateral, medial, and descending vestibular nuclei.

Properties of otolith-related vestibular nuclear neurons in response to bidirectional off-vertical axis rotation of the rat

In decerebrate rats, the responses of tilt-sensitive neurons in the lateral and descending vestibular nuclei were studied during constant velocity 10 degrees off-vertical axis rotations (OVAR) in the clockwise (VW) and counterclockwise (CCW) directions. Seventy three otolith-related units showed a sinusoidal position-dependent discharge modulation to OVAR of both directions; 20 of these showed clipped firing rates in parts of a 360 degree OVAR cycle. With increase in the velocity of rotation (1.75-15 degrees/s), one group of units (n = 36) showed a stable ratio of bidirectional response sensitivity and symmetric response magnitudes to CW and CCW rotations. These units showed gain tuning ratios similar to those of narrowly spatiotemporal-tuned neurons. The other group of OVAR responsive units (n = 13) exhibited velocity-variable and asymmetric bidirectional response sensitivities. Their gain tuning ratios were similar to those of broadly spatiotemporal-tuned neurons. For units with velocity-stable and symmetric bidirectional response sensitivity as well as gain tuning ratio of the narrowly spatiotemporal-tuned neurons, their response gains remained stable with velocity. Some showed stable response phase lead or lag with velocity increase while others showed progressive shifts from response lead of 13 degrees to response lag of -25 degrees. The best response orientations of these units with velocity-stable and symmetric bidirectional response sensitivity were found to point in all directions on the place of rotation. The functional significance of these tilt- and OVAR-sensitive central otolith neurons is discussed.