Quantal and nonquantal transmission in calyx-bearing fibers of the turtle posterior crista

Vestibular afferent neurons develop normally in the absence of quantal/glutamatergic input

The vestibular nerve is comprised of neuron sub-groups with diverse functions related to their intrinsic biophysical properties. This diversity is partly due to differences in the types and numbers of low-voltage-gated potassium channels found in the neurons' membranes. Expression for some low-voltage gated ion channels like KCNQ4 is upregulated during early post-natal development; suggesting that ion channel composition and neuronal diversity may be shaped by hair cell activity. This idea is consistent with recent work showing that glutamatergic input from hair cells is necessary for the normal diversification auditory neurons. To test if biophysical diversity is similarly dependent on glutamatergic input in vestibular neurons, we examined the maturation of the vestibular epithelium and ganglion neurons in Vglut3-ko mice whose hair cell synapses lack glutamate. Despite lacking glutamatergic input, the knockout mice showed no notable balance deficits and crossed challenging balance beams with little difficulty. Immunolabeling of the Vglut3-ko vestibular epithelia showed normal development as indicated by an identifiable striolar zone with calyceal terminals labeled by molecular marker calretinin, and normal expression of KCNQ4 by the end of the second post-natal week. We found similar numbers of Type I and Type II hair cells in the knockout and wildtype animals, regardless of epithelial zone. Thus, the presumably quiescent Type II hair cells are not cleared from the epithelium. Patch-clamp recordings showed that biophysical diversity of vestibular ganglion neurons in the Vglut3-ko mice is comparable to that found in wildtype controls, with a similar range firing patterns at both immature and juvenile ages. However, our results suggest a subtle biophysical alteration to the largest ganglion cells (putative somata of central zone afferents); those in the knockout had smaller net conductance and were more excitable than those in the wild type. Thus, unlike in the auditory nerve, glutamatergic signaling is unnecessary for producing biophysical diversity in vestibular ganglion neurons. And yet, because the input signals from vestibular hair cells are complex and not solely reliant on quantal release of glutamate, whether diversity of vestibular ganglion neurons is simply hardwired or regulated by a more complex set of input signals remains to be determined.

Evidence That Ultrafast Nonquantal Transmission Underlies Synchronized Vestibular Action Potential Generation

Amniotes evolved a unique postsynaptic terminal in the inner ear vestibular organs called the calyx that receives both quantal and nonquantal (NQ) synaptic inputs from Type I sensory hair cells. The nonquantal synaptic current includes an ultrafast component that has been hypothesized to underlie the exceptionally high synchronization index (vector strength) of vestibular afferent neurons in response to sound and vibration. Here, we present three lines of evidence supporting the hypothesis that nonquantal transmission is responsible for synchronized vestibular action potentials of short latency in the guinea pig utricle of either sex. First, synchronized vestibular nerve responses are unchanged after administration of the AMPA receptor antagonist CNQX, while auditory nerve responses are completely abolished. Second, stimulus evoked vestibular nerve compound action potentials (vCAP) are shown to occur without measurable synaptic delay and three times shorter than the latency of auditory nerve compound action potentials (cCAP), relative to the generation of extracellular receptor potentials. Third, paired-pulse stimuli designed to deplete the readily releasable pool (RRP) of synaptic vesicles in hair cells reveal forward masking in guinea pig auditory cCAPs, but a complete lack of forward masking in vestibular vCAPs. Results support the conclusion that the fast component of nonquantal transmission at calyceal synapses is indefatigable and responsible for ultrafast responses of vestibular organs evoked by transient stimuli.SIGNIFICANCE STATEMENT The mammalian vestibular system drives some of the fastest reflex pathways in the nervous system, ensuring stable gaze and postural control for locomotion on land. To achieve this, terrestrial amniotes evolved a large, unique calyx afferent terminal which completely envelopes one or more presynaptic vestibular hair cells, which transmits mechanosensory signals mediated by quantal and nonquantal (NQ) synaptic transmission. We present several lines of evidence in the guinea pig which reveals the most sensitive vestibular afferents are remarkably fast, much faster than their auditory nerve counterparts. Here, we present neurophysiological and pharmacological evidence that demonstrates this vestibular speed advantage arises from ultrafast NQ electrical synaptic transmission from Type I hair cells to their calyx partners.

Otoferlin as a multirole Ca2+ signaling protein: from inner ear synapses to cancer pathways

Humans have six members of the ferlin protein family: dysferlin, myoferlin, otoferlin, fer1L4, fer1L5, and fer1L6. These proteins share common features such as multiple Ca2+-binding C2 domains, FerA domains, and membrane anchoring through their single C-terminal transmembrane domain, and are believed to play a key role in calcium-triggered membrane fusion and vesicle trafficking. Otoferlin plays a crucial role in hearing and vestibular function. In this review, we will discuss how we see otoferlin working as a Ca2+-dependent mechanical sensor regulating synaptic vesicle fusion at the hair cell ribbon synapses. Although otoferlin is also present in the central nervous system, particularly in the cortex and amygdala, its role in brain tissues remains unknown. Mutations in the OTOF gene cause one of the most frequent genetic forms of congenital deafness, DFNB9. These mutations produce severe to profound hearing loss due to a defect in synaptic excitatory glutamatergic transmission between the inner hair cells and the nerve fibers of the auditory nerve. Gene therapy protocols that allow normal rescue expression of otoferlin in hair cells have just started and are currently in pre-clinical phase. In parallel, studies have linked ferlins to cancer through their effect on cell signaling and development, allowing tumors to form and cancer cells to adapt to a hostile environment. Modulation by mechanical forces and Ca2+ signaling are key determinants of the metastatic process. Although ferlins importance in cancer has not been extensively studied, data show that otoferlin expression is significantly associated with survival in specific cancer types, including clear cell and papillary cell renal carcinoma, and urothelial bladder cancer. These findings indicate a role for otoferlin in the carcinogenesis of these tumors, which requires further investigation to confirm and understand its exact role, particularly as it varies by tumor site. Targeting this protein may lead to new cancer therapies.

Using macular velocity measurements to relate parameters of bone conduction to vestibular compound action potential responses

To examine mechanisms responsible for vestibular afferent sensitivity to transient bone conducted vibration, we performed simultaneous measurements of stimulus-evoked vestibular compound action potentials (vCAPs), utricular macula velocity, and vestibular microphonics (VMs) in anaesthetized guinea pigs. Results provide new insights into the kinematic variables of transient motion responsible for triggering mammalian vCAPs, revealing synchronized vestibular afferent responses are not universally sensitive to linear jerk as previously thought. For short duration stimuli (< 1 ms), the vCAP increases magnitude in close proportion to macular velocity and temporal bone (linear) acceleration, rather than other kinematic elements. For longer duration stimuli, the vCAP magnitude switches from temporal bone acceleration sensitive to linear jerk sensitive while maintaining macular velocity sensitivity. Frequency tuning curves evoked by tone-burst stimuli show vCAPs increase in proportion to onset macular velocity, while VMs increase in proportion to macular displacement across the entire frequency bandwidth tested between 0.1 and 2 kHz. The subset of vestibular afferent neurons responsible for synchronized firing and vCAPs have been shown previously to make calyceal synaptic contacts with type I hair cells in the striolar region of the epithelium and have irregularly spaced inter-spike intervals at rest. Present results provide new insight into mechanical and neural mechanisms underlying synchronized action potentials in these sensitive afferents, with clinical relevance for understanding the activation and tuning of neurons responsible for driving rapid compensatory reflex responses.

Expression of hyperpolarization-activated current (Ih) in zonally defined vestibular calyx terminals of the crista

Calyx terminals make afferent synapses with type I hair cells in vestibular epithelia and express diverse ionic conductances that influence action potential generation and discharge regularity in vestibular afferent neurons. Here we investigated the expression of hyperpolarization-activated current (Ih) in calyx terminals in central and peripheral zones of mature gerbil crista slices, using whole cell patch-clamp recordings. Slowly activating Ih was present in >80% calyces tested in both zones. Peak Ih and half-activation voltages were not significantly different; however, Ih activated with a faster time course in peripheral compared with central zone calyces. Calyx Ih in both zones was blocked by 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyrimidinium chloride (ZD7288; 100 µM), and the resting membrane potential became more hyperpolarized. In the presence of dibutyryl-cAMP (dB-cAMP), peak Ih was increased, activation kinetics became faster, and the voltage of half-activation was more depolarized compared with control calyces. In current clamp, calyces from both zones showed three different categories of firing: spontaneous firing, phasic firing where a single action potential was evoked after a hyperpolarizing pulse, or a single evoked action potential followed by membrane potential oscillations. In the absence of Ih, the latency to peak of the action potential increased; Ih produces a small depolarizing current that facilitates firing by driving the membrane potential closer to threshold. Immunostaining showed the expression of HCN2 subunits in calyx terminals. We conclude that Ih is found in calyx terminals across the crista and could influence conventional and novel forms of synaptic transmission at the type I hair cell-calyx synapse.NEW & NOTEWORTHY Calyx afferent terminals make synapses with vestibular hair cells and express diverse conductances that impact action potential firing in vestibular primary afferents. Conventional and nonconventional synaptic transmission modes are influenced by hyperpolarization-activated current (Ih), but regional differences were previously unexplored. We show that Ih is present in both central and peripheral calyces of the mammalian crista. Ih produces a small depolarizing resting current that facilitates firing by driving the membrane potential closer to threshold.

A mathematical model for mechanical activation and compound action potential generation by the utricle in response to sound and vibration

Introduction

Calyx bearing vestibular afferent neurons innervating type I hair cells in the striolar region of the utricle are exquisitely sensitive to auditory-frequency air conducted sound (ACS) and bone conducted vibration (BCV). Here, we present experimental data and a mathematical model of utricular mechanics and vestibular compound action potential generation (vCAP) in response to clinically relevant levels of ACS and BCV. Vibration of the otoconial layer relative to the sensory epithelium was simulated using a Newtonian two-degree-of-freedom spring-mass-damper system, action potential timing was simulated using an empirical model, and vCAPs were simulated by convolving responses of the population of sensitive neurons with an empirical extracellular voltage kernel. The model was validated by comparison to macular vibration and vCAPs recorded in the guinea pig, in vivo.

Results

Transient stimuli evoked short-latency vCAPs that scaled in magnitude and timing with hair bundle mechanical shear rate for both ACS and BCV. For pulse BCV stimuli with durations &lt;0.8 ms, the vCAP magnitude increased in proportion to temporal bone acceleration, but for pulse durations &gt;0.9 ms the magnitude increased in proportion to temporal bone jerk. Once validated using ACS and BCV data, the model was applied to predict blast-induced hair bundle shear, with results predicting acute mechanical damage to bundles immediately upon exposure.

Discussion

Results demonstrate the switch from linear acceleration to linear jerk as the adequate stimulus arises entirely from mechanical factors controlling the dynamics of sensory hair bundle deflection. The model describes the switch in terms of the mechanical natural frequencies of vibration, which vary between species based on morphology and mechanical factors.

Nonquantal transmission at the vestibular hair cell-calyx synapse: KLV currents modulate fast electrical and slow K+ potentials

Vestibular hair cells transmit information about head position and motion across synapses to primary afferent neurons. At some of these synapses, the afferent neuron envelopes the hair cell, forming an enlarged synaptic terminal called a calyx. The vestibular hair cell-calyx synapse supports a mysterious form of electrical transmission that does not involve gap junctions, termed nonquantal transmission (NQT). The NQT mechanism is thought to involve the flow of ions from the presynaptic hair cell to the postsynaptic calyx through low-voltage-activated channels driven by changes in cleft [K⁺] as K⁺ exits the hair cell. However, this hypothesis has not been tested with a quantitative model and the possible role of an electrical potential in the cleft has remained speculative. Here, we present a computational model that captures experimental observations of NQT and identifies features that support the existence of an electrical potential (ϕ) in the synaptic cleft. We show that changes in cleft ϕ reduce transmission latency and illustrate the relative contributions of both cleft [K⁺] and ϕ to the gain and phase of NQT. We further demonstrate that the magnitude and speed of NQT depend on calyx morphology and that increasing calyx height reduces action potential latency in the calyx afferent. These predictions are consistent with the idea that the calyx evolved to enhance NQT and speed up vestibular signals that drive neural circuits controlling gaze, balance, and orientation.

Simultaneous Dual Recordings From Vestibular Hair Cells and Their Calyx Afferents Demonstrate Multiple Modes of Transmission at These Specialized Endings

In the vestibular periphery, transmission via conventional synaptic boutons is supplemented by post-synaptic calyceal endings surrounding Type I hair cells. This review focusses on the multiple modes of communication between these receptors and their enveloping calyces as revealed by simultaneous dual-electrode recordings. Classic orthodromic transmission is accompanied by two forms of bidirectional communication enabled by the extensive cleft between the Type I hair cell and its calyx. The slowest cellular communication low-pass filters the transduction current with a time constant of 10–100 ms: potassium ions accumulate in the synaptic cleft, depolarizing both the hair cell and afferent to potentials greater than necessary for rapid vesicle fusion in the receptor and potentially triggering action potentials in the afferent. On the millisecond timescale, conventional glutamatergic quantal transmission occurs when hair cells are depolarized to potentials sufficient for calcium influx and vesicle fusion. Depolarization also permits a third form of transmission that occurs over tens of microseconds, resulting from the large voltage- and ion-sensitive cleft-facing conductances in both the hair cell and the calyx that are open at their resting potentials. Current flowing out of either the hair cell or the afferent divides into the fraction flowing across the cleft into its cellular partner, and the remainder flowing out of the cleft and into the surrounding fluid compartment. These findings suggest multiple biophysical bases for the extensive repertoire of response dynamics seen in the population of primary vestibular afferent fibers. The results further suggest that evolutionary pressures drive selection for the calyx afferent.

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.

Current Response in Ca V 1.3-/- Mouse Vestibular and Cochlear Hair Cells

Signal transmission by sensory auditory and vestibular hair cells relies upon Ca²⁺-dependent exocytosis of glutamate. The Ca²⁺ current in mammalian inner ear hair cells is predominantly carried through Ca_V1.3 voltage-gated Ca²⁺ channels. Despite this, Ca_V1.3 deficient mice (Ca_V1.3^–/–) are deaf but do not show any obvious vestibular phenotype. Here, we compared the Ca²⁺ current (I_Ca) in auditory and vestibular hair cells from wild-type and Ca_V1.3^–/– mice, to assess whether differences in the size of the residual I_Ca could explain, at least in part, the two phenotypes. Using 5 mM extracellular Ca²⁺ and near-body temperature conditions, we investigated the cochlear primary sensory receptors inner hair cells (IHCs) and both type I and type II hair cells of the semicircular canals. We found that the residual I_Ca in both auditory and vestibular hair cells from Ca_V1.3^–/– mice was less than 20% (12–19%, depending on the hair cell type and age investigated) compared to controls, indicating a comparable expression of Ca_V1.3 Ca²⁺ channels in both sensory organs. We also showed that, different from IHCs, type I and type II hair cells from Ca_V1.3^–/– mice were able to acquire the adult-like K⁺ current profile in their basolateral membrane. Intercellular K⁺ accumulation was still present in Ca_V1.3^–/– mice during I_K,L activation, suggesting that the K⁺-based, non-exocytotic, afferent transmission is still functional in these mice. This non-vesicular mechanism might contribute to the apparent normal vestibular functions in Ca_V1.3^–/– mice.

Recent advances in cochlear hair cell nanophysiology: subcellular compartmentalization of electrical signaling in compact sensory cells

In recent years, genetics, physiology, and structural biology have advanced into the molecular details of the sensory physiology of auditory hair cells. Inner hair cells (IHCs) and outer hair cells (OHCs) mediate two key functions: active amplification and non-linear compression of cochlear vibrations by OHCs and sound encoding by IHCs at their afferent synapses with the spiral ganglion neurons. OHCs and IHCs share some molecular physiology, e.g. mechanotransduction at the apical hair bundles, ribbon-type presynaptic active zones, and ionic conductances in the basolateral membrane. Unique features enabling their specific function include prestin-based electromotility of OHCs and indefatigable transmitter release at the highest known rates by ribbon-type IHC active zones. Despite their compact morphology, the molecular machineries that either generate electrical signals or are driven by these signals are essentially all segregated into local subcellular structures. This review provides a brief account on recent insights into the molecular physiology of cochlear hair cells with a specific focus on organization into membrane domains.

Direct current effects on afferent and hair cell to elicit natural firing patterns

In contrast to the conventional pulsatile neuromodulation that excites neurons, galvanic or direct current stimulation can excite, inhibit, or sensitize neurons. The vestibular system presents an excellent system for studying galvanic neural interface due to the spontaneously firing afferent activity that needs to be either suppressed or excited to convey head motion sensation. We determine the cellular mechanisms underlying the beneficial properties of galvanic vestibular stimulation (GVS) by creating a computational model of the vestibular end organ that elicits all experimentally observed response characteristics to GVS simultaneously. When GVS was modeled to affect the axon alone, the complete experimental data could not be replicated. We found that if GVS affects hair cell vesicle release and axonal excitability simultaneously, our modeling results matched all experimental observations. We conclude that contrary to the conventional belief that GVS affects only axons, the hair cells are likely also affected by this stimulation paradigm.

•

Galvanic vestibular stimulation was shown to evoke naturalistic neural responses

•

Conventional understanding maintains that it affects only afferent axons

•

In contrast, our work suggests that it affects both hair cells and afferents

•

Our work further explains the likely underlying mechanisms of these effects

Cholinergic Modulation of Membrane Properties of Calyx Terminals in the Vestibular Periphery

Vestibular nerve afferents are divided into regular and irregular groups based on the variability of interspike intervals in their resting discharge. Most afferents receive inputs from bouton terminals that contact type II hair cells as well as from calyx terminals that cover the basolateral walls of type I hair cells. Calyces have an abundance of different subtypes of KCNQ (Kv7) potassium channels and muscarinic acetylcholine receptors (mAChRs) and receive cholinergic efferent inputs from neurons in the brainstem. We investigated whether mAChRs affected membrane properties and firing patterns of calyx terminals through modulation of KCNQ channel activity. Patch clamp recordings were performed from calyx terminals in central regions of the cristae of the horizontal and anterior canals in 13-26 day old Sprague-Dawley rats. KCNQ mediated currents were observed as voltage sensitive currents with slow kinetics (activation and deactivation), resulting in spike frequency adaptation so that calyces at best fired a single action potential at the beginning of a depolarizing step. Activation of mAChRs by application of oxotremorine methiodide or inhibition of KCNQ channels by linopirdine dihydrochloride decreased voltage activated currents by ∼30%, decreased first spike latencies by ∼40%, resulted in action potential generation in response to smaller current injections and at lower (i.e., more hyperpolarized) membrane potentials, and increased the number of spikes fired during depolarizing steps. Interestingly, some of the calyces showed spontaneous discharge in the presence of these drugs. Together, these findings suggest that cholinergic efferents can modulate the response properties and encoding of head movements by afferents.

K+ Accumulation and Clearance in the Calyx Synaptic Cleft of Type I Mouse Vestibular Hair Cells

Vestibular organs of Amniotes contain two types of sensory cells, named Type I and Type II hair cells. While Type II hair cells are contacted by several small bouton nerve terminals, Type I hair cells receive a giant terminal, called a calyx, which encloses their basolateral membrane almost completely. Both hair cell types release glutamate, which depolarizes the afferent terminal by binding to AMPA post-synaptic receptors. However, there is evidence that non-vesicular signal transmission also occurs at the Type I hair cell-calyx synapse, possibly involving direct depolarization of the calyx by K⁺ exiting the hair cell. To better investigate this aspect, we performed whole-cell patch-clamp recordings from mouse Type I hair cells or their associated calyx. We found that [K⁺] in the calyceal synaptic cleft is elevated at rest relative to the interstitial (extracellular) solution and can increase or decrease during hair cell depolarization or repolarization, respectively. The change in [K⁺] was primarily driven by G_K,L, the low-voltage-activated, non-inactivating K⁺ conductance specifically expressed by Type I hair cells. Simple diffusion of K⁺ between the cleft and the extracellular compartment appeared substantially restricted by the calyx inner membrane, with the ion channels and active transporters playing a crucial role in regulating intercellular [K⁺]. Calyx recordings were consistent with K⁺ leaving the synaptic cleft through postsynaptic voltage-gated K⁺ channels involving K_V1 and K_V7 subunits. The above scenario is consistent with direct depolarization and hyperpolarization of the calyx membrane potential by intercellular K⁺.

Multiscale modeling of mechanotransduction in the utricle

We review recent progress in using numerical models to relate utricular hair bundle and otoconial membrane (OM) structure to the functional requirements imposed by natural behavior in turtles. The head movements section reviews the evolution of experimental attempts to understand vestibular system function with emphasis on turtles, including data showing that accelerations occurring during natural head movements achieve higher magnitudes and frequencies than previously assumed. The structure section reviews quantitative anatomical data documenting topographical variation in the structures underlying macromechanical and micromechanical responses of the turtle utricle to head movement: hair bundles, OM, and bundle-OM coupling. The macromechanics section reviews macromechanical models that incorporate realistic anatomical and mechanical parameters and reveal that the system is significantly underdamped, contrary to previous assumptions. The micromechanics: hair bundle motion and met currents section reviews work based on micromechanical models, which demonstrates that topographical variation in the structure of hair bundles and OM, and their mode of coupling, result in regional specializations for signaling of low frequency (or static) head position and high frequency head accelerations. We conclude that computational models based on empirical data are especially promising for investigating mechanotransduction in this challenging sensorimotor system.

Semicircular canal biomechanics in health and disease

The semicircular canals are responsible for sensing angular head motion in three-dimensional space and for providing neural inputs to the central nervous system (CNS) essential for agile mobility, stable vision, and autonomic control of the cardiovascular and other gravity-sensitive systems. Sensation relies on fluid mechanics within the labyrinth to selectively convert angular head acceleration into sensory hair bundle displacements in each of three inner ear sensory organs. Canal afferent neurons encode the direction and time course of head movements over a broad range of movement frequencies and amplitudes. Disorders altering canal mechanics result in pathological inputs to the CNS, often leading to debilitating symptoms. Vestibular disorders and conditions with mechanical substrates include benign paroxysmal positional nystagmus, direction-changing positional nystagmus, alcohol positional nystagmus, caloric nystagmus, Tullio phenomena, and others. Here, the mechanics of angular motion transduction and how it contributes to neural encoding by the semicircular canals is reviewed in both health and disease.

Specializations for Fast Signaling in the Amniote Vestibular Inner Ear

During rapid locomotion, the vestibular inner ear provides head-motion signals that stabilize posture, gaze, and heading. Afferent nerve fibers from central and peripheral zones of vestibular sensory epithelia use temporal and rate encoding, respectively, to emphasize different aspects of head motion: central afferents adapt faster to sustained head position and favor higher stimulus frequencies, reflecting specializations at each stage from motion of the accessory structure to spike propagation to the brain. One specialization in amniotes is an unusual nonquantal synaptic mechanism by which type I hair cells transmit to large calyceal terminals of afferent neurons. The reduced synaptic delay of this mechanism may have evolved to serve reliable and fast input to reflex pathways that ensure stable locomotion on land.

Pre- and Postsynaptic Effects of Glutamate in the Frog Labyrinth

The role of glutamate in quantal release at the cytoneural junction was examined by measuring mEPSPs and afferent spikes at the posterior canal in the intact frog labyrinth. Release was enhanced by exogenous glutamate, or dl-TBOA, a blocker of glutamate reuptake. Conversely, drugs acting on ionotropic glutamate receptors did not affect release; the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA-R) blocker CNQX decreased mEPSP size in a dose-dependent manner; the NMDA-R blocker d-AP5 at concentrations <200 µM did not affect mEPSP size, either in the presence or absence of Mg and glycine. In isolated hair cells, glutamate did not modify Ca currents. Instead, it systematically reduced the compound delayed potassium current, IKD, whereas the metabotropic glutamate receptor (mGluR)-II inverse agonist, (2S)-2-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl)propanoic acid (LY341495), increased it. Given mGluR-II decrease cAMP production, these finding are consistent with the reported sensitivity of IKD to protein kinase A (PKA)-mediated phosphorylation. LY341495 also enhanced transmitter release, presumably through phosphorylation-mediated facilitation of the release machinery. The observed enhancement of release by glutamate confirms previous literature data, and can be attributed to activation of mGluR-I that promotes Ca release from intracellular stores. Glutamate-induced reduction in the repolarizing IKD may contribute to facilitation of release. Overall, glutamate exerts both a positive feedback action on mGluR-I, through activation of the phospholipase C (PLC)/IP₃ path, and the negative feedback, by interfering with substrate phosphorylation through G_i/0-coupled mGluRs-II/III. The positive feedback prevails, which may explain the increase in overall rates of release observed during mechanical stimulation (symmetrical in the excitatory and inhibitory directions). The negative feedback may protect the junction from over-activation.

Development and regeneration of vestibular hair cells in mammals

Vestibular sensation is essential for gaze stabilization, balance, and perception of gravity. The vestibular receptors in mammals, Type I and Type II hair cells, are located in five small organs in the inner ear. Damage to hair cells and their innervating neurons can cause crippling symptoms such as vertigo, visual field oscillation, and imbalance. In adult rodents, some Type II hair cells are regenerated and become re-innervated after damage, presenting opportunities for restoring vestibular function after hair cell damage. This article reviews features of vestibular sensory cells in mammals, including their basic properties, how they develop, and how they are replaced after damage. We discuss molecules that control vestibular hair cell regeneration and highlight areas in which our understanding of development and regeneration needs to be deepened.

AMPA receptor-mediated rapid EPSCs in vestibular calyx afferents

In the vestibular periphery neurotransmission between hair cells and primary afferent nerves occurs via specialized ribbon synapses. Type I vestibular hair cells (HCIs) make synaptic contacts with calyx terminals, which enclose most of the HCI basolateral surface. To probe synaptic transmission, whole cell patch-clamp recordings were made from calyx afferent terminals isolated together with their mature HCIs from gerbil crista. Neurotransmitter release was measured as excitatory postsynaptic currents (EPSCs) in voltage clamp. Spontaneous EPSCs were classified as simple or complex. Simple events exhibited a rapid rise time and a fast monoexponential decay (time constant < 1 ms). The remaining events, constituting ~40% of EPSCs, showed more complex characteristics. Extracellular Sr²⁺ greatly increased EPSC frequency, and EPSCs were blocked by the AMPA receptor blocker NBQX. The role of presynaptic Ca²⁺ channels was assessed by application of the L-type Ca²⁺ channel blocker nifedipine (20 µM), which reduced EPSC frequency. In contrast, the L-type Ca²⁺ channel opener BAY K 8644 increased EPSC frequency. Cyclothiazide increased the decay time constant of averaged simple EPSCs by approximately twofold. The low-affinity AMPA receptor antagonist γ-d-glutamylglycine (2 mM) reduced the proportion of simple EPSCs relative to complex events, indicating glutamate accumulation in the restricted cleft between HCI and calyx. In crista slices EPSC frequency was greater in central compared with peripheral calyces, which may be due to greater numbers of presynaptic ribbons in central hair cells. Our data support a role for L-type Ca²⁺ channels in spontaneous release and demonstrate regional variations in AMPA-mediated quantal transmission at the calyx synapse.NEW & NOTEWORTHY In vestibular calyx terminals of mature cristae we find that the majority of excitatory postsynaptic currents (EPSCs) are rapid monophasic events mediated by AMPA receptors. Spontaneous EPSCs are reduced by an L-type Ca²⁺ channel blocker and notably enhanced in extracellular Sr²⁺ EPSC frequency is greater in central areas of the crista compared with peripheral areas and may be associated with more numerous presynaptic ribbons in central hair cells.

Models of utricular bouton afferents: role of afferent-hair cell connectivity in determining spike train regularity

Vestibular bouton afferent terminals in turtle utricle can be categorized into four types depending on their location and terminal arbor structure: lateral extrastriolar (LES), striolar, juxtastriolar, and medial extrastriolar (MES). The terminal arbors of these afferents differ in surface area, total length, collecting area, number of boutons, number of bouton contacts per hair cell, and axon diameter (Huwe JA, Logan CJ, Williams B, Rowe MH, Peterson EH. J Neurophysiol 113: 2420-2433, 2015). To understand how differences in terminal morphology and the resulting hair cell inputs might affect afferent response properties, we modeled representative afferents from each region, using reconstructed bouton afferents. Collecting area and hair cell density were used to estimate hair cell-to-afferent convergence. Nonmorphological features were held constant to isolate effects of afferent structure and connectivity. The models suggest that all four bouton afferent types are electrotonically compact and that excitatory postsynaptic potentials are two to four times larger in MES afferents than in other afferents, making MES afferents more responsive to low input levels. The models also predict that MES and LES terminal structures permit higher spontaneous firing rates than those in striola and juxtastriola. We found that differences in spike train regularity are not a consequence of differences in peripheral terminal structure, per se, but that a higher proportion of multiple contacts between afferents and individual hair cells increases afferent firing irregularity. The prediction that afferents having primarily one bouton contact per hair cell will fire more regularly than afferents making multiple bouton contacts per hair cell has implications for spike train regularity in dimorphic and calyx afferents.NEW & NOTEWORTHY Bouton afferents in different regions of turtle utricle have very different morphologies and afferent-hair cell connectivities. Highly detailed computational modeling provides insights into how morphology impacts excitability and also reveals a new explanation for spike train irregularity based on relative numbers of multiple bouton contacts per hair cell. This mechanism is independent of other proposed mechanisms for spike train irregularity based on ionic conductances and can explain irregularity in dimorphic units and calyx endings.

Muscarinic Acetylcholine Receptors and M-Currents Underlie Efferent-Mediated Slow Excitation in Calyx-Bearing Vestibular Afferents

Stimulation of vestibular efferent neurons excites calyx and dimorphic (CD) afferents. This excitation consists of fast and slow components that differ >100-fold in activation kinetics and response duration. In the turtle, efferent-mediated fast excitation arises in CD afferents when the predominant efferent neurotransmitter acetylcholine (ACh) activates calyceal nicotinic ACh receptors (nAChRs); however, it is unclear whether the accompanying efferent-mediated slow excitation is also attributed to cholinergic mechanisms. To identify synaptic processes underlying efferent-mediated slow excitation, we recorded from CD afferents innervating the turtle posterior crista during electrical stimulation of efferent neurons, in combination with pharmacological probes and mechanical stimulation. Efferent-mediated slow excitation was unaffected by nAChR compounds that block efferent-mediated fast excitation, but were mimicked by muscarine and antagonized by atropine, indicating that it requires ACh and muscarinic ACh receptor (mAChR) activation. Efferent-mediated slow excitation or muscarine application enhanced the sensitivity of CD afferents to mechanical stimulation, suggesting that mAChR activation increases afferent input impedance by closing calyceal potassium channels. These observations were consistent with suppression of a muscarinic-sensitive K⁺-current, or M-current. Immunohistochemistry for putative M-current candidates suggested that turtle CD afferents express KCNQ3, KCNQ4, and ERG1-3 potassium channel subunits. KCNQ channels were favored as application of the selective antagonist XE991 mimicked and occluded efferent-mediated slow excitation in CD afferents. These data highlight an efferent-mediated mechanism for enhancing afferent sensitivity. They further suggest that the clinical effectiveness of mAChR antagonists in treating balance disorders may also target synaptic mechanisms in the vestibular periphery, and that KCNQ channel modulators might offer similar therapeutic value.SIGNIFICANCE STATEMENT Targeting the efferent vestibular system (EVS) pharmacologically might prove useful in ameliorating some forms of vestibular dysfunction by modifying ongoing primary vestibular input. EVS activation engages several kinetically distinct synaptic processes that profoundly alter the discharge rate and sensitivity of first-order vestibular neurons. Efferent-mediated slow excitation of vestibular afferents is of considerable interest given its ability to elevate afferent activity over an extended time course. We demonstrate for the first time that efferent-mediated slow excitation of vestibular afferents is mediated by muscarinic acetylcholine receptor (mAChR) activation and the subsequent closure of KCNQ potassium channels. The clinical effectiveness of some anti-mAChR drugs in treating motion sickness suggest that we may, in fact, already be targeting the peripheral EVS.

Accumulation of K+ in the synaptic cleft modulates activity by influencing both vestibular hair cell and calyx afferent in the turtle

KEY POINTS

In the synaptic cleft between type I hair cells and calyceal afferents, K⁺ ions accumulate as a function of activity, dynamically altering the driving force and permeation through ion channels facing the synaptic cleft. High-fidelity synaptic transmission is possible due to large conductances that minimize hair cell and afferent time constants in the presence of significant membrane capacitance. Elevated potassium maintains hair cells near a potential where transduction currents are sufficient to depolarize them to voltages necessary for calcium influx and synaptic vesicle fusion. Elevated potassium depolarizes the postsynaptic afferent by altering ion permeation through hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, and contributes to depolarizing the afferent to potentials where a single EPSP (quantum) can generate an action potential. With increased stimulation, hair cell depolarization increases the frequency of quanta released, elevates [K⁺ ]_cleft and depolarizes the afferent to potentials at which smaller and smaller EPSPs would be sufficient to trigger APs.

ABSTRACT

Fast neurotransmitters act in conjunction with slower modulatory effectors that accumulate in restricted synaptic spaces found at giant synapses such as the calyceal endings in the auditory and vestibular systems. Here, we used dual patch-clamp recordings from turtle vestibular hair cells and their afferent neurons to show that potassium ions accumulating in the synaptic cleft modulated membrane potentials and extended the range of information transfer. High-fidelity synaptic transmission was possible due to large conductances that minimized hair cell and afferent time constants in the presence of significant membrane capacitance. Increased potassium concentration in the cleft maintained the hair cell near potentials that promoted the influx of calcium necessary for synaptic vesicle fusion. The elevated potassium concentration also depolarized the postsynaptic neuron by altering ion permeation through hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. This depolarization enabled the afferent to reliably generate action potentials evoked by single AMPA-dependent EPSPs. Depolarization of the postsynaptic afferent could also elevate potassium in the synaptic cleft, and would depolarize other hair cells enveloped by the same neuritic process increasing the fidelity of neurotransmission at those synapses as well. Collectively, these data demonstrate that neuronal activity gives rise to potassium accumulation, and suggest that potassium ion action on HCN channels can modulate neurotransmission, preserving the fidelity of high-speed synaptic transmission by dynamically shifting the resting potentials of both presynaptic and postsynaptic cells.

Pharmacologically distinct nicotinic acetylcholine receptors drive efferent-mediated excitation in calyx-bearing vestibular afferents

Electrical stimulation of vestibular efferent neurons rapidly excites the resting discharge of calyx/dimorphic (CD) afferents. In turtle, this excitation arises when acetylcholine (ACh), released from efferent terminals, directly depolarizes calyceal endings by activating nicotinic ACh receptors (nAChRs). Although molecular biological data from the peripheral vestibular system implicate most of the known nAChR subunits, specific information about those contributing to efferent-mediated excitation of CD afferents is lacking. We sought to identify the nAChR subunits that underlie the rapid excitation of CD afferents and whether they differ from α9α10 nAChRs on type II hair cells that drive efferent-mediated inhibition in adjacent bouton afferents. We recorded from CD and bouton afferents innervating the turtle posterior crista during electrical stimulation of vestibular efferents while applying several subtype-selective nAChR agonists and antagonists. The α9α10 nAChR antagonists, α-bungarotoxin and α-conotoxin RgIA, blocked efferent-mediated inhibition in bouton afferents while leaving efferent-mediated excitation in CD units largely intact. Conversely, 5-iodo-A-85380, sazetidine-A, varenicline, α-conotoxin MII, and bPiDDB (N,N-dodecane-1,12-diyl-bis-3-picolinium dibromide) blocked efferent-mediated excitation in CD afferents without affecting efferent-mediated inhibition in bouton afferents. This pharmacological profile suggested that calyceal nAChRs contain α6 and β2, but not α9, nAChR subunits. Selective blockade of efferent-mediated excitation in CD afferents distinguished dimorphic from calyx afferents by revealing type II hair cell input. Dimorphic afferents differed in having higher mean discharge rates and a mean efferent-mediated excitation that was smaller in amplitude yet longer in duration. Molecular biological data demonstrated the expression of α9 in turtle hair cells and α4 and β2 in associated vestibular ganglia.

The quantal component of synaptic transmission from sensory hair cells to the vestibular calyx

Spontaneous and stimulus-evoked excitatory postsynaptic currents (EPSCs) were recorded in calyx nerve terminals from the turtle vestibular lagena to quantify key attributes of quantal transmission at this synapse. On average, EPSC events had a magnitude of ∼ 42 pA, a rise time constant of τ(0) ∼ 229 μs, decayed to baseline with a time constant of τ(R) ∼ 690 μs, and carried ∼ 46 fC of charge. Individual EPSCs varied in magnitude and decay time constant. Variability in the EPSC decay time constant was hair cell dependent and due in part to a slow protraction of the EPSC in some cases. Variability in EPSC size was well described by an integer summation of unitary quanta, with each quanta of glutamate gating a unitary postsynaptic current of ∼ 23 pA. The unitary charge was ∼ 26 fC for EPSCs with a simple exponential decay and increased to ∼ 48 fC for EPSCs exhibiting a slow protraction. The EPSC magnitude and the number of simultaneous unitary quanta within each event increased with presynaptic stimulus intensity. During tonic hair cell depolarization, both the EPSC magnitude and event rate exhibited adaptive run down over time. Present data from a reptilian calyx are remarkably similar to noncalyceal vestibular synaptic terminals in diverse species, indicating that the skewed EPSC size distribution and multiquantal release might be an ancestral property of inner ear ribbon synapses.

Glutamatergic signaling at the vestibular hair cell calyx synapse

In the vestibular periphery a unique postsynaptic terminal, the calyx, completely covers the basolateral walls of type I hair cells and receives input from multiple ribbon synapses. To date, the functional role of this specialized synapse remains elusive. There is limited data supporting glutamatergic transmission, K(+) or H(+) accumulation in the synaptic cleft as mechanisms of transmission. Here the role of glutamatergic transmission at the calyx synapse is investigated. Whole-cell patch-clamp recordings from calyx endings were performed in an in vitro whole-tissue preparation of the rat vestibular crista, the sensory organ of the semicircular canals that sense head rotation. AMPA-mediated EPSCs showed an unusually wide range of decay time constants, from <5 to >500 ms. Decay time constants of EPSCs increased (or decreased) in the presence of a glutamate transporter blocker (or a competitive glutamate receptor blocker), suggesting a role for glutamate accumulation and spillover in synaptic transmission. Glutamate accumulation caused slow depolarizations of the postsynaptic membrane potentials, and thereby substantially increased calyx firing rates. Finally, antibody labelings showed that a high percentage of presynaptic ribbon release sites and postsynaptic glutamate receptors were not juxtaposed, favoring a role for spillover. These findings suggest a prominent role for glutamate spillover in integration of inputs and synaptic transmission in the vestibular periphery. We propose that similar to other brain areas, such as the cerebellum and hippocampus, glutamate spillover may play a role in gain control of calyx afferents and contribute to their high-pass properties.

Exocytotic machineries of vestibular type I and cochlear ribbon synapses display similar intrinsic otoferlin-dependent Ca2+ sensitivity but a different coupling to Ca2+ channels

The hair cell ribbon synapses of the mammalian auditory and vestibular systems differ greatly in their anatomical organization and firing properties. Notably, vestibular Type I hair cells (VHC-I) are surrounded by a single calyx-type afferent terminal that receives input from several ribbons, whereas cochlear inner hair cells (IHCs) are contacted by several individual afferent boutons, each facing a single ribbon. The specificity of the presynaptic molecular mechanisms regulating transmitter release at these different sensory ribbon synapses is not well understood. Here, we found that exocytosis during voltage activation of Ca(2+) channels displayed higher Ca(2+) sensitivity, 10 mV more negative half-maximum activation, and a smaller dynamic range in VHC-I than in IHCs. VHC-I had a larger number of Ca(2+) channels per ribbon (158 vs 110 in IHCs), but their Ca(2+) current density was twofold smaller because of a smaller open probability and unitary conductance. Using confocal and stimulated emission depletion immunofluorescence microscopy, we showed that VHC-I had fewer synaptic ribbons (7 vs 17 in IHCs) to which Cav1.3 channels are more tightly organized than in IHCs. Gradual intracellular Ca(2+) uncaging experiments revealed that exocytosis had a similar intrinsic Ca(2+) sensitivity in both VHC-I and IHCs (KD of 3.3 ± 0.6 μM and 4.0 ± 0.7 μM, respectively). In otoferlin-deficient mice, exocytosis was largely reduced in VHC-I and IHCs. We conclude that VHC-I and IHCs use a similar micromolar-sensitive otoferlin Ca(2+) sensor and that their sensory encoding specificity is essentially determined by a different functional organization of Ca(2+) channels at their synaptic ribbons.

Zonal variations in K+ currents in vestibular crista calyx terminals

We developed a rodent crista slice to investigate regional variations in electrophysiological properties of vestibular afferent terminals. Thin transverse slices of the gerbil crista ampullaris were made and electrical properties of calyx terminals in central zones (CZ) and peripheral zones (PZ) compared with whole cell patch clamp. Spontaneous action potential firing was observed in 25% of current-clamp recordings and was either regular or irregular in both zones. Firing was abolished when extracellular choline replaced Na(+) but persisted when hair cell mechanotransduction channels or calyx AMPA receptors were blocked. This suggests that ion channels intrinsic to the calyx can generate spontaneous firing. In response to depolarizing voltage steps, outward K(+) currents were observed at potentials above -60 mV. K(+) currents in PZ calyces showed significantly more inactivation than currents in CZ calyces. Underlying K(+) channel populations contributing to these differences were investigated. The KCNQ channel blocker XE991 dihydrochloride blocked a slowly activating, sustained outward current in both PZ and CZ calyces, indicating the presence of KCNQ channels. Mean reduction was greatest in PZ calyces. XE991 also reduced action potential firing frequency in CZ and PZ calyces and broadened mean action potential width. The K(+) channel blocker 4-aminopyridine (10-50 μM) blocked rapidly activating, moderately inactivating currents that were more prevalent in PZ calyces. α-Dendrotoxin, a selective blocker of KV1 channels, reduced outward currents in CZ calyces but not in PZ calyces. Regional variations in K(+) conductances may contribute to different firing responses in calyx afferents.

Evidence that protons act as neurotransmitters at vestibular hair cell-calyx afferent synapses

Present data support the conclusion that protons serve as an important neurotransmitter to convey excitatory stimuli from inner ear type I vestibular hair cells to postsynaptic calyx nerve terminals. Time-resolved pH imaging revealed stimulus-evoked extrusion of protons from hair cells and a subsequent buildup of [H(+)] within the confined chalice-shaped synaptic cleft (ΔpH ∼ -0.2). Whole-cell voltage-clamp recordings revealed a concomitant nonquantal excitatory postsynaptic current in the calyx terminal that was causally modulated by cleft acidification. The time course of [H(+)] buildup limits the speed of this intercellular signaling mechanism, but for tonic signals such as gravity, protonergic transmission offers a significant metabolic advantage over quantal excitatory postsynaptic currents--an advantage that may have driven the proliferation of postsynaptic calyx terminals in the inner ear vestibular organs of contemporary amniotes.

Discharge regularity in the turtle posterior crista: comparisons between experiment and theory

Intra-axonal recordings were made from bouton fibers near their termination in the turtle posterior crista. Spike discharge, miniature excitatory postsynaptic potentials (mEPSPs), and afterhyperpolarizations (AHPs) were monitored during resting activity in both regularly and irregularly discharging units. Quantal size (qsize) and quantal rate (qrate) were estimated by shot-noise theory. Theoretically, the ratio, σV/(dμV/dt), between synaptic noise (σV) and the slope of the mean voltage trajectory (dμV/dt) near threshold crossing should determine discharge regularity. AHPs are deeper and more prolonged in regular units; as a result, dμV/dt is larger, the more regular the discharge. The qsize is larger and qrate smaller in irregular units; these oppositely directed trends lead to little variation in σV with discharge regularity. Of the two variables, dμV/dt is much more influential than the nearly constant σV in determining regularity. Sinusoidal canal-duct indentations at 0.3 Hz led to modulations in spike discharge and synaptic voltage. Gain, the ratio between the amplitudes of the two modulations, and phase leads re indentation of both modulations are larger in irregular units. Gain variations parallel the sensitivity of the postsynaptic spike encoder, the set of conductances that converts synaptic input into spike discharge. Phase variations reflect both synaptic inputs to the encoder and postsynaptic processes. Experimental data were interpreted using a stochastic integrate-and-fire model. Advantages of an irregular discharge include an enhanced encoder gain and the prevention of nonlinear phase locking. Regular and irregular units are more efficient, respectively, in the encoding of low- and high-frequency head rotations, respectively.

Tuning and timing in mammalian type I hair cells and calyceal synapses

Afferent nerve fibers in the central zones of vestibular epithelia form calyceal endings around type I hair cells and have phasic response properties that emphasize fast head motions. We investigated how stages from hair-cell transduction to calyceal spiking contribute tuning and timing to central (striolar)-zone afferents of the rat saccular epithelium. In an excised preparation, we deflected individual hair bundles with rigid probes driven with steps and sinusoids (0.5-500 Hz) and recorded whole-cell responses from hair cells and calyces at room temperature and body temperature. In immature hair cells and calyces (postnatal days (P)1-P4), tuning sharpened at each stage. Transducer adaptation and membrane-charging time produced bandpass filtering of the receptor potential with best frequencies of 10-30 Hz and phase leads below 10 Hz. For small stimuli, electrical resonances sharply tuned the hair-cell membrane in the frequency range of 5-40 Hz. The synaptic delay of quantal transmission added a phase lag at frequencies above 10 Hz. The influence of spike thresholds at the calyceal spike initiation stage sharpened tuning and advanced response phase. Two additional mechanisms strongly advanced response phase above 10 Hz when present: (1) maturing (P7-P9) type I hair cells acquired low-voltage-activated channels that shortened the rise time of the receptor potential and (2) some calyces had nonquantal transmission with little synaptic delay. By reducing response time, the identified inner-ear mechanisms (transducer adaptation, low-voltage-activated channels, nonquantal transmission, and spike triggering) may compensate for transmission delays in vestibular reflex pathways and help stabilize posture and gaze during rapid head motions.

Excitatory actions of GABA in developing chick vestibular afferents: effects on resting electrical activity

The aim of this study was to characterize the effect of γ-aminobutyric acid (GABA) in the resting multiunit activity of the vestibular afferents during development using the isolated inner ear of embryonic and postnatal chickens (E15-E21 and P5). GABA (10(-3) to 10(-5) M; n = 133) and muscimol (10(-3) M) elicited an increase in the frequency of the basal discharge of the vestibular afferents. We found that GABA action was dose-dependent and inversely related to animal age. Thus, the largest effect was observed in embryonic ages such as E15 and E17 and decreases in E21 and P5. The GABAA receptor antagonists, bicuculline (10(-5) M; n = 10) and picrotoxin (10(-4) M; n = 10), significantly decreased the excitatory action of GABA and muscimol (10(-3) M). Additionally, CNQX 10(-6) M, MCPG 10(-5) M and 7ClKyn 10(-5) M (n = 5) were co-applied by bath substitution (n = 5). Both the basal discharge and the GABA action significantly decreased in these experimental conditions. The chloride channel blocker 9-AC 0.5 mM produced an important reduction in the effect of GABA 10(-3) (n = 5) and 10(-4) M (n = 5). Thus, our results suggest an excitatory role of GABA in the resting activity of the vestibular afferents that can be explained by changes in the gradient of concentration of Cl(-) during development. We show for the first time that the magnitude of this GABA effect decreases at later stages of embryonic and early postnatal development. Taking into account the results with glutamatergic antagonists, we conclude that GABA has a presynaptic action but is not the neurotransmitter in the vestibular afferent synapses, although it could act as a facilitator of the spontaneous activity and may regulate glutamate release.

Intercellular K⁺ accumulation depolarizes Type I vestibular hair cells and their associated afferent nerve calyx

Mammalian vestibular organs contain two types of sensory receptors, named Type I and Type II hair cells. While Type II hair cells are contacted by several small afferent nerve terminals, the basolateral surface of Type I hair cells is almost entirely enveloped by a single large afferent nerve terminal, called calyx. Moreover Type I, but not Type II hair cells, express a low-voltage-activated outward K(+) current, I(K,L), which is responsible for their much lower input resistance (Rm) at rest as compared to Type II hair cells. The functional meaning of I(K,L) and associated calyx is still enigmatic. By combining the patch-clamp whole-cell technique with the mouse whole crista preparation, we have recorded the current- and voltage responses of in situ hair cells. Outward K(+) current activation resulted in K(+) accumulation around Type I hair cells, since it induced a rightward shift of the K(+) reversal potential the magnitude of which depended on the amplitude and duration of K(+) current flow. Since this phenomenon was never observed for Type II hair cells, we ascribed it to the presence of a residual calyx limiting K(+) efflux from the synaptic cleft. Intercellular K(+) accumulation added a slow (τ>100ms) depolarizing component to the cell voltage response. In a few cases we were able to record from the calyx and found evidence for intercellular K(+) accumulation as well. The resulting depolarization could trigger a discharge of action potentials in the afferent nerve fiber. Present results support a model where pre- and postsynaptic depolarization produced by intercellular K(+) accumulation cooperates with neurotransmitter exocytosis in sustaining afferent transmission arising from Type I hair cells. While vesicular transmission together with the low Rm of Type I hair cells appears best suited for signaling fast head movements, depolarization produced by intercellular K(+) accumulation could enhance signal transmission during slow head movements.

Glutamate transporters EAAT4 and EAAT5 are expressed in vestibular hair cells and calyx endings

Glutamate is the neurotransmitter released from hair cells. Its clearance from the synaptic cleft can shape neurotransmission and prevent excitotoxicity. This may be particularly important in the inner ear and in other sensory organs where there is a continually high rate of neurotransmitter release. In the case of most cochlear and type II vestibular hair cells, clearance involves the diffusion of glutamate to supporting cells, where it is taken up by EAAT1 (GLAST), a glutamate transporter. A similar mechanism cannot work in vestibular type I hair cells as the presence of calyx endings separates supporting cells from hair-cell synapses. Because of this arrangement, it has been conjectured that a glutamate transporter must be present in the type I hair cell, the calyx ending, or both. Using whole-cell patch-clamp recordings, we demonstrate that a glutamate-activated anion current, attributable to a high-affinity glutamate transporter and blocked by DL-TBOA, is expressed in type I, but not in type II hair cells. Molecular investigations reveal that EAAT4 and EAAT5, two glutamate transporters that could underlie the anion current, are expressed in both type I and type II hair cells and in calyx endings. EAAT4 has been thought to be expressed almost exclusively in the cerebellum and EAAT5 in the retina. Our results show that these two transporters have a wider distribution in mice. This is the first demonstration of the presence of transporters in hair cells and provides one of the few examples of EAATs in presynaptic elements.

Quantifying utricular stimulation during natural behavior

The use of natural stimuli in neurophysiological studies has led to significant insights into the encoding strategies used by sensory neurons. To investigate these encoding strategies in vestibular receptors and neurons, we have developed a method for calculating the stimuli delivered to a vestibular organ, the utricle, during natural (unrestrained) behaviors, using the turtle as our experimental preparation. High-speed digital video sequences are used to calculate the dynamic gravito-inertial (GI) vector acting on the head during behavior. X-ray computed tomography (CT) scans are used to determine the orientation of the otoconial layer (OL) of the utricle within the head, and the calculated GI vectors are then rotated into the plane of the OL. Thus, the method allows us to quantify the spatio-temporal structure of stimuli to the OL during natural behaviors. In the future, these waveforms can be used as stimuli in neurophysiological experiments to understand how natural signals are encoded by vestibular receptors and neurons. We provide one example of the method, which shows that turtle feeding behaviors can stimulate the utricle at frequencies higher than those typically used in vestibular studies. This method can be adapted to other species, to other vestibular end organs, and to other methods of quantifying head movements.

Inwardly rectifying potassium channel Kir4.1 is localized at the calyx endings of vestibular afferents

Inwardly rectifying potassium (Kir) channel Kir4.1 (also called Kcnj10) is expressed in various cells such as satellite glial cells. It is suggested that these cells would absorb excess accumulated K(+) from intercellular space which is surrounded by these cell membranes expressing Kir4.1. In the vestibular system, loss of Kir4.1 results in selective degeneration of type I hair cells despite normal development of type II hair cells. The mechanisms underlying this developmental disorder have been unclear, because it was thought that Kir4.1 is only expressed in glial cells throughout the entire nervous system. Here, we show that Kir4.1 is expressed not only in glial cells but also in neurons of the mouse vestibular system. In the vestibular ganglion, Kir4.1 mRNA is transcribed in both satellite cells and neuronal somata, whereas Kir4.1 protein is expressed only in satellite cells. On the other hand, in the vestibular sensory epithelia, Kir4.1 protein is localized at the calyx endings of vestibular afferents, which surround type I hair cells. Kir4.1 protein expression in the vestibular sensory epithelia is detected beginning after birth, and its localization gradually adopts a calyceal shape until type I hair cells are mature. Kir4.1 localized at the calyx endings may play a role in the K(+)-buffering action of vestibular afferents surrounding type I hair cells.

Vestibular hair cells and afferents: two channels for head motion signals

Vestibular epithelia of the inner ear detect head motions over a wide range of amplitudes and frequencies. In mammals, afferent nerve fibers from central and peripheral zones of vestibular epithelia form distinct populations with different response dynamics and spike timing. Central-zone afferents are large, fast conduits for phasic signals encoded in irregular spike trains. The finer afferents from peripheral zones conduct more slowly and encode more tonic, linear signals in highly regular spike trains. The hair cells are also of two types, I and II, but the two types do not correspond directly to the two afferent populations. Zonal differences in afferent response dynamics may arise at multiple stages, including mechanoelectrical transduction, voltage-gated channels in hair cells and afferents, afferent transmission at calyceal and bouton synapses, and spike generation in regular and irregular afferents. In contrast, zonal differences in spike timing may depend more simply on the selective expression of low-voltage-activated ion channels by irregular afferents.

Molecular microdomains in a sensory terminal, the vestibular calyx ending

Many primary vestibular afferents form large cup-shaped postsynaptic terminals (calyces) that envelope the basolateral surfaces of type I hair cells. The calyceal terminals both respond to glutamate released from ribbon synapses in the type I cells and initiate spikes that propagate to the afferent's central terminals in the brainstem. The combination of synaptic and spike initiation functions in these unique sensory endings distinguishes them from the axonal nodes of central neurons and peripheral nerves, such as the sciatic nerve, which have provided most of our information about nodal specializations. We show that rat vestibular calyces express an unusual mix of voltage-gated Na and K channels and scaffolding, cell adhesion, and extracellular matrix proteins, which may hold the ion channels in place. Protein expression patterns form several microdomains within the calyx membrane: a synaptic domain facing the hair cell, the heminode abutting the first myelinated internode, and one or two intermediate domains. Differences in the expression and localization of proteins between afferent types and zones may contribute to known variations in afferent physiology.

Information analysis of posterior canal afferents in the turtle, Trachemys scripta elegans

We have used sinusoidal and band-limited Gaussian noise stimuli along with information measures to characterize the linear and non-linear responses of morpho-physiologically identified posterior canal (PC) afferents and to examine the relationship between mutual information rate and other physiological parameters. Our major findings are: 1) spike generation in most PC afferents is effectively a stochastic renewal process, and spontaneous discharges are fully characterized by their first order statistics; 2) a regular discharge, as measured by normalized coefficient of variation (cv*), reduces intrinsic noise in afferent discharges at frequencies below the mean firing rate; 3) coherence and mutual information rates, calculated from responses to band-limited Gaussian noise, are jointly determined by gain and intrinsic noise (discharge regularity), the two major determinants of signal to noise ratio in the afferent response; 4) measures of optimal non-linear encoding were only moderately greater than optimal linear encoding, indicating that linear stimulus encoding is limited primarily by internal noise rather than by non-linearities; and 5) a leaky integrate and fire model reproduces these results and supports the suggestion that the combination of high discharge regularity and high discharge rates serves to extend the linear encoding range of afferents to higher frequencies. These results provide a framework for future assessments of afferent encoding of signals generated during natural head movements and for comparison with coding strategies used by other sensory systems. This article is part of a Special Issue entitled: Neural Coding.

Potassium accumulation between type I hair cells and calyx terminals in mouse crista

The mode of synaptic transmission in the vestibular periphery, between type I hair cells and their associated calyx terminal, has been the subject of much debate. The close and extensive apposition of pre- and post-synaptic elements has led some to suggest potassium (K(+)) accumulates in the intercellular space and even plays a role in synaptic transmission. During patch clamp recordings from isolated and embedded hair cells in a semi-intact preparation of the mouse cristae, we noted marked differences in whole-cell currents. Embedded type I hair cells show a prominent droop during steady-state activation as well as a dramatic collapse in tail currents. Responses to a depolarizing voltage step (-124 to +16 mV) in embedded, but not isolated, hair cells resulted in a >40 mV shift of the K(+) equilibrium potential and a rise in effective K(+) concentration (>50 mM) in the intercellular space. Together these data suggest K(+) accumulation in the intercellular space accounts for the different responses in isolated and embedded type I hair cells. To test this notion, we exposed the preparation to hyperosmotic solutions to enlarge the intercellular space. As predicted, the K(+) accumulation effects were reduced; however, a fit of our data with a classic diffusion model suggested K(+) permeability, rather than the intercellular space, had been altered by the hyperosmotic change. These results support the notion that under depolarizing conditions substantial K(+) accumulation occurs in the space between type I hair cells and calyx. The extent of K(+) accumulation during normal synaptic transmission, however, remains to be determined.

Ion channels set spike timing regularity of mammalian vestibular afferent neurons

In the mammalian vestibular nerve, some afferents have highly irregular interspike intervals and others have highly regular intervals. To investigate whether spike timing is determined by the afferents' ion channels, we studied spiking activity in their cell bodies, isolated from the vestibular ganglia of young rats. Whole cell recordings were made with the perforated-patch method. As previously reported, depolarizing current steps revealed distinct firing patterns. Transient neurons fired one or two onset spikes, independent of current level. Sustained neurons were more heterogeneous, firing either trains of spikes or a spike followed by large voltage oscillations. We show that the firing pattern categories are robust, occurring at different temperatures and ages, both in mice and in rats. A difference in average resting potential did not cause the difference in firing patterns, but contributed to differences in afterhyperpolarizations. A low-voltage-activated potassium current (I(LV)) was previously implicated in the transient firing pattern. We show that I(LV) grew from the first to second postnatal week and by the second week comprised Kv1 and Kv7 (KCNQ) components. Blocking I(LV) converted step-evoked firing patterns from transient to sustained. Separated from their normal synaptic inputs, the neurons did not spike spontaneously. To test whether the firing-pattern categories might correspond to afferent populations of different regularity, we injected simulated excitatory postsynaptic currents at pseudorandom intervals. Sustained neurons responded to a given pattern of input with more regular firing than did transient neurons. Pharmacological block of I(LV) made firing more regular. Thus ion channel differences that produce transient and sustained firing patterns in response to depolarizing current steps can also produce irregular and regular spike timing.

K+ currents in isolated vestibular afferent calyx terminals

Vestibular hair cells transduce mechanical displacements of their hair bundles into an electrical receptor potential which modulates transmitter release and subsequent action potential firing in afferent neurons. To probe ionic mechanisms underlying sensory coding in vestibular calyces, we used the whole-cell patch-clamp technique to record action potentials and K(+) currents from afferent calyx terminals isolated from the semicircular canals of Mongolian gerbils. Calyx terminals showed minimal current at the mean zero-current potential (-60 mV), but two types of outward K(+) currents were identified at potentials above -50 mV. The first current was a rapidly activating and inactivating K(+) current that was blocked by 4-aminopyridine (4-AP, 2.5 mM) and BDS-I (up to 250 nM). The time constant for activation of this current decreased with membrane depolarization to a minimum value of approximately 1 ms. The 4-AP-sensitive current showed steady-state inactivation with a half-inactivation of approximately -70 mV. A second, more slowly activating current (activation time constant was 8.5 +/- 0.7 ms at -8 mV) was sensitive to TEA (30 mM). The TEA-sensitive current also showed steady-state inactivation with a half-inactivation of -95.4 +/- 1.4 mV, following 500-ms duration conditioning pulses. A combination of 4-AP and TEA blocked approximately 90% of the total outward current. In current clamp, single Na(+)-dependent action potentials were evoked following hyperpolarization to potentials more negative than the resting potential. 4-AP application increased action potential width, whereas TEA both increased the width and greatly reduced repolarization of the action potential.

The septate junction protein caspr is required for structural support and retention of KCNQ4 at calyceal synapses of vestibular hair cells

The afferent innervation contacting the type I hair cells of the vestibular sensory epithelia form distinct calyceal synapses. The apposed presynaptic and postsynaptic membranes at this large area of synaptic contact are kept at a remarkably regular distance. Here, we show by freeze-fracture electron microscopy that a patterned alignment of proteins at the calyceal membrane resembles a type of intercellular junction that is rare in vertebrates, the septate junction (SJ). We found that a core molecular component of SJs, Caspr, colocalizes with the K(+) channel KCNQ4 at the postsynaptic membranes of these calyceal synapses. Immunolabeling and ultrastructural analyses of Caspr knock-out mice reveal that, in the absence of Caspr, the separation between the membranes of the hair cells and the afferent neurons is conspicuously irregular and often increased by an order of magnitude. In these mutants, KCNQ4 fails to cluster at the postsynaptic membrane and appears diffused along the entire calyceal membrane. Our results indicate that a septate-like junction provides structural support to calyceal synaptic contact with the vestibular hair cell and that Caspr is required for the recruitment or retention of KCNQ4 at these synapses.

Ultrastructural analysis of the cristae ampullares in the squirrel monkey (Saimiri sciureus)

Type I hair cells outnumber type II hair cells (HCs) in squirrel monkey (Saimiri sciureus) cristae by a nearly 3:1 ratio. Associated with this type I HC preponderance, calyx fibers make up a much larger fraction of the afferent innervation than in rodents (Fernández et al. [1995] J. Neurophysiol. 73:1253-1269). To study how this affects synaptic architecture, we used disector methods to estimate various features associated with type I and type II HCs in central (CZ) and peripheral (PZ) zones of monkey cristae. Each type I HC makes, on average, 5-10 ribbon synapses with the inner face of a calyx ending. Inner-face synapses outnumber those on calyx outer faces by a 40:1 ratio. Expressed per afferent, there are, on average, 15 inner-face ribbon synapses, 0.38 outer-face ribbons, and 2.6 efferent boutons on calyx-bearing endings. Calyceal invaginations per type I HC range from 19 in CZ to 3 in PZ. For type II HCs, there are many more ribbons and afferent boutons in PZ than in CZ, whereas efferent innervation is relatively uniform throughout the neuroepithelium. Despite outer-face ribbons being more numerous in chinchilla than in squirrel monkey, afferent discharge properties are similar (Lysakowski et al. [1995] J. Neurophysiol. 73:1270-1281), reinforcing the importance of inner-face ribbons in synaptic transmission. Comparisons across mammalian species suggest that the prevalence of type I HCs is a primate characteristic, rather than an arboreal life-style adaptation. Unlike cristae, type II HCs predominate in monkey maculae. Differences in hair-cell counts may reflect the stimulus magnitudes handled by semicircular canals and otolith organs.

Resting discharge patterns of macular primary afferents in otoconia-deficient mice

Vestibular primary afferents in the normal mammal are spontaneously active. The consensus hypothesis states that such discharge patterns are independent of stimulation and depend instead on excitation by vestibular hair cells due to background release of synaptic neurotransmitter. In the case of otoconial sensory receptors, it is difficult to test the independence of resting discharge from natural tonic stimulation by gravity. We examined this question by studying discharge patterns of single vestibular primary afferent neurons in the absence of gravity stimulation using two mutant strains of mice that lack otoconia (OTO-; head tilt, het-Nox3, and tilted, tlt-Otop1). Our findings demonstrated that macular primary afferent neurons exhibit robust resting discharge activity in OTO- mice. Spike interval coefficient of variation (CV = SD/mean spike interval) values reflected both regular and irregular discharge patterns in OTO- mice, and the range of values for rate-normalized CV was similar to mice and other mammals with intact otoconia although there were proportionately fewer irregular fibers. Mean discharge rates were slightly higher in otoconia-deficient strains even after accounting for proportionately fewer irregular fibers [OTO- = 75.4 +/- 31.1(113) vs OTO+ = 68.1 +/- 28.5(143) in sp/s]. These results confirm the hypothesis that resting activity in macular primary afferents occurs in the absence of ambient stimulation. The robust discharge rates are interesting in that they may reflect the presence of a functionally 'up-regulated' tonic excitatory process in the absence of natural sensory stimulation.

Modalities of GABA and glutamate neurotransmission in the vertebrate inner ear vestibule

GABA and glutamate have been postulated as afferent neurotransmitters at the sensory periphery inner ear vestibule in vertebrates. GABA has fulfilled the main criteria to act as afferent neurotransmitter but may also be a putative efferent neurotransmitter, mainly due to cellular localization of its synthesizing enzyme glutamate decarboxylase derived from biochemical, immunocytochemical, in situ hybridization and molecular biological techniques, whereas glutamate afferent neurotransmission role is supported mainly by pharmacological evidences. GABA and Glu could also act as afferent co-neurotransmitters based upon immunocytochemical techniques. This multiplicity was not considered earlier and postulates a peripheral modulation of afferent information being sent to higher vestibular centers. In order to make a definitive cellular assignation to these putative neurotransmitters it is necessary to have evidences derived from immunocytochemical and pharmacological experiments in which both substances are tested simultaneously.