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

The potassium channel subunit KV1.8 (Kcna10) is essential for the distinctive outwardly rectifying conductances of type I and II vestibular hair cells

In amniotes, head motions and tilt are detected by two types of vestibular hair cells (HCs) with strikingly different morphology and physiology. Mature type I HCs express a large and very unusual potassium conductance, gK,L, which activates negative to resting potential, confers very negative resting potentials and low input resistances, and enhances an unusual non-quantal transmission from type I cells onto their calyceal afferent terminals. Following clues pointing to KV1.8 (KCNA10) in the Shaker K channel family as a candidate gK,L subunit, we compared whole-cell voltage-dependent currents from utricular hair cells of KV1.8-null mice and littermate controls. We found that KV1.8 is necessary not just for gK,L but also for fast-inactivating and delayed rectifier currents in type II HCs, which activate positive to resting potential. The distinct properties of the three KV1.8-dependent conductances may reflect different mixing with other KV subunits that are reported to be differentially expressed in type I and II HCs. In KV1.8-null HCs of both types, residual outwardly rectifying conductances include KV7 (KCNQ) channels. Current clamp records show that in both HC types, KV1.8-dependent conductances increase the speed and damping of voltage responses. Features that speed up vestibular receptor potentials and non-quantal afferent transmission may have helped stabilize locomotion as tetrapods moved from water to land.

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.

Maturation of type I and type II rat vestibular hair cells in vivo and in vitro

Vestibular sensory epithelia contain type I and type II sensory hair cells (HCI and HCII). Recent studies have revealed molecular markers for the identification of these cells, but the precise composition of each vestibular epithelium (saccule, utricle, lateral crista, anterior crista, posterior crista) and their postnatal maturation have not been described in detail. Moreover, in vitro methods to study this maturation are not well developed. We obtained total HCI and HCII counts in adult rats and studied the maturation of the epithelia from birth (P0) to postnatal day 28 (P28). Adult vestibular epithelia hair cells were found to comprise ∼65% HCI expressing osteopontin and PMCA2, ∼30% HCII expressing calretinin, and ∼4% HCII expressing SOX2 but neither osteopontin nor calretinin. At birth, immature HCs express both osteopontin and calretinin. P28 epithelia showed an almost adult-like composition but still contained 1.3% of immature HCs. In addition, we obtained free-floating 3D cultures of the epithelia at P1, which formed a fluid-filled cyst, and studied their survival and maturation in vitro up to day 28 (28 DIV). These cultures showed good HC resiliency and maturation. Using an enriched medium for the initial 4 days, a HCI/calretinin+-HCII ratio close to the in vivo ratio was obtained. These cultures are suitable to study HC maturation and mature HCs in pharmacological, toxicological and molecular research.

Comparative biology of the amniote vestibular utricle

The sensory epithelia of the auditory and vestibular systems of vertebrates have shared developmental and evolutionary histories. However, while the auditory epithelia show great variation across vertebrates, the vestibular sensory epithelia appear seemingly more conserved. An exploration of the current knowledge of the comparative biology of the amniote utricle, a vestibular sensory epithelium that senses linear acceleration, shows interesting instances of variability between birds and mammals. The distribution of sensory hair cell types, the position of the line of hair bundle polarity reversal and the properties of supporting cells show marked differences, likely impacting vestibular function and hair cell regeneration potential.

Electrical stimulation of the peripheral and central vestibular system

PURPOSE OF REVIEW

Electrical stimulation of the peripheral and central vestibular system using noninvasive (galvanic vestibular stimulation, GVS) or invasive (intracranial electrical brain stimulation, iEBS) approaches have a long history of use in studying self-motion perception and balance control. The aim of this review is to summarize recent electrophysiological studies of the effects of GVS, and functional mapping of the central vestibular system using iEBS in awake patients.

RECENT FINDINGS

The use of GVS has become increasingly common in the assessment and treatment of a wide range of clinical disorders including vestibulopathy and Parkinson's disease. The results of recent single unit recording studies have provided new insight into the neural mechanisms underlying GVS-evoked improvements in perceptual and motor responses. Furthermore, the application of iEBS in patients with epilepsy or during awake brain surgery has provided causal evidence of vestibular information processing in mostly the middle cingulate cortex, posterior insula, inferior parietal lobule, amygdala, precuneus, and superior temporal gyrus.

SUMMARY

Recent studies have established that GVS evokes robust and parallel activation of both canal and otolith afferents that is significantly different from that evoked by natural head motion stimulation. Furthermore, there is evidence that GVS can induce beneficial neural plasticity in the central pathways of patients with vestibular loss. In addition, iEBS studies highlighted an underestimated contribution of areas in the medial part of the cerebral hemispheres to the cortical vestibular network.

KCNQ2/3 regulates efferent mediated slow excitation of vestibular afferents in mammals

Primary vestibular afferents transmit information from hair cells about head position and movement to the CNS, which is critical for maintaining balance, gaze stability and spatial navigation. The CNS, in turn, modulates hair cells and afferents via the efferent vestibular system (EVS) and its activation of several cholinergic signaling mechanisms. Electrical stimulation of EVS neurons gives rise to three kinetically- and mechanistically-distinct afferent responses including a slow excitation, a fast excitation, and a fast inhibition. EVS-mediated slow excitation is attributed to odd-numbered muscarinic acetylcholine receptors (mAChRs) on the afferent whose activation leads to the closure of a potassium conductance and increased afferent discharge. Likely effector candidates include low-threshold, voltage-gated potassium channels belonging to the KCNQ (Kv7.X) family, which are involved in neuronal excitability across the nervous system and are subject to mAChR modulation. Specifically, KCNQ2/3 heteromeric channels may be the molecular correlates for the M-current, a potassium current that is blocked following the activation of odd-numbered mAChRs. To this end, multiple members of the KCNQ channel family, including KCNQ2 and KCNQ3, are localized to several microdomains within vestibular afferent endings, where they influence afferent excitability and could be targeted by EVS neurons. Additionally, the relative expression of KCNQ subunits appears to vary across the sensory epithelia and among different afferent types. However, it is unclear which KCNQ channel subunits are targeted by mAChR activation and whether that also varies among different afferent classes. Here we show that EVS-mediated slow excitation is blocked and enhanced by the non-selective KCNQ channel blocker XE991 and opener retigabine, respectively. Using KCNQ subunit-selective drugs, we observed that a KCNQ2 blocker blocks the slow response in irregular afferents, while a KCNQ2/3 opener enhances slow responses in regular afferents. The KCNQ2 blockers did not appear to affect resting afferent discharge rates, while KCNQ2/3 or KCNQ2/4 openers decreased afferent excitability. Here, we show pharmacological evidence that KCNQ2/3 subunits are likely targeted by mAChR activation in mammalian vestibular afferents. Additionally, we show that KCNQ3 KO mice have altered resting discharge rate as well as EVS-mediated slow response. These data together suggest that KCNQ channels play a role in slow response and discharge rate of vestibular afferents, which can be modulated by EVS in mammals.

Vestibular Testing-New Physiological Results for the Optimization of Clinical VEMP Stimuli

Both auditory and vestibular primary afferent neurons can be activated by sound and vibration. This review relates the differences between them to the different receptor/synaptic mechanisms of the two systems, as shown by indicators of peripheral function-cochlear and vestibular compound action potentials (cCAPs and vCAPs)-to click stimulation as recorded in animal studies. Sound- and vibration-sensitive type 1 receptors at the striola of the utricular macula are enveloped by the unique calyx afferent ending, which has three modes of synaptic transmission. Glutamate is the transmitter for both cochlear and vestibular primary afferents; however, blocking glutamate transmission has very little effect on vCAPs but greatly reduces cCAPs. We suggest that the ultrafast non-quantal synaptic mechanism called resistive coupling is the cause of the short latency vestibular afferent responses and related results-failure of transmitter blockade, masking, and temporal precision. This "ultrafast" non-quantal transmission is effectively electrical coupling that is dependent on the membrane potentials of the calyx and the type 1 receptor. The major clinical implication is that decreasing stimulus rise time increases vCAP response, corresponding to the increased VEMP response in human subjects. Short rise times are optimal in human clinical VEMP testing, whereas long rise times are mandatory for audiometric threshold testing.

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.

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 <0.8 ms, the vCAP magnitude increased in proportion to temporal bone acceleration, but for pulse durations >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.

Muscarinic Acetylcholine Receptors Modulate HCN Channel Properties in Vestibular Ganglion Neurons

Efferent modulation of vestibular afferent excitability is linked to muscarinic signaling cascades that close low-voltage-gated potassium channels (i.e., KCNQ). Here, we show that muscarinic signaling cascades also depolarize the activation range of hyperpolarization-activated cyclic-nucleotide gated (HCN) channels. We compared the voltage activation range and kinetics of HCN channels and induced firing patterns before and after administering the muscarinic acetylcholine receptor (mAChR) agonist oxotremorine-M (Oxo-M) in dissociated vestibular ganglion neurons (VGNs) from rats of either sex using perforated whole-cell patch-clamp methods. Oxo-M depolarized HCN channels' half-activation voltage (V 1/2) and sped up the rate of activation near resting potential twofold. HCN channels in large-diameter and/or transient firing VGN (putative cell bodies of irregular firing neuron from central epithelial zones) had relatively depolarized V 1/2 in control solution and were less sensitive to mAChR activation than those found in small-diameter VGN with sustained firing patterns (putatively belonging to regular firing afferents). The impact of mAChR on HCN channels is not a direct consequence of closing KCNQ channels since pretreating the cells with Linopirdine, a KCNQ channel blocker, did not prevent HCN channel depolarization by Oxo-M. Efferent signaling promoted ion channel configurations that were favorable to highly regular spiking in some VGN, but not others. This is consistent with previous observations that low-voltage gated potassium currents in VGN are conducted by mAChR agonist-sensitive and -insensitive channels. Connecting efferent signaling to HCN channels is significant because of the channel's impact on spike-timing regularity and nonchemical transmission between Type I hair cells and vestibular afferents.SIGNIFICANCE STATEMENT Vestibular afferents express a diverse complement of ion channels. In vitro studies identified low-voltage activated potassium channels and hyperpolarization-activated cyclic-nucleotide gated (HCN) channels as crucial for shaping the timing and sensitivity of afferent responses. Moreover, a network of acetylcholine-releasing efferent neurons controls afferent excitability by closing a subgroup of low-voltage activated potassium channels on the afferent neuron. This work shows that these efferent signaling cascades also enhance the activation of HCN channels by depolarizing their voltage activation range. The size of this effect varies depending on the endogenous properties of the HCN channel and on cell type (as determined by discharge patterns and cell size). Simultaneously controlling two ion-channel groups gives the vestibular efferent system exquisite control over afferent neuron activity.