I h and HCN channels in murine spiral ganglion neurons: tonotopic variation, local heterogeneity, and kinetic model

The upregulation of K+ and HCN channels in developing spiral ganglion neurons is mediated by cochlear inner hair cells

Spiral ganglion neurons (SGNs) are primary sensory afferent neurons that relay acoustic information from the cochlear inner hair cells (IHCs) to the brainstem. The response properties of different SGNs diverge to represent a wide range of sound intensities in an action-potential code. This biophysical heterogeneity is established during pre-hearing stages of development, a time when IHCs fire spontaneous Ca2+ action potentials that drive glutamate release from their ribbon synapses onto the SGN terminals. The role of spontaneous IHC activity in the refinement of SGN characteristics is still largely unknown. Using pre-hearing otoferlin knockout mice (Otof-/-), in which Ca2+-dependent exocytosis in IHCs is abolished, we found that developing SGNs fail to upregulate low-voltage-activated K+-channels and hyperpolarisation-activated cyclic-nucleotide-gated channels. This delayed maturation resulted in hyperexcitable SGNs with immature firing characteristics. We have also shown that SGNs that synapse with the pillar side of the IHCs selectively express a resurgent K+ current, highlighting a novel biophysical marker for these neurons. RNA-sequencing showed that several K+ channels are downregulated in Otof-/- mice, further supporting the electrophysiological recordings. Our data demonstrate that spontaneous Ca2+-dependent activity in pre-hearing IHCs regulates some of the key biophysical and molecular features of the developing SGNs. KEY POINTS: Ca2+-dependent exocytosis in inner hair cells (IHCs) is otoferlin-dependent as early as postnatal day 1. A lack of otoferlin in IHCs affects potassium channel expression in SGNs. The absence of otoferlin is associated with SGN hyperexcitability. We propose that type I spiral ganglion neuron functional maturation depends on IHC exocytosis.

Chromatin remodeling protein CHD4 regulates axon guidance of spiral ganglion neurons in developing cochlea

The chromodomain helicase binding protein 4 (CHD4) is an ATP-dependent chromatin remodeler. De-novo pathogenic variants of CHD4 cause Sifrim-Hitz-Weiss syndrome (SIHIWES). Patients with SIHIWES show delayed development, intellectual disability, facial dysmorphism, and hearing loss. Many cochlear cell types, including spiral ganglion neurons (SGNs), express CHD4. SGNs are the primary afferent neurons that convey sound information from the cochlea, but the function of CHD4 in SGNs is unknown. We employed the Neurog1(Ngn1) CreERT2 Chd4 conditional knockout animals to delete Chd4 in SGNs. SGNs are classified as type I and type II neurons. SGNs lacking CHD4 showed abnormal fasciculation of type I neurons along with improper pathfinding of type II fibers. CHD4 binding to chromatin from immortalized multipotent otic progenitor-derived neurons was used to identify candidate target genes in SGNs. Gene ontology analysis of CHD4 target genes revealed cellular processes involved in axon guidance, axonal fasciculation, and ephrin receptor signaling pathway. We validated increased Epha4 transcripts in SGNs from Chd4 conditional knockout cochleae. The results suggest that CHD4 attenuates the transcription of axon guidance genes to form the stereotypic pattern of SGN peripheral projections. The results implicate epigenetic changes in circuit wiring by modulating axon guidance molecule expression and provide insights into neurodevelopmental diseases.

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.

Modulation of potassium conductances optimizes fidelity of auditory information

Potassium channels in auditory neurons are rapidly modified by changes in the auditory environment. In response to elevated auditory stimulation, short-term mechanisms such as protein phosphorylation and longer-term mechanisms such as accelerated channel synthesis increase the amplitude of currents that promote high-frequency firing. It has been suggested that this allows neurons to fire at high rates in response to high sound levels. We have carried out simple simulations of the response to postsynaptic neurons to patterns of neurotransmitter release triggered by auditory stimuli. These demonstrate that the amplitudes of potassium currents required for optimal encoding of a low-amplitude auditory signal differ from those for louder sounds. Specifically, the cross-correlation of the output of a neuron with an auditory stimulus is improved by increasing potassium currents as sound amplitude increases. Temporal fidelity for low-frequency stimuli is improved by increasing potassium currents that activate at negative potentials, while that for high-frequency stimuli requires increases in currents that activate at positive membrane potentials. These effects are independent of the firing rate. Moreover, levels of potassium currents that maximize the fidelity of the output of an ensemble of neurons differ from those that maximize fidelity for a single neuron. This suggests that the modulatory mechanisms must coordinate channel activity in groups of neurons or an entire nucleus. The simulations provide an explanation for the modulation of the intrinsic excitability of auditory brainstem neurons by changes in environmental sound levels, and the results may extend to information processing in other neural systems.

Amplification of input differences by dynamic heterogeneity in the spiral ganglion

A defining feature of type I primary auditory afferents that compose ∼95% of the spiral ganglion is their intrinsic electrophysiological heterogeneity. This diversity is evident both between and within unitary, rapid, and slow adaptation (UA, RA, and SA) classes indicative of specializations designed to shape sensory receptor input. But to what end? Our initial impulse is to expect the opposite: that auditory afferents fire uniformly to represent acoustic stimuli with accuracy and high fidelity. Yet this is clearly not the case. One explanation for this neural signaling strategy is to coordinate a system in which differences between input stimuli are amplified. If this is correct, then stimulus disparity enhancements within the primary afferents should be transmitted seamlessly into auditory processing pathways that utilize population coding for difference detection. Using sound localization as an example, one would expect to observe separately regulated differences in intensity level compared with timing or spectral cues within a graded tonotopic distribution. This possibility was evaluated by examining the neuromodulatory effects of cAMP on immature neurons with high excitability and slow membrane kinetics. We found that electrophysiological correlates of intensity and timing were indeed independently regulated and tonotopically distributed, depending on intracellular cAMP signaling level. These observations, therefore, are indicative of a system in which differences between signaling elements of individual stimulus attributes are systematically amplified according to auditory processing constraints. Thus, dynamic heterogeneity mediated by cAMP in the spiral ganglion has the potential to enhance the representations of stimulus input disparities transmitted into higher level difference detection circuitry.NEW & NOTEWORTHY Can changes in intracellular second messenger signaling within primary auditory afferents shift our perception of sound? Results presented herein lead to this conclusion. We found that intracellular cAMP signaling level systematically altered the kinetics and excitability of primary auditory afferents, exemplifying how dynamic heterogeneity can enhance differences between electrophysiological correlates of timing and intensity.

Dynamic Heterogeneity Shapes Patterns of Spiral Ganglion Activity

Neural response properties that typify primary sensory afferents are critical to fully appreciate because they establish and, ultimately represent, the fundamental coding design used for higher-level processing. Studies illuminating the center-surround receptive fields of retinal ganglion cells, for example, were ground-breaking because they determined the foundation of visual form detection. For the auditory system, a basic organizing principle of the spiral ganglion afferents is their extensive electrophysiological heterogeneity establishing diverse intrinsic firing properties in neurons throughout the spiral ganglion. Moreover, these neurons display an impressively large array of neurotransmitter receptor types that are responsive to efferent feedback. Thus, electrophysiological diversity and its neuromodulation are a fundamental encoding mechanism contributed by the primary afferents in the auditory system. To place these features into context, we evaluated the effects of hyperpolarization and cAMP on threshold level as indicators of overall afferent responsiveness in CBA/CaJ mice of either sex. Hyperpolarization modified threshold gradients such that distinct voltage protocols could shift the relationship between sensitivity and stimulus input to reshape resolution. This resulted in an "accordion effect" that appeared to stretch, compress, or maintain responsivity across the gradient of afferent thresholds. cAMP targeted threshold and kinetic shifts to rapidly adapting neurons, thus revealing multiple cochleotopic properties that could potentially be independently regulated. These examples of dynamic heterogeneity in primary auditory afferents not only have the capacity to shift the range, sensitivity, and resolution, but to do so in a coordinated manner that appears to orchestrate changes with a seemingly unlimited repertoire.SIGNIFICANCE STATEMENT How do we discriminate the more nuanced qualities of the sound around us? Beyond the basics of pitch and loudness, aspects, such as pattern, distance, velocity, and location, are all attributes that must be used to encode acoustic sensations effectively. While higher-level processing is required for perception, it would not be unexpected if the primary auditory afferents optimized receptor input to expedite neural encoding. The findings reported herein are consistent with this design. Neuromodulation compressed, expanded, shifted, or realigned intrinsic electrophysiological heterogeneity to alter neuronal responses selectively and dynamically. This suggests that diverse spiral ganglion phenotypes provide a rich substrate to support an almost limitless array of coding strategies within the first neural element of the auditory pathway.

Analog Transmission of Action Potential Fine Structure in Spiral Ganglion Axons

Action potential waveforms generated at the axon initial segment (AIS) are specialized between and within neuronal classes. But is the fine structure of each electrical event retained when transmitted along myelinated axons or is it rapidly and uniformly transmitted to be modified again at the axon terminal? To address this issue action potential axonal transmission was evaluated in a class of primary sensory afferents that possess numerous types of voltage-gated ion channels underlying a complex repertoire of endogenous firing patterns. In addition to their signature intrinsic electrophysiological heterogeneity, spiral ganglion neurons are uniquely designed. The bipolar, myelinated somata of type I neurons are located within the conduction pathway, requiring that action potentials generated at the first heminode must be conducted through their electrically excitable membrane. We utilized this unusual axonal-like morphology to serve as a window into action potential transmission to compare locally-evoked action potential profiles to those generated peripherally at their glutamatergic synaptic connections with hair cell receptors. These comparisons showed that the distinctively-shaped somatic action potentials were highly correlated with the nodally-generated, invading ones for each neuron. This result indicates that the fine structure of the action potential waveform is maintained axonally, thus supporting the concept that analog signaling is incorporated into each digitally-transmitted action potential in the specialized primary auditory afferents.

Encoding sound in the cochlea: from receptor potential to afferent discharge

Ribbon-class synapses in the ear achieve analog to digital transformation of a continuously graded membrane potential to all-or-none spikes. In mammals, several auditory nerve fibres (ANFs) carry information from each inner hair cell (IHC) to the brain in parallel. Heterogeneity of transmission among synapses contributes to the diversity of ANF sound-response properties. In addition to the place code for sound frequency and the rate code for sound level, there is also a temporal code. In series with cochlear amplification and frequency tuning, neural representation of temporal cues over a broad range of sound levels enables auditory comprehension in noisy multi-speaker settings. The IHC membrane time constant introduces a low-pass filter that attenuates fluctuations of the receptor potential above 1-2 kHz. The ANF spike generator adds a high-pass filter via its depolarization-rate threshold that rejects slow changes in the postsynaptic potential and its phasic response property that ensures one spike per depolarization. Synaptic transmission involves several stochastic subcellular processes between IHC depolarization and ANF spike generation, introducing delay and jitter that limits the speed and precision of spike timing. ANFs spike at a preferred phase of periodic sounds in a process called phase-locking that is limited to frequencies below a few kilohertz by both the IHC receptor potential and the jitter in synaptic transmission. During phase-locking to periodic sounds of increasing intensity, faster and facilitated activation of synaptic transmission and spike generation may be offset by presynaptic depletion of synaptic vesicles, resulting in relatively small changes in response phase. Here we review encoding of spike-timing at cochlear ribbon synapses.

Simulated auditory fiber myelination heterogeneity desynchronizes population responses to electrical stimulation limiting inter-aural timing difference representation

Auditory nerve responses to electrical stimulation exhibit aberrantly synchronous response latencies to low-rate pulse trains, nevertheless, cochlear implant users generally have elevated inter-aural timing difference detection thresholds. These findings present an apparent paradox in which single units are unusually precise but downstream within the auditory pathway access to this precision is lost. Auditory nerves innervating a region of cochlea exhibit natural heterogeneity in their diameter, myelination, and other structural properties; a key question is whether this diversity may contribute to the loss of temporal fidelity. In this work, responses of simulated auditory neuron populations with realistic intrinsic diameter and myelination heterogeneity to low-rate pulse trains were produced. By performing a receiver operating characteristic analysis on response latency distributions, ideal-observer interaural timing difference (ITD) detection limits were produced for each population. Fiber heterogeneity produced dispersion of inter-fiber latencies that produced ITD thresholds like that observed in the best performing cochlear implant users. Incorporation of myelin loss into these populations further increased inter-fiber latency variance and elevated ITD detection limits. These findings suggest that the interaction of applied currents with fibers' specific intrinsic properties may introduce fundamental limits on presentation of fine temporal structure in electrical stimulation.

HCN channels in the mammalian cochlea: Expression pattern, subcellular location, and age-dependent changes

Neuronal diversity in the cochlea is largely determined by ion channels. Among voltage‐gated channels, hyperpolarization‐activated cyclic nucleotide‐gated (HCN) channels open with hyperpolarization and depolarize the cell until the resting membrane potential. The functions for hearing are not well elucidated and knowledge about localization is controversial. We created a detailed map of subcellular location and co‐expression of all four HCN subunits across different mammalian species including CBA/J, C57Bl/6N, Ly5.1 mice, guinea pigs, cats, and human subjects. We correlated age‐related hearing deterioration in CBA/J and C57Bl/6N with expression levels of HCN1, −2, and −4 in individual auditory neurons from the same cohort. Spatiotemporal expression during murine postnatal development exposed HCN2 and HCN4 involvement in a critical phase of hair cell innervation. The huge diversity of subunit composition, but lack of relevant heteromeric pairing along the perisomatic membrane and axon initial segments, highlighted an active role for auditory neurons. Neuron clusters were found to be the hot spots of HCN1, −2, and −4 immunostaining. HCN channels were also located in afferent and efferent fibers of the sensory epithelium. Age‐related changes on HCN subtype expression were not uniform among mice and could not be directly correlated with audiometric data. The oldest mice groups revealed HCN channel up‐ or downregulation, depending on the mouse strain. The unexpected involvement of HCN channels in outer hair cell function where HCN3 overlaps prestin location emphasized the importance for auditory function. A better understanding may open up new possibilities to tune neuronal responses evoked through electrical stimulation by cochlear implants.

Gradients in the biophysical properties of neonatal auditory neurons align with synaptic contact position and the intensity coding map of inner hair cells

Sound intensity is encoded by auditory neuron subgroups that differ in thresholds and spontaneous rates. Whether variations in neuronal biophysics contributes to this functional diversity is unknown. Because intensity thresholds correlate with synaptic position on sensory hair cells, we combined patch clamping with fiber labeling in semi-intact cochlear preparations in neonatal rats from both sexes. The biophysical properties of auditory neurons vary in a striking spatial gradient with synaptic position. Neurons with high thresholds to injected currents contact hair cells at synaptic positions where neurons with high thresholds to sound-intensity are found in vivo. Alignment between in vitro and in vivo thresholds suggests that biophysical variability contributes to intensity coding. Biophysical gradients were evident at all ages examined, indicating that cell diversity emerges in early post-natal development and persists even after continued maturation. This stability enabled a remarkably successful model for predicting synaptic position based solely on biophysical properties.

Enhanced Activation of HCN Channels Reduces Excitability and Spike-Timing Regularity in Maturing Vestibular Afferent Neurons

Vestibular ganglion neurons (VGNs) transmit information along parallel neuronal pathways whose signature distinction is variability in spike-timing; some fire at regular intervals while others fire at irregular intervals. The mechanisms driving timing differences are not fully understood but two opposing (but not mutually exclusive) hypotheses have emerged. In the first, regular-spiking is inversely correlated to the density of low-voltage-gated potassium currents (I _KL). In the second, regular spiking is directly correlated to the density of hyperpolarization-activated cyclic nucleotide-sensitive currents (I _H). Supporting the idea that variations in ion channel composition shape spike-timing, VGNs from the first postnatal week respond to synaptic-noise-like current injections with irregular-firing patterns if they have I _KL and with more regular firing patterns if they do not. However, in vitro firing patterns are not as regular as those in vivo Here we considered whether highly-regular spiking requires I _H currents and whether this dependence emerges later in development after channel expression matures. We recorded from rat VGN somata of either sex aged postnatal day (P)9-P21. Counter to expectation, in vitro firing patterns were less diverse, more transient-spiking, and more irregular at older ages than at younger ages. Resting potentials hyperpolarized and resting conductance increased, consistent with developmental upregulation of I _KL Activation of I _H (by increasing intracellular cAMP) increased spike rates but not spike-timing regularity. In a model, we found that activating I _H counter-intuitively suppressed regularity by recruiting I _KL Developmental upregulation in I _KL appears to overwhelm I _H These results counter previous hypotheses about how I _H shapes vestibular afferent responses.SIGNIFICANCE STATEMENT Vestibular sensory information is conveyed on parallel neuronal pathways with irregularly-firing neurons encoding information using a temporal code and regularly-firing neurons using a rate code. This is a striking example of spike-timing statistics influencing information coding. Previous studies from immature vestibular ganglion neurons (VGNs) identified hyperpolarization-activated mixed cationic currents (I _H) as driving highly-regular spiking and proposed that this influence grows with the current during maturation. We found that I _H becomes less influential, likely because maturing VGNs also acquire low-voltage-gated potassium currents (I _KL), whose inhibitory influence opposes I _H Because efferent activity can partly close I _KL, VGN firing patterns may become more receptive to extrinsic control. Spike-timing regularity likely relies on dynamic ion channel properties and complementary specializations in synaptic connectivity.

Genetics of guanylyl cyclase pathways in the cochlea and their influence on hearing

Although hearing loss is the most common sensory deficit in Western societies, there are no successful pharmacological treatments for this disorder. Recent experiments have demonstrated that manipulation of intracellular cyclic guanosine monophosphate (cGMP) concentrations can have both beneficial and harmful effects on hearing. In this review, we will examine the role of cGMP as a key second messenger involved in many aspects of cochlear function and discuss the known functions of downstream effectors of cGMP in sound processing. The nitric oxide-stimulated soluble guanylyl cyclase system (sGC) and the two natriuretic peptide-stimulated particulate GCs (pGCs) will be more extensively covered because they have been studied most thoroughly. The cochlear GC systems are attractive targets for medical interventions that improve hearing while simultaneously representing an under investigated source of sensorineural hearing loss.

A biophysical modelling platform of the cochlear nucleus and other auditory circuits: From channels to networks

Models of the auditory brainstem have been an invaluable tool for testing hypotheses about auditory information processing and for highlighting the most important gaps in the experimental literature. Due to the complexity of the auditory brainstem, and indeed most brain circuits, the dynamic behavior of the system may be difficult to predict without a detailed, biologically realistic computational model. Despite the sensitivity of models to their exact construction and parameters, most prior models of the cochlear nucleus have incorporated only a small subset of the known biological properties. This confounds the interpretation of modelling results and also limits the potential future uses of these models, which require a large effort to develop. To address these issues, we have developed a general purpose, biophysically detailed model of the cochlear nucleus for use both in testing hypotheses about cochlear nucleus function and also as an input to models of downstream auditory nuclei. The model implements conductance-based Hodgkin-Huxley representations of cells using a Python-based interface to the NEURON simulator. Our model incorporates most of the quantitatively characterized intrinsic cell properties, synaptic properties, and connectivity available in the literature, and also aims to reproduce the known response properties of the canonical cochlear nucleus cell types. Although we currently lack the empirical data to completely constrain this model, our intent is for the model to continue to incorporate new experimental results as they become available.

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.

Genetic perturbations suggest a role of the resting potential in regulating the expression of the ion channels of the KCNA and HCN families in octopus cells of the ventral cochlear nucleus

Low-voltage-activated K⁺ (g_KL) and hyperpolarization-activated mixed cation conductances (g_h) mediate currents, I_KL and I_h, through channels of the Kv1 (KCNA) and HCN families respectively and give auditory neurons the temporal precision required for signaling information about the onset, fine structure, and time of arrival of sounds. Being partially activated at rest, g_KL and g_h contribute to the resting potential and shape responses to even small subthreshold synaptic currents. Resting g_KL and g_h also affect the coupling of somatic depolarization with the generation of action potentials. To learn how these important conductances are regulated we have investigated how genetic perturbations affect their expression in octopus cells of the ventral cochlear nucleus (VCN). We report five new findings: First, the magnitude of g_h and g_KL varied over more than two-fold between wild type strains of mice. Second, average resting potentials are not different in different strains of mice even in the face of large differences in average g_KL and g_h. Third, I_KL has two components, one being α-dendrotoxin (α-DTX)-sensitive and partially inactivating and the other being α-DTX-insensitive, tetraethylammonium (TEA)-sensitive, and non-inactivating. Fourth, the loss of Kv1.1 results in diminution of the α-DTX-sensitive I_KL, and compensatory increased expression of an α-DTX-insensitive, tetraethylammonium (TEA)-sensitive I_KL. Fifth, I_h and I_KL are balanced at the resting potential in all wild type and mutant octopus cells even when resting potentials vary in individual cells over nearly 10 mV, indicating that the resting potential influences the expression of g_h and g_KL. The independence of resting potentials on g_KL and g_h shows that g_KL and g_h do not, over days or weeks, determine the resting potential but rather that the resting potential plays a role in regulating the magnitude of either or both g_KL and g_h.

Predictions of the Contribution of HCN Half-Maximal Activation Potential Heterogeneity to Variability in Intrinsic Adaptation of Spiral Ganglion Neurons

Spiral ganglion neurons (SGNs) exhibit a wide range in their strength of intrinsic adaptation on a timescale of 10s to 100s of milliseconds in response to electrical stimulation from a cochlear implant (CI). The purpose of this study was to determine how much of that variability could be caused by the heterogeneity in half-maximal activation potentials of hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels, which are known to produce intrinsic adaptation. In this study, a computational membrane model of cat type I SGN was developed based on the Hodgkin-Huxley model plus HCN and low-threshold potassium (KLT) conductances in which the half-maximal activation potential of the HCN channel was varied and the response of the SGN to pulse train and paired-pulse stimulation was simulated. Physiologically plausible variation of HCN half-maximal activation potentials could indeed determine the range of adaptation on the timescale of 10s to 100s of milliseconds and recovery from adaptation seen in the physiological data while maintaining refractoriness within physiological bounds. This computational model demonstrates that HCN channels may play an important role in regulating the degree of adaptation in response to pulse train stimulation and therefore contribute to variable constraints on acoustic information coding by CIs. This finding has broad implications for CI stimulation paradigms in that cell-to-cell variation of HCN channel properties are likely to significantly alter SGN excitability and therefore auditory perception.

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.

Temporal Considerations for Stimulating Spiral Ganglion Neurons with Cochlear Implants

A wealth of knowledge about different types of neural responses to electrical stimulation has been developed over the past 100 years. However, the exact forms of neural response properties can vary across different types of neurons. In this review, we survey four stimulus-response phenomena that in recent years are thought to be relevant for cochlear implant stimulation of spiral ganglion neurons (SGNs): refractoriness, facilitation, accommodation, and spike rate adaptation. Of these four, refractoriness is the most widely known, and many perceptual and physiological studies interpret their data in terms of refractoriness without incorporating facilitation, accommodation, or spike rate adaptation. In reality, several or all of these behaviors are likely involved in shaping neural responses, particularly at higher stimulation rates. A better understanding of the individual and combined effects of these phenomena could assist in developing improved cochlear implant stimulation strategies. We review the published physiological data for electrical stimulation of SGNs that explores these four different phenomena, as well as some of the recent studies that might reveal the biophysical bases of these stimulus-response phenomena.

Organ of Corti explants direct tonotopically graded morphology of spiral ganglion neurons in vitro

The spiral ganglion is a compelling model system to examine how morphological form contributes to sensory function. While the ganglion is composed mainly of a single class of type I neurons that make simple one-to-one connections with inner hair cell sensory receptors, it has an elaborate overall morphological design. Specific features, such as soma size and axon outgrowth, are graded along the spiral contour of the cochlea. To begin to understand the interplay between different regulators of neuronal morphology, we cocultured neuron explants with peripheral target tissues removed from distinct cochlear locations. Interestingly, these "hair cell microisolates" were capable of both increasing and decreasing neuronal somata size, without adversely affecting survival. Moreover, axon characteristics elaborated de novo by the primary afferents in culture were systematically regulated by the sensory endorgan. Apparent peripheral nervous system (PNS)-like and central nervous system (CNS)-like axonal profiles were established in our cocultures allowing an analysis of putative PNS/CNS axon length ratios. As predicted from the in vivo organization, PNS-like axon bundles elaborated by apical cocultures were longer than their basal counterparts and this phenotype was methodically altered when neuron explants were cocultured with microisolates from disparate cochlear regions. Thus, location-dependent signals within the organ of Corti may set the "address" of neurons within the spiral ganglion, allowing them to elaborate the appropriate tonotopically associated morphological features in order to carry out their signaling function. J. Comp. Neurol. 524:2182-2207, 2016. © 2015 Wiley Periodicals, Inc.

Maturation of NaV and KV Channel Topographies in the Auditory Nerve Spike Initiator before and after Developmental Onset of Hearing Function

Auditory nerve excitation and thus hearing depend on spike-generating ion channels and their placement along the axons of auditory nerve fibers (ANFs). The developmental expression patterns and native axonal locations of voltage-gated ion channels in ANFs are unknown. Therefore, we examined the development of heminodes and nodes of Ranvier in the peripheral axons of type I ANFs in the rat cochlea with immunohistochemistry and confocal microscopy. Nodal structures presumably supporting presensory spiking formed between postnatal days 5 (P5) and P7, including Ankyrin-G, NaV1.6, and Caspr. These immature nodal structures lacked low-voltage-activated KV1.1 which was not enriched at juxtaparanodes until approximately P13, concurrent with the developmental onset of acoustic hearing function. Anatomical alignment of ANF spike-initiating heminodes relative to excitatory input from inner hair cell (IHC) ribbon synapses continued until approximately P30. High-voltage-activated KV3.1b and KV2.2 were expressed in mutually exclusive domains: KV3.1b was strictly localized to nodes and heminodes, whereas KV2.2 expression began at the juxtaparanodes and continued centrally along the first internode. At spike-initiating heminodes in the distal osseous spiral lamina, NaV1.1 partly overlapped NaV1.6 and ankyrin-G. ANFs displayed KV7.2 and KV7.3 at heminodes, nodes, internodes, and the unmyelinated synaptic terminal segments beneath IHCs in the organ of Corti. In response to sound, spikes are initiated at the heminode, which is tightly coupled to the IHC ribbon synapse ∼20-40 μm away. These results show that maturation of nodal alignment and ion channel content may underlie postnatal improvements of ANF excitability and discharge synchrony.

SIGNIFICANCE STATEMENT

Acoustic and electrical hearing depends on rapid, reliable, and precise spike generation in auditory nerve fibers. A limitation of current models and therapies is a lack of information on the identities and topographies of underlying ion channels. We report the developmental profile of the auditory nerve spike generator with a focus on NaV1.1, NaV1.6, KV1.1, KV2.2, KV3.1b, KV7.2, and KV7.3 in relation to the scaffold ankyrin-G. Molecular anatomy of the spike generator matures in the weeks after developmental onset of hearing function. Subcellular positioning of voltage-gated ion channels will enable multicompartmental modeling of auditory nerve responses elicited by afferent chemical neurotransmission from hair cells and modulated by efferent neurotransmitters or evoked by extracellular field stimulation from a cochlear implant.

Identification and modelling of fast and slow Ih current components in vestibular ganglion neurons

Previous experimental data indicates the hyperpolarization‐activated cation (I_h) current, in the inner ear, consists of two components [different hyperpolarization‐activated cyclic nucleotide‐gated (HCN) subunits] which are impossible to pharmacologically isolate. To confirm the presence of these two components in vestibular ganglion neurons we have applied a parameter identification algorithm which is able to discriminate the parameters of the two components from experimental data. Using simulated data we have shown that this algorithm is able to identify the parameters of two populations of non‐inactivated ionic channels more accurately than a classical method. Moreover, the algorithm was demonstrated to be insensitive to the key parameter variations. We then applied this algorithm to I_h current recordings from mouse vestibular ganglion neurons. The algorithm revealed the presence of a high‐voltage‐activated slow component and a low‐voltage‐activated fast component. Finally, the electrophysiological significance of these two I_h components was tested individually in computational vestibular ganglion neuron models (sustained and transient), in the control case and in the presence of cAMP, an intracellular cyclic nucleotide that modulates HCN channel activity. The results suggest that, first, the fast and slow components modulate differently the action potential excitability and the excitatory postsynaptic potentials in both sustained and transient vestibular neurons and, second, the fast and slow components, in the control case, provide different information about characteristics of the stimulation and this information is significantly modified after modulation by cAMP.

Dynamic firing properties of type I spiral ganglion neurons

Spiral ganglion neurons, the first neural element in the auditory system, possess complex intrinsic properties, possibly required to process frequency-specific sensory input that is integrated with extensive efferent regulation. Together with their tonotopically-graded sizes, the somata of these neurons reveal a sophisticated electrophysiological profile. Type I neurons, which make up ~95 % of the ganglion, have myriad voltage-gated ion channels that not only vary along the frequency contour of the cochlea, but also can be modulated by regulators such as voltage, calcium, and second messengers. The resultant developmentally- and tonotopically-regulated neuronal firing patterns conform to three distinct response modes (unitary, rapid, and slow) based on threshold and accommodation. This phenotype, however, is not static for any individual type I neuron. Recent observations have shown that, as neurons become less excitable with age, they demonstrate enhanced plasticity enabling them to change from one response mode to another depending upon resting membrane potential and the presence of neurotrophin-3. Thus, the primary auditory afferents utilized to encode dynamic acoustic stimuli possess the intrinsic specializations that allow them dynamically to alter their firing pattern.