Voltage-gated and two-pore-domain potassium channels in murine spiral ganglion neurons

C-type inactivation and proton modulation mechanisms of the TASK3 channel

The TWIK-related acid-sensitive K+ channel 3 (TASK3) belongs to the two-pore domain (K2P) potassium channel family, which regulates cell excitability by mediating a constitutive "leak" potassium efflux in the nervous system. Extracellular acidification inhibits TASK3 channel, but the molecular mechanism by which channel inactivation is coupled to pH decrease remains unclear. Here, we report the cryo-electron microscopy structures of human TASK3 at neutral and acidic pH. Structural comparison revealed selectivity filter (SF) rearrangements upon acidification, characteristic of C-type inactivation, but with a unique structural basis. The extracellular mouth of the SF was prominently dilated and simultaneously blocked by a hydrophobic gate. His98 protonation shifted the conformational equilibrium between the conductive and C-type inactivated SF toward the latter by engaging a cation-π interaction with Trp78, consistent with molecular dynamics simulations and electrophysiological experiments. Our work illustrated how TASK3 is gated in response to extracellular pH change and implies how physiological stimuli might directly modulate the C-type gating of K2P channels.

Low-voltage Activating K+ Channels in Cochlear Afferent Nerve Fiber Dendrites

Cochlear afferent nerve fibers (ANF) are the first neurons in the ascending auditory pathway. We investigated the low-voltage activating K⁺ channels expressed in ANF dendrites using isolated rat cochlear segments. Whole cell patch clamp recordings were made from the dendritic terminals of ANFs. Outward currents activating at membrane potentials as low as -64 mV were observed in all dendrites studied. These currents were inhibited by 4-aminopyridine (4-AP), a blocker known to preferentially inhibit low-voltage activating K⁺ currents (I_KL) in CNS auditory neurons and spiral ganglion neurons. When the dendritic I_KL was blocked by 4-AP, the EPSP decay time was significantly prolonged, suggesting that dendritic I_KL speeds up the decay of EPSPs and likely modulates action potentials of ANFs. To reveal molecular subtype of dendritic I_KL, α-dendrotoxin (α-DTX), a selective inhibitor for K_v1.1, K_v1.2, and K_v1.6 containing channels, was tested. α-DTX inhibited 23±9% of dendritic I_KL. To identify the α-DTXsensitive and α-DTX-insensitive components of I_KL, immunofluorescence labeling was performed. Strong K_v1.1- and K_v1.2-immunoreactivity was found at unmyelinated dendritic segments, nodes of Ranvier, and cell bodies of most ANFs. A small fraction of ANF dendrites showed K_v7.2- immunoreactivity. These data suggest that dendritic I_KL is conducted through K_v1.1and K_v1.2 channels, with a minor contribution from K_v7.2 and other as yet unidentified channels.

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.

Distribution of Kv3 Subunits in Cochlear Afferent and Efferent Nerve Fibers Implies Distinct Role in Auditory Processing

K_v3 family K⁺ channels, by ensuring speedy repolarization of action potential, enable rapid and high frequency neuronal firing and high precision temporal coding of auditory information in various auditory synapses in the brain. Expression of different K_v3 subtypes within the auditory end organ has been reported. Yet, their precise role at the hair cell synaptic transmission has not been fully elucidated. Using immunolabeling and confocal microscopy we examined the expression pattern of different K_v3 family K⁺ channel subunits in the nerve fibers innervating the cochlear hair cells. K_v3.1b was found in NKA-positive type 1 afferent fibers, exhibiting high signal intensity at the cell body, the unmyelinated dendritic segment, first heminode and nodes of Ranvier. K_v3.3 signal was detected in the cell body and the unmyelinated dendritic segment of NKA-positive type 1 afferent fibers but not in peripherin-positive type 2 afferent. K_v3.4 was found in ChAT-positive LOC and MOC efferent fibers as well as peripherin-positive type 2 afferent fibers. Such segregated expression pattern implies that each K_v3 subunits participate in different auditory tasks, for example, K_v3.1b and K_v3.3 in ascending signaling while K_v3.4 in feedback upon loud noise exposure.

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.

The correlation and role analysis of KCNK2/4/5/15 in Human Papillary Thyroid Carcinoma microenvironment

Background: KCNKs, potassium two pore domain channel family K members, can maintain the resting potential, regulate the amplitude and duration of the plateau of the action potential, and change the membrane potential and membrane excitability. Evidence from many studies indicates that KCNKs is abnormally expressed in many solid tumors and plays a regulatory role in the development and malignant progression of cancer. However, the expression pattern and prognostic value of KCNK factors in papillary thyroid carcinoma have not been reported.

Methods: In this study, we used the data from databases such as ONCOMINE, GEPIA, Kaplan-Meier Plotter, and cBioPortal to perform bioinformatics analysis of KCNK factors in patients with thyroid cancer.

Results: We found that the mRNA expression of KCNK1, KCNK5, KCNK6, KCNK7, and KCNK15 were significantly higher in thyroid cancer tissues than that in normal tissues, while KCNK2, KCNK4, KCNK9, KCNK16 and KCNK17 mRNA levels were decreased compared to normal tissues. And the expression levels of KCNK1/2/4/5/6/7/15 were correlated with the tumor stage. Survival analysis using the Kaplan-Meier Plotter database revealed that KCNK2/3/4/5/12/15 were associated with overall survival (OS) in patients with thyroid cancer.

Conclusion: Finally, the results of ROC curves, immunohistochemical staining, immune cell infiltration and kinase / miRNA / transcription factor regulation showed that KCNK2, KCNK4, KCNK5 and KCNK15 levels could be used as biomarkers for PTC diagnosis. This study implied that KCNK2, KCNK4, KCNK5 and KCNK15 are potential targets of precision therapy for patients with thyroid cancer and these genes are new biomarkers for the therapeutic target for thyroid cancer.

Sodium-activated potassium channels shape peripheral auditory function and activity of the primary auditory neurons in mice

Potassium (K⁺) channels shape the response properties of neurons. Although enormous progress has been made to characterize K⁺ channels in the primary auditory neurons, the molecular identities of many of these channels and their contributions to hearing in vivo remain unknown. Using a combination of RNA sequencing and single molecule fluorescent in situ hybridization, we localized expression of transcripts encoding the sodium-activated potassium channels K_Na1.1 (SLO2.2/Slack) and K_Na1.2 (SLO2.1/Slick) to the primary auditory neurons (spiral ganglion neurons, SGNs). To examine the contribution of these channels to function of the SGNs in vivo, we measured auditory brainstem responses in K_Na1.1/1.2 double knockout (DKO) mice. Although auditory brainstem response (wave I) thresholds were not altered, the amplitudes of suprathreshold responses were reduced in DKO mice. This reduction in amplitude occurred despite normal numbers and molecular architecture of the SGNs and their synapses with the inner hair cells. Patch clamp electrophysiology of SGNs isolated from DKO mice displayed altered membrane properties, including reduced action potential thresholds and amplitudes. These findings show that K_Na1 channel activity is essential for normal cochlear function and suggest that early forms of hearing loss may result from physiological changes in the activity of the primary auditory neurons.

Direct Reprogramming of Spiral Ganglion Non-neuronal Cells into Neurons: Toward Ameliorating Sensorineural Hearing Loss by Gene Therapy

Primary auditory neurons (PANs) play a critical role in hearing by transmitting sound information from the inner ear to the brain. Their progressive degeneration is associated with excessive noise, disease and aging. The loss of PANs leads to permanent hearing impairment since they are incapable of regenerating. Spiral ganglion non-neuronal cells (SGNNCs), comprised mainly of glia, are resident within the modiolus and continue to survive after PAN loss. These attributes make SGNNCs an excellent target for replacing damaged PANs through cellular reprogramming. We used the neurogenic pioneer transcription factor Ascl1 and the auditory neuron differentiation factor NeuroD1 to reprogram SGNNCs into induced neurons (iNs). The overexpression of both Ascl1 and NeuroD1 in vitro generated iNs at high efficiency. Transcriptome analyses revealed that iNs displayed a transcriptome profile resembling that of endogenous PANs, including expression of several key markers of neuronal identity: Tubb3, Map2, Prph, Snap25, and Prox1. Pathway analyses indicated that essential pathways in neuronal growth and maturation were activated in cells upon neuronal induction. Furthermore, iNs extended projections toward cochlear hair cells and cochlear nucleus neurons when cultured with each respective tissue. Taken together, our study demonstrates that PAN-like neurons can be generated from endogenous SGNNCs. This work suggests that gene therapy can be a viable strategy to treat sensorineural hearing loss caused by degeneration of PANs.

TFOS DEWS II pain and sensation report

Pain associated with mechanical, chemical, and thermal heat stimulation of the ocular surface is mediated by trigeminal ganglion neurons, while cold thermoreceptors detect wetness and reflexly maintain basal tear production and blinking rate. These neurons project into two regions of the trigeminal brain stem nuclear complex: ViVc, activated by changes in the moisture of the ocular surface and VcC1, mediating sensory-discriminative aspects of ocular pain and reflex blinking. ViVc ocular neurons project to brain regions that control lacrimation and spontaneous blinking and to the sensory thalamus. Secretion of the main lacrimal gland is regulated dominantly by autonomic parasympathetic nerves, reflexly activated by eye surface sensory nerves. These also evoke goblet cell secretion through unidentified efferent fibers. Neural pathways involved in the regulation of meibomian gland secretion or mucin release have not been identified. In dry eye disease, reduced tear secretion leads to inflammation and peripheral nerve damage. Inflammation causes sensitization of polymodal and mechano-nociceptor nerve endings and an abnormal increase in cold thermoreceptor activity, altogether evoking dryness sensations and pain. Long-term inflammation and nerve injury alter gene expression of ion channels and receptors at terminals and cell bodies of trigeminal ganglion and brainstem neurons, changing their excitability, connectivity and impulse firing. Perpetuation of molecular, structural and functional disturbances in ocular sensory pathways ultimately leads to dysestesias and neuropathic pain referred to the eye surface. Pain can be assessed with a variety of questionaires while the status of corneal nerves is evaluated with esthesiometry and with in vivo confocal microscopy.

Concomitant differentiation of a population of mouse embryonic stem cells into neuron-like cells and schwann cell-like cells in a slow-flow microfluidic device

BACKGROUND

To send meaningful information to the brain, an inner ear cochlear implant (CI) must become closely coupled to as large and healthy a population of remaining spiral ganglion neurons (SGN) as possible. Inner ear gangliogenesis depends on macrophage migration inhibitory factor (MIF), a directionally attractant neurotrophic cytokine made by both Schwann and supporting cells (Bank et al., 2012). MIF-induced mouse embryonic stem cell (mESC)-derived "neurons" could potentially substitute for lost or damaged SGN. mESC-derived "Schwann cells" produce MIF, as do all Schwann cells (Huang et al., a; Roth et al., 2007; Roth et al., 2008) and could attract SGN to a "cell-coated" implant.

RESULTS

Neuron- and Schwann cell-like cells were produced from a common population of mESCs in an ultra-slow-flow microfluidic device. As the populations interacted, "neurons" grew over the "Schwann cell" lawn, and early events in myelination were documented. Blocking MIF on the Schwann cell side greatly reduced directional neurite outgrowth. MIF-expressing "Schwann cells" were used to coat a CI: Mouse SGN and MIF-induced "neurons" grew directionally to the CI and to a wild-type but not MIF-knockout organ of Corti explant.

CONCLUSIONS

Two novel stem cell-based approaches for treating the problem of sensorineural hearing loss are described. Developmental Dynamics 246:7-27, 2017. © 2016 Wiley Periodicals, Inc.

NMDA Receptors Enhance Spontaneous Activity and Promote Neuronal Survival in the Developing Cochlea

Spontaneous bursts of activity in developing sensory pathways promote maturation of neurons, refinement of neuronal connections, and assembly of appropriate functional networks. In the developing auditory system, inner hair cells (IHCs) spontaneously fire Ca(2+) spikes, each of which is transformed into a mini-burst of action potentials in spiral ganglion neurons (SGNs). Here we show that NMDARs are expressed in SGN dendritic terminals and play a critical role during transmission of activity from IHCs to SGNs before hearing onset. NMDAR activation enhances glutamate-mediated Ca(2+) influx at dendritic terminals, promotes repetitive firing of individual SGNs in response to each synaptic event, and enhances coincident activity of neighboring SGNs that will eventually encode similar frequencies of sound. Loss of NMDAR signaling from SGNs reduced their survival both in vivo and in vitro, revealing that spontaneous activity in the prehearing cochlea promotes maturation of auditory circuitry through periodic activation of NMDARs in SGNs.

KCNK5 channels mostly expressed in cochlear outer sulcus cells are indispensable for hearing

In the cochlea, K(+) is essential for mechano-electrical transduction. Here, we explore cochlear structure and function in mice lacking K(+) channels of the two-pore domain family. A profound deafness associated with a decrease in endocochlear potential is found in adult Kcnk5(-/-) mice. Hearing occurs around postnatal day 19 (P19), and completely disappears 2 days later. At P19, Kcnk5(-/-) mice have a normal endolymphatic [K(+)] but a partly lowered endocochlear potential. Using Lac-Z as a gene reporter, KCNK5 is mainly found in outer sulcus Claudius', Boettcher's and root cells. Low levels of expression are also seen in the spiral ganglion, Reissner's membrane and stria vascularis. Essential channels (KCNJ10 and KCNQ1) contributing to K(+) secretion in stria vascularis have normal expression in Kcnk5(-/-) mice. Thus, KCNK5 channels are indispensable for the maintenance of hearing. Among several plausible mechanisms, we emphasize their role in K(+) recycling along the outer sulcus lateral route.

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.

The pre- and post-somatic segments of the human type I spiral ganglion neurons--structural and functional considerations related to cochlear implantation

Human auditory nerve afferents consist of two separate systems; one is represented by the large type I cells innervating the inner hair cells and the other one by the small type II cells innervating the outer hair cells. Type I spiral ganglion neurons (SGNs) constitute 96% of the afferent nerve population and, in contrast to other mammals, their soma and pre- and post-somatic segments are unmyelinated. Type II nerve soma and fibers are unmyelinated. Histopathology and clinical experience imply that human SGNs can persist electrically excitable without dendrites, thus lacking connection to the organ of Corti. The biological background to this phenomenon remains elusive. We analyzed the pre- and post-somatic segments of the type I human SGNs using immunohistochemistry and transmission electron microscopy (TEM) in normal and pathological conditions. These segments were found surrounded by non-myelinated Schwann cells (NMSCs) showing strong intracellular expression of laminin-β2/collagen IV. These cells also bordered the perikaryal entry zone and disclosed surface rugosities outlined by a folded basement membrane (BM) expressing laminin-β2 and collagen IV. It is presumed that human large SGNs are demarcated by three cell categories: (a) myelinated Schwann cells, (b) NMSCs and (c) satellite glial cells (SGCs). Their BMs express laminin-β2/collagen IV and reaches the BM of the sensory epithelium at the habenula perforata. We speculate that the NMSCs protect SGNs from further degeneration following dendrite loss. It may give further explanation why SGNs can persist as electrically excitable monopolar cells even after long-time deafness, a blessing for the deaf treated with cochlear implantation.

Unmasking of spiral ganglion neuron firing dynamics by membrane potential and neurotrophin-3

Type I spiral ganglion neurons have a unique role relative to other sensory afferents because, as a single population, they must convey the richness, complexity, and precision of auditory information as they shape signals transmitted to the brain. To understand better the sophistication of spiral ganglion response properties, we compared somatic whole-cell current-clamp recordings from basal and apical neurons obtained during the first 2 postnatal weeks from CBA/CaJ mice. We found that during this developmental time period neuron response properties changed from uniformly excitable to differentially plastic. Low-frequency, apical and high-frequency basal neurons at postnatal day 1 (P1)-P3 were predominantly slowly accommodating (SA), firing at low thresholds with little alteration in accommodation response mode induced by changes in resting membrane potential (RMP) or added neurotrophin-3 (NT-3). In contrast, P10-P14 apical and basal neurons were predominately rapidly accommodating (RA), had higher firing thresholds, and responded to elevation of RMP and added NT-3 by transitioning to the SA category without affecting the instantaneous firing rate. Therefore, older neurons appeared to be uniformly less excitable under baseline conditions yet displayed a previously unrecognized capacity to change response modes dynamically within a remarkably stable accommodation framework. Because the soma is interposed in the signal conduction pathway, these specializations can potentially lead to shaping and filtering of the transmitted signal. These results suggest that spiral ganglion neurons possess electrophysiological mechanisms that enable them to adapt their response properties to the characteristics of incoming stimuli and thus have the capacity to encode a wide spectrum of auditory information.

Voltage-gated K(+) channels contributing to temporal precision at the inner hair cell-auditory afferent nerve fiber synapses in the mammalian cochlea

To perform auditory tasks such as sound localization in the space, auditory neurons in the brain must distinguish sub-millisecond temporal differences in signals from two ears. Such high temporal resolution is possible when each neuron in the ascending auditory pathway fires brief action potential at very accurate timing. Various pre- and postsynaptic machineries ensuring such high temporal precision of auditory synaptic transmission have been identified. Of particular, in this review, the role of K(+) channels in shortening the duration of synaptic potentials will be discussed. First, the contribution of K(+) channels to AP firing of general auditory neurons will be discussed. Then, the focus will be moved to the inner hair cell (IHC)-auditory afferent nerve fiber (ANF) synapses, the first synapses of ascending auditory pathway. Molecular and immunohistological techniques have revealed various K(+) channels in the cell bodies and their processes of ANFs. Since the development of patch-clamp recordings from the ANF dendrites in 2002, it became possible to monitor the IHC-ANF synaptic transmission in greater detail. As revealed in brain auditory synapses, several different K(+) channels appear to participate in reducing the duration of synaptic potentials at the IHC-ANF synapses. In addition, K(+) channels at the ANF dendrites might act as potential targets of efferent feedback from the brain. The hypothesis is that, upon loud sound exposure, efferent neurotransmitters released onto the ANF dendrites activate certain K(+) channels and prevent excitotoxicity of ANFs. Therefore, K(+) channels of the ANF dendrites might provide potential sites of pharmacological actions to prevent noise-induced hearing loss.

Heterogeneous intrinsic excitability of murine spiral ganglion neurons is determined by Kv1 and HCN channels

The spiral ganglion conveys afferent auditory information predominantly through a single class of type I neurons that receive signals from inner hair cell sensory receptors. These auditory primary afferents, like in other systems (Puopolo and Belluzzi, 1998; Gascon and Moqrich, 2010; Leao et al., 2012) possess a marked diversity in their electrophysiological features (Taberner and Liberman, 2005). Consistent with these observations, when the auditory primary afferents were assessed in neuronal explants separated from their peripheral and central targets it was found that individual neurons were markedly heterogeneous in their endogenous electrophysiological features. One aspect of this heterogeneity, obvious throughout the ganglion, was their wide range of excitability as assessed by voltage threshold measurements (Liu and Davis, 2007). Thus, while neurons in the base differed significantly from apical and middle neurons in their voltage thresholds, each region showed distinctly wide ranges of values. To determine whether the resting membrane potentials (RMPs) of these neurons correlate with the threshold distribution and to identify the ion channel regulatory elements underlying heterogeneous neuronal excitability in the ganglion, patch-clamp recordings were made from postnatal day (P5-8) murine spiral ganglion neurons in vitro. We found that RMP mirrored the tonotopic threshold distribution, and contributed an additional level of heterogeneity in each cochlear location. Pharmacological experiments further indicated that threshold and RMP was coupled through the Kv1 current, which had a dual impact on both electrophysiological parameters. Whereas, hyperpolarization-activated cationic channels decoupled these two processes by primarily affecting RMP without altering threshold level. Thus, beyond mechanical and synaptic specializations, ion channel regulation of intrinsic membrane properties imbues spiral ganglion neurons with different excitability levels, a feature that contributes to primary auditory afferent diversity.

Posthearing Ca(2+) currents and their roles in shaping the different modes of firing of spiral ganglion neurons

Whereas prehearing spiral ganglion neurons (SGNs) rely faithfully on outputs from spontaneously active developing hair cells, the electrical phenotypes of posthearing neurons are shaped by distinct rapid and graded receptor potentials from hair cells. To date, technical difficulties in isolation of fragile posthearing neurons from the rigid bony labyrinth of the inner ear have hindered analyses of the electrical phenotype of SGNs. Therefore, we have recently developed new strategies to isolate posthearing mouse SGNs for functional analyses. Here, we describe the coarse and fine properties of Ca(2+) currents, which sculpt the firing properties of posthearing SGNs. Murine SGNs express multiple Ca(2+) channel currents to enable diverse functions. We have demonstrated that suppression of Ca(2+) currents results in significant hyperpolarization of the resting membrane potential (rmp) of basal SGNs, suggesting that Ca(2+) influx primes rmp for excitation. In contrast, removal of external Ca(2+) has modest effects on rmp of apical SGNs. The blockade of Ca(2+) currents with a mixture of specific blockers attenuates spontaneously active SGNs. Paradoxically, different subtypes of Ca(2+) currents, such as R-type currents, may activate resting outward conductances since blockage of the current results in depolarization of rmp. In keeping with whole-cell current data, single-channel records revealed multiple diverse Ca(2+) channels in SGNs. Additionally, there were differential expressions of distinct Ca(2+) current densities in the apicobasal contour of the adult cochlea. This report provides invaluable insights into Ca(2+)-dependent processes in adult SGNs.

Ototrauma induces sodium channel plasticity in auditory afferent neurons

Exposure to intense sound can cause damage to the delicate sensory and neuronal components of the cochlea leading to hearing loss. Such damage often causes the dendrites of the spiral ganglion neurons (SGN), the neurons that provide the afferent innervation of the hair cells, to swell and degenerate thus damaging the synapse. In models of neuropathic pain, axotomy, another form of afferent nerve damage, is accompanied by altered voltage-gated sodium channel (VGSC) expression, leading to neuronal hyperactivity. In this study, adult Wistar rats were exposed to noise which produced a mild, 20 dB hearing threshold elevation and their VGSC expression was investigated. Quantitative PCR showed decreased Na_V1.1 and Na_V1.6 mRNA expression in the SGN following noise exposure (29% and 56% decrease respectively) while Na_V1.7 mRNA expression increased by approximately 20% when compared to control rats. Immunohistochemistry extended these findings, revealing increased staining for Na_V1.1 along the SGN dendrites and Na_V1.7 in the cell bodies after noise. These results provide the first evidence for selective changes in VGSC expression following moderate noise-induced hearing loss and could contribute to elevated hearing thresholds and to the generation of perceptual anomalies commonly associated with cochlear damage, such as tinnitus and hyperacusis.

Complex distribution patterns of voltage-gated calcium channel α-subunits in the spiral ganglion

As with other elements of the peripheral auditory system, spiral ganglion neurons display specializations that vary as a function of location along the tonotopic axis. Previous work has shown that voltage-gated K(+) channels and synaptic proteins show graded changes in their density that confers rapid responsiveness to neurons in the high frequency, basal region of the cochlea and slower, more maintained responsiveness to neurons in the low frequency, apical region of the cochlea. In order to understand how voltage-gated calcium channels (VGCCs) may contribute to these diverse phenotypes, we identified the VGCC α-subunits expressed in the ganglion, investigated aspects of Ca(2+)-dependent neuronal firing patterns, and mapped the intracellular and intercellular distributions of seven VGCC α-subunits in the spiral ganglion in vitro. Initial experiments with qRT-PCR showed that eight of the ten known VGCC α-subunits were expressed in the ganglion and electrophysiological analysis revealed firing patterns that were consistent with the presence of both LVA and HVA Ca(2+) channels. Moreover, we were able to study seven of the α-subunits with immunocytochemistry, and we found that all were present in spiral ganglion neurons, three of which were neuron-specific (Ca(V)1.3, Ca(V)2.2, and Ca(V)3.3). Further characterization of neuron-specific α-subunits showed that Ca(V)1.3 and Ca(V)3.3 were tonotopically-distributed, whereas Ca(V)2.2 was uniformly distributed in apical and basal neurons. Multiple VGCC α-subunits were also immunolocalized to Schwann cells, having distinct intracellular localizations, and, significantly, appearing to distinguish putative compact (Ca(V)2.3, Ca(V)3.1) from loose (Ca(V)1.2) myelin. Electrophysiological evaluation of spiral ganglion neurons in the presence of TEA revealed Ca(2+) plateau potentials with slopes that varied proportionately with the cochlear region from which neurons were isolated. Because afterhyperpolarizations were minimal or absent under these conditions, we hypothesize that differential density and/or kinetics of one or more of the VGCC α-subunits could account for observed tonotopic differences. These experiments have set the stage for defining the clear multiplicity of functional control in neurons and Schwann cells of the spiral ganglion.

Complex primary afferents: What the distribution of electrophysiologically-relevant phenotypes within the spiral ganglion tells us about peripheral neural coding

Spiral ganglion neurons are the first neural element of the auditory system. They receive precise synaptic signals which represent features of sound stimuli encoded by hair cell receptors and they deliver a digital representation of this information to the central nervous system. It is well known that spiral ganglion neurons are selectively responsive to specific sound frequencies, and that numerous structural and physiological specializations in the inner ear increase the quality of this tuning, beyond what could be accomplished by the passive properties of the basilar membrane. Further, consistent with what we know about other sensory systems, it is becoming clear that the parallel divergent innervation pattern of type I spiral ganglion neurons has the potential to encode additional features of sound stimuli. To date, we understand the most about the sub-modalities of frequency and intensity coding in the peripheral auditory system. Work reviewed herein will address the issue of how intrinsic electrophysiological features of the neurons themselves have the potential to contribute to the precision of coding and transmitting information about these two parameters to higher auditory centers for further processing.

Molecular background of leak K+ currents: two-pore domain potassium channels

Two-pore domain K(+) (K(2P)) channels give rise to leak (also called background) K(+) currents. The well-known role of background K(+) currents is to stabilize the negative resting membrane potential and counterbalance depolarization. However, it has become apparent in the past decade (during the detailed examination of the cloned and corresponding native K(2P) channel types) that this primary hyperpolarizing action is not performed passively. The K(2P) channels are regulated by a wide variety of voltage-independent factors. Basic physicochemical parameters (e.g., pH, temperature, membrane stretch) and also several intracellular signaling pathways substantially and specifically modulate the different members of the six K(2P) channel subfamilies (TWIK, TREK, TASK, TALK, THIK, and TRESK). The deep implication in diverse physiological processes, the circumscribed expression pattern of the different channels, and the interesting pharmacological profile brought the K(2P) channel family into the spotlight. In this review, we focus on the physiological roles of K(2P) channels in the most extensively investigated cell types, with special emphasis on the molecular mechanisms of channel regulation.

Modelled temperature-dependent excitability behaviour of a generalised human peripheral sensory nerve fibre

The objective of this study was to determine if a recently developed human Ranvier node model, which is based on a modified version of the Hodgkin-Huxley model, could predict the excitability behaviour in human peripheral sensory nerve fibres with diameters ranging from 5.0 to 15.0 microm. The Ranvier node model was extended to include a persistent sodium current and was incorporated into a generalised single cable nerve fibre model. Parameter temperature dependence was included. All calculations were performed in Matlab. Sensory nerve fibre excitability behaviour characteristics predicted by the new nerve fibre model at different temperatures and fibre diameters compared well with measured data. Absolute refractory periods deviated from measured data, while relative refractory periods were similar to measured data. Conduction velocities showed both fibre diameter and temperature dependence and were underestimated in fibres thinner than 12.5 microm. Calculated strength-duration time constants ranged from 128.5 to 183.0 micros at 37 degrees C over the studied nerve fibre diameter range, with chronaxie times about 30% shorter than strength-duration time constants. Chronaxie times exhibited temperature dependence, with values overestimated by a factor 5 at temperatures lower than body temperature. Possible explanations include the deviated absolute refractory period trend and inclusion of a nodal strangulation relationship.

Spiral ganglion neurones: an overview of morphology, firing behaviour, ionic channels and function

The spiral ganglion cells provide the afferent innervation of the hair cells of the organ of Corti. Ninety-five percent of these cells (termed type I spiral ganglion neurones) are in synaptic contact with the inner hair cells, whereas about 5% of them are type II cells, which are responsible for the sensory innervation of the outer hair cells. To understand the function of the spiral ganglion neurones, it is important to explore their membrane properties, understand their activity patterns and describe the variety of ionic channels determining their behaviour. In this review, a brief description is given of the various experimental methods that allow the investigation of the spiral ganglion cells, followed by the discussion of their action potential firing patterns and ionic conductances. The presence, distribution and significance of the K(+) currents of the spiral ganglion cells are specifically addressed, along with the introduction of the putative subunit compositions of the relevant voltage-gated K(+) channels.

TWIK-related acid-sensitive K+ channel 1 (TASK1) and TASK3 critically influence T lymphocyte effector functions

Two major K(+) channels are expressed in T cells, (i) the voltage-dependent K(V)1.3 channel and (ii) the Ca(2+)-activated K(+) channel KCa 3.1 (IKCa channel). Both critically influence T cell effector functions in vitro and animal models in vivo. Here we identify and characterize TWIK-related acid-sensitive potassium channel 1 (TASK1) and TASK3 as an important third K(+) conductance on T lymphocytes. T lymphocytes constitutively express TASK1 and -3 protein. Application of semi-selective TASK blockers resulted in a significant reduction of cytokine production and cell proliferation. Interference with TASK channels on CD3(+) T cells revealed a dose-dependent reduction ( approximately 40%) of an outward current in patch clamp recordings indicative of TASK channels, a finding confirmed by computational modeling. In vivo relevance of our findings was addressed in an experimental model of multiple sclerosis, adoptive transfer experimental autoimmune encephalomyelitis. Pretreatment of myelin basic protein-specific encephalitogenic T lymphocytes with TASK modulators was associated with significant amelioration of the disease course in Lewis rats. These data introduce K(2)P channels as novel potassium conductance on T lymphocytes critically influencing T cell effector function and identify a possible molecular target for immunomodulation in T cell-mediated autoimmune disorders.

Volumetric and ionic regulation during the in vitro development of a corneal endothelial barrier

Corneal endothelium is responsible for generating an ion flux between the corneal stroma and the anterior chamber of the eye that is necessary for the cornea to remain transparent. However, the ion transport regulatory mechanisms that develop during the formation of the endothelial barrier are not known. In this study, we determined the influence of cell confluence on cell volume and intracellular ionic content on the corneal endothelial cells of rabbits. Our results demonstrate that non-confluent endothelial cells display a hypertrophic volume increase, with higher intracellular contents of potassium and chlorine than those of confluent cells. In contrast, when cells reach confluence and the endothelial barrier forms, cell volume decreases and the intracellular contents of potassium and chlorine decrease. Our genetic analysis showed a higher expression of CFTR and CA2 genes in non-confluent cells, and of the gene KCNC3 in confluent cells. These results suggest that the normal ionic current that keeps the corneal stroma dehydrated and transparent is regulated by cell-cell contacts and endothelial cell confluence, and could explain why the loss of corneal endothelial cells is often associated with corneal edema and even blindness.

Genome-wide discovery of Pax7 target genes during development

Pax7 plays critical roles in development of brain, spinal cord, neural crest, and skeletal muscle. As a sequence-specific DNA-binding transcription factor, any direct functional role played by Pax7 during development is mediated through target gene selection. Thus, we have sought to identify genes targeted by Pax7 during embryonic development using an unbiased chromatin immunoprecipitation (ChIP) cloning assay to isolate cis-regulatory regions bound by Pax7 in vivo. Sequencing and genomic localization of a library of chromatin-DNA fragments bound by Pax7 has identified 34 candidate Pax7 target genes, with occupancy of a selection confirmed with independent chromatin enrichment tests (ChIP-PCR). To assess the capacity of Pax7 to regulate transcription from these loci, we have cloned alternate transcripts of Pax7 (differing significantly in their DNA binding domain) into expression vectors and transfected cultured cells with these constructs, then analyzed target gene expression levels using RT-PCR. We show that Pax7 directly occupies sites within genes encoding transcription factors Gbx1 and Eya4, the neurogenic cytokine receptor ciliary neurotrophic factor receptor, the neuronal potassium channel Kcnk2, and the signal transduction kinase Camk1d in vivo and regulates the transcriptional state of these genes in cultured cells. This analysis gives us greater insight into the direct functional role played by Pax7 during embryonic development.

Reciprocal regulation of presynaptic and postsynaptic proteins in bipolar spiral ganglion neurons by neurotrophins

A unifying principle of sensory system organization is feature extraction by modality-specific neuronal maps in which arrays of neurons show systematically varied response properties and receptive fields. Only beginning to be understood, however, are the mechanisms by which these graded systems are established. In the peripheral auditory system, we have shown previously that the intrinsic firing features of spiral ganglion neurons are influenced by brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3). We now show that is but a part of a coordinated package of neurotrophin actions that also includes effects on presynaptic and postsynaptic proteins, thus encompassing the input, transmission, and output functions of the spiral ganglion neurons. Using immunocytochemical methods, we determined that proteins targeted to opposite ends of the neuron were organized and regulated in a reciprocal manner. AMPA receptor subunits GluR2 and GluR3 were enriched in base neurons compared with their apex counterparts. This distribution pattern was enhanced by exposure to BDNF but reduced by NT-3. SNAP-25 and synaptophysin were distributed and regulated in the mirror image: enriched in the apex, enhanced by NT-3 and reduced by BDNF. Moreover, we used a novel coculture to identify potential endogenous sources of neurotrophins by showing that sensory receptors from different cochlear regions were capable of altering presynaptic and postsynaptic protein levels in these neurons. From these studies, we suggest that BDNF and NT-3, which are systematically distributed in complementary gradients, are responsible for orchestrating a comprehensive set of electrophysiological specializations along the frequency contour of the cochlea.