The temporal characteristics of Ca2+ entry through L-type and T-type Ca2+ channels shape exocytosis efficiency in chick auditory hair cells during development

Graded spikes differentially signal neurotransmitter input in cerebrospinal fluid contacting neurons of the mouse spinal cord

The action potential and its all-or-none nature is fundamental to neural communication. Canonically, the action potential is initiated once voltage-activated Na⁺ channels are activated, and their rapid kinetics of activation and inactivation give rise to the action potential's all-or-none nature. Here we demonstrate that cerebrospinal fluid contacting neurons (CSFcNs) surrounding the central canal of the mouse spinal cord employ a different strategy. Rather than using voltage-activated Na⁺ channels to generate binary spikes, CSFcNs use two different types of voltage-activated Ca²⁺ channel, enabling spikes of different amplitude. T-type Ca²⁺ channels generate small amplitude spikes, whereas larger amplitude spikes require high voltage-activated Cd²⁺-sensitive Ca²⁺ channels. We demonstrate that these different amplitude spikes can signal input from different transmitter systems; purinergic inputs evoke smaller T-type dependent spikes whereas cholinergic inputs evoke larger spikes that do not rely on T-type channels. Different synaptic inputs to CSFcNs can therefore be signaled by the spike amplitude.

Synaptic transmission at the vestibular hair cells of amniotes

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

SK Current, Expressed During the Development and Regeneration of Chick Hair Cells, Contributes to the Patterning of Spontaneous Action Potentials

Chick hair cells display calcium (Ca2+)-sensitive spontaneous action potentials during development and regeneration. The role of this activity is unclear but thought to be involved in establishing proper synaptic connections and tonotopic maps, both of which are instrumental to normal hearing. Using an electrophysiological approach, this work investigated the functional expression of Ca2+-sensitive potassium [IK(Ca)] currents and their role in spontaneous electrical activity in the developing and regenerating hair cells (HCs) in the chick basilar papilla. The main IK(Ca) in developing and regenerating chick HCs is an SK current, based on its sensitivity to apamin. Analysis of the functional expression of SK current showed that most dramatic changes occurred between E8 and E16. Specifically, there is a developmental downregulation of the SK current after E16. The SK current gating was very sensitive to the availability of intracellular Ca2+ but showed very little sensitivity to T-type voltage-gated Ca2+ channels, which are one of the hallmarks of developing and regenerating hair cells. Additionally, apamin reduced the frequency of spontaneous electrical activity in HCs, suggesting that SK current participates in patterning the spontaneous electrical activity of HCs.

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

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

Functional Postnatal Maturation of the Medial Olivocochlear Efferent-Outer Hair Cell Synapse

The organ of Corti, the auditory mammalian sensory epithelium, contains two types of mechanotransducer cells, inner hair cells (IHCs) and outer hair cells (OHCs). IHCs are involved in conveying acoustic stimuli to the CNS, while OHCs are implicated in the fine tuning and amplification of sounds. OHCs are innervated by medial olivocochlear (MOC) cholinergic efferent fibers. The functional characteristics of the MOC-OHC synapse during maturation were assessed by electrophysiological and pharmacological methods in mouse organs of Corti at postnatal day 11 (P11)-P13, hearing onset in altricial rodents, and at P20-P22 when the OHCs are morphologically and functionally mature. Synaptic currents were recorded in whole-cell voltage-clamped OHCs while electrically stimulating the MOC fibers. A progressive increase in the number of functional MOC-OHC synapses, as well as in their strength and efficacy, was observed between P11-13 and P20-22. At hearing onset, the MOC-OHC synapse presented facilitation during MOC fibers high-frequency stimulation that disappeared at mature stages. In addition, important changes were found in the VGCC that are coupled to transmitter release. Ca²⁺ flowing in through L-type VGCCs contribute to trigger ACh release together with P/Q- and R-type VGCCs at P11-P13, but not at P20-P22. Interestingly, N-type VGCCs were found to be involved in this process at P20-P22, but not at hearing onset. Moreover, the degree of compartmentalization of calcium channels with respect to BK channels and presynaptic release components significantly increased from P11-P13 to P20-P22. These results suggest that the MOC-OHC synapse is immature at the onset of hearing.SIGNIFICANCE STATEMENT The functional expression of both VGCCs and BK channels, as well as their localization with respect to the presynaptic components involved in transmitter release, are key elements in determining synaptic efficacy. In this work, we show dynamic changes in the expression of VGCCs and Ca²⁺-dependent BK K⁺ channels coupled to ACh release at the MOC-OHC synapse and their shift in compartmentalization during postnatal maturation. These processes most likely set the short-term plasticity pattern and reliability of the MOC-OHC synapse on high-frequency activity.

Hair cell maturation is differentially regulated along the tonotopic axis of the mammalian cochlea

Key points

Outer hair cells (OHCs) enhance the sensitivity and the frequency tuning of the mammalian cochlea.

Similar to the primary sensory receptor, the inner hair cells (IHCs), the mature functional characteristics of OHCs are acquired before hearing onset.

We found that OHCs, like IHCs, fire spontaneous Ca²⁺‐induced action potentials (APs) during immature stages of development, which are driven by Ca_V1.3 Ca²⁺ channels.

We also showed that the development of low‐ and high‐frequency hair cells is differentially regulated during pre‐hearing stages, with the former cells being more strongly dependent on experience‐independent Ca²⁺ action potential activity.

Abstract

Sound amplification within the mammalian cochlea depends upon specialized hair cells, the outer hair cells (OHCs), which possess both sensory and motile capabilities. In various altricial rodents, OHCs become functionally competent from around postnatal day 7 (P7), before the primary sensory inner hair cells (IHCs), which become competent at about the onset of hearing (P12). The mechanisms responsible for the maturation of OHCs and their synaptic specialization remain poorly understood. We report that spontaneous Ca²⁺ activity in the immature cochlea, which is generated by Ca_V1.3 Ca²⁺ channels, differentially regulates the maturation of hair cells along the cochlea. Under near‐physiological recording conditions we found that, similar to IHCs, immature OHCs elicited spontaneous Ca²⁺ action potentials (APs), but only during the first few postnatal days. Genetic ablation of these APs in vivo, using Ca_V1.3^−/− mice, prevented the normal developmental acquisition of mature‐like basolateral membrane currents in low‐frequency (apical) hair cells, such as I_K,n (carried by KCNQ4 channels), I_SK2 and I_ACh (α9α10nAChRs) in OHCs and I_K,n and I_K,f (BK channels) in IHCs. Electromotility and prestin expression in OHCs were normal in Ca_V1.3^−/− mice. The maturation of high‐frequency (basal) hair cells was also affected in Ca_V1.3^−/− mice, but to a much lesser extent than apical cells. However, a characteristic feature in Ca_V1.3^−/− mice was the reduced hair cell size irrespective of their cochlear location. We conclude that the development of low‐ and high‐frequency hair cells is differentially regulated during development, with apical cells being more strongly dependent on experience‐independent Ca²⁺ APs.

Outer hair cells (OHCs) enhance the sensitivity and the frequency tuning of the mammalian cochlea.

Similar to the primary sensory receptor, the inner hair cells (IHCs), the mature functional characteristics of OHCs are acquired before hearing onset.

We found that OHCs, like IHCs, fire spontaneous Ca²⁺‐induced action potentials (APs) during immature stages of development, which are driven by Ca_V1.3 Ca²⁺ channels.

We also showed that the development of low‐ and high‐frequency hair cells is differentially regulated during pre‐hearing stages, with the former cells being more strongly dependent on experience‐independent Ca²⁺ action potential activity.

Voltage-Gated Calcium Channels: Key Players in Sensory Coding in the Retina and the Inner Ear

Calcium influx through voltage-gated Ca (Ca_V) channels is the first step in synaptic transmission. This review concerns Ca_V channels at ribbon synapses in primary sense organs and their specialization for efficient coding of stimuli in the physical environment. Specifically, we describe molecular, biochemical, and biophysical properties of the Ca_V channels in sensory receptor cells of the retina, cochlea, and vestibular apparatus, and we consider how such properties might change over the course of development and contribute to synaptic plasticity. We pay particular attention to factors affecting the spatial arrangement of Ca_V channels at presynaptic, ribbon-type active zones, because the spatial relationship between Ca_V channels and release sites has been shown to affect synapse function critically in a number of systems. Finally, we review identified synaptopathies affecting sensory systems and arising from dysfunction of L-type, Ca_V1.3, and Ca_V1.4 channels or their protein modulatory elements.

Tonotopic Variation of the T-Type Ca2+ Current in Avian Auditory Coincidence Detector Neurons

Neurons in avian nucleus laminaris (NL) are binaural coincidence detectors for sound localization and are characterized by striking structural variations in dendrites and axon initial segment (AIS) according to their acoustic tuning [characteristic frequency (CF)]. T-type Ca²⁺ (CaT) channels regulate synaptic integration and firing behavior at these neuronal structures. However, whether or how CaT channels contribute to the signal processing in NL neurons is not known. In this study, we addressed this issue with whole-cell recording and two-photon Ca²⁺ imaging in brain slices of posthatch chicks of both sexes. We found that the CaT current was prominent in low-CF neurons, whereas it was almost absent in higher-CF neurons. In addition, a large Ca²⁺ transient occurred at the dendrites and the AIS of low-CF neurons, indicating a localization of CaT channels at these structures in the neurons. Because low-CF neurons have long dendrites, dendritic CaT channels may compensate for the attenuation of EPSPs at dendrites. Furthermore, the short distance of AIS from the soma may accelerate activation of axonal CaT current in the neurons and help EPSPs reach spike threshold. Indeed, the CaT current was activated by EPSPs and augmented the synaptic response and spike generation of the neurons. Notably, the CaT current was inactivated during repetitive inputs, and these augmenting effects predominated at the initial phase of synaptic activity. These results suggested that dendritic and axonal CaT channels increase the sensitivity to sound at its onset, which may expand the dynamic range for binaural computation in low-CF NL neurons.SIGNIFICANCE STATEMENT Neurons in nucleus laminaris are binaural coincidence detectors for sound localization. We report that T-type Ca²⁺ (CaT) current was prominent at dendrites and the axonal trigger zone in neurons tuned to low-frequency sound. Because these neurons have long dendrites and a closer trigger zone compared with those tuned to higher-frequency sound, the CaT current augmented EPSPs at dendrites and accelerated spike triggers in the neurons, implying a strategic arrangement of the current within the nucleus. This effect was limited to the onset of repetitive inputs due to progressive inactivation of CaT current. The results suggested that the CaT current increases the sensitivity to sound at its onset, which may expand the dynamic range for binaural computation of low-frequency sound.

Different CaV1.3 Channel Isoforms Control Distinct Components of the Synaptic Vesicle Cycle in Auditory Inner Hair Cells

The mechanisms orchestrating transient and sustained exocytosis in auditory inner hair cells (IHCs) remain largely unknown. These exocytotic responses are believed to mobilize sequentially a readily releasable pool of vesicles (RRP) underneath the synaptic ribbons and a slowly releasable pool of vesicles (SRP) at farther distance from them. They are both governed by Ca_v1.3 channels and require otoferlin as Ca²⁺ sensor, but whether they use the same Ca_v1.3 isoforms is still unknown. Using whole-cell patch-clamp recordings in posthearing mice, we show that only a proportion (∼25%) of the total Ca²⁺ current in IHCs displaying fast inactivation and resistance to 20 μm nifedipine, a l-type Ca²⁺ channel blocker, is sufficient to trigger RRP but not SRP exocytosis. This Ca²⁺ current is likely conducted by short C-terminal isoforms of Ca_v1.3 channels, notably Ca_v1.3_42A and Ca_v1.3_43S, because their mRNA is highly expressed in wild-type IHCs but poorly expressed in Otof^-/- IHCs, the latter having Ca²⁺ currents with considerably reduced inactivation. Nifedipine-resistant RRP exocytosis was poorly affected by 5 mm intracellular EGTA, suggesting that the Ca_v1.3 short isoforms are closely associated with the release site at the synaptic ribbons. Conversely, our results suggest that Ca_v1.3 long isoforms, which carry ∼75% of the total IHC Ca²⁺ current with slow inactivation and confer high sensitivity to nifedipine and to internal EGTA, are essentially involved in recruiting SRP vesicles. Intracellular Ca²⁺ imaging showed that Ca_v1.3 long isoforms support a deep intracellular diffusion of Ca²⁺SIGNIFICANCE STATEMENT Auditory inner hair cells (IHCs) encode sounds into nerve impulses through fast and indefatigable Ca²⁺-dependent exocytosis at their ribbon synapses. We show that this synaptic process involves long and short C-terminal isoforms of the Ca_v1.3 Ca²⁺ channel that differ in the kinetics of their Ca²⁺-dependent inactivation and their relative sensitivity to the l-type Ca²⁺ channel blocker nifedipine. The short C-terminal isoforms, having fast inactivation and low sensitivity to nifedipine, mainly control the fast fusion of the readily releasable pool (RRP); that is, they encode the phasic exocytotic component. The long isoforms, with slow inactivation and great sensitivity to nifedipine, mainly regulate the vesicular replenishment of the RRP; that is, the sustained or tonic exocytosis.

Protection of the cochlear hair cells in adult C57BL/6J mice by T-type calcium channel blockers

The aim of the present study was to investigate the protective effect of T-type calcium channel blockers against presbycusis, using a C57BL/6J mice model. The expression of three T-type calcium channel receptor subunits in the cochlea of 6-8-week-old C57BL/6J mice was evaluated using reverse transcription-quantitative polymerase chain reaction. The results confirmed that the three subunits were expressed in the cochlea. In addition, the capacity of T-type calcium channel blockers to protect the cochlear hair cells of 24-26-week-old C57BL/6J mice was investigated in mice treated with mibefradil, benidipine or saline for 4 weeks. Differences in hearing threshold were detected using auditory brainstem recording (ABR), while differences in amplitudes were measured using a distortion product otoacoustic emission (DPOAE) test. The ABR test results showed that the hearing threshold significantly decreased at 24 kHz in the mibefradil-treated and benidipine-treated groups compared with the saline-treated group. The DPOAE amplitudes in the mibefradil-treated group were increased compared with those in the saline-treated group at the F2 frequencies of 11.3 and 13.4 kHz. Furthermore, the DPOAE amplitudes in the benidipine-treated group were increased compared with those in the saline-treated group at an F2 frequency of 13.4 kHz. The loss of outer hair cells (OHCs) was not evident in the mibefradil-treated group; however, the stereocilia of the inner hair cells (IHCs) were disorganised and sparse. In summary, these results indicate that the administration of a T-type calcium channel blocker for four consecutive weeks may improve the hearing at 24 kHz of 24-26-week-old C57BL/6J mice. The function and morphology of the OHCs of the C57BL/6J mice were significantly altered by the administration of a T-type calcium channel blocker; however, the IHCs were unaffected.

Proton-mediated block of Ca2+ channels during multivesicular release regulates short-term plasticity at an auditory hair cell synapse

Synaptic vesicles release both neurotransmitter and protons during exocytosis, which may result in a transient acidification of the synaptic cleft that can block Ca(2+) channels located close to the sites of exocytosis. Evidence for this effect has been reported for retinal ribbon-type synapses, but not for hair cell ribbon synapses. Here, we report evidence for proton release from bullfrog auditory hair cells when they are held at more physiological, in vivo-like holding potentials (Vh = -60 mV) that facilitate multivesicular release. During paired recordings of hair cells and afferent fibers, L-type voltage-gated Ca(2+) currents showed a transient block, which was highly correlated with the EPSC amplitude (or the amount of glutamate release). This effect was masked at Vh = -90 mV due to the presence of a T-type Ca(2+) current and blocked by strong pH buffering with HEPES or TABS. Increasing vesicular pH with internal methylamine in hair cells also abolished the transient block. High concentrations of intracellular Ca(2+) buffer (10 mm BAPTA) greatly reduced exocytosis and abolished the transient block of the Ca(2+) current. We estimate that this transient block is due to the rapid multivesicular release of ∼600-1300 H(+) ions per synaptic ribbon. Finally, during a train of depolarizing pulses, paired pulse plasticity was significantly changed by using 40 mm HEPES in addition to bicarbonate buffer. We propose that this transient block of Ca(2+) current leads to more efficient exocytosis per Ca(2+) ion influx and it may contribute to spike adaptation at the auditory nerve.

T-type channel-mediated neurotransmitter release

Besides controlling a wide variety of cell functions, T-type channels have been shown to regulate neurotransmitter release in peripheral and central synapses and neuroendocrine cells. Growing evidence over the last 10 years suggests a key role of Cav3.2 and Cav3.1 channels in controlling basal neurosecretion near resting conditions and sustained release during mild stimulations. In some cases, the contribution of low-voltage-activated (LVA) channels is not directly evident but requires either the activation of coupled presynaptic receptors, block of ion channels, or chelation of metal ions. Concerning the coupling to the secretory machinery, T-type channels appear loosely coupled to neurotransmitter and hormone release. In neurons, Cav3.2 and Cav3.1 channels mainly control the asynchronous appearance of "minis" [miniature inhibitory postsynaptic currents (mIPSCs) and miniature excitatory postsynaptic currents (mEPSCs)]. The same loose coupling is evident from membrane capacity and amperometric recordings in chromaffin cells and melanotropes where the low-threshold-driven exocytosis possesses the same linear Ca(2+) dependence of the other voltage-gated Ca(2+) channels (Cav1 and Cav2) that is strongly attenuated by slow calcium buffers. The intriguing issue is that, despite not expressing a consensus "synprint" site, Cav3.2 channels do interact with syntaxin 1A and SNAP-25 and, thus, may form nanodomains with secretory vesicles that can be regulated at low voltages. In this review, we discuss all the past and recent issues related to T-type channel-secretion coupling in neurons and neuroendocrine cells.