Multiple voltage-dependent calcium currents in acutely isolated mouse vestibular neurons

Effects of transient, persistent, and resurgent sodium currents on excitability and spike regularity in vestibular ganglion neurons

Vestibular afferent neurons occur as two populations with differences in spike timing regularity that are independent of rate. The more excitable regular afferents have lower current thresholds and sustained spiking responses to injected currents, while irregular afferent neurons have higher thresholds and transient responses. Differences in expression of low-voltage-activated potassium (KLV) channels are emphasized in models of spiking regularity and excitability in these neurons, leaving open the potential contributions of the voltage-gated sodium (NaV) channels responsible for the spike upstroke. We investigated the impact of different NaV current modes (transient, persistent, and resurgent) with whole-cell patch clamp experiments in mouse vestibular ganglion neurons (VGNs), the cultured and dissociated cell bodies of afferents. All VGNs had transient NaV current, many had a small persistent (non-inactivating) NaV current, and a few had resurgent current, which flows after the spike when NaV channels that were blocked are unblocked. A known NaV1.6 channel blocker decreased spike rate and altered spike waveforms in both sustained and transient VGNs and affected all three modes of NaV current. A NaV channel agonist enhanced persistent current and increased spike rate and regularity. We hypothesized that persistent and resurgent currents have different effects on sustained (regular) VGNs vs. transient (irregular) VGNs. Lacking blockers specific for the different current modes, we used modeling to isolate their effects on spiking of simulated transient and sustained VGNs, driven by simulated current steps and noisy trains of simulated EPSCs. In all simulated neurons, increasing transient NaV current increased spike rate and rate-independent regularity. In simulated sustained VGNs, adding persistent current increased both rate and rate-independent regularity, while adding resurgent current had limited impact. In transient VGNs, adding persistent current had little impact, while adding resurgent current increased both rate and rate-independent irregularity by enhancing sensitivity to synaptic noise. These experiments show that the small NaV current modes may enhance the differentiation of afferent populations, with persistent currents selectively making regular afferents more regular and resurgent currents selectively making irregular afferents more irregular.

Effects of transient, persistent, and resurgent sodium currents on excitability and spike regularity in vestibular ganglion neurons

Vestibular afferent neurons occur as two populations with differences in spike timing regularity that are independent of rate. The more excitable regular afferents have lower current thresholds and sustained spiking responses to injected currents, while irregular afferent neurons have higher thresholds and transient responses. Differences in expression of low-voltage-activated potassium (K LV ) channels are emphasized in models of spiking regularity and excitability in these neurons, leaving open the potential contributions of the voltage-gated sodium (Na V ) channels responsible for the spike upstroke. We investigated the impact of different Na V current modes (transient, persistent, and resurgent) with whole-cell patch clamp experiments in mouse vestibular ganglion neurons (VGNs), the cultured and dissociated cell bodies of afferents. All VGNs had transient Na V current, many had a small persistent (non-inactivating) Na V current, and a few had resurgent current, which flows after the spike peak when Na V channels that were blocked are unblocked. Na V 1.6 channels conducted most or all of each Na V current mode, and a Na V 1.6-selective blocker decreased spike rate and altered spike waveforms in both sustained and transient VGNs. A Na V channel agonist enhanced persistent current and increased spike rate and regularity. We hypothesized that persistent and resurgent currents have different effects on sustained (regular) VGNs vs. transient (irregular) VGNs. Lacking blockers specific for the different current modes, we used modeling to isolate their effects on spiking of simulated transient and sustained VGNs, driven by simulated current steps and noisy trains of simulated EPSCs. In all simulated neurons, increasing transient Na V current increased spike rate and rate-independent regularity. In simulated sustained VGNs, adding persistent current increased both rate and rate-independent regularity, while adding resurgent current had limited impact. In transient VGNs, adding persistent current had little impact, while adding resurgent current increased both rate and rate-independent irregularity by enhancing sensitivity to synaptic noise. These experiments show that the small Na V current modes may enhance the differentiation of afferent populations, with persistent currents selectively making regular afferents more regular and resurgent currents selectively making irregular afferents less regular.

Nociceptin/orphanin FQ peptide receptor mediates inhibition of N-type calcium currents in vestibular afferent neurons of the rat

The vestibular system is modulated by various neuromodulators including opioid peptides. The current study was conducted to determine whether activation of nociceptin/orphanin FQ peptide (NOP) receptors modulates voltage-gated calcium currents and action potential discharge of rat vestibular afferent neurons. We performed whole cell patch-clamp recordings on cultured vestibular afferent neurons from P7-P10 Long-Evans rats. Application of nociceptin/orphanin FQ (N/OFQ), a 17-amino acid neuropeptide that is the endogenous ligand for NOP receptor, inhibits the high-voltage activated (HVA) component of the calcium current in a concentration-dependent manner with a half inhibitory concentration of 26 nM. Said inhibitory action on the calcium current is voltage-dependent, which was made clear by the fact that it was reverted in 80% by a depolarizing prepulse. Furthermore, the effect of N/OFQ was blocked by application of the specific NOP-antagonist UFP101, by preincubation with G-protein blocker pertussis toxin, and by coapplication of the specific N-type calcium-current blocker ω-conotoxin-MVIIA. N/OFQ application causes an increase in the duration and maximum rate of repolarization of action potentials. It also decreases repetitive discharge and discharge elicited by sinusoidal stimulation. These results show that in vestibular afferents, NOP receptor activation inhibits N-type calcium current by activating G proteins, mostly through the Gβγ subunit. This suggests that NOP activation produces a presynaptic modulation of primary vestibular afferent neurons' output into the vestibular nuclei, thus taking part in the integration and gain setting of vestibular information in second-order vestibular nucleus neurons.NEW & NOTEWORTHY Our results show that in primary vestibular afferent neurons, activation of the nociceptin/orphanin FQ peptide receptor inhibits the N-type calcium current by a mechanism mediated by G proteins. We propose that calcium current inhibition modulates neurotransmitter release from vestibular afferents, producing a presynaptic modulation of vestibular input to vestibular nuclei, thus contributing to gain control in the vestibular afferent input.

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.

Sodium channel diversity in the vestibular ganglion: NaV1.5, NaV1.8, and tetrodotoxin-sensitive currents

Firing patterns differ between subpopulations of vestibular primary afferent neurons. The role of sodium (NaV) channels in this diversity has not been investigated because NaV currents in rodent vestibular ganglion neurons (VGNs) were reported to be homogeneous, with the voltage dependence and tetrodotoxin (TTX) sensitivity of most neuronal NaV channels. RT-PCR experiments, however, indicated expression of diverse NaV channel subunits in the vestibular ganglion, motivating a closer look. Whole cell recordings from acutely dissociated postnatal VGNs confirmed that nearly all neurons expressed NaV currents that are TTX-sensitive and have activation midpoints between -30 and -40 mV. In addition, however, many VGNs expressed one of two other NaV currents. Some VGNs had a small current with properties consistent with NaV1.5 channels: low TTX sensitivity, sensitivity to divalent cation block, and a relatively negative voltage range, and some VGNs showed NaV1.5-like immunoreactivity. Other VGNs had a current with the properties of NaV1.8 channels: high TTX resistance, slow time course, and a relatively depolarized voltage range. In two NaV1.8 reporter lines, subsets of VGNs were labeled. VGNs with NaV1.8-like TTX-resistant current also differed from other VGNs in the voltage dependence of their TTX-sensitive currents and in the voltage threshold for spiking and action potential shape. Regulated expression of NaV channels in primary afferent neurons is likely to selectively affect firing properties that contribute to the encoding of vestibular stimuli.

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.

Developmental increase in hyperpolarization-activated current regulates intrinsic firing properties in rat vestibular ganglion cells

The primary vestibular neurons convey afferent information from hair cells in the inner ear to the vestibular nuclei and the cerebellum. The intrinsic firing properties of vestibular ganglion cells (VGCs) are heterogeneous to sustained membrane depolarization, and undergo marked developmental changes from phasic to tonic types during the early postnatal period. Previous studies have shown that low-voltage-activated potassium channels, Kv1 and Kv7, play a critical role in determining the firing pattern of VGCs. In the present study, we explored the developmental changes in the properties of hyperpolarization-activated current (Ih) in rat VGCs and the role played by Ih in determining the firing properties of VGCs. Tonic firing VGCs showed a larger current density of Ih as compared to phasic firing VGCs, and tonic firing VGCs became phasic firing in the presence of ZD7288, an Ih channel blocker, indicating that Ih contributes to control the firing pattern of VGCs. The amplitude of Ih increased and the activation kinetics of Ih became faster during the developmental period. Analysis of developmental changes in the expression of hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels revealed that expression of HCN1 protein and its mRNA increased during the developmental period, whereas expression of HCN2-4 protein and its mRNA did not change. Our results suggest that HCN1 channels as well as Kv1 channels are critical in determining the firing pattern of rat VGCs and that developmental up-regulation of HCN1 transforms VGCs from phasic to tonic firing phenotypes.

Activation of μ-opioid receptors inhibits calcium-currents in the vestibular afferent neurons of the rat through a cAMP dependent mechanism

Opioid receptors are expressed in the vestibular endorgans (afferent neurons and hair cells) and are activated by the efferent system, which modulates the discharge of action potentials in vestibular afferent neurons (VANs). In mammals, VANs mainly express the μ opioid-receptor, but the function of this receptors activation and the cellular mechanisms by which they exert their actions in these neurons are poorly studied. To determine the actions of μ opioid receptor (MOR) and cell signaling mechanisms in VANs, we made perforated patch-clamp recordings of VANs that were obtained from postnatal days 7 to 10 (P7–10) rats and then maintained in primary culture. The MOR agonist [D-Ala², N-Me-Phe⁴, Gly⁵-ol]-enkephalin (DAMGO) inhibited the total voltage-gated outward current; this effect was prevented by the perfusion of a Ca²⁺-free extracellular solution. We then studied the voltage-gated calcium current (I_ca) and found that DAMGO Met-enkephalin or endomorphin-1 inhibited the I_Ca in a dose-response fashion. The effects of DAMGO were prevented by the MOR antagonist (CTAP) or by pertussis toxin (PTX). The use of specific calcium channel blockers showed that MOR activation inhibited T-, L- and N-type I_Ca. The use of various enzyme activators and inhibitors and of cAMP analogs allowed us to demonstrate that the MOR acts through a cAMP dependent signaling mechanism. In current clamp experiments, MOR activation increased the duration and decreased the amplitude of the action potentials and modulated the discharge produced by current injection. Pre-incubation with PTX occluded MOR activation effect. We conclude that MOR activation inhibits the T-, L- and N-type I_Ca through activation of a Gα_i/o protein that involves a decrease in AC-cAMP-PKA activity. The modulation of I_Ca may have an impact on the synaptic integration, excitability, and neurotransmitter release from VANs.

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

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

Hyperpolarization-activated current (I(h)) in vestibular calyx terminals: characterization and role in shaping postsynaptic events

Calyx afferent terminals engulf the basolateral region of type I vestibular hair cells, and synaptic transmission across the vestibular type I hair cell/calyx is not well understood. Calyces express several ionic conductances, which may shape postsynaptic potentials. These include previously described tetrodotoxin-sensitive inward Na(+) currents, voltage-dependent outward K(+) currents and a K(Ca) current. Here, we characterize an inwardly rectifying conductance in gerbil semicircular canal calyx terminals (postnatal days 3-45), sensitive to voltage and to cyclic nucleotides. Using whole-cell patch clamp, we recorded from isolated calyx terminals still attached to their type I hair cells. A slowly activating, noninactivating current (I(h)) was seen with hyperpolarizing voltage steps negative to the resting potential. External Cs(+) (1-5 mM) and ZD7288 (100 μM) blocked the inward current by 97 and 83 %, respectively, confirming that I(h) was carried by hyperpolarization-activated, cyclic nucleotide gated channels. Mean half-activation voltage of I(h) was -123 mV, which shifted to -114 mV in the presence of cAMP. Activation of I(h) was well described with a third order exponential fit to the current (mean time constant of activation, τ, was 190 ms at -139 mV). Activation speeded up significantly (τ=136 and 127 ms, respectively) when intracellular cAMP and cGMP were present, suggesting that in vivo I(h) could be subject to efferent modulation via cyclic nucleotide-dependent mechanisms. In current clamp, hyperpolarizing current steps produced a time-dependent depolarizing sag followed by either a rebound afterdepolarization or an action potential. Spontaneous excitatory postsynaptic potentials (EPSPs) became larger and wider when I(h) was blocked with ZD7288. In a three-dimensional mathematical model of the calyx terminal based on Hodgkin-Huxley type ionic conductances, removal of I(h) similarly increased the EPSP, whereas cAMP slightly decreased simulated EPSP size and width.

Postnatal expression of an apamin-sensitive k(ca) current in vestibular calyx terminals

Afferent innervation patterns in the vestibular periphery are complex, and vestibular afferents show a large variation in their regularity of firing. Calyx fibers terminate on type I vestibular hair cells and have firing characteristics distinct from the bouton fibers that innervate type II hair cells. Whole-cell patch clamp was used to investigate ionic currents that could influence firing patterns in calyx terminals. Underlying K(Ca) conductances have been described in vestibular ganglion cells, but their presence in afferent terminals has not been investigated previously. Apamin, a selective blocker of SK-type calcium-activated K(+) channels, was tested on calyx afferent terminals isolated from gerbil semicircular canals during postnatal days 1-50. Lowering extracellular calcium or application of apamin (20-500 nM) reduced slowly activating outward currents in voltage clamp. Apamin also reduced the action potential afterhyperpolarization (AHP) in whole-cell current clamp, but only after the first two postnatal weeks. K(+) channel expression increased during the first postnatal month, and SK channels were found to contribute to the AHP, which may in turn influence discharge regularity in calyx vestibular afferents.

Ion channels set spike timing regularity of mammalian vestibular afferent neurons

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

The firing properties of vestibular ganglion cells in heterozygous BDNF null mice

We examined the firing properties of vestibular ganglion cells (VGCs) acutely isolated from wild or heterozygous brain-derived neurotrophic factor null mice, using the patch-clamp technique. VGCs obtained from wild-type mice showed diverse firing properties during sustained membrane depolarization; approximately half of the neurons exhibited strong adaptation, generating just a single spike or a few spikes (phasic type), whereas approximately one-fourth of the neurons showed moderate adaptation or tonic firing (tonic type). In heterozygous mice, the majority of VGCs belonged to the tonic type, the rate of which was significantly different from that of wild-type. These results suggest that brain-derived neurotrophic factor not only contributes to the survival of the VGCs but also affects their firing properties.

The muscarinic inhibition of the potassium M-current modulates the action-potential discharge in the vestibular primary-afferent neurons of the rat

There is consensus that muscarinic and nicotinic receptors expressed in vestibular hair cells and afferent neurons are involved in the efferent modulation of the electrical activity of the afferent neurons. However the underlying mechanisms of postsynaptic control in neurons are not well understood. In our work we show that the activation of muscarinic receptors in the vestibular neurons modulates the potassium M-current modifying the activity of afferent neurons. Whole-cell patch-clamp recordings were made on vestibular-afferent neurons isolated from Wistar rats (postnatal days 7-10) and held in primary culture (18-24 h). The M-current was studied during its deactivation after depolarizing voltage-clamp pulses. In 68% of the cells studied, those of larger capacitance, the M-current antagonists linopirdine and XE-991 reduced the amplitude of the M-current by 54%+/-7% and 50%+/-3%. The muscarinic-receptor agonist oxotremorine-M also significantly reduced the M-current by 58%+/-12% in the cells. The action of oxotremorine-M was blocked by atropine, thus indicating its cholinergic nature. The erg-channel blocker E-4031 did not significantly modify the M-current amplitude. In current-clamp experiments, linopirdine, XE-991, and oxotremorine-M modified the discharge response to current pulses from single spike to multiple spiking, reducing the adaptation of the electrical discharge. Our results indicate that large soma-size cultured vestibular-afferent neurons (most probably calyx-bearing neurons) express the M-current and that the modulation of this current by activation of muscarinic-receptor reduces its spike-frequency adaptation.

Low-voltage-activated potassium channels underlie the regulation of intrinsic firing properties of rat vestibular ganglion cells

Individual primary vestibular afferents exhibit spontaneous activity the regularity of which can vary from regular to irregular. Different aspects of vestibular responsiveness have been associated with this dimension of regularity of resting discharge. Isolated rat vestibular ganglion cells (VGCs) showed heterogeneous intrinsic firing properties during sustained membrane depolarization: some neurons exhibited a strong adaptation generating just a single or a few spikes (phasic type), whereas other neurons showed moderate adaptation or tonic firing (tonic type). Tonic discharging VGCs were rare at postnatal days 5-7 and increased up to approximately 60% of neurons during postnatal 2-3 wk. To explore the major factors responsible for the discharge regularity of primary vestibular afferents, we investigated the contribution of K+ channels to the firing properties of isolated rat VGCs. Phasic firing became tonic firing in the presence of 4-aminopyridine or alpha-dendrotoxin, indicating that Kv1 potassium channels control the firing pattern of the phasic VGCs. Tetraethylammonium decreased the number of spikes during step current stimuli in all types. Blockade of Ca2+-activated K+ channels decreased the number of spikes in tonic VGCs. Our results suggest that Kv1 channels are critical both in determining the pattern of spike discharge in rat vestibular ganglion neurons and in their proportional change during maturation.

Ion channels in mammalian vestibular afferents may set regularity of firing

Rodent vestibular afferent neurons offer several advantages as a model system for investigating the significance and origins of regularity in neuronal firing interval. Their regularity has a bimodal distribution that defines regular and irregular afferent classes. Factors likely to be involved in setting firing regularity include the morphology and physiology of the afferents' contacts with hair cells, which may influence the averaging of synaptic noise and the afferents' intrinsic electrical properties. In vitro patch clamp studies on the cell bodies of primary vestibular afferents reveal a rich diversity of ion channels, with indications of at least two neuronal populations. Here we suggest that firing patterns of isolated vestibular ganglion somata reflect intrinsic ion channel properties, which in vivo combine with hair cell synaptic drive to produce regular and irregular firing.

Voltage-activated calcium currents in octopus cells of the mouse cochlear nucleus

Octopus cells, neurons in the most posterior and dorsal part of the mammalian ventral cochlear nucleus, convey the timing of synchronous firing of auditory nerve fibers to targets in the contralateral superior paraolivary nucleus and ventral nucleus of the lateral lemniscus. The low input resistances and short time constants at rest that arise from the partial activation of a large, low-voltage-activated K(+) conductance (g(KL)) and a large mixed-cation, hyperpolarization-activated conductance (g(h)) enable octopus cells to detect coincident firing of auditory nerve fibers with exceptional temporal precision. Octopus cells fire conventional, Na(+) action potentials but a voltage-sensitive Ca(2+) conductance was also detected. In this study, we explore the nature of that calcium conductance under voltage-clamp. Currents, carried by Ca(2+) or Ba(2+) and blocked by 0.4 mM Cd(2+), were activated by depolarizations positive to -50 mV and peaked at -23 mV. At -23 mV they reached 1.1 +/- 0.1 nA in the presence of 5 mM Ca(2+) and 1.6 +/- 0.1 nA in 5 mM Ba(2+). Ten micromolar BAY K 8644, an agonist of high-voltage-activated L-type channels, enhanced I(Ba) by 63 +/- 11% (n = 8) and 150 microM nifedipine, an antagonist of L-type channels, reduced the I(Ba) by 65 +/- 5% (n = 5). Meanwhile, 0.5 microM omega-Agatoxin IVA, an antagonist of P/Q-type channels, or 1 microM omega-conotoxin GVIA, an antagonist of N-type channels, suppressed I(Ba) by 15 +/- 4% (n = 5) and 9 +/- 4% (n = 5), respectively. On average 16% of the current remained in the presence of the cocktail of blockers, indicative of the presence of R-type channels. Together these experiments show that octopus cells have a depolarization-sensitive g(Ca) that is largely formed from L-type Ca(2+) channels and that P/Q-, N-, and R-type channels are expressed at lower levels in octopus cells.

Role of high-voltage-activated calcium channels in glucose-regulated beta-cell calcium homeostasis and insulin release

High-voltage-activated (HVA) calcium channels are known to be the primary source of calcium for glucose-stimulated insulin secretion. However, few studies have investigated how these channels can be regulated by chronically elevated levels of glucose. In the present study, we determined the level of expression of the four major HVA calcium channels (N-type, P/Q-type, L(C)-type, and L(D)-type) in rat pancreatic beta-cells. Using quantitative real-time PCR (QRT-PCR), we found the expression of all four HVA genes in rat insulinoma cells (INS-1) and in primary isolated rat islet cells. We then determined the role of each channel in insulin secretion by using channel-selective antagonists. Insulin secretion analysis revealed that N- and L-type channels are both involved in immediate glucose-induced insulin secretion. However, L-type was preferentially coupled to secretion at later time points. P/Q-type channels were not found to play a role in insulin secretion at any stage. It was also found that long-term exposure to elevated glucose increases basal calcium in these cells. Interestingly, chronically elevated glucose decreased the mRNA expression of the channels involved with insulin secretion and diminished the level of stimulated calcium influx in these cells. Using whole cell patch clamp, we found that N- and L-type channel currents increase gradually subsequent to lower intracellular calcium perfusion, suggesting that these channels may be regulated by glucose-induced changes in calcium.

Ca2+-activated K+-current density is correlated with soma size in rat vestibular-afferent neurons in culture

Vestibular-afferent neurons (VANs) transmit information about linear and angular accelerations during head movements from vestibular end organs to vestibular nuclei. In situ, these neurons show heterogeneous discharge patterns that may be produced by differences in their intrinsic properties. However, little is known about the ionic currents underlying their different firing patterns. Using the whole cell patch-clamp technique, we analyzed the expression of Ca(2+) and Ca(2+)-activated K(+) currents (I(KCa)) in primary cultured neurons isolated from young rats (p7-p10). We found two overlapping subpopulations of VANs classified according to low-threshold Ca(2+)-current [low-voltage-activated (LVA)] expression; LVA (-) neurons, formed by small cells, and LVA (+) neurons composed of medium to large cells. The I(KCa) in both cell-groups was carried through channels of high (BK), intermediate (IK), and low conductance (SK), besides a resistant channel to classical blockers (IR). BK was expressed preferentially in LVA (+) cells, whereas IR expression was preferentially in LVA (-) cells. No correlation between SK and IK expression with the soma size was found. Current-clamp experiments showed that BK participates in the adaptation of discharge and in the duration of the action potential, whereas SK and IK did not show a significant contribution to electrical discharge of cultured VANs. However, because of the low number of VANs in culture with repetitive firing it is difficult to interpret our results in terms of discharge patterns. Our results demonstrate that vestibular-afferent neurons possess different Ca(2+)-activated K(+) (K(Ca)) channels and that their expression, heterogeneous among the cells, would contribute to explain some of the differences in the electrical-firing properties of these neurons.

The involvement of Cav3.2/alpha1H T-type calcium channels in excitability of mouse embryonic primary vestibular neurones

Ca2+ influx through voltage-gated calcium channels probably influences neuronal ontogenesis. Many developing neurones transiently express T-type/Cav3 calcium channels that contribute to their electrical activity and potentially to their morphological differentiation. Here we have characterized the electrophysiological properties and the functional role of a large T-type calcium current that is present in mouse developing primary vestibular neurones at embryonic day E17. This T-type current showed fast activation and inactivation, as well as slow deactivation kinetics. The overlap of activation and inactivation parameters produced a window current between -65 and -45 mV. Recovery from short-term inactivation was slow suggesting the presence of the Cav3.2 subunit. This T-type current was blocked by micromolar concentrations of Ni2+ and was inhibited by fast perfusion velocities in a similar fashion to recombinant Cav3.2 T-type channels expressed in HEK-293 cells. More importantly, current clamp experiments have revealed that the T-current could elicit afterdepolarization potentials during the repolarization phase of action potentials, and occasionally generate calcium spikes. Taken together, we demonstrate that the Cav3.2 subunit is likely to be the main T-type calcium channel subunit expressed in embryonic vestibular neurones and should play a key role in the excitability of these neurones during the ontogenesis of vestibular afferentation.

Expression and distribution of μ opioid receptors in the inner ear of the rat

Opioid peptides have demonstrated modulatory effects on the vestibular afferent discharge and are putative vestibular efferent neuromodulators. The distribution of their receptors in the mammalian vestibular epithelia is not known. We used reverse transcriptase-polymerase chain reaction (RT-PCR), in situ hybridization, Western blots and immunohistochemistry to study the expression of mu opioid receptor (MOR) in the Scarpa's ganglia and cristae ampullares of rats. MOR transcript was only detected in the somata of the vestibular afferent neurons. MOR-like immunoreactivity was observed in the somata of vestibular afferents and in nerve terminals in the cristae ampullares epithelia both in the center and peripheral regions. Double labeling of cristae sections with the MOR1 antibody in combination with antibodies against calretinin (a marker for vestibular afferents terminating in calices) and peripherin (a marker for afferents terminating in boutons), respectively showed that MOR1 immunoreactivity was in calyx, dimorphic and bouton vestibular afferents. MOR immunoreactivity was not detected in vestibular efferent fibers identified with choline acetyltransferase immunohistochemistry. These results indicate that MOR may mediate effects of vestibular efferents on afferents.

In vitro reconstitution of signal transmission from a hair cell to the growth cone of a chick vestibular ganglion cell

Signal transmission from a chick hair cell to the growth cone of a vestibular ganglion cell was examined by placing an acutely dissociated hair cell on the growth cone of a cultured vestibular ganglion cell. Electrical stimuli were applied to the hair cell while monitoring the intracellular Ca(2+) concentration ([Ca(2+)](i)) at the growth cone or recording whole-cell currents from the vestibular ganglion cell. Electrical stimulation of the hair cell induced [Ca(2+)](i) increases at the growth cone and inward currents in the vestibular ganglion cell. The [Ca(2+)](i) increase was blocked by 6-cyano-7-nitroquinoxaline (CNQX) (10 microM) but not by 2-amino-5-phosphonovaleric acid (APV; 50 microM). Glutamate (100 nM-300 microM) applied to the vestibular ganglion cell by the Y-tube method induced inward currents which were also antagonized by CNQX, but not by APV. These results indicate that the electrical stimulation of a hair cell induced glutamate or glutamate like agent release from the hair cell, which activated non-N-methyl-D-aspartate receptors at the growth cone of the vestibular ganglion cell, followed by action potentials and [Ca(2+)](i) elevation in the vestibular ganglion cell. This is the first demonstration of in vitro reconstitution of functional signal transmission from a hair cell to a vestibular ganglion cell.

Opioid receptors mediate a postsynaptic facilitation and a presynaptic inhibition at the afferent synapse of axolotl vestibular hair cells

This study was designed to determine the effects of opiate drugs on the electrical activity of afferent neurons and on the ionic currents of hair cells from semicircular canals. Experiments were done on larval axolotls (Ambystoma tigrinum). The multiunit spike activity of afferent neurons was recorded in the isolated inner ear under both resting conditions and mechanical stimulation. Ionic currents were recorded using voltage clamp of hair cells isolated from the semicircular canal. In the isolated inner-ear preparation, microperfusion of either non-specific opioid receptor antagonist naloxone (10 nM to 1 mM), mu receptor agonist [D-Ala(2), N-Me-Phe(4),Gly(5)-ol]-enkephalin (1 pM to 10 microM), or kappa receptor antagonist nor-binaltorphimine (10 nM to 100 microM) elicited a dose-dependent long-lasting (>5 min) increase of the electrical discharge of afferent neurons. The mu receptor agonist funaltrexamine (1 nM to 100 microM) and the kappa receptor agonist U-50488 (1 nM to 10 microM) diminished the basal spike discharge of vestibular afferents. The delta receptor agonist D-Pen(2)-D-Pen(5)-enkephalin (1 nM to 10 mM) and the antagonist naltrindole (1 nM to 10 mM) were without a significant effect. The only drug that displayed a significant action on hair-cell ionic currents was trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]-cyclohexyl) benzeneacetamide methanesulfonate (U-50488) that reduced the Ca(2+) current in a dose-dependent fashion. On its own, mu receptor agonist [D-Ala(2), N-Me-Phe(4),Gly(5)-ol]-enkephalin (0.01 and 10 microM) significantly potentiated the response of afferent neurons to the excitatory amino acid agonist (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (0.1 microM), while synaptic transmission was blocked by the use of high-Mg(2+), low-Ca(2+) solutions. Our data indicate that the activity of vestibular afferent neurons may be regulated in a complex fashion by opioid receptors: mu opioid receptors mediating an excitatory, postsynaptic modulatory input to afferent neurons, and kappa receptors mediating an inhibitory, presynaptic input to hair cells.

Molecular probes of the vestibular nerve. II. Characterization of neurons in Scarpa's ganglion to determine separate populations within the nerve

An unambiguous delineation of the exact numbers and/or proportions of calyx-only, dimorph, and bouton-only vestibular afferents is needed to continue studies concerning vestibular integration in the nervous system. Here, we take advantage of immunocytochemical properties of three groups of vestibular afferents. We utilize calretinin to delineate the calyx-only population, and peripherin to stain the bouton-only afferents. An additional subgroup of afferents that stain with calbindin, but not calretinin is also introduced. The size of the cells that stain with these markers was determined. Cells that are calbindin-positive overlap the sizes of Nissl-stained somata. Cells that stain with peripherin or calretinin are non-overlapping with calretinin cells being the largest and peripherin-positive cells the smallest. Twenty percent of the ganglion cells were peripherin positive, another 20% stained with calretinin antibodies, 30% stained with calbindin, and all cells in Scarpa's ganglion stained with parvalbumin. Most of the calretinin-positive cells also stained with calbindin. One-third of the calbindin-positive population stained only with calbindin. These studies indicate that the calyx- and bouton-only populations of vestibular afferents in gerbil comprise at least 40% of the nerve. In addition, at least 10% of the nerve also stains with calbindin and neither calretinin nor peripherin. Based on indirect evidence, we hypothesize that these are a subpopulation of dimorph afferents. This study has provided an anatomical instrument (in addition to intracellular physiological methods) to study separate populations of vestibular afferents.

Resonance of spike discharge modulation in neurons of the guinea pig medial vestibular nucleus

The modulation of action potential discharge rates is an important aspect of neuronal information processing. In these experiments, we have attempted to determine how effectively spike discharge modulation reflects changes in the membrane potential in central vestibular neurons. We have measured how their spike discharge rate was modulated by various current inputs to obtain neuronal transfer functions. Differences in the modulation of spiking rates were observed between neurons with a single, prominent after hyperpolarization (AHP, type A neurons) and cells with more complex AHPs (type B neurons). The spike discharge modulation amplitudes increased with the frequency of the current stimulus, which was quantitatively described by a neuronal model that showed a resonance peak >10 Hz. Modeling of the resonance peak required two putative potassium conductances whose properties had to be markedly dependent on the level of the membrane potential. At low frequencies (< or =0.4 Hz), the gain or magnitude functions of type A and B discharge rates were similar relative to the current input. However, resting input resistances obtained from the ratio of the membrane potential and current were lower in type B compared with type A cells, presumably due to a higher level of active potassium conductances at rest. The lower input resistance of type B neurons was compensated by a twofold greater sensitivity of their firing rate to changes in membrane potential, which suggests that synaptic inputs on their dendritic processes would be more efficacious. This increased sensitivity is also reflected in a greater ability of type B neurons to synchronize with low-amplitude sinusoidal current inputs, and in addition, their responses to steep slope ramp stimulation are enhanced over the more linear behavior of type A neurons. This behavior suggests that the type B MVNn are moderately tuned active filters that promote high-frequency responses and that type A neurons are like low-pass filters that are well suited for the resting tonic activity of the vestibular system. However, the more sensitive and phasic type B neurons contribute to both low- and high-frequency control as well as signal detection and would amplify the contribution of both irregular and regular primary afferents at high frequencies.

Three types of depolarization-activated potassium currents in acutely isolated mouse vestibular neurons

The nature and electrophysiological properties of Ca(2+)-independent depolarization-activated potassium currents were investigated in vestibular primary neurons acutely isolated from postnatal mice using the whole cell configuration of the patch-clamp technique. Three types of currents were identified. The first current, sensitive to TEA (I(TEA)) and insensitive to 4-aminopyridine (4-AP), activated at -40 mV and exhibited slow activation (tau(ac), 38.4 +/- 7.8 ms at -30 mV, mean +/- SD). I(TEA) had a half activation potential [V(ac(1/2))] of -14.5 +/- 2.6 mV and was inactivated by up to 84.5 +/- 5.7% by 10-s conditioning prepulses with a half inactivation potential [V(inac(1/2))] of -62.4 +/- 0.2 mV. The second current, sensitive to 4-AP (maximum block around 0.5 mM) and to alpha-dendrotoxin (I(DTX)) appeared at -60 mV. Complete block of I(DTX) was achieved using either 20 nM alpha-DTX or 50 nM margatoxin. This current activated 10 times faster than I(TEA) (tau(ac), 3.5 +/- 0.8 ms at -50 mV) with V(ac(1/2)) of -51.2 +/- 0.6 mV, and inactivated only slightly compared with I(TEA) (maximum inactivation, 19.7 +/- 3.2%). The third current, also sensitive to 4-AP (maximum block at 2 mM), was selectively blocked by application of blood depressing substance (BDS-I; maximum block at 250 nM). The BDS-I-sensitive current (I(BDS-I)) activated around -60 mV. It displayed fast activation (tau(ac), 2.3 +/- 0.4 ms at -50 mV) and fast and complete voltage-dependent inactivation. I(BDS-I) had a V(ac(1/2)) of -31.3 +/- 0.4 mV and V(inac(1/2)) of -65.8 +/- 0.3 mV. It displayed faster time-dependent inactivation and recovery from inactivation than I(TEA). The three types of current were found in all the neurons investigated. Although I(TEA) was the major current, the proportion of I(DTX) and I(BDS-I) varied considerably between neurons. The ratio of the density of I(BDS-I) to that of I(DTX) ranged from 0.02 to 2.90 without correlation with the cell capacitances. In conclusion, vestibular primary neurons differ by the proportion rather than the type of the depolarization-activated potassium currents they express.

Low-affinity blockade of neuronal N-type Ca channels by the spider toxin omega-agatoxin-IVA

The recognition of neuronal Ca channel diversity has led to considerable efforts to identify useful classification criteria. Here, we revisit the pharmacological definition of P- and Q-type Ca channels, which is based on their respective high and low sensitivity to the spider omega-agatoxin-IVA (omega-Aga-IVA), using whole-cell recordings of the Ca channel currents carried by 5 mM Ba(2+) in isolated rat subthalamic and sympathetic neurons. In subthalamic neurons, omega-Aga-IVA (1 microM) targeted multiple Ca channels. One population was blocked with high potency. These channels carried 50.4 +/- 3.4% (n = 5) of the control current and showed the same inactivation kinetics and voltage-dependent high affinity for omega-Aga-IVA as do prototypic P-type Ca channels. Other Ca channels were targeted with weaker potency. This heterogeneous population contributed to 14.0 +/- 1.7% (n = 5) of the control current. It included N-type Ca channels as well as high-threshold Ca channels that displayed the pharmacological signature of Q-type Ca channels but resembled P-type Ca channels in their gating properties. N-type Ca current block by omega-Aga-IVA (1 microM) was further investigated in sympathetic neurons, which mainly express this Ca channel type. Block was incomplete ( approximately 30% of the control current). Its relief at positive potentials was consistent with omega-Aga-IVA acting as a channel-gating modifier. These effects did not reflect a complete loss of selectivity, because omega-Aga-IVA (1 microM) had no effect on subthalamic Na and K currents or their T- and L-type Ca currents. Our data confirm that omega-Aga-IVA is a selective P-type Ca channel blocker. However, its diminished selectivity in the micromolar range limits its usefulness for functional studies of Q-type Ca channels.

Efferent function of vestibular afferent endings? Similar localization of N-type calcium channels, synaptic vesicle and synaptic membrane-associated proteins

We investigated the distribution of N-type voltage-dependent calcium channels that mediate Ca(2+) entry initiating transmitter release in the rat vestibular sensory epithelium. We used confocal microscopy to assess the in vitro labeling by fluorescent specific ligand binding, omega-conotoxin-GVIA and also the immunolabeling of presynaptic soluble N-ethylmaleimide-sensitive fusion factor attachment protein receptor (SNARE) proteins, syntaxin, 25,000 mol. wt synaptosome-associated protein and synaptotagmin: components of the neurotransmitter exocytosis machinery. We found that there was a close anatomical association between the voltage-gated calcium channels, the synaptic vesicle and synaptic membrane-associated proteins on the afferent nerve calyces and probably afferent boutons, which are postsynaptic compartments. Our data suggest that these peripheral afferent endings possess the presynaptic Ca(2+) channels and the components of the presynaptic SNARE proteins involved in synaptic vesicle docking and calcium-dependent exocytosis. They provide additional evidence for a secretory function and efferent role of these endings in hair cell neurotransmission.

Properties of Q-type calcium channels in neostriatal and cortical neurons are correlated with beta subunit expression

In brain neurons, P- and Q-type Ca(2+) channels both appear to include a class A alpha1 subunit. In spite of this similarity, these channels differ pharmacologically and biophysically, particularly in inactivation kinetics. The molecular basis for this difference is unclear. In heterologous systems, alternative splicing and ancillary beta subunits have been shown to alter biophysical properties of channels containing a class A alpha1 subunit. To test the hypothesis that similar mechanisms are at work in native systems, P- and Q-type currents were characterized in acutely isolated rat neostriatal, medium spiny neurons and cortical pyramidal neurons using whole-cell voltage-clamp techniques. Cells were subsequently aspirated and subjected to single-cell RT-PCR (scRT-PCR) analysis of calcium channel alpha(1) and beta (beta(1-4)) subunit expression. In both cortical and neostriatal neurons, P- and Q-type currents were found in cells expressing class A alpha(1) subunit mRNA. Although P-type currents in cortical and neostriatal neurons were similar, Q-type currents differed significantly in inactivation kinetics. Notably, Q-type currents in neostriatal neurons were similar to P-type currents in inactivation rate. The variation in Q-type channel biophysics was correlated with beta subunit expression. Neostriatal neurons expressed significantly higher levels of beta(2a) mRNA and lower levels of beta(1b) mRNA than cortical neurons. These findings are consistent with the association of beta(2a) and beta(1b) subunits with slow and fast inactivation, respectively. Analysis of alpha(1A) splice variants in the linker between domains I and II failed to provide an alternative explanation for the differences in inactivation rates. These findings are consistent with the hypothesis that the biophysical properties of Q-type channels are governed by beta subunit isoforms and are separable from toxin sensitivity.

Differential subcellular immunolocalization of voltage-gated calcium channel alpha1 subunits in the chinchilla cristae ampullaris

The immunohistochemical localization of alpha1A, alpha1B, alpha1C, alpha1D and alpha1E voltage-gated calcium channel subunits was investigated in the chinchilla cristae ampullaris and Scarpa's ganglia at the light and electron microscopy level with the use of specific antipeptide antibodies directed against these subunits. The stereocilia membrane of type I and type II hair cells was immunoreactive for alpha1B along its entire length. The basolateral membrane of both types of hair cells was alpha1B, alpha1C and alpha1D immunoreactive. Neurons in the Scarpa's ganglia and afferent nerve terminals in the cristae were immunoreactive for alpha1C and alpha1B. No specific immunoreactivity to alpha1A or alpha1E was seen in the sensory epithelia or ganglia. These findings are consistent with the presence of alpha1B (N-type channel), alpha1C and alpha1D (L-type channels) in the vestibular hair cells, and alpha1B (N-type channel) and alpha1C (L-type channel) in primary vestibular neurons.

Developmental changes in low and high voltage-activated calcium currents in acutely isolated mouse vestibular neurons

1. The development of low voltage-activated (LVA) and high voltage-activated (HVA) calcium currents was studied in neurons acutely dissociated from mouse vestibular ganglia at embryonic stages (E)14, 15, 17 and birth using the whole-cell patch-clamp technique. 2. LVA current was present in almost all neurons tested at stages E14 to E17, although at birth this current was restricted to a few neurons. Two populations of neurons were characterized based on the amplitude of the LVA current. In the first population, LVA current densities decreased between E17 and birth by which time this current tended to disappear in most neurons. A second population of neurons with high density LVA current appeared at E17, and in this group the mean density increased during development. 3. Among HVA currents, the dihydropyridine-sensitive L-type current remained constant between E15 and birth. Over the same period, the density of N- and Q-type currents continuously increased as shown using omega-conotoxin-GVIA (N-type), and high concentrations of omega-agatoxin-IVA (Q-type). The P-type current, sensitive to low concentrations of omega-agatoxin-IVA, transiently increased between E15 and E17, and then both current density and its proportion of the global current decreased. 4. Our results reveal large modifications in the expression of voltage-dependent calcium channels during embryonic development of primary vestibular neurons. The changes in the expression of LVA current and the transient augmentation of P-type HVA current occur during a period characterized by massive neuronal growth and by the beginning of synaptogenesis. These results suggest a specific role of these currents in the ontogenesis of vestibular primary afferents.

Short-term response of postnatal rat vestibular neurons following brain-derived neurotrophic factor or neurotrophin-3 application

The effects of the application of brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) neurotrophins on the intracellular calcium level ([Ca2+]i) were studied in vestibular ganglion neurons (VGNs) from postnatal day 3 (P3) rats cultured for 50 hr. We first assessed the expression of trkB and trkC mRNA receptors in cultured VGNs. Immunobloting and immunocytochemistry confirmed the presence of the neurotrophin receptors on neurons. Both neurotrophins induced transient [Ca2+]i elevations in VGNs: BDNF-treated neurons responded in 65% and NT-3-treated neurons in 56%. The responses could be inhibited by anti-BDNF or anti-NT-3 antibodies. The [Ca2+]i elevation was dependent on extracellular calcium since it was abolished in calcium-free medium but also implicates the release of calcium from intracellular stores as tested by prior depletion with thapsigargin. Our results suggest the implication of a short-term calcium regulation in VGNs, which could reflect specific fast effects of neurotrophins in the early postnatal rat vestibular system.