Persistent and resurgent Na+ currents in vestibular calyx afferents

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.

Inhibition of Ionic Currents by Fluoxetine in Vestibular Calyces in Different Epithelial Loci

Previous studies have suggested a role for selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine (Prozac®) in the treatment of dizziness and inner ear vestibular dysfunction. The potential mechanism of action within the vestibular system remains unclear; however, fluoxetine has been reported to block certain types of K+ channel in other systems. Here, we investigated the direct actions of fluoxetine on membrane currents in presynaptic hair cells and postsynaptic calyx afferents of the gerbil peripheral vestibular system using whole cell patch clamp recordings in crista slices. We explored differences in K+ currents in peripheral zone (PZ) and central zone (CZ) calyces of the crista and their response to fluoxetine application. Outward K+ currents in PZ calyces showed greater inactivation at depolarized membrane potentials compared to CZ calyces. The application of 100 μM fluoxetine notably reduced K+ currents in calyx terminals within both zones of the crista, and the remaining currents exhibited distinct traits. In PZ cells, fluoxetine inhibited a non-inactivating K+ current and revealed a rapidly activating and inactivating K+ current, which was sensitive to blocking by 4-aminopyridine. This was in contrast to CZ calyces, where low-voltage-activated and non-inactivating K+ currents persisted following application of 100 μM fluoxetine. Additionally, marked inhibition of transient inward Na+ currents by fluoxetine was observed in calyces from both crista zones. Different concentrations of fluoxetine were tested, and the EC50 values were found to be 40 µM and 32 µM for K+ and Na+ currents, respectively. In contrast, 100 μM fluoxetine had no impact on voltage-dependent K+ currents in mechanosensory type I and type II vestibular hair cells. In summary, micromolar concentrations of fluoxetine are expected to strongly reduce both Na+ and K+ conductance in afferent neurons of the peripheral vestibular system in vivo. This would lead to inhibition of action potential firing in vestibular sensory neurons and has therapeutic implications for disorders of balance.

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.

Expression of hyperpolarization-activated current (Ih) in zonally defined vestibular calyx terminals of the crista

Calyx terminals make afferent synapses with type I hair cells in vestibular epithelia and express diverse ionic conductances that influence action potential generation and discharge regularity in vestibular afferent neurons. Here we investigated the expression of hyperpolarization-activated current (Ih) in calyx terminals in central and peripheral zones of mature gerbil crista slices, using whole cell patch-clamp recordings. Slowly activating Ih was present in >80% calyces tested in both zones. Peak Ih and half-activation voltages were not significantly different; however, Ih activated with a faster time course in peripheral compared with central zone calyces. Calyx Ih in both zones was blocked by 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyrimidinium chloride (ZD7288; 100 µM), and the resting membrane potential became more hyperpolarized. In the presence of dibutyryl-cAMP (dB-cAMP), peak Ih was increased, activation kinetics became faster, and the voltage of half-activation was more depolarized compared with control calyces. In current clamp, calyces from both zones showed three different categories of firing: spontaneous firing, phasic firing where a single action potential was evoked after a hyperpolarizing pulse, or a single evoked action potential followed by membrane potential oscillations. In the absence of Ih, the latency to peak of the action potential increased; Ih produces a small depolarizing current that facilitates firing by driving the membrane potential closer to threshold. Immunostaining showed the expression of HCN2 subunits in calyx terminals. We conclude that Ih is found in calyx terminals across the crista and could influence conventional and novel forms of synaptic transmission at the type I hair cell-calyx synapse.NEW & NOTEWORTHY Calyx afferent terminals make synapses with vestibular hair cells and express diverse conductances that impact action potential firing in vestibular primary afferents. Conventional and nonconventional synaptic transmission modes are influenced by hyperpolarization-activated current (Ih), but regional differences were previously unexplored. We show that Ih is present in both central and peripheral calyces of the mammalian crista. Ih produces a small depolarizing resting current that facilitates firing by driving the membrane potential closer to threshold.

Characterization in Effective Stimulation on the Magnitude, Gating, Frequency Dependence, and Hysteresis of INa Exerted by Picaridin (or Icaridin), a Known Insect Repellent

Picaridin (icaridin), a member of the piperidine chemical family, is a broad-spectrum arthropod repellent. Its actions have been largely thought to be due to its interaction with odorant receptor proteins. However, to our knowledge, to what extent the presence of picaridin can modify the magnitude, gating, and/or the strength of voltage-dependent hysteresis (Hys(V)) of plasmalemmal ionic currents, such as, voltage-gated Na+ current [INa], has not been entirely explored. In GH3 pituitary tumor cells, we demonstrated that with exposure to picaridin the transient (INa(T)) and late (INa(L)) components of voltage-gated Na+ current (INa) were differentially stimulated with effective EC50's of 32.7 and 2.8 μM, respectively. Upon cell exposure to it, the steady-state current versus voltage relationship INa(T) was shifted to more hyperpolarized potentials. Moreover, its presence caused a rightward shift in the midpoint for the steady-state inactivate curve of the current. The cumulative inhibition of INa(T) induced during repetitive stimuli became retarded during its exposure. The recovery time course from the INa block elicited, following the conditioning pulse stimulation, was satisfactorily fitted by two exponential processes. Moreover, the fast and slow time constants of recovery from the INa block by the same conditioning protocol were noticeably increased in the presence of picaridin. However, the fraction in fast or slow component of recovery time course was, respectively, increased or decreased with an increase in picaridin concentrations. The Hys(V)'s strength of persistent INa (INa(P)), responding to triangular ramp voltage, was also enhanced during cell exposure to picaridin. The magnitude of resurgent INa (INa(R)) was raised in its presence. Picaritin-induced increases of INa(P) or INa(R) intrinsically in GH3 cells could be attenuated by further addition of ranolazine. The predictions of molecular docking also disclosed that there are possible interactions of the picaridin molecule with the hNaV1.7 channel. Taken literally, the stimulation of INa exerted by the exposure to picaridin is expected to exert impacts on the functional activities residing in electrically excitable cells.

Rapid cold hardening increases axonal Na+/K+-ATPase activity and enhances performance of a visual motion detection circuit in Locusta migratoria

Rapid cold hardening (RCH) is a type of phenotypic plasticity that delays the occurrence of chill coma in insects. Chill coma is mediated by a spreading depolarization of neurons and glia in the CNS, triggered by a failure of ion homeostasis. We used biochemical and electrophysiological approaches in the locust, Locusta migratoria, to test the hypothesis that the protection afforded by RCH is mediated by activation of the Na+/K+-ATPase (NKA) in neural tissue. RCH did not affect NKA activity measured in a biochemical assay of homogenized thoracic ganglia. However, RCH hyperpolarized the axon of a visual interneuron (DCMD) and increased the amplitude of an activity-dependent hyperpolarization (ADH) shown previously to be blocked by ouabain. RCH also improved performance of the visual circuitry presynaptic to DCMD to minimize habituation and increase excitability. We conclude that RCH enhances in situ NKA activity in the nervous system but also affects other neuronal properties that promote visual processing in locusts.

Expression and Physiology of Voltage-Gated Sodium Channels in Developing Human Inner Ear

Sodium channel expression in inner ear afferents is essential for the transmission of vestibular and auditory information to the central nervous system. During development, however, there is also a transient expression of Na⁺ channels in vestibular and auditory hair cells. Using qPCR analysis, we describe the expression of four Na⁺ channel genes, SCN5A (Nav1.5), SCN8A (Nav1.6), SCN9A (Nav1.7), and SCN10A (Nav1.8) in the human fetal cristae ampullares, utricle, and base, middle, and apex of the cochlea. Our data show distinct patterns of Na⁺ channel gene expression with age and between these inner ear organs. In the utricle, there was a general trend toward fold-change increases in expression of SCN8A, SCN9A, and SCN10A with age, while the crista exhibited fold-change increases in SCN5A and SCN8A and fold-change decreases in SCN9A and SCN10A. Fold-change differences of each gene in the cochlea were more complex and likely related to distinct patterns of expression based on tonotopy. Generally, the relative expression of SCN genes in the cochlea was greater than that in utricle and cristae ampullares. We also recorded Na⁺ currents from developing human vestibular hair cells aged 10-11 weeks gestation (WG), 12-13 WG, and 14+ WG and found there is a decrease in the number of vestibular hair cells that exhibit Na⁺ currents with increasing gestational age. Na⁺ current properties and responses to the application of tetrodotoxin (TTX; 1 μM) in human fetal vestibular hair cells are consistent with those recorded in other species during embryonic and postnatal development. Both TTX-sensitive and TTX-resistant currents are present in human fetal vestibular hair cells. These results provide a timeline of sodium channel gene expression in inner ear neuroepithelium and the physiological characterization of Na⁺ currents in human fetal vestibular neuroepithelium. Understanding the normal developmental timeline of ion channel gene expression and when cells express functional ion channels is essential information for regenerative technologies.

Similarities in the Biophysical Properties of Spiral-Ganglion and Vestibular-Ganglion Neurons in Neonatal Rats

The membranes of auditory and vestibular afferent neurons each contain diverse groups of ion channels that lead to heterogeneity in their intrinsic biophysical properties. Pioneering work in both auditory- and vestibular-ganglion physiology have individually examined this remarkable diversity, but there are few direct comparisons between the two ganglia. Here the firing patterns recorded by whole-cell patch-clamping in neonatal vestibular- and spiral ganglion neurons are compared. Indicative of an overall heterogeneity in ion channel composition, both ganglia exhibit qualitatively similar firing patterns ranging from sustained-spiking to transient-spiking in response to current injection. The range of resting potentials, voltage thresholds, current thresholds, input-resistances, and first-spike latencies are similarly broad in both ganglion groups. The covariance between several biophysical properties (e.g., resting potential to voltage threshold and their dependence on postnatal age) was similar between the two ganglia. Cell sizes were on average larger and more variable in VGN than in SGN. One sub-group of VGN stood out as having extra-large somata with transient-firing patterns, very low-input resistance, fast first-spike latencies, and required large current amplitudes to induce spiking. Despite these differences, the input resistance per unit area of the large-bodied transient neurons was like that of smaller-bodied transient-firing neurons in both VGN and SGN, thus appearing to be size-scaled versions of other transient-firing neurons. Our analysis reveals that although auditory and vestibular afferents serve very different functions in distinct sensory modalities, their biophysical properties are more closely related by firing pattern and cell size than by sensory modality.

Dopaminergic Inhibition of Na+ Currents in Vestibular Inner Ear Afferents

Inner ear hair cells form synapses with afferent terminals and afferent neurons carry signals as action potentials to the central nervous system. Efferent neurons have their origins in the brainstem and some make synaptic contact with afferent dendrites beneath hair cells. Several neurotransmitters have been identified that may be released from efferent terminals to modulate afferent activity. Dopamine is a candidate efferent neurotransmitter in both the vestibular and auditory systems. Within the cochlea, activation of dopamine receptors may reduce excitotoxicity at the inner hair cell synapse via a direct effect of dopamine on afferent terminals. Here we investigated the effect of dopamine on sodium currents in acutely dissociated vestibular afferent calyces to determine if dopaminergic signaling could also modulate vestibular responses. Calyx terminals were isolated along with their accompanying type I hair cells from the cristae of gerbils (P15-33) and whole cell patch clamp recordings performed. Large transient sodium currents were present in all isolated calyces; compared to data from crista slices, resurgent Na⁺ currents were rare. Perfusion of dopamine (100 μM) in the extracellular solution significantly reduced peak transient Na⁺ currents by approximately 20% of control. A decrease in Na⁺ current amplitude was also seen with extracellular application of the D2 dopamine receptor agonist quinpirole, whereas the D2 receptor antagonist eticlopride largely abolished the response to dopamine. Inclusion of the phosphatase inhibitor okadaic acid in the patch electrode solution occluded the response to dopamine. The reduction in calyx sodium current in response to dopamine suggests efferent signaling through D2 dopaminergic receptors may occur via common mechanisms to decrease excitability in inner ear afferents.