Sodium Current Amplitude Research Articles

Effect of ytterbium on sodium current and its kinetics in rat hippocampal neurons

This study addressed the effects of Yb3+ on voltage-gated sodium currents in rat hippocampal neurons using the whole-cell patch-clamp technique. Voltage-clamp recordings in single neurons were filtered and stored in a computer. Yb3+ increased the amplitude of sodium currents in a concentration-dependent and voltage-dependent manner. The 50 % enhancement concentration of Yb3+ on sodium currents was about 8.97 μmol/L, which was different from the inhibitory effects of Yb3+ on potassium current. The analysis on the activation and inactivation kinetics of Na+ current showed that 100 μmol/L Yb3+ did not change the process of activation and inactivation. In addition, the times reaching the peak of current (t) and inactivated time constant (τ) were voltage dependent. 100 μmol/L Yb3+ significantly prolonged the time to peak at −70 and −80 mV. The effect disappeared at the positive direction of −70 mV. Furthermore, Yb3+ decreased τ values to more positive values than −80 mV. In total, Yb3+ did not change the process of activation, but impelled inactivated process. Yb3+ mainly increased the Na+ current through changing its conductance. It might be one of the mechanisms that Yb3+ affected the hippocampal neurons.

The role of late I Na in development of cardiac arrhythmias.

Late I Na is an integral part of the sodium current, which persists long after the fast-inactivating component. The magnitude of the late I Na is relatively small in all species and in all types of cardiomyocytes as compared with the amplitude of the fast sodium current, but it contributes significantly to the shape and duration of the action potential. This late component had been shown to increase in several acquired or congenital conditions, including hypoxia, oxidative stress, and heart failure, or due to mutations in SCN5A, which encodes the α-subunit of the sodium channel, as well as in channel-interacting proteins, including multiple β subunits and anchoring proteins. Patients with enhanced late I Na exhibit the type-3 long QT syndrome (LQT3) characterized by high propensity for the life-threatening ventricular arrhythmias, such as Torsade de Pointes (TdP), as well as for atrial fibrillation. There are several distinct mechanisms of arrhythmogenesis due to abnormal late I Na, including abnormal automaticity, early and delayed after depolarization-induced triggered activity, and dramatic increase of ventricular dispersion of repolarization. Many local anesthetic and antiarrhythmic agents have a higher potency to block late I Na as compared with fast I Na. Several novel compounds, including ranolazine, GS-458967, and F15845, appear to be the most selective inhibitors of cardiac late I Na reported to date. Selective inhibition of late I Na is expected to be an effective strategy for correcting these acquired and congenital channelopathies.

Altered PKA modulation in the Nav1.1 epilepsy variant I1656M

Genetic epilepsy with febrile seizures plus (GEFS(+)) is an inherited epilepsy that can result from mutations in at least four ion channel subunits. The majority of the known GEFS(+) mutations have been identified in SCN1A, the gene encoding Nav1.1 α-subunit. Protein kinases as critical modulators of sodium channels have been closely related to the genesis of epilepsy. However, little is known about how protein kinases affect the GEFS(+) mutant sodium channel. To gain insight into the protein kinases effect on channel properties and neuronal excitability of SCN1A mutant channels, we investigated the human SCN1A GEFS(+) mutation I1656M by using whole cell patch-clamp technique and an established computational neuron model. The results showed that the PKA inhibition of sodium current amplitude significantly decreased in the I1656M mutant channels, but the PKC inhibition did not. The responses of the voltage-dependent activation and fast inactivation to PKA activator disappeared in the I1656M mutant channels, but the response of the voltage dependence of the slow inactivation did not. Computational model analysis suggested that changes of the I1656M mutant channel gating behaviors in response to PKA activation altered neuronal excitability. These results indicate that altered responses of the mutant channels to PKA signaling may impair the delicate balances between chemical and electrical harmony and lead to abnormal neuronal excitability.

Acetylcholine enhances excitability by lowering the threshold of spike generation in olfactory receptor cells

Olfactory perception is influenced by behavioral states, presumably via efferent regulation. Using the whole cell version of patch-clamp recording technique, we discovered that acetylcholine, which is released from efferent fibers in the olfactory mucosa, can directly affect the signal encoding in newt olfactory receptor cells (ORCs). Under current-clamp conditions, application of carbachol, an acetylcholine receptor agonist, increased the spike frequency of ORCs and lowered their spike threshold. When a 3-pA current to induce near-threshold depolarization was injected into ORCs, 0.0 spikes/s were generated in control solution and 0.5 spikes/s in the presence of carbachol. By strong stimuli of injection of a 13-pA current into ORCs, 9.1 and 11.0 spikes/s were generated in control and carbachol solutions, respectively. A similar result was observed by bath application of 50 μM acetylcholine. Under voltage-clamp conditions, carbachol increased the peak amplitude of a voltage-gated sodium current by 32% and T-type calcium current by 39%. Atropine, the specific muscarinic receptor antagonist, blocked the enhancement by carbachol of the voltage-gated sodium current and T-type calcium current, suggesting that carbachol increases those currents via the muscarinic receptor rather than via the nicotinic receptor. In contrast, carbachol did not significantly change the amplitude of the L-type calcium current or the delayed rectifier potassium current in the ORCs. Because T-type calcium current is known to lower the threshold in ORCs, we suggest that acetylcholine enhance excitability by lowering the threshold of spike generation in ORCs via the muscarinic receptor.

GW24-e1851 An increase of CO2 levels, enhancing late sodium current, is an independent factor of proarrhythmia in the heart

ObjectivesIncreased CO2 levels in the myocardium may be global due to inadequate ventilation/circulation or local due to myocardial ischaemia/infarction (by 4 times higher than the normal), and are associated with...

Differential Effects of TipE and a TipE-Homologous Protein on Modulation of Gating Properties of Sodium Channels from Drosophila melanogaster

β subunits of mammalian sodium channels play important roles in modulating the expression and gating of mammalian sodium channels. However, there are no orthologs of β subunits in insects. Instead, an unrelated protein, TipE in Drosophila melanogaster and its orthologs in other insects, is thought to be a sodium channel auxiliary subunit. In addition, there are four TipE-homologous genes (TEH1-4) in D. melanogaster and three to four orthologs in other insect species. TipE and TEH1-3 have been shown to enhance the peak current of various insect sodium channels expressed in Xenopus oocytes. However, limited information is available on how these proteins modulate the gating of sodium channels, particularly sodium channel variants generated by alternative splicing and RNA editing. In this study, we compared the effects of TEH1 and TipE on the function of three Drosophila sodium channel splice variants, DmNav9-1, DmNav22, and DmNav26, in Xenopus oocytes. Both TipE and TEH1 enhanced the amplitude of sodium current and accelerated current decay of all three sodium channels tested. Strikingly, TEH1 caused hyperpolarizing shifts in the voltage-dependence of activation, fast inactivation and slow inactivation of all three variants. In contrast, TipE did not alter these gating properties except for a hyperpolarizing shift in the voltage-dependence of fast inactivation of DmNav26. Further analysis of the gating kinetics of DmNav9-1 revealed that TEH1 accelerated the entry of sodium channels into the fast inactivated state and slowed the recovery from both fast- and slow-inactivated states, thereby, enhancing both fast and slow inactivation. These results highlight the differential effects of TipE and TEH1 on the gating of insect sodium channels and suggest that TEH1 may play a broader role than TipE in regulating sodium channel function and neuronal excitability in vivo.

μ-Theraphotoxin-An1a: Primary structure determination and assessment of the pharmacological activity of a promiscuous anti-insect toxin from the venom of the tarantula Acanthoscurria natalensis (Mygalomorphae, Theraphosidae)

Tarantulas are included in the mygalomorph spider family Theraphosidae. Although the pharmacological diversity of theraphosid toxins (theraphotoxins) is broad, studies dedicated to the characterization of biologically active molecules from the theraphosid genus Acanthoscurria have been restricted to the investigation of antimicrobial peptides and polyamines produced by the hemocytes of Acanthoscurria gomesiana. The present study reports the purification, primary structure determination and electrophysiological effects of an anti-insect toxin, named μ-theraphotoxin-An1a (μ-TRTX-An1a), from the venom of Acanthoscurria natalensis – a tarantula species occurring in the Brazilian biomes caatinga and cerrado. The analysis of the primary structure of μ-TRTX-An1a revealed the similarity of this toxin to theraphosid toxins bearing a huwentoxin-II-like fold. Electrophysiological experiments showed that μ-TRTX-An1a (100 nM) induces membrane depolarization, increases the spontaneous firing frequency and reduces spike amplitude of cockroach dorsal unpaired median (DUM) neurons. In addition, under voltage-clamp conditions, μ-TRTX-An1a (100 nM) only partially blocks voltage-dependent sodium current amplitudes in DUM neurons without any effect on their voltage dependence. This effect correlates well with the reduction of the spontaneous action potential amplitudes. Altogether, these last results suggest that μ-TRTX-An1a affects insect neuronal voltage-dependent sodium channels, which are among possible channels targeted by this promiscuous toxin.

Varicella-Zoster Viruses Associated with Post-Herpetic Neuralgia Induce Sodium Current Density Increases in the ND7-23 Nav-1.8 Neuroblastoma Cell Line

Post-herpetic neuralgia (PHN) is the most significant complication of herpes zoster caused by reactivation of latent Varicella-Zoster virus (VZV). We undertook a heterologous infection in vitro study to determine whether PHN-associated VZV isolates induce changes in sodium ion channel currents known to be associated with neuropathic pain. Twenty VZV isolates were studied blind from 11 PHN and 9 non-PHN subjects. Viruses were propagated in the MeWo cell line from which cell-free virus was harvested and applied to the ND7/23-Nav1.8 rat DRG x mouse neuroblastoma hybrid cell line which showed constitutive expression of the exogenous Nav 1.8, and endogenous expression of Nav 1.6 and Nav 1.7 genes all encoding sodium ion channels the dysregulation of which is associated with a range of neuropathic pain syndromes. After 72 hrs all three classes of VZV gene transcripts were detected in the absence of infectious virus. Single cell sodium ion channel recording was performed after 72 hr by voltage-clamping. PHN-associated VZV significantly increased sodium current amplitude in the cell line when compared with non-PHN VZV, wild-type (Dumas) or vaccine VZV strains ((POka, Merck and GSK). These sodium current increases were unaffected by acyclovir pre-treatment but were abolished by exposure to Tetrodotoxin (TTX) which blocks the TTX-sensitive fast Nav 1.6 and Nav 1.7 channels but not the TTX-resistant slow Nav 1.8 channel. PHN-associated VZV sodium current increases were therefore mediated in part by the Nav 1.6 and Nav 1.7 sodium ion channels. An additional observation was a modest increase in message levels of both Nav1.6 and Nav1.7 mRNA but not Nav 1.8 in PHN virally infected cells.

Variation in sodium current amplitude between vasopressin and oxytocin hypothalamic supraoptic neurons

Biophysical characteristics of tetrodotoxin-sensitive sodium (Na(+)) currents were studied in vasopressin (VP) and oxytocin (OT) supraoptic neurons acutely isolated from rat hypothalamus. Na(+) current density (pA/pF) was significantly greater in VP neurons than in OT neurons. No significant difference between VP and OT neurons was detected regarding the voltage dependence of activation and steady-state inactivation, or rate of recovery from inactivation of Na(+) currents. In both VP and OT neurons, the macroscopic inactivation of the Na(+) currents was best fitted with a double-exponential expression suggesting two rates of inactivation. Also in both types, the time course of recovery from inactivation proceeded with fast and slow time constants averaging around 8 and 350 ms, respectively, suggesting the presence of multiple pathways of recovery from inactivation. The slower time constant of recovery of inactivation may be involved in the decrease in action potential (AP) amplitude that occurs after the first spike during burst firing in both neuronal types. The larger amplitude of Na(+) currents in VP vs. OT neurons may explain the previous observations that VP neurons exhibit a lower AP threshold and greater AP amplitude than OT neurons, and may serve to differently tune the firing properties and responses to neuromodulators of the respective neuronal types.

GSK3 and β-catenin determines functional expression of sodium channels at the axon initial segment.

Neuronal action potentials are generated through voltage-gated sodium channels, which are tethered by ankyrinG at the membrane of the axon initial segment (AIS). Despite the importance of the AIS in the control of neuronal excitability, the cellular and molecular mechanisms regulating sodium channel expression at the AIS remain elusive. Our results show that GSK3α/β and β-catenin phosphorylated by GSK3 (S33/37/T41) are localized at the AIS and are new components of this essential neuronal domain. Pharmacological inhibition of GSK3 or β-catenin knockdown with shRNAs decreased the levels of phosphorylated-β-catenin, ankyrinG, and voltage-gated sodium channels at the AIS, both "in vitro" and "in vivo", therefore diminishing neuronal excitability as evaluated via sodium current amplitude and action potential number. Thus, our results suggest a mechanism for the modulation of neuronal excitability through the control of sodium channel density by GSK3 and β-catenin at the AIS.

Remodeling of mechanical junctions and of microtubule-associated proteins accompany cardiac connexin43 lateralization

Desmosomes and adherens junctions provide mechanical continuity between cardiac cells, whereas gap junctions allow for cell-cell electrical/metabolic coupling. These structures reside at the cardiac intercalated disc (ID). Also at the ID is the voltage-gated sodium channel (VGSC) complex. Functional interactions between desmosomes, gap junctions, and VGSC have been demonstrated. Separate studies show, under various conditions, reduced presence of gap junctions at the ID and redistribution of connexin43 (Cx43) to plaques oriented parallel to fiber direction (gap junction "lateralization"). To determine the mechanisms of Cx43 lateralization, and the fate of desmosomal and sodium channel molecules in the setting of Cx43 remodeling. Adult sheep were subjected to right ventricular pressure overload (pulmonary hypertension). Tissue was analyzed by quantitative confocal microscopy and by transmission electron microscopy. Ionic currents were measured using conventional patch clamp. Quantitative confocal microscopy demonstrated lateralization of immunoreactive junctional molecules. Desmosomes and gap junctions in lateral membranes were demonstrable by electron microscopy. Cx43/desmosomal remodeling was accompanied by lateralization of 2 microtubule-associated proteins relevant for Cx43 trafficking: EB1 and kinesin protein Kif5b. In contrast, molecules of the VGSC failed to reorganize in plaques discernable by confocal microscopy. Patch-clamp studies demonstrated change in amplitude and kinetics of sodium current and a small reduction in electrical coupling between cells. Cx43 lateralization is part of a complex remodeling that includes mechanical and gap junctions but may exclude components of the VGSC. We speculate that lateralization results from redirectionality of microtubule-mediated forward trafficking. Remodeling of junctional complexes may preserve electrical synchrony under conditions that disrupt ID integrity.

Striking In Vivo Phenotype of a Disease-Associated Human SCN5A Mutation Producing Minimal Changes in Vitro

The D1275N SCN5A mutation has been associated with a range of unusual phenotypes, including conduction disease and dilated cardiomyopathy, as well as atrial and ventricular tachyarrhythmias. However, when D1275N is studied in heterologous expression systems, most studies show near-normal sodium channel function. Thus, the relationship of the variant to the clinical phenotypes remains uncertain. We identified D1275N in a patient with atrial flutter, atrial standstill, conduction disease, and sinus node dysfunction. There was no major difference in biophysical properties between wild-type and D1275N channels expressed in Chinese hamster ovary cells or tsA201 cells in the absence or presence of β1 subunits. To determine D1275N function in vivo, the Scn5a locus was modified to knock out the mouse gene, and the full-length wild-type (H) or D1275N (DN) human SCN5A cDNAs were then inserted at the modified locus by recombinase mediated cassette exchange. Mice carrying the DN allele displayed slow conduction, heart block, atrial fibrillation, ventricular tachycardia, and a dilated cardiomyopathy phenotype, with no significant fibrosis or myocyte disarray on histological examination. The DN allele conferred gene-dose-dependent increases in SCN5A mRNA abundance but reduced sodium channel protein abundance and peak sodium current amplitudes (H/H, 41.0±2.9 pA/pF at -30 mV; DN/H, 19.2±3.1 pA/pF, P<0.001 vs. H/H; DN/DN, 9.3±1.1 pA/pF, P<0.001 versus H/H). Although D1275N produces near-normal currents in multiple heterologous expression experiments, our data establish this variant as a pathological mutation that generates conduction slowing, arrhythmias, and a dilated cardiomyopathy phenotype by reducing cardiac sodium current.

Relative contribution of changes in sodium current versus intercellular coupling on reentry initiation in 2-dimensional preparations of plakophilin-2–deficient cardiac cells

Loss of expression of the desmosomal protein plakophilin-2 (PKP2) leads to decreased gap junction-mediated (GJ) coupling, and alters the amplitude and kinetics of sodium current in cardiac myocytes. Whether these modifications, alone or in combination, are sufficient to act as arrhythmogenic substrates remains undefined. This study sought to characterize arrhythmia susceptibility and reentry dynamics consequent to loss of PKP2 expression, and to assess the relative contribution of cell uncoupling versus alterations in sodium current in generation of reentry. Monolayers of neonatal rat ventricular myocytes were treated with oligonucleotides that either prevented or failed to prevent PKP2 expression. Numerical simulations modeled experimentally observed modifications in I(Na), GJ coupling, or both (models PKP2-Na, PKP2-GJ, and PKP2-KD, respectively). Relative roles of sodium current density versus kinetics were further explored. Loss of PKP2 expression increased incidence of rotors and decreased frequency of rotation. Mathematical simulations revealed that single premature stimuli initiated rotors in models PKP2-Na and PKP2-KD, but not PKP2-GJ. Changes in sodium current kinetics, rather than current density, were key to reentry initiation. Anatomical barriers led to vortex shedding, wavebreaks, and rotors when I(Na) kinetics, but not GJ coupling or I(Na) density, were altered. PKP2-dependent changes in sodium current kinetics lead to slow conduction, increased propensity to functional block, and vortex shedding. Changes in GJ or I(Na) density played only a minor role on reentry susceptibility. Changes in electrical properties of the myocyte caused by loss of expression of PKP2 can set the stage for rotors even if anatomical homogeneity is maintained.

A study on mechanisms of toxic actions of ciguatoxins: existence of functional relationship between CTX3C and charged residues of voltage sensors in Nav1.4 sodium channel

Ciguatoxins, a group of virulent marine toxins, are bound to "site 5" inside voltage-dependent sodium channels and alter their kinetics dramatically; this is probably responsible for clinical symptoms of ciguatoxin poisoning or ciguatera. Although ciguatoxins are known to affect the voltage dependency of both activation and inactivation kinetics, site 5 has not been functionally clarified. This study, therefore, targeted putative voltage sensors as a receptor for ciguatoxins. We constructed mutants, in which 1 of 23 basic residues was substituted with glutamine in the S4 segment for each of four domains. We then examined the effects of a synthetic ciguatoxin congener CTX3C on these mutants. Notably, the suppressive effect of CTX3C on the sodium current (I Na) amplitude of domain 2 mutants, which carried a mutation in the basic S4 residue of domain 2, was either reduced or eliminated. Kinetic analyses of domain 2 mutants in comparison with those of the wild type and other mutants revealed that the negative shift of the steady-state inactivation curve (V 1/2inactΔ) was significantly decreased. The resistance of domain 2 mutants to CTX3C in terms of changes in V 1/2inact is suggested to be closely related to resistance to the suppressive effect of CTX3C on the I Na amplitude. This is the first report to demonstrate the existence of a functional relationship between a ciguatoxin congener and voltage sensor segments in domain 2 of the α-subunit of voltage-dependent sodium channels, although further studies are needed to locate a ciguatoxin receptor.

CHARACTERIZATION OF COMPOUND HETEROZYGOSITY FOR MUTATIONS R269W IN KCNH2 AND P1824A IN SCN5A ASSOCIATED WITH LONG QT SYNDROME AND SINUS NODE DYSFUNCTION

Methods: The functional consequence of the mutations were assessed by whole-cell patch clamp recording of mutant channels in a cultured cell lines. With mathematical models of human ventricular myocytes and peripheral sinoatrial node (SAN) cells, we evaluated the changes in kinetic behavior of IKr and INa on the action potential configuration and pacemaker activity. Results: From genetic analysis for 20 probands who presented bradyarrhythmia and had a family history (e.g. implantation of a pacemaker), we identified two heterozygous missense mutations in the KCNH2 and SCN5A genes (R269W in KCNH2 and P1824A in SCN5A) in a patient with both sinoatrial and atrioventricular block, and marked QT prolongation. Electrophysiological study revealed that the outward tail current density at -50mV after a depolarization to +40mV in CHO cells expressing KCNH2 R269W was 51% of wild type (WT) (p<0.05). V1/2 of steady-state inactivation curve in KCNH2 R269W shifted by 30.7 mV in the negative direction compared to WT (p<0.05). On the other hand, peak sodium current amplitude at -40mV in HEK293 cells expressing SCN5A P1824A was 54% of WT (p<0.05). SCN5A P1824A produced a positive shift in the voltage dependence of activation (+5.0 mV, p<0.05) compared to WT. Simulation study showed that these mutational changes consequently prolonged the midmyocardial action potential duration by 58 % in mutant model of human ventricular myocytes. The study also showed that INa and IKr of the patient peripheral SAN model were much smaller, and increasing gap junction conductance caused arrhythmic dynamics (skipped-beat runs). Conclusions: These results demonstrate that KCNH2 R269W may mainly contribute to QT prolongation, and SCN5A P1824A may contribute to sinus node dysfunction and conduction disease as clinical phenotype expressions.

An important role of a pyrethroid-sensing residue F1519 in the action of the N-alkylamide insecticide BTG 502 on the cockroach sodium channel

Deltamethrin, a pyrethroid insecticide, and BTG 502, an alkylamide insecticide, target voltage-gated sodium channels. Deltamethrin binds to a unique receptor site and causes prolonged opening of sodium channels by inhibiting deactivation and inactivation. Previous 22Na+ influx and receptor binding assays using mouse brain synaptoneurosomes showed that BTG 502 antagonized the binding and action of batrachotoxin (BTX), a site 2 sodium channel neurotoxin. However, the effect of BTG 502 has not been examined directly on sodium channels expressed in Xenopus oocytes. In this study, we examined the effect of BTG 502 on wild-type and mutant cockroach sodium channels expressed in Xenopus oocytes. Toxin competition experiments confirmed that BTG 502 antagonizes the action of BTX and possibly shares a common receptor site with BTX. However, unlike BTX which causes persistent activation of sodium channels, BTG 502 reduces the amplitude of peak sodium current. A previous study showed that BTG 502 was more toxic to pyrethroid-resistant house flies possessing a super-kdr (knockdown resistance) mechanism than to pyrethroid-susceptible house flies. However, we found that the cockroach sodium channels carrying the equivalent super-kdr mutations (M918T and L1014F) were not more sensitive to BTG 502 than the wild-type channel. Instead, a kdr mutation, F1519I, which reduces pyrethroid binding, abolished the action of BTG 502. These results provide evidence the actions of alkylamide and pyrethroid insecticides require a common sodium channel residue.

Neuropathic Nav1.3-Mediated Sensitization to P2X Activation is Regulated by Protein Kinase C

BackgroundIncreased neuronal excitability and spontaneous firing are hallmark characteristics of injured sensory neurons. Changes in expression of various voltage-gated Na+ channels (VGSCs) have been observed under neuropathic conditions and there is evidence for the involvement of protein kinase C (PKC) in sensory hyperexcitability. Here we demonstrate the contribution of PKC to P2X-evoked VGSC activation in dorsal root ganglion (DRG) neurons in neuropathic conditions.ResultsUsing the spinal nerve ligation (SNL) model of neuropathic pain and whole-cell patch clamp recordings of dissociated DRG neurons, we examined changes in excitability of sensory neurons after nerve injury and observed that P2X3 purinoceptor-mediated currents induced by α,β-meATP triggered activation of TTX-sensitive VGSCs in neuropathic nociceptors only. Treatment of neuropathic DRGs with the PKC blocker staurosporine or calphostin C decreased the α,β-meATP-induced Na+ channels activity and reversed neuronal hypersensitivity. In current clamp mode, α,β-meATP was able to evoke action-potentials more frequently in neuropathic neurons than in controls. Pretreatment with calphostin C significantly decreased the proportion of sensitized neurons that generated action potentials in response to α,β-meATP. Recordings measuring VGSC activity in neuropathic neurons show significant change in amplitude and voltage dependence of sodium currents. In situ hybridization data indicate a dramatic increase in expression of embryonic Nav1.3 channels in neuropathic DRG neurons. In a CHO cell line stably expressing the Nav1.3 subunit, PKC inhibition caused both a significant shift in voltage-dependence of the channel in the depolarizing direction and a decrease in current amplitude.ConclusionNeuropathic injury causes primary sensory neurons to become hyperexcitable to ATP-evoked P2X receptor-mediated depolarization, a phenotypic switch sensitive to PKC modulation and mediated by increased activity of TTX-sensitive VGSCs. Upregulation in VGSC activity after injury is likely mediated by increased expression of the Nav1.3 subunit, and the function of the Nav1.3 channel is regulated by PKC.

Difference of Sodium Currents between Pediatric and Adult Human Atrial Myocytes: Evidence for Developmental Changes of Sodium Channels

Voltage-gated calcium currents and potassium currents were shown to undergo developmental changes in postnatal human and animal cardiomocytes. However, so far, there is no evidence whether sodium currents also presented the developmental changes in postnatal human atrial cells. The aim of this study was to observe age-related changes of sodium currents between pediatric and adult atrial myocytes. Human atrial myocytes were acutely isolated and the whole-cell patch clamp technique was used to record sodium currents isolated from pediatric and adult atrial cardiomocytes. The peak amplitude of sodium currents recorded in adult atrial cells was significantly larger than that in pediatric atrial myocytes. However, there was no significant difference of the activation voltage for peak sodium currents between two kinds of atrial myocytes. The time constants for the activation and inactivation of sodium currents were smaller in adult atria than pediatric atria. The further study revealed that the voltage-dependent inactivation of sodium currents were more slow in adult atrial cardiomyocytes than pediatric atrial cells. A significant difference was also observed in the recovery process of sodium channel from inactivation. In summary, a few significant differences were demonstrated in sodium currents characteristics between pediatric and adult atrial myocytes, which indicates that sodium currents in human atria also undergo developmental changes.

β-pompilidotoxin modulates spontaneous activity and persistent sodium currents in spinal networks

The origin of rhythm generation in mammalian spinal cord networks is still poorly understood. In a previous study, we showed that spontaneous activity in spinal networks takes its origin in the properties of certain intrinsically spiking interneurons based on the persistent sodium current (I NaP). We also showed that depolarization block caused by a fast inactivation of the transient sodium current (I NaT) contributes to the generation of oscillatory activity in spinal cord cultures. Recently, a toxin called beta-pompilidotoxin (β-PMTX) that slows the inactivation process of tetrodotoxin (TTX) -sensitive sodium channels has been extracted from the solitary wasp venom. In the present study, we therefore investigated the effect of β-PMTX on rhythm generation and on sodium currents in spinal networks. Using intracellular recordings and multielectrode array (MEA) recordings in dissociated spinal cord cultures from embryonic (E14) rats, we found that β-PMTX reduces the number of population bursts and increases the background asynchronous activity. We then uncoupled the network by blocking all synaptic transmission (APV, CNQX, bicuculline and strychnine) and observed that β-PMTX increases both the intrinsic activity at individual channels and the number of intrinsically activated channels. At the cellular level, we found that β-PMTX has two effects: it switches 58% of the silent interneurons into spontaneously active interneurons and increases the firing rate of intrinsically spiking cells. Finally, we investigated the effect of β-PMTX on sodium currents. We found that this toxin not only affects the inactivation of I NaT but also increases the peak amplitude of the persistent sodium current (I NaP). Altogether, theses findings suggest that β-PMTX acting on I NaP and I NaT enhances intrinsic activity leading to a profound modulation of spontaneous rhythmic activity in spinal networks.

Action potential changes associated with impairment of functional properties of sodium channels in hippocampal neurons induced by melamine

Since the melamine-contamination event happened in September 2008, there have been lots of studies about melamine toxicity, but very limited studies focused on central nervous system (CNS). In the present study, we investigated the effects of melamine (5 × 10 −4, 5 × 10 −5 and 5 × 10 −6 g/ml) on voltage-gated sodium channels (VGSCs) in hippocampal CA1 neurons using whole-cell patch-clamp recordings technique. The results showed that only 5 × 10 −4 g/ml melamine reduced the amplitude of voltage-gated sodium current ( I Na). At the concentrations of 5 × 10 −5 and 5 × 10 −4 g/ml, melamine produced a hyperpolarizing shift in the steady-state activation curve of I Na and also enhanced the steady-state inactivate processing of I Na. Action potential properties and the pattern of repetitive firing were examined using current-clamp recording, which indicated that peak amplitude and overshoot of the evoked single action potential were decreased. The half-width and the firing rate of repetitive firing were increased in a concentration-dependent manner. The data suggest that melamine alters the action potential of hippocampal CA1 neurons by impairing the functional properties of VGSCs, which may be the underlie mechanisms of neurotoxicity induced by melamine.