Distinctive Properties and Powerful Neuromodulation of Nav1.6 Sodium Channels Regulates Neuronal Excitability

Upregulation of Nav1.6 expression in the ventral posterolateral nucleus of thalamus contributes to hyperalgesia in a model of Parkinson's disease

Pain is the most common non-motor manifestation of Parkinson's disease (PD), affecting the quality of life for patients. Nav1.6 is the most abundant subtype of voltage-gated sodium channels (VGSCs) in the brain of adult mammals. Here we investigated the expression patterns of Nav1.6 in the ventral posterolateral (VPL) nucleus of the thalamus and its involvement in the development of hyperalgesia in 6-hydroxydopamine (6-OHDA)-lesioned rats. The results showed a significant increase in Nav1.6 expression in reactive astrocytes of the ipsilateral VPL in 6-OHDA-lesioned rats at 4 weeks post-injection. Moreover, 6-OHDA-lesioned rats exhibited mechanical hyperalgesia, but did not display thermal hyperalgesia in the ipsilateral paw at the same time point. The down-regulation of Nav1.6 in the ipsilateral VPL can reduce mechanical hyperalgesia and improve sensorimotor impairments in 6-OHDA- lesioned rats. Furthermore, the analysis of local field potentials (LFPs) revealed that the increased Nav1.6 may participate in abnormal synchronized oscillations within the thalamocortical loop in 6-OHDA-lesioned rats. These findings suggest that the altered expression of Nav1.6 in astrocytes of the VPL may play an important role in the abnormal processing of pain within the thalamocortical circuit, contributing to the formation of mechanical hyperalgesia in animal models of PD.

Candesartan restores blood-brain barrier dysfunction, mitigates aberrant gene expression, and extends lifespan in a knockin mouse model of epileptogenesis

Blockade of Angiotensin type 1 receptor (AT1R) has potential therapeutic utility in the treatment of numerous detrimental consequences of epileptogenesis, including oxidative stress, neuroinflammation, and blood-brain barrier (BBB) dysfunction. We have recently shown that many of these pathological processes play a critical role in seizure onset and propagation in the Scn8a-N1768D mouse model. Here we investigate the efficacy and potential mechanism(s) of action of candesartan (CND), an FDA-approved angiotensin receptor blocker (ARB) indicated for hypertension, in improving outcomes in this model of pediatric epilepsy. We compared length of lifespan, seizure frequency, and BBB permeability in juvenile (D/D) and adult (D/+) mice treated with CND at times after seizure onset. We performed RNAseq on hippocampal tissue to quantify differences in genome-wide patterns of transcript abundance and inferred beneficial and detrimental effects of canonical pathways identified by enrichment methods in untreated and treated mice. Our results demonstrate that treatment with CND gives rise to increased survival, longer periods of seizure freedom, and diminished BBB permeability. CND treatment also partially reversed or 'normalized' disease-induced genome-wide gene expression profiles associated with inhibition of NF-κB, TNFα, IL-6, and TGF-β signaling in juvenile and adult mice. Pathway analyses reveal that efficacy of CND is due to its known dual mechanism of action as both an AT1R antagonist and a PPARγ agonist. The robust effectiveness of CND across ages, sexes and mouse strains is a positive indication for its translation to humans and its suitability of use for clinical trials in children with SCN8A epilepsy.

Molecular and functional profiling of cell diversity and identity in the lateral superior olive, an auditory brainstem center with ascending and descending projections

The lateral superior olive (LSO), a prominent integration center in the auditory brainstem, contains a remarkably heterogeneous population of neurons. Ascending neurons, predominantly principal neurons (pLSOs), process interaural level differences for sound localization. Descending neurons (lateral olivocochlear neurons, LOCs) provide feedback into the cochlea and are thought to protect against acoustic overload. The molecular determinants of the neuronal diversity in the LSO are largely unknown. Here, we used patch-seq analysis in mice at postnatal days P10-12 to classify developing LSO neurons according to their functional and molecular profiles. Across the entire sample (n = 86 neurons), genes involved in ATP synthesis were particularly highly expressed, confirming the energy expenditure of auditory neurons. Two clusters were identified, pLSOs and LOCs. They were distinguished by 353 differentially expressed genes (DEGs), most of which were novel for the LSO. Electrophysiological analysis confirmed the transcriptomic clustering. We focused on genes affecting neuronal input-output properties and validated some of them by immunohistochemistry, electrophysiology, and pharmacology. These genes encode proteins such as osteopontin, Kv11.3, and Kvβ3 (pLSO-specific), calcitonin-gene-related peptide (LOC-specific), or Kv7.2 and Kv7.3 (no DEGs). We identified 12 "Super DEGs" and 12 genes showing "Cluster similarity." Collectively, we provide fundamental and comprehensive insights into the molecular composition of individual ascending and descending neurons in the juvenile auditory brainstem and how this may relate to their specific functions, including developmental aspects.

Loss of Intracellular Fibroblast Growth Factor 14 (iFGF14) Increases the Excitability of Mature Hippocampal and Cortical Pyramidal Neurons

Mutations in FGF14 , which encodes intracellular fibroblast growth factor 14 (iFGF14), have been linked to spinocerebellar ataxia type 27 (SCA27), a multisystem disorder associated with progressive deficits in motor coordination and cognitive function. Mice ( Fgf14 -/- ) lacking iFGF14 display similar phenotypes, and we have previously shown that the deficits in motor coordination reflect reduced excitability of cerebellar Purkinje neurons, owing to the loss of iFGF14-mediated regulation of the voltage-dependence of inactivation of the fast transient component of the voltage-gated Na + (Nav) current, I NaT . Here, we present the results of experiments designed to test the hypothesis that loss of iFGF14 also attenuates the intrinsic excitability of mature hippocampal and cortical pyramidal neurons. Current-clamp recordings from adult mouse hippocampal CA1 pyramidal neurons in acute in vitro slices, however, revealed that repetitive firing rates were higher in Fgf14 -/- , than in wild type (WT), cells. In addition, the waveforms of individual action potentials were altered in Fgf14 -/- hippocampal CA1 pyramidal neurons, and the loss of iFGF14 reduced the time delay between the initiation of axonal and somal action potentials. Voltage-clamp recordings revealed that the loss of iFGF14 altered the voltage-dependence of activation, but not inactivation, of I NaT in CA1 pyramidal neurons. Similar effects of the loss of iFGF14 on firing properties were evident in current-clamp recordings from layer 5 visual cortical pyramidal neurons. Additional experiments demonstrated that the loss of iFGF14 does not alter the distribution of anti-Nav1.6 or anti-ankyrin G immunofluorescence labeling intensity along the axon initial segments (AIS) of mature hippocampal CA1 or layer 5 visual cortical pyramidal neurons in situ . Taken together, the results demonstrate that, in contrast with results reported for neonatal (rat) hippocampal pyramidal neurons in dissociated cell culture, the loss of iFGF14 does not disrupt AIS architecture or Nav1.6 localization/distribution along the AIS of mature hippocampal (or cortical) pyramidal neurons in situ .

Beyond the Norm: A Report of a Rare Case of Sodium Channel 8 Alpha (SCN8A) Gene-Related Epilepsy Unveiled in a Nine-Year-Old Child

Sodium channel 8 alpha (SCN8A) mutations encompass a spectrum of epilepsy phenotypes with diverse clinical manifestations, posing diagnostic challenges. We present a case of a nine-year-old male with SCN8A gene-associated developmental and epileptic encephalopathies (DEEs), characterized by generalized tonic-clonic seizures (GTCS) since infancy. Despite treatment with multiple antiepileptic drugs (AEDs), including phenytoin, valproate, levetiracetam, carbamazepine, and clobazam, seizure control remained elusive, prompting genetic testing. Whole exome sequencing confirmed a heterozygous mutation (p.Phe210Ser) in SCN8A exon 6, indicative of DEE-13. Functional studies revealed a gain-of-function mechanism in SCN8A variants, resulting in heightened ion channel activity and altered voltage dependence of activation. Despite treatment adjustments, the patient's seizures persisted until topiramate was introduced, offering partial relief. SCN8A, encoding Nav1.6 sodium channels, modulates neuronal excitability, with mutations leading to increased persistent currents and hyperexcitability. Early seizure onset and developmental delays are hallmarks of SCN8A-related DEE. This case highlights the significance of genetic testing in refractory epilepsy management, guiding personalized treatment strategies. Sodium channel blockers like phenytoin and carbamazepine are often first-line therapies, while topiramate presents as a potential adjunctive option in SCN8A-related DEE. Overall, this case underscores the diagnostic and therapeutic complexities of managing SCN8A-related epileptic encephalopathy, emphasizing the importance of long-term monitoring and personalized treatment approaches for optimizing outcomes in refractory epilepsy.

TNFR1 signaling converging on FGF14 controls neuronal hyperactivity and sickness behavior in experimental cerebral malaria

BACKGROUND

Excess tumor necrosis factor (TNF) is implicated in the pathogenesis of hyperinflammatory experimental cerebral malaria (eCM), including gliosis, increased levels of fibrin(ogen) in the brain, behavioral changes, and mortality. However, the role of TNF in eCM within the brain parenchyma, particularly directly on neurons, remains underdefined. Here, we investigate electrophysiological consequences of eCM on neuronal excitability and cell signaling mechanisms that contribute to observed phenotypes.

METHODS

The split-luciferase complementation assay (LCA) was used to investigate cell signaling mechanisms downstream of tumor necrosis factor receptor 1 (TNFR1) that could contribute to changes in neuronal excitability in eCM. Whole-cell patch-clamp electrophysiology was performed in brain slices from eCM mice to elucidate consequences of infection on CA1 pyramidal neuron excitability and cell signaling mechanisms that contribute to observed phenotypes. Involvement of identified signaling molecules in mediating behavioral changes and sickness behavior observed in eCM were investigated in vivo using genetic silencing.

RESULTS

Exploring signaling mechanisms that underlie TNF-induced effects on neuronal excitability, we found that the complex assembly of fibroblast growth factor 14 (FGF14) and the voltage-gated Na+ (Nav) channel 1.6 (Nav1.6) is increased upon tumor necrosis factor receptor 1 (TNFR1) stimulation via Janus Kinase 2 (JAK2). On account of the dependency of hyperinflammatory experimental cerebral malaria (eCM) on TNF, we performed patch-clamp studies in slices from eCM mice and showed that Plasmodium chabaudi infection augments Nav1.6 channel conductance of CA1 pyramidal neurons through the TNFR1-JAK2-FGF14-Nav1.6 signaling network, which leads to hyperexcitability. Hyperexcitability of CA1 pyramidal neurons caused by infection was mitigated via an anti-TNF antibody and genetic silencing of FGF14 in CA1. Furthermore, knockdown of FGF14 in CA1 reduced sickness behavior caused by infection.

CONCLUSIONS

FGF14 may represent a therapeutic target for mitigating consequences of TNF-mediated neuroinflammation.

Acute toxicology report of the emerging marine biotoxin Brevetoxin 3 in mice: Food safety implications

Brevetoxins (PbTxs) are emerging marine toxins that can lead to Neurotoxic Shellfish Poisoning in humans by the ingestion of contaminated seafood. Recent reports on brevetoxin detection in shellfish in regions where it has not been described before, arise the need of updated guidelines to ensure seafood consumers safety. Our aim was to provide toxicological data for brevetoxin 3 (PbTx3) by assessing oral toxicity in mice and comparing it with intraperitoneal administration. We followed an Up-and-Down procedure administering PbTx3 to mice and registering clinical signs, neuromuscular function, histopathology, and blood changes. Neuromuscular dysfunction like seizures and ataxia, as well as loss of limb strength were observed at 6 h. Performance and clinical signs largely improved at 24 h, time at which no blood biochemical or histological alterations were detected independently of the administration route. However, PbTx3 oral administration results in lower toxicity than intraperitoneal administration. Mortality was only observed at 4000 μg/kg bw PbTx3 administered via oral, but we still found toxicity clinical signs at low toxin doses. We could stablish an oral Lowest-Observable-Adverse-Effect-Level for PbTx3 of 100 μg/kg bw and an oral No-Observable-Adverse-Effect-Level of 10 μg/kg bw in mice. The data here reported should be considered in the evaluation of risks of PbTxs for human health.

Role of NaV1.6-mediated persistent sodium current and bursting-pacemaker properties in breathing rhythm generation

Inspiration is the inexorable active phase of breathing. The brainstem pre-Bötzinger complex (preBötC) gives rise to inspiratory neural rhythm, but its underlying cellular and ionic bases remain unclear. The long-standing "pacemaker hypothesis" posits that the persistent Na+ current (INaP) that gives rise to bursting-pacemaker properties in preBötC interneurons is essential for rhythmogenesis. We tested the pacemaker hypothesis by conditionally knocking out and knocking down the Scn8a (Nav1.6 [voltage-gated sodium channel 1.6]) gene in core rhythmogenic preBötC neurons. Deleting Scn8a substantially decreases the INaP and abolishes bursting-pacemaker activity, which slows inspiratory rhythm in vitro and negatively impacts the postnatal development of ventilation. Diminishing Scn8a via genetic interference has no impact on breathing in adult mice. We argue that the Scn8a-mediated INaP is not obligatory but that it influences the development and rhythmic function of the preBötC. The ubiquity of the INaP in respiratory brainstem interneurons could underlie breathing-related behaviors such as neonatal phonation or rhythmogenesis in different physiological conditions.

Voltage-Gated Na+ Channels in Alzheimer's Disease: Physiological Roles and Therapeutic Potential

Alzheimer's disease (AD) is the most common cause of dementia and is classically characterized by two major histopathological abnormalities: extracellular plaques composed of amyloid beta (Aβ) and intracellular hyperphosphorylated tau. Due to the progressive nature of the disease, it is of the utmost importance to develop disease-modifying therapeutics that tackle AD pathology in its early stages. Attenuation of hippocampal hyperactivity, one of the earliest neuronal abnormalities observed in AD brains, has emerged as a promising strategy to ameliorate cognitive deficits and abate the spread of neurotoxic species. This aberrant hyperactivity has been attributed in part to the dysfunction of voltage-gated Na+ (Nav) channels, which are central mediators of neuronal excitability. Therefore, targeting Nav channels is a promising strategy for developing disease-modifying therapeutics that can correct aberrant neuronal phenotypes in early-stage AD. This review will explore the role of Nav channels in neuronal function, their connections to AD pathology, and their potential as therapeutic targets.

Genetic Testing in Children with Developmental and Epileptic Encephalopathies: A Review of Advances in Epilepsy Genomics

Advances in disease-related gene discovery have led to tremendous innovations in the field of epilepsy genetics. Identification of genetic mutations that cause epileptic encephalopathies has opened new avenues for the development of targeted therapies. Clinical testing using extensive gene panels, exomes, and genomes is currently accessible and has resulted in higher rates of diagnosis and better comprehension of the disease mechanisms underlying the condition. Children with developmental disabilities have a higher risk of developing epilepsy. As our understanding of the mechanisms underlying encephalopathies and epilepsies improves, there may be greater potential to develop innovative therapies tailored to an individual’s genotype. This article provides an overview of the significant progress in epilepsy genomics in recent years, with a focus on developmental and epileptic encephalopathies in children. The aim of this review is to enhance comprehension of the clinical utilization of genetic testing in this particular patient population. The development of effective and precise therapeutic strategies for epileptic encephalopathies may be facilitated by a comprehensive understanding of their molecular pathogenesis.

Age-dependent and modality-specific changes in the phenotypic markers Nav1.8, ASIC3, P2X3 and TRPM8 in male rat primary sensory neurons during healthy aging

The effects during healthy aging of the tetrodotoxin-resistant voltage-gated sodium channel 1.8 (Nav1.8), the acid-sensing ion channel-3 (ASIC3), the purinergic-receptor 2X3 (P2X3) and transient receptor potential of melastatin-8 (TRPM8) on responses to non-noxious stimuli are poorly understood. These effects will influence the transferability to geriatric subjects of findings obtained using young animals. To evaluate the involvement of these functional markers in mechanical and cold sensitivity to non-noxious stimuli and their underlying mechanisms, we used a combination of immunohistochemistry and quantitation of immunostaining in sub-populations of neurons of the dorsal root ganglia (DRG), behavioral tests, pharmacological interventions and Western-blot in healthy male Wistar rats from 3 to 24 months of age. We found significantly decreased sensitivity to mechanical and cold stimuli in geriatric rats. These behavioural alterations occurred simultaneously with differing changes in the expression of Nav1.8, ASIC3, P2X3 and TRPM8 in the DRG at different ages. Using pharmacological blockade in vivo we demonstrated the involvement of ASIC3 and P2X3 in normal mechanosensation and of Nav1.8 and ASIC3 in cold sensitivity. Geriatric rats also exhibited reductions in the number of A-like large neurons and in the proportion of peptidergic to non-peptidergic neurons. The changes in normal sensory physiology in geriatric rats we report here strongly support the inclusion of aged rodents as an important group in the design of pre-clinical studies evaluating pain treatments.

CaMKII Inhibition Attenuates Distinct Gain-of-Function Effects Produced by Mutant Nav1.6 Channels and Reduces Neuronal Excitability

Aberrant Nav1.6 activity can induce hyperexcitability associated with epilepsy. Gain-of-function mutations in the SCN8A gene encoding Nav1.6 are linked to epilepsy development; however, the molecular mechanisms mediating these changes are remarkably heterogeneous and may involve post-translational regulation of Nav1.6. Because calcium/calmodulin-dependent protein kinase II (CaMKII) is a powerful modulator of Nav1.6 channels, we investigated whether CaMKII modulates disease-linked Nav1.6 mutants. Whole-cell voltage clamp recordings in ND7/23 cells show that CaMKII inhibition of the epilepsy-related mutation R850Q largely recapitulates the effects previously observed for WT Nav1.6. We also characterized a rare missense variant, R639C, located within a regulatory hotspot for CaMKII modulation of Nav1.6. Prediction software algorithms and electrophysiological recordings revealed gain-of-function effects for R639C mutant channel activity, including increased sodium currents and hyperpolarized activation compared to WT Nav1.6. Importantly, the R639C mutation ablates CaMKII phosphorylation at a key regulatory site, T642, and, in contrast to WT and R850Q channels, displays a distinct response to CaMKII inhibition. Computational simulations demonstrate that modeled neurons harboring the R639C or R850Q mutations are hyperexcitable, and simulating the effects of CaMKII inhibition on Nav1.6 activity in modeled neurons differentially reduced hyperexcitability. Acute CaMKII inhibition may represent a promising mechanism to attenuate gain-of-function effects produced by Nav1.6 mutations.

Dendritic Excitability and Synaptic Plasticity In Vitro and In Vivo

Much of our understanding of dendritic and synaptic physiology comes from in vitro experimentation, where the afforded mechanical stability and convenience of applying drugs allowed patch-clamping based recording techniques to investigate ion channel distributions, their gating kinetics, and to uncover dendritic integrative and synaptic plasticity rules. However, with current efforts to study these questions in vivo, there is a great need to translate existing knowledge between in vitro and in vivo experimental conditions. In this review, we identify discrepancies between in vitro and in vivo ionic composition of extracellular media and discuss how changes in ionic composition alter dendritic excitability and plasticity induction. Here, we argue that under physiological in vivo ionic conditions, dendrites are expected to be more excitable and the threshold for synaptic plasticity induction to be lowered. Consequently, the plasticity rules described in vitro vary significantly from those implemented in vivo.

Glycogen Synthase Kinase 3: Ion Channels, Plasticity, and Diseases

Glycogen synthase kinase 3β (GSK3) is a multifaceted serine/threonine (S/T) kinase expressed in all eukaryotic cells. GSK3β is highly enriched in neurons in the central nervous system where it acts as a central hub for intracellular signaling downstream of receptors critical for neuronal function. Unlike other kinases, GSK3β is constitutively active, and its modulation mainly involves inhibition via upstream regulatory pathways rather than increased activation. Through an intricate converging signaling system, a fine-tuned balance of active and inactive GSK3β acts as a central point for the phosphorylation of numerous primed and unprimed substrates. Although the full range of molecular targets is still unknown, recent results show that voltage-gated ion channels are among the downstream targets of GSK3β. Here, we discuss the direct and indirect mechanisms by which GSK3β phosphorylates voltage-gated Na⁺ channels (Na_v1.2 and Na_v1.6) and voltage-gated K⁺ channels (K_v4 and K_v7) and their physiological effects on intrinsic excitability, neuronal plasticity, and behavior. We also present evidence for how unbalanced GSK3β activity can lead to maladaptive plasticity that ultimately renders neuronal circuitry more vulnerable, increasing the risk for developing neuropsychiatric disorders. In conclusion, GSK3β-dependent modulation of voltage-gated ion channels may serve as an important pharmacological target for neurotherapeutic development.

Naview: A d3.js Based JavaScript Library for Drawing and Annotating Voltage-Gated Sodium Channels Membrane Diagrams

Voltage-gated sodium channels (Nav) are membrane proteins essential to initiating and propagating action potential in neurons and other excitable cells. For a given organism there are often multiple, specialized sodium channels found in different tissues, whose mutations can cause deleterious effects observed in numerous diseases. Consequently, there is high medical and pharmacological interest in these proteins. Scientific literature often uses membrane diagrams to depict important patterns in these channels including the six transmembrane segments (S1-S6) present in four different homologous domains (D1-D4), the S4 voltage sensors, the pore-lining residue segments and the ion selectivity filter residues, glycosylation and phosphorylation residues, toxin binding sites and the inactivation loop, among others. Most of these diagrams are illustrated either digitally or by hand and programs specifically dedicated to the interactive and data-friendly generation of such visualizations are scarce or non-existing. This paper describes Naview, an open-source javascript visualization compatible with modern web browsers for the dynamic drawing and annotation of voltage-gated sodium channels membrane diagrams based on the D3.js library. By using a graphical user interface and combining user-defined annotations with optional UniProt code as inputs, Naview allows the creation and customization of membrane diagrams. In this interface, a user can also map and display important sodium channel properties, residues, regions and their relationships through symbols, colors, and edge connections. Such features can facilitate data exploration and provide fast, high-quality publication-ready graphics for this highly active area of research.

Inhibition of the Akt/PKB Kinase Increases Nav1.6-Mediated Currents and Neuronal Excitability in CA1 Hippocampal Pyramidal Neurons

In neurons, changes in Akt activity have been detected in response to the stimulation of transmembrane receptors. However, the mechanisms that lead to changes in neuronal function upon Akt inhibition are still poorly understood. In the present study, we interrogate how Akt inhibition could affect the activity of the neuronal Na_v channels with while impacting intrinsic excitability. To that end, we employed voltage-clamp electrophysiological recordings in heterologous cells expressing the Na_v1.6 channel isoform and in hippocampal CA1 pyramidal neurons in the presence of triciribine, an inhibitor of Akt. We showed that in both systems, Akt inhibition resulted in a potentiation of peak transient Na+ current (I_Na) density. Akt inhibition correspondingly led to an increase in the action potential firing of the CA1 pyramidal neurons that was accompanied by a decrease in the action potential current threshold. Complementary confocal analysis in the CA1 pyramidal neurons showed that the inhibition of Akt is associated with the lengthening of Na_v1.6 fluorescent intensity along the axonal initial segment (AIS), providing a mechanism for augmented neuronal excitability. Taken together, these findings provide evidence that Akt-mediated signal transduction might affect neuronal excitability in a Na_v1.6-dependent manner.

The Na+/Ca2+ Exchanger 3 Is Functionally Coupled With the NaV1.6 Voltage-Gated Channel and Promotes an Endoplasmic Reticulum Ca2+ Refilling in a Transgenic Model of Alzheimer's Disease

The remodelling of neuronal ionic homeostasis by altered channels and transporters is a critical feature of the Alzheimer's disease (AD) pathogenesis. Different reports converge on the concept that the Na⁺/Ca²⁺ exchanger (NCX), as one of the main regulators of Na⁺ and Ca²⁺ concentrations and signalling, could exert a neuroprotective role in AD. The activity of NCX has been found to be increased in AD brains, where it seemed to correlate with an increased neuronal survival. Moreover, the enhancement of the NCX3 currents (I_NCX) in primary neurons treated with the neurotoxic amyloid β 1-42 (Aβ_1-42) oligomers prevented the endoplasmic reticulum (ER) stress and neuronal death. The present study has been designed to investigate any possible modulation of the I_NCX, the functional interaction between NCX and the Na_V1.6 channel, and their impact on the Ca²⁺ homeostasis in a transgenic in vitro model of AD, the primary hippocampal neurons from the Tg2576 mouse, which overproduce the Aβ_1-42 peptide. Electrophysiological studies, carried in the presence of siRNA and the isoform-selective NCX inhibitor KB-R7943, showed that the activity of a specific NCX isoform, NCX3, was upregulated in its reverse, Ca²⁺ influx mode of operation in the Tg2576 neurons. The enhanced NCX activity contributed, in turn, to increase the ER Ca²⁺ content, without affecting the cytosolic Ca²⁺ concentrations of the Tg2576 neurons. Interestingly, our experiments have also uncovered a functional coupling between NCX3 and the voltage-gated Na_V1.6 channels. In particular, the increased Na_V1.6 currents appeared to be responsible for the upregulation of the reverse mode of NCX3, since both TTX and the Streptomyces griseolus antibiotic anisomycin, by reducing the Na_V1.6 currents, counteracted the increase of the I_NCX in the Tg2576 neurons. In agreement, our immunofluorescence analyses revealed that the NCX3/Na_V1.6 co-expression was increased in the Tg2576 hippocampal neurons in comparison with the WT neurons. Collectively, these findings indicate that NCX3 might intervene in the Ca²⁺ remodelling occurring in the Tg2576 primary neurons thus emerging as a molecular target with a neuroprotective potential, and provide a new outcome of the Na_V1.6 upregulation related to the modulation of the intracellular Ca²⁺ concentrations in AD neurons.

Effective Accentuation of Voltage-Gated Sodium Current Caused by Apocynin (4'-Hydroxy-3'-methoxyacetophenone), a Known NADPH-Oxidase Inhibitor

Apocynin (aPO, 4'-Hydroxy-3'-methoxyacetophenone) is a cell-permeable, anti-inflammatory phenolic compound that acts as an inhibitor of NADPH-dependent oxidase (NOX). However, the mechanisms through which aPO can interact directly with plasmalemmal ionic channels to perturb the amplitude or gating of ionic currents in excitable cells remain incompletely understood. Herein, we aimed to investigate any modifications of aPO on ionic currents in pituitary GH₃ cells or murine HL-1 cardiomyocytes. In whole-cell current recordings, GH₃-cell exposure to aPO effectively stimulated the peak and late components of voltage-gated Na+ current (I_Na) with different potencies. The EC₅₀ value of aPO required for its differential increase in peak or late I_Na in GH₃ cells was estimated to be 13.2 or 2.8 μM, respectively, whereas the K_D value required for its retardation in the slow component of current inactivation was 3.4 μM. The current-voltage relation of I_Na was shifted slightly to more negative potential during cell exposure to aPO (10 μM); however, the steady-state inactivation curve of the current was shifted in a rightward direction in its presence. Recovery of peak I_Na inactivation was increased in the presence of 10 μM aPO. In continued presence of aPO, further application of rufinamide or ranolazine attenuated aPO-stimulated I_Na. In methylglyoxal- or superoxide dismutase-treated cells, the stimulatory effect of aPO on peak I_Na remained effective. By using upright isosceles-triangular ramp pulse of varying duration, the amplitude of persistent I_Na measured at low or high threshold was enhanced by the aPO presence, along with increased hysteretic strength appearing at low or high threshold. The addition of aPO (10 μM) mildly inhibited the amplitude of erg-mediated K+ current. Likewise, in HL-1 murine cardiomyocytes, the aPO presence increased the peak amplitude of I_Na as well as decreased the inactivation or deactivation rate of the current, and further addition of ranolazine or esaxerenone attenuated aPO-accentuated I_Na. Altogether, this study provides a distinctive yet unidentified finding that, despite its effectiveness in suppressing NOX activity, aPO may directly and concertedly perturb the amplitude, gating and voltage-dependent hysteresis of I_Na in electrically excitable cells. The interaction of aPO with ionic currents may, at least in part, contribute to the underlying mechanisms through which it affects neuroendocrine, endocrine or cardiac function.