Going native: voltage-gated potassium channels controlling neuronal excitability

Targeting heterozygous dominant negative variant of KCNA2 using Gapmer ASO for the treatment of drug-resistant epilepsy

A missense mutation c.1220C>G of KCN2A gene was recently identified in an infant with epilepsy. KCNA2 encodes KV1.2 subunits that form voltage-gated potassium channels (VGKC) via tetrameric assembly. The mutation results in amino acid change P407R at the highly conserved PVP motif. Functional characterization revealed that mutant KV1.2_P407R subunits formed loss-of-function channels and suppressed both KV1.2 and KV1.1 channel activities. Hetero-tetrameric assembly of the KV1.2_P407R subunits with other neuronal voltage-gated potassium channels of Shaker subfamily could lead to general deficit of repolarizing potassium current and potentially underlie the enhanced seizure susceptibility. Indeed, expression of human KV1.2_P407R in early postnatal rat cortical neurons or genetically engineered hESC-derived neurons disclosed broadening of action potential duration and early afterdepolarization (EAD), associating with reduced potassium current. We hypothesize that Gapmer antisense oligonucleotides (ASOs) targeted to c.1220C>G mutation will selectively degrade the mutant mRNA while allowing the remaining wild-type (WT) subunits to form functional channels. As a proof of principle, delivery of Gapmer packaged in lipid nanoparticle into cortical neurons selectively suppressed KV1.2_P407R over the WT protein expression, reversing the broadening of action potential duration, abrogating the EAD and leading to overall increase in potassium current.

Molecular logic for cellular specializations that initiate the auditory parallel processing pathways

The cochlear nuclear complex (CN), the starting point for all central auditory processing, comprises a suite of neuronal cell types that are highly specialized for neural coding of acoustic signals, yet molecular logic governing cellular specializations remains unknown. By combining single-nucleus RNA sequencing and Patch-seq analysis, we reveal a set of transcriptionally distinct cell populations encompassing all previously observed types and discover multiple new subtypes with anatomical and physiological identity. The resulting comprehensive cell-type taxonomy reconciles anatomical position, morphological, physiological, and molecular criteria, enabling the determination of the molecular basis of the remarkable cellular phenotypes in the CN. In particular, CN cell-type identity is encoded in a transcriptional architecture that orchestrates functionally congruent expression across a small set of gene families to customize projection patterns, input-output synaptic communication, and biophysical features required for encoding distinct aspects of acoustic signals. This high-resolution account of cellular heterogeneity from the molecular to the circuit level illustrates molecular logic for cellular specializations and enables genetic dissection of auditory processing and hearing disorders with unprecedented specificity.

The involvement of K+ channels in depression and pharmacological effects of antidepressants on these channels

Depression is a common and complex psychiatric illness with multiple clinical symptoms, even leading to the disability and suicide. Owing to the partial understanding of the pathogenesis of depressive-like disorders, available pharmacotherapeutic strategies are developed mainly based on the "monoamine hypothesis", resulting in a limited effectiveness and a number of adverse effects in the clinical practice. The concept of multiple pathogenic factors be helpful for clarifying the etiology of depression and developing the antidepressants. It is well documented that K+ channels serve crucial roles in modulating the neuronal excitability and neurotransmitter release in the brain, and abnormality of these channels participated in the pathogenic process of diverse central nervous system (CNS) pathologies, such as seizure and Alzheimer's disease (AD). The clinical and preclinical evidence also delineates that the involvement of several types of K+ channels in depressive-like behaviors appear to be evident, suggesting these channels being one of the multiple factors in the etiology of this debilitating disorder. Emerging data manifest that diverse antidepressants impact distinct K+ channels, such as Kv, Kir and K2P, meaning the functioning of these drug via a "multi-target" manner. On the other hand, the scenario of antidepressants impinging K+ channels could render an alternative interpretation for the pharmacological effectiveness and numerous side effects in clinical trials. Furthermore, these channels serve to be considered as a "druggable target" to develop novel therapeutic compound to antagonize this psychiatry.

The upregulation of K+ and HCN channels in developing spiral ganglion neurons is mediated by cochlear inner hair cells

Spiral ganglion neurons (SGNs) are primary sensory afferent neurons that relay acoustic information from the cochlear inner hair cells (IHCs) to the brainstem. The response properties of different SGNs diverge to represent a wide range of sound intensities in an action-potential code. This biophysical heterogeneity is established during pre-hearing stages of development, a time when IHCs fire spontaneous Ca2+ action potentials that drive glutamate release from their ribbon synapses onto the SGN terminals. The role of spontaneous IHC activity in the refinement of SGN characteristics is still largely unknown. Using pre-hearing otoferlin knockout mice (Otof-/-), in which Ca2+-dependent exocytosis in IHCs is abolished, we found that developing SGNs fail to upregulate low-voltage-activated K+-channels and hyperpolarisation-activated cyclic-nucleotide-gated channels. This delayed maturation resulted in hyperexcitable SGNs with immature firing characteristics. We have also shown that SGNs that synapse with the pillar side of the IHCs selectively express a resurgent K+ current, highlighting a novel biophysical marker for these neurons. RNA-sequencing showed that several K+ channels are downregulated in Otof-/- mice, further supporting the electrophysiological recordings. Our data demonstrate that spontaneous Ca2+-dependent activity in pre-hearing IHCs regulates some of the key biophysical and molecular features of the developing SGNs. KEY POINTS: Ca2+-dependent exocytosis in inner hair cells (IHCs) is otoferlin-dependent as early as postnatal day 1. A lack of otoferlin in IHCs affects potassium channel expression in SGNs. The absence of otoferlin is associated with SGN hyperexcitability. We propose that type I spiral ganglion neuron functional maturation depends on IHC exocytosis.

Functional properties of a disease mutation for migraine in Kv2.1/6.4 channels

Voltage-gated potassium (Kv) channels are integral to cellular excitability, impacting the resting membrane potential, repolarization, and shaping action potentials in neurons and cardiac myocytes. Structurally, Kv channels are homo or heterotetramers comprising four α-subunits, each with six transmembrane segments (S1-S6). Silent Kv (KvS), includes Kv5.1, Kv6.1-6.4, Kv8.1-8.2, and Kv9.1-9.3, they do not form functional channels on their own but modulate the properties of heteromeric channels. Recent studies have identified the Kv6.4 subunit as a significant modulator within heteromeric channels, such as Kv2.16.4. The Kv2.16.4 heteromer exhibits altered biophysical properties, including a shift in voltage-dependent inactivation and a complex activation. Current genetic studies in migraine patients have revealed a single missense mutation in the Kv6.4 gene. The single missense mutation, L360P is in the highly conserved S4-S5 linker region. This study aims to demonstrate the biophysical effects of the L360P mutation in Kv2.1 6.4 channels with a fixed 2:2 stoichiometry, using monomeric (Kv2.1/6.4) and tandem dimer (Kv2.1_6.4) configurations. Our findings suggest that the L360P mutation significantly impacts the function and regulation of Kv2.1/6.4 channels, providing insights into the molecular mechanisms underlying channel dysfunction in migraine pathology.

Integrate-and-fire-type models of the lateral superior olive

Neurons of the lateral superior olive (LSO) in the auditory brainstem play a fundamental role in binaural sound localization. Previous theoretical studies developed various types of neuronal models to study the physiological functions of the LSO. These models were usually tuned to a small set of physiological data with specific aims in mind. Therefore, it is unclear whether and how they can be related to each other, how widely applicable they are, and which model is suitable for what purposes. In this study, we address these questions for six different single-compartment integrate-and-fire (IF) type LSO models. The models are divided into two groups depending on their subthreshold responses: passive (linear) models with only the leak conductance and active (nonlinear) models with an additional low-voltage-activated potassium conductance that is prevalent among the auditory system. Each of these two groups is further subdivided into three subtypes according to the spike generation mechanism: one with simple threshold-crossing detection and voltage reset, one with threshold-crossing detection plus a current to mimic spike shapes, and one with a depolarizing exponential current for spiking. In our simulations, all six models were driven by identical synaptic inputs and calibrated with common criteria for binaural tuning. The resulting spike rates of the passive models were higher for intensive inputs and lower for temporally structured inputs than those of the active models, confirming the active function of the potassium current. Within each passive or active group, the simulated responses resembled each other, regardless of the spike generation types. These results, in combination with the analysis of computational costs, indicate that an active IF model is more suitable than a passive model for accurately reproducing temporal coding of LSO. The simulation of realistic spike shapes with an extended spiking mechanism added relatively small computational costs.

Effect of Short-Term Restraint Stress on the Hypothalamic Transcriptome Profiles of Rats with Inherited Stress-Induced Arterial Hypertension (ISIAH) and Normotensive Wistar Albino Glaxo (WAG) Rats

Emotional stress is one of the health risk factors in the modern human lifestyle. Stress exposure can provoke the manifestation of various pathological conditions, one of which is a sharp increase in the blood pressure level. In the present study, we analyzed changes in the transcriptome profiles of the hypothalamus of hypertensive ISIAH and normotensive WAG rats exposed to a single short-term restraint stress (the rat was placed in a tight wire-mesh cage for 2 h). This type of stress can be considered emotional stress. The functional annotation of differentially expressed genes allowed us to identify the most significantly altered biological processes in the hypothalamus of hypertensive and normotensive rats. The study made it possible to identify a group of genes that describe a general response to stress, independent of the rat genotype, as well as a hypothalamic response to stress specific to each strain. The alternatively changing expression of the Npas4 (neuronal PAS domain protein 4) gene, which is downregulated in the hypothalamus of the control WAG rats and induced in the hypothalamus of hypertensive ISIAH rats, is suggested to be the key event for understanding inter-strain differences in the hypothalamic response to stress. The stress-dependent ISIAH strain-specific induction of Fos and Jun gene transcription may play a crucial role in neuronal activation in this rat strain. The data obtained can be potentially useful in the selection of molecular targets for the development of pharmacological approaches to the correction of stress-induced pathologies related to neuronal excitability, taking into account the hypertensive status of the patients.

The downregulation of Kv1 channels in Lgi1-/-mice is accompanied by a profound modification of its interactome and a parallel decrease in Kv2 channels

In animal models of LGI1-dependent autosomal dominant lateral temporal lobe epilepsy, Kv1 channels are downregulated, suggesting their crucial involvement in epileptogenesis. The molecular basis of Kv1 channel-downregulation in LGI1 knock-out mice has not been elucidated and how the absence of this extracellular protein induces an important modification in the expression of Kv1 remains unknown. In this study we analyse by immunofluorescence the modifications in neuronal Kv1.1 and Kv1.2 distribution throughout the hippocampal formation of LGI1 knock-out mice. We show that Kv1 downregulation is not restricted to the axonal compartment, but also takes place in the somatodendritic region and is accompanied by a drastic decrease in Kv2 expression levels. Moreover, we find that the downregulation of these Kv channels is associated with a marked increase in bursting patterns. Finally, mass spectrometry uncovered key modifications in the Kv1 interactome that highlight the epileptogenic implication of Kv1 downregulation in LGI1 knock-out animals.

Ion Channel and Transporter Involvement in Chemotherapy-Induced Peripheral Neurotoxicity

The peripheral nervous system can encounter alterations due to exposure to some of the most commonly used anticancer drugs (platinum drugs, taxanes, vinca alkaloids, proteasome inhibitors, thalidomide), the so-called chemotherapy-induced peripheral neurotoxicity (CIPN). CIPN can be long-lasting or even permanent, and it is detrimental for the quality of life of cancer survivors, being associated with persistent disturbances such as sensory loss and neuropathic pain at limb extremities due to a mostly sensory axonal polyneuropathy/neuronopathy. In the state of the art, there is no efficacious preventive/curative treatment for this condition. Among the reasons for this unmet clinical and scientific need, there is an uncomplete knowledge of the pathogenetic mechanisms. Ion channels and transporters are pivotal elements in both the central and peripheral nervous system, and there is a growing body of literature suggesting that they might play a role in CIPN development. In this review, we first describe the biophysical properties of these targets and then report existing data for the involvement of ion channels and transporters in CIPN, thus paving the way for new approaches/druggable targets to cure and/or prevent CIPN.

Mature neurons from iPSCs unveil neurodegeneration-related pathways in mucopolysaccharidosis type II: GSK-3β inhibition for therapeutic potential

Mucopolysaccharidosis (MPS) type II is caused by a deficiency of iduronate-2-sulfatase and is characterized by the accumulation of glycosaminoglycans (GAGs). Without effective therapy, the severe form of MPS II causes progressive neurodegeneration and death. This study generated multiple clones of induced pluripotent stem cells (iPSCs) and their isogenic controls (ISO) from four patients with MPS II neurodegeneration. MPS II-iPSCs were successfully differentiated into cortical neurons with characteristic biochemical and cellular phenotypes, including axonal beadings positive for phosphorylated tau, and unique electrophysiological abnormalities, which were mostly rescued in ISO-iPSC-derived neurons. RNA sequencing analysis uncovered dysregulation in three major signaling pathways, including Wnt/β-catenin, p38 MAP kinase, and calcium pathways, in mature MPS II neurons. Further mechanistic characterization indicated that the dysregulation in calcium signaling led to an elevated intracellular calcium level, which might be linked to compromised survival of neurons. Based on these dysregulated pathways, several related chemicals and drugs were tested using this mature MPS II neuron-based platform and a small-molecule glycogen synthase kinase-3β inhibitor was found to significantly rescue neuronal survival, neurite morphology, and electrophysiological abnormalities in MPS II neurons. Our results underscore that the MPS II-iPSC-based platform significantly contributes to unraveling the mechanisms underlying the degeneration and death of MPS II neurons and assessing potential drug candidates. Furthermore, the study revealed that targeting the specific dysregulation of signaling pathways downstream of GAG accumulation in MPS II neurons with a well-characterized drug could potentially ameliorate neuronal degeneration.

Mono-allelic KCNB2 variants lead to a neurodevelopmental syndrome caused by altered channel inactivation

Ion channels mediate voltage fluxes or action potentials that are central to the functioning of excitable cells such as neurons. The KCNB family of voltage-gated potassium channels (Kv) consists of two members (KCNB1 and KCNB2) encoded by KCNB1 and KCNB2, respectively. These channels are major contributors to delayed rectifier potassium currents arising from the neuronal soma which modulate overall excitability of neurons. In this study, we identified several mono-allelic pathogenic missense variants in KCNB2, in individuals with a neurodevelopmental syndrome with epilepsy and autism in some individuals. Recurrent dysmorphisms included a broad forehead, synophrys, and digital anomalies. Additionally, we selected three variants where genetic transmission has not been assessed, from two epilepsy studies, for inclusion in our experiments. We characterized channel properties of these variants by expressing them in oocytes of Xenopus laevis and conducting cut-open oocyte voltage clamp electrophysiology. Our datasets indicate no significant change in absolute conductance and conductance-voltage relationships of most disease variants as compared to wild type (WT), when expressed either alone or co-expressed with WT-KCNB2. However, variants c.1141A>G (p.Thr381Ala) and c.641C>T (p.Thr214Met) show complete abrogation of currents when expressed alone with the former exhibiting a left shift in activation midpoint when expressed alone or with WT-KCNB2. The variants we studied, nevertheless, show collective features of increased inactivation shifted to hyperpolarized potentials. We suggest that the effects of the variants on channel inactivation result in hyper-excitability of neurons, which contributes to disease manifestations.

Roles of KCNA2 in Neurological Diseases: from Physiology to Pathology

Potassium voltage-gated channel subfamily a member 2 (Kv1.2, encoded by KCNA2) is highly expressed in the central and peripheral nervous systems. Based on the patch clamp studies, gain-of function (GOF), loss-of-function (LOF), and a mixed type (GOF/LOF) variants can cause different conditions/disorders. KCNA2-related neurological diseases include epilepsy, intellectual disability (ID), attention deficit/hyperactive disorder (ADHD), autism spectrum disorder (ASD), pain as well as autoimmune and movement disorders. Currently, the molecular mechanisms for the reported variants in causing diverse disorders are unknown. Consequently, this review brings up to date the related information regarding the structure and function of Kv1.2 channel, expression patterns, neuronal localizations, and tetramerization as well as important cell and animal models. In addition, it provides updates on human genetic variants, genotype-phenotype correlations especially highlighting the deep insight into clinical prognosis of KCNA2-related developmental and epileptic encephalopathy, mechanisms, and the potential treatment targets for all KCNA2-related neurological disorders.

Development and prenatal exposure to androgens alter potassium currents in gonadotropin-releasing hormone neurons from female mice

Pulsatile gonadotropin-releasing hormone (GnRH) release is critical for reproduction. Disruptions to GnRH secretion patterns may contribute to polycystic ovary syndrome (PCOS). Prenatally androgenized (PNA) female mice recapitulate many neuroendocrine abnormalities observed in PCOS patients. PNA and development induce changes in spontaneous GnRH neuron firing rate, response to synaptic input, and the afterhyperpolarization potential of the action potential. We hypothesized potassium currents are altered by PNA treatment and/or development. Whole-cell patch-clamp recordings were made of transient and residual potassium currents of GnRH neurons in brain slices from 3-week-old and adult control and PNA females. At 3 weeks of age, PNA treatment increased transient current density versus controls. Development and PNA altered voltage-dependent activation and inactivation of the transient current. In controls, transient current activation and inactivation were depolarized at 3 weeks of age versus in adulthood. In GnRH neurons from 3-week-old mice, transient current activation and inactivation were more depolarized in control than PNA mice. Development and PNA treatment interacted to shift the time-dependence of inactivation and recovery from inactivation. Notably, in cells from adult PNA females, recovery was prolonged compared to all other groups. Activation of the residual current occurred at more depolarized membrane potentials in 3-week-old than adult controls. PNA depolarized activation of the residual current in adults. These findings demonstrate the properties of GnRH neuron potassium currents change during typical development, potentially contributing to puberty, and further suggest PNA treatment may both alter some typical developmental changes and induce additional modifications, which together may underlie aspects of the PNA phenotype. There was not any clinical trial involved in this work.

KCNA2 IgG autoimmunity in neuropsychiatric diseases

BACKGROUND

Autoantibodies against the potassium voltage-gated channel subfamily A member 2 (KCNA2) have been described in a few cases of neuropsychiatric disorders, but their diagnostic and pathophysiological role is currently unknown, imposing challenges to medical practice.

DESIGN / METHODS

We retrospectively collected comprehensive clinical and paraclinical data of 35 patients with KCNA2 IgG autoantibodies detected in cell-based and tissue-based assays. Patients' sera and cerebrospinal fluid (CSF) were used for characterization of the antigen, clinical-serological correlations, and determination of IgG subclasses.

RESULTS

KCNA2 autoantibody-positive patients (n = 35, median age at disease onset of 65 years, range of 16-83 years, 74 % male) mostly presented with cognitive impairment and/or epileptic seizures but also ataxia, gait disorder and personality changes. Serum autoantibodies belonged to IgG3 and IgG1 subclasses and titers ranged from 1:32 to 1:10,000. KCNA2 IgG was found in the CSF of 8/21 (38 %) patients and in the serum of 4/96 (4.2 %) healthy blood donors. KCNA2 autoantibodies bound to characteristic anatomical areas in the cerebellum and hippocampus of mammalian brain and juxtaparanodal regions of peripheral nerves but reacted exclusively with intracellular epitopes. A subset of four KCNA2 autoantibody-positive patients responded markedly to immunotherapy alongside with conversion to seronegativity, in particular those presenting an autoimmune encephalitis phenotype and receiving early immunotherapy. An available brain biopsy showed strong immune cell invasion. KCNA2 autoantibodies occurred in less than 10 % in association with an underlying tumor.

CONCLUSION

Our data suggest that KCNA2 autoimmunity is clinically heterogeneous. Future studies should determine whether KCNA2 autoantibodies are directly pathogenic or develop secondarily. Early immunotherapy should be considered, in particular if autoantibodies occur in CSF or if clinical or diagnostic findings suggest ongoing inflammation. Suspicious clinical phenotypes include autoimmune encephalitis, atypical dementia, new-onset epilepsy and unexplained epileptic seizures.

Calcium-Sensitive Subthreshold Oscillations and Electrical Coupling in Principal Cells of Mouse Dorsal Cochlear Nucleus

In higher sensory brain regions, slow oscillations (0.5-5 Hz) associated with quiet wakefulness and attention modulate multisensory integration, predictive coding, and perception. Although often assumed to originate via thalamocortical mechanisms, the extent to which subcortical sensory pathways are independently capable of slow oscillatory activity is unclear. We find that in the first station for auditory processing, the cochlear nucleus, fusiform cells from juvenile mice (of either sex) generate robust 1-2 Hz oscillations in membrane potential and exhibit electrical resonance. Such oscillations were absent prior to the onset of hearing, intrinsically generated by hyperpolarization-activated cyclic nucleotide-gated (HCN) and persistent Na+ conductances (NaP) interacting with passive membrane properties, and reflected the intrinsic resonance properties of fusiform cells. Cx36-containing gap junctions facilitated oscillation strength and promoted pairwise synchrony of oscillations between neighboring neurons. The strength of oscillations were strikingly sensitive to external Ca2+, disappearing at concentrations >1.7 mM, due in part to the shunting effect of small-conductance calcium-activated potassium (SK) channels. This effect explains their apparent absence in previous in vitro studies of cochlear nucleus which routinely employed high-Ca2+ extracellular solution. In contrast, oscillations were amplified in reduced Ca2+ solutions, due to relief of suppression by Ca2+ of Na+ channel gating. Our results thus reveal mechanisms for synchronous oscillatory activity in auditory brainstem, suggesting that slow oscillations, and by extension their perceptual effects, may originate at the earliest stages of sensory processing.

Motor control: Snake neurons speed up

How are motor neurons tuned for very different jobs? Classic work has focused on variations in motor neuron size and their premotor networks. New results in rattlesnakes show that shifting a motor neuron's temporal precision can be as simple as changing its potassium channel conductance.

Kv2 channels contribute to neuronal activity within the vagal afferent-nTS reflex arc

Diversity in the functional expression of ion channels contributes to the unique patterns of activity generated in visceral sensory A-type myelinated neurons versus C-type unmyelinated neurons in response to their natural stimuli. In the present study, Kv2 channels were identified as underlying a previously uncharacterized delayed rectifying potassium current expressed in both A- and C-type nodose ganglion neurons. Kv2.1 and 2.2 appear confined to the soma and initial segment of these sensory neurons; however, neither was identified in their central presynaptic terminals projecting onto relay neurons in the nucleus of the solitary tract (nTS). Kv2.1 and Kv2.2 were also not detected in the peripheral axons and sensory terminals in the aortic arch. Functionally, in nodose neuron somas, Kv2 currents exhibited frequency-dependent current inactivation and contributed to action potential repolarization in C-type neurons but not A-type neurons. Within the nTS, the block of Kv2 currents does not influence afferent presynaptic calcium influx or glutamate release in response to afferent activation, supporting our immunohistochemical observations. On the other hand, Kv2 channels contribute to membrane hyperpolarization and limit action potential discharge rate in second-order neurons. Together, these data demonstrate that Kv2 channels influence neuronal discharge within the vagal afferent-nTS circuit and indicate they may play a significant role in viscerosensory reflex function.NEW & NOTEWORTHY We demonstrate the expression and function of the voltage-gated delayed rectifier potassium channel Kv2 in vagal nodose neurons. Within sensory neurons, Kv2 channels limit the width of the broader C-type but not narrow A-type action potential. Within the nucleus of the solitary tract (nTS), the location of the vagal terminal field, Kv2 does not influence glutamate release. However, Kv2 limits the action potential discharge of nTS relay neurons. These data suggest a critical role for Kv2 in the vagal-nTS reflex arc.

High calcium concentrations reduce cellular excitability of mouse MNTB neurons

Calcium, a universal intracellular signaling molecule, plays essential roles in neural functions. Historically, in most in vitro brain slice electrophysiology studies, the extracellular calcium concentration ([Ca2+]e) in artificial cerebrospinal fluid is of a wide range and typically higher than the physiological value. At high [Ca2+]e, synaptic transmission is generally enhanced. However, the effects and the underlying mechanisms of calcium on intrinsic neuronal properties are diverse. Using whole-cell patch clamp in acute brainstem slices obtained from mice of either sex, we investigated the effects and the underlying mechanisms of high [Ca2+]e on intrinsic neuronal properties of neurons in the medial nucleus of the trapezoid body (MNTB), an auditory brainstem component in the sound localization circuitry. Compared to the physiological [Ca2+]e (1.2 mM), high [Ca2+]e at 1.8 and 2.4 mM significantly reduced the cellular excitability of MNTB neurons, resulting in decreased spike firing rate, depolarized spike threshold, and decreased the ability to follow high frequency inputs. High extracellular magnesium concentrations at 1.8 and 2.4 mM produced similar but less robust effects, due to surface charge screening. Upon high calcium application, voltage-gated sodium channel currents remained largely unchanged. Calcium-sensing receptors were detected in MNTB neurons, but blocking these receptors did not eliminate the effects of high calcium on spontaneous spiking. We attribute the lack of significant effects in these last two experiments to the moderate changes in calcium we tested. Our results call for the use of physiological [Ca2+]e in brain slice experiments.

Neuropilin-1 is essential for vascular endothelial growth factor A-mediated increase of sensory neuron activity and development of pain-like behaviors

ABSTRACT

Neuropilin-1 (NRP-1) is a transmembrane glycoprotein that binds numerous ligands including vascular endothelial growth factor A (VEGFA). Binding of this ligand to NRP-1 and the co-receptor, the tyrosine kinase receptor VEGFR2, elicits nociceptor sensitization resulting in pain through the enhancement of the activity of voltage-gated sodium and calcium channels. We previously reported that blocking the interaction between VEGFA and NRP-1 with the Spike protein of SARS-CoV-2 attenuates VEGFA-induced dorsal root ganglion (DRG) neuronal excitability and alleviates neuropathic pain, pointing to the VEGFA/NRP-1 signaling as a novel therapeutic target of pain. Here, we investigated whether peripheral sensory neurons and spinal cord hyperexcitability and pain behaviors were affected by the loss of NRP-1. Nrp-1 is expressed in both peptidergic and nonpeptidergic sensory neurons. A CRIPSR/Cas9 strategy targeting the second exon of nrp-1 gene was used to knockdown NRP-1. Neuropilin-1 editing in DRG neurons reduced VEGFA-mediated increases in CaV2.2 currents and sodium currents through NaV1.7. Neuropilin-1 editing had no impact on voltage-gated potassium channels. Following in vivo editing of NRP-1, lumbar dorsal horn slices showed a decrease in the frequency of VEGFA-mediated increases in spontaneous excitatory postsynaptic currents. Finally, intrathecal injection of a lentivirus packaged with an NRP-1 guide RNA and Cas9 enzyme prevented spinal nerve injury-induced mechanical allodynia and thermal hyperalgesia in both male and female rats. Collectively, our findings highlight a key role of NRP-1 in modulating pain pathways in the sensory nervous system.

WNK1 promotes water homeostasis by acting as a central osmolality sensor for arginine vasopressin release

Maintaining internal osmolality constancy is essential for life. Release of arginine vasopressin (AVP) in response to hyperosmolality is critical. Current hypotheses for osmolality sensors in circumventricular organs (CVOs) of the brain focus on mechanosensitive membrane proteins. The present study demonstrated that intracellular protein kinase WNK1 was involved. Focusing on vascular-organ-of-lamina-terminalis (OVLT) nuclei, we showed that WNK1 kinase was activated by water restriction. Neuron-specific conditional KO (cKO) of Wnk1 caused polyuria with decreased urine osmolality that persisted in water restriction and blunted water restriction-induced AVP release. Wnk1 cKO also blunted mannitol-induced AVP release but had no effect on osmotic thirst response. The role of WNK1 in the osmosensory neurons in CVOs was supported by neuronal pathway tracing. Hyperosmolality-induced increases in action potential firing in OVLT neurons was blunted by Wnk1 deletion or pharmacological WNK inhibitors. Knockdown of Kv3.1 channel in OVLT by shRNA reproduced the phenotypes. Thus, WNK1 in osmosensory neurons in CVOs detects extracellular hypertonicity and mediates the increase in AVP release by activating Kv3.1 and increasing action potential firing from osmosensory neurons.

Chronic intermittent ethanol exposure differentially alters the excitability of neurons in the orbitofrontal cortex and basolateral amygdala that project to the dorsal striatum

Alcohol use disorder is associated with altered neuron function including those in orbitofrontal cortex (OFC) and basolateral amygdala (BLA) that send glutamatergic inputs to areas of the dorsal striatum (DS) that mediate goal and habit directed actions. Previous studies reported that chronic intermittent (CIE) exposure to ethanol alters the electrophysiological properties of OFC and BLA neurons, although projection targets for these neurons were not identified. In this study, we used male and female mice and recorded current-evoked spiking of retrobead labeled DS-projecting OFC and BLA neurons in the same animals following air or CIE treatment. DS-projecting OFC neurons were hyperexcitable 3- and 7-days following CIE exposure and spiking returned to control levels after 14 days of withdrawal. In contrast, firing was decreased in DS-projecting BLA neurons at 3-days withdrawal, increased at 7- and 14-days and returned to baseline at 28 days post-CIE. CIE exposure enhanced the amplitude and frequency of spontaneous excitatory postsynaptic currents (sEPSCs) of DS-projecting OFC neurons but had no effect on inhibitory postsynaptic currents (sIPSCs). In DS-projecting BLA neurons, the amplitude and frequency of sIPSCs was enhanced 3 days post-CIE with no change in sEPSCs while at 7-days post-withdrawal, sEPSC amplitude and frequency were increased and sIPSCs had returned to normal. Finally, in CIE-treated mice, acute ethanol no longer inhibited spike firing of DS-projecting OFC and BLA neurons. Overall, these results suggest that CIE-induced changes in the excitability of DS-projecting OFC and BLA neurons could underlie deficits in behavioral control often observed in alcohol-dependent individuals.

Simulation test for impartment of use-dependent plasticity by inactivation of axonal potassium channels on hippocampal mossy fibers

Modification of axonal excitability directly impacts information transfer through the neuronal networks in the brain. However, the functional significance of modulation of axonal excitability by the preceding neuronal activity largely remains elusive. One remarkable exception is the activity-dependent broadening of action potential (AP) propagating along the hippocampal mossy fibers. The duration of AP is progressively prolonged during repetitive stimuli and facilitated presynaptic Ca2+ entry and subsequent transmitter release. As an underlying mechanism, accumulated inactivation of axonal K+ channels during AP train has been postulated. As the inactivation of axonal K+ channels proceeds on a timescale of several tens of milliseconds slower than the millisecond scale of AP, the contribution of K+ channel inactivation in AP broadening needs to be tested and evaluated quantitatively. Using the computer simulation approach, this study aimed to explore the effects of the removal of the inactivation process of axonal K+ channels in the simple but sufficiently realistic model of hippocampal mossy fibers and found that the use-dependent AP broadening was completely abolished in the model replaced with non-inactivating K+ channels. The results demonstrated the critical roles of K+ channel inactivation in the activity-dependent regulation of axonal excitability during repetitive action potentials, which critically imparts additional mechanisms for robust use-dependent short-term plasticity characteristics for this particular synapse.

Increased intrinsic and synaptic excitability of hypothalamic POMC neurons underlies chronic stress-induced behavioral deficits

Chronic stress exposure induces maladaptive behavioral responses and increases susceptibility to neuropsychiatric conditions. However, specific neuronal populations and circuits that are highly sensitive to stress and trigger maladaptive behavioral responses remain to be identified. Here we investigate the patterns of spontaneous activity of proopiomelanocortin (POMC) neurons in the arcuate nucleus (ARC) of the hypothalamus following exposure to chronic unpredictable stress (CUS) for 10 days, a stress paradigm used to induce behavioral deficits such as anhedonia and behavioral despair [1, 2]. CUS exposure increased spontaneous firing of POMC neurons in both male and female mice, attributable to reduced GABA-mediated synaptic inhibition and increased intrinsic neuronal excitability. While acute activation of POMC neurons failed to induce behavioral changes in non-stressed mice of both sexes, subacute (3 days) and chronic (10 days) repeated activation of POMC neurons was sufficient to induce anhedonia and behavioral despair in males but not females under non-stress conditions. Acute activation of POMC neurons promoted susceptibility to subthreshold unpredictable stress in both male and female mice. Conversely, acute inhibition of POMC neurons was sufficient to reverse CUS-induced anhedonia and behavioral despair in both sexes. Collectively, these results indicate that chronic stress induces both synaptic and intrinsic plasticity of POMC neurons, leading to neuronal hyperactivity. Our findings suggest that POMC neuron dysfunction drives chronic stress-related behavioral deficits.

The unique structural characteristics of the Kir 7.1 inward rectifier potassium channel: a novel player in energy homeostasis control

The inward rectifier potassium channel Kir7.1, encoded by the KCNJ13 gene, is a tetramer composed of two-transmembrane domain-spanning monomers, closer in homology to Kir channels associated with potassium transport such as Kir1.1, 1.2, and 1.3. Compared with other channels, Kir7.1 exhibits small unitary conductance and low dependence on external potassium. Kir7.1 channels also show a phosphatidylinositol 4,5-bisphosphate (PIP2) dependence for opening. Accordingly, retinopathy-associated Kir7.1 mutations mapped at the binding site for PIP2 resulted in channel gating defects leading to channelopathies such as snowflake vitreoretinal degeneration and Leber congenital amaurosis in blind patients. Lately, this channel's role in energy homeostasis was reported due to the direct interaction with the melanocortin type 4 receptor (MC4R) in the hypothalamus. As this channel seems to play a multipronged role in potassium homeostasis and neuronal excitability, we will discuss what is predicted from a structural viewpoint and its possible implications for hunger control.

Mechanisms Underlying the Recruitment of Inhibitory Interneurons in Fictive Swimming in Developing Xenopus laevis Tadpoles

Developing spinal circuits generate patterned motor outputs while many neurons with high membrane resistances are still maturing. In the spinal cord of hatchling frog tadpoles of unknown sex, we found that the firing reliability in swimming of inhibitory interneurons with commissural and ipsilateral ascending axons was negatively correlated with their cellular membrane resistance. Further analyses showed that neurons with higher resistances had outward rectifying properties, low firing thresholds, and little delay in firing evoked by current injections. Input synaptic currents these neurons received during swimming, either compound, unitary current amplitudes, or unitary synaptic current numbers, were scaled with their membrane resistances, but their own synaptic outputs were correlated with membrane resistances of their postsynaptic partners. Analyses of neuronal dendritic and axonal lengths and their activities in swimming and cellular input resistances did not reveal a clear correlation pattern. Incorporating these electrical and synaptic properties into a computer swimming model produced robust swimming rhythms, whereas randomizing input synaptic strengths led to the breakdown of swimming rhythms, coupled with less synchronized spiking in the inhibitory interneurons. We conclude that the recruitment of these developing interneurons in swimming can be predicted by cellular input resistances, but the order is opposite to the motor-strength-based recruitment scheme depicted by Henneman's size principle. This form of recruitment/integration order in development before the emergence of refined motor control is progressive potentially with neuronal acquisition of mature electrical and synaptic properties, among which the scaling of input synaptic strengths with cellular input resistance plays a critical role.SIGNIFICANCE STATEMENT The mechanisms on how interneurons are recruited to participate in circuit function in developing neuronal systems are rarely investigated. In 2-d-old frog tadpole spinal cord, we found the recruitment of inhibitory interneurons in swimming is inversely correlated with cellular input resistances, opposite to the motor-strength-based recruitment order depicted by Henneman's size principle. Further analyses showed the amplitude of synaptic inputs that neurons received during swimming was inversely correlated with cellular input resistances. Randomizing/reversing the relation between input synaptic strengths and membrane resistances in modeling broke down swimming rhythms. Therefore, the recruitment or integration of these interneurons is conditional on the acquisition of several electrical and synaptic properties including the scaling of input synaptic strengths with cellular input resistances.

Plant-derived natural products targeting ion channels for pain

Chronic pain affects approximately one-fifth of people worldwide and reduces quality of life and in some cases, working ability. Ion channels expressed along nociceptive pathways affect neuronal excitability and as a result modulate pain experience. Several ion channels have been identified and investigated as potential targets for new medicines for the treatment of a variety of human diseases, including chronic pain. Voltage-gated channels Na⁺ and Ca²⁺ channels, K⁺ channels, transient receptor potential channels (TRP), purinergic (P2X) channels and acid-sensing ion channels (ASICs) are some examples of ion channels exhibiting altered function or expression in different chronic pain states. Pharmacological approaches are being developed to mitigate dysregulation of these channels as potential treatment options. Since natural compounds of plant origin exert promising biological and pharmacological properties and are believed to possess less adverse effects compared to synthetic drugs, they have been widely studied as treatments for chronic pain for their ability to alter the functional activity of ion channels. A literature review was conducted using Medline, Google Scholar and PubMed, resulted in listing 79 natural compounds/extracts that are reported to interact with ion channels as part of their analgesic mechanism of action. Most in vitro studies utilized electrophysiological techniques to study the effect of natural compounds on ion channels using primary cultures of dorsal root ganglia (DRG) neurons. In vivo studies concentrated on different pain models and were conducted mainly in mice and rats. Proceeding into clinical trials will require further study to develop new, potent and specific ion channel modulators of plant origin.

Neuron devices: emerging prospects in neural interfaces and recognition

Neuron interface devices can be used to explore the relationships between neuron firing and synaptic transmission, as well as to diagnose and treat neurological disorders, such as epilepsy and Alzheimer's disease. It is crucial to exploit neuron devices with high sensitivity, high biocompatibility, multifunctional integration and high-speed data processing. During the past decades, researchers have made significant progress in neural electrodes, artificial sensory neuron devices, and neuromorphic optic neuron devices. The main part of the review is divided into two sections, providing an overview of recently developed neuron interface devices for recording electrophysiological signals, as well as applications in neuromodulation, simulating the human sensory system, and achieving memory and recognition. We mainly discussed the development, characteristics, functional mechanisms, and applications of neuron devices and elucidated several key points for clinical translation. The present review highlights the advances in neuron devices on brain-computer interfaces and neuroscience research.

Functional Effects of Epilepsy Associated KCNT1 Mutations Suggest Pathogenesis via Aberrant Inhibitory Neuronal Activity

KCNT1 (K⁺ channel subfamily T member 1) is a sodium-activated potassium channel highly expressed in the nervous system which regulates neuronal excitability by contributing to the resting membrane potential and hyperpolarisation following a train of action potentials. Gain of function mutations in the KCNT1 gene are the cause of neurological disorders associated with different forms of epilepsy. To gain insights into the underlying pathobiology we investigated the functional effects of 9 recently published KCNT1 mutations, 4 previously studied KCNT1 mutations, and one previously unpublished KCNT1 variant of unknown significance. We analysed the properties of KCNT1 potassium currents and attempted to find a correlation between the changes in KCNT1 characteristics due to the mutations and severity of the neurological disorder they cause. KCNT1 mutations identified in patients with epilepsy were introduced into the full length human KCNT1 cDNA using quick-change site-directed mutagenesis protocol. Electrophysiological properties of different KCNT1 constructs were investigated using a heterologous expression system (HEK293T cells) and patch clamping. All mutations studied, except T314A, increased the amplitude of KCNT1 currents, and some mutations shifted the voltage dependence of KCNT1 open probability, increasing the proportion of channels open at the resting membrane potential. The T314A mutation did not affect KCNT1 current amplitude but abolished its voltage dependence. We observed a positive correlation between the severity of the neurological disorder and the KCNT1 channel open probability at resting membrane potential. This suggests that gain of function KCNT1 mutations cause epilepsy by increasing resting potassium conductance and suppressing the activity of inhibitory neurons. A reduction in action potential firing in inhibitory neurons due to excessively high resting potassium conductance leads to disinhibition of neural circuits, hyperexcitability and seizures.

The Long-Term Pannexin 1 Ablation Produces Structural and Functional Modifications in Hippocampal Neurons

Enhanced activity and overexpression of Pannexin 1 (Panx1) channels contribute to neuronal pathologies such as epilepsy and Alzheimer's disease (AD). The Panx1 channel ablation alters the hippocampus's glutamatergic neurotransmission, synaptic plasticity, and memory flexibility. Nevertheless, Panx1-knockout (Panx1-KO) mice still retain the ability to learn, suggesting that compensatory mechanisms stabilize their neuronal activity. Here, we show that the absence of Panx1 in the adult brain promotes a series of structural and functional modifications in the Panx1-KO hippocampal synapses, preserving spontaneous activity. Compared to the wild-type (WT) condition, the adult hippocampal neurons of Panx1-KO mice exhibit enhanced excitability, a more complex dendritic branching, enhanced spine maturation, and an increased proportion of multiple synaptic contacts. These modifications seem to rely on the actin-cytoskeleton dynamics as an increase in the actin polymerization and an imbalance between the Rac1 and the RhoA GTPase activities were observed in Panx1-KO brain tissues. Our findings highlight a novel interaction between Panx1 channels, actin, and Rho GTPases, which appear to be relevant for synapse stability.

The modulation of potassium channels by estrogens facilitates neuroprotection

Estrogens, the sex hormones, have the potential to govern multiple cellular functions, such as proliferation, apoptosis, differentiation, and homeostasis, and to exert numerous beneficial influences for the cardiovascular system, nervous system, and bones in genomic and/or non-genomic ways. Converging evidence indicates that estrogens serve a crucial role in counteracting neurodegeneration and ischemic injury; they are thereby being considered as a potent neuroprotectant for preventing neurological diseases such as Alzheimer's disease and stroke. The underlying mechanism of neuroprotective effects conferred by estrogens is thought to be complex and multifactorial, and it remains obscure. It is well established that the K+ channels broadly expressed in a variety of neural subtypes determine the essential physiological features of neuronal excitability, and dysfunction of these channels is closely associated with diverse brain deficits, such as ataxia and epilepsy. A growing body of evidence supports a neuroprotective role of K+ channels in malfunctions of nervous tissues, with the channels even being a therapeutic target in clinical trials. As multitarget steroid hormones, estrogens also regulate the activity of distinct K+ channels to generate varying biological actions, and accumulated data delineate that some aspects of estrogen-mediated neuroprotection may arise from the impact on multiple K+ channels, including Kv, BK, KATP, and K2P channels. The response of these K+ channels after acute or chronic exposure to estrogens may oppose pathological abnormality in nervous cells, which serves to extend our understanding of these phenomena.

Conducting Polymer-ECM Scaffolds for Human Neuronal Cell Differentiation

3D cell culture formats more closely resemble tissue architecture complexity than 2D systems, which are lacking most of the cell-cell and cell-microenvironment interactions of the in vivo milieu. Scaffold-based systems integrating natural biomaterials are extensively employed in tissue engineering to improve cell survival and outgrowth, by providing the chemical and physical cues of the natural extracellular matrix (ECM). Using the freeze-drying technique, porous 3D composite scaffolds consisting of poly(3,4-ethylene-dioxythiophene) doped with polystyrene sulfonate (PEDOT:PSS), containing ECM components (i.e., collagen, hyaluronic acid, and laminin) are engineered for hosting neuronal cells. The resulting scaffolds exhibit a highly porous microstructure and good conductivity, determined by scanning electron microscopy and electrochemical impedance spectroscopy, respectively. These supports boast excellent mechanical stability and water uptake capacity, making them ideal candidates for cell infiltration. SH-SY5Y human neuroblastoma cells show enhanced cell survival and proliferation in the presence of ECM compared to PEDOT:PSS alone. Whole-cell patch-clamp recordings acquired from differentiated SHSY5Y cells in the scaffolds demonstrate that ECM constituents promote neuronal differentiation in situ. These findings reinforce the usability of 3D conducting supports as engineered highly biomimetic and functional in vitro tissue-like platforms for drug or disease modeling.

The potassium channel auxiliary subunit Kvβ2 (Kcnab2) regulates Kv1 channels and dopamine neuron firing

Ion channel complexes typically consist of both pore-forming subunits and auxiliary subunits that do not directly conduct current but can regulate trafficking or alter channel properties. Isolating the role of these auxiliary subunits in neurons has proved difficult due to a lack of specific pharmacological agents and the potential for developmental compensation in constitutive knockout models. Here, we use cell-type-specific viral-mediated CRISPR/Cas9 mutagenesis to target the potassium channel auxiliary subunit Kvβ2 (Kcnab2) in dopamine neurons in the adult mouse brain. We find that mutagenesis of Kcnab2 reduces surface expression of Kv1.2, the primary Kv1 pore-forming subunit expressed in dopamine neurons, and shifts the voltage dependence of inactivation of potassium channel currents toward more hyperpolarized potentials. Loss of Kcnab2 broadens the action potential waveform in spontaneously firing dopamine neurons recorded in slice, reduces the afterhyperpolarization amplitude, and increases spike timing irregularity and excitability, all of which is consistent with a reduction in potassium channel current. Similar effects were observed with mutagenesis of the pore-forming subunit Kv1.2 (Kcna2). These results identify Kv1 currents as important contributors to dopamine neuron firing and demonstrate a role for Kvβ2 subunits in regulating the trafficking and gating properties of these ion channels. Furthermore, they demonstrate the utility of CRISPR-mediated mutagenesis in the study of previously difficult to isolate ion channel subunits.NEW & NOTEWORTHY Here, we utilize CRISPR/Cas9-mediated mutagenesis in dopamine neurons in mice to target the gene encoding Kvβ2, an auxiliary subunit that forms a part of Kv1 channel complexes. We find that the absence of Kvβ2 alters action potential properties by reducing surface expression of pore-forming subunits and shifting the voltage dependence of channel inactivation. This work establishes a new function for Kvβ2 subunits and Kv1 complexes in regulating dopamine neuron activity.

Kv3.3 subunits control presynaptic action potential waveform and neurotransmitter release at a central excitatory synapse

Kv3 potassium currents mediate rapid repolarisation of action potentials (APs), supporting fast spikes and high repetition rates. Of the four Kv3 gene family members, Kv3.1 and Kv3.3 are highly expressed in the auditory brainstem and we exploited this to test for subunit-specific roles at the calyx of Held presynaptic terminal in the mouse. Deletion of Kv3.3 (but not Kv3.1) reduced presynaptic Kv3 channel immunolabelling, increased presynaptic AP duration and facilitated excitatory transmitter release; which in turn enhanced short-term depression during high-frequency transmission. The response to sound was delayed in the Kv3.3KO, with higher spontaneous and lower evoked firing, thereby reducing signal-to-noise ratio. Computational modelling showed that the enhanced EPSC and short-term depression in the Kv3.3KO reflected increased vesicle release probability and accelerated activity-dependent vesicle replenishment. We conclude that Kv3.3 mediates fast repolarisation for short precise APs, conserving transmission during sustained high-frequency activity at this glutamatergic excitatory synapse.

Epithelial and sensory mechanisms of nasal hyperreactivity

"Nasal hyperreactivity" is a key feature in various phenotypes of upper airway diseases, whereby reactions of the nasal epithelium to diverse chemical and physical stimuli are exacerbated. In this review, we illustrate how nasal hyperreactivity can result from at least three types of mechanisms: (1) impaired barrier function, (2) hypersensitivity to external and endogenous stimuli, and (3) potentiation of efferent systems. We describe the known molecular basis of hyperreactivity related to the functional impairment of epithelial cells and somatosensory innervation, and indicate that the thermal, chemical, and mechanical sensors determining hyperreactivity in humans remain to be identified. We delineate research directions that may provide new insights into nasal hyperreactivity associated with rhinitis/rhinosinusitis pathophysiology and therapeutics. The elucidation of the molecular mechanisms underlying nasal hyperreactivity is essential for the treatment of rhinitis according to the precepts of precision medicine.

Functions of potassium channels blocked by low micromolar 4-aminopyridine in the crayfish nervous system

4-aminopyridine (4-AP) is a potassium channel blocker that has been used to treat patients with multiple sclerosis and Lambert-Eaton disease. The concentration of this drug in the blood of patients was estimated to be in low or submicromolar range. Animal studies have shown that 4-AP at such low concentration selectively blocks a subset of channels in Kv1 or Kv3 families. The crayfish opener neuromuscular junction and ventral superficial flexor (VSF) preparations were used to examine functions of K⁺ channels blocked by low concentrations of 4-AP. At opener motor axons, intracellular recordings show that 4-AP could increase action potential (AP) amplitude, duration and after-depolarization (ADP) at 10 μM. As 4-AP concentration was increased, in two-fold steps, AP amplitude did not increase further up to 5 mM. AP duration and ADP increased significantly mainly in two concentration ranges, 10-50 μM and 1-5 mM. The effects of 50 μM 4-AP on the VSF were less consistent than that observed at the opener motor axons. 4-AP did not change AP amplitude of motor axons recorded with an extracellular electrode and changes in AP repolarizing potential was observed in ∼25% of the axons. EPSP recorded simultaneously with AP showed an increase in amplitude with 4-AP treatment only in 30% of the axon-EPSP pairs. 4-AP also increased firing frequencies of ∼50% of axons. In four animals, 4-AP "awakened" the firing of APs from an axon that was silent before the drug. The mixture of positive and negative 4-AP effects summarized above was observed in the same VSF preparations in all cases (n = 8). We propose that there is a significant diversity in the density 4-AP sensitive potassium channels among motor axons of the VSF. Functional significance in the differences of 4-AP sensitivity of the two motor systems is discussed. This article is protected by copyright. All rights reserved.

Imbalanced Subthreshold Currents Following Sepsis and Chemotherapy: A Shared Mechanism Offering a New Therapeutic Target?

Both sepsis and treatment of cancer with chemotherapy are known to cause neurologic dysfunction. The primary defects seen in both groups of patients are neuropathy and encephalopathy; the underlying mechanisms are poorly understood. Analysis of preclinical models of these disparate conditions reveal similar defects in ion channel function contributing to peripheral neuropathy. The defects in ion channel function extend to the central nervous system where lower motoneurons are affected. In motoneurons the defect involves ion channels responsible for subthreshold currents that convert steady depolarization into repetitive firing. The inability to correctly translate depolarization into steady, repetitive firing has profound effects on motor function, and could be an important contributor to weakness and fatigue experienced by both groups of patients. The possibility that disruption of function, either instead of, or in addition to neurodegeneration, may underlie weakness and fatigue leads to a novel approach to therapy. Activation of serotonin (5HT) receptors in a rat model of sepsis restores the normal balance of subthreshold currents and normal motoneuron firing. If an imbalance of subthreshold currents also occurs in other central nervous system neurons, it could contribute to encephalopathy. We hypothesize that pharmacologically restoring the proper balance of subthreshold currents might provide effective therapy for both neuropathy and encephalopathy in patients recovering from sepsis or treatment with chemotherapy.

Ion channels and neuronal excitability in polyglutamine neurodegenerative diseases

Polyglutamine (polyQ) diseases are a family composed of nine neurodegenerative inherited disorders (NDDs) caused by pathological expansions of cytosine-adenine-guanine (CAG) trinucleotide repeats which encode a polyQ tract in the corresponding proteins. CAG polyQ repeat expansions produce neurodegeneration via multiple downstream mechanisms; among those the neuronal activity underlying the ion channels is affected directly by specific channelopathies or indirectly by secondary dysregulation. In both cases, the altered excitability underlies to gain- or loss-of-function pathological effects. Here we summarize the repertoire of ion channels in polyQ NDDs emphasizing the biophysical features of neuronal excitability and their pathogenic role. The aim of this review is to point out the value of a deeper understanding of those functional mechanisms and processes as crucial elements for the designing and targeting of novel therapeutic avenues.

Alzheimer's disease therapy based on acetylcholinesterase inhibitor/blocker effects on voltage-gated potassium channels

Alzheimer's disease (AD) is the most prevalent neurodegenerative disorder with progressive loss of memory and other cognitive functions. The pathogenesis of this disease is complex and multifactorial, and remains obscure until now. To enhance the declined level of acetylcholine (ACh) resulting from loss of cholinergic neurons, acetylcholinesterase (AChE) inhibitors are developed and successfully approved for AD treatment in the clinic, with a limited therapeutic effectiveness. At present, it is generally accepted that multi-target strategy is potently useful for designing novel drugs for AD. Accumulated evidence reveals that Kv channels, which are broadly expressed in brain and possess crucial functions in modulating the neuronal activity, are inhibited by several acetylcholinesterase (AChE) inhibitors, such as tacrine, bis(7)-tacrine, donepezil and galantamine. Inhibition of Kv channels by these AChE inhibitors can generate neuroprotective effects by either mitigating Aβ toxicity and neuronal apoptosis, or facilitating cell proliferation. These inhibitory effects provide additional explanations for clinical beneficial effectiveness of AChE inhibitors, meaning that Kv channel is a promising candidate target for novel drugs for AD therapy.

Kv1 channels regulate variations in spike patterning and temporal reliability in the avian cochlear nucleus angularis

Diverse physiological phenotypes in a neuronal population can broaden the range of computational capabilities within a brain region. The avian cochlear nucleus angularis (NA) contains a heterogeneous population of neurons whose variation in intrinsic properties results in electrophysiological phenotypes with a range of sensitivities to temporally modulated input. The low-threshold potassium conductance (G_KLT) is a key feature of neurons involved in fine temporal structure coding for sound localization, but a role for these channels in intensity or spectrotemporal coding has not been established. To determine whether G_KLT affects the phenotypical variation and temporal properties of NA neurons, we applied dendrotoxin-I (DTX), a potent antagonist of Kv1-type potassium channels, to chick brain stem slices in vitro during whole cell patch-clamp recordings. We found a cell-type specific subset of NA neurons that was sensitive to DTX: single-spiking NA neurons were most profoundly affected, as well as a subset of tonic-firing neurons. Both tonic I (phasic onset bursting) and tonic II (delayed firing) neurons showed DTX sensitivity in their firing rate and phenotypical firing pattern. Tonic III neurons were unaffected. Spike time reliability and fluctuation sensitivity measured in DTX-sensitive NA neurons was also reduced with DTX. Finally, DTX reduced spike threshold adaptation in these neurons, suggesting that G_KLT contributes to the temporal properties that allow coding of rapid changes in the inputs to NA neurons. These results suggest that variation in Kv1 channel expression may be a key factor in functional diversity in the avian cochlear nucleus.NEW & NOTEWORTHY The dendrotoxin-sensitive voltage-gated potassium conductance typically associated with neuronal coincidence detection in the timing pathway for sound localization is demonstrated to affect spiking patterns and temporal input sensitivity in the intensity pathway in the avian auditory brain stem. The Kv1-family channels appear to be present in a subset of cochlear nucleus angularis neurons, regulate spike threshold dynamics underlying high-pass membrane filtering, and contribute to intrinsic firing diversity.

Membrane potential instabilities in sensory neurons: mechanisms and pathophysiological relevance

ABSTRACT

Peripheral sensory neurons transduce physicochemical stimuli affecting somatic tissues into the firing of action potentials that are conveyed to the central nervous system. This results in conscious perception, adaptation, and survival, but alterations of the firing patterns can result in pain and hypersensitivity conditions. Thus, understanding the molecular mechanisms underlying action potential firing in peripheral sensory neurons is essential in sensory biology and pathophysiology. Over the past 30 years, it has been consistently reported that these cells can display membrane potential instabilities (MPIs), in the form of subthreshold membrane potential oscillations or depolarizing spontaneous fluctuations. However, research on this subject remains sparse, without a clear conductive thread to be followed. To address this, we here provide a synthesis of the description, molecular bases, mathematical models, physiological roles, and pathophysiological implications of MPIs in peripheral sensory neurons. Membrane potential instabilities have been reported in trigeminal, dorsal root, and Mes-V ganglia, where they are believed to support repetitive firing. They are proposed to have roles also in intercellular communication, ectopic firing, and responses to tonic and slow natural stimuli. We highlight how MPIs are of great interest for the study of sensory transduction physiology and how they may represent therapeutic targets for many pathological conditions, such as acute and chronic pain, itch, and altered sensory perceptions. We identify future research directions, including the elucidation of the underlying molecular determinants and modulation mechanisms, their relation to the encoding of natural stimuli and their implication in pain and hypersensitivity conditions.

Kv3 Channels Contribute to the Excitability of Subpopulations of Spinal Cord Neurons in Lamina VII

Autonomic parasympathetic preganglionic neurons (PGNs) drive contraction of the bladder during micturition but remain quiescent during bladder filling. This quiescence is postulated to be because of recurrent inhibition of PGN by fast-firing adjoining interneurons. Here, we defined four distinct neuronal types within Lamina VII, where PGN are situated, by combining whole cell patch clamp recordings with k-means clustering of a range of electrophysiological parameters. Additional morphologic analysis separated these neuronal classes into parasympathetic preganglionic populations (PGN) and a fast-firing interneuronal population. Kv3 channels are voltage-gated potassium channels (Kv) that allow fast and precise firing of neurons. We found that blockade of Kv3 channels by tetraethylammonium (TEA) reduced neuronal firing frequency and isolated high-voltage-activated Kv currents in the fast-firing population but had no effect in PGN populations. Furthermore, Kv3 blockade potentiated the local and descending inhibitory inputs to PGN indicating that Kv3-expressing inhibitory neurons are synaptically connected to PGN. Taken together, our data reveal that Kv3 channels are crucial for fast and regulated neuronal output of a defined population that may be involved in intrinsic spinal bladder circuits that underpin recurrent inhibition of PGN.

The Perception of Ramped Pulse Shapes in Cochlear Implant Users

The electric stimulation provided by current cochlear implants (CI) is not power efficient. One underlying problem is the poor efficiency by which information from electric pulses is transformed into auditory nerve responses. A novel stimulation paradigm using ramped pulse shapes has recently been proposed to remedy this inefficiency. The primary motivation is a better biophysical fit to spiral ganglion neurons with ramped pulses compared to the rectangular pulses used in most contemporary CIs. Here, we tested the hypotheses that ramped pulses provide more efficient stimulation compared to rectangular pulses and that a rising ramp is more efficient than a declining ramp. Rectangular, rising ramped and declining ramped pulse shapes were compared in terms of charge efficiency and discriminability, and threshold variability in seven CI listeners. The tasks included single-channel threshold detection, loudness-balancing, discrimination of pulse shapes, and threshold measurement across the electrode array. Results showed that reduced charge, but increased peak current amplitudes, was required at threshold and most comfortable levels with ramped pulses relative to rectangular pulses. Furthermore, only one subject could reliably discriminate between equally-loud ramped and rectangular pulses, suggesting variations in neural activation patterns between pulse shapes in that participant. No significant difference was found between rising and declining ramped pulses across all tests. In summary, the present findings show some benefits of charge efficiency with ramped pulses relative to rectangular pulses, that the direction of a ramped slope is of less importance, and that most participants could not perceive a difference between pulse shapes.

Spike firing attenuation of serotonin neurons in learned helplessness rats is reversed by ketamine

Animals suffering from uncontrollable stress sometimes show low effort to escape stress (learned helplessness). Changes in serotonin (5-hydroxytryptamine) signalling are thought to underlie this behaviour. Although the release of 5-hydroxytryptamine is triggered by the action potential firing of dorsal raphe nuclei 5-hydroxytryptamine neurons, the electrophysiological changes induced by uncontrollable stress are largely unclear. Herein, we examined electrophysiological differences among 5-hydroxytryptamine neurons in naïve rats, learned helplessness rats and rats resistant to inescapable stress (non-learned helplessness). Five-week-old male Sprague Dawley rats were exposed to inescapable foot shocks. After an avoidance test session, rats were classified as learned helplessness or non-learned helplessness. Activity-dependent 5-hydroxytryptamine release induced by the administration of high-potassium solution was slower in free-moving learned helplessness rats. Subthreshold electrophysiological properties of 5-hydroxytryptamine neurons were identical among the three rat groups, but the depolarization-induced spike firing was significantly attenuated in learned helplessness rats. To clarify the underlying mechanisms, potassium (K+) channels regulating the spike firing were initially examined using naïve rats. K+ channels sensitive to 500 μM tetraethylammonium caused rapid repolarization of the action potential and the small conductance calcium-activated K+ channels produced afterhyperpolarization. Additionally, dendrotoxin-I, a blocker of Kv1.1 (encoded by Kcna1), Kv1.2 (encoded by Kcna2) and Kv1.6 (encoded by Kcna6) voltage-dependent K+ channels, weakly enhanced the spike firing frequency during depolarizing current injections without changes in individual spike waveforms in naïve rats. We found that dendrotoxin-I significantly enhanced the spike firing of 5-hydroxytryptamine neurons in learned helplessness rats. Consequently, the difference in spike firing among the three rat groups was abolished in the presence of dendrotoxin-I. These results suggest that the upregulation of dendrotoxin-I-sensitive Kv1 channels underlies the firing attenuation of 5-hydroxytryptamine neurons in learned helplessness rats. We also found that the antidepressant ketamine facilitated the spike firing of 5-hydroxytryptamine neurons and abolished the firing difference between learned helplessness and non-learned helplessness by suppressing dendrotoxin-I-sensitive Kv1 channels. The dendrotoxin-I-sensitive Kv1 channel may be a potential target for developing drugs to control activity of 5-hydroxytryptamine neurons.

Changing the firing threshold for normal optic nerve axons by the application of infra-red laser light

Normal optic nerve axons exhibit a temperature dependence, previously explained by a membrane potential hyperpolarization on warming. We now report that near infra-red laser light, delivered via a fibre optic light guide, also affects axonal membrane potential and threshold, at least partly through a photo-thermal effect. Application of light to optic nerve, at the recording site, gave rise to a local membrane potential hyperpolarization over a period of about a minute, and increased the size of the depolarizing after potential. Application near the site of electrical stimulation reversibly raised current-threshold, and the change in threshold recorded over minutes of irradiation was significantly increased by the application of the Ih blocker, ZD7288 (50 µM), indicating Ih limits the hyperpolarizing effect of light. Light application also had fast effects on nerve behaviour, increasing threshold without appreciable delay (within seconds), probably by a mechanism independent of kinetically fast K+ channels and Na+ channel inactivation, and hypothesized to be caused by reversible changes in myelin function.

Subtype maturation of spiral ganglion neurons

PURPOSE OF REVIEW

We review recent progress in the characterization of spiral ganglion neurons (SGNs), the afferent neurons that transmit sound information from mechanosensory hair cells in the inner ear to the central nervous system.

RECENT FINDINGS

Single-cell ribonucleic acid sequencing studies of murine SGNs have demonstrated that SGNs consist of molecularly distinct subtypes. The molecularly defined SGN subtypes likely correspond to SGN subtypes previously identified on the basis of physiological properties, although this has not been experimentally demonstrated. Subtype maturation is completed postnatally in an activity-dependent manner and is impaired in several models of hearing loss.

SUMMARY

The recent molecular studies open new avenues to rigorously test whether SGN subtypes are important for the encoding of different sound features and if they show differential vulnerability to genetic factors and environmental insults. This could have important implications for the development of therapeutic strategies to treat hearing loss.

Sparsely Distributed, Pre-synaptic Kv3 K+ Channels Control Spontaneous Firing and Cross-Unit Synchrony via the Regulation of Synaptic Noise in an Auditory Brainstem Circuit

Spontaneous subthreshold activity in the central nervous system is fundamental to information processing and transmission, as it amplifies and optimizes sub-threshold signals, thereby improving action potential initiation and maintaining reliable firing. This form of spontaneous activity, which is frequently considered noise, is particularly important at auditory synapses where acoustic information is encoded by rapid and temporally precise firing rates. In contrast, when present in excess, this form of noise becomes detrimental to acoustic information as it contributes to the generation and maintenance of auditory disorders such as tinnitus. The most prominent contribution to subthreshold noise is spontaneous synaptic transmission (synaptic noise). Although numerous studies have examined the role of synaptic noise on single cell excitability, little is known about its pre-synaptic modulation owing in part to the difficulties of combining noise modulation with monitoring synaptic release. Here we study synaptic noise in the auditory brainstem dorsal cochlear nucleus (DCN) of mice and show that pharmacological potentiation of Kv3 K⁺ currents reduces the level of synaptic bombardment onto DCN principal fusiform cells. Using a transgenic mouse line (SyG37) expressing SyGCaMP2-mCherry, a calcium sensor that targets pre-synaptic terminals, we show that positive Kv3 K⁺ current modulation decreases calcium influx in a fifth of pre-synaptic boutons. Furthermore, while maintaining rapid and precise spike timing, positive Kv3 K⁺ current modulation increases the synchronization of local circuit neurons by reducing spontaneous activity. In conclusion, our study identifies a unique pre-synaptic mechanism which reduces synaptic noise at auditory synapses and contributes to the coherent activation of neurons in a local auditory brainstem circuit. This form of modulation highlights a new therapeutic target, namely the pre-synaptic bouton, for ameliorating the effects of hearing disorders which are dependent on aberrant spontaneous activity within the central auditory system.

Transmitter and ion channel profiles of neurons in the primate abducens and trochlear nuclei

Extraocular motoneurons initiate dynamically different eye movements, including saccades, smooth pursuit and vestibulo-ocular reflexes. These motoneurons subdivide into two main types based on the structure of the neuro-muscular interface: motoneurons of singly-innervated (SIF), and motoneurons of multiply-innervated muscle fibers (MIF). SIF motoneurons are thought to provoke strong and brief/fast muscle contractions, whereas MIF motoneurons initiate prolonged, slow contractions. While relevant for adequate functionality, transmitter and ion channel profiles associated with the morpho-physiological differences between these motoneuron types, have not been elucidated so far. This prompted us to investigate the expression of voltage-gated potassium, sodium and calcium ion channels (Kv1.1, Kv3.1b, Nav1.6, Cav3.1-3.3, KCC2), the transmitter profiles of their presynaptic terminals (vGlut1 and 2, GlyT2 and GAD) and transmitter receptors (GluR2/3, NMDAR1, GlyR1α) using immunohistochemical analyses of abducens and trochlear motoneurons and of abducens internuclear neurons (INTs) in macaque monkeys. The main findings were: (1) MIF and SIF motoneurons express unique voltage-gated ion channel profiles, respectively, likely accounting for differences in intrinsic membrane properties. (2) Presynaptic glutamatergic synapses utilize vGlut2, but not vGlut1. (3) Trochlear motoneurons receive GABAergic inputs, abducens neurons receive both GABAergic and glycinergic inputs. (4) Synaptic densities differ between MIF and SIF motoneurons, with MIF motoneurons receiving fewer terminals. (5) Glutamatergic receptor subtypes differ between MIF and SIF motoneurons. While NMDAR1 is intensely expressed in INTs, MIF motoneurons lack this receptor subtype entirely. The obtained cell-type-specific transmitter and conductance profiles illuminate the structural substrates responsible for differential contributions of neurons in the abducens and trochlear nuclei to eye movements.

Severe deficiency of the voltage-gated sodium channel NaV1.2 elevates neuronal excitability in adult mice

Scn2a encodes the voltage-gated sodium channel NaV1.2, a main mediator of neuronal action potential firing. The current paradigm suggests that NaV1.2 gain-of-function variants enhance neuronal excitability, resulting in epilepsy, whereas NaV1.2 deficiency impairs neuronal excitability, contributing to autism. However, this paradigm does not explain why ∼20%-30% of individuals with NaV1.2 deficiency still develop seizures. Here, we report the counterintuitive finding that severe NaV1.2 deficiency results in increased neuronal excitability. Using a NaV1.2-deficient mouse model, we show enhanced intrinsic excitability of principal neurons in the prefrontal cortex and striatum, brain regions known to be involved in Scn2a-related seizures. This increased excitability is autonomous and reversible by genetic restoration of Scn2a expression in adult mice. RNA sequencing reveals downregulation of multiple potassium channels, including KV1.1. Correspondingly, KV channel openers alleviate the hyperexcitability of NaV1.2-deficient neurons. This unexpected neuronal hyperexcitability may serve as a cellular basis underlying NaV1.2 deficiency-related seizures.

Genetics in Epilepsy

The presence of unprovoked, recurrent seizures, particularly when drug resistant and associated with cognitive and behavioral deficits, warrants investigation for an underlying genetic cause. This article provides an overview of the major classes of genes associated with epilepsy phenotypes divided into functional categories along with the recommended work-up and therapeutic considerations. Gene discovery in epilepsy supports counseling and anticipatory guidance but also opens the door for precision medicine guiding therapy with a focus on those with disease-modifying effects.

Voltage-gated potassium channel dysfunction in dorsal root ganglia contributes to the exaggerated exercise pressor reflex in rats with chronic heart failure

An exaggerated exercise pressor reflex (EPR) causes excessive sympathoexcitation and exercise intolerance during physical activity in the chronic heart failure (CHF) state. Muscle afferent sensitization contributes to the genesis of the exaggerated EPR in CHF. However, the cellular mechanisms underlying muscle afferent sensitization in CHF remain unclear. Considering that voltage-gated potassium (Kv) channels critically regulate afferent neuronal excitability, we examined the potential role of Kv channels in mediating the sensitized EPR in male rats with CHF. Real-time reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting experiments demonstrate that both mRNA and protein expressions of multiple Kv channel isoforms (Kv1.4, Kv3.4, Kv4.2, and Kv4.3) were downregulated in lumbar dorsal root ganglions (DRGs) of CHF rats compared with sham rats. Immunofluorescence data demonstrate significant decreased Kv channel staining in both NF200-positive and IB4-positive lumbar DRG neurons in CHF rats compared with sham rats. Data from patch-clamp experiments demonstrate that the total Kv current, especially I_A, was dramatically decreased in medium-sized IB4-negative muscle afferent neurons (a subpopulation containing mostly Aδ neurons) from CHF rats compared with sham rats, indicating a potential functional loss of Kv channels in muscle afferent Aδ neurons. In in vivo experiments, adenoviral overexpression of Kv4.3 in lumbar DRGs for 1 wk attenuated the exaggerated EPR induced by muscle static contraction and the mechanoreflex by passive stretch without affecting the blunted cardiovascular response to hindlimb arterial injection of capsaicin in CHF rats. These data suggest that Kv channel dysfunction in DRGs plays a critical role in mediating the exaggerated EPR and muscle afferent sensitization in CHF.NEW & NOTEWORTHY The primary finding of this manuscript is that voltage-gated potassium (Kv) channel dysfunction in DRGs plays a critical role in mediating the exaggerated EPR and muscle afferent sensitization in chronic heart failure (CHF). We propose that manipulation of Kv channels in DRG neurons could be considered as a potential new approach to reduce the exaggerated sympathoexcitation and to improve exercise intolerance in CHF, which can ultimately facilitate an improved quality of life and reduce mortality.