Immunolocalization of NaV1.2 channel subtypes in rat and cat brain and spinal cord with high affinity antibodies

Discovery of Novel Pyrimidine-Based Derivatives as Nav1.2 Inhibitors with Efficacy in Mouse Models of Epilepsy

Dysfunction of voltage-gated sodium channel Nav1.2 causes various epileptic disorders, and inhibition of the channel has emerged as an attractive therapeutic strategy. However, currently available Nav1.2 inhibitors exhibit low potency and limited structural diversity. In this study, a novel series of pyrimidine-based derivatives with Nav1.2 inhibitory activity were designed, synthesized, and evaluated. Compounds 14 and 35 exhibited potent activity against Nav1.2, boasting IC50 values of 120 and 65 nM, respectively. Compound 14 displayed favorable pharmacokinetics (F = 43%) following intraperitoneal injection and excellent brain penetration potency (B/P = 3.6). Compounds 14 and 35 exhibited robust antiepileptic activities in the maximal electroshock test, with ED50 values of 3.2 and 11.1 mg/kg, respectively. Compound 35 also demonstrated potent antiepileptic activity in a 6 Hz (32 mA) model, with an ED50 value of 18.5 mg/kg. Overall, compounds 14 and 35 are promising leads for the development of new small-molecule therapeutics for epilepsy.

Persistent Nav1.1 and Nav1.6 currents drive spinal locomotor functions through nonlinear dynamics

Persistent sodium current (INaP) in the spinal locomotor network promotes two distinct nonlinear firing patterns: a self-sustained spiking triggered by a brief excitation in bistable motoneurons and bursting oscillations in interneurons of the central pattern generator (CPG). Here, we identify the NaV channels responsible for INaP and their role in motor behaviors. We report the axonal Nav1.6 as the main molecular player for INaP in lumbar motoneurons. The inhibition of Nav1.6, but not of Nav1.1, in motoneurons impairs INaP, bistability, postural tone, and locomotor performance. In interneurons of the rhythmogenic CPG region, both Nav1.6 and Nav1.1 equally mediate INaP. Inhibition of both channels is required to abolish oscillatory bursting activities and the locomotor rhythm. Overall, Nav1.6 plays a significant role both in posture and locomotion by governing INaP-dependent bistability in motoneurons and working in tandem with Nav1.1 to provide INaP-dependent rhythmogenic properties of the CPG.

Cerebellar Stellate Cell Excitability Is Coordinated by Shifts in the Gating Behavior of Voltage-Gated Na+ and A-Type K+ Channels

Neuronal excitability in the vertebrate brain is governed by the coordinated activity of both ligand- and voltage-gated ion channels. In the cerebellum, spontaneous action potential (AP) firing of inhibitory stellate cells (SCs) is variable, typically operating within the 5- to 30-Hz frequency range. AP frequency is shaped by the activity of somatodendritic A-type K⁺ channels and the inhibitory effect of GABAergic transmission. An added complication, however, is that whole-cell recording from SCs induces a time-dependent and sustained increase in membrane excitability making it difficult to define the full range of firing rates. Here, we show that whole-cell recording in cerebellar SCs of both male and female mice augments firing rates by reducing the membrane potential at which APs are initiated. AP threshold is lowered due to a hyperpolarizing shift in the gating behavior of voltage-gated Na⁺ channels. Whole-cell recording also elicits a hyperpolarizing shift in the gating behavior of A-type K⁺ channels which contributes to increased firing rates. Hodgkin–Huxley modeling and pharmacological experiments reveal that gating shifts in A-type K⁺ channel activity do not impact AP threshold, but rather promote channel inactivation which removes restraint on the upper limit of firing rates. Taken together, our work reveals an unappreciated impact of voltage-gated Na⁺ channels that work in coordination with A-type K⁺ channels to regulate the firing frequency of cerebellar SCs.

Voltage-gated sodium currents in cerebellar Purkinje neurons: functional and molecular diversity

Purkinje neurons, the sole output of the cerebellar cortex, deliver GABA-mediated inhibition to the deep cerebellar nuclei. To subserve this critical function, Purkinje neurons fire repetitively, and at high frequencies, features that have been linked to the unique properties of the voltage-gated sodium (Nav) channels expressed. In addition to the rapidly activating and inactivating, or transient, component of the Nav current (INaT) present in many types of central and peripheral neurons, Purkinje neurons, also expresses persistent (INaP) and resurgent (INaR) Nav currents. Considerable progress has been made in detailing the biophysical properties and identifying the molecular determinants of these discrete Nav current components, as well as defining their roles in the regulation of Purkinje neuron excitability. Here, we review this important work and highlight the remaining questions about the molecular mechanisms controlling the expression and the functioning of Nav currents in Purkinje neurons. We also discuss the impact of the dynamic regulation of Nav currents on the functioning of individual Purkinje neurons and cerebellar circuits.

Ion Channels in the Paraventricular Hypothalamic Nucleus (PVN); Emerging Diversity and Functional Roles

The paraventricular nucleus of the hypothalamus (PVN) is critical for the regulation of homeostatic function. Although also important for endocrine regulation, it has been referred to as the "autonomic master controller." The emerging consensus is that the PVN is a multifunctional nucleus, with autonomic roles including (but not limited to) coordination of cardiovascular, thermoregulatory, metabolic, circadian and stress responses. However, the cellular mechanisms underlying these multifunctional roles remain poorly understood. Neurones from the PVN project to and can alter the function of sympathetic control regions in the medulla and spinal cord. Dysfunction of sympathetic pre-autonomic neurones (typically hyperactivity) is linked to several diseases including hypertension and heart failure and targeting this region with specific pharmacological or biological agents is a promising area of medical research. However, to facilitate future medical exploitation of the PVN, more detailed models of its neuronal control are required; populated by a greater compliment of constituent ion channels. Whilst the cytoarchitecture, projections and neurotransmitters present in the PVN are reasonably well documented, there have been fewer studies on the expression and interplay of ion channels. In this review we bring together an up to date analysis of PVN ion channel studies and discuss how these channels may interact to control, in particular, the activity of the sympathetic system.

Sodium channels and pain: from toxins to therapies

Voltage-gated sodium channels (NaV channels) are essential for the initiation and propagation of action potentials that critically influence our ability to respond to a diverse range of stimuli. Physiological and pharmacological studies have linked abnormal function of NaV channels to many human disorders, including chronic neuropathic pain. These findings, along with the description of the functional properties and expression pattern of NaV channel subtypes, are helping to uncover subtype specific roles in acute and chronic pain and revealing potential opportunities to target these with selective inhibitors. High-throughput screens and automated electrophysiology platforms have identified natural toxins as a promising group of molecules for the development of target-specific analgesics. In this review, the role of toxins in defining the contribution of NaV channels in acute and chronic pain states and their potential to be used as analgesic therapies are discussed.

LINKED ARTICLES

This article is part of a themed section on Recent Advances in Targeting Ion Channels to Treat Chronic Pain. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.12/issuetoc.

Voltage-gated sodium channels: (NaV )igating the field to determine their contribution to visceral nociception

Chronic visceral pain, altered motility and bladder dysfunction are common, yet poorly managed symptoms of functional and inflammatory disorders of the gastrointestinal and urinary tracts. Recently, numerous human channelopathies of the voltage-gated sodium (NaV ) channel family have been identified, which induce either painful neuropathies, an insensitivity to pain, or alterations in smooth muscle function. The identification of these disorders, in addition to the recent utilisation of genetically modified NaV mice and specific NaV channel modulators, has shed new light on how NaV channels contribute to the function of neuronal and non-neuronal tissues within the gastrointestinal tract and bladder. Here we review the current pre-clinical and clinical evidence to reveal how the nine NaV channel family members (NaV 1.1-NaV 1.9) contribute to abdominal visceral function in normal and disease states.

Activity-dependent differences in function between proximal and distal Schaffer collaterals

Axon conduction fidelity is important for signal transmission and has been studied in various axons, including the Schaffer collateral axons of the hippocampus. Previously, we reported that high-frequency stimulation (HFS) depresses Schaffer collateral excitability when assessed by whole-cell recordings from CA3 pyramidal cells but induces biphasic excitability changes (increase followed by decrease) in extracellular recordings of CA1 fiber volleys. Here, we examined responses from proximal (whole-cell or field-potential recordings from CA3 pyramidal cell somata) and distal (field-potential recordings from CA1 stratum radiatum) portions of the Schaffer collaterals during HFS and burst stimulation in hippocampal slices. Whole-cell and dual-field-potential recordings using 10-100-Hz HFS revealed frequency-dependent changes like those previously described, with higher frequencies producing more drastic changes. Dual-field-potential recordings revealed substantial differences in the response to HFS between proximal and distal regions of the Schaffer collaterals, with proximal axons depressing more strongly and only distal axons showing an initial excitability increase. Because CA3 pyramidal neurons normally fire in short bursts rather than long high-frequency trains, we repeated the dual recordings using 100-1,000-ms interval burst stimulation. Burst stimulation produced changes similar to those during HFS, with shorter intervals causing more drastic changes and substantial differences observed between proximal and distal axons. We suggest that functional differences between proximal and distal Schaffer collaterals may allow selective filtering of nonphysiological activity while maximizing successful conduction of physiological activity throughout an extensive axonal arbor.

Polarised localisation of the voltage-gated sodium channel Na(v)1.2 in cerebellar granule cells

Voltage-gated sodium channels are responsible for action potential initiation and propagation in electrically excitable cells. In this study, we used biochemical, immunohistochemical and quantitative immunoelectron microscopy techniques to reveal the temporal and spatial expression of the Na(v)1.2 channel subunit in granule cells of cerebellum. Using histoblot, we detected Na(v)1.2 widely distributed in the adult brain, but prominently expressed in the cerebellum. During postnatal development, Na(v)1.2 mRNA and protein were detected low during the first and second postnatal week, increased to P15 and then continue to decrease until adult levels. At the light microscopic level, Na(v)1.2 immunoreactivity concentrated in the molecular layer of the cerebellar cortex. Using immunofluorescence, Na(v)1.2 colocalised with VGluT1, but not with VGluT2, demonstrating that the subunit was preferentially present in parallel fibre axons and axon terminals. At the electron microscopic level, Na(v)1.2 immunoparticles were exclusively detected at presynaptic sites in granule cell axons and axon terminals of granule cells, with occasional clustering in their axon initial segment. This was demonstrated using quantitative immunogold analysis. In the axon terminals, the distribution of Na(v)1.2 was relatively uniform along the extrasynaptic plasma membrane and never detected in the active zone. We could not find detectable levels of Na(v)1.2 at postsynaptic elements of granule cells or other cerebellar cell types. The present findings show a polarised distribution of Na(v)1.2 along the neuronal surface of granule cells and suggest its primary involvement in the transmission of information from granule cells to Purkinje cells.

The developmental changes of Na(v)1.1 and Na(v)1.2 expression in the human hippocampus and temporal lobe

Alterations of the genes encoding α1 and α2 subunits of voltage-gated sodium channels (SCN1A, SCN2A) have been reported as causes of various types of epilepsy, most of which occur during the first year of life; as yet, however, the detailed mechanisms are unclear. We suppose that developmental changes of SCN1A and SCN2A in the human brain, which are unknown yet, may play an important role. So here, we studied the developmental changes of their corresponding proteins (Na(v)1.1 and Na(v)1.2) in the human hippocampus and temporal lobe in 28 autopsy cases, which age from 13weeks of gestation (GW) to 63years of age (Y). Using comparative microscopic immunohistochemical (IHC) analysis, we found that Na(v)1.1 and Na(v)1.2 immunoreactivity first appeared at 19GW, simultaneously in the hippocampus and the white matter of temporal lobe. In nearly all age groups, Na(v)1.1 immunoreactivity was weak and relatively homogeneous. In general, Na(v)1.1 immunoreactive (IR) neurons and neurites increased during the late fetal and postnatal periods, reached their peaks 7-9months after birth (M), then decreased and remained stable at a relatively low level during childhood and adulthood. On the other hand, Na(v)1.2 immunoreactivity was strong and heterogeneous. In the hippocampus, Na(v)1.2 IR neurons increased gradually during the late fetal period, reached their peaks at 7-9M, sustained this high level during childhood, and then decreased slightly at adulthood. In the temporal lobe, Na(v)1.2 IR neurons reached a high level during the late fetal period, and maintained that level during subsequent developmental stages; Na(v)1.2 IR neurites also increased to a relatively high level during the late fetal period and continued to increase up to and during adulthood. Using double-staining IHC, we found that Na(v)1.1 and Na(v)1.2 had a relatively high colocalization rate with parvalbumin and showed distinct developmental changes. These findings extend our previous understanding of sodium channels and may help us discover the pathomechanisms of sodium channel-related age-dependent epilepsy.

Expansion of voltage-dependent Na+ channel gene family in early tetrapods coincided with the emergence of terrestriality and increased brain complexity

Mammals have ten voltage-dependent sodium (Nav) channel genes. Nav channels are expressed in different cell types with different subcellular distributions and are critical for many aspects of neuronal processing. The last common ancestor of teleosts and tetrapods had four Nav channel genes, presumably on four different chromosomes. In the lineage leading to mammals, a series of tandem duplications on two of these chromosomes more than doubled the number of Nav channel genes. It is unknown when these duplications occurred and whether they occurred against a backdrop of duplication of flanking genes on their chromosomes or as an expansion of ion channel genes in general. We estimated key dates of the Nav channel gene family expansion by phylogenetic analysis using teleost, elasmobranch, lungfish, amphibian, avian, lizard, and mammalian Nav channel sequences, as well as chromosomal synteny for tetrapod genes. We tested, and exclude, the null hypothesis that Nav channel genes reside in regions of chromosomes prone to duplication by demonstrating the lack of duplication or duplicate retention of surrounding genes. We also find no comparable expansion in other voltage-dependent ion channel gene families of tetrapods following the teleost-tetrapod divergence. We posit a specific expansion of the Nav channel gene family in the Devonian and Carboniferous periods when tetrapods evolved, diversified, and invaded the terrestrial habitat. During this time, the amniote forebrain evolved greater anatomical complexity and novel tactile sensory receptors appeared. The duplication of Nav channel genes allowed for greater regional specialization in Nav channel expression, variation in subcellular localization, and enhanced processing of somatosensory input.

Laminae-specific distribution of alpha-subunits of voltage-gated sodium channels in the adult rat spinal cord

While the voltage-gated sodium channels (VGSCs) are the key molecules for neuronal activities, the precise distribution of them in spinal cord is not clear in previous studies. We examined the expression of mRNAs for alpha-subunits of VGSC (Navs) in adult rat spinal cord before and 7 days after L5 spinal nerve ligation (SPNL) or complete Freund's adjuvant (CFA)-induced paw inflammation by in situ hybridization histochemistry, reverse transcription-polymerase chain reaction, and immunohistochemistry. Nav1.1 and Nav1.6 mRNAs were present in all laminae, except for lamina II, including the spinothalamic tract neurons in lamina I identified by retrograde tracing of Fluoro-gold. Nav1.2 mRNA was predominantly observed in the superficial layers (laminae I, II), and Nav1.3 mRNA was more restricted to these layers. All these transcripts were expressed by the neurons characterized by immunostaining for neuron-specific nuclear protein. Nav1.7 mRNA was selectively expressed by a half of motoneurons in lamina IX. No signals for Nav1.8 or Nav1.9 mRNAs were detected. Immunohistochemistry for Nav1.1, Nav1.2, Nav1.6, and Nav1.7 proteins verified some of these neuronal distributions. L5 SPNL decreased Nav1.1 and Nav1.6 mRNAs, and increased Nav1.3 and Nav1.7 mRNAs in the axotomized spinal motoneurons, without any changes in other laminae of L4-6 spinal segments. Intradermal injection of CFA did not cause any transcriptional change. Our findings demonstrate that spinal neurons have different compositions of VGSCs according to their location in laminae. Pathophysiological changes of spinal neuronal activity may due to post-transcriptional changes of VGSCs. Comparison with our previous data concerning the subpopulation-specific distribution of Nav transcripts in primary afferent neurons provides potentially specific targets for local analgesics at the peripheral nerve and spinal levels.

Differences in Na+ conductance density and Na+ channel functional properties between dopamine and GABA neurons of the rat substantia nigra

Dopamine (DA) neurons and GABA neurons of the substantia nigra (SN) promote distinct functions in the control of movement and have different firing properties and action potential (AP) waveforms. APs recorded from DA and GABA neurons differed in amplitude, maximal rate of rise, and duration. In addition, the threshold potential for APs was higher in DA neurons than in GABA neurons. The activation of voltage-gated Na(+) channels accounts largely for these differences as the application of a low concentration of the voltage-gated Na(+) channel blocker TTX had an effect on all of these parameters. We have examined functional properties of somatic Na(+) channels in nucleated patches isolated from DA and GABA neurons. Peak amplitudes of macroscopic Na(+) currents were smaller in DA neurons in comparison to those in GABA neurons. The mean peak Na(+) conductance density was 24.5 pS microm(-2) in DA neurons and almost twice as large, 41.6 pS microm(-2), in GABA neurons. The voltage dependence of Na(+) channel activation was not different between the two types of SN neurons. Na(+) channels in DA and GABA neurons, however, differed in the voltage dependence of inactivation, the mean mid-point potential of steady-state inactivation curve being more positive in DA neurons than in GABA neurons. The results suggest that specific Na(+) channel gating properties and Na(+) conductance densities in the somatic membrane of SN neurons may have consequences on synaptic signal integration in the soma of both types of neurons and on somatodendritic release of dopamine by DA neurons.

delta-Opioid receptors protect from anoxic disruption of Na+ homeostasis via Na+ channel regulation

Hypoxic/ischemic disruption of ionic homeostasis is a critical trigger of neuronal injury/death in the brain. There is, however, no promising strategy against such pathophysiologic change to protect the brain from hypoxic/ischemic injury. Here, we present a novel finding that activation of delta-opioid receptors (DOR) reduced anoxic Na+ influx in the mouse cortex, which was completely blocked by DOR antagonism with naltrindole. Furthermore, we co-expressed DOR and Na+ channels in Xenopus oocytes and showed that DOR expression and activation indeed play an inhibitory role in Na+ channel regulation by decreasing the amplitude of sodium currents and increasing activation threshold of Na+ channels. Our results suggest that DOR protects from anoxic disruption of Na+ homeostasis via Na+ channel regulation. These data may potentially have significant impacts on understanding the intrinsic mechanism of neuronal responses to stress and provide clues for better solutions of hypoxic/ischemic encephalopathy, and for the exploration of acupuncture mechanism since acupuncture activates opioid system.

Localization and targeting of voltage-dependent ion channels in mammalian central neurons

The intrinsic electrical properties and the synaptic input-output relationships of neurons are governed by the action of voltage-dependent ion channels. The localization of specific populations of ion channels with distinct functional properties at discrete sites in neurons dramatically impacts excitability and synaptic transmission. Molecular cloning studies have revealed a large family of genes encoding voltage-dependent ion channel principal and auxiliary subunits, most of which are expressed in mammalian central neurons. Much recent effort has focused on determining which of these subunits coassemble into native neuronal channel complexes, and the cellular and subcellular distributions of these complexes, as a crucial step in understanding the contribution of these channels to specific aspects of neuronal function. Here we review progress made on recent studies aimed to determine the cellular and subcellular distribution of specific ion channel subunits in mammalian brain neurons using in situ hybridization and immunohistochemistry. We also discuss the repertoire of ion channel subunits in specific neuronal compartments and implications for neuronal physiology. Finally, we discuss the emerging mechanisms for determining the discrete subcellular distributions observed for many neuronal ion channels.

Modulation of sodium channel inactivation gating by a novel lactam: implications for seizure suppression in chronic limbic epilepsy

Epilepsy remains a devastating neurological disorder associated with recurrent, unprovoked, spontaneous epileptic seizures. Current treatments involve seizure suppression using antiepileptic drugs (AEDs); however, many patients remain refractory to current treatments or suffer serious side effects. In view of this continued need for more effective and safer AEDs, we have designed a novel compound, 3-hydroxy-3-(4-methoxyphenyl)-1-methyl-1,3-dihydro-indol-2-one (YWI92), based on a lactam structural class, and evaluated its modulation of human neuronal sodium channel isoform (hNa(v))1.2 currents and hippocampal neuron action potential firing. Furthermore, we have tested its AED activity using a chronic and acute rat seizure model. In a similar manner to lamotrigine, a clinically used AED, YWI92 exhibited tonic block of hNa(v)1.2 channels and caused a hyperpolarizing shift in the steady-state inactivation curve when using a 30-s inactivating prepulse. YWI92 also delayed the time constants of channel repriming after a 30-s inactivating prepulse and exhibited use-dependent block at 20-Hz stimulation frequency. In membrane excitability experiments, YWI92 inhibited burst firing in CA1 neurons of animals with temporal lobe epilepsy at concentrations that had little effect on CA1 neurons from control animals. These actions on neuronal activity translated into AED activity in the maximal electroshock acute seizure model (ED(50) = 22.96 mg/kg), and importantly, in a chronic temporal lobe epilepsy model, in which the mean number of seizures was reduced. Notably, YWI92 exhibited no sedative/ataxic side effects at concentrations up to 500 mg/kg. In summary, greater affinity for inactivated sodium channels, particularly after long depolarizing prepulses, may be important for both anticonvulsant activity and drug tolerability.

Effects of BmK AS on Nav1.2 expressed in Xenopus laevis oocytes

In the present study, the pharmacological effects of BmK AS, a beta-like scorpion toxin on rNav1.2 alpha-subunit expressed in Xenopus laevis oocytes were investigated using a two-electrode voltage-clamp recording. It was found that the voltage dependence of rNav1.2 inactivation was significantly shifted towards positive membrane potential by 500 nM BmK AS, whereas the activation curves of rNav1.2 were unruffled at the same dosage. The inactivation curves of both slow and fast inactivation currents were positively moved about 12.8 and 9.7 mV, respectively. In addition, the persistent currents of rNav1.2 were invariable. The effects of BmK AS on the rNav1.2 inactivation were opposite to the previous results found in the peripheral sensory neurons. The results suggested that Nav1.2 might be the target of BmK AS in the central nervous system, and BmK AS might have an excitatory effect on the central neuron through enhancing Nav1.2.