Action potential initiation in neocortical inhibitory interneurons

Aging-associated weakening of the action potential in fast-spiking interneurons in the human neocortex

Aging is associated with the slowdown of neuronal processing and cognitive performance in the brain; however, the exact cellular mechanisms behind this deterioration in humans are poorly elucidated. Recordings in human acute brain slices prepared from tissue resected during brain surgery enable the investigation of neuronal changes with age. Although neocortical fast-spiking cells are widely implicated in neuronal network activities underlying cognitive processes, they are vulnerable to neurodegeneration. Herein, we analyzed the electrical properties of 147 fast-spiking interneurons in neocortex samples resected in brain surgery from 106 patients aged 11-84 years. By studying the electrophysiological features of action potentials and passive membrane properties, we report that action potential overshoot significantly decreases and spike half-width increases with age. Moreover, the action potential maximum-rise speed (but not the repolarization speed or the afterhyperpolarization amplitude) significantly changed with age, suggesting a particular weakening of the sodium channel current generated in the soma. Cell passive membrane properties measured as the input resistance, membrane time constant, and cell capacitance remained unaffected by senescence. Thus, we conclude that the action potential in fast-spiking interneurons shows a significant weakening in the human neocortex with age. This may contribute to the deterioration of cortical functions by aging.

Disease-causing Slack potassium channel mutations produce opposite effects on excitability of excitatory and inhibitory neurons

The KCNT1 gene encodes the sodium-activated potassium channel Slack (KCNT1, KNa1.1), a regulator of neuronal excitability. Gain-of-function mutations in humans cause cortical network hyperexcitability, seizures, and severe intellectual disability. Using a mouse model expressing the Slack-R455H mutation, we find that Na+-dependent K+ (KNa) and voltage-dependent sodium (NaV) currents are increased in both excitatory and inhibitory cortical neurons. These increased currents, however, enhance the firing of excitability neurons but suppress that of inhibitory neurons. We further show that the expression of NaV channel subunits, particularly that of NaV1.6, is upregulated and that the length of the axon initial segment and of axonal NaV immunostaining is increased in both neuron types. Our study on the coordinate regulation of KNa currents and the expression of NaV channels may provide an avenue for understanding and treating epilepsies and other neurological disorders.

Parvalbumin Interneuron Impairment Leads to Synaptic Transmission Deficits and Seizures in SCN8A Epileptic Encephalopathy

SCN8A epileptic encephalopathy (EE) is a severe epilepsy syndrome resulting from de novo mutations in the voltage-gated sodium channel Na v 1.6, encoded by the gene SCN8A . Na v 1.6 is expressed in both excitatory and inhibitory neurons, yet previous studies have primarily focused on the impact SCN8A mutations have on excitatory neuron function, with limited studies on the importance of inhibitory interneurons to seizure onset and progression. Inhibitory interneurons are critical in balancing network excitability and are known to contribute to the pathophysiology of other epilepsies. Parvalbumin (PV) interneurons are the most prominent inhibitory neuron subtype in the brain, making up about 40% of inhibitory interneurons. Notably, PV interneurons express high levels of Na v 1.6. To assess the role of PV interneurons within SCN8A EE, we used two mouse models harboring patient-derived SCN8A gain-of-function mutations, Scn8a D/+ , where the SCN8A mutation N1768D is expressed globally, and Scn8a W/+ -PV, where the SCN8A mutation R1872W is selectively expressed in PV interneurons. Expression of the R1872W SCN8A mutation selectively in PV interneurons led to the development of spontaneous seizures in Scn8a W/+ -PV mice and seizure-induced death, decreasing survival compared to wild-type. Electrophysiology studies showed that PV interneurons in Scn8a D/+ and Scn8a W/+ -PV mice were susceptible to depolarization block, a state of action potential failure. Scn8a D/+ and Scn8a W/+ -PV interneurons also exhibited increased persistent sodium current, a hallmark of SCN8A gain-of-function mutations that contributes to depolarization block. Evaluation of synaptic connections between PV interneurons and pyramidal cells showed an increase in synaptic transmission failure at high frequencies (80-120Hz) as well as an increase in synaptic latency in Scn8a D/+ and Scn8a W/+ -PV interneurons. These data indicate a distinct impairment of synaptic transmission in SCN8A EE, potentially decreasing overall cortical network inhibition. Together, our novel findings indicate that failure of PV interneuron spiking via depolarization block along with frequency-dependent inhibitory synaptic impairment likely elicits an overall reduction in the inhibitory drive in SCN8A EE, leading to unchecked excitation and ultimately resulting in seizures and seizure-induced death.

Specific afterdischarge properties can enhance the clinical utility of electrical stimulation mapping during intracranial monitoring

OBJECTIVE

Extraoperative electrical cortical stimulation (ECS) facilitates defining the seizure onset zone (SOZ) and eloquent cortex. The clinical relevance of stimulation-induced afterdischarges (ADs) is not well defined.

METHODS

Fifty-five patients who underwent intracranial electroencephalogram evaluations with ECS were retrospectively identified. ADs were identified in these recordings and categorized by pattern, location, and association with stimulation-induced seizures.

RESULTS

ADs were generated in 1774/9285 (19%) trials. Rhythmic spikes and irregular ADs within the stimulated bipolar contact pair were predictive of location within the SOZ compared to non-epileptogenic/non-irritative cortex (rhythmic spikes OR 2.24, p = 0.0098; irregular OR 1.39; p = 0.013). ADs immediately preceding stimulated seizures occurred at lower stimulation intensity thresholds compared to other stimulations (mean 2.94 ± 0.28 mA vs. 4.16 ± 0.05 mA respectively; p = 0.0068).

CONCLUSIONS

Changes in AD properties can provide clinically relevant data in extraoperative stimulation mapping.

SIGNIFICANCE

Although not exclusive to the SOZ, the generation of rhythmic spikes may suggest that a stimulation location is within the SOZ, while decreased stimulation intensity thresholds eliciting ADs may alert clinicians to a heightened probability of seizure generation with subsequent stimulation.

Inversed Effects of Nav1.2 Deficiency at Medial Prefrontal Cortex and Ventral Tegmental Area for Prepulse Inhibition in Acoustic Startle Response

Numerous pathogenic variants of SCN2A gene, encoding voltage-gated sodium channel α2 subunit Nav1.2 protein, have been identified in a wide spectrum of neuropsychiatric disorders including schizophrenia. However, pathological mechanisms for the schizophrenia-relevant behavioral abnormalities caused by the variants remain poorly understood. Here in this study, we characterized mouse lines with selective Scn2a deletion at schizophrenia-related brain regions, medial prefrontal cortex (mPFC) or ventral tegmental area (VTA), obtained by injecting adeno-associated viruses (AAV) expressing Cre recombinase into homozygous Scn2a-floxed (Scn2afl/fl) mice, in which expression of the Scn2a was locally deleted in the presence of Cre recombinase. The mice lacking Scn2a in the mPFC exhibited a tendency for a reduction in prepulse inhibition (PPI) in acoustic startle response. Conversely, the mice lacking Scn2a in the VTA showed a significant increase in PPI. We also found that the mice lacking Scn2a in the mPFC displayed increased sociability, decreased locomotor activity, and increased anxiety-like behavior, while the mice lacking Scn2a in the VTA did not show any other abnormalities in these parameters except for vertical activity which is one of locomotor activities. These results suggest that Scn2a-deficiencies in mPFC and VTA are inversely relevant for the schizophrenic phenotypes in patients with SCN2A variants.

Neuronal Spike Shapes (NSS): A straightforward approach to investigate heterogeneity in neuronal excitability states

The mammalian brain exhibits a remarkable diversity of neurons, contributing to its intricate architecture and functional complexity. The analysis of multimodal single-cell datasets enables the investigation of cell types and states heterogeneity. In this study, we introduce the Neuronal Spike Shapes (NSS), a straightforward approach for the exploration of excitability states of neurons based on their Action Potential (AP) waveforms. The NSS method describes the AP waveform based on a triangular representation complemented by a set of derived electrophysiological (EP) features. To support this hypothesis, we validate the proposed approach on two datasets of murine cortical neurons, focusing it on GABAergic neurons. The validation process involves a combination of NSS-based clustering analysis, features exploration, Differential Expression (DE), and Gene Ontology (GO) enrichment analysis. Results show that the NSS-based analysis captures neuronal excitability states that possess biological relevance independently of cell subtype. In particular, Neuronal Spike Shapes (NSS) captures, among others, a well-characterized fast-spiking excitability state, supported by both electrophysiological and transcriptomic validation. Gene Ontology Enrichment Analysis reveals voltage-gated potassium (K+) channels as specific markers of the identified NSS partitions. This finding strongly corroborates the biological relevance of NSS partitions as excitability states, as the expression of voltage-gated K+ channels regulates the hyperpolarization phase of the AP, being directly implicated in the regulation of neuronal excitability.

Dynamic Foraging Behavior Performance Is Not Affected by Scn2a Haploinsufficiency

Dysfunction in the gene SCN2A, which encodes the voltage-gated sodium channel Nav1.2, is strongly associated with neurodevelopmental disorders including autism spectrum disorder and intellectual disability (ASD/ID). This dysfunction typically manifests in these disorders as a haploinsufficiency, where loss of one copy of a gene cannot be compensated for by the other allele. Scn2a haploinsufficiency affects a range of cells and circuits across the brain, including associative neocortical circuits that are important for cognitive flexibility and decision-making behaviors. Here, we tested whether Scn2a haploinsufficiency has any effect on a dynamic foraging task that engages such circuits. Scn2a +/- mice and wild-type (WT) littermates were trained on a choice behavior where the probability of reward between two options varied dynamically across trials and where the location of the high reward underwent uncued reversals. Despite impairments in Scn2a-related neuronal excitability, we found that both male and female Scn2a +/- mice performed these tasks as well as wild-type littermates, with no behavioral difference across genotypes in learning or performance parameters. Varying the number of trials between reversals or probabilities of receiving reward did not result in an observable behavioral difference, either. These data suggest that, despite heterozygous loss of Scn2a, mice can perform relatively complex foraging tasks that make use of higher-order neuronal circuits.

Structural and functional specializations of human fast-spiking neurons support fast cortical signaling

Fast-spiking interneurons (FSINs) provide fast inhibition that synchronizes neuronal activity and is critical for cognitive function. Fast synchronization frequencies are evolutionary conserved in the expanded human neocortex despite larger neuron-to-neuron distances that challenge fast input-output transfer functions of FSINs. Here, we test in human neurons from neurosurgery tissue, which mechanistic specializations of human FSINs explain their fast-signaling properties in human cortex. With morphological reconstructions, multipatch recordings, and biophysical modeling, we find that despite threefold longer dendritic path, human FSINs maintain fast inhibition between connected pyramidal neurons through several mechanisms: stronger synapse strength of excitatory inputs, larger dendrite diameter with reduced complexity, faster AP initiation, and faster and larger inhibitory output, while Na+ current activation/inactivation properties are similar. These adaptations underlie short input-output delays in fast inhibition of human pyramidal neurons through FSINs, explaining how cortical synchronization frequencies are conserved despite expanded and sparse network topology of human cortex.

The volatile anesthetic isoflurane differentially inhibits voltage-gated sodium channel currents between pyramidal and parvalbumin neurons in the prefrontal cortex

Background

How volatile anesthetics work remains poorly understood. Modulations of synaptic neurotransmission are the direct cellular mechanisms of volatile anesthetics in the central nervous system. Volatile anesthetics such as isoflurane may reduce neuronal interaction by differentially inhibiting neurotransmission between GABAergic and glutamatergic synapses. Presynaptic voltage-dependent sodium channels (Na_v), which are strictly coupled with synaptic vesicle exocytosis, are inhibited by volatile anesthetics and may contribute to the selectivity of isoflurane between GABAergic and glutamatergic synapses. However, it is still unknown how isoflurane at clinical concentrations differentially modulates Na_v currents between excitatory and inhibitory neurons at the tissue level.

Methods

In this study, an electrophysiological recording was applied in cortex slices to investigate the effects of isoflurane on Na_v between parvalbumin (PV⁺) and pyramidal neurons in PV-cre-tdTomato and/or vglut2-cre-tdTomato mice.

Results

Isoflurane at clinically relevant concentrations produced a hyperpolarizing shift in the voltage-dependent inactivation and slowed the recovery time from the fast inactivation in both cellular subtypes. Since the voltage of half-maximal inactivation was significantly depolarized in PV⁺ neurons compared to that of pyramidal neurons, isoflurane inhibited the peak Na_v currents in pyramidal neurons more potently than those of PV⁺ neurons (35.95 ± 13.32% vs. 19.24 ± 16.04%, P = 0.036 by the Mann-Whitney test).

Conclusions

Isoflurane differentially inhibits Na_v currents between pyramidal and PV⁺ neurons in the prefrontal cortex, which may contribute to the preferential suppression of glutamate release over GABA release, resulting in the net depression of excitatory-inhibitory circuits in the prefrontal cortex.

Structure of human NaV1.6 channel reveals Na+ selectivity and pore blockade by 4,9-anhydro-tetrodotoxin

The sodium channel Na_V1.6 is widely expressed in neurons of the central and peripheral nervous systems, which plays a critical role in regulating neuronal excitability. Dysfunction of Na_V1.6 has been linked to epileptic encephalopathy, intellectual disability and movement disorders. Here we present cryo-EM structures of human Na_V1.6/β1/β2 alone and complexed with a guanidinium neurotoxin 4,9-anhydro-tetrodotoxin (4,9-ah-TTX), revealing molecular mechanism of Na_V1.6 inhibition by the blocker. The apo-form structure reveals two potential Na⁺ binding sites within the selectivity filter, suggesting a possible mechanism for Na⁺ selectivity and conductance. In the 4,9-ah-TTX bound structure, 4,9-ah-TTX binds to a pocket similar to the tetrodotoxin (TTX) binding site, which occupies the Na⁺ binding sites and completely blocks the channel. Molecular dynamics simulation results show that subtle conformational differences in the selectivity filter affect the affinity of TTX analogues. Taken together, our results provide important insights into Na_V1.6 structure, ion conductance, and inhibition.

SUMOylation of NaV1.2 channels regulates the velocity of backpropagating action potentials in cortical pyramidal neurons

Voltage-gated sodium channels located in axon initial segments (AIS) trigger action potentials (AP) and play pivotal roles in the excitability of cortical pyramidal neurons. The differential electrophysiological properties and distributions of NaV1.2 and NaV1.6 channels lead to distinct contributions to AP initiation and propagation. While NaV1.6 at the distal AIS promotes AP initiation and forward propagation, NaV1.2 at the proximal AIS promotes the backpropagation of APs to the soma. Here, we show the small ubiquitin-like modifier (SUMO) pathway modulates Na+ channels at the AIS to increase neuronal gain and the speed of backpropagation. Since SUMO does not affect NaV1.6, these effects were attributed to SUMOylation of NaV1.2. Moreover, SUMO effects were absent in a mouse engineered to express NaV1.2-Lys38Gln channels that lack the site for SUMO linkage. Thus, SUMOylation of NaV1.2 exclusively controls INaP generation and AP backpropagation, thereby playing a prominent role in synaptic integration and plasticity.

Disease-causing Slack potassium channel mutations produce opposite effects on excitability of excitatory and inhibitory neurons

KCNT1 encodes the sodium-activated potassium channel Slack (KCNT1, K Na 1.1), an important mediator of neuronal membrane excitability. Gain-of-function (GOF) mutations in humans lead cortical network hyperexcitability and seizures, as well as very severe intellectual disability. Using a mouse model of Slack GOF-associated epilepsy, we found that both excitatory and inhibitory neurons of the cerebral cortex have increased Na + -dependent K + (K Na ) currents and voltage-dependent sodium (Na V ) currents. The characteristics of the increased K Na currents were, however, different in the two cell types such that the intrinsic excitability of excitatory neurons was enhanced but that of inhibitory neurons was suppressed. We further showed that the expression of Na V channel subunits, particularly that of Na V 1.6, is upregulated and that the length of the axon initial segment (AIS) and of axonal Na V immunostaining is increased in both neuron types. We found that the proximity of the AIS to the soma is shorter in excitatory neurons than in inhibitory neurons of the mutant animals, potentially contributing to the different effects on membrane excitability. Our study on the coordinate regulation of K Na currents and the expression of Na V channels may provide a new avenue for understanding and treating epilepsies and other neurological disorders.

In brief

In a genetic mouse model of Na + -activated K + potassium channel gene Slack -related childhood epilepsy, Wu et al . show that a disease-causing gain-of-function (GOF) mutation R455H in Slack channel causes opposite effects on excitability of cortical excitatory and inhibitory neurons. In contrast to heterologous expression systems, they find that the increase in potassium current substantially alters the expression of sodium channel subunits, resulting in increased lengths of axonal initial segments.

Highlights

GOF mutations in Slack potassium channel cause elevated outward K + currents and inward voltage-dependent Na + (Na V ) currents in cortical neurons Slack GOF does not alter the expression of Slack channel but upregulates the expression of Na V channel Slack GOF enhances the excitability of excitatory neurons but suppresses the firing of inhibitory interneuronsSlack GOF alters the length of AIS in both excitatory and inhibitory neuronsProximity of AIS to the soma is different between excitatory neuron and inhibitory neuron.

HCN channels at the cell soma ensure the rapid electrical reactivity of fast-spiking interneurons in human neocortex

Accumulating evidence indicates that there are substantial species differences in the properties of mammalian neurons, yet theories on circuit activity and information processing in the human brain are based heavily on results obtained from rodents and other experimental animals. This knowledge gap may be particularly important for understanding the neocortex, the brain area responsible for the most complex neuronal operations and showing the greatest evolutionary divergence. Here, we examined differences in the electrophysiological properties of human and mouse fast-spiking GABAergic basket cells, among the most abundant inhibitory interneurons in cortex. Analyses of membrane potential responses to current input, pharmacologically isolated somatic leak currents, isolated soma outside-out patch recordings, and immunohistochemical staining revealed that human neocortical basket cells abundantly express hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channel isoforms HCN1 and HCN2 at the cell soma membrane, whereas these channels are sparse at the rodent basket cell soma membrane. Antagonist experiments showed that HCN channels in human neurons contribute to the resting membrane potential and cell excitability at the cell soma, accelerate somatic membrane potential kinetics, and shorten the lag between excitatory postsynaptic potentials and action potential generation. These effects are important because the soma of human fast-spiking neurons without HCN channels exhibit low persistent ion leak and slow membrane potential kinetics, compared with mouse fast-spiking neurons. HCN channels speed up human cell membrane potential kinetics and help attain an input-output rate close to that of rodent cells. Computational modeling demonstrated that HCN channel activity at the human fast-spiking cell soma membrane is sufficient to accelerate the input-output function as observed in cell recordings. Thus, human and mouse fast-spiking neurons exhibit functionally significant differences in ion channel composition at the cell soma membrane to set the speed and fidelity of their input-output function. These HCN channels ensure fast electrical reactivity of fast-spiking cells in human neocortex.

Parvalbumin basket cell myelination accumulates axonal mitochondria to internodes

Parvalbumin-expressing (PV⁺) basket cells are fast-spiking inhibitory interneurons that exert critical control over local circuit activity and oscillations. PV⁺ axons are often myelinated, but the electrical and metabolic roles of interneuron myelination remain poorly understood. Here, we developed viral constructs allowing cell type-specific investigation of mitochondria with genetically encoded fluorescent probes. Single-cell reconstructions revealed that mitochondria selectively cluster to myelinated segments of PV⁺ basket cells, confirmed by analyses of a high-resolution electron microscopy dataset. In contrast to the increased mitochondrial densities in excitatory axons cuprizone-induced demyelination abolished mitochondrial clustering in PV⁺ axons. Furthermore, with genetic deletion of myelin basic protein the mitochondrial clustering was still observed at internodes wrapped by noncompacted myelin, indicating that compaction is dispensable. Finally, two-photon imaging of action potential-evoked calcium (Ca²⁺) responses showed that interneuron myelination attenuates both the cytosolic and mitochondrial Ca²⁺ transients. These findings suggest that oligodendrocyte ensheathment of PV⁺ axons assembles mitochondria to branch selectively fine-tune metabolic demands.

Biophysical Kv3 channel alterations dampen excitability of cortical PV interneurons and contribute to network hyperexcitability in early Alzheimer's

In Alzheimer’s disease (AD), a multitude of genetic risk factors and early biomarkers are known. Nevertheless, the causal factors responsible for initiating cognitive decline in AD remain controversial. Toxic plaques and tangles correlate with progressive neuropathology, yet disruptions in circuit activity emerge before their deposition in AD models and patients. Parvalbumin (PV) interneurons are potential candidates for dysregulating cortical excitability as they display altered action potential (AP) firing before neighboring excitatory neurons in prodromal AD. Here, we report a novel mechanism responsible for PV hypoexcitability in young adult familial AD mice. We found that biophysical modulation of K_v3 channels, but not changes in their mRNA or protein expression, were responsible for dampened excitability in young 5xFAD mice. These K⁺ conductances could efficiently regulate near-threshold AP firing, resulting in gamma-frequency-specific network hyperexcitability. Thus, biophysical ion channel alterations alone may reshape cortical network activity prior to changes in their expression levels. Our findings demonstrate an opportunity to design a novel class of targeted therapies to ameliorate cortical circuit hyperexcitability in early AD.

Paradoxical Hyperexcitability in Disorders of Neurodevelopment

Autism Spectrum Disorder (ASD), Rett syndrome (RTT) and Angelman Syndrome (AS) are neurodevelopmental disorders (NDDs) that share several clinical characteristics, including displays of repetitive movements, developmental delays, language deficits, intellectual disability, and increased susceptibility to epilepsy. While several reviews address the biological basis of non-seizure-related ASD phenotypes, here, I highlight some shared biological mechanisms that may contribute to increased seizure susceptibility. I focus on genetic studies identifying the anatomical origin of the seizure phenotype in loss-of-function, monogenic, mouse models of these NDDs, combined with insights gained from complementary studies quantifying levels of synaptic excitation and inhibition. Epilepsy is characterized by a sudden, abnormal increase in synchronous activity within neuronal networks, that is posited to arise from excess excitation, largely driven by reduced synaptic inhibition. Primarily for this reason, elevated network excitability is proposed to underlie the causal basis for the ASD, RTT, and AS phenotypes. Although, mouse models of these disorders replicate aspects of the human condition, i.e., hyperexcitability discharges or seizures on cortical electroencephalograms, measures at the synaptic level often reveal deficits in excitatory synaptic transmission, rather than too much excitation. Resolving this apparent paradox has direct implications regarding expected outcomes of manipulating GABAergic tone. In particular, in NDDs associated with seizures, cortical circuits can display reduced, rather than normal or increased levels of synaptic excitation, and therefore suggested treatments aimed at increasing inhibition could further promote hypoactivity instead of normality. In this review, I highlight shared mechanisms across animal models for ASD, RTT, and AS with reduced synaptic excitation that nevertheless promote hyperexcitability in cortical circuits.

Developmentally regulated impairment of parvalbumin interneuron synaptic transmission in an experimental model of Dravet syndrome

Dravet syndrome is a neurodevelopmental disorder characterized by epilepsy, intellectual disability, and sudden death due to pathogenic variants in SCN1A with loss of function of the sodium channel subunit Nav1.1. Nav1.1-expressing parvalbumin GABAergic interneurons (PV-INs) from young Scn1a^+/− mice show impaired action potential generation. An approach assessing PV-IN function in the same mice at two time points shows impaired spike generation in all Scn1a^+/− mice at postnatal days (P) 16–21, whether deceased prior or surviving to P35, with normalization by P35 in surviving mice. However, PV-IN synaptic transmission is dysfunctional in young Scn1a^+/− mice that did not survive and in Scn1a^+/− mice ≥ P35. Modeling confirms that PV-IN axonal propagation is more sensitive to decreased sodium conductance than spike generation. These results demonstrate dynamic dysfunction in Dravet syndrome: combined abnormalities of PV-IN spike generation and propagation drives early disease severity, while ongoing dysfunction of synaptic transmission contributes to chronic pathology.

Dravet syndrome is caused by variants in SCN1A with loss of function of Nav1.1 sodium channels. Kaneko et al. use the “mini-slice” to record at two developmental time points. Impaired spike generation of Nav1.1-expressing PV interneurons in Scn1a^+/− mice is transient, while abnormalities of PV interneuron synaptic transmission persist.

Afterhyperpolarization Promotes the Firing of Mitral Cells through a Voltage-Dependent Modification of Action Potential Threshold

In the olfactory bulb, mitral cells (MCs) display a spontaneous firing that is characterized by bursts of action potentials (APs) intermixed with silent periods. Intraburst firing frequency and duration are heterogeneous among MCs and increase with membrane depolarization. By using patch-clamp recording on rat slices, we dissected out the intrinsic properties responsible for this bursting activity. We showed that the threshold of AP generation dynamically changes as a function of the preceding trajectory of the membrane potential. In fact, the AP threshold became more negative when the membrane was hyperpolarized and had a recovery rate inversely proportional to the membrane repolarization rate. Such variations appeared to be produced by changes in the inactivation state of voltage-dependent Na⁺ channels. Thus, AP initiation was favored by hyperpolarizing events, such as negative membrane oscillations or inhibitory synaptic input. After the first AP, the following fast afterhyperpolarization (AHP) brought the threshold to more negative values and then promoted the emission of the following AP. This phenomenon was repeated for each AP of the burst making the fast AHP a regenerative mechanism that sustained the firing, AHP with larger amplitudes and faster repolarizations being associated with larger and higher-frequency bursts. Burst termination was found to be because of the development of a slow repolarization component of the AHP (slow AHP). Overall, the AHP characteristics appeared as a major determinant of the bursting properties.

The diffuse distribution of Nav1.2 on mid-axonal regions is a marker for unmyelinated fibers in the central nervous system

Unmyelinated fibers in the central nervous system are known to exist in hippocampal mossy fibers, cerebellar parallel fibers and striatal projection fibers. Previously, we and others reported diffuse distribution of Nav1.2, a voltage-gated sodium channel α-subunit encoded by the SCN2A gene, on unmyelinated striatal projection fibers. Mutations in the SCN2A gene are associated with epilepsies and autism. In this study, we investigated the distribution of Nav1.2 on the unmyelinated fibers in the corpus callosum and stria terminalis by immunohistochemistry and immunoelectron microscopy analysis, suggesting that diffuse localization of Nav1.2 on mid-axonal regions can be a useful marker for unmyelinated fibers.

Somatostatin-Positive Interneurons Contribute to Seizures in SCN8A Epileptic Encephalopathy

SCN8A epileptic encephalopathy is a devastating epilepsy syndrome caused by mutant SCN8A, which encodes the voltage-gated sodium channel Na_V1.6. To date, it is unclear if and how inhibitory interneurons, which express Na_V1.6, influence disease pathology. Using both sexes of a transgenic mouse model of SCN8A epileptic encephalopathy, we found that selective expression of the R1872W SCN8A mutation in somatostatin (SST) interneurons was sufficient to convey susceptibility to audiogenic seizures. Patch-clamp electrophysiology experiments revealed that SST interneurons from mutant mice were hyperexcitable but hypersensitive to action potential failure via depolarization block under normal and seizure-like conditions. Remarkably, GqDREADD-mediated activation of WT SST interneurons resulted in prolonged electrographic seizures and was accompanied by SST hyperexcitability and depolarization block. Aberrantly large persistent sodium currents, a hallmark of SCN8A mutations, were observed and were found to contribute directly to aberrant SST physiology in computational modeling and pharmacological experiments. These novel findings demonstrate a critical and previously unidentified contribution of SST interneurons to seizure generation not only in SCN8A epileptic encephalopathy, but epilepsy in general.SIGNIFICANCE STATEMENT SCN8A epileptic encephalopathy is a devastating neurological disorder that results from de novo mutations in the sodium channel isoform Na_v1.6. Inhibitory neurons express Na_V1.6, yet their contribution to seizure generation in SCN8A epileptic encephalopathy has not been determined. We show that mice expressing a human-derived SCN8A variant (R1872W) selectively in somatostatin (SST) interneurons have audiogenic seizures. Physiological recordings from SST interneurons show that SCN8A mutations lead to an elevated persistent sodium current which drives initial hyperexcitability, followed by premature action potential failure because of depolarization block. Furthermore, chemogenetic activation of WT SST interneurons leads to audiogenic seizure activity. These findings provide new insight into the importance of SST inhibitory interneurons in seizure initiation, not only in SCN8A epileptic encephalopathy, but for epilepsy broadly.

Enhanced slow inactivation contributes to dysfunction of a recurrent SCN2A mutation associated with developmental and epileptic encephalopathy

KEY POINTS

The recurrent SCN2A mutation R853Q is associated with developmental and epileptic encephalopathy with typical onset after the first months of life. Heterologously expressed R853Q channels exhibit an overall loss-of-function as a result of multiple defects in time- and voltage-dependent channel properties. A previously unrecognized enhancement of slow inactivation is conferred by the R853Q mutation and is a major driver of loss-of-function. Enhanced slow inactivation is potentiated in the canonical splice isoform of the channel and this may explain the later onset of symptoms associated with R853Q.

ABSTRACT

Mutations in voltage gated sodium (NaV ) channel genes, including SCN2A (encoding NaV 1.2), are associated with diverse neurodevelopmental disorders with or without epilepsy that present clinically with varying severity, age-of-onset and pharmacoresponsiveness. We examined the functional properties of the most recurrent SCN2A mutation (R853Q) to determine whether developmentally-regulated alternative splicing impacts dysfunction severity and to investigate effects of the mutation on slow inactivation. We engineered the R853Q mutation into neonatal and adult NaV 1.2 splice isoforms. Channel constructs were heterologously co-expressed in HEK293T cells with human β1 and β2 subunits. Whole-cell patch clamp recording was used to compare time- and voltage-dependent properties of mutant and wild-type channels. The R853Q mutation exhibits an overall loss-of-function attributed to multiple functional defects including a previously undiscovered enhancement of slow inactivation. The mutation exhibited altered voltage dependence of activation and inactivation, slower recovery from inactivation and decreased channel availability during high-frequency depolarizations. More notable were effects on slow inactivation, including a 10-fold slower rate of recovery from slow inactivation exhibited by mutant channels. The impairments in fast inactivation properties were more severe in the neonatal splice isoform, whereas slow inactivation was more pronounced in the splice isoform of the channel expressed predominantly in later childhood. Enhanced later-onset slow inactivation may be a primary driver of the later onset of neurological features associated with this mutation.

Selective inhibition of gamma aminobutyric acid release from mouse hippocampal interneurone subtypes by the volatile anaesthetic isoflurane

BACKGROUND

The cellular and molecular mechanisms by which general anaesthesia occurs is poorly understood. Hippocampal interneurone subpopulations, which are critical regulators of cognitive function, have diverse neurophysiological and synaptic properties, but their responses to anaesthetics are unclear.

METHODS

We used live-cell imaging of fluorescent biosensors expressed in mouse hippocampal neurones to delineate interneurone subtype-specific effects of isoflurane on synaptic vesicle exocytosis. The role of voltage-gated sodium channel (Na_v) subtype expression in determining isoflurane sensitivity was probed by overexpression or knockdown of specific Na_v subtypes in identified interneurones.

RESULTS

Clinically relevant concentrations of isoflurane differentially inhibited synaptic vesicle exocytosis: to 83.1% (11.7%) of control in parvalbumin-expressing interneurones, and to 58.6% (13.3%) and 64.5% (8.5%) of control in somatostatin-expressing interneurones and glutamatergic neurones, respectively. The relative expression of Na_v1.1 (associated with lower sensitivity) and Na_v1.6 (associated with higher sensitivity) determined the sensitivity of exocytosis to isoflurane.

CONCLUSIONS

Isoflurane inhibits synaptic vesicle exocytosis from hippocampal glutamatergic neurones and GABAergic interneurones in a cell-type-specific manner depending on their expression of voltage-gated sodium channel subtypes.

Paradoxical hyperexcitability from NaV1.2 sodium channel loss in neocortical pyramidal cells

Loss-of-function variants in the gene SCN2A, which encodes the sodium channel NaV1.2, are strongly associated with autism spectrum disorder and intellectual disability. An estimated 20%-30% of children with these variants also suffer from epilepsy, with altered neuronal activity originating in neocortex, a region where NaV1.2 channels are expressed predominantly in excitatory pyramidal cells. This is paradoxical, as sodium channel loss in excitatory cells would be expected to dampen neocortical activity rather than promote seizure. Here, we examined pyramidal neurons lacking NaV1.2 channels and found that they were intrinsically hyperexcitable, firing high-frequency bursts of action potentials (APs) despite decrements in AP size and speed. Compartmental modeling and dynamic-clamp recordings revealed that NaV1.2 loss prevented potassium channels from properly repolarizing neurons between APs, increasing overall excitability by allowing neurons to reach threshold for subsequent APs more rapidly. This cell-intrinsic mechanism may, therefore, account for why SCN2A loss-of-function can paradoxically promote seizure.

Effects of general anesthetics on synaptic transmission and plasticity

General anesthetics depress excitatory and/or enhance inhibitory synaptic transmission principally by modulating the function of glutamatergic or GABAergic synapses, respectively, with relative anesthetic agent-specific mechanisms. Synaptic signaling proteins, including ligand- and voltage-gated ion channels, are targeted by general anesthetics to modulate various synaptic mechanisms, including presynaptic neurotransmitter release, postsynaptic receptor signaling, and dendritic spine dynamics to produce their characteristic acute neurophysiological effects. As synaptic structure and plasticity mediate higher-order functions such as learning and memory, long-term synaptic dysfunction following anesthesia may lead to undesirable neurocognitive consequences depending on the specific anesthetic agent and the vulnerability of the population. Here we review the cellular and molecular mechanisms of transient and persistent general anesthetic alterations of synaptic transmission and plasticity.

Does the Brain Function as a Quantum Phase Computer Using Phase Ternary Computation?

Here we provide evidence that the fundamental basis of nervous communication is derived from a pressure pulse/soliton capable of computation with sufficient temporal precision to overcome any processing errors. Signalling and computing within the nervous system are complex and different phenomena. Action potentials are plastic and this makes the action potential peak an inappropriate fixed point for neural computation, but the action potential threshold is suitable for this purpose. Furthermore, neural models timed by spiking neurons operate below the rate necessary to overcome processing error. Using retinal processing as our example, we demonstrate that the contemporary theory of nerve conduction based on cable theory is inappropriate to account for the short computational time necessary for the full functioning of the retina and by implication the rest of the brain. Moreover, cable theory cannot be instrumental in the propagation of the action potential because at the activation-threshold there is insufficient charge at the activation site for successive ion channels to be electrostatically opened. Deconstruction of the brain neural network suggests that it is a member of a group of Quantum phase computers of which the Turing machine is the simplest: the brain is another based upon phase ternary computation. However, attempts to use Turing based mechanisms cannot resolve the coding of the retina or the computation of intelligence, as the technology of Turing based computers is fundamentally different. We demonstrate that that coding in the brain neural network is quantum based, where the quanta have a temporal variable and a phase-base variable enabling phase ternary computation as previously demonstrated in the retina.

Neddylation stabilizes Nav1.1 to maintain interneuron excitability and prevent seizures in murine epilepsy models

The excitability of interneurons requires Nav1.1, the α subunit of the voltage-gated sodium channel. Nav1.1 deficiency and mutations reduce interneuron excitability, a major pathological mechanism for epilepsy syndromes. However, the regulatory mechanisms of Nav1.1 expression remain unclear. Here, we provide evidence that neddylation is critical to Nav1.1 stability. Mutant mice lacking Nae1, an obligatory component of the E1 ligase for neddylation, in parvalbumin-positive interneurons (PVINs) exhibited spontaneous epileptic seizures and premature death. Electrophysiological studies indicate that Nae1 deletion reduced PVIN excitability and GABA release and consequently increased the network excitability of pyramidal neurons (PyNs). Further analysis revealed a reduction in sodium-current density, not a change in channel property, in mutant PVINs and decreased Nav1.1 protein levels. These results suggest that insufficient neddylation in PVINs reduces Nav1.1 stability and thus the excitability of PVINs; the ensuing increased PyN activity causes seizures in mice. Consistently, Nav1.1 was found reduced by proteomic analysis that revealed abnormality in synapses and metabolic pathways. Our findings describe a role of neddylation in maintaining Nav1.1 stability for PVIN excitability and reveal what we believe is a new mechanism in the pathogenesis of epilepsy.

Interneuron Dysfunction in a New Mouse Model of SCN1A GEFS

Advances in genome sequencing have identified over 1300 mutations in the SCN1A sodium channel gene that result in genetic epilepsies. However, it still remains unclear how most individual mutations within SCN1A result in seizures. A previous study has shown that the K1270T (KT) mutation, linked to genetic epilepsy with febrile seizure plus (GEFS+) in humans, causes heat-induced seizure activity associated with a temperature-dependent decrease in GABAergic neuron excitability in a Drosophila knock-in model. To examine the behavioral and cellular effects of this mutation in mammals, we introduced the equivalent KT mutation into the mouse (Mus musculus) Scn1a (Scn1a^KT) gene using CRISPR/Cas9 and generated mutant lines in two widely used genetic backgrounds: C57BL/6NJ and 129X1/SvJ. In both backgrounds, mice homozygous for the KT mutation had spontaneous seizures and died by postnatal day (P)23. There was no difference in mortality of heterozygous KT mice compared with wild-type littermates up to six months old. Heterozygous mutants exhibited heat-induced seizures at ∼42°C, a temperature that did not induce seizures in wild-type littermates. In acute hippocampal slices at permissive temperatures, current-clamp recordings revealed a significantly depolarized shift in action potential threshold and reduced action potential amplitude in parvalbumin (PV)-expressing inhibitory CA1 interneurons in Scn1a^KT/+ mice. There was no change in the firing properties of excitatory CA1 pyramidal neurons. These results suggest that a constitutive decrease in inhibitory interneuron excitability contributes to the seizure phenotype in the mouse model.

Kv1.1 deficiency alters repetitive and social behaviors in mice and rescues autistic-like behaviors due to Scn2a haploinsufficiency

BACKGROUND

Autism spectrum disorder (ASD) and epilepsy are highly comorbid, suggesting potential overlap in genetic etiology, pathophysiology, and neurodevelopmental abnormalities; however, the nature of this relationship remains unclear. This work investigated how two ion channel mutations, one associated with autism (Scn2a-null) and one with epilepsy (Kcna1-null), interact to modify genotype-phenotype relationships in the context of autism. Previous studies have shown that Scn2a+/- ameliorates epilepsy in Kcna1-/- mice, improving survival, seizure characteristics, and brain-heart dynamics. Here, we tested the converse, whether Kcna1 deletion modifies ASD-like repetitive and social behaviors in Scn2a+/- mice.

METHODS

Mice were bred with various combinations of Kcna1 and Scn2a knockout alleles. Animals were assessed for repetitive behaviors using marble burying, grooming, and nestlet shredding tests and for social behaviors using sociability and social novelty preference tests.

RESULTS

Behavioral testing revealed drastic reductions in all repetitive behaviors in epileptic Kcna1-/- mice, but relatively normal social interactions. In contrast, mice with partial Kcna1 deletion (Kcna1+/- ) exhibited increased self-grooming and decreased sociability suggestive of ASD-like features similar to those observed in Scn2a+/- mice. In double-mutant Scn2a+/- ; Kcna1+/- mice, the two mutations interacted to partially normalize ASD-like behaviors associated with each mutation independently.

CONCLUSIONS

Taken together, these findings suggest that Kv1.1 subunits are important in pathways and neural networks underlying ASD and that Kcna1 may be a therapeutic target for treatment of Scn2a-associated ASD.

Sodium channelopathies of skeletal muscle and brain

Voltage-gated sodium channels initiate action potentials in nerve, skeletal muscle, and other electrically excitable cells. Mutations in them cause a wide range of diseases. These channelopathy mutations affect every aspect of sodium channel function, including voltage sensing, voltage-dependent activation, ion conductance, fast and slow inactivation, and both biosynthesis and assembly. Mutations that cause different forms of periodic paralysis in skeletal muscle were discovered first and have provided a template for understanding structure, function, and pathophysiology at the molecular level. More recent work has revealed multiple sodium channelopathies in the brain. Here we review the well-characterized genetics and pathophysiology of the periodic paralyses of skeletal muscle and then use this information as a foundation for advancing our understanding of mutations in the structurally homologous α-subunits of brain sodium channels that cause epilepsy, migraine, autism, and related comorbidities. We include studies based on molecular and structural biology, cell biology and physiology, pharmacology, and mouse genetics. Our review reveals unexpected connections among these different types of sodium channelopathies.

Conduction Velocity Along the Local Axons of Parvalbumin Interneurons Correlates With the Degree of Axonal Myelination

Parvalbumin-containing (PV+) basket cells in mammalian neocortex are fast-spiking interneurons that regulate the activity of local neuronal circuits in multiple ways. Even though PV+ basket cells are locally projecting interneurons, their axons are myelinated. Can this myelination contribute in any significant way to the speed of action potential propagation along such short axons? We used dual whole cell recordings of synaptically connected PV+ interneurons and their postsynaptic target in acutely prepared neocortical slices from adult mice to measure the amplitude and latency of single presynaptic action potential-evoked inhibitory postsynaptic currents. These same neurons were then imaged with immunofluorescent array tomography, the synapses between them identified and a precise map of the connections was generated, with the exact axonal length and extent of myelin coverage. Our results support that myelination of PV+ basket cells significantly increases conduction velocity, and does so to a degree that can be physiologically relevant.

Three-dimensional neuron-astrocyte construction on matrigel enhances establishment of functional voltage-gated sodium channels

This study aimed to investigate and compare cell growth manners and functional differences of primary cortical neurons cultured on either poly-d-lysine (PDL) and or Matrigel, to delineate the role of extracellular matrix on providing resemblance to in vivo cellular interactions in nervous tissue. Primary cortical neurons, obtained from embryonic day 15 mice pups, seeded either on PDL- or Matrigel-coated culture ware were investigated by DIC/bright field and fluorescence/confocal microscopy for their morphology, 2D and 3D structure, and distribution patterns. Patch clamp, western blot, and RT-PCR studies were performed to investigate neuronal firing thresholds and sodium channel subtypes Nav1.2 and Nav1.6 expression. Cortical neurons cultured on PDL coating possessed a 2D structure composed of a few numbers of branched and tortuous neurites that contacted with each other in one to one manner, however, neurons on Matrigel coating showed a more complicated dimensional network that depicted tight, linear axonal bundles forming a 3D interacted neuron-astrocyte construction. This difference in growth patterns also showed a significant alteration in neuronal firing threshold which was recorded between 80 < Iinj > 120 pA on PDL and 2 < Iinj > 160 pA on Matrigel. Neurons grown up on Matrigel showed increased levels of sodium channel protein expression of Nav1.2 and Nav1.6 compared to neurons on PDL. These results have demonstrated that a 3D interacted neuron-astrocyte construction on Matrigel enhances the development of Nav1.2 and Nav1.6 in vitro and decreases neuronal firing threshold by 40 times compared to conventional PDL, resembling in vivo neuronal networks and hence would be a better in vitro model of adult neurons.

Pathophysiological roles and therapeutic potential of voltage-gated ion channels (VGICs) in pain associated with herpesvirus infection

Herpesvirus is ranked as one of the grand old members of all pathogens. Of all the viruses in the superfamily, Herpes simplex virus type 1 (HSV-1) is considered as a model virus for a variety of reasons. In a permissive non-neuronal cell culture, HSV-1 concludes the entire life cycle in approximately 18–20 h, encoding approximately 90 unique transcriptional units. In latency, the robust viral gene expression is suppressed in neurons by a group of noncoding RNA. Historically the lesions caused by the virus can date back to centuries ago. As a neurotropic pathogen, HSV-1 is associated with painful oral lesions, severe keratitis and lethal encephalitis. Transmission of pain signals is dependent on the generation and propagation of action potential in sensory neurons. T-type Ca²⁺ channels serve as a preamplifier of action potential generation. Voltage-gated Na⁺ channels are the main components for action potential production. This review summarizes not only the voltage-gated ion channels in neuropathic disorders but also provides the new insights into HSV-1 induced pain.

An axon-specific expression of HCN channels catalyzes fast action potential signaling in GABAergic interneurons

During high-frequency network activities, fast-spiking, parvalbumin-expressing basket cells (PV⁺-BCs) generate barrages of fast synaptic inhibition to control the probability and precise timing of action potential (AP) initiation in principal neurons. Here we describe a subcellular specialization that contributes to the high speed of synaptic inhibition mediated by PV⁺-BCs. Mapping of hyperpolarization-activated cyclic nucleotide-gated (HCN) channel distribution in rat hippocampal PV⁺-BCs with subcellular patch-clamp methods revealed that functional HCN channels are exclusively expressed in axons and completely absent from somata and dendrites. HCN channels not only enhance AP initiation during sustained high-frequency firing but also speed up the propagation of AP trains in PV⁺-BC axons by dynamically opposing the hyperpolarization produced by Na⁺-K⁺ ATPases. Since axonal AP signaling determines the timing of synaptic communication, the axon-specific expression of HCN channels represents a specialization for PV⁺-BCs to operate at high speed.

The precise subcellular location of ion channels is a key determinant of their functions. Here, subcellular patch-clamp recordings demonstrate that an axon-specific expression of HCN channels facilitates the initiation and propagation of action potentials in parvalbumin-expressing basket cells.

Axonal Tree Morphology and Signal Propagation Dynamics Improve Interneuron Classification

Neurons are diverse and can be differentiated by their morphological, electrophysiological, and molecular properties. Current morphology-based classification approaches largely rely on the dendritic tree structure or on the overall axonal projection layout. Here, we use data from public databases of neuronal reconstructions and membrane properties to study the characteristics of the axonal and dendritic trees for interneuron classification. We show that combining signal propagation patterns observed by biophysical simulations of the activity along ramified axonal trees with morphological parameters of the axonal and dendritic trees, significantly improve classification results compared to previous approaches. The classification schemes introduced here can be utilized for robust neuronal classification. Our work paves the way for understanding and utilizing form-function principles in realistic neuronal reconstructions.

SCN2A channelopathies: Mechanisms and models

Variants in the SCN2A gene, encoding the voltage-gated sodium channel Na_V 1.2, cause a variety of neuropsychiatric syndromes with different severity ranging from self-limiting epilepsies with early onset to developmental and epileptic encephalopathy with early or late onset and intellectual disability (ID), as well as ID or autism without seizures. Functional analysis of channel defects demonstrated a genotype-phenotype correlation and suggested effective treatment options for one group of affected patients carrying gain-of-function variants. Here, we sum up the functional mechanisms underlying different phenotypes of patients with SCN2A channelopathies and present currently available models that can help in understanding SCN2A-related disorders.

Sexually Divergent Mortality and Partial Phenotypic Rescue After Gene Therapy in a Mouse Model of Dravet Syndrome

Dravet syndrome (DS) is a neurodevelopmental genetic disorder caused by mutations in the SCN1A gene encoding the α subunit of the NaV1.1 voltage-gated sodium channel that controls neuronal action potential firing. The high density of this mutated channel in GABAergic interneurons results in impaired inhibitory neurotransmission and subsequent excessive activation of excitatory neurons. The syndrome is associated with severe childhood epilepsy, autistic behaviors, and sudden unexpected death in epilepsy. Here, we compared the rescue effects of an adeno-associated viral (AAV) vector coding for the multifunctional β1 sodium channel auxiliary subunit (AAV-NaVβ1) with a control vector lacking a transgene. We hypothesized that overexpression of NaVβ1 would facilitate the function of residual voltage-gated channels and improve the DS phenotype in the Scn1a^+/− mouse model of DS. AAV-NaVβ1 was injected into the cerebral spinal fluid of neonatal Scn1a^+/− mice. In untreated control Scn1a^+/− mice, females showed a higher degree of mortality than males. Compared with Scn1a^+/− control mice, AAV-NaVβ1-treated Scn1a^+/− mice displayed increased survival, an outcome that was more pronounced in females than males. In contrast, behavioral analysis revealed that male, but not female, Scn1a^+/− mice displayed motor hyperactivity, and abnormal performance on tests of fear and anxiety and learning and memory. Male Scn1a^+/− mice treated with AAV-NaVβ1 showed reduced spontaneous seizures and normalization of motor activity and performance on the elevated plus maze test. These findings demonstrate sex differences in mortality in untreated Scn1a^+/− mice, an effect that may be related to a lower level of intrinsic inhibitory tone in female mice, and a normalization of aberrant behaviors in males after central nervous system administration of AAV-NaVβ1. The therapeutic efficacy of AAV-NaVβ1 in a mouse model of DS suggests a potential new long-lasting biological therapeutic avenue for the treatment of this catastrophic epilepsy.

Role of Voltage-Gated Sodium Channels in the Mechanism of Ether-Induced Unconsciousness

Despite continuous clinical use for more than 170 years, the mechanism of general anesthetics has not been completely characterized. In this review, we focus on the role of voltage-gated sodium channels in the sedative-hypnotic actions of halogenated ethers, describing the history of anesthetic mechanisms research, the basic neurobiology and pharmacology of voltage-gated sodium channels, and the evidence for a mechanistic interaction between halogenated ethers and sodium channels in the induction of unconsciousness. We conclude with a more integrative perspective of how voltage-gated sodium channels might provide a critical link between molecular actions of the halogenated ethers and the more distributed network-level effects associated with the anesthetized state across species.

NaV1.2 haploinsufficiency in Scn2a knock-out mice causes an autistic-like phenotype attenuated with age

Mutations of the SCN2A gene, encoding the voltage gated sodium channel Na_V1.2, have been associated to a wide spectrum of epileptic disorders ranging from benign familial neonatal-infantile seizures to early onset epileptic encephalopathies such as Ohtahara syndrome. These phenotypes may be caused by either gain-of-function or loss-of-function mutations. More recently, loss-of-function SCN2A mutations have also been identified in patients with autism spectrum disorder (ASD) without overt epileptic phenotypes. Heterozygous Scn2a knock-out mice (Scn2a^+/−) may be a model of this phenotype. Because ASD develops in childhood, we performed a detailed behavioral characterization of Scn2a^+/− mice comparing the juvenile/adolescent period of development and adulthood. We used tasks relevant to ASD and the different comorbidities frequently found in this disorder, such as anxiety or intellectual disability. Our data demonstrate that young Scn2a^+/− mice display autistic-like phenotype associated to impaired memory and reduced reactivity to stressful stimuli. Interestingly, these dysfunctions are attenuated with age since adult mice show only communicative deficits. Considering the clinical data available on patients with loss-of-function SCN2A mutations, our results indicate that Scn2a^+/− mice constitute an ASD model with construct and face validity during the juvenile/adolescent period of development. However, more information about the clinical features of adult carriers of SCN2A mutations is needed to evaluate comparatively the phenotype of adult Scn2a^+/− mice.

Biophysical Properties of Somatic and Axonal Voltage-Gated Sodium Channels in Midbrain Dopaminergic Neurons

Spiking activities of midbrain dopaminergic neurons are critical for key brain functions including motor control and affective behaviors. Voltage-gated Na⁺ channels determine neuronal excitability and action potential (AP) generation. Previous studies on dopaminergic neuron excitability mainly focused on Na⁺ channels at the somatodendritic compartments. Properties of axonal Na⁺ channels, however, remain largely unknown. Using patch-clamp recording from somatic nucleated patches and isolated axonal blebs from the axon initial segment (AIS) of dopaminergic neurons in mouse midbrain slices, we found that AIS channel density is approximately 4–9 fold higher than that at the soma. Similar voltage dependence of channel activation and inactivation was observed between somatic and axonal channels in both SNc and VTA cells, except that SNc somatic channels inactivate at more hyperpolarized membrane potentials (V_m). In both SNc and VTA, axonal channels take longer time to inactivate at a subthreshold depolarization V_m level, but are faster to recover from inactivation than somatic channels. Moreover, we found that immunosignals of Nav1.2 accumulate at the AIS of dopaminergic neurons. In contrast, Nav1.1 and Nav1.6 immunosignals are not detectible. Together, our results reveal a high density of Na⁺ channels at the AIS and their molecular identity. In general, somatic and axonal channels of both SNc and VTA dopaminergic neurons share similar biophysical properties. The relatively delayed inactivation onset and faster recovery from inactivation of axonal Na⁺ channels may ensure AP initiation at high frequencies and faithful signal conduction along the axon.

Vasoactive intestinal peptide-expressing interneurons are impaired in a mouse model of Dravet syndrome

Dravet Syndrome (DS) is a severe neurodevelopmental disorder caused by pathogenic loss of function variants in the gene SCN1A which encodes the voltage gated sodium (Na+) channel subunit Nav1.1. GABAergic interneurons expressing parvalbumin (PV-INs) and somatostatin (SST-INs) exhibit impaired excitability in DS (Scn1a+/-) mice. However, the function of a third major class of interneurons in DS - those expressing vasoactive intestinal peptide (VIP-IN) -is unknown. We recorded VIP-INs in brain slices from Scn1a+/-mice and wild-type littermate controls and found prominent impairment of irregular spiking (IS), but not continuous adapting (CA) VIP-INs, in Scn1a+/- mice. Application of the Nav1.1-specific toxin Hm1a rescued the observed deficits. The IS vs. CA firing pattern is determined by expression of KCNQ channels; IS VIP-INs switched to tonic firing with both pharmacologic blockade of M-current and muscarinic acetylcholine receptor activation. These results show that VIP-INs express Nav1.1 and are dysfunctional in DS, which may contribute to DS pathogenesis.

The Autism-Associated Gene Scn2a Contributes to Dendritic Excitability and Synaptic Function in the Prefrontal Cortex

Autism spectrum disorder (ASD) is strongly associated with de novo gene mutations. One of the most commonly affected genes is SCN2A. ASD-associated SCN2A mutations impair the encoded protein Na_V1.2, a sodium channel important for action potential initiation and propagation in developing excitatory cortical neurons. The link between an axonal sodium channel and ASD, a disorder typically attributed to synaptic or transcriptional dysfunction, is unclear. Here we show that Na_V1.2 is unexpectedly critical for dendritic excitability and synaptic function in mature pyramidal neurons in addition to regulating early developmental axonal excitability. Na_V1.2 loss reduced action potential backpropagation into dendrites, impairing synaptic plasticity and synaptic strength, even when Na_V1.2 expression was disrupted in a cell-autonomous fashion late in development. These results reveal a novel dendritic function for Na_V1.2, providing insight into cellular mechanisms probably underlying circuit and behavioral dysfunction in ASD.

Scn2a Haploinsufficiency in Mice Suppresses Hippocampal Neuronal Excitability, Excitatory Synaptic Drive, and Long-Term Potentiation, and Spatial Learning and Memory

Nav1.2, a voltage-gated sodium channel subunit encoded by the Scn2a gene, has been implicated in various brain disorders, including epilepsy, autism spectrum disorder, intellectual disability, and schizophrenia. Nav1.2 is known to regulate the generation of action potentials in the axon initial segment and their propagation along axonal pathways. Nav1.2 also regulates synaptic integration and plasticity by promoting back-propagation of action potentials to dendrites, but whether Nav1.2 deletion in mice affects neuronal excitability, synaptic transmission, synaptic plasticity, and/or disease-related animal behaviors remains largely unclear. Here, we report that mice heterozygous for the Scn2a gene (Scn2a^+/- mice) show decreased neuronal excitability and suppressed excitatory synaptic transmission in the presence of network activity in the hippocampus. In addition, Scn2a^+/- mice show suppressed hippocampal long-term potentiation (LTP) in association with impaired spatial learning and memory, but show largely normal locomotor activity, anxiety-like behavior, social interaction, repetitive behavior, and whole-brain excitation. These results suggest that Nav1.2 regulates hippocampal neuronal excitability, excitatory synaptic drive, LTP, and spatial learning and memory in mice.

Response Properties of Interneurons and Pyramidal Neurons in Macaque MSTd and VPS Areas During Self-Motion

To perceive self-motion perception, the brain needs to integrate multi-modal sensory signals such as visual, vestibular and proprioceptive cues. Self-motion perception is very complex and involves multi candidate areas. Previous studies related to self-motion perception during passive motion have revealed that some of the areas show selective response to different directions for both visual (optic flow) and vestibular stimuli, such as the dorsal subdivision of the medial superior temporal area (MSTd) and the visual posterior sylvian fissure (VPS), although MSTd is dominated by visual signals and VPS is dominated by vestibular signals. However, none of studies related to self-motion perception have distinguished the different neuron types with distinct neuronal properties in cortical microcircuitry, which limited our understanding of the local circuits for self-motion perception. In the current study, we classified the recorded MSTd and VPS neurons into putative pyramidal neurons and putative interneurons based on the extracellular action potential waveforms and spontaneous firing rates. We found that: (1) the putative interneurons exhibited obviously broader direction tuning than putative pyramidal neurons in response to their dominant (visual for MSTd; vestibular for VPS) stimulation type; (2) either in visual or vestibular condition, the putative interneurons were more responsive but with larger variability than the putative pyramidal neurons for both MSTd and VPS areas; and (3) the timing of vestibular and visual peak directional tuning was earlier in the putative interneurons than that of the putative pyramidal neurons for both MSTd and VPS areas. Based on these findings we speculated that, within the microcircuitry, several adjacent putative interneurons with broad direction tuning receive earlier strong but variable signals, which might act feedforward input to shape the direction tuning of the target putative pyramidal neuron, but each interneuron may participate in several microcircuitries, targeting different output neurons.

Afterdischarges elicited by cortical electric stimulation in humans: When do they occur and what do they mean?

INTRODUCTION

Afterdischarges (ADs) are a common and unwanted byproduct of direct cortical stimulation during invasive electroencephalography (EEG) recordings. Brief pulse stimulation (BPS) can sometimes terminate ADs. This study investigated AD characteristics and their relevance for emergence of stimulation seizures. In addition, AD response to BPS was analyzed.

MATERIAL AND METHODS

Invasive EEG recordings including mapping with direct cortical stimulation in patients with refractory epilepsy at the Erlangen Epilepsy Center were retrospectively reviewed. Afterdischarge defined as stimulation-induced rhythmic epileptiform discharges of more than a two-second duration were analyzed regarding incidence, localization, duration, propagation pattern, morphology, and seizure emergence. In addition, the influence of AD characteristics and stimulation settings on BPS success rate was studied.

RESULTS

A number of 4261 stimulation trials in 20 patients were investigated. Afterdischarge occurred in 518 trials (14.2%) and lasted 12.4 s (standard deviation [SD]: 8.6 s) on average. We elicited ADs in the seizure onset zone (SOZ) (n = 64; 19.4%), the irritative zone (n = 105, 20.0%), and outside the irritative area (n = 222, 12.5%). Rhythmic spikes (30.5%) and spike-wave complexes (30.3%) represented predominant morphologies. Afterdischarge morphology in the SOZ and hippocampus differed from other areas with polyspikes and sequential spikes being the most common types there (p = 0.0005; p < 0.0001 respectively). Hippocampal ADs were particularly frequent (n = 50, 38.2%) and long-lasting (mean: 16.6, SD: 8.3 s). Brief pulse stimulation was applied in 18.1% of the AD trials (n = 94) and was successful in 37.4% (n = 40). Success rates were highest when BPS was delivered within 9.5 s (p = 0.0048) and in ADs of spike-wave morphology (p = 0.0004). Fifteen clinical seizures emerged from ADs (3.55%), mostly evolving from sequential spikes. Afterdischarges in patients with stimulation seizures appeared more widespread (p < 0.0001) and lasted longer (mean duration 7.0 s) than in those without (mean duration 21.0 s, p = 0.0054).

CONCLUSION

Afterdischarges appear in the epileptogenic and nonepileptogenic cortex. Duration and propagation patterns can help to quantify the risk of stimulation seizures, with sequential spikes being most susceptible to seizure elucidation. The hippocampus is highly sensitive to AD release. Brief pulse stimulation is a safe and efficacious way to terminate ADs, especially when delivered quickly after AD onset.

Tune it in: mechanisms and computational significance of neuron-autonomous plasticity

The activity of a neural network is a result of synaptic signals that convey the communication between neurons and neuron-based intrinsic currents that determine the neuron’s input-output transfer function. Ample studies have demonstrated that cell-based excitability, and in particular intrinsic excitability, is modulated by learning and that these modifications play a key role in learning-related behavioral changes. The field of cell-based plasticity is largely growing, and it entails numerous experimental findings that demonstrate a large diversity of currents that are affected by learning. The diverse effect of learning on the neuron’s excitability emphasizes the need for a framework under which cell-based plasticity can be categorized to enable the assessment of the computational roles of the intrinsic modifications. We divide the domain of cell-based plasticity into three main categories, where the first category entails the currents that mediate the passive properties and single-spike generation, the second category entails the currents that mediate spike frequency adaptation, and the third category entails a novel learning-induced mechanism where all excitatory and inhibitory synapses double their strength. Curiously, this elementary division enables a natural categorization of the computational roles of these learning-induced plasticities. The computational roles are diverse and include modification of the neuronal mode of action, such as bursting, prolonged, and fast responsive; attention-like effect where the signal detection is improved; transfer of the network into an active state; biasing the competition for memory allocation; and transforming an environmental cue into a dominant cue and enabling a quicker formation of new memories.

Laminar Distribution of Neurochemically-Identified Interneurons and Cellular Co-expression of Molecular Markers in Epileptic Human Cortex

Inhibitory GABAergic interneurons are fundamental elements of cortical circuits and play critical roles in shaping network activity. Dysfunction of interneurons can lead to various brain disorders, including epilepsy, schizophrenia, and anxiety. Based on the electrophysiological properties, cell morphology, and molecular identity, interneurons could be classified into various subgroups. In this study, we investigated the density and laminar distribution of different interneuron types and the co-expression of molecular markers in epileptic human cortex. We found that parvalbumin (PV) and somatostatin (SST) neurons were distributed in all cortical layers except layer I, while tyrosine hydroxylase (TH) and neuropeptide Y (NPY) were abundant in the deep layers and white matter. Cholecystokinin (CCK) neurons showed a high density in layers IV and VI. Neurons with these markers constituted ~7.2% (PV), 2.6% (SST), 0.5% (TH), 0.5% (NPY), and 4.4% (CCK) of the gray-matter neuron population. Double- and triple-labeling revealed that NPY neurons were also SST-immunoreactive (97.7%), and TH neurons were more likely to express SST (34.2%) than PV (14.6%). A subpopulation of CCK neurons (28.0%) also expressed PV, but none contained SST. Together, these results revealed the density and distribution patterns of different interneuron populations and the overlap between molecular markers in epileptic human cortex.

A Transient Developmental Window of Fast-Spiking Interneuron Dysfunction in a Mouse Model of Dravet Syndrome

Dravet syndrome is a severe, childhood-onset epilepsy largely due to heterozygous loss-of-function mutation of the gene SCN1A, which encodes the type 1 neuronal voltage-gated sodium (Na⁺) channel α subunit Nav1.1. Prior studies in mouse models of Dravet syndrome (Scn1a^+/- mice) indicate that, in cerebral cortex, Nav1.1 is predominantly expressed in GABAergic interneurons, in particular in parvalbumin-positive fast-spiking basket cell interneurons (PVINs). This has led to a model of Dravet syndrome pathogenesis in which Nav1.1 mutation leads to preferential dysfunction of interneurons, decreased synaptic inhibition, hyperexcitability, and epilepsy. However, such studies have been implemented at early developmental time points. Here, we performed electrophysiological recordings in acute brain slices prepared from male and female Scn1a^+/- mice as well as age-matched wild-type littermate controls and found that, later in development, the excitability of PVINs had normalized. Analysis of action potential waveforms indirectly suggests a reorganization of axonal Na⁺ channels in PVINs from Scn1a^+/- mice, a finding supported by immunohistochemical data showing elongation of the axon initial segment. Our results imply that transient impairment of action potential generation by PVINs may contribute to the initial appearance of epilepsy, but is not the mechanism of ongoing, chronic epilepsy in Dravet syndrome.SIGNIFICANCE STATEMENT Dravet syndrome is characterized by normal early development, temperature-sensitive seizures in infancy, progression to treatment-resistant epilepsy, developmental delay, autism, and sudden unexplained death due to mutation in SCN1A encoding the Na+ channel subunit Nav1.1. Prior work has revealed a preferential impact of Nav1.1 loss on the function of GABAergic inhibitory interneurons. However, such data derive exclusively from recordings of neurons in young Scn1a^+/- mice. Here, we show that impaired action potential generation observed in parvalbumin-positive fast-spiking interneurons (PVINs) in Scn1a+/- mice during early development has normalized by postnatal day 35. This work suggests that a transient impairment of PVINs contributes to epilepsy onset, but is not the mechanism of ongoing, chronic epilepsy in Dravet syndrome.

Nav1.2 haplodeficiency in excitatory neurons causes absence-like seizures in mice

Mutations in the SCN2A gene encoding a voltage-gated sodium channel Nav1.2 are associated with epilepsies, intellectual disability, and autism. SCN2A gain-of-function mutations cause early-onset severe epilepsies, while loss-of-function mutations cause autism with milder and/or later-onset epilepsies. Here we show that both heterozygous Scn2a-knockout and knock-in mice harboring a patient-derived nonsense mutation exhibit ethosuximide-sensitive absence-like seizures associated with spike-and-wave discharges at adult stages. Unexpectedly, identical seizures are reproduced and even more prominent in mice with heterozygous Scn2a deletion specifically in dorsal-telencephalic (e.g., neocortical and hippocampal) excitatory neurons, but are undetected in mice with selective Scn2a deletion in inhibitory neurons. In adult cerebral cortex of wild-type mice, most Nav1.2 is expressed in excitatory neurons with a steady increase and redistribution from proximal (i.e., axon initial segments) to distal axons. These results indicate a pivotal role of Nav1.2 haplodeficiency in excitatory neurons in epilepsies of patients with SCN2A loss-of-function mutations.

Role of sodium channel subtype in action potential generation by neocortical pyramidal neurons

Neocortical pyramidal neurons express several distinct subtypes of voltage-gated Na⁺ channels. In mature cells, Na_v1.6 is the dominant channel subtype in the axon initial segment (AIS) as well as in the nodes of Ranvier. Action potentials (APs) are initiated in the AIS, and it has been proposed that the high excitability of this region is related to the unique characteristics of the Na_v1.6 channel. Knockout or loss-of-function mutation of the Scn8a gene is generally lethal early in life because of the importance of this subtype in noncortical regions of the nervous system. Using the Cre/loxP system, we selectively deleted Na_v1.6 in excitatory neurons of the forebrain and characterized the excitability of Na_v1.6-deficient layer 5 pyramidal neurons by patch-clamp and Na⁺ and Ca²⁺ imaging recordings. We now report that, in the absence of Na_v1.6 expression, the AIS is occupied by Na_v1.2 channels. However, APs are generated in the AIS, and differences in AP propagation to soma and dendrites are minimal. Moreover, the channels that are expressed in the AIS still show a clear hyperpolarizing shift in voltage dependence of activation, compared with somatic channels. The only major difference between Na_v1.6-null and wild-type neurons was a strong reduction in persistent sodium current. We propose that the molecular environment of the AIS confers properties on whatever Na channel subtype is present and that some other benefit must be conferred by the selective axonal presence of the Na_v1.6 channel.

Fast-spiking Parvalbumin Interneurons are Frequently Myelinated in the Cerebral Cortex of Mice and Humans

Myelination, the insulating ensheathment of axons by oligodendrocytes, is thought to both optimize signal propagation and provide metabolic support. Despite the well-established physiological importance of myelination to neuronal function, relatively little is known about the myelination of GABAergic interneurons in the cerebral cortex. Here, we report that a large fraction of myelin in mouse cerebral cortex ensheaths GABAergic interneurons, reaching up to 80% in hippocampal subregions. Moreover, we find that a very high proportion of neocortical and hippocampal parvalbumin (PV) interneurons exhibit axonal myelination. Using a combination of intracellular recordings and biocytin labeling of ex vivo human neocortex, we also confirm that axons of fast-spiking PV interneurons are extensively myelinated in the human brain. PV interneuron myelination in both mice and humans exhibits a stereotyped topography with a bias towards proximal axonal segments and relatively short internodes (~27 μm) interspersed with branch points. Interestingly, myelin-deficient Shiverer mice exhibit an increased density and more proximal location of en passant boutons, suggesting that myelination might function in part to regulate synapse formation along PV interneuron axons. Taken together, fast-spiking interneuron myelination is likely to have broad implications for cerebral cortex function in health and disease.