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.

Subacute combined degeneration of the spinal cord in a patient with nitrous oxide use and autoimmune atrophic gastritis.

Nitrous oxide is among the most common drugs used by adolescents and young adults, and its neuropsychiatric sequelae are severe but reversible with timely treatment. The causal mechanism relates to impaired metabolism of vitamin B12, which is necessary for the development and maintenance of the myelin sheath. Individuals most susceptible to neuropsychiatric manifestations are those with a secondary cause of vitamin B12 deficiency, including nutritional deficiency and impaired absorption, or an alternative cause of impaired metaboclism. We describe the case of a man in his thirties who developed subacute combined degeneration of the spinal cord and polyneuropathy in the setting of recreational nitrous oxide use and autoimmune atrophic gastritis. Our case highlights clinical pearls for diagnosis and treatment, differential diagnosis, common concomitant aetiologies and the importance of screening for substance use disorder and psychiatric comorbidities.

A multicenter retrospective study of patients treated in the thalamus with responsive neurostimulation.

Introduction: For drug resistant epilepsy patients who are either not candidates for resective surgery or have already failed resective surgery, neuromodulation is a promising option. Neuromodulatory approaches include responsive neurostimulation (RNS), deep brain stimulation (DBS), and vagal nerve stimulation (VNS). Thalamocortical circuits are involved in both generalized and focal onset seizures. This paper explores the use of RNS in the centromedian nucleus of the thalamus (CMN) and in the anterior thalamic nucleus (ANT) of patients with drug resistant epilepsy. Methods: This is a retrospective multicenter study from seven different epilepsy centers in the United States. Patients that had unilateral or bilateral thalamic RNS leads implanted in the CMN or ANT for at least 6 months were included. Primary objectives were to describe the implant location and determine changes in the frequency of disabling seizures at 6 months, 1 year, 2 years, and > 2 years. Secondary objectives included documenting seizure free periods, anti-seizure medication regimen changes, stimulation side effects, and serious adverse events. In addition, the global clinical impression scale was completed. Results: Twelve patients had at least one lead placed in the CMN, and 13 had at least one lead placed in the ANT. The median baseline seizure frequency was 15 per month. Overall, the median seizure reduction was 33% at 6 months, 55% at 1 year, 65% at 2 years, and 74% at >2 years. Seizure free intervals of at least 3 months occurred in nine patients. Most patients (60%, 15/25) did not have a change in anti-seizure medications post RNS placement. Two serious adverse events were recorded, one related to RNS implantation. Lastly, overall functioning seemed to improve with 88% showing improvement on the global clinical impression scale. Discussion: Meaningful seizure reduction was observed in patients who suffer from drug resistant epilepsy with unilateral or bilateral RNS in either the ANT or CMN of the thalamus. Most patients remained on their pre-operative anti-seizure medication regimen. The device was well tolerated with few side effects. There were rare serious adverse events. Most patients showed an improvement in global clinical impression scores.

Intracranial Entrainment Reveals Statistical Learning across Levels of Abstraction.

We encounter the same people, places, and objects in predictable sequences and configurations. Humans efficiently learn these regularities via statistical learning. Importantly, statistical learning creates knowledge not only of specific regularities but also of regularities that apply more generally across related experiences (i.e., across members of a category). Prior evidence for different levels of learning comes from post-exposure behavioral tests, leaving open the question of whether more abstract regularities are detected online during initial exposure. We address this question by measuring neural entrainment in intracranial recordings. Neurosurgical patients viewed a stream of photographs with regularities at one of two levels: In the exemplar-level structured condition, the same photographs appeared repeatedly in pairs. In the category-level structured condition, the photographs were trial-unique but their categories were paired across repetitions. In a baseline random condition, the same photographs repeated but in a scrambled order. We measured entrainment at the frequency of individual photographs, which was expected in all conditions, but critically also at half that frequency-the rate at which to-be-learned pairs appeared in the two structured (but not random) conditions. Entrainment to both exemplar and category pairs emerged within minutes throughout visual cortex and in frontal and temporal regions. Many electrode contacts were sensitive to only one level of structure, but a significant number encoded both levels. These findings suggest that the brain spontaneously uncovers category-level regularities during statistical learning, providing insight into the brain's unsupervised mechanisms for building flexible and robust knowledge that generalizes across input variation and conceptual hierarchies.

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.

Nonquantal transmission at the vestibular hair cell-calyx synapse: KLV currents modulate fast electrical and slow K+ potentials.

Vestibular hair cells transmit information about head position and motion across synapses to primary afferent neurons. At some of these synapses, the afferent neuron envelopes the hair cell, forming an enlarged synaptic terminal called a calyx. The vestibular hair cell-calyx synapse supports a mysterious form of electrical transmission that does not involve gap junctions, termed nonquantal transmission (NQT). The NQT mechanism is thought to involve the flow of ions from the presynaptic hair cell to the postsynaptic calyx through low-voltage-activated channels driven by changes in cleft [K+] as K+ exits the hair cell. However, this hypothesis has not been tested with a quantitative model and the possible role of an electrical potential in the cleft has remained speculative. Here, we present a computational model that captures experimental observations of NQT and identifies features that support the existence of an electrical potential (ϕ) in the synaptic cleft. We show that changes in cleft ϕ reduce transmission latency and illustrate the relative contributions of both cleft [K+] and ϕ to the gain and phase of NQT. We further demonstrate that the magnitude and speed of NQT depend on calyx morphology and that increasing calyx height reduces action potential latency in the calyx afferent. These predictions are consistent with the idea that the calyx evolved to enhance NQT and speed up vestibular signals that drive neural circuits controlling gaze, balance, and orientation.

Temporal Dynamics of Competition between Statistical Learning and Episodic Memory in Intracranial Recordings of Human Visual Cortex.

The function of long-term memory is not just to reminisce about the past, but also to make predictions that help us behave appropriately and efficiently in the future. This predictive function of memory provides a new perspective on the classic question from memory research of why we remember some things but not others. If prediction is a key outcome of memory, then the extent to which an item generates a prediction signifies that this information already exists in memory and need not be encoded. We tested this principle using human intracranial EEG as a time-resolved method to quantify prediction in visual cortex during a statistical learning task and link the strength of these predictions to subsequent episodic memory behavior. Epilepsy patients of both sexes viewed rapid streams of scenes, some of which contained regularities that allowed the category of the next scene to be predicted. We verified that statistical learning occurred using neural frequency tagging and measured category prediction with multivariate pattern analysis. Although neural prediction was robust overall, this was driven entirely by predictive items that were subsequently forgotten. Such interference provides a mechanism by which prediction can regulate memory formation to prioritize encoding of information that could help learn new predictive relationships.SIGNIFICANCE STATEMENT When faced with a new experience, we are rarely at a loss for what to do. Rather, because many aspects of the world are stable over time, we rely on past experiences to generate expectations that guide behavior. Here we show that these expectations during a new experience come at the expense of memory for that experience. From intracranial recordings of visual cortex, we decoded what humans expected to see next in a series of photographs based on patterns of neural activity. Photographs that generated strong neural expectations were more likely to be forgotten in a later behavioral memory test. Prioritizing the storage of experiences that currently lead to weak expectations could help improve these expectations in future encounters.

Remembering the pattern: A longitudinal case study on statistical learning in spatial navigation and memory consolidation.

Distinct brain systems are thought to support statistical learning over different timescales. Regularities encountered during online perceptual experience can be acquired rapidly by the hippocampus. Further processing during offline consolidation can establish these regularities gradually in cortical regions, including the medial prefrontal cortex (mPFC). These mechanisms of statistical learning may be critical during spatial navigation, for which knowledge of the structure of an environment can facilitate future behavior. Rapid acquisition and prolonged retention of regularities have been investigated in isolation, but how they interact in the context of spatial navigation is unknown. We had the rare opportunity to study the brain systems underlying both rapid and gradual timescales of statistical learning using intracranial electroencephalography (iEEG) longitudinally in the same patient over a period of three weeks. As hypothesized, spatial patterns were represented in the hippocampus but not mPFC for up to one week after statistical learning and then represented in the mPFC but not hippocampus two and three weeks after statistical learning. Taken together, these findings suggest that the hippocampus may contribute to the initial extraction of regularities prior to cortical consolidation.

Evaluating consciousness and awareness during focal seizures: responsiveness testing versus recall testing

Objective: The current International League Against Epilepsy (ILAE) guidelines classify focal seizures based on awareness, defined as successful postictal recall of ictal experiences, and exclude the use of responsiveness during seizures for classification. One reason for this exclusion is that responsiveness was thought to not be commonly tested during seizures. Our goal was to determine whether, in at least some settings, responsiveness testing during seizures is relatively common. Methods: We assessed how often responsiveness and recall were each evaluated in patients with focal epilepsy undergoing surface and intracranial EEG-video monitoring. We performed this evaluation by retrospectively reviewing video recordings from 121 seizures from 48 patients during their stay in the epilepsy monitoring unit between September 2012 and November 2019. Results: We found that responsiveness during seizures was tested more frequently than recall of ictal events after seizures. Of 121 seizures in 48 patients, responsiveness was tested in 101 seizures, whereas recall was tested in only 38. Significance: Evaluating if consciousness is impaired during seizures is of critical importance for guiding recommendations for people with epilepsy, such as whether it is safe for them to drive or operate machinery. The ILAE classification guidelines are intended to be broadly useful, but our findings demonstrate that at least in one important clinical setting, responsiveness was used more commonly than recall to evaluate patients during focal seizures. Although our preliminary findings should be replicated in a larger sample and in other patient groups, they suggest that responsiveness testing during focal seizures might be relatively common in at least some clinical practice settings. With further study, this may lead to a re-evaluation of criteria for classifying focal seizures to include both responsiveness and recall of experiences during seizures, as both may provide important information to guide clinical care.

Hippocampal recording via the RNS system reveals marked ipsilateral activation of epileptiform activity during Wada testing.

Wada testing remains an important component of pre-surgical testing to assess the feasibility of temporal lobectomy for patients with intractable epilepsy. In this procedure, an anesthetic is injected into either internal carotid artery while memory and language testing is performed, simulating the effect of temporal lobe resection. The mechanism remains poorly understood because the hippocampal vasculature is predominantly via the posterior circulation. We recorded hippocampal EEG during bilateral methohexital Wada testing in three patients who had previously been implanted with a responsive neurostimulation system (RNS) to determine the effect of the injections on hippocampal activity. In all six injections from three patients, methohexital caused immediate, transient increases in hippocampal spikes. With at least two of these injections, the electrographic changes were consistent with electrographic seizures. In all cases, the epileptiform activity was not apparent on scalp EEG and was without obvious clinical correlate other than the negative findings expected from the anesthetic. The results demonstrate the utility of intracranial EEG during Wada testing and suggest that the elicitation of seizures or continuous spiking might contribute to dysfunction of the hippocampus during the Wada test. We hypothesize that this effect is due to disconnection and disinhibition of medial temporal structures.

Electronic seizure diaries for clinical care and research

Objective: People with epilepsy often record their seizures in diaries. While seizure diaries can be kept using paper and digital notes or calendars, electronic seizure diary services are available on web browsers and mobile apps. Benefits of these electronic diaries include standardized data collection, data backup and protection, long-term trending, and data accessibility to providers or research groups. The major electronic seizure diary services have not recently been systematically evaluated or compared. Methods: We evaluated nine electronic seizure diaries selected based on popularity and prevalence in the literature. Diaries were tested from both the user and provider perspectives. Results: A review of nine commonly used seizure diary services that we could identify showed variability in features offered. All services provide a seizure log with inputs for date and seizure type along with a combination of other features such as duration, intensity, trigger, recovery, mood, and description. Other notable features included in some, but not all diary services are: the ability to track medications, missed doses, sleep, menstrual cycles, and other possible triggers; provider accounts to share data; the ability to report clusters and seizure severity; an option for medication reminders; user-friendly trending of long-term data in relation to changes in treatment; and the ability to export data for research. Significance: Having electronic diaries that allow patients to log seizures quickly and accurately can improve the quantity and quality of data, which can be used both for clinical and research benefit.

Suppression of Kv3.3 channels by antisense oligonucleotides reverses biochemical effects and motor impairment in spinocerebellar ataxia type 13 mice.

Mutations in KCNC3, the gene that encodes the Kv3.3 voltage dependent potassium channel, cause Spinocerebellar Ataxia type 13 (SCA13), a disease associated with disrupted motor behaviors, progressive cerebellar degeneration, and abnormal auditory processing. The Kv3.3 channel directly binds Hax-1, a cell survival protein. A disease-causing mutation, Kv3.3-G592R, causes overstimulation of Tank Binding Kinase 1 (Tbk1) in the cerebellum, resulting in the degradation of Hax-1 by promoting its trafficking into multivesicular bodies and then to lysosomes. We have now tested the effects of antisense oligonucleotides (ASOs) directed against the Kv3.3 channel on both wild type mice and those bearing the Kv3.3-G592R-encoding mutation. Intracerebroventricular infusion of the Kcnc3-specific ASO suppressed both mRNA and protein levels of the Kv3.3 channel. In wild-type animals, this produced no change in levels of activated Tbk1, Hax-1 or Cd63, a tetraspanin marker for late endosomes/multivesicular bodies. In contrast, in mice homozygous for the Kv3.3-G592R-encoding mutation, the same ASO reduced Tbk1 activation and levels of Cd63, while restoring the expression of Hax-1 in the cerebellum. The motor behavior of the mice was tested using a rotarod assay. Surprisingly, the active ASO had no effects on the motor behavior of wild type mice but restored the behavior of the mutant mice to those of age-matched wild type animals. Our findings indicate that, in mature intact animals, suppression of Kv3.3 expression can reverse the deleterious effects of a SCA13 mutation while having little effect on wild type animals. Thus, targeting Kv3.3 expression may prove a viable therapeutic approach for SCA13.

Miglustat Therapy for SCARB2-Associated Action Myoclonus-Renal Failure Syndrome.

OBJECTIVE: We evaluated whether substrate reduction therapy with miglustat could alter the course of action myoclonus-renal failure syndrome (AMRF), a rare, progressive myoclonic epilepsy with early mortality caused by scavenger receptor class B member 2 (SCARB2) gene mutations. METHODS: We identified an AMRF patient with a biallelic combination of SCARB2 mutations determined by whole exome sequencing. SCARB2 encodes a protein that traffics ß-glucocerebrosidase to the lysosomal membrane. Mutations lead to a complex pattern of glucosylceramide accumulation and neurologic symptoms including progressive action myoclonus, seizures, and ataxia. We then evaluated the effect of inhibiting glucosylceramide synthesis, as is used in Gaucher disease. The patient was treated for 3 years with miglustat after several years of steady worsening. RESULTS: Progression of myoclonus halted, dysphagia resolved, some skills were reacquired, and seizures remained well controlled. CONCLUSIONS: The response suggests that neurologic symptoms of SCARB2-associated AMRF could be ameliorated, at least partly, by targeting glycosphingolipid metabolism with available medications.

Patient-detectable responsive neurostimulation as a seizure warning system.

Many people with epilepsy are not aware of their seizures or do not have reliable auras. The responsive neurostimulation system (RNS) delivers stimulation triggered by intracranial epileptiform activity. If an epileptiform pattern continues, the RNS repeats stimulation up to five times per event. The RNS can cause acute stimulation-related symptoms that can be avoided by reducing stimulation. Because each of the five therapies can be programmed independently, it may be possible to program the latter therapies to induce a seizure warning. The goal of this study was to determine what proportion of patients could have tolerable symptoms safely elicited by stimulation, ultimately for the purpose of subjective seizure recognition. Of 18 patients, 12 (67%) had induced symptoms, which were tolerable in 11. Phosphenes were most common. We also present one patient in whom the fifth therapy was set to induce a symptom for early recognition and treatment of clusters of focal impaired awareness seizures, which were previously unrecognized and had led to days of disabling cognitive impairment. This protocol prevented disabling clusters successfully for several years. The findings suggest the RNS can provide a seizure warning, potentially improving safety and quality of life, and leading to prevention of clinical seizures or clusters in select patients.

Electrical cortical stimulation can impair production of the alphabet without impairing counting.

Neurosurgery has the potential to cure patients with drug-resistant focal epilepsy, but carries the risk of permanent language impairment when surgery involves the dominant hemisphere of the brain. This risk can be estimated and minimized using electrical stimulation mapping (ESM), which uses cognitive and linguistic tasks during cortical ESM to differentiate "eloquent" and "resectable" areas in the brain. One such task, counting, is often used to screen and characterize language during ESM in patients whose language abilities are limited. Here we report a patient with drug-resistant epilepsy arising from the language-dominant hemisphere using fMRI. Our patient experienced loss of the ability to recite or write the alphabet, but not to count, during ESM of the dominant left posterior superior temporal gyrus. This selective impairment extended to both spoken and written production. We suggest the need for caution when using counting as a sole means to screen language function and as a method of testing low functioning patients using ESM.

Impaired motor skill learning and altered seizure susceptibility in mice with loss or gain of function of the Kcnt1 gene encoding Slack (KNa1.1) Na+-activated K+ channels.

Gain-of-function mutations in KCNT1, the gene encoding Slack (KNa1.1) channels, result in epilepsy of infancy with migrating focal seizures (EIMFS) and several other forms of epilepsy associated with severe intellectual disability. We have generated a mouse model of this condition by replacing the wild type gene with one encoding Kcnt1R455H, a cytoplasmic C-terminal mutation homologous to a human R474H variant that results in EIMFS. We compared behavior patterns and seizure activity in these mice with those of wild type mice and Kcnt1-/- mice. Complete loss of Kcnt1 produced deficits in open field behavior and motor skill learning. Although their thresholds for electrically and chemically induced seizures were similar to those of wild type animals, Kcnt1-/- mice were significantly protected from death after maximum electroshock-induced seizures. In contrast, homozygous Kcnt1R455H/R455H mice were embryonic lethal. Video-EEG monitoring of heterozygous Kcnt1+/R455H animals revealed persistent interictal spikes, spontaneous seizures and a substantially decreased threshold for pentylenetetrazole-induced seizures. Surprisingly, Kcnt1+/R455H mice were not impaired in tasks of exploratory behavior or procedural motor learning. These findings provide an animal model for EIMFS and suggest that Slack channels are required for the development of procedural learning and of pathways that link cortical seizures to other regions required for animal survival.

Early detection rate changes from a brain-responsive neurostimulation system predict efficacy of newly added antiseizure drugs.

OBJECTIVE: Brain-responsive neurostimulation (RNS System, NeuroPace) is used to treat medically refractory focal epilepsy and also provides long-term ambulatory neurophysiologic data. We sought to determine whether these data could predict the clinical response to antiseizure drugs (ASDs). METHODS: First, newly added medications were identified in RNS System patients followed at a single epilepsy center. Daily detection rates including "episode starts" (predominantly interictal activity) and "long episodes" (often electrographic seizures) were compared before and after ASD initiation. Efficacy was determined from documentation of clinical improvement and medication retention. Next, the analysis was repeated on an independent sample of patients from a multicenter long-term treatment trial, using an efficacy measure of ≥50% reduction in diary-recorded seizure frequency after 3 months. RESULTS: In the single center cohort, long episodes, but not episode starts, had a significantly greater reduction in the first week for clinically efficacious compared to inefficacious medications. In this cohort, having no long episodes in the first week was highly predictive of ASD efficacy. In the multicenter cohort, both long episodes and episode starts had a significantly greater reduction for effective medications starting in the first 1-2 weeks. In this larger dataset, a ≥50% decrease in episode starts was 90% specific for efficacy with a positive predictive value (PPV) of 67%, and a ≥84% decrease in long episodes was 80% specific with a PPV of 48%. Conversely, a <25% decrease in long episodes (including any increase) or a <20% decrease in episode starts had a predictive value for inefficacy of >80%. SIGNIFICANCE: In RNS System patients with stable detection settings, when new ASDs are started, detection rates within the first 1-2 weeks may provide an early, objective indication of efficacy. These data could be used to identify responses to medication trials early to allow more rapid medication adjustments than conventionally possible.

An Epilepsy-Associated KCNT1 Mutation Enhances Excitability of Human iPSC-Derived Neurons by Increasing Slack KNa Currents.

Mutations in the KCNT1 (Slack, KNa1.1) sodium-activated potassium channel produce severe epileptic encephalopathies. Expression in heterologous systems has shown that the disease-causing mutations give rise to channels that have increased current amplitude. It is not known, however, whether such gain of function occurs in human neurons, nor whether such increased KNa current is expected to suppress or increase the excitability of cortical neurons. Using genetically engineered human induced pluripotent stem cell (iPSC)-derived neurons, we have now found that sodium-dependent potassium currents are increased several-fold in neurons bearing a homozygous P924L mutation. In current-clamp recordings, the increased KNa current in neurons with the P924L mutation acts to shorten the duration of action potentials and to increase the amplitude of the afterhyperpolarization that follows each action potential. Strikingly, the number of action potentials that were evoked by depolarizing currents as well as maximal firing rates were increased in neurons expressing the mutant channel. In networks of spontaneously active neurons, the mean firing rate, the occurrence of rapid bursts of action potentials, and the intensity of firing during the burst were all increased in neurons with the P924L Slack mutation. The feasibility of an increased KNa current to increase firing rates independent of any compensatory changes was validated by numerical simulations. Our findings indicate that gain-of-function in Slack KNa channels causes hyperexcitability in both isolated neurons and in neural networks and occurs by a cell-autonomous mechanism that does not require network interactions.SIGNIFICANCE STATEMENTKCNT1 mutations lead to severe epileptic encephalopathies for which there are no effective treatments. This study is the first demonstration that a KCNT1 mutation increases the Slack current in neurons. It also provides the first explanation for how this increased potassium current induces hyperexcitability, which could be the underlining factor causing seizures.