Functional and ionic properties of a slow afterhyperpolarization in ferret perigeniculate neurons in vitro

Thalamic epileptic spikes disrupt sleep spindles in patients with epileptic encephalopathy

In severe epileptic encephalopathies, epileptic activity contributes to progressive cognitive dysfunction. Epileptic encephalopathies share the trait of spike-wave activation during non-REM sleep (EE-SWAS), a sleep stage dominated by sleep spindles, which are brain oscillations known to coordinate offline memory consolidation. Epileptic activity has been proposed to hijack the circuits driving these thalamocortical oscillations, thereby contributing to cognitive impairment. Using a unique dataset of simultaneous human thalamic and cortical recordings in subjects with and without EE-SWAS, we provide evidence for epileptic spike interference of thalamic sleep spindle production in patients with EE-SWAS. First, we show that epileptic spikes and sleep spindles are both predicted by slow oscillations during stage two sleep (N2), but at different phases of the slow oscillation. Next, we demonstrate that sleep-activated cortical epileptic spikes propagate to the thalamus (thalamic spike rate increases after a cortical spike, P ≈ 0). We then show that epileptic spikes in the thalamus increase the thalamic spindle refractory period (P ≈ 0). Finally, we show that in three patients with EE-SWAS, there is a downregulation of sleep spindles for 30 s after each thalamic spike (P < 0.01). These direct human thalamocortical observations support a proposed mechanism for epileptiform activity to impact cognitive function, wherein epileptic spikes inhibit thalamic sleep spindles in epileptic encephalopathy with spike and wave activation during sleep.

Individualized temporal patterns dominate cortical upstate and sleep depth in driving human sleep spindle timing

Sleep spindles are critical for memory consolidation and strongly linked to neurological disease and aging. Despite their significance, the relative influences of factors like sleep depth, cortical up/down states, and spindle temporal patterns on individual spindle production remain poorly understood. Moreover, spindle temporal patterns are typically ignored in favor of an average spindle rate. Here, we analyze spindle dynamics in 1008 participants from the Multi-Ethnic Study of Atherosclerosis using a point process framework. Results reveal fingerprint-like temporal patterns, characterized by a refractory period followed by a period of increased spindle activity, which are highly individualized yet consistent night-to-night. We observe increased timing variability with age and distinct gender/age differences. Strikingly, and in contrast to the prevailing notion, individualized spindle patterns are the dominant determinant of spindle timing, accounting for over 70% of the statistical deviance explained by all of the factors we assessed, surpassing the contribution of slow oscillation (SO) phase (~14%) and sleep depth (~16%). Furthermore, we show spindle/SO coupling dynamics with sleep depth are preserved across age, with a global negative shift towards the SO rising slope. These findings offer novel mechanistic insights into spindle dynamics with direct experimental implications and applications to individualized electroencephalography biomarker identification.

Slack K+ channels limit kainic acid-induced seizure severity in mice by modulating neuronal excitability and firing

Mutations of the Na+-activated K+ channel Slack (KCNT1) are associated with terrible epilepsy syndromes that already begin in infancy. Here we report increased severity of acute kainic acid-induced seizures in adult and juvenile Slack knockout mice (Slack-/-) in vivo. Fittingly, we find exacerbation of cell death following kainic acid exposure in organotypic hippocampal slices as well as dissociated hippocampal cultures from Slack-/- in vitro. Furthermore, in cultured Slack-/- neurons, kainic acid-triggered Ca2+ influx and K+ efflux as well as depolarization-induced tetrodotoxin-sensitive inward currents are higher compared to the respective controls. This apparent changes in ion homeostasis could possibly explain altered action potential kinetics of Slack-/- neurons: steeper rise slope, decreased threshold, and duration of afterhyperpolarization, which ultimately lead to higher action potential frequencies during kainic acid application or injection of depolarizing currents. Based on our data, we propose Slack as crucial gatekeeper of neuronal excitability to acutely limit seizure severity.

Thalamic control of sensory processing and spindles in a biophysical somatosensory thalamoreticular circuit model of wakefulness and sleep

Thalamoreticular circuitry plays a key role in arousal, attention, cognition, and sleep spindles, and is linked to several brain disorders. A detailed computational model of mouse somatosensory thalamus and thalamic reticular nucleus has been developed to capture the properties of over 14,000 neurons connected by 6 million synapses. The model recreates the biological connectivity of these neurons, and simulations of the model reproduce multiple experimental findings in different brain states. The model shows that inhibitory rebound produces frequency-selective enhancement of thalamic responses during wakefulness. We find that thalamic interactions are responsible for the characteristic waxing and waning of spindle oscillations. In addition, we find that changes in thalamic excitability control spindle frequency and their incidence. The model is made openly available to provide a new tool for studying the function and dysfunction of the thalamoreticular circuitry in various brain states.

Double-edged Role of KNa Channels in Brain Tuning: Identifying Epileptogenic Network Micro-Macro Disconnection

Epilepsy is commonly recognized as a disease driven by generalized hyperexcited and hypersynchronous neural activity. Sodium-activated potassium channels (K_Na channels), which are encoded by the Slo 2.2 and Slo 2.1 genes, are widely expressed in the central nervous system and considered as "brakes" to adjust neuronal adaptation through regulating action potential threshold or after-hyperpolarization under physiological condition. However, the variants in K_Na channels, especially gain-of-function variants, have been found in several childhood epileptic conditions. Most previous studies focused on mapping the epileptic network on the macroscopic scale while ignoring the value of microscopic changes. Notably, paradoxical role of K_Na channels working on individual neuron/microcircuit and the macroscopic epileptic expression highlights the importance of understanding epileptogenic network through combining microscopic and macroscopic methods. Here, we first illustrated the molecular and physiological function of K_Na channels on preclinical seizure models and patients with epilepsy. Next, we summarized current hypothesis on the potential role of K_Na channels during seizures to provide essential insight into what emerged as a micro-macro disconnection at different levels. Additionally, we highlighted the potential utility of K_Na channels as therapeutic targets for developing innovative anti-seizure medications.

A design principle of spindle oscillations in mammalian sleep

Neural oscillations are mainly regulated by molecular mechanisms and network connectivity of neurons. Large-scale simulations of neuronal networks have driven the population-level understanding of neural oscillations. However, cell-intrinsic mechanisms, especially a design principle, of neural oscillations remain largely elusive. Herein, we developed a minimal, Hodgkin-Huxley-type model of groups of neurons to investigate molecular mechanisms underlying spindle oscillation, which is synchronized oscillatory activity predominantly observed during mammalian sleep. We discovered that slowly inactivating potassium channels played an essential role in characterizing the firing pattern. The detailed analysis of the minimal model revealed that leak sodium and potassium channels, which controlled passive properties of the fast variable (i.e., membrane potential), competitively regulated the base value and time constant of the slow variable (i.e., cytosolic calcium concentration). Consequently, we propose a theoretical design principle of spindle oscillations that may explain intracellular mechanisms behind the flexible control over oscillation density and calcium setpoint.

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A minimal, Hodgkin-Huxley-type model of spindle oscillations is developed

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The property of delayed rectifier K⁺ channels characterizes spindle oscillations

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The combination of bifurcations specifies spindle oscillations

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Spindle oscillations are controlled by the balance of inward and outward currents

Rodent somatosensory thalamocortical circuitry: Neurons, synapses, and connectivity

As our understanding of the thalamocortical system deepens, the questions we face become more complex. Their investigation requires the adoption of novel experimental approaches complemented with increasingly sophisticated computational modeling. In this review, we take stock of current data and knowledge about the circuitry of the somatosensory thalamocortical loop in rodents, discussing common principles across modalities and species whenever appropriate. We review the different levels of organization, including the cells, synapses, neuroanatomy, and network connectivity. We provide a complete overview of this system that should be accessible for newcomers to this field while nevertheless being comprehensive enough to serve as a reference for seasoned neuroscientists and computational modelers studying the thalamocortical system. We further highlight key gaps in data and knowledge that constitute pressing targets for future experimental work. Filling these gaps would provide invaluable information for systematically unveiling how this system supports behavioral and cognitive processes.

Clinical and experimental insight into pathophysiology, comorbidity and therapy of absence seizures

Absence seizures in children and teenagers are generally considered relatively benign because of their non-convulsive nature and the large incidence of remittance in early adulthood. Recent studies, however, show that 30% of children with absence seizures are pharmaco-resistant and 60% are affected by severe neuropsychiatric comorbid conditions, including impairments in attention, cognition, memory and mood. In particular, attention deficits can be detected before the epilepsy diagnosis, may persist even when seizures are pharmacologically controlled and are aggravated by valproic acid monotherapy. New functional MRI-magnetoencephalography and functional MRI-EEG studies provide conclusive evidence that changes in blood oxygenation level-dependent signal amplitude and frequency in children with absence seizures can be detected in specific cortical networks at least 1 min before the start of a seizure, spike-wave discharges are not generalized at seizure onset and abnormal cortical network states remain during interictal periods. From a neurobiological perspective, recent electrical recordings and imaging of large neuronal ensembles with single-cell resolution in non-anaesthetized models show that, in contrast to the predominant opinion, cortical mechanisms, rather than an exclusively thalamic rhythmogenesis, are key in driving seizure ictogenesis and determining spike-wave frequency. Though synchronous ictal firing characterizes cortical and thalamic activity at the population level, individual cortico-thalamic and thalamocortical neurons are sparsely recruited to successive seizures and consecutive paroxysmal cycles within a seizure. New evidence strengthens previous findings on the essential role for basal ganglia networks in absence seizures, in particular the ictal increase in firing of substantia nigra GABAergic neurons. Thus, a key feature of thalamic ictogenesis is the powerful increase in the inhibition of thalamocortical neurons that originates at least from two sources, substantia nigra and thalamic reticular nucleus. This undoubtedly provides a major contribution to the ictal decrease in total firing and the ictal increase of T-type calcium channel-mediated burst firing of thalamocortical neurons, though the latter is not essential for seizure expression. Moreover, in some children and animal models with absence seizures, the ictal increase in thalamic inhibition is enhanced by the loss-of-function of the astrocytic GABA transporter GAT-1 that does not necessarily derive from a mutation in its gene. Together, these novel clinical and experimental findings bring about paradigm-shifting views of our understanding of absence seizures and demand careful choice of initial monotherapy and continuous neuropsychiatric evaluation of affected children. These issues are discussed here to focus future clinical and experimental research and help to identify novel therapeutic targets for treating both absence seizures and their comorbidities.

Reduced GABAergic Neuron Excitability, Altered Synaptic Connectivity, and Seizures in a KCNT1 Gain-of-Function Mouse Model of Childhood Epilepsy

Gain-of-function (GOF) variants in K+ channels cause severe childhood epilepsies, but there are no mechanisms to explain how increased K+ currents lead to network hyperexcitability. Here, we introduce a human Na+-activated K+ (KNa) channel variant (KCNT1-Y796H) into mice and, using a multiplatform approach, find motor cortex hyperexcitability and early-onset seizures, phenotypes strikingly similar to those of human patients. Although the variant increases KNa currents in cortical excitatory and inhibitory neurons, there is an increase in the KNa current across subthreshold voltages only in inhibitory neurons, particularly in those with non-fast-spiking properties, resulting in inhibitory-neuron-specific impairments in excitability and action potential (AP) generation. We further observe evidence of synaptic rewiring, including increases in homotypic synaptic connectivity, accompanied by network hyperexcitability and hypersynchronicity. These findings support inhibitory-neuron-specific mechanisms in mediating the epileptogenic effects of KCNT1 channel GOF, offering cell-type-specific currents and effects as promising targets for therapeutic intervention.

TRPM4 Conductances in Thalamic Reticular Nucleus Neurons Generate Persistent Firing during Slow Oscillations

During sleep, neurons in the thalamic reticular nucleus (TRN) participate in distinct types of oscillatory activity. While the reciprocal synaptic circuits between TRN and sensory relay nuclei are known to underlie the generation of sleep spindles, the mechanisms regulating slow (<1 Hz) forms of thalamic oscillations are not well understood. Under in vitro conditions, TRN neurons can generate slow oscillations in a cell-intrinsic manner, with postsynaptic Group 1 metabotropic glutamate receptor activation triggering long-lasting plateau potentials thought to be mediated by both T-type Ca²⁺ currents and Ca²⁺-activated nonselective cation currents (I_CAN). However, the identity of I_CAN and the possible contribution of thalamic circuits to slow rhythmic activity remain unclear. Using thalamic slices derived from adult mice of either sex, we recorded slow forms of rhythmic activity in TRN neurons, which were driven by fast glutamatergic thalamoreticular inputs but did not require postsynaptic Group 1 metabotropic glutamate receptor activation. For a significant fraction of TRN neurons, synaptic inputs or brief depolarizing current steps led to long-lasting plateau potentials and persistent firing (PF), and in turn, resulted in sustained synaptic inhibition in postsynaptic relay neurons of the ventrobasal thalamus (VB). Pharmacological approachesindicated that plateau potentials were triggered by Ca²⁺ influx through T-type Ca²⁺ channels and mediated by Ca²⁺- and voltage-dependent transient receptor potential melastatin 4 (TRPM4) channels. Together, our results suggest that thalamic circuits can generate slow oscillatory activity, mediated by an interplay of TRN-VB synaptic circuits that generate rhythmicity and TRN cell-intrinsic mechanisms that control PF and oscillation frequency.SIGNIFICANCE STATEMENT Slow forms of thalamocortical rhythmic activity are thought to be essential for memory consolidation during sleep and the efficient removal of potentially toxic metabolites. In vivo, thalamic slow oscillations are regulated by strong bidirectional synaptic pathways linking neocortex and thalamus. Therefore, in vitro studies in the isolated thalamus offer important insights about the ability of individual neurons and local circuits to generate different forms of rhythmic activity. We found that circuits formed by GABAergic neurons in the thalamic reticular nucleus and glutamatergic relay neurons in the ventrobasal thalamus generated slow oscillatory activity, which was accompanied by persistent firing in thalamic reticular nucleus neurons. Our results identify both cell-intrinsic and synaptic mechanisms that mediate slow forms of rhythmic activity in thalamic circuits.

The Epilepsy of Infancy With Migrating Focal Seizures: Identification of de novo Mutations of the KCNT2 Gene That Exert Inhibitory Effects on the Corresponding Heteromeric KNa1.1/KNa1.2 Potassium Channel

The epilepsy of infancy with migrating focal seizures (EIMFS; previously called Malignant migrating partial seizures of infancy) are early-onset epileptic encephalopathies (EOEE) that associate multifocal ictal discharges and profound psychomotor retardation. EIMFS have a genetic origin and are mostly caused by de novo mutations in the KCNT1 gene, and much more rarely in the KCNT2 gene. KCNT1 and KCNT2 respectively encode the KNa1.1 (Slack) and KNa1.2 (Slick) subunits of the sodium-dependent voltage-gated potassium channel KNa. Functional analyses of the corresponding mutant homomeric channels in vitro suggested gain-of-function effects. Here, we report two novel, de novo truncating mutations of KCNT2: one mutation is frameshift (p.L48Qfs43), is situated in the N-terminal domain, and was found in a patient with EOEE (possibly EIMFS); the other mutation is nonsense (p.K564*), is located in the C-terminal region, and was found in a typical EIMFS patient. Using whole-cell patch-clamp recordings, we have analyzed the functional consequences of those two novel KCNT2 mutations on reconstituted KNa1.2 homomeric and KNa1.1/KNa1.2 heteromeric channels in transfected chinese hamster ovary (CHO) cells. We report that both mutations significantly impacted on KNa function; notably, they decreased the global current density of heteromeric channels by ~25% (p.K564*) and ~55% (p.L48Qfs43). Overall our data emphasize the involvement of KCNT2 in EOEE and provide novel insights into the role of heteromeric KNa channel in the severe KCNT2-related epileptic phenotypes. This may have important implications regarding the elaboration of future treatment.

Sleep Spindles: Mechanisms and Functions

Sleep spindles are burstlike signals in the electroencephalogram (EEG) of the sleeping mammalian brain and electrical surface correlates of neuronal oscillations in thalamus. As one of the most inheritable sleep EEG signatures, sleep spindles probably reflect the strength and malleability of thalamocortical circuits that underlie individual cognitive profiles. We review the characteristics, organization, regulation, and origins of sleep spindles and their implication in non-rapid-eye-movement sleep (NREMS) and its functions, focusing on human and rodent. Spatially, sleep spindle-related neuronal activity appears on scales ranging from small thalamic circuits to functional cortical areas, and generates a cortical state favoring intracortical plasticity while limiting cortical output. Temporally, sleep spindles are discrete events, part of a continuous power band, and elements grouped on an infraslow time scale over which NREMS alternates between continuity and fragility. We synthesize diverse and seemingly unlinked functions of sleep spindles for sleep architecture, sensory processing, synaptic plasticity, memory formation, and cognitive abilities into a unifying sleep spindle concept, according to which sleep spindles 1) generate neural conditions of large-scale functional connectivity and plasticity that outlast their appearance as discrete EEG events, 2) appear preferentially in thalamic circuits engaged in learning and attention-based experience during wakefulness, and 3) enable a selective reactivation and routing of wake-instated neuronal traces between brain areas such as hippocampus and cortex. Their fine spatiotemporal organization reflects NREMS as a physiological state coordinated over brain and body and may indicate, if not anticipate and ultimately differentiate, pathologies in sleep and neurodevelopmental, -degenerative, and -psychiatric conditions.

Genomic discovery of ion channel genes in the central nervous system of the lamprey Petromyzon marinus

The lamprey is a popular animal model for a number of types of neurobiology studies, including organization and operation of locomotor and respiratory systems, behavioral recovery following spinal cord injury (SCI), cellular and synaptic neurophysiology, comparative neuroanatomy, neuropharmacology, and neurodevelopment. Yet relatively little work has been done on the molecular underpinnings of nervous system function in lamprey. This is due in part to a paucity of gene information for some of the most fundamental proteins involved in neural activity: ion channels. We report here 47 putative ion channel sequences in the central nervous system (CNS) of larval lampreys from the predicted coding sequences (CDS) discovered in the P. marinus genome. These include 32 potassium (K+) channels, six sodium (Na+) channels, and nine calcium (Ca2+) channels. Through RT-PCR, we examined the distribution of these ion channels in the anterior (ARRN), middle (MRRN), and posterior (PRRN) rhombencephalic reticular nuclei, as well as the spinal cord (SC). This study lays the foundation for incorporating more advanced molecular techniques to investigate the role of ion channels in the neural networks of the lamprey.

Choline and NMDG directly reduce outward currents: reduced outward current when these substances replace Na+ is alone not evidence of Na+-activated K+ currents

Choline chloride is often, and N-methyl-d-glucamine (NMDG) sometimes, used to replace sodium chloride in studies of sodium-activated potassium channels. Given the high concentrations used in sodium replacement protocols, it is essential to test that it is not the replacement substances themselves, as opposed to the lack of sodium, that cause any observed effects. We therefore compared, in lobster stomatogastric neurons and leech Retzius cells, the effects of applying salines in which choline chloride replaced sodium chloride, and in which choline hydroxide or sucrose was added to normal saline. We also tested, in stomatogastric neurons, the effect of adding NMDG to normal saline. These protocols allowed us to measure the direct effects (i.e., effects not due to changes in sodium concentration or saline osmolarity or ionic strength) of choline on stomatogastric and leech currents, and of NMDG on stomatogastric currents. Choline directly reduced transient and sustained depolarization-activated outward currents in both species, and NMDG directly reduced transient depolarization-activated outward currents in stomatogastric neurons. Experiments with lower choline concentrations showed that adding as little as 150 mM (stomatogastric) or 5 mM (leech) choline reduced at least some depolarization-activated outward currents. Reductions in outward current with choline chloride or NMDG replacement alone are thus not evidence of sodium-activated potassium currents. NEW & NOTEWORTHY We show that choline or N-methyl-d-glucamine (NMDG) directly (i.e., not due to changes in extracellular sodium) decrease outward currents. Prior work studying sodium-activated potassium channels in which sodium was replaced with choline or NMDG without an addition control may therefore be artifactual.

Tapping the Brakes: Cellular and Synaptic Mechanisms that Regulate Thalamic Oscillations

Thalamic oscillators contribute to both normal rhythms associated with sleep and anesthesia and abnormal, hypersynchronous oscillations that manifest behaviorally as absence seizures. In this review, we highlight new findings that refine thalamic contributions to cortical rhythms and suggest that thalamic oscillators may be subject to both local and global control. We describe endogenous thalamic mechanisms that limit network synchrony and discuss how these protective brakes might be restored to prevent absence seizures. Finally, we describe how intrinsic and circuit-level specializations among thalamocortical loops may determine their involvement in widespread oscillations and render subsets of thalamic nuclei especially vulnerable to pathological synchrony.

Differential distribution of the sodium-activated potassium channels slick and slack in mouse brain

The sodium‐activated potassium channels Slick (Slo2.1, KCNT2) and Slack (Slo2.2, KCNT1) are high‐conductance potassium channels of the Slo family. In neurons, Slick and Slack channels are involved in the generation of slow afterhyperpolarization, in the regulation of firing patterns, and in setting and stabilizing the resting membrane potential. The distribution and subcellular localization of Slick and Slack channels in the mouse brain have not yet been established in detail. The present study addresses this issue through in situ hybridization and immunohistochemistry. Both channels were widely distributed and exhibited distinct distribution patterns. However, in some brain regions, their expression overlapped. Intense Slick channel immunoreactivity was observed in processes, varicosities, and neuronal cell bodies of the olfactory bulb, granular zones of cortical regions, hippocampus, amygdala, lateral septal nuclei, certain hypothalamic and midbrain nuclei, and several regions of the brainstem. The Slack channel showed primarily a diffuse immunostaining pattern, and labeling of cell somata and processes was observed only occasionally. The highest Slack channel expression was detected in the olfactory bulb, lateral septal nuclei, basal ganglia, and distinct areas of the midbrain, brainstem, and cerebellar cortex. In addition, comparing our data obtained from mouse brain with a previously published study on rat brain revealed some differences in the expression and distribution of Slick and Slack channels in these species. J. Comp. Neurol. 524:2093–2116, 2016. © 2015 The Authors The Journal of Comparative Neurology Published by Wiley Periodicals, Inc.

Knockout of Slo2.2 enhances itch, abolishes KNa current, and increases action potential firing frequency in DRG neurons

Two mammalian genes, Kcnt1 and Kcnt2, encode pore-forming subunits of Na(+)-dependent K(+) (KNa) channels. Progress in understanding KNa channels has been hampered by the absence of specific tools and methods for rigorous KNa identification in native cells. Here, we report the genetic disruption of both Kcnt1 and Kcnt2, confirm the loss of Slo2.2 and Slo2.1 protein, respectively, in KO animals, and define tissues enriched in Slo2 expression. Noting the prevalence of Slo2.2 in dorsal root ganglion, we find that KO of Slo2.2, but not Slo2.1, results in enhanced itch and pain responses. In dissociated small diameter DRG neurons, KO of Slo2.2, but not Slo2.1, abolishes KNa current. Utilizing isolectin B4+ neurons, the absence of KNa current results in an increase in action potential (AP) firing and a decrease in AP threshold. Activation of KNa acts as a brake to initiation of the first depolarization-elicited AP with no discernible effect on afterhyperpolarizations.

Self-regulation of adult thalamocortical neurons

The thalamus acts as a conduit for sensory and other information traveling to the cortex. In response to continuous sensory stimulation in vivo, the firing rate of thalamocortical neurons initially increases, but then within a minute firing rate decreases and T-type Ca(2+) channel-dependent action potential burst firing emerges. While neuromodulatory systems could play a role in this inhibitory response, we instead report a novel and cell-autonomous inhibitory mechanism intrinsic to the thalamic relay neuron. Direct intracellular stimulation of thalamocortical neuron firing initially triggered a continuous and high rate of action potential discharge, but within a minute membrane potential (Vm) was hyperpolarized and firing rate to the same stimulus was decreased. This self-inhibition was observed across a wide variety of thalamic nuclei, and in a subset firing mode switched from tonic to bursting. The self-inhibition resisted blockers of intracellular Ca(2+) signaling, Na(+)-K(+)-ATPases, and G protein-regulated inward rectifier (GIRK) channels as implicated in other neuron subtypes, but instead was in part inhibited by an ATP-sensitive K(+) channel blocker. The results identify a new homeostatic mechanism within the thalamus capable of gating excitatory signals at the single-cell level.

Adaptation in the visual cortex: influence of membrane trajectory and neuronal firing pattern on slow afterpotentials

The input/output relationship in primary visual cortex neurons is influenced by the history of the preceding activity. To understand the impact that membrane potential trajectory and firing pattern has on the activation of slow conductances in cortical neurons we compared the afterpotentials that followed responses to different stimuli evoking similar numbers of action potentials. In particular, we compared afterpotentials following the intracellular injection of either square or sinusoidal currents lasting 20 seconds. Both stimuli were intracellular surrogates of different neuronal responses to prolonged visual stimulation. Recordings from 99 neurons in slices of visual cortex revealed that for stimuli evoking an equivalent number of spikes, sinusoidal current injection activated a slow afterhyperpolarization of significantly larger amplitude (8.5 ± 3.3 mV) and duration (33 ± 17 s) than that evoked by a square pulse (6.4 ± 3.7 mV, 28 ± 17 s; p<0.05). Spike frequency adaptation had a faster time course and was larger during plateau (square pulse) than during intermittent (sinusoidal) depolarizations. Similar results were obtained in 17 neurons intracellularly recorded from the visual cortex in vivo. The differences in the afterpotentials evoked with both protocols were abolished by removing calcium from the extracellular medium or by application of the L-type calcium channel blocker nifedipine, suggesting that the activation of a calcium-dependent current is at the base of this afterpotential difference. These findings suggest that not only the spikes, but the membrane potential values and firing patterns evoked by a particular stimulation protocol determine the responses to any subsequent incoming input in a time window that spans for tens of seconds to even minutes.

Brain state dependent activity in the cortex and thalamus

Cortical and thalamocortical activity is highly state dependent, varying between patterns that are conducive to accurate sensory-motor processing, to states in which the brain is largely off-line and generating internal rhythms irrespective of the outside world. The generation of rhythmic activity occurs through the interaction of stereotyped patterns of connectivity together with intrinsic membrane and synaptic properties. One common theme in the generation of rhythms is the interaction of a positive feedback loop (e.g., recurrent excitation) with negative feedback control (e.g., inhibition, adaptation, or synaptic depression). The operation of these state-dependent activities has wide ranging effects from enhancing or blocking sensory-motor processing to the generation of pathological rhythms associated with psychiatric or neurological disorders.

Ongoing network state controls the length of sleep spindles via inhibitory activity

Sleep spindles are major transient oscillations of the mammalian brain. Spindles are generated in the thalamus; however, what determines their duration is presently unclear. Here, we measured somatic activity of excitatory thalamocortical (TC) cells together with axonal activity of reciprocally coupled inhibitory reticular thalamic cells (nRTs) and quantified cycle-by-cycle alterations in their firing in vivo. We found that spindles with different durations were paralleled by distinct nRT activity, and nRT firing sharply dropped before the termination of all spindles. Both initial nRT and TC activity was correlated with spindle length, but nRT correlation was more robust. Analysis of spindles evoked by optogenetic activation of nRT showed that spindle probability, but not spindle length, was determined by the strength of the light stimulus. Our data indicate that during natural sleep a dynamically fluctuating thalamocortical network controls the duration of sleep spindles via the major inhibitory element of the circuits, the nRT.

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Coupled excitatory-inhibitory thalamic populations were recorded during spindles

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Spindle termination is preceded by a drop in nRT activity

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Spindles of different lengths have distinct nRT activity trajectories

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Spindle duration is strongly influenced by the initial network state

Novel coupling between TRPC-like and KNa channels modulates low threshold spike-induced afterpotentials in rat thalamic midline neurons

Neurons in thalamic midline and paraventricular nuclei (PVT) display a unique slow afterhyperpolarizing potential (sAHP) following the low threshold spike (LTS) generated by activation of their low voltage Ca(2+) channels. We evaluated the conductances underlying this sAHP using whole-cell patch-clamp recordings in rat brain slice preparations. Initial observations recorded in the presence of TTX revealed a marked dependency of the LTS-induced sAHP on extracellular Na(+): replacing Na(+) with TRIS(+) in the external medium eliminated the LTS-induced sAHP; substitution of Na(+) with either Li(+) or choline(+) in the external medium resulted in a gradual loss of the sAHP and its replacement with a prolonged slow afterdepolarizing potential (sADP). The LTS-induced sAHP was reduced by quinidine and potentiated by loxapine, suggesting involvement of KNa-like channels. Canonical transient receptor potential (TRPC) channels were considered the source for Na(+) based on observations that the sAHP was suppressed by nonselective TRPC channel blockers (2-APB, flufenamic acid and ML204) but unchanged in the presence of TRPV1 channel blocker (SB-366791). In addition, after replacement of Na(+) with Li(+), the isolated LTS-induced sADP was significantly suppressed in the presence of 2-APB or ML204, after replacement of extracellular Ca(2+) with Sr(2+), and by intracellular Ca(2+) chelation with EGTA, data that collectively suggest involvement of Ca(2+)-activated TRPC-like conductances containing TRPC4/5 subunits. The isolated LTS-induced sADP also exhibited a strong voltage dependency, decreasing at hyperpolarizing potentials, further support for involvement of TRPC4/5 subunits. This sADP exhibited neurotransmitter receptor sensitivity, with suppression by 5-CT, a 5-HT7 receptor agonist, and enhancement by the neuropeptide orexin A. These data suggest that LTS-induced slow afterpotentials reflect a simultaneous interplay between KNa and TRPC-like conductances, novel for midline thalamic neurons.

How adaptation currents change threshold, gain, and variability of neuronal spiking

Many types of neurons exhibit spike rate adaptation, mediated by intrinsic slow K(+) currents, which effectively inhibit neuronal responses. How these adaptation currents change the relationship between in vivo like fluctuating synaptic input, spike rate output, and the spike train statistics, however, is not well understood. In this computational study we show that an adaptation current that primarily depends on the subthreshold membrane voltage changes the neuronal input-output relationship (I-O curve) subtractively, thereby increasing the response threshold, and decreases its slope (response gain) for low spike rates. A spike-dependent adaptation current alters the I-O curve divisively, thus reducing the response gain. Both types of an adaptation current naturally increase the mean interspike interval (ISI), but they can affect ISI variability in opposite ways. A subthreshold current always causes an increase of variability while a spike-triggered current decreases high variability caused by fluctuation-dominated inputs and increases low variability when the average input is large. The effects on I-O curves match those caused by synaptic inhibition in networks with asynchronous irregular activity, for which we find subtractive and divisive changes caused by external and recurrent inhibition, respectively. Synaptic inhibition, however, always increases the ISI variability. We analytically derive expressions for the I-O curve and ISI variability, which demonstrate the robustness of our results. Furthermore, we show how the biophysical parameters of slow K(+) conductances contribute to the two different types of an adaptation current and find that Ca(2+)-activated K(+) currents are effectively captured by a simple spike-dependent description, while muscarine-sensitive or Na(+)-activated K(+) currents show a dominant subthreshold component.

Slack, Slick and Sodium-Activated Potassium Channels

The Slack and Slick genes encode potassium channels that are very widely expressed in the central nervous system. These channels are activated by elevations in intracellular sodium, such as those that occur during trains of one or more action potentials, or following activation of non-selective cationic neurotransmitter receptors such as AMPA receptors. This review covers the cellular and molecular properties of Slack and Slick channels and compares them with findings on the properties of sodium-activated potassium currents (termed K_Na currents) in native neurons. Human mutations in Slack channels produce extremely severe defects in learning and development, suggesting that K_Na channels play a central role in neuronal plasticity and intellectual function.

Is slack an intrinsic seizure terminator?

Understanding how epileptic seizures are initiated and propagated across large brain networks is difficult, but an even greater mystery is what makes them stop. Failure of spontaneous seizure termination leads to status epilepticus-a state of uninterrupted seizure activity that can cause death or permanent brain damage. Global factors, like changes in neuromodulators and ion concentrations, are likely to play major roles in spontaneous seizure cessation, but individual neurons also have intrinsic active ion currents that may contribute. The recently discovered gene Slack encodes a sodium-activated potassium channel that mediates a major proportion of the outward current in many neurons. Although given little attention, the current flowing through this channel may have properties consistent with a role in seizure termination.

Low-magnesium medium induces epileptiform activity in mouse olfactory bulb slices

Magnesium-free medium can be used in brain slice studies to enhance glutamate receptor function, but this manipulation causes seizure-like activity in many cortical areas. The rodent olfactory bulb (OB) slice is a popular preparation, and potentially ictogenic ionic conditions have often been used to study odor processing. We studied low Mg(2+)-induced epileptiform discharges in mouse OB slices using extracellular and whole cell electrophysiological recordings. Low-Mg(2+) medium induced two distinct types of epileptiform activity: an intraglomerular delta-frequency oscillation resembling slow sniff-induced activity and minute-long seizure-like events (SLEs) consisting of large negative-going field potentials accompanied by sustained depolarization of output neurons. SLEs were dependent on N-methyl-D-aspartate receptors and sodium currents and were facilitated by α-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors. The events were initiated in the glomerular layer and propagated laterally through the external plexiform layer at a slow time scale. Our findings confirm that low-Mg(2+) medium should be used with caution in OB slices. Furthermore, the SLEs resembled the so-called slow direct current (DC) shift of clinical and experimental seizures, which has recently been recognized as being of great clinical importance. The OB slice may therefore provide a robust and unique in vitro model of acute seizures in which mechanisms of epileptiform DC shifts can be studied in isolation from fast oscillations.

Ca2+-Dependent and Na+-Dependent K+ Conductances Contribute to a Slow AHP in Thalamic Paraventricular Nucleus Neurons: A Novel Target for Orexin Receptors

Thalamic paraventricular nucleus (PVT) neurons exhibit a postburst apamin-resistant slow afterhyperpolarization (sAHP) that is unique to midline thalamus, displays activity dependence, and is abolished in tetrodotoxin. Analysis of the underlying s IAHP confirmed a requirement for Ca2+ influx with contributions from P/Q-, N-, L-, and R subtype channels, a reversal potential near EK+ and a significant reduction by UCL-2077, barium or TEA, consistent with a role for KCa channels. s IAHP was significantly reduced by activation of either the cAMP or the protein kinase C (PKC) signaling pathway. Further analysis of the sAHP revealed an activity-dependent but Ca2+-independent component that was reduced in high [K+]o and blockable after Na+ substitution with Li+ or in the presence of quinidine, suggesting a role for KNa channels. The Ca2+-independent sAHP component was selectively reduced by activation of the PKC signaling pathway. The sAHP contributed to spike frequency adaptation, which was sensitive to activation of either cAMP or PKC signaling pathways and, near the peak of membrane hyperpolarization, was sufficient to cause de-inactivation of low threshold T-Type Ca2+ channels, thus promoting burst firing. PVT neurons are densely innervated by orexin-immunoreactive fibers, and depolarized by exogenously applied orexins. We now report that orexin A significantly reduced both Ca2+-dependent and -independent s IAHP, and spike frequency adaptation. Furthermore orexin A-induced s IAHP inhibition was mediated through activation of PKC but not PKA. Collectively, these observations suggest that KCa and KNa channels have a role in a sAHP that contributes to spike frequency adaptation and neuronal excitability in PVT neurons and that the sAHP is a novel target for modulation by the arousal- and feeding-promoting orexin neuropeptides.

The slack sodium-activated potassium channel provides a major outward current in olfactory neurons of Kv1.3-/- super-smeller mice

The Kv1.3 voltage-dependent potassium channel is expressed at high levels in mitral cells of the olfactory bulb (OB). Deletion of the Kv1.3 potassium channel gene (Kv1.3-/-) in mice lowers the threshold for detection of odors, increases the ability to discriminate between odors, and alters the firing pattern of mitral cells. We have now found that loss of Kv1.3 produces a compensatory increase in Na(+)-activated K(+) currents (K(Na)) in mitral cells. Levels of the K(Na) channel subunit Slack-B determined by Western blotting are substantially increased in the OB from Kv1.3-/- animals compared with those of wildtype animals. In voltage-clamp recordings of OB slices, elevation of intracellular sodium from 0 to 60 mM increased mean outward currents by 15% in mitral cells from wildtype animals and by 40% in cells from Kv1.3-/- animals. In Kv1.3-/- cells, K(Na) current could even be detected with 0 mM Na(+) internal solutions, provided extracellular Na(+) was present, and this current could be abolished by TTX and ZD7288, blockers of Na(+) influx through voltage-dependent Na(+) channels and H-channels, respectively. The role of enhanced expression of Slack subunits in the increase of K(Na) current in Kv1.3-/- cells was also confirmed using an RNA interference (RNA(i)) approach to suppress Slack expression in primary cultures of olfactory neurons. In Kv1.3-/- neurons, treatment with Slack-specific RNA(i) inhibited approximately 75% of the net outward current, whereas in wildtype cells, the same treatment suppressed only about 25% of the total current. Scrambled and mismatched RNA(i) oligonucleotides failed to suppress currents. Our findings raise the possibility that the olfactory phenotype of Kv1.3-/- animals results in part from an enhancement of K(Na) currents.

Spike integration and cellular memory in a rhythmic network from Na+/K+ pump current dynamics

The output of a neural circuit results from an interaction between the intrinsic properties of neurons within the circuit and the features of the synaptic connections between them. The plasticity of intrinsic properties has been primarily attributed to modification of ion channel function and/or number. In this study, we demonstrate a mechanism for intrinsic plasticity in rhythmically active Drosophila neurons that is not conductance-based. Larval motor neurons show a long lasting sodium-dependent afterhyperpolarization (AHP) following bursts of action potentials that is mediated by the electrogenic activity of Na⁺/K⁺ ATPase. This AHP persists for multiple seconds following volleys of action potentials and is able to function as a pattern-insensitive integrator of spike number that is independent of external calcium. This current also interacts with endogenous Shal K⁺ conductances to modulate spike timing for multiple seconds following rhythmic activity, providing a cellular memory of network activity on a behaviorally relevant time scale.

Mechanisms of modulation of AMPA-induced Na+-activated K+ current by mGluR1

Na(+)-activated K(+) (K(Na)) channels can be activated by Na(+) influx via ionotropic receptors and play a role in shaping synaptic transmission. In expression systems, K(Na) channels are modulated by G protein-coupled receptors, but such a modulation has not been shown for the native channels. In this study, we examined whether K(Na) channels coupled to AMPA receptors are modulated by metabotropic glutamate receptors (mGluRs) in lamprey spinal cord neurons. Activation of mGluR1 strongly inhibited the AMPA-induced K(Na) current. However, when intracellular Ca(2+) was chelated with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), the K(Na) current was enhanced by mGluR1. Activation of protein kinase C (PKC) mimicked the inhibitory effect of mGluR1 on the K(Na) current. Blockade of PKC prevented the mGluR1-induced inhibition of the K(Na) current, but did not affect the enhancement of the current seen in BAPTA. Together these results suggest that mGluR1 can differentially modulate AMPA-induced K(Na) current in a Ca(2+)- and PKC-dependent manner.

Properties of a T-type Ca2+channel-activated slow afterhyperpolarization in thalamic paraventricular nucleus and other thalamic midline neurons

Burst firing mediated by a low-threshold spike (LTS) is the hallmark of many thalamic neurons. However, postburst afterhyperpolarizations (AHPs) are relatively uncommon in thalamus. We now report data from patch-clamp recordings in rat brain slice preparations that reveal an LTS-induced slow AHP (sAHP) in thalamic paraventricular (PVT) and other midline neurons, but not in ventrobasal or reticular thalamic neurons. The LTS-induced sAHP lasts 8.9 +/- 0.4 s and has a novel pharmacology, with resistance to tetrodotoxin and cadmium and reduction by Ni(2+) or nominally zero extracellular calcium concentration, which also attenuate both the LTS and sAHP. The sAHP is inhibited by 10 mM intracellular EGTA or by equimolar replacement of extracellular Ca(2+) with Sr(2+), consistent with select activation of LVA T-type Ca(2+) channels and subsequent Ca(2+) influx. In control media, the sAHP reverses near E(K(+)), shifting to -78 mV in 10.1 mM [K(+)](o) and is reduced by Ba(2+) or tetraethylammonium. Although these data are consistent with opening of Ca(2+)-activated K(+) channels, this sAHP lacks sensitivity to specific Ca(2+)-activated K(+) channel blockers apamin, iberiotoxin, charybdotoxin, and UCL-2077. The LTS-induced sAHP is suppressed by a beta-adrenoceptor agonist isoproterenol, a serotonin 5-HT(7) receptor agonist 5-CT, a neuropeptide orexin-A, and by stimulation of the cAMP/protein kinase A pathway with 8-Br-cAMP and forskolin. The data suggest that PVT and certain midline thalamic neurons possess an LTS-induced sAHP that is pharmacologically distinct and may be important for information transfer in thalamic-limbic circuitry during states of attentiveness and motivation.

Maladaptive homeostatic plasticity in a rodent model of central pain syndrome: thalamic hyperexcitability after spinothalamic tract lesions

Central pain syndrome (CPS) is defined as pain associated with a lesion of the CNS and is a common consequence of spinal cord injuries. We generated a rodent model of CPS by making unilateral electrolytic or demyelinating lesions centered on the spinothalamic tract in rats. Thermal hyperalgesia and mechanical allodynia occurred in both hind paws and forepaws by 7 d postlesion and were maintained >31 d. Field potentials in the ventral posterior lateral nucleus (VPL) in thalamic brain slices from lesioned animals displayed an increased probability of burst responses. Ethosuximide, a T-type calcium channel blocker, eliminated busting in lesioned thalamic slices and attenuated lesion-induced hyperalgesia and allodynia. We conclude that CPS in this model results from an increase in the excitability of thalamic nuclei that have lost normal ascending inputs as the result of a spinal cord injury and suggest that ethosuximide will relieve human CPS by restoring normal thalamic excitability.

Amino-termini isoforms of the Slack K+ channel, regulated by alternative promoters, differentially modulate rhythmic firing and adaptation

The rates of activation and unitary properties of Na+-activated K+ (K(Na)) currents have been found to vary substantially in different types of neurones. One class of K(Na) channels is encoded by the Slack gene. We have now determined that alternative RNA splicing gives rise to at least five different transcripts for Slack, which produce Slack channels that differ in their predicted cytoplasmic amino-termini and in their kinetic properties. Two of these, termed Slack-A channels, contain an amino-terminus domain closely resembling that of another class of K(Na) channels encoded by the Slick gene. Neuronal expression of Slack-A channels and of the previously described Slack isoform, now called Slack-B, are driven by independent promoters. Slack-A mRNAs were enriched in the brainstem and olfactory bulb and detected at significant levels in four different brain regions. When expressed in CHO cells, Slack-A channels activate rapidly upon depolarization and, in single channel recordings in Xenopus oocytes, are characterized by multiple subconductance states with only brief transient openings to the fully open state. In contrast, Slack-B channels activate slowly over hundreds of milliseconds, with openings to the fully open state that are approximately 6-fold longer than those for Slack-A channels. In numerical simulations, neurones in which outward currents are dominated by a Slack-A-like conductance adapt very rapidly to repeated or maintained stimulation over a wide range of stimulus strengths. In contrast, Slack-B currents promote rhythmic firing during maintained stimulation, and allow adaptation rate to vary with stimulus strength. Using an antibody that recognizes all amino-termini isoforms of Slack, Slack immunoreactivity is present at locations that have no Slack-B-specific staining, including olfactory bulb glomeruli and the dendrites of hippocampal neurones, suggesting that Slack channels with alternate amino-termini such as Slack-A channels are present at these locations. Our data suggest that alternative promoters of the Slack gene differentially modulate the properties of neurones.

Increasing extracellular potassium results in subthalamic neuron activity resembling that seen in a 6-hydroxydopamine lesion

Abnormal neuronal activity in the subthalamic nucleus (STN) plays a crucial role in the pathophysiology of Parkinson's disease (PD). Although altered extracellular potassium concentration ([K+]o) and sensitivity to [K+]o modulates neuronal activity, little is known about the potassium balance in the healthy and diseased STN. In vivo measurements of [K+]o using ion-selective electrodes demonstrated a twofold increase in the decay time constant of lesion-induced [K+]o transients in the STN of adult Wistar rats with a unilateral 6-hydroxydopamine (6-OHDA) median forebrain bundle lesion, employed as a model of PD, compared with nonlesioned rats. Various [K+]o concentrations (1.5-12.5 mM) were applied to in vitro slice preparations of three experimental groups of STN slices from nonlesioned control rats, ipsilateral hemispheres, and contralateral hemispheres of lesioned rats. The majority of STN neurons of nonlesioned rats and in slices contralateral to the lesion fired spontaneously, predominantly in a regular pattern, whereas those in slices ipsilateral to the lesion fired more irregularly or even in bursts. Experimentally increased [K+]o led to an increase in the number of spontaneously firing neurons and action potential firing rates in all groups. This was accompanied by a decrease in the amplitude of post spike afterhyperpolarization (AHP) and the amplitude and duration of the posttrain AHP. Lesion effects in ipsilateral neurons at physiological [K+]o resembled the effects of elevated [K+]o in nonlesioned rats. Our data suggest that changed potassium sensitivity due to conductivity alterations and delayed clearance may be critical for shaping STN activity in parkinsonian states.

Sodium-dependent potassium channels of a Slack-like subtype contribute to the slow afterhyperpolarization in lamprey spinal neurons

The slow afterhyperpolarization (sAHP) following the action potential is the main determinant of spike frequency regulation. The sAHP after single action potentials in neurons of the lamprey locomotor network is largely due to calcium-dependent K+channels (80%), activated by calcium entering the cell during the spike. The residual (20%) component becomes prominent during high level activity (50% of the sAHP). It is not Ca2+ dependent, has a reversal potential like that of potassium, and is not affected by chloride injection. It is not due to rapid activation of Na+/K+-ATPase. This non-KCa-sAHP is reduced markedly in amplitude when sodium ions are replaced by lithium ions, and is thus sodium dependent. Quinidine also blocks this sAHP component, further indicating an involvement of sodium-dependent potassium channels (KNa). Modulators tested do not influence the KNa-sAHP amplitude. Immunofluorescence labelling with an anti-Slack antibody revealed distinct immunoreactivity of medium-sized and large neurons in the grey matter of the lamprey spinal cord, suggesting the presence of a Slack-like subtype of KNa channel. The results strongly indicate that a KNa potassium current contributes importantly to the sAHP and thereby to neuronal frequency regulation during high level burst activity as during locomotion. This is, to our knowledge, the first demonstration of a functional role for the Slack gene in contributing to the slow AHP.

Dual GABAergic synaptic response of fast excitation and slow inhibition in the medial habenula of rat epithalamus

We report here a novel action of GABAergic synapses in regulating tonic firing in the mammalian brain. By using gramicidin-perforated patch recording in rat brain slices, we show that cells of the medial habenula of the epithalamus generate tonic firing in basal conditions. The GABAergic input onto these cells at postnatal days 18-25 generates a combinatorial activation of fast excitation and slow inhibition. The fast excitation, mediated by gamma-aminobutyric acid type A receptors (GABA A Rs), is alone capable of triggering robust action potentials to increase cell firing. This excitatory influence of GABAergic input results from the Cl(-) homeostasis that maintains intracellular Cl(-) at high levels. The GABA A excitation is often followed by a slow inhibition mediated by GABA B Rs that suppresses tonic firing. Interestingly, in a subpopulation of the cells, the GABA B inhibition exhibits a remarkably low threshold for synaptic activation in that low-strength GABAergic input often activates selectively the GABA B slow inhibition, whereas the GABA A excitation requires further increases in stimulus strength. Our study demonstrates that the dual activation of GABAergic excitation and inhibition through GABA A Rs and GABA B Rs generates distinct temporal patterns of cell firing that alter the cellular output in an activity-dependent manner.

Contribution of persistent sodium current to locomotor pattern generation in neonatal rats

The persistent sodium current (I(NaP)) is known to play a role in rhythm generation in different systems. Here, we investigated its contribution to locomotor pattern generation in the neonatal rat spinal cord. The locomotor network is mainly located in the ventromedial gray matter of upper lumbar segments. By means of whole cell recordings in slices, we characterized membrane and I(NaP) biophysical properties of interneurons located in this area. Compared with motoneurons, interneurons were more excitable, because of higher input resistance and membrane time constant, and displayed lower firing frequency arising from broader spikes and longer AHPs. Ramp voltage-clamp protocols revealed a riluzole- or TTX-sensitive inward current, presumably I(NaP), three times smaller in interneurons than in motoneurons. However, in contrast to motoneurons, I(NaP) mediated a prolonged plateau potential in interneurons after reducing K(+) and Ca(2+) currents. We further used in vitro isolated spinal cord preparations to investigate the contribution of I(NaP) to locomotor pattern. Application of riluzole (10 muM) to the whole spinal cord or to the upper lumbar segments disturbed fictive locomotion, whereas application of riluzole over the caudal lumbar segments had no effect. The effects of riluzole appeared to arise from a specific blockade of I(NaP) because action potential waveform, dorsal root-evoked potentials, and miniature excitatory postsynaptic currents were not affected. This study provides new functional features of ventromedial interneurons, with the first description of I(NaP)-mediated plateau potentials, and new insights into the operation of the locomotor network with a critical implication of I(NaP) in stabilizing the locomotor pattern.

Characterization of Na+-Activated K+ Currents in Larval Lamprey Spinal Cord Neurons

Potassium channels play an important role in controlling neuronal firing and synaptic interactions. Na+-activated K+ ( KNa) channels have been shown to exist in neurons in different regions of the CNS, but their physiological function has been difficult to assess. In this study, we have examined if neurons in the spinal cord possess KNa currents. We used whole cell recordings from isolated spinal cord neurons in lamprey. These neurons display two different KNa currents. The first was transient and activated by the Na+ influx during the action potentials, and it was abolished when Na+ channels were blocked by tetrodotoxin. The second KNa current was sustained and persisted in tetrodotoxin. Both KNa currents were abolished when Na+ was substituted with choline or N-methyl-d-glucamine, indicating that they are indeed dependent on Na+ influx into neurons. When Na+ was substituted with Li+, the amplitude of the inward current was unchanged, whereas the transient KNa current was reduced but not abolished. This suggests that the transient KNa current is partially activated by Li+. These two KNa currents have different roles in controlling the action potential waveform. The transient KNa appears to act as a negative feedback mechanism sensing the Na+ influx underlying the action potential and may thus be critical for setting the amplitude and duration of the action potential. The sustained KNa current has a slow kinetic of activation and may underlie the slow Ca2+-independent afterhyperpolarization mediated by repetitive firing in lamprey spinal cord neurons.

A postsynaptic negative feedback mediated by coupling between AMPA receptors and Na+-activated K+ channels in spinal cord neurones

Na+-activated K+ channels (K(Na)) exist in different types of neurones and their activation has been shown to depend on Na+ influx via voltage-activated channels. However, one major route for Na+ influx into neurones is through ionotropic receptors and its role in activating K(Na) is still unclear. We have examined whether Na+ influx induced by activation of AMPA receptors can activate K(Na) in lamprey spinal cord neurones. Our results showed that the application of AMPA induced not only the characteristic inward current but also produced an outward current outlasting the activation of the receptors. This outward current was mediated by K+ and was abolished when Na+ was substituted with Li+. The AMPA-mediated K(Na) current was completely blocked by quinidine but was not modulated by increased intracellular Cl- concentration or ATP. Thus, Na+ influx via AMPA receptor channels activates K(Na) with properties similar to Slack channels. The AMPA-activated K(Na) may act as an inherent negative feedback mechanism to regulate the homeostasis of excitation.

Sodium-activated potassium current in guinea pig gastric myocytes

This study was designed to identify and characterize Na+-activated K+ current (I(K(Na))) in guinea pig gastric myocytes under whole-cell patch clamp. After whole-cell configuration was established under 110 mM intracellular Na+ concentration ([Na+]i) at holding potential of -60 mV, a large inward current was produced by external 60 mM K+([K+]degrees). This inward current was not affected by removal of external Ca2+. K+ channel blockers had little effects on the current (p>0.05). Only TEA (5 mM) inhibited steady-state current to 68+/-2.7% of the control (p<0.05). In the presence of K+ channel blocker cocktail (mixture of Ba2+, glibenclamide, 4-AP, apamin, quinidine and TEA), a large inward current was activated. However, the amplitude of the steady-state current produced under [K+]degrees (140 mM) was significantly smaller when Na+ in pipette solution was replaced with K+- and Li+ in the presence of K+ channel blocker cocktail than under 110 mM [Na+]i. In the presence of K+ channel blocker cocktail under low Cl- pipette solution, this current was still activated and seemed K+-selective, since reversal potentials (E(rev)) of various concentrations of [K+]degrees-induced current in current/voltage (I/V) relationship were nearly identical to expected values. R-56865 (10-20 microM), a blocker of I(K(Na)), completely and reversibly inhibited this current. The characteristics of the current coincide with those of I(K(Na)) of other cells. Our results indicate the presence of I(K(Na)) in guinea pig gastric myocytes.

Alterations in Burst Firing of Thalamic VPL Neurons and Reversal by Nav1.3 Antisense After Spinal Cord Injury

We recently showed that spinal cord contusion injury (SCI) at the thoracic level induces pain-related behaviors and increased spontaneous discharges, hyperresponsiveness to innocuous and noxious peripheral stimuli, and enlarged receptive fields in neurons in the ventral posterolateral (VPL) nucleus of the thalamus. These changes are linked to the abnormal expression of Nav1.3, a rapidly repriming voltage-gated sodium channel. In this study, we examined the burst firing properties of VPL neurons after SCI. Adult male Sprague–Dawley rats underwent contusion SCI at the T9 level. Four weeks later, when Nav1.3 protein was upregulated within VPL neurons, extracellular unit recordings were made from VPL neurons in intact animals, those with SCI, and in SCI animals after receiving lumbar intrathecal injections of Nav1.3 antisense or mismatch oligodeoxynucleotides for 4 days. After SCI, VPL neurons with identifiable peripheral receptive fields showed rhythmic oscillatory burst firing with changes in discrete burst properties, and alternated among single-spike, burst, silent, and spindle wave firing modes. Nav1.3 antisense, but not mismatch, partially reversed alterations in burst firing after SCI. These results demonstrate several newly characterized changes in spontaneous burst firing properties of VPL neurons after SCI and suggest that abnormal expression of Nav1.3 contributes to these phenomena.

Neuronal basis of the slow (<1 Hz) oscillation in neurons of the nucleus reticularis thalami in vitro

During deep sleep and anesthesia, the EEG of humans and animals exhibits a distinctive slow (<1 Hz) rhythm. In inhibitory neurons of the nucleus reticularis thalami (NRT), this rhythm is reflected as a slow (<1 Hz) oscillation of the membrane potential comprising stereotypical, recurring "up" and "down" states. Here we show that reducing the leak current through the activation of group I metabotropic glutamate receptors (mGluRs) with either trans-ACPD [(+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid] (50-100 microM) or DHPG [(S)-3,5-dihydroxyphenylglycine] (100 microM) instates an intrinsic slow oscillation in NRT neurons in vitro that is qualitatively equivalent to that observed in vivo. A slow oscillation could also be evoked by synaptically activating mGluRs on NRT neurons via the tetanic stimulation of corticothalamic fibers. Through a combination of experiments and computational modeling we show that the up state of the slow oscillation is predominantly generated by the "window" component of the T-type Ca2+ current, with an additional supportive role for a Ca2+-activated nonselective cation current. The slow oscillation is also fundamentally reliant on an Ih current and is extensively shaped by both Ca2+- and Na+-activated K+ currents. In combination with previous work in thalamocortical neurons, this study suggests that the thalamus plays an important and active role in shaping the slow (<1 Hz) rhythm during deep sleep.

Bursting of thalamic neurons and states of vigilance

This article addresses the functional significance of the electrophysiological properties of thalamic neurons. We propose that thalamocortical activity, is the product of the intrinsic electrical properties of the thalamocortical (TC) neurons and the connectivity their axons weave. We begin with an overview of the electrophysiological properties of single neurons in different functional states, followed by a review of the phylogeny of the electrical properties of thalamic neurons, in several vertebrate species. The similarity in electrophysiological properties unambiguously indicates that the thalamocortical system must be as ancient as the vertebrate branch itself. We address the view that rather than simply relays, thalamic neurons have sui generis intrinsic electrical properties that govern their specific functional dynamics and regulate natural functional states such as sleep and vigilance. In addition, thalamocortical activity has been shown to be involved in the genesis of several neuropsychiatric conditions collectively described as thalamocortical dysrhythmia syndrome.

Layer-specific properties of the persistent sodium current in sensorimotor cortex

We evaluated the characteristics of the persistent sodium current (I(NaP)) in pyramidal neurons of layers II/III and V in slices of rat sensorimotor cortex using whole cell patch-clamp recordings. In both layers, I(NaP) began activating around -60 mV and was half-activated at -43 mV. The I(NaP) peak amplitude and density were significantly higher in layer V. The voltage-dependent I(NaP) steady-state inactivation occurred at potentials that were significantly more positive in layer V (V(1/2): -42.3 +/- 1.1 mV) than in layer II/III (V(1/2): -46.8 +/- 1.6 mV). In both layers, a current fraction corresponding to about 25% of the maximal peak amplitude did not inactivate. The time course of I(NaP) inactivation and recovery from inactivation could be fitted with a biexponential function. In layer V pyramidal neurons the faster time constant of development of inactivation had variable values, ranging from 158.0 to 1,133.8 ms, but it was on average significantly slower than that in layer II/III (425.9 +/- 80.5 vs. 145.8 +/- 18.2 ms). In both layers, I(NaP) did not completely inactivate even with very long conditioning depolarizations (40 s at -10 mV). Recovery from inactivation was similar in the two layers. Layer V intrinsically bursting and regular spiking nonadapting neurons showed particularly prolonged depolarized plateau potentials when Ca2+ and K+ currents were blocked and slower early phase of I(NaP) development of inactivation. The biexponential kinetics characterizing the time-dependent inactivation of I(NaP) in layers II/III and V indicates a complex inactivating process that is incomplete, allowing a residual "persistent" current fraction that does not inactivate. Moreover, our data indicate that I(NaP) has uneven inactivation properties in pyramidal neurons of different layers of rat sensorimotor cortex. The higher current density, the rightward shifted voltage dependency of inactivation as well the slower kinetics of inactivation characterizing I(NaP) in layer V with respect to layer II/III pyramidal neurons may play a significant role in their ability to fire recurrent action potential bursts, as well in the high susceptibility to generate epileptic events.

Slow Adaptation in Fast-Spiking Neurons of Visual Cortex

Fast-spiking (FS) neurons are a class of inhibitory interneurons classically characterized as having short-duration action potentials (<0.5 ms at half height) and displaying little to no spike-frequency adaptation during short (<500 ms) depolarizing current pulses. As a consequence, the resulting injected current intensity versus firing frequency relationship is typically steep, and they can achieve firing frequencies of ≤1 kHz. Here we have investigated the properties of FS neurons discharges on a longer time scale. Twenty second discharges were induced in electrophysiologically identified FS neurons by means of current injection either with sinusoidal current or with square pulses. We found that virtually all FS neurons recorded in cortical slices do show spike-frequency adaptation but with a slow time course (τ = 2–19 s). This slow time course has precluded the observation of this property in previous studies that used shorter pulses. Contrary to the classical view of FS neurons functional properties, long-duration discharges were followed by a slow afterhyperpolarization lasting ≤23 s. During this postadaptation period, the excitability of the neurons was decreased on average for 16.7 ± 6.8 s, therefore rendering the cell less responsive to subsequent afferent inputs. Slow adaptation is also reported here for FS neurons recorded in vivo. This longer time scale of adaptation in FS neurons may be critical for balancing excitation and inhibition as well as for the understanding of cortical network computations.

Single-column thalamocortical network model exhibiting gamma oscillations, sleep spindles, and epileptogenic bursts

To better understand population phenomena in thalamocortical neuronal ensembles, we have constructed a preliminary network model with 3,560 multicompartment neurons (containing soma, branching dendrites, and a portion of axon). Types of neurons included superficial pyramids (with regular spiking [RS] and fast rhythmic bursting [FRB] firing behaviors); RS spiny stellates; fast spiking (FS) interneurons, with basket-type and axoaxonic types of connectivity, and located in superficial and deep cortical layers; low threshold spiking (LTS) interneurons, which contacted principal cell dendrites; deep pyramids, which could have RS or intrinsic bursting (IB) firing behaviors, and endowed either with nontufted apical dendrites or with long tufted apical dendrites; thalamocortical relay (TCR) cells; and nucleus reticularis (nRT) cells. To the extent possible, both electrophysiology and synaptic connectivity were based on published data, although many arbitrary choices were necessary. In addition to synaptic connectivity (by AMPA/kainate, NMDA, and GABA(A) receptors), we also included electrical coupling between dendrites of interneurons, nRT cells, and TCR cells, and--in various combinations--electrical coupling between the proximal axons of certain cortical principal neurons. Our network model replicates several observed population phenomena, including 1) persistent gamma oscillations; 2) thalamocortical sleep spindles; 3) series of synchronized population bursts, resembling electrographic seizures; 4) isolated double population bursts with superimposed very fast oscillations (>100 Hz, "VFO"); 5) spike-wave, polyspike-wave, and fast runs (about 10 Hz). We show that epileptiform bursts, including double and multiple bursts, containing VFO occur in rat auditory cortex in vitro, in the presence of kainate, when both GABA(A) and GABA(B) receptors are blocked. Electrical coupling between axons appears necessary (as reported previously) for persistent gamma and additionally plays a role in the detailed shaping of epileptogenic events. The degree of recurrent synaptic excitation between spiny stellate cells, and their tendency to fire throughout multiple bursts, also appears critical in shaping epileptogenic events.

Membrane bistability in thalamic reticular neurons during spindle oscillations

The thalamic reticular (RE) nucleus is a major source of inhibition in the thalamus. It plays a crucial role in regulating the excitability of thalamocortical networks and in generating some sleep rhythms. Current-clamp intracellular recordings of RE neurons in cats under barbiturate anesthesia revealed the presence of membrane bistability in approximately 20% of neurons. Bistability consisted of two alternate membrane potentials, separated by approximately 17-20 mV. While non-bistable (common) RE neurons fired rhythmic spike-bursts during spindles, bistable RE neurons fired tonically, with burst modulation, throughout spindle sequences. Bistability was strongly voltage dependent and only expressed under resting conditions (i.e. no current injection). The transition from the silent to the active state was a regenerative event that could be activated by brief depolarization, whereas brief hyperpolarizations could switch the membrane potential from the active to the silent state. These effects outlasted the current pulses. Corticothalamic stimulation could also switch the membrane potential from silent to active states. Addition of QX-314 in the recording micropipette either abolished or disrupted membrane bistability, suggesting I(Na(p)) to be responsible for its generation. Thalamocortical cells presented various patterns of spindling that reflected the membrane bistability in RE neurons. Finally, experimental data and computer simulations predicted a role for RE neurons' membrane bistability in inducing various patterns of spindling in target thalamocortical cells. We conclude that membrane bistability of RE neurons is an intrinsic property, likely generated by I(Na(p)) and modulated by cortical influences, as well as a factor that determines different patterns of spindle rhythms in thalamocortical neurons.

A Ca2+-independent slow afterhyperpolarization in substantia nigra compacta neurons

The discharge properties of dopaminergic neurons in substantia nigra are influenced by slow adaptive responses, which have not been fully identified. The present study describes, in a slice preparation from the rat, a complex afterhyperpolarization (AHP), elicited by action potential trains. The AHP could be subdivided into a fast component (AHP(f)), which was generated near action potential threshold, relaxed within approximately 1 s, and had highest amplitude when evoked by short-lasting (0.1 s) depolarizations, and a slow component (AHP(s)), which lasted several seconds, was evoked from subthreshold potentials, and required prolonged depolarizing stimuli (>0.1 s). A large proportion of the AHP(f) was sensitive to (i) 0.1 microM apamin, (ii) the Ca(2+) antagonists, Cd(2+) (0.2 mM) and Ni(2+) (0.3 mM), (iii) low (0.2 mM) extracellular Ca(2+) concentration, and (iv), Ca(2+) chelation with intracellular EGTA. The AHP(s) was resistant to the above treatments, and it was insensitive to 25 microM dantrolene or prolonged exposure to 1 microM thapsigargin. The reversal potential of the AHP(s) (-97 mV) was close to the K(+) equilibrium potential. It was significantly inhibited by 5 mM 4-aminopyridine, 5 microM haloperidol, 10 microM terfenadine, or high extracellular Mg(2+) (10 mM), but not by 30 mM tetraethylammonium chloride, 50 microM carbachol, 0.5 microM glipizide, 2 microM (-)sulpiride, 100 microM N-allyl-normetazocine, or 100 microM pentazocine. Haloperidol reduced the post-stimulus inhibitory period seen during spontaneous discharge, but had no detectable effect on spike frequency adaptation. It is concluded that the SK-type Ca(2+)-activated K(+) channels underlies a major component of the AHP(f), whereas the AHP(s) is Ca(2+)-independent and relies, in part, on a voltage-dependent K(+) current with properties resembling the ether-a-go-go-related gene K(+) channel. The latter component exerts a slow, spike-independent, inhibitory influence on repetitive discharge and contributes to the prolonged decrease in excitability following sustained depolarizing stimuli.

Histamine modulates thalamocortical activity by activating a chloride conductance in ferret perigeniculate neurons

In the mammalian central nervous system only gamma-aminobutyric acid (GABA) and glycine have been firmly linked to inhibition of neuronal activity through increases in membrane Cl(-) conductance, and these responses are mediated by ionotropic receptors. Iontophoretic application of histamine can also cause inhibitory responses in vivo, although the mechanisms of this inhibition are unknown and may involve pre- or postsynaptic factors. Here, we report that application of histamine to the GABAergic neurons of the thalamic perigeniculate nucleus (PGN), which is innervated by histaminergic fibers from the tuberomammillary nucleus of the hypothalamus, causes a slow membrane hyperpolarization toward a reversal potential of -73 mV through a relatively small increase in membrane conductance to Cl(-). This histaminergic action appears to be mediated by the H(2) subclass of histaminergic receptors and inhibits the single-spike activity of these PGN GABAergic neurons. Application of histamine to the PGN could halt the generation of spindle waves, indicating that increased activity in the tuberomammillary histaminergic system may play a functional role in dampening thalamic oscillations in the transition from sleep to arousal.

Slick (Slo2.1), a rapidly-gating sodium-activated potassium channel inhibited by ATP

Neuronal stressors such as hypoxia and firing of action potentials at very high frequencies cause intracellular Na+ to rise and ATP to be consumed faster than it can be regenerated. We report the cloning of a gene encoding a K+ channel, Slick, and demonstrate that functionally it is a hybrid between two classes of K+ channels, Na+-activated (KNa) and ATP-sensitive (KATP) K+ channels. The Slick channel is activated by intracellular Na+ and Cl- and is inhibited by intracellular ATP. Slick is widely expressed in the CNS and is detected in heart. We identify a consensus ATP binding site near the C terminus of the channel that is required for ATP and its nonhydrolyzable analogs to reduce open probability. The convergence of Na+, Cl-, and ATP sensitivity in one channel may endow Slick with the ability to integrate multiple indicators of the metabolic state of a cell and to adjust electrical activity appropriately.