Gap junctions modulate seizures in a mean-field model of general anesthesia for the cortex

During slow-wave sleep, general anesthesia, and generalized seizures, there is an absence of consciousness. These states are characterized by low-frequency large-amplitude traveling waves in scalp electroencephalogram. Therefore the oscillatory state might be an indication of failure to form coherent neuronal assemblies necessary for consciousness. A generalized seizure event is a pathological brain state that is the clearest manifestation of waves of synchronized neuronal activity. Since gap junctions provide a direct electrical connection between adjoining neurons, thus enhancing synchronous behavior, reducing gap-junction conductance should suppress seizures; however there is no clear experimental evidence for this. Here we report theoretical predictions for a physiologically-based cortical model that describes the general anesthetic phase transition from consciousness to coma, and includes both chemical synaptic and direct electrotonic synapses. The model dynamics exhibits both Hopf (temporal) and Turing (spatial) instabilities; the Hopf instability corresponds to the slow (≲8 Hz) oscillatory states similar to those seen in slow-wave sleep, general anesthesia, and seizures. We argue that a delicately balanced interplay between Hopf and Turing modes provides a canonical mechanism for the default non-cognitive rest state of the brain. We show that the Turing mode, set by gap-junction diffusion, is generally protective against entering oscillatory modes; and that weakening the Turing mode by reducing gap conduction can release an uncontrolled Hopf oscillation and hence an increased propensity for seizure and simultaneously an increased sensitivity to GABAergic anesthesia.     

Seizure-induced plasticity of h channels in entorhinal cortical layer III pyramidal neurons

The entorhinal cortex (EC) provides the predominant excitatory drive to the hippocampal CA1 and subicular neurons in chronic epilepsy. Discerning the mechanisms underlying signal integration within EC neurons is essential for understanding network excitability alterations involving the hippocampus during epilepsy. Twenty-four hours following a single seizure episode when there were no behavioral or electrographic seizures, we found enhanced spontaneous activity still present in the rat EC in vivo and in vitro. The increased excitability was accompanied by a profound reduction in I(h) in EC layer III neurons and a significant decline in HCN1 and HCN2 subunits that encode for h channels. Consequently, dendritic excitability was enhanced, resulting in increased neuronal firing despite hyperpolarized membrane potentials. The loss of I(h) and the increased neuronal excitability persisted for 1 week following seizures. Our results suggest that dendritic I(h) plays an important role in determining the excitability of EC layer III neurons and their associated neural networks.     

Neural mass models as a tool to investigate neural dynamics during seizures

Epilepsy is one of the most common neurological disorders and is characterized by recurrent seizures. We use theoretical neuroscience tools to study brain dynamics during seizures. We derive and simulate a computational model of a network of hippocampal neuronal populations. Each population within the network is based on a model that has been shown to replicate the electrophysiological dynamics observed during seizures. The results provide insights into possible mechanisms for seizure spread. We observe that epileptiform activity remains localized to a pathological region when a global connectivity parameter is less than a critical value. After establishing the critical value for seizure spread, we explored how to correct the effect by altering particular synaptic gains. The spreading of seizures is quantified using numerical methods for seizure detection. The results from this study provide a new avenue of exploration for seizure control.     

Neuronal models for sleep-wake regulation and synaptic reorganization in the sleeping hippocampus

In this article, we discuss mathematical models that address the control of sleep-wake behavior in the infant and adult rodent and a model that addresses changes in single-cell firing patterns in the hippocampus across wake and rapid eye movement (REM) sleep states. Each of the models describes the dynamics of experimentally identified neuronal components--either the firing activity of wake-and sleep-promoting neuronal populations or the spiking activity of hippocampal pyramidal neurons. Our discussion of each model illustrates how a mathematical model that describes the temporal dynamics of the modeled neuronal components can reveal specifics about proposed neuronal mechanisms that underlie sleep-wake regulation or sleep-specific firing patterns. For example, the dynamics of the models developed for sleep-wake regulation in the infant rodent lend insight into the involved brain-stem neuronal populations and the evolution of the network during maturation. The results of the model for sleep-wake regulation in the adult rodent suggest distinct properties of the involved neuronal populations and their interactions that account for long-lasting and brief waking bouts. The dynamics of the model for sleep-specific hippocampal neural activity proposes neural mechanisms to account for observed activity changes that can invoke synaptic reorganization associated with learning and memory consolidation.     

Optogenetic Delay of Status Epilepticus Onset in an In Vivo Rodent Epilepsy Model

Epilepsy is a devastating disease, currently treated with medications, surgery or electrical stimulation. None of these approaches is totally effective and our ability to control seizures remains limited and complicated by frequent side effects. The emerging revolutionary technique of optogenetics enables manipulation of the activity of specific neuronal populations in vivo with exquisite spatiotemporal resolution using light. We used optogenetic approaches to test the role of hippocampal excitatory neurons in the lithium-pilocarpine model of acute elicited seizures in awake behaving rats. Hippocampal pyramidal neurons were transduced in vivo with a virus carrying an enhanced halorhodopsin (eNpHR), a yellow light activated chloride pump, and acute seizure progression was then monitored behaviorally and electrophysiologically in the presence and absence of illumination delivered via an optical fiber. Inhibition of those neurons with illumination prior to seizure onset significantly delayed electrographic and behavioral initiation of status epilepticus, and altered the dynamics of ictal activity development. These results reveal an essential role of hippocampal excitatory neurons in this model of ictogenesis and illustrate the power of optogenetic approaches for elucidation of seizure mechanisms. This early success in controlling seizures also suggests future therapeutic avenues.     

Does phenobarbital protect against trimethyltin-induced neuropathology of limbic structures?

Because of the similarity in the pattern of limbic sites damaged by both compounds, it has been suggested that trimethyltin (TMT) may be an excitotoxin like kainic acid (KA). KA produces seizures which eventually result in neuronal damage similar to that found in epilepsy. Anticonvulsants reduce both the seizures and pathology associated with KA. Because TMT may also produce seizures, we undertook to determine whether or not some of the TMT-induced limbic neuropathology could result from seizure activity. To do this, a single dose of TMT chloride (either 7.5 or 15 mg/kg) was given per os to rats, and then phenobarbital (30 mg/kg) was administered subcutaneously in repeated doses. Treatment with phenobarbital did not prevent pathologic changes in the hippocampus, dentate gyrus, and pyriform or prepyriform cortex. Since phenobarbital did not protect against TMT-induced neuronal damage, as it has been reported by others to protect against KA-induced damage, the present findings suggest that these two toxicants probably produce hippocampal pathology via different mechanisms and that the TMT-induced pathologic changes do not require sustained electrical seizure activity.     

Hippocampal neurons and glia in epileptic EL mice

Reactive changes in hippocampal astrocytes are frequently encountered in association with temporal lobe epilepsy in humans and with drug or kindling-induced seizures in animal models. These reactive changes generally involve increases in astrocyte size and number and often occur together with neuronal loss and synaptic rearrangements. In addition to producing astrocytic changes, seizure activity can also produce reactive changes in microglia, the resident macrophages of brain. In this study, we examined the effects of recurrent seizure activity on hippocampal neurons and glia in the epileptic EL mouse, a natural model of human multifactorial idiopathic epilepsy and complex partial seizures. Timm staining was used to evaluate infrapyramidal mossy fiber organization and the optical dissector method was used to count Nissl-stained neurons in hippocampus of adult (about one year of age) EL mice and nonepileptic C57BL/6J (B6) and DDY mice. Immunostaining forglial fibrillary acidic protein (GFAP) and Iba1, an actin cross-linking molecule restricted to macrophages and microglia, was used to evaluate astrocytes and microglia, respectively. The EL mice experienced about 25–30 complex partial seizures with secondary generalization during routine weekly cage changing. No significant differences were found among the mouse strains for Timm staining scores or for neuronal counts in the CA1 and CA3 pyramidal fields or in the hilus. However, the number of GFAP-positive astrocytes was significantly elevated in the stratum radiatum and hilus of EL mice, while microglia appeared hyper-ramified and were more intensely stained in EL mice than in the B6 or DDY mice in the hilus, parietal cortex, and pyriform cortex. The results indicate that recurrent seizure activity in EL mice is associated with abnormalities in hippocampal astrocytes and brain microglia, but is not associated with obvious neuronal loss or mossy fiber synaptic rearrangements. The EL mouse can be a useful model for evaluating neuron-glia interactions related to idiopathic epilepsy.     

Transient Morphological Alterations in the Hippocampus After Pentylenetetrazole-Induced Seizures in Rats

The relationships between seizures, neuronal death, and epilepsy remain one of the most disputed questions in translational neuroscience. Although it is broadly accepted that prolonged and repeated seizures cause neuronal death and epileptogenesis, whether brief seizures can produce a mild but similar effect is controversial. In the present work, using a rat pentylenetetrazole (PTZ) model of seizures, we evaluated how a single episode of clonic–tonic seizures affected the viability of neurons in the hippocampus, the area of the brain most vulnerable to seizures, and morphological changes in the hippocampus up to 1 week after PTZ treatment (recovery period). The main findings of the study were: (1) PTZ-induced seizures caused the transient appearance of massively shrunken, hyperbasophilic, and hyperelectrondense (dark) cells but did not lead to detectable neuronal cell loss. These dark neurons were alive, suggesting that they could cope with seizure-related dysfunction. (2) Neuronal and biochemical alterations following seizures were observed for at least 1 week. The temporal dynamics of the appearance and disappearance of dark neurons differed in different zones of the hippocampus. (3) The numbers of cells with structural and functional abnormalities in the hippocampus after PTZ-induced seizures decreased in the following order: CA1?>?CA3b,c?>?hilus?>?dentate gyrus. Neurons in the CA3a subarea were most resistant to PTZ-induced seizures. These results suggest that even a single seizure episode is a potent stressor of hippocampal neurons and that it can trigger complex neuroplastic changes in the hippocampus.     

Notch Signaling Activation Promotes Seizure Activity in Temporal Lobe Epilepsy

Notch signaling in the nervous system is often regarded as a developmental pathway. However, recent studies have suggested that Notch is associated with neuronal discharges. Here, focusing on temporal lobe epilepsy, we found that Notch signaling was activated in the kainic acid (KA)-induced epilepsy model and in human epileptogenic tissues. Using an acute model of seizures, we showed that DAPT, an inhibitor of Notch, inhibited ictal activity. In contrast, pretreatment with exogenous Jagged1 to elevate Notch signaling before KA application had proconvulsant effects. In vivo, we demonstrated that the impacts of activated Notch signaling on seizures can in part be attributed to the regulatory role of Notch signaling on excitatory synaptic activity in CA1 pyramidal neurons. In vitro, we found that DAPT treatment impaired synaptic vesicle endocytosis in cultured hippocampal neurons. Taken together, our findings suggest a correlation between aberrant Notch signaling and epileptic seizures. Notch signaling is up-regulated in response to seizure activity, and its activation further promotes neuronal excitation of CA1 pyramidal neurons in acute seizures.     

Failure of adaptive self-organized criticality during epileptic seizure attacks

Critical dynamics are assumed to be an attractive mode for normal brain functioning as information processing and computational capabilities are found to be optimal in the critical state. Recent experimental observations of neuronal activity patterns following power-law distributions, a hallmark of systems at a critical state, have led to the hypothesis that human brain dynamics could be poised at a phase transition between ordered and disordered activity. A so far unresolved question concerns the medical significance of critical brain activity and how it relates to pathological conditions. Using data from invasive electroencephalogram recordings from humans we show that during epileptic seizure attacks neuronal activity patterns deviate from the normally observed power-law distribution characterizing critical dynamics. The comparison of these observations to results from a computational model exhibiting self-organized criticality (SOC) based on adaptive networks allows further insights into the underlying dynamics. Together these results suggest that brain dynamics deviates from criticality during seizures caused by the failure of adaptive SOC.     

The Role of Cell Volume in the Dynamics of Seizure,Spreading Depression,and Anoxic Depolarization

Cell volume changes are ubiquitous in normal and pathological activity of the brain. Nevertheless, we know little of how cell volume affects neuronal dynamics. We here performed the first detailed study of the effects of cell volume on neuronal dynamics. By incorporating cell swelling together with dynamic ion concentrations and oxygen supply into Hodgkin-Huxley type spiking dynamics, we demonstrate the spontaneous transition between epileptic seizure and spreading depression states as the cell swells and contracts in response to changes in osmotic pressure. Our use of volume as an order parameter further revealed a dynamical definition for the experimentally described physiological ceiling that separates seizure from spreading depression, as well as predicted a second ceiling that demarcates spreading depression from anoxic depolarization. Our model highlights the neuroprotective role of glial K buffering against seizures and spreading depression, and provides novel insights into anoxic depolarization and the relevant cell swelling during ischemia. We argue that the dynamics of seizures, spreading depression, and anoxic depolarization lie along a continuum of the repertoire of the neuron membrane that can be understood only when the dynamic ion concentrations, oxygen homeostasis,and cell swelling in response to osmotic pressure are taken into consideration. Our results demonstrate the feasibility of a unified framework for a wide range of neuronal behaviors that may be of substantial importance in the understanding of and potentially developing universal intervention strategies for these pathological states.     

Canonical Transient Receptor Potential Channel 3 Contributes to Febrile Seizure Inducing Neuronal Cell Death and Neuroinflammation

Febrile seizure (FS) counts as the most common seizures symptom in children undergoing recurrent seizures, posing a high risk to developing subsequent temporal lobe epilepsy. Canonical transient receptor potential channel (TRPC) members are identified as the FS-related genes in hyperthermia prone rats. However, the role of TRPC3 in hyperthermia-induced FS rats remains unclear. In the present study, we investigated whether TRPC3 functionally contributes to the development of FSs. Elevated TRPC3 mRNA and protein levels was detected in hyperthermia-induced FS rats and rat hippocampal neuron cells. The specific inhibitor of TRPC3, Pyr3, remarkably attenuated the susceptibility and severity of seizures, neuronal cell death, and neuroinflammation in FS rats. Conversely, NCX3 activation was apparently suppressed in rats subjected to recurrent FS and rat hippocampal neuron cells. The expression of NCX3 was up-regulated after TRPC3 inhibition in vivo and in vitro. Furthermore, an interaction between TRPC3 and NCX3 was detected by co-immunoprecipitation. Inhibition of TRPC3 suppressed intracellular Ca²⁺ levels in hyperthermia-treated hippocampal neuronal cells. In conclusion, our findings supported that TRPC3 functions as a critical regulator of seizure susceptibility and targeting TRPC3 may be a new therapeutic strategy for FS.     

Roles of prostaglandin synthesis in excitotoxic brain diseases

Cyclooxygenase (COX) is a rate-limiting enzyme in prostaglandin synthesis. COX consists of two isoforms, constitutive COX-1 and inducible COX-2. We have first found that COX-2 expression in the brain is tightly regulated by neuronal activity under physiological conditions, and electroconvulsive seizure robustly induces COX-2 mRNA in the brain. Our recent in-depth studies reveal COX-2 expression is divided into two phases, early in neurons and late in non-neuronal cells, such as endothelial cells or astrocytes. In this review, we present that early synthesized COX-2 facilitates the recurrence of hippocampal seizures in rapid kindling model, and late induced COX-2 stimulates hippocampal neuron loss after kainic acid treatment. Hence, we consider the potential role of COX-2 inhibitors as a new therapeutic drug for a neuronal loss after seizure or focal cerebral ischemia. The short-term and sub-acute medication of selective COX-2 inhibitors that suppresses an elevation of prostaglandin E(2) (PGE(2)) may be an effective treatment to prevent neuronal loss after onset of neuronal excitatory diseases. This review also discusses a novel role of vascular endothelial cells in brain diseases. We found that these cells produce PGE(2) by synthesizing COX-2 and microsomal prostaglandin E synthase-1 (mPGES-1) in response to excitotoxicity and neuroinflammation. We also show a possible mechanisms of neuronal damage associated with seizure via astrocytes and endothelial cells. Further analysis of the interaction among neurons, astrocytes and endothelial cells may provide a better understanding of the processes of neuropathological disorders, as well as facilitating the development of new treatments.     

Remote excitation of neuronal circuits using low-intensity, low-frequency ultrasound

Possessing the ability to noninvasively elicit brain circuit activity yields immense experimental and therapeutic power. Most currently employed neurostimulation methods rely on the somewhat invasive use of stimulating electrodes or photon-emitting devices. Due to its ability to noninvasively propagate through bone and other tissues in a focused manner, the implementation of ultrasound (US) represents a compelling alternative approach to current neuromodulation strategies. Here, we investigated the influence of low-intensity, low-frequency ultrasound (LILFU) on neuronal activity. By transmitting US waveforms through hippocampal slice cultures and ex vivo mouse brains, we determined LILFU is capable of remotely and noninvasively exciting neurons and network activity. Our results illustrate that LILFU can stimulate electrical activity in neurons by activating voltage-gated sodium channels, as well as voltage-gated calcium channels. The LILFU-induced changes in neuronal activity were sufficient to trigger SNARE-mediated exocytosis and synaptic transmission in hippocampal circuits. Because LILFU can stimulate electrical activity and calcium signaling in neurons as well as central synaptic transmission we conclude US provides a powerful tool for remotely modulating brain circuit activity.     

Representing episodes in the mammalian brain

Memory lets the past inform the present so that we can attain future goals. In many species, these abilities require the hippocampus. Recent experiments, in which memory demand was varied while overt behavior and the environment were kept constant, have revealed firing patterns of hippocampal neurons that corresponded with memory demands and predicted performance. Although the active population appeared to be 'place cells' that signalled location, it actually included cells the activity patterns of which distinguished the recent or pending history of behavior during identical actions that occurred in the same place. Different populations of hippocampal cells fired as a rat walked along the same spatial path on the way to different goals, and coded past, present and pending events. Other experiments provide converging data that neuronal activity is modulated by goal-directed behavioral episodes. Together, these firing patterns suggest a testable mechanism of episodic memory coding: that hippocampal dynamics encode a temporally extended, hierarchically organized representation of goal-directed behavior.     

BDNF-induced TrkB activation down-regulates the K+-Cl- cotransporter KCC2 and impairs neuronal Cl- extrusion

Pathophysiological activity and various kinds of traumatic insults are known to have deleterious long-term effects on neuronal Cl- regulation, which can lead to a suppression of fast postsynaptic GABAergic responses. Brain-derived neurotrophic factor (BDNF) increases neuronal excitability through a conjunction of mechanisms that include regulation of the efficacy of GABAergic transmission. Here, we show that exposure of rat hippocampal slice cultures and acute slices to exogenous BDNF or neurotrophin-4 produces a TrkB-mediated fall in the neuron-specific K+-Cl- cotransporter KCC2 mRNA and protein, as well as a consequent impairment in neuronal Cl- extrusion capacity. After kindling-induced seizures in vivo, the expression of KCC2 is down-regulated in the mouse hippocampus with a spatiotemporal profile complementary to the up-regulation of TrkB and BDNF. The present data demonstrate a novel mechanism whereby BDNF/TrkB signaling suppresses chloride-dependent fast GABAergic inhibition, which most likely contributes to the well-known role of TrkB-activated signaling cascades in the induction and establishment of epileptic activity.     

Abnormal interictal gamma activity may manifest a seizure onset zone in temporal lobe epilepsy

Even though recent studies have suggested that seizures do not occur suddenly and that before a seizure there is a period with an increased probability of seizure occurrence, neurophysiological mechanisms of interictal and pre-seizure states are unknown. The ability of mathematical methods to provide much more sensitive tools for the detection of subtle changes in the electrical activity of the brain gives promise that electrophysiological markers of enhanced seizure susceptibility can be found even during interictal periods when EEG of epilepsy patients often looks 'normal'. Previously, we demonstrated in animals that hippocampal and neocortical gamma-band rhythms (30-100 Hz) intensify long before seizures caused by systemic infusion of kainic acid. Other studies in recent years have also drawn attention to the fast activity (>30 Hz) as a possible marker of epileptogenic tissue. The current study quantified gamma-band activity during interictal periods and seizures in intracranial EEG (iEEG) in 5 patients implanted with subdural grids/intracranial electrodes during their pre-surgical evaluation. In all our patients, we found distinctive (abnormal) bursts of gamma activity with a 3 to 100 fold increase in power at gamma frequencies with respect to selected by clinicians, quiescent, artifact-free, 7-20 min "normal" background (interictal) iEEG epochs 1 to 14 hours prior to seizures. Increases in gamma activity were largest in those channels which later displayed the most intensive electrographic seizure discharges. Moreover, location of gamma-band bursts correlated (with high specificity, 96.4% and sensitivity, 83.8%) with seizure onset zone (SOZ) determined by clinicians. Spatial localization of interictal gamma rhythms within SOZ suggests that the persistent presence of abnormally intensified gamma rhythms in the EEG may be an important tool for focus localization and possibly a determinant of epileptogenesis.     

Modulation of neuronal excitability by intracellular calcium buffering: from spiking to bursting

We have investigated the detailed regulation of neuronal firing pattern by the cytosolic calcium buffering capacity using a combination of mathematical modeling and patch-clamp recording in acute slice. Theoretical results show that a high calcium buffer concentration alters the characteristic regular firing of cerebellar granule cells and that a transition to various modes of oscillations occurs, including bursting. Using bifurcation analysis, we show that this transition from spiking to bursting is a consequence of the major slowdown of calcium dynamics. Patch-clamp recordings on cerebellar granule cells loaded with a high concentration of the fast calcium buffer BAPTA (15 mM) reveal dramatic alterations in their excitability as compared to cells loaded with 0.15 mM BAPTA. In high calcium buffering conditions, granule cells exhibit all bursting behaviors predicted by the model whereas bursting is never observed in low buffering. These results suggest that cytosolic calcium buffering capacity can tightly modulate neuronal firing patterns leading to generation of complex patterns and therefore that calcium-binding proteins may play a critical role in the non-synaptic plasticity and information processing in the central nervous system.     

Microarray Noninvasive Neuronal Seizure Recordings from Intact Larval Zebrafish

Zebrafish epilepsy models are emerging tools in experimental epilepsy. Zebrafish larvae, in particular, are advantageous because they can be easily genetically altered and used for developmental and drug studies since agents applied to the bath penetrate the organism easily. Methods for electrophysiological recordings in zebrafish are new and evolving. We present a novel multi-electrode array method to non-invasively record electrical activity from up to 61 locations of an intact larval zebrafish head. This method enables transcranial noninvasive recording of extracellular field potentials (which include multi-unit activity and EEG) to identify epileptic seizures. To record from the brains of zebrafish larvae, the dorsum of the head of an intact larva was secured onto a multi-electrode array. We recorded from individual electrodes for at least three hours and quantified neuronal firing frequency, spike patterns (continuous or bursting), and synchrony of neuronal firing. Following 15 mM potassium chloride- or pentylenetetrazole-infusion into the bath, spike and burst rate increased significantly. Additionally, synchrony of neuronal firing across channels, a hallmark of epileptic seizures, also increased. Notably, the fish survived the experiment. This non-invasive method complements present invasive zebrafish neurophysiological techniques: it affords the advantages of high spatial and temporal resolution, a capacity to measure multiregional activity and neuronal synchrony in seizures, and fish survival for future experiments, such as studies of epileptogenesis and development.     

Hub GABA neurons mediate gamma-frequency oscillations at ictal-like event onset in the immature hippocampus

Gamma-frequency oscillations (GFOs, >40?Hz) are a general network signature at seizure onset at all stages of development, with possible deleterious consequences in the immature brain. At early developmental stages, the simultaneous occurrence of GFOs in different brain regions suggests the existence of a long-ranging synchronizing mechanism at seizure onset. Here, we show that hippocamposeptal (HS) neurons, which are GABA long-range projection neurons, are mandatory to drive the firing of hippocampal interneurons in a high-frequency regime at the onset of epileptiform discharges in the intact, immature septohippocampal formation. The synchronized firing of interneurons in turn produces GFOs, which are abolished after the elimination of a small number of HS neurons. Because they provide the necessary fast conduit for pacing large neuronal populations and display intra- and extrahippocampal long-range projections, HS neurons appear to belong to the class of hub cells that play a crucial role in the synchronization of developing networks.