Cortical Tonic Inhibition Gates the Expression of Spike-and-Wave Discharges Associated with Absence Epilepsy.

OBJECTIVE: Absence seizures result from aberrant thalamocortical processing that confers synchronous, bilateral spike-and-wave discharges (SWDs) and behavioral arrest. Previous work has demonstrated that SWDs can result from enhanced thalamic tonic inhibition, consistent with the mechanism of first-line antiabsence drugs that target thalamic low-voltage-activated calcium channels. However, nearly half of patients with absence epilepsy are unresponsive to first-line medications. In this study we evaluated the role of cortical tonic inhibition and its manipulation on absence seizure expression. METHODS: We used video-electroencephalogram (EEG) monitoring to show that mice with a γ-aminobutyric acid type A (GABAA) receptor mutation (γ2R43Q) display absence seizures. Voltage-clamp recordings in brain slices from wild type and γ2R43Q mice were used to evaluate the amount of tonic inhibition and its selective pharmacological modulation. Finally, we determined whether modulating tonic inhibition controls seizure expression. RESULTS: γ2R43Q mice completely lack tonic inhibition in principal neurons of both layer 2/3 cortex and ventrobasal thalamus. Blocking cortical tonic inhibition in wild type mice is sufficient to elicit SWDs. Tonic inhibition in slices from γ2R43Q mice could be rescued in a dose-dependent fashion by the synthetic neurosteroid ganaxolone. Low-dose ganaxolone suppressed seizures in γ2R43Q mice. CONCLUSIONS: Our data suggest that reduced cortical tonic inhibition promotes absence seizures and that normal function can be restored via selective pharmacological rescue. These results, together with previous findings, suggest that deviations of tonic inhibition either above or below an optimal set point can contribute to absence epilepsy. Returning the thalamocortical system to this set point may provide a novel treatment for refractory absence epilepsy.

Higher-order thalamic nuclei facilitate the generalization and maintenance of spike-and-wave discharges of absence seizures.

Spike-and-wave discharges (SWDs), generated by the cortico-thalamo-cortical (CTC) network, are pathological, large amplitude oscillations and the hallmark of absence seizures (ASs). SWDs begin in a cortical initiation network in both humans and animal models, including the Genetic Absence Epilepsy Rats from Strasbourg (GAERS), where it is located in the primary somatosensory cortex (S1). The behavioral manifestation of an AS occurs when SWDs spread from the cortical initiation site to the whole brain, however, the mechanisms behind this rapid propagation remain unclear. Here we investigated these processes beyond the principal CTC network, in higher-order (HO) thalamic nuclei (lateral posterior (LP) and posterior (PO) nuclei) since their diffuse connectivity and known facilitation of intracortical communications make these nuclei key candidates to support SWD generation and maintenance. In freely moving GAERS, multi-site LFP in LP, PO and multiple cortical regions revealed a novel feature of SWDs: during SWDs there are short periods (named SWD-breaks) when cortical regions far from S1, such the primary visual cortex (V1), become transiently unsynchronized from the ongoing EEG rhythm. Inactivation of HO nuclei with local muscimol injections or optogenetic perturbation of HO nuclei activity increased the occurrence of SWD-breaks and the former intervention also increased the SWD propagation-time from S1. The neural underpinnings of these findings were explored further by silicon probe recordings from single units of PO which uncovered two previously unknown groups of excitatory neurons based on their burst firing dynamics at SWD onset. Moreover, a switch from tonic to burst firing at SWD onset was shown to be an important feature since it was much less prominent for non-generalized events, i.e. SWDs that remained local to S1. Additionally, one group of neurons showed a reverse of this switch during SWD-breaks, demonstrating the importance of this firing pattern throughout the SWD. In summary, these results support the view that multiple HO thalamic nuclei are utilized at SWD onset and contribute to cortical synchrony throughout the paroxysmal discharge.

Electroencephalographic investigation of the effects of Ginkgo biloba on spike-wave discharges in rats with genetic absence epilepsy.

OBJECTIVE: EGb 761, a plant extract obtained from the leaves of the Ginkgo biloba tree, is widely used in modern medicine and traditional medicine applications in the treatment of many diseases. However, in some clinical case reports, it has been suggested that G. biloba causes epileptic seizures. A limited number of experimental animal studies related to the effects of G. biloba on epileptic seizures do not provide sufficient information on the solution of a serious clinical problem with contrasting findings. We aimed to investigate the effects of EGb 761 administered in different doses to adult male Wistar Albino Glaxo/Rijswijk (WAG/Rij) rats which is the genetic animal model of absence epilepsy, on absence seizures using in vivo electrophysiological method. In addition, the effects of EGb 761 doses on locomotor behavior of WAG/Rij rats were evaluated with open-field and rotarod behavioral tests. METHODS: 50, 100, 200, and 400 mg/kg doses of EGb 761 were administered to male WAG/Rij rats with implanted EEG electrodes by oral gavage for 28 days. Evaluation of absence seizures was performed on spike-wave discharges (SWDs) in EEG recorded for 4 h each week. The number of SWDs, the total duration of SWDs, and the mean duration of SWD were determined for the analysis. RESULTS: In the group treated with 400 mg/kg EGb 761, the number of SWDs and the mean duration of SWD at the 1st and 7th doses and the total duration of SWDs at the 1st, 7th and 14th doses were significantly increased (p < 0.05). In all experimental groups treated with EGb 761 doses, there was no significant change in locomotor activity in the open-field and the rotarod tests. CONCLUSION: Ginkgo biloba extract EGb 761 increased the epileptic SWD parameters of WAG/Rij rats at high doses (400 mg/kg), causing a pro-epileptic effect on absence seizures. It should be noted that in patients with epilepsy and in high-dose applications, G. biloba extract EGb 761 may lead to an increase in neuronal excitability.

Optogenetic activation of the reticular nucleus of the thalamus attenuates limbic seizures via inhibition of the midline thalamus.

OBJECTIVE: The nucleus reticularis of the thalamus (nRT) is most studied in epilepsy for its role in the genesis of absence seizures; much less is known regarding its role in other seizure types, including those originating in limbic structures and the temporal lobe. As it is a major source of inhibitory input to higher order thalamic nuclei, stimulation of the nRT may be an effective strategy to disrupt seizure activity that requires thalamic engagement. METHODS: We recorded single unit activity from the nRT prior to and after infusion of bicuculline into the area tempestas. We monitored single unit activity time-locked with interictal spikes. We optogenetically activated the nRT in both the area tempestas and amygdala kindling models. We tested a role for projections from the nRT to higher order midline thalamic nuclei through the use of retrogradely trafficked viral vector. RESULTS: Mean firing rate in the nRT was decreased after infusion of bicuculline into the area tempestas as compared to the preinfusion baseline. nRT unit firing in response to interictal spikes was heterogeneous, with an approximately equal proportion of neurons displaying (1) no change in firing, (2) increased firing, and (3) decreasing firing. Optogenetic activation of the nRT significantly suppressed seizure activity in both the area tempestas and amygdala kindling models. Optogenetic activation of contralaterally targeting projections but not ipsilaterally targeting projections from the nRT to the midline thalamus significantly suppressed seizures in the kindling model. SIGNIFICANCE: Although the nRT is typically thought of in the context of absence seizures, our data show that it may be a viable target for other seizure types. In two models that recapitulate the seizure types seen in temporal lobe epilepsy, nRT activation suppressed both electrographic and behavioral seizures. These data suggest that the nRT should be considered more broadly in the context of epilepsy.

Single-pulse stimulation of cerebellar nuclei stops epileptic thalamic activity.

BACKGROUND: Epileptic (absence) seizures in the cerebral cortex can be stopped by pharmacological and optogenetic stimulation of the cerebellar nuclei (CN) neurons that innervate the thalamus. However, it is unclear how such stimulation can modify underlying thalamo-cortical oscillations. HYPOTHESIS: Here we tested whether rhythmic synchronized thalamo-cortical activity during absence seizures can be desynchronized by single-pulse optogenetic stimulation of CN neurons to stop seizure activity. METHODS: We performed simultaneous thalamic single-cell and electrocorticographical recordings in awake tottering mice, a genetic model of absence epilepsy, to investigate the rhythmicity and synchronicity. Furthermore, we tested interictally the impact of single-pulse optogenetic CN stimulation on thalamic and cortical recordings. RESULTS: We show that thalamic firing is highly rhythmic and synchronized with cortical spike-and-wave discharges during absence seizures and that this phase-locked activity can be desynchronized upon single-pulse optogenetic stimulation of CN neurons. Notably, this stimulation of CN neurons was more effective in stopping seizures than direct, focal stimulation of groups of afferents innervating the thalamus. During interictal periods, CN stimulation evoked reliable but heterogeneous responses in thalamic cells in that they could show an increase or decrease in firing rate at various latencies, bi-phasic responses with an initial excitatory and subsequent inhibitory response, or no response at all. CONCLUSION: Our data indicate that stimulation of CN neurons and their fibers in thalamus evokes differential effects in its downstream pathways and desynchronizes phase-locked thalamic neuronal firing during seizures, revealing a neurobiological mechanism that may explain how cerebellar stimulation can stop seizures.

Adult loss of Cacna1a in mice recapitulates childhood absence epilepsy by distinct thalamic bursting mechanisms.

Inborn errors of CACNA1A-encoded P/Q-type calcium channels impair synaptic transmission, producing early and lifelong neurological deficits, including childhood absence epilepsy, ataxia and dystonia. Whether these impairments owe their pathologies to defective channel function during the critical period for thalamic network stabilization in immature brain remains unclear. Here we show that mice with tamoxifen-induced adult-onset ablation of P/Q channel alpha subunit (iKOp/q) display identical patterns of dysfunction, replicating the inborn loss-of-function phenotypes and, therefore demonstrate that these neurological defects do not rely upon developmental abnormality. Unexpectedly, unlike the inborn model, the adult-onset pattern of excitability changes believed to be pathogenic within the thalamic network is non-canonical. Specifically, adult ablation of P/Q channels does not promote Cacna1g-mediated burst firing or T-type calcium current (IT) in the thalamocortical relay neurons; however, burst firing in thalamocortical relay neurons remains essential as iKOp/q mice generated on a Cacna1g deleted background show substantially diminished seizure generation. Moreover, in thalamic reticular nucleus neurons, burst firing is impaired accompanied by attenuated IT. Interestingly, inborn deletion of thalamic reticular nucleus-enriched, human childhood absence epilepsy-linked gene Cacna1h in iKOp/q mice reduces thalamic reticular nucleus burst firing and promotes rather than reduces seizure, indicating an epileptogenic role for loss-of-function Cacna1h gene variants reported in human childhood absence epilepsy cases. Together, our results demonstrate that P/Q channels remain critical for maintaining normal thalamocortical oscillations and motor control in the adult brain, and suggest that the developmental plasticity of membrane currents regulating pathological rhythmicity is both degenerate and age-dependent.

Exogenous Ketones Lower Blood Glucose Level in Rested and Exercised Rodent Models.

Diseases involving inflammation and oxidative stress can be exacerbated by high blood glucose levels. Due to tight metabolic regulation, safely reducing blood glucose can prove difficult. The ketogenic diet (KD) reduces absolute glucose and insulin, while increasing fatty acid oxidation, ketogenesis, and circulating levels of ß-hydroxybutyrate (ßHB), acetoacetate (AcAc), and acetone. Compliance to KD can be difficult, so alternative therapies that help reduce glucose levels are needed. Exogenous ketones provide an alternative method to elevate blood ketone levels without strict dietary requirements. In this study, we tested the changes in blood glucose and ketone (ßHB) levels in response to acute, sub-chronic, and chronic administration of various ketogenic compounds in either a post-exercise or rested state. WAG/Rij (WR) rats, a rodent model of human absence epilepsy, GLUT1 deficiency syndrome mice (GLUT1D), and wild type Sprague Dawley rats (SPD) were assessed. Non-pathological animals were also assessed across different age ranges. Experimental groups included KD, standard diet (SD) supplemented with water (Control, C) or with exogenous ketones: 1, 3-butanediol (BD), ßHB mineral salt (KS), KS with medium chain triglyceride/MCT (KSMCT), BD acetoacetate diester (KE), KE with MCT (KEMCT), and KE with KS (KEKS). In rested WR rats, the KE, KS, KSMCT groups had lower blood glucose level after 1 h of treatment, and in KE and KSMCT groups after 24 h. After exercise, the KE, KSMCT, KEKS, and KEMCT groups had lowered glucose levels after 1 h, and in the KEKS and KEMCT groups after 7 days, compared to control. In GLUT1D mice without exercise, only KE resulted in significantly lower glucose levels at week 2 and week 6 during a 10 weeks long chronic feeding study. In 4-month and 1-year-old SPD rats in the post-exercise trials, blood glucose was significantly lower in KD and KE, and in KEMCT groups, respectively. After seven days, the KSMCT group had the most significantly reduced blood glucose levels, compared to control. These results indicate that exogenous ketones were efficacious in reducing blood glucose levels within and outside the context of exercise in various rodent models of different ages, with and without pathology.

Impaired cortico-striatal excitatory transmission triggers epilepsy.

STXBP1 and SCN2A gene mutations are observed in patients with epilepsies, although the circuit basis remains elusive. Here, we show that mice with haplodeficiency for these genes exhibit absence seizures with spike-and-wave discharges (SWDs) initiated by reduced cortical excitatory transmission into the striatum. Mice deficient for Stxbp1 or Scn2a in cortico-striatal but not cortico-thalamic neurons reproduce SWDs. In Stxbp1 haplodeficient mice, there is a reduction in excitatory transmission from the neocortex to striatal fast-spiking interneurons (FSIs). FSI activity transiently decreases at SWD onset, and pharmacological potentiation of AMPA receptors in the striatum but not in the thalamus suppresses SWDs. Furthermore, in wild-type mice, pharmacological inhibition of cortico-striatal FSI excitatory transmission triggers absence and convulsive seizures in a dose-dependent manner. These findings suggest that impaired cortico-striatal excitatory transmission is a plausible mechanism that triggers epilepsy in Stxbp1 and Scn2a haplodeficient mice.

CaV 3.2 drives sustained burst-firing, which is critical for absence seizure propagation in reticular thalamic neurons.

OBJECTIVE: Genetic alterations have been identified in the CACNA1H gene, encoding the CaV 3.2 T-type calcium channel in patients with absence epilepsy, yet the precise mechanisms relating to seizure propagation and spike-wave-discharge (SWD) pacemaking remain unknown. Neurons of the thalamic reticular nucleus (TRN) express high levels of CaV 3.2 calcium channels, and we investigated whether a gain-of-function mutation in the Cacna1h gene in Genetic Absence Epilepsy Rats from Strasbourg (GAERS) contributes to seizure propagation and pacemaking in the TRN. METHODS: Pathophysiological contributions of CaV 3.2 calcium channels to burst firing and absence seizures were assessed in vitro using acute brain slice electrophysiology and quantitative real-time polymerase chain reaction (PCR) and in vivo using free-moving electrocorticography recordings. RESULTS: TRN neurons from GAERS display sustained oscillatory burst-firing that is both age- and frequency-dependent, occurring only in the frequencies overlapping with GAERS SWDs and correlating with the expression of a CaV 3.2 mutation-sensitive splice variant. In vivo knock-down of CaV 3.2 using direct thalamic injection of lipid nanoparticles containing CaV 3.2 dicer small interfering (Dsi) RNA normalized TRN burst-firing, and in free-moving GAERS significantly shortened seizures. SIGNIFICANCE: This supports a role for TRN CaV 3.2 T-type channels in propagating thalamocortical network seizures and setting the pacemaking frequency of SWDs.

Modeling pathogenesis and treatment response in childhood absence epilepsy.

OBJECTIVE: Childhood absence epilepsy (CAE) is a genetic generalized epilepsy syndrome with polygenic inheritance, with genes for γ-aminobutyric acid (GABA) receptors and T-type calcium channels implicated in the disorder. Previous studies of T-type calcium channel electrophysiology have shown genetic changes and medications have multiple effects. The aim of this study was to use an established thalamocortical computer model to determine how T-type calcium channels work in concert with cortical excitability to contribute to pathogenesis and treatment response in CAE. METHODS: The model is comprised of cortical pyramidal, cortical inhibitory, thalamocortical relay, and thalamic reticular single-compartment neurons, implemented with Hodgkin-Huxley model ion channels and connected by AMPA, GABAA , and GABAB synapses. Network behavior was simulated for different combinations of T-type calcium channel conductance, inactivation time, steady state activation/inactivation shift, and cortical GABAA conductance. RESULTS: Decreasing cortical GABAA conductance and increasing T-type calcium channel conductance converted spindle to spike and wave oscillations; smaller changes were required if both were changed in concert. In contrast, left shift of steady state voltage activation/inactivation did not lead to spike and wave oscillations, whereas right shift reduced network propensity for oscillations of any type. SIGNIFICANCE: These results provide a window into mechanisms underlying polygenic inheritance in CAE, as well as a mechanism for treatment effects and failures mediated by these channels. Although the model is a simplification of the human thalamocortical network, it serves as a useful starting point for predicting the implications of ion channel electrophysiology in polygenic epilepsy such as CAE.

Postnatal expression of thalamic GABAA receptor subunits in the stargazer mouse model of absence epilepsy.

Absence seizures are known to originate from disruptions within the corticothalamocortical network; however, the precise underlying cellular and molecular mechanisms that induce hypersynchronicity and hyperexcitability are debated and likely to be complex and multifactorial. Recent studies implicate impaired thalamic GABAergic inhibition as a common feature in multiple animal models of absence epilepsy, including the well-established stargazer mouse model. Recently, we demonstrated region-specific increases in the whole tissue and synaptic levels of GABAA receptor (GABAAR) subunits α1 and ß2, within the ventral posterior region of the thalamus in adult epileptic stargazer mice compared with nonepileptic control littermates. The objective of this study was to investigate whether such changes in GABAAR subunits α1 and ß2 can be observed before the initiation of seizures, which occur around postnatal (PN) days 16-18 in stargazers. Semiquantitative western blotting was used to analyze the relative tissue level expression of GABAAR α1 and ß2 subunits in the thalamus of juvenile stargazer mice compared with their nonepileptic control littermates at three different time points before the initiation of seizures. We show that there is a statistically significant increase in the expression of α1 and ß2 subunits in the thalamus of stargazer mice, at the PN7-9 stage, compared with the control littermates, but not at PN10-12 and PN13-15 stages. These results suggest that an aberrant expression of GABAAR subunits α1 and ß2 in the stargazers does not occur immediately before seizure onset and therefore is unlikely to directly contribute to the initiation of absence seizures.

Regulation of Thalamic and Cortical Network Synchrony by Scn8a.

Voltage-gated sodium channel (VGSC) mutations cause severe epilepsies marked by intermittent, pathological hypersynchronous brain states. Here we present two mechanisms that help to explain how mutations in one VGSC gene, Scn8a, contribute to two distinct seizure phenotypes: (1) hypoexcitation of cortical circuits leading to convulsive seizure resistance, and (2) hyperexcitation of thalamocortical circuits leading to non-convulsive absence epilepsy. We found that loss of Scn8a leads to altered RT cell intrinsic excitability and a failure in recurrent RT synaptic inhibition. We propose that these deficits cooperate to enhance thalamocortical network synchrony and generate pathological oscillations. To our knowledge, this finding is the first clear demonstration of a pathological state tied to disruption of the RT-RT synapse. Our observation that loss of a single gene in the thalamus of an adult wild-type animal is sufficient to cause spike-wave discharges is striking and represents an example of absence epilepsy of thalamic origin.

Developmental changes in Notch1 and NLE1 expression in a genetic model of absence epilepsy.

Childhood absence epilepsy (CAE) is an epilepsy syndrome with seizures occurring in the early childhood, highlighting that seizures susceptibility in CAE is dependent on brain development. The Notch 1 signalling pathway is important in brain development, yet the role of the Notch1 signalling pathway in CAE remains elusive. We here explored Notch1 and its modulator notchless homologue 1 (NLE1) expression in WAG/Rij and control rats using immunohistochemistry. Functional Notch 1 effects were assessed in WAG/Rij rats in vivo. WAG/Rij rats lack the developmental increase in cortical Notch1 and NLE 1 mRNA expression seen in controls, and Notch 1 and NLE1 mRNA and protein expression were lower in somatosensory cortices of WAG/Rij rats when compared to controls. This coincided with an overall decreased cortical GFAP expression in the early development in WAG/Rij rats. These effects were region-specific as they were not observed in thalamic tissues. Neuron-to-glia ratio as a marker of the impact of Notch signalling on differentiation was higher in layer 4 of somatosensory cortex of WAG/Rij rats. Acute application of Notch 1 agonist Jagged 1 suppressed, whereas DAPT, a Notch antagonist, facilitated spike and wave discharges (SWDs) in WAG/Rij rats. These findings point to Notch1 as an important signalling pathway in CAE which likely shapes architectural organization of the somatosensory cortex, a region critically involved in developmental epileptogenesis in CAE. More immediate effects of Notch 1 signalling are seen on in vivo SWDs in CAE, pointing to the Notch 1 pathway as a possible treatment target in CAE.

Timing of high-frequency cortical stimulation in a genetic absence model.

INTRODUCTION: Seizure control is one of the ultimate aims of epileptology: here acute and prolonged effects of closed loop high-frequency stimulation of the somatosensory cortex on the expression of spontaneously occurring spike-wave discharges (SWD) were investigated in a genetic absence model. Effects of closed loop stimulation in the experimental group were compared with a yoked control group allowing to investigate the effect of timing related to SWD occurrence, while controlling for amount and intensity of stimulation. METHODS: WAG/Rij rats were implanted with stimulation electrodes in the deep layers of the somatosensory cortex, and recording electrodes in the cortex and thalamus. Closed-loop and yoked stimulation (1 sec trains, biphasic 0.4 msec pulses, 130 Hz) sessions lasted 24h. The stimulation sessions were preceded and followed by baseline and post stimulation 24-h recordings. RESULTS: Closed-loop stimulation interrupted SWD and duration of SWD was shortened. Both types of stimulation resulted in a reduction in SWD number during stimulation sessions. Closed-loop stimulation also resulted in less SWD during the last eight hours of the post-stimulation recording session. Sometimes yoked stimulation induced low-frequency afterdischarges. DISCUSSION: SWD can be aborted by closed-loop stimulation of the somatosensory cortex, and at the same time the number of SWD was reduced. It can be regarded as a relatively safe neuromodulatory technique without habituation. The reduction of SWD during yoked stimulation session might be caused by 3 Hz afterdischarges. The reduction of SWD on the stimulation and post-stimulation sessions demonstrates the critical relevance of timing for the induction of longer lasting neuromodulatory effects: it suggests that absence seizures themselves might be involved in their reoccurrence.

Isolated P/Q Calcium Channel Deletion in Layer VI Corticothalamic Neurons Generates Absence Epilepsy.

Generalized spike-wave seizures involving abnormal synchronization of cortical and underlying thalamic circuitry represent a major category of childhood epilepsy. Inborn errors of Cacna1a, the P/Q-type voltage-gated calcium channel α subunit gene, expressed throughout the brain destabilize corticothalamic rhythmicity and produce this phenotype. To determine the minimal cellular lesion required for this network disturbance, we used neurotensin receptor 1 (Ntsr1) cre-driver mice to ablate floxed Cacna1a in layer VI pyramidal neurons, which supply the sole descending cortical synaptic input to thalamocortical relay cells and reticular interneurons and activate intrathalamic circuits. Targeted Cacna1a ablation in layer VI cells resulted in mice that display a robust spontaneous spike-wave absence seizure phenotype accompanied by behavioral arrest and inhibited by ethosuximide. To verify the selectivity of the molecular lesion, we determined that P/Q subunit proteins were reduced in corticothalamic relay neuron terminal zones, and confirmed that P/Q-mediated glutamate release was reduced at these synapses. Spike-triggered exocytosis was preserved by N-type calcium channel rescue, demonstrating that evoked release at layer VI terminals relies on both P/Q and N-type channels. Whereas intrinsic excitability of the P/Q channel depleted layer VI neurons was unaltered, T-type calcium currents in the postsynaptic thalamic relay and reticular cells were dramatically elevated, favoring rebound bursting and seizure generation. We find that an early P/Q-type release defect, limited to synapses of a single cell-type within the thalamocortical circuit, is sufficient to remodel synchronized firing behavior and produce a stable generalized epilepsy phenotype. SIGNIFICANCE STATEMENT: This study dissects a critical component of the corticothalamic circuit in spike-wave epilepsy and identifies the developmental importance of P/Q-type calcium channel-mediated presynaptic glutamate release at layer VI pyramidal neuron terminals. Genetic ablation of Cacna1a in layer VI neurons produced synchronous spike-wave discharges in the cortex and thalamus that were inhibited by ethosuximide. These mice also displayed N-type calcium channel compensation at descending thalamic synapses, and consistent with other spike-wave models increased low-threshold T-type calcium currents within postsynaptic thalamic relay and reticular neurons. These results demonstrate, for the first time, that preventing the developmental homeostatic switch from loose to tightly coupled synaptic release at a single class of deep layer cortical excitatory output neurons results in generalized spike-wave epilepsy.

Reduction of thalamic and cortical Ih by deletion of TRIP8b produces a mouse model of human absence epilepsy.

Absence seizures occur in several types of human epilepsy and result from widespread, synchronous feedback between the cortex and thalamus that produces brief episodes of loss of consciousness. Genetic rodent models have been invaluable for investigating the pathophysiological basis of these seizures. Here, we identify tetratricopeptide-containing Rab8b-interacting protein (TRIP8b) knockout mice as a new model of absence epilepsy, featuring spontaneous spike-wave discharges on electroencephalography (EEG) that are the electrographic hallmark of absence seizures. TRIP8b is an auxiliary subunit of the hyperpolarization-activated cyclic-nucleotide-gated (HCN) channels, which have previously been implicated in the pathogenesis of absence seizures. In contrast to mice lacking the pore-forming HCN channel subunit HCN2, TRIP8b knockout mice exhibited normal cardiac and motor function and a less severe seizure phenotype. Evaluating the circuit that underlies absence seizures, we found that TRIP8b knockout mice had significantly reduced HCN channel expression and function in thalamic-projecting cortical layer 5b neurons and thalamic relay neurons, but preserved function in inhibitory neurons of the reticular thalamic nucleus. Our results expand the known roles of TRIP8b and provide new insight into the region-specific functions of TRIP8b and HCN channels in constraining cortico-thalamo-cortical excitability.

Carbamazepine aggravates absence seizures in two dedicated mouse models.

BACKGROUND: The aim of this study was to evaluate the effect of carbamazepine (CBZ) upon chemically induced absence seizures and in a genetic absence seizures model in the mouse. METHODS: The γ-butyrolactone (GBL)-induced acute absence seizures and the stargazer spontaneous absence seizures mice models were used to characterize the aggravation of absence seizures induced by oral CBZ treatment. The effect of CBZ upon GABA inward-currents in Ltk cells expressing human recombinant α1ß2γ2, α2ß2γ2, α3ß2γ2 and α5ß2γ2 GABAA receptors was evaluated by means of patch clamp. RESULTS: GBL administration induced motor impairment in NMRI mice. High dose CBZ (25mg/kg body weight) had no effect on motor performance but exacerbated the behavioral incoordination observed for GBL. Also, coadministration of a high dose CBZ and GBL impaired spontaneous locomotion. Moreover, CBZ was investigated after oral administration to evaluate the potential to aggravate GBL-induced acute spike-and-wave discharges (SWD) in the electroencephalogram. High dose CBZ significantly aggravated SWD induced by GBL. Likewise, in the stargazer mouse model of genetic spontaneous absence seizures, CBZ significantly aggravated SWD frequency and duration. Pre-treatment with the T-type Ca(2+) channel blocker ethosuximide (200mg/kg body weight) prevented the CBZ aggravation of SWD induced by GBL and in the stargazer mouse. CBZ increased in a concentration dependent manner sub-maximal α1ß2γ2 and α3ß2γ2 GABA currents. CONCLUSION: CBZ aggravates absence seizures as assessed in two dedicated mouse models of absence seizures. Facilitation of sub-maximal α1ß2γ2, and α3ß2γ2 GABA currents by CBZ may play a role in CBZ-induced GABA-mediated aggravation of absence seizures.

Optogenetic control of thalamus as a tool for interrupting penicillin induced seizures.

Penicillin epilepsy model, whose discharge resembles that of human absence epilepsy, is one of the most useful acute experimental epilepsy models. Though closed-loop optogenetic strategy of interrupting seizures was proved sufficient to switch off epilepsy by controlling thalamus in the post-lesion partial chronic epilepsy model, doubts still exist in absence epilepsy attenuation through silencing thalamus. Here we directly arrested the thalamus to modulate penicillin-induced absence seizures through pseudorandom responsive stimulation on eNpHR-transfected rats. Our data suggested that the duration of epileptiform bursts under light conditions, compared with no light conditions, did not increase or decrease when modulated specific eNpHR-expressing neurons in thalamus.