Thalamocortical oscillations in a genetic model of absence seizures

Isolation of the murine Glut1 deficient thalamocortical circuit: wavelet characterization and reverse glucose dependence of low and gamma frequency oscillations

Glucose represents the principal brain energy source. Thus, not unexpectedly, genetic glucose transporter 1 (Glut1) deficiency (G1D) manifests with encephalopathy. G1D seizures, which constitute a prominent disease manifestation, often prove refractory to medications but may respond to therapeutic diets. These seizures are associated with aberrant thalamocortical oscillations as inferred from human electroencephalography and functional imaging. Mouse electrophysiological recordings indicate that inhibitory neuron failure in thalamus and cortex underlies these abnormalities. This provides the motivation to develop a neural circuit testbed to characterize the mechanisms of thalamocortical synchronization and the effects of known or novel interventions. To this end, we used mouse thalamocortical slices on multielectrode arrays and characterized spontaneous low frequency oscillations and less frequent 30-50 Hz or gamma oscillations under near-physiological bath glucose concentration. Using the cortical recordings from layer IV among other regions recorded, we quantified oscillation epochs via an automated wavelet-based algorithm. This method proved analytically superior to power spectral density, short-time Fourier transform or amplitude-threshold detection. As expected from human observations, increased bath glucose reduced the lower frequency oscillations while augmenting the gamma oscillations, likely reflecting strengthened inhibitory neuron activity, and thus decreasing the low:high frequency ratio (LHR). This approach provides an ex vivo method for the evaluation of mechanisms, fuels, and pharmacological agents in a crucial G1D epileptogenic circuit.

Isolation of the murine Glut1 deficient thalamocortical circuit: wavelet characterization and reverse glucose dependence of low and gamma frequency oscillations

Glucose represents the principal brain energy source. Thus, not unexpectedly, genetic glucose transporter 1 (Glut1) deficiency (G1D) manifests with encephalopathy. G1D seizures, which constitute a prominent disease manifestation, often prove refractory to medications but may respond to therapeutic diets. These seizures are associated with aberrant thalamocortical oscillations as inferred from human electroencephalography and functional imaging. Mouse electrophysiological recordings indicate that inhibitory neuron failure in thalamus and cortex underlies these abnormalities. This provides the motivation to develop a neural circuit testbed to characterize the mechanisms of thalamocortical synchronization and the effects of known or novel interventions. To this end, we used mouse thalamocortical slices on multielectrode arrays and characterized spontaneous low frequency oscillations and less frequent 30-50 Hz or gamma oscillations under near-physiological bath glucose concentration. Using the cortical recordings from layer IV, we quantified oscillation epochs via an automated wavelet-based algorithm. This method proved analytically superior to power spectral density, short-time Fourier transform or amplitude-threshold detection. As expected from human observations, increased bath glucose reduced the lower frequency oscillations while augmenting the gamma oscillations, likely reflecting strengthened inhibitory neuron activity. This approach provides an ex vivo method for the evaluation of mechanisms, fuels, and pharmacological agents in a crucial G1D epileptogenic circuit.

Stiripentol inhibits spike‐and‐waves discharges in animal models of absence seizures, a new mechanism of action involving T‐type calcium channels?

Objective

Stiripentol (STP; Diacomit®) is an antiepileptic drug indicated for Dravet syndrome that has been identified as a γ‐aminobutyric acid (GABAergic) positive allosteric modulator. Dravet syndrome is characterized by multiple seizure types: generalized tonic–clonic, focal, myoclonic, and absence seizures. In addition to its antiepileptic effects on tonic–clonic seizures, STP has also been reported to reduce the frequency of atypical absence seizures in patients. Our study focused on STP potential effects on absence seizures, to better characterize its full spectrum of mechanisms of action.

Methods

STP effects on absence seizures were quantified by electroencephalographic recording in two animal models: rats treated with a low dose of pentylenetetrazol (20 mg/kg ip) and rats from the WAG/Rij strain. In addition, we characterized STP effects on T‐type calcium channel activity. Peak currents were recorded with manual patch clamp on cells transfected with cDNA encoding for the human isoform for Ca_v3.1, Ca_v3.2, and Ca_v3.3.

Results

STP administered before pentylenetetrazol almost completely abolished the generation of spike‐and‐wave discharges (SWDs) at the dose of 300 mg/kg. At this dose, STP also statistically significantly decreased SWD cumulated duration and number in WAG/Rij rats. Its antiepileptic effect was maintained in WAG/Rij rats, whose seizures were aggravated by the GABA agonist THIP (gaboxadol hydrochloride). Furthermore, electrophysiological recordings showed that STP inhibits T‐type calcium channel peak activity, with a higher specificity for the Ca_v3.3 subtype.

Significance

In addition to its previously characterized anticonvulsive properties, these data highlight a new mechanism of action of STP on abnormal thalamocortical activity. This strong antiabsence effect on seizures is correlated with an inhibition of T‐type calcium channels. This new mechanism of action could be implicated in the specificity of STP therapeutic effects in Dravet syndrome.

Separating Neural Oscillations from Aperiodic 1/f Activity: Challenges and Recommendations

Electrophysiological power spectra typically consist of two components: An aperiodic part usually following an 1/f power law [Formula: see text] and periodic components appearing as spectral peaks. While the investigation of the periodic parts, commonly referred to as neural oscillations, has received considerable attention, the study of the aperiodic part has only recently gained more interest. The periodic part is usually quantified by center frequencies, powers, and bandwidths, while the aperiodic part is parameterized by the y-intercept and the 1/f exponent [Formula: see text]. For investigation of either part, however, it is essential to separate the two components. In this article, we scrutinize two frequently used methods, FOOOF (Fitting Oscillations & One-Over-F) and IRASA (Irregular Resampling Auto-Spectral Analysis), that are commonly used to separate the periodic from the aperiodic component. We evaluate these methods using diverse spectra obtained with electroencephalography (EEG), magnetoencephalography (MEG), and local field potential (LFP) recordings relating to three independent research datasets. Each method and each dataset poses distinct challenges for the extraction of both spectral parts. The specific spectral features hindering the periodic and aperiodic separation are highlighted by simulations of power spectra emphasizing these features. Through comparison with the simulation parameters defined a priori, the parameterization error of each method is quantified. Based on the real and simulated power spectra, we evaluate the advantages of both methods, discuss common challenges, note which spectral features impede the separation, assess the computational costs, and propose recommendations on how to use them.

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.

Abolishing cAMP sensitivity in HCN2 pacemaker channels induces generalized seizures

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are dually gated channels that are operated by voltage and by neurotransmitters via the cAMP system. cAMP-dependent HCN regulation has been proposed to play a key role in regulating circuit behavior in the thalamus. By analyzing a knockin mouse model (HCN2EA), in which binding of cAMP to HCN2 was abolished by 2 amino acid exchanges (R591E, T592A), we found that cAMP gating of HCN2 is essential for regulating the transition between the burst and tonic modes of firing in thalamic dorsal-lateral geniculate (dLGN) and ventrobasal (VB) nuclei. HCN2EA mice display impaired visual learning, generalized seizures of thalamic origin, and altered NREM sleep properties. VB-specific deletion of HCN2, but not of HCN4, also induced these generalized seizures of the absence type, corroborating a key role of HCN2 in this particular nucleus for controlling consciousness. Together, our data define distinct pathological phenotypes resulting from the loss of cAMP-mediated gating of a neuronal HCN channel.

Modulation of thalamocortical oscillations by TRIP8b, an auxiliary subunit for HCN channels

Hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels have important functions in controlling neuronal excitability and generating rhythmic oscillatory activity. The role of tetratricopeptide repeat-containing Rab8b-interacting protein (TRIP8b) in regulation of hyperpolarization-activated inward current, I h, in the thalamocortical system and its functional relevance for the physiological thalamocortical oscillations were investigated. A significant decrease in I h current density, in both thalamocortical relay (TC) and cortical pyramidal neurons was found in TRIP8b-deficient mice (TRIP8b-/-). In addition basal cAMP levels in the brain were found to be decreased while the availability of the fast transient A-type K+ current, I A, in TC neurons was increased. These changes were associated with alterations in intrinsic properties and firing patterns of TC neurons, as well as intrathalamic and thalamocortical network oscillations, revealing a significant increase in slow oscillations in the delta frequency range (0.5-4 Hz) during episodes of active-wakefulness. In addition, absence of TRIP8b suppresses the normal desynchronization response of the EEG during the switch from slow-wave sleep to wakefulness. It is concluded that TRIP8b is necessary for the modulation of physiological thalamocortical oscillations due to its direct effect on HCN channel expression in thalamus and cortex and that mechanisms related to reduced cAMP signaling may contribute to the present findings.

Decrease of neocortical paired-pulse depression in GAERS and possible implication of gap junctions

Thalamocortical slices are widely used to study thalamocortical relationships and absence epilepsy. However, it is still not known whether (1) intracortical synaptic transmission, in particular neocortical paired-pulse depression (PPD), is maintained in these slices and (2) whether PPD is altered in the Genetic Absence Epilepsy Rat from Strasbourg (GAERS, a model of absence epilepsy for which cortico-thalamic loops are involved). Furthermore, while the involvement of gap junctions (GJ) in the mechanisms leading to epileptiform discharges has been intensively studied, little is known about their effect on intracortical transmission. We first studied intracortical connection efficacy and PPD in thalamocortical slices from GAERS and non-epileptic rats (NER). We then investigated the effects of GJ blockers (carbenoxolone and quinidine) on intracortical response following single or paired-pulse stimulations in coronal slices from Wistar rats. We show that the efficacy of intracortical connections is not impaired in GAERS. We also show that neocortical PPD is preserved in thalamocortical slices of NER, but that its efficacy is strongly decreased in GAERS. Moreover, a NMDA antagonist strongly reduced the PPD in NER but had no effect in GAERS. Cortical responses to white matter stimulation were not modified by quinidine or carbenoxolone in coronal slices of Wistar rats. PPD was recorded in these slices and was decreased by carbenoxolone but not by quinidine. We hypothesize that the decrease of PPD observed in GAERS might be due to a decrease in function of (1) NMDA receptors and/or (2) astrocytic GJ's.

Diminution of the NMDA receptor NR2B subunit in cortical and subcortical areas of WAG/Rij rats

Modulation of glutamatergic NMDA receptors affects the synchronization of spike discharges in in WAG/Rij rats, a valid genetic animal model of absence epilepsy. In this study, we describe the alteration of NR2B subunit of NMDA receptors expression in WAG/Rij rats in different somatosensory cortical layers and in hippocampal CA1 area. Experimental groups were divided into four groups of six rats of both WAG/Rij and Wistar strains with 2 and 6 months of age. The distribution of NR2B receptors was assessed by immunohistochemical staining in WAG/Rij and compared with age-matched Wistar rats. The expression of NR2B subunit was significantly decreased in different somatosensory cortical layers in 2- and 6-month-old WAG/Rij rats. In addition, the distribution of NR2B in hippocampal CA1 area was lower in 6-month-old WAG/Rij compared with age-matched Wistar rats. The reduction of NR2B receptors in different brain areas points to disturbance of glutamate receptors expression in cortical and subcortical areas in WAG/Rij rats. An altered subunit assembly of NMDA receptors may underlie cortical hyperexcitability in absence epilepsy.

Thalamocortical relationships and network synchronization in a new genetic model "in mirror" for absence epilepsy

Electroencephalographic generalized spike and wave discharges (SWD), the hallmark of human absence seizures, are generated in thalamocortical networks. However, the potential alterations in these networks in terms of the efficacy of the reciprocal synaptic activities between the cortex and the thalamus are not known in this pathology. Here, the efficacy of these reciprocal connections is assessed in vitro in thalamocortical slices obtained from BS/Orl mice, which is a new genetic model of absence epilepsy. These mice show spontaneous SWD, and their features can be compared to that of BR/Orl mice, which are free of SWD. In addition, since gap junctions may modulate the efficacy of these connections, their implications in pharmacologically-induced epileptiform discharges were studied in the same slices. The thalamus and neocortex were independently stimulated and the electrically-evoked responses in both structures were recorded from the same slice. The synaptic efficacy of thalamocortical and corticothalamic connections were assessed by measuring the dynamic range of synaptic field potential changes in response to increasing stimulation strengths. The connection efficacy was weaker in epileptic mice however, this decrease in efficacy was more pronounced in thalamocortical afferents, thus introducing an imbalance in the reciprocal connections between the cortex and thalamus. However, short-term facilitation of the thalamocortical responses were increased in epileptic mice compared to non-epileptic animals. These features may favor occurrence of rhythmical activities in thalamocortical networks. In addition, carbenoxolone (a gap junction blocker) decreased the cumulative duration of 4-aminopyridine-induced ictal-like activities, with a slower time course in epileptic mice. However, the 4-aminopyridine-induced GABA-dependent negative potentials, which appeared to trigger the ictal-like activities, remained. Our results show that the balance of the reciprocal connections between the thalamus and cortex is altered in favor of the corticothalamic connections in epileptic mice, and suggest that gap junctions mediate a stronger cortical synchronization in this strain.

NIPA2 located in 15q11.2 is mutated in patients with childhood absence epilepsy

While pathogenic copy number variations (CNVs) in 15q11.2 were recently identified in Caucasian patients with idiopathic generalized epilepsies (IGEs), the epilepsy-associated gene(s) in this region is/are still unknown. Our study investigated whether the CNVs in 15q11.2 are associated with childhood absence epilepsy (CAE) in Chinese patients and whether the selective magnesium transporter NIPA2 gene affected by 15q11.2 microdeletions is a susceptive gene for CAE. We assessed IGE-related CNVs by Affymetrix SNP 5.0 microarrays in 198 patients with CAE and 198 controls from northern China, and verified the identified CNVs by high-density oligonucleotide-based CGH microarrays. The coding region and exon-intron boundaries of NIPA2 were sequenced in all 380 patients with CAE and 400 controls. 15q11.2 microdeletions were detected in 3 of 198 (1.5%) patients and in no controls. Furthermore, we identified point mutations or indel in a heterozygous state of the NIPA2 gene in 3 out of 380 patients, whereas they were absent in 700 controls (P = 0.043). These mutations included two novel missense mutations (c.532A>T, p.I178F; c.731A>G, p.N244S) and one small novel insertion (c.1002_1003insGAT, p.N334_335EinsD). No NIPA2 mutation was found in 400 normal controls. We first identified that NIPA2, encoding a selective magnesium transporter, is a susceptible gene of CAE, and 15q11.2 microdeletions are important pathogenic CNVs for CAE with higher frequency in Chinese populations than that previously reported in Caucasians. The haploinsufficiency of NIPA2 may be a mechanism underlying the neurological phenotypes of 15q11.2 microdeletions.

Cortical hyperexcitability and epileptogenesis: Understanding the mechanisms of epilepsy - part 2

Epilepsy encompasses a diverse group of seizure disorders caused by a variety of structural, cellular and molecular alterations of the brain primarily affecting the cerebral cortex, leading to recurrent unprovoked epileptic seizures. In this two-part review we examine the mechanisms underlying normal neuronal function and those predisposing to recurrent epileptic seizures starting at the most basic cellular derangements (Part 1, Volume 16, Issue 3) and working up to the highly complex epileptic networks and factors that modulate the predisposition to seizures (Part 2). We attempt to show that multiple factors can modify the epileptic process and that different mechanisms underlie different types of epilepsy, and in most situations there is an interplay between multiple genetic and environmental factors.

Diminished presynaptic GABA(B) receptor function in the neocortex of a genetic model of absence epilepsy

Changes in GABA(B) receptor subunit expression have been recently reported in the neocortex of epileptic WAG/Rij rats that are genetically prone to experience absence seizures. These alterations may lead to hyperexcitability by downregulating the function of presynaptic GABA(B) receptors in neocortical networks as suggested by a reduction in paired-pulse depression. Here, we tested further this hypothesis by analyzing the effects induced by the GABA(B) receptor agonist baclofen (0.1-10 microM) on the inhibitory events recorded in vitro from neocortical slices obtained from epileptic (>180 day-old) WAG/Rij and age-matched, non-epileptic control (NEC) rats. We found that higher doses of baclofen were required to depress pharmacologically isolated, stimulus-induced IPSPs generated by WAG/Rij neurons as compared to NEC. We also obtained similar evidence by comparing the effects of baclofen on the rate of occurrence of synchronous GABAergic events recorded by WAG/Rij and NEC neocortical slices treated with 4-aminopyridine + glutamatergic receptor antagonists. In conclusion, these data highlight a decreased function of presynaptic GABA(B) receptors in the WAG/Rij rat neocortex. We propose that this alteration may contribute to neocortical hyperexcitability and thus to absence seizures.

Detecting epileptic seizures in long-term human EEG: a new approach to automatic online and real-time detection and classification of polymorphic seizure patterns

Epileptic seizures can cause a variety of temporary changes in perception and behavior. In the human EEG they are reflected by multiple ictal patterns, where epileptic seizures typically become apparent as characteristic, usually rhythmic signals, often coinciding with or even preceding the earliest observable changes in behavior. Their detection at the earliest observable onset of ictal patterns in the EEG can, thus, be used to start more-detailed diagnostic procedures during seizures and to differentiate epileptic seizures from other conditions with seizure-like symptoms. Recently, warning and intervention systems triggered by the detection of ictal EEG patterns have attracted increasing interest. Since the workload involved in the detection of seizures by human experts is quite formidable, several attempts have been made to develop automatic seizure detection systems. So far, however, none of these found widespread application. Here, we present a novel procedure for generic, online, and real-time automatic detection of multimorphologic ictal-patterns in the human long-term EEG and its validation in continuous, routine clinical EEG recordings from 57 patients with a duration of approximately 43 hours and additional 1,360 hours of seizure-free EEG data for the estimation of the false alarm rates. We analyzed 91 seizures (37 focal, 54 secondarily generalized) representing the six most common ictal morphologies (alpha, beta, theta, and delta- rhythmic activity, amplitude depression, and polyspikes). We found that taking the seizure morphology into account plays a crucial role in increasing the detection performance of the system. Moreover, besides enabling a reliable (mean false alarm rate<0.5/h, for specific ictal morphologies<0.25/h), early and accurate detection (average correct detection rate>96%) within the first few seconds of ictal patterns in the EEG, this procedure facilitates the automatic categorization of the prevalent seizure morphologies without the necessity to adapt the proposed system to specific patients.

Zn2+ Slows Down CaV3.3 Gating Kinetics: Implications for Thalamocortical Activity

We employed whole cell patch-clamp recordings to establish the effect of Zn2+ on the gating the brain specific, T-type channel isoform CaV3.3 expressed in HEK-293 cells. Zn2+ (300 μM) modified the gating kinetics of this channel without influencing its steady-state properties. When inward Ca2+ currents were elicited by step depolarizations at voltages above the threshold for channel opening, current inactivation was significantly slowed down while current activation was moderately affected. In addition, Zn2+ slowed down channel deactivation but channel recovery from inactivation was only modestly changed. Zn2+ also decreased whole cell Ca2+ permeability to 45% of control values. In the presence of Zn2+, Ca2+ currents evoked by mock action potentials were more persistent than in its absence. Furthermore, computer simulation of action potential generation in thalamic reticular cells performed to model the gating effect of Zn2+ on T-type channels (while leaving the kinetic parameters of voltage-gated Na+ and K+ unchanged) revealed that Zn2+ increased the frequency and the duration of burst firing, which is known to depend on T-type channel activity. In line with this finding, we discovered that chelation of endogenous Zn2+ decreased the frequency of occurrence of ictal-like epileptiform discharges in rat thalamocortical slices perfused with medium containing the convulsant 4-aminopyridine (50 μM). These data demonstrate that Zn2+ modulates CaV3.3 channel gating thus leading to increased neuronal excitability. We also propose that endogenous Zn2+ may have a role in controlling thalamocortical oscillations.

Reduced GABAB receptor subunit expression and paired-pulse depression in a genetic model of absence seizures

Neocortical networks play a major role in the genesis of generalized spike-and-wave (SW) discharges associated with absence seizures in humans and in animal models, including genetically predisposed WAG/Rij rats. Here, we tested the hypothesis that alterations in GABA(B) receptors contribute to neocortical hyperexcitability in these animals. By using Real-Time PCR we found that mRNA levels for most GABA(B(1)) subunits are diminished in epileptic WAG/Rij neocortex as compared with age-matched non-epileptic controls (NEC), whereas GABA(B(2)) mRNA is unchanged. Next, we investigated the cellular distribution of GABA(B(1)) and GABA(B(2)) subunits by confocal microscopy and discovered that GABA(B(1)) subunits fail to localize in the distal dendrites of WAG/Rij neocortical pyramidal cells. Intracellular recordings from neocortical cells in an in vitro slice preparation demonstrated reduced paired-pulse depression of pharmacologically isolated excitatory and inhibitory responses in epileptic WAG/Rij rats as compared with NECs; moreover, paired-pulse depression in NEC slices was diminished by a GABA(B) receptor antagonist to a greater extent than in WAG/Rij rats further suggesting GABA(B) receptor dysfunction. In conclusion, our data identify changes in GABA(B) receptor subunit expression and distribution along with decreased paired-pulse depression in epileptic WAG/Rij rat neocortex. We propose that these alterations may contribute to neocortical hyperexcitability and thus to SW generation in absence epilepsy.

Synchronous high-voltage spindles in the cortex-basal ganglia network of awake and unrestrained rats

Synchronous oscillations in various frequency ranges have been recorded in several nuclei of the basal ganglia (BG) and are thought to be an information processing mechanism. High-voltage spindles (HVSs) are 5-13 Hz spike-and-wave oscillations, which are commonly recorded in rats and which have been reported in some recent studies where their occurrence in the BG has been investigated. We recorded single neurons and local field potentials (LFPs) simultaneously in the motor cortex, striatum and substantia nigra pars reticulata (SNr) of the freely moving rat. We took advantage of the high level of synchronization observed during HVSs to study signal transmission in the cortex-BG network in the awake animals. The results show that LFPs are synchronized in the motor cortex, striatum and SNr during HVS episodes and that the latter propagate from the cortex to the SNr via the striatum. Moreover, > 50% of single neurons in each of these structures are triggered by the HVS. Following the discharge of cortical cells, SNr neurons are first inhibited after approximately 19 ms and then activated after approximately 45 ms. This response is probably driven by the direct and indirect pathways, respectively, without any involvement of the hyperdirect pathway. Here, it is shown that cortex-BG connectivity can be studied using physiological signals in the freely moving animal as opposed to artificial stimulation under anaesthetized conditions. This opens the door to further studies under various experimental conditions, such as animal models of basal ganglia disorders.

Neocortical hyperexcitability in a genetic model of absence seizures and its reduction by levetiracetam

PURPOSE

To study the effect of the antiepileptic drug levetiracetam (LEV) on the patterns of intrinsic optical signals (IOSs) generated by slices of the somatosensory cortex obtained from 3- and 6-month-old WAG/Rij and age-matched, nonepileptic control (NEC) rats.

METHODS

WAG/Rij and NEC animals were anesthetized with enfluorane and decapitated. Brains were quickly removed, and neocortical slices were cut coronally with a vibratome, transferred to a submerged tissue chamber, and superfused with oxygenated artificial cerebrospinal fluid (aCSF). Slices were illuminated with a dark-field condensor and examined with a x2.5 objective; images were processed with a real time digital video image-enhancement system. Images were acquired before (background) and during electrical stimulation with a temporal resolution of 10 images/s and were displayed in pseudocolors. Extracellular stimuli (200 micros; <4 V) were delivered through bipolar stainless steel electrodes placed in the white matter.

RESULTS

IOSs recorded in NEC slices bathed in control aCSF became less intense and of reduced size with age (p < 0.05); this trend was not seen in WAG/Rij slices. Age-dependent decreases in IOS intensity and area size were also seen in NEC slices superfused with aCSF containing the convulsant 4-aminopyridine (4-AP, 5 microM); in contrast, significant increases in both parameters occurred with age in 4-AP-treated WAG/Rij slices (p < 0.05). Under any of these conditions, the IOS intensity and area size slices were larger in WAG/Rij than in NEC slices. LEV (50-500 microM) application to WAG/Rij slices caused dose-dependent IOS reductions that were evident both in control and in 4-AP-containing aCSF and were more pronounced in 6-month-old tissue.

CONCLUSIONS

These data demonstrate age-dependent IOS modifications in NEC and WAG/Rij rat slices and identify a clear pattern of hyperexcitability that occurs in 6-month-old WAG/Rij neocortical tissue, an age when absence seizures occur in all animals. The ability of LEV to reduce these patterns of network hyperexcitability supports the potential use of this new antiepileptic drug in primary generalized epileptic disorders.

Metabotropic glutamate receptors as a strategic target for the treatment of epilepsy

Epilepsy is a chronic neurological disorder that has many known types, including generalized epilepsies that involve cortical and subcortical structures. A proportion of patients have seizures that are resistant to traditional anti-epilepsy drugs, which mainly target ion channels or postsynaptic receptors. This resistance to conventional therapies makes it important to identify novel targets for the treatment of epilepsy. Given the involvement of the neurotransmitter glutamate in the etiology of epilepsy, targets that control glutamatergic neurotransmission are of special interest. The metabotropic glutamate receptors (mGluRs) are of a family of eight G-protein-coupled receptors that serve unique regulatory functions at synapses that use the neurotransmitter glutamate. Their distribution within the central nervous system provides a platform for both presynaptic control of glutamate release, as well as postsynaptic control of neuronal responses to glutamate. In recent years, substantial efforts have been made towards developing selective agonists and antagonists which may be useful for targeting specific receptor subtypes in an attempt to harness the therapeutic potential of these receptors. We examine the possibility of intervening at these receptors by considering the specific example of absence seizures, a form of generalized, non-convulsive seizure that involves the thalamus. Views of the etiology of absence seizures have evolved over time from the "centrencephalic" concept of a diffuse subcortical pacemaker toward the "cortical focus" theory in which cortical hyperexcitability leads the thalamus into the 3-4 Hz rhythms that are characteristic of absence seizures. Since the cortex communicates with the thalamus via a massive glutamatergic projection, ionotropic glutamate receptor (iGluR) blockade has held promise, but the global nature of iGluR intervention has precluded the clinical effectiveness of drugs that block iGluRs. In contrast, mGluRs, because they modulate iGluRs at glutamatergic synapses only under certain conditions, may quell seizure activity by selectively reducing hyperactive glutamatergic synaptic communication within the cortex and thalamus without significantly affecting normal response rates. In this article, we review the circuitry and events leading to absence seizure generation within the corticothalamic network, we present a comprehensive review of the synaptic location and function of mGluRs within the thalamus and cerebral cortex, and review the current knowledge of mGluR modulation and seizure generation. We conclude by reviewing the potential advantages of Group II mGluRs, specifically mGluR2, in the treatment of both convulsive and non-convulsive seizures.

Effects of some neurosteroids injected into some brain areas of WAG/Rij rats, an animal model of generalized absence epilepsy

Neurosteroids are synthesized in the brain and have been demonstrated to modulate various cerebral functions. Allopregnanolone (3alpha-hydroxy-5alpha-pregnan-20-one), a naturally occurring neurosteroid, and ganaxolone (3alpha-hydroxy-3beta-methyl-5alpha-pregnan-20-one), a synthetic derivative, are two neurosteroids acting as positive allosteric modulators of the GABA(A) receptor complex acting on a specific steroid recognition site. Both agents antagonize generalized tonic-clonic seizures in various animal models of epilepsy. Pregnenolone sulphate (3beta-hydroxy-5alpha-pregnen-20-one 3-sulphate; PS) is a negative allosteric modulator of GABA(A) receptors and a positive modulator of the NMDA receptors. We have evaluated the effects of such compounds in a genetic animal model of absence epilepsy, the WAG/Rij rat. Animals were chronically implanted with five frontoparietal cortical electrodes for electrocorticogram (EEG) recordings and bilateral guide cannulae into specific brain areas of the cortico-thalamic circuit in order to evaluate the effects of these compounds on the number and duration of epileptic spike-wave discharges (SWDs). The focal and bilateral microinjection of the two GABA(A) positive modulators into some thalamic nuclei (nucleus ventralis posteromedialis, nucleus reticularis thalami, nucleus ventralis posterolateralis was usually able to significantly worsen the occurrence of SWDs in WAG/Rij rats. Whereas both compounds were able to reduce the number and duration of SWDs when microinjected into the peri-oral region of the primary somatosensory cortex. The effects of PS were more complex depending on both the dose and the site of administration, generally, at low doses in thalamic nuclei and cortex, PS induced an increase of absence activity and a reduction at higher doses. These findings suggest that neurosteroids might play a role in absence epilepsies and that it might depend on the involvement of specific neuronal areas.

Synaptic hyperexcitability of deep layer neocortical cells in a genetic model of absence seizures

We used sharp-electrode, intracellular recordings in an in vitro brain slice preparation to study the excitability of neocortical neurons located in the deep layers (>900 microm from the pia) of epileptic (180-210-days old) Wistar Albino Glaxo/Rijswijk (WAG/Rij) and age-matched, non-epileptic control (NEC) rats. Wistar Albino Glaxo/Rijswijk rats represent a genetic model of absence seizures associated with generalized spike and wave (SW) discharges in vivo. When filled with neurobiotin, these neurons had a typical pyramidal shape with extensive apical and basal dendritic trees; moreover, WAG/Rij and NEC cells had similar fundamental electrophysiological and repetitive firing properties. Sequences of excitatory postsynaptic potentials (EPSPs) and hyperpolarizing inhibitory postsynaptic potentials (IPSPs) were induced in both the strains by electrical stimuli delivered to the underlying white matter or within the neocortex; however, in 24 of 55 regularly firing WAG/Rij cells but only in 2 of 25 NEC neurons, we identified a late EPSP that (1) led to action potential discharge and (2) was abolished by the N-methyl-D-aspartate (NMDA) receptor antagonist 3,3-(2-carboxypiperazine-4-yl)-propyl-1-phosphonate (20 microM; n = 8/8 WAG/Rij cells). Finally, we found that the fast and slow components of the stimulus-induced IPSPs recorded during the application of glutamatergic receptor antagonists had similar reversal potentials in the two strains, while the peak conductance of the fast IPSP was significantly reduced in WAG/Rij cells. These findings document an increase in synaptic excitability that is mediated by NMDA receptors, in epileptic WAG/Rij rat neurons located in neocortical deep layers. We propose that this mechanism may be instrumental for initiating and maintaining generalized SW discharges in vivo.

NMDA-NR1 and AMPA-GluR4 receptor subunit immunoreactivities in the absence epileptic WAG/Rij rat

From an age of 2-3 months onwards, the WAG/Rij rat, a genetic model for absence epilepsy, develops spike-wave discharges (SWD). SWD start in the peri-oral somatosensory cortex (POsc), whereas the rostral reticular thalamic nucleus (rRTN) contributes to synchronizing the thalamo-cortical oscillations. We hypothesize that N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoazolepropionic acid (AMPA) receptors in the POsc and rRTN are involved in, respectively, the initiation and synchronization of SWD activity. As a first step to test this hypothesis, 3 months old non-epileptic and 6 months old absence epileptic WAG/Rij rats were compared with age-matched non-epileptic ACI control rats. The presence of NMDA and AMPA receptors was assessed by quantifying immunostaining for the NMDA-NR1 subunit and the AMPA-GluR4 subunit, respectively. In the POsc, WAG/Rij rats of both ages showed less NMDA-NR1 (-14.7%) and AMPA-GluR4 (-8.7%) subunit staining than ACI rats. From 3 to 6 months, AMPA-GluR4 subunit staining more strongly increased in the rRTN of WAG/Rij rats than of ACI rats. Further studies should support our assumption that in the POsc of the WAG/Rij rat, SWD start as a result of reduced NMDA- and AMPA-mediated glutamatergic stimulation, and that AMPA-GluR4 containing neurons in the rRTN of this rat strain contribute to synchronization of thalamic and cortical neurons.

Effects in vitro and in vivo of a gap junction blocker on epileptiform activities in a genetic model of absence epilepsy

We investigated the effects of carbenoxolone (CBX), a gap junctions (GJ) blocker, on epileptiform activities in vivo and in vitro. In a first series of experiments, i.p. CBX decreased the cumulative duration of cortical spike-wave discharges (SWD) in adult Genetic Absence Epilepsy Rats from Strasbourg (GAERS) without reduction in the SW amplitude or frequency. Since SWD are generated in thalamocortical networks, we studied the effect of CBX on thalamic and cortical activities elicited by 4-aminopyridine (4AP) in thalamocortical slices from GAERS or non-epileptic rats (NER). Spontaneous ictal-like activities (ILA) were recorded simultaneously in thalamus and somatosensory cortex. However, experiments where these structures were surgically separated showed that ILA were generated in the cortex and recorded by volume conduction in the thalamus. GABA-dependent negative field potentials were also recorded in the cortex, either isolated or initiating ILA. After bath-applying CBX (100 microM), the frequency and cumulative duration of ILA decreased but less rapidly in GAERS than in NER slices and they disappeared at a time point when GABA-dependent negative potentials remained. These data suggest that GJ do not mediate the 4AP induced interneuronal synchronisation but may be implicated in the spreading of the synchronised activities from interneuronal networks to principal neurones. Our results show that CBX exerts an antiepileptic action in vivo, and that GJ blockers limits spread of synchronised activities in vitro. They may represent an appropriate target for development of new antiepileptic drugs.

Increased thalamus levels of glutamate and glutamine (Glx) in patients with idiopathic generalised epilepsy

OBJECTIVE

Abnormal thalamo-cortical oscillations underlie idiopathic generalised epilepsy (IGE). Although thalamic involvement has long been indicated by electrophysiological data, it has only recently become feasible to test this with independent methods. In this magnetic resonance (MR) study, we investigated the metabolic and structural integrity of the thalamus. Possible changes in glutamine and glutamate concentrations and signs of neuronal damage were of particular interest.

METHOD

Forty three IGE patients and 38 age and sex matched healthy controls were investigated. Quantitative single volume MR spectroscopy (MRS, 1.5 T) was used to measure concentrations of glutamate and glutamine (Glx) and N-acetyl aspartate (NAA) in thalamus and occipital cortex. We also measured thalamic volumes on high resolution gradient-echo images and estimated fractions of thalamic grey and white matter with voxel based morphometry (VBM).

RESULTS

IGE patients showed elevated Glx and reduced NAA concentrations in the thalamus compared to controls (12.2+/-2.6 v 8.9+/-4.1 mM, p = 0.0022 for Glx, and 9.9+/-1.0 v 10.7+/-0.9 mM, p = 0.017 for NAA). Thalamic grey matter fraction was reduced in IGE patients, and white matter fraction was increased with the greatest increase in the dorso-medial thalamus. Mean thalamic volume was reduced in patients (6.7+/-0.7 v 7.2+/-0.6 ml in controls, p = 0.0001), as was mean cerebral volume (1163+/-128 v 1250+/-102 ml, p = 0.0003). Patients' thalamus/whole brain ratios were normal.

CONCLUSION

Quantitative MRS and VBM provide further evidence for involvement of the thalamus in IGE. The observed elevation of Glx levels together with reductions in NAA levels and grey matter fractions are consistent with epilepsy related excitoxicity as a possible underlying mechanism.

Cellular and network mechanisms of spike-wave seizures

Spike-wave seizures are often considered a relatively "pure" form of epilepsy, with a uniform defect present in all patients and involvement of the whole brain homogeneously. Here, we present evidence against these common misconceptions. Rather than a uniform disorder, spike-wave rhythms arise from the normal inherent network properties of brain excitatory and inhibitory circuits, where they can be provoked by many different insults in several different brain networks. Here we discuss several different cellular and molecular mechanisms that may contribute to the generation of spike-wave seizures, particularly in idiopathic generalized epilepsy. In addition, we discuss growing evidence that electrical, neuroimaging, and molecular changes in spike-wave seizures do not involve the entire brain homogeneously. Rather, spike-wave discharges occur selectively in some thalamocortical networks, while sparing others. It is hoped that improved understanding of the heterogeneous defects and selective brain regions involved will ultimately lead to more effective treatments for spike-wave seizures.

Morphometric Golgi study of cortical locations in WAG/Rij rats: the cortical focus theory

Recently it was demonstrated that for the absence epilepsy characteristic spike-wave discharges initially emerge from the somatosensory cortex and quickly involve the rest of the cortex and cortico-thalamic network. This has led to the development of the focal theory of absence epilepsy. In this experiment, this theory was further investigated by studying the neuronal organization of the cortical focal zone, a non-focal zone in genetic epileptic WAG/Rij rats and functional related areas in non-epileptic age matched control rats. A classical Golgi staining technique was used to visualize whole cortical neurons with dendritic and axon arborisation. Apical dendrites of pyramidal cells in epileptic rats were often split, declined and were running in non-perpendicular directions. Quantitative differences between the strains were found for the length of neurons, between focal and control areas mainly for dendritic arborization. A significant "strain-zone" interaction was found for the maximal distance between two points of dendritic arborization, the mean length of a dendritic segment and the number of free terminations of apical dendrites. All this demonstrates that properties of dendrites in the cortical focal area of WAG/Rij rats were at variance with dendritic characteristics outside the focal area and with functional similar areas in non-epileptic controls. These features might reflect the hyperexcitability of somatosensory neurons, which underlie the initiation and spreading of spike-wave discharges in WAG/Rij rats. Finally, these results are in line with the cortical focus theory of absence epilepsy.

FMRI of brain activation in a genetic rat model of absence seizures

PURPOSE

EEG-triggered functional magnetic resonance imaging (fMRI) was used to identify areas of brain activation during spontaneous spike-and-wave discharges (SWDs) in an epileptic rat strain under awake conditions.

METHODS

Spontaneous absence seizures from 10 WAG/Rij rats were imaged by using T2*-weighted echo planar imaging at 4.7 Tesla. fMRI of the blood-oxygenation-level-dependent (BOLD) signal was triggered based on EEG recordings during imaging. Images obtained during spontaneous SWDs were compared with baseline images.

RESULTS

Significant positive BOLD signal changes were apparent in several areas of the cortex and several important nuclei of the thalamus. In addition, no negative BOLD signal was found in any brain area.

CONCLUSIONS

We have shown that EEG-triggered BOLD fMRI can be used to detect cortical and thalamic activation related to the spontaneous SWDs that characterize absence seizures in awake WAG/Rij rats. These results draw an anatomic correlation between areas in which increased BOLD signal is found and those in which SWDs have been recorded. In addition, no negative BOLD signal was found to be associated with these spontaneous SWDs. We also demonstrated the technical feasibility of using EEG-triggered fMRI in a genetic rat model of absence seizure.

Neocortical seizures: initiation, development and cessation

Different forms of electrical paroxysms in experimental animals mimic the patterns of absence seizures associated with spike-wave complexes at approximately 3 Hz and of Lennox-Gastaut seizures with spike-wave or polyspike-wave complexes at approximately 1.5-2.5 Hz, intermingled with fast runs at 10-20 Hz. Both these types of electrical seizures are preferentially generated during slow-wave sleep. Here, we challenge the hypothesis of a subcortical pacemaker that would account for suddenly generalized spike-wave seizures as well as the idea of an exclusive role of synaptic excitation in the generation of paroxysmal depolarizing components, and we focus on three points, based on multiple intracellular and field potential recordings in vivo that are corroborated by some clinical studies: (a) the role of neocortical bursting neurons, especially fast-rhythmic-bursting neurons, and of very fast oscillations (ripples, 80-200 Hz) in seizure initiation; (b) the cortical origin of both these types of electrical paroxysms, the synaptic propagation of seizures from one to other, local and distant, cortical sites, finally reaching the thalamus, where the synchronous cortical firing excites thalamic reticular inhibitory neurons and thus leads to steady hyperpolarization and phasic inhibitory postsynaptic potentials in a majority of thalamocortical neurons, which might explain the obliteration of signals from the external world and the unconsciousness during absence seizures; and (c) the cessation of seizures, whose cellular mechanisms have only begun to be investigated and remain an open avenue for research.

Pathway-specific action of gamma-hydroxybutyric acid in sensory thalamus and its relevance to absence seizures

The systemic injection of gamma-hydroxybutyric acid (GHB) elicits spike and wave discharges (SWDs), the EEG hallmark of absence seizures, and represents a well established, widely used pharmacological model of this nonconvulsive epilepsy. Despite this experimental use of GHB, as well as its therapeutic use in narcolepsy and its increasing abuse, however, the precise cellular mechanisms underlying the different pharmacological actions of this drug are still unclear. Because sensory thalamic nuclei play a key role in the generation of SWDs and sleep rhythms, and because direct injection of GHB in the ventrobasal (VB) thalamus elicits SWDs, we investigated GHB effects on corticothalamic EPSCs and GABAergic IPSCs in VB thalamocortical (TC) neurons. GHB (250 microm-10 mm) reversibly decreased the amplitude of electrically evoked EPSCs and GABAA IPSCs via activation of GABAB receptors; however, approximately 60% of the IPSCs were insensitive to low (250 microm-1.0 mm) GHB concentrations. The putative GHB receptor antagonist NSC 382 applied alone had a number of unspecific effects, whereas it either had no action on, or further increased, the GHB-elicited effects on synaptic currents. Low GHB concentrations (250 microm) were also effective in increasing absence-like intrathalamic oscillations evoked by cortical afferent stimulation. These results indicate that low concentrations of GHB, similar to the brain concentrations that evoke SWDs in vivo, differentially affect excitatory and inhibitory synaptic currents in TC neurons and promote absence-like intrathalamic oscillations. Furthermore, the present data strengthen previous suggestions on the GHB mechanism of sleep promotion and will help focus future studies on the cellular mechanisms underlying its abuse.

Cortical-area specific block of genetically determined absence seizures by ethosuximide

Absence epilepsy is characterised by a paroxysmal loss of consciousness, of abrupt onset and termination, and is associated with a bilateral synchronous spike and wave discharge (SWD) on the electroencephalogram. Absence seizures involve an interplay between thalamic and cortical structures, although most research has so far focussed on sensory thalamic nuclei and the reticular thalamic nucleus (RTN). Thus, microinfusion of ethosuximide (ETX), a first choice anti-absence drug, into either the ventrobasal thalamus or RTN of the genetic absence epilepsy rat from Strasbourg (GAERS), a validated rat model of absence epilepsy, does not produce immediate cessation of seizure activity, as is seen following systemic administration. As recent evidence indicates a seizure initiation site within the peri-oral region of the primary somatosensory cortex (S1po), we have now applied ETX into S1po as well as the somatosensory cortex forelimb region (S1FL) and the motor cortex (M1) of freely moving GAERS. Microinfusion of 10 or 20 nmol/side of ETX into S1po produced an immediate cessation of seizure activity. A less marked response was produced when even a higher dose (200 nmol/side) was infused into S1FL. No reduction of SWD was seen when ETX was infused into M1. Microinfusion of CGP 36742 (5 nmol/side), a GABA(B) antagonist, produced immediate cessation of seizure activity in both S1po and M1 and a delayed effect in S1FL. These data suggest that the ability of ETX to abolish genetically determined absence seizures is cortical-area specific and support the involvement of S1po in the initiation of SWDs.