On-demand low-frequency stimulation for seizure control: efficacy and behavioural implications

Mesial temporal lobe epilepsy (MTLE), the most common form of focal epilepsy in adults, is often refractory to medication and associated with hippocampal sclerosis. Deep brain stimulation represents an alternative treatment option for drug-resistant patients who are ineligible for resective brain surgery. In clinical practice, closed-loop stimulation at high frequencies is applied to interrupt ongoing seizures, yet has (i) a high incidence of false detections; (ii) the drawback of delayed seizure-suppressive intervention; and (iii) limited success in sclerotic tissue. As an alternative, low-frequency stimulation (LFS) has been explored recently in patients with focal epilepsies. In preclinical epilepsy models, hippocampal LFS successfully prevented seizures when applied continuously. Since it would be advantageous to reduce the stimulation load, we developed a protocol for on-demand LFS. Given the importance of the hippocampus for navigation and memory, we investigated potential consequences of LFS on hippocampal function. To this end, we used the intrahippocampal kainate mouse model, which recapitulates the key features of MTLE, including spontaneous seizure activity and hippocampal sclerosis. Specifically, our online detection algorithm monitored epileptiform activity in hippocampal local field potential recordings and identified short epileptiform bursts preceding focal seizure clusters, triggering hippocampal LFS to stabilize the network state. To probe behavioural performance, we tested the acute influence of LFS on anxiety-like behaviour in the light-dark box test, spatial and non-spatial memory in the object location memory and novel object recognition test, as well as spatial navigation and long-term memory in the Barnes maze. On-demand LFS was almost as effective as continuous LFS in preventing focal seizure clusters but with a significantly lower stimulation load. When we compared the behavioural performance of chronically epileptic mice to healthy controls, we found that both groups were equally mobile, but epileptic mice displayed an increased anxiety level, altered spatial learning strategy and impaired memory performance. Most importantly, with the application of hippocampal LFS before behavioural training and test sessions, we could rule out deleterious effects on cognition and even show an alleviation of deficits in long-term memory recall in chronically epileptic mice. Taken together, our findings may provide a promising alternative to current therapies, overcoming some of their major limitations, and inspire further investigation of LFS for seizure control in focal epilepsy syndromes.

Ramelteon attenuates hippocampal neuronal loss and memory impairment following kainate-induced seizures

Evidence suggests that the neuroprotective effects of melatonin involve both receptor-dependent and -independent actions. However, little is known about the effects of melatonin receptor activation on the kainate (KA) neurotoxicity. This study examined the effects of repeated post-KA treatment with ramelteon, a selective agonist of melatonin receptors, on neuronal loss, cognitive impairment, and depression-like behaviors following KA-induced seizures. The expression of melatonin receptors decreased in neurons, whereas it was induced in astrocytes 3 and 7 days after seizures elicited by KA (0.12 μg/μL) in the hippocampus of mice. Ramelteon (3 or 10 mg/kg, i.p.) and melatonin (10 mg/kg, i.p.) mitigated KA-induced oxidative stress and impairment of glutathione homeostasis and promoted the nuclear translocation and DNA binding activity of Nrf2 in the hippocampus after KA treatment. Ramelteon and melatonin also attenuated microglial activation but did not significantly affect astroglial activation induced by KA, despite the astroglial induction of melatonin receptors after KA treatment. However, ramelteon attenuated KA-induced proinflammatory phenotypic changes in astrocytes. Considering the reciprocal regulation of astroglial and microglial activation, these results suggest ramelteon inhibits microglial activation by regulating astrocyte phenotypic changes. These effects were accompanied by the attenuation of the nuclear translocation and DNA binding activity of nuclear factor κB (NFκB) induced by KA. Consequently, ramelteon attenuated the KA-induced hippocampal neuronal loss, memory impairment, and depression-like behaviors; the effects were comparable to those of melatonin. These results suggest that ramelteon-mediated activation of melatonin receptors provides neuroprotection against KA-induced neurotoxicity in the mouse hippocampus by activating Nrf2 signaling to attenuate oxidative stress and restore glutathione homeostasis and by inhibiting NFκB signaling to attenuate neuroinflammatory changes.

Presynaptic Kainate Receptors onto Somatostatin Interneurons Are Recruited by Activity throughout Development and Contribute to Cortical Sensory Adaptation

Somatostatin (SST) interneurons produce delayed inhibition because of the short-term facilitation of their excitatory inputs created by the expression of metabotropic glutamate receptor 7 (mGluR7) and presynaptic GluK2-containing kainate receptors (GluK2-KARs). Using mice of both sexes, we find that as synaptic facilitation at layer (L)2/3 SST cell inputs increases during the first few postnatal weeks, so does GluK2-KAR expression. Removal of sensory input by whisker trimming does not affect mGluR7 but prevents the emergence of presynaptic GluK2-KARs, which can be restored by allowing whisker regrowth or by acute calmodulin activation. Conversely, late trimming or acute inhibition of Ca2+/calmodulin-dependent protein kinase II is sufficient to reduce GluK2-KAR activity. This developmental and activity-dependent regulation also produces a specific reduction of L4 GluK2-KARs that advances in parallel with the maturation of sensory processing in L2/3. Finally, we find that removal of both GluK2-KARs and mGluR7 from the synapse eliminates short-term facilitation and reduces sensory adaptation to repetitive stimuli, first in L4 of somatosensory cortex, then later in development in L2/3. The dynamic regulation of presynaptic GluK2-KARs potentially allows for flexible scaling of late inhibition and sensory adaptation.SIGNIFICANCE STATEMENT Excitatory synapses onto somatostatin (SST) interneurons express presynaptic, calcium-permeable kainate receptors containing the GluK2 subunit (GluK2-KARs), activated by high-frequency activity. In this study we find that their presence on L2/3 SST synapses in the barrel cortex is not based on a hardwired genetic program but instead is regulated by sensory activity, in contrast to that of mGluR7. Thus, in addition to standard synaptic potentiation and depression mechanisms, excitatory synapses onto SST neurons undergo an activity-dependent presynaptic modulation that uses GluK2-KARs. Further, we present evidence that loss of the frequency-dependent synaptic components (both GluK2-KARs and mGluR7 via Elfn1 deletion) contributes to a decrease in the sensory adaptation commonly seen on repetitive stimulus presentation.

Virtual Screening-Accelerated Discovery of a Phosphodiesterase 9 Inhibitor with Neuroprotective Effects in the Kainate Toxicity In Vitro Model

In the central nervous system, some specific phosphodiesterase (PDE) isoforms modulate pathways involved in neuronal plasticity. Accumulating evidence suggests that PDE9 may be a promising therapeutic target for neurodegenerative diseases. In the current study, computational techniques were used to identify a nature-inspired PDE9 inhibitor bearing the scaffold of an isoflavone, starting from a database of synthetic small molecules using a ligand-based approach. Furthermore, docking studies supported by molecular dynamics investigations allowed us to evaluate the features of the ligand-target complex. In vitro assays confirmed the computational results, showing that the selected compound inhibits the enzyme in the nanomolar range. Additionally, we evaluated the expression of gene and protein levels of PDE9 in organotypic hippocampal slices, observing an increase following exposure to kainate (KA). Importantly, the PDE9 inhibitor reduced CA3 damage induced by KA in a dose-dependent manner in organotypic hippocampal slices. Taken together, these observations strongly support the potential of the identified nature-inspired PDE9 inhibitor and suggest that such a molecule could represent a promising lead compound to develop novel therapeutic tools against neurological diseases..

3D X-ray Histology for the Investigation of Temporal Lobe Epilepsy in a Mouse Model

The most common form of epilepsy among adults is mesial temporal lobe epilepsy (mTLE), with seizures often originating in the hippocampus due to abnormal electrical activity. The gold standard for the histopathological analysis of mTLE is histology, which is a two-dimensional technique. To fill this gap, we propose complementary three-dimensional (3D) X-ray histology. Herein, we used synchrotron radiation-based phase-contrast microtomography with 1.6 μm-wide voxels for the post mortem visualization of tissue microstructure in an intrahippocampal-kainate mouse model for mTLE. We demonstrated that the 3D X-ray histology of unstained, unsectioned, paraffin-embedded brain hemispheres can identify hippocampal sclerosis through the loss of pyramidal neurons in the first and third regions of the Cornu ammonis as well as granule cell dispersion within the dentate gyrus. Morphology and density changes during epileptogenesis were quantified by segmentations from a deep convolutional neural network. Compared to control mice, the total dentate gyrus volume doubled and the granular layer volume quadrupled 21 days after injecting kainate. Subsequent sectioning of the same mouse brains allowed for benchmarking 3D X-ray histology against well-established histochemical and immunofluorescence stainings. Thus, 3D X-ray histology is a complementary neuroimaging tool to unlock the third dimension for the cellular-resolution histopathological analysis of mTLE.

Animal Models of Drug-Resistant Epilepsy as Tools for Deciphering the Cellular and Molecular Mechanisms of Pharmacoresistance and Discovering More Effective Treatments

In the last 30 years, over 20 new anti-seizure medicines (ASMs) have been introduced into the market for the treatment of epilepsy using well-established preclinical seizure and epilepsy models. Despite this success, approximately 20-30% of patients with epilepsy have drug-resistant epilepsy (DRE). The current approach to ASM discovery for DRE relies largely on drug testing in various preclinical model systems that display varying degrees of ASM drug resistance. In recent years, attempts have been made to include more etiologically relevant models in the preclinical evaluation of a new investigational drug. Such models have played an important role in advancing a greater understanding of DRE at a mechanistic level and for hypothesis testing as new experimental evidence becomes available. This review provides a critical discussion of the pharmacology of models of adult focal epilepsy that allow for the selection of ASM responders and nonresponders and those models that display a pharmacoresistance per se to two or more ASMs. In addition, the pharmacology of animal models of major genetic epilepsies is discussed. Importantly, in addition to testing chemical compounds, several of the models discussed here can be used to evaluate other potential therapies for epilepsy such as neurostimulation, dietary treatments, gene therapy, or cell transplantation. This review also discusses the challenges associated with identifying novel therapies in the absence of a greater understanding of the mechanisms that contribute to DRE. Finally, this review discusses the lessons learned from the profile of the recently approved highly efficacious and broad-spectrum ASM cenobamate.

Neuroglobin deficiency increases seizure susceptibility but does not affect basal behavior in mice

Neuroglobin (Ngb) is found in the neurones of several different brain areas and is known to bind oxygen and other gaseous molecules and reactive oxygen species (ROS) in vitro, but it does not seem to act as a respiratory molecule for neurones. Using male and female Ngb‐knockout (KO) mice, we addressed the role of Ngb in neuronal brain activity using behavioral tests but found no differences in general behaviors, memory processes, and anxiety−/depression‐like behaviors. Oxidative stress and ROS play key roles in epileptogenesis, and oxidative injury produced by an excessive production of free radicals is involved in the initiation and progression of epilepsy. The ROS binding properties led us to hypothesize that lack of Ngb could affect central coping with excitatory stimuli. We consequently explored whether exposure to the excitatory molecule kainate (KA) would increase severity of seizures in mice lacking Ngb. We found that the duration and severity of seizures were increased, while the latency time to develop seizures was shortened in Ngb‐KO compared to wildtype adult female mice. Consistently, c‐fos expression after KA was significantly increased in Ngb‐KO mice in the amygdala and piriform cortex, regions rich in Ngb and known to be centrally involved in seizure generation. Moreover, the measured c‐fos expression levels were correlated with seizure susceptibility. With these new findings combined with previous studies we propose that Ngb could constitute an intrinsic defense mechanism against neuronal hyperexcitability and oxidative stress by buffering of ROS in amygdala and other Ngb‐containing brain regions.

(S)-2-Mercaptohistidine: A First Selective Orthosteric GluK3 Antagonist

The development of tool compounds for the ionotropic glutamate receptors (iGluRs) remains an important research objective, as these are essential for the study and understanding of the roles of these receptors in health and disease. Herein, we report on the pharmacological characterization of (S)-2-hydroxyhistidine (2a) and (S)-2-mercaptohistidine (2b) as mediators of glutamatergic neurotransmission. While 2a displayed negligible binding affinity or activity at all glutamate receptors and transporters investigated, 2b displayed selectivity for homomeric GluK3 with binding affinities in the low micromolar range (K_i = 6.42 ± 0.74 μM). The iGluR subtype selectivity ratio for 2b was calculated at ∼30-fold for GluK1/GluK3, GluA3/GluK3, and GluA4/GluK3 and >100-fold for GluK2/GluK3, GluA1/GluK3, and GluA2/GluK3. Unexpectedly, functional characterization of 2b revealed that the compound is an antagonist (K_b = 7.6 μM) at homomeric GluK3 receptors while exhibiting only weak agonist activity at GluA2 (EC₅₀ = 3.25 ± 0.55 mM). The functional properties of 2b were explored further in electrophysiological recordings of mouse hippocampal neurons.

Resilient Hippocampal Gamma Rhythmogenesis and Parvalbumin-Expressing Interneuron Function Before and After Plaque Burden in 5xFAD Alzheimer's Disease Model

Recent studies have implicated impaired Parvalbumin Fast-Spiking Interneuron (PVIN) function as a precipitating factor underlying abnormalities in network synchrony, oscillatory rhythms, and cognition associated with Alzheimer's disease (AD). However, a complete developmental investigation of potential gamma deficits, induced by commonly used carbachol or kainate in ex vivo slice preparations, within AD model mice is lacking. We examined gamma oscillations using field recordings in acute hippocampal slices from 5xFAD and control mice, through the period of developing pathology, starting at 3 months of age, when there is minimal plaque presence in the hippocampus, through to 12+ months of age, when plaque burden is high. In addition, we examined PVIN participation in gamma rhythms using targeted cell-attached recordings of genetically-reported PVINs, in both wild type and mutant mice. In parallel, a developmental immunohistochemical characterisation probing the PVIN-associated expression of PV and perineuronal nets (PNNs) was compared between control and 5xFAD mice. Remarkably, this comprehensive longitudinal evaluation failed to reveal any obvious correlations between PVIN deficits (electrical and molecular), circuit rhythmogenesis (gamma frequency and power), and Aβ deposits/plaque formation. By 6-12 months, 5xFAD animals have extensive plaque formation throughout the hippocampus. However, a deficit in gamma oscillatory power was only evident in the oldest 5xFAD animals (12+ months), and only when using kainate, and not carbachol, to induce the oscillations. We found no difference in PV firing or phase preference during kainate-induced oscillations in younger or older 5xFAD mice compared to control, and a reduction of PV and PNNs only in the oldest 5xFAD mice. The lack of a clear relationship between PVIN function, network rhythmicity, and plaque formation in our study highlights an unexpected resilience in PVIN function in the face of extensive plaque pathology associated with this model, calling into question the presumptive link between PVIN pathology and Alzheimer's progression.

Neurotensin receptor 2 is induced in astrocytes and brain endothelial cells in relation to neuroinflammation following pilocarpine-induced seizures in rats

Neurotensin (NT) acts as a primary neurotransmitter and neuromodulator in the CNS and has been involved in a number of CNS pathologies including epilepsy. NT mediates its central and peripheral effects by interacting with the NTSR1, NTSR2, and Sort1/NTSR3 receptor subtypes. To date, little is known about the precise expression of the NT receptors in brain neural cells and their regulation in pathology. In the present work, we studied the cellular distribution of the NTSR2 protein in the rat hippocampus and questioned whether its expression was modulated in conditions of neuroinflammation using a model of temporal lobe epilepsy induced by pilocarpine. This model is characterized by a rapid and intense inflammatory reaction with reactive gliosis in the hippocampus. We show that NTSR2 protein is expressed in hippocampal astrocytes and its expression increases together with astrocyte reactivity following induction of status epilepticus. NTSR2 immunoreactivity is also increased in astrocytes proximal to blood vessels and their end-feet, and in endothelial cells. Proinflammatory factors such as IL1β and LPS induced NTSR2 mRNA and protein in cultured astroglial cells. Antagonizing NTSR2 with SR142948A decreased NTSR2 expression as well as astroglial reactivity. Together, our results suggest that NTSR2 is implicated in astroglial and gliovascular inflammation and that targeting the NTSR2 receptor may open new avenues in the regulation of neuroinflammation in CNS diseases.

β-amyloid monomers drive up neuronal aerobic glycolysis in response to energy stressors

Research on cerebral glucose metabolism has shown that the aging brain experiences a fall of aerobic glycolysis, and that the age-related loss of aerobic glycolysis may accelerate Alzheimer’s disease pathology. In the healthy brain, aerobic glycolysis, namely the use of glucose outside oxidative phosphorylation, may cover energy demand and increase neuronal resilience to stressors at once. Currently, the drivers of aerobic glycolysis in neurons are unknown. We previously demonstrated that synthetic monomers of β-amyloid protein (Aβ) enhance glucose uptake in neurons, and that endogenous Aβ is required for depolarization-induced glucose uptake in cultured neurons. In this work, we show that cultured cortical neurons increased aerobic glycolysis in response to the inhibition of oxidative phosphorylation by oligomycin or to a kainate pulse. Such an increase was prevented by blocking the endogenous Aβ tone and re-established by the exogenous addition of synthetic Aβ monomers. The activity of mitochondria-bound hexokinase-1 appeared to be necessary for monomers-stimulated aerobic glycolysis during oxidative phosphorylation blockade or kainate excitation. Our data suggest that, through Aβ release, neurons coordinate glucose uptake with aerobic glycolysis in response to metabolic stressors. The implications of this new finding are that the age-related drop in aerobic glycolysis and the susceptibility to Alzheimer’s disease could be linked to factors interfering with release and functions of Aβ monomers.

Targeting Ionotropic Glutamate Receptors in the Treatment of Epilepsy

Background

A dysfunction in glutamate neurotransmission is critical for seizure. Glutamate is the major excitatory drive in the cerebral cortex, where seizures occur. Glutamate acts via (i) ionotropic (iGlu) receptors, which are ligand-gated ion channels mediating fast excitatory synaptic transmission; and (ii) G proteins coupled metabotropic (mGlu) receptors.

Objective

To overview the evidence on the role of iGlu receptors in the onset, duration, and severity of convulsive and non-convulsive seizures to lay the groundwork for novel strategies for drug-resistant epilepsy.

Methods

We used PubMed crossed-search for “glutamate receptor and epilepsy” (sorting 3,170 reports), searched for “ionotropic glutamate receptors”, “AMPA receptors”, “NMDA receptors”, “kainate receptors”, “convulsive seizures”, “absence epilepsy”, and selected those papers focusing this Review’s scope.

Results

iGlu receptor antagonists inhibit, whereas agonists worsen experimental seizures in various animal species. Clinical development of iGlu receptor antagonists has been limited by the occurrence of adverse effects caused by inhibition of fast excitatory synaptic transmission. To date, only one drug (perampanel) selectively targeting iGlu receptors is marketed for the treatment of focal epilepsy. However, other drugs, such as topiramate and felbamate, inhibit iGlu receptors in addition to other mechanisms.

Conclusion

This review is expected to help dissect those steps induced by iGlu receptors activation, which may be altered to provide antiepileptic efficacy without altering key physiological brain functions, thus improving the safety and tolerability of iGlu-receptor directed antiepileptic agents. This effort mostly applies to drug resistant seizures, which impact the quality of life and often lead to status epilepticus, which is a medical urgency.

Mossy fiber sprouting into the hippocampal region CA2 in patients with temporal lobe epilepsy

Hippocampal sclerosis (HS) in Temporal Lobe Epilepsy (TLE) shows neuronal death in cornu ammonis (CA)1, CA3, and CA4. It is known that granule cells and CA2 neurons survive and their axons, the mossy fibers (MF), lose their target cells in CA3 and CA4 and sprout to the granule cell layer and molecular layer. We examined in TLE patients and in a mouse epilepsy model, whether MF sprouting is directed to the dentate gyrus or extends to distant CA regions and whether sprouting is associated with death of target neurons in CA3 and CA4. In 319 TLE patients, HS was evaluated by Wyler grade and International League against Epilepsy (ILAE) types using immunohistochemistry against neuronal nuclei (NeuN). Synaptoporin was used to colocalize MF. In addition, transgenic Thy1-eGFP mice were intrahippocampally injected with kainate and sprouting of eGFP-positive MFs was analyzed together with immunocytochemistry for regulator of G-protein signaling 14 (RGS14). In human HS Wyler III and IV as well as in ILAE 1, 2, and 3 specimens, we found synaptoporin-positive axon terminals in CA2 and even in CA1, associated with the extent of granule cell dispersion. Sprouting was seen in cases with cell death of target neurons in CA3 and CA4 (classical severe HS ILAE type 1) but also without this cell death (atypical HS ILAE type 2). Similarly, in epileptic mice eGFP-positive MFs sprouted to CA2 and beyond. The presence of MF terminals in the CA2 pyramidal cell layer and in CA1 was also correlated with the extent of granule cell dispersion. The similarity of our findings in human specimens and in the mouse model highlights the importance and opens up new chances of using translational approaches to determine mechanisms underlying TLE.

The Perils of Navigating Activity-Dependent Alternative Splicing of Neurexins

Neurexins are presynaptic cell-adhesion molecules essential for synaptic function that are expressed in thousands of alternatively spliced isoforms. Recent studies suggested that alternative splicing at splice site 4 (SS4) of Nrxn1 is tightly regulated by an activity-dependent mechanism. Given that Nrxn1 alternative splicing at SS4 controls NMDA-receptor-mediated synaptic responses, activity-dependent SS4 alternative splicing would suggest a new synaptic plasticity mechanism. However, conflicting results confound the assessment of neurexin alternative splicing, prompting us to re-evaluate this issue. We find that in cortical cultures, membrane depolarization by elevated extracellular K⁺-concentrations produced an apparent shift in Nrxn1-SS4 alternative splicing by inducing neuronal but not astroglial cell death, resulting in persistent astroglial Nrxn1-SS4+ expression and decreased neuronal Nrxn1-SS4– expression. in vivo, systemic kainate-induced activation of neurons in the hippocampus produced no changes in Nrxn1-SS4 alternative splicing. Moreover, focal kainate injections into the mouse cerebellum induced small changes in Nrxn1-SS4 alternative splicing that, however, were associated with large decreases in Nrxn1 expression and widespread DNA damage. Our results suggest that although Nrxn1-SS4 alternative splicing may represent a mechanism of activity-dependent synaptic plasticity, common procedures for testing this hypothesis are prone to artifacts, and more sophisticated approaches will be necessary to test this important question.

Hippocampal low-frequency stimulation prevents seizure generation in a mouse model of mesial temporal lobe epilepsy

Mesial temporal lobe epilepsy (MTLE) is the most common form of focal, pharmacoresistant epilepsy in adults and is often associated with hippocampal sclerosis. Here, we established the efficacy of optogenetic and electrical low-frequency stimulation (LFS) in interfering with seizure generation in a mouse model of MTLE. Specifically, we applied LFS in the sclerotic hippocampus to study the effects on spontaneous subclinical and evoked generalized seizures. We found that stimulation at 1 Hz for 1 hr resulted in an almost complete suppression of spontaneous seizures in both hippocampi. This seizure-suppressive action during daily stimulation remained stable over several weeks. Furthermore, LFS for 30 min before a pro-convulsive stimulus successfully prevented seizure generalization. Finally, acute slice experiments revealed a reduced efficacy of perforant path transmission onto granule cells upon LFS. Taken together, our results suggest that hippocampal LFS constitutes a promising approach for seizure control in MTLE.

Systematic evaluation of rationally chosen multitargeted drug combinations: a combination of low doses of levetiracetam, atorvastatin and ceftriaxone exerts antiepileptogenic effects in a mouse model of acquired epilepsy

Epileptogenesis, the gradual process that leads to epilepsy after brain injury or genetic mutations, is a complex network phenomenon, involving a variety of morphological, biochemical and functional brain alterations. Although risk factors for developing epilepsy are known, there is currently no treatment available to prevent epilepsy. We recently proposed a multitargeted, network-based approach to prevent epileptogenesis by rationally combining clinically available drugs and provided first proof-of-concept that this strategy is effective. Here we evaluated eight novel rationally chosen combinations of 14 drugs with mechanisms that target different epileptogenic processes. The combinations consisted of 2-4 different drugs per combination and were administered systemically over 5 days during the latent epileptogenic period in the intrahippocampal kainate mouse model of acquired temporal lobe epilepsy, starting 6 h after kainate. Doses and dosing intervals were based on previous pharmacokinetic and tolerability studies in mice. The incidence and frequency of spontaneous electrographic and electroclinical seizures were recorded by continuous (24/7) video linked EEG monitoring done for seven days at 4 and 12 weeks post-kainate, i.e., long after termination of drug treatment. Compared to vehicle controls, the most effective drug combination consisted of low doses of levetiracetam, atorvastatin and ceftriaxone, which markedly reduced the incidence of electrographic seizures (by 60%; p<0.05) and electroclinical seizures (by 100%; p<0.05) recorded at 12 weeks after kainate. This effect was lost when higher doses of the three drugs were administered, indicating a synergistic drug-drug interaction at the low doses. The potential mechanisms underlying this interaction are discussed. We have discovered a promising novel multitargeted combination treatment for modifying the development of acquired epilepsy.

Expression of the ionotropic glutamate receptors on neuronostatin neurons in the periventricular nucleus of the hypothalamus

BACKGROUND

Neuronostatin, a newly identified peptide, is accepted as an anorexigenic peptide since it suppresses food intake when given intracerebroventricularly. Although the effect mechanisms of neuronostatin have been shown in different studies, there are no reports in the literature describing the mechanisms controlling neuronostatin neurons. In this study, we aimed to determine the presence of the ionotropic glutamate receptor subunits (iGluRs) in neuronostatin neurons in the periventricular nucleus of the hypothalamus.

MATERIALS AND METHODS

The presence of glutamate receptors in neuronostatin neurons was investigated by dual immunohistochemistry. Immunohistochemistry was performed on 40 µm thick coronal brain sections with antibodies against AMPA (GluA1-4), kainate (GluK1/2/3, and GluK5), and NMDA (GluN1 and GluN2A) receptor subunits.

RESULTS

The results showed that the neuronostatin neurons expressed most of the NMDA and non-NMDA receptor subunits. The neuronostatin neurons in the anterior hypothalamic periventricular nucleus were particularly immunopositive for GluA1, GluA4, GluK1/2/3, GluK5 and GluN1 antibodies. No expression was observed for GluA2, GluA3 and GluN2A antibodies.

CONCLUSIONS

For the first time in the literature, our study demonstrated that the neuronostatin neurons express glutamate receptor subunits which may form homomeric or heteromeric functional receptor complexes. Taken together, these results suggest that multiple subunits of iGluRs are responsible for glutamate transmission on neuronostatin neurons in the anterior hypothalamic periventricular nucleus.

Dietary omega-3 fatty acids prevent neonatal seizure-induced early alterations in the hippocampal glutamatergic system and memory deficits in adulthood

OBJECTIVE

We investigated the influence of dietary omega-3 polyunsaturated fatty acids (n-3 PUFAs) on glutamatergic system modulation after a single episode of neonatal seizures and their possible effects on seizure-induced long-lasting behavioral deficits.

METHODS

Male Wistar rats receiving an omega-3 diet (n-3) or an n-3 deficient diet (D) from the prenatal period were subjected to a kainate-induced seizure model at P7. Glutamate transporter activity and immunocontents (GLT-1 and GLAST) were assessed in the hippocampus at 12, 24, and 48 h after the seizure episode. Fluorescence intensity for glial cells (GFAP) and neurons (NeuN) was assessed 24 h after seizure in the hippocampus. Behavioral analysis (elevated-plus maze and inhibitory avoidance memory task) was performed at 60 days of age.

RESULTS

The D group showed a decrease in glutamate uptake 24 h after seizure. In this group only, the GLT1 content increased at 12 h, followed by a decrease at 24 h. GLAST increased up to 24 h after seizure. GFAP fluorescence was higher, and NeuN fluorescence decreased, in the D group independent of seizures. In adulthood, the D group presented memory deficits independent of seizures, but short-term memory (1.5 h after a training session) was abolished in the D group treated with kainate.

SIGNIFICANCE

N-3 PUFA positively influenced the glutamatergic system during seizure and prevented seizure-related memory deficits in adulthood.

Reduced Interhemispheric Coherence after Cerebellar Vermis Output Perturbation

Motor coordination and motor learning are well-known roles of the cerebellum. Recent evidence also supports the contribution of the cerebellum to the oscillatory activity of brain networks involved in a wide range of disorders. Kainate, a potent analog of the excitatory neurotransmitter glutamate, can be used to induce dystonia, a neurological movement disorder syndrome consisting of sustained or repetitive involuntary muscle contractions, when applied on the surface of the cerebellum. This research aims to study the interhemispheric cortical communication between the primary motor cortices after repeated kainate application on cerebellar vermis for five consecutive days, in mice. We recorded left and right primary motor cortices electrocorticograms and neck muscle electromyograms, and quantified the motor behavior abnormalities. The results indicated a reduced coherence between left and right motor cortices in low-frequency bands. In addition, we observed a phenomenon of long-lasting adaptation with a modification of the baseline interhemispheric coherence. Our research provides evidence that the cerebellum can control the flow of information along the cerebello-thalamo-cortical neural pathways and can influence interhemispheric communication. This phenomenon could function as a compensatory mechanism for impaired regional networks.

Melatonin Suppresses the Kainate Receptor-Mediated Excitation on Gonadotropin-Releasing Hormone Neurons in Female and Male Prepubertal Mice

Melatonin, a pineal gland secretion, is an amphiphilic neurohormone involved in the biological and physiologic regulation of bodily functions. Numerous studies have shown the effects of melatonin on the release of gonadotropins and their actions at one or several levels of the hypothalamic–pituitary–gonadal axis. However, direct melatonin action on gonadotropin-releasing hormone (GnRH) neurons and its mechanism of action remain unclear. Here, plasma melatonin levels were measured and the effect of melatonin on GnRH neurons was assessed using brain slice patch clamp techniques. The plasma melatonin levels in prepubertal mice were higher than those in the adults. Melatonin itself did not change the firing activity of GnRH neurons. Interestingly, the kainate receptor-mediated responses but not the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)- and N-methyl-D-aspartic acid (NMDA)-induced responses were suppressed by melatonin in both the voltage clamp and current clamp modes. The inhibitory effects of the kainate-induced response by melatonin tended to increase with higher melatonin concentrations and persisted in the presence of tetrodotoxin, a voltage-sensitive Na⁺ channel blocker, or luzindole, a non-selective melatonin receptor antagonist. However, the response was completely abolished by pretreatment with pertussis toxin. These results suggest that melatonin can regulate GnRH neuronal activities in prepubertal mice by partially suppressing the excitatory signaling mediated by kainate receptors through pertussis toxin-sensitive G-protein-coupled receptors.

Histological Correlates of Diffusion-Weighted Magnetic Resonance Microscopy in a Mouse Model of Mesial Temporal Lobe Epilepsy

Mesial temporal lobe epilepsy (MTLE) is the most common type of focal epilepsy. It is frequently associated with abnormal MRI findings, which are caused by underlying cellular, structural, and chemical changes at the micro-scale. In the current study, it is investigated to which extent these alterations correspond to imaging features detected by high resolution magnetic resonance imaging in the intrahippocampal kainate mouse model of MTLE. Fixed hippocampal and whole-brain sections of mouse brain tissue from nine animals under physiological and chronically epileptic conditions were examined using structural and diffusion-weighted MRI. Microstructural details were investigated based on a direct comparison with immunohistochemical analyses of the same specimen. Within the hippocampal formation, diffusion streamlines could be visualized corresponding to dendrites of CA1 pyramidal cells and granule cells, as well as mossy fibers and Schaffer collaterals. Statistically significant changes in diffusivities, fractional anisotropy, and diffusion orientations could be detected in tissue samples from chronically epileptic animals compared to healthy controls, corresponding to microstructural alterations (degeneration of pyramidal cells, dispersion of the granule cell layer, and sprouting of mossy fibers). The diffusion parameters were significantly correlated with histologically determined cell densities. These findings demonstrate that high-resolution diffusion-weighted MRI can resolve subtle microstructural changes in epileptic hippocampal tissue corresponding to histopathological features in MTLE.

Kainate receptor regulation of synaptic inhibition in the hippocampus

Kainate receptors (KARs) are glutamate-type receptors that mediate both canonical ionotropic currents and non-canonical metabotropic signalling. While KARs are expressed widely throughout the brain, synaptic KAR currents have only been recorded at a limited set of synapses, and the KAR currents that have been recorded are relatively small and slow, which has led to the question, what is the functional significance of KARs? While the KAR current itself is relatively modest, its impact on inhibition in the hippocampus can be profound. In the CA1 region of the hippocampus, presynaptic KAR activation bidirectionally regulates γ-aminobutyric acid (GABA) release in a manner that depends on the glutamate concentration; lower levels of glutamate facilitate GABA release via an ionotropic pathway, while higher levels of glutamate depress GABA release via a metabotropic pathway. Postsynaptic interneuron KAR activation increases spike frequency through an ionotropic current, which in turn can strengthen inhibition. In the CA3 region, postsynaptic KAR activation in pyramidal neurons also strengthens inhibition, but in this case through a metabotropic pathway which regulates the neuronal chloride gradient and hyperpolarizes the reversal potential for GABA (E_GABA ). Taken together, the evidence for KAR-mediated regulation of the strength of inhibition via pre- and postsynaptic mechanisms provides compelling evidence that KARs are ideally positioned to regulate excitation-inhibition balance - through sensing the excitatory tone and concomitantly tuning the strength of inhibition.

Zinc Modulates Olfactory Bulb Kainate Receptors

Kainate receptors (KARs) are glutamate receptors with ionotropic and metabotropic activity composed of the GluK1-GluK5 subunits. We previously reported that KARs modulate excitatory and inhibitory transmission in the olfactory bulb (OB). Zinc, which is highly concentrated in the OB, also appears to modulate OB synaptic transmission via actions at other ionotropic glutamate receptors (i.e., AMPA, NMDA). However, few reports of effects of zinc on recombinant and/or native KARs exist and none have involved the OB. In the present study, we investigated the effects of exogenously applied zinc on OB KARs expressed by mitral/tufted (M/T) cells. We found that 100 µM zinc inhibits currents evoked by various combinations of KAR agonists (kainate or SYM 2081) and the AMPA receptor antagonist SYM 2206. The greatest degree of zinc-mediated inhibition was observed with coapplication of zinc with the GluK1- and GluK2-preferring agonist SYM 2081 plus SYM 2206. This finding is consistent with prior reports of zinc's inhibitory effects on some recombinant (homomeric GluK1 and GluK2 and heteromeric GluK2/GluK4 and GluK2/GluK5) KARs, although potentiation of other (GluK3, GluK2/3) KARs has also been described. It is also of potential importance given our previously reported molecular data suggesting that OB neurons express relatively high levels of GluK1 and GluK2. Our present findings suggest that a physiologically relevant concentration of zinc modulates KARs expressed by M/T cells. As M/T cells are targets of zinc-containing olfactory sensory neurons, synaptically released zinc may influence odor information-encoding synaptic circuits in the OB via actions at KARs.

Design and Synthesis of 2,3- trans-Proline Analogues as Ligands for Ionotropic Glutamate Receptors and Excitatory Amino Acid Transporters

Development of pharmacological tools for the ionotropic glutamate receptors (iGluRs) is imperative for the study and understanding of the role and function of these receptors in the central nervous system. We report the synthesis of 18 analogues of (2 S,3 R)-2-carboxy-3-pyrrolidine acetic acid (3a), which explores the effect of introducing a substituent on the ε-carbon (3c-q). A new synthetic method was developed for the efficient synthesis of racemic 3a and applied to give expedited access to 13 racemic analogues of 3a. Pharmacological characterization was carried out at native iGluRs, cloned homomeric kainate receptors (GluK1-3), NMDA receptors (GluN1/GluN2A-D), and excitatory amino acid transporters (EAAT1-3). From the structure-activity relationship studies, several new ligands emerged, exemplified by triazole 3p-d1, GluK3-preferring (GluK1/GluK3 K_i ratio of 15), and the structurally closely related tetrazole 3q-s3-4 that displayed 4.4-100-fold preference as an antagonist for the GluN1/GluN2A receptor ( K_i = 0.61 μM) over GluN1/GluN2B-D ( K_i = 2.7-62 μM).

Ripple-related firing of identified deep CA1 pyramidal cells in chronic temporal lobe epilepsy in mice

OBJECTIVE

Temporal lobe epilepsy (TLE) is often associated with memory deficits. Reactivation of memory traces in the hippocampus occurs during sharp-wave ripples (SWRs; 140-250 Hz). To better understand the mechanisms underlying high-frequency oscillations and cognitive comorbidities in epilepsy, we evaluated how rigorously identified deep CA1 pyramidal cells (dPCs) discharge during SWRs in control and TLE mice.

METHODS

We used the unilateral intraamygdala kainate model of TLE in video-electroencephalography (EEG) verified chronically epileptic adult mice. Local field potential and single-cell recordings were performed using juxtacellular recordings from awake control and TLE mice resting on a spherical treadmill, followed by post hoc identification of the recorded cells.

RESULTS

Hippocampal SWRs in TLE mice occurred with increased intraripple frequency compared to control mice. The frequency of SWR events was decreased, whereas the overall frequency of SWRs, interictal epileptiform discharges, and high-frequency ripples (250-500 Hz) together was not altered. CA1 dPCs in TLE mice showed significantly increased firing during ripples as well as between the ripple events. The strength of ripple modulation of dPC discharges increased in TLE without alteration of the preferred phase of firing during the ripple waves.

SIGNIFICANCE

These juxtacellular electrophysiology data obtained from identified CA1 dPCs from chronically epileptic mice are in general agreement with recent findings indicating distortion of normal firing patterns during offline SWRs as a mechanism underlying deficits in memory consolidation in epilepsy. Because the primary seizure focus in our experiments was in the amygdala and we recorded from the CA1 region, these results are also in agreement with the presence of altered high-frequency oscillations in areas of secondary seizure spread.

Conditional Knock-out of mGluR5 from Astrocytes during Epilepsy Development Impairs High-Frequency Glutamate Uptake

Astrocyte expression of metabotropic glutamate receptor 5 (mGluR5) is consistently observed in resected tissue from patients with epilepsy and is equally prevalent in animal models of epilepsy. However, little is known about the functional signaling properties or downstream consequences of astrocyte mGluR5 activation during epilepsy development. In the rodent brain, astrocyte mGluR5 expression is developmentally regulated and confined in expression/function to the first weeks of life, with similar observations made in human control tissue. Herein, we demonstrate that mGluR5 expression and function dramatically increase in a mouse model of temporal lobe epilepsy. Interestingly, in both male and female mice, mGluR5 function persists in the astrocyte throughout the process of epileptogenesis following status epilepticus. However, mGluR5 expression and function are transient in animals that do not develop epilepsy over an equivalent time period, suggesting that patterns of mGluR5 expression may signify continuing epilepsy development or its resolution. We demonstrate that, during epileptogenesis, astrocytes reacquire mGluR5-dependent calcium transients following agonist application or synaptic glutamate release, a feature of astrocyte-neuron communication absent since early development. Finally, we find that the selective and conditional knock-out of mGluR5 signaling from astrocytes during epilepsy development slows the rate of glutamate clearance through astrocyte glutamate transporters under high-frequency stimulation conditions, a feature that suggests astrocyte mGluR5 expression during epileptogenesis may recapitulate earlier developmental roles in regulating glutamate transporter function.SIGNIFICANCE STATEMENT In development, astrocyte mGluR5 signaling plays a critical role in regulating structural and functional interactions between astrocytes and neurons at the tripartite synapse. Notably, mGluR5 signaling is a positive regulator of astrocyte glutamate transporter expression and function, an essential component of excitatory signaling regulation in hippocampus. After early development, astrocyte mGluR5 expression is downregulated, but reemerges in animal models of temporal lobe epilepsy (TLE) development and patient epilepsy samples. We explored the hypothesis that astrocyte mGluR5 reemergence recapitulates earlier developmental roles during TLE acquisition. Our work demonstrates that astrocytes with mGluR5 signaling during TLE development perform faster glutamate uptake in hippocampus, revealing a previously unexplored role for astrocyte mGluR5 signaling in hypersynchronous pathology.

Oscillatory Cortical Activity in an Animal Model of Dystonia Caused by Cerebellar Dysfunction

The synchronization of neuronal activity in the sensorimotor cortices is crucial for motor control and learning. This synchrony can be modulated by upstream activity in the cerebello-cortical network. However, many questions remain over the details of how the cerebral cortex and the cerebellum communicate. Therefore, our aim is to study the contribution of the cerebellum to oscillatory brain activity, in particular in the case of dystonia, a severely disabling motor disease associated with altered sensorimotor coupling. We used a kainic-induced dystonia model to evaluate cerebral cortical oscillatory activity and connectivity during dystonic episodes. We performed microinjections of low doses of kainic acid into the cerebellar vermis in mice and examined activities in somatosensory, motor and parietal cortices. We showed that repeated applications of kainic acid into the cerebellar vermis, for five consecutive days, generate reproducible dystonic motor behavior. No epileptiform activity was recorded on electrocorticogram (ECoG) during the dystonic postures or movements. We investigated the ECoG power spectral density and coherence between motor cortex, somatosensory and parietal cortices before and during dystonic attacks. During the baseline condition, we found a phenomenon of permanent adaptation with a change of baseline locomotor activity coupled to an ECoG gamma band increase in all cortices. In addition, after kainate administration, we observed an increase in muscular activity, but less signs of dystonia together with modulations of the ECoG power spectra with an increase in gamma band in motor, parietal and somatosensory cortices. Moreover, we found reduced coherence in all measured frequency bands between the motor cortex and somatosensory or parietal cortices compared to baseline. In conclusion, examination of cortical oscillatory activities in this animal model of chronic dystonia caused by cerebellar dysfunction reveals a disruption in the coordination of neuronal activity across the cortical sensorimotor/parietal network, which may underlie motor skill deficits.

Attenuating M-current suppression in vivo by a mutant Kcnq2 gene knock-in reduces seizure burden and prevents status epilepticus-induced neuronal death and epileptogenesis

OBJECTIVES

The M-current is a low-threshold voltage-gated potassium current generated by Kv7 subunits that regulates neural excitation. It is important to note that M-current suppression, induced by activation of Gq-coupled neurotransmitter receptors, can dynamically regulate the threshold of action-potential firing and firing frequency. Here we sought to directly examine whether M-current suppression is involved in seizures and epileptogenesis.

METHODS

Kv7.2 knock-in mice lacking the key protein kinase C (PKC) phosphorylation acceptor site for M-current suppression were generated by introducing an alanine substitution at serine residue 559 of mouse Kv7.2, mKv7.2(S559A). Basic electrophysiologic properties of the M-current between wild-type and Kv7.2(S559A) knock-in mice were analyzed in primary cultured neurons. Homozygous Kv7.2(S559A) knock-in mice were used to evaluate the protective effect of mutant Kv7.2 channel against chemoconvulsant-induced seizures. In addition, pilocarpine-induced neuronal damage and spontaneously recurrent seizures were evaluated after equivalent chemoconvulsant-induced status epilepticus was achieved by coadministration of the M-current-specific channel inhibitor, XE991.

RESULT

Neurons from Kv7.2(S559A) knock-in mice showed normal basal M-currents. Knock-in mice displayed reduced M-current suppression when challenged by a muscarinic agonist, oxotremorine-M. Kv7.2(S559A) mice were resistant to chemoconvulsant-induced seizures with no mortality. Administration of XE991 transiently exacerbated seizures in knock-in mice equivalent to those of wild-type mice. Valproate, which disrupts neurotransmitter-induced M-current suppression, showed no additional anticonvulsant effect in Kv7.2(S559A) mice. After experiencing status epilepticus, Kv7.2(S559A) knock-in mice did not show seizure-induced cell death or spontaneous recurring seizures.

SIGNIFICANCE

This study provides evidence that neurotransmitter-induced suppression of M-current generated by Kv7.2-containing channels exacerbates behavioral seizures. In addition, prompt recovery of M-current after status epilepticus prevents subsequent neuronal death and the development of spontaneously recurrent seizures. Therefore, prompt restoration of M-current activity may have a therapeutic benefit for epilepsy.

Position- and Time-Dependent Arc Expression Links Neuronal Activity to Synaptic Plasticity During Epileptogenesis

In mesial temporal lobe epilepsy (mTLE) an initial precipitating injury can trigger aberrant wiring of neuronal circuits causing seizure activity. While circuit reorganization is known to be largely activity-dependent, the interactions between neuronal activity and synaptic plasticity during the development of mTLE remain poorly understood. Therefore, the present study aimed at delineating the spatiotemporal relationship between epileptic activity, activity-dependent gene expression and synaptic plasticity during kainic acid-induced epileptogenesis in mice. We show that during epileptogenesis the sclerotic hippocampus differed from non-sclerotic regions by displaying a consistently lower power of paroxysmal discharges. However, the power of these discharges steadily increased during epileptogenesis. This increase was paralleled by the upregulation of the activity-related cytoskeleton protein (Arc) gene expression in dentate granule cells (DGCs) of the sclerotic hippocampus. Importantly, we found that Arc mRNA-upregulating DGCs exhibited increased spine densities and spine sizes, but at the same time decreased AMPA-type glutamate receptor (AMPAR) densities. Finally, we show that in vivo optogenetic stimulation of DGC synapses evoked robust seizure activity in epileptic mice, but failed to induce dendritic translocation of Arc mRNA as under healthy conditions, supporting the theory of a breakdown of the dentate gate in mTLE. We conclude that during epileptogenesis epileptic activity emerges early and persists in the whole hippocampus, however, only the sclerotic part shows modulation of discharge amplitudes accompanied by plasticity of DGCs. In this context, we identified Arc as a putative mediator between seizure activity and synaptic plasticity.

Status Epilepticus: Behavioral and Electroencephalography Seizure Correlates in Kainate Experimental Models

Various etiological factors, such as head injury, chemical intoxication, tumors, and gene mutation, can induce epileptogenesis. In animal models, status epilepticus (SE) triggers epileptogenesis. In humans, convulsive SE for >30 min can be a life-threatening medical emergency. The duration and severity of convulsive SE are highly variable in chemoconvulsant animal models. A continuous video-electroencephalography (EEG) recording, and/or diligent direct observation, facilitates quantification of exact duration of different stages of convulsive seizures (Racine stages 3–5) to determine the severity of SE. A continuous convulsive SE for >30 min usually causes high mortality in some rodents and results in widespread brain damage in the surviving animals, in spite of treating with antiepileptic drugs (AEDs). AEDs control behavioral seizures but not EEG seizures. The severity of initial SE impacts epileptogenesis and cognitive function; therefore, quantitative assessment of behavioral SE and EEG in animal models will help to understand the impact of SE severity on epileptogenesis. There are several excellent reviews on experimental models of seizure/SE/epilepsy. This review focusses on the comparison of induction and characterization of behavioral SE and EEG correlates in mice and rats induced by kainate. We also discuss the advantages of repeated low dose of kainate (i.p. route), which minimizes variability in the initial SE severity between animals and reduces mortality rate. A refined approach to induce SE with kainate also addresses the two of the 3Rs (i.e., refinement and reduction), the guiding principles for ethical and scientific standpoint of animal research.

The Glutamatergic System in Primary Somatosensory Neurons and Its Involvement in Sensory Input-Dependent Plasticity

Glutamate is the most common neurotransmitter in both the central and the peripheral nervous system. Glutamate is present in all types of neurons in sensory ganglia, and is released not only from their peripheral and central axon terminals but also from their cell bodies. Consistently, these neurons express ionotropic and metabotropic receptors, as well as other molecules involved in the synthesis, transport and release of the neurotransmitter. Primary sensory neurons are the first neurons in the sensory channels, which receive information from the periphery, and are thus key players in the sensory transduction and in the transmission of this information to higher centers in the pathway. These neurons are tightly enclosed by satellite glial cells, which also express several ionotropic and metabotropic glutamate receptors, and display increases in intracellular calcium accompanying the release of glutamate. One of the main interests in our group has been the study of the implication of the peripheral nervous system in sensory-dependent plasticity. Recently, we have provided novel evidence in favor of morphological changes in first- and second-order neurons of the trigeminal system after sustained alterations of the sensory input. Moreover, these anatomical changes are paralleled by several molecular changes, among which those related to glutamatergic neurotransmission are particularly relevant. In this review, we will describe the state of the art of the glutamatergic system in sensory ganglia and its involvement in input-dependent plasticity, a fundamental ground for advancing our knowledge of the neural mechanisms of learning and adaptation, reaction to injury, and chronic pain.

Glial source of nitric oxide in epileptogenesis: A target for disease modification in epilepsy

Epileptogenesis is the process of developing an epileptic condition and/or its progression once it is established. The molecules that initiate, promote, and propagate remarkable changes in the brain during epileptogenesis are emerging as targets for prevention/treatment of epilepsy. Epileptogenesis is a continuous process that follows immediately after status epilepticus (SE) in animal models of acquired temporal lobe epilepsy (TLE). Both SE and epileptogenesis are potential therapeutic targets for the discovery of anticonvulsants and antiepileptogenic or disease-modifying agents. For translational studies, SE targets are appropriate for screening anticonvulsive drugs prior to their advancement as therapeutic agents, while targets of epileptogenesis are relevant for identification and development of therapeutic agents that can either prevent or modify the disease or its onset. The acute seizure models do not reveal antiepileptogenic properties of anticonvulsive drugs. This review highlights the important components of epileptogenesis and the long-term impact of intervening one of these components, nitric oxide (NO), in rat and mouse kainate models of TLE. NO is a putative pleotropic gaseous neurotransmitter and an important contributor of nitro-oxidative stress that coexists with neuroinflammation and epileptogenesis. The long-term impact of inhibiting the glial source of NO during early epileptogenesis in the rat model of TLE is reviewed. The importance of sex as a biological variable in disease modification strategies in epilepsy is also briefly discussed.

Early tissue damage and microstructural reorganization predict disease severity in experimental epilepsy

Mesial temporal lobe epilepsy (mTLE) is the most common focal epilepsy in adults and is often refractory to medication. So far, resection of the epileptogenic focus represents the only curative therapy. It is unknown whether pathological processes preceding epilepsy onset are indicators of later disease severity. Using longitudinal multi-modal MRI, we monitored hippocampal injury and tissue reorganization during epileptogenesis in a mouse mTLE model. The prognostic value of MRI biomarkers was assessed by retrospective correlations with pathological hallmarks Here, we show for the first time that the extent of early hippocampal neurodegeneration and progressive microstructural changes in the dentate gyrus translate to the severity of hippocampal sclerosis and seizure burden in chronic epilepsy. Moreover, we demonstrate that structural MRI biomarkers reflect the extent of sclerosis in human hippocampi. Our findings may allow an early prognosis of disease severity in mTLE before its first clinical manifestations, thus expanding the therapeutic window.

Synaptic Remodeling of Entorhinal Input Contributes to an Aberrant Hippocampal Network in Temporal Lobe Epilepsy

The hippocampus is reciprocally connected with the entorhinal cortex. Although several studies emphasized a role for the entorhinal cortex in mesial temporal lobe epilepsy (MTLE), it remains uncertain whether its synaptic connections with the hippocampus are altered. To address this question, we traced hippocampo-entorhinal and entorhino-hippocampal projections, assessed their connectivity with the respective target cells and examined functional alterations in a mouse model for MTLE. We show that hippocampal afferents to the dorsal entorhinal cortex are lost in the epileptic hippocampus. Conversely, entorhino-dentate projections via the medial perforant path (MPP) are preserved, but appear substantially altered on the synaptic level. Confocal imaging and 3D-reconstruction revealed that new putative contacts are established between MPP fibers and dentate granule cells (DGCs). Immunohistochemical identification of pre- and postsynaptic elements indicated that these contacts are functionally mature synapses. On the ultrastructural level, pre- and postsynaptic compartments of MPP synapses were strongly enlarged. The length and complexity of postsynaptic densities were also increased pointing to long-term potentiation-related morphogenesis. Finally, whole-cell recordings of DGCs revealed an enhancement of evoked excitatory postsynaptic currents. In conclusion, the synaptic rearrangement of excitatory inputs to DGCs from the medial entorhinal cortex may contribute to the epileptogenic circuitry in MTLE.

Molecular Mechanism of AMPA Receptor Modulation by TARP/Stargazin

AMPA receptors (AMPARs) mediate the majority of fast excitatory transmission in the brain and critically contribute to synaptic plasticity and pathology. AMPAR trafficking and gating are tightly controlled by auxiliary transmembrane AMPAR regulatory proteins (TARPs). Here, using systematic domain swaps with the TARP-insensitive kainate receptor GluK2, we show that AMPAR interaction with the prototypical TARP stargazin/γ2 primarily involves the AMPAR membrane domains M1 and M4 of neighboring subunits, initiated or stabilized by the AMPAR C-tail, and that these interactions are sufficient to enable full receptor modulation. Moreover, employing TARP chimeras disclosed a key role in this process also for the TARP transmembrane domains TM3 and TM4 and extracellular loop 2. Mechanistically, our data support a two-step action in which binding of TARP to the AMPAR membrane domains destabilizes the channel closed state, thereby enabling an efficient opening upon agonist binding, which then stabilizes the open state via subsequent interactions.

A study of cannabinoid-1 receptors during the early phase of excitotoxic damage to rat spinal locomotor networks in vitro

Endocannabinoids acting on cannabinoid-1 receptors (CB1Rs) are proposed to protect brain and spinal neurons from excitotoxic damage. The ability to recover from spinal cord injury (SCI), in which excitotoxicity is a major player, is usually investigated at late times after modulation of CB1Rs whose role in the early phases of SCI remains unclear. Using the rat spinal cord in vitro as a model for studying SCI initial pathophysiology, we investigated if agonists or antagonists of CB1Rs might affect SCI induced by the excitotoxic agent kainate (KA) within 24h from a transient (1h) application of this glutamate agonist. The CB1 agonist anandamide (AEA or pharmacological block of its degradation) did not limit excitotoxic depolarization of spinal networks: cyclic adenosine monophosphate (cAMP) assay demonstrated that CB1Rs remained functional 24h later and similarly expressed among dead or survived cells. Locomotor-like network activity recorded from ventral roots could not recover with such treatments and was associated with persistent depression of synaptic transmission. Motoneurons, that are particularly vulnerable to KA, were not protected by AEA. Application of 2-arachidonoylglycerol also did not attenuate the electrophysiological and histological damage. The intensification of damage by the CB1 antagonist AM251 suggested that endocannabinoids were operative after excitotoxic stimulation, yet insufficient to contrast it efficiently. The present data indicate that the early phases of excitotoxic SCI could not be arrested by pharmacologically exploiting the endocannabinoid system, consistent with the notion that AEA and its derivatives are more useful to treat late SCI phases.

Seizure-Induced Motility of Differentiated Dentate Granule Cells Is Prevented by the Central Reelin Fragment

Granule cell dispersion (GCD) represents a pathological widening of the granule cell layer in the dentate gyrus and it is frequently observed in patients with mesial temporal lobe epilepsy (MTLE). Recent studies in human MTLE specimens and in animal epilepsy models have shown that a decreased expression and functional inactivation of the extracellular matrix protein Reelin correlates with GCD formation, but causal evidence is still lacking. Here, we used unilateral kainate (KA) injection into the mouse hippocampus, an established MTLE animal model, to precisely map the loss of reelin mRNA-synthesizing neurons in relation to GCD along the septotemporal axis of the epileptic hippocampus. We show that reelin mRNA-producing neurons are mainly lost in the hilus and that this loss precisely correlates with the occurrence of GCD. To monitor GCD formation in real time, we used organotypic hippocampal slice cultures (OHSCs) prepared from mice which express enhanced green fluorescent protein (eGFP) primarily in differentiated dentate granule cells. Using life cell microscopy we observed that increasing doses of KA resulted in an enhanced motility of eGFP-positive granule cells. Moreover, KA treatment of OHSC resulted in a rapid loss of Reelin-producing interneurons mainly in the hilus, as observed in vivo. A detailed analysis of the migration behavior of individual eGFP-positive granule cells revealed that the majority of these neurons actively migrate toward the hilar region, where Reelin-producing neurons are lost. Treatment with KA and subsequent addition of the recombinant R3–6 Reelin fragment significantly prevented the movement of eGFP-positive granule cells. Together, these findings suggest that GCD formation is indeed triggered by a loss of Reelin in hilar interneurons.

SIRT5 Deficiency Enhances Susceptibility to Kainate-Induced Seizures and Exacerbates Hippocampal Neurodegeneration not through Mitochondrial Antioxidant Enzyme SOD2

Epilepsy is a common and serious neurological disorder characterized by occurrence of recurrent spontaneous seizures, and emerging evidences support the association of mitochondrial dysfunction with epilepsy. Sirtuin 5 (SIRT5), localized in mitochondrial matrix, has been considered as an important functional modulator of mitochondria that contributes to ageing and neurological diseases. Our data shows that SIRT5 deficiency strikingly increased mortality rate and severity of response to epileptic seizures, dramatically exacerbated hippocampal neuronal loss and degeneration in mice exposed to Kainate (KA), and triggered more severe reactive astrogliosis. We found that the expression of mitochondrial SIRT5 of injured hippocampus was relatively up-regulated, indicating its potential contribution to the comparably increased survival of these cells and its possible neuroprotective role. Unexpectedly, SIRT5 seems not to apparently alter the decline of antioxidant enzymes superoxide dismutase 2 (SOD2) and glutathione peroxidase (GPx) in hippocampus caused by KA exposure in our paradigm, which indicates the protective role of SIRT5 on seizures and cellular degeneration might through different regulatory mechanism that would be explored in the future. In the present study, we provided strong evidences for the first time to demonstrate the association between SIRT5 and epilepsy, which offers a new understanding of the roles of SIRT5 in mitochondrial functional regulation. The neuroprotection of SIRT5 in KA-induced epileptic seizure and neurodegeneration will improve our current knowledge of the nature of SIRT5 in central nervous system (CNS) and neurological diseases.

Mossy fiber sprouting and pyramidal cell dispersion in the hippocampal CA2 region in a mouse model of temporal lobe epilepsy

Dentate granule cells and the hippocampal CA2 region are resistant to cell loss associated with mesial temporal lobe epilepsy (MTLE). It is known that granule cells undergo mossy fiber sprouting in the dentate gyrus which contributes to a recurrent, proepileptogenic circuitry in the hippocampus. Here it is shown that mossy fiber sprouting also targets CA2 pyramidal cell somata and that the CA2 region undergoes prominent structural reorganization under epileptic conditions. Using the intrahippocampal kainate mouse model for MTLE and the CA2-specific markers Purkinje cell protein 4 (PCP4) and regulator of G-Protein signaling 14 (RGS14), it was found that during epileptogenesis CA2 neurons survive and disperse in direction of CA3 and CA1 resulting in a significantly elongated CA2 region. Using transgenic mice that express enhanced green fluorescent protein (eGFP) in granule cells and mossy fibers, we show that the recently described mossy fiber projection to CA2 undergoes sprouting resulting in aberrant large, synaptoporin-expressing mossy fiber boutons which surround the CA2 pyramidal cell somata. This opens up the potential for altered synaptic transmission that might contribute to epileptic activity in CA2. Indeed, intrahippocampal recordings in freely moving mice revealed that epileptic activity occurs concomitantly in the dentate gyrus and in CA2. Altogether, the results call attention to CA2 as a region affected by MTLE-associated pathological restructuring.

Role of brain glycogen in the response to hypoxia and in susceptibility to epilepsy

Although glycogen is the only carbohydrate reserve of the brain, its overall contribution to brain functions remains unclear. It has been proposed that glycogen participates in the preservation of such functions during hypoxia. Several reports also describe a relationship between brain glycogen and susceptibility to epilepsy. To address these issues, we used our brain-specific Glycogen Synthase knockout (GYS1^Nestin-KO) mouse to study the functional consequences of glycogen depletion in the brain under hypoxic conditions and susceptibility to epilepsy. GYS1^Nestin-KO mice presented significantly different power spectra of hippocampal local field potentials (LFPs) than controls under hypoxic conditions. In addition, they showed greater excitability than controls for paired-pulse facilitation evoked at the hippocampal CA3–CA1 synapse during experimentally induced hypoxia, thereby suggesting a compensatory switch to presynaptic mechanisms. Furthermore, GYS1^Nestin-KO mice showed greater susceptibility to hippocampal seizures and myoclonus following the administration of kainate and/or a brief train stimulation of Schaffer collaterals. We conclude that brain glycogen could play a protective role both in hypoxic situations and in the prevention of brain seizures.

Ontogeny of kainate-induced gamma oscillations in the rat CA3 hippocampus in vitro

GABAergic inhibition, which is instrumental in the generation of hippocampal gamma oscillations, undergoes significant changes during development. However, the development of hippocampal gamma oscillations remains largely unknown. Here, we explored the developmental features of kainate-induced oscillations (KA-Os) in CA3 region of rat hippocampal slices. Up to postnatal day P5, the bath application of kainate failed to evoke any detectable oscillations. KA-Os emerged by the end of the first postnatal week; these were initially weak, slow (20-25 Hz, beta range) and were poorly synchronized with CA3 units and synaptic currents. Local field potential (LFP) power, synchronization of units and frequency of KA-Os increased during the second postnatal week to attain gamma (30-40 Hz) frequency by P15-21. Both beta and gamma KA-Os are characterized by alternating sinks and sources in the pyramidal cell layer, likely generated by summation of the action potential-associated currents and GABAergic synaptic currents, respectively. Blockade of GABA(A) receptors with gabazine completely suppressed KA-Os at all ages indicating that GABAergic mechanisms are instrumental in their generation. Bumetanide, a NKCC1 chloride co-transporter antagonist which renders GABAergic responses inhibitory in the immature hippocampal neurons, failed to induce KA-Os at P2-4 indicating that the absence of KA-Os in neonates is not due to depolarizing actions of GABA. The linear developmental profile, electrographic features and pharmacological properties indicate that CA3 hippocampal beta and gamma KA-Os are fundamentally similar in their generative mechanisms and their delayed onset and developmental changes likely reflect the development of perisomatic GABAergic inhibition.

KCC2 activity is critical in limiting the onset and severity of status epilepticus

The K(+)/Cl(-) cotransporter (KCC2) allows adult neurons to maintain low intracellular Cl(-) levels, which are a prerequisite for efficient synaptic inhibition upon activation of γ-aminobutyric acid receptors. Deficits in KCC2 activity are implicated in epileptogenesis, but how increased neuronal activity leads to transporter inactivation is ill defined. In vitro, the activity of KCC2 is potentiated via phosphorylation of serine 940 (S940). Here we have examined the role this putative regulatory process plays in determining KCC2 activity during status epilepticus (SE) using knockin mice in which S940 is mutated to an alanine (S940A). In wild-type mice, SE induced by kainate resulted in dephosphorylation of S940 and KCC2 internalization. S940A homozygotes were viable and exhibited comparable basal levels of KCC2 expression and activity relative to WT mice. However, exposure of S940A mice to kainate induced lethality within 30 min of kainate injection and subsequent entrance into SE. We assessed the effect of the S940A mutation in cultured hippocampal neurons to explore the mechanisms underlying this phenotype. Under basal conditions, the mutation had no effect on neuronal Cl(-) extrusion. However, a selective deficit in KCC2 activity was seen in S940A neurons upon transient exposure to glutamate. Significantly, whereas the effects of glutamate on KCC2 function could be ameliorated in WT neurons with agents that enhance S940 phosphorylation, this positive modulation was lost in S940A neurons. Collectively our results suggest that phosphorylation of S940 plays a critical role in potentiating KCC2 activity to limit the development of SE.

Augmentation of Ca(2+) signaling in astrocytic endfeet in the latent phase of temporal lobe epilepsy

Astrocytic endfeet are specialized cell compartments whose important homeostatic roles depend on their enrichment of water and ion channels anchored by the dystrophin associated protein complex (DAPC). This protein complex is known to disassemble in patients with mesial temporal lobe epilepsy and in the latent phase of experimental epilepsies. The mechanistic underpinning of this disassembly is an obvious target of future therapies, but remains unresolved. Here we show in a kainate model of temporal lobe epilepsy that astrocytic endfeet display an enhanced stimulation-evoked Ca(2+) signal that outlast the Ca(2+) signal in the cell bodies. While the amplitude of this Ca(2+) signal is reduced following group I/II metabotropic receptor (mGluR) blockade, the duration is sustained. Based on previous studies it has been hypothesized that the molecular disassembly in astrocytic endfeet is caused by dystrophin cleavage mediated by Ca(2+) dependent proteases. Using a newly developed genetically encoded Ca(2+) sensor, the present study bolsters this hypothesis by demonstrating long-lasting, enhanced stimulation-evoked Ca(2+) signals in astrocytic endfeet.

Overexpression of C-terminal fragment of glutamate receptor 6 prevents neuronal injury in kainate-induced seizure via disassembly of GluR6-PSD-95-MLK3 signaling module

Our previous study showed that when glutamate receptor (GluR)6 C terminus-containing peptide conjugated with the human immunodeficiency virus Tat protein (GluR6)-9c is delivered into hippocampal neurons in a brain ischemic model, the activation of mixed lineage kinase 3 (MLK3) and c-Jun NH2-terminal kinase (JNK) is inhibited via GluR6-postsynaptic density protein 95 (PSD95). In the present study, we investigated whether the recombinant adenovirus (Ad) carrying GluR6c could suppress the assembly of the GluR6-PSD95-MLK3 signaling module and decrease neuronal cell death induced by kainate in hippocampal CA1 subregion. A seizure model in Sprague-Dawley rats was induced by intraperitoneal injections of kainate. The effect of Ad-Glur6-9c on the phosphorylation of JNK, MLK3 and mitogen-activated kinase kinase 7 (MKK7) was observed with western immunoblots and immunohistochemistry. Our findings revealed that overexpression of GluR6c inhibited the interaction of GluR6 with PSD95 and prevented the kainate-induced activation of JNK, MLK3 and MKK7. Furthermore, kainate-mediated neuronal cell death was significantly suppressed by GluR6c. Taken together, GluR6 may play a pivotal role in neuronal cell death.

Acquisition of conditioned fear is followed by region-specific changes in RNA editing of glutamate receptors

Adenosine (A) to inosine (I) RNA editing is a post-transcriptional modification process that can affect synaptic function. Transcripts encoding the kainate GRIK1 and AMPA GluA2 glutamate receptor subunits undergo editing that leads to a glycine/arginine (Q/R) exchange and reduced Ca(2+) permeability. We hypothesized that editing at these sites could be experience-dependent, temporally dynamic and region-specific. We trained C57/Bl6 mice in trace and contextual fear conditioning protocols, and examined editing levels at GRIK1 and GluA2 Q/R sites in the amygdala (CeA) and hippocampus (CA1 and CA3), at two time points after training. We also examined experience-dependent changes in the expression of RNA editing enzymes and editing targets. Animals trained in the trace fear conditioning protocol exhibited a transient increase in unedited GRIK1 RNA in the amygdala, and their learning efficiency correlated with unedited RNA levels in CA1. In line with previous reports, GluA2 RNA editing levels were nearly 100%. Additionally, we observed experience-dependent changes in mRNA expression of the RNA editing enzymes ADAR2 and ADAR1 in amygdala and hippocampus, and a learning-dependent increase in the alternatively spliced inactive form of ADAR2 in the amygdala. Since unedited transcripts code for Ca(2+)-permeable receptor subunits, these findings suggest that RNA editing at Q/R sites of glutamate receptors plays an important role in experience-dependent synaptic modification processes.

GABA-A and NMDA receptor subunit mRNA expression is altered in the caudate but not the putamen of the postmortem brains of alcoholics

Chronic consumption of alcohol by humans has been shown to lead to impairment of executive and cognitive functions. Here, we have studied the mRNA expression of ion channel receptors for glutamate and GABA in the dorsal striatum of post-mortem brains from alcoholics (n = 29) and normal controls (n = 29), with the focus on the caudate nucleus that is associated with the frontal cortex executive functions and automatic thinking and on the putamen area that is linked to motor cortices and automatic movements. The results obtained by qPCR assay revealed significant changes in the expression of specific excitatory ionotropic glutamate and inhibitory GABA-A receptor subunit genes in the caudate but not the putamen. Thus, in the caudate we found reduced levels of mRNAs encoding the GluN2A glutamate receptor and the δ, ε, and ρ2 GABA-A receptor subunits, and increased levels of the mRNAs encoding GluD1, GluD2, and GABA-A γ1 subunits in the alcoholics as compared to controls. Interestingly in the controls, 11 glutamate and 5 GABA-A receptor genes were more prominently expressed in the caudate than the putamen (fold-increase varied from 1.24 to 2.91). Differences in gene expression patterns between the striatal regions may underlie differences in associated behavioral outputs. Our results suggest an altered balance between caudate-mediated voluntarily controlled and automatic behaviors in alcoholics, including diminished executive control on goal-directed alcohol-seeking behavior.

Adenosine kinase, glutamine synthetase and EAAT2 as gene therapy targets for temporal lobe epilepsy

Astrocytes are an attractive cell target for gene therapy, but the validation of new therapeutic candidates is needed. We determined whether adeno-associated viral (AAV) vector-mediated overexpression of glutamine synthetase (GS) or excitatory amino-acid transporter 2 (EAAT2), or expression of microRNA targeting adenosine kinase (miR-ADK) in hippocampal astrocytes in the rat brain could modulate susceptibility to kainate-induced seizures and neuronal cell loss. Transgene expression was found predominantly in astrocytes following direct injection of glial-targeting AAV9 vectors by 3 weeks postinjection. ADK expression in miR-ADK vector-injected rats was reduced by 94-96% and was associated with an ~50% reduction in the duration of kainate-induced seizures and greater protection of dentate hilar neurons but not CA3 neurons compared with miR-control vector-injected rats. In contrast, infusion of AAV-GS and EAAT2 vectors did not afford any protection against seizures or neuronal damage as the level of transcriptional activity of the glial fibrillary acidic promoter was too low to drive any significant increase in transgenic GS or EAAT2 relative to the high endogenous levels of these proteins. Our findings support ADK as a prime therapeutic target for gene therapy of temporal lobe epilepsy and suggest that alternative approaches including the use of stronger glial promoters are needed to increase transgenic GS and EAAT2 expression to levels that may be required to affect seizure induction and propagation.

The role of glutamate and its receptors in autism and the use of glutamate receptor antagonists in treatment

Glutamate is the major excitatory neurotransmitter in the brain and may be a key neurotransmitter involved in autism. Literature pertaining to glutamate and autism or related disorders (e.g., Fragile X syndrome) is reviewed in this article. Interest in glutamatergic dysfunction in autism is high due to increasing convergent evidence implicating the system in the disorder from peripheral biomarkers, neuroimaging, protein expression, genetics and animal models. Currently, there are no pharmaceutical interventions approved for autism that address glutamate deficits in the disorder. New treatments related to glutamatergic neurotransmission, however, are emerging. In addition, older glutamate-modulating medications with approved indications for use in other disorders are being investigated for re-tasking as treatments for autism. This review presents evidence in support of glutamate abnormalities in autism and the potential for translation into new treatments for the disorder.