Initial loss but later excess of GABAergic synapses with dentate granule cells in a rat model of temporal lobe epilepsy

Rat strain differences in seizure frequency and hilar neuron loss after systemic treatment with pilocarpine

At least 3 months after systemic treatment with pilocarpine to induce status epilepticus, Long-Evans and Sprague-Dawley rats were video-EEG monitored for seizures continuously for 1 month. Rats were then perfused, hippocampi were processed for Nissl staining, and hilar neurons were quantified. Seizure frequency in Long-Evans rats was 1/10th of that in Sprague-Dawley rats, and more variable. Hilar neuron loss was also less severe in Long-Evans rats. However, there was no correlation between hilar neuron loss and seizure frequency in either strain. The low and variable seizure frequency suggests limited usefulness of pilocarpine-treated Long-Evans rats for some epilepsy experiments.

Molecular Genetics of Acquired Temporal Lobe Epilepsy

An epilepsy diagnosis reduces a patient's quality of life tremendously, and it is a fate shared by over 50 million people worldwide. Temporal lobe epilepsy (TLE) is largely considered a nongenetic or acquired form of epilepsy that develops in consequence of neuronal trauma by injury, malformations, inflammation, or a prolonged (febrile) seizure. Although extensive research has been conducted to understand the process of epileptogenesis, a therapeutic approach to stop its manifestation or to reliably cure the disease has yet to be developed. In this review, we briefly summarize the current literature predominately based on data from excitotoxic rodent models on the cellular events proposed to drive epileptogenesis and thoroughly discuss the major molecular pathways involved, with a focus on neurogenesis-related processes and transcription factors. Furthermore, recent investigations emphasized the role of the genetic background for the acquisition of epilepsy, including variants of neurodevelopmental genes. Mutations in associated transcription factors may have the potential to innately increase the vulnerability of the hippocampus to develop epilepsy following an injury-an emerging perspective on the epileptogenic process in acquired forms of epilepsy.

Activation of hypoactive parvalbumin-positive fast-spiking interneuron restores dentate inhibition to prevent epileptiform activity in the mouse intrahippocampal kainate model of temporal lobe epilepsy

Parvalbumin-positive (PV+) GABAergic interneurons in the dentate gyrus provide powerful perisomatic inhibition of dentate granule cells (DGCs) to prevent overexcitation and maintain the stability of dentate gyrus circuits. Most dentate PV+ interneurons survive status epilepticus, but surviving PV+ interneuron mediated inhibition is compromised in the dentate gyrus shortly after status epilepticus, contributing to epileptogenesis in temporal lobe epilepsy. It is uncertain whether the impaired activity of dentate PV+ interneurons recovers at later times or if it continues for months following status epilepticus. The development of compensatory modifications related to PV+ interneuron circuits in the months following status epilepticus is unknown, although reduced dentate GABAergic inhibition persists long after status epilepticus. We employed PV immunostaining and whole-cell patch-clamp recordings from dentate PV+ interneurons and DGCs in slices from male and female sham controls and intrahippocampal kainate (IHK) treated mice that developed spontaneous seizures months after status epilepticus to study epilepsy-associated changes in dentate PV+ interneuron circuits. We found that the number of dentate PV+ cells was reduced in IHK treated mice. Electrical recordings showed that: 1) Action potential firing rates of dentate PV+ interneurons were reduced in IHK treated mice up to four months after status epilepticus; 2) Spontaneous inhibitory postsynaptic currents (sIPSCs) in DGCs exhibited reduced frequency but increased amplitude in IHK treated mice; and 3) The amplitude of evoked IPSCs in DGCs by optogenetic activation of dentate PV+ cells was upregulated without changes in short-term plasticity. Video-EEG recordings revealed that IHK treated mice showed spontaneous epileptiform activity in the dentate gyrus and that chemogenetic activation of PV+ interneurons abolished the epileptiform activity. Our results suggest not only that the compensatory changes in PV+ interneuron circuits develop after IHK treatment, but also that increased PV+ interneuron mediated inhibition in the dentate gyrus may compensate for cell loss and reduced intrinsic excitability of dentate PV+ interneurons to stop seizures in temporal lobe epilepsy.

Highlights

Reduced number of dentate PV+ interneurons in TLE micePersistently reduced action potential firing rates of dentate PV+ interneurons in TLE miceEnhanced amplitude but decreased frequency of spontaneous IPSCs in the dentate gyrus in TLE miceIncreased amplitude of evoked IPSCs mediated by dentate PV+ interneurons in TLE miceChemogenetic activation of PV+ interneurons prevents epileptiform activity in TLE mice.

Transient Seizure Clusters and Epileptiform Activity Following Widespread Bilateral Hippocampal Interneuron Ablation

Interneuron loss is a prominent feature of temporal lobe epilepsy in both animals and humans and is hypothesized to be critical for epileptogenesis. As loss occurs concurrently with numerous other potentially proepileptogenic changes, however, the impact of interneuron loss in isolation remains unclear. For the present study, we developed an intersectional genetic approach to induce bilateral diphtheria toxin-mediated deletion of Vgat-expressing interneurons from dorsal and ventral hippocampus. In a separate group of mice, the same population was targeted for transient neuronal silencing with DREADDs. Interneuron ablation produced dramatic seizure clusters and persistent epileptiform activity. Surprisingly, after 1 week seizure activity declined precipitously and persistent epileptiform activity disappeared. Occasional seizures (≈1/day) persisted to the end of the experiment at 4 weeks. In contrast to the dramatic impact of interneuron ablation, transient silencing produced large numbers of interictal spikes, a significant but modest increase in seizure occurrence and changes in EEG frequency band power. Taken together, findings suggest that the hippocampus regains relative homeostasis-with occasional breakthrough seizures-in the face of an extensive and abrupt loss of interneurons.

Preferential pruning of inhibitory synapses by microglia contributes to alteration of the balance between excitatory and inhibitory synapses in the hippocampus in temporal lobe epilepsy

BACKGROUND

A consensus has formed that neural circuits in the brain underlie the pathogenesis of temporal lobe epilepsy (TLE). In particular, the synaptic excitation/inhibition balance (E/I balance) has been implicated in shifting towards elevated excitation during the development of TLE.

METHODS

Sprague Dawley (SD) rats were intraperitoneally subjected to kainic acid (KA) to generate a model of TLE. Next, electroencephalography (EEG) recording was applied to verify the stability and detectability of spontaneous recurrent seizures (SRS) in rats. Moreover, hippocampal slices from rats and patients with mesial temporal lobe epilepsy (mTLE) were assessed using immunofluorescence to determine the alterations of excitatory and inhibitory synapses and microglial phagocytosis.

RESULTS

We found that KA induced stable SRSs 14 days after status epilepticus (SE) onset. Furthermore, we discovered a continuous increase in excitatory synapses during epileptogenesis, where the total area of vesicular glutamate transporter 1 (vGluT1) rose considerably in the stratum radiatum (SR) of cornu ammonis 1 (CA1), the stratum lucidum (SL) of CA3, and the polymorphic layer (PML) of the dentate gyrus (DG). In contrast, inhibitory synapses decreased significantly, with the total area of glutamate decarboxylase 65 (GAD65) in the SL and PML diminishing enormously. Moreover, microglia conducted active synaptic phagocytosis after the formation of SRSs, especially in the SL and PML. Finally, microglia preferentially pruned inhibitory synapses during recurrent seizures in both rat and human hippocampal slices, which contributed to the synaptic alteration in hippocampal subregions.

CONCLUSIONS

Our findings elaborately characterize the alteration of neural circuits and demonstrate the selectivity of synaptic phagocytosis mediated by microglia in TLE, which could strengthen the comprehension of the pathogenesis of TLE and inspire potential therapeutic targets for epilepsy treatment.

The role of dendritic spines in epileptogenesis

Epilepsy is a chronic central nervous system (CNS) disease associated with high morbidity. To date, there is no known disease-modifying therapy for epilepsy. A leading hypothesis for a mechanism of epileptogenesis is the generation of aberrant neuronal networks. Although the underlying biological mechanism is not clear, scientific evidence indicates that it is associated with a hyperexcitable synchronous neuronal network and active dendritic spine plasticity. Changes in dendritic spine morphology are related to altered expression of synaptic cytoskeletal proteins, inflammatory molecules, neurotrophic factors, and extracellular matrix signaling. However, it remains to be determined if these aberrant dendritic spine formations lead to neuronal hyperexcitability and abnormal synaptic connections or whether they constitute an underlying mechanism of seizure susceptibility. Focusing on dendritic spine machinery as a potential target for medications could limit or reverse the development of epilepsy.

Adult-born neurons in critical period maintain hippocampal seizures via local aberrant excitatory circuits

Temporal lobe epilepsy (TLE), one common type of medically refractory epilepsy, is accompanied with altered adult-born dentate granule cells (abDGCs). However, the causal role of abDGCs in recurrent seizures of TLE is not fully understood. Here, taking advantage of optogenetic and chemogenetic tools to selectively manipulate abDGCs in a reversible manner, combined with Ca²⁺ fiber photometry, trans-synaptic viral tracing, in vivo/vitro electrophysiology approaches, we aimed to test the role of abDGCs born at different period of epileptogenic insult in later recurrent seizures in mouse TLE models. We found that abDGCs were functionally inhibited during recurrent seizures. Optogenetic activation of abDGCs significantly extended, while inhibition curtailed, the seizure duration. This seizure-modulating effect was attributed to specific abDGCs born at a critical early phase after kindled status, which experienced specific type of circuit re-organization. Further, abDGCs extended seizure duration via local excitatory circuit with early-born granule cells (ebDGCs). Repeated modulation of "abDGC-ebDGC" circuit may easily induce a change of synaptic plasticity, and achieve long-term anti-seizure effects in both kindling and kainic acid-induced TLE models. Together, we demonstrate that abDGCs born at a critical period of epileptogenic insult maintain seizure duration via local aberrant excitatory circuits, and inactivation of these aberrant circuits can long-termly alleviate severity of seizures. This provides a deeper and more comprehensive understanding of the potential pathological changes of abDGCs circuit and may be helpful for the precise treatment in TLE.

In the fast lane: Receptor trafficking during status epilepticus

Status epilepticus (SE) remains a significant cause of morbidity and mortality and often is refractory to standard first-line treatments. A rapid loss of synaptic inhibition and development of pharmacoresistance to benzodiazepines (BZDs) occurs early during SE, while NMDA and AMPA receptor antagonists remain effective treatments after BZDs have failed. Multimodal and subunit-selective receptor trafficking within minutes to an hour of SE involves GABA-A, NMDA, and AMPA receptors and contributes to shifts in the number and subunit composition of surface receptors with differential impacts on the physiology, pharmacology, and strength of GABAergic and glutamatergic currents at synaptic and extrasynaptic sites. During the first hour of SE, synaptic GABA-A receptors containing γ2 subunits move to the cell interior while extrasynaptic GABA-A receptors with δ subunits are preserved. Conversely, NMDA receptors containing N2B subunits are increased at synaptic and extrasynaptic sites, and homomeric GluA1 ("GluA2-lacking") calcium permeant AMPA receptor surface expression also is increased. Molecular mechanisms, largely driven by NMDA receptor or calcium permeant AMPA receptor activation early during circuit hyperactivity, regulate subunit-specific interactions with proteins involved with synaptic scaffolding, adaptin-AP2/clathrin-dependent endocytosis, endoplasmic reticulum (ER) retention, and endosomal recycling. Reviewed here is how SE-induced shifts in receptor subunit composition and surface representation increase the excitatory to inhibitory imbalance that sustains seizures and fuels excitotoxicity contributing to chronic sequela such as "spontaneous recurrent seizures" (SRS). A role for early multimodal therapy is suggested both for treatment of SE and for prevention of long-term comorbidities.

Hippocampal transplants of fetal GABAergic progenitors regulate adult neurogenesis in mice with temporal lobe epilepsy

GABAergic interneurons play a role in regulating adult neurogenesis within the dentate gyrus (DG) of the hippocampus. Neurogenesis occurs within a stem cell niche in the subgranular zone (SGZ) of the DG. In this niche, populations of neural progenitors give rise to granule cells that migrate radially into the granule cell layer of the DG. Altered neurogenesis in temporal lobe epilepsy (TLE) is linked to a transient increase in the proliferation of new neurons and the abnormal inversion of Type 1 progenitors, resulting in ectopic migration of Type 3 progenitors into the hilus of the DG. These ectopic cells mature into granule cells in the hilus that become hyperexcitable and contribute to the development of spontaneous recurrent seizures. To test whether grafts of GABAergic cells in the DG restore synaptic inhibition, prior work focused on transplanting GABAergic progenitors into the hilus of the DG. This cell-based therapeutic approach was shown to alter the disease phenotype by ameliorating spontaneous seizures in mice with pilocarpine-induced TLE. Prior optogenetic and immunohistochemical studies demonstrated that the transplanted GABAergic interneurons increased levels of synaptic inhibition by establishing inhibitory synaptic contacts with adult-born granule cells, consistent with the observed suppression of seizures. Whether GABAergic progenitor transplantation into the DG ameliorates underlying abnormalities in adult neurogenesis caused by TLE is not known. As a first step to address this question, we compared the effects of GABAergic progenitor transplantation on Type 1, Type 2, and Type 3 progenitors in the stem cell niche using cell type-specific molecular markers in naïve, non-epileptic mice. The progenitor transplantation increased GABAergic interneurons in the DG and led to a significant reduction in Type 2 progenitors and a concomitant increase in Type 3 progenitors. Next, we compared the effects of GABAergic interneuron transplantation in epileptic mice. Transplantation of GABAergic progenitors resulted in reductions in inverted Type 1, Type 2, and hilar ectopic Type 3 cells, concomitant with an increase in the radial migration of Type 3 progenitors into the GCL (Granule Cell Layer). Thus, in mice with Pilocarpine induced TLE, hilar transplants of GABA interneurons may reverse abnormal patterns of adult neurogenesis, an outcome that may ameliorate seizures.

Tetrodotoxin prevents heat-shock induced granule cell dispersion in hippocampal slice cultures

Granule cell dispersion (GCD) has been associated as a pathological feature of temporal lobe epilepsy (TLE). Early-life epileptiform activity such as febrile seizures has been proposed to have a causal link to developing chronic TLE. During postnatal development, the hippocampus may be particularly vulnerable to hyperexcitability-induced insults since neuronal migration and differentiation are still ongoing in the hippocampus. Further, the extracellular matrix (ECM), here in particular the protein reelin, has been implicated in the pathophysiology of GCD. Thus, loss of reelin-expressing cells, Cajal-Retzius cells and subsets of interneurons, may be related to GCD. To study the possible role of febrile seizures, we previously induced GCD in vitro by subjecting hippocampal slice cultures to a transient heat-shock, which was not accompanied by loss of Cajal-Retzius cells. In order to examine the mechanisms involved in heat-shock induced GCD, the present study aimed to determine whether such dispersion could be prevented by blocking cellular electrical activity. Here we show that the extent of heat-shock induced GCD could be significantly reduced by treatment with the sodium channel blocker tetrodotoxin (TTX), suggesting that electrical activity is an important factor involved in heat-shock induced GCD.

Cannabinoid receptor 1-labeled boutons in the sclerotic dentate gyrus of epileptic sea lions

Pathology in the dentate gyrus, including sclerosis, is a hallmark of temporal lobe epilepsy, and reduced inhibition to dentate granule cells may contribute to epileptogenesis. The perisomatic-targeting axonal boutons of parvalbumin-expressing interneurons decrease in proportion with granule cells in temporal lobe epilepsy. In contrast, dendrite-targeting axonal boutons of somatostatin-expressing interneurons sprout exuberantly in temporal lobe epilepsy. A third major class of GABAergic interneurons expresses cannabinoid receptor type 1 (CB1) on their terminal boutons, but there is conflicting evidence as to whether these boutons are increased or decreased in temporal lobe epilepsy. Naturally occurring temporal lobe epilepsy in California sea lions, with unilateral or bilateral sclerosis, offers the benefit of neuroanatomy and neuropathology akin to humans, but with the advantage that the entirety of both hippocampi from control and epileptic brains can be studied. Stereological quantification in the dentate gyrus revealed that sclerotic hippocampi from epileptic sea lions had fewer CB1-labeled boutons than controls. However, the reduction in the number of granule cells was greater, resulting in increased CB1-labeled boutons per granule cell in sclerotic hippocampi at temporal levels. This suggests that although CB1-expressing boutons are decreased in sclerotic dentate gyri, surviving cells have enhanced innervation from these boutons in epileptic sea lions.

Non-invasive, neurotoxic surgery reduces seizures in a rat model of temporal lobe epilepsy

Surgery can be highly effective for treating certain cases of drug resistant epilepsy. The current study tested a novel, non-invasive, surgical strategy for treating seizures in a rat model of temporal lobe epilepsy. The surgical approach uses magnetic resonance-guided, low-intensity focused ultrasound (MRgFUS) in combination with intravenous microbubbles to open the blood-brain barrier (BBB) in a transient and focal manner. During the period of BBB opening, a systemically administered neurotoxin (Quinolinic Acid: QA) that is normally impermeable to the BBB gains access to a targeted area in the brain, destroying neurons where the BBB has been opened. This strategy is termed Precise Intracerebral Non-invasive Guided Surgery (PING). Spontaneous recurrent seizures induced by pilocarpine were monitored behaviorally prior to and after PING or under control conditions. Seizure frequency in untreated animals or animals treated with MRgFUS without QA exhibited expected seizure rate fluctuations frequencies between the monitoring periods. In contrast, animals treated with PING targeting the intermediate-temporal aspect of the hippocampus exhibited substantial reductions in seizure frequency, with convulsive seizures being eliminated entirely in two animals. These findings suggest that PING could provide a useful alternative to invasive surgical interventions for treating drug resistant epilepsy, and perhaps for treating other neurological disorders in which aberrant neural circuitries play a role.

Proteomic differences in the hippocampus and cortex of epilepsy brain tissue

Epilepsy is a common neurological disorder affecting over 70 million people worldwide, with a high rate of pharmaco-resistance, diverse comorbidities including progressive cognitive and behavioural disorders, and increased mortality from direct (e.g. sudden unexpected death in epilepsy, accidents, drowning) or indirect effects of seizures and therapies. Extensive research with animal models and human studies provides limited insights into the mechanisms underlying seizures and epileptogenesis, and these have not translated into significant reductions in pharmaco-resistance, morbidities or mortality. To help define changes in molecular signalling networks associated with seizures in epilepsy with a broad range of aetiologies, we examined the proteome of brain samples from epilepsy and control cases. Label-free quantitative mass spectrometry was performed on the hippocampal cornu ammonis 1-3 region (CA1-3), frontal cortex and dentate gyrus microdissected from epilepsy and control cases (n = 14/group). Epilepsy cases had significant differences in the expression of 777 proteins in the hippocampal CA1 - 3 region, 296 proteins in the frontal cortex and 49 proteins in the dentate gyrus in comparison to control cases. Network analysis showed that proteins involved in protein synthesis, mitochondrial function, G-protein signalling and synaptic plasticity were particularly altered in epilepsy. While protein differences were most pronounced in the hippocampus, similar changes were observed in other brain regions indicating broad proteomic abnormalities in epilepsy. Among the most significantly altered proteins, G-protein subunit beta 1 (GNB1) was one of the most significantly decreased proteins in epilepsy in all regions studied, highlighting the importance of G-protein subunit signalling and G-protein-coupled receptors in epilepsy. Our results provide insights into common molecular mechanisms underlying epilepsy across various aetiologies, which may allow for novel targeted therapeutic strategies.

Modulation of neuropathology and cognitive deficits by lipopolysaccharide preconditioning in a mouse pilocarpine model of status epilepticus

Recent studies indicate that microglia may play a critical role in epileptogenesis during the early post-status epilepticus (SE) period. In this study, we aimed to elucidate the effects of preconditioning of microglia with lipopolysaccharide (LPS) on neuropathology and cognitive deficits in a mouse pilocarpine model of SE. Mice were treated with an intraperitoneal injection of LPS 24 h before SE induction. The open field test at 13 days after SE showed that LPS preconditioning suppressed SE-induced hyperactivity. The Y-maze test at 14 days after SE showed that LPS preconditioning ameliorated SE-induced working memory impairment. The extent of neuronal damage was decreased by LPS preconditioning in the hippocampus of mice euthanized at 15 days after SE. Gene profile analysis of hippocampal microglia at 15 days after SE showed that the expression level of interleukin-1β was increased by SE induction but decreased by LPS preconditioning. By contrast, SE induction increased the expression levels of phagocytosis-related genes, and LPS preconditioning further enhanced their expression. Interestingly, LPS preconditioning increased the numerical density of hippocampal microglia expressing the 5D4 keratan sulfate epitope, a population of cells known to be involved in phagocytosis. The voxel density of glutamatergic synapses was increased by SE induction but decreased by LPS preconditioning, while GABAergic synapses were not affected by these procedures. Our findings indicate that LPS preconditioning may in part alleviate SE-related abnormal synaptogenesis and cognitive deficits, and also suggest that modulation of microglial activation during the early post-SE period may be a novel strategy for epilepsy treatment.

A Tisket a Tasket: Chandelier ≠ Basket (Cell Bouton Density in Sclerotic DG)

GABA bouton subpopulations in the human dentate gyrus are differentially altered in mesial temporal lobe epilepsy Alhourani A, Fish KN, Wozny TA, Sudhakar V, Hamilton RL, Richardson RM. J Neuro . 2020;123(4):392-406. doi:10.1152/jn.00523.2018 Medically intractable temporal lobe epilepsy is a devastating disease, for which surgical removal of the seizure-onset zone is the only known cure. Multiple studies have found evidence of abnormal dentate gyrus network circuitry in human mesial temporal lobe epilepsy (MTLE). Principal neurons within the dentate gyrus gate entorhinal input into the hippocampus provide a critical step in information processing. Crucial to that role are GABA-expressing neurons, particularly parvalbumin (PV)-expressing basket cells (PVBCs) and chandelier cells (PVChCs), which provide strong, temporally coordinated inhibitory signals. Alterations in PVBC and PVChC boutons have been described in epilepsy, but the value of these studies has been limited due to methodological hurdles associated with studying human tissue. We developed a multilabel immunofluorescence confocal microscopy and a custom segmentation algorithm to quantitatively assess PVBC and PVChC bouton densities and to infer relative synaptic protein content in the human dentate gyrus. Using en bloc specimens from MTLE subjects with and without hippocampal sclerosis, paired with nonepileptic controls, we demonstrate the utility of this approach for detecting cell-type specific synaptic alterations. Specifically, we found increased density of PVBC boutons, while PVChC boutons decreased significantly in the dentate granule cell layer of subjects with hippocampal sclerosis compared with matched controls. In contrast, bouton densities for either PV-positive cell type did not differ between epileptic subjects without sclerosis and matched controls. These results may explain conflicting findings from previous studies that have reported both preserved and decreased PV bouton densities and establish a new standard for quantitative assessment of interneuron boutons in epilepsy.

Induction of Temporal Lobe Epilepsy in Mice with Pilocarpine

In the pilocarpine model of temporal lobe epilepsy (TLE) in rodents, systemic injections of pilocarpine induce continuous, prolonged limbic seizures, a condition termed "Status Epilepticus" (SE). With appropriate doses, many inbred strains of mice show behavioral seizures within an hour after pilocarpine is injected. With the behavioral scoring system based on a modification of the original Racine scale, one can monitor the seizures behaviorally, as they develop into more prolonged seizures and SE. SE is typically associated with damage to subsets of hippocampal neurons and other structural changes in the hippocampus and generally subsides on its own. However, more precise control of the duration of SE is commonly achieved by injecting a benzodiazepine into the mouse 1 to 3 h after the onset of SE to suppress the seizures. Several days following pilocarpine-induced SE, electrographic and behavioral seizures begin to occur spontaneously. The goal of this protocol is to reliably generate mice that develop spontaneous recurrent seizures (SRS) and show the typical neuropathological changes in the brain characteristic of severe human mesial temporal lobe epilepsy (mTLE), without high mortality. To reduce mortality, multiple subthreshold injections of pilocarpine are administered, which increases the percentage of mice developing SE without concomitant mortality. Precise control of the duration of SE (1 or 3 h) is achieved by suppressing SE with the benzodiazepine Midazolam (Versed). We have found that this protocol is an efficient means for generating mice that subsequently develop characteristics of human mTLE including high-frequency interictal spike and wave activity and SRS. In addition, we and others have shown that this protocol produces mice that show excitotoxic cell death of subsets of hippocampal GABAergic interneurons, particularly in the dentate gyrus and compensatory sprouting of excitatory projections from dentate granule cells (mossy fiber sprouting). Aspects of this protocol have been described in several of our previous publications.

Chronic exercise buffers the cognitive dysfunction and decreases the susceptibility to seizures in PTZ-treated rats

Epilepsy is a serious neurological disorder posing a severe burden to our society. Cognitive deficits are very common comorbidities of epilepsy. It is known that enhanced cognition has been demonstrated as an indicator for successful treatment of epilepsy. Physical exercise shows a positive consequence on cognition in healthy individuals and improves health and life conditions in people with epilepsy. However, there is no direct evidence to determine the role and the potential mechanism of physical exercise on the cognitive impairment and the relationship of susceptibility to seizures. The goal of the current investigation was to explore whether sustained physical exercise improves the cognitive dysfunction and simultaneously decreases the susceptibility to seizures in rats with epilepsy. Rats were treated with pentylenetetrazole (PTZ) (35 mg/kg, i.p. [intraperitoneally]) for 36 days to induce chronic epilepsy. During the induction period, rats were exposed to voluntary wheel running or forced swimming 30 min prior to each PTZ injection from the 16th day. The cognition of rats was evaluated by object recognition test and passive avoidance test. The susceptibility to seizures was evaluated by seizure frequency and duration. The levels of synaptic-related proteins including PSD95 (postsynaptic density 95), Synapsin, GluA1, and BDNF (brain-derived neurotrophic factor) were measured to evaluate the hippocampal synaptic plasticity. Furthermore, the GAD67 (glutamic acid decarboxylase) levels and GABA (γ-aminobutyric acid)ergic function in PTZ-treated rats were also determined. Finally, antagonist of GABA_AR (GABA_A receptors) bicuculline was used to explore the reversal effects of physical activity on seizures and cognition. The results showed that rats subjected to voluntary wheel running or forced swimming showed a significant reduction of seizure frequency and duration in PTZ-treated group relative to rats without running or swimming. In addition, both running and swimming improved cognitive function as measured by enhanced performance in object recognition test and passive avoidance test. Furthermore, the reduced levels of synaptic-related proteins and GABAergic function were reversed by exercise compared with rats without exercise. Moreover, antagonism of hippocampal CA3 (cornu ammonis 3) GABAergic neurons blocks the reversal effects of physical activity on seizures and cognition in PTZ-treated rats. These data showed that chronic physical exercise reduced the frequency of seizures and improved the cognitive function in a rat model of chronic epilepsy through normalization of CA3 synaptic plasticity and GABAergic function. Our findings suggest that chronic physical exercise has beneficial effects on controlling seizure through enhancement of cognition and highlights the possibility to translate into reduced seizure recurrence in people with epilepsy.

Altered Synaptic Drive onto Birthdated Dentate Granule Cells in Experimental Temporal Lobe Epilepsy

Dysregulated adult hippocampal neurogenesis occurs in many temporal lobe epilepsy (TLE) models. Most dentate granule cells (DGCs) generated in response to an epileptic insult develop features that promote increased excitability, including ectopic location, persistent hilar basal dendrites (HBDs), and mossy fiber sprouting. However, some appear to integrate normally and even exhibit reduced excitability compared to other DGCs. To examine the relationship between DGC birthdate, morphology, and network integration in a model of TLE, we retrovirally birthdated either early-born [EB; postnatal day (P)7] or adult-born (AB; P60) DGCs. Male rats underwent pilocarpine-induced status epilepticus (SE) or sham treatment at P56. Three to six months after SE or sham treatment, we used whole-cell patch-clamp and fluorescence microscopy to record spontaneous excitatory and inhibitory currents from birthdated DGCs. We found that both AB and EB populations of DGCs recorded from epileptic rats received increased excitatory input compared with age-matched controls. Interestingly, when AB populations were separated into normally integrated (normotopic) and aberrant (ectopic or HBD-containing) subpopulations, only the aberrant populations exhibited a relative increase in excitatory input (amplitude, frequency, and charge transfer). The ratio of excitatory-to-inhibitory input was most dramatically upregulated for ectopically localized DGCs. These data provide definitive physiological evidence that aberrant integration of post-SE, AB DGCs contributes to increased synaptic drive and support the idea that ectopic DGCs serve as putative hub cells to promote seizures.SIGNIFICANCE STATEMENT Adult dentate granule cell (DGC) neurogenesis is altered in rodent models of temporal lobe epilepsy (TLE). Some of the new neurons show abnormal morphology and integration, but whether adult-generated DGCs contribute to the development of epilepsy is controversial. We examined the synaptic inputs of age-defined populations of DGCs using electrophysiological recordings and fluorescent retroviral reporter birthdating. DGCs generated neonatally were compared with those generated in adulthood, and adult-born (AB) neurons with normal versus aberrant morphology or integration were examined. We found that AB, ectopically located DGCs exhibit the most pro-excitatory physiological changes, implicating this population in seizure generation or progression.

The expression of keratan sulfate reveals a unique subset of microglia in the mouse hippocampus after pilocarpine-induced status epileptics

Induction of keratan sulfate in microglia has been found in several animal models of neurological disorders. However, the significance of keratan sulfate-expressing microglia is not fully understood. To address this issue, we analyzed the characteristics of microglia labeled by the 5D4 epitope, a marker of high-sulfated keratan sulfate, in the mouse hippocampus during the latent period after pilocarpine-induced status epilepticus (SE). Only 5D4-negative (5D4^- ) microglia were found in the CA1 region of vehicle-treated controls and pilocarpine-treated mice at 1 day after SE onset. A few 5D4⁺ microglia appeared in the strata oriens and radiatum at 5 days post-SE, and they were distributed into the stratum pyramidale at 14 days post-SE. The expressions of genes related to both anti- and pro-inflammatory cytokines were higher in 5D4⁺ cells than in 5D4^- cells at 5 but not 14 days post-SE. The expressions of genes related to phagocytosis were higher in 5D4⁺ cells than in 5D4^- cells throughout the latent period. The phagocytic activity of microglia, as measured by engulfment of the zymosan bioparticles, was higher in 5D4⁺ cells than in 5D4^- cells. The contact ratios between excitatory synaptic boutons and microglia were also higher in 5D4⁺ cells than in 5D4^- cells at 5 and 14 days post-SE. The excitatory/inhibitory ratios of synaptic boutons within the microglial domain were lower in 5D4⁺ cells than in 5D4^- cells at 14 days post-SE. Our findings indicate that 5D4⁺ microglia may play some role in epileptogenesis via pruning of excitatory synapses during the latent period after SE.

Proportional loss of parvalbumin-immunoreactive synaptic boutons and granule cells from the hippocampus of sea lions with temporal lobe epilepsy

One in 26 people develop epilepsy and in these temporal lobe epilepsy (TLE) is common. Many patients display a pattern of neuron loss called hippocampal sclerosis. Seizures usually start in the hippocampus but underlying mechanisms remain unclear. One possibility is insufficient inhibition of dentate granule cells. Normally parvalbumin-immunoreactive (PV) interneurons strongly inhibit granule cells. Humans with TLE display loss of PV interneurons in the dentate gyrus but questions persist. To address this, we evaluated PV interneuron and bouton numbers in California sea lions (Zalophus californianus) that naturally develop TLE after exposure to domoic acid, a neurotoxin that enters the marine food chain during harmful algal blooms. Sclerotic hippocampi were identified by the loss of Nissl-stained hilar neurons. Stereological methods were used to estimate the number of granule cells and PV interneurons per dentate gyrus. Sclerotic hippocampi contained fewer granule cells, fewer PV interneurons, and fewer PV synaptic boutons, and the ratio of granule cells to PV interneurons was higher than in controls. To test whether fewer boutons was attributable to loss versus reduced immunoreactivity, expression of synaptotagmin-2 (syt2) was evaluated. Syt2 is also expressed in boutons of PV interneurons. Sclerotic hippocampi displayed proportional losses of syt2-immunoreactive boutons, PV boutons, and granule cells. There was no significant difference in the average numbers of PV- or syt2-positive boutons per granule cell between control and sclerotic hippocampi. These findings do not address functionality of surviving synapses but suggest reduced granule cell inhibition in TLE is not attributable to anatomical loss of PV boutons.

Alterations in GABAA-Receptor Trafficking and Synaptic Dysfunction in Brain Disorders

GABA_A receptors (GABA_AR) are the major players in fast inhibitory neurotransmission in the central nervous system (CNS). Regulation of GABA_AR trafficking and the control of their surface expression play important roles in the modulation of the strength of synaptic inhibition. Different pieces of evidence show that alterations in the surface distribution of GABA_AR and dysregulation of their turnover impair the activity of inhibitory synapses. A diminished efficacy of inhibitory neurotransmission affects the excitatory/inhibitory balance and is a common feature of various disorders of the CNS characterized by an increased excitability of neuronal networks. The synaptic pool of GABA_AR is mainly controlled through regulation of internalization, recycling and lateral diffusion of the receptors. Under physiological condition these mechanisms are finely coordinated to define the strength of GABAergic synapses. In this review article, we focus on the alteration in GABA_AR trafficking with an impact on the function of inhibitory synapses in various disorders of the CNS. In particular we discuss how similar molecular mechanisms affecting the synaptic distribution of GABA_AR and consequently the excitatory/inhibitory balance may be associated with a wide diversity of pathologies of the CNS, from psychiatric disorders to acute alterations leading to neuronal death. A better understanding of the cellular and molecular mechanisms that contribute to the impairment of GABAergic neurotransmission in these disorders, in particular the alterations in GABA_AR trafficking and surface distribution, may lead to the identification of new pharmacological targets and to the development of novel therapeutic strategies.

Docosanoids and elovanoids from omega-3 fatty acids are pro-homeostatic modulators of inflammatory responses, cell damage and neuroprotection

The functional significance of the selective enrichment of the omega-3 essential fatty acid docosahexaenoic acid (DHA; 22C and 6 double bonds) in cellular membrane phospholipids of the nervous system is being clarified by defining its specific roles on membrane protein function and by the uncovering of the bioactive mediators, docosanoids and elovanoids (ELVs). Here, we describe the preferential uptake and DHA metabolism in photoreceptors and brain as well as the significance of the Adiponectin receptor 1 in DHA retention and photoreceptor cell (PRC) survival. We now know that this integral membrane protein is engaged in DHA retention as a necessary event for the function of PRCs and retinal pigment epithelial (RPE) cells. We present an overview of how a) NPD1 selectively mediates preconditioning rescue of RPE and PR cells; b) NPD1 restores aberrant neuronal networks in experimental epileptogenesis; c) the decreased ability to biosynthesize NPD1 in memory hippocampal areas of early stages of Alzheimer's disease takes place; d) NPD1 protection of dopaminergic circuits in an in vitro model using neurotoxins; and e) bioactivity elicited by DHA and NPD1 activate a neuroprotective gene-expression program that includes the expression of Bcl-2 family members affected by Aβ42, DHA, or NPD1. In addition, we highlight ELOVL4 (ELOngation of Very Long chain fatty acids-4), specifically the neurological and ophthalmological consequences of its mutations, and their role in providing precursors for the biosynthesis of ELVs. Then we outline evidence of ELVs ability to protect RPE cells, which sustain PRC integrity. In the last section, we present a summary of the protective bioactivity of docosanoids and ELVs in experimental ischemic stroke. The identification of early mechanisms of neural cell survival mediated by DHA-synthesized ELVs and docosanoids contributes to the understanding of cell function, pro-homeostatic cellular modulation, inflammatory responses, and innate immunity, opening avenues for prevention and therapeutic applications in neurotrauma, stroke and neurodegenerative diseases.

Total Number Is Important: Using the Disector Method in Design-Based Stereology to Understand the Structure of the Rodent Brain

The advantages of using design-based stereology in the collection of quantitative data, have been highlighted, in numerous publications, since the description of the disector method by Sterio (1984). This review article discusses the importance of total number derived with the disector method, as a key variable that must continue to be used to understand the rodent brain and that such data can be used to develop quantitative networks of the brain. The review article will highlight the huge impact total number has had on our understanding of the rodent brain and it will suggest that neuroscientists need to be aware of the increasing number of studies where density, not total number, is the quantitative measure used. It will emphasize that density can result in data that is misleading, most often in an unknown direction, and that we run the risk of this type of data being accepted into the collective neuroscience knowledge database. It will also suggest that design-based stereology using the disector method, can be used alongside recent developments in electron microscopy, such as serial block-face scanning electron microscopy (SEM), to obtain total number data very efficiently at the ultrastructural level. Throughout the article total number is discussed as a key parameter in understanding the micro-networks of the rodent brain as they can be represented as both anatomical and quantitative networks.

Cx36 in the mouse hippocampus during and after pilocarpine-induced status epilepticus

Gap junctions play an important role in the synchronization activity of coupled cells. Hippocampal inhibitory interneurons are involved in epileptogenesis and seizure activity, and express gap junction protein connexin (Cx) 36. Cx36 is also localized in the axons (mossy fibers) of granule cells in the dentate gyrus. While it has been documented that Cx36 is involved in epileptogenesis, there are still controversies regarding the expression levels of Cx36 at different developmental stages of human and animal models of epileptogenesis. In this study, the expression of Cx36 was investigated in the mouse hippocampus at 1 h, 4 h during pilocarpine-induced status epilepticus (PISE) and 1 week, 2 months after PISE. We found that Cx36 was down-regulated in neurons at different time points during and after PISE, whereas it was increased significantly in the stratum lucidum of CA3 area at 2 months after PISE. Double immunofluorescence indicated that Cx36 was localized in parvalbumin (PV) immunopositive interneuron in CA1 area and in mossy fibers and their terminals in the stratum lucidum of CA3 area. It suggests that decreased expression of Cx36 in interneurons may be related to less effective inhibitory control of excitatory activity of hippocampal principal neurons. However, the increased Cx36 immunopositive product in mossy fibers at the chronic stage after PISE may enhance the contacts between granule cells in the dentate gyrus and pyramidal neurons in CA3 area. The two different changes of Cx36 may be implicated in the epileptogenesis.

Inhibitory or excitatory? Optogenetic interrogation of the functional roles of GABAergic interneurons in epileptogenesis

Alteration in the excitatory/inhibitory neuronal balance is believed to be the underlying mechanism of epileptogenesis. Based on this theory, GABAergic interneurons are regarded as the primary inhibitory neurons, whose failure of action permits hyperactivity in the epileptic circuitry. As a consequence, optogenetic excitation of GABAergic interneurons is widely used for seizure suppression. However, recent evidence argues for the context-dependent, possibly “excitatory” roles that GABAergic cells play in epileptic circuitry. We reviewed current optogenetic approaches that target the “inhibitory” roles of GABAergic interneurons for seizure control. We also reviewed interesting evidence that supports the “excitatory” roles of GABAergic interneurons in epileptogenesis. GABAergic interneurons can provide excitatory effects to the epileptic circuits via several distinct neurological mechanisms. (1) GABAergic interneurons can excite postsynaptic neurons, due to the raised reversal potential of GABA receptors in the postsynaptic cells. (2) Continuous activity in GABAergic interneurons could lead to transient GABA depletion, which prevents their inhibitory effect on pyramidal cells. (3) GABAergic interneurons can synchronize network activity during seizure. (4) Some GABAergic interneurons inhibit other interneurons, causing disinhibition of pyramidal neurons and network hyperexcitability. The dynamic, context-dependent role that GABAergic interneurons play in seizure requires further investigation of their functions at single cell and circuitry level. New optogenetic protocols that target GABAergic inhibition should be explored for seizure suppression.

Disrupted Co-activation of Interneurons and Hippocampal Network after Focal Kainate Lesion

GABAergic interneurons are known to control activity balance in physiological conditions and to coordinate hippocampal networks during cognitive tasks. In temporal lobe epilepsy interneuron loss and consecutive network imbalance could favor pathological hypersynchronous epileptic discharges. We tested this hypothesis in mice by in vivo unilateral epileptogenic hippocampal kainate lesion followed by in vitro recording of extracellular potentials and patch-clamp from GFP-expressing interneurons in CA3, in an optimized recording chamber. Slices from lesioned mice displayed, in addition to control synchronous events, larger epileptiform discharges. Despite some ipsi/contralateral and layer variation, interneuron density tended to decrease, average soma size to increase. Their membrane resistance decreased, capacitance increased and contralateral interneuron required higher current intensity to fire action potentials. Examination of synchronous discharges of control and larger amplitudes, revealed that interneurons were biased to fire predominantly with the largest population discharges. Altogether, these observations suggest that the overall effect of reactive cell loss, hypertrophy and reduced contralateral excitability corresponds to interneuron activity tuning to fire with larger population discharges. Such cellular and network mechanisms may contribute to a runaway path toward epilepsy.

Neural mechanisms underlying GABAergic regulation of adult hippocampal neurogenesis

Within the dentate gyrus of the adult hippocampus is the subgranular zone, which contains a neurogenic niche for radial-glia like cells, the most primitive neural stem cells in the adult brain. The quiescence of neural stem cells is maintained by tonic gamma-aminobutyric acid (GABA) released from local interneurons. Once these cells differentiate into neural progenitor cells, GABA continues to regulate their development into mature granule cells, the principal cell type of the dentate gyrus. Here, we review the role of GABA circuits, signaling, and receptors in regulating development of adult-born cells, as well as the molecular players that modulate GABA signaling. Furthermore, we review recent findings linking dysregulation of adult hippocampal neurogenesis to the altered GABAergic circuitry and signaling under various pathological conditions.

Disease Modifying Effects of the Spider Toxin Parawixin2 in the Experimental Epilepsy Model

(1) Background: Temporal lobe epilepsy (TLE) is the most common type of epilepsy in adults. It is also the one with the highest percentage of drug-resistance to the current available anti-epileptic drugs (AED). Additionaly, most antiepileptic drugs are only able to control seizures in epileptogenesis, but do not decrease the hippocampal neurodegenerative process. TLE patients have a reduced population of interneuronal cells, which express Parvalbumin (PV) proteins. This reduction is directly linked to seizure frequency and severity in the chronic period of epilepsy. There is therefore a need to seek new therapies with a disease-modifying profile, and with efficient antiepileptic and neuroprotective properties. Parawixin2, a compound isolated from the venom of the spider Parawixia bistriata, has been shown to inhibit GABA transporters (GAT) and to have acute anticonvulsant effects in rats. (2) Methods: In this work, we studied the effects of Parawixin2 and Tiagabine (an FDA- approved GAT inhibitor), and compared these effects in a TLE model. Rats were subjected to lithium-pilocarpine TLE model and the main features were evaluated over a chronic period including: (a) spontaneous recurrent seizures (SRS), (b) neuronal loss, and (c) PV cell density in different regions of the hippocampus (CA1, CA3, DG and Hilus). (3) Results: Parawixin2 treatment reduced SRS frequency whereas Tiagabine did not. We also found a significant reduction in neuronal loss in CA3 and in the hilus regions of the hippocampus, in animals treated with Parawixin2. Noteworthy, Parawixin2 significantly reversed PV cell loss observed particularly in DG layers. (4) Conclusions: Parawixin2 exerts a promising neuroprotective and anti-epileptic effect and has potential as a novel agent in drug design.

Seizure frequency correlates with loss of dentate gyrus GABAergic neurons in a mouse model of temporal lobe epilepsy

Epilepsy occurs in one of 26 people. Temporal lobe epilepsy is common and can be difficult to treat effectively. It can develop after brain injuries that damage the hippocampus. Multiple pathophysiological mechanisms involving the hippocampal dentate gyrus have been proposed. This study evaluated a mouse model of temporal lobe epilepsy to test which pathological changes in the dentate gyrus correlate with seizure frequency and help prioritize potential mechanisms for further study. FVB mice (n = 127) that had experienced status epilepticus after systemic treatment with pilocarpine 31-61 days earlier were video-monitored for spontaneous, convulsive seizures 9 hr/day every day for 24-36 days. Over 4,060 seizures were observed. Seizure frequency ranged from an average of one every 3.6 days to one every 2.1 hr. Hippocampal sections were processed for Nissl stain, Prox1-immunocytochemistry, GluR2-immunocytochemistry, Timm stain, glial fibrillary acidic protein-immunocytochemistry, glutamic acid decarboxylase in situ hybridization, and parvalbumin-immunocytochemistry. Stereological methods were used to measure hilar ectopic granule cells, mossy cells, mossy fiber sprouting, astrogliosis, and GABAergic interneurons. Seizure frequency was not significantly correlated with the generation of hilar ectopic granule cells, the number of mossy cells, the extent of mossy fiber sprouting, the extent of astrogliosis, or the number of GABAergic interneurons in the molecular layer or hilus. Seizure frequency significantly correlated with the loss of GABAergic interneurons in or adjacent to the granule cell layer, but not with the loss of parvalbumin-positive interneurons. These findings prioritize the loss of granule cell layer interneurons for further testing as a potential cause of temporal lobe epilepsy.

Massively augmented hippocampal dentate granule cell activation accompanies epilepsy development

In a mouse model of temporal lobe epilepsy, multicellular calcium imaging revealed that disease emergence was accompanied by massive amplification in the normally sparse, afferent stimulation-induced activation of hippocampal dentate granule cells. Patch recordings demonstrated reductions in local inhibitory function within the dentate gyrus at time points where sparse activation was compromised. Mimicking changes in inhibitory synaptic function and transmembrane chloride regulation was sufficient to elicit the dentate gyrus circuit collapse evident during epilepsy development. Pharmacological blockade of outward chloride transport had no effect during epilepsy development, and significantly increased granule cell activation in both control and chronically epileptic animals. This apparent occlusion effect implicates reduction in chloride extrusion as a mechanism contributing to granule cell hyperactivation specifically during early epilepsy development. Glutamine plays a significant role in local synthesis of GABA in synapses. In epileptic mice, sparse granule cell activation could be restored by glutamine application, implicating compromised GABA synthesis. Glutamine had no effect on granule cell activation earlier, during epilepsy development. We conclude that compromised feedforward inhibition within the local circuit generates the massive dentate gyrus circuit hyperactivation evident in animals during and following epilepsy development. However, the mechanisms underlying this disinhibition diverge significantly as epilepsy progresses.

Normal and epilepsy-associated pathologic function of the dentate gyrus

The dentate gyrus plays critical roles both in cognitive processing, and in regulation of the induction and propagation of pathological activity. The cellular and circuit mechanisms underlying these diverse functions overlap extensively. At the cellular level, the intrinsic properties of dentate granule cells combine to endow these neurons with a fundamental reluctance to activate, one of their hallmark traits. At the circuit level, the dentate gyrus constitutes one of the more heavily inhibited regions of the brain, with strong, fast feedforward and feedback GABAergic inhibition dominating responses to afferent activation. In pathologic states such as epilepsy, a number of alterations within the dentate gyrus combine to compromise the regulatory properties of this circuit, culminating in a collapse of its normal function. This epilepsy-associated transformation in the fundamental properties of this critical regulatory hippocampal circuit may contribute both to seizure propensity, and cognitive and emotional comorbidities characteristic of this disease state.

Hilar somatostatin interneuron loss reduces dentate gyrus inhibition in a mouse model of temporal lobe epilepsy

OBJECTIVE

In patients with temporal lobe epilepsy, seizures usually start in the hippocampus, and dentate granule cells are hyperexcitable. Somatostatin interneurons are a major subpopulation of inhibitory neurons in the dentate gyrus, and many are lost in patients and animal models. However, surviving somatostatin interneurons sprout axon collaterals and form new synapses, so the net effect on granule cell inhibition remains unclear.

METHODS

The present study uses optogenetics to activate hilar somatostatin interneurons and measure the inhibitory effect on dentate gyrus perforant path-evoked local field potential responses in a mouse model of temporal lobe epilepsy.

RESULTS

In controls, light activation of hilar somatostatin interneurons inhibited evoked responses up to 40%. Epileptic pilocarpine-treated mice exhibited loss of hilar somatostatin interneurons and less light-induced inhibition of evoked responses.

SIGNIFICANCE

These findings suggest that severe epilepsy-related loss of hilar somatostatin interneurons can overwhelm the surviving interneurons' capacity to compensate by sprouting axon collaterals.

Modulation of axonal sprouting along rostro-caudal axis of dorsal hippocampus and no neuronal survival in parahippocampal cortices by long-term post-lesion melatonin administration in lithium-pilocarpine model of temporal lobe epilepsy

Feature outcome of hippocampus and extra-hippocampal cortices was evaluated in melatonin treated lithium-pilocarpine epileptic rats during early and chronic phases of temporal lobe epilepsy (TLE). After status epilepticus (SE) induction, 5 and 20 mg/kg melatonin were administered for 14 days or 60 days. All animals were killed 60 days post SE induction and the histological features of the rosrto-caudal axis of the dorsal hippocampus, piriform and entorhinal cortices were evaluated utilizing Nissl, Timm, and synapsin I immunoflorescent staining. Melatonin (20 mg/kg) effect on CA1 and CA3 neurons showed a region-specific pattern along the rostro-caudal axis of the dorsal hippocampus. The number of counted granular cells by melatonin (20 mg/kg) treatment increased along the rostro-caudal axis of the dorsal hippocampus in comparison to the untreated epileptic group. The density of Timm granules in the inner molecular layer of the dentate gyrus decreased significantly in all melatonin treated groups in comparison to the untreated epileptic animals. The increased density of synapsin I immunoreactivity in the outer molecular layer of the dentate gyrus of untreated epileptic rats showed a profound decrease following melatonin treatment. There was no neuronal protection in the piriform and entorhinal cortices whatever the melatonin treatment. Long-term melatonin administration as a co-adjuvant probably could reduce the post-lesion histological consequences of TLE in a region-specific pattern along the rostro-caudal axis of the dorsal hippocampus.

The role of inhibition in epileptic networks

Inhibition plays many roles in cortical circuits, including coordination of network activity in different brain rhythms and neuronal clusters, gating of activity, gain control, and dictating the manner in which activity flows through the network. This latter is particularly relevant to epileptic states, when extreme hypersynchronous discharges can spread across cortical territories. We review these different physiological and pathological roles and discuss how inhibition can be compromised and why this predisposes the network to seizures.

More Docked Vesicles and Larger Active Zones at Basket Cell-to-Granule Cell Synapses in a Rat Model of Temporal Lobe Epilepsy

Temporal lobe epilepsy is a common and challenging clinical problem, and its pathophysiological mechanisms remain unclear. One possibility is insufficient inhibition in the hippocampal formation where seizures tend to initiate. Normally, hippocampal basket cells provide strong and reliable synaptic inhibition at principal cell somata. In a rat model of temporal lobe epilepsy, basket cell-to-granule cell (BC→GC) synaptic transmission is more likely to fail, but the underlying cause is unknown. At some synapses, probability of release correlates with bouton size, active zone area, and number of docked vesicles. The present study tested the hypothesis that impaired GABAergic transmission at BC→GC synapses is attributable to ultrastructural changes. Boutons making axosomatic symmetric synapses in the granule cell layer were reconstructed from serial electron micrographs. BC→GC boutons were predicted to be smaller in volume, have fewer and smaller active zones, and contain fewer vesicles, including fewer docked vesicles. Results revealed the opposite. Compared with controls, epileptic pilocarpine-treated rats displayed boutons with over twice the average volume, active zone area, total vesicles, and docked vesicles and with more vesicles closer to active zones. Larger active zones in epileptic rats are consistent with previous reports of larger amplitude miniature IPSCs and larger BC→GC quantal size. Results of this study indicate that transmission failures at BC→GC synapses in epileptic pilocarpine-treated rats are not attributable to smaller boutons or fewer docked vesicles. Instead, processes following vesicle docking, including priming, Ca(2+) entry, or Ca(2+) coupling with exocytosis, might be responsible.

SIGNIFICANCE STATEMENT

One in 26 people develops epilepsy, and temporal lobe epilepsy is a common form. Up to one-third of patients are resistant to currently available treatments. This study tested a potential underlying mechanism for previously reported impaired inhibition in epileptic animals at basket cell-to-granule cell (BC→GC) synapses, which normally are reliable and strong. Electron microscopy was used to evaluate 3D ultrastructure of BC→GC synapses in a rat model of temporal lobe epilepsy. The hypothesis was that impaired synaptic transmission is attributable to smaller boutons, smaller synapses, and abnormally low numbers of synaptic vesicles. Results revealed the opposite. These findings suggest that impaired transmission at BC→GC synapses in epileptic rats is attributable to later steps in exocytosis following vesicle docking.

Dentate cannabinoid-sensitive interneurons undergo unique and selective strengthening of mutual synaptic inhibition in experimental epilepsy

Altered inhibition is a salient feature of hippocampal network reorganization in epilepsy. Hippocampal pyramidal cells and dentate granule cells show specific reduction in cannabinoid receptor type 1 (CB1R)-sensitive GABAergic inputs in experimental epilepsy. In the dentate gyrus, CB1Rs regulate synaptic release from accommodating interneurons (AC-INs) with adapting firing characteristics and axonal projections in the molecular layer, but not from fast-spiking basket cells (FS-BCs). However, it is not known whether the intrinsic physiology and synaptic inhibition of AC-INs responsible for CB1R-sensitive inhibition is altered in epilepsy. Using the pilocarpine-induced status epilepticus (SE) model of epilepsy, we find that the basic physiological characteristics of AC-INs in epileptic rats are not different from age-matched controls. In paired interneuronal recordings, the amplitude of unitary inhibitory synaptic currents (uIPSCs) between AC-INs doubled after SE. Non-stationary noise analysis revealed that the post-SE strengthening of synapses between AC-INs resulted from an increase in postsynaptic receptors. Baseline synaptic release and CB1R antagonist enhancement of release at synapses between AC-INs were not different between control and post-SE rats. Additionally, uIPSC amplitude in FS-BCs to AC-INs pairs was unchanged after SE indicating input-specific microcircuit alterations in inhibitory inputs to AC-INs. At the network level, AC-INs showed no reduction in spontaneous and miniature inhibitory synaptic current (sIPSC or mIPSC) frequency or amplitude after SE. However, AC-IN mIPSC amplitude was persistently enhanced in post-SE and epileptic rats. CB1R agonist reduced the amplitude and suppressed a greater proportion of sIPSCs in AC-INs from post-SE and epileptic rats demonstrating a novel, cell-type specific increase in CB1R-sensitive inhibition of AC-INs after SE. This unique post-SE strengthening of inhibition between AC-INs could lead to activity-dependent suppression of AC-IN firing and compromise dentate CB1R-sensitive inhibition in epilepsy.

Temporal progression of evoked field potentials in neocortical slices after unilateral hypoxia-ischemia in perinatal rats: Correlation with cortical epileptogenesis

Infarcts of the neonatal cerebral cortex can lead to progressive epilepsy, which is characterized by time-dependent increases in seizure frequency after the infarct and by shifts in seizure-onset zones from focal to multi-focal. Using a rat model of unilateral perinatal hypoxia-ischemia (PHI), where long-term seizure monitoring had previously demonstrated progressive epilepsy, evoked field potentials (EFPs) were recorded in layers II/III of coronal neocortical slices to analyze the underlying time-dependent, network-level alterations ipsilateral vs. contralateral to the infarct. At 3weeks after PHI, EFPs ipsilateral to the infarct were normal in artificial cerebrospinal fluid (ACSF); however, after blocking GABAA receptors with bicuculline methiodide (BMI, 30μM), the slices with an infarct were more hyperexcitable than slices without an infarct. At 3weeks, contralateral PHI slices had responses indistinguishable from controls. Six months after PHI in normal ACSF, both ipsi- and contralateral slices from rats with cortical infarcts showed prolonged afterdischarges, which were only slightly augmented in BMI. These data suggest that the early changes after PHI are localized to the ipsilateral infarcted cortex and masked by GABA-mediated inhibition; however, after 6months, progressive epileptogenesis results in generation of robust bilateral hyperexcitability. Because these afterdischarges were only slightly prolonged by BMI, a time-dependent reduction of GABAergic transmission is hypothesized to contribute to the pronounced hyperexcitability at 6months. These changes in the EFPs coincide with the seizure semiology of the epilepsy and therefore offer an opportunity to study the mechanisms underlying this form of progressive pediatric epilepsy.

Decrease in CA3 inhibitory network activity during Theiler's virus encephalitis

Viral infections of the central nervous system are often associated with seizures, and while patients usually recover from the infection and the seizures cease, there is an increased lifetime incidence of epilepsy. These viral infections can result in mesial temporal sclerosis, and, subsequently, a type of epilepsy that is difficult to treat. In previous work, we have shown that Theiler's murine encephalomyelitis virus (TMEV) infections in C57B/6 mice, an animal model of virus-induced epilepsy, results in changes in excitatory currents of CA3 neurons both during the acute infection and two months later, at a time when seizure thresholds are reduced and when spontaneous seizures can occur. The changes in the excitatory system differ at these two time points, suggesting different mechanisms for seizure generation. In the present paper, we examine GABAergic mediated inhibition in CA3 pyramidal cells at these two time points following TMEV infection. We found that amplitudes of sIPSCs and mIPSCs were reduced during the acute infection, but recovered at the two-month time point. These observations are consistent with previous measurements of excitatory currents suggesting different mechanisms of seizure generation during the acute infection and during chronic epilepsy.

Evolution of Network Synchronization during Early Epileptogenesis Parallels Synaptic Circuit Alterations

In secondary epilepsy, a seizure-prone neural network evolves during the latent period between brain injury and the onset of spontaneous seizures. The nature of the evolution is largely unknown, and even its completeness at the onset of seizures has recently been challenged by measures of gradually decreasing intervals between subsequent seizures. Sequential calcium imaging of neuronal activity, in the pyramidal cell layer of mouse hippocampal in vitro preparations, during early post-traumatic epileptogenesis demonstrated rapid increases in the fraction of neurons that participate in interictal activity. This was followed by more gradual increases in the rate at which individual neurons join each developing seizure, the pairwise correlation of neuronal activities as a function of the distance separating the pair, and network-wide measures of functional connectivity. These data support the continued evolution of synaptic connectivity in epileptic networks beyond the latent period: early seizures occur when recurrent excitatory pathways are largely polysynaptic, while ongoing synaptic remodeling after the onset of epilepsy enhances intranetwork connectivity as well as the onset and spread of seizure activity.

Functional Reduction in Cannabinoid-Sensitive Heterotypic Inhibition of Dentate Basket Cells in Epilepsy: Impact on Network Rhythms

Strong perisomatic inhibition by fast-spiking basket cells (FS-BCs) regulates dentate throughput. Homotypic FS-BC interconnections that support gamma oscillations, and heterotypic inputs from diverse groups of interneurons that receive extensive neurochemical regulation, together, shape FS-BC activity patterns. However, whether seizures precipitate functional changes in inhibitory networks and contribute to abnormal network activity in epilepsy is not known. In the first recordings from dentate interneuronal pairs in a model of temporal lobe epilepsy, we demonstrate that status epilepticus (SE) selectively compromises GABA release at synapses from dentate accommodating interneurons (AC-INs) to FS-BCs, while efficacy of homotypic FS-BC synapses is unaltered. The functional decrease in heterotypic cannabinoid receptor type 1 (CB₁R)-sensitive inhibition of FS-BCs resulted from enhanced baseline GABA_B-mediated suppression of synaptic release after SE. The frequency of CB₁R-sensitive inhibitory synaptic events in FS-BCs was depressed early after SE induction and remained reduced in epileptic rats. In biologically based simulations of heterogeneous inhibitory networks and excitatory-inhibitory cell networks, experimentally identified decrease in reliability of AC-IN to FS-BCs synaptic release reduced theta power and theta-gamma coupling and enhanced gamma coherence. Thus, the experimentally identified functional reduction in heterotypic inhibition of FS-BCs can contribute to compromised network oscillations in epilepsy and could precipitate memory and cognitive co-morbidities.

Epileptogenesis

Epileptogenesis is a chronic process that can be triggered by genetic or acquired factors, and that can continue long after epilepsy diagnosis. In 2015, epileptogenesis is not a treatment indication, and there are no therapies available in clinic to treat individuals at risk of epileptogenesis. However, thanks to active research, a large number of animal models have become available for search of molecular mechanisms of epileptogenesis. The first glimpses of treatment targets and biomarkers that could be developed to become useful in clinic are in sight. However, the heterogeneity of the epilepsy condition, and the dynamics of molecular changes over the course of epileptogenesis remain as challenges to overcome.

Molecular alterations in areas generating fast ripples in an animal model of temporal lobe epilepsy

The molecular basis of epileptogenesis is poorly characterized. Studies in humans and animal models have identified an electrophysiological signature that precedes the onset of epilepsy, which has been termed fast ripples (FRs) based on its frequency. Multiple lines of evidence implicate regions generating FRs in epileptogenesis, and FRs appear to demarcate the seizure onset zone, suggesting a role in ictogenesis as well. We performed gene expression analysis comparing areas of the dentate gyrus that generate FRs to those that do not generate FRs in a well-characterized rat model of epilepsy. We identified a small cohort of genes that are differentially expressed in FR versus non-FR brain tissue and used quantitative PCR to validate some of those that modulate neuronal excitability. Gene expression network analysis demonstrated conservation of gene co-expression between non-FR and FR samples, but examination of gene connectivity revealed changes that were most pronounced in the cm-40 module, which contains several genes associated with synaptic function and the differentially expressed genes Kcna4, Kcnv1, and Npy1r that are down-regulated in FRs. We then demonstrate that the genes within the cm-40 module are regulated by seizure activity and enriched for the targets of the RNA binding protein Elavl4. Our data suggest that seizure activity induces co-expression of genes associated with synaptic transmission and that this pattern is attenuated in areas displaying FRs, implicating the failure of this mechanism in the generation of FRs.

Seizure-related regulation of GABAA receptors in spontaneously epileptic rats

In this study, we analyzed the impact that spontaneous seizures might have on the plasma membrane expression, composition and function of GABAA receptors (GABAARs). For this, the tissue of chronically epileptic rats was collected within 3h of seizure occurrence (≤3h group) or at least 24h after seizure occurrence (≥24h group). A retrospective analysis of seizure frequency revealed that selecting animals on the bases of seizure proximity also grouped animals in terms of overall seizure burden with a higher seizure burden observed in the ≤3h group. A biochemical analysis showed that although animals with more frequent/recent seizures (≤3h group) had similar levels of GABAAR at the plasma membrane they showed deficits in inhibitory neurotransmission. By contrast, the tissue obtained from animals experiencing infrequent seizures (≥24h group) had increased plasma membrane levels of GABAAR and showed no deficit in inhibitory function. Together, our findings offer an initial insight into the molecular changes that might help to explain how alterations in GABAAR function can be associated with differential seizure burden. Our findings also suggest that increased plasma membrane levels of GABAAR might act as a compensatory mechanism to more effectively maintain inhibitory function, repress hyperexcitability and reduce seizure burden. This study is an initial step towards a fuller characterization of the molecular events that trigger alterations in GABAergic neurotransmission during chronic epilepsy.

Impaired neuronal operation through aberrant intrinsic plasticity in epilepsy

OBJECTIVE

Patients with temporal lobe epilepsy often display cognitive comorbidity with recurrent seizures. However, the cellular mechanisms underlying the impairment of neuronal information processing remain poorly understood in temporal lobe epilepsy. Within the hippocampal formation neuronal networks undergo major reorganization, including the sprouting of mossy fibers in the dentate gyrus; they establish aberrant recurrent synapses between dentate granule cells and operate via postsynaptic kainate receptors. In this report, we tested the hypothesis that this aberrant local circuit alters information processing of perforant path inputs constituting the major excitatory afferent pathway from entorhinal cortex to dentate granule cells.

METHODS

Experiments were performed in dentate granule cells from control rats and rats with temporal lobe epilepsy induced by pilocarpine hydrochloride treatment. Neurons were recorded in patch clamp in whole cell configuration in hippocampal slices.

RESULTS

Our present data revealed that an aberrant readout of synaptic inputs by kainate receptors triggered a long-lasting impairment of the perforant path input-output operation in epileptic dentate granule cells. We demonstrated that this is due to the aberrant activity-dependent potentiation of the persistent sodium current altering intrinsic firing properties of dentate granule cells.

INTERPRETATION

We propose that this aberrant activity-dependent intrinsic plasticity, which lastingly impairs the information processing of cortical inputs in dentate gyrus, may participate in hippocampal-related cognitive deficits, such as those reported in patients with epilepsy.

Cellular plasticity induced by anti-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor encephalitis antibodies

Objective

Autoimmune-mediated anti–α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) encephalitis is a severe but treatment-responsive disorder with prominent short-term memory loss and seizures. The mechanisms by which patient antibodies affect synapses and neurons leading to symptoms are poorly understood.

Methods

The effects of patient antibodies on cultures of live rat hippocampal neurons were determined with immunostaining, Western blot, and electrophysiological analyses.

Results

We show that patient antibodies cause a selective decrease in the total surface amount and synaptic localization of GluA1- and GluA2-containing AMPARs, regardless of receptor subunit binding specificity, through increased internalization and degradation of surface AMPAR clusters. In contrast, patient antibodies do not alter the density of excitatory synapses, N-methyl-D-aspartate receptor (NMDAR) clusters, or cell viability. Commercially available AMPAR antibodies directed against extracellular epitopes do not result in a loss of surface and synaptic receptor clusters, suggesting specific effects of patient antibodies. Whole-cell patch clamp recordings of spontaneous miniature postsynaptic currents show that patient antibodies decrease AMPAR-mediated currents, but not NMDAR-mediated currents. Interestingly, several functional properties of neurons are also altered: inhibitory synaptic currents and vesicular γ-aminobutyric acid transporter (vGAT) staining intensity decrease, whereas the intrinsic excitability of neurons and short-interval firing increase.

Interpretation

These results establish that antibodies from patients with anti-AMPAR encephalitis selectively eliminate surface and synaptic AMPARs, resulting in a homeostatic decrease in inhibitory synaptic transmission and increased intrinsic excitability, which may contribute to the memory deficits and epilepsy that are prominent in patients with this disorder.

Long-term seizure suppression and optogenetic analyses of synaptic connectivity in epileptic mice with hippocampal grafts of GABAergic interneurons

Studies in rodent epilepsy models suggest that GABAergic interneuron progenitor grafts can reduce hyperexcitability and seizures in temporal lobe epilepsy (TLE). Although integration of the transplanted cells has been proposed as the underlying mechanism for these disease-modifying effects, prior studies have not explicitly examined cell types and synaptic mechanisms for long-term seizure suppression. To address this gap, we transplanted medial ganglionic eminence (MGE) cells from embryonic day 13.5 VGAT-Venus or VGAT-ChR2-EYFP transgenic embryos into the dentate gyrus (DG) of adult mice 2 weeks after induction of TLE with pilocarpine. Beginning 3-4 weeks after status epilepticus, we conducted continuous video-electroencephalographic recording until 90-100 d. TLE mice with bilateral MGE cell grafts in the DG had significantly fewer and milder electrographic seizures, compared with TLE controls. Immunohistochemical studies showed that the transplants contained multiple neuropeptide or calcium-binding protein-expressing interneuron types and these cells established dense terminal arborizations onto the somas, apical dendrites, and axon initial segments of dentate granule cells (GCs). A majority of the synaptic terminals formed by the transplanted cells were apposed to large postsynaptic clusters of gephyrin, indicative of mature inhibitory synaptic complexes. Functionality of these new inhibitory synapses was demonstrated by optogenetically activating VGAT-ChR2-EYFP-expressing transplanted neurons, which generated robust hyperpolarizations in GCs. These findings suggest that fetal GABAergic interneuron grafts may suppress pharmacoresistant seizures by enhancing synaptic inhibition in DG neural circuits.

Hippocampal neuropathology of domoic acid-induced epilepsy in California sea lions (Zalophus californianus)

California sea lions (Zalophus californianus) are abundant human-sized carnivores with large gyrencephalic brains. They develop epilepsy after experiencing status epilepticus when naturally exposed to domoic acid. We tested whether sea lions previously exposed to DA (chronic DA sea lions) display hippocampal neuropathology similar to that of human patients with temporal lobe epilepsy. Hippocampi were obtained from control and chronic DA sea lions. Stereology was used to estimate numbers of Nissl-stained neurons per hippocampus in the granule cell layer, hilus, and pyramidal cell layer of CA3, CA2, and CA1 subfields. Adjacent sections were processed for somatostatin immunoreactivity or Timm-stained, and the extent of mossy fiber sprouting was measured stereologically. Chronic DA sea lions displayed hippocampal neuron loss in patterns and extents similar but not identical to those reported previously for human patients with temporal lobe epilepsy. Similar to human patients, hippocampal sclerosis in sea lions was unilateral in 79% of cases, mossy fiber sprouting was a common neuropathological abnormality, and somatostatin-immunoreactive axons were exuberant in the dentate gyrus despite loss of immunopositive hilar neurons. Thus, hippocampal neuropathology of chronic DA sea lions is similar to that of human patients with temporal lobe epilepsy.

Blockade of excitatory synaptogenesis with proximal dendrites of dentate granule cells following rapamycin treatment in a mouse model of temporal lobe epilepsy

Inhibiting the mammalian target of rapamycin (mTOR) signaling pathway with rapamycin blocks granule cell axon (mossy fiber) sprouting after epileptogenic injuries, including pilocarpine-induced status epilepticus. However, it remains unclear whether axons from other types of neurons sprout into the inner molecular layer and synapse with granule cell dendrites despite rapamycin treatment. If so, other aberrant positive-feedback networks might develop. To test this possibility stereological electron microscopy was used to estimate the numbers of excitatory synapses in the inner molecular layer per hippocampus in pilocarpine-treated control mice, in mice 5 days after pilocarpine-induced status epilepticus, and after status epilepticus and daily treatment beginning 24 hours later with rapamycin or vehicle for 2 months. The optical fractionator method was used to estimate numbers of granule cells in Nissl-stained sections so that numbers of excitatory synapses in the inner molecular layer per granule cell could be calculated. Control mice had an average of 2,280 asymmetric synapses in the inner molecular layer per granule cell, which was reduced to 63% of controls 5 days after status epilepticus, recovered to 93% of controls in vehicle-treated mice 2 months after status epilepticus, but remained at only 63% of controls in rapamycin-treated mice. These findings reveal that rapamycin prevented excitatory axons from synapsing with proximal dendrites of granule cells and raise questions about the recurrent excitation hypothesis of temporal lobe epilepsy.