Increased excitatory synaptic input to granule cells from hilar and CA3 regions in a rat model of temporal lobe epilepsy

Neuronal deletion of phosphatase and tensin homolog in mice results in spatial dysregulation of adult hippocampal neurogenesis

Adult neurogenesis is a persistent phenomenon in mammals that occurs in select brain structures in both healthy and diseased brains. The tumor suppressor gene, phosphatase and tensin homolog deleted on chromosome 10 (Pten) has previously been found to restrict the proliferation of neural stem/progenitor cells (NSPCs) in vivo. In this study, we aimed to provide a comprehensive picture of how conditional deletion of Pten may regulate the genesis of adult NSPCs in the dentate gyrus of the hippocampus and the subventricular zone bordering the lateral ventricles. Using conventional markers and stereology, we quantified multiple stages of neurogenesis, including proliferating cells, immature neurons (neuroblasts), and apoptotic cells in several regions of the dentate gyrus, including the subgranular zone (SGZ), outer granule cell layer (oGCL), molecular layer, and hilus at 4 and 10 weeks of age. Our data demonstrate that conditional deletion of Pten in mice produces successive increases in dentate gyrus proliferating cells and immature neuroblasts, which confirms the known negative roles Pten has on cell proliferation and maturation. Specifically, we observe a significant increase in Ki67+ proliferating cells in the neurogenic SGZ at 4 weeks of age, but not 10 weeks of age. We also observe a delayed increase in neuroblasts at 10 weeks of age. However, our study expands on previous work by providing temporal, subregional, and neurogenesis-stage resolution. Specifically, we found that Pten deletion initially increases cell proliferation in the neurogenic SGZ, but this increase spreads to non-neurogenic dentate gyrus areas, including the hilus, oGCL, and molecular layer, as mice age. We also observed region-specific increases in apoptotic cells in the dentate gyrus hilar region that paralleled the regional increases in Ki67+ cells. Our work is accordant with the literature showing that Pten serves as a negative regulator of dentate gyrus neurogenesis but adds temporal and spatial components to the existing knowledge.

Glial-mediated dysregulation of neurodevelopment in Fragile X Syndrome

Astrocytes are highly involved in a multitude of developmental processes that are known to be dysregulated in Fragile X Syndrome. Here, we examine these processes individually and review the roles astrocytes play in contributing to the pathology of this syndrome. As a growing area of interest in the field, new and exciting insight is continually emerging. Understanding these glial-mediated roles is imperative for elucidating the underlying molecular mechanisms at play, not only in Fragile X Syndrome, but also other ASD-related disorders. Understanding these roles will be central to the future development of effective, clinically-relevant treatments of these disorders.

IRAK-M Ablation Promotes Status Epilepticus-Induced Neuroinflammation via Activating M1 Microglia and Impairing Excitatory Synaptic Function

Epilepsy is one of the most common neurological disorders. The pro-epileptic and antiepileptic roles of microglia have recently garnered significant attention. Interleukin-1 receptor-associated kinase (IRAK)-M, an important kinase in the innate immune response, is mainly expressed in microglia and acts as a negative regulator of the TLR4 signaling pathway that mediates the anti-inflammatory effect. However, whether IRAK-M exerts a protective role in epileptogenesis as well as the molecular and cellular mechanisms underlying these processes are yet to be elucidated. An epilepsy mouse model induced by pilocarpine was used in this study. Real-time quantitative polymerase chain reaction and western blot analysis were used to analyze mRNA and protein expression levels, respectively. Whole-cell voltage-clamp recordings were employed to evaluate the glutamatergic synaptic transmission in hippocampal neurons. Immunofluorescence was utilized to show the glial cell activation and neuronal loss. Furthermore, the proportion of microglia was analyzed using flow cytometry. Seizure dynamics influenced the expression of IRAK-M. Its knockout dramatically exacerbated the seizures and the pathology in epilepsy and increased the N-methyl-d-aspartate receptor (NMDAR) expression, thereby enhancing glutamatergic synaptic transmission in hippocampal CA1 pyramidal neurons in mice. Furthermore, IRAK-M deficiency augmented hippocampal neuronal loss via a possible mechanism of NMDAR-mediated excitotoxicity. IRAK-M deletion promotes microglia toward the M1 phenotype, which resulted in high levels of proinflammatory cytokines and was accompanied by a visible increase in the expressions of key microglial polarization-related proteins, including p-STAT1, TRAF6, and SOCS1. The findings demonstrate that IRAK-M dysfunction contributes to the progression of epilepsy by increasing M1 microglial polarization and glutamatergic synaptic transmission. This is possibly related to NMDARs, particularly Grin2A and Grin2B, which suggests that IRAK-M could serve as a novel therapeutic target for the direct alleviation of epilepsy.

Human cortical interneurons optimized for grafting specifically integrate, abort seizures, and display prolonged efficacy without over-inhibition

Previously, we demonstrated the efficacy of human pluripotent stem cell (hPSC)-derived GABAergic cortical interneuron (cIN) grafts in ameliorating seizures. However, a safe and reliable clinical translation requires a mechanistic understanding of graft function, as well as the assurance of long-term efficacy and safety. By employing hPSC-derived chemically matured migratory cINs in two models of epilepsy, we demonstrate lasting efficacy in treating seizures and comorbid deficits, as well as safety without uncontrolled growth. Host inhibition does not increase with increasing grafted cIN densities, assuring their safety without the risk of over-inhibition. Furthermore, their closed-loop optogenetic activation aborted seizure activity, revealing mechanisms of graft-mediated seizure control and allowing graft modulation for optimal translation. Monosynaptic tracing shows their extensive and specific synaptic connections with host neurons, resembling developmental connection specificity. These results offer confidence in stem cell-based therapy for epilepsy as a safe and reliable treatment for patients suffering from intractable epilepsy.

Seizure-induced hilar ectopic granule cells in the adult dentate gyrus

Epilepsy is a chronic neurological disorder characterized by hypersynchronous spontaneous recurrent seizures, and affects approximately 50 million people worldwide. Cumulative evidence has revealed that epileptogenic insult temporarily increases neurogenesis in the hippocampus; however, a fraction of the newly generated neurons are integrated abnormally into the existing neural circuits. The abnormal neurogenesis, including ectopic localization of newborn neurons in the hilus, formation of abnormal basal dendrites, and disorganization of the apical dendrites, rewires hippocampal neural networks and leads to the development of spontaneous seizures. The central roles of hilar ectopic granule cells in regulating hippocampal excitability have been suggested. In this review, we introduce recent findings about the migration of newborn granule cells to the dentate hilus after seizures and the roles of seizure-induced ectopic granule cells in the epileptic brain. In addition, we delineate possible intrinsic and extrinsic mechanisms underlying this abnormality. Finally, we suggest that the regulation of seizure-induced ectopic cells can be a promising target for epilepsy therapy and provide perspectives on future research directions.

Deficits in Behavioral and Neuronal Pattern Separation in Temporal Lobe Epilepsy

In temporal lobe epilepsy, the ability of the dentate gyrus to limit excitatory cortical input to the hippocampus breaks down, leading to seizures. The dentate gyrus is also thought to help discriminate between similar memories by performing pattern separation, but whether epilepsy leads to a breakdown in this neural computation, and thus to mnemonic discrimination impairments, remains unknown. Here we show that temporal lobe epilepsy is characterized by behavioral deficits in mnemonic discrimination tasks, in both humans (females and males) and mice (C57Bl6 males, systemic low-dose kainate model). Using a recently developed assay in brain slices of the same epileptic mice, we reveal a decreased ability of the dentate gyrus to perform certain forms of pattern separation. This is because of a subset of granule cells with abnormal bursting that can develop independently of early EEG abnormalities. Overall, our results linking physiology, computation, and cognition in the same mice advance our understanding of episodic memory mechanisms and their dysfunction in epilepsy.SIGNIFICANCE STATEMENT People with temporal lobe epilepsy (TLE) often have learning and memory impairments, sometimes occurring earlier than the first seizure, but those symptoms and their biological underpinnings are poorly understood. We focused on the dentate gyrus, a brain region that is critical to avoid confusion between similar memories and is anatomically disorganized in TLE. We show that both humans and mice with TLE experience confusion between similar situations. This impairment coincides with a failure of the dentate gyrus to disambiguate similar input signals because of pathologic bursting in a subset of neurons. Our work bridges seizure-oriented and memory-oriented views of the dentate gyrus function, suggests a mechanism for cognitive symptoms in TLE, and supports a long-standing hypothesis of episodic memory theories.

A critical period of neuronal activity results in aberrant neurogenesis rewiring hippocampal circuitry in a mouse model of epilepsy

In the mammalian hippocampus, adult-born granule cells (abGCs) contribute to the function of the dentate gyrus (DG). Disruption of the DG circuitry causes spontaneous recurrent seizures (SRS), which can lead to epilepsy. Although abGCs contribute to local inhibitory feedback circuitry, whether they are involved in epileptogenesis remains elusive. Here, we identify a critical window of activity associated with the aberrant maturation of abGCs characterized by abnormal dendrite morphology, ectopic migration, and SRS. Importantly, in a mouse model of temporal lobe epilepsy, silencing aberrant abGCs during this critical period reduces abnormal dendrite morphology, cell migration, and SRS. Using mono-synaptic tracers, we show silencing aberrant abGCs decreases recurrent CA3 back-projections and restores proper cortical connections to the hippocampus. Furthermore, we show that GABA-mediated amplification of intracellular calcium regulates the early critical period of activity. Our results demonstrate that aberrant neurogenesis rewires hippocampal circuitry aggravating epilepsy in mice.

Hyperoside alleviates epilepsy-induced neuronal damage by enhancing antioxidant levels and reducing autophagy

ETHNOPHARMACOLOGICAL RELEVANCE

Hypericum perforatum L. (genus Hypericum, family Hypericaceae), a plant commonly used in traditional Chinese medicine, is believed to confer a wide range of benefits, including fever reduction, detoxification, calming, and pain relief via decoctions of its stems and leaves. Hyperoside (HYP), a natural compound extracted from Hypericum perforatum L., has been shown to demonstrate a wide array of bioactivities including antioxidative, anti-inflammatory, and anti-apoptotic effects. In this study, we investigated the effects of HYP on epilepsy-induced neuronal damage in mice and the associated regulatory factors.

AIM OF THE STUDY

This study examined the potential therapeutic use of HYP for the treatment of neuronal damage in a mouse model of epilepsy and explored the relationships of the potential neuroprotective effects of HYP pretreatment with antioxidant levels and autophagy.

MATERIALS AND METHODS

ICR mice were randomly divided into six groups: sham group, sham-HYP group, KA group, KA-HYP group, KA-HYP-DDC group and KA-CQ group. Immunohistochemical staining was used to assess changes in NeuN, IBA-1, and GFAP expression in the CA3 region of the hippocampus. Immunofluorescence staining was used to assess the effects of HYP on the number of autophagosomes that accumulated in neurons in the hippocampal CA3 region. The levels of SOD1, SOD2, LC3I/II, Beclin1, and PI3K/AKT and MAPK signaling-related proteins were detected by Western blot.

RESULTS

Pretreatment with 50 mg/kg HYP protected against epilepsy-induced neuronal damage in the hippocampal CA3 region. Additionally, HYP enhanced antioxidant levels and reduced the levels of autophagy-related proteins via the PI3K/AKT and MAPK pathways.

CONCLUSION

HYP protected the hippocampal CA3 region against epilepsy-induced neuronal damage via enhancing antioxidant levels and reducing autophagy. The mechanism of action may be related to the maintenance of antioxidant levels and the suppression of autophagy via the PI3K/Akt and MAPK pathways.

Semilunar Granule Cells Are the Primary Source of the Perisomatic Excitatory Innervation onto Parvalbumin-Expressing Interneurons in the Dentate Gyrus

We analyzed the origin and relevance of the perisomatic excitatory inputs on the parvalbumin interneurons of the granule cell layer in mouse. Confocal analysis of the glutamatergic innervation showed that it represents ∼50% of the perisomatic synapses that parvalbumin cells receive. This excitatory input may originate from granule cell collaterals, the mossy cells, or even supramammillary nucleus. First, we assessed the input from the mossy cells on parvalbumin interneurons. Axon terminals of mossy cells were visualized by their calretinin content. Using multicolor confocal microscopy, we observed that less than 10% of perisomatic excitatory innervation of parvalbumin cells could originate from mossy cells. Correlative light and electron microscopy revealed that innervation from mossy cells, although present, was indeed infrequent, except for those parvalbumin cells whose somata were located in the inner molecular layer. Second, we investigated the potential input from supramammillary nucleus on parvalbumin cell somata using anterograde tracing or immunocytochemistry against vesicular glutamate transporter 2 (VGLUT2) and found only occasional contacts. Third, we intracellularly filled dentate granule cells in acute slice preparations using whole-cell recording and examined whether their axon collaterals target parvalbumin interneurons. We found that typical granule cells do not innervate the perisomatic region of these GABAergic cells. In sharp contrast, semilunar granule cells (SGCs), a scarce granule cell subtype often contacted the parvalbumin cell soma and proximal dendrites. Our data, therefore, show that perisomatic excitatory drive of parvalbumin interneurons in the granular layer of the dentate gyrus is abundant and originates primarily from SGCs.

Early Chronic Carbamazepine-in-Food Administration to MAM/Pilocarpine Rats Does Not Affect Convulsive Motor Seizures

Antiepileptic drug-resistance is a major health problem in patients with cortical dysplasia (CD). Whether drug-resistant epilepsy is associated with progressive brain damage is still debated. We previously generated a rat model of acquired CD, the methylazoxymethanol-pilocarpine (MP) rat, in which the occurrence of status epilepticus and subsequent spontaneous seizures induce progressive brain damage (Nobili et al., 2015). The present study tested the outcome of early-chronic carbamazepine (CBZ) administration on both seizure activity and brain damage in MP rats. We took advantage of the non-invasive CBZ-in-food administration protocol, established by Ali (2012), which proved effective in suppressing generalized convulsive seizures in kainic acid rat model of epilepsy. MP rats were treated immediately after the onset of the first spontaneous seizure with 300 mg/kg/day CBZ formulated in pellets for a two-months-trial. CBZ-treated rats were continuously video-monitored to detect seizure activity and were compared with untreated epileptic MP rats. Despite CBZ serum levels in treated rats were within the suggested therapeutic range for humans, CBZ affected spontaneous convulsive seizures in 2 out of 10 treated rats (responders), whereas the remaining animals (non-responders) did not show any difference when compared to untreated MP rats. Histological analysis revealed cortical thinning paralleled by robust staining of Fluoro-Jade⁺ (FJ⁺) degenerating neurons and diffuse tissue necrosis in CBZ-non-responder vs CBZ-responder rats. Data reported here suggest that MP rat model represents suitable experimental setting where to investigate mechanisms of CD-related drug-resistant epilepsy and to verify if modulation of seizures, with appropriate treatment, may reduce seizure-induced brain damage.

Circuit-based interventions in the dentate gyrus rescue epilepsy-associated cognitive dysfunction

Temporal lobe epilepsy is associated with significant structural pathology in the hippocampus. In the dentate gyrus, the summative effect of these pathologies is massive hyperexcitability in the granule cells, generating both increased seizure susceptibility and cognitive deficits. To date, therapeutic approaches have failed to improve the cognitive symptoms in fully developed, chronic epilepsy. As the dentate's principal signalling population, the granule cells' aggregate excitability has the potential to provide a mechanistically-independent downstream target. We examined whether normalizing epilepsy-associated granule cell hyperexcitability-without correcting the underlying structural circuit disruptions-would constitute an effective therapeutic approach for cognitive dysfunction. In the systemic pilocarpine mouse model of temporal lobe epilepsy, the epileptic dentate gyrus excessively recruits granule cells in behavioural contexts, not just during seizure events, and these mice fail to perform on a dentate-mediated spatial discrimination task. Acutely reducing dorsal granule cell hyperactivity in chronically epileptic mice via either of two distinct inhibitory chemogenetic receptors rescued behavioural performance such that they responded comparably to wild type mice. Furthermore, recreating granule cell hyperexcitability in control mice via excitatory chemogenetic receptors, without altering normal circuit anatomy, recapitulated spatial memory deficits observed in epileptic mice. However, making the granule cells overly quiescent in both epileptic and control mice again disrupted behavioural performance. These bidirectional manipulations reveal that there is a permissive excitability window for granule cells that is necessary to support successful behavioural performance. Chemogenetic effects were specific to the targeted dorsal hippocampus, as hippocampal-independent and ventral hippocampal-dependent behaviours remained unaffected. Fos expression demonstrated that chemogenetics can modulate granule cell recruitment via behaviourally relevant inputs. Rather than driving cell activity deterministically or spontaneously, chemogenetic intervention merely modulates the behaviourally permissive activity window in which the circuit operates. We conclude that restoring appropriate principal cell tuning via circuit-based therapies, irrespective of the mechanisms generating the disease-related hyperactivity, is a promising translational approach.

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.

Specificity, Versatility, and Continual Development: The Power of Optogenetics for Epilepsy Research

Optogenetics is a powerful and rapidly expanding set of techniques that use genetically encoded light sensitive proteins such as opsins. Through the selective expression of these exogenous light-sensitive proteins, researchers gain the ability to modulate neuronal activity, intracellular signaling pathways, or gene expression with spatial, directional, temporal, and cell-type specificity. Optogenetics provides a versatile toolbox and has significantly advanced a variety of neuroscience fields. In this review, using recent epilepsy research as a focal point, we highlight how the specificity, versatility, and continual development of new optogenetic related tools advances our understanding of neuronal circuits and neurological disorders. We additionally provide a brief overview of some currently available optogenetic tools including for the selective expression of opsins.

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.

Kappa opioid receptors regulate hippocampal synaptic homeostasis and epileptogenesis

OBJECTIVE

Homeostatic synaptic plasticity (HSP) serves as a gain control mechanism at central nervous system (CNS) synapses, including those between the dentate gyrus (DG) and CA3. Improper circuit control of DG-CA3 synapses is hypothesized to underlie epileptogenesis. Here, we sought to (1) identify compounds that preferentially modulate DG-CA3 synapses in primary neuronal culture and (2) determine if these compounds would delay or prevent epileptogenesis in vivo.

METHODS

We previously developed and validated an in vitro assay to visualize the behavior of DG-CA3 synapses and predict functional changes. We used this "synapse-on-chip" assay (quantification of synapse size, number, and type using immunocytochemical markers) to dissect the mechanisms of HSP at DG-CA3 synapses. Using chemogenetic constructs and pharmacological agents we determined the signaling cascades necessary for gain control at DG-CA3 synapses. Finally, we tested the implicated cascades (using kappa opioid receptor (OR) agonists and antagonists) in two models of epileptogenesis: electrical amygdala kindling in the mouse and chemical (pentylenetetrazole) kindling in the rat.

RESULTS

In vitro, synapses between DG mossy fibers (MFs) and CA3 neurons are the primary homeostatic responders during sustained periods of activity change. Kappa OR signaling is both necessary and sufficient for the homeostatic elaboration of DG-CA3 synapses, induced by presynaptic DG activity levels. Blocking kappa OR signaling in vivo attenuates the development of seizures in both mouse and rat models of epilepsy.

SIGNIFICANCE

This study elucidates mechanisms by which synapses between DG granule cells and CA3 pyramidal neurons undergo activity-dependent homeostatic compensation, via OR signaling in vitro. Modulation of kappa OR signaling in vivo alters seizure progression, suggesting that breakdown of homeostatic closed-loop control at DG-CA3 synapses contributes to seizures, and that targeting endogenous homeostatic mechanisms at DG-CA3 synapses may prove useful in combating epileptogenesis.

Rabies tracing of birthdated dentate granule cells in rat temporal lobe epilepsy

OBJECTIVE

To understand how monosynaptic inputs onto adult-born dentate granule cells (DGCs) are altered in experimental mesial temporal lobe epilepsy (mTLE) and whether their integration differs from early-born DGCs that are mature at the time of epileptogenesis.

METHODS

A dual-virus tracing strategy combining retroviral birthdating with rabies virus-mediated putative retrograde trans-synaptic tracing was used to identify and compare presynaptic inputs onto adult-born and early-born DGCs in the rat pilocarpine model of mTLE.

RESULTS

Our results demonstrate that hilar ectopic DGCs preferentially synapse onto adult-born DGCs after pilocarpine-induced status epilepticus (SE), whereas normotopic DGCs synapse onto both adult-born and early-born DGCs. We also find that parvalbumin^- and somatostatin^- interneuron inputs are greatly diminished onto early-born DGCs after SE. However, somatostatin^- interneuron inputs onto adult-born DGCs are maintained, likely due to preferential sprouting. Intriguingly, CA3 pyramidal cell backprojections that specifically target adult-born DGCs arise in the epileptic brain, whereas axons of interneurons and pyramidal cells in CA1 appear to sprout across the hippocampal fissure to preferentially synapse onto early-born DGCs.

INTERPRETATION

These data support the presence of substantial hippocampal circuit remodeling after an epileptogenic insult that generates prominent excitatory monosynaptic inputs, both local recurrent and widespread feedback loops, onto DGCs. Both adult-born and early-born DGCs are targets of new inputs from other DGCs as well as from CA3 and CA1 pyramidal cells after pilocarpine treatment, changes that likely contribute to epileptogenesis in experimental mTLE. Ann Neurol 2017;81:790-803.

Association between mossy fiber sprouting and expression of semaphorin-3f protein in dentate gyrus of hippocampus in lithium-pilocarpine-induced status epilepticus mouse model

OBJECTIVES

Mossy fiber sprouting is involved in the pathogenesis of mesial temporal lobe epilepsy. But the exact mechanism of formation of mossy fiber sprouting is still unclear. Semaphorin-3f protein could inhibit the growth of neuron axons. The aim of this research is to evaluate the association between semaphorin-3f expression and mossy fiber sprouting.

METHODS

We established pilocarpine-induced status epilepticus (PISE) models firstly. Then, mossy fiber sprouting in the hippocampus of PISE models was examined by Timm staining. Expression of semaphorin-3f was evaluated by western blot analysis and immunohistochemical examination. Expression of semaphorin-3f protein in different subregions of hippocampus and its relationship with mossy fiber sprouting were studied.

RESULTS

We found that in PISE group, mossy fiber sprouting appeared in dentate gyrus (DG) region. It started to develop in the latent phase of PISE group and increased significantly in the chronic phase. Expression of semaphorin-3f protein in DG region started to decrease in the latent phase, and stayed at low level in the chronic phase. No such change was found in the other groups.

CONCLUSIONS

These results indicate that the decrease in semaphorin-3f expression in DG region was in parallel to the change of mossy fiber sprouting in PISE models, suggesting that mossy fiber sprouting is closely associated with reduced expression of semaphorin-3f in this model.

LSPS/Optogenetics to Improve Synaptic Connectivity Mapping: Unmasking the Role of Basket Cell-Mediated Feedforward Inhibition

Neocortical pyramidal cells (PYRs) receive synaptic inputs from many types of GABAergic interneurons. Connections between parvalbumin (PV)-positive, fast-spiking interneurons (“PV cells”) and PYRs are characterized by perisomatic synapses and high-amplitude, short-latency IPSCs. Here, we present novel methods to study the functional influence of PV cells on layer 5 PYRs using optogenetics combined with laser-scanning photostimulation (LSPS). First, we examined the strength and spatial distribution of PV-to-PYR inputs. To that end, the fast channelrhodopsin variant AAV5-EF1α-DIO-hChR2(E123T)-eYFP (ChETA) was expressed in PV cells in somatosensory cortex of mice using an adeno-associated virus-based viral construct. Focal blue illumination (100–150 µm half-width) was directed through the microscope objective to excite PV cells along a spatial grid covering layers 2–6, while IPSCs were recorded in layer 5 PYRs. The resulting optogenetic input maps showed evoked PV cell inputs originating from an ∼500-μm-diameter area surrounding the recorded PYR. Evoked IPSCs had the short-latency/high-amplitude characteristic of PV cell inputs. Second, we investigated how PV cell activity modulates PYR output in response to synaptic excitation. We expressed halorhodopsin (eNpHR3.0) in PV cells using the same strategy as for ChETA. Yellow illumination hyperpolarized eNpHR3.0-expressing PV cells, effectively preventing action potential generation and thus decreasing the inhibition of downstream targets. Synaptic input maps onto layer 5 PYRs were acquired using standard glutamate-photolysis LSPS either with or without full-field yellow illumination to silence PV cells. The resulting IPSC input maps selectively lacked short-latency perisomatic inputs, while EPSC input maps showed increased connectivity, particularly from upper layers. This indicates that glutamate uncaging LSPS-based excitatory synaptic maps will consistently underestimate connectivity.

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.

Axonal plasticity of age-defined dentate granule cells in a rat model of mesial temporal lobe epilepsy

Dentate granule cell (DGC) mossy fiber sprouting (MFS) in mesial temporal lobe epilepsy (mTLE) is thought to underlie the creation of aberrant circuitry which promotes the generation or spread of spontaneous seizure activity. Understanding the extent to which populations of DGCs participate in this circuitry could help determine how it develops and potentially identify therapeutic targets for regulating aberrant network activity. In this study, we investigated how DGC birthdate influences participation in MFS and other aspects of axonal plasticity using the rat pilocarpine-induced status epilepticus (SE) model of mTLE. We injected a retrovirus (RV) carrying a synaptophysin-yellow fluorescent protein (syp-YFP) fusion construct to birthdate DGCs and brightly label their axon terminals, and compared DGCs born during the neonatal period with those generated in adulthood. We found that both neonatal and adult-born DGC populations participate, to a similar extent, in SE-induced MFS within the dentate gyrus inner molecular layer (IML). SE did not alter hilar MF bouton density compared to sham-treated controls, but adult-born DGC bouton density was greater in the IML than in the hilus after SE. Interestingly, we also observed MF axonal reorganization in area CA2 in epileptic rats, and these changes arose from DGCs generated both neonatally and in adulthood. These data indicate that both neonatal and adult-generated DGCs contribute to axonal reorganization in the rat pilocarpine mTLE model, and indicate a more complex relationship between DGC age and participation in seizure-related plasticity than was previously thought.

TLR1 expression in mouse brain was increased in a KA-induced seizure model

OBJECTIVE

Toll-like receptors (TLRs) that mediate inflammatory responses play an important role in epilepsy; however, whether TLR1 is also involved in epileptogenesis remains unclear. Thus, in this study, we investigated the extent and pattern of TLR1 expression in epileptic tissues.

METHODS

One-hundred and thirty-two mice were intra-cerebroventricularly injected with PBS or kainic acid (KA) and were examined at 1, 3, 8 and 24 h. The expression pattern and distribution of TLR1 were examined by reverse-transcriptase polymerase chain reaction (RT-PCR), western blot analysis and immunohistochemistry staining.

RESULTS

The mRNA and protein levels of TLR1 were significantly upregulated in the hippocampus and temporal cortex of epileptic mice compared with those of controls. TLR1 expression was increased as early as 1 h following KA treatment and peaked at 8 and 24 h. Immunohistochemistry staining demonstrated that TLR1 was distributed in the CA1-3, dentate gyrus and hilus regions of the hippocampus and different cortical regions. Immunofluorescent staining further revealed that TLR1 was primarily expressed in the neurons, microglia, and astrocytes of epileptogenic tissue.

SIGNIFICANCE

These results demonstrate that cortical and hippocampal sub-regional expression of TLR1 is altered during epileptogenesis in a time- and location-specific manner, suggesting a close association with the process of epilepsy.

In vivo evaluation of the dentate gate theory in epilepsy

The dentate gyrus is a region subject to intense study in epilepsy because of its posited role as a 'gate', acting to inhibit overexcitation in the hippocampal circuitry through its unique synaptic, cellular and network properties that result in relatively low excitability. Numerous changes predicted to produce dentate hyperexcitability are seen in epileptic patients and animal models. However, recent findings question whether changes are causative or reactive, as well as the pathophysiological relevance of the dentate in epilepsy. Critically, direct in vivo modulation of dentate 'gate' function during spontaneous seizure activity has not been explored. Therefore, using a mouse model of temporal lobe epilepsy with hippocampal sclerosis, a closed-loop system and selective optogenetic manipulation of granule cells during seizures, we directly tested the dentate 'gate' hypothesis in vivo. Consistent with the dentate gate theory, optogenetic gate restoration through granule cell hyperpolarization efficiently stopped spontaneous seizures. By contrast, optogenetic activation of granule cells exacerbated spontaneous seizures. Furthermore, activating granule cells in non-epileptic animals evoked acute seizures of increasing severity. These data indicate that the dentate gyrus is a critical node in the temporal lobe seizure network, and provide the first in vivo support for the dentate 'gate' hypothesis.

Microcircuits and their interactions in epilepsy: is the focus out of focus?

Epileptic seizures represent dysfunctional neural networks dominated by excessive and/or hypersynchronous activity. Recent progress in the field has outlined two concepts regarding mechanisms of seizure generation, or ictogenesis. First, all seizures, even those associated with what have historically been thought of as 'primary generalized' epilepsies, appear to originate in local microcircuits and then propagate from that initial ictogenic zone. Second, seizures propagate through cerebral networks and engage microcircuits in distal nodes, a process that can be weakened or even interrupted by suppressing activity in such nodes. We describe various microcircuit motifs, with a special emphasis on one that has been broadly implicated in several epilepsies: feed-forward inhibition. Furthermore, we discuss how, in the dynamic network in which seizures propagate, focusing on circuit 'choke points' remote from the initiation site might be as important as that of the initial dysfunction, the seizure 'focus'.

Revisiting hippocampal sclerosis in mesial temporal lobe epilepsy according to the "two-hit" hypothesis

Hippocampal sclerosis (HS) is the most common neuropathological pattern observed in pharmacoresistant epilepsy and represents a critical feature in mesial temporal lobe epilepsy syndrome. However, its pathophysiological mechanisms and neuropathological consequences on seizures remain mostly unresolved. The new international classification of hippocampal sclerosis aims at standardizing its description to allow comparisons between different clinical studies. However, several aspects are not considered in this classification (granule cell dispersion, sprouting, glial modifications…). In this chapter, we discuss these different features associated with hippocampal sclerosis in perspective with the "two-hit" hypothesis and propose mechanisms that could be involved in the modulation of some specific neuropathological aspects like early life stress, hyperthermic seizures, brain lesions or hormonal modifications.

Suppression of adult neurogenesis increases the acute effects of kainic acid

Adult neurogenesis, the generation of new neurons in the adult brain, occurs in the hippocampal dentate gyrus (DG) and the olfactory bulb (OB) of all mammals, but the functions of these new neurons are not entirely clear. Originally, adult-born neurons were considered to have excitatory effects on the DG network, but recent studies suggest a net inhibitory effect. Therefore, we hypothesized that selective removal of newborn neurons would lead to increased susceptibility to the effects of a convulsant. This hypothesis was tested by evaluating the response to the chemoconvulsant kainic acid (KA) in mice with reduced adult neurogenesis, produced either by focal X-irradiation of the DG, or by pharmacogenetic deletion of dividing radial glial precursors. In the first 4 hrs after KA administration, when mice have the most robust seizures, mice with reduced adult neurogenesis had more severe convulsive seizures, exhibited either as a decreased latency to the first convulsive seizure, greater number of convulsive seizures, or longer convulsive seizures. Nonconvulsive seizures did not appear to change or they decreased. Four-21 hrs after KA injection, mice with reduced adult neurogenesis showed more interictal spikes (IIS) and delayed seizures than controls. Effects were greater when the anticonvulsant ethosuximide was injected 30 min prior to KA administration; ethosuximide allows forebrain seizure activity to be more easily examined in mice by suppressing seizures dominated by the brainstem. These data support the hypothesis that reduction of adult-born neurons increases the susceptibility of the brain to effects of KA.

Structural organization of glial cells at the border between the neurotransplant and recipient brain

Integration of fetal hippocampal dentate fascia neurotransplants with the neocortical somatosensory region in adult rats was studied by electron microscopy. The growth of nerve fiber through the neurotransplant/brain border formed by the glial cells was studied. The interface zone was organized by various astrocyte subpopulations and ependymocytes forming multilamellar accumulations in some sites of the interface. These conglomerations of the glial cells and their processes did not prevent the growth of axonal and axodendritic bundles; moreover, fibrous astrocyte axons accompanied them. Under conditions of immature nervous tissue transplantation to the focus of mature brain damage, the glial cells created a substrate in the interface permeable for nerve fibers, thus promoting the functional integration of the neurotransplant.

Surviving mossy cells enlarge and receive more excitatory synaptic input in a mouse model of temporal lobe epilepsy

Numerous hypotheses of temporal lobe epileptogenesis have been proposed, and several involve hippocampal mossy cells. Building on previous hypotheses we sought to test the possibility that after epileptogenic injuries surviving mossy cells develop into super-connected seizure-generating hub cells. If so, they might require more cellular machinery and consequently have larger somata, elongate their dendrites to receive more synaptic input, and display higher frequencies of miniature excitatory synaptic currents (mEPSCs). To test these possibilities pilocarpine-treated mice were evaluated using GluR2-immunocytochemistry, whole-cell recording, and biocytin-labeling. Epileptic pilocarpine-treated mice displayed substantial loss of GluR2-positive hilar neurons. Somata of surviving neurons were 1.4-times larger than in controls. Biocytin-labeled mossy cells also were larger in epileptic mice, but dendritic length per cell was not significantly different. The average frequency of mEPSCs of mossy cells recorded in the presence of tetrodotoxin and bicuculline was 3.2-times higher in epileptic pilocarpine-treated mice as compared to controls. Other parameters of mEPSCs were similar in both groups. Average input resistance of mossy cells in epileptic mice was reduced to 63% of controls, which is consistent with larger somata and would tend to make surviving mossy cells less excitable. Other intrinsic physiological characteristics examined were similar in both groups. Increased excitatory synaptic input is consistent with the hypothesis that surviving mossy cells develop into aberrantly super-connected seizure-generating hub cells, and soma hypertrophy is indirectly consistent with the possibility of axon sprouting. However, no obvious evidence of hyperexcitable intrinsic physiology was found. Furthermore, similar hypertrophy and hyper-connectivity has been reported for other neuron types in the dentate gyrus, suggesting mossy cells are not unique in this regard. Thus, findings of the present study reveal epilepsy-related changes in mossy cell anatomy and synaptic input but do not strongly support the hypothesis that mossy cells develop into seizure-generating hub cells.

Distinct physiological and developmental properties of hippocampal CA2 subfield revealed by using anti-Purkinje cell protein 4 (PCP4) immunostaining

The hippocampal CA2 subfield was initially identified by Lorente de Nó as an anatomically distinct region based on its cytoarchitectural features. Although there is an enormous body of literature on other hippocampal subfields (CA1 and CA3), relatively little is known about the physiological and developmental properties of CA2. Here we report identification of the CA2 region in the mouse by immunostaining with a Purkinje cell protein 4 (PCP4) antibody, which effectively delineates CA3/CA2 and CA2/CA1 borders and agrees well with previous cytoarchitectural definitions of CA2. The PCP4 immunostaining–delineated CA2 neurons have distinguishable differences in cell morphology, physiology, and synaptic circuit connections compared with distal CA3 and proximal CA1 regions. The average somatic sizes of excitatory cells differ across CA1–3, with the smallest to largest somatic size being CA1<CA2<CA3. CA2 excitatory cells have dense dendritic spines, but do not have thorny excrescences associated with bordering CA3 neurons. Photostimulation functional circuit mapping shows that CA2 excitatory neurons receives extensive synaptic input from CA3, but no detectable input from the dentate gyrus. CA2 excitatory cells also differ significantly from CA3 cells in intrinsic electrophysiological parameters, such as membrane capacitance and spiking rates. Although CA2 neurons differ from CA1 neurons for PCP4 and other marker expressions, these neurons have less distinct neurophysiological and morphological properties. Developmental examination revealed that PCP4 immunostaining first appears at postnatal day 4–5 and becomes successively more refined around CA2 until reaching adult form by postnatal day 21. J. Comp. Neurol. J. Comp. Neurol. 522:1333–1354, 2014. © 2013 Wiley Periodicals, Inc.

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.

Widespread activation of microglial cells in the hippocampus of chronic epileptic rats correlates only partially with neurodegeneration

Activation of microglial cells (brain macrophages) soon after status epilepticus has been suggested to be critical for the pathogenesis of mesial temporal lobe epilepsy (MTLE). However, microglial activation in the chronic phase of experimental MTLE has been scarcely addressed. In this study, we questioned whether microglial activation persists in the hippocampus of pilocarpine-treated, epileptic Wistar rats and to which extent it is associated with segmental neurodegeneration. Microglial cells were immunostained for the universal microglial marker, ionized calcium-binding adapter molecule-1 and the activation marker, CD11b (also known as OX42, Mac-1). Using quantitative morphology, i.e., stereology and Neurolucida-based reconstructions, we investigated morphological correlates of microglial activation such as cell number, ramification, somatic size and shape. We find that microglial cells in epileptic rats feature widespread, activation-related morphological changes such as increase in cell number density, massive up-regulation of CD11b and de-ramification. The parameters show heterogeneity in different hippocampal subregions. For instance, de-ramification is most prominent in the outer molecular layer of the dentate gyrus, whereas CD11b expression dominates in hilus. Interestingly, microglial activation only partially correlates with segmental neurodegeneration. Major neuronal death in the hilus, CA3 and CA1 coincides with strong up-regulation of CD11b. However, microglial activation is also observed in subregions that do not feature neurodegeneration, such as the molecular and granular layer of the dentate gyrus. This in vivo study provides solid experimental evidence that microglial cells feature widespread heterogeneous activation that only partially correlates with hippocampal segmental neuronal loss in experimental MTLE.

Structural signs of dynamic state of synaptic contacts between neurotransplant and brain

We studied ultrastructure of synaptic connections between long-living dentate fascia transplants and somatosensory area of the neocortex in rats. Giant synaptic terminals of granular neurons upon contact with unusual neuronal targets in the neocortex reproduced their specific constitutive morphological features. At the same time, developing synapses with signs of active structural and metabolic reorganization were revealed. This is indicative of a dynamic state and instability of functional connections between the transplant and brain despite long time after transplantation.

Effective connectivity of hippocampal neural network and its alteration in Mg2+-free epilepsy model

Understanding the connectivity of the brain neural network and its evolution in epileptiform discharges is meaningful in the epilepsy researches and treatments. In the present study, epileptiform discharges were induced in rat hippocampal slices perfused with Mg2+-free artificial cerebrospinal fluid. The effective connectivity of the hippocampal neural network was studied by comparing the normal and epileptiform discharges recorded by a microelectrode array. The neural network connectivity was constructed by using partial directed coherence and analyzed by graph theory. The transition of the hippocampal network topology from control to epileptiform discharges was demonstrated. Firstly, differences existed in both the averaged in- and out-degree between nodes in the pyramidal cell layer and the granule cell layer, which indicated an information flow from the pyramidal cell layer to the granule cell layer during epileptiform discharges, whereas no consistent information flow was observed in control. Secondly, the neural network showed different small-worldness in the early, middle and late stages of the epileptiform discharges, whereas the control network did not show the small-world property. Thirdly, the network connectivity began to change earlier than the appearance of epileptiform discharges and lasted several seconds after the epileptiform discharges disappeared. These results revealed the important network bases underlying the transition from normal to epileptiform discharges in hippocampal slices. Additionally, this work indicated that the network analysis might provide a useful tool to evaluate the neural network and help to improve the prediction of seizures.

Progressive brain damage, synaptic reorganization and NMDA activation in a model of epileptogenic cortical dysplasia

Whether severe epilepsy could be a progressive disorder remains as yet unresolved. We previously demonstrated in a rat model of acquired focal cortical dysplasia, the methylazoxymethanol/pilocarpine - MAM/pilocarpine - rats, that the occurrence of status epilepticus (SE) and subsequent seizures fostered a pathologic process capable of modifying the morphology of cortical pyramidal neurons and NMDA receptor expression/localization. We have here extended our analysis by evaluating neocortical and hippocampal changes in MAM/pilocarpine rats at different epilepsy stages, from few days after onset up to six months of chronic epilepsy. Our findings indicate that the process triggered by SE and subsequent seizures in the malformed brain i) is steadily progressive, deeply altering neocortical and hippocampal morphology, with atrophy of neocortex and CA regions and progressive increase of granule cell layer dispersion; ii) changes dramatically the fine morphology of neurons in neocortex and hippocampus, by increasing cell size and decreasing both dendrite arborization and spine density; iii) induces reorganization of glutamatergic and GABAergic networks in both neocortex and hippocampus, favoring excitatory vs inhibitory input; iv) activates NMDA regulatory subunits. Taken together, our data indicate that, at least in experimental models of brain malformations, severe seizure activity, i.e., SE plus recurrent seizures, may lead to a widespread, steadily progressive architectural, neuronal and synaptic reorganization in the brain. They also suggest the mechanistic relevance of glutamate/NMDA hyper-activation in the seizure-related brain pathologic plasticity.

Does mossy fiber sprouting give rise to the epileptic state?

Many patients with temporal lobe epilepsy display structural changes in the seizure initiating zone, which includes the hippocampus. Structural changes in the hippocampus include granule cell axon (mossy fiber) sprouting. The role of mossy fiber sprouting in epileptogenesis is controversial. A popular view of temporal lobe epileptogenesis contends that precipitating brain insults trigger transient cascades of molecular and cellular events that permanently enhance excitability of neuronal networks through mechanisms including mossy fiber sprouting. However, recent evidence suggests there is no critical period for mossy fiber sprouting after an epileptogenic brain injury. Instead, findings from stereological electron microscopy and rapamycin-delayed mossy fiber sprouting in rodent models of temporal lobe epilepsy suggest a persistent, homeostatic mechanism exists to maintain a set level of excitatory synaptic input to granule cells. If so, a target level of mossy fiber sprouting might be determined shortly after a brain injury and then remain constant. Despite the static appearance of synaptic reorganization after its development, work by other investigators suggests there might be continual turnover of sprouted mossy fibers in epileptic patients and animal models. If so, there may be opportunities to reverse established mossy fiber sprouting. However, reversal of mossy fiber sprouting is unlikely to be antiepileptogenic, because blocking its development does not reduce seizure frequency in pilocarpine-treated mice. The challenge remains to identify which, if any, of the many other structural changes in the hippocampus are epileptogenic.

Abrupt and gradual transitions between low and hyperexcited firing frequencies in neuronal models with fast synaptic excitation: a comparative study

Hyperexcitability of neuronal networks is one of the hallmarks of epileptic brain seizure generation, and results from a net imbalance between excitation and inhibition that promotes excessive abnormal firing frequencies. The transition between low and high firing frequencies as the levels of recurrent AMPA excitation change can occur either gradually or abruptly. We used modeling, numerical simulations, and dynamical systems tools to investigate the biophysical and dynamic mechanisms that underlie these two identified modes of transition in recurrently connected neurons via AMPA excitation. We compare our results and demonstrate that these two modes of transition are qualitatively different and can be linked to different intrinsic properties of the participating neurons.

High-dose rapamycin blocks mossy fiber sprouting but not seizures in a mouse model of temporal lobe epilepsy

PURPOSE

The role of granule cell axon (mossy fiber) sprouting in temporal lobe epileptogenesis is unclear and controversial. Rapamycin suppresses mossy fiber sprouting, but its reported effects on seizure frequency are mixed. The present study used high-dose rapamycin to more completely block mossy fiber sprouting and to measure the effect on seizure frequency.

METHODS

Mice were treated with pilocarpine to induce status epilepticus. Beginning 24 h later and continuing for 2 months, vehicle or rapamycin (10 mg/kg/day) was administered. Starting 1 month after status epilepticus, mice were monitored by video 9 h per day, every day, for 1 month to measure the frequency of spontaneous motor seizures. At the end of seizure monitoring, a subset of mice was prepared for anatomic analysis. Mossy fiber sprouting was measured as the proportion of the granule cell layer and molecular layer that displayed black labeling in Timm-stained sections.

KEY FINDINGS

Extensive mossy fiber sprouting developed in mice that experienced status epilepticus and were treated with vehicle. In rapamycin-treated mice, mossy fiber sprouting was blocked almost to the level of naive controls. Seizure frequency was similar in vehicle-treated and rapamycin-treated mice.

SIGNIFICANCE

These findings suggest that mossy fiber sprouting is not necessary for epileptogenesis in the mouse pilocarpine model. They also reveal that rapamycin does not have antiseizure or antiepileptogenic effects in this model.

GABAergic transmission facilitates ictogenesis and synchrony between CA3, hilus, and dentate gyrus in slices from epileptic rats

The impact of regional hippocampal interactions and GABAergic transmission on ictogenesis remain unclear. Cortico-hippocampal slices from pilocarpine-treated epileptic rats were compared with controls to investigate associations between seizurelike events (SLE), GABAergic transmission, and neuronal synchrony within and between cortico-hippocampal regions. Multielectrode array recordings revealed more prevalent hippocampal SLE in epileptic tissue when excitatory transmission was enhanced and GABAergic transmission was intact [removal of Mg(2+) (0Mg)] than when GABAergic transmission was blocked [removal of Mg(2+) + bicuculline methiodide (0Mg+BMI)]. When activity within individual regions was analyzed, spectral and temporal slow oscillation/SLE correlations and cross-correlations were highest within the hilus of epileptic tissue during SLE but were similar in 0Mg and 0Mg+BMI. GABAergic facilitation of spectral "slow" oscillation and ripple correlations was most prominent within CA3 of epileptic tissue during SLE. When activity between regions was analyzed, slow oscillation and ripple coherence was highest between the hilus and dentate gyrus as well as between the hilus and CA3 of epileptic tissue during SLE and was significantly higher in 0Mg than 0Mg+BMI. High 0Mg-induced SLE cross-correlations between the hilus and dentate gyrus as well as between the hilus and CA3 were reduced or abolished in 0Mg+BMI. SLE cross-correlation lag measurements provided evidence for a monosynaptic connection from the hilus to the dentate gyrus during SLE. Findings implicate the hilus as an oscillation generator, whose impact on other cortico-hippocampal regions is mediated by GABAergic transmission. Data also suggest that GABAA receptor-mediated transmission facilitates back-propagation from CA3/hilus to the dentate gyrus and that this back-propagation augments SLE in epileptic hippocampus.

Mechanisms of epileptogenesis: a convergence on neural circuit dysfunction

Epilepsy is a prevalent neurological disorder associated with significant morbidity and mortality, but the only available drug therapies target its symptoms rather than the underlying cause. The process that links brain injury or other predisposing factors to the subsequent emergence of epilepsy is termed epileptogenesis. Substantial research has focused on elucidating the mechanisms of epileptogenesis so as to identify more specific targets for intervention, with the hope of preventing epilepsy before seizures emerge. Recent work has yielded important conceptual advances in this field. We suggest that such insights into the mechanisms of epileptogenesis converge at the level of cortical circuit dysfunction.

One hour of pilocarpine-induced status epilepticus is sufficient to develop chronic epilepsy in mice, and is associated with mossy fiber sprouting but not neuronal death

Determining the minimal duration of status epilepticus (SE) that leads to the development of subsequent spontaneous seizures (i.e., epilepsy) is important, because it provides a critical time-window for seizure intervention and epilepsy prevention. In the present study, male ICR (Imprinting Control Region) mice were injected with pilocarpine to induce acute seizures. SE was terminated by diazepam at 10 min, 30 min, 1 h, 2 h and 4 h after seizure onset. Spontaneous seizures occurred in the 1, 2 and 4 h SE groups, and the seizure frequency increased with the prolongation of SE. Similarly, the Morris water maze revealed that the escape latency was significantly increased and the number of target quadrant crossings was markedly decreased in the 1, 2 and 4 h SE groups. Robust mossy fiber sprouting was observed in these groups, but not in the 10 or 30 min group. In contrast, Fluoro-Jade B staining revealed significant cell death only in the 4 h SE group. The incidence and frequency of spontaneous seizures were correlated with Timm score (P = 0.004) and escape latency (P = 0.004). These data suggest that SE longer than one hour results in spontaneous motor seizures and memory deficits, and spontaneous seizures are likely associated with robust mossy fiber sprouting but not neuronal death.

Spatially clustered neuronal assemblies comprise the microstructure of synchrony in chronically epileptic networks

Epilepsy is characterized by recurrent synchronizations of neuronal activity, which are both a cardinal clinical symptom and a debilitating phenomenon. Although the temporal dynamics of epileptiform synchronizations are well described at the macroscopic level using electrophysiological approaches, less is known about how spatially distributed microcircuits contribute to these events. It is important to understand the relationship between micro and macro network activity because the various mechanisms proposed to underlie the generation of such pathological dynamics are united by the assumption that epileptic activity is recurrent and hypersynchronous across multiple scales. However, quantitative analyses of epileptiform spatial dynamics with cellular resolution have been hampered by the difficulty of simultaneously recording from multiple neurons in lesioned, adult brain tissue. We have overcome this experimental limitation and used two-photon calcium imaging in combination with a functional clustering algorithm to uncover the functional network structure of the chronically epileptic dentate gyrus in the mouse pilocarpine model of temporal lobe epilepsy. We show that, under hyperexcitable conditions, slices from the epileptic dentate gyrus display recurrent interictal-like network events with a high diversity in the activity patterns of individual neurons. Analysis reveals that multiple functional clusters of spatially localized neurons comprise epileptic networks, and that network events are composed of the coactivation of variable subsets of these clusters, which show little repetition between events. Thus, these interictal-like recurrent macroscopic events are not necessarily recurrent when viewed at the microcircuit scale and instead display a patterned but variable structure.

Status epilepticus enhances tonic GABA currents and depolarizes GABA reversal potential in dentate fast-spiking basket cells

Temporal lobe epilepsy is associated with loss of interneurons and inhibitory dysfunction in the dentate gyrus. While status epilepticus (SE) leads to changes in granule cell inhibition, whether dentate basket cells critical for regulating granule cell feedforward and feedback inhibition express tonic GABA currents (I(GABA)) and undergo changes in inhibition after SE is not known. We find that interneurons immunoreactive for parvalbumin in the hilar-subgranular region express GABAA receptor (GABA(A)R) δ-subunits, which are known to underlie tonic I(GABA). Dentate fast-spiking basket cells (FS-BCs) demonstrate baseline tonic I(GABA) blocked by GABA(A)R antagonists. In morphologically and physiologically identified FS-BCs, tonic I(GABA) is enhanced 1 wk after pilocarpine-induced SE, despite simultaneous reduction in spontaneous inhibitory postsynaptic current (sIPSC) frequency. Amplitude of tonic I(GABA) in control and post-SE FS-BCs is enhanced by 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP), demonstrating the contribution of GABA(A)R δ-subunits. Whereas FS-BC resting membrane potential is unchanged after SE, perforated-patch recordings from FS-BCs show that the reversal potential for GABA currents (E(GABA)) is depolarized after SE. In model FS-BCs, increasing tonic GABA conductance decreased excitability when E(GABA) was shunting and increased excitability when E(GABA) was depolarizing. Although simulated focal afferent activation evoked seizurelike activity in model dentate networks with FS-BC tonic GABA conductance and shunting E(GABA), excitability of identical networks with depolarizing FS-BC E(GABA) showed lower activity levels. Thus, together, post-SE changes in tonic I(GABA) and E(GABA) maintain homeostasis of FS-BC activity and limit increases in dentate excitability. These findings have implications for normal FS-BC function and can inform studies examining comorbidities and therapeutics following SE.

Buyang Huanwu decoction enhances cell membrane fluidity in rats with cerebral ischemia/reperfusion

After bilateral carotid artery occlusion for 30 minutes and reperfusion for 2 hours, distinct pathological changes presented in the cerebral cortex and cerebellum of rats. Compared with normal rats, nerve cell membrane fluidity significantly decreased in ischemia/reperfusion rats as detected by spin-labeling electron spin resonance, consistent with order parameter S and rotational correlation time τc measurements. Brain nerve cells from rats with ischemia/reperfusion injury were cultured with 1–100 mg/mL Buyang Huanwu decoction. Results showed that Buyang Huanwu decoction gradually increased membrane fluidity dose-dependently to normal levels, and eliminated hydroxide (OH^·) and superoxide ( O₂^·) free radicals dose-dependently. These findings suggest that Buyang Huanwu decoction can protect against cell membrane fluidity changes in rats with ischemia/ reperfusion injury by scavenging free radicals.

Prevention of posttraumatic axon sprouting by blocking collapsin response mediator protein 2-mediated neurite outgrowth and tubulin polymerization

Epileptogenesis following traumatic brain injury (TBI) is likely due to a combination of increased excitability, disinhibition, and increased excitatory connectivity via aberrant axon sprouting. Targeting these pathways could be beneficial in the prevention and treatment of posttraumatic epilepsy. Here, we tested this possibility using the novel anticonvulsant (R)-N-benzyl 2-acetamido-3-methoxypropionamide ((R)-lacosamide [LCM]), which acts on both voltage-gated sodium channels and collapsin response mediator protein 2 (CRMP2), an axonal growth/guidance protein. LCM inhibited CRMP2-mediated neurite outgrowth, an effect phenocopied by CRMP2 knockdown. Mutation of LCM-binding sites in CRMP2 reduced the neurite inhibitory effect of LCM by ∼8-fold. LCM also reduced CRMP2-mediated tubulin polymerization. Thus, LCM selectively impairs CRMP2-mediated microtubule polymerization, which underlies its neurite outgrowth and branching. To determine whether LCM inhibits axon sprouting in vivo, LCM was injected into rats subjected to partial cortical isolation, an animal model of posttraumatic epileptogenesis that exhibits axon sprouting in cortical pyramidal neurons. Two weeks following injury, excitatory synaptic connectivity of cortical layer V pyramidal neurons was mapped using patch clamp recordings and laser scanning photostimulation of caged glutamate. In comparison with injured control animals, there was a significant decrease in the map size of excitatory synaptic connectivity in LCM-treated rats, suggesting that LCM treatment prevented enhanced excitatory synaptic connectivity due to posttraumatic axon sprouting. These findings suggest, for the first time, that LCM's mode of action involves interactions with CRMP2 to inhibit posttraumatic axon sprouting.