Lithium pilocarpine-induced status epilepticus in postnatal day 20 rats results in greater neuronal injury in ventral versus dorsal hippocampus

Cell-free DNA-based liquid biopsies in neurology

This article reviews recent developments in the application of cell-free DNA-based liquid biopsies to neurological diseases. Over the past few decades, an explosion of interest in the use of accessible biofluids to identify and track molecular disease has revolutionized the fields of oncology, prenatal medicine and others. More recently, technological advances in signal detection have allowed for informative analysis of biofluids that are typically sparse in cells and other circulating components, such as CSF. In parallel, advancements in epigenetic profiling have allowed for novel applications of liquid biopsies to diseases without characteristic mutational profiles, including many degenerative, autoimmune, inflammatory, ischaemic and infectious disorders. These events have paved the way for a wide array of neurological conditions to benefit from enhanced diagnostic, prognostic, and treatment abilities through the use of liquid biomarkers: a 'liquid biopsy' approach. This review includes an overview of types of liquid biopsy targets with a focus on circulating cell-free DNA, methods used to identify and probe potential liquid biomarkers, and recent applications of such biomarkers to a variety of complex neurological conditions including CNS tumours, stroke, traumatic brain injury, Alzheimer's disease, epilepsy, multiple sclerosis and neuroinfectious disease. Finally, the challenges of translating liquid biopsies to use in clinical neurology settings-and the opportunities for improvement in disease management that such translation may provide-are discussed.

EGCG Attenuates CA1 Neuronal Death by Regulating GPx1, NF-κB S536 Phosphorylation and Mitochondrial Dynamics in the Rat Hippocampus following Status Epilepticus

Epigallocatechin-3-gallate (EGCG) is an antioxidant that directly scavenges reactive oxygen species (ROS) and inhibits pro-oxidant enzymes. Although EGCG protects hippocampal neurons from status epilepticus (SE, a prolonged seizure activity), the underlying mechanisms are not fully understood. As the preservation of mitochondrial dynamics is essential for cell viability, it is noteworthy to elucidate the effects of EGCG on impaired mitochondrial dynamics and the related signaling pathways in SE-induced CA1 neuronal degeneration, which are yet unclear. In the present study, we found that EGCG attenuated SE-induced CA1 neuronal death, accompanied by glutathione peroxidase-1 (GPx1) induction. EGCG also abrogated mitochondrial hyperfusion in these neurons by the preservation of extracellular signal-regulated kinase 1/2 (ERK1/2)-dynamin-related protein 1 (DRP1)-mediated mitochondrial fission, independent of c-Jun N-terminal kinase (JNK) activity. Furthermore, EGCG abolished SE-induced nuclear factor-κB (NF-κB) serine (S) 536 phosphorylation in CA1 neurons. ERK1/2 inhibition by U0126 diminished the effect of EGCG on neuroprotection and mitochondrial hyperfusion in response to SE without affecting GPx1 induction and NF-κB S536 phosphorylation, indicating that the restoration of ERK1/2-DRP1-mediated fission may be required for the neuroprotective effects of EGCG against SE. Therefore, our findings suggest that EGCG may protect CA1 neurons from SE insults through GPx1-ERK1/2-DRP1 and GPx1-NF-κB signaling pathways, respectively.

Adenosine Kinase Isoforms in the Developing Rat Hippocampus after LiCl/Pilocarpine Status Epilepticus

LiCl/pilocarpine status epilepticus (SE) induced in immature rats leads, after a latent period, to hippocampal hyperexcitability. The excitability may be influenced by adenosine, which exhibits anticonvulsant activity. The concentration of adenosine is regulated by adenosine kinase (ADK) present in two isoforms—ADK-L and ADK-S. The main goal of the study is to elucidate the changes in ADK isoform expression after LiCl/pilocarpine SE and whether potential changes, as well as inhibition of ADK by 5-iodotubercidin (5-ITU), may contribute to changes in hippocampal excitability during brain development. LiCl/pilocarpine SE was elicited in 12-day-old rats. Hippocampal excitability in immature rats was studied by the model of hippocampal afterdischarges (ADs), in which we demonstrated the potential inhibitory effect of 5-ITU. ADs demonstrated significantly decreased hippocampal excitability 3 days after SE induction, whereas significant hyperexcitability after 20 days compared to controls was shown. 5-ITU administration showed its inhibitory effect on the ADs in 32-day-old SE rats compared to SE rats without 5-ITU. Moreover, both ADK isoforms were examined in the immature rat hippocampus. The ADK-L isoform demonstrated significantly decreased expression in 12-day-old SE rats compared to the appropriate naïve rats, whereas increased ADK-S isoform expression was revealed. A decreasing ADK-L/-S ratio showed the declining dominance of ADK-L isoform during early brain development. LiCl/pilocarpine SE increased the excitability of the hippocampus 20 days after SE induction. The ADK inhibitor 5-ITU exhibited anticonvulsant activity at the same age. Age-related differences in hippocampal excitability after SE might correspond to the development of ADK isoform levels in the hippocampus.

Spatio-temporal heterogeneity in hippocampal metabolism in control and epilepsy conditions

The hippocampus's dorsal and ventral parts are involved in different operative circuits, the functions of which vary in time during the night and day cycle. These functions are altered in epilepsy. Since energy production is tailored to function, we hypothesized that energy production would be space- and time-dependent in the hippocampus and that such an organizing principle would be modified in epilepsy. Using metabolic imaging and metabolite sensing ex vivo, we show that the ventral hippocampus favors aerobic glycolysis over oxidative phosphorylation as compared to the dorsal part in the morning in control mice. In the afternoon, aerobic glycolysis is decreased and oxidative phosphorylation increased. In the dorsal hippocampus, the metabolic activity varies less between these two times but is weaker than in the ventral. Thus, the energy metabolism is different along the dorsoventral axis and changes as a function of time in control mice. In an experimental model of epilepsy, we find a large alteration of such spatiotemporal organization. In addition to a general hypometabolic state, the dorsoventral difference disappears in the morning, when seizure probability is low. In the afternoon, when seizure probability is high, the aerobic glycolysis is enhanced in both parts, the increase being stronger in the ventral area. We suggest that energy metabolism is tailored to the functions performed by brain networks, which vary over time. In pathological conditions, the alterations of these general rules may contribute to network dysfunctions.

Variability in sub-threshold signaling linked to Alzheimer's disease emerges with age and amyloid plaque deposition in mouse ventral CA1 pyramidal neurons

The hippocampus is vulnerable to deterioration in Alzheimer's disease (AD). It is, however, a heterogeneous structure, which may contribute to the differential volumetric changes along its septotemporal axis during AD progression. Here, we investigated amyloid plaque deposition along the dorsoventral axis in two strains of transgenic AD (ADTg) mouse models. We also used patch-clamp physiology in these mice to probe for functional consequences of AD pathogenesis in ventral hippocampus, which we found bears significantly higher plaque burden in the aged ADTg group compared to corresponding dorsal regions. Despite dorsoventral differences in amyloid load, ventral CA1 pyramidal neurons of aged ADTg mice exhibited subthreshold physiological changes similar to those previously reported in dorsal neurons, indicative of an HCN channelopathy, but lacked exacerbated suprathreshold accommodation. Additionally, HCN channel function could be rescued by pharmacological manipulation of the endoplasmic reticulum. These observations suggest that an AD-linked HCN channelopathy emerges in both dorsal and ventral CA1 pyramidal neurons, but that the former encounter an additional integrative obstacle in the form of reduced intrinsic excitability.

Detection of Brain Somatic Mutations in Cerebrospinal Fluid from Refractory Epilepsy Patients

Brain mosaic mutations are a major cause of refractory focal epilepsies with cortical malformations such as focal cortical dysplasia, hemimegalencephaly, malformation of cortical development with oligodendroglial hyperplasia in epilepsy, and ganglioglioma. Here, we collected cerebrospinal fluid (CSF) during epilepsy surgery to search for somatic variants in cell-free DNA (cfDNA) using targeted droplet digital polymerase chain reaction. In 3 of 12 epileptic patients with known somatic mutations previously identified in brain tissue, we here provide evidence that brain mosaicism can be detected in the CSF-derived cfDNA. These findings suggest future opportunities for detecting the mutant allele driving epilepsy in CSF. ANN NEUROL 2021;89:1248-1252.

The Kainic Acid Models of Temporal Lobe Epilepsy

Experimental models of epilepsy are useful to identify potential mechanisms of epileptogenesis, seizure genesis, comorbidities, and treatment efficacy. The kainic acid (KA) model is one of the most commonly used. Several modes of administration of KA exist, each producing different effects in a strain-, species-, gender-, and age-dependent manner. In this review, we discuss the advantages and limitations of the various forms of KA administration (systemic, intrahippocampal, and intranasal), as well as the histologic, electrophysiological, and behavioral outcomes in different strains and species. We attempt a personal perspective and discuss areas where work is needed. The diversity of KA models and their outcomes offers researchers a rich palette of phenotypes, which may be relevant to specific traits found in patients with temporal lobe epilepsy.

Adult neurogenesis in the mouse dentate gyrus protects the hippocampus from neuronal injury following severe seizures

Previous studies suggest that reducing the numbers of adult-born neurons in the dentate gyrus (DG) of the mouse increases susceptibility to severe continuous seizures (status epilepticus; SE) evoked by systemic injection of the convulsant kainic acid (KA). However, it was not clear if the results would be the same for other ways to induce seizures, or if SE-induced damage would be affected. Therefore, we used pilocarpine, which induces seizures by a different mechanism than KA. Also, we quantified hippocampal damage after SE. In addition, we used both loss-of-function and gain-of-function methods in adult mice. We hypothesized that after loss-of-function, mice would be more susceptible to pilocarpine-induced SE and SE-associated hippocampal damage, and after gain-of-function, mice would be more protected from SE and hippocampal damage after SE. For loss-of-function, adult neurogenesis was suppressed by pharmacogenetic deletion of dividing radial glial precursors. For gain-of-function, adult neurogenesis was increased by conditional deletion of pro-apoptotic gene Bax in Nestin-expressing progenitors. Fluoro-Jade C (FJ-C) was used to quantify neuronal injury and video-electroencephalography (video-EEG) was used to quantify SE. Pilocarpine-induced SE was longer in mice with reduced adult neurogenesis, SE had more power and neuronal damage was greater. Conversely, mice with increased adult-born neurons had shorter SE, SE had less power, and there was less neuronal damage. The results suggest that adult-born neurons exert protective effects against SE and SE-induced neuronal injury.

Epilepsy-Induced Reduction in HCN Channel Expression Contributes to an Increased Excitability in Dorsal, But Not Ventral, Hippocampal CA1 Neurons

CA1 neurons in epileptic animals are vulnerable to selective changes in ion channel expression, called acquired channelopathies, which can increase the excitability of a neuron. Under normal conditions there is a gradient of ion channel expression and intrinsic excitability along the longitudinal, dorsoventral axis of hippocampal area CA1 of the rodent. Many of these channels, including M-channels, GIRK channels and HCN channels, all have dorsoventral expression gradients that might be altered in rodent models of epilepsy. Here, we show that the excitability of dorsal, but not ventral CA1 neurons, had an increased firing rate, reduced interspike interval (ISI) and increased input resistance in a status epilepticus (SE) model of temporal lobe epilepsy (TLE). As a result, the excitability of CA1 neurons became uniform across the dorsoventral axis of the rat hippocampus post-SE. Using current clamp recordings with pharmacology and immunohistochemistry, we demonstrate that the expression of HCN channels was downregulated in the dorsal CA1 region post-SE, while the expression of M and GIRK channels were unchanged. We did not find this acquired channelopathy in ventral CA1 neurons post-SE. Our results suggest that the excitability of dorsal CA1 neurons post-SE increase to resemble the intrinsic properties of ventral CA1 neurons, which likely makes the hippocampal circuit more permissible to seizures, and contributes to the cognitive impairments associated with chronic epilepsy.

D-type potassium channels normalize action potential firing between dorsal and ventral CA1 neurons of the mouse hippocampus

Specific memory processes and neurological disorders can be ascribed to different dorsoventral regions of the hippocampus. Recently, differences in the anatomical and physiological properties between dorsal and ventral hippocampal CA1 neurons were described for both the rat and mouse hippocampus and have greatly contributed to our understanding of these processes. While differences in the subthreshold properties were similar between rat and mouse neurons, differences in action potential output between dorsal and ventral neurons were strikingly less divergent in mouse compared with rat CA1 neurons. Here, we investigate the mechanism underlying the lack of difference in action potential firing between dorsal and ventral CA1 pyramidal neurons in mouse hippocampus. Consistent with rat, we found that ventral CA1 neurons had a more depolarized resting membrane potential and higher input resistance than dorsal CA1 neurons in the mouse hippocampus. Despite these differences, action potential output in response to current injection was not significantly different. We found that ventral neurons have a more depolarized action potential threshold compared with dorsal neurons and that threshold in ventral neurons was more sensitive to block of KV1 channels compared with dorsal neurons. Outside-out voltage-clamp recordings found that slowly inactivating K+ currents were larger in ventral CA1 neurons. These results suggest that, despite differences in subthreshold properties between dorsal and ventral CA1 neurons, action potential output is normalized by the differential functional expression of D-type K+ channels. NEW & NOTEWORTHY Understanding differences in neurons within a brain region is integral in the reliable interpretation of comparative studies. Our findings identify a novel mechanism by which D-type potassium channels normalize action potential firing between dorsal and ventral CA1 neurons of mouse hippocampus despite differences in subthreshold intrinsic properties. Action potential threshold in ventral neurons is influenced by a greater functional expression of D-type potassium channels resulting in a depolarized action potential threshold compared with dorsal hippocampus.

Switching between persistent firing and depolarization block in individual rat CA1 pyramidal neurons

The hippocampal formation plays a role in mnemonic tasks and epileptic discharges in vivo. In vitro, these functions and malfunctions may relate to persistent firing (PF) and depolarization block (DB), respectively. Pyramidal neurons of the CA1 field have previously been reported to engage in either PF or DB during cholinergic stimulation. However, it is unknown whether these cells constitute disparate populations of neurons. Furthermore, it is unclear which cell-specific peculiarities may mediate their diverse response properties. However, it has not been shown whether individual CA1 pyramidal neurons can switch between PF and DB states. Here, we used whole cell patch clamp in the current clamp mode on in vitro CA1 pyramidal neurons from acutely sliced rat tissue to test various intrinsic properties which may provoke individual cells to switch between PF and DB. We found that individual cells could switch from PF to DB, in a cholinergic agonist concentration dependent manner and depending on the parameters of stimulation. We also demonstrate involvement of TRPC and potassium channels in this switching. Finally, we report that the probability for DB was more pronounced in the proximal than in the distal half of CA1. These findings offer a potential mechanism for the stronger spatial modulation in proximal, compared to distal CA1, as place field formation was shown to be affected by DB. Taken together, our results suggest that PF and DB are not mutually exclusive response properties of individual neurons. Rather, a cell's response mode depends on a variety of intrinsic properties, and modulation of these properties enables switching between PF and DB.

Seizure severity-dependent selective vulnerability of the granule cell layer and aberrant neurogenesis in the rat hippocampus

The pilocarpine-induced status epilepticus rodent model has been commonly used to analyze the mechanisms of human temporal lobe epilepsy. Recent studies using this model have demonstrated that epileptic seizures lead to increased adult neurogenesis of the dentate granule cells, and cause abnormal cellular organization in dentate neuronal circuits. In this study, we examined these structural changes in rats with seizures of varying severity. In rats with frequent severe seizures, we found a clear loss of Prox1 and NeuN expression in the dentate granule cell layer (GCL), which was confined mainly to the suprapyramidal blade of the GCL at the septal and middle regions of the septotemporal axis of the hippocampus. In the damaged suprapyramidal region, the number of immature neurons in the subgranular zone was markedly reduced. In contrast, in rats with less frequent severe seizures, there was almost no loss of Prox1 and NeuN expression, and the number of immature neurons was increased. In rats with no or slight loss of Prox1 expression in the GCL, ectopic immature neurons were detected in the molecular layer of the suprapyramidal blade in addition to the hilus, and formed chainlike aggregated structures along the blood vessels up to the hippocampal fissure, suggesting that newly generated neurons migrate at least partially along blood vessels to the hippocampal fissure. These results suggest that seizures of different severity cause different effects on GCL damage, neurogenesis, and the migration of new neurons, and that these structural changes are selective to subdivisions of the GCL and the septotemporal axis of the hippocampus.

Targeted Interneuron Ablation in the Mouse Hippocampus Can Cause Spontaneous Recurrent Seizures

The death of GABAergic interneurons has long been hypothesized to contribute to acquired epilepsy. These experiments tested the hypothesis that focal interneuron lesions cause acute seizures [i.e., status epilepticus (SE)] and/or chronic epilepsy [i.e., persistent spontaneous recurrent seizures (SRSs)]. To selectively ablate interneurons, Gad2-ires-Cre mice were injected unilaterally in the CA1 area of the dorsal hippocampus with an adeno-associated virus containing the diphtheria toxin receptor (DTR). Simultaneously, an electrode, connected to a miniature telemetry device, was positioned at the injection site for chronic recordings of local field potentials (LFPs). Two weeks after virus transfection, intraperitoneal injection of DT consistently caused focal, specific, and extensive ablation of interneurons. Long-term, continuous monitoring revealed that all mice with DT-induced interneuron lesions had SRSs. Seizures lasted tens of seconds and interseizure intervals were several hours (or days); therefore, these interneuron lesions did not induce SE. The SRSs occurred 3-5 d after DT treatment, which is the estimated time required for DT-induced cell death; therefore, induction of SRSs occurred without the latent period typical of acquired epilepsy. In five of six DT-treated mice, SRSs stopped within days, suggesting that the DT-induced interneuron lesions did not usually cause epilepsy. In one mouse, however, SRSs occurred for ≥34 d after interneuron ablation, similar to epilepsy after experimental SE. Sham control mice had no detectable seizures, confirming that the SRSs were due to ablation of interneurons. These data show that selective interneuron ablation consistently caused SRSs but not SE; and, at least under the conditions used here, interneuron lesions rarely led to persistent SRSs (i.e., epilepsy).

Effects of protease-activated receptor 1 inhibition on anxiety and fear following status epilepticus

Protease-activated receptor 1 (PAR1) is an important contributor to the pathogenesis of a variety of brain disorders associated with a risk of epilepsy development. Using the lithium-pilocarpine model of temporal lobe epilepsy (TLE), we recently showed that inhibition of this receptor during the first ten days after pilocarpine-induced status epilepticus (SE) results in substantial anti-epileptogenic and neuroprotective effects. As PAR1 is expressed in the central nervous system regions of importance for processing emotional reactions, including amygdala and hippocampus, and TLE is frequently associated with a chronic alteration of the functions of these regions, we tested the hypothesis that PAR1 inhibition could modulate emotionally driven behavioral responses of rats experiencing SE. We showed that SE induces a chronic decrease in the animals' anxiety-related behavior and an increase of locomotor activity. PAR1 inhibition after SE abolished the alteration of the anxiety level but does not affect the increase of locomotor activity in the open field and elevated plus maze tests. Moreover, while PAR1 inhibition produces an impairment of memory recall in the context fear conditioning paradigm in the control group, it substantially improves contextual and cued fear learning in rats experiencing SE. These data suggest that PAR1-dependent signaling is involved in the mechanisms underlying emotional disorders in epilepsy.

Widespread neuronal injury in a model of cholinergic status epilepticus in postnatal day 7 rat pups

OBJECTIVE

Status Epilepticus (SE) is common in neonates and infants, and is associated with neuronal injury and adverse developmental outcomes. However, the role of SE in this injury is uncertain. Until now, we have lacked an animal model in which seizures result in neuronal injury in rodent models at ages below postnatal day 12 (P12) unless seizures are combined with inflammatory stressors.

METHODS

We induced SE with high-dose lithium and pilocarpine in P7 rats, which are developmentally close to human neonates. Several EEG measures and O2 saturation were recorded during the 6h following initiation of SE. We assessed neuronal injury at 6 and 24h post-SE onset using Fluoro-Jade B staining (FJB) and caspase-3a immunoreactivity (IR).

RESULTS

EEGs showed continuous polyspikes activity for 54.3 ± 6.7 min, while O2 saturation showed no significant hypoxemia. By 24h after SE onset, significant neuronal injury was observed in CA1/subiculum, CA3, dentate gyrus, thalamus, neocortex, amygdala, piriform cortex, lateral entorhinal cortex, hypothalamus, caudate putamen, globus pallidus, ventral pallidum, and nucleus accumbens. At 24h post-SE, caspase-3a IR was significantly increased in CA1/subiculum, thalamus, and neocortex compared to sham, and caspase-3a IR neurons had fragmented nuclei, suggesting that SE triggered an irreversible form of cell injury.

SIGNIFICANCE

In conclusion, we have developed a model of cholinergic SE in P7 rat pups, which combines high survival (69.9% survival at 24h) and widespread brain injury. These studies suggest that the immature brain is vulnerable to severe forms of SE.

Co-administration of subtherapeutic diazepam enhances neuroprotective effect of COX-2 inhibitor, NS-398, after lithium pilocarpine-induced status epilepticus

RATIONALE

Seizures during status epilepticus (SE) cause neuronal death and induce cyclooxygenase-2 (COX-2). Pilocarpine-induced SE was used to determine if COX-2 inhibition with NS-398, when administered alone or with diazepam, decreases the duration and/or intensity of SE and/or reduces neuronal injury in the rat hippocampus.

METHODS

Electroencephalogram (EEG) electrodes were implanted in male Sprague-Dawley rats. SE was induced with lithium-pilocarpine, and continuous EEG and video monitoring were performed for 24 h. Rats were divided into four groups (n=8-14 rats/group) and received NS-398, diazepam, NS-398 and diazepam, or vehicle 30 min after the first motor seizure. Six hours later, NS-398 injection was repeated in the NS-398 and in the NS-398+diazepam groups. The duration of SE (continuous spiking) and the EEG power in the γ-band were analyzed. FluoroJade B staining in the dorsal hippocampus at 24h after SE was analyzed semi-quantitatively in the CA1, CA3 and hilus.

RESULTS

The duration and intensity of electrographic SE was not significantly different across the four groups. In rats treated with NS-398 alone, compared to vehicle-treated rats, neuronal damage was significantly lower compared to vehicle-treated rats in the CA3 (27%) and hilus (27%), but neuroprotection was not detected in the CA1. When NS-398 was administered with diazepam, decreased neuronal damage was further obtained in all areas investigated (CA1: 61%, CA3: 63%, hilus: 60%).

CONCLUSIONS

NS-398, when administered 30 min after the onset of SE with a repeat dose at 6h, decreased neuronal damage in the hippocampus. Administration of diazepam with NS-398 potentiates the neuroprotective effect of the COX-2 inhibitor. These neuroprotective effects occurred with no detectable effect on electrographic SE.

Status epilepticus increases mature granule cells in the molecular layer of the dentate gyrus in rats

Enhanced neurogenesis in the dentate gyrus of the hippocampus following seizure activity, especially status epilepticus, is associated with ectopic residence and aberrant integration of newborn granule cells. Hilar ectopic granule cells may be detrimental to the stability of dentate circuitry by means of their electrophysiological properties and synaptic connectivity. We hypothesized that status epilepticus also increases ectopic granule cells in the molecular layer. Status epilepticus was induced in male Sprague-Dawley rats by intraperitoneal injection of pilocarpine. Immunostaining showed that many doublecortin-positive cells were present in the molecular layer and the hilus 7 days after the induction of status epilepticus. At least 10 weeks after status epilepticus, the estimated number of cells positive for both prospero homeobox protein 1 and neuron-specific nuclear protein in the hilus was significantly increased. A similar trend was also found in the molecular layer. These findings indicate that status epilepticus can increase the numbers of mature and ectopic newborn granule cells in the molecular layer.

NMDA and AMPA receptors: development and status epilepticus

Glutamate is the main excitatory neurotransmitter in the brain and ionotropic glutamate receptors mediate the majority of excitatory neurotransmission (Dingeldine et al. 1999). The high level of glutamatergic excitation allows the neonatal brain (the 2(nd) postnatal week in rat) to develop quickly but it also makes it highly prone to age-specific seizures that can cause lifelong neurological and cognitive disability (Haut et al. 2004). There are three types of ionotropic glutamate receptors (ligand-gated ion channels) named according to their prototypic agonists: N-methyl-D-aspartate (NMDA), 2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl) propanoic acid (AMPA) and kainate (KA). During early stages of postnatal development glutamate receptors of NMDA and AMPA type undergo intensive functional changes owing to modifications in their subunit composition (Carter et al. 1988, Watanabe et al. 1992, Monyer et al. 1994, Wenzel et al. 1997, Sun et al. 1998, Lilliu et al. 2001, Kumar et al. 2002, Matsuda et al. 2002, Wee et al. 2008, Henson et al. 2010, Pachernegg et al. 2012, Paoletti et al. 2013). Participation and role of these receptors in mechanisms of seizures and epilepsy became one of the main targets of intensive investigation (De Sarro et al. 2005, Di Maio et al. 2012, Rektor 2013). LiCl/Pilocarpine (LiCl/Pilo) induced status epilepticus is a model of severe seizures resulting in development temporal lobe epilepsy (TLE). This review will consider developmental changes and contribution of NMDA and AMPA receptors in LiCl/Pilo model of status epilepticus in immature rats.

Advantages of repeated low dose against single high dose of kainate in C57BL/6J mouse model of status epilepticus: behavioral and electroencephalographic studies

A refined kainate (KA) C57BL/6J mouse model of status epilepticus (SE) using a repeated low dose (RLD) of KA (5 mg/kg, intraperitoneal; at 30 min intervals) was compared with the established single high dose (SHD) of KA (20 mg/kg, intraperitoneal) model. In the RLD group, increased duration of convulsive motor seizures (CMS, Racine scale stage ≥3) with a significant reduction in mortality from 21% to 6% and decreased variability in seizure severity between animals/batches were observed when compared to the SHD group. There was a significant increase in the percentage of animals that reached stage-5 seizures (65% versus 96%) in the RLD group. Integrated real-time video-EEG analysis of both groups, using NeuroScore software, revealed stage-specific spikes and power spectral density characteristics. When the seizures progressed from non-convulsive seizures (NCS, stage 1-2) to CMS (stage 3-5), the delta power decreased which was followed by an increase in gamma and beta power. A transient increase in alpha and sigma power marked the transition from NCS to CMS with characteristic 'high frequency trigger' spikes on the EEG, which had no behavioral expression. During SE the spike rate was higher in the RLD group than in the SHD group. Overall these results confirm that RLD of KA is a more robust and consistent mouse model of SE than the SHD of KA mouse model.

When and how do seizures kill neurons, and is cell death relevant to epileptogenesis?

The effect of seizures on neuronal death and the role of seizure-induced neuronal death in acquired epileptogenesis have been debated for decades. Isolated brief seizures probably do not kill neurons; however, severe and repetitive seizures (i.e., status epilepticus) certainly do. Because status epilepticus both kills neurons and also leads to chronic epilepsy, neuronal death has been proposed to be an integral part of acquired epileptogenesis. Several studies, particularly in the immature brain, have suggested that neuronal death is not necessary for acquired epileptogenesis; however, the lack of neuronal death is difficult if not impossible to prove, and more recent studies have challenged this concept. Novel mechanisms of cell death, beyond the traditional concepts of necrosis and apoptosis, include autophagy, phagoptosis, necroptosis, and pyroptosis. The traditional proposal for why neuronal death may be necessary for epileptogenesis is based on the recapitulation of development hypothesis, where a loss of synaptic input from the dying neurons is considered a critical signal to induce axonal sprouting and synaptic-circuit reorganization. We propose a second hypothesis - the neuronal death pathway hypothesis, which states that the biochemical pathways causing programmed neurodegeneration, rather than neuronal death per se, are responsible for or contribute to epileptogenesis. The reprogramming of neuronal death pathways - if true - is proposed to derive from necroptosis or pyroptosis. The proposed new hypothesis may inform on why neuronal death seems closely linked to epileptogenesis, but may not always be.

Neuronal degeneration is observed in multiple regions outside the hippocampus after lithium pilocarpine-induced status epilepticus in the immature rat

Although hippocampal sclerosis is frequently identified as a possible epileptic focus in patients with temporal lobe epilepsy, neuronal loss has also been observed in additional structures, including areas outside the temporal lobe. The claim from several researchers using animal models of acquired epilepsy that the immature brain can develop epilepsy without evidence of hippocampal neuronal death raises the possibility that neuronal death in some of these other regions may also be important for epileptogenesis. The present study used the lithium pilocarpine model of acquired epilepsy in immature animals to assess which structures outside the hippocampus are injured acutely after status epilepticus. Sprague-Dawley rat pups were implanted with surface EEG electrodes, and status epilepticus was induced at 20 days of age with lithium pilocarpine. After 72 h, brain tissue from 12 animals was examined with Fluoro-Jade B, a histochemical marker for degenerating neurons. All animals that had confirmed status epilepticus demonstrated Fluoro-Jade B staining in areas outside the hippocampus. The most prominent staining was seen in the thalamus (mediodorsal, paratenial, reuniens, and ventral lateral geniculate nuclei), amygdala (ventral lateral, posteromedial, and basomedial nuclei), ventral premammillary nuclei of hypothalamus, and paralimbic cortices (perirhinal, entorhinal, and piriform) as well as parasubiculum and dorsal endopiriform nuclei. These results demonstrate that lithium pilocarpine-induced status epilepticus in the immature rat brain consistently results in neuronal injury in several distinct areas outside of the hippocampus. Many of these regions are similar to areas damaged in patients with temporal lobe epilepsy, thus suggesting a possible role in epileptogenesis.

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.

Variability of sclerosis along the longitudinal hippocampal axis in epilepsy: a post mortem study

Detailed neuropathological studies of the extent of hippocampal sclerosis (HS) in epilepsy along the longitudinal axis of the hippocampus are lacking. Neuroimaging studies of patients with temporal lobe epilepsy support that sclerosis is not always localised. The extent of HS is of relevance to surgical planning and poor outcomes may relate to residual HS in the posterior remnant. In 10 post mortems from patients with long histories of drug refractory epilepsy and 3 controls we systematically sampled the left and right hippocampus at seven coronal anatomical levels along the body to the tail. We quantified neuronal densities in CA1 and CA4 subfields at each level using Cresyl Violet (CV), calretinin (CR), calbindin (CB) and Neuropeptide Y (NPY) immunohistochemistry. In the dentate gyrus we graded the extent of granule cell dispersion, patterns of CB expression, and synaptic reorganisation with CR and NPY at each level. We identified four patterns of HS based on patterns of pyramidal and interneuronal loss and dentate gyrus reorganisation between sides and levels as follows: (1) symmetrical HS with anterior-posterior (AP) gradient, (2) symmetrical HS without AP gradient, (3) asymmetrical HS with AP gradient and (4) asymmetrical cases without AP gradient. We confirmed in this series that HS can extend into the tail. The patterns of sclerosis (classical versus atypical or none) were consistent between all levels in less than a third of cases. In conclusion, this series highlights the variability of HS along the longitudinal axis. Further studies are required to identify factors that lead to focal versus diffuse HS.

Recording EEG in immature rats with a novel miniature telemetry system

Serial EEG recordings from immature rat pups are extremely difficult to obtain but important for analyzing animal models of neonatal seizures and other pediatric neurological conditions as well as normal physiology. In this report, we describe the features and applications of a novel miniature telemetry system designed to record EEG in rat pups as young as postnatal day 6 (P6). First, we have recorded electrographic seizure activity in two animal models of neonatal seizures, hypoxia- and kainate-induced seizures at P7. Second, we describe a viable approach for long-term continuous EEG monitoring of naturally reared rat pups implanted with EEG at P6. Third, we have used serial EEG recordings to record age-dependent changes in the background EEG signal as the animals matured from P7 to P11. The important advantages of using miniature wireless EEG technology are: 1) minimally invasive surgical implantation; 2) a device form-factor that is compatible with housing of rat pups with the dam and littermates; 3) serial recordings of EEG activity; and 4) low power consumption of the unit, theoretically allowing continuous monitoring for up to 2 yr without surgical reimplantation. The miniature EEG telemetry system provides a technical advance that allows researchers to record continuous and serial EEG recordings in neonatal rodent models of human neurological disorders, study the progression of the disease, and then assess possible therapies using quantitative EEG as an outcome measure. This new technical approach should improve animal models of human conditions that rely on EEG monitoring for diagnosis and therapy.

Intrinsic excitability of CA1 pyramidal neurones from the rat dorsal and ventral hippocampus

The hippocampus has a central role in learning and memory. Although once considered a relatively homogenous structure along the longitudinal axis, it has become clear that the rodent hippocampus can be anatomically and functionally divided into a dorsal component generally associated with spatial navigation, and a ventral component primarily associated with non-spatial functions that involve an emotional component. The ventral hippocampus (VHC) is also more sensitive to epileptogenic stimuli than the dorsal hippocampus (DHC), and seizures tend to originate in the VHC before spreading to other brain regions. Although synaptic and biochemical differences in DHC and VHC have been investigated, the intrinsic excitability of individual neurones from the DHC and VHC has received surprisingly little attention. In this study, we have characterized the intrinsic electrophysiological properties of CA1 pyramidal neurones from the DHC and the VHC using the whole-cell current-clamp method. Our results demonstrate that somatic current injections of equal magnitude elicit significantly more action potentials in VHC neurones than DHC neurones, and that this difference stems from the more depolarized resting membrane potential (RMP; 7 mV) and higher input resistance (R(in); 46 M measured from RMP) observed in VHC neurones. These differences in RMP and R(in) were also observed in dendritic whole-cell current-clamp recordings. Furthermore, morphological reconstructions of individual neurones revealed significant differences in the dendritic branching pattern between DHC and VHC neurones that could, in principle, contribute to the lower somatic R(in) of DHC neurones. Together, our results highlight significant differences in the intrinsic electrophysiological properties of CA1 pyramidal neurones across the longitudinal hippocampal axis, and suggest that VHC neurones are intrinsically more excitable than DHC neurones. This difference is likely to predispose the VHC to hyperexcitability.

Remodeling of hippocampal network in pilocarpine-treated mice expressing synaptopHluorin in the mossy fiber terminals

Pilocarpine-induced seizures induce an ectopic projection of hippocampal mossy fibers (MFs). Here, the sprouting was directly examined using TV-42 mice that express synaptopHluorin (SpH) selectively in the MF boutons. The SpH was ectopically expressed in the inner molecular layer (IML) of the dentate gyrus in typical mice after seizures, but were not always accompanied by the zinc fluorescence. The expression of SpH also has a tendency to be enhanced in layers of the CA3a region. It is suggested that the abnormal connection of neurons is more widespread than expected based on the previous zinc-detecting methods.