Seizure-induced changes in place cell physiology: relationship to spatial memory

Distinct changes to hippocampal and medial entorhinal circuits emerge across the progression of cognitive deficits in epilepsy

Temporal lobe epilepsy (TLE) causes pervasive and progressive memory impairments, yet the specific circuit changes that drive these deficits remain unclear. To investigate how hippocampal-entorhinal dysfunction contributes to progressive memory deficits in epilepsy, we performed simultaneous in vivo electrophysiology in hippocampus (HPC) and medial entorhinal cortex (MEC) of control and epileptic mice 3 or 8 weeks after pilocarpine-induced status epilepticus (Pilo-SE). We found that HPC synchronization deficits (including reduced theta power, coherence, and altered interneuron spike timing) emerged within 3 weeks of Pilo-SE, aligning with early-onset, relatively subtle memory deficits. In contrast, abnormal synchronization within MEC and between HPC-MEC emerged later, by 8 weeks after Pilo-SE, when spatial memory impairment was more severe. Furthermore, a distinct subpopulation of MEC layer 3 excitatory neurons (active at theta troughs) was specifically impaired in epileptic mice. Together, these findings suggest that hippocampal-entorhinal circuit dysfunction accumulates and shifts as cognitive impairment progresses in TLE.

Mechanisms for Cognitive Impairment in Epilepsy: Moving Beyond Seizures

There has been a major emphasis on defining the role of seizures in the causation of cognitive impairments like memory deficits in epilepsy. Here we focus on an alternative hypothesis behind these deficits, emphasizing the mechanisms of information processing underlying healthy cognition characterized as rate, temporal and population coding. We discuss the role of the underlying etiology of epilepsy in altering neural networks thereby leading to both the propensity for seizures and the associated cognitive impairments. In addition, we address potential treatments that can recover the network function in the context of a diseased brain, thereby improving both seizure and cognitive outcomes simultaneously. This review shows the importance of moving beyond seizures and approaching the deficits from a system-level perspective with the guidance of network neuroscience.

Status epilepticus and early development: neuronal injury, neurodegeneration, and their consequences

Evidence showing that the immature brain is vulnerable to seizure-induced damage has been accumulating for decades. Clinical data have always suggested that some early-life seizures are associated with negative sequelae, but clinical observations are frequently obscured by multiple uncontrolled contributing factors and can rarely establish causality. Determining with certainty that seizures, per se, can cause neuronal death and can irreversibly disrupt critical developmental processes, required the development of suitable model systems. Several experimental seizure models clearly show that the immature brain can sustain neuronal injury as a result of uncontrolled seizure activity and that even in the absence of observable neuronal death, the developing brain is selectively vulnerable to interruptions of required growth programs. Severe early-life seizures inhibit DNA, RNA, and protein synthesis, and they can reduce the accumulation of myelin and synaptic markers in the developing nervous system, leading to functional delays in development. Depending on the seizure pathway involved, and the developmental period under study, classic neurodegeneration, excitotoxicity, and apoptosis can result in permanent damage to critical neural networks in the temporal lobe and in many other brain regions. This conclusion is further supported by recent clinical studies showing that prolonged febrile status epilepticus can lead to hippocampal injury, which evolves into hippocampal atrophy and hippocampal sclerosis. A growing body of experimental data demonstrates that the metabolic compromise and cellular loss produced by seizures during critical phases of brain development negatively affect later hippocampal physiology including learning and memory functions in maturity.

Dysregulation of Npas4 and Inhba expression and an altered excitation-inhibition balance are associated with cognitive deficits in DBA/2 mice

Differences in the learning associated transcriptional profiles between mouse strains with distinct learning abilities could provide insight into the molecular basis of learning and memory. The inbred mouse strain DBA/2 shows deficits in hippocampus-dependent memory, yet the transcriptional responses to learning and the underlying mechanisms of the impairments are unknown. Comparing DBA/2J mice with the reference standard C57BL/6N mouse strain we verify an enhanced susceptibility to kainic acid induced seizures, confirm impairments in hippocampus-dependent spatial memory tasks and uncover additional behavioral abnormalities including deficits in hippocampus-independent learning. Surprisingly, we found no broad dysfunction of the DBA/2J strain in immediate early gene (IEG) activation but instead report brain region-specific and gene-specific alterations. The learning-associated IEGs Arc, c-Fos, and Nr4a1 showed no DBA/2J deficits in basal or synaptic activity induced gene expression in hippocampal or cortical primary neuronal cultures or in the CA1, CA3, or retrosplenial cortex following spatial object recognition (SOR) training in vivo. However, the parietal cortex showed reduced and the dentate gyrus showed enhanced SOR-evoked induction of most IEGs. All DBA/2J hippocampal regions exhibited elevated basal expression of inhibin β A (Inhba) and a learning-associated superinduction of the transcription factor neuronal Per-Arnt-Sim domain protein 4 (Npas4) known to regulate the synaptic excitation-inhibition balance. In line with this, CA1 pyramidal neurons of DBA/2J mice showed fewer inhibitory and more excitatory miniature postsynaptic currents but no alteration in most other electrophysiological properties or gross dendritic morphology. The dysregulation of Npas4 and Inhba expression and synaptic connectivity may underlie the cognitive deficits and increased susceptibility to seizures of DBA/2J mice.

An inventory of basic research in temporal lobe epilepsy

Temporal lobe epilepsy is a severe neurological disease, characterized by seizure occurrence and invalidating cognitive co-morbidities, which affects up to 1% of the adults. Roughly one third of the patients are resistant to any conventional pharmacological treatments. The last option in that case is the surgical removal of the epileptic focus, with no guarantee for clinical symptom alleviation. This state of affairs requests the identification of cellular or molecular targets for novel therapeutic approaches with limited side effects. Here we review some generalities about the disease as well as some of the most recent discoveries about the cellular and molecular mechanisms of TLE, and the latest perspectives for novel treatments.

Brains, complex systems and therapeutic opportunities in epilepsy

The treatment of epilepsy remains extremely challenging for the thirty percent of people that do not become seizure free. This is despite the introduction of multiple new drugs over that last several decades, highlighting the need for new approaches to identifying novel therapeutic strategies. Conceptualizing the brain as a complex adaptive system and applying the tools that are used in addressing such systems provides an opportunity for expanding the space in which to search for new therapies. Epilepsy has long been considered a network disease at the level of whole brain connectivity, but the application of the concepts to gene and protein expression networks as well as to the dynamic behaviors of microcircuits has been underexplored. These levels of the brain complex adaptive system will be reviewed and a case made for the epilepsy community to embrace these concepts in order to reap to enormous potential rewards.

Alterations of Neuronal Dynamics as a Mechanism for Cognitive Impairment in Epilepsy

Epilepsy is commonly associated with cognitive and behavioral deficits that dramatically affect the quality of life of patients. In order to identify novel therapeutic strategies aimed at reducing these deficits, it is critical first to understand the mechanisms leading to cognitive impairments in epilepsy. Traditionally, seizures and epileptiform activity in addition to neuronal injury have been considered to be the most significant contributors to cognitive dysfunction. In this review we however highlight the role of a new mechanism: alterations of neuronal dynamics, i.e. the timing at which neurons and networks receive and process neural information. These alterations, caused by the underlying etiologies of epilepsy syndromes, are observed in both animal models and patients in the form of abnormal oscillation patterns in unit firing, local field potentials, and electroencephalogram (EEG). Evidence suggests that such mechanisms significantly contribute to cognitive impairment in epilepsy, independently of seizures and interictal epileptiform activity. Therefore, therapeutic strategies directly targeting neuronal dynamics rather than seizure reduction may significantly benefit the quality of life of patients.

Modulation of Brain Hyperexcitability: Potential New Therapeutic Approaches in Alzheimer's Disease

People with Alzheimer's disease (AD) have significantly higher rates of subclinical and overt epileptiform activity. In animal models, oligomeric Aβ amyloid is able to induce neuronal hyperexcitability even in the early phases of the disease. Such aberrant activity subsequently leads to downstream accumulation of toxic proteins, and ultimately to further neurodegeneration and neuronal silencing mediated by concomitant tau accumulation. Several neurotransmitters participate in the initial hyperexcitable state, with increased synaptic glutamatergic tone and decreased GABAergic inhibition. These changes appear to activate excitotoxic pathways and, ultimately, cause reduced long-term potentiation, increased long-term depression, and increased GABAergic inhibitory remodelling at the network level. Brain hyperexcitability has therefore been identified as a potential target for therapeutic interventions aimed at enhancing cognition, and, possibly, disease modification in the longer term. Clinical trials are ongoing to evaluate the potential efficacy in targeting hyperexcitability in AD, with levetiracetam showing some encouraging effects. Newer compounds and techniques, such as gene editing via viral vectors or brain stimulation, also show promise. Diagnostic challenges include identifying best biomarkers for measuring sub-clinical epileptiform discharges. Determining the timing of any intervention is critical and future trials will need to carefully stratify participants with respect to the phase of disease pathology.

Status Epilepticus in Children

For various reasons, status epilepticus in children is different than in adults. Pediatric specificities include status epilepticus epidemiology, underlying etiologies, pathophysiological mechanisms, and treatment options. Relevant data from the literature are presented for each of them, and questions remaining open for future studies on status epilepticus in childhood are listed.

Disrupted Spatial Maps in Epilepsy Highlight the Importance of Timing in Neural Codes

Breakdown of Spatial Coding and Interneuron Synchronization in Epileptic Mice

Shuman T, Aharoni D, Cai DJ, et al. Nat Neurosci. 2020;23:229-238. doi:10.1038/s41593-019-0559-0.

Temporal lobe epilepsy causes severe cognitive deficits, but the circuit mechanisms remain unknown. Interneuron death and reorganization during epileptogenesis may disrupt the synchrony of hippocampal inhibition. To test this, we simultaneously recorded from the CA1 and dentate gyrus in pilocarpine-treated epileptic mice with silicon probes during head-fixed virtual navigation. We found desynchronized interneuron firing between the CA1 and dentate gyrus in epileptic mice. Since hippocampal interneurons control information processing, we tested whether CA1 spatial coding was altered in this desynchronized circuit, using a novel wire-free miniscope. We found that CA1 place cells in epileptic mice were unstable and completely remapped across a week. This spatial instability emerged around 6 weeks after status epilepticus, well after the onset of chronic seizures and interneuron death. Finally, CA1 network modeling showed that desynchronized inputs can impair the precision and stability of CA1 place cells. Together, these results demonstrate that temporally precise intrahippocampal communication is critical for spatial processing.

Breakdown of spatial coding and interneuron synchronization in epileptic mice

Temporal lobe epilepsy causes severe cognitive deficits, but the circuit mechanisms remain unknown. Interneuron death and reorganization during epileptogenesis may disrupt the synchrony of hippocampal inhibition. To test this, we simultaneously recorded from the CA1 and dentate gyrus in pilocarpine-treated epileptic mice with silicon probes during head-fixed virtual navigation. We found desynchronized interneuron firing between the CA1 and dentate gyrus in epileptic mice. Since hippocampal interneurons control information processing, we tested whether CA1 spatial coding was altered in this desynchronized circuit, using a novel wire-free miniscope. We found that CA1 place cells in epileptic mice were unstable and completely remapped across a week. This spatial instability emerged around 6 weeks after status epilepticus, well after the onset of chronic seizures and interneuron death. Finally, CA1 network modeling showed that desynchronized inputs can impair the precision and stability of CA1 place cells. Together, these results demonstrate that temporally precise intrahippocampal communication is critical for spatial processing.

Breakdown of spatial coding and interneuron synchronization in epileptic mice

Temporal lobe epilepsy causes severe cognitive deficits, but the circuit mechanisms remain unknown. Interneuron death and reorganization during epileptogenesis may disrupt the synchrony of hippocampal inhibition. To test this, we simultaneously recorded from the CA1 and dentate gyrus in pilocarpine-treated epileptic mice with silicon probes during head-fixed virtual navigation. We found desynchronized interneuron firing between the CA1 and dentate gyrus in epileptic mice. Since hippocampal interneurons control information processing, we tested whether CA1 spatial coding was altered in this desynchronized circuit, using a novel wire-free miniscope. We found that CA1 place cells in epileptic mice were unstable and completely remapped across a week. This spatial instability emerged around 6 weeks after status epilepticus, well after the onset of chronic seizures and interneuron death. Finally, CA1 network modeling showed that desynchronized inputs can impair the precision and stability of CA1 place cells. Together, these results demonstrate that temporally precise intrahippocampal communication is critical for spatial processing.

Long-lasting disruption of spatial memory by GABAB receptor antagonist induced seizures

The objective of this project was to test whether a drug-induced model of temporal lobe seizures, namely seizures induced by a gamma aminobutyric acid (GABA_B) receptor antagonist, CGP35348, result in long-term disruption of hippocampal memory function. Seizures were induced in experimental rats by intracerebroventricular (i.c.v.) injection of CGP35348 (0.64 μmol in 3 μL) for three consecutive days; control rats received no injection. Rats were first trained to criterion on an open radial arm maze (RAM) with 4 of the 8 arms baited, then received seizure and control treatment, and tested again on the RAM during the first week (days 1-5) and fourth week (days 22-29) after the last injection. An initial i.c.v. CGP35348 injection induced a mean of 4.4 seizures in the hippocampus, often accompanied with stages 3-5 convulsions, and sometimes with jumping; three daily CGP35348 injections induced 10.4 ± 1.8 (n = 7 rats) seizures in total. In two separate experiments, seizure-treated rats performed worse than control rats in working memory (WM) during both the 1st and 4th weeks after seizures. Reference memory (RM) deficit during the 1st week after seizures was observed in only one experiment in which RM was acquired >2 weeks ago. The memory deficits were not accompanied by gross neuronal loss in the hippocampus. In conclusion, i.c.v. injection of a GABA_B receptor antagonist in adult rats induced brief, multiple, focal hippocampal seizures that induced deficits in spatial memory for up to 4 weeks.

The impact of pathological high-frequency oscillations on hippocampal network activity in rats with chronic epilepsy

In epilepsy, brain networks generate pathological high-frequency oscillations (pHFOs) during interictal periods. To understand how pHFOs differ from normal oscillations in overlapping frequency bands and potentially perturb hippocampal processing, we performed high-density single unit and local field potential recordings from hippocampi of behaving rats with and without chronic epilepsy. In epileptic animals, we observed two types of co-occurring fast oscillations, which by comparison to control animals we could classify as ‘ripple-like’ or ‘pHFO’. We compared their spectral characteristics, brain state dependence, and cellular participants. Strikingly, pHFO occurred irrespective of brain state, were associated with interictal spikes, engaged distinct subnetworks of principal neurons compared to ripple-like events, increased the sparsity of network activity, and initiated both general and immediate disruptions in spatial information coding. Taken together, our findings suggest that events that result in pHFOs have an immediate impact on memory processes, corroborating the need for proper classification of pHFOs to facilitate therapeutic interventions that selectively target pathological activity.

Transient Cognitive Impairment in Epilepsy

Impairments of the dialog between excitation and inhibition (E/I) is commonly associated to neuropsychiatric disorders like autism, bipolar disorders and epilepsy. Moderate levels of hyperexcitability can lead to mild alterations of the EEG and are often associated with cognitive deficits even in the absence of overt seizures. Indeed, various testing paradigms have shown degraded performances in presence of acute or chronic non-ictal epileptiform activity. Evidences from both animal models and the clinics suggest that anomalous activity can cause cognitive deficits by transiently disrupting cortical processing, independently from the underlying etiology of the disease. Here, we will review our understanding of the influence of an abnormal EEG activity on brain computation in the context of the available clinical data and in genetic or pharmacological animal models.

Treatment of early life status epilepticus: What can we learn from animal models?

Treatment of status epilepticus (SE) in infants and children is challenging. There is a recognition that a broad set of developmental processes need to be considered to fully appreciate the physiologic complexity of severe seizures, and seizure outcomes, in infants and children. The development and use of basic models to elucidate important mechanisms will help further our understanding of these processes. Here we review some of the key experimental models and consider several areas relevant to treatment that could lead to productive translational research. Terminating seizures quickly is essential. Understanding pharmacoresistance of SE as it relates to receptor trafficking will be critical to seizure termination. Once a severe seizure is terminated, how will the developing brain respond? Basic studies suggest that there are important acute and long‐term histopathologic, and pathophysiologic, consequences that, if left unaddressed, will produce long‐lasting deficits on the form and function of the central nervous system. To fully utilize the evidence that basic models produce, age‐ and development‐ and model‐specific frameworks have to be considered carefully. Studies have demonstrated that severe seizures can cause perturbations to developmental processes during critical periods of development that lead to life‐long deficits. Unfortunately, some of the drugs that are commonly used to treat seizures may also produce negative outcomes by enhancing Cl^‐‐mediated depolarization, or by accelerating programmed cell death. More research is needed to understand these phenomena and their relevance to the human condition, and to develop rational drugs that protect the developing brain from severe seizures to the fullest extent possible.

Long-Term Effects of Anterior Thalamic Nucleus Deep Brain Stimulation on Spatial Learning in the Pilocarpine Model of Temporal Lobe Epilepsy

INTRODUCTION AND OBJECTIVES

Cognitive impairment is a significant comorbidity of temporal lobe epilepsy that is associated with extensive hippocampal cell loss. Deep brain stimulation (DBS) of the anterior thalamic nucleus (ANT) has been used for the treatment of refractory partial seizures. In the pilocarpine model of epilepsy, ANT DBS applied during status epilepticus (SE) reduces hippocampal inflammation and apoptosis. When given to chronic epileptic animals it reduces hippocampal excitability and seizure frequency. Here, we tested whether ANT DBS delivered during SE and the silent phase of the pilocarpine model would reduce cognitive impairment when animals became chronically epileptic.

MATERIALS AND METHODS

SE was induced by a systemic pilocarpine injection (320 mg/kg). Immediately after SE onset, rats were assigned to receive DBS during the first six hours of SE (n = 8; DBSa group) or during SE + the silent period (i.e., 6 h/day until the animals developed the first spontaneous recurrent seizure; n = 10; DBSs group). Four months following SE, animals underwent water maze testing and histological evaluation. Nonstimulated chronic epileptic animals (n = 13; PCTL group) and age-matched naïve rats (n = 11, CTL group) were used as controls. Results were analyzed by repeated-measures analyses of variance (RM_ANOVA) and one-way ANOVAs, followed by Newman-Keuls post hoc tests.

RESULTS

Although all groups learned the spatial task, epileptic animals with or without DBS spent significantly less time in the platform quadrant, denoting a spatial memory deficit (p < 0.02). Despite these negative behavioral results, we found that animals given DBS had a significantly higher number of cells in the CA1 region and dentate gyrus. Mossy fiber sprouting was similar among all epileptic groups.

CONCLUSIONS

Despite lesser hippocampal neuronal loss, ANT DBS delivered either during SE or during SE and the silent phase of the pilocarpine model did not mitigate memory deficits in chronic epileptic rats.

Calpastatin Overexpression Preserves Cognitive Function Following Seizures, While Maintaining Post-Injury Neurogenesis

In the adult mammalian brain, new neurons continue to be produced throughout life in two main regions in the brain, the subgranular zone (SGZ) in the hippocampus and the subventricular zone in the walls of the lateral ventricles. Neural stem cells (NSCs) proliferate in these niches, and migrate as neuroblasts, to further differentiate in locations where new neurons are needed, either in normal or pathological conditions. However, the endogenous attempt of brain repair is not very efficient. Calpains are proteases known to be involved in neuronal damage and in cell proliferation, migration and differentiation of several cell types, though their effects on neurogenesis are not well known. Previous work by our group has shown that the absence of calpastatin (CAST), the endogenous inhibitor of calpains, impairs early stages of neurogenesis. Since the hippocampus is highly associated with learning and memory, we aimed to evaluate whether calpain inhibition would help improve cognitive recovery after lesion and efficiency of post-injury neurogenesis in this region. For that purpose, we used the kainic acid (KA) model of seizure-induced hippocampal lesion and mice overexpressing CAST. Selected cognitive tests were performed on the 3rd and 8th week after KA-induced lesion, and cell proliferation, migration and differentiation in the dentate gyrus (DG) of the hippocampus of adult mice were analyzed using specific markers. Cognitive recovery was evaluated by testing the animals for recognition, spatial and associative learning and memory. Cognitive function was preserved by CAST overexpression following seizures, while modulation of post-injury neurogenesis was similar to wild type (WT) mice. Calpain inhibition could still be potentially able to prevent the impairment in the formation of new neurons, given that the levels of calpain activity could be reduced under a certain threshold and other harmful effects from the pathological environment could also be controlled.

Melatonin Alleviates the Epilepsy-Associated Impairments in Hippocampal LTP and Spatial Learning Through Rescue of Surface GluR2 Expression at Hippocampal CA1 Synapses

Epilepsy-associated cognitive impairment is common, and negatively impacts patients' quality of life. However, most antiepileptic drugs focus on the suppression of seizures, and fewer emphasize treatment of cognitive dysfunction. Melatonin, an indolamine synthesized primarily in the pineal grand, is reported to be neuroprotective against several central nervous system disorders. In this study, we investigated whether melatonin could reverse cognitive dysfunction in lithium-pilocarpine treated rats. Chronic treatment with melatonin (8 mg/kg daily for 15 days) after induction of status epilepticus significantly alleviated seizure severity, reduced neuronal death in the CA1 region of the hippocampus, improved spatial learning (as measured by the Morris water maze test), and reversed LTP impairments, compared to vehicle treatment. Furthermore, we found that melatonin rescued the decreased surface levels of GluR2 in the CA1 region observed in epilepsy, which might be the underlying mechanism of the neuroprotective and synapse-modulating function of melatonin. Our study provides experimental evidence for the possible clinical utility of melatonin as an adjunctive therapy to prevent epilepsy-associated cognitive impairments.

Pilocarpine-Induced Status Epilepticus Is Associated with Changes in the Actin-Modulating Protein Synaptopodin and Alterations in Long-Term Potentiation in the Mouse Hippocampus

Epilepsy is a complex neurological disorder which can severely affect neuronal function. Some patients may experience status epilepticus, a life-threatening state of ongoing seizure activity associated with postictal cognitive dysfunction. However, the molecular mechanisms by which status epilepticus influences brain function beyond seizure activity remain not well understood. Here, we addressed the question of whether pilocarpine-induced status epilepticus affects synaptopodin (SP), an actin-binding protein, which regulates the ability of neurons to express synaptic plasticity. This makes SP an interesting marker for epilepsy-associated alterations in synaptic function. Indeed, single dose intraperitoneal pilocarpine injection (250 mg/kg) in three-month-old male C57BL/6J mice leads to a rapid reduction in hippocampal SP-cluster sizes and numbers (in CA1 stratum radiatum of the dorsal hippocampus; 90 min after injection). In line with this observation (and previous work using SP-deficient mice), a defect in the ability to induce long-term potentiation (LTP) of Schaffer collateral-CA1 synapses is observed. Based on these findings we propose that status epilepticus could exert its aftereffects on cognition at least in part by perturbing SP-dependent mechanisms of synaptic plasticity.

Why Are Children With Epileptic Encephalopathies Encephalopathic?

The epileptic encephalopathies are devastating conditions characterized by frequent seizures, severely abnormal electroencephalograms (EEGs), and cognitive slowing or regression. The cognitive impairment in the epileptic encephalopathies may be more concerning to the patient and parents than the epilepsy itself. There is increasing recognition that the cognitive comorbidity can be both chronic, primarily due to the underlying etiology of the epilepsy, and dynamic or evolving because of recurrent seizures, interictal spikes, and antiepileptic drugs. Much of scholars' understanding of the neurophysiological underpinnings of cognitive dysfunction in the epileptic encephalopathies comes from rodent studies. Frequent seizures and interictal EEG discharges in rats lead to considerable spatial and social-cognitive deficits. Paralleling these cognitive deficits are dyscoordination of dynamic neural activity within and between the neural networks that subserve normal cognitive processes.

Effects of pharmacological treatments on hippocampal NCAM1 and ERK2 expression in epileptic rats with cognitive dysfunction

The present study aimed to investigate the effects of various pharmacological agents on the hippocampal expression of neural cell adhesion molecule 1 (NCAM1) and extracellular signal-regulated kinase 2 (ERK2) in epileptic rats with cognitive dysfunction. The experiments were conducted using 120 Wistar rats: 20 controls and 100 with pilocarpine-induced status epilepticus (SE). The SE rats were randomly assigned to 5 groups (n=20/group) that received daily treatments for 1 month with one of the following: (i) saline (no effect on epilepsy); (ii) carbamazepine (an anticonvulsant); (iii) oxcarbazepine (an anticonvulsant); (iv) aniracetam (a nootropic); or (v) donepezil (an acetylcholinesterase inhibitor). Spatial learning and memory were assessed using a Morris Water Maze (MWM). Hippocampal tissue was assessed for NCAM1 and ERK2 messenger RNA (mRNA) expression by reverse transcription polymerase chain reaction, and protein expression by immunochemistry. The results revealed that SE rats had significantly poorer MWM performances compared with controls (P<0.01). Performance in SE rats was improved with donepezil treatment (P<0.01), but declined with carbamazepine (P<0.01). Compared with controls, saline-treated SE rats exhibited increased hippocampal NCAM1 mRNA expression (P<0.01). Among SE rats, NCAM1 mRNA expression was highest in those treated with donepezil, followed by aniracetam-, saline-, oxcarbazepine- and carbamazepine-treated rats. Compared to controls, saline-treated SE rats exhibited decreased hippocampal ERK2 mRNA expression (P<0.01). Among SE rats, ERK2 mRNA expression was highest in those treated with donepezil, followed by aniracetam, saline, oxcarbazepine and carbamazepine. NCAM1 and ERK2 protein expression levels were parallel to those of the mRNA. In saline-treated SE rats, hippocampal ERK2 expression was decreased and NCAM1 expression was increased; thus, these two molecules may be involved in the impairment of spatial memory. Carbamazepine augmented this impairment, whereas donepezil was found to ameliorate the dysfunction associated with epilepsy. In conclusion, ERK2 and NCAM1 have significant roles in impairment of spatial memory in SE rats. Carbamazepine may increase this impairment, while donepezil may decrease this impairment.

Spatial cognition following early-life seizures in rats: Performance deficits are dependent on task demands

Cognitive impairment is a common comorbidity in childhood epilepsy. Studies in rodents have demonstrated that frequent seizures during the first weeks of life result in impaired spatial cognition when the rats are tested as juvenile or adults. To determine if spatial cognitive deficits following early-life seizures are task-specific or similar across spatial tasks, we compared the effects of early-life seizures in two spatial assays: 1) the Morris water maze, a hippocampal-dependent task of spatial cognition and 2) the active avoidance task, a task that associates an aversive shock stimulus with a static spatial location that requires intact hippocampal-amygdala networks. Rats with early-life seizures tested as adults did not differ from control rats in the water maze. However, while animals with early-life seizures showed some evidence of learning the active avoidance task, they received significantly more shocks in later training trials, particularly during the second training day, than controls. One possibility for the performance differences between the tasks is that the active avoidance task requires multiple brain regions and that interregional communication could be affected by alterations in white matter integrity. However, there were no measurable group differences with regard to levels of myelination. The study suggests that elucidation of mild cognitive deficits seen following early-life seizures may be dependent on task features of active avoidance.

Effect of Seizures on the Developing Brain and Cognition

Epilepsy is a complex disorder, which involves much more than seizures, encompassing a range of associated comorbid health conditions that can have significant health and quality-of-life implications. Of these comorbidities, cognitive impairment is one of the most common and distressing aspects of epilepsy. Clinical studies have demonstrated that refractory seizures, resistant to antiepileptic drugs, and occurring early in life have significant adverse effects on cognitive function. Much of what has been learned about the neurobiological underpinnings of cognitive impairment following early-life seizures has come from animal models. Although early-life seizures in rodents do not result in cell loss, seizures cause in changes in neurogenesis and synaptogenesis and alteration of excitatory or inhibitory balance, network connectivity and temporal coding. These morphological and physiological changes are accompanied by parallel impairment in cognitive skills. This increased understanding of the pathophysiological basis of seizure-induced cognitive deficits should allow investigators to develop novel targets for therapeutic interventions.

Interictal epileptiform discharges induce hippocampal-cortical coupling in temporal lobe epilepsy

Interactions between the hippocampus and cortex are critical for memory. Interictal epileptiform discharges (IEDs) identify epileptic brain regions and can impair memory, but how they interact with physiological patterns of network activity is mostly undefined. We show in a rat model of temporal lobe epilepsy that spontaneous hippocampal IEDs correlate with impaired memory consolidation and are precisely coordinated with spindle oscillations in the prefrontal cortex during NREM sleep. This coordination surpasses the normal physiological ripple-spindle coupling and is accompanied by decreased ripple occurrence. IEDs also induce spindles during REM sleep and wakefulness, behavioral states that do not naturally express these oscillations, by generating a cortical ‘DOWN’ state. We confirm a similar correlation of temporofrontal IEDs with spindles over anatomically restricted cortical regions in a pilot clinical examination of four subjects with focal epilepsy. These findings imply that IEDs may impair memory via misappropriation of physiological mechanisms for hippocampal-cortical coupling, suggesting a target to treat memory impairment 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.

T2 relaxation time post febrile status epilepticus predicts cognitive outcome

Evidence from animal models and patient data indicates that febrile status epilepticus (FSE) in early development can result in permanently diminished cognitive abilities. To understand the variability in cognitive outcome following FSE, we used MRI to measure dynamic brain metabolic responses to the induction of FSE in juvenile rats. We then compared these measurements to the ability to learn an active avoidance spatial task weeks later. T2 relaxation times were significantly lower in FSE rats that were task learners in comparison to FSE non-learners. While T2 time in whole brain held the greatest predictive power, T2 in hippocampus and basolateral amygdala were also excellent predictors. These signal differences in response to FSE indicate that rats that fail to meet metabolic and oxygen demand are more likely to develop spatial cognition deficits. Place cells from FSE non-learners had significantly larger firing fields and higher in-field firing rate than FSE learners and control animals and imply increased excitability in the pyramidal cells of FSE non-learners. These findings suggest a mechanistic cause for the spatial memory deficits in active avoidance and are relevant to other acute neurological insults in early development where cognitive outcome is a concern.

Impaired neuronal operation through aberrant intrinsic plasticity in epilepsy

OBJECTIVE

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

METHODS

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

RESULTS

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

INTERPRETATION

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

Early life seizures: evidence for chronic deficits linked to autism and intellectual disability across species and models

Recent work in Exp Neurol by Lugo et al. (2014b) demonstrated chronic alterations in sociability, learning and memory following multiple early life seizures (ELS) in a mouse model. This work adds to the growing body of evidence supporting the detrimental nature of ELS on the developing brain to contribute to aspects of an autistic phenotype with intellectual disability. Review of the face validity of behavioral testing and the construct validity of the models used informs the predictive ability and thus the utility of these models to translate underlying molecular and cellular mechanisms into future human studies.

Brain morphometry of Dravet syndrome

The aim of this study was to identify differential global and local brain structural patterns in Dravet Syndrome (DS) patients as compared with a control subject group, using brain morphometry techniques which provide a quantitative whole-brain structural analysis that allows for specific patterns to be generalized across series of individuals. Nine patients with the diagnosis of DS that tested positive for mutation in the SCN1A gene and nine well-matched healthy controls were investigated using voxel brain morphometry (VBM), cortical thickness and cortical gyrification measurements. Global volume reductions of gray matter (GM) and white matter (WM) were related to DS. Local volume reductions corresponding to several white matter regions in brainstem, cerebellum, corpus callosum, corticospinal tracts and association fibers (left inferior fronto-occipital fasciculus and left uncinate fasciculus) were also found. Furthermore, DS showed a reduced cortical folding in the right precentral gyrus. The present findings describe DS-related brain structure abnormalities probably linked to the expression of the SCN1A mutation.

Cognitive impairment in temporal lobe epilepsy: role of online and offline processing of single cell information

Cognitive impairment is a common comorbidity in temporal lobe epilepsy (TLE) and is often considered more detrimental to quality of life than seizures. While it has been previously shown that the encoding of memory during behavior is impaired in the pilocarpine model of TLE in rats, how this information is consolidated during the subsequent sleep period remains unknown. In this study, we first report marked deficits in spatial memory performance and severe cell loss in the CA1 layer of the hippocampus lower spatial coherence of firing in TLE rats. We then present the first evidence that the reactivation of behavior-driven patterns of activity of CA1 place cells in the hippocampus is intact in TLE rats. Using a template-matching method, we discovered that real-time (3-5 s) reactivation structure was intact in TLE rats. Furthermore, we estimated the entropy rate of short time scale (∼250 ms) bursting activity using block entropies and found that significant, extended temporal correlations exist in both TLE and control rats. Fitting a first-order Markov Chain model to these bursting time series, we found that long sequences derived from behavior were significantly enriched in the Markov model over corresponding models fit on randomized data confirming the presence of replay in shorter time scales. We propose that the persistent consolidation of poor spatial information in both real time and during bursting activity may contribute to memory impairments in TLE rats.

The epileptic encephalopathies

The term epileptic encephalopathy refers to the condition where epileptic activity, clinical or subclinical, is thought to be responsible for any disturbance of cognition, behavior, or motor control. Although currently described as a concept that may occur in any of the epilepsies, children with the severe early onset epilepsies are thought to be more at risk than others. These epilepsies have been termed the "epileptic encephalopathies." The degree to which epileptic activity is responsible for neurodevelopmental compromise may be variable in each individual case, and the degree to which this may be reversible unclear. Data from the laboratory and the clinic may provide greater insight into the degree to which epileptic activity may contribute in individual syndromes, although much is yet to be learnt. The aim in epilepsy management remains one of seizure control; in some specific circumstances this may include subclinical epileptic activity. However, avoidance of treatment that may lead to deterioration of seizure control may be equally important.

Disentangling the relationship between epilepsy and its behavioral comorbidities - the need for prospective studies in new-onset epilepsies

It has been long recognized that there is more to epilepsy than seizures. The prevalence of such neurobehavioral abnormalities as cognitive and mood disorders, autism spectrum disorder, and attention deficit and hyperactivity disorder (ADHD) is significantly higher among patients with epilepsy than in the general population. A long-held view that comorbidities of epilepsy represent mere epiphenomena of seizures has undergone substantial transformation during the past decade, as emerging clinical evidence and experimental evidence suggest the involvement of specific neurobiological mechanisms in the evolution of neurobehavioral deficits in patients with epilepsy. Developmental aspects of both epilepsy and its comorbidities, as well as the frequently reported reciprocal connection between these disorders, both add other dimensions to the already complex problem. In light of progress in effective seizure management in many patients with epilepsy, the importance of neurobehavioral comorbidities has become acute, as the latter are frequently more detrimental to patients' quality of life compared with seizures. This calls for a serious increase in efforts to effectively predict, manage, and ideally cure these comorbidities. Coordinated multicenter clinical, translational, and basic research studies focusing on epidemiology, neuropsychology, neurophysiology, imaging, genetics, epigenetics, and pharmacology of neurobehavioral comorbidities of epilepsy are absolutely instrumental for ensuring tangible progress in the field. Clinical research should focus more on new-onset epilepsy and put particular emphasis on longitudinal studies in large cohorts of patients and groups at risk, while translational research should primarily focus on the development of valid preclinical systems which would allow investigating the fundamental mechanism of epilepsy comorbidities. The final goal of the described research efforts would lie in producing an armamentarium of evidence-based diagnostic tools and therapeutic interventions which would at minimum mitigate and at maximum prevent or abolish neurobehavioral comorbidities of epilepsy and, thus, improve the quality of life of those patients with epilepsy who suffer from the said comorbidities.

Transient impact of spike on theta rhythm in temporal lobe epilepsy

Epileptic spike is an indicator of hyper-excitability and hyper-synchrony of neural networks. While cognitive deficit in epilepsy is a common observation, how spikes transiently influence brain oscillations, especially those essential for cognitive functions, remains obscure. Here we aimed to quantify the transient impacts of sporadic spikes on theta oscillations and investigate how such impacts may evolve during epileptogenesis. Longitudinal depth EEG data were recorded in the CA1 area of pilocarpine temporal lobe epilepsy (TLE) rat models. Phase stability, a measure of synchrony, and theta power were estimated around spikes as well as in the protracted spike-free periods (FP) at least 1h after spike bursts. We found that the change in theta power did not correlate with the change in phase stability. More importantly, the impact of spikes on theta rhythm was highly time-dependent. While theta power decreased abruptly after spikes both in the latent and chronic stages, changes of theta phase stability demonstrated opposite trends in the latent and chronic stages, potentially due to the substantial reorganization of neural circuits along epileptogenesis. During FP, theta phase stability was significantly higher than the baseline level before injections, indicating that hyper-synchrony remained even hours after the spike bursts. We concluded that spikes have transient negative effects on theta rhythm, however, impacts are different during latent and chronic stages, implying that its influence on cognitive processes may also change over time during epileptogenesis.

Continuous EEG monitoring in the neonatal intensive care unit

Continuous EEG monitoring provides an opportunity to both accurately identify seizures and monitor the neurologic status of critically ill neonates in the intensive care unit. The incidence of seizures is higher in the neonatal period than at any other time in life. Seizures and abnormalities of EEG background are associated with significant risk of mortality and long-term neurodevelopmental morbidities. In the neonatal population the majority of seizures are not clinically evident and go undetected without EEG monitoring. We review the incidence and risk factors for neonatal seizures, and the utility of continuous EEG monitoring in the neonatal intensive care unit for seizure detection and for analysis of background to allow prognostication. We consider the role of amplitude-integrated EEG in the neonatal population. We consider the utility of continuous EEG for frequently encountered neurologic indications and discuss the outcome data and some new developments in continuous EEG monitoring.

Speed modulation of hippocampal theta frequency correlates with spatial memory performance

Hippocampal theta rhythm is believed to play a critical role in learning and memory. In animal models of temporal lobe epilepsy (TLE), there is evidence that alterations of hippocampal theta oscillations are involved in the cognitive impairments observed in this model. However, hippocampal theta frequency and amplitude at both the local field potential (LFP) and single unit level are strongly modulated by running speed, suggesting that the integration of locomotor information into memory processes may also be critical for hippocampal processing. Here, we investigate whether hippocampal speed-theta integration influences spatial memory and whether it could account for the memory deficits observed in TLE rats. LFPs were recorded in both Control (CTR) and TLE rats as they were trained in a spatial alternation task. TLE rats required more training sessions to perform the task at CTR levels. Both theta frequency and power were significantly lower in the TLE group. In addition, speed/theta frequency correlation coefficients and regression slopes varied from session to session and were worse in TLE. Importantly, there was a strong relationship between speed/theta frequency parameters and performance. Our analyses reveal that speed/theta frequency correlation with performance cannot merely be explained by the direct influence of speed on behavior. Therefore, variations in the coordination of theta frequency with speed may participate in learning and memory processes. Impairments of this function could explain at least partially memory deficits in epilepsy.

Standardized environmental enrichment supports enhanced brain plasticity in healthy rats and prevents cognitive impairment in epileptic rats

Environmental enrichment of laboratory animals influences brain plasticity, stimulates neurogenesis, increases neurotrophic factor expression, and protects against the effects of brain insult. However, these positive effects are not constantly observed, probably because standardized procedures of environmental enrichment are lacking. Therefore, we engineered an enriched cage (the Marlau™ cage), which offers: (1) minimally stressful social interactions; (2) increased voluntary exercise; (3) multiple entertaining activities; (4) cognitive stimulation (maze exploration), and (5) novelty (maze configuration changed three times a week). The maze, which separates food pellet and water bottle compartments, guarantees cognitive stimulation for all animals. Compared to rats raised in groups in conventional cages, rats housed in Marlau™ cages exhibited increased cortical thickness, hippocampal neurogenesis and hippocampal levels of transcripts encoding various genes involved in tissue plasticity and remodeling. In addition, rats housed in Marlau™ cages exhibited better performances in learning and memory, decreased anxiety-associated behaviors, and better recovery of basal plasma corticosterone level after acute restraint stress. Marlau™ cages also insure inter-experiment reproducibility in spatial learning and brain gene expression assays. Finally, housing rats in Marlau™ cages after severe status epilepticus at weaning prevents the cognitive impairment observed in rats subjected to the same insult and then housed in conventional cages. By providing a standardized enriched environment for rodents during housing, the Marlau™ cage should facilitate the uniformity of environmental enrichment across laboratories.

Epileptogenesis in the developing brain

The neonatal brain has poorly developed GABAergic circuits, and in many of them GABA is excitatory, favoring ictogenicity. Frequently repeated experimental seizures impair brain development in an age-dependent manner. At critical ages, they delay developmental milestones, permanently lower seizure thresholds, and can cause very specific cognitive and learning deficits, such as the permanent impairment of neuronal spatial maps. Some types of experimental status epilepticus cause neuronal necrosis and apoptosis, and are followed by chronic epilepsy with spontaneous recurrent seizures, others appear relatively benign, so that seizure-induced neuronal injury and epileptogenesis are highly age-, seizure model-, and species-dependent. Experimental febrile seizures can be epileptogenic, and hyperthermia aggravates both neuronal injury and epileptogenicity. Antiepileptic drugs, the mainstay of treatment, have major risks of their own, and can, at therapeutic or near-therapeutic doses, trigger neuronal apoptosis, which is also age-, drug-, cell type-, and species-dependent. The relevance of these experimental results to human disease is still uncertain, but while their brains are quite different, the basic biology of neurons in rodents and humans is strikingly similar. Further research is needed to elucidate the molecular mechanisms of epileptogenesis and of seizure- or drug-induced neuronal injury, in order to prevent their long-term consequences.

Dravet syndrome: the main issues

Dravet syndrome (DS) is a severe form of infantile onset epilepsy characterized by multiple seizure types, prolonged convulsive seizures and frequent episodes of status epilepticus. Seizures precipitated by fever are a main characteristic. Affected children exhibit normal early development. Cognitive impairment, behavioral disturbances with hyperactivity and sometimes autistic traits occur after seizure onset. Seizures persist into adulthood but become less frequent. In about 85% of patients, a mutation of the SCN1A gene is present. DS fully illustrates the concept of epileptic encephalopathy. However, it is difficult to determine the causative role of the underlying sodium channel dysfunction and that of the consequent seizures in influencing cognitive outcome. An overwhelmingly high number of SCN1A mutations have been associated with DS. Intragenic or whole gene deletions, duplications and amplifications are additional rare molecular mechanisms. Most mutations are de novo, but familial mutations also occur. Somatic mosaic mutations should be considered when estimating the recurrence. MRI imaging is usually normal, and no neuropathologic signature of the condition seems to exist. In heterozygous Scn1a+/- mice, GABAergic interneurons exhibit substantially reduced sodium current density with reduced ability for sustained action potential firing. GABAergic output is reduced and excitability of downstream synaptic targets increased. Stiripentol was effective in combination with valproate and clobazam in two pivotal phase III trials. Phenytoin, carbamazepine, and lamotrigine can worsen seizures and should be avoided. Prospective studies will clarify to what extent earlier diagnosis and efforts at seizure control with the most appropriate drug combinations will reduce clinical deterioration.

Hippocampal desynchronization of functional connectivity prior to the onset of status epilepticus in pilocarpine-treated rats

Status epilepticus (SE), a pro-epileptogenic brain insult in rodent models of temporal lobe epilepsy, is successfully induced by pilocarpine in some, but not all, rats. This study aimed to identify characteristic alterations within the hippocampal neural network prior to the onset of SE. Sixteen microwire electrodes were implanted into the left hippocampus of male Sprague-Dawley rats. After a 7-day recovery period, animal behavior, hippocampal neuronal ensemble activities, and local field potentials (LFP) were recorded before and after an intra-peritoneal injection of pilocarpine (350 mg/kg). The single-neuron firing, population neuronal correlation, and coincident firing between neurons were compared between SE (n = 9) and nonSE rats (n = 12). A significant decrease in the strength of functional connectivity prior to the onset of SE, as measured by changes in coincident spike timing between pairs of hippocampal neurons, was exclusively found in SE rats. However, single-neuron firing and LFP profiles did not show a significant difference between SE and nonSE rats. These results suggest that desynchronization in the functional circuitry of the hippocampus, likely associated with a change in synaptic strength, may serve as an electrophysiological marker prior to SE in pilocarpine-treated rats.

Computational modeling of epilepsy

With the rapid rise in our knowledge about the structural and functional properties of neuronal microcircuits and the exponentially increasing power of computers, it has become possible to closely integrate experimental findings with large-scale, anatomically and biophysically realistic computational simulations of control and epileptic neuronal networks with unprecedented precision and predictive power. In this paper, we discuss the biological basis of model development, and outline specific applications, including exciting new computational and experimental results concerning the roles of aberrant hyperconnected hub-like neurons in seizures.

Promise of resveratrol for easing status epilepticus and epilepsy

Resveratrol (RESV; 3,5,4'-tri-hydroxy stilbene), a naturally occurring phytoalexin, is found at a high concentration in the skin of red grapes and red wine. RESV mediates a wide-range of biological activities, which comprise an increased life span, anti-ischemic, anti-cancer, antiviral, anti-aging and anti-inflammatory properties. Studies in several animal prototypes of brain injury suggest that RESV is an effective neuroprotective compound. Ability to enter the brain after a peripheral administration and no adverse effects on the brain or body are other features that are appealing for using this compound as a therapy for brain injury or neurodegenerative diseases. The goal of this review is to discuss the promise of RESV for treating acute seizures, preventing the acute seizure or status epilepticus induced development of chronic epilepsy, and easing the chronic epilepsy typified by spontaneous recurrent seizures and cognitive dysfunction. First, the various beneficial effects of RESV on the normal brain are discussed to provide a rationale for considering RESV treatment in the management of acute seizures and epilepsy. Next, the detrimental effects of acute seizures or status epilepticus on the hippocampus and the implications of post-status epilepticus changes in the hippocampus towards the occurrence of chronic epilepsy and cognitive dysfunction are summarized. The final segment evaluates studies that have used RESV as a neuroprotective compound against seizures, and proposes studies that are critically needed prior to the clinical application of RESV as a prophylaxis against the development of chronic epilepsy and cognitive dysfunction after an episode of status epilepticus or head injury.

Impaired cognition in rats with cortical dysplasia: additional impact of early-life seizures

One of the most common and serious co-morbidities in patients with epilepsy is cognitive impairment. While early-life seizures are considered a major cause for cognitive impairment, it is not known whether it is the seizures, the underlying neurological substrate or a combination that has the largest impact on eventual learning and memory. Teasing out the effects of seizures from pre-existing neurological disorder is critical in developing therapeutic strategies. We therefore investigated the additional cognitive effects of seizures on rodents with malformations of cortical development induced with methylazoxymethanol acetate. Pregnant rats were injected with saline or methylazoxymethanol acetate at embryonic Day 15 or 17 to induce differing malformation severity. From the day of birth to 9 days of age, half the pups received 50 flurothyl-induced seizures. All rats underwent testing in the Morris water maze to test spatial memory at 25 days of age (immediate post-weaning) or during adolescence at 45 days of age. Post-weaning rats had severe spatial cognitive deficits in the water maze and seizures worsened performance. In contrast, in animals tested during adolescence, there was no longer an additional adverse effect of seizures. We also investigated whether the severity of the structural abnormality and seizures impacted brain weight, cortical thickness, hippocampal area and cell dispersion area. The mean brain weight in control animals was greater than in rats exposed to methylazoxymethanol acetate at embryonic Day 17, which was greater than rats exposed to methylazoxymethanol acetate at embryonic Day 15. Rats exposed to methylazoxymethanol acetate at embryonic Day 15 had a thinner cortical mantle compared with rats exposed at embryonic Day 17 and control animals. The hippocampal area was similar in rats exposed at embryonic Days 15 and 17 but was smaller compared with controls. Methylazoxymethanol at embryonic Day 17 caused dispersion of the CA1-4 cell layers in the hippocampus, whereas methylazoxymethanol at embryonic Day 15 caused focal nodules in or above the CA1 layer, but the CA1-4 layers were intact and similar to control. Early-life seizures did not have a significant impact on any of these parameters. These observations indicate that the major factor responsible for the cognitive impairment in the rats with cortical dysplasia was the underlying brain substrate, not seizures. These findings have significant implications for the understanding of cognitive impairments in childhood epilepsy and suggest that early aggressive therapy of seizures alone may not be an adequate strategy for minimizing cognitive effects.