Early life seizures cause long-standing impairment of the hippocampal map

Altered short-term plasticity in the prefrontal cortex after early life seizures

Seizures during development are a relatively common occurrence and are often associated with poor cognitive outcomes. Recent studies show that early life seizures alter the function of various brain structures and have long-term consequences on seizure susceptibility and behavioral regulation. While many neocortical functions could be disrupted by epileptic seizures, we have concentrated on studying the prefrontal cortex (PFC) as disturbance of PFC functions is involved in numerous co-morbid disorders associated with epilepsy. In the present work we report an alteration of short-term plasticity in the PFC in rats that have experienced early life seizures. The most robust alteration occurs in the layer II/III to layer V network of neurons. However short-term plasticity of layer V to layer V network was also affected, indicating that the PFC function is broadly influenced by early life seizures. These data strongly suggest that repetitive seizures early in development cause substantial alteration in PFC function, which may be an important component underlying cognitive deficits in individuals with a history of seizures during development.

SCN1A mutations in Dravet syndrome: impact of interneuron dysfunction on neural networks and cognitive outcome

Dravet syndrome (DS) is a childhood disorder associated with loss-of-function mutations in SCN1A and is characterized by frequent seizures and severe cognitive impairment. Animal studies have revealed new insights into the mechanisms by which mutations in this gene, encoding the type I voltage-gated sodium channel (Na(v)1.1), may lead to seizure activity and cognitive dysfunction. In this review, we further consider the function of fast-spiking GABAergic neurons, one cell type particularly affected by these mutations, in the context of the temporal coordination of neural activity subserving cognitive functions. We hypothesize that disruptions in GABAergic firing may directly contribute to the poor cognitive outcomes in children with DS, and discuss the therapeutic implications of this possibility.

Seizures in early life suppress hippocampal dendrite growth while impairing spatial learning

Impaired learning and memory are common in epilepsy syndromes of childhood. Clinical investigations suggest that the developing brain may be particularly vulnerable to the effects of intractable seizure disorders. Magnetic resonance imaging (MRI) studies have demonstrated reduced volumes in brain regions involved in learning and memory. The earlier the onset of an epilepsy the larger the effects seem to be on both brain anatomy and cognition. Thus, childhood epilepsy has been proposed to interfere in some unknown way with brain development. Experiments reported here explore these ideas by examining the effects of seizures in infant mice on learning and memory and on the growth of CA1 hippocampal pyramidal cell dendrites. Fifteen brief seizures were induced by flurothyl between postnatal days 7 and 11 in mice that express green fluorescent protein (GFP) in hippocampal pyramidal cells. One to 44days later, dendritic arbors were reconstructed to measure growth. Spatial learning and memory were also assessed in a water maze. Our results show that recurrent seizures produced marked deficits in learning and memory. Seizures also dramatically slowed the growth of basilar dendrites while neurons in littermate control mice continued to add new dendritic branches and lengthen existing branches. When experiments were performed in older mice, seizures had no measureable effects on either dendrite arbor complexity or spatial learning and memory. Our results suggest that the recurring seizures of intractable childhood epilepsy contribute to associated learning and memory deficits by suppressing dendrite growth.

Enhanced oscillatory activity in the hippocampal-prefrontal network is related to short-term memory function after early-life seizures

Neurological insults during development are associated with later impairments in learning and memory. Although remedial training can help restore cognitive function, the neural mechanisms of this recovery in memory systems are largely unknown. To examine this issue, we measured electrophysiological oscillatory activity in the hippocampus (both CA3 and CA1) and prefrontal cortex of adult rats that had experienced repeated seizures in the first weeks of life, while they were remedially trained on a delayed-nonmatch-to-sample memory task. Seizure-exposed rats showed initial difficulties learning the task but performed similarly to control rats after extra training. Whole-session analyses illustrated enhanced theta power in all three structures while seizure rats learned response tasks before the memory task. While performing the memory task, dynamic oscillation patterns revealed that prefrontal cortex theta power was increased among seizure-exposed rats. This enhancement appeared after the first memory-training steps using short delays and plateaued at the most difficult steps, which included both short and long delays. Further, seizure rats showed enhanced CA1-prefrontal cortex theta coherence in correct trials compared with incorrect trials when long delays were imposed, suggesting increased hippocampal-prefrontal cortex synchrony for the task in this group when memory demand was high. Seizure-exposed rats also showed heightened gamma power and coherence among all three structures during the trials. Our results demonstrate the first evidence of hippocampal-prefrontal enhancements following seizures in early development. Dynamic compensatory changes in this network and interconnected circuits may underpin cognitive rehabilitation following other neurological insults to higher cognitive systems.

Epilepsy and brain inflammation

During the last decade, experimental research has demonstrated a prominent role of glial cells, activated in brain by various injuries, in the mechanisms of seizure precipitation and recurrence. In particular, alterations in the phenotype and function of activated astrocytes and microglial cells have been described in experimental and human epileptic tissue, including modifications in potassium and water channels, alterations of glutamine/glutamate cycle, changes in glutamate receptor expression and transporters, release of neuromodulatory molecules (e.g. gliotransmitters, neurotrophic factors), and induction of molecules involved in inflammatory processes (e.g. cytokines, chemokines, prostaglandins, complement factors, cell adhesion molecules) (Seifert et al., 2006; Vezzani et al., 2011; Wetherington et al., 2008). In particular, brain injury or proconvulsant events can activate microglia and astrocytes to release a number of proinflammatory mediators, thus initiating a cascade of inflammatory processes in brain tissue. Proinflammatory molecules can alter neuronal excitability and affect the physiological functions of glia by paracrine or autocrine actions, thus perturbing the glioneuronal communications. In experimental models, these changes contribute to decreasing the threshold to seizures and may compromise neuronal survival (Riazi et al., 2010; Vezzani et al., 2008). In this context, understanding which are the soluble mediators and the molecular mechanisms crucially involved in glio-neuronal interactions is instrumental to shed light on how brain inflammation may contribute to neuronal hyperexcitability in epilepsy. This review will report the clinical observations in drug-resistant human epilepsies and the experimental findings in adult and immature rodents linking brain inflammation to the epileptic process in a causal and reciprocal manner. By confronting the clinical evidence with the experimental findings, we will discuss the role of specific soluble inflammatory mediators in the etiopathogenesis of seizures, reporting evidence for both their acute and long term effects on seizure threshold. The possible contribution of these mediators to co-morbidities often described in epilepsy patients will be also discussed. Finally, we will report on the anti-inflammatory treatments with anticonvulsant actions in experimental models highlighting possible therapeutic options for treating drug-resistant seizures and for prevention of epileptogenesis.

Early-life seizures produce lasting alterations in the structure and function of the prefrontal cortex

Early-life seizures (ELS) are associated with long-term behavioral disorders including autism and ADHD, suggesting that frontal lobe structures may be permanently affected. We tested whether ELS produce structural alterations in the prefrontal cortex (PFC) and impair PFC-mediated function using an operant task of behavioral flexibility in rats. Adult rats that had been exposed to 75 flurothyl seizures during postnatal days 1-10 showed decreased behavioral flexibility in the task compared to controls over multiple behavioral sessions, measured as a lever preference asymmetry (p<0.001) and a decreased efficiency of attaining food rewards (p<0.05). ELS rats also showed an increased thickness of the PFC (p<0.01), primarily attributed to layer V (p<0.01) with no differences in cell density. These structural changes correlated with lever preference behavioral impairments (p<0.05). This study demonstrates that the consequences of ELS extend to the PFC, which may help explain the high prevalence of comorbid behavioral disorders following ELS.

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.

Development of later life spontaneous seizures in a rodent model of hypoxia-induced neonatal seizures

PURPOSE

To study the development of epilepsy following hypoxia-induced neonatal seizures in Long-Evans rats and to establish the presence of spontaneous seizures in this model of early life seizures.

METHODS

Long-Evans rat pups were subjected to hypoxia-induced neonatal seizures at postnatal day 10 (P10). Epidural cortical electroencephalography (EEG) and hippocampal depth electrodes were used to detect the presence of seizures in later adulthood (> P60). In addition, subdermal wire electrode recordings were used to monitor age at onset and progression of seizures in the juvenile period, at intervals between P10 and P60. Timm staining was performed to evaluate mossy fiber sprouting in the hippocampi of P100 adult rats that had experienced neonatal seizures.

KEY FINDINGS

In recordings made from adult rats (P60-180), the prevalence of epilepsy in cortical and hippocampal EEG recordings was 94.4% following early life hypoxic seizures. These spontaneous seizures were identified by characteristic spike and wave activity on EEG accompanied by behavioral arrest and facial automatisms (electroclinical seizures). Phenobarbital injection transiently abolished spontaneous seizures. EEG in the juvenile period (P10-60) showed that spontaneous seizures first occurred approximately 2 weeks after the initial episode of hypoxic seizures. Following this period, spontaneous seizure frequency and duration increased progressively with time. Furthermore, significantly increased sprouting of mossy fibers was observed in the CA3 pyramidal cell layer of the hippocampus in adult animals following hypoxia-induced neonatal seizures. Notably, Fluoro-Jade B staining confirmed that hypoxic seizures at P10 did not induce acute neuronal death.

SIGNIFICANCE

The rodent model of hypoxia-induced neonatal seizures leads to the development of epilepsy in later life, accompanied by increased mossy fiber sprouting. In addition, this model appears to exhibit a seizure-free latent period, following which there is a progressive increase in the frequency of electroclinical seizures.

Sleep deprivation prevents stimulation-induced increases of levels of P-CREB and BDNF: protection by caffeine

It is well known that caffeine and sleep deprivation have opposing effects on learning and memory; therefore, this study was undertaken to determine the effects of chronic (4wks) caffeine treatment (0.3g/l in drinking water) on long-term memory deficit associated with 24h sleep deprivation. Animals were sleep deprived using the modified multiple platform method. The results showed that chronic caffeine treatment prevented the impairment of long-term memory as measured by performance in the radial arm water maze task and normalized L-LTP in area CA1 of the hippocampi of sleep-deprived anesthetized rats. Sleep deprivation prevents the high frequency stimulation-induced increases in the levels of phosphorylated-cAMP response element binding protein (P-CREB) and brain-derived neurotrophic factor (BDNF) seen during the expression of late phase long-term potentiation (L-LTP). However, chronic caffeine treatment prevented the effect of sleep-deprivation on the stimulated levels of P-CREB and BDNF. The results suggest that chronic caffeine treatment may protect the sleep-deprived brain probably by preserving the levels of P-CREB and BDNF.

Interictal spikes in developing rats cause long-standing cognitive deficits

Frequent interictal spikes are a common finding in the electroencephalograms of children with epileptic encephalopathies. While it is well recognized that interictal spikes are a biological marker of seizures and can lead to transitory cognitive impairment, whether interictal spikes can result in long-standing adverse effects on learning and memory in children is not known. Here we investigated the consequences of interictal spikes in rat pups without seizures on long-term learning and memory. Rat pups were given a low dose of flurothyl for 4h for 10 days during continuous electroencephalographic monitoring. Rats developed interictal spikes without seizures while age-matched controls under similar testing conditions had few interictal spikes. When rats were tested as adults, there was impairment in reference memory in the probe test of the Morris water maze, reference memory impairment in the four-trial radial-arm water maze and impaired long-term potentiation. Early-life interictal spikes resulted in impaired new cell formation and decreased cell counts in the hippocampus but did not cause an increase in apoptosis. This study, for the first time demonstrates that interictal spikes in rat pups without seizures can result in long-standing spatial cognitive impairment. Our findings suggest that suppressing IIS may be as important as treating seizures during brain development.

Characterization of developing rat cortical neurons after epileptiform discharges

The developing brain undergoes major reorganization in response to early environmental changes. The elevated excitation that allows the neonatal brain to develop quickly also makes it highly vulnerable to age-specific seizures that can cause lifelong cognitive and neurological disability. However, it is not yet clear how seizures interfere with the developmental program and how epileptogenesis actualize. Here, by using an in vitro model, we report a global abnormal status of cortical cells after epileptiform activity was induced: more NR2B is targeted on the neuronal surface with less NR2A. Dendrotoxicity including dendritic beading, distortion and simplification of dendritic branching patterns were observed. Early-life seizure-like insults also exert effects on the excitatory synaptic size and interactions between PSD-95 and NR2A or NR2B receptor subunits. Our findings support an abnormal development or, worse, cellular degeneration that resembles immature cells, which may enlighten better understanding of the pathological mechanism of early-life seizures and its related injury.

Chronic psychosocial stress accelerates impairment of long-term memory and late-phase long-term potentiation in an at-risk model of Alzheimer's disease

Although it is generally agreed that Aβ contributes to the pathogenesis of AD, its precise role in AD and the reason for the varying intensity and time of onset of the disease have not been elucidated. In addition to genetic factors, environmental issues such as stress may also play a critical role in the etiology of AD. This study examined the effect of chronic psychosocial stress in an at-risk (treatment with a subpathogenic dose of Aβ; "subAβ") rat model of AD on long-term memory by three techniques: memory tests in the radial arm water maze, electrophysiological recordings of synaptic plasticity in anesthetized rats, and immunoblot analysis of learning- and long-term memory-related signaling molecules. Chronic psychosocial stress was induced using a rat intruder model. The subAβ rat model of AD was induced by continuous infusion of 160 pmol/day Aβ(1-42) via a 14-day i.c.v. osmotic pump. All tests showed that subAβ rats were not different from control rats. Result from behavioral tests and electrophysiological recordings showed that infusion of subAβ in chronically stressed rats (stress/subAβ group) caused significant impairment of cognitive functions and late-phase long-term potentiation (L-LTP). Molecular analysis of various signaling molecules after expression of L-LTP, revealed an increase in the levels of p-CREB in control, stress, and subAβ rats, but not in the stress/subAβ rats. These findings suggest that the chronic stress-induced molecular alteration may accelerate the impairment of cognition and synaptic plasticity in individuals "at-risk" for AD.

Recurrent neonatal seizures result in long-term increases in neuronal network excitability in the rat neocortex

Neonatal seizures are associated with a high likelihood of adverse neurological outcomes, including mental retardation, behavioral disorders, and epilepsy. Early seizures typically involve the neocortex, and post-neonatal epilepsy is often of neocortical origin. However, our understanding of the consequences of neonatal seizures for neocortical function is limited. In the present study, we show that neonatal seizures induced by flurothyl result in markedly enhanced susceptibility of the neocortex to seizure-like activity. This change occurs in young rats studied weeks after the last induced seizure and in adult rats studied months after the initial seizures. Neonatal seizures resulted in reductions in the amplitude of spontaneous inhibitory postsynaptic currents and the frequency of miniature inhibitory postsynaptic currents, and significant increases in the amplitude and frequency of spontaneous excitatory postsynaptic currents (sEPSCs) and in the frequency of miniature excitatory postsynaptic currents (mEPSCs) in pyramidal cells of layer 2/3 of the somatosensory cortex. The selective N-methyl-D-aspartate (NMDA) receptor antagonist D-2-amino-5-phosphonovalerate eliminated the differences in amplitude and frequency of sEPSCs and mEPSCs in the control and flurothyl groups, suggesting that NMDA receptors contribute significantly to the enhanced excitability seen in slices from rats that experienced recurrent neonatal seizures. Taken together, our results suggest that recurrent seizures in infancy result in a persistent enhancement of neocortical excitability.

The long-term effects of neonatal seizures

The highest incidence of seizures occurs during the first hours to days after birth. The immature brain is prone to seizures because of reduced inhibition. GABA, which is the primary inhibitory neurotransmitter in the mature brain, is depolarizing and excitatory in the immature brain. Seizures are an ominous sign, indicating either an acquired brain insult or a genetic abnormality. While the primary outcome determinant of neonatal seizures is etiology, whether seizures can result in long-term adverse consequences independently is not clear. While the clinical data is uncertain, there is now a considerable body of evidence indicating that in animals, neonatal seizures can adversely alter the developing brain. Animal data indicates that the sequelae of seizures are strongly age dependent; seizures will affect the developing and plastic neuronal circuitry much differently than the fixed circuitry of the mature brain. Seizures at an early developmental stage can dramatically affect the construction of networks, resulting in severe and permanent handicaps in some patients. In the young brain, the long-lasting detrimental consequences of seizures are caused by an alteration of developmental programs rather than by neuronal cell loss, as occurs in adults. In animal models, neonatal seizures result in decreases in neurogenesis, sprouting of mossy fibers, and long-standing changes in signaling properties. Seizures in rat pups are also associated with abnormalities in firing patterns of single cells in the hippocampus. Furthermore, these anatomic and physiologic changes correlate well with behavioral dysfunction.

The 2008 Judith Hoyer lecture: epilepsy in children: listening to mothers

The incidence of epilepsy is significantly higher in children than adults. When faced with the diagnosis of epilepsy, parents have many questions regarding cause, treatment, and prognosis. Although the majority of children with epilepsy have an excellent prognosis and respond well to therapy, some children are refractory to therapy and suffer from cognitive decline. Animal models are now providing insights into the mechanisms responsible for the high incidence of seizures during development and age-dependent seizure-induced damage. One of the causes of the increased susceptibility of the young brain to seizures is the depolarizing effects of GABA secondary to high intracellular concentrations of chloride in young neurons. Although cell loss is not a feature of seizures in the young brain, recurrent seizures do result in aberrant sprouting of mossy fibers, reduce neurogenesis, and alter excitatory and inhibitory neurotransmitter receptor structure and function. Behavioral consequences of early-life seizures include impaired spatial cognition, which now can be assessed using single-cell recordings from the hippocampus. Antiepileptic drugs have had a tremendous positive influence in epilepsy management, although there are now a number of studies demonstrating that antiepileptic drugs at therapeutic concentrations can impair cognition and result in increased apoptosis. While clinical judgment and experience are paramount when discussing the consequences of seizures and their treatment, awareness of studies from animals can provide the clinician with guidance in addressing these important issues with parents.