Local circuit abnormalities in chronically epileptic rats after intrahippocampal tetanus toxin injection in infancy

Neocortical slices from adult chronic epileptic rats exhibit discharges of higher voltages and broader spread

Much of the current understanding of epilepsy mechanisms has been built on data recorded with one or a few electrodes from temporal lobe slices of normal young animals stimulated with convulsants. Mechanisms of adult, extratemporal, neocortical chronic epilepsy have not been characterized as much. A more advanced understanding of epilepsy mechanisms can be obtained by recording epileptiform discharges simultaneously from multiple points of an epileptic focus so as to define their sites of initiation and pathways of spreading. Brain slice recordings can characterize epileptic mechanisms in a simpler, more controlled preparation than in vivo. Yet, the intrinsic hyper-excitability of a chronic epileptic focus may not be entirely preserved in slices following the severing of connections in slice preparation. This study utilizes recordings of multiple electrode arrays to characterize which features of epileptic hyper-excitability present in in vivo chronic adult neocortical epileptic foci are preserved in brain slices. After tetanus toxin somatosensory cortex injections, adult rats manifest chronic spontaneous epileptic discharges both in the injection site (primary focus) and in the contralateral side (secondary focus). We prepared neocortical slices from these epileptic animals. When perfused with 4-Aminopyridine in a magnesium free medium, epileptic rat slices exhibit higher voltage discharges and broader spreading than control rat slices. Rates of discharges are similar in slices of epileptic and normal rats, however. Ictal and interictal discharges are distributed over most cortical layers, though with significant differences between primary and secondary foci. A chronic neocortical epileptic focus in slices does not show increased spontaneous pacemakers initiating epileptic discharges but shows discharges with higher voltages and broader spread, consistent with an enhanced synchrony of cellular and synaptic generators over wider surfaces.

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.

Involvement of the thalamic parafascicular nucleus in mesial temporal lobe epilepsy

Mesial temporal lobe epilepsy (MTLE) is characterized by focal seizures, associated with hippocampal sclerosis, and often resistance to antiepileptic drugs. The parafascicular nucleus (PF) of the thalamus is involved in the generation of physiological oscillatory rhythms. It receives excitatory inputs from the cortex and inhibitory inputs from the basal ganglia, a system implicated in the control of epileptic seizures. The aim of this study was to examine the involvement of the PF in the occurrence of hippocampal paroxysmal discharges (HPDs) in a chronic animal model of MTLE in male mice. We recorded the local field potential (LFP) and the extracellular and intracellular activity of hippocampal and PF neurons during spontaneous HPDs in vivo. The end of the HPDs was concomitant with a slow repolarization in hippocampal neurons leading to an electrical silence. In contrast, it was associated in the PF with a transient increase in the power of the 10-20 Hz band in LFPs and a depolarization of PF neurons resulting in a sustained firing. We tested the role of the PF in the control of HPDs by single 130 Hz electrical stimulation of this nucleus and bilateral intra-PF injection of NMDA and GABA(A) antagonist and agonist. High-frequency PF stimulation interrupted ongoing HPDs at an intensity devoid of behavioral effects. NMDA antagonist and GABA(A) agonist suppressed hippocampal discharges in a dose-dependent way, whereas NMDA agonist and GABA(A) antagonist increased HPDs. Altogether, these data suggest that the PF nucleus plays a role in the modulation of MTLE seizures.

Increased seizure susceptibility and up-regulation of nNOS expression in hippocampus following recurrent early-life seizures in rats

This study aimed to determine the long-term change of seizure susceptibility and the role of nNOS on brain development following recurrent early-life seizures in rats. Video-EEG recordings were conducted between postnatal days 50 and 60. Alterations in seizure susceptibility were assayed on day 22 or 50 using the flurothyl method. Changes in nNOS expression were determined by quantitative immunoblotting on day 50. On average, rats had 8.4+/-2.7 seizures during 10 daily 1 hr behavioral monitoring sessions. As adults (days 50-60), all rats displayed interictal spikes in the hippocampus and/or overlying cortex. Brief electrographic seizures were recorded in only one of five animals. Rats appeared to progress from a period of marked seizure susceptibility (day 22) to one of lessened seizure susceptibility (day 50). Up-regulation of nNOS expression following early-life recurrent seizures was observed on day 50. In conclusion, these data suggested that recurrent early-life seizures had the long-term effects on seizure susceptibility late in life and up-regulatory nNOS expression on the hippocampus during brain development, and nNOS appeared to contribute to the persistent changes in seizure susceptibility, and epileptogenesis.

The impact of chronic network hyperexcitability on developing glutamatergic synapses

The effects recurring seizures have on the developing brain are an important area of debate because many forms of human epilepsy arise in early life when the central nervous system is undergoing dramatic developmental changes. To examine effects on glutamatergic synaptogenesis, epileptiform activity was induced by chronic treatment with GABAa receptor antagonists in slice cultures made from infant rat hippocampus. Experiments in control cultures showed that molecular markers for glutamatergic and GABAergic synapses recapitulated developmental milestones reported previously in vivo. Following a 1-week treatment with bicuculline, the intensity of epileptiform activity that could be induced in cultures was greatly diminished, suggesting induction of an adaptive response. In keeping with this notion, immunoblotting revealed the expression of NMDA and AMPA receptor subunits was dramatically reduced along with the scaffolding proteins, PSD95 and Homer. These effects could not be attributed to neuronal cell death, were reversible, and were not observed in slices taken from older animals. Co-treating slices with APV or TTX abolished the effects of bicuculline suggesting that effects were dependent on NMDA receptors and neuronal activity. Neurophysiological recordings supported the biochemical findings and demonstrated decreases in both the amplitude and frequency of NMDA and AMPA receptor-mediated miniature EPSCs (mEPSCs). Taken together these results suggest that neuronal network hyperexcitability interferes with the normal maturation of glutamatergic synapses, which could have implications for cognitive deficits commonly associated with the severe epilepsies of early childhood.

Recurrent seizures and the molecular maturation of hippocampal and neocortical glutamatergic synapses

Recurrent seizures in animal models of early-onset epilepsy have been shown to produce deficits in spatial learning and memory. While neuronal loss does not appear to underlie these effects, dendritic spine loss has been shown to occur. In experiments reported here, seizures induced either by tetanus toxin or flurothyl during the second postnatal week were found to reduce the expression of NMDA receptor subunits in both the hippocampus and neocortex. Most experiments focused on alterations in the expression of the NR2A subunit and its associated scaffolding protein, PSD95, since their expression is developmentally regulated. Results suggest that the depression in expression can be delayed by at least 5 days but persists for at least 3-4 weeks. These effects were dependent on the number of seizures experienced, and were not observed when seizures were induced in adult mice. Taken together, the results suggest that recurrent seizures in infancy may interrupt synapse maturation and produce persistent decreases in molecular markers for glutamatergic synapses - particularly components of the NMDA receptor complex implicated in learning and memory.

The tetanus toxin model of chronic epilepsy

In experimental models of epilepsy, single and recurrent seizures are often used in an attempt to determine the effects of the seizures themselves on mammalian brain function. These models attempt to emulate as many features as possible of their human disease counterparts without many of the confounding factors such as underlying disease processes and medication effects. Numerous models have been used in the past to address different questions. Nevertheless, the basic questions are often the same: 1. Do seizures cause long-term damage? 2. Do seizures predispose to chronic epilepsy (epileptogenesis), that is long-term spontaneous repetitive seizures? 3. Are these results developmentally regulated? 4. Are the underlying mechanisms of epileptogenesis and brain damage related? In pursuing these questions, the goal is to determine how seizures exert their effects and to minimize any side effects from the methods employed to induce the seizures themselves. This requires a detailed characterization of the methods used to induce seizures. In this chapter, we will review the literature regarding the tetanus toxin model of chronic epilepsy with regard to its mechanisms of action, clinical comparisons, how it is experimentally implemented and the results obtained thus far. These results will be compared to other models of chronic epilepsy in order to make generalizations about the effects of repetitive seizures in adult and early life. At this time, it appears that repetitive seizures cause long-term changes in learning ability and may cause a predisposition to chronic seizures at all ages. In younger animals, both features of learning impairment and epilepsy are not typically associated with cell loss as they are in adult animals. At all ages, some form of synaptic reorganization has been demonstrated to occur.

Effects of brief seizures during development

The effects of brief seizures during development depend on multiple factors such as underlying brain pathology, specific age of occurrence and frequency. Studies in rats are frequently used to determine the consequences of seizures in the developing brain. The shorter prepubertal development and life span of the rat compared to humans may suggest that brief seizures in the rat are not necessarily equivalent to brief seizures in humans. Nevertheless, there is substantial evidence that in the rat, the consequences of seizures are age-dependent. The immature brain is relatively resistant to morphological damage, especially in the hippocampus, and functional changes as measured by electrophysiology and behavior. Developmental kindling can be used as a model to study brief seizures early in life. Kindling permanently alters the brain so that rats stimulated again in adulthood require only few kindling stimuli for fully kindled seizures to occur although there are no apparent morphological and functional changes in the hippocampus resulting from kindling early in life. The appreciation that kindling can alter brain function without any discrete (to date) morphological changes may lead to the development of effective neuroprotective strategies to alter the process, but it is not clear that all kindling-induced changes are detrimental to the brain.

Spatial learning deficits without hippocampal neuronal loss in a model of early-onset epilepsy

Studies were undertaken to examine the effects recurrent early-life seizures have on the ability of rats to acquire spatial memories in adulthood. A minute quantity of tetanus toxin was injected unilaterally into the hippocampus on postnatal day 10. Within 48 h, rats developed recurrent seizures that persisted for 1 week. Between postnatal days 57 and 61, rats were trained in a Morris water maze. Toxin-injected rats were markedly deficient in learning this task. While these rats showed gradual improvement in escape latencies over 20 trials, their performance always lagged behind that of controls. Poor performance could not be explained by motor impairments or motivational difficulties since swimming speed was similar for the groups. Only eight of 16 toxin-injected animals showed focal interictal spikes in the hippocampus during electroencephalographic recordings. This suggests that learning deficiencies and chronic epilepsy may be independent products of recurrent early-life seizures. A quantitative analysis of hippocampus revealed a significant decrease in neuronal density in stratum pyramidale of experimental rats. However, the differences were largely explained by a concomitant increase in the area of stratum pyramidale. Studies of glial fibrillary acidic protein expression and spread of horseradish peroxidase-conjugated tetanus toxin in the hippocampus suggest that the dispersion of cell bodies in stratum pyramidale can neither be explained by a reactive gliosis nor the direct action of the toxin itself. Taken together, we suggest that recurrent seizures beginning in early life can lead to a significant deficiency in spatial learning without ongoing hippocampal synchronized network discharging or a substantial loss of hippocampal pyramidal cells.

Reduced excitatory drive onto interneurons in the dentate gyrus after status epilepticus

Impaired GABAergic inhibition may contribute to the development of hyperexcitability in epilepsy. We used the pilocarpine model of epilepsy to demonstrate that regulation of excitatory synaptic drive onto GABAergic interneurons is impaired during epileptogenesis. Synaptic input from granule cells (GCs), perforant path, and CA3 inputs onto hilar border interneurons of the dentate gyrus were examined in rat hippocampal slices during the latent period (1-8 d) after induction of status epilepticus (SE). Short-term depression (STD) of GC inputs to interneurons induced by brief (500-800 msec), repetitive (5-20 Hz) stimulation, as well as paired-pulse depression at both GC and CA3 inputs to interneurons, were significantly (p < 0.05) enhanced in SE-experienced rats. In contrast, we found no significant differences between SE-experienced and age-matched control rats in the properties of minimal EPSCs evoked at low frequency (0.3 Hz). Consistent with reduced GABAergic inhibition onto granule cells, paired-pulse depression of perforant path-evoked granule cell population spikes was lost in SE-experienced rats. Enhanced STD was partially mediated by group II metabotropic glutamate receptors, because the selective antagonist, 2S-2-amino-2-(1S,2S-2-carboxycyclopropyl-1-yl)-3-(xanth-9-yl)propanoic acid, attenuated STD in SE-experienced rats but had no effect on STD of GC inputs in the normal adult rat. The group II mGluR agonist, (2S',1R',2R',3R')-2-(2,3-dicarboxylcyclopropyl) glycine (1 micrometer), produced a greater depression of GC input to hilar border interneurons in SE-experienced rats than in controls. These results indicate that, in the SE-experienced rat, excitatory drive to hilar border inhibitory interneurons is weakened through a use-dependent mechanism involving group II metabotropic glutamate receptors.

Neuronal activity and the establishment of normal and epileptic circuits during brain development

The question we attempted to address in this chapter is: Do brief but recurrent seizures in early life alter the ontogeny of hippocampal networks in ways that produce epileptic circuits? Results from the tetanus toxin model suggest that this is likely the case. Following seizures in Postnatal Weeks 2 and 3, most adult rats have a focal epilepsy that arises from hippocampus. Recordings from hippocampal slices support this conclusion since they demonstrated the occurrence of spontaneous network discharges in normal artificial cerebrospinal fluid. Moreover, when GABA-A receptor-mediated synaptic transmission was suppressed, slices from adult epileptic rats produced prolonged electrographic seizures which are never observed in control rats. This suggests that hyperexcitable recurrent excitatory networks contribute to hippocampal seizures in this model. In light of this, anatomical results from biocytin-filled neurons were surprising. Results suggest that recurrent axon arbors neither sprout additional branches as a result of seizure activity nor maintain their exuberant branching patterns of early life. Thus, excessive connectivity cannot explain seizure generation. Axon arbors either remodel in normal ways or prune additional collaterals as a result of ongoing epileptiform discharging. At the same time that axon arbors remodel, the dendrites of these cells have decreased dendritic spine density, suggesting a partial deafferentation. While a complete understanding of the origins of spine loss requires further investigation, we hypothesize that this loss is a product of a partial deafferentation that occurs due to excessive and abnormal selection of synaptic connections. Network-induced heterosynaptic LTD of noncoincidentally active afferants may be one mechanism that leads to a loss of synapses. Moreover, competition among and selection between individual recurrent excitatory synapses may contribute to spine loss as well. The "winners" of this competition, the most potent and effective early-formed recurrent excitatory synapses, are likely key contributors to seizure generation in this model and possibly in humans with early-onset temporal lobe epilepsy.

Spine loss and other dendritic abnormalities in epilepsy

Studies of neurons from human epilepsy tissue and comparable animal models of focal epilepsy have consistently reported a marked decrease in dendritic spine density on hippocampal and neocortical pyramidal cells. Spine loss is often accompanied by focal varicose swellings or beading of dendritic segments. An ongoing excitotoxic injury of dendrites (dendrotoxicity), produced by excessive release of glutamate during seizures, is often assumed to produce these abnormalities. Indeed, application of glutamate receptor agonists to dendrites can produce both spine loss and beading. However, the cellular mechanisms underlying the two processes appear to be different. One recent study suggests NMDA-induced spine loss is produced by Ca2+-mediated alterations of the spine cytoskeleton. In contrast, dendritic beading is not dependent on extracellular Ca2+; instead, it appears to be produced by the movement of Na+ and Cl- intracellularly and an obligate movement of water to maintain osmolarity. A decrease in dendritic spine density was recently reported in a model of recurrent focal seizures in early life. Unlike results from other models, dendritic beading was not observed, and other signs of neuronal injury and death were absent. Thus, additional mechanisms to those of excitotoxicity may produce dendritic spine loss in epileptic tissue. A hypothesis is presented that spine loss can be a product of a partial deafferentation of pyramidal cells, resulting from an activity-dependent pruning of neuronal connectivity induced by recurring seizures. The dendritic abnormalities observed in epilepsy are commonly suggested to be a product and not a cause of epilepsy. However, anatomical remodeling may be accompanied by alterations in molecular expression and targeting of both voltage- and ligand-gated channels in dendrites. It is conceivable that such changes could contribute to the neuronal hyperexcitability of epilepsy.

Cellular abnormalities and synaptic plasticity in seizure disorders of the immature nervous system

The nervous system has an enhanced capacity to generate seizures during a restricted phase of postnatal development. Studies in animals and particularly in in vitro brain slices from hippocampus and neocortex have been instrumental in furthering an understanding of the underlying processes. Developmental alterations in glutaminergic excitatory synaptic transmission appear to play a key role in the enhanced seizure susceptible of rodents during the second and third week of life. Prior to this period, the number of excitatory synapses is relatively low. The scarcity of connections and the inability of the existing synapses to release glutamate when activated at high frequencies likely contribute importantly to the resistance of neonates to seizures. However, at the beginning of week 2, a dramatic outgrowth of excitatory synapses occurs, and these synapses are able to faithfully follow activation at high frequencies. These changes, coupled with the prolonged nature of synaptic potentials in early life, likely contribute to the ease of seizure generation. After this time, seizure susceptibility declines, patterns of local synaptic connectivity remodel, and some synapses are pruned. Concurrently, the duration of excitatory postsynaptic potentials shortens due at least in part to a switch in the subunit composition of postsynaptic receptors. Other studies have examined the mechanisms underlying chronic epilepsy initiated in early life. Models of both cortical dysplasia and recurrent early-life seizures suggest that alterations in the normal development of excitatory synaptic transmission can contribute importantly to chronic epileptic conditions. In the recurrent early-life seizure model, abnormal use-dependent selection of subpopulations of excitatory synapses may play a role. In experimental cortical dysplasia, alterations in the molecular composition of postsynaptic receptor are observed that favor subunit combinations characteristic of infancy.

Multiple pilocarpine-induced status epilepticus in developing rats: a long-term behavioral and electrophysiological study

PURPOSE

Animal models are useful for the study of status epilepticus (SE)-induced epileptogenesis and neurological sequelae, especially during early brain development. Here, we show several permanent abnormalities in animals subjected to multiple SE during early development.

METHODS

Wistar pup rats (7 to 9 days old) were subjected to three consecutive episodes of SE induced by systemic pilocarpine injections. To study the long-lasting consequences of early-induced SE. chronic electroencephalographic recordings were made from the hippocampus and cortex and several behavioral tests (inhibitory step-down avoidance, rota-rod, open field, elevated plus-maze, and Skinner box) were performed at postnatal days 30 to 90. We also investigated in vitro electrophysiological responses of the CA1 area using extracellular recordings in hippocampal slices. A histological analysis was done using cresyl violet staining 24 hours and several months after SE induction. Apoptotic cell death was evaluated by terminal deoxynucleotidyl transferase dUTP nick-end labeling (TUNEL staining) 24 hours after the last SE episode.

RESULTS

Electroencephalographic recordings from 30- to 90-day-old rats that had been subjected to multiple SE episodes in early life showed marked changes compared with those from nontreated controls. These included frequent episodes of continuous complex spiking activity and high-voltage ictal discharges, with a small percentage of these rats presenting spontaneous behavioral seizures. These animals also presented evidence of severe cognitive deficit in adulthood. In vitro, a persistent hyperexcitability of the CA1 area was detected in experimental animals. Histological analysis of the brains did not reveal any major long-term pathological changes. Nevertheless, an increased number of TUNEL-positive nuclei were present in some animals in both the hippocampus and the thalamus.

CONCLUSIONS

These data show persistent abnormalities in animals subjected to multiple SE episodes during early postnatal development. SE may result in important plastic changes in critical periods of brain maturation leading to long-lasting epileptogenesis, as manifested by electrographic epileptiform discharges, behavioral deficits, and in vitro hyperexcitability of hippocampal networks.

Granule-like neurons at the hilar/CA3 border after status epilepticus and their synchrony with area CA3 pyramidal cells: functional implications of seizure-induced neurogenesis

A group of neurons with the characteristics of dentate gyrus granule cells was found at the hilar/CA3 border several weeks after pilocarpine- or kainic acid-induced status epilepticus. Intracellular recordings from pilocarpine-treated rats showed that these "granule-like" neurons were similar to normal granule cells (i. e., those in the granule cell layer) in membrane properties, firing behavior, morphology, and their mossy fiber axon. However, in contrast to normal granule cells, they were synchronized with spontaneous, rhythmic bursts of area CA3 pyramidal cells that survived status epilepticus. Saline-treated controls lacked the population of granule-like cells at the hilar/CA3 border and CA3 bursts. In rats that were injected after status epilepticus with bromodeoxyuridine (BrdU) to label newly born cells, and also labeled for calbindin D(28K) (because it normally stains granule cells), many double-labeled neurons were located at the hilar/CA3 border. Many BrdU-labeled cells at the hilar/CA3 border also were double-labeled with a neuronal marker (NeuN). Taken together with the recent evidence that granule cells that are born after seizures can migrate into the hilus, the results suggest that some newly born granule cells migrate as far as the CA3 cell layer, where they become integrated abnormally into the CA3 network, yet they retain granule cell intrinsic properties. The results provide insight into the physiological properties of newly born granule cells in the adult brain and suggest that relatively rigid developmental programs set the membrane properties of newly born cells, but substantial plasticity is present to influence their place in pre-existing circuitry.

Immature rat convulsions and long-term effects on hippocampal cholinergic neurotransmission

Generalized tonic-clonic convulsions were induced on 2 consecutive days by pentylenetetrazol (PTZ) in immature rats (postnatal days 10 and 20), and hippocampal slices were prepared at different intervals post-injection. The anticholinesterase eserine provoked interictal-like discharges in the CA3 area of PTZ-injected rats (19/33), but not in controls (0/15), an effect mimicked by carbachol and reversed by atropine. This enhanced response to eserine was recorded in slices from 25-100% of the PTZ-injected rats, the percentage varying with the age at injection and post-injection interval. These results suggest that seizures in immature brain may have long-term consequences in cholinergic neurotransmission, converting a rise in endogenous ACh into an epileptogenic stimulus, which in turn would presumably facilitate the recurrence of seizures.

Insights into the tetanus toxin model of early-onset epilepsy from long-term video monitoring during anticonvulsant therapy

Video monitoring studies were undertaken to determine if the anticonvulsant, carbamazepine (CBZ), could prevent seizures in infant rats that had been intrahippocampally injected with tetanus toxin (TNTX). In control rats, seizure frequency peaked 5-6 days after injection and rapidly declined by postinjection day 9. Twice-daily CBZ treatments dramatically suppressed behavioral seizures for 7 days. However, despite increasing the dosage of CBZ, rats experienced more behavioral seizures during the second week after TNTX injection. Paradoxically, tetanus-toxin-injected control rats had very few seizures at this time. Results not only suggest that this TNTX model may be useful in screening drugs for treating intractable focal epilepsy of infancy but also provide some insight into the processes that may contribute to the rapid decline in behavioral seizure frequency that occurs during the acute phase of epileptogenesis in this model.

Febrile seizures in the developing brain result in persistent modification of neuronal excitability in limbic circuits

Febrile (fever-induced) seizures affect 3-5% of infants and young children. Despite the high incidence of febrile seizures, their contribution to the development of epilepsy later in life has remained controversial. Combining a new rat model of complex febrile seizures and patch clamp techniques, we determined that hyperthermia-induced seizures in the immature rat cause a selective presynaptic increase in inhibitory synaptic transmission in the hippocampus that lasts into adulthood. The long-lasting nature of these potent alterations in synaptic communication after febrile seizures does not support the prevalent view of the 'benign' nature of early-life febrile convulsions.

Acute and chronic effects of seizures in the developing brain: experimental models

Clinical experience suggests two major components to the relationship between brain development and epilepsy. First, the maturational state of the immature brain appears to generally decrease seizure threshold and contribute to a different seizure phenotype from the adult. Second, certain forms of seizures, when present during development, may modify brain maturation to result in chronic epilepsy and/or other neurocognitive deficits. Maturational studies in animals suggest there are numerous factors developmentally regulated in such a way as to increase excitability in immature neuronal networks in the forebrain. The developing brain appears to exhibit a transient overexpression of glutamate receptors, glutamate receptor subunit composition permissive of enhanced excitatory neurotransmission, a relative lack of GABAergic inhibitory transmission, and ion channel expression and homeostasis which enhance neuronal excitability. The increased excitatory "drive" that is likely to be critical for normal brain development may share common mechanisms with those responsible for rendering the immature brain more susceptible to seizures, seizure induced plasticity (epileptogenesis), and neuronal injury. Furthermore, the coincidence of seizures during early postnatal brain development may modify many of these parameters, which in turn may promote long term epilepsy.

A chronic focal epilepsy with mossy fiber sprouting follows recurrent seizures induced by intrahippocampal tetanus toxin injection in infant rats

Studies were conducted to characterize a chronic epileptic condition that follows recurrent seizures induced by intrahippocampal tetanus toxin injection in infancy. Wistar rat pups received a single injection of tetanus toxin in the right CA3 region on postnatal day 10. Animals were monitored for epileptiform activity by video electroencephalographic or visual observation during the following three to five days. Repeat evaluation six months later demonstrated interictal discharges in 79% (11 of 14) and electrographic seizures in 42% (six of 14) of adult rats with tetanus toxin-induced seizures in infancy. Five of the animals had interictal activity which occurred focally in either the left (n = 2) or right (n = 3) hippocampus. One animal had focal interictal activity independently in these regions and in the left and right cortical regions. The remaining five animals had interictal activity in the hippocampus and synchronously in the ipsilateral cortex or the contralateral hippocampus. Electrographic seizures were focal (nine of 14) or bilateral (five of 14) in onset. The behaviors that accompanied these seizures were quite variable. Clonic face and forelimb movements were observed in some animals. However, a significant portion of rats had electrographic seizures with no associated behavioral change. Timm staining was performed on hippocampal sections from experimental and control animals. There was a significantly greater Timm score (aberrant Timm granules) in the inner molecular layer of the dentate gyrus in tetanus toxin-treated rats than in control rats. Our findings suggest that intrahippocampal tetanus toxin injection in infant rats results in a chronic focal epilepsy that persists for at least six months and is associated with aberrant mossy fiber sprouting in the dentate gyrus. The model described here contributes significantly to the evidence for chronic effects of recurrent seizures in early life, and provides a model for investigation of the molecular and cellular events that contribute to the development of chronic epilepsy.

Newly formed excitatory pathways provide a substrate for hyperexcitability in experimental temporal lobe epilepsy

Temporal lobe epilepsy (TLE) in humans and animals is associated with axonal sprouting of glutamatergic neurons and neosynaptogenesis in the hippocampal formation. We examined whether this plasticity of excitatory pathways contributes to an increased level of glutamatergic excitation in the CA1 region of rats experiencing chronic spontaneous limbic seizures following kainic acid or pilocarpine treatment. In chronic cases, we report an extensive axonal sprouting of CA1 pyramidal neurons, with many axonal branches entering the pyramidal cell layer and stratum radiatum, regions that are not innervated by axonal collaterals of CA1 pyramidal neurons in control animals. Concurrently with this anatomical reorganization, a large increase of the spontaneous glutamatergic drive is observed in the dendrites and somata of CA1 pyramidal cells. Furthermore, electrical activation of the reorganized CA1 associational pathway evokes epileptiform bursts in CA1 pyramidal cells. These findings suggest that reactive plasticity could contribute to the hyperexcitability of CA1 pyramidal neurons and to the propagation of seizures in these two models of TLE.

Long-term depression of perforant path excitatory postsynaptic potentials following synchronous network bursting in area CA3 of immature hippocampus

Various forms of synaptic long-term potentiation and depression have been studied in detail in hippocampus and neocortex. Each are produced by specific patterns of synaptic activation and rely on electrical stimulation of afferents for their induction. Few studies have explored the ability of activity produced by cortical networks themselves to generate similar long-term changes in synaptic efficacy. Experiments have shown that periods of synchronized network bursting in both slices and dissociated cultures of hippocampus can lead to persistent network discharges. Conversely, one study has reported that network discharging can disrupt the induction of long-term potentiation in hippocampus. Whether long-term depression can be induced by synchronous network discharging is unknown. In experiments reported here, we examined the long-term effects of synchronized activity within hippocampal CA3 networks on synaptic potentials produced by the converging perforant path. Prior to induction of network bursting, slices were incubated in a perfusate containing picrotoxin and elevated (4 mM) Ca2+ and Mg2+. To induce network discharges, the concentration of both divalent cations were reduced to normal levels (1.5 mM). Following 20 min of network bursting, perforant path synapses, that did not participate in network discharging, underwent a 30% non-decrementing long-term depression. At the same time, synchronized network discharges, that were absent prior to induction, persisted upon return to preinduction conditions. The antagonist of the N-methyl-D-aspartate receptor, D(-)2-amino-5-phosphonovalerate, blocked both long-term depression of perforant path excitatory postsynaptic potentials and persistent network discharging. Results suggest that activity generated by hippocampal networks is able to produce long-term depression of non-coincidentally active synapses.

Spine loss and other persistent alterations of hippocampal pyramidal cell dendrites in a model of early-onset epilepsy

To explore the anatomical substrates for network hyperexcitability in adult rats that become chronically epileptic after recurrent seizures in infancy, the dendritic and axonal arbors of biocytin-filled hippocampal pyramidal cells were reconstructed. On postnatal day 10, tetanus toxin was unilaterally injected into the hippocampus and produced brief but recurrent seizures for 1 week. Later, hippocampal slices taken from these rats exhibited synchronized network bursts in area CA3C. Both the apical and basilar dendritic arbors of CA3C pyramidal cells were markedly abnormal in these epileptic rats. There was a considerable reduction in the density of dendrite spines, although the extent of this loss could vary among dendritic segments. Spine density on terminal segments of the basilar and apical dendrites was reduced on average by 35 and 20%, respectively. In addition, the diameters of these same dendritic segments were markedly reduced. Dendritic spine loss has previously been suggested to indicate a partial deafferentation of epileptic neurons, but this interpretation is difficult to reconcile with the critical role recurrent excitatory synaptic transmission plays in the generation of synchronized network burst. In this study, axonal arbors of CA3C pyramidal cells exhibited normal branching patterns, branching complexity, and varicosity density. This suggests that if deafferentation occurs, synapses other than recurrent excitatory ones are lost. The morphological abnormalities reported here would be expected to significantly alter electrical signaling within dendrites that may contribute importantly to seizures and other behavioral sequelae of early-onset epilepsy.