Tetanus toxin-induced seizures in infant rats and their effects on hippocampal excitability in adulthood

A companion to the preclinical common data elements for rodent models of pediatric acquired epilepsy: A report of the TASK3-WG1B, Pediatric and Genetic Models Working Group of the ILAE/AES Joint Translational Task Force

Epilepsy syndromes during the early years of life may be attributed to an acquired insult, such as hypoxic-ischemic injury, infection, status epilepticus, or brain trauma. These conditions are frequently modeled in experimental rodents to delineate mechanisms of epileptogenesis and investigate novel therapeutic strategies. However, heterogeneity and subsequent lack of reproducibility of such models across laboratories is an ongoing challenge to maintain scientific rigor and knowledge advancement. To address this, as part of the TASK3-WG1B Working Group of the International League Against Epilepsy/American Epilepsy Society Joint Translational Task Force, we have developed a series of case report forms (CRFs) to describe common data elements for pediatric acquired epilepsy models in rodents. The "Rodent Models of Pediatric Acquired Epilepsy" Core CRF was designed to capture cohort-general information; while two Specific CRFs encompass physical induction models and chemical induction models, respectively. This companion manuscript describes the key elements of these models and why they are important to be considered and reported consistently. Together, these CRFs provide investigators with the tools to systematically record critical information regarding their chosen model of acquired epilepsy during early life, for improved standardization and transparency across laboratories. These outcomes will support the ultimate goal of such research; that is, to understand the childhood onset-specific biology of epileptogenesis after acquired insults, and translate this knowledge into therapeutics to improve pediatric patient outcomes and minimize the lifetime burden of epilepsy.

De Novo and Inherited SETD1A Variants in Early-onset Epilepsy

Early-onset epilepsy is a neurological abnormality in childhood, and it is especially common in the first 2 years after birth. Seizures in early life mostly result from structural or metabolic disorders in the brain, and the genetic causes of idiopathic seizures have been extensively investigated. In this study, we identified four missense mutations in the SETD1A gene (SET domain-containing 1A, histone lysine methyltransferase): three de novo mutations in three individuals and one inherited mutation in a four-generation family. Whole-exome sequencing indicated that all four of these mutations were responsible for the seizures. Mutations of SETD1A have been implicated in schizophrenia and developmental disorders, so we examined the role of the four mutations (R913C, Q269R, G1369R, and R1392H) in neural development. We found that their expression in mouse primary cortical neurons affected excitatory synapse development. Moreover, expression of the R913C mutation also affected the migration of cortical neurons in the mouse brain. We further identified two common genes (Neurl4 and Usp39) affected by mutations of SETD1A. These results suggested that the mutations of SETD1A play a fundamental role in abnormal synaptic function and the development of neurons, so they may be pathogenic factors for neurodevelopmental disorders.

Models and detection of spontaneous recurrent seizures in laboratory rodents

Epilepsy, characterized by spontaneous recurrent seizures (SRS), is a serious and common neurological disorder afflicting an estimated 1% of the population worldwide. Animal experiments, especially those utilizing small laboratory rodents, remain essential to understanding the fundamental mechanisms underlying epilepsy and to prevent, diagnose, and treat this disease. While much attention has been focused on epileptogenesis in animal models of epilepsy, there is little discussion on SRS, the hallmark of epilepsy. This is in part due to the technical difficulties of rigorous SRS detection. In this review, we comprehensively summarize both genetic and acquired models of SRS and discuss the methodology used to monitor and detect SRS in mice and rats.

Asynchronous Distributed Multielectrode Microstimulation Reduces Seizures in the Dorsal Tetanus Toxin Model of Temporal Lobe Epilepsy

BACKGROUND

Electrical brain stimulation has shown promise for reducing seizures in drug-resistant epilepsy, but the electrical stimulation parameter space remains largely unexplored. New stimulation parameters, electrode types, and stimulation targets may be more effective in controlling seizures compared to currently available options.

HYPOTHESIS

We hypothesized that a novel electrical stimulation approach involving distributed multielectrode microstimulation at the epileptic focus would reduce seizure frequency in the tetanus toxin model of temporal lobe epilepsy.

METHODS

We explored a distributed multielectrode microstimulation (DMM) approach in which electrical stimulation was delivered through 15 33-µm-diameter electrodes implanted at the epileptic focus (dorsal hippocampus) in the rat tetanus toxin model of temporal lobe epilepsy.

RESULTS

We show that hippocampal theta (6-12 Hz brain oscillations) is decreased in this animal model during awake behaving conditions compared to control animals (p < 10(-4)). DMM with biphasic, theta-range (6-12 Hz/electrode) pulses delivered asynchronously on the 15 microelectrodes was effective in reducing seizures by 46% (p < 0.05). When theta pulses or sinusoidal stimulation was delivered synchronously and continuously on the 15 microelectrodes, or through a single macroelectrode, no effects on seizure frequency were observed. High frequency stimulation (>16.66 Hz/per electrode), in contrast, had a tendency to increase seizure frequency.

CONCLUSIONS

These results indicate that DMM could be a new effective approach to therapeutic brain stimulation for reducing seizures in epilepsy.

Neonatal Seizures: Impact on Neurodevelopmental Outcomes

Neonatal period is the most vulnerable time for the occurrence of seizures, and neonatal seizures often pose a clinical challenge both for their acute management and frequency of associated long-term co-morbidities. Etiologies of neonatal seizures are known to play a primary role in the anti-epileptic drug responsiveness and the long-term sequelae. Recent studies have suggested that burden of acute recurrent seizures in neonates may also impact chronic outcomes independent of the etiology. However, not many studies, either clinical or pre-clinical, have addressed the long-term outcomes of neonatal seizures in an etiology-specific manner. In this review, we briefly review the available clinical and pre-clinical research for long-term outcomes following neonatal seizures. As the most frequent cause of acquired neonatal seizures, we focus on the studies evaluating long-term effects of HIE-seizures with the goal to evaluate (1) what parameters evaluated during acute stages of neonatal seizures can reliably be used to predict long-term outcomes? and (2) what available clinical and pre-clinical data are available help determine importance of etiology vs. seizure burdens in long-term sequelae.

Do seizures and epileptic activity worsen epilepsy and deteriorate cognitive function?

Relevant to the definition of epileptic encephalopathy (EE) is the concept that the epileptic activity itself may contribute to bad outcomes, both in terms of epilepsy and cognition, above and beyond what might be expected from the underlying pathology alone, and that these can worsen over time. The review of the clinical and experimental evidence that seizures or interictal electroencephalography (EEG) discharges themselves can induce a progression toward more severe epilepsy and a regression of brain function leads to the following conclusions: The possibility of seizure-dependent worsening is by no means a general one but is limited to some types of epilepsy, namely mesial temporal lobe epilepsy (MTLE) and EEs. Clinical and experimental data concur in indicating that prolonged seizures/status epilepticus (SE) are a risky initial event that can set in motion an epileptogenic process leading to persistent, possibly drug-refractory epilepsies. The mechanisms for SE-related epileptogenic process are incompletely known; they seem to involve inflammation and/or glutamatergic transmission. The evidence of the role of recurrent individual seizures in sustaining epilepsy progression is ambiguous. The correlation between high seizure frequency and bad outcome does not necessarily demonstrate a cause-effect relationship, rather high seizure frequency and bad outcome can both depend on a particularly aggressive epileptogenic process. The results of EE studies challenge the idea of a common seizure-dependent mechanism for epilepsy progression/intellectual deterioration.

Neonatal seizures: controversies and challenges in translating new therapies from the lab to the isolette

Neonatal seizures have unique properties that have proved challenging for both clinicians and basic science researchers. Clinical therapies aimed at neonatal seizures have proven only partially effective and new therapies are slow to develop. This article will discuss neonatal seizures within the framework of the barriers that exist to the development of new therapies, and the challenges inherent in bringing new therapies from the bench to the bedside. With the European Union and USA creating national collaborative project infrastructure, improved collaborative resources should advance clinical research on urgently needed new therapies for this disorder.

Animal models

Epilepsy accounts for a significant portion of the dis-ease burden worldwide. Research in this field is fundamental and mandatory. Animal models have played, and still play, a substantial role in understanding the patho-physiology and treatment of human epilepsies. A large number and variety of approaches are available, and they have been applied to many animals. In this chapter the in vitro and in vivo animal models are discussed,with major emphasis on the in vivo studies. Models have used phylogenetically different animals - from worms to monkeys. Our attention has been dedicated mainly to rodents.In clinical practice, developmental aspects of epilepsy often differ from those in adults. Animal models have often helped to clarify these differences. In this chapter, developmental aspects have been emphasized.Electrical stimulation and chemical-induced models of seizures have been described first, as they represent the oldest and most common models. Among these models, kindling raised great interest, especially for the study of the epileptogenesis. Acquired focal models mimic seizures and occasionally epilepsies secondary to abnormal cortical development, hypoxia, trauma, and hemorrhage.Better knowledge of epileptic syndromes will help to create new animal models. To date, absence epilepsy is one of the most common and (often) benign forms of epilepsy. There are several models, including acute pharmacological models (PTZ, penicillin, THIP, GBL) and chronic models (GAERS, WAG/Rij). Although atypical absence seizures are less benign, thus needing more investigation, only two models are so far available (AY-9944,MAM-AY). Infantile spasms are an early childhood encephalopathy that is usually associated with a poor out-come. The investigation of this syndrome in animal models is recent and fascinating. Different approaches have been used including genetic (Down syndrome,ARX mutation) and acquired (multiple hit, TTX, CRH,betamethasone-NMDA) models.An entire section has been dedicated to genetic models, from the older models obtained with spontaneous mutations (GEPRs) to the new engineered knockout, knocking, and transgenic models. Some of these models have been created based on recently recognized patho-genesis such as benign familial neonatal epilepsy, early infantile encephalopathy with suppression bursts, severe myoclonic epilepsy of infancy, the tuberous sclerosis model, and the progressive myoclonic epilepsy. The contribution of animal models to epilepsy re-search is unquestionable. The development of further strategies is necessary to find novel strategies to cure epileptic patients, and optimistically to allow scientists first and clinicians subsequently to prevent epilepsy and its consequences.

Progressive, potassium-sensitive epileptiform activity in hippocampal area CA3 of pilocarpine-treated rats with recurrent seizures

Rat hippocampal area CA3 pyramidal cells synchronously discharge in rhythmic bursts of action potentials after acute disinhibition or convulsant treatment in vitro. These burst discharges resemble epileptiform activity, and are of interest because they may shed light on mechanisms underlying limbic seizures. However, few studies have examined CA3 burst discharges in an animal model of epilepsy, because a period of prolonged, severe seizures (status epilepticus) is often used to induce the epileptic state, which can lead to extensive neuronal loss in CA3. Therefore, the severity of pilocarpine-induced status epilepticus was decreased with anticonvulsant treatment to reduce damage. Rhythmic burst discharges were recorded in the majority of slices from these animals, between two weeks and nine months after status epilepticus. The incidence and amplitude of bursts progressively increased with time after status, even after spontaneous behavioral seizures had begun. The results suggest that modifying the pilocarpine models of temporal lobe epilepsy to reduce neuronal loss leads to robust network synchronization in area CA3. The finding that these bursts increase long after spontaneous behavioral seizures begin supports previous arguments that temporal lobe epilepsy exhibits progressive pathophysiology.

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.

Epileptogenesis in the immature brain: emerging mechanisms

Epileptogenesis is defined as the process of developing epilepsy-a disorder characterized by recurrent seizures-following an initial insult. Seizure incidence during the human lifespan is at its highest in infancy and childhood. Animal models of epilepsy and human tissue studies suggest that epileptogenesis involves a cascade of molecular, cellular and neuronal network alterations. Within minutes to days following the initial insult, there are acute early changes in neuronal networks, which include rapid alterations to ion channel kinetics as a result of membrane depolarization, post-translational modifications to existing functional proteins, and activation of immediate early genes. Subacute changes occur over hours to weeks, and include transcriptional events, neuronal death and activation of inflammatory cascades. The chronic changes that follow over weeks to months include anatomical changes, such as neurogenesis, mossy fiber sprouting, network reorganization, and gliosis. These epileptogenic processes are developmentally regulated and might contribute to differences in epileptogenesis between adult and developing brains. Here we review the factors responsible for enhanced seizure susceptibility in the developing brain, and consider age-specific mechanisms of epileptogenesis. An understanding of these factors could yield potential therapeutic targets for the prevention of epileptogenesis and also provide biomarkers for identifying patients at risk of developing epilepsy or for monitoring disease progression.

Role of Ca2+/calmodulin-dependent protein kinase II in dendritic spine remodeling during epileptiform activity in vitro

Epileptiform activity (EA) in vivo and in vitro induces a loss of dendritic spines and synapses. Because CaMKII has been implicated in synaptogenesis and synaptic plasticity, we investigated the role of CaMKII in the effects of EA on spines, using rat hippocampal slice cultures. To visualize dendrites and postsynaptic densities (PSDs) in pyramidal neurons in the slices, we used biolistic transfection to express either free GFP or a PSD95-YFP construct that specifically labels PSDs. This allowed us to distinguish two classes of dendritic protrusions: spines that contain PSDs, and filopodia that lack PSDs and that are, on average, longer than spines. By these criteria, 48 hr of EA caused a decrease specifically in the number of spines. Immunoblots showed that EA increased CaMKII activity in the slices. Inhibition of CaMKII by expression of AIP, a specific peptide inhibitor of CaMKII, reduced spine number under basal conditions and failed to prevent EA-induced spine loss. However, under EA conditions, AIP increased the number of filopodia and the number of PSDs on the dendritic shaft. These data show at least two roles for CaMKII activity in maintenance and remodeling of dendritic spines under basal or EA conditions. First, CaMKII activity promotes the maintenance of spines and spine PSDs. Second, CaMKII activity suppresses EA-induced formation of filopodia and suppresses an increase in shaft PSDs, apparently by promoting translocation of PSDs from dendritic shafts to spines and/or selectively stabilizing spine rather than shaft PSDs.

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.

Changes of cortical interhemispheric responses after status epilepticus in immature rats

PURPOSE

To study cortical excitability after status epilepticus induced in two age groups of immature rats.

METHODS

Lithium-pilocarpine status epilepticus was elicited in 12- (SE12) or 25-day-old (SE25) rats. Control siblings received saline instead of pilocarpine. Interhemispheric responses were elicited by stimulation of sensorimotor region of cerebral cortex 3, 6, 9, 13, or 26 days after status. Single biphasic pulses with intensities from 0.2 to 4 mA were used for stimulation; eight responses were always averaged. Amplitude of the first positive and negative waves (i.e., monosynaptic transcallosal responses) was measured and used for construction of input-output (I/O) curves. FluoroJade B was used to visualize degenerating neurons 24 h after status in both age groups.

RESULTS

No significant changes were found at short intervals, but only a tendency to lower amplitudes 3 days after status in SE12 group. Marked changes appeared 26 days after status. The younger group exhibited lower amplitudes than did control rats, whereas SE25 animals generated responses with higher amplitude than did controls (i.e., the I/O curve was steeper. FluoroJade B-positive neurons were scarce in SE12 rats, whereas a substantial number of positive neurons was found in SE25 animals. The positive neurons exhibited characteristics of interneurons, and their distribution in cortical layers differed in the two groups.

CONCLUSIONS

Status epilepticus resulted in neuronal death in both SE12 and SE25 animals. Changes in transcallosal evoked potentials were opposite in the two age groups. Augmented amplitude of responses in SE25 rats may indicate an increased cortical excitability.

Ontogeny of feline temporal lobe epilepsy in amygdala-kindled kittens: an update

This report describes amygdala-kindled seizure development and the post-kindling course in 58 cats (29 males and 29 females), including 40 preadolescents between 2.5 and 6.5 months of age and 18 adults >1 year of age at the beginning of kindling. The results extend previous findings, as follows: (1) Youngest animals (<5.0 months, n=30) are far more likely than older kittens and adults to exhibit spontaneous epilepsy, indexed by seizures that occur >1 h after stimulus-evoked seizures. (2) The youngest animals also exhibit accelerated kindling rates and rapid post-kindling onset of multifocal spontaneous epilepsy with a catastrophic clinical course. The profile includes a variety of EEG and/or clinical seizure manifestations and a progressive increase in the number and density of convulsive seizure clusters. Behavioral sequelae accompany seizure clusters and can range from sensory or motor deficits (visual agnosia, sensory hypersensitivity, atonic episodes, restricted mobility) to social isolation and placidity. (3) Onset of spontaneous epilepsy with developmental deterioration is substantially enhanced by recurrent evoked seizures early in the post-kindling course. The post-kindling progression can be stopped or minimized by suspension of evoked seizure trials and/or by management of frequent spontaneous convulsions (>1 per hour) with anticonvulsants. (4) In older cats, many more evoked seizures are required to generate fewer spontaneous seizures with relatively innocuous behavioral disorders. The findings suggest a 'critical period' in kindled kittens for onset of spontaneous temporal lobe epilepsy with severe behavioral consequences and a favorable prognosis for the young following early detection and intervention.

Epilepsy and synaptic reorganization in a perinatal rat model of hypoxia-ischemia

PURPOSE

One of the potential consequences of perinatal hypoxia-ischemia (H-I) is the development of epilepsy, and synaptic reorganization in the hippocampus has been associated with epilepsy after an injury. We tested the hypothesis that perinatal H-I will induce spontaneous motor seizures, hippocampal lesions, and synaptic reorganization in the dentate gyrus.

METHODS

The right common carotid artery of 7-day-old rats was permanently ligated, and the rats were placed for 120 min into a chamber filled with 8% oxygen (37 degrees C). Animals were directly observed for chronic motor seizures for 7 to 24 months after the H-I insult.

RESULTS

Nearly half of the rats (i.e., eight of 20) were seen to have spontaneous motor seizures after the H-I injury. The ipsilateral hippocampi from both the rats with seizures and the rats not seen to have seizures had hippocampal lesions and increased amounts of Timm stain in the inner molecular layer (IML) compared with controls. The contralateral hippocampi from the rats with seizures, but not the hippocampi from the rats not seen to have seizures, had significantly increased amounts of Timm stain in the IML.

CONCLUSIONS

These results suggest that perinatal H-I can induce epilepsy, ipsilateral hippocampal lesions, and mossy fiber sprouting in the lesioned and contralateral hippocampus.

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.

Status epilepticus in immature rats leads to behavioural and cognitive impairment and epileptogenesis

It remains under dispute whether status epilepticus (SE) in the perinatal period or early childhood or the underlying neuropathology is the cause of functional impairment later in life. The present study examined whether SE induced by LiCl-pilocarpine in normal immature brain (at the age of 12 or 25 days; P12 or P25) causes cognitive decline and epileptogenesis, and the data were compared to those of rats undergoing SE as adults. Rats in the P12 group had impaired memory (repeated exposure to open-field paradigm) and emotional behaviour (lower proportion of open-arm entries and higher incidence of risk assessment period in elevated plus-maze) when assessed 3 months after SE, although not as severe as in the older age groups. Importantly, video-electroencephalography monitoring 3 months after SE demonstrated that 25% of rats in the P12 and 50% in P25 group developed spontaneous seizures. Only nonconvulsive seizures (ictal activity in hippocampus accompanied by automatisms) were recorded in the P12 group whereas rats in the P25 group exhibited clonic convulsions. The present findings indicate that SE is harmful to the immature brain as early as P12, which might be compared with early infancy in humans.

The effects of seizures on the connectivity and circuitry of the developing brain

Recurring seizures in infants and children are often associated with cognitive deficits, but the reason for the learning difficulties is unclear. Recent studies in several animal models suggest that seizures themselves may contribute in important ways to these deficits. Other studies in animals have shown that recurring seizures result in dendritic spine loss. This change, coupled with a down-regulation in NMDA receptor subunit expression, suggests that repetitive seizures may interrupt the normal development of glutamatergic synaptic transmission. We hypothesize that homeostatic, neuroprotective processes are induced by recurring early-life seizures. These processes, by diminishing glutamatergic synaptic transmission, are aimed at preventing the continuation of seizures. However, by preventing the normal development of glutamatergic synapses, and particularly NMDA receptor-mediated synaptic transmission, such homeostatic processes also reduce synaptic plasticity and diminish the ability of neuronal circuits to learn and store memories.

In vivo evidence for an activity-independent maturation of AMPA/NMDA signaling in the developing hippocampus

Correlated pre- and postsynaptic activity is thought to promote maturation of excitatory synapses in the developing brain by directing AMPA receptors to pure NMDA synapses. However, this hypothesis has not been tested in vivo. Here, we have performed such test by inhibiting correlated neural activity in vivo using a single injection of tetanus toxin into the rat hippocampal CA1 area at postnatal day 1. When examined in the acute slice preparation (1-7 days post-injection), there was a strong reduction, down to 20% of control level, in the frequency of glutamatergic and GABAergic spontaneous postsynaptic currents (sPSCs). This activity deprivation led to a growth retardation of CA1 pyramidal neurons and to markedly faster decay kinetics of NMDA sPCSs. However, it did not alter the relationship between AMPA and NMDA sPSCs with respect to either their frequency or amplitude. Thus, although critical for certain aspects of neuronal development, correlated neural activity in the neonatal hippocampus does not seem to promote incorporation of AMPA receptors at pure NMDA synapses.

Temporal Lobe Epileptogenesis and Epilepsy in the Developing Brain: Bridging the Gap Between the Laboratory and the Clinic. Progression, But in What Direction?

The origins of human mesial temporal lobe epilepsy and hippocampal sclerosis are still not well understood. Hippocampal sclerosis and temporal lobe epileptogenesis involve a series of pathologies including hippocampal neuronal loss and gliosis, axonal reorganization, and maybe hippocampal neoneurogenesis. However, the causality of these events is unclear as well as their relation to the factors that may precipitate epileptogenesis. Significant differences between temporal lobe epileptogenesis in the adult and immature brain may require differential approaches. Hereditary factors also may participate in some cases of hippocampal sclerosis. The key point is to identify the significance of these age-dependent changes and to design preventive treatments. Novel strategies for the prevention and treatment of mesial temporal lobe epilepsy and hippocampal sclerosis may include rational use of neuroprotective agents, hormonotherapy, immunizations, and immunotherapy.

Animal model of dementia induced by entorhinal synaptic damage and partial restoration of cognitive deficits by BDNF and carnitine

A rat dementia model with cognitive deficits was generated by synapse-specific lesions using botulinum neurotoxin (BoNTx) type B in the entorhinal cortex. To detect cognitive deficits, different tasks were needed depending upon the age of the model animals. Impaired learning and memory with lesions were observed in adult rats using the Hebb-Williams maze, AKON-1 maze and a continuous alternation task in T-maze. Cognitive deficits in lesioned aged rats were detected by a continuous alternation and delayed non-matching-to-sample tasks in T-maze. Adenovirus-mediated BDNF gene expression enhanced neuronal plasticity, as revealed by behavioral tests and LTP formation. Chronic administration of carnitine over time pre- and post-lesions seemed to partially ameliorate the cognitive deficits caused by the synaptic lesion. The carnitine-accelerated recovery from synaptic damage was observed by electron microscopy. These results demonstrate that the BoNTx-lesioned rat can be used as a model for dementia and that cognitive deficits can be alleviated in part by BDNF gene transfer or carnitine administration.

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.

Recent experimental studies of the effects of seizures on brain development

Results from experimental studies in the past have suggested that early-life seizures have little consequence on brain development. However, the creation of new animal models is altering this view. This chapter briefly summarizes these new findings and suggests avenues for future study.

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.

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.

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.

Blockade of neuronal activity during hippocampal development produces a chronic focal epilepsy in the rat

During brain development, neuronal activity can transform neurons characterized by widely ranging axonal projections to ones with more restricted patterns of synaptic connectivity. Previous studies have shown that an exuberant outgrowth of local recurrent excitatory axons occurs in hippocampal area CA3 during postnatal weeks 2 and 3. Axons are remodeled with maturation, and nearly half of the branches are eliminated. Postnatal weeks 2 and 3 also coincide with a "critical" period of development, when CA3 networks have a marked propensity to generate electrographic seizures. In an attempt to prevent axonal remodeling, local circuit activity was blocked unilaterally in dorsal hippocampus by continuous infusion of tetrodotoxin (TTX). Field potential recordings from behaving animals were dramatically altered when TTX infusion was initiated at the beginning of the critical period, week 2, but not later in life. Spontaneous, synchronized spikes and electrographic seizures with behavioral accompaniments were observed after 4 weeks of TTX infusion and persisted into adulthood. When recordings were made during TTX infusion, synchronized spiking was recorded in ventral hippocampus as early as 2 weeks after infusate introduction. At this same time, extracellular field recordings from in vitro slices demonstrated spontaneous network-driven "mini-bursts" arising from ventral hippocampal slices. These were abolished by glutamate receptor antagonists. Whole-cell recordings from CA3 neurons revealed bursts of excitatory synaptic potentials coincident with the network bursts recorded extracellularly. Thus, local assemblies of mutually excitatory CA3 pyramidal cells are hyperexcitable in these rats. Whether alterations in developmental axonal remodeling mediate these effects awaits further studies.

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.

Clostridial toxins as therapeutic agents: benefits of nature's most toxic proteins

Toxins are increasingly being used as valuable tools for analysis of cellular physiology, and some are used medicinally for treatment of human diseases. In particular, botulinum toxin, the most poisonous biological substance known, is used for treatment of a myriad of human neuromuscular disorders characterized by involuntary muscle contractions. Since approval of type-A botulinum toxin by the US Food and Drug Administration in December 1989 for three disorders (strabismus, blepharospasm, and hemifacial spasm), the number of indications being treated has increased greatly to include numerous focal dystonias, spasticity, tremors, cosmetic applications, migraine and tension headaches, and other maladies. Many of these diseases were previously refractory to pharmacological and surgical treatments. The remarkable therapeutic utility of botulinum toxin lies in its ability to specifically and potently inhibit involuntary muscle activity for an extended duration. The clostridia produce more protein toxins than any other bacterial genus and are a rich reservoir of toxins for research and medicinal uses. Research is underway to use clostridial toxins or toxin domains for drug delivery, prevention of food poisoning, and the treatment of cancer and other diseases. The remarkable success of botulinum toxin as a therapeutic agent has created a new field of investigation in microbiology.

Seizure-like events in disinhibited ventral slices of adult rat hippocampus

Epileptic discharges lasting 2-90 s, were studied in vitro in slices from the ventral hippocampus of adult rats, in which inhibition was blocked acutely with bicuculline methiodide (BMI, 5-30 microM) and potassium ([K(+)](o)) raised to 5 mM. These seizure-like events (SLEs) comprised three distinct phases, called here primary, secondary, and tertiary bursts. Primary bursts lasted 90-150 ms. Secondary bursts lasted a further 70-250 ms, comprising a short series of afterdischarges riding on the same depolarization as the primary burst. Finally a train of tertiary bursts started with a peak frequency of 5-10 Hz and could last >1 min. Slices from the ventral hippocampus showed significantly higher susceptibility to SLEs than did dorsal slices. SLEs proved sensitive to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonists. They were insensitive to N-methyl-D-aspartate (NMDA) receptor antagonists; 50 microl D-2-amino-5-phosphonopentanoic acid (D-AP5) did block the transient secondary bursts selectively. SLEs were restricted to the hippocampus proper even if the entorhinal cortex was present. Entorhinal bursts could last <2 s and were only coupled with hippocampal bursts in a minority of slices. Reentry of epileptic bursts occasionally occurred during interictal discharges, but not during the later stages of SLEs. Full-length SLEs always started in CA3 region and could be recorded in minislices containing CA3 plus dentate hilus. Ion-sensitive microelectrodes revealed that interictal discharges were followed by short (2-3 s) [K(+)](o) waves, peaking at approximately 7.5 mM. SLEs were always accompanied by increases in [K(+)](o) reaching approximately 8.5 mM at the start of tertiary bursts; [K(+)](o) then increased more slowly to a ceiling of 11-12 mM. After the end of each SLE, [K(+)](o) fell back to baseline within 10-15 s. SLEs were accompanied by significant increase in synaptic activity, compared with baseline and/or interictal activity, estimated by the variance of the intracellular signal in the absence of epileptic bursts and action potentials (0. 38 mV(2), compared with 0.13 mV(2), and 0.1 mV(2), respectively). No significant increases were observed in the interval preceding spontaneous interictal activity. These studies show that focal assemblies of hippocampal neurons, without long reentrant loops, are sufficient for the generation of SLEs. We propose that a key factor in the transition from interictal activity to SLEs is an increase in axonal and terminal excitability, resulting, at least in part, from elevations in [K(+)](o).

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.

Effects of tetanus toxin on functional inhibition after injection in separate cortical areas in rat

Tetanus Toxin is widely used as a model of chronic focal epilepsy and is assumed to act by blocking neurotransmitter release with high selectivity for inhibitory synapses. However, the exact mechanisms are not fully understood, since, e.g., GABA release is only temporarily decreased although epileptiform activity persists pointing towards a change in the interplay of excitation and inhibition. Furthermore there have been reports about different effects of tetanus toxin after injection in separate brain areas. Therefore, we investigated the functional inhibition after injecting tetanus toxin either in the motor or sensory cortex of adult rats by using a paired-pulse paradigm as a measure of excitatory and inhibitory drive. Tetanus toxin injection into the motor cortex (n=10) induced a marked, long-lasting reduction in inhibition which was highly significant in most parts of the injected cortical area. Injections into the sensory cortex, however, showed less marked changes in inhibition which were more widespread and significant only in 3 of 14 animals injected. These results give further evidence for a prominent effect of tetanus toxin on functional inhibition and strengthen the idea of a differential effect in separate cortical areas. They may be accounted for by the different cytoarchitecture of cortical areas with variable inhibitory and excitatory intracortical connections.

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.

Autism and Clostridium tetani

Autism is a severe developmental disability believed to have multiple etiologies. This paper outlines the possibility of a subacute, chronic tetanus infection of the intestinal tract as the underlying cause for symptoms of autism observed in some individuals. A significant percentage of individuals with autism have a history of extensive antibiotic use. Oral antibiotics significantly disrupt protective intestinal microbiota, creating a favorable environment for colonization by opportunistic pathogens. Clostridium tetani is an ubiquitous anaerobic bacillus that produces a potent neurotoxin. Intestinal colonization by C. tetani, and subsequent neurotoxin release, have been demonstrated in laboratory animals which were fed vegetative cells. The vagus nerve is capable of transporting tetanus neurotoxin (TeNT) and provides a route of ascent from the intestinal tract to the CNS. This route bypasses TeNT's normal preferential binding sites in the spinal cord, and therefore the symptoms of a typical tetanus infection are not evident. Once in the brain, TeNT disrupts the release of neurotransmitters by the proteolytic cleavage of synaptobrevin, a synaptic vesicle membrane protein. This inhibition of neurotransmitter release would explain a wide variety of behavioral deficits apparent in autism. Lab animals injected in the brain with TeNT have exhibited many of these behaviors. Some children with autism have also shown a significant reduction in stereotyped behaviors when treated with antimicrobials effective against intestinal clostridia. When viewed as sequelae to a subacute, chronic tetanus infection, many of the puzzling abnormalities of autism have a logical basis. A review of atypical tetanus cases, and strategies to test the validity of this paper's hypothesis, are included.

Mechanisms and experimental models of seizure generation

Over the past year evidence has accumulated against the idea that seizures require re-entrant activity between spatially separate structures. Seizures in vivo typically do involve interconnected, spatially separate brain regions, but they often show no net phase lag around the putative circuit. In many cases seizure-like events can arise from localized regions such as the entorhinal cortex or hippocampus proper, through mechanisms that are starting to be identified.

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

In vitro slice experiments were undertaken in adult rats to investigate the physiological origins of a chronic epileptic condition that was initiated in infancy. A unilateral injection of a minute quantity of tetanus toxin into hippocampus on postnatal day 10 produced a severe convulsive syndrome characterized by brief but repeated seizures that lasted for 5-7 days. Hippocampal slices were then taken from these rats in adulthood because at this time previous studies have shown the occurrence of electrographic and behavioral seizures. Dramatic alterations in local circuit functioning were observed. In normal artificial cerebrospinal fluid (ACSF), spontaneous epileptiform network bursts were recorded in a majority (73%) of experimental rats. Network bursts occurred in area CA3 of both the injected and contralateral hippocampus. These consisted of intracellular depolarization shifts that were coincident with extracellularly recorded network bursts. Often they occurred at frequencies of 0.05-0.1 Hz and although variable in amplitude and duration, had all-or-none-like qualities. These events appeared to arise largely from local circuits in the CA3C subfield. Network bursts were rarely recorded in area CA1 and were never observed in the dentate gyrus. However in 31% of rats, a novel, higher frequency (2-8 Hz) field potential was recorded in area CA1. This was coincident with rhythmic, intracellularly recorded, inhibitory postsynaptic potentials (IPSPs). These summated IPSPs blocked action potential firing and reversed polarity near -75 mV. To understand the origins of network bursting in area CA3C, comparisons were made of the fundamental neurophysiological properties of pyramidal cells in epileptic and control rats. Of the passive and active membrane properties examined, all appeared normal. Unusually prolonged bursts of action potentials were observed in a small subset of pyramidal cells. However on average the duration of intrinsic bursts were unaltered in the CA3 neurons analyzed from experimental rats. To explore the role that alterations in CA3 recurrent excitatory network excitability may play in epileptiform discharges, picrotoxin was bath applied. On blockade of gamma-aminobutyric acid (GABAA) receptors, slices from experimental rats underwent prolonged electrographic seizures that were up to 10 s in duration. In contrast, slices from control rats produced only brief 100-ms network bursts. These results suggest that a change in excitability within CA3C recurrent excitatory networks likely contributes to seizures in chronically epileptic rats. However, at the same time, this hyperexcitability is controlled to an important degree by functional GABAA-mediated synaptic inhibition.

A new protocol for the transmission of physiological signals by digital telemetry

A method has been devised to transmit physiological signals from the brains of rats using a new digital telemetry transmission protocol. The chief advantage of the present system over existing systems is that the circuit only consumes power during the transmission of a brief pulse of information. This results in a very much extended battery life allowing the device to be implanted and recordings to be carried out over a longer period of time.

Increased susceptibility to tetanus toxin-induced seizures in immature rats

Studies were undertaken to compare tetanus toxin-induced electrographic and behavioral seizure activity in immature and adult rats. Multifocal epileptiform activity was induced by unilateral hippocampal injection of tetanus toxin in 10-day-old immature and in 90-120-day-old adult rats. Bilateral cortical surface electrodes and, in some animals, hippocampal depth electrodes were placed and serial video EEG was recorded for 2 h during the week following tetanus toxin injection. Tetanus toxin-induced interictal spike frequency was significantly higher and seizure duration was significantly longer in immature compared to adult rats. Immature rats tended to have more frequent seizures compared with adults, but the difference was not statistically significant. Ictal behaviors were different for the two groups with immature rats commonly demonstrating wild running seizures which were not seen in adult rats. These findings suggest that there are age-dependent differences in electrographic and behavioral seizure expression in the tetanus toxin seizure model and that the epileptic syndrome in immature rats is more severe.