Tetanus toxin induces long-term changes in excitation and inhibition in the rat hippocampal CA1 area

Deciphering temporal gene expression dynamics during epilepsy development using a rat model of focal neocortical epilepsy

OBJECTIVE

Epilepsy involves significant changes in neural cells during epileptogenesis. Although the molecular mechanism of epileptogenesis remains obscure, changes in gene regulation play a crucial role in the evolution of epilepsy. This study aimed to compare changes in a subset of specific genes during epilepsy development, focusing on the period after the first spontaneous seizure, to identify critical time windows for targeting different regulators.

METHODS

Using a rat model of acquired focal neocortical epilepsy induced by tetanus toxin, we characterized gene expression at acute, subacute, and chronic stages (48-72 h, 2 weeks, and 30 days after first spontaneous seizure, respectively), focusing on genes' potential contribution to epilepsy progression.

RESULTS

We observed dynamic changes in the expression of these genes throughout the period after the first spontaneous seizure. Astrocytic reactions primarily occur early, before epilepsy is well established. Changes in Mtor (mammalian target of rapamycin) and Rest (repressor element 1 silencing transcription factor) signaling pathways are highly dynamic and correlated with the progression of epilepsy development. Ccl2 (chemokine C-C-motif ligand) is upregulated at the chronic stage, indicating activation of the neuroinflammatory pathway. Finally, Gabra5 (γ-aminobutyric acidergic signaling) is downregulated at the late stage after epilepsy is established. Surprisingly, changes in the expression of specific genes are linked to the time since the first seizure, rather than seizure frequency or duration.

SIGNIFICANCE

These results suggest that the regulation of specific genes is essentially stage-dependent during the development of epilepsy, highlighting the importance of targeting specific genes at appropriate stages of epilepsy development.

An update on the seizures beget seizures theory

Seizures beget seizures is a longstanding theory that proposed that seizure activity can impact the structural and functional properties of the brain circuits in ways that contribute to epilepsy progression and the future occurrence of seizures. Originally proposed by Gowers, this theory continues to be quoted in the pathophysiology of epilepsy. We critically review the existing data and observations on the consequences of recurrent seizures on brain networks and highlight a range of factors that speak for and against the theory. The existing literature demonstrates clearly that ictal activity, especially if recurrent, induces molecular, structural, and functional changes including cell loss, connectivity reorganization, changes in neuronal behavior, and metabolic alterations. These changes have the potential to modify the seizure threshold, contribute to disease progression, and recruit wider areas of the epileptic network into epileptic activity. Repeated seizure activity may, thus, act as a pathological positive-feedback mechanism that increases seizure likelihood. On the other hand, the time course of self-limited epilepsies and the presence of seizure remission in two thirds of epilepsy cases and various chronic epilepsy models oppose the theory. Experimental work showed that seizures could induce neural changes that increase the seizure threshold and decrease the risk of a subsequent seizure. Due to the complex nature of epilepsies, it is wrong to consider only seizures as the key factor responsible for disease progression. Epilepsy worsening can be attributed to the various forms of interictal epileptiform activity or underlying disease mechanisms. Although seizure activity can negatively impact brain structure and function, the "seizures beget seizures" theory should not be used dogmatically but with extreme caution.

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.

Altered expression of the voltage-gated calcium channel subunit α₂δ-1: a comparison between two experimental models of epilepsy and a sensory nerve ligation model of neuropathic pain

The auxiliary α2δ-1 subunit of voltage-gated calcium channels is up-regulated in dorsal root ganglion neurons following peripheral somatosensory nerve damage, in several animal models of neuropathic pain. The α2δ-1 protein has a mainly presynaptic localization, where it is associated with the calcium channels involved in neurotransmitter release. Relevant to the present study, α2δ-1 has been shown to be the therapeutic target of the gabapentinoid drugs in their alleviation of neuropathic pain. These drugs are also used in the treatment of certain epilepsies. In this study we therefore examined whether the level or distribution of α2δ-1 was altered in the hippocampus following experimental induction of epileptic seizures in rats, using both the kainic acid model of human temporal lobe epilepsy, in which status epilepticus is induced, and the tetanus toxin model in which status epilepticus is not involved. The main finding of this study is that we did not identify somatic overexpression of α2δ-1 in hippocampal neurons in either of the epilepsy models, unlike the upregulation of α2δ-1 that occurs following peripheral nerve damage to both somatosensory and motor neurons. However, we did observe local reorganization of α2δ-1 immunostaining in the hippocampus only in the kainic acid model, where it was associated with areas of neuronal cell loss, as indicated by absence of NeuN immunostaining, dendritic loss, as identified by areas where microtubule-associated protein-2 immunostaining was missing, and reactive gliosis, determined by regions of strong OX42 staining.

Endocrine-disrupting chemicals: elucidating our understanding of their role in sex and gender-relevant end points

Endocrine-disrupting chemicals (EDCs) are diverse and pervasive and may have significant consequence for health, including reproductive development and expression of sex-/gender-sensitive parameters. This review chapter discusses what is known about common EDCs and their effects on reproductively relevant end points. It is proposed that one way that EDCs may exert such effects is by altering steroid levels (androgens or 17-estradiol, E₂) and/or intracellular E₂ receptors (ERs) in the hypothalamus and/or hippocampus. Basic research findings that demonstrate developmentally sensitive end points to androgens and E₂ are provided. Furthermore, an approach is suggested to examine differences in EDCs that diverge in their actions at ERs to elucidate their role in sex-/gender-sensitive parameters.

Rapid eye movement sleep and hippocampal theta oscillations precede seizure onset in the tetanus toxin model of temporal lobe epilepsy

Improved understanding of the interaction between state of vigilance (SOV) and seizure onset has therapeutic potential. Six rats received injections of tetanus toxin (TeTX) in the ventral hippocampus that resulted in chronic spontaneous seizures. The distribution of SOV before 486 seizures was analyzed for a total of 19 d of recording. Rapid eye movement sleep (REM) and exploratory wake, both of which express prominent hippocampal theta rhythm, preceded 47 and 34%, for a total of 81%, of all seizures. Nonrapid eye movement sleep (NREM) and nonexploratory wake, neither of which expresses prominent theta, preceded 6.8 and 13% of seizures. We demonstrate that identification of SOV yields significant differentiation of seizure susceptibilities, with the instantaneous seizure rate during REM nearly 10 times higher than baseline and the rate for NREM less than half of baseline. Survival analysis indicated a shorter duration of preseizure REM bouts, with a maximum transition to seizure at ∼90 s after the onset of REM. This study provides the first analysis of a correlation between SOV and seizure onset in the TeTX model of temporal lobe epilepsy, as well as the first demonstration that hippocampal theta rhythms associated with natural behavioral states can serve a seizure-promoting role. Our findings are in contrast with previous studies suggesting that the correlations between SOV and seizures are primarily governed by circadian oscillations and the notion that hippocampal theta rhythms inhibit seizures. The documentation of significant SOV-dependent seizure susceptibilities indicates the potential utility of SOV and its time course in seizure prediction and control.

Structural and functional substrates of tetanus toxin in an animal model of temporal lobe epilepsy

The effects of tetanus toxin (TeNT) both in the spinal cord, in clinical tetanus, and in the brain, in experimental focal epilepsy, suggest disruption of inhibitory synapses. TeNT is a zinc protease with selectivity for Vesicle Associated Membrane Protein (VAMP; previously synaptobrevin), with a reported selectivity for VAMP2 in rats. We found spatially heterogeneous expression of VAMP1 and VAMP2 in the hippocampus. Inhibitory terminals in stratum pyramidale expressed significantly more VAMP1 than VAMP2, while glutamatergic terminals in stratum radiatum expressed significantly more VAMP2 than VAMP1. Intrahippocampal injection of TeNT at doses that induce epileptic foci cleaved both isoforms in tissue around the injection site. The cleavage was modest at 2 days after injection and more substantial and extensive at 8 and 16 days. Whole-cell recordings from CA1 pyramidal cells close to the injection site, made 8-16 days after injection, showed that TeNT decreases spontaneous EPSC frequency to 38 % of control and VAMP2 immunoreactive axon terminals to 37 %. In contrast, TeNT almost completely abolished both spontaneous and evoked IPSCs while decreasing VAMP1 axon terminals to 45 %. We conclude that due to the functional selectivity of the toxin to the relative sparing of excitatory synaptic transmission shifts the network to pathogenically excitable state causing epilepsy.

Dentate gyrus progenitor cell proliferation after the onset of spontaneous seizures in the tetanus toxin model of temporal lobe epilepsy

Temporal lobe epilepsy alters adult neurogenesis. Existing experimental evidence is mainly from chronic models induced by an initial prolonged status epilepticus associated with substantial cell death. In these models, neurogenesis increases after status epilepticus. To test whether status epilepticus is necessary for this increase, we examined precursor cell proliferation and neurogenesis after the onset of spontaneous seizures in a model of temporal lobe epilepsy induced by unilateral intrahippocampal injection of tetanus toxin, which does not cause status or, in most cases, detectable neuronal loss. We found a 4.5 times increase in BrdU labeling (estimating precursor cells proliferating during the 2nd week after injection of toxin and surviving at least up to 7days) in dentate gyri of both injected and contralateral hippocampi of epileptic rats. Radiotelemetry revealed that the rats experienced 112±24 seizures, lasting 88±11s each, over a period of 8.6±1.3days from the first electrographic seizure. On the first day of seizures, their duration was a median of 103s, and the median interictal period was 23min, confirming the absence of experimentally defined status epilepticus. The total increase in cell proliferation/survival was due to significant population expansions of: radial glial-like precursor cells (type I; 7.2×), non-radial type II/III neural precursors in the dentate gyrus stem cell niche (5.6×), and doublecortin-expressing neuroblasts (5.1×). We conclude that repeated spontaneous brief temporal lobe seizures are sufficient to promote increased hippocampal neurogenesis in the absence of status epilepticus.

Reduced threshold for induction of LTP by activation of dopamine D1/D5 receptors at hippocampal CA1-subiculum synapses

The phasic release of dopamine in the hippocampal formation has been shown to facilitate the encoding of novel information. There is evidence that the subiculum operates as a detector and distributor of sensory information, which incorporates the novelty and relevance of signals received from CA1. The subiculum acts as the final hippocampal relay station for outgoing information. Subicular pyramidal cells have been classified as regular- and burst-spiking neurons. The goal of the present study was to study the effect of dopamine D1/D5 receptor activation on synaptic transmission and plasticity in the subicular regular-spiking neurons of 4–6 week old Wistar rats. We demonstrate that prior activation of D1/D5 receptors reduces the threshold for the induction of long-term potentiation (LTP) in subicular regular-spiking neurons. Our results indicate that D1/D5 receptor activation facilitates a postsynaptic form of LTP in subicular regular-spiking cells that is NMDA receptor-dependent, relies on postsynaptic Ca²⁺ signaling, and requires the activation of protein kinase A. The enhanced propensity of subicular regular-spiking cells to express postsynaptic LTP after activation of D1/D5 receptors provides an intriguing mechanism for the encoding of hippocampal output information.

Optogenetic and potassium channel gene therapy in a rodent model of focal neocortical epilepsy

Neocortical epilepsy is frequently drug-resistant. Surgery to remove the epileptogenic zone is only feasible in a minority of cases, leaving many patients without an effective treatment. We report the potential efficacy of gene therapy in focal neocortical epilepsy using a rodent model in which epilepsy is induced by tetanus toxin injection in the motor cortex. By applying several complementary methods that use continuous wireless electroencephalographic monitoring to quantify epileptic activity, we observed increases in high frequency activity and in the occurrence of epileptiform events. Pyramidal neurons in the epileptic focus showed enhanced intrinsic excitability consistent with seizure generation. Optogenetic inhibition of a subset of principal neurons transduced with halorhodopsin targeted to the epileptic focus by lentiviral delivery was sufficient to attenuate electroencephalographic seizures. Local lentiviral overexpression of the potassium channel Kv1.1 reduced the intrinsic excitability of transduced pyramidal neurons. Coinjection of this Kv1.1 lentivirus with tetanus toxin fully prevented the occurrence of electroencephalographic seizures. Finally, administration of the Kv1.1 lentivirus to an established epileptic focus progressively suppressed epileptic activity over several weeks without detectable behavioral side effects. Thus, gene therapy in a rodent model can be used to suppress seizures acutely, prevent their occurrence after an epileptogenic stimulus, and successfully treat established focal epilepsy.

Mechanisms of physiological and epileptic HFO generation

High frequency oscillations (HFO) have a variety of characteristics: band-limited or broad-band, transient burst-like phenomenon or steady-state. HFOs may be encountered under physiological or under pathological conditions (pHFO). Here we review the underlying mechanisms of oscillations, at the level of cells and networks, investigated in a variety of experimental in vitro and in vivo models. Diverse mechanisms are described, from intrinsic membrane oscillations to network processes involving different types of synaptic interactions, gap junctions and ephaptic coupling. HFOs with similar frequency ranges can differ considerably in their physiological mechanisms. The fact that in most cases the combination of intrinsic neuronal membrane oscillations and synaptic circuits are necessary to sustain network oscillations is emphasized. Evidence for pathological HFOs, particularly fast ripples, in experimental models of epilepsy and in human epileptic patients is scrutinized. The underlying mechanisms of fast ripples are examined both in the light of animal observations, in vivo and in vitro, and in epileptic patients, with emphasis on single cell dynamics. Experimental observations and computational modeling have led to hypotheses for these mechanisms, several of which are considered here, namely the role of out-of-phase firing in neuronal clusters, the importance of strong excitatory AMPA-synaptic currents and recurrent inhibitory connectivity in combination with the fast time scales of IPSPs, ephaptic coupling and the contribution of interneuronal coupling through gap junctions. The statistical behaviour of fast ripple events can provide useful information on the underlying mechanism and can help to further improve classification of the diverse forms of HFOs.

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.

High-frequency activity in experimental and clinical epileptic foci

Pathological high-frequency electrographic activity (pHFA, >80Hz) represents one of the major discoveries in epilepsy research over the past few decades. In this review we focus on the high-frequency activity recorded in vivo in chronic models of epilepsy. The presence of HFA particularly of fast ripples (250-600Hz)reflects epileptogenic reorganization of brain tissue, endogenous epileptogenicity and ability to generate spontaneous seizures. The spatial distribution of epileptic HFA can be used to localize epileptic foci. In some regions of brain the localizing value of epileptic HFA is weakened by frequency overlap with physiological HFA. In this situation, only detailed knowledge of the regional physiological activity may provide relevant information which frequencies provide localizing information. In the epileptic hippocampus, the activity from 250Hz to 600Hz frequency band (fast ripples) is always epileptic and can be used as reliable marker of epileptic tissue in all hippocampal subregions. The localizing value of HFA in the identification of the epileptic focus is discussed from an experimental and clinical perspective; as the information provided by HFA can improve presurgical diagnosis and surgical outcome. Finally, research into HFA has contributed to improved understanding and new insights into the cellular and network organization of epileptic foci and the pathophysiology of epilepsy.

Altered intrinsic neuronal excitability and reduced Na+ currents in a mouse model of Alzheimer's disease

Transgenic mice that overproduce beta-amyloid (Aβ) peptides can exhibit central nervous system network hyperactivity. Patch clamp measurements from CA1 pyramidal cells of PSAPP and wild type mice were employed to investigate if altered intrinsic excitability could contribute to such network hyperfunction. At approximately 10 months, when PSAPP mice have a substantial central nervous system Aβ load, resting potential and input resistance were genotype-independent. However, PSAPP mice exhibited a substantially more prominent action potential (AP) burst close to the onset of weak depolarizing current stimuli. The spike afterdepolarization (ADP) was also larger in PSAPP mice. The rate of rise, width and height of APs were reduced in PSAPP animals; AP threshold was unaltered. Voltage-clamp recordings from nucleated macropatches revealed that somatic Na(+) current density was depressed by approximately 50% in PSAPP mice. K(+) current density was unaltered. All genotype-related differences were absent in PSAPP mice aged 5-7 weeks which lack a substantial Aβ load. We conclude that intrinsic neuronal hyperexcitability and changes to AP waveforms may contribute to neurophysiological deficits that arise as a consequence of Aβ accumulation.

Advances in understanding basic mechanisms of epilepsy and seizures

Sixty years ago the clinical neurophysiology of epilepsy had progressed to the stage that it posed questions that could be addressed by major advances in cellular electrophysiology made around the that time. However, it took about 25-30 years to build up serious momentum in understanding the mechanisms of epileptic discharges. Over the past 2-3 decades developments in pharmacology and molecular biology have substantially increased the depth and complexity of our insights into the nervous system in general and the epileptic brain in particular. One of the biggest advances in our understanding of the brain is in its plasticity in the adult - that is its ability to modify its structure and function. The current state of play is that for most chronic epileptic foci it is possible to identify multiple differences from normal brain tissue in both the structure and function of neurons, neuronal networks and glia. This review will chart some of this progress to give an idea of the pace of advances over the decades.

Epileptic high-frequency network activity in a model of non-lesional temporal lobe epilepsy

High-frequency cortical activity, particularly in the 250–600 Hz (fast ripple) band, has been implicated in playing a crucial role in epileptogenesis and seizure generation. Fast ripples are highly specific for the seizure initiation zone. However, evidence for the association of fast ripples with epileptic foci depends on animal models and human cases with substantial lesions in the form of hippocampal sclerosis, which suggests that neuronal loss may be required for fast ripples. In the present work, we tested whether cell loss is a necessary prerequisite for the generation of fast ripples, using a non-lesional model of temporal lobe epilepsy that lacks hippocampal sclerosis. The model is induced by unilateral intrahippocampal injection of tetanus toxin. Recordings from the hippocampi of freely-moving epileptic rats revealed high-frequency activity (>100 Hz), including fast ripples. High-frequency activity was present both during interictal discharges and seizure onset. Interictal fast ripples proved a significantly more reliable marker of the primary epileptogenic zone than the presence of either interictal discharges or ripples (100–250 Hz). These results suggest that fast ripple activity should be considered for its potential value in the pre-surgical workup of non-lesional temporal lobe epilepsy.

Exposure to 56Fe-particle radiation accelerates electrophysiological alterations in the hippocampus of APP23 transgenic mice

Abstract An unavoidable complication of space travel is exposure to high-charge, high-energy (HZE) particles. In animal studies, exposure of the CNS to HZE-particle radiation leads to neurological alterations similar to those seen in aging or Alzheimer's disease. In this study we examined whether HZE-particle radiation accelerated the age-related neuronal dysfunction that was previously described in transgenic mice overexpressing human amyloid precursor protein (APP). These APP23 transgenic mice exhibit age-related behavioral abnormalities and deficits in synaptic transmission. We exposed 7-week-old APP23 transgenic males to brain-only (56)Fe-particle radiation (600 MeV/nucleon; 1, 2, 4 Gy) and recorded synaptic transmission in hippocampal slices at 2, 6, 9, 14 and 18-24 months. We stimulated Schaeffer collaterals and recorded field excitatory postsynaptic potentials (fEPSP) and population spikes (PS) in CA1 neurons. Radiation accelerated the onset of age-related fEPSP decrements recorded at the PS threshold from 14 months of age to 9 months and reduced synaptic efficacy. At 9 months, radiation also reduced PS amplitudes. At 6 months, we observed a temporary deficit in paired-pulse inhibition of the PS at 2 Gy. Radiation did not significantly affect survival of APP23 transgenic mice. We conclude that irradiation of the brain with HZE particles accelerates Alzheimer's disease-related neurological deficits.

Seizure entrainment with polarizing low-frequency electric fields in a chronic animal epilepsy model

Neural activity can be modulated by applying a polarizing low-frequency (<<100 Hz) electric field (PLEF). Unlike conventional pulsed stimulation, PLEF stimulation has a graded, modulatory effect on neuronal excitability, and permits the simultaneous recording of neuronal activity during stimulation suitable for continuous feedback control. We tested a prototype system that allows for simultaneous PLEF stimulation with minimal recording artifact in a chronic tetanus toxin animal model (rat) of hippocampal epilepsy with spontaneous seizures. Depth electrode local field potentials recorded during seizures revealed a characteristic pattern of field postsynaptic potentials (fPSPs). Sinusoidal voltage-controlled PLEF stimulation (0.5-25 Hz) was applied in open-loop cycles radially across the CA3 of ventral hippocampus. For stimulated seizures, fPSPs were transiently entrained with the PLEF waveform. Statistical significance of entrainment was assessed with Thomson's harmonic F-test, with 45/132 stimulated seizures in four animals individually demonstrating significant entrainment (p < 0.04). Significant entrainment for multiple presentations at the same frequency (p < 0.01) was observed in three of four animals in 42/64 stimulated seizures. This is the first demonstration in chronically implanted freely behaving animals of PLEF modulation of neural activity with simultaneous recording.

Viral clostridial light chain gene-based control of penicillin-induced neocortical seizures

Restraining excitatory neurotransmission within a seizure focus provides a nondestructive treatment strategy for intractable neocortical epilepsy. Clostridial toxin light chain (LC) inhibits synaptic transmission by digesting a critical vesicle-docking protein, synaptobrevin, without directly altering neuronal health. This study tests the treatment efficacy of adenoviral vector delivered LC (AdLC) on a model of seizures in rats induced by motor cortex penicillin (PCN) injection. LC expression significantly reduced electroencephalogram (EEG) frequency, amplitude, duration, and latency compared to control groups injected with either an adenoviral vector bearing green fluorescent protein (AdGFP) or phosphate buffered solution (PBS). Correspondingly, LC gene expression improved behavioral manifestations including seizure severity and latency. There was no statistical difference in motor function before and after vector administration between treatment and control groups. Histological analysis revealed spatially discrete LC expression with corresponding synaptobrevin depletion in the cortex surrounding the injection site. Thus, vector-mediated LC gene delivery is capable of improving both the EEG and behavioral manifestations of PCN-induced focal neocortical seizures through synaptobrevin depletion.

Characterization of the tetanus toxin model of refractory focal neocortical epilepsy in the rat

PURPOSE

To characterize in detail a model of focal neocortical epilepsy.

METHODS

Chronic focal epilepsy was induced by injecting 25-50 ng of tetanus toxin or vehicle alone (controls) into the motor neocortex of rats. EEG activity was recorded from electrodes implanted at the injection site, along with facial muscle electromyographic (EMG) activity and behavioral monitoring intermittently for up to 5 months in some animals. Drug responsiveness was assessed by using the antiepileptic drugs (AEDs) diazepam (DZP) and phenytoin (PHT) delivered systemically, while 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide (NBQX), a competitive antagonist at AMPA receptors, was administered directly to the brain to investigate the potential benefits of focal drug delivery.

RESULTS

Tetanus toxin induced mild behavioral seizures that persisted indefinitely in all animals. EEG spiking activity, occurring up to 80% of the time, correlated with clinical seizures consisting of interrupted behavioral activity, rhythmic bilateral facial twitching, and periods of abrupt motor arrest. Seizures were refractory to systemic administration of DZP and PHT. However, focal delivery of NBQX to the seizure site reversibly reduced EEG and behavioral seizure activity without detectable side effects.

CONCLUSIONS

This study provides a long-term detailed characterisation of the tetanus toxin model. Spontaneous, almost continuous, well-tolerated seizures occur and persist, resembling those seen in neocortical epilepsy, including cortical myoclonus and epilepsia partialis continua. The seizures appear to be similarly resistant to conventional AEDs. The consistency, frequency, and clinical similarity of the seizures to refractory epilepsy in humans make this an ideal model for investigation of both mechanisms of seizure activity and new therapeutic approaches.

Models and mechanisms of experimental epilepsies

PURPOSE

To review the value of experimental models of epilepsy to help us understand the underlying basic neuronal mechanisms.

METHODS

The methods addressed here include a variety of acute and chronic experimental epilepsies and their investigation in vitro, in vivo, and in computer simulations.

RESULTS

Epileptic discharges are emergent properties of neuronal networks, depending crucially on intrinsic neuronal properties, and on the structural and functional organisation of the synaptic networks that interconnect them. Fast oscillations are another emergent property of these networks; while they are involved in normal function, they can play a crucial role in the initiation of at least some kinds of focal seizure.

DISCUSSION

Brief "interictal" epileptic events are now relatively well understood. Rather less so are the mechanisms of chronic epileptic foci, and particularly of the prolonged hypersynchronous hyperexcitability characteristic of focal seizures, but here too progress is being made, offering the prospect of better targeted treatments.