Synaptic inhibition in primary and secondary chronic epileptic foci induced by intrahippocampal tetanus toxin in the rat

Dissociation Between the Epileptogenic Lesion and Primary Seizure Onset Zone in the Tetanus Toxin Model of Temporal Lobe Epilepsy

Despite extensive temporal lobe epilepsy (TLE) research, understanding the specific limbic structures' roles in seizures remains limited. This weakness can be attributed to the complex nature of TLE and the existence of various TLE subsyndromes, including non-lesional TLE. Conventional TLE models like kainate and pilocarpine hinder precise assessment of the role of individual limbic structures in TLE ictogenesis due to widespread limbic damage induced by the initial status epilepticus. In this study, we used a non-lesional TLE model characterized by the absence of initial status and cell damage to determine the spatiotemporal profile of seizure initiation and limbic structure recruitment in TLE. Epilepsy was induced by injecting a minute dose of tetanus toxin into the right dorsal hippocampus in seven animals. Following injection, animals were implanted with bipolar recording electrodes in the amygdala, dorsal hippocampus, ventral hippocampus, piriform, perirhinal, and entorhinal cortices of both hemispheres. The animals were video-EEG monitored for four weeks. In total, 140 seizures (20 seizures per animal) were analyzed. The average duration of each seizure was 53.2+/-3.9 s. Seizure could initiate in any limbic structure. Most seizures initiated in the ipsilateral (41 %) and contralateral (18 %) ventral hippocampi. These two structures displayed a significantly higher probability of seizure initiation than by chance. The involvement of limbic structures in seizure initiation varied between individual animals. Surprisingly, only 7 % of seizures initiated in the injected dorsal hippocampus. The limbic structure recruitment into the seizure activity wasn't random and displayed consistent patterns of early recruitment of hippocampi and entorhinal cortices. Although ventral hippocampus represented the primary seizure onset zone, the study demonstrated the involvement of multiple limbic structures in seizure initiation in a non-lesional TLE model. The study also revealed the dichotomy between the primary epileptogenic lesion and main seizure onset zones and points to the central role of ventral hippocampi in temporal lobe ictogenesis.

Modulation of locomotor behaviors by location-specific epileptic spiking and seizures

INTRODUCTION

Epilepsy is a debilitating neurological condition characterized by spontaneous seizures as well as significant comorbid behavioral abnormalities. In addition to seizures, epileptic patients exhibit interictal spikes far more frequently than seizures, often, but not always observed in the same brain areas. The exact relationship between spiking and seizures as well as their respective effects on behavior are not well understood. In fact, spiking without overt seizures is seen in various psychiatric conditions including attention-deficit hyperactivity disorder.

METHODS

In order to study the effects of spiking and seizures on behavior in an epileptic animal model, we used long-term video-electroencephalography recordings at six cortical recording sites together with behavioral activity monitoring. Animals received unilateral injections of tetanus toxin into either the somatosensory or motor cortex.

RESULTS

Somatosensory cortex-injected animals developed progressive spiking ipsilateral to the injection site, while those receiving the injection into the motor cortex developed mostly contralateral spiking and spontaneous seizures. Animals with spiking but no seizures displayed a hyperactive phenotype, while animals with both spiking and seizures displayed a hypoactive phenotype. Not all spikes were equivalent as spike location strongly correlated with distinct locomotor behaviors including ambulatory distance, vertical movements, and rotatory movement.

CONCLUSIONS

Together, our results demonstrate relationships between brain region-specific spiking, seizures, and behaviors in rodents that could translate into a better understanding for patients with epileptic behavioral comorbidities and other neuropsychiatric disorders.

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

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

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.

Involvement of GABA in mirror focus: a case report

Mirror focus (MF) is a cortical epileptogenic lesion that is posited to develop in the contralateral site to a cortical primary focus (PF) by secondary epileptogenic mechanisms. Previous animal evidence supports the implication of gamma-aminobutyric acid (GABA) in this phenomenon, but this contention has not yet been substantiated by clinical findings. Here we report for the first time clinical evidence suggesting the involvement of GABAergic cortical transmission in MF pathogenesis, in a 37-year-old man affected by a lesional PF in the right frontal lobe and a homotopic MF in the contralateral hemisphere, triggered by hyperventilation. One year after surgical excision of the PF, the electric activity of the MF remained unchanged, but was accompanied by a significant increase in the density of GABA(A)/benzodiazepine receptor binding in the left frontal lobe, as measured by (123)I-Iomazenil SPECT. These results extend previous evidence on the involvement of GABAergic signaling in MF pathophysiology.

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.

Propagation of low calcium non-synaptic induced epileptiform activity to the contralateral hippocampus in vivo

Recent experiments show that non-synaptic epileptiform activity can be induced by high K+ and low Ca2+ solution in vivo in the hippocampal CA1 region when synaptic transmission is blocked. However, the ability of this type of epileptiform activity to propagate to other brain areas is unknown. Presumably, this epileptiform activity should propagate and project along the axons to remote brain areas. This hypothesis was tested in vivo by inducing non-synaptic seizures in the left hippocampus and by recording spontaneous and evoked field potentials in both left and right hippocampi. The results show that one type of non-synaptic epileptiform activity, late bursts, observed in the left exposed CA1 and CA3 regions could propagate to the contralateral intact CA1 and induce seizures with onsets of high-frequency rhythm. A cut of the commissural fibers near the midline of the brain prevented this propagation. In addition, the measurement of time delays between the exposed left CA3 and contralateral right CA1, as well as between the two recording electrodes in the right CA1, showed that the burst activity propagated through the commissural pathways. Experimental data also showed that these late bursts in the left hippocampus were first generated in the Schaffer collaterals of the CA1 region, traveled to the ipsilateral CA3 region and then propagated through the commissural fibers to the other side. These results suggest that non-synaptic epileptiform activity can propagate along axon projections to intact brain area causing seizure activity. This non-synaptic activity propagating through axonal pathway provides a possible mechanism for the generation of high-frequency low-amplitude onset activity observed commonly in human epileptic EEGs.

Detection of abnormal cerebral excitability by coincident stimulation and recording

OBJECTIVE

A method for mapping brain excitability and detecting abnormalities, by concurrently stimulating and recording 'focal' compound responses through one microelectrode, was evaluated in three rat epilepsy models in comparison with distal stimulation of perforant path afferents.

METHODS

A fixed trajectory from neocortex to dentate gyrus was mapped under halothane anesthesia. Several weeks earlier, tetanus toxin or vehicle was microinjected into the dentate polymorphic layer, or else rats were genetically epilepsy-prone (GEPR-9) or epilepsy-resistant (GERR-0). Other (unmapped) rats received acute penicillin microinjections within the dentate granular layer.

RESULTS

Focal responses, although widespread, proved largest in the dentate (>+/-0.5 mV). Tetanus toxin diminished focal responses near the microinjection site versus vehicle-microinjected (66%) or contralateral controls (55%), but enhanced them elsewhere in the dentate. It enhanced distal responses at all hippocampal locations. Focal but not distal responses were higher in GEPR-9 than in GERR-0 rats at widespread forebrain locations (mean 233%). Penicillin facilitated both focal and distal dentate responses, but the focal facilitation peaked sooner (about 75 versus 180 min).

CONCLUSIONS

Focal responses better uncover pervasive or discrete excitability differences.

SIGNIFICANCE

Focal mapping may aid in diagnostic imaging and intraoperative targeting, offering high resolution, rapid performance, low stimulus currents and minimal invasion.

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.

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.

Amygdala-kindling induces alterations in neuronal density and in density of degenerated fibers

Kindling is characterized by a progressive intensification of seizure activity culminating in generalized seizures following repeated administration of an initially subconvulsive electrical or chemical stimulus. Since it is known that epilepsy induces morphological alterations in the limbic system, we examined the neuropathological consequences of kindling with a sensitive silver-staining method for the visualization of damaged neurons and Nissl staining for the estimation of the neuronal densities in different limbic areas. Wistar rats implanted with electrodes in the left basolateral nucleus were stimulated until 15 consecutive stage V seizures (scale of Racine). Amygdala-kindled animals had reduced cell density in the amygdala and increased density of fragments of degenerated axons. Reduced neuronal density and the occurrence of degenerated axons in kindled animals were more prominent in the ipsilateral than in the contralateral hemisphere. In addition, more degenerated axons were found in cortical structures of kindled than sham-operated animals. These results indicate that kindling induced morphological alterations that were not restricted to either the ipsilateral hemisphere or the stimulated region. These morphological changes might be responsible for the emotional and behavioral disturbances that can accompany epilepsy.

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

Intrahippocampal tetanus toxin induces a period of chronic recurrent limbic seizures in adult rats, associated with a failure of inhibition in the hippocampus. The rats normally gain remission from their seizures after 6-8 weeks, but show persistent cognitive impairment. In this study we assessed which changes in cellular and network properties could account for the enduring changes in this model, using intracellular and extracellular field recordings in hippocampal slices from rats injected with tetanus toxin or vehicle, 5 months previously. In CA1 pyramidal neurones from toxin-injected rats, the slope of the action potential upstroke was reduced by 32%, the fast afterhyperpolarisation by 32% and the slow afterhyperpolarisation by 54%, suggesting changes in voltage-dependent conductances. The excitatory postsynaptic potential slope was reduced by 60% and the population synaptic potential slope was reduced at all stimulus intensities, suggesting a reduced afferent input in CA1. Paired-pulse stimulation showed an increase of the excitability ratio and an increase of cellular excitability only for the second pulse, suggesting a reduced inhibition. The polysynaptic inhibitory postsynaptic potential was reduced by 34%, whereas neither the inhibitory postsynaptic potential at subthreshold stimulus intensities,nor the pharmacologically isolated monosynaptic inhibitory postsynaptic potential were different in toxin-injected rats, suggesting a reduced synaptic excitation of interneurones. Stratum radiatum stimuli in toxin-injected rats, and not in controls, evoked antidromic activation of CA1 neurones, demonstrating axonal sprouting into areas normally devoid of CA1 pyramidal cell axons.We conclude that this combination of enduring changes in cellular and network properties, both pro-epileptic (increased recurrent excitatory connectivity, reduced recurrent inhibition and reduced afterhyperpolarisations) and anti-epileptic (impaired firing and reduced excitation), reaches a balance that allows remission of seizures, perhaps at the price of persistent cognitive impairment.

Investigation of the neuronal aggregate generating seizures in the rat tetanus toxin model of epilepsy

A key question in epilepsy is the organization and size of the neuronal networks necessary for generating seizures. Hypotheses include: a single focal neuronal network drives seizure discharges across the brain, which may or may not be identical with the circuits that generate interictal spikes; or multiple neuronal networks link together in re-entrant loops or other long-range networks. It remains unclear whether any of these hypotheses apply to spontaneous seizures in freely moving animals. We used the tetanus toxin chronic model of epilepsy to test the different predictions made by each hypothesis about the propagation and interaction of epileptic discharges during seizures. Seizures could start in either the injected or noninjected dorsal hippocampus, suggesting that seizures have multifocal onsets in the tetanus toxin model. During seizures, individual bursts propagated in either direction, both between the right and left dorsal hippocampi, and between CA3 and the dentate gyrus in the same hippocampus. These findings argue against one site "driving" seizures or seizures propagating around a limbic loop. Specifically, the side leading each burst switched a median of three times during the first 20 s of a seizure. Analysis of bursts during seizures suggested that the network at each recording site acted like a neuronal oscillator. Coupling of population spikes in right and left CA3 increased during the early part of seizures, but the cross-correlation of their whole-discharge waveforms changed little over the same period. Furthermore, the polarity of the phase difference between population spikes did not follow the phase difference for complete discharges. We concluded that the neuronal aggregate necessary for seizures in our animals comprises multiple spatially distributed neuronal networks and that the increased synchrony of the output (population spike firing) of these networks during the early part of seizures may contribute to seizure generation.

Network and pharmacological mechanisms leading to epileptiform synchronization in the limbic system in vitro

Seizures in patients presenting with mesial temporal lobe epilepsy result from the interaction among neuronal networks in limbic structures such as the hippocampus, amygdala and entorhinal cortex. Mesial temporal lobe epilepsy, one of the most common forms of partial epilepsy in adulthood, is generally accompanied by a pattern of brain damage known as mesial temporal sclerosis. Limbic seizures can be mimicked in vitro using preparations of combined hippocampus-entorhinal cortex slices perfused with artificial cerebrospinal fluid containing convulsants or nominally zero Mg(2+), in order to produce epileptiform synchronization. Here, we summarize experimental evidence obtained in such slices from rodents. These data indicate that in control animals: (i) prolonged, NMDA receptor-dependent epileptiform discharges, resembling electrographic limbic seizures, originate in the entorhinal cortex from where they propagate to the hippocampus via the perforant path-dentate gyrus route; (ii) the initiation and maintenance of these ictal discharges is paradoxically contributed by GABA (mainly type A) receptor-mediated mechanisms; and (iii) CA3 outputs, which relay a continuous pattern of interictal discharge at approximately 1Hz, control rather than sustain ictal discharge generation in entorhinal cortex. Recent work indicates that such a control is weakened in the pilocarpine model of epilepsy (presumably as a result of CA3 cell damage). In addition, in these experiments electrographic seizure activity spreads directly to the CA1-subiculum regions through the temporoammonic pathway. Studies reviewed here indicate that these changes in network interactions, along with other mechanisms of synaptic plasticity (e.g. axonal sprouting, decreased activation of interneurons, upregulation of bursting neurons) can confer to the epileptic, damaged limbic system, the ability to produce recurrent limbic seizures as seen in patients with mesial temporal lobe epilepsy.

A comparison of the efficacy of carbamazepine and the novel anti-epileptic drug levetiracetam in the tetanus toxin model of focal complex partial epilepsy

1. The tetanus toxin seizure model, which is associated with spontaneous and intermittent generalized and non-generalized seizures, is considered to reflect human complex partial epilepsy. The purpose of the present study was to investigate and compare the anticonvulsant effects of carbamazepine with that of levetiracetam, a new anti-epileptic drug in this model. 2. One microl of tetanus toxin solution (containing 12 mLD(50) microl(-1) of tetanus toxin) was placed stereotactically into the rat left hippocampus resulting in generalized and non-generalized seizures. 3. Carbamazepine (4 mg kg(-1) h(-1)) and levetiracetam (8 and 16 mg kg(-1) h(-1)) were administered during a 7 day period via an osmotic minipump which was placed in the peritoneal cavity. Carbamazepine (4 mg kg(-1) h(-1)) exhibited no significant anticonvulsant effect, compared to control, when the entire 7 day study period was evaluated but the reduction in generalized seizures was greater (35.5%) than that for non-generalized seizures (12.6%). However, during the first 2 days of carbamazepine administration a significant reduction in both generalized seizure frequency (90%) and duration (25%) was observed. Non-generalized seizures were unaffected. This time-dependent anticonvulsant effect exactly paralleled the central (CSF) and peripheral (serum) kinetics of carbamazepine in that steady-state concentrations declined over time, with the highest concentrations achieved during the first 2 days. Also there was a significant 27.3% reduction in duration of generalized seizures during the 7 day study period (P=0.0001). 4. Levetiracetam administration (8 and 16 mg kg(-1) h(-1)) was associated with a dose-dependent reduction in the frequency of both generalized (39 v 57%) and non-generalized (36 v 41%) seizures. However, seizure suppression was more substantial for generalized seizures. Also a significant dose-dependent reduction in overall generalized seizure duration was observed. 5. These data provide experimental evidence for the clinical efficacy of levetiracetam for the management of patients with complex partial seizures. Furthermore, levetiracetam probably does not act by preventing ictogenesis per se but acts to reduce seizure severity and seizure generalization.

Altered dentate filtering during the transition to seizure in the rat tetanus toxin model of epilepsy

The dentate gyrus is thought to be a key area in containing the spread of seizure discharges in temporal lobe epilepsy. We investigated whether it actively contributes to the transition to seizure in vivo using the tetanus toxin chronic experimental epilepsy. Brief epileptic discharges lasted <2 s in freely moving animals and were clearly distinguishable from spontaneous seizures that lasted tens of seconds. This suggested that the changes underpinning the transition to seizure started within the first few seconds of seizure onset. During this period, we found that the amplitude of dentate gyrus population spikes depressed initially, but from 1.1 s after seizure onset, they potentiated. The amplitude and number of CA3 population spikes paralleled the pattern found in the dentate gyrus. We used hippocampal slices to study dentate filtering in more detail. The perforant pathway was stimulated repetitively at the frequency of field postsynaptic potentials found during epileptic discharges in vivo. The amplitude of dentate gyrus population spikes decreased to a steady state in naïve hippocampal slices. In hippocampal slices prepared from rats previously injected with tetanus toxin, population spike amplitude decreased transiently and then potentiated. We found that the biphasic profile and rate of potentiation of dentate population spikes in vivo can be reproduced in naïve hippocampal slices by blocking GABA(B) receptors. We conclude that the filtering properties of the dentate gyrus are altered in the tetanus toxin model of epilepsy and propose how this contributes to the transition to seizure in our animals.

Do fits really beget fits? The effect of previous epileptic activity on the subsequent induction of the tetanus toxin model of limbic epilepsy in the rat

The effect of pretreatment with either tetanus toxin (in ventral hippocampus) or kainic acid (into dorsal hippocampus, with or without suppression of seizures by phenobarbital) on the subsequent development of epilepsy in rats injected with tetanus toxin (into ventral hippocampus) has been studied. Both treatments advanced the timing of the development of the subsequent epilepsy by a few days but did not affect the severity of the syndrome. The fits stopped after 3 weeks in all the rats but recurred in 6 of 20 of those given kainic acid, with or without phenobarbital, but not in those given only tetanus toxin. It is concluded that while fits make the brain more sensitive to a further epileptogenic stimulus they do not themselves increase their severity or persistence. It is the destruction of the CA3/4 area of the hippocampus which results in this advance and in the predisposition to permanent epilepsy.

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

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

Kindling induces transient fast inhibition in the dentate gyrus--CA3 projection

The granule cells of the dentate gyrus (DG) send a strong glutamatergic projection, the mossy fibre tract, toward the hippocampal CA3 field, where it excites pyramidal cells and neighbouring inhibitory interneurons. Despite their excitatory nature, granule cells contain small amounts of GAD (glutamate decarboxylase), the main synthetic enzyme for the inhibitory transmitter GABA. Chronic temporal lobe epilepsy results in transient upregulation of GAD and GABA in granule cells, giving rise to the speculation that following overexcitation, mossy fibres exert an inhibitory effect by release of GABA. We therefore stimulated the DG and recorded synaptic potentials from CA3 pyramidal cells in brain slices from kindled and control rats. In both preparations, DG stimulation caused excitatory postsynaptic potential (EPSP)/inhibitory postsynaptic potential (IPSP) sequences. These potentials could be completely blocked by glutamate receptor antagonists in control rats, while in the kindled rats, a bicuculline-sensitive fast IPSP remained, with an onset latency similar to that of the control EPSP. Interestingly, this IPSP disappeared 1 month after the last seizure. When synaptic responses were evoked by high-frequency stimulation, EPSPs in normal rats readily summate to evoke action potentials. In slices from kindled rats, a summation of IPSPs overrides that of the EPSPs and reduces the probability of evoking action potentials. Our data show for the first time that kindling induces functionally relevant activity-dependent expression of fast inhibition onto pyramidal cells, coming from the DG, that can limit CA3 excitation in a frequency-dependent manner.

Reversible deafferentation of abducens motoneurons and internuclear neurons with tetanus neurotoxin

Tetanus neurotoxin (TeNT) is a blocker of synaptic vesicle exocytosis in central synapses with preferential affinity for inhibitory neurotransmission. Following its intramuscular injection, TeNT is retrogradely and trans-synaptically transported towards the premotor terminals. Therefore, we have used TeNT as a tool to study the consequences of functional deafferentation on motoneurons following its peripheral administration. For this, we injected the toxin into the lateral rectus muscle at doses of 5 or 0.5 ng/kg and recorded the discharge activity of abducens motoneurons and internuclear neurons in the alert cat. Our results showed that: (i) TeNT blocked selectively the afferent inhibitory signals on abducens neurons only when used at a low dose, whereas both excitatory and inhibitory synaptic drive was lost after the high dose treatment; (ii) all effects were reversible within one month; and (iii) strikingly, the internuclear neurons of the abducens nucleus showed similar discharge alterations to the motoneurons, suggesting a TeNT action on shared common afferences.

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

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

Ultrastructural localization of the binding fragment of tetanus toxin in putative gamma-aminobutyric acidergic terminals in the intermediolateral cell column: a potential basis for sympathetic dysfunction in generalized tetanus

Tetanus toxin (TeTx) causes sympathetic hyperactivity, a major cause of mortality in generalized tetanus, apparently by obstructing the inhibition of sympathetic preganglionic neurons (SPNs). Neuroanatomic tracing and immunohistochemistry were used to investigate whether axon terminals in the intermediolateral cell column (IML) that synapse on SPNs and use the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) may be infected transsynaptically with TeTx. The binding fragment of TeTx (TTC; an atoxic surrogate of TeTx) and the cholera toxin B subunit (CTB; a retrograde tracer) were injected into the rat superior cervical ganglion and, over 16-48 hours, were transported to the ipsilateral IML in the caudal half of the last cervical and first three thoracic spinal cord segments. With light microscopy, diffuse CTB immunolabeling extended throughout SPN perikarya and dendrites. Punctate TTC and GABA immunolabeling were accumulated densely in the neuropil between and surrounding SPN processes. With electron microscopy, 54% of the axon terminals in the IML (n = 1,337 terminals) were TTC immunolabeled (TTC(+)), and 25% contained putative neurotransmitter levels of GABA immunolabeling (GABA(+)). On average, GABA(+) terminals had a 76% chance of also being TTC(+) and a 62% greater chance of being TTC(+) than GABA(-) terminals (P < 0.000001). Axon terminals were just as likely to be TTC(+) and/or GABA(+) regardless of whether the dendrites they synapsed on were large (>1 microM) or small in cross-sectional area or were labeled retrogradely. Sympathetic hyperactivity in tetanus may involve 1) retrograde and transsynaptic transport of TeTx by SPNs and 2) at least in part, an infection of GABAergic terminals in the IML.

9-16 Hz oscillation precedes secondary generalization of seizures in the rat tetanus toxin model of epilepsy

Unilateral intrahippocampal injection of tetanus toxin results in a chronic syndrome of intermittent epileptic seizures. During some of these seizures, rats develop a stereotypic, pathological motor behavior that indicates secondary generalization of epileptic activity. We report that secondary generalization was preceded by a 9-16 Hz oscillation of field potentials which was synchronized between the right and left dorsal hippocampi. The oscillation was associated with increased synchrony of population spike firing in right and left CA1 subregions which form the major output of the hippocampi. Cutting the ventral commissure abolished synchrony across the hippocampi and reduced the probability that the 9-16 Hz activity would be followed by secondary generalization. We concluded that a bilaterally synchronous 9-16 Hz hippocampal oscillation played a role in the secondary generalization of focal seizures in this chronic model of limbic epilepsy.

Increased pyramidal excitability and NMDA conductance can explain posttraumatic epileptogenesis without disinhibition: a model

Partially isolated cortical islands prepared in vivo become epileptogenic within weeks of the injury. In this model of chronic epileptogenesis, recordings from cortical slices cut through the injured area and maintained in vitro often show evoked, long- and variable-latency multiphasic epileptiform field potentials that also can occur spontaneously. These events are initiated in layer V and are synchronous with polyphasic long-duration excitatory and inhibitory potentials (currents) in neurons that may last several hundred milliseconds. Stimuli that are significantly above threshold for triggering these epileptiform events evoke only a single large excitatory postsynaptic potential (EPSP) followed by an inhibitory postsynaptic potential (IPSP). We investigated the physiological basis of these events using simulations of a layer V network consisting of 500 compartmental model neurons, including 400 principal (excitatory) and 100 inhibitory cells. Epileptiform events occurred in response to a stimulus when sufficient N-methyl-D-aspartate (NMDA) conductance was activated by feedback excitatory activity among pyramidal cells. In control simulations, this activity was prevented by the rapid development of IPSPs. One manipulation that could give rise to epileptogenesis was an increase in the threshold of inhibitory interneurons. However, previous experimental data from layer V pyramidal neurons of these chronic epileptogenic lesions indicate: upregulation, rather than downregulation, of inhibition; alterations in the intrinsic properties of pyramidal cells that would tend to make them more excitable; and sprouting of their intracortical axons and increased numbers of presumed synaptic contacts, which would increase recurrent EPSPs from one cell onto another. Consistent with this, we found that increasing the excitability of pyramidal cells and the strength of NMDA conductances, in the face of either unaltered or increased inhibition, resulted in generation of epileptiform activity that had characteristics similar to those of the experimental data. Thus epileptogenesis such as occurs after chronic cortical injury can result from alterations of intrinsic membrane properties of pyramidal neurons together with enhanced NMDA synaptic conductances.

Hormonal effects on the brain

Changes in seizure frequency over the course of the menstrual cycle (i.e., catamenial epilepsy) have long been documented. Ovarian steroid hormones have a number of important short- and long-term effects on the brain that may contribute to this phenomenon. In particular, estrogen induces structural and functional changes in hippocampal neurons which may contribute significantly to increasing seizure susceptibility. This article reviews the mechanisms of action of steroid hormones on the basis of findings in animal models, with particular emphasis on the effects of estrogen on the hippocampus.

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.

Interneurons in area CA1 stratum radiatum and stratum oriens remain functionally connected to excitatory synaptic input in chronically epileptic animals

Past work has demonstrated a reduction of stimulus-evoked inhibitory input to hippocampal CA1 pyramidal cells in chronic models of temporal lobe epilepsy (TLE). It has been postulated that this reduction in inhibition results from impaired excitation of inhibitory interneurons. In this report, we evaluate the connectivity of area CA1 interneurons to their excitatory afferents in hippocampal-parahippocampal slices obtained from a rat model of chronic TLE. Rats were made chronically epileptic by a period of continuous electrical stimulation of the hippocampus, which establishes an acute condition of self-sustained limbic status epilepticus (SSLSE). This period of SSLSE is followed by a development of chronic recurrent spontaneous limbic seizures that are associated with chronic neuropathological changes reminiscent of those encountered in human TLE. Under visual control, whole cell patch-clamp recordings of interneurons and pyramidal cells were obtained in area CA1 of slices taken from adult, chronically epileptic post-SSLSE rats. Neurons were activated by means of electrodes positioned in stratum radiatum. Intrinsic membrane properties, including resting membrane potential, action potential (AP) threshold, AP half-height width, and membrane impedance, were unchanged in interneurons from chronically epileptic (post-SSLSE) tissue compared with control tissue. Single stimuli delivered to stratum radiatum evoked depolarizing excitatory postsynaptic potentials and APs in interneurons, whereas paired-pulse stimulation evoked facilitation of the postsynaptic current (PSC) in both control and post-SSLSE tissue. No differences between interneurons in control versus post-SSLSE tissue could be found with respect to the mean stimulus intensity or mean stimulus duration needed to evoke an AP. A multiple linear regression analysis over a range of stimulus intensities demonstrated that a greater number of APs could be evoked in interneurons in post-SSLSE tissue compared with control tissue. Spontaneous PSCs were observed in area CA1 interneurons in both control and post-SSLSE tissue and were markedly attenuated by glutamatergic antagonists. In conclusion, our data suggest that stimulus-evoked and spontaneous excitatory synaptic input to area CA1 interneurons remains functional in an animal model of chronic temporal lobe epilepsy. These findings suggest, therefore, that the apparent decrease of polysynaptic inhibitory PSPs in CA1 pyramidal cells in epileptic tissue is not due to a deficit in excitatory transmission from Schaffer collaterals to interneurons in stratum radiatum and straum oriens.

Parvalbumin- and calbindin-containing neurons express c-fos protein in primary and secondary (mirror) epileptic foci of the rat neocortex

The present experiments aimed at the description and further immunocytochemical characterization of activated neocortical neurons expressing the c-fos gene. Focal seizures were induced by the topical application of isotonic, isohydric 4-aminopyridine solution to the frontal neocortex of adult anesthetized Wistar rats. The EEG of both hemispheres was recorded from the surface of the skull. The animals were perfused with fixative, coronal plane vibratome sections were cut and stained with cocktails containing polyclonal c-fos and monoclonal calbindin or parvalbumin antibodies. The polyclonal c-fos antibody was tested with Western blotting and the diffusion of 4-aminopyridine investigated with autoradiography of [3H]4-aminopyridine. The c-fos protein was detected in every layer of the neocortex (primary focus) and in some allocortical areas of the treated hemisphere. Scattered immunostained nuclei were observed in layers II, III, IV and VI of the contralateral neocortex (mirror focus). Several parvalbumin- and calbindin-positive neurons contained the c-fos protein in both foci. The medium-sized non-pyramidal parvalbumin neurons were found in layers II-IV and VI of the neocortex and in stratum multiforme of the prepiriform cortex. The c-fos protein was colocalized with calbindin mainly in layers II and III in small and medium-sized non-pyramidal neurons. The results prove that focal epileptiform activity of the neocortex activates diverse inhibitory neuronal populations. As concluded, the inhibitory control is probably more effective in the contralateral hemisphere (mirror focus) than on the side of 4-APY treatment (primary focus).

Review of the role of inhibitory neurons in chronic epileptic foci induced by intracerebral tetanus toxin

Blocking inhibition provides one of the most common experimental means of triggering epileptic activity in hippocampus and neocortex. However, it has proved much more difficult to show that chronic models of epilepsies are due to disinhibition. One problem is knowing how much inhibition needs to be blocked to provide a sufficient mechanism for epileptic activity. We have found that inhibitory (GABAA) transmission, estimated from evoked monosynaptic IPSCs, must be reduced to 17% of their control amplitude (by 4-7 microM bicuculline) before hippocampal slices generate all-or-none epileptic discharges. Similar estimates of inhibition in chronic epileptic foci induced by intrahippocampal injection of tetanus toxin showed that monosynaptic IPSCs dropped to 10% of control in the injected hippocampus during the first 2 weeks after injection. At all other stages of the active epileptic foci in the two hippocampi the reduction in IPSCs was not alone sufficient for epileptic activity; at 4-6 weeks IPSCs were normal despite continued epileptic activity. One likely mechanism for the late epileptic activity is a reduction of either the intrinsic excitability, or the synaptic excitation, of inhibitory interneurons so they fail to be recruited normally. Alternative mechanisms include the formation of new excitatory connections, as found at modest levels in the dentate gyrus. Several mechanisms may play a part in chronic foci such as those induced by tetanus toxin, either acting together, or sequentially during the progression of the epileptic focus.

The development of psychosis in epilepsy: a re-examination of the kindling hypothesis

It is generally acknowledged that psychosis occurs with increased frequency within epileptic populations. There are several possible explanations for this over-representation: (1) psychosis may develop as a result of anti-epileptic drug or surgical treatment, or as a result of the psychosocial effects of living with epilepsy; (2) epilepsy and psychosis may, in some cases, have a common cause: and (3) chronic seizure activity may sometimes cause psychosis. The objective of this review is to evaluate the hypothesis that focal seizure activity may lead to the development of psychosis through a kindling process. There is some evidence to suggest that secondary epileptogenesis may develop following the spread of seizure activity from a primary focus, possible via a kindling mechanism. Although it has been suggested that long-term potentiation (LTP) may result in the development of secondary epileptic foci; LTP is not necessarily implicated. The kindling hypothesis of the development of psychosis in epilepsy must address the neural mechanism by which the spread of seizures might result in psychosis. At present, the neurochemical mechanisms by which psychosis could result from epilepsy are unclear.

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

A new experimental model of developmental epilepsy is reported. Behavioral and EEG features of seizures produced by unilateral intrahippocampal injection of tetanus toxin in postnatal day 9-11 rats, are described. Within 24-72 h of tetanus toxin injection, rat pups developed frequent and often prolonged seizures which included combinations of repetitive wet dog shakes, and wild running-jumping seizures. Intrahippocampal and cortical surface EEG recordings showed that coincident with these behaviors, electrographic seizures occurred not only in the injected hippocampus, but also in the contralateral hippocampus and bilaterally in the neocortex. Analysis of the interictal EEG revealed multiple independent spike foci. One week following tetanus toxin injection, the number of seizures markedly decreased; however, interictal spiking persisted. After injection rats were allowed to mature some were observed to have unprovoked behavioral seizures and/or epileptiform EEG activity. Mature animals were also studied using in vitro slice techniques. Recordings from hippocampal slices demonstrated spontaneous epileptiform burst discharges in the majority of rats which had tetanus toxin induced seizures as infants. These events occurred in area CA3 and consisted of interictal spikes and intracellularly recorded paroxysmal depolarization shifts (PDSs). On rarer occasions, electrographic seizures were recorded. The use of the tetanus toxin model in developing rats may facilitate a better understanding of the unique features of epileptogenesis in the developing brain and the consequences early-life seizures have on brain maturation and the genesis of epileptic conditions in later life.

Chronic focal epilepsy induced by intracerebral tetanus toxin

A single, minute dose of tetanus toxin injected into mammalian cerebral cortex induces a chronic epileptic syndrome. Seizures lasting up to 3 minutes occur spontaneously and intermittently for several weeks to months. The cellular mechanisms of this model have been studied in detail using brain slices in vitro. Initially the release of the inhibitory neurotransmitter, GABA, is blocked, but after 2-4 weeks, other mechanisms take over. Intrahippocampal tetanus toxin models human complex partial seizures (temporal lobe epilepsy). It results in consistent behavioural changes analogous with those seen clinically, in spite of the limited neuronal loss found in only 10-30% of rats. Treatment with carbamazepine ameliorates both the seizures and their behavioural consequences. Tetanus toxin provides a versatile and long-lasting model of focal epilepsies.

Epileptic activity outlasts disinhibition after intrahippocampal tetanus toxin in the rat

1. A single dose of tetanus toxin, injected under anaesthesia into one dorsal hippocampus of the rat, produces chronic epileptic foci involving both hippocampi. Generalized seizures occurred 1-6 weeks after injection and epileptic discharges were found in hippocampal slices in vitro. Here we measured the time course of decay of epileptic activity and the level of GABAA receptor-mediated inhibition in hippocampal slices 1-16 weeks after toxin injection in vivo. 2. Epileptic activity peaked in the dentate granule cell and CA3 pyramidal cell layers 2 weeks after toxin injection and at 4 weeks in CA1. Thresholds for evoking epileptic activity were lowest in the suprapyramidal blade of the dentate gyrus and area CA3c. Recovery from epileptic activity occurred more rapidly in the contralateral hippocampus. Polyspike activity ceased by 8 weeks and interictal activity by 16 weeks. Epileptic discharges could still be evoked from CA1 16 weeks after toxin injection. 3. The maximal monosynaptic fast inhibitory postsynaptic current (IPSC) conductance changes (gIPSC) decreased to < 10% of control values at the time of peak epileptic activity and remained lower than controls for 4 weeks ipsilaterally. In the contralateral hippocampus, gIPSC fell to ca 50% of control values for the first 2 weeks. Responses to exogenous GABA remained unchanged. 4. After 8 weeks dentate granule cells had gIPSC significantly larger than controls. No increase in gIPSC occurred in CA3. Epileptic activity persisted 8-10 weeks after recovery from disinhibition ipsilaterally and 4 weeks contralaterally. 5. Epileptic activity was seen when monosynaptic GABAA receptor-mediated IPSCs were normal or supranormal. At these times polysynaptic inhibition was still profoundly reduced. These observations provide strong evidence for long-term changes in the pattern of synaptic excitation contributing to a chronic epileptic syndrome syndrome following disinhibitory insult, and are consistent with weakened excitation of inhibitory neurones.

Prion protein is necessary for normal synaptic function

The prion diseases are neurodegenerative conditions, transmissible by inoculation, and in some cases inherited as an autosomal dominant disorder. They include Creutzfeldt-Jakob disease in humans and scrapie and bovine spongiform encephalopathy in animals. The prion consists principally of a post-translationally modified form of a host-encoded glycoprotein (PrPC), designated PrPSc (ref. 1); the normal cellular function of PrPC is, however, unknown. Although PrP is highly conserved among mammals and widely expressed in early embryogenesis, mice homozygous for disrupted PrP genes appear developmentally and behaviourally normal. PrP is a protein anchored to the neuronal surface by glycosylphosphatidylinositol, suggesting a role in cell signalling or adhesion. Here we report that hippocampal slices from PrP null mice have weakened GABAA (gamma-aminobutyric acid type A) receptor-mediated fast inhibition and impaired long-term potentiation. This impaired synaptic inhibition may be involved in the epileptiform activity seen in Creutzfeldt-Jakob disease and we argue that loss of function of PrPC may contribute to the early synaptic loss and neuronal degeneration seen in these diseases.

Cellular mechanisms of neocortical secondary epileptogenesis

Firing activity, membrane parameters and postsynaptic responses were studied by recording intracellularly from different types of neurons during the development of a secondary neocortical epileptiform focus (mirror focus, Mf) contralateral to the site of an aminopyridine-induced focus (primary focus, Pf) in anesthetized rats. Three different stages in the development of secondary epileptogenesis were observed. (i) in the Pf stage epileptiform discharges appeared only in the ECoG recorded from the Pf, but neurons in the Mf showed reduced firing activity; (ii) in the Pf + Mf stage, synchronous ictal epileptiform activity occurred in the Pf and Mf. Changes in the balance between inhibition and excitation, appearance of novel electrophysiological phenomena (e.g. antidromic like action potentials, PDS (paroxysmal depolarization shift) potentials, rebound bursts), enhanced intrinsic bursting, and a transition from regular spiking to bursting were observed at the cellular level; (iii) in the Pf/Mf stage in 10% of the animals, the surface epileptic discharges were in synchrony with cellular activity in the Mf but were temporally independent of Pf activity, suggesting that during secondary epileptogenesis the Pf and the Mf can have underlying epileptogenic mechanisms which are different in origin.

Are there unifying principles underlying the generation of epileptic afterdischarges in vitro?

To find general principles in the cellular mechanisms of epileptogenesis, one must analyze experimental epilepsy models and determine what exists in common between them. We consider here afterdischarges in hippocampal slices induced using either (1) GABAA blockade (e.g. with bicuculline), (2) a bathing solution lacking Mg2+ ions (low Mg-induced epilepsy), or (3) 4-aminopyridine (4AP). By 'afterdischarge' we mean an event that lasts hundreds of milliseconds or more, involving the synchronous firing of all the neurons in a population, shaped into a long initial burst and a series of one or more secondary bursts, and terminating in a prolonged afterhyperpolarization (AHP). We propose that the following features exists in common between these three experimental epilepsies: (1) recurrent excitatory synaptic connections; (2) sustained dendritic synaptic excitation, mediated by either AMPA or NMDA receptors, or both; (3) an intrinsic cellular response to sustained excitation, consisting of rhythmical dendritic bursts, primarily mediated by Ca spikes. In conclusion, if the picture outlined here proves correct, then the stereotypic appearance of epileptic afterdischarges--consisting of synchronized population bursts in series, whatever the network alteration leading to seizures--does indeed reflect a common set of mechanisms. The mechanisms cannot, apparently, be formulated in simple terms of this receptor or that receptor. Rather, we suggest, the recurrent excitatory synapses are able, under diverse circumstances, collectively to produce sustained dendritic conductances in neuronal populations. Pyramidal neurons, by virtue of their normal intrinsic membrane properties, respond to such sustained conductances with rhythmical bursts. The recurrent synapses, in a dual role, serve to maintain the synchrony of these bursts, and so shape the activity into a synchronized oscillation.

Injection of tetanus toxin into the neocortex elicits persistent epileptiform activity but only transient impairment of GABA release

Focal injection of a minute quantity of tetanus toxin into the rat neocortex induces chronic epileptogenesis. Within a day, spontaneous and stimulus-evoked paroxysmal discharges appear in widespread regions of both hemispheres and this lasts for at least nine months. Tetanus toxin blocks transmitter release, apparently by catalysing the breakdown of synaptobrevin, a synaptic protein. It specifically binds to neuronal membranes but its potent epileptogenic properties have been ascribed to a higher affinity for inhibitory neurons. Following focal injection of tetanus toxin into the hippocampus a long-lasting epileptic syndrome also develops. During the early part of the syndrome GABA release is depressed in slices from the injected side, but not in slices from the contralateral, secondary focus. In the present experiments on neocortex, release of radiolabelled GABA was measured from primary and secondary epileptic foci induced by unilateral focal injection of tetanus toxin into the parietal cortex. By four weeks after the injection, no differences were detected in GABA release from any neocortical site in control or toxin-injected animals, despite the persistence of profound epileptic activity in slices from the latter. At earlier times (1.5 days) after the toxin injection, however, release was significantly depressed in both hemispheres. The results indicate that at first, the toxin induces focal neocortical epileptogenesis by directly impeding GABAergic synaptic transmission but that with time there is a recovery from this initial effect. We propose, as has also been suggested for other models, that the initial epileptogenesis leaves in its wake a long-lasting change in the local functional connectivity, such that the neocortex is rendered permanently epileptic.

Epileptic focus induced in rat by intrahippocampal cholera toxin: neuronal properties in vitro

Injecting 0.5-1.0 microgram of cholera toxin into rat hippocampus induces a chronic epileptic focus which generates interictal discharges and brief epileptic seizures intermittently over the following seven to 10 days. Here we examined the electrophysiological properties of hippocampal slices prepared from these rats three to four days after injection, at the height of the epileptic syndrome. These slices generated epileptic discharges in response to electrical stimulation of afferent pathways. In many cases epileptic discharges occurred spontaneously in the CA3 subregion; these usually lasted < 200 ms, but they could last < 0.6 s. Intracellular recordings from pyramidal layer cells revealed depolarization shifts synchronous with the epileptic field potentials. These depolarization shifts had slow onsets compared with those induced by blocking inhibition with bicuculline (depolarizations started a mean of 57 ms before, and reached 5.2 mV by, the onset of the cholera toxin epileptic field potential, compared with 12 ms and 3.6 mV respectively for 70 microM bicuculline methiodide). Extracellular unit recordings showed that the slow predepolarization seen in the cholera toxin focus was associated with an acceleration of the firing of other pyramidal layer neurons. The epileptic activity in this model cannot be attributed to the loss of synaptic inhibition, because inhibitory postsynaptic potentials could be evoked when the synchronous bursts were blocked by increasing [Ca2+]o from 2 to 8 mM. Observations of monosynaptic inhibitory postsynaptic currents isolated by application of 20 microM 6-cyano-7-nitroquinoxaline-2,3-dione, 50 microM DL-2-amino-5-phosphonovaleric acid and 100-200 microM 3-amino-2-(4-chlorophenyl)-2-hydroxy-propylsulphonic acid showed a small effect of the toxin only on the time course of the inhibitory postsynaptic current. On the other hand, there were significant changes in the intrinsic properties of individual neurons. The membrane potentials of cells in the cholera toxin focus did not differ from those in slices from rats injected with vehicle solution, but their input resistances were significantly increased. Unlike the other cellular changes in this model, the increase in input resistance was not seen in slices exposed acutely to 1 micrograms/ml cholera toxin for 30 min, suggesting there may be morphological changes in the chronic focus. Action potential accommodation and the slow afterhyperpolarization were depressed in both acute and chronic epileptic tissue, indicating impairments of Ca(2+)- and/or voltage-dependent K+ currents, and we conclude that these provide the most likely basis for cholera toxin epileptogenesis.