Different reactions of control and epileptic rats to administration of APV or muscimol on thalamic or CA3-induced CA1 responses

The physiology and pharmacology of CA1 is changed in epilepsy. There is evidence that the thalamic input to CA1 has a somewhat different physiological effect compared with the CA3 input. In this study we sought to determine whether this difference in physiology persists in epilepsy, and whether there are changes in the pharmacologic profile of these responses. Under urethane two stimulating electrodes were placed in mid to ventral CA3 and in the midline thalamus of control or epileptic rats. One glass micropipette electrode was placed into CA1 for recording. After the baseline acquisition of CA1-evoked responses to single- or paired-pulse stimulation, the stimuli were repeated with local application of either the GABAA agonist muscimol or the NMDA antagonist dl-2-amino-5-phosphonovalerate (APV). The CA1 response of epileptic rats had a smaller population postsynaptic potential (PSP) and spike amplitudes, longer PSP duration, multiple spikes, and the paired-pulse (at 20-ms intervals) facilitation in contrast to the paired-pulse depression seen in control and kindled rats. The duration of the PSP as well as the amplitude and number of spikes were reduced by administration of APV or muscimol into CA1 in both control and epileptic rats. In control rats, APV enhanced the depression induced by maximal paired thalamic or CA3 stimulation at 20-ms intervals and reduced the facilitation of threshold stimulation into no change. In contrast, muscimol in control rats reversed the depression induced by paired maximal stimulation into a mild facilitation and reduced the facilitation of threshold stimulation. In epileptic rats neither APV nor muscimol had a significant effect on the changes of the CA1 responses induced by maximal or threshold paired stimulation. This initial in vivo study demonstrated that the physiology and pharmacology of CA1 in epileptic rats are different from control rats. Although there are physiological differences in the evoked responses that are linked to the site of stimulation in the control and epileptic group, the pharmacology in each condition is independent of the site of stimulation.

Diminished allopregnanolone enhancement of GABA(A) receptor currents in a rat model of chronic temporal lobe epilepsy

1. Neurosteroid modulation of GABA(A) receptors present on dentate granule cells (DGCs) acutely isolated from epileptic (epileptic DGCs) or control rats (control DGCs) was studied by application of GABA with or without the modulators and by measuring the amplitude of peak whole-cell currents. 2. In epileptic DGCs, GABA efficacy (1394 +/- 277 pA) was greater than in control DGCs (765 +/- 38 pA). 3. Allopregnanolone enhanced GABA-evoked currents less potently in epileptic DGCs (EC50 = 92.7 +/- 13.4 nM) than in control DGCs (EC50 = 12.9 +/- 2.3 nM). 4. Pregnenolone sulfate inhibited GABA-evoked currents with similar potency and efficacy in control and epileptic DGCs. 5. Diazepam enhanced GABA-evoked currents less potently in epileptic (EC50 = 69 +/- 14 nM) compared to the control DGCs (EC50 = 29.9 +/- 5.7 nM). 6. There were two different patterns of zolpidem modulation of GABA(A) receptor currents in the epileptic DGCs. In one group, zolpidem enhanced GABA(A) receptor currents but with reduced potency compared to the control DGCs (EC50 = 134 +/- 20 nM vs. EC50 = 52 +/- 13 nM). In the second group of epileptic DGCs zolpidem inhibited GABA(A) receptor currents, an effect not observed in control DGCs. 7. Epileptic DGCs were more sensitive to Zn2+ inhibition of GABA(A) receptor currents (IC50 = 19 +/- 6 microM) compared to control (IC50 = 94.7 +/- 7.9 microM). 8. This study demonstrates significant differences between epileptic and control DGCs. We conclude that (1) diminished sensitivity of GABA(A) receptors of epileptic DGCs to allopregnanolone can increase susceptibility to seizures; (2) reduced sensitivity to diazepam and zolpidem, and increased sensitivity to Zn2+ indicate that loss of allopregnanolone sensitivity is likely to be due to altered subunit expression of postsynaptic GABA(A) receptors present on epileptic DGCs; and (3) an inverse effect of zolpidem in some epileptic DGCs demonstrates the heterogeneity of GABA(A) receptors present on epileptic DGCs.

Photothrombotic brain infarction results in seizure activity in aging Fischer 344 and Sprague Dawley rats

This study was designed to determine whether photothrombotic brain infarction could result in epileptic seizures in adult animals. Male Fischer 344 (F344) rats at 2, 6, 12, 24, and 30 months of age and male Sprague Dawley (SD) rats at 2 and 6 months of age underwent photothrombotic brain infarction with the photosensitive dye rose bengal by focusing a wide (6 mm) or narrow (3 mm) diameter white light beam on the skull overlying left hemisphere anterior frontal, midfrontal, frontoparietal, or parietal areas. Animals were monitored with video and EEG recordings. Morphological analysis of infarct size was performed with a computer-assisted image analysis system. The primary finding of this study was that epileptic seizures were recorded in post-mature rats 2 months after lesioning the frontoparietal cortex with large photothrombotic infarcts that extended to the cortical-subcortical interface. These seizures were characterized behaviorally by motor arrest, appeared to originate in the periinfarct area, and could be distinguished from inherited spontaneous bilateral cortical discharges by the morphology, frequency, duration, and laterality of the ictal discharges. Small cortical lesions were ineffective in producing seizures except for one animal that demonstrated recurrent prolonged focal discharges unaccompanied by behavioral change. Stage 3 seizures were observed in a small number of mid-aged and aged animals lesioned with large infarcts in anterior frontal and frontoparietal areas. These results suggest that the technique of photothrombosis can be used to produce neocortical infarction as a means to study mechanisms of secondary epileptogenesis.

Seizures induce phase shifts of rat circadian rhythms

RATIONALE

Epileptic seizures may alter neuroendocrinological cycles. Light pulses induce phase shifts in circadian rhythms. Using hippocampal-kindled rats to ensure maximal clinical expression, we determined if seizures likewise induce phase shifts.

METHODS

We monitored the circadian rhythm of temperature (CRT) with intraperitoneal radiotelemetry in rats (n=21) isolated from time cues and light for 3-week trials. Seizures were triggered with hippocampal electrical stimulation at different circadian phases. Optimized, least-error phase shifts were calculated from preictal and postictal CRTs. Induced seizures were referenced to CRT (t(max)=00:00, 24-h circadian cycle).

RESULTS

Phase shifts (individual responses=57) differed across the circadian cycle. Rather than forming a clear phase-response curve, phase shifts were especially variable between 00:00 and 06:00 h.

CONCLUSIONS

This study demonstrates that electrically-induced seizures induce advances and delays in CRT in a phase-dependent fashion but in a pattern different from typical light-induced phase shifts. Disorders of circadian regulation may contribute to some of the altered endogenous cycles associated with epilepsy.

The midline thalamus: alterations and a potential role in limbic epilepsy

PURPOSE

In limbic or mesial temporal lobe epilepsy, much attention has been given to specific regions or cell populations (e.g., the hippocampus or dentate granule cells). Epileptic seizures may involve broader changes in neural circuits, and evidence suggests that subcortical regions may play a role. In this study we examined the midline thalamic regions for involvement in limbic seizures, changes in anatomy and physiology, and the potential role for this region in limbic seizures and epilepsy.

METHODS

Using two rat models for limbic epilepsy (hippocampal kindled and chronic spontaneous limbic epilepsy) we examined the midline thalamus for evidence of involvement in seizure activity, alterations in structure, changes in the basic in vitro physiology of the thalamic neurons. We also explored how this region may influence limbic seizures.

RESULTS

The midline thalamus was consistently involved with seizure activity from the onset, and there was significant neuronal loss in the medial dorsal and reuniens/rhomboid nuclei. In addition, thalamic neurons had changes in synaptically mediated and voltage-gated responses. Infusion of lidocaine into the midline thalamus significantly shortened afterdischarge duration.

CONCLUSIONS

These observations suggest that this thalamic region is part of the neural circuitry of limbic epilepsy and may play a significant role in seizure modulation. Local neuronal changes can enhance the excitability of the thalamolimbic circuits.

Propofol and midazolam in the treatment of refractory status epilepticus

PURPOSE

To explore outcome differences between propofol and midazolam (MDL) therapy for refractory status epilepticus (RSE).

METHODS

Retrospective chart review of consecutive patients treated for RSE between 1995 and 1999.

RESULTS

We found 14 patients treated primarily with propofol and six with MDL. Propofol and MDL therapy achieved 64 and 67% complete clinical seizure suppression, and 78 and 67% electrographic seizure suppression, respectively. Overall mortality, although not statistically significant, was higher with propofol (57%) than with MDL (17%) (p = 0.16). Subgroup mortality data in propofol and MDL patients based on APACHE II (Acute Physiology and Chronic Health Evaluation) score did not show statistically significant differences except for propofol-treated patients with APACHE II score > or = 20, who had a higher mortality (p = 0.05). Reclassifying the one patient treated with both agents to the MDL group eliminated this statistically significant difference (p = 0.22).

CONCLUSIONS

In our small sample of RSE patients, propofol and MDL did not differ in clinical and electrographic seizure control. Seizure control and overall survival rates, with the goal of electrographic seizure elimination or burst suppression rather than latter alone, were similar to previous reports. In RSE patients with APACHE II score > or = 20, survival with MDL may be better than with propofol. A large multicenter, prospective, randomized comparison is needed to clarify these data. If comparable efficacy of these agents in seizure control is borne out, tolerance with regard to hemodynamic compromise, complications, and mortality may dictate the choice of RSE agents.

Aberrant neuronal physiology in the basal nucleus of the amygdala in a model of chronic limbic epilepsy

Limbic epilepsy is a chronic condition associated with a broad zone of seizure onset and pathology. Studies have focused mainly on the hippocampus, but there are indications that changes occur in other regions of the limbic system. This study used in vitro intracellular recording and histology to examine alterations to the physiology and anatomy of the basal nucleus of the amygdala in a rat model of chronic limbic epilepsy characterized by spontaneously recurring seizures. Epileptic pyramidal neuron responses evoked by stria terminalis stimulation revealed hyperexcitability characterized by multiple action potential bursts and no evident inhibitory potentials. In contrast, no hyperexcitability was observed in amygdalar neurons from kindled (included as a control for seizure activity) or control rats. Blockade of ionotropic glutamate receptors unmasked inhibitory postsynaptic potentials in epileptic pyramidal neurons. Control, kindled and epileptic inhibitory potentials were predominantly biphasic, with fast and slow components, but a few cells exhibited only the fast component (2/12 in controls, 0/3 in kindled, 3/10 in epileptic). Epileptic fast inhibitory potentials had a more rapid onset and shorter duration than control and kindled. Approximately 40% of control neurons exhibited spontaneous inhibitory potentials; no spontaneous inhibitory potentials were observed in neurons from kindled or epileptic rats. A preliminary histological examination revealed no gross alterations in the basal amygdala from epileptic animals. These results extend previous findings from this laboratory that hyperexcitability is found in multiple epileptic limbic regions and may be secondary to multiple alterations in excitatory and inhibitory efficacy. Because there were no differences between control and kindled animals, the changes observed in the epileptic animals are unlikely to be secondary to recurrent seizures.

Anticonvulsant effects of gamma surgery in a model of chronic spontaneous limbic epilepsy in rats

OBJECT

The management of intractable epilepsy remains a challenge, despite advances in its surgical and nonsurgical treatment. The identification of low-risk, low-cost therapeutic strategies that lead to improved outcome is therefore an important ongoing goal of basic and clinical research. Single-dose focal ionizing beam radiation delivered at necrosis-inducing and subnecrotic levels was investigated for its effects on seizure activity by using an established model of chronic recurrent spontaneous limbic seizures in rats.

METHODS

A single 90-minute period of repetitive electrical stimulation (inducing stimulus) of the hippocampus in rats elicited a single episode of status epilepticus, followed by a 2- to 4-week seizure-free period. Spontaneous recurrent seizures developed subsequently and persisted for the duration of monitoring (2-10 months). Simultaneous computerized electroencephalography and video recording were used to monitor the animals. After the establishment of spontaneous recurrent seizures, bilateral radiation centered in the ventral hippocampal formation was administered with the Leksell gamma knife, aided by a stereotactic device custom made for small animals. A center dose of 10, 20, or 40 Gy was administered using a 4-mm collimator. Control animals were subjected to the same seizure-inducing stimulus but underwent a sham treatment instead of gamma irradiation. In a second experiment, the authors examined the effects of gamma irradiation on the proclivity of hippocampal neurons to display epileptiform discharges. Naive animals were irradiated with a single 40-Gy dose, as already described. Slices of the hippocampus were prepared from animals killed between 1 and 178 days postirradiation. Sensitivity to penicillin-induced epileptiform spiking was examined in vitro in slices prepared from control and irradiated rat brains.

CONCLUSIONS

In the first experiment, single doses of 20 or 40 Gy (but not 10 Gy) reduced substantially, and in some cases eliminated, behaviorally and electrographically recognized seizures. Significant reductions in both the frequency and duration of spontaneous seizures were observed during a follow-up period of up to 10 months postradiation. Histological examination of the targeted region did not reveal signs of necrosis. These findings indicate that single-dose focal ionizing beam irradiation at subnecrotic dosages reduces or eliminates repetitive spontaneous seizures in a rat model of temporal lobe epilepsy. In the second experiment, synaptically driven neuronal firing was shown to be intact in hippocampal neurons subjected to 40-Gy doses. However, the susceptibility to penicillin-induced epileptiform activity was reduced in the brain slices of animals receiving 40-Gy doses, compared with those from control rats that were not irradiated. The results provide rational support for the utility of subnecrotic gamma irradiation as a therapeutic strategy for treating epilepsy. These findings also provide evidence that a functional increase in the seizure threshold of hippocampal neurons contributes to the anticonvulsant influence of subnecrotic gamma irradiation.

The pathological substrate of limbic epilepsy: neuronal loss in the medial dorsal thalamic nucleus as the consistent change

PURPOSE

The focus of research in limbic epilepsy has been the hippocampus because of its well-known pathology of hippocampal atrophy and sclerosis as well as the strong propensity for this structure to seize under a variety of circumstances. There is ample evidence, however, for pathological alterations in other regions of the limbic system in limbic/mesial temporal lobe epilepsy, including the amygdala, the entorhinal cortex, and, in some cases, the thalamus. In this preliminary evaluation of the pathological substrate for limbic epilepsy, we wished to determine if there was consistent anatomic change at extrahippocampal sites.

METHODS

We compared paraffin sections of brains from rats with chronic spontaneous limbic epilepsy and age-matched controls to determine the consistency of the pathology at five sites: the hippocampus, amygdala, entorhinal cortex, piriform cortex, and medial dorsal thalamus.

RESULTS

In a qualitative evaluation of these sections taken from standardized positions, we found that the medial dorsal thalamic nucleus in the epileptic animals was the site that was consistently involved with neuronal loss. With all other sites, at least several animals had qualitatively normal tissue.

CONCLUSIONS

This finding suggests that neuronal loss in the medial dorsal thalamus may be the consistent pathology in limbic epilepsy, at least in an animal model of the disorder. The presence of a structurally abnormal subcortical region with broad connections to the limbic sites involved with chronic epilepsy may have implications for our understanding of the pathophysiology of this disorder.

Effects of circadian regulation and rest-activity state on spontaneous seizures in a rat model of limbic epilepsy

PURPOSE

Circadian regulation via the suprachiasmatic nuclei and rest-activity state may influence expression of limbic seizures.

METHODS

Male rats (n = 14) were made epileptic by electrical stimulation of the hippocampus, causing limbic status epilepticus and subsequent seizures. We monitored seizures with intrahippocampal electrodes in 12-12-h light/dark (LD) cycles and in continuous dark (DD). We used radiotelemetry monitoring of activity to measure state and body temperature to determine circadian phase. Cosinor analysis and chi2 tests determined whether seizures occurred rhythmically when plotted by phase. State was defined as inactive or active in 10-min epochs based on whether activity count was below or above a cut-off value validated from video observation.

RESULTS

In LD, the peak seizure occurrence was 14:59 h after circadian temperature peak (95% confidence limit, 13:37-16:19). Phasic seizure occurrence persisted in DD for 14:05 (12:31-15:38), p < 0.0001, against uniform mean distribution. In LD, 14,787 epochs contained 1, 268 seizures; seizures preferentially occurred during inactive epochs (965 observed, 878 expected in proportion to the overall distribution of inactive versus active epochs; p < 0.001). In DD, 20, 664 epochs contained 1,609 seizures; seizures had no preferential occurrence by state (999 observed, 1,025 expected; p = 0.16).

CONCLUSIONS

Limbic seizures occurred with an endogenous circadian rhythm. Seizures preferentially struck during inactivity during entrainment to the light-dark cycle.

Hypothalamic neuronal loss and altered circadian rhythm of temperature in a rat model of mesial temporal lobe epilepsy

PURPOSE

Numerous dysfunctions in endogenous hypothalamic function have been associated with mesial temporal lobe epilepsy (MTLE). One endogenous activity is the circadian rhythm of temperature (CRT). In this study we examined whether hypothalamically mediated function is altered in the electrically induced, self-sustained, limbic status epilepticus model of MTLE. We then wished to determine whether there was a structural basis for regulatory alterations.

METHODS

We measured CRT with peritoneal temperature telemetry obtained in light-entrained (LD) and in free-running, constant-dark (DD) conditions. CRT from epileptic and controls of normal animals and kindled animals were quantized by fast Fourier transform-nonlinear least squares analysis to determine rhythmic complexity.

RESULTS

The circadian component of CRT was preserved in all animals. In DD, CRTs of epileptic animals were more complex than those of normal animals. CRT of kindled animals showed no increased complexity after electrically induced seizures. Neuronal density was decreased in regions of the anterior and posterior hypothalamus but not in the suprachiasmatic nuclei from the epileptic rats.

CONCLUSIONS

Alterations in CRT due to the epileptic state were independent of isolated seizures. Altered circadian thermoregulation in epileptic rats corresponded to regional hypothalamic neuronal loss. Structural changes of the hypothalamus may explain alterations in endogenous rhythms in MTLE.

Thalamic excitation of hippocampal CA1 neurons: a comparison with the effects of CA3 stimulation

CA1 is the major output area for the hippocampus, and current evidence shows that it is excited primarily from ipsilateral and contralateral CA3 pyramidal cells in the rat. Direct connections from the midline thalamic nuclei to the hippocampus have been described anatomically, but the physiological role of these connections has not been reported until the recent observation that these inputs may have a mild excitatory effect (subthreshold for population spikes). In this study, we report a more powerful excitatory effect of thalamic stimulation on the response of the CA1 neurons in the urethane-anesthetized rat. Electrical stimulation to the midline thalamus induced responses similar to responses from stimulation of the contralateral hippocampus (CA3), with well-developed field excitatory postsynaptic potentials and large population spikes. The latency of the CA1 response suggested that the thalamic connection was monosynaptic, and there was a laminar CA1 response profile that depended on the site of stimulation (contralateral CA3 or thalamus). In an initial examination of possible differences in the physiological effects of these two pathways on the CA1 region, we tested both sites for long-term potentiation of CA1, for the effects of repetitive stimulation on CA1 responses (e.g., possible augmenting responses) and for the effect of paired-pulse stimulation. In these three measures, there were clear and statistically significant differences between the effects of CA3 and thalamic stimulation on CA1 responses. This study demonstrates that the well-described thalamic connection to the hippocampus allows for the direct and powerful excitation of the CA1 region. This thalamohippocampal connection bypasses the trisynaptic/commissural pathway that has been thought to be the exclusive excitatory drive to CA1. In addition, preliminary data indicate that the thalamus and CA3 inputs have different physiological effects on CA1 pyramidal cells.

Functional anatomy of limbic epilepsy: a proposal for central synchronization of a diffusely hyperexcitable network

The limbic/mesial temporal lobe epilepsy syndrome has been defined as a focal epilepsy, with the implication that there is a well defined focus of onset, traditionally centered around the hippocampus. The pathology of the hippocampus in this syndrome has been well described and a number of physiological abnormalities have been defined in this structure in animal models and humans with epilepsy. However, anatomical and physiological abnormalities have also been described in other limbic sites in this form of epilepsy. Previous studies have shown broadly synchronized or multifocal seizure onset within the limbic system of the animal models and human patients. We hypothesized that the epileptogenic circuit for the initiation of seizures was distributed throughout the limbic system with a possible central synchronizing process. In vitro studies showed that multiple limbic sites in epileptic animals (hippocampus, entorhinal cortex, piriform cortex and amygdala) have epileptiform changes with prolonged depolarizations and multiple superimposed action potentials. In vivo studies revealed that thalamic stimulation yields short latency excitatory responses in the entorhinal cortex and hippocampus. In addition, in epileptic animals, thalamic stimulation caused epileptiform responses in the hippocampus. Based on the findings of this study and on previous anatomy and physiology reports, we hypothesize that the process of seizure initiation involves broad circuit interactions involving multiple independent limbic structures, and that the midline thalamus may act as a physiological synchronizer. We offer a new proposal for the functional anatomy of limbic epilepsy that takes widespread hyperexcitability in the limbic system and the potential for thalamic synchronization into consideration.

Hippocampal AMPA and NMDA mRNA levels and subunit immunoreactivity in human temporal lobe epilepsy patients and a rodent model of chronic mesial limbic epilepsy

This study compared temporal lobe epilepsy patients, along with kindled animals and self sustained limbic status epilepticus (SSLSE) rats for parallels in hippocampal AMPA and NMDA receptor subunit expression. Hippocampal sclerosis patients (HS), non-HS cases, and autopsies were studied for: hippocampal AMPA GluR1-3 and NMDAR1&2b mRNA levels using in situ hybridization: GluR1, GluR2/3, NMDAR1, and NMDAR2(a&b) immunoreactivity (IR); and neuron densities. Similarly, spontaneously seizing rats after SSLSE, kindled rats, and control animals were studied for: fascia dentata neuron densities: GluR1 and NMDAR2(a&b) IR; and neo-Timm's staining. In HS and non-HS cases, the mRNA hybridization densities per granule cell, as well as molecular layer IR, showed increased GluR1 (relative to GluR2/3) and increased NMDAR2b (relative to NMDAR1) compared to autopsies. Likewise, the molecular layer of SSLSE rats with spontaneous seizures demonstrated more neo-Timm's staining, and higher levels of GluR1 and NMDAR2(a&b) IR compared to kindled animals and controls. These results indicate that hippocampal AMPA and NMDA receptor subunit mRNAs and their proteins are differentially increased in association with spontaneous, but not kindled, seizures. Furthermore, there appears to be parallels in fascia dentata AMPA and NMDA receptor subunit expression between HS (and non-HS) epileptic patients and SSLSE rats. This finding supports the hypothesis that spontaneous seizures in humans and SSLSE rats involve differential alterations in hippocampal ionotrophic glutamate receptor subunits. Moreover, non-HS hippocampi were more like HS cases than hippocampi from kindled animals with respect to glutamate receptors; therefore, hippocampi from kindled rats do not accurately model human non-HS cases, despite some similarities in neuron densities and mossy fiber axon sprouting.

Ontogeny of altered synaptic function in a rat model of chronic temporal lobe epilepsy

In the limbic status model of chronic temporal lobe epilepsy, hippocampal stimulation induces acute status epilepticus in rats; recurrent, spontaneous seizures develop following an asymptomatic silent period lasting several weeks. Previous work has shown increased excitability and decreased inhibition in CA1 pyramidal neurons in chronically epileptic animals. To determine the relationship of altered cellular responses to seizure onset, in vitro intracellular recording was used to follow the evolution of changes in synaptic physiology occurring during the seizure-free silent period. Pyramidal cells displayed increasing epileptiform activity throughout the period investigated, 3-14 days following status; the mean number of evoked action potentials from 1.1+/-0.05 in control cells to 2.4+/-0.4 early (3 days after status) and 4. 3+/-0.7 late (14 days) in the silent period. Monosynaptic inhibitory postsynaptic potentials mediated by gamma-aminobutyric acid-A receptors in silent period cells differed markedly from controls. Area, rise time, and duration of these potentials decreased by 40-60% within 3 days following status and to values commensurate with chronically epileptic animals in 7 to 10 days. gamma-Aminobutyric acid-B receptor-mediated IPSPs diminished more gradually in the silent period, reaching a minimum at day 14. In contrast, presynaptic gamma-aminobutyric acid-B receptor function showed maximum impairment 3 days after status. The benzodiazepine type 1 receptor agonist zolpidem reduced hyperexcitability in both silent period and chronically epileptic cells, but was more effective at unmasking the underlying IPSP in silent period neurons. The results indicate that changes in different components of pyramidal cell inhibitory synaptic physiology associated with chronic epilepsy in this model evolve individually at different rates, but are all complete before seizure onset. Although the results do not imply causality, they do suggest that the development of physiological changes in CA1 pyramidal cells may contribute to the lag period preceding the onset of chronic seizures.

Responses of deep entorhinal cortex are epileptiform in an electrogenic rat model of chronic temporal lobe epilepsy

We investigated whether entorhinal cortex (EC) layer IV neurons are hyperexcitable in the post-selfsustaining limbic status epilepticus (post-SSLSE) animal model of temporal lobe epilepsy. We studied naive rats (n = 44), epileptic rats that had experienced SSLSE resulting in spontaneous seizures (n = 45), and electrode controls (n = 7). There were no differences between electrode control and naive groups, which were pooled into a single control group. Intracellular and extracellular recordings were made from deep layers of EC, targeting layer IV, which was activated by stimulation of the superficial layers of EC or the angular bundle. There were no differences between epileptic and control neurons in basic cellular characteristics, and all neurons were quiescent under resting conditions. In control tissue, 77% of evoked intracellular responses consisted of a short-duration [8.6 +/- 1.3 (SE) ms] excitatory postsynaptic potential and a single action potential followed by gamma-aminobutyric acid-A (GABAA) and GABAB inhibitory post synaptic potentials (IPSPs). Ten percent of controls did not contain IPSPs. In chronically epileptic tissue, evoked intracellular responses demonstrated prolonged depolarizing potentials (256 +/- 39 ms), multiple action potentials (13 +/- 4), and no IPSPs. Ten percent of epileptic responses were followed by rhythmic "clonic" depolarizations. Epileptic responses exhibited an all-or-none response to progressive increases in stimulus intensity and required less stimulation to elicit action potentials. In both epileptic and control animals, intracellular responses correlated precisely in morphology and duration with extracellular field potentials. Severing the hippocampus from the EC did not alter the responses. Duration of intracellular epileptic responses was reduced 22% by the N-methyl--aspartate (NMDA) antagonist (-)-2-amino-5-phosphonovaleric acid (APV), but they did not return to normal and IPSPs were not restored. Epileptic and control responses were abolished by the non-NMDA antagonist 6, 7-dinitroquinoxaline-2-3-dione (DNQX). A monosynaptic IPSP protocol was used to test connectivity of inhibitory interneurons to primary cells by direct activation of interneurons with a stimulating electrode placed near the recording electrode in the presence of APV and DNQX. Using this protocol, IPSPs similar to control (P > 0.05) were seen in epileptic cells. The findings demonstrate that deep layer EC cells are hyperexcitable or "epileptiform" in this model. Hyperexcitability is not due to interactions with the hippocampus. It is due partially to augmented NMDA-mediated excitation. The lack of IPSPs in epileptic neurons may suggest inhibition is impaired, but we found evidence that inhibitory interneurons are connected to their target cells and are capable of inducing IPSPs.

Temporal distribution of partial seizures: comparison of an animal model with human partial epilepsy

Seizures do not often strike randomly but may occur in circadian patterns. We compared daily times of partial seizures determined by continuous electroencephalography among patients with mesial temporal lobe epilepsy (MTLE; n = 64), those with extratemporal lobe (XTLE; n = 26) or lesional temporal lobe epilepsy (LTLE; n = 8), and a rat model similar to MTLE in which rats become epileptic after electrically induced limbic status epilepticus (postlimbic status [PLS]; n = 20). Rats were maintained on a 12-hour light/dark cycle with lights on at 0700 hours. The distributions of seizures were fitted by cosinor analysis to determine time of peak seizure incidence +/- 95% confidence interval (95% CI). The mean fraction +/- SD of seizures recorded during light was 63 +/- 17% in PLS animals and 60 +/- 21% in humans. Peak incidence of seizures for PLS rats (547 seizures) was 1645 (95% CI = 1448,1830) and for MTLE subjects (774 seizures) was 1500 (95% CI = 1324,1636). Seizures from XTLE (465 seizures) and LTLE (48 seizures) did not fit a cosinor model and occurred no more frequently during light than dark. In conclusion, limbic seizures in humans and PLS rats occur more often during light than dark and have similar cosinor daily distributions. The chronological similarity between human MTLE and PLS rat epilepsy suggests that limbic seizure occurrence has a relation to the circadian regulatory system.

Design and construction of a long-term continuous video-EEG monitoring unit for simultaneous recording of multiple small animals

In recent years several new rat models of human limbic/mesial temporal lobe epilepsy have been described [1,2,4-7,11,15-17]. Unlike earlier models such as kindling in which the seizures are induced by an exogenous stimulus, these new models are characterized by seizures that occur spontaneously at random intervals. Although the spontaneity of the seizures makes these models more like human epilepsy, documentation of these seizures by direct observation is highly inefficient, and sub-behavioral electrographic seizures could be missed. Continuous paper EEG and video recording have been used [5-7,15], but these techniques are resource intensive. The slow paper speed required by long-term paper recordings limits the ability to differentiate between true seizure activity and electrical artifact. Subtle behavioral seizures are likely to be missed during rapid review of video recordings alone [16]. Ambulatory cassette EEG recordings have been used [3], but the systems require expensive proprietary hardware, and the systems have limited channels for recording (8-16). To improve the utility of the models, we developed a long-term EEG/video monitoring system to detect the electrographic seizures and document their behavioral accompaniment. The system is based on commercially available components, including a computerized EEG seizure detection system that was initially developed for human seizure monitoring [8,9,13]. Seizures are reliably detected and the data are reduced so that 24 h of recording can be reviewed in 30-90 min. Although the computer program is accurate, special care must be taken in system design and construction to reduce sources of electrical artifact that can cause false detections when multiple animals are recorded simultaneously on a single EEG machine. During data review it is necessary to differentiate between electrical artifact induced by animal activity from true seizure activity by key EEG patterns. Certain seizure patterns (less than 3 hz. low amplitude) will not be detected by the seizure detection program, but the system is highly effective for typical limbic seizures and may be useful for the animal models of absence epilepsy [12,14]. It can also be used as a continuous or intermittent EEG/physiological recording device for experiments that examine animals' spontaneous behavior and the EEG correlate (e.g. sleep/wake cycles, learning and memory tasks).

Shortened-duration GABA(A) receptor-mediated synaptic potentials underlie enhanced CA1 excitability in a chronic model of temporal lobe epilepsy

Intracellular recording techniques were used to examine GABA(A) receptor-mediated synaptic inhibition in pyramidal cells of the CA1 region of the rat hippocampus in the post-self sustaining limbic status epilepticus model of temporal lobe epilepsy. Orthodromically evoked, monosynaptic inhibitory postsynaptic potentials were recorded in vitro following pharmacological blockade of ionotropic glutamate and GABA(B) receptors. Inhibitory postsynaptic potentials from epileptic tissue were kinetically altered relative to controls; both the 10-90% rise-time and width (measured at half-maximum amplitude) were reduced by approximately 50% resulting in significant shortening of duration. The degree of pyramidal cell hyperexcitability, assessed before pharmacological treatment as the number of action potentials evoked by maximum intensity afferent stimulation, correlated significantly with the magnitude of synaptic potential duration reduction determined following blockade of glutamatergic neurotransmission. Bath application of the benzodiazepine type 1 receptor agonist zolpidem reduced post-self sustaining limbic status epilepticus CA1 pyramidal cell hyperexcitability substantially (but not completely) via a marked increase in inhibitory postsynaptic potential area. Post-self-sustaining limbic status epilepticus inhibitory postsynaptic potentials which exhibited the most pronounced shortening were augmented by zolpidem to a greater degree than longer duration synaptic potentials. In contrast, zolpidem-induced augmentation of control inhibitor, postsynaptic potential area was much less robust. It is suggested that a deficiency in post-self-sustaining limbic status epilepticus GABA(A) receptor-mediated synaptic inhibition contributes to a state of partial disinhibition which is a major factor in enhanced CA1 excitability in chronic limbic epilepsy. Possible mechanisms underlying post-self-sustaining limbic status epilepticus kinetic abnormalities are discussed.

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.

Volumetric magnetic resonance imaging evidence of bilateral hippocampal atrophy in mesial temporal lobe epilepsy

PURPOSE

We measured absolute volumes and volume differences of hippocampi in patients with mesial temporal lobe epilepsy (MTLE) using volumetric magnetic resonance imaging (MRI) to determine the extent of bilateral atrophy in MTLE and to relate hippocampal volumes (HV) to outcome of temporal lobectomy.

METHODS

HV and hippocampal differences (HD) were measured in 40 patients with MTLE determined by pathology of hippocampal sclerosis (HS) and compared with those of age-matched controls. Results were matched with surgical outcome.

RESULTS

Hippocampi contralateral to lobectomy (right hippocampi 2.96 +/- 0.49 cm3, left 3.14 +/- 0.51 cm3) were significantly smaller than those of controls (right hippocampi 3.73 +/- 0.52 cm3, left 3.60 +/- 0.51 cm3) but were significantly larger than hippocampi ipsilateral to lobectomy (right hippocampi 2.63 +/- 0.61 cm3, 2.18 cm3) as compared across groups by analysis of variance (ANOVA: F = 27.2, p < 0.0001). The smaller hippocampus was ipsilateral to lobectomy in 39 of 40 cases. Seven of 40 MTLE patients (18%) had bilateral atrophy, defined by volumes of each hippocampi 2 SD lower than control means. Surgical outcome was independent of hippocampal asymmetry and bilateral atrophy measured by chi-square and Fisher's exact tests.

CONCLUSIONS

We determined that most patients with MTLE have some degree of bilateral, asymmetric hippocampal pathology. However, asymmetry and bilateral atrophy have no clear relation to surgical outcome.

Longitudinal distribution of hippocampal atrophy in mesial temporal lobe epilepsy

Patients with mesial temporal lobe epilepsy (MTLE) have asymmetric hippocampal volumes with atrophy that sometimes by visual inspection appears to favor different regions along the longitudinal axis of the affected hippocampus. Histological studies suggest that cell loss may affect the anterior hippocampus preferentially, and that hippocampal sclerosis (HS) limited to the anterior of the hippocampus may indicate better surgical outcome. We used volumetric magnetic resonance imaging (MRI): (1) to objectively describe the distribution of volume loss in HS; and (2) to relate this distribution to outcome of temporal lobectomy. Hippocampal volumes and anterior and posterior subvolumes (AHV, PHV) were measured from MP-RAGE MRI in 43 temporal lobectomy patients with MTLE determined by pathological findings of HS and compared to 23 age-matched controls. Atrophy was defined as 'anterior', 'diffuse', 'posterior', or 'normal' depending on position of AHV and PHV relative to the mean +/- 2 S.D. of regional volumes of control hippocampi. Anterior to posterior ratios (APR = AHV/PHV) were also calculated. Mean APR of hippocampi ipsilateral to lobectomy cannot be distinguished from hippocampi contralateral to lobectomy or from controls. AHV and PHV from hippocampi contralateral to temporal lobectomy were smaller than controls but larger than hippocampi ipsilateral to lobectomy. Surgical outcome was independent of longitudinal distribution of atrophy. We determined that overall volume loss in HS is diffuse, neither clearly favoring the head nor body-tail. Surgical outcome for MTLE is not related to the longitudinal distribution of atrophy revealed by volumetric MRI.

In contrast to kindled seizures, the frequency of spontaneous epilepsy in the limbic status model correlates with greater aberrant fascia dentata excitatory and inhibitory axon sprouting, and increased staining for N-methyl-D-aspartate, AMPA and GABA(A) receptors

This study determined whether there were differences in hippocampal neuron loss and synaptic plasticity by comparing rats with spontaneous epilepsy after limbic status epilepticus and animals with a similar frequency of kindled seizures. At the University of Virginia, Sprague-Dawley rats were implanted with bilateral ventral hippocampal electrodes and treated as follows; no stimulation (electrode controls; n=5): hippocampal stimulation without status (stimulation controls; n=5); and limbic status from continuous hippocampal stimulation (n=12). The limbic status group were electrographically monitored for a minimum of four weeks. Four rats had no recorded chronic seizures (status controls), and all three control groups showed no differences in hippocampal pathology and were therefore incorporated into a single group (controls). Eight limbic status animals eventually developed chronic epilepsy (spontaneous seizures) and an additional eight rats were kindled to a similar number and frequency of stage 5 seizures (kindled) as the spontaneous seizures group. At the University of California (UCLA) the hippocampi were processed for: (i) Niss1 stain for densitometric neuron counts; (ii) neo-Timm's histochemistry for mossy fiber sprouting; and (iii) immunocytochemical staining for glutamate decarboxylase, N-methyl-D-aspartate receptor subunit 2, AMPA receptor subunit 1 and the GABA(A) receptor. In the fascia dentata inner and outer molecular layers the neo-Timm's stain and immunoreactivity was quantified as gray values using computer image analysis techniques. Statistically significant results (P<0.05) showed the following. Compared to controls and kindled animals, rats with spontaneous seizures had: (i) lower neuron counts for the fascia dentata hilus, CA3 and CA1 stratum pyramidale; (ii) greater supragranular inner molecular layer mossy fiber staining; and (iii) greater glutamate decarboxylase immunoreactivity in both molecular layers. Greater supragranular excitatory mossy fiber and GABAergic axon sprouting correlated with: (i) increases in N-methyl-D-aspartate receptor subunit 2 inner molecular layer staining; (ii) more AMPA receptor subunit 1 immunoreactivity in both molecular layers; and (iii) greater outer than inner molecular layer GABA(A) immunoreactivity. Furthermore, in contrast to kindled animals, rats with spontaneous seizures showed that increasing seizure frequency per week and the total number of natural seizures positively correlated with greater Timm's and GABAergic axon sprouting, and with increases in N-methyl-D-aspartate receptor subunit 2 and AMPA receptor subunit 1 receptor staining. In this rat limbic status model these findings indicate that chronic seizures are associated with hippocampal neuron loss, reactive axon sprouting and increases in excitatory receptor plasticity that differ from rats with an equal frequency of kindled seizures and controls. The hippocampal pathological findings in the limbic status model are similar to those in humans with hippocampal sclerosis and mesial temporal lobe epilepsy, and support the hypothesis that synaptic reorganization of both excitatory and inhibitory systems in the fascia dentata is an important pathophysiological mechanism that probably contributes to or generates chronic limbic seizures.

Histological and 1H magnetic resonance spectroscopic imaging analysis of quinolinic acid-induced damage to the rat striatum

NAA has been described as a neuron-specific compound. NAA levels as determined by magnetic resonance spectroscopic imaging (MRSI) have been used to determine degree of neuronal loss in several neurological diseases, but there has been limited work to document the accuracy and reliability of this technique. This study addresses this question quantitatively with histological analysis of cell viability and tissue shrinkage in quinolinic acid (QA)-induced damage of the rat striatum compared with 1H MRSI measurement of N-acetyl aspartate (NAA) as a noninvasive measure of neuronal loss. Both 1H MRSI and histology detect damage to the lesioned striatum; however, there are differences in the degree of damage as assessed by the two methods. Although partial-volume effects and tissue shrinkage may decrease the sensitivity of MR to such damage, the sparing of axons by QA may be another important factor in the differences in assessment. These results indicate that further studies of NAA metabolism and its distribution within neurons are warranted.

Functional anatomy of spontaneous seizures in a rat model of limbic epilepsy

PURPOSE

The substrate of seizure initiation in partial epilepsy is not well understood. Although many studies imply a focus that is will localized to a single structure, some information suggests that seizures arise regionally with simultaneous or near-simultaneous onset at several separate sites. To determine the physiological pattern of seizure onset, recordings were made of spontaneous seizures from multiple limbic structures in a rat model of limbic epilepsy.

METHODS

Seventeen rats with chronic spontaneous limbic seizures that occurred after an episode of electrical hippocampal stimulation-induced status epilepticus (SE) underwent chronic recording of seizures with a minimum of four electrodes placed bilaterally in the midventral hippocampus and either the amygdala or piriform cortex. As many as 15 of the first recorded spontaneous seizures (mean of 12 seizures, range 2-15) was evaluated for each animal with regard to pattern of seizure onset (focal hippocampal, focal nonhippocampal, or diffuse). The results were evaluated for number of seizure patterns displayed by each animal and the potential change in the pattern of onset with successive seizures (from the first to the fifteenth).

RESULTS

In all, 210 seizures were evaluated beginning 1 week after SE and with monitoring lasting < or = 18 weeks (mean recording duration 6 weeks) to record a maximum of 15 seizures for each animal. Overall, 54% of the seizures were of diffuse onset, 25% were of nonhippocampal onset, and 21% were of hippocampal onset. Eight of the 17 (47%) animals had seizures with all three patterns of onset. Evaluation of the evolution of the pattern of onset from the first to the fifteenth seizure recorded for the animals showed a clear tendency for the later seizures to have diffuse onsets (regression analysis r = 0.737, p < 0.002). Qualitative histologic analysis demonstrated neuronal loss in the recorded regions.

CONCLUSIONS

These findings indicate that the epileptogenic zone is broad in this model of limbic "focal" epilepsy and suggest that the substrate for seizure generation is distributed over several structures.

Epilepsy surgery in children and adolescents

There has been a considerable increase in interest in the surgical management of intractable epilepsy in children. Better methods of diagnosis and improved knowledge of natural history and results of surgery have been major factors in this phenomenon. The results of surgery in 76 patients who were 19 years of age or younger are reported. Success rates, defined as no postoperative seizures with loss of consciousness (Engel Grades I and II) were 78% in 52 patients treated by temporal lobectomy, 75% in eight patients who had extratemporal resections, 80% in 10 children treated by hemispherectomy, and 33% in three patients treated by corpus callosotomy.

The evolution of a rat model of chronic spontaneous limbic seizures

The evolution of untreated partial epilepsy is unknown. This study uses a newly developed model of chronic limbic epilepsy to determine whether seizures inexorably worsen in duration, frequency and behavioral accompaniment. The seizures begin following an episode of limbic status epilepticus induced by continuous electrical stimulation of the hippocampus, and they persist for more than a year (longest duration followed). We monitored 10 rats continuously with combined EEG and closed circuit television for 24 weeks following the first recorded spontaneous seizure. Seizure duration, behavioral accompaniment and frequency all intensified during the early stages, but the last 12-16 weeks of the study were characterized by a plateau for all measures. The results showed significant increases that occurred over the first 12 weeks only (P < 0.01 for duration and behavioral accompaniment, P < 0.05 for seizure frequency). These findings suggest that untreated epilepsy will undergo an early maturation process, but that once the seizures mature they remain stable over a prolonged period. It was also noted that 67% (P < 0.00001) of the seizures occurred during the day, suggesting that the sleep-wake cycle has a strong influence on the occurrence of seizures in this model of limbic epilepsy.

The ontogeny of seizures in a rat model of limbic epilepsy: evidence for a kindling process in the development of chronic spontaneous seizures

Kindling is an experimental model for epilepsy in which repeated stimuli induce longer electrographic seizures and eventually cause behavioral convulsions. Although kindling has some features that are similar to chronic human epilepsy, it is not known whether this process plays a role in the development of chronic seizure disorders. We have recently described a rat model of chronic spontaneous limbic seizures that has a number of similarities to human limbic epilepsy. To determine whether a kindling process is involved in the ontogeny of the seizures in this animal model, we continuously monitored 16 rats with EEG and closed circuit television until they had experienced a minimum of 5 and as many as 10 seizures following the first motor seizure. All animals had at least one non-motor seizure before the first motor event (mean 5.1 +/- 0.9 S.E.M. initial non-motor seizures, range 1-12). In addition the seizures significantly lengthened in duration with succeeding events (mean 90 s for the first motor seizure to mean 110 s for the tenth subsequent seizure). These data demonstrate that there is a kindling process involved in the early development of chronic limbic seizures.

Morphometric effects of intermittent kindled seizures and limbic status epilepticus in the dentate gyrus of the rat

The effect of recurrent seizures on the hippocampus has been controversial for many years. To determine the effect different seizure paradigms had on the structure of the dentate gyrus, we conducted histological studies on the dentate gyrus (DG) from three groups of rats: (1) those that had experienced 1500 intermittent kindled seizures; (2) those that had experienced a single episode of limbic status epilepticus (SE); and (3) control rats that had been implanted with electrodes. When compared to controls the DG of SE rats was overall slightly, but non-significantly, smaller, but the DG of rats with 1500 kindled seizures was significantly larger. The decrease of size following SE was attributable to a significant atrophy of the molecular layer. The increase in area associated with kindling was the result of an enlargement of the molecular layer and the hilus. Absolute neuronal counts showed a decrease in the hilus after SE but no change following kindling, but both groups had decreased neuronal densities in the hilus when compared to controls. The decreased density after SE was secondary to neuronal loss, but the decrease in neuronal density following kindling was the result of the expansion of the hilar neuropil without change in the number of neurons. This study extends our previous findings in Ammon's horn and indicates that SE induces significant neuronal loss, but numerous intermittent kindled seizures have no effect on neuronal numbers in the DG.

Epileptogenic effects of status epilepticus

Determining whether and under what conditions status epilepticus (SE) leads to undesirable long-term sequelae has major clinical ramifications. In addition to structural brain damage and enduring neurological deficits following SE, it has been suggested that SE can establish a chronic condition of active epilepsy. These three residua (epileptic brain damage, neurological deficits, and epilepsy) have been especially linked to protracted SE. The older clinical literature indicates that these sequelae are especially likely if SE occurs in an immature brain, but this point has been challenged in recent studies. Clinical and animal model work that examines the issue of chronic nervous system deficits arising as a consequence of SE is reviewed, with particular attention to the question of the epileptogenic effect of SE. Because of the inherent problem of not being able to exclude occult neurological disease antecedent to SE in brain, animal model work promises to be especially relevant to the issues at hand. Work done on adult rats has shown that a previously normal brain can be "converted" after a bout of SE to an epileptic brain, as manifest both by epileptic brain damage resembling that found in the hippocampus of patients with intractable temporal lobe epilepsy and by spontaneous recurrent seizures registered in the hippocampus. A two-step model is proposed: morphological brain injury takes place first and this change, in turn, promotes seizures. This model is offered as one way in which chronic active epilepsy can be established by a transient episode of SE. Although some findings from work with animal models have been interpreted as not supporting the idea that the immature brain is sensitive to a chronic epileptogenic influence initiated by SE, the majority of such work is consistent with this idea. On the other hand, a considerable amount of animal work indicates that the brains of immature animals are quite resistant to SE-induced brain damage, in contrast to those of adults. Thus, under these circumstances, a different process of epileptogenesis than the two-step model may be operational. It is concluded that, under appropriate conditions, SE does exert an epileptogenic effect that persists.

Ambulatory EEG cassette recorders for prolonged electroencephalographic monitoring in animals

We describe the use of ambulatory cassette EEG recorders for monitoring in vivo neurophysiologic signals from multiple animals over prolonged periods of time. This technique centers around a simple interface device that attenuates the intracerebral signals to the input range of the recording device and around a common indifferent input for all animals. The resulting analog recordings have the advantage of good signal resolution and rapid review of 24 h of recorded data.

Recurrent spontaneous hippocampal seizures in the rat as a chronic sequela to limbic status epilepticus

A period of continuous hippocampal stimulation (CHS) establishes an acute condition of self-sustaining limbic status epilepticus (SSLSE) which is followed by chronic neuropathological changes reminiscent of hippocampal sclerosis encountered in epileptic patients. In the chronic (greater than or equal to 1 month) condition following CHS-induced SSLSE, extended electrographic monitoring in the hippocampus revealed spontaneous recurrent paroxysmal discharges. All 6 animals studied had persistent interictal spiking; 3 had multiple fully developed electrographic seizures. There was a marked diminution of paired pulse inhibition, demonstrated by a protocol known to reflect the potency of inhibition mediated by GABAA receptors. Hippocampal slices from animals that had previously experienced CHS-induced SSLSE demonstrated an increased excitability relative to slices from control animals as evidenced by epileptiform bursting in increased extracellular potassium ([K+]0) and decreased extracellular calcium ([Ca2+]0). These studies establish that CHS-induced SSLSE in rats provides an experimental model with recurrent spontaneous hippocampal seizures. Based on electrophysiological data we suggest that a decrease in GABA-mediated inhibition and/or altered sensitivity to extracellular ions may play roles in the development of such seizures.

NMDA receptor antagonists and limbic status epilepticus: a comparison with standard anticonvulsants

Status epilepticus (SE) evolves through several stages when untreated. The later stages of SE are less responsive to standard anticonvulsants and may require general anesthesia to suppress seizures. Antagonists acting at the N-methyl-D-aspartate (NMDA) subclass of glutamate (excitatory) receptors have been demonstrated to exert antiepileptic activity in some seizure models. We report experiments performed to determine if NMDA receptor antagonists are effective in stopping seizures in the late stages of SE. A model of limbic SE induced by 90 min of 'continuous' electrical stimulation of the hippocampus in rats was employed. Three NMDA receptor antagonists, one 'competitive' (CPP) and two 'non-competitive' (ketamine and MK-801), were compared to 3 standard antiepileptic drugs (diazepam, phenobarbital, and phenytoin) for their ability to suppress seizures at a physiologically defined stage of SE. All NMDA receptor antagonists, diazepam and phenobarbital were effective in suppressing behavioral and electrographic seizures for varying periods of time. Phenytoin had no effect on SE. Ketamine and MK-801 induced a paradoxical enhancement of electrographic seizures that preceded SE suppression. We believe that NMDA-receptor antagonists offer a novel approach for treating the late stages of SE.

The hippocampus in experimental chronic epilepsy: a morphometric analysis

The effect of intermittent seizures on the pyramidal neurons of the hippocampus is largely unknown. To determine whether recurrent seizures centered in the hippocampus can produce neuronal loss in this region, a morphometric analysis was performed from standardized sections of hippocampus using 5 groups of animals: (1) surgical control subjects, (2) rats kindled by the rapidly recurring hippocampal seizure (RRHS) paradigm, (3) kindled rats with a few additional limbic seizures (528 +/- 66 seizures), (4) kindled rats with many limbic seizures (1,523 +/- 130 seizures), and (5) rats experiencing limbic status epilepticus (SE) induced by "continuous" hippocampal stimulation. The RRHS and SE protocols induced significant neuronal loss in the CA1 region, but no evidence was found for additional cell loss with increasing numbers of intermittent seizures. These intermittent seizures were, however, associated with a significant thickening of the basal and apical dendritic fields of the CA1 region. These findings indicate that intermittent seizures produce no significant hippocampal neuronal loss and may result in a hypertrophy of CA1 dendritic fields.

Self-sustaining limbic status epilepticus induced by 'continuous' hippocampal stimulation: electrographic and behavioral characteristics

A model of status epilepticus centered in the limbic system and elicited by 'continuous' focal electrical stimulation of the hippocampus is presented. Under appropriate conditions, the status epilepticus persisted for many hours after discontinuing the electrical stimulus. The critical determinant for the establishment of this self-sustaining limbic status epilepticus (SSLSE) was the length of stimulation, rather than the side (left vs. right) of stimulation or kindling before stimulation. Observations, obtained from stimulus-free intervals spaced regularly during the stimulus protocol and from the period after stimulation had been completed, revealed a distinct and stereotyped electrographic progression of SSLSE though several stages. Brief monitoring periods throughout the stimulus protocol yielded electrographic criteria that predicted which animals would experience experience SSLSE. The presence of synchronous, stimulus-independent seizure activity bilaterally in the hippocampi during stimulation was necessary to establish SSLSE. Intense motor seizure activity, like that seen with kindled motor seizures, occurred intermittently during SSLSE. However, 'limbic' behavioral seizures identical to those seen after low doses of kainic acid or during the early stages of kindling were nearly continuous. These studies indicate that there is a predictable course to limbic status epilepticus and point to the hippocampus as a key element involved in initiating and maintaining this syndrome.