Epileptiform activity induced by 4-aminopyridine in entorhinal cortex hippocampal slices of rats with a genetically determined absence epilepsy (GAERS)

Repeated application of 4-aminopyridine provoke an increase in entorhinal cortex excitability and rearrange AMPA and kainate receptors

Entorhinal cortex is a highly epilepsy-prone brain region. Effects of repetitive seizures on ionotropic glutamate receptors (iGluRs) were investigated in rat entorhinal cortex slices. Seizures were induced by daily administration of 4-aminopyridine (4-AP). Electrophysiological, pharmacological and histological investigations were carried out to determine changes in synaptic efficacy and in sensitivity of iGluRs due to recurring seizures. Repeated 4-AP-induced seizures increased the amplitude of evoked synaptic field responses in rat entorhinal cortical slices. While vulnerability to inhibition of AMPA receptors by the specific antagonist GYKI 52466 was slightly reduced, responsiveness to NMDA receptor antagonist APV remained unaffected. Testing of bivalent cation permeability of iGluRs revealed reduced Ca(2+)-influx through non-NMDA receptors. According to the semi-quantitative histoblot analysis GluA1-4, GluA1, GluA2, GluK5, GluN1 and GluN2A subunit protein expression differently altered. While there was a marked decrease in the level of GluA1-4, GluA2 and GluK5 receptor subunits, GluA1 and GluN2A protein levels moderately increased. The results indicate that brief convulsions, repeated daily for 10 days can increase overall entorhinal cortex excitability despite a reduction in AMPA/kainate receptor activity, probably through the alteration of local network susceptibility.

Ethosuximide modifies network excitability in the rat entorhinal cortex via an increase in GABA release

Ethosuximide is the drug of choice for treating generalized absence seizures, but its mechanism of action is still a matter of debate. It has long been thought to act by disrupting a thalamic focus via blockade of T-type channels and, thus, generation of spike-wave activity in thalamocortical pathways. However, there is now good evidence that generalized absence seizures may be initiated at a cortical focus and that ethosuximide may target this focus. In the present study we have looked at the effect ethosuximide on glutamate and GABA release at synapses in the rat entorhinal cortex in vitro, using two experimental approaches. Whole-cell patch-clamp studies revealed an increase in spontaneous GABA release by ethosuximide concurrent with no change in glutamate release. This was reflected in studies that estimated global background inhibition and excitation from intracellularly recorded membrane potential fluctuations, where there was a substantial rise in the ratio of network inhibition to excitation, and a concurrent decrease in excitability of neurones embedded in this network. These studies suggest that, in addition to well-characterised effects on ion channels, ethosuximide may directly elevate synaptic inhibition in the cortex and that this could contribute to its anti-absence effects. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Compensatory network alterations upon onset of epilepsy in synapsin triple knock-out mice

Adult synapsin triple-knockout mice exhibit epilepsy that manifests as generalized tonic-clonic seizures. Because in vitro recordings have shown a reduction in quantal release from inhibitory neurons, an inherent excitation-inhibition imbalance has been hypothesized as the direct culprit for epilepsy in these mice. We critically assessed this hypothesis by examining neurotransmission during the emergence of epilepsy. Using long-term video and telemetric EEG monitoring we found that synapsin triple-knockout mice exhibit an abrupt transition during early adulthood from a seizure-free presymptomatic latent state to a consistent symptomatic state of sensory-induced seizures. Electrophysiological recordings showed that during the latent period larger field responses could be elicited in slices from mutant mice. However, only after the transition to a symptomatic state in the adult mice did evoked epileptiform activity become prevalent. This state was characterized by resistance to the epileptiform-promoting effects of 4-aminopyridine, by marked hypersensitivity to blockage of GABAA receptors, and by the emergence of unresponsiveness to NMDA receptor antagonism, all of which were not observed during the latent period. Importantly, enhancement in inhibitory transmission was associated with upregulation of GAD67 expression without affecting the number of inhibitory neurons in the same brain areas where epileptiform activity was recorded. We therefore suggest that while deletion of the synapsins initially increases cortical network activity, this enhanced excitability is insufficient to elicit seizures. Rather, compensatory epileptogenic mechanisms are activated during the latent period that lead to an additional almost-balanced enhancement of both the excitatory and inhibitory components of the network, finally culminating in the emergence of epilepsy.

Variations of dopamine, serotonin, and amino acid concentrations in Noda epileptic rat (NER) retina

Noda epileptic rats (NER) exhibit frequent spontaneous tonic-clonic convulsions which represent a valuable model of human epilepsy. If implication of brain neurotransmitters was largely reported, little is known about retina. However, it has been reported that human epilepsy syndrome varies not only with the location of seizure foci but also according to rhythmic patterns, for which retina has a major role in the transmission of external light-dark cycle information. The purpose of this work was to evaluate dopamine (DA), DA metabolites, serotonin (5-HT), and amino acid [glutamate, aspartate, glycine, gamma aminobutyric acid (GABA), and taurine] level variations in retina from NER, at two different nycthemeral periods (11 a.m. and 11 p.m.) and at different ages (2, 6, and 12 months). In NER, retinal dopaminergic function was decreased as soon as 2 months, whereas GABA levels were increased, even if no differences among the different ages could be distinguished. These variations were associated to a slight increase in 5-HT. Other amino acids tested were not affected by epilepsy, whereas taurine decreased with aging in NER as well as in control rats. Retinal 5-HT occurs principally as a precursor of melatonin (MEL). A triangular interaction may be hypothesized: MEL could decrease DA synthesis or release by enhancing GABA activity. Taken together, these results suggest that the retinal physiology is affected by the epileptic status and that information transmitted from retina to the brain should be affected by epilepsy in NER.

Electrophysiologic changes in the lateral and basal amygdaloid nuclei in temporal lobe epilepsy: an in vitro study in epileptic rats

The functional consequences of neuronal loss during epileptogenesis in the lateral and basal amygdaloid nuclei are poorly understood. The present study tested the hypothesis that electrical responsiveness varies in different amygdaloid nuclei in the chronically epileptic amygdala. Further, we examined the amygdaloid region most prone to seizure initiation. Epileptogenesis was triggered in 20 rats by inducing status epilepticus (SE) with electrical stimulation of the lateral nucleus of the amygdala. Electrode-implanted non-stimulated rats served as controls. The occurrence and duration of spontaneous seizures were monitored with video-electroencephalography (EEG) at 8-9 weeks after SE. Thereafter, animals were killed and extracellular recordings were made from slices of both amygdalas. In the lateral nucleus of epileptic animals, the frequency of spontaneous responses was reduced compared with controls (P < 0.05). The amplitudes of evoked field responses were reduced (P < 0.01), whereas paired pulse (PP) facilitation was enhanced (P < or = 0.05). In the basal nucleus of the epileptic animals, PP facilitation was enhanced (P < 0.05) and sensitivity to 4-aminopyridine (4-AP)-induced epileptiform activity was increased compared with controls (P < 0.05). In the epileptic animals, the basal nucleus was also more sensitive than the lateral nucleus to 4-AP-induced epileptiform activity (P < 0.05). Correlation analysis indicated that longer SE duration was associated with longer half widths (P = 0.001) and smaller slopes (P < 0.05) of evoked responses as well as with attenuated PP facilitation (P<0.01). Moreover, a higher frequency of spontaneous seizures was associated with longer half widths (P < 0.05) and smaller slopes (P < 0.05) of evoked responses as well as with enhanced PP facilitation (P < 0.05). These data suggest that there is a reduced release of glutamate and reduced inhibition in the lateral and basal amygdaloid nuclei in epileptic animals. Further, the basal nucleus is more prone to epileptic activity than the lateral nucleus. Finally, the severity of SE and spontaneous seizures in vivo is associated with electrophysiologic alterations in vitro.

Immunocytochemical analysis of glutamate and GABA in hippocampus of genetic absence epilepsy rats (GAERS)

In the present study, we used an immunocytochemical technique at the electron microscopic level to determine if there are changes in the glutamate and GABA neurotransmitter content of the hippocampus of genetic absence epilepsy rats from Strasbourg (GAERS). We also investigated if there was mossy fiber reorganization. After perfusion fixation, brains were removed and cryostat sections were stained according to the neo-Timm's procedure. High-resolution electron microscopy was used for ultrastructural examination of the hippocampus of GAERS and non-epileptic control Wistar animals. For ultrastructural and immunocytochemical studies, ultrathin-cut sections were obtained and immunolabeled with anti-glutamate and anti-GABA antibodies. The number of gold particles per nerve terminal was counted and the area of the nerve terminal was determined using the program NIH Image Analysis. No mossy fiber sprouting was detected in the hippocampus of GAERS. GABA and glutamate immunoreactivity were observed in the mossy fiber terminals of both the control and GAERS groups. Glutamate density in the CA3 region of GAERS hippocampus was found to be significantly increased compared to the control group. However, there was no difference in the GABA density of nerve terminals and in areas of GABAergic and mossy terminals between GAERS and the control group. The difference in glutamate level may merely be due to strain differences between the GAERS strain and the original Wistar strain or it is also possible that it appears after seizures have started.

Involvement of Ca(2+) channels in abnormal excitability of hippocampal CA3 pyramidal cells in noda epileptic rats

Noda epileptic rat (NER) is a mutant rat, which spontaneously exhibits a tonic-clonic convulsion from 14 weeks of age. An intracellular recording study was performed to elucidate the abnormal excitability of NER hippocampal CA3 neurons. The recorded neurons were classified into two groups, group A and B neurons, according to the responses to a single stimulation of mossy fibers. In group A neurons, a stimulus elicited a long-lasting depolarization shift accompanying repetitive firings followed by after-hyperpolarization. In group B neurons, the same stimulus elicited a single spike without a long-lasting depolarization shift. Bath application of 1 mM Cd(2+), a nonselective Ca(2+) channel blocker, completely inhibited the abnormal excitation in group A neurons. We further examined the character of Ca(2+) spikes in NER CA3 neurons. Ca(2+) spikes were completely blocked by 10 microM Cd(2+) in group A neurons, but not in either group B or control neurons, suggesting that Ca(2+) channels in NER group A neurons have the hypersensitivity to Cd(2+). Analysis using subtype specific blockers of Ca(2+) channel raised the possible involvement of T-type Ca(2+) channels. These results suggest that Ca(2+) channel dysfunction is involved in the abnormal excitability of CA3 pyramidal neurons and pathogenesis of epilepsy in NER.

Epileptiform burst discharges in hippocampal CA3 neurons of young but not mature Noda epileptic rats (NER)

Noda epileptic rat (NER), originally found in a colony of Crj; Wistar rats, shows spontaneous tonic-clonic convulsion characterized by the appearance of high voltage polyspikes in cortical and hippocampal EEG once every 2-3 days after 2-4 months of age. Electrophysiological studies using hippocampal slice preparations of NER were performed to determine whether hippocampal neurons have abnormal excitability. When a single stimulus (1-25 V) was delivered to the mossy fibers of NER at 4-6 weeks old before they showed any seizures, a long-lasting depolarization shift (DS) accompanied by repetitive firings and after-hyperpolarization following the abnormal firing was observed in seven of 14 hippocampal CA3 neurons. A lower stimulation intensity evoked DS and abnormal firing in three of nine CA3 neurons of NER at 10-15 weeks old which had already showed seizures at 10-15 weeks of age. However, the abnormal firing was not observed in any 10 neurons of the animals at more than 20 weeks old nor in Wistar rats. The input impedances of CA3 neurons in NER with abnormal firing were lower than those without abnormal firing and those in Wistar rats. The abnormal excitability obtained in NER at an age when it did not display any seizures suggests that the hippocampus may play a role in epileptogenicity in NER.

Paired pulse facilitation is turned into paired pulse depression in hippocampal slices after epilepsy induced by 4-aminopyridine in vivo

Modifications in synaptic plasticity seem to play a key role in the origin and persistence of epilepsy. 4-Aminopyridine (4-AP) induces intense and long lasting epileptic seizures and neurodegeneration when applied into the hippocampus in vivo, effects that seem to be mediated by overactivation of glutamate receptors due to the enhancement of glutamate release from nerve endings. We have studied presynaptic modifications of CA1 responses, using the paired pulse paradigm, in hippocampal slices obtained from 4-AP-treated rats killed during epileptic activity (ex vivo). The paired pulse facilitation (PPF) observed in control slices with interstimulus intervals of 10-30 ms was changed into paired pulse depression (PPD) after 100 microM 4-AP added in vitro. A strikingly similar change was observed in the ex vivo slices even though 4-AP was no longer present in the tissue. We conclude that the facilitation of glutamate release induced by 4-AP becomes chronic after a transient exposure to the drug. This suggests that the facilitated neurotransmitter release induced by 4-AP triggers a more permanent plastic change that may be responsible for the persistence of epilepsy.

Spike-and-wave discharges of absence seizures as a transformation of sleep spindles: the continuing development of a hypothesis

OBJECTIVES

This review aims to offer a critical account of recent scientific developments relevant to the hypothesis which Pierre Gloor proposed in the 1970s for the generation of spike and wave discharges (SWDs) of primary generalized absence seizures.

RESULTS

According to this hypothesis SWDs develop in the same circuits, which normally generate sleep spindles, by an initially cortical transformation of one every two or more spindle waves to a 'spike' component of SWDs, while the next one or more spindle waves are eliminated and replaced by a slow negative wave. This hypothesis was based on experiments in feline generalized penicillin epilepsy showing the possibility of transition from spindles to SWDs, when cortical neurons become hyper-responsive to thalamocortical volleys, which normally induce spindles, and thus engage feedback cortical inhibition, rebound excitation, recurrent intracortical dissemination of excitation during the 'spike' and strong excitation of thalamus for further augmentation of a brain wide synchronous oscillation. In the 1980s, electrophysiological studies in vitro and in vivo revealed the basic features of spindle rhythm generation by neurons in nucleus reticularis thalami and thalamocortical-corticothalamic oscillatory reverberations.

CONCLUSIONS

In the light of this knowledge, experimental studies in several genetic and pharmacological animal models of absence seizures, clinical observations and theoretical studies in computer models have considered, tested, modified and challenged this hypothesis. It may still be found useful in the era of dynamic digital EEG analysis of SWDs and its current sources.