Hippocampal extracellular amino acids and EEG spectral analysis in a genetic rat model of absence epilepsy

Atypical absence seizures and gene variants: A gene-based review of etiology, electro-clinical features, and associated epilepsy syndrome

Atypical absence seizures are generalized non-convulsive seizures that often occur in children with cognitive impairment. They are common in refractory epilepsy and have been recognized as one of the hallmarks of developmental epileptic encephalopathies. Notably, pathogenic variants associated with AAS, such as GABRG2, GABRG3, SLC6A1, CACNB4, SCN8A, and SYNGAP1, are also linked to developmental epileptic encephalopathies. Atypical absences differ from typical absences in that they are frequently drug-resistant and the prognosis is dependent on the etiology or related epileptic syndromes. To improve clinicians' understanding of atypical absences and provide novel perspectives for clinical treatment, we have reviewed the electro-clinical characteristics, etiologies, treatment, and prognosis of atypical absences, with a focus on the etiology of advancements in gene variants, shedding light on potential avenues for improved clinical management.

Pallidal GABA B receptors: involvement in cortex beta dynamics and thalamic reticular nucleus activity

The external globus pallidus (GP) firing rate synchronizes the basal ganglia-thalamus-cortex network controlling GABAergic output to different nuclei. In this context, two findings are significant: the activity and GABAergic transmission of the GP modulated by GABA B receptors and the presence of the GP-thalamic reticular nucleus (RTn) pathway, the functionality of which is unknown. The functional participation of GABA B receptors through this network in cortical dynamics is feasible because the RTn controls transmission between the thalamus and cortex. To analyze this hypothesis, we used single-unit recordings of RTn neurons and electroencephalograms of the motor cortex (MCx) before and after GP injection of the GABA B agonist baclofen and the antagonist saclofen in anesthetized rats. We found that GABA B agonists increase the spiking rate of the RTn and that this response decreases the spectral density of beta frequency bands in the MCx. Additionally, injections of GABA B antagonists decreased the firing activity of the RTn and reversed the effects in the power spectra of beta frequency bands in the MCx. Our results proved that the GP modulates cortical oscillation dynamics through the GP-RTn network via tonic modulation of RTn activity.

NETWORK ANALYSIS REVEALS A ROLE OF THE HIPPOCAMPUS IN ABSENCE SEIZURES: THE EFFECTS OF A CANNABINOID AGONIST

The role of the hippocampus (Hp) in absence epileptic networks and the effect of endocannabinoid system on this network remain enigmatic. Here, using adapted nonlinear Granger causality, we compared the differences in network strength in four intervals (baseline or interictal, preictal, ictal and postictal) in two hours before (Epoch 1) and six hours (epochs 2, 3 and 4) after the administration of three different doses of the endocannabinoid agonist WIN55,212-2 (WIN) or solvent. Local field potentials were recorded for eight hours in 23 WAG/Rij rats in the Frontal (FC), Parietal PC), Occipital Cortex (OC) and in the hippocampus (Hp). The four intervals were visually marked by an expert neurophysiologist and the strength of couplings between electrode pairs were calculated in both directions. Ictally, a strong decrease in coupling strength was found between Hp and FC, as well as a large increase bidirectionally between PC and FC and unidirectionally from FC and PC to OC, and from FC to Hp over all epochs. The highest dose of WIN increased the couplings strength from FC to Hp and from OC to PC during 4 and 2 hr respectively in all intervals, and decreased the FC to PC coupling strength postictally in epoch 2. A single rat showed generalized convulsive seizures after the highest dose: this rat shared not only coupling changes with the other rats in the same condition, but showed many more. WIN reduced SWD number in epoch 2 and 3, their mean duration increased in epochs 3 and 4. Conclusions:during SWDs FC and PC are strongly coupled and drive OC, while at the same time the influence of Hp to FC is diminished. The first is in agreement with the cortical focus theory, the latter demonstrates an involvement of the hippocampus in SWD occurrence and that ictally the hippocampal control of the cortico-thalamo-cortical system is lost. WIN causes dramatic network changes which have major consequences for the decrease of SWDs, the occurrence of convulsive seizures, and the normal cortico-cortical and cortico-hippocampal interactions.

Electron microscopic GABA evaluation in hippocampal mossy terminals of genetic absence epilepsy rats receiving kindling stimulations

Objective: The hypotheses related to the fact of epileptic mechanisms are mainly based on excitation-inhibition imbalance in central nervous system. GAERS (Genetic Absence Epilepsy Rats from Strasbourg) is a well-known animal model of absence epilepsy, and frequently used in experimental studies. In the present study, we aimed to examine possible morphological and gamma-aminobutyric acid (GABA) density changes in GAERS hippocampus after electrical kindling stimulations. Methods: All control and test group rats received 6 kindling stimulations. Rats were decapitated 1 h after the last stimulation. Ultrastructural GABA immunocytochemistry was used to evaluate GABA density quantitatively in mossy terminals of hippocampal CA3 region. Results: GABA levels were less in kindling groups compared to their controls, and in GAERS groups compared to Wistar groups; mitochondrial and dendritic spine area ratios were greater in GAERS groups compared to Wistar groups, although all these evaluations were statistically nonsignificant. Depletion of synaptic vesicles was evident in the mossy terminals of kindling groups. Conclusion: The reason of decreased levels of GABA found in the present study might be that GABA has been released from the synaptic pool rapidly at an early time period after the last stimulation, for compansation mechanisms. Depletion of synaptic vesicles observed in kindling groups shows that even 6 kindling stimulations have an impact of changing hippocampal morphology in trisynaptic cycle. The increased mitochondrial area in GAERS might be related to the increased mitochondrial activity. The increased dendritic spine area might be related to the increased performance of learning in GAERS. Our findings indicating that absence epilepsy and temporal lobe epilepsy have different mechanisms of epileptogenesis might be a basis for further experimental studies

Changes in corticocortical and corticohippocampal network during absence seizures in WAG/Rij rats revealed with time varying Granger causality

PURPOSE

Spike-and-wave discharges (SWDs) recorded in the cortical EEGs of WAG/Rij rats are the hallmark for absence epilepsy in this model. Although this type of epilepsy was long regarded as a form of primary generalized epilepsy, it is now recognized that there is an initiation zone - the perioral region of the somatosensory cortex. However, networks involved in spreading the seizure are not yet fully known. Previously, the dynamics of coupling between different layers of the perioral cortical region and between these zones and different thalamic nuclei was studied in time windows around the SWDs, using nonlinear Granger causality. The aim of the present study was to investigate, using the same method, the coupling dynamics between different regions of the cortex and between these regions and the hippocampus.

METHODS

Local field potentials were recorded in the frontal, parietal, and occipital cortices and in the hippocampus of 19 WAG/Rij rats. To detect changes in coupling reliably in a short time window, in order to provide a good temporal resolution, the innovative adapted time varying nonlinear Granger causality method was used. Mutual information function was calculated in addition to validate outcomes. Results of both approaches were tested for significance.

RESULTS

The SWD initiation process was revealed as an increase in intracortical interactions starting from 3.5s before the onset of electrographic seizure. The earliest preictal increase in coupling was directed from the frontal cortex to the parietal cortex. Then, the coupling became bidirectional, followed by the involvement of the occipital cortex (1.5s before SWD onset). There was no driving from any cortical region to hippocampus, but a slight increase in coupling from hippocampus to the frontoparietal cortex was observed just before SWD onset. After SWD onset, an abrupt drop in coupling in all studied pairs was observed. In most of the pairs, the decoupling rapidly disappeared, but driving force from hippocampus and occipital cortex to the frontoparietal cortex was reduced until the SWD termination.

CONCLUSION

Involvement of multiple cortical regions in SWD initiation shows the fundamental role of corticocortical feedback loops, forming coupling architecture and triggering the generalized seizure. The results add to the ultimate aim to construct a complete picture of brain interactions preceding and accompanying absence seizures in rats.

Increased inhibitory synaptic activity in the hippocampus (CA1) of genetic absence epilepsy rats: Relevance of kindling resistance

PURPOSE

Genetic absence epilepsy rats from Strasbourg (GAERS), a well-validated genetic rat model for typical absence epilepsy, are known to manifest a resistance to secondary generalization of abnormal focal electrical activity evoked by kindling. The mechanism of this resistance is still unclear. In order to understand the possible mechanism of kindling resistance, we investigated for the first time, the differences of short-term synaptic plasticity by using a paired-pulse paradigm as an indicator of GABAergic activity in CA1 region of hippocampus in GAERS and non-epileptic Wistar rats in-vivo.

METHODS

Rats were subjected to kindling process, basolateral amygdala was stimulated twice a day, with a supra-threshold current, until they displayed limbic or convulsive seizures. One hour after the last kindling stimulation, evoked field potentials from CA1 pyramidal layer of hippocampus were recorded in-vivo under urethane anesthesia.

RESULTS

In response to supra-threshold kindling stimulations GAERS showed a significantly delayed kindling progression and displayed a significant increase in hippocampal excitability at early stages of kindling that is the critical for the development of convulsive seizures. In control rats that were not received kindling stimulation, paired-pulse depression (PPD) was significantly pronounced in GAERS with respect to the Wistar group. During the kindling course, PPD was gradually reduced in the Wistar rats as kindling progression was advanced. However in GAERS, PPD ratios were not significantly changed at early stages of kindling. When GAERS reached convulsive stage, their PPD ratios became similar to that of Wistar rats.

DISCUSSION

The increased inhibition in paired-pulse responses at early stages of kindling in GAERS suggests the role of augmented GABAergic activity as one of the underlying mechanisms of kindling resistance observed in genetic rat models of absence epilepsy.

Levodopa-induced dyskinesias are associated with transient down-regulation of cAMP and cGMP in the caudate-putamen of hemiparkinsonian rats: reduced synthesis or increased catabolism?

Second messenger cAMP and cGMP represent a key step in the action of dopamine that modulates directly or indirectly their synthesis. We aimed to verify whether levodopa-induced dyskinesias are associated with changes of the time course of levodopa/dopamine stimulated cAMP and cGMP levels, and/or with changes of their catabolism by phosphodiesterase activity in rats with experimental hemiparkinsonism. Microdialysis and tissue homogenates of the striatal tissues demonstrated that extracellular and intracellular cAMP/cGMP levels were lower in dyskinetic animals during the increasing phase of dyskinesias compared to eukinetic animals, but cAMP/cGMP levels increased in dyskinetic animals during the phase of decreasing and extinction of dyskinesias. Dyskinesias and the abnormal lowering of striatal cGMP and cAMP after levodopa were prevented by pretreatment with the multipotent drug amantadine, outlining the inverse relationship of cAMP/cGMP to dyskinesias. Moreover, dyskinetic animals showed higher striatal hydrolyzing cGMP-phosphodiesterase but not hydrolyzing cAMP-phosphodiesterase activity, suggesting that low cGMP but not cAMP levels could be due to increased catabolism. However, expressions of isozyme phosphodiesterase-1B and -10A highly and specifically located in the basal ganglia were not changed after levodopa in dyskinetic and eukinetic animals: accordingly, selective inhibitors of phosphodiesterase-1B and -10A were ineffective on levodopa dyskinesias. Therefore, the isozyme(s) expressing higher cGMP-phosphodiesterase activity in the striatum of dyskinetic animal should be determined. These observations suggest that dopamine-mediated processes of synthesis and/or degradation of cAMP/cGMP could be acutely impaired in levodopa dyskinesias, opening new ways to understanding physiopathology and treatment.

Ultrastructural GABA immunocytochemistry in the mossy fiber terminals of Wistar and genetic absence epileptic rats receiving amygdaloid kindling stimulations

The existence of absence epilepsy and temporal lobe epilepsy in the same patient is not common in clinical practice. The reason why both types of seizures are rarely seen in the same patient is not well understood. Therefore, we aimed to investigate kindling in a well known model of human absence epilepsy, genetic absence epilepsy rats from Strasbourg (GAERS). In the present study, we analyzed whether the GABA content of GAERS that received kindling stimulations was altered in the hippocampal mossy fiber terminals compared to non-epileptic control (NEC) Wistar rats. For this purpose, we used an immunocytochemical technique at the ultrastructural level. Ultrathin sections were immunolabeled with anti-GABA antibody and transmission electron microscopy was used for the ultrastructural examination. The number of gold particles per nerve terminal was counted and the area of the nerve terminal was determined using NIH image analysis program. The GABA density was found to be higher in sham-operated GAERS than sham-operated Wistar rats. The density was increased in kindling Wistar group compared to sham-operated Wistar and kindling GAERS groups. No statistical difference was observed between sham-operated GAERS and kindling GAERS groups. The increase in GABA levels in stimulated Wistar rats may be a result of a protective mechanism. Furthermore, there may be strain differences between Wistar rats and GAERS and our findings addressing different epileptogenesis mechanisms in these strains might be a basis for future experimental studies.

The ketogenic diet has no effect on the expression of spike-and-wave discharges and nutrient transporters in genetic absence epilepsy rats from Strasbourg

The genetic absence epilepsy rat from Strasbourg is considered an isomorphic, predictive, and homologous model of typical childhood absence epilepsy. It is characterized by the expression of spike-and-wave discharges (SWDs) in the thalamus and cortex. The ketogenic diet (KD) is successfully used in humans and animals with various types of seizures, but was not effective in children with intractable atypical absence epilepsy. Here, we studied its potential impact on the occurrence of SWDs in genetic absence epilepsy rat from Strasbourg. Rats were fed the KD for 3 weeks during which they were regularly subjected to the electroencephalographic recording of SWDs. The KD did not influence the number and duration of SWDs despite a 15-22% decrease in plasma glucose levels and a large increase in beta-hydroxybutyrate levels. Likewise, the KD did not affect the level of expression of the blood-brain barrier glucose transporter GLUT1 or of the monocarboxylate transporters, MCT1 and MCT2. This report extends the observation in humans that the KD does not appear to show effectiveness in intractable atypical absence epilepsy to this model of typical childhood absence epilepsy which responds to specific antiepileptic drugs.

Spatiotemporal characteristics of astroglial death in the rat hippocampo-entorhinal complex following pilocarpine-induced status epilepticus

Recently we reported that astroglial loss and subsequent gliogenesis in the dentate gyrus play a role in epileptogenesis following pilocarpine-induced status epilepticus (SE). In the present study we investigated whether astroglial damages in the hippocampo-entorhinal complex following SE are relevant to pathological or electrophysiological properties of temporal lobe epilepsy. Astroglial loss/damage was observed in the entorhinal cortex and the CA1 region at 4 weeks and 8 weeks after SE, respectively. These astroglial responses in the hippocampo-entorhinal cortex were accompanied by hyperexcitability of the CA1 region (impairment of paired-pulse inhibition and increase in excitability ratio). Unlike the dentate gyrus and the entorhinal cortex, CA1 astroglial damage was protected by conventional anti-epileptic drugs. alpha-Aminoadipic acid (a specific astroglial toxin) infusion into the entorhinal cortex induced astroglial damage and changed the electrophysiological properties in the CA1 region. Astroglial regeneration in the dentate gyrus and the stratum oriens of the CA1 region was found to originate from gliogenesis, while that in the entorhinal cortex and stratum radiatum of the CA1 region originated from in situ proliferation. These findings suggest that regional specific astroglial death/regeneration patterns may play an important role in the pathogenesis of temporal lobe epilepsy.

Prevention of Glutamate Accumulation and Upregulation of Phospho-Akt may Account for Neuroprotection Afforded by Bergamot Essential Oil against Brain Injury Induced by Focal Cerebral Ischemia in Rat

The effects of bergamot essential oil (BEO; Citrus bergamia, Risso) on brain damage caused by permanent focal cerebral ischemia in rat were investigated. Administration of BEO (0.1-0.5 ml/kg but not 1 ml/kg, given intraperitoneally 1 h before occlusion of the middle cerebral artery, MCAo) significantly reduced infarct size after 24 h permanent MCAo. The most effective dose (0.5 ml/kg) resulted in a significant reduction of infarct extension throughout the brain, especially in the medial striatum and the motor cortex as revealed by TTC staining of tissue slices. Microdialysis experiments show that BEO (0.5 ml/kg) did not affect basal amino acid levels, whereas it significantly reduced excitatory amino acid, namely aspartate and glutamate, efflux in the frontoparietal cortex typically observed following MCAo. Western blotting experiments demonstrated that these early effects were associated, 24 h after permanent MCAo, to a significant increase in the phosphorylation and activity of the prosurvival kinase, Akt. Indeed, BEO significantly enhanced the phosphorylation of the deleterious downstream kinase, GSK-3beta, whose activity is negatively regulated via phosphorylation by Akt.

Effect of stage 2 kindling on local cerebral blood flow rates in rats with genetic absence epilepsy

PURPOSE

Genetic absence epilepsy rats from Strasbourg (GAERS) are resistant to the progression of kindling seizures. We studied local cerebral blood flow (LCBF) changes in brain regions involved in seizures in both GAERS and nonepileptic rats (NEC) to map the differences that may be related to the resistance to kindling.

METHODS

Electrodes were implanted in the amygdala of adult NEC and GAERS male rats, which were stimulated to reach stage 2. Quantitative autoradiographic measurements of LCBF were performed by the [(14)C]-iodoantipyrine ([(14)C]IAP) autoradiographic technique allowing the precise mapping of regional perfusion changes. LCBF rates were measured bilaterally in 43 brain regions. The tracer infusion lasted for 60 s and started at 15 s before seizure induction.

RESULTS

Rates of LCBF increased in stimulated GAERS and NEC groups compared to nonstimulated controls. The LCBF increase in stimulated GAERS was larger and more widespread than that observed in stimulated NEC. The LCBF increase in the somatosensory cortex, ventrobasal and anterior thalamic nuclei, hypothalamus, subthalamic nucleus, piriform, entorhinal and perirhinal cortex, amygdala, CA2 region of hippocampus, and substantia nigra was statistically significantly larger in stimulated GAERS compared to stimulated NEC rats.

CONCLUSION

The results show that more brain regions are activated by kindling stimulation in GAERS. This widespread activation in GAERS involves the somatosensory cortex and thalamus, which are both known to be involved in the expression of absence seizures as well as numerous limbic regions thought not to play a role in the expression of absence seizures, suggesting an interaction between corticothalamocortical and limbic circuitries.

The role of the inherited genetic background on the consequences of lithium-pilocarpine status epilepticus: study in Genetic Absence Epilepsy Rats from Strasbourg and Wistar audiogenic rats

The susceptibility of rats with genetically inherited epilepsy to the genesis and consequences of secondary temporal lobe epilepsy is unknown. Here, we induced lithium-pilocarpine status epilepticus (SE) in Genetic Absence Epilepsy Rats from Strasbourg (GAERS) or in Wistar audiogenic sensitive (AS) rats. Wistar AS needed less pilocarpine than GAERS and Non-Epileptic Rats (NERs) to develop SE. Sixty six, 40 and 5% of Wistar AS, GAERS and NERs, respectively, died within 24 h after SE. In GAERS, SE prevented the occurrence of absence seizures for 5 days. Thereafter a limited number of absence seizures with low amplitude and short duration were recorded. Wistar AS developed limbic epilepsy within 9 days after SE while GAERS and NERs needed 36-39 days to develop spontaneous motor seizures. Neuronal loss consecutive to SE was similar in the three strains and particularly marked in limbic forebrain and parahippocampal cortices. In conclusion, the development of focal limbic epilepsy in GAERS largely impairs the expression of absence seizures. The genetic background underlying the expression of audiogenic seizures sensitizes strongly the rats to a further insult and compromises their survival.

Anti-glutamatergic effect of riluzole: comparison with valproic acid

Riluzole, an anti-amyotrophic lateral sclerosis drug, known to decrease presynaptic glutamate release, is viewed as a candidate supplementary medication for epilepsy. In the present study, we compared the effects of riluzole and valproate (VPA) in the pilocarpine-induced limbic seizure model and in the gamma-hydroxybutyrate lactone (GBL)-induced absence seizure model. We applied immunohistochemical study for vesicular transporter 1 (VGLUT1) and extracellular recording in the rat dentate gyrus of both pilocarpine- and GBL-induced seizure models to measure effects of riluzole and VPA. Both VPA and riluzole treatments reduced VGLUT1 immunoreactivity. Riluzole treatment completely inhibited pre-ictal spikes and spike-wave discharges in the pilocarpine- and GBL-induced epilepsy models, whereas VPA partially inhibited these phenomena. In both seizure models, the anti-epileptic effects of VPA and riluzole are basically related to anti-glutamatergic (reducing field excitatory postsynaptic potential slope and excitability ratio), not GABAergic (paired-pulse inhibition) effect. Riluzole was more effective at reducing seizure activity in both epilepsy models than VPA. These results suggest that riluzole is a potential antiepileptic drug with activity against limbic seizure and absence seizure.

Typical versus atypical absence seizures: network mechanisms of the spread of paroxysms

PURPOSE

Typical absence seizures differ from atypical absence seizures in terms of semiology, EEG morphology, network circuitry, and cognitive outcome, yet have the same pharmacological profile. We have compared typical to atypical absence seizures, in terms of the recruitment of different brain areas. Our initial question was whether brain areas that do not display apparent paroxysmal discharges during typical absence seizures, are affected during the ictal event in terms of synchronized activity, by other, distant areas where seizure activity is evident. Because the spike-and-wave paroxysms in atypical absence seizures invade limbic areas, we then asked whether an alteration in inhibitory processes in hippocampi may be related to the spread seizure activity beyond thalamocortical networks, in atypical seizures.

METHODS

We used two models of absence seizures in rats: one of typical and the other of atypical absence seizures. We estimated phase synchronization, and evaluated inhibitory transmission using a paired-pulse paradigm.

RESULTS

In typical absence seizures, we observed an increase in synchronization between hippocampal recordings when spike-and-wave discharges occurred in the cortex and thalamus. This indicates that seizure activity in the thalamocortical circuitry enhances the propensity of limbic areas to synchronize, but is not sufficient to drive hippocampal circuitry into a full paroxysmal discharge. Lower paired-pulse depression was then found in hippocampus of rats that displayed atypical absence seizures.

CONCLUSIONS

These observations suggest that circuitries in brain areas that do not display apparent seizure activity become synchronized as seizures occur within thalamocortical circuitry, and that a weakened hippocampal inhibition may predispose to develop synchronization into full paroxysms during atypical absence seizures.

GABAB receptor blockade enhances theta and gamma rhythms in the hippocampus of behaving rats

The participation of GABA(B) receptors in hippocampal EEG generation was studied by intracerebroventricular (icv) and intracerebral infusions of GABA(B) receptor antagonist p-(3-aminopropyl)-p-diethoxymethyl-phosphinic acid (CGP35348) in freely behaving rats. During awake-immobility, icv CGP35348 induced a theta rhythm and increased gamma waves (30-100 Hz) in the hippocampus. The immobility theta peaked at 6-7 Hz and had a theta phase in CA1 stratum radiatum of approximately 160 degrees with reference to the theta at the alveus, when compared with approximately 130 degrees during walking. Immobility theta power peaks at 6-7 Hz was also found in normal rats, and it was detected in 27% of the EEG segments during immobility. Incidence of immobility theta increased to 87.5% after 480 nmol of CGP35348 icv. Muscarinic antagonist scopolamine (5 mg/kg, ip) suppressed the induction of immobility theta and the gamma power increase after icv CGP35348. CGP35348 icv did not significantly change the hippocampal theta power at 7-8 Hz during walking (theta fundamental), but it increased power at 12-15 Hz, at the second harmonic of theta. CGP35348 icv also increased 30-50 Hz gamma power during walking. Medial septal infusion of CGP35348 (12 nmol in 0.4 microl) increased the power and the frequency of the hippocampal theta second harmonic during walking, but did not increase gamma activity. Infusion of CGP35348 (8 nmol in 0.4 microl) in the hippocampus increased the local gamma activity at 30-100 Hz, but did not induce immobility theta or affect the walking theta rhythm. In conclusion, icv GABA(B) receptor blockade increased an atropine-sensitive input that generated an immobility theta rhythm, while GABA(B) receptor blockade of the medial septum increased atropine-resistant theta harmonics possibly generated by apical dendritic spikes. GABA(B) receptor blockade may enhance cognitive task performance by activating hippocampal theta and gamma rhythms in behaving rats.

The essential oil of bergamot enhances the levels of amino acid neurotransmitters in the hippocampus of rat: implication of monoterpene hydrocarbons

The effects of bergamot essential oil (BEO) on the release of amino acid neurotransmitters in rat hippocampus have been studied by in vivo microdialysis and by in vitro superfusion of isolated nerve terminals. Intraperitoneal administration of BEO (100microl/kg) significantly elevated the extracellular concentration of aspartate, glycine and taurine in a Ca(2+)-dependent manner. A dose-relation study generated a bell-shaped curve. When perfused into the hippocampus via the dialysis probe (20microl/20min), BEO produced a significant increase of extracellular aspartate, glycine, taurine as well as of GABA and glutamate. The augmentation of all amino acids was Ca(2+)-independent. Focally injected 1:1 diluted BEO preferentially caused extracellular increase of glutamate. Interestingly, this release appeared to be strictly Ca(2+)-dependent. BEO concentration-dependently enhanced the release of [(3)H]D-aspartate from superfused hippocampal synaptosomes. Similar results were obtained by monitoring the BEO-evoked release of endogenous glutamate. At relatively high concentrations, the BEO-induced [(3)H]d-aspartate release was almost entirely prevented by the glutamate transporter blocker dl-threo-beta-benzyloxyaspartic acid (DL-TBOA) and was Ca(2+)-independent. At relatively low concentrations the release of [(3)H]D-aspartate was only in part ( approximately 50%) DL-TBOA-sensitive and Ca(2+)-independent; the remaining portion of release was dependent on extracellular Ca(2+). Interestingly, the monoterpene hydrocarbon-free fraction of the essential oil appeared to be inactive while the bergapten-free fraction superimposed the releasing effect of BEO supporting the deduction that psoralens may not be implicated. To conclude, BEO contains into its volatile fraction still unidentified monoterpene hydrocarbons able to stimulate glutamate release by transporter reversal and/or by exocytosis, depending on the dose administered.

Cortical glutamate metabolism is enhanced in a genetic model of absence epilepsy

Disturbances in GABAergic and glutamatergic neurotransmission in the thalamocortical loop are involved in absence seizures. Here, we examined potential disturbances in metabolism and interactions between neurons and glia in 5-month-old genetic absence epilepsy rats from Strasbourg (GAERS) and nonepileptic rats (NER). Animals received [1-(13)C]glucose and [1,2-(13)C]acetate, the preferential substrates of neurons and astrocytes, respectively. Extracts from cerebral cortex, thalamus, and hippocampus were analyzed by (13)C nuclear magnetic resonance spectroscopy. Most changes were detected in the cortex. Pyruvate metabolism was enhanced as evidenced by increases of lactate, and labeled and unlabeled alanine. Neuronal mitochondrial metabolism was also enhanced as detected by elevated amounts of N-acetylaspartate and nicotinamide adenine dinucleotide as well as increased incorporation of label from [2-(13)C]acetyl CoA into glutamate, glutamine, and aspartate. Likewise, mitochondrial metabolism in astrocytes was increased. Changes in thalamus were restricted to increased concentration and labeling of glutamine. Changes in the hippocampus were similar to those in the cortex. This increase in glutamate-glutamine metabolism in cortical neurons and astrocytes accompanied by a decreased gamma aminobyturic acid level may lead to impaired thalamic filter function. Hence, reduced sensory input to cortex could allow the occurrence of spike-and-wave discharges in the thalamocortical loop. Increased glutamatergic output from the cortex to hippocampus may be the underlying cause of improved learning in GAERS.

The Effects of Ethosuximide on Amino Acids in Genetic Absence Epilepsy Rat Model

Genetic absence epilepsy rats from Strasbourg (GAERS), a selectively inbred strain of Wistar rats, has been validated as an experimental model for human absence epilepsy. In this model, systemic administration of ethosuximide (ETX) was shown to reduce the spike and wave discharges (SWD). In this study, gamma-aminobutyric acid (GABA) and L-glutamic acid levels in response to ETX injections (i.p., 100 mg/kg) were measured in the microdialysis samples collected from the ventrolateral thalamus (VLT) and the primary motor cortex (M1) area of Wistar rats and GAERS by using HPLC with fluorescent detection. Throughout the microdialysis procedure, continuous EEG recording was performed where ETX was shown to suppress the SWD activity. We demonstrated increased basal GABA levels in the M1 and VLT of GAERS, and ETX treatment did not produce any effect on higher GABA levels in the VLT, but suppressed the increased GABA levels significantly in the M1 of GAERS. All these findings denote the importance of corticothalamic circuitry and the role of increased GABA tonus in primary motor cortex and thalamus of GAERS. The primary motor cortex also seems to be involved in the SWD activity and ETX exerts, at least partially, its neurotransmitter effects through it.

Altered glutamate and GABA release within thalamocortical circuitry in metabotropic glutamate receptor 4 knockout mice

The metabotropic glutamate receptor 4 is highly expressed presynaptically on thalamocortical neurons that are involved in the pathogenesis of generalized absence seizures. Mutant mice devoid of metabotropic glutamate receptor 4 are completely resistant to absence seizures induced by low doses of GABA type A receptor antagonists. The purpose of this study was to test the hypothesis that there is altered glutamate and GABA release within thalamocortical circuitry in mice devoid of metabotropic glutamate receptor 4. Extracellular GABA and glutamate release were determined in ventrobasal thalamus, the nucleus reticularis thalami and laminae I-III, and IV-VI of cerebral cortex (laminae I-III of cerebral cortex, and laminae IV-VI of cerebral cortex) using in vivo microdialysis techniques on awake, free moving mice. A significant increase of both basal and K(+)-evoked glutamate release was detected in the ventrobasal thalamus, the nucleus reticularis thalami and laminae IV-VI of cerebral cortex of mice devoid of metabotropic glutamate receptor 4 mice. There also was a significant increase in both basal and K(+)-evoked GABA release in the mice devoid of metabotropic glutamate receptor 4, but a significant decrease of GABA release in laminae IV-VI of cerebral cortex. However, there was no alteration of either GABA or glutamate release in laminae I-III of cerebral cortex, cortical laminae that are not involved in absence seizures. These data indicate that deletion of the metabotropic glutamate receptor 4 gene results in a selective perturbation of glutamate and GABA release within the thalamocortical circuitry involved in the pathogenesis of absence seizures.

Glutamate and GABA immunocytochemical electron microscopy in the hippocampal dentate gyrus of normal and genetic absence epilepsy rats

It is generally accepted that absence epilepsy results from the impairment of GABAergic and glutamatergic neurotransmission. In particular, besides excessive GABA mediation within the thalamo-cortico-thalamic circuit in absence epilepsy, neuronal networks of the hippocampus have recently received attention. In the present study, we examined the density of glutamate and GABA neurotransmitter immunolabeling in the dentate gyrus of the hippocampus of genetic absence epilepsy rats from Strasbourg (GAERS) compared to the control group. GABA and glutamate were found to exist in synaptic vesicles of the mossy fiber terminals of the control and GAERS groups. The density of glutamate immunolabeling within the mossy fiber terminals in the hilar region of GAERS hippocampus was found to be significantly decreased compared to the control group. There was no difference in the density of immunolabeling within GABA nerve terminals between GAERS and control group. The findings of this study suggest that mechanisms underlying absence seizures in GAERS may also manifest themselves in other brain regions such as the hippocampus. The presence of GABA within synaptic vesicles of mossy fiber terminals, as revealed by high resolution ultrastructural immunocytochemistry, has provided additional evidence to the possible modulatory role of GABA on synaptic transmission between the mossy fiber and the target cell.

Chronic treatment with fluoxetine decreases seizure threshold in naïve but not in rats exposed to the learned helplessness paradigm: Correlation with the hippocampal glutamate release

The proconvulsive effect of the new generation of antidepressants remains controversial. The authors investigated in naïve rats the effect of chronic treatment with fluoxetine (FLX) on the convulsive threshold and on two parameters of the hippocampal glutamatergic neurotransmission: the in vitro glutamate release and the binding of [3H] MK801 to NMDA receptors. While the acute treatment with FLX provoked no change either in seizure susceptibility or in the glutamate release, the chronic treatment decreased the convulsive threshold in coincidence with an increment in the in vitro glutamate release. No significant effects on the binding of [3H] MK801 to NMDA receptors were found to be attributable to the FLX treatment. We also assessed the effect of the chronic treatment with FLX on the seizure threshold in rats exposed to an experimental model of depression, the learned helplessness paradigm (LH). While a decrease in the K+-stimulated glutamate release was observed in non treated LH animals, when they were chronically injected with FLX, no changes in the epileptic susceptibility and no increments in the glutamate release were found. Our results indicate that chronic treatment with FLX decreases the epileptic threshold in naïve but not in LH rats and that this effect correlates with the levels of the hippocampal glutamate release.

Hippocampal afterdischarges after GABA(B)-receptor blockade in the freely moving rat

PURPOSE

To determine whether hippocampal afterdischarges (ADs) and excitability changes were induced by gamma-aminobutyric acid (GABA)(B)-receptor blockade in adult, freely moving rats.

METHODS

A specific GABA(B)-receptor antagonist CGP35348, CGP55845A, or CGP55699A was injected intracerebroventricularly (i.c.v.), and EEGs and behaviors of rats were analyzed.

RESULTS

CGP35348 (56-110 microg, i.c.v.) induced afterdischarges (ADs) approximately 60% of the time, starting at the hippocampus or neocortex. Neocortical-onset ADs began with sporadic discharges and were <3 mV. Hippocampal-onset ADs were bilateral, >5 mV, and spread to the entorhinal cortex and amygdala, often ending in a rebound AD and accompanied with stereotypic jumping, forelimb clonus, and wet-dog shakes. The CGP35348-induced hippocampal AD had an onset frequency (5-9 Hz) that was higher than the electrically evoked AD (2-4 Hz). CGP35348 i.c.v. also increased the mean starting frequency of an electrically evoked hippocampal AD from 3.6 Hz to 5.3 Hz. Hippocampal gamma activity (25-80 Hz) increased up to twofold for 30 min after a hippocampal but not a neocortical AD. A single dose of CGP35348 induced repeated ADs of increasing duration. Paired-pulse inhibition of the evoked potentials in CA1, at interpulse interval of <100 ms, was decreased after but not before a hippocampal AD. CGP56999A (i.c.v.) gave results similar to those with CGP35348, whereas CGP55845A (i.c.v.) rarely induced ADs.

CONCLUSIONS

GABA(B)-receptor blockade increases seizure susceptibility by reducing AD threshold and increasing the frequency and spread of a hippocampal AD. Hippocampal excitability (based on paired-pulse test) and gamma activity increased after but not before a hippocampal AD.

Amygdala amino acid and monoamine levels in genetically Fast and Slow kindling rat strains during massed amygdala kindling: a microdialysis study

We investigated the neurochemistry of epileptic seizures in rats selectively bred to be seizure-prone (Fast) vs. seizure-resistant (Slow) to amygdala kindling. Microdialysis was used to measure levels of amino acids [glutamate, aspartate and gamma-aminobutyric acid (GABA)] and monoamines (noradrenaline, dopamine and serotonin) during 'massed' stimulation (MS) (every 6 min) of the ipsilateral amygdala for a total of 40 stimulation trials. Behavioral seizure profiles together with their afterdischarge thresholds (ADTs) and associated durations were assessed during the procedure, and subsequently were redetermined 1, 7 and 14 days later. Then normal 'daily' kindling commenced and continued until the animal reached the fully kindled state. During MS, several generalized seizures were triggered in Fast rats that were associated with long afterdischarge (AD) durations and intermittent periods of elevated thresholds, but in Slow rats, most stimulations were associated with stable ADTs and short ADs. Progressively increasing extracellular glutamate and decreasing GABA was observed in Fast rats during the MS, whereas Slow rats showed levels similar to baseline values. Levels of noradrenaline and dopamine, but not of serotonin, were also increased in both strains throughout the MS treatment. In Fast rats, a dramatic lengthening of AD durations occurred 7 and 14 days following MS, as well as subsequent strong positive transfer to daily kindling, all of which were not seen in Slow rats. Together, these results show that repeated, closely spaced stimulations of the amygdala can differentially alter excitatory and/or inhibitory transmitter levels in a seizure network, and that sensitivity to this manipulation is genetically determined.

The influence of gender on the aggravation of absence seizures by carbamazepine in the low-dose pentylenetetrazol rat model

OBJECTIVES

To determine whether carbamazepine (CBZ) aggravates absence seizures in the low-dose pentylenetetrazol (PTZ) rat model in both male and female animals, and investigate for gender differences.

METHODS

Inbred Sprague-Dawley rats were implanted with EEG electrodes. Seven days later PTZ (20 mg/kg, i.p.) was administered following pre-treatment with vehicle or CBZ (20 mg/kg, i.p.) and the occurrence of spike-and-wave discharges (SWDs) on the EEG quantified.

RESULTS

The cumulative SWD for 90-minute post-PTZ was higher in the CBZ versus vehicle pre-treatment arm for both female (mean 110 seconds vs. 62 seconds; P = 0.03) and male (mean 89 seconds vs. 60 seconds; P = 0.03) rats. The increase in SWD duration in the CBZ arm was greater in female rats for the first five 15-minute intervals, but none attained statistical significance (P > 0.05). CBZ pre-treatment resulted in reductions in both SWD frequency (Hz) (male, P = 0.003; female, P < 0.0001) and latency to onset of SWD (male, P = 0.002). The frequency of SWD in CBZ pre-treated rats was lower in females (5.8 Hz vs. 6.1 Hz, P = 0.002) as was the decrease in the SWD burst duration following CBZ versus vehicle pre-treatment (-0.05 seconds vs. -0.25 seconds, P = 0.046).

CONCLUSIONS

CBZ consistently aggravates absence seizures in the low-dose PTZ model in both female and male rats. However, while some gender differences were found, the results failed to support the hypothesis that females are significantly more susceptible to aggravation of the number or duration of absence seizures by CBZ.

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.

Changes in rectal temperature and ECoG spectral power of sensorimotor cortex elicited in conscious rabbits by i.c.v. injection of GABA, GABA(A) and GABA(B) agonists and antagonists

1. In order to ascertain whether both GABA(A) and GABA(B), or only GABA(B) receptors, directly modulate thermoregulation in conscious rabbits, GABA(A)/GABA(B) agonist and antagonist agents were injected intracerebroventricularly in conscious rabbits while monitoring changes in rectal temperature (RT), gross motor behaviour (GMB) and electrocorticogram (ECoG) power spectra (ps) from sensorimotor cortices. 2. GABA (48 micromol), nipecotic acid (50 nmol), THIP (60 nmol), muscimol (18 nmol) and baclofen (8 nmol) induced hypothermia (-deltaRTmax values of 1.70+/-0.1, 1.4+/-0.2, 1.0+/-0.4, 1.1+/-0.2 and 1.6+/-0.3 degrees C, respectively), accompanied by inhibition of GMB and ECoG synchronization. THIP increased ps at delta frequency band (1.1-3.3 Hz), while GABA, nipecotic acid, muscimol and baclofen did the same at both delta and (4.6-6.5 Hz) frequency bands. ECoG ps changes were concomitant or even preceded hypothermia. 3. Bicuculline (1.8 nmol) induced hyperthermia (deltaRTmax 1.2+/-0.5 degrees C) and slight excitation of GMB, while CGP35348 (1.2 micromol) did not affect RT nor GMB. Both compounds did not affect ECoG ps. 4. Bicuculline potentiated muscimol-induced hypothermia, inhibition of GMB and synchronization of ECoG, while CGP35348 fully antagonized these effects. 5. In conclusion, the present results, while confirming the prevailing role of GABA(B), also outline a direct involvement of GABA(A) receptors in the central mechanisms of thermoregulation. Ascending inhibition towards discrete cortical areas controlling muscular activity and thermogenesis may result from GABA receptor activation in neurones proximal to the ventricles, thus contributing to hypothermia, although hypothermia-induced reduction of neuronal activity of these cortical areas cannot be ruled out.

Amino acid composition of brain cysts: levels of excitatory amino acids in cyst fluid fail to predict seizures

A recent study describing two epileptic patients with brain cysts has suggested that elevated concentrations of excitatory amino acids in cysts may play a role in induction and maintenance of epileptogenesis [Epilepsy Res. 28 (1997) 245]. Here, we report that only in 3 out of 22 patients with brain cysts undergoing brain surgery cyst fluids displayed highly increased amounts of the excitatory amino acids aspartate and/or glutamate. Two of these patients experienced epileptic seizures prior to neurosurgical intervention. Thus, highly increased excitatory amino acid levels are present only in a subset of patients with brain cysts. Our observation that one patient with highly increased glutamate and aspartate concentrations in the cyst did not display seizures or typical epileptiform potentials in the EEG questions that these excitatory amino acids in the cyst fluid are directly involved in epileptogenicity. This patient displayed an increased level of adenosine in the cyst fluid, which is known to have anticonvulsant properties and might provide protection from seizures. In summary, there is no evidence for a close correlation between excitatory amino acids in brain cysts and the occurrence of seizures.

Pilocarpine-induced seizure-like activity with increased BNDF and neuropeptide Y expression in organotypic hippocampal slice cultures

Organotypic hippocampal slice cultures were treated with the muscarinic agonist pilocarpine to study induced seizure-like activity and changes in neurotrophin and neuropeptide expression. For establishment of a seizure-inducing protocol, 2-week-old cultures derived from 6-8-day-old rats were exposed to 0.1 mM to 5 mM of pilocarpine for 4 h to 7 days. Other cultures were treated with pilocarpine for 7 days and left for 7-14 days in normal medium. Age-matched, non-treated cultures served as controls. Intracellular recordings from CA1 pyramidal cells revealed increased spontaneous activity in 31 of 35 cultures superfused with 0.1 or 5 mM pilocarpine. Epileptiform discharges were recorded in 17 of the 31 cultures, and 19 displayed frequencies specifically in the 6-12-Hz (Theta rhythm) range when superfused with pilocarpine. The pilocarpine effect was blocked by simultaneous superfusion with the muscarinic receptor antagonist atropine (100 microM). Regardless of dose and exposure time, the pilocarpine treatment induced very limited neuronal cell death, recorded as cellular propidium iodide uptake. Cultures exposed to 5 mM pilocarpine for up to 7 days displayed increased BDNF expression when analyzed by Western blot and ELISA. This BDNF increase correlated with increased neuropeptide Y immunoreactivity, known to accompany seizure activity. Addition of BDNF (200 ng/ml) to otherwise untreated cultures also upregulated NPY expression. The pilocarpine-induced seizure-like activity in hippocampal slice cultures, with concomitant increase in BDNF and NPY expression, is compared with in vivo observations and discussed in terms of the potential use of the easily accessible slice cultures in experimental seizure research.

Use of epileptic animals for adverse effect testing

A crucial parameter deciding the clinical utility of new antiepileptic drugs (AEDs) is the therapeutic index expressing the margin between anticonvulsant and adverse effects. The latter is commonly quantified during preclinical testing in the rotarod test in normal, healthy rodents. However, the validity of using normal animals for adverse effect predictions in epilepsy patients is questionable. Limbic kindling of rodents induced by corneal kindling of mice and amygdala kindling of rats confirm that epileptic animals are more susceptible to the behavioral and cognitive alterations following acute administration of NMDA antagonists and certain established AEDs. This appears to represent a permanent reactivity specific for limbic kindling since it is absent in rats after chemical kindling with pentylenetetrazole. Animal species with inborn epilepsy, including audiogenic and photosensitive animals, are not revealing an enhanced susceptibility to the behavioral alterations induced by NMDA antagonists. In contrast, these induce severe adverse effects in genetic absence epilepsy rats where certain AEDs also are associated with a more marked deterioration of motor function than in normal animals. This appears in line with several complications with AED use in man being linked to an interaction with the dysfunction of the brain imposed by the epileptic condition. Thus, it is important to involve epileptic animals in preclinical adverse effect testing, in particular when evaluating new AED candidates with novel or unknown mechanisms. In that respect, limbic kindling appears to represent a sensitive and relevant approach.