Involvement of nigral glutamatergic inputs in the control of seizures in a genetic model of absence epilepsy in the rat

Molecular Mechanisms Underlying the Generation of Absence Seizures: Identification of Potential Targets for Therapeutic Intervention

Understanding the molecular mechanisms underlying the generation of absence seizures is crucial for developing effective, patient-specific treatments for childhood absence epilepsy (CAE). Currently, one-third of patients remain refractive to the antiseizure medications (ASMs), previously called antiepileptic drugs (AEDs), available to treat CAE. Additionally, these ASMs often produce serious side effects and can even exacerbate symptoms in some patients. Determining the precise cellular and molecular mechanisms directly responsible for causing this type of epilepsy has proven challenging as they appear to be complex and multifactorial in patients with different genetic backgrounds. Aberrant neuronal activity in CAE may be caused by several mechanisms that are not fully understood. Thus, dissecting the causal factors that could be targeted in the development of precision medicines without side effects remains a high priority and the ultimate goal in this field of epilepsy research. The aim of this review is to highlight our current understanding of potential causative mechanisms for absence seizure generation, based on the latest research using cutting-edge technologies. This information will be important for identifying potential targets for future therapeutic intervention.

Chronic intermittent convection-enhanced delivery of vigabatrin to the bilateral subthalamic nucleus in an acute rat seizure model

Targeted intracerebral drug delivery is an attractive experimental approach for the treatment of drug-resistant epilepsies. In this regard, the subthalamic nucleus (STN) represents a focus-independent target involved in the remote modulation and propagation of seizure activity. Indeed, acute and chronic pharmacological inhibition of the STN with vigabatrin (VGB), an irreversible inhibitor of GABA transaminase, has been shown to produce antiseizure effects. This effect, however, is lost over time as tolerance develops with chronic, continuous intracerebral pharmacotherapy. Here we investigated the antiseizure effects of chronic intermittent intra-STN convection-enhanced delivery of VGB in an acute rat seizure model focusing on circumventing tolerance development and preventing adverse effects. Timed intravenous pentylenetetrazol (PTZ) seizure threshold testing was conducted before and after implantation of subcutaneous drug pumps and bilateral intra-STN cannulas. Drug pumps infused vehicle or VGB twice daily (0.4 µg) or once weekly (2.5 µg, 5 µg) over three weeks. Putative adverse effects were evaluated and found to be prevented by intermittent compared to previous continuous VGB delivery. Clonic seizure thresholds were more clearly raised by intra-STN VGB compared to myoclonic twitch. Both twice daily and once weekly intra-STN VGB significantly elevated clonic seizure thresholds depending on dose and time point, with responder rates of up to 100% observed at tolerable doses. However, tolerance could not be completely avoided, as tolerance rates of 40-75% were observed with chronic VGB treatment. Results indicate that the extent of tolerance development after intermittent intra-STN VGB delivery varies depending on infusion dose and regimen.

Long-lasting antiseizure effects of chronic intrasubthalamic convection-enhanced delivery of valproate

Intracerebral drug delivery is an experimental approach for the treatment of drug-resistant epilepsies that allows for pharmacological intervention in targeted brain regions. Previous studies have shown that targeted pharmacological inhibition of the subthalamic nucleus (STN) via modulators of the GABAergic system produces antiseizure effects. However, with chronic treatment, antiseizure effects are lost as tolerance develops. Here, we report that chronic intrasubthalamic microinfusion of valproate (VPA), an antiseizure medication known for its wide range of mechanisms of action, can produce long-lasting antiseizure effects over three weeks in rats. In the intravenous pentylenetetrazole seizure-threshold test, seizure thresholds were determined before and during chronic VPA application (480 μg/d, 720 μg/d, 960 μg/d) to the bilateral STN. Results indicate a dose-dependent variation in VPA-induced antiseizure effects with mean increases in seizure threshold of up to 33%, and individual increases of up to 150%. The lowest VPA dose showed a complete lack of tolerance development with long-lasting antiseizure effects. Behavioral testing with all doses revealed few, acceptable adverse effects. VPA concentrations were high in STN and low in plasma and liver. In vitro electrophysiology with bath applied VPA revealed a reduction in spontaneous firing rate, increased background membrane potential, decreased input resistance and a significant reduction in peak NMDA, but not AMPA, receptor currents in STN neurons. Our results suggest an advantage of VPA over purely GABAergic modulators in preventing tolerance development with chronic intrasubthalamic drug delivery and provide first mechanistic insights in intracerebral pharmacotherapy targeting the STN.

Acute and chronic convection-enhanced muscimol delivery into the rat subthalamic nucleus induces antiseizure effects associated with high responder rates

Intracerebral drug delivery is an emerging treatment strategy aiming to manage seizures in patients with systemic drug-resistant epilepsies. In rat seizure and epilepsy models, the GABA_A receptor agonist muscimol has shown powerful antiseizure potential when injected acutely into the subthalamic nucleus (STN), known for its capacity to provide remote control of different seizure types. However, chronic intrasubthalamic muscimol delivery required for long-term seizure suppression has not yet been investigated. We tested the hypothesis that chronic convection-enhanced delivery (CED) of muscimol into the STN produces long-lasting antiseizure effects in the intravenous pentylenetetrazole seizure threshold test in female rats. Acute microinjection was included to verify efficacy of intrasubthalamic muscimol delivery in this seizure model and caused significant antiseizure effects at 30 and 60 ng per hemisphere with a dose-dependent increase of responders and efficacy and only mild adverse effects compared to controls. For the chronic study, muscimol was bilaterally infused into the STN over three weeks at daily doses of 60, 300, or 600 ng per hemisphere using an implantable pump and cannula system. Chronic intrasubthalamic CED of muscimol caused significant long-lasting antiseizure effects for up to three weeks at 300 and 600 ng daily. Drug responder rate increased dose-dependently, as did drug tolerance rates. Transient ataxia and body weight loss were the main adverse effects. Drug distribution was comparable (about 2-3 mm) between acute and chronic delivery. This is the first study providing proof-of-concept that not only acute, but also chronic, continuous CED of muscimol into the STN raises seizure thresholds.

Not Part of the Temporal Lobe, but Still of Importance? Substantia Nigra and Subthalamic Nucleus in Epilepsy

The most researched brain region in epilepsy research is the temporal lobe, and more specifically, the hippocampus. However, numerous other brain regions play a pivotal role in seizure circuitry and secondary generalization of epileptic activity: The substantia nigra pars reticulata (SNr) and its direct input structure, the subthalamic nucleus (STN), are considered seizure gating nuclei. There is ample evidence that direct inhibition of the SNr is capable of suppressing various seizure types in experimental models. Similarly, inhibition via its monosynaptic glutamatergic input, the STN, can decrease seizure susceptibility as well. This review will focus on therapeutic interventions such as electrical stimulation and targeted drug delivery to SNr and STN in human patients and experimental animal models of epilepsy, highlighting the opportunities for overcoming pharmacoresistance in epilepsy by investigating these promising target structures.

Bypassing the Blood-Brain Barrier: Direct Intracranial Drug Delivery in Epilepsies

Epilepsies are common chronic neurological diseases characterized by recurrent unprovoked seizures of central origin. The mainstay of treatment involves symptomatic suppression of seizures with systemically applied antiseizure drugs (ASDs). Systemic pharmacotherapies for epilepsies are facing two main challenges. First, adverse effects from (often life-long) systemic drug treatment are common, and second, about one-third of patients with epilepsy have seizures refractory to systemic pharmacotherapy. Especially the drug resistance in epilepsies remains an unmet clinical need despite the recent introduction of new ASDs. Apart from other hypotheses, epilepsy-induced alterations of the blood-brain barrier (BBB) are thought to prevent ASDs from entering the brain parenchyma in necessary amounts, thereby being involved in causing drug-resistant epilepsy. Although an invasive procedure, bypassing the BBB by targeted intracranial drug delivery is an attractive approach to circumvent BBB-associated drug resistance mechanisms and to lower the risk of systemic and neurologic adverse effects. Additionally, it offers the possibility of reaching higher local drug concentrations in appropriate target regions while minimizing them in other brain or peripheral areas, as well as using otherwise toxic drugs not suitable for systemic administration. In our review, we give an overview of experimental and clinical studies conducted on direct intracranial drug delivery in epilepsies. We also discuss challenges associated with intracranial pharmacotherapy for epilepsies.

Clinical and experimental insight into pathophysiology, comorbidity and therapy of absence seizures

Absence seizures in children and teenagers are generally considered relatively benign because of their non-convulsive nature and the large incidence of remittance in early adulthood. Recent studies, however, show that 30% of children with absence seizures are pharmaco-resistant and 60% are affected by severe neuropsychiatric comorbid conditions, including impairments in attention, cognition, memory and mood. In particular, attention deficits can be detected before the epilepsy diagnosis, may persist even when seizures are pharmacologically controlled and are aggravated by valproic acid monotherapy. New functional MRI-magnetoencephalography and functional MRI-EEG studies provide conclusive evidence that changes in blood oxygenation level-dependent signal amplitude and frequency in children with absence seizures can be detected in specific cortical networks at least 1 min before the start of a seizure, spike-wave discharges are not generalized at seizure onset and abnormal cortical network states remain during interictal periods. From a neurobiological perspective, recent electrical recordings and imaging of large neuronal ensembles with single-cell resolution in non-anaesthetized models show that, in contrast to the predominant opinion, cortical mechanisms, rather than an exclusively thalamic rhythmogenesis, are key in driving seizure ictogenesis and determining spike-wave frequency. Though synchronous ictal firing characterizes cortical and thalamic activity at the population level, individual cortico-thalamic and thalamocortical neurons are sparsely recruited to successive seizures and consecutive paroxysmal cycles within a seizure. New evidence strengthens previous findings on the essential role for basal ganglia networks in absence seizures, in particular the ictal increase in firing of substantia nigra GABAergic neurons. Thus, a key feature of thalamic ictogenesis is the powerful increase in the inhibition of thalamocortical neurons that originates at least from two sources, substantia nigra and thalamic reticular nucleus. This undoubtedly provides a major contribution to the ictal decrease in total firing and the ictal increase of T-type calcium channel-mediated burst firing of thalamocortical neurons, though the latter is not essential for seizure expression. Moreover, in some children and animal models with absence seizures, the ictal increase in thalamic inhibition is enhanced by the loss-of-function of the astrocytic GABA transporter GAT-1 that does not necessarily derive from a mutation in its gene. Together, these novel clinical and experimental findings bring about paradigm-shifting views of our understanding of absence seizures and demand careful choice of initial monotherapy and continuous neuropsychiatric evaluation of affected children. These issues are discussed here to focus future clinical and experimental research and help to identify novel therapeutic targets for treating both absence seizures and their comorbidities.

Impaired cortico-striatal excitatory transmission triggers epilepsy

STXBP1 and SCN2A gene mutations are observed in patients with epilepsies, although the circuit basis remains elusive. Here, we show that mice with haplodeficiency for these genes exhibit absence seizures with spike-and-wave discharges (SWDs) initiated by reduced cortical excitatory transmission into the striatum. Mice deficient for Stxbp1 or Scn2a in cortico-striatal but not cortico-thalamic neurons reproduce SWDs. In Stxbp1 haplodeficient mice, there is a reduction in excitatory transmission from the neocortex to striatal fast-spiking interneurons (FSIs). FSI activity transiently decreases at SWD onset, and pharmacological potentiation of AMPA receptors in the striatum but not in the thalamus suppresses SWDs. Furthermore, in wild-type mice, pharmacological inhibition of cortico-striatal FSI excitatory transmission triggers absence and convulsive seizures in a dose-dependent manner. These findings suggest that impaired cortico-striatal excitatory transmission is a plausible mechanism that triggers epilepsy in Stxbp1 and Scn2a haplodeficient mice.

Spike and wave discharge (SWD) activity is seen during absence seizures and is thought to be thalamocortical in origin. Here, the authors show that SWDs are initiated through the impaired corticostriatal excitatory transmissions onto striatal fast spiking interneurons.

The role of the basal ganglia in the control of seizure

Epilepsy is a network disorder and each type of seizure involves distinct cortical and subcortical network, differently implicated in the control and propagation of the ictal activity. The role of the basal ganglia has been revealed in several cases of focal and generalized seizures. Here, we review the data that show the implication of the basal ganglia in absence, temporal lobe, and neocortical seizures in animal models (rodent, cat, and non-human primate) and in human. Based on these results and the advancement of deep brain stimulation for Parkinson's disease, basal ganglia neuromodulation has been tested with some success that can be equally seen as promising or disappointing. The effect of deep brain stimulation can be considered promising with a 76% in seizure reduction in temporal lobe epilepsy patients, but also disappointing, since only few patients have become seizure free and the antiepileptic effects have been highly variable among patients. This variability could probably be explained by the heterogeneity among the patients included in these clinical studies. To illustrate the importance of specific network identification, electrophysiological activity of the putamen and caudate nucleus has been recorded during penicillin-induced pre-frontal and motor seizures in one monkey. While an increase of the firing rate was found in putamen and caudate nucleus during pre-frontal seizures, only the activity of the putamen cells was increased during motor seizures. These preliminary results demonstrate the implication of the basal ganglia in two types of neocortical seizures and the necessity of studying the network to identify the important nodes implicated in the propagation and control of each type of seizure.

Evaluation of GAD67 immunoreactivity in the region of substantia nigra pars reticulata in resistance to development of convulsive seizure in genetic absence epilepsy rats

OBJECTIVE:

Nonconvulsive absence epilepsy and convulsive epilepsy seizures are rarely seen in the same patient. It has been demonstrated that there is a resistance to development of convulsive seizures in genetic absence epilepsy models. The present study investigated glutamic acid decarboxylase (GAD) immunoreactivity in the brain region related to the interaction of these two seizure types, namely substantia nigra pars reticulata (SNR) subregions, SNR_anterior and SNR_posterior.

METHODS:

Nonepileptic adult male Wistar rats and Genetic Absence Epilepsy Rats from Strasbourg (GAERS) were used. Experimental groups of Wistar and GAERS were electrically stimulated for kindling model to induce convulsive epileptic seizures. An electrical stimulation cannula was stereotaxically implanted to the basolateral amygdala and recording electrodes were placed on the cortex. Sagittal sections of SNR were used to evaluate immunohistochemical reaction. Sections were incubated with anti-GAD67 antibody. Densitometric analysis of GAD67 immunoreactive neurons was performed using photographs of stained sections. One-way analysis of variance and post hoc Bonferroni test were used for statistical analysis of the data.

RESULTS:

There was no difference in GAD67 immunoreactivity of SNR subregions of control Wistar and control GAERS. An increase in GAD67 immunoreactivity was detected in SNR_posterior subregion of stimulated Wistar rats, whereas there was a decrease in GAD67 immunoreactivity in SNR_posterior of stimulated GAERS. The difference in GAD67 immunoreactivity between these two groups was statistically significant.

CONCLUSION:

Level of synthetized gamma-aminobutyric acid in SNR_posterior subregion plays an important role in the interaction of nonconvulsive absence epilepsy seizures and convulsive epilepsy seizures.

Anticonvulsant Effects by Bilateral and Unilateral Transplantation of GABA-Producing Cells into the Subthalamic Nucleus in an Acute Seizure Model

Neural transplantation of GABA-producing cells into key structures within seizure-suppressing circuits holds promise for medication-resistant epilepsy patients not eligible for resection of the epileptic focus. The substantia nigra pars reticulata (SNr), a basal ganglia output structure, is well known to modulate different seizure types. A recent microinjection study by our group indicated that the subthalamic nucleus (STN), which critically regulates nigral activity, might be a more promising target for focal therapy in epilepsies than the SNr. As a proof of principle, we therefore assessed the anticonvulsant efficacy of bilateral and unilateral allografting of GABA-producing cell lines into the STN using the timed intravenous pentylenetetrazole seizure threshold test, which allows repeated seizure threshold determinations in individual rats. We observed (a) that grafted cells survived up to the end of the experiments, (b) that anticonvulsant effects can be induced by bilateral transplantation into the STN using immortalized GABAergic cells derived from the rat embryonic striatum and cells additionally transfected to obtain higher GABA synthesis than the parent cell line, and (c) that anticonvulsant effects were observed even after unilateral transplantation into the STN. Neither grafting of control cells nor transplantation outside the STN induced anticonvulsant effects, emphasizing the site and cell specificity of the observed anticonvulsant effects. To our knowledge, the present study is the first showing anticonvulsant effects by grafting of GABA-producing cells into the STN. The STN can be considered a highly promising target region for modulation of seizure circuits and, moreover, has the advantage of being clinically established for functional neurosurgery.

The subcortical hidden side of focal motor seizures: evidence from micro-recordings and local field potentials

Focal motor seizures are characterized by transient motor behaviour that occurs simultaneously with paroxystic activity in the controlateral motor cortex. The implication of the basal ganglia has already been shown for generalized seizure but the propagation pathways from the motor cortex towards the basal ganglia during focal motor seizures are largely unknown. With a better knowledge of those pathways, a therapeutic modulation for reducing drug resistant motor epilepsy could be considered. Here, we recorded single-unit activities and local field potentials in the basal ganglia of two Macaca fascicularis in which acute focal motor seizures were induced by the injection of penicillin over the arm motor cortex territory. Each neuron was characterized using its mean firing rate and its type of firing pattern during interictal periods and seizures. Time-frequency analyses of local field potentials and electroencephalographic signals were used to assess dynamic changes occurring during seizure at a larger spatial level. The firing rate of neurons of input stages of basal ganglia (subthalamic nucleus and putamen) and those from the external part of the globus pallidus were significantly higher during seizures as compared to interictal periods. During seizures, the proportion of oscillatory neurons in subthalamic nucleus (71%), external globus pallidus (45%) and putamen (53%) significantly increased in comparison to interictal periods. Rhythmic activity was synchronized with ictal cortical spikes in external globus pallidus and subthalamic nucleus, but not in the putamen which oscillated faster than motor cortex. In contrast, no significant modification of the firing rate of the output stages of basal ganglia (internal part of the globus pallidus, substantia nigra pars reticulata) could be found during seizures. The local field potentials of subthalamic nucleus and external globus pallidus changed abruptly at the onset of the seizure, showing synchronization with the cortical activity throughout the seizure. In putamen, the synchronization appeared only by the end of seizures and for the two output structures, despite some increase of the oscillatory activity, the synchronization with the cortex was not significant. Our results suggest that the subthalamo-(external)-pallidal pathway is the main subcortical route involved during ictal motor seizures. Surprisingly, ictal activity did not propagate to the output structure of basal ganglia in that model. This finding may be important for clinical decisions of targeting when considering anti-epileptic neuromodulation in human beings suffering from disabling, drug resistant motor epilepsy.

Neural adaptation to responsive stimulation: a comparison of auditory and deep brain stimulation in a rat model of absence epilepsy

BACKGROUND

Responsive deep brain stimulation (rDBS) has been recently proposed to block epileptic seizures at onset. Yet, long-term stability of brain responses to such kind of stimulation is not known.

OBJECTIVE

To quantify the neural adaptation to repeated rDBS as measured by the changes of anti-epileptic efficacy of bilateral DBS of the substantia nigra pars reticulata (SNr) versus auditory stimulation, in a rat model of spontaneous recurrent absence seizures (GAERS).

METHODS

Local field potentials (LFP) were recorded in freely moving animals during 1 h up to 24 h under automated responsive stimulations (SNr-DBS and auditory). Comparison of seizure features was used to characterise transient (repetition-suppression effect) and long-lasting (stability of anti-epileptic efficacy, i.e. ratio of successfully interrupted seizures) effects of responsive stimulations.

RESULTS

SNr-DBS was more efficient than auditory stimulation in blocking seizures (97% vs. 52% of seizures interrupted, respectively). Sensitivity to minimal interstimulus interval was much stronger for SNr-DBS than for auditory stimulation. Anti-epileptic efficacy of SNr-DBS was remarkably stable during long-term (24 h) recordings.

CONCLUSIONS

In the GAERS model, we demonstrated the superiority of SNr-DBS to suppress seizures, as compared to auditory stimulation. Importantly, we found no long-term habituation to rDBS. However, when seizure recurrence was frequent, rDBS lack anti-epileptic efficacy because responsive stimulations became too close (time interval < 40 s) suggesting the existence of a refractory period. This study thus motivates the use of automated rDBS in patients having transient seizures separated by sufficiently long intervals.

Roles of the subthalamic nucleus and subthalamic HCN channels in absence seizures

Absence seizures consist of a brief and sudden impairment of consciousness. They are characterized by bilaterally synchronized spike and wave discharges (SWDs), which reflect abnormal oscillations in the thalamocortical loops. Recent studies have suggested that the basal ganglia are involved in generation of the SWDs, but their roles are poorly understood at the molecular and cellular levels. Here we studied the pathophysiological roles of the basal ganglia, using in vivo and in vitro measurements of tottering mice, a well-established model of absence epilepsy. We found that the membrane excitability in subthalamic nucleus (STN) neurons was enhanced in tottering mice, which resulted from reduced hyperpolarization-activated cyclic nucleotide-gated (HCN) channel activity. Pharmacological blockade and activation of HCN channel activity in vitro bidirectionally altered the membrane excitability of the STN neurons. Furthermore, these pharmacological modulations of HCN channel activity in the STN in vivo bidirectionally altered the mean SWD duration. In addition, STN deep brain stimulation modulated SWDs in a frequency-dependent manner. These results indicate that STN is involved in the rhythm maintenance system of absence seizures.

GABAergic neuron deficit as an idiopathic generalized epilepsy mechanism: the role of BRD2 haploinsufficiency in juvenile myoclonic epilepsy

Idiopathic generalized epilepsy (IGE) syndromes represent about 30% of all epilepsies. They have strong, but elusive, genetic components and sex-specific seizure expression. Multiple linkage and population association studies have connected the bromodomain-containing gene BRD2 to forms of IGE. In mice, a null mutation at the homologous Brd2 locus results in embryonic lethality while heterozygous Brd2+/− mice are viable and overtly normal. However, using the flurothyl model, we now show, that compared to the Brd2+/+ littermates, Brd2+/− males have a decreased clonic, and females a decreased tonic-clonic, seizure threshold. Additionally, long-term EEG/video recordings captured spontaneous seizures in three out of five recorded Brd2+/− female mice. Anatomical analysis of specific regions of the brain further revealed significant differences in Brd2+/− vs +/+ mice. Specifically, there were decreases in the numbers of GABAergic (parvalbumin- or GAD67-immunopositive) neurons along the basal ganglia pathway, i.e., in the neocortex and striatum of Brd2+/− mice, compared to Brd2+/+ mice. There were also fewer GABAergic neurons in the substantia nigra reticulata (SNR), yet there was a minor, possibly compensatory increase in the GABA producing enzyme GAD67 in these SNR cells. Further, GAD67 expression in the superior colliculus and ventral medial thalamic nucleus, the main SNR outputs, was significantly decreased in Brd2+/− mice, further supporting GABA downregulation. Our data show that the non-channel-encoding, developmentally critical Brd2 gene is associated with i) sex-specific increases in seizure susceptibility, ii) the development of spontaneous seizures, and iii) seizure-related anatomical changes in the GABA system, supporting BRD2's involvement in human IGE.

Involvement of the thalamic parafascicular nucleus in mesial temporal lobe epilepsy

Mesial temporal lobe epilepsy (MTLE) is characterized by focal seizures, associated with hippocampal sclerosis, and often resistance to antiepileptic drugs. The parafascicular nucleus (PF) of the thalamus is involved in the generation of physiological oscillatory rhythms. It receives excitatory inputs from the cortex and inhibitory inputs from the basal ganglia, a system implicated in the control of epileptic seizures. The aim of this study was to examine the involvement of the PF in the occurrence of hippocampal paroxysmal discharges (HPDs) in a chronic animal model of MTLE in male mice. We recorded the local field potential (LFP) and the extracellular and intracellular activity of hippocampal and PF neurons during spontaneous HPDs in vivo. The end of the HPDs was concomitant with a slow repolarization in hippocampal neurons leading to an electrical silence. In contrast, it was associated in the PF with a transient increase in the power of the 10-20 Hz band in LFPs and a depolarization of PF neurons resulting in a sustained firing. We tested the role of the PF in the control of HPDs by single 130 Hz electrical stimulation of this nucleus and bilateral intra-PF injection of NMDA and GABA(A) antagonist and agonist. High-frequency PF stimulation interrupted ongoing HPDs at an intensity devoid of behavioral effects. NMDA antagonist and GABA(A) agonist suppressed hippocampal discharges in a dose-dependent way, whereas NMDA agonist and GABA(A) antagonist increased HPDs. Altogether, these data suggest that the PF nucleus plays a role in the modulation of MTLE seizures.

Lack of effect of intranigral transplants of a GABAergic cell line on absence seizures

The substantia nigra pars reticulata (SNpr) is involved in controlling a variety of seizure phenomena. Intranigral transplants of GABAergic cells have been shown to decrease the severity of already established epileptic seizures, but the effects observed have been short-lived. This study evaluated the ability of intranigral transplants of GABA-producing cells to reduce spontaneous absence seizures in a genetic animal model for periods up to 3 months after transplantation. Intranigral transplants did not induce any behavioral deficits in the animals, and they did not form tumors; however, the transplants failed to decrease absence seizures in the genetic model. The assumed increase in intranigral levels of GABA after the transplants may be insufficient to counteract all the factors involved in generating the absence seizures; in this animal model, it may be necessary to further decrease nigral activity by implanting GABAergic cells in another area. These results bear down on the fact that cell transplants need to be tailored for each type of convulsive disorder in terms of the type of cells delivered and the location of the transplants.

Controlling seizures is not controlling epilepsy: A parametric study of deep brain stimulation for epilepsy

Pharmacological inhibition and high-frequency stimulation (HFS) of the substantia nigra pars reticulata (SNr) suppress seizures in different animal models of epilepsy. The aim of the present study was to determine the optimal parameters of HFS to control spontaneous seizures in a genetic model of absence epilepsy in the rat. Single SNr stimulation that was bilateral, bipolar and monophasic at 60 Hz frequency and with 60-micros pulse width was optimal. However, when used for repeated stimulations, long-term suppression did not occur and even the number of seizures increased. A delay of at least 60 s between stimulations was necessary to be fully effective. Although single HFS of the SNr can be used to suppress ongoing seizures, repeated HFS is ineffective and could even aggravate seizures in our model. Thus investigations of accurate stimulation procedures are still needed.

Treatment of late lithium-pilocarpine-induced status epilepticus with diazepam

PURPOSE

It is well known that status epilepticus (SE) becomes increasingly difficult to control over time. Previous studies have indicated that the electroencephalographic pattern at the time of intervention is predictive of the probability of successful treatment. However, these findings are based on studies limited to the first 2h of SE onset. Little data is available on the efficacy of treating SE at later time points.

METHODS

The aim of the present study was to investigate the efficacy of diazepam (DZP) treatment given at two different phases of SE in a lithium-pilocarpine rat model: during continuous ictal discharges (CIDs, phase 3), and during late periodic epileptiform discharges (late PEDs, phase 5). Changes in cortical and hippocampal electroencephalographs (EEGs) were observed continuously during the phases of SE, as well as at 24, 36, 48, and 72h after SE onset. The effects of DZP treatment during CIDs or during late PEDs were compared to control DZP-untreated rats.

RESULTS

In all three groups, hippocampal and cortical EEGs displayed five distinct phases of SE. There were no statistical differences in the duration of phases 1 and 2 among the three groups. Although DZP administration during CIDs did not terminate CIDs in most rats, it did significantly shorten the duration of phases 3 and 4 of SE. Importantly, DZP given during phase 5 successfully ended behavioral and electrographic seizures in most rats.

CONCLUSIONS

Hippocampal and cortical EEGs displayed five distinct phases of SE that were similarly responsive to DZP treatment. Termination of electrographic seizures with DZP treatment was more effective in the last phase of SE (late PEDs) than in phase 3 (CIDs). These findings suggest that previous reports of DZPs decrease in efficacy over time may not be applicable to DZP treatment at 4h-post onset.

Activity of ventral medial thalamic neurons during absence seizures and modulation of cortical paroxysms by the nigrothalamic pathway

Absence seizures are characterized by bilaterally synchronous spike-and-wave discharges (SWDs) in the electroencephalogram, which reflect abnormal oscillations in corticothalamic networks. Although it was suggested that basal ganglia could modulate, via their feedback circuits to the cerebral cortex, the occurrence of SWDs, the cellular and network mechanisms underlying such a subcortical control of absence seizures remain unknown. The GABAergic projections from substantia nigra pars reticulata (SNR) to thalamocortical neurons of the ventral medial (VM) thalamic nucleus provide a potent network for the control of absence seizures by basal ganglia. The present in vivo study provides the first description of the activity of VM thalamic neurons during seizures in the genetic absence epilepsy rats from Strasbourg, a well established model of absence epilepsy. Cortical paroxysms were accompanied in VM thalamic neurons by rhythmic bursts of action potentials. Pharmacological blockade of excitatory inputs of nigrothalamic neurons led to a transient interruption of SWDs, correlated with a change in the activity of thalamic cells, which was increased in frequency and converted into a sustained arrhythmic firing pattern. Simultaneously, cortical neurons exhibited a decrease in their firing rate that was associated with an increase in membrane polarization and a decrease in input resistance. These new findings demonstrate that an inhibition of SNR neurons changes the activity of their thalamic targets, which in turn could affect cortical neurons excitability and, consequently, the generation of cortical epileptic discharges. Thus, the nigro-thalamo-cortical pathway may provide an on-line system control of absence seizures.

Deep brain stimulation of the subthalamic nucleus as adjunct treatment for refractory epilepsy

PURPOSE

We studied the efficacy and safety of bilateral subthalamic deep brain stimulation (DBS) for refractory partial-onset epilepsy in two cases.

METHODS

This was an open treatment pilot study for subjects who had failed numerous medications and had seizure injuries. Seizure counts and adverse events were collected during a 3-4 month baseline, and for 26-32 months after DBS surgery, with AEDs held constant.

RESULTS

Case 1, age 45, with bitemporal seizures, had about half the seizure frequency but still fell with injuries. Case 2, age 46, with left frontal encephalomalacia, had a frequency reduction of about one-third, but a more meaningful reduction of seizure severity and injuries.

CONCLUSIONS

Subthalamic DBS partly reduced partial-onset seizures, but the quality of life was more affected by seizure-related injuries.

Experimental manipulations of the subthalamic nucleus fail to suppress tonic seizures in the electroshock model of epilepsy

Recently, it has been shown that the subthalamic nucleus (STN) has anticonvulsant effects on epileptic seizures originating from the forebrain. The aim of the present study was to determine whether the anticonvulsant properties of the STN extend to the suppression of tonic seizures originating from the brainstem elicited by electroshock in rats. Three different procedures were used to manipulate activity in the STN and in each case the duration of tonic hindlimb extension elicited by electroshock was used as a measure of seizure-severity. Under general anesthesia, two groups of rats received chronic implants of either bilateral stainless steel guide cannulae or bilateral bipolar stimulating electrodes stereotaxically implanted and aimed at the STN. After 3 days of recovery, each rat in the first group was tested with electroshock on three consecutive days after having received 220 nl bilateral microinjections into the STN of either 200 or 400 pmol of muscimol (a GABA agonist) dissolved in saline or the same volume of normal saline. In the second group the electroshock test was conducted, again on three consecutive days, immediately following high frequency electrical stimulation (HFS) of the STN at 130 or 260 Hz or a no current control condition. In the third group, rats were tested with electroshock before and after bilateral excitotoxic lesions of the STN with either kainic or ibotenic acids. None of these manipulations produced significant suppression of the tonic hind limb extension elicited by electroshock compared with the relevant control conditions. This suggests that, within the limitations of the current procedures, the anticonvulsant properties of the STN appear to be ineffective against tonic seizures originating in the brainstem.

Rhythmic bursting in the cortico-subthalamo-pallidal network during spontaneous genetically determined spike and wave discharges

Absence seizures are characterized by impairment of consciousness associated with bilaterally synchronous spike-and-wave discharges (SWDs) in the electroencephalogram (EEG), which reflect paroxysmal oscillations in thalamocortical networks. Although recent studies suggest that the subthalamic nucleus (STN) provides an endogenous control system that influences the occurrence of absence seizures, the mechanisms of propagation of cortical epileptic discharges in the STN have never been explored. The present study provides the first description of the electrophysiological activity in the cortico-subthalamo-pallidal network during absence seizures in the genetic absence epilepsy rats from Strasbourg, a well established model of absence epilepsy. In corticosubthalamic neurons, the SWDs were associated with repetitive suprathreshold depolarizations correlated with EEG spikes. These cortical paroxysms were reflected in the STN by synchronized, rhythmic, high-frequency bursts of action potentials. Intracellular recordings revealed that the intraburst pattern in STN neurons was sculpted by an early depolarizing synaptic potential, followed by a short hyperpolarization and a rebound of excitation. The rhythmic hyperpolarizations in STN neurons during SWDs likely originate from a subpopulation of pallidal neurons exhibiting rhythmic bursting temporally correlated with the EEG spikes. The repetitive discharges in STN neurons accompanying absence seizures might convey powerful excitation to basal ganglia output nuclei and, consequently, may participate in the control of thalamocortical SWDs.

Modifications of local cerebral glucose utilization in thalamic structures following injection of a dopaminergic agonist in the nucleus accumbens—involvement in antiepileptic effects?

Dopaminergic transmission in the nucleus accumbens (NAcc) is implicated in different aspects of reward and motivational mechanisms. More recently, it has been suggested that this nucleus could also be involved in the modulation of generalized epileptic seizures. In particular, microinjection of dopaminergic agonists in the NAcc suppresses the occurrence of epileptic seizures in a model of absence seizures, the GAERS (generalized absence epileptic rats from Strasbourg). The aim of this study was to identify the structures involved in this effect. Local cerebral metabolic rates for glucose utilization (LCMRglc) were measured in different parts of the basal ganglia and output structures after apomorphine injection in the NAcc in GAERS and in the inbred non-epileptic rats (NE), concomitantly with seizure suppression. Apomorphine injection in the NAcc induced a significant increase of glucose intake in the anteromedial, mediodorsal and ventrolateral nuclei of the thalamus in NE rats, while no significant changes were observed in the basal ganglia structures (globus pallidus, subthalamic nucleus, substantia nigra). Furthermore, microinjections of muscimol (100 and 200 pmol/side) in the mediodorsal nucleus of the thalamus in GAERS rats suppressed seizures. These results suggest that the mediodorsal nucleus of the thalamus could be involved in absence seizures modulation. Along with data from the literature, our data suggest that this nucleus could participate in the control of the basal ganglia over generalized epileptic seizures.

Emergent properties of CNS neuronal networks as targets for pharmacology: application to anticonvulsant drug action

CNS drugs may act by modifying the emergent properties of complex CNS neuronal networks. Emergent properties are network characteristics that are not predictably based on properties of individual member neurons. Neuronal membership within networks is controlled by several mechanisms, including burst firing, gap junctions, endogenous and exogenous neuroactive substances, extracellular ions, temperature, interneuron activity, astrocytic integration and external stimuli. The effects of many CNS drugs in vivo may critically involve actions on specific brain loci, but this selectivity may be absent when the same neurons are isolated from the network in vitro where emergent properties are lost. Audiogenic seizures (AGS) qualify as an emergent CNS property, since in AGS the acoustic stimulus evokes a non-linear output (motor convulsion), but the identical stimulus evokes minimal behavioral changes normally. The hierarchical neuronal network, subserving AGS in rodents is initiated in inferior colliculus (IC) and progresses to deep layers of superior colliculus (DLSC), pontine reticular formation (PRF) and periaqueductal gray (PAG) in genetic and ethanol withdrawal-induced AGS. In blocking AGS, certain anticonvulsants reduce IC neuronal firing, while other agents act primarily on neurons in other AGS network sites. However, the NMDA receptor channel blocker, MK-801, does not depress neuronal firing in any network site despite potently blocking AGS. Recent findings indicate that MK-801 actually enhances firing in substantia nigra reticulata (SNR) neurons in vivo but not in vitro. Thus, the MK-801-induced firing increases in SNR neurons observed in vivo may involve an indirect effect via disinhibition, involving an action on the emergent properties of this seizure network.

Single-unit Analysis of Substantia Nigra Pars Reticulata Neurons in Freely Behaving Rats with Genetic Absence Epilepsy

PURPOSE

The substantia nigra pars reticulata (SNpr) is assumed to be involved in the control of several kinds of epileptic seizures, an assumption based mostly on neuropharmacologic evidence. However, only very few neurophysiological recordings from the basal ganglia support neuropharmacologic data. We investigated the electrophysiologic activity of SNpr neurons in rats with genetic absence epilepsy.

METHODS

Electrocorticography (ECoG) and multi-unit recordings using permanently implanted tetrodes were obtained in freely behaving rats. After spike sorting, auto- and cross-correlation analysis was used to detect oscillatory neuronal activities and synchronizations.

RESULTS

During interictal periods, neither oscillation nor synchronization could be observed in the firing patterns of SNpr neurons. At the beginning of the absence seizure, the firing rate increased significantly. The SNpr neurons started firing in bursts of action potentials. Bursts were highly correlated to the spike-and-wave discharges (SWDs) in the ECoG, mainly after the spike component of the cortical spike-and-wave complex. Moreover, pairs of SNpr neurons tended to fire synchronously. Before the end of the seizure, the firing rate decreased progressively, and the burst-firing pattern ended at or before the end of the SWDs. Once the SWDs had stopped, the SNpr neurons resumed their basal firing pattern as before the seizure onset.

CONCLUSIONS

These results provide electrophysiologic evidence that firing patterns and synchronization of SNpr neurons are in phase with the occurrence of SWDs. The findings support the concept that nigral control mechanisms are involved in modulating the propagation of an ongoing generalized seizure.

The effect of electrical stimulation of the subthalamic nucleus on seizures is frequency dependent

PURPOSE

Animal studies and anecdotal human case reports have indicated that the subthalamic nucleus (STN) may be a site of anticonvulsant action.

METHODS

We tested the hypothesis that continuous electrical stimulation of the STN inhibits seizures acutely. We determined the effects of three stimulation frequencies, 130 Hz, 260 Hz, and 800 Hz, on generalized clonic and tonic-clonic flurothyl seizures. Adult male rats were implanted with concentric bipolar stimulating electrodes in the STN bilaterally. After recovery, rats underwent flurothyl seizures to compare the effects of each stimulation frequency on seizure threshold. Rats were tested 4 times, twice in the stimulated condition, and twice in the unstimulated condition. The order of trials was random, except that stimulation trials alternated with control trials. Flurothyl seizure thresholds under each stimulation condition were compared with control values from the same animal.

RESULTS

Bilateral stimulation of the STN at 130 Hz produced a significant increase in the seizure threshold for clonic flurothyl seizures, whereas stimulation at 260 Hz did not appear to have any effect on seizures. STN stimulation at 800 Hz significantly lowered seizure threshold for tonic-clonic seizures.

CONCLUSIONS

We conclude that electrical stimulation of the STN can be anticonvulsant, but the effects appear to depend on the stimulation frequency and the type of seizure.

Mu-opioid receptors in seizure-controlling brain structures are altered by prenatal morphine exposure and by male and female gonadal steroids in adult rats

The present study used autoradiography to examine the effect of prenatal morphine exposure on mu-opioid receptor density in epileptic seizure-controlling brain structures including the substantia nigra pars compacta (SNC), substantia nigra pars reticulata (SNR), superior colliculus (SC), and subthalamic nucleus (STN) of adult male and female rats. The results demonstrate that prenatal morphine exposure increases the mu-opioid receptor density in the SNC and STN, but not in the SNR or in the SC of gonadally intact adult male rats. The density of mu-opioid receptors in the SNC and STN is, however, decreased following gonadectomy in morphine-exposed males, and testosterone treatment fails to restore this decrease to the level of gonadally intact males. Further, in the SC, the density of mu receptors was lower in both saline-exposed, gonadectomized (GNX) and GNX, TP-treated males and in morphine-exposed, GNX, TP-treated males relative to gonadally intact saline- and morphine-exposed males, respectively. In ovariectomized (OVX) female rats, the same prenatal morphine exposure increases the mu-opioid receptor density in the SNC and SNR, but decreases it in the STN. The density of mu-opioid receptors is also decreased in the SNC and SC of OVX estrogen-treated females and in the SNR and SC of OVX, progesterone-treated females. Thus, the present study demonstrates that mu-opioid receptors in seizure-controlling brain structures are sex-specifically altered by prenatal morphine exposure in adult progeny. Further, prenatal morphine exposure alters gonadal hormone effects on the density of mu receptors in adult, OVX females.

Superior colliculus firing changes after lesion or electrical stimulation of the subthalamic nucleus in the rat

Recent data have suggested a critical role for the basal ganglia in the remote control of epileptic seizures. In particular, it has been shown that inhibition of either substantia nigra pars reticulata or subthalamic nucleus as well as activation of the superior colliculus suppresses generalized seizures in several animal models. It was previously shown that high frequency stimulation of the subthalamic nucleus, thought to act as functional inhibition, stopped ongoing non-convulsive generalized seizures in rats. In order to determine whether high frequency stimulation of the subthalamic nucleus involved an activation of superior colliculus neurons, we examined the effects of subthalamic nucleus manipulation, by either high frequency stimulation or chemical lesion, on the spontaneous electrical activity of superior colliculus neurons. Acute high frequency stimulation of the subthalamic nucleus (frequency 130 Hz) induced an immediate increase of unitary activity in 70% of responding cells, mainly located within the deep layers, whereas a reduction was observed in the remaining 30%. The latter responses are dependent on the intensity and frequency of the stimulation. Unilateral excitotoxic lesion of the subthalamic nucleus induced a delayed and transient decrease of superior colliculus activity. Our data suggest that high frequency stimulation of the subthalamic nucleus suppresses generalised epileptic seizures through superior colliculus activation.

Antiepileptic effect of high-frequency stimulation of the subthalamic nucleus (corpus luysi) in a case of medically intractable epilepsy caused by focal dysplasia: a 30-month follow-up: technical case report

OBJECTIVE AND IMPORTANCE

Currently, some forms of epilepsy are resistant to both pharmacological and surgical interventions. As a result, there is a need for new therapeutic strategies. Because the nigral system modulates neuronal excitability in animal models of epilepsy, we considered therapeutic high-frequency stimulation of the subthalamic nucleus (STN). We were encouraged by the known relationship between the STN and the nigral system, as well as by our experience with high-frequency stimulation of the STN in Parkinsonian patients.

CLINICAL PRESENTATION

A 5-year-old girl with pharmacologically resistant, inoperable epilepsy caused by focal centroparietal dysplasia underwent implantation with a permanent electrode in the left STN and was chronically stimulated. To date, we have followed up this patient for 30 months postoperatively.

TECHNIQUE

High-frequency stimulation of the STN induced a significant voltage-dependent reduction (by 80%) in the number and severity of seizures. In addition, consistent improvement in both motor and cognitive functions was noted as a result of reduced postictal states. The effect was more prominent for seizures occurring in clusters (89% reduction) and during the day (88% reduction) than for those that occurred during sleep (53% reduction).

CONCLUSION

This is the first report of epilepsy control using chronic high-frequency stimulation of the STN. Preliminary observations in three other operated patients (at 2, 12, and 18 mo) confirm these data. We think that high-frequency stimulation of the STN may hold significant future potential as a treatment for epilepsy, similar to its established role in the treatment of Parkinson's disease. This finding opens completely new experimental and therapeutic avenues for the treatment of surgically and medically intractable epilepsy.

Electrical stimulation of substantia nigra pars reticulata is anticonvulsant in adult and young male rats

Electrical stimulation of deep brain structures has been used for pain relief and treatment of refractory Parkinson's disease. Recently, stimulation of the subthalamic nucleus or anterior nuclei of the thalamus was introduced for the treatment of refractory epilepsy when other treatments failed. The substantia nigra pars reticulata (SNR) is another crucial site involved in the control of seizures. We studied the effects of continuous electrical stimulation of the SNR as a function of age in male rats. Adult [postnatal day (PN) 60] and young (PN 15) rats with electrodes symmetrically implanted in the SNR were used. The rats were stimulated with continuous constant current pulses (130 Hz) and simultaneously challenged with flurothyl to induce seizures. Control rats had the electrodes implanted but were not stimulated. High-frequency electrical stimulation of the SNR had anticonvulsant effects in both age groups. However, we identified age-specific features: In PN 60 rats, both unilateral and bilateral stimulation of the anterior region of the SNR produced anticonvulsant effects against clonic seizures, while stimulation of the posterior region of the SNR was ineffective. Stimulation of either SNR region had no effects on tonic-clonic seizures. In PN 15 rats, irrespective of the stimulation site within the SNR, bilateral stimulations of the SNR produced anticonvulsant effects against both clonic and tonic-clonic flurothyl-induced seizures, while unilateral stimulation was without effect. The data suggest that the SNR may be a candidate site for deep brain stimulation for the treatment of epilepsy.

Deep brain stimulation in epilepsy

Since the pioneering studies of Cooper et al. to influence epilepsy by cerebellar stimulation, numerous attempts have been made to reduce seizure frequency by stimulation of deep brain structures. Evidence from experimental animal studies suggests the existence of a nigral control of the epilepsy system. It is hypothesized that the dorsal midbrain anticonvulsant zone in the superior colliculi is under inhibitory control of efferents from the substantia nigra pars reticulata. Inhibition of the subthalamic nucleus (STN) could release the inhibitory effect of the substantia nigra pars reticulata on the dorsal midbrain anticonvulsant zone and thus activate the latter, raising the seizure threshold. Modulation of the seizure threshold by stimulation of deep brain structures-in particular, of the STN-is a promising future treatment option for patients with pharmacologically intractable epilepsy. Experimental studies supporting the existence of the nigral control of epilepsy system and preliminary results of STN stimulation in animals and humans are reviewed, and alternative mechanisms of seizure suppression by STN stimulation are discussed.

Inhibition of the substantia nigra suppresses absences and clonic seizures in audiogenic rats, but not tonic seizures: evidence for seizure specificity of the nigral control

GABAergic inhibition of the substantia nigra pars reticulata has been shown to suppress seizures in most models of epilepsy involving forebrain networks, such as absences or clonic seizures. No such antiepileptic effects were observed, however, in genetically audiogenic rats exhibiting tonic seizures generated in the brainstem. This suggests a constitutive dysfunction of the nigral GABAergic neurotransmission in this strain of rat or a selective action of the nigral control on specific networks. In the present study, we first confirmed that bilateral injection of muscimol (700 pmol/side) in the substantia nigra had no effect in Wistar rats with audiogenic seizures (Wistar AS). [3H]Muscimol autoradiography suggested a 40% reduced density of GABA(A) receptors in the substantia nigra of Wistar AS, whereas no change was observed in the cortex and the superior colliculus (superficial and intermediate layers), as compared to control animals. In Wistar AS where 40 repetitions of audiogenic stimulations progressively induced generalised convulsive seizures with both tonic and clonic components, bilateral injection of muscimol (350 pmol/side) in the substantia nigra suppressed the clonic component but had no effect on tonic seizures. In hybrid rats issued from cross-breeding between Wistar AS and rats with spontaneous absence seizures, bilateral injection of muscimol (18 pmol/side) in the substantia nigra abolished cortical spike-and-wave discharges, but had no effect on tonic audiogenic seizures at doses up to 700 pmol/side. These results show that despite a decreased number of GABA(A) receptors in the substantia nigra, inhibition of this structure in Wistar AS still leads to inhibition of seizures involving forebrain structures. These results confirm that GABAergic inhibition of the substantia nigra has antiepileptic effects through the control of forebrain circuits. They suggest that this control mechanism has no inhibitory effect on circuits underlying audiogenic tonic seizures.

Surgical treatment of epilepsy

Approximately 30-40% of patients with focal epilepsy continue to have seizures despite appropriate medical therapy. Surgical treatments should be considered in this important subset of patients. Recent advances in neuroimaging technology have revolutionized the identification and evaluation of surgical candidates. The goal of the presurgical evaluation (video EEG monitoring, neuroimaging, and neuropsychological assessment) is to delineate the epileptogenic zone. Surgery is recommended when this has been adequately identified and the proposed procedure is expected to result in a high likelihood of seizure freedom and a low risk of neurologic and cognitive morbidity.

Synaptic localization of ionotropic glutamate receptors in the rat substantia nigra

Glutamatergic neurotransmission in the substantia nigra pars compacta and pars reticulata is mediated through N-methyl-D-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxaline propionic acid/kainate (AMPA) type receptors as well as other glutamate receptors and is critical for basal ganglia functioning. A major glutamatergic input to the substantia nigra originates in the subthalamic nucleus, and the long-lasting stimulation of the dopaminergic cells of the substantia nigra pars compacta by the subthalamic neurons has been implicated in the pathophysiology of Parkinson's disease. The objectives of the present study were to determine the subcellular and subsynaptic localization of subunits of the N-methyl-D-aspartate and AMPA receptors in the substantia nigra, and also to determine whether co-localization of N-methyl-D-aspartate and AMPA receptor subunits occur at individual synapses. To achieve this, pre-embedding and post-embedding immunocytochemistry was applied to sections of substantia nigra using antibodies that recognize the NR1 and NR2A/B subunits of the N-methyl-D-aspartate receptor, and GluR2/3 subunits of the AMPA receptor. In both regions of the substantia nigra, immunolabelling for each of the subunits was observed in numerous perikarya and proximal dendrites. At the subcellular level, silver-intensified immunogold particles localizing N-methyl-D-aspartate and AMPA receptor subunits were most commonly present within dendrites where they were associated with a variety of intracellular organelles and with the internal surface of the plasma membrane. Post-embedding immunogold labelling revealed immunoparticles labelling for NR1, NR2A/B and GluR2/3 to be enriched at asymmetric synaptic specializations, although a large proportion of asymmetric synapses were immunonegative. Double immunolabelling revealed, in addition to single-labelled synapses, the co-localization of subunits of the N-methyl-D-aspartate receptor and subunits of the AMPA receptor at individual asymmetric synapses. Similarly, double immunolabelling also revealed the co-localization of the NRl and NR2A/B subunits of the N-methyl-D-aspartate receptor at individual asymmetric synapses. Labelling for NR1 and GluR2/3 was, on average, relatively evenly distributed across the width of the synapse with a gradual reduction towards the periphery when analysed in single sections. In summary, the present results demonstrate that AMPA and N-methyl-D-aspartate receptors are selectively localized at a subpopulation of asymmetric synapses in the substantia nigra pars compacta and reticulata and that the two receptor types, at least partially co-localize at individual synapses. It is concluded that glutamatergic transmission in the substantia nigra pars compacta and pars reticulata occurs primarily at asymmetric synapses and, at least in part, is mediated by both N-methyl-D-aspartate and AMPA receptors.

Dopamine in the striatum modulates seizures in a genetic model of absence epilepsy in the rat

Inhibition of the substantia nigra pars reticulata has been shown to suppress seizures in different animal models of epilepsy. The striatum is the main input of the substantia nigra pars reticulata. The aim of the present study was to examine the role of dopaminergic neurotransmission within the striatum in the control of absence seizures in a genetic model in the rat. Injections of mixed dopaminergic D1/D2 or of selective D1 or D2 agonists or antagonists in the dorsal parts of the striatum led to suppression of absence seizures associated with strong behavioral and electroencephalographic side-effects. When injected in the ventral part of the striatum (i.e. the nucleus accumbens core), all these agonists and antagonists respectively decreased and increased absence seizures without behavioral or electroencephalographic side-effects. Combined injections of low doses of a D1 and a D2 agonist in the core of the nucleus accumbens had an additive effect in absence seizures suppression. Furthermore, combined injections of low doses of a GABA(A) agonist and a N-methyl-D-aspartate antagonist in the substantia nigra also had cumulative effects in absence seizures suppression. These results show that dopamine neurotransmission in the core of the nucleus accumbens is critical in the control of absence seizures. The modulatory and additive effects on absence seizures of dopaminergic neurotransmission through both the D1 and D2 receptors in the core of the nucleus accumbens further suggest that ventral pathways of the basal ganglia system are involved in the modulation of absence seizures.

Postural and anticonvulsant effects of inhibition of the rat subthalamic nucleus

The subthalamic nucleus (STN) plays a crucial role as a regulator of basal ganglia outflow by providing excitatory glutamatergic input into the two output nuclei of the basal ganglia, substantia nigra pars reticulata (SNpr), and entopeduncular nucleus. This study examined the effects of suppressing activity in the STN of the awake, behaving rat. Specifically, we evaluated the effects of unilateral and bilateral focal inhibition of STN on posture, locomotion, and susceptibility to limbic motor seizures. Unilateral microinjection of a GABA(A) receptor agonist (muscimol, 200 pmol) into STN produced a site-dependent contralaterally directed postural asymmetry without locomotor activation. This effect differed from responses produced by the same dose of muscimol placed into SNpr, which included locomotor activation in addition to contralaterally directed postural asymmetry. Locomotor activation and postural asymmetry were obtained also after blockade of glutamate transmission in SNpr by the unilateral application of kynurenate (100 nmol). Our observation that STN inhibition did not induce the locomotor activation characteristic of SNpr inhibition suggests that there are glutamatergic inputs to SNpr, other than those from STN, that are responsible for controlling locomotion. Bilateral, but not unilateral, injection of muscimol (200 pmol) into STN protected against limbic motor seizures evoked either by intravenous bicuculline or by focal application of bicuculline into anterior piriform cortex (area tempestas). These results demonstrate that focal inhibition of STN reproduces the postural asymmetry and anticonvulsant actions that are obtained with the inhibition of SNpr. This provides behavioral support for the concept that STN contributes a crucial tonic excitatory (glutamatergic) drive to the rat SNpr.

Special considerations in treating children with epilepsy

The incidence of seizures is high in infants and children. Many epileptic syndromes have their onset early in life. The increase in seizure susceptibility of the immature brain may be due to several factors, including an imbalance between excitatory and inhibitory processes, age-specific differences in ionic transport and clearance systems, high incidence of epileptogenic stimuli early in life, and the age-specific expression of pre- and perinatal brain anomalies. All of these factors must be taken into account when developing safe and effective age-specific antiepileptic drugs (AEDs). The use of developmental epilepsy models, followed by clinical trials in children, may help identify such AEDs.

Sleep and epilepsy: A key role for nitric oxide?

PURPOSE

It has been suggested that nitric oxide (NO) is involved in sleep mechanisms and in the pathophysiology of epilepsy. Data are, however, controversial because it is not clear whether NO facilitates sleep or waking, or whether it exerts pro-or antiepileptic influences.

METHODS

The question was considered through NO voltammetric measurements and electroencephalographic recordings performed in GAERS rats (Genetic Absence Epilepsy Rat from Strasbourg): an experimental model of "petit-mal" human disease. Regulatory processes of sleep and epilepsy were studied after administration of a NO synthase inhibitor [l-arginine-p-nitroanilide (l-ANA) 100 mg/kg i.p.], a NO donor (SIN-1 100 ng/2 microl i.c.v.), and the antiepileptic drugs used in clinic [valproate (VPA 200 mg/kg i.p.) and ethosuximide (ESM 100 mg/kg i.p.)].

RESULTS

In GAERS rats, spontaneous circadian organizations of spike-wave discharges and paradoxical sleep (PS) occur in an opposite way; spontaneous NO concentrations are higher during seizures than during wakefulness, slow-wave sleep, and PS, respectively. l-ANA induces a disappearance of NO peak, an epileptic induction, and a loss of PS while SIN-1 induces opposite effects. Antiepileptic effects of VPA and ESM are associated with a PS increase and a significant release of NO.

CONCLUSIONS

These results indicate that NO could be, in GAERS rats, a central piece in the reciprocal inhibitory mechanisms regulating the induction of PS and spike-wave discharges. NO could prevent absence epilepsy and act as an antiepileptic substance in facilitating PS. Antiepileptic efficiency of VPA and ESM may work through their ability to release NO. A track for a new treatment of petit-mal disease in children can be envisioned.

Activation of group II metabotropic glutamate receptors inhibits synaptic excitation of the substantia Nigra pars reticulata

Loss of nigrostriatal dopaminergic neurons in Parkinson's disease (PD) leads to increased activity of glutamatergic neurons in the subthalamic nucleus (STN). Recent studies reveal that the resultant increase in STN-induced excitation of basal ganglia output nuclei is responsible for the disabling motor impairment characteristic of PD. On the basis of this, it is possible that any manipulation that reduces activity at excitatory STN synapses onto basal ganglia output nuclei could be useful in the treatment of PD. We now report that group II metabotropic glutamate receptors (mGluRs) are presynaptically localized on STN terminals and that activation of these receptors inhibits excitatory transmission at STN synapses. In agreement with the hypothesis that this could provide a therapeutic benefit in PD, a selective agonist of group II mGluRs induces a dramatic reversal of catalepsy in a rat model of PD. These results raise the exciting possibility that selective agonists of group II mGluRs could provide an entirely new approach to the treatment of PD. These novel therapeutic agents would provide a noninvasive pharmacological treatment that does not involve the manipulation of dopaminergic systems, thus avoiding the problems associated with current therapies.

Glial glutamate transporter mRNAs in the genetically absence epilepsy rat from Strasbourg

Recent studies support a critical role for the glutamatergic system and glutamate transporters in the pathogenesis of epilepsy. The glial glutamate transporters GLT-1 (L-glutamate transporter) and GLAST (L-glutamate/L-aspartate transporter) are known to be responsible for the majority of glutamate reuptake from the synaptic cleft and constitute one mechanism by which extracellular glutamate levels may be controlled. The present study therefore compared GLT-1 and GLAST mRNA levels in the genetically absence epilepsy rat from Strasbourg (GAERS) with those of age-matched non-epileptic controls. The GAERS rat has been proposed as an animal model of inherited human absence epilepsy, displaying recurrent, generalised, non-convulsive seizures that originate from thalamic and cortical structures. In situ hybridisation with 35S-labelled oligonucleotide probes demonstrated substantial and significant increases in GLT-1 mRNA levels in the ventromedial nucleus of the thalamus (VM) and the subthalamic nucleus (STN) of GAERS rats. Increases in GLAST mRNA were found in the primary somatosensory cortex (SS1) and temporal cortex (Te) of GAERS. These data, along with previous studies, suggest that regional imbalances in GABAergic and glutamatergic systems may be associated with the pathogenesis of absence seizures in GAERS.