Seizure suppression in kindling epilepsy by intracerebral implants of GABA- but not by noradrenaline-releasing polymer matrices

Emerging therapeutic targets for epilepsy: Preclinical insights

INTRODUCTION

Around 30% of patients with epilepsy suffer from drug-resistant seizures. Drug-resistant seizures may have significant consequences such as sudden death in epilepsy, injuries, memory disturbances, and childhood learning and developmental problems. Conventional and newer available antiepileptic drugs (AEDs) work via numerous mechanisms - mainly through inhibition of voltage-operated Na⁺ and/or Ca²⁺ channels, excitation of K⁺ channels, enhancement of GABA-mediated inhibition and/or blockade of glutamate-produced excitation. However, the discovery and development of novel brain targets may improve the future pharmacological management of epilepsy and hence is of pivotal importance.

AREAS COVERED

This article examines novel drug targets such as brain multidrug efflux transporters and inflammatory pathways; it progresses to discuss possible strategies for the management of drug-resistant seizures. Reduction of the consequences of blood brain barrier dysfunction and enhancement of anti-oxidative defense are discussed.

EXPERT OPINION

Novel drug targets comprise brain multidrug efflux transporters, TGF-β, Nrf2-ARE or m-TOR signaling and inflammatory pathways. Gene therapy and antagomirs seem the most promising targets. Epileptic foci may be significantly suppressed by viral-vector-mediated gene transfer, leading to an increased in situ concentration of inhibitory factors (for instance, galanin). Also, antagomirs offer a promising possibility of seizure inhibition by silencing micro-RNAs involved in epileptogenesis and possibly in seizure generation.

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.

Ultrastructural GABA immunogold labeling in the substantia nigra pars reticulata of kindled genetic absence epilepsy rats

Genetic Absence Epilepsy Rats from Strasbourg (GAERS) is a well-known animal model of absence epilepsy and they are resistant to electrical kindling stimulations. The present study aimed to examine possible differences in gamma-aminobutyric acid (GABA) levels and synapse counts in the substantia nigra pars reticulata anterior (SNRa) and posterior (SNRp) regions between GAERS and Wistar rats receiving kindling stimulations. Animals in the kindling group either received six stimulations in the amygdala and had grade 2 seizures or they were kindled, having grade five seizures. Rats were decapitated one hour after the last stimulation. SNR regions were obtained after vibratome sectioning of the brain tissue. GABA immunoreactivity was detected by immunogold method and synapses were counted. Sections were observed by transmission electron microscope and analyzed by Image J program. GABA density in the SNRa region of fully kindled GAERS and Wistar groups increased significantly compared to that of their corresponding grade 2 groups. The number of synapses increased significantly in kindled and grade 2 GAERS groups, compared to kindled and grade 2 Wistar groups, respectively, in the SNRa region. GABA density in the SNRp region of kindled GAERS group increased significantly compared to that of GAERS grade 2 group. In the SNRp region, both kindled and grade 2 GAERS groups were found to have increased number of synapses compared to that of GAERS control group. We concluded that both SNRa and SNRp regions may be important in modulating resistance of GAERS to kindling stimulations.

Advanced fabrication approaches to controlled delivery systems for epilepsy treatment

INTRODUCTION

Epilepsy is a chronic brain disease characterized by unprovoked seizures, which can have severe consequences including loss of awareness and death. Currently, 30% of epileptic patients do not receive adequate seizure alleviation from oral routes of medication. Over the last decade, local drug delivery to the focal area of the brain where the seizure originates has emerged as a potential alternative and may be achieved through the fabrication of drug-loaded polymeric implants for controlled on-site delivery.

AREAS COVERED

This review presents an overview of the latest advanced fabrication techniques for controlled drug delivery systems for refractory epilepsy treatment. Recent advances in the different techniques are highlighted and the limitations of the respective techniques are discussed.

EXPERT OPINION

Advances in biofabrication technologies are expected to enable a new paradigm of local drug delivery systems through offering high versatility in controlling drug release profiles, personalized customization and multi-drug incorporation. Tackling some of the current issues with advanced fabrication methods, including adhering to GMP-standards and industrial scale-up, together with innovative solutions for complex designs will see to the maturation of these techniques and result in increased clinical research into implant-based epilepsy treatment.

ABBREVIATIONS

GMP: Good manufacturing process; DDS(s): Drug delivery system(s); 3D: Three-dimensional; AEDs: Anti-epileptic drugs; BBB: Blood brain barrier; PLA: Polylactic acid; PLGA: Poly(lactic-co-glycolic acid); PCL: poly(ɛ-caprolactone); ESE: Emulsification solvent evaporation; O/W: Oil-in-water; W/O/W: Water-in-oil-in-water; DZP: Diazepam; PHT: Phenytoin; PHBV: Poly(hydroxybutyrate-hydroxyvalerate); PEG: Polyethylene glycol; SWD: Spike-and-wave discharges; CAD: Computer aided design; FDM: Fused deposition modeling; ABS: Acrylonitrile butadiene styren; eEVA: Ethylene-vinyl acetate; GelMA: Gelatin methacrylate; PVA: Poly-vinyl alcohol; PDMS: Polydimethylsiloxane; SLA: Stereolithography; SLS: Selective laser sintering.

Injectable phenytoin loaded polymeric microspheres for the control of temporal lobe epilepsy in rats

PURPOSE

Epilepsy is a prevalent neurological disorder with a high frequency of drug resistance. While significant advancements have been made in drug delivery systems to overcome anti-epileptic drug resistance, efficacies of materials in biological systems have been poorly studied. The purpose of the study was to evaluate the anti-epileptic effects of injectable poly(epsilon-caprolactone) (PCL) microspheres for controlled release of an anticonvulsant, phenytoin (PHT), in an animal model of epilepsy.

METHODS

PHT-PCL and Blank-PCL microspheres formulated using an oil-in-water (O/W) emulsion solvent evaporation method were evaluated for particle size, encapsulation efficiency, surface morphology and in-vitro drug release profile. Microspheres with the most suitable morphology and release characteristics weresubsequently injected into the hippocampus of a rat tetanus toxin model of temporal lobe epilepsy. Electrocorticography (ECoG)from the cerebral cortex were recorded for all animals. The number of seizure events, severity of seizures, and seizure duration were then compared between the two treatment groups.

RESULTS

We have shown that small injections of drug-loaded microspheres are biologically tolerated and released PHT can control seizures for the expected period of time that is in accord with in-vitro release data.

CONCLUSION

The study demonstrated the feasibility of polymer-based delivery systems incontrolling focal seizures.

Implantable and transdermal polymeric drug delivery technologies for the treatment of central nervous system disorders

The complexity of the brain and the membranous blood-brain barrier (BBB) has proved to be a significant limitation to the systemic delivery of pharmaceuticals to the brain rendering them sub-therapeutic and ineffective in the treatment of neurological diseases. Apart from this, lack of innovation in product development to counteract the problem is also a major contributing factor to a poor therapeutic outcome. Various innovative strategies show potential in treating some of the neurological disorders; however, drug delivery remains the most popular. To attain therapeutic drug levels in the central nervous system, large, intolerable systemic doses are generally administered. The major factors responsible for the success maintenance therapy of neurological diseases included controlled and sustained release of neurotherapeutics, reduced frequency of administration, higher bioavailability, and patient compliances. Conventional oral or injectable formulations cannot satisfy all the requirements in many circumstances. This article reviews the therapeutic implantable polymeric and transdermal devices employed in an attempt to effectively achieve therapeutic quantities of drug across the BBB over a prolonged period, to improve patient disease prognosis.

GABA-ergic cell therapy for epilepsy: Advances, limitations and challenges

Diminution in the number of gamma-amino butyric acid positive (GABA-ergic) interneurons and their axon terminals, and/or alterations in functional inhibition are conspicuous brain alterations believed to contribute to the persistence of seizures in acquired epilepsies such as temporal lobe epilepsy. This has steered a perception that replacement of lost GABA-ergic interneurons would improve inhibitory synaptic neurotransmission in the epileptic brain region and thereby reduce the occurrence of seizures. Indeed, studies using animal prototypes have reported that grafting of GABA-ergic progenitors derived from multiple sources into epileptic regions can reduce seizures. This review deliberates recent advances, limitations and challenges concerning the development of GABA-ergic cell therapy for epilepsy. The efficacy and limitations of grafts of primary GABA-ergic progenitors from the embryonic lateral ganglionic eminence and medial ganglionic eminence (MGE), neural stem/progenitor cells expanded from MGE, and MGE-like progenitors generated from human pluripotent stem cells for alleviating seizures and co-morbidities of epilepsy are conferred. Additional studies required for possible clinical application of GABA-ergic cell therapy for epilepsy are also summarized.

Reduction of epileptiform activity through local valproate-implants in a rat neocortical epilepsy model

PURPOSE

Pharmacotherapy of epilepsies is limited due to low concentrations at epileptogenic foci, side effects of high systemic doses and that some potentially efficient substances do not pass the blood-brain barrier. To overcome these limitations, we tested the efficacy of local valproate (VPA)-containing polymer implants in a model of necocortical injected tetanus toxin (TeT) in the rat.

METHODS

Tetanus toxin was injected intracortically and cobalt (II) chloride (CoCl2) was applied on the cortical surface. Video-electrocorticography recordings with intracortical electrodes were performed. VPA-containing polymers were implanted above the cortical focus. Antiepileptic effects were evaluated as reductions of epileptiform potentials (EPs) per hour in comparison to saline (NaCl)-containing polymer implants.

RESULTS

Triple 50ng TeT injections plus CoCl2 application (20/10mg) showed consistent EPs. NaCl-implanted animals (n=6) showed a mean of 10.5EPs/h after the first week, the EP frequency increased to 53.5EPs/h after the second week. VPA-implant animals (n=5) showed a reduction in EP frequency from 71.6 to 4.8EPs/h after the second week. The EP frequency after the second week was higher in the NaCl-implanted animals than in the VPA-implanted (p=0.0303). The mean EPs/h increase in NaCl-implanted rats (+42.9EPs/h) was different (p=0.0087) from the mean EPs/h decrease in VPA-implanted rats (-66.8EPs/h).

CONCLUSION

Despite former publications no clear seizures could be reproduced but it was possible to establish focal EPs, which proved to be a reliable marker for epileptic activity. Local antiepileptic therapy with VPA has shown efficacy in decreasing EP frequency.

Novel methods of antiepileptic drug delivery -- polymer-based implants

Epilepsy is a neurological disorder characterised by spontaneous seizures. Over one third of patients receive insufficient benefit from oral anti-epileptic drug (AED) therapy, and continue to experience seizures whilst on medication. Epilepsy researchers are consequently seeking new ways to deliver AEDs directly to the seizure focus in the brain in order to deliver higher, more effective doses to the seizure focus whilst bypassing the remainder of the brain and body to prevent side effects. The focus of this review will be polymer-based implants, which are polymeric devices loaded with AED that are designed for implantation at the seizure focus in order to achieve gradual, continuous release of AED direct into the region of the brain responsible for seizures. Polymer-based implants produced for epilepsy to date are based on a range of polymers, both biodegradable and non-biodegradable, and range from simple materials development studies through to investigations of implants in animal models of seizures and epilepsy, with varying degrees of success. This review describes the range of methods employed to manufacture polymer-based implants and compares their advantages and potential appeal to industry, and describes and compares the results and successes of polymer-based materials and devices produced to date for the treatment of epilepsy. We also discuss disadvantages and hurdles to be overcome in the field, and describe our predictions for advances to be made in the field in the coming decade.

Neuromodulation in Epilepsy

Neuromodulation strategies have been proposed to treat a variety of neurological disorders, including medication-resistant epilepsy. Electrical stimulation of both central and peripheral nervous systems has emerged as a possible alternative for patients who are not deemed to be good candidates for resective procedures. In addition to well-established treatments such as vagus nerve stimulation, epilepsy centers around the world are investigating the safety and efficacy of neurostimulation at different brain targets, including the hippocampus, thalamus, and subthalamic nucleus. Also promising are the preliminary results of responsive neuromodulation studies, which involve the delivery of stimulation to the brain in response to detected epileptiform or preepileptiform activity. In addition to electrical stimulation, novel therapeutic methods that may open new horizons in the management of epilepsy include transcranial magnetic stimulation, focal drug delivery, cellular transplantation, and gene therapy. We review the current strategies and future applications of neuromodulation in epilepsy.

Glutamatergic Excitation and GABA Release from a Transplantable Cell Line

The cell line M213-2O CL-4 was derived from cell line M213-2O and further modified to express human glutamate decarboxylase (hGAD-67), the enzyme that synthesizes GABA. Brain transplants of this cell line in animal models of epilepsy have been shown to modulate seizures. However, the mechanisms that underlie such actions are unknown. The purpose of the present study was to characterize this cell line and its responsiveness to several depolarizing conditions, in order to better understand how these cells exert their effects. Intracellular GABA levels were 34-fold higher and GAD activity was 16-fold higher in clone M213-2O CL-4 than in M213-2O. Both cell lines could take up [3H]GABA in vitro, and this uptake was prevented by nipecotic acid. By combining GABA release measurements and calcium imaging in vitro, we found that high extracellular K+, zero Mg2+, or glutamate activated M213-2O CL-4 cells and resulted in GABA release. The response to glutamate appeared to be mediated by AMPA/NMDA-like receptors. High KCl-induced GABA release was prevented when a Ca2+-free Krebs solution was used, suggesting an exocytotic-like mechanism. These results indicate that the cell line M213-2O CL-4 synthesizes, releases, and takes up GABA in vitro, and can be activated by depolarizing stimuli.

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.

Transplant of GABAergic Precursors Restores Hippocampal Inhibitory Function in a Mouse Model of Seizure Susceptibility

Defects in GABAergic function can cause epilepsy. In the last years, cell-based therapies have attempted to correct these defects with disparate success on animal models of epilepsy. Recently, we demonstrated that medial ganglionic eminence (MGE)-derived cells grafted into the neonatal normal brain migrate and differentiate into functional mature GABAergic interneurons. These cells are able to modulate the local level of GABA-mediated synaptic inhibition, which suggests their suitability for cell-based therapies. However, it is unclear whether they can integrate in the host circuitry and rescue the loss of inhibition in pathological conditions. Thus, as proof of principle, we grafted MGE-derived cells into a mouse model of seizure susceptibility caused by specific elimination of GABAergic interneuron subpopulations in the mouse hippocampus after injection of the neurotoxic saporin conjugated to substance P (SSP-Sap). This ablation was associated with significant decrease in inhibitory postsynaptic currents (IPSC) on CA1 pyramidal cells and increased seizure susceptibility induced by pentylenetetrazol (PTZ). Grafting of GFP+ MGE-derived cells in SSP-Sap-treated mice repopulates the hippocampal ablated zone with cells expressing molecular markers of mature interneurons. Interestingly, IPSC kinetics on CA1 pyramidal cells of ablated hippocampus significantly increased after transplantation, reaching levels similar to the normal mice. More importantly, this was associated with reduction in seizure severity and decrease in postseizure mortality induced by PTZ. Our data show that MGE-derived cells fulfill most of the requirements for an appropriate cell-based therapy, and indicate their suitability for neurological conditions where a modulation of synaptic inhibition is needed, such as epilepsy.

Evolution and prospects for intracranial pharmacotherapy for refractory epilepsies: the subdural hybrid neuroprosthesis

Intracranial pharmacotherapy is a novel strategy to treat drug refractory, localization-related epilepsies not amenable to resective surgery. The common feature of the method is the use of some type of antiepileptic drug (AED) delivery device placed inside the cranium to prevent or stop focal seizures. This distinguishes it from other nonconventional methods, such as intrathecal pharmacotherapy, electrical neurostimulation, gene therapy, cell transplantation, and local cooling. AED-delivery systems comprise drug releasing polymers and neuroprosthetic devices that can deliver AEDs into the brain via intraparenchymal, ventricular, or transmeningeal routes. One such device is the subdural Hybrid Neuroprosthesis (HNP), designed to deliver AEDs, such as muscimol, into the subdural/subarachnoid space overlaying neocortical epileptogenic zones, with electrophysiological feedback from the treated tissue. The idea of intracranial pharmacotherapy and HNP treatment for epilepsy originated from multiple sources, including the advent of implanted medical devices, safety data for intracranial electrodes and catheters, evidence for the seizure-controlling efficacy of intracerebral AEDs, and further understanding of the pathophysiology of focal epilepsy. Successful introduction of intracranial pharmacotherapy into clinical practice depends on how the intertwined scientific, engineering, clinical, neurosurgical and regulatory challenges will be met to produce an effective and commercially viable device.

Adenosine augmentation therapies (AATs) for epilepsy: prospect of cell and gene therapies

Deficiencies in the brain's own adenosine-based seizure control system contribute to seizure generation. Consequently, reconstitution of adenosinergic neuromodulation constitutes a rational approach for seizure control. This review will critically discuss focal adenosine augmentation strategies and their potential for antiepileptic and disease modifying therapy. Due to systemic side effects of adenosine focal adenosine augmentation--ideally targeted to an epileptic focus--becomes a therapeutic necessity. This has experimentally been achieved in kindled seizure models as well as in post-status epilepticus models of spontaneous recurrent seizures using three different therapeutic strategies that will be discussed here: (i) polymer-based brain implants that were loaded with adenosine; (ii) brain implants comprised of cells engineered to release adenosine and embedded in a cell-encapsulation device; (iii) direct transplantation of stem cells engineered to release adenosine. To meet the therapeutic goal of focal adenosine augmentation, genetic disruption of the adenosine metabolizing enzyme adenosine kinase (ADK) in rodent and human cells was used as a molecular strategy to induce adenosine release from cellular brain implants, which demonstrated antiepileptic and neuroprotective properties. New developments and therapeutic challenges in using AATs for epilepsy therapy will critically be evaluated.

Intraventricular and intracerebral delivery of anti-epileptic drugs in the kindling model

A means to avoid the pharmacokinetic problems affecting the anti-epileptic drugs may be their direct intracerebroventricular (ICV) or intracerebral delivery. This approach may achieve a greater drug concentration at the epileptogenic area while minimizing it in other brain or systemic areas, and thus it could be an interesting therapeutic alternative in drug-resistant epilepsies. The objective of this article is to review a series of experiments, ranging from actute ICV injection to continuous intracerebral infusion of anti-epileptic drugs or grafting of neurotransmitter producing cells, in experimental models, especially in the kindling model of epilepsy in the rat. Acute ICV injection of phenytoin, phenobarbital or carbamacepine is able to diminish the intensity of kindling seizures, but it is also associated with a high neurologic toxicity, especially phenobarbital. Continuous ICV infusion of anti-epileptic drugs can effectively control seizures, but neurologic toxicity is not improved compared with systemic delivery. However, systemic toxicity may be improved, as in the case of valproic acid, whose continuous ICV infusion results in very low plasmatic or hepatic drug concentrations. Continuous intracerebral infusion at the epileptogenic area was studied as an alternative to minimize neurologic toxicity. Thus, intra-amygdalar infusion of gamma-aminobutyric acid (GABA) controls seizures with minimal neurotoxicity in amygdala-kindled rats. Similarly, continuous infusion of GABA into the dorsomedian nucleus of the thalamus improves seizure spread, while not affecting the local epileptogenic activity at the amygdala. Grafting of GABA releasing cells may reduce kindling parameter severity without behavioral side effects. We may conclude that ICV or intracerebral delivery of anti-epileptic drugs or neurotransmitters may be a useful technique to modulate epilepsy.

Transplantation of GABA-producing cells for seizure control in models of temporal lobe epilepsy

A high percentage of patients with temporal lobe epilepsy (TLE) are refractory to conventional pharmacotherapy. The progressive neurodegenerative processes associated with a lifetime of uncontrolled seizures mandate the development of alternative approaches to treat this disease. Transplantation of inhibitory cells has been suggested as a potential therapeutic strategy to achieve seizure suppression in humans with intractable TLE. Preclinical investigations over 20 years have demonstrated that multiple cell types from several sources can produce anticonvulsant, and antiepileptogenic, effects in animal models of TLE. Transplanting GABA-producing cells, in particular, has been shown to reduce seizures in several well-established models. This review addresses experimentation using different sources of transplantable GABAergic cells, highlighting progress with fetal tissue, neural cell lines, and stem cells. Regardless of the source of the GABAergic cells used in seizure studies, common challenges have emerged. Several variables influence the anticonvulsant potential of GABA-producing cells. For example, tissue availability, graft survival, immunogenicity, tumorigenicity, and varying levels of cell migration, differentiation, and integration into functional circuits and the microenvironment provided by sclerotic tissue all contribute to the efficacy of transplanted cells. The challenge of understanding how all of these variables work in concert, in a disease process that has no well-established etiology, suggests that there is still much basic research to be done before rational cell-based therapies can be developed for TLE.

Convection-enhanced delivery in the treatment of epilepsy

Convection-enhanced delivery (CED) is a novel drug-delivery technique that uses positive hydrostatic pressure to deliver a fluid containing a therapeutic substance by bulk flow directly into the interstitial space within a localized region of the brain parenchyma. CED circumvents the blood-brain barrier and provides a wider, more homogenous distribution than bolus deposition (focal injection) or other diffusion-based delivery approaches. A potential use of CED is for the local delivery of antiseizure agents, which would provide an epilepsy treatment approach that avoids the systemic toxicities of orally administered antiepileptic drugs and bystander effects on nonepileptic brain regions. Recent studies have demonstrated that brief CED infusions of nondiffusible peptides that inhibit the release of excitatory neurotransmitters, including omega-conotoxins and botulinum neurotoxins, can produce long-lasting (weeks to months) seizure protection in the rat amygdala-kindling model. Seizure protection is obtainable without detectable neurological or behavioral side effects. Although conventional diffusible antiepileptic drugs do confer seizure protection when administered locally by CED, the effect is transitory. CED is a potential approach for seizure protection that could represent an alternative to resective surgery in the treatment of focal epilepsies that are resistant to orally-administered antiepileptic drugs. The prolonged duration of action of nondiffusible toxins would allow seizure protection to be maintained chronically with infrequent reinfusions.

Nanotechnology for delivery of drugs to the brain for epilepsy

Epilepsy results from aberrant electrical activity that can affect either a focal area or the entire brain. In treating epilepsy with drugs, the aim is to decrease seizure frequency and severity while minimizing toxicity to the brain and other tissues. Antiepileptic drugs (AEDs) are usually administered by oral and intravenous routes, but these drug treatments are not always effective. Drug access to the brain is severely limited by a number of biological factors, particularly the blood-brain barrier, which impedes the ability of AEDs to enter and remain in the brain. To improve the efficacy of AEDs, new drug delivery strategies are being developed; these methods fall into the three main categories: drug modification, blood-brain barrier modification, and direct drug delivery. Recently, all three methods have been improved through the use of drug-loaded nanoparticles.

Benefits and risks of intranigral transplantation of GABA-producing cells subsequent to the establishment of kindling-induced seizures

Neural transplantation has been investigated experimentally and clinically for the purpose of developing new treatment options for intractable epilepsy. In the present study we assessed the anticonvulsant efficacy and safety of bilateral allotransplantation of genetically engineered striatal GABAergic rat cell lines into the substantia nigra pars reticulata (SNr). Rats with previously-established seizures, induced by amygdala kindling, were used as a model of temporal lobe epilepsy. Three cell lines were transplanted: (1) immortalized GABAergic cells (M213-2O) derived from embryonic rat striatum; (2) M213-2O cells (CL4) transfected with human GAD67 cDNA to obtain higher GABA synthesis than the parent cell line; and (3) control cells (121-1I), also derived from embryonic rat striatum, but which did not show GAD expression. A second control group received injections of medium alone. Transplantation of M213-2O cells into the SNr of kindled rats resulted in significant but transient anticonvulsant effects. Neither control cells nor medium induced anticonvulsant effects. Strong tissue reactions were, however, induced in the host brain of kindled but not of non-kindled rats, and only in animals that received grafts of genetically modified CL4 cells. These tissue reactions included graft rejection, massive infiltration of inflammatory immune cells, and gliosis. The anticonvulsant effect of M213-2O cells emphasizes the feasibility of local manipulations of seizures by intranigral transplantation of GABA-producing cells. On the other hand, the present data suggest that kindling-induced activation of microglia in the SNr can enhance immune reactions to transplanted cells. In this case, under conditions of further immunological stimulation by CL4 cells, transfected with a human cDNA, substantial immune reactions occurred. Thus, it appears that the condition of the host brain and the production of foreign proteins by transplanted cells have to be considered in estimating the risks of rejection of transplants into the brain.

Grafting of striatal precursor cells into hippocampus shortly after status epilepticus restrains chronic temporal lobe epilepsy

Status epilepticus (SE) typically progresses into temporal lobe epilepsy (TLE) typified by complex partial seizures. Because sizable fraction of patients with TLE exhibit chronic seizures that are resistant to antiepileptic drugs, alternative therapies that are efficient for diminishing SE-induced chronic epilepsy have great significance. We hypothesize that bilateral grafting of appropriately treated striatal precursor cells into hippocampi shortly after SE is efficacious for diminishing SE-induced chronic epilepsy through long-term survival and differentiation into GABA-ergic neurons. We induced SE in adult rats via graded intraperitoneal injections of kainic acid, bilaterally placed grafts of striatal precursors (pre-treated with fibroblast growth factor-2 and caspase inhibitor) into hippocampi at 4 days post-SE, and examined long-term effects of grafting on spontaneous recurrent motor seizures (SRMS). Analyses at 9-12 months post-grafting revealed that, the overall frequency of SRMS was 67-89% less than that observed in SE-rats that underwent sham-grafting surgery and epilepsy-only controls. Graft cell survival was approximately 33% of injected cells and approximately 69% of surviving cells differentiated into GABA-ergic neurons, which comprised subclasses expressing calbindin, parvalbumin, calretinin and neuropeptide Y. Grafting considerably preserved hippocampal calbindin but had no effects on aberrant mossy fiber sprouting. The results provide novel evidence that bilateral grafting of appropriately treated striatal precursor cells into hippocampi shortly after SE is proficient for greatly reducing the frequency of SRMS on a long-term basis in the chronic epilepsy period. Presence of a large number of GABA-ergic neurons in grafts further suggests that strengthening of the inhibitory control in host hippocampi likely underlies the beneficial effects mediated by grafts.

Intranigral transplants of a GABAergic cell line produce long-term alleviation of established motor seizures

We have previously shown that intranigral transplants of immortalized GABAergic cells decrease the number of kainic acid-induced seizures [Castillo CG, Mendoza S, Freed WJ, Giordano M. Intranigral transplants of immortalized GABAergic cells decrease the expression of kainic acid-induced seizures in the rat. Behav Brain Res 2006;171:109-15] in an animal model. In the present study, recurrent spontaneous behavioral seizures were established by repeated systemic injections of this excitotoxin into male Sprague-Dawley rats. After the seizures had been established, cells were transplanted into the substantia nigra. Animals with transplants of control cells (without hGAD67 expression) or with sham transplants showed a death rate of more than 40% over the 12 weeks of observation, whereas in animals with M213-2O CL-4 transplants, the death rate was reduced to less than 20%. The M213-2O CL-4 transplants significantly reduced the percentage of animals showing behavioral seizures; animals with these transplants also showed a lower occurrence of stage V seizures than animals in the other groups. In vivo and in vitro analyses provided evidence that the GABAergic cells show sustained expression of both GAD67 and hGAD67 cDNA, as well as increased gamma-aminobutyric acid (GABA) levels in the ventral mesencephalon of transplanted animals. Therefore, transplantation of GABA-producing cells can produce long-term alleviation of behavioral seizures in an animal model.

The problems of delivering neuroactive molecules to the CNS

At present, the aetiologies of many neurological and neurodegenerative diseases are unknown. However, emergence of a better understanding of these diseases, at both cellular and molecular levels, opens up the possibility of replacement therapies. The presence of the blood-brain barrier complicates the delivery of molecules to the central nervous system. Numerous attempts have been made to bypass this barrier either by delivering the drugs directly into the brain or by transplanting cells to produce the missing molecules in situ. This review explores several methods for delivering bioactive molecules into the CNS, including the use of permeabilizers, osmotic pumps, slow polymer release systems and transplantation of cells with or without the use of the encapsulation technology.

Adenosine-based cell therapy approaches for pharmacoresistant epilepsies

Despite recent medical advances pharmacoresistant epilepsy continues to be a major health problem. The knowledge of endogenous protective mechanisms of the brain may lead to the development of rational therapies tailored to a patient's needs. Adenosine has been identified as an endogenous neuromodulator with antiepileptic and neuroprotective properties. However, the therapeutic use of adenosine or its receptor agonists is largely precluded by strong peripheral and central side effects. Thus, local delivery of adenosine to a critical site of the brain may provide a solution for the therapeutic use of adenosine. The following rationale for the local augmentation of the adenosine system as a novel therapeutic principle in the treatment of epilepsy has been established: (1) Deficits in the adenosinergic system are associated with epileptogenesis and these deficits promote seizures. Thus, reconstitution of an inhibitory adenosinergic tone is a rational therapeutic approach. (2) The focal paracrine delivery of adenosine from encapsulated cells suppresses seizures in kindled rats without overt side effects. (3) The anticonvulsant activity of locally released adenosine is maintained in models of epilepsy which are resistant to major antiepileptic drugs. This review summarizes the rationale and recent approaches for adenosine-based cell therapies for pharmacoresistant epilepsies.

Role of Endogenous Neural Stem Cells in Neurological Disease and Brain Repair

These examples show that stem-cell-based therapy of neuro-psychiatric disorders will not follow a single scheme, but rather include widely different approaches. This is in accordance with the notion that the impact of stem cell biology on neurology will be fundamental, providing a shift in perspective, rather than introducing just one novel therapeutic tool. Stem cell biology, much like genomics and proteomics, offers a "view from within" with an emphasis on a theoretical or real potential and thereby the inherent openness, which is central to the concept of stem cells. Thus, stem cell biology influences many other, more traditional therapeutic approaches, rather than introducing one distinct novel form of therapy. Substantial advances have been made i n neural stemcell research during the years. With the identification of stem and progenitor cells in the adult brain and the complex interaction of different stem cell compartments in the CNS--both, under physiological and pathological conditions--new questions arise: What is the lineage relationship between t he different progenitor cells in the CNS and how much lineage plasticity exists? What are the signals controlling proliferation and differentiation of neural stem cells and can these be utilized to allow repair of the CNS? Insights in these questions will help to better understand the role of stem cells during development and aging and the possible relation of impaired or disrupted stem cell function and their impact on both the development and treatment of neurological disease. A number o f studies have indicated a limited neuronal and glial regeneration certain pathological conditions. These fundamental observations have already changed our view on understanding neurological disease and the brain's capacity for endogenous repair. The following years will have to show how we can influence andmodulate endogenous repair nisms by increasing the cellular plasticity in the young and aged CNS.

Refractory epilepsy: mechanisms and solutions

Pharmacoresistance remains a major challenge in epilepsy management. The availability of ten new antiepileptic drugs since the late 1980s has not dramatically improved the outcome of refractory epilepsy. This article provides an overview of the contemporary understanding of epilepsy and the limitation of current treatment modalities, discusses putative biological mechanisms of medical intractability and reviews some of the novel strategies under investigation to overcome the challenge of pharmacoresistance.

Focal treatment for refractory epilepsy: hope for the future?

Despite advances in anti-epileptic drug therapy and epilepsy surgery in recent years, intractable epilepsy remains a large clinical problem. Surgical resection, which can have an excellent outcome, is appropriate for only a minority of patients in whom an identifiable focus in non-eloquent brain can be identified. Systemic drug delivery is inevitably limited by the potential for unwanted side effects, due to actions both outside the CNS and in non-epileptic brain regions. Thus for a substantial number of patients novel treatment approaches are urgently needed. Both focal drug delivery and neuronal stem cell grafting have been evaluated in a variety of experimental epilepsy models in recent years, targeting either the seizure focus or key propagation pathways. The literature in this field is critically reviewed and considered in a clinical context. Studies in both areas are hampered by the limitations of available animal models, and by uncertainties in discerning which changes in the epileptic brain directly promote seizures, and which are compensatory. However, in many cases promising, though short-term, results have been obtained. Before such studies could be considered in humans further investigations that include long-term seizure and behavioural outcomes, in clinically relevant experimental models, are required. However, the current literature does provide proof in principle for a focal treatment approach, which may offer hope for many currently intractable patients for whom drug developments and surgical advances have proved disappointing.

Transplants of Cells Engineered to Produce GABA Suppress Spontaneous Seizures

PURPOSE

Cell transplantation into the brain is an aggressive clinical alternative. The hopes of treating diseases like intractable temporal lobe epilepsy have been subdued because the preclinical successes thus far have shown only slowing of epileptogenesis, or suppression of electrically induced seizures. Because the hallmark of epilepsy is spontaneous seizures, the clinical relevance of these studies has been questioned. The purpose of this study was to establish that cells genetically engineered to produce gamma-aminobutyric acid (GABA) could suppress spontaneous seizures in an accepted model of temporal lobe epilepsy.

METHODS

Conditionally immortalized neurons were engineered to produce GABA under the control of tetracycline. These cells were transplanted into the substantia nigra of spontaneously seizing animals. After transplantation, the animals were monitored for 3 days immediately after surgery and again for 3 days beginning 7-8 days after surgery. Seizures and epileptiform spikes were recorded and later analyzed with detection software combined with video monitoring.

RESULTS

Animals that received genetically engineered GABA-producing cells had significantly fewer spontaneous seizures than did animals that received control cells, or animals that received GABA-producing cells plus doxycycline at the observation period starting 1 week after transplantation. A significant suppression of epileptiform spikes also was noted between the group that received GABA-producing cells and the group that received the same cells but were given doxycycline. The engineered cells show evidence of integration with the host but limited survival.

CONCLUSIONS

These data demonstrate that genetically engineered cells have the ability to suppress spontaneous seizures when transplanted into seizure-modulating nuclei. This is an important step toward defining a clinical potential for this approach in epilepsy. The fact that the gene of interest can be regulated suggests that individualizing transplant therapy may be possible.

Artificial Niches for Human Adult Neural Stem Cells: Possibility for Autologous Transplantation Therapy

Cellular transplantation therapy is thought to play a central role in the concept of restorative neurosurgery, which aims to restore function to the damaged nervous system. Stem cells represent a potentially renewable source of transplantable cells. However, control of the behavior of these cells, both in the process of clonogenic expansion and post-transplantation, represents formidable challenges. Stem cell behavior is thought to be directed by extracellular signals in their in vivo niches, many of which are protein or peptide based. As only one example, activation of Notch plays an important role in normal development and is the strongest known signal for stem cells to choose glial over neuronal fates. Therefore, artificial extracellular matrix proteins represent a potentially powerful tool to custom design artificial niches to strategically control stem cell behavior. We have developed a family of aECM proteins that incorporate the active domains of the DSL ligands to the Notch receptor into an elastin-based backbone. The development of our DSL-elastin artificial proteins demonstrates the design strategy and methodology for the production of bioactive artificial extracellular matrix proteins aimed at modulating stem cell behavior, and this method can be used to design other bioactive aECM proteins. In addition, we have developed a method for the isolation and characterization of adult human neural stem cells from periventricular tissue harvested from living patients. This paper reviews cellular transplantation therapy from the clinical perspective and summarizes ongoing work aimed at exploring the intriguing possibility of autologous transplantation, whereby neural stem cells can be harvested from adult patients, expanded or modified in vitro in artificial niches, and retransplanted into the original patient.

Potential new methods for antiepileptic drug delivery

Use of novel drug delivery methods could enhance the efficacy and reduce the toxicity of antiepileptic drugs (AEDs). Slow-release oral forms of medication or depot drugs such as skin patches might improve compliance and therefore seizure control. In emergency situations, administration via rectal, nasal or buccal mucosa can deliver the drug more quickly than can oral administration. Slow-release oral forms and rectal forms of AEDs are already approved for use, nasal and buccal administration is currently off-label and skin patches for AEDs are an attractive but currently hypothetical option. Therapies under development may result in the delivery of AEDs directly to the regions of the brain involved in seizures. Experimental protocols are underway to allow continuous infusion of potent excitatory amino acid antagonists into the CSF. In experiments with animal models of epilepsy, AEDs have been delivered successfully to seizure foci in the brain by programmed infusion pumps, acting in response to computerised EEG seizure detection. Inactive prodrugs can be given systemically and activated at the site of the seizure focus by locally released compounds. One such drug under development is DP-VPA (or DP16), which is cleaved to valproic acid (sodium valproate) by phospholipases at the seizure focus. Liposomes and nanoparticles are engineered micro-reservoirs of a drug, with attached antibodies or receptor-specific binding agents designed to target the particles to a specific region of the body. Liposomes in theory could deliver a high concentration of an AED to a seizure focus. Penetration of the blood-brain barrier can be accomplished by linking large particles to iron transferrin or biological toxins that can cross the barrier. In the near future, it is likely that cell transplants that generate neurotransmitters and neuromodulators will accomplish renewable endogenous drug delivery. However, the survival and viability of transplanted cells have yet to be demonstrated in the clinical setting. Gene therapy also may play a role in local drug delivery with the use of adenovirus, adeno-associated virus, herpesvirus or other delivery vectors to induce brain cells to produce local modulatory substances. New delivery systems should significantly improve the therapeutic/toxic ratio of AEDs.

The role of catecholamines in seizure susceptibility: new results using genetically engineered mice

The catecholamines norepinephrine and dopamine are abundant in the CNS, and modulate neuronal excitability via G-protein-coupled receptor signaling. This review covers the history of research concerning the role of catecholamines in modulating seizure susceptibility in animal models of epilepsy. Traditionally, most work on this topic has been anatomical, pharmacological, or physiological in nature. However, the recent advances in transgenic and knockout mouse technology provide new tools to study catecholamines and their roles in seizure susceptibility. New results from genetically engineered mice with altered catecholamine signaling, as well as possibilities for future experiments, are discussed.

Preliminary study of the behavioral effects of LBS-neuron implantation on seizure susceptibility following middle cerebral artery occlusion in the rats

Neural transplantation is a promising treatment strategy that can restore the motor, sensory and cognitive functions in the rat middle cerebral artery occlusion (MCAO) model of stroke. In particular, neuronal cells derived from a human teratocarcinoma cell line, called hNT neurons or LBS neurons (clinical grade preparation), are effective in improving behavioral recovery after stroke. In the elderly, epilepsy is a common sequela of stroke, especially if the infarction involves cerebral cortex. However, the effect of implanting neural cells on seizure susceptibility in the MCAO model has not yet been determined. The purpose of this study was to determine the susceptibility to pentylenetetrazol (PTZ)-induced seizures in normal, MCAO-lesioned and MCAO-lesioned rats in which the LBS neurons were injected. Adult, male Sprague-Dawley rats were subjected to 60 min of MCAO using the intraluminal filament technique followed 3-4 weeks later by transplantation of 80,000 LBS-neurons into the ipsilateral cortex. Susceptibility to PTZ-induced seizures was tested 4-6 weeks post-transplant at doses of 35, 50 and 70 mg/kg, administered subcutaneously. Latency to the first lethal response, latency to first generalized seizure, duration of the first generalized seizure, and the number of generalized seizures in an hour post-PTZ treatment observation period was determined. Even thought there was a tendency for groups that underwent MCAO to be more susceptible to seizures, there were no statistically significant differences between the groups and no differences between MCAO alone and MCAO animals in which cells had been implanted. While grafted cells were identified in all but one injected animal, the results suggest that the grafts may not have been healthy either from immunological rejection or PTZ-induced injury. These results suggest that while placing cells within the cortex does not reduce seizure susceptibility, it also does not increase the incidence of seizures. Further investigations are warranted.

The intracerebral administration of phenytoin using controlled-release polymers reduces experimental seizures in rats

PURPOSE

An alternative strategy for the treatment of intractable seizures may be to administer anticonvulsants directly into the brain near the site of a seizure focus using controlled-release polymers. We describe the pharmacokinetics of a phenytoin-ethylene-vinyl acetate (EVAc) controlled-release polymer and report the reduction of seizures in a cobalt-induced rat model of epilepsy with the intracerebral delivery of phenytoin using surgically implanted polymers.

METHODS

In the pharmacokinetics study, the drug release rate of 50%-loaded phenytoin-EVAc polymers (n=3) was determined in vitro over 15 weeks initially and then several months later (over a 2-week period after 1 year of in vivo release). In the efficacy study, 85 rats underwent implantation of skull-mounted cortical electrodes for electrocorticography (ECoG) and then underwent application of cobalt chloride to the cerebral cortex for the induction of seizures. Rats in the treatment group (n=9) underwent surgical implantation of phenytoin-EVAc polymers and rats in the control group (n=10) underwent implantation of empty EVAc polymers. In the morbidity study, the potential histologic pathology of the intracerebral delivery of increasing doses of phenytoin from the polymer (10, 20, 30, and 50% loading) was assessed.

RESULTS

Phenytoin was released in vitro from EVAc polymers in a controlled fashion with an initial release of 0.20% of the total loaded dose per week and a continued release of 0.70% of the total loaded dose per week after 365 days of implantation in the brain. The intracerebral controlled-release of phenytoin resulted in a statistically significant reduction in seizure activity in the treatment group as evidenced by lower Racine scores. The four groups of rats (n=5 per group) that underwent intracerebral implantation of 10, 20, 30, or 50%-loaded phenytoin-EVAc polymers displayed expected average weight gain and normal behavior over 365 days. One rat in the 50% group, however, died 354 days after polymer implantation for undetermined reasons.

CONCLUSIONS

The intracerebral delivery of phenytoin using an EVAc polymer, which will release this drug for a calculated period of 3.5 years, resulted in a significant reduction in seizures in a rat model of cobalt-induced epilepsy by both behavioral and ECoG criteria. In rats, the long-term interstitial delivery of phenytoin in the brain was not associated with any deleterious effects.

Cell delivery to the central nervous system

A dysfunctional central nervous system (CNS) resulting from neurological disorders and diseases impacts all of humanity. The outcome presents a staggering health care issue with a tremendous potential for developing interventive therapies. The delivery of therapeutic molecules to the CNS has been hampered by the presence of the blood-brain barrier (BBB). To circumvent this barrier, putative therapeutic molecules have been delivered to the CNS by such methods as pumps/osmotic pumps, osmotic opening of the BBB, sustained polymer release systems and cell delivery via site-specific transplantation of cells. This review presents an overview of some of the CNS delivery technologies with special emphasis on transplantation of cells with and without the use of polymer encapsulation technology.

Cell replacement therapies for central nervous system disorders

In animal models, immature neural precursors can replace lost neurons, restore function and promote brain self-repair. Clinical trials in Parkinson's disease suggest that similar approaches may also work in the diseased human brain. But how realistic is it that cell replacement can be developed into effective clinical therapy?

Conditionally immortalized cell lines, engineered to produce and release GABA, modulate the development of behavioral seizures

Transplantation of genetically engineered cells can provide sustained focal delivery of naturally occurring molecules, including neurotransmitters and growth factors. We have engineered immortalized mouse cortical neurons and glia to deliver GABA by driving GAD(65) expression. Engineered cell lines showed GAD(65) mRNA expression, enzymatic activity, and GABA release. In vitro, basal flux of GABA was approximately 20% of total cellular GABA. We transplanted these GABA-producing cells bilaterally into either the anterior or the posterior substantia nigra of 43 rats. The rats were subsequently kindled through an electrode placed in the entorhinal cortex. GABA-producing cells, but not beta-galactosidase-producing cells, affected kindling rates. The number of stimulations needed to reach the first stage-5 seizure and to achieve full kindling differed significantly between the anterior and posterior transplantation sites when GAD(65)-producing cells were transplanted but not when beta-galactosidase-producing cells were transplanted. Our data show that transplanted engineered cells can make and release GABA at physiologically meaningful concentrations.

Long-term survival of fetal porcine lateral ganglionic eminence cells in the hippocampus of rats

Embryonic porcine brain tissue from the lateral ganglionic eminence was transplanted into the adult rat hippocampus to determine whether fetal striatal cells could survive, differentiate, and integrate in a heterotopic site. The hippocampus, a common site of epileptic seizure activity, was chosen to determine if fetal striatal cells could supply inhibitory GABAergic neurons that may serve to block seizures. Cells were either implanted with a single deposit using a standard metal cannula or by five smaller disseminated deposits with a glass micropipette. At 20-24 weeks, animals immunosuppressed with cyclosporin showed long-term survival of porcine cells in the adult hippocampus. Analysis by immunohistochemistry and in situ hybridization showed that the grafts contained glial and neuronal cell types, including GABAergic neurons within graft core and networks of porcine neuronal fibers extending from the graft into the host parenchyma. In addition, a marker of porcine presynaptic terminals, synaptobrevin, was abundant within the grafts and was found associated with hippocampal structures and cell layers suggesting functional integration of grafted cells within the host. The survival of xenografts in the hippocampus and potential integration of inhibitory components provides evidence that these grafts may serve as an internal negative feedback mechanism to quench epileptiform activity.

Role of the subthalamo-nigral input in the control of amygdala-kindled seizures in the rat

The substantia nigra pars reticulata (SNpr) is a critical site for the control of epileptic seizures. Potentiation of the inhibitory GABAergic input from the striatum to the SNpr suppresses primary or secondary generalized seizures in the rat. The purpose of this study was to examine the possible involvement of the excitatory glutamatergic input from the subthalamic nucleus to the SNpr in the control of both the electroencephalographic and the motor components of amygdala-kindled seizures in the rat. Microinjections of either an N-methyl-D-aspartate (NMDA) antagonist in the substantia nigra or a GABAA agonist in the subthalamic nucleus, significantly reduced motor seizures but did not modified the afterdischarges. These results provide evidence for the involvement of the subthalamo-nigral projection in the modulation and the propagation of the motor components of amygdala-kindled seizures.

The problem of intractability: the continuing need for new medical therapies in epilepsy

Treatment of epilepsy, one of the most common neurologic disorders, has evolved from "institutional" polytherapy to "dogmatic" monotherapy, and, most recently, to "rational" polypharmacy. The introduction of bromides for the treatment of epilepsy was followed first by phenobarbital and then by phenytoin as therapeutic options. Although attempts to combine medications were legion, none was supported by studies that demonstrated the benefit of such combinations. The issue of adverse effects became a principal argument in favor of monotherapy. Monotherapy, using newly developed drugs, avoided problems due to drug interactions but was ineffective in 20-30% of patients. A greater understanding of basic disease mechanisms and developments in molecular biology have led to an increased number of effective drugs for the estimated 6-12% of patients with epilepsy whose condition is intractable. Clinical research continues to build on the work of basic scientists in attempting to develop treatments based on a desire to move beyond the palliative and to affect the causative mechanisms of the disease. Novel medical approaches now under exploration include the use of drugs with complementary mechanisms of action, stimulation of various components of the nervous system, biochemical manipulations, focal intracerebral drug perfusion, and gene therapy.

Seizure suppression in kindling epilepsy by grafts of fetal GABAergic neurons in rat substantia nigra

Compared with studies on models of neurodegenerative diseases, considerably less work has been performed with neural grafts in experimental epilepsy. The potential value of this approach, however, is already shown by evidence that noradrenergic grafts implanted bilaterally into the hippocampus or amygdala-piriform cortex can suppress seizure development in the kindling model of temporal lobe epilepsy. We previously showed that amygdala kindling results in a significant decrease of GABA and its synthesizing enzyme glutamate decarboxylase in substantia nigra (SN), i.e., a region thought to be critically involved in seizure propagation in various models of epilepsy. Thus, transplantation of fetal GABAergic neurons into SN might be an effective means of permanently blocking seizure generalization in kindling epilepsy and probably also other types of epilepsy. To test this hypothesis, three groups of female Wistar rats (n = 10 per group) were kindled by electrical stimulation via a bipolar electrode in the basolateral amygdala. After all rats were fully kindled, one group was implanted with GABA-rich cells prepared from the striatal eminence of Wistar rat fetuses at embryonic day 14. The striatal neurons were bilaterally microinjected at various sites over the anterior-posterior axis of the SN, aimed at the pars reticulata. The second group received microinjections of spinal cord cell preparations, whereas the third group received microinjections of cell-free medium only. In all rats, the threshold for focal discharges (afterdischarge threshold [ADT]) as well as afterdischarge duration and severity and duration of seizures occurring at ADT current were determined once weekly before and after transplantation. Eleven to 12 weeks following transplantation, the rats were killed, and location and integration of grafts were examined by immunohistological methods. Rats with GABAergic grafts in SN exhibited a significant increase in ADT and marked reduction in seizure severity compared with pretransplantation values, whereas no such alteration was seen in the other groups. However, the seizure-suppressing effect of GABAergic grafts was not permanent but slowly disappeared over the weeks after transplantation. Although the data indicate that intranigral transplantation of GABA-producing cells is no effective means of inducing long-lasting anticonvulsant effects in experimental epilepsy, this approach may be an initial step to develop more efficient strategies for seizure suppression.

Suppression of kindling epileptogenesis in rats by intrahippocampal cholinergic grafts

Selective immunolesioning of the basal forebrain cholinergic system by 192 IgG-saporin, which leads to a dramatic loss of the cholinergic innervation in cortical and hippocampal regions, facilitates the development of hippocampal kindling in rats. The aim of the present study was to explore whether grafted cholinergic neurones are able to reverse the lesion-induced increase of seizure susceptibility. Intraventricular 192 IgG-saporin was administered to rats which 3 weeks later were implanted with rat embryonic, acetylcholine-rich septal-diagonal band tissue ('cholinergic grafts') or cortical tissue/vehicle ('sham grafts') bilaterally into the hippocampal formation. After 3 months, the grafted animals as well as non-lesioned control rats were subjected to daily hippocampal kindling stimulations. In the animals with cholinergic grafts, which had reinnervated the hippocampus and dentate gyrus bilaterally, there was a marked suppression of the development of seizures as compared with the hyperexcitable, sham-grafted rats. This effect was significantly correlated to the density of the graft-derived cholinergic innervation of the host hippocampal formation. The kindling rate in the rats with cholinergic grafts was similar to that in non-lesioned controls. These results provide further evidence that the intrinsic basal forebrain cholinergic system dampens kindling epileptogenesis and demonstrate that this function can be exerted also by grafted cholinergic neurones.