Abnormal GABAA receptors from the human epileptic hippocampal subiculum microtransplanted to Xenopus oocytes

Calsyntenin-1 expression and function in brain tissue of lithium-pilocarpine rat seizure models

To present the expression of calsyntenin-1 (Clstn1) in the brain and investigate the potential mechanism of Clstn1 in lithium-pilocarpine rat seizure models. Thirty-five male SD adult rats were induced to have seizures by intraperitoneal injection of lithium chloride pilocarpine. Rats exhibiting spontaneous seizures were divided into the epilepsy (EP) group (n = 15), whereas those without seizures were divided into the control group (n = 14). Evaluate the expression of Clstn1 in the temporal lobe of two groups using Western blotting, immunohistochemistry, and immunofluorescence. Additionally, 55 male SD rats were subjected to status epilepticus (SE) using the same induction method. Rats experiencing seizures exceeding Racine's level 4 (n = 48) were randomly divided into three groups: SE, SE + control lentivirus (lentiviral vector expressing green fluorescent protein [LV-GFP]), and SE + Clstn1-targeted RNA interference lentivirus (LV-Clstn1-RNAi). The LV-GFP group served as a control for the lentiviral vector, whereas the LV-Clstn1-RNAi group received a lentivirus designed to silence Clstn1 expression. These lentiviral treatments were administered via hippocampal stereotactic injection 2 days after SE induction. Seven days after SE, Western blot analysis was performed to evaluate the expression of Clstn1 in the hippocampus and temporal lobe. Meanwhile, we observed the latency of spontaneous seizures and the frequency of spontaneous seizures within 8 weeks among the three groups. The expression of Clstn1 in the cortex and hippocampus of the EP group was significantly increased compared to the control group (p < .05). Immunohistochemistry and immunofluorescence showed that Clstn1 was widely distributed in the cerebral cortex and hippocampus of rats, and colocalization analysis revealed that it was mainly co expressed with neurons in the cytoplasm. Compared with the SE group (11.80 ± 2.17 days) and the SE + GFP group (12.40 ± 1.67 days), there was a statistically significant difference (p < .05) in the latency period of spontaneous seizures (15.14 ± 2.41 days) in the SE + Clstn1 + RNAi group rats. Compared with the SE group (4.60 ± 1.67 times) and the SE + GFP group (4.80 ± 2.05 times), the SE + Clstn1 + RNAi group (2.0 ± .89 times) showed a significant reduction in the frequency of spontaneous seizures within 2 weeks of chronic phase in rats (p < .05). Elevated Clstn1 expression in EP group suggests its role in EP onset. Targeting Clstn1 may be a potential therapeutic approach for EP management.

Xenopus Oocytes as a Powerful Cellular Model to Study Foreign Fully-Processed Membrane Proteins

The use of Xenopus oocytes in electrophysiological and biophysical research constitutes a long and successful story, providing major advances to the knowledge of the function and modulation of membrane proteins, mostly receptors, ion channels, and transporters. Earlier reports showed that these cells are capable of correctly expressing heterologous proteins after injecting the corresponding mRNA or cDNA. More recently, the Xenopus oocyte has become an outstanding host-cell model to carry out detailed studies on the function of fully-processed foreign membrane proteins after their microtransplantation to the oocyte. This review focused on the latter overall process of transplanting foreign membrane proteins to the oocyte after injecting plasma membranes or purified and reconstituted proteins. This experimental approach allows for the study of both the function of mature proteins, with their native stoichiometry and post-translational modifications, and their putative modulation by surrounding lipids, mostly when the protein is purified and reconstituted in lipid matrices of defined composition. Remarkably, this methodology enables functional microtransplantation to the oocyte of membrane receptors, ion channels, and transporters from different sources including human post-mortem tissue banks. Despite the large progress achieved over the last decades on the structure, function, and modulation of neuroreceptors and ion channels in healthy and pathological tissues, many unanswered questions remain and, most likely, Xenopus oocytes will continue to help provide valuable responses.

Molecular Mechanisms of Epilepsy: The Role of the Chloride Transporter KCC2

Epilepsy is a neurological disease characterized by abnormal or synchronous brain activity causing seizures, which may produce convulsions, minor physical signs, or a combination of symptoms. These disorders affect approximately 65 million people worldwide, from all ages and genders. Seizures apart, epileptic patients present a high risk to develop neuropsychological comorbidities such as cognitive deficits, emotional disturbance, and psychiatric disorders, which severely impair quality of life. Currently, the treatment for epilepsy includes the administration of drugs or surgery, but about 30% of the patients treated with antiepileptic drugs develop time-dependent pharmacoresistence. Therefore, further investigation about epilepsy and its causes is needed to find new pharmacological targets and innovative therapeutic strategies. Pharmacoresistance is associated to changes in neuronal plasticity and alterations of GABA_A receptor-mediated neurotransmission. The downregulation of GABA inhibitory activity may arise from a positive shift in GABA_A receptor reversal potential, due to an alteration in chloride homeostasis. In this paper, we review the contribution of K⁺-Cl^--cotransporter (KCC2) to the alterations in the Cl^- gradient observed in epileptic condition, and how these alterations are coupled to the increase in the excitability.

Mammalian Brain Ca2+ Channel Activity Transplanted into Xenopus laevis Oocytes

Several mutations on neuronal voltage-gated Ca²⁺ channels (VGCC) have been shown to cause neurological disorders and contribute to the initiation of epileptic seizures, migraines, or cerebellar degeneration. Analysis of the functional consequences of these mutations mainly uses heterologously expressed mutated channels or transgenic mice which mimic these pathologies, since direct electrophysiological approaches on brain samples are not easily feasible. We demonstrate that mammalian voltage-gated Ca²⁺ channels from membrane preparation can be microtransplanted into Xenopus oocytes and can conserve their activity. This method, originally described to study the alteration of GABA receptors in human brain samples, allows the recording of the activity of membrane receptors and channels with their native post-translational processing, membrane environment, and regulatory subunits. The use of hippocampal, cerebellar, or cardiac membrane preparation displayed different efficacy for transplanted Ca²⁺ channel activity. This technique, now extended to the recording of Ca²⁺ channel activity, may therefore be useful in order to analyze the calcium signature of membrane preparations from unfixed human brain samples or normal and transgenic mice.

GABAA Receptor Subunit Composition Drives Its Sensitivity to the Insecticide Fipronil

Fipronil (FPN) is a worldwide-used neurotoxic insecticide, targeting, and blocking GABA_A receptors (GABA_ARs). Beyond its efficiency on insect GABA_ARs, FPN causes neurotoxic effects in humans and mammals. Here, we investigated the mode of action of FPN on mammalian α6-containing GABA_ARs to understand its inhibitory effects on GABA-induced currents, as a function of the synaptic or extrasynaptic localization of GABA_ARs. We characterized the effects of FPN by electrophysiology using Xenopus oocytes which were microtransplanted with cerebellum membranes or injected with α6β3, α6β3γ2S (synaptic), and α6β3δ (extrasynaptic) cDNAs. At micromolar concentrations, FPN dose-dependently inhibited cerebellar GABA currents. FPN acts as a non-competitive antagonist on ternary receptors. Surprisingly, the inhibition of GABA-induced currents was partial for extra-synaptic (α6β3δ) and binary (α6β3) receptors, while synaptic α6β3γ2S receptors were fully blocked, indicating that the complementary γ or δ subunit participates in FPN-GABA_AR interaction. FPN unexpectedly behaved as a positive modulator on β3 homopentamers. These data show that FPN action is driven by the subunit composition of GABA_ARs-highlighting the role of the complementary subunit-and thus their localization within a physiological synapse. We built a docking model of FPN on GABA_ARs, which reveals two putative binding sites. This is consistent with a double binding mode of FPN on GABA_ARs, possibly one being of high affinity and the other of low affinity. Physiologically, the γ/δ subunit incorporation drives its inhibitory level and has important significance for its toxicity on the mammalian nervous system, especially in acute exposure.

Synaptic Reshaping and Neuronal Outcomes in the Temporal Lobe Epilepsy

Temporal lobe epilepsy (TLE) is one of the most common types of focal epilepsy, characterized by recurrent spontaneous seizures originating in the temporal lobe(s), with mesial TLE (mTLE) as the worst form of TLE, often associated with hippocampal sclerosis. Abnormal epileptiform discharges are the result, among others, of altered cell-to-cell communication in both chemical and electrical transmissions. Current knowledge about the neurobiology of TLE in human patients emerges from pathological studies of biopsy specimens isolated from the epileptogenic zone or, in a few more recent investigations, from living subjects using positron emission tomography (PET). To overcome limitations related to the use of human tissue, animal models are of great help as they allow the selection of homogeneous samples still presenting a more various scenario of the epileptic syndrome, the presence of a comparable control group, and the availability of a greater amount of tissue for in vitro/ex vivo investigations. This review provides an overview of the structural and functional alterations of synaptic connections in the brain of TLE/mTLE patients and animal models.

Dissecting the role of subiculum in epilepsy: Research update and translational potential

The subiculum serves as the strategic core output of the hippocampus, through which neural activity exits the hippocampal proper and targets the entorhinal cortex and other more distant subcortical and cortical areas. The past decade has witnessed a growing interest in the subiculum, owing to discoveries revealing its critical role in regulating many physiological and pathophysiological processes. Notably, accumulating evidence from both clinical and experimental studies suggests that the subiculum plays a vital role in seizure initiation and propagation, in epilepsy. In this review, we briefly describe the structure and connectivity of the subiculum and then summarize the molecular and cellular mechanisms in the subiculum underlying the epileptic brain, in both epilepsy patients and animal models. Next, we review some translational approaches targeting the malfunctioned subiculum to treat epilepsy. Finally, we pose open questions for future research in the subiculum and their clinical translation challenges.

Phytocannabinoids in Neurological Diseases: Could They Restore a Physiological GABAergic Transmission?

γ-Aminobutyric acid type A receptors (GABAARs) are the main inhibitory mediators in the central nervous system (CNS). GABAARs are pentameric ligand gated ion channels, and the main subunit composition is usually 2α2βγ, with various isotypes assembled within a set of 19 different subunits. The inhibitory function is mediated by chloride ion movement across the GABAARs, activated by synaptic GABA release, reducing neuronal excitability in the adult CNS. Several studies highlighted the importance of GABA-mediated transmission during neuro-development, and its involvement in different neurological and neurodevelopmental diseases, from anxiety to epilepsy. However, while it is well known how different classes of drugs are able to modulate the GABAARs function (benzodiazepines, barbiturates, neurosteroids, alcohol), up to now little is known about GABAARs and cannabinoids interaction in the CNS. Endocannabinoids and phytocannabinoids are lately emerging as a new class of promising drugs for a wide range of neurological conditions, but their safety as medication, and their mechanisms of action are still to be fully elucidated. In this review, we will focus our attention on two of the most promising molecules (Δ9-tetrahydrocannabinol; Δ9-THC and cannabidiol; CBD) of this new class of drugs and their possible mechanism of action on GABAARs.

Role of NKCC1 and KCC2 in Epilepsy: From Expression to Function

As a main inhibitory neurotransmitter in the central nervous system, γ-aminobutyric acid (GABA) activates chloride-permeable GABAa receptors (GABAa Rs) and induces chloride ion (Cl⁻) flow, which relies on the intracellular chloride concentration ([Cl⁻]_i) of the postsynaptic neuron. The Na-K-2Cl cotransporter isoform 1 (NKCC1) and the K-Cl cotransporter isoform 2 (KCC2) are two main cation-chloride cotransporters (CCCs) that have been implicated in human epilepsy. NKCC1 and KCC2 reset [Cl⁻]_i by accumulating and extruding Cl⁻, respectively. Previous studies have shown that the profile of NKCC1 and KCC2 in neonatal neurons may reappear in mature neurons under some pathophysiological conditions, such as epilepsy. Although increasing studies focusing on the expression of NKCC1 and KCC2 have suggested that impaired chloride plasticity may be closely related to epilepsy, additional neuroelectrophysiological research aimed at studying the functions of NKCC1 and KCC2 are needed to understand the exact mechanism by which they induce epileptogenesis. In this review, we aim to briefly summarize the current researches surrounding the expression and function of NKCC1 and KCC2 in epileptogenesis and its implications on the treatment of epilepsy. We will also explore the potential for NKCC1 and KCC2 to be therapeutic targets for the development of novel antiepileptic drugs.

Using Xenopus oocytes in neurological disease drug discovery

Introduction: Neurological diseases present a difficult challenge in drug discovery. Many of the current treatments have limited efficiency or result in a variety of debilitating side effects. The search of new therapies is of a paramount importance, since the number of patients that require a better treatment is growing rapidly. As an in vitro model, Xenopus oocytes provide the drug developer with many distinct advantages, including size, durability, and efficiency in exogenous protein expression. However, there is an increasing need to refine the recent breakthroughs.Areas covered: This review covers the usage and recent advancements of Xenopus oocytes for drug discovery in neurological diseases from expression and functional measurement techniques to current applications in Alzheimer's disease, painful neuropathies, and amyotrophic lateral sclerosis (ALS). The existing limitations of Xenopus oocytes in drug discovery are also discussed.Expert opinion: With the rise of aging population and neurological disorders, Xenopus oocytes, will continue to play an important role in understanding the mechanism of the disease, identification and validation of novel molecular targets, and drug screening, providing high-quality data despite the technical limitations. With further advances in oocytes-related techniques toward an accurate modeling of the disease, the diagnostics and treatment of neuropathologies will be becoming increasing personalized.

Erythropoietin Increases GABAA Currents in Human Cortex from TLE Patients

Erythropoietin (EPO) is a hematopoietic growth factor that has an important role in the erythropoiesis. EPO and its receptor (EPO-R) are expressed all over in the mammalian brain. Furthermore, it has been reported that EPO may exert neuroprotective effect in animal models of brain disorders as ischemia and epilepsy. Here, we investigate whether EPO could modulate the GABA-evoked currents (I_GABA) in both human epileptic and non-epileptic control brain tissues. Therefore, we transplanted in Xenopus oocytes cell membranes obtained from autoptic and surgical brain tissues (cortex) of seven temporal lope epilepsy (TLE) patients and of five control patients. Two microelectrodes voltage-clamp technique has been used to record I_GABA. Moreover, qRT-PCR assay was performed in the same human tissues to quantify the relative gene expression levels of EPO/EPO-R. To further confirm experiments in oocytes, we performed additional experiments using patch-clamp recording in slices obtained from rat cerebellum. We show that exposure to EPO significantly increased the amplitude of the I_GABA in all the patients analyzed. No differences in the expression of EPO and EPO-R in both TLE and control patients have been found. Notably, the increase of I_GABA has been recorded also in rat cerebellar slices. Our findings show a new modulatory action of EPO on GABA_A receptors (GABA_A-Rs). This effect could be relevant to balance the GABAergic dysfunction in human TLE. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

Neurosteroid regulation of GABAA receptors: A role in catamenial epilepsy

The female reproductive hormones progesterone and estrogen regulate network excitability. Fluctuations in the circulating levels of these hormones during the menstrual cycle cause frequent seizures during certain phases of the cycle in women with epilepsy. This seizure exacerbation, called catamenial epilepsy, is a dominant form of drug-refractory epilepsy in women of reproductive age. Progesterone, through its neurosteroid derivative allopregnanolone, increases γ-aminobutyric acid type-A receptor (GABAR)-mediated inhibition in the brain and keeps seizures under control. Catamenial seizures are believed to be a neurosteroid withdrawal symptom, and it was hypothesized that exogenous administration of progesterone to maintain its levels high during luteal phase will treat catamenial seizures. However, in a multicenter, double-blind, phase III clinical trial, progesterone treatment did not suppress catamenial seizures. The expression of GABARs with reduced neurosteroid sensitivity in epileptic animals may explain the failure of the progesterone clinical trial. The expression of neurosteroid-sensitive δ subunit-containing GABARs is reduced, and the expression of α4γ2 subunit-containing GABARs is upregulated, which alters the inhibition of dentate granule cells in epilepsy. These changes reduce the endogenous neurosteroid control of seizures and contribute to catamenial seizures.

A novel action of lacosamide on GABAA currents sets the ground for a synergic interaction with levetiracetam in treatment of epilepsy

Epilepsy is one of the most common chronic neurological diseases, and its pharmacological treatment holds great importance for both physicians and national authorities, especially considering the high proportion of drug-resistant patients (about 30%). Lacosamide (LCM) is an effective and well-tolerated new-generation antiepileptic drug (AED), currently licensed as add-on therapy for partial-onset seizures. However, LCM mechanism of action is still a matter of debate, although its effect on the voltage sensitive sodium channels is by far the most recognized. This study aimed to retrospectively analyze a cohort of 157 drug-resistant patients treated with LCM to describe the most common and effective therapeutic combinations and to investigate if the LCM can affect also GABA_A-mediated neurotransmission as previously shown for levetiracetam (LEV). In our cohort, LEV resulted the compound most frequently associated with LCM in the responder subgroup. We therefore translated this clinical observation into the laboratory bench by taking advantage of the technique of "membrane micro-transplantation" in Xenopus oocytes and electrophysiological approaches to study human GABA_A-evoked currents. In cortical brain tissues from refractory epileptic patients, we found that LCM reduces the use-dependent GABA impairment (i.e., "rundown") that it is considered one of the specific hallmarks of drug-resistant epilepsies. Notably, in line with our clinical observations, we found that the co-treatment with subthreshold concentrations of LCM and LEV, which had no effect on GABA_A currents on their own, reduced GABA impairment in drug-resistant epileptic patients, and this effect was blocked by PKC inhibitors. Our findings demonstrate, for the first time, that LCM targets GABA_A receptors and that it can act synergistically with LEV, improving the GABAergic function. This novel mechanism might contribute to explain the clinical efficacy of LCM-LEV combination in several refractory epileptic patients.

Perampanel, an antagonist of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors, for the treatment of epilepsy: studies in human epileptic brain and nonepileptic brain and in rodent models

Perampanel [Fycompa, 2-(2-oxo-1-phenyl-5-pyridin-2-yl-1,2-dihydropyridin-3-yl)benzonitrile hydrate 4:3; Eisai Inc., Woodcliff Lake, NJ] is an AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor antagonist used as an adjunctive treatment of partial-onset seizures. We asked whether perampanel has AMPA receptor antagonist activity in both the cerebral cortex and hippocampus associated with antiepileptic efficacy and also in the cerebellum associated with motor side effects in rodent and human brains. We also asked whether epileptic or nonepileptic human cortex is similarly responsive to AMPA receptor antagonism by perampanel. In rodent models, perampanel decreased epileptic-like activity in multiple seizure models. However, doses of perampanel that had anticonvulsant effects were within the same range as those engendering motor side effects. Perampanel inhibited native rat and human AMPA receptors from the hippocampus as well as the cerebellum that were reconstituted into Xenopus oocytes. In addition, with the same technique, we found that perampanel inhibited AMPA receptors from hippocampal tissue that had been removed from a patient who underwent surgical resection for refractory epilepsy. Perampanel inhibited AMPA receptor-mediated ion currents from all the tissues investigated with similar potency (IC50 values ranging from 2.6 to 7.0 μM). Cortical slices from the left temporal lobe derived from the same patient were studied in a 60-microelectrode array. Large field potentials were evoked on at least 45 channels of the array, and 10 μM perampanel decreased their amplitude and firing rate. Perampanel also produced a 33% reduction in the branching parameter, demonstrating the effects of perampanel at the network level. These data suggest that perampanel blocks AMPA receptors globally across the brain to account for both its antiepileptic and side-effect profile in rodents and epileptic patients.

Does epilepsy cause a reversion to immature function?

Seizures have variable effects on brain. Numerous studies have examined the consequences of seizures, in light of the way that these may alter the susceptibility of the brain to seizures, promote epileptogenesis, or functionally alter brain leading to seizure-related comorbidities. In many -but not all- situations, seizures shift brain function towards a more immature state, promoting the birth of newborn neurons, altering the dendritic structure and neuronal connectivity, or changing neurotransmitter signaling towards more immature patterns. These effects depend upon many factors, including the seizure type, age of seizure occurrence, sex, and brain region studied. Here we discuss some of these findings proposing that these seizure-induced immature features do not simply represent rejuvenation of the brain but rather a de-synchronization of the homeostatic mechanisms that were in place to maintain normal physiology, which may contribute to epileptogenesis or the cognitive comorbidities.

Changes in the sensitivity of GABAA current rundown to drug treatments in a model of temporal lobe epilepsy

The pharmacological treatment of mesial temporal lobe epilepsy (mTLE), the most common epileptic syndrome in adults, is still unsatisfactory, as one-third of the patients are or become refractory to antiepileptic agents. Refractoriness may depend upon drug-induced alterations, but the disease per se may also undergo a progressive evolution that affects the sensitivity to drugs. mTLE has been shown to be associated with a dysfunction of the inhibitory signaling mediated by GABA_A receptors. In particular, the repetitive activation of GABA_A receptors produces a use-dependent decrease (rundown) of the evoked currents (I_GABA), which is markedly enhanced in the hippocampus and cortex of drug-resistant mTLE patients. This phenomenon has been also observed in the pilocarpine model, where the increased I_GABA rundown is observed in the hippocampus at the time of the first spontaneous seizure, then extends to the cortex and remains constant in the chronic phase of the disease. Here, we examined the sensitivity of I_GABA to pharmacological modulation. We focused on the antiepileptic agent levetiracetam (LEV) and on the neurotrophin brain-derived neurotrophic factor (BDNF), which were previously reported to attenuate mTLE-induced increased rundown in the chronic human tissue. In the pilocarpine model, BDNF displayed a paramount effect, decreasing rundown in the hippocampus at the time of the first seizure, as well as in the hippocampus and cortex in the chronic period. In contrast, LEV did not affect rundown in the hippocampus, but attenuated it in the cortex. Interestingly, this effect of LEV was also observed on the still unaltered rundown observed in the cortex at the time of the first spontaneous seizure. These data suggest that the sensitivity of GABA_A receptors to pharmacological interventions undergoes changes during the natural history of mTLE, implicating that the site of seizure initiation and the timing of treatment may highly affect the therapeutic outcome.

Robust photoregulation of GABA(A) receptors by allosteric modulation with a propofol analogue

Photochemical switches represent a powerful method for improving pharmacological therapies and controlling cellular physiology. Here we report the photo-regulation of GABA_A receptors (GABA_ARs) by a derivative of propofol (2,6-diisopropylphenol), a GABA_AR allosteric modulator, that we have modified to contain photo-isomerizable azobenzene. Using α₁β₂γ₂ GABA_ARs expressed in Xenopus laevis oocytes and native GABA_ARs of isolated retinal ganglion cells, we show that the trans-azobenzene isomer of the new compound (trans-MPC088), generated by visible light (wavelengths ~440 nm), potentiates the GABA-elicited response and at higher concentrations directly activates the receptors. cis-MPC088, generated from trans-MPC088 by UV light (~365 nm), produces little if any receptor potentiation/activation. In cerebellar slices, MPC088 co-applied with GABA affords bidirectional photo-modulation of Purkinje cell membrane current and spike-firing rate. The findings demonstrate photo-control of GABA_ARs by an allosteric ligand and open new avenues for fundamental and clinically oriented research on GABA_ARs, a major class of neurotransmitter receptors in the central nervous system.

Microtransplantation of cellular membranes from squid stellate ganglion reveals ionotropic GABA receptors

The squid has been the most studied cephalopod, and it has served as a very useful model for investigating the events associated with nerve impulse generation and synaptic transmission. While the physiology of squid giant axons has been extensively studied, very little is known about the distribution and function of the neurotransmitters and receptors that mediate inhibitory transmission at the synapses. In this study we investigated whether γ-aminobutyric acid (GABA) activates neurotransmitter receptors in stellate ganglia membranes. To overcome the low abundance of GABA-like mRNAs in invertebrates and the low expression of GABA in cephalopods, we used a two-electrode voltage clamp technique to determine if Xenopus laevis oocytes injected with cell membranes from squid stellate ganglia responded to GABA. Using this method, membrane patches containing proteins and ion channels from the squid's stellate ganglion were incorporated into the surface of oocytes. We demonstrated that GABA activates membrane receptors in cellular membranes isolated from squid stellate ganglia. Using the same approach, we were able to record native glutamate-evoked currents. The squid's GABA receptors showed an EC(50) of 98 μmol l(-1) to GABA and were inhibited by zinc (IC(50) = 356 μmol l(-1)). Interestingly, GABA receptors from the squid were only partially blocked by bicuculline. These results indicate that the microtransplantation of native cell membranes is useful to identify and characterize scarce membrane proteins. Moreover, our data also support the role of GABA as an ionotropic neurotransmitter in cephalopods, acting through chloride-permeable membrane receptors.

Anomalous levels of Cl- transporters cause a decrease of GABAergic inhibition in human peritumoral epileptic cortex

PURPOSE

Several factors contribute to epileptogenesis in patients with brain tumors, including reduced γ-aminobutyric acid (GABA)ergic inhibition. In particular, changes in Cl(-) homeostasis in peritumoral microenvironment, together with alterations of metabolism, are key processes leading to epileptogenesis in patients afflicted by glioma. It has been recently proposed that alterations of Cl(-) homeostasis could be involved in tumor cell migration and metastasis formation. In neurons, the regulation of intracellular Cl(-) concentration ([Cl(-) ](i) ) is mediated by NKCC1 and KCC2 transporters: NKCC1 increases while KCC2 decreases [Cl(-) ](i) . Experiments were thus designed to investigate whether, in human epileptic peritumoral cortex, alterations in the balance of NKCC1 and KCC2 activity may decrease the hyperpolarizing effects of GABA, thereby contributing to epileptogenesis in human brain tumors.

METHODS

Membranes from peritumoral cortical tissues of epileptic patients afflicted by gliomas (from II to IV WHO grade) and from cortical tissues of nonepileptic patients were injected into Xenopus oocytes leading to the incorporation of functional GABA(A) receptors. The GABA-evoked currents were recorded using standard two-microelectrode voltage-clamp technique. In addition, immunoblot analysis and immunohistochemical staining were carried out on membranes and tissues from the same patients.

KEY FINDINGS

We found that in oocytes injected with epileptic peritumoral cerebral cortex, the GABA-evoked currents had a more depolarized reversal potential (E(GABA) ) compared to those from nonepileptic healthy cortex. This difference of E(GABA) was abolished by the NKCC1 blocker bumetanide or unblocking of KCC2 with the Zn(2+) chelator TPEN. Moreover, Western blot analysis revealed an increased expression of NKCC1, and more modestly, of KCC2 transporters in epileptic peritumoral tissues compared to nonepileptic control tissues. In addition, NKCC1 immunoreactivity was strongly increased in peritumoral cortex with respect to nonepileptic cortex, with a prominent expression in neuronal cells.

SIGNIFICANCE

We report that the positive shift of E(GABA) in epileptic peritumoral human cortex is due to an altered expression of NKCC1 and KCC2, perturbing Cl(-) homeostasis, which might lead to a consequent reduction in GABAergic inhibition. These findings point to a key role of Cl(-) transporters KCC2 and NKCC1 in tumor-related epilepsy, suggesting a more specific drug therapy and surgical approaches for the epileptic patients afflicted by brain tumors.

Functional rundown of gamma-aminobutyric acid(A) receptors in human hypothalamic hamartomas

OBJECTIVE

Human hypothalamic hamartomas (HHs) are highly associated with treatment-resistant gelastic seizures. HHs are intrinsically epileptogenic, although the basic cellular mechanisms responsible for seizure activity are unknown. Altered gamma-aminobutyric acid (GABA) function can contribute to epileptogenesis in humans and animal models. Recently, functional GABA(A) receptor (GABA(A) R) rundown has been described in surgically resected human temporal lobe epilepsy tissue. We asked whether functional GABA(A) R rundown also occurs in human HH neurons.

METHODS

GABA(A) R-mediated currents were measured using perforated patch-clamp recordings in single neurons acutely dissociated from surgically resected HH tissue. In addition, functional GABA(A) Rs were expressed in Xenopus oocytes after microinjection with membrane fractions from either HH or control hypothalamus, and were studied with 2-electrode voltage-clamp recordings.

RESULTS

Perforated patch-clamp recordings in dissociated HH neurons showed that repetitive exposure to GABA (5 consecutive exposures to 0.1 mM GABA with 1-second duration and at 20-second intervals) induced a time-dependent rundown of whole-cell currents in small HH neurons, whereas large HH neurons showed much less rundown using the same protocol. Functional rundown was not observed in HH neurons with repetitive exposure to glycine or glutamate. Two-electrode voltage-clamp recordings (6 consecutive exposures to 1 mM GABA with 10-second duration and at 40-second intervals) induced GABA current rundown in Xenopus oocytes microinjected with HH membrane proteins, but not in the oocytes expressing hypothalamic membrane proteins derived from human autopsy controls. Functional rundown of GABA currents was significantly attenuated by intracellular application of adenosine triphosphate or the nonspecific phosphatase inhibitor, okadaic acid.

INTERPRETATION

Neurons from surgically resected human HH demonstrate functional rundown of GABA(A) R-mediated transmembrane currents in response to GABA agonist exposure. Rundown may be a marker for impaired GABAergic function and a contributing mechanism for seizure genesis within HH tissue.

Medicinal chemistry of ρ GABAC receptors

The inhibitory neurotransmitter, GABA, is a low-molecular-weight molecule that can achieve many low-energy conformations, which are recognized by GABA receptors and transporters. In this article, we assess the structure–activity relationship profiles of GABA analogs at the ionotropic ρ GABAC receptor. Such studies have significantly contributed to the design and development of potent and selective agonists and antagonists for this subclass of GABA receptors. With these tools in hand, the role of ρ GABAC receptors is slowly being realized. Of particular interest is the development of selective phosphinic acid analogs of GABA and their potential use in sleep disorders, inhibiting the development of myopia, and in improving learning and memory.

Altered GABA(A) receptor subunit expression and pharmacology in human Angelman syndrome cortex

The neurodevelopmental disorder Angelman syndrome is most frequently caused by deletion of the maternally derived chromosome 15q11-q13 region, which includes not only the causative UBE3A gene, but also the beta(3)-alpha(5)-gamma(3) GABA(A) receptor subunit gene cluster. GABAergic dysfunction has been hypothesized to contribute to the occurrence of epilepsy and cognitive and behavioral impairments in this condition. In the present study, analysis of GABA(A) receptor subunit expression and pharmacology was performed in cerebral cortex from four subjects with Angelman syndrome and compared to that from control tissue. The membrane fraction of frozen postmortem neocortical tissue was isolated and subjected to quantitative Western blot analysis. The ratios of beta(3)/beta(2) and alpha(5)/alpha(1) subunit protein expression in Angelman syndrome cortex were significantly decreased when compared with controls. An additional membrane fraction was injected into Xenopus oocytes, resulting in incorporation of the brain membrane vesicles with their associated receptors into the oocyte cellular membrane. Two-electrode voltage-clamp analysis of GABA(A) receptor currents was then performed. Studies of GABA(A) receptor pharmacology in Angelman syndrome cortex revealed increased current enhancement by the alpha(1)-selective benzodiazepine-site agonist zolpidem and by the barbiturate phenobarbital, while sensitivity to current inhibition by zinc was decreased. GABA(A) receptor affinity and modulation by neurosteroids were unchanged. This shift in GABA(A) receptor subunit expression and pharmacology in Angelman syndrome is consistent with impaired extrasynaptic but intact to augmented synaptic cortical GABAergic inhibition, which could contribute to the epileptic, behavioral, and cognitive phenotypes of the disorder.

Enhancement of GABA(A)-current run-down in the hippocampus occurs at the first spontaneous seizure in a model of temporal lobe epilepsy

Refractory temporal lobe epilepsy (TLE) is associated with a dysfunction of inhibitory signaling mediated by GABA(A) receptors. In particular, the use-dependent decrease (run-down) of the currents (I(GABA)) evoked by the repetitive activation of GABA(A) receptors is markedly enhanced in hippocampal and cortical neurons of TLE patients. Understanding the role of I(GABA) run-down in the disease, and its mechanisms, may allow development of medical alternatives to surgical resection, but such mechanistic insights are difficult to pursue in surgical human tissue. Therefore, we have used an animal model (pilocarpine-treated rats) to identify when and where the increase in I(GABA) run-down occurs in the natural history of epilepsy. We found: (i) that the increased run-down occurs in the hippocampus at the time of the first spontaneous seizure (i.e., when the diagnosis of epilepsy is made), and then extends to the neocortex and remains constant in the course of the disease; (ii) that the phenomenon is strictly correlated with the occurrence of spontaneous seizures, because it is not observed in animals that do not become epileptic. Furthermore, initial exploration of the molecular mechanism disclosed a relative increase in alpha4-, relative to alpha1-containing GABA(A) receptors, occurring at the same time when the increased run-down appears, suggesting that alterations in the molecular composition of the GABA receptors may be responsible for the occurrence of the increased run-down. These observations disclose research opportunities in the field of epileptogenesis that may lead to a better understanding of the mechanism whereby a previously normal tissue becomes epileptic.

Adenosine receptor antagonists alter the stability of human epileptic GABAA receptors

We examined how the endogenous anticonvulsant adenosine might influence gamma-aminobutyric acid type A (GABA(A)) receptor stability and which adenosine receptors (ARs) were involved. Upon repetitive activation (GABA 500 microM), GABA(A) receptors, microtransplanted into Xenopus oocytes from neurosurgically resected epileptic human nervous tissues, exhibited an obvious GABA(A)-current (I(GABA)) run-down, which was consistently and significantly reduced by treatment with the nonselective adenosine receptor antagonist CGS15943 (100 nM) or with adenosine deaminase (ADA) (1 units/ml), that inactivates adenosine. It was also found that selective antagonists of A2B (MRS1706, 10 nM) or A3 (MRS1334, 30 nM) receptors reduced I(GABA) run-down, whereas treatment with the specific A1 receptor antagonist DPCPX (10 nM) was ineffective. The selective A2A receptor antagonist SCH58261 (10 nM) reduced or potentiated I(GABA) run-down in approximately 40% and approximately 20% of tested oocytes, respectively. The ADA-resistant, AR agonist 2-chloroadenosine (2-CA) (10 microM) potentiated I(GABA) run-down but only in approximately 20% of tested oocytes. CGS15943 administration again decreased I(GABA) run-down in patch-clamped neurons from either human or rat neocortex slices. I(GABA) run-down in pyramidal neurons was equivalent in A1 receptor-deficient and wt neurons but much larger in neurons from A2A receptor-deficient mice, indicating that, in mouse cortex, GABA(A)-receptor stability is tonically influenced by A2A but not by A1 receptors. I(GABA) run-down from wt mice was not affected by 2-CA, suggesting maximal ARs activity by endogenous adenosine. Our findings strongly suggest that cortical A2-A3 receptors alter the stability of GABA(A) receptors, which could offer therapeutic opportunities.

Loss of GABAergic neurons in the subiculum and its functional implications in temporal lobe epilepsy

Clinical and experimental evidence suggest that the subiculum plays an important role in the maintenance of temporal lobe seizures. Using the pilocarpine-model of temporal lobe epilepsy (TLE), the present study examines the vulnerability of GABAergic subicular interneurons to recurrent seizures and determines its functional implications. In the subiculum of pilocarpine-treated animals, the density of glutamic acid decarboxylase (GAD) mRNA-positive cells was reduced in all layers. Our data indicate a substantial loss of parvalbumin-immunoreactive neurons in the pyramidal cell and molecular layer whereas calretinin-immunoreactive cells were predominantly reduced in the molecular layer. Though the subiculum of pilocarpine-treated rats showed an increased intensity of GAD65 immunoreactivity, the density of GAD65 containing synaptic terminals in the pyramidal cell layer was decreased indicating an increase in the GAD65 intensity of surviving synaptic terminals. We observed a decrease in evoked inhibitory post-synaptic currents that mediate dendritic inhibition as well as a decline in the frequency of miniature inhibitory post-synaptic currents (mIPSCs) that are restricted to the perisomatic region. The decrease in mIPSC frequency (-30%) matched with the reduced number of perisomatic GAD-positive terminals (-28%) suggesting a decrease of pre-synaptic GABAergic input onto pyramidal cells in epileptic animals. Though cell loss in the subiculum has not been considered as a pathogenic factor in human and experimental TLE, our data suggest that the vulnerability of subicular GABAergic interneurons causes an input-specific disturbance of the subicular inhibitory system.

GABA(A)-current rundown of temporal lobe epilepsy is associated with repetitive activation of GABA(A) "phasic" receptors

A study was made of the "rundown" of GABA(A) receptors, microtransplanted to Xenopus oocytes from surgically resected brain tissues of patients afflicted with drug-resistant human mesial temporal lobe epilepsy (mTLE). Cell membranes, isolated from mTLE neocortex specimens, were injected into frog oocytes that rapidly incorporated functional GABA(A) receptors. Upon repetitive activation with GABA (1 mM), "epileptic" GABA(A) receptors exhibited a GABA(A)-current (I(GABA)) rundown that was significantly enhanced by Zn(2+) (</=250 microM), and practically abolished by the high-affinity GABA(A) receptor inverse agonist SR95531 (gabazine; 2.5-25 microM). Conversely, I(GABA) generated by "control" GABA(A) receptors microtransplanted from nonepileptic temporal lobe, lesional TLE, or authoptic disease-free tissues remained stable during repetitive stimulation, even in oocytes treated with Zn(2+). We conclude that rundown of mTLE epileptic receptors depends on the presence of "phasic GABA(A) receptors" that have low sensitivity to antagonism by Zn(2+). Additionally, we found that GABA(A) receptors, microtransplanted from the cerebral cortex of adult rats exhibiting recurrent seizures, caused by pilocarpine-induced status epilepticus, showed greater rundown than control tissue, an event also occurring in patch-clamped rat pyramidal neurons. Rundown of epileptic rat receptors resembled that of human mTLE receptors, being enhanced by Zn(2+) (40 microM) and sensitive to the antiepileptic agent levetiracetam, the neurotrophin brain-derived neurotrophic factor, and the phosphatase blocker okadaic acid. Our findings point to the rundown of GABA(A) receptors as a hallmark of TLE and suggest that modulating tonic and phasic mTLE GABA(A) receptor activity may represent a useful therapeutic approach to the disease.

The Antiepileptic Drug Levetiracetam Stabilizes the Human Epileptic GABAAReceptors upon Repetitive Activation

PURPOSE

GABAA receptors from the brain of patients afflicted with mesial temporal lobe epilepsy (MTLE) become less efficient (run-down) when repetitively activated by GABA. Experiments were designed to investigate whether the antiepileptic drug, levetiracetam (LEV), which is used as an adjunctive treatment for medically intractable MTLE, counteracts the GABAA receptor run-down.

METHODS

GABAA receptors were microtransplanted from the brains of patients afflicted with MTLE into Xenopus oocytes. The GABA-current run-down, caused by repetitive applications of GABA, was investigated using the standard two-microelectrode voltage-clamp technique. Additionally, the GABA-current run-down was investigated directly on pyramidal neurons in human MTLE cortical slices.

RESULTS

It was found that, in oocytes injected with membranes isolated from the MTLE neocortex, the GABA-current run-down was inhibited by a 3-h pretreatment with 0.5-100 microM LEV. Moreover, the GABAA receptors of pyramidal neurons in human neocortical slices exhibited a current run-down that was significantly reduced by 1 microM LEV. Interestingly, the run-down in oocytes injected with membranes isolated from the MTLE hippocampal subiculum was not affected by LEV.

CONCLUSIONS

We report that the antiepileptic LEV strengthens GABA inhibition of neuronal circuits by blocking the receptor run-down in the cortex whilst leaving the run-down of GABAA receptors in the hippocampal subiculum unaltered. These findings point to the GABAA receptor run-down as an important event in epileptogenesis and as a possible target for testing and screening antiepileptic drugs.

Etiology and management of refractory epilepsies

The epilepsies are an important, common and diverse group of symptom complexes characterized by recurrent spontaneous seizures. Although many patients with epilepsy have their seizures controlled effectively by antiepileptic drugs (AEDs), about one-third of patients continue to have seizures, despite trying a range of AEDs. Such patients bear the heaviest burden of epilepsy, with increased morbidity and risk of premature mortality. Our current understanding of the refractory epilepsies--the most common of which are focal--is limited; even their definition is problematic. Standard treatments for refractory epilepsies include optimization of existing AED regimens, trials of further AEDs, and, for some patients, therapeutic resective neurosurgery. Recent basic research has explored possible underlying causes of refractory epilepsy, and two main hypotheses have emerged to account for the failure of AED treatment. According to one hypothesis, AEDs might fail because of alterations in the properties of their usual targets. Alternatively, they might fail because multidrug transporter mechanisms limit concentrations of the drugs at their targets. The refractory epilepsies can be viewed as offering remarkable insights into biological processes in the epilepsies, and their effective treatment remains an important aim; treatment would potentially bring much-needed relief to hundreds of thousands of patients across the world.

Electrophysiological properties and subunit composition of GABAA receptors in patients with gelastic seizures and hypothalamic hamartoma

Abnormalities in GABA(A) receptor structure and/or function have been associated with various forms of epilepsy in both humans and animals. Whether this is true for patients with gelastic seizures and hypothalamic hamartoma (HH) is unknown. In this study, we characterized the pharmacological properties and native subunit composition of GABA(A) receptors on acutely dissociated single neurons from surgically resected HH tissues using patch-clamp, immunocytochemical, and RT-PCR techniques. We found that 1) GABA induced an inward current (I(GABA)) at a holding potential of -60 mV; 2) I(GABA) was mimicked by the GABA(A) receptor agonist muscimol and blocked by the GABA(A) receptor antagonist bicuculline, suggesting that I(GABA) was mediated principally through the GABA(A) receptor; 3) the EC(50) and Hill coefficient derived from the I(GABA) concentration-response curve were 6.8 muM and 1.9, respectively; 4) the current-voltage curve was linear at a reversal potential close to zero; and 5) I(GABA) exhibited low sensitivity to zinc and diazepam but higher sensitivity to pentobarbital and pregnanolone. Additionally, using Xenopus oocytes microtransplanted with normal human hypothalamic tissue, we confirmed that the functional properties of GABA(A) receptors were similar to those seen in small isolated HH neurons. Finally, the expression profile of GABA(A) receptor subunits obtained from normal control human hypothalamic tissue was identical to that from surgically resected human HH tissue. Taken together, our data indicate that GABA(A) receptors on small HH neurons exhibit normal pharmacosensitivity and subunit composition. These findings bear relevance to a broader understanding of inhibitory neurotransmission in human HH tissue.

Repeated intracerebroventricular infusion of nicotine prevents kainate-induced neurotoxicity by activating the α7 nicotinic acetylcholine receptor

We examined whether (-)-nicotine infusion can affect kainic acid (KA)-induced neurotoxicity in rats. Although treatment with a single nicotine infusion (0.5 or 1.0 microg/side, i.c.v.) failed to attenuate KA-induced neurotoxicity, repeated nicotine infusions (1.0 microg/side/day for 10 days) attenuated the seizures, the severe loss of cells in hippocampal regions CA1 and CA3, the increase in activator protein (AP)-1 DNA binding activity, and mortality after KA administration. alpha-Bungarotoxin and mecamylamine blocked the neuroprotective effects of nicotine. These results suggest that repeated nicotine treatment provides alpha7 nicotinic acetylcholine receptor-mediated neuroprotection against KA toxicity.

Properties of glutamate receptors of Alzheimer's disease brain transplanted to frog oocytes

It is known that Alzheimer's disease (AD) is a synaptic disease that involves various neurotransmitter systems, particularly those where synaptic transmission is mediated by acetylcholine or glutamate (Glu). Nevertheless, very little is known about the properties of neurotransmitter receptors of the AD human brain. We have shown previously that cell membranes, carrying neurotransmitter receptors from the human postmortem brain, can be transplanted to frog oocytes, and their receptors will still be functional. Taking advantage of this fact, we have now studied the properties of Glu receptors (GluRs) from the cerebral cortices of AD and non-AD brains and found that oocytes injected with AD membranes acquired GluRs that have essentially the same functional properties as those of oocytes injected with membranes from non-AD brains. However, the amplitudes of the currents elicited by Glu were always smaller in the oocytes injected with membranes from AD brains. Western blot analyses of the same membrane preparations used for the electrophysiological studies showed that AD membranes contained significantly fewer GluR2/3 subunit proteins. Furthermore, the corresponding mRNAs were also diminished in the AD brain. Therefore, the smaller amplitude of membrane currents elicited by Glu in oocytes injected with membranes from an AD brain is a consequence of a reduced number of GluRs in cell membranes transplanted from the AD brain. Thus, using the comparatively simple method of microtransplantation of receptors, it is now possible to determine the properties of neurotransmitter receptors of normal and diseased human brains. That knowledge may help to decipher the etiology of the diseases and also to develop new treatments.

GABAergic synaptic inhibition is reduced before seizure onset in a genetic model of cortical malformation

Malformations of the neocortex are a common cause of human epilepsy; however, the critical issue of how disturbances in cortical organization render neurons epileptogenic remains controversial. The present study addressed this issue by studying inhibitory structure and function before seizure onset in the telencephalic internal structural heterotopia (tish) rat, which is a genetic model of heightened seizure susceptibility associated with a prominent neocortical malformation. Both normally positioned (normotopic) and misplaced (heterotopic) pyramidal neurons in the tish neocortex exhibited lower resting membrane potentials and a tendency toward higher input resistance compared with pyramidal neurons from control brains. GABAergic synaptic transmission was attenuated in the tish cortex, characterized by significant reductions in the frequency of spontaneous IPSCs (sIPSCs) and miniature IPSCs recorded from pyramidal neurons. In addition, the amplitudes of sIPSCs were reduced in the tish neocortex, an effect that was more profound in the normotopic cells. Immunohistochemical assessment of presynaptic GABAergic terminals showed a reduction in terminals surrounding pyramidal cell somata in normotopic and heterotopic tish neocortex. The attenuation of inhibitory innervation was more prominent for normotopic neurons and was associated with a reduction in a subset of GABAergic interneurons expressing the calcium-binding protein parvalbumin. Together, these findings indicate that key facets of inhibitory GABAergic neurotransmission are disturbed before seizure onset in a brain predisposed to developing seizures. Such alterations represent a rational substrate for reduced seizure thresholds associated with certain cortical malformations.

A GABAA receptor mutation causing generalized epilepsy reduces benzodiazepine receptor binding

Understanding the consequences of newly discovered single gene mutations causing human epilepsy has the potential to yield new insights into the underlying mechanisms of this disorder. A mutation of the gamma2 subunit of the GABA(A) receptor, which substitutes glutamine for arginine at position 43 (R43Q) has been found in a familial generalized epilepsy. We tested the hypothesis that individuals affected by the GABRG2(R43Q) mutation have reduced binding to the GABA(A) receptor complex using positron emission tomography (PET) and the benzodiazepine receptor ligand [(11)C]-flumazenil. Fourteen subjects with the GABRG2(R43Q) mutation and 20 controls were studied. Benzodiazepine receptor binding was reduced in subjects with the mutation (mean whole brain binding potential for [(11)C]-flumazenil: GABA(A) mutation 0.66+/-0.1; controls 0.89+/-0.1; P<0.003). The greatest change in benzodiazepine binding occurred anteriorly, with peak differences in insular and anterior cingulate cortices revealed by statistical parametric mapping. Our findings provide in vivo evidence of reduced benzodiazepine receptor binding in subjects with the mutation. As synaptic inhibition in the human brain is largely mediated by the GABA(A) receptor, these findings are likely to represent an important clue to the mechanisms linking this gene defect and the epilepsy phenotype.

Anomalous levels of Cl- transporters in the hippocampal subiculum from temporal lobe epilepsy patients make GABA excitatory

The mRNA levels of NKCC1, an inwardly directed Na(+), K(+)-2Cl(-) cotransporter that facilitates the accumulation of intracellular Cl(-), and of KCC2, an outwardly directed K(+)-Cl(-) cotransporter that extrudes Cl(-), were studied in surgically resected brain specimens from drug-resistant temporal lobe (TL) epilepsy (TLE) patients. Quantitative RT-PCR analyses of the mRNAs extracted from the human TLE-associated brain regions revealed an up-regulation of NKCC1 mRNA and a down-regulation of KCC2 mRNA in the hippocampal subiculum, compared with the hippocampus proper or the TL neocortex, suggesting an abnormal transcription of Cl(-) transporters in the TLE subiculum. In parallel experiments, cell membranes isolated from the same TLE-associated brain regions were injected into Xenopus oocytes that rapidly incorporated human GABA(A) receptors into their surface membrane. The GABA currents elicited in oocytes injected with membranes from the subiculum had a more depolarized reversal potential (E(GABA)) compared with the hippocampus proper or the neocortex. The NKCC1 blocker bumetanide or a temperature decrease of 10 degrees C shifted the GABA-current E(GABA) more negative in oocytes injected with membranes from TLE hippocampal subiculum, matching the E(GABA) of TL neocortex-injected oocytes. We conclude that the anomalous expression of both Cl(-) transporters, NKCC1 and KCC2 [corrected] in TLE hippocampal subiculum probably causes altered Cl(-) transport in the "epileptic" neurons, as revealed in the microtransplanted Xenopus oocytes, and renders GABA aberrantly "exciting," a feature that may contribute to the precipitation of epileptic seizures.

Cellular and molecular mechanisms of epilepsy in the human brain

Animal models have provided invaluable data for identifying the pathogenesis of epileptic disorders. Clearly, the relevance of these experimental findings would be strengthened by the demonstration that similar fundamental mechanisms are at work in the human epileptic brain. Epilepsy surgery has indeed opened the possibility to directly study the functional properties of human brain tissue in vitro, and to analyze the mechanisms underlying seizures and epileptogenesis. Here, we summarize the findings obtained over the last 40 years from electrophysiological, histochemical and molecular experiments made with the human brain tissue. In particular, this review will focus on (i) the synaptic and non-synaptic properties of neocortical neurons along with their ability to produce synchronous activity; (ii) the anatomical and functional alterations that characterize limbic structures in patients presenting with mesial temporal lobe epilepsy; (iii) the issue of antiepileptic drug action and resistance; and (iv) the pathophysiology of seizure genesis in Taylor's type focal cortical dysplasia. Finally, we will address some of the problems that are inherent to this type of experimental approach, in particular the lack of proper controls and possible strategies to obviate this limitation.

Rundown of GABA type A receptors is a dysfunction associated with human drug-resistant mesial temporal lobe epilepsy

Pharmacotherapeutic strategies have been difficult to develop for several forms of temporal lobe epilepsy, which are consequently treated by surgical resection. To examine this problem, we have studied the properties of transmitter receptors of tissues removed during surgical treatment. We find that when cell membranes, isolated from the temporal neocortex of patients afflicted with drug-resistant mesial temporal lobe epilepsy (TLE), are injected into frog oocytes they acquire GABA type A receptors (GABA(A)-receptors) that display a marked rundown during repetitive applications of GABA. In contrast, GABA(A)-receptor function is stable in oocytes injected with cell membranes isolated from the temporal lobe of TLE patients afflicted with neoplastic, dysgenetic, traumatic, or ischemic temporal lesions (lesional TLE, LTLE). Use-dependent GABA(A)-receptor rundown is also found in the pyramidal neurons of TLE neocortical slices and is antagonized by BDNF. Pyramidal neurons in cortical slices of a traumatic LTLE patient did not show GABA(A)-receptor rundown. However, the apparent affinity of GABA(A)-receptor in oocytes microtransplanted with membranes from all of the epileptic patients studied was smaller than the affinity of receptors transplanted from the nonepileptic brain. We conclude that the use-dependent rundown of neocortical GABA(A)-receptor represents a TLE-specific dysfunction, whereas the reduced affinity may be a general feature of brains of both TLE and LTLE patients, and we speculate that our findings may help to develop new treatments for TLE and LTLE.