Glutamate transporters prevent the generation of seizures in the developing rat neocortex

NMDARs regulate the excitatory-inhibitory balance within neural circuits

Excitatory-inhibitory (E/I) balance is essential for normal neural development, behavior and cognition. E/I imbalance leads to a variety of neurological disorders, such as autism and schizophrenia. NMDA receptors (NMDARs) regulate AMPAR-mediated excitatory and GABAAR-mediated inhibitory synaptic transmission, suggesting that NMDARs play an important role in the establishment and maintenance of the E/I balance. In this review, we briefly introduced NMDARs, AMPARs and GABAARs, summarized the current studies on E/I balance mediated by NMDARs, and discussed the current advances in NMDAR-mediated AMPAR and GABAAR development. Specifically, we analyzed the role of NMDAR subunits in the establishment and maintenance of E/I balance, which may provide new therapeutic strategies for the recovery of E/I imbalance in neurological disorders.

Hippocampal astrocytic neogenin regulating glutamate uptake, a critical pathway for preventing epileptic response

Epilepsy, a common neurological disorder, is featured with recurrent seizures. Its underlying pathological mechanisms remain elusive. Here, we provide evidence for loss of neogenin (NEO1), a coreceptor for multiple ligands, including netrins and bone morphological proteins, in the development of epilepsy. NEO1 is reduced in hippocampi from patients with epilepsy based on transcriptome and proteomic analyses. Neo1 knocking out (KO) in mouse brains displays elevated epileptiform spikes and seizure susceptibility. These phenotypes were undetectable in mice, with selectively depleted NEO1 in excitatory (NeuroD6-Cre⁺) or inhibitory (parvalbumin⁺) neurons, but present in mice with specific hippocampal astrocytic Neo1 KO. Additionally, neurons in hippocampal dentate gyrus, a vulnerable region in epilepsy, in mice with astrocyte-specific Neo1 KO show reductions in inhibitory synaptic vesicles and the frequency of miniature inhibitory postsynaptic current(mIPSC), but increase of the duration of miniature excitatory postsynaptic current and tonic NMDA receptor currents, suggesting impairments in both GABAergic transmission and extracellular glutamate clearance. Further proteomic and cell biological analyses of cell-surface proteins identified GLAST, a glutamate-aspartate transporter that is marked reduced in Neo1 KO astrocytes and the hippocampus. NEO1 interacts with GLAST and promotes GLAST surface distribution in astrocytes. Expressing NEO1 or GLAST in Neo1 KO astrocytes in the hippocampus abolishes the epileptic phenotype. Taken together, these results uncover an unrecognized pathway of NEO1-GLAST in hippocampal GFAP⁺ astrocytes, which is critical for GLAST surface distribution and function, and GABAergic transmission, unveiling NEO1 as a valuable therapeutic target to protect the brain from epilepsy.

LSD1 is an environmental stress-sensitive negative modulator of the glutamatergic synapse

Along with neuronal mechanisms devoted to memory consolidation –including long term potentiation of synaptic strength as prominent electrophysiological correlate, and inherent dendritic spines stabilization as structural counterpart– negative control of memory formation and synaptic plasticity has been described at the molecular and behavioral level. Within this work, we report a role for the epigenetic corepressor Lysine Specific Demethylase 1 (LSD1) as a negative neuroplastic factor whose stress-enhanced activity may participate in coping with adverse experiences. Constitutively increasing LSD1 activity via knocking out its dominant negative splicing isoform neuroLSD1 (neuroLSD1^KO mice), we observed extensive structural, functional and behavioral signs of excitatory decay, including disrupted memory consolidation. A similar LSD1 increase, obtained with acute antisense oligonucleotide-mediated neuroLSD1 splicing knock down in primary neuronal cultures, dampens spontaneous glutamatergic transmission, reducing mEPSCs. Remarkably, LSD1 physiological increase occurs in response to psychosocial stress-induced glutamatergic signaling. Since this mechanism entails neuroLSD1 splicing downregulation, we conclude that LSD1/neuroLSD1 ratio modulation in the hippocampus is instrumental to a negative homeostatic feedback, restraining glutamatergic neuroplasticity in response to glutamate. The active process of forgetting provides memories with salience. With our work, we propose that softening memory traces of adversities could further represent a stress-coping process in which LSD1/neuroLSD1 ratio modulation may help preserving healthy emotional references.

Astrocytes of the early postnatal brain

In the rodent forebrain, the majority of astrocytes are generated during the early postnatal phase. Following differentiation, astrocytes undergo maturation which accompanies the development of the neuronal network. Neonate astrocytes exhibit a distinct morphology and domain size which differs to their mature counterparts. Moreover, many of the plasma membrane proteins prototypical for fully developed astrocytes are only expressed at low levels at neonatal stages. These include connexins and Kir4.1, which define the low membrane resistance and highly negative membrane potential of mature astrocytes. Newborn astrocytes moreover express only low amounts of GLT-1, a glutamate transporter critical later in development. Furthermore, they show specific differences in the properties and spatio-temporal pattern of intracellular calcium signals, resulting from differences in their repertoire of receptors and signalling pathways. Therefore, roles fulfilled by mature astrocytes, including ion and transmitter homeostasis, are underdeveloped in the young brain. Similarly, astrocytic ion signalling in response to neuronal activity, a process central to neuron-glia interaction, differs between the neonate and mature brain. This review describes the unique functional properties of astrocytes in the first weeks after birth and compares them to later stages of development. We conclude that with an immature neuronal network and wider extracellular space, astrocytic support might not be as demanding and critical compared to the mature brain. The delayed differentiation and maturation of astrocytes in the first postnatal weeks might thus reflect a reduced need for active, energy-consuming regulation of the extracellular space and a less tight control of glial feedback onto synaptic transmission.

Advances in the Potential Biomarkers of Epilepsy

Epilepsy is a group of chronic neurological disorders characterized by recurrent, spontaneous, and unpredictable seizures. It is one of the most common neurological disorders, affecting tens of millions of people worldwide. Comprehensive studies on epilepsy in recent decades have revealed the complexity of epileptogenesis, in which immunological processes, epigenetic modifications, and structural changes in neuronal tissues have been identified as playing a crucial role. This review discusses the recent advances in the biomarkers of epilepsy. We evaluate the possible molecular background underlying the clinical changes observed in recent studies, focusing on therapeutic investigations, and the evidence of their safety and efficacy in the human population. This article reviews the pathophysiology of epilepsy, including recent reports on the effects of oxidative stress and hypoxia, and focuses on specific biomarkers and their clinical implications, along with further perspectives in epilepsy research.

General Pathophysiology of Astroglia

Astroglial cells are involved in most if not in all pathologies of the brain. These cells can change the morpho-functional properties in response to pathology or innate changes of these cells can lead to pathologies. Overall pathological changes in astroglia are complex and diverse and often vary with different disease stages. We classify astrogliopathologies into reactive astrogliosis, astrodegeneration with astroglial atrophy and loss of function, and pathological remodelling of astrocytes. Such changes can occur in neurological, neurodevelopmental, metabolic and psychiatric disorders as well as in infection and toxic insults. Mutation in astrocyte-specific genes leads to specific pathologies, such as Alexander disease, which is a leukodystrophy. We discuss changes in astroglia in the pathological context and identify some molecular entities underlying pathology. These entities within astroglia may repent targets for novel therapeutic intervention in the management of brain pathologies.

The brain in flux: Genetic, physiologic, and therapeutic perspectives on transporters in the CNS

The brain has specific properties that make it uniquely dependent upon transporters. This is the 3rd edition of a biennial special issue that originates from a scientific meeting devoted to studies of transporters and their relationship to brain function and to neurodevelopmental, neurologic, and psychiatric disorders. The field continues to rapidly evolve with advances in studies of structure that inform mechanism, with genetic analyses in humans revealing surprising aspects of biology, and with integrated cellular to whole animal analyses of the role of transporters in their control of physiology and pathophysiology. This special issue includes a sampling of review articles that address timely questions of the field followed by several primary research articles.

Region-specific deletions of the glutamate transporter GLT1 differentially affect seizure activity and neurodegeneration in mice

Glial glutamate transporter GLT1 plays a key role in the maintenance of extracellular glutamate homeostasis. Recent human genetic studies have suggested that de novo mutations in GLT1 (EAAT2) cause early-onset epilepsy with multiple seizure types. Consistent with these findings, global GLT1 null mice show lethal spontaneous seizures. The consequences of GLT1 dysfunction vary between different brain regions, suggesting that the role of GLT1 dysfunction in epilepsy may also vary with brain regions. In this study, we generated region-specific GLT1 knockout mice by crossing floxed-GLT1 mice with mice that express the Cre recombinase in a particular domain of the ventricular zone. Selective deletion of GLT1 in the diencephalon, brainstem and spinal cord is sufficient to reproduce the phenotypes (excess mortality, decreased body weight, and lethal spontaneous seizure) of the global GLT1 null mice. By contrast, dorsal forebrain-specific GLT1 knockout mice showed nonlethal complex seizures including myoclonic jerks, hyperkinetic running, spasm and clonic convulsion via the activation of NMDA receptors during a limited period from P12 to P14 and selective neuronal death in cortical layer II/III and the hippocampus. Thus, GLT1 dysfunction in the dorsal forebrain is involved in the pathogenesis of infantile epilepsy and GLT1 in the diencephalon, brainstem and spinal cord may play a critical role in preventing seizure-induced sudden death.

Astroglia as a cellular target for neuroprotection and treatment of neuro-psychiatric disorders

Astrocytes are key homeostatic cells of the central nervous system. They cooperate with neurons at several levels, including ion and water homeostasis, chemical signal transmission, blood flow regulation, immune and oxidative stress defense, supply of metabolites and neurogenesis. Astroglia is also important for viability and maturation of stem-cell derived neurons. Neurons critically depend on intrinsic protective and supportive properties of astrocytes. Conversely, all forms of pathogenic stimuli which disturb astrocytic functions compromise neuronal functionality and viability. Support of neuroprotective functions of astrocytes is thus an important strategy for enhancing neuronal survival and improving outcomes in disease states. In this review, we first briefly examine how astrocytic dysfunction contributes to major neurological disorders, which are traditionally associated with malfunctioning of processes residing in neurons. Possible molecular entities within astrocytes that could underpin the cause, initiation and/or progression of various disorders are outlined. In the second section, we explore opportunities enhancing neuroprotective function of astroglia. We consider targeting astrocyte-specific molecular pathways which are involved in neuroprotection or could be expected to have a therapeutic value. Examples of those are oxidative stress defense mechanisms, glutamate uptake, purinergic signaling, water and ion homeostasis, connexin gap junctions, neurotrophic factors and the Nrf2-ARE pathway. We propose that enhancing the neuroprotective capacity of astrocytes is a viable strategy for improving brain resilience and developing new therapeutic approaches.

The conversion of glutamate by glutamine synthase in neocortical astrocytes from juvenile rat is important to limit glutamate spillover and peri/extrasynaptic activation of NMDA receptors

Glutamate transporters (EAATs) are important to maintain spatial and temporal specificity of synaptic transmission. Their efficiency to uptake and transport glutamate into the intracellular space depends on several parameters including the intracellular concentrations of Na⁺ and glutamate, the elevations of which may slow down the cycling rate of EAATs. In astrocytes, glutamate is maintained at low concentration due to the presence of specific enzymes such as glutamine synthase (GS). GS inhibition results in cytosolic accumulation of glutamate suggesting that the conversion of glutamate by GS is important for EAATs operation. Here we recorded astrocytes from juvenile rat neocortical slices and analyzed the consequences of elevated intracellular glutamate concentrations and of GS inhibition on the time course of synaptically evoked transporter current (STC). In slices from rats treated with methionine sulfoximine (MSO), a GS inhibitor, STC evoked by short burst of high frequency stimulation (HFS; 100 Hz for 100 ms) but not by low frequency stimulation (LFS; 0.1 Hz) was twice slower than STC evoked from saline injected rats. Same results were obtained for astrocytes recorded with pipette containing 3-10 mM glutamate and compared with cells recorded with 0 or1 mM glutamate in the patch pipette. We also showed that HFS elicited significantly larger NMDAR-excitatory postsynaptic currents (EPSCs) with a stronger peri/extrasynaptic component in pyramidal cells from MSO-treated compared with saline treated rats. Taken together our data demonstrate that the conversion of glutamate by GS is fundamental to ensure an efficient clearance of glutamate by EAATs and to prevent glutamate spillover. GLIA 2017;65:401-415.

Neuron-glia cross talk revealed in reverberating networks by simultaneous extracellular recording of spikes and astrocytes' glutamate transporter and K+ currents

In neocortex networks, we simultaneously captured spikes and the slower astrocytes' K⁺and glutamate transporter (GluT) currents with the use of individual MEA electrodes. Inward and outward K⁺currents in different regions of the glial syncytium suggested that spatial buffering was operant. Moreover, in organotypic slices from ventral tegmental area and prefrontal cortex, the large GluT current amplitudes allowed to measure transporter currents with a single electrode. Our method allows direct study of the dynamic interplay of different cell types in excitable and nonexcitable tissue.

Astrocytes uptake synaptically released glutamate with electrogenic transporters (GluT) and buffer the spike-dependent extracellular K⁺ excess with background K⁺ channels. We studied neuronal spikes and the slower astrocytic signals on reverberating neocortical cultures and organotypic slices from mouse brains. Spike trains and glial responses were simultaneously captured from individual sites of multielectrode arrays (MEA) by splitting the recorded traces into appropriate filters and reconstructing the original signal by deconvolution. GluT currents were identified by using dl-threo-β-benzyloxyaspartate (TBOA). K⁺ currents were blocked by 30 μM Ba²⁺, suggesting a major contribution of inwardly rectifying K⁺ currents. Both types of current were tightly correlated with the spike rate, and their astrocytic origin was tested in primary cultures by blocking glial proliferation with cytosine β-d-arabinofuranoside (AraC). The spike-related, time-locked inward and outward K⁺ currents in different regions of the astrocyte syncytium were consistent with the assumptions of the spatial K⁺ buffering model. In organotypic slices from ventral tegmental area and prefrontal cortex, the GluT current amplitudes exceeded those observed in primary cultures by several orders of magnitude, which allowed to directly measure transporter currents with a single electrode. Simultaneously measuring cell signals displaying widely different amplitudes and kinetics will help clarify the neuron-glia interplay and make it possible to follow the cross talk between different cell types in excitable as well as nonexcitable tissue.

Translational potential of astrocytes in brain disorders

Fundamentally, all brain disorders can be broadly defined as the homeostatic failure of this organ. As the brain is composed of many different cells types, including but not limited to neurons and glia, it is only logical that all the cell types/constituents could play a role in health and disease. Yet, for a long time the sole conceptualization of brain pathology was focused on the well-being of neurons. Here, we challenge this neuron-centric view and present neuroglia as a key element in neuropathology, a process that has a toll on astrocytes, which undergo complex morpho-functional changes that can in turn affect the course of the disorder. Such changes can be grossly identified as reactivity, atrophy with loss of function and pathological remodeling. We outline the pathogenic potential of astrocytes in variety of disorders, ranging from neurotrauma, infection, toxic damage, stroke, epilepsy, neurodevelopmental, neurodegenerative and psychiatric disorders, Alexander disease to neoplastic changes seen in gliomas. We hope that in near future we would witness glial-based translational medicine with generation of deliverables for the containment and cure of disorders. We point out that such as a task will require a holistic and multi-disciplinary approach that will take in consideration the concerted operation of all the cell types in the brain.

Soluble Aβ oligomers impair hippocampal LTP by disrupting glutamatergic/GABAergic balance

Epileptic activity may be more prevalent in early stage Alzheimer's disease (AD) than previously believed. Several studies report spontaneous seizures and interictal discharges in mouse models of AD undergoing age-related Aβ accumulation. The mechanism by which Aβ-induced neuronal excitability can trigger epileptiform activity remains unknown. Here, we systematically examined field excitatory postsynaptic potentials (fEPSP) in stratum radiatum and population spikes (PSs) in the adjacent stratum pyramidale of CA1 in wild-type mouse hippocampal slices. Soluble Aβ oligomers (oAβ) blocked hippocampal LTP and EPSP-spike (E-S) potentiation, and these effects were occluded by prior treatment with the glutamate uptake inhibitor TBOA. In accord, oAβ elevated glutamate levels in the hippocampal slice medium. Recording the PS revealed that oAβ increased PS frequency and reduced LTP, and this LTP deficit was occluded by pretreatment with the GABAA antagonist picrotoxin. Whole-cell recordings showed that oAβ significantly increased spontaneous EPSC frequency. Decreasing neuronal activity by increasing GABA tone or partially blocking NMDAR activity prevented oAβ impairment of hippocampal LTP. Finally, treating slices with two antiepileptic drugs rescued the LTP inhibition induced by oAβ. We conclude that soluble Aβ oligomers at the low nanomolar levels present in AD brain increase neuronal excitability by disrupting glutamatergic/GABAergic balance, thereby impairing synaptic plasticity.

The domain of neuronal firing on a plane of input current and conductance

The activation of neurotransmitter receptors increases the current flow and membrane conductance and thus controls the firing rate of a neuron. In the present work, we justified the two-dimensional representation of a neuronal input by voltage-independent current and conductance and obtained experimentally and numerically a complete input-output (I/O) function. The dependence of the steady-state firing rate on the input current and conductance was studied as a two-parameter I/O function. We employed the dynamic patch clamp technique in slices to get this dependence for the whole domain of two input signals that evoke stationary spike trains in a single neuron (Ω-domain). As found, the Ω-domain is finite and an additional conductance decreases the range of spike-evoking currents. The I/O function has been reproduced in a Hodgkin-Huxley-like model. Among the simulated effects of different factors on the I/O function, including passive and active membrane properties, external conditions and input signal properties, the most interesting were: the shift of the right boundary of the Ω-domain (corresponding to the exCitation block) leftwards due to the decrease of the maximal potassium conductance; and the reduction of the Ω-domain by the decrease of the maximal sodium concentration. As found in experiments and simulations, the Ω-domain is reduced by the decrease of extracellular sodium concentration, by cooling, and by adding slow potassium currents providing interspike interval adaptation; the Ω-domain height is increased by adding color noise. Our modeling data provided a generalization of I/O dependencies that is consistent with previous studies and our experiments. Our results suggest that both current flow and membrane conductance should be taken into account when determining neuronal firing activity.

Novel astrocyte targets: new avenues for the therapeutic treatment of epilepsy

During the last 20 years, it has been well established that a finely tuned, continuous crosstalk between neurons and astrocytes not only critically modulates physiological brain functions but also underlies many neurological diseases. In particular, this novel way of interpreting brain activity is markedly influencing our current knowledge of epilepsy, prompting a re-evaluation of old findings and guiding novel experimentation. Here, we review recent studies that have unraveled novel and unique contributions of astrocytes to the generation and spread of convulsive and nonconvulsive seizures and epileptiform activity. The emerging scenario advocates an overall framework in which a dynamic and reciprocal interplay among astrocytic and neuronal ensembles is fundamental for a fuller understanding of epilepsy. In turn, this offers novel astrocytic targets for the development of those really novel chemical entities for the control of convulsive and nonconvulsive seizures that have been acknowledged as a key priority in the management of epilepsy.

Adenosine kinase, glutamine synthetase and EAAT2 as gene therapy targets for temporal lobe epilepsy

Astrocytes are an attractive cell target for gene therapy, but the validation of new therapeutic candidates is needed. We determined whether adeno-associated viral (AAV) vector-mediated overexpression of glutamine synthetase (GS) or excitatory amino-acid transporter 2 (EAAT2), or expression of microRNA targeting adenosine kinase (miR-ADK) in hippocampal astrocytes in the rat brain could modulate susceptibility to kainate-induced seizures and neuronal cell loss. Transgene expression was found predominantly in astrocytes following direct injection of glial-targeting AAV9 vectors by 3 weeks postinjection. ADK expression in miR-ADK vector-injected rats was reduced by 94-96% and was associated with an ~50% reduction in the duration of kainate-induced seizures and greater protection of dentate hilar neurons but not CA3 neurons compared with miR-control vector-injected rats. In contrast, infusion of AAV-GS and EAAT2 vectors did not afford any protection against seizures or neuronal damage as the level of transcriptional activity of the glial fibrillary acidic promoter was too low to drive any significant increase in transgenic GS or EAAT2 relative to the high endogenous levels of these proteins. Our findings support ADK as a prime therapeutic target for gene therapy of temporal lobe epilepsy and suggest that alternative approaches including the use of stronger glial promoters are needed to increase transgenic GS and EAAT2 expression to levels that may be required to affect seizure induction and propagation.

Glutamatergic system controls synchronization of spontaneous neuronal activity in the murine neonatal entorhinal cortex

Synchronized spontaneous neuronal activity is a characteristic feature of the developing brain. Rhythmic network discharges in the neonatal medial entorhinal cortex (mEC) in vitro depend on activation of ionotropic glutamate receptors, but spontaneously active neurons are required for their initiation. Field potential recordings revealed synchronized neuronal activity in the mEC in vivo developmentally earlier than in vitro. We suggested that not only ionotropic receptors, but also other components of the glutamatergic system modulate neuronal activity in the mEC. Ca(2+) imaging was used to record neuronal activity in neonatal murine brain slices. Two types of spontaneous events were distinguished: global synchronous discharges (synchronous activity) and asynchronously (not synchronized with global discharges) active cells (asynchronous activity). AMPA receptor blockade strongly reduced the frequency of synchronous discharges, while NMDA receptor inhibition was less effective. AMPA and NMDA receptor blockade or activation of group 2/3 metabotropic glutamate receptors (mGluR2/3) completely suppressed synchronous discharges and increased the number of active cells. Blockade of glutamate transporters with DL-TBOA led to NMDA receptor-mediated hyper-synchronization of neuronal activity. Inhibition of NMDA receptors in the presence of DL-TBOA failed to restore synchronous discharges. The latter were partially reestablished only after blockade of mGluR2/3. We conclude that the glutamatergic system can influence neuronal activity via different receptors/mechanisms. As both NMDA and mGluR2/3 receptors have a high affinity for glutamate, changes in extracellular glutamate levels resulting for instance from glutamate transporter malfunction can balance neuronal activity in the mEC, affecting in turn synapse and network formation.

Occasional seizures, epilepsy, and inborn errors of metabolism

Seizures are a common paediatric problem, with inborn errors of metabolism being a rare underlying aetiology. The clinical presentation of inborn errors of metabolism is often associated with other neurological symptoms, such as hypotonia, movement disorders, and cognitive disturbances. However, the occurrence of epilepsy associated with inborn errors of metabolism represents a major challenge that needs to be identified quickly; for some cases, specific treatments are available, metabolic decompensation might be avoided, and accurate counselling can be given about recurrence risk. Some clinical presentations are more likely than others to point to an inborn error of metabolism as the cause of seizures. Knowledge of important findings at examination, and appropriate biochemical investigation of children with seizures of uncertain cause, can aid the diagnosis of an inborn error of metabolism and ascertain whether or not the seizures are amenable to specific metabolic treatment.

The role of neuronal activity and transmitter release on synapse formation

The long history of probing the role of neuronal activity in the development of nervous system circuitry has recently taken an interesting turn. Although undoubtedly activity plays a critical part in the maintenance and refinement of synaptic connections, often via competitive mechanisms, evidence is building that it also drives the process of synapse formation itself. Perhaps predictably, this turns out not to be a uniform process. It seems that different circuits, indeed specific synaptic connections, are differentially sensitive to the effects of activity. We examine possible ways in which neurotransmitter may drive synapse formation, and speculate on how the environment of the developing brain may allow a different spatiotemporal range for neuronal activity to operate in the generation of connectivity.

Brain maturation and epilepsy

At full term, both glutamate and gamma-amino-butyric acid (GABA) are excitatory; cortical synapses are beginning to appear, there is little myelin in the cerebral hemispheres, and long tracts hardly start to develop. Neonatal myoclonic encephalopathy can result from premature activation of N-methyl-D-aspartate (NMDA) transmission. Benign neonatal seizures and migrating partial seizures in infancy could involve excessive or premature excitability of deep cortical layers. Benign rolandic epilepsy and continuous spike waves in slow sleep are consistent with an excess of both excitatory and inhibitory cortical synapses. West and Lennox-Gastaut syndromes express age-related diffuse cortical hyperexcitability, the pattern depending on the age of occurrence; synchronization of spikes is becoming possible with maturation of the myelin. Idiopathic generalized epilepsy is itself modulated by maturation that causes frontal hyperexcitability generating myoclonic-astatic seizures, between the ages of infantile and juvenile myoclonic epilepsies. Physiological delay of hippocampo-neocortical pathways maturation could account for the delayed occurrence of mesial temporal epilepsy following infantile damage, whereas premature maturation could contribute to fronto-temporal damage characteristic of fever-induced epileptic encephalopathy in school-age children, a dramatic school-age epileptic encephalopathy.

Genetic deletion of the neuronal glutamate transporter, EAAC1, results in decreased neuronal death after pilocarpine-induced status epilepticus

Excitatory amino acid carrier 1 (EAAC1 also called EAAT3) is a Na(+)-dependent glutamate transporter expressed by both glutamatergic and GABAergic neurons. It provides precursors for the syntheses of glutathione and GABA and contributes to the clearance of synaptically released glutamate. Mice deleted of EAAC1 are more susceptible to neurodegeneration in models of ischemia, Parkinson's disease, and aging. Antisense knock-down of EAAC1 causes an absence seizure-like phenotype. Additionally, EAAC1 expression increases after chemonvulsant-induced seizures in rodent models and in tissue specimens from patients with refractory epilepsy. The goal of the present study was to determine if the absence of EAAC1 affects the sensitivity of mice to seizure-induced cell death. A chemoconvulsant dose of pilocarpine was administered to EAAC1(-/-) mice and to wild-type controls. Although EAAC1(-/-) mice experienced increased latency to seizure onset, no significant differences in behavioral seizure severity or mortality were observed. We examined EAAC1 immunofluorescence 24h after pilocarpine administration and confirmed that pilocarpine causes an increase in EAAC1 protein. Forty-eight hours after induction of seizures, cell death was measured in hippocampus and in cortex using Fluoro-Jade C. Surprisingly, there was ∼2-fold more cell death in area CA1 of wild-type mice than in the corresponding regions of the EAAC1(-/-) mice. Together, these studies indicate that absence of EAAC1 results in either a decrease in pilocarpine-induced seizures that is not detectable by behavioral criteria (surprising, since EAAC1 provides glutamate for GABA synthesis), or that the absence of EAAC1 results in less pilocarpine/seizure-induced cell death, possible explanations as discussed.

Spontaneous Network Activity and Synaptic Development

Throughout development, the nervous system produces patterned spontaneous activity. Research over the past two decades has revealed a core group of mechanisms that mediate spontaneous activity in diverse circuits. Many circuits engage several of these mechanisms sequentially to accommodate developmental changes in connectivity. In addition to shared mechanisms, activity propagates through developing circuits and neuronal pathways (i.e., linked circuits in different brain areas) in stereotypic patterns. Increasing evidence suggests that spontaneous network activity shapes synaptic development in vivo Variations in activity-dependent plasticity may explain how similar mechanisms and patterns of activity can be employed to establish diverse circuits. Here, I will review common mechanisms and patterns of spontaneous activity in emerging neural networks and discuss recent insights into their contribution to synaptic development.

Glutamate transporters: a key piece in the glutamate puzzle of major depressive disorder

Glutamatergic therapies are emerging as the new path for the treatment of Major Depression Disorder. Recent reports reviewing the use of glutamate activity modulators in the treatment of resistant depression advocate the importance of understanding the alterations of the diverse components of this complex system in mood disorders. In this postmortem study we used in situ hybridization and microarray analysis to evaluate the gene expression of the membrane transporters SLC1A2 and SLCA3 and the vesicular transporter SLCA17A7 in the hippocampus of Major Depressive Disorder (MDD) and Bipolar Disorder (BPD) subjects. Samples from 8 controls, 11 MDD and 6 BPD subjects were processed for in situ hybridization using cRNA probes for SLC1A2, SLC1A3 and SLC17A7. Laser capture microdissection was used to collect tissue from adjacent sections for microarray analysis. The results showed that the expression of the membrane transporters SLC1A2 and SLC1A3 was diminished in the MDD group compared to controls. The expression of the vesicular glutamate transporter SLC17A7 on the other hand was increased in MDD subjects. As for the BPD group, all three transporters showed trends similar to those observed in MDD, but the changes observed did not reach significance. We hypothesize that the decreased expression of the membrane glutamate transporters and the increased expression of the vesicular transporter in the hippocampus would affect the balance of the glutamatergic circuitry of the hippocampus, and that this effect may be a major contributor to depressive symptoms.

Mechanisms and effects of seizures in the immature brain

The developing immature brain is not simply a small adult brain but rather possesses unique physiological properties. These include neuronal ionic currents that differ markedly from those in the adult brain, typically being longer-lasting and less selective. This enables immature heterogeneous neurons to connect and fire together but at the same time, along with other features may contribute to the enhanced propensity of the developing brain to become epileptic. Indeed, immature neurons tend to readily synchronize and thus generate seizures. Here, we review the differences between the immature and adult brain, with particular focus on the developmental sequence of γ-aminobutyric acid that excites immature neurons while being inhibitory in the normal adult brain. We review the mechanisms underlying the developmental changes to intracellular chloride levels, as well as how epileptiform activity can drive pathologic changes to chloride balance in the brain. We show that regulation of intracellular chloride is one important factor that underlies both the ease with which seizures can be generated and the facilitation of further seizures. We stress in particular the importance of understanding normal developmental sequences and how they are interrupted by seizures and other insults, and how this knowledge has led to the identification of potential novel treatments for conditions such as neonatal seizures.

Epilepsy in inborn errors of metabolism

Epilepsies associated with inborn errors of metabolism (IEM) represent a major challenge. Seizures rarely dominate the clinical presentation, which is more frequently associated with other neurological symptoms, such as hypotonia and/or cognitive disturbances. Although epilepsy in IEM can be classified in various ways according to pathogenesis, age of onset, or electroclinical presentation, the most pragmatic approach is determined by whether they are accessible to specific treatment or not. The main potentially treatable causes comprise vitamin B6 (pyridoxine deficiency), biotine, and GLUT1 deficiency (GLUT1DS) syndromes. Folinic acid-dependent seizures are allelic with pyridoxine dependency. Incompletely treatable IEMs include pyridoxal phosphate, serine, and creatine deficiencies. The main IEMs that present with epilepsy but offer no specific treatment are nonketotic hyperglycinemia, mitochondrial disorders, sulfite oxidase deficiency, ceroid-lipofuscinosis, Menkes disease, and peroxisomal disorders.

Inhibitors of glutamate dehydrogenase block sodium-dependent glutamate uptake in rat brain membranes

We recently found evidence for anatomic and physical linkages between the astroglial Na(+)-dependent glutamate transporters (GLT-1/EAAT2 and GLAST/EAAT1) and mitochondria. In these same studies, we found that the glutamate dehydrogenase (GDH) inhibitor, epigallocatechin-monogallate (EGCG), inhibits both glutamate oxidation and Na(+)-dependent glutamate uptake in astrocytes. In the present study, we extend this finding by exploring the effects of EGCG on Na(+)-dependent l-[(3)H]-glutamate (Glu) uptake in crude membranes (P2) prepared from rat brain cortex. In this preparation, uptake is almost exclusively mediated by GLT-1. EGCG inhibited l-[(3)H]-Glu uptake in cortical membranes with an IC50 value of 230 μM. We also studied the effects of two additional inhibitors of GDH, hexachlorophene (HCP) and bithionol (BTH). Both of these compounds also caused concentration-dependent inhibition of glutamate uptake in cortical membranes. Pre-incubating with HCP for up to 15 min had no greater effect than that observed with no pre-incubation, showing that the effects occur rapidly. HCP decreased the V max for glutamate uptake without changing the K m, consistent with a non-competitive mechanism of action. EGCG, HCP, and BTH also inhibited Na(+)-dependent transport of d-[(3)H]-aspartate (Asp), a non-metabolizable transporter substrate, and [(3)H]-γ-aminobutyric acid (GABA). In contrast to the forebrain, glutamate uptake in crude cerebellar membranes (P2) is likely mediated by GLAST (EAAT1). Therefore, the effects of these compounds were examined in cerebellar membranes. In this region, none of these compounds had any effect on uptake of either l-[(3)H]-Glu or d-[(3)H]-Asp, but they all inhibited [(3)H]-GABA uptake. Together these studies suggest that GDH is preferentially required for glutamate uptake in forebrain as compared to cerebellum, and GDH may be required for GABA uptake as well. They also provide further evidence for a functional linkage between glutamate transport and mitochondria.

New vistas on astroglia in convulsive and non-convulsive epilepsy highlight novel astrocytic targets for treatment

Our current knowledge of the role of astrocytes in health and disease states supports the view that many physiological brain functions and neurological diseases are finely tuned, and in certain cases fully determined, by the continuous cross-talk between astrocytes and neurons. This novel way of interpreting brain activity as a dynamic and reciprocal interplay between astrocytic and neuronal networks has also influenced our understanding of epilepsy, not only forcing a reinterpretation of old findings, but also being a catalyst for novel experimentation. In this review, we summarize some of the recent studies that highlight these novel distinct contributions of astrocytes to the expression of convulsive and non-convulsive epileptiform discharges and seizures. The emerging picture suggests a general framework based on bilateral signalling between astrocytes and neurons for a fuller understanding of epileptogenic and epileptic mechanisms in the brain network. Astrocytes potentially represent targets for the development of those novel chemical entities with improved efficacy for the treatment of convulsive and non-convulsive epilepsy that expert groups have recognized as one of the key priorities for the management of epilepsy.

Glutamate transporters and presynaptic metabotropic glutamate receptors protect neocortical Cajal-Retzius cells against over-excitation

Cajal-Retzius (CR) cells, early generated neurons in the marginal zone of developing neocortex, are reported to be highly vulnerable to excitotoxic damage. Because extracellular glutamate concentration in the central nervous system is mainly controlled by glutamate transporters (EAATs), we studied the effects of EAAT blockade on CR cells. DL: -TBOA, a specific EAAT antagonist, induced NMDA receptor-dependent bursting discharges in layer 2/3 pyramidal neurons, indicating that EAATs operate in the uptake mode and their blockade leads to elevation of extracellular glutamate concentration. In CR cells, however, DL: -TBOA failed to change either the membrane resistance or holding current, and moreover, it reduced the frequency of spontaneous GABAergic postsynaptic currents. DL: -TBOA decreased the mean amplitude and increased paired-pulse ratio of evoked GABAergic postsynaptic currents, indicating the presynaptic locus of its action. Indeed, LY379268, a specific agonist of group II metabotropic glutamate receptors (mGluR-II), mimicked the DL: -TBOA-mediated effects, and LY341495, an unspecific mGluR antagonist, eliminated the DL: -TBOA-induced effects. As dihydrokainic acid, a specific EAAT2 blocker, failed to affect evoked GABAergic postsynaptic currents, whereas TFB-TBOA, a selective blocker of EAAT1 and EAAT2, produced effects similar to that of DL: -TBOA, extracellular glutamate concentration in the marginal zone is mainly controlled by EAAT1 (GLAST). Thus, even though CR cells are highly vulnerable to excitotoxic damage, a number of mechanisms serve to protect them against excessive extracellular glutamate concentration including a lack of functional glutamatergic synapses, Mg(2+) blockade of NMDA receptors, and presynaptic mGluRs that inhibit transmission at GABAergic synapses.

Increased glial glutamate transporter EAAT2 expression reduces epileptogenic processes following pilocarpine-induced status epilepticus

Several lines of evidence indicate that glutamate plays a crucial role in the initiation of seizures and their propagation; abnormal glutamate release causes synchronous firing of large populations of neurons, leading to seizures. In the present study, we investigated whether enhanced glutamate uptake by increased glial glutamate transporter EAAT2, the major glutamate transporter, could prevent seizure activity and reduce epileptogenic processes. EAAT2 transgenic mice, which have a 1.5-2 fold increase in EAAT2 protein levels as compared to their non-transgenic counterparts, were tested in a pilocarpine-induced status epilepticus (SE) model. Several striking phenomena were observed in EAAT2 transgenic mice compared with their non-transgenic littermates. First, the post-SE mortality rate and chronic seizure frequency were significantly decreased. Second, neuronal degeneration in hippocampal subfields after SE were significantly reduced. Third, the SE-induced neurogenesis and mossy fiber sprouting were significantly decreased. The severity of cell loss in epileptic mice was positively correlated with that of mossy fiber sprouting and chronic seizure frequency. Our results suggest that increased EAAT2 expression can protect mice against SE-induced death, neuropathological changes, and chronic seizure development. This study suggests that enhancing EAAT2 protein expression is a potential therapeutic approach.

Bmi-1 absence causes premature brain degeneration

Bmi-1, a polycomb transcriptional repressor, is implicated in cell cycle regulation and cell senescence. Its absence results in generalized astrogliosis and epilepsy during the postnatal development, but the underlying mechanisms are poorly understood. Here, we demonstrate the occurrence of oxidative stress in the brain of four-week-old Bmi-1 null mice. The mice showed various hallmarks of neurodegeneration including synaptic loss, axonal demyelination, reactive gliosis and brain mitochondrial damage. Moreover, astroglial glutamate transporters and glutamine synthetase decreased in the Bmi-1 null hippocampus, which might contribute to the sporadic epileptic-like seizures in these mice. These results indicate that Bmi-1 is required for maintaining endogenous antioxidant defenses in the brain, and its absence subsequently causes premature brain degeneration.

Bidirectional neuron-glia interactions triggered by deficiency of glutamate uptake at spinal sensory synapses

Bidirectional interactions between neurons and glial cells are crucial to the genesis of pathological pain. The mechanisms regulating these interactions and the role of this process in relaying synaptic input in the spinal dorsal horn remain to be established. We studied the role of glutamate transporters in the regulation of such interactions. On pharmacological blockade of glutamate transporters, slow inward currents (SICs) appeared spontaneously and/or were evoked by peripheral synaptic input in the spinal superficial dorsal horn neurons, including the spinothalamic tract neurons. We showed that the SICs were induced by the release of glutamate from glial cells. On inhibition of glutamate uptake, the stimulation-induced, synaptically released glutamate activated glial cells and caused glial cells to release glutamate. Glial-derived glutamate acted on extrasynaptic N-methyl-d-aspartate (NMDA) receptors mainly composed of NR2B receptors and generated SICs, which led to depolarization and action potential generation in superficial spinal dorsal horn neurons. Thus glutamate transporters regulate glutamatergic neuron-glia interactions at spinal sensory synapses. When glutamate uptake is impaired, glial cells function like excitatory interneurons-they are activated by peripheral synaptic input and release glutamate to activate postsynaptic neurons in spinal pain pathways.

GABAergic control of neurite outgrowth and remodeling during development and adult neurogenesis: general rules and differences in diverse systems

During development, Gamma-aminobutyric acidergic (GABAergic) neurons mature at early stages, long before excitatory neurons. Conversely, GABA reuptake transporters become operative later than glutamate transporters. GABA is therefore not removed efficiently from the extracellular domain and it can exert significant paracrine effects. Hence, GABA-mediated activity is a prominent source of overall neural activity in developing CNS networks, while neurons extend dendrites and axons, and establish synaptic connections. One of the unique features of GABAergic functional plasticity is that in early development, activation of GABA_A receptors results in depolarizing (mainly excitatory) responses and Ca²⁺ influx. Although there is strong evidence from several areas of the CNS that GABA plays a significant role in neurite growth not only during development but also during adult neurogenesis, surprisingly little effort has been made into putting all these observations into a common framework in an attempt to understand the general rules that regulate these basic and evolutionary well-conserved processes. In this review, we discuss the current knowledge in this important field. In order to decipher common, universal features and highlight differences between systems throughout development, we compare findings about dendritic proliferation and remodeling in different areas of the nervous system and species, and we also review recent evidence for a role in axonal elongation. In addition to early developmental aspects, we also consider the GABAergic role in dendritic growth during adult neurogenesis, extending our discussion to the roles played by GABA during dendritic proliferation in early developing networks versus adult, well established networks.

Dynamics of glutamatergic signaling in the mushroom body of young adult Drosophila

Background

The mushroom bodies (MBs) are paired brain centers located in the insect protocerebrum involved in olfactory learning and memory and other associative functions. Processes from the Kenyon cells (KCs), their intrinsic neurons, form the bulk of the MB's calyx, pedunculus and lobes. In young adult Drosophila, the last-born KCs extend their processes in the α/β lobes as a thin core (α/β cores) that is embedded in the surrounding matrix of other mature KC processes. A high level of L-glutamate (Glu) immunoreactivity is present in the α/β cores (α/βc) of recently eclosed adult flies. In a Drosophila model of fragile X syndrome, the main cause of inherited mental retardation, treatment with metabotropic Glu receptor (mGluR) antagonists can rescue memory deficits and MB structural defects.

Results

To address the role of Glu signaling in the development and maturation of the MB, we have compared the time course of Glu immunoreactivity with the expression of various glutamatergic markers at various times, that is, 1 hour, 1 day and 10 days after adult eclosion. We observed that last-born α/βc KCs in young adult as well as developing KCs in late larva and at various pupal stages transiently express high level of Glu immunoreactivity in Drosophila. One day after eclosion, the Glu level was already markedly reduced in the α/βc neurons. Glial cell processes expressing glutamine synthetase and the Glu transporter dEAAT1 were found to surround the Glu-expressing KCs in very young adults, subsequently enwrapping the α/β lobes to become distributed equally over the entire MB neuropil. The vesicular Glu transporter DVGluT was detected by immunostaining in processes that project within the MB lobes and pedunculus, but this transporter is apparently never expressed by the KCs themselves. The NMDA receptor subunit dNR1 is widely expressed in the MB neuropil just after eclosion, but was not detected in the α/βc neurons. In contrast, we provide evidence that DmGluRA, the only Drosophila mGluR, is specifically expressed in Glu-accumulating cells of the MB α/βc immediately and for a short time after eclosion.

Conclusions

The distribution and dynamics of glutamatergic markers indicate that newborn KCs transiently accumulate Glu at a high level in late pupal and young eclosed Drosophila, and may locally release this amino acid by a mechanism that would not involve DVGluT. At this stage, Glu can bind to intrinsic mGluRs abundant in the α/βc KCs, and to NMDA receptors in the rest of the MB neuropil, before being captured and metabolized in surrounding glial cells. This suggests that Glu acts as an autocrine or paracrine agent that contributes to the structural and functional maturation of the MB during the first hours of Drosophila adult life.

Mechanisms underlying spontaneous patterned activity in developing neural circuits

Patterned, spontaneous activity occurs in many developing neural circuits, including the retina, the cochlea, the spinal cord, the cerebellum and the hippocampus, where it provides signals that are important for the development of neurons and their connections. Despite there being differences in adult architecture and output across these various circuits, the patterns of spontaneous network activity and the mechanisms that generate it are remarkably similar. The mechanisms can include a depolarizing action of GABA (gamma-aminobutyric acid), transient synaptic connections, extrasynaptic transmission, gap junction coupling and the presence of pacemaker-like neurons. Interestingly, spontaneous activity is robust; if one element of a circuit is disrupted another will generate similar activity. This research suggests that developing neural circuits exhibit transient and tunable features that maintain a source of correlated activity during crucial stages of development.

Early NMDA receptor-driven waves of activity in the developing neocortex: physiological or pathological network oscillations?

Several patterns of coherent activity have been described in developing cortical structures, thus providing a general framework for network maturation. A detailed timely description of network patterns at circuit and cell levels is essential for the understanding of pathogenic processes occurring during brain development. Disturbances in the expression timetable of this pattern sequence are very likely to affect network maturation. This review focuses on the maturation of coherent activity patterns in developing neocortical structures. It emphasizes the intrinsic and synaptic cellular properties that are unique to the immature neocortex and, in particular, the critical role played by extracellular glutamate in controlling network excitability and triggering synchronous network waves of activity.

Glutamate uptake shapes low-[Mg2+] induced epileptiform activity in juvenile rat hippocampal slices

A wide range of data support a role for ambient glutamate (Glu) in epilepsy, although temporal patterns associated with the cellular uptake of Glu have not been addressed in detail. We report on the effects of Glu uptake inhibitors on recurrent seizure-like events (SLEs) evoked by low-[Mg(2+)] condition in juvenile rat hippocampal slices. Effects were compared for inhibitors such as L-trans-pyrrolidine-2,4-dicarboxylate (tPDC), DL-threo-beta-benzyloxyaspartate (DL-TBOA) and dihydrokainic acid (DHK), representing different transporter specificity and transportability profiles. Latency to the first SLE after drug application was shortened by the inhibitors (in % of control: 500 microM tPDC: 54+/-7, 15 microM DL-TBOA: 74+/-5, 50 microM dl-TBOA: 70+/-6, 100 microM DHK: 69+/-4, 300 microM DHK: 71+/-7). Further SLEs were frequently aborted by higher inhibitor concentrations applied (500 microM tPDC: 2/6, 50 microM TBOA: 5/5, 100 microM DHK: 6/8, 300 microM DHK: 3/3). Simultaneous field potential and whole-cell voltage recordings showed depolarization-induced inactivation of CA3 pyramidal neurons during inhibitor application. In the presence of inhibitors, the amplitude of forthcoming SLE was also decreased (in % of control: 500 microM tPDC: 66+/-9, 15 microM dl-TBOA: 88+/-5, 50 microM dl-TBOA: 59+/-6, 100 microM DHK: 67+/-4, 300 microM DHK: 68+/-1). Dependent on type and concentration of the inhibitor, the duration of the first SLE of drug application either increased (100 microM DHK: 375+/-90 %; 100 microM tPDC: 137+/-13 %) or decreased (50 microM TBOA: 62+/-13 %; 300 microM DHK: 60+/-15 %) reflecting differences in subtype-specificity or mechanism of action of the inhibitors. Our findings suggest a role for ambient Glu in the genesis and maintenance of recurrent epileptiform discharges.

Synaptic and extrasynaptic factors governing glutamatergic retinal waves

In the few days prior to eye-opening in mice, the excitatory drive underlying waves switches from cholinergic to glutamatergic. Here, we describe the unique synaptic and spatiotemporal properties of waves generated by the retina's glutamatergic circuits. First, knockout mice lacking vesicular glutamate transporter type 1 do not have glutamatergic waves, but continue to exhibit cholinergic waves, demonstrating that the two wave-generating circuits are linked. Second, simultaneous outside-out patch and whole-cell recordings reveal that retinal waves are accompanied by transient increases in extrasynaptic glutamate, directly demonstrating the existence of glutamate spillover during waves. Third, the initiation rate and propagation speed of retinal waves, as assayed by calcium imaging, are sensitive to pharmacological manipulations of spillover and inhibition, demonstrating a role for both signaling pathways in shaping the spatiotemporal properties of glutamatergic retinal waves.

Glutamate transporters prevent excessive activation of NMDA receptors and extrasynaptic glutamate spillover in the spinal dorsal horn

Activation of N-methyl-d-aspartate (NMDA) receptors in the spinal dorsal horn neurons is a key process related to sensory transmission, neural plasticity, and pathogenesis of pain. In this study, we investigated how activation of NMDA receptors in spinal substantia gelatinosa neurons is regulated by glutamate re-uptake through glutamate transporters located in the astrocytic and neuronal plasma membranes. Using visualized whole cell patch recording techniques, NMDA excitatory postsynaptic currents evoked by graded peripheral inputs in spinal substantia gelatinosa neurons of spinal slices from young adult rats were analyzed before and after combined inhibition of glial and neuronal glutamate transporters by d-threo-beta-benzyloxyaspartate (TBOA). Blockade of glutamate transporters increased the number and duration of NMDA receptors activated by weak and by strong primary afferent inputs as well as by exogenous glutamate. The enhancement in activation of NMDA receptors induced by TBOA was greater in neurons that have weaker synaptic input at baseline. Impaired glutamate uptake increased the open probability of NMDA channels and caused glutamate spillover outside the active synapses, leading to activation of extrasynaptic NMDA receptors and/or receptors located in neighboring synapses. Finally, blockade of glutamate transporters resulted in an increased proportion of NR2B subunit activation induced by peripheral input, and this increase was further augmented by stronger afferent input. These data indicate that glutamate transporters regulate spatiotemporal and intensity coding for sensory input and prevent excessive activation of glutamate receptors in the spinal dorsal horn. It is suggested that remedying dysfunctional glutamate transporters may be a potential new avenue to prevent the pathogenesis of pain.

Sequential generation of two distinct synapse-driven network patterns in developing neocortex

Developing cortical networks generate a variety of coherent activity patterns that participate in circuit refinement. Early network oscillations (ENOs) are the dominant network pattern in the rodent neocortex for a short period after birth. These large-scale calcium waves were shown to be largely driven by glutamatergic synapses albeit GABA is a major excitatory neurotransmitter in the cortex at such early stages, mediating synapse-driven giant depolarizing potentials (GDPs) in the hippocampus. Using functional multineuron calcium imaging together with single-cell and field potential recordings to clarify distinct network dynamics in rat cortical slices, we now report that the developing somatosensory cortex generates first ENOs then GDPs, both patterns coexisting for a restricted time period. These patterns markedly differ by their developmental profile, dynamics, and mechanisms: ENOs are generated before cortical GDPs (cGDPs) by the activation of glutamatergic synapses mostly through NMDARs; cENOs are low-frequency oscillations (approximately 0.01 Hz) displaying slow kinetics and gradually involving the entire network. At the end of the first postnatal week, GABA-driven cortical GDPs can be reliably monitored; cGDPs are recurrent oscillations (approximately 0.1 Hz) that repetitively synchronize localized neuronal assemblies. Contrary to cGDPs, cENOs were unexpectedly facilitated by short anoxic conditions suggesting a contribution of glutamate accumulation to their generation. In keeping with this, alterations of extracellular glutamate levels significantly affected cENOs, which are blocked by an enzymatic glutamate scavenger. Moreover, we show that a tonic glutamate current contributes to the neuronal membrane excitability when cENOs dominate network patterns. Therefore, cENOs and cGDPs are two separate aspects of neocortical network maturation that may be differentially engaged in physiological and pathological processes.

Decreased glutamate transport enhances excitability in a rat model of cortical dysplasia

Glutamate transporters function to maintain low levels of extracellular glutamate and play an important role in synaptic transmission at many synapses. Disruption of glutamate transporter function or expression can result in increased extracellular glutamate levels. Alterations in glutamate transporter expression have been reported in human epilepsy and animal seizure models. Functional electrophysiological changes that occur when transporter expression is disrupted in chronic epilepsy models have not been examined. Here, we used a freeze-induced model of cortical dysplasia to test the role of glutamate transporters in synaptic hyperexcitability. We report that inhibiting glutamate transporters with the non-selective antagonist, DL-threo-beta-benzylozyaspartic acid (TBOA) preferentially prolongs postsynaptic currents (PSCs) and decreases the threshold for evoking epileptiform activity in lesioned compared to control cortex. The effect of inhibiting uptake is mediated primarily by the glia glutamate transporter (GLT-1) since the selective antagonist dihydrokainate (DHK) mimicked the effects of TBOA. The effect of uptake inhibition is mediated by activation of N-methyl-D-aspartate (NMDA) receptors since D-(-)-2-amino-5-phosphonovaleric acid (APV) prevents TBOA-induced effects. Neurons in lesioned cortex also have a larger tonic NMDA current. These results indicate that chronic changes in glutamate transporters and NMDA receptors contribute to hyperexcitability in cortical dysplasia.

Glutamate transporters regulate lesion-induced plasticity in the developing somatosensory cortex

Glutamate transporters are involved in neural differentiation, neuronal survival, and synaptic transmission. In the present study, we examined glutamate transporter 1 (GLT1) expression in the neonatal somatosensory cortex of C57BL/6 mice, and pursued its role in somatosensory development by comparing barrel development between GLT1 knock-out and control mice. During the first few neonatal days, a critical period for barrels, GLT1 expression is strikingly upregulated in cortical astrocytes, whereas it was downregulated in neuronal elements to below the detection threshold. GLT1 knock-out neonates developed normally in terms of body growth, cortical histoarchitecture, barrel formation, and critical period termination. However, when row C whiskers were lesioned during the critical period, reduction of lesioned row C barrels and reciprocal expansion of intact row B/D barrels were both milder in GLT1 knock-out mice than in control littermates. Accordingly, the map plasticity index, calculated as (B + D)/2C, was significantly lowered in GLT1 knock-out mice. We also found that extracellular glutamate levels in the neonatal somatosensory cortex were significantly elevated in GLT1 knock-out mice. Diminished lesion-induced plasticity was further found in mutant mice lacking glutamate-aspartate transporter (GLAST), an astrocyte-specific glutamate transporter throughout development. Therefore, glutamate transporters regulate critical period plasticity by enhancing expansion of active barrels and shrinkage of inactive barrels. Because cortical contents of glutamate receptors and GLAST were unaltered in GLT1 knock-out mice, this action appears to be mediated, at least partly, by keeping the ambient glutamate level low. Considering an essential role of glutamate receptors in the formation of whisker-related thalamocortical synapse patterning, glutamate transporters thus facilitate their activity-dependent remodeling.

Astrocytic function and its alteration in the epileptic brain

Currently available anticonvulsant drugs and complementary therapies are insufficient to control seizures in about a third of epileptic patients. Thus, there is an urgent need for new treatments that prevent the development of epilepsy and control it better in patients already afflicted with the disease. A prerequisite to reach this goal is a deeper understanding of the cellular basis of hyperexcitability and synchronization in the affected tissue. Epilepsy is often accompanied by massive reactive gliosis. Although the significance of this alteration is poorly understood, recent findings suggest that modified astroglial function may have a role in the generation and spread of seizure activity. Here we summarize properties of astrocytes as well as their changes that can be associated with epileptic tissue. The goal is to provide an understanding of the current knowledge of these cells with the long-term view of providing a foundation for the development of novel hypotheses about the role of glia in epilepsy.