Extracellular metabolites in the cortex and hippocampus of epileptic patients

Glutamate release from astrocyte cell-line GL261 via alterations in the intracellular ion environment

Astrocytes modify and maintain neural activity and functions via gliotransmitter release such as, glutamate. They also change their properties and functions in response to alterations of ion environment resulting from neurotransmission; however, the direct evidence for whether intracellular ion alteration in astrocytes triggers gliotransmitter release is not indicated. Recent studies have reported that channelrhodopsin-2 (ChR2) is useful for alteration of intracellular ion environment in several types of cells with blue light exposure. Here, we show that ChR2-expressing GL261 (GLChR2) cells, clonal astrocytes, change their properties by photo-activation. Increased intracellular sodium and calcium ion concentrations and an altered membrane potential were observed in GLChR2 cells with blue light exposure. Alterations in the intracellular ion environment caused intracellular acidification and the inhibition of proliferation. In addition, it triggered glutamate release from GLChR2 cells. Glutamate from GLChR2 cells acted on N18 cells, clonal neuronal cells, as both a transmitter and neurotoxin depending on photo-activation. Our results show that the properties of ChR2-expressing astrocytes can be controlled by blue light exposure, and cation influx through photo-activated ChR2 might trigger functional cation influx via endogenous channels and result in the increase of glutamate release. Further, our results suggest that ChR2-expressing glial cells could become a useful tool in understanding the roles of glial cell activation and neural communication in the regulation of brain functions.

Electrographic seizures are significantly reduced by in vivo inhibition of neuronal uptake of extracellular glutamine in rat hippocampus

Rats were given unilateral kainate injection into hippocampal CA3 region, and the effect of chronic electrographic seizures on extracellular glutamine (GLNECF) was examined in those with low and steady levels of extracellular glutamate (GLUECF). GLNECF, collected by microdialysis in awake rats for 5h, decreased to 62±4.4% of the initial concentration (n=6). This change correlated with the frequency and magnitude of seizure activity, and occurred in the ipsilateral but not in contralateral hippocampus, nor in kainate-injected rats that did not undergo seizure (n=6). Hippocampal intracellular GLN did not differ between the Seizure and No-Seizure Groups. These results suggested an intriguing possibility that seizure-induced decrease of GLNECF reflects not decreased GLN efflux into the extracellular fluid, but increased uptake into neurons. To examine this possibility, neuronal uptake of GLNECF was inhibited in vivo by intrahippocampal perfusion of 2-(methylamino)isobutyrate, a competitive and reversible inhibitor of the sodium-coupled neutral amino acid transporter (SNAT) subtypes 1 and 2, as demonstrated by 1.8±0.17 fold elevation of GLNECF (n=7). The frequency of electrographic seizures during uptake inhibition was reduced to 35±7% (n=7) of the frequency in pre-perfusion period, and returned to 88±9% in the post-perfusion period. These novel in vivo results strongly suggest that, in this well-established animal model of temporal-lobe epilepsy, the observed seizure-induced decrease of GLNECF reflects its increased uptake into neurons to sustain enhanced glutamatergic epileptiform activity, thereby demonstrating a possible new target for anti-seizure therapies.

Homeostatic control of brain function - new approaches to understand epileptogenesis

Neuronal excitability of the brain and ongoing homeostasis depend not only on intrinsic neuronal properties, but also on external environmental factors; together these determine the functionality of neuronal networks. Homeostatic factors become critically important during epileptogenesis, a process that involves complex disruption of self-regulatory mechanisms. Here we focus on the bioenergetic homeostatic network regulator adenosine, a purine nucleoside whose availability is largely regulated by astrocytes. Endogenous adenosine modulates complex network function through multiple mechanisms including adenosine receptor-mediated pathways, mitochondrial bioenergetics, and adenosine receptor-independent changes to the epigenome. Accumulating evidence from our laboratories shows that disruption of adenosine homeostasis plays a major role in epileptogenesis. Conversely, we have found that reconstruction of adenosine's homeostatic functions provides new hope for the prevention of epileptogenesis. We will discuss how adenosine-based therapeutic approaches may interfere with epileptogenesis on an epigenetic level, and how dietary interventions can be used to restore network homeostasis in the brain. We conclude that reconstruction of homeostatic functions in the brain offers a new conceptual advance for the treatment of neurological conditions which goes far beyond current target-centric treatment approaches.

Brain mitochondrial metabolic dysfunction and glutamate level reduction in the pilocarpine model of temporal lobe epilepsy in mice

Although certain metabolic characteristics such as interictal glucose hypometabolism are well established for temporal lobe epilepsy (TLE), its pathogenesis still remains unclear. Here, we performed a comprehensive study of brain metabolism in a mouse model of TLE, induced by pilocarpine-status epilepticus (SE). To investigate glucose metabolism, we injected mice 3.5-4 weeks after SE with [1,2-(13)C]glucose before microwave fixation of the head. Using (1)H and (13)C nuclear magnetic resonance spectroscopy, gas chromatography-mass spectrometry and high-pressure liquid chromatography, we quantified metabolites and (13)C labeling in extracts of cortex and hippocampal formation (HF). Hippocampal levels of glutamate, glutathione and alanine were decreased in pilocarpine-SE mice compared with controls. Moreover, the contents of N-acetyl aspartate, succinate and reduced nicotinamide adenine dinucleotide (phosphate) NAD(P)H were decreased in HF indicating impairment of mitochondrial function. In addition, the reduction in (13)C enrichment of hippocampal citrate and malate suggests decreased tricarboxylic acid (TCA) cycle turnover in this region. In cortex, we found reduced (13)C labeling of glutamate, glutamine and aspartate via the pyruvate carboxylation and pyruvate dehydrogenation pathways, suggesting slower turnover of these amino acids and/or the TCA cycle. In conclusion, mitochondrial metabolic dysfunction and altered amino-acid metabolism is found in both cortex and HF in this epilepsy model.

Regulation of astrocyte glutamine synthetase in epilepsy

Astrocytes play a crucial role in regulating and maintaining the extracellular chemical milieu of the central nervous system under physiological conditions. Moreover, proliferation of phenotypically altered astrocytes (a.k.a. reactive astrogliosis) has been associated with many neurologic and psychiatric disorders, including mesial temporal lobe epilepsy (MTLE). Glutamine synthetase (GS), which is found in astrocytes, is the only enzyme known to date that is capable of converting glutamate and ammonia to glutamine in the mammalian brain. This reaction is important, because a continuous supply of glutamine is necessary for the synthesis of glutamate and GABA in neurons. The known stoichiometry of glutamate transport across the astrocyte plasma membrane also suggests that rapid metabolism of intracellular glutamate via GS is a prerequisite for efficient glutamate clearance from the extracellular space. Several studies have indicated that the activity of GS in astrocytes is diminished in several brain disorders, including MTLE. It has been hypothesized that the loss of GS activity in MTLE leads to increased extracellular glutamate concentrations and epileptic seizures. Understanding the mechanisms by which GS is regulated may lead to novel therapeutic approaches to MTLE, which is frequently refractory to antiepileptic drugs. This review discusses several known mechanisms by which GS expression and function are influenced, from transcriptional control to enzyme modification.

Glutaraldehyde cross-linked glutamate oxidase coated microelectrode arrays: selectivity and resting levels of glutamate in the CNS

Glutaraldehyde is widely used as a cross-linking agent for enzyme immobilization onto microelectrodes. Recent studies and prior reports indicate changes in enzyme activity and selectivity with certain glutaraldehyde cross-linking procedures that may jeopardize the performance of microelectrode recordings and lead to falsely elevated responses in biological systems. In this study, the sensitivity of glutaraldehyde cross-linked glutamate oxidase-based microelectrode arrays to 22 amino acids was tested and compared to glutamate. As expected, responses to electroactive amino acids (Cys, Tyr, Trp) were detected at both nonenzyme-coated and enzyme-coated microelectrodes sites, while the remaining amino acids yielded no detectable responses. Electroactive amino acids were effectively blocked with a m-phenylene diamine (mPD) layer and, subsequently, no responses were detected. Preliminary results on the use of poly(ethylene glycol) diglycidyl ether (PEGDE) as a potentially more reliable cross-linking agent for the immobilization of glutamate oxidase onto ceramic-based microelectrode arrays are reported and show no significant advantages over glutaraldehyde as we observe comparable selectivities and responses. These results support that glutaraldehyde-cross-linked glutamate oxidase retains sufficient enzyme specificity for accurate in vivo brain measures of tonic and phasic glutamate levels when immobilized using specific "wet" coating procedures.

Enzymes in the glutamate-glutamine cycle in the anterior cingulate cortex in postmortem brain of subjects with autism

BACKGROUND

Accumulating evidence suggests that dysfunction in the glutamatergic system may underlie the pathophysiology of autism. The anterior cingulate cortex (ACC) has been implicated in autism as well as in glutamatergic neurotransmission. We hypothesized that alterations in the glutamate-glutamine cycle in the ACC might play a role in the pathophysiology of autism.

METHODS

We performed Western blot analyses for the protein expression levels of enzymes in the glutamate-glutamine cycle, including glutamine synthetase, kidney-type glutaminase, liver-type glutaminase, and glutamate dehydrogenases 1 and 2, in the ACC of postmortem brain of individuals with autism (n = 7) and control subjects (n = 13).

RESULTS

We found that the protein levels of kidney-type glutaminase, but not those of the other enzymes measured, in the ACC were significantly lower in subjects with autism than in controls.

CONCLUSION

The results suggest that reduced expression of kidney-type glutaminase may account for putative alterations in glutamatergic neurotransmission in the ACC in autism.

Neuroprotective effects of electrical stimulation of the anterior nucleus of the thalamus for hippocampus neurons in intractable epilepsy

Epilepsy and Parkinson's disease (PD) are common neurological disorders. Both epilepsy and PD are potentially progressive disabling diseases that can be treated with the established therapy of deep brain stimulation (DBS). The difference in therapy is target selection and stimulation parameter modulation. The anterior nucleus of the thalamus (ANT) is chosen for intractable epilepsy and the subthalamic nucleus (STN) is chosen for PD. Long-term stable symptom control of STN-DBS can be seen in PD patients while the positive effect of ANT-DBS can be observed in epilepsy patients. Experimental data and clinical evidence have been reported that indicate the neuroprotective property of STN-DBS could be found in PD patients. Therefore, we hypothesize that the neuroprotective benefits of ANT-DBS may be present in epilepsy patients.

In vivo MRS and histochemistry of status epilepticus-induced hippocampal pathology in a juvenile model of temporal lobe epilepsy

Childhood status epilepticus (SE) initiates an epileptogenic process that leads to spontaneous seizures and hippocampal pathology characterized by neuronal loss, gliosis and an imbalance between excitatory and inhibitory neurotransmission. It remains unclear whether these changes are a cause or consequence of chronic epilepsy. In this study, in vivo MRS was used in a post-SE juvenile rat model of temporal lobe epilepsy (TLE) to establish the temporal evolution of hippocampal injury and neurotransmitter imbalance. SE was induced in P21 rats by injection of lithium and pilocarpine. Four and eight weeks after SE, in vivo (1) H and γ-aminobutyric acid (GABA)-edited MRS of the hippocampus was performed in combination with dedicated ex vivo immunohistochemistry for the interpretation and validation of MRS findings. MRS showed a 12% decrease (p<0.0001) in N-acetylaspartate and a 15% increase (p=0.0226) in choline-containing compound concentrations, indicating neuronal death and gliosis, respectively. These results were confirmed by FluoroJade and vimentin staining. Furthermore, severe and progressive decreases in GABA (-41%, p<0.001) and glutamate (Glu) (-17%, p<0.001) were found. The specific severity of GABAergic cell death was confirmed by parvalbumin immunoreactivity (-68%, p<0.001). Unexpectedly, we found changes in glutamine (Gln), the metabolic precursor of both GABA and Glu. Gln increased at 4 weeks (+36%, p<0.001), but returned to control levels at 8 weeks. This decrease was consistent with the simultaneous decrease in glutamine synthase immunoreactivity (-32%, p=0.037). In vivo MRS showed gliosis and (predominantly GABAergic) neuronal loss. In addition, an increase in Gln was detected, accompanied by a decrease in glutamine synthase immunoreactivity. This may reflect glutamine synthase downregulation in order to normalize Gln levels. These changes occurred before spontaneous recurrent seizures were present but, by creating a pre-epileptic state, may play a role in epileptogenesis. MRS can be applied in a clinical setting and may be used as a noninvasive tool to monitor the development of TLE.

Gene expression of glutamate metabolizing enzymes in the hippocampal formation in human temporal lobe epilepsy

PURPOSE

Increased interictal concentrations of extracellular hippocampal glutamate have been implicated in the pathophysiology of temporal lobe epilepsy (TLE). Recent studies suggest that perturbations of the glutamate metabolizing enzymes glutamine synthetase (GS) and phosphate activated glutaminase (PAG) may underlie the glutamate excess in TLE. However, the molecular mechanism of the enzyme perturbations remains unclear. A better understanding of the regulatory mechanisms of GS and PAG could facilitate the discovery of novel therapeutics for TLE.

METHODS

We used in situ hybridization on histologic sections to assess the distribution and quantity of messenger RNA (mRNA) for GS and PAG in subfields of hippocampal formations from the following: (1) patients with TLE and concomitant hippocampal sclerosis, (2) patients with TLE and no hippocampal sclerosis, and (3) nonepilepsy autopsy subjects.

KEY FINDINGS

GS mRNA was increased by ~50% in the CA3 in TLE patients without hippocampal sclerosis versus in TLE patients with sclerosis and in nonepilepsy subjects. PAG mRNA was increased by >100% in the subiculum in both TLE patient categories versus in nonepilepsy subjects. PAG mRNA was also increased in the CA1, CA2, CA3, and dentate hilus in TLE without hippocampal sclerosis versus in TLE with sclerosis. Finally, PAG mRNA was increased in the dentate gyrus in TLE with sclerosis versus in nonepilepsy subjects, and also increased in the hilus in TLE without sclerosis versus in TLE with sclerosis.

SIGNIFICANCE

These findings demonstrate complex changes in the expression of mRNAs for GS and PAG in the hippocampal formation in TLE, and raise the possibility that both transcriptional and posttranscriptional mechanisms may underlie the regulation of GS and PAG proteins in the epileptic brain.

Roles of glutamine synthetase inhibition in epilepsy

Glutamine synthetase (GS, E.C. 6.3.1.2) is a ubiquitous and highly compartmentalized enzyme that is critically involved in several metabolic pathways in the brain, including the glutamine-glutamate-GABA cycle and detoxification of ammonia. GS is normally localized to the cytoplasm of most astrocytes, with elevated concentrations of the enzyme being present in perivascular endfeet and in processes close to excitatory synapses. Interestingly, an increasing number of studies have indicated that the expression, distribution, or activity of brain GS is altered in several brain disorders, including Alzheimer's disease, schizophrenia, depression, suicidality, and mesial temporal lobe epilepsy (MTLE). Although the metabolic and functional sequelae of brain GS perturbations are not fully understood, it is likely that a deficiency in brain GS will have a significant biological impact due to the critical metabolic role of the enzyme. Furthermore, it is possible that restoration of GS in astrocytes lacking the enzyme could constitute a novel and highly specific therapy for these disorders. The goals of this review are to summarize key features of mammalian GS under normal conditions, and discuss the consequences of GS deficiency in brain disorders, specifically MTLE.

Glutamate biosensor imaging reveals dysregulation of glutamatergic pathways in a model of developmental cortical malformation

Cortical malformations can cause intractable epilepsy, but the underlying epileptogenic mechanisms are poorly understood. We used high-speed glutamate biosensor imaging to ask how glutamatergic signaling is altered in cortical malformations induced by neonatal freeze-lesions (FL). In non-lesion neocortical slices from 2 to 8week old rats, evoked glutamate signals were symmetrical in the medio-lateral axis and monotonic, correlating with simple, brief (≈50ms) local field potentials (LFPs). By contrast, in FL cortex glutamate signals were prolonged, increased in amplitude, and polyphasic, which paralleled a prolongation of the LFP. Using glutamate biosensor imaging, we found that glutamate signals propagated throughout large areas of FL cortex and were asymmetric (skewed toward the lesion). Laminar analysis demonstrated a shift in the region of maximal glutamate release toward superficial layers in FL cortex. The ability to remove exogenous glutamate was increased within the FL itself but was decreased in immediately adjacent regions. There were corresponding alterations in astrocyte density, with an increase within the lesion and a decrease in deep cortical layers surrounding the lesion. These findings demonstrate both network connectivity and glutamate metabolism are altered in this cortical malformation model and suggests that the regional ability of astrocytes to remove released glutamate may be inversely related to local excitability.

Glutamate transporter EAAT2: a new target for the treatment of neurodegenerative diseases

Glutamate is the primary excitatory amino acid neurotransmitter in the CNS. The concentration of glutamate in the synaptic cleft is tightly controlled by interplay between glutamate release and glutamate clearance. Abnormal glutamate release and/or dysfunction of glutamate clearance can cause overstimulation of glutamate receptors and result in neuronal injury known as excitotoxicity. The glial glutamate transporter EAAT2 plays a major role in glutamate clearance. Dysfunction or reduced expression of EAAT2 has been documented in many neurodegenerative diseases. In addition, many studies in animal models of disease indicate that increased EAAT2 expression provides neuronal protection. Here, we summarize these studies and suggest that EAAT2 is a potential target for the prevention of excitotoxicity. EAAT2 can be upregulated by transcriptional or translational activation. We discuss current progress in the search for EAAT2 activators, which is a promising direction for the treatment of neurodegenerative diseases.

Increased expression of TRPV1 in the cortex and hippocampus from patients with mesial temporal lobe epilepsy

Transient receptor potential vanilloid type-1 (TRPV1) is a ligand-gated nonselective cation channel that has been well characterized in peripheral pain pathway. Recent evidence from animal models of temporal lobe epilepsy (TLE) has supported the important role of TRPV1 in epileptogenesis. In this study, we investigated the expression and cellular distribution of TRPV1 in the temporal cortex (CTX) and hippocampus (HPC) from 26 patients with mesial TLE (MTLE) compared with 12 histologically normal samples. Reverse transcription-PCR and Western blotting revealed up-regulated mRNA and protein levels of TRPV1 in the MTLE group versus the control group. Immunohistochemistry data demonstrated that TRPV1 was mainly distributed in the cell bodies and dendrites of neurons. Double-labeled immunofluorescence further revealed that TRPV1 was localized on NeuN-positive neurons and GFAP-positive astrocytes, but not on HLA-positive microglia. In addition, its co-localization with glutamate and gamma-aminobutyric acid (GABA) indicated that TRPV1 was distributed on both glutamatergic and GABAergic neurons. Moreover, nerve growth factor, a sensitizing factor for TRPV1, was showed a higher expression pattern in MTLE patients. Taken together, our findings suggest that the overexpression and distribution patterns of TRPV1 might be involved in the pathogenesis and epileptogenesis of human MTLE.

Astroglial networks and implications for therapeutic neuromodulation of epilepsy

Epilepsy is a common chronic neurologic disorder affecting approximately 1% of the world population. More than one-third of all epilepsy patients have incompletely controlled seizures or debilitating medication side effects in spite of optimal medical management. Medically refractory epilepsy is associated with excess injury and mortality, psychosocial dysfunction, and significant cognitive impairment. Effective treatment options for these patients can be limited. The cellular mechanisms underlying seizure activity are incompletely understood, though we here describe multiple lines of evidence supporting the likely contribution of astroglia to epilepsy, with focus on individual astrocytes and their network functions. Of the emerging therapeutic modalities for epilepsy, one of the most intriguing is the field of neuromodulation. Neuromodulatory treatment, which consists of administering electrical pulses to neural tissue to modulate its activity leading to a beneficial effect, may be an option for these patients. Current modalities consist of vagal nerve stimulation, open and closed-loop stimulation, and transcranial magnetic stimulation. Due to their unique properties, we here present astrocytes as likely important targets for the developing field of neuromodulation in the treatment of epilepsy.

Clinical microdialysis in neuro-oncology: principles and applications

Clinical microdialysis allows a discrete volume of the brain to be sampled for neurochemical analysis of neurotransmitters, metabolites, biomarkers, and drugs. The technique can be safely used in humans intraoperatively, in the intensive care unit, and in ambulatory settings. Microdialysis probes, micropumps, and analytical equipment are commercially available and have been used extensively for neurochemical monitoring in traumatic brain injury, stroke, and subarachnoid hemorrhage. There has been very limited use of microdialysis in neuro-oncology, but this technique has great promise in the study of the basic neurochemistry of brain tumors, alterations in neurochemistry in response to therapy, and the pharmacokinetics of chemotherapeutic agents. Microdialysis probes may also be used to deliver drugs while simultaneously permitting monitoring of neurochemical changes induced by this therapy.

Stimulation of the anterior nucleus of the thalamus induces changes in amino acids in the hippocampi of epileptic rats

We investigated the changes in the levels of amino acids during high frequency stimulation (HFS) of the anterior nucleus of the thalamus (ANT) in epileptic rats, which had seizures induced by unilaterally stereotactic administration of kainic acid (KA). Thirty-six adult male Wistar rats were divided into three groups: the KA-stim group (KA rats received ipsilateral ANT stimulation), the KA-sham group (KA rats received sham stimulation) and the control group, which underwent stereotactic administration of saline and received ipsilateral ANT stimulation. Microdialysis probes were unilaterally lowered into the CA3 region of the hippocampus, but probes were implanted bilaterally in the KA-stim group. The concentrations of glutamate (Glu), taurine (Tau), aspartate (Asp) and γ-aminobutyric acid (GABA) in the dialysate samples were determined by high-performance liquid chromatography. The concentrations of Glu, Asp and Tau in the hippocampi of KA rats were significantly higher than that found in control rats; however, no difference in the concentrations of GABA were found. In the ipsilateral hippocampi (KA-injected) of rats in the KA-stim group, stimulation of the ANT caused decreases in concentrations of Glu and Asp, an increase in the concentration of GABA and no significant change in the concentration of Tau. Unilateral ANT stimulation did not influence the amino acids in the contralateral hippocampus. In control rats, extracellular Tau significantly increased during and after stimulation. This study demonstrated that unilateral ANT stimulation inhibited the hyperactivation of the excitatory process and promoted the inhibitory process in the ipsilateral hippocampus of KA rats.

Astrocytic regulation of glutamate homeostasis in epilepsy

Astrocytes play a critical role in regulation of extracellular neurotransmitter levels in the central nervous system. This function is particularly prominent for the excitatory amino acid glutamate, with estimates that 80-90% of extracellular glutamate uptake in brain is through astrocytic glutamate transporters. This uptake has significance both in regulation of the potential toxic accumulation of extracellular glutamate and in normal resupply of inhibitory and excitatory synapses with neurotransmitter. This resupply of neurotransmitter is accomplished by astroglial uptake of glutamate, transformation of glutamate to glutamine by the astrocytic enzyme glutamine synthetase (GS), and shuttling of glutamine back to excitatory and inhibitory neurons via specialized transporters. Once in neurons, glutamine is enzymatically converted back to glutamate, which is utilized for synaptic transmission, either directly, or following decarboxylation to γ-aminobutyric acid. Many neurologic and psychiatric conditions, particularly epilepsy, are accompanied by the development of reactive gliosis, a pathology characterized by anatomical and biochemical plasticity in astrocytes, accompanied by proliferation of these cells. Among the biochemical changes evident in reactive astrocytes is a downregulation of several of the important regulators of the glutamine-glutamate cycle, including GS, and possibly also glutamate transporters. This downregulation may have significance in contributing both to the aberrant excitability and to the altered neuropathology characterizing epilepsy. In the present review, we provide an overview of the normal function of astrocytes in regulating extracellular glutamate homeostasis, neurotransmitter supply, and excitotoxicity. We further discuss the potential role reactive gliosis may play in the pathophysiology of epilepsy.

The role of astroglia in the epileptic brain

Epilepsies comprise a family of multifactorial neurological disorders that affect at least 50 million people worldwide. Despite a long history of neurobiological and clinical studies the mechanisms that lead the brain network to a hyperexcitable state and to the intense, massive neuronal discharges reflecting a seizure episode are only partially defined. Most epilepsies of genetic origin are related to mutations in ionic channels that cause neuronal hyperexcitability. However, idiopathic epilepsies of unclear origin represent the majority of these brain disorders. A large body of evidence suggests that in the epileptic brain neurons are not the only players. Indeed, the glial cell astrocyte is known to be morphologically and functionally altered in different types of epilepsy. Although it is unclear whether these astrocyte dysfunctions can have a causative role in epileptogenesis, the hypothesis that astrocytes contribute to epileptiform activities recently received a considerable experimental support. Notably, currently used antiepileptic drugs, that act mainly on neuronal ion channels, are ineffective in a large group of patients. Clarifying astrocyte functions in the epileptic brain tissue could unveil astrocytes as novel therapeutic targets. In this review we present first a short overview on the role of astrocytes in the epileptic brain starting from the "historical" observations on their fundamental modulation of brain homeostasis, such as the control of water content, ionic equilibrium, and neurotransmitters concentrations. We then focus our review on most recent studies that hint at a distinct contribution of these cells in the generation of focal epileptiform activities.

Evidence for astrocytes as a potential source of the glutamate excess in temporal lobe epilepsy

Increased extracellular brain glutamate has been implicated in the pathophysiology of human refractory temporal lobe epilepsy (TLE), but the cause of the excessive glutamate is unknown. Prior studies by us and others have shown that the glutamate degrading enzyme glutamine synthetase (GS) is deficient in astrocytes in the epileptogenic hippocampal formation in a subset of patients with TLE. We have postulated that the loss of GS in TLE leads to increased glutamate in astrocytes with elevated concentrations of extracellular glutamate and recurrent seizures as the ultimate end-points. Here we test the hypothesis that the deficiency in GS leads to increased glutamate in astrocytes. Rats were chronically infused with methionine sulfoximine (MSO, n=4) into the hippocampal formation to induce GS deficiency and recurrent seizures. A separate group of rats was infused with 0.9% NaCl (saline) as a control (n=6). At least 10days after the start of infusion, once recurrent seizures were established in the MSO-treated rats, the concentration of glutamate was assessed in CA1 of the hippocampal formation by immunogold electron microscopy. The concentration of glutamate was 47% higher in astrocytes in the MSO-treated vs. saline-treated rats (p=0.02), and the ratio of glutamate in astrocytes relative to axon terminals was increased by 74% in the MSO-treated rats (p=0.003). These data support our hypothesis that a deficiency in GS leads to increased glutamate in astrocytes. We additionally propose that the GS-deficient astrocytes in the hippocampal formation in TLE lead to elevated extracellular brain glutamate either through decreased clearance of extracellular glutamate or excessive release of glutamate into the extracellular space from these cells, or a combination of the two.

Redistribution of monocarboxylate transporter 2 on the surface of astrocytes in the human epileptogenic hippocampus

Emerging evidence points to monocarboxylates as key players in the pathophysiology of temporal lobe epilepsy (TLE) with hippocampal sclerosis (mesial temporal lobe epilepsy, MTLE). Monocarboxylate transporters (MCTs) 1 and 2, which are abundantly present on brain endothelial cells and perivascular astrocyte endfeet, respectively, facilitate the transport of monocarboxylates and protons across cell membranes. Recently, we reported that the density of MCT1 protein is reduced on endothelial cells and increased on astrocyte plasma membranes in the hippocampal formation in patients with MTLE and in several animal models of the disorder. Because the perivascular astrocyte endfeet comprise an important part of the neurovascular unit, we now assessed the distribution of the MCT2 in hippocampal formations in TLE patients with (MTLE) or without hippocampal sclerosis (non-MTLE). Light microscopic immunohistochemistry revealed significantly less perivascular MCT2 immunoreactivity in the hippocampal formation in MTLE (n = 6) than in non-MTLE (n = 6) patients, and to a lesser degree in non-MTLE than in nonepilepsy patients (n = 4). Immunogold electron microscopy indicated that the loss of MCT2 protein occurred on perivascular astrocyte endfeet. Interestingly, the loss of MCT2 on astrocyte endfeet in MTLE (n = 3) was accompanied by an upregulation of the protein on astrocyte membranes facing synapses in the neuropil, when compared with non-MTLE (n = 3). We propose that the altered distribution of MCT1 and MCT2 in TLE (especially MTLE) limits the flux of monocarboxylates across the blood-brain barrier and enhances the exchange of monocarboxylates within the brain parenchyma.

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.

Roles of glutamine in neurotransmission

Glutamine (Gln) is found abundantly in the central nervous system (CNS) where it participates in a variety of metabolic pathways. Its major role in the brain is that of a precursor of the neurotransmitter amino acids: the excitatory amino acids, glutamate (Glu) and aspartate (Asp), and the inhibitory amino acid, γ-amino butyric acid (GABA). The precursor-product relationship between Gln and Glu/GABA in the brain relates to the intercellular compartmentalization of the Gln/Glu(GABA) cycle (GGC). Gln is synthesized from Glu and ammonia in astrocytes, in a reaction catalyzed by Gln synthetase (GS), which, in the CNS, is almost exclusively located in astrocytes (Martinez-Hernandez et al., 1977). Newly synthesized Gln is transferred to neurons and hydrolyzed by phosphate-activated glutaminase (PAG) to give rise to Glu, a portion of which may be decarboxylated to GABA or transaminated to Asp. There is a rich body of evidence which indicates that a significant proportion of the Glu, Asp and GABA derived from Gln feed the synaptic, neurotransmitter pools of the amino acids. Depolarization-induced-, calcium- and PAG activity-dependent releases of Gln-derived Glu, GABA and Asp have been observed in CNS preparations in vitro and in the brain in situ. Immunocytochemical studies in brain slices have documented Gln transfer from astrocytes to neurons as well as the location of Gln-derived Glu, GABA and Asp in the synaptic terminals. Patch-clamp studies in brain slices and astrocyte/neuron co-cultures have provided functional evidence that uninterrupted Gln synthesis in astrocytes and its transport to neurons, as mediated by specific carriers, promotes glutamatergic and GABA-ergic transmission. Gln entry into the neuronal compartment is facilitated by its abundance in the extracellular spaces relative to other amino acids. Gln also appears to affect neurotransmission directly by interacting with the NMDA class of Glu receptors. Transmission may also be modulated by alterations in cell membrane polarity related to the electrogenic nature of Gln transport or to uncoupled ion conductances in the neuronal or glial cell membranes elicited by Gln transporters. In addition, Gln appears to modulate the synthesis of the gaseous messenger, nitric oxide (NO), by controlling the supply to the cells of its precursor, arginine. Disturbances of Gln metabolism and/or transport contribute to changes in Glu-ergic or GABA-ergic transmission associated with different pathological conditions of the brain, which are best recognized in epilepsy, hepatic encephalopathy and manganese encephalopathy.

Phosphorus magnetic resonance spectroscopy in malformations of cortical development

PURPOSE

The aim of this study was to evaluate phospholipid metabolism in patients with malformations of cortical development (MCDs).

METHODS

Thirty-seven patients with MCDs and 31 control subjects were studied using three-dimensional phosphorus magnetic resonance spectroscopy ((31)P-MRS) at 3.0 T. The voxels in the lesions and in the frontoparietal cortex of the control subjects were compared (the effective volumes were 12.5 cm(3)). Robust quantification methods were applied to fit the time-domain data to the following resonances: phosphoethanolamine (PE); phosphocholine (PC); inorganic phosphate (Pi); glycerophosphoethanolamine (GPE); glycerophosphocholine (GPC); phosphocreatine (PCr); and α-, β-, and γ-adenosine triphosphate (ATP). We also estimated the total ATP (ATP(t) = α-+β-+γ-ATP), phosphodiesters (PDE = GPC+GPE), phosphomonoesters (PME = PE+PC), and the PME/PDE, PCr/ATP(t) and PCr/Pi ratios. The magnesium (Mg(2+)) levels and pH values were calculated based on PCr, Pi, and β-ATP chemical shifts.

KEY FINDINGS

Compared to controls and assuming that a p-value < 0.05 indicates statistical significance, the patients with MCDs exhibited significantly lower pH values and higher Mg(2+) levels. In addition, the patients with MCDs had lower GPC and PDE and an increased PME/PDE ratio.

SIGNIFICANCE

Mg(2+) and pH are important in the regulation of bioenergetics and are involved in many electrical activity pathways in the brain. Our data support the idea that neurometabolic impairments occur during seizure onset and propagation. The GPC, PDE, and PME/PDE abnormalities also demonstrate that there are membrane turnover disturbances in patients with MCDs.

13C MRS studies of neuroenergetics and neurotransmitter cycling in humans

In the last 25 years, (13)C MRS has been established as the only noninvasive method for the measurement of glutamate neurotransmission and cell-specific neuroenergetics. Although technically and experimentally challenging, (13)C MRS has already provided important new information on the relationship between neuroenergetics and neuronal function, the energy cost of brain function, the high neuronal activity in the resting brain state and how neuroenergetics and neurotransmitter cycling are altered in neurological and psychiatric disease. In this article, the current state of (13)C MRS as it is applied to the study of neuroenergetics and neurotransmitter cycling in humans is reviewed. The focus is predominantly on recent findings in humans regarding metabolic pathways, applications to clinical research and the technical status of the method. Results from in vivo (13)C MRS studies in animals are discussed from the standpoint of the validation of MRS measurements of neuroenergetics and neurotransmitter cycling, and where they have helped to identify key questions to address in human research. Controversies concerning the relationship between neuroenergetics and neurotransmitter cycling and factors having an impact on the accurate determination of fluxes through mathematical modeling are addressed. We further touch upon different (13)C-labeled substrates used to study brain metabolism, before reviewing a number of human brain diseases investigated using (13)C MRS. Future technological developments are discussed that will help to overcome the limitations of (13)C MRS, with special attention given to recent developments in hyperpolarized (13)C MRS.

Glutamate abnormalities in obsessive compulsive disorder: neurobiology, pathophysiology, and treatment

Obsessive compulsive disorder is prevalent, disabling, incompletely understood, and often resistant to current therapies. Established treatments consist of specialized cognitive-behavioral psychotherapy and pharmacotherapy with medications targeting serotonergic and dopaminergic neurotransmission. However, remission is rare, and more than a quarter of OCD sufferers receive little or no benefit from these approaches, even when they are optimally delivered. New insights into the disorder, and new treatment strategies, are urgently needed. Recent evidence suggests that the ubiquitous excitatory neurotransmitter glutamate is dysregulated in OCD, and that this dysregulation may contribute to the pathophysiology of the disorder. Here we review the current state of this evidence, including neuroimaging studies, genetics, neurochemical investigations, and insights from animal models. Finally, we review recent findings from small clinical trials of glutamate-modulating medications in treatment-refractory OCD. The precise role of glutamate dysregulation in OCD remains unclear, and we lack blinded, well-controlled studies demonstrating therapeutic benefit from glutamate-modulating agents. Nevertheless, the evidence supporting some important perturbation of glutamate in the disorder is increasingly strong. This new perspective on the pathophysiology of OCD, which complements the older focus on monoaminergic neurotransmission, constitutes an important focus of current research and a promising area for the ongoing development of new therapeutics.

Excitatory action of GABA on immature neurons is not due to absence of ketone bodies metabolites or other energy substrates

Brain slices incubated with glucose have provided most of our knowledge on cellular, synaptic, and network driven mechanisms. It has been recently suggested that γ-aminobutyric acid (GABA) excites neonatal neurons in conventional glucose-perfused slices but not when ketone bodies metabolites, pyruvate, and/or lactate are added, suggesting that the excitatory actions of GABA are due to energy deprivation when glucose is the sole energy source. In this article, we review the vast number of studies that show that slices are not energy deprived in glucose-containing medium, and that addition of other energy substrates at physiologic concentrations does not alter the excitatory actions of GABA on neonatal neurons. In contrast, lactate, like other weak acids, can produce an intracellular acidification that will cause a reduction of intracellular chloride and a shift of GABA actions. The effects of high concentrations of lactate, and particularly of pyruvate (4-5 mm), as used are relevant primarily to pathologic conditions; these concentrations not being found in the brain in normal "control" conditions. Slices in glucose-containing medium may not be ideal, but additional energy substrates neither correspond to physiologic conditions nor alter GABA actions. In keeping with extensive observations in a wide range of animal species and brain structures, GABA depolarizes immature neurons and the reduction of the intracellular concentration of chloride ([Cl(-)](i)) is a basic property of brain maturation that has been preserved throughout evolution. In addition, this developmental sequence has important clinical implications, notably concerning the higher incidence of seizures early in life and their long-lasting deleterious sequels. Immature neurons have difficulties exporting chloride that accumulates during seizures, leading to permanent increase of [Cl(-)](i) that converts the inhibitory actions of GABA to excitatory and hampers the efficacy of GABA-acting antiepileptic drugs.

Altered expression of brain monocarboxylate transporter 1 in models of temporal lobe epilepsy

Monocarboxylate transporter 1 (MCT1) facilitates the transport of monocarboxylate fuels (lactate, pyruvate and ketone bodies) and acidic drugs, such as valproic acid, across cell membranes. We recently reported that MCT1 is deficient on microvessels in the epileptogenic hippocampal formation in patients with medication-refractory temporal lobe epilepsy (TLE). To further define the role of MCT1 in the pathophysiology of TLE, we used immunohistochemistry and stereological analysis to localize and quantify the transporter in the hippocampal formation in three novel and highly relevant rat models of TLE and in nonepileptic control animals. One model utilizes methionine sulfoximine to induce brain glutamine synthetase deficiency and recurrent limbic seizures, while two models employ an episode of perforant pathway stimulation to cause epilepsy. MCT1 was lost on microvessels and upregulated on astrocytes in the hippocampal formation in all models of TLE. Notably, the loss of MCT1 on microvessels was not due to a reduction in microvessel density. The similarities in MCT1 expression among human subjects with TLE and several animal models of the disease strongly suggest a critical role of this molecule in the pathogenesis of TLE. We hypothesize that the downregulation of MCT1 may promote seizures via impaired uptake of ketone bodies and antiepileptic drugs by the epileptogenic brain. We also propose that the overexpression of MCT1 on astrocytes may lead to increased uptake or release of monocarboxylates by these cells, with important implications for brain metabolism and excitability. These hypotheses can now be rigorously tested in several animal models that replicate key features of human TLE.

Intracranial depth electrodes implantation in the era of image-guided surgery

The advent of modern image-guided surgery has revolutionized depth electrode implantation techniques. Stereoelectroencephalography (SEEG), introduced by Talairach in the 1950s, is an invasive method for three-dimensional analysis on the epileptogenic zone based on the technique of intracranial implantation of depth electrodes. The aim of this article is to discuss the principles of SEEG and their evolution from the Talairach era to the image-guided surgery of today, along with future prospects. Although the general principles of SEEG have remained intact over the years, the implantation of depth electrodes, i.e. the surgical technique that enables this method, has undergone tremendous evolution over the last three decades, due the advent of modern imaging techniques, computer systems and new stereotactic techniques. The use of robotic systems, the constant evolution of imaging and computing techniques and the use of depth electrodes together with microdialysis probes will open up enormous prospects for applying depth electrodes and SEEG both for investigative use and for therapeutic use. Brain stimulation of deep targets and the construction of "smart" electrodes may, in the near future, increase the need to use this method.

Effects of high frequency electrical stimulation and R-verapamil on seizure susceptibility and glutamate and GABA release in a model of phenytoin-resistant seizures

The present study was focused to characterize the effects of intrahippocampal application of R-verapamil, a P-glycoprotein blocker, and High Frequency Electrical Stimulation (HFS) at 130 Hz, on seizure susceptibility and extracellular concentrations of glutamate and γ-aminobutyric acid (GABA) in hippocampus of kindled rats with drug-resistant seizures. Fully kindled rats classified in responsive and non-responsive to phenytoin were used for this purpose. In contrast with responsive animals, non-responsive rats showed lower afterdischarge threshold (ADT) values in pre-kindling conditions and required less number of kindling trials to achieve the kindled state. Once the animals attained the kindled state, both epileptic groups presented high glutamate and low GABA interictal release, effect more evident in non-responsive rats. In hippocampus of responsive animals, GABA levels demonstrated two increases at 120 and 240 min after the ictal event, a situation no detected for non-responsive rats. Kindled animals receiving hippocampal HFS showed augmented ADT, an effect associated with enhanced GABA release in responsive rats. Intrahippocampal perfusion of R-verapamil (5 mM) decreased the seizure susceptibility (high ADT values), enhanced the interictal GABA release and the postictal levels of glutamate and GABA in responsive and non-responsive rats. It is conclude that alterations of glutamate and GABA release in the epileptic hippocampus of non-responsive animals resemble those found in hippocampus of patients with refractory TLE. In addition, intrahippocampal application of HFS and R-verapamil modifies the amino acid release and reduces the seizure susceptibility of both, responsive and non-responsive rats.

Lactate Effectively Covers Energy Demands during Neuronal Network Activity in Neonatal Hippocampal Slices

Although numerous experimental data indicate that lactate is efficiently used for energy by the mature brain, the direct measurements of energy metabolism parameters during neuronal network activity in early postnatal development have not been performed. Therefore, the role of lactate in the energy metabolism of neurons at this age remains unclear. In this study, we monitored field potentials and contents of oxygen and NAD(P)H in correlation with oxidative metabolism during intense network activity in the CA1 hippocampal region of neonatal brain slices. We show that in the presence of glucose, lactate is effectively utilized as an energy substrate, causing an augmentation of oxidative metabolism. Moreover, in the absence of glucose lactate is fully capable of maintaining synaptic function. Therefore, during network activity in neonatal slices, lactate can be an efficient energy substrate capable of sustaining and enhancing aerobic energy metabolism.

The biochemical and cellular basis for nutraceutical strategies to attenuate neurodegeneration in Parkinson's disease

Future therapeutic intervention that could effectively decelerate the rate of degeneration within the substantia nigra pars compacta (SNc) could add years of mobility and reduce morbidity associated with Parkinson’s disease (PD). Neurodegenerative decline associated with PD is distinguished by extensive damage to SNc dopaminergic (DAergic) neurons and decay of the striatal tract. While genetic mutations or environmental toxins can precipitate pathology, progressive degenerative succession involves a gradual decline in DA neurotransmission/synaptic uptake, impaired oxidative glucose consumption, a rise in striatal lactate and chronic inflammation. Nutraceuticals play a fundamental role in energy metabolism and signaling transduction pathways that control neurotransmission and inflammation. However, the use of nutritional supplements to slow the progression of PD has met with considerable challenge and has thus far proven unsuccessful. This review re-examines precipitating factors and insults involved in PD and how nutraceuticals can affect each of these biological targets. Discussed are disease dynamics (Sections 1 and 2) and natural substances, vitamins and minerals that could impact disease processes (Section 3). Topics include nutritional influences on α-synuclein aggregation, ubiquitin proteasome function, mTOR signaling/lysosomal-autophagy, energy failure, faulty catecholamine trafficking, DA oxidation, synthesis of toxic DA-quinones, o-semiquinones, benzothiazolines, hyperhomocyseinemia, methylation, inflammation and irreversible oxidation of neuromelanin. In summary, it is clear that future research will be required to consider the multi-faceted nature of this disease and re-examine how and why the use of nutritional multi-vitamin-mineral and plant-based combinations could be used to slow the progression of PD, if possible.

Glial cell-mediated deterioration and repair of the nervous system after traumatic brain injury in a rat model as assessed by positron emission tomography

Traumatic brain injury (TBI) is one of the most acute degenerative pathologies in the central nervous system, and in vivo indices enabling an assessment of TBI on a mechanistic basis have yet to be established. The aim of this work was to pursue neuroinflammatory changes and their link to functional disruptions of traumatically-damaged neurons in a rat model of TBI by longitudinal positron emission tomographic (PET) assays. TBI was induced in the unilateral frontal cortex of craniotomied rats according to a lateral fluid percussion brain injury protocol. The use of [(18)F]fluoroethyl-DAA1106 as a PET tracer for translocator protein (TSPO) permitted demonstration of the inflammatory response to the injury, peaking at 1 week after impact. This alteration was parallel to metabolic deficits assessed by PET with [(18)F]fluorodeoxyglucose, but the difference in TSPO levels between impacted and non-impacted frontal cortices was more than threefold of the interlateral metabolic difference, indicating superiority of TSPO imaging for sensitive detection of post-traumatic pathologies. Comparative PET, autoradiographic. and immunohistochemical investigations illustrated the primary contribution of hypertrophic microglia and macrophages to acute TSPO signals in the vicinity of the impact. Astrocytes also formed a TSPO-positive glial scar encompassing necrotic inflammation, and were clustered with PET-detectable TSPO signals in the bilateral external and internal capsules at late stages, putatively reacting with diffuse axonal injury. These observations support the applicability of TSPO-PET as an imaging-based preclinical and clinical biomarker assay in TBI, and indicate its potential capability to clarify aggressive and protective roles of glial responses to injury when combined with emerging anti-inflammatory and immunomodulatory treatments.

Chronic electrographic seizure reduces glutamine and elevates glutamate in the extracellular fluid of rat brain

Effects of spontaneous seizures on extracellular glutamate and glutamine were studied in the kainate-induced rat model of epilepsy in the chronic phase. Extracellular fluid from the CA1-CA3 regions of the hippocampus was collected with a 2-mm microdialysis probe every 2 min for 5h. EEG seizures with no or mild behavioral components caused 2- to 6-fold elevation of extracellular glutamate. Concomitantly, extracellular glutamine decreased at t=5h to 48% of the initial value (n=6). The changes in extracellular glutamate and glutamine correlated with the frequency and magnitude of seizure activity. In contrast, no change in either metabolite was observed in kainate-injected rats that did not undergo seizure during microdialysis (n=6). In hippocampal tissue (9.4 ± 1.1mg) that contained the region sampled by microdialysis and the site of kainate injection, intracellular glutamine concentration was significantly reduced in the seizure group, compared to that in no-seizure group. The observed elevation of extracellular glutamate strongly suggests that neurotransmitter glutamate was released at a rate faster than the rate of its uptake into glia, possibly due to down-regulation of the transporter. This reduces the availability of substrate glutamate for glutamine synthesis, as corroborated by the observed reduction of intracellular glutamine. This is likely to reduce the rate of glutamine efflux from glia and result in the observed decrease of extracellular glutamine. There remains an intriguing possibility that seizure-induced decrease of extracellular glutamine also reflects its increased uptake into neurons to replenish neurotransmitter glutamate during enhanced epileptiform activity.

Astrocytes and epilepsy

Astrocytes form a significant constituent of seizure foci in the human brain. For a long time it was believed that astrocytes play a significant role in the causation of seizures. With the increase in our understanding of the unique biology of these cells, their precise role in seizure foci is receiving renewed attention. This article reviews the information now available on the role of astrocytes in the hippocampal seizure focus in patients with temporal lobe epilepsy with hippocampal sclerosis. Our intent is to try to integrate the available data. Astrocytes at seizure foci seem to not be a homogeneous population of cells, and in addition to typical glial fibrillary acidic protein, positive reactive astrocytes also include a population of neuron glia-2-like cells The astrocytes in sclerotic hippocampi differ from those in nonsclerotic hippocampi in their membrane physiology, having elevated Na+ channels and reduced inwardly rectifying potassium ion channels, and some having the capacity to generate action potentials. They also have reduced glutamine synthetase and increased glutamate dehydrogenase activity. The molecular interface between the astrocyte and microvasculature is also changed. The astrocytes are also associated with increased expression of many molecules normally concerned with immune and inflammatory functions. A speculative mechanism postulates that neuron glia-2-like cells may be involved in creating a high glutamate environment, whereas the function of more typical reactive astrocytes contribute to maintain high extracellular K+ levels; both factors contributing to the hyperexcitability of subicular neurons to generate epileptiform activity. The functions of the astrocyte vascular interface may be more critical to the processes involved in epileptogenesis.

Development of intraoperative electrochemical detection: wireless instantaneous neurochemical concentration sensor for deep brain stimulation feedback

Deep brain stimulation (DBS) is effective when there appears to be a distortion in the complex neurochemical circuitry of the brain. Currently, the mechanism of DBS is incompletely understood; however, it has been hypothesized that DBS evokes release of neurochemicals. Well-established chemical detection systems such as microdialysis and mass spectrometry are impractical if one is assessing changes that are happening on a second-to-second time scale or for chronically used implanted recordings, as would be required for DBS feedback. Electrochemical detection techniques such as fast-scan cyclic voltammetry (FSCV) and amperometry have until recently remained in the realm of basic science; however, it is enticing to apply these powerful recording technologies to clinical and translational applications. The Wireless Instantaneous Neurochemical Concentration Sensor (WINCS) currently is a research device designed for human use capable of in vivo FSCV and amperometry, sampling at subsecond time resolution. In this paper, the authors review recent advances in this electrochemical application to DBS technologies. The WINCS can detect dopamine, adenosine, and serotonin by FSCV. For example, FSCV is capable of detecting dopamine in the caudate evoked by stimulation of the subthalamic nucleus/substantia nigra in pig and rat models of DBS. It is further capable of detecting dopamine by amperometry and, when used with enzyme linked sensors, both glutamate and adenosine. In conclusion, WINCS is a highly versatile instrument that allows near real-time (millisecond) detection of neurochemicals important to DBS research. In the future, the neurochemical changes detected using WINCS may be important as surrogate markers for proper DBS placement as well as the sensor component for a "smart" DBS system with electrochemical feedback that allows automatic modulation of stimulation parameters. Current work is under way to establish WINCS use in humans.

Three-dimensional relationships between perisynaptic astroglia and human hippocampal synapses

Perisynaptic astroglia are critical for normal synaptic development and function. Little is known, however, about perisynaptic astroglia in the human hippocampus. When mesial temporal lobe epilepsy (MTLE) is refractory to medication, surgical removal is required for seizure quiescence. To investigate perisynaptic astroglia in human hippocampus, we recovered slices for several hours in vitro from three surgical specimens and then quickly fixed them to achieve high-quality ultrastructure. Histological samples from each case were found to have mesial temporal sclerosis with Blumcke Type 1a (mild, moderate) or 1b (severe) pathology. Quantitative analysis through serial section transmission electron microscopy in CA1 stratum radiatum revealed more synapses in the mild (10/10 microm(3)) than the moderate (5/10 microm(3)) or severe (1/10 microm(3)) cases. Normal spines occurred in mild and moderate cases, but a few multisynaptic spines were all that remained in the severe case. Like adult rat hippocampus, perisynaptic astroglial processes were preferentially associated with larger synapses in the mild and moderate cases, but rarely penetrated the cluster of axonal boutons surrounding multisynaptic spines. Synapse perimeters were only partially surrounded by astroglial processes such that all synapses had some access to substances in the extracellular space, similar to adult rat hippocampus. Junctions between astroglial processes were observed more frequently in moderate than mild case, but were obscured by densely packed intermediate filaments in astroglial processes of the severe case. These findings suggest that perisynaptic astroglial processes associate with synapses in human hippocampus in a manner similar to model systems and are disrupted by severe MTLE pathology.

Glutamate spillover augments GABA synthesis and release from axodendritic synapses in rat hippocampus

Tight coupling between gamma-aminobutyric acid (GABA) synthesis and vesicle filling suggests that the presynaptic supply of precursor glutamate could dynamically regulate inhibitory synapses. Although the neuronal glutamate transporter excitatory amino acid transporter 3 (EAAT3) has been proposed to mediate such a metabolic role, highly efficient astrocytic uptake of synaptically released glutamate normally maintains low-extracellular glutamate levels. We examined whether axodendritic inhibitory synapses in stratum radiatum of hippocampal area CA1, which are closely positioned among excitatory glutamatergic synapses, are regulated by synaptic glutamate release via presynaptic uptake. Under conditions of spatially and temporally coordinated release of glutamate and GABA within pyramidal cell dendrites, blocking glial glutamate uptake enhanced quantal release of GABA in a transporter-dependent manner. These physiological findings correlated with immunohistochemical studies revealing expression of EAAT3 by interneurons and uptake of D-asparate into putative axodendritic inhibitory terminals only when glial uptake was blocked. These results indicate that spillover of glutamate between adjacent excitatory and inhibitory synapses can occur under conditions when glial uptake incompletely clears synaptically released glutamate. Our anatomical studies also suggest that perisomatic inhibitory synapses, unlike synapses within dendritic layers of hippocampus, are not capable of glutamate uptake and therefore transporter-mediated dynamic regulation of inhibition is a unique feature of axodendritic synapses that may play a role in maintaining a homeostatic balance of inhibition and excitation.

Metabolic correlatives of brain activity in a FOS epilepsy patient

The correlation and the interactions between neuronal activity and underlying metabolic dynamics are still a matter of debate, especially in pathological conditions. This study reports findings obtained on a subject suffering from fixation-off sensitivity (FOS) epilepsy, exploited as a model system of triggerable anomalous electrical activity. Functional Magnetic Resonance Spectroscopy was used to investigate the metabolic response to visual spike-inducing stimuli in a single voxel placed in the temporo-occipital lobe of a FOS epilepsy patient. MRS measurements were additionally performed on a control group of five healthy volunteers. The FOS patient also underwent an EEG session with the same stimulus paradigm. Uniquely in the FOS patient, glutamate and glutamine concentration increased during the first 10 min of stimulation and then returned to baseline. On the other hand, FOS-induced epileptic activity (spiking) endured throughout all the stimulation epoch. The observed metabolic dynamics may be likely linked to a complex interplay between alterations of the metabolic pathways of glutamate and modulation of the neuronal activity.

Mitochondria, oxidative stress, and temporal lobe epilepsy

Mitochondrial oxidative stress and dysfunction are contributing factors to various neurological disorders. Recently, there has been increasing evidence supporting the association between mitochondrial oxidative stress and epilepsy. Although certain inherited epilepsies are associated with mitochondrial dysfunction, little is known about its role in acquired epilepsies such as temporal lobe epilepsy (TLE). Mitochondrial oxidative stress and dysfunction are emerging as key factors that not only result from seizures, but may also contribute to epileptogenesis. The occurrence of epilepsy increases with age, and mitochondrial oxidative stress is a leading mechanism of aging and age-related degenerative disease, suggesting a further involvement of mitochondrial dysfunction in seizure generation. Mitochondria have critical cellular functions that influence neuronal excitability including production of adenosine triphosphate (ATP), fatty acid oxidation, control of apoptosis and necrosis, regulation of amino acid cycling, neurotransmitter biosynthesis, and regulation of cytosolic Ca(2+) homeostasis. Mitochondria are the primary site of reactive oxygen species (ROS) production making them uniquely vulnerable to oxidative stress and damage which can further affect cellular macromolecule function, the ability of the electron transport chain to produce ATP, antioxidant defenses, mitochondrial DNA stability, and synaptic glutamate homeostasis. Oxidative damage to one or more of these cellular targets may affect neuronal excitability and increase seizure susceptibility. The specific targeting of mitochondrial oxidative stress, dysfunction, and bioenergetics with pharmacological and non-pharmacological treatments may be a novel avenue for attenuating epileptogenesis.

Real-time glutamate measurements in the putamen of awake rhesus monkeys using an enzyme-based human microelectrode array prototype

Commonly used for research studies in the central nervous system, microdialysis has revealed a link between dysregulation of the excitatory neurotransmitter glutamate and ischemia and seizure, however limitations like slow temporal resolution have stalled the advancement of microdialysis as a diagnostic tool. We have developed and extensively characterized an enzyme-based microelectrode array technology for second-by-second in vivo amperometric measurements of glutamate in the mammalian CNS. The current studies demonstrated the ability of a human microelectrode array prototype (Spencer-Gerhardt-2 (SG-2)) to measure tonic and phasic glutamate neurotransmission in the putamen of unanesthetized non-human primates. We also showed that the SG-2 remains functional following sterilization. Ability to monitor dynamic changes in glutamate in real-time may assist the development of clinical algorithms to potentially alert care-providers prior to onset of overt ischemia or seizure, or provide neurosurgeons with second-by-second measurements of rapid changes in extracellular glutamate which could help guide surgical procedures or aid in interventional strategies.

The development of recurrent seizures after continuous intrahippocampal infusion of methionine sulfoximine in rats: a video-intracranial electroencephalographic study

Glutamine synthetase is deficient in astrocytes in the epileptogenic hippocampus in human mesial temporal lobe epilepsy (MTLE). To explore the role of this deficiency in the pathophysiology of MTLE, rats were continuously infused with the glutamine synthetase inhibitor methionine sulfoximine (MSO, 0.625 microg/h) or 0.9% NaCl (saline control) unilaterally into the hippocampus. The seizures caused by MSO were assessed by video-intracranial electroencephalogram (EEG) monitoring. All (28 of 28) of the MSO-treated animals and none (0 of 12) of the saline-treated animals developed recurrent seizures. Most recurrent seizures appeared in clusters of 2 days' duration (median; range, 1 to 12 days). The first cluster was characterized by frequent, predominantly stage I seizures, which presented after the first 9.5 h of infusion (median; range, 5.5 to 31.7 h). Subsequent clusters of less-frequent, mainly partial seizures occurred after a clinically silent interval of 7.1 days (median; range, 1.8 to 16.2 days). The ictal intracranial EEGs shared several characteristics with recordings of partial seizures in humans, such as a distinct evolution of the amplitude and frequency of the EEG signal. The neuropathology caused by MSO had similarities to hippocampal sclerosis in 23.1% of cases, whereas 26.9% of the animals had minimal neuronal loss in the hippocampus. Moderate to severe diffuse neuronal loss was observed in 50% of the animals. In conclusion, the model of intrahippocampal MSO infusion replicates key features of human MTLE and may represent a useful tool for further studies of the cellular, molecular and electrophysiological mechanisms of this disorder.

Overview of brain microdialysis

The technique of microdialysis enables sampling and collecting of small-molecular-weight substances from the interstitial space. It is a widely used method in neuroscience and is one of the few techniques available that permits quantification of neurotransmitters, peptides, and hormones in the behaving animal. More recently, it has been used in tissue preparations for quantification of neurotransmitter release. This unit provides a brief review of the history of microdialysis and its general application in the neurosciences. The authors review the theoretical principles underlying the microdialysis process, methods available for estimating extracellular concentration from dialysis samples (i.e., relative recovery), the various factors that affect the estimate of in vivo relative recovery, and the importance of determining in vivo relative recovery to data interpretation. Several areas of special note, including impact of tissue trauma on the interpretation of microdialysis results, are discussed. Step-by-step instructions for the planning and execution of conventional and quantitative microdialysis experiments are provided.

Neonatal seizures: do they damage the brain?

Seizures are an early sign of brain injury in newborns. These seizures are in most cases repetitive or associated with asymptomatic electrographic seizures. Despite the relative resistance of the immature brain to seizure-induced brain damage, there is more and more evidence that neonatal seizures impair normal brain development. This review addresses the changes associated with neonatal seizures and discusses current and future potential neuroprotective strategies.

Hippocampal extracellular GABA correlates with metabolism in human epilepsy

As the major inhibitory neurotransmitter in human brain, GABA is an important modulator of hyperexcitability in epilepsy patients. Given the high energetic cost of neurotransmission and synaptic activity, GABA concentrations may be hypothesized to correlate with metabolic function. We studied human epilepsy patients undergoing intracranial EEG monitoring for seizure localization to examine microdialysis measures of extracellular GABA (ecGABA), pre-operative MR spectroscopic measures of neuronal mitochondrial function (NAA/Cr), and wherever possible, neuropathology and hippocampal volumetry. Two groups undergoing intracranial monitoring for seizure localization were studied: surgically treated hippocampal epilepsy (MTLE) and neocortical (non-hippocampal seizure onset) epilepsy. All data are hippocampal and thus these groups allow comparisons between the epileptogenic and non-epileptogenic regions. ecGABA was measured using in vivo microdialysis performed during intracranial monitoring. Pre-operative in vivo MR spectroscopic imaging was performed to measure the ratio of N-acetyl aspartate (NAA) to creatine. Standard methods for neuropathology and hippocampal volumetry were used. In the neocortical group, increased ecGABA correlated with greater NAA/Cr (R = +0.70, p < 0.015, n = 12) while in the MTLE group, increased ecGABA linked with decreased NAA/Cr (R = -0.94, p < 0.001, n = 8). In MTLE, ecGABA (increased) and NAA/Cr (decreased) correlated with increased glial cell numbers (R = +0.71, p < 0.01, n = 12, R = -0.76 p < 0.03 respectively). No relationship was seen between ecGABA and hippocampal volumes in either group. In epilepsy, ecGABA increases occur across a range of metabolic function. Outside the seizure focus, ecGABA and NAA/Cr increase together; in contrast, within the seizure focus, ecGABA increases with declining mitochondrial function.