Interactions between glutamate transporters and metabotropic glutamate receptors at excitatory synapses in the cerebellar cortex

Developmental impact of glutamate transporter overexpression on dopaminergic neuron activity and stereotypic behavior

Obsessive-compulsive disorder (OCD) is a disabling condition that often begins in childhood. Genetic studies in OCD have pointed to SLC1A1, which encodes the neuronal glutamate transporter EAAT3, with evidence suggesting that increased expression contributes to risk. In mice, midbrain Slc1a1 expression supports repetitive behavior in response to dopaminergic agonists, aligning with neuroimaging and pharmacologic challenge studies that have implicated the dopaminergic system in OCD. These findings suggest that Slc1a1 may contribute to compulsive behavior through altered dopaminergic transmission; however, this theory has not been mechanistically tested. To examine the developmental impact of Slc1a1 overexpression on compulsive-like behaviors, we, therefore, generated a novel mouse model to perform targeted, reversible overexpression of Slc1a1 in dopaminergic neurons. Mice with life-long overexpression of Slc1a1 showed a significant increase in amphetamine (AMPH)-induced stereotypy and hyperlocomotion. Single-unit recordings demonstrated that Slc1a1 overexpression was associated with increased firing of dopaminergic neurons. Furthermore, dLight1.1 fiber photometry showed that these behavioral abnormalities were associated with increased dorsal striatum dopamine release. In contrast, no impact of overexpression was observed on anxiety-like behaviors or SKF-38393-induced grooming. Importantly, overexpression solely in adulthood failed to recapitulate these behavioral phenotypes, suggesting that overexpression during development is necessary to generate AMPH-induced phenotypes. However, doxycycline-induced reversal of Slc1a1/EAAT3 overexpression in adulthood normalized both the increased dopaminergic firing and AMPH-induced responses. These data indicate that the pathologic effects of Slc1a1/EAAT3 overexpression on dopaminergic neurotransmission and AMPH-induced stereotyped behavior are developmentally mediated, and support normalization of EAAT3 activity as a potential treatment target for basal ganglia-mediated repetitive behaviors.

Rapid Regulation of Glutamate Transport: Where Do We Go from Here?

Glutamate is the predominant excitatory neurotransmitter in the mammalian central nervous system (CNS). A family of five Na⁺-dependent transporters maintain low levels of extracellular glutamate and shape excitatory signaling. Shortly after the research group of the person being honored in this special issue (Dr. Baruch Kanner) cloned one of these transporters, his group and several others showed that their activity can be acutely (within minutes to hours) regulated. Since this time, several different signals and post-translational modifications have been implicated in the regulation of these transporters. In this review, we will provide a brief introduction to the distribution and function of this family of glutamate transporters. This will be followed by a discussion of the signals that rapidly control the activity and/or localization of these transporters, including protein kinase C, ubiquitination, glutamate transporter substrates, nitrosylation, and palmitoylation. We also include the results of our attempts to define the role of palmitoylation in the regulation of GLT-1 in crude synaptosomes. In some cases, the mechanisms have been fairly well-defined, but in others, the mechanisms are not understood. In several cases, contradictory phenomena have been observed by more than one group; we describe these studies with the goal of identifying the opportunities for advancing the field. Abnormal glutamatergic signaling has been implicated in a wide variety of psychiatric and neurologic disorders. Although recent studies have begun to link regulation of glutamate transporters to the pathogenesis of these disorders, it will be difficult to determine how regulation influences signaling or pathophysiology of glutamate without a better understanding of the mechanisms involved.

Astrocyte-neuron signaling in the mesolimbic dopamine system: the hidden stars of dopamine signaling

Astrocytes are fundamental components of brain information processing and possess the ability to respond to synaptic signaling with increases in cytoplasmic calcium and modulate neuronal activity with the subsequent release of neuroactive transmitters. Dopamine signaling is essential for brain physiology and pathology, participating in learning and memory, motor control, neurological diseases, and psychiatric diseases, and astrocytes are emerging as a key cellular target of dopamine signaling. The present review will examine evidence revealing that astrocytes respond to dopamine and modulate information processing in the primary brain regions implicated in the mesolimbic dopamine system. Astrocytes exhibit circuit-specific modulation of neuronal networks and have the potential to serve as a therapeutic target for interventions designed for dopamine pathologies.

The Neuronal Glutamate Transporter EAAT3 in Obsessive-Compulsive Disorder

Obsessive compulsive disorder (OCD) is a heterogeneous psychiatric disorder affecting 1%–3% of the population worldwide. About half of OCD afflicted individuals do not respond to currently available pharmacotherapy, which is mainly based on serotonin reuptake inhibition. Therefore, there is a critical need to search novel and improved therapeutic targets to treat this devastating disorder. In recent years, accumulating evidence has supported the glutamatergic hypothesis of OCD, and particularly pointing a potential role for the neuronal glutamate transporter EAAT3. This mini-review summarizes recent findings regarding the neurobiological basis of OCD, with an emphasis on the glutamatergic neurotransmission and EAAT3 as a key player in OCD etiology.

Proteomics analysis of the amygdala in rats with CFA-induced pain aversion with electro-acupuncture stimulation

Background

Clinical patients suffering from pain usually exhibit aversion to pain-associated environments (pain aversion). Electro-acupuncture (EA) has been proven to be effective for the treatment of pain aversion in our previous studies. The amygdala could have substantial consequences on emotion and pain consolidation as well as general pain aversion behavior, however, the underlying mechanism remains unclear.

Purpose

The current study was performed to investigate Isobaric tags for relative and absolute quantitation (iTRAQ) based quantitative proteomic analysis of the amygdala in rats with complete Freund’s adjuvant (CFA)-induced pain aversion, and comprehensive analysis of protein expression were performed to explore the underlying mechanism by which EA affects pain aversion.

Materials and methods

Inflammatory pain was induced with an intraplantar injection of 100 μL of CFA in the plantar surface of the left hind paw of the male Spragure-Dawley (SD) rats. Then the CFA-induced conditioned place aversion (C-CPA) test was performed. EA stimulation on the bilateral Zusanli and Sanyinjiao acu-points was used for 14 days and the EA stimulation frequency is 2 Hz. Based on iTRAQ-based proteomics analysis, we investigated the protein expression in the amygdala.

Results

EA can increase the paw withdrawal threshold in inflammatory pain induced by noxious stimulation. A total of 6319 proteins were quantified in amygdala. Of these identified proteins, 123 were identified in the pain aversion group relative to those in the saline group, and 125 significantly altered proteins were identified in the pain aversion + EA group relative to the pain aversion group. A total of 11 proteins were found to be differentially expressed in the amygdala of pain aversion and EA-treated rats. The expression of three proteins, glyceraldehyde-3-phosphate dehydrogenase, glutamate transporter-1, and p21-activated kinase 6, were confirmed to be consistent with the results of the proteome.

Conclusion

Our investigation demonstrated the possible mechanism of central nerve system by which EA intervetion on pain aversion.

Glutamate Transporters in Hippocampal LTD/LTP: Not Just Prevention of Excitotoxicity

Glutamate uptake is a process mediated by sodium-dependent glutamate transporters, preventing glutamate spillover from the synapse. Typically, astrocytes express higher amounts of glutamate transporters, thus being responsible for most of the glutamate uptake; nevertheless, neurons can also express these transporters, albeit in smaller concentrations. When not regulated, glutamate uptake can lead to neuronal death. Indeed, the majority of the studies regarding glutamate transporters have focused on excitotoxicity and the subsequent neuronal loss. However, later studies have found that glutamate uptake is not a static process, evincing a possible correlation between this phenomenon and the efficiency of synaptic transmission and plasticity. In this review, we will focus on the role of the increase in glutamate uptake that occurs during long-term potentiation (LTP) in the hippocampus, as well as on the impairment of long-term depression (LTD) under the same conditions. The mechanism underpinning the modulatory effect of glutamate transporters over synaptic plasticity still remains unascertained; yet, it appears to have a more prominent effect over the N-methyl-D-aspartate receptor (NMDAR), despite changes in other glutamate receptors may also occur.

OCD candidate gene SLC1A1/EAAT3 impacts basal ganglia-mediated activity and stereotypic behavior

Obsessive-compulsive disorder (OCD) is a chronic, disabling condition with inadequate treatment options that leave most patients with substantial residual symptoms. Structural, neurochemical, and behavioral findings point to a significant role for basal ganglia circuits and for the glutamate system in OCD. Genetic linkage and association studies in OCD point to SLC1A1, which encodes the neuronal glutamate/aspartate/cysteine transporter excitatory amino acid transporter 3 (EAAT3)/excitatory amino acid transporter 1 (EAAC1). However, no previous studies have investigated EAAT3 in basal ganglia circuits or in relation to OCD-related behavior. Here, we report a model of Slc1a1 loss based on an excisable STOP cassette that yields successful ablation of EAAT3 expression and function. Using amphetamine as a probe, we found that EAAT3 loss prevents expected increases in (i) locomotor activity, (ii) stereotypy, and (iii) immediate early gene induction in the dorsal striatum following amphetamine administration. Further, Slc1a1-STOP mice showed diminished grooming in an SKF-38393 challenge experiment, a pharmacologic model of OCD-like grooming behavior. This reduced grooming is accompanied by reduced dopamine D₁ receptor binding in the dorsal striatum of Slc1a1-STOP mice. Slc1a1-STOP mice also exhibit reduced extracellular dopamine concentrations in the dorsal striatum both at baseline and following amphetamine challenge. Viral-mediated restoration of Slc1a1/EAAT3 expression in the midbrain but not in the striatum results in partial rescue of amphetamine-induced locomotion and stereotypy in Slc1a1-STOP mice, consistent with an impact of EAAT3 loss on presynaptic dopaminergic function. Collectively, these findings indicate that the most consistently associated OCD candidate gene impacts basal ganglia-dependent repetitive behaviors.

GLT-1: The elusive presynaptic glutamate transporter

Historically, glutamate uptake in the CNS was mainly attributed to glial cells for three reasons: 1) none of the glutamate transporters were found to be located in presynaptic terminals of excitatory synapses; 2) the putative glial transporters, GLT-1 and GLAST are expressed at high levels in astrocytes; 3) studies of the constitutive GLT-1 knockout as well as pharmacological studies demonstrated that >90% of glutamate uptake into forebrain synaptosomes is mediated by the operation of GLT-1. Here we summarize the history leading up to the recognition of GLT-1a as a presynaptic glutamate transporter. A major issue now is understanding the physiological and pathophysiological significance of the expression of GLT-1 in presynaptic terminals. To elucidate the cell-type specific functions of GLT-1, a conditional knockout was generated with which to inactivate the GLT-1 gene in different cell types using Cre/lox technology. Astrocytic knockout led to an 80% reduction of GLT-1 expression, resulting in intractable seizures and early mortality as seen also in the constitutive knockout. Neuronal knockout was associated with no obvious phenotype. Surprisingly, synaptosomal uptake capacity (Vmax) was found to be significantly reduced, by 40%, in the neuronal knockout, indicating that the contribution of neuronal GLT-1 to synaptosomal uptake is disproportionate to its protein expression (5-10%). Conversely, the contribution of astrocytic GLT-1 to synaptosomal uptake was much lower than expected. In contrast, the loss of uptake into liposomes prepared from brain protein from astrocyte and neuronal knockouts was proportionate with the loss of GLT-1 protein, suggesting that a large portion of GLT-1 in astrocytic membranes in synaptosomal preparations is not functional, possibly because of a failure to reseal. These results suggest the need to reinterpret many previous studies using synaptosomal uptake to investigate glutamate transport itself as well as changes in glutamate homeostasis associated with normal functions, neurodegeneration, and response to drugs.

Activity dependent internalization of the glutamate transporter GLT-1 mediated by β-arrestin 1 and ubiquitination

GLT-1 is the main glutamate transporter in the brain and undergoes trafficking processes that control its concentration on the cell surface thereby shaping glutamatergic neurotransmission. We have investigated how the traffic of GLT-1 is regulated by transporter activity. We report that internalization of GLT-1 from the cell surface is accelerated by transportable substrates like glutamate or aspartate, as well as by the transportable inhibitor L-trans-2,4-PDC, but not by the non-substrate inhibitor WAY 213613 in primary mixed cultures and in transiently transfected HEK293 cells. Analysis of the mechanism of endocytosis in HEK293 cells revealed that glutamate promoted the association with the transporter of the adaptor protein β-arrestin and the ubiquitin ligase Nedd4-2. The addition of glutamate is accompanied by an increase in the transporter ubiquitination, and the internalization is suppressed by an ubiquitination inhibitor (PYR41), and in a mutant defective in C-terminal lysines. The glutamate triggered endocytosis was also suppressed by siRNA for β-arrestin. This regulatory mechanism might be relevant in controlling the amount of transporter on the cell surface in conditions such as ischemia or traumatic brain injury, where extracellular concentrations of glutamate are persistently elevated.

Transcriptional Regulation of Glutamate Transporters: From Extracellular Signals to Transcription Factors

Glutamate is the predominant excitatory neurotransmitter in the mammalian CNS. It mediates essentially all rapid excitatory signaling. Dysfunction of glutamatergic signaling contributes to developmental, neurologic, and psychiatric diseases. Extracellular glutamate is cleared by a family of five Na(+)-dependent glutamate transporters. Two of these transporters (GLAST and GLT-1) are relatively selectively expressed in astrocytes. Other of these transporters (EAAC1) is expressed by neurons throughout the nervous system. Expression of the last two members of this family (EAAT4 and EAAT5) is almost exclusively restricted to specific populations of neurons in cerebellum and retina, respectively. In this review, we will discuss our current understanding of the mechanisms that control transcriptional regulation of the different members of this family. Over the last two decades, our understanding of the mechanisms that regulate expression of GLT-1 and GLAST has advanced considerably; several specific transcription factors, cis-elements, and epigenetic mechanisms have been identified. For the other members of the family, little or nothing is known about the mechanisms that control their transcription. It is assumed that by defining the mechanisms involved, we will advance our understanding of the events that result in cell-specific expression of these transporters and perhaps begin to define the mechanisms by which neurologic diseases are changing the biology of the cells that express these transporters. This approach might provide a pathway for developing new therapies for a wide range of essentially untreatable and devastating diseases that kill neurons by an excitotoxic mechanism.

Astroglial glutamate transporters coordinate excitatory signaling and brain energetics

In the mammalian brain, a family of sodium-dependent transporters maintains low extracellular glutamate and shapes excitatory signaling. The bulk of this activity is mediated by the astroglial glutamate transporters GLT-1 and GLAST (also called EAAT2 and EAAT1). In this review, we will discuss evidence that these transporters co-localize with, form physical (co-immunoprecipitable) interactions with, and functionally couple to various 'energy-generating' systems, including the Na(+)/K(+)-ATPase, the Na(+)/Ca(2+) exchanger, glycogen metabolizing enzymes, glycolytic enzymes, and mitochondria/mitochondrial proteins. This functional coupling is bi-directional with many of these systems both being regulated by glutamate transport and providing the 'fuel' to support glutamate uptake. Given the importance of glutamate uptake to maintaining synaptic signaling and preventing excitotoxicity, it should not be surprising that some of these systems appear to 'redundantly' support the energetic costs of glutamate uptake. Although the glutamate-glutamine cycle contributes to recycling of neurotransmitter pools of glutamate, this is an over-simplification. The ramifications of co-compartmentalization of glutamate transporters with mitochondria for glutamate metabolism are discussed. Energy consumption in the brain accounts for ∼20% of the basal metabolic rate and relies almost exclusively on glucose for the production of ATP. However, the brain does not possess substantial reserves of glucose or other fuels. To ensure adequate energetic supply, increases in neuronal activity are matched by increases in cerebral blood flow via a process known as 'neurovascular coupling'. While the mechanisms for this coupling are not completely resolved, it is generally agreed that astrocytes, with processes that extend to synapses and endfeet that surround blood vessels, mediate at least some of the signal that causes vasodilation. Several studies have shown that either genetic deletion or pharmacologic inhibition of glutamate transport impairs neurovascular coupling. Together these studies strongly suggest that glutamate transport not only coordinates excitatory signaling, but also plays a pivotal role in regulating brain energetics.

Modulation of glutamate transport and receptor binding by glutamate receptor antagonists in EAE rat brain

The etiology of multiple sclerosis (MS) is currently unknown. However, one potential mechanism involved in the disease may be excitotoxicity. The elevation of glutamate in cerebrospinal fluid, as well as changes in the expression of glutamate receptors (iGluRs and mGluRs) and excitatory amino acid transporters (EAATs), have been observed in the brains of MS patients and animals subjected to experimental autoimmune encephalomyelitis (EAE), which is the predominant animal model used to investigate the pathophysiology of MS. In the present paper, the effects of glutamatergic receptor antagonists, including amantadine, memantine, LY 367583, and MPEP, on glutamate transport, the expression of mRNA of glutamate transporters (EAATs), the kinetic parameters of ligand binding to N-methyl-D-aspartate (NMDA) receptors, and the morphology of nerve endings in EAE rat brains were investigated. The extracellular level of glutamate in the brain is primarily regulated by astrocytic glutamate transporter 1 (GLT-1) and glutamate-aspartate transporter (GLAST). Excess glutamate is taken up from the synaptic space and metabolized by astrocytes. Thus, the extracellular level of glutamate decreases, which protects neurons from excitotoxicity. Our investigations showed changes in the expression of EAAT mRNA, glutamate transport (uptake and release) by synaptosomal and glial plasmalemmal vesicle fractions, and ligand binding to NMDA receptors; these effects were partially reversed after the treatment of EAE rats with the NMDA antagonists amantadine and memantine. The antagonists of group I metabotropic glutamate receptors (mGluRs), including LY 367385 and MPEP, did not exert any effect on the examined parameters. These results suggest that disturbances in these mechanisms may play a role in the processes associated with glutamate excitotoxicity and the progressive brain damage in EAE.

Widespread microRNA dysregulation in multiple system atrophy - disease-related alteration in miR-96

MicroRNA (miRNA) are short sequences of RNA that function as post-transcriptional regulators by binding to target mRNA transcripts resulting in translational repression. A number of recent studies have identified miRNA as being involved in neurodegenerative disorders including Alzheimer's disease, Parkinson's disease and Huntington's disease. However, the role of miRNA in multiple system atrophy (MSA), a progressive neurodegenerative disorder characterized by oligodendroglial accumulation of alpha-synuclein remains unexamined. In this context, this study examined miRNA profiles in MSA cases compared with controls and in transgenic (tg) models of MSA compared with non-tg mice. The results demonstrate a widespread dysregulation of miRNA in MSA cases, which is recapitulated in the murine models. The study employed a cross-disease, cross-species approach to identify miRNA that were either specifically dysregulated in MSA or were commonly dysregulated in neurodegenerative conditions such as Alzheimer's disease, dementia with Lewy bodies, progressive supranuclear palsy and corticobasal degeneration or the tg mouse model equivalents of these disorders. Using this approach we identified a number of miRNA that were commonly dysregulated between disorders and those that were disease-specific. Moreover, we identified miR-96 as being up-regulated in MSA. Consistent with the up-regulation of miR-96, mRNA and protein levels of members of the solute carrier protein family SLC1A1 and SLC6A6, miR-96 target genes, were down-regulated in MSA cases and a tg model of MSA. These results suggest that miR-96 dysregulation may play a role in MSA and its target genes may be involved in the pathogenesis of MSA.

Excitatory amino acid transporters: roles in glutamatergic neurotransmission

Excitatory amino acid transporters or EAATs are the major transport mechanism for extracellular glutamate in the nervous system. This family of five carriers not only displays an impressive ability to regulate ambient extracellular glu concentrations but also regulate the temporal and spatial profile of glu after vesicular release. This dynamic form of regulation mediates several characteristic of synaptic, perisynaptic, and spillover activation of ionotropic and metabotropic receptors. EAATs function through a secondary active, electrogenic process but also possess a thermodynamically uncoupled ligand gated anion channel activity, both of which have been demonstrated to play a role in regulation of cellular activity. This review will highlight the inception of EAATs as a focus of research, the transport and channel functionality of the carriers, and then describe how these properties are used to regulate glutamatergic neurotransmission.

Functional studies and rare variant screening of SLC1A1/EAAC1 in males with obsessive-compulsive disorder

Several studies have found that the neuronal glutamate transporter gene SLC1A1/EAAC1 is associated with obsessive-compulsive disorder (OCD), with a stronger association in males. Previous studies have primarily focused on common single-nucleotide polymorphisms, rather than rare functional variants that are likely to have larger effects. We screened 184 males with OCD for rare variation in SLC1A1 exons; however, no new coding variation was found. When combined with previous screens, only one SLC1A1 amino acid variant has been detected among the 841 individuals screened, which is less than for other neurotransmitter transporter genes (P=0.0001). We characterized the function of the one SLC1A1 missense variant reported previously in OCD, Thr164Ala, and found that the Ala164 allele leads to decreased Vmax and Km (P<0.0001) in transfected human embryonic kidney cells. Further work will be necessary to understand the impact of this rare SLC1A1/EAAC1 Ala164 variant on neuronal function and circuitry relevant to OCD.

Glutamate transporter expression and function in a striatal neuronal model of Huntington's disease

Excitotoxicity may contribute to the pathogenesis of Huntington's disease. High affinity Na+ dependent glutamate transporters, residing in the plasma membrane, clear glutamate from the extracellular space and are the primary means of protection against excitotoxicity. Many reports suggest that Huntington's disease is associated with a decrease in the expression and function of glutamate transporters. We studied the expression and function of these transporters in a cellular model of Huntington's disease, STHdh(Q111/Q111) and STHdh(Q7/Q7) cells. We found that only GLT-1b and EAAC1 were expressed in these cell lines and only EAAC1 significantly contributed to the glutamate uptake. Surprisingly, there was an increase in Na+-dependent glutamate uptake in STHdh(Q111/Q111) cells accompanied by an increase in surface expression of EAAC1. We studied the influence of the Akt pathway on EAAC1 mediated uptake, since EAAC1 surface expression is influenced by Akt and previous studies have shown increased Akt expression in STHdh(Q111/Q111) cells. Glutamate uptake was inhibited by Akt pathway inhibitors in both the STHdh(Q7/Q7) and the STHdh(Q111/Q111) cell lines. We found no difference in Akt activation between the two cell lines under our conditions of culture. Therefore a difference in Akt activation does not seem to explain the increase in EAAC1 mediated uptake in the STHdh(Q111/Q111) cells.

Metabotropic glutamate 1 receptor: current concepts and perspectives

Almost 25 years after the first report that glutamate can activate receptors coupled to heterotrimeric G-proteins, tremendous progress has been made in the field of metabotropic glutamate receptors. Now, eight members of this family of glutamate receptors, encoded by eight different genes that share distinctive structural features have been identified. The first cloned receptor, the metabotropic glutamate (mGlu) receptor mGlu1 has probably been the most extensively studied mGlu receptor, and in many respects it represents a prototypical subtype for this family of receptors. Its biochemical, anatomical, physiological, and pharmacological characteristics have been intensely investigated. Together with subtype 5, mGlu1 receptors constitute a subgroup of receptors that couple to phospholipase C and mobilize Ca(2+) from intracellular stores. Several alternatively spliced variants of mGlu1 receptors, which differ primarily in the length of their C-terminal domain and anatomical localization, have been reported. Use of a number of genetic approaches and the recent development of selective antagonists have provided a means for clarifying the role played by this receptor in a number of neuronal systems. In this article we discuss recent advancements in the pharmacology and concepts about the intracellular transduction and pathophysiological role of mGlu1 receptors and review earlier data in view of these novel findings. The impact that this new and better understanding of the specific role of these receptors may have on novel treatment strategies for a variety of neurological and psychiatric disorders is considered.

Influence of environmental enrichment on steady-state mRNA levels for EAAC1, AMPA1 and NMDA2A receptor subunits in rat hippocampus

Interaction with the environment has a key role in refining the neuronal circuitry required for normal brain function throughout life. Profound effects of enriched environment have been shown on neuronal structure and chemistry in experimental animals. Epidemiological studies imply that this is true also in man, thus cognitive stimulation has a protective effect on neurodegeneration, e.g., in Alzheimer's disease. Glutamatergic pathways are imperative for cognitive functions, such as memory, learning and long-term potentiation, and relies on the AMPA and NMDA glutamate receptors and the hippocampus, with its specific subregions, is an important anatomical substrate in this. The glutamate signalling is also dependent on a fine-tuned transport system, in the hippocampus primarily achieved by the glutamate transporter EAAC1. In this study we show how environmental enrichment modulates these parts of the glutamatergic system using quantitative in situ hybridisation. This work demonstrates for the first time that environmental enrichment modulates the mRNA expression of EAAC1 which is significantly and region specifically decreased in the hippocampus. We also provide evidence for regional and hemisphere-specific upregulation of NMDA mRNA in the hippocampus after environmental enrichment. The current work also shows that AMPA mRNA of the hippocampus is not per se changed by environmental enrichment in adult animals. Taken together, our results extend the knowledge of the glutamatergic system of specific regions of the hippocampus and its modulation by environmental enrichment and could contribute to the development of strategies aimed at limiting pathological changes associated with glutamatergic dysfunctions.

Focal reduction of neuronal glutamate transporters in human neocortical epilepsy

PURPOSE

To study the differential expression of excitatory amino acid transporters (EAATs) at localized epileptic foci compared to nonepileptic regions in human neocortical epilepsy. Decreased expression of EAATs, the predominant mechanism to remove synaptic-released glutamate, may explain mechanisms of heightened excitability at these epileptic foci.

METHODS

The differential expression of EAAT1-4 at the mRNA and protein levels was measured in electrically mapped human neocortical tissues using quantitative real-time PCR and immunoblotting. This required a novel way to prevent aggregation of EAAT proteins through cold solubilization. Layer-specific neuronal densities were measured to control for potential differences in neuronal density.

RESULTS

While focal epileptic brain regions show marked increases in immediate early genes, they have significant reductions in the neuronal glutamate transporter mRNAs (EAAT3 and EAAT4). These changes were not associated with changes in relative neuronal density, suggesting a reduction in EAAT mRNA per neuron. Immunohistochemical staining of epileptic human neocortex confirmed the presence of EAAT1 and EAAT2 proteins in astroglial cells and EAAT3 and EAAT4 proteins in human cortical neurons. At the protein level, western blots of the same epileptic and nonepileptic regions for a subset of these patients showed a similar decrease of EAAT3 and EAAT4. Despite no change in EAAT2 mRNA, EAAT2 protein expression was significantly reduced at epileptic foci.

CONCLUSION

Regional reductions in EAAT expression at human neocortical epileptic foci could produce increased local glutamate levels that in turn may contribute to both hyperexcitability and the spontaneous generation of epileptic discharges that characterize human epileptic foci.

Origins of blood volume change due to glutamatergic synaptic activity at astrocytes abutting on arteriolar smooth muscle cells

The cellular mechanisms that couple activity of glutamatergic synapses with changes in blood flow, measured by a variety of techniques including the BOLD signal, have not previously been modelled. Here we provide such a model, that successfully accounts for the main observed changes in blood flow in both visual cortex and somatosensory cortex following their stimulation by high-contrast drifting grating or by single whisker stimulation, respectively. Coupling from glutamatergic synapses to smooth muscle cells of arterioles is effected by astrocytes releasing epoxyeicosatrienoic acids (EETs) onto them, following glutamate stimulation of the astrocyte. Coupling of EETs to the smooth muscle of arterioles is by means of potassium channels in their membranes, leading to hyperpolarization, relaxation and hence an increase in blood flow. This model predicts a linear increase in blood flow with increasing numbers of activated astrocytes, but a non-linear increase with increasing glutamate release.

Modulation of excitation by metabotropic glutamate receptors

Metabotropic glutamate receptors, in contrast to ionotropic glutamate receptors, do not form ion channels but instead affect intracellular chemical messenger systems. They couple via GTP-binding proteins ("G-proteins") to a variety of effectors such as ion channels and thus give glutamate, the major excitatory transmitter in the CNS, the ability to modulate processes involved in excitatory synaptic transmission. Therefore, excitatory synaptic transmission is regulated not only by the conventional GABAergic but also by the glutamatergic mechanisms themselves. Many metabotropic glutamate receptors are localized outside the immediate vicinity of transmitter release sites, thereby setting specific requirements for their activation, such as cooperation between synapses, burst activity, and glial involvement in the regulation of ambient glutamate levels.

Transporters for L-glutamate: an update on their molecular pharmacology and pathological involvement

L-Glutamate (Glu) is the major excitatory neurotransmitter in the mammalian CNS and five types of high-affinity Glu transporters (EAAT1-5) have been identified. The transporters EAAT1 and EAAT2 in glial cells are responsible for the majority of Glu uptake while neuronal EAATs appear to have specialized roles at particular types of synapses. Dysfunction of EAATs is specifically implicated in the pathology of neurodegenerative conditions such as amyotrophic lateral sclerosis, epilepsy, Huntington's disease, Alzheimer's disease and ischemic stroke injury, and thus treatments that can modulate EAAT function may prove beneficial in these conditions. Recent advances have been made in our understanding of the regulation of EAATs, including their trafficking, splicing and post-translational modification. This article summarises some recent developments that improve our understanding of the roles and regulation of EAATs.

Acute regulation of sodium-dependent glutamate transporters: a focus on constitutive and regulated trafficking

The acidic amino acid glutamate activates a family of ligand-gated ion channels to mediate depolarization that can be as short-lived as a few milliseconds and activates a family of G protein-coupled receptors that couple to both ion channels and other second messenger pathways. Glutamate is the predominant excitatory neurotransmitter in the mammalian central nervous system and is required for essentially all motor, sensory, and cognitive functions. In addition, glutamate-mediated signaling is required for development and the synaptic plasticity thought to underlie memory formation and retrieval. The levels of glutamate in brain approach 10 mmol/kg and most cells in the CNS express at least one of the receptor subtypes. Unlike acetylcholine that mediates "rapid" excitatory neurotransmission at the neuromuscular junction, there is no evidence for extracellular inactivation of glutamate. Instead, glutamate is cleared by a family of Na(+)-dependent transport systems that are found on glial processes that sheath the synapse and found on the pre- and postsynaptic elements of neurons. These transporters ensure crisp excitatory transmission by maintaining synaptic concentrations below those required for tonic activation of glutamate receptors under baseline conditions (approximately 1 microM) and serve to limit activation of glutamate receptors after release. During the past few years, it has become clear that like many of the other neurotransmitter transporters discussed in this volume of Handbook of Experimental Pharmacology, the activity of these transporters can be rapidly regulated by a variety of effectors. In this chapter, a broad overview of excitatory signaling will be followed by a brief introduction to the family of Na(+)-dependent glutamate transporters and a detailed discussion of our current understanding of the mechanisms that control transporter activity. The focus will be on our current understanding of the mechanisms that could regulate transporter activity within minutes, implying that this regulation is independent of transcriptional or translational control mechanisms. The glutamate transporters found in forebrain are regulated by redistributing the proteins to or from the plasma membrane; the signals involved and the net effects on transporter activity are being defined. In addition, there is evidence to suggest that the intrinsic activity of these transporters is also regulated by mechanisms that are independent of transporter redistribution; less is known about these events. As this field progresses, it should be possible to determine how this regulation affects physiologic and pathologic events in the CNS.

Suppression of neuronal network excitability and seizure-like events by 2-methyl-4-oxo-3H-quinazoline-3-acetyl piperidine in juvenile rat hippocampus: involvement of a metabotropic glutamate receptor

We present data on the antiepileptic potency of 2-methyl-4-oxo-3H-quinazoline-3-acetyl piperidine (Q5) in juvenile (P9-13) rat hippocampal slices and in particular Q5's action mechanism and target. Q5 (200-500 microM), but not alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/Kainate receptor antagonists blocked low-[Mg2+]-induced seizure-like events (SLE) in the CA3 region. Q5 (100 microM) decreased Glu-induced [35S]guanosine 5'-O-(3-thiotriphosphate) binding enhancement in brain homogenates, without interaction with ionotropic Glu receptor sites and Glu transport. In voltage-clamped CA3 pyramidal cells, Q5 (500 microM) depressed activities of spontaneous excitatory and inhibitory postsynaptic currents without affecting miniature inhibitory currents. Metabotropic Glu receptor (mGluR) subtype antagonists affected network excitability dissimilarly. Intracellular Ca2+ ion transients induced by the mGluR agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) were suppressed by Q5. Agreeing predictions obtained by modelling Q5 binding to different experimental conformations of mGlu1, Q5 was bound partially to an mGluR binding site in the presence of 1mM ACPD. Findings suggest the apparent involvement of a novel phenotype of action or a new mGluR subtype in the specific suppression of epileptiform activity by Q5 through the depression of network excitability.

PKC-dependent stimulation of EAAT3 glutamate transporter does not require the integrity of actin cytoskeleton

The activity and the membrane expression of EAAT3 glutamate transporter are stimulated upon PKC activation by phorbol esters in C6 rat glioma cells. To investigate the role of cytoskeleton in these effects, we have employed actin-perturbing toxins and found that the perturbation of actin cytoskeleton inhibits basal but not phorbol-stimulated EAAT3 activity and membrane trafficking. In the absence of phorbols, latrunculin A, a toxin that disassembles actin cytoskeleton, produced a rapid inhibition of EAAT3 activity, due to a decrease in transport V(max). The inhibitory effect was fully reversible and was not detected for other sodium dependent transport systems for amino acids. However, latrunculin did not prevent the increase in transport caused by phorbol esters and, moreover, cells pre-treated with phorbols were resistant to the inhibitory effect of the toxin on EAAT3 activity. Biotinylation experiments indicated that the inhibitory effect of latrunculin was attributable to a decreased expression of the carrier on the membrane, while the toxin did not suppress the PKC-dependent increase in EAAT3 membrane abundance. Latrunculin A effects on EAAT3 were shared by cytochalasin D, a toxin that disorganizes actin filaments with a distinct mechanism of action. On the contrary, a small, but significant, increase of EAAT3 activity was observed upon incubation with jasplakinolide, a drug that stabilizes actin microfilaments. Also jasplakinolide, however, did not hinder phorbol-dependent stimulation of aspartate transport. Colchicine, a toxin that disrupts microtubules, also lowered EAAT3 activity without preventing transport stimulation by phorbols, while microtubule stabilization by paclitaxel led to an increase in aspartate transport. It is concluded that, in C6 cells, the PKC-mediated stimulatory effects on EAAT3 are cytoskeleton-independent, while in the absence of phorbols, the transporter is partially inhibited by the disorganization of either actin microfilaments or microtubules. These results suggest that EAAT3 trafficking in C6 cells involves different pools of transporters.

Expression of GFP-tagged neuronal glutamate transporters in cerebellar Purkinje neurons

Of the five excitatory amino acid transporters (EAATs) identified, two genes are expressed by neurons (EAAT3 and EAAT4) and give rise to transporters confined to neuronal cell bodies and dendrites. At an ultrastructural level, EAAT3 and EAAT4 proteins are clustered at the edges of postsynaptic densities of excitatory synapses. This pattern of localization suggests that postsynaptic EAATs may help to limit spillover of glutamate from excitatory synapses. In an effort to study transporter localization in living neurons and ultimately to manipulate uptake at intact synapses, we have developed viral reagents encoding neuronal EAATs tagged with GFP. We demonstrate that these fusion proteins are capable of Na(+)-dependent glutamate uptake, that they generate ionic conductances indistinguishable from their wild-type counterparts, and that GFP does not alter their glutamate dose-dependence. Two-photon microscopy was used to examine fusion protein expression in Purkinje neurons in acute cerebellar slices. Both EAAT3-GFP and EAAT4-GFP were observed at high levels in the dendritic spines of transfected Purkinje neurons. These findings indicate that functional EAAT fusion proteins can be synthesized and appropriately trafficked to postsynaptic compartments. Furthermore, they validate a powerful system for looking at EAAT function in situ.

Effects of glutamate transporter and receptor ligands on neuronal glutamate uptake

The excitatory amino acids (EAAs) transporters regulate the balance between physiological and pathological signaling over stimulation of the glutamatergic system pathway. The effect of transportable substrates and glutamate (Glu) receptor agonists on Glu uptake in neuronal cells was assessed at different conditions. Cells pre-incubated with Glu, L- or D-aspartate (Asp) and washed presented an inhibition on [(3)H]-Glu uptake and this effect was not mimicked by Glu receptors agonists. The effects of L- and D-Asp were not altered by the presence of N-methyl-d-aspartate (NMDA) receptor antagonists. Thus, the reduction on Glu uptake induced by EAAs is probably linked to the transporter activity. In contrast, the presence of NMDA or (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (SR-ACPD) during the pre-incubation and the [(3)H]-Glu uptake assay period increased Glu uptake, whilst kainic acid (KA) had no effect. The NMDA effect was not altered by its antagonists (+/-)-2-amino-5-phosphonopentanoic acid (AP-5) or dizocilpine (MK-801). The SR-ACPD effect was due to the activation of metabotropic Glu receptor, since it was abolished by its antagonist, L(+/-)-2-amino-3-phosphonopropionic acid (L-AP3). Thus, the current studies suggest that the neuronal EAAs transporter is regulated in different manner by transportable substrates and Glu receptor agonists. The possible involvement of this modulation after certain neurotoxicity insults is discussed.

Early and delayed glutamate effects in rat primary cortical neurons

Glutamate-induced changes in the subcellular distribution of protein kinase C isoforms and in the intracellular calcium concentration were investigated in rat primary cortical neurons. Western blot analysis of protein kinase C isoforms (alpha, beta1, beta2, gamma, delta, epsilon, zeta and theta), performed 30 min after a 10 min treatment with 30 microM glutamate, revealed a decrease in the total beta1 (-24%) and beta2 (-40%) isoform levels, without any significant change in any of the other isozymes. All conventional isoforms translocated to the membrane compartment, while delta, epsilon, zeta and theta; maintained their initial subcellular distribution. Twenty-four hours after glutamate treatment, the total protein kinase C labelling had increased, particularly the epsilon isoform, which accounted for 34% of the total densitometric signal. At this time, protein kinase C beta1, delta, epsilon and zeta isoforms were mainly detected in the membrane compartment, while gamma and theta; signals were displayed almost solely in the cytosol. Basal intracellular calcium concentration (FURA 2 assay) was concentration-dependently increased (maximum effect +77%) 30 min, but not 24h after a 10 min glutamate (10-100 microM) treatment, while the net increase induced by electrical stimulation (10 Hz, 10s) was consistently reduced (maximum effect -64%). The N-methyl-d-aspartate receptor antagonist, MK-801, 1 microM, prevented glutamate action both 30 min and 24 h after treatment, while non-selective protein kinase C inhibitors, ineffective at 30 min, potentiated it at 24 h. These findings show that protein kinase C isoforms are differently activated and involved in the early and delayed glutamate actions, and that the prevailing effect of their activation is neuroprotective.