Adipose Tissue-Derived Mesenchymal Stem Cell Concentrated Conditioned Medium Alters the Expression Pattern of Glutamate Regulatory Proteins and Aquaporin-4 in the Retina after Mild Traumatic Brain Injury.

Concentrated conditioned media from adipose tissue-derived mesenchymal stem cells (ASC-CCM) show promise for retinal degenerative diseases. In this study, we hypothesized that ASC-CCM could rescue retinal damage and thereby improve visual function by acting through Müller glia in mild traumatic brain injury (mTBI). Adult C57Bl/6 mice were subjected to a 50-psi air pulse on the left side of the head, resulting in an mTBI. After blast injury, 1 µL (∼100 ng total protein) of human ASC-CCM was delivered intravitreally and followed up after 4 weeks for visual function assessed by electroretinogram and histopathological markers for Müller cell-related markers. Blast mice that received ASC-CCM, compared with blast mice that received saline, demonstrated a significant improvement in a- and b-wave response correlated with a 1.3-fold decrease in extracellular glutamate levels and a concomitant increase in glutamine synthetase (GS), as well as the glutamate transporter (GLAST) in Müller cells. Additionally, an increase in aquaporin-4 (AQP4) in Müller cells in blast mice received saline restored to normal levels in blast mice that received ASC-CCM. In vitro studies on rMC-1 Müller glia exposed to 100 ng/mL glutamate or RNA interference knockdown of GLAST expression mimicked the increased Müller cell glial fibrillary acidic protein (a marker of gliosis) seen with mTBI, and suggested that an increase in glutamate and/or a decrease in GLAST might contribute to the Müller cell activation in vivo. Taken together, our data suggest a novel neuroprotective role for ASC-CCM in the rescue of the visual deficits and pathologies of mTBI via restoration of Müller cell health.

Functional glutamate transporters are expressed in the carotid chemoreceptor.

BACKGROUND: The carotid body (CB) plays a critical role in cyclic intermittent hypoxia (CIH)-induced chemosensitivity; however, the underlying mechanism remains uncertain. We have demonstrated the presence of multiple inotropic glutamate receptors (iGluRs) in CB, and that CIH exposure alters the level of some iGluRs in CB. This result implicates glutamatergic signaling in the CB response to hypoxia. The glutamatergic neurotransmission is not only dependent on glutamate and glutamate receptors, but is also dependent on glutamate transporters, including vesicular glutamate transporters (VGluTs) and excitatory amino acid transporters (EAATs). Here, we have further assessed the expression and distribution of VGluTs and EAATs in human and rat CB and the effect of CIH exposure on glutamate transporters expression. METHODS: The mRNA of VGluTs and EAATs in the human CB were detected by RT-PCR. The protein expression of VGluTs and EAATs in the human and rat CB were detected by Western blot. The distribution of VGluT3, EAAT2 and EAAT3 were observed by immunohistochemistry staining and immunofluorescence staining. Male Sprague-Dawley (SD) rats were exposed to CIH (FIO2 10-21%, 3 min/3 min for 8 h per day) for 2 weeks. The unpaired Student's t-test was performed. RESULTS: Here, we report on the presence of mRNAs for VGluT1-3 and EAAT1-3 in human CB, which is consistent with our previous results in rat CB. The proteins of VGluT1 and 3, EAAT2 and 3, but not VGluT2 and EAAT1, were detected with diverse levels in human and rat CB. Immunostaining showed that VGluT3, the major type of VGluTs in CB, was co-localized with tyrosine hydroxylase (TH) in type I cells. EAAT2 and EAAT3 were distributed not only in type I cells, but also in glial fibrillary acidic protein (GFAP) positive type II cells. Moreover, we found that exposure of SD rats to CIH enhanced the protein level of EAAT3 as well as TH, but attenuated the levels of VGluT3 and EAAT2 in CB. CONCLUSIONS: Our study suggests that glutamate transporters are expressed in the CB, and that glutamate transporters may contribute to glutamatergic signaling-dependent carotid chemoreflex to CIH.

Glutamate transporters: Gene expression regulation and signaling properties.

Glutamate is the major excitatory neurotransmitter in the vertebrate central nervous system. During synaptic activity, glutamate is released and binds to specific membrane receptors and transporters activating, in the one hand, a wide variety of signal transduction cascades, while in the other hand, its removal from the synaptic cleft. Extracellular glutamate concentrations are maintained within physiological levels mainly by glia glutamate transporters. Inefficient clearance of this amino acid is neurotoxic due to a prolonged hyperactivation of its postsynaptic receptors, exacerbating a wide array of intracellular events linked to an ionic imbalance, that results in neuronal cell death. This process is known as excitotoxicity and is the underlying mechanisms of an important number of neurodegenerative diseases. Therefore, it is important to understand the regulation of glutamate transporters function. The transporter activity can be regulated at different levels: gene expression, transporter protein targeting and trafficking, and post-translational modifications of the transporter protein. The identification of these mechanisms has paved the way to our current understanding the role of glutamate transporters in brain physiology and will certainly provide the needed biochemical information for the development of therapeutic strategies towards the establishment of novel therapeutic approaches for the treatment and/or prevention of pathologies associated with excitotoxicity insults. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

A toll-like receptor 9 antagonist restores below-level glial glutamate transporter expression in the dorsal horn following spinal cord injury.

Spinal cord (SC) trauma elicits pathological changes at the primary lesion and in regions distant from the injury epicenter. Therapeutic agents that target mechanisms at the injury site are likely to exert additional effects in these remote regions. We previously reported that a toll-like receptor 9 (TLR9) antagonist, oligodeoxynucleotide 2088 (ODN 2088), improves functional deficits and modulates the milieu at the epicenter in mice sustaining a mid-thoracic contusion. The present investigations use the same paradigm to assess ODN 2088-elicited alterations in the lumbar dorsal horn (LDH), a region remote from the injury site where SCI-induced molecular alterations have been well defined. We report that ODN 2088 counteracts the SCI-elicited decrease in glial glutamate aspartate transporter (GLAST) and glutamate transporter 1 (GLT1) levels, whereas the levels of the neuronal glutamate transporter excitatory amino acid carrier 1 (EAAC1) and astroglial GABA transporter 3 (GAT3) were unaffected. The restoration of GLAST and GLT1 was neither paralleled by a global effect on astrocyte and microglia activation nor by changes in the expression of cytokines and growth factors reported to regulate these transporters. We conclude that the effects of intrathecal ODN 2088 treatment extend to loci beyond the epicenter by selectively targeting glial glutamate transporters.

17ß-estradiol and tamoxifen protect mice from manganese-induced dopaminergic neurotoxicity.

Chronic exposure to manganese (Mn) causes neurotoxicity, referred to as manganism, with common clinical features of parkinsonism. 17ß-estradiol (E2) and tamoxifen (TX), a selective estrogen receptor modulator (SERM), afford neuroprotection in several neurological disorders, including Parkinson's disease (PD). In the present study, we tested if E2 and TX attenuate Mn-induced neurotoxicity in mice, assessing motor deficit and dopaminergic neurodegeneration. We implanted E2 and TX pellets in the back of the neck of ovariectomized C57BL/6 mice two weeks prior to a single injection of Mn into the striatum. One week later, we assessed locomotor activity and molecular mechanisms by immunohistochemistry, real-time quantitative PCR, western blot and enzymatic biochemical analyses. The results showed that both E2 and TX attenuated Mn-induced motor deficits and reversed the Mn-induced loss of dopaminergic neurons in the substantia nigra. At the molecular level, E2 and TX reversed the Mn-induced decrease of (1) glutamate aspartate transporter (GLAST) and glutamate transporter 1 (GLT-1) mRNA and protein levels; (2) transforming growth factor-α (TGF-α) and estrogen receptor-α (ER-α) protein levels; and (3) catalase (CAT) activity and glutathione (GSH) levels, and Mn-increased (1) malondialdehyde (MDA) levels and (2) the Bax/Bcl-2 ratio. These results indicate that E2 and TX afford protection against Mn-induced neurotoxicity by reversing Mn-reduced GLT1/GLAST as well as Mn-induced oxidative stress. Our findings may offer estrogenic agents as potential candidates for the development of therapeutics to treat Mn-induced neurotoxicity.

Therapeutic Effect of Astroglia-like Mesenchymal Stem Cells Expressing Glutamate Transporter in a Genetic Rat Model of Depression.

Recent studies have proposed that abnormal glutamatergic neurotransmission and glial pathology play an important role in the etiology and manifestation of depression. It was postulated that restoration of normal glutamatergic transmission, by enhancing glutamate uptake, may have a beneficial effect on depression. We examined this hypothesis using unique human glial-like mesenchymal stem cells (MSCs), which in addition to inherent properties of migration to regions of injury and secretion of neurotrophic factors, were differentiated to express high levels of functional glutamate transporters (excitatory amino acid transporters; EAAT). Additionally, gold nanoparticles (GNPs), which serve as contrast agents for CT imaging, were loaded into the cells for non-invasive, real-time imaging and tracking of MSC migration and final location within the brain. MSC-EAAT (2×105; 10 µl) were administered (i.c.v.) to Flinder Sensitive Line rats (FSLs), a genetic model for depression, and longitudinal behavioral and molecular changes were monitored. FSL rats treated with MSC-EAAT showed attenuated depressive-like behaviors (measured by the forced swim test, novelty exploration test and sucrose self-administration paradigm), as compared to controls. CT imaging, Flame Atomic Absorption Spectroscopy analysis and immunohistochemistry showed that the majority of MSCs homed specifically to the dentate gyrus of the hippocampus, a region showing structural brain changes in depression, including loss of glial cells. mRNA and protein levels of EAAT1 and BDNF were significantly elevated in the hippocampus of MSC-EAAT-treated FSLs. Our findings indicate that MSC-EAATs effectively improve depressive-like manifestations, possibly in part by increasing both glutamate uptake and neurotropic factor secretion in the hippocampus.

Regulation of Glutamate Transporter Expression in Glial Cells.

Glutamate (Glu) is the major excitatory neurotransmitter in the vertebrate central nervous system. During synaptic activity, Glu is released into the synaptic cleft and binds to Glu receptors activating a wide variety of signal transduction cascades. Extracellular Glu concentrations are maintained exclusively within physiological levels mainly by glial Glu transporters. Inefficient clearance of synaptic Glu may be neurotoxic owing to prolonged hyperactivation of postsynaptic Glu receptors, causing a multitude of intracellular events in the postsynaptic neuron, which ultimately results in neuronal cell death. This phenomenon is known as excitotoxicity and is the underlying mechanisms of a number of neurodegenerative diseases. Therefore, it is important to understand the regulation of Glu transporters' function. Transporter activity can be regulated in different ways, including gene expression, transporter protein targeting and trafficking, and posttranslational modifications of the transporter protein. The identification of these mechanisms has allowed to understand the role of Glu transporters during pathology and will aid in the development of therapeutic strategies for treating or preventing pathologies associated with excitotoxicity.

Effects of chronic inhalation of electronic cigarettes containing nicotine on glial glutamate transporters and α-7 nicotinic acetylcholine receptor in female CD-1 mice.

Alteration in glutamate neurotransmission has been found to mediate the development of drug dependence, including nicotine. We and others, through using western blotting, have reported that exposure to drugs of abuse reduced the expression of glutamate transporter-1 (GLT-1) as well as cystine/glutamate antiporter (xCT), which consequently increased extracellular glutamate concentrations in the mesocorticolimbic area. However, our previous studies did not reveal any changes in glutamate/aspartate transporter (GLAST) following exposure to drugs of abuse. In the present study, for the first time, we investigated the effect of chronic exposure to electronic (e)-cigarette vapor containing nicotine, for one hour daily for six months, on GLT-1, xCT, and GLAST expression in frontal cortex (FC), striatum (STR), and hippocampus (HIP) in outbred female CD1 mice. In this study, we also investigated the expression of alpha-7 nicotinic acetylcholine receptor (α-7 nAChR), a major pre-synaptic nicotinic receptor in the glutamatergic neurons, which regulates glutamate release. We found that inhalation of e-cigarette vapor for six months increased α-7 nAChR expression in both FC and STR, but not in the HIP. In addition, chronic e-cigarette exposure reduced GLT-1 expression only in STR. Moreover, e-cigarette vapor inhalation induced downregulation of xCT in both the STR and HIP. We did not find any significant changes in GLAST expression in any brain region. Finally, using liquid chromatography-tandem mass spectrometry (LC-MS/MS) techniques, we detected high concentrations of nicotine and cotinine, a major metabolite of nicotine, in the FC tissues of e-cigarette exposed mice. These data provide novel evidence about the effects of chronic nicotine inhalation on the expression of key glial glutamate transporters as well as α-7 nAChR. Our work may suggest that nicotine exposure via chronic inhalation of e-cigarette vapor may be mediated in part by alterations in the glutamatergic system.

Increased gene expression of selected vesicular and glial glutamate transporters in the frontal cortex in rats exposed to voluntary wheel running.

Though positive effects of exercise on mood and well being are well recognised, the central regulatory mechanisms are still not fully understood. The present study was aimed to testing the hypothesis that voluntary wheel running activates the gene expression of glutamate transporters in the brain cortex of rats. The animals were assigned to the control and voluntary wheel running groups. Voluntary wheel running rats had free access to a stainless steel activity wheel for 3 weeks. The daily running distance gradually increased to 6.21 ± 1.05 km by day 21. Vesicular glutamate transporter 3 (VGLUT3) mRNA levels in the frontal cortex were significantly elevated in the group of running animals compared to the values in sedentary controls, while the expression of other vesicular transporters were unchanged. The concentrations of mRNA coding for glial glutamate transporter 1 (GLT-1), but not glutamate aspartate transporter (GLAST) were increased by running. Voluntary wheel running resulted in an elevation of plasma corticosterone and increased expression of brain derived neurotrophic factor (BDNF) in the frontal cortex. In conclusion, chronic voluntary wheel running results in increased gene expression of VGLUT3 and GLT-1 in the brain cortex without changes in other glutamate transporter subtypes.

Efficacy of phosphodiesterase-4 inhibitors in juvenile Batten disease (CLN3).

OBJECTIVE: Juvenile neuronal ceroid lipofuscinosis (JNCL), or juvenile Batten disease, is a pediatric lysosomal storage disease caused by autosomal recessive mutations in CLN3, typified by blindness, seizures, progressive cognitive and motor decline, and premature death. Currently, there is no treatment for JNCL that slows disease progression, which highlights the need to explore novel strategies to extend the survival and quality of life of afflicted children. Cyclic adenosine monophosphate (cAMP) is a second messenger with pleiotropic effects, including regulating neuroinflammation and neuronal survival. Here we investigated whether 3 phosphodiesterase-4 (PDE4) inhibitors (rolipram, roflumilast, and PF-06266047) could mitigate behavioral deficits and cell-specific pathology in the Cln3Δex7/8 mouse model of JNCL. METHODS: In a randomized, blinded study, wild-type (WT) and Cln3Δex7/8 mice received PDE4 inhibitors daily beginning at 1 or 3 months of age and continuing for 6 to 9 months, with motor deficits assessed by accelerating rotarod testing. The effect of PDE4 inhibitors on cAMP levels, astrocyte and microglial activation (glial fibrillary acidic protein and CD68, respectively), lysosomal pathology (lysosomal-associated membrane protein 1), and astrocyte glutamate transporter expression (glutamate/aspartate transporter) were also examined in WT and Cln3Δex7/8 animals. RESULTS: cAMP levels were significantly reduced in the Cln3Δex7/8 brain, and were restored by PF-06266047. PDE4 inhibitors significantly improved motor function in Cln3Δex7/8 mice, attenuated glial activation and lysosomal pathology, and restored glutamate transporter expression to levels observed in WT animals, with no evidence of toxicity as revealed by blood chemistry analysis. INTERPRETATION: These studies reveal neuroprotective effects for PDE4 inhibitors in Cln3Δex7/8 mice and support their therapeutic potential in JNCL patients. Ann Neurol 2016;80:909-923.

Glia plasma membrane transporters: Key players in glutamatergic neurotransmission.

Glutamate, the main excitatory amino acid in the central nervous system, elicits its functions through the activation of specific membrane receptors that are expressed in neurons and glial cells. The re-cycling of this amino acid is carried out mostly through a continuous interplay between neurons and glia cells, given the fact that the removal of glutamate from the synaptic cleft depends mainly on glial glutamate transporters. Therefore, a functional and physical interaction between membrane transporters links glutamate uptake, transformation to glutamine and its release to the extra-synaptic space and its uptake to the pre-synaptic terminal. This sequence of events, best known as the glutamate/glutamine shuttle is central to glutamatergic transmission. In this sense, the uptake process triggers a complex series of biochemical cascades that modify the physiology of glial cells in the immediate, short and long term so as to be capable to take up, transform and release these amino acids in a regulated amount and in an appropriate time frame to sustain glutamatergic neurotransmission. Among the signaling cascades activated in glial cells by glutamate transporters, a sustained Na(+) and Ca(2+) influx, protein posttranslational modifications and gene expression regulation at the transcriptional and translational levels are present. Therefore, it is clear that the pivotal role of glial cells in the context of excitatory transmission has been constantly underestimated.

Effect of adenosine and adenosine receptor antagonist on Müller cell potassium channel in Rat chronic ocular hypertension models.

Müller cells are principal glial cells in rat retina and have attracted much attention in glaucoma studies. However, it is not clear whether adenosine and adenosine receptor (AR) antagonists play any roles in the regulation of potassium channels in Müller cells and subsequently in the promotion of glutamine synthetase (GS) and L-Glutamate/L-Aspartate Transporter (GLAST) functions. We found that chronic ocular hypertension (COH) in rat down-regulated Müller cells Kir2.1, Kir4.1, TASK-1, GS and GLAST expressions and attenuated the peak of inward potassium current. Retinal ganglion cells (RGC) count was lower in the COH rats than that in the sham operation animals. Intravitreal injection of selective A2A AR antagonist SCH442416 up-regulated Müller cell Kir4.1, TASK-1, GS and GLAST expressions and enhanced inward potassium currents compared with those in the COH rats with vehicle control. Meanwhile, the RGC count was higher following intravitreal injection of SCH442416 in the COH rats than that after vehicle injection. The fact that PKA inhibitor H-89 blocked these SCH442416 effects suggested that the PKA signaling pathway was involved in the observed ocular responses following the intravitreal SCH442416 injection.

GLAST/EAAT1 regulation in cultured Bergmann glia cells: role of the NO/cGMP signaling pathway.

Glutamate, the major excitatory amino acid, activates a wide variety of signal transduction cascades. Ionotropic and metabotropic glutamate receptors are critically involved in long-term synaptic changes, although recent findings suggest that the electrogenic Na(+)-dependent glutamate transporters, responsible for its removal from the synaptic cleft participate in the signaling transactions triggered by this amino acid. Glutamate transporters are profusely expressed in glia therefore most of its uptake occurs in this cellular compartment. In the cerebellar cortex, Bergmann glial cells enwrap glutamatergic synapses and participate in the recycling of its neurotransmitter through the glutamate/glutamine shuttle. It has long been acknowledged that glutamatergic transmission in the cerebellar molecular layer results in cGMP accumulation within Bergmann glia cells. In this context, we decided to investigate a plausible role of the nitric oxide/cGMP-signaling pathway in the regulation of Bergmann glia glutamate transporters. To this end, the well-established model of primary cultures of chick cerebellar Bergmann glial cells was used. Confluent monolayers were exposed to the nitric oxide donor, sodium nitroprusside, or to the non-hydrolysable cGMP analog dbcGMP and the [(3)H] D-aspartate uptake activity measured. An increase in uptake activity, related to an augmentation in VMax, was detected with both treatments. The signaling cascade includes NO/cGMP/PKG and Ca(2+) influx through the Na(+)/Ca(2+) exchanger and might be related to the plasma membrane glutamate transporters turnover. Interestingly enough, an inhibitor of the cGMP dependent protein kinase was capable to abolish the sodium nitroprusside induced Ca(2+) influx. These results provide an insight into the physiological role of cGMP in the cerebellum.

Regional regulation of glutamate signaling during cuprizone-induced demyelination in the brain.

Glutamate excitotoxicity is associated with a wide range of neurodegenerative disorders and also seems to be involved in the pathology of demyelinating disorders such as multiple sclerosis (MS). Cuprizone-induced toxic demyelination shows clear characteristics of MS such as demyelination and axonal damage without the involvement of the innate immune system. In this study, we have evaluated glutamate signaling during cuprizone-induced demyelination in the white and gray matter of mouse brain by studying the expression of ionotropic and metabotropic glutamate-receptors and -transporters by Affymetrix gene array analysis, followed by real-time PCR and western blot analysis. Cellular localization of glutamate transporters was investigated by fluorescence double-labeling experiments. Comparing white and gray matter areas, the expression of glutamate receptors was region-specific. Among NMDA receptor subunits, NR2A was up-regulated in the demyelinated corpus callosum (CC), whereas the metabotropic glutamate receptor mGluR2 was down-regulated in demyelinated gray matter. Glutamate-aspartate transporter (GLAST) co-localizing with GFAP(+) astrocytes was increased in both demyelinated CC and telencephalic cortex, whereas Slc1a4 transporter was up-regulated only in CC. Our data indicate that cuprizone treatment affects glutamate-receptors and -transporters differently in gray and white matter brain areas revealing particularly regulation of GLAST and Slc1a4 compared with other genes. This might have an important influence on brain-region selective sensitivity to neurotoxic compounds and the progression of demyelination as has been reported for MS and other demyelinating neurological diseases.

Evaluation of cell-free protein synthesis for the crystallization of membrane proteins--a case study on a member of the glutamate transporter family from Staphylothermus marinus.

Cell-free in vitro synthesis of proteins using coupled transcription/translation is considered to be a powerful alternative to the use of traditional cell-based expression systems. Recently, promising developments have been reported applying cell-free production to membrane proteins for structural biology and in particular for NMR spectroscopy. However, the general applicability of this system to produce large amounts of stable, functional and homogeneous membrane proteins as required for X-ray crystallography remains to be determined. Here, we present a systematic study comparing structural and functional properties of membrane proteins produced using Escherichia coli derived in vitro and in vivo expression systems. The function of the target membrane protein, a previously uncharacterized bacterial glutamate transporter homolog from Staphylothermus marinus, was analyzed using ligand binding and transport assays. In addition, the protein structure was investigated with respect to its overall fold and oligomeric state in different detergents. We found that the protein synthesized in vitro is highly stable and monodisperse. However, in contrast to the protein produced using an in vivo system, it was not able to assemble into the native trimeric state nor to bind substrate. We thus conclude that cell-free expression systems can compromise folding and function of such complex secondary active transporters. The expression product has to be carefully characterized prior to biophysical investigations like crystallography of membrane proteins.

A novel cross-talk between endothelin and ErbB receptors controlling glutamate transporter expression in astrocytes.

The endothelin and epidermal growth factor (EGF) systems are central to the control of reactive brain processes and are thought to partly exert these tasks by endothelin-induced transactivation of the epidermal growth factor receptor (EGFR) Here we show that beyond EGFR transactivation, endothelins prevent the ligand-induced internalization of the EGFR. We unravel that endothelins abrogate internalization of the EGFR by either promoting the formation of "internalization-deficient" EGFR/ErB2-heterodimers or by activating c-Abl kinase, a negative regulator of EGFR internalization. We further provide evidence that this cross-talk is operational in the control of astrocytic glutamate transport. Specifically, we establish that the inhibitory effects exerted by endothelins on basal as well as EGF-induced expression of the major astroglial glutamate transporter subtype, glutamate transporter 1, are a direct consequence of the endothelin-dependent retention of the EGFR at the cell surface. Together our findings unravel a previously unknown cross-talk between endothelin and epidermal growth factor receptors, which may have implications for a variety of pathological conditions.

Clustered burst firing in FMR1 premutation hippocampal neurons: amelioration with allopregnanolone.

Premutation CGG repeat expansions (55-200 CGG repeats; preCGG) within the fragile X mental retardation 1 (FMR1) gene cause fragile X-associated tremor/ataxia syndrome (FXTAS). Defects in neuronal morphology and migration have been described in a preCGG mouse model. Mouse preCGG hippocampal neurons (170 CGG repeats) grown in vitro develop abnormal networks of clustered burst (CB) firing, as assessed by multielectrode array recordings and clustered patterns of spontaneous Ca(2+) oscillations, neither typical of wild-type (WT) neurons. PreCGG neurons have reduced expression of vesicular GABA and glutamate (Glu) transporters (VGAT and VGLUT1, respectively), and preCGG hippocampal astrocytes display a rightward shift on Glu uptake kinetics, compared with WT. These alterations in preCGG astrocytes and neurons are associated with 4- to 8-fold elevated Fmr1 mRNA and occur despite consistent expression of fragile X mental retardation protein levels at ∼50% of WT levels. Abnormal patterns of activity observed in preCGG neurons are pharmacologically mimicked in WT neurons by addition of Glu or the mGluR1/5 agonist, dihydroxyphenylglycine, to the medium, or by inhibition of astrocytic Glu uptake with dl-threo-ß-benzyloxyaspartic acid, but not by the ionotropic Glu receptor agonists, α-2-amino-3-(5-methyl-3-oxo-1,2-oxazol-4-yl) propanoic acid or N-methyl-d-aspartic acid. The mGluR1 (7-(hydroxyimino)cyclopropa [b]chromen-1a-carboxylate ethyl ester) or mGluR5 (2-methyl-6-(phenylethynyl)pyridine hydrochloride) antagonists reversed CB firing. Importantly, the acute addition of the neurosteroid allopregnanolone mitigated functional impairments observed in preCGG neurons in a reversible manner. These results demonstrate abnormal mGluR1/5 signaling in preCGG neurons, which is ameliorated by mGluR1/5 antagonists or augmentation of GABA(A) receptor signaling, and identify allopregnanolone as a candidate therapeutic lead.

Ceftriaxone upregulates the glutamate transporter in medial prefrontal cortex and blocks reinstatement of methamphetamine seeking in a condition place preference paradigm.

Glutamate signaling plays an essential role in drug-seeking behavior. Using reinstatement of conditioned place preference (CPP), we determined whether ceftriaxone, a ß-lactam antibiotic known to increase the expression and activity of the glutamate transporter (EAAT2) on glial cells, blocks methamphetamine-triggered reinstatement of CPP. Rats acquired methamphetamine CPP following 7 consecutive days of conditioning, during which each animal received pairings of alternating morning methamphetamine (2.5 mg/kg, IP) and afternoon saline (IP). Animals showing CPP were successfully extinguished with repeated twice daily saline administration over a 7-day period. Ceftriaxone (200 mg/kg, IP) was administered (vs. saline) once a day for 7 days during the extinction period. Upon successful extinction, animals received a single dose of methamphetamine (2.5 mg/kg, IP) for reinstatement and were tested for CPP one day later. Using real time PCR, EAAT2 mRNA levels in the nucleus accumbens (NAc) and medial prefrontal cortex (mPFC) were quantified in response to ceftriaxone. Ceftriaxone blocked methamphetamine-triggered reinstatement of CPP and significantly increased EAAT2 mRNA levels in the mPFC, with a trend towards significance in the NAc. In conclusion, Ceftriaxone modulated the expression of the glutamate transporter in a critical region of the cortico-striatal addiction circuitry and attenuated drug-seeking behavior in rats. Further research is needed to test the efficacy of compounds targeting the EAAT2 in human methamphetamine-dependent users.

EAAT expression by macrophages and microglia: still more questions than answers.

Glutamate is the main excitatory amino acid, but its presence in the extracellular milieu has deleterious consequences. It may induce excitotoxicity and also compete with cystine for the use of the cystine-glutamate exchanger, blocking glutathione neosynthesis and inducing an oxidative stress-induced cell death. Both mechanisms are critical in the brain where up to 20% of total body oxygen consumption occurs. In normal conditions, the astrocytes ensure that extracellular concentration of glutamate is kept in the micromolar range, thanks to their coexpression of high-affinity glutamate transporters (EAATs) and glutamine synthetase (GS). Their protective function is nevertheless sensitive to situations such as oxidative stress or inflammatory processes. On the other hand, macrophages and microglia do not express EAATs and GS in physiological conditions and are the principal effector cells of brain inflammation. Since the late 1990s, a number of studies have now shown that both microglia and macrophages display inducible EAAT and GS expression, but the precise significance of this still remains poorly understood. Brain macrophages and microglia are sister cells but yet display differences. Both are highly sensitive to their microenvironment and can perform a variety of functions that may oppose each other. However, in the very particular environment of the healthy brain, they are maintained in a repressed state. The aim of this review is to present the current state of knowledge on brain macrophages and microglial cells activation, in order to help clarify their role in the regulation of glutamate under pathological conditions as well as its outcome.

In vitro effect of adenosine A2A receptor antagonist SCH 442416 on the expression of glutamine synthetase and glutamate aspartate transporter in rat retinal Müller cells at elevated hydrostatic pressure.

The aim of this study was to investigate the effect of an adenosine A2A receptor antagonist on the expression of glutamine synthetase (GS) and glutamate aspartate transporter (GLAST) in rat retinal Müller cells at elevated hydrostatic pressure in vitro. Immunofluorescence staining of GS and GFAP was used for the identification of Müller cells. The expression of GS and GLAST in different hydrostatic pressure (0, 20, 40, 60, 80 mmHg/24 h) was examined by real-time PCR and Western blotting to identify the most suitable pressure. Müller cells treated with 0.1, 1, 10 µM SCH 442416 (A2A receptor antagonist) in the most suitable pressure, and the levels of GS and GLAST were examined by real-time PCR and Western blotting. Significantly increased expression of GS and GLAST at 40 mmHg pressure was observed in Müller cells and treatment with 10 µM SCH 442416 in 40 mmHg pressure further promoted the expression of GS and GLAST. A2A receptor antagonist increased the expression of GLAST and GS of Müller cells and accelerated the clearance of extracellular glutamate.