Abnormal metabotropic glutamate receptor expression and signaling in the cerebral cortex in diffuse Lewy body disease is associated with irregular alpha-synuclein/phospholipase C (PLCbeta1) interactions

Diffuse Lewy body disease (DLBD) is a degenerative disease of the nervous system, involving the brain stem, diencephalic nuclei and cerebral cortex, associated with abnormal a-synuclein aggregation and widespread formation of Lewy bodies and Lewy neurites. DLBD presents as pure forms (DLBDp) or in association with Alzheimer disease (AD) in the common forms (DLBDc). Several neurotransmitter abnormalities have been reported including those of the nigrostriatal and mesocorticolimbic dopaminergic system, and central noradrenergic, serotoninergic and cholinergic pathways. The present work examines metabotropic glutamate receptor (mGluR) expression and signaling in the frontal cortex of DLBDp and DLBDc cases in comparison with age-matched controls. Abnormal L-[3H]glutamate specific binding to group I and II mGluRs, and abnormal mGluR1 levels have been found in DLBD. This is associated with reduced expression levels of phospholipase C beta1 (PLCbeta1), the effector of group I mGluRs following protein G activation upon glutamate binding. Additional modification in the solubility of PLCbeta1 and reduced PLCbeta1 activity in pure and common DLBD further demonstrates for the first time abnormal mGluR signaling in the cerebral cortex in DLBD. In order to look for a possible link between abnormal mGluR signaling and a-synuclein accumulation in DLBD, immunoprecipitation studies have shown alpha-synuclein/PLCbeta1 binding in controls and decreased alpha-synuclein/PLCbeta1 binding in DLBD. This is accompanied by a shift in the distribution of a-synuclein, but not of PLCbeta1, in DLBD when compared with controls. Together, these results support the concept that abnormal a-synuclein in DLBD produces functional effects on cortical glutamatergic synapses, which are associated with reduced alpha-synuclein/PLCbeta1 interactions, and, therefore, that mGluRs are putative pharmacological targets in DLBD. Finally, these results emphasize the emergence of a functional neuropathology that has to be explored for a better understanding of the effects of abnormal protein interactions in degenerative diseases of the nervous system.

Glutamate uptake by oligodendrocytes: Implications for excitotoxicity in multiple sclerosis

BACKGROUND

Excitotoxic damage is a common pathologic event in a number of neurologic diseases occurring after accumulation of excess extracellular glutamate in the CNS and subsequent overstimulation of glutamate receptors. In gray matter, astrocytes take up synaptically released glutamate and are thus key cells in maintaining glutamate homeostasis. In white matter, oligodendrocytes have been shown to express glutamate transporters, but their role in extracellular glutamate removal is unclear.

OBJECTIVE

To investigate whether cultured human fetal oligodendrocytes functionally express the main glutamate transporters EAAT-1 and EAAT-2.

METHODS

Cultures of fetal human oligodendrocytes were examined by immunocytochemistry and [3H]glutamate uptake, and the findings were correlated with glutamate transporter expression in normal and multiple sclerosis (MS) CNS tissue.

RESULTS

Both EAAT-1 and EAAT-2 were expressed by human oligodendrocytes in vitro. Incubation of oligodendrocytes with the proinflammatory cytokine tumor necrosis factor-alpha (TNFalpha) reduced EAAT-1 expression and inhibited glutamate uptake by >75%. Furthermore, in normal human white matter, oligodendrocytes were found to be the predominant cells to express EAAT-1 and EAAT-2, both at the mRNA and at the protein level. A small number of astrocytes in white matter expressed these receptors, more so EAAT-1 than EAAT-2. In MS white matter, oligodendrocytes lost expression of EAAT-1 and EAAT-2 receptors in the lesion vicinity.

CONCLUSIONS

Oligodendrocytes appear to be predominant cells for glutamate clearance in human white matter. Glutamate receptor expression and glutamate removal were defective in MS white matter, possibly mediated by TNFalpha, changes that might underlie high extracellular glutamate and an increased risk for glutamate excitotoxicity.

Pontine intertrigeminal region attenuates sleep apneas in rats

STUDY OBJECTIVES

To determine whether the pontine intertrigeminal region (ITR), with recently described anatomic connections and an effect on vagally induced reflex apnea, has an impact on spontaneous sleep apneas in rats.

DESIGN

Respiration, electroencephalogram (EEG), and electromyogram (EMG) were recorded in rats with lesions of the pontine ITR and in control animals.

PARTICIPANTS

9 adult male Sprague-Dawley rats.

INTERVENTIONS

Rats were implanted with EEG and EMG electrodes and were polygraphically recorded for 6 hours, and their respiration was monitored by placing each animal inside a single-chamber plethysmograph. Subsequently, a respiratory-related intertrigeminal site was identified by probing on dorsoventral tracks with 2 to 5 nL glutamate (10 nL, 10 mmol) injections from a multibarrel glass pipette. This site was then lesioned by injecting ibotenic acid (10 nL, 50 mmol) from a second pipette barrel. Animals were again recorded for 6 hours on days 2, 7, and 14 after the lesion.

MEASUREMENTS AND RESULTS

ITR lesions exerted no impact on mean respiratory pattern during any sleep-wake state, compared to baseline recordings. In contrast, apnea frequency during non-rapid eye movement sleep increased following ITR lesion, more than doubling by day 14.

CONCLUSIONS

This study demonstrates that a small and well-localized unilateral lesion of the ITR region in the lateral pons can increase sleep apnea expression in freely moving rats over a 2-week period. The present findings are in agreement with the general modulatory role of pontine structures in activities including respiration, heart rate, and regulation of blood pressure.

Expression of apical membrane L-glutamate transporters in neonatal porcine epithelial cells along the small intestinal crypt-villus axis

Enteral l-glutamate is extensively utilized as an oxidative fuel by the gut mucosa in the neonate. To identify major uptake pathways and to understand uptake regulation, we examined transport kinetics and molecular identities of apical membrane l-glutamate transporters in epithelial cells sequentially isolated along the small intestinal crypt-villus axis from milk protein-fed, 16-day-old pigs. The distended intestinal sac method was used to isolate 12 sequential cell fractions from the tip villus to the bottom crypt. Initial rates and kinetics of l-glutamate uptake were measured with l-[G-(3)H]glutamate by fast filtration in apical membrane vesicles prepared by Mg(2+) precipitation and differential centrifugation, with membrane potential clamped by SCN(-). Initial l-glutamate uptake results suggested the presence of B(o) and X(AG)(-) transport systems, but the X(AG)(-) system was predominant for uptake across the apical membrane. Kinetic data suggested that l-glutamate uptake through the X(AG)(-) system was associated with higher maximal transport activity but lower transporter affinity in crypt than in villus cells. Molecular identity of the X(AG)(-) glutamate transporter, based on immunoblot and RT-PCR analysis, was primarily the defined excitatory amino acid carrier (EAAC)-1. EAAC-1 expression was increased with cell differentiation and regulated at transcription and translation levels from crypt to upper villus cells. In conclusion, efficiency and capacity of luminal l-glutamate uptake across the apical membrane are regulated by changing expression of the X(AG)(-) system transporter gene EAAC-1 at transcription and translation levels as well as maximal uptake activity and transporter affinity along the intestinal crypt-villus axis in the neonate.

Modulation of Na(+),K(+) pumping and neurotransmitter uptake by beta-amyloid

Micromolar concentrations of beta-amyloid (Abeta), a 40/42-amino-acid-long proteolytic fragment (Abeta(1-40/42)) of the amyloid precursor protein, was shown previously to play a crucial role in pathogenesis of Alzheimer's disease. We used the Xenopus oocyte expression system to investigate specific effects of micromolar concentrations of Abeta(1-42) on the neurotransmitter transporters for gamma-aminobutyric acid (GABA), GAT1, and for the excitatory amino acid glutamate, EAAC1, which are driven by the transmembrane Na(+) gradient that is regulated by the Na(+),K(+)-ATPase. Brief treatment with Abeta(1-42), up to 80 min, leads to a significant inhibition of ion translocation by the Na(+),K(+)-ATPase (30-40%); also glutamate uptake is inhibited (20%) while GABA uptake is not affected. Since reduced glutamate uptake will result in elevated, neurotoxic concentrations of extracellular glutamate, we investigated the effects of Abeta(1-42) and the smaller fragments, Abeta(12-28) and Abeta(25-35), on EAAC1 in more detail. Prolonged incubation in 1 microM Abeta(1-42) leads to further, strong inhibition of glutamate uptake and EAAC1-mediated current (after 4 h inhibition amounts to more than 80%). Abeta(12-28) is less effective with 50% inhibition after 4 h of incubation at 20 microM. Abeta(1-42) and Abeta(12-28) affect EAAC1-mediated current to a similar extent as the rate of glutamate uptake. The effects on EAAC1-mediated current are irreversible if Abeta were applied for longer time periods. Peptides directly microinjected into the oocyte are ineffective suggesting that the observed effect were mediated by extracellular proteins. Abeta(25-35) hardly affects EAAC1-mediated current or glutamate uptake. The results demonstrate that Abeta specifically inhibits the Na(+),K(+) pump and EAAC1. The domain between amino acids 12 and 28 of Abeta seems to play a crucial role for inhibition of EAAC1. The inhibition of EAAC1 by neurotoxic, elevated extracellular glutamate levels may contribute to Alzheimer's pathogenesis.

Transdermal reverse iontophoresis of valproate: a noninvasive method for therapeutic drug monitoring

PURPOSE

The objectives of this work were (a) to explore the potential of transdermal reverse iontophoresis for therapeutic drug monitoring and (b) to develop an "internal standard" calibration procedure so as to render the technique completely noninvasive.

METHODS

A series of in vitro iontophoresis experiments was performed in which the subdermal concentration of sodium valproate was varied from 21 microM to 1 mM. Glutamic acid was also introduced into the subdermal donor at a fixed concentration to act as an "internal standard" for the calibration method.

RESULTS

Both valproate and glutamate anions were recovered, as expected, at the anodal receptor chamber. The iontophoretic extraction flux of valproate was linearly correlated with the subdermal concentration. Glutamate flux was constant. It follows that the ratio of extracted fluxes (valproate/glutamate) was directly dependent upon (a) the subdermal valproate concentration and (b) the subdermal concentration ratio (valproate/glutamate), offering a means, thereby, to a completely noninvasive methodology.

CONCLUSIONS

This work demonstrates the potential of reverse iontophoresis for noninvasive therapeutic monitoring. The simultaneous quantification of the analyte of interest and of an "internal standard" renders the withdrawal of a blood sample unnecessary.

Riluzole enhances glutamate uptake in rat astrocyte cultures

1. Riluzole is used for the treatment of amyotrophic lateral sclerosis and reported to have neuroprotective effects in animal models of Parkinson's disease, Huntington's disease, and brain ischemia. The neuroprotective action of riluzole has been attributed to its ability to inhibit glutamate release (A. Doble, Neurology 47(4):233S-241S, 1996). 2. The effect of riluzole on L-[2,3-3H] glutamate uptake was investigated in rat cortical astrocyte cultures. 3. Riluzole showed a biphasic concentration-dependent effect on basal glutamate uptake. At low concentrations (1 and 10 microM) riluzole significantly increased glutamate uptake, whereas from 100 microM promoted a slight reduction. 4. Considering the large range of glutamate levels in the synaptic cleft, we studied the 1 microM riluzole effect on uptake of glutamate at different concentrations (1-1000 microM). Riluzole was more effective at low glutamate concentrations (10 microM), enhancing the basal glutamate uptake up to 42%. 5. The action of riluzole on astrocytic glutamate uptake could be an additional mechanism to its neuroprotective role, perhaps suggesting a modulatory action on glutamatergic system involving glutamate clearance from synaptic cleft.

Mutations in the extracellular domains of glutamate-gated chloride channel alpha3 and beta subunits from ivermectin-resistant Cooperia oncophora affect agonist sensitivity

Two full-length glutamate-gated chloride channel (GluCl) cDNAs, encoding GluClalpha3 and GluClbeta subunits, were cloned from ivermectin-susceptible (IVS) and -resistant (IVR) Cooperia oncophora adult worms. The IVS and IVR GluClalpha3 subunits differ at three amino acid positions, while the IVS and IVR GluClbeta subunits differ at two amino acid positions. The aim of this study was to determine whether mutations in the IVR subunits affect agonist sensitivity. The subunits were expressed singly and in combination in Xenopus laevis oocytes. Electrophysiological whole-cell voltage-clamp recordings showed that mutations in the IVR GluClalpha3 caused a modest but significant threefold loss of sensitivity to glutamate, the natural ligand for GluCl receptors. As well, a significant decrease in sensitivity to the anthelmintics ivermectin and moxidectin was observed in the IVR GluClalpha3 receptor. Mutations in the IVR GluClbeta subunit abolished glutamate sensitivity. Co-expressing the IVS GluClalpha3 and GluClbeta subunits resulted in heteromeric channels that were more sensitive to glutamate than the respective homomeric channels, demonstrating co-assembly of the subunits. In contrast, the heteromeric IVR channels were less sensitive to glutamate than the homomeric IVR GluClalpha3 channels. The heteromeric IVS channels were significantly more sensitive to glutamate than the heteromeric IVR channels. Of the three amino acids distinguishing the IVS and IVR GluClalpha3 subunits, only one of them, L256F, accounted for the differences in response between the IVS and IVR GluClalpha3 homomeric channels.

Differentiation of substrate and non-substrate inhibitors of transport system xc(-): an obligate exchanger of L-glutamate and L-cystine

In addition to the well-characterized sodium-dependent excitatory amino acid transporters (EAATs) present in the mammalian CNS, a chloride-dependent, sodium-independent transporter has also been identified that is capable of mediating the uptake of L-glutamate. Named system x(c)(-), this transporter is an obligate exchanger that normally couples the export of intracellular L-glutamate with the import of extracellular L-cystine. Two cell lines that express high levels of system x(c)(-) are used to delineate the pharmacology of the transporter and demonstrate that it is distinct from both the EAATs and EAA ionotropic receptors. Potent competitive inhibitors of system x(c)(-) include: L-homocysteate, ibotenate, L-serine-O-sulphate, (RS)-4-bromohomoibotenate, quisqualate, and (S)-4-carboxyphenylglycine. A fluorescent-based assay that allows system x(c)(-)-mediated exchange of L-glutamate and L-cystine to be followed in real time is used to assess substrate activity. Interestingly, those compounds that proved to be the most potent competitive inhibitors (e.g. L-quisqualate and 4-S-CPG) also proved to be the least active as substrates, suggesting that distinct structural features may control binding and translocation. Lastly, the finding that a number of system x(c)(-) inhibitors are also commonly used as probes of excitotoxic pathology (e.g., L-quisqualate, ibotenate and L-homocysteate) raises some interesting questions regarding the mechanisms through which these analogues produce CNS damage.

Effects of L-glutamate/D-aspartate and monensin on lactic acid production in retina and cultured retinal Müller cells

We have investigated the dependence of the rate of lactic acid production on the rate of Na(+) entry in cultured transformed rat Müller cells and in normal and dystrophic (RCS) rat retinas that lack photoreceptors. To modulate the rate of Na(+) entry, two approaches were employed: (i) the addition of L-glutamate (D-aspartate) to stimulate coupled uptake of Na(+) and the amino acid; and (ii) the addition of monensin to enhance Na(+) exchange. Müller cells produced lactate aerobically and anaerobically at high rates. Incubation of the cells for 2-4 h with 0.1-1 mM L-glutamate or D-aspartate did not alter the rate of production of lactate. ATP content in the cells at the end of the incubation period was unchanged by addition of L-glutamate or D-aspartate to the incubation media. Na(+)-dependent L-glutamate uptake was observed in the Müller cells, but the rate of uptake was very low relative to the rate of lactic acid production. Ouabain (1 mM) decreased the rate of lactic acid production by 30-35% in Müller cells, indicating that energy demand is enhanced by the activity of the Na(+)-K(+) pump or depressed by its inhibition. Incubation of Müller cells with 0.01 mM monensin, a Na(+) ionophore, caused a twofold increase in aerobic lactic acid production, but monensin did not alter the rate of anaerobic lactic acid production. Aerobic ATP content in cells incubated with monensin was not different from that found in control cells, but anaerobic ATP content decreased by 40%. These results show that Na(+)-dependent L-glutamate/D-aspartate uptake by cultured retinal Müller cells causes negligible changes in lactic acid production, apparently because the rates of uptake are low relative to the basal rates of lactic acid production. In contrast, the marked stimulation of aerobic lactic acid production caused by monensin opening Na(+) channels shows that glycolysis is an effective source of ATP production for the Na(+)-K(+) ATPase. A previous report suggests that coupled Na(+)-L-glutamate transport stimulates glycolysis in freshly dissociated salamander Müller cells by activation of glutamine synthetase. The Müller cell line used in this study does not express glutamine synthetase; consequently these cells could only be used to examine the linkage between Na(+) entry and the Na(+) pump. As normal and RCS retinas express glutamine synthetase, the role of this enzyme was examined by coapplication of L-glutamate and NH(4) (+) in the presence and absence of methionine sulfoximine, an inhibitor of glutamine synthetase. In normal retinas, neither the addition of L-glutamate alone or together with NH(4) (+) caused a significant change in the glycolytic rate, an effect linked to the low rate of uptake of this amino acid relative to the basal rate of retinal glycolysis. However, incubation of the RCS retinas in media containing L-glutamate and NH(4)(+) did produce a small (15%) increase in the rate of glycolysis above the rate found with L-glutamate alone and controls. It is unlikely that this increase was the result of conversion of L-glutamate to L-glutamine, as it was not suppressed by inhibition of glutamine synthetase with 5 mm methionine sulfoximine. It appears that the magnitude of Müller cell glycolysis required to sustain the coupled transport of Na(+) and L-glutamate and synthesis of L-glutamine is small relative to the basal glycolytic activity in a rat retina.

Effects of dehydroevodiamine exposure on glutamate release and uptake in the cultured cerebellar cells

Dehydroevodiamine has been reported to have neuroprotective and antiamnesic effects. This study examined the effects of dehydroevodiamine on glutamate release and uptake in cultured cerebellar cells. Chronic dehydroevodiamine exposure decreased the viability of granule cells. The basal and N-methyl-D-aspartate (NMDA)-induced release of glutamate from granule cells were decreased (26 and 14%) by dehydroevodiamine. The NMDA-induced release of glutamate was concentration-dependently inhibited in the granule cells. The basal and NMDA-induced releases of glutamate in chronically dehydroevodiamine-preexposed granule cells were unaffected by dehydroevodiamine. Glutamate uptake in the glial cells incubated without and with cAMP was inhibited (31% and 8%, respectively) by dehydroevodiamine. In the chronically dehydroevodiamine-preexposed glial cells, glutamate uptake was increased (8%) in the cAMP-coexposed glial cells by dehydroevodiamine but was unaffected in the naive cells. In addition, dehydroevodiamine potentiated (from 20% to 34%) the inhibition of L-pyrollidine-2,4-dicarboxylic acid (PDC) on glutamate uptake in naive glial cells, but this inhibition was reduced (from 41% to 26%) in cAMP-coexposed glial cells. These results suggest that dehydroevodiamine inhibits glutamate uptake and release. Furthermore, the results suggest that the characteristics of glutamate release and uptake in granule and glial cells may be altered by chronic exposure to dehydroevodiamine.

In vitro ischemic tolerance involves upregulation of glutamate transport partly mediated by the TACE/ADAM17-tumor necrosis factor-alpha pathway

A short ischemic event [ischemic preconditioning (IPC)] can result in a subsequent resistance to severe ischemic injury (ischemic tolerance). Although tumor necrosis factor-alpha (TNF-alpha) contributes to the brain damage found after cerebral ischemia, its expression and neuroprotective role in models of IPC have also been described. Regarding the role of TNF-alpha convertase (TACE/ADAM17), we have recently shown its upregulation in rat brain after IPC induced by transient middle cerebral artery occlusion and that subsequent TNF-alpha release accounts for at least part of the neuroprotection found in this model. We have now used an in vitro model of IPC using rat cortical cultures exposed to sublethal oxygen-glucose deprivation (OGD) to investigate TACE expression and activity after IPC and the subsequent mechanisms of ischemic tolerance. OGD-induced cell death was significantly reduced in cells exposed to IPC by sublethal OGD 24 hr before, an effect that was inhibited by the TACE inhibitor BB3103 (1 microm) and anti-TNF-alpha antibody (2 microg/ml) and that was mimicked by TNF-alpha (10 pg/ml) preincubation. Western blot analysis showed that TACE expression is increased after IPC. IPC caused TNF-alpha release, an effect that was blocked by the selective TACE inhibitor BB-3103. In addition, IPC diminished the increase in extracellular glutamate caused by OGD and increased cellular glutamate uptake and expression of EAAT2 and EAAT3 glutamate transporters; however, only EAAT3 upregulation was mediated by increased TNF-alpha. These data demonstrate that neuroprotection induced by IPC involves upregulation of glutamate uptake partly mediated by TACE overexpression.

An Evaluation of Erythrocytes as Plasma Glutamate Scavengers for Enhanced Brain-to-Blood Glutamate Efflux

Several acute brain pathological conditions are characterized by the presence of excess glutamate in brain interstitial fluid. We have previously shown that decreasing blood glutamate levels increases the driving force for an enhanced brain-to-blood efflux of glutamate. The present study investigated the glutamate pumping ability of glutamate-depleted erythrocytes both in vitro and in vivo to determine whether the latter could potentially be used in a blood exchange procedure for neuroprotection. We have observed that glutamate is taken up in red blood cells only via a passive diffusive process with a diffusion constant of 0.144/h. When glutamate-depleted blood cells resuspended in 6% hetastarch were injected into recipient rats, using a blood exchange protocol, a decrease of blood glutamate was observed but attributed to plasma dilution. These observations are discussed in light of a novel neuroprotective strategy based on blood glutamate scavenging.

Identification of glutamate transporters and receptors in mouse testis

AIM

To investigate the presence of glutamate transporters and receptors in mouse testis.

METHODS

Glutamate uptake analysis was performed to study the function of glutamate transporters in mouse testis. Comparative RT-PCR technique and sequencing analysis were used to study the expression of glutamate receptors and transporters in mouse testis.

RESULTS

Mouse testis possessed glutamate uptake capacity with sodium-dependence. Vmax value of glutamate uptake was (1.60 +/- 0.21) pmol/min per mg protein and Km value of glutamate uptake was (11.0+/-1.6) micromol/L in mouse testis according to saturation analysis. Furthermore, the uptake activity could be inhibited by DHK (GLT1 selective inhibitor) and THA (glutamate uptake inhibitor). In addition, RT-PCR results revealed that glutamate transporters (GLT1 and EAAC1) and ionotropic glutamate receptors (NR1, NR2B, GluR6 and KA2) were expressed in mouse testis.

CONCLUSION

Glutamate transporters and receptors do exist in mouse testis.

Glutamatergic transmission in the rat brain and gravitational stress

The effects of hypergravity stress on L-[14C]-glutamate release from synaptosomes obtained from the rat brain and on the kinetic parameters of high-affinity glutamate transport activity were investigated. We found that hypergravity stress affected only the Ca(2+)-dependent component of L-[14C]-glutamate release. It did not modify the transporter affinity, but the maximum rate of uptake dropped from 12.5 +/- 3.2 to 5.6 +/- 0.9 nmol/min/mg of protein (in control rats and in animals subjected to hypergravity, respectively).

Presenilin-1 and intracellular calcium stores regulate neuronal glutamate uptake

Glutamate uptake by high affinity glutamate transporters is essential for preventing excitotoxicity and maintaining normal synaptic function. We have discovered a novel role for presenilin-1 (PS1) as a regulator of glutamate transport. PS1-deficient neurons showed a decrease in glutamate uptake of approximately 50% compared to wild-type neurons. Gamma-secretase inhibitor treatment mimicked the effects of PS1 deficiency on glutamate uptake. PS1 loss-of-function, accomplished by PS1 deficiency or gamma-secretase inhibitor treatment, caused a corresponding decrease in cell surface expression of the neuronal glutamate transporter, EAAC1. PS1 deficiency is known to reduce intracellular calcium stores. To explore the possibility that PS1 influences glutamate uptake via regulation of intracellular calcium stores, we examined the effects of treating neurons with caffeine, thapsigargin, and SKF-96365. These compounds depleted intracellular calcium stores by distinct means. Nonetheless, each treatment mimicked PS1 loss-of-function by impairing glutamate uptake and reducing EAAC1 expression at the cell surface. Blockade of voltage-gated calcium channels, activation and inhibition of protein kinase C (PKC), and protein kinase A (PKA) all had no effect on glutamate uptake in neurons. Taken together, these findings indicate that PS1 and intracellular calcium stores may play a significant role in regulating glutamate uptake and therefore may be important in limiting glutamate toxicity in the brain.

Glial soluble factors regulate the activity and expression of the neuronal glutamate transporter EAAC1: implication of cholesterol

A co-ordinated regulation between neurons and astrocytes is essential for the control of extracellular glutamate concentration. Here, we have investigated the influence of astrocytes and glia-derived cholesterol on the regulation of glutamate transport in primary neuronal cultures from rat embryonic cortices. Glutamate uptake rate and expression of the neuronal glutamate transporter EAAC1 were low when neurons were grown without astrocytes and neurons were unable to clear extracellular glutamate. Treatment of the neuronal cultures with glial conditioned medium (GCM) increased glutamate uptake Vmax, EAAC1 expression and restored the capacity of neurons to eliminate extracellular glutamate. Thus, astrocytes up-regulate the activity and expression of EAAC1 in neurons. We further showed that cholesterol, present in GCM, increased glutamate uptake activity when added directly to neurons and had no effect on glutamate transporter expression. Furthermore, part of the GCM-induced effect on glutamate transport activity was lost when cholesterol was removed from GCM (low cholesterol-GCM) and was restored when cholesterol was added to low cholesterol-GCM. This demonstrates that glia-derived cholesterol regulates glutamate transport activity. With these experiments, we provide new evidences for neuronal glutamate transport regulation by astrocytes and identified cholesterol as one of the factors implicated in this regulation.

Exposure of animals to artificial gravity conditions leads to the alteration of the glutamate release from rat cerebral hemispheres nerve terminals

The biochemical basis underlying the effects of altered gravity on the process of nervous signal transmission is not clear. We have investigated the effect of hypergravity stress (created by centrifugation of rats at l0 g for 1 h) on the basal and stimulated release of L-[14C]glutamate (a chemical transmitter of excitatory signals) from isolated rat brain nerve terminals (synaptosomes). It has been shown that the hypergravity stress exerted a different influence on the Ca(2+)-dependent and the Ca(2+)-independent component of neurotransmitter release. The Ca(2+)-dependent L-[14C]glutamate release evoked by potassium chloride was equal to 14.4 +/- 0.7% of total synaptosomal label for control animals and 6.2 +/- 1.9% for animals, exposed to hypergravity (P < or = 0.05) and was more than twice decreased as a result of the hypergravity stress. We observed no statistically significant difference in the Ca(2+)-independent component of L-[14C]glutamate release. For control group and animals exposed to the hypergravity stress it was equal to 7.7 +/- 2.8% and 12.9 +/- 2.0%, respectively. We have also investigated the effect of the hypergravity stress on the activity of high-affinity Na(+)-dependent glutamate transporters. Km and Vmax of L-[14C]glutamate uptake have been determined. The maximal velocity of glutamate uptake was decreased as a result of hypergravity loading, but no difference in the Km values between control rats and hypergravity exposed animals was observed. These findings indicate that hypergravity stress alters neurotransmitter reuptake and exocytotic neurotransmitter release processes.

Identification of glutamate receptors and transporters in mouse and human sperm

gamma-Aminobutyric acid (GABA) and glutamate (Glu) are considered as the predominant inhibitory and excitatory neurotransmitters in mammalian central nervous systems (CNS), respectively. The presence of the GABA system and metabotropic glutamate receptors in sperm prompted us to explore the existence of ionotropic glutamate receptors and glutamate transporters in sperm. Immunofluorescent analysis was used to investigate the existence and location of glutamate, glutamate receptor (NR2B), and glutamate transporter (GLT1) in mouse and human sperm. Our present results showed that NR2B was located in the midpiece of sperm, whereas GLT1 mainly existed in the head. Moreover, glutamate uptake activity was detected in mouse sperm and it could be blocked by dihydrokainic acid (DHK, GLT1-selective inhibitor) and DL-threo-beta-hydroxyaspartic acid (THA, nonselective inhibitor). In addition, reverse transcription-polymerase chain reaction technique and sequencing analysis revealed that glutamate transporters (GLT1 and EAAC1) and ionotropic glutamate receptors (NR1, NR2B, GluR6, and KA2) existed in mouse sperm as well as in human sperm. The present findings are the first direct evidence for the existence of ionotropic glutamate receptors and glutamate transporters in sperm. It also indicates that, in sperm, glutamate receptors and transporters might have functions other than neurotransmission.

Allosteric modulation of [3H]dizocilpine binding to N-methyl-d-aspartate receptor by an endogenous Na+, K+-ATPase inhibitor: dependence on receptor activation

An endogenous Na(+), K(+)-ATPase inhibitor, termed endobain E, has been isolated from rat brain and proved to decrease [3H]dizocilpine binding to cerebral cortex N-methyl-D-aspartate (NMDA) receptor, an effect independent of sodium pump activity. The purpose of this study was to disclose the mechanism of [3H]dizocilpine binding reduction by endobain E by performing saturation, kinetic and competitive assays. In saturation binding assays, endobain E increased K(d) without modifying B(max) value. To determine whether competitive or allosteric interaction was involved, kinetics of [3H]dizocilpine binding to cerebral cortex membranes was studied. Endobain E increased [3H]dizocilpine dissociation rate constant and induced an initial fast phase, without modifying association rate constant, indicating an allosteric interaction. In competitive [3H]dizocilpine binding assays, no additive effect was observed with endobain E plus competitive antagonists for glutamate or glycine sites (2-amino-5-phosphonopentanoic acid (AP-5) and 7-chlorokynurenic acid, respectively), indicating that coagonist site blockade interferes with endobain E effect. However, the higher glutamate and glycine concentration, the greater its effect. Endobain E binding reduction was partially additive with that induced by ketamine or Mg(2+) (receptor-associated channel blockers). Results suggest that the greater the channel activation by glutamate and glycine, the greater endobain E allosteric effect. Furthermore, as ketamine and Mg(2+) interfere with endobain E effect, this factor most likely binds to the inner surface of the NMDA associated channel.

Cannabinoids decrease corticostriatal synaptic transmission via an effect on glutamate uptake

Activation of cannabinoid CB1 receptors reduces glutamatergic synaptic transmission in the rodent striatum and is involved in the normal control of motor function by the basal ganglia. Here we investigated CB1 receptor regulation of glutamate release and uptake and synaptic transmission in the rat striatum. We show that CB1 receptor activation reduces both the release and uptake of [3H]glutamate in striatal slices. We also demonstrate that both activation of CB1 receptors and inhibition of glutamate uptake reduce corticostriatal synaptic transmission in a mutually occlusive manner and that both forms of depression are dependent on metabotropic glutamate receptor (mGluR) activation. We propose that CB1 receptor activation in the striatum decreases glutamate transporter activity and that the resulting increase in synaptic cleft glutamate concentration causes the activation of presynaptic mGluRs, which then decrease glutamate release.

Activation of group III metabotropic glutamate receptors in selected regions of the basal ganglia alleviates akinesia in the reserpine-treated rat

1. This study examined whether group III metabotropic glutamate (mGlu) receptor agonists injected into the globus pallidus (GP), substantia nigra pars reticulata (SNr) or intracerebroventricularly (i.c.v.) could reverse reserpine-induced akinesia in the rat. 2. Male Sprague-Dawley rats, cannulated above the GP, SNr or third ventricle, were rendered akinetic with reserpine (5 mg kg(-1) s.c.). 18 h later, behavioural effects of the group III mGlu receptor agonists L-serine-O-phosphate (L-SOP) or L-(+)-2-amino-4-phosphonobutyric acid (L-AP4) were examined. 3. In reserpine-treated rats, unilateral injection of L-SOP (2000 and 2500 nmol in 2.5 microl) into the GP produced a significant increase in net contraversive rotations compared to vehicle, reaching a maximum of 83+/-21 rotations 120 min(-1) (n=8). Pretreatment with the group III mGlu receptor antagonist methyl-serine-O-phosphate (M-SOP; 250 nmol in 2.5 microl) inhibited the response to L-SOP (2000 nmol) by 77%. Unilateral injection of L-SOP (250-1000 nmol in 2.5 microl) into the SNr of reserpine-treated rats produced a dose-dependent increase in net contraversive rotations, reaching a maximum of 47+/-6 rotations 30 min(-1) (n=6). M-SOP (50 nmol in 2.5 microl) inhibited the response to L-SOP (500 nmol) by 78%. 4. Following i.c.v. injection, L-SOP (2000-2500 nmol in 2.5 microl) or L-AP4 (0.5-100 nmol in 2 microl) produced a dose-dependent reversal of akinesia, attaining a maximum of 45+/-17 (n=8) and 72+/-3 (n=9) arbitrary locomotor units 30 min(-1), respectively. 6. These studies indicate that injection of group III mGlu receptor agonists into the GP, SNr or cerebral ventricles reverses reserpine-induced akinesia, the mechanism for which remains to be established.

Ebselen protects against methylmercury-induced inhibition of glutamate uptake by cortical slices from adult mice

Methylmercury (MeHg) is a highly neurotoxic compound and the inhibition of glutamate uptake by astrocytes has been pointed as an important mechanism involved in MeHg-induced glutamate excitotoxicity. We examined the effect of oral exposure to MeHg (10 and 40 mg/l in drinking water) on glutamate uptake by brain cortical slices of adult mice. Moreover, the possible protective role of ebselen (20 mg/kg, subcutaneously) against MeHg effect was also examined. In addition, it was measured the glutathione peroxidase and catalase activities in mice brain. Our results demonstrated, for the first time, that in vivo exposure to MeHg causes a dose-dependent decrease in glutamate uptake and that ebselen, which did not affect the uptake per se, reverted this effect. MeHg decreased glutathione peroxidase activity and increased catalase activity, effects which were also prevented by ebselen. These results may indirectly indicate that: (i) the in vivo inhibitory effect of MeHg on glutamate uptake could be probably related to overproduction of H(2)O(2); (ii) the protective effect of ebselen on MeHg-induced inhibition of glutamate uptake could be related to its ability to detoxify H(2)O(2).

Blood-mediated scavenging of cerebrospinal fluid glutamate

The maintenance of brain extracellular glutamate (Glu) at levels below its excitotoxic threshold is performed by Glu transporters present on glia and neurons as well as on brain capillary endothelial cells which remove brain Glu into blood. The feasibility of accelerating the naturally occurring brain-to-blood Glu efflux was studied using paradigms based on the fate of Glu present in the cerebrospinal fluid or infused into the brain ventricles and monitored before, during, and after decreasing blood Glu levels with pyruvate and oxaloacetate, the respective Glu co-substrates of the blood resident enzymes glutamate-pyruvate transaminase and glutamate-oxaloacetate transaminase. Results from cerebroventricular perfusions with [3H]Glu, intracerebroventricular injections of [3H]Glu, and measurements of the basal CSF Glu levels point out to the same conclusion that the intravenous administration of pyruvate and oxaloacetate which decreases blood Glu levels accelerates the brain-to-blood Glu efflux. We conclude that the brain extracellular Glu levels can be controlled in part by the blood Glu levels. The results may provide not only a rational explanation for the inhibition of Glu release and neuroprotective effects of parentally administered pyruvate in hemorrhagic shock and forebrain ischemia but could also outline a potential strategy for the removal of excess Glu in various neurodegenerative disorders.

Glutamate transport, glutamine synthetase and phosphate-activated glutaminase in rat CNS white matter. A quantitative study

Glutamatergic signal transduction occurs in CNS white matter, but quantitative data on glutamate uptake and metabolism are lacking. We report that the level of the astrocytic glutamate transporter GLT in rat fimbria and corpus callosum was approximately 35% of that in parietal cortex; uptake of [3H]glutamate was 24 and 43%, respectively, of the cortical value. In fimbria and corpus callosum levels of synaptic proteins, synapsin I and synaptophysin were 15-20% of those in cortex; the activities of glutamine synthetase and phosphate-activated glutaminase, enzymes involved in metabolism of transmitter glutamate, were 11-25% of cortical values, and activities of aspartate and alanine aminotransferases were 50-70% of cortical values. The glutamate level in fimbria and corpus callosum was 5-6 nmol/mg tissue, half the cortical value. These data suggest a certain capacity for glutamatergic neurotransmission. In optic and trigeminal nerves, [3H]glutamate uptake was < 10% of the cortical uptake. Formation of [14C]glutamate from [U-14C]glucose in fimbria and corpus callosum of awake rats was 30% of cortical values, in optic nerve it was 13%, illustrating extensive glutamate metabolism in white matter in vivo. Glutamate transporters in brain white matter may be important both physiologically and during energy failure when reversal of glutamate uptake may contribute to excitotoxicity.