RETRACTED: mPEG-b-P(Glu-co-Phe) nanoparticles increase gastric retention time and gastric ulcer treatment efficacy of 20(S)-ginsenoside Rg3

This article has been retracted: please see Elsevier Policy on Article Withdrawal (http://www.elsevier.com/locate/withdrawalpolicy). On behalf of all authors, the corresponding author, Jiannan Li, is retracting the above article. The authors informed the journal that they mistakenly provided inappropriate H&E and EGFR immunohistochemical images for the Rg3-NPs group in Fig. 9 of the published article. The results in Fig. 9D cannot be reproduced as originally published. Importantly, in the present version, Rg3-NPs groups do not show an EGFR promotion effect compared to Rg3 and Cimetidine groups. Therefore, their final results and conclusions are not supported. The authors sincerely apologise to the editors and journal readership for these oversights and inconvenience that this may have caused.

Admissible daily intake for glutamate

PURPOSE OF REVIEW

Total glutamate (Glu) intake is 5-20 g/day in adults and about 40 mg/kg in breast-fed infant. Glu intake is constituted by Glu from protein and free Glu from certain foods and flavor-enhancing additive. The admissible intake of free Glu additive is addressed.

RECENT FINDING

In the gut, Glu is actively metabolized by enterocytes and because of this metabolism, the systemic availability of ingested Glu remains relatively low. Human studies are preferred to assess the transfer in blood of dietary free Glu salts and their possible risks. When human data are not available, experimental animal models provide the basis to assess the risks to humans but toxicity studies in rodents remain for a part controversial. A No Observable Adverse Effect Level (NOAEL) in rodent of 3200 mg/kg/day and an uncertainty factor of 100 lead to an acceptable daily intake (ADI) of 30 mg/kg/day for free Glu salts used as additives, whereas a NOAEL higher than 6000 mg/kg/day and an uncertainty factor of 25 leads to an ADI of 240 mg/kg/day for free Glu salts.

SUMMARY

Current discussions indicate an ADI from 30 to 240 mg/kg/day depending on the chosen NOAEL in animal model and compound-specific uncertainty factor (from 25 to 100).

Brush border enzyme-cleavable linkers: Evaluation for reducing renal uptake of radiolabeled prostate-specific membrane antigen inhibitors

INTRODUCTION

Radiolabeled, low-molecular-weight prostate-specific membrane antigen (PSMA) inhibitors based on the Glu-ureido pharmacophore show promise for the detection and treatment of castration-resistant prostate cancer; however, high renal retention of activity, related in part to overexpression of PSMA in kidneys can be problematic. The goal of the current study was to investigate the use of brush border enzyme-cleavable linkers as a strategy for reducing kidney activity levels from radiolabeled PSMA inhibitors.

METHODS

PSMA-769 (6), a derivative of the prototypical PSMA inhibitor (((S)‑1‑carboxy‑5‑(4‑iodobenzamido)pentyl)carbamoyl)glutamate (12) modified to contain a Gly-Tyr linker, and its protected tin precursor (11) were synthesized starting from the basic pharmacophore molecule Lys-urea-Glu. An analogue of 6 containing d‑tyrosine in lieu of l‑tyrosine (PSMA-769-d-tyrosine) and the corresponding tin precursor (d-11) also were synthesized. Both radioiodinated and ²¹¹At-labeled 6 were synthesized by radiohalogenation of 11 and deprotection in situ. Similarly, radioiodinated d-6 was synthesized from d-11. Paired label biodistribution of [¹²⁵I]12 and [¹³¹I]6 was performed in normal mice and in SCID mice bearing both PC3 PIP (PSMA+) and PC3 flu (PSMA-) subcutaneous prostate carcinoma xenografts. The biodistribution of [¹³¹I]6 and [²¹¹At]6 was also evaluated in this tumor model. Biodistribution of the two radioiodinated diastereomers of 6 was evaluated in normal mice and urine samples were analyzed for the presence of 4‑iodohippuric acid.

RESULTS

Compounds [¹³¹I]6 and [²¹¹At]6 were synthesized from 11 in overall radiochemical yields of 32.5 ± 0.1% (n = 4) and 22% (n = 1), respectively; radiochemical purity was >95%. In normal mice, renal uptake of [¹³¹I]6 was 1.4-, 2.8- and 161-fold lower than that seen for co-injected [¹²⁵I]12 at 1 h, 4 h and 21 h, respectively. In tumor-bearing mice, kidney uptake of [¹³¹I]6 was similar to that for [¹²⁵I]12 (P > 0.05) at 1 h and 4 h but was 6- to 7-fold lower at 21 h; however, [¹³¹I]6 uptake in PC3 PIP tumors was also lower than that seen for [¹²⁵I]12 at 21 h (12.6 ± 3.4%ID/g vs. 36.8 ± 12.4%ID/g). Uptake of [²¹¹At]PSMA-769 in PC3 PIP tumors was slightly higher than that seen for [¹³¹I]PSMA-769 at 4 h (9.6 ± 1.6%ID/g versus 7.8 ± 1.6%ID/g; P = 0.002); its uptake in a number of normal tissues also was higher. In normal mice, kidney uptake of [¹²⁵I]PSMA-769 at 4 h was about 73% of that seen for [¹³¹I]PSMA-769-d-tyrosine. Activity in the urine of mice receiving [¹²⁵I]PSMA-769 contained mainly 4‑[¹²⁵I]iodohippuric acid while unmetabolized intact molecule was present in the case of [¹²⁵I]PSMA-769-d-tyrosine.

CONCLUSION

Use of this brush border enzyme-cleavable linker reduced kidney uptake and resulted in improved tumor:kidney uptake ratios. Although further structural improvements are needed, this linker approach might be useful as a component in strategies for reducing renal uptake of radiolabeled PSMA inhibitors.

Impact of oral supplementation of Glutamate and GABA on memory performance and neurochemical profile in hippocampus of rats

Glutamate (GLU) and gamma-amino butyric acid (GABA) are essential amino acids (AA) for brain function serving as excitatory and inhibitory neurotransmitter respectively. Their tablets are available in market for improving gut function and muscle performance. Despite of having a major role during memory formation and processing, effects of these tablets on brain functioning like learning and memory have not been investigated. Therefore, present study is aimed to investigate the effects of orally supplemented GLU and GABA on learning and memory performance and further to monitor related effects of these orally supplemented GLU and GABA on brain levels of these AA. Three groups of rats were supplemented orally with drinking water (control group) or suspension of tablets of GABA and Glutamate, respectively for four weeks. Cognitive performance was determined using behavioral tests (Novel object recognition test, Morris water maze, Passive avoidance test) measuring recognition, spatial reference and aversive memory. Levels of GLU, GABA and acetylcholine (ACh) were estimated in rat hippocampus. Results showed that chronic oral administration of GLU and GABA tablets has a significant impact on brain function and can alter GLU and GABA content in rat hippocampus. Compared to GABA, GLU supplementation specifically enhances memory performance via increasing ACh. Thus, GLU can be suggested as a useful supplement for improving learning and memory performance and neurochemical status of brain and in future could be effective in the treatment of neurological disorders affecting learning and memory performance.

Novel bicyclo[3.1.0]hexane analogs as antagonists of metabotropic glutamate 2/3 receptors for the treatment of depression

Negative modulators of metabotropic glutamate 2 & 3 receptors demonstrate antidepressant-like activity in animal models and hold promise as novel therapeutic agents for the treatment of major depressive disorder. Herein we describe our efforts to prepare and optimize a series of conformationally constrained 3,4-disubstituted bicyclo[3.1.0]hexane glutamic acid analogs as orthosteric (glutamate site) mGlu_2/3 receptor antagonists. This work led to the discovery of a highly potent and efficacious tool compound 18 (hmGlu₂ IC₅₀ 46±14.2nM, hmGlu₃ IC₅₀=46.1±36.2nM). Compound 18 showed activity in the mouse forced swim test with a minimal effective dose (MED) of 1mg/kg ip. While in rat EEG studies it exhibited wake promoting effects at 3 and 10mg/kg ip without any significant effects on locomotor activity. Compound 18 thus represents a novel tool molecule for studying the impact of blocking mGlu_2/3 receptors both in vitro and in vivo.

[ABSOLUTE AND RELATIVE BIOAVAILABILITY OF GLUTARON--A NEW DERIVATIVE OF GLUTAMIC ACID]

The pharmacokinetics of studies of 3-phenylglutamic acid hydrochloride (glutaron) has been studied in rats. The main pharmacokinetic parameters show low values of the half-life (T1/2 = 3.75 h), mean retention time in the body (MRT = 5.77 h). The medium rate of drug concentration decrease in the blood plasma leads to a low value of the area under pharmacokinetic curve (AUC = 41.18 mg · h/mL). The general volume of distribution (Vd = 3.42 L/kg) is 3.5 times greater than the volume of extracellular fluid in the rat body. These data indicate a high ability of the glutaron to be distributed and accumulated in animal tissues. The value of absolute bioavailability is 84%, and the relative bioavailabity is 100%.

Diphenyl ditelluride induces anxiogenic-like behavior in rats by reducing glutamate uptake

Anxiety-related disorders are a common public health issue. Several lines of evidence suggest that altered glutamatergic neurotransmission underlies anxiety. The present study evaluated the effect of diphenyl ditelluride [(PhTe)2] exposure on the behavioral performance of rats and examined whether the behavioral effects could be attributed to changes in the modulation of glutamatergic function. Rats were exposed to (PhTe)2 (subcutaneously) during 8 weeks-final dose one third LD50 (124 μg/kg). The testing schedule included elevated plus-maze, open-field, T-maze, rotorod, and Morris water maze tests. Synaptosomal basal [(3)H] glutamate release and uptake were also evaluated. The time spent in the open arm and the ratio of time spent in the open arm/total were decreased in the (PhTe)2 group. Furthermore, the [(3)H] glutamate uptake was decreased in this experimental group. The results suggest that exposure to (PhTe)2 did not change motor abilities whereas it may result in anxiogenic-like behavior, induced by changes in the glutamatergic system at the pre-synaptic level.

Measurement of absolute cell volume, osmotic membrane water permeability, and refractive index of transmembrane water and solute flux by digital holographic microscopy

A dual-wavelength digital holographic microscope to measure absolute volume of living cells is proposed. The optical setup allows us to reconstruct two quantitative phase contrast images at two different wavelengths from a single hologram acquisition. When adding the absorbing dye fast green FCF as a dispersive agent to the extracellular medium, cellular thickness can be univocally determined in the full field of view. In addition to the absolute cell volume, the method can be applied to derive important biophysical parameters of living cells including osmotic membrane water permeability coefficient and the integral intracellular refractive index (RI). Further, the RI of transmembrane flux can be determined giving an indication about the nature of transported solutes. The proposed method is applied to cultured human embryonic kidney cells, Chinese hamster ovary cells, human red blood cells, mouse cortical astrocytes, and neurons.

Nitrogen in dietary glutamate is utilized exclusively for the synthesis of amino acids in the rat intestine

Although previous studies have shown that virtually the entire carbon skeleton of dietary glutamate (glutamate-C) is metabolized in the gut for energy production and amino acid synthesis, little is known regarding the fate of dietary glutamate nitrogen (glutamate-N). In this study, we hypothesized that dietary glutamate-N is an effective nitrogen source for amino acid synthesis and investigated the fate of dietary glutamate-N using [(15)N]glutamate. Fischer male rats were given hourly meals containing [U-(13)C]- or [(15)N]glutamate. The concentration and isotopic enrichment of several amino acids were measured after 0-9 h of feeding, and the net release of each amino acid into the portal vein was calculated. Most of the dietary glutamate-C was metabolized into CO(2), lactate, or alanine (56, 13, and 12% of the dietary input, respectively) in the portal drained viscera (PDV). Most of the glutamate-N was utilized for the synthesis of other amino acids such as alanine and citrulline (75 and 3% of dietary input, respectively) in the PDV, and only minor amounts were released into the portal vein in the form of ammonia and glutamate (2 and 3% of the dietary input, respectively). Substantial incorporation of (15)N into systemic amino acids such as alanine, glutamine, and proline, amino acids of the urea cycle, and branched-chain amino acids was also evident. These results provide quantitative evidence that dietary glutamate-N distributes extensively to amino acids synthesized in the PDV and, consequently, to circulating amino acids.

The accessibility in the external part of the TM5 of the glutamate transporter EAAT1 is conformationally sensitive during the transport cycle

Background

Excitatory amino acid transporter 1 (EAAT1) is a glutamate transporter which is a key element in the termination of the synaptic actions of glutamate. It serves to keep the extracellular glutamate concentration below neurotoxic level. However the functional significance and the change of accessibility of residues in transmembrane domain (TM) 5 of the EAAT1 are not clear yet.

Methodology/Principal Findings

We used cysteine mutagenesis with treatments with membrane-impermeable sulfhydryl reagent MTSET [(2-trimethylammonium) methanethiosulfonate] to investigate the change of accessibility of TM5. Cysteine mutants were introduced from position 291 to 300 of the cysteine-less version of EAAT1. We checked the activity and kinetic parameters of the mutants before and after treatments with MTSET, furthermore we analyzed the effect of the substrate and blocker on the inhibition of the cysteine mutants by MTSET. Inhibition of transport by MTSET was observed in the mutants L296C, I297C and G299C, while the activity of K300C got higher after exposure to MTSET. V_max of L296C and G299C got lower while that of K300C got higher after treated by MTSET. The L296C, G299C, K300C single cysteine mutants showed a conformationally sensitive reactivity pattern. The sensitivity of L296C to MTSET was potentiated by glutamate and TBOA,but the sensitivity of G299C to MTSET was potentiated only by TBOA.

Conclusions/Significance

All these facts suggest that the accessibility of some positions of the external part of the TM5 is conformationally sensitive during the transport cycle. Our results indicate that some residues of TM5 take part in the transport pathway during the transport cycle.

Specification of transplantable astroglial subtypes from human pluripotent stem cells

Human pluripotent stem cells (hPSCs) have been differentiated efficiently to neuronal cell types. However, directed differentiation of hPSCs to astrocytes and astroglial subtypes remains elusive. In this study, hPSCs were directed to nearly uniform populations of immature astrocytes (>90% S100β(+) and GFAP(+)) in large quantities. The immature human astrocytes exhibit similar gene expression patterns as primary astrocytes, display functional properties such as glutamate uptake and promotion of synaptogenesis, and become mature astrocytes by forming connections with blood vessels after transplantation into the mouse brain. Furthermore, hPSC-derived neuroepithelia, patterned to rostral-caudal and dorsal-ventral identities with the same morphogens used for neuronal subtype specification, generate immature astrocytes that express distinct homeodomain transcription factors and display phenotypic differences of different astroglial subtypes. These human astroglial progenitors and immature astrocytes will be useful for studying astrocytes in brain development and function, understanding the roles of astrocytes in disease processes and developing novel treatments for neurological disorders.

Amino acid prodrugs of acyclovir as possible antiviral agents against ocular HSV-1 infections: Interactions with the neutral and cationic amino acid transporter on the corneal epithelium

PURPOSE

The aim of this study was to explore the feasibility of improvement of ocular bioavailability of the antiviral agent acyclovir by designing amino acid prodrugs targeted to the amino acid transporters on the rabbit cornea.

MATERIALS AND METHODS

Transcorneal flux of two water-soluble amino acid ester prodrugs of acyclovir (ACV), gamma-glutamate-ACV (EACV) and L-tyrosine-ACV (YACV), was studied across freshly excised rabbit cornea. Chemical and enzymatic hydrolysis studies of the two prodrugs were also conducted.

RESULTS

EACV inhibited the uptake of [(3)H]L-Arg in rabbit primary corneal epithelial cells (rPCECs). The compound also exhibited longer half-life (t(1/2)) in cornea in comparison to YACV. Transcorneal flux of EACV was observed to be concentration-, energy-, and sodium-dependent and independent of pH within the range studied. EACV transport was inhibited by neutral and cationic amino acids, L-ornithine (specific for cationic amino acids), and BCH (2-aminobicyclo-[2,2,1]-heptane-2-carboxylic-acid) (specific inhibitor for L-type system and B(0,+) system). On the other hand, YACV was not recognized by this amino acid transporter as it failed to inhibit the uptake of [(3)H]Arg, and also its transport across cornea was not inhibited by arginine. YACV and EACV exhibited excellent antiviral activity against HSV-1 and 2 and Varicella-Zoster Virus (VZV) in comparison to ACV.

CONCLUSIONS

Analyses of the inhibition pattern of EACV transport suggests the involvement of a single transport system; namely, B(0,+). Design of amino acid prodrugs seems to be an attractive strategy to enhance the solubility of the otherwise poorly aqueous soluble compounds and also to afford a targeted and possibly enhanced delivery of the active drug.

Effect of L-arginine on synaptosomal mitochondrial function

OBJECTIVE

Specific aim of this study is to elucidate the direct effects of L-arginine on the synaptosomal neurotransmission related to the mitochondrial respiratory function.

METHODS

Using isolated endbrains from wild-type mice (ICR), crude synaptosome was analyzed for their concentration of gamma-aminobutyric acid (GABA) and glutamate (Glu) with/without addition of L-arginine. We analyzed the contents of releasing amino acids evoked by high potassium condition and uptake of them in three separated fractions (cytosol, vesicles, and intact mitochondria). The oxygen consumption was also measured by oxygen electrode.

RESULTS

The entire uptakes of GABA and Glu were inhibited by rotenone (about 30 nmol/mg protein) with dose-dependent manner and showed a plateau at about 70% of total uptake. L-arginine inhibited the uptake of Glu logarithmically, however it showed no change in uptake of GABA. The contents of GABA and Glu in synaptosome were decreased in the presence of L-arginine. L-arginine enhanced the respiration of state II by succinate on synaptosomal respiration, although the respiration of synaptosomal mitochondrial fraction and the respiratory chains enzyme activities were almost unaffected by L-arginine. In the presence of rotenone, L-arginine decreased the uptake of Glu without changing the uptake of GABA, increased the releasing of GABA, and may modulate the excitability of neuronal state on the cytosol, cytomembrane, and/or organelles except for mitochondria.

CONCLUSIONS

L-arginine may modulate excitation by neurotransmitters at nerve endings, in relation to potentiated respiratory metabolism of succinate in synaptosomes. Such effects might contribute to alleviation of stroke-like symptoms in MELAS.

Resveratrol increases glutamate uptake, glutathione content, and S100B secretion in cortical astrocyte cultures

Resveratrol (3,5,4'-trihydroxy-trans-stilbene) is a polyphenol present in grapes and red wine, which has antioxidant properties and a wide range of other biological effects. In this study, we investigated the effect of resveratrol, in a concentration range of 10-250 microM, on primary cortical astrocytes; evaluating cell morphology, parameters of glutamate metabolism such as glutamate uptake, glutamine synthetase activity and glutathione total content, and S100B secretion. Astrocyte cultures were prepared of cerebral cortex from neonate Wistar rats. Morphology was evaluated by phase-contrast microscopy and immunocytochemistry for glial fibrillary acidic protein (GFAP). Glutamate uptake was measured using L-[2,3-3H]glutamate. Glutamine synthetase and content of glutathione were measured by enzymatic colorimetric assays. S100B content was determined by ELISA. Typical polygonal morphology becomes stellated when astrocyte cultures were exposed to 250 microM resveratrol for 24 h. At concentration of 25 microM, resveratrol was able to increase glutamate uptake and glutathione content. Conversely, at 250 microM, resveratrol decreased glutamate uptake. Unexpectedly, resveratrol at this high concentration increased glutamine synthetase activity. Extracellular S100B increased from 50 microM upwards. Our findings reinforce the protective role of this compound in some brain disorders, particularly those involving glutamate toxicity. However, the underlying mechanisms of these changes are not clear at the moment and it is necessary caution with its administration because elevated levels of this compound could contribute to aggravate these conditions.

Splanchnic oxidation is the major metabolic fate of dietary glutamate in enterally fed preterm infants

The intestine is a major site of amino acid metabolism, especially in neonates. The energy needed for the metabolic processes in neonatal animals is derived from dietary glucose and amino acids. No data are available showing that dietary amino acids function as intestinal fuel source in human neonates as well. We hypothesized that preterm infants show a high splanchnic first-pass glutamate metabolism and the primary metabolic fate of glutamate is oxidation. Five preterm infants (birth weight 1.2+/-0.2 kg, gestational age 29+/-1 wk) were studied by dual tracer ([U-(13)C]glutamate and [D3]glutamate) techniques on two study days (within postnatal d 14-19). Splanchnic and whole-body glutamate kinetics were assessed by plasma isotopic enrichment of [U-(13)C]glutamate and [D3]glutamate and breath CO2 enrichment. Fractional first-pass glutamate uptake was 77+/-18% on d 1, and 70+/-7% on d 2, mean 74+/-13%. Almost all (86+/-7%) of the glutamate used in the first pass is directed toward oxidation. There is a high splanchnic fractional first-pass uptake and a high oxidation rate of glutamate in preterm infants. Glutamate is an important source of energy for the splanchnic tissues in preterm infants receiving full enteral feeding.

Glycine stimulates the release of labeled acetylcholine but not dopamine nor glutamate from superfused rat striatal tissue

Glycine is known as an inhibitory neurotransmitter in the spinal cord and forebrain but its precise role in the forebrain is largely overlooked. This investigation evaluated whether glycine alters acetylcholine, glutamate or dopamine release from striatal tissue using an in vitro approach. We observed that while glycine induced a robust (3)H-acetylcholine release ((3)H-ACh) from superfused striatal tissue, it failed at releasing (3)H-glutamate or (3)H-dopamine. Glycine stimulated (3)H-ACh release in a dose- and calcium-dependent manner (EC(50)=69 microM). Tetrodotoxin (1 microM) inhibited about 75% of the release demonstrating a predominant dendritic and cell body location of glycine receptors. The prototypical glycine receptor antagonist strychnine at 10 microM completely abolished (3)H-ACh release. To further characterize the role of striatal glycine receptors in (3)H-ACh release we examined glycine effects after in vivo treatment with Haloperidol-decanoate (HD). Treatment for 30 days or more with HD decreased maximal glycine-stimulated release of (3)H-ACh suggesting a non-competitive inhibition. After 30 days of washout release parameters did not return to vehicle-treated levels. The glutamate agonist NMDA also stimulated acetylcholine release but showed slightly different behavior in HD-treated striatal tissue. These effects could be attributed to changes in chloride transporters expressed in the giant striatal cholinergic cell as well as glycine receptor subunit composition and finally, GABA/glycine co-release in this tissue.

Understanding safety of glutamate in food and brain

Glutamate is ubiquitous in nature and is present in all living organisms. It is the principal excitatory neurotransmitter in central nervous system. Glutamate is being used as food additive for enhancing flavour for over last 1200 years imparting a unique taste known as "umami" in Japanese. It is being marketed for about last 100 years. The taste of umami is now recognized as the fifth basic taste. Many of the foods used in cooking for enhancing flavour contain high amount of glutamate. Breast milk has the highest concentration of glutamate amongst all amino acids. Glutamate in high doses as gavage or parenteral injection have been reported to produce neurodegeneration in infant rodents. The neurodegeneration was not produced when gluamate was given with food. The Joint FAO/WHO Expert Committee on Food Additives, based on enumerable scientific evidence, has declared that, "glutamate as an additive in food" is not an health hazard to human being. Glutamate is used as signaling molecule not only in neuronal but also in non-neuronal tissues. Excessive accumulation of glutamate in the synaptic cleft has been associated with excitotoxicty and glutamate is implicated in number of neurological disorders. Excessive accumulation could be attributed to increase release, failure of transport system for uptake mechanism, neuronal injury due to hypoxia-ischemia, trauma and associated metabolic failures. The role blood brain barrier, vesicular glutamate and sodium dependent excitatory amino acid transporters in glutamate homeostasis are emphasized in the review.

In vitro and in vivo evaluation of 64Cu-labeled DOTA-linker-bombesin(7-14) analogues containing different amino acid linker moieties

The gastrin-releasing peptide receptor (GRPR) is overexpressed on a variety of tumor types and has been targeted with radiolabeled peptides for detection and therapy of these cancers. Analogues of the 14 amino acid bombesin (BN) peptide have been radiolabeled with both gamma- and positron-emitting radionuclides for detection of GRPR-expressing tumors. We have previously evaluated BN analogues radiolabeled with the positron-emitter, copper-64 (64Cu), that contained various aliphatic linkers placed between the BN peptide and the 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) chelator. These studies showed that the analogues could be used for positron-emission tomographic (PET) imaging of GRPR-positive tumors in mice but clinical translation would be hindered by significant uptake in background tissues. Therefore, the purpose of this study was to determine if the use of amino acid linkers placed between the DOTA chelate and the BN peptide would reduce nontarget tissue uptake, while maintaining good prostate tumor uptake. The linkers studied utilized three amino acid combinations of glycine (G), serine (S), or glutamic acid (E). In vitro assays in PC-3 cells showed that the glutamic acid-containing linkers had poor binding and internalization, while the other analogues had IC50 values <100 nM and good internalization. In vivo, these same analogues demonstrated tumor-specific uptake and good imaging characteristics that were comparable to, or better than the previously reported 64Cu-labeled DOTA-BN analogues. Overall, this study shows that BN analogues containing amino acid linkers can be used for the PET imaging of GRPR-expressing prostate cancer and that these linkers lead to lower background tissue uptake.

Selective dysfunction of hippocampal CA1 astrocytes contributes to delayed neuronal damage after transient forebrain ischemia

Transient global ischemia, as with cardiac arrest, causes loss of CA1 hippocampal neurons 2-4 d later, whereas nearby dentate gyrus (DG) neurons are relatively resistant. Whether differential astrocyte vulnerability in ischemic injury contributes to CA1 neuronal death is uncertain. Here, we find that CA1 astrocytes are more sensitive to ischemia than DG astrocytes. In rats subjected to transient forebrain ischemia, CA1 astrocytes lose glutamate transport activity and immunoreactivity for GFAP, S100beta, and glutamate transporter GLT-1 within a few hours of reperfusion, but without astrocyte cell death. Oxidative stress may contribute to the observed selective CA1 changes, because CA1 astrocytes show early increases in mitochondrial free radicals and reduced mitochondrial membrane potential. Similar changes were not observed in DG astrocytes. Upregulation of GLT-1 expression in astrocytes with ceftriaxone protected CA1 neurons from forebrain ischemia. We suggest that greater oxidative stress and loss of GLT-1 function selectively in CA1 astrocytes is central to the well known delayed death of CA1 neurons.

Synthesis, characterization and biological activity of Schiff base analogues of indole-3-carboxaldehyde

Eight novel heterocyclic Schiff bases derived from the condensation reactions of indole 3-carboxaldehyde with different l-amino acids (histidine, glutamic acid, aspartic acid, leucine, valine) as well as with some aminophenols, have been synthesized and characterized by various spectroscopic methods (IR, MS, (1)H NMR). Schiff base derivatives of indole 3-carboxaldehyde were labeled with (99m)Tc and radiochemical purity was above 97% which is ascertained by instant thin layer chromatography using different solvent conditions. Stability studies of all the derivatives of indole 3-carboxaldehyde was determined under physiological conditions and were stable for more than 24h. Blood clearance showed a quick wash out from the circulation and biological half life was found to be t((1/2))(F)=1h 15min; t((1/2))(S)=10h 05min. Excellent quality radioimages of tumor bearing mice were recorded showing rapid clearance of background activity, visualization of tumor at 3h and clearance from kidneys of histidine analogue which was further evidenced in biodistribution studies. Antimicrobial activity of these Schiff base compounds was evaluated against Bacillus subtilis, Pseudomonas fluorescence, Staphylococcus aureus, Aspergillus niger, Candida albicans and Trichophyton rubrum.

Effects of hypoxia/reperfusion injury on drug disposition in the rat isolated perfused liver

1. Ischaemia-reperfusion injury is known to be associated with a range of functional and structural alterations in the liver. However, the effect of this injury on drug disposition is not well understood. The present study was designed to examine the effects of hypoxia/reperfusion on the disposition of glutamate and propranolol in the rat isolated perfused liver. Both glutamate and propranolol are mainly metabolised in the pericentral region of the liver. 2. Hypoxia/reperfusion was established using the slow flow-reflow method of perfusion in both anterograde and retrograde perfusion. Glutamate metabolism was measured by the recovery of [(14)C]-glutamic acid and [(14)C]-labelled metabolites in a single pass in both anterograde and retrograde perfusion in the presence of a steady state concentration of unlabelled glutamic acid. Propranolol disposition, mean transit time and normalized variance were assessed from the outflow concentration-time profile of unchanged [(3)H]-propranolol determined after a bolus injection of [(3)H]-propranolol using HPLC and liquid scintillation counting. 3. Hypoxia/reperfusion of livers did not affect oxygen consumption, but caused significant changes in enzyme release, lignocaine hepatic availability and bile flow. 4. Hypoxia/reperfusion did not affect the hepatic metabolism of glutamate to carbon dioxide or the hepatic extraction of propranolol. Small but significant changes were evident in the distribution parameters of mean transit time and vascular disposition for the hypoxic-ischaemic liver. 5. It is concluded that reperfusion injury induced by slow flow-reflow perfusion did not influence the extraction of glutamate or propranolol, but may have affected pericentral morphology and solute distribution.

Intrathecal glutamate promotes glycinergic neuronal activity and inhibits the micturition reflex in urethane-anesthetized rats

OBJECTIVES

In order to clarify the role of glutamate in the micturition reflex and in glutamatergic and glycinergic neuronal activity, we examined the effects of intrathecal (IT) injection of glutamate or MK-801 (an N-methyl-D-aspartate receptor antagonist) on bladder activity and on the glutamate and glycine levels in the lumbosacral cord of female rats with or without acute lower thoracic spinal cord injury (SCI).

METHODS

Under urethane anesthesia, isovolumetric cystometry was performed in rats with or without SCI before and after IT injection of glutamate or MK-801 at the lumbosacral cord level. The glutamate and glycine levels of the whole lumbosacral cord were measured after IT injection of glutamate or MK-801 in both groups.

RESULTS

In intact rats, IT glutamate (100 microg) prolonged the interval between bladder contractions and decreased the amplitude of contractions. IT MK-801 (3-100 microg) also prolonged the interval between bladder contractions and decreased the amplitude in intact rats. In SCI rats, cystometry demonstrated the disappearance of bladder contractions, and the glycine level in the lumbosacral cord was elevated. In intact rats, IT glutamate (0.3-100 microg) increased the glycine level in the lumbosacral cord. On the other hand, IT MK-801 (3-100 microg) decreased both glutamate and glycine levels in intact and SCI rats.

CONCLUSIONS

These results suggest that glutamatergic neurons have stimulatory projections to both glutamatergic and glycinergic neurons in the lumbosacral cord, and that glutamatergic neurons inhibit the micturition reflex by stimulating glycinergic neurons.

Metabotropic glutamate receptor/phospholipase C system in female rat heart

Glutamate is the main excitatory neurotransmitter in the central nervous system. This amino acid mediates learning and memory processes acting through ionotropic and metabotropic receptor binding. Metabotropic glutamate receptors (mGluRs) are G protein-coupled receptors that stimulate phospholipase C (PLC) or inhibit adenylyl cyclase (AC). MGluRs have been widely described in CNS. However, little is known about these receptors in peripheral system. The present work describes the mGluR/PLC pathway in membranes from pregnant and non-pregnant rat heart by radioligand binding, Western-blot assays and PLC activity determination. Furthermore, mRNA coding mGluR1, mGluR5, alphaGq/11 and PLCbeta1 was identified by RT-PCR. Binding assays indicated total mGlu receptor numbers of 4.7+/-0.2 pmol/mg protein and 4.2+/-1.0 pmol/mg protein in non-pregnant and pregnant rats respectively, and their corresponding KD values were 545.3+/-85.6 nM and 1062.8+/-393.6 nM. Western blots revealed bands corresponding to mGluR1 and mGluR5 receptors, confirming that these receptors are expressed in heart. The beta1 isoform of PLC, which mediates group I mGluRs (mGluR I) response, was also expressed in rat heart. Moreover, PLC activity was modulated by calcium in a dose-dependent manner. Finally, specific agonists for mGluRs increased the PLC activity and the increase was prevented by specific mGluR antagonists. These results demonstrate the presence of group I mGlu receptors and their functional coupling to the PLC stimulation in female rat heart, suggesting a possible role of mGluR/PLC pathway in this tissue.

The uptake of amino acids by the cyanobacterium Planktothrix rubescens is stimulated by light at low irradiances

The rates of uptake of five amino acids--alanine, glutamate, glycine, leucine and serine--by axenic cultures of the cyanobacterium Planktothrix rubescens were measured over a range of irradiances using the (14)C-labelled amino acids at the nanomolar concentrations observed in Lake Zürich. The rates in the light exceeded the dark rates by as much as two- to ninefold. The light-affinity constants for stimulation were similar, indicating a similar process for each of the five amino acids. The E(k) (light saturation irradiance) for light stimulation was only 1 micromol m(-2) s(-1), less than the compensation point for photosynthesis and autotrophic growth, and much lower than the E(k) for either process. The E(k) for amino acid uptake was also less than the irradiance at which filaments obtain neutral buoyancy, which determines the depth at which they stratify and the irradiance they receive. This indicates that stimulation of amino acid uptake by light of low irradiances provides a mechanism for supplementing growth of filaments stratifying deep in the metalimnion, which, while able to grow at low irradiances, are often left with insufficient light to sustain them. Acetate uptake was also stimulated by light, but the kinetics differed.

The glutamate-aspartate transporter GLAST mediates glutamate uptake at inner hair cell afferent synapses in the mammalian cochlea

Ribbon synapses formed between inner hair cells (IHCs) and afferent dendrites in the mammalian cochlea can sustain high rates of release, placing strong demands on glutamate clearance mechanisms. To investigate the role of transporters in glutamate removal at these synapses, we made whole-cell recordings from IHCs, afferent dendrites, and glial cells adjacent to IHCs [inner phalangeal cells (IPCs)] in whole-mount preparations of rat organ of Corti. Focal application of the transporter substrate D-aspartate elicited inward currents in IPCs, which were larger in the presence of anions that permeate the transporter-associated anion channel and blocked by the transporter antagonist D,L-threo-beta-benzyloxyaspartate. These currents were produced by glutamate-aspartate transporters (GLAST) (excitatory amino acid transporter 1) because they were weakly inhibited by dihydrokainate, an antagonist of glutamate transporter-1 (excitatory amino acid transporter 2) and were absent from IPCs in GLAST-/- cochleas. Furthermore, D-aspartate-induced currents in outside-out patches from IPCs exhibited larger steady-state currents than responses elicited by L-glutamate, a prominent feature of GLAST, and examination of cochlea from GLAST-Discosoma red (DsRed) promoter reporter mice revealed that DsRed expression was restricted to IPCs and other supporting cells surrounding IHCs. Saturation of transporters by photolysis of caged D-aspartate failed to elicit transporter currents in IHCs, as did local application of D-aspartate to afferent terminals, indicating that neither presynaptic nor postsynaptic membranes are major sites for glutamate removal. These data indicate that GLAST in supporting cells is responsible for transmitter uptake at IHC afferent synapses.

Resveratrol increases glutamate uptake and glutamine synthetase activity in C6 glioma cells

Resveratrol, a phytoalexin found mainly in grapes, is a promising natural product with anti-cancer and cardio-protective activities. Here, we investigated, in C6 glioma cells, the effect of resveratrol on some specific parameters of astrocyte activity (glutamate uptake, glutamine synthetase and secretion of S100B, a neurotrophic cytokine) commonly associated with the protective role of these cells. Cell proliferation was significantly decreased by 8% and 26%, following 24h of treatment with 100 and 250 microM resveratrol. Extracellular S100B increased after 48 h of resveratrol exposure. Short-term resveratrol exposure (from 1 to 100 microM) induced a linear increase in glutamate uptake (up to 50% at 100 microM resveratrol) and in glutamine synthetase activity. Changes in these glial activities can contribute to the protective role of astrocytes in brain injury conditions, reinforcing the putative use of this compound in the therapeutic arsenal against neurodegenerative diseases and ischemic disorders.

Glutamate metabolism in HIV-infected macrophages: implications for the CNS

Central nervous system disorders are still a common complication of human immunodeficiency virus (HIV) infection and can lead to dementia and death. They are mostly the consequences of an inflammatory macrophagic activation and relate to glutamate-mediated excitotoxicity. However, recent studies also suggest neuroprotective aspects of macrophage activation through the expression of glutamate transporters and glutamine synthetase. We thus aimed to study whether HIV infection or activation of macrophages could modulate glutamate metabolism in these cells. We assessed the effect of HIV infection on glutamate transporter expression as well as on glutamate uptake by macrophages and showed that glutamate transport was partially decreased in the course of virus replication, whereas excitatory amino acid transporter-2 (EAAT-2) gene expression was dramatically increased. The consequences of HIV infection on glutamine synthetase were also measured and for the first time we show the functional expression of this key enzyme in macrophages. This expression was repressed during virus production. We then quantified EAAT-1 and EAAT-2 gene expression as well as glutamate uptake in differentially activated macrophages and show that the effects of HIV are not directly related to pro- or anti-inflammatory mediators. Finally, this study shows that glutamate transport by macrophages is less affected than what has been described in astrocytes. Macrophages may thus play a role in neuroprotection against glutamate in the infected brain, through their expression of both EAATs and glutamine synthetase. Because glutamate metabolism by activated macrophages is sensitive to both HIV infection and inflammation, it may thus be of potential interest as a therapeutic target in HIV encephalitis.

Effect of glutamate ingestion on whole-body glutamate turnover in healthy elderly and patients with chronic obstructive pulmonary disease

OBJECTIVE

Decreased whole-body glutamate turnover is found in healthy elderly and in patients with chronic obstructive pulmonary disease (COPD). Glutamate supplementation as an option to increase whole-body glutamate turnover and, hence, glutamate availability has never been investigated. In the present study, we developed a protocol based on repeated glutamate ingestion to increase plasma glutamate concentration to a steady-state level without inducing toxic side effects and to evaluate the effect of repeated glutamate ingestion on whole-body glutamate turnover in patients with COPD and healthy elderly.

METHODS

In part 1, the response of plasma glutamate concentration was determined in young healthy volunteers who repeatedly ingested a glutamate solution. The tolerance of the glutamate drink was evaluated in 26 healthy volunteers by a food tolerance questionnaire. In part 2, eight male patients with COPD and eight healthy elderly ingested the glutamate drink, an isomolar amount of a glutamine drink, or only water to test the effect on plasma glutamate concentration and whole-body glutamate turnover.

RESULTS

In part 1, repeated ingestion of 30 mg of glutamate per kilogram of body weight every 20 min increased plasma glutamate concentration five-fold to steady-state level within 80 min and without any side effects. In part 2, repeated ingestion of glutamate significantly increased whole-body glutamate turnover in healthy controls and patients with COPD, although the increase was smaller in patients with COPD than in controls.

CONCLUSION

We found that repeated ingestion of 30 mg of glutamate per kilogram of body weight every 20 min can increase glutamate availability in healthy elderly and patients with COPD, who are likely more dependent on external glutamate ingestion than are young adults.

Regulation of astrocytic glutamate transporter expression by Akt: evidence for a selective transcriptional effect on the GLT-1/EAAT2 subtype

In the nervous system, astrocytes express different ratios of the two glial glutamate transporters, glutamate transporter subtype 1 (GLT-1) and glutamate/aspartate transporter (GLAST), but little is known about the signaling pathways that independently regulate their expression. Treatment with dibutyryl-cAMP, epidermal growth factor (EGF) or other growth factors both induces expression of GLT-1 and increases expression of GLAST in astrocyte cultures. The induction of GLT-1 is correlated with morphological and biochemical changes that are consistent with astrocyte maturation. Pharmacological studies suggest that phosphatidylinositol 3-kinase (PI-3K) and the nuclear transcription factor-kappaB (NF-kappaB) may be involved in the induction of GLT-1 expression. In several signaling systems Akt, also known as protein kinase B (PKB), functions downstream of PI-3K. In these present studies we used lentiviral vectors engineered to express dominant-negative (DN), constitutively active (CA), or null variants of Akt to study the possible involvement of Akt in the regulation of GLT-1. Expression of DN-Akt attenuated the EGF-dependent induction of GLT-1. Expression of CA-Akt caused a dose- and time-dependent increase in GLT-1 protein, increased GLT-1 mRNA levels, increased dihydrokainate-sensitive (presumably GLT-1 mediated) transport activity, and caused a change in astrocyte morphology to a more stellate shape, but had no effect on GLAST protein levels. Finally, the expression of CA-Akt increased the expression of a reporter construct containing a putative promoter fragment from the human homolog of GLT-1, called EAAT2. From these studies, we conclude that Akt induces the expression of GLT-1 through increased transcription and that Akt can regulate GLT-1 expression without increasing GLAST expression in astrocytes.

Galphao2 regulates vesicular glutamate transporter activity by changing its chloride dependence

Classical neurotransmitters, including monoamines, acetylcholine, glutamate, GABA, and glycine, are loaded into synaptic vesicles by means of specific transporters. Vesicular monoamine transporters are under negative regulation by alpha subunits of trimeric G-proteins, including Galpha(o2) and Galpha(q). Furthermore, glutamate uptake, mediated by vesicular glutamate transporters (VGLUTs), is decreased by the nonhydrolysable GTP-analog guanylylimidodiphosphate. Using mutant mice lacking various Galpha subunits, including Galpha(o1), Galpha(o2), Galpha(q), and Galpha11, and a Galpha(o2)-specific monoclonal antibody, we now show that VGLUTs are exclusively regulated by Galpha(o2). G-protein activation does not affect the electrochemical proton gradient serving as driving force for neurotransmitter uptake; rather, Galpha(o2) exerts its action by specifically affecting the chloride dependence of VGLUTs. All VGLUTs show maximal activity at approximately 5 mm chloride. Activated Galpha(o2) shifts this maximum to lower chloride concentrations. In contrast, glutamate uptake by vesicles isolated from Galpha(o2-/-) mice have completely lost chloride activation. Thus, Galpha(o2) acts on a putative regulatory chloride binding domain that appears to modulate transport activity of vesicular glutamate transporters.

Downregulation of Kir4.1 inward rectifying potassium channel subunits by RNAi impairs potassium transfer and glutamate uptake by cultured cortical astrocytes

Glial cell-mediated potassium and glutamate homeostases play important roles in the regulation of neuronal excitability. Diminished potassium and glutamate buffering capabilities of astrocytes result in hyperexcitability of neurons and abnormal synaptic transmission. The role of the different K+ channels in maintaining the membrane potential and buffering capabilities of cortical astrocytes has not yet been definitively determined due to the lack of specific K+ channel blockers. The purpose of the present study was to assess the role of the inward-rectifying K+ channel subunit Kir4.1 on potassium fluxes, glutamate uptake and membrane potential in cultured rat cortical astrocytes using RNAi, whole-cell patch clamp and a colorimetric assay. The membrane potentials of control cortical astrocytes had a bimodal distribution with peaks at -68 and -41 mV. This distribution became unimodal after knockdown of Kir4.1, with the mean membrane potential being shifted in the depolarizing direction (peak at -45 mV). The ability of Kir4.1-suppressed cells to mediate transmembrane potassium flow, as measured by the current response to voltage ramps or sequential application of different extracellular [K+], was dramatically impaired. In addition, glutamate uptake was inhibited by knock-down of Kir4.1-containing channels by RNA interference as well as by blockade of Kir channels with barium (100 microM). Together, these data indicate that Kir4.1 channels are primarily responsible for significant hyperpolarization of cortical astrocytes and are likely to play a major role in potassium buffering. Significant inhibition of glutamate clearance in astrocytes with knock-down of Kir4.1 highlights the role of membrane hyperpolarization in this process.

Vesicular storage and secretion of L-glutamate from glucagon-like peptide 1-secreting clonal intestinal L cells

Vesicular glutamate transporter (VGLUT) is responsible for the vesicular storage of l-glutamate, and plays an essential role in glutamate-mediated intercellular signal transmission in the CNS and in some neuroendocrine cells. Intestinal L cells are the glucose-responsive neuroendocrine cells responsible for the secretion of glucagon-like peptide 1 (GLP-1). We have shown that intestinal L cells express VGLUT2, a VGLUT isoform, which suggests that L cells secrete L-glutamate. In the present study, we investigated this possibility using GLUTag mouse clonal L cells. RT-PCR and northern blot analyses revealed expression of the VGLUT1 and VGLUT2 genes, but not of the VGLUT3 gene. Western blot analysis revealed immunological counterparts for VGLUT2, whereas an immunological counterpart of VGLUT1 was not detected. Indirect immunofluorescence microscopy revealed a punctate distribution of VGLUT2 immunoreactivity throughout the cells, which co-localized with GLP-1. Double-labeling immunoelectronmicroscopy confirmed the association of VGLUT2 with GLP-1-containing secretory granules. The membrane fraction exhibited ATP-dependent L-glutamate uptake, which was sensitive to bafilomycin A1 (a vacuolar proton ATPase inhibitor) and Evans blue (a VGLUT inhibitor) but insensitive to D,L-aspartate. Upon depolarization with KCl, GLUTag cells secreted appreciable amounts of L-glutamate and GLP-1. D-Glucose and methyl-alpha-D-glucopyranoside, stimulators of exocytosis of GLP-1, also triggered the secretion of L-glutamate. The L-glutamate secretion was partially dependent on Ca2+ and sensitive to bafilomycin A1. These results demonstrated that GLUTag cells stored L-glutamate in secretory granules and secreted it with GLP-1 by exocytosis. As GLUTag cells and intestinal L cells express kainate receptors and plasma membrane glutamate transporters, these results support the concept of L-glutamate-mediated intercellular signaling in the vicinity of intestinal L cells.

The in vitro uptake of glutamate in GLAST and GLT-1 transfected mutant CHO-K1 cells is inhibited by manganese

In the central nervous system (CNS), extracellular concentrations of amino acids (e.g., aspartate, glutamate) and divalent metals (e.g., zinc, copper, manganese) are primarily regulated by astrocytes. Adequate glutamate homeostasis and control over extracellular concentrations of these excitotoxic amino acids are essential for the normal functioning of the brain. Not only is glutamate of central importance for nitrogen metabolism but, along with aspartate, it is the primary mediator of excitatory pathways in the brain. Similarly, the maintenance of proper Mn levels is important for normal brain function. Brain glutamate is removed from the extracellular fluid mainly by astrocytes via high affinity astroglial Na+-dependent excitatory amino acid transporters, glutamate/aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1). The effects of Mn on specific glutamate transporters have yet to be determined. As a first step in this process, we examined the effects of Mn on the transport of [D-2, 3-3H]D-aspartate, a non-metabolizable glutamate analog, in Chinese hamster ovary cells (CHO) transfected with two glutamate transporter subtypes, GLAST (EAAT1) or GLT-1 (EAAT2). Mn-mediated inhibition of glutamate transport in the CHO-K1 cell line DdB7 was pronounced in both the GLT-1 and GLAST transfected cells. This resulted in a statistically significant inhibition (p<0.05) of glutamate uptake compared with transfected control in the absence of Mn treatment. These studies suggest that Mn accumulation in the CNS might contribute to dysregulation of glutamate homeostasis.

MRS assessment of glutamate clearance in a novel masticatory muscle pain model

The injection of 1.0 M glutamate into the masseter (jaw-closer) muscle results in a short period of muscle pain (5-10 min) and a prolonged period of mechanical sensitization (> 30 min). It is unclear, however, whether there is a temporal relationship between intramuscular glutamate concentration and either muscle pain or mechanical sensitization. In the present study, (1)H MRS and electrophysiological recording of masticatory muscle nerve fibers were performed in order to monitor glutamate clearance and nerve fiber activity, respectively, after injection of glutamate into rat masticatory muscles. Glutamate signal amplitude was found to decay rapidly (half-life t 1/2 = 108 +/- 42 s), and became indistinguishable from the baseline 10 min after the injection. Glutamate-evoked nerve fiber activity was also found to decay rapidly (t 1/2 = 76 +/- 28 s). These results suggest that glutamate clearance correlates well with the time course of glutamate-evoked muscle pain fiber discharge.

Effect of chronic gestational treatment with caffeine or theophylline on Group I metabotropic glutamate receptors in maternal and fetal brain

Pregnant rats were treated throughout the gestational period with either caffeine or theophylline, and its effect on the metabotropic glutamate receptor (mGluRs) signal transduction pathway was studied in both maternal and fetal brain. In maternal brain, radioligand binding assays showed that chronic treatment with methylxanthines caused a significant decrease in the total number of mGluRs. This decrease was accompanied by an increase in receptor affinity. Immunodetection showed that mGluR1a and phospholipase C beta1 (PLCbeta1) were significantly decreased in response to chronic methylxanthine treatment, whereas alphaG(q/11) was not affected. A loss was also detected of PLC stimulation mediated by (S)-3,5-dihydroxyphenylglycine (DHPG), a selective Group I mGluR agonist, suggesting desensitization of the mGluR/PLC pathway. In fetal brain, a loss in total mGluRs was observed in fetuses from mothers treated with caffeine or theophylline, without variation in receptor affinity. A decrease in mGluR1a, alphaG(q/11) and PLCbeta1 levels was also observed in response to treatment. However, changes detected in this immature tissue were not associated with variations in PLC activity. These results suggest that chronic caffeine or theophylline treatment down-regulates several mGluR/PLC transduction pathway components in both maternal and fetal brain, causing a loss of receptor responsiveness only in maternal brain.

Guanosine Enhances Glutamate Transport Capacity in Brain Cortical Slices

1. The effect of guanosine on L-[3H] glutamate uptake was investigated in brain cortical slices within physio-pathological range of glutamate(1-1000 microM). In these conditions, glutamate uptake was significantly enhanced in slices treated with 100 microM guanosine only at 100 and 300 microM glutamate (44 and 52%, respectively). 2. Evaluation of kinetic parameters showed that guanosine affected significantly only uptake Vmax (23%). 3. The guanosine withdrawal did not abolish its significant effect on glutamate uptake when 100 or 300 microM glutamate were used (an increase of 66 and 35%, respectively). 4. These results support the hypothesis of a protective role for guanosine during excitotoxic conditions when glutamate levels are enhanced (e.g. brain ischemia and seizures), possibly by activating glutamate uptake. Moreover, our results may contribute to understand the antiexcitotoxic mechanism of guanosine on glutamate transport, giving new information concerning its mechanism of action.

Cystine and glutamate transport in renal epithelial cells transfected with human system x(-) (c)

BACKGROUND

System x(-) (c) is a heterodimeric transporter, comprised of a light chain, xCT, and heavy chain, 4F2hc, which mediates the sodium-independent exchange of cystine and glutamate at the plasma membrane. In the current study we tested the hypothesis that stable transfection of Madin-Darby canine kidney (MDCK) cells with human xCT and 4F2hc results in the expression of functional system x(-) (c).

METHODS

MDCK cells were transfected stably with human clones for xCT and 4F2hc. Analyses of time- and temperature-dependence, saturation kinetics, and substrate specificity of l-cystine and l-glutamate transport were carried out in control and xCT-4F2hc-transfected MDCK cells. We also measured the uptake of l-cystine in Xenopus oocytes expressing human xCT and/or 4F2hc or xCT and/or rBAT (a heavy chain homologous to 4F2hc).

RESULTS

All of the different sets of data revealed that transport of l-cystine and l-glutamate increased significantly (twofold to threefold) in the MDCK cells subsequent to transfection with xCT-4F2hc. Moreover, uptake of l-cystine also increased (about tenfold) in Xenopus oocytes expressing hxCT and h4F2hc. Biochemical analyses of l-cystine uptake in oocytes verified our findings in the transfected MDCK cells. Interestingly, in oocytes injected with rBAT with or without xCT, uptake of l-cystine was significantly greater than that in water-injected oocytes.

CONCLUSION

Our findings indicate that stable transfection of MDCK cells with xCT and 4F2hc results in a cell-line expressing a functional system x(-) (c) transporter that can utilize l-cystine and l-glutamate as substrates. This study apparently represents the first stable transfection of a mammalian cell line with system x(-) (c).

Substrate-induced modulation of glutamate uptake in human platelets

In the central nervous system (CNS), glutamate rapidly upregulates the activities of different excitatory amino-acid transporter subtypes (EAATs) in order to help protect neurons from excitotoxicity. Since human platelets display a specific sodium-dependent glutamate uptake activity, and express the three major glutamate transporters, which may be affected in neurological disorders, we investigated whether platelets are subject to substrate-induced modulation as described for CNS. A time- and dose-dependent upregulation of [3H]-glutamate uptake (up to two-fold) was observed in platelets preincubated with glutamate. There was an increase in maximal velocity rate without affinity changes. Glutamate receptor agonists and antagonists did not modulate this upregulation and preincubation with glutamate analogues failed to mimic the glutamate effect. Only aspartate preincubation increased the uptake, albeit approximately 35% less with respect to glutamate. The effect of glutamate preincubation on the expression of the three major transporters was studied by Western blotting, showing an increase of approximately 70% in EAAT1 immunoreactivity that was completely blocked by cycloheximide (CEM). However, L-serine-O-sulphate, at a concentration (200 microM) known to block EAAT1/3 selectively, did not completely inhibit the effect of glutamate stimulation, indicating the possible involvement of EAAT2. In fact, glutamate stimulation was completely abolished only when, following CEM pre-incubation, the experiment was run in the presence of the selective EAAT2 inhibitor dihydrokainic acid. Since surface biotinylation experiments failed to show evidence of EAAT2 translocation, our results suggest the existence of a different way of regulating EAAT2 activity. These findings indicate that human platelets display a substrate-dependent modulation of glutamate uptake mediated by different molecular mechanisms and confirm that ex vivo platelets are a reliable model to investigate the dysfunction of glutamate uptake regulation in patients affected by neurological disorders.

Fibroblast glutamate transport in aging and in AD: correlations with disease severity

Altered glutamate transport and aberrant EAAT1 expression were shown in Alzheimer's disease (AD) brains. It is presently unknown whether these modifications are a consequence of neurodegeneration or play a pathogenetic role. However, recent findings of decreased glutamate uptake, EAAT1 protein and mRNA in AD platelets suggest that glutamate transporter modifications may be systemic and might explain the decreased glutamate uptake. We now used primary fibroblast cultures from 10 AD patients to further investigate the specific involvement of glutamate transporters in this disorder and in normal aging. Decreased glutamate uptake (p<0.001), EAAT1 expression (p<0.05) and mRNA (p<0.01) were observed in aged people, compared to younger controls. In AD fibroblasts, compared to age-matched controls, we observed further reductions of glutamate uptake (p<0.0005) and EAAT1 expression (p<0.005), while EAAT1 mRNA increase (p<0.001) was shown. EAAT1 parameters were mutually correlated (p<0.01) and correlations were shown with dementia severity (p<0.05 MMSE-expression, p<0.005 MMSE-mRNA). We suggest fibroblast cultures as possible ex vivo peripheral model to study the glutamate involvement and possible molecular and therapeutic targets in AD.

Differential effects of aging on NMDA receptors in the intermediate versus the dorsal hippocampus

There are discrepancies in the literature about the effects of aging on NMDA receptor expression in the hippocampus. The present study was designed to determine whether there are regional differences in how NMDA receptors are affected by aging. Brains from male C57BL/6 mice from three different age groups (3, 10, and 30 months of age) were sectioned coronally through the dorsal hippocampus and horizontally through the intermediate hippocampus. Sections were processed and analyzed for [3H]glutamate binding to NMDA receptors using receptor autoradiography and for mRNA for the zeta1 (NR1), epsilon1 (NR2A), and epsilon2 (NR2B) subunits of the NMDA receptor using in situ hybridization. There were more significant effects of aging within the intermediate hippocampus in NMDA-displaceable [3H]glutamate binding and epsilon2 mRNA densities than were seen in the dorsal hippocampus. There was no significant effect of age on densities of either the zeta1 or epsilon1 subunit mRNA. These results suggest that the aging process affects NMDA receptors more in the intermediate hippocampus than the dorsal.

Glutamate uptake is attenuated in spinal deep dorsal and ventral horn in the rat spinal nerve ligation model

Alteration of glutamatergic (GLU) neurotransmission within the spinal cord contributes to hyperalgesic and allodynic responses following nerve injury. In particular, changes in expression and efficacy of glutamate transporters have been reported. Excitatory, pain transmitting primary afferent neurons utilizing glutamate as an excitatory neurotransmitter project to both superficial (I-II) and deep (III-V) laminae of the dorsal horn. These experiments were designed to examine changes in glutamate uptake occurring concomitantly within the spinal deep dorsal and ventral horn in situ after experimentally induced neuropathic pain. In vivo voltammetry, using microelectrode arrays configured for enzyme-based detection of GLU were employed. Sprague-Dawley rats had either sham surgery or tight ligation of L5 and L6 spinal nerves (SNL). Four to six weeks later, the L4-L6 spinal cord of chloral hydrate-anesthetized animals was exposed, and ceramic-based glutamate microelectrodes equipped with glass micropipettes 50 microm from the recording surfaces were placed stereotaxically at sites within the spinal cord. Pressure ejection of GLU into the ipsilateral L5-L6 spinal cord resulted in a 72% reduction of GLU uptake in SNL rats compared to sham controls in the ipsilateral L5-L6 deep dorsal horn and a 96% reduction in the ventral horn. In contrast, in the same animals, the contralateral L5-L6 or the ipsilateral L4 spinal cord showed no change in glutamate uptake. The data suggest that spinal nerve ligation produced attenuated glutamate uptake activity extending into the deep dorsal and ventral horn. The study suggests that plasticity related to spinal nerve injury produces widespread alteration in glutamate transporter function that may contribute to the pathophysiology of neuropathic pain.

Impaired mitochondrial glutamate transport in autosomal recessive neonatal myoclonic epilepsy

Severe neonatal epilepsies with suppression-burst pattern are epileptic syndromes with either neonatal onset or onset during the first months of life. These disorders are characterized by a typical electroencephalogram pattern--namely, suppression burst, in which higher-voltage bursts of slow waves mixed with multifocal spikes alternate with isoelectric suppression phases. Here, we report the genetic mapping of an autosomal recessive form of this condition to chromosome 11p15.5 and the identification of a missense mutation (p.Pro206Leu) in the gene encoding one of the two mitochondrial glutamate/H(+) symporters (SLC25A22, also known as "GC1"). The mutation cosegregated with the disease and altered a highly conserved amino acid. Functional analyses showed that glutamate oxidation in cultured skin fibroblasts from patients was strongly defective. Further studies in reconstituted proteoliposomes showed defective [(14)C]glutamate uniport and [(14)C]glutamate/glutamate exchange by mutant protein. Moreover, expression studies showed that, during human development, SLC25A22 is specifically expressed in the brain, within territories proposed to contribute to the genesis and control of myoclonic seizures. These findings provide the first direct molecular link between glutamate mitochondrial metabolism and myoclonic epilepsy and suggest potential insights into the pathophysiological bases of severe neonatal epilepsies with suppression-burst pattern.

Lipopolysaccharide increases microglial GLT-1 expression and glutamate uptake capacity in vitro by a mechanism dependent on TNF-α

This study investigates the effect of microglial activation on microglial glutamate transporters in vitro. Stimuli known to activate microglia and/or to be associated with pathological conditions with an impaired astroglial glutamate uptake were compared. Morphological changes and marked increases in ED1 antigen expression were found after 8-h incubation of rat microglia in 56 mM KCl, 1 ng/ml lipopolysaccharide (LPS), and 100 microM glutamate, as well as in acidic and basic conditions, showing that the cells were activated. Of the stimuli used, only LPS induced a significant release of the proinflammatory cytokines tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6), and was the only stimulus that increased microglial GLT-1 expression and glutamate uptake capacity after 12-h incubation. This effect was probably mediated by TNF-alpha, since this cytokine mimicked the effect of LPS. Furthermore, the effect of LPS was blocked by thalidomide, an inhibitor of TNF-alpha synthesis. Additionally, neutralizing antibodies against TNF-alpha also blocked the increase, indicating TNF-alpha as an inducer of GLT-1 expression in microglia. It was also found that preincubation with glutamate dose-dependently inhibited the microglial glutamate uptake. This could suggest different physiological functions for microglial and astroglial glutamate uptake and might indicate a reciprocal control of GLT-1 expression between microglia and astrocytes.

In vitro uptake of glutamate in GLAST- and GLT-1-transfected mutant CHO-K1 cells is inhibited by the ethylmercury-containing preservative thimerosal

Thimerosal, also known as thimersal, Merthrolate, or sodiumethyl-mercurithiosalicylate, is an organic mercurial compound that is used in a variety of commercial as well as biomedical applications. As a preservative, it is used in a number of vaccines and pharmaceutical products. Its active ingredient is ethylmercury. Both inorganic and organic mercurials are known to interfere with glutamate homeostasis. Brain glutamate is removed mainly by astrocytes from the extracellular fluid via high-affinity astroglial Na+-dependent excitatory amino acid transporters, glutamate/ aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1). The effects of thimerosal on glutamate homeostasis have yet to be determined. As a first step in this process, we examined the effects of thimerosal on the transport of [3H]-d-aspartate, a nonmetabolizable glutamate analog, in Chinese hamster ovary (CHO) cells transfected with two glutamate transporter subtypes, GLAST (EAAT1) and GLT-1 (EAAT2). Additionally, studies were undertaken to determine the effects of thimerosal on mRNA and protein levels of these transporters. The results indicate that thimerosal treatment caused significant but selective changes in both glutamate transporter mRNA and protein expression in CHO cells. Thimerosal-mediated inhibition of glutamate transport in the CHO-K1 cell line DdB7 was more pronounced in the GLT-1-transfected cells compared with the GLAST- transfected cells. These studies suggest that thimerosal accumulation in the central nervous system might contribute to dysregulation of glutamate homeostasis.

Acute reserpine and subchronic haloperidol treatments change synaptosomal brain glutamate uptake and elicit orofacial dyskinesia in rats

Reserpine- and haloperidol-induced orofacial dyskinesia are putative animal models of tardive dyskinesia (TD) whose pathophysiology has been related to free radical generation and oxidative stress. In the present study, the authors induced orofacial dyskinesia by acute reserpine and subchronic haloperidol administration to rats. Reserpine injection (one dose of 1 mg/kg s.c.) every other day for 3 days caused a significant increase in vacuous chewing, tongue protrusion and duration of facial twitching, compared to the control. Haloperidol administration (one dose of 12 mg/kg once a week s.c.) for 4 weeks caused an increase in vacuous chewing, tongue protrusion and duration of facial twitching observed in four weekly evaluations. After the treatments and behavioral observation, glutamate uptake by segments of the brain was analyzed. A decreased glutamate uptake was observed in the subcortical parts of animals treated with reserpine and haloperidol, compared to the control. Importantly, a decrease in glutamate uptake correlates negatively with an increase in the incidence of orofacial diskinesia. These results indicate that early changes in glutamate transport may be related to the development of vacuous chewing movements in rats.

Kinetic and Pharmacological Characterization of Desensitizing and Non-desensitizing Glutamate-gated Chloride Channels in Cockroach Neurons

Glutamate-gated chloride channels (GluCls) are found only in invertebrate nerve and muscle, where they mediate inhibitory synaptic transmission, and are important target sites of insecticides. Two GluCl subtypes have previously been distinguished in isolated cockroach CNS neurons based on differential pharmacology. The present study characterizes the kinetics and pharmacological properties of desensitizing and non-desensitizing GluCls. Both types of GluCls were sensitive to glutamate and ibotenic acid. The non-desensitizing GluCl subtype was elicited by glutamate with an EC(50) of 115.8 microM and a Hill coefficient of 2.6 and was also sensitive to the agonist ibotenic acid with an EC(50) of 42 microM and a Hill coefficient of 1.7. The desensitizing and non-desensitizing currents were carried by chloride ions, and occurred either separately or in combination in individual neurons. The GluCls were also found to coexist with and function independently of the GABA-activated chloride channels. The desensitizing and non-desensitizing GluCls exhibited different sensitivities to the ligand-gated channel blocker picrotoxinin. The desensitizing GluCls were blocked only 8% by 30 microM picrotoxinin, whereas the non-desensitizing GluCls were potently blocked by picrotoxinin with an IC(50) of 4.1 microM. The insecticides fipronil and dieldrin at 1 microM inhibited the desensitizing currents by 56 and 13%, respectively, and the non-desensitizing currents by 98 and 43%, respectively. It is concluded that the two types of GluCls found in cockroach neurons exhibit significantly different electrophysiological and pharmacological characteristics.

Hydrogen peroxide potentiates volume-sensitive excitatory amino acid release via a mechanism involving Ca2+/calmodulin-dependent protein kinase II

Excessive excitatory amino acid (EAA) release in cerebral ischemia is a major mechanism responsible for neuronal damage and death. A substantial fraction of ischemic EAA release occurs via volume-regulated anion channels (VRACs). Hydrogen peroxide (H2O2), which is abundantly produced during ischemia and reperfusion, activates a number of protein kinases critical for VRAC functioning and has recently been reported to activate VRACs. In the present study, we explored the effects of H2O2 on volume-dependent EAA release in cultured astrocytes, measured as the release of preloaded D-[3H]aspartate. 100-1,000 microm H2O2 enhanced swelling-induced EAA release by approximately 2.5-3-fold (EC50 approximately 10 microM). The VRAC blockers ATP, phloretin, and 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) potently inhibited both control swelling-induced and the H2O2-potentiated release, suggesting a role for VRACs. The H2O2-induced component of EAA release was attenuated by the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester (BAPTA-AM) and completely eliminated by the calmodulin antagonists trifluoperazine and W-7 and the Ca2+/calmodulin-dependent protein kinase II (CaMKII) inhibitor KN-93. Inhibitors of tyrosine kinases, protein kinase C, and the myosin light chain kinase were ineffective in blocking the H2O2 response. H2O2 treatment of swollen astrocytes, but not swelling alone, resulted in CaMKII activation that was inhibited by KN-93, as determined by a phospho-Thr286 CaMKII antibody. These data demonstrate that H2O2 strongly up-regulates astrocytic volume-sensitive EAA release via a CaMKII-dependent mechanism and in this way may potently promote pathological EAA release and brain damage in ischemia.

Biosynthesis of NAAG by an enzyme-mediated process in rat central nervous system neurons and glia

The peptide transmitter N-acetylaspartylglutamate (NAAG) is present in millimolar concentrations in mammalian spinal cord. Data from the rat peripheral nervous system suggest that this peptide is synthesized enzymatically, a process that would be unique for mammalian neuropeptides. To test this hypothesis in the mammalian CNS, rat spinal cords were acutely isolated and used to study the incorporation of radiolabeled amino acids into NAAG. Consistent with the action of a NAAG synthetase, inhibition of protein synthesis did not affect radiolabel incorporation into NAAG. Depolarization of spinal cords stimulated incorporation of radiolabel. Biosynthesis of NAAG by cortical astrocytes in cell culture was demonstrated by tracing incorporation of [3H]-glutamate by astrocytes. In the first test of the hypothesis that NAA is an immediate precursor in NAAG biosynthesis, [3H]-NAA was incorporated into NAAG by isolated spinal cords and by cell cultures of cortical astrocytes. Data from cerebellar neurons and glia in primary culture confirmed the predominance of neuronal synthesis and glial uptake of NAA, leading to the hypothesis that while neurons synthesize NAA for NAAG biosynthesis, glia may take it up from the extracellular space. However, cortical astrocytes in serum-free low-density cell culture incorporated [3H]-aspartate into NAAG, a result indicating that under some conditions these cells may also synthesize NAA. Pre-incubation of isolated spinal cords and cultures of rat cortical astrocytes with unlabeled NAA increased [3H]-glutamate incorporation into NAAG. In contrast, [3H]-glutamine incorporation in spinal cord was not stimulated by unlabeled NAA. These results are consistent with the glutamate-glutamine cycle greatly favoring uptake of glutamine into neurons and glutamate by glia and suggest that NAA availability may be rate-limiting in the synthesis of NAAG by glia under some conditions.

Quinolinic acid promotes seizures and decreases glutamate uptake in young rats: reversal by orally administered guanosine

Quinolinic acid (QA) has been used as a model for experimental overstimulation of the glutamatergic system. Glutamate uptake is the main mechanism involved in the maintenance of extracellular glutamate below toxic levels. Guanosine systemically administered prevents quinolinic acid-induced seizures in adult mice and increases basal glutamate uptake by cortical astrocyte culture and slices from young rats. The immature brain differs from the adult brain in its susceptibility to seizures, seizure characteristics, and responses to antiepileptic drugs (AED). Here we investigated the effect of guanosine p.o. on QA-induced seizures in young rats (P12-14) and upon ex vivo glutamate uptake by cortical slices from these animals. I.c.v. infusion of 250 nmol QA induced seizures in all animals and decreased glutamate uptake. I.p. injection of MK-801 and phenobarbital 30 min before QA administration prevented seizures in all animals. Guanosine (7.5 mg/kg) 75 min before QA prevented seizures in 50% of animals as well as prevented the decrease of glutamate uptake in the protected animals. To investigate if the anticonvulsive effect of guanosine was specific for QA-induced seizures, the picrotoxin-induced seizures model was also performed. Pretreatment with phenobarbital i.p. (60 mg/kg-30 min) prevented picrotoxin-induced seizures in all animals, whereas guanosine p.o. (7.5 mg/kg-75 min) and MK-801 i.p. (0.5 mg/kg-30 min) had no effect. Thus, guanosine protection on the QA-induced seizures in young rats and on the decrease of glutamate uptake showed some specificity degree towards the QA-induced toxicity. This points that guanosine could be considered for treatments of epilepsy, and possibly other neurological disorders in children.