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Notes: Metabotropic glutamate receptors (mGluR) modulate neuronal function. Here, we tested the effect on metabolism of a range of Group I and II mGluR ligands in Guinea pig brain cortical tissue slices, applying 13C NMR spectroscopy and metabolomic analysis using multivariate statistics. The effects of Group I agonists (S)-3,5-dihydroxyphenylglycine (DHPG) and (RS)-2-chloro-5-hydroxyphenylglycine (CHPG) depended upon concentration and were mostly stimulatory, increasing both net metabolic flux through the Krebs cycle and glutamate/glutamine cycle activity. Only the higher (50 µm) concentrations of CHPG had the opposite effect. The Group I antagonist (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA), consistent with its neuroprotective role, caused significant decreases in metabolism. With principal components analysis of the metabolic profiles generated by these ligands, the effects could be separated by two principal components. Agonists at Group II mGluR [(2S,2′R,3′R)-2-(2′,3′-dicarboxycyclopropyl)glycine (DCG IV) and 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate (APDC)] generally stimulated metabolism, including glutamate/glutamine cycling, although this varied with concentration. The antagonist (2S)-α-ethylglutamic acid (EGLU) stimulated astrocyte metabolism with minimal impact on glutamate/glutamine cycling. (RS)-1-Aminophosphoindan-1-carboxylic acid (APICA) decreased metabolism at 5 µm but had a stimulatory effect at 50 µm. All ligand effects were separated from control and from each other using two principal components. The ramifications of these findings are discussed.

Notes: 1. It has been suggested that Na+/K+-ATPase and Na+-dependent glutamate transport (GluT) are tightly linked in brain tissue. In the present study, we have investigated Na+/K+-ATPase activity using Rb+ uptake by ‘minislices’ (prisms) of the cerebral cortex. This preparation preserves the morphology of neurons, synapses and astrocytes and is known to possess potent GluT that has been well characterized. Uptake of Rb+ was determined by estimating Rb+ in aqueous extracts of the minislices, using atomic absorption spectroscopy.2. We determined the potencies of several known substrates/inhibitors of GluT, such as l-trans-pyrrolidine-2,4-dicarboxylate (LtPDC), dl-threo-3-benzyloxyaspartic acid, (2S,3S,4R)-2-(carboxycyclopropyl)-glycine (L-CCG III) and l-anti,endo-3,4-methanopyrrolidine dicarboxylic acid, as inhibitors of [3H]-l-glutamate uptake by cortical prisms. In addition, we established the susceptibility of GluT, measured as [3H]-l-glutamate uptake in brain cortical prisms, to the inhibition of Na+/K+-ATPase by ouabain. Then, we tested the hypothesis that the Na+/K+-ATPase (measured as Rb+ uptake) can respond to changes in the activity of GluT produced by using GluT substrates as GluT-specific pharmacological tools.3. The Na+/K+-ATPase inhibitor ouabain completely blocked Rb+ uptake (IC50 = 17 µmol/L), but it also potently inhibited a fraction of GluT (approximately 50% of [3H]-l-glutamate uptake was eliminated; IC50 〈 1 µmol/L).4. None of the most commonly used GluT substrates and inhibitors, such as l-aspartate, d-aspartate, L-CCG III and LtPDC (all at 500 µmol/L), produced any significant changes in Rb+ uptake.5. The N-methyl-d-aspartate (NMDA) receptor agonists (R,S)-(tetrazol-5-yl)-glycine and NMDA decreased Rb+ uptake in a manner compatible with their known neurotoxic actions.6. None of the agonists or antagonists for any of the other major classes of glutamate receptors caused significant changes in Rb+ uptake.7. We conclude that, even if a subpopulation of glutamate transporters in the rat cerebral cortex may be intimately linked to a fraction of Na+/K+-ATPase, it is not possible, under the present experimental conditions, to detect regulation of Na+/K+-ATPase by GluT.

Notes: The role of glutamine and alanine transport in the recycling of neurotransmitter glutamate was investigated in Guinea pig brain cortical tissue slices and prisms, and in cultured neuroblastoma and astrocyte cell lines. The ability of exogenous (2 mm) glutamine to displace 13C label supplied as [3-13C]pyruvate, [2-13C]acetate, l-[3-13C]lactate, or d-[1-13C]glucose was investigated using NMR spectroscopy. Glutamine transport was inhibited in slices under quiescent or depolarising conditions using histidine, which shares most transport routes with glutamine, or 2-(methylamino)isobutyric acid (MeAIB), a specific inhibitor of the neuronal system A. Glutamine mainly entered a large, slow turnover pool, probably located in neurons, which did not interact with the glutamate/glutamine neurotransmitter cycle. This uptake was inhibited by MeAIB. When [1-13C]glucose was used as substrate, glutamate/glutamine cycle turnover was inhibited by histidine but not MeAIB, suggesting that neuronal system A may not play a prominent role in neurotransmitter cycling. When transport was blocked by histidine under depolarising conditions, neurotransmitter pools were depleted, showing that glutamine transport is essential for maintenance of glutamate, GABA and alanine pools. Alanine labelling and release were decreased by histidine, showing that alanine was released from neurons and returned to astrocytes. The resultant implications for metabolic compartmentation and regulation of metabolism by transport processes are discussed.