2-Oxo-[14C]glutarate is taken up by glutamatergic nerve terminals in the rat striatum

Rates of pyruvate carboxylase, glutamate and GABA neurotransmitter cycling, and glucose oxidation in multiple brain regions of the awake rat using a combination of [2-13C]/[1-13C]glucose infusion and 1H-[13C]NMR ex vivo

Anaplerosis occurs predominately in astroglia through the action of pyruvate carboxylase (PC). The rate of PC (Vpc) has been reported for cerebral cortex (or whole brain) of awake humans and anesthetized rodents, but regional brain rates remain largely unknown and, hence, were subjected to investigation in the current study. Awake male rats were infused with either [2-¹³C]glucose or [1-¹³C]glucose (n = 27/30) for 8, 15, 30, 60 or 120 min, followed by rapid euthanasia with focused-beam microwave irradiation to the brain. Blood plasma and extracts of cerebellum, hippocampus, striatum, and cerebral cortex were analyzed by ¹H-[¹³C]-NMR to establish ¹³C-enrichment time courses for glutamate-C4,C3,C2, glutamine-C4,C3, GABA-C2,C3,C4 and aspartate-C2,C3. Metabolic rates were determined by fitting a three-compartment metabolic model (glutamatergic and GABAergic neurons and astroglia) to the eighteen time courses. Vpc varied by 44% across brain regions, being lowest in the cerebellum (0.087 ± 0.004 µmol/g/min) and highest in striatum (0.125 ± 0.009) with intermediate values in cerebral cortex (0.106 ± 0.005) and hippocampus (0.114 ± 0.005). Vpc constituted 13-19% of the oxidative glucose consumption rate. Combination of cerebral cortical data with literature values revealed a positive correlation between Vpc and the rates of glutamate/glutamine-cycling and oxidative glucose consumption, respectively, consistent with earlier observations.

Glutamate-synthesizing enzymes in GABAergic neurons of the neocortex: a double immunofluorescence study in the rat

L-Glutamate is the immediate precursor of the inhibitory transmitter GABA, and considered to be supplied from alpha-ketoglutarate through a transamination reaction or from glutamine through a glutaminase reaction. In the present study, the localization of aspartate aminotransferase and glutaminase in GABAergic neurons was investigated in the rat neocortex by a double immunofluorescence method. Immunoreactivities for both soluble and mitochondrial aspartate aminotransferases were detected in more than 90% of GABA-positive neurons, whereas glutaminase immunoreactivity was not found in GABA-positive neurons. All neocortical neurons with soluble aspartate aminotransferase immunoreactivity were immunopositive for GABA, but none for glutaminase. Neurons with mitochondrial aspartate aminotransferase immunoreactivity showed either glutaminase or GABA immunoreactivity. Under confocal laser scan microscopy, immunoreactivity for soluble aspartate aminotransferase was observed in many axons and axon terminals showing immunoreactivity for glutamic acid decarboxylase, whereas immunoreactivity for mitochondrial aspartate aminotransferase was seen in only a few axons displaying immunoreactivity for glutamic acid decarboxylase. The present results indicate that soluble aspartate aminotransferase is selectively localized to cell bodies and axon terminals of GABAergic non-pyramidal neurons in the cerebral neocortex. This suggests that glutamate is supplied from alpha-ketoglutarate via transamination and works as the immediate precursor for GABA in axon terminals of GABAergic neurons. The absence of glutaminase immunoreactivity in GABAergic neurons indicates that glutamine is a "metabolically remote" precursor for GABA. Mitochondrial aspartate aminotransferase was located in perikarya, rather than in axon terminals of GABAergic neurons, suggesting a transmitter-irrelevant role of this enzyme in neurons.

The NMDA receptor complex

The synaptic responses elicited by glutamate and aspartate in the CNS are mediated by distinct groups of receptors which include the ionotropic NMDA receptor. The NMDA receptor is activated by high-strength synaptic input and produces relatively sustained depolarization which can lead to repetitive burst firing. These characteristics allow it to be involved in the maintainance of rhythmic neuronal activity and in the modulation of synaptic efficacy and plasticity. Overstimulation of the NMDA receptor appears to play a pivotal role in the physiopathology of ischemic brain injury. The NMDA receptor contains an integral cationic channel which is highly permeable to Ca2+ as well as to Na+ and K+. This receptor has several domains in addition to the NMDA recognition site: i) a divalent cation binding site within the channel pore, at which Mg2+ ions bind, ii) a binding site recognized by dissociative anesthetics and MK-801 within the channel; and iii) modulatory sites sensitive to glycine, Zn2+ and polyamines. The NMDA receptor is strictly controlled by Mg2+ ions in a voltage-dependent manner. Moreover, it is modulated by protons, by changes in the redox state and by endogenous physiological substances, eg NO and arachidonic acid. Selective antagonists now exist for the NMDA recognition site and glycine and polyamine modulatory sites. Molecular cloning of the NMDA receptor has identified a subunit termed NMDA-R1 and four additional subunits (NMDA-R2A through NMDA-R2D). Functionally distinct NMDA receptor subtypes are formed by heteromeric assembly of NMDA-R1 with NMDA-R2 subunits. NMDA receptor subunits contain consensus phosphorylation sites for protein kinases at the cytoplasmic domain. The high Ca2+ permeability and sensitivity to channel block by Mg2+ are imparted by asparagine residues in a putative channel-forming segment of the protein transmembrane 2. The knowledge of the molecular structure of the NMDA receptor will help to understand the molecular mechanisms responsible for its regulatory features and the molecular bases of neurotoxicity.

Uptake and metabolism of glutamate and aspartate by astroglial and neuronal preparations of rat cerebellum

Astrocytes, neuronal perikarya and synaptosomes were prepared from rat cerebellum. Kinetics of high and low affinity uptake systems of glutamate and aspartate, nominal rates of 14CO2 production from [U-14C]glutamate, [U-14C]aspartate and [1-14C]glutamate and activities of enzymes of glutamate metabolism were studied in these preparations. The rate of uptake and the nomial rate of production of 14CO2 from these amino acids was higher in the astroglia than neuronal perikarya and synaptosomes. Activities of glutamine synthetase and glutamate dehydrogenase were higher in astrocytes than in neuronal perikarya and synaptosomes. Activities of glutaminase and glutamic acid decarboxylase were observed to be highest in neuronal perikarya and synaptosomes respectively. These results are in agreement with the postulates of theory of metabolic compartmentation of glutamate while others (presence of glutaminase in astrocytes and glutamine synthetase in synaptosomes) are not. Results of this study also indicated that (i) at high extracellular concentrations, glutamate/aspartate uptake may be predominantly into astrocytes while at low extracellular concentrations, it would be into neurons (ii) production of alpha-ketoglutarate from glutamate is chiefly by way of transamination but not by oxidative deamination in these three preparations and (iii) there are topographical differences glutamate metabolism within the neurons.

Aspartate aminotransferase, malate dehydrogenase, and pyruvate carboxylase activities in rat cerebral synaptic and nonsynaptic mitochondria: effects of in vitro treatment with ammonia, hyperammonemia and hepatic encephalopathy

The effects of in vitro treatment with ammonium chloride, hepatic encephalopathy (HE) due to thioacetamide (TAA) induced liver failure and chronic hyperammonemia produced by i.p. administration of ammonium acetate on the activity of the two malate-aspartate shuttle enzymes: aspartate aminotransferase (AAT), malate dehydrogenase (MDH), and on the pyruvate carboxylase (PC) activity were examined in synaptic and nonsynaptic mitochondria from rat brain. With regard to the shuttle enzymes the response to ammonium ions in vitro (3mM NH4Cl) was observed in nonsynaptic mitochondria only, and was manifested by a 27% decrease of AAT activity and a 16% decrease in MDH activity. By contrast, both in vivo conditions primarily affected the synaptic mitochondrial enzymes: TAA-induced HE produced a 26% decrease of synaptic mitochondrial AAT and a 50% decrease of synaptic mitochondrial MDH. Hyperammonemia inhibited synaptic mitochondrial AAT by 30% and synaptic mitochondrial MDH by 45%. HE produced no effect at all in nonsynaptic mitochondria while hyperammonemia produced a 30% increase in the AAT activity, but no changes in MDH. All the experimental conditions affected the nonsynaptic mitochondria PC: ammonium chloride in vitro produced a 20% decrease, TAA-induced HE--a 30% decrease, whereas hyperammonemia inhibited the enzyme by 53%. The PC activity in synaptic mitochondria was very low (about 2% of that measured in nonsynaptic mitochondria), which is consistent with the primarily astrocytic localization of the enzyme.

Activation of arginine metabolism to glutamate in rat brain synaptosomes in thioacetamide-induced hepatic encephalopathy: an adaptative response?

Crude (P2) synaptosomes derived from rats with acute hepatic encephalopathy (HE) induced with thioacetamide showed a slightly increased uptake of radiolabeled arginine (ARG) and a 2.5-fold enhanced conversion of newly taken-up ARG to both glutamate (GLU) and gamma-aminobutyric acid (GABA) as compared with control synaptosomes. Pulse treatment of the preloaded synaptosomes with a high potassium medium decreased their radioactive GLU and GABA content without affecting the content of the precursor ARG. This result, which was identical with control or HE preparations, appears to indicate that ARG contributes at least, in part, to the synthesis of neurotransmitter GLU or GABA. As measured in purified synaptosomal preparations, HE increased by about 50% the activities of arginase and ornithine-delta-aminotransferase--the two enzymes of the ARG to GLU shunt. It is postulated that increased conversion of ARG to GLU may compensate for excessive utilization of the latter amino acid as an ammonia trap during HE and, as such, may be considered as an adaptative response of the synaptic compartment to this pathological condition.

Aspartate aminotransferase for synthesis of transmitter glutamate in the medulla oblongata: effect of aminooxyacetic acid and 2-oxoglutarate

The effects of aminooxyacetic acid (AOAA), a transaminase inhibitor, and 2-oxoglutarate, a precursor to glutamate by the activity of aspartate aminotransferase (AAT), on slices of rat medulla oblongata, cerebellum, cerebral cortex, and hippocampus were studied. The slices were superfused and electrically stimulated. There was a Ca2+-dependent stimulus-evoked release of endogenous glutamate, gamma-aminobutyric acid (GABA), and beta-alanine in all regions examined. AOAA (10(-4) and 10(-3) M) decreased the release of glutamate in the medulla oblongata and cerebellum but not in the hippocampus. L-Canaline, a specific inhibitor of ornithine aminotransferase, did not affect the glutamate release in the medulla. 2-Oxoglutarate (10(-3) M) increased the release of glutamate in the medulla oblongata and cerebellum but not in the cerebral cortex and hippocampus. Treatment with AOAA (10(-4) M) almost abolished the activities of AAT in all regions studied. AOAA (10(-4) and 10(-3) M) increased the stimulus-evoked release of GABA in the cerebellum, cerebral cortex, and hippocampus, whereas the stimulus-evoked release of beta-alanine was decreased by this agent in all regions studied. These results suggest the participation of AAT in the synthesis of the transmitter glutamate in the medulla oblongata and cerebellum of the rat.