Cellular Origins of Endogenous Amino Acids Released into the Extracellular Fluid of the Rat Striatum During Severe Insulin-Induced Hypoglycemia

The effect of severe insulin-induced hypoglycemia on the extracellular levels of endogenous amino acids in the rat striatum was examined using the brain microdialysis technique. A characteristic pattern of alterations consisting of a 9-12-fold increase in aspartate (Asp), and more moderate increases in glutamate (Glu), taurine (Tau), and gamma-aminobutyric acid (GABA), was noted following cessation of electroencephalographic activity (isoelectricity). Glutamine (Gln) levels were reduced both during and after the isoelectric period and there was a delayed increase in extracellular phosphoethanolamine (PEA) content. The effects of decortication and excitotoxin lesions on the severe hypoglycemia-evoked efflux of endogenous amino acids in the striatum were also examined. Decortication reduced the release of Glu and Asp both 1 week and 1 month post-lesion. The efflux of other neuroactive amino acids was not affected significantly. In contrast, GABA, Tau, and PEA efflux was attenuated in kainate-lesioned striata. Glu and Asp release was also reduced under these conditions, and a smaller decrease in extracellular Gln was noted. These data suggest that GABA, Glu, and Asp are released primarily from their transmitter pools during severe hypoglycemia. The releasable pools of Tau and PEA appear to be located in kainate-sensitive striatal neurons. The significance of these results is discussed with regard to the excitotoxic theory of hypoglycemic cell death.     

Extracellular Concentrations of Taurine,Glutamate, and Aspartate in the Cerebral Cortex of Rats at the Asymptomatic Stage of Thioacetamide-Induced Hepatic Failure: Modulation by Ketamine Anesthesia

Subclinical hepatic encephalopathy (SHE) was produced in rats by two intraperitoneal injections of TAA at 24 h intervals and the animals were examined 21 days later. Concentrations of the neuroactive amino acids taurine (Tau), glutamate (Glu) and aspartate (Asp), were measured in the cerebral cortical microdialysates of thioacetamide (TAA)-treated and untreated control rats. During microdialysis some animals were awake while others were anesthetized with ketamine plus xylazine. There was no difference in the water content of cerebral cortical slices isolated from control and SHE rats, indicating a recovery from cerebral cortical edema that accompanies the acute, clinical phase of hepatic encephalopathy in this model. When microdialysis was carried out in awake rats, dialysate concentrations of all the three amino acids were 30% to 50% higher in SHE rats than in control rats. Ketamine anesthesia caused a 2.2% increase of water content of cerebral cortical slices and increased Asp, Glu, and Tau concentration in microdialysates of control rats. In SHE rats, ketamine anesthesia produced a similar degree of cerebral edema, however, it did not alter Asp and Glu concentrations in the microdialysates. These data may reflect on one hand a neuropathological process of excitotoxic neuronal damage related to increased Glu and Asp, on the other hand neuroprotection from neuronal swelling indicated by Tau redistribution in the cerebral cortex. The reduction of the effects of SHE on Glu and Asp content in ketamine-anesthesized rats is likely to be due to interference of ketamine with the NMDA receptor-mediated component of the SHE-evoked excitatory neurotransmitter efflux and/or reuptake of the two amino acids. By contrast, the SHE-related increase of Tau content was not affected by ketamine anesthesia, indicating that the mechanism(s) underlying SHE-evoked accumulation of Tau must be different from the mechanism causing release of excitatory amino acids. The results with ketamine advocate caution when using this anesthetic in studies employing the cerebral microdialysis technique for measurement of extracellular amino acids.     

Neuronal dependence of extracellular dopamine,acetylcholine, glutamate,aspartate and gamma-aminobutyric acid (GABA) measured simultaneously from rat neostriatum using in vivo microdialysis: reciprocal interactions

Summary The neuronal origin of extracellular levels of dopamine (DA), acetylcholine (ACh), glutamate (Glu), aspartate (Asp) and gamma-aminobutyric acid (GABA) simultaneously collected from the neostriatum of halothane anaesthetized rats with in vivo microdialysis was studied. The following criteria were applied (1) sensitivity to K⁺-depolarization; (2) sensitivity to inhibition of synaptic inactivation mechanisms; (3) sensitivity to extracellular Ca²⁺; (4) neuroanatomical regionality; sensitivity to selective lesions and (5) sensitivity to chemical stimulation of the characterized pathways.It was found that: (1) Extracellular DA levels found in perfusates collected from the neostriatum fulfills all the above criteria and therefore the changes in extracellular DA levels measured with microdialysis reflect actual release from functionally active nerve terminals, and so reflect ongoing synaptic transmission. (2) Changes in neostriatal ACh levels reflect neuronal activity, provided that a ACh-esterase inhibitor is present in the perfusion medium. (3) Extracellular Glu, Asp and GABA could be measured in different perfusion media in the rat neostriatum and probably reflect metabolic as well as synaptic release. However, (4) the majority of the extracellular GABA levels found in perfusates collected from the neostriatum may reflect neuronal release, since GABA levels were increased, in a Ca²⁺-dependent manner, by K⁺-depolarization, and could be selectively decreased by an intrinsic neostriatal lesion. (5) It was not possible to clearly distinguish between the neuronal and the metabolic pools of Glu and Asp, since neostriatal Glu and Asp levels were only slightly increased by K⁺-depolarization, and no changes were seen after decortication. A blocker of Glu re-uptake, DHKA, had to be included in the perfusion medium in order to monitor the effect of K⁺-depolarization on Glu and Asp levels. Under this condition, it was found (6) that neostriatal Glu and Asp levels were significantly increased by K⁺-depolarization, although only increases in the Glu levels were sensitive to Ca²⁺ in the perfusion medium, suggesting that Glu but not Asp is released from vesicular pools. (7) Evidence is provided that selective stimulations of nigral DA cell bodies may lead to changes in release patterns from DA terminals in the ipsilateral neostriatum, which are in turn followed by discrete changes in extracellular levels of GABA and Glu in the same region. Finally, some methodological considerations are presented to clarify the contribution of neuronal release to extracellular levels of amino acid neurotransmitters in the rat neostriatum.     

Regional changes in the concentrations of glutamate,glycine, taurine,and GABA in the vitamin B-6 deficient developing rat brain: Association with neonatal seizures

It is well known that a dietary restriction of vitamin B-6 during gestation and lactation produces spontaneous seizures in neonatal animals. Since pyridoxal phosphate, one of the biologically active forms of vitamin B-6, is the cofactor for GAD the neonatal seizures have been attributed to low levels of brain GABA as a result of cofactor depletion. Although GABA levels are significantly lower in B-6 restricted neonatal rats with spontaneous seizures, seizure activity is not present in B-6 deficient adult rats or 28 day old rats in the present study, despite significantly low levels of brain GABA. These facts suggest that depletion of GABA is not the only biochemical alteration essential for the emergence of seizures. In the present study, the effect of vitamin B-6 undernutrition on the concentrations of the neuroactive amino acids, Glu, Gly, Tau, and GABA was determined in selected regions of the developing rat brain. The results show that the concentrations of Glu, Tau, and GABA were significantly lower and GLY significantly higher in selected brain regions of the B-6 restricted 14 day old rat compared to control tissue. Most of these changes were unique to 14 days of age, the time when spontaneous seizures are observed, and not present at 28 or 56 days of age when seizures are absent. This pattern of amino acid changes in the brain and the magnitude of the changes was consistent with those measured in a variety of chemically-induced animal models of epilepsy and in human epileptic foci. The regional distribution of amino acid changes was associated with brain regions which have been suggested to be responsible for the initiation and propagation of seizure activity. Two unique findings were also made in this study. First, there was a regional brain heterogeneity in the age-associated loss of brain Tau concentrations with the pons/medulla and substantia nigra appearing to be highly vulnerable and the hippocampus quite resistant to the loss of Tau. A second finding was the normalization of the neonatal GABA deficit in most brain regions by 56 days of age. The normalization of brain GABA was present in the face of continued dietary vitamin B-6 restriction. In summary, this study shows that the neuroactive amino acids Glu, Gly, Tau, and GABA are markedly altered in the seizure-prone vitamin B-6 restricted neonatal rat brain. The alterations in the brain concentration of Glu, Gly, and Tau may play an equally important role as GABA in the underlying mechanism of seizures associated with this condition.Abbreviations GAD Glutamic acid decarboxylase - GABA gamma-aminobutyric acid - Glu glutamate - Gly glycine - Tau taurine - CNS central nervous system - CTX cortex - HIPP hippocampus - C/P caudate/putamen - SN substantia nigra - Cb cerebellum - P/M pons/medulla     

Extracellular Overflow of Neuroactive Amino Acids During Severe Insulin-Induced Hypoglycemia: In Vivo Dialysis of the Rat Hippocampus

Hypoglycemia-evoked changes in levels of extracellular excitatory and inhibitory amino acids were studied using the microdialysis technique. A newly designed dialysis probe was inserted stereotaxically into the rat hippocampus. Animals were then subjected to insulin-induced hypoglycemia; then blood glucose levels were restored by glucose injections after a 30-min period of isoelectric electroencephalography. Dialysates were collected before, during, and after the isoelectric period. Amino acids in the dialysates were analyzed by liquid chromatography and fluorescence detection following automatic precolumn derivatization with o-phthaldialdehyde. During the isoelectric phase, the concentration of aspartate increased 15-fold, whereas glutamate, gamma-amino-butyric acid, taurine, and phosphoethanolamine levels were elevated three- to sixfold. Smaller increases were observed for nonneuroactive amino acids such as asparagine, alanine, and phenylalanine. In contrast to all other amino acids, the glutamine content was reduced to less than 30% of preisoelectric values. The concentrations of the neuroactive amino acids were restored to normal in the post-isoelectric phase. These data demonstrate that there is an extracellular overflow of neuroactive amino acids, especially aspartate, during severe hypoglycemia.     

Amino Acids in Rat Striatal Dialysates: Methodological Aspects and Changes After Electroconvulsive Shock

Extracellular levels of amino acids were estimated in dialysates of the rat striatum that were collected 1, 2, and/or more than 5 days after surgery, before (resting release) and during exposure to high K concentrations (50 mM) or electroconvulsive shocks. The resting release of several amino acids (Glu, Asn, Thr, Tau, Tyr, Gly, and Ala) was higher 9 days as compared to 1 day after surgery. In the 1-day preparation the resting release correlated highly with that observed with push-pull cannulas. The correlation with the tissue content of the amino acids was high only when they were divided into two groups (putative transmitters and metabolic intermediates). High K exposure produced increased output of Ala, ethanolamine (Eam), Asp, Glu, Tau, and Gly and a decrease in the egress of Gln 1 or 2 days after surgery. The effects on Asp and Glu had disappeared, and that on Gln reversed after 4-9 days. Electrically induced convulsions produced increased output of Ala, Gln, and Eam 1 or 2 days and 2 weeks after implantation of the probe. Changes were seen not only during but also (and some cases even more prominent) after the seizure. This study shows the usefulness of dialysis to monitor extracellular transmitter amino acids in the striatum of conscious rats (also bilateral dialysis was possible) for only a limited time after implantation of the probe. The dialysis method is suitable for longer time, when metabolic changes in amino acids are to be followed. In addition to transmitter release, glycolysis can be monitored by the measurement of Ala in the dialysate.     

γ-氨基丁酸 (GABA) 毛叶茶品质成分分析

以毛叶茶为研究对象,通过真空厌氧处理将其制作成γ 氨基丁酸(GABA)毛叶茶,探求毛叶茶经厌氧处理后的品质成分变化。结果表明：(1)厌氧处理后的毛叶茶,其GABA含量显著提高,达到GABA茶标准。游离氨基酸、黄酮和生物碱含量也显著升高,但茶多酚和水浸出物含量降低。同时,真空处理还能促进儿茶素的转化。简单儿茶素含量呈降低趋势,ECG和CG含量显著提高,EGCG、GCG含量及酯型儿茶素总量却先增加后降低,最终总量与对照样无明显差异。(2) 毛叶茶中除含有一般的蛋白质氨基酸外,还含有普通茶树品种所特有的特征氨基酸Thea,以及微量的GABA。游离氨基酸中含量较高的有Thea、Glu、Asp,较低的是Met、Cit、α ABA、Tau、Gly。Cysthi和EOHNH2是GABA毛叶茶中特有氨基酸。在真空厌氧条件下,GABA毛叶茶的游离氨基酸由于蛋白质发生降解而总量增加。其中P Ser、Thr、Ser、Asn、Pro、Gly、Cit、α ABA、Val、Cysthi、Ile、Leu、Tyr、Phe、GABA、Trp、Lys、His含量上升,Asp、Glu和α AAA含量均降低,而Ala 和Arg含量却呈现先增后降的趋势,Thea、Cys、Met等游离氨基酸含量在真空处理后无明显变化。     

Amino acid neurotransmitter alterations in three sublines of Rb mice differing by their susceptibility to audiogenic seizures

The levels of inhibitory amino acids (Tau, Gly), or excitatory amino acids (Glu, Asp) and Gln, precursor of GABA, have been determined, under resting conditions, in 17 brain areas of 3 sublines of inbred Rb mice displaying different responses to an acoustic stimulus. Rb1 mice were clonictonic seizure-prone, Rb2 mice were clonic seizure-prone and Rb3 mice were seizure resistant. Profile of distribution in the brain of each one of these amino acids differed. Maximum to minimum level ratio was higher for Tau (3.8) than for Glu or Asp or Gln (2). The level of Gly was similar in 13 out of the 17 areas examined. Multiple inter-subline differences were recorded for each amino acid. These differences have been analyzed considering the seizure susceptibility or severity of the three Rb sublines. Common lower levels (approximately –20%: Rb1/Rb3, Rb2/Rb3) of Gln in Temporal Cortex may be implicated in seizure susceptibility. Seirure severity (Rb1/Rb2) seems to correlate, in some areas, with additional lower amounts of GABA already reported and, to a lower extent, of Asp (–19% in striatum, inferior colliculus and cerebellum), of Tau and Gly; a tendency for a rise in Gln content was observed in certain others (10–20% in olfactory bulb, thalamus, hypothalamus, substantia nigra, and frontal, temporal and occipital cortex). The data and correlations recorded provide guidelines for further investigations for synaptosomal and metabolic alterations in the three sublines of the same strain of Rb mice.Abbreviations used GABA 4-aminobutyrate - Tau taurine - Gly glycine - Asp aspartate - Glu glutamate - Gln glutamine - GEPR genetically epilepsy-prone rat - OB olfactory bulbs - OT olfactory tubercles - Sr striatum - Se septum - Hy hypothalamus - Hi hippocampus - Th thalamus - A amygdala - SC superior colliculus - IC interior colliculus - SN substantia nigra - FCx frontal cortex - TCx temporal cortex - OCx occipital cortex - C cerebellum - P pons - Ra raphe     

γ-氨基丁酸（GABA）毛叶茶品质成分分析

以毛叶茶为研究对象,通过真空厌氧处理将其制作成γ-氨基丁酸（GABA）毛叶茶,探求毛叶茶经厌氧处理后的品质成分变化。结果表明：（1）厌氧处理后的毛叶茶,其GABA含量显著提高,达到GABA茶标准。游离氨基酸、黄酮和生物碱含量也显著升高,但茶多酚和水浸出物含量降低。同时,真空处理还能促进儿茶素的转化。简单儿茶素含量呈降低趋势,ECG和CG含量显著提高,EGCG、GCG含量及酯型儿茶素总量却先增加后降低,最终总量与对照样无明显差异。（2）毛叶茶中除含有一般的蛋白质氨基酸外,还含有普通茶树品种所特有的特征氨基酸Thea,以及微量的GABA。游离氨基酸中含量较高的有Thea、Glu、Asp,较低的是Met、Cit、α-ABA、Tau、Gly。Cysthi和EOHNH2是GABA毛叶茶中特有氨基酸。在真空厌氧条件下,GABA毛叶茶的游离氨基酸由于蛋白质发生降解而总量增加。其中P-Set、Thr、Ser、Asn、Pro、Gly、Cit、α-ABA、Val、Cysthi、Ile、Leu、Tyr、Phe、GABA、Trp、Lys、His含量上升,Asp、Glu和仪．AAA含量均降低,而Ala和Arg含量却呈现先增后降的趋势,Thea、Cys、Met等游离氨基酸含量在真空处理后无明显变化。     

Levels of glutamate,aspartate, GABA,and taurine in different regions of the cerebellum after X-irradiation-induced neuronal loss

The levels of glutamate (Glu), aspartate (Asp), -amino-n-butyric acid (GABA), and taurine (Tau) were determined in the cortex, molecular layer, and deep nuclei of cerebella of adult rats exposed to X-irradiation at 12–15 days following birth (to prevent the acquisition of late-forming granule cells; 12–15x group) and 8–15 days following birth (to prevent the acquisition of granule and stellate cells; 8–15x group). Also, the levels of the four amino acids were measured in the crude synaptosomal fraction (P₂) isolated from the whole cerebella of the control, 12–15x, and 8–15x groups. The level of Glu was significantly decreased by (1) 6–20% in the cerebellar cortex; (2) 15–20% in the molecular layer; and (3) 25–50% in the P₂ fraction of the X-irradiated groups relative to control values. The content of Glu in the deep nuclei was not changed by X-irradiation treatment. Regional levels of Asp were unchanged by X-irradiation, while its level in P₂ decreased by 15–30% after treatment. The levels of GABA and Tau in the molecular layer, deep nuclei, or P₂ were not changed in the experimental groups. However, there was a 15% increase in the levels of GABA and Tau in the cerebellar cortex of the 8–15x group relative to control values. The data support the proposed role of glutamate as the excitatory transmitter released from the cerebellar granule cells but are inconclusive regarding a transmitter role for either Tau or GABA from cerebellar stellate cells.     

Somatostatin Potently Stimulates In Vivo Striatal Dopamine and γ-Aminobutyric Acid Release by a Glutamate-Dependent Action

Abstract: We have used in vivo microdialysis in anaesthetised rats to investigate whether somatostatin (SRIF) can play a neuromodulatory role in the striatum. When 100 n M SRIF was retrodialysed for 15 min, it increased concentrations of dopamine (DA) by 28-fold, γ-aminobutyric acid (GABA) by eightfold, and glutamate (Glu) by sixfold as well as those of aspartate (Asp) and taurine (Tau). These effects were both calcium- and tetrodotoxin-sensitive. Lower (10 or 50 n M ) and higher (1 µ M ) SRIF concentrations were less effective. Rapid sampling showed that whereas Asp and Glu concentrations were raised for 3 min at the start of 15-min SRIF infusions, those of DA were increased for 12 min. A second 15-min application of 100 n M SRIF given 135 min after the first application failed to increase transmitter release. An NMDA receptor antagonist, 2-amino-5-phosphonopentanoic acid (200 µ M ), blocked SRIF (100 n M )-evoked Asp, Glu, Tau, and GABA release and reduced that of DA. An α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate antagonist, 6,7-dinitroquinoxaline-2,3-dione (100 µ M ), blocked SRIF-induced DA and Tau release and reduced that of Asp, Glu, and GABA. These results show that SRIF increases DA, Glu, Asp, GABA, and Tau release in the rat striatum and suggest that its actions on DA and GABA release are mainly mediated through increased excitatory amino acid release.     

Effects of dorsal root section and occlusion of dorsal spinal artery on the neurotransmitter candidates in rat spinal cord

In order to obtain further evidence of putative neurotransmitters in primary sensory neurons and interneurons in the dorsal spinal cord, we have studied the effects of unilateral section of dorsal roots and unilateral occlusion of the dorsal spinal artery on cholinergic enzyme activity and on selected amino acid levels in the spinal cord. One week after sectioning dorsal roots from caudal cervical (C₇) to cranial thoracic (T₂) levels, the specific activity of choline acetyltransferase (ChAT) was significantly decreased and acetylcholinesterase (AChE) showed a tendency to decrease in the dorsal quadrant on the operated side of the spinal cord. Dorsal root sectioning had little effect on the levels of free glutamic acid or other amino acids in the dorsal spinal cord. These results suggest that primary sensory neurons may include some cholinergic axons, and that levels of putative amino acid transmitters are not regulated by materials supplied by axonal transport from the dorsal root ganglia. By contrast, one week following unilateral occlusion of the dorsal spinal artery, the activities of ChAT and AChE were unchanged in the operated quadrant of the spinal cord, while decreases of Asp, Glu, and GABA, and an increase in Tau were detected. These findings are consistent with the proposals that such amino acids, but not ACh, may function as neurotransmitter candidates in interneurons of the dorsal spinal cord.Abbreviation used ACh acetylcholine - AChE acetylcholinesterase - Asp aspartic acid - ChAT choline acetyltransferase - GABA -aminobutyric acid - Glu glutamic acid - Gly glycine - SP substance P - Tau taurine     

Modulation of In Vivo Striatal Transmitter Release by Nitric Oxide and Cyclic GMP

Abstract: The effects of nitric oxide (NO) and cyclic GMP on in vivo transmitter release in the rat striatum were investigated using microdialysis sampling in urethane-anaesthetised animals. The NO release-inducing substances S -nitrosoacetylpenicillamine (SNAP), S -nitrosoglutathione (SNOG), and sodium nitroprusside (SNP) increased extracellular concentrations of aspartate (Asp), glutamate (Glu), γ-aminobutyric acid (GABA), taurine (Tau), acetylcholine (ACh), and serotonin (5-HT). Dopamine (DA) concentrations were decreased by SNAP but were increased by SNOG and SNP. An NO scavenger, haemoglobin, blocked or reduced the effects of SNAP on transmitter release. However, the control carrier compounds for SNAP, SNOG, and SNP (penicillamine, glutathione, and potassium ferricyanide, respectively, which do not induce release of NO) also increased GABA, Tau, DA, and 5-HT concentrations. When NO gas was given directly by dissolving it in degassed Ringer's solution, DA concentrations decreased significantly, and those of Asp, Glu, GABA, Tau, ACh, and 5-HT increased. These effects of NO gas were all inhibited by coadministration of haemoglobin and for GABA, Tau, ACh, and DA showed some calcium dependency. The cyclic GMP agonists 8-bromo-cyclic GMP and dibutryl-cyclic GMP stimulated dose-dependent increases in Asp, Glu, GABA, Tau, ACh, DA, and 5-HT concentrations. Increased striatal transmitter release in response to NO may therefore be mediated by its stimulatory action on cyclic GMP formation. NO inhibition of DA release may be mediated indirectly through its stimulation of local cholinergic and GABAergic neurones.     

Time-Related Cortical Amino Acid Changes After Basal Forebrain Lesion: A Microdialysis Study

Abstract: Cholinergic basal forebrain (BF) lesions in experimental animals have been used as a potential model for cholinergic deficits in cortex and hippocampus that occur in normal aging and Alzheimer's disease (AD). Glutamatergic cortical neurons are also affected in AD and could be part of the neurodegenerative process. In the present study, the effect of bilateral BF lesion with ibotenic acid microinjection on cortical extracellular amino acid levels was determined. Samples were collected every 20 min with microdialysis probes in awake, freely moving rats under basal and potassium stimulation conditions and measured by HPLC with fluorescence detection. Microdialysis experiments were performed 13 days, 21 days, and 30 days after BF lesion. The effectiveness of the lesion was shown by a significant 30% depletion in acetyl-CoA:choline O -acetyltransferase (EC 2.3.1.6) activity in the frontal cortex. Under basal conditions at 13 days only extracellular levels of taurine (Tau) and Glu were significantly reduced. Tau and Glu levels were recovered after 21 days and 30 days, respectively. In contrast, increase in Gly levels reaches its significance only at 30 days after lesion. Significant increases of Gln levels were observed at 21 days and 30 days. Asp and Ser levels remained constant throughout the period studied. Potassium stimulation led to increased Asp, Glu, Gly, and Tau levels, whereas Gln content decreased and Ser remained unaltered. As Ser is not believed to be a neurotransmitter, its lack of variation in any of the experimental conditions studied supports specific neuronal changes of the other amino acids. Results are discussed with reference to data observed in AD patients and possible mechanisms underlying the changes are suggested.     

Changes in free amino acid concentrations in serum,brain, and CSF throughout embryogenesis

Using the developing chick embryo as a model and a very sensitive micromethod for amino acid analysis, a complete analysis is presented of the developmental changes in free amino acid concentration in the blood, in the CSF, and in two different brain regions (optic lobe and frontal lobe) of the chick embryo (from day 4 of incubation, until day 5 post hatching). The developmental profile of Lys is the only one that is almost identical in all three compartments. The developmental profiles of the serum and of the brain are very similar for Arg and Phe, less so for Leu and Gly, and towards the end of the embryonic period, similar also for Val, Ile, Trp, and Met. The amino acid concentrations in the CSF are either much lower than in serum and brain already at the earliest stages, or they progressively decline to levels lower than those in brain and serum, most rapidly between day 6 and 8 of embryonic life. The concentrations of neuroactive amino acids (Gln, Glu, Asp, GABA, Tau, and Gly) in both brain regions begin to increase very early, and continue to rise, except Tau, which goes through a maximum at day 8. Comparative analysis of the developmental profiles of each amino acid in serum, brain, and CSF reveals that the blood supply and the cellular uptake, retention, and metabolism by neural cells are the major determinants of the free amino acid pool of the developing brain.     

Endogenous Glutamate Increases Extracellular Concentrations of Dopamine, GABA, and Taurine Through NMDA and AMPA/Kainate Receptors in Striatum of the Freely Moving Rat: A Microdialysis Study

Abstract: Interactions between glutamate (Glu), dopamine (DA), GABA, and taurine (Tau) were investigated in striatum of the freely moving rat by using microdialysis. Intrastriatal infusions of the selective Glu uptake inhibitor l - trans -pyrrolidine-3,4-dicarboxylic acid (PDC) were used to increase the endogenous extracellular [Glu]. Correlations between extracellular [Glu] and extracellular [DA], [GABA], and [Tau], and the effects of a selective blockade of ionotropic Glu receptors, were studied. PDC (1, 2, and 4 m M ) produced a dose-related increase in extracellular [Glu]. At the highest dose of PDC, [Glu] increased from 1.55 ± 0.35 to 6.11 ± 0.88 µ M . PDC also increased extracellular [DA], [GABA], and [Tau]. The increasing [Glu] was correlated significantly with increasing [DA], [GABA], and [Tau]. PDC also decreased extracellular concentrations of DA metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and 4-hydroxy-3-methoxyphenylacetic acid (HVA). Perfusion with the NMDA-receptor antagonist 3-[( R )-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid (1 m M ) or the AMPA/kainate-receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) (1 m M ) attenuated the increases produced by PDC (4 m M ) on [DA], [GABA], and [Tau], and decreases in [DOPAC] and [HVA]. DNQX also attenuated the increases in [Glu] induced by PDC. These data show that endogenous Glu plays a role in modulating the extracellular concentrations of DA, GABA, and Tau in striatum of the freely moving rat.     

Roles of glutamine in neurotransmission

Glutamine (Gln) is found abundantly in the central nervous system (CNS) where it participates in a variety of metabolic pathways. Its major role in the brain is that of a precursor of the neurotransmitter amino acids: the excitatory amino acids, glutamate (Glu) and aspartate (Asp), and the inhibitory amino acid, γ-amino butyric acid (GABA). The precursor-product relationship between Gln and Glu/GABA in the brain relates to the intercellular compartmentalization of the Gln/Glu(GABA) cycle (GGC). Gln is synthesized from Glu and ammonia in astrocytes, in a reaction catalyzed by Gln synthetase (GS), which, in the CNS, is almost exclusively located in astrocytes (Martinez-Hernandez et al., 1977). Newly synthesized Gln is transferred to neurons and hydrolyzed by phosphate-activated glutaminase (PAG) to give rise to Glu, a portion of which may be decarboxylated to GABA or transaminated to Asp. There is a rich body of evidence which indicates that a significant proportion of the Glu, Asp and GABA derived from Gln feed the synaptic, neurotransmitter pools of the amino acids. Depolarization-induced-, calcium- and PAG activity-dependent releases of Gln-derived Glu, GABA and Asp have been observed in CNS preparations in vitro and in the brain in situ. Immunocytochemical studies in brain slices have documented Gln transfer from astrocytes to neurons as well as the location of Gln-derived Glu, GABA and Asp in the synaptic terminals. Patch-clamp studies in brain slices and astrocyte/neuron co-cultures have provided functional evidence that uninterrupted Gln synthesis in astrocytes and its transport to neurons, as mediated by specific carriers, promotes glutamatergic and GABA-ergic transmission. Gln entry into the neuronal compartment is facilitated by its abundance in the extracellular spaces relative to other amino acids. Gln also appears to affect neurotransmission directly by interacting with the NMDA class of Glu receptors. Transmission may also be modulated by alterations in cell membrane polarity related to the electrogenic nature of Gln transport or to uncoupled ion conductances in the neuronal or glial cell membranes elicited by Gln transporters. In addition, Gln appears to modulate the synthesis of the gaseous messenger, nitric oxide (NO), by controlling the supply to the cells of its precursor, arginine. Disturbances of Gln metabolism and/or transport contribute to changes in Glu-ergic or GABA-ergic transmission associated with different pathological conditions of the brain, which are best recognized in epilepsy, hepatic encephalopathy and manganese encephalopathy.     

A Possible Interrelationship Between Extracellular Taurine and Phosphoethanolamine in the Hippocampus

The effect of guanidinoethane sulfonic acid (GES), an inhibitor of taurine uptake, was examined with respect to endogenous amino acids in the hippocampus of the freely moving rabbit. GES increased the extracellular levels of both taurine and phosphoethanolamine (PEA), other amino acids being unaffected. However, long-term oral administration of GES selectively reduced endogenous taurine levels. The effect of GES on PEA appeared to be a consequence of the elevated extracellular taurine as exogenously administered taurine per se increased PEA levels in the extracellular space. The findings are discussed in conjunction with the proposed membrane-stabilizing effects of taurine.     

衰老对大鼠脑区氨基酸水平的影响

本文测定了正常青龄组（３月龄）和老龄组（２０月龄）大鼠不同脑区（皮层、小脑海马、纹状体和下丘脑）谷氨酸、天门冬氨酸、甘氨酸、ｒ－氨基丁酸和牛磺酸的含量。结果表明：在衰老过程中大鼠某些脑区谷氨酸、天门冬氨酸、甘氨酸和牛磺酸水平显著降低;而纹状体γ－氨基丁酸含量则显著升高。     

Plasma amino acids in 3/sec spike-wave epilepsy

Plasma levels of 14 amino acids were determined in 44 probands with 3/sec spike-wave epilepsy, 27 of their first-degree relatives, and 22 controls. Six ratios of metabolically related amino acids were also calculated. Statistically significant differences were found for 7/20 variables when the experimental and control probands were compared, and for 6/20 variables when the relatives were compared with controls. Tau, Asp, and Tau/Glu were decreased and Glu was increased in both the experimental probands and their first degree relatives. When discriminant analysis was used, age- and sex-matched epileptic probands could be distinguished from the age- and sex-matched control probands with 100% accuracy employing only 3 amino acids (Asp, Glu, Val) and one ratio (Thr/Ser).