Effect of Insulin-Induced Hypoglycemia on the Concentrations of Glutamate and Related Amino Acids and Energy Metabolites in the Intact and Decorticated Rat Neostriatum

Abstract The glutamate (Glu) terminals in rat neostriatum were removed by a unilateral frontal decortication. One to two weeks later the effects of insulin-induced hypoglycemia on the steady-state levels of amino acids [Glu, glutamine (Gin), aspartate (Asp), γ-aminobutyric acid (GABA), tau-rine] and energy metabolites (glucose, glycogen, α-ketoglu-tarate, pyruvate, lactate, ATP, ADP, AMP, phosphocre-atine) were examined in the intact and decorticated neostriatum from brains frozen in situ. The changes in the metabolite levels were examined during normoglycemia, hypoglycemia with burst-suppression (BS) EEG, after 5 and 30 min of hypoglycemic coma with isoelectric EEG, and 1 h of recovery following 30 min of isoelectric EEG. In normoglycemia Glu decreased and Gin and glycogen increased significantly on the decorticated side. During the BS period no significant differences in the measured compounds were noted between the two sides. After 5 min of isoelectric EEG Glu, Gin, GABA, and ATP levels were significantly lower and Asp higher on the intact than on the decorticated side. No differences between the two sides were found after 30 min of isoelectric EEG. After 1 h of recovery from 30 min of isoelectric EEG Glu, Gin, and glycogen had not reached their control levels. Glu was significantly lower, and Gin and glycogen higher on the decorticated side. The Asp and GABA levels were not significantly different from control levels. The results indicate that the turnover of Glu is higher in the intact than in decorticated neostriatum during profound hypoglycemia.     

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.     

The Acute Effect of Ammonium Acetate on Levels of Amino Acids in the Intact and Decorticated Rat Neostriatum

Unilateral frontal cortex ablations were performed in rats so that the glutamate terminals in the ipsilateral rostral neostriatum were removed. At 1 or 7 days later, intraperitoneal injections of ammonium acetate induced different changes in amino acid concentrations in the intact and deafferentated neostriatum. After 1 day, the level of glutamate decreased only in the intact side, whereas that of glutamine increased and that of aspartate decreased to the same extent on both sides following ammonia injection. After 7 days, the glutamate level decreased more in the intact than the decorticated side in both nonconvulsing and convulsing rats. The concentration of alanine increased most in the intact neostriatum, whereas glutamine levels increased and aspartate levels decreased to the same extent on both sides in nonconvulsing and convulsing rats. The results indicate that ammonia has a more pronounced effect on neuronal than glial glutamate pools.     

Analysis of Glutamine Accumulation in Rat Brain Mitochondria in the Presence of a Glutamine Uptake Inhibitor,Histidine, Reveals Glutamine Pools With a Distinct Access to Deamidation

Rat cerebral nonsynaptic mitochondria were incubated in medium containing 2 mM glutamine (Gln) or 2 mM glutamate (Glu), in the presence of a Gln uptake inhibitor histidine (His) as well as other basic amino acids, lysine and arginine (Lys, Arg) not inhibiting Gln uptake. Subsequently, the mitochondrial contents of Glu and Gln were determined by HPLC. Incubation in the presence of Glu alone increased the Glu content from 3.5 to 15 nmol/mg protein, without affecting the Gln content. On the other hand, incubation with Gln increased the content of Gln from 1.5 to 12 nmol/mg, and that of Glu to 10 nmol/mg. As expected, addition of His did not alter the Glu and Gln content resulting from incubation with Glu. However, His significantly decreased to almost the preincubation level the content of Glu in mitochondria incubated with Gln, without affecting the content of Gln. No other amino acid had any effect on these parameters. The results point to the existence of distinct Gln pools, one of which is accessible to external Gln via a His-sensitive transporter and is accessible for deamidation in the mitochondria.Special issue dedicated to Dr. Lawrence F. Eng.     

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.     

In Vivo Studies on the Extracellular, and Veratrine-Releasable, Pools of Endogenous Amino Acids in the Rat Striatum: Effects of Corticostriatal Deafferentation and Kainic Acid Lesion

The effects of corticostriatal deafferentation (decortication) and destruction of intrinsic neurons (intrastriatal kainate injection) on the extracellular concentration, and veratrine-releasable pools, of endogenous amino acids in the rat striatum were examined using the in vivo brain dialysis technique. Intracellular amino acid content was also determined. Decortication reduced selectively intra- and extracellular levels of glutamate (Glu) and aspartate (Asp). Extracellular changes were more pronounced than those in tissue content. gamma-Aminobutyric acid (GABA), taurine (Tau), and phosphoethanolamine (PEA) levels were not affected, whereas nonneuroactive amino acids were increased at 1 week but not at 1 month post-lesion. The intracellular pool of Glu and Asp was also reduced in kainate-lesioned striata. However, extracellular levels of these compounds were not affected significantly by this treatment. The tissue content of all other amino acids was decreased, the most prominent change being in the concentration of GABA. Extracellular GABA concentration was also reduced dramatically, whereas the concentrations of noneuroactive amino acids were increased to varying degrees. These data suggest that transmitter pools of neuroactive amino acids are an important supply for their extracellular pools. Lesion-induced alterations in nonneuroactive amino acids are discussed with regard to the loss of metabolic pools, glial reactivity, and changes in blood-brain barrier transport. Veratrine induced a massive release of neuroactive amino acids such as Glu, Asp, GABA, and Tau into the extracellular fluid, and a delayed increase in PEA. Extracellular levels of neuroactive amino acids were raised slightly. Decortication reduced, selectively, the amounts of Glu and Asp released by veratrine.(ABSTRACT TRUNCATED AT 250 WORDS)     

Release of endogenous excitatory amino acids in the neostriatum of the rat under physiological and pharmacologically-induced conditions

Summary There is immunohistochemical evidence suggesting that glutamate (Glu) is released from nerve terminals and acts, via several receptor subtypes, as a major excitatory neurotransmitter in the cortico-striatal pathway of the rat. Aspartate (Asp) is also present in cortico-striatal neurons, but its role as a neurotransmitter has been questioned, since, in contrast to Glu, it has not been demonstrated in presynaptic vesicles. Glu and Asp can be found at subM concentrations in the extracellular compartment of most areas of the basal ganglia. Their concentrations are largely regulated by transport mechanisms, but also by a synaptotagmin-dependent exocytotic release, and are sufficiently high to occupy junctional and extrajunctional receptors.We have investigated whether Glu and Asp release in the neostriatum can be selectively modulated by different neuronal systems. Dopamine (DA) and cholecystokinin (CCK) selectively stimulate Asp release, via D₁ and CCK_B receptor subtypes, respectively. Also opioid -agonists increase Asp release. We propose that the selective modulation of Asp release by D_1–, CCK_B- and agonists involves striatal neurons containing Asp, but not Glu. In contrast, local perfusion with the ,-opioid antagonist D-Phe-Cys-Tyr-D-Trp-Orn-ThrPen-Thr-NH₂ (CTOP) increases both Glu and Asp release. This effect is probably exerted on cortico-striatal terminals, via presynaptic inhibitory -receptors. Thus, these results demonstrate that extracellular levels of Glu and Asp are modulated differentially by different neuronal systems, and suggest that in the neostriatum of the rat there are neuronal populations using Glu and/or Asp as messenger(s).     

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.     

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.     

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     

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.     

Uptake and metabolism ofl-[3H]glutamate andl-[3H]glutamine in adult rat cerebellar slices

Using very low concentrations (1 mumol range) of L-2-3-[3H]glutamate, (3H-Glu) or L-2-3-[3H]glutamine (3H-Gln), we have previously shown by autoradiography that these amino acids were preferentially taken up in the molecular layer of the cerebellar cortex. Furthermore, the accumulation of 3H-Glu was essentially glial in these conditions. We report here experiments in which uptake and metabolism of either (3H-Glu) or (3H-Gln) were studied in adult rat cerebellar slices. Both amino acids were rapidly converted into other metabolic compounds: after seven minutes of incubation in the presence of exogenous 3H-Glu, 70% of the tissue accumulated radioactivity was found to be in compounds other than glutamate. The main metabolites were Gln (42%), alpha-ketoglutarate (25%) and GABA (1,4%). In the presence of exogenous 3H-Gln the rate of metabolism was slightly slower (50% after seven minutes of incubation) and the metabolites were also Glu (29%), alpha-ketoglutarate (15%) and GABA (5%). Using depolarizing conditions (56 mM KCl) with either exogenous 3H-Glu or 3H-Gln, the radioactivity was preferentially accumulated in glutamate compared to control. From these results we conclude: i) there are two cellular compartments for the neurotransmission-glutamate-glutamine cycle; one is glial, the other neuronal; ii) these two cellular compartments contain both Gln and Glu; iii) transmitter glutamate is always in equilibrium with the so-called "metabolic" pool of glutamate; iv) the regulation of the glutamate-glutamine cycle occurs at least at two different levels: the uptake of glutamate and the enzymatic activity of the neuronal glutaminase.     

Study of the importance of Glu361 in the active site of cholesterol oxidase from<Emphasis Type="Italic">Rhodococcus</Emphasis> sp. PTCC 1633 by site-directed mutagenesis

Cholesterol oxidase is a monomeric flavoenzyme which catalyzes the oxidation and isomerization of cholesterol to cholest-4-en-3-one. ThechoR gene was cloned in pET23a and used as the starting plasmid for Glu361Asn, Glu361Gln and Glu361Asp site-directed mutagenesis. The purified mutant proteins like the wild-type have a molecular mass of 55 kD. The specific activities of Glu361Gln and Glu361Asn mutants were 28 and 35 times less than the wild-type. Glu361Asp mutant showed nearly no catalytic activity and was not purified. These experiments clearly demonstrated the importance of Glu361 for the enzymatic reactions of cholesterol oxidaseRhodococcus sp.     

Analysis of essential leucine residue for catalytic activity of novel thermostable chitosanase by site-directed mutagenesis

Bacterial chitosanases share weak amino acid sequence similarities at certain regions of each enzyme. These regions have been assumed to be important for catalytic activities of the enzyme. To verify this assumption, the functional importance of the conserved region in a novel thermostable chitosanase (TCH-2) from Bacillus coagulans CK108 was investigated. Each of the conserved amino acid residues (Leu64, Glu80, Glu94, Asp98, and Gly108) was changed to aspartate and glutamine or asparagine and glutamate by site-directed mutagenesis, respectively. Kinetic parameters for colloidal chitosan hydrolysis were determined with wild-type and 10 mutant chitosanases. The Leu64 Arg and Leu64 Gln mutations were essentially inactive and kinetic parameters such as V _max and k _cat were approximately 1/10⁷ of those of the wild-type enzyme. The Asp98 Asn mutation did not affect the K _m value significantly, but decreased k _cat to 15% of that of wild-type chitosanase. On the other hand, the Asp98 srarr; Glu mutation affected neither K _m nor k _cat. The observation that approximately 15% of activity remained after the substitution of Asp98 by Asn indicated that the carboxyl side chain of Asp98 is not absolutely required for catalytic activity. These results indicate that the Leu64 residue is directly involved in the catalytic activity of TCH-2.     

Microdialysis of excitatory amino acids in the periaqueductal gray of the rat after unilateral peripheral inflammation

Summary This study measured the release of glutamate (Glu) and aspartate (Asp) amino acid transmitters in the ventrocaudal compartment of the rat periaqueductal gray (PAG) following exposure to unilateral peripheral inflammation. The release of endogenous Glu and Asp from the rat ventrocaudal PAG was monitored with the microdialysis technique in unanesthetized, unrestrained rats. There was significant increase (1,300%) in the basal concentrations of Glu release in the 7 days Complete Freund's Adjuvant (CFA) treated group compared to 24h mineral oil control group. Amino acid release was induced by infusing veratridine (75M, a sodium channel activator) directly through the 1 mm long dialysis probe. Perfusion of veratridine into the ventrocaudal PAG resulted in significant elevation of Glu and Asp amino acids. In the 24h and 7 days CFA treated rats, veratridine-evoked release of Glu was significantly decreased in the lateral ventrocaudal PAG compared to control rats injected with mineral oil (CFA vehicle). The peak minus baseline concentrations of Glu in 24h and 7 days CFA treated groups decreased 55.7% and 43.9%, respectively. In contrast, The basal and the peak minus baseline concentrations of Asp showed no significant change between control group and 24h and 7 days CFA treated animals. The results provide direct evidence that Glu excitatory amino acid may be involved in nociception/nociception modulation pathway in the ventrocaudal PAG.     

Comparison of Transmitter Amino Acid Levels in Rat Globus Pallidus and Neostriatum During Hypoglycemia or After Treatment with Methionine Sulfoximine or γ-Vinyl γ-Aminobutyric Acid

The levels of amino acids in globus pallidus, a structure heavily innervated with gamma-aminobutyric acid (GABA)-ergic terminals but few glutamergic terminals, were compared with the levels in neostriatum, a structure richly innervated with glutamergic terminals but intermediate in GABAergic terminals. The level of glutamate in neostriatum was twice as high as in globus pallidus whereas the level of GABA in globus pallidus was three times higher than in neostriatum. The level of aspartate was similar in both regions whereas the level of glutamine was correlated with the level of glutamate. Methionine sulfoximine, a glutamine synthetase inhibitor, reduced the level of glutamine to 10-20% of control in both structures. This reduction was accompanied by the largest decrease in the level of glutamate in neostriatum, indicating that transmitter glutamate turns over more rapidly than other glutamate pools. Likewise, insulin decreased the levels of glutamate and glutamine more in neostriatum than in globus pallidus. gamma-Vinyl GABA increased the level of GABA in globus pallidus more than in neostriatum although the percent increase was largest in neostriatum. Treatment with gamma-vinyl GABA was accompanied by a large reduction in the level of GABA, indicating that a substantial proportion of the glutamine pool is linked to GABA metabolism.     

Contribution of active site residues to the activity and thermal stability of ribonuclease Sa

We have used site-specific mutagenesis to study the contribution of Glu 74 and the active site residues Gln 38, Glu 41, Glu 54, Arg 65, and His 85 to the catalytic activity and thermal stability of ribonuclease Sa. The activity of Gln38Ala is lowered by one order of magnitude, which confirms the involvement of this residue in substrate binding. In contrast, Glu41Lys had no effect on the ribonuclease Sa activity. This is surprising, because the hydrogen bond between the guanosine N1 atom and the side chain of Glu 41 is thought to be important for the guanine specificity in related ribonucleases. The activities of Glu54Gln and Arg65Ala are both lowered about 1000-fold, and His85Gln is totally inactive, confirming the importance of these residues to the catalytic function of ribonuclease Sa. In Glu74Lys, k(cat) is reduced sixfold despite the fact that Glu 74 is over 15 A from the active site. The pH dependence of k(cat)/K(M) is very similar for Glu74Lys and wild-type RNase Sa, suggesting that this is not due to a change in the pK values of the groups involved in catalysis. Compared to wild-type RNase Sa, the stabilities of Gln38Ala and Glu74Lys are increased, the stabilities of Glu41Lys, Glu54Gln, and Arg65Ala are decreased and the stability of His85Gln is unchanged. Thus, the active site residues in the ribonuclease Sa make different contributions to the stability.     

Phenylsuccinate reduces KCL-induced release of GABA evidence for the participation of the ketodicarboxylate carrier in the biosynthesis of transmitter-GABA

Summary The present study investigates the effects of phenylsuccinate (PS), an inhibitor of the mitochondrial ketodicarboxylate carrier (KCC), on release of-aminobutyric acid (GABA), glutamate (Glu), glutamine (Gln), and glycine (Gly), induced by potassium chloride (KCl) and by cardiac arrest caused by a halothane overdose. Microdialysates were collected from the hippocampus of anaesthetized rats, and analyzed by HPLC. Continuous perfusion of 50 mM PS through the dialysis probe, reduces release of GABA induced by KCl (50 mM for 10 min through the dialysis probe) by up to 72%. In addition, PS abolished KCl-induced release of Glu. Release of GABA during cardiac arrest was not reduced by PS, whereas PS reduced release of Glu in the early stage of cardiac arrest. PS furthermore increased the basal level of Gln, and reversed a decrease of Gln induced by cardiac arrest.It is proposed that the KCC is present in GABA'ergic neurons of the rat hippocampus, and that GABA, released by KCl, can be synthesized in a KCC dependent manner. It is also suggested that ischemia-induced release of GABA, to some extent, has a non-transmitter origin. The results furthermore indicate that uptake of Gln into GABA'ergic and Glu'ergic neurons is not regulated by simple demand mechanisms.Abbreviations PS phenylsuccinate - KCC ketodicarboxylate carrier - GABA -aminobutyric acid - Glu glutamate - Gln glutamine - Gly glycine - -KG -ketoglutarate - Mal malate - KRB-buffer Krebs-Ringer bicarbonate-buffer - HPLC high pressure liquid chromatography     

Functional investigation of conserved membrane-embedded glutamate residues in the proton-coupled peptide transporter YjdL

Proton-dependent oligopeptide transporters (POTs) are secondary active symporters that utilize the proton gradient to drive the inward translocation of di- and tripeptides. We have mutated two highly conserved membraneembedded glutamate residues (Glu20 and Glu388) in the E. coli POT YjdL to probe their possible functional roles, in particular if they were involved/implicated in recognition of the substrate N-terminus. The mutants (Glu20Asp, Glu20Gln, Glu388Asp, and Glu388Gln) were tested for substrate uptake, which indicated that both the negative charge and the side chain length were important for function. The IC50 values of dipeptides with lack of or varying N-terminus (Ac-Lys, Gly- Lys, β-Ala-Lys, and 4-GABA-Lys), showed that Gly-Lys and β-Ala-Lys ranged between ~0.1 to ~1.0 mM for wild type and Glu20 mutants. However, for Glu388Gln the IC50 increased to ~2.0 and > 10 mM for Gly-Lys and β-Ala-Lys, respectively, suggesting that Glu388, and not Glu20, is able to sense the position of the N-terminus and important for the interaction. Furthermore, uptake as a function of pH showed that the optimum at around pH 6.5 for wild type YjdL shifted to 7.0-7.5 for the Glu388Asp/Gln mutants while the Glu20Asp retained the wild type optimum. Uptake by the Glu20Gln on the other hand was completely unaffected by the bulk pH in the range tested, which indicated a possible role of Glu20 in proton translocation.     

Amphetamine Promotes Neostriatal Ascorbate Release via a Nigro-Thalamo-Cortico-Neostriatal Loop

Abstract: In the neostriatum, amphetamine and other dopamine agonists elevate the extracellular level of ascorbate, which is known to modulate neostriatal function. Although both D₁ and D₂ receptors have been linked to neostriatal ascorbate release, ample evidence suggests it is controlled by areas outside the neostriatum. The present series of experiments used selective lesions and intracerebral drug infusions to probe the involvement of the ventromedial thalamus and substantia nigra pars reticulata. Our results implicate both of these sites in amphetamine-induced increases in the release of neostriatal ascorbate. Thus, whereas unilateral electrolytic lesions of the substantia nigra pars reticulata completely abolished the ability of systemic amphetamine (2.5 mg/kg) to increase extracellular ascorbate in ipsilateral neostriatum, intranigral infusions of this drug (10 and 30 µg/µl) elevated neostriatal ascorbate release. This infusion effect, moreover, was blocked by electrolytic lesions of the ipsilateral ventromedial thalamus, which receives input from the substantia nigra pars reticulata and projects to the cerebral cortex. These results, combined with previous evidence implicating cortical projections to neostriatum as the source of extracellular ascorbate, suggest that neostriatal ascorbate release is regulated, at least in part, by a nigro-thalamo-cortico-neostriatal pathway.