Glutamate as a CNS transmitter. I. Evaluation of glucose and glutamine as precursors for the synthesis of preferentially released glutamate

Effects of ammonium salts on synaptic transmission to hippocampal CA1 and CA3 pyramidal cells in vivo

The effects of ammonium acetate or chloride, perfused through the lateral ventricle, were studied on the hippocampal formation of the rat. During perfusion with ammonia, the population spikes, evoked by stimuli delivered to the fimbria, were first increased and then reduced. On the other hand, the late positive wave gradually decreased throughout the application of ammonia. The inhibition, studied by the paired-pulse test, was found to be reduced when the population spike was transiently enhanced, indicating that disinhibition could be responsible for the enhancement of synaptically evoked responses. Neither antidromically evoked population spikes nor the typical effects of iontophoretically applied glutamate, aspartate or gamma-aminobutyrate were changed by ammonia. These findings can be accounted for by a single action of ammonia, a depression of excitatory synaptic transmission, the excitatory synapses on inhibitory interneurons being more readily depressed than those on the pyramidal cells. Both effects, early hyperexcitability and late depression, are probably due to a reduction in the release of the excitatory neurotransmitter, glutamate and/or aspartate. We tentatively suggest that these mechanisms are responsible for some of the symptoms observed during the development of hyperammonemic encephalopathies.

Isolation of astrocytes, neurons, and synaptosomes of rat brain cortex: distribution of enzymes of glutamate metabolism

A simplified method was developed for the bulk separation of neuronal perikarya and astroglial cells from adult rat brain without the involvement of density gradients. Activities of various enzymes involved in glutamate metabolism were estimated and compared with those of synaptosomes. The activities of glutamate dehydrogenase and aspartate aminotransferase were higher in synaptosomes than in neuronal perikarya or glia. Glutamine synthetase was distributed in all the three fractions while glutaminase activity was higher in astrocytes than in synaptosomes and was not detectable in neuronal perikarya. The significance of these results in relation to metabolic compartmentation was discussed.

Effect of cortico-striate pathway lesion on the activities of enzymes involved in synthesis and metabolism of amino acid neurotransmitters in the striatum

The activities of several enzymes involved in the metabolism of aspartate and glutamate were measured in striatal (nucleus caudatus and putamen) homogenates 2-3, 6-7, and 35-40 days following frontoparietal and frontal cortical ablation. The activity of glutamine synthetase (GS) was substantially increased (46-48%) on the operated side 6-7 days following the lesion whereas smaller changes were observed at 2-3 and 35-40 days after lesion. In contrast, decreased levels of glutaminase and malate dehydrogenase (MDH) were observed by 6-7 days while no significant change was found at either 2-3 or 35-40 after the lesion. The activities of glutamate dehydrogenase (GDH) and glutamate decarboxylase (GAD) were elevated after 35-40 days whereas no changes in the levels of either GDH or aspartate aminotransferase (ASAT) were found at 2-3 or 6-7 days after the fronto-parietal decortication. When only the frontal cortex was removed quantitatively similar changes were observed in striatal GS and glutaminase activity. The content of glutamate and glutamine in the denervated striatum followed qualitatively the changes in glutaminase and GS. The results indicate that the degeneration of cortico-striatal terminals causes a profound glial reaction in the striatum, and both glutaminase and MDH are present in relatively high concentrations in the corticostriatal terminals.

Proline, glutamate and glutamine metabolism in mouse brain synaptosomes

In nerve terminals, glutamate (Glu) may serve as precursor of the inhibitory neurotransmitter, GABA, and the putative excitatory transmitter, aspartate (Asp), in addition to exerting its own excitatory neurotransmitter role in brain. Glu carbon can originate from glucose through glycolysis and the Krebs cycle, from glutamine (Gln) subsequent to uptake, and from proline (Pro) and ornithine (Orn). Orn, but not Glu, is an effective precursor in nerve terminals of Pro, a putative inhibitory neurotransmitter. [3H]Arg can be converted in mouse brain nerve terminals to Orn, which in turn gives rise to Glu, Pro and GABA. In the present study, the conversion subsequent to uptake of labeled Glu, Gln and Pro to other amino acids was studied in unfractionated and subfractionated synaptosomal particles which layered, respectively, on 1.0 M, 1.2 M, 1.3 M and 1.5 M sucrose after centrifugation in a discontinuous gradient (fractions 1-4, respectively). Fraction 1 contained small synaptosomal fragments with vesicles and almost no mitochondria. Fractions 2 and 3 showed numerous normal-appearing mitochondria-containing synaptosomes, and fraction 4 contained large synaptosomes and more free mitochondria than the other fractions. Glu was readily taken up in all fractions and converted to Asp, Gln and GABA, the greatest formation of Asp from Glu occurring in fractions 2 and 3 and of Gln in fraction 4. In contrast, Gln was taken up poorly in fraction 1 and not metabolized, converted extensively to Glu and GABA in fractions 2-4, giving rise only to very small amounts of Asp in fractions 2 and 3. Although Pro was taken up to the greatest extent in fraction 2, it was by far most readily converted to Glu, Gln and GABA in fraction 1, showing only small amounts of Asp formation in fractions 1-3 and none in 4. There was no significant production of Pro from Glu or Gln or of Arg and Orn from any of the 3 precursors studied. The above results suggest that Glu, Gln and Pro may be taken up largely in different classes of synaptosomes which are distributed among the centrifugally separated fractions and which possess differing transport and metabolic characteristics. Determination of glutamate decarboxylase activity (GAD) indicated that GABA-forming nerve terminals were present in all synaptosomal fractions studied. Amino acid determinations by HPLC in the subfractionated synaptosomes showed a similar distribution for Glu, Asp and GABA contents, peaking in fraction 2, and an inverse relationship of the latter 3 with Arg contents.(ABSTRACT TRUNCATED AT 400 WORDS)

Alteration of striatal glutamate release after glutamine synthetase inhibition

The effect of the glutamine synthetase (GS) inhibitor, methionine sulfoximine (MSO), on glutamate levels in, and glutamate release from, rat striatal tissue was examined. Tissue levels of glutamate were unchanged 24 h after an intraventricular injection of MSO, but tissue glutamine levels were decreased 50%. Calcium-dependent, potassium-stimulated glutamate release was diminished in tissue prisms from animals pretreated with MSO compared to controls. The decreased release of glutamate correlated over time with the inhibition of GS following an intraventricular injection of MSO. The maximum diminution of calcium-dependent, potassium-stimulated glutamate release (50%) and the maximum inhibition of GS activity (51%) were observed 24 h after MSO. The addition of 0.5 mM glutamine to the perfusion medium completely reversed the effects of MSO pretreatment on calcium-dependent, potassium-stimulated glutamate release. Since GS is localized in glial cells and the measured glutamate release is presumed to occur from neurons, the data support the contention that astroglial glutamine synthesis is an important contributor to normal neuronal neurotransmitter release.

Regional synthesis of 14C-amino acids in cerebral cortex of the unanesthetized rat: simultaneous analysis by "push-pull" perfusion

Rats were surgically prepared with two sets of guide cannulae positioned bilaterally to rest below the superficial layer of the prefrontal, parietal and occipital regions of the cerebral cortex. Following post-operative recovery, two cortical sites were labeled simultaneously by microinjection of a 1.0 microliter volume of 5.0 microCi of D-[U-14C]-glucose. After a 20 min interval for incorporation into amino acids, two sites were concurrently perfused over an interval of 20 min by means of a push-pull cannula system with an artificial CSF solution at a rate of 25 microliters/min. A thin-layer chromatographic assay for separation of amino acid neurotransmitters was used to estimate the content in each sample of perfusate of labeled gamma-amino-butyric acid (GABA), glutamate, glutamine, aspartate and glycine. The results show that recovery of three amino acids, GABA, aspartate and glycine, was uniform among the three areas of the rat's cortex. However, the synthesis of labeled glutamate and glutamine was significantly lower within the parietal cortex in comparison to that of the prefrontal and occipital regions. Overall, the recovery of GABA was proportionally higher whereas glycine was lower than that of the other amino acids. The difference in the results from previous observations can be attributed to the distribution of amino acids in neurons and their terminals in deeper layers of cortex, which may give a different profile of release when epidural measurements are obtained.

The uptake and release of glutamate at the crayfish neuromuscular junction

The abdominal slow flexor muscle of the crayfish (Cambarus clarkii) was pre-incubated in deuterium-labelled glutamate (glutamate-d5) solution, and the release of glutamate, glutamate-d5 and aspartate in the bath solution was measured by mass fragmentography. The glutamate-d5 was taken up into the preparation and released together with endogenous glutamate. The resting release of glutamate-d5 was 8.2 +/- 0.96 pmol/10 min after the incubation with 0.5 mM-glutamate-d5, and those of endogenous glutamate and aspartate were 7.4 +/- 1.19 and 2.8 +/- 0.81 pmol/10 min, respectively. The release of glutamate-d5 was not significantly increased by nerve stimulation, while that of endogenous glutamate was increased by about 9.7 pmol/10 min above the resting release. The application of high-K solution induced a net increase of 1.9 pmol/10 min in glutamate-d5 release, 6.1 pmol/10 min in endogenous glutamate release and 7.9 pmol/10 min in aspartate release. The relative amount of glutamate-d5 accumulated into the preparation during pre-incubation with 0.5 mM-glutamate-d5 was a few per cent of the endogenous glutamate in the preparation. It is concluded that the exogenous glutamate is taken up mainly into a compartment which differs from that related to nerve evoked transmitter release and that high-K solution has effects on the amino acid release which differ from those of nerve stimulation.

Suppression of the enzymes of glutamate metabolism in cortical synaptosomes in methionine sulfoximine toxicity

Enzymes of glutamate metabolism were studied in synaptosomes prepared from normal rats and those treated with acute (300 mg/kg) and subacute (150 mg/kg) doses of the convulsant methionine sulfoximine (MSO). The activities of glutamine synthetase, glutamate dehydrogenase and aspartate aminotransferase were inhibited in the synaptosomes of drug treated animals. It is suggested that MSO would suppress the formation of glutamine and glutamate and consequently the releasable pool of glutamate, aspartate and GABA. These neurotransmitters would be depleted from the nerve endings. It is also indicated that the ammonia accumulated would affect the cerebral functioning by interfering with the maintenance of ionic gradients.

Variable CA II compartmentalization in vertebrate retina

We have generated a series of polyclonal and monoclonal antibodies to mammalian, avian, and osteichthian CA II for the purpose of studying its distribution in vertebrate nervous systems. In mature chicken retina, CA II is immunohistochemically detectable only in Müller glial cells. However, during embryonic development, CA II expression is suddenly "switched-on" early as a general constituent of all retinoblasts, later becoming restricted to Müller cells and transiently to a distinct type of amacrine neuron. A similar developmental pattern occurs in mouse. However, at maturity high CA II levels remain in certain amacrine neurons in addition to Müller cells. Comparative analyses of mature retinas of lower vertebrates show that reptiles parallel chicken with high CA II only in Müller cells, certain amphibians show CA II staining in Müller cells, amacrine neurons as in mouse, and in horizontal neurons, teleost and elasmobranch fish possess high CA II in Müller cells and the horizontal neurons, and lamprey eel shows CA II staining primarily in horizontal cells. An evolutionary sequence that will be discussed is thus suggested.

Kinetic properties of glutaminase from cerebral cortex

The rates of phosphate activated glutaminase activity in finely homogenised cerebral cortex and synaptosomes were measured. Activity was 25-50% higher at pH 7.0 than at pH 8.0. Glutamate inhibited activity with a Ki of 2-3 mM while aspartate had little effect. Calcium (1 mM) activated the enzyme but magnesium was without action. The pH profiles of the effects of these modulators of glutaminase activity in these finely ground preparations showed that all agents were more effective at pH 7.0 than at pH 8.0.

Ornithine-delta-aminotransferase exhibits different kinetic properties in astrocytes, cerebral cortex interneurons, and cerebellar granule cells in primary culture

To evaluate a possible role of ornithine-delta-aminotransferase (EC 2.6.1.13; Orn-T) as a rate-limiting enzyme for the synthesis of transmitter glutamate and gamma-aminobutyric acid (GABA), respectively, its activity and kinetic properties were analyzed in cultured astrocytes as well as in neuronal cultures consisting mainly of glutamatergic neurons (cerebellar granule cells) or GABAergic neurons (cerebral cortex interneurons). For comparison the activity and kinetics of Orn-T were also assayed in mouse brain homogenates. The highest activity of Orn-T was found in astrocytes and in cerebral cortical neurons (5.3 +/- 0.5 and 5.3 +/- 0.4 nmol X mg-1 X min-1, respectively) whereas the activities of Orn-T in cerebellar granule cell cultures and in mouse brain were found to be about half of these values (3.1 +/- 0.3 and 2.8 +/- 0.1 nmol X min-1 X mg-1, respectively). From a kinetic study of Orn-T in the different preparations only a relatively low affinity for the enzyme with respect to ornithine was found in cerebellar granule cells, astrocytes, and whole brain [apparent Km values (at 0.5 mM alpha-ketoglutarate): 4.7 +/- 0.9, 4.3 +/- 2.2, and 6.8 +/- 2.2 mM, respectively] whereas the corresponding Km value for Orn-T in cerebral cortex interneurons was found to be significantly lower (apparent Km: 0.8 +/- 0.3 mM). The enzyme was not found to be inhibited by GABA (range 0.1 - 10 mM) in any of the preparations.

Glutamine oxidation by dissociated cells and homogenates of rat brain: kinetics and inhibitor studies

The rates of [U-14C]glutamine oxidation to 14CO2 were determined under a variety of experimental conditions using whole homogenates and dissociated cells from rat brain. The pattern of glutamine oxidation by homogenates differed from that by dissociated brain cells in several respects. The rates of glutamine oxidation by dissociated brain cells showed saturation kinetics with an apparent Km of 0.30 mM. Lineweaver-Burk plots of glutamine oxidation by homogenates revealed two linear segments with two apparent Km values (0.58 mM and 3.0 mM). In the presence of aminooxyacetate, however, the Lineweaver-Burk plots for homogenates were linear with a single Km of 0.47 mM. The oxidation of glutamine by homogenates was inhibited by both rotenone and antimycin A (80-85%), as were glutamate and glucose oxidation, suggesting that a significant amount of glutamine is oxidized via the tricarboxylic acid cycle. In the presence of aminooxyacetate, glutamine oxidation was inhibited less than 40%, whereas the oxidation of glutamate was inhibited 75%; in contrast, glucose oxidation was enhanced 50%. The rates of glutamine oxidation by homogenates were highest in the presence of high levels of potassium (50 mM) and low levels of sodium (2.5 mM). Varying ionic composition, however, had little or no effect on the rates of glutamine oxidation by dissociated brain cells. Measurements of glutamine oxidation by homogenates prepared from 2-, 10-, 15-, 25-, and 90-day-old rats revealed little or no age-dependent difference. In contrast, the oxidation by dissociated brain cells from 2-day-old animals was significantly less than that obtained for animals 10 days or older (7.76 vs. 15.6 nmol/h/mg).(ABSTRACT TRUNCATED AT 250 WORDS)

Cellular compartmentalization of carbonic anhydrase-C and glutamine synthetase in developing and mature mouse neural retina

Using immunohistochemical methods, we have determined the cellular localization of the enzymes, glutamine synthetase (GS) and carbonic anhydrase-C (CA-C), in mouse neural retina during development and in the mature tissue. GS is always confined exclusively to the Müller glial cells; it is first detectable in these cells post-natally on about day 12, i.e. shortly before the eyes open. Also CA-C in the mature retina is localized in the Müller cells but, in addition, it is found in certain amacrine neurons as well. CA-C is first detectable in the retina already several days before birth; at that time it is found in most of the cells, with the exception of the emerging ganglion cells. However, with advancing differentiation, CA-C becomes progressively restricted to Müller cells and to a sub-category of amacrine neurons, and persists only in these cells in the mature retina. The present results extend our previous studies on these enzymes in the avian retina; they demonstrate that also in mammalian retina, different temporal and cellular patterns of GS and CA-C expression and localization earmark distinct phases of structural and functional differentiation of the retina. The striking developmental changes in the cellular localization of CA-C, and the finding of this enzyme in certain amacrine neurons as well as in Müller cells, raise questions about the role of CA-C in the retina, and about mechanisms regulating its expression in specific cell types.

Localization of cysteine sulfinic acid uptake sites in rat brain by quantitative autoradiography

In vitro autoradiography was employed to localize and quantify Na-dependent binding sites of [35S]cysteic acid (CA), an analog of cysteine sulfinic acid (CSA). The heterogeneous anatomical distribution and pharmacological specificity of [35S]CA differs from that of the glutamate/aspartate marker D-[3H]aspartate, and appears to represent a specific uptake site for CSA. These results suggest that CSA may act as an excitatory transmitter in the central nervous system.

Storage and release of endogenous and labelled GABA formed from [3H]glutamine and [14C]glucose in hippocampal slices: effect of depolarization

To study the effect of depolarization on the synthesis, storage and release of GABA, hippocampal slices were incubated in 0.25 mM [3H]glutamine and 2.5 mM [14C]glucose in the presence of 3 or 50 mM K+. Total and labelled glutamine, glutamate and GABA contents were measured by high-performance liquid chromatography. Depolarization in the presence of Ca2+ led to a two-fold increase of labelled glutamate and a 3-fold increase of labelled GABA content originating from both labelled precursors. In the absence of Ca2+ and in the presence of 10 mM Mg2+, depolarization failed to increase labelled glutamate content and labelled GABA formation was increased by only 30%. Following superfusion with unlabelled 0.25 mM glutamine and 2.5 mM glucose a second depolarization with 50 mM K+ released twice as much labelled GABA from slices that had been incubated in the presence of 50 mM K+, than from those incubated in 3 mM K+. This difference remained unchanged in slices that were superfused with 1 mM aminooxyacetic acid, an inhibitor of GABA synthesis. The contribution of labelled GABA, especially of GABA derived from [3H]glutamine, to released GABA was significantly higher than to GABA stored in the slices. Results suggest that depolarization in the presence of Ca2+ results in increased glutamate and GABA synthesis from both glutamine and glucose and that part of GABA released by high K+ originates from preformed GABA stores.

Preferential glutamine uptake by cochlear hair cells: implications for the afferent cochlear transmitter

The cochlear uptake of amino acids which are putative neurotransmitters, or closely-related compounds, was examined autoradiographically in the gerbil. Hair cells showed no preferential uptake of most compounds tested. However, preferential accumulation of glutamine by cochlear hair cells was striking. Vestibular hair cells showed no affinity for this amino acid. Glutamine uptake by cochlear hair cells may play an important role in afferent synaptic transmission, by providing transmitter precursor and/or by clearing the synaptic cleft.

Autoradiographic localization and depolarization-induced release of acidic amino acids in differentiating cerebellar granule cell cultures

Granule cells from 8-day-old rat cerebella were grown in basal Eagle's medium with 10% fetal calf serum, for 2,5,8 or 12 days in vitro (DIV), in conditions giving a purity greater than 90%. The results obtained can be summarized as follows: (1) Light microscopic autoradiography showed that cultured granule cells and their processes can accumulate the glutamate analog [3H]D-aspartate once they have reached an advanced degree of morphological differentiation (8 and 12 DIV), but, even then, only a limited number of cells was heavily labeled. In contrast, astrocytes were heavily labeled at all stages. (2) Calcium-dependent, high [K+]-induced release, or tetrodotoxin-sensitive, veratridine-induced release of [3H]D-aspartate from granule cell-enriched cultures was detectable only in cultures of 8 or 12 DIV. (3) When subject to 3 consecutive depolarizations, cultured granule cells maintained their ability to release [3H]D-aspartate and endogenous glutamate almost unchanged. (4) Newly synthesized [3H]glutamate was autoradiographically localized in both neurons and astrocytes (the latter, however, were not preferentially labeled as with [3H]D-aspartate), but was specifically released from neuronal structures (perikarya and processes) by depolarizing stimuli.

In vivo release by vagal stimulation of L-[3H] glutamic acid in the nucleus tractus solitarius preloaded with L-[3H] glutamine

In anesthetized and paralyzed rats, using a push-pull perfusion technique, we examined the effect of bilateral vagal stimulation on the release of L-[3H] glutamic acid (L- [3H] Glu) from the nucleus tractus solitarius (NTS), after preloading the tissue either with L-[3H] Glu or L-[3H] glutamine (L- [3H] Gln). Vagal stimulation sufficient to produce a maximum fall of arterial pressure (AP) evoked release of L- [3H] Glu from the NTS when the tissue was preloaded with either 3H-Glu or 3H-Gln, and of D-[3H] aspartic acid (D-[3H] Asp) when this stable Glu analogue was used to preloaded with either 3H-Glu or 3H-Gln, and of D-[3H] precursor L-Gln is a good marker of the releasable pool of L-Glu in vivo and are consistent with the hypothesis that L-[3H] Glu is a neurotransmitter in the NTS, mediating the vasodepressor response from cardiopulmonary mechanoreceptors.

Potential locus and mechanism of blockade of conditioned avoidance responding by neuroleptics

In order to assess the possible loci of action of neuroleptics in blocking the acquisition of a one-way conditioned avoidance response, microinjections of three neuroleptics and seven putative neurotransmitters were made into several brain regions and their effects on this behavior were assessed. When injected into the amygdala, the ED50 values for haloperidol (0.128 nmol), chlorpromazine (1.04 nmol) and thioridazine (1.41 nmol) were appropriate in relation to their clinical potency. Injections of neurotransmitters were without effect except in a few cases. Most significantly, the intra-amygdaloid administration of glutamate diethyl ester (an antagonist at quisqualate-type receptors) produced a blockade of avoidance acquisition which, as in the case of the neuroleptics, was not diminished by pretreatment with atropine. Following intraperitoneal injection of chlorpromazine, a statistically-significant blockade of avoidance acquisition and of glutamate, released from slices of amygdala, was obtained at doses of 2 mg/kg or more. With haloperidol, comparable behavioral effects and release of glutamate were found at doses of 0.05 mg/kg or more. The depression of release of glutamate from amygdaloid slices could be attributed to glutamate derived from glutamine. These data suggest a possible role for glutamatergic transmission in the effects of neuroleptics.

Glial contribution to amino acid content and metabolism of the deafferented dentate gyrus

The time course of tissue content and evoked release of endogenous amino acids was analyzed in the partially deafferented dentate gyrus of the rat hippocampus 2-24 days following unilateral lesion of the perforant path. Amino acids in tissue extracts and perfusates were determined after precolumn derivatization and hplc separation. The astrocytic glial cell reaction was monitored with immunohistochemistry of S-100. The tissue content of glutamate decreased significantly on the lesioned side, whereas only a moderate reduction in taurine, aspartate, and alanine occurred. Glutamine was significantly elevated at 7 days. The evoked efflux of glutamate was reduced at 2 and 7 days, whereas no change was seen at longer survival periods. The evoked release of GABA and aspartate increased on the denervated side after 12 and 24 days. The rate of carbon utilization into amino acid pools was followed with 14C-glucose and 14C-acetate. The incorporation of acetate showed a peak 2-9 days following lesion, which paralleled in time the hypertrophic glial cells. The incorporation of glucose decreased during this period. The metabolic events are discussed in relation to the morphological changes in synapses and glial cells.

Effects of hypoglycemia on the transmitter pool and the metabolic pool of glutamate in rat brain

The glutamate terminals in rat neostriatum were removed by unilateral frontal decortication. Insulin-induced hypoglycemia was accompanied by different changes in amino acids in rostral neostriatum on the intact and lesion side. Thus the glutamate/aspartate ratio, glutamate concentration and glutamine concentration were significantly more reduced and the aspartate concentration more elevated on the non-operated than on the operated side. The results show a higher turnover of glutamate in the non-operated side indicating a higher turnover of the transmitter pool than of the metabolic pool of glutamate. The combination of brain lesions with drugs inhibiting the metabolism points to a new method to investigate the compartmentation of glutamate in the mammalian brain.

Comparative investigations of glutaminase development in cerebral cortex of chick embryo and in primary cultures of neurons and glial cells

Glutaminase activity was determined in pure cultures of neurons, glial cells and in mixed cultures obtained from chick embryo brain. The development of this enzyme was observed periodically over time and compared to its evolution in corresponding cerebral hemispheres during embryonic and postnatal development. The specific activity of brain glutaminase increased between the twelfth and sixteenth day of embryogenesis. A similar increase was observed in cultures of neuroblasts during the corresponding period of time, although the activity in culture was about one-third lower than in vivo. In contrast to neurons, there was no significant increase of glutaminase activity in glial cells before the fifteenth day of culture. The enzyme level in glial cells between the thirteenth and fifteenth days of culture was approximately 25% of that in 7- and 8-day-old neurons. The different development of glutaminase activity in neurons and glial cells was demonstrated in both pure and mixed cultures. The results support the hypothesis that there is a glutamine shunt from glial cells to neurons.

Postnatal changes in the activity of glutamate dehydrogenase and aspartate aminotransferase in the rat nervous system with special reference to the glutamate transmitter metabolism

The activities of aspartate aminotransferase (AAT) and glutamate dehydrogenase (GIDH), the major glutamate metabolizing enzymes, were studied in hippocampal formation, cerebellar cortex, dorsal root ganglia, superior cervical ganglia and liver as a function of postnatal development. At birth, in all these nervous tissues the enzyme activities were quite low and showed similar levels (AAT 7-15 U/g wet weight; 0.18-0.23 U/mg protein; GIDH 3.4-13 U/g wet weight; 0.07-0.18 U/mg protein). Based on protein, AAT activity increased during the postnatal period studied 5.8 and 3.8 times in the hippocampal formation and cerebellar cortex, respectively, while the respective GIDH rise was 5.2 and 2.3 times. During postnatal maturation, enzyme activities in dorsal root ganglia showed only minor changes. In superior cervical ganglia, AAT and GIDH were remarkably constant. In liver the enzyme activities changed during postnatal development, but the activity curve profile was quite distinct from those obtained for brain regions. The steep rise of AAT and GIDH activities in brain regions is discussed as being a consequence of the maturation of preferably glutamatergic structures. Glutamatergic transmission processes obviously do not take place in superior cervical ganglia and dorsal root ganglia, and certainly not in liver. The present results suggest a quantitatively significant participation of glutamate transmitter metabolism in proportion to the whole glutamate metabolism of the CNS.

Uptake and metabolism of L-[3H]glutamate and L-[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.

In vivo release from cerebral cortex of [14C]glutamate synthesized from [U-14C]glutamine

Awake, unrestrained, and behaviourally normal animals with superfusion cannulae implanted over the sensorimotor cortex were used in a study of the capacity of infused [U-14C]glutamine for labelling glutamate and other amino acids released by depolarising stimuli. A spontaneous background release of [14C]glutamate was detected. This was increased by tityustoxin (1 microM). The specific radioactivity of glutamate increased eightfold during the evoked-release period. [14C]Aspartate was also detected and showed increased release, but not increased specific labelling, in response to depolarisation. Evoked gamma-aminobutyric acid (GABA) release occurred but only small amounts of [14C]GABA were detected. Glutamine showed increased rates of uptake to the sensorimotor cortex during stimulation periods, suggesting an accelerated breakdown via glutaminase.

[15N] leucine as a source of [15N] glutamate in organotypic cerebellar explants

Approximately 26.0% of the [15N] glutamate and [alpha 15N] glutamine formed in organotypic cerebellar explants was derived from [15N] leucine. Approximately 14.0% of the 15NH3 and [amide 15N] glutamine synthesized came from leucine nitrogen. Another 4.0% of the alpha nitrogen of both glutamate and glutamine was derived from [15N] valine. These results suggest that branched-chain amino acids, particularly leucine, may be important for the synthesis of glutamic acid by the brain.

Selective inhibition of glial versus neuronal uptake of L-glutamic acid by SITS

SITS, an inhibitor of anion exchange, was found to be a potent and selective inhibitor of L-glutamic acid uptake by cultured LRM55 glioma cells and rat brain astrocytes. Synaptosomal uptake of glutamate was relatively insensitive to inhibition by SITS. This differential effect indicates that the glutamate transport system in glia differs from that in neurons and that SITS may provide a tool for investigating the exclusive neuronal transport and metabolism of L-glutamic acid.

Hormonal induction of glutamine synthetase in cultures of embryonic retina cells: requirement for neuron-glia contact interactions

Cortisol induces glutamine synthetase (GS) in gliocytes of chick embryo neural retina. Using adherent cultures of retina cells we have demonstrated that responsiveness of the gliocytes to GS induction by the hormone requires contact with neurons. GS is not inducible in high-density cultures depleted of neurons and consisting only of gliocytes. In neuron-containing cultures, induced GS was detected immunohistochemically only in those gliocytes that were closely juxtaposed with clusters of neurons. Unlike the induction of GS, the expression of carbonic anhydrase-C (which does not require cortisol) persisted in these glia cells also in the absence of neurons. The nature and role of glia-neuron interactions in the hormonal induction of GS are briefly discussed.

Release of endogenous amino acids from superior colliculus of the rabbit: in vitro studies after retinal ablation

Tissue slices from the superior colliculi (SC) of the rabbit were superfused and investigated 1 week after unilateral eye removal. Amino acid levels were determined both in the tissue slices and in the medium after chemical depolarisation (56 mM K). The amino acid determinations were done fluorimetrically by precolumn derivation and HPLC separation. Colliculi contralateral to the enucleation exhibited a 16% reduction in glutamate compared with the ipsilateral colliculi. The Ca-dependent release of glutamate or other amino acids tested was not appreciably affected by enucleation. However, both the total and the Ca-independent release of glutamate was lower from contralateral SC slices compared with the ipsilateral slices. The results do not favour glutamate as the major optic nerve transmitter in the rabbit, but do not rule out glutamate as a transmitter in a minor population of retinal fibres.

Selective uptake of [3H]glutamine and [3H]glutamate into neurons and satellite cells of dorsal root ganglia in vitro

The uptake of [3H]glutamate and [3H]glutamine into rat dorsal root ganglia has been examined by autoradiography and thin-layer chromatography. [3H]glutamate was selectively accumulated by satellite glial cells and after 10 min, 53% of this had been converted to [3H]glutamine. [3H]glutamine, on the other hand, entered neuronal perikarya and 40% was converted to [3H]glutamate. It is suggested that these selective uptake processes provide supporting evidence for the existence of a neuronal-glial glutamine cycle in dorsal root ganglia. Small dark (B) cells accumulated 6 times as much [3H]glutamine as did large light (A) cells. The reasons for this marked difference in the metabolism of the two main types of dorsal root ganglion neurone are discussed.

Glutamine and glucose as precursors of transmitter amino acids: ex vivo studies

Radiolabelled glutamine and glucose were infused into lateral ventricles of rats in order to label transmitter amino acid pools in vivo. Brain regions close to the lateral ventricle (hippocampus, corpus striatum, hypothalamus) were labelled more effectively than more distant structures such as cerebral cortex or cerebellum. All regions were labelled to much the same extent over 30-150 min by [U-14C]glucose, [U-14C]glutamine, or [3H]glutamine administered alone or together in double-label experiments when allowance was made for any differences in precursor specific radioactivities. Slices of cerebral cortex or hippocampus from brains labelled in vivo were incubated and stimulated in vitro with veratrine (75 microM); tetrodotoxin (1 microM) was present in the control medium. Single-label experiments showed that [U-14C]glutamine was more effective than [U-14C]glucose for labelling releasable glutamate and GABA. Double-label experiments showed that [3H]glutamine and [U-14C]glucose given together in vivo labelled glutamate and GABA releasable in vitro to a similar extent. Both types of experiment emphasise the large contribution made by glutamine in vivo to pools of transmitter glutamate and GABA.

Calcium stimulation of glutamine hydrolysis in synaptosomes from rat brain

Calcium stimulates the hydrolysis of glutamine in synaptosomes prepared from rat brain both by the sucrose- (12) and the Ficoll/sucrose-gradient techniques (13). The calcium activation is phosphate-dependent and maximal effect is obtained at a calcium concentration of 0.5-1.0 mM. It is reduced by increasing the numbers of synaptosomes in the incubation mixture, and abolished by the product inhibitors of glutaminase, glutamate and ammonia, but unaffected by the uncoupler 2,4-dinitrophenol which inhibits the mitochondrial proton pump. Moreover, since the hydrolysis of glutamine is mediated by glutaminase (EC 3.5.1.2), and calcium does not activate the purified enzyme, an indirect phosphate-dependent effect of calcium on glutaminase is most likely. Calcium activates preferentially the N-ethylmaleimide insensitive fraction of glutaminase. The calcium activation is not dependent on synaptosomal membranes as it is found in synaptosomes subject to previous freezing. It is also found in isolated synaptosomal mitochondria and is thus a property of nerve endings. The calcium activation of glutaminase is unaffected by potassium in depolarizing concentrations, and may not be directly involved in the neurotransmission processes, but possibly in replenishing depleted stores of transmitter glutamate.

Selective release of glutamate from cerebellar granule cells differentiating in culture

The aim of the present study was to assess whether endogenous and newly synthesized glutamate can be released from differentiating cultured cerebellar granule cells in a way compatible with a neurotransmitter role. Granule cells from 8-day-old rat cerebella were grown in basal Eagle's medium with 10% fetal calf serum for 2-12 days in vitro (DIV), then washed with Krebs-Ringer medium, and labeled for 45 min with tracer amounts of radioactive glutamine. Subsequently, the release of endogenous glutamate and of newly formed radioactive glutamate was measured in basal conditions and upon depolarization with elevated K(+) concentration or veratridine. At 2 DIV, the release of endogenous and newly synthesized glutamate evoked by high K(+) concentration was small and Ca(2+) independent, but it progressively and steadily increased (up to 8- to 10-fold) and became Ca(2+) dependent (up to 80-85%) at later stages (4, 8, and 12 DIV). Veratridine was almost ineffective with cells at 2 DIV but greatly increased glutamate release (endogenous and neosynthesized) at 8 DIV, and its action was totally antagonized by tetrodotoxin. The level and synthesis of glutamate remained fairly constant in cells from 2 to 12 DIV. gamma-Aminobutyric acid synthesis from radioactive glutamine was about 3% of that of glutamate, and gamma-aminobutyric acid release (endogenous and neosynthesized) was not measurable. Aspartate synthesis was about 10% of that of glutamate, and the high K(+) concentration-evoked release of this amino acid was modest and scarcely affected by Ca(2+). Neither high K(+) concentration nor veratridine was able to induce glutamate release from confluent cerebellar astrocyte cultures at 14 DIV, although the level and synthesis of the amino acid were comparable to those in granule cells. In conclusion, the data show that a stimulus-coupled release of endogenous and neosynthesized glutamate is progressively expressed by cerebellar granule cells differentiating in culture, and this strongly supports the concept that glutamate is the neurotransmitter of these cells.

In vitro release of endogenous beta-alanine, GABA, and glutamate, and electrophysiological effect of beta-alanine in pigeon optic tectum

The efflux of 20 amino acids, induced by either high K+ concentration or veratrine, was determined in pigeon tectal slices. Ca2+-dependent, K+-induced release of beta-alanine, gamma-aminobutyric acid (GABA), and glutamate was observed. Veratrine caused release of the same amino acids plus glycine in a tetrodotoxin-sensitive manner. beta-Alanine had a strong inhibitory effect on the activity of tectal neurons which was blocked by strychnine but not by bicuculline. The results indicated a transmitter function for beta-alanine in the optic tectum, and were consistent with the previously proposed transmitter role of GABA and glutamate in this structure.