Content of amino acids in axons from the CNS of the lobster.

The contents of alanine, proline, glycine, GABA, glutamate, and aspartate were measured in four bundles of axons (designated areas A through D) from the circumesophageal connective of the lobster (Homarus americanus). The contents of these amino acids were also determined in individual axons within specific bundles and in the external sheath covering the circumesophageal connective. Within the nerve bundles the levels of aspartate were highest of the amino acids measured, ranging from 1.95 +/- 0.12 mumol/mg protein in area C to 7.55 +/- 0.54 mumol/mg protein in area B. On the other hand, GABA had the lowest value in the four bundles; its highest level was found in area C (0.083 +/- 0.006 mu mol/mg protein) and the lowest in area B (none detected). The content of glycine ranged from 1.63 +/- 0.14 (area C) to 2.52 +/- 0.32 mumol/mg protein in area A; that for glutamate ranged from 0.390 +/- 0.019 (area C) to 1.01 +/- 1.03 (area B). The contents of alanine and proline changed relatively little from bundle-to-bundle. The content of aspartate was the highest of any of the amino acids assayed in individual axons (with diameters in the range of 40 to 65 mu) dissected from areas B and C. Glycine had the next highest content followed in order by glutamate, proline, and alanine. GABA was not detected in these axons. With the exception of GABA (which could not be detected), aspartate had the lowest level (0.066 +/- 0.017) and glycine had the highest level (2.00 +/- 0.498 mumol/mg protein) in the external sheath covering the the circumesophageal connective.     

The distribution of several amino acids in specific ganglia and nerve bundles of the lobster

Using the technique of measuring DNP-amino acid methyl esters by gas-liquid chromatography, the distribution of alanine, proline, glycine, GABA, glutamate and aspartate was determined in individual ganglia and the associated nerve bundles between these ganglia after isolation from the nervous system of the lobster, Homarus americanus. The brain or supraesophageal ganglion (27.2 mg) and the next 5 thoracic ganglia (varying from 24 to 10 mg in a rostral–caudal direction) as well as the nerve bundles connecting these ganglia were used. GABA and aspartate values varied the most among the individual ganglia; highest values were found in the second and third thoracic ganglia. The levels of alanine, proline, glycine and glutamate varied very little from ganglion to ganglion; however, the values for these amino acids did exhibit some variability among the individual connectives. The highest value for each was in the nerve bundle between the first and second thoracic ganglion. Glycine was present at the highest level of any of the amino acids whereas GABA was at the lowest level in the individual structures assayed.     

Contents of several amino acids in the cerebellum, brain stem and cerebrum of the 'staggerer', 'weaver' and 'nervous' neurologically mutant mice.

The content of glutamate, GABA, aspartate, glycine and alanine was determined in the cerebellum, brain stem and cerebrum of three different mutant mice which have been named ‘staggerer’, ‘weaver’ and ‘nervous’ on the basis of neurological symptoms. In the ‘staggerer’ and ‘weaver’ mutants there is an almost complete absence of granule cells in the cerebellar cortex while in the ‘nervous’ mutant there is a loss of Purkinje cells (and to a lesser extent a loss of granule cells) in the cerebellar cortex. In the cerebellum of the ‘weaver’ mutant, the content of glutamate was signficantly lower (P < 0.025) than control values (8.77 ± 0.76 vs 12.0 ± 1.3 μmol/g tissue wet wt) and the contents of GABA and glycine were significantly greater than normal levels. In the cerebellum of the ‘staggerer’ mutant, the content of glutamate was significantly lower (6.62 ± 0.70 μmol/g) and the contents of glycine and alanine significantly higher than control values. In the cerebrum and brain stem regions of the staggerer mutant, weaver mutant and the normals the contents of the five amino acids were the same. The contents of glycine and alanine in the cerebellum, GARA and glycine in the brain stem and GABA and alanine in the cerebrum of the nervous mutants were higher than control values. The data are discussed in terms of a possible role for glutamate functioning as an excitatory transmitter when released from the cerebellar granule cells.     

Levels of free amino acids in excitatory, inhibitory and sensory axons of the walking limbs of the lobster

Using a gas chromatography procedure, the levels of several amino acids were determined in individual excitatory and inhibitory axons, in bundles of sensory fibers, and in muscle tissue from the walking limb of the lobster, $\text{Homarus}$$\text{americanus}$. In addition, the levels of amino acids in the hemolymph were also determined. Of the amino acids assayed in the excitatory and inhibitory axons and in the sensory fibers the level of aspartate was highest whereas in hemolymph and muscle, aspartate had one of the lowest values. The levels of glutamate, glycine and proline were significantly higher in the excitatory axons than in the inhibitory axons. GABA was present in inhibitor axons and in the muscle tissue which these axons innervate and was not detected in the other axons assayed nor in the hemolymph. β-Alanine was present at low levels in hemolymph and in muscle but was not detected in the excitatory nor in the inhibitory axons.     

INCORPORATION OF 14C FROM [U-14C]GLUCOSE INTO FREE AMINO ACIDS IN MOUSE BRAIN REGIONS UNDER CYANIDE INTOXICATION

—During anoxia induced by the administration of potassium cyanide, [U-¹⁴C]glucose was injected intraperitoneally into adult mice and they were decapitated at 5, 15 and 30 min after the injection. After freeze-drying in vacuo, differences in the uptake of radioactive carbon from [U-¹⁴C]glucose into free amino acids (glutamate + glutamine, aspartate + asparagine, GABA, alanine and glycine) in mouse cerebral neocortex, cerebellar hemisphere, caudate nucleus, thalamus, hypothalamus and medulla oblongata were investigated (by macroautoradiography and GLC separation) and compared with those obtained under normal conditions. (1) During anoxia, autoradiographical densities in the thalamus and medulla oblongata were higher than that in the cerebral neocortex and caudate nucleus. (2) Among specific radioactivities (d.p.m./μmol) of free amino acids, alanine gave the highest value during anoxia, except in the cerebellar hemisphere and hypothalamus at 5 min and the medulla oblongata at 30 min. (3) During anoxia, the specific radioactivities of alanine and glycine in each brain region did not significantly decrease at 15 and 30 min compared with those under normal conditions. During anoxia, the specific radioactivity of glutamate + glutamine in the cerebellar hemisphere and hypothalamus did not significantly decrease compared with the normal conditions, while that of GABA, aspartate + asparagine and glutamate + glutamine in the cerebral neocortex, caudate nucleus, thalamus and medulla oblongata showed an increase. (4) The percentage decrease of glutamate + glutamine and aspartate + asparagine at 5 and 15 min was highly significant in the cerebral neocortex and caudate nucleus.     

A Comparative Study of Excitatory and Inhibitory Amino Acids in Three Different Brainstem Nuclei

This study was designed to shed more light onto the three different brainstem regions which are implicated in the pain pathway for the level of various excitatory and inhibitory neurotransmitters before and following neuronal stimulation. The in vivo microdialysis technique was used in awake, freely moving adult Sprague-Dawley rats. The neurotransmitters studied included aspartate, glutamate, GABA, glycine, and taurine. The three brainstem regions examined included the mid-brain periaqueductal gray (PAG), the medullary nucleus raphe magnus (NRM), and the spinal trigeminal nucleus (STN). Neuronal stimulation was achieved following the administration of the sodium channel activator veratridine. The highest baseline levels of glutamate (P < 0.0001), aspartate (P < 0.0001), GABA (P < 0.01), taurine (P < 0.0001), and glycine (P < 0.001) were seen in the NRM. On the other hand, the lowest baseline levels of glutamate, GABA, glycine, and taurine were found in the PAG, while that of aspartate was found in the STN. Following the administration of veratridine, the highest release of the above neurotransmitters except for the aspartate and glycine was found in the PAG where the level of glutamate increased by 1,310 ± 293% (P < 0.001), taurine by 1,008 ± 143% (P < 0.01), and GABA by 10,358 ± 1,920% (P < 0.0001) when comparison was performed among the three brainstem regions and in relation to the baseline levels. The highest release of aspartate was seen in the STN (2,357 ± 1,060%, P < 0.001), while no significant difference was associated with glycine. On the other hand, the lowest release of GABA and taurine was found in the STN (696 ± 91 and 305 ± 25%, respectively), and glutamate and aspartate in the NRM (558 ± 200 and 874 ± 315%, respectively). Our results indicate, and for the first time, that although some differences are seen in the baseline levels of the above neurotransmitters in the three regions studied, there are quite striking variations in the level of release of these neurotransmitters following neuronal stimulation in these regions. In our opinion this is the first study to describe the pain activation/modulation related changes of the excitatory and inhibitory amino acids profile of the three different brainstem areas.     

Regional changes in amino acid levels of the neonate rat brain during anoxia and recovery

The aim of this study was to compare the changes in amino acids (alanine, aspartate, GABA, glutamate, glutamine, glycine, serine taurine) that are produced in different regions of the neonate brain (telencephalon, diencephalon cerebellum, brain stem) following a survivable period of anoxia and after the re-establishment of air respiration. Anoxia provoked different responses in the different regions. The changes during the anoxic period were as follows. In the brain stem there was a decrease in aspartate, in the telencephalon there was a significant increase in GABA and alanine and a decrease in aspartate, in the diencephalon, glutamate and GABA increased, and in the cerebellum, glycine and alanine levels were enhanced. The changes during recovery were even more dissimilar. Here the greatest shifts were seen in the brain stem with increases in glutamine, GABA, aspartate, glycine, serine, alanine, and taurine. In the telencephalon glutamate fell and alanine increased, in the diencephalon GABA increased, and in the cerebellum, glutamate fell while glycine and alanine increased. In none of the major brain regions did the pattern of changes in neurotransmitters correspond to that seen in anoxic tolerant species.     

Effects of 3-acetylpyridine on the levels of several amino acids in different CNS regions of the rat

The effects of one intraperitoneal injection of 60–65 mg/kg of 3-acetylpyridine (3-AP) on the levels of aspartate, glutamate, GABA, taurine, glycine, and alanine in the cerebellum, medulla, telencephalon, and diencephalon-mesencephalon of the rat were studied at various times (4–28 days) after injection. In the first 4–7 days, the levels of glutamate, GABA, glycine, and alanine in the cerebellum were 10–30% higher in the 3-AP-treated rats than in the control animals. By day 14, the levels of these four amino acids were normal (in the case of glutamate and glycine) or below normal (for GABA and alanine). By day 21, the values for GABA and alanine returned to normal. In the first 7 days, the level of aspartate in the cerebellum was the same in both the 3-AP- and saline-injected groups. From days 14 to 28, the level of aspartate in the cerebellum was 10–20% lower in the 3-AP-injected group than in the saline-treated animals. The level of taurine in the cerebellum was 15–30% lower in the 3-AP group than in the control group from days 7 to 28. The pattern of changes observed in the medulla in the first 7 days was similar to that found in the cerebellum for this period. However, unlike the data for the cerebellum, the level of aspartate in the medulla was unchanged by the 3-AP injection from day 14 to day 28, and the level of glutamate in the medulla remained higher (10–15%) from days 14 to 28 in the 3-AP-injected animals with respect to control values. The levels of taurine in the medulla were lower (10–15%) from day 7 to day 28 in the 3-AP injected group with respect to control values. The injection of 3-AP did not alter the levels of aspartate, glutamate, GABA, taurine, glycine, or alanine in the telencephalon on days 7, 14, 21, or 28 and in the diencephalon-mesencephalon on day 21 with respect to control levels.     

In Vitro Release of Endogenous Amino Acids from Granule Cell-, Stellate Cell-, and Climbing Fiber-Deficient Cerebella

Abstract: The K⁺-stimulated, Ca²⁺-dependent release of glutamate, aspartate, -γ-aminobutyric acid (GABA), alanine, taurine, and glycine was measured in slices of cerebella obtained from control, and granule cell-, granule cell plus stellate cell-, or climbing fiber-deficient cerebella of the rat. The 55 mm -K⁺-stimulated release of glutamate and GABA was 10-fold greater in the presence of Ca²⁺ than in its absence. The stimulated release of aspartate was 4-fold higher when Ca²⁺ was present in the bathing media, while the value for alanine was twice as high as the amount obtained in the absence of Ca²⁺. There was no stimulated release of either taurine or glycine from the cerebellar slices. Increasing the Mg²⁺ concentration to 16 HIM inhibited the K⁺-stimulated, Ca²⁺-dependent release of glutamate, GABA, aspartate, and alanine 85% or more. The K⁺-stimulated, Ca²⁺ dependent release of glutamate, aspartate, and alanine from x-irradiated cerebella deficient in granule cells was reduced to 50–57% of control value. Additional x-irradiation treatment, which further reduced the cerebellar granule cell population and also prevented the acquisition of stellate cells, decreased the release of glutamate by 77%, aspartate by 66%, alanine by 91%, and, in addition, decreased the release of GABA by 55%. The K⁺-stimulated, Ca²⁺-dependent release of glutamate, aspartate, GABA, and alanine was not changed in climbing fiber-deficient cerebella obtained from 3-acetylpyridine-treated rats. The data support a transmitter role for GABA and glutamate in the cerebellum, but do not support a similar function for either taurine or glycine. The data also suggest that alanine and aspartate may be co-released along with glutamate from granule cells.     

INCORPORATION OF 14C FROM [U-14C]GLUCOSE INTO FREE AMINO ACIDS IN MOUSE BRAIN LOCI IN VIVO UNDER NORMAL CONDITIONS

By macroautoradiography and by GLC separation, differences in the uptake of radioactive carbon from [U-¹⁴C]glucose into free amino acids (glutamate + glutamine, aspartate + asparagine, GABA, alanine and glycine) in mouse cerebral neocortex, hippocampus, thalamus and hypothalamus were investigated. (1) The autoradiographical densities in the thalamus, cerebral neocortex and hippocampus measured with a microdensitometer were higher than that in the hypothalamus at 5 min after subcutaneous injection. At 180 min, densities in the cerebral neocortex, hippocampus and hypothalamus were higher than that in thalamus. (2) The free amino acid levels determined by GLC varied with each brain region. (3) The specific radioactivity (d.p.m./μmol) of alanine in each brain region was higher than that of the other amino acids at 5 min after the injection. The specific radioactivity of GABA in the brain regions was clearly higher than that of (glutamate + glutamine), (aspartate + asparagine) and glycine at 5 and 15 min. (4) The autoradiographical data were in good agreement with the chemical data at 5 min but were different at 180 min. (5) Variations in specific radioactivity of each free amino acid among brain regions at 5 min were influenced greatly by existing free amino acid concentrations in each region.     

Glucose and amino acid metabolism in neonatal rat skeletal muscle in tissue culture

The characteristics of glucose and amino acid metabolism over a 98-hour incubation period were studied in a primary culture of neonatal rat skeletal muscle cells. The cells formed large myotubes in culture, were spontaneously highly contractile, and had cell phosphocreatine levels exceeding ATP concentrations. Medium glucose fell from 7.2±0.2 to 1.5±0.1 mM between 0 and 98 hours; intracellular glucose was readily detectable, indicating glycolysis was limited by phosphorylation, not glucose transport. Alanine levels in the medium increased from 0.06±0.01 to 0.82±0.04 mM between 0 and 48 hours and decreased to 0.72±0.04 mM by 98 hours. The period of net alanine production correlated with the rise in the cell mass action ratio of the alanine aminotransferase reaction. Cell aspartate, glutamate, and calculated oxalacetate levels were inversely related to the cell NADH/NAD⁺ ratio, as represented by the intracellular lactate/pyruvate ratio (r=0.78–0.88). The branched chain amino acids (leucine, isoleucine, valine) were actively utilized, e.g., medium leucine fell from 0.70±0.01 to 0.30±0.06 mM between 0 and 98 hours. In addition, arginine and serine consumption was observed in conjunction with ornithine, proline, and glycine production. Conclusions: (1) A major driving force for the high rates of alanine production by skeletal muscle cells in tissue culture is the active utilization of branched chain amino acids. (2) Intracellular aspartate and glutamate pools are linked, probably via the malate-aspartate shuttle, to the cell NADH/NAD⁺ redox state. (3) Muscle cells in tissue culture metabolize significant amounts of arginine and serine in association with the production of ornithine and proline, and these pathways may possibly be related to creatine production.     

Biosynthesis of amino acids in Clostridium pasteurianum

1. Clostridium pasteurianum was grown on a synthetic medium with the following carbon sources: (a) (14)C-labelled glucose, alone or with unlabelled aspartate or glutamate, or (b) unlabelled glucose plus (14)C-labelled aspartate, glutamate, threonine, serine or glycine. The incorporation of (14)C into the amino acids of the cell protein was examined. 2. In both series of experiments carbon from exogenous glutamate was incorporated into proline and arginine; carbon from aspartate was incorporated into glutamate, proline, arginine, lysine, methionine, threonine, isoleucine, glycine and serine. Incorporations from the other exogenous amino acids indicated the metabolic sequence: aspartate --> threonine --> glycine right harpoon over left harpoon serine. 3. The following activities were demonstrated in cell-free extracts of the organism: (a) the formation of aspartate by carboxylation of phosphoenolpyruvate or pyruvate, followed by transamination; (b) the individual reactions of the tricarboxylic acid route to 2-oxoglutarate from oxaloacetate; glutamate dehydrogenase was not detected; (c) the conversion of aspartate into threonine via homoserine; (d) the conversion of threonine into glycine by a constitutive threonine aldolase; (e) serine transaminase, phosphoserine transaminase, glycerate dehydrogenase and phosphoglycerate dehydrogenase. This last activity was abnormally high. 4. The combined evidence indicates that in C. pasteurianum the biosynthetic role of aspartate and glutamate is generally similar to that in aerobic and facultatively aerobic organisms, but that glycine is synthesized from glucose via aspartate and threonine.     

Levels of several amino acids in the cerebellar peduncles and four medullary nuclei of control and 3-acetylpyridine-treated rats

The levels of aspartate, glutamate, GABA, taurine, glycine, and alanine were determined in the inferior (ICP), middle (MCP) and superior (SCP) cerebellar peduncles, and in the inferior olive (ION), lateral reticular (LRN), lateral vestibular (LVN) and descending trigeminal (DTN) nuclei of control rats and of rats given a single intraperitoneal injection of 65 mg/kg of 3-acetylpyridine (3-AP). The content of glutamate in the MCP and SCP was 30% higher than that found for the ICP. The content of GABA was 4 to 6-fold greater in the SCP than in either the ICP or MCP. The level of taurine in the SCP was 25% lower than the value in the MCP but was not significantly lower than the value for the ICP. The levels of aspartate, glycine, and alanine were evenly distributed among the three peduncles. The contents of aspartate, glutamate, taurine, and alanine were evenly distributed among the four medullary nuclei. The level of glycine was significantly greater in the DTN than in either the LRN or LVN. The content of GABA in the ION and LVN was significantly greater than the value found for the LRN. Injection of 3-AP caused a decrease in the level of taurine of 10% in the ION and LRN, 15% in the LVN, and 25% in the DTN. No other statistically significant differences were found in the levels of the amino acids in the peduncles or medullary nuclei following 3-AP treatment. The present data do not support the idea that asparate and/or taurine are present in relatively high concentrations in the cerebellar climbing fibers.     

PUTATIVE TRANSMITTERS IN DENERVATED OLFACTORY CORTEX

Olfactory bulb removal and the consequent degeneration of the lateral olfactory tract led to a selective change in putative synaptic transmitters of the denervated olfactory cortex of guinea-pigs. Nine days after bulbectomy there was a significant decrease in content of aspartate (31%) and glutamate (20%) in olfactory cortex but no change in neocortex. The content of GABA, alanine and ACh in olfactory cortex was unchanged (± 3 per cent of control). The content of 5-HT, dopamine, NE and glycine was increased by 10-18 per cent. The decrease in aspartate and glutamate content of the olfactory cortex after bulbectomy suggests that these two amino acids have a high concentration in fibres of the lateral olfactory tract. This tissue may prove to be useful in determining the possible role of these acidic amino acids in neuronal function.     

Release of Endogenous Amino Acids from the Striatum from Developing and Adult Mice in Ischemia

In most other studies the release of amino acid neurotransmitters and modulators in vitro has been studied mostly using labeled preloaded compounds. For several reasons the estimated release may not reliably reflect the release of endogenous compounds. The magnitudes of the release cannot thus be quite correctly estimated using radioactive labels. The basal and K⁺-evoked release of the neuroactive endogenous amino acids γ-aminobutyrate (GABA), glycine, taurine, glutamate and aspartate was now studied in slices from the striatum from 7-day-old to 3-month-old mice under control (normoxic) and ischemic conditions. The release of alanine, threonine and serine was assessed as control. GABA and glutamate release was much greater in 3-month-old than in 7-day-old mice, whereas with taurine the situation was the opposite. Ischemia markedly enhanced the release of all these three amino acids. The release of aspartate and glycine was markedly enhanced as well whereas no effects were discernible in the release of glutamine, alanine, serine and threonine. K⁺ stimulation (50 mM) enhanced the release of GABA, glutamate, taurine, aspartate and glycine in most cases, except with taurine in 3-month-old mice under the ischemic conditions and with aspartate in 7-day-old mice under the control conditions. K⁺ stimulation did not affect the release of glutamine, alanine, serine or threonine. The results on endogenous amino acids are qualitatively similar to those obtained in our earlier experiments with labeled preloaded amino acids. In conclusion, in developing mice only inhibitory taurine is released in such amounts that may counteract the harmful effects of excitatory amino acids in ischemia.     

The synthesis of glucose and ammonia by kidney tubules isolated from suckling and early-weaned lambs

Glucose and ammonia production were examined in kidney tubules isolated from suckling and early-weaned lambs, on days 10-30 after birth, with abrupt weaning occurring at day 14. There were no differences in the rates of glucose or ammonia production for a given substrate by tubules isolated from any of the lambs, regardless of age or stage of weaning. The preferred substrates for gluconeogenesis were glycerol = lactate greater than propionate = pyruvate = fructose = proline greater than alanine greater than glutamate greater than glutamine greater than aspartate greater than glycine greater than serine, and for ammoniagenesis were glutamine much greater than alanine greater than aspartate much greater than serine greater than glycine = glutamate = proline.     

Turnover rates of amino acid neurotransmitters in regions of rat cerebellum

The turnover rates of aspartate, gamma-aminobutyric acid (GABA), glutamate, glutamine, alanine, serine, and glycine were measured in five regions of rat cerebellum. Turnover rates of the putative neurotransmitters (aspartate, glutamate, and GABA) were 2-20-fold higher than those of alanine and serine, and generally consistent with the proposed neurotransmitter functions for these amino acids. However, glutamate turnover was high and similar in magnitude in the deep nuclei and granule layer, suggesting possible release, not only from parallel fibers, but from mossy fibers as well. The differential distribution of turnover rates for GABA supports its neuronal release by Purkinje, stellate, basket, and Golgi cells, whereas aspartate may be released by both climbing and mossy fibers. The distribution of glycine turnover rates is consistent with release from Golgi cells, whereas alanine may be released from granule cell parallel fibers. Turnover rates measured in two other motor areas, the striatum and motor cortex, indicated that utilization of these amino acid neurotransmitters is differentially distributed in brain motor regions. The data indicate that turnover rate measurements may be useful in identifying neurotransmitter function where content measurements alone are insufficient.     

The rate of turnover of cortical GABA from [1-13C]glucose is reduced in rats treated with the GABA-transaminase inhibitor vigabatrin (γ-vinyl GABA)

Brain GABA levels rise and plateau following prolonged administration of the irreversible GABA-transaminase inhibitor vigabatrin (γ-vinylGABA). Recently it has been shown that increased GABA levels reduces GAD₆₇ protein, one of two major isoforms of glutamic acid decarboxylase (GAD). The effects of GABA elevation on GABA synthesis were assessed in vivo using¹H and¹³C-edited NMR spectroscopy. Rates of turnover of cortical glutamate and GABA from intravenously administered [1-¹³C]glucose were measured in α-chloralose anesthetized rats 24 hours after receiving vigabatrin (500 mg/kg, i.p.) and in non-treated controls. GABA concentration was increased 2-fold at 24 hours (from 1.3±0.4 to 2.7±0.9 μmol/g) and GABA-T activity was inhibited by 60%. Tricarboxylic acid cycle flux was not affected by vigabatrin treatment compared to non-treated rats (0.47±0.19 versus 0.52±0.18 μmol/g, respectively). GABA-C2 fractional enrichment (FE) measured in acid extracts rose more slowly in vigabatrin-treated compared to nontreated rats, reaching >90% of the glutamate FE after 3 hours. In contrast, GABA FE≥glutamate FE in non-treated rats. A metabolic model consisting of a single glutamate pool failed to account for the rapid labeling of GABA from glutamate. Metabolic modelling analysis based on two (non-communicating) glutamate pools revealed a ∼70% decrease in the rate of GABA synthesis following vigabatrin-treatment, from 0.14 (non-treated) to 0.04 μmol/g/min (vigabatrin-treated). These findings, in conjunction with the previously reported differential effects of elevated GABA on the GAD isoforms, suggests that GAD₆₇ may account for a major fraction of cortical GABA synthesis in the α-chloralose anesthetized rat brain in vivo. Special issue dedicated to Dr. Herman Bachelard.     

THE FORMATION OF GLUTAMATE, ASPARTATE AND GABA IN THE RAT RETINA; GLUCOSE AND GLUTAMINE AS PRECURSORS

Abstract— The distribution of the neuroactive amino acids taurine, GABA, glycine, glutamate and aspartate, together with glutamine, have been studied in the rat retina. Peak levels of taurine were found in photoreceptor cells and of GABA and glycine in a retinal fraction enriched in amacrine cells and, synaptic terminals. In vitro , GABA formation from [³H]glutamine and [¹⁴C]glucose was also most prominent in this fraction; at 500 μ m [³H]glutamine was the better precursor.
Observations on metabolism in the photoreceptor cell layer of the tissue suggest an active turnover of glutamate, aspartate and GABA, and show that glutamine may serve as an alternative substrate to glucose here, perhaps via the GABA bypath.     

Effects of X-irradiation induced loss of cerebellar granule cells on the synaptosomal levels and the high affinity uptake of amino acids.

Abstract— Crude synaptosomal (P₂) preparations were obtained from the cerebella of rats in which the granule cell population had been selectively reduced by X-irradiation treatment and from the cerebella of control animals. In the P₂ fraction from control cerebella, the level of glutamate was greater than any other of the 5 amino acids measured and was 2-fold higher than taurine, which was present at the next highest level. The content of taurine was slightly higher than that found for aspartate and was 3-fold greater than that observed for GABA. Alanine and glycine were present in the lowest amounts. The levels of glutamate and aspartate were significantly (P < 0.05) lower by 25 and 15%, respectively, in the P₂ fraction isolated from the X-irradiated cerebella in comparison to control values. The content of taurine, GABA, glycine, and alanine were not changed by the X-irradiation treatment. The uptake of 1.0 μm -l -[³H]glutamate and l -[³H]aspartate was reduced approx 20% by X-irradiation treatment, whereas the uptake of 1.0 μm -[³H]GABA and [³H]taurine was unchanged. A more detailed kinetic analysis of l -[³H]glutamate uptake revealed there was a 20% decrease in the V_max value with X-irradiation treatment and no change in the apparent K_m value. In a second study, the uptake of l -[³H]glutamate, l -[³H]aspartate and [³H]GABA was measured using P₂ fractions obtained from the cerebella of rats in which the population of granule, stellate and basket cells had been reduced by X-irradiation treatment. The uptake of 1.0μm -l -[³H]glutamate, l -[³H]aspartate and [³H]GABA was significantly (P < 0.05) reduced to 57, 68, and 59%, respectively, of control values. A more detailed kinetic analysis of [³H]GABA uptake revealed no significant change in the apparent K_m and a 35% decrease in the V_max value. The data are discussed in terms of glutamate being the excitatory neurotransmitter released from granule cells and GABA being the inhibitory neurotransmitter released from basket cells.