THE RELEASE OF AMINO ACIDS FROM THE HEMISECTED SPINAL CORD DURING STIMULATION

Abstract— Isolated frog or toad hemicords were incubated for 40 min with either [¹⁴C]glycine, [³H]GABA, l -[¹⁴C]glutamate. l -[¹⁴C]aspartate, l -[¹⁴C]serine, l [¹⁴C]threonine or l -[³H]leucine, and the release of these compounds from the cord was measured under resting conditions and during electrical stimulation. Stimulation of spinal roots produced no significant change in the efflux of any of the compounds tested. Direct stimulation of the rostral cord however, produced a large increase in the efflux of [¹⁴C]glycine, [³H]GABA, l -[¹⁴C]glutamate and l -[¹⁴C]aspartate. These increased effluxes were calcium dependent, the effects of stimulation being reduced in a calcium-free, or magnesium-supplemented (10 mM) medium. Stimulation failed to produce an increase in the efflux of l -[¹⁴C]serine, l -[¹⁴C]threonine, l -[¹⁴H]leucine, [¹⁴C]mannitol or [¹⁴C]urea. These results are consistent with the suggestions that glycine, GABA, glutamate and aspartate may be synaptic transmitters in the spinal cord.     

EFFECT OF INSULIN ON LEVELS AND TURNOVER OF INTERMEDIATES OF BRAIN CARBOHYDRATE METABOLISM IN VIVO

Abstract— –The rates of incorporation of ¹⁴C from [U-^l4C]glucose into intermediary metabolites have been measured in rat brain in vivo. The time course of labelling of glycogen was similar to that of glutamate and of glucose, which were all maximally labelled between 20 and 40min, but different from lactate, which lost radioactivity rapidly after 20min. The extent of labelling of glycogen (d.p.m./ μ mol of glucose) was of the same order as that of glutamate at 20 and 40 min after injection of [¹⁴C]glucose. However, calculations of turnover rates showed that glutamate turns over some 8-10 times faster than glycogen. Insulin, intracisternally applied, produced after 4-5 h a 60 per cent increase in glucose-6-P and a 50 per cent increase in glycogen. There was no change in the levels of glucose, glutamate or lactate, nor in the activity or properties of the particulate and soluble hexokinase of the brain. The injection of insulin affected neither the glycogen nor glucose contents of skeletal muscle from the same animals. The effects of insulin on the incorporation of ^l4C into the metabolites contrasted with its effects on their levels. The specific activities of glycogen and glucose were unchanged and there was a slight but non-significant increase in the specific activity of glutamate. The time course of incorporation into lactate was unaffected up to 20 min, but a significant delay in the loss of ¹⁴C after 20 min occurred as a result of the insulin injection. At 40 min, the specific activity of cerebral lactate was 60 per cent higher in insulin-treated animals than in control animals. The results are interpreted in terms of an effect of insulin on glucose uptake to the brain, with possibly an additional effect on a subsequent stage in metabolism, which involves lactate.     

Oligodendroglial Glycerophospholipid Synthesis: Incorporation of Radioactive Precursors into Ethanolamine Glycerophospholipids by Calf Oligodendroglia Prepared by a Percoll Procedure and Maintained in Suspension Culture

Abstract: Oligodendroglia prepared from minced calf cerebral white matter by trypsinization at pH 7.4, screening, and isosmotic Percoll (polyvinylpyr-rolidone-coated silica gel) density gradient centrifugation survived in culture on polylysine-coated glass, extending processes and maintaining phenotypic characteristics of oligodendroglia. In the present study, ethanolamine glycerophospholipid (EGP) metabolism of the freshly isolated cells was examined during short-term suspension culture by dual label time course and substrate concentration dependence experiments with [2-³H]glycerol and either [1,2-¹⁴C]ethanolamine or L-[U-¹⁴C]serine. Rates of incorporation of ³H from the glycerol and of ¹⁴C from the ethanolamine into EGP were constant for 14 h. In medium containing 3 mM-[1,2-¹⁴C]ethanolamine and 4.8 mM-[2-³H]glycerol, rates of incorporation of ¹⁴C and ³H into diacyl glycerophosphoethanolamine (diacyl GPE) were similar. Under the same conditions, ³H specific activities of alkylacyl GPE and alkenylacyl GPE were much lower than ¹⁴C specific activities, likely as a result of the loss of tritium during synthesis of these forms of EGP via dihydroxyacetone phosphate. L-[U-¹⁴C]serine was incorporated into serine glycerophospholipid (SGP) by base exchange rather than de novo synthesis. ¹⁴C from L-[U-¹⁴C]serine also appeared in EGP after an initial lag period of several hours. Methylation of oligodendroglial EGP to choline glycerophospholipid (CGP) was not detected.     

Glutamine and Glucose as Precursors of Transmitter Amino Acids: Ex Vivo Studies

Abstract: 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 doublelabel 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 μ M ); tetrodotoxin (1 μ M ) 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 empbasise the large contribution made by glutamine in vivo to pools of transmitter glutamate and GABA.     

Exchange-mediated dilution of brain lactate specific activity: implications for the origin of glutamate dilution and the contributions of glutamine dilution and other pathways

The magnitude of metabolic activation is greatly underestimated in autoradiographic studies using [1- or 6-¹⁴C]glucose compared to parallel assays with [¹⁴C]deoxyglucose indicating that most of the label corresponding to the additional [¹⁴C]glucose consumed during activation compared to rest is quickly released from activated structures. Label could be lost by net release of [¹⁴C]lactate from brain or via lactate exchange between blood and brain. These possibilities were distinguished by comparison of glucose and lactate specific activities in arterial blood and brain before, during, and after generalized sensory stimulation and during spreading cortical depression. Over a wide range of brain lactate concentrations, lactate specific activity was close to the theoretical maximum, i.e. half that of [6-¹⁴C]glucose, indicating that exchange-mediated dilution of lactate is negligible and that efflux of [¹⁴C]lactate probably accounts for most of the label loss. Low lactate dilution also indicates that dilution of glutamate C4 fractional enrichment in [¹³C]glucose studies, currently ascribed predominantly to lactate exchange, arises from other unidentified pathways or factors. Alternative explanations for glutamate dilution (presented in Supporting Information) include poorly labeled amino acid pools and oxidative metabolism of minor substrates in astrocytes to first dilute the astrocytic glutamine pool, followed by dilution of glutamate via glutamate–glutamine cycling.     

Brain Carbohydrate Metabolism in Developing Rats During Hypercapnia

Abstract: Brain glucose metabolism was studied in developing rats at ages 10 and 20 days postnatal under normal and hypercapnic conditions. Brains were removed and frozen within 1 s with a freeze-blowing apparatus. Glucose utilization was measured with [2-¹⁴C]glucose and [³H]deoxyglucose as tracers. Metabolites were determined by standard enzymatic techniques. Data from [³H]deoxyglucose phosphorylation indicated that normal brain glucose utilization increased almost threefold between the 10th and 20th postnatal days. From the relative rates of utilization of the two isotopes in the 20-day-old control group, it appeared that about 25% of ¹⁴C label derived from metabolism of [2-¹⁴C]glucose was lost from brain (probably as lactate) rather than entering the Krebs cycle. Under hypercapnic conditions (20% CO₂-21% O₂-59% N₂), rates of glucose utilization by brain were decreased by one-half at both ages and there were progressive decreases in the concentrations of many intermediary metabolites. The bases for concluding that these metabolites were used to supplement glucose as a fuel for respiration, rather than being lost by leakage into blood, are discussed. Despite the differences in brain glucose metabolism between 10-day-old and 20-day-old rats, their responses to hypercapnia are remarkably similar: Rates of glucose utilization are reduced to approximately the same proportion of the original rate by 20% CO₂, and endogenous metabolites (particularly glutamate and lactate) appear to be oxidized as replacement fuels.     

NEUROTRANSMITTER UPTAKE INTO SLICES OF RAT CEREBRAL CORTEX IN VITRO: EFFECT OF SLICE SIZE

Abstract— The uptake of [¹⁴C]GABA, [¹⁴C]taurine, [³H] β -alanine and [¹⁴C]dopamine was compared in slices of rat cerebral cortex of three different sizes (0.1 × 0.1 × 2 mm, 0.2 × 0.2 × 2 mm and 0.4 × 0.4 × 2 mm prepared with a mechanical tissue chopper). [¹⁴C]Taurine and [³H] β -alanine uptake increased whereas [¹⁴C]GABA uptake decreased with increasing slice size. [¹⁴C]Dopamine uptake was optimal in 0.2 × 0.2 × 2 mm slices. Increasing slice size was shown to decrease inhibition of [³H] β -alanine and [¹⁴C]GABA uptake by l -2,4-diaminobutyric acid. Lactate dehydrogenase activity increased with increasing slice size indicating decreased tissue damage or increased cellular integrity. The possibility that varying slice size can be used to distinguish between neuronal and glial uptake is discussed. It is suggested that taurine uptake in the cerebral cortex is predominantly glial.     

Partitioning of CO2 Production Between Glucose and Lactate in Excised Sympathetic Ganglia, with Implications for Brain

Abstract: Chains of lumbar sympathetic ganglia from 15-day-old chicken embryos were incubated for 4 h at 36°C in a bicarbonate-buffered salt solution equilibrated with 5% CO₂-95% O₂. Glucose (1–10 m M ), lactate (1–10 m M ), [U-¹⁴C]glucose, [1-¹⁴C]glucose, [6-¹⁴C]glucose, and [U-¹⁴C]lactate were added as needed. ¹⁴CO₂ output was measured continuously by counting the radioactivity in gas that had passed through the incubation chamber. Lactate reduced the output of CO₂ from [U-¹⁴C]glucose, and glucose reduced that from [U-¹⁴C]lactate. When using uniformly labeled substrates in the presence of 5.5 m M glucose, the output of CO₂ from lactate exceeded that from glucose when the lactate concentration was >2 m M . The combined outputs at each concentration tested were greater than those from either substrate alone. The ¹⁴CO₂ output from [1-¹⁴C]glucose always exceeded that from [6-¹⁴C]glucose, indicating activity of the hexose monophosphate shunt. Lactate reduced both of these outputs, with the maximum difference between them during incubation remaining constant as the lactate concentration was increased, suggesting that lactate may not affect the shunt. Modeling revealed many details of lactate metabolism as a function of its concentration. Addition of a blood-brain barrier to the model suggested that lactate can be a significant metabolite for brain during hyperlactemia, especially at the high levels reached physiologically during exercise.     

Metabolism of Cysteine in Astroglial Cells: Synthesis of Hypotaurine and Taurine

Abstract: The synthesis of hypotaurine and taurine was investigated in astroglia-rich primary cultures obtained from brains of neonatal Wistar rats using ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy. Cell extracts of astroglial cultures analyzed by ¹H NMR spectroscopy show prominent signals of hypotaurine. To identify cysteine as precursor for hypotaurine and taurine synthesis in astroglial cells, primary cultures were incubated with [3-¹³C]cysteine for 24 or 72 h. Cell extracts and incubation media were then analyzed with ¹³C NMR spectroscopy. Labeled hypotaurine, taurine, glutathione, and lactate were identified in the cell extracts. Within 72 h, 35.0% of the total intracellular hypotaurine and 22.5% of taurine were newly synthesized from [3-¹³C]cysteine. The presence of [1-¹³C]hypotaurine and [1-¹³C]taurine in the incubation medium proves the release of those products of cysteine metabolism into the medium. Minor amounts of the [3-¹³C]cysteine were used for the synthesis of glutathione in astroglial cells or metabolized to [3-¹³C]lactate, which was found in cell extracts and media. These results indicate that the formation of hypotaurine and taurine is a major pathway of cysteine metabolism in astroglial cells.     

CITRATE AS THE PRECURSOR OF THE ACETYL MOIETY OF ACETYLCHOLINE

Abstract— Rat brain cortex slices were incubated with glucose labeled with either ³H or ¹⁴C in the 6-position. The ³H/¹⁴C ratios and the incorporation of radioactivity into lactate, citrate, malate and acetylcholine were determined. While the ³H/¹⁴C ratio of lactate was close to that of glucose, the ratios in the acetyl moiety of acetylcholine and the acetyl (C-4,5) portion of citrate decreased in a similar proportion. This was interpreted as indirect evidence for the participation of citrate as a precursor to the acetyl moiety of acetylcholine. Two inhibitors of the citrate cleavage pathway: n -butylmalonate, an inhibitor of citrate transport and (-)-hydroxycitrate, an inhibitor of ATP-citrate lyase were studied for their effect on acetylcholine synthesis. N -butylmalonate (10 mM) and (-)-hydroxycitrate (7.5 mM) led to a decrease in the per cent of ¹⁴C recovered as acetylcholine. In each instance the ³H/¹⁴C ratio in acetylcholine was higher in the presence of inhibitor while the corresponding ratios in lactate and citrate (C-4.5) remained unchanged. From the results, it is suggested that citrate is involved in the transport mechanism of acetyl units from its site of synthesis in mitochondria to the site of acetylcholine synthesis in the cytosol.     

Effect of Chronic Lead Ingestion by Rats on Glucose Metabolism and Acetylcholine Synthesis in Cerebral Cortex Slices

Abstract: The effect of chronic low-level lead (Pb²⁺) ingestion on the metabolic pathways leading to the acetyl moiety of acetylcholine (ACh) was examined. Cerebral cortex slices, prepared from untreated or Pb²⁺-exposed rats (600 ppm lead acetate in the drinking water for 20 days), were incubated in Krebs-Ringer bicarbonate buffer with 10 m M glucose and tracer amounts of [6-³H]glucose and either [6-¹⁴C]glucose or [3-¹⁴C] β -hydroxybutyrate. Altering the concentration of Pb²⁺ in the drinking water produced a dose-related increase in blood and brain lead levels. When tissue from Pb²⁺-exposed rats was incubated with mixed-labeled glucose, incorporation into lacate, citrate, and ACh was considerably decreased, although no changes occurred in the ³H/¹⁴C ratios. Similar effects of Pb²⁺ were found when ¹⁴C-labeled β -hydroxy-butyrate was substituted for the [¹⁴C]glucose. It appears from these data that Pb²⁺ exerts a generalized effect on energy metabolism and not on a specific step in glucose metabolism. The impairment of glucose metabolism may explain partially the Pb²⁺-induced changes observed in cholinergic function.     

CAPABILITY OF TUBULIN AND MICROTUBULES TO INCORPORATE AND TO RELEASE TYROSINE AND PHENYLALANINE AND THE EFFECT OF THE INCORPORATION OF THESE AMINO ACIDS ON TUBULIN ASSEMBLY

Abstract— Incorporation of [¹⁴C]tyrosine into the C-terminal position of α-tubulin of rat brain cytosol was 10-fold higher for non-assembled than for assembled tubulin. The incorporation into tubulin from disassembled microtubules was higher than into non-assembled tubulin; therefore, the low incorporation into microtubules was not due to a lower acceptor capacity of their tubulin constituent.
[¹⁴C]Tyrosine was released from assembled and non-assembled [¹⁴C]tyrosinated tubulin by the action of an endogenous carboxypeptidase. Release from non-assembled tubulin was shown by incubating a tubulinyl-[¹⁴C]tyrosine preparation in the presence of CaCl₂ at a concentration that abolished microtubule formation. Release from microtubules was inferred from the observation that the percentages of [¹⁴C]tyrosine released and the decrease of the specific radioactivity of the recovered microtubules were practically identical and did not change after a 10-fold dilution of the incubated microtubules.
[³H]Phenylalanine was released from a preparation of tubulinyl-[³H]phenylalanine also by an enzymatic activity.
The capacity of a tubulin preparation to incorporate tyrosine was increased 43% by pre-treatment with endogenous carboxypeptidase.
Tubulin tyrosinated in vitro was assembled to the same extent as native tubulin. After a mixture of tubulinyl-[¹⁴C]tyrosine and tubulinyl-[³H]phenylalanine was partially assembled, the ratio of ¹⁴C/³H found in the microtubules was the same as in the non-assembled tubulin fraction.     

Further evidence for dolichyl phosphate-mediated glycosyl translocation through membranes

Abstract Mannosyl residues from GDP-[¹⁴C]Man are transferred into liposomes and onto internal GDP if the liposomal membranes contain dolichyl phosphate and the enzyme GDP-Man: Dol-P mannosyl transferase. Experiments using a mixture of GDP-[¹⁴C]Man and [³H]GDP-Man excluded the possibility that the transmembrane translocation of [¹⁴C]mannose is due to a GDP-[¹⁴C]Man_outside ag GDP_inside exchange reaction.     

RELEASE OF [3H]GAMMA-AMINOBUTYRIC ACID FROM GLIAL CELLS IN RAT DORSAL ROOT GANGLIA.

Abstract— Desheathed rat dorsal root ganglia were incubated in a medium containing amino-oxyacetic acid and [³H]GABA. Under these conditions, [³H]GABA is taken up exclusively by the satellite glial cells in the ganglia. Efflux of [³H]GABA from the tissue was measured after passing the ganglia through a series of wash solutions. The spontaneous efflux of radioactivity, mostly [³H]GABA, was more rapid in the absence of amino-oxyacetic acid in the incubation and wash media.
Raising the potassium concentration in the wash media caused an increase in the efflux of [³H]GABA. This increase was sigmoidally related to the potassium concentration in the wash media, reaching a maximum at 64 m m -K⁺. The releasing effect of K⁺ was inhibited by removing calcium from the media. Reducing the calcium and raising the magnesium concentration in the wash solutions inhibited the increased efflux of [³H]GABA due to 64 m m -K⁺ by 48 per cent, while 5 mM-La³⁺ and diphenylhydantoin (0·005 and 0·5 m m ) had no effect on this increase.
Only a small increase in the efflux of [¹⁴C]glutamate was produced by 64 m m -K⁺ and it had no effect upon the effluxes of [³H]glycine, [³H]alanine or [³H]leucine. The efflux of lactate dehydrogenase was similarly unaffected by 64 mM-K⁺. The results suggest that glial cells in spinal ganglia can respond to depolarizing concentrations of potassium by releasing GABA in a calcium-dependent process.     

SUBCELLULAR DISTRIBUTION OF ENDOGENOUS AND [3H] γ-AMINOBUTYRIC ACID IN RAT CEREBRAL CORTEX

Abstract— Slices of rat cerebral cortex were labelled by incubation with [³H]γ-aminobutyric acid (GABA) and homogenized in isotonic sucrose. The subcellular distributions of endogenous GAB A, [³H]GABA and glutamate decarboxylase (GAD) were studied by density gradient centrifugation. The subcellular distributions of the labelled and endogenous amino acid were remarkably similar, indicating that [³H]GABA is taken up into the endogenous GABA pool. About 40 per cent of both endogenous and [³H]GABA were recovered in particles which were tentatively identified as synaptosomes from their equilibrium density and sensitivity to osmotic shock. In slices labelled with [³H]GABA and [¹⁴C]α-aminoisobutyric (AIB) acid, significantly more [³H]GABA was recovered in paniculate fractions than [¹⁴C]AIB. About 80 per cent of the enzyme GAD was also recovered in the same particle fractions which contained [³H]GABA and endogenous GABA. Evidence is presented which suggests that a loss of particle-bound GABA occurs during subcellular fractionation procedures.     

Tumour Cell Recruitment of the Jb-1 and L 1210 Ascites Tumour Determined Directly By Double Labelling With [14C]- and [3H]-Thymidine

Abstract. Tumour cell recruitment of the JB-1 and L 1210 ascites tumour has been demonstrated directly by a double-labelling method with [¹⁴C]- and [³H]-thymidine (TdR). After [¹⁴C]-labelling of all proliferating tumour cells by multiple injections of [¹⁴C]TdR, recruitment of resting cells was stimulated by removal of the majority of tumour cells, i.e. by maximum aspiration of ascitic fluid. the number of recruited resting cells in the remaining tumour that re-enter the cell cycle after stimulation was demonstrated directly by a single injection of [³H]TdR given at different times after stimulation. the increase in the percentage of purely [³H]-labelled cells, i.e. recruited cells, with increasing time after stimulation, shows that recruitment is not a synchronous but a continuous process, the maximum of which occurs earlier in the case of the L 1210 than the JB-1 tumour. This suggests that there seems to be a relationship between the time required for maximum recruitment and the corresponding cell cycle parameters of the unperturbed tumour. There is a transitory increase of the growth fraction to about 100% and a considerable shortening of the cycle time at the maximum of recruitment.     

THE EFFECT OF ELECTRICAL STIMULATION AND HIGH POTASSIUM CONCENTRATIONS ON THE EFFLUX OF [3H] γ-AMINOBUTYRIC ACID FROM BRAIN SLICES

Abstract— Brain slices were incubated with [³H]GABA in a medium containing aminooxyacetic acid to prevent metabolism of [³H]GABA by GABA-glutamate transaminase. The slices, which rapidly accumulated radioactivity, were then continuously perfused and the efflux of [³H]GABA from the tissue was measured. The spontaneous efflux of [³H]GABA consisted of an initial rapid phase followed by a much slower release of [³[H]GABA. After 40 min perfusion 90 per cent of the radioactivity remained in the tissue.
The slices were depolarized by electrical stimulation or by perfusion with a medium containing a high potassium concentration (40 mM). These procedures caused a striking increase in the efflux of [³H]GABA. The increased efflux produced by potassium, but not that produced by electrical stimulation, was dependent on calcium ions in the medium. The effect of electrical stimulation on [³H]GABA release was considerably reduced by a raised concentration (10 mM) of magnesium in the medium.
High potassium concentrations and electrical stimulation did not cause an increase in the efflux of [¹⁴C]urea, L-[³H]leucine or [¹⁴C]α-amino-isobutyric acid from brain slices. These results are consistent with the suggestion that GABA may be an inhibitory transmitter in the cerebral cortex.     

Effect of Tetanus Toxin on Transmitter Release from the Substantia Nigra and Striatum In Vitro

Abstract: The effect of tetanus toxin on the uptake and release of radiolabelled transmitters from slices prepared from substantia nigra (SN) and striatum of rats has been investigated. Tetanus toxin-500–750 mouse lethal doses (MLD)-injected into the SN 6 h before preparing the slices significantly reduced the calcium-dependent, potassium-evoked release of [³H]GABA. Endogenous GABA levels in the SN and [³H]GABA uptake by nigral slices were unaffected by pretreatment with the toxin. Injections of tetanus toxin (1000–2000 MLD) into the striatum significantly reduced the calcium-dependent, potassium-evoked release of [¹⁴C]GABA and also [³H]dopamine, but had no effect on the K⁺-evoked release of [³H]5-hydroxytryptamine or [¹⁴C]acetylcholine. It is concluded that tetanus toxin inhibits GABA release directly and not by interference with synthesis or inactivation processes.     

[2-3H]GLYCEROL AS A PRECURSOR OF PHOSPHOLIPIDS IN RAT BRAIN: EVIDENCE FOR LACK OF RECYCLING

Abstract— When [2-³H]glycerol was injected intracranially into young rats, it was presented as a pulse label, leaving the brain rapidly and giving up much of its labelled hydrogen to water. [2-³H]glycerol was efficiently incorporated into brain lipids, especially into choline and ethanolamine phospholipids. Following injection of a mixture of [³H]- and [¹⁴C]-labelled glycerol, the ratio of ³H to ¹⁴C in the phospholipids of both whole brain and the microsomal fraction decreased as a function of time after injection. This finding indicated less recycling of the tritium label. This lack of recycling was further indicated by the finding that 94 per cent of the tritium label of phosphatidyl choline was in the glycerol portion of the molecule rather than in the fatty acids. At 2 weeks following injection with [³H]glycerol, 93 per cent of the total radioactivity in brain appeared in the lipid fraction. In contrast, following injection with [¹⁴C]glycerol, only 57 per cent of the radioactivity appeared in lipid, with about 20 per cent in protein.     

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.