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

Regulation of glutaminase by exogenous glutamate, ammonia and 2-oxoglutarate in synaptosomal enriched preparation from rat brain

Phosphate activated glutaminase in synaptosomal enriched preparation from rat brain is very sensitive to inhibition by low concentration of glutamate, ammonia and 2-oxoglutarate when added to the incubation medium at pH 7.6. By increasing the concentration of either of these compounds up to 0.5 mM a pronounced initial inhibition is followed by little or no further effect when the concentration is increased beyond this level. By lowering the pH of the reaction mixture to 7.0, the inhibition by glutamate is almost abolished and that of ammonia reduced. Glutamate inhibits mainly the N-ethylmaleimide-sensitive fraction of glutaminase which previously is suggested to be localized to the outer phase of the mitochondrial inner membrane whereas ammonia inhibits both the N-ethylmaleimide-sensitive and -insensitive fraction. Evidence has been produced to show that the inhibition by 2-oxoglutarate is caused by glutamate formation by aminotransferase reactions. Since 2-oxoglutarate is produced by the tricarboxylic acid cycle, the operation of this cycle may regulate the glutaminase reaction by controlling glutamate formation via the aminotransferase reactions.

A new synaptosomal biosynthetic pathway of glutamate and GABA from ornithine and its negative feedback inhibition by GABA

In sonicates of mouse brain synaptosomes, we demonstrated that gamma-aminobutyric acid (GABA) can be formed when L-ornithine (Orn) through L-glutamic acid (Glu), but not through putrescine (Put). Incubation of these sonicates with [3H]ORN yielded not only [3H]Glu and [3H]L-proline (Pro) but also produced [3H]GABA from the [3H]Glu. Formation of each of these three major amino acids from [3H]Orn was strongly inhibited by the addition of GABA (1-5 mM). The likely enzymatic site of this negative feedback inhibition by GABA appeared to be ornithine delta-aminotransferase (OAT). A radiometric procedure was employed to study the effects of the three amino acids cited above and of others found in the free form in brain on the activity of a 30-fold-purified OAT from rat brain. Enzyme activity was measured in the presence of low concentrations of Orn, such as might occur in vivo. OAT was inhibited by GABA to a considerably greater extent than by Glu, L-glutamine, or Put; no inhibition was found with Pro, glycine, aspartarte, taurine, or beta-alanine. The inhibition of GABA was competitive with Orn. These results clearly show that one of the molecular mechanisms underlying the negative feedback inhibition of synaptosomal GABA biosynthesis from Orn is a competitive inhibition by GABA of the brain OAT activity that is responsible for the formation of L-glutamic-gamma-semialdehyde in equilibrium with L-delta 1-pyrroline-5-carboxylic acid from Orn. Thus, the results suggest that GABA may play an important role in restricting the metabolic flow from Orn to Glu and thence to GABA. It is confirmed that L-canaline (delta-aminooxy-L-alpha-aminobutyric acid) is a potent and specific inhibitor of brain OAT whereas much weaker inhibition was observed with two other carbonyl-trapping agents, aminooxyacetic acid and hydrazine.

Putative acidic amino acid transmitters in the cerebellum. I. Depolarization-induced release

In the present investigation we studied the autoradiographic localization and the characteristics of the depolarization-induced release of acidic amino acids in in vitro rat cerebellar preparations. Light microscopy autoradiography of cerebellar slices preincubated in the presence of the non-metabolized glutamate analogue D-[3H]aspartate showed a large accumulation of radioactivity over glial cells, and very little labelling of the granule cells, whose putative neurotransmitter may be glutamate. In spite of its predominant localization in glia, D-[3H]aspartate (and [14C]glutamate) was released from cerebellar slices depolarized with high [K+] in a Ca2+-dependent way, and the release elicited by veratrine was prevented by TTX. These findings, together with the observation that freshly isolated or cultured glial cells did not show any Ca2+-dependent, depolarization-induced release of D-[3H]aspartate, suggest that the radioactive amino acid released from slices has a neuronal origin. The high [K+]-induced release of exogenous radioactive acidic amino acids from superfused cerebellar synaptosomal preparations exhibited, as best, a modest Ca2+-dependence, a result probably due to the existence of a substantial non-Ca2+-dependent release of the amino acid from glial fragments contaminating the preparation. However, both the K+-evoked release of endogenous glutamate, and that of [14C]glutamate previously synthesized from [14C]glutamine were largely Ca2+-dependent, suggesting that nerve endings are the main sites involved in the stimulus-coupled secretion. In the experiments in which synaptosomes had been prelabelled with [14C]glutamine, a study of the specific radioactivity of the glutamate released and of that present in synaptosomes at the beginning and at the end of superfusion period provided evidence in favour of a preferential release of the newly synthesized [14C]glutamate. In contrast to glutamate, endogenous aspartate was not released in a Ca2+-dependent manner, and the efflux of newly formed [14C]aspartate was only slightly potentiated by Ca2+, which suggests that glutamate and aspartate are not released from the same sites. Studies on preparations (slices and synaptosomes) from immature, 8-day-old cerebella showed that neither the K+-evoked release of D-[3H]aspartate, nor that of endogenous glutamate was Ca2+-dependent. In conclusion, the data presented are consistent with the proposition that glutamate has a neurotransmitter role in the cerebellum.U

Glutamine and alpha-ketoglutarate uptake and metabolism by nerve terminal enriched material from mouse cerebellum

In order to provide evidence relevant to the hypothesis that nonsynaptically derived alpha-ketoglutarate serves as a metabolic precursor of the neurotransmitter pools of glutamate and GABA the uptake and metabolism of alpha-ketoglutarate by nerve terminal enriched material was studied and compared to corresponding data for glutamine. Both alpha-ketoglutarate and glutamine were transported across the cell membrane by high affinity and low affinity carriers. Under conditions prevailing in vivo alpha-ketoglutarate probably is transported primarily by its high affinity carrier, whereas gluatmine should be transported primarily by one or more low affinity carriers. Based upon reciprocal uptake inhibition experiments glutamine appeared to be transported by the alanine preferring system, and to a lesser extent by the basic amino acid and large neutral amino acid carriers. A comparison of the rate of uptake by different cellular preparations enriched in either nerve terminals or cell bodies indicated that alpha-ketoglutarate is transported selectively by nerve terminals. Both substrates were rapidly converted to glutamate; however, glutamine was more readily metabolized to GABA. The results of our study are consistent with the concept that both glutamine and alpha-ketoglutarate derived from extra-neuronal sources are taken up by nerve terminals and utilized to replenish the neurotransmitter pools of glutamate and GABA.

The activities in different neural cell types of certain enzymes associated with the metabolic compartmentation glutamate

The cellular distribution of certain enzymes associated with the metabolic compartmentation of glutamate was estimated in ultrastructurally preserved and metabolically competent perikarya fractions that were enriched in astrocytes, granule cells and Purkinje cells and derived from 8-day-old rat cerebellum, and in monolayer cultures (14 days in vitro) composed principally of interneurones or astrocytes. The neuronal activities of glutamine synthetase and glutamate dehydrogenase were respectively about 4- to 8-fold and 2- to 5-fold lower than in astrocytes, depending upon the class of neurone and the type of preparation used for comparison. By contrast glutaminase activity was about 3- to 12-fold higher in neuronal than in astroglial preparations. Estimations of the specific activity of succinate dehydrogenase differed less between cell types, indicating that the differences in glutamate dehydrogenase and glutaminase were not simply related to variations in the concentration of mitochondria relative to the other cellular constituents. The findings presented provide direct evidence in support of our model assigning the 'small' glutamate compartment, where most of the labelled glutamine is synthesized, to glial cells, and the 'large' compartment to neurones, and also underline the metabolic interaction between these two cell types in the brain.

L-glutamate and L-glutamine uptake in adult rat cerebellum: an autoradiographic study

The compartmentation of L-glutamate in the central nervous system has been extensively studied and L-glutamine is believed to be the precursor of the neuronal releasable pool of the L-glutamate. In order to localize the sites of uptake of both L-glutamate and L-glutamine, autoradiography was used in tissue slices of adult rat cerebellum, where granule cells are considered to be glutamatergic. Incubation of the tissue with low concentrations of [3H]L-glutamate or [3H]L-glutamine produces in both cases a heavy labelling of the molecular layer. [3H]L-glutamate uptake seems to be essentially glial (Golgi epithelial cells and Bergmann fibres) while [3H]L-glutamine is more diffusely distributed over the molecular layer. Although no conclusions can be drawn on the nature of L-glutamine uptake, these results are in agreement with the model which considers L-glutamate uptake by glial cells to be the inactivating process of glutamatergic synapses.

Distribution and uptake of glycine, glutamate and gamma-aminobutyric acid in the vagal nuclei and eight other regions of the rat medulla oblongata

In order to study the central neurochemical control of the vagus nerve, the contents of glycine, GABA, glutamate and five other amino acids have been measured in ten anatomically distinct regions of the rat medulla oblongata. Additionally, the high affinity uptake of glycine, GABA, glutamate, and leucine were measured in the same ten medullary regions. The data support published evidence for glutamatergic and GABAergic transmission in the nucleus of the tractus solitarius (NTS), and glycinergic inhibition in the hypoglossal nucleus. The data also lead to the suggestion that GABA and glutamate may be taken up into glial cells which exist along fiber tracts.

Effect of motor and premotor cortex ablation on concentrations of amino acids, monoamines, and acetylcholine and on the ultrastructure in rat striatum. A confirmation of glutamate as the specific cortico-striatal transmitter

At 1, 2, and 4 weeks after unilateral premotor and motor cortex ablation in rats, a significant and lasting decrease in glutamate levels in the ipsilateral versus contralateral striatum was observed. A significant corresponding fall in aspartate was seen only after 1 week. In contrast, there was a large increase in the striatal concentrations of lysine, threonine, alanine, and glutamine 1 week after the cortical ablation. This correlates with the extensive glial proliferation in the deafferented ipsilateral striatum. Four weeks after cortical ablation the GABA concentration was significantly increased. There was no decrease in other putative transmitters (dopamine, serotonin, acetylcholine, glycine and taurine), nor was a glutamate decrease observed in the hippocampus or in the hypothalamus, which do not receive direct premotor and motor cortical inputs. Both biochemical and morphological evidence for a minor contralateral cortico-striatal projection was obtained. Correlating with the fall in glutamate, ultrastructural observations indicated the degeneration of two types of striatal synapses, i.e., those of the axo-spinous type III and of the axo-dendritic type VII. Frontal cortex ablation clearly affects, in opposite directions, the metabolism of various striatal amino acids but not that of acetylcholine and the monoamine transmitters. The results strongly support the view that glutamate is the transmitter of the cortico-striatal fibers.

Evidence in favor of a neurotransmitter role of glycine in the rat cerebral cortex

In the present study we analyze whether glycine satisfies some electrophysiological and biochemical criteria to consider it as a putative transmitter in the rat cerebral cortex. Intracellular recordings from rat sensory-motor cortex showed that in 15-20% of the tested neurons glycine hyperpolarized the cell membrane, decreased the firing rate and flattened the evoked EPSP-IPSP sequence by increasing the membrane conductance. The iontophoretic application of strychnine antagonized the block of 'spontaneous' firing and the membrane hyperpolarization induced by glycine. Moreover, in a group of neurons, strychnine decreased the amplitude and duration of the IPSP and brought back the membrane potential to resting values. Previously accumulated [3H]glycine and endogenous glycine were released from cortical synaptosomal preparations by depolarizing stimuli in a Ca2+-dependent way. The release pattern of glycine was qualitatively similar in cortical and in spinal synaptosomes. [14C]Glycine was rapidly synthetized from [14C]serine in cortical synaptosomal preparations, and the newly formed [14C]glycine was released by depolarizing stimuli in a Ca2+-dependent way. It is concluded that glycine, which is generally considered as an inhibitory neurotransmitter in the spinal cord, medulla and pons, may also have a transmitter role in a discrete number of cortical neurons of some mammalian species.U

Alteration in neuronal-glial metabolism of glutamate by the neurotoxin kainic acid

The effect of the excitotoxin kainic acid on glutamate and glutamine metabolism was studied in cerebellar slices incubated with D-[2-14C]glucose, [U-14C]gamma-aminobutyric acid, [3H]acetate, [U-14C]glutamate, and [U-14C]glutamine as precursors. Kainic acid (1 mM) strongly inhibited the labeling of glutamine relative to that of glutamate from all precursors except [2-14C]glucose and [U-14C]glutamine. Kainic acid did not inhibit glutamine synthetase directly. The data indicate that in the cerebellum kainic acid inhibits the synthesis of glutamine from the small pool of glutamate that is thought to be associated with glial cells. Kainic acid also markedly stimulated the efflux of glutamate from cerebellar slices and this release was not sensitive to tetrodotoxin. Kainic acid stimulated efflux of both glucose- and acetate-labeled glutamate. In contrast, veratridine released glucose-labeled glutamate preferentially via a tetrodotoxin-sensitive mechanism. Kainic acid did not release [U-14C]glutamate from synaptosomal fractions. These results suggest that the bulk of the glutamate released from cerebellar slices by kainic acid comes from nonsynaptic pools.

Topographic and quantitative characteristics of glutamate dehydrogenase of the hippocampus formation during the postnatal development of the rat brain. Comparative studies on succinate and alpha-glycerophosphate dehydrogenase with special reference to putatively glutamatergic structures

As topochemically demonstrated, glutamate dehydrogenase is present within the dendritic layers of the rat hippocampus formation. Its activity increased in parallel to the postnatal maturation of synaptic structures. The results point to a contribution of glutamate dehydrogenase in the synthesis of transmitter glutamate. concomitantly, the activity of succinate and alpha-glycerophosphate dehydrogenase increased in the hippocampal dendritic areas during the postnatal development which is discussed expressing of metabolic activity. The results of the formazan elution technique reflect the topochemical findings in a quantitative manner.

Carbonic anhydrase C in the neural retina: transition from generalized to glia-specific cell localization during embryonic development

The developmental profile and cellular localization of carbonic anhydrase C (carbonate dehydratase; carbonate hydro-lyase, EC 4.2.1.1) in the neural retina of chicken embryos and adults were investigated by immunochemical and immunohistochemical methods. Carbonic anhydrase C is present in the retina by the 3rd day of embryonic development. In the undifferentiated retina, it is detectable in virtually all the cells; however, as cell specialization progresses, its level declines rapidly in the emerging neurons and increases in Müller glia cells. An exception is certain amacrine neurons that contain carbonic anhydrase C to about the 16th day of development. In the adult retina, the enzyme is confined exclusively to Müller cells (the only gliocytes in the retina). Their identification was confirmed by immunostaining for glutamine synthase, an established Müller cell "marker." The presence in the mature retina of both these enzymes in Müller cells indicates that retinal gliocytes combine functional features that, in the brain, are segregated in astrocytes and oligodendrocytes. In the embryonic retina, carbonic anhydrase C and glutamine synthase differ markedly in their developmental profiles, cellular distribution, and susceptibility to regulation by cortisol and by cell interactions. Such differences make these two enzymes an attractive "marker team" for studying developmental mechanisms in embryonic retina and specific functions of Müller cells.

A new type of lesion-induced synaptogenesis: I. Synaptic turnover in non-denervated zones of the dentate gyrus in young adult rats

It is well established that partial denervation causes the formation of new synapses within denervated areas. It is also possible that synapse formation and remodeling occurs outside denervated zones. In this study we evaluate this possibility by examining the effect of a unilateral entorhinal lesion on the number and characteristics of synapses in non-denervated zones of the dentate gyrus within the hippocampal formation. A unilateral entorhinal lesion massively denervates the outer two-thirds of the ipsilateral dentate molecular layer and also causes a minor loss of synapses in the outer two-thirds of the contralateral dentate molecular layer. The inner one-third of the molecular layer is not denervated on either side. In the ipsilateral inner molecular layer the number of synapses rapidly decreases by about 20% and recovers by 10 days post-lesion. Similarly, in the contralateral inner molecular layer, synapses are lost and replaced, but the time course is slower. Loss is maximal at 60 days post-lesion and this recovers by 180 days post-lesion. Thus, a complete cycle of turnover occurs in both of the inner molecular layers. No degenerating terminals of any type were seen throughout the time course in these layers. Small synapses with non-complex synaptic junctions appear to account for most of the changes. Also the outer two-thirds of the contralateral molecular layer, which has lost less than 5% of its input, loses about 37% of its synapses and replaces the majority of them over time. However, the total number of synapses in the contralateral molecular layer never fully attains the value of unoperated animals. The total synaptic population reaches a value such that the ipsilateral and contralateral molecular layers are nearly equivalent. These changes, achieved through synaptic turnover, may represent a homeostatic response to nearby denervation which may facilitate restoration of bilateral function in the dentate gyrus.

Perforant path lesion depletes glutamate content of fascia dentata synaptosomes

Destruction of the hippocampal perforant path fibers reduces the glutamate content of a crude synaptosomal fraction of the rat fascia dentata by about 40%, but does not reduce the aspartate content. This result supports the hypothesis that the perforant path fibers use glutamate as their transmitter. Since a perforant path lesion reduces the glutamate content of dentate homogenates to a much lesser degree, the reduction in synaptosomal glutamate may be offset by an increase in extraterminal glutamate stores. Thus, when evaluating the possible transmitter role of glutamate or aspartate, one can probably gain more definitive information by measuring the glutamate and aspartate content of a synaptosomal preparation than of the target region as a whole.

Evidence for compartmentalization of glutamate in rat brain synaptosomes using the glutamate sensitivity of phosphate-activated glutaminase as a functional test

The glutamate content of rat brain synaptosomes was measured by high performance liquid chromatography to be 39 micromol/g protein. If uncompartmentalized this glutamate (4 mM) would inhibit phosphate-activated glutaminase considerably. Since the action of any endogenous effector on the enzyme is assumed to be negligible following disruption of the synaptosomes, due to dilution with the incubation medium, the inhibition by glumate and activation by phosphate were compared in intact and disrupted synaptosomes. The inhibition by endogenous glutamate in intact synaptosomes was found to correspond to less than that of 0.5 mM of added glutamate to disrupted synaptosomes, indicating that the major fraction of synaptosomal glutamate is compartmentalized.

Micromolar L-2-amino-4-phosphonobutyric acid selectively inhibits perforant path synapses from lateral entorhinal cortex

Transverse slices of the rat hippocampus were used to examine the ability of phosphonate analogues of acidic amino acids to inhibit perforant path synaptic transmission. Micromolar concentrations of L-2-amino-4-phosphonobutyric acid (APB), an analogue of L-glutamic acid, inhibited transmission from the lateral entorhinal cortex. Two other less-sensitive components were detected in projections from the medial entorhinal cortex. The component from the lateral entorhinal cortex showed high stereospecificity for the L-isomer of APB and was relatively insensitive to phosphonate homologues of shorter and longer chain length.

Glutamate as a putative transmitter in the cerebellum: stimulation by GABA of glutamic acid release from specific pools

The aim of the present paper was to determine whether the release of glutamate from putative "glutamergic" terminals in the cerebellum is influenced by gamma-aminobutyric acid (GABA). In a group of preliminary experiments, we present biochemical evidence in favour of a neurotransmitter role of glutamate in the cerebellum: (1) endogenous glutamate was released from depolarized cerebellar synaptosomal preparations in a Ca2+-dependent away; (2) [14C]glutamate was synthesized from [14C]glutamine in cerebellar synaptosomes, and the newly synthesized [14C]glutamate was released released in a Ca2+-dependent way; (3) the elevation of cyclic GMP elicited by depolarization of cerebellar slices in the presence of Ca2+ was partly reversed by the glutamate antagonist glutamic acid diethyl ester, which probably prevented the interaction of endogenously released glutamate with postsynaptic receptors. GABA and muscimol at low concentrations (2--20 micrometers) potentiated the depolarization-induced release of D-[3H]aspartate (a glutamate analogue which labels the glutamate "reuptake pool") from cerebellar synaptosomes. The effect was concentration dependent and was largely prevented by two GABA antagonists, bicuculline and picrotoxin. The stimulation of D-[3H]aspartate release evoked by muscimol was linearly related to the logarithm of K+ concentration in the depolarizing medium. GABA did not affect the overall release of endogenous glutamate, but potentiated, in a picrotoxin-sensitive manner, the depolarization-evoked release of [14C]glutamate previously synthesized from [14C]glutamine. Since nerve endings are the major site of glutamate synthesis from glutamine, GABA and muscimol appear to exert their stimulatory effect at the level of "glutamergic" nerve terminals, probably after interacting with presynaptic GABA receptors. The possible functional significance of these findings is briefly discussed.

Evidence for compartmentation of synaptosomal phosphate-activated glutaminase

Phosphate-activated glutaminase (EC 3.5.1.2) in synaptosomal preparations is inhibited 40-60% by the sulphydryl group reagent N-ethylmaleimide (NEM), forming the basis for distinction between NEM-sensitive and NEM-insensitive glutaminases. The NEM effect cannot be explained by differential effects on distinct glutaminases because other glutaminases have not been detected, and the synaptosomal glutaminase activity can be fully accounted for by the activity of phosphate-activated glutaminase. By fractionation of mitochondria isolated from synaptosomal preparations, which are preincubated with and without NEM, both NEM-sensitive and NEM-insensitive glutaminases are found to be localized to the inner mitochondrial membrane. Variations in pH (7.0-7.6) and the phosphate concentration (5-10 mM) affect chiefly NEM-sensitive glutaminase, demonstrating that this glutaminase may be subject to regulation by compounds in the cytosol having restricted permeability to the inner mitochondrial membrane. Since p-hydroxymercuribenzoate, which is known to be impermeable to the inner mitochondrial membrane, inhibits glutaminase similarly to NEM, phosphate-activated glutaminase is assumed to be compartmentalized within the inner mitochondrial membrane. Thus, NEM-sensitive glutaminase is localized to the outer face and NEM-insensitive glutaminase to the inner region of this membrane and probably also to the matrix region.

K+-evoked Caa+-dependent release of D-[3H]aspartate from terminals of the cortico-pontine pathway

Pontine nuclei dissected from rat brain slices released previously accumulated D-[3H]aspartate (D-Asp) and [14C]gamma-aminobutyrate (GABA) Ca-dependently when exposed to 50 mM K. These efflux rates were substantially increased by including 0.5 mg/ml bovine serum albumin in the superfusion fluid. Degeneration of the cortico-pontine fibres 5 days after cutting the crus cerebri caused an 80% reduction in the fractional rate of Ca-dependent D-Asp release and a 60% reduction in uptake. The fractional rate of GABA efflux was significantly less reduced than that of D-Asp efflux, and GABA uptake was nearly unchanged.

Enhancement by GABA of glutamate depolarization-induced release from cerebellar nerve endings

Cerebellar synaptosomes were superfused in the presence of D-[3H]aspartate (to label the glutamate 'reuptake pool') and [14C]glutamine (to label the 'new synthesis pool'). The depolarization-induced release of D-[3H]aspartate and of newly synthesized [14C]glutamate were potentiated by low concentrations of GABA (2--20 microM) or muscimol. The effect was probably mediated by the interaction of GABA with presynaptic GABA receptors localized in 'glutamergic' nerve endings, since it was antagonized by the GABA antagonists picrotoxin and bicuculline.

Effect of asparagine, glutamine and insulin on cerebral amino acid neurotransmitters

Treatment of rats with asparagine or glutamine caused substantial increases in glutamine concentrations in cerebellum and medulla oblongata. Insulin treatment caused a diminution of glutamate and GABA in these regions of brain. Since it is now well-established that glutamine is a very efficient precursor of the neurotransmitter pool of glutamate in mammalian brain, treatment with asparagine or glutamine could be of therapeutic (replacement) value in the treatment of neurological disorders such as Friedreich's ataxia, in which cerebral glutamate concentrations have been found to be diminished.

Asparagine as precursor for transmitter aspartate in corticostriatal fibres

The role of asparagine as precursor for the neurotransmitter aspartate was investigated in rat striatum in vitro. 14C-asparagine incubated with striatal slices is converted to a great extent to 14C-aspartate which is released in a calcium-dependent manner by high KCl. Furthermore, a frontoparietal cortex ablation of two weeks produces a decrease of more than 70% in the striatal release of newly synthetized 14C-aspartate, whereas the striatal GABA release is unaffected. This suggests that asparagine is a possible precursor in vitro for transmitter aspartate in the striatum. This reaction is dependent on intact corticostriatal fibres.

Endogenous amino acid release from rat cerebellum in vitro

The release of six endogenous amino acids from superfused rat cerebellar slices was investigated. Only L-glutamate, and GABA exhibited a calcium-dependent, potassium-stimulated release. Glutamine release was significantly inhibited during the stimulation period. Experiments were also performed with slices from cerebella which had received previously, injections of 2 micrograms kainic acid. This procedure failed to modify the release of any amino acid. At this time point (24 h), cell bodies of inhibitory neurons utilising GABA as their transmitter, have largely been destroyed; thus, the lack of effect on GABA release in particular, may indicate that the mechanisms for transmitter release from terminals are still operative under these conditions.

The synthesis of neuroactive amino acids from radioactive glucose and glutamine in the rat retina: effects of light stimulation

The effect of light stimulation in vitro on the labelling of neuroactive amino acids derived from [14C]glucose or [14C]glutamine in the rat retina has been studied. [14C]Glutamine, at 700 microM, provided about 50% of the tissue pools of glutamate, aspartate, and GABA; and the labelling of these decreased on light stimulation, both in the photoreceptor cells (glu and asp) and in the inner retina (glu, asp, and GABA). In contrast, there were no significant changes in the entry of label derived from [14C]glucose, although similar trends were apparent in the data obtained for the photoreceptor cell layer. The pools may, therefore, be separate. Other results support the contention that glucose is the principal energy source for the retina, its entry into non-amino acid derivates being decreased on light stimulation.

Glutamate as a CNS transmitter. II. Regulation of synthesis in the releasable pool

Slices of the molecular layer of the dentate gyrus of rat hippocampus were used to study the regulation of glutamate synthesis and release. Net glutamate synthesis increased dramatically during conditions which stimulated the release of glutamate. The rate of glutamine incorporation into glutamate released into the medium was increased almost immediately upon stimulation with a 56 mM KCl, 3 mM CaCl2 medium. Synthesis appeared to be regulated both by glutamine uptake and the activity og glutaminase. Glutamine uptake was stimulated in the presence of 56 mM KCl and 3 mM CaCl2. The increased glutamine uptake was not due to a decrease in efflux, was unrelated to tissue glutamate levels, and could be dissociated from the rate of glutamate biosynthesis. The presence of Ca2+ ions and depolarization seemed necessary. Glutaminase activity was regulated by end product inhibition: increased levels of tissue glutamate resulted in a decrease in glutamate synthesis. Glutamine in the presence of 56 mM KCl increased the rate of glucose incorporation into glutamate over that seen without glutamine.