Stimulus evoked increase in the biosynthesis of the putative neurotransmitter glutamate in the hippocampus

Dysfunctional glia: contributors to neurodegenerative disorders

Astrocytes are integral components of the central nervous system, where they are involved in numerous functions critical for neuronal development and functioning, including maintenance of blood-brain barrier, formation of synapses, supporting neurons with nutrients and trophic factors, and protecting them from injury. These roles are markedly affected in the course of chronic neurodegenerative disorders, often before the onset of the disease. In this review, we summarize the recent findings supporting the hypothesis that astrocytes play a fundamental role in the processes contributing to neurodegeneration. We focus on α-synucleinopathies and tauopathies as the most common neurodegenerative diseases. The mechanisms implicated in the development and progression of these disorders appear not to be exclusively neuronal, but are often related to the astrocytic-neuronal integrity and the response of astrocytes to the altered microglial function. A profound understanding of the multifaceted functions of astrocytes and identification of their communication pathways with neurons and microglia in health and in the disease is of critical significance for the development of novel mechanism-based therapies against neurodegenerative disorders.

Molecular, Structural, Functional, and Pharmacological Sites for Vesicular Glutamate Transporter Regulation

Vesicular glutamate transporters (VGLUTs) control quantal size of glutamatergic transmission and have been the center of numerous studies over the past two decades. VGLUTs contain two independent transport modes that facilitate glutamate packaging into synaptic vesicles and phosphate (Pi) ion transport into the synaptic terminal. While a transmembrane proton electrical gradient established by a vacuolar-type ATPase powers vesicular glutamate transport, recent studies indicate that binding sites and flux properties for chloride, potassium, and protons within VGLUTs themselves regulate VGLUT activity as well. These intrinsic ionic binding and flux properties of VGLUTs can therefore be modulated by neurophysiological conditions to affect levels of glutamate available for release from synapses. Despite their extraordinary importance, specific and high-affinity pharmacological compounds that interact with these sites and regulate VGLUT function, distinguish between the various modes of transport, and the different isoforms themselves, are lacking. In this review, we provide an overview of the physiologic sites for VGLUT regulation that could modulate glutamate release in an over-active synapse or in a disease state.

The history of the pharmacology and cloning of ionotropic glutamate receptors and the development of idiosyncratic nomenclature

In this article, the beginnings of glutamate pharmacology are traced from the early doubts about 'non-specific' excitatory effects, through glutamate- and aspartate-preferring receptors, to NMDA, quisqualate/AMPA and kainate subtypes, and finally to the cloning of genes for these receptor subunits. The development of selective antagonists, crucial to the subtype classification, allowed the fundamental importance of glutamate receptors to synaptic activity throughout the CNS to be realised. The ability to be able to express and manipulate cloned receptor subunits is leading to huge advances in our understanding of these receptors. Similarly the tortuous path of the nomenclature is followed from naming with reference to exogenous agonists, through abortive early attempts at generic schemes, and back to the NC-IUPHAR system based on the natural agonist, the defining exogenous agonist and the gene names.

Localization of putative transmitters in the hippocampal formation: with a note on the connections to septum and hypothalamus

Biochemical assays on microdissected samples, denervation studies, subcellular fractionation, and light and electron microscopic autoradiography of high affinity uptake have been performed to study the cellular localization of transmitter candidates in the rat hippocampal formation. High affinity uptake of glutamate and aspartate is localized in the terminals of several excitatory systems, such as the entorhino-dentate fibres (perforant path), mossy fibres (from granular cells) and pyramidal cell axons. Thus, in stratum radiatum and oriens of CA1, 85% of glutamate and asparate uptake and 40% of glutamate and aspartate content are lost after lesions of ipsilateral plus commissural fibres from CA3/CA4. Hippocampal efferents also take up aspartate and glutamate, since these activities are heavily reduced in the lateral septum and mamillary bodies after transection of fimbria and the dorsal fornix. The synthesis (by glutamic acid decarboxylase), content and high affinity uptake of gamma-aminobutyrate (GABA) are not reduced after lesions of these or other projection fibre systems. A localization in intrinsic neurons is confirmed by a selective loss of glutamic acid decarboxylase after local injections of kainic acid. Peak concentrations of the enzyme occur near the pyramidal and granular cell bodies, corresponding to the site of the inhibitory basket cell terminals, and in the outer parts of the molecular layers. Some 85% of glutamic acid decarboxylase is situated in 'nerve ending particles'. Acetylcholine synthesis (by choline acetyltransferase) disappears after lesions of septo-hippocampal fibres. Since 80% of the hippocampal choline acetyltransferase is in 'nerve ending particles', the characteristic topographical distribution of this enzyme should reflect the distribution of cholinergic septo-hippocampal afferents. Serotonin, noradrenaline, dopamine and histamine are located/synthesized in afferent fibre systems. Some monoamine-containing afferents to the hippocampal formation pass via the septal area, others via the amygdala. The hippocampal formation also contains nerve elements reacting with antibodies against neuroactive peptides, such as enkephalin, substance P, somatostatin and gastrin/cholecystokinin.

Aspartate- and Glutamate-like Immunoreactivities in Rat Hippocampal Slices: Depolarization-induced Redistribution and Effects of Precursors

The light microscopic localization of aspartate-like immunoreactivity (Asp-LI) was compared to that of glutamate-like immunoreactivity (Glu-LI) in hippocampal slices by means of specific polyclonal antibodies recognizing the amino acids fixed by glutaraldehyde. After incubation in Krebs' solution with normal (5 mM) or depolarizing concentrations of K+, and various additives, the slices were fixed with glutaraldehyde, resectioned and processed according to the peroxidase - antiperoxidase procedure. At 5 mM K+, Glu-LI was localized in nerve-terminal like dots with a conspicuous laminar distribution, the highest Glu-LI concentrations coinciding with the terminal fields of major excitatory pathways thought to use glutamate or aspartate as transmitters. The localization of Asp-LI showed some similarity to that of Glu-LI, but the laminar distribution was less differentiated and the immunoreactivity was much weaker. At 40 and 55 mM K+ the nerve terminal localizations of Glu-LI and Asp-LI were strongly reduced. Concomitantly, both immunoreactivities appeared in astroglial cells. These changes were Ca2+-dependent. The nerve ending staining patterns of Asp-LI and Glu-LI could be sustained during depolarization if the medium was supplemented with glutamine (0.5 mM). Under these conditions Asp-LI became more intense and its distribution approached that of Glu-LI. This suggests that, when stimulated, some nerve endings can increase their reservoir of releasable aspartate. The presence of glutamine during depolarization strongly reduced glial Asp-LI and Glu-LI, possibly due to its providing nitrogen for conversion of glutamate to glutamine. alpha-Ketoglutarate, another glia-derived precursor of neuronal glutamate, was virtually ineffective in supporting Glu-LI and Asp-LI in nerve endings, and did not suppress Glu-LI or Asp-LI in glia. Our findings provide morphological support for the view that excitatory nerve endings under certain conditions can contain high levels of both aspartate and glutamate (possibly in the same terminals), and that aspartate as well as glutamate can be released synaptically. Further, they underline the importance of the glial supply of the nerve endings with precursor glutamine, which allows them to build up and sustain high concentrations of transmitter amino acids during release.

Effects of high-potassium-induced depolarization on amino acid chemistry of the dorsal cochlear nucleus in rat brain slices

High K+ was used to depolarize glia and neurons in order to study the effects on amino acid release from and concentrations within the dorsal cochlear nucleus (DCN) of brain slices. The release of glutamate, gamma-aminobutyrate (GABA) and glycine increased significantly during exposure to 50 mM K+, while glutamine and serine release decreased significantly during and/or after exposure, respectively. After 10 min of exposure to 50 mM K+, glutamine concentrations increased in all three layers of DCN slices, to more than 5 times the values in unexposed slices. In the presence of a glutamate uptake blocker, L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC), glutamine concentrations in all layers did not increase as much during 50 mM K+. Similar but smaller changes occurred for serine. Mean ATP concentrations were lower in 50 mM K(+)-exposed slices compared to control. The results suggest that depolarization, such as during increased neural activity, can greatly affect amino acid metabolism in the cochlear nucleus.

Molecular biology of glutamate receptors in the central nervous system and their role in excitotoxicity, oxidative stress and aging

Forty years of research into the function of L-glutamic acid as a neurotransmitter in the vertebrate central nervous system (CNS) have uncovered a tremendous complexity in the actions of this excitatory neurotransmitter and an equally great complexity in the molecular structures of the receptors activated by L-glutamate. L-Glutamate is the most widespread excitatory transmitter system in the vertebrate CNS and in addition to its actions as a synaptic transmitter it produces long-lasting changes in neuronal excitability, synaptic structure and function, neuronal migration during development, and neuronal viability. These effects are produced through the activation of two general classes of receptors, those that form ion channels or "ionotropic" and those that are linked to G-proteins or "metabotropic". The pharmacological and physiological characterization of these various forms over the past two decades has led to the definition of three forms of ionotropic receptors, the kainate (KA), AMPA, and NMDA receptors, and three groups of metabotropic receptors. Twenty-seven genes are now identified for specific subunits of these receptors and another five proteins are likely to function as receptor subunits or receptor associated proteins. The regulation of expression of these protein subunits, their localization in neuronal and glial membranes, and their role in determining the physiological properties of glutamate receptors is a fertile field of current investigations into the cell and molecular biology of these receptors. Both ionotropic and metabotropic receptors are linked to multiple intracellular messengers, such as Ca2+, cyclic AMP, reactive oxygen species, and initiate multiple signaling cascades that determine neuronal growth, differentiation and survival. These cascades of complex molecular events are presented in this review.

Glutamate metabolic pathways in displaced ganglion cells of the chicken retina

Glutamate (E) is the putative amino acid neurotransmitter used by ganglion cells, photoreceptors, and bipolar cells. Aspartate (D) and glutamine (Q) are potential precursors of glutamate, and glutamate-utilizing neurons may use one or more of these amino acids to sustain production of glutamate. We used post-embedding immunocytochemistry for several amino acid neurotransmitters to characterize the amino acid signatures for displaced ganglion cells of the avian retina. We found two neurochemical signatures for displaced ganglion cells, EQ and EDQ, in mid-peripheral and far-peripheral retina, respectively. Differences in neurochemical signatures cannot be explained by the existence of two ganglion cell populations, and we propose that the two signature categories for the large-diameter displaced ganglion cells reflect variations in the aspartate precursor pool. The transamination reaction involved in glutamate production, aspartate/oxaloacetate and alpha-ketoglutarate/glutamate, requires an active TCA cycle, since the carbon skeleton of glutamate is derived from alpha-ketoglutarate, a TCA intermediary. We hypothesized that aspartate levels vary in the normal chicken retina because eccentricity-dependent differences in oxygen availability result in changes of alpha-ketoglutarate levels, and hence, alterations in the equilibrium of the transamination reaction. We tested this hypothesis by incubating isolated chicken retinas in anaerobic conditions and found elevated aspartate immunoreactivity in subpopulations of glutamate-utilizing neurons in the central retina. Under aerobic conditions, or in retinas placed directly into fixative, retinal samples from the central edge of the pecten did not show differential cellular staining for aspartate. We have, therefore, identified differences in neurochemical signatures for retinal neurons involving changes in active maintenance of precursor pools.

Glutamate, GABA and epilepsy

The nature and value of various animal models of epilepsy for the study and understanding of the human epilepsies are reviewed, with special reference to the ILAE classification of seizures. Kindling as a model of complex-partial seizures with secondary generalisation is treated in detail, dwelling principally on the evidence that the neurotransmitters glutamate and GABA are centrally involved in the kindling process. Kindling in the entorhinal cortex-hippocampus system and its relationship to LTP are analysed in detail. Changes in amino acid content in animal and human brain tissue following onset of the epileptic state are reviewed with special reference to glutamate and GABA. Studies of changes in the extent of basal and stimulus-evoked release of glutamate and GABA both in vivo (microdialysis) and in vitro (brain slices) are evaluated. This includes both kindling and other models of epilepsy, and microdialysis of human patients with epilepsy. Experiments which study the influence of pre-synaptic metabotropic glutamate receptors on glutamate release, and consequently on the extent of electrical kindling, are described. This pre-synaptic control of glutamate release can be studied using synaptosomes. The significance of the ability of focal intracerebrally injected glutamate and NMDA to cause (chemical) kindling and the strong sensitivity of this process to pre-treatment with NMDA receptor antagonists is analysed. Electrical and chemical kindling effects are additive, indicating the existence of mechanisms in common. They are both sensitive to NMDA antagonists and the common mechanism is probably NMDA receptor activation due to the presence of exogenous (chemical) or endogenous (electrically-released) extracellular glutamate. The participation of the NMDA receptor in the generation of the spontaneous hyperactivity which characterises the chronic epileptic state is reviewed. This includes the entry of Ca2+ to stimulate various post-synaptic phosphorylation processes, and possible modulation of NMDA receptor population size and sensitivity. The question of whether neurotransmitter glutamate is involved in initiation and/or spread of seizures is discussed.

Effect of anoxia on glutamate formation from glutamine in cultured neurons: dependence on neuronal subtype

Synthesis and release of glutamate formed from labeled glutamine were studied in primary cultures of the glutamatergic cerebellar granule cells and of the mainly GABAergic cerebral cortical neurons under anoxic conditions and under normoxic control conditions. Under both control and anoxic conditions cerebellar granule cells synthesized and released glutamate more intensely than cerebral cortical neurons, but this difference was enhanced under anoxic conditions. Thus, under normoxic conditions synthesis of intracellular labeled glutamate from glutamine was twice as high in cerebellar granule cell neurons as in cerebral cortical neurons during 30 min of incubation, but the release of newly synthesized labeled glutamate to the extracellular medium from cerebellar granule cell neurons was more than 4 times higher than the release from cerebral cortical neurons during 30 min of incubation. Based on these observations it is suggested that a major reason for the increase in extracellular glutamate concentration during brain ischemia may be enhanced production and release of glutamate, especially in glutamatergic neurons.

Neurochemical signatures revealed by glutamine labeling in the chicken retina

Postembedding immunocytochemistry was used to determine the retinal distribution of the amino acid glutamine, and characterize amino acid signatures in the avian retinal ganglion cell layer. Glutamine is a potential precursor of glutamate and some glutamatergic neurons may use this amino acid to sustain production of glutamate for neurotransmission. Ganglion cells, cells in the inner nuclear layer, and some photoreceptors exhibited glutamine immunoreactivity of varying intensity. Ganglion cells demonstrated the highest level of immunoreactivity which indicates either slow glutamine turnover or active maintenance of a large standing glutamine pool relative to other glutamatergic neurons. Müller's cells in the avian retina are involved in glutamate uptake and carbon recycling by the rapid conversion of glutamate to glutamine, thus explaining the low glutamate and high glutamine immunoreactivity found throughout Müller's cells. Most chicken retinal ganglion cells are glutamate (E) and glutamine (Q) immunoreactive but display diverse signatures with presumed functional subsets of cells displaying admixtures of E and Q with GABA (gamma) and/or glycine (G). The four major ganglion cell signatures are (1) EQ; (2) EQ gamma; (3) EQG; and (4) EQ gamma G.

13C NMR spectroscopy study of cortical nerve cell cultures exposed to hypoxia

Primary cultures of cerebral cortical GABA-ergic neurons growing on top of a preformed layer of astrocytes (co-cultures) were incubated with [1-13C]glucose and exposed to a low oxygen atmosphere (2% O2) for 17 hr. 13C, 1H, and 31P nuclear magnetic resonance (NMR) spectroscopy was performed on perchloric acid (PCA) extracts of cells and of media collected from these cultures. In the control groups incorporation of 13C label into glutamine, citrate, and lactate could be demonstrated in both cell extracts and culture media. Labeled GABA and glutamate were only observed in cell extracts. During hypoxia high energy phosphates decreased but lactate production and glucose consumption increased. There was a decreased amount of citrate and glutamine in cell extracts and media of the hypoxic co-cultures. There was a change in distribution of the 13C label within the GABA molecule, with an increase of labeling in the C-2 position. This change in 13C distribution was not found in glutamine present in the media where it is a precursor for GABA in neurons. Instead a decrease in the corresponding C-4 position was observed. These results suggest that energy depletion during hypoxia leads to reduced export from the astrocytic tricarboxylic acid (TCA) cycle as demonstrated by a decreased amount of citrate and changed distribution of 13C in glutamine. The change in the distribution of label in GABA from cell extracts as compared to glutamine in the medium may indicate that neurons are synthesizing GABA using precursors supplied from their own TCA cycle and not from precursors supplied by astrocytes.

Uptake and metabolism of malate in neurons and astrocytes in primary cultures

Uptake and oxidative metabolism of [14C]malate as well as its incorporation into aspartate, glutamate, glutamine, and GABA were studied in cultured cerebral cortical neurons (GABAergic), cerebellar granule neurons (glutamatergic), and cerebral cortical astrocytes. All cell types exhibited high affinity uptake of malate (Km 10-85 microM) with slightly higher Vmax values in neurons (0.1-0.2 nmol x min-1 x mg-1) than in astrocytes (0.06 nmol x min-1 x mg-1). Malate was oxidatively metabolized in all three cell types with nominal rates of 14CO2 production of 2-15 pmol x min-1 x mg-1. The oxidation of malate was only slightly inhibited by 5 mM aminooxyacetic acid (AOAA). In granule cell preparations [14C]malate was incorporated into aspartate and glutamate and, to a much less extent, into glutamine. This incorporation was blocked by 5 mM AOAA. Astrocytes exhibited slightly higher incorporation rates into aspartate and glutamate, but in these cells glutamine was labelled to a considerable extent. AOAA (5 mM) inhibited the incorporation by 60-70%. In cultures of cerebral cortical neurons, very low levels of radioactivity derived from [14C]malate were found in aspartate and glutamate, and GABA was not labelled at all. Glutamine had the same specific activity as glutamate, indicating that the low rates of incorporation of radioactivity into amino acids in this preparation is likely to exclusively represent metabolism of malate in the small population of astrocytes (5% of total cell number), contaminating the neuronal cultures. The findings suggest that exogenous malate to a quantitatively limited extent may serve as a precursor for transmitter glutamate in glutamatergic neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Release of GABA and taurine from brain slices

In brain slices the mechanisms of release of GABA have been extensively studied, but those of taurine markedly less. The knowledge acquired from studies on GABA is, nevertheless, still fragmentary, not to speak of that obtained from the few studies on taurine, and firm conclusions are difficult, even impossible, to draw. This is mainly due to methodological matters, such as the diversity and pitfalls of the techniques applied. Brain slices are relatively easy to prepare and they represent a preparation that may most closely reflect relations prevailing in vivo, since the tissue structure and cellular integrity are largely preserved. In our opinion the most recommendable method at present is to superfuse freely floating agitated slices in continuously oxygenated medium. Taurine is metabolically rather inert in the brain, whereas the metabolism of GABA must be taken into account in all release studies. The use of inhibitors of GABA catabolism is discouraged, however, since a block in GABA metabolism may distort relations between different releasable pools of GABA in tissue. It is not known for sure how well, and homogeneously, incubation of slices with radioactive taurine labels the releasable pools but at least in the case of GABA there may prevail differences in the behavior of labeled and endogenous GABA. It is suggested therefore that the results obtained with radioactive GABA or taurine should be frequently checked and confirmed by analyzing the release of respective endogenous compounds. The spontaneous efflux of both GABA and taurine from brain slices is very slow. The magnitude of stimulation of GABA release by homoexchange is greater than that of taurine under the same experimental conditions. However, the release of both amino acids is generally enhanced by a great number of structural analogs, the most potent being those which are simultaneously the most potent inhibitors of uptake. This may result in part from inhibition of reuptake of amino acid molecules released from slices but the findings may also signify that the efflux of GABA and taurine is at least partially mediated by the membrane carriers operating in an outward direction. It is thus advisable not to interpret that stimulation of release in the presence of uptake inhibitors solely results from the block of reuptake of exocytotically released molecules, since changes in the carrier-mediated transport are also likely to occur upon stimulation. The electrical and K+ stimulation evoke the release of both GABA and taurine. The evoked release of GABA is several-fold greater than that of taurine in slices from the adult brain.(ABSTRACT TRUNCATED AT 400 WORDS)

Interrelationship between retinal ischaemic damage and turnover and metabolism of putative amino acid neurotransmitters, glutamate and GABA

Conditions causing a reduction of oxygen availability (anoxia), such as stroke or diabetes, result in drastic changes in ion movements, levels of neurotransmitters and metabolites and subsequent neural death. Currently, there is no clinically available treatment for anoxia induced neural cell death resulting in drastic and permanent central nervous system dysfunction. However, there have been some exciting developments in experimentally induced anoxic conditions where several classes of drugs appear to significantly reduce neural cell death. This report aims to provide the foundations for understanding both the basic mechanisms involved in retinal ischaemic damage and experimental treatments used to prevent such damage. We discuss the normal release, actions and uptake of the fast retinal neurotransmitters, glutamate and GABA, in the vertebrate retina. Immunocytochemistry is used to demonstrate that both glutamate and GABA are found in the macaque retina. Following this is a discussion on how ischaemia may enhance neurotransmitter release or disrupt its uptake, thus causing an increase in extracellular concentration of these neurotransmitters and subsequent neuronal damage. The mechanisms involved in glutamate neurotoxicity are reviewed, because excess glutamate is the likely cause of retinal ischaemic damage. Finally, the mechanisms behind four possible modes of treatment of neurotransmitter toxicity and their advantages and disadvantages are discussed. Hopefully, further research in this area will lead to the development of a rational therapy for retinal, as well as cerebral ischaemia.

Effect of perforant path lesion on pattern of glutamate-like immunoreactivity in rat dentate gyrus

To investigate the relation between perforant path and the pattern of glutamate-like immunoreactivity in its target regions in the rat hippocampal formation, unilateral lesions of various size and location were placed to interrupt certain contingents of these afferent fibers. Postembedding immunohistochemistry at the level of light microscopy yielded the same pattern of immunoreactivity in the hippocampal formation contralateral to the lesion as in untreated animals. On the ipsilateral side, however, extensive transections of the perforant path led to a drastic loss of glutamate-immunoreactive terminal-like elements in the outer part of the dentate molecular layer. More restricted lesions induced a loss of punctate glutamate-like immunoreactivity in narrower bands within this zone. The width and the location of the affected bands appeared to depend on the extent of the transections and their topographical relation to the perforant path fiber system. These results and those obtained using a postembedding immunogold method at the level of electron microscopy strongly indicate that perforant path terminals in the dentate molecular layer of the rat contain high levels of glutamate and, thus, provide further support for an already well-documented role of this excitatory amino acid as neurotransmitter in this system.

Electrical stimulation-evoked release of endogenous aspartate from rat medulla oblongata slices. Effects of inhibitors of aspartate aminotransferase and GABA transaminase

The effects of aminooxyacetic acid (AOAA), an aspartate aminotransferase (AAT) inhibitor, L-canaline, an ornithine aminotransferase inhibitor, and gamma-acetylenic GABA and gabaculine, both gamma-aminobutyric acid transaminase (GABA-T) inhibitors, on the release of aspartate from slices of rat medulla oblongata and hippocampus were studied. The slices were superfused and electrically stimulated. There was a Ca2(+)-dependent stimulus-evoked release of endogenous aspartate. AOAA (10(-4) and 10(-3) M) decreased the evoked release of aspartate in the medulla oblongata but not in the hippocampus. In addition, AOAA produced a decrease in the spontaneous efflux and tissue content of aspartate in the medulla oblongata. L-Canaline (5 x 10(-5) M), gamma-acetylenic GABA (10(-4) M) and gabaculine (10(-5) M) did not affect the evoked release of aspartate in the medulla oblongata, while these agents produced a decrease in spontaneous efflux and tissue content of aspartate. These findings suggest that AAT participates in the synthesis of transmitter aspartate in the medulla oblongata of the rat. It appears that there are the pools of transmitter aspartate and non-transmitter aspartate in the rat medulla oblongata.

Neurochemical and electrophysiological studies on the inhibitory effect of ammonium ions on synaptic transmission in slices of rat hippocampus: evidence for a postsynaptic action

To elucidate the mechanisms involved in the inhibition of synaptic transmission by ammonium ions, the effects of NH4Cl on glutamate release and on synaptic transmission from Schaffer collaterals to CA1 pyramidal cells were measured in fully submerged slices of rat hippocampus. The large, Ca(2+)-dependent release of glutamate evoked by electrical-field stimulation or by 56 mM K+ was not reduced by 5 mM NH4Cl. In contrast, 5 mM NH4Cl decreased the smaller, field stimulation-induced release of glutamate observed in the presence of low concentrations of Ca2+ (0.1 mM), as well as the spontaneous release of glutamate both in normal and low Ca2+. Unlike the Ca(2+)-dependent release of glutamate, synaptic transmission was reversibly depressed even by 1 mM NH4 Cl. Firing of CA1 pyramidal cells evoked by iontophoretically applied glutamate was significantly inhibited by 2 or 5 mM NH4Cl. This depression was increased in the presence of 25 microM bicuculline. Results suggest that ammonium ions do not depress the Ca(2+)-dependent release of glutamate originating from synaptic vesicles, which is involved in synaptic transmission. Rather, ammonium ions inhibit synaptic transmission by a postsynaptic action, a conclusion strengthened by the inhibitory effect of NH4Cl on glutamate-induced firing. However, NH4Cl may inhibit the formation of cytoplasmic glutamate, the source of spontaneous and Ca(2+)-independent release.

Aspartate aminotransferase for synthesis of transmitter glutamate in the medulla oblongata: effect of aminooxyacetic acid and 2-oxoglutarate

The effects of aminooxyacetic acid (AOAA), a transaminase inhibitor, and 2-oxoglutarate, a precursor to glutamate by the activity of aspartate aminotransferase (AAT), on slices of rat medulla oblongata, cerebellum, cerebral cortex, and hippocampus were studied. The slices were superfused and electrically stimulated. There was a Ca2+-dependent stimulus-evoked release of endogenous glutamate, gamma-aminobutyric acid (GABA), and beta-alanine in all regions examined. AOAA (10(-4) and 10(-3) M) decreased the release of glutamate in the medulla oblongata and cerebellum but not in the hippocampus. L-Canaline, a specific inhibitor of ornithine aminotransferase, did not affect the glutamate release in the medulla. 2-Oxoglutarate (10(-3) M) increased the release of glutamate in the medulla oblongata and cerebellum but not in the cerebral cortex and hippocampus. Treatment with AOAA (10(-4) M) almost abolished the activities of AAT in all regions studied. AOAA (10(-4) and 10(-3) M) increased the stimulus-evoked release of GABA in the cerebellum, cerebral cortex, and hippocampus, whereas the stimulus-evoked release of beta-alanine was decreased by this agent in all regions studied. These results suggest the participation of AAT in the synthesis of the transmitter glutamate in the medulla oblongata and cerebellum of the rat.

Glutamate-like immunoreactivity revealed in rat olfactory bulb, hippocampus and cerebellum by monoclonal antibody and sensitive staining method

Although there is good evidence favoring L-glutamate as a major excitatory amino acid transmitter, relatively little is known about the distribution of nerve terminals using this substance. A method visualizing glutamate-like immunoreactivity at the light microscopic level by means of a monoclonal antibody, mAb 2D7, is described. --The antigen used for immunization was a glutaraldehyde-linked glutamate-BSA conjugate, and hybridomas were differentially screened by ELISA for production of antibodies recognizing glutamate- but not aspartate-BSA. The crossreactivity of 'anti-glutamate' mAb 2D7 as estimated in absorption tests was low even with conjugates closely related to glutamate-BSA.--Semithin sections from rapidly perfusion-fixed, plastic-embedded rat brain tissues were etched and stained by a combination of the peroxidase-antiperoxidase method and silver enhancement of the diaminobenzidine reaction product. Only this amongst several other immunohistochemical methods tried produced labeling patterns which showed terminal-like elements in brain regions such as olfactory bulb, hippocampus and cerebellum, and which were mostly consistent with already available information on systems using glutamate as neurotransmitter. Particularly striking was the staining of elements reminiscent of mossy fiber terminals in hippocampus and cerebellum as well as of cerebellar parallel fiber terminals.

Effect of glutamine on glutamate release from hippocampal slices induced by high K+ or by electrical stimulation: interaction with different Ca2+ concentrations

To characterize the effect of glutamine on the release of glutamate, aspartate, and gamma-aminobutyric acid (GABA), rat hippocampal slices were superfused with different concentrations of glutamine or Ca2+. Amino acids released and retained were analyzed by HPLC. Glutamine (0.5 mmol/L) increased more than threefold the release of glutamate evoked by 50 mmol/L K+ in the presence of 2.6 mmol/L Ca2+ without a corresponding increase in glutamate content, while the release of aspartate was increased less and that of GABA not at all by glutamine. The evoked release of all three amino acids, including the enhanced release of glutamate in the presence of glutamine, was strongly dependent on Ca2+ concentrations between 0.1 and 2.6 mmol/L. The potentiation of glutamate release by glutamine reached a plateau at 0.25 mmol/L glutamine. Intermittent electrical field stimulation increased the release of only glutamate and this release was nearly doubled by glutamine. The increased release was Ca2+ dependent and tetrodotoxin (TTX) sensitive. Results suggest that extracellular glutamine promotes primarily the formation of releasable glutamate and this enhancement is dependent on extracellular Ca2+.

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.

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.

Distribution of neuropeptides in the limbic system of the rat: the hippocampus

The distribution of several neuropeptides (vasoactive intestinal polypeptide, cholecystokinin octapeptide, substance P, neurotensin, methionine-enkephalin and somatostatin) in the hippocampal formation has been studied with immunocytochemical techniques. Numerous vasoactive intestinal polypeptide, cholecystokinin-octapeptide and somatostatin-positive cell bodies were found within the hippocampus and subiculum. Neurotensin-positive cell bodies were found within the subiculum, but no substance P or methionine-enkephalin-containing cell bodies were seen in either hippocampus proper or subiculum. Vasoactive intestinal polypeptide and cholecystokinin-octapeptide-positive cell bodies were predominantly located in the stratum moleculare and stratum radiatum of CA 1-2 regions and dentate gyrus, whilst somatostatin-containing cell bodies were found mainly in the stratum oriens. Nerve fibres containing each of the six peptides were found within the hippocampus. Large numbers of vasoactive intestinal polypeptide, cholecystokinin-octapeptide and somatostatin fibres innervated the pyramidal and granule cell layers, with smaller numbers in the stratum radiatum and fewer still in the stratum moleculare and stratum oriens. Other than a moderately dense neurotensin-positive fibre plexus in the dorsal subiculum, fewer neurotensin, substance P and methionine-enkephalin fibres were present. However, when present, these three peptides had a distribution restricted to a region close to the pyramidal layer in the CA 2/3 region and to the stratum moleculare of the CA 1 region. Peptide-containing fibres were identified entering or leaving the hippocampus in three ways, via (i) the fornix (all six peptides), (ii) the dorsal subiculum (neurotensin-positive fibres projecting to the cingulate cortex: somatostatin, vasoactive intestinal polypeptide, and cholecystokinin-octapeptide present in fibres running between the dorsal subiculum and occipito-parietal cortex) and (iii) the ventral subiculum (vasoactive intestinal polypeptide, cholecystokinin-octapeptide and somatostatin in fibres running between entorhinal cortex and hippocampus, and all six peptides in fibres crossing the amygdalo-hippocampal border). These findings indicate a major distinction between those peptides (vasoactive intestinal polypeptide, cholecystokinin-octapeptide, somatostatin, neurotensin) which are found in cell bodies intrinsic to the hippocampal formation and those peptides (substance P, methionine-enkephalin) which are found only in hippocampal afferents.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrical stimulation of the stratum radiatum increases the release and neosynthesis of aspartate, glutamate, and gamma-aminobutyric acid in rat hippocampal slices

The release of endogenous aspartic, glutamic, and gamma-aminobutyric acids (Asp, Glu, GABA, respectively) was measured in the effluent from superfused hippocampal slices using a new and sensitive mass spectrometric method. The stimulation of the stratum radiatum of the rat dorsal hippocampus caused a Ca2+-dependent increase in the release of these amino acids. This release was accompanied by an increase in the incorporation of [13C2] from [13C]glucose into Asp, Glu, and GABA, suggesting an increase in their neosynthesis. The removal of Ca2+ from the superfusion fluid brought about a marked decrease in Asp and Glu release at rest, and prevented their stimulation-evoked release and the appearance of population spikes. The results support the hypothesis that Asp and Glu are excitatory neurotransmitters in intrinsic hippocampal circuits and are possibly released from the Schaffer collaterals and commissural fibres. The increase in GABA release and neosynthesis during stimulation of the stratum radiatum could be related to recurrent inhibition evoked by transsynaptic stimulation of the pyramidal cells.

Behavior during hippocampal microinfusions. IV. Transmitter interactions

The interaction between noradrenergic, cholinergic, and GABAergic receptor stimulation in the dentate gyrus of the rat was investigated at the behavioral level. Awake, unrestrained rats in a holeboard/activity apparatus received continuous 40-min infusions into the dentate hilus of combined solutions of norepinephrine, the cholinergic agonist carbachol, and the GABA antagonist picrotoxin. Infusions into the dentate gyrus of either carbachol or picrotoxin have been found to produce a comparable locomotor activation of rats that is probably due to the excitation of dentate granule cells. Low dose hippocampal infusions of norepinephrine have been shown to significantly affect the quality but not the quantity of the locomotor activity or rats. Co-infusion of norepinephrine potentiated the effects of picrotoxin and blocked the effects of carbachol. It is hypothesized that the noradrenergic input to the area dentata increases the efficacy of extrinsic afferents while also facilitating recurrent granule cell inhibition mediated by GABAergic interneurons.

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.

Organization of substance P fibers within the hippocampal formation demonstrated with a biotin-avidin immunoperoxidase technique

The distribution of substance P-containing fibers within the hippocampal formation of the cat was examined using an immunohistochemical approach. A new, indirect immunoperoxidase method based on the high affinity binding of vitamin H (biotin) by avidin was developed to demonstrate substance P-like immunoreactive fibers. The neocortex contained only occasional single substance P fibers. In contrast, the archicortical structures contained a well organized substance P innervation. The entorhinal area of the parahippocampal gyrus contained a delicate network of varicose axons which appeared to ascend from the subcortical white matter to terminate predominantly in layer II. The subiculum contained a network of positive varicosities throughout the neuropil of both the pyramidal and the molecular layers. These substance P terminals appeared to arise, at least in part, from fibers entering laterally across the angular bundle. Within the ventral hippocampus the substance P innervation was predominantly in the pyramidal cell layer, and was heaviest in field CA3 and weakest in CA1. Some fibers were also observed in strata oriens, lucidum, and radiatum. Occasional varicose fibers could be seen entering the hippocampus from the fimbria. Within the dentate gyrus the substance P fibers were most concentrated in the supragranular layer and among the more superficial of the granule cells. A moderately dense plexus of fibers also occurred in the neuropil of the hilus among the polymorphic cells. In addition, a distinct, narrow band of varicosities was found at the outer edge of the inner molecular layer, while the outer molecular layer did not contain any positive elements. It is concluded that the hippocampal formation receives a distinct, well organized substance P innervation. Substance P fibers appear to enter the hippocampus both laterally across the angular bundle and ventrally through the fimbria. From this detailed examination of the distribution of the substance P fibers within the hippocampus in relation to the other known inputs to this area it is suggested that many of the substance P fibers arise in or, more probably, pass through the septal area to innervate the hippocampus.

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 role of epileptic activity in hippocampal and "remote" cerebral lesions induced by kainic acid

Kainic acid (KA) was injected systemically, intracerebroventricularly (i.c.v.) and focally in the amygdala and other deep brain structures in the rat. EEG and behavioral changes were studied in relation to the neuropathology which developed subsequently. Following intra-amygdaloid KA injection, diazepam blocked the epileptic events induced by the toxin, and abolished the neuronal loss usually seen in the lateral septum, claustrum, and contralateral cortex and hippocampus. The lesions in medial thalamic structures and ipsilateral hippocampus were also reduced by diazepam. Prior transection of the perforant path ipsilateral to the KA injection also decreased the severity of the electrographic and motor effects of the toxin and similarly reduced the extent of distant ("remote") pathological brain damage. Neither diazepam nor perforant path transection reduced the damage at the site of KA injection. Kainic acid (0.4-2 microgram) injected into the bed nucleus of the stria terminalis (BST) or the medial septum produced seizures with a longer latency and little brain damage outside the injection site. In contrast, intrastriatal KA injections were followed by ipsilateral hippocampal lesions. i.c.v. Injection of KA (0.4-1.6 microgram) produced a complex syndrome which included bilateral exophthalmos, mydriasis, foaming, tremor of the vibrissae, and paw and body tremor. The pattern of brain damage resembled that seen following intra-amygdaloid administration of the toxin. In addition, however, there was a bilateral necrosis of the pyriform and prepyriform cortices up to the rhinal fissure. Systemic administration of diazepam (i.p.) reduced the extent of the damage and in particular completely prevented the cortical damage. Systemic administration of KA (9-15 mg/kg i.p.) readily produced motor and EEG seizures similar to those seen after intra-amygdaloid injection of the toxin. The pattern of brain damage was however more symmetrical than that which followed focal i.c.v. injection of the toxin and included necrosis of the pyriform cortex. It is concluded that spread of seizure activity from the injection site plays a crucial role in the induction of "remote" brain damage after focal intracerebral injections.

Analysis of short-term plasticity at the perforant path-granule cell synapse

Short-term plasticity was investigated at the perforant path-granule cell synapse in the hippocampal slice preparation. A successive decrement in the amplitude of the extracellular EPSP was obtained at all stimulus frequencies above 0.05 Hz. This effect of repetitive stimulation has previously been shown to fulfill the requirements for habituation processes. If each stimulus within an habituation train was followed by a second identical test stimulus the response to the test stimulus was larger than that to the paired conditioning stimulus. This short-term plasticity has been called paired pulse potentiation. The test response potentiated only with respect to the paired conditioning response and not with respect to previous test responses. Neither form of plasticity appeared to result from changes in the amplitude of the afferent fiber volley. Both habituation and paired pulse potentiation result from an interaction of at least three changes in the efficacy of transmission after a conditioning stimulus: (1) an initial depression, (2) an intermediate relative potentiation and (3) a late depression which decays slowly. Paired pulse potentiation could be demonstrated only if the interpair interval corresponded to the period of maximal late depression and the interstimulus interval to the period of relative potentiation. The amplitudes of intermediate relative potentiation and late depression (and inhibition of transmission by 2-amino-4-phosphonobutyric acid (APB)) were inversely related to the control response amplitude. This relationship likely derives from nonlinear stimulation of postsynaptic ionic currents at higher stimulus intensities. In contrast, the initial depression increased with response amplitude. This is consistent with a mechanism dependent on the postsynaptic membrane potential, such as refractoriness to succeeding stimuli. When the response amplitudes in the presence and absence of 2.5 mM APB were equalized by adjusting the stimulus intensity, no difference was found in the magnitude of either form of plasticity. Since APB probably inhibits transmission at this site through competitive antagonism at the postsynaptic receptor, this observation suggests that habituation and paired pulse potentiation are generated presynaptically.

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

Slices of the molecular layer of the dentate gyrus of the hippocampal formation were incubated with either [14C]glucose, [14C]pyruvate or 14C glutamine and the efflux of endogenous and radioactive glutamate was monitored under various conditions. After prelabeling with either [14C]glutamine or [14C]glucose elevation of K+ concentration to 56 mM (Ca2+ free) increased efflux of endogenous and [14C]glutamate. Introduction of Ca2+ into the elevated K+ medium further increased the efflux of endogenous glutamate and radioactive glutamate derived from any of the precursors tested. In glutamine containing media, the increase in glutamate efflux as well as basal efflux was considerably higher than in the absence of glutamine and the specific activity of glutamate release was higher than that in tissue. Thus glutamine was superior to glucose or pyruvate as precursor and most specifically labeled the putative transmitter pool of glutamate. Similar experiments were carried out 4 and 14 days after a unilateral lesion in the entorhinal cortex which provides about 60% of the total synaptic input to the dentate granule cells. The Ca2+ dependent release of glutamate derived from either glucose or glutamine was markedly reduced on the operated side. This result suggests that the transmitter pool of glutamate is in perforant path terminals and can be synthesized from glucose or glutamine.