Release of glutamate, aspartate, and gamma-aminobutyric acid from isolated nerve terminals

Inhibition of SKF 89976-A of the gamma-aminobutyric acid release from primary neuronal chick cultures

Neuronal cultures were made from the 8-d-old embryonic chick telencephalon. The primary culture model was further improved, the medium composition was modified, and the cells grown for 10 d, which allowed the development of relatively differentiated neurones. A superfusion protocol was developed and applied to study the release of [3H]-gamma-aminobutyric acid ([3H]GABA). High endogenous activity levels of glutamate decarboxylase (GAD) and of a Ca-dependent potassium stimulated [3H]GABA release were used as criteria for GABAergic differentiation. The influence of the non-substrate inhibitor of GABA transport, SKF 89976-A, on the GABA release, was studied using the primary neuronal culture. The release was found to be inhibited by SKF 89976-A at higher concentrations (= 400 microM).

Effect of oxidative stress on the release of [3H]GABA in cultured chick retina cells

The effect of ascorbate (1.5 mM)/Fe2+ (7.5 microM)-induced oxidative stress on the release of pre-accumulated [3H]gamma-aminobutyric acid ([3H]GABA) from cultured chick retina cells was studied. Depolarization of control cells with 50 mM K+ increased the release of [3H]GABA by 1.01 +/- 0.16% and 2.5 +/- 0.3% of the total, in the absence and in the presence of Ca2+, respectively. Lipid peroxidation increased the release of [3H]GABA to 2.07 +/- 0.31% and 3.6 +/- 0.39% of the total, in Ca(2+)-free or in Ca(2+)-containing media, respectively. The inhibitor of the GABA carrier, 1-(2-(((diphenylmethylene)amino)oxy)ethyl)-1,2,5,6-tetrahydro-3-py ridine- carboxylic acid hydrochloride (NNC-711) blocked almost completely the release of [3H]GABA due to K(+)-depolarization in the absence of Ca2+, but only 65% of the release occurring in the presence of Ca2+ in control and peroxidized cells. Under oxidative stress retina cells release more [3H]GABA than control cells, being the Ca(2+)-independent mechanism, mediated by the reversal of the Na+/GABA carrier, the most affected. MK-801 (1 microM), a non-competitive antagonist of the NMDA receptor-channel complex, blocked by 80% the release of [3H]GABA in peroxidized cells, whereas in control cells the inhibitory effect was of 48%. The non-selective blocker of the non-NMDA glutamate receptors, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), inhibited the release of [3H]GABA by 30% and 70% in control and peroxidized cells, respectively. Glycine (5 microM) stimulated [3H]GABA release evoked by 50 mM K+-depolarization in control but not in peroxidized cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucocorticoids increase extracellular [3H]D-aspartate overflow in hippocampal cultures during cyanide-induced ischemia

Glucocorticoids (GCs), the adrenal steroid hormones secreted during stress, exacerbate neuronal death in the hippocampus during ischemia. Since ischemia brain damage is ascribed to an elevated level of extracellular excitatory amino acids (EAAs), this study was undertaken to investigate the effect of GCs on EAA homeostasis in hippocampal cell cultures during the insult of cyanide exposure. Using D-[2,3-3H]aspartic acid ([3H]D-Asp) as a tracer, we found that corticosterone (CORT, the physiological GC in rats) increased the accumulation of extracellular [3H]D-Asp by 25% in hippocampal cultures during cyanide-induced ischemia. CORT had no effect on the release of [3H]D-Asp. Instead, analysis of [3H]D-Asp uptake kinetics indicates that CORT decreased the maximum uptake rate and the Michaelis constant by 44% and 50%, respectively, in cells treated with cyanide. It is concluded that, during cyanide-induced ischemia, CORT might enhance extracellular overflow of [3H]D-Asp by decreasing its uptake, thereby endangering neurons.

Cholinergic modulation of spontaneous activity in rat dorsal cochlear nucleus

Extracellular recordings were made from brain stem slices to test the effects of bath application of cholinergic agonists and antagonists on the firing rates of spontaneously active dorsal cochlear nucleus neurons. About 90% of neurons responded to carbachol. A higher proportion responded to muscarine than to nicotine. Muscarine elicited larger responses at lower concentrations than nicotine. Responses to either carbachol or muscarine were always blocked by atropine or scopolamine. The nicotinic antagonists d-tubocurarine, hexamethonium, and mecamylamine blocked the responses to nicotine, but did not decrease the responses to carbachol. Regularly firing neurons showed only increases of firing rate during exposure to cholinergic agonists. About half of responsive bursting neurons showed increased firing; half showed increased followed by decreased firing to 10 microM carbachol or muscarine. All phases of the responses of most bursting neurons were greatly decreased or abolished in low calcium, high magnesium medium, while responses of regular neurons were not detectably affected. Thus, cholinergic agonists appear to act directly on regularly firing neurons, while their actions on bursting neurons may require synaptic activity. The data suggest that cholinergic transmission in the dorsal cochlear nucleus is predominantly muscarinic, and that most regularly firing spontaneously active neurons have muscarinic receptors.

Glutamate agonists and [3H]GABA release from rat hippocampal slices: involvement of metabotropic glutamate receptors in the quisqualate-evoked release

The effects of glutamate agonists and their selective antagonists on the Ca(2+)-dependent and independent releases of [3H]GABA from rat coronal hippocampal slices were studied in a superfusion system. The Ca(2+)-dependent release evoked by glutamate, kainate and N-methyl-D-aspartate (NMDA) gradually declined with time despite the continuous presence of the agonists. Quisqualate (QA) caused a sustained release which exhibited no tendency to decline within the 20-min period of stimulation. This release was enhanced in Ca(2+)-free medium. The release evoked by QA in Ca(2+)-containing medium was significantly inhibited by (+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohept-5,10-imine hydrogen maleate (MK-801) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), showing that QA activates NMDA receptors directly or indirectly through (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors. The inhibition of MK-801 was slightly diminished and that of CNQX totally abolished in Ca(2+)-free medium. Verapamil inhibited the QA-activated release in both Ca(2+)-containing and Ca(2+)-free media. The effect of QA but not that of AMPA was blocked in Ca(2+)-free medium by L(+)-2-amino-3-phosphonopropionate (L-AP3), a selective antagonist of the metabotropic glutamate receptor. It is suggested that the sustained release of GABA is also mediated partly by activation of metabotropic receptors and mobilization of Ca2+ form intracellular stores.

Nitric oxide participates in excitotoxic mechanisms induced by chemical hypoxia

Changes in the activity of the NMDA receptor-gated ionic channels induced by potassium cyanide were studied in rat hippocampal slices utilizing a [3H]MK-801 binding technique. A 30-min exposure of slices to potassium cyanide (KCN) increased MK-801 binding by 252%. Co-application of N omega-nitro-L-arginine (NNLA), a competitive antagonist of nitric oxide (NO) synthase, reduced this increase by 72%. This inhibition by NNLA was completely reversed by an excess of L-arginine, a substrate for NO synthase, suggesting that the KCN-induced increase in MK-801 binding is mediated by NO synthase activity. KCN had no effect on MK-801 binding in synaptic membranes. In Ca(2+)-containing medium, KCN increased the release of glutamate, aspartate and glycine by 4- to 5-fold, and this was blocked by application of NNLA. NNLA inhibition was reversed by an excess of L-arginine, indicating that KCN-stimulated release of these amino acids is mediated by NO synthase activity. In Ca(2+)-free medium, a KCN-induced increase in MK-801 binding and in excitatory amino acid release was also observed, however, this increase was not influenced by NO-related agents, suggesting that these changes were not mediated by NO synthase activation. NNLA given after the end of exposure to KCN did not reverse the increase in MK-801 binding. These findings suggest that NO is involved in the initial activation of NMDA receptor-gated ionic channels and in the enhanced amino acid transmitter release induced by KCN, but that KCN can also induce some of these effects by a Ca(2+)- and NO-independent mechanism.

Subcellular fractionation on Percoll gradient of mossy fiber synaptosomes: evoked release of glutamate, GABA, aspartate and glutamate decarboxylase activity in control and degranulated rat hippocampus

Using discontinuous density gradient centrifugation in isotonic Percoll sucrose, we have characterized two subcellular fractions (PII and PIII) enriched in mossy fiber synaptosomes and two others (SII and SIII) enriched in small synaptosomes. These synaptosomal fractions were compared with those obtained from adult hippocampus irradiated at neonatal stage to destroy granule cells and their mossy fibers. Synaptosomes were viable as judged by their ability to release aspartate, glutamate and GABA upon K+ depolarization. After irradiation, compared to the control values, the release of glutamate and GABA was decreased by 57 and 74% in the PIII fraction, but not in the other fractions and the content of glutamate, aspartate and GABA was also decreased in PIII fraction by 62, 44 and 52% respectively. These results suggest that mossy fiber (MF) synaptosomes contain and release glutamate and GABA. Measurement of the GABA synthesizing enzyme, glutamate decarboxylase, exhibited no significant difference after irradiation, suggesting that GABA is not synthesized by this enzyme in mossy fibers.

Free radicals enhance basal release of D-[3H]aspartate from cerebral cortical synaptosomes

Excessive generation of free radicals has been implicated in several pathological conditions. We demonstrated previously that peroxide-generated free radicals decrease calcium-dependent high K(+)-evoked L[3H]-glutamate release from synaptosomes while increasing calcium-independent basal release. The present study evaluates the nonvesicular release of excitatory amino acid neurotransmitters, using D-[3H]aspartate as an exogenous label of the cytoplasmic pool of L-glutamate and L-aspartate. Isolated presynaptic nerve terminals from the guinea pig cerebral cortex were used to examine the actions and interactions of peroxide, iron, and desferrioxamine. Pretreatment with peroxide, iron alone, or peroxide with iron significantly increased the calcium-independent basal release of D-[3H]aspartate. Pretreatment with desferrioxamine had little effect on its own but significantly limited the enhancement by peroxide. High K(+)-evoked release in the presence of Ca2+ was enhanced by peroxide but not by iron. These data suggest that peroxide increases nonvesicular basal release of excitatory amino acids through Fenton-generated hydroxyl radicals. This release could cause accumulation of extracellular excitatory amino acids and contribute to the excitotoxicity associated with some pathologies.

Dopamine D1 and D2 receptor antagonism differentially modulates stimulation of striatal neurotransmitter levels by N-methyl-D-aspartic acid

The effects of local perfusion with the dopamine D1 receptor antagonist SCH 23390 and the dopamine D2 receptor antagonist raclopride on basal and N-methyl-D-aspartate (NMDA) stimulated dopamine, gamma-aminobutyric acid (GABA) and glutamate levels in the dorsolateral striatum were monitored using in vivo microdialysis in the halothane anaesthetized rat. Local perfusion (90 min) with SCH 23390 and raclopride (1 and 10 microM) dose dependently increased basal striatal dopamine release whereas GABA and glutamate dialysate levels were unaffected. Local perfusion (10 min) with the 1 mM concentration of NMDA increased striatal dopamine, GABA and glutamate outflow. However, when perfused together with SCH 23390 (1 microM) NMDA inhibited glutamate efflux. Moreover, in the presence of SCH 23390 (10 microM) the NMDA induced rise in dopamine and GABA levels was partly prevented. On the other hand, raclopride 1 microM enhanced the NMDA stimulated GABA efflux while at 10 microM it partly counteracted the NMDA induced dopamine release. These data demonstrate that NMDA induced changes in striatal neurotransmitter outflow are effectively modified by dopamine D1 and D2 receptor blockade.

Effects of an A1 adenosine receptor agonist on the neurochemical, behavioral and histological consequences of ischemia

Untreated rats and rats given the A1 receptor adenosine agonist, R-phenylisopropyladenosine (R-PIA), were subjected to four vessel ischemia. The effect of R-PIA on hippocampal amino acid release, hippocampal neuronal damage, exploratory behavior, learning capacity and global neurological score were evaluated. R-PIA decreased by half the glutamate released during ischemia and improved the global neurological scores 3, 24, 48, 78 h and 7 days after ischemia. But R-PIA had no effect on taurine/GABA release (during ischemia), hippocampal neuronal damage (7 days post-ischemia), exploratory behavior (48 h post-ischemia) or learning capacity (7 days post-ischemia). Thus, a decrease in glutamate release by R-PIA is not systematically correlated with an improvement of histological damage or learning capacity. Reduced glutamate release is not therefore a sufficient criterion on which to evaluate the neuroprotective capacity of a drug.

Effects of diverse omega-conopeptides on the in vivo release of glutamic and gamma-aminobutyric acids

omega-Conopeptides are antagonists of subtypes of neuronal calcium channels. Two omega-conopeptides, SVIB and MVIIC, have recently been identified which have a novel specificity for these ionophores. We have tested the actions these peptides, as well as the more selective MVIIA, on the release of glutamic acid and gamma-aminobutyric acid (GABA) in the hippocampus in vivo. For the assay of peptide effects on release, we used microdialysis to deliver multiple pulses of elevated potassium to the brain extracellular fluid. Peptide effects were quantitated from the decrement of the release with peptide perfused through the probes, in comparison to that in control experiments. Synthetic MVIIC caused a 40-50% decrement in the release of both glutamate and GABA at a probe concentration of about 200 nM. Synthetic SVI-B caused a 50% block at about 20-40 microM, while about 200 microM of MVIIA was required for 50% block. Chromatographic experiments showed that differences in potency between MVIIC and MVIIA were not explained by differential degradation. Blockade of release was also observed in the thalamus. MVIIC provides a tool for investigating the role of calcium mediated release of glutamate and GABA in physiological and pathological processes in the mammalian brain in vivo.

Compartmentation of cerebral glutamate in situ as detected by 1H/13C n.m.r

Incorporation of 13C label from either [1-13C]glucose to glutamate C-4 and lactate C-3 or from [2-13C]acetate to glutamate C-4 was monitored in situ in a superfused brain slice preparation by using 1H-detected/13C-edited (1H/13C) n.m.r. spectroscopy. The fractional enrichments of both metabolites were determined by this means in both brain slices and acid extracts of the preparations in order to assess their 1H-n.m.r. detectabilities. The 1H/13C satellite resonances from glutamate C-4 and lactate C-3 in brain tissue were followed from 4 min onwards in the presence of 5 mM [1-13C]glucose. Fractional enrichment of glutamate C-4 in the slice preparations was higher than in their acid extracts throughout the incubation of 100 min; at 30 min the enrichment was 15.9 +/- 0.6% in the slice preparations and 10.6 +/- 0.9% in extracts and at 100 min 24.5 +/- 1.7% compared with 19.7 +/- 0.4%, respectively. In contrast, lactate C-3 reached a steady-state fractional enrichment of approx. 43% by 15 min and there was no difference between the values determined in the slice preparations and the acid extracts. There was a significant difference between the glutamate C-4 fractional enrichments in the brain slices (7.4 +/- 0.6%) and extracts (5.1 +/- 0.3%) after 60 min of incubation with [2-13C]acetate. Thus 13C label from both glucose and exogenous acetate enters a pool of glutamate that is more amenable to 1H n.m.r. detection than total acid-extracted brain biochemical glutamate, whereas lactate is labelled with full 1H n.m.r. visibility. The results are discussed in the light of the biochemical factors that affect glutamate 1H-n.m.r. susceptibility and thus its n.m.r. visibility.

Inhibition of neurotransmitter release by clostridial neurotoxins correlates with specific proteolysis of synaptosomal proteins

Rat brain synaptosomes were used to study the effect of several clostridial neurotoxins on the neurotransmitter release. In this system the blockade of transmitter release correlated with the proteolytic activity of the toxins. Blockade of glutamate release was linked to selective proteolysis of one of the following synaptic proteins: synaptobrevin (BoNT/D, BoNT/F); SNAP-25 (BoNT/A, BoNT/E), or HPC-1/syntaxin (BoNT/C1). All the toxins used had an inhibitory effect on synaptosomes with the exception of BoNT/F. BoNT/F cleaved synaptobrevin in permeabilized synaptosomes but failed to produce the same effect on intact synaptosomes.

Unchanged exocytotic release of glutamic acid in cortex and neostriatum of the rat during aging

The Ca(2+)-dependent release of glutamate induced by 4-aminopyridine in synaptosomes prepared both from the cerebral cortex and basal ganglia was unchanged in aged rats (27-30 months) when compared to adults rats (3 months). Consistent with the absence of changes in glutamate exocytosis during aging, the rise in the cytosolic free Ca2+ concentration, [Ca2+]c, induced by depolarization in synaptosomes from aged rats was similar to that found in control adult rats. The results suggest that during aging the nerve terminals from the cerebral cortex and basal ganglia maintain an intact ability to release glutamate by exocytosis.

Characterization of glutamate, aspartate, and GABA release from ischemic rat cerebral cortex

The purpose of this study was to evaluate potential mechanisms of ischemia-evoked amino acid transmitter release. Changes in extracellular levels of transmitter amino acids and lactic acid dehydrogenase (LDH) in rat cerebral cortex during and following four-vessel occlusion elicited global cerebral ischemia were examined using a cortical cup technique. Ischemia-evoked release of glutamate, aspartate and gamma-amino-butyric acid (GABA) was compared in control vs. drug-treated animals. Tetrodotoxin and antagonists of glutamate receptors (DNQX, MK-801, and AP-3) depressed the initial rate of increase in extracellular glutamate and aspartate without altering the total amount of these amino acids collected in the cortical superfusates. Cobalt, a calcium channel antagonist, failed to alter efflux. Acidic amino acid transport inhibitors (dihydrokainate, L-trans-PDC) depressed the rate of onset of glutamate and aspartate release and dihydrokainate depressed total release by 44%. PD 81723, an allosteric enhancer at the A1 adenosine receptor, depressed glutamate efflux, as did L-NAME, an inhibitor of nitric oxide synthase. Extracellular increases in GABA levels were depressed by tetrodotoxin and L-trans-PDC. The GABA transport inhibitor, nipecotic acid, increased the initial rate of onset of GABA release. Increases in LDH levels in the extracellular fluid became apparent during the period of ischemia and continued to increase during the subsequent 90 min of reperfusion. These results suggest that ischemia evokes a release of neurotransmitter amino acids that is only partially dependent upon Ca2+ influx activation or the reversal of amino acid transporters. Nonselective mechanisms, resulting from the disruption of plasma membrane integrity, may contribute significantly to the total ischemia-evoked release of excitatory amino acids.

GABA uptake and release by a mammalian cell line stably expressing a cloned rat brain GABA transporter

In order to facilitate study of the neuronal GABA transporter and provide a convenient system for potential drug screening, we have established a CHO cell line, designated 1F9, which stably expresses the cloned GABA transporter from rat brain (GAT-1). 1F9 cells transport GABA at levels approximately 300-fold higher than untransfected CHO cells, and GABA transport in these cells has the following properties: (1) a dependence on sodium and chloride ions; (2) higher sensitivity to neuronal subtype uptake inhibitors (DABA and ACHC) than to glial subtype inhibitors (beta-alanine and THPO); and (3) Km (2.5 microM) and IC50 values for various competitive ligands that are comparable with values determined in synaptosomes and brain slices. Given the fidelity with which the 1F9 cell line expresses these characteristics of the native neuronal GABA transporter, we have used it to further address GABA transporter activity. [3H]GABA uptake by 1F9 cells is inhibited approximately 50% by the chloride transport blockers DIDS and SITS. The GABA receptor agonists muscimol and baclofen also inhibit GABA transport; however, the receptor antagonists bicuculline and phaclofen have no effect. 1F9 cells also show release of [3H]GABA release is calcium independent, and is differentially affected by changes in the ion gradient, as well as by the presence of external substrates and uptake blockers. These experiments indicate that 1F9 cells provide a convenient system for the screening of GABA transport inhibitors.

Na+ influx through Ca2+ channels can promote striatal GABA efflux in Ca(2+)-deficient conditions in response to electrical field depolarization

Electrical field depolarization releases gamma-aminobutyric acid (GABA) in rat striatal slices in the absence of external Ca2+. omega-Conotoxin GVIA (omega-CgTx; 1-50 nM), a neuronal Ca2+ channel blocker, inhibits electrically evoked efflux of newly taken up [3H]GABA in a concentration-dependent manner in either normal or Ca(2+)-free medium. This suggests that ion influx occurs through Ca2+ channels in the absence of external Ca2+ and contributes to the efflux of GABA. Reducing external Na+ concentration to 27.25 mM (low [Na+]o medium) by equimolarly substituting choline chloride for sodium chloride has differential effects on electrically evoked GABA efflux depending on the external Ca2+ concentrations. In normal Ca2+ medium, electrically evoked GABA efflux increases whereas, in Ca(2+)-free medium, it is greatly inhibited when [Na+]o is reduced to 27.25 mM. In low [Na+]o medium, GABA efflux is largely tetrodotoxin (TTX)-sensitive, however, spike firing evoked by antidromic stimulation of striatal cells is inhibited. In Na(+)-free medium, resting GABA efflux increases 17-fold whereas evoked GABA efflux diminishes. In Ca(2+)-free medium, 70 min of incubation with 1-2-bis-(1-aminophenoxy)ethane-N,N,N',N' tetraacetoxy methyl ester (BATPA-AM, 1 microM), an intracellular calcium chelator, increases both resting GABA efflux and electrically evoked GABA overflow by approximately 100%. These results suggest that: (1) in Ca(2+)-free conditions, Na+ permeability of cells increases via Ca2+ channels and this profoundly affects GABA efflux. (2) Electrical field depolarization is likely to release GABA by directly depolarizing axon terminals. (3) Ca(2+)-independent GABA efflux is not promoted by an increase in intracellular free Ca2+ concentration via Na+/Ca2+ exchange processes from internal pools.

Injection of tetanus toxin into the neocortex elicits persistent epileptiform activity but only transient impairment of GABA release

Focal injection of a minute quantity of tetanus toxin into the rat neocortex induces chronic epileptogenesis. Within a day, spontaneous and stimulus-evoked paroxysmal discharges appear in widespread regions of both hemispheres and this lasts for at least nine months. Tetanus toxin blocks transmitter release, apparently by catalysing the breakdown of synaptobrevin, a synaptic protein. It specifically binds to neuronal membranes but its potent epileptogenic properties have been ascribed to a higher affinity for inhibitory neurons. Following focal injection of tetanus toxin into the hippocampus a long-lasting epileptic syndrome also develops. During the early part of the syndrome GABA release is depressed in slices from the injected side, but not in slices from the contralateral, secondary focus. In the present experiments on neocortex, release of radiolabelled GABA was measured from primary and secondary epileptic foci induced by unilateral focal injection of tetanus toxin into the parietal cortex. By four weeks after the injection, no differences were detected in GABA release from any neocortical site in control or toxin-injected animals, despite the persistence of profound epileptic activity in slices from the latter. At earlier times (1.5 days) after the toxin injection, however, release was significantly depressed in both hemispheres. The results indicate that at first, the toxin induces focal neocortical epileptogenesis by directly impeding GABAergic synaptic transmission but that with time there is a recovery from this initial effect. We propose, as has also been suggested for other models, that the initial epileptogenesis leaves in its wake a long-lasting change in the local functional connectivity, such that the neocortex is rendered permanently epileptic.

Demonstration of glutamate/aspartate uptake activity in nerve endings by use of antibodies recognizing exogenous D-aspartate

Nerve terminals as well as glial cells are thought to possess high-affinity Na(+)-dependent transport sites for excitatory amino acids. However, recent immunocytochemical results with antibodies against such a transporter isolated from rat brain showed a selective labelling of glial cells [Danbolt et al. (1992) Neuroscience 51, 295-310]. Critical evaluation of the literature indicates that previous evidence for nerve terminal uptake of acidic amino acids might possibly be attributed to glia. To find out whether there is indeed a glutamate transporter in nerve endings, we incubated hippocampal slices with D-aspartate (10 and 50 microM), a metabolically inert substrate for the high-affinity glutamate transport system. After fixation by glutaraldehyde/formaldehyde the slices were processed immunocytochemically with specific polyclonal antibodies raised against D-aspartate coupled to albumin by glutaraldehyde/formaldehyde. The electron-microscopic postembedding immunogold technique demonstrated a large accumulation of gold particles in nerve terminals making asymmetrical synapses, compared to their postsynaptic dendritic spines, as well as in glial cell processes. The labelled terminals include those of the glutamatergic Schaffer collaterals. Axosomatic boutons appeared unlabelled. Comparison with a test conjugate with known concentration of fixed D-aspartate (94 mM) suggests that the concentration attained in the terminals after incubation with 50 microM D-aspartate was in the lower millimolar range. The uptake was totally dependent on Na+, blocked by L-threo-3-hydroxyaspartate, and had a high affinity for D-aspartate (apparent Km about 20 microM). There was no labelling in slices incubated without D-aspartate. Compared to glia, the nerve terminals had a higher D-aspartate density and accounted for a much higher proportion of the total tissue uptake, but this relationship may be different in vivo. At the light-microscopic level the D-aspartate-like immunoreactivity showed a distinct laminar distribution, identical to that shown autoradiographically for D-[3H]aspartate and L-[3H]glutamate uptake sites [Taxt and Storm-Mathisen (1984) Neuroscience 11, 79-100], and corresponding to the terminal fields of the major excitatory fibre systems in the hippocampal formation. The novel approach described here establishes that glutamatergic nerve terminals as well as glia do sustain sodium-dependent high-affinity transport of excitatory amino acids, implying that more than one glutamate transporter must be present in the brain. Immunogold detection of D-aspartate gives a much higher anatomical resolution than electron microscopic autoradiography of D-[3H]aspartate or L-[3H]glutamate uptake, the only method that has been available previously for ultrastructural demonstration of uptake activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Carrier-mediated release of neurotransmitters

There is growing evidence that neurotransmitters can be released not only by exocytosis but also through the membrane carriers responsible for transmitter reuptake. Giulio Levi and Maurizio Raiteri review the in vitro and in vivo evidence supporting the existence of a carrier-mediated release for different classes of transmitters. While the physiological significance of carrier-mediated release remains speculative, widely used drugs such as sympathomimetic amines, the anorectic drug fenfluramine and some drugs of abuse act in part by stimulating monoamine carrier-mediated release. Moreover, antidepressants known to inhibit monoamine reuptake, can block carrier-mediated release. This mechanism may also come into play in pathological conditions such as ischaemia.

The chemical nature of the main central excitatory transmitter: a critical appraisal based upon release studies and synaptic vesicle localization

The chemical nature of the central transmitter responsible for fast excitatory events and other related phenomena is analysed against the historical background that has progressively clarified the structure and function of central synapses. One of the problems posed by research in this field has been whether one or more of the numerous excitatory substances endogenous to the brain is responsible for fast excitatory synaptic transmission, or if such a substance is, or was, a previously unknown one. The second question is related to the presence in the CNS of three main receptor types related to fast excitatory transmission, the so-called alpha-amino-3-hydroxy-5-methylisoxazole propionic acid, kainate and N-methyl-D-aspartate receptors. This implies the possibility that each receptor type might have its own endogenous agonist, as has sometimes been suggested. To answer such questions, an analysis was done of how different endogenous substances, including L-glutamate, L-aspartate, L-cysteate, L-homocysteate, L-cysteine sulfinate, L-homocysteine sulfinate, N-acetyl-L-aspartyl glutamate, quinolinate, L-sulfoserine, S-sulfo-L-cysteine, as well as possible unknown compounds, were able to fulfil the more important criteria for transmitter identification, namely identity of action, induced release, and presence in synaptic vesicles. The conclusion of this analysis is that glutamate is clearly the main central excitatory transmitter, because it acts on all three of the excitatory receptors, it is released by exocytosis and, above all, it is present in synaptic vesicles in a very high concentration, comparable to the estimated number of acetylcholine molecules in a quantum, i.e. 6000 molecules. Regarding a possible transmitter role for aspartate, for which a large body of evidence has been presented, it seems, when this evidence is carefully scrutinized, that it is either inconclusive, or else negative. This suggests that aspartate is not a classical central excitatory transmitter. From this analysis, it is suggested that the terms alpha-amino-3-hydroxy-5-methylisoxazole propionic acid, kainate and N-methyl-D-aspartate receptors, should be changed to that of glutamate receptors, and, more specifically, to GLUA, GLUK and GLUN receptors, respectively. When subtypes are described, a Roman numeral may be added, as in GLUNI, GLUNII, and so on.

The stimulus-evoked release of glutamate and GABA from brain subregions following transient forebrain ischemia in the rat

The release of glutamate and GABA in response to K+ depolarization was determined for tissue prisms prepared from brain subregions removed from rats following 30 min of forebrain ischemia or recirculation periods up to 24 h. There were statistically significant effects of this treatment on release of both amino acids from samples of the dorsolateral striatum, an area developing selective neuronal degeneration. However, for at least the first 3 h of recirculation the calcium-dependent and calcium-independent release of both amino acids in this region were similar to pre-ischemic values. Differences were observed under some conditions at longer recirculation times. In particular there was a decrease in calcium-dependent GABA release at 24 h of recirculation and a trend towards increased release of glutamate at 6 h of recirculation and beyond. No statistically significant differences were seen in samples from the paramedian neocortex, a region resistant to post-ischemic damage. These results suggest that changes in the ability to release glutamate and GABA in response to stimulation are not necessary for the development of neurodegeneration in the striatum but rather that release of these amino acids may be modified as a result of the degenerative process.

Sodium-dependent release of exogenous glycine from preloaded rat hippocampal synaptosomes

The effect of high potassium, veratridine, and ouabain stimulation upon the release of exogenously-loaded [3H]glycine was evaluated in crude synaptosomal preparations from rat hippocampi by means of a superfusion technique in the presence of media with different ionic compositions and of tetrodotoxin (TTX). Four minute superfusion of synaptosomes with 30 mM KCl, 10 microM veratridine or 0.4 mM ouabain caused a significant increase in the [3H]glycine efflux which averaged 6.6 +/- 0.2, 25.5 +/- 1.0, and 8.9 +/- 1.0% of the total radioactivity present in the synaptosomes, respectively. The omission of Ca2+ ions in the superfusion medium markedly decreased K(+)-evoked [3H]glycine efflux (2.5 +/- 0.5%), did not appreciably modify that evoked by veratridine (24.2 +/- 2.0%) and significantly increased that evoked by ouabain (18.5 +/- 0.5%). The superfusion of synaptosomes with Na(+)-free media always resulted in a drastic decrease of the depolarization-stimulated [3H]glycine efflux, whereas the omission of Cl- generally resulted in a moderate increase of [3H]glycine efflux. TTX (0.8 microM) markedly affected the stimulatory effect of veratridine (2.5 +/- 0.9%) and ouabain (2.2 +/- 0.5%), but failed to modify significantly that evoked by high potassium (6.5 +/- 0.7%). Finally, [3H]glycine was seen readily to exchange in a partially sodium-dependent way with unlabelled glycine present in the medium. On the whole these findings appear to be consistent with the neurotransmitter character of the glycine release from hippocampal synaptosomes.

Vesicular and carrier-mediated depolarization-induced release of [3H]GABA: inhibition by amiloride and verapamil

The Ca(2+)-dependent, presumably exocytotic fraction of the [3H]GABA released by depolarization is dissected from the depolarization-induced Na(+)-dependent, carrier-mediated fraction of [3H]GABA release in mouse brain synaptosomes. GABA homoexchange is prevented by the [3H]GABA carrier blocker, DABA. The absence of external Na+ completely abolishes the release of the carrier-mediated, presumably cytoplasmic release of [3H]GABA induced by homoexchange and heteroexchange with GABA and DABA, respectively. The carrier-mediated, Na(+)-dependent fraction of the depolarization-induced release of [3H]GABA is resistant to tetrodotoxin (TTX) but is sensitive to amiloride and verapamil. The Ca(2+)-dependent fraction of the [3H]GABA released by high K+ depolarization is also completely abolished by amiloride (from 300 microM) and sensitive to verapamil (30 microM), but in contrast is insensitive to the absence of external Na+ and to DABA. On the basis of these results we conclude that amiloride and verapamil inhibit high K(+)-induced release of [3H]GABA by antagonizing the entrance of Ca2+ (and possibly Na+ when external Ca2+ is absent) through a population of voltage sensitive presynaptic Ca2+ channels activated by depolarization.

Glial plasmalemmal vesicles: a subcellular fraction from rat hippocampal homogenate distinct from synaptosomes

By a Percoll density-gradient centrifugation of rat hippocampal homogenate, we found a novel subcellular fraction (specific gravity approximately 1.046 g/ml), besides synaptosomes (approximately 1.060 g/ml), which showed a high activity of Na(+)-dependent glutamate uptake. The initial rate of the glutamate uptake in this fraction was as high as twice that in synaptosomes. Activities of choline acetyltransferase and high affinity choline uptake were, on the other hand, much lower. gamma-Aminobutyric acid uptake activity was nearly equivalent in both fractions. Electron microscopic observations revealed that the fraction was morphologically different from synaptosomal or myelin fractions, but mainly consisted of two different types of empty membrane vesicles; irregular (0.3-0.8 micron in diameter) and spheroid type (0.2 micron). The immunoreactivity to glial fibrillary acidic protein was appreciably high in this fraction. The marker enzyme analysis showed the fraction was rich in plasma membranes. On the basis of these results, the fraction is termed glial plasmalemmal vesicles (GPV). We analyzed kinetically the reaction of Na(+)-dependent glutamate uptake by GPV comparing with that by synaptosomes. Km values for glutamate in GPV was 4.7 microM and Vmax was 33 nmol/mg/min, while in synaptosomes 11 microM and 17 nmol/mg/min, respectively. Hill coefficients of Na+ activation in GPV and synaptosomes were 1.1 and 2.0, respectively. Thus, the mechanism or transporter molecule in glial cells for Na(+)-dependent glutamate transport is likely to be different from that in neurons.

Thirty years of synaptosome research

Detached synapses (synaptosomes), first isolated by the author in 1958 and identified as such in 1960, are sealed presynaptic nerve terminals often with a portion of the target cell--sometimes amounting to a complete dendritic spine--adhering to their external surface. They can be prepared in high yield from brain tissue and also in decreasing yield from spinal cord, retina, sympathetic ganglia, myenteric plexus and electric organs. They are sealed structures which, under metabolizing conditions, respire, take up oxygen and glucose, extrude Na+, accumulate K+, maintain a normal membrane potential and, on depolarization, release transmitter in a Ca(2+)-dependent manner. They thus provide an excellent preparation with which to investigate synaptic function without the complications encountered with synapses in situ. They also serve as the parent fraction for preparations of synaptic vesicles and other synaptic components.

Modulation of calcium-dependent and -independent components of veratridine-evoked release of glutamate from rat cerebellum

The entry of Ca2+ into the presynaptic neuronal terminal is considered to be a prerequisite for exocytosis. However, reports suggest that a Ca(2+)-independent component of release can exist for some neurotransmitters. In this study we have used veratridine-stimulated release of glutamate from rat cerebellar slices to investigate Ca(2+)-dependent and -independent release. A 1-min pulse of veratridine (10 microM) induced release of glutamate in both Ca(2+)-replete and Ca(2+)-free ACSF. Both modes of release, however, could be elicited in a sequential manner following a single application of veratridine in Ca(2+)-free ACSF, with return to Ca(2+)-replete conditions 5 min post-pulse. This separation permitted the modulation of either, or both, phases of release. Apamin and dihydrokainate had little effect on Ca(2+)-independent release but produced enhancement of the Ca(2+)-dependent phase. Tetrodotoxin abolished both phases of release when applied with the veratridine pulse, but had no effect on the Ca(2+)-dependent phase alone. The Ca(2+)-dependent phase was partially sensitive to Co2+, although the Ca2+ channel blockers verapamil, amiloride, omega-conotoxin and ruthenium red were ineffective, suggesting a lack of involvement of L-, N- or T-type channels. The possible mechanisms mediating the Ca(2+)-dependent and -independent components of endogenous glutamate release from cerebellar slices are discussed.

Rat-1 fibroblasts engineered with GAD65 and GAD67 cDNAs in retroviral vectors produce and release GABA

We have used a retroviral cDNA expression system to drive the expression of the different forms of glutamic acid decarboxylase (GAD65, GAD67, or both). Individual clones of engineered Rat-1 cells make the appropriate GAD mRNAs and GAD polypeptides, show GAD enzymatic activity, and make GABA. Clones expressing GAD65 had higher enzymatic activity than those expressing GAD67. As is the case for brain GADs and for GADs produced in engineered bacteria, the enzymatic activity of GAD65 is more responsive to added pyridoxal phosphate than that of GAD67. Immunostaining for both GADs is scattered throughout the cytoplasm. GAD65 immunostaining is less homogeneous than that of GAD67 and also appears to be associated with the surfaces of large vesicle-like structures. Cells expressing GAD65 and GAD67 showed similar immunostaining patterns with anti-GABA antibodies and contained substantial amounts of GABA (ranging from 7 to 18 pmol of GABA/10(6) cells), which was roughly proportional to their levels of GAD activity. GABA is released from the engineered cells into the surrounding medium under resting conditions, suggesting that cells programmed with GAD cDNAs might serve as effective sources of GABA in cell transplantation experiments.

Barium-evoked glutamate release from guinea-pig cerebrocortical synaptosomes

Ba2+ has multiple effects on presynaptic terminals. The ion inhibits the K+ channels responsible for stabilizing the plasma membrane potential in the same way as previously reported for dendrotoxin and 4-aminopyridine. Secondly, the ion can substitute fully for Ca2+ in supporting KCl-evoked release of glutamate from guinea-pig cerebrocortical synaptosomes. In the latter case, the kinetics of glutamate release in the presence of saturating Ca2+ or Ba2+ are essentially identical. Substantially lower external concentrations of Ba2+ are required to achieve the same release kinetics as with Ca2+. The average internal free Ba2+ concentration attained during KCl depolarization is some 10-fold higher than that for Ca2+. However, because the fura-2 signal reflects predominantly the overflow of divalent cation after dissociation from the release trigger, it is not the valid parameter to compare effectiveness of the cations in triggering glutamate exocytosis. In view of the established inability of Ba2+ to interact with calmodulin, these results are discussed in relation to theories in which Ca2+/calmodulin-dependent protein kinase-mediated phosphorylation is a prerequisite for synaptic vesicle exocytosis.

Depolarization by K+ and glutamate activates different neurotransmitter release mechanisms in GABAergic neurons: vesicular versus non-vesicular release of GABA

Neurotransmitter release and changes in the concentration of intracellular free calcium ([Ca++]i) were studied in cultured GABAergic cerebral cortical neurons, from mice, upon depolarization with either an unphysiologically high potassium concentration (55 mM) or the physiological excitatory neurotransmitter glutamate (100 microM). Both depolarizing stimuli exerted prompt increases in the release of preloaded [3H]GABA as well as in [Ca++]i. However, the basic properties of transmitter release and the increase in [Ca++]i under a variety of conditions were different during stimulation with K+ or glutamate. Potassium-evoked release of [3H]GABA consisted of two phases, a rapid, large and transient phase followed by a smaller, more persistent second phase. The rapid phase was inhibited (60%) by nocodazole which reduced the number of vesicles in the neurites by 80%. This rapid phase of the GABA release was also reduced by organic (verapamil) and inorganic (Co++) Ca++ channel blockers but was insensitive to the GABA transport inhibitor SKF 89976A. In contrast, the second phase was less sensitive to nocodazole and Ca++ channel antagonists but could be inhibited by SKF 89976A. The glutamate-induced [3H]GABA release, which was mainly mediated by N-methyl-D-aspartate receptors, consisted of a single, sustained phase. This was insensitive to nocodazole, partly inhibited by verapamil and could be blocked by Co++ as well as SKF 89976A. The action of Co++ could be attributed to a block of N-methyl-D-aspartate-associated ion channels. These findings strongly suggest that the majority of the K(+)-stimulated GABA release is dependent upon vesicles whereas the glutamate induced release is non-vesicular and mediated by a depolarization-dependent reversal of the direction of high-affinity GABA transport. The basic differences in the mode of action of the two depolarizing stimuli were reflected in the properties of the increase in [Ca++]i elicited by 55 mM K+ and 100 microM glutamate, respectively. The K(+)-induced increase in [Ca++]i was reduced by both verapamil and Ca(++)-free media whereas the corresponding glutamate response was only sensitive to Ca(++)-free conditions. Exposure of the cells to nocodazole or SKF 89976A had no effect on the ability of K+ or glutamate to increase [Ca++]i. Altogether, the results clearly demonstrate that K(+)-induced transmitter release from these GABAergic neurons is vesicular in nature whereas that induced by the neurotransmitter glutamate is not.

The secretion of amino acid transmitters from cerebellar primary cultures probed by alpha-latrotoxin

Using amino acid transmitter release, inositol phosphate production and 45Ca2+ accumulation as cellular responses to alpha-latrotoxin treatment, we studied the susceptibility of rat primary cerebellar granule cultures to toxin. Binding studies as well as immunofluorescence of toxin-treated cells, allow an evaluation of the expression, properties and distribution of toxin-binding sites. After binding, the toxin develops its releasing action on amino acid transmitters, in a manner reminiscent of the events described for cholinergic and catecholaminergic cell systems. Although few peculiarities emerge, toxin-dependent release of amino acid transmitters from granule cells in culture appears to be a fully exocytotic process.

Synaptic inhibition in primary and secondary chronic epileptic foci induced by intrahippocampal tetanus toxin in the rat

1. Injecting twelve mouse minimum lethal doses of tetanus toxin into one hippocampus of a rat leads to the development of chronic epileptic foci in both hippocampi. These generate intermittent epileptic discharges for 6-8 weeks. Here we compare GABAergic inhibition, 10-18 days after injection, in slices prepared from the injected and contralateral hippocampi (respectively the primary and the secondary or 'mirror' foci), using both neurochemical and electrophysiological methods. 2. Epileptic activity was recorded from slices of both hippocampi from all tetanus toxin-injected rats. Evoked epileptic discharges were similar on the two sides, but spontaneous epileptic discharges were more common contralaterally. 3. Ca(2+)-dependent, K(+)-stimulated (synaptic) release of radiolabelled GABA was depressed in slices from the injected hippocampus, compared with vehicle-injected controls. In contrast, slices from the contralateral hippocampus had normal levels of Ca(2+)-dependent, K(+)-stimulated GABA release, even though adjacent slices were epileptogenic. 4. Intracellular recordings revealed that both fast and slow stimulus-evoked inhibitory postsynaptic potentials (IPSPs) were abolished in CA3 pyramidal cells in the primary focus. In the secondary focus, however, fast IPSPs were seen in seven of twenty-five cells, and slow IPSPs were seen in all cells if the stimulus was strong enough. 5. Monosynaptic IPSPs were isolated pharmacologically by blocking glutamatergic excitatory postsynaptic potentials (EPSPs) with 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and D(-)-2-amino-5-phosphopentanoic acid (AP-5). No monosynaptic IPSPs were uncovered in cells from the primary focus at any stimulus strength. Monosynaptic IPSPs were evoked in all cells from both the secondary focus and control slices. The estimated conductances of monosynaptic fast IPSPs were similar in cells from the secondary focus and from the controls, although the former required twice the stimulus strength. 6. Slow IPSPs were found in the secondary focus and in controls, but not in the primary focus. They were sensitive to 3-amino-2-(4-chlorophenyl)-2-hydroxy-propylsulphonic acid (2-OH saclofen). The estimated conductances of slow IPSPs evoked by weak stimuli in the secondary focus were much smaller than in the controls. However, stimuli that could trigger epileptic discharges in the secondary focus, evoked 2-OH saclofen-sensitive slow IPSPs with estimated conductances approaching the controls. This marked increase in the slow IPSP did not occur when EPSPs, and epileptic bursts, were blocked with CNQX and AP-5, suggesting that a strong barrage of excitation is needed to generate full-sized slow IPSPs in the secondary focus.(ABSTRACT TRUNCATED AT 400 WORDS)

Omega-conotoxin sensitive calcium channels in cerebellar granule cells are not coupled to [3H]glutamate release

We have studied the biochemical and functional aspects of omega-conotoxin GVIA (omega-CgTx)-sensitive calcium channels in cerebellar granule cells in vitro. 125I-omega-Conotoxin GVIA (125I-omega-CgTx) binding sites were detected in intact cultured cerebellar granule cells and binding parameters were measured (Bmax: 134 fmol/mg protein; kinetic association constant kappa: 3.10(6) M-1.s-1). [3H]Glutamate release was assessed under different release paradigms (namely release triggered by calcium, voltage, and sodium channel agonists) and different times (15 s and 2 min). However, in all cases, [3H]glutamate release was found to be completely insensitive to omega-CgTx. Conversely, voltage-dependent release was inhibited in a dose-dependent fashion by cadmium chloride, with total inhibition at 10(-4) M. These results indicate that N-type calcium channels are not involved in glutamate secretion from granule neurons.

Glutamate increases the [Ca2+]i but stimulates Ca(2+)-independent release of [3H]GABA in cultured chick retina cells

The effect of glutamate on [Ca2+]i and on [3H] gamma-aminobutyric acid (GABA) release was studied on cultured chick embryonic retina cells. It was observed that glutamate (100 microM) increases the [Ca2+]i by Ca2+ influx through Ca2+ channels sensitive to nitrendipine, but not to omega-conotoxin GVIA (omega-Cg Tx) (50%), and by other channels insensitive to either Ca2+ channel blocker. Mobilization of Ca2+ by glutamate required the presence of external Na+, suggesting that Na+ mobilization through the ionotropic glutamate receptors is necessary for the Ca2+ channels to open. The increase in [Ca2+]i was not related to the release of [3H]GABA induced by glutamate, suggesting that the pathway for the entry of Ca2+ triggered by glutamate does not lead to exocytosis. In fact, the glutamate-induced release of [3H]GABA was significantly depressed by Ca(2+)o, but it was dependent on Na(+)o, just as was observed for the [3H]GABA release induced by veratridine (50 microM). The veratridine-induced release could be fully inhibited by TTX, but this toxin had no effect on the glutamate-induced [3H]GABA release. Both veratridine- and glutamate-induced [3H]GABA release were inhibited by 1-(2-(((diphenylmethylene)amino)oxy)ethyl)-1,2,5,6-tetrahydro-3-py ridine- carboxylic acid (NNC-711), a blocker of the GABA carrier. Blockade of the NMDA and non-NMDA glutamate receptors with MK-801 and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), respectively, almost completely blocked the release of [3H]GABA evoked by glutamate.(ABSTRACT TRUNCATED AT 250 WORDS)

N-methyl-D-aspartic acid differentially regulates extracellular dopamine, GABA, and glutamate levels in the dorsolateral neostriatum of the halothane-anesthetized rat: an in vivo microdialysis study

The effects of local perfusion with the glutamate receptor agonist NMDA and the noncompetitive NMDA receptor antagonist dizolcipine (MK-801) on extracellular dopamine (DA), GABA, and glutamate (Glu) levels in the dorsolateral striatum were monitored using in vivo microdialysis in the halothane-anesthetized rat. In addition, the sensitivity of both the basal and NMDA-induced increases in levels of these neurotransmitter substances to perfusion with tetrodotoxin (TTX: 10(-5) M) and a low Ca2+ concentration (0.1 mM) was studied. The results show that the local perfusion (10 min) with both the 10(-3) and 10(-4) M dose of NMDA increased striatal DA and GABA outflow, whereas only the (10(-3) M) dose of NMDA was associated with a small and delayed increase in extracellular Glu levels. The NMDA-induced effects were dose-dependently counteracted by simultaneous perfusion with MK-801 (10(-6) and 10(-5) M). Both the basal and NMDA (10(-3) M)-induced increase in extracellular striatal DA content was reduced in the presence of TTX and a low Ca2+ concentration, whereas both basal and NMDA-stimulated GABA levels were unaffected by these treatments. Both the basal and NMDA-stimulated Glu levels were enhanced following TTX treatment, whereas perfusion with a low Ca2+ concentration reduced basal Glu levels and enhanced and prolonged the NMDA-induced stimulation. These data support the view that NMDA receptor stimulation plays a role in the regulation of extracellular DA, GABA, and Glu levels in the dorso-lateral neostriatum and provide evidence for a differential effect of NMDA receptor stimulation on these three striatal neurotransmitter systems, possibly reflecting direct and indirect actions mediated via striatal NMDA receptors.

Relation of exocytotic release of gamma-aminobutyric acid to Ca2+ entry through Ca2+ channels or by reversal of the Na+/Ca2+ exchanger in synaptosomes

The specific inhibitor of the gamma-aminobutyric acid (GABA) carrier, NNC-711, (1-[(2-diphenylmethylene)amino]oxyethyl)- 1,2,5,6-tetrahydro-3-pyridine-carboxylic acid hydrochloride, blocks the Ca(2+)-independent release of [3H]GABA from rat brain synaptosomes induced by 50 mM K+ depolarization. Thus, in the presence of this inhibitor, it was possible to study the Ca(2+)-dependent release of [3H]GABA in the total absence of carrier-mediated release. Reversal of the Na+/Ca2+ exchanger was used to increase the intracellular free Ca2+ concentration ([Ca2+]i) to test whether an increase in [Ca2+]i alone is sufficient to induce exocytosis in the absence of depolarization. We found that the [Ca2+]i may rise to values above 400 nM, as a result of Na+/Ca2+ exchange, without inducing release of [3H]GABA, but subsequent K+ depolarization immediately induced [3H]GABA release. Thus, a rise of only a few nanomolar Ca2+ in the cytoplasm induced by 50 mM K+ depolarization, after loading the synaptosomes with Ca2+ by Na+/Ca2+ exchange, induced exocytotic [3H]GABA release, whereas the rise in cytoplasmic [Ca2+] caused by reversal of the Na+/Ca2+ exchanger was insufficient to induce exocytosis, although the value for [Ca2+]i attained was higher than that required for exocytosis induced by K+ depolarization. The voltage-dependent Ca2+ entry due to K+ depolarization, after maximal Ca2+ loading of the synaptosomes by Na+/Ca2+ exchange, and the consequent [3H]GABA release could be blocked by 50 microM verapamil. Although preloading the synaptosomes with Ca2+ by Na+/Ca2+ exchange did not cause [3H]GABA release under any conditions studied, the rise in cytoplasmic [Ca2+] due to Na+/Ca2+ exchange increased the sensitivity to external Ca2+ of the exocytotic release of [3H]GABA induced by subsequent K+ depolarization. Thus, our results show that the vesicular release of [3H]GABA is rather insensitive to bulk cytoplasmic [Ca2+] and are compatible with the view that GABA exocytosis is triggered very effectively by Ca2+ entry through Ca2+ channels near the active zones.

Adenosine modulation of amino acid release in rat hippocampus during ischemia and veratridine depolarization

This study was undertaken to determine whether endogenous adenosine modulates 'in vivo' neurotransmitter amino acid release via its presynaptic receptors. Two conditions were compared: neuronal depolarization by local infusion of veratridine (600 microM), and transient global ischemia by four-vessel occlusion. Both stimuli were applied for 20 min. Extracellular amino acid (glutamate, taurine/GABA, glycine) variations in concentration were determined in the rat hippocampus by microdialysis and HPLC. Modulation of adenosine receptor activity was objectified by continuous local infusion of an adenosine agonist (R-phenylisopropyladenosine R-PIA) or an antagonist (theophylline), starting one hour before stimulation of amino acid release. R-PIA (100 microM) significantly decreased the glutamate release (50%) evoked by veratridine, whereas it did not significantly modify the ischemia-induced glutamate release. In contrast, theophylline did not significantly affect veratridine-induced glutamate release, but it significantly potentiated glutamate efflux (400%) under ischemic conditions. Neither treatment altered the release of the other amino acids. These data suggest that endogenous adenosine appearing in the extracellular space during veratridine-induced depolarization cannot control glutamate release. In contrast, ischemia-induced glutamate release was strongly inhibited by the concomitant increase in extracellular adenosine.

1H nuclear magnetic resonance spectroscopy study of cerebral glutamate in an ex vivo brain preparation of guinea pig

Cerebral glutamate was monitored in a superfused cerebral cortical preparation by 1H NMR spectroscopy using a semiselective spin-echo sequence N-acetyl aspartate (NAA) as an internal concentration reference. During controlled metabolic conditions, the cerebral 1H NMR-detected glutamate-to-NAA ratio was approximately 20-30% lower than expected from the ratio of neutralized perchloric acid extracts of the preparations. Inhibition of respiration in the presence of glucose did not change the 1H NMR glutamate-to-NAA ratio in brain slice preparation. In contrast, either complete depletion of ATP during cyanide poisoning together with 0 mM glucose, anoxia in the absence of glucose, or treatment with nigericin or with a protonophore, carbonyl cyanide-m-fluorophenylhydrazone, increased 1H NMR-detected glutamate/NAA in the cerebral preparations without a change in the relative and absolute concentration ratios determined from the tissue acid extracts. Spin-spin relaxation times of glutamate and NAA peaks in anoxic slices were 749 +/- 89 and 729 +/- 94 ms, respectively, and thus, the portion of glutamate that could not be detected by 1H NMR was quantified in absolute terms. It was calculated that an increase in the glutamate-to-NAA ratio from 0.55 +/- 0.02 to 0.67 +/- 0.02 during aglycemic anoxia corresponded to some 6 mmol/kg of tissue dry weight of glutamate from the total concentration of 28 mmol/kg dry weight. It is suggested that this 22% of total glutamate pool is present in a noncytoplasmic compartment during controlled metabolic state.

Galanin reduces release of endogenous excitatory amino acids in the rat hippocampus

Galanin is a 29-amino acid peptide widely distributed in the central nervous system where it modulates the release of several neurotransmitters and neurohormones. Because previous data had postulated that galanin could inhibit the release of excitatory amino acids and protect hippocampal neurons against anoxia, we have investigated the effect of galanin on the release of endogenous glutamate and aspartate evoked by potassium depolarization in rat hippocampal slices. Galanin, added to a concentration of 0.3 microM, produced a 50-60% reduction in the release of endogenous glutamate and aspartate as evoked by 40 mM K+. This effect was concentration-dependent with half-maximal effective galanin concentrations (EC50) of 1.7 and 5.9 nM for glutamate and aspartate, respectively. Such an effect was found to occur preferentially in the ventral rather than in the dorsal region of the hippocampus. The inhibitory effect of galanin on the K(+)-evoked release of excitatory amino acids was reversed by the specific galanin antagonist M-15 (0.3 microM), and by the ATP-sensitive potassium channel blocker glibenclamide (10 microM). Furthermore, M-15 alone increased the basal and the K(+)-evoked release of glutamate and aspartate from hippocampal slices. It is concluded that galanin exerts a tonic inhibition of excitatory glutamate/aspartate neurotransmission in the rat ventral hippocampus. The efficacy of glibenclamide in antagonizing the effect of galanin suggests the involvement of ATP-sensitive or -insensitive potassium channels in such a regulation.

Relationship between extracellular neurotransmitter amino acids and energy metabolism during cerebral ischemia in rats monitored by microdialysis and in vivo magnetic resonance spectroscopy

The time-course of changes in extracellular glutamate and energy metabolism during 30 or 60 min of complete cerebral ischemia and 60-90 min of reperfusion was investigated by microdialysis and magnetic resonance spectroscopy in parallel groups of rats. During the first 10 min of ischemia, adenosine triphosphate (ATP) was completely depleted, and lactate increased 10-fold; after 30 min, intracellular pH had decreased to 6.33 +/- 0.11. ATP and lactate did not change further between 30 and 60 min of ischemia. Glutamate increased 30-fold between 10 and 30 min of ischemia and continued to increase in the 60-min ischemia group. After 30 min of reperfusion, glutamate had returned to pre-ischemic levels in both groups. The cellular energy state recovered within 50-60 min after 30 min of ischemia but never returned to more than 60% of baseline values after 60 min of ischemia. The continued increase in extracellular glutamate after total depletion of ATP suggests that glutamate release during ischemia is not entirely energy dependent. Ca(2+)-independent glutamate release and failure of energy-dependent glutamate re-uptake mechanisms may result in continued increase in extracellular glutamate. The rapid normalization of extracellular glutamate after 30 and 60 min of ischemia despite differences in the recovery of energy metabolism suggests that the glutamate levels were reduced by an energy-independent mechanism, such as diffusion into the restored circulation.

Release of endogenous glutamic and aspartic acids from cerebrocortex synaptosomes and its modulation through activation of a gamma-aminobutyric acidB (GABAB) receptor subtype

The depolarization-evoked release of endogenous glutamate (GLU) and -aspartate (ASP) and its modulation mediated by gamma-aminobutyric acid (GABA) heteroreceptors was investigated in superfused rat cerebrocortical synaptosomes. Exposure to 12 mM K+ enhanced the release of GLU and ASP. The K(+)-evoked overflow of both amino acids was largely Ca(2+)-dependent. Exogenous GABA inhibited the K(+)-evoked overflow of GLU (EC50 2.8 microM) and ASP (EC50 2.7 microM). The effect of GABA was mimicked by the GABAB receptor agonist (-)-baclofen (EC50 2.0 microM for GLU and 1.3 microM for ASP release) but not by the GABAA receptor agonist muscimol, up to 100 microM. Accordingly, the GABA-induced inhibition of GLU and ASP release was not affected by the GABAA receptor antagonists, bicuculline or picrotoxin, but was antagonized by the GABAB receptor antagonist, 3-amino-propyl(diethoxymethyl)phosphinic acid (CGP 35348). The GABA effect was, however, insensitive to another GABAB receptor antagonist, phaclofen, up to 1,000 microM. It can be concluded that GABA heteroreceptors of the GABAB type regulating the depolarization-evoked release of GLU and ASP are present on cortical GLU/ASP-releasing nerve terminals. These receptors may be classified as a phaclofen-insensitive GABAB receptor subtype.

Presynaptic alpha 2 adrenoceptors inhibit glutamate release from rat spinal cord synaptosomes

The presynaptic regulation of amino acid release from nerve terminals was investigated using synaptosomes prepared from the rat spinal cord. The basal releases of endogenous glutamate (Glu), aspartate (Asp), and gamma-amino-butyric acid (GABA) were 34.6, 21.5, and 10.0 pmol/min/mg of protein, respectively. Exposure to a depolarizing concentration of KCl (30 mM) evoked 2.7-, 1.5-, and 2.9-fold increases in Glu, Asp, and GABA release, respectively. Clonidine reduced the K(+)-evoked overflow of Glu to 56% of the control overflow with a potency (IC50) of 17 nM, but it did not affect K(+)-evoked overflow of Asp, GABA, and their basal releases. Similarly, noradrenaline inhibited the K(+)-evoked overflow of Glu, although phenylephrine and isoproterenol showed no effect. The inhibitory effect of clonidine was counteracted by alpha 2-adrenoceptor antagonists, rauwolscine, yohimbine, and idazoxan, regardless of the imidazoline structures. Because Glu is considered a neurotransmitter of primary afferents that transmit both nociceptive and nonnociceptive stimuli in the spinal cord, these data suggest that part of Glu release may be regulated by the noradrenergic system through alpha 2 adrenoceptors localized on the primary afferent terminals.

Effects of short-term hypoxia on [3H]glutamate release from preloaded hippocampal and cortical synaptosomes

The effect of short-term hypoxia on the release of [3H]glutamate from preloaded hippocampal and cortical synaptosomes was studied in a rapid superfusion system. The technique minimised the loss of released glutamate by reuptake. The results indicated that the effects of short term hypoxia were qualitatively similar to those reported in previous studies using more long-term hypoxia, but were significantly smaller. The non-Ca(2+)-dependent efflux of glutamate from cortical synaptosomes was increased by hypoxia as was the Ca(2+)-dependent release from hippocampal tissue. Possible mechanisms for these findings were discussed. The small amplitude of these changes in comparison to the effects seen in slowly perfused tissue in vitro and in vivo indicated that the contribution made by changes in neuronal efflux to the overall increase in extracellular glutamate seen in hypoxia is relatively minor.

35 mM K(+)-stimulated 45Ca2+ uptake in cerebellar granule cell cultures mainly results from NMDA receptor activation

In primary cultures of cerebellar granule cells, the Ca2+ influx resulting from K+ depolarization (35 mM) was equal to one-third of that observed with 100 microM N-methyl-D-aspartate (NMDA) and was reduced in a major part (90%) by NMDA receptor antagonists. The rank order of potency of these competitive and non-competitive NMDA receptor antagonists was very close to their affinity for the NMDA and phencyclidine sites respectively. Granular cell depolarization with 35 mM K+ also induced a large increase in the extracellular glutamate concentration. Repeated washes of the culture wells, addition of glutamate pyruvate transaminase (+2 mM pyruvate), or pretreatment of the cells with tetanus toxin resulted in a parallel reduction of the extracellular glutamate concentration and 45Ca2+ uptake measured after a 35 mM K+ stimulation. Dihydropyridine (BAY K-8644) stimulated the release of glutamate in a nifedipine-sensitive manner in the presence of 15 mM K+. However, nifedipine (1 microM), which decreased by 60% the K(+)-induced 45Ca2+ uptake, did not reduce the 35 mM K(+)-evoked glutamate release. Taken together, these results demonstrated that in cerebellar granule cell cultures, 90% of the 35 mM K(+)-stimulated 45Ca2+ influx resulted from the release of glutamate and the consecutive activation of NMDA receptors. Activation of these glutamate receptors then allows Ca2+ influx to occur through L-type voltage-operated Ca2+ channels.

Lack of involvement of [Ca2+]i in the external Ca(2+)-independent release of acetylcholine evoked by veratridine, ouabain and alpha-latrotoxin: possible role of [Na+]i

Synaptosomes were challenged by veratridine, ouabain and alpha-latrotoxin, and the release of 14C-acetylcholine (ACh) was measured in the absence of external Ca2+. We wished to test whether Ca2+ mobilized from internal stores triggered the ACh release that was independent of external Ca2+. We found that none of the agents altered the [Ca2+]i in a Ca(2+)-free medium. Buffering the intracellular Ca2+ concentration with BAPTA did not prevent the increase in release of 14C-ACh by veratridine or ouabain in the absence of Ca2+, however, it greatly reduced the release evoked in a Ca(2+)-containing medium. In parallel samples the release of ACh and the change in the internal Na+ concentration ([Na+]i) were measured. It was found that veratridine, ouabain and alpha-latrotoxin all enhanced [Na+]i in a concentration-dependent manner and a good quantitative relationship existed between the increase in [Na+]i and the release of ACh.