Compartmentation of glutamate and aspartate within cerebral cortical synaptosomes: evidence for a non-cytoplasmic origin for the calcium-releasable pool of glutamate

L-aspartate as an amino acid neurotransmitter: mechanisms of the depolarization-induced release from cerebrocortical synaptosomes

The role of L-aspartate as a classical neurotransmitter of the CNS has been a matter of great debate. In this study, we have characterized the main mechanisms of its depolarization-induced release from rat purified cerebrocortical synaptosomes in superfusion and compared them with those of the well known excitatory neurotransmitter L-glutamate. High KCl and 4-aminopyridine were used as depolarizing agents. At 15 mM KCl, the overflows of both transmitters were almost completely dependent on external Ca2+. At 35 and 50 mM KCl, the overflows of L-aspartate, but not those of L-glutamate, became sensitive to DL-threo-b-benzyloxy aspartic acid (DL-TBOA), an excitatory amino acid transporter inhibitor. In the presence of DL-TBOA, the 50 mM KCl-evoked release of L-aspartate was still largely external Ca2+-dependent. The DL-TBOA insensitive,external Ca2+-independent component of the 50 mM KCl-evoked overflows of L-aspartate and L-glutamate was significantly decreased by the mitochondrial Na+/Ca2+ exchanger blocker CGP 37157. The Ca2+-dependent, KCl-evoked overflows of L-aspartate and L-glutamate were diminished by botulinum neurotoxin C, although to a significantly different extent. The 4-aminopyridine-induced L-aspartate and L-glutamate release was completely external Ca2+-dependent and never affected by DL-TBOA. Superimposable results have been obtained by pre-labeling synaptosomes with [3H]D aspartate and [3H]L-glutamate. Therefore, our data showing that L-aspartate is released from nerve terminals by calcium dependent,exocytotic mechanisms support the neurotransmitter role of this amino acid.

Properties of synaptic vesicle pools in mature central nerve terminals

Readily releasable and reserve pools of synaptic vesicles play different roles in neurotransmission, and it is important to understand their recycling and interchange in mature central synapses. Using adult rat cerebrocortical synaptosomes, we have shown that 100 mosm hypertonic sucrose caused complete exocytosis of only the readily releasable pool (RRP) of synaptic vesicles containing glutamate or gamma-aminobutyric acid. Repetitive hypertonic stimulations revealed that this pool recycled (and reloaded the neurotransmitter from the cytosol) fully in <30 s and did so independently of the reserve pool. Multiple rounds of exocytosis could occur in the constant absence of extracellular Ca(2+). However, although each vesicle cycle includes a Ca(2+)-independent exocytotic step, some other stage(s) critically require an elevation of cytosolic [Ca(2+)], and this is supplied by intracellular stores. Repetitive recycling also requires energy, but not the activity of phosphatidylinositol 4-kinase, which maintains the normal level of phosphoinositides. By varying the length of hypertonic stimulations, we found that approximately 70% of the RRP vesicles fused completely with the plasmalemma during exocytosis and could then enter silent pools, probably outside active zones. The rest of the RRP vesicles underwent very fast local recycling (possibly by kiss-and-run) and did not leave active zones. Forcing the fully fused RRP vesicles into the silent pool enabled us to measure the transfer of reserve vesicles to the RRP and to show that this process requires intact phosphatidylinositol 4-kinase and actin microfilaments. Our findings also demonstrate that respective vesicle pools have similar characteristics and requirements in excitatory and inhibitory nerve terminals.

Aspartate release from rat hippocampal synaptosomes

Certain excitatory pathways in the rat hippocampus can release aspartate along with glutamate. This study utilized rat hippocampal synaptosomes to characterize the mechanism of aspartate release and to compare it with glutamate release. Releases of aspartate and glutamate from the same tissue samples were quantitated simultaneously. Both amino acids were released by 25 mM K(+), 300 microM 4-aminopyridine (4-AP) and 0.5 and 1 microM ionomycin in a predominantly Ca(2+)-dependent manner. For a roughly equivalent quantity of glutamate released, aspartate release was significantly greater during exposure to elevated [K(+)] than to 4-AP and during exposure to 0.5 than to 1 microM ionomycin. Aspartate release was inefficiently coupled to P/Q-type voltage-dependent Ca(2+) channels and was reduced by KB-R7943, an inhibitor of reversed Na(+)/Ca(2+) exchange. In contrast, glutamate release depended primarily on Ca(2+) influx through P/Q-type channels and was not significantly affected by KB-R7943. Pretreatment of the synaptosomes with tetanus toxin and botulinum neurotoxins C and F reduced glutamate release, but not aspartate release. Aspartate release was also resistant to bafilomycin A(1), an inhibitor of vacuolar H(+)-ATPase, whereas glutamate release was markedly reduced. (+/-) -Threo-3-methylglutamate, a non-transportable competitive inhibitor of excitatory amino acid transport, did not reduce aspartate release. Niflumic acid, a blocker of Ca(2+)-dependent anion channels, did not alter the release of either amino acid. Exogenous aspartate and aspartate recently synthesized from glutamate accessed the releasable pool of aspartate as readily as exogenous glutamate and glutamate recently synthesized from aspartate accessed the releasable glutamate pool. These results are compatible with release of aspartate from either a vesicular pool by a "non-classical" form of exocytosis or directly from the cytoplasm by an as-yet-undescribed Ca(2+)-dependent mechanism. In either case, they suggest aspartate is released mainly outside the presynaptic active zones and may therefore serve as the predominant agonist for extrasynaptic N-methyl-D-aspartate receptors.

A biochemical approach to study sub-second endogenous release of diverse neurotransmitters from central nerve terminals

Exocytosis in central nerve terminals is rapidly triggered by the influx of calcium through high voltage sensitive Ca2+ -channels. Mainly due to their small size, studies in which neurotransmitter release from these terminals was determined at the sub-second time-scale are still rather limited. Here we describe the use of a pneumatic rapid mixing device, allowing application of short (> or = 50 ms) K+ -depolarizing pulses to purified nerve terminals, synaptosomes, to trigger endogenous release of different transmitter types. A consistent, Ca2+ -dependent exocytotic release of the amino acid transmitters, glutamate and GABA, from synaptosomes purified from rat and mouse brain was observed after 100 ms depolarization. For determination of amino acid release after longer depolarizations (> 100 ms), transporter blockers had to be added to prevent clearance of the vesicularly released transmitters. Ca2+ -dependent release of the neuropeptide cholecystokinin occured only after 250 ms depolarization. In addition, the time-courses of amino acid and cholecystokinin release were clearly different. The fast Ca2+ -dependent release of all transmitters was selectively and strongly inhibited by the P/Q-type Ca2+ -channel blocker omega-Agatoxin IVA. In conclusion, this approach allows direct measurement of Ca2+ -dependent release of diverse endogenous neurotransmitters from central nerve terminals upon depolarization pulses at a physiologically relevant, sub-second, time scale.

alpha-Latrotoxin, acting via two Ca2+-dependent pathways, triggers exocytosis of two pools of synaptic vesicles

alpha-Latrotoxin stimulates three types of [(3)H]gamma-aminobutyric acid and [(14)C]glutamate release from synaptosomes. The Ca(2+)-independent component (i) is insensitive to SNAP-25 cleavage or depletion of vesicle contents by bafilomycin A1 and represents transmitter efflux mediated by alpha-latrotoxin pores. Two other components of release are Ca(2+)-dependent and vesicular but rely on distinct mechanisms. The fast receptor-mediated pathway (ii) involves intracellular Ca(2+) stores and acts upon sucrose-sensitive readily releasable vesicles; this mechanism is insensitive to inhibition of phosphatidylinositol 4-kinase (PI 4-kinase). The delayed pore-dependent exocytotic component (iii) is stimulated by Ca(2+) entering through alpha-latrotoxin pores; it requires PI 4-kinase and occurs mainly from depot vesicles. Lanthanum perturbs alpha-latrotoxin pores and blocks the two pore-mediated components (i, iii) but not the receptor-mediated release (ii). alpha-Latrotoxin mutant (LTX(N4C)) cannot form pores and stimulates only the Ca(2+)-dependent receptor-mediated amino acid exocytosis (ii) (detectable biochemically and electrophysiologically). These findings explain experimental data obtained by different laboratories and implicate the toxin receptors in the regulation of the readily releasable pool of synaptic vesicles. Our results also suggest that, similar to noradrenergic vesicles, amino acid-containing vesicles at some point in their cycle require PI 4-kinase.

Lidocaine reduces the hypoxia-induced release of an excitatory amino acid analog from rat striatal slices in superfusion

1. Lidocaine has been extensively investigated as a potential neuroprotective drug against ischemia-induced neurodegeneration without reaching any satisfactory conclusion. 2. The present work evaluates the effect of lidocaine -17 microM- on the hypoxia-induced release of tritiated D-aspartate from rat striatal slices in superfusion. 3. Hypoxia resulted in a significant increase in the amount of D-aspartate released from striatal slices preloaded with the tritiated excitatory amino acid analog. 4. The addition of lidocaine to the superfusion solution resulted in a drastic reduction in the amount of D-aspartate release evoked by hypoxia, rendering it close to normal values.

Effects of in vitro ethanol and chronic ethanol consumption on the release of excitatory amino acids in the rat hippocampus

In CA1-CA3 hippocampal slices, in vitro ethanol (EtOH) (10-100 mM) evoked, as a function of EtOH concentration, a differential release of aspartate (Asp) and glutamate (Glu). Omission of Ca2+ ions from the superfusion media completely abolished the EtOH-induced release of Asp but not that of Glu. In addition, at 20 mM, EtOH enhanced K(+)-evoked release only of Asp. Finally, delayed changes were observed on NMDA-evoked release of [3H]noradrenaline (NA) in the dentate gyrus (DG) after withdrawal from EtOH for 30 days.

Changes in extracellular levels of glutamate and aspartate in rat substantia nigra induced by dopamine receptor ligands: in vivo microdialysis studies

The microdialysis technique was utilized to study the local effects of D1 and D2 family type dopamine (DA) receptor (R) ligands on the in vivo release of endogenous glutamate (GLU) and aspartate (ASP) from rat substantia nigra (SN). Addition to the dialysis perfusion solution of either D1-R and D2-R agonists, such as SKF-38393 (50 and 100 microM) and Quinpirole (5 and 10 microM), resulted in dose-dependent increases in extracellular concentrations of GLU and ASP, respectively. The SKF-38393 and Quinpirole-induced effects were reduced by SCH-23390 (0.5 microM), a D1-R antagonist, and by Spiperone (1.0 microM), a D2-R antagonist, respectively. However, SCH-23390 and Spiperone did increase GLU and ASP extracellular concentrations. Local infusion with Tetrodotoxin (TTX) (1.0 microM), a blocker of voltage-dependent Na+ channels, increased basal extracellular levels of GLU. In addition, co-infusion of TTX and SKF-38393 evoked increases in extracellular GLU levels higher than those observed after SKF-38393 alone. Finally, chemical lesions of nigral DA cells with 6-OH-DA increased the basal extracellular levels of GLU. It is proposed that the release of GLU and ASP from SN may be regulated by D1- and D2-receptors present in this basal ganglia structure. In addition, part of the D1 receptors present in SN might be located presynaptically on GLU-containing nerve endings.

Evidence for glutamate as neurotransmitter in trigemino-and spinothalamic tract terminals in the nucleus submedius of cats

The nucleus submedius in the medial thalamus of cats is an important termination site for lamina I trigemino-and spinothalamic tract (TSTT) neurons, many of which are nociceptive-specific, and the nucleus submedius has been proposed to be a dedicated nociceptive substrate involved in the affective aspect of pain. In the present study, the distribution of glutamate was examined by immunocytochemical methods in order to evaluate the possible role of this amino acid as a neurotransmitter in TSTT terminals in the nucleus submedius. TSTT terminals were identified by anterograde transport of horseradish peroxidase and wheatgerm agglutinin-horseradish peroxidase conjugate from the spinal cord or the medullary dorsal horn. Quantitative analysis of immunogold labelling revealed that TSTT terminals contain about twice the tissue average of glutamate-like immunoreactivity. A strong positive correlation was found between the density of synaptic vesicles and the density of gold particles in these terminals, whereas no relationship was seen between these variables in GABAergic presynaptic dendrites. Enrichment of glutamate-like immunoreactivity (approximately 250% of the tissue average) was also observed in terminals of presumed cortical origin. Presynaptic dendrites and neuron cell bodies in the nucleus submedius were found to contain relatively low levels of glutamate-like immunoreactivity, at or below the tissue average. These observations provide evidence that glutamate is a neurotransmitter in lamina I TSTT terminals in the nucleus submedius. The findings also suggest glutamatergic neurotransmission between cortical afferents and nucleus submedius neurons. Glutamate is therefore likely to be an important mediator of nociceptive processing in the medial thalamus.

Quantitative immunogold evidence for enrichment of glutamate but not aspartate in synaptic terminals of retino-geniculate, geniculo-cortical, and cortico-geniculate axons in the cat

A postembedding immunogold procedure was used on thin sections of the dorsal lateral geniculate nucleus (LGN) and perigeniculate nucleus (PGN) of the cat to estimate qualitatively and quantitatively, at the electron-microscopic (EM) level, the intensity of glutamate or aspartate immunoreactivities on identifiable synaptic terminals and other profiles of the neuropil. On sections incubated with a glutamate antibody, terminals of retinal and cortical axons in the LGN, and of collaterals of geniculo-cortical axons in the PGN, contain significantly higher density of immunogold particles than GABAergic terminals, glial cells, dendrites, and cytoplasm of geniculate cells. By contrast, in sections incubated with an aspartate antibody, terminals of retino-geniculate, cortico-geniculate, and geniculo-cortical axons did not show a selective enrichment of immunoreactivity, but instead the density of immunogold particles was generally low in the different profiles of the neuropil, with the exception of nucleoli. These results suggest that glutamate, but not aspartate, is a neurotransmitter candidate in the retino-geniculo-cortical pathways.

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.

Release of amino acids into regional superfusates of the spinal cord by mechano-stimulation of the reproductive tract

Based on pharmacological evidence that inhibitory amino acids mediate vaginocervical mechano-stimulation produced analgesia (VSPA), we hypothesized that inhibitory amino acids would be released endogenously in the spinal cord in response to vaginocervical mechano-stimulation (VS). This hypothesis was tested by HPLC analysis of the amino acid content of 5-min superfusates of the spinal cord before, during and after VS (400 g force applied against the cervix) in urethane-anesthetized rats. Utilizing an in vivo push-pull superfusion method, artificial cerebrospinal fluid was continuously superfused over the spinal cord through the intrathecal space surrounding the sacral-lower thoracic region. In addition, concentrations of amino acids in the superfusate were measured in response to KCl stimulation (increasing the superfusion medium from 3.4 to 40.0 mM KCl to produce non-specific depolarization), and noxious hind paw mechano-stimulation (pinching the hind paw to produce a sustained flexor response in ipsilateral hind leg). There was a significant increase in the concentration of Gly, Tau, Asp, Glu and Lys in the superfusate in response to VS (n = 8) and to KCl (n = 8), but not to hind paw stimulation (n = 5). Also, GABA concentrations increased in response to KCl, and the concentration of Ala, Ser, Gln, Thr, Arg and Phe increased in response to VS, however, GABA levels were sometimes below the limits of detection. In contrast, there was no significant change in any amino acid concentration in response to hind paw pinch stimulation, and VS did not significantly affect the concentrations of Tyr, His, Ile, Leu, Met, Trp or Val. The present findings support our hypothesis that VS releases inhibitory amino acids in the spinal cord. Moreover, other amino acids, including 'excitatory' amino acids, are released into the superfusate. The profile of amino acid release in response to VS differs from that in response to paw pinch or KCl administration.

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.

Extracellular neuroactive amino acids in the rat striatum during ischaemia: comparison between penumbral conditions and ischaemia with sustained anoxic depolarisation

Changes in the extracellular levels of excitatory and inhibitory amino acid transmitters were studied in the rat striatum during penumbral ischaemia using intracerebral microdialysis. Effects of penumbral forebrain ischaemia were compared with those of ischaemia with sustained anoxic depolarisation and K+ (100 mM). Comparisons were also made between different groups of animals at 2 and 24 h after dialysis probe implantation. The K+ stimulus did not provoke any release of excitatory amino acids in the 24-h group, probably reflecting a decrease of functional synapses adjacent to the probe. During 30 min of penumbral ischaemia, excitatory amino acids did not reach critical concentrations in the extracellular fluid, and increases in levels of inhibitory/modulatory amino acids were similar. On the other hand, severe transient ischaemia resulted in massive synchronous release of many neuroactive excitatory and inhibitory compounds, in both the 2- and 24-h groups. These and other data suggest that changes during severe ischaemia may arise from both neurotransmitter and metabolic pools. It is concluded that ischaemic damage in the penumbra may not be related to extracellular neuroactive amino acid changes generated within this region.

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.

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.

Regulation of excitatory amino acid release by N-methyl-d-aspartate receptors in rat striatum: in vivo microdialysis studies

The microdialysis technique was utilized to study the effects of N-methyl-D-aspartate (NMDA) receptor ligands on the in vivo release of endogenous glutamate (Glu) and aspartate (Asp) from the rat striatum. Addition of NMDA (250 and 500 microM) to the dialysis perfusion solution resulted in a striking dose-dependent increase in extracellular concentrations of Glu and Asp in the striatum. The NMDA-induced effects were reduced in a dose-related way by prior perfusion with 75 microM dizocilpine (MK-801), a non-competitive NMDA receptor antagonist. MK-801, at 75 microM, produced no changes on basal levels of Glu and Asp. However, 100 microM MK-801 did increase Glu and Asp extracellular concentrations. Local infusion with 500 microM D-serine, an agonist at the glycine site associated to the NMDA receptor, significantly increased basal level of Glu, but not Asp. Such D-serine-induced effects were reduced by 7-Cl-kynurenic acid (200 microM), a selective blocker of the glycine site present in the NMDA receptor. It is proposed that activation of NMDA receptors by endogenous Glu and Asp enhances the subsequent release of these excitatory amino acids in the striatum. Part of these NMDA receptors might be located presynaptically on cortico-striatal nerve endings. In addition, postsynaptic NMDA receptors present in the striatum may also indirectly modulate the release of Glu and Asp, through trans-synaptic mechanism.

Perfusion of Immobilized Isolated Nerve Terminals as a Model for the Regulation of Transmitter Release: Release of Different, Endogenous Transmitters, Repeated Stimulation, and High Time Resolution

To study the release of neurotransmitters, i.e., the recruitment of transmitters for release and the regulation of the release process, isolated nerve terminals (synaptosomes) of the rat forebrain were immobilized in Sephadex gel inside a perfusion chamber. In this way, the following were achieved: (a) A very limited pressure stress was exerted on the synaptosomes, so that these remained viable for long periods (greater than 30 min) inside the chamber and did not elute from the chamber, which allowed long-term experiments with repeated stimulations; (b) estimation of the release of various endogenous transmitters, both in a Ca(2+)-dependent (exocytotic) and Ca(2+)-independent manner; (c) a step-like stimulation with depolarizing agents (rise time, 3-4 s) and a high time resolution (600-ms sampling); and (d) negligible reuptake of transmitter into the terminals or extracellular breakdown. It is concluded that this perfusion setup helps to provide new insights in the presynaptic stimulus-secretion coupling, co-transmission, and the exo-endocytosis cycle.

Presynaptic modulation of amino acid release from synaptosomes

Using synaptosomes prepared from whole rat brain, the spontaneous, calcium-independent, and calcium-dependent release of glutamate and GABA was assessed. Time intervals of 1-30 seconds were studied. Spontaneous release of glutamate (but not GABA) was elevated by 10 microM NMDA or AMPA by thirty seconds. This stimulation was partially calcium-dependent. Calcium-dependent release induced by 30 mM KCl was biphasic, confirming previous findings. This release was stimulated at all time periods by the presence of 10 microM NMDA or AMPA in an antagonist-sensitive manner. These data suggest that glutamate and GABA are released from vesicular stores in rat synaptosomes and that some of this release is modulated by presynaptic glutamate receptors.

Autoreceptor regulation of glutamate and aspartate release from slices of the hippocampal CA1 area

Slices of hippocampal area CA1 were employed to test the hypothesis that the release of glutamate and aspartate is regulated by the activation of excitatory amino acid autoreceptors. In the absence of added Mg2+, N-methyl-D-aspartate (NMDA)-receptor antagonists depressed the release of glutamate, aspartate, and gamma-aminobutyrate evoked by 50 mM K+. Conversely, the agonist NMDA selectively enhanced the release of aspartate. The latter action was observed, however, only when the K+ stimulus was reduced to 30 mM. Actions of the competitive antagonists 3-[(+/- )-2-carboxypiperazin-4-yl]-propyl-l-phosphonic acid (CPP) and D-2-amino-5-phosphonovalerate (D-AP5) differed, in that the addition of either 1.2 mM Mg2+ or 0.1 microM tetrodotoxin to the superfusion medium abolished the depressant effect of CPP without diminishing the effect of D-AP5. These results suggest that the activation of NMDA receptors by endogenous glutamate and aspartate enhances the subsequent release of these amino acids. The cellular mechanism may involve Ca2+ influx through presynaptic NMDA receptor channels or liberation of a diffusible neuromodulator linked to the activation of postsynaptic NMDA receptors. (RS)-alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, a selective quisqualate receptor agonist, and kainate, an agonist active at both kainate and quisqualate receptors, selectively depressed the K(+)-evoked release of aspartate. Conversely, 6-cyano-7-nitro-quinoxaline-2,3-dione, an antagonist active at both quisqualate and kainate receptors, selectively enhanced aspartate release. These results suggest that glutamate can negatively modulate the release of aspartate by activating autoreceptors of the quisqualate, and possibly also of the kainate, type. Thus, the activation of excitatory amino acid receptors has both presynaptic and postsynaptic effects.

Differential release of amino acids, neuropeptides, and catecholamines from isolated nerve terminals

We have investigated transmitter release from small and large dense-core vesicles in nerve terminals isolated from guinea pig hippocampus. Small vesicles are found in clusters near the active zone, and large dense-core vesicles are located at ectopic sites. The abilities of Ca2+ channel activation and uniform elevation of Ca2+ concentration (with ionophores) to evoke secretion of representative amino acids, catecholamines, and neuropeptides were compared. For a given increase in Ca2+ concentration, ionophore was less effective than Ca2+ channel activation in releasing amino acids, but not in releasing cholecystokinin-8. Titration of the average Ca2+ concentration showed that the Ca2+ affinity for cholecystokinin-8 secretion was higher than that for amino acids. Catecholamine release showed intermediate behavior. It is concluded that neuropeptide release is triggered by small elevations in the Ca2+ concentration in the bulk cytoplasm, whereas secretion of amino acids requires higher elevations, as produced in the vicinity of Ca2+ channels.

Cortical ablation reduces veratridine evoked release of endogenous glutamate from superfused substantia nigra slices

A brain slice superfusion system and HPLC coupled to fluorometric detection were used to study depolarization-evoked release of endogenous glutamate (Glu) and aspartate (Asp) from substantia nigra (SN) slices. Veratridine (15-25 microM) and K+ (50 mM) evoked both a Ca2(+)-dependent and Ca2(+)-dependent release of Glu and Asp. However, the pattern of Glu release was different from that of Asp. In addition, cortical ablation significantly reduced the evoked release of Glu but not that of Asp. There results indicate the existence in SN of functional Glu- and Asp-releasing nerve terminals. Some of the Glu-enriched nerve terminals might belong to a cortico-nigral neuronal pathway.

Transmitter glutamate release from isolated nerve terminals: evidence for biphasic release and triggering by localized Ca2+

The kinetics of Ca2(+)-dependent release of glutamate from guinea-pig cerebrocortical synaptosomes evoked by KCl or 4-aminopyridine are investigated using a continuous fluorimetric assay. Release by both agents is biphasic, with a rapid phase complete within 2 s followed by a more extensive slow phase with a half-maximal release in 52 s for KCl-evoked release and greater than 120 s for 4-aminopyridine-evoked release. The two phases of glutamate release may reflect a dual localization of releasable vesicles at the active zone and in the bulk cytoplasm. Decreasing depolarization depresses the extent rather than increasing the time for half-maximal Ca2(+)-dependent release. Both the fast and the slow phases of glutamate release require external Ca2+ and cytoplasmic ATP. KCl depolarization produces a transient "spike" of cytoplasmic free Ca2+ [( Ca2+]c), which recovers to a plateau; the major component of glutamate release occurs during this plateau. Predepolarization in the absence of added external Ca2+, to inhibit transient Ca2+ channels, does not affect the subsequent glutamate release evoked by Ca2+ readdition. Thus, release involves primarily noninactivating Ca2+ channels. For a given increase in [Ca2+]c, KCl and 4-aminopyridine cause equal release of glutamate, while ionomycin releases much less glutamate. This lowered efficiency is not due to ATP depletion. It is concluded that glutamate exocytosis is evoked by localized Ca2+ entering through noninactivating voltage-dependent Ca2+ channels and that nonlocalized Ca2+ entry with ionomycin is inefficient.

Activity-dependent recruitment of endogenous glutamate for exocytosis

The Ca(2+)-dependent release of the neurotransmitter glutamate from purified nerve terminals (synaptosomes) of the rat hippocampus was studied in a rapid perfusion apparatus. The response of the terminals was investigated with respect to the kinetics and duration of the release of endogenous glutamate upon brief and sustained stimulation and upon repetitive stimulation. The synaptosomes were stimulated by sustained chemical depolarization (0.5-3 min 30 mM K+). The cellular levels of glutamate, free Ca2+ and ATP in the nerve terminals were measured. The Ca(2+)-dependent release of glutamate showed an immediate elevation upon K(+)-depolarization. When the stimulation was maintained, a prolonged phase of glutamate release was observed. After 3 min, the Ca(2+)-dependent release stopped, although K(+)-depolarization was still effective. When synaptosomes were stimulated again after a relatively short stimulation period (30 s), the second response was similar to the previous one. After a longer stimulation period, maintained until termination of release, the second response did not show the immediate initial elevation of Ca(2+)-dependent glutamate release. Only 30 s after stimulation the release developed with a time profile comparable to the first response. This initial lack of response was not due to low cytosolic levels of glutamate or ATP or to changes in cellular Ca(2+)-buffering. It can be concluded that the capacity to release glutamate after brief depolarizations is fully restored during the repolarization period. However, if stimulation periods are of long duration (until termination of release), this capacity is no longer fully restored, especially with respect to a fast component of release. New glutamate is recruited only during the subsequent depolarization and with a delay.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha-latrotoxin releases both vesicular and cytoplasmic glutamate from isolated nerve terminals

alpha-Latrotoxin causes a massive release of endogenous glutamate from guinea-pig cerebrocortical synaptosomes. There appear to be two components to the release. In the first 2 min following addition of 1.3 nM alpha-latrotoxin, glutamate release is largely energy dependent. Superimposed upon this release is a more slowly developing but ultimately much more extensive release of cytoplasmic glutamate together with gamma-aminobutyric acid and nonvesicular amino acids such as aspartate and alpha-aminoisobutyrate. In parallel with this cytoplasmic release there is an extensive depletion of ATP, a massive rise in cytoplasmic free Ca2+ concentration, and a severe restriction of synaptosomal respiratory capacity. The cytoplasmic release is only partially Na+ dependent, eliminating a simple reversal of the plasma membrane acidic amino acid carrier. It is concluded that alpha-latrotoxin releases both transmitter and cytoplasmic pools of amino acids in synaptosomes and causes a major disruption of terminal integrity.

The release and uptake of excitatory amino acids

In this article, David Nicholls and David Attwell describe recent advances in our understanding of the mechanisms by which excitatory amino acids are released from cells, and of the way in which a low extracellular glutamate concentration is maintained. Glutamate can be released from cells by two mechanism: either by Ca2(+)-dependent vesicular release or, in pathological conditions, by reversal of the plasma membrane uptake carrier. The contrasting pharmacology and ionic dependence of the glutamate uptake carriers in the vesicle membrane and in the plasma membrane explain how glutamate (but probably not aspartate) can function as a neurotransmitter, and why the extracellular glutamate concentration rises to neurotoxic levels in brain anoxia.

Glutamine and aspartate loading of synaptosomes: a reevaluation of effects on calcium-dependent excitatory amino acid release

Guinea-pig cerebral cortical synaptosomes were preincubated for 60 min with 100 microM D-aspartate, L-aspartate, or L-glutamate. The total D- plus L-aspartate content of the synaptosomal fraction increased to 235%, 195%, or 164%, respectively, of the control. Despite this no increase was seen in the very low KCl evoked, Ca2+-dependent release of aspartate. Preincubation with the three amino acids changed the synaptosomal glutamate content to 78% (D-aspartate), 149% (L-aspartate), or 168% (L-glutamate) of control. However there was no statistically significant effect of these preincubations on the extent of Ca2+-dependent glutamate release. Thus the Ca2+-dependent release of aspartate and glutamate is not determined by the total synaptosomal content of these amino acids. The addition of 0.1-0.5 mM glutamine to the incubation caused a massive appearance of glutamate in the extrasynaptosomal medium. Analysis of specific activities showed that glutamine was hydrolysed directly by an extrasynaptosomal glutaminase, and that intrasynaptosomal glutamate was predominantly labelled by uptake of this glutaminase-derived glutamate. No increase was seen in the extent of Ca2+-dependent release of glutamate (by fluorimetry) either after preincubation with glutamine or in the continued presence of glutamine. Thus we are unable to confirm reports that glutamine expands the transmitter pool of glutamate. The extrasynaptosomal glutaminase activity in the synaptosomal preparation was inhibited by Ca2+ and activated by phosphate. Identical kinetics were obtained with "free" brain mitochondria, confirming the origin of the glutamine-derived glutamate.

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.