3H-D-aspartate release from cerebellar granule neurons is differentially regulated by glutamate- and K+-stimulation

Glucose is necessary to maintain neurotransmitter homeostasis during synaptic activity in cultured glutamatergic neurons

Glucose is the primary energy substrate for the adult mammalian brain. However, lactate produced within the brain might be able to serve this purpose in neurons. In the present study, the relative significance of glucose and lactate as substrates to maintain neurotransmitter homeostasis was investigated. Cultured cerebellar (primarily glutamatergic) neurons were superfused in medium containing [U-13C]glucose (2.5 mmol/L) and lactate (1 or 5 mmol/L) or glucose (2.5 mmol/L) and [U-13C]lactate (1 mmol/L), and exposed to pulses of N-methyl-D-aspartate (300 micromol/L), leading to synaptic activity including vesicular release. The incorporation of 13C label into intracellular lactate, alanine, succinate, glutamate, and aspartate was determined by mass spectrometry. The metabolism of [U-13C]lactate under non-depolarizing conditions was high compared with that of [U-13C]glucose; however, it decreased significantly during induced depolarization. In contrast, at both concentrations of extracellular lactate, the metabolism of [U-13C]glucose was increased during neuronal depolarization. The role of glucose and lactate as energy substrates during vesicular release as well as transporter-mediated influx and efflux of glutamate was examined using preloaded D-[3H]aspartate as a glutamate tracer and DL-threo-beta-benzyloxyaspartate to inhibit glutamate transporters. The results suggest that glucose is essential to prevent depolarization-induced reversal of the transporter (efflux), whereas vesicular release was unaffected by the choice of substrate. In conclusion, the present study shows that glucose is a necessary substrate to maintain neurotransmitter homeostasis during synaptic activity and that synaptic activity does not induce an upregulation of lactate metabolism in glutamatergic neurons.

Role of glutamine and neuronal glutamate uptake in glutamate homeostasis and synthesis during vesicular release in cultured glutamatergic neurons

Glutamate exists in a vesicular as well as a cytoplasmic pool and is metabolically closely related to the tricarboxylic acid (TCA) cycle. Glutamate released during neuronal activity is most likely to a large extent accumulated by astrocytes surrounding the synapse. A compensatory flux from astrocytes to neurons of suitable precursors is obligatory as neurons are incapable of performing a net synthesis of glutamate from glucose. Glutamine appears to play a major role in this context. Employing cultured cerebellar granule cells, as a model system for glutamatergic neurons, details of the biosynthetic machinery have been investigated during depolarizing conditions inducing vesicular release. [U-13C]Glucose and [U-13C]glutamine were used as labeled precursors for monitoring metabolic pathways by nuclear magnetic resonance (NMR) spectroscopy and liquid chromatography-mass spectrometry (LC-MS) technologies. To characterize release mechanisms and influence of glutamate transporters on maintenance of homeostasis in the glutamatergic synapse, a quantification was performed by HPLC analysis of the amounts of glutamate and aspartate released in response to depolarization by potassium (55 mM) in the absence and presence of DL-threo-beta-benzyloxyaspartate (TBOA) and in response to L-trans-pyrrolidine-2,4-dicarboxylate (t-2,4-PDC), a substrate for the glutamate transporter. Based on labeling patterns of glutamate the biosynthesis of the intracellular pool of glutamate from glutamine was found to involve the TCA cycle to a considerable extent (approximately 50%). Due to the mitochondrial localization of PAG this is unlikely only to reflect amino acid exchange via the cytosolic aspartate aminotransferase reaction. The involvement of the TCA cycle was significantly lower in the synthesis of the released vesicular pool of glutamate. However, in the presence of TBOA, inhibiting glutamate uptake, the difference between the intracellular and the vesicular pool with regard to the extent of involvement of the TCA cycle in glutamate synthesis from glutamine was eliminated. Surprisingly, the intracellular pool of glutamate was decreased after repetitive release from the vesicular pool in the presence of TBOA indicating that neuronal reuptake of released glutamate is involved in the maintenance of the neurotransmitter pool and that 0.5 mM glutamine exogenously supplied is inadequate to sustain this pool.

Characterization of depolarization-coupled release of glutamate from cultured mouse cerebellar granule cells using DL-threo-beta-benzyloxyaspartate (DL-TBOA) to distinguish between the vesicular and cytoplasmic pools

Release of preloaded [3H]D-aspartate in response to depolarization induced by N-methyl-D-aspartate (NMDA) or the endogenous agonist glutamate was characterized using cultured glutamatergic cerebellar granule neurons. Release from the vesicular and the cytoplasmic glutamate pools, respectively, was distinguished employing the competitive, non-transportable glutamate transport inhibitor DL-threo-beta-benzyloxyaspartate (DL-TBOA). NMDA (300 microM)-induced release was enhanced (50%) by a simultaneous elevation of the extracellular potassium concentration to 15 mM, which lifts the voltage-dependent magnesium block of the NMDA receptors. This NMDA/K(+)-induced release was not sensitive to DL-TBOA (100 microM) but was inhibited by 75% in the presence of the unspecific calcium channel antagonist La(3+) (100 microM). Glutamate (100 microM) induced a large fractional release of the preloaded [3H]D-aspartate and in the presence of DL-TBOA the release was reduced by approximately 50%. In contrast, release evoked by 25 microM glutamate was not inhibited by DL-TBOA. These results indicate that the release elicited by 100 microM glutamate is comprised of a significant glutamate transporter-mediated component in addition to the vesicular release while the NMDA/K(+)-induced release is vesicular in nature. It is likely that the high glutamate concentration (100 microM) may facilitate heteroexchange of the preloaded [3H]D-aspartate.

Pharmacology and toxicology of astrocyte-neuron glutamate transport and cycling

The interaction between astrocytes and neurons is examined from the standpoint of glutamate neurotoxicity. The review details 1. the distribution of glutamate transporters on astrocytes and neurons, provoking a reformulation of the interdependence between these two cell types in removing extracellular glutamate and preventing excitotoxic injury; 2. the potential involvement of aberrant glutamate transporter function in the etiology of neuropathological conditions; 3. the role of astrocyte-neuron interaction in widely divergent aspects of brain energetics; 4. the role of astrocytes in the process of glutamate recycling within the context of anesthetic treatment with pentobarbital and thiopental.

Neuroprotective effects of an adenoviral vector expressing the glucose transporter: a detailed description of the mediating cellular events

Considerable knowledge exists concerning the events mediating neuron death following a necrotic insult; prompted by this, there have now been successful attempts to use gene therapy approaches to protect neurons from such necrotic injury. In many such studies, however, it is not clear what sequence of cellular events connects the overexpression of the transgene with the enhanced survival. We do so, exploring the effects of overexpressing the Glut-1 glucose transporter with an adenoviral vector in hippocampal cultures challenged with the excitotoxin kainic acid (KA). Such overexpression enhanced glucose transport, attenuated the decline in ATP concentrations, decreased the release of excitatory amino acid neurotransmitters, and decreased the total free cytosolic calcium load. Commensurate with these salutary effects, neuronal survival was enhanced with this gene therapy intervention. Thus, the neuroprotective effects of this particular gene therapy occurs within the known framework of the mechanisms of necrotic neuronal injury.

Comparison of effects of DL-threo-beta-benzyloxyaspartate (DL-TBOA) and L-trans-pyrrolidine-2,4-dicarboxylate (t-2,4-PDC) on uptake and release of [3h]D-aspartate in astrocytes and glutamatergic neurons

Uptake and release processes in cerebellar astrocytes and granule neurons (glutamatergic) for glutamate were investigated by the use of [3H]D-aspartate, a non-metabolizable glutamate analog. The effects of DL-threo-beta-benzyloxyaspartate (DL-TBOA) and L-trans-pyrrolidine-2,4-dicarboxylate (t-2,4-PDC) on uptake and release of [3H]D-aspartate were studied. Both compounds inhibited potently uptake of [3H]D-aspartate in neurons and astrocytes (IC50 values 10-100 microM), DL-TBOA being slightly more potent than t-2,4-PDC. Release of preloaded [3H]D-aspartate from neurons or astrocytes could be stimulated by addition of excess t-2,4-PDC whereas addition of DL-TBOA had no effect on [3H]D-aspartate efflux. Moreover, DL-TBOA inhibited significantly the depolarization-induced (55 mM KCI) release of preloaded [3H]D-aspartate in the neurons. The results reflect the fact that DL-TBOA is not transported by the glutamate carriers while t-2,4-PDC is a substrate which may heteroexchange with [3H]D-aspartate. It is suggested that DL-TBOA may be used to selectively inhibit depolarization coupled glutamate release mediated by reversal of the carriers.

Depolarization-induced release of [(3)H]D-aspartate from GABAergic neurons caused by reversal of glutamate transporters

Cultured neocortical neurons, which predominantly consist of GABAergic neurons exhibit a pronounced stimulus-coupled GABA release. Since the cultures may contain a small population of glutamatergic neurons and the GABAergic neurons have a high content of glutamate it was of interest to examine if glutamate in addition to gamma-aminobutyric acid (GABA) could be released from these cultures. The neurons were preloaded with [(3)H]D-aspartate and subsequently its release was followed during depolarization induced by a high potassium concentration or the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor agonists, AMPA and kainate. Depolarization of the neurons with 55 mM potassium increased the release of [(3)H]D-aspartate by more than 10-fold. When the non-specific calcium-channel blockers cobalt or lanthanum were included in the stimulation buffer with potassium, the release of [(3)H]D-aspartate was decreased by about 40%. These results indicated that some of the released [(3)H]D-aspartate might originate from a vesicular pool. When AMPA was applied to the neurons, the release of [(3)H]D-aspartate was increased 2-fold and could not be prevented or decreased by addition of cobalt. Since AMPA has a rapid desensitizing effect on AMPA receptors, it was examined whether AMPA under non-desensitizing conditions was able to induce an increased release of [(3)H]D-aspartate as compared to the conditions of applying AMPA alone. The desensitization of AMPA receptors was blocked by 6-chloro-3,4-dihydro-3-(2-norbornen-5-yl)-2H-1,2, 4-benzothiadiazine-7-sulphonamide-1,1-dioxide (cyclothiazide). Under the non-desensitizing conditions, the AMPA-induced release of [(3)H]D-aspartate was highly enhanced showing about a 10-fold increase over basal release. Addition of cobalt or lanthanum did not decrease the amount of [(3)H]D-aspartate released, indicating that the release originated from a cytoplasmic pool. Kainate, which induces an almost non-desensitizing effect on AMPA receptors, showed similar results as observed for AMPA under non-desensitizing conditions. The NMDA receptor antagonist (5R,10 S)-(+)-5-methyl-10, 11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine (MK-801) had only minor effects on the [(3)H]D-aspartate release induced by AMPA and kainate. Thus, the depolarization-induced release of [(3)H]D-aspartate from cultured GABAergic neurons appears to be caused mainly by reversal of the glutamate transporters.

Functional properties of glycine receptors expressed in primary cultures of mouse cerebellar granule cells

Expression of the glycine receptor was investigated in membranes prepared from primary cultures of mouse cerebellar granule cells and postnatal mouse cerebellum using the antagonist [3H]strychnine for ligand binding. Scatchard analysis of the binding data obtained from P17 cerebellum showed a single population of binding sites (K(D) approximately 6 nM) and [3H]strychnine binding to membranes prepared from cultured neurons and P17 cerebellum was found to have the same sensitivity to the glycinergic agonists glycine, beta-alanine and taurine. The development of [3H]strychnine binding sites in cultured cerebellar granule cells and cerebellum showed opposing profiles. [3H]strychnine binding to primary cultures increased significantly during the culture period whereas during development in vivo the number of binding sites decreased over time and was hardly detectable in the adult cerebellum. Release of preloaded D-[3H]aspartate evoked by 40 mM K+ from granule cells cultured for seven days was inhibited by glycine by about 50%. Beginning after seven days in culture the ability of glycine to inhibit transmitter release declined to no inhibition after 17 days in culture. Experiments with the non-competitive antagonist, picrotoxinin, showed no blocking effect of 150 microM picrotoxinin on the glycine-induced inhibition of transmitter release. This contrasted with the inhibitory effect of 100 microM picrotoxinin in whole-cell patch-clamp recordings on responses to 500 microM glycine (56% block). Furthermore, it was demonstrated that the amplitude of the glycine activated peak current had the same size after six to seven days and after 16-17 days in culture. Northern blot analysis, and co-injection of messenger RNA plus antisense oligonucleotides into Xenopus oocytes revealed glycine receptor alpha2 and beta messenger RNAs in the cultured granule cells. These findings suggest that granule cells in culture express glycine receptor isoforms containing alpha2 picrotoxinin-sensitive and alpha2/beta picrotoxinin-insensitive receptors.

Regulation of D-aspartate release and uptake in adult brain stem auditory nuclei after unilateral middle ear ossicle removal and cochlear ablation

In young adult guinea pigs, the effects of unilateral ossicle removal and cochlear ablation were determined on transmitter release from glutamatergic presynaptic endings and glutamate inactivation via uptake. (i) D-[3H]Aspartate release and uptake were measured in subdivisions of the cochlear nucleus (CN) and in nuclei of the superior olive (SOC) and auditory midbrain (MB) up to 145 days after placing the lesions. Activities were compared to those from age-matched unlesioned controls. Fiber degeneration was visualized histologically. (ii) In the ipsilateral CN, changes in release and uptake were governed by the type of lesion. Ossicle removal produced sparse pruning of fibers only after 112 days and decreased release and uptake at 145 days, consistent with regulatory weakening of excitatory glutamatergic transmission. Cochlear ablation deafferented the CN, producing deficient release and uptake at 2 days and abundant fiber degeneration at 7 days. Subsequently, the residual release and uptake increased in magnitude, consistent with strengthening of excitatory glutamatergic transmission. (iii) In the contralateral CN, after either lesion, changes in release and uptake usually matched those in the ipsilateral CN. Thus, the auditory pathway associated with the lesioned ear probably provided cues for the regulation of synaptic strength in the contralateral CN. (iv) Both lesions increased release in the SOC and MB, and uptake in the SOC, consistent with strengthening of excitatory glutamatergic transmission. Sparse fiber degeneration, suggesting axonal pruning, appeared in the SOC and MB after cochlear ablation. (v) The strengthening of excitatory glutamatergic transmission may facilitate and maintain symptoms such as loudness recruitment and tinnitus which often accompany hearing loss.

Presynaptic calcium channels and field-evoked transmitter exocytosis from cultured cerebellar granule cells

Regulated exocytosis from cultured rat cerebellar granule cells can be localized by the vesicle specific marker FM2-10 to specific sites, the highest density of which are at visible varicosities coinciding with neurite-neurite contacts. Exocytosis can be evoked by uniform electrical field pulses, which initiate tetrodotoxin-sensitive action potentials, or by elevated KCl. [3H]D-Aspartate is an authentic false transmitter in this preparation, judged by sensitivity of release to bafilomycin A1 and tetanus toxin. The coupling of presynaptic voltage-activated Ca2+ channels to [3H]D-aspartate exocytosis was determined during field stimulation. The peak cytoplasmic free Ca2+ concentration achieved in the varicosities was proportional to Ca2+ entry during a 10 strain of pulses. L-type Ca2+ channels did not contribute to either Ca2+ entry or [3H]D-aspartate exocytosis. The P-type Ca2+ channel antagonist omega-agatoxin-IVA (30 nM) only inhibited at 75% of the varicosities, although a mean 15% inhibition of Ca2+ entry caused a 39% inhibition of exocytosis. In contrast the N-type Ca2+ channel inhibitor omega-conotoxin-GVIA (1 microM), which inhibited at virtually all varicosities, caused mean inhibitions of Ca2+ entry and exocytosis of 26% and 24% respectively. The toxin omega-conotoxin-MVIIC (5 microM), which inhibits N-, P- and Q-type Ca2+ channels, was effective at all varicosities. The Q-type component of Ca2+ entry was calculated to be only 5-10%; however, the additional inhibition of exocytosis was 30%. Thus P-type and particularly Q-type channels appear to be more closely coupled to exocytosis than N-type Ca2+ channels. The residual Ca2+ entry following 5 microM omega-conotoxin-MVIIC is scarcely coupled to release. The omega-agatoxin-IVA and omega-conotoxin-GVIA inhibitions of both Ca2+ entry and exocytosis were additive and varied stochastically between individual varicosities. These results demonstrate that both Q- and P-type Ca2+ channels are highly efficient in their coupling to amino acid exocytosis, with N-type less efficient, and L-type channels not at all. The Ca2+ channel types coupled to exocytosis are also able to support exocytosis when evoked by either brief field-evoked action potentials or prolonged depolarization with KCl, indicating that these presynaptic channels, in contrast to those on the somata of the cells, can respond to widely different patterns of activation.

PrP and beta-amyloid fragments activate different neurotoxic mechanisms in cultured mouse cells

Alzheimer's disease and prion diseases such as Creutzfeldt-Jakob disease are caused by as yet undefined metabolic disturbances of normal cellular proteins, the amyloid precursor protein and the prion protein (PrP). Synthetic fragments of both proteins, beta-amyloid 25-35 (betaA25-35) and PrP106-126, have been shown to be toxic to neurons in culture. Cell death in both cases occurs by apoptosis. Here we show that there are considerable differences in the mechanisms involved. Thus, PrP106-126 is not toxic to cortical cell cultures of PrP knockout mouse neurons whereas betaA25-35 is. The toxicity of both peptides involves Ca2+ uptake through voltage-sensitive Ca2+ channels but only PrP106-126 toxicity involves the activity of NMDA receptors. The toxicity of betaA25-35, but not PrP106-126, is attenuated by the action of forskolin. These results indicate that PrP106-126 and PA25-35 induce neuronal apoptosis through different mechanisms.

Pharmacological characterisation of voltage-sensitive calcium channels and neurotransmitter release from mouse cerebellar granule cells in culture

Using subtype-specific Ca-channel blockers, we have characterised the voltage-sensitive Ca2+ currents as well as neurotransmitter release from cultured mouse cerebellar granule cells. The whole cell version of the patch clamp technique was adapted to monitor the isolated Ca-channel currents. The currents were activated at potentials more positive than -40 mV and were composed of at least four pharmacological distinct components being sensitive to nifedipine (35%), omega-conotoxin GVIA (10%), and omega-agatoxin IVA (42%) corresponding to L-, N-, and P-channel-mediated currents. The insensitive fraction (13%) possibly represented R channels. High potassium-evoked release of 3H-D-aspartate was used as a model of synaptic release. These studies were performed at relatively mild stimulation conditions (30 mM K+, 0.4 mM Ca2+), and 85% of the evoked release was Ca2+ dependent as well as tetrodotoxin and Cd2+ sensitive. Nifedipine and omega-agatoxin IVA dose dependently (IC50 values of 10 nM and 0.7 nM, respectively) blocked most of the release, whereas omega-conotoxin MVIIA (IC50 = 5 nM) caused partial blockage. The results indicate that several subtypes of voltage-sensitive Ca channels are present in mouse cerebellar granule cells. Furthermore, the data suggest that L, N, and P channels act in concert in the neurotransmitter release process.

Kainic acid, AMPA, and dihydrokainic acid effect on uptake and efflux of D-[3H] aspartic acid in cerebellar slices

In this study we show that the glutamate ionotropic agonist kainate (KA) stimulates the efflux of preloaded D-[3H]aspartate (D-[3H]Asp) and inhibits the uptake of this amino acid in cerebellar slices. The effect of this agonist on the efflux of D-[3H]Asp is sensitive to (i) 6-nitro-7-sulphamoylbenzo(f)quinoxaline-2-3-dione (NBQX), indicating the involvement of KA/(RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, and is (ii) partially tetrodotoxin (TTX)-sensitive, indicating that pre-(TTX-insensitive) and post-synaptic (TTX-sensitive) KA/AMPA receptors are involved. In contrast, the effect on uptake is NBQX- and TTX-insensitive indicating a direct interaction with glutamate transporters. AMPA inhibited D-[3H]Asp uptake and had no effect on D-[3H]Asp efflux. In the same system, the uptake but not the efflux of D-[3H]Asp was affected by dihydrokainate (DHK). The DHK-induced uptake inhibition occurred in the presence of TTX. NBQX inhibited DHK-induced effect at 5 mM but not at 1 mM DHK concentrations.

On-line detection of glutamate release from cultured chick retinospheroids

A continuous fluorometric assay was adapted to measure the release of endogenous glutamate from cultured chick retinospheroids. The results obtained with this technique are compared with the release of [3H]D-aspartate from monolayer cultures of chick retina cells. It is shown that although excitatory amino acids may be released in a Ca(2+)-dependent manner, most of the neurotransmitter release from cultured retina cells occurs by reversal of the glutamate transporter. The presence of extracellular Ca2+ may actually inhibit glutamate release by the cells present in the retinospheroids, or the [3H]D-aspartate release by cells in monolayers, when veratridine is the depolarizing agent.

Glutamate release evoked by glutamate receptor agonists in cultured chick retina cells: modulation by arachidonic acid

We studied the effect of ionotropic glutamate receptor agonists on the release of endogenous glutamate or of [3H]D-aspartate from reaggregate cultures (retinospheroids) or from monolayer cultures of chick retinal cells, respectively. Kainate increased the fluorescence ratio of the Na+ indicator SBFI and stimulated a dose-dependent release of glutamate in low (0.1 mM) Ca2+ medium, as measured using a fluorometric assay. Under the same experimental conditions, the release evoked by N-methyl-D-aspartate (NMDA; 400 microM) was about half of that evoked by the same kainate concentration; alpha-amino-3-hydroxy-5-methyl-4-isoxasolepropionic acid (AMPA; 400 microM) did not trigger a significant response. In the presence of 1 mM CaCl2, all of the agonists increased the [Ca2+]i, as determined with the fluorescence dye Indo-1, but the glutamate release evoked by NMDA and kainate was significantly lower than that measured in 0.1 mM CaCl2 medium. Inhibition by Ca2+ of the kainate-stimulated release of glutamate was partially reversed by the phospholipase A2 inhibitor oleiloxyethyl phosphorylcholine (OPC), suggesting that the effect was mediated by the release of arachidonic acid, which inhibits the glutamate carrier. Accordingly, kainate, NMDA, and AMPA stimulated a Ca(2+)-dependent release of [3H]arachidonic acid, and the direct addition of the exogenous fatty acid to the medium decreased the release of glutamate evoked by kainate in low (0.1 mM) CaCl2 medium. In monolayer cultures, we showed that NMDA, kainate, and AMPA also stimulated the release of [3H]D-aspartate, but in this case release in the presence of 1 mM CaCl2 was significantly higher than that evoked in media with no added Ca2+. The ranking order of efficacy for stimulation of Ca(2+)-dependent release of [3H]D-aspartate was NMDA > > kainate > AMPA.

Effectors of D-[3H]aspartate release from rat cerebellum

The effect of aminooxyacetic acid (AOAA), NH4+, phenylsuccinate (Phs), ketone bodies (KB) and glutamine (Gln), that might interfere with the biosynthesis of neurotransmitter glutamate on the K(+)-evoked Ca(2+)-dependent release of D-[3H]aspartate from rat cerebellar slices was studied. Therefore slices were preincubated in a Krebs-Ringer-bicarbonate-glucose (KR) buffer, loaded with D-[3H]aspartate and superfused in the presence of Ca2+ or when Ca2+ was replaced by Mg2+ or in some cases by EGTA. AOAA, NH4+ and Phs increase the K(+)-evoked Ca(2+)-dependent release of radioactivity by 30%, 68% and 188% compared to the control respectively indicating that these agents are inhibitors of the K(+)-evoked Ca(2+)-dependent release of glutamate. KB and Gln had no effect on the Ca(2+)-dependent release of radioactivity. AOAA, NH4+, Phs and KB but not Gln increase the total release of radioactivity by 43%, 69%, 139%, and 37% respectively. AOAA, NH4+ and KB but not Phs or Gln increase the Ca(2+)-independent release (Mg2+ replacing Ca2+) of radioactivity by 71%, 71% and 108% respectively. The present results indicate that in the cerebellum: 1) Neurotransmitter glutamate is mostly synthesized through the phosphate activated glutaminase (PAG) reaction 2) It is further supported that glutamate released in Ca(2+)-dependent manner before entering its pool in the cytosol has to move into the mitochondrial matrix.

Ca2+ influx through glutamate receptor-associated channels in retina cells correlates with neuronal cell death

We studied the effect of glutamate, N-methyl-D-aspartate (NMDA), kainate or K+ depolarization, on neurotoxicity in cultured chick retinal cells, under conditions in which we could discriminate between Ca2+ entering through ionotropic glutamate receptors and voltage-sensitive Ca2+ channels (VSCCs). When neurons were challenged with NMDA, kainate or glutamate, in Na(+)-containing medium, a decrease in cell survival was observed, whereas K+ depolarization did not affect the viability of the cells. The Mg2+ ion completely prevented the toxic effect mediated by the NMDA receptor, and had a small but significant protective effect at the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid/kainate (AMPA/kainate) receptor-induced cell death. We observed that, in a Na(+)-free N-methyl-D-glucamine (NMG) medium, to avoid the activation of VSCCs indirectly by the glutamate receptor agonists, stimulation of the glutamate receptors causes Ca2+ influx only through NMDA and AMPA/kainate receptor-associated channels, and that Ca2+ entry correlates well with subsequent cell death. These results show that the activation of NMDA or AMPA/kainate receptors can cause excitotoxicity in retinal neurons by mechanisms not involving Na+ influx, but rather depending on the permeation of Ca2+ through glutamate receptor-associated channels. For small Ca2+ loads the entry of Ca2+ through the NMDA receptor-associated channel was more efficient in triggering cell death than the influx of Ca2+ through the AMPA/kainate receptor.

Characterization of a chemical anoxia model in cerebellar granule neurons using sodium azide: protection by nifedipine and MK-801

Induction of chemical anoxia, using sodium azide in cerebellar granule cells maintained in primary culture, was evaluated as an in vitro assay for screening of potential neuroprotective compounds. The purpose of this study was to evaluate sodium azide as an alternative to cyanide salts, compounds which, despite their unfavorable characteristics, are often used in assays for chemical anoxia. The viability of neuronal cultures after treatment with azide, with or without preincubation with calcium channel blockers, tetrodotoxin (TTX), or glutamate receptor antagonists, was monitored by subsequent incubation with the tetrazolium dye MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide), followed by isopropanol extraction and spectrophotometric quantification of cellularly reduced MTT. The azide-induced degeneration of neurons was shown to be dependent on the concentration as well as on the duration of incubation with submaximal concentrations of azide. Incubation of the neurons with nifedipine, a blocker of L-type voltage-sensitive calcium channels (L-VSCC), or with the noncompetitive N-methyl-D-aspartate (NMDA) subtype glutamate receptor antagonist MK-801, prior to addition of submaximal concentrations of azide, significantly attenuated azide-induced neuronal death. Blockers of N-type and Q-type VSCC (omega-conotoxin MVIIA and MVIIC, respectively) and the P-type VSCC blocker omega-agatoxin IVA had no effect in this assay. The sodium channel blocker TTX was without effect when added to neurons under depolarizing conditions, but potently and effectively protected cells when experiments were performed in a nondepolarizing buffer. The results show that chemical anoxia induced by incubation of cultured neurons with azide leads to detrimental effects, which may be quantitatively monitored by the capability of the cells to reduce MTT. This procedure is a suitable method for screening of compounds for possible protective effects against neuronal death induced by energy depletion. In addition, the results suggest involvement of L-type VSCC as well as of glutamate receptors in the pathways leading to neuronal degradation induced by energy depletion in cerebellar granule neurons. This would further support the notion that these pathways might be important in neurodegeneration induced by cerebral ischemia or anoxia.

Glutamate receptor agonists evoked Ca(2+)-dependent and Ca(2+)-independent release of [3H]D-aspartate from cultured chick retina cells

We studied the release of [3H]D-aspartate evoked by glutamate receptor agonists from monolayer cultures of chick retina cells, and found that activation of the glutamate receptors can evoke both Ca(2+)-dependent and Ca(2+)-independent release of [3H]D-aspartate. In Ca(2+)-free (no added Ca2+) Na+ medium, the agonists of the glutamate receptors induced the release of [3H]D-aspartate with the following rank order of potency: kainate > alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) approximately N-methyl-D-aspartate (NMDA). In media containing 1 mM CaCl2 the release of [3H]D-aspartate evoked by NMDA, kainate and AMPA was increased by about 112 percent, 20 percent and 39 percent, respectively, as compared to the release evoked by the same agonists in Ca(2+)-free medium. NMDA was the most potent agonist in stimulating the Ca(2+)-dependent release of [3H]D-aspartate, possibly by exocytosis, and AMPA was as potent as kainate. The Ca(2+)-dependent release of [3H]D-aspartate evoked by kainate was dependent on the influx of Ca2+ through the receptor associated channel, as well as through the N-(omega-Conotoxin GVIA-sensitive) and L- (nitrendipine-sensitive) type voltage-sensitive Ca2+ channels (VSCC). The exocytotic release of [3H]D-aspartate evoked by AMPA relied exclusively on Ca2+ entry through the L-type VSCC, whereas the effect of NMDA was partially mediated by the influx of Ca2+ through the receptor-associated channel, but not through L- or N-type VSCC. Thus, activation of these different glutamate receptors under physiological conditions is expected to cause the release of cytosolic and vesicular glutamate, and the routes of Ca2+ entry modulating vesicular release may be selectively recruited.

Effect of K+- and kainate-mediated depolarization on survival and functional maturation of GABAergic and glutamatergic neurons in cultures of dissociated mouse cerebellum

The effect of the depolarizing agents, an elevated potassium concentration (25 mM) or kainic acid (50 microM) on neuronal survival and differentiation was investigated in cultures of dissociated neurons from cerebella of 7-day-old mice. When maintained in the presence of an antimitotic agent such cultures consist primarily of glutamatergic and GABAergic neurons. Cell survival was monitored by measurement of DNA, and differentiation by determining uptake and depolarization coupled release of glutamate (D-aspartate as label) and GABA. The depolarizing agents were added separately or together either from the start of the culture period (7-8 days) or at day 5 in culture. The main findings are that K+ depolarization is important for differentiation of glutamatergic neurons but not for GABAergic neurons. This depolarizing signal is important during the early phase of development in culture. For glutamatergic neurons, kainate may replace K+ as a depolarizing signal whereas in case of the GABAergic neurons, kainate was toxic particularly during the late phase of development. It was further observed that the glutamatergic neurons when maintained in a medium with 5 mM K+ during the first 5 days in culture became sensitive to kainate toxicity when this amino acid was added at day 5. This was not the case when the medium contained 25 mM K+ from the start of the culture period.

Differences in the release of L-glutamate and D-aspartate from primary neuronal chick cultures

Primary neuronal cultures were made from eight-day-old embryonic chick telencephalon. Ten-day-old cultures were used to study the release of D-[3H]aspartate and L-[3H]glutamate. The D[3H]aspartate release was stimulated by increasing potassium concentrations, but it was not calcium dependent. In contrast, the potassium dependent L-[3H]glutamate release was calcium dependent, and furthermore L-[3H]glutamate release was optimal at potassium concentrations < 30 mM. The inhibitors of glutamate uptake, dihydrokainate and 1-aminocyclobutane-trans-1,3-dicarboxylic acid (CACB), also referred to as cis-1 -aminocyclobutane-1,3-dicarboxylate, were used in the release experiments. Dihydrokainate had no effect on aspartate release, whereas CACB increased both the basal efflux of D-[3H]aspartate and the potassium evoked release. CACB had no effect on the potassium stimulated L-glutamate release. We believe that L-glutamate is released mainly by a vesicular mechanism from the presumably glutamatergic neurons present in our culture. D-aspartate release observed by us, could be mediated by a transporter protein. The cellular origin of this release remains to be assessed.

Striatal NMDA receptor subtypes: the pharmacology of N-methyl-D-aspartate-evoked dopamine, gamma-aminobutyric acid, acetylcholine and spermidine release

We have examined the inhibitory potencies of MK 801, memantine, dextromethorphan, Mg2+ and of strychnine-insensitive glycine site antagonists on the N-methyl-D-aspartate (NMDA)-evoked (300 microM) release of [14C]acetylcholine and [3H]spermidine or [14C] gamma-aminobutyric acid [14C]GABA and [3H]dopamine from rat striatal slices. MK 801, dextromethorphan and all glycine antagonists examined (7-chlorokynurenate, L-689,560 ((+/-)-trans-2-carboxy-5,7-dichlorotetrahydroquinoline-4-phenylure a), 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), 6,7-dichloroquinoxaline-2,3-dione (DNQX), and (+)-HA966 ((3-amino-1-hydroxypyrrolidin-2-one) more potently inhibited NMDA-evoked dopamine and GABA release than acetylcholine and spermidine release by a factor of 3-21. MgCl2, which does not inhibit NMDA-evoked spermidine release, and memantine which only weakly antagonised NMDA-evoked spermidine release, inhibited NMDA-evoked dopamine, acetylcholine and GABA release with similar potencies. No pharmacological differences were observed between NMDA-evoked dopamine and GABA release. These findings extend those suggesting that NMDA-evoked acetylcholine and spermidine release are mediated by different NMDA receptor subtypes in the striatum and suggest a third native subtype with a distinct pharmacology that regulates striatal dopamine and GABA release.

Taurine deficiency in dissociated mouse cerebellar cultures affects neuronal migration

The role of taurine in the process of neuronal migration was studied in a microwell cell culture system. Immunocytochemical analysis of the cellular composition of this culture system revealed the presence of the astrocytic marker GFAP in some structures such as the aggregates of neuronal bodies and in those cables used for migration, resembling what is described in vivo. The neuronal marker gamma-enolase stained practically all structures, including the aggregates and all cables. The intracellular taurine concentration was reduced by 60% in mouse cerebellar granule cells treated with a blocker of taurine transport, guanidinoethane sulfonate (GES). Under these conditions cell migration was markedly reduced to approximately 50% of that in untreated cultures. Both, taurine depletion and impairment of cell migration induced by GES were prevented by adding taurine to the culture medium. Taurine deficiency similarly affected different morphological parameters such as the number of cables suitable for neuronal migration as well as the number of migrating neurons. The number of aggregates of neuronal bodies was significantly increased, by about 30%, as a consequence of the reduced migration. Taurine alone did not exert any effect on the parameters evaluated. GES treatment of granule cells did not affect mitochondrial metabolism or K(+)-stimulated Ca(2+)-dependent [3H]-D-aspartate release. This suggests that the described effects of taurine deficiency were not due to an alteration of neuronal viability and that the action of GES was not simply due to unspecific and deleterious effects. These results are in agreement with those obtained in in vivo studies. This approach represents a useful model to investigate the role played by taurine in the process of neuronal migration.

Citrate modulates the regulation by Zn2+ of N-methyl-D-aspartate receptor-mediated channel current and neurotransmitter release

The effect of the two metal-ion chelators EDTA and citrate on the action of N-methyl-D-aspartate (NMDA) receptors was investigated by use of cultured mouse cerebellar granule neurons and Xenopus oocytes, respectively, to monitor either NMDA-evoked transmitter release or membrane currents. Transmitter release from the glutamatergic neurons was determined by superfusion of the cells after preloading with the glutamate analogue D-[3H]aspartate. The oocytes were injected with mRNA isolated from mouse cerebellum and, after incubation to allow translation to occur, currents mediated by NMDA were recorded electrophysiologically by voltage clamp at a holding potential of -80 mV. It was found that citrate as well as EDTA could attenuate the inhibitory action of Zn2+ on NMDA receptor-mediated transmitter release from the neurons and membrane currents in the oocytes. These effects were specifically related to the NMDA receptor, since the NMDA receptor antagonist MK-801 abolished the action and no effects of Zn2+ and its chelators were observed when kainate was used to selectively activate non-NMDA receptors. Since it was additionally demonstrated that citrate (and EDTA) preferentially chelated Zn2+ rather than Ca2+, the present findings strongly suggest that endogenous citrate released specifically from astrocytes into the extracellular space in the brain may function as a modulator of NMDA receptor activity. This is yet another example of astrocytic influence on neuronal activity.

Differences of the electrical and nicotinic receptor stimulation-evoked liberation of norepinephrine from chicken sympathetic neurons in culture: possible involvement of different pools of the transmitter

We studied the release of [3H]norepinephrine from chicken sympathetic neurons in culture evoked by nicotinic and electrical stimulation with an intention to establish functional identity or nonidentity of the two stimuli in investigations of neurotransmitter release. Nicotinic stimulation evoked extracellular calcium dependent release of [3H]norepinephrine and the rise of intracellular calcium concentration. The release was completely blocked by nicotinic antagonists hexamethonium (100 mumol/l) and mecamylamine (10 mumol/l), and decreased by tetrodotoxin (0.3 mumol/l) and omega-conotoxin (0.1 mumol/l) to 17% and 27%, resp. The intracellular calcium response was decreased by nicotinic antagonists and tetrodotoxin, but not changed by omega-conotoxin. The electrical stimulation-evoked release was blocked by both tetrodotoxin and omega-conotoxin, and decreased by previous electrical, but not nicotinic, stimulation. The differential sensitivity to omega-conotoxin and tetrodotoxin,and the inability of nicotinic stimulation to decrease the liberation by following electrical stimulation may suggest the mobilization of different pools of the transmitter.

Haloperidol reduces K(+)-evoked Ca(2+)-dependent D-[3H]aspartate release from rat hippocampal slices

Rat hippocampal slices preloaded with D-[3H]aspartate, a non metabolizable analogue of L-glutamate, were superfused with artificial CSF. Depolarization was induced by 53.5 mM K+, in the presence of Ca2+ (1.3 mM) or Mg2+ (5 mM) to determine the Ca2+ dependent release. Haloperidol added in the superfusion medium at 100 microM reduced by about 60% the Ca2+ dependent release of D-[3H]aspartate. This drug at 20 microM or 100 microM inhibited the non-activated glutamate dehydrogenase (GDH) but had no effect on GDH activated by ADP (2 mM) or leucine (5 mM). In addition no effect was observed on phosphate activated glutaminase (PAG) in the presence either of 20 mM or 5 mM phosphate. These results indicate that the effect of haloperidol is exerted on presynaptic mechanisms regulating neurotransmitter release.

Regulatory sites and effectors of D-[3H]aspartate release from rat cerebral cortex

To study the effect of agents interfering with the biosynthesis and/or the K(+)-evoked Ca(2+)-dependent release of neurotransmitter glutamate, rat cerebral slices were preincubated with Krebs-Ringer-HEPES-glucose-glutamine buffer (KRH buffer), loaded with D-[3H]aspartate and superfused with the preincubation medium in the presence or in the absence of Ca2+. The difference in radioactivity release divided by the basal release per min under the two conditions represented the K(+)-evoked Ca(2+)-dependent release. The agents used were: 1) Aminooxyacetic acid (AOAA), the inhibitor of transaminases, 2) Leucine (Leu), the inhibitor of phosphate activated glutaminase (PAG), 3) NH4+, the inhibitor of PAG, 4) Phenylsuccinic acid (Phs), the inhibitor of the mitochondrial ketodicarboxylate carrier, 5) ketone bodies, the inhibitors of glycolysis, 6) the absence of glutamine, the substrate of PAG. The results show that Leu, NH4+, Phs and the absence of Gln significantly increase the K(+)-evoked Ca(2+)-dependent release of radioactivity by 64%, 200%, 95% and 147% respectively, indicating that these agents are inhibitors of the K(+)-evoked Ca(2+)-dependent release of glutamate. Ketone bodies and AOAA had no effect. These results indicate that the major if not the exclusive biosynthetic pathway of neurotransmitter glutamate in rat cerebral cortex is through the PAG reaction and support a model for the pathway followed by neurotransmitter glutamate i.e. glutamate formed outside the inner mitochondrial membrane has to enter the mitochondrial matrix or is formed within it from where it can be extruded to supply the transmitter pool in exchange of GABA.

Role of GABAB receptors in intracellular Ca2+ homeostasis and possible interaction between GABAA and GABAB receptors in regulation of transmitter release in cerebellar granule neurons

The expression of GABAB receptors in cultured mouse cerebellar granule cells was investigated in binding experiments using [3H](S,R)-baclofen as well as in functional assessment of the ability of (R)-baclofen to interact with depolarization (15-40 mM KCl) coupled changes in intracellular Ca2+ homeostasis and neurotransmitter release. In the latter case a possible functional coupling between GABAA and GABAB receptors was investigated. The binding studies showed that the granule cells express specific binding sites for (R)-baclofen. The number of binding sites could be increased by exposure of the cells to the GABAA receptor agonist THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) during the culture period. Pretreatment of the neurons with pertussis toxin showed that the GABAB receptors are coupled to G-proteins. This coupling was, however, less pronounced when the cells had been cultured in the presence of THIP. When 45Ca2+ uptake was measured or the intracellular Ca2+ concentration ([Ca2+]i) determined using the fluorescent Ca2+ chelator Fluo-3 it could be demonstrated that culturing the neurons in THIP influences intracellular Ca2+ homeostasis. Moreover, this homeostasis was found to be functionally coupled to the GABAB receptors as (R)-baclofen inhibited depolarization-induced increases in 45Ca2+ uptake and [Ca2+]i. (R)-Baclofen also inhibited K(+)-induced transmitter release from the neurons as monitored by the use of [3H]D-aspartate which labels the neurotransmitter pool of glutamate. Using the selective GABAA receptor agonist isoguvacine it could be demonstrated that the GABAB receptors are functionally coupled to GABAA receptors in the neurons leading to a disinhibitory action of GABAB receptor agonists.

omega-Conotoxin binding sites and regulation of transmitter release in cerebellar granule neurons

The protective action of Ca2+ and a series of other divalent cations on heat inactivation (48 degrees C, 30 min) of [125I]omega-conotoxin (CTX) binding sites was investigated in membranes prepared from rat forebrain. Moreover, the influence of GABA (500 microM) on this protection was studied. Binding of [125I]CTX as well as its inhibitory action on K+ (55 mM) stimulated, Ca(2+)-dependent transmitter release were studied in rat cerebellar granule neurons cultured in the presence or absence of the GABAA receptor against THIP (4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol). In cells cultured in the presence of THIP (150 microM) it was investigated whether the ability of THIP to inhibit evoked transmitter release could be influenced by CTX. Ca2+ and other divalent cations could effectively protect against heat inactivation of [125I]CTX binding sites in rat forebrain membranes, but this protective action was not influenced by the presence of 500 microM GABA. The cultured cerebellar granule neurons exhibited specific binding sites for [125I]CTX, the number of which was independent of exposure of the cells to THIP during the culture period. Evoked transmitter release was inhibited by CTX with an IC50 value of 13 nM. In neurons cultured in the presence of 150 microM THIP, THIP could dose-dependently inhibit evoked transmitter release, but this inhibitory action was not influenced by CTX (20 nM). The results show that cerebellar granule neurons exhibit functionally meaningful CTX binding sites. An association between such sites and GABA receptors is not apparent.

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.

Identification and function of glycine receptors in cultured cerebellar granule cells

Poly(A)+ mRNA was isolated from cultured mouse cerebellar granule cells and injected into Xenopus oocytes. This led to the expression of receptors that evoked large membrane currents in response to glycine. Current-responses were also obtained after application of beta-alanine and taurine, but these were very low relative to that of glycine (maximal beta-alanine and taurine responses were 8 and 3% of that of glycine, respectively). The role of glycine receptors on K(+)-evoked transmitter release in cultured cerebellar granule cells was also assayed. Release of preloaded D-[3H]aspartate evoked by 40 mM K+ was dose dependently inhibited by glycine, and the concentration producing half-maximal inhibition was 50 microM. Taurine, beta-alanine, and the specific GABAA receptor agonist isoguvacine also inhibited K(+)-evoked release, and the maximal inhibition was similar for all agonists (approximately 40%). The EC50 value was 200 microM for taurine, 70 microM for beta-alanine, and 4 microM for isoguvacine. Bicuculline (150 microM) antagonized the inhibitory effect of isoguvacine (150 microM) but not that of glycine (1 mM). In contrast, strychnine (20 microM) antagonized the inhibitory effect of glycine (1 mM) but not that of isoguvacine (150 microM). The pharmacology of the responses to beta-alanine and taurine showed that these agonists activate both glycine and GABAA receptors. The results indicate that cultured cerebellar granule cells translate the gene for the glycine receptor and that activation of glycine receptors produces neuronal inhibition.

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.

A developmental study of lactate dehydrogenase isozyme and aspartate aminotransferase activity in organotypic rat hippocampal slice cultures and primary cultures of mouse neocortical and cerebellar neurons

The development of enzyme activity and isozyme distribution of lactate dehydrogenase (LDH) was studied in murine organotypic hippocampal slice cultures and dissociated cultures of neocortical neurons and cerebellar granule cells and compared with that of the respective brain regions in vivo. In the hippocampal slice cultures and the hippocampus in vivo, the activity of aspartate aminotransferase (AAT) was also measured. During development in culture the specific activity of LDH increased in all types of cultures reaching values similar to that found in the corresponding brain areas in vivo. However, significant differences in the isozyme distribution were observed between the preparations in vitro and in vivo. During development in vivo, the LDH isozyme pattern changed from a preferential M-subunit composition to a preferential H-subunit composition regardless of the brain area. This shift was not observed in the respective cultures where the M4-isozyme prevailed at all culture periods examined accounting for 30-45% of the total LDH activity. The cultured cerebellar granule cells did not express the H4-isozyme at all, while in the hippocampal slice cultures and the cultured neo-cortical neurons this isozyme accounted for about 5% of the total LDH activity. The activity of AAT in the hippocampal organotypic slice cultures increased considerably during the culture period in parallel with the increase in AAT activity during postnatal development of hippocampus in vivo. The activity of AAT in the slice cultures was, however, consistently lower than the corresponding activity in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Excitatory amino acid-mediated cytotoxicity and calcium homeostasis in cultured neurons

A large body of evidence suggests that disturbances of Ca2+ homeostasis may be a causative factor in the neurotoxicity induced by excitatory amino acids (EAAs). The route or routes by which an increase in intracellular calcium concentration ([Ca2+]i) is mediated in vivo are presently not clarified. This may partly reflect the complexity of intact nervous tissue in combination with the relative unspecific action of the available "calcium antagonists," e.g., blockers of voltage-sensitive calcium channels. By using primary cultures of cortical neurons as a model system, it has been found that all EAAs stimulate increases in [Ca2+]i but via different mechanisms. By using the drug dantrolene, it has been shown that 2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)propionate (AMPA) apparently exclusively stimulates Ca2+ influx through agonist-operated calcium channels and voltage-operated calcium channels. Increased [Ca2+]i due to exposure to kainate (KA) is for the major part caused by influx, as in the case of AMPA, but a small part of the increase in [Ca2+]i may be attributed to a release of Ca2+ from intracellular stores. Quisqualate (QA) stimulates Ca2+ release from an intracellular store that is independent of Ca2+ influx; presumably this store is activated by inositol phosphates. The increase in [Ca2+]i due to exposure to glutamate or N-methyl-D-aspartate (NMDA) may be compartmentalized into three components, one of which is related to influx and the other two to Ca2+ release from internal stores. Only one of the latter stores is dependent on Ca2+ influx with regard to release of Ca2+, whereas the other is activated by some other second messengers or, alternatively, directly coupled to the receptor. In muscles dantrolene is known to inhibit Ca2+ release from the sarcoplasmic reticulum, and also in neurons dantrolene inhibits an equivalent release from one or more hitherto unidentified internal Ca2+ pool(s). By using this drug it has been possible to show to what extent these Ca2+ stores are involved in the toxicity observed subsequent to exposure to the EAAs. It turned out that dantrolene, even under conditions allowing Ca2+ influx, inhibited toxicity induced by QA, NMDA, and glutamate, whereas that induced by AMPA or KA was unaffected. In combination with the findings that dantrolene inhibited release from the intracellular stores activated by QA, NMDA, and glutamate, it may be concluded that Ca2+ influx per se is not the primary event causing toxicity following exposure to these EAAs in these neurons. However, it may certainly be involved in the cases of toxicity induced by AMPA and KA.(ABSTRACT TRUNCATED AT 400 WORDS)