Glutamate release from cerebellar granule cells differentiating in culture: Modulation of the K+-stimulated release by inhibitory amino acids

Changes in the striatal extracellular levels of dopamine and dihydroxyphenylacetic acid evoked by ammonia and N-methyl-D-aspartate: modulation by taurine

Acute hyperammonemia is associated with motor disturbances that are thought to involve striatal dopaminergic dysfunction. Discharge of striatal dopaminergic neurons is controlled by N-methyl-D-aspartate (NMDA) receptors, the excessive activation of which contributes to ammonia neurotoxicity. Here we show that ammonium chloride ("ammonia", extracellular concentration 5 mM) or NMDA (1 mM), when directly administered to the rat striatum via a microdialysis probe, evoke a prompt accumulation of dopamine (DA) in the microdialysates. However, while ammonia increases, NMDA decreases, the extracellular dihydroxyphenylacetate (DOPAC) level. The results point to the NMDA receptor-mediated enhancement of DA release and increased DA metabolism as two independent ways by which ammonia affects the striatal dopaminergic system. Taurine (extracellular concentration 10 mM) attenuated the NMDA- and ammonia-evoked DA release and ammonia-induced accumulation of DOPAC, reflecting two different neuroprotective mechanisms of this amino acid.

Enhanced taurine release in cultured cerebellar granule cells in cell-damaging conditions

The release of taurine from cultured cerebellar granule neurons was studied in different cell-damaging conditions, including hypoxia, hypoglycemia, ischemia, oxidative stress and in the presence of free radicals. The effects of both ionotropic and metabotropic glutamate receptor agonists on the release were likewise investigated. The release of [3H]taurine from the glutamatergic granule cells was increased by K+ (50 mM) and veratridine (0.1 mM), the effect of veratridine being the greater. Hypoxia and ischemia produced an initial increase in release compared to normoxia but resulted in a diminished response to K+. Hypoglycemia, oxidative stress and free radicals enhanced taurine release, and subsequent K+ treatment exhibited a correspondingly greater stimulation. A common feature of taurine release in all the above conditions was a slow response to the stimulus evoked by K+ and particularly to that evoked by veratridine. All ionotropic glutamate receptor agonists potentiated taurine release, but only the action of kainate seemed to be receptor-mediated. Metabotropic receptor agonists of group I slightly stimulated the release. The prolonged taurine release seen in both normoxia and cell-damaging conditions may be of importance in maintaining homeostasis in the cerebellum and reducing excitability for a longer period than other neuroprotective mechanisms.

Interference of S-alkyl derivatives of glutathione with brain ionotropic glutamate receptors

The effects of glutathione, glutathione sulfonate and S-alkyl derivatives of glutathione on the binding of glutamate and selective ligands of ionotropic N-methyl-D-aspartate (NMDA) and non-NMDA receptors were studied with mouse synaptic membranes. The effects of glutathione and its analogues on 45Ca2+ influx were also estimated in cultured rat cerebellar granule cells. Reduced and oxidized glutathione, glutathione sulfonate, S-methyl-, -ethyl-, -propyl-, -butyl- and -pentylglutathione inhibited the Na+-independent binding of L-[3H]glutamate. They strongly inhibited also the binding of (S)-2-amino-3-hydroxy-5-[3H]methyl-4-isoxazolepropionate [3H]AMPA (IC50 values: 0.8-15.9 microM). S-Alkylation of glutathione rendered the derivatives unable to inhibit [3H]kainate binding. The NMDA-sensitive binding of L-[3H]glutamate and the binding of 3-[(R)-2-carboxypiperazin-4-yl][1,2-(3)H]propyl-1-phosphonate ([3H]CPP, a competitive antagonist at NMDA sites) were inhibited by the peptides at micromolar concentrations. The strychnine-insensitive binding of the NMDA coagonist [3H]glycine was attenuated only by oxidized glutathione and glutathione sulfonate. All peptides slightly enhanced the use-dependent binding of [3H]dizocilpine (MK-801) to the NMDA-gated ionophores. This effect was additive with the effect of glycine but not with that of saturating concentrations of glutamate or glutamate plus glycine. The glutamate- and NMDA-evoked influx of 45Ca2+ into cerebellar granule cells was inhibited by the S-alkyl derivatives of glutathione. We conclude that besides glutathione the endogenous S-methylglutathione and glutathione sulfonate and the synthetic S-alkyl derivatives of glutathione act as ligands of the AMPA and NMDA receptors. In the NMDA receptor-ionophore these glutathione analogues bind preferably to the glutamate recognition site via their gamma-glutamyl moieties.

The release of glutamate and aspartate from rat brain synaptosomes in response to domoic acid (amnesic shellfish toxin) and kainic acid

Kainic acid is known to stimulate the release of glutamate (GLU) and aspartate (ASP) from presynaptic neurons. It has been suggested that the enhanced release of these endogenous EAA's plays a significant role in the excitotoxic effects of KA. Domoic acid (DOM), a shellfish toxin, is structurally similar to KA, and has been shown to be 3-8 times more toxic than KA. In this study, effects of KA and DOM on the release of GLU and ASP from rat brain synaptosomes were investigated. Amino acid analysis was performed by the reversed phase HPLC, following derivatization with 9-fluorenylmethyl chloroformate (FMOC). Potassium chloride (40 mM) was used as a positive control, and stimulated GLU release from rat brain synaptosomes in presence or absence of Ca2+. DOM enhanced the release of GLU, whereas KA stimulated the release of both GLU and ASP from synaptosomes in the presence of Ca2+. However, their potency to stimulate GLU and ASP release was enhanced in absence of Ca2+. These results indicate that different mechanisms may be involved in the release of GLU and ASP in response to DOM and KA, and that neurotransmitter release appeared to be highly specific for these agonists. It would appear that DOM and KA may interact with different receptors on the presynaptic nerve terminal, and/or activate different subtypes of voltage-dependent Ca2+ channels to promote influx of Ca2+ which is targeted for different pools neurotransmitters.

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.

Regulation of D-aspartate release by glutamate and GABA receptors in cerebral cortical slices from developing and ageing mice

The basal release of D-[3H]aspartate, an unmetabolized analogue of glutamate, from cerebral cortical slices remained at the same level from three-day-old to 24-month-old mice, but the response to K+ stimulation (50 mM) was smaller in young than in adult or aged mice. Kainate, N-methyl-D-aspartate and quisqualate (0.1 mM) stimulated the basal release of D-aspartate in the cerebral cortex of seven-day-old mice, the effects of kainate and N-methyl-D-aspartate being reduced by their antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and dizocilpine maleate, respectively, indicating that in the immature cerebral cortex the kainate and N-methyl-D-aspartate types of the glutamate receptor are involved in the basal release. The K(+)-stimulated release was not affected by glutamate agonists in developing mice, though they markedly attenuated the evoked release in adults. The inhibitory amino acids GABA, taurine and glycine depressed the K(+)-stimulated release only in the adult cerebral cortex. The action of GABA was abolished by bicuculline, demonstrating the involvement of presynaptic GABAA receptors. The glycine effect was strychnine-insensitive, characteristic of the glycine modulatory site in the N-methyl-D-aspartate receptor. This kind of regulation by both kainate and N-methyl-D-aspartate receptors could be of physiological significance, particularly in the immature cerebral cortex.

Interactions of gamma-L-glutamyltaurine with excitatory aminoacidergic neurotransmission

The in vitro effects of gamma-L-glutamyltaurine on different stages of excitatory aminoacidergic neurotransmission were tested with gamma-D-glutamyltaurine as reference. gamma-L-Glutamyltaurine enhanced the K(+)-stimulated release of [3H]glutamate from cerebral cortical slices (25% at 0.1 mM) and slightly inhibited the uptake by crude brain synaptosomal preparations (about 10% at 1 mM). gamma-L-Glutamyltaurine was also a weak displacer of glutamate and its agonists from their binding sites in brain synaptic membrane preparations, being, however, less selective to quisqualate (QA) sites than gamma-D-glutamyltaurine. The basal influx of Ca2+ into cultured cerebellar granular cells was not affected by 1 mM gamma-L-glutamyltaurine, but the glutamate- and its agonist-activated influx was significantly inhibited in low-Mg2+ (0.1 mM) and Mg(2+)-free media. The glutamate-evoked increase in free intracellular Ca2+ and the kainate-activated formation of cGMP in cerebellar slices were both markedly inhibited by 0.1 mM gamma-L-glutamyltaurine. We propose that gamma-L-glutamyltaurine may act as endogenous modulator in excitatory aminoacidergic neurotransmission.

Pharmacological characterization of glutamate binding sites in cultured cerebellar granule cells and cortical astrocytes

Membranes prepared from cerebellar granule cells and cortical astrocytes exhibited specific, saturable binding of L-[3H]glutamate. The apparent binding constant KD was 135 nM and 393 nM and the maximal binding capacity Bmax 42 and 34 mumol/kg in granule cells and astrocytes, respectively. In granule cells the binding was strongly inhibited by the glutamate receptor agonists kainate, quisqualate, N-methyl-D-aspartate (NMDA), L-homocysteate and ibotenate, and the antagonist DL-5-aminophosphonovalerate. In astrocytes, only quisqualate among these was effective. L-Aspartate, L-cysteate, L-cysteinesulphinate and gamma-D-glutamylglycine were inhibitors in both cell types. The binding was totally displaced in both cell types by L-cysteinesulphinate with IC50 in the micromolar range. In astrocytes the binding was also totally displaced by quisqualate, but in granule cells only partially by NMDA, kainate and quisqualate in turn. It is concluded from the relative potencies of agonists and antagonists in [3H]glutamate binding that cerebellar granule cells express the NMDA, kainate and quisqualate types of the glutamate receptor, while only the quisqualate-sensitive binding seems to be present in cortical astrocytes.

Long-term GABA treatment elicits supersensitivity of quisqualate-preferring metabotropic glutamate receptor in cultured rat cerebellar neurons

In primary cultures of rat cerebellar granule neurons, GABA treatment (50 microM, 7 days) caused a withdrawal supersensitivity selective for the metabotropic glutamate receptors that mainly prefer L-glutamate, quisqualate and, to a lesser extent, kainate. The withdrawal supersensitivity was absent when 10 microM SR-95531 was coadministered with GABA during the treatment period, an event that suggests the GABAA receptors primarily produced the GABA treatment effect. This was supported further by the inability of baclofen treatment to mimic completely the treatment effect of GABA. Withdrawal from 7 days of baclofen treatment only produced a slight increase in the metabotropic effect of L-glutamate and carbachol. In addition, in untreated neurons, baclofen had no acute effect, whereas GABA inhibited the effect of L-glutamate and carbachol. The inhibitory effect of GABA was reversed by SR-95531 and was absent in neurons treated with GABA. These observations suggest the involvement of GABAA receptors and the apparent development of tolerance to GABA, respectively. Also, dependence on GABA may have occurred; the metabotropic effects of glutamate, kainate, and quisqualate were not altered in neurons maintained with GABA treatment.

Glutathione modulates the N-methyl-D-aspartate receptor-activated calcium influx into cultured rat cerebellar granule cells

The effects of reduced (GSH) and oxidized (GSSG) glutathione and dithiothreitol (DTT) and L-cysteine on the influx of 45Ca2+ were studied with cultured cerebellar granule cells. DTT slightly enhanced the basal influx but strongly activated the influx stimulated by glutamate or N-methyl-D-aspartate (NMDA). The effects on the kainate- or quisqualate-induced influx were less pronounced. Extracellular GSH had no effect on the basal influx of Ca2+. A concentration of 0.5 mM GSH slightly activated the glutamate- and NMDA-induced influx while GSSG was inhibitory. The enhancement by DTT and cysteine of the responses to excitatory amino acids was attenuated by GSH and GSSG. We propose that both the accessibility and redox state of the functional sulfhydryl groups in NMDA receptor-ionophores may be regulated by endogenous glutathione. These effects are attributed to the gamma-glutamyl moiety and sulfhydryl group in the tripeptide molecule.

Uptake and release of beta-alanine in cerebellar granule cells in primary culture: regulation of release by glutamatergic and GABAergic receptors

The uptake and release of beta-[3H]alanine were studied in cultured glutamatergic cerebellar granule cells of the rat. The uptake of beta-alanine was saturable and sodium-dependent, comprising one high-affinity transport component. It was inhibited by hypotaurine, taurine, GABA and homotaurine but not by glycine or glutamate. The release was enhanced by homoexchange, veratridine and high K+ concentrations (50 mM). The K(+)-stimulated release was at least partially Ca(2+)-dependent. The release was shown to be subject to regulation by GABAA receptors and glutamate receptors of the kainate type. The results signify that beta-alanine may have a functional role in cerebellar granule cells.

Effect of magnesium on calcium influx activated by glutamate and its agonists in cultured cerebellar granule cells

The effects of Mg2+ on the glutamate-, kainate-, N-methyl-D-aspartate- and quisqualate-induced influx of 45Ca2+ were studied in cultured cerebellar granule cells. The N-methyl-D-aspartate- and quisqualate-evoked influx was totally and the kainate- and glutamate-evoked influx partially blocked in 1.3 mM extracellular Mg2+. The increase in influx induced by kainate, quisqualate and glutamate was maximal at 0.1 mM Mg2+, whereas N-methyl-D-aspartate was most effective in totally Mg(2+)-free media. D-2-Amino-5-phosphonovalerate blocked partially and phencyclidine completely the enhancement of Ca2+ influx by 1 mM quisqualate in 0.1-mM Mg2+ medium. The effect of 10 microM quisqualate was also significantly inhibited by antagonists specific for different glutamate receptor subtypes, including N-methyl-D-aspartate, (RS) alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate and metabotropic receptors. This evidences a heterogeneous action of quisqualate, mediated by different glutamate receptor subtypes in 0.1 mM Mg2+ medium. The efficacy of quisqualate in inducing influx of Ca2+ and the selectivity of antagonists for different receptors are also modified by extracellular Mg2+.

Modulation of glutamate agonist-induced influx of calcium into neurons by gamma-L-glutamyl and beta-L-aspartyl dipeptides

Gamma-L-Glutamate and beta-L-aspartate dipeptides, present in the mammalian brain with a yet unknown function, were shown to affect the influx of Ca2+ into cultured cerebellar granule cells. The most active peptides, gamma-L-glutamyl-L-aspartate, gamma-L-glutamyl-L-glutamate and gamma-L-glutamylglycine, enhanced the basal influx but inhibited the glutamate-activated influx of Ca2+ in a dose-dependent manner. Gamma-L-Glutamyl-L-aspartate, the strongest inhibitor of the glutamate-activated influx of Ca2+, exhibited selective Mg(2+)-dependent antagonism in the N-methyl-D-aspartate (NMDA)-activated influx of Ca2+. This finding may explain its previously shown deleterious effects on the long-term memory. On the other hand, gamma-L-glutamyl-L-aspartate enhanced alone the entry of Ca2+ into neurons. This effect was antagonized by the non-NMDA antagonists 6-nitro-7-cyanoquinoxaline-2,3-dione (CNQX) and 6,7-dinitroquinoxaline-2,3-dione (DNQX), suggesting a non-NMDA receptor-mediated action, that may also be involved in excitotoxicity in some neurodegenerative disorders.

Paradoxical potentiation by low extracellular Ca2+ of acute chemical anoxic neuronal injury in cerebellar granule cell culture

Acute chemical anoxic injury was produced in primary cerebellar granule cell cultures incubated with iodoacetate (IAA) alone or IAA combined with potassium cyanide (KCN). Cytotoxicity was assessed using Trypan blue exclusion or LDH release. Four millimolars of KCN induced approx 30% neuron death at 3 h, whereas greater than 50% cell death was produced by 0.2 mM IAA. No potentiation of cytotoxicity was observed by IAA + KCN. A total of 0.2 mM IAA produced an early major reduction of intracellular ATP prior to the onset of neuron injury or reduction in intracellular glutathione (GSH). Medium Na+ replacement by choline, K+, or methylglucamine protected against IAA-induced neuronal injury, reduced the rate of decline of intracellular ATP but had no effect on intracellular GSH. Some 80% neuronal survival was obtained when Na+ was deleted from the medium even after the intracellular ATP had been reduced to less than 10% of control. Removal of Ca2+ from the medium had no effect on control culture, Trypan blue exclusion, GSH, or ATP, but potentiated the onset and magnitude of IAA-induced cytotoxicity. ATP and GSH decline. Loading of granule cells with the Ca2+ chelator Fura-2 did not influence IAA-induced cytotoxicity in control or low Ca2+ media. Addition of 50 microM glutamate had a minimal cytotoxic effect over 3 h and the combined addition of 0.2 mM IAA plus 50 microM glutamate did not potentiate IAA-induced injury. The glutamate receptor antagonists, D-2-amino-5-phosphonovaleric acid (APV) or kynurenate did not block IAA-induced injury in control medium but inhibited the potentiation of toxicity seen in the low Ca2+ medium. This study suggests the use of IAA as a chemical anoxic agent in cerebellar granule cell culture. The early, dose-dependent decline in ATP may be dissociated from GSH change. Acute IAA-induced injury is Na+/Cl- dependent but paradoxically potentiated in low Ca2+ medium. The low Ca2+ potentiated component was sensitive to glutamate/NMDA receptor antagonists and associated with reduction of intracellular GSH.

Effects of inhibitors of protein synthesis and intracellular transport on the gamma-aminobutyric acid agonist-induced functional differentiation of cultured cerebellar granule cells

The effect of inhibitors of protein synthesis (actinomycin D, cycloheximide), proteases (leupeptin), and intracellular transport (colchicine, monensin) on the gamma-aminobutyric acid (GABA) agonist [4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP)]-induced changes in morphological differentiation and GABA receptor expression was investigated in cultured cerebellar granule cells. After 4 days in culture the neurons were exposed to the inhibitors for 6 h in the simultaneous presence of THIP. Subsequently, cultures were either fixed for electron microscopic examination or used for preparation of membranes for [3H]GABA binding assays. In some experiments the functional activity of the newly induced low-affinity GABA receptors was assessed by investigation of the ability of GABA to inhibit neurotransmitter release from the neurons. These experiments were performed to differentiate between an intracellular and a plasma membrane localization of the receptors. In all experiments cultures treated with THIP alone served as controls. The inhibitors of protein synthesis totally abolished the ability of THIP to induce low-affinity GABA receptors. In contrast, the inhibitors of intracellular transport as well as the protease inhibitor did not affect this parameter. However, studies of effects of GABA on transmitter release from monensin-treated cultures showed that transmitter release could not be inhibited by GABA in these cells in spite of the presence of low-affinity GABA receptors in the membrane preparations. This indicates that the low-affinity receptors were not located in the plasma membrane. This is in good agreement with the corresponding morphological findings, that monensin treatment led to an intense vacuolization of the Golgi apparatus, thereby preventing intracellular transport of the newly synthesized GABA receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Low calcium-induced release of glutamate results in autotoxicity of cerebellar granule cells

Primary cultures of cerebellar rat granule neurons were grown for 18-22 days in vitro in the absence of antibiotics. When the cultures were placed in a low calcium (no EGTA) balanced salt solution at room temperature, rapid cell death occurred usually within 30 min of placing cells in the buffer. Changes in the cells were evident within 10 min and included an apparent cellular granulation with a partial loss of cell body birefringence at 10 x magnification which was complete by 30 min. This rapid death was prevented by (1) replacing chloride in the buffer with acetate; (2) increasing the osmolarity of the buffer by 30% with sucrose; (3) the addition of the selective excitatory amino acid (EAA) antagonist, 2-amino-7-phosphonoheptanoic acid (APH, 200 microM) but not by the selective kainate-quisqualate antagonist, glutamylaminomethylsulfonic acid (GAMS, 400 microM); or (4) the addition of one of the following calcium channel antagonists, verapamil (400 microM) diltiazem (150 microM) or lanthanum (5 microM). Placing cells in low calcium buffer resulted in a 3.7- and 3.2-fold increase in the non-selective secretion of aspartate and glutamate (as well as other amino acids) over baseline secretion (same buffer except containing 2.5 mM calcium). This increase was partially prevented by verapamil, but not by APH or chloride deletion. Verapamil only partially prevented the efflux of glutamate in buffer containing 1 mM EGTA. These results indicate that placing cells in low calcium buffer results in neurotoxicity secondary to both the influx of chloride and water in conjunction with the efflux of amino acids, some of which stimulate an excitatory amino acid receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship of GABAa receptor heterogeneity to regional differences in drug response

Molecular biological approaches to the GABAa receptor have resulted in new insights into the structure and pharmacology of this complex. It is known that the GABAa complex is a hetero-oligomer composed of multiple subunits which contain binding sites for the GABA, benzodiazepines and barbiturates. These subunits also contain regulatory sites for phosphorylation by intracellular kinases. There appear to be regional differences in the expression of the various subunits for the GABAa receptor complex. The functional significance of molecular heterogeneity is not yet known but it is expected that regional differences may result in pharmacologically diverse responses. Studies on the effects of chronic administration of diazepam have clearly delineated such regional differences. Chronic benzodiazepine administration results in the development of subsensitivity to the electrophysiological actions of GABA in the dorsal raphe, but not in GABA receptive neurons of the substantia nigra pars reticulata. Such data is consistent with regional heterogeneity in response to chronic benzodiazepine exposure. It is hoped that by understanding GABAa receptor heterogeneity, and its molecular basis, we can improve the existing receptor subtype specificity and pharmacology of the benzodiazepines.

D-aspartate release from cerebellar astrocytes: modulation of the high K-induced release by neurotransmitter amino acids

The properties of D-aspartate release were studied in cerebellar astrocytes (14-15 DIV) in primary cultures in the rat. The spontaneous release of D-aspartate from astrocytes was fast, being further enhanced in Na- and Ca-free (EDTA-containing) media. Kainate, quisqualate, D-aspartate and L-glutamate stimulated the release, whereas L-glutamatediethylester was inhibitory. The release was enhanced by veratridine and high K (50 mM). Substitution of chloride by acetate in the experimental medium did not change the basal release but slightly decreased the potassium-induced release, indicating that the high K-induced D-aspartate release is primarily due to depolarization of cells. The K-stimulated release was independent of extracellular Ca2+ and potentiated by kainate and quisqualate. The effect of kainate was reduced by kynurenate, and that of quisqualate by L-glutamatediethylester. Glycine, taurine and GABA were equally effective in depressing the stimulated release of D-aspartate. The inhibition of GABA could be blocked by GABA antagonists. The results suggest that inhibitory amino acids may be involved in the regulation of glutamate release from cerebellar astrocytes. A further implication is that cerebellar astrocytes possess functional glutamate receptors of kainate and quisqualate subtypes.

Uptake and release of glycine in cerebellar granule cells and astrocytes in primary culture: potassium-stimulated release from granule cells is calcium-dependent

The properties of [3H]glycine uptake and release were studied with cerebellar granule cells, 7-9 days in vitro, (DIV) and astrocytes, 14-15 DIV, in primary cultures. The uptake of glycine in both cell types consisted of a saturable high-affinity transport and nonsaturable diffusion. The transport constant (Km) and maximal velocity (V) were significantly higher in granule cells than in astrocytes. Uptake was strictly Na+-dependent and also markedly diminished in low-Cl medium. The specificity of the uptake was similar in both cell types. The spontaneous release of glycine from granule cells and astrocytes was fast. Homoexchange with extracellularly added glycine in granule cells suggests that the efflux is at least partly mediated via membrane transport sites in these cells. Kainate stimulated the release more effectively in neurons than in glial cells, the effect apparently being mediated by specific kainate-sensitive receptors in both cell types. The release was enhanced by veratridine and by depolarization of cell membranes by high K (50 mM) in both neurons and astrocytes. The potassium-stimulated release was partially Ca-dependent in neurons but Ca-independent in glial cells. The results suggest a functional role for glycine in both cerebellar astrocytes and glutamatergic granule cells.

Effects of taurine on cell morphology and expression of low-affinity GABA receptors in cultured cerebellar granule cells

Effects of taurine and THIP were studied on the development of cultured cerebellar granule cells with regard to GABA receptor expression and morphological development. Culturing in the presence of taurine or THIP led to the formation of low affinity GABA receptors as revealed from Scatchard analysis of [3H]GABA binding. This formation of receptors was susceptible to inhibition upon culturing in the simultaneous presence of taurine and bicuculline demonstrating the involvement of the high affinity GABA receptors which are present on the cells regardless of the culture condition. Superfusion experiments on cells cultured under the different conditions demonstrated that the low affinity GABA receptors expressed after culturing in the presence of THIP or taurine mediated an inhibition by GABA of evoked transmitter release from the granule cells. Cells cultured in either plain culture media or in the presence of taurine were indistinguishable with respect to the number of neurite extending cells observed after 4 days in culture. In contrast, culturing in the presence of THIP increased the number of neurite extending cells by 8% relative to the controls.

Baclofen-induced, calcium-dependent stimulation of in vivo release of D-[3H]aspartate from rat hippocampus monitored by intracerebral microdialysis

The release of D-[3H]aspartate (used as a tracer for endogenous glutamate and aspartate) was studied at high K+ (100 mM) and under ischemia in rats implanted with 0.3 mm diameter dialysis tubing through the hippocampus. The effect on the D-[3H]aspartate release of the two gamma-aminobutyric acid (GABA) agonists 4,5,6,7-tetrahydroisoxazolo[5,4-c]-pyridin-3-ol (THIP) and (+/-)-beta-(p-chlorophenyl)GABA (baclofen), which specifically activate GABAA and GABAB receptors, respectively, was studied. Initial experiments employing HPLC analysis showed a coincident increase in the amounts of glutamate, aspartate and the amount of radioactivity following introduction of K+ (100 mM) or a period of ischemia suggesting that the D-[3H]aspartate labels the transmitter pools of the two amino acids under the present experimental conditions. The presence of 10 mM baclofen or 10 mM THIP in the perfusion medium did not inhibit ischemia induced D-[3H]aspartate release. On the contrary, 10 mM baclofen alone (but not 0.1 or 1 mM) in the perfusion medium induced release of D-[3H]aspartate in a calcium dependent manner, whereas 10 mM THIP had no significant releasing effect.

Glutamate receptor agonists increase intracellular Ca2+ independently of voltage-gated Ca2+ channels in rat cerebellar granule cells

Changes in membrane potential and cytosolic free Ca2+ concentrations, [Ca2+]i, in response to L-glutamate and glutamate receptor agonists were measured in rat cerebellar granule cells grown on coverslips. The membrane was depolarized by the application of L-glutamate and kainate, and by elevating the extracellular K+ concentration, as determined by using the membrane potential probe bisoxonol (DiBA-C4-(3)). The [Ca2+]i as measured with fura-2 was 220 nM on average under resting conditions and increased by raising the extracellular K+ and by applying L-glutamate, kainate, quisqualate or N-methyl-D-aspartate (NMDA). Verapamil and nifedipine reduced the high-K+ induced rise in [Ca2+]i but did not significantly affect the responses produced by NMDA, quisqualate and kainate, suggesting that the increase in intracellular Ca2+ in response to glutamate receptor agonists is primarily due to Ca2+ influx through receptor-coupled ion channels.

86Rubidium release from cultured primary astrocytes: effects of excitatory and inhibitory amino acids

The effects of high K+, glutamate and its analogue, kainate, on K+ release were studied in primary astrocyte cultures prepared from newborn rat brains using 86Rb+ as a tracer for K+. An increase in 86Rb+ release was observed when the extracellular K+ concentration was elevated (10-40 mM). Glutamate and kainate stimulated the release in a dose-dependent manner, 100 microM concentrations being about as equally effective as high K+ (40 mM). Both compounds also caused an increase in the absorbance of the cyanine dye, 3,3'-diethylthiadicarbocyanine, indicating depolarization of the membrane. No significant Na+-dependent uptake of [3H]kainate occurred in the cells, thus excluding the possibility that depolarization was due to electrogenic uptake of amino acid into the cells. GABA and taurine significantly depressed the high K+- and glutamate-induced 86Rb+ release. Taurine itself caused a small increase in 86Rb+ release and the membrane was depolarized, judging from the increase in the absorbance of the cyanine dye, 3,3'-diethylthiadicarbocyanine. No effect of taurine was observed when the Cl- concentration was reduced in the experimental medium. The results suggest that cultured astrocytes respond by membrane depolarization to high external K+ and to glutamate and kainate. The degree of this depolarization can be modified by the inhibitory amino acids GABA, taurine and glycine, the effect of taurine probably being mediated by an increase in Cl- conductance across the cell membrane. The role of functional receptors for amino acid transmitters and the effects observed are discussed.

Release of taurine from cultured cerebellar granule cells and astrocytes: co-release with glutamate

The properties of the release of preloaded [3H]taurine and endogenous taurine were studied with cultured cerebellar granule cells (7-8 days in vitro) and astrocytes (14-15 days in vitro) from the rat. The spontaneous release of taurine from both cell types was slow. The release from both neurons and astrocytes was significantly enhanced by 0.1 mM veratridine, the stimulatory effect being more pronounced in granule cells than in astrocytes. No homo or heteroexchange with extracellularly added taurine or its structural analogues could be detected, suggesting that the efflux is probably not mediated via the membrane transport sites. Kainate stimulated the release more from granule cells than from astrocytes, the effect apparently being mediated by kainate-sensitive receptors. Depolarization of cell membranes by 50 mM K+ induced co-release of endogenous taurine and glutamate from both cell types. Preloaded [3H]taurine was readily released from astrocytes by potassium stimulation. Stimulated release occurred from granule cells if they had been cultured for 4 days with the label but not from the cells preloaded for only 15 min.