Modification of taurine and hypotaurine uptake systems in cultured primary astrocytes by serum-free medium and dibutyryl cyclic AMP treatment

Taurine release is enhanced in cell-damaging conditions in cultured cerebral cortical astrocytes

The release of preloaded [3H]taurine from cultured cerebral cortical astrocytes was studied under various cell-damaging conditions, including hypoxia, ischemia, aglycemia and oxidative stress, and in the presence of free radicals. Astrocytic taurine release was enhanced by K+ (50 mM), veratridine (0.1 mM) and the ionotropic glutamate receptor agonist kainate (1.0 mM). Metabotropic glutamate receptor agonists had only weak effects on taurine release. Similarly to the swelling-induced taurine release the efflux in normoxia seems to be mediated mainly by DIDS-(diisothiocyanostilbene-2,2'-disulphonate) and SITS-(4-acetamido-4'-isothiocyanostilbene-2,2'-disulphonate) sensitive CI- channels, since these blockers were able to reduce both basal and K+ -stimulated release. The basal release of taurine was moderately enhanced in hypoxia and ischemia, whereas the potentiation in the presence of free radicals was marked. The small basal release from astrocytes signifies that taurine release from brain tissue in ischemia may originate from neurons rather than glial cells. On the other hand, the release evoked by K+ in hypoxia and ischemia was greater than in normoxia, with a very slow time-course. The enhanced release of the inhibitory amino acid taurine from astrocytes in ischemia may be beneficial to surrounding neurons, outlasting the initial stimulus and counteracting overexcitation.

Astrocyte membrane Na+, K(+)-ATPase and Mg(2+)-ATPase as targets of organic solvent impact

Astrocytes serve to maintain the proper homeostatic environment for neurons. In these regulations, the astrocyte membrane ATPase plays an important role. In the present study, the astrocyte were cultured in the modified Minimum Essential Medium (MEM), and the isolated cell membranes were exposed to solvents at different concentrations (3, 6 and 9 mM) for one hour. The activities of the membrane-bound Na+, K(+)-ATPase and Mg(2+)-ATPase were determined after exposure to aromatic, alicyclic and aliphatic hydrocarbons. Aromatic hydrocarbons decreased the enzyme activities dose-dependently according to their lipid solubilities. The slight enzyme-inhibiting effect of aliphatic and alicyclic hydrocarbons was not dependent on the lipophilicity of these solvents. Their molecular structure may have determined the solvent impact. In the present study, the activity of ouabain sensitive Na+, K(+)-ATPase in astrocytes was only 20-15% of the total ATPase activity, and in neurons 45-55%. The enzyme inhibition caused by organic solvents may disturb the homeostatic regulatory functions of astrocytes and thus have a toxic effect on the CNS.

Taurine transport at the blood-brain barrier: an in vivo brain perfusion study

Taurine transport into six brain regions of equithesin-anesthetized rats was studied by the in situ brain perfusion technique. This technique gives both accurate measurements of cerebrovascular amino acid transport and allows complete control of the perfusate amino acid composition. Final wash procedure showed that taurine efflux occurred rapidly from endothelial cells. The taurine influx into endothelial cells was sodium and chloride dependent suggesting that the sodium and chloride gradients are the principal source of energy for taurine transport into endothelial cells. Taurine transport could be fitted by a model with saturable components. The kinetic constants in the parietal cortex were 1.4 x 10(-4) mumol/s/g for the apparent Vmax and 0.078 mM for the apparent Km. Competition experiments in the presence of sodium ions showed that [14C]taurine uptake was strongly inhibited by the structural analogs of taurine, hypotaurine and beta-alanine.

The Neurotoxicity of Organic Solvents Studied Using Synaptosomes and Neural Cell Cultures

The most common acute neurotoxic effect of organic solvents is their central nervous system (CNS) depressant effect. The molecular mechanism underlying this effect is not known. The purpose of our studies has been to evaluate the adverse effects of organic solvents on the CNS in vitro. Synaptosomal membranes, whole brain reaggregate and astrocyte cultures were studied. Our results suggest that cell membrane integral proteins are targets for solvent impact, but that there are differences among various cell types. In addition to lipophilicity, the structure of solvent molecules seems to be important when considering CNS toxicity. Organic solvents increase the fluidity of the membranes, which may disturb the lipid-protein interaction and the optimal functioning of the enzyme. However, direct effects of solvents on proteins cannot be excluded. In vitro cell models can be used in methods designed to predict acute neurotoxic effects of foreign compounds, and in studies of neurotoxic mechanisms.

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.

Single-channel characteristics of the large-conductance anion channel in rat cortical astrocytes in primary culture

Cultured rat cortical astrocytes, in addition to a variety of voltage-sensitive potassium channels, also express anion channels. However, the behavior and regulation of these anion channels have been far less studied. This paper describes a patch-clamp study on a voltage-sensitive 200-300 pS high-conductance single-channel anion current, which seems to possess at least five different open sublevels or, alternatively, be formed from five or more small-conductance ion channels linked together. This channel is voltage dependent, showing a bell-shaped open probability curve with highest open probability close to the reversal potential (zero-current). Although potassium channels are commonly detected in astrocytes in cell-attached and excised patches with both normal osmolarity and hypoosmotic solutions, the occurrence of the anion channel is clearly increased in isolated patches when hypoosmotic bath solutions are used. Also, cell aging in culture and the preparation of secondary cell cultures by trypsinization seem to increase the rate of occurrence of the anion channel. Though this channel is more routinely seen when a membrane patch is excised from the cell, occasionally cell-attached configurations with instant channel activity can be formed. While the modulation of this anion channel was being studied, it was found to be blocked by an anion transport inhibitor, L-644,711, reported to affect cell volume regulation in astrocytes.

Adrenergic receptor regulation of amino acid neurotransmitter uptake in astrocytes

Using culture techniques it has been demonstrated that astroglia possess uptake carriers for amino acid neurotransmitters and enzyme systems for inactivation of several neurotransmitters. They express membrane receptors functionally coupled to second messenger systems and they can regulate the extracellular ionic milieu including a clearing of K+ from the extracellular space. With these specific functional characteristics and their strategic anatomy the cells might influence the passage of information between neurons.

Heterogeneity among astroglial cells with respect to 5HT-evoked cytosolic Ca2+ responses. A microspectrofluorimetric study on single cells in primary culture

The effect of 5-hydroxytryptamine (5HT) on cytoplasmic Ca2+ concentration was examined at the single cell level in astroglial enriched primary cultures from newborn rat cerebral cortex. Type 1 astroglial cells were identified and the Ca2+ indicator dye fura 2/AM was used in a microspectrofluorimetric system. Pharmacological studies indicated that the Ca2+ responses were mediated by 5HT2 receptors. Four different patterns of 5HT evoked cytosolic Ca2+ responses were identified including two different types of spike patterns and two types of Ca2+ oscillations (low amplitude and base-line spiking behaviour). In addition, cells with spontaneous Ca2+ oscillations were of two types, those responding to 5HT and those not responding to 5HT. The different responses were identified already on day 7 in culture and were followed up to day 21 with a concomitant increase in the number of responding cells, although the response patterns did not differ during culture. The triggering 5HT concentration was 1 microM. The results suggest that subpopulations of astrocytes exist with respect to 5HT2-evoked cytosolic Ca2+ mobilization.

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.

Alpha-receptor and cholinergic receptor-linked changes in cytosolic Ca2+ and membrane potential in primary rat astrocytes

Both phenylephrine and carbachol caused a sustained increase in Ca2+ influx and intracellular free Ca2+ of primary astrocytes as measured with 45Ca2+ and fura-2. The responses to phenylephrine and carbachol were additive, suggesting that they use different releasable pools of Ca2+. If extracellular Ca2+ was removed by EGTA only a transient rise in cytosolic Ca2+ was seen upon application of the agonists. Both compounds caused depolarization of the astrocyte membrane as determined with the optical probe 3,3-diethylthiadicarboxyamineiodide. Activation of protein kinase C with 12-tetradecanoylphorbol myristate acetate (TPA) or the diacylglycerol analogue dioctanoylglycerol (DiC8) also depolarized the cells. A prior activation of protein kinase C with TPA or DiC8 abolished the depolarizing effect of phenylephrine suggesting that they act through the same mediators. If the cells were made ideally permeable to K+ with the ionophore valinomycin, or the K+ channels had been blocked with Ba2+, neither TPA nor phenylephrine had any significant effect on the membrane potential. Neither TPA nor phenylephrine had any effect on the 86Rb+ equilibrium potential across the cell membrane. The results suggest that the depolarizing effect of these substances could be through a blocking of K+ channels.

Effect of HEPES on the taurine uptake by cultured glial cells

HEPES inhibited the taurine uptake in glial cells. A different kind of inhibition was observed when HEPES was present in the culture medium or in the incubation medium used for the taurine-uptake measurement. As an example of a possible interference of HEPES in pharmacological experiments, we have studied the effect of this buffer on the modulation of taurine uptake by beta agonists or ionic concentration.

Properties of single potassium channels in cultured primary astrocytes

The patch-clamp technique was used to characterize the single channel ion currents in primary cultures of rat astrocytes. The most dominant channel type, which was found in over half of the inside-out membrane patches, was a potassium channel. The measured reversal potential was -67 mV, which is close to the calculated Nernst potential for potassium ions (-80 mV). These potassium channels activated with bursts of very brief openings. Once activated the channels did not inactivate. The measured probabilities of the channels to be closed showed at least 3 different modes of channel behaviour: one voltage-independent and two voltage-dependent modes. During each activity-mode a 'main' conductance level plus two other conductance levels were observed. In some recordings a pronounced outward rectification could also be seen.

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.

Glutamate receptor-linked changes in membrane potential and intracellular Ca2+ in primary rat astrocytes

Kainate-, quisqualate- and glutamate-induced depolarization and mobilization of intracellular Ca2+ was determined in primary cultured astrocytes using the fluorescent probes DiBa-C4-(3) and fura-2, respectively. All three receptor agonists depolarized the cells in a Na+-dependent manner and increased the intracellular Ca2+ concentration. The glutamate- and quisqualate-induced increase in cytosolic Ca2+ was only partially inhibited by removal of extracellular Ca2+, whereas the response to kainate was totally dependent on extracellular Ca2+. The mechanisms for depolarization and increases in cytosolic Ca2+ appeared to be independent of each other, as extracellular Ca2+ removal or intracellular Ca2+ buffering with entrapped BAPTA did not affect the depolarization. Removal of extracellular Na+ did not affect the agonist-induced increase in Ca2+. If quisqualate was added after kainate, the cells were hyperpolarized in a Ca2+- and K+-dependent manner. This could be due to effects on a Ca2+-dependent K+ channel, the effects of which are normally hidden by the greater effect on Na+ channels as a response to quisqualate.

Activators of protein kinase C and phenylephrine depolarize the astrocyte membrane by reducing the K+ permeability

The membrane potential of astrocytes has been measured by monitoring the absorbance of a cyanine dye DiS-C2-(5). Ba2+, the phorbol ester 12-tetradecanoylphorbol myristateacetate (TPA) and the diglyceride, dioctanoylglycerol (DiC8) depolarize the membrane. Valinomycin which makes the membrane potential dependent on the K+ electrochemical potential evokes a hyperpolarization when added subsequently. The alpha-adrenergic receptor agonist phenylephrine was blocked by Ba2+, TPA, DiC8 and valinomycin. The results suggest that a protein kinase C-mediated reduction in the K+ permeability is responsible for the depolarizing effect of TPA, DiC8 and phenylephrine.

The effect of K+ and glutamate receptor agonists on the membrane potential of suspensions of primary cultures of rat astrocytes as measured with a cyanine dye, DiS-C2-(5)

The cyanine dye DiS-C2-(5) was used to investigate the effect of K+ and glutamate receptor agonists on the membrane potential of whole populations of primary rat astrocytes in suspension. Increasing the external K+ concentration from 5 to 40 mM caused a depolarization of the cells. Ba2+ blocked the response to K+, whereas 4-aminopyridine had no effect on the depolarization. The effect of added external K+ was enhanced by the addition of the neutral K+ ionophore valinomycin. This supports the view that the membrane potential of primary astrocytes is dependent of the K+ gradient, and suggests that the membrane is not ideally permeable to K+ ions. Glutamate caused a depolarization of the cells which was not affected by Ba2+. In the presence of veratridine and ouabain no effect of glutamate was seen. The cells were also depolarized by the glutamate receptor agonists quisqualate, kainate and N-methyl-D-aspartate (NMDA). The response to kainate was blocked by kynurenate, which also diminished the depolarization caused by glutamate. NMDA was effective when added after kainate. The effect of the glutamate receptor agonists tested was generally smaller than that of glutamate itself, and a prior addition of one of the agonists diminished the response to glutamate. The results obtained suggest that cyanine dyes are well suited for investigating the behavior of whole populations of cultured primary astrocytes.

Taurine and beta-alanine uptake in primary astrocytes differentiating in culture: effects of ions

The effects of ions on taurine and beta-alanine uptake were studied in astrocytes during cellular differentiation in primary cultures. The uptakes were strictly Na+-dependent and also inhibited by the omission of K+ and in the presence of ouabain suggesting that their transport is fuelled mainly by these cation gradients. Two sodium ions were associated in the transport of one taurine and beta-alanine molecule across cell membranes. A reduction in Cl- concentration also markedly inhibited the uptake of both amino acids, indicating that this anion is of importance in the transport processes. The similar ion dependency profiles of taurine and beta-alanine uptake corroborate the assumption that the uptake of these amino acids in astrocytes is mediated by the same carrier. In Na+- and K+-free media both taurine and beta-alanine uptakes were reduced significantly more in 14-day-old or older than in 7-day-old cultures. No significant changes occurred in the coupling ratio between Na+ and taurine or beta-alanine as a function of spontaneous cellular differentiation or upon dBcAMP treatment. These results suggest that the uptake systems of these structurally related amino acids in astrocytes have reached a relatively high degree of functional maturity by two weeks in culture.

Glucose deprivation depolarizes plasma membrane of cultured astrocytes and collapses transmembrane potassium and glutamate gradients

Primary cultures of astrocytes were used to investigate the effects of glucose deprivation on plasma membrane potential, on the respiration and on the energy status of these cells. Plasma membrane potential, as monitored with a cyanine dye, 3,3'-diethylthiadicarbocyanine, hyperpolarized by about 100% when glucose was added to substrate-deprived cells. The effect of glucose was prevented by iodoacetate or ouabain. In the absence of glucose, cellular adenosine triphosphate/adenosine diphosphate ratio was extensively reduced and pyruvate was unable either to restore energy status or to hyperpolarize the plasma membrane of astrocytes, although it was the preferential substrate for mitochondria within the cells. Glucose deprivation and inhibition of glycolysis or respiration in the presence of glucose caused dramatic decrease in transmembrane potassium ion and L-glutamate gradients. The gradients were not restored in the presence of pyruvate. Thus, aerobic glycolysis, rather than oxidation of pyruvate, is required to maintain maximal plasma membrane potential, adenosine triphosphate/adenosine diphosphate ratios as well as K+ and L-glutamate gradients. This evidence, together with the unresponsiveness of astrocyte respiration to ouabain, indicates a functional dissociation between energy dissipation at the plasma membrane and mitochondrial synthesis of adenosine triphosphate. The results are discussed with regard to the vulnerability of glia at low levels of blood glucose and the contribution of glial dysfunction to development of hypoglycaemic encephalopathy.

Sodium-dependent high-affinity uptake of taurine in cultured cerebellar granule cells and astrocytes

Taurine uptake in cultured cerebellar granule cells and astrocytes consisted of a saturable high-affinity component and nonsaturable diffusion. The transport constant (Km) was significantly lower and the maximal velocity (V) higher in granule cells than in astrocytes. The uptakes were strictly sodium dependent and also moderately decreased in potassium-free medium. The specificity profile of taurine uptake was similar in both cell types, hypotaurine, beta-alanine, and guanidinoethanesulphonic acid being the most potent inhibitors, followed by GABA and homotaurine. Glutamate inhibited taurine uptake more in astrocytes than in granule cells. In principle, the uptake systems were similar in granule cells and astrocytes, exhibiting features characteristic of uptake of a neurotransmitter or -modulator.

High-affinity uptake of taurine and beta-alanine in primary cultures of rat astrocytes

The kinetics and specificity of taurine and beta-alanine uptake were studied in primary cultures of rat astrocytes under identical experimental conditions. The uptake consisted of nonsaturable penetration and saturable high-affinity transport that was strictly sodium dependent. The cells accumulated taurine more effectively than beta-alanine, both the affinity and uptake capacity being greater for taurine. Taurine uptake was competitively inhibited by beta-alanine and GABA, the former being more potent. Also, hypotaurine and 2-guanidinoethanesulphonic acid strongly reduced taurine uptake, but L-2,4-diaminobutyric acid had no significant effect. beta-Alanine uptake was also competitively inhibited by GABA, but the most potent inhibitors were hypotaurine and 2-guanidinoethanesulphonic acid. L-2,4-Diaminobutyric acid was moderately active. The uptake systems for taurine and beta-alanine were thus in principle similar, and they exhibited certain characteristics typical for a neurotransmitter amino acid. The inhibition studies further suggest the existence of only one common transport system for taurine, beta-alanine, and GABA in cultured primary astrocytes. The same uptake system may also be used for hypotaurine.

Free amino acids of rat astrocytes in primary culture: changes during cell maturation

The concentrations of free amino acids were analysed in cultured primary astrocytes during cell maturation and in the starting material, i.e. the cerebral hemispheres of newborn rats. Taurine was the most abundant amino acid in all samples, the content of glutamine being comparable only in immature astrocytes (7 days in culture). The intracellular levels of most amino acids significantly decreased during the first 2 weeks in culture, remaining fairly stable during the third week.

Release of preloaded taurine and hypotaurine from astrocytes in primary culture: stimulation by calcium-free media

The spontaneous and stimulated release of taurine and hypotaurine from astrocytes in primary cultures were investigated. Spontaneous efflux was slow, less than one half of preloaded labeled taurine and hypotaurine still remaining in the cells after a 60-min efflux period. The release processes of both amino acids were in principle similar. No homo- or heteroexchange with extracellularly added taurine, hypotaurine or GABA could be detected, and depolarizing potassium concentrations failed to stimulate taurine or hypotaurine release. On the other hand, omission of calcium ions from medium increased efflux of taurine and hypotaurine about three- and twofold, respectively, in both high-K+ and normal-K+ media.