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

Effects of glucose deficiency on glutamate/aspartate release and excitatory synaptic responses in the hippocampal CA1 area in vitro

The effects of glucose deficiency on (1) the K+-evoked release of glutamate and aspartate and (2) excitatory synaptic transmission were studied in the Schaffer collateral-commissural-ipsilateral associational (SCCIA) projection to area CA1 of the rat hippocampal formation in vitro. Compared with 1 or 10 mM glucose, superfusion of CA1 slices with 0.1 mM glucose enhanced the K+-evoked release of both glutamate and aspartate, increased the ratio of aspartate release to glutamate release and did not affect the release of GABA. With both high and low glucose concentrations, the K+-evoked release of glutamate and aspartate originated predominantly from a Ca2+-sensitive store associated with the SCCIA projection. Superfusion with glucose-deficient medium abolished the inhibitory effect of adenosine on glutamate and aspartate release, but augmented the enhancing effect of the adenosine antagonist 8-phenyltheophylline. These results suggest that enough endogenous adenosine was released from the slices under these conditions to saturate the presynaptic A1 receptors. Despite its facilitatory effect on excitatory transmitter release, glucose-deficient medium inhibited transmission at Schaffer collateral-commissural synapses. Even when the postsynaptic response to a single electrical pulse was abolished, however, a substantial response could still be evoked through paired-pulse or frequency potentiation and the inhibition promptly reversed upon superfusion with 10 mM glucose. The increased ratio of aspartate release to glutamate release appears to reflect changes in the tissue content of these amino acids. The enhanced release of both excitants is suggested to result partly from a rise in intraterminal Ca2+ concentration and partly from inhibition of glutamate/aspartate uptake. Enhanced aspartate release may be particularly relevant to hypoglycemic damage in the CA1 area, because aspartate is a more potent hippocampal excitotoxin than glutamate.

Interrelationships between glucose metabolism, energy state, and the cytosolic free calcium concentration in cortical synaptosomes from the guinea pig

The stoichiometries of glycolysis and pyruvate oxidation were determined in cortical synaptosomes under varying rates of ATP consumption. Glycolysis was measured by using D-3-[3H]glucose as a marker and pyruvate oxidation by using D-3,4-[14C]glucose, which has to be metabolized to 1-[14C]pyruvate before being decarboxylated by the pyruvate dehydrogenase complex of intrasynaptosomal mitochondria. Cytosolic free Ca2+ concentration [( Ca2+]c) was determined in parallel and was manipulated by using EGTA in the incubation. The results show that in nonstimulated synaptosomes glycolysis and pyruvate oxidation are tightly coupled and stoichiometric. In the absence of Ca2+, when [Ca2+]c drops from 260 nM to 40 nM, glucose utilization increases, following the increase in energy demand, which has been shown to be due to elevated Na+ cycling. KCl depolarization, veratridine, and a mitochondrial uncoupler, carbonyl cyanide m-chlorophenylhydrazone, all stimulate glycolysis and pyruvate oxidation stoichiometrically, independently of the presence of external Ca2+. A rise in [Ca2+]c, therefore, is not required to regulate mitochondrial pyruvate metabolism. It is concluded that synaptosomes exhibit a high degree of respiratory control, that they rely on glucose oxidation for their energetics, and that stimulation of energy production can be achieved independently of changes in [Ca2+]c.

The glial drug methionine sulfoximine reduces goldthioglucose lesions in mice

A toxic form of glucose, goldthioglucose (GTG), damages the arcuate nucleus and nucleus of the solitary tract (NTS) after passage through permeable circumventricular capillaries in adjacent structures, but the mechanism of action is unclear. To see if GTG initially acts upon glia to cause lesions, adult male mice were given either saline or a drug--methionine sulfoximine (MS)--that causes glia, but not neurons, to generate intracellular deposits of glycogen from glutamate. One day later, mice were given 0.3 mg/g of GTG and the subsequent severity of GTG-induced arcuate and NTS lesions was examined. The MS-treated group had significantly smaller GTG lesions in both brain sites, suggesting that a MS-induced alteration in glial metabolism produced a diminished uptake of GTG. Uptake of GTG via a glial glucose transport system may lead to lesions by disturbing the heme metabolism of a specialized subset of glia present in these regions. Factors affecting glial glucose transport in these areas may thus modulate both effects of GTG and effects of glucose upon the regulation of feeding.

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.

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.