The influence of the extracellular matrix on the morphology and intracellular pH of cultured astrocytes exposed to media lacking bicarbonate

Thyroid Hormone and Astrocyte Differentiation

Thyroid hormones (THs) have important contributions to the development of the mammalian brain, targeting its actions on both neurons and glial cells. Astrocytes, which constitute about half of the glial cells, characteristically undergo dramatic changes in their morphology during development and such changes become necessary for the proper development of the brain. Interestingly, a large number of studies have suggested that THs play a profound role in such morphological maturation of the astrocytes. This review discusses the present knowledge on the mechanisms by which THs elicit progressive differentiation and maturation of the astrocytes. As a prelude, information on astrocyte morphology during development and its regulations, the role of THs in the various functions of astrocyte shall be dealt with for a thorough understanding of the subject of this review.

Methylglyoxal Induces Changes in the Glyoxalase System and Impairs Glutamate Uptake Activity in Primary Astrocytes

The impairment of astrocyte functions is associated with diabetes mellitus and other neurodegenerative diseases. Astrocytes have been proposed to be essential cells for neuroprotection against elevated levels of methylglyoxal (MG), a highly reactive aldehyde derived from the glycolytic pathway. MG exposure impairs primary astrocyte viability, as evaluated by different assays, and these cells respond to MG elevation by increasing glyoxalase 1 activity and glutathione levels, which improve cell viability and survival. However, C6 glioma cells have shown strong signs of resistance against MG, without significant changes in the glyoxalase system. Results for aminoguanidine coincubation support the idea that MG toxicity is mediated by glycation. We found a significant decrease in glutamate uptake by astrocytes, without changes in the expression of the major transporters. Carbenoxolone, a nonspecific inhibitor of gap junctions, prevented the cytotoxicity induced by MG in astrocyte cultures. Thus, our data reinforce the idea that astrocyte viability depends on gap junctions and that the impairment induced by MG involves glutamate excitotoxicity. The astrocyte susceptibility to MG emphasizes the importance of this compound in neurodegenerative diseases, where the neuronal damage induced by MG may be aggravated by the commitment of the cells charged with MG clearance.

Transient acidification and subsequent proinflammatory cytokine stimulation of astrocytes induce distinct activation phenotypes

The foot processes of astrocytes cover over 60% of the surface of brain microvascular endothelial cells, regulating tight junction integrity. Retraction of astrocyte foot processes has been postulated to be a key mechanism in pathology. Therefore, movement of an astrocyte in response to a proinflammatory cytokine or even limited retraction of processes would result in leaky junctions between endothelial cells. Astrocytes lie at the gateway to the CNS and are instrumental in controlling leukocyte entry. Cultured astrocytes typically have a polygonal morphology until stimulated. We hypothesized that cultured astrocytes which were induced to stellate would have an activated phenotype compared with polygonal cells. We investigated the activation of astrocytes derived from adult macaques to the cytokine TNF-α under resting and stellated conditions by four parameters: morphology, intermediate filament expression, adhesion, and cytokine secretion. Astrocytes were stellated following transient acidification; resulting in increased expression of GFAP and vimentin. Stellation was accompanied by decreased adhesion that could be recovered with proinflammatory cytokine treatment. Surprisingly, there was decreased secretion of proinflammatory cytokines by stellated astrocytes compared with polygonal cells. These results suggest that astrocytes are capable of multiple phenotypes depending on the stimulus and the order stimuli are applied.

Morphological plasticity of rodent astroglia

In the past two decades, there has been an explosion of research on the role of neuroglial interactions in the control of brain homeostasis in both physiological and pathological conditions. Astrocytes, a subtype of glia in the central nervous system, are dynamic signaling elements that regulate neurogenesis and development of brain circuits, displaying intimate dynamic relationships with neurons, especially at synaptic sites where they functionally integrate the tripartite synapse. When astrocytes are isolated from the brain and maintained in culture, they exhibit a polygonal shape unlike their precursors in vivo. However, cultured astrocytes can be induced to undergo morphological plasticity leading to process formation, either by interaction with neurons or by the influence of pharmacological agents. This review highlights studies on the molecular mechanisms underlying morphological plasticity in astrocyte cultures and intact brain tissue, both in situ and in vivo.

In vitro S100B secretion is reduced by apomorphine: effects of antipsychotics and antioxidants

Astrocytes express dopamine receptors and respond to dopamine stimulation. However, the role of astrocytes in psychiatric disorders and the effects of antipsychotics on astroglial cells have only been investigated recently. S100B is a glial-derived protein, commonly used as a marker of astroglial activation in psychiatric disorders, particularly schizophrenia. We investigated S100B secretion in three different rat brain preparations (fresh hippocampal slices, C6 glioma cells and primary astrocyte cultures) exposed to apomorphine and antipsychotics (haloperidol and risperidone), aiming to evaluate, ex vivo and in vitro, whether dopamine activation and dopaminergic antagonists modulate astroglial activation, as measured by changes in the extracellular levels of S100B. The serum S100B elevation observed in schizophrenic patients is not reflected by the in vitro decrease of S100B secretion that we observed in hippocampal slices, cortical astrocytes and C6 glioma cells treated with apomorphine, which mimics dopaminergic hyperactivation. This decrease in S100B secretion can be explained by a stimulation of D2 receptors negatively coupled to adenyl cyclase. Antipsychotic medications and antioxidant supplementation were able to prevent the decline in S100B secretion. Findings reinforce the benefits of antioxidant therapy in psychiatric disorders. Based on our results, in hippocampal slices exposed to apomorphine, it may be suggested that antipsychotics could help to normalize S100B secretion by astrocytes.

Gap junction inhibitors modulate S100B secretion in astrocyte cultures and acute hippocampal slices

Astrocytes sense, integrate, and respond to stimuli generated by neurons or neural injury; this response involves gap junction (GJ) communication. Neuronal vulnerability to injury increased when cocultures of astrocytes and neurons were exposed to GJ inhibitors. However, GJ uncoupling could limit the extension of a lesion. We investigated a possible link between GJ communication and S100B secretion. S100B is a calcium-binding protein of 21 kDa that is predominantly expressed and secreted by astrocytes, which has trophic paracrine activity on neurite growth, glial proliferation, and neuronal survival. GJ inhibitors were analyzed in isolated astrocytes in primary cultures from hippocampus, acute hippocampal slices, and C6 glioma cells, which were used as a negative control. Our data indicate that GJ blocking stimulates S100B secretion in astrocyte cultures and acute hippocampal slices. Different assays were used to confirm cell integrity during exposure to GJ inhibitors. S100B secretion was observed with different types of GJ inhibitors; the resulting event was dependent on time, the nature of the inhibitor, its putative molecular target of GJ blocking, and/or the cell preparation used. Only carbenoxolone induced a fast and persistent increase in S100B secretion in both preparations. Endothelin-1 increased S100B secretion in astrocyte cultures at 1 hr, but a decrease was observed at 6 hr or in acute hippocampal slices. Physiologically, a local GJ closure associated with release of S100B in injury conditions favors the idea of a common mechanism available to limit the extension of lesion and increase the chances of cell survival.

Three-dimensional bioprinting of rat embryonic neural cells

We present a direct cell printing technique to pattern neural cells in a three-dimensional (3D) multilayered collagen gel. A layer of collagen precursor was printed to provide a scaffold for the cells, and the rat embryonic neurons and astrocytes were subsequently printed on the layer. A solution of sodium bicarbonate was applied to the cell containing collagen layer as nebulized aerosols, which allowed the gelation of the collagen. This process was repeated layer-by-layer to construct the 3D cell-hydrogel composites. Upon characterizing the relationship between printing resolutions and the growth of printed neural cells, single/multiple layers of neural cell-hydrogel composites were constructed and cultured. The on-demand capability to print neural cells in a multilayered hydrogel scaffold offers flexibility in generating artificial 3D neural tissue composites.

Antioxidants attenuate hyperglycaemia-mediated brain endothelial cell dysfunction and blood-brain barrier hyperpermeability

AIMS

Hyperglycaemia (HG), in stroke patients, is associated with worse neurological outcome by compromising endothelial cell function and the blood-brain barrier (BBB) integrity. We have studied the contribution of HG-mediated generation of oxidative stress to these pathologies and examined whether antioxidants as well as normalization of glucose levels following hyperglycaemic insult reverse these phenomena.

METHODS

Human brain microvascular endothelial cell (HBMEC) and human astrocyte co-cultures were used to simulate the human BBB. The integrity of the BBB was measured by transendothelial electrical resistance using STX electrodes and an EVOM resistance meter, while enzyme activities were measured by specific spectrophotometric assays.

RESULTS

After 5 days of hyperglycaemic insult, there was a significant increase in BBB permeability that was reversed by glucose normalization. Co-treatment of cells with HG and a number of antioxidants including vitamin C, free radical scavengers and antioxidant enzymes including catalase and superoxide dismutase mimetics attenuated the detrimental effects of HG. Inhibition of p38 mitogen-activated protein kinase (p38MAPK) and protein kinase C but not phosphoinositide 3 kinase (PI3 kinase) also reversed HG-induced BBB hyperpermeability. In HBMEC, HG enhanced pro-oxidant (NAD(P)H oxidase) enzyme activity and expression that were normalized by reverting to normoglycaemia.

CONCLUSIONS

HG impairs brain microvascular endothelial function through involvements of oxidative stress and several signal transduction pathways.

S100B secretion is stimulated by IL-1beta in glial cultures and hippocampal slices of rats: Likely involvement of MAPK pathway

S100B is an astrocyte-derived cytokine implicated in the IL-1beta-triggered cytokine cycle in Alzheimer's disease. However, the secretion of S100B following stimulation by IL-1beta has not been directly demonstrated. We investigated S100B secretion in cortical primary astrocyte cultures, C6 glioma cells and acute hippocampal slices exposed to IL-1beta. S100B secretion was induced by IL-1beta in all preparations, involving MAPK pathway and, apparently, NF-small ka, CyrillicB signaling. Astrocytes and C6 cells exhibited different sensitivities to IL-1beta. These results suggest that IL-1beta-induced S100B secretion is a component of the neuroinflammatory response, which would support the involvement of S100B in the genesis of neurodegenerative diseases.

Resveratrol increases glutamate uptake, glutathione content, and S100B secretion in cortical astrocyte cultures

Resveratrol (3,5,4'-trihydroxy-trans-stilbene) is a polyphenol present in grapes and red wine, which has antioxidant properties and a wide range of other biological effects. In this study, we investigated the effect of resveratrol, in a concentration range of 10-250 microM, on primary cortical astrocytes; evaluating cell morphology, parameters of glutamate metabolism such as glutamate uptake, glutamine synthetase activity and glutathione total content, and S100B secretion. Astrocyte cultures were prepared of cerebral cortex from neonate Wistar rats. Morphology was evaluated by phase-contrast microscopy and immunocytochemistry for glial fibrillary acidic protein (GFAP). Glutamate uptake was measured using L-[2,3-3H]glutamate. Glutamine synthetase and content of glutathione were measured by enzymatic colorimetric assays. S100B content was determined by ELISA. Typical polygonal morphology becomes stellated when astrocyte cultures were exposed to 250 microM resveratrol for 24 h. At concentration of 25 microM, resveratrol was able to increase glutamate uptake and glutathione content. Conversely, at 250 microM, resveratrol decreased glutamate uptake. Unexpectedly, resveratrol at this high concentration increased glutamine synthetase activity. Extracellular S100B increased from 50 microM upwards. Our findings reinforce the protective role of this compound in some brain disorders, particularly those involving glutamate toxicity. However, the underlying mechanisms of these changes are not clear at the moment and it is necessary caution with its administration because elevated levels of this compound could contribute to aggravate these conditions.

S100B levels in the cerebrospinal fluid of rats are sex and anaesthetic dependent

1. S100B is a calcium-binding protein that acts as a neurotrophic cytokine and is expressed in the central nervous system, predominantly by astrocytes. At nanomolar concentrations, S100B stimulates neurite outgrowth and glial glutamate uptake, as well as protecting neurons against glutamate excitoxicity. 2. Peripheral S100B concentrations, particularly in the serum and cerebrospinal fluid (CSF), have been used as a parameter of glial activation or death in several physiological and pathological conditions. 3. In the present study, we investigated the effect of anaesthetics (thiopental, ketamine and halothane) on CSF concentrations of S100B, as well as a possible sex dependence, because several studies have suggested astrocytes as putative targets for oestrogen. 4. Higher levels of CSF S100B were found when rats were anaesthetized with thiopental; these levels, independently of anaesthetic, were sex dependent. Conversely, no effect of anaesthetic or sex was observed on serum concentrations of S100B. 5. The increase in CSF concentrations of S100B induced by thiopental was confirmed in non-anaesthetized neonatal rats and cortical astrocyte cultures. 6. Assuming CSF S100B as a marker of development, glial activation or even brain damage, investigations regarding the sex dependence of its concentration may be useful in gaining an understanding of sex variations in the behaviour and the pathological course of, as well as susceptibility to, many brain disorders. The findings of the present study reinforce the sex effect on synaptic plasticity and suggest a sex dependence of neural communication mediated by extracellular S100B without restricting the influence of astrocytes on the developmental phase.

A pharmacological activator of AMP-activated protein kinase (AMPK) induces astrocyte stellation

AMP-activated protein kinase (AMPK) represents a key energy-sensing molecule in many cell types. Because astrocytes are key mediators of metabolic signaling in the brain, we have initiated studies on the expression and activation of AMPK in these cells. Treatment of cultured rat cortical astrocytes with a pharmacological AMPK activator, AICA-riboside (AICAR) resulted in a time- and concentration-dependent increase in phosphorylation of AMPK and acetyl-CoA carboxylase (ACC), a direct substrate. AICAR treatment also induced a transition from epithelioid to stellate morphology in a time- and concentration-dependent manner. As stellation is indicative of actin cytoskeletal reorganization, the formation of stress fibers and focal adhesions in response to AICAR was assessed. AICAR-induced stellation correlated with F-actin disassembly and focal adhesion dispersal. Furthermore, transient transfection of an activated RhoA construct prevented AICAR-induced stellation, indicating a mechanism upstream of RhoA. Use of pharmacological inhibitor compound C prevented AICAR-induced stellation demonstrating necessity of AMPK activity for the response. Our findings suggest that AMPK mediates morphological alterations of astrocytes in response to energy depletion.

Protective effects of resveratrol on hydrogen peroxide induced toxicity in primary cortical astrocyte cultures

It is well established that the brain is particularly susceptible to oxidative damage due to its high consumption of oxygen and that astrocytes are involved in a variety of important activities for the nervous system, including a protective role against damage induced by reactive oxygen species (ROS). The use of antioxidant compounds, such as polyphenol resveratrol found in red wine, to improve endogenous antioxidant defenses has been proposed for neural protection. The aim of this study is to evaluate the putative protective effect of resveratrol against acute H2O2-induced oxidative stress in astrocyte cultures, evaluating ROS production, glutamate uptake activity, glutathione content and S100B secretion. Our results confirm the ability of resveratrol to counteract oxidative damage caused by H2O2, not only by its antioxidant properties, but also through the modulation of important glial functions, particularly improving glutamate uptake activity, increasing glutathione content and stimulating S100B secretion, which all contribute to the functional recovery after brain injury.

S100B content and secretion decrease in astrocytes cultured in high-glucose medium

S100B is an astrocyte calcium-binding protein that plays a regulatory role in the cytoskeleton and cell cycle. Moreover, extracellular S100B, a marker of glial activation in several conditions of brain injury, has a trophic or apoptotic effect on neurons, depending on its concentration. Hyperglycemic rats show changes in glial parameters, including S100B expression. Here, we investigated cell density, morphological and biochemical alterations in primary cortical astrocytes from rats and C6 glioma cells cultured in high-glucose medium. Astrocytes and C6 glioma cells have a reduced content of S100B and glial fibrillary acidic protein when cultured in a high-glucose environment, as well as a reduced content of glutathione and cell proliferation rate. Although these cells have been used indistinctly to study S100B secretion, we observed a contrasting profile of S100B secretion in a high-glucose medium: a decrease in primary astrocytes and an increase in C6 glioma cells. Based on the in vitro neurotrophic effects of the S100B protein, our data suggest that chronic elevated glucose levels affect astrocyte activity, reducing extracellular secretion of S100B and that this, in turn, could affect neuronal activity and survival. Such astrocyte alterations could contribute to cognitive deficit and other impairments observed in diabetic patients.

Resveratrol increases glutamate uptake and glutamine synthetase activity in C6 glioma cells

Resveratrol, a phytoalexin found mainly in grapes, is a promising natural product with anti-cancer and cardio-protective activities. Here, we investigated, in C6 glioma cells, the effect of resveratrol on some specific parameters of astrocyte activity (glutamate uptake, glutamine synthetase and secretion of S100B, a neurotrophic cytokine) commonly associated with the protective role of these cells. Cell proliferation was significantly decreased by 8% and 26%, following 24h of treatment with 100 and 250 microM resveratrol. Extracellular S100B increased after 48 h of resveratrol exposure. Short-term resveratrol exposure (from 1 to 100 microM) induced a linear increase in glutamate uptake (up to 50% at 100 microM resveratrol) and in glutamine synthetase activity. Changes in these glial activities can contribute to the protective role of astrocytes in brain injury conditions, reinforcing the putative use of this compound in the therapeutic arsenal against neurodegenerative diseases and ischemic disorders.

Role of Rho GTPase in astrocyte morphology and migratory response during in vitro wound healing

Small Rho GTPases are key regulators of the cytoskeleton in a great variety of cells. Rho function mediates morphological changes as well as locomotor activity. Using astrocyte cultures established from neonatal mice we investigated the role of Rho in process formation during astrocyte stellation. Using a scratch-wound model, we examined the impact of Rho on a variety of morphological and functional variables such as stellation and migratory activity during wound healing. C3 proteins are widely used to study cellular Rho functions. In addition, C3 derived from Clostridium botulinum (C3bot) is considered selectively to promote neuronal regeneration. Because the latter requires a balanced activity of neurones and glial cells, the effects of C3 protein on glial cells such as astrocytes have to be considered carefully. Low nanomolar concentrations of C3 proteins significantly promoted process outgrowth and increased process branching. Besides enzymatic inactivation of Rho by ADP-ribosylation, changes in protein levels of the various Rho GTPases may also contribute to the observed effects. Furthermore, incubation of scratch-wounded astrocyte cultures with C3bot accelerated wound healing. By inhibiting the Rho downstream effector ROCK with the selective inhibitor Y27632 we were able to demonstrate that the accelerated wound closure resulted from both enhanced polarized process formation and increased migratory activity of astrocytes into the lesion site. These results suggest that Rho negatively regulates astrocytic process growth and migratory responses after injury and that its inactivation by C3bot in nanomolar concentrations promotes astrocyte migration.

Involvement of the S100B in cAMP-induced cytoskeleton remodeling in astrocytes: a study using TRTK-12 in digitonin-permeabilized cells

1. Stellation of astrocytes in culture involves a complex rearrangement of microfilaments, intermediate filaments, and microtubules, which reflects in part the plasticity of these cells observed during development or after injury. 2. An astrocytic calcium-binding protein, S100B, has been implicated in the regulation of plasticity due to its ability to interact with cytoskeletal proteins. 3. We used digitonin-permeabilized astrocytes to introduce TRTK-12, a peptide that binds to the C-terminal of S100B and blocks its interaction with cytoskeletal proteins. 4. TRTK-12 was able to block cAMP-induced astrocyte stellation and this effect was dependent on the concentration of the peptide. These results support the idea that S100B has a modulatory role on astrocyte morphology.

An endothelial and astrocyte co-culture model of the blood-brain barrier utilizing an ultra-thin, nanofabricated silicon nitride membrane

The endothelial cells comprising brain capillaries have extremely tight intercellular junctions which form an essentially impermeable barrier to passive transport of water soluble molecules between the blood and brain. Several in vitro models of the blood-brain barrier (BBB) have been studied, most utilizing commercially available polymer membranes affixed to plastic inserts. There is mounting evidence that direct contact between endothelial cells and astrocytes, another cell type found to have intimate interaction with the brain side of BBB capillaries, is at least partially responsible for the development of the tight intercellular junctions between BBB endothelial cells. However, the membranes commonly used for BBB in vitro models are lacking certain attributes that would permit a high degree of direct contact between astrocytes and endothelial cells cultured on opposing sides. This work is based on the hypothesis that co-culturing endothelial and astrocyte cells on opposite sides of an ultra-thin, highly porous membrane will allow for increased direct interaction between the two cell types and therefore result in a better model of the BBB. We used standard nanofabrication techniques to make membranes from low-stress silicon nitride that are at least an order of magnitude thinner and at least two times more porous than commercial membrane inserts. An experimental survey of pore sizes for the silicon nitride membranes suggested pores approximately 400 nm in diameter are adequate for restricting astrocyte cell bodies to the seeded side while allowing astrocyte processes to pass through the pores and interact with endothelial cells on the opposite side. The inclusion of a spun-on, cross-linked collagen membrane allowed for astrocyte attachment and culture on the membranes for over two weeks. Astrocytes and endothelial cells displayed markers specific to their cell types when grown on the silicon nitride membranes. The transendothelial electrical resistances, a measure of barrier tightness, of endothelial and astrocyte co-cultures on the silicon nitride membranes were comparable to the commercial membranes, but neither system showed synergy between the two cell types in forming a tighter barrier. This lack of synergy may have been due to the loss of ability of commercially available primary bovine brain microvascular endothelial cells to respond to astrocyte differentiating signals.

Expression of the Vacuolar H+-ATPase 16-kDa Subunit Results in the Triton X-100-insoluble Aggregation of β1 Integrin and Reduction of Its Cell Surface Expression

Vacuolar H(+)-ATPase functions as a vacuolar proton pump and is responsible for acidification of intracellular compartments such as the endoplasmic reticulum, Golgi, lysosomes, and endosomes. Previous reports have demonstrated that a 16-kDa subunit (16K) of vacuolar H(+)-ATPase via one of its transmembrane domains, TMD4, strongly associates with beta(1) integrin, affecting beta(1) integrin N-linked glycosylation and inhibiting its function as a matrix adhesion receptor. Because of this dramatic inhibition of beta(1) integrin-mediated HEK-293 cell motility by 16K expression, we investigated the mechanism by which 16 kDa was having this effect. Using HT1080 cells whose alpha(5)beta(1) integrin-mediated adhesion to fibronectin has been extensively studied, the expression of 16 kDa also resulted in reduced cell spreading on fibronectin-coated substrates. A pulse-chase study of beta(1) integrin biosynthesis indicated that 16K expression down-regulated the level of the 110-kDa biosynthetic form of beta(1) integrin (premature form) and, consequently, the level of the 130-kDa form of beta(1) integrin (mature form). Further experiments showed that the normal levels of association between the premature beta(1) integrin form and calnexin were significantly decreased by the expression of either 16 kDa or TMD4. Expression of 16 kDa also resulted in a Triton X-100-insoluble aggregation of an unusual 87-kDa form of beta(1) integrin. Interestingly, both Western blotting and a pulse-chase experiment showed co-immunoprecipitation of calnexin and 16K. These results indicate that 16K expression inhibits beta(1) integrin surface expression and spreading on matrix by a novel mechanism that results in reduced levels of functional beta(1) integrin.

Nutrition during brain activation: does cell-to-cell lactate shuttling contribute significantly to sweet and sour food for thought?

Functional activation of astrocytic metabolism is believed, according to one hypothesis, to be closely linked to excitatory neurotransmission and to provide lactate as fuel for oxidative metabolism in neighboring neurons. However, review of emerging evidence suggests that the energetic demands of activated astrocytes are higher and more complex than recognized and much of the lactate presumably produced by astrocytes is not locally oxidized during activation. In vivo activation studies in normal subjects reveal that the rise in consumption of blood-borne glucose usually exceeds that of oxygen, especially in retina compared to brain. When the contribution of glycogen, the brain's major energy reserve located in astrocytes, is taken into account the magnitude of the carbohydrate-oxygen utilization mismatch increases further because the magnitude of glycogenolysis greatly exceeds the incremental increase in utilization of blood-borne glucose. Failure of local oxygen consumption to equal that of glucose plus glycogen in vivo is strong evidence against stoichiometric transfer of lactate from astrocytes to neighboring neurons for oxidation. Thus, astrocytes, not nearby neurons, use the glycogen for energy during physiological activation in normal brain. These findings plus apparent compartmentation of metabolism of glycogen and blood-borne glucose during activation lead to our working hypothesis that activated astrocytes have high energy demands in their fine perisynaptic processes (filopodia) that might be met by glycogenolysis and glycolysis coupled to rapid lactate clearance. Tissue culture studies do not consistently support the lactate shuttle hypothesis because key elements of the model, glutamate-induced increases in glucose utilization and lactate release, are not observed in many astrocyte preparations, suggesting differences in their oxidative capacities that have not been included in the model. In vivo nutritional interactions between working neurons and astrocytes are not as simple as implied by "sweet (glucose-glycogen) and sour (lactate) food for thought."