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

Apoptosis, Autophagy, Necrosis and Their Multi Galore Crosstalk in Neurodegeneration

The progression of neurodegenerative disorders is mainly characterized by immense neuron loss and death of glial cells. The mechanisms which are active and regulate neuronal cell death are namely necrosis, necroptosis, autophagy and apoptosis. These death paradigms are governed by a set of molecular determinants that are pivotal in their performance and also exhibit remarkable overlapping functional pathways. A large number of such molecules have been demonstrated to be involved in the switching of death paradigms in various neurodegenerative diseases. In this review, we discuss various molecules and the concurrent crosstalk mediated by them. According to our present knowledge and research in neurodegeneration, molecules like Atg1, Beclin1, LC3, p53, TRB3, RIPK1 play switching roles toggling from one death mechanism to another. In addition, the review also focuses on the exorbitant number of newer molecules with the potential to cross communicate between death pathways and create a complex cell death scenario. This review highlights recent studies on the inter-dependent regulation of cell death paradigms in neurodegeneration, mediated by cross-communication between pathways. This will help in identifying potential targets for therapeutic intervention in neurodegenerative diseases.

Non-preferential fuelling of the Na(+)/K(+)-ATPase pump

There is abundant evidence that glycolysis and the Na(+)/K(+)-ATPase pump are functionally coupled, and it is thought that the nature of the coupling is energetic, with glycolysis providing the ATP that fuels the pump. This notion has been instrumental to current models of brain energy metabolism. However, structural and biophysical considerations suggest that the pump should also have access to mitochondrial ATP, which is much more abundant. In the present study, we have investigated the source of ATP that fuels the Na(+) pump in astrocytes, taking advantage of the high temporal resolution of recently available FRET nanosensors for glucose, lactate and ATP. The activity of the Na(+) pump was assessed in parallel with the Na(+)-sensitive dye SBFI AM (Na(+)-binding benzofuran isophthalate acetoxymethyl ester). OXPHOS (oxidative phosphorylation) inhibition resulted in bulk ATP depletion and a 5-fold stimulation of glycolytic flux, in spite of which Na(+) pumping was inhibited by 90%. Mathematical modelling of ATP dynamics showed that the observed pump failure is inconsistent with preferential fuelling of the Na(+) pump by glycolytic ATP. We conclude that the nature of the functional coupling between the Na(+) pump and the glycolytic machinery is not energetic and that the pump is mainly fuelled by mitochondrial ATP.

Necrotic cell death in Caenorhabditis elegans

Similar to other organisms, necrotic cell death in the nematode Caenorhabditis elegans is manifested as the catastrophic collapse of cellular homeostasis, in response to overwhelming stress that is inflicted either in the form of extreme environmental stimuli or by intrinsic insults such as the expression of proteins carrying deleterious mutations. Remarkably, necrotic cell death in C. elegans and pathological cell death in humans share multiple fundamental features and mechanistic aspects. Therefore, mechanisms mediating necrosis are also conserved across the evolutionary spectrum and render the worm a versatile tool, with the capacity to facilitate studies of human pathologies. Here, we overview necrotic paradigms that have been characterized in the nematode and outline the cellular and molecular mechanisms that mediate this mode of cell demise. In addition, we discuss experimental approaches that utilize C. elegans to elucidate the molecular underpinnings of devastating human disorders that entail necrosis.

Dissociated expression of mitochondrial and cytosolic creatine kinases in the human brain: a new perspective on the role of creatine in brain energy metabolism

The phosphocreatine/creatine kinase (PCr/CK) system in the brain is defined by the expression of two CK isozymes: the cytosolic brain-type CK (BCK) and the ubiquitous mitochondrial CK (uMtCK). The system plays an important role in supporting cellular energy metabolism by buffering adenosine triphosphate (ATP) consumption and improving the flux of high-energy phosphoryls around the cell. This system is well defined in muscle tissue, but there have been few detailed studies of this system in the brain, especially in humans. Creatine is known to be important for neurologic function, and its loss from the brain during development can lead to mental retardation. This study provides the first detailed immunohistochemical study of the expression pattern of BCK and uMtCK in the human brain. A strikingly dissociated pattern of expression was found: uMtCK was found to be ubiquitously and exclusively expressed in neuronal populations, whereas BCK was dominantly expressed in astrocytes, with a low and selective expression in neurons. This pattern indicates that the two CK isozymes are not widely coexpressed in the human brain, but rather are selectively expressed depending on the cell type. These results suggest that the brain cells may use only certain properties of the PCr/CK system depending on their energetic requirements.

Oxidatively modified glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and Alzheimer's disease: many pathways to neurodegeneration

Recently, the oxidoreductase, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), has become a subject of interest as more and more studies reveal a surfeit of diverse GAPDH functions, extending beyond traditional aerobic metabolism of glucose. As a result of multiple isoforms and cellular locales, GAPDH is able to come in contact with a variety of small molecules, proteins, membranes, etc., that play important roles in normal and pathologic cell function. Specifically, GAPDH has been shown to interact with neurodegenerative disease-associated proteins, including the amyloid-beta protein precursor (AbetaPP). Studies from our laboratory have shown significant inhibition of GAPDH dehydrogenase activity in Alzheimer's disease (AD) brain due to oxidative modification. Although oxidative stress and damage is a common phenomenon in the AD brain, it would seem that inhibition of glycolytic enzyme activity is merely one avenue in which AD pathology affects neuronal cell development and survival, as oxidative modification can also impart a toxic gain-of-function to many proteins, including GAPDH. In this review, we examine the many functions of GAPDH with respect to AD brain; in particular, the apparent role(s) of GAPDH in AD-related apoptotic cell death is emphasized.

Non-apoptotic cell death in Caenorhabditis elegans

The simple nematode worm Caenorhabditis elegans has been instrumental in deciphering the molecular mechanisms underlying apoptosis. Beyond apoptosis, several paradigms of non-apoptotic cell death, either genetically or extrinsically triggered, have also been described in C. elegans. Remarkably, non-apoptotic cell death in worms and pathological cell death in humans share numerous key features and mechanistic aspects. Such commonalities suggest that similarly to apoptosis, non-apoptotic cell death mechanisms are also conserved, and render the worm a useful organism, in which to model and dissect human pathologies. Indeed, the genetic malleability and the sophisticated molecular tools available for C. elegans have contributed decisively to advance our understanding of non-apoptotic cell death. Here, we review the literature on the various types of non-apoptotic cell death in C. elegans and discuss the implications, relevant to pathological conditions in humans.

Multifunctional roles of enolase in Alzheimer's disease brain: beyond altered glucose metabolism

Enolase enzymes are abundantly expressed, cytosolic carbon-oxygen lyases known for their role in glucose metabolism. Recently, enolase has been shown to possess a variety of different regulatory functions, beyond glycolysis and gluconeogenesis, associated with hypoxia, ischemia, and Alzheimer's disease (AD). AD is an age-associated neurodegenerative disorder characterized pathologically by elevated oxidative stress and subsequent damage to proteins, lipids, and nucleic acids, appearance of neurofibrillary tangles and senile plaques, and loss of synapse and neuronal cells. It is unclear if development of a hypometabolic environment is a consequence of or contributes to AD pathology, as there is not only a significant decline in brain glucose levels in AD, but also there is an increase in proteomics identified oxidatively modified glycolytic enzymes that are rendered inactive, including enolase. Previously, our laboratory identified alpha-enolase as one the most frequently up-regulated and oxidatively modified proteins in amnestic mild cognitive impairment (MCI), early-onset AD, and AD. However, the glycolytic conversion of 2-phosphoglycerate to phosphoenolpyruvate catalyzed by enolase does not directly produce ATP or NADH; therefore it is surprising that, among all glycolytic enzymes, alpha-enolase was one of only two glycolytic enzymes consistently up-regulated from MCI to AD. These findings suggest enolase is involved with more than glucose metabolism in AD brain, but may possess other functions, normally necessary to preserve brain function. This review examines potential altered function(s) of brain enolase in MCI, early-onset AD, and AD, alterations that may contribute to the biochemical, pathological, clinical characteristics, and progression of this dementing disorder.

Differential effects of iodoacetamide and iodoacetate on glycolysis and glutathione metabolism of cultured astrocytes

Iodoacetamide (IAA) and iodoacetate (IA) have frequently been used to inhibit glycolysis, since these compounds are known for their ability to irreversibly inhibit the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH). However, the consequences of a treatment with such thiol reagents on the glutathione (GSH) metabolism of brain cells have not been explored. Exposure of astroglia-rich primary cultures to IAA or IA in concentrations of up to 1 mM deprived the cells of GSH, inhibited cellular GAPDH activity, lowered cellular lactate production and caused a delayed cell death that was detectable after 90 min of incubation. However, the two thiol reagents differed substantially in their potential to deprive cellular GSH and to inhibit astrocytic glycolysis. IAA depleted the cellular GSH content more efficiently than IA as demonstrated by half-maximal effects for IAA and IA that were observed at concentrations of about 10 and 100 μM, respectively. In contrast, IA was highly efficient in inactivating GAPDH and lactate production with half-maximal effects observed already at a concentration below 100 μM, whereas IAA had to be applied in 10 times higher concentration to inhibit lactate production by 50%. These substantial differences of IAA and IA to affect GSH content and glycolysis of cultured astrocytes suggest that in order to inhibit astrocytic glycolysis without substantially compromising the cellular GSH metabolism, IA – and not IAA – should be used in low concentrations and/or for short incubation periods.

Non-developmentally programmed cell death in Caenorhabditis elegans

The simple nematode worm Caenorhabditis elegans has played a pivotal role in deciphering the molecular mechanisms of apoptosis. Precisely 131 somatic cells undergo programmed apoptotic death during development to contour the 959-cell adult organism. In addition to developmental cell death, specific genetic manipulations and extrinsic factors can trigger non-programmed cell death that is morphologically and mechanistically distinct from apoptosis. Here, we survey paradigms of cell death that is not developmentally programmed in C. elegans and review the molecular mechanisms involved. Furthermore, we consider the potential of the nematode as a platform to investigate pathological cell death. The striking extent of conservation between apoptotic pathways in worms and higher organisms including humans, holds promise that similarly, studies of non-programmed cell death in C. elegans will yield significant new insights, highly relevant to human pathology.

Extracellular taurine in the substantia nigra: Taurine-glutamate interaction

Taurine has been proposed as an inhibitory transmitter in the substantia nigra (SN), but the mechanisms involved in its release and uptake remain practically unexplored. We studied the extracellular pool of taurine in the rat's SN by using microdialysis methods, paying particular attention to the taurine-glutamate (GLU) interaction. Extracellular taurine increased after cell depolarization with high-K(+) in a Ca(2+)-dependent manner, being modified by the local perfusion of GLU, GLU receptor agonists, and zinc. Nigral administration of taurine increased the extracellular concentration of gamma-aminobutyric acid (GABA) and GLU, the transmitters of the two main inputs of the SN. The modification of the glial metabolism with fluocitrate and L-methionine sulfoximine also changed the extracellular concentration of taurine. The complex regulation of the extracellular pool of taurine, its interaction with GABA and GLU, and the involvement of glial cells in its regulation suggest a volume transmission role for taurine in the SN.

Pyruvate ameliorates post ischemic injury of rat astrocytes and protects them against PARP mediated cell death

This in vitro study was designed to examine the efficacy of exogenous pyruvate and glucose as a fuel substrate to protect rat astrocytes from post-ischemic injury. Astrocytes were incubated in Kreb's buffer deprived of oxygen and glucose for 6 h (ischemia) followed by incubation with added pyruvate or glucose and normoxia for the next 6 h (reperfusion). The transformation of reactive astrocytes in response to various treatments was examined by immunostaining with glial fibrillary acidic protein. The extent of cell damage was evaluated in terms of lactate dehydrogenase leakage from the cells and altered intracellular redox status. The mechanism of cell death was determined by immunoblotting with cytochrome C, caspase-3 and PARP antibodies. The mechanism of the action of pyruvate was determined by measuring the activity of pyruvate dehydrogenase complex, and cellular metabolic status by measuring ATP levels. In comparison to glucose, supply of exogenous pyruvate restored the morphological integrity of post-ischemic astrocytes and prevented gliosis. Pyruvate prevented the cell death of post-ischemic astrocytes by inhibiting the leakage of lactate dehydrogenase, decreasing the redox ratio and restraining the activation of apoptotic events such as release of mitochondrial cytochrome c and fragmentation of caspase-3 and PARP. This study also suggests that pyruvate may accelerate its own metabolism by increasing the activity of pyruvate dehydrogenase and thus restores the cellular ATP levels in post-ischemic astrocytes. Use of pyruvate as an alternate fuel substrate may provide a possibility for the novel therapeutic approach to the treatment of cerebral ischemia.

Astrocytes degenerate in frontotemporal dementia: possible relation to hypoperfusion

To understand the extent and specificity of astrocyte pathology in sporadic frontotemporal dementia (FTD), we examined several FTD cases for molecular and morphologic characteristics of astrocyte degeneration. We quantified reactive and degenerating astrocytes in sections of frontal, temporal, parietal, and occipital cortex identified using glial fibrillary acidic protein (GFAP) immunoreactivity, terminal deoxynucleotidyl transferase (TdT) labeling, and morphological characteristics and compared them with nondemented, age-matched control brains. Conventional and confocal microscopy revealed that a subpopulation of GFAP(+) astrocytes exhibited positive TdT labeling and beading of their processes in the frontal, temporal, and parietal cortices in 5 of 7 FTD cases that also exhibited gliosis. This morphology was reproduced in cultured astrocytes using ischemic insults. Degenerating astrocytes in FTD correlated inversely with cerebral blood flow as measured by single photon emission computed tomography (SPECT) analysis of (133)Xe inhalation (r = 0.55, p < 0.05). Furthermore, areas of significant astrogliosis corresponded to areas of SPECT hypoperfusion, suggesting that astrocytes may be affected by or perhaps have a causal role in the disturbances of cerebral perfusion in FTD.

Requirement of glycolytic and mitochondrial energy supply for loading of Ca(2+) stores and InsP(3)-mediated Ca(2+) signaling in rat hippocampus astrocytes

A major consequence of brain hypoxia and hypoglycemia, which induces the detrimental effects of stroke, is impaired ATP supply. However, it is not yet clear to which degree reduced cellular ATP production affects Ca(2+) homeostasis and Ca(2+) signaling of glia cells. Here we studied in cultured hippocampal astrocytes the influence of inhibition of cellular energy supply on Ca(2+) load of intracellular stores. Inhibition of glycolysis in the presence of substrates for mitochondrial respiration resulted in an average drop of intracellular ATP levels by 35%. Inhibition of oxidative phosphorylation reduced intracellular ATP on average by 16%. With inhibition of both glycolysis and mitochondrial ATP production, intracellular ATP level was drastically reduced (84%). In astrocytes in Ca(2+)-free buffer, cytosolic [Ca(2+)](i) was dramatically increased due to inhibition of glycolysis, even in the presence of mitochondrial substrates. However, only a minor increase of [Ca(2+)](i) was observed with inhibitors of mitochondrial ATP synthesis. Remarkably, the moderate reduction of ATP levels found with inhibitors of glycolysis caused a severe loss of Ca(2+) from cyclopiazonic acid (CPA)-sensitive Ca(2+) stores. Consequently, inhibition of glycolysis reduced P2Y receptor- or thrombin receptor-evoked Ca(2+) responses on average by 95%, whereas a reduction of only 26% was found with mitochondrial inhibitors. In conclusion, glycolysis is the most important source of ATP for the maintenance of Ca(2+) load in stores that are required for transmitter-induced signaling. These results are consistent with the concept that a local ATP source in the vicinity of endoplasmic reticulum Ca(2+) pumps is required.

Neuronal-glial interactions and behaviour

Both neurons and glia interact dynamically to enable information processing and behaviour. They have had increasingly intimate, numerous and differentiated associations during brain evolution. Radial glia form a scaffold for neuronal developmental migration and astrocytes enable later synapse elimination. Functionally syncytial glial cells are depolarised by elevated potassium to generate slow potential shifts that are quantitatively related to arousal, levels of motivation and accompany learning. Potassium stimulates astrocytic glycogenolysis and neuronal oxidative metabolism, the former of which is necessary for passive avoidance learning in chicks. Neurons oxidatively metabolise lactate/pyruvate derived from astrocytic glycolysis as their major energy source, stimulated by elevated glutamate. In astrocytes, noradrenaline activates both glycogenolysis and oxidative metabolism. Neuronal glutamate depends crucially on the supply of astrocytically derived glutamine. Released glutamate depolarises astrocytes and their handling of potassium and induces waves of elevated intracellular calcium. Serotonin causes astrocytic hyperpolarisation. Astrocytes alter their physical relationships with neurons to regulate neuronal communication in the hypothalamus during lactation, parturition and dehydration and in response to steroid hormones. There is also structural plasticity of astrocytes during learning in cortex and cerebellum.

Potassium signalling in the brain: its role in behaviour

This paper examines evidence that glial cells respond to changes in extracellular potassium ([K+]e) in ways that contribute to modulation of neuronal activity and thereby behaviour. Glial cells spatially (and probably directionally) redistribute potassium from regions of increasing concentration to those with a lesser concentration. This redistribution is largely responsible for slow potential shifts associated with behavioural responses of animals. These slow shifts are related in amplitude to the level of 'arousal' of an animal, and its motivational state. In addition, glia, especially astrocytes, respond to changes in [K+]e, the presence of transmitters like nor-adrenaline and glutamate and at least some hormones with changes in their metabolism and/or the morphological characteristics of the cell. The ionic, metabolic and morphological responses of glia to changes in extracellular potassium after neuronal activity have been associated with at least some forms of learning, including habituation, one trial passive avoidance learning and changes associated with enriched environments. The implication of these effects of potassium signalling in the brain is that there is considerable involvement of glia in a number of processes crucial to neuronal activity. Glia may also form another route for information distribution in the brain that is at least bi-directional, though less specific than its neuronal counterparts. It is evident that the Neuroscience of the future will have to incorporate much more study of neuron-glial interactions than hitherto.

Delayed treatment with an adenosine kinase inhibitor, GP683, attenuates infarct size in rats with temporary middle cerebral artery occlusion

BACKGROUND AND PURPOSE

Brain ischemia is associated with a marked increase in extracellular adenosine levels. This results in activation of cell surface adenosine receptors and some degree of neuroprotection. Adenosine kinase is a key enzyme controlling adenosine metabolism. Inhibition of this enzyme enhances the levels of endogenous brain adenosine already elevated as a result of the ischemic episode. We studied a novel adenosine kinase inhibitor (AKI), GP683, in a rat focal ischemia model.

METHODS

Four groups of 10 adult Sprague-Dawley rats were exposed to 90 minutes of temporary middle cerebral artery (MCA) occlusion. Animals were injected intraperitoneally with vehicle, 0.5 mg/kg, 1.0 mg/kg, or 2.0 mg/kg of GP683 30, 150, and 270 minutes after the induction of ischemia by a researcher blinded to treatment group. The animals were euthanatized 24 hours after MCA occlusion, and brains were stained with 2,3,5-triphenyltetrazolium chloride. We measured brain temperatures in a separate group of 6 rats before and after administration of 1.0 mg/kg GP683.

RESULTS

All treated groups showed a reduction in infarct volumes, but a significant effect was observed only in the 1.0 mg/kg-dose group (44% reduction, P=0.0077). Body weight, physiological parameters, neurological scores, and mortality did not differ among the 4 groups. No apparent behavioral side effects were observed. Brain temperatures did not change after drug injection.

CONCLUSIONS

Our results indicate that the use of AKIs offers therapeutic potential and may represent a novel approach to the treatment of acute brain ischemia. The therapeutic effect observed was not caused by a decrease in brain temperature.

Effects of glucose deprivation, chemical hypoxia, and simulated ischemia on Na+ homeostasis in rat spinal cord astrocytes

A steep inwardly directed Na+ gradient is essential for glial functions such as glutamate reuptake and regulation of intracellular ion concentrations. We investigated the effects of glucose deprivation, chemical hypoxia, and simulated ischemia on intracellular Na+ concentration ([Na+]i) in cultured spinal cord astrocytes using fluorescence ratio imaging with sodium-binding benzofuran isophthalate (SBFI) AM. Glucose removal or chemical hypoxia (induced by 10 mM NaN3) for 60 min increased [Na+]i from a baseline of 8.3 to 11 mM. Combined glycolytic and respiratory blockage by NaN3 and 0 glucose saline caused [Na+]i to increase by 20 mM, similar to the [Na+]i increases elicited by blocking the Na+/K+-ATPase with ouabain. Recovery from large [Na+]i increases (>15 mM) induced by the glutamatergic agonist kainate was attenuated during glucose deprivation or NaN3 application and was blocked in NaN3 and 0 glucose. To mimic in vivo ischemia, we exposed astrocytes to NaN3 and 0 glucose saline containing L-lactate and glutamate with increased [K+] and decreased [Na+], [Ca2+], and pH. This induced an [Na+]i decrease followed by an [Na+]i rise and a further [Na+]i increase after reperfusion with standard saline. Similar multiphasic [Na+]i changes were observed after NaN3 and 0 glucose saline with only reduced [Na+]e. Our results suggest that the ability to maintain a low [Na+]i enables spinal cord astrocytes to continue uptake of K+ and/or glutamate at the onset of energy failure. With prolonged energy failure, however, astrocytic [Na+]i rises; with loss of their steep transmembrane Na+ gradient, astrocytes may aggravate metabolic insults by carrier reversal and release of acid, K+, and/or glutamate into the extracellular space.

Dexamethasone augments ischemia-induced extracellular accumulation of glutamate in gerbil hippocampus

Glucocorticoids exacerbate neuronal damage due to hypoxia, ischemia, seizure and hypoglycemia. Because the release of glutamate is closely involved in neuronal damage, the effects of dexamethasone on the ischemia-induced accumulation of extracellular amino acids (aspartate, glutamate, and glycine) were investigated in the gerbil hippocampal CA1 region by a microdialysis-high-performance liquid chromatography procedure in vivo. There were no differences in the extracellular concentrations of amino acids before ischemia between the control group and the dexamethasone (3m microg, i.c.v.)-injected group. The concentration of glutamate reached 246% of that before ischemia within 2.5 min of transient forebrain ischemia. Dexamethasone augmented the increase in glutamate to 508% of that before ischemia. This finding suggests that glucocorticoids aggravate ischemic neuronal damage by causing glutamate to accumulate in the extracellular space.

Endogenous monocarboxylates sustain hippocampal synaptic function and morphological integrity during energy deprivation

The ability to fuel neurons via glycogenolysis is believed to be an important function of glia. Indeed, the slow, rather than immediate, depression of synaptic transmission in hippocampal slices during exogenous glucose deprivation suggests that intrinsic energy reservoirs help to sustain neurotransmission. It is believed that glia fuel neighboring neurons via diffusible monocarboxylates such as pyruvate and lactate, although a role for glucose has been proposed also. Using alpha-cyano-4-hydroxycinnamate (4-CIN) to inhibit monocarboxylate transport and cytochalasin B (CCB) to inhibit glucose transport, we examined the role of glucose and monocarboxylates in supporting the functional and morphological integrity of hippocampal neurons during glucose deprivation. Although 200 microM 4-CIN failed to depress EPSPs supported by 10 mM glucose, pretreatment with 4-CIN accelerated the depression of EPSPs during glucose deprivation. 4-CIN also accelerated the decline in glucose-supported EPSPs after administration of 50 microM CCB, whereas CCB failed to alter the slow decay of pyruvate-supported EPSPs during pyruvate deprivation. 4-CIN did not alter the morphology of pyramidal neurons in the presence of 10 mM glucose but produced significant damage during glucose deprivation or CCB administration. These results suggest that endogenous monocarboxylates rather than glucose maintain neuronal integrity during energy deprivation. Furthermore, EPSPs supported by 2-3.3 mM glucose were sensitive to 4-CIN, suggesting that endogenous monocarboxylates are involved in maintaining neuronal function even under conditions of mild glucose deprivation.

Spontaneous induction of nitric oxide- and prostaglandin E2-release by hypoxic astroglial cells is modulated by interleukin 1 beta

The effect of 0, 30, 60, 120, 240, 360 min hypoxia on the release of NO and PGE2 was investigated in human cultured astroglial cells. Exposure of astroglial cells to hypoxic injury produced a dose-dependent increase of the nitrite (the breakdown product of NO) level in the cell supernatant. In addition, a significant activation of the inducible isoform of NO synthase was seen, demonstrating that the enhancement on NO release produced by hypoxic injury was related to an increased biosynthesis of NO-generating enzyme(s). This effect was strongly antagonised by pretreating cells with dexamethasone (20 microM). The increase in NO release by hypoxic astroglial cells was accompanied by sustained release of PGE2, which was antagonised by the cyclooxygenase inhibitor indomethacin (10 microM), and partially attenuated by L-NAME (100 microM), a nitric oxide synthase inhibitor, showing that the release of PGE2 was driven by NO. Finally, inducible NOS activity elicited by hypoxic injury, was antagonised by incubating astroglial cells with antibodies directed against type 2 receptor for IL1 beta. In conclusion, hypoxia stimulates cytokine network in astroglial cells leading to enhanced release of NO and prostanoids and this may represent a key mechanism in cerebral blood flow disturbances.

Adenosine kinase inhibition protects brain against transient focal ischemia in rats

Endogenous adenosine released locally during cerebral ischemia is neuroprotective, and agents which decrease adenosine inactivation may potentiate its protective effects. The effects of 5'-deoxy-5-iodotubercidin (5'd-5IT), an inhibitor of the adenosine-catabolizing enzyme, adenosine kinase, were studied in male Wistar rats subjected to 2 h of transient middle cerebral artery occlusion. 5'd-5IT or the vehicle (10% DMSO in saline) was administered i.p. 30 min before, and 2 h and 6 h after the induction of middle cerebral artery occlusion. The infarct volume was determine using 2,3,5-triphenyltetrazolium chloride staining 48 h after middle cerebral artery occlusion. The infarct volume was significantly reduced in rats treated with 1.85 mg/kg x 3 (57% reduction, P < 0.001) or 1.0 mg/kg x 3 (34% reduction, P < 0.05), but not 0.3 mg/kg x 3 5'd-5IT compared to vehicle-treated rats. The reduction of infarct volume was accompanied by a significant improvement in behavioral measures of neurological deficit. These data further support a role of adenosine in neuroprotection and suggest that adenosine kinase inhibition may be a useful approach to the treatment of focal cerebral ischemia.

Stimulation of glutamate uptake and Na,K-ATPase activity in rat astrocytes exposed to ischemia-like insults

The postsynaptic actions of glutamate are rapidly terminated by high affinity glutamate uptake into glial cells. In this study we demonstrate the stimulation of both glutamate uptake and Na,K-ATPase activity in rat astrocyte cultures in response to sublethal ischemia-like insults. Primary cultures of neonatal rat cortical astrocytes were subjected to hypoxia, or to serum- and glucose-free medium, or to both conditions (ischemia). Cell death was assessed by propidium iodide staining of cell nuclei. To measure sodium pump activity and glutamate uptake, 3H-glutamate and 86Rb were both simultaneously added to the cell culture in the presence or absence of 2 mM ouabain. Na,K-ATPase activity was defined as ouabain-sensitive 86Rb uptake. Concomitant transient increases (2-3 times above control levels) of both Na,K-ATPase and glutamate transporter activities were observed in astrocytes after 4-24 h of hypoxia, 4 h of glucose deprivation, and 2-4 h of ischemia. A 24 h ischemia caused a profound loss of both activities in parallel with significant cell death. The addition of 5 mM glucose to the cells after 4 h ischemia prevented the loss of both sodium pump activity and glutamate uptake and rescued astrocytes from death observed at the end of 24 h ischemia. Reoxygenation after the 4 h ischemic event caused the selective inhibition of Na,K-ATPase activity. The observed increases in Na,K-ATPase activity and glutamate uptake in cultured astrocytes subjected to sublethal ischemia-like insults may model an important functional response of astrocytes in vivo by which they attempt to maintain ion and glutamate homeostasis under restricted energy and oxygen supply.

Recovery potential in glucose deprived astrocytes

D-glucose deprivation for a 45 min period reduces the ATP and creatine phosphate concentrations of astrocytes. Recovery experiments were initiated by reincubating the cells with D-glucose and glucose replacement metabolites. No recovery of ATP concentration could be obtained even after 1 h of reincubation with the replacement metabolites. After a 45 min incubation period without D-glucose, 14CO2 production fell to 36% and 21% of controls when the cells were reincubated respectively with D-[U-14C]-glucose and L-[2-14C]-pyruvate as substrate marker. When reincubated for 1 h in the presence of L-malate (1 mM)+L-pyruvate (10 mM) with L-[2-14C]-pyruvate as marker, a total recovery of 14CO2 production was ascertained. Reincubation of the glucose deprived cells in the presence of D-glucose (10 mM) did not increase the 14CO2 production indicating that the cells were unable to use D-glucose for oxidative purposes. As pyruvate concentration was dramatically decreased in glucose deprived cells, astrocytes were treated with alpha-ketovalerate (25 mM) which led to an 8-fold increase in pyruvate concentration. In these conditions 14CO2 production did not increase when the cells were incubated in the presence of L-malate (1 mM). O2 consumption of State 4 in astrocytes, submitted to glucose deprivation, decreased. These cells treated with FCCP could not be uncoupled and when reincubated in the presence of replacement metabolites only a 20% increase of oxygen consumption took place.

Effect of glucose deprivation on rat glutamine synthetase in cultured astrocytes

Glutamine synthetase was purified from the cerebral cortex of adult rats and characterized. Polyclonal rabbit antibodies were raised against the enzyme, purified and their specific anti-(glutamine synthetase) activity determined. A primary astroglial culture was prepared from newborn Sprague-Dawley rats. Astrocytes at different ages of development were incubated in the presence and absence of glucose. In glucose-deprived conditions the specific activity of glutamine synthetase decreased. This decrease was more pronounced in 8-day-old than in 21-day-old cultures. Kinetic analysis demonstrated that the reduction in activity was mainly related to a decrease in Vmax. By immunoprecipitation, it was shown that the number of enzyme molecules in astrocytes was decreased in glucose-deprived conditions. On addition of glucose, a total recovery of glutamine synthetase was obtained after 36 h in 8-day-old culture. Rates of degradation and synthesis were investigated. When compared with an incubation in the presence of glucose, glucose deprivation increased enzyme turnover, as estimated from the first-order disappearance of radioactivity from glutamine synthetase. Synthesis rate was estimated from the incorporation of [35S]methionine during a 2 h incubation period and was decreased in glucose-deprived conditions. Trichloroacetate-precipitable proteins changed only slightly in the experimental conditions, and total protein did not vary significantly during the experimental period. A mathematical model is presented which attempts to integrate degradation and synthesis in our experimental model.

Astrocyte-neuron interaction during one-trial aversive learning in the neonate chick

During two specific stages of the Gibbs-Ng model of one-trial aversive learning in the neonate chick, we have recently found unequivocal evidence for a crucial involvement of astrocytes. This evidence is metabolic (utilization of the astrocyte-specific energy store, glycogen, during normal learning and inhibition of memory formation by the astrocyte specific metabolic inhibitors, fluoroacetate and methionine sulfoximine) as well as physiological (abolition of memory formation in the presence of ethacrynic acid, an astrocyte-specific inhibitor of cellular reaccumulation of potassium ions). These findings are discussed in the present review in the framework of a more comprehensive description of metabolic and physiological neuronal-astrocytic interactions across an interstitial (extracellular) space bounded by minute processes from either cell type.

Pharmacological characteristics of potassium-induced, glycogenolysis in astrocytes

Elevated extracellular concentrations of the potassium ion ([K+]o) stimulate glycogenolysis in primary cultures of mouse astrocytes that have been grown in the presence of dibutyryl cyclic AMP but not in corresponding cultures which have not been treated in this manner. The response is potently inhibited by nifedipine, suggesting that it is evoked by entry of calcium ions through voltage dependent L-channels. The benzodiazepine midazolam, which is known to enhance calcium entry at concentrations of [K+]o causing submaximum calcium entry, increases the glycogenolytic effect by such levels of [K+]o.

Herpes simplex virus vectors overexpressing the glucose transporter gene protect against seizure-induced neuron loss

We have generated herpes simplex virus (HSV) vectors vIE1GT and v alpha 4GT bearing the GLUT-1 isoform of the rat brain glucose transporter (GT) under the control of the human cytomegalovirus ie1 and HSV alpha 4 promoters, respectively. We previously reported that such vectors enhance glucose uptake in hippocampal cultures and the hippocampus. In this study we demonstrate that such vectors can maintain neuronal metabolism and reduce the extent of neuron loss in cultures after a period of hypoglycemia. Microinfusion of GT vectors into the rat hippocampus also reduces kainic acid-induced seizure damage in the CA3 cell field. Furthermore, delivery of the vector even after onset of the seizure is protective, suggesting that HSV-mediated gene transfer for neuroprotection need not be carried out in anticipation of neurologic crises. Using the bicistronic vector v alpha 22 beta gal alpha 4GT, which coexpresses both GT and the Escherichia coli lacZ marker gene, we further demonstrate an inverse correlation between the extent of vector expression in the dentate and the amount of CA3 damage resulting from the simultaneous delivery of kainic acid.

Energy metabolism in hypoxic astrocytes: protective mechanism of fructose-1,6-bisphosphate

The protective effects of fructose-1,6-biphosphate (FBP) during hypoxia/ischemia are thought to result from uptake and utilization of FBP as a substrate for glycolysis or from stimulation of glucose metabolism. To test these hypotheses, we measured CO2 and lactate production from [6-14C]glucose, [1-14C]glucose, and [U-14C]FBP in normoxic and hypoxic cultured astrocytes with and without FBP present. FBP had little effect on CO2 production by glycolysis, but increased CO2 production by the pentose phosphate pathway. Labeled FBP produced very small amounts of CO2. Lactate production from [1-, and 6-14C]glucose increased similarly during hypoxic hypoxia; the increase was independent of added FBP. Labeled lactate from [U-14C]FBP was minimal. We conclude that exogenous FBP is not used by astrocytes as a substrate for glycolysis and that FBP alters glucose metabolism.

High extracellular potassium concentrations stimulate oxidative metabolism in a glutamatergic neuronal culture and glycolysis in cultured astrocytes but have no stimulatory effect in a GABAergic neuronal culture

Rates of deoxyglucose accumulation and of CO2 production from [U-14C]glucose, or from [U-14C]lactate or [2-14C]pyruvate (as a determination of tricarboxylic acid (TCA) cycle activity) were determined in primary cultures of either astrocytes, cerebellar granule cell neurons (utilizing glutamate as their transmitter) or cerebral cortical interneurons (utilizing GABA as their transmitter) during control ('resting') conditions and during exposure to an elevated extracellular potassium concentration, mimicking functional activity. The elevation of the extracellular potassium concentration increased the rate of deoxyglucose accumulation, but not of TCA cycle activity in astrocytes and both deoxyglucose accumulation and TCA cycle activity in cerebellar granule cells, but had no stimulatory effect in cerebral cortical neurons. Based on these observations it is suggested that the increase in energy metabolism in the CNS in vivo during functional activity mainly reflects increased active accumulation of potassium ions and extrusion of sodium ions in neurons receiving excitatory input and in adjacent astrocytes in order to re-establish pre-stimulus ion distribution across cell membranes.

Ethanol and glucose-deprivation neurotoxicity in cortical cell cultures

The toxic effects of ethanol on rat cortical cell cultures were compared with neuronal damage induced by glucose deprivation. Exposure to decreased glucose concentrations produced dose-dependent neuronal injury, as indicated by the release of lactate dehydrogenase (LDH) into the culture medium. Complete glucose deprivation resulted in mean LDH release that was more than 60% greater than that from sister cultures incubated in the presence of 5.5 mmol/L glucose. Exposure to ethanol (25, 50, or 100 mmol/L) similarly resulted in dose-related LDH release. The degree of injury resulting from complete glucose deprivation or 100 mmol/L ethanol approximated that produced by exposure to 100 mmol/L glutamic acid. Ethanol did not significantly alter LDH release from cultures consisting of only astrocytes. Both effects were inhibited by the N-methyl-D-aspartate (NMDA) receptor antagonist, D,L-2-amino-5-phosphonovaleric acid (APV). Glutamate levels were increased in the culture medium to 191% +/- 8% of the control value after glucose deprivation (P < .001) and to 186% +/- 16% after exposure to 100 mmol/L ethanol (P < .01). 3H-glutamate uptake by cultured astrocytes was reduced by glucose deprivation and by ethanol. This range of ethanol concentrations has previously been shown to inhibit hexose uptake by cultured astrocytes. The present results suggest that decreased glucose uptake by astrocytes in the presence of ethanol impairs their uptake of glutamate, which contributes to excitotoxic neuronal injury.

Glucocorticoids increase extracellular [3H]D-aspartate overflow in hippocampal cultures during cyanide-induced ischemia

Glucocorticoids (GCs), the adrenal steroid hormones secreted during stress, exacerbate neuronal death in the hippocampus during ischemia. Since ischemia brain damage is ascribed to an elevated level of extracellular excitatory amino acids (EAAs), this study was undertaken to investigate the effect of GCs on EAA homeostasis in hippocampal cell cultures during the insult of cyanide exposure. Using D-[2,3-3H]aspartic acid ([3H]D-Asp) as a tracer, we found that corticosterone (CORT, the physiological GC in rats) increased the accumulation of extracellular [3H]D-Asp by 25% in hippocampal cultures during cyanide-induced ischemia. CORT had no effect on the release of [3H]D-Asp. Instead, analysis of [3H]D-Asp uptake kinetics indicates that CORT decreased the maximum uptake rate and the Michaelis constant by 44% and 50%, respectively, in cells treated with cyanide. It is concluded that, during cyanide-induced ischemia, CORT might enhance extracellular overflow of [3H]D-Asp by decreasing its uptake, thereby endangering neurons.

Cell death in primary cultures of mouse neurons and astrocytes during exposure to and 'recovery' from hypoxia, substrate deprivation and simulated ischemia

Effects of hypoxia, substrate deprivation and simulated ischemia (combined hypoxia and substrate deprivation) on cell survival during the insult itself and during a 24 h 'recovery' period were studied in primary cultures of mouse astrocytes and in cerebral cortical neuronal-astrocytic co-cultures. Cell death was determined by release of the cytosolic high molecular enzyme lactate dehydrogenase (LDH) as well as morphologically (retention of staining with rhodamine 123 and lack of staining with propidium iodide as an indicator of live cells). Glutamate concentrations were measured in the incubation media at the end of the metabolic insults. Astrocytes were very resistant to hypoxia, but less so to simulated ischemia; under both conditions the glutamate concentrations in the media remained low. Cerebral cortical neurons were almost equally susceptible to damage by hypoxia and by simulated ischemia, although hypoxia had a faster deleterious effects on some of the neurons and simulated ischemia during a long-term insult (9 h) killed all neurons, whereas a non-negligible neuronal subpopulation survived 9 h of hypoxia. Neuronal cell death after long-term hypoxia (but not after simulated ischemia) was correlated with high concentrations of glutamate in the incubation media. After certain insults, most notably relatively short lasting simulated ischemia (3 h) in neurons (which caused no increased cell death during the insult), there was a large release of LDH during the 'recovery' period.

Glutamate uptake and glutamate content in primary cultures of mouse astrocytes during anoxia, substrate deprivation and simulated ischemia under normothermic and hypothermic conditions

During brain ischemia in vivo the extracellular concentration of the excitotoxic amino acid, glutamate, increases. This increase could be caused either by an enhanced formation rate of glutamate (from glutamine) or by an impaired re-uptake (or both). This re-uptake occurs to a large extent in astrocytes. In the present study we have determined glutamate uptake and the ability of the cells to maintain their glutamate content during exposure to anoxia, substrate deprivation and combined substrate deprivation and anoxia ('simulated ischemia') for a duration of up to 4 h. Isolated anoxia had no significant effect, whereas both substrate deprivation alone and 'simulated ischemia' reduced glutamate uptake and glutamate content by one-half after 2 h. Under hypothermic conditions (incubation at 32 degrees C), which in in vivo experiments exerts some protection against ischemic cell death in neurons, ischemia of intermediate duration (2 h) decreased glutamate uptake and glutamate content to a less extent than at 37 degrees C. Hypothermia did not have a similar effect during exposure to isolated substrate deprivation.

Astrocyte glutamate uptake during chemical hypoxia in vitro

Glutamate uptake was measured in primary rat cortical astrocyte cultures exposed to sodium azide, 2,4-dinitrophenol, or antimycin A to assess the ability of astrocytes to function under hypoxic conditions. Uptake was maintained at 54-63% of control values despite maximal inhibition of oxidative ATP production. In contrast, the glycolytic inhibitor sodium fluoride (20 mM) reduced glutamate uptake by more than 95% when glucose was the only available substrate. These data suggest that glutamate uptake is largely maintained during hypoxia provided glucose remains available. Astrocyte glutamate uptake may aid neuronal survival during conditions such as incomplete ischemia where oxygen but not glucose is depleted.

Corticosterone accelerates hypoxia- and cyanide-induced ATP loss in cultured hippocampal astrocytes

Glucocorticoids potentiate injury to the rodent hippocampus following a variety of metabolic insults, including hypoxia/ischemia, both in vitro and in vivo. We have examined whether corticosterone (CORT), the principal glucocorticoid in the rat, could exacerbate hypoxic energy failure in cultured hippocampal astrocytes. Exposure to 6 h of atmospheric hypoxia (100% N2) or to 30 min of cyanide did not cause any detectable cell injury, although moderate astrocyte damage did occur alter 6 h of hypoxia in the absence of glucose. Both cyanide and hypoxia significantly reduced astrocyte ATP content, a decline that was further reduced when glucose was omitted. A 30 min exposure to 100 microM glutamate elevated ATP content under normoxic conditions but enhanced the cyanide-induced loss of ATP. A 24 h pre-treatment with CORT did not influence normoxic ATP levels but potentiated the loss of ATP following both cyanide and hypoxia. CORT also exacerbated the loss of ATP seen after combined exposure to cyanide and glutamate, as well as that following cyanide + 0 mM glucose. These results indicate that both CORT and glutamate can potentiate hypoxia-induced energy failure in hippocampal astrocytes, albeit by different mechanisms.

Glucocorticoids exacerbate hypoxic and hypoglycemic hippocampal injury in vitro: biochemical correlates and a role for astrocytes

The acute secretion of glucocorticoids is critical for responding to physiological stress. Under normal circumstances these hormones do not cause acute neuronal injury, but they have been shown to enhance ischemic and seizure-induced neuronal injury in the rat brain. Using fetal rat hippocampal cultures, we asked whether hypoxic and hypoglycemic cell damage in vitro could be exacerbated by direct exposure to corticosterone (CORT). Each of these insults alone damaged neuronal cells, whereas 4-6 h of hypoxic treatment could damage age-matched astrocytes if glucose was reduced or omitted. Ischemic-like injury to both cell types could be attenuated by pretreatment with high (30 mM) glucose. Exposure to 100 nM CORT did not affect cell viability under control conditions but enhanced both hypoxic and hypoglycemic neuronal injury. In both cases, pretreatment with high glucose abolished this CORT-mediated synergy. In astrocyte cultures, CORT exacerbated both hypoxic and hypoglycemic injury and this effect was also attenuated by high-glucose pretreatment. Identical 24-h CORT treatment caused a 13% reduction in glucose uptake in astrocytes and a 38% reduction in glycogen content, without affecting the level of intracellular glucose. Thus, CORT could endanger both neurons and astrocytes in mixed hippocampal cultures and this effect emerged only under conditions of substrate depletion. The metabolic disruption in astrocytes by CORT further suggests that the ability of CORT to exacerbate neuronal injury may be due in part to impaired glial cell function.

Developmental and regional differences in the vulnerability of rat hippocampal slices to lack of glucose

Field excitatory postsynaptic potentials were recorded in stratum radiatum of CA1 and CA3 in submerged hippocampal slices from adult or newborn (postnatal days 5-25) Wistar rats. In adult slices, excitatory postsynaptic potentials were depressed by glucose removal ("aglycemia") more rapidly and to a greater extent in CA1 than in CA3 [respective mean times to 50% reduction in peak amplitude were 7.5 +/- 0.83 (standard error) min and 12.5 +/- 0.27 (standard error) min]. Subsequent recovery of excitatory postsynaptic potentials in normoglycemic medium was correspondingly quicker in CA3 than in CA1. Transmission failure at the synapses was indicated by the preservation of the afferent volley, and sharp depression of synaptic input-output plots. In the early postnatal period, CA1 excitatory postsynaptic potentials were much more resistant to aglycemia, substantially persisting for as long as 75 min, with full subsequent recovery in normoglycemic medium. The higher resistance of slices from newborn rats progressively disappeared over the first two postnatal weeks.

Metabolic inhibition and electrical properties of type-1-like cortical astrocytes

Type-1-like cortical mouse astrocytes were studied in homogeneous cultures. Membrane input resistance and membrane potential were measured during drug-induced inhibition of glycolysis (sodium fluoride), mitochondrial respiration (antimycin-a) and Na+/K+ pump activity (ouabain). It was found that the electrical properties of the astrocytes recovered after a 60 min period with inhibited glycolysis or mitochondrial respiration, exhibiting only small reversible depolarizations. A 60 min period of high K(+)-induced depolarization, of cell swelling or of Na+/K+ pump inhibition does not lead to irreversible changes. Total block of energy metabolism, however, causes (1) a large depolarization, which is mainly mediated by external calcium, and (2) a 10-fold increase in input resistance, suggestive of an uncoupling of gap junctions. After an exposure period ranging between 45 and 60 min these conditions lead to irreversible damage. This damage appears to be independent of extracellular calcium and the degree of depolarization and to be specifically mediated by events occurring after the 60-min period of inhibited cell metabolism, that is during the recovery period.

Glucocorticoids inhibit glucose transport and glutamate uptake in hippocampal astrocytes: implications for glucocorticoid neurotoxicity

Glucocorticoids (GCs), the adrenal steroid hormones secreted during stress, can damage the hippocampus and impair its capacity to survive coincident neurological insults. This GC endangerment of the hippocampus is energetic in nature, as it can be prevented when neurons are supplemented with additional energy substrates. This energetic endangerment might arise from the ability of GCs to inhibit glucose transport into both hippocampal neurons and astrocytes. The present study explores the GC inhibition in astrocytes. (1) GCs inhibited glucose transport approximately 15-30% in both primary and secondary hippocampal astrocyte cultures. (2) The parameters of inhibition agreed with the mechanisms of GC inhibition of glucose transport in peripheral tissues: A minimum of 4 h of GC exposure were required, and the effect was steroid specific (i.e., it was not triggered by estrogen, progesterone, or testosterone) and tissue specific (i.e., it was not triggered by GCs in cerebellar or cortical cultures). (3) Similar GC treatment caused a decrease in astrocyte survival during hypoglycemia and a decrease in the affinity of glutamate uptake. This latter observation suggests that GCs might impair the ability of astrocytes to aid neurons during times of neurologic crisis (i.e., by impairing their ability to remove damaging glutamate from the synapse).

Effects of ethanol on hexose uptake by cultured rat brain cells

The effects of ethanol on hexose uptake by glial cells was investigated using primary cultures prepared from term rat fetuses. Specific 3H 2-deoxy-D-glucose (2DG) uptake was significantly reduced by a 4-hr exposure to ethanol at concentrations of 25, 50, and 100 mM, but not 200 or 300 mM. The inhibitory effect of 50 mM ethanol increased with the duration of exposure, with 2DG uptake inhibited by 36% after 18 hr. Astrocytes cultured from the brains of term fetuses of rats fed ethanol during pregnancy showed essentially the same 2DG uptake response to in vitro ethanol treatment. Kinetics of 2DG uptake showed a significant decrease of Vmax in the presence of ethanol. No interaction was found between ethanol and insulin, which stimulated 2DG uptake and protein content of the cultures. The data suggest that ethanol can modulate hexose uptake by astrocytes cultured from fetal rat brain. However, insulin actions on glial cells were not affected by ethanol.

Regulation of glutamate and aspartate release from the Schaffer collaterals and other projections of CA3 hippocampal pyramidal cells

Excitatory synaptic transmission in the CNS can be modulated by endogenous substances and metabolic states that alter release of the transmitter, usually glutamate and/or aspartate. To explore this issue, we have studied the release of endogenous glutamate and aspartate from synaptic terminals of the CA3-derived Schaffer collateral, commissural and ipsilateral associational fibers in slices of hippocampal area CA1. These terminals release glutamate and aspartate in about a 5:1 ratio. The release process is modulated by adenosine, by the transmitters themselves and by nerve terminal metabolism. Adenosine inhibits the release of both amino acids by acting upon an A1 receptor. The transmitters, once released, can regulate their further release by acting upon both an NMDA and a non-NMDA (quisqualate/kainate) receptor. Activation of the NMDA receptor enhances the release of both glutamate and aspartate, whereas activation of the non-NMDA receptor depresses the release of aspartate only. Superfusion of CA1 slices with a glucose-deficient medium increases the release of both amino acids and reduces the glutamate/aspartate ratio. These results have implications for the regulation of excitatory synaptic transmission in the CA1 area and for the mechanism of hypoglycemic damage to CA1 pyramidal 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.