Influence of Na+, K+, and Ca2+ on glutamine synthesis and distribution in rat brain cortex slices: a possible linkage of glutamine synthetase with cerebral transport processes and energetics in the astrocytes

Characterization of reduced astrocyte creatine kinase levels in Alzheimer's disease

The creatine-phosphocreatine cycle serves as a crucial temporary energy buffering system in the brain, regulated by brain creatine kinase (CKB), in maintaining Adenosine triphosphate (ATP) levels. Alzheimer's disease (AD) has been linked to increased CKB oxidation and loss of its regulatory function, although specific pathological processes and affected cell types remain unclear. In our study, cerebral cortex samples from individuals with AD, dementia with Lewy bodies (DLB), and age-matched controls were analyzed using antibody-based methods to quantify CKB levels and assess alterations associated with disease processes. Two independently validated antibodies exclusively labeled astrocytes in the human cerebral cortex. Combining immunofluorescence (IF) and mass spectrometry (MS), we explored CKB availability in AD and DLB cases. IF and Western blot analysis demonstrated a loss of CKB immunoreactivity correlated with increased plaque load, severity of tau pathology, and Lewy body pathology. However, transcriptomics data and targeted MS demonstrated unaltered total CKB levels, suggesting posttranslational modifications (PTMs) affecting antibody binding. This aligns with altered efficiency at proteolytic cleavage sites indicated in the targeted MS experiment. These findings highlight that the proper function of astrocytes, understudied in the brain compared with neurons, is highly affected by PTMs. Reduction in ATP levels within astrocytes can disrupt ATP-dependent processes, such as the glutamate-glutamine cycle. As CKB and the creatine-phosphocreatine cycle are important in securing constant ATP availability, PTMs in CKB, and astrocyte dysfunction may disturb homeostasis, driving excitotoxicity in the AD brain. CKB and its activity could be promising biomarkers for monitoring early-stage energy deficits in AD.

Astrocyte regulation of synaptic signaling in psychiatric disorders

Over the last 15 years, the field of neuroscience has evolved toward recognizing the critical role of astroglia in shaping neuronal synaptic activity and along with the pre- and postsynapse is now considered an equal partner in tripartite synaptic transmission and plasticity. The relative youth of this recognition and a corresponding deficit in reagents and technologies for quantifying and manipulating astroglia relative to neurons continues to hamper advances in understanding tripartite synaptic physiology. Nonetheless, substantial advances have been made and are reviewed herein. We review the role of astroglia in synaptic function and regulation of behavior with an eye on how tripartite synapses figure into brain pathologies underlying behavioral impairments in psychiatric disorders, both from the perspective of measures in postmortem human brains and more subtle influences on tripartite synaptic regulation of behavior in animal models of psychiatric symptoms. Our goal is to provide the reader a well-referenced state-of-the-art understanding of current knowledge and predict what we may discover with deeper investigation of tripartite synapses using reagents and technologies not yet available.

Astrocytes: The Housekeepers and Guardians of the CNS

Astroglia are a diverse group of cells in the central nervous system. They are of the ectodermal, neuroepithelial origin and vary in morphology and function, yet, they can be collectively defined as cells having principle function to maintain homeostasis of the central nervous system at all levels of organisation, including homeostasis of ions, pH and neurotransmitters; supplying neurones with metabolic substrates; supporting oligodendrocytes and axons; regulating synaptogenesis, neurogenesis, and formation and maintenance of the blood-brain barrier; contributing to operation of the glymphatic system; and regulation of systemic homeostasis being central chemosensors for oxygen, CO₂ and Na⁺. Their basic physiological features show a lack of electrical excitability (inapt to produce action potentials), but display instead a rather active excitability based on variations in cytosolic concentrations of Ca²⁺ and Na⁺. It is expression of neurotransmitter receptors, pumps and transporters at their plasmalemma, along with transports on the endoplasmic reticulum and mitochondria that exquisitely regulate the cytosolic levels of these ions, the fluctuation of which underlies most, if not all, astroglial homeostatic functions.

Physiology of Astroglial Excitability

Classic physiology divides all neural cells into excitable neurons and nonexcitable neuroglia. Neuroglial cells, chiefly responsible for homeostasis and defense of the nervous tissue, coordinate their complex homeostatic responses with neuronal activity. This coordination reflects a specific form of glial excitability mediated by complex changes in intracellular concentration of ions and second messengers organized in both space and time. Astrocytes are equipped with multiple molecular cascades, which are central for regulating homeostasis of neurotransmitters, ionostasis, synaptic connectivity, and metabolic support of the central nervous system. Astrocytes are further provisioned with multiple receptors for neurotransmitters and neurohormones, which upon activation trigger intracellular signals mediated by Ca²⁺, Na⁺, and cyclic AMP. Calcium signals have distinct organization and underlying mechanisms in different astrocytic compartments thus allowing complex spatiotemporal signaling. Signals mediated by fluctuations in cytosolic Na⁺ are instrumental for coordination of Na⁺ dependent astrocytic transporters with tissue state and homeostatic demands. Astroglial ionic excitability may also involve K⁺, H^+, and Cl^-. The cyclic AMP signalling system is, in comparison to ions, much slower in targeting astroglial effector mechanisms. This evidence review summarizes the concept of astroglial intracellular excitability.

Sodium Fluctuations in Astroglia and Their Potential Impact on Astrocyte Function

Astrocytes are the main cell type responsible for the regulation of brain homeostasis, including the maintenance of ion gradients and neurotransmitter clearance. These processes are tightly coupled to changes in the intracellular sodium (Na⁺) concentration. While activation of the sodium-potassium-ATPase (NKA) in response to an elevation of extracellular K⁺ may decrease intracellular Na⁺, the cotransport of transmitters, such as glutamate, together with Na⁺ results in an increase in astrocytic Na⁺. This increase in intracellular Na⁺ can modulate, for instance, metabolic downstream pathways. Thereby, astrocytes are capable to react on a fast time scale to surrounding neuronal activity via intracellular Na⁺ fluctuations and adjust energy production to the demand of their environment. Beside the well-documented conventional roles of Na⁺ signaling mainly mediated through changes in its electrochemical gradient, several recent studies have identified more atypical roles for Na⁺, including protein interactions leading to changes in their biochemical activity or Na⁺-dependent regulation of gene expression. In this review, we will address both the conventional as well as the atypical functions of astrocytic Na⁺ signaling, presenting the role of transporters and channels involved and their implications for physiological processes in the central nervous system (CNS). We will also discuss how these important functions are affected under pathological conditions, including stroke and migraine. We postulate that Na⁺ is an essential player not only in the maintenance of homeostatic processes but also as a messenger for the fast communication between neurons and astrocytes, adjusting the functional properties of various cellular interaction partners to the needs of the surrounding network.

NCX and EAAT transporters in ischemia: At the crossroad between glutamate metabolism and cell survival

Energy metabolism impairment is a central event in the pathophysiology of ischemia. The limited availability of glucose and oxygen strongly affects mitochondrial activity, thus leading to ATP depletion. In this setting, the switch to alternative energy sources could ameliorate cells survival by enhancing ATP production, thus representing an attractive strategy for ischemic treatment. In this regard, some studies have recently re-evaluated the metabolic role of glutamate and its potential to promote cell survival under pathological conditions. In the present review, we discuss the ability of glutamate to exert an "energizing role" in cardiac and neuronal models of hypoxia/reoxygenation (H/R) injury, focusing on the Na⁺/Ca²⁺ exchanger (NCX) and the Na⁺-dependent excitatory amino acid transporters (EAATs) as key players in this metabolic pathway.

Physiology of Astroglia

Astrocytes are principal cells responsible for maintaining the brain homeostasis. Additionally, these glial cells are also involved in homocellular (astrocyte-astrocyte) and heterocellular (astrocyte-other cell types) signalling and metabolism. These astroglial functions require an expression of the assortment of molecules, be that transporters or pumps, to maintain ion concentration gradients across the plasmalemma and the membrane of the endoplasmic reticulum. Astrocytes sense and balance their neurochemical environment via variety of transmitter receptors and transporters. As they are electrically non-excitable, astrocytes display intracellular calcium and sodium fluctuations, which are not only used for operative signalling but can also affect metabolism. In this chapter we discuss the molecules that achieve ionic gradients and underlie astrocyte signalling.

Osmotic Demyelination: From an Oligodendrocyte to an Astrocyte Perspective

Osmotic demyelination syndrome (ODS) is a disorder of the central myelin that is often associated with a precipitous rise of serum sodium. Remarkably, while the myelin and oligodendrocytes of specific brain areas degenerate during the disease, neighboring neurons and axons appear unspoiled, and neuroinflammation appears only once demyelination is well established. In addition to blood‒brain barrier breakdown and microglia activation, astrocyte death is among one of the earliest events during ODS pathology. This review will focus on various aspects of biochemical, molecular and cellular aspects of oligodendrocyte and astrocyte changes in ODS-susceptible brain regions, with an emphasis on the crosstalk between those two glial cells. Emerging evidence pointing to the initiating role of astrocytes in region-specific degeneration are discussed.

Ionic signalling in astroglia beyond calcium

Astrocytes are homeostatic and protective cells of the central nervous system. Astroglial homeostatic responses are tightly coordinated with neuronal activity. Astrocytes maintain neuronal excitability through regulation of extracellular ion concentrations, as well as assisting and modulating synaptic transmission by uptake and catabolism of major neurotransmitters. Moreover, they support neuronal metabolism and detoxify ammonium and reactive oxygen species. Astroglial homeostatic actions are initiated and controlled by intercellular signalling of ions, including Ca²⁺ , Na⁺ , Cl^- , H⁺ and possibly K⁺ . This review summarises current knowledge on ionic signals mediated by the major monovalent ions, which occur in microdomains, as global events, or as propagating intercellular waves and thereby represent the substrate for astroglial excitability.

Prolyl hydroxylase domain inhibitors: can multiple mechanisms be an opportunity for ischemic stroke?

Stroke and cerebrovascular disease are now the fifth most common cause of death behind other diseases such as heart, cancer and respiratory disease and accounts for approximately 40-50 fatalities per 100,000 people each year in the United States. Currently the only therapy for acute stroke, is intravenous administration of tissue plasminogen activator which was approved in 1996 by the FDA. Surprisingly no new treatments have come on the market since, although endovascular mechanical thrombectomy is showing promising results in trials. Recently focus has shifted towards a preventative therapy rather than trying to reverse or limit the amount of damage occurring following stroke onset. During one of the components of ischemia, hypoxia, a number of physiological changes occur within neurons which include the stabilization of hypoxia-inducible factors. The activity of these proteins is regulated by O₂, Fe²⁺, 2-OG and ascorbate-dependant hydroxylases which contain prolyl-4-hydroxylase domains (PHDs). PHD inhibitors are capable of pharmacologically activating the body's own endogenous adaptive response to low levels of oxygen and have therefore become an attractive therapeutic target for treating ischemia. They have been widely used in the periphery and have been shown to have a preconditioning and protective effect against a later and more severe ischemic insult. Currently there are a number of these agents in phase 1, 2 and 3 clinical trials for the treatment of anemia. In this review we assess the neuroprotective effects of PHD inhibitors, including dimethyloxalylglycine and deferoxamine and suggest that not all of their effects in the CNS are HIF-dependent. Unravelling new roles and a better understanding of the function of PHD inhibitors in the CNS may be of great benefit especially when investigating their use in the treatment of stroke and other ischemic diseases.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

The Glutamate-Glutamine (GABA) Cycle: Importance of Late Postnatal Development and Potential Reciprocal Interactions between Biosynthesis and Degradation

The gold standard for studies of glutamate-glutamine (GABA) cycling and its connections to brain biosynthesis from glucose of glutamate and GABA and their subsequent metabolism are the elegant in vivo studies by (13)C magnetic resonance spectroscopy (NMR), showing the large fluxes in the cycle. However, simpler experiments in intact brain tissue (e.g., immunohistochemistry), brain slices, cultured brain cells, and mitochondria have also made important contributions to the understanding of details, mechanisms, and functional consequences of glutamate/GABA biosynthesis and degradation. The purpose of this review is to attempt to integrate evidence from different sources regarding (i) the enzyme(s) responsible for the initial conversion of α-ketoglutarate to glutamate; (ii) the possibility that especially glutamate oxidation is essentially confined to astrocytes; and (iii) the ontogenetically very late onset and maturation of glutamine-glutamate (GABA) cycle function. Pathway models based on the functional importance of aspartate for glutamate synthesis suggest the possibility of interacting pathways for biosynthesis and degradation of glutamate and GABA and the use of transamination as the default mechanism for initiation of glutamate oxidation. The late development and maturation are related to the late cortical gliogenesis and convert brain cortical function from being purely neuronal to becoming neuronal-astrocytic. This conversion is associated with huge increases in energy demand and production, and the character of potentially incurred gains of function are discussed. These may include alterations in learning mechanisms, in mice indicated by lack of pairing of odor learning with aversive stimuli in newborn animals but the development of such an association 10-12 days later. The possibility is suggested that analogous maturational changes may contribute to differences in the way learning is accomplished in the newborn human brain and during later development.

Sodium fluxes and astroglial function

Astrocytes exhibit their excitability based on variations in cytosolic Ca(2+) levels, which leads to variety of signalling events. Only recently, however, intracellular fluctuations of more abundant cation Na(+) are brought in the limelight of glial signalling. Indeed, astrocytes possess several plasmalemmal molecular entities that allow rapid transport of Na(+) across the plasma membrane: (1) ionotropic receptors, (2) canonical transient receptor potential cation channels, (3) neurotransmitter transporters and (4) sodium-calcium exchanger. Concerted action of these molecules in controlling cytosolic Na(+) may complement Ca(2+) signalling to provide basis for complex bidirectional astrocyte-neurone communication at the tripartite synapse.

Sodium dynamics: another key to astroglial excitability?

Astroglial excitability is largely mediated by fluctuations in intracellular ion concentrations. In addition to generally acknowledged Ca²⁺ excitability of astroglia, recent studies have demonstrated that neuronal activity triggers transient increases in the cytosolic Na⁺ concentration ([Na⁺](i)) in perisynaptic astrocytes. These [Na⁺](i) transients are controlled by multiple Na⁺-permeable channels and Na⁺-dependent transporters; spatiotemporally organized [Na⁺](i) dynamics in turn regulate diverse astroglial homeostatic responses such as metabolic/signaling utilization of lactate and glutamate, transmembrane transport of neurotransmitters and K⁺ buffering. In particular, near-membrane [Na⁺](i) transients determine the rate and the direction of the transmembrane transport of GABA and Ca²⁺. We discuss here the role of Na⁺ in the regulation of various systems that mediate fast bidirectional communication between neurones and glia at the single synapse level.

Ionotropic receptors in neuronal-astroglial signalling: what is the role of "excitable" molecules in non-excitable cells

Astroglial cells were long considered to serve merely as the structural and metabolic supporting cast and scenery against which the shining neurones perform their illustrious duties. Relatively recent evidence, however, indicates that astrocytes are intimately involved in many of the brain's functions. Astrocytes possess a diverse assortment of ionotropic transmitter receptors, which enable these glial cells to respond to many of the same signals that act on neurones. Ionotropic receptors mediate neurone-driven signals to astroglial cells in various brain areas including neocortex, hippocampus and cerebellum. Activation of ionotropic receptors trigger rapid signalling events in astroglia; these events, represented by local Ca(2+) or Na(+) signals provide the mechanism for fast neuronal-glial signalling at the synaptic level. Since astrocytes can detect chemical transmitters that are released from neurones and can release their own extracellular signals, gliotransmitters, they are intricately involved in homocellular and heterocellular signalling mechanisms in the nervous system. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

In vivo visualization of reactive gliosis using manganese-enhanced magnetic resonance imaging

Reactive astrogliosis occurs after diverse central nervous system (CNS) insults. While astrogliosis provides protection against inflammation, it is also obstructive in the progress of neuranagenesis after CNS insults. Thus, a method that enables in vivo visualization and tissue characterization for gliosis would be invaluable for studies of CNS insults and corresponding treatments. Manganese has proven to be a useful MRI contrast agent that enters cells via Ca(2+) channels and has been applied to manganese-enhanced MRI (MEMRI) for neuronal functional mapping. This study investigated whether MEMRI can detect astrogliosis after focal ischemia in vivo. Rats were divided into groups according to the number of days after either transient middle cerebral artery occlusion or a sham. Ring- or crescent-shaped enhancement of MEMRI corresponded to the GFAP-positive astroglia observed in the peripheral region of the ischemic core 11 days after middle cerebral artery occlusion. This indicates that MEMRI enhancement predominantly reflects reactive astrogliosis after stroke.

Brain energy metabolism in a sub-acute rat model of manganese neurotoxicity: an ex vivo nuclear magnetic resonance study using [1-13C]glucose

Ex vivo high-resolution NMR spectroscopy combined with in vivo injection of [1-13C]glucose was applied to gain insight into the mechanism(s) leading to energy failure in manganese neurotoxicity. In rats treated for 4 days with 50mg/kg MnCl(2) (intraperitoneally, i.p.), the concentration of 13C-labeled lactate increased to 154% compared to control rats. Changes in the absolute amounts of lactate were much less, resulting in increased fractional 13C-enrichments in lactate (indicating relative changes of de novo synthesis from glucose via the glycolytic pathway) to 143% of control values (P < 0.001). Analysis of samples obtained from blood plasma and peripheral organs demonstrate a selective increase of lactate synthesis from [1-13C]glucose in the brain, which is released into the circulation. In parallel, manganese treatment resulted in stimulation of flux through pyruvate dehydrogenase (PDH), leading to accumulation of [4-13C]glutamate, [4-13C]glutamine and [2-13C]GABA to 168, 247 and 144% of control, respectively. The relative flux of glucose through astrocytic pyruvate carboxylase (PC), on the other hand, was impaired by manganese, as evident from a decreased ratio of [2-13C]/[4-13C]glutamate or [2-13C]/[4-13C] glutamine. Consistent with stimulated glucose oxidative metabolism, the fractional 13C-enrichment in [2-13C]acetyl-CoA entering the tricarboxylic acid (TCA) cycle and contributing to glutamate and glutamine synthesis increased to 138 and 156% of control, respectively (P < 0.001). In parallel, the TCA cycling ratio increased to 134% compared to control rats, prior to the label ending up in glutamate. In contrast, glutamine is synthesized mainly during the first TCA cycle turn. The present data provide new evidence in support of changes in brain energy metabolism playing an important role in manganese neurotoxicity. In particular, increased glycolytic flux and lactate synthesis may contribute to the deleterious effects of manganese in the brain. Furthermore, stimulated astrocytic glucose oxidation and glutamine synthesis may be associated with astrocytic pathology and altered astrocytic-neuronal metabolic trafficking in manganese neurotoxicity.

Glutamate-glutamine cycling in the epileptic human hippocampus

PURPOSE

Several findings suggest that energy metabolism and the glutamate-glutamine cycle may be impaired in epilepsy. Positron emission tomography often shows interictal hypometabolism of the epileptogenic hippocampus. In vivo microdialysis studies show that seizure-associated glutamate release is doubled, and clearance is slowed. We hypothesized that the glutamate-glutamine cycle between neurons and glia may be decreased in the epileptic human hippocampus.

METHODS

A 20% solution of 2-13C-glucose was infused before resection of the epileptogenic hippocampus. Blood glucose isotopic fractions were measured every 30 min. Blood and brain specimens were frozen quickly; perchloric acid extracts of the small metabolites were prepared and analyzed by proton and carbon magnetic resonance spectroscopy (MRS) at 11.75 Tesla.

RESULTS

Standard histology showed 12 with hippocampal sclerosis and five with minimal neuron loss. The relative rates of glutamate-glutamine cycling with respect to glutamate synthesis were decreased in biopsies affected by hippocampal sclerosis (mean, 0.08; 95% confidence interval, 0.04-0.12) compared with those with minimal neuron loss (0.52; 95% CI, 0.30-0.75). Mean cellular glutamate concentrations were higher in minimal neuron loss (8.9 mM; 95% CI, 7.4-10.4) than hippocampal sclerosis (7.3 mM; 95% CI, 5.9-8.7). Cellular glutamine concentrations (mean, 2.8 mM; 95% CI, 2.4-3.2; n = 17) were the same in all groups.

CONCLUSIONS

The epileptogenic, gliotic human hippocampus appears to be characterized metabolically by slow rates of glutamate-glutamine cycling, decreased glutamine content, and a relative increase in glutamate content. We hypothesize that the low rate of glutamate-glutamine cycling that results from a failure of glial glutamate detoxification could account for slow glutamate clearance from synapses and continuing low-grade excitotoxicity.

Alterations in K+ evoked profiles of neurotransmitter and neuromodulator amino acids after focal ischemia-reperfusion

Secondary elevations in extracellular amino acids occur during reperfusion after transient cerebral ischemia. The delayed accumulation of excitatory amino acids may contribute to the progressive development of neuronal injury. In this study, we explored the mechanisms that may be involved in this phenomenon. Microdialysis samples from probes located in rabbit cortex were analysed with a chiral amino acid procedure. Concentrations of neurotransmitters (L-Glu, GABA), N-methyl-D-aspartate receptor modulators (D-Ser, Gly), an inhibitory neuromodulator (Tau), the lipid component phosphoethanolamine, and L-Gln, L-Ser and L-Ala were measured. Depolarization via perfusion with potassium was used to assess the status of release/reuptake systems at 2 and 4 h reperfusion after 2 h transient focal ischemia. Background experiments classified potassium evoked responses as calcium dependent or calcium-independent by inclusion of 30 microM omega-conopeptide MVIIC or by inclusion of 20 mM magnesium and ommision of calcium. During ischemia, large elevations of almost all amino acids occurred. During reperfusion, secondary elevations in transmitter amino acids (L-Glu, GABA) and N-methyl-D-aspartate receptor modulators (D-Ser, Gly) occurred. Tau remained slightly elevated whereas the lipid component phosphoethanolamine remained high and stable during reperfusion. Reperfusion significantly potentiated the potassium response for amino acids with calcium-dependent responses (L-Glu and GABA). In contrast, calcium-independent responses (Tau, phosphoethanolamine, L-Gln) were significantly attenuated. Intermediate behavior was observed with Gly, while no potassium responses were observed for D-Ser, L-Ser or L-Ala. These data demonstrate that perturbations in evoked amino acid profiles after ischemia-reperfusion are selective. Reduction of calcium-independent responses implicate a general decline in efficacy of transporter mechanisms that restore transmembrane gradients of ions and transmitters. Decreased efficacy of transporter systems may reduce transmitter reuptake and account for the amplified release of L-Glu and GABA, thus contributing to progressive neural dysfunction after cerebral ischemia.

Neuronal and glial handling of glutamate and glutamine during hypoosmotic stress: a biochemical and quantitative immunocytochemical analysis using the rat cerebellum as a model

Biochemical and immunocytochemical analyses were performed to resolve how glutamate and glutamine are handled in rat cerebellar cortex in acute hypoosmotic stress. Rats were subjected to a 15-20% reduction in plasma osmolality by intraperitoneal injection of distilled water and then perfusion fixed after 4 or 8 h survival. Some rats in the latter group had their plasma isoosmolality restored by injections of hypertonic saline 4 h prior to perfusion. Water loading caused a pronounced increase in the tissue level of glutamine and an equimolar decrease in the level of glutamate after 4 h survival. The increase in glutamine was transient, as judged by analyses at 8 h survival. Light microscopic immunocytochemistry revealed a pronounced enhancement of the glutamine immunolabelling of glial cells (Golgi epithelial cells and astrocytes), including their perivascular end feet, and quantitative immunogold analyses at the electron microscopic level showed that this enhancement reflected a 50% increase in the intracellular concentration of fixed glutamine. Since water loading was associated with glial swelling this change corresponded to a several-fold increase in the glial content of glutamine. There was a modest reduction in the overall staining intensity for glutamate. The biochemical and immunocytochemical changes were reversed upon restoration of plasma osmolality by hypertonic saline. These findings suggest that hypoosmotic stress causes an increased conversion of glutamate to glutamine in glial cells and that the latter amino acid is subsequently lost from the tissue. The flux of glutamate carbon skeletons through the glutamine synthetase pathway in glia, prior to an efflux to the systemic circulation, may explain how glutamate, and excitatory transmitter and potential toxin, can be used as an organic osmolyte in brain tissue.

Potentiation of excito-toxicity by glutamate uptake inhibitor rather than glutamine synthetase inhibitor

The neuroprotective functions of glia cells in the presence of excessive amounts of extracellular glutamate (Glu) were examined using glia-rich and glia-poor cultured cerebellar granule cells that contained the same number of neurons. In order to focus on the metabolic enzyme glutamine synthetase (GS) and the uptake system in glia cells, selective inhibitors such as L-methionine sulfoximine (MSO) and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS) were used as pharmacological tools. The increased amount of lactate dehydrogenase (LDH) leakage induced by 50 microM Glu and SITS was equivalent to that of 1 mM Glu. However, the simultaneous treatment with 50 microM Glu and 5 microM MSO did not increase the LDH leakage. The larger quantities of extracellular Glu were sustained in both glia-rich and glia-poor cultures. After the administration of Glu and MSO, however, the larger quantities of Glu were not sustained. Taking these results into consideration, the Glu uptake system in glia cells seems to be more important than the Glu metabolic enzyme system in the regulation of neuronal protection from Glu toxicity.

Effect of fructose-1,6-bisphosphate on glutamate uptake and glutamine synthetase activity in hypoxic astrocyte cultures

Astrocytes are important in regulating the microenvironment of neurons both by catabolic and synthetic pathways. The glutamine synthetase (GS) activity observed in astrocytes affects neurons by removing toxic substances, NH3 and glutamate; and by providing an important neuronal substrate, glutamine. This glutamate cycle might play a critical role during periods of hypoxia and ischemia, when an increase in extracellular excitatory amino acids is observed. It was previously shown in our laboratory that fructose-1,6-bisphosphate (FBP) protected cortical astrocyte cultures from hypoxic insult and reduced ATP loss following a prolonged (18-30 hrs) hypoxia. In the present study we established the effects of FBP on the level of glutamate uptake and GS activity under normoxic and hypoxic conditions. Under normoxic conditions, [U-14C]glutamate uptake and glutamine production were independent of FBP treatment; whereas under hypoxic conditions, the initial increase in glutamate uptake and an overall increase in glutamine production in astrocytes were FBP-dependent. Glutamine synthetase activity was dependent on FBP added during the 22 hours of either normoxic- or hypoxic-treatment, hence significant increases in activity were observed due to FBP regardless of the oxygen/ATP levels in situ. These studies suggest that activation of GS by FBP may provide astrocytic protection against hypoxic injury.

Glial calcium

This review summarizes current knowledge relating intracellular calcium and glial function. During steady state, glia maintain a low cytosolic calcium level by pumping calcium into intracellular stores and by extruding calcium across the plasma membrane. Glial Ca2+ increases in response to a variety of physiological stimuli. Some stimuli open membrane calcium channels, others release calcium from intracellular stores, and some do both. The temporal and spatial complexity of glial cytosolic calcium changes suggest that these responses may form the basis of an intracellular or intercellular signaling system. Cytosolic calcium rises effect changes in glial structure and function through protein kinases, phospholipases, and direct interaction with lipid and protein constituents. Ultimately, calcium signaling influence glial gene expression, development, metabolism, and regulation of the extracellular milieu. Disturbances in glial calcium homeostasis may have a role in certain pathological conditions. The discovery of complex calcium-based glial signaling systems, capable of sensing and influencing neural activity, suggest a more integrated neuro-glial model of information processing in the central nervous system.

Neuronal activity triggers calcium waves in hippocampal astrocyte networks

The recent discovery that the neurotransmitter glutamate can trigger actively propagating Ca2+ waves in the cytoplasm of cultured astrocytes suggests the possibility that synaptically released glutamate may trigger similar Ca2+ waves in brain astrocytes in situ. To explore this possibility, we used confocal microscopy and the Ca2+ indicator fluo-3 to study organotypically cultured slices of rat hippocampus, where astrocytic and neuronal networks are intermingled in their normal tissue relationships. We find that astrocytic Ca2+ waves are present under these circumstances and that these waves can be triggered by the firing of glutamatergic neuronal afferents with latencies as short as 2 s. The Ca2+ waves closely resemble those previously observed in cultured astrocytes: they propagate both within and between astrocytes at velocities of 7-27 microns/s at 21 degrees C. The ability of tissue astrocyte networks to respond to neuronal network activity suggests that astrocytes may have a much more dynamic and active role in brain function than has been generally recognized.

Decreased glucose utilization in discrete brain regions of rat in thioacetamide-induced hepatic encephalopathy as measured with [3H]-deoxyglucose

To evaluate the possible contribution of bioenergetic failure in the particular brain regions to the pathomechanism of hepatic encephalopathy (HE), local cerebral metabolic rate for glucose (LCMRglue) was evaluated from [3H]-deoxyglucose uptake in frontal, visual and auditory cortex, striatum, cerebellum and medulla oblongata of rats with acute HE induced with a hepatotoxin--thioacetamide (TAA). HE caused a decrease of LCMRglue in all the regions studied. The strongest decrease (about 65%) was noted in hippocampus and cerebral cortex--the two regions rich in glutamatergic neurons. The results indicate a possible link between decreased energy metabolism and impaired excitatory, glutamatergic neurotransmission--the two factors whose contribution to HE has so far been implicated separately.

Effect of ammonia on calcium homeostasis in primary astrocyte cultures

Calcium influx, accumulation and efflux were studied in primary cultures of rat astrocytes treated with ammonium chloride. Treatment of the cells for 3 days with 10 mMN4Cl resulted in a 35% reduction in 45Ca influx. The decrease in calcium influx was dose-dependent between 2 and 10 mM NH4Cl. Short-term (30 min) exposure to ammonia had no effect on calcium influx. Calcium accumulation, as measured by 20-min exposure to 45Ca, decreased after treating cultures with 10 mM NH4Cl for one or 3 days; a greater effect was observed after the 3-day treatment. Studies with lanthanum, an inhibitor of calcium transport, indicated that the effect of ammonia was not due to non-specific leakage of calcium. Calcium efflux was not affected by exposure of the cultures to ammonium chloride. Purinergic-evoked calcium influx and mobilization was not altered by ammonia. While the mechanism(s) of calcium homeostasis affected by long-term hyperammonemia remain to be defined, these results suggest that reduced astrocytic calcium may be related to the pathogenesis of ammonia-related disorders such as hepatic encephalopathy.

Interrelationship between glutamate and membrane-bound ATPases in nerve cells

Plasma membrane potential generated by Na+, K(+)-ATPase provides the driving force for high-affinity, Na(+)-dependent uptake of glutamate into the cytoplasm of glutamatergic nerve endings and glial cells. Ca2(+)-calmodulin-dependent ATPase in the plasma membrane and Ca2(+)-ATPase in the endoplasmic reticulum influence the intracellular [Ca2+] and, therefore, the exocytotic release of neurotransmitter glutamate. The membrane potential across the membrane of the synaptic vesicles, generated by a H(+)-ATPase, provides the driving force for synaptic vesicular uptake of glutamate as well as that of GABA and glycine. Hypoxia and ischemia lead to release of glutamate, perhaps in consequence of an increased endogenous pool of glutamate and/or lack of substrate (ATP) for the ATPases. This release, rather than being exocytotic, is believed to result mainly from a reversal of the Na(+)-dependent high-affinity glutamate transporter in the plasma membrane.

Reduction of glutamine synthetase specific activity in cultured astroglia by ferrous chloride

Immature and mature rat astroglia in culture were assayed for glutamine synthetase (GS) activity after a single exposure to the epileptogen FeCl2. Cells were cultured with both standard and elevated extracellular potassium or glutamate (Glu) concentrations. FeCl2 reduced GS activity below control levels, whereas high Glu increased GS activity. However, stimulation by high Glu was significantly attenuated in cultures given both FeCl2 and high Glu, indicating that cells treated with FeCl2 were not able to respond as effectively to increased extracellular glutamate by increasing their GS activity. The significance of these findings is that glial regulation of the neuronal environment may be impaired, based on the proposed importance of GS in ammonia detoxification in the brain.

Low level lead neurotoxicity in a pregnant guinea pigs model: neuroglial enzyme activities and brain trace metal concentrations

Specific activities of the astroglial marker glutamine synthetase (GS), and the oligodendroglial marker glycerol-3-phosphate dehydrogenase (GPDH) were measured in the spinal cord of fetal guinea pigs and their dams following chronic exposure to low levels of lead (Pb) during gestation. In addition, the effects of Pb on intracellular trace metals (Cu, Fe, Zn) were measured in the blood, cerebellum and forebrain. Aminolevulinic acid dehydratase (ALAD) and zinc protoporphyrin IX (ZPP) were measured in order to monitor established parameters of Pb-exposure. Pregnant guinea pigs were orally administered 0, 5.5 or 11 mg Pb/kg body weight for 30 or 40 days commencing on day 22 of gestation. Blood Pb levels produced in dams and fetuses were at or near the currently identified "no effect" levels for children (10-30 micrograms/dl). These Pb blood levels produced a significant (P less than 0.05) dose-dependent decrease in GS and GPDH activity in the dams and fetuses. Fe and Zn concentrations in blood, cerebellum and forebrain of both dams and fetuses were significantly (P less than 0.05) decreased in a dose-dependent manner. However, Cu concentrations in the blood, cerebellum and forebrain were decreased in the dams but increased in the fetuses in a dose-dependent fashion. The alteration of trace metal concentrations is a proposed mechanism of Pb neurotoxicity. Blood ALAD activity was significantly (P less than 0.05) decreased and ZPP levels were significantly (P less than 0.05) increased, as expected in Pb-exposed animals. This study presents the first biochemical evidence for the alteration of neuroglial function at low levels of Pb exposure and focuses attention on the fetus as an important Pb target.