Glycogen synthesis and immunocytochemical study of fructose-1,6-biphosphatase in methionine sulfoximine epileptogenic rodent brain

State-Dependent Changes in Brain Glycogen Metabolism

A fundamental understanding of glycogen structure, concentration, polydispersity and turnover is critical to qualify the role of glycogen in the brain. These molecular and metabolic features are under the control of neuronal activity through the interdependent action of neuromodulatory tone, ionic homeostasis and availability of metabolic substrates, all variables that concur to define the state of the system. In this chapter, we briefly describe how glycogen responds to selected behavioral, nutritional, environmental, hormonal, developmental and pathological conditions. We argue that interpreting glycogen metabolism through the lens of brain state is an effective approach to establish the relevance of energetics in connecting molecular and cellular neurophysiology to behavior.

Status Epilepticus due to Intraperitoneal Injection of Vehicle Containing Propylene Glycol in Sprague Dawley Rats

Published reports of status epilepticus due to intraperitoneal injection containing propylene glycol in rats are sparse. In fact, there are no reports specifying a maximum safe dose of propylene glycol through intraperitoneal administration. We report here a case of unexpected seizures in Sprague Dawley rats after receiving an intraperitoneal injection containing propylene glycol. Nine-week-old, 225–250 gram male rats were reported to experience tremor progressing to seizures within minutes after given injections of resveratrol (30 mg/kg) dissolved in a 40 : 60 propylene glycol/corn oil vehicle solution by direct intraperitoneal (IP) slow bolus injection or via a preplaced intraperitoneal catheter. The World Health Organization suggests a maximum dose of 25 mg/kg/day of propylene glycol taken orally and no more than 25 mg/dL in blood serum, whereas the animals used in our study got a calculated maximum 0.52 g/kg (25 times lower dose). Blood tests from the seizing rat support a diagnosis of hemolysis and lactic acidosis which may have led to the seizures, all of which appeared to be a consequence of the propylene glycol administration. These findings are consistent with oral and intravenous administration of propylene glycol toxicity as previously reported in other species, including humans. To our knowledge, this report represents the first published case of status epilepticus due to an IP injection containing propylene glycol.

Does abnormal glycogen structure contribute to increased susceptibility to seizures in epilepsy?

Epilepsy is a family of brain disorders with a largely unknown etiology and high percentage of pharmacoresistance. The clinical manifestations of epilepsy are seizures, which originate from aberrant neuronal synchronization and hyperexcitability. Reactive astrocytosis, a hallmark of the epileptic tissue, develops into loss-of-function of glutamine synthetase, impairment of glutamate-glutamine cycle and increase in extracellular and astrocytic glutamate concentration. Here, we argue that chronically elevated intracellular glutamate level in astrocytes is instrumental to alterations in the metabolism of glycogen and leads to the synthesis of polyglucosans. Unaccessibility of glycogen-degrading enzymes to these insoluble molecules compromises the glycogenolysis-dependent reuptake of extracellular K(+) by astrocytes, thereby leading to increased extracellular K(+) and associated membrane depolarization. Based on current knowledge, we propose that the deterioration in structural homogeneity of glycogen particles is relevant to disruption of brain K(+) homeostasis and increased susceptibility to seizures in epilepsy.

Physiological bases of the K+ and the glutamate/GABA hypotheses of epilepsy

Epilepsy is a heterogeneous family of neurological disorders that manifest as seizures, i.e. the hypersynchronous activity of large population of neurons. About 30% of epileptic patients do not respond to currently available antiepileptic drugs. Decades of intense research have elucidated the involvement of a number of possible signaling pathways, however, at present we do not have a fundamental understanding of epileptogenesis. In this paper, we review the literature on epilepsy under a wide-angle perspective, a mandatory choice that responds to the recurrent and unanswered question about what is epiphenomenal and what is causal to the disease. While focusing on the involvement of K+ and glutamate/GABA in determining neuronal hyperexcitability, emphasis is given to astrocytic contribution to epileptogenesis, and especially to loss-of-function of astrocytic glutamine synthetase following reactive astrogliosis, a hallmark of epileptic syndromes. We finally introduce the potential involvement of abnormal glycogen synthesis induced by excess glutamate in increasing susceptibility to seizures.

Roles of glutamine synthetase inhibition in epilepsy

Glutamine synthetase (GS, E.C. 6.3.1.2) is a ubiquitous and highly compartmentalized enzyme that is critically involved in several metabolic pathways in the brain, including the glutamine-glutamate-GABA cycle and detoxification of ammonia. GS is normally localized to the cytoplasm of most astrocytes, with elevated concentrations of the enzyme being present in perivascular endfeet and in processes close to excitatory synapses. Interestingly, an increasing number of studies have indicated that the expression, distribution, or activity of brain GS is altered in several brain disorders, including Alzheimer's disease, schizophrenia, depression, suicidality, and mesial temporal lobe epilepsy (MTLE). Although the metabolic and functional sequelae of brain GS perturbations are not fully understood, it is likely that a deficiency in brain GS will have a significant biological impact due to the critical metabolic role of the enzyme. Furthermore, it is possible that restoration of GS in astrocytes lacking the enzyme could constitute a novel and highly specific therapy for these disorders. The goals of this review are to summarize key features of mammalian GS under normal conditions, and discuss the consequences of GS deficiency in brain disorders, specifically MTLE.

Glycogen as a Putative Target for Diagnosis and Therapy in Brain Pathologies

Brain glycogen, a glucose polymer, is now considered as a functional energy store to the brain. Indeed, when neurons outpace their own possibilities to provide themselves with energy, astrocytic metabolism is in charge of feeding neurons, since brain glycogen synthesis is mainly due to astrocyte. Therefore, malfunctions or perturbations of astrocytic glycogen content, synthesis, or mobilization may be involved in processes of brain pathologies. This is the case, for example, in epilepsies and gliomas, two different situations in which, brain needs high level of energy during acute or chronic conditions. The purpose of the present paper is to demonstrate how brain glycogen might be relevant in these two pathologies and to pinpoint the possibilities of considering glycogen as a tool for diagnostic and therapeutic approaches in brain pathologies.

Pharmacological, neurochemical, and behavioral profile of JB-788, a new 5-HT1A agonist

A novel pyridine derivative, 8-{4-[(6-methoxy-2,3-dihydro-[1,4]dioxino[2,3-b]pyridine-3-ylmethyl)-amino]-butyl}-8-aza-spiro[4.5]decane-7,9-dione hydrochloride, termed JB-788, was designed to selectively target 5-HT(1A) receptors. In the present study, the pharmacological profile of JB-788 was characterized in vitro using radioligands binding tests and in vivo using neurochemical and behavioural experiments. JB-788 bound tightly to human 5-HT(1A) receptor expressed in human embryonic kidney 293 (HEK-293) cells with a K(i) value of 0.8 nM. Its binding affinity is in the same range as that observed for the (+/-)8-OH-DPAT, a reference 5HT(1A) agonist compound. Notably, JB-788 only bound weakly to 5-HT(1B) or 5-HT(2A) receptors and moreover the drug displayed only weak or indetectable binding to muscarinic, alpha(2), beta(1) and beta(2) adrenergic receptors, or dopaminergic D(1) receptors. JB-788 was found to display substantial binding affinity for dopaminergic D(2) receptors and, to a lesser extend to alpha(1) adrenoreceptors. JB-788 dose-dependently decreased forskolin-induced cAMP accumulation in HEK cells expressing human 5-HT(1A), thus acting as a potent 5-HT(1A) receptor agonist (E(max.) 75%, EC(50) 3.5 nM). JB-788 did not exhibit any D(2) receptor agonism but progressively inhibited the effects of quinpirole, a D(2) receptor agonist, in the cAMP accumulation test with a K(i) value of 250 nM. JB-788 induced a weak change in cAMP levels in mouse brain but, like some antipsychotics, transiently increased glycogen contents in various brain regions. Behavioral effects were investigated in mice using the elevated plus-maze. JB-788 was found to increase the time duration spent by animals in anxiogenic situations. Locomotor hyperactivity induced by methamphetamine in mouse, a model of antipsychotic activity, was dose-dependently inhibited by JB-788. Altogether, these results suggest that JB-788 displays pharmacological properties, which could be of interest in the area of anxiolytic and antipsychotic drugs.

The development of recurrent seizures after continuous intrahippocampal infusion of methionine sulfoximine in rats: a video-intracranial electroencephalographic study

Glutamine synthetase is deficient in astrocytes in the epileptogenic hippocampus in human mesial temporal lobe epilepsy (MTLE). To explore the role of this deficiency in the pathophysiology of MTLE, rats were continuously infused with the glutamine synthetase inhibitor methionine sulfoximine (MSO, 0.625 microg/h) or 0.9% NaCl (saline control) unilaterally into the hippocampus. The seizures caused by MSO were assessed by video-intracranial electroencephalogram (EEG) monitoring. All (28 of 28) of the MSO-treated animals and none (0 of 12) of the saline-treated animals developed recurrent seizures. Most recurrent seizures appeared in clusters of 2 days' duration (median; range, 1 to 12 days). The first cluster was characterized by frequent, predominantly stage I seizures, which presented after the first 9.5 h of infusion (median; range, 5.5 to 31.7 h). Subsequent clusters of less-frequent, mainly partial seizures occurred after a clinically silent interval of 7.1 days (median; range, 1.8 to 16.2 days). The ictal intracranial EEGs shared several characteristics with recordings of partial seizures in humans, such as a distinct evolution of the amplitude and frequency of the EEG signal. The neuropathology caused by MSO had similarities to hippocampal sclerosis in 23.1% of cases, whereas 26.9% of the animals had minimal neuronal loss in the hippocampus. Moderate to severe diffuse neuronal loss was observed in 50% of the animals. In conclusion, the model of intrahippocampal MSO infusion replicates key features of human MTLE and may represent a useful tool for further studies of the cellular, molecular and electrophysiological mechanisms of this disorder.

Epilepsy, regulation of brain energy metabolism and neurotransmission

Seizures are the result of a sudden and temporary synchronization of neuronal activity, the reason for which is not clearly understood. Astrocytes participate in the control of neurotransmitter storage and neurotransmission efficacy. They provide fuel to neurons, which need a high level of energy to sustain normal and pathological neuronal activities, such as during epilepsy. Various genetic or induced animal models have been developed and used to study epileptogenic mechanisms. Methionine sulfoximine induces both seizures and the accumulation of brain glycogen, which might be considered as a putative energy store to neurons in various animals. Animals subjected to methionine sulfoximine develop seizures similar to the most striking form of human epilepsy, with a long pre-convulsive period of several hours, a long convulsive period during up to 48 hours and a post convulsive period during which they recover normal behavior. The accumulation of brain glycogen has been demonstrated in both the cortex and cerebellum as early as the pre-convulsive period, indicating that this accumulation is not a consequence of seizures. The accumulation results from an activation of gluconeogenesis specifically localized to astrocytes, both in vivo and in vitro. Both seizures and brain glycogen accumulation vary when using different inbred strains of mice. C57BL/6J is the most "resistant" strain to methionine sulfoximine, while CBA/J is the most "sensitive" one. The present review describes the data obtained on methionine sulfoximine dependent seizures and brain glycogen in the light of neurotransmission, highlighting the relevance of brain glycogen content in epilepsies.

Peripheral glucose metabolism is altered by epileptic seizures

The aim of the present study was to investigate the status of jejunal absorption and peripheral metabolism of glucose in Wistar Audiogenic Rats (WAR), a genetic model of epilepsy, after seizures induced by intensive sound exposure. The jejunal loop of rats was isolated and infused (0.5 mL min(-1)) with Tyrode solution containing twice the normal concentrations of glucose, sodium, and potassium. Samples were taken at 5 or 10-min intervals over a 40-min period. At the end of the experiment, samples of liver and gastrocnemius muscle were taken to measure the levels of glycogen, glucose-6-phosphate, fructose-6-phosphate and glucose transporter-4 (GLUT4). Hepatic glucose-6-phosphate increased in WAR submitted to audiogenic seizure (21.90 +/- 3.08) as compared to non-susceptible Wistar rats (8.12 +/- 0.87) and to WAR not submitted to audiogenic stimulation (5.17 +/- 0.97). In addition, an increase in hepatic fructose-6-phosphate, an intermediate metabolite of the glycolytic pathway, was observed in WAR submitted to audiogenic seizure (5.98 +/- 0.99) compared to non-susceptible Wistar rats (2.38 +/- 0.53). According to the present results, jejunal absorption of glucose was not changed by seizures. However, generalized tonic-clonic seizures produced by sound stimulation resulted in a decrease in muscle glycogen content. In addition, our results demonstrated that the concentration of GLUT4 in the gastrocnemius muscle of WAR was 1.6-fold higher than that observed in resistant rats and that the audiogenic stimulus led to decreased concentration of this receptor in the muscle of WAR animals.

Abnormal organelles in cultured astrocytes are largely enhanced by streptomycin and intensively by gentamicin

The effects of two aminoglycoside antibiotics on cultured astrocyte organelles were investigated in rat, sheep, and human cultured astrocytes using transmission electron microscopy. Marked changes in mitochondrial shapes were observed in cultured or subcultured astrocytes obtained from three species, including humans. As well, new types of organelles were observed: (i) numerous concentric membranes forming vesicles, which were termed multilamellar vesicles; and (ii) many vesicles gathering into membranous structures, which were termed multivesicular myeloid bodies. The number of abnormalities increased proportionally with increasing concentrations of the two aminoglycosides (streptomycin and gentamicin). The incorporation of peroxidase or albumin-gold complex in the abnormal vesicles showed that the endolysosomal system was involved in the formation of these vesicles. Our results show that: abnormal organelles are present in cultured astrocytes; these abnormalities are enhanced by streptomycin and gentamicin; and gentamicin induces more abnormalities than streptomycin. The binding of aminoglycosides to membrane phospholipids may explain the formation of the observed abnormalities in rat, sheep, and human cultured astrocytes.

Astrocytic amino acid metabolism under control conditions and during oxygen and/or glucose deprivation

Amino acid contents were measured in 1- and 3-week-old primary cultures of astrocytes and in their incubation media, an amino acid-free salt solution with or without glucose, during 3-h incubation under normoxic or anoxic conditions. Most essential amino acids were rapidly released to the medium during the beginning of the incubation. A subsequent slow medium increase reflected proteolysis. Glutamate and aspartate were absent from the media during all conditions, indicating fueling of their uptake by either glycolytically or oxidatively derived energy. The total content of glutamine increased, except during incubation in glucose-deprived media, when it declined or remained constant. Changes in aspartate were negligible, suggesting oxidative degradation of aspartate-derived oxaloacetate during normoxia and its reduction to succinate during anoxia, driving regeneration of NAD+ from NADH. An increase of alanine was reduced in glucose-free media, whereas serine showed especially large increase during isolated glucose deprivation, suggesting its production from glutamine via 3-phosphoglycerate.

In vivo and in vitro glycogenic effects of methionine sulfoximine are different in two inbred strains of mice

We investigated the relationship between brain glycogen anabolism and methionine sulfoximine (MSO)-induced seizures in two inbred mouse strains that presented differential susceptibility to the convulsant. CBA/J was considered a MSO-high-reactive strain and C57BL/6J a MSO-low-reactive strain. Accordingly, the dose of MSO needed to induce seizures in CBA/J mice is lower than that in C57BL/6J mice, and CBA/J mice which had seizures, died during the first convulsion. In addition, the time--course of the MSO effect is faster in CBA/J mice than that in C57BL/6J mice. Analyses were performed in C57BL/6J and CBA/J mice after administration of 75 (subconvulsive dose) and 40 mg/kg of MSO (subconvulsive dose, not lethal dose), respectively. In the preconvulsive period, MSO induced an increase in the brain glycogen content of C57BL/6J mice only. Twenty-four hours after MSO administration, the brain glycogen content increased in both strains. The activity and expression of fructose-1,6-bisphosphatase, the last key enzyme of the gluconeogenic pathway, were increased in MSO-treated C57BL/6J mice as compared to control mice, at all experimental time points, whereas they were increased in CBA/J mice only 24 h after MSO administration. These latter results correspond to CBA/J mice that did not have seizures. Interestingly, the differences observed in vivo were consistent with results in primary cultured astrocytes from the two strains. This data suggests that the metabolism impairment, which was not a consequence of seizures, could be related to the difference in seizure susceptibility between the two strains, depending on their genetic background.

Chronic inhibition of glutamine synthetase is not associated with impairment of learning and memory in mice

The convulsant methionine sulfoximine (MSO) is a byproduct of the agenized flour commonly used for feeding domestic animals decades ago. MSO is a powerful glycogenic and epileptogenic agent, and it is an irreversible inhibitor of glutamine synthetase. This latter effect was hypothesized to be responsible for the increase in the incidence of some neuropathologies in humans, such as Alzheimer's disease or Parkinson's disease. In order to test this hypothesis, we chronically administered MSO to two inbred strains of mice, C57BL/6J and BALB/cJ, and analyzed possible alterations in learning and memory features of these mice. Mice were given 20 mg/kg of MSO three times a week for 10 weeks. Spatial learning capabilities assessed with a radial maze were not affected by the long-term MSO treatment, although activity was significantly decreased in BALB/cJ mice. Thus, our data suggest that long-term administration of non-convulsive and non-glycogenic doses of MSO do not alter the spatial memory of mice. Our results do not support the hypothesis that chronic treatment with MSO influences hippocampus-dependent learning abilities in mice.

Expression of fructose-1,6-bisphosphatase mRNA isoforms in normal and basal forebrain cholinergic lesioned rat brain

Fructose-1,6-bisphosphatase is one of the key enzymes in the gluconeogenic pathway predominantly occurring in liver, kidney and muscle. In the brain, fructose-1,6-bisphosphatase has been suggested to be an astrocyte-specific enzyme but the functional importance of glyconeogenesis in the brain is still unclear. To further elucidate the cellular source of fructose-1,6-bisphosphatase in the brain, non-radioactive in situ hybridizations were performed using digoxigenin-labeled RNA probes based on the sequence of recently cloned rat liver and muscle fructose-1,6-bisphosphatase cDNAs. In situ hybridization using a riboprobe for the liver isoform revealed a location of the hybridization signal mainly in neurons, while rat muscle fructose-1,6-bisphosphatase mRNA was detected in both neurons and astrocytes in the hippocampal formation and in layer I of the cerebral cortex.RT-PCR using RNA preparations of rat astrocytes, neurons, and adult whole brain demonstrated a localization of liver fructose-1,6-bisphosphatase mRNA isoform in neurons but not in astrocytes. The muscle fructose-1,6-bisphosphatase mRNA isoform could be detected by RT-PCR in total rat brain, astrocytic, and neuronal mRNA preparations. The isoforms of fructose-1,6-bisphosphatase mRNA seemingly demonstrate a distinct cellular expression pattern in rat brain suggesting a role of glyconeogenesis in both neurons and glial cells.

Glucocorticoids induce glutamine synthetase in folliculostellate cells of rat pituitary glands in vivo and in vitro

Glutamine synthetase (GS) is a glucocorticoid-inducible enzyme that has a key role for glutamate metabolism in the central and peripheral nervous system. In this study GS activity was measured and the amount of immunoreactive GS (ir-GS) cells in the rat anterior pituitary gland was quantified as a function of age. In addition, the effects of GS inhibitors, glucocorticoid administration, and adrenalectomy on GS activity were examined. Some of the ir-GS cells were also immunoreactive for S100 protein (ir-S100) which is a known marker for folliculostellate cells (FS) in the anterior pituitary. FS cells expressing GS were first detected in 3-d-old rats, and this cell population, expressed as the immunostained cell area divided by a standard unit area, increased as a function of age. The percentages of FS cells also expressing GS were 0.2, 6.4, 25 and 74% at 3 d, 30 d, 60 d and 2 y of age, respectively. GS enzyme activity also increased in parallel with the increase of ir-GS cell population maturation. The subcutaneous injection of methionine sulphoximine, a GS and gamma-glutamylcysteine synthetase inhibitor, reduced pituitary GS activity by 83%, but increased the population of ir-GS cells 3.5-fold in 30-d-old rats. Buthionine sulphoximine, a specific inhibitor of y-glutamylcysteine synthetase, had little effect on GS activity or the ir-GS cell population. Neither methionine sulphoximine nor buthionine sulphoximine changed the population of ir-S100 protein cells (FS cells). Dexamethasone and hydrocortisone increased the population of ir-GS cells by 3.1 and 4.2-fold, respectively, within 12 h after administration. A significant increase of GS activity due to the injection of glucocorticoids was observed in the anterior pituitary, but not in the brain, retina or liver of immature rats. Adrenalectomy did not cause decrease of pituitary GS activity, and dexamethasone administration increased GS activity in both adrenalectomised and intact rats. In the monolayer culture of anterior pituitary cells, glucocorticoids increased GS activity by x 1.5, and methionine sulphoximine reduced the activity by over 94%. These results demonstrate that GS in folliculostellate cells is a glucocorticoid-inducible enzyme in vivo and in vitro, and that the age-dependent increase of GS activity is independent of endogenous adrenal glucocorticoids.

Isolation of carbohydrate metabolic clones from cultured astrocytes

Astrocytes are the principal sites of glycogen synthesis in the nervous tissue. Growing evidence shows that there are many types of astrocytes. The aim of the present investigation was to isolate different types of astrocytes that display different carbohydrate anabolism. Astrocytes from newborn rat brain were directly cloned from primary cultures without a previous transformation. Many clones were obtained, and they were termed CP clones. Another series of clones, termed SV clones, were obtained after the transfection of the primary cultures by the SV40 T antigen. The effectiveness of the transfection was verified by the rate of DNA synthesis using flow cytometry and by the presence of plasmid DNA in the genomic DNA of the astrocytes using the Southern blot method. After the transfection, the growth velocity increased greatly. The size and shape of the astrocytes were the same for each cell in a given clone, regardless of the cloning method utilized. However, these sizes and shapes could be different from one clone to another in CP clones, whereas all the astrocytes of all the SV clones looked like each other. All the clones obtained stained positively with anti-glial fibrillary acidic protein antibodies. Glycogen stained in the clones using concanavalin A-horseradish peroxidase. The glycogen content was also measured using biochemical analysis. Concordant results obtained using two methods showed that some clones contained an important quantity of glycogen while other clones contained a small amount, in the CP series as well as in the SV series. This property was the same for the intracellular glucose concentrations. The activity of the gluconeogenic enzyme fructose-1, 6-bisphosphatase was measured in each clone using spectrophotometry. This activity was also significantly different from one clone to another. The clones containing large amounts of glycogen had important fructose-1,6-bisphosphatase activity. The present results show that it is possible to clone astrocytes either directly from primary cultures without immortalization or after their transformation. When analyzing these clones, it appears that carbohydrate anabolism can be significantly different from one astrocyte to another. This difference may also exist in vivo.

Glycogen accumulated in the brain following insults is not degraded during a subsequent period of ischemia

The primary objective of this study was to attempt to induce excessive intraglial acidosis during ischemia by subjecting rats to an initial insult which leads to post insult accumulation of glycogen, presumed to accumulate primarily in astrocytes. The initial insults were 15 min of transient forebrain ischemia, 30 min of hypoglycemic coma, and intraperitonial injection of methionine-sulphoximine (MSO). In the first two of these insults, glycogen content in neocortex increased to 6-7 mM kg(-1) after 6 h of recovery, and in MSO-treated animals even higher values were measured 24 h after administration ( 12 mM kg(-1)). In spite of this glycogen loading, the amount of lactate formed during a subsequent ischemic insult (of 5-30 min duration) did not exceed values usually obtained during complete ischemia in animals with normal glycogen contents (tissue lactate contents of 15 mM kg(-1)) This was because appreciable amounts of glycogen (3-7 mM kg(-1)) remained undegraded even after 30 min of ischemia. The undigested part largely reflected the extra amount of glycogen accumulated after the initial insults. It is discussed whether this part is unavailable to degradation by phosphorylase.

Regulation of fructose-1,6-bisphosphatase activity in primary cultured astrocytes

In the gluconeogenic pathway, fructose-1,6-bisphosphatase (EC 3. 1. 3. 11) is the last key-enzyme before the synthesis of glucose-6-phosphate. The extreme diversity of cells present in the whole brain does not facilitate in vivo study of this enzyme and makes it difficult to understand the regulatory mechanisms of the related carbohydrate metabolism. It is for instance difficult to grasp the actual effect of ions like potassium, magnesium and manganese on the metabolic process just as it is difficult to grasp the effect of different pH values and the influence of glycogenic compounds such as methionine sulfoximine. The present investigation attempts to study the expression and regulation of fructose-1,6-bisphosphatase in cultured astrocytes. Cerebral cortex of new-born rats was dissociated into single cells that were then plated. The cultured cells were flat and roughly polygonal and were positively immunostained by anti-glial fibrillary acidic protein antibodies. Cultured astrocytes are able to display the activity of fructose-1,6-bisphosphatase. This activity was much higher than that in brain tissue in vivo. Fructose-1,6-bisphosphatase in cultured astrocytes did not require magnesium ions for its activity. The initial velocity observed when the activity was measured in standard conditions was largely increased when the enzyme was incubated with Mn2+. This increase was however followed by a decrease in absorbance resulting in the induction, by the manganese ions, of a singular kinetics in the enzyme activity. Potassium ions also stimulated fructose-1,6-bisphosphatase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Colocalization of fructose-1,6-bisphosphatase and glial fibrillary acidic protein in rat brain

Immunofluorescence studies of rat brain sections demonstrated an exclusive colocalization of the gluconeogenic key enzyme fructose-1,6-bisphosphatase (FBPase) with the astroglial marker glial fibrillary acidic protein, indicating FBPase in brain as an astrocyte-specific enzyme. This conclusion was supported by the presence of FBPase activity in astroglia-rich but not neuron-rich primary cultures derived from rat brain.

Some aspects of carbohydrate metabolism in the brain

A convenient physiology of the nervous system closely depends on the availability of glucose, the lack of which quickly results in syncope and death. Carbohydrate metabolism in the brain was long thought of as being specific and different from liver carbohydrate metabolism. The present report tries to summarize current data and advances in our knowledge about carbohydrate metabolism. Glucose is brought to the brain by blood flowing through a special network of arteries and is quickly catabolized by the glycolytic and tricarboxylic acid cycle pathways to synthesize energy. It is also used in the synthesis of numerous amino acids, nucleotides and NADPH. Glucose can be polymerized into glycogen in the brain. The nerve tissue is capable of synthesizing glucose-6-phosphate in the gluconeogenic pathway since the fructose-1,6-bisphosphatase, the key enzyme believed to be absent, is actually active and has been purified up to electrophoretic homogeneity. Moreover, the possibility of free glucose synthesis by astrocytes exists. Although the exact role of glycogen in the brain is not totally clear, it is known that the polysaccharide content generally decreases when the functioning of the brain is stimulated and increases in sedative state. This carbohydrate can therefore serve as an indicator for the level of brain activity. Through the administration of methionine sulfoximine, it is possible to increase the amount of glycogen in the brain massively and obtain particles similar to those found in the liver. These in vivo findings have been confirmed by studies based on cultured astrocytes. It has been shown with cultured astrocytes that glutamate increases glycogen synthesis in a pathway which still remains to be elucidated. Brain carbohydrate metabolism is thus in many ways similar to liver carbohydrate metabolism. The astrocyte constitutes the main cell implicated in this metabolism. Improvement in our knowledge about brain carbohydrate metabolism should spread the use of brain glucose metabolism in the diagnosis of certain diseases.

Modification of hypoxia-induced injury in cultured rat astrocytes by high levels of glucose

BACKGROUND AND PURPOSE

Preexisting hyperglycemia exacerbates central nervous system injury after transient global and focal cerebral ischemia. Increased anaerobic metabolism with resultant lactic acidosis has been shown to cause the hyperglycemic, neuronal injury. The contribution of astrocytes in producing lactic acidosis under hyperglycemic/ischemic conditions is unclear, whereas the protective role of astrocytes in ischemic-induced neuronal injury has been documented. The ability of astrocytes to maintain energy status and ion homeostasis under hyperglycemic conditions could ultimately reduce neuronal injury. Therefore, we determined the effects of increased glucose concentrations on glucose utilization, lactate production, extracellular pH, and adenosine triphosphate concentrations in hypoxia-treated astrocyte cultures.

METHODS

Primary astrocytes were prepared from neonatal rat cerebral cortices. After 35 days in vitro, cultures were incubated with 0-60 mmol/L glucose and subjected to hypoxic conditions at 95% N2/5% CO2 for 24 hours. In addition, under high-glucose conditions (30 mmol/L), astrocytes were exposed to up to 72 hours of hypoxia. Determination of lactate dehydrogenase efflux, adenosine triphosphate concentrations, and extracellular lactate concentrations defined astrocyte status. Equiosmolar levels of mannitol were added in place of high glucose concentrations to distinguish hyperosmotic effect.

RESULTS

When physiological concentrations of glucose (7.5 mmol/L) or lower concentrations were used, significant cell damage occurred with 24 hours of hypoxia, as determined by increased efflux of lactate dehydrogenase and loss of cell protein. When higher glucose concentrations (15-60 mmol/L) were used, efflux of lactate dehydrogenase was similar to that observed in normoxic cultures, despite an increased utilization of glucose. Lactate concentrations in the media at low or normal glucose concentrations exceeded normoxic levels, but higher glucose concentrations (15-30 mmol/L) failed to increase lactate levels further. Values of adenosine triphosphate for hypoxic astrocytes treated with high glucose concentrations were significantly higher than those of astrocytes with zero or low glucose levels. In cultures exposed to hypoxia and high glucose levels (30 mmol/L), no cellular injury was observed before 48 hours of hypoxia. Lactate concentrations in the media increased during the first 24 hours of hypoxia and reached steady state. The pH of the media decreased to 6.4 after 24 hours and 5.5 at 48 hours. The latter pH was concomitant with a marked increase in extracellular lactate dehydrogenase activity. Hyperosmotic mannitol failed to protect cultured astrocytes against hypoxia.

CONCLUSIONS

Hypoxic injury to mature astrocytes was reduced by the presence of 15-60 mmol/L glucose in the medium during 24-30 hours of hypoxia. Injury occurred when the pH of the medium was < 5.5. This protection was not afforded by the hyperosmotic effect of high glucose concentrations, nor was the hypoxic injury at later time periods with 30 mmol/L glucose mediated solely by lactate accumulation.

Incorporation of radioactivity from [14C]lactate into the glycogen of cultured mouse astroglial cells. Evidence for gluconeogenesis in brain cells

A pure population of astroglial cells was selected from heterogeneous astroglia-rich primary cultures in a medium containing sorbitol instead of glucose. It was shown that astroglial cells synthesize glycogen when they are returned to a glucose-containing medium, and that when [14C]lactate is also present the synthesized glycogen is radioactively labelled. Compared with the degree of incorporation of radioactivity in the presence of tritiated glucose, the incorporation of radioactivity from lactate was small but significant. After incubation of astroglial cells with radioactively labelled lactate, the glycogen was isolated and enzymatically hydrolysed to glucose, which was found to be radioactively labelled. Astrocytes are therefore able to convert lactate to glucosyl residues, a metabolic pathway known as gluconeogenesis. It is proposed that astrocytic gluconeogenesis may consume lactic acid formed in neighboring cells such as neurons, during anaerobic glycolysis at times of high energy demand.

Biochemical and ultrastructural study of glycogen in cultured astrocytes submitted to the convulsant methionine sulfoximine

The convulsant methionine sulfoximine is a potent glycogenic agent in the central nervous system of rodents in vivo. This investigation was undertaken to look for the basic mechanism underlying this property. Astrocytes were cultivated from newborn rat neopallium and glycogen was studied by both biochemical and ultrastructural methods. When the astrocytes were incubated in a medium containing 5.55 mM glucose, methionine sulfoximine (0.55 mM) induced a significant increase in their glycogen content. Glucose content did not change in astrocytes, but it diminished in the medium in all cases. When the decrease in glucose level in the medium was limited, the same glycogenic effects of methionine sulfoximine were observed, but the glycogen contents were higher. The augmentation of the concentration of the convulsant enhanced its glycogenic effect, but this was not directly dose dependent. When the flat and polygonal astrocytes were transformed into process-bearing astrocytes by dibutyryl cyclic AMP methionine sulfoximine always induced an increase in glycogen content. In this case, the values of glycogen contents were lower. In electron microscopy, no glycogen particles were present in the astrocytes even after methionine sulfoximine treatment, contrary to the case in vivo. These results show that the convulsant does not need the presence of neuronal cells to induce glycogen accumulation and that astrocytes may be the direct cell targets. The apparent discrepancy between the biochemical and ultrastructural data is probably due to the relatively low concentration of glycogen in cultured astrocytes.

Correlation between carbohydrate and catecholamine level impairments in methionine sulfoximine epileptogenic rat brain

This work shows that the convulsant methionine sulfoximine induces an increase in glucose and glycogen levels and a parallel decrease in norepinephrine and dopamine levels in rat brain. Among the epileptogenic agents, methionine sulfoximine is known to have a glycogenic property in the central nervous system. The aim of this work is to look for the neurochemical mechanism underlying this property. For this, catecholamines, glucose, and glycogen were measured at the same time in different areas of the brain in rats submitted to methionine sulfoximine. The convulsant induced an increase in glucose and glycogen levels as previously described and a decrease in dopamine and norepinephrine levels in all the areas of the rat brain. These changes were roughly dose dependent. When L-dihydroxyphenylalanine and benserazide (a decarboxylase inhibitor) were administered with methionine sulfoximine, the latter failed to induce seizures in rat up to 8 h after dosing. Moreover, the glucose and glycogen amounts did not increase. In all these experiments, there was an obvious evidence of parallelism between seizures, increase in carbohydrate levels, and decrease in catecholamine levels. These results allow to conclude that the glycogenic property of methionine sulfoximine in the central nervous system probably results from its ability to decrease norepinephrine and dopamine levels. Because the effect of the convulsant on the catecholamine levels persisted for long, it is normal that glucose and glycogen levels increased during preconvulsive, convulsive and postconvulsive period. Methionine sulfoximine is probably glycogenic in rat brain because it decreases catecholamine levels for a long time.

Possible involvement of indolamines in the glycogenic effect of the convulsant methionine sulfoximine in rat brain

The aim of the present investigation was to look for the mechanisms causing disturbances in carbohydrate metabolism during the action of the epileptogenic agent methionine sulfoximine. The levels of glucose, glycogen, and indolamines were measured in seven different regions of rat brain. Methionine sulfoximine induced a decrease in serotonin level which was roughly dose-dependent. There were no obvious changes in tryptophan and 5-hydroxyindoleacetic levels in any area. Methionine sulfoximine induced the known increase in glucose and glycogen levels. The direct precursor of serotonin. 5-hydroxytryptophan, and benserazide (a decarboxylase inhibitor) were then injected into rats in association with methionine sulfoximine. In this case, methionine sulfoximine failed to induce seizures. Moreover, the serotonin level was unchanged and the carbohydrate content did not significantly increase. There was only a rise in 5-hydroxyindoleacetic acid level. This work shows a striking parallelism between serotonin decrease and glycogen increase.