Effects of fluoroacetate and fluorocitrate on the metabolic compartmentation of tricarboxylic acid cycle in rat brain slices

Key Metabolic Enzymes Underlying Astrocytic Upregulation of GABAergic Plasticity

GABAergic plasticity is recognized as a key mechanism of shaping the activity of the neuronal networks. However, its description is challenging because of numerous neuron-specific mechanisms. In particular, while essential role of glial cells in the excitatory plasticity is well established, their involvement in GABAergic plasticity only starts to emerge. To address this problem, we used two models: neuronal cell culture (NC) and astrocyte-neuronal co-culture (ANCC), where we chemically induced long-term potentiation at inhibitory synapses (iLTP). iLTP could be induced both in NC and ANCC but in ANCC its extent was larger. Importantly, this functional iLTP manifestation was accompanied by an increase in gephyrin puncta size. Furthermore, blocking astrocyte Krebs cycle with fluoroacetate (FA) in ANCC prevented enhancement of both mIPSC amplitude and gephyrin puncta size but this effect was not observed in NC, indicating a key role in neuron-astrocyte cross-talk. Blockade of monocarboxylate transport with α-Cyano-4-hydroxycinnamic acid (4CIN) abolished iLTP both in NC and ANCC and in the latter model prevented also enlargement of gephyrin puncta. Similarly, blockade of glycogen phosphorylase with BAYU6751 prevented enlargement of gephyrin puncta upon iLTP induction. Finally, block of glutamine synthetase with methionine sulfoxide (MSO) nearly abolished mIPSC increase in both NMDA stimulated cell groups but did not prevent enlargement of gephyrin puncta. In conclusion, we provide further evidence that GABAergic plasticity is strongly regulated by astrocytes and the underlying mechanisms involve key metabolic enzymes. Considering the strategic role of GABAergic interneurons, the plasticity described here indicates possible mechanism whereby metabolism regulates the network activity.

Inhibition of astrocyte metabolism is not the primary mechanism for anaesthetic hypnosis

Astrocytes have been promoted as a possible mechanistic target for anaesthetic hypnosis. The aim of this study was to explore this using the neocortical brain slice preparation. The methods were in two parts. Firstly, multiple general anaesthetic compounds demonstrating varying in vivo hypnotic potency were analysed for their effect on "zero-magnesium" seizure-like event (SLE) activity in mouse neocortical slices. Subsequently, the effect of astrocyte metabolic inhibition was investigated in neocortical slices, and compared with that of the anaesthetic drugs. The rationale was that, if suppression of astrocytes was both necessary and sufficient to cause hypnosis in vivo, then inhibition of astrocytic metabolism in slices should mimic the anaesthetic effect. In vivo anaesthetic potency correlated strongly with the magnitude of reduction in SLE frequency in neocortical slices (R(2) 37.7 %, p = 0.002). Conversely, SLE frequency and length were significantly enhanced during exposure to both fluoroacetate (23 and 20 % increase, respectively, p < 0.01) and aminoadipate (12 and 38 % increase, respectively, p < 0.01 and p < 0.05). The capacity of an anaesthetic agent to reduce SLE frequency in the neocortical slice is a good indicator of its in vivo hypnotic potency. The results do not support the hypothesis that astrocytic metabolic inhibition is a mechanism of anaesthetic hypnosis.

Neuron-astrocyte interaction enhance GABAergic synaptic transmission in a manner dependent on key metabolic enzymes

Gamma aminobutric acid (GABA) is the major inhibitory neurotransmitter in the adult brain and mechanisms of GABAergic inhibition have been intensely investigated in the past decades. Recent studies provided evidence for an important role of astrocytes in shaping GABAergic currents. One of the most obvious, but yet poorly understood, mechanisms of the cross-talk between GABAergic currents and astrocytes is metabolism including neurotransmitter homeostasis. In particular, how modulation of GABAergic currents by astrocytes depends on key enzymes involved in cellular metabolism remains largely unknown. To address this issue, we have considered two simple models of neuronal culture (NC): nominally astrocyte-free NC and neuronal-astrocytic co-cultures (ANCC). Miniature Inhibitory Postsynaptic Currents (mIPSCs) were recorded in control conditions and in the presence of different enzyme blockers. We report that enrichment of NC with astrocytes results in a marked increase in mIPSC frequency. This enhancement of GABAergic activity was accompanied by increased number of GAD65 and vGAT puncta, indicating that at least a part of the frequency enhancement was due to increased number of synaptic contacts. Inhibition of glutamine synthetase (Glns) (with MSO) strongly reduced mIPSC frequency in ANCC but had no effect in NC. Moreover, treatment of ANCC with inhibitor of glycogen phosphorylase (Gys) (BAYU6751) or with selective inhibitor of astrocytic Krebs cycle, fluoroacetate, resulted in a marked reduction of mIPSC frequency in ANCC having no effect in NC. We conclude that GABAergic synaptic transmission strongly depends on neuron-astrocyte interaction in a manner dependent on key metabolic enzymes as well as on the Krebs cycle.

Preseizure increased gamma electroencephalographic activity has no effect on extracellular potassium or calcium

Extracellular ion concentrations change during seizures in seizure models. [K(+)](o) increases and [Ca(2+)](o) decreases, resulting from population discharges, enhanced neuronal excitability, though not obviously before seizure onset. In acute pharmacological epilepsy models, there are striking increases in preictal high-frequency (gamma) electroencephalographic (EEG) activity. It is not known whether enhanced gamma EEG results in ionic changes, because gamma and ions have not been measured simultaneously. In this study, unanesthetized, paralyzed rats were given intravenous injections of kainic acid or picrotoxin to induce EEG discharges. Changes in EEG, [K(+)](o), and [Ca(2+)](o) in cortex and hippocampus were recorded. Kainic acid caused small [K(+)](o) fluctuations, without a temporal relationship of these with increased gamma EEG or with onset of discharges. Gamma EEG increases after picrotoxin also failed to affect [K(+)](o) and [Ca(2+)](o). Picrotoxin-induced electrical discharges led to [K(+)](o) rises of >9 mM and [Ca(2+)](o) falls of 0.1-0.2 mM. Kainic acid-induced discharges generated only moderate (2-3 mM) rises in [K(+)](o) and no changes in [Ca(2+)](o). In both models, there were large potassium rises (15-80 mM) and calcium falls (>0.5 mM), suggesting spreading depressions. Small [K(+)](o) fluctuations after kainic acid are consistent with disruption in potassium homeostasis, possibly because of depolarization of astrocytes. To reveal possible latent [K(+)](o) or [Ca(2+)](o) changes, we injected fluorocitrate intracortically to impair astrocytic function, before administering picrotoxin. Even fluorocitrate did not cause gamma-related ion changes but did cause low-magnitude, transient, potassium increases and slower potassium homeostasis during discharges, minor changes consistent with involvement of both astrocytes and neurons in [K(+)](o) regulation. (c) 2007 Wiley-Liss, Inc.

Characteristics of monocarboxylates as energy substrates other than glucose in rat brain slices and the effect of selective glial poisoning - a 31P NMR study

In rat brain slices we examined the differences in the levels of high-energy phosphates in the presence of various energy substrates by using 31P NMR with a time resolution of 4 min at 25 degrees C. In parallel experiments we recorded population excitatory postsynaptic potentials (EPSPs) from granule cells in rat hippocampal slices. During high K(+) stimulation (8 min) phosphocreatine (PCr) decreased to a low level and recovered to the control level in standard artificial cerebrospinal fluid (ACSF) in about 10 min. Population EPSPs disappeared following high-K(+) stimulation and recovered in standard ACSF. In iodoacetic acid (IAA)-pretreated slices, whereas glucose was unable to support energy metabolism, the PCr level, which decreased following high-K(+) stimulation, recovered in ACSF containing lactate or pyruvate. The half-time of recovery of PCr levels in ACSF containing lactate was longer than that containing glucose. Population EPSPs in standard ACSF were maintained for more than 1 h, but those in ACSF containing lactate decreased gradually by about half in 40 min. In IAA-pretreated slices, when further treated with fluorocitrate (100 microM) for 2 h, the recovery of the PCr level in ACSF containing lactate after high-K(+) stimulation was completely abolished, whereas the recovery of the PCr level in ACSF containing pyruvate was unaffected. These results indicate that neurons can utilize pyruvate as well as glucose, but not lactate, as exogenous energy substrates, and that lactate may be metabolized to pyruvate in glial cells and transported to neurons to be utilized as an energy substrate.

Trafficking of amino acids between neurons and glia in vivo. Effects of inhibition of glial metabolism by fluoroacetate

Glial-neuronal interchange of amino acids was studied by 13C nuclear magnetic resonance spectroscopy of brain extracts from fluoroacetate-treated mice that received [1,2-(13)C]acetate and [1-(13)C]glucose simultaneously. [13C]Acetate was found to be a specific marker for glial metabolism even with the large doses necessary for nuclear magnetic resonance spectroscopy. Fluoroacetate, 100 mg/kg, blocked the glial, but not the neuronal tricarboxylic acid cycles as seen from the 13C labeling of glutamine, glutamate, and gamma-aminobutyric acid. Glutamine, but not citrate, was the only glial metabolite that could account for the transfer of 13C from glia to neurons. Massive glial uptake of transmitter glutamate was indicated by the labeling of glutamine from [1-(13)C]glucose in fluoroacetate-treated mice. The C-3/C-4 enrichment ratio, which indicates the degree of cycling of label, was higher in glutamine than in glutamate in the presence of fluoroacetate, suggesting that transmitter glutamate (which was converted to glutamine after release) is associated with a tricarboxylic acid cycle that turns more rapidly than the overall cerebral tricarboxylic acid cycle.

Contribution of glial metabolism to neuronal damage caused by partial inhibition of energy metabolism in retina

Glial cells are relatively resistant to energy impairment, although little is known of the extent to which glial metabolism is affected during partial energy impairment and how this influences neurons. Fluorocitrate has been shown to be a glial specific metabolic inhibitor. Its selective effect on chick retinal Müller cells was verified by measuring incorporation of radiolabel from 3H-acetate and U-14C-glucose into glutamate and glutamine following exposure of isolated embryonic day 15-18 chick retina to 20 microm fluorocitrate. Fluorocitrate significantly reduced the incorporation of radiolabel from acetate and glucose into glutamine, with less effect on incorporation of label from acetate into glutamate and no reduction of label from glucose into glutamate. The relative specific activity (RSA; ratio of glutamine to glutamate) increased between embryonic day 15 and 18 consistent with the increase in glutamine synthetase activity that occurs in Müller cells at this time in chick retinal development. As with previous findings, mild energy stress produced by inhibiting glycolysis with the general inhibitor iodoacetate (IOA) for up to 45 min, caused acute neuronal damage that was predominately NMDA receptor mediated and occurred in the absence of a net efflux of excitatory amino acids. Acute NMDA-mediated toxicity in this preparation is characterized by the selective damage to amacrine and ganglion cells and quantitatively, by GABA release into the medium. When IOA was combined with fluorocitrate, acute toxicity was potentiated and temporally accelerated. Acute damage was first noted at 15 min, occurred throughout all retinal layers and was accompanied by an overflow of excitatory amino acids at 30 and 45 min. Blocking NMDA receptors with MK-801 during IOA plus fluorocitrate exposure attenuated the rise in excitatory amino acids and prevented the swelling in neuronal, but not Müller cells. Following incorporation of radiolabel from acetate and glucose into glutamate and glutamine after different times of exposure to IOA showed that while the effects of incorporation of label from glucose were immediate, glutamine synthesis from acetate was preserved for a longer period of time. These findings suggest that during a partial energy impairment, neuronal metabolism is affected to a greater extent than is glial metabolism. Glial cells may play a protective role in this situation, and can delay the onset of acute neuronal damage.

Fluorocitrate and fluoroacetate effects on astrocyte metabolism in vitro

The Krebs cycle inhibitor fluorocitrate (FC) and its precursor fluoroacetate (FA) are taken up in brain preferentially by glia. These compounds are used experimentally to inhibit glial metabolism in situ. The actions of these agents have been attributed to both the disruption of carbon flux through the Krebs cycle and to impairment of ATP production. We used primary astrocyte cultures to evaluate these two possible modes of action. Astrocyte ATP levels exhibited little or no reduction during incubation with 0.5 mM FC or 25 mM FA. Correspondingly, FC and FA caused less than 30% reductions in glutamate uptake (P > 0.05), an important energy-dependent astrocyte function. Carbon flux through the Krebs cycle was assessed by measuring astrocyte glutamine production in the absence of exogenous glutamate or aspartate. Under these conditions, glutamine production was reduced 65 +/- 5% by 0.5 mM FC and 61 +/- 3% by 25 mM FA (P < 0.01). In contrast, FC and FA had no effect on glutamine production when 50 microM glutamate was provided in the media. These findings suggest that the metabolic effects of FC and FA on astrocytes in vivo result from impairment of carbon flux through the Krebs cycle, and not from impairment of oxidative ATP production.

NMR spectroscopy of cultured astrocytes: effects of glutamine and the gliotoxin fluorocitrate

Glial synthesis of glutamine, citrate, and other carbon skeletons, as well as metabolic effects of the gliotoxin fluorocitrate, were studied in cultured astrocytes with 13C and 31P NMR spectroscopy. [2-13C]Acetate and [1-13C]glucose were used as labeled precursors. In some experiments glutamine (2.5 mM) was added to the culture medium. Fluorocitrate (20 microM) inhibited the tricarboxylic acid (TCA) cycle without affecting the level of ATP. The net export of glutamine was reduced significantly, and that of citrate increased similarly, consistent with an inhibition of aconitase. Fluorocitrate (100 microM) inhibited TCA cycle activity even more and (without addition of glutamine) caused a 40% reduction in the level of ATP. In the presence of 2.5 mM glutamine, 100 microM fluorocitrate did not affect ATP levels, although glutamine synthesis was nearly fully blocked. The consumption of the added glutamine increased with increasing concentrations of fluorocitrate, whereas the consumption of glucose decreased. This shows that glutamine fed into the TCA cycle, substituting for glucose as an energy substrate. These findings may explain how fluorocitrate selectively lowers the level of glutamine and inhibits glutamine formation in the brain in vivo, viz., not by depleting glial cells of ATP, but by causing a rerouting of 2-oxoglutarate from glutamine synthesis into the TCA cycle during inhibition of aconitase. Analysis of the 13C labeling of the C-2 versus the C-4 positions in glutamine obtained with [2-13C]acetate revealed that 57% of the TCA cycle intermediates were lost per turn of the cycle.(ABSTRACT TRUNCATED AT 250 WORDS)

Seizures induced by fluoroacetic acid and fluorocitric acid may involve chelation of divalent cations in the spinal cord

Fluoroacetic and fluorocitric acid toxicity is often characterized by seizures, however the mechanism of this activity is unknown. Intrathecal (i.t.) injection of fluorocitrate in mice resulted in seizures after an average latency of 15 s, while intracerebroventricular (i.c.v.) injection produced seizures after 36.5 min, and required higher doses to achieve this effect. This indicates the probable site of fluoroacetate and fluorocitrate neurotoxicity is the spinal cord. To mimic citrate accumulation, characteristic of fluoroacetate and fluorocitrate poisoning, citric acid was injected i.t. and also found to produce seizures. The structurally unrelated compounds EDTA, EGTA, glutamic acid and lactic acid also produced seizures identical to fluorocitrate. The ability of these compounds to chelate Ca2+ correlates well with their ability to cause seizures when administered i.t. and coadministration of calcium greatly attenuated the neurotoxicity of these compounds as well as fluoroacetate and fluorocitrate. In contrast, Ca2+ was unable to inhibit seizures elicited by strychnine, suggesting calcium's ability to inhibit chelators of divalent cations is not due to a general anticonvulsant effect. These results suggest that changes in Ca2+ concentration in the spinal cord may be responsible for some forms of seizure activity.

The effect of fluorocitrate on transmitter amino acid release from rat striatal slices

In order to study the role of glutamine from glial cells for the synthesis of transmitter amino acids, the effect of the gliotoxic-substance fluorocitrate on amino acid release from slices was investigated. In vivo treatment with 1 nmol fluorocitrate reduced the Ca2+ dependent K+ evoked release of endogenous glutamate and GABA from the slices, whereas the glutamine efflux decreased and alanine efflux increased. The K+ evoked release of [3H]D-aspartate increased during fluorocitrate treatment. The latter is consistent with an inhibited uptake of D-aspartate into glial cells. Incubation of striatal slices with fluorocitrate (0.1 mM) decreased the glutamine efflux and increased the alanine efflux. Similar to the in vivo condition, fluorocitrate increased the K+ evoked [3H]D-aspartate release, but the K+ evoked release of endogenous glutamate and GABA increased rather than decreased. The ratio between the K+ evoked release of exogenous D-aspartate to endogenous glutamate increased in both cases. The results suggest an important role of glial cells in the synthesis and inactivation of transmitter amino acids.

The toxin kainic acid: a study of avian nerve and glial cell response utilizing tritiated kainic acid and electron microscopic autoradiography

Three questions are asked regarding the toxin kainic acid (KA). Does it destroy specific glial cells as well as neurons? Does KA gain access to the cytoplasm in intact cells and to which organelles does it bind? Intracerebral injections of tritiated KA into the pigeon (Columba livia) paleostriatal complex (basal ganglia) coupled with electron microscopic autoradiography revealed the following major points. Kainic acid destroyes oligodendrocytes, with pathophysiology apparent by 30 min after challenge with KA leading to cell destruction by 4 h. The response of astrocytes at the longest observation period (4 h) involves swelling of perivascular endfeet and processes in the neuropil. Reactive microglial-like cells show an accumulation of label in their cytoplasm, but no apparent morphological changes. The label appears in the cytoplasm of intact cells, both glia and neurons early after challenge with the toxin. Label is associated (bound) with mitochondria at an incidence significantly above chance at 30 min, 2 and 4 h after challenge with KA. Two hours after exposure to KA is the critical period where metabolic, physiological and morphological changes occur that lead to cell death. Cell destruction may be a consequence of KA-induced energy depletion. Kainate may interfere with adequate energy production by uncoupling glycolysis and the Krebs cycle in the mitochondria.

An in vivo model for studying function of brain tissue temporarily devoid of glial cell metabolism: the use of fluorocitrate

The effect of intrastriatal injection of fluorocitrate on amino acid pattern, cell enzyme markers, and ultrastructural appearance was investigated. A dose of 1 nmol of fluorocitrate resulted in temporarily decreased levels of glutamine, glutamate, and aspartate, whereas the level of alanine was increased. The glutamine level was severely reduced after 4 h but was reversed after 24 h. The activity of different cellular enzyme markers did not change markedly after this dose. Ultrastructural changes in glial cells were observed, concomitant with the biochemical changes. A dose of greater than or equal to 2 nmol of fluorocitrate resulted in more marked and irreversible changes in amino acid levels. By 24-72 h after the injection of this dose, several marker enzyme activities decreased markedly. The ultrastructural changes affected the neurons as well as the glial cells and were not reversible. The use of microinjection of 1 nmol of fluorocitrate into the neostriatum of the rat to provide a model for studying transmitter amino acid metabolism in brain devoid of glial cell activity is discussed.