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

Glial scarring limits recovery following decompressive surgery in rats with syringomyelia

Syringomyelia is a neurological disease that is difficult to cure, and treatments often have limited effectiveness. In this study, a rat model of syringomyelia induced by epidural compression was used to investigate the factors that limit the prognosis of syringomyelia. After we treated syringomyelia rats with surgical decompression alone, MRI revealed that the syringomyelia rats did not show the expected therapeutic effect. Through cerebrospinal fluid (CSF) tracing experiments, we found that the CSF flow in the subarachnoid space (SAS) of rats was restored after decompression. This shows that the poor prognosis of syringomyelia rats in this study is not caused by CSF circulation disorders, suggesting the existence of other factors. Further, immunofluorescence revealed that there were extensive glial scars characterized by increased expression of glial fibrillary acidic protein (GFAP) and chondroitin sulfate proteoglycans (CSPGs) around the syrinx in the non-improved group compared to the improved group. To verify the limiting role of glial scarring in the prognosis of syringomyelia, we intervened with the selective astrocyte inhibitor fluorocitrate (FC). Intrathecal injection of FC significantly inhibited the formation of glial scar after decompression in syringomyelia rats and promoted the reduction of syrinx. This scar-inhibiting effect significantly improved neuronal survival, promoted axonal and myelin recovery, and showed better recovery in sensory function and fine motor control functions. These findings suggest that glial scarring around syrinx is a key factor limiting recovery of syringomyelia. By inhibiting glial scar formation, the prognosis of syringomyelia can be significantly improved, which provides a new strategy for improving clinical treatment effects.

Disruption of the sodium-dependent citrate transporter SLC13A5 in mice causes alterations in brain citrate levels and neuronal network excitability in the hippocampus

In addition to tissues such as liver, the plasma membrane sodium-dependent citrate transporter, NaCT (SLC13A5), is highly expressed in brain neurons, but its function is not understood. Loss-of-function mutations in the human SLC13A5 gene have been associated with severe neonatal encephalopathy and pharmacoresistant seizures. The molecular mechanisms of these neurological alterations are not clear. We performed a detailed examination of a Slc13a5 deletion mouse model including video-EEG monitoring, behavioral tests, and electrophysiologic, proteomic, and metabolomic analyses of brain and cerebrospinal fluid. The experiments revealed an increased propensity for epileptic seizures, proepileptogenic neuronal excitability changes in the hippocampus, and significant citrate alterations in the CSF and brain tissue of Slc13a5 deficient mice, which may underlie the neurological abnormalities. These data demonstrate that SLC13A5 is involved in brain citrate regulation and suggest that abnormalities in this regulation can induce seizures. The present study is the first to (i) establish the Slc13a5-knockout mouse model as a helpful tool to study the neuronal functions of NaCT and characterize the molecular mechanisms by which functional deficiency of this citrate transporter causes epilepsy and impairs neuronal function; (ii) evaluate all hypotheses that have previously been suggested on theoretical grounds to explain the neurological phenotype of SLC13A5 mutations; and (iii) indicate that alterations in brain citrate levels result in neuronal network excitability and increased seizure propensity.

Astrocytes in the nucleus of the solitary tract: Contributions to neural circuits controlling physiology

The nucleus of the solitary tract (NTS) is the primary brainstem centre for the integration of physiological information from the periphery transmitted via the vagus nerve. In turn, the NTS feeds into downstream circuits regulating physiological parameters. Astrocytes are glial cells which have key roles in maintaining CNS tissue homeostasis and regulating neuronal communication. Recently an increasing number of studies have implicated astrocytes in the regulation of synaptic transmission and physiology. This review aims to highlight evidence for a role for astrocytes in the functions of the NTS. Astrocytes maintain and modulate NTS synaptic transmission contributing to the control of diverse physiological systems namely cardiovascular, respiratory, glucoregulatory, and gastrointestinal. In addition, it appears these cells may have a role in central control of feeding behaviour. As such these cells are a key component of signal processing and physiological control by the NTS.

Zinc signaling and epilepsy

Evidence from both preclinical and clinical studies suggest the importance of zinc homeostasis in seizures/epilepsy. Undoubtedly, zinc, via modulation of a variety of targets, is necessary for maintaining the balance between neuronal excitation and inhibition, while an imbalance between excitation and inhibition underlies seizures. However, the relationship between zinc signaling and seizures/epilepsy is complex as both extracellular and intracellular zinc may produce either protective or detrimental effects. This review provides an overview of preclinical/behavioral, functional and molecular studies, as well as clinical data on the involvement of zinc in the pathophysiology and treatment of seizures/epilepsy. Furthermore, the potential of targeting elements associated with zinc signaling or homeostasis and zinc levels as a therapeutic strategy for epilepsy is discussed.

Response of catecholaminergic neurons in the mouse hindbrain to glucoprivic stimuli is astrocyte dependent

Hindbrain catecholaminergic (CA) neurons are required for critical autonomic, endocrine, and behavioral counterregulatory responses (CRRs) to hypoglycemia. Recent studies suggest that CRR initiation depends on hindbrain astrocyte glucose sensors (McDougal DH, Hermann GE, Rogers RC. Front Neurosci 7: 249, 2013; Rogers RC, Ritter S, Hermann GE. Am J Physiol Regul Integr Comp Physiol 310: R1102-R1108, 2016). To test the proposition that hindbrain CA responses to glucoprivation are astrocyte dependent, we utilized transgenic mice in which the calcium reporter construct (GCaMP5) was expressed selectively in tyrosine hydroxylase neurons (TH-GCaMP5). We conducted live cell calcium-imaging studies on tissue slices containing the nucleus of the solitary tract (NST) or the ventrolateral medulla, critical CRR initiation sites. Results show that TH-GCaMP5 neurons are robustly activated by a glucoprivic challenge and that this response is dependent on functional astrocytes. Pretreatment of hindbrain slices with fluorocitrate (an astrocytic metabolic suppressor) abolished TH-GCaMP5 neuronal responses to glucoprivation, but not to glutamate. Pharmacologic results suggest that the astrocytic connection with hindbrain CA neurons is purinergic via P2 receptors. Parallel imaging studies on hindbrain slices of NST from wild-type C57BL/6J mice, in which astrocytes and neurons were prelabeled with a calcium reporter dye and an astrocytic vital dye, show that both cell types are activated by glucoprivation but astrocytes responded significantly sooner than neurons. Pretreatment of these hindbrain slices with P2 antagonists abolished neuronal responses to glucoprivation without interruption of astrocyte responses; pretreatment with fluorocitrate eliminated both astrocytic and neuronal responses. These results support earlier work suggesting that the primary detection of glucoprivic signals by the hindbrain is mediated by astrocytes.

Editor's Highlight: Formulation and Toxicology Evaluation of the Intrathecal AYX1 DNA-Decoy in Sprague Dawley Rats

The longevity of pain after surgery is debilitating and limits the recovery of patients. AYX1 is a double-stranded, unprotected, 23 base-pair oligonucleotide designed to reduce acute post-surgical pain and prevent its chronification with a single intrathecal perioperative dose. AYX1 mimics the DNA sequence normally bound by EGR1 on chromosomes, a transcription factor transiently induced in the dorsal root ganglia-spinal cord network following a noxious input. AYX1 binds to EGR1 and prevents it from launching waves of gene regulation that are necessary to maintain pain over time. A formulation suitable for an intrathecal injection of AYX1 was developed, including a specific ratio of AYX1 and calcium so the ionic homeostasis of the cerebrospinal fluid is maintained and no impact on neuromuscular control is produced upon injection. A GLP toxicology study in naïve Sprague Dawley rats was conducted using 3 dose levels up to the maximum feasible dose. Clinical observations, neurobehavioral observations, clinical pathology and histopathology of the nervous system and peripheral tissues were conducted. An additional nonGLP study was conducted in the spared nerve injury model of chronic neuropathic pain in which EGR1 is induced in the dorsal root ganglia and spinal cord. Similar testing was performed, including a modified Irwin test to assess a potential impact of AYX1 on autonomic nervous system responses, locomotion, activity, arousal, sensorimotor, and neuromuscular function. No AYX1-related adverse events were observed in any of the studies and the no-observed-adverse-effect-level was judged to be the maximum feasible dose.

Plasma Membrane Na⁺-Coupled Citrate Transporter (SLC13A5) and Neonatal Epileptic Encephalopathy

SLC13A5 is a Na⁺-coupled transporter for citrate that is expressed in the plasma membrane of specific cell types in the liver, testis, and brain. It is an electrogenic transporter with a Na⁺:citrate³⁻ stoichiometry of 4:1. In humans, the Michaelis constant for SLC13A5 to transport citrate is ~600 μM, which is physiologically relevant given that the normal concentration of citrate in plasma is in the range of 150–200 μM. Li⁺ stimulates the transport function of human SLC13A5 at concentrations that are in the therapeutic range in patients on lithium therapy. Human SLC13A5 differs from rodent Slc13a5 in two important aspects: the affinity of the human transporter for citrate is ~30-fold less than that of the rodent transporter, thus making human SLC13A5 a low-affinity/high-capacity transporter and the rodent Slc13a5 a high-affinity/low-capacity transporter. In the liver, SLC13A5 is expressed exclusively in the sinusoidal membrane of the hepatocytes, where it plays a role in the uptake of circulating citrate from the sinusoidal blood for metabolic use. In the testis, the transporter is expressed only in spermatozoa, which is also only in the mid piece where mitochondria are located; the likely function of the transporter in spermatozoa is to mediate the uptake of citrate present at high levels in the seminal fluid for subsequent metabolism in the sperm mitochondria to generate biological energy, thereby supporting sperm motility. In the brain, the transporter is expressed mostly in neurons. As astrocytes secrete citrate into extracellular medium, the potential function of SLC13A5 in neurons is to mediate the uptake of circulating citrate and astrocyte-released citrate for subsequent metabolism. Slc13a5-knockout mice have been generated; these mice do not have any overt phenotype but are resistant to experimentally induced metabolic syndrome. Recently however, loss-of-function mutations in human SLC13A5 have been found to cause severe epilepsy and encephalopathy early in life. Interestingly, there is no evidence of epilepsy or encephalopathy in Slc13a5-knockout mice, underlining the significant differences in clinical consequences of the loss of function of this transporter between humans and mice. The markedly different biochemical features of human SLC13A5 and mouse Slc13a5 likely contribute to these differences between humans and mice with regard to the metabolic consequences of the transporter deficiency. The exact molecular mechanisms by which the functional deficiency of the citrate transporter causes epilepsy and impairs neuronal development and function remain to be elucidated, but available literature implicate both dysfunction of GABA (γ-aminobutyrate) signaling and hyperfunction of NMDA (N-methyl-d-aspartate) receptor signaling. Plausible synaptic mechanisms linking loss-of-function mutations in SLC13A5 to epilepsy are discussed.

Hindbrain cytoglucopenia-induced increases in systemic blood glucose levels by 2-deoxyglucose depend on intact astrocytes and adenosine release

The hindbrain contains critical neurocircuitry responsible for generating defensive physiological responses to hypoglycemia. This counter-regulatory response (CRR) is evoked by local hindbrain cytoglucopenia that causes an autonomically mediated increase in blood glucose, feeding behavior, and accelerated digestion; that is, actions that restore glucose homeostasis. Recent reports suggest that CRR may be initially triggered by astrocytes in the hindbrain. The present studies in thiobutabarbital-anesthetized rats show that exposure of the fourth ventricle (4V) to 2-deoxyglucose (2DG; 15 μmol) produced a 35% increase in circulating glucose relative to baseline levels. While the 4V application of the astrocytic signal blocker, fluorocitrate (FC; 5 nmol), alone, had no effect on blood glucose levels, 2DG-induced increases in glucose were blocked by 4V FC. The 4V effect of 2DG to increase glycemia was also blocked by the pretreatment with caffeine (nonselective adenosine antagonist) or a potent adenosine A1 antagonist (8-cyclopentyl-1,3-dipropylxanthine; DPCPX) but not the NMDA antagonist (MK-801). These results suggest that CNS detection of glucopenia is mediated by astrocytes and that astrocytic release of adenosine that occurs after hypoglycemia may cause the activation of downstream neural circuits that drive CRR.

Astrocytes restrict discharge duration and neuronal sodium loads during recurrent network activity

Influx of sodium ions into active neurons is a highly energy-expensive process which must be strictly limited. Astrocytes could play an important role herein because they take up glutamate and potassium from the extracellular space, thereby dampening neuronal excitation. Here, we performed sodium imaging in mouse hippocampal slices combined with field potential and whole-cell patch-clamp recordings and measurement of extracellular potassium ([K(+)]o). Network activity was induced by Mg(2+)-free, bicuculline-containing saline, during which neurons showed recurring epileptiform bursting, accompanied by transient increases in [K(+)]o and astrocyte depolarizations. During bursts, neurons displayed sodium increases by up to 22 mM. Astrocyte sodium concentration increased by up to 8.5 mM, which could be followed by an undershoot below baseline. Network sodium oscillations were dependent on action potentials and activation of ionotropic glutamate receptors. Inhibition of glutamate uptake caused acceleration, followed by cessation of electrical activity, irreversible sodium increases, and swelling of neurons. The gliotoxin NaFAc (sodium-fluoroacetate) resulted in elevation of astrocyte sodium concentration and reduced glial uptake of glutamate and potassium uptake through Na(+) /K(+)-ATPase. Moreover, NaFAc extended epileptiform bursts, caused elevation of neuronal sodium, and dramatically prolonged accompanying sodium signals, most likely because of the decreased clearance of glutamate and potassium by astrocytes. Our experiments establish that recurrent neuronal bursting evokes sodium transients in neurons and astrocytes and confirm the essential role of glutamate transporters for network activity. They suggest that astrocytes restrict discharge duration and show that an intact astrocyte metabolism is critical for the neurons' capacity to recover from sodium loads during synchronized activity.

Astrocytes potentiate GABAergic transmission in the thalamic reticular nucleus via endozepine signaling

Emerging evidence indicates that diazepam-binding inhibitor (DBI) mediates an endogenous benzodiazepine-mimicking (endozepine) effect on synaptic inhibition in the thalamic reticular nucleus (nRT). Here we demonstrate that DBI peptide colocalizes with both astrocytic and neuronal markers in mouse nRT, and investigate the role of astrocytic function in endozepine modulation in this nucleus by testing the effects of the gliotoxin fluorocitrate (FC) on synaptic inhibition and endozepine signaling in the nRT using patch-clamp recordings. FC treatment reduced the effective inhibitory charge of GABAA receptor (GABAAR)-mediated spontaneous inhibitory postsynaptic currents in WT mice, indicating that astrocytes enhance GABAAR responses in the nRT. This effect was abolished by both a point mutation that inhibits classical benzodiazepine binding to GABAARs containing the α3 subunit (predominant in the nRT) and a chromosomal deletion that removes the Dbi gene. Thus, astrocytes are required for positive allosteric modulation via the α3 subunit benzodiazepine-binding site by DBI peptide family endozepines. Outside-out sniffer patches pulled from neurons in the adjacent ventrobasal nucleus, which does not contain endozepines, show a potentiated response to laser photostimulation of caged GABA when placed in the nRT. FC treatment blocked the nRT-dependent potentiation of this response, as did the benzodiazepine site antagonist flumazenil. When sniffer patches were placed in the ventrobasal nucleus, however, subsequent treatment with FC led to potentiation of the uncaged GABA response, suggesting nucleus-specific roles for thalamic astrocytes in regulating inhibition. Taken together, these results suggest that astrocytes are required for endozepine actions in the nRT, and as such can be positive modulators of synaptic inhibition.

Intoxicação por monofluoroacetato em animais

O monofluoroacetato (MF) ou ácido monofluoroacético é utilizado na Austrália e Nova Zelândia no controle populacional de mamíferos nativos ou exóticos. O uso desse composto é proibido no Brasil, devido ao risco de intoxicação de seres humanos e de animais, uma vez que a substância permanece estável por décadas. No Brasil casos recentes de intoxicação criminosa ou acidental têm sido registrados. MF foi identificado em diversas plantas tóxicas, cuja ingestão determina "morte súbita"; de bovinos na África do Sul, Austrália e no Brasil. O modo de ação dessa substância baseia-se na formação do fluorocitrato, seu metabólito ativo, que bloqueia competitivamente a aconitase e o ciclo de Krebs, o que reduz produção de ATP. As espécies animais têm sido classificadas nas quatro Categorias em função do efeito provocado por MF: (I) no coração, (II) no sistema nervoso central (III) sobre o coração e sistema nervoso central ou (IV) com sintomatologia atípica. Neste trabalho, apresenta-se uma revisão crítica atualizada sobre essa substância. O diagnóstico da intoxicação por MF é realizado pelo histórico de ingestão do tóxico, pelos achados clínicos e confirmado por exame toxicológico. Uma forma peculiar de degeneração hidrópico-vacuolar das células epiteliais dos túbulos uriníferos contorcidos distais tem sido considerada como característica dessa intoxicação em algumas espécies. O tratamento da intoxicação por MF é um desafio, pois ainda não se conhece um agente capaz de reverte-la de maneira eficaz; o desfecho geralmente é fatal

Glutamate released spontaneously from astrocytes sets the threshold for synaptic plasticity

Astrocytes exhibit spontaneous calcium oscillations that could induce the release of glutamate as gliotransmitter in rat hippocampal slices. However, it is unknown whether this spontaneous release of astrocytic glutamate may contribute to determining the basal neurotransmitter release probability in central synapses. Using whole-cell recordings and Ca(2+) imaging, we investigated the effects of the spontaneous astrocytic activity on neurotransmission and synaptic plasticity at CA3-CA1 hippocampal synapses. We show here that the metabolic gliotoxin fluorocitrate (FC) reduces the amplitude of evoked excitatory postsynaptic currents and increases the paired-pulse facilitation, mainly due to the reduction of the neurotransmitter release probability and the synaptic potency. FC also decreased intracellular Ca(2+) signalling and Ca(2+) -dependent glutamate release from astrocytes. The addition of glutamine rescued the effects of FC over the synaptic potency; however, the probability of neurotransmitter release remained diminished. The blockage of group I metabotropic glutamate receptors mimicked the effects of FC on the frequency of miniature synaptic responses. In the presence of FC, the Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N ',N '-tetra-acetate or group I metabotropic glutamate receptor antagonists, the excitatory postsynaptic current potentiation induced by the spike-timing-dependent plasticity protocol was blocked, and it was rescued by delivering a stronger spike-timing-dependent plasticity protocol. Taken together, these results suggest that spontaneous glutamate release from astrocytes contributes to setting the basal probability of neurotransmitter release via metabotropic glutamate receptor activation, which could be operating as a gain control mechanism that regulates the threshold of long-term potentiation. Therefore, endogenous astrocyte activity provides a novel non-neuronal mechanism that could be critical for transferring information in the central nervous system.

Behaviour and time to unconsciousness of brushtail possums (Trichosurus vulpecula) after a lethal or sublethal dose of 1080

Context. Sodium fluoroacetate (1080) is a vertebrate pesticide used in several countries, including New Zealand, where it is the main pesticide for controlling brushtail possums (Trichosurus vulpecula) – a significant agricultural and conservation pest. There is growing concern internationally about the impacts of vertebrate pesticides, including 1080, on pest animal welfare. Aims. Behavioural changes and time to loss of consciousness of possums after 1080 ingestion in carrot baits were determined to compare the animal welfare impacts with other possum pesticides. Methods. Eight lethally dosed possums and eight that consumed a dose intended to be sublethal were observed until death or recovery but not handled. Another nine lethally dosed possums were handled to determine responses to stimuli, indicating time to loss of consciousness. Key results. Unhandled, lethally dosed possums died after 11 h 26 min ± 1 h 55 min (mean ± s.e.m.). Half had abnormal appearances and postures 1 h 50 min ± 9 min after consuming baits. Seven showed retching, and three vomited, over 27 ± 12 min from 2 h 53 min ± 13 min. Lack of coordination began 3 h 37 min ± 32 min after dosing, then possums spent most of the time until death lying, showing spasms and tremors. Five showed seizures while lying prostrate. Possums receiving a nominally sublethal dose all showed signs of poisoning, including abnormal postures, lethargy, lack coordination, retching, spasms or tremors, and a cessation of grooming, feeding and activity. One died 18 h 15 min after dosing, experiencing two seizures within 30 min of death. Response to handling, indicating total loss of consciousness, was lost in two possums before death. Conclusions. Possums ingesting a lethal dose of 1080 experienced ~9.5 h of changed behaviour and lost consciousness close to death, although awareness was likely reduced sometime beforehand. Possums ingesting a nominally sublethal dose experienced some effects of poisoning. Implications. These impacts are intermediate compared with other possum pesticides in New Zealand. Nevertheless, the potential for consciousness during and after retching and seizures is undesirable for animal welfare. Further work is encouraged on alternatives to 1080 and means of reducing its impact on animal welfare.

Supraspinal glial-neuronal interactions contribute to descending pain facilitation

Spinal glial reaction and proinflammatory cytokine induction play an important role in the development of chronic pain states after tissue and nerve injury. The present study investigated the cellular and molecular mechanisms underlying descending facilitation of neuropathic pain with an emphasis on supraspinal glial-neuronal relationships. An early and transient reaction of microglia and prolonged reaction of astrocytes were found after chronic constriction injury (CCI) of the rat infraorbital nerve in the rostral ventromedial medulla (RVM), a major component of brainstem descending pain modulatory circuitry. There were prolonged elevations of cytokines tumor necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta) after CCI, and they were expressed in RVM astrocytes at 14 d after injury. Intra-RVM injection of microglial and astrocytic inhibitors attenuated mechanical hyperalgesia and allodynia at 3 and 14 d after CCI, respectively. Moreover, TNFR1 and IL-1R, receptors for TNF-alpha and IL-1beta, respectively, were expressed primarily in RVM neurons exhibiting immunoreactivity to the NMDA receptor (NMDAR) subunit NR1. CCI increased TNFR1 and IL-1R levels and NR1 phosphorylation in the RVM. Neutralization of endogenous TNF-alpha and IL-1beta in the RVM significantly reduced CCI-induced behavioral hypersensitivity and attenuated NR1 phosphorylation. Finally, intra-RVM administration of recombinant TNF-alpha or IL-1beta upregulated NR1 phosphorylation and caused a reversible and NMDAR-dependent allodynia in normal rats, further suggesting that TNF-alpha and IL-1beta couple glial hyperactivation with NMDAR function. These studies have addressed a novel contribution of supraspinal astrocytes and associated cytokines as well as central glial-neuronal interactions to the enhancement of descending facilitation of neuropathic pain.

Sodium fluoroacetate poisoning

Sodium fluoroacetate was introduced as a rodenticide in the US in 1946. However, its considerable efficacy against target species is offset by comparable toxicity to other mammals and, to a lesser extent, birds and its use as a general rodenticide was therefore severely curtailed by 1990. Currently, sodium fluoroacetate is licensed in the US for use against coyotes, which prey on sheep and goats, and in Australia and New Zealand to kill unwanted introduced species. The extreme toxicity of fluoroacetate to mammals and insects stems from its similarity to acetate, which has a pivotal role in cellular metabolism. Fluoroacetate combines with coenzyme A (CoA-SH) to form fluoroacetyl CoA, which can substitute for acetyl CoA in the tricarboxylic acid cycle and reacts with citrate synthase to produce fluorocitrate, a metabolite of which then binds very tightly to aconitase, thereby halting the cycle. Many of the features of fluoroacetate poisoning are, therefore, largely direct and indirect consequences of impaired oxidative metabolism. Energy production is reduced and intermediates of the tricarboxylic acid cycle subsequent to citrate are depleted. Among these is oxoglutarate, a precursor of glutamate, which is not only an excitatory neurotransmitter in the CNS but is also required for efficient removal of ammonia via the urea cycle. Increased ammonia concentrations may contribute to the incidence of seizures. Glutamate is also required for glutamine synthesis and glutamine depletion has been observed in the brain of fluoroacetate-poisoned rodents. Reduced cellular oxidative metabolism contributes to a lactic acidosis. Inability to oxidise fatty acids via the tricarboxylic acid cycle leads to ketone body accumulation and worsening acidosis. Adenosine triphosphate (ATP) depletion results in inhibition of high energy-consuming reactions such as gluconeogenesis. Fluoroacetate poisoning is associated with citrate accumulation in several tissues, including the brain. Fluoride liberated from fluoroacetate, citrate and fluorocitrate are calcium chelators and there are both animal and clinical data to support hypocalcaemia as a mechanism of fluoroacetate toxicity. However, the available evidence suggests the fluoride component does not contribute. Acute poisoning with sodium fluoroacetate is uncommon. Ingestion is the major route by which poisoning occurs. Nausea, vomiting and abdominal pain are common within 1 hour of ingestion. Sweating, apprehension, confusion and agitation follow. Both supraventricular and ventricular arrhythmias have been reported and nonspecific ST- and T-wave changes are common, the QTc may be prolonged and hypotension may develop. Seizures are the main neurological feature. Coma may persist for several days. Although several possible antidotes have been investigated, they are of unproven value in humans. The immediate, and probably only, management of fluoroacetate poisoning is therefore supportive, including the correction of hypocalcaemia.

Role of enteric glia in intestinal physiology: effects of the gliotoxin fluorocitrate on motor and secretory function

The role of enteric glia in gastrointestinal physiology remains largely unexplored. We examined the actions of the gliotoxin fluorocitrate (FC) on intestinal motility, secretion, and inflammation after assessing its efficacy and specificity in vitro. FC (100 microM) caused a significant decrease in the phosphorylation of the glucose analog 2-[N-(7-nitrobenz-2-oxa-1,3-diaz-4-yl)amino]-2-deoxyglucose in enteric glial cultures and a reduction in glial uptake of the fluorescent dipeptide Ala-Lys-7-amino-4-methylcoumarin-3-acetic acid in both the ileum and colon. Dipeptide uptake by resident murine macrophages or guinea pig myenteric neurons was unaffected by FC. Incubation of isolated guinea pig ileal segments with FC caused a specific and significant increase in glial expression of the phosphorylated form of ERK-1/2. Disruption of enteric glial function with FC in mice reduced small intestinal motility in vitro, including a significant decrease in basal tone and the amplitude of contractility in response to electrical field stimulation. Mice treated with 10 or 20 micromol/kg FC twice daily for 7 days demonstrated a concentration-dependent decrease in small intestinal transit. In contrast, no changes in colonic transit or ion transport in vitro were observed. There were no changes in glial or neuronal morphology, any signs of inflammation in the FC-treated mice, or any change in the number of myenteric nitric oxide synthase-expressing neurons. We conclude that FC treatment causes enteric glial dysfunction, without causing intestinal inflammation. Our data suggest that enteric glia are involved in the modulation of enteric neural circuits underlying the regulation of intestinal motility.

Toxicology of fluoroacetate: a review, with possible directions for therapy research

Fluoroacetate (FA; CH2FCOOR) is highly toxic towards humans and other mammals through inhibition of the enzyme aconitase in the tricarboxylic acid cycle, caused by 'lethal synthesis' of an isomer of fluorocitrate (FC). FA is found in a range of plant species and their ingestion can cause the death of ruminant animals. Some fluorinated compounds -- used as anticancer agents, narcotic analgesics, pesticides or industrial chemicals -- metabolize to FA as intermediate products. The chemical characteristics of FA and the clinical signs of intoxication warrant the re-evaluation of the toxic danger of FA and renewed efforts in the search for effective therapeutic means. Antidotal therapy for FA intoxication has been aimed at preventing fluorocitrate synthesis and aconitase blockade in mitochondria, and at providing citrate outflow from this organelle. Despite a greatly improved understanding of the biochemical mechanism of FA toxicity, ethanol, if taken immediately after the poisoning, has been the most acceptable antidote for the past six decades. This review deals with the clinical signs and physiological and biochemical mechanisms of FA intoxication to provide an explanation of why, even after decades of investigation, has no effective therapy to FA intoxication been elaborated. An apparent lack of integrated toxicological studies is viewed as a limiter of progress in this regard. Two principal ways of developing effective therapies for FA intoxication are considered. Firstly, competitive inhibition of FA interaction with CoA and of FC interaction with aconitase. Secondly, channeling the alternative metabolic pathways by orienting the fate of citrate via cytosolic aconitase, and by maintaining the flux of reducing equivalents into the TCA cycle via glutamate dehydrogenase.

Central Proinflammatory Cytokines and Pain Enhancement

Enhanced pain is a component of the 'sickness response' which is an evolutionarily adaptive constellation of responses that enhance the survival of the host. Proinflammatory cytokines mediate these sickness behaviors, and whether proinflammatory cytokines are involved in exaggerated pain has become an intriguing question. Studies suggest that spinal cord glial cells (astrocytes and microglia) are activated in conditions that lead to enhanced pain. Not only is glial activation associated with enhanced pain, but it is also integral to the induction and maintenance of these pain states. Proinflammatory cytokines can be released by activated astrocytes and microglia within the central nervous system. This review will discuss the role of proinflammatory cytokines in experimental models of prolonged pain states. Administration of exogenous proinflammatory cytokines facilitates pain, and agents that antagonize proinflammatory cytokine actions have been shown to block and/or reverse enhanced pain. These findings suggest that blocking the synthesis and/or release of proinflammatory cytokines may be viable strategies for the treatment of pathological pain. Gene therapy to augment the endogenous anti-inflammatory cytokine, interleukin-10, is one of the more promising therapies currently under study.

Inhibition of aconitase in astrocytes increases the sensitivity to chemical convulsants

Although there is evidence that astrocytes support neuronal function, the contribution of astrocytes to seizure onset and termination is not known. To determine whether there are changes in seizure susceptibility or neuronal damage when the ability of astrocytes to generate ATP is reduced, 0.5 nmol of fluorocitrate (FC) was injected into the right ventricle. Injection of FC alone did not produce electrographic or behavioral seizures and did not stress or injure neurons or astrocytes, as measured with silver stain and immunohistochemistry for HSP32 or HSP72. However, in animals pretreated with FC, administration of kainic acid, at a dose that does not initiate seizures in control animals (7 mg/kg), caused wet dog shakes and neuronal damage in the hilus. Wet dog shakes did not cause any neuronal damage in control animals. If the dose of FC was increased to 0.75 nmol, then subsequent administration of the same dose of kainic acid (7 mg/kg) caused stage 3-5 seizures. Injection of FC also reduced the dose of pilocarpine needed to produce seizures. Given simultaneously with FC, isocitrate, which bypasses the biochemical inhibition of aconitase, blocked the effects of FC in both kainic acid and pilocarpine treated animals. The data demonstrate that inhibition of aconitase in astrocytes lowers the doses of both kainic acid and pilocarpine that will cause behavioral seizures and may increase neuronal vulnerability to seizures.

Fluorocitrate-mediated astroglial dysfunction causes seizures

A role for astroglia in epileptogenesis has been hypothesised but is not established. Low doses of fluorocitrate specifically and reversibly disrupt astroglial metabolism by blocking aconitase, an enzyme integral to the tricarboxylic acid cycle. We used cerebral cortex injections of fluorocitrate, at a dose that we demonstrated to inhibit astroglial metabolism selectively, to determine whether astroglial disturbances lead to seizures. Rats were halothane-anesthetized, and 0.8 nmol of sodium fluorocitrate was injected into the cerebral cortex. Extradural electroencephalogram (EEG) electrodes were implanted, after which the anesthesia was ceased and the animals were observed. In all experiments, 14 of 15 fluorocitrate-treated animals exhibited epileptiform EEG discharges, with some animals exhibiting convulsive seizures. Discharges commenced as early as 30 min postfluorocitrate injection. Intraperitoneal octanol, but not halothane by inhalation, given to test the possible participation of gap junctions in EEG discharge generation, blocked or delayed the occurrence of discharges after fluorocitrate. These results indicate that focal cerebrocortical astroglial dysfunction leads to focal epileptiform discharges and sometimes to convulsive seizures and that the process possibly depends on effects mediated by gap junctions.

Citrate, beneficial or deleterious in the CNS?

Cerebellar granule neurons were incubated with or without glucose (3 mM) in the presence or absence of citrate (20 mM) using normoxic and/or hypoxic incubation conditions. During 4 h of hypoglycemia and also during hypoxia plus hypoglycemia, citrate increased lactate dehydrogenase (LDH) leakage from the cells and decreased mitochondrial activity, the latter was also the case in the presence of glucose. After 24 h of hypoglycemia, however, citrate decreased LDH leakage slightly, possibly due to its metabolism in the tricarboxylic acid cycle under these conditions. It should be noted that during mild hypoxia plus hypoglycemia a reduced LDH leakage was observed when compared to hypoglycemia alone. The 4 h low oxygen period did protect the neurons also during the 20 h re-oxygenation period. The present study might indicate that incubation of brain cell cultures in an atmosphere of air (30% oxygen) and 5% CO2, which is used in most laboratories, can be toxic and that oxygen concentration should be lowered considerably to mimic conditions in the brain.

Metabolic differences between primary cultures of astrocytes and neurons from cerebellum and cerebral cortex. Effects of fluorocitrate

Astrocytes and neurons cultured from mouse cerebellum and cerebral cortex were analyzed with respect to content and synthesis of amino acids as well as export of metabolites to the culture medium and the response to fluorocitrate, an inhibitor of aconitase. The intracellular levels of amino acids were similar in the two astrocytic populations. The release of citrate, lactate and glutamine, however, was markedly higher from cerebellar than from cortical astrocytes. Neurons contained higher levels of glutamate, aspartate and GABA than astrocytic cultures. Cortical neurons were especially high in GABA and aspartate, and the level of aspartate increased specifically when the extracellular level of glutamine was elevated. Fluorocitrate inhibited the TCA cycle in the astrocytes, but was less effective in cerebellar neurons. Whereas neurons responded to fluorocitrate with an increase in the formation of lactate, reflecting glycolysis, astrocytes decreased the formation of lactate in the presence of fluorocitrate, indicating that astrocytes to a high degree synthesize pyruvate and hence lactate from TCA cycle intermediates.

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.

Astrocytes implicated in the energizing of intermediate memory processes in neonate chicks

Day-old chicks trained in a single trial passive avoidance task develop three sequentially dependent stages of discrimination memory. The second intermediate stage is made up of two phases: the initial A phase being susceptible to inhibition of oxidative metabolism in the tricarboxcylic acid (TCA) system with 2,4-dinitrophenol (DNP), and a second DNP-insensitive B phase. The studies reported in this paper found that doses of the metabolic toxins fluoroacetate (0.2 mM) and fluorocitrate (0.1 mM) previously reported to disrupt the astrocytic TCA cycle only, also disrupt the A (but not the B) phase of intermediate memory, suggesting an interaction between the astrocytic and neuronal oxidative systems may be required to meet the metabolic demands of this earlier phase. The B phase, on the other hand, was not expressed in the presence of the glycolytic inhibitor iodoacetate (1 mM), suggesting that glycolysis (known to be more efficient in astrocytes) and glycogenolysis (which may be exclusive to astrocytes) may support this second phase of intermediate memory. In this regard, the rise in forebrain noradrenaline levels previously reported to occur before the appearance of the B phase is particularly relevant. Given that noradrenaline has been shown to be capable of enhancing glycogenolysis in astrocyte-enriched cell cultures, it is conceivable that noradrenaline exerts an effect on memory by stimulating the glycolytic system in astrocytes, thereby providing energy or metabolites (e.g. pyruvate) needed to sustain the cellular processes operating during the B phase of intermediate memory.

Uptake, release, and metabolism of citrate in neurons and astrocytes in primary cultures

Synthesis, uptake, release, and oxidative metabolism of citrate were investigated in neurons and astrocytes cultured from cerebral cortex or cerebellum. In addition, the possible role of citrate as a donor of the carbon skeleton for biosynthesis of neurotransmitter glutamate was studied. All cell types expressed the enzyme citrate synthase at a high activity, the cerebellar granule neurons containing the enzyme at a higher activity than that found in the astrocytes from the two brain regions or the cortical neurons. Saturable citrate uptake could not be detected in any of the cell types, but the astrocytes, and, in particular, those of cerebellar origin, had a very active de novo synthesis and release of citrate (approximately 70 nmol x h-1 x mg of protein-1). The rate of release of citrate from neurons was < 5% of this value. Using [14C]citrate it could be shown that citrate was oxidatively metabolized to 14CO2 at a modest rate (approximately 1 nmol x h-1 x mg-1 of protein) with slightly higher rates in astrocytes compared with neurons. Experiments designed to investigate the ability of exogenously supplied citrate to serve as a precursor for synthesis of transmitter glutamate in cerebellar granule neurons failed to demonstrate this. Rather than citrate serving this purpose it may be suggested that astrocytically released citrate may regulate the extracellular concentration of Ca2+ and Mg2+ by chelation, thereby modulating neuronal excitability.

Genetically epilepsy-prone rodents show some changes of ion levels in the brain

In the present study the water and ion (Na+, K+, Ca2+, Fe3+, Se4+, Mg2+, Mn2+, Mn2, Se4+, Cu2+) content in the brain of genetically epilepsy-prone rats (GEPRs) and of 21-, 45-, and 60-day-old DBA/2 mice were determined, and compared with those measured in normal controls (Sprague-Dawley rats and Swiss mice), to verify whether the predisposition to audiogenic seizures (AGS) may be partially related to changes in the cerebral osmotic and ionic state. Our findings clearly evidenziate two points: a) a more complex shift in brain ionic balance (rather than a peculiar modification in the concentration of a single ion) seems very likely involved in AGS susceptibility; (b) brain Ca2+ and Se4+ amounts, together with the water content, appear to be really important factors to which a role in abnormal seizure predisposition may be attributed.

Fluoroacetate and fluorocitrate: mechanism of action

The concept of lethal synthesis as suggested by Peters is reviewed in the light of the more recent work in this area. It is suggested that fluorocitrate is a "suicide" substrate for aconitase rather than a competitive inhibitor as originally suggested. The use of these substances to study glial-neuronal relationships is considered.