Enhancement of NMDA-mediated responses by cyanide

The two faces of cyanide: an environmental toxin and a potential novel mammalian gasotransmitter

Cyanide is traditionally viewed as a cytotoxic agent, with its primary mode of action being the inhibition of mitochondrial Complex IV (cytochrome c oxidase). However, recent studies demonstrate that the effect of cyanide on Complex IV in various mammalian cells is biphasic: in lower concentrations (nanomolar to low micromolar) cyanide stimulates Complex IV activity, increases ATP production and accelerates cell proliferation, while at higher concentrations (high micromolar to low millimolar) it produces the previously known (‘classic’) toxic effects. The first part of the article describes the cytotoxic actions of cyanide in the context of environmental toxicology, and highlights pathophysiological conditions (e.g., cystic fibrosis with Pseudomonas colonization) where bacterially produced cyanide exerts deleterious effects to the host. The second part of the article summarizes the mammalian sources of cyanide production and overviews the emerging concept that mammalian cells may produce cyanide, in low concentrations, to serve biological regulatory roles. Cyanide fulfills many of the general criteria as a ‘classical’ mammalian gasotransmitter and shares some common features with the current members of this class: nitric oxide, carbon monoxide, and hydrogen sulfide.

Can isosorbide dinitrate oral spray serve as an immediate bridging therapy for a mass cyanide poisoning?

OBJECTIVE

In this proof-of-concept study, the aim was to evaluate the short-term clinical effectiveness of isosorbide dinitrate (ISDN) oral spray in non-anaesthetized cyanide-poisoned swine.

METHODS

A comparative study was conducted using domestic swine. Animals were intravenously poisoned with potassium cyanide (KCN), either 2 mg/kg or 4 mg/kg dose. Two control groups (one for each cyanide dose) were not further treated. Two other groups (one for each cyanide dose) were treated within 1 min after poisoning with ISDN oral spray: 3 spray actuations (averaging a total of 3.75 mg) after the lower cyanide dose and 4 spray actuations (averaging a total of 5.0 mg) after the higher dose. The study outcomes were clinical score, time to death, and blood tests including pH, lactate, and methemoglobin levels.

RESULTS

All the animals started to convulse within 20 to 30 sec after KCN poisoning, then became unresponsive and hemodynamically depressed after another 20 to 30 sec. After the KCN 2 mg/kg dose, 3 of 4 control animals survived, while all treated animals survived. Compared with control animals, ISDN-treated animals displayed significantly better clinical scores starting 5 min after KCN poisoning. Acidosis was significantly more pronounced in the untreated animals. After the KCN 4 mg/kg dose, similar survival rates were observed for control and ISDN-treated groups (1/4), but treated animals had longer time to death and better pH and lactate levels.

CONCLUSION

ISDN oral spray administration following KCN poisoning in this porcine model did not result in statistically significant increased survival. However, based on clinical scores and clinical laboratory values, ISDN may benefit as a bridging countermeasure until currently-available specific cyanide antidotes can be administered. Further research is warranted to better characterize this potential role of ISDN in cyanide poisoning.

Effectiveness of isosorbide dinitrate in cyanide poisoning as a function of the administration timing

BACKGROUND

Better and safer antidotes against cyanide poisoning are needed. Prior study has shown a favorable effect of isosorbide dinitrate (ISDN) on the survival of cyanide-poisoned rabbits when administered as early as 1 min after poisoning. The aim of the current study was to further evaluate the efficacy of intravenous ISDN administered in clinically relevant timing for first responders.

METHODS

A comparative animal study using 24 rabbits in 4 randomized study groups was performed. Animals were poisoned with intravenous potassium cyanide (1 mg/kg). Animals in Group 1 served as controls and received no treatment. Groups 2-4 animals were treated intravenously with ISDN (50 μg/kg) after poisoning; one group after 3 min, another group after 5 min and the last after 7 min. Animals were observed for 30 min after poisoning. The study endpoints included survival rate, clinical status, blood pressure, pulse per minute, blood lactate and pH.

RESULTS

Five of 6 animals (83.3%) from every treatment group survived the whole observation period while all control untreated animals died. All the rabbits collapsed immediately after exposure, showing rapidly deteriorated vital signs with lactic metabolic acidosis (peak blood lactate levels of 18.1 to 19.0 mmol/L on average at 10 min post exposure). Vital signs, clinical scores, and blood gases of treated rabbits gradually improved.

CONCLUSION

Poisoned rabbits showed improved short-term survival following the administration of ISDN up to 7 min after lethal cyanide poisoning of. We see a potential for ISDN as an antidote against cyanide poisoning.

Early administration of isosorbide dinitrate improves survival of cyanide-poisoned rabbits

CONTEXT

More effective, rapidly delivered, safer antidotes are needed for cyanide poisoning. Previous study has demonstrated a beneficial effect of isosorbide dinitrate on the survival of cyanide-poisoned mice.

OBJECTIVE

To evaluate the effectiveness of isosorbide dinitrate compared with that of sodium nitrite in cyanide poisoning.

MATERIALS AND METHODS

A comparative animal study was performed using 18 rabbits, randomized into 3 study groups. Animals were poisoned intravenously with potassium cyanide (1 mg/kg). The first group was not given any further treatment. The second and third groups were treated intravenously 1 min after poisoning with sodium nitrite (6 mg/kg) and isosorbide dinitrate (50 μg/kg), respectively. The primary outcome was short-term survival of up to 30 min. Secondary outcomes included time to death, a clinical score, mean blood pressure, pulse, blood pH, and lactate and methemoglobin levels.

RESULTS

Rabbits treated with isosorbide dinitrate or sodium nitrite survived while only one untreated rabbit survived. Median time to death of the 5 poisoned and untreated animals was 10 min. All the animals collapsed soon after poisoning, exhibiting rapidly disturbed vital signs and developed lactic metabolic acidosis; average peak blood lactate levels were 15.5-19.1 mmol/L at 10 min after poisoning. The treated animals improved gradually with practically full recovery of the clinical scores, vital signs, and blood gas levels. Sodium nitrite administration raised methemoglobin to an average peak of 7.9%, while isosorbide dinitrate did not change methemoglobin levels.

CONCLUSION

Early administration of isosorbide dinitrate improved the short-term survival of cyanide-poisoned rabbits. Isosorbide dinitrate shows potential as an antidote for cyanide poisoning and may exert its effect using a nitric-oxide-dependent mechanism.

Effect of alpha-ketoglutarate and N-acetyl cysteine on cyanide-induced oxidative stress mediated cell death in PC12 cells

Cyanide is a mitochondrial poison, which is ubiquitously present in the environment. Cyanide-induced oxidative stress is known to play a key role in mediating the neurotoxicity and cell death in rat pheochromocytoma (PC12) cells. PC12 cells are widely used as a model for neurotoxicity assays in vitro. In the present study, we investigated the protective effects of alpha-ketoglutarate (A-KG), a potential cyanide antidote, and N-acetyl cysteine (NAC), an antioxidant against toxicity of cyanide in PC12 cells. Cells were treated with various concentrations (0.625—1.25 mM) of potassium cyanide (KCN) for 4 hours, in the presence or absence of simultaneous treatment of A-KG (0.5 mM) and NAC (0.25 mM). Cyanide caused marked decrease in the levels of cellular antioxidants like superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), and glutathione reductase (GR). Lipid peroxidation indicated by elevated levels of malondialdehyde (MDA) was found to be accompanied by decreased levels of reduced glutathione (GSH) and total antioxidant status (TAS) of the cells. Cyanide-treated cells showed notable increase in caspase-3 activity and induction of apoptotic type of cell death after 24 hours. A-KG and NAC alone were very effective in restoring the levels of GSH and TAS, but together they significantly resolved the effects of cyanide on antioxidant enzymes, MDA levels, and caspase-3 activity. The present study reveals that combination of A-KG and NAC has critical role in abbrogating the oxidative stress-mediated toxicity of cyanide in PC12 cells. The results suggest potential role of A-KG and NAC in cyanide antagonism.

A preferential role for glycolysis in preventing the anoxic depolarization of rat hippocampal area CA1 pyramidal cells

During brain anoxia or ischemia, a decrease in the level of ATP leads to a sudden decrease in transmembrane ion gradients [anoxic depolarization (AD)]. This releases glutamate by reversing the operation of glutamate transporters, which triggers neuronal death. By whole-cell clamping CA1 pyramidal cells, we investigated the energy stores that delay the occurrence of the AD in hippocampal slices when O2 and glucose are removed. With glycolytic and mitochondrial ATP production blocked in P12 slices, the AD occurred in approximately 7 min at 33 degrees C, reflecting the time needed for metabolic activity to consume the existing ATP and phosphocreatine, and for subsequent ion gradient decrease. Allowing glycolysis fueled by glycogen, in the absence of glucose, delayed the AD by 5.5 min, whereas superfused glucose prevented the AD for >1 h. With glycolysis blocked, the latency to the AD was 6.5 min longer when mitochondria were allowed to function, demonstrating that metabolites downstream of glycolysis (pyruvate, citric acid cycle intermediates, and amino acid oxidation) provide a significant energy store for oxidative phosphorylation. With glycolysis blocked but mitochondria functioning, superfusing lactate did not significantly delay the AD, showing that ATP production from lactate is much less than that from endogenous metabolites. These data demonstrate a preferential role for glycolysis in preventing the AD. They also define a hierarchy of pool sizes for hippocampal energy stores and suggest that brain ATP production from glial lactate may not be significant in conditions of energy deprivation.

Effect of anoxia and pharmacological anoxia on whole-cell NMDA receptor currents in cortical neurons from the western painted turtle

The mammalian brain undergoes rapid cell death during anoxia that is characterized by uncontrolled Ca(2+) entry via N-methyl-D-aspartate receptors (NMDARs). In contrast, the western painted turtle is extremely anoxia tolerant and maintains close-to-normal [Ca(2+)](i) during periods of anoxia lasting from days to months. A plausible mechanism of anoxic survival in turtle neurons is the regulation of NMDARs to prevent excitotoxic Ca(2+) injury. However, studies using metabolic inhibitors such as cyanide (NaCN) as a convenient method to induce anoxia may not represent a true anoxic stress. This study was undertaken to determine whether turtle cortical neuron whole-cell NMDAR currents respond similarly to true anoxia with N(2) and to NaCN-induced anoxia. Whole-cell NMDAR currents were measured during a control N(2)-induced anoxic transition and a control NaCN-induced transition. During anoxia with N(2) normalized, NMDAR currents decreased to 35.3%+/-10.8% of control values. Two different NMDAR current responses were observed during NaCN-induced anoxia: one resulted in a 172%+/-51% increase in NMDAR currents, and the other was a decrease to 48%+/-14% of control. When responses were correlated to the two major neuronal subtypes under study, we found that stellate neurons responded to NaCN treatment with a decrease in NMDAR current, while pyramidal neurons exhibited both increases and decreases. Our results show that whole-cell NMDAR currents respond differently to NaCN-induced anoxia than to the more physiologically relevant anoxia with N(2).

Preconditioning-induced protection against cyanide-induced neurotoxicity is mediated by preserving mitochondrial function

The central nervous system is one of the main target organs in cyanide toxicity. In this study, primary cultures of chick embryonic neurons were used to characterize sodium cyanide (NaCN)-induced cell death and to investigate the mechanism of NaCN-mediated preconditioning. After treatment of the cells with 1mM NaCN for 1h followed by a NaCN-free incubation period of 23 h, we observed features of apoptosis such as a reduction in nuclear size, chromatin condensation and nuclear fragmentation as evaluated by nuclear staining with Hoechst 33258 and electron microscopy. In addition, NaCN-induced neurotoxicity was reduced by the protein synthesis inhibitor cycloheximide (CHX) suggesting an active type of cell death. Most of the neurons with condensed chromatin and a shrunken nuclei also showed membrane damage at a late stage. Mitochondrial membrane potential as well as the protein levels of Bcl-2 and Bcl-x(L) decreased 15-60 min and 1-3 h after the exposure to NaCN (1mM, 1h), respectively. Preconditioning caused by incubating chick neurons with 100 microM NaCN for 30 min followed by a NaCN-free interval of 24h significantly protected the neurons against subsequent NaCN (1mM, 1h)-induced damage. Preconditioning prevented NaCN-induced decrease in the mitochondrial membrane potential as well as in the protein levels of Bcl-2 and Bcl-x(L) suggesting that preconditioning-induced neuroprotection is mediated by preserving mitochondrial function.

Cyanide interaction with redox modulatory sites enhances NMDA receptor responses

Activation of NMDA receptors plays an important role in cyanide neurotoxicity. Cyanide indirectly activates the receptor by inducing neuronal release of glutamate and also enhances receptor-mediated responses by a direct interaction with the receptor complex. This study investigated the mechanism in cerebellar granule cells by which cyanide enhances NMDA-induced Ca2+ influx. Cyanide (50 microM) increased the influx of Ca2+ over the NMDA concentration range of 0.5-500 microM. Experiments showed that cyanide does not interact with the receptor's glycine or PKC mediated phosphorylation regulatory sites. N-ethylmaleimide, a thiol alkylating agent which inactivates the redox regulatory sites of the receptor, blocked the enhancing effect of cyanide. Pretreatment of cells with 5,5-dithio-bis-2-nitrobenzoic acid (DTNB), a compound that oxidizes the receptor redox sites, had no effect on the response to cyanide. On the other hand, the nonpermeant reducing agents, dithiothreitol or cysteine, further increased the cyanide effect. These observations can be explained by cyanide interacting with redox sensitive disulfide groups that are not accessible to the non-permeant reducing agents. It is proposed that cyanide interacts with a redox site(s) located either on the intracellular receptor domain or in the transmembrane hydrophobic domain. Furthermore the enhancement by cyanide of the excitotoxic actions of NMDA involves receptor sites that are sensitive to oxidation/reduction and this interaction contributes to the neurotoxic action of cyanide.

Food toxins, ampa receptors, and motor neuron diseases

Environmental chemicals involved in the etiology of human neurodegenerative disorders are challenging to identify. Described here is research designed to determine the etiology and molecular pathogenesis of nerve cell degeneration in two little known corticomotoneuronal diseases with established environmental triggers. Both conditions are toxic-nutritional disorders dominated by persistent spastic weakness of the legs and degeneration of corresponding corticospinal pathways. Lathyrism, a disease caused by dietary dependence on grass pea (Lathyrus sativus), is mediated by a stereospecific plant amino acid (beta-N-oxalylamino-L-alanine) that serves as a potent agonist at the (RS)-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) subclass of neuronal glutamate receptors. A neurologically similar disorder, konzo ("tied legs"), is found among protein-poor African communities that rely for food on cyanogen-containing cassava roots. Thiocyanate, the principal metabolite of cyanide, is an attractive etiologic candidate for konzo because it selectively promotes the action of glutamate at AMPA receptors. Studies are urgently needed to assess the health effects of cassava and other cyanogenic plants, components of which are widely used as food.

Subunit-specific interactions of cyanide with the N-methyl-D-aspartate receptor

Cyanide can potentiate N-methyl-D-aspartate receptor-mediated physiological responses in neurons. Here we show that this phenomenon may be attributable to a subunit-specific chemical modification of the receptor directly by the toxin. N-Methyl-D-aspartate (30 microM)-induced whole cell responses in mature (22-29 days in vitro) rat cortical neurons were potentiated nearly 2-fold by a 3-5-min treatment with 2 mM potassium cyanide, as did a similar treatment with 4 mM dithiothreitol. A 1-min incubation with the thiol oxidant 5,5'-dithiobis(2-nitrobenzoic acid) (0.5 mM) readily reversed the potentiation induced by either cyanide or dithiothreitol. Cyanide did not increase further currents previously potentiated by dithiothreitol nor was it able to potentiate responses during brief co-application with the agonist. Transient expression studies in Chinese hamster ovary cells with wild-type and mutated recombinant N-methyl-D-aspartate subunits (NR) demonstrated that cyanide selectively potentiated NR1/NR2A receptors, presumably via the chemical reduction of NR2A. In contrast, currents mediated by NR1/NR2B receptors were somewhat diminished by the metabolic inhibitor. Some of the effects of cyanide on NR1/NR2B receptors may be mediated by the formation of a thiocyanate adduct with a cysteine residue located in NR1. Cyanide thus is able to distinguish chemically between two different N-methyl-D-aspartate receptor subtypes and produce diametrically opposing functional effects.

Metabolic inhibition increases glutamate susceptibility on a PC12 cell line

The effect of energetic metabolism compromise, obtained by chemical induction of hypoglycaemia (glucose deprivation), hypoxia (mitochondrial respiratory chain inhibition), and ischaemia (hypoglycaemia plus hypoxia), on glutamate toxicity was analyzed on PC12 cells. The respiratory status of these cells, measured by the MTT [3-(4,5-dimethylthiazol-2-yl)2,5-diphenyltetrazolium bromide] assay, was significantly decreased after metabolic inhibition induced by ischaemia, but it was not affected by both hypoglycaemia and hypoxia. Under hypoglycaemia, but not under hypoxia, ATP levels were significantly reduced (from 12.67+/-0.48 to 5.38+/-1.41 nmol/mg protein). However, ischaemic-like conditions greatly potentiated the decline of ATP levels (95% decrease) observed after hypoglycaemia. The influence of metabolic inhibition on glutamate-induced cytotoxicity was also analyzed. When the cells were preincubated under conditions that deplete ATP (hypoglycaemia and ischaemia), the inhibition of MTT reduction, measured after glutamate incubation, was potentiated. This effect could be reverted when vitamin E and idebenone were present during the induction of metabolic inhibition. The ATP levels above which glutamate susceptibility was enhanced were also determined. These results indicate that glutamate toxicity on PC12 cells, which occurs by a mechanism independent of N-methyl-D-aspartate (NMDA) receptor activation, can be enhanced by the depletion of intracellular ATP upon metabolic stress; it is dependent on the extent of ATP depletion and seems to involve the generation of free radicals. It can be concluded that under ischaemic conditions, the deleterious effects of glutamate can be potentiated by the energetic compromise associated with this pathologic situation.

Hydrogen cyanide generation by mu-opiate receptor activation: possible neuromodulatory role of endogenous cyanide

Hydrogen cyanide, a gaseous molecule, is produced by white blood cells during phagocytosis. The present study examined the possibility that neuronal-like cells may also produce cyanide following activation. Rat pheochromocytoma (PC12) cells exhibited a low level of cyanide generation that was significantly increased by mu-opiate agonists (hydromorphone, morphine) and blocked by naloxone. A variety of other agonists including bradykinin, nicotine and glutamate did not generate cyanide in PC12 cells. Systemic administration of hydromorphone to rats increased brain cyanide levels by 61% after 15 min. Using microdialysis probes implanted in the cortical-hippocampal areas of the anesthetized rat or in the hypothalamus of the conscious hamster, a 2- to 5-fold increase in cyanide generation was seen after hydromorphone administration and this increase was blocked by naloxone. To determine whether cyanide release by hydromorphone has functional significance in a neuronal system, cyanide enhancement of N-methyl-D-aspartate (NMDA)-induced increased [Ca2+]i was measured in rat cerebellar granule cells. Hydromorphone enhanced the response to NMDA similar to cyanide and the hydromorphone effect was blocked by cyanide scavengers. These data show that cyanide generation is increased in neuronal tissue by a mu-opiate receptor agonist and it is proposed that endogenous cyanide may modulate the NMDA receptor response.

'Chemical ischemia' in cultured retina cells: the role of excitatory amino acid receptors and of energy levels on cell death

In this study, we determined whether the retina cell death observed in response to an ischemic-like insult is related to an overactivation of the ionotropic glutamate receptors and/or to a collapse of the energy levels. Cultured chick retina cells were submitted to 'chemical ischemia' by metabolic inhibition with sodium cyanide and iodoacetic acid, which block oxidative phosphorylation and glycolysis, respectively. The assessment of neuronal injury was made spectrophotometrically by quantification of cellularly reduced MTT, which gives information about mitochondrial function, or by staining with fluorescein diacetate (FDA), which correlates with changes in the plasma membrane permeability. 'Chemical ischemia' induced both an acute and a delayed time-dependent degeneration of chick retina cells. We observed that 2 min after the ischemic insult, the levels of ATP were reduced to a minimum. On the other hand, the metabolic inhibition induced the release of aspartate, glutamate and gamma-aminobutyric acid, and the activation of AMPA/kainate receptors during the period of metabolic arrest was partially responsible for the loss of mitochondrial function. However, the NMDA and non-NMDA receptor antagonists (MK-801 and CNQX) did not prevent the plasma membrane damage caused by sodium cyanide and iodoacetic acid. The results show that the collapse of the energy levels, rather than the increase in excitatory amino acids, appears to underlie the observed cell injury, suggesting an important relationship between ischemia-induced depletion of high-energy metabolites and retina cell degeneration.

Enhanced GABA release in cell-damaging conditions in the adult and developing mouse hippocampus

The release of [3H]GABA from hippocampal slices from adult (3-month-old) and developing (7-day-old) mice was studied in cell-damaging conditions in vitro using a superfusion system. Cell damage was induced by modified superfusion media, including hypoxia, hypoglycemia, ischemia, the presence of Free radicals and oxidative stress. The basal release of GABA from the immature and mature hippocampus was generally markedly increased in all cell-damaging conditions. In 7-day-old mice the release was enhanced most in the presence of free radicals. 1.0 mM NaCN and ischemia, whereas in the adults 1.0 mM NaCN provoked the largest release of GABA, followed by ischemia and free radical-containing media. Potassium stimulation (50 mM K+) was still able to potentiate the release in all cell-damaging conditions in both age groups. It was shown by superfusing the slices in Ca- and Na-free media that ischemia-induced GABA release was Ca-independent, occurring by a reversed operation of Na-dependent cell membrane carriers in both adult and developing hippocampus. Glutamate and its receptor agonists, N-methyl-D-aspartate (NMDA), kainate and 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), potentiated GABA release only in the immature hippocampus by a receptor-mediated mechanism. The enhancement by kainate and AMPA receptors also operated under ischemic conditions. The massive amount of GABA released simultaneously with excitatory amino acids in the mature and immature hippocampus may be an important protective mechanism against excitotoxicity, counteracting harmful effects that lead to neuronal death. The GABA release induced by activation of presynaptic glutamate receptors may contribute particularly to the maintenance of homeostasis in the hippocampus upon impending hyperexcitation.

MK-801 prevents cyanide-induced changes of Fos levels in rat brain

The effect of acute cyanide intoxication on levels of transcriptional regulatory proteins Fos and c-Jun in rat cortex, hippocampus, cerebellum and brain stem was studied. Western blot analysis showed a differential effect of cyanide on Fos levels in the selected brain areas. The most prominent changes were seen 60 min. following ip. injection of KCN in all brain areas except the brain stem, which showed the maximal change 120 min. following cyanide. Fos levels were doubled in cortex and cerebellum and decreased to below 70% of the control levels in hippocampus. Levels of c-Jun were not altered 60 min. following cyanide treatment. Pretreatment with the NMDA receptor antagonist, MK-801, prevented the cyanide-induced changes of Fos. The differential effect of cyanide on Fos levels in different brain areas and the blockade of these changes by MK-801 suggest involvement of multiple neuronal pathways, including the excitatory amino acid (EAA) neurotransmitter system. It is concluded that cyanide alters levels of the transcriptional regulatory protein Fos through activation of the EAA neurotransmitter system and, thus, may affect gene expression in neuronal or glia cells.