Characterization of suppression of nitric oxide production by carbon monoxide poisoning in the striatum of free-moving rats, as determined by in vivo brain microdialysis

Carbon Monoxide (CO), Nitric Oxide, and Hydrogen Sulfide Signaling During Acute CO Poisoning

Major toxic effects of acute carbon monoxide (CO) poisoning result from increases in reactive oxygen species (ROS) and reactive nitrogen species (RNS) producing oxidative stress. The importance of altered nitric oxide (NO) signaling in evoking increases in RNS during CO poisoning has been established. Although there is extensive literature describing NO and hydrogen sulfide (H2S) signaling in different types of cells under normal conditions, how CO poisoning-evoked deregulation of additional NO signaling pathways and H2S signaling pathways could result in cell injury has not been previously considered in detail. The goal of this article was to do this. The approach was to use published data to describe signaling pathways driven by CO bonding to different ferroproteins and then to collate data that describe NO and H2S signaling pathways that could interact with CO signaling pathways and be important during CO poisoning. Arteriolar smooth muscle cells—endothelial cells located in the coronary and some cerebral circulations—were used as a model to illustrate major signaling pathways driven by CO bonding to different ferroproteins. The results were consistent with the concept that multiple deregulated and interacting NO and H2S signaling pathways can be involved in producing cell injury evoked during acute CO poisoning and that these pathways interact with CO signaling pathways.

Coronary and cerebral metabolism-blood flow coupling and pulmonary alveolar ventilation-blood flow coupling may be disabled during acute carbon monoxide poisoning

Current evidence indicates that the toxicity of carbon monoxide (CO) poisoning results from increases in reactive oxygen species (ROS) generation plus tissue hypoxia resulting from decreases in capillary Po₂ evoked by effects of increases in blood [carboxyhemoglobin] on the oxyhemoglobin dissociation curve. There has not been consideration of how increases in Pco could influence metabolism-blood flow coupling, a physiological mechanism that regulates the uniformity of tissue Po₂, and alveolar ventilation-blood flow coupling, a mechanism that increases the efficiency of pulmonary O₂ uptake. Using published data, I consider hypotheses that these coupling mechanisms, triggered by O₂ and CO sensors located in arterial and arteriolar vessels in the coronary and cerebral circulations and in lung intralobar arteries, are disrupted during acute CO poisoning. These hypotheses are supported by calculations that show that the Pco in these vessels can reach levels during CO poisoning that would exert effects on signal transduction molecules involved in these coupling mechanisms.NEW & NOTEWORTHY This article introduces and supports a postulate that the tissue hypoxia component of carbon monoxide poisoning results in part from impairment of physiological adaptation mechanisms whereby tissues can match regional blood flow to O₂ uptake, and the lung can match regional blood flow to alveolar ventilation.

The Use of Intracerebral Microdialysis to Elucidate Environmentally Induced Neurotoxic Mechanisms

The technique of microdialysis permits the assessment of neurotransmitter activity and the monitoring of other cellular entities in tissue extracellular fluid. The method is widely used for quantifying biogenic amine and amino acid transmitters, peptides, administered drugs, and other molecules in response to various experimental treatments. This article provides an overview of the manner in which the methodology of intracerebral microdialysis is utilized in the field of neurotoxicology to elucidate the actions of environmental agents. The technique is employed in a variety of creative ways to address specific experimental goals involving myriad toxicants. With appropriate consideration of method parameters, investigators have also been able to address mechanistic issues in their studies. These investigations consist of sampling of neurotransmitters in extracellular fluid after various protocols of environmental metal exposure as well as assessments of blood-brain barrier permeability, the detection of reactive oxygen species, and description of the toxicodynamics of environmental agents. The purpose of this examination is not to review the investigational findings, per se, but to highlight the various approaches utilized with this methodology and the experimental questions that have been addressed. © 2019 by John Wiley & Sons, Inc.

Possible role of antioxidants and nitric oxide inhibitors against carbon monoxide poisoning: Having a clear conscience because of their potential benefits

Carbon monoxide poisoning is one of the important emergency situations manifested by primarily acute and chronic anoxic central nervous system (CNS) injuries and other organ damages. Current descriptions and therapeutic approaches have been focused on the anoxic pathophysiology. However, this point of view incompletely explains some of the outcomes and needs to be investigated extensively. Considering this, we propose that reactive oxygen species (ROS) including especially nitric oxide (NO) are likely to be a key concept to understand the emergency related to CO poisoning and to discover new therapeutic modalities in CO toxicity. If we consider the hypothesis that ROS is involved greatly in acute and chronic toxic effects of CO on CNS and some other vital organs such as heart, it follows that the antioxidant and anti-NO therapies might give the clinicians more opportunities to prevent deep CNS injury. In support of this, we review the subject in essence and summarize clinical and experimental studies that support a key role of ROS in the explanation of pathophysiology of CO toxicity as well as new treatment modalities after CO poisoning.

Salvianolic acids attenuate rat hippocampal injury after acute CO poisoning by improving blood flow properties

Carbon monoxide (CO) poisoning causes the major injury and death due to poisoning worldwide. The most severe damage via CO poisoning is brain injury and mortality. Delayed encephalopathy after acute CO poisoning (DEACMP) occurs in forty percent of the survivors of acute CO exposure. But the pathological cause for DEACMP is not well understood. And the corresponding therapy is not well developed. In order to investigate the effects of salvianolic acid (SA) on brain injury caused by CO exposure from the view point of hemorheology, we employed a rat model and studied the dynamic of blood changes in the hemorheological and coagulative properties over acute CO exposure. Compared with the groups of CO and 20% mannitol + CO treatments, the severe hippocampal injury caused by acute CO exposure was prevented by SA treatment. These protective effects were associated with the retaining level of hematocrit (Hct), plasma viscosity, fibrinogen, whole blood viscosities and malondialdehyde (MDA) levels in red blood cells (RBCs). These results indicated that SA treatment could significantly improve the deformation of erythrocytes and prevent the damage caused by CO poisoning. Meanwhile, hemorheological indexes are good indicators for monitoring the pathological dynamic after acute CO poisoning.

The role of reactive oxygen species and oxidative stress in carbon monoxide toxicity: an in-depth analysis

The underlying mechanism of the central nervous system (CNS) injury after acute carbon monoxide (CO) poisoning is interlaced with multiple factors including apoptosis, abnormal inflammatory responses, hypoxia, and ischemia/reperfusion-like problems. One of the current hypotheses with regard to the molecular mechanism of CO poisoning is the oxidative injury induced by reactive oxygen species, free radicals, and neuronal nitric oxide. Up to now, the relevant mechanism of this injury remains poorly understood. The weakening of antioxidant systems and the increase of lipid peroxidation in the CNS have been implicated, however. Accordingly, in this review, we will highlight the relationship between oxidative stress and CO poisoning from the perspective of forensic toxicology and molecular toxicology.

Induction of heme oxygenase-1 with hemin attenuates hippocampal injury in rats after acute carbon monoxide poisoning

Carbon monoxide (CO) poisoning is a major cause of brain injury and mortality; delayed neurological syndrome (DNS) is encountered in survivors of acute CO exposure. The toxic effects of CO have been attributed to oxidative stress induced by hypoxia. Heme oxygenase-1 (HO-1) is the inducible heme oxygenase isoform, and its induction acts as an important cellular defense mechanism against oxidative stress, cellular injury and disease. In this study, we examined the functional roles of HO-1 induction in a rat model of CO-exposured hippocampal injury. We report that acute CO exposure produces severe hippocampal injury in rats. However, hemin pretreatment reduced both the CO-induced rise in hippocampal water content and levels of neuronal damage in the hippocampus; survival rates at 24 h were significantly improved. Upregulation of HO-1 by hemin pretreatment resulted in a significant decrease in hippocampal levels of malondialdehyde (MDA), a marker of oxidative stress; levels of pro-apoptotic caspase-3 were also reduced. In contrast, inhibition of HO activity by administration of tin protoporphyrin IX (SnPP, a specific inhibitor of HO) abolished the neuroprotective effects of HO-1 induction. These data suggested that the upregulation of endogenous HO-1 expression therefore plays a pivotal protective role in CO neurotoxicity. Though the precise mechanisms underlying hemin-mediated HO-1 induction and neuroprotection are not known, these may involve the anti-oxidant and anti-apoptotic effects of HO-1 enzyme activity.

Role of nitric oxide system in hydroxyl radical generation in rat striatum due to carbon monoxide poisoning, as determined by microdialysis

We explored the possible role of the nitric oxide (NO) system in hydroxyl radical (*OH) generation induced by carbon monoxide (CO) poisoning in rat striatum by means of microdialysis with the use of NO synthase (NOS) inhibitors, N(G)-nitro-L-arginine methyl ester (L-NAME) and N(G)-monomethyl-L-arginine (L-NMMA), as well as L-arginine (L-Arg; the NOS substrate) and D-arginine (D-Arg). The CO-induced *OH generation was suppressed by both L-Arg and D-Arg. It was also suppressed by L-NAME, which inhibits generation of reactive oxygen species (ROS) via neuronal NOS (nNOS) and inducible NOS, but not via endothelial NOS. In contrast, L-NMMA, which inhibits only ROS generation via inducible NOS, potentiated the *OH generation. L-Arg completely reversed the L-NAME effect and partly reversed the L-NMMA effect. D-Arg reversed the L-NAME effect more potently than did L-Arg, resulting in much more *OH generation than was observed with CO alone, and also potentiated the L-NMMA effect. On the other hand, W-7, an antagonist of calmodulin, which is critical for nNOS activity, had no effect on the CO-induced *OH generation. These findings suggest that complex mechanisms operate in *OH generation in rat striatum upon CO poisoning and that the NO system might not be included among those mechanisms.

Nitric oxide-independent cGMP efflux in the striatum of rats exposed to carbon monoxide as determined by microdialysis

Extracellular cGMP in the striatum of rats exposed to 3000 ppm carbon monoxide (CO) or 8% O2 was decreased during the early period of exposure. Thereafter, extracellular cGMP in rats exposed to CO, but not 8% O2, was transiently increased. A nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine, strongly reduced the steady-state level of extracellular cGMP in the striatum, indicating a primary role of NO in cGMP production. However, it failed to suppress the CO-induced increase in extracellular cGMP in the striatum. These findings suggest that CO may stimulate cGMP production in rat striatum independently of NO and hypoxia.

Excessive nitric oxide attenuates leptin-mediated signal transducer and activator of transcription 3 activation

The mechanisms of leptin resistance observed in most cases of human obesity are poorly understood. Therefore, we evaluated the effects of nitric oxide (NO) on the leptin-induced activation of Janus kinase/signal transducer and activator of transcription 3 (JAK/STAT3) pathways and on the leptin receptor (LEPR) expression using SH-SY5Y cells. Here, we show that the NO donor spermine/NONOate inhibited leptin-induced activation of STAT3 in vitro. The inhibition of leptin-mediated STAT3 phosphorylation caused by excessive NO was partially prevented by a sulfhydryl reducing agent, ascorbic acid. Cellular experiments show that reduced expression of long form leptin receptor (LEPR-b) and STAT3 protein instability induced by NO may be mechanisms of the NO-mediated inhibition of leptin-STAT3 signaling. We also present data showing that the hypothalamic NO content of high-fat (HF)-diet-induced obese mice was higher than that of control mice; this is likely caused by decreased caveolin-1 expression and increased nNOS expression induced by HF diet over 19 weeks. Concurrently with the overproduction of NO, the decrease of hypothalamic LEPR-b in obese mice also supports these in vitro data. Combined results suggest that excess of NO can induce the attenuation of leptin-mediated STAT3 activation through reduced expression of LEPR-b mRNA and instability of STAT3 protein at least in part. Furthermore, our in vivo data indicate that long-term HF diet induces hypothalamic overproduction of NO, which may be related with leptin insensitivity. However, further study is required to warrant direct in vivo evidence of a causal relationship between endogenous excess of hypothalamic NO and central leptin resistance.

Microdialysis as a tool in local pharmacodynamics

In many cases the clinical outcome of therapy needs to be determined by the drug concentration in the tissue compartment in which the pharmacological effect occurs rather than in the plasma. Microdialysis is an in vivo technique that allows direct measurement of unbound tissue concentrations and permits monitoring of the biochemical and physiological effects of drugs throughout the body. Microdialysis was first used in pharmacodynamic research to study neurotransmission, and this remains its most common application in the field. In this review, we give an overview of the principles, techniques, and applications of microdialysis in pharmacodynamic studies of local physiological events, including measurement of endogenous substances such as acetylcholine, catecholamines, serotonin, amino acids, peptides, glucose, lactate, glycerol, and hormones. Microdialysis coupled with systemic drug administration also permits the more intensive examination of the pharmacotherapeutic effect of drugs on extracellular levels of endogenous substances in peripheral compartments and blood. Selected examples of the physiological effects and mechanisms of action of drugs are also discussed, as are the advantages and limitations of this method. It is concluded that microdialysis is a reliable technique for the measurement of local events, which makes it an attractive tool for local pharmacodynamic research.

Different response to exogenous l-arginine in nitric oxide production between hippocampus and striatum of conscious rats: a microdialysis study

We previously showed that systemic administration of a nitric oxide (NO) precursor, L-arginine (L-Arg), failed to reverse suppression by NO synthase (NOS) inhibitors of chemically induced shaking behavior in rats, leading to the hypothesis that exogenous L-Arg might be non-uniformly supplied to brain regions susceptible to NOS inhibitors. In the present study, therefore, we examined the effect of exogenous L-Arg on the extracellular levels of the oxidative nitric oxide (NO) products, nitrite (NO2-) and nitrate (NO3-), in two different brain regions, the hippocampus and the striatum, of conscious rats by means of in vivo brain microdialysis. The basal NO2- levels in the two brain regions were comparable, while the NO3- level was significantly lower in the hippocampus than the striatum. The addition of 10 mM L-Arg, but not D-Arg, to the perfusing solution significantly increased NO2- and NO3- in the hippocampus and NO2- alone in the striatum. These increases were abolished by 1 mM N(omega)-nitro-L-arginine, an NOS inhibitor. L-Arg at 1mM was able to significantly increase NO2-, but not NO3-, in the hippocampus to a level comparable with that at 10 mM L-Arg, while it had no effect in the striatum. L-Arg (500 mg/kg, i.p.) induced a significant increase in NO2- and NO3- in the hippocampus, but not in the striatum. These results suggest that the striatum may have a lower ability to enhance NO production by utilising exogenous L-Arg than the hippocampus, despite higher basal NO production.

Characterization of hydroxyl radical generation in the striatum of free-moving rats due to carbon monoxide poisoning, as determined by in vivo microdialysis

Carbon monoxide (CO) poisoning caused by CO exposure at 3000 ppm for 40 min resulted in stimulation of hydroxyl radical (*OH) generation (estimated by measuring 2,3-dihydroxybenzoic acid (2,3-DHBA) production from salicylic acid) in the striatum of free-moving rats, as determined by means of brain microdialysis. Pretreatment with a voltage-dependent Na+ channel blocker, tetrodotoxin (TTX), lowered the basal level of 2,3-DHBA and strongly suppressed the increase in 2,3-DHBA induced by CO poisoning. CO poisoning significantly, though only slightly, increased extracellular glutamate in the striatum, and glutamate (Glu) receptor antagonists, such as MK-801 (dizocilpine) and NBQX, failed to suppress the CO-induced increase in 2,3-DHBA. These findings suggest that CO poisoning may induce Na+ influx via the voltage-dependent Na+ channels, resulting in stimulation of *OH generation in rat striatum. This effect may be independent of Glu receptor activation by increased extracellular Glu.

Neuronal nitric oxide synthase and N-methyl-D-aspartate neurons in experimental carbon monoxide poisoning

We measured changes in nitric oxide (NO) concentration in the cerebral cortex during experimental carbon monoxide (CO) poisoning and assessed the role for N-methyl-d-aspartate receptors (NMDARs), a glutamate receptor subtype, with progression of CO-mediated oxidative stress. Using microelectrodes, NO concentration was found to nearly double to 280 nM due to CO exposure, and elevations in cerebral blood flow, monitored as laser Doppler flow (LDF), were found to loosely correlate with NO concentration. Neuronal nitric oxide synthase (nNOS) activity was the cause of the NO elevation based on the effects of specific NOS inhibitors and observations in nNOS knockout mice. Activation of nNOS was inhibited by the NMDARs inhibitor, MK 801, and by the calcium channel blocker, nimodipine, thus demonstrating a link to excitatory amino acids. Cortical cyclic GMP concentration was increased due to CO poisoning and shown to be related to NO, versus CO, mediated guanylate cyclase activation. Elevations of NO were inhibited when rats were infused with superoxide dismutase and in rats depleted of platelets or neutrophils. When injected with MK 801 or 7-nitroindazole, a selective nNOS inhibitor, rats did not exhibit CO-mediated nitrotyrosine formation, myeloperoxidase (MPO) elevation (indicative of neutrophil sequestration), or impaired learning. Similarly, whereas CO-poisoned wild-type mice exhibited elevations in nitrotyrosine and myeloperoxidase, these changes did not occur in nNOS knockout mice. We conclude that CO exposure initiates perivascular processes including oxidative stress that triggers activation of NMDA neuronal nNOS, and these events are necessary for the progression of CO-mediated neuropathology.