Bio-imaging of nitric oxide-producing neurones in slices of rat brain using 4,5-diaminofluorescein

Relaxin peptides reduce cellular damage in cultured brain slices exposed to transient oxygen–glucose deprivation: an effect mediated by nitric oxide

The effect of treatment with human relaxins on cell death was studied in oxygen- and glucose-deprived brain slices. In addition, involvement of nitric oxide and the relaxin receptor, RXFP3, was studied. Brain slices ( n = 12–18/group) were cultured under standard conditions for two weeks and then exposed to: ( i) an oxygenated balanced salt solution, ( ii) a deoxygenated, glucose-free balanced salt solution (OGD media), or ( iii) OGD media containing 10−7 mol/L H2 relaxin, 10−7 mol/L H2 relaxin with 50 μmol/L L-NIL, 10−7 mol/L H3 relaxin, or 10−7 mol/L H3 relaxin with 50 μmol/L L-NIL. Cell death was assessed using propidium iodide fluorescence. In a separate experiment, 10−5 mol/L R3 B1-22R (an antagonist of RXFP3) was added to both H2 and H3 relaxin treatments. H2 and H3 relaxin treatment reduced cell damage or death in OGD slices and L-NIL partially attenuated the effect of H3 relaxin. Antagonism of RXFP3 blocked the effect of H3 but not H2 relaxin. These data increase our understanding of the role of relaxin ligands and their receptors in protecting tissues throughout the body from ischemia and reperfusion injury.

Effects of selective inhibition of nNOS and iNOS on neuropathic pain in rats

Various animal models have been employed to understand the pathogenic mechanism of neuropathic pain. Nitric oxide (NO) is an important molecule in nociceptive transmission and is involved in neuropathic pain. However, its mechanistic actions remain unclear. The aim of this study was to better understand the involvement of neuronal and inducible isoforms of nitric oxide synthase (nNOS and iNOS) in neuropathic pain induced by chronic constriction injury (CCI) of the sciatic nerve in rats. We evaluated pain sensitivity (mechanical withdrawal thresholds using Randall and Selitto, and von Frey tests, and thermal withdrawal thresholds using Hargreaves test) prior to CCI surgery, 14 days post CCI and after intrathecal injections of selective nNOS or iNOS inhibitors. We also evaluated the distribution of NOS isozymes in the spinal cord and dorsal root ganglia (DRG) by immunohistochemistry, synthesis of iNOS and nNOS by Western blot, and NO production using fluorescent probe DAF-2 DA (DA). Our results showed higher number of nNOS and iNOS-positive neurons in the spinal cord and DRG of CCI compared to sham rats, and their reduction in CCI rats after treatment with selective inhibitors compared to non-treated groups. Western blot results also indicated reduced expression of nNOS and iNOS after treatment with selective inhibitors. Furthermore, both inhibitors reduced CCI-evoked mechanical and thermal withdrawal thresholds but only nNOS inhibitor was able to efficiently lower mechanical withdrawal thresholds using von Frey test. In addition, we observed higher NO production in the spinal cord and DRG of injured rats compared to control group. Our study innovatively shows that nNOS may strongly modulate nociceptive transmission in rats with neuropathic pain, while iNOS may partially participate in the development of nociceptive responses. Thus, drugs targeting nNOS for neuropathic pain may represent a potential therapeutic strategy.

Free cholesterol accumulation impairs antioxidant activities and aggravates apoptotic cell death in menadione-induced oxidative injury

Although the relationship between hypercholesterolemia and oxidative stress has been extensively investigated, direct evidence regarding to the roles of cholesterol accumulation in the generations of reactive oxygen species (ROS) and apoptotic cell death under oxidative stress is lack. In this study, we investigated productions of superoxide anions (O(2)(-)) and nitric oxide (NO), and apoptotic cell death in wild type Chinese hamster ovary (CHO) cells and cholesterol accumulated CHO cells genetically and chemically. Oxidative stress was induced by menadione challenge. The results revealed that abundance of free cholesterol (FC) promoted menadione-induced O(2)(-) and NO productions. FC accumulation down-regulated eNOS expression but up-regulated NADPH oxidases, and inhibited the activities of superoxide dismutase (SOD) and catalase. Treatment of menadione increased the expressions of iNOS and qp91 phox, enhanced the activities of SOD and catalase in the wild-type CHO cells but inhibited the activity of glutathione peroxidase in the cholesterol accumulated CHO cells. Moreover, FC abundance promoted apoptotic cell death in these cells. Taken together, those results suggest that free cholesterol accumulation aggravates menadione-induced oxidative stress and exacerbates apoptotic cell death.

Endogenous presynaptic nitric oxide supports an anterograde signaling in the central nervous system

The source size and density determine the extent of nitric oxide (NO) diffusion which critically influences NO signaling. In the brain, NO released from postsynaptic somas following NMDA-mediated activation of neuronal nitric oxide synthase (nNOS) retrogradely affects smaller presynaptic targets. By contrast, in guinea pig trigeminal motor nucleus (TMN), NO is produced presynaptically by tiny and disperse nNOS-containing terminals that innervate large nNOS-negative motoneurons expressing the soluble guanylyl-cyclase (sGC); consequently, it is uncertain whether endogenous NO supports an anterograde signaling between pre-motor terminals and postsynaptic trigeminal motoneurons. In retrogradely labeled motoneurons, we indirectly monitored NO using triazolofluorescein (DAF-2T) fluorescence, and evaluated sGC activity by confocal cGMP immunofluorescence. Multiple fibers stimulation enhanced NO content and cGMP immunofluorescence into numerous nNOS-negative motoneurons; NOS inhibitors prevented depolarization-induced effects, whereas NO donors mimicked them. Enhance of cGMP immunofluorescence required extracellular Ca(2+), a nNOS-physiological activator, and was prevented by inhibiting sGC, silencing neuronal activity or impeding NO diffusion. In conclusion, NO released presynaptically from multiple cooperative tiny fibers attains concentrations sufficient to activate sGC in many motoneurons despite of the low source/target size ratio and source dispersion; thus, endogenous NO is an effective anterograde neuromodulator. By adjusting nNOS activation, presynaptic Ca(2+) might modulate the NO diffusion field in the TMN.

Physiologic role for "inducible" nitric oxide synthase: a new form of astrocytic-neuronal interface

Nitric oxide (NO) has been long recognized as an atypical neuronal messenger affecting excitatory synaptic transmission, but its cellular source has remained unresolved as the neuronal isoform of NO synthase (nNOS) in many brain regions is expressed only by small subsets of inhibitory neurons. It is generally believed that the glial NO-producing isoform (iNOS) is not expressed in the normal brain, but rather it undergoes a transcription-mediated up-regulation following an immunological challenge. Therefore, the involvement of iNOS in modulating normal neuronal functions has been largely ignored. Here I review evidence to the contrary: I summarize data pointing to the existence of a functioning iNOS in normal undisturbed mammalian brains, and experimental results tracing this expression to astrocytes. Finally, I review recent findings asserting that iNOS-dependent NO modulates synaptic release from presynaptic terminals. Based on these data, I propose that astrocytes express basal levels of iNOS. Flanking synaptic elements, astrocytes are perfectly positioned to release NO and affect synaptic transmission.

Multimodality imaging of nitric oxide and nitric oxide synthases

Nitric oxide (NO) and NO synthases (NOSs) are crucial factors in many pathophysiological processes such as inflammation, vascular/neurological function, and many types of cancer. Noninvasive imaging of NO or NOS can provide new insights in understanding these diseases and facilitate the development of novel therapeutic strategies. In this review, we will summarize the current state-of-the-art multimodality imaging in detecting NO and NOSs, including optical (fluorescence, chemiluminescence, and bioluminescence), electron paramagnetic resonance (EPR), magnetic resonance (MR), and positron emission tomography (PET). With continued effort over the last several years, these noninvasive imaging techniques can now reveal the biodistribution of NO or NOS in living subjects with high fidelity which will greatly facilitate scientists/clinicians in the development of new drugs and/or patient management. Lastly, we will also discuss future directions/applications of NO/NOS imaging. Successful development of novel NO/NOS imaging agents with optimal in vivo stability and desirable pharmacokinetics for clinical translation will enable the maximum benefit in patient management.

Endogenous nitric oxide is a key promoting factor for initiation of seizure-like events in hippocampal and entorhinal cortex slices

Nitric oxide (NO) modulates synaptic transmission, and its level is elevated during epileptic activity in animal models of epilepsy. However, the role of NO for development and maintenance of epileptic activity is controversial. We studied this aspect in rat organotypic hippocampal slice cultures and acute hippocampal-entorhinal cortex slices from wild-type and neuronal NO synthase (nNOS) knock-out mice combining electrophysiological and fluorescence imaging techniques. Slice cultures contained nNOS-positive neurons and an elaborated network of nNOS-positive fibers. Lowering of extracellular Mg(2+) concentration led to development of epileptiform activity and increased NO formation as revealed by NO-selective probes, 4-amino-5-methylamino-2',7'-difluorofluorescein and 1,2-diaminoanthraquinone sulfate. NO deprivation by NOS inhibitors and NO scavengers caused depression of both EPSCs and IPSCs and prevented initiation of seizure-like events (SLEs) in 75% of slice cultures and 100% of hippocampal-entorhinal cortex slices. This effect was independent of the guanylyl cyclase/cGMP pathway. Suppression of SLE initiation in acute slices from mice was achieved by both the broad-spectrum NOS inhibitor N-methyl-L-arginine acetate and the nNOS-selective inhibitor 7-nitroindazole, whereas inhibition of inducible NOS by aminoguanidine was ineffective, suggesting that nNOS activity was crucial for SLE initiation. Additional evidence was obtained from knock-out animals because SLEs developed in a significantly lower percentage of slices from nNOS(-/-) mice and showed different characteristics, such as prolongation of onset latency and higher variability of SLE intervals. We conclude that enhancement of synaptic transmission by NO under epileptic conditions represents a positive feedback mechanism for the initiation of seizure-like events.

Different effect of chronic stress on learning and memory in BALB/c and C57BL/6 inbred mice: Involvement of hippocampal NO production and PKC activity

Nitric oxide (NO) has been involved in many pathophysiological brain processes. Recently, we showed that neuronal nitric oxide synthase (nNOS)-mediated decrease in NO production is involved in memory impairment induced by chronic mild stress (CMS) in BALB/c mice. Two genetically different inbred murine strains, C57BL/6 and BALB/c, show distinct behavioral responses, neurodevelopmental and neurochemical parameters. Here, we perform a comparative study on CMS effects upon learning and memory in both strains, analyzing the role of NO production and its regulation by protein kinase C (PKC). Stressed BALB/c, but not C57Bl/6 mice, showed a poor learning performance in both the open field and passive avoidance inhibitory tasks. Also, CMS induced a diminished NO production by nNOS, associated with an increment in gamma and zeta PKC isoenzymes in BALB/c mice. In C57BL/6 mice, CMS had no effect on NO production, but increased delta and decreased betaI PKC isoforms. In vivo administration of a NOS inhibitor induced behavioral alterations in both strains. These results suggest a differential effect of stress, with BALB/c being more vulnerable to stress than C57BL/6 mice. This effect could be related to a differential regulation of NOS and PKC isoenzymes, pointing to an important role of NO in learning and memory.

Protective effects of brain-derived neurotrophic factor against neurotoxicity of 3-nitropropionic acid in rat cortical neurons

Brain-derived neurotrophic factor (BDNF) deficiency has been implicated in pathogenesis of Huntington's disease (HD). 3-Nitropropionic acid (3-NP), an irreversible mitochondrial complex II inhibitor, has been commonly used as a pharmacological model recapitulating HD phenotypes in rodents and nonhuman primates. Herein we test whether BDNF may exert neuroprotective effects against mitochondrial dysfunction caused by 3-NP in primary culture of fetal rat cortical neurons. Preconditioning of neuronal cells with BDNF (100 ng/ml for 8h) attenuated 3-NP toxicity (2.5 mM for additional 24h) based on Hoechst and propidium iodide (PI) staining. BDNF effects can be inhibited by the nitric oxide synthase (NOS) inhibitor L-nitroarginine methylester (L-NAME, 100 microM), the cGMP-dependent protein kinase (PKG) inhibitor KT5823 (2 microM), the thioredoxin reductase inhibitor 1-chloro-2,4-dinitrobenzene (DNCB, 5 microM), and a membrane-permeable Bcl-2 inhibitor (12.5 microM). 8-Br-cGMP is a cGMP analogue capable of activating PKG independent of NO. Exogenous application of 8-Br-cGMP (3-30 microM) and purified thioredoxin (3-5 microM) partially mimicked BDNF effects in conferring 3-NP resistance to cortical cells. These results, together with our previous report showing NO donor S-nitrosoglutathione (GSNO)-mediated neuroprotective effects against 3-NP toxicity, suggest that BDNF may protect neurons from mitochondrial dysfunction at least partly via activation of the signaling cascades involving NOS/NO, PKG, thioredoxin and Bcl-2.

The physiologic aggresome mediates cellular inactivation of iNOS

Nitric Oxide (NO), produced by inducible nitric oxide synthase (iNOS), has been implicated in the pathogenesis of various biological and inflammatory disorders. Recent evidence suggests that aggresome formation is a physiologic stress response not limited to misfolded proteins. That stress response, termed "physiologic aggresome," is exemplified by aggresome formation of iNOS, an important host defense protein. The functional significance of cellular formation of the iNOS aggresome is hitherto unknown. In this study, we used live cell imaging, fluorescence microscopy, and intracellular fluorescence NO probes to map the subcellular location of iNOS and NO under various conditions. We found that NO production colocalized with cytosolic iNOS but aggresomes containing iNOS were distinctly devoid of NO production. Further, cells expressing iNOS aggresomes produced significantly less NO as compared with cells not expressing aggresomes. Importantly, primary normal human bronchial epithelial cells, stimulated by cytokines to express iNOS, progressively sequestered iNOS to the aggresome, a process that correlated with marked reduction of NO production. These results suggest that bronchial epithelial cells used the physiologic aggresome mechanism for iNOS inactivation. Our studies reveal a novel cellular strategy to terminate NO production via formation of the iNOS aggresome.

Hypothyroidism induces selective oxidative stress in amygdala and hippocampus of rat

The effects of hypothyroidism on lipid peroxidation (LP), reactive oxygen species (ROS), and nitric oxide synthase (NOS), levels and expression, in rat brain were examined. Hypothyroidism was induced by administering methimazole in drinking water (60 mg/kg/day). In striatum, motor cortex and cerebellum of hypothyroid rats LP was not modified, whereas LP and ROS increased in amygdala and hippocampus of hypothyroid rats at the third week of treatment with methimazole as compared to euthyroid group values. Regarding NOS participation, only hippocampal constitutive-NOS activity was increased, accompanied by an augmentation in nNOS expression. Results show that hypothyroidism induces selective oxidative stress in both the hippocampus and amygdala, where the nitrergic system is involved.

Simultaneous nitric oxide and dehydroascorbic acid imaging by combining diaminofluoresceins and diaminorhodamines

Spatial measurements of nitric oxide (NO) production are important to understand the function and metabolism of this molecule. The reagent, 4,5-diaminofluorescein (DAF-2) and several structurally similar probes are widely used for detection and imaging of NO. However, DAF-2 also reacts with dehydroascorbic acid (DHA) in biological samples, with both products having nearly indistinguishable fluorescence spectra. Measurements using fluorimetry and fluorescence microscopy cannot easily differentiate NO-related fluorescent signals from DHA-related signals. While DAFs and the structurally related diaminorhodamines (DARs) both react with NO and DHA, they do so to different extents. We report a multiderivatization method to image NO and DHA simultaneously by using both DAF and DAR. Specifically, DAF-2 and DAR-4M are used to image NO and DHA concentrations; after reaction, the solutions are excited, at 495 nm to measure fluorescence emission from DAF-2, and at 560 nm to measure fluorescence emission from DAR-4M. Using the appropriate calibrations, images are created that depend either on the relative NO or the relative DHA concentration, even though each probe reacts to both compounds. The method has been validated by imaging NO production in both undifferentiated and differentiated pheochromocytoma (PC12) cells.

Enhanced astrocytic nitric oxide production and neuronal modifications in the neocortex of a NOS2 mutant mouse

Background

It has been well accepted that glial cells in the central nervous system (CNS) produce nitric oxide (NO) through the induction of a nitric oxide synthase isoform (NOS2) only in response to various insults. Recently we described rapid astroglial, NOS2-dependent, NO production in the neocortex of healthy mice on a time scale relevant to neuronal activity. To explore a possible role for astroglial NOS2 in normal brain function we investigated a NOS2 knockout mouse (B6;129P2-Nos2^tm1Lau/J, Jackson Laboratory). Previous studies of this mouse strain revealed mainly altered immune responses, but no compensatory pathways and no CNS abnormalities have been reported.

Methodology/Principal Findings

To our surprise, using NO imaging in brain slices in combination with biochemical methods we uncovered robust NO production by neocortical astrocytes of the NOS2 mutant. These findings indicate the existence of an alternative pathway that increases basal NOS activity. In addition, the astroglial mutation instigated modifications of neuronal attributes, shown by changes in the membrane properties of pyramidal neurons, and revealed in distinct behavioral abnormalities characterized by an increase in stress-related parameters.

Conclusions/Significance

The results strongly indicate the involvement of astrocytic-derived NO in modifying the activity of neuronal networks. In addition, the findings corroborate data linking NO signaling with stress-related behavior, and highlight the potential use of this genetic model for studies of stress-susceptibility. Lastly, our results beg re-examination of previous studies that used this mouse strain to examine the pathophysiology of brain insults, assuming lack of astrocytic nitrosative reaction.

A potentiating effect of endogenous NO in the physiologic secretion from airway submucosal glands

It is known that several second messengers, such as Ca(2+) or cAMP, play important roles in the intracellular pathway of electrolyte secretion in tracheal submucosal gland. However, the participation of cGMP, and therefore nitric oxide (NO), is not well understood. To investigate the physiologic role of NO, we first examined whether tracheal glands can synthesize NO in response to acetylcholine (ACh), and then whether endogenous NO has some effects on the ACh-triggered ionic currents. From the experiments using the NO-specific fluorescent indicator 4,5-diaminofluorescein diacetate salt (DAF-2DA), we found that a physiologically relevant low dose of ACh (100 nM) stimulated the endogenous NO synthesis, and it was almost completely suppressed in the presence of the nonspecific NO synthase (NOS) inhibitor Nomega-Nitro-L-arginine Methyl Ester Hydrochloride (L-NAME) or the neuronal NOS (nNOS)-specific inhibitor 7-Nitroindazole (7-NI). Patch-clamp experiments revealed that both the NOS inhibitors (L-NAME or 7-NI) and cGK inhibitors (KT-5823 or Rp-8-Br-cGMP) partially decreased ionic currents induced by 30 nM of ACh, but not in the case of 300 nM of ACh. Our results indicate that NO can be synthesized through the activation of nNOS endogenously and has potentiating effects on the gland secretion, under a physiologically relevant ACh stimulation. When cells were stimulated by an inadequately potent dose of ACh, which caused an excess elevation in [Ca(2+)](i), the cells were desensitized. Therefore, due to NO, gland cells become more sensitive to calcium signaling and are able to maintain electrolyte secretion without desensitization.

Loss of hippocampal neuronal nitric oxide synthase contributes to the stress-related deficit in learning and memory

Nitric oxide (NO) has been involved in many pathophysiological brain processes. However, the exact role of NO in the cognitive deficit associated to chronic stress exposure has not been elucidated. In this study, we investigated the participation of hippocampal NO production and their regulation by protein kinase C (PKC) in the memory impairment induced in mice subjected to chronic mild stress model (CMS). CMS mice showed a poor learning performance in both open field and passive avoidance inhibitory task respect to control mice. Histological studies showed a morphological alteration in the hippocampus of CMS mice. On the other hand, chronic stress induced a diminished NO production by neuronal nitric oxide synthase (nNOS) correlated with an increment in gamma and zeta PKC isoenzymes. Partial restoration of nNOS activity was obtained after PKC activity blockade. NO production by inducible nitric oxide synthase isoform was not detected. The magnitude of oxidative stress, evaluated by reactive oxygen species production, after excitotoxic levels of NMDA was increased in hippocampus of CMS mice. Moreover, ROS formation was higher in the presence of nNOS inhibitor in both control and CMS mice. Finally, treatment of mice with nNOS inhibitors results in behavioural alterations similar to those observed in CMS animals. These findings suggest a novel role for nNOS showing protective activity against insults that trigger tissue toxicity leading to memory impairments.

Mitochondria and neuronal activity

Mitochondria are central for various cellular processes that include ATP production, intracellular Ca(2+) signaling, and generation of reactive oxygen species. Neurons critically depend on mitochondrial function to establish membrane excitability and to execute the complex processes of neurotransmission and plasticity. While much information about mitochondrial properties is available from studies on isolated mitochondria and dissociated cell cultures, less is known about mitochondrial function in intact neurons in brain tissue. However, a detailed description of the interactions between mitochondrial function, energy metabolism, and neuronal activity is crucial for the understanding of the complex physiological behavior of neurons, as well as the pathophysiology of various neurological diseases. The combination of new fluorescence imaging techniques, electrophysiology, and brain slice preparations provides a powerful tool to study mitochondrial function during neuronal activity, with high spatiotemporal resolution. This review summarizes recent findings on mitochondrial Ca(2+) transport, mitochondrial membrane potential (DeltaPsi(m)), and energy metabolism during neuronal activity. We will first discuss interactions of these parameters for experimental stimulation conditions that can be related to the physiological range. We will then describe how mitochondrial and metabolic dysfunction develops during pathological neuronal activity, focusing on temporal lobe epilepsy and its experimental models. The aim is to illustrate that 1) the structure of the mitochondrial compartment is highly dynamic in neurons, 2) there is a fine-tuned coupling between neuronal activity and mitochondrial function, and 3) mitochondria are of central importance for the complex behavior of neurons.

Differential regulation of the neuronal isoform of nitric oxide synthase in the superior colliculus and dorsal lateral geniculate nucleus of the adult rat brain following eye enucleation

Nitric oxide has been shown to play various physiological and pathological roles in the visual system. We studied here the expression of the neuronal isoform of nitric oxide synthase in the rat superior colliculus and in the dorsal lateral geniculate nucleus after unilateral enucleation, by means of immunohistochemistry, immunoblotting, and real-time PCR. Immunohistochemistry revealed an increase of nitric oxide synthase-positive neurons in specific layers of the superior colliculus and in the dorsal lateral geniculate nucleus between 1 and 30 days post-lesion. Immunoblotting analyses confirmed that the neuronal isoform of nitric oxide synthase is upregulated in the superior colliculus and in the dorsal lateral geniculate nucleus after retinal removal. Diaminofluorescein histochemistry suggested that nitric oxide production was increased in both deafferented retinorecipient areas. Our real-time PCR results indicated that nitric oxide synthase transcript levels in the superior colliculus were not significantly altered after monocular enucleation, although an upregulation of the enzyme transcription was detected into the deafferented dorsal lateral geniculate nucleus. These findings indicated that neuronal nitric oxide synthase may undergo different forms of regulation in the adult deafferented visual system.

Nitric oxide-dependent long-term potentiation revealed by real-time imaging of nitric oxide production and neuronal excitation in the dorsal horn of rat spinal cord slices

Nitric oxide (NO) is thought to be involved in the central mechanism of hyperalgesia and allodynia at the spinal level. Recently, we reported that NO played an important role in the induction of long-term potentiation (LTP) of synaptic strength in spinal dorsal horn, which is believed to underlie hyperalgesia and allodynia. In this study, to elucidate the relationship of NO to LTP in spinal dorsal horn, we measured the spatiotemporal distribution of NO signal with the NO-sensitive dye, DAR-4M, and neuronal excitation with the voltage-sensitive dye, RH482, in rat spinal cord slices, elicited by dorsal root stimulation. In superficial dorsal horn, neuronal excitation evoked by C fiber-activating dorsal root stimulation was potentiated for more than 2 h after low-frequency conditioning stimulation (LFS, 240 pulses at 2 Hz for 2 min). In the same slices that exhibited LTP, NO was produced and distributed in the superficial dorsal horn during the delivery of LFS, and the amplitude of LTP and amount of NO production showed close correlation from slice to slice. LTP and production of NO were inhibited in the presence of the NO synthase inhibitors and an inhibitor of heme oxygenase, the synthetic enzyme for carbon monoxide (CO). These results suggest that production and distribution of NO is necessary for the induction of LTP in spinal dorsal horn, and that CO contributes to the LTP induction and NO production by LFS.

The challenge of real-time measurements of nitric oxide release in the brain

Nitric oxide (NO) acts as a signalling molecule in the brain. NO has been implicated in a variety of central functions such as learning, plasticity and neurodegeneration. It is also involved in regulation of autonomic homeostasis at different levels of neuraxis including the nucleus tractus solitarii. In spite of the ample evidence for NO-mediated signalling many aspects of its mechanism of action the brain remain unknown largely due to the difficulties of NO detection in real time coupled with its unique ability to freely cross cellular membranes. Here we give a brief overview of the currently available options for NO detection in the brain (such as electrochemistry, fluorescent indicators, electron-paramagnetic resonance) and consider some of their limitations. We conclude that it would be extremely useful to develop a highly sensitive probe for NO detection with some kind of build-in amplification which would magnify the changes triggered by NO to allow its detection within microdomains of the brain tissue in real time.

Rapid and reactive nitric oxide production by astrocytes in mouse neocortical slices

Nitric oxide (NO), a cellular signaling molecule, is produced in the brain by both neurons and astrocytes. While neurons are capable of rapid release of small amounts of NO serving as neurotransmitter, astrocytic NO production has been demonstrated mainly as a slow reaction to various stress stimuli. Little is known about the role of astrocyte-produced NO. Using the NO indicator 4,5-diaminofluorescein-2 diacetate (DAF-2DA) and acute slices from mouse brain, we distinguished neurons from astrocytes based on their different fluorescence kinetics and pattern, cellular morphology, electrophysiology, and responses to selective nitric oxide synthase (NOS) inhibitors. Typically, astrocytic fluorescence followed neuronal fluorescence with a delay of 1-2 min and was dependent on the inducible NOS isoform (iNOS) activity. Western blot analysis established the presence of functional iNOS in the neocortex. An assay for cell death revealed that most DAF-2DA-positive neurons, but not astrocytes, were damaged. Whole cell recordings from astrocytes confirmed that these cells maintained their membrane potential and passive properties during illumination and afterward. Induction of excitotoxicity by brief application of glutamate triggered an immediate and intense astrocytic response, while high-frequency electrical stimulation failed to do so. The present study demonstrates, for the first time, rapid and massive iNOS-dependent NO production by astrocytes in situ, which appears to be triggered by acute neuronal death. These data may bear important implications for our theoretical understanding and practical management of acute brain insults.

NOSIP and its interacting protein, eNOS, in the rat trachea and lung

Endothelial nitric oxide synthase (eNOS), the major nitric oxide (NO)-generating enzyme of the vasculature, is regulated through multiple interactions with proteins, including caveolin-1, Hsp90, Ca2+-calmodulin, and the recently discovered eNOS-interacting protein, NOSIP. Previous studies indicate that NOSIP may contribute to the intricate regulation of eNOS activity and availability. Because eNOS has been shown to be abundantly expressed in the airways, we determined the expression and cellular localization of NOSIP in rat trachea and lung by RT-PCR and immunohistochemistry and examined the interaction of NOSIP with eNOS in lung by coimmunoprecipitation. In tracheal epithelium and lung, NOSIP mRNA expression was prevalent, as shown by RT-PCR, and the corresponding protein interacted with eNOS, as demonstrated by coimmunoprecipitation. Using immunohistochemistry, we found both NOSIP and eNOS immunoreactivity in ciliated epithelial cells of trachea and bronchi, while Clara cells showed immunoreactivity for NOSIP only. NOSIP and eNOS were present in vascular and bronchial smooth muscle cells of large arteries and airways, whereas endothelial cells, as well as bronchiolar and arteriolar smooth muscle cells, exclusively stained for NOSIP. Our results point to functional role(s) of NOSIP in the control of airway and vascular diameter, mucosal secretion, NO synthesis in ciliated epithelium, and, therefore, of mucociliary and bronchial function.

Performance of diamino fluorophores for the localization of sources and targets of nitric oxide

An emergent approach to the detection of nitric oxide (NO) in tissues relies on the use of fluorescence probes that are activated by products of NO autoxidation. Here we explore the performance of the widely used NO probe 4,5-diaminofluorescein diacetate (DAF-2 DA) for the localization of sources of NO in rat aortic tissue, either from endogenous NO synthesis or from chemically or photolytically released NO from targets of nitrosation/nitrosylation. Of importance toward understanding the performance of this probe in tissues is the finding that, with incubation conditions commonly used in the literature (10 microM DAF-2 DA), intracellular DAF-2 accumulates to concentrations that approach the millimolar range. Whereas such high probe concentrations do not interfere with NO release or signaling, they help to clarify why DAF-2 nitrosation is possible in the presence of endogenous nitrosation scavengers (e.g., ascorbate and glutathione). The gain attained with such elevated concentrations is, however, mitigated by associated high levels of background autofluorescence from the probe. This, together with tissue autofluorescence, limits the sensitivity of the probe to low-micromolar levels of accumulated DAF-2 triazole (DAF-2 T), the activated form of the probe, which is higher than the concentrations of most endogenous nitrosation/nitrosylation products found in tissues. We further show that the compartmentalization of DAF-2 around elastic fibers further limits its potential to characterize the site of NO production at the subcellular level. Moreover, we find that reaction of DAF-2 with HgCl(2) and other commonly employed reagents is associated with spectral changes that may be misinterpreted as NO signals. Finally, UV illumination can lead to high levels of nitrosating species that interfere with NO detection from enzymatic sources. These findings indicate that while DAF-2 may still represent an important tool for the localization of NO synthesis, provided important pitfalls and limitations are taken into consideration, it is not suited for the detection of basally generated nitrosation/nitrosylation products.

Cellular sources, targets and actions of constitutive nitric oxide in the magnocellular neurosecretory system of the rat

Nitric oxide (NO) is a key activity-dependent modulator of the magnocellular neurosecretory system (MNS) during conditions of high hormonal demand. In addition, recent studies support the presence of a functional constitutive NO tone. The aim of this study was to identify the cellular sources, targets, signalling mechanisms and functional relevance of constitutive NO production within the supraoptic nucleus (SON). Direct visualization of intracellular NO, along with neuronal nitric oxide synthase (nNOS) and cGMP immunohistochemistry, was used to study the cellular sources and targets of NO within the SON, respectively. Our results support the presence of a strong NO basal tone within the SON, and indicate that vasopressin (VP) neurones constitute the major neuronal source and target of basal NO. NO induced-fluorescence and cGMP immunoreactivity (cGMPir) were also found in the glia and microvasculature of the SON, suggesting that they contribute as sources/targets of NO within the SON. cGMPir was also found in association with glutamic acid decarboxylase 67 (GAD67)- and vesicular glutamate transporter 2 (VGLUT2)-positive terminals. Glutamate, acting on NMDA and possibly AMPA receptors, was found to be an important neurotransmitter driving basal NO production within the SON. Finally, electrophysiological recordings obtained from SON neurones in a slice preparation indicated that constitutive NO efficiently restrains ongoing firing activity of these neurones. Furthermore, phasically active (putative VP) and continuously firing neurones appeared to be influenced by NO originating from different sources. The potential roles for basal NO as an autocrine signalling molecule, and one that bridges neuronal-glial-vascular interactions within the MNS are discussed.

Increased expression of nitric oxide synthase in visual structures of the chick brain after retinal removal

Nitric oxide (NO) seems to act as a retrograde messenger in the establishment and refinement of synaptic connections during development and in neural plasticity processes in adult life. Previous studies have shown that the expression of NO synthase (NOS) in the optic tectum of developing chicks is regulated by the retinal innervation. The aim of this study was to observe the effects of unilateral retinal lesions upon the expression of NOS in central visual areas of the adult chick brain. After different survival times (1-30 days), the chick brains were submitted to immunohistochemical, immunoblotting, and NO imaging procedures to evaluate NOS expression and activity. Our results indicate that NOS expression in visual areas is also regulated by retinal innervation in the adult chick. However, differently from the case in the developing animal, the deafferentation seems to generate an increase of the NOS expression in retinorecipient visual areas. Our results suggest that NOS expression in visual structures of the adult chick brain may be down-regulated by the retinal innervation. Alternatively, the increase of NOS expression observed after retinal removal could be an indicative of a role of the NO system in plasticity processes.

Direct evidence that induced nitric oxide production in hepatocytes prevents liver damage during lipopolysaccharide tolerance in rats

BACKGROUND

The role of nitric oxide (NO) in lipopolysaccharide (LPS) tolerance in the liver has been investigated in a number of previous studies, but it is still not clear whether NO is cytotoxic or cytoprotective. The aims of this study were to investigate whether low-dose LPS (LLPS)-induced hepatic production of NO is beneficial and to clarify the origins of cytoprotective NO-producing cells in the liver during LPS tolerance.

MATERIALS AND METHODS

Male Wistar rats received saline or LLPS intraperitoneally (i.p.; 0.01-1000 microg/kg) followed by a high dose of LPS (HLPS, 5 mg/kg) at various time intervals (4-16 h). NG-nitro-L-arginine methyl ester (L-NAME) was used to investigate the effects of inhibition of NOS. 4,5-Diaminofluorescein (DAF-2) was used to identify NO-producing cells in isolated liver cells in vitro. At various time points (4-16 h) after saline or LLPS (1 microg/kg, i.p.) injection, hepatocytes and Kupffer cells were isolated, incubated in 7 microm DAF-2 diacetate, and perfused with Krebs solution. Illumination at 495 nm revealed DAF-fluorescence (515 nm) in isolated cells under confocal laser fluorescence microscopy. The NO production in hepatocytes and Kupffer cells was assessed by the number of labeled cells per 1000 cells or per 100 cells, respectively.

RESULTS

Pretreatment with LLPS (0.1-100 microg/kg) resulted in a significant reduction (maximal at 8 h) of the HLPS-induced liver damage. L-NAME abolished the LLPS-induced protection. The NO production in hepatocytes was significantly increased and reached a maximum of 84% of all cells 8 h after LLPS administration. By contrast, the NO production in Kupffer cells remained constant at 95%, even following preinjection of LLPS.

CONCLUSION

LLPS-induced NO in hepatocytes, but not in Kupffer cells, exhibits cytoprotective effects on HLPS-induced liver damage, suggesting that NO has a beneficial role in the induction of the early phase of LPS tolerance.

Impaired nitric oxide production of the myenteric plexus in colitis detected by a new bioimaging system

Direct measurement of the release of nitric oxide (NO) from the myenteric plexus has been extremely difficult to date, due to the lack of suitable methodologies. We have developed a new bioimaging system to visualize the nitrergic neurons of the myenteric plexus and investigated whether NO production is impaired in dextran sulfate sodium (DSS)-induced colitis. Longitudinal muscle layers with the myenteric plexus intact were obtained from the rat colon and were incubated with 4,5-diaminofluorescein-2-diacetate (DAF-2DA) (7 microm) for 30 min. Illumination at 450-490 nm revealed the fluorescence in the myenteric plexus. Confocal laser microscopy and three-dimensional reconstruction techniques were used to quantify the changes in the amount of NO production by the myenteric plexus. Fluorescent double-labeled immunostaining for nNOS was performed to confirm the colocalization of nNOS in 4,5-diaminofluorescein (DAF-2)-positive cells. DAF-2 fluorescence was abolished by pretreatment with N(G)-nitro-l-arginine methyl ester (l-NAME; a nonselective NOS inhibitor), 1-(2-trifluoromethylphenyl) imidazole (TRIM; a selective neuronal NOS inhibitor), and omega-conotoxin GVIA (an N-type Ca(2+) channel blocker), but not by nifedipine (an l-type Ca(2+) channel blocker). Fluorescent double-labeled immunostaining showed that DAF-2-positive cells colocalized with nNOS-positive cells. Oral administration of 5% DSS for 7 days induced distal colitis and the number of DAF-2-positive neurons were significantly reduced to 55 +/- 17% of control. DAF-2 offers a sensitive indicator for visualizing production of NO with high spatial resolution. This new system may contribute to the study of the pathophysiological role of the nitrergic pathway in the gastrointestinal tract.

Nitric oxide imaging in living neuronal tissues using fluorescent probes

Nitric oxide (NO) is a major modulator of neural functions. Since NO is a gaseous molecule with very short half-life, the spatial distribution of NO and its relationship to neuronal activity are difficult to resolve. Non-invasive and direct visualization of NO in neuronal tissues had been hampered by the lack of a suitable method to identify NO directly. A fluorescent indicator, which directly detects NO under physiological conditions, would be advantageous. Several indicators for direct detection of NO have been developed, which react with NO by forming a fluorescent complex. However, some of these dyes have cytotoxic properties or have been found to be rather unspecific under certain conditions. Fortunately, some of the indicators, which change their fluorescent pattern in the presence of NO, appear to be promising for the visualization of NO. Since little is known about the spatial spread and the temporal aspects of NO release after a specific stimulus, the use of the specific and non-toxic fluorescent NO indicators could provide a potentially powerful tool to study these aspects of NO release in neuronal tissues in vitro and in vivo. Such measurements, especially in combination with electrophysiological recordings, would greatly further NO research. In addition, based on their fluorescent pattern, these NO-sensitive dyes can be distinguished from the calcium-sensitive dye Fura-2, which allows NO-imaging together with calcium-imaging. This article summarizes recent advances and current trends in the visualization of NO in living neuronal tissues.

Nitric Oxide Production Within Rat Urothelial Cells

PURPOSE

Recent studies have suggested that nitric oxide (NO) synthase (NOS) may be localized in the urothelium of the proximal part of the mammalian ureter. We investigated endogenous NO production in the proximal half of the rat ureter, localized its cellular source, characterized the NOS isoforms involved and assessed the impact of NO on ureteral motility.

MATERIALS AND METHODS

Direct detection of NO production was performed on primary cultures of living rat ureteral cells with the fluorescent indicator diaminofluorescein. Cultures were incubated with the NO precursor L-arginine or the NOS inhibitors L-NAME (N-nitro-L-arginine-methyl ester) and 1400W. NOS expression was determined by immunofluorescence and Western blot analysis. The functional effects of NO donors were assessed on isolated ureters.

RESULTS

Significant basal NO production was demonstrated by the high fluorescence level detected in diaminofluorescein treated cell cultures. NO production was strictly limited to urothelial cells since no fluorescence was seen in smooth muscle cells. Pretreatment with L-NAME or 1400W resulted in a significant decrease in fluorescence. Constitutive and inducible NOS isoforms were detected in urothelial cultured cells and in lysates of the urothelial layer. NO donors inhibited in a concentration dependent manner the agonist induced contractile activity of isolated ureters.

CONCLUSIONS

These results suggest that NO production stems from the urothelium and the NO pathway inhibits contractile activity in the proximal half of the rat ureter. Hence, the nitrergic pathway may be an important target for drugs producing relaxation of the mammalian ureter.

Proteasome activation and nNOS down-regulation in neuroblastoma cells expressing a Cu,Zn superoxide dismutase mutant involved in familial ALS

Reactive oxygen and nitrogen species have emerged as predominant effectors of neurodegeneration. We demonstrated that expression of the fully active G93A Cu,Zn superoxide dismutase mutant in neuroblastoma cells is associated with an increased level of oxidatively modified proteins, in terms of carbonylated residues. A parallel increase in proteasome activity was detected and this was mandatory in order to assure cell viability. In fact, proteasome inhibition by lactacystin or MG132 resulted in programmed cell death. Nitrosative stress was not involved in the oxidative unbalance, as a decrease in neuronal nitric oxide production and down-regulation of neuronal nitric oxide synthase (nNOS) level were detected. The nNOS down-regulation was correlated to increased proteolytic degradation by proteasome, because comparable levels of nNOS were detected in G93A and parental cells upon treatment with lactacystin. The altered rate of proteolysis observed in G93A cells was specific for nNOS as Cu,Zn superoxide dismutase (Cu,Zn SOD) degradation by proteasome was influenced neither by its mutation nor by increased proteasome activity. Treatment with the antioxidant 5,5'-dimethyl-1-pyrroline N-oxide resulted in inhibition of protein oxidation and decrease in proteasome activity to the basal levels. Overall these results confirm the pro-oxidant activity of G93A Cu,Zn SOD mutant and, at the same time, suggest a cross-talk between reactive oxygen and nitrogen species via the proteasome pathway.

L-arginyl-3,4-spermidine is neuroprotective in several in vitro models of neurodegeneration and in vivo ischaemia without suppressing synaptic transmission

1. Stroke is the third most common cause of death in the world, and there is a clear need to develop new therapeutics for the stroke victim. To address this need, we generated a combinatorial library of polyamine compounds based on sFTX-3.3 toxin from which L-Arginyl-3,4-Spermidine (L-Arg-3,4) emerged as a lead neuroprotective compound. In the present study, we have extended earlier results to examine the compound's neuroprotective actions in greater detail. 2. In an in vitro ischaemia model, L-Arg-3,4 significantly reduced CA1 cell death when administered prior to induction of 60 min of ischaemia as well as when administered immediately after ischaemia. Surprisingly, L-Arg-3,4 continued to prevent cell death significantly when administration was delayed for as long as 60 min after ischaemia. 3. L-Arg-3,4 significantly reduced cell death in excitotoxicity models mediated by glutamate, NMDA, AMPA, or kainate. Unlike glutamate receptor antagonists, 300 microM L-Arg-3,4 did not suppress synaptic transmission as measured by evoked responses in acute hippocampal slices. 4. L-Arg-3,4 provided significant protection, in vitro, in a superoxide mediated injury model and prevented an increase of superoxide production after AMPA or NMDA stimulation. It also decreased nitric oxide production after in vitro ischaemia and NMDA stimulation, but did so without inhibiting nitric oxide synthase directly. 5. Furthermore, L-Arg-3,4 was significantly neuroprotective in an in vivo model of global forebrain ischaemia, without any apparent neurological side-effects. 6. Taken together, these results demonstrate that L-Arg-3,4 is protective in several models of neurodegeneration and may have potential as a new therapeutic compound for the treatment of stroke, trauma, and other neurodegenerative diseases.

Nitric oxide modulates low-Mg2+-induced epileptiform activity in rat hippocampal-entorhinal cortex slices

The production of nitric oxide (NO) during low-Mg2+-induced epileptiform activity in rat hippocampal-entorhinal cortex slices was investigated by real-time monitoring using 1,2-diaminoanthraquinone (DAQ). NO reacts with the aromatic amino groups of DAQ at neutral pH and in the presence of oxygen to form the fluorescence product 1H-anthra-[1,2d]-[1,2,3]triazole-6,11-dione (ATD). The DAQ-induced formation of ATD required NO and was insensitive to radical oxygen species. Removal of Mg2+ ions from the artificial cerebrospinal fluid (ACSF) induced a significant elevation in the ATD fluorescence signal. The application of L-arginine (2 mM), a substrate of nitric oxide synthase (NOS), caused a comparable increase in the ATD fluorescence signal. Furthermore, ATD signal increase induced either by low-Mg2+ ACSF or by L-arginine was sensitive to N-nitro-L-arginine methyl ester (L-NAME), a NOS inhibitor. The application of L-NAME (200 microM) caused a complete blockade of low-Mg2+-induced epileptiform activity. Under this condition, increasing NO concentration by addition of the NO donor S-nitroso-N-acetylpenicillamine (200 microM) reinduced the epileptiform activity. It has been concluded that onset and maintenance of low-Mg2+-induced spontaneous epileptiform activity are modulated by NO concentration. Further NO imaging studies may help to elucidate the role of NO in detail and may bring to light new means for epilepsy therapy.

Detection of reactive oxygen and nitrogen species in tissues using redox-sensitive fluorescent probes

The take-home message of this chapter is that the fluorescent probes for ROS and RNS have great potential in improving our understanding of redox behavior within cells and tissues. However, data obtained from studies using these probes must be expressed in the context of the limitations of the chemistry of the probes in the cellular microenvironment, which may change under different conditions, such as cell stress or injury. In most cases, as suggested, results should be described in a general context of reflecting an increase in oxidizing reactions within the cell and not as a quantitative measure of the production of a specific oxidant species. It is highly recommended that results be verified, when possible, with alternative fluorescent probes or preferably using alternative methods, such as electron spin resonance or other newly emerging technology.

Oxidative stress in glial cultures: detection by DAF-2 fluorescence used as a tool to measure peroxynitrite rather than nitric oxide

4,5-diaminofluorescein diacetate (DAF-2DA) is widely used as a fluorescent probe to detect endogenously produced nitric oxide (NO). Recent reports that refer to the high sensitivity of DAF-2 toward NO prompted us to test its efficiency and specificity in a mixed murine primary glial culture model, in which the NO-synthesizing enzyme inducible nitric oxide synthase (iNOS) is expressed by stimulation with lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma). Cultures were loaded with DAF-2DA and the fluorescence was measured using confocal microscopy. NO production in the cultures was determined using the ozone/chemiluminescence technique. Due to the extremely high photosensitivity of DAF-2, low laser intensities were used to avoid artifacts. No difference in DAF-2 fluorescence was observed in NO-producing cultures compared to control cultures, whereas the NO/peroxynitrite-sensitive dye 2,7-dihydrodichlorofluorescein (DCF) showed a significant fluorescence increase specifically in microglia cells. A detectable gain in fluorescence was seen when NO-containing buffer was added to the DAF-2DA-loaded cells with a minimum NO concentration at 7.7 microM. An additional gain of DAF-2 fluorescence was obtained when the cells were depleted of glutathione (GSH) with L-buthionine S,R-sulfoximine (BSO). Hence, we monitored the change in DAF-2 fluorescence intensity in the presence of NO and O(-*)(2) in a cell-free solution. The fluorescence due to NO was indeed larger when O(-*)(2) was added, implying a higher sensitivity of DAF-2 for peroxynitrite. Nevertheless, our results also indicate that measurement of DCF fluorescence is a better tool for monitoring intracellular changes in the levels of NO and/or peroxynitrite than DAF-2.

A fluorescence-based method for measuring nitric oxide in extracts of skeletal muscle

We describe here a fluorescence assay for nitric oxide synthase activity in skeletal muscle based on a new indicator, 4,5-diaminofluorescein (DAF-2). The rapid and irreversible binding of DAF-2 to oxidized NO allows real-time measurement of NO production. The method is safer and more convenient than the usual citrulline radioassay and can be used with crude muscle extracts. Rabbit fast tibialis anterior (TA) muscle had a nitric oxide synthase (NOS) activity of 44.3 +/- 3.5 pmol/min/mg muscle. Addition of NOS blocker N(G)-allyl-L-arginine reduced this activity by 43%. Slow soleus muscle displayed NOS activity of 7.3 +/- 2.5 pmol/min/mg muscle, 16% that of the TA muscle. Continuous stimulation of TA muscle at 10 Hz for 3 weeks reduced NOS activity by 47% to an intermediate value consistent with the associated conversion of the muscle phenotype from fast to slow.

Orthogonality of calcium concentration and ability of 4,5-diaminofluorescein to detect NO

We have developed diaminofluoresceins (DAFs) and diaminorhodamines as fluorescent indicators for NO based on the specific reaction of the aromatic vicinal diamines with NO. Among them, 4,5-diaminofluorescein (DAF-2) is widely used for real-time biological imaging of NO in cultured cells or tissues by many researchers. Contrary to a recent report of divalent cation sensitivity and photoactivation of DAF-2 (Broillet, M. C., Randin, O., and Chatton, J. Y. (2001) FEBS Lett. 491, 227-232), our study using NO gas itself reveals that the reaction of DAF-2 and NO is completely independent of Ca2+ and Mg2+ at physiological concentrations. Ca2+ enhances not the conversion of DAF-2 into its fluorescent product (DAF-2 triazole) but the release of NO from NO donors. Therefore it is concluded that DAF-2 can provide reliable information on NO production in biological systems regardless of the dynamic changes of Ca2+ concentration.

Characterization of basal nitric oxide production in living cells

Nitric oxide (NO) is an important modulator of immune, endocrine and neuronal functions; however, measuring physiological levels of NO in cell cultures is generally difficult because of the lack of suitable methodologies. We have selected three cell lines from different origins: the neuroblastoma-derived Neuro2A (N2A), the cholinergic SN56 and the non-neuronal COS-1. We first demonstrated the presence of NADPH-diaphoretic activity, a potential marker of the NO-synthesizing (NOS) enzyme. By immunocytochemistry, using specific antibodies for each NOS subtype, we observed that subtype I was present in all cell lines and that subtype II was present in COS-1 and N2A cell lines. The presence of these NOS subtypes was further verified by Western blot analysis. Control cells treated with DAF-2 DA exhibited significant fluorescent levels corresponding to basal NO production. The subcellular distribution of the synthesizing enzyme was consistent with the NO-fluorescence signal; whereas, fixation affected the subcellular pattern of NO fluorescence signal. Addition of NOS inhibitors or NO scavengers to the incubation medium reduced the intensity of the NO fluorescence signal in a concentration-dependent manner. Conversely, increasing concentrations of a NO donor, or incident light, increased the fluorescence intensity. Our observation of NO production and distribution using the DAF-2 method has a direct impact on studies using these cell lines.

Distinction between nitrosating mechanisms within human cells and aqueous solution

The quintessential nitrosating species produced during NO autoxidation is N(2)O(3). Nitrosation of amine, thiol, and hydroxyl residues can modulate critical cell functions. The biological mechanisms that control reactivity of nitrogen oxide species formed during autoxidation of nano- to micromolar levels of NO were examined using the synthetic donor NaEt(2)NN(O)NO (DEA/NO), human tumor cells, and 4,5-diaminofluorescein (DAF). Both the disappearance of NO and formation of nitrosated product from DAF in aerobic aqueous buffer followed second order processes; however, consumption of NO and nitrosation within intact cells were exponential. An optimal ratio of DEA/NO and 2-phenyl-4,4,5,5-tetramethylimidazole-1-oxyl 3-oxide (PTIO) was used to form N(2)O(3) through the intermediacy of NO(2). This route was found to be most reflective of the nitrosative mechanism within intact cells and was distinct from the process that occurred during autoxidation of NO in aqueous media. Manipulation of the endogenous scavengers ascorbate and glutathione indicated that the location, affinity, and concentration of these substances were key determinants in dictating nitrosative susceptibility of molecular targets. Taken together, these findings suggest that the functional effects of nitrosation may be organized to occur within discrete domains or compartments. Nitrosative stress may develop when scavengers are depleted and this architecture becomes compromised. Although NO(2) was not a component of aqueous NO autoxidation, the results suggest that the intermediacy of this species may be a significant factor in the advent of either nitrosation or oxidation chemistry in biological systems.

Nitric oxide generation by isolated descending vasa recta

Nitric oxide (NO) generation by the outer medullary descending vasa recta (OMDVR) was measured with the fluorescent dye 4,5-diaminofluoroscein (DAF-2) during 30-min incubations. Addition of 0.1 or 1.0 mM L-arginine to the incubation buffer increased the DAF-2 signal by 8.7 and 13.6% (P = 0.08 and P < 0.05), respectively. Compared with L-arginine alone (0.1 mM), bradykinin (BK; 1 x 10(-7) M) enhanced the DAF-2 signal by 11.1% (P < 0.05). The NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (0.1 mM) reversed the BK-stimulated NO generation as measured with either DAF-2 or by the oxidation of Fe(2+) hemoglobin. Using 1 mM 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (tempol), a cell-permeant superoxide dismutase mimetic, we tested whether reduction of superoxide anion increases intracellular NO. Tempol increased DAF-2 fluorescence by 12 and 23.3%, respectively, over BK-stimulated or control vessels. Tempol also vasodilated ANG II (1 x 10(-8) M)-preconstricted OMDVR (P < 0.05). We conclude that NO generation by isolated OMDVR can be increased by L-arginine, that the endothelium-dependent vasodilator BK enhances NO production, and that NO consumption by superoxide plays a role in the determination of cellular NO concentrations.

Increased nitric oxide production in nasal epithelial cells from allergic patients--RT-PCR analysis and direct imaging by a fluorescence indicator: DAF-2 DA

BACKGROUND

Nitric oxide (NO) is believed to participate in the regulation of airway clearance and non-specific cellular immunity. Recent studies have suggested that airway epithelial cells of allergic and non-allergic individuals may differ in their ability to produce this molecule.

OBJECTIVE

The aim of this study was to detect the difference in NO production in human nasal epithelial cells between normal subjects and patients with perennial allergic rhinitis (AR), and to assess the relationship between the expression of nitric oxide synthase (NOS) isoforms and the severity of the disease.

METHODS

Nasal epithelial cells were obtained from the inferior turbinate. The expression of mRNAs encoding constitutive endothelial NOS (eNOS) and inducible NOS (iNOS) was studied by reverse transcription-polymerase chain reaction (RT-PCR). Direct NO production in living cells was visualized and quantified by a fluorescent indicator, DAF-2 DA.

RESULTS

RT-PCR analysis demonstrated that AR patients with a RAST score of 5 or 6 showed significant increases in the levels of iNOS mRNA and slight reductions in those of eNOS mRNA. Patients with a RAST score of 2-4 also revealed the same tendency however, the difference was not significant. DAF-2 DA imaging demonstrated that epithelial cells, especially the ciliated cells, produced a larger amount of NO than non-epithelial inflammatory cells. Preincubation with L-NAME resulted in an approximate 40% decrease in both groups.

CONCLUSION

These results directly indicate that nasal epithelial cells of AR patients overall produce higher levels of NO through the concomitant expression of different NOS isoforms. Continuous NO production by the epithelial cells in normal subjects further support the hypothesis that NO derived from epithelium may play dual roles in the regulation of nasal airway clearance and in the host defense. In addition, the use of DAF-2 DA provides a reliable method to visualize and quantify the direct NO production of living cells.

Photoactivation and calcium sensitivity of the fluorescent NO indicator 4,5-diaminofluorescein (DAF-2): implications for cellular NO imaging

The fluorescent indicator of nitric oxide (NO), 4,5-diaminofluorescein (DAF-2), and its membrane-permeable derivative (DAF-2 diacetate) have been recently developed to perform real-time biological imaging of NO. In this study, we show that DAF-2 is strongly influenced by factors other than the concentration of NO itself. Using measurements with a fluorimeter as well as fluorescence microscopy, we found that the divalent cation concentration in the medium, as well as the incident light, strongly affects the ability of DAF-2 to detect NO. Calcium, in particular, enhanced the signal detection of NO released by NO donors by up to 200 times. With multiple and longer exposures to light, no bleaching of the dye was observed but, instead, a potentiation of the fluorescence response could be measured. While these two properties will affect the use and interpretation of the hitherto acquired data with this fluorescent compound, they may also open up new possibilities for its application.

Determination and bioimaging method for nitric oxide in biological specimens by diaminofluorescein fluorometry

A simple and sensitive assay and a cellular bioimaging method for nitric oxide (NO) were developed using a novel diaminofluorescein DAF-FM and its diacetate. DAF-FM is converted via an NO-specific mechanism to an intensely fluorescent triazole derivative. For the measurement of NO, the triazole derivative of DAF-FM was determined by reversed-phase high-performance liquid chromatography with fluorescence detection. In the presence of 1 microM DAF-FM, the concentrations of NOR-1, an NO donor, in the range of 2-200 nM were linearly related to the fluorescence intensity. This sensitive NO assay enabled us to detect the spontaneous and substance P-induced NO release from isolated porcine coronary arteries, both of which were dependent entirely on the NO synthase activity in vascular endothelial cells. We also obtained fluorescence images of cultured smooth muscle cells of the rat urinary bladder after loading with DAF-FM diacetate. In the cells pretreated with cytokines, the fluorescence intensity increased with time after DAF-FM loading. This increase in the fluorescence intensity was blocked by prior treatment of the muscle cells with an NO synthase inhibitor, N(G)-nitro-l-arginine methyl ester. Therefore, the present novel diaminofluorescein fluorometry should be useful not only for sensitive NO assay, but also for NO imaging in a variety of biological specimens.

Fluorescent imaging of nitric oxide production in neuronal varicosities associated with intraparenchymal arterioles in rat hippocampal slices

The fluorescent indicator 4,5-diaminofluorescein (DAF-2) has been used to investigate the production of nitric oxide in the vicinity of intraparenchymal cerebral blood vessels. Slices of rat hippocampus 300-350 microm thick, were loaded with 5 microM DAF-2 diacetate. On exposure to light of 450-490 nm wavelength, point sources of fluorescence, 1.8+/-0.2 microm in diameter (mean+/-SEM), were observed in close apposition to the outer surface of the vascular smooth muscle wall of 10/15 arterioles. In fixed slices, resectioned and processed for nicotinamide adenine dinucleotide phosphate-dependent diaphorase, stained varicose fibres were also seen in close association with the smooth muscle wall of small arterioles. These findings suggest that tonic activity in perivascular nitrergic nerve fibres lying in close proximity to intraparenchymal microvessels may be a source of dilator tone within the parenchyma.

Direct evidence of nitric oxide presence within mitochondria

Nitric oxide (NO) has been implicated in the modulation of mitochondrial respiration, membrane potential, and subsequently in apoptosis. Although the presence of a mitochondrial NO synthase (mtNOS) has been described, there is no direct evidence in vivo of the presence of NO within mitochondria. It was the aim of this study to demonstrate the in vivo production of NO within mitochondria. Using the novel fluorometric NO detection system, 4,5-diaminofluorescein diacetate (DAF-2/DA), we observed the presence of NO production in PC12 and COS-1 cells by conventional and confocal fluorescence microscopy. Part of the overall NO signal was colocalized within a subpopulation of mitochondria, labeled with the potential-dependent probe MitoTracker red. These findings demonstrate for the first time that the subcellular distribution of NO production is consistent with the presence of a mitochondrial NOS. Our results provide a new tool to directly study the modulatory role of NO in mitochondrial respiration and membrane potential, in vivo.