Dynamics of nitric oxide during simulated ischaemia-reperfusion in rat striatal slices measured using an intrinsic biosensor, soluble guanylyl cyclase

Pretreatment with 2-(4-methoxyphenyl)ethyl-2-acetamido-2-deoxy-β-D-pyranoside attenuates cerebral ischemia/reperfusion-induced injury in vitro and in vivo

Salidroside, extracted from the root of Rhodiola rosea L, is known for its pharmacological properties, in particular its neuroprotective effects. 2-(4-Methoxyphenyl) ethyl-2-acetamido-2-deoxy-β-D-pyranoside (GlcNAc-Sal), an analog of salidroside, was recently synthesized and shown to possess neuroprotective properties. The purpose of the current study was to investigate the neuroprotective effects of GlcNAc-Sal against oxygen-glucose deprivation-reperfusion (OGD-R)-induced neurotoxicity in vitro and global cerebral ischemia-reperfusion (GCI-R) injury in vivo. Cell viability tests and Hoechst 33342 staining confirmed that GlcNAc-Sal pretreatment markedly attenuated OGD-R induced apoptotic cell death in immortalized mouse hippocampal HT22 cells. Western blot, immunofluorescence and PCR analyses revealed that GlcNAc-Sal pretreatment restored the balance of pro- and anti-apoptotic proteins and inhibited the activation of caspase-3 and PARP induced by OGD-R treatment. Further analyses showed that GlcNAc-Sal pretreatment antagonized reactive oxygen species (ROS) generation, iNOS-derived NO production and NO-related apoptotic cell death during OGD-R stimulation. GCI-R was induced by bilateral common carotid artery occlusion (BCCAO) and reperfusion in mice in vivo. Western blot analysis showed that GlcNAc-Sal pretreatment decreased the expression of caspase-3 and increased the expression of Bcl-2 (B-cell lymphoma 2)/Bax (Bcl-2-associated X protein) induced by GCI-R treatment. Our findings suggest that GlcNAc-Sal pretreatment prevents brain ischemia reperfusion injury by the direct or indirect suppression of cell apoptosis and GlcNAc-Sal could be developed as a broad-spectrum agent for the prevention and/or treatment of cerebral ischemic injury.

Picomolar nitric oxide signals from central neurons recorded using ultrasensitive detector cells

Nitric oxide (NO) is a widespread signaling molecule with potentially multifarious actions of relevance to health and disease. A fundamental determinant of how it acts is its concentration, but there remains a lack of coherent information on the patterns of NO release from its sources, such as neurons or endothelial cells, in either normal or pathological conditions. We have used detector cells having the highest recorded NO sensitivity to monitor NO release from brain tissue quantitatively and in real time. Stimulation of NMDA receptors, which are coupled to activation of neuronal NO synthase, routinely generated NO signals from neurons in cerebellar slices. The average computed peak NO concentrations varied across the anatomical layers of the cerebellum, from 12 to 130 pm. The mean value found in the hippocampus was 200 pm. Much variation in the amplitudes recorded by individual detector cells was observed, this being attributable to their location at variable distances from the NO sources. From fits to the data, the NO concentrations at the source surfaces were 120 pm to 1.4 nm, and the underlying rates of NO generation were 36–350 nm/s, depending on area. Our measurements are 4–5 orders of magnitude lower than reported by some electrode recordings in cerebellum or hippocampus. In return, they establish coherence between the NO concentrations able to elicit physiological responses in target cells through guanylyl cyclase-linked NO receptors, the concentrations that neuronal NO synthase is predicted to generate locally, and the concentrations that neurons actually produce.

Nitric oxide alters GABAergic synaptic transmission in cultured hippocampal neurons

Nitric oxide (NO) production increases during hypoxia/ischemia-reperfusion in the immature brain and is associated with neurotoxicity. NO at physiologic concentrations has been shown to modulate GABAergic (gamma-aminobutyric acid) synaptic transmission in the adult brain. However, the effects of neurotoxic concentrations of NO (relevant to hypoxia-ischemia) on GABAergic synaptic transmission remain unknown. The present study tests the hypothesis that nNOS is expressed at GABAergic synapses and that exposure to neurotoxic concentrations of NO results in enhanced GABAergic synaptic transmission in cultured hippocampal neurons (days-in-vitro 10-14) prepared from fetal rats. Using double immunocytochemistry techniques, we were able to demonstrate that nNOS is co-localized to both presynaptic and postsynaptic markers of GABAergic synapses. The effects of NO on GABAergic synaptic transmission were then studied using whole cell patch-clamp electrophysiology. Spontaneous and miniature inhibitory postsynaptic currents (sIPSCS and mIPSCs) were recorded prior to and after exposure to 250 microM of the NO donor diethyleneamine/nitric oxide adduct (DETA-NO). Exposure to DETA-NO resulted in increased sIPSCs and mIPSCs frequency, indicating that neurotoxic concentrations of NO enhance GABAergic synaptic transmission in cultured hippocampal neurons. Because GABA synapses appear to be excitatory in the immature brain, this effect may contribute to overall enhanced synaptic transmission and hyperexcitability. We speculate that NO represents one of the mechanisms by which hypoxia-ischemia increases seizure susceptibility in the immature brain.

What is the real physiological NO concentration in vivo?

Clarity about the nitric oxide (NO) concentrations existing physiologically is essential for developing a quantitative understanding of NO signalling, for performing experiments with NO that emulate reality, and for knowing whether or not NO concentrations become abnormal in disease states. A decade ago, a value of about 1 μM seemed reasonable based on early electrode measurements and a provisional estimate of the potency of NO for its guanylyl cyclase-coupled receptors, which mediate physiological NO signal transduction. Since then, numerous efforts to measure NO concentrations directly using electrodes in cells and tissues have yielded an irreconcilably large spread of values. In compensation, data from several alternative approaches have now converged to provide a more coherent picture. These approaches include the quantitative analysis of NO-activated guanylyl cyclase, computer modelling based on the type, activity and amount of NO synthase enzyme contained in cells, the use of novel biosensors to monitor NO release from single endothelial cells and neurones, and the use of guanylyl cyclase as an endogenous NO biosensor in tissue subjected to a variety of challenges. All these independent lines of evidence suggest the physiological NO concentration range to be 100 pM (or below) up to ∼5 nM, orders of magnitude lower than was once thought.

Concepts of neural nitric oxide-mediated transmission

As a chemical transmitter in the mammalian central nervous system, nitric oxide (NO) is still thought a bit of an oddity, yet this role extends back to the beginnings of the evolution of the nervous system, predating many of the more familiar neurotransmitters. During the 20 years since it became known, evidence has accumulated for NO subserving an increasing number of functions in the mammalian central nervous system, as anticipated from the wide distribution of its synthetic and signal transduction machinery within it. This review attempts to probe beneath those functions and consider the cellular and molecular mechanisms through which NO evokes short- and long-term modifications in neural performance. With any transmitter, understanding its receptors is vital for decoding the language of communication. The receptor proteins specialised to detect NO are coupled to cGMP formation and provide an astonishing degree of amplification of even brief, low amplitude NO signals. Emphasis is given to the diverse ways in which NO receptor activation initiates changes in neuronal excitability and synaptic strength by acting at pre- and/or postsynaptic locations. Signalling to non-neuronal cells and an unexpected line of communication between endothelial cells and brain cells are also covered. Viewed from a mechanistic perspective, NO conforms to many of the rules governing more conventional neurotransmission, particularly of the metabotropic type, but stands out as being more economical and versatile, attributes that presumably account for its spectacular evolutionary success.

Tonic and phasic nitric oxide signals in hippocampal long-term potentiation

Nitric oxide (NO) participates in long-term potentiation (LTP) and other forms of synaptic plasticity in many different brain areas but where it comes from and how it acts remain controversial. Using rat and mouse hippocampal slices, we tested the hypothesis that tonic and phasic NO signals are needed and that they derive from different NO synthase isoforms. NMDA increased NO production in a manner that was potently inhibited by three different neuronal NO synthase (nNOS) inhibitors. Tonic NO could be monitored after sensitizing guanylyl cyclase-coupled NO receptors, allowing the very low ambient NO concentrations to be detected by cGMP measurement. The levels were unaffected by inhibition of NMDA receptors, nNOS, or the inducible NO synthase (iNOS). iNOS was also undetectable in protein or activity assays. Tonic NO was susceptible to agents inhibiting endothelial NO synthase (eNOS) and was missing in eNOS knock-out mice. The eNOS knock-outs exhibited a deficiency in LTP resembling that seen in wild-types treated with a NO synthase inhibitor. LTP in the knock-outs could be fully restored by supplying a low level of NO exogenously. Inhibition of nNOS also caused a major loss of LTP, particularly of late-LTP. Again, exogenous NO could compensate, but higher concentrations were needed compared with those restoring LTP in the eNOS knock-outs. It is concluded that tonic and phasic NO signals are both required for hippocampal LTP and the two are generated, respectively, by eNOS and nNOS, the former in blood vessels and the latter in neurons.

Effects of N-methyl-d-aspartate on extracellular citrulline level in the rat nucleus accumbens

In vivo microdialysis combined with high-performance liquid chromatography and electrochemical detection was used to study effects of intraaccumbal infusion of N-methyl-D-aspartate (NMDA) on the content of extracellular citrulline (a nitric oxide co-product) in the medial nucleus accumbens of Sprague-Dawley rats. The intraaccumbal NMDA infusion (10-1000 microM) dose-dependently increased the local dialysate citrulline levels (193+/-7% and 258+/-7% versus basal for the 100 and 1000 microM, respectively). The NMDA-induced increase of extracellular citrulline was completely prevented by intraaccumbal infusions through the dialysis probe both of 50 microM dizocilpine maleate (an NMDA antagonist) and of 0.5 mM N-nitro-L-arginine (a nitric oxide synthase inhibitor). Local infusion of N-nitro-L-arginine (0.5 mM) slightly decreased basal citrulline levels in the nucleus accumbens throughout the entire period of the infusion, whereas dizocilpine maleate (50 microM) had no long-lasting effect. These results suggest that NMDA receptor stimulation of the medial nucleus accumbens might cause a local nitric oxide synthase activation resulting in nitric oxide production in this brain area.

Normoxic resuscitation after cardiac arrest protects against hippocampal oxidative stress, metabolic dysfunction, and neuronal death

Resuscitation and prolonged ventilation using 100% oxygen after cardiac arrest is standard clinical practice despite evidence from animal models indicating that neurologic outcome is improved using normoxic compared with hyperoxic resuscitation. This study tested the hypothesis that normoxic ventilation during the first hour after cardiac arrest in dogs protects against prelethal oxidative stress to proteins, loss of the critical metabolic enzyme pyruvate dehydrogenase complex (PDHC), and minimizes subsequent neuronal death in the hippocampus. Anesthetized beagles underwent 10 mins ventricular fibrillation cardiac arrest, followed by defibrillation and ventilation with either 21% or 100% O2. At 1 h after resuscitation, the ventilator was adjusted to maintain normal blood gas levels in both groups. Brains were perfusion-fixed at 2 h reperfusion and used for immunohistochemical measurements of hippocampal nitrotyrosine, a product of protein oxidation, and the E1alpha subunit of PDHC. In hyperoxic dogs, PDHC immunostaining diminished by approximately 90% compared with sham-operated dogs, while staining in normoxic animals was not significantly different from nonischemic dogs. Protein nitration in the hippocampal neurons of hyperoxic animals was 2-3 times greater than either sham-operated or normoxic resuscitated animals at 2 h reperfusion. Stereologic quantification of neuronal death at 24 h reperfusion showed a 40% reduction using normoxic compared with hyperoxic resuscitation. These results indicate that postischemic hyperoxic ventilation promotes oxidative stress that exacerbates prelethal loss of pyruvate dehydrogenase and delayed hippocampal neuronal cell death. Moreover, these findings indicate the need for clinical trials comparing the effects of different ventilatory oxygen levels on neurologic outcome after cardiac arrest.

Mechanism of nitric oxide action on inhibitory GABAergic signaling within the nucleus tractus solitarii

The cellular mechanisms mediating nitric oxide (NO) modulation of the inhibitory transmission in the nucleus tractus solitarii (NTS) remain unclear, even though this could be extremely important for various physiological and pathological processes. Specifically, in the NTS NO-evoked glutamate and gamma-aminobutyric acid (GABA) release might contribute to pathological hypertension. In cultured rat brainstem slices, NTS GABAergic neurons were targeted using an adenoviral vector to express enhanced green fluorescent protein and studied with a combination of patch clamp and confocal microscopy. Low nanomolar concentrations of NO increased intracellular Ca2+ concentration ([Ca2+]i) in somata, dendrites, and putative axons of GABAergic neurons, with axons being the most sensitive compartment. This effect was cGMP mediated and not related to depolarization or indirect presynaptic effects on glutamatergic transmission. Blockade of the cyclic adenosine diphosphate ribose (cADPR)/ryanodine-sensitive stores but not the inositol triphosphate-sensitive stores, inhibited NO effect. Since cADPR/ryanodine-sensitive stores are implicated in the Ca2+-induced Ca2+ release, NO can be expected to potentiate GABA release. In support of this notion, a cADPR antagonist abolished the NO-induced potentiation of GABAergic inhibitory postsynaptic potentials in the NTS. Thus, the NO-cGMP-cADPR-Ca2+ pathway, previously described in sea urchin eggs, also operates in mammalian GABAergic neurons. Potentiation of GABA release by NO may have implications for numerous brain functions.

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.

Effects of chemical ischemia in cerebral cortex slices. Focus on nitric oxide

Superfused rat cerebral cortex slices were submitted to a continuous electrical (5 Hz) stimulation and treated with sodium azide (1-10 mM) in the presence of 2 mM 2-deoxyglucose ("chemical ischemia"). Presynaptic cholinergic activity, evaluated as acetylcholine release, was inhibited depending on the sodium azide concentrations and on the length of application (5-30 min). Following a 5-min treatment with 10 mM sodium azide, acetylcholine release was reduced to 45+/-2.3%; simultaneously, there was a 15- and 10-fold increase in glutamate and nitric oxide effluxes, respectively. After restoring normal superfusion conditions, acetylcholine release recovered to 70+/-3.1% of the controls; the N-methyl-D-aspartate receptor antagonist MK-801 (10 microM) as well as the nitric oxide scavengers, haemoglobin (20 microM) and 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-l-oxyl-3-oxide (150 microM), improved the recovery in presynaptic activity, showing that both glutamate and nitric oxide play detrimental roles in chemical ischemia. On the other hand, the post-ischemic recovery was worsened by the guanylylcyclase inhibitor 1H-[l,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (10 microM), suggesting that the activation of such a pathway plays a neuroprotective role and that the nitric oxide-induced harmful effects depend on different mechanisms. Chemical ischemia-evoked nitric oxide efflux partly derived from its calcium-dependent endogenous synthesis, since both the intracellular calcium chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (1 mM), and the nitric oxide synthase inhibitor, N(omega)-nitro-L-arginine methyl ester (100 microM), substantially prevented sodium azide effects. Nitric oxide efflux was only weakly reduced by MK-801 and was not modified by either the L-type calcium channel blocker, nifedipine (10 microM) or the N-type calcium channel blocker omega-conotoxin (0.5 microM), thus suggesting a prevailing intracellular calcium-dependence of nitric oxide production, although a partial extracellular calcium source cannot be ruled out. These findings show that sodium azide plus 2-deoxyglucose treatment is a useful protocol to induce brain ischemia in vitro and underline the involvement of nitric oxide in the complex events following the ischemic insult.

On the regulation of NMDA receptors by nitric oxide

Nitric oxide (NO) is generated in central synapses on activation of N-methyl-D-aspartate (NMDA) receptors and exerts physiological effects by changing cGMP levels. NO has frequently also been claimed to engage a different mechanism, namely the covalent modification of thiol residues (S-nitrosation), and thereby exert a negative feedback on NMDA receptors. Tests of this hypothesis were conducted by recording NMDA receptor-mediated synaptic potentials in the CA1 area of rat hippocampal slices. Manipulations designed to increase or decrease endogenous NO levels had no effect. Addition of exogenous NO using a NONOate donor in concentrations up to 30-fold higher than those needed to evoke maximal cGMP accumulation also had no effect. Nevertheless, in agreement with previous findings, photolysis of a caged NO derivative with UV light led to an enduring block of synaptic NMDA receptors. To address these contradictory results, NMDA receptor-mediated currents were recorded from HEK-293 cells transfected with NR1 and NR2A subunits. As found in slices, photolysis of caged NO inhibited the currents whereas perfusion of NO (up to 5 microM) was ineffective. However, when NO was supplied at a concentration found to be effective when released photolytically (5 microM) and the cells simultaneously exposed to the UV light used for photolysis, NMDA receptor-mediated currents were inhibited. This effect was not observed at more physiological NO concentrations (10 nM range). The results indicate that neither endogenous NO nor exogenous NO in supra-physiological concentration inhibits synaptic NMDA receptors; the combination of high NO concentration and UV light can give an artifactual result.

The use of nitric oxide in neonatal care

Knowledge of NO and its role in the human body currently is limited. Further scientific research involving this unique molecule will expand its clinical usefulness. It is an exciting era in research,involving numerous body processes and systems. The initial work on pulmonary vascular response in newborns who have PPHN has opened the door to seemingly endless possibilities involving many aspects of health.

Hippocampal neurons in organotypic slice culture are highly resistant to damage by endogenous and exogenous nitric oxide

Nitric oxide (NO) has been proposed to mediate neurodegeneration arising from NMDA receptor activity, but the issue remains controversial. The hypothesis was re-examined using organotypic slice cultures of rat hippocampus, with steps being taken to avoid known artefacts. The NO-cGMP signalling pathway was well preserved in such cultures. Brief exposure to NMDA resulted in a concentration-dependent delayed neuronal death that could be nullified by administration of the NMDA antagonist MK801 (10 microm) given postexposure. Two inhibitors of NO synthesis failed to protect the slices, despite fully blocking NMDA-induced cGMP accumulation. By comparing NMDA-induced cGMP accumulation with that produced by an NO donor, toxic NMDA concentrations were estimated to produce only physiological NO concentrations (2 nm). In studies of the vulnerability of the slices to exogenous NO, it was found that continuous exposure to up to 4.5 microm NO failed to affect ATP levels (measured after 6 h) or cause damage during 24 h, whereas treatment with the respiratory inhibitors myxothiazol or cyanide caused ATP depletion and complete cell death within 24 h. An NO concentration of 10 microm was required for ATP depletion and cell death, presumably through respiratory inhibition. It is concluded that sustained activity of neuronal NO synthase in intact hippocampal tissue can generate only low nanomolar NO concentrations, which are unlikely to be toxic. At the same time, the tissue is remarkably resistant to exogenous NO at up to 1000-fold higher concentrations. Together, the results seriously question the proposed role of NO in NMDA receptor-mediated excitotoxicity.

Essential cellular regulatory elements of oxidative stress in early and late phases of apoptosis in the central nervous system

The generation of reactive oxygen species and subsequent oxidative stress in the central nervous system is now considered to be one of the primary etiologies of a host of neurodegenerative disorders, such as Alzheimer disease, Parkinson disease, and cerebral ischemia. On a cellular level, oxidative stress leads to an apoptotic early phase that involves cellular membrane phosphatidylserine (PS) exposure and a late phase that pertains to the degradation of genomic DNA. The translocation of membrane PS from the inner cellular membrane to the surface is a critical component for both microglial activation and cellular disposal of injured cells. During oxidative stress, this early phase of apoptosis is intimately controlled by neuronal PS exposure and microglial PS receptor expression. The late phase of apoptosis that involves a loss of genomic DNA integrity can result as a function of an ill-fated attempt to enter the cell cycle in postmitotic neurons. By using a cascade of pathways that involve cysteine proteases to modulate programmed cell death, protein kinase B (Akt) surfaces as a key regulatory element of both extrinsic pathways of inflammation and intrinsic pathways of cellular integrity. Further understanding of the cellular mechanisms modulating neuronal cellular integrity and phagocytic cell disposal during oxidative stress may form the basis for the future development of cytoprotective strategies in the nervous system.

Nitric oxide in brain: diffusion, targets and concentration dynamics in hippocampal subregions

Nitric oxide (NO(*)) is a diffusible regulatory molecule involved in a wide range of physiological and pathological events. At the tissue level, a local and temporary increase in NO(*) concentration is translated into a cellular signal. From our current knowledge of biological synthesis and decay, the kinetics and mechanisms that determine NO(*) concentration dynamics in tissues are poorly understood. Generally, NO(*) mediates its effects by stimulating (e.g., guanylate cyclase) or inhibiting (e.g., cytochrome oxidase) transition metal-containing proteins and by post-translational modification of proteins (e.g., formation of nitrosothiol adducts). The borderline between the physiological and pathological activities of NO(*) is a matter of controversy, but tissue redox environment, supramolecular organization and compartmentalisation of NO(*) targets are important features in determining NO(*) actions. In brain, NO(*) synthesis in the dependency of glutamate NMDA receptor is a paradigmatic example; the NMDA-subtype glutamate receptor triggers intracellular signalling pathways that govern neuronal plasticity, development, senescence and disease, suggesting a role for NO(*) in these processes. Measurements of NO(*) in the different subregions of hippocampus, in a glutamate NMDA receptor-dependent fashion, by means of electrochemical selective microsensors illustrate the concentration dynamics of NO(*) in the sub-regions of this brain area. The analysis of NO(*) concentration-time profiles in the hippocampus requires consideration of at least two interrelated issues, also addressed in this review. NO(*) diffusion in a biological medium and regulation of NO(*) activity.

Positive N-methyl-d-aspartate receptor modulation by selective glycine transporter-1 inhibition in the rat dorsal spinal cord in vivo

In this study we have employed the selective glycine transporter-1 (GlyT-1) and GlyT-2 transporter inhibitors R-(-)-N-methyl-N-[3-[(4-trifluoromethyl)phenoxy]-3-phenyl-propyl]glycine (1:1) lithium salt (Org 24598) and 4-benzyloxy-3,5-dimethoxy-N-[1-(dimethylaminocyclopently)methyl]benzamide (Org 25543), respectively, and microdialysis perfusion to determine the effect of GlyT transporter inhibition on extracellular amino acid concentrations in the lumbar dorsal spinal cord of the halothane-anaesthetised rat. Reverse dialysis of Org 24598 (0.1-10 microM) induced a concentration-related increase in extracellular glycine accompanied by a progressive increase in citrulline, but not aspartate, glutamate or GABA, efflux. Org 25543 (10 microM) by the same route induced a similar increase in glycine levels without affecting the efflux of other amino acids quantified. To test the hypothesis that the increase in citrulline efflux resulted from activation of the N-methyl-D-aspartate receptor (NMDA-R)/nitric oxide synthase (NOS) signalling cascade, the sensitivity was determined of GlyT-1 inhibition-induced effects to NMDA-R antagonism or NOS inhibition. Co-administration by reverse dialysis of the selective NMDA-R channel blocker MK-801 (0.5 mM) or the selective antagonist of the strychnine-insensitive glycine site, 7-chlorokynurenic acid (1 mM), with Org 24598 (10 microM) did not affect the uptake inhibition-induced increase in glycine efflux, but did significantly attenuate the increase in extracellular citrulline. Similarly, co-administration with Org 24598 of the isoform non-selective and selective neuronal NOS inhibitors Nomega-nitro-L-arginine methyl ester (1 mM) or 1-(2-trifluoromethylphenyl)imidazole (0.2 mM), respectively, prevented Org 24598-induced citrulline efflux with no effect on increased glycine efflux. These data provide evidence that the observed increased in extracellular citrulline is a consequence of positive modulation of NMDA-R, secondary to increased extracellular glycine and support a protective role for GlyT-1 against fluctuations in extracellular glycine uptake at glutamatergic synapses in the dorsal spinal cord. Such a mechanism could be important to NMDA-R-mediated synaptic plasticity in the spinal cord and be of relevance to the clinical usage of GlyT-1 inhibitors.

Nitrosation, Nitration, and Autoxidation of the Selective Estrogen Receptor Modulator Raloxifene by Nitric Oxide, Peroxynitrite, and Reactive Nitrogen/Oxygen Species

The regulation of estrogenic and antiestrogenic effects by selective estrogen receptor modulators (SERMs) provides the basis for use in long-term therapy in cancer chemoprevention and postmenopausal osteoporosis. However, the evidence for carcinogenic properties within this class requires study of potential pathways of toxicity. There is strong evidence for the elevation of cellular levels of NO in tissue treated with SERMs, including the benzothiophene derivative, raloxifene, in part via up-regulation of nitric oxide synthases. Therefore, the reactions of 17beta-estradiol (E(2)), raloxifene, and an isomer with NO, peroxynitrite, and reactive nitrogen/oxygen species (RNOS) generated from NO(2)(-)/H(2)O(2) systems were examined. Peroxynitrite from bolus injection or slow release from higher concentrations of 3-morpholinosydnonimine (SIN-1) reacted with the benzothiophenes and E(2) to give aromatic ring nitration, whereas peroxynitrite, produced from the slow decomposition of lower concentrations of SIN-1, was relatively unreactive toward E(2) and yielded oxidation and nitrosation products with raloxifene and its isomer. The oxidation and nitrosation products formed were characterized as a dimer and quinone oxime derivative. Interestingly, the reaction of the benzothiophenes with NO in aerobic solution efficiently generated the same oxidation products. Stable quinone oximes are not unprecedented but have not been previously reported as products of RNOS-mediated metabolism. The reaction of glutathione (GSH) with the quinone oxime gave both GSH adducts from Michael addition and reduction to the corresponding o-aminophenol. The ready autoxidation of raloxifene, observed in the presence of NO, is the first such observation on the reactivity of SERMs and is potentially a general phenomenon of significance to SERM chemical toxicology.

On the role of nitric oxide in hippocampal long-term potentiation

Nitric oxide (NO) functions in several types of synaptic plasticity, including hippocampal long-term potentiation (LTP), in which it may serve as a retrograde messenger after postsynaptic NMDA receptor activation. In accordance with a prediction of this hypothesis, and with previous findings using guinea pig tissue, exogenous NO, when paired with a short tetanus (ST) to afferent fibers, generated a stable NMDA receptor-independent potentiation of rat CA1 hippocampal synaptic transmission that occluded LTP. Contrary to predictions, however, the pairing-induced potentiation was abolished in the presence of NO synthase inhibitors, indicating that endogenous NO is required for exogenous NO to facilitate LTP. Periodic application of NO while endogenous NO synthesis was blocked indicated that a tonic low level is necessary on both sides of the NO-ST pairing for the plasticity to occur. A similar dependence on tonic NO seems to extend to LTP, because application of an NO synthase inhibitor 5 min after tetanic stimulation blocked LTP as effectively as adding it beforehand. The posttetanus time window during which NO operated was restricted to <15 min. Inhibition of the guanylyl cyclase-coupled NO receptor indicated that the potentiation resulting from NO-ST pairing and the NO signal transduction pathway during early LTP are both through cGMP. We conclude that NO does not function simply as an acute signaling molecule in LTP induction but has an equally important role outside this phase. The results resonate with observations concerning the role of the hippocampal NO-cGMP pathway in certain types of learning behavior.

Kinetics of nitric oxide-cyclic GMP signalling in CNS cells and its possible regulation by cyclic GMP

Physiologically, nitric oxide (NO) signal transduction occurs through soluble guanylyl cyclase (sGC), which catalyses cyclic GMP (cGMP) formation. Knowledge of the kinetics of NO-evoked cGMP signals is therefore critical for understanding how NO signals are decoded. Studies on cerebellar astrocytes showed that sGC undergoes a desensitizing profile of activity, which, in league with phosphodiesterases (PDEs), was hypothesized to diversify cGMP responses in different cells. The hypothesis was tested by examining the kinetics of cGMP in rat striatal cells, in which cGMP accumulated in neurones in response to NO. Based on the effects of selective PDE inhibitors, cGMP hydrolysis following exposure to NO was attributed to a cGMP-stimulated PDE (PDE 2). Analysis of NO-induced cGMP accumulation in the presence of a PDE inhibitor indicated that sGC underwent marked desensitization. However, the desensitization kinetics determined under these conditions described poorly the cGMP profile observed in the absence of the PDE inhibitor. An explanation shown plausible theoretically was that cGMP determines the level of sGC desensitization. In support, tests in cerebellar astrocytes indicated an inverse relationship between cGMP level and recovery of sGC from its desensitized state. We suggest that the degree of sGC desensitization is related to the cGMP concentration and that this effect is not mediated by (de)phosphorylation.

Differential sensitivity of guanylyl cyclase and mitochondrial respiration to nitric oxide measured using clamped concentrations

Nitric oxide (NO) signal transduction may involve at least two targets: the guanylyl cyclase-coupled NO receptor (NO(GC)R), which catalyzes cGMP formation, and cytochrome c oxidase, which is responsible for mitochondrial O(2) consumption and which is inhibited by NO in competition with O(2). Current evidence indicates that the two targets may be similarly sensitive to NO, but quantitative comparison has been difficult because of an inability to administer NO in known, constant concentrations. We addressed this deficiency and found that purified NO(GC)R was about 100-fold more sensitive to NO than reported previously, 50% of maximal activity requiring only 4 nm NO. Conversely, at physiological O(2) concentrations (20-30 microM), mitochondrial respiration was 2-10-fold less sensitive to NO than estimated beforehand. The two concentration-response curves showed minimal overlap. Accordingly, an NO concentration maximally active on the NO(GC)R (20 nm) inhibited respiration only when the O(2) concentration was pathologically low (50% inhibition at 5 microM O(2)). Studies on brain slices under conditions of maximal stimulation of endogenous NO synthesis suggested that the local NO concentration did not rise above 4 nm. It is concluded that under physiological conditions, at least in brain, NO is constrained to target the NO(GC)R without inhibiting mitochondrial respiration.