Search

Nitric oxide (•NO) is an intercellular messenger implicated in memory formation and neurodegeneration in the hippocampus. Owing to its physical and chemical properties, the concentration dynamics of •NO is a critical issue in determining its bioactivity as a signaling molecule. Its production is closely related to glutamate N-methyl- D-aspartate (NMDA) receptors, following a rise in intracellular calcium levels. However, that dependent on α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors remains elusive and controversial, despite reports describing a role for these receptors in other brain regions, largely because of lack of quantitative and dynamic measurements of •NO. Using a •NO-selective microsensor inserted in the diffusional spread of •NO in the CA1 region of rat hippocampal slices, we measured its real-time endogenous production, following activation of ionotropic glutamate receptors and under tissue physiological oxygen tension. Both NMDA and AMPA stimulation resulted in a concentration-dependent •NO production but encompassing distinct kinetics for lag phases and slower rates of •NO production were observed for AMPA stimulation. Robustness of the results was achieved instrumentally and pharmacologically, by means of nitric oxide synthase (NOS) inhibitors and antagonists of NMDA ( D-(−)-2-amino-5-phosphonopentanoic acid, AP5) and AMPA (2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulfonamide, NBQX) receptors. When using glutamate as a stimulus, •NO production was of lower magnitude in the presence of AP5 plus NBQX than with AP5 alone, suggesting that even when NMDA receptors are inhibited Ca2+ rises to levels to induce a peak of •NO from the background. Whereas extracellular Ca2+ was required for the •NO signals, Philanthotoxin-4,3,3 (PhTX-4,3,3) a toxin used to target Ca2+-permeable AMPA receptors, attenuated •NO production. These observations are interpreted on basis of a distinct coupling between the glutamate receptors and neuronal NOS. A role for Ca2+-permeable AMPA receptors in the Ca2+ activation of neuronal NOS is suggested. © 2008 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

This study aimed to characterize the redox interaction between 3,4-dihydroxyphenylacetic acid (DOPAC) and nitric oxide (·NO), and to assess the reductive and oxidative decay pathways of the DOPAC semiquinone originating from this interaction. The reaction between DOPAC and ·NO led to the formation of the DOPAC semiquinone radical, detected by electron paramagnetic resonance (EPR) and stabilized by Mg[sup 2+], and the nitrosyl anion detected as nitrosylmyoglobin. The EPR signal corresponding to the DOPAC semiquinone was modulated as follows: (i) it was suppressed by glutathione and ascorbic acid with the formation of new EPR spectra corresponding to the glutathionyl and ascorbyl radical, respectively; (ii) it was enhanced by Cu,Zn-superoxide dismutase; the enzyme also accelerated the decay of the semiquinone species to DOPAC quinone. These results are interpreted as a one-electron oxidation of DOPAC by ·NO; the reductive decay of the semiquinone back to DOPAC was facilitated by reducing agents, such as glutathione and ascorbate, whereas the oxidative decay to DOPAC quinone was facilitated by superoxide dismutase. The latter effect is understood in terms of a reversible conversion of nitrosyl anion to ·NO by the enzyme. The biological relevance of these reactions is also discussed in terms of the reactivity of peroxynitrite towards DOPAC as a model with implications for aerobic conditions. [ABSTRACT FROM AUTHOR]

Nitrite has been viewed essentially as an inert metabolic endpoint of nitric oxide (•NO). However, under certain conditions, nitrite can be a source of •NO. In the brain, this alternative source of •NO production independent of nitric oxide synthase activity may be particularly relevant in ischemia/reperfusion (I/R), where low oxygen availability limits enzymatic production of •NO. Notably, in vivo concentration of nitrite can be easily increased with diet, through the ingestion of nitrate-rich foods, opening the window for a therapeutic intervention based on diet. Considering the modulation of mitochondrial respiration by •NO, we have hypothesized that the protective action of nitrite in I/R may also result from modulation of mitochondrial function. We used high-resolution respirometry to evaluate the effects of nitrite in two in vitro models of I/R. In both cases, an increase in oxygen flux was observed following reoxygenation, a phenomenon that has been coined "oxidative burst". The amplitude of this "oxidative burst" was decreased by nitrite in a concentration-dependent manner. Additionally, a pilot in vivo study in which animals received a nitrate-rich diet as a strategy to increase circulating and tissue levels of nitrite also revealed that the "oxidative burst" was decreased in the nitrate-treated animals. These results may provide mechanistic support to the observation of a protective effect of nitrite in situations of brain ischemia. [Display omitted] • Ischemia/Reperfusion in hippocampal slices reveals oxidative burst upon reperfusion at the tissue level; • Nitrite reduces oxidative burst upon reperfusion in hippocampal slices; • In vivo nitrate supplementation reduces oxidative burst upon reperfusion in hippocampal slices. [ABSTRACT FROM AUTHOR]

The small and diffusible free radical nitric oxide (•NO) has fascinated biological and medical scientists since it was promoted from atmospheric air pollutant to biological ubiquitous signaling molecule. Its unique physical chemical properties expand beyond its radical nature to include fast diffusion in aqueous and lipid environments and selective reactivity in a biological setting determined by bioavailability and reaction rate constants with biomolecules. In the brain, •NO is recognized as a key player in numerous physiological processes ranging from neurotransmission/neuromodulation to neurovascular coupling and immune response. Furthermore, changes in its bioactivity are central to the molecular pathways associated with brain aging and neurodegeneration. The understanding of •NO bioactivity in the brain, however, requires the knowledge of its concentration dynamics with high spatial and temporal resolution upon stimulation of its synthesis. Here we revise our current understanding of the role of neuronal-derived •NO in brain physiology, aging and degeneration, focused on changes in the extracellular concentration dynamics of this free radical and the regulation of bioenergetic metabolism and neurovascular coupling. Image 1 • Changes in the bioactivity of •NO are central to the molecular pathways associated with brain aging and neurodegeneration. • Bioactivity of •NO is shaped by the redox milieu in cells and tissues, which impacts neurometabolism, neurovascular coupling and neuroinflammation. • •NO concentration Dynamics in brain tissue can be monitoried using electrochemical methods coupled to microelectrodes. [ABSTRACT FROM AUTHOR]

Abstract: Nitric oxide ( NO) is a diffusible intercellular messenger, acting via volume signaling in the brain and, therefore, the knowledge of its temporal dynamics is determinant to the understanding of its neurobiological role. However, such an analysis in vivo is challenging and indirect or static approaches are mostly used to infer NO bioactivity. In the present work we measured the glutamate-dependent NO temporal dynamics in vivo in the hippocampus (CA1, CA3 and DG subregions), cerebral cortex and striatum, using NO selective microelectrodes. Concurrently, the immunolocalization of nNOS was evaluated in each region. A transitory increase in NO levels occurred at higher amplitudes in the striatum and hippocampus relatively to the cortex. In the hippocampus, subtle differences in the profiles of NO signals were observed along the trisynaptic loop, with CA1 exhibiting the largest signals. The topography of NO temporal dynamics did not fully overlap with the pattern of the density of nNOS expression, suggesting that, complementary to the distribution of nNOS, the local regulation of NO synthesis as well as the decay pathways critically determine the effective NO concentration sensed by a target within the diffusional spread of this free radical. In sum, the rate and pattern of NO changes here shown, by incorporating regulatory mechanisms and processes that affect NO synthesis and decay, provide refined information critical for the understanding of NO multiple actions in the brain. [Copyright &y& Elsevier]