Oxidative Stress and the CNS

Re^3: Symposium 783

Margaret E. Rice

On Mon Dec 14, Bernhard H.J. Juurlink wrote
>Dr, Rice,
>     One thing that puzzles me is why neurons which have a poor ability to reduce oxidized-ascorbate (because of their relatively oxidized cytosol) should have an order of magnitude more ascorbate than astrocytes which have a very good ability to reduce oxidized-ascorbate. Is there any evidence for ascorbate metabolic coupling between neurons and astrocytes with oxidized-ascorbate transferred from neurons to astrocytes and reduced-ascorbate transferred from astrocytes to neuron?  It is known that the glucose transporters transport oxidized-ascorbate and there would be a concentration gradient differential tending to move oxidized-ascorbate out of the neuron to the extracellular space and from there to the astrocyte.  What about the movement of reduced-ascorbate in the other direction?

>Why should there be so much ascorbate in neurons?  Does it have a function other than reducing oxidized-tocopherol?  What would this be?  BJ

Dr. Juurlink – excellent questions!  First I must clarify your statement that the cytosol of neurons is “relatively oxidized.”  Yes, compared to that of less metabolically active glia (astrocytes), this might be true.  BUT, the overall intracellular environment of both neurons and glia is reducing.  This is indicated by tissue levels of ascorbate and glutathione (GSH) which are roughly 99% in the reduced state in rapidly frozen samples (the ratio in specific subcellular organelles can differ from this, of course).  

Localization of ascorbate (10 mM) to the cytosol of neurons (at a concentration of 10 mM), therefore, puts it exactly the right place to act as an important component of the neuronal antioxidant network.  As you note, ascorbate can help maintain tocopherol in the reduced state.  But other studies have shown that it can also directly reduce a variety of reactive oxygen species, including H2O2, hydroxyl radical, superoxide, and nitric oxide.  It has been proposed that ascorbate (and GSH) are particularly important for neutralizing the highly reactive hydroxyl radical, since there are no specific enzymes for this molecule.  When cytosolic ascorbate is oxidized in this way, it will be readily re-reduced by GSH, which our data suggest is 2.5 mM in neurons.  Oxidized GSH, in turn, will be rapidly returned to its reduced state by GSH-reductase, so that redox balance is maintained.  Perhaps your data showing that a small increase in GSH levels markedly enhances cellular reducing capacity (Juurlink, this symposium) includes a positive effect on ascorbate recycling.

Consequently, I do not think shuttling of oxidized ascorbate (dehydroascorbate, DHA) between neurons and glia is either required or likely, although it is an intriguing suggestion.  

You asked what ascorbate might be doing in neurons besides acting as an antioxidant.  We have previously reported that the brains of anoxia-tolerant species, like pond turtles, have very high levels of ascorbate (Rice et al. 1995).  We have postulated that these levels are necessary to maintain redox balance and prevent oxidative damage upon reoxygenation after a hypoxic dive.  Interesting, GSH is not elevated in turtle brain tissue.  Other postulated functions for elevated ascorbate levels might include serving as an alternative metabolic substrate or as an organic osmolyte for cell volume regulation.  However no data from our laboratory or others have yet demonstrated such functions.  Ascorbate DOES have actions as a neuromodulator as reviewed by Rebec and Pierce (1994).  It is also a specific cofactor for norepinephrine synthesis.  Perhaps most importantly, it is released from brain tissue by glutamate in a dose-dependent fashion via glutamate/ascorbate heteroexchange (O’Neill, Fillenz, et al. 1984;  Cammack et al. 1991).  Nontheless, the ability of ascorbate to act as an antioxidant and scavenger of reactive oxygen species suggests that these actions comprise its most important function in the CNS.

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