Hell, blood levels of glutamate don't even matter, as it doesn't even pass the blood-brain barrier! http://jn.nutrition.org/cgi/content/full/130/4/1016S This is a major peer reviewed journal.
Take another look at the journal you linked. It does not say "it doesn't even pass the blood-brain barrier"
It is classified similar to L-aspartate which you should know does in fact cross the BBB. Glutamate may need a transporter, may not have a high uptake under "normal" conditions and is manufactured in the brain, but it certainly does cross the blood brain barrier.
The first transport systems to be proposed for the BBB were identified on the basis of results from in vivo uptake studies (Oldendorf 1971Citation , Oldendorf and Szabo 1976Citation , Pardridge 1979Citation ). These transporters include the following:...4) System x-, which mediates sodium-independent, high affinity uptake of amino acids with anionic side chains, including L-glutamate and L-aspartate.
In contrast, L-glutamate and L-aspartate, which can be synthesized readily in brain, show much lower rates of uptake into brain at the BBB (Al-Sarraf et al. 1995Citation and 1997bCitation , Benrabh and Lefauconnier 1996Citation ). For these "dietary nonessential" amino acids, brain supply is governed more by intracerebral synthesis and breakdown.
Alhough L-glutamine shows measurable affinity for System L, Ennis et al. (1998)Citation recently reported that glutamine is actually taken up into brain by a separate sodium-dependent mechanism at the BBB, which they identified tentatively as System N. In other tissues, System N mediates the sodium-dependent transport of L-glutamine, L-histidine and L-asparagine.
Just because certain conditions are required for it to happen, does not allow one to say it does not cross the barrier. It may be somewhat independent of plasma levels and needs the right conditions, but those conditions do occur so it does make it's way into the brain. Additionally there are some areas that are not even subject to the Blood brain barrier:
Although the BBB helps protect most of the brain from changes in circulating plasma L-glutamate, there are a few brain areas that do not contain a BBB (Fig. 2Citation ) and do allow rapid L-glutamate uptake from the circulation (Hawkins et al. 1995Citation ). These are known collectively as "the circumventricular organs" and include the median eminence, area postrema, subfornical organ, subcommissural organ, pineal gland, neurohypophysis and organum vasculosum of the lamina terminalis (Gross and Weindel 1987Citation ). Brain uptake rates for small solutes in these areas exceed those of normal brain by 10- to 1000-fold (Gross et al. 1987Citation , Gross 1991Citation , Hawkins et al. 1995Citation , Shaver et al. 1992Citation ). Once within brain extracellular fluid, solutes can move into adjacent brain areas via intercellular diffusion or via flow along the Virchow-Robin spaces. Such movement has been documented for glutamate and aspartate in animals after high dose amino acid administration (Price et al. 1981Citation and 1984Citation ). The net result is that certain areas of the brain are vulnerable to acute fluctuations in plasma glutamate concentration of large magnitude as a result of "flooding" from the circumventricular organs.
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