What is absolute from a read of the reference ABSTRACT is as follows: ...glutamate is a non-essential amino acid whose brain content is much greater than plasma...
Free transfer of glutamate across the intact BBB is unlikely. The brain has to sequester glutamate from the plasma (systemic circulation) and store it in its parenchyma in any case...
Brain Glu levels are tightly regulated since excess levels can have deleterious effects such as neurotoxicity (Lau and Tymianski, 2010) and potentially also affect sensory transmission (Laurse et al. 2014).
Extracellular Glu levels are continuously maintained at 1-2μM by uptake of Glu through amino acid transporters 1 and 2 (EAAT1 and EAAT2), that are expressed by glial cells, mainly astrocytes (Anderson and Swanson 2000; Danbolt 2001).