According to the cited article,
The major route by which ascorbate enters the CNS involves transport from plasma to the CSF across the epithelium of the choroid plexus.
But a different study The Influx of Ascorbic Acid into the Rat’s Brain
The idea, proposed by Spector & Lorenzo (1973), that the choroid plexus plays an important role in the transport of ascorbic acid into the brain must now be reconsidered in the light of our finding that there is a carrier-mediated transport process for this vitamin across the cerebral capillaries…when ascorbic acid in the plasma is within the normal physiological concentration (58 uM), the influx across the brain is approximately 10 times higher than that through the choroid plexus…Another reason for believing that the choroid plexus only plays a small part in the entry of ascorbic acid into the tissue of the brain is that when the ascorbic acid has passed through the choroid plexus into the cerebrospinal fluid (CSF) it has a long pathway to traverse before it reaches the cerebral cells. It must be carried in the CSF from the lateral ventricles, over the convexity of the hemispheres and then alongside the penetrating vessels of the brain into the cerebral tissue. In this study, we have shown that the labelled vitamin was found in the brain tissue within 1 min. The rapidity of this accumulation in the cerebral tissue makes it most unlikely that the choroid plexus is a major route of entry.
And another different study Vitamin C Crosses the Blood–Brain Barrier in the Oxidized Form Through the Glucose Transporters
We identified the chemical form of vitamin C that readily crosses the blood–brain barrier, and the mechanism of this process. Ascorbic acid was not able to cross the blood–brain barrier in our studies. In contrast, the oxidized form of vitamin C, dehydroascorbic acid (oxidized ascorbic acid), readily entered the brain and was retained in the brain tissue in the form of ascorbic acid…The findings define the transport of dehydroascorbic acid by GLUT1 as a mechanism by which the brain acquires vitamin C, and point to the oxidation of ascorbic acid as a potentially important regulatory step in accumulation of the vitamin by the brain.
So, the form of vitamin C that enters the brain, and the mechanism of its entry into the brain under normal
conditions, remains controversial and there is by no means any general agreement or consensus among scientists.
However, it is well-known and agreed that when DHAA is injected into the bloodstream, vitamin C accumulates in the brain at a rate and in an amount many, many times greater than when an equivalent amount of AA is administered. This has been shown in numerous studies. The most recent such study that I’m aware of is this one in which the comparative accumulation is demonstrated with remarkable images
of the brain: (11C)Ascorbic and (11C)Dehydroascorbic Acid, An Endogenous Redox Pair for Sensing Reactive Oxygen Species Using Positron Emission Tomography
Thus for the purpose of pharmacological
intervention with exogenous vitamin C for the potential treatment of Alzheimer’s, Parkinson’s, stroke, brain cancers, etc., it seems to me that the choice of DHAA versus AA should be…well, a no-brainer!
Douglas Q. Kitt, founder of ReCverin LLC, sellers of stabilized dehydroascorbic acid solutions.