Margaret E. Rice
Cellular ascorbate levels are maintained by active uptake from the extracellular fluid, rather than intracellular ascorbate synthesis. Consequently, higher expression of an ascorbate transporter in neurons than in glia might underlie the differences in intracellular levels. Your point about differences in surface-to-volume ratio among cell types is well taken. This could contribute to differences in intracellualr concentration. However, I doubt that's the main controlling factor for two reasons: 1) granule cells in the cerebellum have a much higher surface-to-volume ratio than pyramidal cells in cerebral cortex, yet our data indicate that ascorbate levels are similar in both cell populations; and 2) surface-to-volume ratio differences among neuron populations are likely to vary as greatly as between the "average" neuron and the "average" glial cell.
More likely, as suggested above, the differences are caused by differences in expression level of ascorbate tranporters, or by modulation of activity by phosphorylation state or other regulatory mechanisms, or even by expression of distinct neuron and glial transport proteins. Unfortunately, little is known about about ascorbate transporter(s) at the molecular level, so these comments remain speculative at present.