Low‐affinity excitatory amino acid uptake in hippocampal astrocytes: A possible role of Na⁺/dicarboxylate cotransporters

Abstract

The excitatory amino acid transporters (EAATs) underlie the so-called "high affinity" uptake of glutamate, which is well characterized. In contrast, the "low-affinity" uptake of glutamate remains poorly defined, and it has been discussed whether it may represent a mere in vitro artifact. Here we have visualized "low-affinity" excitatory amino acid uptake sites by incubating rat hippocampal slices with the glutamate analogue D-aspartate in the presence of PMB-TBOA, which blocks the EAATs. After fixation of the slices, D-aspartate taken up into the tissue was localized with the use of light microscopic immunoperoxidase and electron microscopic immunogold methods, exploiting highly specific antibodies against D-aspartate. PMB-TBOA blocked uptake of both low and high exogenous D-aspartate concentrations (0.01-1.0 mM) into nerve terminals, as well as the uptake of 0.01 mM D-aspartate into astrocytes. Interestingly, there was a residual PMB-TBOA insensitive uptake of D-aspartate in astrocytes at higher exogenous D-aspartate concentrations (0.05-1.0 mM), strongly suggesting that astrocytes have "low-affinity" uptake sites for excitatory amino acid. The PMB-TBOA insensitive D-aspartate uptake in astrocytes was sodium dependent and inhibited by succinate and to certain extent by homocysteate, but not by cystine or DIDS. We suggest that excitatory amino acid is transported into astrocytes in a "low-affinity" fashion by sodium/dicarboxylate transporters.

The sodium‐dependent di‐ and tricarboxylate transporter, NaCT, is not responsible for the uptake of D‐, L‐2‐hydroxyglutarate and 3‐hydroxyglutarate into neurons

“…However, glutamate neither induced inward currents nor inhibited succinate uptake in hNaDC3-expressing oocytes, and 1 mM glutamate reduced succinate uptake only by 19.9 ± 8.5 % (N.S.). Therefore, hNaDC3 can be excluded as a transporter facilitating glutamate uptake into astrocytes as opposed to recent speculation (Frizzo et al 2004; Holten et al 2008). …”

“…Uptake of glutamate into astrocytes occurs by distinct excitatory amino acid transporters (EAATs) and possibly by low affinity sodium-dependent transport systems (Danboldt 2001; Holten et al 2008). One such candidate transporter may be NaDC3, recently localized to astrocytes (Yodoya et al 2006).…”