Localization of the Na+-coupled neutral amino acid transporter 2 in the cerebral cortex

We studied the distribution and cellular localization of Na+-coupled neutral amino acid transporter 2, a member of the system A family of amino acid transporters, in the rat and human cerebral cortex using immunocytochemical methods. Na+-coupled neutral amino acid transporter 2-positive neurons were pyramidal and non-pyramidal, and Na+-coupled neutral amino acid transporter 2/GABA double-labeling studies revealed that Na+-coupled neutral amino acid transporter 2 was highly expressed by GABAergic neurons. Double-labeling studies with the synaptophysin indicated that rare axon terminals express Na+-coupled neutral amino acid transporter 2. Na+-coupled neutral amino acid transporter 2-immunoreactivity was also found in astrocytes, leptomeninges, ependymal cells and choroid plexus. Electron microscopy showed robust Na+-coupled neutral amino acid transporter 2-immunoreactivity in the somato-dendritic compartment of neurons and in glial processes, but, as in the case of double-labeling studies, failed to reveal Na+-coupled neutral amino acid transporter 2-immunoreactivity in terminals. To rule out the possibility that the absence of Na+-coupled neutral amino acid transporter 1-[Melone M, Quagliano F, Barbaresi P, Varoqui H, Erickson JD, Conti F (2004) Localization of the glutamine transporter SNAT1 in rat cerebral cortex and neighboring structures, with a note on its localization in human cortex. Cereb Cortex 14:562–574] and Na+-coupled neutral amino acid transporter 2-positive terminals was due to insufficient antigen detection, we evaluated Na+-coupled neutral amino acid transporter 1/synaptophysin and Na+-coupled neutral amino acid transporter 2/synaptophysin coexpression using non-standard immunocytochemical procedures and found that Na+-coupled neutral amino acid transporter 1 and Na+-coupled neutral amino acid transporter 2+ terminals were rare in all conditions. These findings indicate that Na+-coupled neutral amino acid transporter 1 and Na+-coupled neutral amino acid transporter 2 are virtually absent in cortical terminals, and suggest that they do not contribute significantly to replenishing the Glu and GABA transmitter pools through the glutamate–glutamine cycle. The strong expression of Na+-coupled neutral amino acid transporter 2 in the somato-dendritic compartment and in non-neuronal elements that are integral parts of the blood–brain and brain–cerebrospinal fluid barrier suggests that Na+-coupled neutral amino acid transporter 2 plays a role in regulating the levels of Gln and other amino acids in the metabolic compartment of cortical neurons.