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.