Na + -coupled carboxylate transporters (NaCs) mediate the uptake of tricarboxylic acid cycle intermediates in mammalian tissues. Of these transporters, NaC3 (formerly known as Na + -coupled dicarboxylate transporter 3, NaDC3/SDCT2) and NaC2 (formerly known as Na + -coupled citrate transporter, NaCT) have been shown to be expressed in brain. There is, however, little information available on the precise distribution and function of both transporters in the CNS. In the present study, we investigated the functional characteristics of Na uptake by unlabeled succinate, N-acetyl-L-aspartate and citrate was 15.9, 155 and 764 lM respectively. In primary cultures of neurons, uptake of citrate was also Na + dependent and saturable with a K t value of 16.2 lM, which was different from that observed in astrocytes, suggesting that different Na + -dependent citrate transport systems are expressed in neurons and astrocytes. RT-PCR and immunocytochemistry revealed that NaC3 and NaC2 are expressed in cerebrocortical astrocytes and neurons respectively. These results are in good agreement with our previous reports on the brain distribution pattern of NaC2 and NaC3 mRNA using in situ hybridization. This is the first report of the differential expression of different NaCs in astrocytes and neurons. These transporters might play important roles in the trafficking of tricarboxylic acid cycle intermediates and related metabolites between glia and neurons.

We investigated in the present study the transport characteristics of N-acetyl-L-aspartate in primary cultures of astrocytes from rat cerebral cortex and the involvement of NA + -coupled high-affinity carboxylate transporter NaC3 (formerly known as NaDC3) responsible for N-acetyl-L-aspartate transport. N-acetyl-L-aspartate transport was NA + -dependent and saturable with a Michaelis-Menten constant (K m ) of 110 lM. NA + -activation kinetics revealed that the NA + to-N-acetyl-L-aspartate stoichiometry was 3 : 1 and concentration of Na + necessary for half-maximal transport (K NA m ) was 70 mM. NA + -dependent N-acetyl-L-aspartate transport was competitively inhibited by succinate with an inhibitory constant (K i ) of 14.7 lM, which was comparable to the K m value of NA + -dependent succinate transport (29.4 lM). L-Aspartate also inhibited NA + -dependent [ 14 C]N-acetyl-L-aspartate transport with relatively low affinity (K i ¼ 2.2 mM), whereas N-acetyl-L-aspartate was not able to inhibit NA + -dependent aspartate transport in astrocytes. In addition, Li + was found to have a significant inhibitory effect on the NA + -dependent N-acetyl-L-aspartate transport in a concentration-dependent manner. Furthermore, RT-PCR and western blot analyses revealed that NaC3 is expressed in primary cultures of astrocytes. Taken collectively, these results indicate that NaC3 expressed in rat cerebrocortical astrocytes is responsible for NA + -dependent N-acetyl-L-aspartate transport. This transporter is likely to be an essential prerequisite for the metabolic role of N-acetyl-L-aspartate in the process of myelination.