Molecular characterization of four pharmacologically distinct α-aminobutyric acid transporters in mouse brain

Two novel γ-aminobutyric acid (GABA) transporters, GAT3 and GAT4, were cloned from the mouse neonatal brain cDNA library and expressed in Xenopus oocytes. Sequence analysis indicated they were members of the Na⁺-dependent neurotransmitter transporter family. The GABA uptake activities were measured in cRNA injected Xenopus oocytes. The K_m for GABA uptake by GAT3 was 18 μM and by GAT4 was 0.8 μM. GAT3 also transports β-alanine and taurine with K_m of 28 and 540 μM, respectively. Similarly, GAT4 transports β-alanine with K_m of 99 μM and taurine with a K_m of 1.4 mM. The newly cloned GABA transporters were compared with two previously cloned GABA transporters, GAT1 and GAT2, in terms of molecular and pharmacological properties. While GAT1 and GAT4 gene expression were neural specific, GAT2 and GAT3 mRNAs were detected in other tissues such as liver and kidney, in which GAT3 mRNA was especially abundant. The expression of GAT3 mRNA in mouse brain is developmentally regulated, and its mRNA is abundant in neonatal brain but not in adult brain. High affinity GABA transporters GAT1 and GAT4 were more sensitive to inhibition by nipecotic acid. Low affinity GABA transporters GAT2 and GAT3 were inhibited most effectively by betaine and β-alanine, respectively. The differential tissue distribution and distinct pharmacological properties of those four GABA transporters suggest functional specialization in the mechanisms of GABA transmission termination.