Localization and functional relevance of system A neutral amino acid transporters in cultured hippocampal neurons

Glutamine and alanine are important precursors for the synthesis of glutamate. Provided to neurons by neighboring astrocytes, these amino acids are internalized by classical system A amino acid carriers. In particular, System A transporter (SAT1) is a highly efficient glutamine transporter, whereas SAT2 exhibits broad specificity for neutral amino acids with a preference for alanine. We investigated the localization and the functional relevance of SAT1 and SAT2 in primary cultures of hippocampal neurons. Both carriers have been expressed since early developmental stages and are uniformly distributed throughout all neuronal processes. However, whereas SAT1 is present in axonal growth cones and can be detected at later developmental stages at the sites of synaptic contacts, SAT2 does not appear to be significantly expressed in these compartments. The non-metabolizable amino acid analogue alpha-(methylamino)-isobutyric acid, a competitive inhibitor of system A carriers, significantly reduced miniature excitatory postsynaptic current amplitude in neurons growing on top of astrocytes, being ineffective in pure neuronal cultures. alpha-(Methylamino)-isobutyric acid did not alter neuronal responsitivity to glutamate, thus excluding a postsynaptic effect. These data indicate that system A carriers are expressed with a different subcellular distribution in hippocampal neurons and play a crucial role in controlling the astrocyte-mediated supply of glutamatergic neurons with neurotransmitter precursors.