Sodium-coupled antiporter of neutral amino acids. In a tri- substrate transport cycle, exchanges neutral amino acids between the extracellular and intracellular compartments, coupled to the inward cotransport of at least one sodium ion (PubMed:24808181, PubMed:27272177, PubMed:34630942, PubMed:8910405). Exchanges neutral amino acids such as L- and D-serine, L-threonine, L-asparagine and L- alanine in a bidirectional way (PubMed:14502423, PubMed:26041762, PubMed:27272177, PubMed:34630942, PubMed:8101838, PubMed:8340364, PubMed:8910405). Involved in homeostasis of D-serine, a coagonist of synaptic NMDA receptors. In astrocytes at excitatory synapses, mediates electrogenic D-serine influx coupled to L-serine efflux and L-serine shuttling to neurons for de novo D-serine synthesis (By similarity). Displays sodium- and amino acid-dependent but uncoupled channel-like anion conductance with a preference SCN(-) > NO3(-) > I(-) > Cl(-) (PubMed:8910405). {ECO:0000250|UniProtKB:O35874, ECO:0000269|PubMed:14502423, ECO:0000269|PubMed:24808181, ECO:0000269|PubMed:26041762, ECO:0000269|PubMed:27272177, ECO:0000269|PubMed:34630942, ECO:0000269|PubMed:8101838, ECO:0000269|PubMed:8340364, ECO:0000269|PubMed:8910405}. This is the function of Ensembl gene identifier ENSG00000115902 (SLC1A4, solute carrier family 1 member 4).