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D-serine is a physiologic coagonist of NMDA receptors (NMDARs) required for synaptic plasticity, but mechanisms that terminate D-serine signaling are unclear. In particular, the identity of unidirectional plasma membrane transporters that mediate D-serine reuptake has remained elusive. We report that D-serine and glutamine share the same neuronal transport system, consisting of the classic system A transporters Slc38a1 and Slc38a2. We show that these transporters are not saturated with glutamine in vivo and regulate the extracellular levels of D-serine and NMDAR activity. Glutamine increased the NMDAR-dependent long-term potentiation and the isolated NMDAR potentials at the Schaffer collateral-CA1 synapses, but without affecting basal neurotransmission in male mice. Glutamine did not increase the NMDAR potentials in slices from serine racemase knock-out mice, which are devoid of D-serine, indicating that the effect of glutamine is caused by outcompeting D-serine for a dual glutamine-D-serine transport system. Inhibition of the system A reduced the uptake of D-serine in synaptosomes and neuronal cultures of mice of either sex, while increasing the extracellular D-serine concentration in slices and in vivo by microdialysis. When compared with Slc38a2, the Slc38a1 transporter displayed more favorable kinetics toward the D-enantiomer. Biochemical experiments with synaptosomes from Slc38a1 knock-down mice of either sex further support its role as a D-serine reuptake system. Our study identifies the first concentrative and electrogenic transporters mediating D-serine reuptake in vivo In addition to their classical role in the glutamine-glutamate cycle, system A transporters regulate the synaptic turnover of D-serine and its effects on NMDAR synaptic plasticity. SIGNIFICANCE STATEMENT Despite the plethora of roles attributed to D-serine, the regulation of its synaptic turnover is poorly understood. We identified the system A transporters Slc38a1 and Slc38a2 as the main pathway for neuronal reuptake of D-serine. These transporters are not saturated with glutamine in vivo and provide an unexpected link between the serine shuttle pathway, responsible for regulating D-serine synaptic turnover, and the glutamine-glutamate cycle. Our observations suggest that Slc38a1 and Slc38a2 have a dual role in regulating neurotransmission. In addition to their classical role as the glutamine providers, the system A transporters regulate extracellular D-serine and therefore affect NMDAR-dependent synaptic plasticity. Higher glutamine export from astrocytes would increase extracellular D-serine, providing a feedforward mechanism to increase synaptic NMDAR activation., (Copyright © 2020 the authors.)