Phenylglycine analogs are inhibitors of the neutral amino acid transporters ASCT1 and ASCT2 and enhance NMDA receptor-mediated LTP in rat visual cortex slices
The N-methyl-D-aspartate receptor (NMDA) co-agonist D-serine is a substrate for the neutral amino acid transporters ASCT1 (SLC1A4) and ASCT2 (SLC1A5). We identified L-phenylglycine (PG) and its analogs as inhibitors of ASCT1 and ASCT2. PG analogs were shown to be non-substrate inhibitors of ASCT1 and ASCT2 with a range of activities relative to other amino acid transport systems, including sodium-dependent glutamate transporters, the sodium-independent D-serine transporter asc-1 and system L. L-4-chloroPG was the most potent and selective ASCT1/2 inhibitor identified. The PG analogs facilitated theta-burst induced long-term potentiation in rat visual cortex slices in a manner that was dependent on extracellular D-serine. For structurally-related PG analogs, there was an excellent correlation between ASCT1/2 transport inhibition and enhancement of LTP which was not the case for inhibition of asc-1 or system L. The ability of PG analogs to enhance LTP is likely due to inhibition of D-serine transport by ASCT1/2, leading to elevated extracellular levels of D-serine and increased NMDA receptor activity. These results suggest that ASCT1/2 may play an important role in regulating extracellular D-serine and NMDA receptor-mediated physiological effects and that ASCT1/2 inhibitors have the potential for therapeutic benefit.