The effect of L-theanine and S-ketamine on D-serine cellular uptake

Decreased extracellular level of D-Serine (D-Ser), a co-agonist of the N-methyl-D-aspartate (NMDA) receptors was connected to receptor hypofunction in the brain and the related deficit of cognitive functions. Extracellular D-Ser concentration is modulated by ASCT neutral amino acid transporters. L-Theanine (L-Tea), a neutral amino acid component of green tea was reported to improve cognitive functions. We thus intended to investigate the possible inhibitory effect of L-Tea on the D-Ser uptake of SH-SY5Y neuroblastoma cells, which was previously found as a good model of D-Ser transport into astrocytes. Cells were incubated with D-Ser and various concentrations of L-Tea or the reference compound S-ketamine (S-Ket). The effect on the uptake was assessed by measuring the intracellular D-Ser concentration using a capillary electrophoresis – laser induced fluorescence detection method. L-Tea competitively inhibited D-Ser uptake into SH-SY5Y cells with an IC50 value of 9.68 mM. Having previously described as an inhibitor of ASCT-2 transporter, S-Ket was intended to be used as a positive control. However, no acute inhibition of D-Ser transport by S-Ket was observed. Its long-term effect on the transport was also examined. No significant difference in D-Ser uptake in control and S-Ket-treated cells was found after 72 h treatment, although the intracellular D-Ser content of the 50 μM S-Ket pre-treated cells was significantly higher. L-Tea was found to be a weak competitive inhibitor of the ASCT transporters, while S-Ket did not directly affect D-Ser uptake or modify the uptake kinetics after a long-term incubation period. © 2020 The Author(s)