Effects of (S)-nicotine and (R,S)-DHNK on the intracellular D-Ser levels. PC-12 and 1321N1 cells were incubated with increasing concentrations of S-nicotine (0 – 10 μM) (A) or (R,S)-DHNK (0.05 – 0.45 μM) (C) for 36h followed by the determination of intracellular D-Ser content. EC50 values for (S)-nicotine in PC-12 and 1321N1 cells were 1.9 ± 0.5 μM and 1.8 ± 0.2 μM, respectively; IC50 values for (R,S)-DHNK in PC-12 and 1321N1 cells were 0.115 and 0.035 μM, respectively. B, Cells were incubated with either MLA (0.05 μM), (S)-nicotine (10 μM) or the combination of MLA + (S)-nicotine for 12h. Results are shown as average ± SD, where the experiments were done in triplicate on 3 independent days. All of the observed decreases in intracellular D-Ser were significant at p < 0.05.