D -serine levels in microglial culture medium measured by HPLC. A. Chromatographic separation of amino acid standards. 1: L-Asp, Rt = 22 min; 2: L-Glu, Rt = 24.7'; 3: L-Ser, Rt = 26.2'; 4: D-Ser, Rt = 27.8'; 5: L-Gln, Rt = 29.3'; 6: Gly, Rt = 30.9'; 7: L-Arg, Rt = 33.2'. B. Chromatographic separation of actual microglia-conditioned medium. C. Primary microglia were incubated 20 h with no addition (Con) or 15 μM Aβ1–42. Tracings are shown for aliquots of media from duplicates of each treatment. D. D-serine values are represented as the mean ± SEM of triplicates (*p < 0.01), and results are representative of three separate experiments.