Elevations in apparent D -serine detected by neuronal bioassay. Primary hippocampal neurons were monitored for [Ca2+]i during the application of conditioned medium (CM) from microglia (for the period indicated by the lower bar). DCKA (100 μM) was added as indicated by the bar thus labeled. A. Microglia were cultured 20 h in the absence (evenly dashed line) or presence of Aβ1–42 for 20 h. CM from Aβ-treated cultures was added to the neurons at either a 1:100 or 1:18 dilution. B. Microglia were cultured 20 h in the absence (dashed line) or presence (solid line) of 300 ng/mL LPS. Both samples were added to neurons at a dilution of 1:10. C. CM from Aβ-treated cultures was incubated with DAAOx or a control buffer, then applied to neurons at a 1:18 dilution. Similar results were obtained with conditioned medium from LPS-stimulated microglia.