NMDA receptor blockade slows dendritic arbor development in simple neurons. Time-lapse in vivoconfocal images of four DiI-labeled tectal neurons from control animals or animals exposed to 100 μm APV, 20 μmCNQX, or 1 μm TTX. Images in the top rowof each set (0h) were collected before drug treatment. Images in the bottom row of each set (4h) were collected after 4 hr in rearing solution (CONTROL) or drug solution, as indicated. Control neurons show increases in branch length and dynamic changes in the branch tip numbers and arrangement. Cells from the treated animals show only modest morphological changes. Growth cones are represented by dotted outlines.