(A) Images of dendrites from EGFP-expressing neurons at 7–12 DIV before and after the addition of vehicle, MG132, or lactacystin at t = 0 (black arrow). Yellow arrows indicate new spines. (B) MG132 (10 μM) decreased the rate of spine outgrowth (green bar; 34 spines, 5 cells) as compared to vehicle control (black bar; 68 spines, 6 cells; p < 0.05). (C) Lactacystin (10 μM) decreased the rate of spine outgrowth (light green bar; 33 spines, 6 cells) as compared to vehicle control (black bar; 131 spines, 8 cells; p < 0.01). Doubling the concentration of lactacystin (20 μM) inhibited spine outgrowth to a comparable extent (dark green bar; 25 spines, 6 cells; p < 0.001). (D) New spine growth was significantly reduced within 5 min after addition of lactacystin (10 μM) and remained significantly reduced for all subsequent time points (p < 0.05). Error bars represent SEM.