Exogenous expression of NR2B blocks an activity-dependent increase in growth rate. A, Images and drawings of representative neurons imaged 1 d after single-cell electroporation. The neurons were imaged before and after a 4 h period in the dark and then after a 4 h period of visual stimulation. Axons present in the images are not shown in the drawings. B, Growth rates in the dark and light of all analyzed cells are shown in gray with the average highlighted in black. C, Average growth rates for the groups of cells imaged in the dark and in the light [dark and light, respectively (in μm): control, 60.99 ± 11.67, 95.27 ± 12.73, *p < 0.05; NR2A, 82.35 ± 17.36, 77.53 ± 14.58; NR2B, 46.98 ± 12.27, 45.92 ± 15.18; n = 15 cells per group]. D, Total branch length additions and retractions in the dark and in the light. The average branch length added by new branches or retracted through the loss of old branches is highlighted in gray. The remaining difference to the total represents the branch length that was added or retracted through extension or shortening of existing branches [dark and light, respectively (in μm): total branch length addition, control, 162.72 ± 10.55, 223.17 ± 21.06; NR2A, 177.59 ± 15.40, 181.56 ± 15.42; NR2B, 151.4 ± 18.61, 174.95 ± 20.63, *p < 0.02; total branch length retraction, control, −172.31 ± 15.4, −204.97 ± 21.92; NR2A, −183.67 ± 17.82, −200.23 ± 18.61; NR2B, −164.04 ± 18.65, −190.47 ± 18.19]. D, Dark, L, light. Error bars are SEM. The Wilcoxon signed-rank test was used to compare growth rates and the Mann–Whitney U test to compare branch length additions and retractions.