Figure 3.
Hippocampal dendrites and synapses develop normally in arg−/− mice through P21 but are reduced as the mice age. A, B, Mean total length (A) and branchpoint number (B) of apical dendrite arbors from WT and arg−/− hippocampal CA1 pyramidal neurons at P18–P21, P30–P31, 6–8 weeks of age (6–8 wk), or 3.5–4 months of age (3.5–4 mo). Error bars indicate mean ± SE. Data from 6- to 8-week-old mice from Figure 1 are used for comparison. Data for P18–P21 mice are as follows: WT, n = 13 mice, 20 neurons; arg−/−, n = 13 mice, 25 neurons. Data for P30–P31 are as follows: WT, n = 5 mice, 11 neurons; arg−/−, n = 4 mice, 11 neurons. Data for 3.5–4 months are as follows: WT, n = 6 mice, 13 neurons; arg−/−, n = 6 mice, 15 neurons. ANOVA for length, p = 0.0043; ANOVA for branchpoints, p = 0.0017; post hoc pairwise Student's t test for significance, *p < 0.05. C, D, Electron micrographs of hippocampal area CA1 from adult WT (C) and arg−/− (D) mice. Dendritic spines (S) are indicated. Scale bars, 200 nm. E, Mean SC-CA1 synapse density (number per square micrometer) in WT and arg−/− mice at P21 and P42. Error bars indicate mean ± SE. Quantitation was performed on the following mice: P21 WT, n = 10 sections, 3 mice; P21 arg−/−, n = 10 sections, 3 mice; P31 WT, n = 27 sections, 2 mice; P31 arg−/−, n = 25 sections, 2 mice; P42 WT, n = 10 sections, 3 mice; P42 arg−/−, n = 10 sections, 3 mice. ANOVA between all genotypes and time points, p < 0.0001; post hoc Student's t test for significance, *p < 0.05.