Fig. 2. Disruption of neuronal F-actin and its effect on GluR1-labeled spines. Neurons were stained at 3 weeks in culture with rhodamine phalloidin to label F-actin (A,C, E, G, I,K, M, O) and with an antibody against GluR1 (B, D,F, H, J, L,N, P). The smaller boxes(C, D, G,H, K, L, O,P) show enlarged regions from the neurons above (arrowheads represent spines). There were many spines with concentrations of both F-actin and GluR1 in control neurons (A–D) and after treatment with 10 μg/ml cytochalasin D for 24 hr (E–H). Although much of the cortical actin was disrupted by cytochalasin D, the spines were still positive for both F-actin (E,G) and GluR1 (F,H). In contrast, after a 24 hr treatment with 5 μm latrunculin A, most of the F-actin was depolymerized (I, K) with a corresponding loss of GluR1-labeled spines (J, L). Some neurons exhibited apparently “deflated” spines after latrunculin A treatment, protrusions close to the shafts lacking F-actin (M, O) but still containing concentrations of GluR1 (N, P). Scale bars, 10 μm.