Fig. 4. The effect of activin on spine morphology does not require de novo protein synthesis or RNA synthesis. (A) The effect of cycloheximide on spine lengthening induced by activin (100 ng/ml). Cultured hippocampal neurons were treated with the indicated compounds for 6 hours. (B) The protein-synthesis inhibitors cycloheximide (3 µM) and emetine (0.1 µM), and the RNA-synthesis inhibitor actinomycin D (1 µM), did not have a significant effect on average spine length. Cultured hippocampal neurons were treated with the indicated compounds for 6 hours. The average spine length (µm) of control neurons was 1.49 (cycloheximide), 1.58 (emetine) and 1.58 (actinomycin D). Control versus experimental groups: ^*P<0.05; ^**P<0.01. The total numbers of spines and neurons that were measured in each experiment were: >700 and 8 (cycloheximide), >600 and 6 (emetine), and >500 and 6 (actinomycin), respectively. These experiments were carried out five times (cycloheximide), three times (emetine) and twice (actinomycin), and representative data sets are shown.