Forskolin pretreatment promotes activity-dependent synaptic growth in Syn(+) but not in Syn(−) preparations. A, Prominent outgrowth in pretreated Syn(+) preparations. Arrows on the right panel (stimulated) indicate new boutons. B, Modest outgrowth in pretreated Syn(−) preparations. C, Forskolin pretreatment induces significant (p < 0.03 per 1-way ANOVA followed by Tukey's post hoc test) increase in synaptic growth in Syn(+) but not in Syn(−) preparations. Data collected from n = 40 Syn(+) and n = 55 Syn(−) forskolin-treated preparations (≥7 larvae per line). Dotted lines correspond to untreated preparations (the same as in Fig. 4D, shown for comparison). D, Cumulative frequency distribution of Syn(+) pretreated preparations is shifted to the right, indicating enhanced outgrowth. The cumulative frequency distributions for Syn(−) are similar for treated and untreated preparations.