Fig. 8. Application of crustacean cardioactive peptide (CCAP) results in a significant increase in the activity of the synaptic endings on the anterior foreguts of early-molt stage larvae. For these experiments the density of FM1-43 loaded puncta was used to estimate the endogenous activity of the presynaptic terminals found on the anterior foregut musculature. (A) Under control conditions there were relatively few FM1-43-labeled puncta found on the esophageal dilator muscles of the anterior foregut of an early-molt larva. (B) However, following a 15 min incubation in 10^–8 mol l^–1 CCAP, there is a dramatic increase in the density of FM1-43-labeled puncta on the muscle. (C) A plot summarizing the FM1-43 results. When loading occurred in control saline the density of FM1-43 puncta on the early-molt stage anterior foreguts was only about a quarter of the value calculated for intermolt larvae. Following exposure to 10^–8 mol l^–1 CCAP for 15 min, the density of labeled puncta on the anterior foregut muscles of early-molt stage larvae was similar to that observed in anterior foreguts isolated from intermolt larvae (P>0.05, unpaired t-test). *Significant difference from both intermolt saline level (P<0.005, unpaired t-test) and from the 10^–8 mol l^–1 CCAP level (P<0.005, paired t-test). Scale bar, 20 µm.