Behavioral sensitization in the mollusk Aplysia is mediated in part by serotonin (5HT). 5HT facilitates transmission at sensorimotor synapses by mobilizing protein kinases A and C (PKC). Imaging experiments show that 5HT translocates the non-calcium-dependent PKC isoform Apl-II, but not the calcium-dependent Apl-I, from cytosol to membranes in living sensory neurons (Zhao et al., SFN abst., 2004). Consistent with this difference, introduction of a dominant negative form of Apl-II, but not of Apl-I, into sensory neurons inhibits 5HT-induced facilitation under some conditions. To measure Apl-I tranlsocation we used a fluorescent Apl-I-EGFP construct. In SF9 cells only very high concentrations of diacylglycerol (DAG) translocated Apl-I, while low concentrations translocated Apl-II. However, calcium greatly decreased the requirement for DAG, suggesting that Apl-I could be a sensor of the combination of DAG and calcium. We tested this hypothesis by imaging Apl-I-EGFP expressed in sensory neurons. The cellular d...