Behavioral, cellular, and molecular analysis of memory in Aplysia I: Intermediate-term memory

Serotonin (5HT) induces short-term and long-term synaptic facilitation (STF and LTF, respectively) at sensory neuron to motor neuron (SN-MN) synapses in Aplysia, and these forms of plasticity are thought to contribute to short-term and long-term memory for behavioral sensitization. Recent evidence in Aplysia has identified a third phase of synaptic facilitation - intermediate-term facilitation (ITF) - that is temporally and mechanistically distinct from STF and LTF. Here, we review the findings of recent studies that have examined this unique intermediate-term phase at molecular, cellular, and behavioral levels. The results indicate that, at tail SN-MN synapses, multiple forms of ITF can be distinguished; they are induced via distinct mechanisms and use parallel molecular pathways for their expression. Moreover, we have incorporated the temporal and molecular features of these different forms of ITF at tail SN-MN synapses into behavioral analyses, and found that they accurately predict distinct forms of intermediate-term memory for sensitization of the tail-elicited siphon withdrawal reflex. These findings indicate that different types of experiences engage distinct molecular pathways in the service of memory retention over the same time domain.