Molecular mechanisms underlying a unique intermediate phase of memory in Aplysia

Short- and long-term synaptic facilitation induced by serotonin at Aplysia sensory-motor (SN-MN) synapses has been widely used as a cellular model of short- and long-term memory for sensitization. In recent years, a distinct intermediate phase of synaptic facilitation (ITF) has been described at SN- MN synapses. Here, we identify a novel intermediate phase of behavioral mem ory (ITM) for sensitization in Aplysia and demonstrate that it shares the t emporal and mechanistic features of ITF in the intact CNS: (1) it declines completely prior to the onset of LTM, (2) its induction requires protein bu t not RNA synthesis, and (3) its expression requires the persistent activat ion of protein kinase A. Thus, in Aplysia, the same temporal and molecular characteristics that distinguish ITF from other phases of synaptic plastici ty distinguish ITM from other phases of behavioral memory.