SHERFF, C.M.*; CAREW, T.J.: Mechanistic analysis of multiple memory phases in Aplysia II: Long-term memory

Two different patterns of 5HT exposure induce different forms of long-term facilitation (LTF, >24h) at sensorimotor synapses in Aplysia. Five spaced 5HT pulses (5x5HT) to both the presynaptic soma and the synapse induces LTF that is independent of postsynaptic protein synthesis, while a single, 25 min continuous application of 5HT to the soma coincident with 5 min synaptic 5HT (“coincident 5HT”) induces LTF that requires postsynaptic protein synthesis (Sherff and Carew, 1999). We have recently found that coincident LTF also requires both presynaptic and postsynaptic RNA synthesis. Because these two forms of LTF involve different cellular mechanisms, in the present study we asked whether they were equally capable of inducing another, intermediate-term form of facilitation (ITF, ~1-3h). Confirming previous results (Ghirardi et al., 1995; Sutton and Carew, 2000), 5x5HT induced ITF. However, we found that coincident 5HT did not induce ITF, demonstrating that LTF can be induced independently of ITF. Next we searched for the site of ITF induction by 5x5HT. To explore this question we perfused 5x5HT either to the presynaptic soma or to the synapse alone. We found that while LTF was induced by both protocols, ITF was only induced by synaptic 5HT but not by somatic 5HT. These results further dissociate the induction of ITF and LTF since somatic 5HT does not induce ITF but is known to induce LTF (Emptage and Carew, 1993), and they raise interesting predictions for the induction of different forms of long-term memory in Aplysia.