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

In animals ranging from humans to honeybees memories exist in different temporal phases. One system that has been particularly useful for studying the cellular and molecular mechanisms underlying different memory phases is the siphon and tail withdrawal reflex of Aplysia. This reflex exhibits short-term (minutes) and long-term (days) memory for sensitization. These temporal domains of memory correspond well with short-term facilitation (STF: <15 min) and long-term facilitation (LTF: >24h) induced by serotonin (5HT) at tail sensory-motor (SN-MN) synapses in the CNS. In addition to STF and LTF, a novel intermediate-term synaptic facilitation (1-3h) has also been identified (Ghirardi et al., 1995; Mauelshagen et al., 1998). We recently found that there are at least two forms of ITF: (1) Activity-independent ITF requires 5 spaced pulses of 5HT, protein synthesis for induction, and PKA for expression; (2) Activity-dependent ITF requires a single 5HT pulse together with SN activity, does not depend on protein synthesis, and requires PKC for expression. These cellular observations raised the question of whether unique forms of memory might be mediated by these two different intermediate-term processes. We explored this issue in behavioral experiments and have identified two novel forms of intermediate-term memory that correspond directly to the induction and expression requirements of activity-independent and activity-dependent ITF. Thus our initial observations at cellular and molecular levels accurately predicted important features of behavioral memory for sensitization in Aplysia.