Parallel molecular pathways mediate expression of distinct forms of intermediate-term facilitation at tail sensory-motor synapses in Aplysia

Three distinct temporal phases of synaptic facilitation (short-, intermedia te-, and long-term) are induced by serotonin (5-HT) at sensory (SN) to moto r (MN) synapses in Aplysia. Here, we characterize two mechanistically disti nct forms of intermediate-term faciliation (ITF) at tail SN-MN synapses. On e form, activity-independent ITF, is produced by five spaced pulses of 5-HT in the absence of SN activity. Its induction requires protein synthesis, a nd its expression requires persistent activation of PKA but not PKC. The ot her form, activity-dependent ITF, is produced by a single pulse of 5-HT coi ncident with SN activation. Its induction does not require protein synthesi s, and its expression requires persistent activation of PKC but not PKA. Th ese results demonstrate that SN-MN synapses can exhibit two distinct forms of ITF that are mediated by parallel molecular pathways.