Parallel Molecular Pathways Mediate Expression of Distinct Forms of Intermediate-Term Facilitation at Tail Sensory–Motor Synapses in Aplysia

AbstractThree distinct temporal phases of synaptic facilitation (short-, intermediate-, and long-term) are induced by serotonin (5-HT) at sensory (SN) to motor (MN) synapses in Aplysia. Here, we characterize two mechanistically distinct forms of intermediate-term faciliation (ITF) at tail SN–MN synapses. One form, activity-independent ITF, is produced by five spaced pulses of 5-HT in the absence of SN activity. Its induction requires protein synthesis, and its expression requires persistent activation of PKA but not PKC. The other form, activity-dependent ITF, is produced by a single pulse of 5-HT coincident with SN activation. Its induction does not require protein synthesis, and its expression requires persistent activation of PKC but not PKA. These results demonstrate that SN–MN synapses can exhibit two distinct forms of ITF that are mediated by parallel molecular pathways