Developmental dissociation of serotonin-induced spike broadening and synaptic facilitation in Aplysia sensory neurons

In sensory neurons (SNs) of adult Aplysia, serotonin (5-HT)-induced spike b roadening has long been implicated as important for synaptic facilitation [ spike duration-dependent (SDD) facilitation], particularly at nondepressed synapses. At depressed synapses, spike broadening has less impact on synapt ic facilitation; under these conditions, 5-HT induces a spike duration-inde pendent (SDI) form of facilitation (Byrne and Kandel, 1996). it has been di fficult to dissociate clearly the cellular mechanisms underlying these two forms of facilitation. However, the observation that a major form of spike broadening emerges late in juvenile development (Marcus and Carew, 1998) pr ovides a unique opportunity to examine the relationship between spike broad ening and synaptic facilitation in juvenile Aplysia. We have identified thr ee forms of synaptic plasticity in juvenile Aplysia: homosynaptic depressio n, SDD facilitation, and SDI facilitation. We show that homosynaptic depres sion is fully developed in the juvenile and that 5-HT reliably induces syna ptic facilitation at depressed synapses. However, in nondepressed synapses, 5-HT-induced facilitation is not reliable.