Differential induction of long-term synaptic facilitation by spaced and massed applications of serotonin at sensory neuron synapses of Aplysia californica

Serotonin (5HT)-induced facilitation of synaptic transmission from tail sen sory neurons (SNs) to motor neurons (MNs) in the marine mollusc Aplysia pro vides a cellular model of short- and long-term memory for behavioral sensit ization of the tail withdrawal reflex. Synaptic facilitation at these synap ses occurs in three temporal phases: short-term (STF, lasting minutes), int ermediate-term (ITF, lasting more than an hour), and long-term (LTF, lastin g >24 hr). STF, ITF, and LTF differ in their induction requirements: A sing le brief exposure of 5HT induces STF, whereas five applications are require d for ITF and LTF. Moreover, STF and LTF can be induced independently. Different forms of memory often show differential sensitivity to the patter n of training trials. To begin to explore this effect at a cellular level, we examined ITF and LTF induced by one of two patterns of 5HT application: a spaced pattern (five 5-min exposures with an interval of 15 min) or a mas sed pattern (one continuous 25-min application). The spaced and massed patt erns both induced ITF; however, spaced 5HT application was significantly mo re reliable at inducing LTF than was massed application. Thus, whereas indu ction of ITF and LTF require similar amounts of 5HT, the cellular mechanism s underlying the induction of LTF are more sensitive to the pattern of the induction trials. In the massed group, further analysis revealed a relation ship between the expression of ITF and the subsequent expression of LTF, su ggesting that these two processes may be mechanistically related.