DYNAMICS OF INDUCTION AND EXPRESSION OF LONG-TERM SYNAPTIC FACILITATION IN APLYSIA

Serotonin (5HT)-induced short-term facilitation and long-term facilita tion (STF and LTF) of the monosynaptic connection between tail sensory neurons (SNs) and motor neurons (MNs) in Aplysia have been useful in delineating possible cellular mechanisms contributing to short-term an d long-term memory. Previous work from our laboratory showed that LTF can be produced in the absence of STF, suggesting that these processes may be functionally independent. In the present study, we explored th is hypothesis by examining the temporal relationship between STF and L TF. We recorded intracellularly from pairs of monosynaptically connect ed SNs and MNs in isolated pleural-pedal ganglia. In the first experim ental series, we followed the time course of LTF across a 24 hr period after its induction by five applications of 10 mu M 5HT. STF complete ly decayed to baseline several hours before the expression of LTF. Thi s biphasic expression profile of STF and LTF further supports the hypo thesis that LTF is not a simple elaboration of STF. In the second expe rimental series, we monitored the immediate expression of facilitation during and after different numbers of 5HT applications. We identified a rapidly decaying STF (lasting 15-30 min) after one to four pulses o f 50 mu M 5HT and a unique, prolonged intermediate-term facilitation ( ITF; lasting up to 90 min) after five pulses of 50 mu M 5HT. These res ults raise the possibility that STF, ITF, and LTF may reflect componen ts of different memory phases in the intact animal.