LONG-TERM SYNAPTIC FACILITATION IN THE ABSENCE OF SHORT-TERM FACILITATION IN APLYSIA NEURONS

Serotonin (5-HT) induces both short-term and long-term facilitation of the identified synaptic connections between sensory and motor neurons of Aplysia. Three independent experimental approaches showed that lon g-term facilitation can normally be expressed in the absence of short- term facilitation: (i) The 5-HT antagonist cyproheptadine blocked the induction of short-term but not long-term facilitation; (ii) concentra tions of 5-HT below threshold for the induction of short-term facilita tion nonetheless induced long-term facilitation; and (iii) localized a pplication of 5-HT to the sensory neuron cell body and proximal synaps es induced long-term facilitation in distal synapses that were not exp osed to 5-HT and had not expressed short-term facilitation. These resu lts suggest that short-term and long-term synaptic facilitation are in duced in parallel in the sensory neurons and that the short-term proce ss, because it is induced and expressed at the synapse, can occur loca lly, but the long-term process, because of its dependence on a nuclear signal, is expressed throughout the neuron.