Presynaptic morphological changes associated with long-term synaptic facilitation are triggered by actin polymerization at preexisting varicosities

Morphological changes are thought to contribute to the expression of long-t erm synaptic plasticity, a cellular basis for learning and memory. The mech anisms mediating the initiation and maintenance of the morphological change s are poorly understood. We repeatedly imaged the axonal arbors of mechanos ensory neurons of Aplysia as they formed new synaptic varicosities and axon al branches after applications of serotonin that cause long-term synaptic f acilitation. New varicosities formed exclusively from preexisting varicosit ies, by splitting or branch outgrowth. These changes were prevented by cyto chalasin D, which blocks actin polymerization and the turnover of actin fil aments. The suppression of the morphological changes by cytochalasin D did not impair their expression when cytochalasin D was removed 24 hr after exp osure to serotonin. These results imply that serotonin induces persistent e ffects at preexisting presynaptic varicosities, which enhance actin polymer ization, and that this is essential for presynaptic morphological changes o f long-term facilitation.