GENETIC DISSECTION OF STRUCTURAL AND FUNCTIONAL COMPONENTS OF SYNAPTIC PLASTICITY .2. FASCICLIN-II CONTROLS PRESYNAPTIC STRUCTURAL PLASTICITY

Increased neuronal activity (eag Shaker mutants) and cAMP concentratio n (dunce mutants) lead to increased synaptic structure and function at the Drosophila neuromuscular junction. Here, we show that the increas e in synaptic growth is accompanied by an similar to 50% decrease in s ynaptic levels of the cell adhesion molecule Fasciclin II (Fas II). Th is decrease in Fas II is both necessary and sufficient for presynaptic sprouting; FasII mutants that decrease Fas II levels by similar to 50 % lead to sprouting similar to eag Shaker and dunce, while transgenes that maintain synaptic Fas II levels suppress sprouting in eag Shaker and dunce. However, FasII mutants that cause a 50% increase in bouton number do not alter synaptic strength; rather, evoked release from sin gle boutons has a reduced quantal content, suggesting that the wild-ty pe amount of release machinery is distributed throughout more boutons.