Synaptic vesicle pools and plasticity of synaptic transmission at the Drosophila synapse

Our knowledge on the Drosophila neuromuscular synapse is rapidly expanding. Thus, this synapse offers an excellent model for studies of the molecular mechanism of synaptic transmission and synaptic plasticity. Two synaptic vesicle (SV) pools have been identified and characterized using a fluorescent styryl dye, FM1-43, to stain SVs. They are termed the exo/endo cycling pool (ECP), which corresponds to the readily releasable pool (RRP) defined electrophysiologically, and the reserve pool (RP). These two pools were identified first in a temperature-sensitive paralytic mutant, shibire, and subsequently confirmed in wild-type larvae. The ECP participates in synaptic transmission during low frequency firing of presynaptic nerves and locates in the periphery of presynaptic boutons in the vicinity of release sites, while SVs in the RP spread toward the center of boutons and are recruited only during tetanic stimulation. These two pools are separately replenished by endocytosis. Cyclic AMP facilitates recruitment of SVs from the RP to the ECP. Activation of presynaptic metabotropic glutamate receptors recruits SVs from the RP and enhances SV release by elevation of the cAMP level. Memory mutants that have defects in the cAMP/PKA cascade, dunce and rutabaga, exhibit reduced levels of recruitment of synaptic SVs from the RP to the ECP and have limited short-term synaptic plasticity. SV mobilization between the two pools could be a key step for changes in synaptic efficacy. Since a variety of mutants that have distinct defects in synaptic transmission are available for detailed studies of synaptic function, this direction of approach in Drosophila seems promising.