The sizes and contents of transmitter-filled vesicles have been shown to vary depending on experimental manipulations resulting in altered quantal sizes. However, whether such a presynaptic regulation of quantal size can be induced under physiological conditions as a potential alternative mechanism to alter the strength of synaptic transmission is unknown. Here we show that presynaptic vesicles of glutamatergic synapses of Drosophila neuromuscular junctions increase in size as a result of high natural crawling activities of larvae, leading to larger quantal sizes and enhanced evoked synaptic transmission. We further show that these larger vesicles are formed during a period of enhanced replenishment of the reserve pool of vesicles, from which they are recruited via a PKA- and actin-dependent mechanism. Our results demonstrate that natural behavior can induce the formation, recruitment, and release of larger vesicles in an experience-dependent manner and hence provide evidence for an additional mechanism of synaptic potentiation.

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Receptor-linked protein tyrosine phosphatases (RPTPs) regulate axon guidance and synaptogenesis in Drosophila embryos and larvae. We describe DPTP52F, the sixth RPTP to be discovered in Drosophila. Our genomic analysis indicates that there are likely to be no additional RPTPs encoded in the fly genome. Five of the six Drosophila RPTPs have C. elegans counterparts, and three of the six are also orthologous to human RPTP subfamilies. DPTP52F, however, has no clear orthologs in other organisms. The DPTP52F extracellular domain contains five fibronectin type III repeats and it has a single phosphatase domain. DPTP52F is selectively expressed in the CNS of late embryos, as are DPTP10D, DLAR, DPTP69D and DPTP99A. To define developmental roles of DPTP52F, we used RNA interference (RNAi)-induced phenotypes as a guide to identify Ptp52F alleles among a collection of EMS-induced lethal mutations. Ptp52F single mutant embryos have axon guidance phenotypes that affect CNS longitudinal tracts. This phenotype is suppressed in Dlar Ptp52F double mutants, indicating that DPTP52F and DLAR interact competitively in regulating CNS axon guidance decisions. Ptp52F single mutations also cause motor axon phenotypes that selectively affect the SNa nerve. DPTP52F, DPTP10D and DPTP69D have partially redundant roles in regulation of guidance decisions made by axons within the ISN and ISNb motor nerves.