synj^LY/Df(2R)X58-7 mutant neuromuscular junctions show reliable homeostatic compensation (increase in quantal content) after treatment with philanthotoxin-433 for 10 minutes.

synj^LY/Df(2R)X58-7 mutants exhibit structural defects in synaptic growth. These mutants exhibit on average approximately 12.2 satellite boutons per synapse, compared to on average approximately 3.1 in controls. This 4-fold increase is statistically significant. The satellite boutons are capable of activity-dependent exocytosis and endocytosis of synaptic vesicles.

synj^LY/Df(2R)X58-7 mutant synapses exhibit a 22% increase in active zones compared to controls. Active zones are about 50% more densely spaced over the plasma membrane of synj^LY/Df(2R)X58-7 mutants than contains. The active zones did not seem to cluster in the mutants, but appeared spaced to maximize their spacing distance. In wild-type terminals, most active zones contain a single dense body. In synj^LY/Df(2R)X58-7 synapses, there are, on average, more dense bodies per active zone. This difference is attributable to more synj^LY/Df(2R)X58-7 active zones having two dense bodies and fewer having one or none. Roughly 10% of synj^LY/Df(2R)X58-7 active have three dense bodies, a phenomenon not encountered in wild-type boutons. Commensurate with their increased number, the overall crowding of dense bodies is about twice as high in synj^LY/Df(2R)X58-7 terminals as in wild-type ones. There is a tendency towards clustering of dense bodies within individual mutant active zones, although the active zones themselves are randomly distributed.

synj^LY photoreceptors differentiate correctly and form structurally normal synapses in the lamina. However, the photoreceptor terminals contain synaptic vesicles that are more clustered than in wild-type terminals.

synj^LY/Df(2R)X58-7 neuromuscular varicosities have fewer synaptic vesicles than controls. Normal-amplitude synaptic responses can be evoked, although there is a slight decrease in miniature excitatory junction potential (mEJP) frequency compared to controls. In synj^LY/Df(2R)X58-7 synaptic terminals, some vesicles close to the T bar ribbon of an active zone are of an unusually large diameter and vesicles are generally more clustered than in controls.

Stimulation of synj^LY/Df(2R)X58-7 larvae at 10 Hz for 10 minutes results in the release of a lower total number of quanta compared to wild-type larvae (~100,000 vs ~350,000 quanta). Like wild-type larvae, synj^LY/Df(2R)X58-7 larvae show a two step decline of EJP amplitudes during this time, but the second phase is more pronounced and rapid in the mutants. Monitoring of the recovery of the vesicle pool following stimulation shows that synj^LY/Df(2R)X58-7 vesicle pools are able to recover, but at a much slower rate than wild-type pools; responses in wild-type larvae are ~92% within 10 seconds of the prestimulus amplitude, while responses in mutant larvae are ~68% after 10 minutes.

The mEJP amplitude distribution is higher in synj^LY/Df(2R)X58-7 larvae immediately following a tetanus challenge than in wild-type larvae. After a 10 minute rest, synj^LY/Df(2R)X58-7 amplitudes are even larger due to the appearance of a subpopulation of particularly large mEJPs.

synj^LY/Df(2R)X58-7 synaptic vesicles are able to load lipophilic dyes, but at a greatly reduced rate compared to wild-type vesicles, indicative of slowed endocytosis. Endocytotic rates increase from 1 to 5 Hz in synj^LY/Df(2R)X58-7 synapses, but are saturated by ~7 Hz stimulation, with synapses consistently recycling ~80 vesicles/s from 7 to 20 Hz. In contrast, endocytotic rates increase from 1 to 20 Hz stimulation in wild-type synapses with recycling up to at least 360 vesicles/s.