The amplitude of excitatory junctional potentials (EJPs) in lap¹/lap^sd-3 mutants is drastically reduced relative to controls.

When larvae expressing lap^{N.4C.T:Zzzz\FLAG,T:Zzzz\TC} in a lap¹/lap^sd-3 mutant background are pretreated with FlAsH-FALI and photoinactivated to inactive lap, they show synaptic vesicle endocytosis defects (reduced dye internalisation) in the NMJ when stimulated with potassium. Unlike in wild type and unactivated controls, only small punctae of dye are detected within the synaptic boutons. Inactivation of lap has no effect on synaptic vesicle exocytosis or transmitter release. When lap is inactivated just prior to potassium stimulation boutons are seen that display acute synaptic membrane protrusions ("pseudopodia"), and these are not observed in controls. Normal sized synaptic vesicles are rarely observed and two categories of phenotype are seen 1) boutons packed with multiple intermediate-sized cisternae or 2) boutons exhibiting massive synaptic vesicle depletion, but containing very large cisternae and membrane infoldings. Large balloon-like membrane infoldings that remain contiguous with the plasma membrane and may also contain smaller vesicle-like structures, are also seen.

Mutant neuromuscular junctions show reliable homeostatic compensation (increase in quantal content) after treatment with philanthotoxin-433 for 10 minutes.

lap¹ mutant third instar larvae have 12.09 satellite boutons per synapse, a 4-fold increase over wild-type.

lap¹ type 1b boutons have larger synaptic vesicles and larger quantal size than wild type. However, quantal variance is not significantly different.

Homozygotes die during the early larval stages, with few pupal or adult escapers. Third instar larvae are severely uncoordinated and sluggish. Adult escapers do not walk properly and die within a few days. Synaptic vesicle endocytosis is impaired in homozygous larvae. The amplitude of excitatory junction potentials at the larval neuromuscular junction is reduced compared to controls. Synaptic vesicle density at the neuromuscular junction in homozygous or hemizygous larvae is reduced to approximately one-third of that seen in control larvae and most synaptic vesicles are located near the peripheral area in proximity to the presynaptic membrane. Cisternae are readily visible and their number is significantly increased compared to wild-type. The size of the synaptic vesicles is variable and enlarged compared to wild-type. Quantal size or miniature junctional current amplitude is more variable and significantly larger than in controls.