The neuromuscular junctions of Eps-15e75/Eps-15Δ29 third instar larvae show impaired FM1-43FX dye uptake compared to wild type, indicating a defect in synaptic vesicle endocytosis.
The average number of boutons per neuromuscular junction is increased in Eps-15e75/Eps-15Δ29 and Eps-15Δ29/Df(2R)Dll-MP mutant larvae. In addition, the mutant neuromuscular junctions have more boutons with two branches and more boutons with three or more branches than controls.
Spontaneous miniature excitatory junctional potentials (mEJPs) at the NMJ of Eps-15e75/Eps-15Δ29 larvae have a similar amplitude, but higher frequency, than those of controls. When the NMJ of Eps-15e75/Eps-15Δ29 larvae is stimulated at a low frequency, evoked EJPs have a similar amplitude to those of controls. However, under high-frequency stimulation conditions (10Hz for 10 minutes), the mutant NMJs show strong synaptic depression to approximately 40% of the initial EJP amplitude (in contrast to control NMJs, which are able to maintain EJPs at approximately 90% of the initial amplitude).
Homozygous and Eps-15e75/Eps-15Δ29 third instar larvae show severely reduced dye uptake at the neuromuscular junction during nerve stimulation compared to controls, indicating a defect in endocytosis.
The mean vesicle density in Eps-15e75/Eps-15Δ29 larval boutons at rest is not significantly different from that of controls, although there is an increased proportion of abnormally large vesicles or cisternae in the resting mutant boutons. During a 90mM K[+] stimulation, the mutant boutons show a reduction in mean vesicle density compared to controls and there is an increased proportion of large cisternae that are greater than 85nm in size (some of these cisternae appear to be large membrane invaginations which are contiguous with the plasma membrane). When the mutant boutons are allowed to recover for 1 minute after stimulation, the mean vesicle density shows a poor recovery, being reduced compared to resting levels (in contrast to wild-type boutons which recover the vesicle density to resting levels). The size distribution of boutons after a 1 minute recovery from stimulation is normal in the mutants.