Neurons expressing Rab5^{N142I.Scer\UAS} under the control of Scer\GAL4^ppk.1.9 extend axons and major dendritic branches, however, they display greatly simplified dendritic arbors and decreased number of terminal branches compared with that found for wild-type neurons.

After the expression of Rab5^{N142I.Scer\UAS} is driven by Scer\GAL4^hs.PS the number of multivesicular bodies in the somatic domain of photoreceptor cells decreases to approximately 25% of that found in the wild-type, whereas the number of Golgi complexes is not affected. This suggests that Rab5^{N142I.Scer\UAS} functions as a specific inhibitor of endogenous Rab5. In contrast to wild-type flies, when Rab5^{N142I.Scer\UAS} is expressed under the control of Scer\GAL4^ninaE.PH, a number of enlarged synaptic vesicles are distributed throughout the nerve terminal. These structures are mostly spherical, although some have narrow tubular or tubulo-vesicular structures. Enlargement of synaptic vesicles with Rab5^{N142I.Scer\UAS} is markedly enhanced when vesicles are slowly recycled in the dark (under the control of Scer\GAL4^ninaE.PH). Such enhancement is not seen in wild-type. This suggests that Rab5^{N142I.Scer\UAS} causes homotypic fusion of synaptic vesicles, as this would occur more often in a slowly-recycled environment. This suggests that Rab5 on synaptic vesicles contributes to the prevention of homotypic fusion between the vesicles. After in-vitro incubation of Rab5^{N142I.Scer\UAS} synaptic vesicles in cytosol at 37^oC for 15 minutes, vesicle size dramatically increases, demonstrating that synaptic vesicles can be homotypically fused in the presence of Rab5^{N142I.Scer\UAS} protein. Velocity sedimentation analysis on a sucrose gradient also shows the incubation shifts the synaptic vesicles from lower to upper fractions, the latter containing vesicles of larger diameters. Neither alteration in vesicle size nor sedimentation velocity shift occurs in the wild-type fly, suggesting that Rab5 prevents homotypic vesicle fusion. Electroretinograms (ERGs) from Rab5^{N142I.Scer\UAS} flies contain no on-transient response. This indicates there is no synaptic transmission between the photoreceptor cells and the lamina neurons. ERG of the light-adapted mutant fly (exhibiting fast endocytic recycling) show a small off-transient response, which disappears when the fly is dark-adapted. Electron microscopy indicates that Rab5^{N142I.Scer\UAS} flies contain enlarged synaptic vesicles in light, which enlarge further during dark adaptation. These experiments suggest that the enlargement of synaptic vesicles reduces the efficiency of synaptic transmission, although the vesicles are still able to fuse to the presynaptic membrane. In Rab5^{N142I.Scer\UAS} flies, the number of flies exhibiting a stop-walk response is significantly reduced. The stop-walk response can be used to measure the synaptic transmission between photoreceptor cells and lamina neurons. A stop-walk response, where flies suddenly stop walking and exhibit an unusual jump response to a light-off stimulus is closely associated with the amplitude of the off-transient response observed on an electroretinogram. This experiment indicates that the enlargement of synaptic vesicles lowers the efficiency of synaptic transmission.