Try to think in pinocitosys, even in the axon there is no naked neural memebarne, glia always surrround it. I have no experiencen with the subject but with proteins, they enter via pinocytosys and from there can get to the neurons.

This is rather a complicated question. Just to clarify that retro-beads are typically made of a synthetic aromatic hydrocarbon polymers, i.e. essentially they are plastic beads.

With their typical diameter of 0.02-0.2 micron, they can enter into neurons through several pathways, including synaptic vesicle cycle as well as via membrane damage induced by injection. Particles such as these are typically internalized by cells via pinocytosis, phagocytosis as well as clathrin- and caveole-dependent and independent pathways.

The fact that retro-beads can retrogradely label neurons suggests that they are internalized via mechanisms and pathways mediating the uptake and transport of neurotrophic factors, i.e. receptor independent pinocytosis or caveole dependent endocytosis.

Hope this is helpful. Please also check: Article Circumventing Brain Barriers: Nanovehicles for Retroaxonal T...