  A model simulating transport of dense core vesicles (DCVs) in type II axonal
terminals of Drosophila motoneurons has been developed. The morphology of type
II terminals is characterized by the large number of en passant boutons. The
lack of both scaled up DCV transport and scaled down DCV capture in boutons
results in a less efficient supply of DCVs to distal boutons. Furthermore, the
large number of boutons that DCVs pass as they move anterogradely, until they
reach the most distal bouton, may lead to the capture of a majority of DCVs
before they turn around in the most distal bouton to move in the retrograde
direction. This may lead to a reduced retrograde flux of DCVs and a lack of DCV
circulation in type II terminals. The developed model simulates DCV
concentrations in boutons, DCV fluxes between the boutons, age density
distributions of DCVs, and the mean age of DCVs in various boutons. Unlike
published experimental observations, our model predicts DCV circulation in type
II terminals after these terminals are filled to saturation. This disagreement
is likely because experimentally observed terminals were not at steady-state,
but rather were accumulating DCVs for later release. Our estimates show that
the number of DCVs in the transiting state is much smaller than that in the
resident state. DCVs traveling in the axon, rather than DCVs transiting in the
terminal, may provide a reserve of DCVs for replenishing boutons after a
release. The techniques for modeling transport of DCVs developed in our paper
can be used to model the transport of other organelles in axons.
