  In neurons, neuropeptides are synthesized in the soma and are then
transported along the axon in dense core vesicles (DCVs). DCVs are captured in
varicosities located along the axon terminal called en passant boutons, which
are active terminal sites that accumulate and release neurotransmitters.
Recently developed experimental techniques allow for the estimation of the age
of DCVs in various locations in the axon terminal. Accurate simulation of the
mean age of DCVs in boutons requires the development of a model that would
account for resident, transiting-anterograde, and transiting-retrograde DCV
populations. In this paper, such a model is developed. The model is applied to
simulating DCV transport in Drosophila type II motoneurons. The model simulates
DCV transport and capture in the axon terminals and makes it possible to
predict the age density distribution of DCVs in en passant boutons as well as
DCV's mean age in boutons. The predicted prevalence of older organelles in
distal boutons may explain the "dying back" pattern of axonal degeneration
observed in dopaminergic neurons in Parkinson's disease. The predicted
difference of two hours between the age of older DCVs residing in distal
boutons and the age of younger DCVs residing in proximal boutons is consistent
with an approximate estimate of age difference deduced from experimental
observations. The age density of resident DCVs is found to be bimodal, which is
because DCVs are captured from two transiting states: the anterograde
transiting state that contains younger DCVs and the retrograde transiting state
that contains older DCVs.
