The common brush-tailed possum Trichosurus
vulpecula is a small diprotodont marsupial common
to both urban and natural environments. This is the first
analysis of the neurotransmitter content of its retinal
cells and, as the possum is a nocturnal forager, it was
appropriate to begin with the dopaminergic amacrine cells
that form an essential link in the modulation of the rod
pathways subserving nocturnal vision. These results were
compared with those from another diprotodont, the marsupial
wallaby or quokka (Dann, 1996) to establish whether the
dopaminergic systems were similar between these two diprotodont
marsupials and also to compare these findings with those
of other mammals. This study describes a population of
amacrine cells in the possum retina that were immunolabelled
with an antibody raised against tyrosine hydroxylase (TH).
These TH-immunoreactive (IR) cells were located within
the inner nuclear layer (INL) and their dendrites predominantly
ramified within the most sclerad layers of the inner plexiform
layer (IPL). The TH-IR amacrines formed a sparse cell population,
of around 2400 cells, distributed over the entire retina.
There was little evidence of a concentration gradient except
for a slight elevation in density in the naso-temporal
axis in dorsal retina. The formation of rings within the
dendritic plexus, a feature common to TH-IR cells in other
species, was also present in the possum and these appeared
relatively frequently. This latter finding was rather unexpected
since, in the marsupial quokka (Dann, 1996), the TH-IR
dendrites formed fewer rings despite having the same density
of TH-IR amacrines as the possum. This suggests that there
may be subtle differences in the way the rod pathways are
interconnected even within the same marsupial group and
may also be a reflection of relative rod dominance across
species.