Dopaminergic amacrine cells in the retina of the possum, Trichosurus vulpecula

Abstract

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.